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Russian Chemical Reviews 70 (5) 357 ± 372 (2001) Chemical chromaticity: potential of the method, application areas and future prospects VMIvanov, O V Kuznetsova Contents I. Introduction II. The main chromaticity characteristics and relationships between them III. The theoretical aspects of the chemical chromaticity method IV. Practical applications of the chromaticity method V. Conclusion Abstract. in method chromaticity the of application the on Data Data on the application of the chromaticity method in chemical analysis and in various branches of industry are sur- chemical analysis and in various branches of industry are sur- veyed. The potential of the method is demonstrated and the veyed. The potential of the method is demonstrated and the prospects of its further development are discussed.The biblio- prospects of its further development are discussed. The biblio- graphy includes 196 references graphy includes 196 references. I. Introduction Chromaticity is a science that deals with methods for colour measurement and its quantitative characterisation. The chroma- ticity method involving calculation of colour characteristics from the available spectral parameters of a test sample makes it possible to both differentiate between spectrally similar compounds and obtain additional information about them. In view of this, researchers are interested in the informative potential of the method, its field of application and limitations. A great number of studies on the elaboration of visual detection systems and image processing have been published to date; most of these are comprehensive specialised monographs that mostly deal with the traditional methods for colour measure- ment and data processing.1± 5 However, so far this perfect mathematical apparatus has been little employed in analytical chemistry, although there are a number of computerised spectro- scopic analytical procedures that use colour measuring systems.4 The appearance of a characteristic colour upon a chemical reaction is normally used in qualitative analysis for the detection of elements.It is well known that our vision is very sensitive to small absolute light intensity and distinguishes hues rather well. Not only can an eye compare two radiations but it can also determine which one is more (or less) intense.Colorimetric systems are designed and perfected for the quantitative evaluation of radiation. For some time, their practical use was hindered by the complexity of the hardware required. The need for determi- nation of the compositions of complex dye mixtures used in the textile industry has served as an impetus for the development of the chromaticity method. The solution of this problem has made it possible not only to develop compact light-measuring instruments but also to create a mathematical apparatus for the calculation of VMIvanov,OV Kuznetsova Department of Chemistry,M V Lomonosov Moscow State University, Leninskie Gory, 119899 Moscow, Russian Federation. Fax (7-095) 932 88 46.Tel. (7-095) 939 22 77 Received 16 November 2000 Uspekhi Khimii 70 (5) 411 ± 428 (2001); translated by S S Veselyi #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n05ABEH000636 357 358 360 366 370 concentration dependences from the colour changes in the test samples. There are two basic methods for colour measurement which differ in the calculation procedure: one is based on the spectral composition of radiation and the other is based on the number of individual colours in a mixture resulting in the colour to be determined. According to this classification, all colorimetric instruments and procedures for the calculation of chromaticity coordinates can arbitrarily be divided into two types, depending on the quantity to be measured: 1.The chromaticity coordinates are calculated from the known spectral composition of radiation from a test compound (absorption and reflectance spectra, etc.). This method is believed to be the most accurate.6 2. The measured general function of colour addition (which in many cases corresponds to the sensitivities of photocells) is used to determine directly the chromaticity coordinates of the required component. The development of this method is hampered due to the difficulty of manufacturing photocells with the required spectral sensitivities. The measurement inaccuracy of a colorimetric instrument is predicted as a function of the principle of its operation.7± 10 Chromaticity measurements are successfully used in spectro- photometric titration,11 ± 13 as well as for characterisation of the forms in which indicators exist.14 ± 17 Chromaticity measurements are even more promising in chemical analysis in the absence or with minor use of instrumentation.Owing to the development of visual-detection express methods, the problem arises of determi- nation of the content of a compound from the overall change in the colour of a test sample (the so-called test-methods); this problem can only be solved by chromaticity measurement meth- ods which approximate most exactly the characteristics of human vision. The methods of objective colour measurement with computer data processing are popular in many branches of industry, e.g., in the printing and paint industries,18, 19 textile industry,20 ± 22 glass industry 23, 24 and pharmaceutical industry.25, 26 In addition, col- our is an important parameter of the quality of food.27, 28 There are so many application fields of chromaticity measurement methods that they cannot all be listed.To our knowledge, only one review on the application of the chromaticity method has appeared over the last ten years.14 However, more than 200 publications on various aspects of the method have been published over the same period. These vast literature data are scattered, which hampers the studies, especially for those scientists who just start working in this field. In view of this, the purpose of our review is to discuss the analytical358 capabilities and practical applications of the chromaticity method and to estimate the major trends and prospects in this field.II. The main chromaticity characteristics and relationships between them In essence, spectroscopic methods of analysis involve the detec- tion of optical radiation parameters of a test sample. As a rule, these are the space ± time distributions of radiation, its amplitude, frequency, phase, polarisation and coherence ratio. Measurement of the analytical signal is based on changes in these parameters due to interaction of the optical radiation with the object that is analysed (light absorption, reflection, dispersion) and on changes in the characteristics of the object itself upon exposure to light due to photochromism, electrooptical effects, luminescence and other phenomena.6 The main informative parameters of optical mon- itoring in analytical chemistry include amplitude-related parame- ters, such as optical density (A), diffuse reflection coefficient (R), luminescence intensity (Ilum), etc.Integral characteristics (the total colour differences measured photometrically) are almost never used for this purpose. According to Grassman's laws,1 colour determination requires that three parameters be measured independently; two of them characterise the colour and the third one, the intensity. However, these parameters are used extremely seldom in analyt- ical chemistry because of certain specific features of quantitative measurements. Spectroscopic analysis does not allow direct determination of the chemical composition of a compound.It gives indirect information in the form of a suitable measurable physical quan- tity, such as optical density, diffuse reflectance, etc. An additional independent parameter, such as wavelength or frequency, is introduced in order to describe the spectrum. Chemical analytical techniques are mostly based on the amplitude-related character- istics and the values of these independently measured parameters. A similar approach is used for measurement of chromaticity characteristics.29 The mathematical description of colour in colorimetry is based on the experimental fact that each colour can be represented as a mixture (sum) of certain amounts of three linearly independ- ent colours.2 The basic colours include red (R), green (G) and blue (B), i.e., three monochromatic radiations with wavelengths (l) of 700, 546.1 and 435.8 nm, respectively.30 The colour gamut diagram in this system is shown in Fig.1. The point S1 corresponds to a colour within the colour gamut, while S2 corresponds to a colour outside the gamut. It is most convenient to illustrate this representation geometrically. The three basic colours serve as axes in the orthogonal coordinate system, while any colour determined by three chromaticity coordinates is reflected by a vector S (see Fig. 1) S = RR+GG+BB, (1) where R, G and B are the red, green and blue chromaticity coordinates, respectively; R, G and B are the unit vectors of these colours that are mixed.Connection of the unit basic colour points (R=1, G=1, B=1) with each other gives a triangle located in the unit plane. A remarkable feature of this plane is that the colour vectors having different lengths but the same direction cross it in the same point S, which characterises the colour saturation. In order to establish the position of the point S on the unit plane, the chromaticity coordinates r, g, b are introduced; these are related to the chromaticity coordinates R, G, B as follows: r à R á G á B ; R b à R á G á B . (2) B g à R á G á B ; G VM Ivanov, O V Kuznetsova B S2 S1 G R Figure 1. The RGB three-coordinate colour space.30 It follows from relationships (2) that r+g+b=1, i.e., two chromaticity coordinates are sufficient to determine unambigu- ously the position of a point on the unit plane.The colour perception by humans is taken into account based on experiments with colour mixing. In these experiments, one has to equalise visually pure spectral colours of equal intensity (which correspond to monochromatic lights with various wavelengths) with mixtures of the three basic colours produced by an analytical instrument. To make the comparison, pure spectral light and a mixture of the three basic colours are arranged alongside on two halves of the photometric comparison field. Upon equalisation, the amounts of the three basic colours are measured and their ratios to the unit amounts of these colours are determined. The values obtained are the chromaticity coordinates of the colour that is equalised in the selected chromaticity coordinate system.This procedure does not allow equalisation of the majority of pure spectral colours with mixtures of the three basic colours provided by the instrument. In such cases, a certain amount of one of the basic colours is added to the colour to be equalised, i.e., the chromaticity coordinate is allowed to be negative. Based on the estimates given by several observers, one calculates the averaged amounts of the three basic colours (specific chromaticity coor- dinates) which produce a mixture visually indistinguishable from the pure spectral colour. The graphic plots of the basic colour amounts on the wavelength represent the so-called colour addi- tion curves.Based on these curves, one can calculate the fractions of the basic colours required to obtain a mixture visually indis- tinguishable from a colour with a complex spectral composition. For this purpose, the colour of a complex radiation is represented as a sum of pure spectral colours corresponding to its mono- chromatic components (with account of their intensities). The possibility of such representation is based on one of Grassman's laws, according to which the chromaticity coordinates of the colour of a mixture are equal to the sums of the corresponding coordinates of all of the colours mixed. The addition functions are positive for the colours included in the RGB colour gamut. However, if a colour is outside the gamut, the addition functions become negative, which is inconvenient for calculations. In order to overcome this drawback, the Interna- tional Commission on Illumination (CIE) summarised the known colour properties and formulas for colour variation calculations.As a result, a non-orthogonal three-colour system (XYZ) was created, which describes the spectral properties of a test object as three-dimensional vector coordinates.2 The integral separation of the light radiation intensity into three components is convenient for correlation with human vision (due to the existence of red-, green- and blue-sensitive fibres in the eye retina). The XYZ coordinate system is based on the following assumptions: � the XYZ chromaticity coordinates are always positive for all real colours; �the Y coordinate defines the colour brightness;Chemical chromaticity: potential of the method, application areas and future prospects Y Z X Figure 2.The XYZ three-coordinate colour space.2 (3) (4) (5) � the chromaticity coordinates of white equal-energy radia- tion correspond to the centre of gravity of a triangle located in the unit plane (Fig. 2). The following relationships are used for conversion from the RGB system of basic colours to the XYZ system: X=XRR+XGG+XBB, Y=YRR+YGG+YBB, Z=ZRR+ZGG+ZBB, where XR, XG, XB, YR, YG, YB, ZR, ZG and ZB are coordinates of the basic colours in the three-coordinate RGB space. It should be noted that these values have also been standardised.The chroma- ticity coordinates in the new colour space will have the form x à X X á Y á Z , y à X á Y á Z , Y (6) x+y+z=1. z à X á Y á Z , Z A unit plane is a right-angled triangle called the chromaticity plot; its vertices are located in the intersection points with the vectors of the basic colours. The vectors of spectral colours cross the unit plane along the pure spectral colour line (Fig. 3), which limits the region of existence of the real colours. Y 520S 560 500 W 700 X 380 Figure 3. A unit plane in the XYZ system and the line of pure spectral colours (the solid curve).2 The S andWpoints correspond to the chromaticity coordinates of a given colour and white colour. The numbers indicate the irradiation wave- lengths (l /nm).In addition to the X, Y, Z coordinates, there are other characteristics such as hue [T(l)] and colour purity (P ), which define unambiguously the radiation chromaticity. Obviously, any real colour can be obtained by additive mixing of white radiation with the appropriate monochromatic radiation. The hue is deter- mined by the intersection point of the straight line which passes through the chromaticity coordinates of a given colour and the white colour (see points S andW in Fig. 3) with the pure spectral colour line (Fig. 4). The colour purity shows the ratio in which the monochromatic and white colours are mixed; the purity of monochromatic radiation is equal to unity and that of white radiation is equal to zero.It should be noted that the use of the colour plot in the XYZ system (CIE, 1931) meets certain difficulties. For example, some 359 Y520 530540 510 500 560 570 490 590600690 ± 780 470 X 380 ± 410 Figure 4. A colour diagram in the XYZ system.2 The numbers indicate the irradiation wavelengths (l /nm). distances between pairs of points in the plot (see Fig. 4) do not correlate with the visual perception of humans, since the scale of the plot is non-uniform. In addition, it does not give information regarding the colour lightness. The systems considered above provide only a quantitative estimate of the colour, but one cannot estimate visually the difference between two colours using these systems, since the distances between points in the colour space do not correspond to a difference reliably visible by eye.Several attempts have been undertaken to create such an equicontrast space where the distance between two points would correlate with visible colour variation. However, none of these attempts have been successful to date. The International Commission on Illumi- nation has proposed several quasi-equal contrast systems of this kind; CIELAB (1976) is most popular of these systems. The L, A, B coordinates in this system are related to the X, Y, Z coordinates by the followingionships: (7) L=116 (Y/YW)1/3716, (8) A=500 (X/XW)1/37(Y/YW)1/3, (9) B=200 (Y/YW)1/37(Z/ZW)1/3, where L is lightness;Aand B are the chromaticity coordinates (Ais used for the red-green axis; B, for the yellow-blue axis); XW, YW, ZW are the coordinates of the white colour in the XYZ system; the axes A and B lie in a plane perpendicular to the L axis (Fig.5). The distance between two points in the three-coordinate LAB colour space characterises the total colour difference (DE ) calcu- lated using the formula (10) DE=qÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ ÖDLÜ2 á ÖDAÜ2 á ÖDBÜ2. The hue (T ) and saturation (S ) are determined by the following relationships: S 2=A2+B2, T=arctg(B/A). (11) L a 70 50 b 0 c d A e 750 f 100 0 50 750 B Figure 5. The LAB three-coordinate colour space. The white (a), green (b), grey (c), yellow (d ), purple (e) and blue (f ) regions.30360 The yellowness (G) and whiteness (W) are expressed by the equations (12) 100O1:28X ¢§ 1:06ZU , W=1007DE.Y G a The CIELAB colour space is most suitable for calculations, since equal distances between the points corresponding to differ- ent colours in any part of this coordinate system correlate with visual perception, enabling one to introduce a convenient measure for the quantitative determination of colour differences. In practice, the chromaticity coordinates in the XYZ and CIELAB systems and the colour characteristics of the objects of interest are obtained, e.g., from the diffusion reflectance spectrum, as follows: X a k SOlUxOlUROlUDl , Y a k SOlUyOlUROlUDl , XXl (13) (14) (15) Z a k SOlUzOlUROlUDl , l Xl where k is the normalising coefficient; S(l) is the radiation spectral density from one of the standard light sources A, C or D65 in relative units; x, y and zare the colour addition functions; R(l) is the measured diffuse reflection coefficient.For self-luminous objects: k a SOlUyOlUDl . 100 l X Upon introduction of the factor k, the chromaticity coordinates for non-self-luminous objects are expressed in the scale where Y=100. Thus, chromaticity characteristics are more informative than the spectral ones, although they cannot be used without analysis of diffuse reflection spectra. The use of chromaticity character- istics in analytical chemistry expands the possibilities of studying processes that occur in test samples. The colorimetric instruments and calculation methods created for this purpose make it possible to detect colour changes and to use colour measurements for the analysis of objects.31 ¡¾ 33 III.The theoretical aspects of the chemical chromaticity method The most common task in the broad range of analyses carried out with the use of chromaticity involves the determination of metal ions. It is easy to understand that the essence of the method requires that the metals be transformed into coloured compounds, in most cases into complexes with organic ligands. These com- pounds can be analysed both in solution and in solid state (in the form of sorbates); in the latter case, the detection limit can be lowered by pre-concentration. Metals are mostly transformed into coloured compounds using the classical organic reagents employed in conventional spectrophotometry.Chromaticity stud- ies on the properties of these organic reagents (optical and chromaticity parameters, etc.) is an important but insufficiently studied stage in the development of analytical procedures. 1. Studies of the state of organic reagents in solution and in the sorbent phase Studies of the properties of organic reagents are important for expanding the analytical capabilities of chromaticity.34 In view of this, the optical and chromaticity characteristics of Arsenazo III in solution and in the adsorbed state on an AV-17 anion-exchange resin have been studied. The chromaticity functions (CF) of Arsenazo III, namely the chromaticity coordinates x, y, z in the XYZ system, lightness (L), chromaticity coordinates A, B in the CIELAB system, colour saturation (S ), yellowness (G), hue (T ), as well as colour differences with respect to saturation (DS ), VM Ivanov, O V Kuznetsova lightness (DL) and tint (DT), were measured relative to a white colour standard with a diffuse reflection coefficient R equal to 0.98. It is assumed for an absolutely white body that R=1, W=100, and both DE and G equal zero.The functions R, W, L, X, Y, Z should be lower and DE should be higher for coloured compounds that absorb and reflect light simultaneously; these differences are the more significant the more intense the colora- tion of compounds. In agreement with this, the values of the functions R,W, L, X, Y, Z for sorbates, the coloration of which is more intense than that of solutions due to concentration, are lower, namely, they vary from 20 to 40 (for R in the range 0.2 ¡¾ 0.4), whereas for solutions they vary from 80 to 70 (for R in the range 0.8 ¡¾ 0.7).Of the CF, the yellowness provides the most abundant colour information. For an absolutely white body, G=0; for yellow and red bodies, G>0, while for green and blue ones, G<0. The inflexion points in the plots (Fig. 6) correspond to regions of transition between the forms. Changes in the magnitude and sign of G in the inflexion points can indicate the state of the organic reagent in the test medium. In Arsenazo III solutions at pH<5, the yellowness becomes positive as they contain H8R molecules and ions such as H7R7 (pH<2), H6R27, H4R47 (pH&4.6), which are red.At pH>9.6, the yellowness is negative (710), since the blue ions (HR77 and R87) dominate in the solution. On the contrary, the G values are positive and rather high (from 40 to 120) for all sorbates and reach a maximum (120) at pH 12. It was inferred from these data that only the red ions, most likely the single-charged H7R7 ions, are sorbed on the anion-exchange resin. Sorption of two- and multi- charged ions is hampered or impossible due to steric hindrance. Ivanov et al.34 suggested the use of molar CF coefficients (the ratio of the signal of a given CF to the concentration of the complexing agent expressed in mol litre71) which increase in the series: R<G<B<L, X<S, A<W<DE<Z<Y.It was also found that the molar CF coefficients are 1.5 ¡¾ 2.5 orders higher than the molar absorption coefficients (e). This suggests that the Yellow B a 13 1 12 2 35 10 11 7 3 6 4 25 5 b 7.5 8 6 9 1 15 2 13 41 10 c Green 5 12 8 10 11 11 5 13 12 `Grey point' Red A 0 10 710 Blue Figure 6. Changes in the A and B chromaticity coordinates (CIELAB) of 4-(2-pyridylazo)resorcinol (PAR) (a), 4-(2-thiazolylazo)resorcinol (TAR) (b) and 1-(2-pyridylazo)-2-naphthol (PAN) (c) solutions as functions of the medium acidity.41 Solution concentrations (105c /mol litre71): (a) 5, (b) 1.5, (c) 2.5. Acetone concentration (vol.%): (a) 0, (b) 10, (c) 20; the ionic strength (I) equals 0.1.The numbers indicate the pH of the solutions.Chemical chromaticity: potential of the method, application areas and future prospects use of CF is promising for an increase in the sensitivity of determinations. 2. Studies of acid ¡¾ base properties of reagents In the majority of studies, spectrophotometric methods are used to determine the dissociation constants of organic reagents. In these methods, dependences of optical density on the solution pH are obtained and then processed graphically or by semi- or bilogarithmic computational methods. A correct choice of wave- lengths is very important when measuring the optical density. They normally correspond to the maxima of light absorption by various ionic and molecular forms of the reagents.The situation becomes more complicated if the positions of the maxima differ insignificantly and especially if tautomeric equilibria exist and are imposed on the dissociation equilibria. Changes in the properties of the equilibrium states of a reagent can be recorded using the chromaticity coordinates of the CIELAB equicontrast chromaticity system.30 These values pro- vide data on the integral distribution of light absorption in an equilibrium system tested in a broad range of wavelengths (380 ¡¾ 720 nm). This allows one to avoid inaccuracies associated with a wrong choice of the range of the reagent absorption spectrum where the measurements are carried out.Thus, the reliability of the results obtained increases considerably. Two methods exist for the determination of the equilibrium constants (pKa). 1. Colour saturation (S ) is plotted versus the solution pH at various reagent concentrations, then pKa is determined graphi- cally or calculated using the equation pK (16) OpH2 ¢§ pH1UODS10 ¢§ DS1U a a pH1 a ODS2 ¢§ DS20U a ODS10 ¢§ DS1U , where pH1 and pH2 are pH values around the assumed pK value; DS1, DS2, DS 01 and DS 02 are the changes in saturation depending on whether the equilibrium is shifted from alkaline to acid or from acid to alkaline medium, respectively.35 2. Differential plots of the reagent colour on the medium acidity are used: SCD= DS DpH , where the SCD (specific colour discrimination) parameter char- acterises the change in the colour saturation upon a change in pH.The latter method is more accurate and has proved itself well in titrimetric methods.36 ¡¾ 39 Its application in complexonometric titration is discussed in detail elsewhere.30, 40 Of considerable interest was the use of the differential method for the determi- nation of dissociation constants in spectroscopic analytical methods (diffuse reflection spectroscopy, solid-phase spectro- photometry). Heterocyclic azo compounds, namely, PAN, PAR and TAR, have been chosen,41 since these are well studied as metallochromic indicators. a. Determination of dissociation constants in solutions It is known that PAR, TAR and PAN can exist in three main forms, viz., protonated at the nitrogen atom, neutral (molecular) and anionic (formed due to dissociation of the hydroxy groups) depending on the medium acidity.42 The variations of the chromaticity coordinates A and B (CIELAB) of model solutions of PAR, TAR and PAN at various pH values are shown in Fig.6. It follows from the general character of these dependences that there are discrete pH values where the colour changes proportionally to the content of the form in which the reagent exists in the solution, and the differ- ential plots of the colour of PAR, TAR and PAN versus the medium acidity should contain maxima corresponding to the pK values of the reagents. Differential curves of this type are usually analysed using the SCD parameter.43, 44 Morozko et al.41 made Table 1.Dissociation constants of PAR, TAR and PAN in water ¡¾ acetone solutions determined by chromaticity method and the method of isosbestic points (I=0.1). Constant pKi obtained by Reagent Acetone concentration (vol.%) PAR pKNH pKp-OH pKo-OH TAR pKNH pKp-OH pKo-OH pKNH pKp-OH pKo-OH pKNH pKp-OH pKo-OH PAN pKNH pKOH pKNH pKOH pKNH pKOH 0 b 0 b 0 b 10 10 10 20 20 20 40 40 40 10 10 20 20 40 40 a Reported data;41 b measurements in water. calculations based on the numerical values of colour saturation. The dissociation constants found by the graphical method were found to be close to those obtained by the method of isosbestic points (Table 1).In a number of studies on acid ¡¾ base equilibria in solutions of heterocyclic azo compounds,53, 54 it was noted that the region of existence of the reactive molecular form of an organic reagent expands with a decrease in the dielectric constant (e) of the medium. This statement has been checked by the chromaticity method for model water ¡¾ acetone solutions of PAN, PAR and TAR.41 Figure 7 shows these dependences for TAR. The charac- ter of changes in the pKNH and pKOH constants makes it possible to conclude that as the content of the organic solvent increases, the reaction occurs in a more acidic medium (pKNH decreases with a decrease in e), whereas the stability of the complex increases (pKOH increases with a decrease in e). The effect of the solvent on the pKNH and pKOH values for TAR was demonstrated by measuring the half-widths of SCD peaks (Table 2).The narrower the peak the closer the pK value SCD 11975 1 3 2 2 1 1 3 3 4 Figure 7. Variation of SCD of a TAR solution (c=1075 mol litre71) as a function of acetone concentration and pH.41 Acetone concentration (vol.%): (1) 10, (2) 20, (3) 40. 361 chromati- method of city method a isosbestic points 2.550.15 (see 42) 5.470.09 (see 45) 11.360.08 (see 46) 75.9 (see 47) 9.4 (see 48) 76.150.22 (see 49) 9.680.19 (see 49) 77.330.17 (see 45) 12.320.18 (see 45) 2.320.04 (see 50) 12.00.03 (see 50) 1.900.03 (see 51) 12.220.05 (see 51) 2.090.08 (see 52) 12.63 0.06 (see 52) 2.70.1 5.30.2 11.50.2 1.00.2 5.80.2 9.50.2 1.00.1 6.30.1 9.80.2 0.80.2 7.10.2 12.10.2 2.10.2 11.90.2 1.90.2 12.00.2 1.70.2 12.20.2 1 2 3 8 12 pH362 Table 2.Peak half-widths SCD (Dp0.5Hi) as a function of acetone concentration.41 Parameter Dp0.5Hi at acetone concentrations (vol.%)40 20 10 Dp0.5HNH Dp0.5Hp-OH Dp0.5Ho-OH 0.30.1 0.50.2 0.60.2 1.20.2 1.20.2 1.30.2 0.40.1 1.00.2 1.20.2 found graphically to that found by calculation. The results obtained suggest a sharper colour change of the reagent forms existing in solution upon both the variation of the medium acidity and the increase in the content of the polar solvent. b. Determination of dissociation constants in the sorbent phase The chromaticity approach to the determination of the state of organic reagents has been well proved for solutions.55, 56 On the other hand, studies of protolytic reactions of these organic compounds on sorbent surfaces are no less important for inves- tigation of the properties of the modified sorbents.First, proto- lytic equilibria characterise the reactivities of compounds, because this type of reaction is restricted to a larger extent than, e.g., complexation processes. Hence, the data obtained allow one to estimate rather reliably the reactivities of different functional groups of the sorbent and of the organic compound immobilised thereon. In addition, the knowledge of the acid ± base properties of reagents is necessary for the quantitative description of the stability of complexes on sorbent surfaces. The state of TARin the phases of silica (Silochrom C-120) and an anion-exchange resin with styrene ± divinylbenzene matrix (AV-1768) was studied 41 by diffuse reflectance spectroscopy (DRS) and by solid-phase spectrophotometry (SPS).The TAR dissociation constants have been found using the graphical and chromaticity calculation methods based on Fig. 8: the values of pKNH, pKp-OH and pKo-OH were 1.50.2, 6.60.2 and 10.80.2 for Silochrom C-120 and 1.00.2, 5.70.2 and 9.40.2 for the anion-exchange resin AV-1768, respectively. Let us note that the above values of the TAR dissociation constants on AV-1768 are close to those obtained for solutions.For TAR sorbed on silica, the dissociation equilibrium is shifted to the more alkaline region in comparison with solutions. This can probably be explained by specific interactions in the sorb- ate ± sorbent system and by destruction of the silica skeleton (at SCD 1072 SCD 3 20 1 2 2 2 1 10 1 1 2 pH 0 4 8 12 Figure 8. Variation of SCD of modified sorbents as a function of medium acidity.41 Systems: (1) SG±TAR, c=20 mmol g71 (DRS method); (2) AV-1768± TAR, c=10 mmol g71 (SPS method). Sorbent fractions 200 ± 350 mm. VM Ivanov, O V Kuznetsova pH>10) 57 and possible dissociation of silicic acid (pK1=9.9, pK2=11.8, pK3=13.7).58 It can be concluded that the quantitative description of equilibrium in the sorbent phase is a much more complicated task which usually involves a number of simplifications and assumptions that cannot be verified strictly.In spite of this, studies of equilibria on solid phase surfaces by the chromaticity method not only facilitate experimental procedures but are also of undoubted interest for researchers, as complex-forming sorbents are widely used in analytical chemistry. 3. Increasing the sensitivity of determination of metal ions It is important to know the chromaticity parameters of complexes in order to expand the determination capabilities of chromaticity methods for elements. The use of chromaticity coordinates can increase both the determination selectivity and its sensitivity, as CF vary over broader limits than optical density or the diffuse reflectance coefficient. Complexation of a series of transition metals, viz., CuII, ZnII, NiII, MnII, FeIII, CrIII, CoII, CoIII, with PAN immobilised on silica gel (SG ± PAN) was studied.59, 60 It was found that the chromaticity coordinates of the SG±PAN surface in the air-dry state are strictly constant, viz., A=77.00.5 and B=34.00.5.Basically, the chromaticity coordinates should remain constant. However, since various masking substances (for example, sodium thiosulfate for masking copper and sodium fluoride for masking iron) were added and the medium acidity was changed during the experiments, the initial chromaticity coordinates of SG±PAN changed also. These changes were particularly pronounced in the case of MnII and CrIII, because the determination was carried out at pH 10.0, i.e., near the critical acidity of the sorbent (pH 11.0) where it undergoes decomposition (depolymerisation).Owing to cleavage of hydrogen bonds and desorption of PAN, the chroma- ticity coordinates in the reference experiment are close to the `grey point'. An increase in the absolute content of a metal ion in the solution changes the chromaticity coordinates of the complex proportionally and linearly; the saturation point of SG±PAN remains virtually constant. This means that the saturation point can serve as an individual characteristic for the detection of elements in solution. This point is usually determined using such parameters as the hue and saturation.In this case, the hue allows one to judge to which primary colour the given colour corre- sponds. For example, the hue of zinc is 625 nm, i.e., a red colour. Saturation shows to what degree the hue is expressed in the given colour. The higher the saturation the brighter the colour of a complex adsorbed by SG± PAN. The optimum conditions for the complexation of rare-earth metal (REM) ions, viz., La, Nd, Sm, Tb, Dy, Ho, Er, Yb, with PAR were determined.61 For all of the complexes, correlation equations for the dependence of CF on the REMconcentration in the range of 0.4 ± 2.0 mg ml71 were obtained. As an example, Fig. 9 presents such dependences for erbium pyridylazoresorci- nate; for the other REM studied, similar dependences were observed. It was shown that of all CF studied, the chromaticity coordinate A, the yellowness (G) and the hue (T) have the maximum magnitudes; the use of these quantities allows one to increase the determination sensitivity 35 ± 50-fold in comparison with the sensitivity that can be achieved by spectrophotometry. Unfortunately, no correlations between the optical (such as molar absorption coefficients) or chromaticity characteristics and the properties of metal ions (for example, ionic radii) could be found.61 Later,62 the features of complexation of some REM with organic reagents of various classes were studied.In particular, the complexation of lanthanum, terbium and erbium with 2-(5- bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PAAP) was studied with and without cationic (dodecyltrimethylammonium bromide), anionic (sodium dodecylsulfate) and non-ionic (OP-7) surfactants at various concentrations of organic solvents.The majority of CF depend linearly on the element concentration inChemical chromaticity: potential of the method, application areas and future prospects A, Z,W R510 1 X, Y, L, B, S, T, G 2 5 130 16 3 8 110 0.7 0 90 0.5 6, 7 8 78 9 70 0.3 4 716 50 0.1 30 10 11 cEr /mg per 25 ml 10 20 40 Figure 9. Dependence of the colour functions of erbium 4-(2-pyridyla- zo)resorcinate on the erbium concentration:61 (1) whiteness (W), (2) the chromaticity coordinate A in the CIELAB system, (3) the chromaticity coordinate X in the XYZ system, (4) diffuse reflectance coefficient at l=510 nm (R510), (5) yellowness (G) (6) the chromaticity coordinate B in the CIELAB system, (7) saturation (S ), (8) the L coordinate in the CIELAB system, (9) hue (T), (10) the X coordinate and (11) the Y coordinate in the XYZ system.solution. However, for some functions, e.g., for T, Z, A and W, this proportionality is violated, though the reasons for these anomalies are as yet unclear. The molar absorption coefficients were compared to the molar CF coefficients of the complexes under optimal complexation conditions. The G, S and B values were found to be the most sensitive characteristics. For complexes of lanthanum, erbium and terbium, the molar CF coefficients decrease in the series G>S>B>W>Y>X>Z>L, B>G>S>Y>A>W>X>L and B>S>W>Y>A > X>L, respectively.It was also shown that the use of CF provides an essential increase in the sensitivity of REM determination, viz., by a factor of 25 ± 100 in comparison with photometric methods (cf. Ref. 61). Similar studies were carried out for complexes of erbium with Arsenazo I, Arsenazo III and Chlorophosphonazo III.63 The molar CF coefficients for these complexes decrease in the series G>B>Y>X>Z>L>W>A for Arsenazo I, T>A> S>G>W>X>B>Y>L for Arsenazo III and T>G> S>A>W>B>Y>X for Chlorophosphonazo III. A study on the dependence of colour functions on the optical path length showed that all of them, except hue, change propor- tionally with the increase in the optical path, but the proportion- ality coefficient does not correlate with the molar coefficients of these functions.In addition, the sign of the change in a CF does not always coincide with the sign of its molar coefficient. These anomalies are explained 63 by the existence of complex non-linear relationships between the CF. Thus, the CF should be measured at the same layer thicknesses rather than be referred to 1 cm, as is common in photometry according to the Bouguer ± Lambert ± Be- er law.64 The main chemical-analytical characteristics of com- plexes in solutions used for the determination of metals by the chromaticity method are listed in Table 3.A study of indium complexation with PAR,66 PAN67 and 5-Br-PAAP 68, 69 showed that the CF depend linearly on the In content in the same concentration range (0.07 ± 0.53 mg ml71) as in a study by the DRS method. Eriochrome cyanine R (ECR) was used for the determination of trace amounts of aluminium 70 ± 72 and beryllium 73, 74 by DRS and chromaticity methods. Study on the effect of the sorbent nature on the sorption of complexes showed that the sorption ability of aluminium complexes increases in the following series of sorbents: AV-1768<silica gel<Chromaton-N-Super<cellu- lose. The sensitivity and reproducibility are better for colour functions than for the diffuse reflectance coefficient. The sensitiv- ity of functions in the aluminium determination decreases in the 363 Table 3.Chemical and analytical parameters of complexes in solutions in metal determination by chromaticity (cmin=0.1 mg litre71). Ref. Element pH Reagent CPa range /mg litre71 PAR Arsenazo I Arsenazo III 0.4 ± 2.0 0.4 ± 2.0 0.4 ± 2.0 0.4 ± 2.0 0.4 ± 2.0 0.4 ± 2.0 0.4 ± 2.0 0.4 ± 2.0 0.4 ± 2.4 0.4 ± 1.6 0.025 ± 0.800 0.23 ± 0.90 0.23 ± 0.90 La Nd Sm Tb Dy Ho Er Yb Er Er Chlorophosphonazo III Er b MoVI Mo Lumogallion Magnezone 61 61 61 61 61 61 61 61 63 63 63 65 65 8.1 ± 8.9 8.0 ± 9.0 8.3 ± 9.1 8.0 ± 8.6 8.0 ± 9.0 8.5 ± 9.1 8.0 ± 9.5 7.0 ± 8.0 6.0 ± 8.5 2.5 ± 4.5 2.0 ± 5.0 2.0 ± 3.5 2.0 ± 4.9 aCP is the linear region of the calibrating plot; b cmin=0.01 mg litre71.series G>Y>L>B>W>S>A>R for Chromaton-N- Super and G>Y>B>L>S>W>A>R for cellulose. Beryllium interferes in the determination of aluminium; how- ever, the beryllium ±ECR complex is not sorbed by cellulose, which allows selective determination of aluminium in the presence of beryllium. The detection limit for aluminium with cellulose as the sorbent was found to be 0.004 mg litre71. The procedures developed were used for the determination of aluminium in natural water and in sodium acetate. The complexation of nickel with dimethylglyoxime (DMG) and benzyldioxime (BD) was studied.75 The complexes obtained were sorbed on silica gel. It was found that the optical and chromaticity characteristics form the following series in terms of determination sensitivity for nickel: R<S<A<L<Y for SG± BD± Ni and R<L&S<A<Y&T for SG±DMG± Ni.Thus, the diffuse reflectance coefficient is the least sensitive colour function of coloured immobilised nickel complexes. The highest error of nickel determination is observed if the chroma- ticity coordinate A and saturation are used. The sorption of nickel complexes with DMG and BD on microcrystalline cellulose (MCC) was also studied.76 It was found that the sensitivity of the CF increased as follows: R<B<G<L<S<T<A<W= DE<Y for MCC±DMG±Ni and R<B<S< L<T<A<W=DE<G<Y for MCC±BD± Ni.One can see that if MCC is used, a greater number of the CF depend linearly on the nickel concentration and the order in the sensitivity series changes. Simultaneously, the detection limit becomes lower. The CF of molybdenum(VI) complexes with o,o 0-dihydroxy- azo compounds [lumogallion (LG) and IREA Magnezon (MG)] and with heterocyclic azo compounds (PAR, 5-Br-PAAP) in the presence or in the absence of a third component (hydroxylamine) were determined.65 In all cases, the three-component systems are not inferior, and in most cases superior, to two-component systems with respect to optical characteristics. Microquantities of CoII, NiII, FeII and FeIII in the form of complexes with nitroso-R-salt (NRS) were determined by meas- urement of chromaticity coordinates of a membrane filter after concentration of metal complexes on it; the detection limits were 0.1 ± 0.4 mg.77 This reagent was also used to find conditions for the separate determination of FeII and FeIII ions (see Ref.78). The chemical and analytical characteristics of adsorbed metal com- plexes determined by the chromaticity method are listed in Table 4. Three-coordinate chromaticity was used in a study of the behaviour of the Fe(bipy)2(CN)2 complex (where bipy indicates bipyridyl) in 17 different organic solvents.81 The chromaticity364 Table 4. Conditions for the determination of metal ions in a sorbent phase by chromaticity. Reagent PAN PAR 5-Br-PAAP TANb TAR LG MG DMG BD NRS ECR a Indium oxide was used as the subject of the study; b 1-(2-thiazolylazo)-2-naphtol; c natural water was used as the subject of the study.Reagent structure N N N HO OH N N N HO Br N N NEt2 N HO S N N N HO S OH N N N HO HO SO3H HO N N OH Cl OH HO SO3Na N N Cl Me C NOH NOH Me C Ph C NOH NOH Ph C NaO3S SO3Na OH NO Me Me OH O NaO2C CO2Na SO3H pH Sorbent Element 3.0 ± 5.0 3.0 ± 5.0 5.0 ± 8.0 2.5 ± 4.5 5.0 ± 6.5 4.5 ± 4.8 5.0 ± 7.5 6.0 ± 9.0 3.0 ± 4.5 7.5 ± 10.0 CoII CoIII CuII CrVI FeIII InIII MnII NiII PdII ZnII SG-120 """"""""" CoIII InIII MoVI MoVI PdII 0.5 ± 1.0 4.8 ± 5.2 4.7 ± 5.8 4.6 ± 6.1 0.5 ± 3.0 """AV-17 SG-120 3.8 ± 5.2 3.5 ± 5.0 5.0 ± 6.0 InIII (see a) InIII MoVI """ 4.0 ± 6.0 3.0 ± 4.5 CoIII PdII "" 4.0 ± 8.0 0.6 ± 1.5 6.5 ± 8.5 5.2 ± 6.8 0.5 ± 2.0 5.6 ± 7.4 CoII CoIII CuII NiII PdII ZnII """""" 2.0 ± 3.5 AV-17 MoVI 2.0 ± 4.9 " MoVI 9.5 ± 10.5 9.5 ± 10.5 MCC SG-120 NiII NiII 7.8 ± 8.8 8.5 ± 10.0 NiII (see c) NiII "MCC CoII FeII FeIII NiII membrane 7 " 7 " 7 " 75.9 ± 6.2 MCC AlIII (see c) cmin /mg litre71 0.04 0.01 1.0 0.1 0.3 0.04 0.05 0.03 0.02 0.03 0.02 0.01 0.03 0.3 0.1 0.0003 0.002 0.04 0.02 0.1 0.005 0.006 0.005 0.005 0.04 0.01 0.1 0.1 0.2 0.6 0.3 0.1 0.1 0.3 0.4 0.2 0.004 VM Ivanov, O V Kuznetsova Ref.CP range /mg litre71 59 79 60 60 60 67 60 60 79 60 0.5 ± 5.0 0.03 ± 0.30 0.5 ± 5.0 5.0 ± 50.0 10.0 ± 100.0 0.2 ± 0.6 5.0 ± 50.0 0.5 ± 5.0 0.06 ± 1.60 5.0 ± 50.0 79 66 65 65 79 0.06 ± 1.00 0.04 ± 0.30 0.1 ± 0.9 0.95 ± 3.80 0.3 ± 2.0 68 69 65 0.008 ± 0.600 0.008 ± 0.640 0.05 ± 0.19 79 79 0.06 ± 1.00 0.3 ± 2.0 80 79 80 80 79 80 0.05 ± 1.00 0.05 ± 1.00 0.05 ± 1.00 0.05 ± 1.00 0.13 ± 1.60 0.15 ± 5.00 65 0.23 ± 0.90 65 0.23 ± 0.90 76 75 0.7 ± 2.5 0.6 ± 3.3 75 76 0.3 ± 1.3 0.3 ± 1.3 77 77 77 77 0.3 ± 3.0 1.0 ± 10.0 1.5 ± 20.0 0.5 ± 5.0 76 0.004 ± 0.040Chemical chromaticity: potential of the method, application areas and future prospects coordinates allow one to compare all spectral changes occurring in a system on going from one solvent to another. In particular, the hue was used as the basic characteristic.Equations relating the T values with the acceptor numbers of the solvents (AN), the Dimroth ± Reichardt parameter (Et) and the Kosover parameter (z) were found: AN=(729.153.68)+(0.200.01)T, T=(151.668.79)+(4.710.30)AN, T=(73.2958.24)+(75.551.22)Et, (17) (18) (19) (20) T=(167.0255.89)+(6.150.77) z. The results obtained make it possible to predict the acid ± base properties of solvents based on the hue. Thus, the chromaticity method provides a considerable increase in the determination sensitivity of microquantities of metals, especially if pre-concentration is used.In view of this, it would be of interest to survey the chromaticity characteristics of complexes used in analytical chemistry, first of all, in photometry. Based on this systematisation, one could try to find regularities of the effect of the reagent structure on chromaticity functions (as has already been made in photometry 42) in order to select the most valuable organic reagents and hence to develop new deter- mination procedures. In our opinion, this task is made much easier owing to the fact that, with rare exceptions, the optimum complexation conditions found by optical methods (including DRS) coincide with the optimum conditions for chromaticity analysis.4. Increasing the determination selectivity of metal ions Photometric methods are promising for the determination of mixtures of elements which form complexes with the same reagent either under different conditions (pH, temperature) or with differ- ent positions of their light absorption maxima. In the latter case, derivative spectrometry or two-wave spectrometry is used. Some- times, this lowers the detection limit;82 the latter can also be lowered by pre-concentration of complexes, including coloured compounds, on various sorbents or by changing the method for signal recording (for example, DRS). The greatest effect is usually achieved by a combination of analytical procedures with selection of the method for signal recording. However, selective signal recording is still an issue of topical interest.On the other hand, the chromaticity method makes it possible to increase the determination selectivity of metal ions. For example, the chromaticity method enables separate determination of microquantities of copper and zinc in water after concentration by sorption on SG± PAN.83 The equations for the calculation of the content of metal ions are derived using vector addition of the analytical signals (21) 2i DAS à i rÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ XDA , where DAi is the colour difference with respect to lightness (DL), saturation (DS ) or shading (DE ). This set of fourth-order equa- tions is solved by the decahotomy method using the SVI.LAB program created by Morozko and Ivanov,83 which allows deter- mination of no more than two metals based on the total change in the chromaticity coordinates of a sample. Quite satisfactory results were obtained; however, determination of copper and zinc should be carried out in separate aliquots of the test solution.In a development of this approach, the possibility of separate determination of cobalt and palladium in mixtures after concen- tration by chromaticity analysis with subsequent calculation of the amounts of these metals using the equations by Ivanov et al.79 was studied. Cobalt and palladium were chosen due to the anomaly of the colours of their complexes with PAN (green to bluish-green) in comparison with complexes of other elements (reddish-violet) rather than by the similarity of their chemical and 365 analytical properties, and also because they can be used to demonstrate the advantages of chromaticity as an analytical method.Due to the closeness of the absorption (reflection) maxima of both complexes, cobalt and palladium cannot be determined separately by the DRS method because of overlap of the signals. Therefore, the effect of the sorption conditions on their chroma- ticity characteristics was studied. Heterocyclic azo compounds immobilised on silica form green complexes with cobalt and palladium which differ in hue (yellow- ish-green and bluish-green, respectively). Recording of the colour change versus the content of metal ions in a solution with the use of the chromaticity coordinates A and B provides the possibility of using these differences for the separate determination of the ions in a mixture by measuring the signal at one wavelength.For example, Fig. 10 shows such dependences for complexes adsorbed on SG± PAN. If the absolute content of a metal ion in the solution (and, hence, in the sorbate) increases, the chromaticity coordi- nates of the sorbate surface change proportionally; the saturation point of the sorbate with respect to each metal does not change upon further increase in concentration of the corresponding ion in the solution. B 50 1 40 30 2 20 10 A 0 720 710 Figure 10. Effect of cobalt (1) and palladium (2) concentration on the chromaticity coordinates A and B of sorbates on SG ±PAN at pH 3.79 The arrows show the increasing metal concentration.The equations derived for the change in the chromaticity coordinates of the sample (Table 5) are used to find conditions for separate determination of metal ions in solutions. It was suggested to use the signal separation factor (a) as a measure of completeness of separation; it is calculated using the following equations: a=|arctgKM17arctgKM2 |, if |arctgKM17arctgKM2|<908; (22) a=18087|arctgKM17arctgKM2 |, if |arctgKM17arctgKM2 |590 8, and KM are the slopes of plots of the chromaticity 2 1 where KM coordinates A and B of the sorbent on the concentration of metal ionsM1 andM2, respectively. The magnitude of a is affected by the error of determination of the analytical signal (chromaticity coordinates).It was found experimentally for the Spectroton colorimeter that separate determination of two metals is satisfactory for a525 8. For all of the sorbents studied, dependences of the chroma- ticity coordinates A and B on the content of cobalt and palladium in solution were obtained and the following signal separation factors were found: SG ±PAN 27 8 SG ±TAN SG ±TAR 15 8 SG ±PAR 21 8 20 8 Sorbent a366 Table 5. Chromaticity coordinates of cobalt(III) and palladium(II) complexes on the sorbent surface. Equation of the sorbent colour change Reagent Ion CoIII TAR TAN PAR B=(280.1)A+(28.30.7) B=(1.80.05)A+(29.50.4) B=(4.70.1)A+(19.60.3) B=(3.90.05)A+(62.30.2) PdII PAN TAR TAN B=(1.150.05)A+(38.50.5) B=(1.10.1)A+(30.10.6) B=(1.50.1)A+(23.70.1) B=(1.20.05)A+(55.60.2) PAR PAN a Saturation was used as the reference experiment. It was concluded on the basis of the data obtained that the separation factor increases on going from thiazole azo compounds to pyridine ones and from resorcinol to naphthol.Only SG±PAN complies with the criterion of satisfactory separation (a525 8). So this sorbent was chosen for the concentration of cobalt and palladium followed by their determination by the chromaticity method. The concentrations were calculated using the following set of equations: DL2S =(a11+b11x1)2+(a12+b12x2)2, (23) DS2S =(a21+b21x1)2+(a22+b22x2)2, where DLS and DSS are the lightness and saturation, respectively, for the sum of the two metals; aij and bij (i, j=1, 2) are the factors of the calibration plot for the dependence of the chromaticity coordinates A and B on the metal ion concentrations (see Table 5); x1 and x2 are the unknown concentrations of the cobalt and palladium ions, respectively.This fourth-order set of equations has four different parame- ters (four x1 and x2 pairs) which, in the general, case satisfy the solution. The sets of equations were solved on a personal computer using the Newton method. The amounts of cobalt and palladium (in 20 ml of solution) were varied from 0.5 to 5.0 mg and from 2.0 to 20.0 mg, respectively. Satisfactory results of separate determination of these metals in model mixtures were obtained; they were close to the data of analysis by the DRS method on the SG±TAR sorbent.Of the greatest interest is the possibility of increasing the selectivity in the determination of metal ions which form reddish- violet complexes, since it is impossible to determine individual metals in a mixture in this spectral region by other methods (DRS, SPS) without masking or separation. TAR was selected as the reagent.80 The possibility of separate determination was studied for copper, nickel and zinc in the presence of iron and cobalt. The optimum conditions for the sorption of these metals are indicated in Table 4. The detectability of colour changes depending on the content of metal ions in solution with the use of chromaticity coordinates A and B implies that it is possible to carry out separate determi- nation of ions in two- and three-component mixtures.Figure 11 shows such dependences for sorbates of complexes on SG± TAR. The data obtained were processed by the least-squares method. Based on the calibration plots for the dependences of the chromaticity coordinatesAand B on the content of copper, nickel, zinc and cobalt, the following signal separation factors were found: Zn ±Ni 37 8 Cu ± Ni 13 8 Cu ± Zn 23 8 M1±M2 aOne can see that the nickel ± zinc pair satisfies the criterion of separate determination. It should be noted that copper interferes in the simultaneous separate determination of nickel and zinc, Range of the sorbent colour change,a (Ai, Bi)7(Aj, Bj) (7.0, 48.0) ± (77.5, 8.0) (70.5, 28.5) ± (714.0, 4.0) (1.3, 25.5) ± (72.5, 8.0) (72.5, 52.5) ± (712.5, 13.0) (7.0, 48.0) ± (718.0, 11.0) (70.5, 28.5) ± (719.0, 9.0) (1.3, 25.5) ± (710.0, 8.5) (72.5, 52.5) ± (714.0, 4.0) B 30 20 5 10 10 0 710 720 Figure 11.Dependence of the chromaticity coordinates A and B of sorbates on SG ±TAR on the content of cobalt(II) (1), nickel (2), copper (3), zinc (4) at pH 7.0 and cobalt(III) (5) at pH 2.0.80 The arrows show the increasing metal concentration. therefore it is masked with sodium thiosulfate. The DL and DS values obtained were used to calculate nickel and zinc concen- trations using the set of Eqns (23). Separate determinations of nickel and zinc in the presence of copper and cobalt were carried out similarly, except that the concentrations were calculated using a modified set of equations: DL2S =(a11+b11x1)2+(a12+b12x2)2+d1 , DS2S =(a21+b21x1)2+(a22+b22x2)2+d2 , where x1 and x2 are the unknown concentrations of nickel and zinc ions, respectively; d1 and d2 are constants describing the contribu- tion of cobalt to the set of equations.The contribution of cobalt was estimated by determining its content at l=610 nm. The procedures described above were used to determine nickel, zinc and cobalt in a standard soil sample SP-3. The above version of chromaticity analysis allows separate determination of 0.03 ± 0.30 mg ml71 cobalt and 0.013 ± 0.130 mg ml71 palladium, as well as 0.05 ± 1.00 mg ml71 nickel and 0.15 ± 5.00 mg ml71 zinc.The advantages of chromaticity for increasing the selectivity of determination of metal ions in two-, three- and four-component mixtures have been shown. IV. Practical applications of the chromaticity method 1. Application of the chromaticity method in chemical analysis Let us dwell on the most interesting practical applications of chromaticity. At present, the test procedures are becoming more and more popular. A test metal ion is usually identified by the colour of a complex it forms with a test reagent.84 In the case of VM Ivanov, O V Kuznetsova Saturation point /mg of ion 10.97 18.00 8.40 18.00 21.10 21.00 13.10 35.60 4 2 3 1 A 30 20 (24)Chemical chromaticity: potential of the method, application areas and future prospects acid ± base indicators, the colour change upon decrease or increase in the medium pH is recorded.General-purpose test paper (pH 1 ± 10) or reactive test papers used for qualitative and semi- quantitative analysis can serve as obvious examples.84, 85 A colour change of the paper (sorbent) after a reaction indicates the presence of the ion in question in the test solution. Only the colour change is usually observed while the laws of this process are not recorded. Construction of calibration test scales for the visual determi- nation using the known dependence of the total colour difference (DE ) on the content of the test element allows minimisation of inaccuracy of a test technique and quantitative description of not only the intensity but also of the colour change.Since one colour discrimination threshold corresponds to one DE unit in the equicontrast colorimetric system (CIELAB), the concentrations of the test element are chosen in accordance with a certain DE step to be determined most precisely by a human eye. Even before the equicontrast CIELAB system had been developed, it was noticed that the difference in intensity of luminous fluxes is felt reliably if they differ by approximately 10% (see Ref. 6). The total colour difference is measured in the range of 0 ± 100 arbitrary units, hence 10% corresponds to a step of DE=10. If colour is to be recognised from the surface of a reactive paper (sorbent) by a human, it is recommended to create test scales with DE510 (see Ref.86). If a test sample loses its own colour or discolours due to the analytical reaction and the colour of the test sample support is close to white (for example, in the case of silica), the test scale should better be constructed using the inverse quantity, viz., the whiteness of the test sample,W=100 ± DE (see Ref. 68). The dependence of the variation of theWand DE parameters of a test sample on the content of the test microelement has the shape of a logarithmic curve.59 Thus, for a constant step DE=10 the concentration of a given microelement in a test sample changes in a geometrical progression, which is often suggested for the construction of a visual test scale.87 In this case, the total colour difference of 10 arbitrary units between the original test sample (E0) and a test sample following reaction of the microelement to be tested (E) is considered as the lowest determination threshold: E7E0=10 (orW7W0=10).It was shown convincingly that the use of heterocyclic azo compounds immobilised on various sorbents makes it possible to achieve low detection limits in sorption-spectroscopic analyses in combination with high sensitivity and good reproducibility of results. In some cases, selective express techniques for the deter- mination of microelements can be created using such reagents. According to the general ideology of test methods for the environmental control, three basic rules were used as a guide in the development of sorption-spectroscopic procedures.86 1.The identification and determination of microelements in the environmental objects should be carried out quickly. This is achieved by comparing the indicator test sample with a standard scale. 2. It is necessary to provide the documentation of the test or the possibility of performing post-instrumental chemical analysis, which is realistic only for stable test samples. 3. The reactions should be contrasting. The chromaticity method allows one to create an equicontrast chromaticity test scale well discernible by the human's eye. Based on the functional relationship between DE and the content of a test element in an object with a colour difference threshold DE510, it is possible to construct a calibration test scale with a minimum error.Therefore, the descriptions of sorption-spectroscopic deter- mination procedures involve the ln(DE)=f(c) dependences in a mathematically processed form. The non-selective colour difference parameter based on light- ness (DL) is applicable for the post-instrumental determination of the content of a microelement in a test sample, provided that the chemical reactions which can be used for the sorption-spectro- scopic analysis occur selectively and the reaction products are 367 extracted quantitatively. The latter can be predicted to a certain extent based on the regularities of the sorption extraction of reagents by sorbent surfaces that we have studied. The potential of the chromaticity method with the use of chromaticity coordinates is demonstrated in this study (Ref.86) for the development of selective sorption-spectroscopic proce- dures for the separate simultaneous determination of cobalt(III) and palladium(II), copper(II) and zinc(II) using TAR immobilised on silica gel (SG ± TAR). The capacity of the sorbent with respect to the reagent determined from saturation curves for metals was 20 mmol g71 (see Refs 79, 80). An advantage of the sorption version of testing is that it allows one to change the sensitivity of determination of a microelement by varying the sample volume, which is impossible if reagent test papers are used. When a coloured compound formed and the sorbent sedi- mented, its colour was compared with the standard calibration colour scale for microelement determination, using the step DE=10 for making the test scale.The sorbates for the test scale were placed in test tubes with a diameter of 4 mm. The test scale obtained in this way is stable for at least five years. 2. Application of the chromaticity method in other fields of science and industry a. Food chemistry The greatest number of studies deal with the use of the chroma- ticity method for monitoring the production of foodstuffs, quality of cultures grown and in wine-making. The effect of the growth conditions (greenhouse or open air) of hot pepper on its chromaticity characteristics 88 showed that the amount of pigments in pepper grown in a greenhouse can be predicted using the chromaticity coordinates A and B and the ratio A/B.For pepper grown in the open air, a similar correlation is observed for the values L and A/L. The CIELAB system was used to estimate the quality of tomatoes,89 tinned mushrooms 90 and potato chips.91 It was found that the angle between the coordinates A and B depends on the colour of vegetables. A similar correlation with the L coordinate is not always observed. In particular, a study on the darkening of cut-off lettuce versus time 92 did not show such a dependence. TheXYZchromaticity system can be used to classify honeys 93 and to estimate the natural sweetener from the cactus tree.94 In this case, some other characteristics (viscosity, glucose content, etc.) were used along with the chromaticity coordinates.The chromaticity coordinates were also used to monitor production of sausages,95, 96 in particular, to study the effect of the content of soybean proteins on the colour characteristics of the product.95 The three-coordinate colour system was used to study the first phase of milk turning 97, 98 and to estimate the colour of milk prepared from powder.99 A theory allowing the preparation of emulsions with certain colour characteristics has been devel- oped.100 In this case, the chromaticity coordinates are calculated from the diffuse reflectance coefficient (R). The theoretical calculations are in good correlation with the experimental results. The variations of the chromaticity characteristics of soybean milk during the grinding of soy beans have also been studied.101 The chromaticity systems CIEXYZ, CIELUV and CIELAB were successfully employed for the characterisation of anthocyans isolated from red grapes,28, 102 raspberry 103 and strawberry.104 The effect of an atmosphere enriched with carbon dioxide on the decomposition of strawberry after harvesting was studied.104 Two hundred and twenty four model solutions of cyanine copigmented with rutin were studied.The procedure is based on measurements of the total colour difference and the copig- ment : pigment ratio. The strongest effect is reached at pH 3 ± 5 and low copigment : pigment ratio (1 : 1).105 The chromaticity system CIELAB is the best to describe the colour changes observable by a human eye.368 The kinetics of colour changes in apples, bananas, beets and potatoes in the course of drying (convective and vacuum drying at 50, 70 and 90 8C) was studied.106 The effect of pasteurisation on the chromaticity characteristics of grapefruit juice 107 and pear juice 108 was studied.It was shown that the strongest changes are observed for the saturation and the chromaticity coordinate B, in particular, this allows one to monitor the decrease in the content of vitamin C and the discol- oration of orange juice during storage at 4 ± 24 8C (Ref. 109). Two reviews are devoted to the spectrophotometry of wines.110, 111 It was noted that the CIELAB chromaticity system is most promising as it allows one to create any colour scales, quickly compare the results of different operators in various flow points and is advantageous as it requires simple equipment.For port as an example, the effects of temperature, pH, sugar content and time on the colour characteristics of wines was studied.111, 112 A simpler method for the calculation of the chromaticity coor- dinates of white, red and pink wines and brandy 27, 33 based on the measurement of the reflectance signal at three fixed wavelengths (440, 540 and 610 nm) was suggested. The method was tested for 690 wine samples. The effect of pH and content of anthocyans on the chroma- ticity characteristics of a number of wines was studied.113 ± 117 Satisfactory correlations were found between the degree of ionisation of anthocyans and the wavelength of absorption maximum (lmax) and between the degree of ionisation of anthocyans and the colour intensity.114 It was noted that a decrease in the medium acidity increases the colour intensity and the degree of ionisation of anthocyans.113 The effect of pretreatment of red table wine with sulfur dioxide on their colour, phenolic composition and quality was studied.118 It was shown that wines untreated with SO2 darken faster than the treated wines.b. Textile industry The chromaticity method is used for the selection of colours in the dyeing of natural and synthetic fibres, for the control of fading of coloured cloths and fibres and for monitoring the formation of brown spots on paper-based materials. The observed colours are produced by subtractive and addi- tive methods.In subtractive synthesis, colour appears due to selective absorption of a portion of radiation from a light beam falling on an object. This occurs upon successive passage of a light beam through a series of differently coloured media (for example, colour films) or upon passage of light through a solution contain- ing several dyes. Subtractive synthesis is the basis of the modern methods of colour photography, cinematography and three- colour printing. Required colours of materials formed by several dyes are obtained using this method. The subtractive synthesis method makes it possible to blend various colours by using triads of dyes whose colours correspond to three linearly independent colours in various ratios.The painting compounds most suitable for these purposes have yellow, light-blue and purple colours.Aparticularly wide scale is obtained in the case where these are `ideal' colours. A characteristic feature of the spectra of such dyes is that radiation of each of the three spectral zones is either absorbed completely or transmitted completely. The dyes that have such spectra are ideal for various processes of colour reproduction and allow one to blend virtually any colour. If real dyestuffs are used, their transmission spectra are characterised by the absence of complete transmission in any of the zones, and the palette of the colours obtained becomes narrower and reproduction of high-purity colour is impossible. Therefore, a number of studies was devoted to the development of dyes the properties of which approached those of ideal dyes.Several regression models for the optimal selection of dyes for various fibres have been developed.20, 22, 119, 120 Three 9-phenylxanthene derivatives were synthesised and used for dyeing polyacrylonitrile and polyamide cloths. All of VM Ivanov, O V Kuznetsova the products were subjected to chromaticity characterisation.121 Aqueous extracts of black tea were used for dyeing cotton and jute fibres.21 The fibre colours were monitored using the CIELAB system. The effect of the solvent system CCl3COOH±CH2Cl2 on the ability of cotton yarn to undergo dyeing was studied. The chromaticity characteristics of dyes were studied by varying the concentration and duration of treatment of yarn with the sol- vent.122 A number of natural dyes were used for the colouring of synthetic fibres (polyester, nylon, acrylic and acetate fibre).123 It was noted that the fibre colour saturation increased with the increase in the treatment temperature and this effect is especially strong in the case of nylon fibres.The dependence of the chromaticity characteristics of nylon fibres on the conditions of its heating was studied.124 The temper- ature was varied from 150 to 180 8C; the time was varied from 2 to 10 h. The resistance of 15 dyes for cotton fibres and their mixtures to fading was studied.125 The samples were irradiated with a xenon arc lamp. It was shown that the fibres dyed with a combination of dyes always faded less than the samples dyed with just one dye. The effect of pH of the starting cochineal solution on the chromaticity characteristics of fibres was studied.126 The pH values were set using solutions of hydrochloric, sulfuric, nitric and phosphoric acids.It was found that the chromaticity coor- dinate B decreases with the increase in pH of the starting solution; in addition, the coordinates L, A, B decrease regularly on storing the solutions from 1 to 21 days. This effect was most pronounced if phosphoric acid was used for setting the pH. New synthetic polyoxyalkylene ± polyurethane water-repel- lent fibres were obtained.127 Polarising fibres from poly(vinyl alcohol) which possess constant light absorption in the visible region were described.128 Such fibres can be used for making liquid-crystal displays and polarising screens.The so-called fluorescent (or optical) whiteners are known, i.e., compounds that absorb radiation with wavelengths of 340 ± 400 nm and transform it into radiation with wavelengths of 415 ± 466 nm (in the visible region of the spectrum). Depending on the wavelength of the absorbed light, fluorescence of different colours is observed, viz., violet (the fluorescence maximum is observed at 415 ± 429 nm), dark blue (at 430 ± 440 nm) and light- blue (at 466 nm). The most popular optical whiteners have light- blue fluorescence (radiation with wavelengths of 400 ± 500 nm). Unlike optical whiteners, fluorescent dyes absorb light in the visible spectrum region, like ordinary dyes.As a consequence of fluorescence of dyes, their brightness increases. Optical whiteners are used like ordinary dyes and are often referred to as white dyes. White paper, cellulose and other cloths and fibres partially absorb light in the shortwave part of the spectrum and thus become yellowish. If optical whiteners are applied on such yellowed objects, the former radiate light with wavelengths of 400 ± 500 nm and hence compensate for the absorbed part of visible light and complement the spectrum of the reflected light. As a result, the degree of whiteness of the objects increases, i.e., an optical (fluorescent) whitening effect is observed. Optical whiten- ers are widely used for whitening natural and synthetic materials (cotton, wool, viscose, hair, silk, cellulose acetates, polyamides, polyurethanes, polyacrylonitrile, polypropylene, polyester), paper, plastics, for improvement of colour of dyed cloths, etc.The CIELAB system was used to study the reasons for the appearance of brown spots on materials based on paper (ancient books, paintings). Optical whiteners are added to washing powders and compo- sitions for dry cleaning. They should have good light resistance, resistance against acids and bases, oxidants and reducing agents and should not form complexes with metals. Bleaching with such substances is similar to dyeing with direct and acid dyes: cellulose fibres are treated with suitable optical whiteners in neutral or weakly alkaline media, while polyamide fibre, silk and wool areChemical chromaticity: potential of the method, application areas and future prospects treated in weakly acidic media.Dispersion whiteners are used for polyester and acetate fibres, and cationic whiteners, for polyacry- lonitrile fibres. The synthesis and properties of some unsubstituted triazinyl- stilbenes as fluorescent whitening agents were described.129 The spectral and thermal characteristics, the whitening effect for cotton and the chromaticity coordinates were determined. Fluo- rescent dyes based on naphthalene-1,8-dicarboxylic anhydride suitable for the dyeing of polyamide fibres and epoxy resins were synthesised. The relationship between the dye structure and the colour characteristics was discussed.130 The effect of fluorescent additives on the colour saturation of fibres was studied.131 For this purpose, a cotton fibre was dyed using eight dye types with five various saturation levels.The fibres obtained were treated with two types of fluorescent additives. It was shown that the addition of fluorescent compounds increased the saturation of red and dark blue hued fibres and decreased that of yellow hued fibres. The total colour difference was used to characterise a coated antibacterial polyester fibre.132 A new composition based on esters was suggested for dry cleaning of cotton cloths.133 The efficiency of laundry disks in the removal of spots from cotton and polyester fibres with the use of detergents in the absence of water was studied.134 It was shown using chromaticity characteristics that these detergents are not superior to the compounds used for `wet cleaning'.c. Geochemistry and mineralogy The CIELAB system allows one to distinguish the hues of precious stones (the alexandrite effect).135 Depending on the hue, precious stones can be grouped as follows: type 1 � alexandrite, fluorite, spinel, tourmaline; type 2 � glass; type 3 � sapphire; type 4 � garnet. Such functions as lightness and saturation can be used for documentary characterisation of the differences observed. The microscopy and photometry of ores over the period of 1890 ± 1998 was reviewed.136 In particular, it was emphasised that identification of ore materials includes plotting of chromaticity diagrams as an obligatory stage.The chromaticity characteristics of a rare sulfide deposit with the composition Cu5FeS6 were obtained.137 The chromaticity coordinates ware applied to determine the nature of the colour of the Mozambiquean beryls. It was found that the green colour appears due to the presence of chromium ions in the aluminium crystal lattice, while the purple colour is due to the presence of manganese ions.138 A method for non-destructive determination of metal corro- sion products with the use of the chromaticity characteristics of the CIELAB system was suggested.139 Studies of alloys based on copper, zinc and aluminium showed that an increase in the content of zinc and aluminium first increases the lightness and yellowness to a certain value and then decreases these values.140, 141 The results obtained have enabled the creation of differently coloured alloys, viz., copperish-red, goldish-yellow, brown, purple and, finally, silverish-grey.The colour characteristics have been utilised for the descrip- tion of protective and decorative coatings from chromium and titanium142 ± 144 and for painted steel panels.145 d. Production of glasses and ceramics The chromaticity characteristics were used to control the colour range of glasses 146 ± 149 and ceramics.150 ± 152 Twenty five antique glass samples were studied. It was shown that the three fragment types, i.e., bluish-green, light-blue and yellowish-green, result from the combinations FeII±FeIII, FeII±CoII and FeII ± FeS, respectively.148 It was suggested to make grey and bronze glasses 149 by using combinations of such additives as Co3O4, Nd2O3 and V2O5, MnO2, Er2O3.The resulting glass colour was monitored using chromaticity characteristics. A series of studies was devoted to the effect of temperature ± time modes on the synthesis of pigments.23, 150 ± 153 It was found 369 that the heat treatment temperature is the most important factor determining the colour of the pigments obtained. An optimal temperature should hinder the formation of colourless or weakly coloured compounds. The formation of significant amount of a glassy phase resulting in dissolution of coloured crystal phases is also undesirable.23, 150 ± 152 A procedure for the preparation of ceramics by coprecipita- tion of hydroxides was suggested.The particle sizes of the resulting polymeric hydroxides are comparable to those of colloid species. Such compounds can be transformed into ceramics of a certain colour at lower temperatures compared to those employed in the oxide technology.152 A study on the effect of temperature on the colour stability of series of differently coloured ceramic samples showed that the yellowish-brown ceramics are most unstable.24 e. Pharmacology and fabrication of cosmetics In the pharmaceutical industry, chromaticity is used as a reliable method to control the production of dental compositions, to check the uniformity of coatings on pills, etc.For example, the hue of silver-containing alloys was made similar to the colour of human tooth enamel by adding cerium dioxide, which discolours silver, to the alloys. The addition of 0.1% CeO2 causes an essential (threefold) discoloration of the alloys without any loss of strength.25 The relationship between the colour of dental cement and paste was studied using chromaticity characteristics.154 Metal ceramics for tooth crowns were created and characterised by chromaticity.155, 156 The value of total colour difference was utilised to control the uniformity of coatings on pills.157 The absorption and diffuse reflection spectra of structural isomers of riboflavin tetrabutyrates were obtained.26 The specific feature of these compounds is that their colour characteristics in the solid state differ depending on the type of the structural isomer, whereas in solution, all of the spectra are identical.This is explained by intramolecular reactions occurring in the solid phase, which result in hydrogen bond formation. The chromatic- ity method allows one to distinguish one isomer from another. Tinted powders for correction of face colour were created; they consist of micas covered with granular metal oxides (tita- nium, iron and their mixtures) and have definite values of the chromaticity coordinates.158 Studies of the reaction of Methylene Blue with natural white mica and with synthetic sodium-type mica 159 showed that Methylene Blue reacts with both types of mica on heating in the absence of a solvent; as a result, the chromaticity coordinates change strongly (the purple colour changes to light-blue).The resulting compounds are water- insoluble due to immobilisation of Methylene Blue on mica. Chromaticity characteristics were used to compare a number ofens.160 f. Electronic industry, preparation of colour coatings, radio- engineering and photo-engineering A series of studies dealt with the development of light-emitting diodes.161 ± 168 It was found that green sources provide a long life of electron-beam tubes and improve the radiating character- istics.163 A chromaticity-based method for the correction of the electron-beam tube characteristics was developed.164 It was suggested to use bathocuproin 161 and triphenyl amine derivatives 169 as blocking layers for organic light-emitting diodes. The diodes were characterised by the chromaticity coordinates X and Y; it was proven on this basis that the source has pure red, dark blue or green radiation.170 ± 172 New sources for three-coordinate chromaticity 173 containing red and green reverse lenses were created.Sources based on polymers and oligomers 174 cover a broad spectral range, viz., from dark blue through white to green. A series of powder radiation sources were synthesised.175 ± 178 The effect of the con- tent of SmIII (see Ref. 179), ZnII (see Ref. 180) and TmIII (see370 Ref. 181) added to fluorescent powders on the chromaticity characteristics of irradiation was studied.Resins based on an aromatic polyester with improved colour characteristics were created;182, 183 the advantages of copolymers of N-vinylcarbazole and N-phenyl-2-(2-thienyl-5-vinyl)-5-(20- thienyl)pyrrole were shown.184 The synthesis of new copolymers containing electron- and hole-conducting elements (oxadiazole and carbazole, respectively) and an irradiator (naphthalimide) was reported.185 Their lumi- nescence spectra were studied and the chromaticity coordinates were calculated. A new process for obtaining a photographic image consisting of an absorbing dye and a sensitive dye was created; the dyes were selected in such a manner that their lmax values were equal. This made it possible to improve the image characteristics in the red spectral region.186Auniform image in the blue spectral region was obtained using silver halide grains (95 mol.% AgCl).187 A mate- rial containing photosensitive microcapsules that included fuch- sine was used to form the purple layer of the photographic image.188 Coatings for steel sheets resistant against changes in weather conditions were developed;189 they can be suitable for roofs, cars, doors and mechanisms. A mixture of yellow pigments for traffic signs with good reflection properties has been developed.190 The theory and practice of colour measurement for powder coatings were covered in a review 18 which provided recommendations on controlling the colour of these coatings.Amethod for the creation of coloured powders by mixing dry powders was suggested.191 Blending the required powder colour is simplified by plotting the colour diagrams.192 A computer-aided method for blending the colour of paints was reported.193 The panels are coated with various mixtures of a colour and white paints; the reflection signal is measured, and the chromaticity coordinates are calculated. The values obtained are compared with those in a standard database.Such a technique simplifies the colour blending and increases its accuracy. The optical properties of paint coatings formed on a surface depend on the nature of the pigment and the film-forming agent, on the volume concentration of the pigment and on the surface geometry.19 The changes in the spectral composition of the light reflected by samples upon increase in their brightness due to application of a polymeric layer were estimated by changes in the optical characteristics in the CIELAB system (L, S, T, DE).Upon application of the first layer 10 ± 20 mm thick, a sharp decrease in reflection coefficients and increase in lightness are observed; further increase in the layer thickness affects these characteristics only slightly. For light-blue and light-green coatings, the hue decreases continuously with the increase in the layer thickness: the colour vector turns clockwise. The hue of orange samples remains unchanged. This results from the fact that light dispersion is stronger in the short-wave spectral region. V. Conclusion Over many years, the progress in colour-measuring systems was restrained by the low technological level.Therefore, simultane- ously with development of the equicontrast colorimetric system (CIELAB) by the International Commission on Illumination, colorimetric instruments and calculation methods were created which made it possible to record colour and use colour measure- ments in the analysis. Today, the chromaticity method, especially combined with computer programs for processing spectral data, has become widely popular in polygraphy and in the paint, textile, glass, pharmaceutical and food industries and in a number of other branches of industry. One can hope that the chromaticity method will be used more widely in analytical chemistry, especially in the cases where the analytical signal is directly related to the colour of the compound to be determined, for example, in the approximate evaluation of the content of a compound from the colour intensity.Test VM Ivanov, O V Kuznetsova methods, which basically do not require complicated instruments, are promising for fast batch analysis. The use of chromaticity systems which approach as much as possible the colour character- istics of human vision should favour the development of express analysis methods. In our opinion, chromaticity will also be used more widely in spectroscopic analysis, since the use of optical radiation as the information carrier seems to be undoubtedly promising. 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ISSN:0036-021X    

出版商:RSC    

年代:2001

数据来源: RSC

摘要:

Russian Chemical Reviews 70 (5) 373 ± 403 (2001) X-Ray synchrotron radiation in physicochemical studies Ya V Zubavichus, Yu L Slovokhotov Contents I. Introduction II. Design of a storage ring and parameters of synchrotron radiation III. Interaction of X-rays with matter IV. X-Ray methods of physicochemical analysis V. The main trends of applied studies using synchrotron radiation VI. Conclusion Abstract. meth- X-ray conventional the of modifications Modern Modern modifications of the conventional X-ray meth- ods of physicochemical analysis (X-ray and electron spectroscopy, ods of physicochemical analysis (X-ray and electron spectroscopy, diffraction methods, using techniques combined new and diffraction methods, etc.) and new combined techniques using X-ray synchrotron radiation are considered.Unique character- X-ray synchrotron radiation are considered. Unique character- istics namely, noted, are radiation synchrotron of istics of synchrotron radiation are noted, namely, continuous continuous spectrum, polarisation, intensity, high exceptionally spectrum, exceptionally high intensity, polarisation, coherence, coherence, pulse methods combined of use the in trends key pulse nature, nature, etc. The The key trends in the use of combined methods in of studies including science materials and chemistry in chemistry and materials science including studies of substances substances under extreme conditions, the study of rapid processes, studies under extreme conditions, the study of rapid processes, studies with in achievements The discussed.are resolution spatial high with high spatial resolution are discussed. The achievements in the the applications by illustrated are radiation synchrotron of applications of synchrotron radiation are illustrated by examples examples from The sciences. related and chemistry of fields various from various fields of chemistry and related sciences. The biblio- biblio- graphy references. 405 includes graphy includes 405 references.. I. Introduction Development of modern science relies to a great extent upon the set of available investigation tools. Many methods of physico- chemical analysis commonly applied in chemistry are based on interaction of a substance under study with electromagnetic radiation of various spectral ranges.Several independent groups of methods can be distinguished based on the spectral range used, e.g., infrared, visible, ultraviolet and X-ray radiation. Radiation in the required spectral range is usually generated using special sources (such as gas discharge tubes, lamps, lasers, X-ray tubes) characteristic of classical versions of each method. Each source type requires its own experimental methodology, which takes into account the specific features of the interaction of this radiation with matter. Integration of natural sciences over the last decade of the XXth century affected the techniques of physicochemical analysis stimulating their convergence. A unified approach to the gener- ation of electromagnetic radiation of different spectral ranges is based upon a fundamental principle of electrodynamics: the electromagnetic radiation is generated by a charged particle Ya V Zubavichus, Yu L Slovokhotov A N Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul.Vavilova 28, 117813 Moscow, Russian Federation. Fax (7-095) 135 50 85. Tel. (7-095) 135 93 04. E-mail: yan@xafs.ineos.ac.ru (Ya V Zubavichus) E-mail: slov@xafs.ineos.ac.ru (Yu L Slovokhotov) Received 1 February 2001 Uspekhi Khimii 70 (5) 429 ± 463 (2001); translated by Ya V Zubavichus #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n05ABEH000656 373 374 378 380 393 396 moving with an acceleration. In particular, electrons or positrons, which move along a curved trajectory in a magnetic field with a velocity close to the velocity of light, generate a flux of photons in a broad spectral range, i.e., synchrotron radiation (SR) (Fig.1). S SR e7 N Figure 1. Generation of synchrotron radiation upon movement of an electron in a magnetic field. Accelerators of charged elementary particles (typically, elec- trons or positrons) operating in a stationary regime of a storage ring can conveniently serve as powerful SR sources. In such large- scale facilities, ultrarelativistic electrons,{ moving along a closed- loop orbit in an evacuated chamber of a storage ring with curvature radius of a few tens of meters, emit extremely intense electromagnetic radiation with a continuous spectrum ranging from far infrared to hard X-ray (Fig.2). The energies of electrons in storage rings of different constructional designs can vary from hundreds of eV up to several GeV. Upon an incremental increase in the electron energy, the spectral maximum of the SR generated shifts from ultraviolet through hard ultraviolet (vacuum ultra- violet, VUV) and soft X-ray up to hard X-ray, respectively.1 ±8 Modern SR sources gradually become collective use research centres. Such centres accommodate up to a few tens of exper- imental stations, which function in the 2467 mode shift by shift servicing specialists from various institutions and fields of science � from nuclear physics to molecular biology and materi- als science. Along with synchrotron modifications of established and widely accepted methods using traditional X-ray sources, unique purely synchrotron-based specialised techniques are rap- idly developing.Applications of SR in industry are also expand- { The term ultrarelativistic particles is commonly referred to those, the kinetic energy of which E 44 m0c2, wherem0 is the rest mass of the particle and c is the velocity of light. Velocities of such species are virtually constant and very close to the velocity of light; thus any increase in their kinetic energies leads to the increase in their relativistic masses.374 E /eV l /m 1074 1072 Microwaves 1073 1073 1072 1074 IR 1071 1075 100 1076 Visible light 101 1077 UV Vacuum UV 102 1078 Soft X-rays 103 1079 Hard X-rays 104 10710 g-Rays 10711 105 Figure 2.Spectral ranges of electromagnetic radiation, which can be generated by modern synchrotron radiation sources. ing, in particular, in the manufacture of microelectronics (micro- chips, printed circuit boards, etc.) and micromechanics (micro- motors, micropumps) devices. In this connection, applied industrial methods, such as deep X-ray lithography and LIGA technology (German acronym for a 3-stage technological process of microfabrication including lithography, electrodeposition and moulding), have made tremendous progress.9 Medical applica- tions utilising SR are evolving as well, especially ones aimed at early diagnostics and radiotherapy of various diseases, including cancer and at imaging of human internal organs and tissues with high contrast and spatial resolution (mammography, angiogra- phy, radiography).10 Synchrotron techniques are successfully used in environmental studies, e.g., for detection of trace amounts of heavy metal pollutants in industrial wastes, aerosols, soils and in biological objects�plants, living tissues, etc.11 Currently, there are about 40 operating SR sources world- wide and more than 10 new sources are under construction.In Russia, starting from the late 70's, the Siberian Synchrotron Radiation Centre (SSRC) continuously functions on the basis of the VEPP-3 { storage ring (2 GeV) at the G I Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences in Novosibirsk.12 Recently, a new upgraded VEPP-4 storage ring (6 GeV) has been put into operation in the SSRC.Construction of a dedicated storage ring `Sibir'-2' (2.5 GeV) has been finished at the Kurchatov Synchrotron Radiation Source (KSRS) in the Russian Research Centre `Kurchatov Institute' in Moscow;13 two new synchrotron centres in Dubna and Zeleno- grad are in the construction phase. An important benefit of storage rings is their environmental safety,} which distinguishes such facilities advantageously from, for example, neutron sources, viz., nuclear reactors. Even a major accident on an electron storage ring cannot result in any radiation pollution of the environmenork at a synchrotron radiation centre does also not imply any health risk for researchers provided that they obey elementary safety rules and regulations.All of the main methods utilising electromagnetic radiation are represented at the modern synchrotron radiation centres. A substantial organisational advantage of the SR sources is that all { Russian acronym for electron-positron colliding beams. } In this respect, it is indicative that several storage rings are constructed in residential areas of large cities, e.g., BESSY (Berlin) and KSRS (Moscow). Ya V Zubavichus, Yu L Slovokhotov the state-of-the-art techniques and advanced instrumentation are concentrated in one centre and even in one building. This makes it possible to conduct comprehensive studies with the use of a broad range of complementary experimental techniques as well as stimulates mutual influence and integration of different experi- mental approaches and ideas. This review is devoted to applications of SR in chemistry and related disciplines, viz., chemical materials science, biochemistry and molecular biology. Since it is virtually impossible to cover all modern trends in the SR applications, we have confined ourselves to discussion of only instrumental techniques based on X-ray part of the synchrotron radiation spectrum.For the same reason, we did not consider such technological applications of SR as X-ray lithography and the methods in which synchrotron radiation is used for triggering physicochemical processes, e.g., photochem- ical reactions and radiation-induced degradation (to get familiar with this field one may see, for example, Ref.14, where synthesis of amino acids from a gaseous mixture of nitrogen, water and carbon dioxide at atmospheric pressure under irradiation by intense photon beams is described). For a detailed review of modern technological applications of SR see Ref. 15. Among non-X-ray synchrotron methods we should mention various types of UV spectroscopy 5 and synchrotron IR spectroscopy, which have been actively developed over the last years, especially its varieties with high spatial (microspectroscopy) 16 and time reso- lution.17 X-Ray-based instrumental techniques, which are most impor- tant for chemistry, include X-ray diffraction methods (in partic- ular, single-crystal X-ray crystallography), X-ray and electron spectroscopy.In `classical' laboratory versions of these methods, allowing detailed exploration of atomic and electronic structures of substances, traditional X-ray sources, viz., X-ray tubes, are used. However, synchrotron radiation sources have several sig- nificant advantages. Synchrotron X-ray beams are more intense than radiation of the high-energy X-ray tubes with rotating anodes by several orders of magnitude. Such a gain in intensity allows one to shorten the data acquisition time and investigate fine effects which would require very lengthy experiments with tradi- tional sources. A considerable portion of energy emitted by an X-ray tube lies in a narrow spectral range (anode characteristic radiation), whereas the high intensity of synchrotron radiation is uniformly distributed over the entire X-ray spectral range.With `white' synchrotron radiation, it is possible to select the wave- length most appropriate in each particular case or scan over a certain wavelength range during the experiment, which extends significantly the potential of traditional X-ray techniques. Yet another advantage of SR is relatively small cross-section and low divergence of X-ray beams, which can further be focused and collimated using X-ray optics. This approach is successfully applied in X-ray microprobe methods and in studies of substances under extreme } conditions (it has to be mentioned, however, that focusing X-ray optics is now frequently used in laboratory devices as well).Pulse structure, polarisation and coherence of synchro- tron radiation are other factors, which are of great interest and potential for applications. II. Design of a storage ring and parameters of synchrotron radiation 1. Building blocks of a storage ring A storage ring is an accelerator of charged light particles (electron or positrons) in which ultrarelativistic particles move along a closed-loop trajectory with velocity close to the velocity of light emitting an intense photon flux. The energy lost by electrons during each revolution as emitted radiation is compensated in a special electromagnetic acceleration system, viz., radiofrequency } The term extreme condition refers here to the limiting achievable (high and low) temperatures and superhigh pressures, which are created using specialised instruments to apply to substances under study.X-Ray synchrotron radiation in physicochemical studies 1 10 9 5 2 4 8 3 7 6 Figure 3.Design of a storage ring, the SR source: (1) is an electron gun, (2) is a linear preaccelerator (linac), (3) is a ring preaccelerator (booster), (4) is a bending magnet, (5) is a system of `magnetic lenses', (6) is a radiofrequency resonator, (7) is a straight section suitable for accommo- dation of specialised insertion devices, (8) is a channel, (9) is an exper- imental station, (10) is a biological protection concrete wall. resonator. The storage ring is designed to maintain a stationary regime of particles' movement rather than to generate a single pulse of particles with the maximum energy (as in the case of elementary particle accelerators in nuclear physics).A storage ring includes the following indispensable compo- nents (Fig. 3): � an electron source (1), which is, as a rule, a standard high- power electron gun based on the thermoelectric emission; � a system for preacceleration and injection of electrons: linear (linac) (2) or ring (booster) (3) preaccelerators; � a hollow torroidal chamber of the accelerator where the beam of ultrarelativistic electrons moves. High vacuum has to be maintained in this chamber (1079± 10710 Torr) in order to reduce energy losses of the electrons due to collisions with background molecules; �a system of bending magnets (magnetic dipoles) (4), which define closed-loop trajectory of the electrons' movement.Between the bending magnets the particles move linearly. It is in the bending magnets that the synchrotron radiation directed tangen- tially to the beam trajectory is generated (see Fig. 1); �a system of magnetic lenses (5), viz., magnetic quadrupoles and higher-order magnets, which is used for the electron beam focusing; �an electromagnetic system (6) is used for the acceleration of electrons up to ultrarelativistic velocities as well as for compensa- tion of energy losses due to emitted radiation (radiofrequency resonator); � specialised magnetic devices (insertion devices) (7), viz., undulators and wigglers, are inserted in straight sections between the bending magnets.These devices are used for the generation of SR with enhanced characteristics compared to the bending magnet radiation. Though these are optional components of a storage ring, they are installed in the majority of modern storage rings; � a biological protection concrete wall (10) adsorbs back- ground radiation of the storage ring. Photon beams are guided by evacuated channels (8) through the biological protection wall to the experimental station (9), where experimental stations and all devices for investigations and other works based on SR utilisation are situated. 375 2. Design parameters of a storage ring and characteristics of synchrotron radiation The main parameters of a storage ring as an SR source are as follows: the orbit radius R, along which the electrons move (in modern facilities, it is typically 10 ± 30 m), electron energy E (1 ± 6 GeV), magnetic field (B) in bending magnets (1 ± 2 T) and their number and electron current I, which is an integral measure of the total number of electrons in the storage ring chamber (50 ± 500 mA).These parameters are interlinked by the following relations:4±7 (1) R à E eB , where e is the electron charge; (2) I à 2peNeR , c where c is the velocity of light and Ne is the total number of electrons in the chamty of an electron (or positron) { beam is character- ised by a beam lifetime, which is the time required for the electron current to decrease by a factor of *2.7 (it corresponds to a decrease in ln I by unity).Beam lifetime depends primarily upon the vacuum quality in the chamber and can range from a few to a few tens hours. This governs the frequency of electron re-injec- tions to the storage ring chamber required in order to maintain the intensity of SR at the appropriate level. Design parameters of a storage ring determine the character- istics of SR it generates. Spectral distribution of the SR is commonly characterised by a critical wavelength lc or critical energy Ec=hc , lc (h is the Plank constant); lc is chosen so that the total energy emitted by the source at the wavelengths longer than lc was equal to the total energy emitted at shorter wavelengths.Size of a synchrotron radiation source is defined by the emittance e; this quantity is the product of a linear size of the emitting area of the electron beam by the divergence angle of the photon beam. Typical values of the vertical and horizontal emittances of modern SR sources lie in the range of 10710 ± 1078 and 1078 ± 1076 m rad, respectively. The intensity of SR is characterised by its brilliance, which is determined as number of photons emitted in 1 s from the emitting surface unity into the solid angle unity within the certain photon energy bandwidth (BW). Usually, the bandwidth is chosen to be equal to 0.1% of Ep (Ep is the photon energy), the so-called 0.1% BW. Thus, the unit of measure of the brilliance is pho- ton s71 mm72 rad72 (0.1%BW)71.Other intensity character- istics include spectral flux, which is obtained by multiplication of the brilliance by horizontal and vertical emittances, and total flux, which is obtained by integration of the spectral flux over the entire range of the wavelengths emitted. Total power W emitted by the electron beam per revolution and the critical wavelength depend on the electron energy in the beam, electron current and the orbit radius: (3) W* I g4 R , where g=E/m0c2 is the relativistic (Lorentz) factor of an electron (m0 is the rest mass of the electron) and { Positron beams are used, for example, in the DCI storage ring (Orsay, France). The design of all storage rings in the SSRC (VEPP series) allows physical experiments using colliding electron ± positron beams (which are accelerated in opposite directions within the same chamber).Similar principles are implied in the German PETRA accelerator (Hamburg, Germany): its name stands for Positron-Electron Tandem Ring Acceler- ator.376 (4) lc à 4peR . 3g3 Most widespread types of insertion devices are undulators and wigglers. These are multipole magnetic systems with an intense magnetic field B (0.1 ± 10 T depending on the device type) and alternating polarity. The trajectory of electrons becomes strongly curved in the magnetic field of insertion devices, thus the synchro- tron radiation with specially selected characteristics can be generated (Fig. 4). Spectral distribution of synchrotron radiation emitted by a bending magnet, wiggler and undulator is shown in Fig.5; the spectrum from a `classical' X-ray tube is also given for comparison. Yet another type of specialised magnetic insertion devices is long undulators or free-electron lasers (FEL), which act as sources of coherent monochromatic SR. Currently, more than 100 insertion devices are in operation in the synchrotron radiation centres world-wide. b a S N S N N N S S N S S e7 e7 S S N N SNN N S N S Figure 4. Specialised insertion devices: wiggler (a) and undulator (b) Brilliance /photon s71 mm72 rad72 (0.1% BW)71 Harmonics of an undulator 1020 1016 2 1012 3 1 4 108 103 105 E /eV 101 Figure 5. Spectral distribution of SR generated by bending magnet (1), undulator (2) and wiggler (3).For comparison, emission spectrum of an X-ray tube is also shown (4). Adjacent curved sections of the electrons' trajectory within an N-pole wiggler emit independently, which leads to an increase in the intensity of SR by a factor of N compared to radiation of a bending magnet. Yet another advantage of wigglers is that they allow for creation of magnetic fields much stronger (5 ± 10 T) than in a bending magnet since in the latter the value of B is linked to the orbit radius [see Eqn (1)], which results in a shift of the critical wavelength to higher energy. This is achieved by using super- conducting magnets, which induce transverse `beats' of the electrons' trajectory with amplitude of a few tenths mm.One wiggler may contain from several up to several tens of magnetic poles. Compared to wiggler, an undulator is characterised by a larger number of magnetic poles, smaller separation between them and a weaker magnetic field (*1 T). The operation parameters of an Ya V Zubavichus, Yu L Slovokhotov undulator are chosen so as to achieve interference of the emission by adjacent curved trajectory sections. As a result of this interfer- ence, spectral distribution of the SR generated exhibits intensity maxima, i.e., the so-called undulator harmonics. The intensity of the harmonics, which is proportional to N2 is much higher than that generated by both bending magnets and wigglers. Further- more, the radiation emitted by an undulator is more directed, i.e., its emittance is diminished.18 Due to differences in physical back- ground and underlying mathematical formalism, synchrotron and undulator radiation are considered separately in the high-energy physics.An important parameter of the SR is its polarisation, i.e., the existence of preferred directions of the electric field vector. There are two types of the `pure' polarisation, viz., linear polarisation (the electric field vector oscillates in a certain plane) and circular one (the electric field vector precesses clockwise or counter-clock- wise in the plane perpendicular to the direction of the wave propagation). In contrast to X-rays generated by X-ray tubes, synchrotron radiation is always completely polarised. The bend- ing magnet radiation is linearly polarised in the plane of electrons' movement.Upon deviations from this plane up or down, the radiation becomes partially circularly polarised (elliptic polar- isation). The synchrotron radiation generated in insertion devices (wigglers or undulators) also exhibits linear or circular polar- isation depending on the design of the corresponding device (for example, for generation of circularly polarised SR, helical undu- lators or elliptic multipole wigglers can be used). Yet another important characteristic of SR is its coherence, i.e., correlation in oscillations of the electromagnetic field at different space points and at different time moments. The coher- ence determines the ability of radiation to produce an interference pattern; the planned fourth-generation SR sources approach lasers in this respect.There are two types of coherence, viz., temporal (longitudinal) coherence, which depends mostly on monochromaticity of the SR beam and is characterised by the longitudinal coherence length, and spatial (transverse) coherence, which is, first of all, determined by the emittance of the source. Synchrotron radiation has a pulse structure. Due to relativ- istic effects, the electron beam in the storage ring is split into separate moving clusters (bunches), each bunch is of a few cm in length. Therefore, electron current through a bending magnet is not continuous and the SR generated contains periodic pulses with duration of a few tens ps with nanosecond intervals between them.Under certain injection conditions, it is possible to achieve the state where only one bunch of electrons moves in the storage ring (single-bunch mode). In this case, both pulse durations and intervals between them are highly regular, which can be of great importance for time-resolved studies of fast processes. The key parameter of a SR source is its emittance. It is the emittance, which depends upon perfection of the magnetic system of the source, that reflects most clearly the level of industrial technology involved in the construction of the storage ring. As a rule, the emittance is correlated with the brilliance: the smaller the emittance the higher the brilliance. In addition brilliance depends on the electron current.According to the value of emittance, all the operating synchrotron centres (Table 1) can be conventionally subdivided into generations. First-generation storage rings built in the 60's have the emittance of 100 ± 500 nm rad. They were designed and constructed within the scopes of nuclear physics research programmes and thus were not optimised for applica- tions based on utilisation of the synchrotron radiation and such applications took only small portion of the all available beamtime. SR studies on the first-generation storage rings were often conducted in parallel to the main studies on high-energy physics of the electron, positron, or electron ± positron colliding beams (in the so-called `parasitic' mode). In the 80's, the second-generation storage rings with emittance of the order of 50 ± 150 nm rad appeared.These accelerators were specialised for SR applications, so they are often referred to as dedicated sources. Most of storage rings which are in operation nowadays belong to the secondX-Ray synchrotron radiation in physicochemical studies Table 1. Characteristics of several storage rings. Storage ring name (city, country) Energy /GeV Circumfer- ence /m 4.5 2.0 2.0 1.85 2.5 2.6 6.0 2.2 1.9 2.0 1.5 1.5 6.0 2.0 7.0 1.37 8.0 2.5 a In parentheses, the year of last large update of the storage ring is specified. 289.2 74.4 96 94.6 187 170.1 366 65 196.8 280.6 90 120 844 259.2 1104 93.2 1436 124.1 DORIS III (Hamburg, Germany) VEPP-3 (Novosibirsk, Russia) SRS (Daresbury, UK) DCI (Orsay, France) Photon Factory (Tsukuba, Japan) NSLS (Stoney Brook, USA) VEPP-4 (Novosibirsk, Russia) BEPC (Bejing, PRC) ALS (Berkeley, USA) PLS (Pohang, South Korea) MAX II (Lund, Sweden) SRRC (Hsinchu, Taiwan) ESRF (Grenoble, France) ELETTRA (Trieste, Italy) APS (Chicago, USA) LNLS (Campinas, Brazil) SPRing-8 (Harima, Japan) Sibir'-2 (Moscow, Russia) generation.Several most modern storage rings built in the 90's are classified as third-generation storage rings. They are characterised by emittance of the order of 1 ± 10 nm rad and, as a constructive design feature, contain many straight sections in their vacuum chamber between bending magnets for installation of insertion devices.Brilliance of SR sources increases by 2 ± 3 orders of magnitude with each new generation.19 For example, brilliance [photon s71 mm72 rad72 (0.1% BW)71] of first-generation SR sources is 109 ± 1012, of second-generation SR sources, it is 1012 ± 1015, of third-generation SR sources, it is 1018± 1023 and >1024 as expected for X-ray free-electron lasers. For comparison, brilliance of standard X-ray tubes and X-ray tubes with rotating anode is only 106 ± 107. The modern SR sources generate radiation, which is more intense than that of X-ray tubes by a factor of 108± 1014. Further modernisation of third-generation storage rings will soon lead to a `diffraction limit' of a single electron emittance (e0) e0= l 2p (where l is the wavelength of emitted photons), which is equal to 16 pm at l=1 A (E=12 keV).Thus the currently exploited SR sources are close to the physical limit of brilliance 1024 pho- ton s71 mm72 rad72 (0.1%BW)71. The advent of fourth-gen- eration sources with emittance of the order of 0.01 nm rad is related to new technological schemes, such as utilisation of linear accelerators or multi-ring recuperator accelerators and FELs.12 3. Components of experimental stations Special channels guide synchrotron radiation generated in bend- ing magnets, wigglers or undulators to experimental stations (see. Fig. 3). High vacuum should be maintained in these channels, as in the main chamber, in order to minimise losses and radiation background due to absorption of photons by molecules of gases within the channel.Though exact instrumentation of each station largely depends on the specificity of the method used, several main components are common for most stations. These are X-ray optics elements, X-ray monochromators and X-ray detectors.20 Virtually in all the cases, optical transformation of the primary SR beam is necessary for experiments, viz., collimation (transformation of a divergent beam into a parallel one) and focusing (transformation of a divergent or parallel beam into a convergent one). X-Ray beams with small linear sizes are widely Emittance /nm rad Maximum current /mA 404 270 104 1600 36 100 1200 766 128.8 19478.2 1005.6 76 120 (e+) 250 (e7) 250 (e7) 300 (e+) 770 (e7) 300 (e7) 80 (e+, e7) 100 (e+, e7) 400 (e7) 100 (e7) 200 (e7) 240 (e7) 200 (e7) 300 (e7) 300 (e7) 160 (e7) 100 (e7) 300 (e7) used in X-ray microprobe techniques, X-ray microscopy and microspectroscopy.X-Ray monochromators, which allow one to extract a narrow spectral band with certain (and variable in most cases) wavelength from `white' synchrotron radiation, are also often considered as X-ray optical elements. The main tasks of the X-ray optics development include the increase in the energy and spatial resolution and the decrease in the intensity losses associated with the interaction of the respective elements with SR beams.Optical schemes for focusing and monochromatisation of visible light beams (and also, to a great extent, in IR and softUVspectral ranges), viz., lenses, prisms, etc., are based on the dispersion phenomenon, i.e., the dependence of the refraction coefficient on the wavelength. However, the refrac- tion coefficient for X-rays in most of materials is very close to unity and virtually independent of the wavelength (see below). Besides, soft X-rays are strongly absorbed by virtually all materi- als (including air). Thus alternative basic principles are used in X-ray optics, such as total external reflection at incidence angles less than 0.1 8 (grazing incidence), Fresnel scattering and Bragg diffraction. Most typical optical elements aimed at collimating and focus- ing the synchrotron X-ray beams include: grazing incidence concave mirrors (toroidal, spherical, cylindrical, etc.), curved single crystals, Fresnel zone plates (FZP), Bragg Fresnel lenses (Bragg Fresnel optics, BFO) and tapered capillaries (Fig. 6).Minimal linear size of an X-ray beam achieved in modern SR centres using microcapillary techniques is 20 ± 50 nm. The dis- advantage of tapered capillaries is strong divergence of the focused beam after the focal point, so they should be mounted very closely to the sample under investigation. FZP and BFO allow one to produce virtually parallel X-ray beams with linear sizes of the order of 1 mm. In applications which do not require very high spatial resolution, concave X-ray mirrors are most commonly used.Generally, an X-ray mirror represents a glass or silicon support coated with a fine and extremely smooth layer of non-oxidisable noble metals (Rh, Pt or Au). A focusing mirror allows amplification of SR beam intensity by 1 ± 2 orders of magnitude at the costs of decrease in its linear size. Some of the aforementioned optical elements (curved crystals, FZP, BFO) can also play a role of a monochromator. In addition, multilayer periodical structures and coatings as well as gratings are often used as monochromators for soft X-rays. Multilayer X-ray mirrors produce sufficiently high intensity of the reflected 377 Startup year a Number of stations 1974 (1995) 1975 1980 1980 1982 (1997) 1982 1979 (1992) 1988 1993 1994 1994 1994 1994 1995 1996 1996 1997 1999 429 39 21 21 7077 19778 509 689 614378 abcd Figure 6.Main types of optical elements for collimating and focusing X-ray SR: (a) X-ray mirror, (b) Bragg trensel lens, (c) focusing multilayer mirror, (d ) tappered capillary. beam even at usual incidence angles (up to a few degrees) and simultaneously focus and monochromatise the primary beam. In the hard X-ray range, single crystals oriented in a diffract- ing position by one of its crystallographic planes are most typical monochromators. Among such monocromators, flat single-crys- tal plates, channel-cut monochromators and more advanced double crystal monochromators with two independent drives are well known.Upon variation of an incident radiation wavelength, the exit-axis of a channel-cut monochromator can drift by a few millimeters, whilst the independent drives of two single crystals make it possible to fix the spatial direction of the monochroma- tised beam. The most important characteristic of a monochromator is its energy (or wavelength) resolution DE/E (or Dl/l). For example, energy resolution of a monochromator based on a flat mosaic graphite crystal with the diffracting plane [002] is *1072; for double crystal monochromator with Si [111] crystals, this value is *1074. Higher resolution can be achieved by utilising low- intensity reflections from higher-order crystallographic planes along directions almost perpendicular to the crystal surface (back reflections).For instance, monochromators with Si [13 13 13] crystals in the back reflection geometry allow one to reach a Dl/l resolution of &1078. For experiments requiring very high energy resolution, systems of nested double crystal monochromators are often applied (Fig. 7). A very promising direction in the field of X-ray monochromatisation is the utilisa- a Figure 7. Examples of X-ray monochromators: (a) channel-cut mono- chromator, (b) double-crystal monochromator with independent drives for both crystals, (c) a system of two nested double-crystal monochroma- tors. c b Ya V Zubavichus, Yu L Slovokhotov tion of the resonance nuclear diffraction phenomenon, which allows one to reach resolutions down to 10710.It has to be noted that monochromatisation of a `white' SR beam (extraction of a narrow spectral band) is always accompanied by great losses in intensity. An X-ray beam monochromatised using Bragg diffraction from a single crystal is always contaminated with higher harmon- ics, i.e., radiation components with an energy multiple of the main component. In order to reject these harmonics, grazing incidence mirrors are often used with parameters adjusted so that the total reflection condition holds for the main component only; higher harmonics in this case pass without changing direction. In double- crystal monochromators, the harmonic rejection can be achieved by slight detuning of the two diffracting crystalline planes (this is however accompanied by noticeable losses in the main component intensity).In some methods, (e.g., in the energy-dispersive modification of XAFS spectroscopy, see below), similar X-ray optical elements based on Bragg diffraction (in particular, curved single crystals) are used not for monochromatisation but for decomposition of the `white' beam into spectral components and defining certain spatial directions for photons with different energies (crystal- polychromators). For control and modification of SR polarisation properties, a specialised polarised optics is used: crystal-polarisers, X-ray quarter-wave plates, phase retarders, etc. The state-of-the-art of modern X-ray optics is discussed in detail in a number of reviews (see, for example, Refs 1 ± 4, 20 ± 22).Virtually any experiment utilising X-ray radiation requires measurement of beam intensities (primary, transmitted through a sample, fluorescent, etc.). For this purpose, X-ray detectors are used. Basic characteristics of an X-ray detector are sensitivity, linearity range, maximum load, counting rate, energy and spatial resolution. The following types of detectors are most often used in modern synchrotron radiation centres: photographic films and plates, ionisation chambers, proportional counters, scintillation detectors, solid state semiconducting detectors and charge- coupled devices (CCD). Depending on spatial resolution, all detectors can be subdi- vided into point and position-sensitive detectors (PSD).In turn, the latter can be linear (1D) or two-dimensional (2D). PSDs allow registration of X-ray intensity distribution over a certain spatial area. Of them, multichannel (multiwire) proportional counters, detectors of the type Image Plates (which transform X-rays into visible light in recurring cycles `energy storage as crystalline lattice defects ± stimulated emission in the visible spectral range upon laser irradiation') and CCDs are most popular. Scintillation counters and ionisation chambers are `classical' X-ray detectors. They possess extremely high sensitivity (down to registration of single photons). Ionisation chambers are often used as primary beam monitors due to their wide linearity range and ability to operate under conditions of very intense X-ray beams.For some specific tasks (such as imaging, small angle scattering, sample adjustment in a beam), photofilm and photosensitive paper are still in use. Solid-state semiconducting detectors exhibit high energy res- olution. They are applied in cases where a spectral distribution analysis of the detected X-ray radiation is required. Avalanche photodiodes (APD), where each photon coming into the semi- conducting diode generates an `avalanche' of electron ± hole pairs are the fastest of the modern X-ray detectors. Such detectors allow intensity measurements for very short pulses of SR (*0.1 ns), so they are widely adopted in time-resolved studies of fast processes. III. Interaction of X-rays with matter Upon interaction with the matter, X-ray radiation can be scat- tered or absorbed. These processes as well as combined phenom- ena of inelastic and anomalous scattering form the basis of X-ray methods of physicochemical analysis.X-Ray synchrotron radiation in physicochemical studies When a photon is scattered on a particle of a medium and changes its directions, its energy can either be conserved (elastic scattering) or partially transferred to the matter (inelastic scatter- ing).Inelastic scattering can be due to various channels of energy transfer including excitation of collective vibrations of atomic nuclei in a crystalline lattice (phonons) or charge carriers, viz., electrons and holes in a conduction zone (plasmons); excitation of electrons from valence zone to vacant energy levels with creation of electron ± hole pairs; ionisation of core level electrons of light elements (the so-called X-ray Raman scattering).For standard energy of a primary beam of*10 keV, inelastic losses due to phonon excitations have the order of a few meV, plasmon losses and losses due to electron ± hole pairs creation lie in the range of 0.5 ± 10 eV, ionisation of core electron levels may lead to losses of several tens or hundred eV (depending on the binding energy of the excited level). In the case of very hard X-ray radiation, the energy of which is much larger than the binding energies of electrons in atoms, the dominant channel of inelastic scattering is the Compton scattering of X-ray photons on quasi- free electrons (without quantum restrictions on the momentum and energy transfer). Harder g-radiation with energy starting from several MeV (which usually is not produced by standard SR sources and thus is not considered in detail in this review {) interacts with the matter through the mechanisms of elementary particles' transformation, for example, by generation of electron ± positron pairs.Elastic (coherent) scattering of X-ray photons depends pri- marily on their interaction with electron shells of atoms (Thomson scattering). Since the energies (i.e., the wavelengths) of all the scattered photons are the same, elastic scattering can lead to diffraction, i.e., a spatial redistribution of the scattered beam intensity as a result of interference of waves coming from different atoms.When atomic arrangement has a translational symmetry (as in crystals), diffraction reflections, i.e., narrow maxima of the scattered wave intensity arise at certain directions. Furthermore, a persistence of a mean local atomic order even in amorphous substances gives rise to sinusoidal oscillations of the smooth (without sharp maxima) elastically scattered background as a function of the scattering angle. Diffraction phenomena form the basis of a number of experimental techniques which allow studies of fine details in the structure of the matter, including local atomic environment, supramolecular arrangement and periodicity ele- ments (types of partial ordering).23, 24 Absorption of X-ray photons leading to attenuation of the photon beam as it propagates in a medium is primarily related to the photoelectric effect, i.e., photoionisation of inner electronic shells of atoms.In this case, a vacancy (hole) in the respective core electron level and a free electron are generated. All the methods of X-ray absorption spectroscopy are based on the analysis of X-ray absorption coefficient as a function of the energy of incident photons. Energy and spatial distribution of the photoelectrons generated in this process are the main subject of X-ray photo- electron spectroscopy techniques.25 The excited state with a vacancy in the core level (X-ray term), to which an atom transfers upon absorption of an X-ray photon, is metastable with a lifetime not longer than 10715 ±10716 s.The photoionised atom tends to diminish its energy by filling the core level hole with an electron from higher-lying levels. This can be achieved either through emission of an X-ray photon of lower energy (X-ray fluorescence) or through a radiationless two- electron process involving a transition of one electron from a higher electronic level to the core level vacancy with simultaneous detachment of a second electron (the Auger process). X-Ray fluorescence is the dominant direction of decay of an X-ray term with a K-shell hole (K-term) for atoms with atomic numbers Z>20 and with Z>90 for an L-term (Fig. 8). { Recently, generation of hard g-rays using Compton back scattering of a laser light on ultrarelativistic electrons has been developed in several synchrotron radiation centres (see, for example, Ref. 12).379 e7 hn0 Photoionisation hn1 e7 X-Ray fluorescence The Auger process Secondary processes Figure 8. The main processes occurring in matter upon absorption of an X-ray photon. X-Ray fluorescence and the Auger process result in new holes and thus evoke a cascade of secondary processes, such as emission of secondary electrons, fluorescence in the longer wavelength region, etc. In particular, secondary processes can lead to the appearance of luminescence in the visible light range. This phenomenon forms the physical background of the XEOL (X-ray excited optical luminescence) method.Energy transferred to the system from the absorbed X-ray photon can result in cleavage of chemical bonds and detachment of molecular frag- ments (photostimulated ion desorption, PSID). X-Ray absorp- tion by semiconducting materials is accompanied by an increase in concentration of charge carriers in the conduction zone and thus by an increase in their conductivities. Secondary processes ini- tiated by absorption of X-ray photons are widely utilised in various techniques like X-ray fluorescence spectroscopy, Auger electron spectroscopy, secondary electron spectroscopy, etc. Absorption of X-ray photons in matter can also occur due to nuclear excitations. In particular, the MoÈ ssbauer spectroscopy is based on this phenomenon. Generally, this technique is not considered as a sort of X-ray spectroscopy but rather referred to as a distinct field, viz., g-spectroscopy.However such a discrim- ination is arbitrary: the energy of the nuclear transition in the 57Fe isotope, most often exploited in the MoÈ ssbauer spectroscopy (14.4 keV) is close and even slightly less than the energy of characteristic radiation produced by an MoKa X-ray tube, which is most popular in X-ray diffraction experiments (17.4 keV). The energy of nuclear excitations for many other isotopes commonly studied by MoÈ ssbauer spectroscopy lies also in the range of 10 ± 100 keV attainable with modern SR sources. As in the case of electron excitations, excited nuclear states are metastable (however their lifetimes are substantially longer than that of electronic states, viz., *1077 s) and the excited nucleus reverts back to its ground state with emission of an X-ray photon of the corresponding energy.The energy of emitted photon may be equal to (elastic or coherent nuclear scattering), or slightly less (inelastic scattering) than, that of the initial radiation. Due to longer lifetime of excited nuclear states, absorption and emission bands associated with nuclear transitions are exceptionally nar- row.26 When X-ray beam strikes an interface, X-ray reflection occurs with the reflection angle equal to the incidence angle. However, in contrast to, for example, visible light, the probability of the X-ray reflection decreases rapidly with an increase in the incidence angle and for large incidence angle common for visible light optics, the X-ray beam simply crosses the interface not changing its direction.Noticeable X-ray reflection can be observed only at very small grazing incidence angles (*0.1 8) under conditions of the so-called total external reflection. Under this condition, the penetration380 a b m Grazing incidence Usual incidence a Incident beam 907a<0.18 Reflected beam Beam passed trough the interface Figure 9. Reflection of an X-ray beam from a surface under conditions of usual (a) and grazing (b) incidence. depth of X-ray radiation into the matter is only of a few atomic layers (Fig. 9). This phenomenon has to be taken into consid- eration in design of X-ray mirrors and focusing microcapillaries; moreover, it is the basis of a number of surface investigation techniques.IV. X-Ray methods of physicochemical analysis All X-ray-based research methods can be classified based on the processes which accompany interaction of X-rays with the matter. According to this principle, the following groups of methods can be formulated: X-ray spectroscopy, X-ray electron spectroscopy, X-ray diffraction and X-ray inelastic scattering. Additional inde- pendent groups include methods utilising effect of the resonance nuclear scattering (MoÈ ssbauer spectroscopy) and imaging techni- ques. 1. X-Ray spectroscopy a. X-Ray absorption spectroscopy The subject of X-ray absorption spectroscopy is the analysis of the X-ray absorption coefficient (m) as a function of the incident photons energy m=m(E).In the most common transmission mode, the linear absorption coefficient is determined by the formula (5) m à ln I0 , It where I0 and It are intensities of the incident X-ray beam and the beam transmitted through the sample. This formula coincides with the respective relations for linear absorption coefficients in other spectral ranges (UV, visible, IR), since X-ray absorption also obeys the Bouguer ± Lambert ± Beer law, which links attenu- ation and optical path of radiation propagating in a medium. The major part of modern X-ray absorption spectroscopy studies is performed using SR, since they require scanning the incident beam energy over a broad spectral range.The X-ray absorption coefficient increases sharply for certain values of incident radiation energy superimposed on a smooth m(E) curve monotonically decreasing with an increase in the energy. This resonance absorption induced by photoionisation of atoms of a certain element (see above) define `triangular' shape of X-ray absorption bands (Fig. 10 a). Near the absorption edge, the m(E) manifests a fine structure. The fine structure is subdivided into two types, viz., X-ray absorption near-edge structure XANES (Fig. 10,b) and extended X-ray absorption fine structure expressed as c(k) EXAFS (Fig. 10 c). XANES covers the energy range from *50 eV before the absorption edge up to 100 ± 150 eV after it. This fine structure is composed of relatively narrow resonance bands generated by electron transitions from the core level to vacant energy levels up to complete ionisation (i.e., photoionisa- tion) along with broader bands associated with electron transi- tions to quasi-bound states (multiple scattering).The oscillatory structure EXAFS, which appears as a result of a free photo- electron scattering on the local atomic environment, is observed in the energy range of 100 ± 1000 eV above the absorption edge. Ya V Zubavichus, Yu L Slovokhotov a b12650 12700 E /eV 12600 13000 E /eV 12800c d k3 w(k) 0 2 4 6 8R /A0 4 8 12 Wave vector module k /A71 Figure 10. Steps of an EXAFS data processing: (a) an experimental X-ray absorption spectrum; (b) the amplified XANES region; (c) the normalised EXAFS curve, i.e., the oscillating part of the X- ray absorption coefficient; (d) the Fourier transform (FT) of the normal- ised EXAFS curve, the maxima of the Fourier transform correspond to the coordination spheres around the central atom. XANES spectroscopy is used in studies of electronic structure of substances, including determination of the energy and symme- try of vacant orbitals in molecules or unoccupied zones above the Fermi level in solids.In particular, using this method, information on the oxidation state and coordination symmetry of the central atom can be obtained.27, 28 Analysis of EXAFS gives additional structural information on the local order around the absorbing atom, including the type and the number of closest neighbours as well as interatomic distances within a sphere with the radius of 5 ± 6 A (see Fig. 10 d).29, 30 In addition to the interatomic distan- ces, bond angles can be found using such modern approaches as the assessment of multiple photoelectron scattering or simulta- neous fitting of spectra measured at the absorption edges of several elements in the sample.In modern X-ray absorption spectroscopy, a common term XAFS (or XAS), which covers both EXAFS and XANES, is often used. Since the intensities of all the secondary processes are deter- mined by the number of X-ray photons absorbed (i.e., by the number of holes generated in the core level), the quantum yields of these processes are proportional to the X-ray absorption coef- ficient and their energy dependencies exhibit XAFS-like fine structures.For example, such a fine structure is manifested in the dependence of the X-ray fluorescence intensity If on the energy of incident photons. Due to substantially higher signal-to-back- ground ratio, the fluorescence yield defined as the ratio of If to the intensity of the excitation radiation I0 m(E)= If , I0 is 10 to 100-fold more sensitive to the oxidation state and local atomic environment of the photoionised atom than the X-ray absorption coefficient measured in the transmission mode. XAFS in the fluorescence yield mode can be used for the analysis of the oxidation state and coordination of admixture atoms (in concentrations down to 0.1% ± 0.2%).31 Registration of the total or the Auger electron yield FT amplitudeX-Ray synchrotron radiation in physicochemical studies m(E)= ie , I0 (where ie is the electron current) as a function of incident photon energy leads to enhancement of EXAFS sensitivity to surface layers of the sample since the escape depth of electrons does not exceed 50 ± 100 A.32 The optical luminescence yield in XEOL33 or the ion yield in PSID 34 can be used in a similar way.XAFS allows investigations of substances in any aggregate state. Therefore, it is applicable to the solution of a broad range of structural chemical problems related to the electronic state and local atomic surroundings of a certain element. Methods of X-ray absorption spectroscopy proved to be efficient in structural investigations of semiconductors,35 superconductors,36 amor- phous glasses and alloys,37 polymeric electrolytes,38 ceramics,39 metallocentres in biomolecules,40 ± 42 supported catalysts,43 ± 45 metallic nanoclusters,46 ± 48 etc.XAFS studies of solutions reveal information on structural rearrangements of molecules,49 solva- tion mechanisms and structures of solvate shells of ions.50, 51 This method can be used for structural monitoring of reactions in solutions (e.g., electrochemical processes).52 Structural studies of melts give important information on the nature of the interatomic interactions.53 In the soft X-ray range at the absorption edges of light elements, such as B, C, N, O and F (the excitation energies from 100 to 1000 eV), the X-ray absorption fine structure is usually referred to as NEXAFS (near-edge X-ray absorption fine struc- ture).54 Soft X-rays are strongly absorbed in matter (photon free path is only a few tens of nm), therefore NEXAFS is a surface- sensitive technique.The corresponding spectra are generally recorded in vacuum or in a helium atmosphere in the total electron yield mode. The NEXAFS spectroscopy is widely applied for characterisation of polymers,55 chemisorbed molecules,56 adsorbed monolayers,57 Langmuir ± Blodgett films, etc. For example, thin films of higher fullerenes, which are available only in minute quantities, have been characterised using this method.58 Though extended oscillating structure is not very prominent in NEXAFS spectra, they are sensitive to the long-range atomic ordering,57, 58 which allows identification of, for example, con- 1s?p 1s?s 280 300 290 Figure 11.Carbon K-edge NEXAFS spectra for a series of fullerenes Cn (n=60 ± 96). Electronic transitions making principal contributions into the respective absorption bands are shown.58 Graphite C96 C94 C92 C90 C88 C86 C84 C78 C76 C70 C60 Ef /eV 381 formations of long-chain hydrocarbon fragments or individual atomic clusters Cn with n496 (Fig. 11).58 Recently, significant attention has been drawn to XAFS spectroscopy in the energy range of 1.5 ± 3 keV, which is inter- mediate between hard (>5 keV) and soft (< 1 keV) X-rays.59 Such techniques require expensive ultrahigh vacuum equipment, which is generally utilised in the soft X-ray region, but methodo- logical approaches developed for hard X-ray studies can be applied to them.As an example, SOEXAFS (soft EXAFS) may be mentioned. The aforementioned energy range covers K absorp- tion edges of such key elements as magnesium (*1.30 keV), aluminium (*1.56 keV) and silicon (*1.84 keV), which form the base for many building materials and composites, natural minerals, supports for catalysts, organometallic reagents, etc.60 ± 62 Furthermore, K-edges of phosphorus (2.14 keV) and sulfur (2.47 keV), which are constituent of many organic mole- cules, also belong to this region.63 b. X-Ray fluorescence spectroscopy Methods of X-ray fluorescence (emission) spectroscopy (XRF) are based on detection and spectral analysis of the secondary radiation emitted by a substance following absorption of mono- chromatic or `white' synchrotron radiation.They are in active use for elemental analysis of specimens of various origin.64 An X-ray emission spectrum in XRF analysis measured usually in a broad energy range (typically up to a few tens keV) presents a set of characteristic emission lines corresponding to electron transitions from occupied levels to core holes. Modern synchrotron radia- tion-based X-ray fluorescence analysis is one of the most sensitive and accurate non-destructive methods for the determination of trace element concentration. In routine multi-element analysis, the lowest detection limit for a broad range of elements from calcium to actinides reaches the level of 0.1 ppm (i.e., 1077 g g71);65 *0.1 mg of a sample is required to carry out the measurement and the measurement itself takes a few minutes.This enables compositional analysis of samples available only in very small amounts, such as in the case of microelement distribution in lunar rock probes (Fig. 12).66 In dedicated studies optimised for quantitative analysis of a single element or a limited range of elements, the sensitivity as high as*1 ppb (i.e., 1079 g g71 ) and even several ppt (i.e., 10712 g g71) in the analysis of aerosols pumped through a filter, can be achieved.67 High-resolution X-ray fluorescence spectroscopy is an estab- lished technique for characterisation of the electronic structure of matter, viz., for the determination of energy distribution of occupied electronic states below the Fermi level. Information on the electronic structure collected by this method is complementary a Y /ppm b 300 Y /ppm 200 100 100 123456 123 0 1000 Zr /ppm 500 0 500 Zr /ppm Figure 12.Zirconium vs. yttrium concentration correlations in samples of lunar rocks collected in `Apollo' (a) and `Luna' (b) space missions as determined using synchrotron X-ray fluorescence analysis.66 (a): (1) Apollo-11, (2) Apollo-12, (3) Apollo-14, (4) Apollo-15, (5) Apollo- 16, (6) Apollo-17; (b): (1) Luna-16, (2) Luna-20, (3) Luna-24.382 4s 15 10 Figure 13.High-resolution soft X-ray fluorescence (CKa andOKa) of CO adsorbed on the (100) face of Ni single crystal.70 X-ray CKa fluorescence: (1) perpendicular to the surface of Ni(100) face (p-components make major contribution); (2) parallel to the adsorbed layer. X-ray OKa fluorescence: (3) parallel to the adsorbed layer; (4) perpendicular to the adsorbed layer; (5) usual XPS spectrum. to that obtained using X-ray absorption spectroscopy. Exact positions of X-ray fluorescence lines recorded with high precision depend upon the oxidation state and coordination environment of the element under study.68 High-resolution XRF spectroscopy can thus be considered as an analogue of the X-ray photoelectron spectroscopy (see next section), which does not require recourse to ultrahigh vacuum equipment.X-Ray fluorescence spectroscopy in the soft X-ray region is particularly informative in studies of organic molecules.69 ± 71 Resonant photoionisation of atoms of a certain element achieved using monochromatic radiation with the specially adjusted energy (resonant X-ray emission) enhances selectively the intensities of transitions to the vacant level gener- ated in this process and of all the transitions in the cascade of associated secondary processes. This leads to a substantial increase in the sensitivity.72 For instance, in a study of CO chemisorbed on metallic Ni, this approach allowed one to demonstrate experimentally donation of electrons from the Ni atoms to the 2p* molecular orbital of CO (Fig.13 and Scheme 1). CO LUMO HOMO2p*(2p C, O) HOMO (LUMO is the lowest unoccupied molecular orbital;HOMO is the highest occupied molecular orbital). Intensity (arb. u.) CO/Ni(100) 1p*(2p O, C) 5s(2s C) 4s(2p O) 3s(2s O) Intensity (arb. u.) Ya V Zubavichus, Yu L Slovokhotov 2p* 5s+1p Recent achievements in the field of applications and instrumenta- tion of synchrotron X-ray fluorescence spectroscopy are discussed in reviews.64, 73, 74 123450 5 Binding energy /eV c. X-Ray electron spectroscopy In traditional methods of X-ray electron spectroscopy, the energy distribution of electrons knocked out of the sample by an X-ray beam is analysed. There are several types of X-ray electron spectroscopy, such as X-ray photoelectron spectroscopy (XPS, sometimes also referred to as ESCA, i.e., electron spectroscopy for chemical analysis), Auger electron spectroscopy (AES), secondary electron spectroscopy (SES), etc.All the electron spectroscopy techniques are used for surface studies since the electron escape depth in the energy range typical of these methods does not exceed 50 ± 100 A. XPS and AES are common methods for quantitative analysis of the surface chemical composition. Positions of lines in the XPS spectra correspond to binding energies of the corresponding electron levels, i.e., they give information on the electronic state of atoms at the surface (chemical shift of core levels) and energy structure of the valence zone. Several other effects are manifested in XPS and give additional information, viz., spin-orbit splitting of lines into multiplets, two-electron excitation processes (low energy shake-up and shake-off satellites), inelastic energy losses of photoelectrons (characteristic bulk and surface plasmons).The use of SR increases the sensitivity and energy resolution of the method noticeably due to high intensity of SR and availability of strictly monochromatic X-ray beams, respectively.75 Variation of the excitation energy, for example, allows one to distinguish photoelectron and the Auger lines in complex spectra (the positions of the Auger lines, in contrast to photoelectron ones, do not depend on the excitation energy).25, 76 As in the case of XRF, additional information can be obtained from XPS spectra provided that the excitation energy is chosen close to one of the edges of resonance absorption (resonant photoemission).This method, in particular, can be used for the determination of contributions of certain atomic orbitals to molecular orbitals, which is important for chemical bonding description, for example, in transition metal complexes with p-ligands (Fig. 14).77, 78 By varying energy of the incident beam, incidence angle or electron escape angle, one may reconstruct concentration vs. depth profiles of the elements under study. A new promising branch of photoelectron spectroscopy, viz., angular-resolved X-ray photoelectron spectroscopy (ARXPS),79 a b A B C 21 Photoionisation cross-section (arb.u.) 123 Scheme 1 75 55 20 35 5 10 15 Binding energy /eV 15Excitation energy /eV Figure 14. Valence zone photoelectron spectra of the p-complex Ti(Z7-C7H7)(Z5-C5H5) for two different excitation energies (a) and dependence of relative photoionisation cross-sections for bands A, B and C upon the excitation energy (b). (a) Excitation energy /eV: (1) 33, (2) 48; (b) band: (1) A, (2) B, (3) C. Sharp increase in the band A intensity for the excitation energy of*48 eV due to approach of the excitation energy to the MII,III absorption edge of Ti (3p?3d resonance) indicates that this band is mostly composed of Ti 3d atomic orbital; bands B and C correspond to molecular orbitals with significant contribution of the ligand orbitals.77X-Ray synchrotron radiation in physicochemical studies Electrostatic electron energy analyser + 7 e7 Electron detector Azimuthal angle Polar angle SR Sample Figure 15.A scheme of a photoelectron spectrometer for recording angular-resolved spectra. is actively developing in modern SR centres over the last years. This method is based on registration of a spatial distribution of photoelectrons with certain kinetic energies (Fig. 15). Analysis of this distribution allows one to extract the contribution of photo- electron scattering on local atoms surrounding the photoionised atom, i.e., the effect of the photoelectron diffraction. In contrast to other techniques utilising electron diffraction, such as low- energy electron diffraction (LEED) or reflected high-energy electron diffraction (RHEED), the photoelectron diffraction is observed in systems both with and without long-range atomic ordering of a surface.ARXPS, like XAFS, allows studies of local atomic environment of photoionised atoms in samples with disordered or partially ordered surface structure. In particular, this method proved its effectiveness in studies of sorption ± desorption processes. This allows one to reveal positions and orientations of diatomic or more complex molecules adsorbed on crystallographic faces of metal single crystals and to estimate adlayer ± support distances as a function of adsorbate concen- tration. Photoelectron diffraction can be applied to structural characterisation of various surface systems, viz., thin films, epitaxial deposits, surface alloys, metal ± semiconductor interfa- ces, oxide layers, supported metal nanoclusters, products of ion- implantation, as well as processes like growth of interfaces and surface phase transitions including melting.80 ± 82 Further development of the theory and instrumentation of ARXPS has lead to appearance of a new method for surface structural studies, viz., photoelectron holography.Photoelectron holography is based on mathematical processing of large body of data on the intensities of photoelectron lines obtained by varying the sample orientation relative to the incident beam and detector (both azimuthal and polar scans) and excitation X-ray radiation energy.Holographic processing of ARXPS data allows complete reconstruction of spatial arrangement of atoms within the sphere with a radius of 5 ± 20 A around the photoionised atom with an accuracy in atomic positions of 0.02 ± 0.05 A.83 Similar approaches are being developed in Auger electron spectroscopy and X-ray fluorescence spectroscopy.84 2. X-Ray diffraction methods a. Single crystal X-ray diffraction X-Ray diffractometry is a most important experimental method of modern structural chemistry. This allows one to determine the atomic structure of a single crystal (including unit cell parameters, space group, coordinates and types of atoms within the unit cell and parameters of their thermal vibrations). Based on atomic coordinates, any structural characteristics like interatomic dis- tances, bond and torsion angles, etc., can easily be calculated.In contrast to the majority of structural methods, X-ray diffractom- etry allows complete objective determination of a crystal structure without prior knowledge of any of its structural characteristics and even chemical composition. The set of experimental data measured by X-ray diffracto- metry includes angle coordinates and intensities of reflections 383 generated by diffraction of X-rays on a single crystal. The scattering of X-rays occurs primarily on electron shells; thus, mathematical processing of the experimental data (Eulerian angles and intensities of reflections) allows reconstruction of the electron density distribution r(r) in the unit cell.In routine diffraction experiments, distinct maxima of the r(r) function corresponding to the centres of atoms, are revealed with the accuracy of 0.01 ± 0.02 A. Based on high-precision data, fine features of the electron density distribution functions, such as valence electrons redistribution due to formation of chemical bonds, as well as other physicochemical characteristics of a crystal related to r(r) can be analysed. Qualitatively, the redistribution of electron density can be visualised in deformational electron density maps Dr(r) obtained by subtraction of the promolecule's electron density [composed of spherically symmetric non-interact- ing atoms whose positions are determined from the `high-angle' refinement of the structure versus reflections with high (siny)/l ratio, where y is the diffraction angle and l is the wavelength] from the total experimental r(r) function.Currently, the most popular technique in chemical X-ray crystallography is the conventional single crystal diffractometry based on diffraction of monochromatic X-ray beam on a single crystal. X-Ray tubes are most often used as an X-ray source. A serious disadvantage of this method is very strict requirements to the quality of a sample under study. It has to be a well-formed single crystal with linear dimensions of approximately 0.1 ± 1 mm and a sufficient diffraction power at a fixed wavelength (typically, characteristic MoKa l=0.71 A or CuKa l=1.54 A radiation). If a crystal is smaller than the dimensions mentioned or it is of poor quality (due to domain misorientation, static or dynamic disorder of molecules or molecular fragments in the unit cell, etc.), the routine structure solution becomes very complicated or even impossible.Such accompanying phenomena as X-ray absorption, anomalous dispersion, multiple diffraction and extinction (devia- tions of X-ray reflection intensities from the values predicted by the kinematic X-ray scattering theory) further decrease accuracy of the X-ray diffraction results.23, 24 The use of SR in X-ray crystallography eliminates many restrictions of this method. Due to high intensity of SR beams with a capability of their further focusing and collimation by X-ray optics, it is possible to collect reflection sets of satisfactory quality for weakly diffracting and very small crystals.Low divergence of SR beams leads to improvement of angle resolution in high-precision diffraction experiments.85 ± 88 Routine synchrotron X-ray crystallographic studies of low- molecular-weight compounds are now developing in two principal directions, viz., investigations of samples not applicable to labo- ratory diffractometry (e.g., due to a weak diffraction power) 89, 90 and experimental investigations of electron density distribution in crystals.91 Nowadays, crystals with linear dimensions down to several micrometers can be characterised by X-ray diffraction in synchrotron centres.92, 93 Such crystals are commonly formed in a chemical synthesis 94 and they are particularly typical in geology and mineralogy.95, 96 For mounting of such small crystals onto a diffractometer, special methods and devices (for example, on the stage of an electron microscope) are required.97, 98 In the 90's, systematic studies of electron density distribution in crystals based on high-precision diffraction experiments were started in synchrotron radiation centres.Data acquisition time required for these studies shortened down to 15 ± 20 hours from 10 ± 15 days needed with laboratory equipment. Furthermore, requirements for the quality of single crystals for the studies also became not so strict as, for example, in high-precision studies (Fig. 16).99 ± 102 Application of hard X-ray radiation reduces absorption and extinction and increases the resulting accuracy due to larger number of observed reflections with high (siny)/l ratio.In high-precision laboratory experiments, AgKa line of X-ray tubes (l=0.56 A) is used whereas several studies with synchrotron radiation wavelengths as short as 0.3 ± 0.1 A have been reported.103 ± 105384 b a O O C C O O O O Figure 16. Static deformational electron density maps Dr(r) recon- structed in high-precision X-ray diffraction studies of MnCO3 single crystal (section in the plane of the carbonate group, solid lines correspond to positive and dashed lines to negative deformational electron density; contours are drawn with a step of 0.1 e A73): (a) data collected on a laboratory diffractometer, (b) SR data (Photon Factory, Tsukuba, Japan).100 In processing of high-precision synchrotron X-ray diffraction data, the most advanced computational techniques are applied, viz., multipole refinement and topological analysis of the r(r) function.99, 106 Comparison of SR-based and laboratory approaches shows that the accuracy of synchrotron diffraction experiments is at least not lower than that obtained using laboratory diffractometers and is noticeably higher in many cases (see Fig.14).100, 107, 108 For example, one of the most accurate structures ever solved from single crystal X-ray diffrac- tion data is racemic DL-serine: data collected at HASYLAB (Hamburg, Germany) at the wavelength of 0.45 A.99 Currently, X-ray crystallography is widely applied in studies of biological macromolecules (proteins, nucleic acids, polysac- charides, viruses, etc.).109 Single crystals of biopolymers possess large unit cells with parameters of 50 ± 500 A containing up to several hundred thousand independent atoms.For this reason, measurement and structure solution techniques applied in this field have to be noticeably altered. The `classical' X-ray diffraction study allows one to reconstruct the electron density map of the crystal of a biopolymer with a relatively low resolution (3 ± 5 A), in which typical functional groups or fragments rather than single atoms can be seen on a partially resolved background of hydration water molecules (which can occupy 15%± 20% of the crystal volume). Furthermore, large and well formed single crystals of biopolymers are extremely difficult to grow and they easily decompose under X-ray irradiation, since the standard data acquisition can take several weeks on a laboratory diffractometer. Distinct advantages of synchrotron radiation-based techni- ques caused a very fast growth of their application to crystallo- graphic studies of proteins and other biopolymers.100 ± 113 According to Biosync (an international organisation of SR users in the field of structural biology), the fraction of SR in all diffraction studies of new crystal structures of biological macro- molecules has grown from 18% in 1990 to 44% in 1996.114 Presently, almost a half of all studies in protein crystallography is performed using SR sources.In a number of cases, the use of SR allowed one to achieve the resolution comparable to the standard resolution in small-molecule X-ray crystallography. Thus in an X-ray diffraction study of the protein concanavalin A (molecular weight 25 000), the resolution of 0.93 A has been reached, which made it possible to reveal and refine the positions of all 2134 non- hydrogen atoms within the unit cell in the anisotropic approx- imation.115 Since the ratio of the number of independent reflec- tions vs. refined parameters was relatively high, accuracy comparable to that of small-molecule crystallography has been obtained (Fig. 17). Even higher resolution of 0.54 A has been achieved in the diffraction study of the oligopeptide cramine and Ya V Zubavichus, Yu L Slovokhotov Asp His Tyr 2.469(6) Glu 2.233(7) 2.153(5) H2O 2.383(5) Mn 2.166(5) 2.513(5) 2.405(6) 2.360(9) Ca 2.192(5) 2.182(8) 2.350(6) Asp H2O 2.261(8) 2.428(8) H2O Asn H2O Figure 17.Geometry of the metallocentre in the protein concanavalin A determined using synchrotron X-ray single-crystal diffraction with accu- racy typical of low molecular-weight compounds (the synchrotron centre CHESS, Ithaca, USA, l=0.92 A). Only the atoms of amino acid frag- ments coordinated to the metal atoms are shown. The crystallographic parameters are as follows: orthorhombic, a=89.55(2) A, b= 86.46(2) A, c=62.11(1) A, space group I222, 116923 observed reflec- tions, 19158 least-squares parameters refined for 2134 independent non- hydrogen atoms, Rf=12.2%.the deformation electron density map has been analysed for the first time for a protein molecule (data collected at HASYLAB).116 As a recognition of great importance of structural biology for the modern science, the 1997 Nobel prize in chemistry was awarded to the international team including J E Walker (UK), P D Boyer (USA) and J C Skou (Dennmark) for the determina- tion of the enzymatic mechanism of the intercellular synthesis of adenosine triphosphate (ATP). The key stage of the investigation was the single crystal structure study of the enzyme complex F1-ATPase using SR diffraction data collected in Daresbury Laboratory (UK). This enzyme is a constituent of the ATP-synthase complex, which catalyses the last stage of the intercellular ATP synthesis by oxidative phosporylation of adenosine diphosphate.The solution of the structure of F1-ATPase (molecular weight 371 000, ortho- rhombic, unit cell parameters 28561086140 A, space group P212121) took 12 years. To date, it is the largest non-centrosym- metric crystal structure solved from X-ray diffraction data.117 In addition to conventional diffractometry, the Laue diffrac- tion method is widely applied in synchrotron crystallographic studies of proteins. In this method, the diffraction pattern is obtained by `white' X-ray beam scattering on a rotationally fixed single crystal. Combined utilisation of intense synchrotron radia- tion and area detectors allows investigations of extremely small single crystals (with a volume of*0.1 mm3) with a very short data acquisition time (several ns).The unprecedented volume for a single crystal of gold studied by this method 118 is *1073 mm3. During recent years, special techniques for data recording and processing have been developed in this field, which allow at present the ab initio (i.e., without any a priori structural models) solution of relatively simple structures solely from Laue diffrac- tion data 119, 120 and refinement of structures of any complex- ities.121 Yet another direction in the development of synchrotron X-ray crystallography is magnetic scattering. The incorporation of relativistic effects gives corrections of the X-ray scattering amplitude due to interaction of X-ray photons with magnetic moments of electrons.In a classical X-ray diffraction studies, scattering of photons on magnetic moments is several orders of magnitude weaker than that on charge density. However, the magnetic component increases in the case of very hard X-rays (80 ± 100 keV). Furthermore, the magnetic scattering intensity can be substantially enhanced (by 3 ± 4 orders of magnitude) when the scattered wavelength approaches an absorption edge of an ele- ment with unpaired electrons in the sample (the so-called X-rayX-Ray synchrotron radiation in physicochemical studies resonance exchange scattering). In this case, the intensity of magnetic scattering of photons achieves the intensity of scattering on electron charge density.122 Magnetic X-ray scattering leads to the appearance of forbid- den diffraction maxima (extinction condition violations) and superstructure reflections corresponding to the sublattice of magnetic atoms.With synchrotron X-ray sources of high inten- sities it is possible to record magnetic reflections both in the resonance region and far from the absorption edges.123, 124 It allows investigations of magnetic structures of paramagnetic crystals. In this respect, magnetic scattering of SR turns into an important alternative for the traditional technique in this field, viz., magnetic scattering of polarised neutrons. Magnetic X-ray scattering is widely used for studies of magnetic ordering in compounds of f-elements (lanthanides and actinides).125 One of the first demonstrations of research capabilities of the technique has been the confirmation of helical magnetic structure (i.e., heliomagnetism, a particular case of antiferromagnetic ordering) in a number of rare-earth metals, in particular, in holmium.126 Yet another emerging trend of synchrotron X-ray crystallog- raphy is the so-called perturbation crystallography, covering structural investigations of crystals in an external perturbing field, such as mechanical deformation, electrostatic or magnetic field and crystals of substances in excited states.127 For instance, an X-ray diffraction study has been performed for a single crystal of 2-methyl-4-nitroaniline (exhibiting non-linear optical and pie- zoelectric properties) in an external electric field (NSLS, Stony Brook, USA).Changes in unit cell parameters and intensities of selected reflections for the crystal placed into the electrostatic field of 3.96106 V m71 have been explained by rotation of the molecules tending to align their dipole moments with the direction of the external field.128 Changes in unit cell parameters and electron charge density redistribution have been observed in a crystal of deuterated potassium dihydrophosphate KD2PO4 (manifesting segnetoelectric and piezoelectric properties) in the electrostatic field of 1.36106 V m71. Upon application of the external field, a phase transition with alteration of space group and syngony (I42d?Fdd2) has been detected.129 A combined study using IR and X-ray diffraction data for a single crystal of the Na2[Ru(NO2)4(NO)OH] .2H2O photochro- mic complex with excitation by a laser pulse has been carried out in ESRF (Grenoble, France).130 An excited state of the molecule (with relative population of 0.2 ± 0.3) has been detected and structurally characterised. The excitation is accompanied by elongation of the N7O bond by 0.19(5) A and decrease in the Ru7N7Obond angle by 6(2) 8. Earlier, a non-zero population of an excited state with changed geometry of theMNOfragment has been observed for (Z5-C5H5)NiNO by XAFS spectroscopy.131 b. X-Ray powder diffraction Techniques of X-ray powder diffraction (XRD) or X-ray phase analysis are the main fast methods for structural characterisation of polycrystalline samples.In XRD, the dependence of scattering intensity as a function of a single scattering angle 2y, is measured. Compared to single crystal X-ray diffractometry, X-ray powder diffraction methods are much more versatile allowing studies of a broader range of samples including compact workpieces, partially ordered multiphase systems, polymers, thin films, layered nano- composites, minerals and so on. XRD is applicable to solution of such problems as qualitative identification and semi-quantitative analysis of crystalline phases in mixtures, estimation of degree of crystallinity and sizes of crystallites in polymers, analysis of preferred orientation and textures in materials, investigations of phase transitions and equilibria in solids, determination of unit cell parameters as a function of the phase chemical composition or external conditions (pressure, temperature), structural monitor- ing of solid-phase reactions, etc.XRD is routinely used in laboratories with X-ray tubes as the X-ray source. However, during the last decade measurement of XRD patterns in synchrotron centres (especially for samples with Diffraction intensity (arb. u.) 17 5 10 Figure 18. X-Ray powder diffraction patterns for radical-ion salt BEDTTTF+.C60 . I¡3 (BEDTTTF stands for bis-ethylenedithiotetrathia- fulvalene). (1)7SR pattern (VEPP-3 storage ring, Novosibirsk, l=1.5402 A, angle step 0.01 8, exposure time 1 h); (2) 7 laboratory pattern (DRON-3 diffractometer, X-ray tube CuKa , l&1.5418 A, 45 kV620 mA, angle step 0.02 8, exposure time 20 h).low diffracting power and substances available in small amounts) become more and more common (Fig. 18). Most important areas of the SR X-ray powder diffraction are high-precision and high- resolution diffractometry, which allows ab initio structure solu- tion in combination with the structure refinement using the full- profile analysis (Rietveld method) solely from powder diffraction data; diffraction investigations of partially ordered and amor- phous samples; time-resolved studies of processes in sol- ids.12, 20, 132 As in synchrotron X-ray crystallography, position- sensitive detectors are widely utilised in SR-basedXRD(typically, curved linear 1D PSDs capable of simultaneous full pattern registration in the 2y range from 0 8 up to 165 ± 170 8).Depending on the instrumental setup, two methods of XRD can be distinguished: conventional angle-dispersive diffraction (when a fixed-wavelength monochromatic X-ray beam is used) and energy-dispersive diffraction (when the scattering intensity is measured at the constant scattering angle and the wavelength of the incident beam is varied).133 Energy-dispersive X-ray powder diffraction can be accomplished only on SR sources; this mod- ification of XRD is very useful in structural investigations of matter under extreme conditions. In the case of angle-resolved diffractometry, due to high collimation and degree of monochro- maticity of X-ray beams, resolution of weak reflections is sub- stantially better than that obtained using laboratory diffractometers; the profiles of reflections are narrower and more regular.Using SR, it becomes possible to achieve good signal-to- noise statistics for very short time intervals (down to ms); thus it allows registration of minor crystalline phases present in the sample in very low concentration (*0.1%).134 All these factors substantially facilitate data processing including full-profile anal- ysis of powder patterns by the Rietveld method. The latter is used to refine positions of atoms within the unit cells by fitting the calculated intensity profile I(2y) to the experimental curve; the 385 12 21 19 2y /deg 12 15 20 2y /deg386 accuracy of this procedure may approach that of single crystal X-ray crystallography.135 The main unsolved problem in the powder crystallography is the selection of the initial approximation, i.e., structure solution.In synchrotron XRD patterns, up to a few hundred individual reflections can be revealed. Thus it turns possible to solve ab initio, in a routine mode, relatively complex structures with up to several tens of independent atoms in the unit cell. For structure solution, both algorithms widely adopted in crystallography of small- molecular-weight compounds (direct methods, the Patterson method) and ad hoc techniques (maximum entropy, maximum likelihood and Monte Carlo methods, trial-and-error search among sterically allowed orientations of a fragment with known geometry in the unit cell with variation of selected structural parameters such as torsion angles, etc.) are used.The reliability of the Rietveld refinement results can be enhanced by fitting the structural model versus several independent experimental data sets (for example, vs. synchrotron diffraction patterns measured at different wavelengths). Recently, the Rietveld refinement com- bined with trial-and-error search for most probable atomic con- figurations proved to be very efficient in structure solution from powder diffraction data.136 ± 139 The number of various structures of organic,140 ± 142 inor- ganic,143 ± 146 and organometallic 147, 148 compounds solved ab initio based on synchrotron XRD data has now reached several hundreds. Synchrotron XRD is actively employed in structural studies of fullerene derivatives,149 such as endohedral metalloful- lerenes Sc@C82,150 Y@C82, Sc2@C84 (which are available only in a few mg),151 dimeric azafullerene (C59N)2,152 low-temperature modification of fullerene bromide C60Br24 .2Br2 (see Ref. 153), etc. In certain cases, this method allows one to solve independently several components of a multiphase system without their separa- tion. For example, it was possible to determine crystal structures of two polymorph modification of cyclopentadienyl rubidium from synchrotron powder diffraction pattern of their mixture (Fig. 19).154 The electron density distribution revealing features at the subatomic level for a series of simple inorganic solids, viz., BN, Mg, Be, was successfully reconstructed based on synchrotron X-ray powder diffraction and using the maximum entropy method (MEM).155 ± 157 Structural investigations of textured samples, which can be considered as an intermediate step between a single crystal and a polycrystalline powder with random orientation of crystallites, can also be facilitated if SR is used up to the possibility of routine structure solutions by direct methods.158 In this case, a typical routine includes measurement of an XRD pattern at different orientations of the sample in the X-ray beam.In contrast to synchrotron experiments, such a procedure with the laboratory diffractometers usually leads to a substantial line broadening due to high divergence of the incident beam produced by an X-ray tube.c. X-Ray scattering by amorphous and partially ordered samples When X-ray radiation is scattered by a completely amorphous solid or by a liquid, the dependence of intensity on the scattering angle exhibits no sharp diffraction maxima. The Fourier trans- formation of smooth oscillations observed in this dependence gives rise to a radial distribution function (RDF). The maxima in this curve correspond to interatomic distances. A classical version of this method } has a limited applicability, since the interpretation of results for systems more complex than monocomponent or binary is often ambiguous. For example, noncrystalline scattering of synchrotron X-rays with the energy of 101.2 keV (HASYLAB centre) has been utilised for the analysis of RDF in normal and heavy water. In this study, small systematic differences in intera- tomic distances between oxygen atoms forming H-bonds with } This method is also often referred to as the method of radial electron density (RED), wide-angle X-ray scattering (WAXS), or partial distribu- tion functions (PDF).Ya V Zubavichus, Yu L Slovokhotov I Rb C b 0 ca II Rb C c b 0 a Figure 19. Crystal structures of two polymorphic modifications of Rb(C5H5) differing from each other in spatial arrangement of infinite Cp ±Rb ± Cp chains. The structures have been solved ab initio based on the synchrotron powder diffraction pattern for their mixture.154 (I) Space group Pnma, a=10.799 A, b=8.962 A, c=5.706 A (chains are arranged along the a axis); (II) space group Pbcm, a=9.340 A, b=10.967 A, c=10.549 A (chains are arranged along both the b and c axes).different hydrogen isotopes (the Ubbelohde effect) have been revealed.159 Noncrystalline scattering of hard X-rays is a promising technique for structural characterisation of glasses and other inorganic materials with disordered structures, since in this case, the amplitudes of scattering on light atoms become negligible and thus only interatomic distances related to heavy atoms are seen in RDF, which facilitates substantially their interpretation. Analysis of oscillations in the noncrystalline X-ray scattering curves up to high values of the diffraction vector Q à 4p sin y l allows determination of interatomic distances with the accuracy of ca. 0.01 A.160, 161 For instance, in a study of a short-range order around barium atoms in BaSi2O5 using non-crystalline scatteringX-Ray synchrotron radiation in physicochemical studies of synchrotron X-rays with the energy of 150 keV over the Q range of 0.8 ± 38.7 A71, long Ba7Ba distances at 4.5 ± 4.8 A have been revealed.162 For measurements of this kind, intense SR beams produced by insertion devices (wigglers and undulators) in third-generation storage rings are most suitable.In substances with partially ordered structures (defect-rich, disordered and dynamic phases, liquid crystals, thin films, inter- calation compounds, polymers, etc.) as well as in phases with non- crystallographic ordering (modulated and incommensurate phases, quasi-crystals), the types of spatial symmetry can be different for different directions and/or components of the sam- ple.Intercalation compounds of layered matrices (such as tran- sition metal dichalcogenides, graphite, etc.) may be mentioned as examples of partially ordered systems. They often exhibit a strictly parallel stacking of alternating `guest/host' layers, long-range order within host layers combined with the possibilities of phase transitions in two-dimensional guest layers and orientational disorder of adjacent layers. Powder diffraction patterns for such systems often contain relatively narrow reflections together with wide diffuse features.Important structural information can be extracted in this case from the analysis of shapes and widths of diffraction lines 133, 135 and diffuse scattering.163, 164 A variety of partial ordering is particularly typical of biopoly- mers and other biological systems.} In SR-based studies of such systems, two-coordinate detectors are conveniently used. The measured patterns are usually adjusted to fit some a priori model, which corresponds to a structure of a segment of polymer chain, conformation of the chain, fibre orientation, etc.165, 166 d. Small-angle X-ray scattering Small-angle X-ray scattering (SAXS) is based on the scattering of X-rays on optical inhomogeneities of the sample (atomic aggre- gates, cavities) with the size of the order of several tens of nm.Experimentally, scattering intensity is registered as a function of scattering angle in the region of small angles from several angular minutes up to a few degrees, i.e., for diffraction vector moduli 0<Q40.2 (see Refs 167 ± 169). In such small angle diffraction patterns, usual diffraction maxima corresponding to reflections from long-periodicity atomic planes with interplanar distances 10 ± 50 nm (100 ± 500 A) can be observed. A periodic structure can also be formed by packing of supramolecular objects (polymer globules, nanoparticles, etc.). By analysing the scattering intensity decrease function for a sample with completely irregular structure, one may retrieve information on the mean particle size (or inhomogeneity regions) in the sample as well as estimate the shape and size distribution of such particles (Fig.20). SAXS plays a very important role in characterisation of polymer morphology (including biopolymers), conformations of macromolecules in solutions,170, 171 colloid systems,172 submicro- and nanoparticles as well as in studies of such processes as phase } Studies of biological objects using X-ray crystallography, non-crystal- line diffraction, small angle scattering (see below) and XAFS are often merged under the common term `structural biology'. lnI The Guinier region lnI*R2g Q2 Transient region I*Q7D The region of diffraction reflections The Porod region I*Q74 1/R2 1/R1 lnQ Figure 20.General view of a small angle X-ray scattering curve on an inhomogeneous sample. R1 is radius of aggregates; R2 is radius of particles composing fractal aggregates;Rg is gyration radius of aggregates;Dis fractal dimensionality. 387 segregation in amorphous glasses, gelation, nucleation, crystal growth, amorphisation, etc.173 ± 175 Small divergence of synchro- tron X-ray beams extends the capabilities of the technique enabling collection of scattering data at very small angles corre- sponding to interplanar distances up to *1000 nm [ultra-small angle X-ray scattering (USAXS)].176 ± 178 3. Inelastic X-ray scattering Analysis of energy distribution of scattered X-rays forms the basis of a number of methods.179 ± 181 As has been mentioned above (see Section III), inelastic losses accompanying X-ray scattering are due to several distinct physical processes (Fig.21). Processing of phonon and plasmon loss spectra allows one to reconstruct dynamic structural factors (which describe the collective dynamics of a multiparticle system) for nuclei and electrons, respectively. The dynamic factor of nuclei is linked with many important characteristics of materials, such as strength, compressibility, sound velocity, etc. I Elastic scattering Phonon losses Excitation of core electrons Plasmon losses Excitation of valence electrons 2 1071 1072 1073 1 10 10 E ±E0 /eV Figure 21. Characteristic energy losses in inelastic X-ray scattering spec- tra.Phonon spectra of solids are close analogues of vibrational molecular IR spectra. For studies of phonon structure of crystals, inelastic neutron scattering is traditionally used. However, appli- cation of synchrotron X-rays has a number of advantages, in particular, for small crystals as well as for amorphous or liquid samples.182, 183 In order to study atomic vibrations using inelastic X-ray scattering, a very high degree of the incident beam mono- chromaticity is required, so development of this method has become possible after the appearance of third-generation SR sources, systems of nested monochromators and monochromati- sation techniques based on resonance nuclear diffraction (see Section IV.4).Spectra of plasmon vibrations of electrons in the conduction zone for such solids as metals, semi- and superconductors contain important information on microscopic mechanisms of conductiv- ity and other electrophysical properties. Spectra of energy losses corresponding to electron excitations allow determination of electron transitions, i.e., studies of energy band structure of solids (similarly to the analysis of electronic states of molecules by UV spectroscopy).184, 185 The Compton scattering profiles can be transformed into the Fermi surface maps, which reveal the distribution of electron moments in the conduction zone.186 Many electrophysical characteristics of metals and semiconduc- tors can be calculated based on this distribution. X-Ray Raman scattering becomes increasingly important in chemical studies.187, 188 Similarly to conventional Raman scatter- ing employed in Raman spectroscopy, X-ray Raman satellites are produced by a quantified transfer of energy from photons to the sample under study.However, it is phonon loss spectra that are the closest analogues of molecular Raman spectra, whereas X-ray Raman scattering has several special features. First of all, the energy lost by an X-ray photon is consumed to induce photo-a Scattering intensity (arb. u.) 388 1s?p* 1s?s* 200 180 1s?p* 1s?s* 420 400 380 Figure 22. Boron (a) and nitrogen (b) K-edge X-ray absorption fine structures according to X-ray Raman scattering on a boron nitride single crystal in transmission (1) and reflection (2) modes.180 (1) The scattering vector is parallel to the c-axis of the crystal; (2) scattering vector is perpendicular to the c-axis of the crystal; excitation energy *7 keV.ionisation of light elements rather than excitation of vibrational modes and thus the respective amount of energy transfer exceeds the energy of vibrations by 5 ± 6 orders of magnitude. Second, only low-energy satellites are observed in X-ray Raman scattering which represent triangular-shaped extended bands with an edge rather than narrow resonance maxima (Fig. 22). Low-energy Raman satellites observed in inelastic scattering of a hard monochromatic X-ray beam (*10 keV) due to photo- ionisation of 1s shells manifest XAFS-like fine structure.Thus, X-ray Raman scattering allows one to measure XAFS spectra for the elements, the absorption edges of which lie in the soft X-ray region (50 ± 500 eV), if one operates with ordinary hard X-ray radiation in air. This technique allows one to obtain the same information on the electronic state and local atomic environment of light elements as is given by NEXAFS or electron energy loss spectroscopy (EELS) under ultrahigh vacuum conditions. This technique is particularly promising for investigations of substan- ces under extreme conditions, since such experiments usually require sample environment non-transparent for soft X-rays. X-Ray Raman scattering has been applied to study electronic structure of light elements, such as Li, Be, C, e.g., in graphite intercalation compounds with alkaline metals (carbon 1s-edge) as well as in LiC6 intercalate (lithium 1s-edge).189 ± 191 When the energy of inelastically scattered X-ray radiation is close to resonance absorption of one of elements in the sample, the so-called resonant inelastic X-ray scattering (RIXS) is observed.The physical background of RIXS is very similar to that of resonant X-ray emission (see above). The difference in these two processes is reflected in their quantum description: the RIXS is a one-stage process; meanwhile in the resonant X-ray emission, two stages can be distinguished, viz., absorption and re-emission (during lifetime of the excited state, part of the information on absorbed photon, such as its initial moment, can be lost due to internal conversion processes).The RIXS method can be effi- ciently used to explore fine features of electronic structures of solids: many electron transitions (excitation channels) non-detect- 220 E ±E0 /eV b 12 440 E± E0 /eV Ya V Zubavichus, Yu L Slovokhotov able in absorption or non-resonant inelastic scattering spectra clearly manifest themselves in RIXS.192, 193 4. Synchrotron radiation MoÈ ssbauer spectroscopy (6) Transitions of atomic nuclei to low-lying quantum excited states for many elements lie in the hard X-ray region of the energy range 10 ± 100 keV. When the energy of the incident X-ray radiation coincides with the energy of a nuclear transition, resonance nuclear absorption occurs, also known as nuclear g-resonance or the MoÈ ssbauer effect.Each MoÈ ssbauer isotope (possessing low- lying excited states) is characterised with its own distinct set of nuclear transitions. Nuclear absorption bands are very narrow, their widths are usually 1078 ±10710 eV (DE/E&10714). For comparison, the width of absorption bands associated with valence electron transitions is *1077 eV (DE/E&1078) and that with core electron transitions is 1 ± 10 eV (DE/E&1073). In this connection, measurements of resonance nuclear absorption are possible only with X-ray beams of the extremely high degree of monochromaticity. In the conventional laboratory modification of MoÈ ssbauer spectroscopy, excited nuclei of the isotope under study produced upon a radioactive decay are normally used as an excitation energy source; for example: 57Co?57Fe*?57Fe+g. Other techniques of strictly monochromatic g-rays genera- tion, such as target activation with a neutron beam or collision of atoms with high-kinetic-energy ions (Coulomb nuclei activation) are available in nuclear research centres.{ Isotopic (or chemical) shift of the nuclear excited level energy and quadrupole splitting of the level are the parameters that are measured in MoÈ ssbauer spectra.Furthermore, hyperfine mag- netic splitting of spectral lines is observed for magnetically ordered systems. These parameters represent fingerprints for each individual compound; so these allow rather straightforward and unambiguous determination of the oxidation states of an element, symmetry of coordination environment and degree of its distortion. A major disadvantage of the conventional MoÈ ssbauer spec- troscopy is a limited number of monochromatic g-sources avail- able.The predominant part of all MoÈ ssbauer studies is performed for a few elements which have convenient g-active isotopes, such as 57Fe, 119Sn, 121Sb, 151Eu. In principle, utilisation of SR eliminates this restriction since X-ray radiation with continuous energy distribution up to 100 keV and higher is made available (this is particularly true for the third-generation SR sources and insertion devices). TheMoÈ ssbauer effect on synchrotron radiation was first observed in the mid 80's.194 Now, experimental stations for the experiments in this field are installed in several SR centres, such as HASYLAB, ESRF, APS, SPring-8.195, 196 Single crystal X-ray monochromators do not provide the degree of monochromaticity required for the convenient modifi- cation ofMoÈ ssbauer absorption spectroscopy.In this relation, the so-called nuclear forward scattering (NFS) technique is under active development in the field of synchrotron MoÈ ssbauer spec- troscopy. In this method, a sample is irradiated by a very fast pulse of monochromatised SR with energy close to the energy of resonance nuclear absorption and photons scattered in the direc- tion of the primary beam (forward scattering) are registered.197 Under such experimental conditions, a fast-decaying back- ground signal of X-rays elastically scattered by electrons is observed immediately after the excitation pulse followed by a coherent nuclear scattering (nuclear fluorescence) with a delay of the order of 100 ns, corresponding to the lifetime of a nuclear { In the classical version of MoÈ ssbauer spectroscopy, the energy difference between the incident radiation emitted by nuclei of a source and resonance absorption of target nuclei is compensated by movement of a sample due to the Doppler effect; thus shifts of MoÈ ssbauer spectral lines are usually measured in velocity units (mm s71).26X-Ray synchrotron radiation in physicochemical studies 5 6 4 3 2 1 Figure 23.A scheme of a device for measurement of MoÈ ssbauer spectra in the NFS mode with utilisation of pulsed synchrotron radiation. (1) Pulsed SR source, (2) pre-monochromator (DE/E ^ 1074), (3) high- resolution monochromator (DE/E ^ 1077), (4) sample, (5) APD, (6) delay circuit (*100 ns).excited state (Fig. 23). In this signal extracted with a specialised delay circuit and measured with a fast-counting detector, such as an avalanche photodiode (APD), decaying oscillations of the scattered beam intensity (quantum beats) are observed. Quanti- tative parameters of these oscillations can be transformed into normal MoÈ ssbauer characteristics 197 implying that the informa- tion similar to that of conventional MoÈ ssbauer spectra can be obtained (sometimes with higher accuracy). Up to now, first experiments have been conducted on `exotic' MoÈ ssbauer nuclei, which however are potentially important for chemistry, such as 67Zn, 61Ni, 40K, 73Ge and a number of lanthanides and actinides.Synchrotron MoÈ ssbauer spectroscopy is a promising research tool in bioinorganic chemistry 196, 197 and structural materials science (materials under high pres- sures,198 ± 200 time-resolved studies of fast processes, magnetic properties 201, 202). Coherence of nuclear scattering can be violated as a result of various dynamic processes involving excited nuclei.203 For instance, diffusion of excited nuclei leads to a faster decay of quantum beats. This phenomenon (quasi-elastic nuclear scatter- ing 204 ± 206) is used in NFS studies of single crystals for determi- nation of length and direction of elementary diffusion jumps.Energy distribution of nuclear fluorescence at non-zero scattering angles relative to the excitation beam (inelastic nuclear scatter- ing 207) gives information on phonon structure of a crystal lattice, i.e., on collective dynamics of nuclei in the time scale comparable to the lifetime of excited states.208 Yet another important trend in development of synchrotron MoÈ ssbauer spectroscopy is the so-called resonance nuclear dif- fraction.209 When a single crystal containing aMoÈ ssbauer isotope (for example, yttrium iron garnet enriched with 57Fe isotope) is irradiated by a synchrotron X-rays with the energy close to resonance excitation of the nucleus (under Bragg's conditions), all the inelastic nuclear scattering processes are suppressed and the beam reflected becomes highly monochromatic.X-Ray mono- chromators based on the resonance nuclear diffraction provide the highest energy resolution available to date allowing one to extract exceptionally narrow spectral lines corresponding to energies of MoÈ ssbauer nuclei resonance absorption from incident beams of a medium degree of monochromaticity. Such high- resolution monochromators are used in MoÈ ssbauer studies and in phonon structure studies of solids by means of inelastic X-ray scattering.210 5. Imaging techniques Synchrotron X-ray radiation (as well as electromagnetic radiation of other spectral regions) can be used for production of magnified images of various objects.SR-based imaging techniques include X-ray microscopy, X-ray diffraction topography, computed tomography along with methods widely adopted in medical applications, such as radiography, subtraction angiography, mammography, etc. With respect to spatial resolution, X-ray microscopy occupies an intermediate position between optical and electron micro- scopy. Synchrotron radiation-based X-ray microscopy benefits 389 from the possibility of varying such characteristics of the incident X-ray beam as energy and polarisation. Besides, utilisation of hard X-rays (7 ± 30 keV) does not require an ultrahigh vacuum analytical chamber (in contrast to all modifications of electron microscopy). This enables in vivo studies of biological systems and strongly facilitates investigations of large objects.Standard spatial resolution of hard X-ray microscopy is *0.25 mm. Utilisation of soft X-rays (0.2 ± 1.2 keV) in combination with advanced X-ray optics, such as FZP and BFO, makes spatial resolution of 30 ± 50 nm attainable. Penetration depth of soft X-rays into a condensed matter is higher than that of electrons, thus, the samples which are opaque for transmission electron microscope can be studied by X-ray microscopy. Furthermore, the use of electron beams induces greater radiation damage of the sample compared to synchrotron X-rays. Yet another advantage of X-ray microscopy is the possibility to achieve element and even valence contrast by varying the incident radiation wavelength and using absorption edges.Studies of biological objects are usually performed in the energy range referred to as the `water window', viz., 280 ± 550 eV. In this range, absorption by oxygen atoms (and thus by water molecules always abundantly present in biological samples) is relatively weak. By varying the beam energy within the `water window' it becomes possible to achieve contrast in visualisation of, for example, carbon and nitrogen atoms and thus independently estimate the spatial distribution of proteins and nucleic acids in the sample. X-Ray microscopy can be subdivided into contact, projection and scanning modifications depending on mutual orientation of SR source, sample and detector.211 ± 214 Recent achievements in the field of synchrotron X-ray microscopy are considered in detail in the monograph.215 X-Ray diffraction topography is a method of defect imaging in crystals.It is based on the analysis of intensity distribution of scattered radiation within the diffraction spot.216 Using X-ray topography it is possible to visualise point defects and dislocations with spatial resolution of 2 ± 3 mm, to analyse their dynamics upon application of a mechanical stress (for example, in piezoelectric crystals), as well as to study local distortions, deformations, inhomogeneities and microcracks in nearly perfect crystals.217 Diffraction topography is sensitive to gradients of unit cell parameter deformations Da/a of the order of*1077. This method can be used efficiently in studies of large single crystals which are opaque for visible light.The principal field of application of this technique is technological control of perfect crystals, for example, in manufacturing semiconductors. Utilisation of SR in this field leads to increase in the spatial resolution and decrease in exposure times (and therefore radiation damage to the sample).216 ± 219 X-Ray topography can be utilised as a fast preliminary test of single crystal quality. This can be particularly important in protein crystallography where diffraction experiments are very expensive and time-consuming. X-Ray computed tomography allows visualisation of internal structure of samples in different cross-sections. The image is reconstructed by mathematical processing of transmitted intensity profiles for various mutual orientations of the sample and radiation source.220, 221 This method is used for characterisation of such objects as technological workpieces, biological and geo- logical samples (e.g., microcavities and micropores in minerals).For image reconstruction, parallel or divergent, `white' or mono- chromatic synchrotron X-ray beams can be used. The spatial resolution of X-ray tomography is a few micrometers.222 As in the case of X-ray microscopy, the contrast of reconstructed images can be noticeably enhanced if incident radiation wavelength is chosen close to the absorption edges of certain elements in the sample. For instance, distribution of light and heavy elements throughout the sample may be independently reconstructed from X-ray tomography data collected at two strongly different wave- lengths [dual photon absorptiometry ± computed tomography (DPA-CT)].390 In modern imaging techniques, utilisation of the coherent properties of SR acquires increasing importance.In this connec- tion, coherent synchrotron X-ray radiation of third-generation SR sources allows one to achieve phase contrast besides standard absorption contrast in images. Absorption contrast is obtained as a result of differences in X-ray absorption coefficients of different points in the sample, whereas phase contrast is due to phase shifts of scattered coherent radiation, which gives rise to an interference pattern superimposed on the sample image.This interference pattern depends upon sample-to-detector distance and therefore very high contrast images may be obtained by adjusting this distance.223 ¡¾ 225 Coherent properties are also utilised in a new SR-based visualisation technique, viz., X-ray photon correlation spectro- scopy (XPCS) or intensity fluctuation spectroscopy. This method is an X-ray analogue of visible light dynamic scattering, which is widely used in studies of colloid systems. The X-ray photon correlation spectroscopy is based on observation of a real-time evolution of the speckle interference pattern produced by scatter- ing of coherent X-ray radiation on disordered samples. Using this method, it is possible to study low-frequency dynamic processes (1073¡¾ 106 Hz) in the sample and visualise short-range density fluctuations within the distances range of the order of 1 nm.226 This method has been successfully applied to study Brownian motion of metal nanoparticles 227 and copolymer micelles in solutions,228 dynamics of charge density waves, critical fluctua- tions near the point of order ¡¾ disorder phase transitions in amorphous alloys,229 conformational dynamics of polymers upon glassification,230 etc.6. Other methods a. Utilisation of anomalous scattering In the theoretical description of X-ray scattering on atoms, the atomic scattering factor f0 (sometimes also referred to as atomic form factor) is of key importance. The atomic scattering factor is a smooth function of scattering angle y, monotonically decreasing with an increase in y from 0 8 to 180 8.Atomic scattering factors for adjacent elements in the D I Mendeleev Periodic Table are close. Over a wide range of wavelengths, f0 is virtually independent of incident radiation energy. However, if incident radiation energy approaches an X-ray absorption edge of an element in the sample, a wavelength- dependent correction to f0 has to be introduced into the scattering formalism. This correction term is commonly referred to as anomalous dispersion (Fig. 24). In the energy range of X-ray resonance absorption, scattering is accompanied by intensity attenuation. In this case, the total atomic scattering factor f(y,l) formally acquires a complex value (7) f (y,l)=f0(y)+f 0 (y,l)+i f 00(y,l), where f0 is the wavelength-independent part of atomic scattering factor (rapidly decreasing with an increase in scattering angle); f 00 f 0 E0 E E E0 Figure 24.Real (f 0) and imaginary (f 00) parts of anomalous dispersion correction to the atomic scattering factor as functions of the radiation energy. Ya V Zubavichus, Yu L Slovokhotov f 0 and f 00 are real and imaginary parts of the anomalous disper- sion, respectively. The imaginary part of the anomalous disper- sion is directly proportional to the X-ray absorption cross-section, i.e., to X-ray absorption coefficient m, and its real part is linked with the imaginary part by the Kramers ¡¾ Kronig dispersion relation (8) f 0OoU a 2p O?0 o2 ¢§ Oo0U2 do0, o0f 00Oo0U where o is the radiation frequency.An anomalous dispersion contribution may substantially distort scattering intensities compared to values calculated on the base of f0 only. In contrast to f0, the f 0 and f 00 terms only weakly depend on the scattering angle, so the relative contribution of anomalous dispersion is higher for large scattering angles. As a rule, the energies of X-ray absorption edges for adjacent elements in the D I Mendeleev Periodic Table differ from each other by a few hundreds of eV, consequently, account for anomalous dis- persion leads to substantial differences in atomic scattering factors (and thus to different X-ray scattering patterns) for such elements when the scattered radiation wavelength is close to an X-ray absorption edge of one of these two elements.Moreover, due to different chemical shifts of X-ray absorption edges in different compounds of one element, atomic scattering factors can become noticeably different even for atoms of one element in different chemical environment or charge state (valence contrast). Advanced techniques based on application of anomalous scatter- ing and respective experimental diffraction results are thoroughly discussed in a number of recent publications.231 ¡¾ 233 The anomalous scattering combines X-ray absorption and scattering phenomena. In order to utilise it in diffraction experi- ments, it is necessary to collect diffraction data with incident radiation of continuously variable wavelength (which is possible only with synchrotron sources) or to carry out a series of diffraction experiments at several wavelengths, at least one of which being close to an X-ray absorption edge of an element in the sample.In the latter case, the difference between two diffraction patterns in the proximity of the edge and apart from it will be primarily caused by scattering on the absorbing atoms. Anomalous scattering of synchrotron radiation is widely used in powder X-ray diffraction. Using this approach, atoms of certain elements can be precisely located in the unit cell, which substantially facilitates structure solution. This technique is often referred to as resonance X-ray diffraction. In studies of amor- phous samples using anomalous wide-angle X-ray scattering (AWAXS), partial radial distribution functions may be deter- mined separately for each heavy element.232 This makes interpre- tation of experimental results much more reliable and unambiguous.A similar approach is adopted in anomalous small-angle X-ray scattering (ASAXS), for example, for determi- nation of mean size and size distribution of particles in supported metal catalysts and nanoparticles stabilised in polymer matrices or for analysis of metal distribution in bimetallic nanoclus- ters.234 ¡¾ 236 In conventional single crystal X-ray crystallography, account for anomalous dispersion can be used in determination of the absolute configuration of a non-centrosymmetrical (chiral) crystal and its constituent molecules.When SR is used, data collection at the wavelength of resonance absorption allows unambiguous establishment of the chemical nature of anomalously scattering atoms as well as charge states for crystallographically non- equivalent atoms of one element in mixed-valence complexes, the so-called valence contrast X-ray crystallography. Capabilities of this technique has been demonstrated for a series of inorganic substances in which atoms of one element in different oxidation states occupy crystallographically non-equivalent positions, e.g., Fe3O4,237 GaCl2,238 NbSe3,239 as well as for a number of mixed- valence polynuclear complexes.240, 241X-Ray synchrotron radiation in physicochemical studies Utilisation of anomalous dispersion in X-ray crystallography is most logically implemented in the multi-wavelength anomalous diffraction (MAD) method, which becomes increasingly popular in protein crystallography.242 ± 245 For a single crystal containing atoms of heavy elements, it is possible to estimate experimentally the phase of any reflection based on its intensity at three different wavelengths before, directly at and above the X-ray absorption edge of the corresponding element.Thus, MAD helps to over- come the key problem of single crystal X-ray crystallography, the so-called problem of initial phases: in order to transform the set of diffraction data into the target electron density distribution map, it is necessary to assign certain phases (which cannot be directly determined from standard diffraction experiment) to experimen- tally measured intensities of reflections.In small-molecule crys- tallography, reliable direct methods of statistical estimation of the initial phases have been developed, whereas in protein crystallog- raphy, the search for initial phases remains very complicated and not always solvable problem. Determination of reflection phases using MAD is a much more laborious procedure than common diffraction measure- ments, however the information obtained is often worth the time spent. The MAD method gradually displaces multiple isomor- phous replacement (MIR) method in protein crystallography. This formerly popular but much less versatile method is based on comparison of diffraction data sets for crystallographically isomorphous derivatives of a protein with different heavy sub- stituents.Many biological macromolecules in their native form contain heavy elements (iron, molybdenum, zinc, copper, man- ganese, etc.) and thus the MAD-phasing procedure is directly applicable to them. Recently, first experiments have been under- taken, which demonstrated in principle the possibility of MAD application in the soft X-ray region at the absorption edges of sulfur and phosphorus. However technical problems arose related to rapid radiation-induced decomposition of the sample due to strong X-ray absorption.246, 247 Heavy anomalously scattering atoms can be introduced into samples under study by chemical methods. For instance, calcium or magnesium ions in biological molecules can be replaced by heavier lanthanide ions; zinc ions, by mercury, etc.In this case, the substituted derivative does not have to be crystallographically isomorphous to the native protein (which is a prerequisite for MIR), it is sufficient that the primary amino acid sequence and the main conformation of the protein molecule are conserved. Heavy elements can be introduced into biological molecules as labelling fragments, which are chemical analogues of their native building blocks. Among such labels, selenomethionine for proteins (MAD at the SeK-edge) and brominated uracil for nucleic acids (MAD at the BrK-edge) are most often used. Yet another route for incorporating heavy elements into crystals is to perform a dif- fraction experiment under pressure of Xe.In this case, Xe atoms replace water molecules in cavities of the crystal lattice without disturbing the general structure of the biopolymer, and MAD- phasing at XeK-edge is then applied.248 Anomalous dispersion can be efficiently used to enhance research capabilities of X-ray absorption spectroscopy.249 In this connection, the diffraction anomalous fine structure (DAFS) method, which can be considered as a diffraction modification of XAFS, is actively developing. In the DAFS method, the intensity of a selected diffraction reflection is measured while varying the incident radiation energy so as to pass across an X-ray absorption edge of a certain element.Since imaginary part of the anomalous dispersion is directly proportional to the X-ray absorption coef- ficient m, the dependence of reflection intensity upon the incident radiation energy manifests XAFS-like fine structure. The DAFS method can be applied to studies of both single crystals and polycrystalline powders. In particular, it is useful for investigations of mixtures of polycrystalline powders since it allows registration of XAFS spectra at the edge for a certain element separately for each phase. Furthermore, since crystallo- graphically non-equivalent atoms of one element make different 391 contributions to the intensity of different reflections, DAFS data measured for several diffraction reflections allow one to extract contributions into XAFS spectrum from each crystallographically independent group of atoms of a certain element.Capabilities of DAFS have been demonstrated on such classes of substances as spinels, complex oxides, superconductors, etc.250 ± 252 b. X-Ray standing waves In addition to anomalous dispersion, X-ray absorption and diffraction phenomena are manifested simultaneously in the method of X-ray standing waves (XSW).253 Under conditions of a specular reflection or Bragg's diffraction in a nearly perfect single crystal, the intensity of the electromagnetic wave is redis- tributed so as to form a standing wave. Differences in local intensities of the electromagnetic field in points of nodes and antinodes of the standing wave lead to different absorption and thus to different yield of all secondary processes, such as X-ray florescence, photo- and Auger electron emission, etc.in the corresponding atomic layers.254, 255 Experimentally, it is mani- fested in a strong dependence of intensity of respective secondary processes on the incidence angle of the X-rays on the crystal or on the wavelength of the X-rays at constant incidence angle. In the latter case, the geometry of normal incidence of radiation to the surface of the crystal is often selected; this technique is referred to as normal incidence X-ray standing waves (NIXSW).256 The XSW method, among very few other instrumental tech- niques, allows structural investigations of amorphous near-sur- face layers of crystals with thicknesses from several nm to several mm.It is also used for precise measurement of layer thickness in multilayer periodic structures as well as for localisation of impurity atoms in crystal lattices, e.g., dopant atoms in semi- conducting single crystals, implanted ions, guest metal ions in biomembranes, etc.257 ± 259 For example,XSWhas been applied to study gallium arsenide single crystals with a surface doped with silicon using various techniques. As has been shown in the case of molecular beam epitaxy, all silicon atoms occupy the gallium positions of the crystal lattice. In contrast, in the case of ion implantation followed by annealing (or thermally activated dif- fusion), only 30% of Si atoms regularly occupy the gallium positions, the rest of Si atoms occupy random positions and of them *6% occupy the As positions.260 Modern theoretical concepts underlying the XSW method, its instrumentation and applications are discussed in recent reviews.256, 261 c.Surface study methods in the geometry of grazing incidence X-Ray reflectometry method is based on the analysis of X-ray reflectivity coefficient as a function of the X-ray incidence angle. In a number of cases, this method gives important information on the atomic structure of near-surface layers.262 As has been mentioned above (see Section III), under conditions of total external reflection, the ratio of reflected radiation approaches unity and the incident radiation does not penetrate deep into the sample interacting only with the thin near-surface layer (*10 nm).This effect can be used in order to increase surface sensitivity of `classical' bulk X-ray methods (such as X-ray spectroscopy and diffraction). In particular, total reflection X-ray fluorescence (TRXRF) is used for quantitative analysis of trace atoms (including light elements starting from boron) in near- surface regions of semiconductors, materials modified by ion implantation, etc.263 ± 265 Due to small penetration depth of X-rays into the sample under total reflection conditions, the method is extremely sensitive: the lowest detection limit expressed as absolute mass of an admixture is of the order of femtograms (10715 g).266 This method is used for admixture level control in such very pure objects as semiconducting single crystals.267 Grazing-incidence X-ray diffraction (GI-XRD) is used in studies of single crystal surfaces and other materials with atomi- cally ordered surfaces, in particular, those with specific two- dimensional ordering.Recently, this method has been applied to such objects as self-assembled monomolecular layers, Langmuir ±392 Blodgett, epitaxial, atomic deposition and CVD films, as well as two-dimensional crystals of lipophilic systems (fatty acids and their salts, o-amino acids, phospholipids) at the water ± air inter- face.268 ± 275 Grazing incidence small-angle X-ray scattering is used for characterisation of thin films and supported catalysts.276 Meas- urement of X-ray reflectivity coefficient as a function of incident radiation energy at the incidence angle close to the angle of total external reflection allows registration of XAFS spectra for near- surface layers: X-ray reflectivity coefficient is correlated with complex refraction coefficient, imaginary part of which is directly proportional to the absorption coefficient m(E) [see Eqn (7)].This technique is commonly referred to as Refl-EXAFS. The term SEXAFS (surface EXAFS) is also sometimes used but it combines an entire set of surface-sensitive techniques for EXAFS registra- tion including, in addition Refl-EXAFS, total electron yield, yield of photon stimulated ion desorption and so on.277 ± 279 d. The use of polarisation of synchrotron radiation The use of polarisation of electromagnetic radiation in physico- chemical analysis of substances with anisotropic atomic structures is based on dichroism, i.e., the dependence of optical properties of substances on the direction of the electric vector in the electro- magnetic wave propagating through the substance. For instance, plane-of-polarisation rotation angle per optical path unit is a key characteristic of substances with asymmetric (chiral) atomic structures; meanwhile, circular dichroism (CD) spectra give important information on mutual orientation of chromophore groups in molecules.In contrast to X-ray radiation produced by traditional sour- ces, viz., X-ray tubes, synchrotron radiation is always completely polarised. Similarly to visible light optics, two types of dichroism, viz., linear and circular, can be distinguished in the X-ray region.Polarisation of SR is most widely used in X-ray spectroscopy, since it primarily affects the probability of electron transitions. In the case of molecule excitation by a linearly polarised SR, the probability of an electronic transition of a certain symmetry depends on the mutual orientation of the electronic transition dipole moment (difference between dipole moments of the mole- cule in the ground and excited states) and polarisation direction of the excitation radiation. Upon resonant photoionisation of an atom by linearly polarised X-rays, the photoelectron wave gen- erated does not possess spherical symmetry: the probability of the photoelectron propagation is the maximum in the plane of polar- isation and is equal to zero in the perpendicular plane.In EXAFS, in particular, this results in the situation where atoms from the local environment lying exactly in this plane dominate in the Fourier transforms.280 A prerequisite for manifestation of linear X-ray dichroism is ordering and strict orientation of the sample under study relative to the incident SR beam. For example, adsorbed monolayers, which exhibit strong anisotropy of properties in the directions parallel to the layer (interaction within the layer) and perpendic- ular to the layer (adsorbate ± support interactions), can be effi- ciently studied using polarisation-dependent NEXAFS spectroscopy. This anisotropy causes a distinct dependence of X- ray absorption spectra on the incidence angle of the polarised SR beam or on the polarisation direction at the constant incidence angle. In particular, in the case of grazing-incidence, s and p polarisations of the incident beam are often used, which probe interactions parallel and perpendicular to the sample surface, respectively.281 Utilisation of linear polarisation of SR in X-ray microscopy with element contrast allows one to determine real- space orientation of atomic fragments in polymeric fibres, since the shape of the absorption edge (NEXAFS spectrum) becomes sensitive to the orientation of certain chemical bonds, such as C=C and C=O, which have a pronounced p*-reso- nance.54, 212, 282 Intercalation compounds into layered matrices (such as graphite and its chemical derivatives, transition metal dichalcoge- 2p3/2 2p1/2 Absorption coefficient Ya V Zubavichus, Yu L Slovokhotov nides, phosphates, etc.) constitute another group of objects for which the application of X-ray spectroscopy with linearly polar- ised SR is particularly informative.These systems in many cases manifest virtually ideal texture with the layers of matrix strictly parallel to the surface of the sample. Thus, using polarisation- dependent XAFS it is possible to determine, for instance, orienta- tion of molecules within the intercalated layers.283, 284 Analysis of polarisation-dependent EXAFS spectra for potas- sium niobate (perovskite type) made it possible to determine crystallographic directions of niobium atom shifts out of centres of octahedra formed by oxygen atoms in the phase transition.285 Studies of distortions in planar square CuO4 fragments in single crystals of high-temperature superconducting phases gave addi- tional information on the mechanism of superconductivity.286, 287 In quantum mechanical description of polarised light proper- ties, the term photon spin is often used.A photon, being a boson, possesses spin quantum number S=1 and two allowed spin moment projectionsmS=1 (the projectionmS=0 is prohibited for a photon due to its zero rest mass). A beam of photons with mS=1 corresponds to the right and with mS=71 to the left circular polarisation. The electrons with a defined spin direction (coinciding with the photon spin) are excited upon absorption of circularly polarised photons (spin polarisation).If in the sub- stance under study, all atomic magnetic moments are co-aligned (this is the case for a one-domain ferromagnetic or for a para- magnetic placed in a magnetic field), a distinct dependence of X-ray absorption coefficient on mutual orientation of the sample macroscopic magnetic moment and circular polarisation direction of the synchrotron radiation is manifested in XANES spectra (Fig. 25). This phenomenon first observed in 1987 288 was called X-ray magnetic circular dichroism (XMCD). Over recent years, the field ofXMCDexperienced rapid development since using this technique it is possible to analyse magnetic properties of materials at the atomic level, viz., with element specificity determine magnetic moments of atoms and identify spin and orbital con- tributions.289, 290 XMCD (its realisation is possible only in synchrotron radia- tion centres) has already become an established technique for characterisation of magnetic materials: thin films of 3d-metals a External magnetic field 3d b LIII LII Photon energy /eV Figure 25.Scheme of the origin of circular dichroism in X-ray absorption spectra (a) and typical XMCD spectrum at LII,III-edges of 3d- and 4d- metals (b). Circular polarisation of the incident beam is chosen so as to excite electrons with spins parallel (1) and antiparallel (2) to the external magnetic field.(arb. u.) EF Circularly polarised SR 12X-Ray synchrotron radiation in physicochemical studies (Fe, Co, Ni), lanthanide and actinide derivatives, materials with giant magnetoresistance (strong dependence of conductivity on the applied magnetic field), etc. Giant magnetoresistance is typical, for instance, of multilayers with alternating ferro- and diamagnetic layers. Such materials are promising in information storage technology.291 ± 294 In 1995, X-ray natural circular dichroism XNCD (i.e., X-ray circular dichroism, which is manifested without application of external magnetic field) was observed in some chemical systems, e.g., in chiral paramagnetic crystals.295 ± 297 Soft X-ray circular dichroism spectroscopy is widely used for characterisation of helical protein structures.298 The EXAFS spectra measured with circularly polarised SR produce RDF curves where atoms with non-zero magnetic moments dominate (spin-polarised or `magnetic' EXAFS) (Fig.26).299, 300 The X-ray circular dichroism is actively employed in modern versions of spin-resolved photoelectron spectro- scopy.301, 302 1 2 3 4 I II 0 2 4 6 8 R /A Figure 26. Normal (1) and magnetic (2) EXAFS spectra taken at HoLII- edge for Ho3Fe5 O12. (1) Ho7O 2.3 ± 2.4 A; (2) Ho7Fe 3.0 ± 3.7 A; (3) Ho7Ho 3.7 A; (4) Ho7Ho 5.6 A.299 In contrast to X-ray spectroscopy, polarisation properties of SR have not found broad applications in X-ray diffraction techniques, though many phenomena accompanying diffraction exhibit polarisation dependence, which can be used for extraction of additional information.Besides, polarisation has to be taken into consideration in development and optimisation of X-ray optical elements designed for experiments with SR. Thus when linearly polarised X-rays with energy close to an X-ray absorption edge of an element in the sample is scattered by a low-symmetry single crystal, the dependence of atomic scattering factor on the scattering angle becomes anisotropic. This anisotropy is due to the fact that the absorption coefficient for the crystal (and thus corrections to the atomic scattering factor owing to the anomalous dispersion) will be different for non-equivalent crystallographic directions. In particular, this leads to the appearance of diffraction reflections forbidden by extinction conditions.This phenomenon is called anisotropic anomalous scattering (AAS) or forbidden reflection near-edge diffraction (FRNED). It can be used for determination of positions of anomalously scattering atoms within the unit cell.303 ± 305 For such forbidden reflections, the polarisation type and direction of the scattered wave can be different from those of the incident SR, this change being depend- ent on the specific crystal structure.306 Utilisation of circularly polarised SR in X-ray crystallography makes determination of absolute configurations of chiral molecules in single crystals more FT amplitude (arb. u.) 393 reliable. X-Ray topography with circularly polarised SR is used for visualisation of magnetic domains.307 Polarisation dependence of reflection intensities in magnetic scattering allows separation of total magnetic moment density into spin and orbital contribu- tions.308 V.The main trends of applied studies using synchrotron radiation 1. Combined X-ray techniques In majority of cases, application of only one, even quite powerful instrumental method, appears insufficient to get insight into the atomic and electronic structure of real objects (a probable exception is X-ray crystallography of small molecules). Therefore, complex SR-based studies with involvement of a number of complementary X-ray techniques are more desirable. The organ- isational structure of synchrotron centres itself (concentration of highly skilled staff and advanced instrumentation in one building) promotes such a convergence stimulating interchange of ideas between different experimental techniques.Moreover, a limited number of experimental stations installed at modern international synchrotron radiation centres is used by numerous research groups with diverse scientific interests. Under such operational conditions, each experimental station has to be sufficiently flexible and versatile and allow solution of a wide range of research problems with minimum hardware modifications.309 ± 312 Among many advanced combined techniques routinely applied in SR centres to the studies of short- and long-range atomic order in various materials, the following combinations are most popular: XRD/XAFS (simultaneous registration of powder diffraction pattern and EXAFS spectrum) and XRD/DAFS (simultaneous registration of powder diffraction pattern and diffraction anomalous fine structure spectrum).313 ± 315 Specialised software has been developed for simultaneous refinement of crystal structure parameters from the experimental data of both methods.316 For structural characterisation of semi-crystalline and amorphous polymers, SAXS/XRD or SAXS/WAXS combi- nations are used.317 Protein crystallography is often supplemented with EXAFS data in order to determine exact geometry of metal centres.318 Besides, in experiments based on the MAD method, measurement of X-ray absorption spectrum from the very same crystal as used in the diffraction experiment is required for the calculation of f 0 and f 00 anomalous dispersion terms [see Eqn (8)] used in the phasing procedure.An ultrahigh vacuum chamber is the most expensive part of any device designed for surface studies. In order to use it with maximum efficiency, the respective device should be equipped with a broad range of functionality, for example, for utilisation of angular resolved photoelectron spec- troscopy, X-ray standing waves, SEXAFS, etc.319 A similar consideration is applicable to techniques based on utilisation of circularly polarised SR, where combination of magnetic scatter- ing, XMCD, spin-polarised EXAFS, spin-resolved X-ray photo- electron spectroscopy, etc., in one device could be required. Synchrotron radiation-based methods are often combined with traditional laboratory analytical methods, for example, XRD can be combined with differential scanning calorimetry for studies of phase transitions,317, 320 the combination `Quick- EXAFS/IR' can be used for the characterisation of matrix syn- thesis products.321 Laboratories for chemical, biological and materials science studies in modern SR centres are equipped with the modern instruments for realisation of traditional analytical techniques.The availability of traditional facilities (chromato- graphs, lasers, presses, scanning tunnelling and transmission electron microscopes, etc.) is particularly important for investiga- tions of such complex objects as surfaces, advanced materials, biological systems, etc.The trend to combine several different techniques in one device is realised in a new SR-based method, viz., event correlation or coincidence spectroscopy. In this method, several types of secondary emissions (fluorescent photons, photoelectrons, AugerÊà 394 electrons, secondary ions) initiated by resonance absorption of X-ray photons are simultaneously detected using specialised electronic circuits. As an example of this approach, mention can be made of photoelectron-photoion coincidence (PEPICO) spec- troscopy, which implies simultaneous registration of photoelec- trons and ions (with a time-of-flight mass spectrometer) produced by photoionisation of a free molecule by an X-ray photon.Other event correlation methods, such as Auger-photoelectron coinci- dence, photoelectron-fluorescent photon coincidence, etc., are also developing. These methods are used in investigations of photochemical reactions, determination of electronic structure and lifetime of excited states, studies of dynamics of photoionisa- tion and photodissociation of chemical bonds in free molecules or ions in the gas phase.322 ± 327 2. Studies of substances under extreme conditions Studies of matter under extreme conditions are of great impor- tance for many fields of modern materials science, condensed state physics, geochemistry, mineralogy and geology.Akey component of devices for high-pressure researches is a diamond anvil cell (DAC), which allows production of pressures up to a few hundreds GPa at areas of several square micrometers (Fig.27). The choice of diamond as the material for such a high-pressure cell is dictated by its unique properties, such as extremely high mechanical strength combined with transparency for hard X-rays. A disadvantage in this case is unavoidable presence of glitches due to Bragg's reflections from walls of the cell. For local heating of the sample, a laser beam can be used and for cooling, a flow of liquid helium. The temperature and pressure inside the DAC can be monitored using structural (unit cell parameters, bond lengths) or spectral characteristics of an internal standard.A ruby crystal is often used as an internal standard with the shift of optical luminescence line as a parameter enabling monitoring.Experimental stations capable to achieve conditions (T&6000 K, P&200 GPa) close to that in the Earth's core (T&5000 K, P&300 GPa) are in routine operation for several years in a number of synchrotron radiation centres.328 ± 333 Along with static studies, dynamic investigations of substan- ces and materials under microsecond impacts or in propagating blast waves are of significant practical importance. The usefulness of synchrotron radiation in studies of this kind relates primarily to its high intensity (which allows beam penetration through the sample environment) and to the temporal beam modulation in the single-bunch mode of the storage ring operation.XRD (especially its energy-dispersive modification), X-ray crystallography, EXAFS and X-ray topography are most com- monly applied for characterisation of substances under high pressures. High-pressure studies using synchrotron MoÈ ssbauer spectroscopy are also under rapid development over recent years.199, 200 It has to be stressed that on most of specialised devices for high-pressure studies, several physical techniques are 7 6 4 1 5 5 3 2 7 Figure 27. General view of a high-pressure cell. (1) A sample under study, (2) a ruby crystal (internal standard used for pressure monitoring), (3) gasket, (4) tapered diamond anvils, (5) diamond anvil holder made of hard alloy, (6) steel housing, (7) pressure regulation mechanism (mechanic, pneumatic, hydraulic, etc.).Ya V Zubavichus, Yu L Slovokhotov 87654321 8 16 14 10 12 2y /deg Figure 28. X-Ray powder diffraction patterns of solid oxygen at room temperature in the pressure range of 77 ± 116 GPa (ESRF, Grenoble, France).341 P /GPa: (1) 77, (2) 85, (3) 88, (4) 96, (5) 101, (6) 105, (7) 110, (8) 116. At P*96 GPa, a structural phase transition associated with oxygen metal- lisation occurs. realised.333, 334 Recently, equations of state for such substances as molecular hydrogen, helium, oxygen, carbon and water have been determined up to pressures>100 GPa (Fig. 28) based on results obtained using SR. Phase diagrams of compounds which form the Earth's mantle and thus are very important in order to achieve understanding of geochemical processes (metallic iron, silicon, quartz, silicates and aluminosilicates) have been studied in a broad range of temperatures and pressures.335 ± 337 Results on structural monitoring of catalytic graphite ± diamond transformation in the presence of magnesium carbonates and metallic nickel 338 and structural transition of carbon dioxide into polymeric quartz-like structure and further to metallic state have been reported.339 In the last years, metallic states of substances and their mixtures which are gaseous under normal conditions (H2, NH3, CH4, etc.), have become attainable, which allows simulation of the state of the matter in cores of distant planets of the Solar system: Saturn, Uranus and Neptune.340 ± 342 A large part of all experiments with substances under extreme conditions is devoted to detection of unusual chemical properties and to studies of structure and properties of compounds not existing under normal temperatures and pressures.In particular, single crystals of molecular complexes formed in the hydrogen ± methane system under high pressures, viz., (CH4)2H2 and (CH4)(H2)2 , have been studied by synchrotron X-ray crystallo- graphy.342 In another study, structural behaviour of potassium under high pressures has been investigated.343 It has been shown thatKforms intermetallides with 3d-metals, which implies that its chemical properties approach those of transition metals. Pressure- induced phase transitions in semiconducting, superconducting and magnetic materials often become subjects of SR-based studies.344 ± 347 Such transitions are sometimes accompanied by a change of coordination type or charge state of one of elements in the sample with preservation of overall stoichiometry.348, 349 Structures and transformations of substances in supercritical media are widely studied.In particular, in a series of stud- ies,350 ± 352 structural changes of hydration shells of such ions as Rb+, Sr2+ and Br7 upon transition of water into a supercritical state have been monitored and a decrease of mean coordination number in the first coordination sphere has been revealed as a common effect. 3. Studies with high spatial resolution Modern X-ray optics is capable of producing soft X-ray beams with linear size of several tens nm and hard X-ray beams of Diffraction intensity (arb.u.)X-Ray synchrotron radiation in physicochemical studies submicron size. Technical progress in this field stimulates develop- ment of X-ray microprobe techniques: spatial resolution of such techniques has approached that of classical microprobe techni- ques utilising beams of charged particles, viz., electrons, protons or ions.353 ± 355 At the same time, synchrotron radiation gives access to a broader range of experimental techniques due to the possibility of detecting secondary radiation of various types, varying excitation energy, using polarisation and so on.{ Microdiffraction (especially in the Laue modification) is used in studies of single grains and intergrain regions in compact polycrystalline materials (metals, composites, ceramics, semi- and superconductors),362, 363 in analysis of local ordering areas in liquids or systems with a heavy disorder, in X-ray crystallo- graphic investigations of submicron single crystals (clays, micas, zeolites) 364 and in structural characterisation of microscopic mineral inclusions in biological tissues.365 Single fibre molecules are studied with synchrotron X-ray microdiffraction.366 Spatially resolved X-ray fluorescence spectroscopy allows one to map the distribution of elements in a sample with resolution of the order of micrometers.Microprobe modifications of X-ray absorption spectroscopy (EXAFS, XANES, NEXAFS) give additional infor- mation on the chemical state and local environment of atoms in the sample under study (Fig. 29).212, 367 Microprobe modification of XPS (sometimes referred to as `Super-ESCA') is actively used for characterisation of minute amounts of hardly accessible or hazardous substances.For instance, an XPS study of curium oxide available in the amount of*1 mg has been performed at the ALS synchrotron radiation centre (Berkley, USA).368 The average beam size in this experiment was *50 mm and the mass of the sample actually irradiated and contributing to the spectrum is estimated by the authors as only 4 ng. Spatially resolved informa- tion on supramolecular organisation of various objects can be obtained using microprobe modification of SAXS.369, 370 12 295 290 E /eV 300 285 Figure 29.Micro-NEXAFS spectra (carbon K-edge) for thin polymeric films with area of 0.1 mm2 on a scanning X-ray microscope (NSLS, Stony Brook, USA).212 (1) Polyethylene terephthalate; (2) polycarbonate. Synchrotron X-ray microprobe techniques are routinely used for characterisation of spatially inhomogeneous objects including biological systems (cells, living tissues, bones), natural samples (microinclusions in minerals, coals, soils), advanced materials (ceramics, nanocomposites) as well as for environmental control of industrial wastes. Recently, the use of X-ray microdiffraction has been suggested as an express diagnostic tool for multilayer optoelectronics devices.371 A set of synchrotron microprobe techniques (X-ray fluorescence, diffraction, XANES) with spatial { Instrumental realisation of X-ray microprobe techniques in modern synchrotron radiation centres and results of recent applications of these techniques are thoroughly discussed in a special issue of Journal of Electron Spectroscopy and Related Phenomena,356 as well as in a number of books and reviews (see, for example, Refs 215, 357 ± 361).Absorption coefficient (arb. u.) 395 resolution of 3620 mm2 has been used for determination of chemical composition, crystal structure and charge states of atoms in thin phosphor films of MAlO3 (M is a rare-earth metal) grown by combinatorial synthesis.372 Convergence of SR-based microprobe spectroscopic techniques and X-ray microscopy (see Section IV) during the last decade has lead to emergence of a new research area, viz., spectromicroscopy.356 4.Time-resolved studies of processes Evolution of SR sources and instrumentation, viz., gain in bright- ness, focusing optics optimisation, design of fast and sensitive detectors, development of specialised techniques for data collec- tion and processing, has lead to a substantial decrease in the time required for conduction of an X-ray experiment with SR. Collec- tion of XRF, EXAFS or SAXS data on facilities of modern SR centres may take only a few milliseconds. This approach is of great interest for chemistry, since it makes real-time studies of reaction dynamics possible. SR intensity is sufficiently high to obtain reliable results when the beam passes through walls of a chemical reactor (reaction chamber) or sample environment of another type. That is why most of synchrotron radiation-based methods can be realised in situ (see, for example, a special issue of MRS Bulletin dedicated to in situ studies of materials using synchrotron techniques 373).Real-time diffraction (the term `diffraction cinema' is some- times used) allows one to investigate dynamics of solid-state reactions, to detect formation of crystalline intermediates and, in the case of heterogeneous reactions, to follow the movement of the reaction front (interface).20 Hydrothermal synthesis is one of the most promising fields in modern inorganic chemistry. This allows simulation of geochemical processes of natural mineral synthesis and preparation of new micro- and mesoporous materials, which can be applied as molecular sieves, ion-exchange resins, adsorb- ents, etc.The applicability of synchrotron real-time X-ray powder diffraction to studies of hydrothermal synthesis reactions is discussed in Refs 374, 375. Changes in local environment of a reacting centre in the course of a chemical reaction can be analysed by means of Quick- EXAFS. In particular, such an approach has been utilised for detection of changes in local atomic order in active centres of catalysts.376, 377 Time-resolved EXAFS can be used as well for structural monitoring of chemical transformations in solu- tions.378, 379 Under the term Quick-EXAFS, the techniques with a rapid rotation of a standard crystal monochromator or so-called energy-dispersive EXAFS with a fixed crystal polychromator and a linear detector, are usually meant.380 ± 382 Recently,383 a new method aimed at a decrease in the XAFS data acquisition time has been proposed.It is based on a variation of the electron beam trajectory within insertion devices by adjust- ing operational parameters of the electromagnetic focusing system (Fig. 30) In this case, the crystal monochromator remains immo- bile and the beam intensity can be measured with a `point' detector, i.e., ionisation chamber, which has better characteristics (such as counting rate, accuracy and sensitivity) compared to modern linear detectors. As an alternative of the aforementioned instrumental techniques of Quick-EXAFS, a stop-flow method can be used.This implies withdrawal of samples from a reaction mixture at required time intervals followed by their instant freezing and investigation using standard techniques.384 Time-resolved X-ray photoelectron spectroscopy enables investigations of chemical reactions at surfaces. For instance, this technique has been applied in studies of kinetics of NO reduction by molecular hydrogen on a single crystal face (553) of rhodium.385 Data collected at the Super-ESCA station of the ELETTRA synchrotron centre allowed authors to detect inter- mediate unstable species (atomic nitrogen, atomic hydrogen, NHx) and suggest a mechanism explaining the oscillatory dynam- ics of this reaction.Time-resolved studies of biological processes give insights into mechanisms of key biochemical processes. For example, over the396 b Electron beam Trajectory 3 Trajectory 2 Trajectory 1 Wiggler Electromagnetic lenses for correction of electron beam orbit Figure 30. Experimental schemes for Quick-XAFS registration.383 (a) Energy-dispersive XAFS, (b) fast registration of XAFS by varying the electron trajectory within the channel of a storage ring. last years, a substantial progress has been reached in under- standing molecular mechanisms of muscle functioning thanks to SR-based studies of conformational transitions and supramolec- ular organisation changes of muscles under contraction and relaxation.Systematic real-time diffraction and SAXS studies of muscles are carried out in several international SR centres.386, 387 A combination of powerful third-generation SR sources and fast-counting 2D position-sensitive detectors opens new avenues in dynamic structural studies by means of Laue diffraction. Within this approach, a method of single crystal X-ray crystallog- raphy with time resolution down to tens of picoseconds is realised for biological macromolecules.388, 389 In this modification of the `diffraction cinema', a series of Laue patterns is registered during successive pulses of SR with a storage ring operating in the single- bunch mode. This approach is of great potential importance for investigations of enzymatic reactions. As one of the first applica- tions of this technique, structural changes occurring in a single crystal of carboxy-myoglobin upon visible light-induced decar- bonylation (Fig. 31) have been investigated.390 ± 392 Capabilities offered by storage rings operating in the single- bunch mode are far from full realisation.A traditional method in Fe Figure 31. Migration of CO (dashed lines) in the initial stages of carboxy- myoglobin decarbonylation initiated by a visible light flash (ESRF). Atomic positions are determined by refinement of a common initial structural model versus a series of successive Laue-diffractograms meas- ured for a single crystal of the substance with time resolution of*10 ps.392 Ya V Zubavichus, Yu L Slovokhotov a Linear detector White SR E3 Crystal polychromator E2 E1 Sample Ionisation chamber Sample SR Monochromator E1 E2 E3 this field is optical luminescence excited by X-ray pulse: its spectrum and decay dynamics are the key parameters in character- isation of new semiconducting and scintillating materials.393 With a high level of confidence, we may suggest that in the immediate future, the interest of researchers in temporally modulated SR will increase, since this can supplement research capabilities of many of the aforementioned techniques with time resolution.VI. Conclusion Research capabilities of SR in chemistry and related sciences are wide and yet not fully realised. Over the last 30 years, introduction of synchrotron radiation into physicochemical analysis has resulted in substantial improvement of the accuracy and sensitiv- ity of a few tens of physical methods.New synchrotron radiation centres are being constructed and the user community of the existing ones expands rapidly. The number of research papers reporting results obtained with the use of SR is thousands per year. Starting from 1994, the International Union of Crystallogra- phy publishes a dedicated Journal of Synchrotron Radiation, in which over 1000 papers on various synchrotron radiation-related topics have already been published. Key international and Russian national conferences with a dominant or major portion of synchrotron studies in their scope that took place over the period of 1994 ± 2000, are listed in Table 2.In certain fields of modern science (such as protein crystallog- raphy or XAFS), the synchrotron techniques has taken a leading position. And in such directions as development of MAD appli- cations in crystallographic studies of biological polymers or time- resolved structural studies using Laue-diffraction, the last few years can be considered as a real breakthrough. Investigations of magnetic properties of materials using circularly polarised X-ray SR play an important role in modern technology. Instrumentation of SR studies is under constant upgrade and sophistication. SR sources are evolving leading to even higher brilliance and stability and lower divergence of SR beams. According to predictions of machine R&D groups from several synchrotron centres [SSRL (Stanford, USA), ESRF, NSLS], fourth-generation SR sources may come into reality in the immediate future.In parallel, new ideas in the field of X-ray optics are emerging, which allows production of highly intense narrow-focus beams. Progress in this field is based to a greatX-Ray synchrotron radiation in physicochemical studies Table 2. Large conferences held in 1994 ± 2000 with a substantial portion of synchrotron topics in their scopes. Conference name Year 1994 1994 1994 1995 1996 1996 1996 1996 1997 1997 1997 1997 1997 1998 1998 1998 The Fifth International Conference on Synchrotron Radiation Instrumentation (SRI'94) The Tenth All-Russian Conference on Synchrotron Radiation (SR-94) The Eighth International Conference on X-ray Absorption Fine Structure (XAFS-8) The 16th European Crystallography Meeting (ECM-16) The First International Conference on Application of SR in Materials Science (SRMS-1) XAFS-9 SR-96 The 17th Symposium of the International Union of Crystallography (IUCr-17) SRI'97 European Conference `SR and Surface' International Conference Frontiers in Synchrotron Radiation Spectroscopy' (FSRS'97) The First All-Russian Conference on the Application of X-rays, Synchrotron Radiation, Neutrons and Electrons to Studies of Materials (XRSNE-97) ECM-17 SRMS-2 The Sixth International Conference on Applications of SR in Biophysics The Fourth European Conference on High-Resolution X-ray Diffraction and Topography (XTOP98) International School-Symposium `SR in Natural Sciences' XAFS-10 ECM-18 SR-98 IUCr-18 XRSNE-99 SRI'2000 XAFS-11 ECM-19 SR-2000 1998 1998 1998 1998 1999 1999 2000 2000 2000 2000 a References to conference proceedings published in refereed journals.extent on the latest achievements of X-ray lithography applica- tions, which is also under active development in synchrotron centres. New fast, precise and sensitive X-ray detectors as well as X-ray monochromators with higher energy resolution appear. With improvement of coherent properties of SR, essentially new directions, such as X-ray holography, become possible. A break- through in imaging techniques can be expected with development of X-ray free-electron lasers. Therefore, we may conclude that synchrotron radiation is one of the most promising points of growth for modern science.Starting from the 90's, the main SR centres turned into a new type of economic enterprises aimed at production of complex physical data. The overwhelming part of studies in modern centres have applied character and are tightly linked to technology. Despite the essentially profitless nature of `photon factories' (for instance, costs of 1 h of beamtime on an EXAFS station, which is one of the `cheapest' synchrotron techniques, is US $200 ± 400), contribution of synchrotron studies into modern competitive production is substantial. It explains, why the bulk of synchrotron studies in developed countries steadily increases and why many commercial companies tend to construct own stations at third- generation SR sources (for example, see review 405 on the use of synchrotron methods for characterisation of microelectronics materials prepared by the IBM R&D division on the basis of experimental results obtained at the NSLS).Up to the 80's, the Soviet Union has been one of the world leaders in the field of synchrotron radiation theory and instru- mentation. Many important steps in maturation of synchrotron radiation as a research tool have been made in the USSR. One of the first synchrotrons was constructed in 1964, experimental observation of synchrotron radiation dates back to 1971, undu- lators were designed in 1977, superconducting multipole wigglers in 1979 and helical undulators � sources of circularly polarised quasi-monochromatic radiation in 1983.Presently, G I Budker Institute of Nuclear Physics, where Siberian Synchrotron Radia- tion Centre (SSRC) operates, maintains a leading position in this field. Insertion devices, position-sensitive detectors and other high-tech devices designed and produced in BINP are used in many SR centres over the world: CAMD (Baton Rouge, USA), HASYLAB (Hamburg, Germany), Photon Factory (Tsukuba, Japan), PLS (Pohang, South Korea). Meanwhile, the level and the volume of the applied synchro- tron studies in Russia is noticeably lower than those in the largest world centres in USA, Japan and Western Europe. To the date of publication of this review, there is only one routinely operating national SR centre, Siberian synchrotron radiation centre (SSRC) in Novosibirsk, with a first-generation storage ring VEPP-3.Kurchatov synchrotron radiation source with the dedicated `Sibir'-2' storage ring is on the stage of startup already for several years. Neither of the advanced complex techniques discussed above is involved in routine studies, which directly determine the level of modern science instrumentation. The authors hope that this review will draw attention of Russian researchers to this hot area of modern science. This review has been written with the financial support of the Russian Foundation for Basic Research (Project No. 99-03- City New York Novosibirsk Berlin Lund Chicago Grenoble Novosibirsk Seattle Himeji Castelvecco Pascoli Tokyo Dubna Lisbon Kobe Chicago Durham Ustron-Jazovec Chicago Prague Novosibirsk Glasgow Moscow Berlin Ako City Nancy Novosibirsk 397 Country Ref.a USA 394 Russia Germany 395 396 Sweden USA 77397 398 France Russia USA 399 Japan Italy Japan 400 Russia 7 Portugal Japan USA England 7777401 402 Poland USA Chech Republic Russia Scotland Russia Germany Japan France Russia 403 777404 77398 32810), International Center for Diffraction Data (ICDD) and INTAS (Y V Z Grant YSF00-4095).We would like to thank our colleagues A A Ulyanov (Department of Geology, Moscow State University) and V A Strel'tsov (University of West Australia), who have kindly provided the results of their studies used as illustrations in this review, and O A Belyakova who provided us with materials of School on Synchrotron Radiation (Trieste, Italy, December 1997). 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年代:2001

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Russian Chemical Reviews 70 (5) 405 ± 424 (2001) Regio- and stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to alkenes R P Litvinovskaya, V A Khripach Contents VIII. Vinyl- and allylphosphine oxides in cycloaddition reactions with nitrile oxides I. Introduction II. Cycloaddition of nitrile oxides to allylic and homoallylic alcohols and their derivatives III. Cycloaddition of nitrile oxides to unsaturated amines, amides and related compounds IV. Cycloaddition of nitrile oxides to acrylates, crotonates and acrylamides V. Cycloaddition of nitrile oxides to alkenes in the presence of Lewis acids VI. Cycloaddition of nitrile oxides to unsaturated aldehyde derivatives VII. Alkenylboronates in cycloaddition reactions with nitrile oxides IX. The nitrile oxide method in carbohydrate chemistry X.Steroid alkenes as dipolarophiles in cycloaddition reactions with nitrile oxides XI. Other examples of cycloaddition of nitrile oxides to acyclic alkenes Abstract. intermolecular of chemistry the on data published The The published data on the chemistry of intermolecular 1,3-dipolar types different to oxides nitrile of cycloaddition 1,3-dipolar cycloaddition of nitrile oxides to different types of of alkene stereo- of aspects Various systematised. are derivatives alkene derivatives are systematised. Various aspects of stereo- and and regiochemistry of this reaction are considered. The bibliography regiochemistry of this reaction are considered. The bibliography includes 182 references includes 182 references. I.Introduction 1,3-Dipolar cycloaddition of nitrile oxides to unsaturated systems is commonly used in the synthesis of heterocyclic compounds.1 ±5 This is due to at least two reasons. First, nitrile oxides react with a great diversity of dipolarophiles by virtue of their high reactiv- ities 1 and, second, the reaction products, viz., isoxazoles and 4,5- dihydroisoxazoles, can be used as starting compounds for various synthetic transformations including synthesis of natural com- pounds and their analogues.2, 3 This review summarises the published data on 1,3-dipolar cycloaddition of nitrile oxides to alkenes. Problems of regio- and stereochemistry of intermolecular cycloaddition (as a rule, to non- cyclic dipolarophiles) are considered in more detail.Nitrile oxides are usually generated by treating hydroximoyl chlorides with triethylamine, by dehydrating primary nitro deriv- atives with isocyanates or by oxidising aldoximes with N-chloro- succinimide.6 ±8 New effective approaches to the generation of nitrile oxides have been developed in recent years, e.g., from aldoximes under the action of sodium hypochlorite,9 from hydroximoyl chlorides in the presence of silver(I) acetate 10 or from conjugated nitroalkenes under the action of titanium(IV) chloride.11 The latter approach allows preparation of a-function- alised nitrile oxides. R P Litvinovskaya, V A Khripach Institute of Bioorganic Chemistry, National Academy of Sciences of Belarus, ul. Akad. Kuprevicha 5/2, 220141 Minsk, Belarus.Fax (37-517) 264 86 47. Tel. (37-517) 263 76 15. E-mail: litvin@ns.iboch.ac.by (R P Litvinovskaya) Tel. (37-517) 264 76 48. E-mail: khripach@ns.iboch.ac.by (V A Khripach) Received 7 December 2000 Uspekhi Khimii 70 (5) 464 ± 485 (2001); translated by R L Birnova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n05ABEH000644 405 406 407 409 412 414 415 416 416 417 420 Usually, it is only regiochemistry that is the matter of discussion in the cycloaddition of nitrile oxides to terminal alkenes, viz., the formation of a single regioisomer, i.e., 3,5- disubstituted (R1=H) or 3,5,5-trisubstituted (R1=Alk, Ar, etc.) 4,5-dihydroisoxazole.12 ± 17 R3 R1R2C CH2+R3CNO N R1 R2 O The analysis of stereochemical aspects of these reactions points to their low stereoselectivities 18, 19 unless carried out under special conditions (see below). Non-symmetrical 1,2-disubstituted alkenes usually give two regioisomers.20 ± 24 However, reactions of certain compounds manifest high regioselectivities with nitrile oxides.Thus the addition of acetonitrile oxide to the coumarin derivative 1 gave the regioisomeric dihydroisoxazoles 2a and 3a in a 75 : 3 ratio, whereas 4-methoxybenzonitrile oxide afforded exclusively the regioisomer 3b.25 R CO2Et N O H H CO2Et H CO2Et H R RCNO N OH H + O O O O O O 1 3a,b 2a,b R=Me (a), 4-MeOC6H4 (b). Attempts at performing asymmetric 1,3-dipolar cycloaddition using chiral alkenes or nitrile oxides have been undertaken, however, stereoselective introduction of a new asymmetric centre using chiral nitrile oxides was only partly successful.26 ± 29 Thus the reaction of the optically active nitrile oxide 4 with the alkene 5 afforded a mixture of diastereomers 6 in a 2.5 : 1 ratio.30 Reactions with other nitrile oxides yield nearly equimolar mixtures of diastereomers.406 Pri MeN CH2OR2 OR1 CNO + Me N Me R2O(CH2)3 4 5 CO2Et Pri MeN CH2OR2 Me (CH2)3OR2 OR1 Me N O N 6 CO2Et R1=CH2OMe; R2=ButMe2Si.II. Cycloaddition of nitrile oxides to allylic and homoallylic alcohols and their derivatives Allylic alcohols and their esters with a chiral centre in the a-position are often used as dipolarophiles; however, the diaster- eoselectivity of these reactions is not always high.As a rule, reactions of nitrile oxides with allylic alcohols give predominantly syn-isomers.31, 32 Cycloaddition of alkenes containing protected hydroxy groups in the a-position to the double bond increases the stereoselectivity.33 ± 36 The reaction of the nitrile oxide 7a with (S)-(+)-O-isopropylidenebut-3-ene-1,2-diol (8) is among the first examples of regioselective cycloaddition of nitrile oxides to allylic alcohols; this resulted in the anti-adduct 9a (9a : 10a=4:1) as the main product.35 In this case, the stereochemistry of the cyclo- addition reaction is governed by the oxygen-containing substitu- ent in the allylic position.Me O Me H RCNO + O 7a ± g 8 Me Me O O Me Me H H O O R R + H H 10a ± g O N 9a ± g O N , Ph (d), R=EtO2C (a), Me (b), THPOCH2 (c) THP= O O MeO CH2 (g). CH2 (e), Et (f), O Nitrile oxides 7b ± g also react with compound 8 with high diastereoselectivities with the predominant formation of anti- addition products (80% ± 94%); the selectivity of this reaction is only weakly dependent on the structure of the nitrile oxide.34 But- 3-ene-1,2-diol containing unprotected hydroxy groups manifests low diastereoselectivity in reactions with nitrile oxides, this is somewhat higher with dioxolanes analogous to compound 8.33 The highest diastereoselectivity is achieved with spiroketals of vicinal diols.36 It is assumed 34 that the cycloaddition proceeds predominantly via the transition state corresponding to the con- former A (the Kozikovsky model).Nitrile oxide attacks the C=C bond on the side opposite to the C7O bond (indicated by an arrow). Thus, the allylic oxygen atom displays an anti-directing (the so-called antiperiplanar) effect. According to the calculations R P Litvinovskaya, V A Khripach made by Houk et al.,37 this is due to minimisation of secondary interactions of the non-bonding orbitals. Me Me O O H CH2 H A The cycloadditions of pivalo- and benzonitrile oxides to a-hydroxyallylsilane (11a) and its O-derivatives 11b ± g have been studied.31, 38 Compound 11a yields a mixture of syn- and anti-adducts with a small excess of the syn-isomer 13a; with different acylated derivatives, the stereoselectivity of this reaction is weakly affected, whereas silyl ether 11b gives predominantly the anti-addition products 12b (yield 94%).SiEt3 R2CNO OR1 11a ± g N O N O SiEt3 R2 SiEt3 + R1 OR1 OR1 13a ± g 12a ± g R1=H(a), SiMe2But (b), COMe (c), CO2Me (d), 4-MeOC6H4CO (e), COPh (f), 4-NO2C6H4CO (g); R2=But, Ph. It was shown 31 that the syn-selectivity of cycloaddition of nitrile oxides, which is characteristic of the majority of allylic alcohols studied so far, decreases with the increase in the proton- accepting capacity of the solvent used. The stereochemistry of cycloaddition of benzonitrile oxides to allylic alcohols and their ethers, allylic chlorides and 3,3-diphe- nylprop-1-ene has been studied in detail.39, 40 N O N O R ArCNO R R + Ar Ar X X X 14b 14a R=Me, Ph, Et, But; X=OH, OMe, OBn, OTHP, OSiMe2But, Cl, Ph, Me; Ar=Ph, 4-NO2C6H4 .In this case, too, allylic ethers react with nitrile oxides in a stereoselective manner to give the anti-addition products 14a (up to 95% for X=OMe, OSiMe2But); the selectivity of this reaction increases with the increase in the size of the substituent R. Alcohols yield predominantly the syn-addition products 14b, albeit with low selectivity. The replacement of the ether allylic substituent by Cl, Ph or Me results in a complete or partial loss of stereoselectivity. It is of note that the ratio of the diastereomers strongly depends on the nature of the substituent in the nitrile oxide.Such a behaviour of allylic halides has been described by other authors.41, 42 Based on the experimental data 37, 43, 44 and the calculations of transition state models of 1,3-dipolar cycloaddition of nitrile oxides to alkenes, it was suggested that the majority of transition states are structurally close to that shown in Fig. 1a. In this case, the substituents in the allyl position can be localised outside, inside or in the anti-position with respect to the C7O bond formed. The main product is formed from the transition state A where the largest substituent (L) is in the anti-position, whereas the medium substituent (M) is located inside (Fig. 1b). The transition state B (Fig. 1c) yields the minor isomer. The outside position is more sensitive to steric factors due to the interaction of nitrile oxide with the oxygen atom.The electronic characteristics of substituents play a prominent role. Since cycloaddition of nitrile oxides is related to weakly electrophilic reactions, the donor substituent in the allylic position of the transition state is pref- erably in the anti-position, while the acceptor substituents areRegio- and stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to alkenes b c a N N N O O O inside S M outside M S H H H anti L L B A Figure 1. The transition state models in the reaction of 1,3-dipolar addition of nitrile oxides to alkenes.43, 44 Designations for substituents: L is large, Mis medium, S is small. localised either inside or outside (in order to minimise electronic interactions).39 This transition state model commonly referred to as the Houk model provides a rationale for the effects of substituents on the stereochemistry of 1,3-dipolar cycloaddition. The formation of a hydrogen bond between the oxygen atom of the nitrile oxide and the hydroxy groups of cyclic allylic alcohols results in stabilisation of the threo-transition state A (Fig.1b). The relatively low stereoselectivities in the case of acyclic allylic alcohols can be assigned to the fact that conforma- tions A and B compete with one another in the transition state.43, 44 It is of note that the increase in stereoselectivity with the increase in the size of the substituent R in the allylic alcohol fits well the Houk model, but cannot be explained in terms of the Kozikovsky transition state model.34 Thus the cycloaddition of nitrile oxides to terminal alkenes with different sizes of substituents in the allylic positions gives predominantly the diastereomers 15.The stereoselectivity depends on the relative sizes of the medium (M) and large (L) groups and varies from very low (M=Me, L=Et, 15 : 16=1:1) to moderate (M=Me, L=But, 15 : 16=4 : 1). The same ten- dency is observed where M=AlkO group, although the stereo- selectivity is somewhat higher in this case. The anti-diastereomer 17 usually predominates over the syn-diastereomer 18 (*3 : 1). For R2=Bui, it is the anti-isomer that is predominantly formed. The ratios of syn- to anti-products in these reactions change little with the change in steric and electronic factors in both the nitrile oxide substituent R3 and the oxygen atom substituent R1.39 N O N O S RCNO + L L R R M L M S S M 16 15 N O N O H R3CNO R2 R2 + R3 R3 R1O R2 OR1 OR1 18 17 Assuming that the steric and electronic effects of the trime- thylsilyl group 43 in the allylic position favour the formation of anti-cycloadducts, the behaviour of silanes of the type 19 has been studied in the reaction with nitrile oxides.45 N O N O Me RCNO SiMe3 SiMe3 R +R 19 SiMe3 Me Me 21 20 R=Ph, But.However, the diastereoselectivity of this reaction was low, although the main product 20 (60% ± 65%) was formed in accord with the Houk model.Thus, high diastereoselectivity is difficult to achieve in the absence of an oxygen-containing substituent in the allylic position,. Recent studies have shown 46 that the ratio of 5- and 4-silyl- substituted adducts 22a and 22b obtained by cycloaddition of 407 nitrile oxides to trialkoxy(vinyl)silanes depends on the nature of substituents at the silicon atom and the method used to generate nitrile oxides. This reaction is especially selective for R1=Me3SiO and R2=Me (22a :22b=10 : 1). R13 Si R13 Si R2CNO + R13 SiCH CH2 O O R2 R2 N 22b N 22a R1=MeO, EtO, Me3SiO, NCH2CH2O; R2=Me, Et, Ph. It was shown 47 that cycloaddition of nitrile oxides to 1,1- disubstituted alkenes 23 containing a hydroxy- or an alkoxy group in the allylic position (the so-called Baylis ± Hillman adducts) gives predominantly the syn-isomers 24, the stereoselectivity of the reaction being dependent on the substituent R.OX OX OX CO2Me CO2Me R R CO2Me PhCNO + O O R N N Ph 24 Ph 25 23a ± d X R Ratio 24 : 25 Ratio 24 : 25 Com- pound 23 Com- R X pound 23 c a Me HAc b 3 : 1 ButMe2Si 6 : 1 1 : 1 PrnH 12:1 d ButMe2Si 7 : 1 3 : 1 PriH 32:17 : 1 9 : 1 2 : 1 4 : 1 1 : 1 ButMe2Si Ac Ph HButMe2Si Ac Ac III.Cycloaddition of nitrile oxides to unsaturated amines, amides and related compounds The directing effect of the nitrogen atom in the allylic position has been studied in less detail than that of the oxygen atom.Cyclo- additions of nitrile oxides to vinylglycine and 2-aminobut-3-en-1- ol have been described;48 ± 50 the selectivities of these reactions were low. Reactions of nitrile oxides with (R,S)-4-vinyl-2-phenyl- 4,5-dihydrooxazole (26a), N-benzyloxycarbonyl-(4R)-vinyl-2,2- dimethyloxazolidine (26b) and (2R)-(N-benzyloxycarbonyl)but- 3-enol (26c) have been studied.51 It was found that the selectivities of the reactions of the cyclic compounds 26a,b are higher than that of the acyclic analogue 26c; the erythro-isomers of the 4,5- dihydroisoxazoles 27 are the main products. The reaction of alkene 26a with EtO2CCNO oxide occurs with the highest selectivity (the 27 : 28 ratio is 82 : 18). O N O N R2CNO + R1 R2 R1 R2 R1 26a ± c 28 27 CH2OH O (c); R1= N (a), BocN NHBoc Ph OMe (b), Me R2=Ph, EtO2C, Br.The effect of introduction of an asymmetric centre into the nitrile oxide component has been studied.52 Cycloaddition of, e.g., 4,5-dihydrooxazole-4-carbonitrile oxide 29, to styrene yields a 1 : 1 mixture of 5-phenyl-4,5-dihydroisoxazoles 30 and 31. This phe- nomenon is attributed to the large distance between the asym- metric centre of the molecule and the newly formed centre.408 ONC O PhCH CH2+ N29 Ph N O N O Ph Ph O O + N N 31 30 Ph Ph A series of 3-substituted cyclopentenes 32a ± p including amides 32f ± p have been studied as dipolarophiles;53 variation of the amide acyl group made it possible to change the acidity of the proton in the amide substituent Ph N N O R PhCNO O Ph R+ R + 32a ± p 34a ± p 33a ± p Ph N O N O Ph R R + + 36a ± p 35a ± p R=Me (a), NMe2 (b), OMe (c), OAc (d), OH (e), NHCOPh (f), NMeCOPh (g), NHCOMe (h), NHCOCF3 (i), NHCOC4F9 ( j), NHCSMe (k), NHCOC6H4OMe-4 (l), NHCOC6H4CF3-3 (m), NHCOC6H4NO2-4 (n), NHSO2Me (o), NHSO2CF3 (p).The content of cycloadducts formed as a result of the nitrile oxide attack on the alkene on the anti-side with respect to the substituent R exceeded 92% for compounds containing a methyl (32a), a dimethylamino (32b), a methoxy (32c) or an acetoxy (32d) group in the allylic position. The reaction of compound 32e containing an allylic hydroxy group in diethyl ether afforded the adduct 33e in 30% yield. This adduct was formed due to the formation of a hydrogen bond in the transition state of the reaction.If the reaction is carried out in benzene, which binds a proton more weakly than ether, the yield of the product 33e increases to 50%. It is assumed that the hydroxy derivative 32e forms a hydrogen bond with the solvent in ether and with nitrile oxide in benzene. The cycloaddition of the allylic benzamide 32f yielded the adducts 33f and 36f (9 : 1), which can also be ascribed to the formation of a hydrogen bond in the transition stateCof the cycloaddition. N C O Ph HNCOR C The model proposed sheds more light on the effects of solvents and amide substituents on the reaction rate. The product 33f is predominantly formed in accordance with the model C, since the energy of its transition state decreases due to the formation of a hydrogen bond, while the energies of the transition states leading to the other three products are not changed.Therefore, the alkene 32f should be more reactive than compounds unable to form hydrogen bonds with the nitrile oxide. Indeed, experiments on competitive reactions of compounds 32f,g with pivalonitrile oxide yielded three products all of which were formed by cycloaddition of nitrile oxide to the alkene 32f and the yield of the adduct of the type 33 reached 85%. R P Litvinovskaya, V A Khripach Competitive cycloadditions in benzene, 1,2-dimethoxyethane, dimethylformamide and hexamethylphosphoramide (HMPA) have been studied in order to establish the role of the solvent in this reaction.54 It was found that the tertiary amide 32g is much less reactive than the amide 32f.In the case of the amide 32g, the nature of the solvent influences neither the degree of conversion of the starting compounds, nor the ratios of the reaction products. Contrariwise, in the case of the amide 32f the concentration of the reaction product 33f increases significantly with the increase in the proton-donor capacity of the solvent; while the degree of con- version of the starting compounds changes only slightly. When the reaction was carried out in HMPA, the distribution of the reaction products was similar for both amides, which can be explained within the framework of the model C. Substitution of nitrile oxide for the solvent as a hydrogen-bonding acceptor decreases the rate of formation of the adduct 33f, but the rates of formation of other reaction products change little, as a result of which the total degree of conversion decreases. The model C suggests also that the yields of cycloadducts of the type 33 depend on the ability of the amide to form a hydrogen bond.Thus studies of amides 32h ± p revealed a marked tendency of an increase in the reaction selectivity with increase in the acidity of the amide proton. It is of interest that the sulfonamide group for which the acidity of the proton is the highest in this series, does not manifest high selectivity. In all probability, the bulky sulfonyl group prevents the formation of the transition state C.The effects of other factors should not be ruled out either. These data show that the introduction of a disubstituted amide substituent into the allyl position of alkenes can change both the regio- and stereoselectivity of cycloaddition of nitrile oxides. The formation of hydrogen bonds in the transition state also plays a significant role in this effect. It is of note that the selectivities of cycloaddition reactions with acyclic allylic amides studied earlier 55 ± 58 are low. However, it was shown 59 that amide substituents influence the selectivity of cycloaddition stronger than the allylic hydroxy groups, this effect being especially well-pronounced in the case of cyclic amides. The stereoselectivity of cycloaddition of the homoallylic amides 37a ± d to nitrile oxides is low in all cases, being compara- ble to that of the homoallylic alcohol 37e and amine 37f.R ButCNO Pri 37a ± f N O N O R R + But But Pri Pri R=NHCOMe (a), NHCOCF3 (b), NHSO2Me (c), NHSO2CF3 (d), OH (e), NH2 (f). The regioselectivity of cycloaddition of pivalonitrile oxide to allylic and homoallylic acyclic amides containing a disubstituted double bond is also low, e.g., the regioisomers 38a and 38b are formed in a*2 : 1 ratio.59 Me N Ph ButCNO n O O O Me Me n n N Ph N Ph + O O But But N N 38b 38a n=1, 2. At 20 8C, the cycloaddition occurs slowly; after 5 days, 70% of the original alkene remains unconsumed. At 80 8C, the yield is higher, but the regioselectivity is still very low. It was concluded 59 that the formation of a hydrogen bond in the transition state of acyclic systems is not critical, otherwise the amount of the regioisomer 38a would have been much higher.Regio- and stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to alkenes The hydroxy group plays the role of a proton donor when hydroxamic acid 39 is used as a dipolarophile. This reaction gave three isomers (40 : 41 : 42=11 : 2 : 4).This suggests that the hydroxy group of hydroxamic acid is a better directing group than the hydroxy groups of alcohols and produces an effect comparable to that of the NH groups.59 OH ButCNO N COPh 39 N N N But But But O O O + + OH OH OH N N N COPh COPh COPh 42 41 40 IV.Cycloaddition of nitrile oxides to acrylates, crotonates and acrylamides An efficient approach to optically active dihydroisoxazoles might become both an alternative to the well-known aldol-type reactions and a new method for the synthesis of enantiomerically pure primary amines. Two approaches to asymmetric induction in the cycloaddition of nitrile oxides were used, viz., the introduction of an allylic asymmetric centre into a dipolarophile 34, 35 or the use of a chiral nitrile oxide.27 The attempts at developing efficient procedures for diastereoselective 1,3-dipolar cycloaddition of nitrile oxides using menthyl ethers of allylic alcohols 60 were unsuccessful. Thus 1,3-dipolar cycloaddition of 4-nitrobenzonitrile oxide with optically active acrylate 43a and allylic ether 43b yielded a mixture of the diastereomers 44 and 45, the diastereomeric excess in favour of the latter was only 4%.The selectivity of the reaction of 4-nitrobenzonitrile oxide with the bornyl ester 43c was some- what higher. Pronounced stereoselectivity was observed only in the reaction of the sulfonamide derivatives 43d,e with isobutyr- onitrile oxide and cyclohexanonitrile oxide. The diastereomers 44 having S-configurations of the newly formed chiral centres were the main isomers in these reactions.61 X R2CNO R1 O Y 43a ± e X X R1 R1 R2 R2 O O + Y Y N O N O 45 44 Y R2 X R1 Ratio 44 : 45 Com- pound 43 Pri 1 : 1 1 : 1 4-NO2C6H4 4-NO2C6H4 O H a CH2 H b Me Me Me OBn 1 : 1 O H c 4-NO2C6H4 Me Me MeCH2SO2NR32 1 : 3 1 : 4 1 : 2 1 : 1 1 : 1 O H d O H d O Me e O Me e O Me e But Ph 4-NO2C6H4 But Ph R3=Pri, C6H11 409 It is assumed 61 that the major diastereomers 44 result from the cycloaddition of the nitrile oxide to s-cis-confomers A of acrylates 43d,e, whereas minor diastereomers are formed by addition to the s-trans-conformers B.O O CH2SO2NR O CH2SO2NR O B A Later it was demonstrated 5 that the acrylates 43d,e containing sulfonamide groups enter into other cycloaddition reactions predominantly in the s-cis-conformations. Optically active 4,5-dihydroisoxazoles were synthesised by Akiyama et al.62, 63 who added nitrile oxides to chiral acrylates where cyclitols played the role of auxiliary chiral substituents. The acrylates 46a ± d gave mixtures of dihydroisoxazoles 47 and 48 in the reactions with benzonitrile oxide (R2=Ph).The diastereose- lectivities in the case of esters containing methoxymethyl (46a) and benzyl (46b) substituents were low, whereas the presence of bulky tert-butyldimethyl- and tert-butyldiphenylsilyl substituents in esters 46c,d provided high p-facial selectivity. This stereochemical result is due to the fact that the major isomer 47 is formed from the s-cis-conformer of the acrylate. The bulky tert-butyldiphenylsilyl group effectively blocks the attack from the opposite side of the double bond. The maximum diastereoselectivity was achieved when the reaction was carried out in benzene.O R2CNO R1 O46a ± d O O R1 R1 R2 R2 O O + 48a ± d 47a ± d N O O N O O OX R1= O O R2 R2 Ratio Ratio 47 : 48 Com- X 47 : 48 pound 46 Com- X pound 46 Bn CH2OMe Ph Ph ButMe2Si Ph abcd 1 : 2 1 : 1 5 : 1 ButPh2Si Ph 19 : 1 10 : 1 9 : 1 9 : 1 10 : 1 9 : 1 ButPh2Si Me ButPh2Si Et ButPh2Si n-C5H11 ButPh2Si But ButPh2Si 4-ClC6H4 ddddd The cycloaddition of 4-cyanobenzonitrile oxide to isobutyl vinylacetate affords a racemic mixture of dihydroisoxazoles.64 It was found that the hydrolysis of this mixture using the lipase PS30 gave optically pure [4,5-dihydro-3-(4-cyanophenyl)isoxazol- 5-yl]acetic acid with the R-configuration of the chiral centre.4-NCC6H4CNO CH2C(O)OBui CH2CO2H CH2C(O)OBui O O 4-NCC6H4 4-NCC6H4 N N The addition of nitrile oxides to bornyl crotonates 49 (R2=Me) gave two regioisomeric cycloadducts 50 and 51, each representing a mixture of diastereomers.65410 R3 N O N R2 R1O R3CNO R1O R1O O+ R3 O 49 O O R2 R2 51 50 Yield of adducts (%) R3 R2 R1 51 50 Me Me 22 25 65 75 Ph Me Me Me Me H Me Me Ph Me Me Me 26 25 79 56 46 H Me 7 Me Ph Me Me Ph Me Me Me Me 28 24 94 63 76 H Me 7 The diastereoselectivity of this reaction is>80%. The dia- stereomers were separated and enantiomerically pure 4,5-di- hydroisoxazoles were obtained after the cleavage of the ester bond. The regioselectivity of this reaction does not depend on either the substituent in the bornyl residue or the nature of nitrile oxide.The addition of nitrile oxides to acrylates 49 (R2=H) results in an almost overriding formation of the sterically preferrable products 51. The diastereoselectivity of this reaction is compara- ble to that of crotonates, which points to the similarity of the transition states generated by the attack of the dipole at the least shielded side of the alkene having an s-cis-conformation.66 The reaction of methyl cinnamate 52a with nitrile oxides affords a mixture of regioisomeric dihydroisoxazoles 53a and 54a (4 : 1).67 Quite unexpectedly, the use of other cinnamic acid derivatives, e.g., the tertiary amides 52b ± f, changed the regiose- lectivity of this reaction, whereas in the case of primary and secondary amines the regioselectivity was the same as with esters.It is also surprising that the substituent at the nitrogen atom virtually did not affect the regiochemistry of the reaction. Hence, its regioselectivity can be varied using different cinnamic acid derivatives and different nitrile oxides. O O Ph R R Y Y RCNO + Y N N Ph Ph 52 O54 O53 O Ratio 53 : 54 Y Compound 52 4 : 1 1 : 3 1 : 2 1 : 2 see a 1 : 3 abcdef OMe NEt2 N(Me)CH2CO2Me N(Ph)CH2CO2Me L-proline methyl ester N(CH2CO2Et)2 a Compound 54 is the main reaction product. The use of chiral acrylamides as dipolarophiles has been reported;68 ± 73 acrylamides can have two low-energy rotamers due to rotation around the C7N bond.Cycloaddition of nitrile oxides to the optically active acryloylsultam 55 gives a mixture of diastereomers 56 and 57; however, the diastereoselectivity of this reaction is rather high, especially in non-polar solvents.68 O RCNO N H SO2 55 O N SO2 O 56 N Solvent R C6H14 PhMe CH2Cl2 C6H14 C6H14 C6H14 But But But Ph Me Et O Me PhH (CH2)2 O These results prompted a transition state model the existence of which was confirmed by X-ray diffraction analysis of com- pound 55.68 According to calculations,74 the dipole ± dipole interaction of the C=O and S=O groups and, as a consequence, steric destabi- lisation, are absent only in the s-trans-conformer.However, this model cannot be applied to reactions promoted by Lewis acids, since the predominant conformation changes upon chelation.74 The results of quantum-mechanical calculations of the energy and geometrical parameters of various transition states suggest 75 that Coulomb interactions are the main factor which determines diastereofacial differentiation. The Coulomb interactions of the oxygen atom of the nitrile oxide with the sultam oxygen endo- atom and, to a lesser degree, with the sulfur atom determine the energy predominance of the transition states during the approach of nitrile oxide to the double bond above the sultam ring which affords compounds of the type 56 (although the approach of the nitrile oxide to the double bond under the sultam ring is sterically more favourable and results in the formation of isomers 57).It was shown 21,76 that methacryloylsultam 58a is inactive in the reaction with nitrile oxides and provides low asymmetric induction, whereas this reaction is completely regiospecific (the ratio of diastereomers 59a is *2 : 1; the stereochemistry of the major reaction product was not determined). O R3CNO R2 N R1 SO258a,b O NR1 SO2 O N 59a,b R1=Me, R2=H (a); R1= H, R2=Me (b); R3= Et, But, Ph. R P Litvinovskaya, V A Khripach O N + SO2 O R R 57 N Ratio 56 : 57 19 : 1 9 : 1 4 : 1 19 : 1 9 : 1 9 : 1 7 : 1 O N R2 R2 + R1 SO2 O R3 N R3 60a,bRegio- and stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to alkenes Crotonoylsultam 58b ensures high diastereoselectivity (the diastereomer ratio is 9 : 1); however, the regioselectivity of this reaction is rather low and four products are formed.The regioisomers 59b and 60b are formed in nearly equal amounts. High selectivity is also reached with the Rebek benzooxazoli- mides 61,77 which manifest a unique ability to control the selectivity of the reaction owing to the U-shape of the molecule. The ratio of the regioisomers 62 : 63 reaches 99 : 1. R2 R1 O ButCNO N O NO 61 N But O R1 O R2 N + O NO 62 R1=H, Me; R2=H, Me, Ph, CO2Et. The addition of nitrile oxides to acrylamides 69 and a,b- unsaturated esters 78 containing a symmetrical chiral imidazoli- dine substituent in the b-position is characterised by extremely high diastereoselectivity.However, the former are difficult to obtain, while the latter manifest low regioselectivities. The reac- tions of 2,2-dimethyl-(4S,5S)-diphenyl-N,N0-diacryloylimidazo- lidine (64) and 2,2-dimethyl (4R)-phenyl-N-acryloylisoxazolidine (65) with benzonitrile oxide are highly diastereoselective.79 Com- pound 64 yielded a mixture of the diastereomeric cycloadducts 66a and 66b in a*5 : 1 ratio, whereas the reaction with compound 65 afforded a mixture of dihydroisoxazoles 67a and 67b (*4.5 : 1). Ph Ph PhCNO N N O Me Me O 64 Ph Ph Ph N N N O O Me Me O 66a Ph Ph PhN N N + O O Me Me O 66b Ph PhCNO N OMe Me O 65 Ph Ph + N O N O Me Me O 67a N But O R1 O R2 N O NO 63 Ph + N O Ph N O Ph Ph N N O O Me Me O 67b 411 In both cases, hydrolysis of the reaction products leads to the corresponding dihydroisoxazoles possessing high optical purity. Based on the structure of the products 66a,b and 67a,b, it may be concluded that acrylamides 64 and 65 have an s-cis-conformation in the transition state, and benzonitrile oxide attacks the double bond from the side opposite to the phenyl substituent.Studies of reactions of unsaturated compounds with benzoni- trile oxide by the method of competing reactions 21 made it possible to construct an approximate scale of alkene reactivities, viz., activated monosubstituted>activated 1,1-disubstituted> non-activated 1,1-disubstituted=activated trans-1,2-disubsti- tuted.A high level of diastereoselectivity was achieved using com- pounds 68 which represent Kemp's triacid derivatives with chiral centres at one of the nitrogen atoms as dipolarophiles.69, 76 The adducts 69 with the R-configuration of the newly formed chiral centre are formed in more than 98% diastereomeric excess. It was noted that the pseudoenantiomeric alkene gives a C(5 0)-isomeric dihydroisoxazole. N Ph O O H O N PhCNO O N R N O R N 68 O 69 O R=(S)-1-phenylethyl, 2-naphthyl. 1,3-Dipolar cycloaddition of nitrile oxides to the chiral acrylamides 70a ± d derived from (S)-proline and (S)-indoline-2- carboxylic acid yields chiral 4,5-dihydroisoxazoles 71 and 72.80 The stereoselectivity of this reaction is largely determined by the structure of the dipolarophile and is practically independent of the solvent used but varies depending on the nitrile oxide structure.The dipolarophiles 70a ± d differ in their steric characteristics. The largest steric factor is inherent in compound 70c due to the presence of two phenyl rings, whereas the steric factor in the rigid molecule of alkene 70d is determined by the effect of the cyclohexane ring. In the latter case, the stereoselectivity is the highest (up to 95% of the isomer 71d is formed). R1 R1 O O R1 N N R2CNO O + O O R2 R2 72 71 70a ± d R1= CO2Me (b), CO2Me (a), N N (c), CO2Me (d); C(OH)Ph2 N N R2=Me, Et, Ph, 3,5-Cl2C6H3 , 4-MeC6H4.The asymmetric cycloaddition to a,b-unsaturated esters 73a ± c containing chiral imidazolidine substituents in the b-posi- tion has been studied.78 With methyl acrylate 73a, the regio- and stereoselectivity of the reaction is low, viz., the trans-isomers 74 are formed in small excess. The addition of the unsaturated esters 73b,c containing a symmetrically substituted imidazolidine unit was regio- or diastereoselective depending on the nature of the substituent at the nitrogen atom of the heterocycle. Thus the reaction of the acrylate 73b was regiospecific and afforded differ- ent regioisomers depending on the nature of the nitrile oxide. However, in all cases studied the products represented mixtures of diastereomers [*(3 ± 2) : 1].With compound 73c, this reaction occurred with low regioselectivity [the ratio of the regioisomers412 74 : 75 was (3 ± 2) : 1]; however, each regioisomer in this mixture represented an individual (4S,5S)-diastereomer. R2CNO R1 CO2Me 73a ± c CO2Me R1 O N 74 Ph (a), R1= N PhN NPh H R2=Ph, 4-MeOC6H4 . The reactions of acrylates 76 with a (4R,7S)-7-isopropyl-4- methyl-4,5,6,7-tetrahydro-2H-indazol-3-yl fragment as an auxil- iary chiral substituent have been studied.81 Mixtures of two regioisomers 77 and 78 were obtained, the regioselectivity decreased if the reaction was carried out in more polar solvents. The nature of the substituent R2 in the dipolarophile weakly affected the yield and the regioselectivity.82 The acrylate 76 (R2=H) gives predominantly the cycloadduct 77 with benzoni- trile oxide, whereas the corresponding cinnamate (76, R2=Ph) yields a mixture of the regioisomers 77 and 78.R2 R1 R3CNO R1 O 76 Ph R1= N N ; R2=H, Me, Ph, 4-ClC6H4 , 4-MeC6H4 . R3=Me, Et, Pri, Bus, But, Bn, PhCH(Me), PhCH(Me)CH2 . Me Me Reactions of nitrile oxides prepared from oximes 79 or 80 with N-acryloyl-(2R)- or -(2S)-bornane-10,2-sultams 81a,b have been successfully used in the synthesis of pseudodipeptides.83 (5R)-Dia- stereomers of dihydroisoxazoles 82 or 83 containing a chiral fragment XCO in position 5 were formed predominantly from 89% to 100%. R1 H N R2 C NOH a, b R3 O79 R4R1 O N R2 N R3 O 82 R4 H BocHN C NOH a, b R5 H 80 R1 CO2Me R2 + O R2 N 75 Ph Ph Ph (c); (b), NMe MeN NPh Ph N O N O + R1 Ph O O R2 78 R2 77 R1 N O COX N R2 COX + R3 O R4 R P Litvinovskaya, V A Khripach N O N O BocHN BocHN COX COX + H R5 H R5 83 R1=Boc, OCOBn; R2=H, R3=But; R2=R3=Me; R4=H, Me; R5=OSiButMe2, CHMe2 , Ph; Me Me Me Me (a) N-bromosuccinimide; (b) XCOCH=CH2 (81a,b), where X= N (b).N (a), SO2 SO2 As can be seen from the data presented, in the majority of cases the addition of nitrile oxides to substituted alkenes results in the adducts containing substituents in position 5. However, there are exceptions to this rule. Thus 1,3-dipolar cycloaddition of 4-tert-butylbenzonitrile oxide to 6A-acrylamido-6A-deoxy-b- cyclodextrin gives predominantly 4-substituted dihydroisoxa- zole 84 (the regioisomeric ratio is 4 : 1).This effect is due to the formation of an intermediate `host ± guest' complex between the cyclodextrin fragment and nitrile oxide. An efficient method for regio- and stereocontrol over the addition of nitrile oxides to the double bond of a,b-unsaturated diketones which makes use of chiral S-oxides of their cyclic dithioketals, viz., the diastereomeric 2-R-2-crotonoyl-1,3-dithiane s-oxides 84, has been proposed.85 The reaction with nitrile oxides is regioselective for both syn-84 and anti-84; the former reacts with benzonitrile oxide to give products 85a and 85b in a 5 : 1 ratio, whereas for the latter the ratio is 1 : 3. 7O O R1 +S R2CNO Me S 84 7 Me Me O 7O R1 O R1 O + + S S R2 R2 + S S O N O N 85b 85a R1=Me, Et; R2 =Me, Bui, Ph, Mes.V. Cycloaddition of nitrile oxides to alkenes in the presence of Lewis acids Lewis acids catalyse cycloaddition reactions of nitrile oxides to alkenes.86 ± 89 Thus the reaction of hydroximoyl chlorides with Grignard reagents yields the nitrile oxide complexes 86. The first step includes O-metallation to give the intermediate 87 which undergoes 1,3-elimination of metal halide to give the nitrile oxide which further reacts with metal halide to give the complex 86. R1(Cl)C NOH R2MgBr CH2Cl2,778 8C R1(Cl)C NOMgBr 7MgClBr 87 MgClBr MgClBr R1CNO R1CNO 86 In the presence of ethylmagnesium bromide 86 the reaction rate of hydroximoyl chlorides with allylic alcohols 88 increases; no side products, e.g., nitrile oxide dimers, are hereby formed.Cycloaddition occurs in a syn-selective manner (up to 95%). Free nitrile oxides generated under the action of triethylamine manifested only weak syn-selectivities.Regio- and stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to alkenes OH R2(Cl)C NOH EtMgBr 88 R1 N O N O + R1 R1 R2 R2 OH OH R1=Me, Et; R2=Ph, 4-MeOC6H4. The nitrile oxide complexes of the type 86 can be generated under the action of other organometallic compounds (e.g., BunLi, Et2Zn, Et3Al); however, stereoselectivities of reactions are much lower in these cases. The complexes formed in the reaction with Et2AlCl and EtAlCl2 did not react with allylic alcohols 88 at all.86 The cycloaddition of nitrile oxides coordinated to Lewis acids to 1,1-disubstituted alkenes 89a ± e has been studied.89 The formation of 4,5-dihydroisoxazoles always proceeds with high yields.The cycloaddition of acrylates 89a ± d with hydroxy groups in their chiral centres to free nitrile oxides is accompanied by the formation of a mixture of the diastereomers 90a ± d and 91a ± d, the anti-addition products 91a ± d being the main isomers. The proportion of the anti-adducts 91 decreases if nitrile oxides are used as complexes with LiCl, whereas magnesium and zinc complexes yield predominantly the cycloadducts 90a ± d. The highest syn-selectivity (98%) is observed in the presence of a twofold excess of magnesium alkoxide.CO2Me R3CNO MX OR2 R1 89a ± e N O N O CO2Me R1 CO2Me R1 R3 + R3 OR2 OR2 91a ± e 90a ± e MX=MgBr2 , ZnEt2 , LiCl, Mg(OAlk)2 .R3 Compound 89 R2 R1 Ph 4-MeOC6H4 But Ph Ph abcde Me Me Me Et Me HHHHSiMe3 A transition state model which accounts for the formation of syn- and anti-cyloaddition products has been proposed (Fig. 2).89 According to this model, the metal atom in the nitrile oxide ± Lewis acid complex is coordinated to the hydroxy group of the dipolarophile. The transition state represents a system of condensed five-membered rings bent along the C(2)7O bond. Localisation of the substituent R1 at the chiral centre inside the bent condensed system (Fig. 2 a) produces significant steric hin- drances.Therefore, this reaction occurs via a sterically less a b N N MX C MX C O O 3 3 E E 2 OH OH 2 1 1 R3HH R3HH R1 H H R1 E=CO2Me. Figure 2. The transition states of the reaction of cycloaddition of nitrile oxides to chiral homoallylic alcohols 89a ± d.89 (a) anti-transition state, (b) syn-transition state; E=CO2Me. 413 hindered syn-transition state (Fig. 2 b). This model explains the effect of the coordinative capacity of the metal atom on the stereoselectivity of this reaction. Lithium and zinc form no stable complexes. In the presence of magnesium salts, dipolarophiles undergo ionisation to produce stronger complexes with metals, as a result of which their syn-selectivities increase.If the hydroxy group is protected by a bulky trimethylsilyl substituent (compound 89e), the formation of a chelate complex in the transition state is less favourable; therefore, the alkene 89e yields the anti-isomers 91 with moderate stereoselectivities (73% ± 79%) irrespective of the method used for the generation of nitrile oxides. An alternative approach to increase in the stereoselectivity of nitrile oxide addition to alkenes is the use of magnesium alkoxides obtained by the reaction of allylic alcohols 86 or a,b-unsaturated ketones 90 with organomagnesium compounds. This reaction occurs in a syn-stereoselective manner. The maximum selectivity is reached with 2 equivalents of the alkoxide and decreases when the reaction is carried out in THF.In the case of magnesium alkoxides 92 prepared from the corresponding allylic alcohols, the cycloaddition is regioselective and affords the cycloadducts 93 and 94; with 2 equivalents of alkoxides 92 the syn-addition products 93 are predominantly formed.86 R2 OMgBr R3(Cl)C NOH 92 R1 N O N O R2 R2 R1 R1 R3 + R3 OH OH 94 93 Ratio 93 : 94 R3 R2 R1 HHH 96 : 4 99 : 1 89 : 11 97:3 96 : 4 Ph Ph 4-MeOC6H4 H PhPh Me Et Et Pri Bun Me The application of this methodology to alkoxides of homo- allylic alcohols resulted in the exclusive formation of the regioisomers 96 and 97 from terminal alkenes with no or very small diastereoselectivity.86 R Ph(Cl)C NOH OMgBr 95 O N N O OH OH + Ph Ph R R 97 96 R=Me, Pri.It should be noted that the control over the selectivity of 1,3- dipolar cycloaddition by coordination to metals proved to be efficient in those cases where internal alkenes were used as dipolarophiles. The cycloaddition of nitrile oxides to allylic alcohols 98 in the presence of ethylmagnesium bromide is regio- specific with the predominant formation of the syn-adduct 99 (94%), whereas the nitrile oxide generated under the action of triethylamine gave a mixture of compounds 99 and 100 in a 55 : 12 ratio and up to 18% of diastereomers of 4,5-dihydroisoxazole 101, the total yield of the reaction products being much lower. Me Me Ph(Cl)C=NOH, EtMgBr or Et3N OH 98414 N O N O O N Ph Me+Ph Me+Ph Me OH OH CH(OH)Me 101 Me 100 Me 99 The cycloaddition of nitrile oxides to dienes which contain a hydroxy group in both the allylic and homoallylic positions has been studied.91 Thus the reaction of hexa-1,5-dien-3-ol (102a) with benzonitrile oxide generated under the action of Et3N gives a mixture of the cycloadducts 103a and 104a (the diastereomeric ratios were 2 : 1 and 3 : 2, respectively), i.e., under these conditions, the double bond of the allylic fragment was only a little more reactive than that of the homoallylic fragment. OR Ar(Cl)C NOH EtMgBr or Et3N 102a ± c O N N O OR + Ar Ar OR 104a ± c 103a ± c R=H(a), Ac (b), SiMe2But (c); Ar=Ph, 2,6-Cl2C6H3 .When EtMgBr is used to generate nitrile oxide, the reaction is chemoselective with respect to the allylic fragment, the syn : anti- diastereomeric ratio in compound 103 being 99 : 1.Thus the magnesium alkoxide methodology proved to be more efficient for allylic alcohols than for homoallylic alcohols. The influence of hydrogen bond formation on the cyclo- addition of 2,6-dichlorobenzonitrile oxide to compounds 102a ± c has been studied. In weakly polar solvents, such as benzene or dichloromethane, nitrile oxide forms a hydrogen bond with the hydroxy group of the alcohol 102a and coordinates to the double bond of the allylic fragment (the transition state A is more favourable than B), compound 103a being the main product. The use of polar solvents, e.g., DMF, prevents the formation of the hydrogen bond, and thus the product 104a becomes predominant.In the case of a protected hydroxy group (compounds 102b,c), no hydrogen bond can be formed; therefore, acetate 102b gives predominantly the adduct 104b irrespective of the polarity of the solvent used. In addition, the homoallylic double bond in the acetoxy derivative 102b becomes more reactive as a dipolarophile due to steric repulsion between the acetoxy group and nitrile oxide. For compound 102c, the yield of dihydroisoxazole 104c increases even more due to significant steric hindrances. N H H C O N O O R C O R A B High syn-selectivity in the addition to the double bond of the allylic fragment of magnesium alkoxide prepared from the alcohol 102a can be explained in terms of the reaction mechanism.Magnesium alkoxide forms five-membered intermediates C or D, the latter being much less stable due to steric hindrances created by the propenyl substituent. N N MgBr O C MgBr C O R R O O D C Thus the allylic double bond is relatively more reactive in the reaction of 1,3-dipolar cycloaddition of nitrile oxides to hexa-1,5- dien-3-ols than the homoallylic bond due to the formation of a R P Litvinovskaya, V A Khripach hydrogen bond. In acetyl or silyl derivatives of the alcohol 102a, it is the isolated homoallylic bond that is more reactive. Chelation with magnesium derivatives results in high regio- and diastereo- selectivity. It should be noted that the products of simultaneous addition at two bonds were also isolated.Reactions of 1,3-dipolar cycloaddition of benzonitrile oxide with 3,3-disubstituted allylic alcohols 105 proceed with very low regioselectivity; the nature of the substituent practically does not influence the ratio of the cycloadducts 106 and 107.92 In the case of magnesium alkoxides (X=MgBr), the reaction rate increases considerably and so does the regioselectivity,93, 94 this effect being observed exclusively with more than 1 equivalent of magnesium alkoxide. Similar results were obtained for mesitylenecarbonitrile oxide.94 OX R1 PhCNO R2 105 O N O N R2 + Ph Ph CH2OH R1 R1 R2 CH2OH 107 106 R1=H,Me, Pri, Ph; R2= H, Me, Prn; X = H, MgBr. The cycloaddition of 2,6-dichlorobenzonitrile oxide to mag- nesium alkoxides 108 obtained by treatment of a,b-unsaturated aldehydes and ketones with Grignard reagents has been studied.90 This reaction is regio- and diastereoselective and results in the syn- adducts 109.O OMgX R3 R3MgX R4CNO R2 R2 R1 R1 108 N R4 O R1 R3 R2 HO 109 R1=H, Me; R2=H, R17R2=CH2CH2; R3=Me, Et; R4=2,6-Cl2C6H3 . Encouraging results were also obtained with cyclopentenone; however, the attempts to carry out an analogous reaction with cyclohexenone were without success. VI. Cycloaddition of nitrile oxides to unsaturated aldehyde derivatives Cycloaddition to cinnamaldehyde is one of few examples of cycloaddition of aceto- and benzonitrile oxides to a,b-unsaturated aldehydes.95 In addition to the expected products, e.g., 4-formyl- 5-phenyl-3-R-4,5-dihydrooxazoles, bis-adducts were isolated due to the cycloaddition of nitrile oxide to the carbonyl group of the primary cycloadducts.Therefore, in subsequent experiments the carbonyl groups of a,b-unsaturated aldehydes were protected prior to their cycloaddition to nitrile oxides. The reactions of cyclic acetals 110 with nitrile oxides 96 gave predominantly the adducts 111. The desired regioselectivity can be attained by varying the nitrile oxide structure. For example, for R1=CO2Et and R2=Ph the 111 : 112 ratio is 99 : 1. R1 R1 O O O O R2CNO R2 O O H O + H O N H R1 110 112 N R2 111 R1=Ph, CO2Et, Et; R2=Ph,Me, Pri.Regio- and stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to alkenes The compositions of cycloaddition products in the case of b-substituted a,b-unsaturated aldehyde derivatives depend on the nature of the protective groups.97 The cycloaddition to cyclic acetals 113a,b yields predominantly the regioisomers with an acetal substituent in position 4, whereas for dithioacetals 113c,d the regioselectivity is opposite.Calculations led to a conclusion that the orientation in this case of the dithioacetal group is determined by the steric factor, whereas that in the case of the acetal group is determined by the interaction of the frontal orbitals. X O O R1 R1 N X N R2CNO X + X X 113a ± d R2 R1 R2 115 114 X Ratio 114 : 115 X R2 Compound 113 R1 abc 91 : 9 17 : 1 9 : 1 17 : 83 27 : 73 3 : 7 Ph 4-MeOC6H4 PhCH2 Ph 4-MeOC6H4 PhCH2 Ph O Ph O Me O Ph S Ph S Me S d Thus, by varying the protecting group in the a,b-unsaturated aldehyde, one can change the regioselectivity of the reaction resulting in substituted 4,5-dihydroisoxazoles with the formyl substituent in different positions.High stereoselectivity is achieved in the addition of nitrile oxides to 2-hydroxybut-3-enal dithioacetal derivatives.98 Thus the diastereoselectivity of the cycloaddition of nitrile oxides to 2-silyl- oxybut-3-enal dithioacetals 116 virtually does not depend on either the size of the protective group at the oxygen or the sulfur atoms or the nature of the substituent.The anti-isomers 117 are formed as the main products and the maximum diastereoselectiv- ity (96%) is achieved with the bulky dithioacetal group (R2=But). However, the yield of this reaction depends, to some extent, on the nature of the nitrile oxide; the total yield of the cycloadducts 117 and 118 is low (25% ± 26%) with R1= SiMe2But, R2=But and R3=CO2Et or CH2OBn. SR2 R3CNO SR2 OR1 116 N O N O SR2 SR2 + R3 R3 SR2 SR2 OR1 OR1 118 117 R1=Bn, SiMe2But; R2=Ph, (CH2)3, But; R3=Me, Ph, 4-MeOC6H4, N , CO2Et, CH2OBn. S Deprotection of the aldehyde group occurs with high yields and without epimerisation of the chiral centres, which apparently makes this method especially attractive from the synthetic point of view.98 VII.Alkenylboronates in cycloaddition reactions with nitrile oxides Organoboron derivatives, particularly alkenylboronate deriva- tives are readily available and are often used in synthetic practice as convenient building blocks.99 The possibility of the use of alkenylboronates as dipolarophiles in cycloaddition with nitrile oxides has been studied.100 ± 102 It is of note that an attempt at enhancing the reactivity of the double bond in alkenylboranes by 415 introducing stronger electron-withdrawing groups, such as bora- bicyclononyl, catecholboryl or BHal2 groups, has not led to the desired effect.103 It was found that the vinylboronates 119a ± g react with nitrile oxides in a regiospecific manner to give 4,5-dihydroisoxazoles 120a ± g, their yields being only little dependent on the changes in the nitrile oxide structure.a-Substituted alkenylboronates 119h,i gave also 4,5-dihydroisoxazoles 120h,i containing a boronate residue in position 5 as the reaction products, while the trisub- stituted compound 119j gave no cycloadducts. b-Substituted alkenylboronates 119k ±m do not afford products similar to compounds 120; this reaction gave 4,5-dihydroisoxazoles 121k,l or isoxazole 122 containing no boronate substituent. BL R1=H R2 O R3 N 120a ± i LB R1 R1 BL R3CNO R2=H O O R1 R3 R3 R2 119a ±m N 121k,l N 123k,l R1=Ts R2=H O R3 N 122 O Me Me . B Hereinafter BL is O Me Me Reaction product R3 Substrate 119 R2 R1 H 4-ClC6H4 t H H H Me H H Bu H H CO2 Et COMe HH HH Me Ph HH H 120a 120b 120c 120d 120e (CH2)3CO2Me 120f H H CH2 CH(OMe)2 120g 120h 120i 7121k 121l 122 4-ClC6H4 4-ClC6H4 4-ClC6H4 4-ClC6H4 4-ClC6H4 4-ClC6H4 (CH2)4 Bu MeOCO HH abcdefghijklm Ts Amechanism of cycloaddition has been proposed 101 based on the results of the reaction of vinylboronates with diazo com- pounds.104 According to this mechanism, nitrile oxide is added regioselectively to cis-alkenylboronates 119k ± l to give 4,5-dihy- droisoxazoles 123 with a boronate residue in position 4.Similar compounds were isolated and characterised in previous studies.105 Spontaneous 1,3-migration of the boronate residue and subse- quent hydrolysis give 4,5-dihydroisoxazoles 121.Elimination of TsOH in the case where R1=Ts results in isoxazole 122 (R3=4-ClC6H4).106 An effective procedure for the synthesis of 4-hydroxy-4,5- dihydroisoxazoles using vinylboronates has been proposed.105 Thus 1,3-dipolar cycloaddition of nitrile oxides to 2-substituted trans-alkenylboronates 124 gives 4,5-dihydroisoxazoles 125 with a boronate substituent in position 4 as the main regioisomers. Treatment of the reaction mixture with ButO2H gives trans- hydroxy-4,5-dihydro-4-isoxazoles 126 in high yields (75% ± 88%). LB R1 R2CNO BL R1 O R2 124 N 125416 R1 HO 1) R2CNO 124 2) ButO2H O R2 N 126 R1=Prn, CH2OAc, Ph, Bun, CO2Me, (CH2)3Cl; R2=Ph, 4-MeOC6H4 .The cycloaddition of nitrile oxides to allylboranes 127 has been studied.101 As in the case of alkenylboranes, one regioisomer 128 is formed. Allylboranes containing a chiral centre in the a-position yield a nearly equimolar mixture of diastereomers with nitrile oxides; their stereochemistry has not been established. R2 R1 R2 R3CNO BL R1 O BL R3 N 128 127 R1=H, Me; R2=H, Cl, C6H13, C5H9; R3=But, 4-ClC6H4 . Thus, 1,3-dipolar cycloaddition of nitrile oxides to alkenyl- or allylboronates and subsequent oxidative deborylation of the adducts formed is a direct route to 4-hydroxy- or 5-hydroxy- alkyl-4,5-dihydroisoxazoles. The reaction of [3-R-4,5-dihydroisoxazol-2-yl]boranes 120a ± h with diiodomethane in the presence of MeLi yields their homologues.107 BL CH2BL RCNO 1) CH2I2 , THF BL 2) MeLi O O R R 119a N N 120a ± h R=4-ClC6H4 (a), Me (b), But (c), CO2Et (d), COMe (e), Ph (f), (CH2)2CO2Me (g), CMe2OSiMe3 (h).Cycloaddition of nitrile oxides to the optically active alkenyl- boronate 129 gives four products; the total yield of 4,5-dihydro- isoxazoles 130a,b is*50%, whereas that of (4S,5S)- and (4R,5R)- 4-hydroxy-4,5-dihydroisoxazoles 130c,d is *45%, erythro-iso- mers being predominant in both cases.108 Thus, this reaction occurs in a regiospecific manner but with moderate stereoselec- tivity. Compounds 130a,b are formed as a result of deborylation of the primary 4,5-dihydroisoxazoles which contain a boronate substituent in position 4 (like compounds 123a,b).Me Me Me Me O O + O O 1) RCNO 2) NaOH R BL 129 130a O N Me Me Me Me Me Me O O O O O O OH OH + + R R R + O N O N O N 130d 130c 130b R=Ph, 4-ClC6H4. Compounds 130c,d were the only reaction products (130c : 130d&4 : 1) when tert-butyl peroxide was used for the generation of nitrile oxide. R P Litvinovskaya, V A Khripach VIII. Vinyl- and allylphosphine oxides in cycloaddition reactions with nitrile oxides 1,3-Dipolar cycloaddition of nitrile oxides to racemic methylphe- nylvinylphosphine oxide results in phosphorus-containing 4,5- dihydroisoxazoles 131a,b and 132a,b in good yields.109 The isomers 131a,b with a phosphine oxide substituent in position 5 either predominate or are the only reaction products. The cyclo- addition occurs with a significant (*40%) diastereofacial selec- tivity.According to X-ray diffraction data, compound 131a, which exists in an erythro-configuration, has a conformation in which the C7O and P=O bonds are in the anti-position. N N R R O O Ph O RCNO P + H H Me H P H H Me Ph Ph Me H PO 132a,b O 131a,b R=Ph, Me. 1-R-Propenyldiphenylphosphine oxides 133 were studied in the reaction of 1,3-dipolar cycloaddition.110 O Ph R2CNO P Ph133 R1 O O O N O N Ph Ph P P + R2 R2 Ph Ph R1 R1 134b 134a R1=Me, Et, Pr, C6H11, C11H23, CO2Et, (CH2)nCO2Me; R2=H, Me, Et, Pr, Bui. Cycloadducts are not formed, if nitrile oxides contain a bulky substituent R2. In other cases, the reaction proceeds at a very slow rate.The anti-addition products 134a are predominant in all cases. This is one of the few examples of cycloaddition reactions where the substituent in the nitrile oxide has a stronger influence than the substituent R1 in the allylic position. It was suggested 110 that the extremely bulky phosphorus-containing substituent in the Houk transition state occupies a position opposite to that of the ring which is formed, whereas the R1 group and the allylic hydrogen atom are localised inside and outside, respectively. Branched substituents R2, e.g., Pri, are too bulky for these positions; there- fore, alkenes 133 do not form cycloadducts with such nitrile oxides. IX. The nitrile oxide method in carbohydrate chemistry Cycloaddition of nitrile oxides to alkenes is used in the synthesis of various sugar derivatives.The nitrile oxide method proved to be a convenient procedure for the elongation of a carbon chain in the synthesis of higher sugars. The 1,3-dipolar cycloaddition of the simplest nitrile oxides to the double bonds of the sugar derivatives 135a ± d has been studied.111 ± 116 The reaction of compounds 135a,b,d with nitrile oxides is regioselective and gives 4,5-dihy- droisoxazoles 136 and 137 which contain the sugar moiety in position 5 (74 : 26 and 97 : 3, respectively). The major diastereom- ers of the adducts 136 have the R-configurations of the newly formed chiral centres. Quite a different result is obtained for the corresponding D-ribose derivative 135c,113, 115 which represents an epimer of compound 135a as regards the homoallylic C(3) atom.In this case, the syn- and anti-adducts 136 and 137 are formed in approximately equal amounts. This reaction is the firstRegio- and stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to alkenes example of the dependence of the composition of nitrile oxide cycloaddition products on the configuration of the chiral centre in the homoallylic position. R2CNO R1 135a ± d N O N O 5(6) + O R2 O R2 4(5) 137 136 O O (b), (a), R1=R3O OMe O O O O Me Me Me Me O O O Me Me R3O (d) (c), O O O O O Me Me Me Me R2=Ph, CO2Et; R3=H, OH, OMe, OBn, Ac, Ar. The 1,3-dipolar addition of o-unsaturated heptoses and hexoses 138a,b to the nitrile oxide 139 was used in the synthesis of 7-deoxytrideca- and 6-deoxydodecadialdoses.117 It should be noted that this reaction is one of the few examples of asymmetric induction by a chiral nitrile oxide.Both alkenes react regiospecifi- cally and with high diastereoselectivity to give predominantly the anti-adducts 141a,b (141 : 140=4 : 1 and 14 : 3, respectively). O ONC O Me Me + O O R138a,b O Me 139 Me O O N N R R O O O O Me Me + Me Me O O O O O O Me Me 141a,b 140a,b Me Me O O O Me (b). R= (a), BnO Me O O O O O Me Me Me Me The stereo-controlled cycloaddition of the alkene 138b to nitrile oxides derived from D-xylose (142a) or D-arabinose (142b) has been carried out. In both cases, the reaction is regiospecific and diastereoselective with the predominance (up to 80%) of the diastereomers 143 having R-configurations of the newly formed chiral centres.118 N O O CNO O O AcO OAc OAc O 138b+ AcO OAc BnO OAc O 143 Me Me (5S,6S )-142a; (5R,6R)-142b.417 Similar results were obtained with L- or D-arabinonitrile (145),119 oxide derivatives 144 and glyceronitrile oxide (146).115 Me Me Me Me CNO O O O O O O CNO CNO O O O O Me Me146 Me 145 Me Me 144 Me The 1,3-dipolar cycloaddition of nitrile oxides to nitroalkenes and a,b-unsaturated carbonyl compounds 147a ± d containing a sugar moiety has been studied. For alkenes 147b ± d, this reaction is regio- and stereoselective and yields exclusively the diastereo- mers 148 with the trans-configuration of substituents at the C(4) and C(5) atoms.120 R2 N R1 O H OAc R1 OAc R2CNO AcO AcO AcO AcO OAc OAc OAc OAc 147a ± d 148a ± d R1=NO2 (a), Ac (b), CO2Me (c), CO2Et (d); R2=Me, Ph, Mes, Br.Based on molecular-mechanical models and the structure of the reaction products, it was suggested 120 that in this case it is the stable transition state A that is realised. This conclusion is in good agreement with the anti-periplanar effect of the acetoxy group in the allylic position. N R2 O OAc H R1 A The regioselective cycloaddition of tetrahydropyranyloxyace- tonitrile oxide to the nitrogen bases 149 containing vinyl groups results in the nucleoside aza analogues 150 possessing antiviral activities.121, 122 B +THPOCH2CNO 149 B B HOCH2 THPOCH2 H+ N O N O 150 B=6-chloropurine, thymine, uracil, adenosine.X. Steroid alkenes as dipolarophiles in cycloaddition reactions with nitrile oxides 1,3-Dipolar cycloaddition of nitrile oxides to steroid alkenes has been studied using compounds with a double bond in both the cyclic part of the molecule (e.g., D5(6),16(17)-steroids) and the side chain. The addition of nitrile oxides to the disubstituted double bond of the steroids 151a ± d occurs at a slow rate and sometimes requires a large excess of nitrile oxide; its result does not change upon modification of the steroid skeleton but strongly depends on the nature of the substituent R2 of the nitrile oxide.123 No cyclo- addition products were found for R2=Pri.418 N O O N Me R2CNO Me Me + R2 R2 R1 R1 R1 151a ± d 153 152 (R3=Ac (a), Me (b)), R1=R3O (c), (d); R2=Me, Bui. OMe O This addition is regiospecific leading exclusively to the for- mation of 4,5-dihydroisoxazol-5-yl derivatives and stereoselective (152 : 153=3 : 1).The main epimer is formed as a result of an attack of the nitrile oxide at the double bond from the sterically less hindered side (the a-region of the steroid). 1,3-Dipolar cycloaddition of nitrile oxides to the 17b-hydroxy-17a-vinyl estrone derivative 154a proceeds in a regiospecific and stereoselective manner [155 : 156=(10 ± 15) : 1]. The stereoselectivity of the reaction decreases on going to the steroids of the androstane series 154b,c (155:156=1:1) for R2=Me and 3 : 1 for R2=Pri.124, 125 OH RCNO A B 154a ± c OH OH R R+ N A N O A O B B 156 155 (X=H (b), Ac (c)); (a), A=MeO XO B R=Me, Pri.The steroid derivatives 157a ± i containing a but-2-enyl sub- stituent in the ring D have been studied in the reaction of 1,3- dipolar cycloaddition to nitrile oxides.126 ± 132 In all cases, this O N O N R2CNO + R2 R2 R1 R1 R1 159 157a ± i 158 (c), (R3=Ac (a), Me (b)), R1= R3O O O (e), (d), (f), O O O O O OMe N (i); (g), AcO O N (h),AcO AcO H N O Ph R2=Me, Pri, Ph. R P Litvinovskaya, V A Khripach reaction gave a mixture of the 50-epimeric 4,5-dihydroisoxazol-5- yl steroids 158 and 159. The ratio of the epimers (50R)-158 and (50S)-158 depends on the structures of both the nitrile oxide and the steroid.In some cases, the reaction possessed pronounced stereoselectivity, e.g., the 158 : 159 ratio in the reaction of com- pound 157h with benzonitrile oxide is 4.5 : 1, whereas the reaction of the alkene 157f with isobutyronitrile oxide is not selective. The reaction of compound 157g with nitrile oxides is a special case, for it gives only one reaction product 158 (R2=Me, Pri). 1,3-Dipolar cycloaddition of nitrile oxides to steroidal allylic alcohols 160a ± c 126, 132, 133 affords the adducts 161 and 162; the threo-isomers 161 are formed with high stereoselectivities. The cycloaddition of EtO2CCNO to compound 160b gave only one diasteromer, viz., 4,5-dihydroisoxazole 161b (R2=CO2Et), albeit in a low yield.O N O N OH OH OH R2CNO R2 R2 + R1 R1 162a ± c 161a ± c R1 160a ± c (c); (b), (a), R1=Ac OMe O R2 =Pri, Me, CO2Et. The stereochemistry and the ratio of the reaction products in the cycloaddition of nitrile oxides to the D23-steroids 163 is determined by the nature of the oxygen-containing substituents in the a- and b-positions to the double bond and the configuration of the C(20) and C(22) centres in the starting dipolarophile (Table 1).134 ± 138 OR2 Me MeCNO R1 163a ± g OR2 OR2 Me Me Me Me + R1 R1 O N O N 165a ± g 164a ± g The cycloaddition of acetonitrile oxide to the steroid 163a containing a (22S)-hydroxy group occurs in a regio- and stereo- selective manner with the predominant formation of the threo- isomers 164a.The ratio of the epimers formed in the reaction with (22S)-acetoxy derivative 163b was 1 : 1 (with a concomitant decrease in the degree of conversion). The use of the (20S,22R)- hydroxy derivative 163c also resulted in the predominant forma- tion of the threo-isomer 165c, whereas in the case of the (22R)- acetoxy derivative 165d the epimers were formed in a 1 : 1 ratio and the degree of conversion was also decreased. This reaction becomes stereospecific upon protection of the hydroxy group by a bulky tert-butyldimethylsilyl substituent (compound 163e), how- ever, the (22R,50S)-diastereomer 165e is formed in a low yield. The (22S,50S)-diastereomer 165f was predominant upon cycloaddition of the (20R,22R)-22-hydroxy derivative 163f, but the stereoselec- tivity of this reaction was not very high. When the (20R,22R)-22- acetoxy derivative 163g is used as a dipolarophile, the syn-cyclo- addition product 165g is formed in a large excess which is in contrast to the reactions with the acetoxy derivatives 163b and 163d.These reaction patterns differ basically from those for non- steroid compounds and suggest the possible formation of a transition state (an intermediate complex) including nitrile oxide and the allylic hydroxy group of the steroid molecule.2 The structure of the transition state depends on the nature of theRegio- and stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to alkenes Table 1.The stereochemistry and ratios of reaction products in the reaction of acetonitrile oxide with the steroids 163. R2 R1 HAc O O HAc SiMe2But e O HAc O a The conversion is 41%. b The conversion is 38%. c 20 8C. d Only the adduct 165 is formed. e 60 8C. f The conversion is 40%. steroid fragment and the configuration of the allylic and homo- allylic centres. The reaction of acetonitrile oxide with the 20-hydroxy-20- propenyl steroids 166b,d containing the homoallylic hydroxy groups occurs without any noticeable stereoselectivity to afford approximately equal amounts of the epimeric 4,5-dihydroisoxa- zoles 167b,d and 168b,d.139 X Me R 166a ± d X Me R167a ± d R= OMe R= AcO This reaction is also non-stereoselective with the steroid alkenes 166a,c devoid of hydroxy groups.140 The first attempts to use steroidal nitrile oxides 169a,b and 170a ± d141 ± 144 as dipoles in reactions with alkenes revealed that the cycloaddition which occurs in a regioselective manner yields one regioisomer in the form of epimeric pairs of 4,5-dihydroisox- azol-3-yl steroids.However, this reaction is non-stereoselective for both C(22)- (169a,b) and C(23)-nitrile oxides (170a ± d). Con- Com- Total pound figuration yield 163 Ratio of adducts 164 and 165 of the C(20) and C(22) centres 4:1 1 : 1 (20S,22S) 88 (20S,22S) 81a abcd 1:4 1 : 1 see d (20S,22R) 88 (20S,22R) 90b (20S,22S) 16c 25 e 1:2 1 : 4 (20R,22R) 70 (20R,22S) 78f fg MeCNO X Me Me Me + O N O N R168a ± d : X = H (a), OH (b); : X = H (c), OH (d).419 CNO Me H2C CHR2 R1 169a,b N O N O + Me Me R2 R2 R1 R1 (b); (a), R1= N AcO O N N OMe O Ph R2=CH2OH, CH2OAc, CMe2OH, Ac, CH2Br, Pri. X Me H2C CR2R3 CNO R1 170a ± d X X R2 R2 Me Me + R3 R3 R1 R1 N O N O : X =H(a), OAc (b); R1= O O R1= : X = H (c), OAc (d); OMe R2=H,R3=Me; R2=Me, R3=Pri. The reactions of 3-methylidene-17-acetoxyandrostane (171) with aceto- and benzonitrile oxides gave mixtures of 3a- and 3b- isomers of the spiro derivatives 172 and 173 with the predom- inance of the former isomers (2.5 ± 3) : 1.145, 146 The reactions of the steroid 171 with nitrile oxides RCNO, where R=EtCO2, 2,4,6-Me3C6H2, always give the only cycloaddition products, viz., the 3a-isomers 172 (yields 60% and 25%, respectively).The addition of triphenylacetonitrile oxide to alkene 171 yields a 1 : 1 mixture of two epimeric 4,5-dihydroisoxazoles in a total yield of 87%.147 OAc RCNO OAc 171 OAc + O N R O N 173 172 R R=Me, Ph, Mes, CO2Et, Ph3C.420 It was reported 145 that 1,3-dipolar cycloaddition of nitrile oxides to 17-methylidene steroids gives regioisomeric 4,5-dihy- droisoxazoles spiro-conjugated to the steroid fragment in posi- tion 17. However, the configuration of the C(17) centre of the steroid has not been established (yield 60%± 70%).145 The stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to steroidal compounds with an endocyclic double bond is usually determined by the attack of the nitrile oxide from the less protected a-region of the steroid. Thus cycloaddition of arene- carbonitrile oxides to the spirost-5-ene derivatives 148 is regio- and stereoselective and results in 3 0-substituted [5a,6a-d]-4,5-dihy- droisoxazoles.The published data concerning the addition of nitrile oxides to the D16-bond of steroids are controversial.149 ± 155 In some cases, only [17a,16a-d]-4,5-dihydroisoxazoles or [16a,17a-d]-4,5-dihy- droisoxazoles are exclusively formed, but the formation of two regioisomers was also reported. The composition of reaction products depends on the structure of the 1,3-dipole and the method used for its generation.It is of note that sometimes the D5-bond and the cross-conjugated (3-oxo-1,4-diene) system are not involved in this reaction. XI. Other examples of cycloaddition of nitrile oxides to acyclic alkenes It was noted 105, 156, 157 that 1,3-dipolar cycloaddition of some achiral vinyl sulfones to nitrile oxides studied previously proceeds without any noticeable regioselectivity. OR2 R3CNO R1 PhSO2 174 PhO2S PhO2S OR2 OR2 R3 + R3 R1 R1 N O N O 175 R1=Et, Pri; R2=Ac, MOM, ButMe2Si; R3=Me, Ph. The reaction of the chiral vinyl sulfones 174 with nitrile oxides does not go to completion even in the presence of a large excess of the 1,3-dipole (the conversion is 36%± 68%).158 In this case, only the regioisomers with the phenylsulfonyl group at C(4) are formed.The stereoselectivity of this reaction depends on the nature of substituents in both the alkene and nitrile oxide, however, the anti-adducts 175 are always predominant and in the case of the bulky substituents R1 and R2 they are virtually the only isolated products,158 which is in agreement with the Houk model. The first attempts at cycloaddition of nitrile oxides to the homochiral vinyl ethers 176a ± f 159 resulted in reaction products in low yields and with low diastereoselectivities.160 O O O O O R2CNO R1 N N R1 R1 + R2 R2 178 177 176a ± f R1=(7)-menthyl (a), 8-phenylmenthyl (b), (1S)-endo-bornyl (c), (R)-PhCH(CO2Me) (d), (S)-1-(2-naphthyl)ethyl (e), (S)-1-phenylbutyl (f); R2=Me, Et, Prn, But, Ph, Ph(CH2)2, 4-NO2C6H4.The best results were obtained for the vinyl ether 176e,161 viz., the ratio of the isomers 177 : 178 is (3 ± 4) : 1 in reactions with almost all nitrile oxides. X-Ray diffraction data suggest that compounds 177 with the R-configurations of the newly formed chiral centres at the C(5) atoms are the predominant isomers. 1,3-Dipolar cycloaddition of nitrile oxides to allenes yields several reaction products.162 The reaction of N,N-diarylamino R P Litvinovskaya, V A Khripach allenes 179 with 1 equiv. of 3,5-dichloro-2,4,6-trimethylbenzoni- trile oxide gives the adduct 180 with the involvement of the a,b-double bond and a small amount of the spirobisadduct 181.163 ArCNO CHN C H2C R1 R2R3 179a ± d Ar N O Ar Ar N N R1 R2 R1 R2 + N O N O R3 R3 181 180a ± d R1=R2=R3=H(a); R17R2=(CH2)2, R3 = H (b); R17R2=CH=CH, R3 = H (c); R1=R2=H,R3=Me (d); Ar=3,5-Cl2-2,4,6-Me3C6 .The reaction of the allenes 179 with 2 equivs. of the nitrile oxide gives only spiro compounds 181. The a,b-double bond in the allenes 179 is more reactive due to the activating and polarisation effects of the nitrogen-containing substituent, which also controls the regio- and stereoselectivity of this reaction. It was shown 164 that the fragment of proline benzyl ester plays the role of an effective chiral auxiliary group in the asymmetric addition of nitrile oxides to the alkenes 182. This gives a mixture of the isomers 183 and 184 with the predominance of the former isomer.The regioselectivity does not depend on the reaction temperature and the nature of the substituent R1. The reaction of the alkene 185 with benzo- and pivalonitrile oxides yields the adducts 186 and 187 in 10 : 1 and 9 : 1 ratios, respectively.164 R1 R1 COR2 COR2 COR2 R3CNO + O O R3 R3 N N R1 182 184 183 COR2 R2OC COR2 R2OC COR2 R3CNO + O O R2OC R3 R3 185 N 186 N 187 ; R3=But, Ph. R1=H, Me; R2= CO2Bn N 3-Vinylcephalosporins 188 were used in the synthesis of novel 3-substituted cephalosporins 189.165 It was found that compounds 188 do not react with nitrile oxides generated from nitroalkanes under the action of phenyl isocyanate in the presence of triethyl- amine or N,N,N0,N0-tetramethylurea. S S R1 R1 R2CNO N O N N O O R2 188 189 CO2Me CO2Me R1=BnCONH, PhOCH2CONH; R2=Me, Et.4,5-Dihydroxyisoxazoles 189 could be synthesised by the reaction of alkenes 188 with silyl nitronates as synthetic equiv- alents of nitrile oxides. These reactions result in N-silyloxyisox- azolidines which undergo spontaneous elimination of the trimethylsilanol group to yield 4,5-dihydroisoxazoles 189; this reaction is regio- and stereoselective.Regio- and stereochemistry of 1,3-dipolar cycloaddition of nitrile oxides to alkenes The simultaneous presence of C=C and C=N bonds in the molecule poses the problem of chemoselectivity.166 Thus benzoni- trile oxide and pivalonitrile oxide react with the heterodienes 190a,b containing a terminal C=C bond to give the substituted 4,5-dihydroisoxazoles 191 in good yields.The cycloaddition of benzonitrile oxide to the b-substituted substrates 190c ± e involves exclusively the C=Nbond resulting in 4,5-dihydro-1,2,4-oxadia- zoles 192. In this case, E-isomers react faster than Z-isomers. Compounds 190c ± f do not react with pivalonitrile oxide, appa- rently due to steric hindrance.167 R4 N R1 O R1 R2 R3O2C R2 Ph N R4CNO Ph Ph 191 R3O2C R1 N 190a ± f Ph Ph R2 PhN O R3O2C N R4 192 R1=R2=H:R3=Me (a), Et (b); R1=Me, R2=H, R3=Me (c); R1=H,R2=R3=Me (d); R1=Ph, R2=H,R3=Me (e); R1=H,R2=Ph, R3=Me (f); R4=Ph, But. The irontricarbonyl complexes 193a ± d can be used to prepare 4,5-dihydroisoxazoles with a high degree of stereoselectivity.168 This reaction results in the predominant formation of the epimers 194 [194 : 195=(7 ± 9) : 1].It is noteworthy that the formation of the isomers 195 is explained 168 by the presence of cis-rotamers in the original complexes 193. R2CNO R1 Fe(CO)3 193a ± d O O N N R1 + R1 R2 R2 Fe(CO)3 195 Fe(CO)3 194 R1= Me (a), Et (b), But (c), Ph (d); R2= Me, CO2Me, CH2OSiPh2But. The chiral synthesis of (+)-(S)-[6]-gingerol was performed using the complex 193a.169 It was shown that the use of the chromiumtricarbonyl com- plexes of the styrene derivatives 196 in the reaction with 3,5- dichloro-2,4,6-trimethyl-benzonitrile oxide results in the stereo- selective synthesis of the adducts 197 with the S-configurations of the newly formed chiral centres at the C(5) atoms.170 It is of note that the reaction of nitrile oxides with styrene proceeds in a non- stereoselective manner.R2CNO R1 (CO)3Cr 196 R2 R2 N N + O O R1 R1 (CO)3Cr (CO)3Cr 198 197 R1=OMe, Me, Cl; R2=3,5-Cl2-2,4,6-Me3C6 . Ratio 197 : 198 R1 80 : 20 98 : 2 96 : 4 OMe Me Cl 421 The cycloaddition of nitrile oxides to the Z1-allyliron com- plexes 199 171 yields mixtures of 4,5-dihydroisoxazoles 200. In the case of the chiral complexes 199b,c, mixtures of diastereomeric 4,5-dihydroisoxazoles 200b,c,e,f are formed in (59 ± 93) : (41 ± 7) ratios. O N RCNO Fe Fe R OC OC 199 L L 200 O P R=Ph: L=CO (a), (b), PPh3 (c); R=Mes: L=CO (d); O OO P (e), PPh3 (f).OOThe 1,3-dipolar addition of arenecarbonitrile oxides to the chiral vinyl sulfoxides 201 occurs with high regio- and stereo- selectivity and results in (4S,5R,RS)-4,5-dihydroisoxazoles 202.172, 173 N Ar O R 4-MeC6H4 ArCNO S 4-MeC6H4 OMe O S R OMe 201 O 202 R=CH2F, CHF2, CF3, C2F5 ; Ar=Ph, 2,6-Cl2C6H3 , 3,5-Cl2-2,4,6-Me3C6 . The effect of the sulfinyl group is much less pronounced if the latter is in the b-position relative to the double bond.174 This reaction is characterised by full regioselectivity but moderate stereoselectivity and results in the formation of chiral 4,5-dihy- droisoxazoles. Thus, the addition of 3,5-dichloro-2,4,6-trimethyl- benzonitrile oxide to the allylic sulfoxide 203 results in the diastereomers 204 and 205 in a 1.4 : 1 ratio.174 X CH2F ArCNO S O 203 N N Ar Ar O O H H X X + S S H CH2F O O H CH2F 205 204 X=4-MeC6H4; Ar=3,5-Cl2-2,4,6-Me3C6 .A mixture of the diastereomers 207 and 208 (2 : 1) is formed in the reaction of (Z)-allylic sulfoxide 206 with 3,5-dichloro-2,4,6- dimethylbenzonitrile oxide. X CH2F S ArCNO OMeO2C H 206 N N Ar Ar O O H MeO2C X + X S S MeO2C O O CH2F 208 H CH2 F 207 X=4-MeC6H4; Ar=3,5-Cl2-2,4,6-Me3C6 . The dipolarophiles 209 have two chiral centres, viz., the sulfinyl group and the oxygen-containing substituent in the allylic position. Both functional groups influence the stereochemistry of the 1,3-dipolar cycloaddition.175 The reactions of compounds (3S,RS)-209 with arenecarbonitrile oxides give mixtures of 4,5- dihydroisoxazoles 210 and 211. In all cases studied, the epimers 210 were predominant (up to 67%).422 R OEt X ArCNO S O OH 209 Ar Ar N N H H O X O X + S S R R OEt OEt O O H H HO211 HO210 X=4-MeC6H4; R = CF3, CF2Cl; Ar=3,5-Cl2-2,4,6-Me3C6 , 2,6-Cl2C6H3 .It was found 176, 177 that baker's yeast Saccharomyces cerevi- siae are effective chiral catalysts in reactions of asymmetric cyclo- addition of nitrile oxides. Thus the addition of nitrile oxides to alkenes 212a ± c in the presence of baker's yeast and cyclodextrin gives 4,5-dihydroisoxazoles 213a ± c with high stereoselectivity. R1 N O R3CNO R1 R3 R2 R2 212a ± e 213a ± e (b), (c); (a), R1=H:R2= N N N R1=Me: R2 =CN (d), CO2Me (e); R3=Mes, 2,6-Cl2C6H3 , 2,4,6-(MeO)3C6H2 ; The electron-deficient dipolarophiles 212d,e also gave opti- cally active 4,5-dihydroisoxazoles 213d,e.178 However, some authors 179 argue against this conclusion assuming that the isolation of only one enantiomer is due to the loss of the second enantiomer in the recrystallisation.It was noted that cycloaddition of nitrile oxides to captodative alkenes 214 occurs in a regioselective manner and results in the isoxazoles 215.180 Here, alkenes 214 behave as acetylene equiv- alents. Ph CO2Me PhCNO N CO2Me OCOAr 214 O 215 * * * Thus, 1,3-dipolar cycloaddition of nitrile oxides to the double bond can be applied to various classes of compounds. In the majority of cases, this occurs with good yields and high regio- and, sometimes, stereoselectivity which allows its use for the solution of complex problems of organic synthesis.The reaction products, viz., 4,5-dihydroisoxazoles, present interest from the standpoint of their biological activities.181, 182 In our opinion, further investiga- tions into the cycloaddition of nitrile oxides to alkenes will be associated with the search for novel catalysts and chiral substitu- ents for regio- and stereo-controlled addition. References 1. C Grundmann, P Grunanger The Nitrile Oxides (New York: Springer, 1971) 2. A P Kozikowski Acc. Chem. Res. 17 410 (1984) 3. P G Baraldi, A Barco, S Benetti, G P Pollini, D Simoni Synthesis 857 (1987) 4.F A Lakhvich, E V Koroleva, A A Akhrem Khim. Geterotsikl. Soedin. 435 (1989) a R P Litvinovskaya, V A Khripach 5. D P Curran Adv. Cycloadd. 1 129 (1988) 6. A A Akhrem, V A Khripach, F A Lakhvich,M I Zavadskaya, O A Drachenova, I A Zorinayu Zh. Org. 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ISSN:0036-021X    

出版商:RSC    

年代:2001

数据来源: RSC

摘要:

Russian Chemical Reviews 70 (5) 425 ± 447 (2001) Problems of control of the reactivity of macroradicals and the growth of polymer chains D F Grishin, L L Semyonycheva Contents I. Introduction II. Coordination interactions of organic compounds of Group II and III elements with macroradicals and their influence on chain propagation III. Growth of the polymer chain in the presence of stable radicals and iniferters IV. Characteristic features of homo- and co-polymerisation of vinyl monomers in the presence of organoelement compounds derived from Group V elements V. Conclusion Abstract. iniferters, radicals, stable of influence the on Data Data on the influence of stable radicals, iniferters, organoelement on additives active other and compounds organoelement compounds and other active additives on the the reactivity of steps elementary the and macroradicals of reactivity of macroradicals and the elementary steps of the the (co)polymerisation monomers other some and acrylic of (co)polymerisation of acrylic and some other monomers are are generalised. of lifetime the of control the to approaches New generalised.New approaches to the control of the lifetime of the the polymer are stage propagation chain the of and chain polymer chain and of the chain propagation stage are analysed. analysed. The problems of synthesis of compositionally homogeneous The problems of synthesis of compositionally homogeneous homo- and copolymers by radical polymerisation are considered. homo- and copolymers by radical polymerisation are considered.The bibliography includes 247 references The bibliography includes 247 references. I. Introduction Polymerisation involving radical species has long been and still remains the key method for the synthesis of polymers. Obvious advantages of radical polymerisation are the methodical and technical simplicity of its implementation and good reproducibil- ity of the results. A significant drawback of the method which somewhat delays its practical use is the difficulty of controlling the kinetics of polymerisation and compositional homogeneneity of the products, caused by the fact that the reactivity of free radicals participating in the polymerisation barely depends on the medium composition or other conditions.1±4 In this connection, development of effective methods for controlling the growth and the lifetime of a polymer chain is a topical task.This problem could be solved if it were possible to change the reactivity of the chain-propagating radicals in the required manner. In recent years, several new approaches for controlling the growth of a polymer chain under the conditions of radical polymerisation have been proposed. These approaches include reversible inhibition by stable radicals, the use of iniferters as controlling initiators of a new type and the use of catalytic amounts of stable radicals or coordinatively unsaturated organo- element compounds (OEC) as reactive additives, etc. These D F Grishin, L L Semyonycheva Research Institute of Chemistry, N I Lobachevsky Nizhnii Novgorod State University, prosp.Gagarina 23/5, 603600 Nizhnii Novgorod, Russian Federation. Fax (7-831) 235 64 80. Tel. (7-831) 265 81 62. E-mail: grishin@ichem.unn.runnet.ru (D F Grishin) Tel. (7-831) 265 74 51 (L L Semyonycheva) Received 13 November 2000 Uspekhi Khimii 70 (5) 486 ± 510 (2001); translated by Z P Bobkova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n05ABEH000635 425 425 435 441 443 methods have been used to develop effective methods for the synthesis of compositionally uniform polymeric materials with a specified set of properties. This review is devoted to the analysis of these methods. II. Coordination interactions of organic compounds of Group II and III elements with macroradicals and their influence on chain propagation The search for the ways of target-directed change in the reactivity of monomers and chain-propagating radicals directly during polymerisation is one of the most important aspects in the studies on the methods of synthesis of polymeric materials of a particular structure with specified properties.In the 1970 ± 1980s, the vigorous studies concerning the use of protic and aprotic Lewis acids (halides of Group II and III elements and some transition metals) for controlling the reactivity of vinyl and, especially, acrylic monomers carried out in a number of leading scientific schools of the world gave rise to a new line of research in the synthetic chemistry of polymers, namely, coordi- nated radical polymerisation.5±7 It was found that Lewis acids, which are strong electron acceptors, can substantially influence the electron density distri- bution over the monomer p-bond and functional group and are thus able to change the kinetic parameters of homo- and copoly- merisation and the relative activities of monomers.On the basis of this approach, original methods for the synthesis of alternating and compositionally homogeneous copolymers, physiologically active high-molecular-mass compounds, graft and block copoly- mers, and a number of promising composite materials have been developed. The main drawback of this method for modification of polymeric materials is that the controlling additive is introduced, as a rule, in a substantial excess with respect to the initiator (1% ± 5% relative to the monomer).This leads to contamination of the resulting polymer. The cost of its purification is high; therefore, this method for the control of the composition and structure of polymers does not have prospects in industry. The use of OEC as initiators has laid the foundations of a new stage in the development of the synthetic chemistry of polymers. The Ziegler ± Natta catalysts proposed for the synthesis of homo- and copolymers of alkenes and dienes are of limited utility for (co)polymerisation of vinyl monomers, especially acrylates,426 because these monomers are highly reactive towards the compo- nents of these organometallic catalysts.8 Conversely, initiating systems based on alkyl derivatives of non-transition elements and oxidants including organic and organoelement peroxides (OEP) not only initiate efficiently rad- ical homo- and copolymerisation of a broad range of monomers but also control the composition of copolymers and the kinetics of processes; they also change noticeably the properties of polymers.Based on the analysis of the characteristic features of polymer- isation kinetics and unusual properties of macromolecules syn- thesised in the presence of organoelement initiators, it was initially suggested and later convincingly proved that the features men- tioned above are related to the coordination interaction of the OEC with the growing radical. This interaction promotes an increase in the electron-withdrawing properties (electrophilicity) of the macroradical.9 ±11 1.The effect of coordinatively unsaturated organoelement compounds on the polymerisation kinetics of vinyl monomers and on the molecular-mass characteristics of polymers Among organoelement initiators of the redox type, it is necessary first of all to distinguish compositions based on alkyl derivatives of boron and aluminium, which possess a number of advantages over the conventional radical initiators.11 ± 20 In particular, boron and aluminium alkyl derivatives in combination with organic and organoelement peroxides permit one to carry out polymerisation of vinyl monomers, which occurs at a high initial rate over a broad temperature range,{ from 0 to 50 8C,13 ± 15 and copolymerisation of vinyl monomers with alkenes and allylic monomers.23 ± 25 Polymers and copolymers prepared in the presence of these organoelement initiators have unusual properties such as stability against thermal treatment and unusual molecular-mass character- istics; copolymers possess a number of unique mechanical proper- ties.13 ± 29 In view of the fact that the rates of chain propagation during the synthesis are high and based on the studies of some phys- icochemical properties of the prepared polymers, it has been suggested 14, 27 ± 29 that components of an organoelement initiator not only participate in the stage of generation of chain-propagat- ing radicals but also influence the chain propagation and termi- nation stages.It is known that unsaturated compounds including monomers are able to be coordinated to OEP.30 ± 32 In these compounds, monomers act as electron density donors. On the one hand, complex formation increases the rate of peroxide decomposition and, hence, increases the initiation rate, while, on the other hand, it favours redistribution of electron density on the monomer p-bond and thus changes the reactivity of the monomer including that with respect to radicals. Similar complexes of vinyl monomers with Lewis acids (metal halides) have long been known and studied in detail.5 It was shown reliably by IR 33 ± 36 and NMR37 spectroscopy that metal halides are coordinated to the carbonyl group of the monomer. The constants of complex formation of organic compounds of Group III elements with methyl methacryl- ate (MMA) and acrylonitrile (AN) were estimated taking organo- aluminium compounds as examples.38 If the concentration of the OEC used as the initiator amounts to 0.01 mol.% ± 0.05 mol.%, the formation of OEC complexes with radicals,39 ± 41 including polymeric radicals, appears more likely.28, 33 Upon the OEC± radical coordination interaction, the electron-withdrawing properties of the growing macroradical are enhanced and its reactivity with respect to the multiple bonds in monomers increases. This interaction results in a higher rate constant for chain propagation. In addition, due to steric factors and the enhanced electrophilicity of the species, the rate constant for chain termination decreases.{ For vinyl chloride (VC), this range is760 to 50 8C.15, 19, 21, 22 D F Grishin, L L Semyonycheva In a series of studies,17 ± 19, 40, 41 it was demonstrated that the addition of catalytic amounts of OEC derived from Group II and III elements in a polymerising mixture which contains peroxide and azo initiators entails an increase in the rate of polymerisation of (meth)acrylic and some vinyl monomers and markedly influ- ences the properties of the resulting polymers (Table 1). Table 1. Influence of triisobutylborane (TIBB) on the rate of polymer- isation (V) of vinyl monomers and the molecular mass (MM) of polymers (T = 313 K). Monomer TIBB (mol.%) Initiator (0.1 mol.%) AIBN VA DCHPD BA AIBN AN DCHPD MMA 0.0 0.2 0.3 0.0 0.2 0.4 0.6 0.8 0.0 0.5 1.0 1.5 0.0 0.2 0.4 DCHPD AA (in THF) DCHPD St 0.0 0.015 0.045 0.060 0.0 0.2 0.3 Note.The following designations are used: VA is vinyl acetate, AIBN is azobisisobutyronitrile; BA is butyl acrylate, DCHPD is dicyclohexyl peroxydicarbonate, AA is acrylamide, St is styrene. Thus the rate of polymerisation ofANand butyl acrylate (BA) increases by almost two orders of magnitude upon introduction of trialkylborane in the system. Meanwhile, although organoboron compounds are capable of initiating homolytic decomposition of hydroperoxides,42, 43 they do not exert catalytic action on the decomposition of acyl peroxides [in particular, benzoyl peroxide (BP)44 or dicyclohexyl peroxydicarbonate (DCHPD)],18 or other compounds,45 used in the studies cited as initiators.The research- ers concluded that the accelerating influence of trialkylboranes on the polymerisation of polar vinyl monomers can be due exclu- sively to the coordination interaction of the growing macro- radicals with the boron atom in the OEC. This interaction occurs either directly at the reaction centre (with participation of the unpaired electron), or at the functional group CH2 Y CH2 C.+BR3 X CH2 (whereXis a functional group,Yis the hydrogen atom or a methyl group), or simultaneously at both sites with delocalisation of the unpaired electron 1073MM 104 V /mol litre71 s71 737 614 503 0.3 1.7 6.0 0.3 19.9 2680 610 440 380 200 7777 610 586 5609.4 777.7 21.0 159.2 295.1 237.3 0.7 1.1 1.5 1.8 0.09 0.22 0.56 1.12 3.4 3.6 3.7 1800 1780 1660 BR3 (1) YC.XYC. X BR3Problems of control of the reactivity of macroradicals and the growth of polymer chains Y Y (2) C CH2 CH2 BR3 . C.+BR3 X X On the basis of studies of copolymerisation of MMA with electron-donating monomers involving binary trialkylborane ± OEP initiators,11, 14 it was suggested that the Lewis acid might be involved in the coordination interaction with the poly(methyl methacrylate) (PMMA) radical; this hypotheisis was proved by EPR spectroscopy 28, 46 and confirmed by quantum-chemical calculations.47 In addition, it was shown by quantum-chemical calculations 48 that in the case of radicals containing aC=Ogroup in the a-position to the reaction centre, the reaction centre is delocalised over three atoms.Unlike metal halides (Lewis acids), organometallic additives introduced in the system in amounts comparable with the initiator concentration (0.1 mol.% ± 0.5 mol.% with respect to the mono- mer), influence appreciably the growth of the polymer chain. This fact indicates unambiguously that an increase in the polymer- isation rate in the presence of these compounds is due to complex formation of the OEC exactly with the macroradical rather than with the monomer. It has been suggested 49 that oneOECmolecule can participate consecutively in several chain propagation steps, while retaining the growing macroradical in its coordination sphere.In this case, chain propagation occurs in a cyclic reaction complex which incorporates the growing macroradical, the monomer, and the complex-forming agent. H H C C CH2 BR3 CH2 + C O C O H3CO H3CO H C H CH2 C CH2 (3) C OCH3 C B O H3CO O H H C C CH2 BR3 CH2 C O O C H3CO H3CO A similar mechanism involving an eight-membered reaction complex has been proposed by Kabanov 50 to interpret the increase in the propagation rate constant in the polymerisation of MMA in the presence of metal halides and by Smirnova and coworkers,51, 52 who studied copolymerisation of acrylic acid with VA (vinyl acetate) in the presence of germanium salts.In our opinion, the above-presented reaction complex [see Eqn (3)], which includes the allylic delocalised macroradical is thermo- dynamically and geometrically more stable. It was found 53 that the addition of trialkylborane accelerates processes with monomers containing a polar substituent. In the case of styrene, organoelement additives have little influence on the polymerisation. Apparently, this is due to the fact that the St macroradical contains no functional groups capable of being coordinated to trialkylborane; therefore, the additive almost does not change the electrophilicity of the growing macroradical. The coordination directly to the reaction centre, namely, to the carbon atom carrying the unpaired electron is relatively unlikely.Moreover, this interaction can create steric hindrance to chain propagation. It is not by chance that the St polymerisation rate is lower in the presence of some organic compounds of Group III elements than without any complex-forming additives.49 427 The features of electronic and spatial structures of the growing macroradical play a crucial role in the chain propagation stage of coordinated radical polymerisation. On the one hand, coordina- tion of an OEC molecule to a macroradical increases the electro- philicity of the latter and thus enhances its reactivity with respect to the monomer p-bond. It is evident that this effect is more pronounced in the case of polymerisation of sterically non- hindered monomers such as BA, AN and VA.On the other hand, upon the coordination interaction of the radical with OEC, the size of the reaction centre increases and additional steric hindrance to the chain propagation can arise. In the case of bulky radicals, for example, those of PPMA or polystyrene (PS), steric factors are, naturally, more significant in the chain propagation step than in the case of the BA or VA macroradicals. This is why the activating effect of Group III element alkyl derivatives is manifested most clearly in the polymerisation of esters and other functional derivatives of acrylic acid (amides, nitriles). The effect involved in the polymerisation of methacrylates, for example, MMA and butyl methacrylate (BMA) is much less pronounced.The structure of OEC also influences the reactivity of the chain-propagating radicals and, hence, the kinetic parameters of polymerisation of acrylic monomers. For example, replacement of the alkyl radical in the trialkylborane molecule by a halogen atom [dibutylboron bromide (DBBB)] deteriorates the coordination capacity of boron due to the partial blocking of its vacant p orbital (pp ± pp conjugation). As a consequence, DBBB is less active as a controller of acrylate polymerisation than TIBB (triisobutyl- borane).49 Similar regularities can be observed in the case of phenyl derivatives of boron. Triphenylborane and dibutylphenylbor- ane 53 were found to accelerate polymerisation of acrylic mono- mers to a smaller extent than TIBB.This can be due both to the steric restrictions involved in the coordination of triphenylborane to the growing polymer radical and to the conjugation of one phenyl group of the substituent with the vacant p orbital of boron. Consequently, as in the case of DBBB, the electron-withdrawing capacity of the central atom in the OEC decreases. These results do not contradict the published data on the relationship of the reactivities of alkyl and phenyl derivatives of boron.54 The alkyl derivatives of boron analogues � aluminium, gallium and indium � also increase somewhat the rate of polymerisation of acrylic monomers and VA;49 however, as in the case of DBBB, their influence on the chain propagation stage is less pronounced than that of TIBB.The difference between the effects of organoboron compounds, on the one hand, and organo- aluminium, -gallium and -indium compounds, on the other hand, is, apparently, due to different coordination capacities of the central atoms in the OEC and, as a consequence, their dissimilar influence on the reactivity of the macroradical. A similar consid- eration can be used to interpret the lower efficiency, compared to that of trialkylboranes,53 of zinc and cadmium alkyl derivatives as controllers of the polymer chain growth in the polymerisation of acrylic monomers. Thus, alkyl derivatives of boron exert a greater influence on chain propagation in the radical polymerisation of vinyl mono- mers than other coordinatively unsaturated OEC derived from Group II and III elements.54 The participation of OEC in the elementary steps of polymer- isation (chain propagation, transfer, termination) opens broad opportunities for controlling the MM of the resulting polymers.The introduction in the polymerisate of boron alkyl derivatives in amounts commensurable with the concentration of the initiator decreases substantially the MM. The calculated constants for the chain transfer to TIBB are 161072 for MMA and 2.261072 for VA.53 The addition of TIBB also influences markedly theMMof poly(butyl acrylate) when polymerisation is carried out in an ethyl acetate solution.49 The molecular mass of the polymer decreases with an increase in the OEC concentration; upon the introduction of 0.8 mol%of trialkylborane, it decreases by more than an order428 of magnitude.The calculated constant for chain transfer to the organoboron compound is 7.461072; this is somewhat higher than the constant for chain transfer to TIBB during MMA polymerisation and appreciably higher than the chain transfer constant to BA (0.361074).55 The high constants of chain transfer to trialkylboranes can be due to two processes involving the macroradical and the OEC. One process is free-radical substitution of the growing polymer radical for the alkyl radical in OECCH CH CH2 CH2 (4) C C RO RO O O CH CH CH2 CH2 (5) C C + (R0) +BR03 RO RO O OBR02 R, R0 is alkyl (isobutyl, ethyl, n-butyl). Using EPR spectroscopy and quantum-chemical calculations, it has been shown 48 that (meth)acrylate macroradicals are prone to exist in a state typical of alkyl radicals, i.e., the reaction centre is delocalised to give both carbon- and oxygen-centred radicals [Eqn (4)].However, the rate constant for the replacement of alkyl radicals in trialkylborane by oxygen-centred radicals [Eqn (5)] is rather high,*105 litre mol71 s71.56 The second process which is also favourable for the control of the polymer MM is the abstraction of a hydrogen atom from the a-methylene unit of an alkyl group in OEC by the growing macroradical.CH CH2 C +Me2CHCH2BBun2 RO O (6) CH2 CH2 C +Me2CHCHBBun2 O RO The occurrence of this reaction was proved unambiguously by EPR using the spin trap technique in a study of the influence of small additives of methyl acrylate (MA) and other acrylic mono- mers on the polymerisation of MMA with organoelement initia- tors.57 The rate constant for this reaction in the case of triethylborane is*104 litre mol71 s71.56 In the polymerisation of styrene (St), the MM of the product barely changes, which is consistent with the results of kinetic measurements and indicates that OEC does not interact with the growing macroradical of PS.49, 53 The influence of added OEC on the molecular-mass distribu- tion (MMD) of polymers has been studied by gel permeation chromatography (GPC).58 In the presence of AIBN as the initiator, a unimodal MMD curve is observed; however, when TIBB is added to the system, the curve becomes clearly bimodal. As a consequence, the polydispersity of samples increases some- what.Evidently, in the presence of TIBB, the growth of the polymer chain occurs in parallel by two mechanisms, namely, coordinated radical and radical mechanisms. On the one hand, OEC is coordinated to the growing macro- radical, thus increasing its electrophilicity and, hence, its reactivity with respect to the monomer. As noted above, chain propagation takes place in a cyclic reaction complex [see, for example, Eqn (2)]. This interaction changes the chain propagation and termination rate constants and decreases theMMof the polymer. On the other hand, growth of the polymer chain occurs simultaneously by the classical radical mechanism D F Grishin, L L Semyonycheva Me Me Me Me C C C.C. CH2 CH2 CH2 (7) CH2 + O C O C O C O C OMe OMe OMe OMe A bimodalMMDcurve is also observed when polymerisation of MMA is carried out with a binary initiator, TIBB ± di-tert- butylperoxytriphenylantimony (DPA), which is an efficient low- temperature initiator controlling polymerisation of vinyl mono- mers.13, 15, 19, 22, 32, 44 The conclusions drawn confirm indirectly the results of studies of the MMDof PS samples synthesised in the presence of various OEC derived from Group II and III elements.52 The introduction of these compounds barely influences both the general pattern of theMMDcurve and the degree of polydispersity of samples. It has been noted above that OEC do not react with the growing polystyrene radical due to electronic and steric factors.49, 58 There- fore, in the polymerisation of St, chain propagation follows only the traditional radical mechanism without participation of the complex-forming agent.Thus, the data outlined above indicate that organic com- pounds of Group III elements provide effective control of the molecular-mass characteristics of polyacrylates. This way of controlling polymerisation of acrylic monomers opens up new opportunities for the synthesis of polymers with novel physico- chemical and practical properties. A certain drawback of the proposed method is the higher polydispersity of the resulting polymers, caused by the fact that polymerisation in the presence of organoelement additives follows two parallel mechanisms.2. Specific influence of phenolic inhibitors on homo- and copolymerisation of vinyl monomers in the presence of organic compounds of Group III elements Hydroquinone, ionol, diphenylolpropane and some other phenols are known inhibitors of radical polymerisation; they are tradi- tionally used as stabilisers for the storage of vinyl monomers. The inhibiting efficiency substantially increases in the presence of oxygen; this may be due to oxidation of phenols to quinones, which retard chain propagation more appreciably.59 According to the most popular viewpoint,59, 60 the inhibitory influence of hydroquinone is due to the easy abstraction of the active hydrogen atom from the phenol mole giving rise to a phenoxyl radical.(8) R.+ArOH R7H+ArO.. The phenoxyl radical, which exhibits a low reactivity towards the monomer multiple bond, is unable to initiate a radical process (polymerisation). Back in the 1960s, it was found 61 ± 64 that the addition of some OEC derived from Group III elements makes it possible to carry out polymerisation of MMA in the presence of BP or AIBN to a high degree of conversion, despite the presence of comparable amounts of hydroquinone. Apparently,56 in the presence of OEC, the phenoxyl radicals undergo a fast SR2 substitution reaction.(9) R.+ArOMR2 ArO.+MR3 Mis a Group II or III metal. The alkyl radicals formed in this reaction can subsequently initiate polymerisation. It is in these processes that organoboron compounds can act as chain transfer agents in the polymerisation of acrylates.As noted above, the rate constants for the chain transfer in peroxide-initiated polymerisation of MMA are equal to 0.015 for tri-n-butylborane and 0.01 for TIBB.20 A somewhat different interpretation of the absence of inhib- itory influence of phenols on polymerisation in the presence of OEC was proposed by Arimoto,62 who suggested that trialkylbor- ane is coordinated to the growing macroradical through the unpaired electron of the latter.Problems of control of the reactivity of macroradicals and the growth of polymer chains C C BR3 (10) C C +BR3 C C C C C C BR3 As a consequence, the reaction centre of the radical becomes blocked and the interaction with phenol is made impossible.It should be noted that Grotewold et al.61 cast doubt on the experimental results obtained by Arimoto. In some studies published in the 1990s,16 ± 18, 65 it was shown that diphenylolpropane and hydroquinone are not merely inactive as inhibitors of radical polymerisation of acrylates in the presence of trialkylboranes but, conversely, accelerate it. Indeed, the rates of MMA and AN polymerisation markedly increase in the presence of organic compounds of Group III elements, the effect becoming more pronounced with an increase in the contents of OEC and the above-mentioned diphenols. Similar data have also been obtained for reactions carried out in the presence of alkylalkoxy boron derivatives.44 This unusual accelerating effect can be due to some extent to the occurrence of chain transfer to the OEC [Eqn (9)].However, this fact alone cannot account for the absence of the inhibiting effect in the polymerisation of MMA, AN, and MA in the presence of substantial concentrations of hydroquinone and diphenylolpropane (an order of magnitude higher than the initiator concentration) or for the absence of inhibiting effect after the introduction of Lewis acids in the system which under- goes polymerisation.66 ± 68 The unusual behaviour of phenols can be explained in terms of the coordinated radical mechanism of (co)polymerisation of acrylic monomers on organoelement initia- tors. In the presence of electron acceptors such as Lewis acids, the macroradical and the monomer are coordinated to components of the initiator. The complex formation of OEC with the monomer activates the monomer;58 this enables initiation of the polymer- isation by phenoxyl radicals.CX CX CH2 CH2 (11) +MR3 Y Y MR3 CX ArO CH2 (12) Y MR3 ArO.+CH2 CX Y MR3 Mis a Group III metal. The coordinated macroradical, unlike the non-coordinated one, exhibits enhanced electrophilicity with respect to the mono- mer p-bond, which results in a higher propagation rate con- stant.5, 11, 18 The occurrence of reaction (12) is also confirmed by the fact that IR- and UV-spectroscopic studies show the presence of phenoxy groups in the MMA polymer. This view on the kinetics of polymerisation of acrylates in the presence of OEC and inhibitors is indirectly supported by the absence or attenuation of the inhibiting effect inMMA polymerisation in the presence of Group III metal halides.66 ± 68 In this case, SR2 reactions [Eqn (9)] are impossible in principle; therefore, the effect observed can be interpreted in terms of either the Arimoto scheme, or reactions (11) and (12), or the cyclic mechanism of chain propagation [see Eqn (3)].7, 20, 49 The same trialkylborane molecule can be involved in several consecutive chain propagation events, while retaining the growing macroradical in the coordination sphere of the boron atom and `receiving' (coordinating) new monomer molecules.Chain propagation in the cyclic reaction complex hampers the interaction of the growing macroradical with the inhibitor.It is the processes considered above that account for the increase in the rate of MMA, AN and MA polymerisation in the presence of organic compounds of Group III elements and phenols such as hydroquinone and diphenylolpropane. However, in the case of ionol, which is a sterically hindered phenol, polymer- 429 isation rate somewhat decreases.7, 16 This appears to be due to the fact that reactions (9) and (12) are much less efficient for ionol than for hydroquinone or diphenylolpropane. In view of the foregoing, it can be assumed that the vacant orbitals of TIBB accept electron density through coordination to the macroradical. This increases the electrophilicity of the coor- dinated poly(methyl methacrylate) radicals; therefore, they become relatively `inert' with respect to the abstraction of a hydrogen atom from phenol.In addition, chain propagation takes place in the cyclic reaction complex, which also hampers the reaction of the macroradical with the active hydrogen atom of the phenol hydroxy group. As a consequence, polymerisation of MMA proceeds up to a high degree of conversion in the presence of a 10-fold excess of the inhibitor with respect to OEC. Phenolic inhibitors have also a certain influence on the polymer MM.7, 16, 49, 65 In particular, the molecular mass of PMMA decreases upon the introduction of phenols. However, this influence is not so pronounced (as could be expected) as that observed in the absence of OEC or in the presence of conventional chain transfer agents.Oxygen plays an important role in the mechanism of inhib- ition of radical polymerisation by various phenols. According to one view,59 the influence of oxygen reduces to the oxidation of phenol to the corresponding stable radical or quinone. In the presence of organic compounds of boron or aluminium or of benzoquinone, polymerisation of MMA proceeds without an induction effect.40, 61 ± 64 The inhibitory effect of benzoquinone after the addition of TIBB depends on the [TIBB] : [benzoquinone] molar ratio. If the amount of benzoquinone is smaller than that of TIBB, the inhibitory action has virtually no effect on the polymer- isation kinetics. The process occurs up to a high degree of conversion and the molecular mass of PMMA changes insignif- icantly.In the absence of OEC, benzoquinone efficiently inhibits polymerisation of acrylates: the rate of polymerisation in the initial stages is close to zero, the limiting degree of conversion is less than 0.5%.49, 58 As the benzoquinone concentration increases, the rate of polymerisation decreases. However, even with a twofold excess of benzoquinone with respect to alkylborane, it is markedly higher than that in the absence of OEC. Analysis of the publications cited and data on the reactivity of organoboron compounds and quinones point to efficient consumption of trialkylborane and quinone.69 ± 74 OBR2 O R (13) O R3B+O 4-ROC6H4OBR2 4-ROC6H4O R R O (14) R.+O O O O O.R is the initiator radical. Me Me (15) C C OC6H4O CH2 CH2 O +O C C OMe O OMe O Reaction (14) and the regularities of inhibition of alternating copolymerisation by quinones have been studied comprehensively by EPR spectroscopy by Golubev,70 who used the reactions of benzoquinone, naphthoquinone, and fluoranil with organic rad- icals of different electrophilicity as examples to demonstrate that the abnormally slight influence of quinoid-type radical inhibitors on the alternating copolymerisation is related to substantial polar effects involved in this reaction. In accordance with this concept, the increase in the electrophilicity of macroradicals upon the coordination to OEC should substantially influence the reactivity430 of the macroradical with respect to the quinone [chain termina- tion, Eqns (13) ± (15)].It is beyond doubt that in the presence of OEC in a polymer- ising system, the factor of polarity would play a vital part in the attenuation of the inhibiting effect of quinones. In particular, the phenoxyl radicals generated in processes (13) ± (15) enter into SR2 substitution to give alkyl radicals able to initiate polymerisation once again [see Eqn (9)]. Judging from the average chain length, two chain transfers to the trialkylborane molecule take place.63 The influence of organoaluminium compounds on the poly- merisation of acrylates in the presence of benzoquinone is equally efficient.58 The rate of polymerisation increases with an increase in the [triethylaluminium] : [benzoquinone] ratio, passes through a maximum, and then decreases.This was attributed 64 to the occurrence of two mechanisms of initiation; one of them is related to the decomposition of the initiator into radicals and the other, to the generation of radical centres upon decomposition of the triethylaluminium complex with quinone. In view of the analysis of products of the reaction of organoaluminum compounds with quinones, the latter mechanism seems quite debatable. Apparently, in this case, as for trialkylborane, both the heterolytic reaction [Eqns (13) ± (15)] and the homolytic transfer of the reaction centre to the metal atom in OEC [see Eqn (9)] take place. In addition, whereas in the case of TIBB, the reaction can involve two alkyl groups, in the case of triethylaluminium, only one group is involved 64 (the rate constant for chain transfer is equal to 89 litre mol71 s71; for comparison, the rate constant for chain propagation is 576 mol litre71 s71.75).Organoelement compounds formed by Group III elements and the inhibitors chosen accelerate not only homo- but also co- polymerisation of MMA with a number of vinyl monomers, including St, VA, and methacrylic acid (MAA).7 Similar features have also been found for theMAA±VA pair of monomers. It was shown in relation to the copolymerisation of these monomers in the presence of hydroquinone and OEC that the inhibitor partic- ipates in the chain propagation and termination stages and influences the dependence of the copolymer composition on the composition of the monomer mixture.17 ForMAAcontents in the monomer mixture ranging from 20 mol.% to 80 mol %, the addition of hydroquinone virtually does not change the composi- tion of the MAA±VA copolymer, which does not depend on the composition of the monomer mixture.In the Kelen ± Tudoc calculation of the copolymerisation constants, the intermediate calculated parameters (Z and c) are found to deviate from the linear dependence; this is evidence supporting the coordinated radical mechanism of copolymerisation. 3. The coordinated radical copolymerisation of vinyl and some other monomers in the presence of organoelement initiators At present, copolymerisation is an important method for the preparation of polymers with a specified set of chemical and physical properties because it is difficult in most cases to control the properties of homopolymers.Meanwhile the need to control these properties is a topical task in preparing materials for various purposes.4, 76 Copolymerisation normally affords random copolymers, the composition and properties of which are determined by the composition of the monomer mixture and the reactivity of monomers and chain-propagating radicals. However, there exist monomer pairs copolymerisation of which gives polymers with regular alternation of the units. As a rule, strict alternation of monomer units is observed if one monomer is a clear-cut donor of electron density, and the other one is an acceptor.Since the number of monomers with clearly defined electron-withdrawing properties is small [maleic anhydride and its derivatives, sulfur(IV) dioxide], the number of monomer pairs that can form alternating copolymers is also few. The main method for controlling the composition of copoly- mers is introduction of special additives, usually, protic and D F Grishin, L L Semyonycheva aprotic acids, into the system. The latter are coordinated to the monomers or to the chain-propagating radicals and thus enhance their electron-withdrawing properties and provide the possibility of preparing alternating copolymers.5, 77, 78 The main drawback of this method of modification of polymer materials is that the controlling additive is introduced in a large excess with respect to the initiator (1 ± 3 orders of magnitude), which results in contam- ination of the polymer formed.The cost of polymer purification is high; therefore, the use of this method to control the polymer composition on an industrial scale is non-expedient. The use of organoelement initiators based on Group III element organic compound and peroxides eliminates this draw- back because the controlling influence of these additives on chain propagation manifests itself, as noted above, when catalytic amounts of the initiator are used (0.01 mol%± 1.00 mol %). The complex formation effects in the radical copolymerisation in the presence of Lewis acids are largely determined by the electronic structure of the monomer.6, 79 In the possible limiting cases, either one monomer or both monomers react with the complex-forming agent.Both situations are analysed below. It has been shown in relation to a number of monomer pairs that an organoelement initiator influences directly the variation of the copolymer composition and the copolymerisation rate vs. the composition of the monomer mixture. The controlling influence of OEC is most pronounced in the copolymerisation of (meth)- acrylates with electron-donating monomers.7, 58 For instance, in the copolymerisation of MMA with allyl chloride (AC) with a classical radical initiator AIBN, the copolymer is enriched in the more active monomer, MMA, in each section of the composition curve (the relative activities of the monomers are rMMA=3.05; rAC=0.05).23, 77 The rate of copolymerisation decreases with an increase in the content of AC in the monomer mixture.When a binary initiator, TIBB ± DPA, is used,80 the composition curve is S-shaped. A section in which the copolymer composition does not depend on the composition of the monomer mixture can be clearly seen in the curve; in this case, the trend for the alternation of monomer units is close to its limiting value. The relative activities of both monomers become much less different and smaller than unity (Table 2); the product r1r2 ? 0, pointing to a coordinated radical mechanism of (co)polymerisa- tion.5, 6 This is also indicated by the deviation from linearity of the calculated parameters in the determination of the copolymerisa- tion constants by the Kelen ± Tudoc method.Organoelement initiators, even present in low concentrations (0.01 mol%± 0.10 mol %), have a noticeable controlling influ- ence on the composition and structure of the MMA±AC copoly- mer. This is due to the coordination of the growing macroradical to the components of the initial system. Studies of the separate effects of the initiator components on the dependence of the copolymer composition on the composition of the monomer mixture indicate that TIBB, which contains a boron atom with a vacant p orbital, plays the most active role in the complex formation with the macroradical.80 Its coordination to the grow- ing poly(methyl methacrylate) radical induces electron density redistribution in the radical, increases the radical electrophilicity, and enhances its reactivity towards the p-bond of the electron- donating monomer AC.The variation of the rate of copolymerisation of AC with MMA as a function of the initial composition of the monomer mixture has a clear-cut maximum at a monomer ratio close to the equimolar ratio. This is evidence in favour of the coordinated radical mechanism of copolymerisation and also provides grounds for believing that the monomers and macroradicals form a cyclic reaction complex in the chain propagation stage. Me CH CH2 C CH2 CH2 B Cl C MeO OProblems of control of the reactivity of macroradicals and the growth of polymer chains Table 2. Effective values for the relative activity of monomers.Monomers r2 r1 1 2VA AN AC AN MMA AN VC AN VDC AN MAA AN St AN MAA MMA AC MMA St MMA 0.06 0.20 0.04 0.05 0.03 0.42 0.05 0.15 1.35 1.10 1.53 0.75 0.67 0.64 0.04 0.05 0.05 0.05 0.37 0.14 5.50 0.43 0.40 0.45 0.26 0.27 1.40 0.99 0.62 0.85 0.28 0.03 0.03 0.52 0.57 4.05 0.97 0.90 0.85 0.40 0.35 3.05 3.78 0.15 0.15 0.04 0.15 0.26 0.10 2.80 2.20 0.86 0.74 0.91 0.12 0.15 0.15 0.04 0.19 0.14 0.04 0.58 0.95 1.07 1.10 0.45 48.10 0.57 0.46 0.37 Note. The following designations are used: VDC is vinylidene chloride, AA is acrylic acid, VPD is vinylpyrrolidone, TBP is tert-butyl peroxide; TBPT is tert-butylperoxytriethyltin; TBPG is tert-butylperoxytriethylgermane; TBPS is tert-butylperoxytriphenylantimony; TBPS is tert-butylperoxytrimethyl- silane.The cyclic mechanism of propagation prevents the destructive chain transfer to the allylic monomer; this accounts for the high degree of conversion in the copolymerisation of AC with acrylates. The presence of fragments with double bonds in the MMA±AC copolymer (*4%± 5%) can be explained by assum- ing that allyl-type radicals formed upon abstraction of a hydrogen atom from AC can again initiate polymerisation in the presence of OEC by the following scheme: PnH +CH2 CH CH2Cl Pn +CH2 CH CH CH2 CH2 CH Cl +BR3 CHCl CH CH2 CH Cl BR3 CMe CH2 CH CH2 + C CH Cl R3B OMe O ClCH CH(CH2)2CMe C BR3 O MeO Monomers Initiator 1MMA MMA MMA VA VA VA VPD St St Ethylene Ethylene MMA Propene Hex-1-ene Hex-1-ene AIBN TIBB ± TBP TIBB ± TBPT TIBB ±TBPG TIBB ± TBPA TIBB ±DPA AIBN TIBB ±DPA PB TIBB ± oxygen TIBB ± TPB TIBB ± TBPT TIBB ±TBPG TIBB ±DPA AIBN TIBB ± oxygen TIBB ±DPA TIBB ± TBPA AIBN TIBB ±DPA AIBN TIBB ±DPA AIBN TBPS TBPT TIBB ±DPA BP AIBN ± TIBB AIBN ±DPA AIBN ± TBPS TIBB ±DPA AIBN TIBB ±DPA DCHPD TIBB ±DPA Coordination of OEC to an allyl type radical increases the radical electrophilicity and reactivity with respect to the p-bond of the monomer.The coordinated allylic radical, unlike a non- coordinated one, is able to re-initiate polymerisation [Eqn (16b)].The possibility of initiating polymerisation by an allylic macroradical in the presence of Lewis acids (metal halides) was also mentioned in other publications. 5, 81, 82 CH CHCl The NMR spectra of MMA±AC copolymers and syndiotac- tic PMMA coincide,80 which confirms the radical mechanism of the copolymerisation of MMA with AC. (16a) BR3 A thermomechanical study of MMA±AC copolymers showed 80 a sharp decrease in the glass transition temperature and a decrease in the ultimate strength and in the modulus of elasticity of copolymers with respect to the corresponding char- acteristics of the MMA homopolymer prepared both with the DPA± TIBB initiating system 83 and with a classical radical initiator.84 Hence, the presence of AC (10 mol %± 15 mol %) in the copolymer has a substantial influence on its properties.(16b) The decrease in the glass transition temperature of the MMA±AC copolymer noted above extends the temperature range of the highly elastic state of the copolymer and thus increases its frost resistance, which is significant for practical purposes. r2 r1 2VDC VC AA VDC VC VPD St AA MAA VA VC St 0.50 0.06 0.02 0.02 0.03 3.60 0.20 1.68 0.97 3.02 0.19 0.15 0.28 5.20 0.96 0.94 0.25 0.04 0.70 0.03 1.30 1.20 1.10 3.60 0.50 2.0 0.33 0.60 2.50 1.04 15.0 6.7 1.03 0.03 0.12 0.23 0.58 0.20 0.32 0.17 0.30 0.05 0.05 0.29 0.15 0.87 0.15 0.27 1.00 0.80 0.95 0.24 0.22 0.20 0.32 0.50 VC VC AN 0.39 0.65 0.88 0.94 12.2 4.60 0.28 0.05 0.06 0.13 0.0 0.10 431 Initiator AIBN TIBB ±DPA AIBN TIBB ± TBPA TIBB ± TBPS AIBN TIBB ±DPA AIBN TIBB ±DPA AIBN TIBB ± TBP TIBB ±TBPG TIBB ±DPA DCHPD TIBB ±DPA TIBB ±TBPG BP TIBB ± TBPA BP TIBB ± TBPA AIBN TIBB ±DPA TIBB ± TBPS AIBN TIBB ±DPA Ziegler ± Natta DCHPD±DPA TIBB ± oxygen TIBB ±DPA TIBB ±DPA TIBB ± TBPA TIBB ± TBPS AIBN TIBB ±DPA432 Copolymerisation of AC with the electron-donating VA monomer in the presence of organoelement initiators proceeds to a lower degree of conversion (less than 10% over a period of 7 days) 85, 86 than copolymerisation of AC with acrylates.This copolymer was also found to contain fragments with double bonds, which can serve as evidence for initiation of the polymer- isation by allyl-type radicals in the presence of OEC [see Eqns (16a,b)]. In the copolymerisation of AN and AC, the formation of a cyclic reaction complex is impossible for steric reasons;87 there- fore, the influence of an organoelement initiator on the depend- ences of the copolymer composition and copolymerisation rate on the composition of the monomer mixture is less pronounced.80 In all sections of the composition curve, the copolymer is enriched in AN, which is more reactive (r1>1, r2<1), although the relative activities of the monomers found in AIBN- and OEP-initiated reactions differ somewhat from each other.Conversely, copolymerisation of MMA with an electron- donating monomer (St) is subject to regularities 27, 88 similar to those observed in the copolymerisation of the MMA±AC pair, namely, the variation of copolymerisation rate as a function of the initial composition of the monomer mixture passes through an extremum (this may also imply the formation of a reaction complex in the chain propagation stage) and the relative activities of both monomers are much smaller than unity. According to NMR study of the MMA± St copolymers synthesised using DCHPD and organoelement initiators, the microstructures of the specimens obtained are markedly differ- ent.88 The copolymer synthesised in the presence of DCHPD shows a good agreement of the calculated content of triads with that found experimentally.However, in the case of the copolymer prepared in the presence of an organoelement initiator, the discrepancy between the experimental and theoretical values is substantial, indicating that the Mayo ± Lewis scheme is inapplic- able under these conditions. The ratio of triads calculated from the relative activities in accordance with the pre-terminal unit model showed that the Mertz ± Alfrey ± Goldfinger equation describes the polymerisation process much more adequately. It can be seen from the data presented that the effect of the pre-terminal unit is not the only factor able to change the copolymer composition.This indicates once again that complex formation between the monomer and the initiator and especially, between the initiator and the macroradical contributes significantly to the change in the monomer relative activities. Analysis of the dependences of the instantaneous and gross compositions of MMA± St copolymers on the degree of conver- sion shows that the product of copolymerisation in the presence of DCHPD is enriched in MMA at high degrees of conversion. Initiation by the TIBB ±OEP system gives rise to a copolymer with a narrower composition distribution of fractions.88 Hirai et al.,89, 90 who studied the controlling influence of boron compounds on the composition of the MMA± St copolymer, demonstrated that photoinitiation in the presence of 1 mol % of boron trichloride or ethylboron dichloride gives rise to an alter- nating St ±MMA copolymer.On the basis of kinetic measure- ments and study of the microtacticity of copolymers, it was concluded that copolymerisation involves the ternary complex `MMA± St ± organoboron compound'. It was noted that diethyl- boron chloride and triethylborane have a weaker effect on the composition of theMMA± St copolymer than trialkylboranes. In the presence of boron trifluoride, homopolymerisation of styrene takes place. It has been proposed 91 to use a mixture of alkylboron halides for the synthesis of alternating copolymers of acrylates with alkenes over a broad temperature range (7150 to 100 8C) and at a relatively low pressure (<100 atm).The degree of conversion reaches 70% over 5 ± 6 h. However, the controlling effect of the organoelement additive shows itself only when the amount of the additive is comparable with the monomer concentration. In this case, the influence of OEC on the copolymer composition is likely D F Grishin, L L Semyonycheva to be due to the coordination interaction of the OEC with the monomer rather than with the macroradical. Blends based on organoelement derivatives of Group II elements in combination with Group IV element halides influence the composition of copolymers in a similar way.92 An efficient controlling influence of the binary initiator trialkylborane ±OEP can be traced in the copolymerisation of AN with VA.14, 93, 94 In the case of the classical radical initiator AIBN, the AN±VA copolymer is enriched in AN, which is the more reactive monomer (rAN=4.05, rVA=0.06) at any compo- sition of the monomer mixture.When the organoelement initiator TIBB ±OEP is employed, the composition curve is S-shaped. A section in which the copolymer composition barely depends on the composition of the monomer mixture can be clearly distinguished on the curve; this suggests the trend for alternation of the monomer units in the copolymer and points to the coordinated radical mechanism of copolymerisation. The same is indicated by equalisation of the relative activities of the monomers: rAN=0.35; rVA=0.42 (for comparison, r1 and r2 found in terms of the Q7e scheme are 4.27 and 0.06, respectively) and by the deviation from linear dependence of the calculated parameters in the Kelen ± Tudoc determination of the copolymerisation constants.In our opinion, the very pronounced influence of low concen- trations of the initiator (0.1 mol %± 1.0 mol %) is due to the coordination interaction of the growing macroradical with the components of the initiating system. Important roles should be played by both trialkylborane, which has a vacant orbital at the boron atom, and the organoelement peroxide. This interaction results in electron density redistribution and contributes to the stabilisation of the species CH2 CH CH2 C C N BR3 C N BR3 H The decrease in the relative activity of AN, found as the ratio of the rate constant for the reaction of the radical with the like monomer (K11) to the rate constant for its reaction with the foreign monomer (K12) rAN=K11, K12 in the presence of OEC can be related to either a decrease in K11 , or an increase K12 , or a simultaneous change of both rate constants in the above-indicated directions.These changes can be brought about by an increase in the electron-withdrawing properties of the polyacrylonitrile radical coordinated to the OEC. It is evident that an increase in the electrophilicity decreases the reactivity of the macroradical towards the p-bond of AN (the monomer with a strong electron-withdrawing group) and increases the reactivity towards VA (the electron-donating mono- mer). CH2 K11 ... CH CH2 CH2 + K12 C N ...CH C N CH O C Me O 7d +d CH CH2 K11 ... +d 7d CH CH2 + 7d C N +CHd CH2 EOC C N K12 ... O C Me O K11<K11 , K12>K12 . These changes result in a substantial decrease in the relative activity of acrylonitrile (rAN<rAN).Problems of control of the reactivity of macroradicals and the growth of polymer chains The rate constant for the addition to VA of the poly(vinyl acetate) radical coordinated to the OEC increases, while that for the addition to AN decreases. As a consequence, the relative activities of the monomers which undergo copolymerisation (AN and VA) become nearly equal and smaller than unity (see Table 2), which generates the trend for alternation of the mono- mer units. The relative activity of AN changes more sharply than the copolymerisation constant of VA; this indicates that complex formation of OEC with the polyacrylonitrile radical is the governing factor in this system.The dependence of the rate of copolymerisation of AN with VA in the presence of organoelement initiator on the composition of the monomer mixture has a clear-cut maximum, as in the case of the MMA±AC monomer pair.14 This points to a coordinated radical character of the process. Similar features have been found for the copolymerisation of AN with MAA, acrylic acid and MMA.7 As for the AN±VA pair of monomers, the composition curve has a section in which the copolymer composition virtually does not depend on the composition of the monomer mixture.It can be seen from the data of Table 2 that the relative activities of the monomers in the presence of organelement initiators differ markedly from the values calculated using the Q7e scheme or published data. The observed levelling of the relative activities of monomers entails the compositional uniformity of the resulting polymer. The influence of the components of organelement initiators on the dependence of the copolymer composition on the composition of the monomer mixture in the case of MMA±MAA monomer pair is equally significant.95 When copolymerisation of these monomers is carried out without OEC additives (AIBN initiation, see Table 2), the copolymer is enriched in the more active mono- mer, MAA, at any composition of the monomer mixture.In the case of a binary organoelement initiator, TIBB ± DPA, the com- position curve follows an S-shaped pattern, and the relative activities of both monomers are substantially smaller than unity. The product of the copolymerisation constants tends to zero, pointing to the alternation of monomer units in the copolymer. Studies of the separate influence of components of the binary organoelement initiator on the chain propagation 27 demon- strated that the addition of 0.2 mol%of TIBB (the concentration of the AIBN initiator is 0.1 mol %) results in substantial levelling of the relative activities of the monomers (see Table 2). The addition of an organosilicon peroxide to AIBN also influences appreciably the reactivities of the monomers and the chain- propagating radicals � the copolymer is enriched in MMA in each section of the curve.The effective values of the relative activities of MMA and MAA are also subject to the controlling influence of DPA, which is an efficient co-initiator for trialkylbor- anes.13 ± 15 Thus, both components of the organoelement initiator (alkylborane and OEP) have a controlling influence on the copolymerisation of acrylic monomers. Copolymerisation ofMMAwith an electron-donating mono- mer (St) is influenced by components of an organoelement initiator in a different way.88 For the polymerisation of these monomers initiated by organoelement initiators, the composition curve is also S-shaped. The relative activities of both monomers are smaller than unity, their numerical values being slightly dependent on the initiator.However, for theMMA± St monomer pair with the use of organoelement initiators, the dependence of the copolymerisation rate on the composition of the monomer mixture has a clear-cut maximum at a nearly equimolar monomer ratio, which is indicative of a coordinated radical copolymerisa- tion mechanism and of the participation of the initiator compo- nents in the propagation stage. The different effects of OEC as weak Lewis acids on the copolymerisation of MMA with MAA and with St are due to the fact that in the former case, the two monomers (MMA and MAA) are similar in structure and electron-withdrawing properties and thus both can react with the complex-forming agent, whereas in 433 the latter case(MMAand St), only one monomer, namely,MMA, reacts with the Lewis acid. These conclusions are in good agreement with the results of kinetic studies of the copolymerisation of AN with VA and with (meth)acrylates.7, 93 ± 95 The effect of the heteroatom of the organoelement peroxide on the copolymer composition and the copolymerisation rate was also found for the AN± St pair of monomers at 343 ± 353 K,96 although it is less pronounced than in the case of a binary organoelement inittor.A quantitative estimate of the change in the reactivity (includ- ing electrophilicity) of macroradicals and monomers induced by coordination to the components of an organoelement initiator is provided by the Q7e parameters of the Alfrey ± Price equa- tion.97, 98 These parameters are known to have a clear physical meaning and to correlate with the energy of localisation, the order and the p-electron density of the monomer double bond.Com- parative analysis of the published Q and e values for various vinyl monomers and the values of these parameters calculated using the relative activities of monomers for the case of organoelement initiators shows that the introduction of organoelement additives changes these values appreciably. Indeed, judging by the Q and e parameters, in the presence of the binary TIBB ±OEP initiator, N-vinylpyrrolidone acts as an electron density acceptor (e=0.94) rather than an electron density donor (e=71.0), as usual.97 Similar regularities have been observed for other vinyl mono- mers (Table 3): the parameter e, which characterises the electronic structure of the monomer, substantially increases.Table 3. The parameters Q and e for several vinyl monomers. Q e Q e Monomer Monomer MA St VC AN VPD VDC AA MAA VA 0.40 0.45 0.20 0.97 0.36 1.07 0.94 0.34 d 1.50 e 70.22 70.06 0.74 0.76 0.04 0.94 0.22 0.29 0.33 0.50 d 0.50 e 0.03 0.85 0.8 0.8 1.20 1.33 71.00 1.89 a 0.77 1.03 0.68 1.39 0.40 b 0.53 c 1.0 1.0 0.60 0.90 0.20 1.31 a 1.15 0.34 1.50 0.86 0.74 b 1.15 c Note. The values obtained in the absence of initiators are given above the stroke; those in the presence of the organoelement initiator TIBB ±DPA are below the stroke.Additives: a tin tetrachloride, b dimethylformamide, c Lewis base, d TBPG± TIBB, e acetic acid. An increase in the numerical value of e in (meth)acrylates attests to an increase in the energy of conjugation and a decrease in the electrophilicity of the double bond. This means an increase in the reactivity of the monomer p-bond towards elctrophilic radi- cals.It has been found within the empirical Alfrey ± Price scheme that an increase in the parameter e, which is related to the polarity of the monomer double bond, entails an increase in the rate constant for propagation and, hence, in the rate of homo- and copolymerisation.97, 98 As noted above, this is actually observed in the study of the kinetics of homo- and copolymerisation of (meth)acrylates in the presence of catalytic amounts of organo- element compounds.The efficient controlling influence of organoelement initiators based on trialkylboranes and Group IV ±V element peroxides is also manifested in those cases where both monomers involved in copolymerisation are electron donors but contain functional434 groups able to be coordinated to Lewis acids. These monomers can be represented, for example, by VPD and VA.99 The results of studies of the kinetic dependences for the composition and structure of copolymers point unambiguously to a coordinated radical mechanism of copolymerisation. The monomers form a reaction complex with the components of the initiator during the chain propagation stage: the dependence of the copolymerisation rate on the composition of the monomer mixture has an extremum in the region of equimolar amounts of monomers in the initial mixture.An organoelement initiator has a substantial influence on the copolymerisation of VPD with VDC, which are `naturally alter- nating' comonomers.99, 100 The composition curve is S-shaped when DCHPD is used as an initiator. When copolymerisation of this pair is carried out with a binary initiator, TIBB ± OEP, the resulting copolymer is enriched in VDC. Organosilicon peroxides have a controlling influence on the copolymerisation of acrylates in the presence of sulfuryl chloride and phosphorus trichloride;101, 102 however, for the monomer pairs considered in the studies cited, this influence is less pro- nounced than that in the case of Group III element OEC; this is undoubtedly due to the difference between the electron-with- drawing properties of the modifiers. Thus, catalytic amounts of coordinatively unsaturated OEC not only effectively initiate low-temperature polymerisation of vinyl monomers but also have a controlling effect on the kinetic parameters of copolymerisation of a broad range of monomers and the composition of copolymers.The compositional uniformities of macromolecules for one± and two-centre copolymerisation are substantially different.103 Therefore, important information on the chain propagation mechanism can be gained by performing copolymerisation of a broad range of monomers.It was found in relation to copolymerisation of MMA with MAA that the integral curves for the distribution over composi- tion forMMA±MAAcopolymers synthesised in the presence of a conventional organic (DCHPD)104 and organoelement (TIBB ± DPA)105 initiators are different. The curve for DCHPD is unim- odal,104 while that obtained with organoelement initiators is bimodal.105 When acrylates are made to copolymerise with alkenes, the controlling effect of OEC is not so pronounced as in the copoly- merisation of vinyl monomers with each other. The rates of copolymerisation of AN with cyclohexene and hex-1-ene, MMA with ethylene, and VA with ethylene 106 decrease linearly with an increase in the alkene content in the monomer mixture. The degree of conversion does not exceed 30%.The relative activities still differ somewhat from those found for classical radical initiators but this difference is less significant than in the case of copoly- merisation of two vinyl monomers.7 Apparently, this is due to the fact that alkenes and, especially, chain-propagating radicals derived from them, are less prone to form complexes than monomers containing functional groups or atoms with lone electron pairs. In should be noted that low-pressure radical copolymerisation of ethylene and a-olefins with polar vinyl monomers is quite a topical problem in polymer chemistry. This is especially true for the synthesis of VC copolymers with a-olefins. Vinyl chloride and a-olefins are known to undergo efficient polymerisation at low pressure according to different mecha- nisms, namely, polymerisation of VC follows a radical mecha- nism, while ethylene and its homologues polymerise by a coordination mechanism in the presence of Ziegler ± Natta cata- lysts.107, 108 It is impossible to synthesise VC± olefin copolymers with a high degree of conversion under technologically beneficial conditions in the presence of conventional initiators.The highest degree of conversion on the Ziegler ± Natta catalysts does not exceed 24% (in the presence of electron-donor additives, 30%),109 that in the presence of AIBN is 17%.110 D F Grishin, L L Semyonycheva It should be noted that physicochemical properties of VC copolymers with ethylene or a-olefins, e.g., good processability, great elongation at rupture, enhanced specific impact strength, etc., make them promising as plastics.In addition, the introduc- tion of even 5%± 10% of ethylene units in poly(vinyl chloride) markedly decreases the cost of manufacture of the polymer. The absence of efficient initiators holds up large-scale production of these copolymers. As shown above, the coordination-radical initiators based on OEC with Group III elements and OEP with Group IV elements, due to the specific features of their structure, occupy an inter- mediate position between the traditional radical initiators and Ziegler ± Natta catalysts. With these initiators, copolymerisation of VC with ethylene and with a-olefins can be carried out at high rates and high degrees of conversion with minimum energy expenditure.106, 111, 112 The degree of conversion in the homopolymerisation of alkenes initiated by these compounds does not exceed 3%± 5%, which may be due to the specific features of radical formation in the TIBB ±OEP systems.57, 113 However, copolymerisation of ethylene and other alkenes with vinyl monomers can be performed in the presence of the TIBB ±OEP systems over a broad range of compositions and with high yields.Using the vinyl acetate ± ethylene and AN± hex-1-ene mono- mer pairs as examples, the activation energies for copolymerisa- tion on the TIBB ± tert-butylperoxytetraphenylantimony binary initiator were estimated to be 25.8 and 36.4 kJ mol71, respec- tively.29, 106 These values are much lower than the activation energies found for the use of diazo compounds and acyl peroxides as initiators and are close to the data for homopolymerisation of vinyl monomers with organoelement initiators.The rate of copolymerisation of alkenes with vinyl monomers and the degree of conversion depend substantially on the compo- sition of the monomer mixture and the number of carbon atoms in the alkene molecule. Thus in the case of the VC± ethylene pair, the yield of the copolymer reaches 90% over a period of 5 ± 6 h.112 As the alkyl chain length in the alkene increases, the rate of polymerisation and the degree of conversion decrease; in the case of oct-1-ene, the yield of the copolymer does not exceed 20%.Additional introduction of an initiator results in only a slight increase in the yield. The reason for the decrease in the degree of conversion on passing from ethylene to a-olefins is probably related to the chain termination involving the active hydrogen atoms in the a-position relative to the alkene double bond. In the case where traditional radical initiators or Ziegler ± Natta catalysts are used in the VC copolymerisation with alkenes, the possibility of varying the copolymer composition is quite limited. The course of the composition curves and the relative activities of the monomers indicate that the copolymer is enriched in one monomer at any composition of the initial mixture. This is due to the considerable difference between the reactivities of VC and alkenes.The use of the trialkylborane ±OEP systems results in substantial equalisation of the copolymerisation constants. Thus the composition curve for VC copolymerisation with ethylene or other alkenes follows an S-shaped pattern; for alkene contents in the monomer mixture ranging from 30 mol.% to 70 mol.%, the copolymer composition is virtually constant and does not depend on the composition of the mixture.112 The introduction of alkene units in vinyl polymers influences the physicochemical properties of the product in a definite manner. In particular, in the case of the ethylene ±VA pair of monomers, the intrinsic viscosity of the copolymer increases with an increase in the alkene fraction.Conversely, the intrinsic viscosity (and MM) of the ethylene ±VC copolymer decreases with an increase in the ethylene mole fraction. We calculated the Fickentcher constant (KF), characterising the MM of poly(vinyl chloride) for a number of VC± alkene copolymer specimens.114 The constant proved to be much lower (KF=40 ± 70) than in the case of poly(vinyl chloride) synthesised with these initiators.22 A decrease in the Fickentcher constant is known to facilitateProblems of control of the reactivity of macroradicals and the growth of polymer chains processing and to enhance the plasticity of the polymeric prod- uct.14 It is worthy of note that an increase in the concentration of the initiator for an invariant ratio of the monomers in the mixture decreases the viscosity of the copolymers and slightly (by 5%± 6%) increases the degree of conversion.106 Thus, the use of coordinatively unsaturated OEC able to change the reactivity of macroradicals in a desired manner is an efficient technique for controlling the growth of the polymer chain under conditions of radical polymerisation.One more way of controlling the lifetime of a polymer chain aimed at synthesising compositionally uniform polymers is the use of stable radicals. III. Growth of the polymer chain in the presence of stable radicals and iniferters In recent years, a new concept of controlled processes called `pseudo-living'-chain { (or 'living'-chain) radical polymerisation has been vigorously developing. This polymerisation is of consid- erable interest as a simple and reliable method for the synthesis of polymers with a definite structure and specified properties.The essence of the `pseudo-living' polymerisation is as fol- lows.76, 117 ± 120 The polymerisate contains catalytic amounts of active additives of a specific type (for example, stable radicals or their sources, complex ions of variable-valence metals, and other chain propagation controllers), able to undergo a reversible reaction with chain-propagating active radicals (R.) to give a labile adduct [R±X], which decomposes under certain conditions to recover the same chain-propagating macroradical. (17) RX. R.+.X An important feature of `pseudo-living' radical processes is alternation of the periods of chain propagation, termination and re-initiation.An obvious advantage of this approach is that the polymer chain grows step-by-step, i.e., it becomes possible to control the lifetime of the growing radical. This brings about the following specific features of both the polymerisation kinetics and the properties of the resulting polymer: � the number of polymer chains and active centres remains constant at any degree of conversion; � starting from a low degree of conversion, the polymerMM continuously increases, the number-average MM value (Mn) increasing linearly with the degree of conversion; � a narrow polydispersity (Mw/Mn , whereMw is the weight- average MM) of the polymer is observed at any degree of conversion; � the isolated products of polymerisation can also act as macroinitiators; the addition of a new portion of the monomer results in further polymerisation and an increase in the polymer MM; � when two or more monomers are added successively, block copolymers are formed.Although the `pseudo-living' radical polymerisation is a relatively new line in the research dealing with controlled radical processes, quite a few organic and organoelement compounds which permit radical polymerisation to be carried out in the `living' regime have been proposed to date. Currently, more than a hundred publications devoted to this topic appear annually, and this flow does not decrease.} { The process has been called `pseudo-living' radical polymerisation because it has some features peculiar to the ionic `living' polymerisation in which no chain termination takes place and chain propagation is continued until the monomer is exhausted.115, 116 } Suffice it to say that in the section `Radical Polymerisation' of the World Congress `IUPAC MACRO-2000,' the problems of `pseudo-living' poly- merisation were the most vigorously discussed topic: of 36 oral reports and 51 posters, 17 reports and 20 posters were related to `pseudo-living' polymerisation; and 18 posters were presented at the separate poster session on `pseudo-living' polymerisation.121 435 Depending on the active additives (chain propagation con- trollers) used, the processes carried out in the `living'-chain regime can be classified into the following types: � polymerisation involving stable carbon-centred radicals and nitroxides, first of all, 2,2,4,4-tetramethyl-1-piperidinoxyl (TEMPO) and its analogues; � polymerisation in the presence of spin traps and other potential sources of stable radicals; � polymerisation using iniferters, controlling initiators of a new type; � polymerisation in the presence of OEC, mainly, transition metal halides (atom transfer radical polymerisation�ATRP).1. `Pseudo-living'-chain polymerisation with participation of carbon-centred radicals and nitroxides Examples of using stable radicals for controlling the kinetic parameters of radical polymerisation are well known and have been described fairly comprehensively not only in original pub- lications but in relevant monographs.76 Stable radicals, for example, galvinoxyl, phenoxyl and nitroxide spin adducts are active acceptors of free radicals, including polymeric radicals, over a broad temperature range.In recent years, in relation to the develible inhibition concept,122 ± 124 the interest in these compounds, especially nitroxides, has substan- tially increased as they can be used to control the chain propaga- tion.An important feature of some nitroxides is reversible inter- action with a number of growing polymeric radicals at temper- atures above 100 8C. This opens up some prospects for controlling the growth and, mainly, the lifetime of the polymeric chain. Over a fairly short period, the studies on the influence of nitr- oxides 76, 118 ± 120, 125 ± 136 and alkoxyamines 76, 118 ± 120, 137 ± 139 have occupied a prominent place in the research on `pseudo-living' polymerisation.The compounds used most often to control the lifetime of the growing chain include TEMPO125 ± 133 and its substituted analogues such as 4-methoxy-2,2,6,6-tetramethyl-1-piperidin- oxyl,137 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinoxyl,135 etc. These nitroxides are used both as such (in the case of thermally initiated polymerisation) and in combination with traditional radical initiators (BP, AIBN, etc.).125 ± 136, 140 `Pseudo-living' polymerisation in the presence of nitroxides based on TEMPO is normally carried out at temperatures of 95 ± 130 8C; it is most typical of St and its derivatives (4-bromostyrene,137 3- and 4-chloromethylstyrenes,140 etc.), as well as some acrylic mono- mers.127, 135 However, it should be noted that in the case of acrylates, nitroxide spin adducts are much less efficient.In all probability, this is due to the fact that the bond (C7O7N) formed according to reaction (17) between the growing macro- radical and the nitroxide is stronger, and hence, less labile in the case of polyacrylate radicals than in the case of styrene. Thus, dissociation of this bond requires more energy and, correspond- ingly, a higher temperature. However, raising the temperature of the synthesis can result in irreversible transformation of the terminal fragment into hydroxylamine.120, 141 This may account for the fact that in the case of acrylates and other vinyl monomers, `pseudo-living' mechanism could not be induced by increasing polymerisation temperature to 150 8C.130, 135, 142, 143 Nevertheless, poly(butyl acrylate) with Mn=10 500 ± 27 000 and Mw/Mn= 1.53 has been prepared in the presence of 2,2,6,6-tetramethyl-4- hydroxy-1-piperidinoxyl at 150 8C.135 Copolymerisation of St with BA in the presence of TEMPO using an azeotropic monomer mixture (St :BA=7 : 3) at 120 8C also proceeds by a `pseudo- living' mechanism.144 Kuchanov 124 has studied the effect of nitroxides on block polymerisation of St carried out at 120 8C, with different initial concentrations of TEMPO and without initiators.124 In the presence of an initiator, polymerisation follows a `pseudo-living' mechanism,129, 144, 145 as indicated by the absence of a gel effect and low polydispersity coefficients of the resulting polymer as well436 as by the increase in MM following an increase in the degree of conversion.`Pseudo-living' copolymerisation of St with BA was carried out under similar conditions.144 In the presence of TEMPO, block copolymers of St with MMA and with butyl methacrylate have been synthesised.128, 146 It was shown that PS macromolecules are capable of re-initiating polymerisation. Yoshida and Sugita 147 combined cationic and radical polymerisation to prepare a biterminal polymer, poly- (styrene-b-tetrahydrofuran), with two terminal TEMPO radicals, which subsequently undergoes copolymerisation with St to give triblock copolymer.A detailed and thoroughly justified description of the kinetic features of the `living'-chain polymerisation in the presence of nitroxides has been reported.120 In addition to nitroxides, stable radicals of other classes have also found use in `pseudo-living' radical polymerisation processes. In particular, the influence of the triphenylmethyl (trityl) radical, formed upon homolytic decomposition of hexaphenylethane (HPE) (18) 2Ph3C., Ph3C7CPh3 on the polymerisation of acrylic monomers has been studied. It was shown 148, 149 in relation to the AIBN-initiated polymerisation of MMA at 80 8C that in the presence of excess HPE ([HPE] : [AIBN]>5), the process switches to the `pseudo-living' regime, which is characterised by successive increase in the polymer MM and suppression of the gel effect at high degrees of conversion. However, unlike nitroxides (TEMPO and its ana- logues), which do not react with the monomer double bond, the triphenylmethyl radical can itself initiate polymerisation.148, 150 Therefore, radical polymerisation of MMA in the presence of HPE has a number of specific features.It was shown 148, 150 that polymerisation of MMA at 80 8C with the addition of HPE and in the absence of AIBN proceeds very slowly (polymerisation time up to a high degree of conversion at [HPE]=0.1 mol % is *50 h; for the same AIBN concentra- tion, it is 2 ± 3 h) and exhibits features of a `pseudo-living' process. In particular, gel effect is scarcely observed, the MM of the polymer linearly increases with an increase in the degree of conversion, and the polydispersity decreases with an increase in the degree of conversion of the monomer.The researchers cited 150 convincingly demonstrated that in this case, polymerisation of MMA follows a two-stage `pseudo-living' mechanism. The first stage is due to fast consumption of the trityl radicals in chain initiation and termination. At this stage, a reactive PMMA oligomer containing trityl groups at both ends of the chain is formed and rapidly accumulated (19) Ph3C.+MMA Ph3C7MMA., (20) Ph3C7MMA7CPh3 . Ph3C7MMA.+.CPh3 The terminal bond in this oligomer remains labile. The PMMA oligomer re-initiates polymerisation at the second stage of the process, giving rise to a polymeric product in the system.During polymerisation at the second stage, the average MM continuously increases due to the formation of the polymer with a higher molecular mass and gradual consumption of the oligomer. By analysis of the molecular-mass characteristics of the synthesised PMMA, interesting features of this process were identified.148, 150 The products formed at the first stage show a bimodal MMD. The position of the low-molecular-mass mode corresponding to oligomers virtually does not change during the process; however, its area gradually decreases. The high-molec- ular-mass mode shifts to higherMMwith an increase in the degree of conversion, and its area increases. At high degrees of conver- sion, the oligomer mode virtually disappears, and the chromato- gram becomes unimodal.Similar changes in theMMDof the final product � PMMA � were also observed when MMA polymer- D F Grishin, L L Semyonycheva isation was carried out in the presence of specially synthesised and isolated oligomer. Taking into account the experimental data considered above on the polymerisation of MMA in the presence of HPE, the following mechanism of the AIBN-initiated formation of PMMA was proposed.148, 150 Hexaphenylethane dissociates at 80 8C to afford trityl radicals, which initiate polymerisation; the rate constant for this reaction is 161073 litre mol71 s71 (the rate of initiation at [HPE]=5.061073 mol litre71 is 561075 mol litre71 s71).In parallel with this process, initiation at a rate of 561078 mol litre71 s71 due to the thermal decomposition of AIBN takes place. It is clear that initiation caused by decom- position of AIBN at the beginning of the process can be neglected. The resulting oligomers re-initiate polymerisation; the parallel polymerisation due to initiation by AIBN also continues. The ratio of the rates of these processes is determined by the ratio of the initial concentrations of HPE and AIBN (the rate constants of these reactions are close 1). If excess HPE is used (for example, [HPE] : [AIBN]>50), the mechanism of this process is close to the `pseudo-living' polymerisation mechanism. In the opinion of the researchers cited,148 ± 150 HPE can be regarded as an iniferter because in the absence of a reactive competing agent, initiator (in this case, AIBN), the relely inert trityl radical has time to attack the monomer and participates in the step of initiation of polymerisation; however, the rate of polymerisation with HPE is much lower than the rate of the AIBN-initiated process.ethane 117, 151 ± 154 and The diphenylmethyl and (trimethylsilyloxy)diphenylmethyl radicals, formed upon decomposition of tetraphenyl- bis(trimethylsilyloxy)tetraphenyl- ethane,117, 155 respectively, behave similarly to the trityl radical in the `pseudo-living' thermal polymerisation. In the first stage consisting of initiation and termination of the growing chains, the radicals formed upon thermal decomposition are rapidly consumed, and the reactive oligomeric product is accumulated. This product re-initiates polymerisation in the second stage of the process, which gives rise to the polymeric product.The polymers formed in the first stage are characterised by a bimodal MMD. The position of the low-molecular-mass mode, corresponding to oligomers, barely changes during the process; however, its area gradually decreases. Study of copolymerisation of vinyl monomers in the presence of stable carbon-centred radicals is of obvious interest. The first data on the mechanism of copolymerisation involving carbon- centred radicals were obtained back in 1985.156, 157 The use of O,O0-diisopropyl xanthogen sulfide (DXS) as a controlling additive made it possible to perform `pseudo-living' copolymerisation of St with MA at 120 ± 140 8C.158, 159 A specific feature of this process is a substantial decrease in the copolymer- isation rate.Apparently, this is due to the fact that the rate of re- initiation in this system is lower than the rate of initiation;159 this is obviously due to the difference between the energies of the S7S bond in the dissociating DXS and the C7S bond in the terminal group of the copolymer (21) PriO C S S C OPri 2 .S C OPri S S S S C OPri CH2CH CH2CH.+ .S C OPri (22) S R S R It can be seen from Eqn (22) that under these conditions, the stable S-centred radical formed acts as both an initiator and a chain propagation controller, similarly to carbon-centred radicals such as the trityl 148 ± 150 or diphenylmethyl radical.151 ± 155 The copolymers considered are gradient polymers the properties of which differ from the properties of random polymers.159Problems of control of the reactivity of macroradicals and the growth of polymer chains 2.Controlled radical polymerisation in the presence of spin traps and other sources of stable radicals radical (*Pn). In particular, the energy of the R17O bond in the R17O7Nbridge of theR17O7N(R3)R2 type nitroxide adducts formed according to Eqn (23) depends appreciably on the nature of radicals surrounding the N7O group, especially on their spatial structure. Thus in the case of TEMPO, a known controller of chain propagation, or the methyl radical, the bond energy is *30 kcal mol71.Apparently, this is why TEMPO and its analogues are able to control efficiently the radical polymerisation of St and some acrylic monomers only at relatively high temperatures (100 ± 130 8C). For a more bulky ethyl radical, the energy of this bond is *25 kcal mol71, and in the case of sterically hindered tert-butyl or polyacrylate radicals, it is even lower (*20 kcal mol71). When vinyl monomers are made to polymerise in the presence of PBN (or MNP), the nitrogen atom in the R17O7N(R3)R2 nitroxide adduct is linked to even more bulky and sterically hindered radicals. For example, in the case of MMA and MNP, it is linked to a tert-butyl radical (from MNP) and two poly- acrylate radicals, having a high-molecular-mass `tail'.MeCH2 As noted above, a substantial drawback of the control of chain propagation by stable radicals is that the use of these species allows one to perform `pseudo-living' radical polymerisation only at relatively high temperatures, above 100 8C. It is clear that development of effective methods for conducting polymerisation in the `living'-chain regime at lower temperatures, normal for radical polymerisation of vinyl monomers under industrial con- ditions (50 ± 80 8C), is a topical problem of synthetic polymer chemistry. In this respect, the use of peroxides and azo compounds (typical radical initiators) in combination with active additives able to generate stable radicals, chain propagation controllers, directly in the polymerisation system (in situ) appears to be promising for performing controlled polymerisation.Nitroso compounds, nitrones, and other acceptors of free radicals used as spin traps in the studies of homolytic processes in organic and polymer chemistry can be used for this purpose.160 For example, the use of C-phenyl-N-tert-butylnitrone (PBN) and 2-methyl-2- nitrosopropane (MNP) as effective controllers of the lifetime of a polymer chain for a number of vinyl monomers of various structures has been proposed.161 ± 169 These compounds exert an effective controlling influence on the polymerisation of various monomers in the temperature range of 50 ± 80 8C. On the basis of analysis of experimental data on the reactivity The strength of this bond is much lower than that in the case of TEMPO; naturally, it will dissociate rather easily at lower temper- atures to give the initial radicals.of nitrones and nitroso compounds, the following scheme of chain propagation in the presence of these additives was pro- posed.161 ± 163 At the first stage, MNP (or PBN) react with the growing macroradical (*Pn) to give a stable nitroxide. N ButA Pn Pn + But N O O (23) Ph C N Bun CH N Bun A Pn Moreover, since the nitroxide adduct B is formed initially upon the interaction of two bulky radicals directly during the synthesis of the polymer at a temperature of 50 ± 80 8C, it cannot be ruled out that the chemical bond between the chain-propagat- ing radical (*Pn) and the nitroxide spin adduct (A ) is a vibra- tionally excited molecule (an analogue of a radical pair in the [*Pn A] solid matrix) even at the instant of formation.Pn + H O O Ph The nitroxide spin adduct A further reacts with the chain- propagating radical*Pn to give a labile bond (24) *Pn+.A [*Pn A]. The reversible reaction (24) results in alternation of the To date, `pseudo-living' polymerisation in the presence of PBN has been accomplished for a number of vinyl monomers, namely, MMA,161 ± 164, 166 ± 168 butyl methacrylate, BA, St, and VC.165 ± 167 The main process parameters and the properties of polymers prepared in the presence of PBN are listed in Table 4. Some common features of the mechanism of the `pseudo-living' radical polymerisation in the presence of PBN can be distin- guished.`sleeping' and 'living' periods of the polymeric radical: the mono- mer (M) molecules add successively to the macroradical; thus, the polymer chain grows. (25) A]. A]+M [*Pn+1 [*Pn A] The lability (relative instability) of the resulting [*Pn+1 In particular, the introduction of a controlling additive results in an appreciable decrease, down to complete suppression, of the gel effect and allows conduction of the polymerisation to high degrees of conversion (80% ± 90%). It is very important for practical purposes that the time of polymerisation increases insignificantly (with respect to that for the process without an additive), as a rule, not more than twofold (see Table 4).The duration of polymerisation of St andMMAby the `pseudo-living' bond under conditions of polymerisation of vinyl monomers (50 ± 80 8C) is due to both high stability of the nitroxide spin adduct (A.) and specific features of the structure of the polymeric Table 4. Some kinetic parameters for the polymerisation of vinyl monomers in the presence of PBN and molecular-mass characteristics of polymers (initiator AIBN, 0.1 mol.%). Monomer T/8C Degree of monomer conversion (%) Time /h [PBN] (mol.%) 104 V /mol litre71 s71 MMA BA 8 1727 BMA VC 91 75 98 65 99 78 92 89 50 50 65 65 50 50 50 50 70.80 70.02 70.2 70.01 12 2435 1.2 0.8 2.1 0.1 a 1.1 0.8 a 9.1 0.6 a The rates are given for [PBN] = 0.1 mol.%.b The Mn value. c The Fickentcher constants are given. 437 Me But C O C C N O C CH2 OOMe OMe B Ref. Mw/Mn 106MZ 161, 162 169 169 165 >2.0 1.7 >2.0 1.7 772.5 1.7 4.3 1.2 70.7 b 10.7 6.5 70 c 66 c438 radical mechanism is known to increase severalfold in the presence of TEMPO,125 ± 136 triphenylmethyl radicals 148 ± 150 or other rad- icals. The PBN concentration at which the gel effect is suppressed depends on the structures of the monomer and, especially, the growing macroradical; in any case, this concentration is commen- surable with the content of the initiator (0.01 mol.% ± 0.80 mol.%).161, 162, 164, 169 The number-average and viscosity-average (MZ) molecular masses of the polymer prepared in the presence of PBN follow a linear dependence on the degree of conversion, which is typical of polymerisation by the `pseudo-living' mechanism.The shift of the low-molecular-mass shoulder and the whole mode to higher molecular masses with an increase in the degree of conversion of the monomer can be clearly seen in the MMD curves.161, 162, 169 An important feature of the proposed controller is the fact that stable radicals able to control chain propagation are formed directly during polymerisation as a result of interaction of the macroradicals with an additive introduced to the system, which effectively controls the synthesis of macromolecules even at 50 ± 80 8C.A shortcoming of this method for controlling the growth of a polymer chain is that the MMD of the resulting polymer is broader than, for example, in the case of ATRP.76, 119, 120 In our opinion, this is due to the inhomogeneity of high-molecular-mass nitroxides formed by reaction (23) as chain propagation control- lers.It should be noted that the structures of the macroradical and the monomer have a substantial influence on the polymerisation kinetics and the polymer MM under the conditions of reversible inhibition in the presence of PBN. Thus comparison of acrylic monomers, namely, MMA, butyl methacrylate and BA, shows that the process involving BA occurs at higher temperature (>65 8C) and at much lower concentrations of the active additive and has a longer induction period than the processes with MMA or butyl methacrylate.166 ± 169 The differences in the regularities of polymerisation of MMA and butyl methacrylate, on the one hand, and BA, on the other hand, are undoubtedly due to specific features of the structure of growing macroradicals and the reactivity of the macroradicals towards nitroxide spin adducts.Thus, from a thermodynamic standpoint, sterically hindered PMMA and poly(butyl methacry- late) macroradicals (tertiary radicals) form a weaker and, hence, a more labile bond with the nitroxide used as a chain propagation controller [reaction (23)]. In the case of polymerisation of BA, the bond between the nitroxide spin adduct and the growing macro- radical of poly(butyl acrylate) (secondary radical) is stronger and more stable against dissociation.As a consequence, in the case of polymerisation of BA, the `living'-chain mechanism can be accomplished at a higher temperature. In addition, the rate constant for the acceptance of secondary and tertiary radicals by a spin trap (PBN) are known to differ by almost an order of magnitude. In our opinion, this can account for the long induction period in BA polymerisation and for the possibility of using lower concentrations of nitrone as a controller of the chain propagation than those required in the polymerisation of MMA and butyl methacrylate.166 ± 169 The foregoing is confirmed by the data on the polymerisation of VC165, 166, 168 and St 166, 168 in the presence of PBN: the growing polymer radicals derived from these monomers are secondary radicals; hence, as in the case of BA, the controlled process takes place at a lower PBN concentration.It was shown in relation to the polymerisation of MMA that the use of MNP as a controlling additive entails effects similar to those observed with PBN; in the latter case, the control of the polymer chain propagation requires lower concentrations of the additive (0.005 mol.% ± 0.01 mol.%). Apparently, this is due to higher constants for the acceptance of macroradicals by nitroso compounds than by nitrones.163, 166 ± 169 D F Grishin, L L Semyonycheva 3. Iniferters as controlling initiators of polymerisation of a new type Yet another elegant method for performing controlled radical polymerisation proposed in recent years is based on the use of a special class of initiators called iniferters.} These compounds decompose rather readily to give free radicals (A.) and (B.) (26) A7B A.+B..One radical is reactive (A.), it can react with the monomer and thus initiate the chain propagation. The second radical (B.) is reactive only with respect to the growing macroradical and can react with it according to the reversible inhibition pattern (27) A.+M A7M., (28) A7M.+.B A7M7B. Chain termination via recombination of the growing macro- radical with the primary radical of the iniferter [reaction (28)] occurs much faster than chain termination via recombination or disproportionation of the growing radicals.In addition, iniferter participates in chain transfer reactions. In terms of their chemical nature, the iniferters known to date can be conventionally divided into three groups. In the iniferters of the first group, homolytic decomposition of both the initial and the resulting macromolecular initiator involves a labile bond of the same chemical nature. Examples of these iniferters are alkoxyamines 76, 118 ± 120, 137 ± 142 and alkylthio- carbamates.170 ± 174 In the case of second-group iniferters, the steps of decom- position of the initial and macromolecular initiators involve bonds of different chemical natures. This group of iniferters includes, for example, phenylazotriphenylmethane 117, 148, 175, 176 and some thio iniferters.117 Binary redox systems based on organic and organoelement compounds and the corresponding oxidants (sterically hindered quinones, oxytriazenes, etc.) can be classified as the third type of iniferters.177 ± 182 Alkoxyamines, which are used most often as iniferters, are compounds of the R7Mn7X type, where R is the radical formed upon decomposition of a conventional initiator of radical poly- merisation (BP, AIBN, DCHPD135, 183 ± 189), M is the monomer (most often, St), X is a nitroxide (as a rule, TEMPO or its analogue).It has been noted above that the parameters of polymerisation in the presence of TEMPO are close to the parameters of `living' ionic processes; this is due to the high re-initiation constant by reaction (17).In this connection, vigorous studies into the use of alkoxyamines as iniferters are being carried out. Numerous publications have been devoted to the synthesis of new alkoxyamines 189 ± 195 and their application in the polymer- isation processes. The results of these studies can be used to select the compounds R7Mn7X the half-life of which is shorter than the period of complete polymerisation transformation by factors of tens or even hundreds;120 this, in turn, can increase the efficiency of a nitroxide as a `pseudo-living' polymerisation agent. An important indication of a `living' process is polydisper- sity. If ordinary quadratic termination does not take place, the polydispersity of the polymer should be equal to unity.Polystyr- ene samples with polydispersity values close to unity have been obtained with a number of alkoxyamines.120 The results of kinetic studies of the mechanism of `living' radical polymerisation of St in the presence of alkoxyamines and of analytical calculation studies were considered in detail in a review.120 Polymerisation in the presence of alkoxyamines occurs by the following scheme: } The term iniferter is composed of the names of elementary steps of radical polymerisation: ini (initiation), fer (transfer), ter (termina- tion).117, 124Problems of control of the reactivity of macroradicals and the growth of polymer chains R7R 2 (initiation), R . R7X R.+X., (29) R.+X. R7X, R.+M R7M., R.+R. R7R.The process kinetics has the following specific features: the rate of polymerisation is proportional to the monomer concen- tration and does not depend on the alkoxyamine concentration; the polymer MM increases linearly with an increase in the degree of polymerisation (this regularity holds, at least, up to Mn=20 000 ± 30 000); the polydispersity of the polymer normally first decreases with an increase in the degree of conversion and then increases; the polydispersity coefficient fluctuates depending on the process conditions but, as a rule, it remains less than 1.3; the concentration of the stable nitroxide (EPR detection) is equal to 0.1% ± 1.0% relative to the initial concentration of the alkoxy- amine throughout the whole process.196 Computer analysis of the kinetic data and the molecular-mass characteristics 197±199 made it possible to establish correlations between reaction parameters (temperature, concentration of the reactants) and the properties of `living' polymerisation agents [the rate constants for the forward and back reactions (28)].These results can be used to choose the optimum conditions for performing polymerisation of St with a particular iniferter.However, it should be noted that, similarly to the polymer- isation in the presence of TEMPO (and, evidently, for the same reasons), these compounds can be used at temperatures above 100 8C and only for polymerisation of St and its ana- logues 137, 140, 198, 200, 201 and copolymerisation or preparation of block copolymers of St with other monomers, most commonly, acrylates.202 ± 204 Dissociation of alkoxyamines at high temperatures (above 135 8C) has been studied in detail in relation to 1-methylbenzyl- oxy-2,2,6,6-tetramethyl-1-piperidine 205 and its derivatives result- ing from decomposition of BP in St.141 The process follows two pathways, one to give a reactive hydrocarbon radical and a stable nitroxide (it is this reaction that is responsible for the `living' polymerisation mechanism) and the other, giving rise to an unsaturated hydrocarbon and hydroxylamine (hydride transfer).Alkoxyamines based on TEMPO have been used 206 ± 209 to synthesise densely cross-linked glassy polymers by `pseudo-living' radical three-dimensional polymerisation; copolymerisation of St with MA was taken as an example.Due to the `pseudo-living' polymerisation mechanism, the point at which a continuous network was formed was shifted appreciably towards higher degrees of conversion (>17% ± 35%) compared to that observed for conventional three-dimensional radical polymerisation. Block copolymerisation of St and its analogues with acrylates has proved to be a more efficient method for the extension of the scope of application of alkoxyamines. Block copolymers based on PS or on St copolymers with acrylates with high St contents prepared in the presence of alkoxyamines have been considered in detail in reviews 120, 210 and monographs.76, 119 The use of derivatives of thiocarbamates as photoiniferters has been studied extensively.124, 170 ± 174 Unlike the iniferters which decompose at high temperatures, thiocarbamates tend to decom- pose on exposure to UV radiation at much lower temperatures than alkoxyamines.The polymerisation of St in the presence of these reagents has all the main features of `pseudo-living' polymer- isation.173, 174 An EPR study of the photo decomposition of S-benzyl N,N- diethyldithiocabamate and PS containing a terminal dithio- carbamate group induced by UV radiation (l=365 nm, 25 8C) has shown 120, 211 that decomposition follows the same mechanism with rupture of the C7S bond to give reactive benzyl (or polystyrene) and stable N,N-diethyldithiyl radicals. 439 hn (30) BnSCNEt2 Bn + SCNEt2 S S (31) CH2CHSCNEt2 CH2CH + SCNEt2 S Ph Ph S By assuming that decomposition of these compounds occurs by the same mechanism, one can explain the fact that the regularities of polymerisation in the presence of low- and high- molecular-mass iniferters�S-benzylN,N-diethyldithiocabamate and the PS containing a terminal dithiocabamate group � prove to be qualitatively the same.Research into the photo and thermal decomposition of thio iniferters showed 212 ± 215 that decomposition of symmetric mole- cules involves either the S7S bond hn (32) 2RS. RSSR R=Ph, C(S)NAlk2 , or both the S7S and C7S bonds (33) 2RS., RSSR hn RSSR hn R.+.SSR R=C(O)Ph. In the case of nonsymmetric dithiocarbamates and dithiocar- bonates R(CH2)nSC(S)NAlk2 and R(CH2)nSC(S)OAlk (R=Ph, R0CO), the formation of dithiyl (stable) radicals needed to induce `pseudo-living' polymerisation takes place only for the com- pounds BnSC(S)NAlk2 , BnSC(S)OAlk and AlkCOO..CH2SC(S)NAlk2 .216 ± 218 ethane 117, 151 ± 154 and Other compounds that can be attributed to the class of iniferters in question include HPE,148 ± 150 tetraphenyl- bis(trimethylsilyloxy)tetraphenyl- ethane,117, 155 which do decompose under polymerisation conditions to yield symmetric inert radicals but only in the case where they are added without an initiator. In the absence of an active competing agent, initiator, the low-reactivity radical has time to attack the monomer and participates in the initiation of polymerisation.However, the rate of polymerisation in the presence of HPE or related tetraphenylethane or bis(trimethyl- silyloxy)tetraphenylethane is much lower than that of the process initiated by AIBN or an organic peroxide. Another type of iniferters includes phenyl azotriphenylme- thane (PAT); the use of this compound as the initiator of `pseudo- living' polymerisation of MMA was first described by Otsu and Matsumoto.117 The thermal decomposition of PAT occurs at a high rate with rupture of the C7N bond; it is accompanied by evolution of nitrogen and gives rise to phenyl and triphenylmethyl radicals according to the following scheme Ph.+N2+.CPh3 . Ph7N=N7CPh3 The reactive phenyl radicals initiate polymerisation, while the relatively stable triphenylmethyl radicals participate only in the chain termination by macroradicals to give a terminal bond.As has already been noted, the terminal bonds are fairly labile thermally and their homolytic cleavage results in re-initiation of polymerisation. It was found that up to a degree of conversion of *2%, the polymerisation rate of MMA148, 175 and MA176 increases (as in the presence of HPE); subsequently, in the transient region up to degrees of conversion of 5%± 15%, the rate markedly decreases; finally, gradual self-acceleration of the reaction starts. The dependence of the MM on the degree of conversion for polymer samples is linear up to 50% degree of conversion. The MMD follows a specific pattern of variation during polymerisation, namely, the products formed at degrees of con-440 version ranging from1%to7%exhibit a unimodalMMDand are oligomeric products. As the degree of conversion increases, the MMDcurves become bimodal: a second mode arises, which shifts to higher MM as polymerisation advances, and its area continu- ously increases.The position of the oligomeric mode does not change; however, its area gradually decreases. When the degree of conversion is high (nearly limiting), the oligomeric mode com- pletely disappears, and the MMD of the resulting PMMA and poly(methyl acrylate) becomes again unimodal. A similar pattern of variation of theMMDis observed for `pseudo-living' polymer- isation of MMA involving trityl radicals.148 ± 150 Evidently, the mechanism of formation of PMMA in the presence of PAT is underlain by the same processes as in the initiation by AIBN with HPE148 ± 150 (see the previous Section).An interesting feature ofMMApolymerisation in the presence of PAT has been identified.148, 175 Starting with some degree of conversion (40% ± 50%), theMMDmode stops moving to higher molecular masses, i.e., the contribution of the `pseudo-living' polymerisation to the formation of the polymer markedly decreases. In the opinion of the researchers cited, this may be due to the fact that the trityl radical is capable of initiating polymerisation of the acrylate, giving rise to a conventional `dead' polymer, thiscess being more active at high degrees of conversion.In addition, accumulation of the `dead' polymer in the system can be due to side reactions, resulting in deactivation of the labile bond in the macroiniferter, namely, isomerisation of the triphenylmethyl radical followed by its addition to the macro- radical, which occurs in parallel with the formation of the labile bond 219 Me C CPh3 CH2 Me COOMe C. +Ph3C. CH2 Me (34) Ph COOMe C C CH2 Ph MeOOC H and disproportionation of these radicals 219 Me CH2 (35) C C. CH2 CH2 +Ph3C. + HCPh3 . COOMe COOMe As noted above, decomposition of the symmetric molecules of thio iniferters involves either the S7S bond or both the S7S and C7S bonds [see reactions (32) and (33)]. The former route yields symmetric S-centred radicals, which can initiate polymerisation without AIBN or peroxide.We have already discussed data on the use of DXS as a controlling additive in the copolymerisation of vinyl monomers.158, 159 The stable S-centred radical arising according to reactions (32) and (33) under the indicated con- ditions acts as both an initiator and a chain propagation controller and can be attributed to the class of iniferters under consideration because the labile bond in the terminal group of the copolymer is the C7S bond. Original binary initiators and chain propagation controllers based on oxytriazenes and oxidants (metal oxides, organic and inorganic peroxides and hydroperoxides) have been pro- posed.179 ± 182 It was found by EPR spectroscopy that, owing to the simultaneous generation of reactive phenyl radicals (chain propagation initiators) and nitroso compounds, which can react with chain-propagating radicals to give nitroxides (which control the chain propagation), these systems are able, on the one hand, to initiate radical polymerisation, and, on the other hand, to act as process controllers.D F Grishin, L L Semyonycheva e7 RO C OO C OR+Ph N N N But OH O O + Ph N N N But RO C OO C OR HO O O (36) + RO C O Ph N N +HO N But +7O C OR O O Ph+N2 RO +CO2 + N O (37) But RO C OH+ HO N But+7O C OR O O R=C16H33 . The possibility of realisation of a similar single-electron transfer mechanism in the reaction of acyl peroxides with sub- stituted amines has been demonstrated previously.220 In addition, it cannot be ruled out that secondary reactions take place between triazene and the alkoxide radicals derived from the peroxide, which results in triazene decomposition.Ph N N N But+RO. 7ROH (38) OH Ph N N N But Ph.+N2+But N O O. The presence of nitroxides in the polymerisate was detected by EPR using the spin trap technique [a triplet with the splitting constant at nitrogen aN=15.6 Oe corresponding to the adduct of the nitroso compound with the poly(methyl methacrylate) radi- cal].Polymerisation of MMA in the presence of oxytriazene, like that in the presence of MNP,163, 166 ± 168 proceeds under relatively mild temperature conditions (50 ± 60 8C) and exhibits features peculiar to `pseudo-living' polymerisation, namely, the absence of gel effect in the polymerisation up to high degrees of conversion, linear increase in the MM following an increase in the degree of conversion, relatively low polydispersity, etc.179 ± 182 Thus, these systems can be regarded as a sort of binary iniferter.Yet another example of unusual initiators and controllers of polymer chain propagation is provided by compositions based on sterically hindered quinones and OEC. Thus it has been found 177, 178 that 3,5-di-tert-butylquinone, used traditionally as the inhibitor of radical processes, when combined with trialkyl- borane, is able not only to initiate effectively radical polymer- isation of various monomers but also to control the propagation of the polymeric chain according to the `pseudo-living' mecha- nism.It was found by EPR spectroscopy and by kinetic measure- ments that the rate of MMApolymerisation is comparable in this case with the rate of polymerisation of acrylic monomers in the presence of traditional radical initiators, AIBN and acyl perox- ides, and in some cases, even exceeds it. Analysis of the data obtained shows that several processes occur in the system. One process is the reaction of quinone, for example, with TIBB, which gives rise to radicals carrying the polymerisation. But But O O (39) +R. +BR3 BR2 O But But OProblems of control of the reactivity of macroradicals and the growth of polymer chains (40) R C C. R.+ C C The occurrence of reaction (39) has also been assumed in other studies 221, 222 (this assumption was made taking account of the results of EPR investigations of the reaction of quinones with alkyl derivatives of Group III elements, in particular, gallium).In addition, the chain-propagating radicals can react with quinones to give oxygen-centred radicals of the phenoxyl type, which react with TIBB to generate new alkyl radicals,223 able to initiate polymerisation. But But OP O (41) P. + But But O O. But But OP OP (42) +R. +BR3 But But O. OBR2 P is the chain-propagating (polymeric) radical. The dependence of MM of poly(methyl methacrylate) syn- thesised in the presence of the 3,5-di-tert-butylquinone ± TIBB system on the degree of conversion is linear over a broad range of MM values, from 17 000 to 200 000.As the degree of conversion increases, the polymerMMDmode moves continuously to higher molecular masses. This type of dependence of the polymerMMon the degree of conversion is characteristic of `pseudo-living' chain propagation, including that in the presence of iniferters. It can be seen from Eqn (39) that the reaction of the components of the 3,5- di-tert-butylquinone ± TIBB binary initiator furnishes the reactive alkyl radical R., capable of initiating polymerisation, and a stable radical (A ), which can be coordinated to the chain-propagating radical to afford a labile bond. (43) *P.+.A [*P. .A]. The polymer chain grows according to a stepwise pattern as a result of consecutive steps of cleavage of the terminal labile bond [*P A] and consecutive addition of some number of monomer units to give a new labile bond according to the scheme P C C..A . (44) P. .A + C C Thus, quinones combined with OEC act as a sort of low- temperature binary iniferters. In this case, the reactive radical (R.) which initiates polymerisation and the stable radical (A ) which controls the chain propagation are generated directly in the reaction system upon the reaction of the components of a binary initiator with each other [see Eqns (39) ± (43)]. Similar regularities are also characteristic of some other quinones and OEC derived from Group III�V elements.224 Original initiators-chain propagation controllers based on sterically hindered trialkylboranes and oxidants have been pro- posed by Chung et al.225 These investigators were the first to use stable boron-containing radicals produced upon oxidation of 1-octyl-9-borabicyclononane to control the chain propagation in the polymerisation of MMA.The use of this additive brings about the features typical of `pseudo-living' radical polymerisation. It is of fundamental importance that, unlike most known systems, with this system the process occurs at room temperature which allows one to eliminate the undesirable thermal constituent of the process, which markedly deteriorates the MMD of the polymer. 441 4. Radical polymerisation in the `living' chain regime in the presence of organoelement compounds The use of OEC as chain propagation controllers in the `pseudo- living' radical polymerisation of vinyl monomers opens up pros- pects for modelling `pseudo-living' processes using individual features of the metal or non-metal atom in OEC.Processes in which organic compounds of transition metal halides are used to control the growth of a polymer chain have been studied to date.76, 119, 226 ± 228 It is known 229 that transition metal halides with the formula Xn7Ym7Lx (X is a transition metal, n is the valence of the metal, Y is chloride or bromide, L is an organic ligand, most often, a Lewis base) containing metal in a lower oxidation state react with alkyl halides (RX) at*100 8Cto give the all radicalsR.capable of initiating polymerisation. Simultaneously, oxidation of the transition metal takes place according to the reversible Kharasch reaction.229 (45) R.+Xn+17Ym7Lx , R7X+Xn7Ym7Lx (46) R.+xM R7Mx. , Mis monomer. When this reaction was induced in the medium of a vinyl monomer, stepwise growth of the polymer chain was observed, i.e., a `pseudo-living' chain process took place. Currently, a broad range of OEC(copper, iron, palladium and other compounds) are used as controlling additives; the organic ligands most often contained in these systems are 2,20-bipyri- dine,230, 231 4,40-di(non-5-yl)-2,20-bipyridine,232 ± 236 triphenyl- phosphine,226 ± 228, 237 etc. The alkyl halides normally used for this purpose include alkyl(or aryl) bromides and dialkyl(or diaryl)- alkylaryl bromides.226, 230, 232, 233, 235, 236 For a series of vinyl monomers, namely, St,226, 228, 232 ± 235 MA,233, 234 MMA,226, 232, 234, 237 ± 239 AN230 and so on, the ATRP process has been accomplished [see redox reaction (45)].The rate of this process is rather high; therefore, all the polymer chains are initiated almost simultaneously; this allows the manufacture of polymers with a polydispersity close to unity (1.05 ± 1.30).227, 228, 230, 232 ± 239 Unlike the controlling additives on the basis of stable radicals considered above, the ATRP agents are not incorporated in the polymer chain and, hence, they can be regarded as analogues of catalysts. In addition to transition metal compounds (for example, as in ATRP), other OEC are also employed to induce `pseudo-living' radical polymerisation. As early as in the 1970s, AcademicianGA Razuvaev and co-workers 240, 241 synthesised block copolymers of MMA with AN using radical initiation by organo-mercury and - tin compounds.It was established by emission spectroscopy that PMMA contained mercury in the terminal groups, and subse- quently these macromolecules can act as peculiar macroinitiators. The molecular mass of the secondary polymer is twice as high as theMMof the initial one, which confirms indirectly the possibility of a `pseudo-living' chain mechanism. IV. Characteristic features of homo- and co-polymerisation of vinyl monomers in the presence of organoelement compounds derived from Group V elements The coordinatively unsaturated OEC containing Group II and, especially, Group III elements participate directly in the chain propagation step.The addition of compounds derived from Group IV and V elements also influences the elementary steps in polymerisation. These compounds have vacant d orbitals, which stipulates their electron density acceptor properties. Tin and germanium tetrachlorides can influence significantly co-polymerisation of MAA esters with MMA.51, 52 The introduc- tion of 1 mol.% of these compounds to a polymerisate gives rise to alternating copolymers. Conversely, the addition of tetrabutyltin442 barely influences either polymerisation rate or the MM of polymers. Apparently, this is due to the fact that vacant d orbitals of the tin atom are removed from the nucleus and, therefore, the electron-acceptor capacity of these orbitals is too low to activate the chain-propagating radical.Electron-donating alkyl radicals do not promote the coordinating capacity of OEC either. Data on the copolymerisation of MMA with MAA in the presence of tetrabutyltin additives confirm the conclusion that the activities of monomers and growing macroradicals virtually do not change. Meanwhile, the controlling effect of Group IV element halides on the copolymerisation of acrylates, described by Smirnova and coworkers,51, 52 can be explained by the fact that inorganic Lewis acids are more prone to form intermolecular complexes than organic compounds of the same elements.A specific position is occupied by OEC containing Group V elements because they contain both a lone electron pair in the p orbital suitable for a nucleophilic attack and vacant d orbitals, which can give rise to electron-withdrawing properties. It has been shown 242, 243 that the addition of alkyl derivatives of bismuth and antimony influences directly the kinetic parame- ters of polymerisation of vinyl monomers. Thus the rate ofBAand VA polymerisation increases by more than an order of magnitude in the presence of catalytic amounts of triisobutylbismuth. The addition of triethylantimony markedly accelerates not only bulk polymerisation of acrylates but also solution polymerisation in ethyl acetate, which is a polar solvent and can be regarded as a hydrogenated analogue of MMA in terms of its electronic structure; like the monomer, it contains an ester group able to be coordinated to electron density acceptors. Organic compounds of bismuth and antimony, like those of Group III elements, do not exert any substantial catalytic effect on the decomposition of an initiator such as AIBN either in inert hydrocarbons or in the presence of vinyl monomers.53 Thus, the data presented above indicate unambiguously that acceleration of polymerisation of acrylic monomers in the pres- ence of OEC containing Group V elements is due to the direct participation of these additives in the chain propagation stage.(47) [*P..E .R3], *P.+E..R3 It should be noted that catalytic amounts of OEC accelerate chain propagation in the polymerisation of VA and BA much more efficiently than in the polymerisation of MMA, which is in good agreement with the results of investigations into the influ- ence of Group III element compounds on the polymerisation of acrylates.The phenyl derivatives of Group V elements stable against oxidation by atmospheric oxygen also activate somewhat the process of MMA polymerisation. The accelerating effect of these derivatives decreases in the series Ph3Bi>Ph3Sb>Ph3As, which is in line with the variation of nucleophilic properties of these compounds. It is clear that the influence of organic derivatives of Group V elements on the polymerisation of acrylic monomers is directly related to the presence of the lone electron pair in the p orbital.This conclusion is supported by the fact that introduction of pentaphenylantimony in the polymerisate barely changes the polymerisation rate and the polymer MM.242, 243 Apparently, during its growth, a macroradical reacts with the metal atom in the OEC to give a four-coordinate radical complex where *P. is the growing macroradical; E is a Group V element (antimony, bismuth); R=Et, Ph, Bui. It is known 56 that, owing to the lone electron pair in the p orbital and free d orbitals, Group V elements are capable of expanding their electron shells. It has been shown by EPR 56 that compounds of trivalent Group V elements can react with free oxygen- and carbon-centred radicals; as this takes place, the coordination number increases to four and the corresponding intermediates are formed [reaction (47)].The [*P ER3 ] bond thus formed between the growing macroradical and a Group V element is labile and can be cleaved D F Grishin, L L Semyonycheva either yielding the initial compounds (OEC and the chain-prop- agating radical, which can continue polymerisation) or with the `ejection' of a radical from the organoelement compound (SR2 substitution). P.+ER3 (48) P. .ER3 PER2+R. Thus, one of the variants for the participation of OEC in the chain transfer stage and in the control ofMMis realised. It cannot be ruled out that in the presence of coordinatively unsaturated OEC, chain propagation takes place via insertion of the monomer at the labile [*P...ER3] bond to give a new coordinated macroradical. P ER3 P. ER3 +CH2 CH CH CH2 X X (49) CH P ER3 CH2 X Thus, OEC is coordinated to the growing macroradical and thus participates directly in chain propagation and prevents bimolecular chain termination upon recombination of growing macroradicals. This type of process can be considered as a variant of `pseudo-living' polymerisation. It has been noted above that an increase in the rate of complex- radical polymerisation in the presence of metal halides can be explained by assuming the cyclic mechanism of chain propaga- tion.5 Similarly to OEC containing Group III elements, the alkyl derivatives of antimony and bismuth have a more pronounced influence on the chain propagation in the polymerisation of VA and BA than in the polymerisation of MMA.242, 243 Since the distributions of electron density in the molecules of alkyl acrylates and methacrylates (for example, inMMAand BA) differ insignif- icantly,79 it can be suggested that steric factors make the crucial contribution to the change in the kinetic parameters of polymer- isation (in particular, the rate constants for chain propagation).The absence of substituents at the central carbon atom in the butyl acrylate radical makes the reaction centre more readily accessible for the coordination to OEC and subsequent chain propagation than in the case of MMA (tertiary radical). The mechanism of chain propagation proposed above is also supported by the absence of inhibitory effect upon joint introduc- tion of hydroquinone and triethylantimony in the polymerisate.53 The organoelement additive enters into the coordination inter- action with the growing macroradical and thus `blocks' the reaction centre and prevents its interaction with the inhibitor.Consequently, the rate of polymerisation and the MM of poly- mers virtually do not change in the presence of hydroquinone and triethylantimony. The character of influence of benzoquinone (a more effective inhibitor of radical polymerisation) on the kinetics of the proc- esses studied is determined by the quinone concentration in the monomer mixture.53 When the ratio [benzoquinone] : [triethylan- timony]<1, the rate of polymerisation virtually does not change.Conversely, with an excess of the quinone, polymerisation is retarded. This pattern of variation indicates, apparently, that at least two independent processes take place. One process is fast reaction of triethylantimony with benzoquinone in which quinone is reduced to hydroquinone.244, 245 As noted above, hydroquinone influences only slightly the kinetics of polymerisation of acrylic monomers in the presence of organic compounds of Group V elements. In addition, when [benzoquinone] : [triethylanti- mony]>1, the excess benzoquinone reacts with the growing macroradical to give stable phenoxyl radicals; as a consequence, polymerisation also slows down.The substantial decrease in the molecular mass of PMMA upon introduction of the quinoneProblems of control of the reactivity of macroradicals and the growth of polymer chains confirms the assumption that it participates in the chain termi- nation step.243 Organoelement compounds containing Group V elements not only influence the rate of polymerisation of vinyl monomers but also control the MM of polymers, the effect being more pro- nounced than that induced by the addition of trialkylboranes.53 The addition of triethylantimony in amounts comparable with the concentration of the initiator markedly decreases the MM of poly(butyl acrylate) both for bulk polymerisation and for solution polymerisation in ethyl acetate.243 The molecular mass of the polymer decreases with an increase in the OEC concentration: on introduction of 0.8 mol.% of triethylantimony, it decreases by more than an order of magnitude.The rate constant for the chain transfer to triethylantimony amounts to 0.118, which markedly exceeds the corresponding value for the chain transfer to the monomer (0.003). The rate constants for the chain transfer to triisobutylbismuth in the polymerisation of MMA and VA calculated from exper- imental data are equal to 0.011 and 0.71, respectively.242, 243 The values for the chain transfer to antimony and bismuth alkyl derivatives considerably exceed the corresponding parameters for TIBB (161072 and 2.261072 for MMA and VA, respec- tively).20, 53 The very high rate constants for chain transfer in the polymerisation of BA and VA can be due to the occurrence of two processes involving the macroradical and OEC.One process, namely, free-radical substitution of the growing polymeric radical for the alkyl radical in organo-antimony and -bismuth com- pounds, occurs by a scheme similar to that presented above for trialkylboranes [see reactions (4) and (5)]. The rate constant for the substitution of oxygen-centred radicals for the alkyl radicals in triethylantimony is*105 litre mol71 s71 (see Refs 246 and 247). The second process, which also contributes to the control of the polymer MM, is abstraction of a hydrogen atom from the alkyl group of the OEC by the growing macroradical [similarly to Eqn (6)]. The occurrence of this reaction was proved unambigu- ously by EPR using the spin trap technique in a study of the influence of minor additives of methyl acrylate and other acrylic monomers on the control of the PMMA molecular mass during polymerisation involving organoelement initiators.113 It was found that, like trialkylboranes, organic compounds of antimony and bismuth also influence the composition of copoly- mers.Thus in the case of theMMA±MAApair of monomers, the addition of triethylantimony in amounts commensurable with the concentration of the initiator induces transition from the copoly- mer enriched in one component (rMAA=1.40, rMMA=0.08) to strict alternation of units in the copolymer.243 The appearance of alternation of units can be explained by an increase in the rate of cross propagation as a result of the formation of a cyclic reaction complex involving the monomer, the growing macroradical and the OEC.It is noteworthy that, as in the case of TIBB, the flat section in the composition curve corresponds to an MAA content in the copolymer, equal to 0.66 mol.%. This indicates that MAA is involved in the chain propagation step as a dimer.243 Triethyl- antimony also influences somewhat the regularities of copolymer- isation of MAA with the electron-donor St monomer:53 the introduction of triethylantimony substantially increases the rate of copolymerisation; however, neither the composition of the copolymers, nor the relative activities of the monomers change noticeably. The difference from the copolymerisation of the MAA± MMA pair of monomers is caused by the different in kind structures of MMA (electron density acceptor) and St (electron density donor), and the corresponding macroradicals. Thus, OEC containing Group V elements participate directly in the elementary steps of polymerisation of vinyl monomers and control efficiently polymerisation kinetics and the MM and composition of copolymers prepared both in the bulk and in solution.443 V. Conclusion The effective methods for controlling the reactivity of macro- radicals developed to date permit the preparation of homo- and copolymers uniform in composition and molecular mass. The control is attained via active influence on the elementary processes of polymerisation including the chain propagation step.The introduction of very small amounts of active additives makes it possible to achieve substantial and, what is more important, targeted change in the reactivity of the macroradicals and to control systematically the polymer chain propagation. It is of prime importance that some OEC, spin traps, iniferters and other controllers of chain propagation considered above are active when present in amounts comparable with the concentration of the initiator. Hence, in the future, they could be used in the industrial synthesis of polymers. 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