摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2643-2648 EPR Spin-labelling and Spin-trapping Study of Proteins in Reverse Micelles Graham S. Timmins, Michael J. Davies and Bruce C. Gilbert* Department of Chemistry, University of York, York, UK YO1 5DD Horia Caldararu* Romanian Academy, Institute of Physical Chemistry, Splaiul lndependentei 202,77208 Bucharest, Romania EPR spectroscopy has been used to study the motions of several spin-labelled and spin-trapped proteins (a-chymotrypsin, cytochrome c and myoglobin) enclosed within reverse micelles formed by sodium bis-(2-ethyl- hexyl) sulfosuccinate (AOT) in isooctane. In several cases the spectra obtained from the encapsulated protein are significantly different from those observed in bulk solution. The motions of the labelled proteins, inferred from the anisotropy of the EPR spectra (A values), vary with the amount of solubilized water (W,) and hence the physical size of the water-pool in the reverse micelles; it is suggested that the level of solvation and the solvent structure in the water-pool of the reverse micelle cause the observed changes in motion.These results support the previously postulated ' water-shell ' model of proteins contained in reverse micelles. The study of proteins, especially enzymes, encapsulated within reverse micelles (RM) has attracted increasing atten- tion, because of their use as models of biological membranes and their potential applications in biotechnology. *-' Despite this work, there remains considerable doubt as to how the RM's structural and chemical properties affect, and are affected by, intra-micellar proteins, and how incorporation within RMs may cause changes in enzyme stability, kinetics and substrate specificity.For example, it would be of particu- lar interest to determine whether the water present inside the RM (the water-pool) possesses different physico-chemical parameters (such as viscosity and relative permittivity) from bulk water, and if so, whether these changes may modify protein dynamics, conformation and activity. These questions have been addressed in several theoretical models of protein- containing RMs.'.' A variety of techniques have been used to study proteins in RMs including ultracentrifugation, circular dichroism, fluo- rescence and NMR spectroscopy, dynamic light scattering and small-angle X-ray and neutron ~cattering.'~~~'EPR spectroscopy, although extensively used for studies of protein dynamics in bulk solution (because of the sensitivity of EPR spectra to alterations in molecular motion), has been scarcely utilised thus far to study proteins within RMs.' '-14 As a result of the development of specific spin-labels which can be attached to particular sites of a pr~tein,'~and our recent studies in which stable free radicals have been generated on proteins through radical-induced damage and subsequent spin-trapping,' we have examined whether the specificity of EPR spectroscopy for radical species and its sensitivity to their molecular motions can provide information on the nature of the water-pool and its influence upon proteins con- tained within RMs.Recent studies by Marzola and co-workers of spin-labelled chymotrypsin and human serum albumin encapsulated within RMs formed by sodium bis(2-ethylhexyl) sul-fosuccinate (AOT) in is~octane,'~.'~ showed that such an approach is realistic: changes in EPR spectral parameters provided information on the conformation and dynamics of proteins in bulk water and within RMs. The current study has aimed to characterize further the interactions between RMs and a number of proteins solvated in AOT-isooctane reverse micelles using both spin-labelled proteins (a-chy- motrypsin spin-labelled at two specific amino acid side-chain sites, Met-192 and Ser-195, located at the surface of the protein)" and spin-trapped proteins formed as a result of radical-induced damage' (with these generated either before incorporation into the RM or within the RM).Investigation of the motion of these proteins with attached free radicals in both bulk solution and within RMs with different sized water-pools, from small RMs (W, = 3) to large RMs (W, = 40),where the molar ratio W, = [H,O]/[AOT], has allowed us to study how changes in water-pool size, and subsequent solvation parameters, affect the molecular dynamics of the labelled proteins. Results and Discussion Spin-trapped Proteins Spin-trapping in Bulk Solution Reaction of HO' (generated by the Fe2+/H,02 couple) with cytochrome c in bulk water in the presence of the spin-trap 3,5-dibromo-4-nitrosobenzene-sulfonic acid (DBNBS) resulted in an EPR spectrum of a partially immobilized spin- adduct formed via the reaction of the hydroxyl radical with the protein and subsequent trapping of the protein radical by the spin-trap DBNBS, as described previously'6 [Fig. l(a)]; these species have been shown to be stable for several hours.16 The broad, anisotropic, nature of the observed spec- trum is consistent with that of a slowly tumbling, partially immobilized, radical-adduct with some local freedom of motion around the aminooxy radical centre.Information regarding the molecular motion of the observed species (and similarly for spin-labelled proteins as described below) may be readily obtained from the splitting between the outermost features of the spectrum (denoted as 2AII; this value is char- acteristic of the extent of immobilization of the species, with increasing motion tending to decrease A II until isotropic spectra, indicative of rapid protein tumbling and/or rapid molecular motion of the aminooxy group, are obtained).Incorporation of these pre-formed cytochrome c-DBNBS radical-adducts within RMs of varying W, (between 33 and 73, gave spectra similar to those obtained in bulk water [Fig. l(b)-(e)]. At higher W,, All could be easily measured, whereas at lower W, (< 12) this was not possible owing to the poor signal-to-noise ratios (the amount of trapped species cannot be readily increased in these cases, since its addition J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 -Fig. 1 EPR spectra of the cytochrome c-DBNBS radical-adduct formed upon attack of cytochrome c by the hydroxyl radical: (a)in water (pH 3.5); (b)-(e) the adduct obtained in (a)injected into AOT reverse micelles of W, = 33, 15, 12, 7.5, respectively. The measure- ment of 2AII is indicated in (a). in solution to the AOT-isooctane mixture determines the W, value) and underlying baseline drift due to a component of the iron species. For micelles with W, = 33 and 15, the All value (2.80 mT) is somewhat smaller than that found in bulk water (2.91 mT). This unexpected decrease in All in this range suggests that incorporation of the spin-adduct into the reverse micelle results in an increase in the molecular motion of the aminooxy group.This may be due to protein confor- mational changes, resulting in increased local freedom of motion about the aminooxy group, thereby reducing the All. value: interaction between the surface lysine residues of the protein (which are positively charged and basic in nature) with the (negatively charged) head groups of the AOT mol- ecules in the RMs might be responsible for the postulated conformational change. This type of interaction is known to direct the binding of cytochrome c to the membrane-bound cytochrome c oxidase complex in mitochondria.’ Chymotrypsin-DBNBS radical-adducts produced in a similar manner in bulk water gave rise to EPR spectra [Fig. 2(a)] that appear to contain at least two components, one of which is partially immobilised and similar to that obtained with cytochrome c (with All 73.03 mT) and one which gives rise to a much more isotropic spectrum. This suggests that there are several sites of radical formation upon the protein and hence several possible sites of spin-adduct formation on the chymotrypsin molecule, with differences in the nature of these species and hence local aminooxy radical mobilities.This is not unexpected since the HO’ radicals which generate the initial species are highly reactive, and will be generated across the protein surface in an approximately random manner (the iron-EDTA complex has a stability constant” -\\ 7-‘-1 1 mT-Fig. 2 EPR spectra of the chymotrypsin-DBNBS radical-adduct formed upon attack of chymotrypsin by the hydroxyl radical: (a) in water (pH 6); (b)-(e) the adduct obtained in (a) injected into AOT reverse micelles of W, = 40,20, 6, 3, respectively.The measurement of 2AI,is indicated in (a). of 1.6 x 1014 and hence this complex is free in solution and the iron is not bound to specific sites on the protein); differ- ences in the surface ‘topography’ could therefore result in differing freedom of motions of the DBNBS adduct. Incorporation of these radical-adducts into AOT RMs of varying W, resulted in the observation of the spectra shown in Fig. 2(b)-(e). As observed with cytochrome c, the value of All for the species giving the more anisotropic spectrum is smaller (2.87 mT) in large water-pools (W, = 40 and 20) than the value in bulk water (3.03 mT); however, on incorporation into RMs with smaller water-pools (W, = 10 and 6) an increase in All to 2.91 mT and 3.02 mT, respectively, is observed, though these values do not exceed those obtained in bulk solution.The reason for the decrease in A and hence the increase in mobility, upon incorporation into RMs of larger W, could again involve interactions of basic surface residues of the protein with AOT head groups resulting in conformational changes in the protein as postulated for cytochrome c, although it is known that cytochrome c and chymotrypsin appear differentially associated with the water-pool inter- face.” The increase in All with decreasing size of the water- pool (W,) can probably be assigned to an increase of the microviscosity of the water in its core (for a fuller discussion, see later); alternatively, conformational changes of the protein structure might also be brought about by the size of the water-pool within the RM, with the protein being ‘squeezed’ or altered in conformation at lower W, values. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Spin-trapping within Reverse Micelles Attempts to trap protein-derived radicals from cytochrome c or chymotrypsin subjected to HO' attack in the presence of DBNBS within the water-pool of the RM, resulted in the detection of isotropic spectra from two radical species which were also observed in the absence of the protein. This sug- gests that HO' preferentially attacks the hydrocarbon chains of the surfactant, producing two adduct species, one of which gives rise to a triplet spectrum (with aN= 1.36 mT), due to the trapping of a tertiary radical, and the other to a triplet of doublets (aN= 1.40 mT; aH = 0.80 mT), due to the trapping of a secondary radical; attempts to form protein radical-adducts preferentially with either protein by manipulation of the reagent concentrations failed.The presence of either protein in this system did, however, result in an increase in the rate of decay of the adduct species, implying that the adduct may interact with the protein, possibly through one- electron oxidation or reduction of the radical centre by protein-bound species (which could either be thiol groups or reactive species produced on the proteins ; radical-damaged proteins are known to contain such species2').To minimise radical attack on the surfactant chains by HO', myoglobin (a haem-containing protein) encapsulated within RMs was reacted with H20, and DBNBS in the absence of added iron; the observed spin-adduct arises from the trapping of a radical species on the protein resulting from the reaction of H,02 at the haem centre followed by an intramolecular electron-transfer reaction, without the forma- tion of H0'.'6 Fig. 3 shows the EPR spectra of myoglobin- DBNBS adducts, generated in this manner, in RMs of varying W,. The spectral shape of these adducts formed in RMs of W, = 40 and the All value (2.64 mT) are almost iden- \\ i I tical to those found in bulk water (2.61 mT).I6 However, at lower W, values an additional isotropic species was obtained ; this could be due to either the interaction of the protein with the AOT head groups, which may result in alteration of the protein structure, as observed with a-chymotrypsin and cyto- chrome c, or transfer of the damage from the protein to the AOT molecules and subsequent trapping of these radicals. No variation in the All of the anisotropic species with changes in W, was observed for myoglobin (in contrast to a-chymotrypsin and cytochrome c); this is believed to be due to the position of the spin-adduct on the protein with, in this case, the aminooxy group buried sufficiently deeply within the protein structure to be unaffected by changes in the local freedom at the protein surface owing to differences in water- pool size, except when the protein surface itself may be dis- rupted at very low W, values.Spin-labelled Proteins Reaction of a-chymotrypsin with the stable aminooxy radical 1 in bulk solution followed by purification, as described in the Experimental section, resulted in the formation of the spin-labelled protein with the spin-label attached at the Met-192 residue. The EPR spectrum of this material in bulk water, which is similar to that previously reported2' (see Fig. 4), corresponds to the region of fast motion and is indicative of moderately rapid rotation of the spin-label, which being at an exposed surface site, is only slightly motionally restricted by the protein structure.21*22 Incorporation of this labelled protein into RMs of varying W, resulted in alterations in the observed spectra [Fig.4(a)-(c)].Compared with the spectra in bulk water, these suggest increasing restriction of motion Fig. 3 EPR spectra of equine myoglobin-DBNBS radical-adducts formed by reaction with H202 in the absence of Fe": (a)in a blank experiment; (b)-(d) within the AOT reverse micelles of W, = 40, 15 and 10, respectively. Experimental settings were the same for all Fig. 4 EPR spectra of a-chymotrypsin spin-labelled with 1 in: (a)-samples except field modulation width in (b)and (b')is 0.32 mT and (c)AOT reverse micelles of W, = 3, 6 and 20, respectively; (d)buffer, 0.5 mT, respectively. pH 6.Experimental settings were identical for all samples. 2646 with decreasing W, , indicated by their increased linewidth (and hence greater z,). At high W, values (> 20) the linewidth remains constant never reaching the breadth observed in bulk solution. 0 II 0 F--P--OCH&H3II I H3C, ,N+-(CH.&CH3 Br-N H CC H*Br 0 I I H3C I000I I I 0-0-0-1 2 3 Spin-labelling the same protein with a second stable aminooxy radical 2, which is known to react at the nearby Ser-195 residue, resulted in a partially immobilized species in bulk solution, though in this case a weak isotropic signal is also observed in the EPR spectrum; this signal is believed to be due to the free spin-label formed by hydrolysis of the phosphate ester linking the spin-label to the protein, as observed previo~sly,~~.~~ and in accord with this suggestion its signal intensity increased with time after preparation of the labelled protein (data not shown).Previous crystallo-graphic and other studies have shown that the Ser-195 residue (to which the aminooxy radical is attached) lies in the active site of the enzyme in a shallow hydrophobic depression on the surface of the pr~tein;~~*~~ the partial immobilization of the aminooxy radical on binding to the protein, as indi- cated by its EPR spectrum, is believed to be due to its accom- modation within this po~ket.'~,'~Incorporation of this second labelled protein (immediately after its preparation) into RMs of varying W, gave the spectra shown in Fig.5(a)-(d); these spectra indicate varying degrees of mobility of the aminooxy radical, with lower W, micelles resulting in greater immobilization. In the case of RMs of W, b 10, an additional isotropic component was observed; since an increase in W, resulted in a greater proportion of this isotropic species, and the proportion of this species at fixed W, (except W, = 3) increased with time, this isotropic species is again attributed to the hydrolysed spin-label. The All values observed for this labelled protein in RMs of varying W, are summarised in Fig. 6; note that no further decrease in All was observed with W, > 20, and that the observed All value (2.91 mT) for these micelles indicates a greater degree of spin-label immobil- ization than in bulk water.The observed dependence of the spectral parameters on W, is most likely to derive from changes in the microviscosity of the solvent surrounding the protein (which alters the rate of local molecular motion of the spin label) or changes in polarity (which can change the nitrogen hyperfine splitting).? Studies of the microviscosity and polarity within RMs identi- cal to those used in the present study (but in the absence of protein) with the charged spin probe 3 give rotational corre- lation times (z,) and nitrogen hyperfine splittings (aN) of 7 Contributions to changes in spectral parameters from overall micellar and/or protein tumbling may be ruled out because: (i) non- protein-containing RMs of W, 2 5 have a hydrodynamic radius of 30 A'** this being above the threshold value of 20 A above which the contribution to anisotropicity by the rate of micellar tumbling is outside the range detectable by X-band EPR measurement^;^^*^^ incorporation of proteins will increase the RM size yet further.(ii) The differences in EPR parameters of chymotrypsin in bulk water labelled with 1 and 2 indicate that it is local spin-label motions, and not overall protein tumbling, that are significant in determining spectra in this system. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 7 1, ,' I Fig. 5 EPR spectra of a-chymotrypsin spin-labelled with 2 in AOT reverse micelles: (a) and (a') Wo= 3, (6) and (6') Wo= 6, (c) and (c') Wo= 10, (d)and (d') Wo = 20.Experimental settings were the same for all samples except field modulation width in (a) 0.4 mT, (a') 0.63 mT, (b) 0.125 mT, (6') 0.32 mT, (c) 0.125 mT, (c') 0.4 mT, (d) 0.125 mT, (d')0.32 mT. 0.7 x lo-'' s and 1.67 mT, respectively, for bulk water, and 15.5 x lo-'' s and 1.51 mT, respectively, for RMs with W, = 3.5*27 Although this positively charged aminooxy radical may be anchored at the AOT/water interface (by elec- trostatic interaction), the differences in z, values indicate that there are differences in the microviscosity of the various water-pools and also of bulk water. When the dependence of z, of 3 and AI, of the Ser-195 labelled a-chymotrypsin are plotted against W, (Fig.6) it can be seen that the two are quite similar, with smaller z, values found at higher W, ;since changes in z, have been associated with the microviscosity of water within the RM,25327 a similar explanation would seem 3.3 17 3.2 I-3.1 --T 3.0 -0 10 20 30 40 wo Fig. 6 Variation of A,, of cr-chymotrypsin spin-labelled with 2 (0) and z, of spin probe 3 (m) in AOT reverse micelles as a function of WO J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 appropriate for the observed changes in All for the spin- labelled proteins in this study. These data are in accord with those obtained by Marzola et ~1.'~with spin-labelled human serum albumin, where a weakly immobilized species (believed to be due to attachment to a surface lysine residue28) and a more strongly immobilized species underwent similar changes in mobility in RMs when W, was changed from 2.2 to 7.2; this behaviour has been attributed to an increased depth of solvating water molecules around the protein as W, increases.In order to obtain further evidence to support the sugges- tion that it is the microviscosity within RMs which modu- lates the motion of the spin-label with the labelled proteins, their EPR spectra were recorded in buffer solutions contain- ing varying amounts of glycerol to alter their viscosity (from 1 :1 to 1 :4 w/w, whose relative viscosity values range from ca. 6 to 60, re~pectively).~~ The spectra observed with these mixtures (Fig. 7) are similar to those observed with RMs of varying W, (cf:Fig.4and 5), with increasing amounts of glyc- erol giving increasingly anisotropic spectra (with larger A II values). Attempts to use the EPR spin-probe technique to detect any structural changes in the RMs themselves brought about by encapsulation of the proteins failed (using spin-probe 3 or 5-doxylstearic acid [2-(3-carboxypropyl)-4,4-dimethyl-2-tridecyl-3-oxazolidin-l-yloxyl]},filled RMs only represent a low proportion of the total RMs (owing to the limited solu- bility of the proteins in the added water). Thus it was not possible to distinguish between RMs that did or did not contain protein;30 similar results have been obtained with other method^.^^'^,^' Conclusions Two models for the solubilization of proteins within RMs have been developed; the 'water-shell' model in which it is proposed that protein solubilization gives rise to larger Fig.7 EPR spectra of buffer solution containing a-chymotrypsin spin-labelled with 1 in buffer-glycerol mixtures (w/w): (a) 1:4, (b) 1 :1 and similarly, a-chymotrypsin spin-labelled with 2 (c) 1:3 and (d) 1 :1. Experimental settings were the same for all samples. ~~~1-3,32,33and an alternative model which suggests that no increase in RM size occurs.5,6*34,35 In the water-shell model, which is believed to be particularly suited to hydro- philic proteins and the one-protein-per micelle proposal,'Y2 it is supposed that the protein is surrounded by a shell of water with.in the RM.The dependence of the aminooxy radical motion for the spin-labelled a-chymotrypsin on the microviscosity/polarity and hence W, (at least for W, d 15-20) would seem to support the water-shell model in this case. This hypothesis is also in agreement with the observed dependence of the rate of hydrolysis of a-chymotrypsin labelled with 2 upon W,, with the very slow rate of hydro- lysis in RMs with W, = 3 indicating that at these values most of the water molecules are essentially bound to the protein and/or inner-RM surfaces. It has also been shown that the enzymic stability of a-chymotrypsin in AOT RMs is enhanced at low W, ,presumably for similar reasons.36 The decrease in the microviscosity of the water-pool as W, is increased is believed to result from the increased depth of the solvating water layer at higher W, causing a decrease in the short-range order of the water layer induced by its inter- actions (e.g.electrostatic) with the protein and AOT surfaces.For RMs with W, 2 20, where the protein radius (ca. 22 A) is much smaller than the radii of the water-pools (between 35 and 60 A, for W, = 20 and 40, respectively), the lack of observed changes in mobility of the aminooxy group attached to a-chymotrypsin for both labels with increasing W, probably results from little (or no) change in the micro- viscosity of the water-pool with water layer thickness, i.e. there is a threshold value above which little change occurs; it has previously been reported that little change in the struc- ture of the micelles occurs over this threshold W, 8.37.38 Experimental All chemicals, which were of the highest commercially avail- able quality, were obtained from either Sigma or Aldrich, and were used without further purification ; deionized water was used in the preparation of all solutions.The AOT-isooctane (<0.005% water) solution consisted of 0.1 mol dm-3 AOT in isooctane (W, = 0). Reverse micelles of varying W, values, containing spin-labelled or spin-trapped proteins were obtained by the injection method and gently stirred until completely transparent.',2 All sample flasks and EPR sample tubes were sealed to prevent changes in W, caused by evapo- ration; aqueous solutions were made up in 0.01 mol dm-3 ammonium acetate buffer, pH 6, unless otherwise stated.All the concentrations of added materials refer to those in the water-pool of the RM only, and are final concentrations. a-Chymotrypsin was spin-labelled at the Met-192 residue with 4-(2-bromoacetamido)-2,2,6,6-tetramethylpiperidin-l-y1-oxyl 1.' Separation of unreacted spin-label from labelled protein (total volume <2 cm3) was achieved by chromatog- raphy on Sephadex G-25 columns (100 mm x 10 mm) in ammonium acetate buffer and the protein fraction re-chromatographed (usually twice) until it contained no free spin-label, as ascertained by the absence of an isotropic EPR signal; the samples were then frozen in aliquots at -20°C until used. Protein concentrations were determined by the Biuret method using a kit obtained from Sigma.The Ser-195 residue of a-chymotrypsin was spin-labelled with 4-(ethoxy- fluorophosphinyloxy) -2,2,6,6- tetramethylpiperidin- 1 -yl -oxyl 2," purification and storage was as described above. The spin-labelled protein solution was subsequently injected in AOT-isooctane (W, = 0) solution to give RMs of differing W, values. Final protein concentrations within the water-pool of the RMs were between 5 x lo6 and 7 x lo-' mol dm-3. 2648 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Spin-trapped proteins, produced by radical damage, 13 P. Marzola, C. Pinzino and C. A. Veracini, Langmuir, 1991, 7, were either produced by reaction (at 293 K) of the protein (cytochrome c, 0.5 mmol dmP3; cc-chymotrypsin 1 mmol dm -3, with equimolar concentrations of FeSO,-thylene- diaminetetraacetic acid (EDTA) and H20, in the presence of the spin-trap 3,5-dibromo-4-nitrosobenzene sulfonic acid (2 mmol dm-3; DBNBS) in bulk solution; or they were pro- 14 15 16 238.P. Marzola, C. Forte, C. Pinzino and C. A. Veracini, FEBS Lett., 1991,289, 29. J. D. Morrisett, in Spin Labelling, Theory and Applications, ed. L. J. Berliner, Academic Press, New York, 1976, p. 274. M. J. Davies, B. C. Gilbert and R. M. Haywood, Free Radical Res. Commun., 1991, 15, 111. duced within RMs by injection of all components (as above) into the AOT-isooctane mixture with the H202 added last. In the case of radical-adducts produced from myoglobin (0.8 mmol dm-3) a similar procedure was adopted except that the FeSO, was omitted.EPR spectra were recorded at 291 K on a JEOL JES-RElX spectrometer with 100 kHz field modulation using X-band frequencies. For the reverse micelle experiments round silica sample tubes (internal diameter 4 mm) were used, whilst for aqueous samples, standard aqueous flat-cells were employed. Hyperfine coupling constants and A values were 17 18 19 20 21 22 V. M. Darley-Usmar, R. A. Capaldi, S. Takamiya, F. Millet, M. T. Wilson, F. Malatesta and P. Sarti, in Mitochondria, A Practi- cal Approach, ed. V. M. Darley-Usmar, D. Rickwood and M. T. Wilson, IRL Press, Oxford, 1987, p. 113. L. G. Sillen, E. Hogfeldt, A. E. Martell and R. M. Smith, in Stability Constants Supplement No. I, Special Publication No. 25, The Chemical Society, London, 1971.M. P. Pileni, T. Zemb and C. Petit, Chem. Phys. Lett., 1985, 118, 414. R. T. Dean, S. Gieseg and M. J. 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ISSN:0956-5000    

DOI:10.1039/FT9949002643

出版商:RSC    

年代:1994

数据来源: RSC

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J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2649-2652 EPR Data do not support the P=O Representation for Trialkyl Phosphates and Phosphine Oxides or Sulfides Uma S. Rai Department of Chemistry, Banaras Hindu University, Varanasi 221005,India Martyn C. R. Symons* Department of Chemistry and Biological Chemistry, University of Essex, Wivenhoe Park, Colchester, Essex, UK C043SQ Almost all representations of phosphates [(RO),P=O], trialkyl phosphine oxides (R,P=O) and related species include a double bond between phosphorus and oxygen. However, we strongly oppose the double-bond repre- sentation and maintain that it is misleading. EPR evidence for electron-loss centres are shown to be in accord with the structures (R),+P-0', with small, negative spin density on phosphorus.Also, electron addition shows that there are no low-lying vacant 3d orbitals available for the excess electrons, which occupy a largely non- bonding CT orbital with high spin-density in the 3s orbital on phosphorus. These results cannot be reconciled with any of the concepts which lead to the P=O formulation. It is almost universal practice to represent bonding between phosphorus and oxygen, sulfur, selenium or tellurium (X) as R,P=X. In a recent review, a variety of models for this bonding were described, and, despite much uncertainty as to the proper description of the 'real' nature of the bonding, it was firmly concluded that the double-bond formulation is correct: 'The PO bond is a double bond, formulated as P=O'.' Much weight is placed on the idea that these bonds are shorter and stronger than P-0 single bonds.NMR results are said to show that there is only a small positive charge on phosphorus, and IR results are also said to accord with expectation for the P=O formulation.' However, certain evidence was not cited in this review, nor elsewhere in this book on phosphorus compounds, namely, EPR data for related radicals. In our early studies, we argued that these results preclude significant double but these and later results have been ignored. The aim of the present study was to generate more radicals of the type R,P-X', (or R,P=X') having an unpaired elec- tron in a px orbital on ligand X. The results give further strong evidence for almost complete localisation on X, with no significant delocalisation, by any route, onto phosphorus. Results are also discussed for electron adducts, (R3PX)- which give no evidence for 3d-orbital participation, and which also disfavour the double-bond formulation.Experimental All compounds were of the highest grades available, and were not further purified. CFCl, was purified by passing it down an alumina column, followed by drying and deoxygenating it using oxygen-free nitrogen. Dilute solutions (ca. 1 : lo00 mole fraction) were irradiated at 77 K with doses in the region of lo3 Gy. EPR spectra were recorded on a Varian El09 X-band spectrom- eter using 100 kHz modulation. This was interfaced to an Archimedes computer. Samples were annealed in situ, with continuous monitoring of the EPR spectra, and recooled to 77 K whenever significant changes were observed.Results and Discussion The present results, together with EPR results from related studies, are given in Table 1. A typical EPR spectrum for Ph,PS'+ radical cations, is shown in Fig. 1. These results are -f This paper was presented at the 27th International ESR Con-ference at the University of Wales, Cardiff, 21st-25th March, 1994. unexceptional, and conform with our expectations for the species concerned. In this study, the Ph3PS'+ radical cations were formed by exposing dilute solutions of Ph,PS in CFC1, to y-rays at 77 K, since this is a standard procedure for pre- paring radical and no other product is expected. In our related studies of phosphine oxide radical cations, R,PO'+, hyperfine coupling to a single solvent chlorine nucleus was observed in addition to 31Pcoupling.In the present work, two centres were found for radical cations of Ph,PS, one showing a clear 31P doublet with a large Ag,, as expected for non-complexed Ph3PS'+, and the other, with a greatly reduced g-shift, showing complex features from hyper- fine coupling to 35Cl and 37Cl (Table 1). Thus, in this case, a borderline situation seems to apply. We have previously shown that, as the ionization potential decreases, the ten- dency to bond to chlorine is reduced.' So the sulfides should form weaker complexes than the oxides. The key result is that, for the uncomplexed radical cation (Table 1 and Fig.2), the ,'P hyperfine coupling is very small (ca. 22 G), and almost isotropic. This can be understood entirely in terms of a localised orbital on sulfur with spin polarisation of the P-S CT electrons (structure I). \ I p'-pl.. For irradiated (CH,),PO, well defined features for H2cP(0)(CH,), radicals were detected on annealing. The 'H and 31P hyperfine coupling constants were 22 and 33 G, Table 1 31P Hyperfine splittings for a range of R,P-X' centres ~~ 31P hyperfine coupling constants/G ~ ~~ Ph PS' + 22" ca. 20 ca. 20 ca. 20.7 +Ph3PS(CFC13)' ca. 25b ca. 25 25 ca. 25 (j5Cl) 56, (37)C1)46 +H$P(CH,)~SH 35 35 35 35 ('H) 27 18 18 21 18Sb 20.0 18.7 19.1 16 16 16 16 (H3C)zNPCN(CH3)2120C ('HI ---27.5 (I4N) 36 ca.0 ca. 0 ca. 12 g, = 2.13; g,, z g, z 2.002. * Single-crystal study (R. A. Serway, S. A. Mar-shall, J. A. Marshall and W. D. Ohlsen, J. Chem. Phys., 1969, 51, 4978). Irradiated hexamethyl phosphoramide (ref. 12). hl Fig. 1 First-derivative X-band EPR spectrum for a dilute solution of triphenylphosphine sulfide in CFC1, after exposure to ionizing radiation at 77 K, showing features assigned to (a)the parent radical cations, Ph,PS'+, (b) the solvent adducts thereof and (c) central fea- tures assigned to the phenyl-based cations, Ph'+ -P(Ph),S respectively, again establishing almost complete localisation of the singly occupied molecular orbital (SOMO) on carbon; this can be compared with the CH,cH, radical in which the CH, protons have hyperfine splitting constant, aiso= 22 G 1 3230 G 20 G 'H -1 0 +1 -1 0 +1 Fig.2 First-derivative X-band EPR spectrum for a dilute solution of trimethylphosphine sulfide after exposure to ionizing radiation and annealing, showing features assigned to H,c-P(CH,),SH+ radicals J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 whilst for 'CH,, a = 23 G, showing that delocalisation is indeed small. Evidence for Spin Localisation The importance of the EPR results given in Table 1 is that they all suggest strong localisation of the SOMO on the ligand (-0, -S, -cH2) rather than delocalisation onto phosphorus. In particular, for the -CH, unit, the 'H coup- ling constants suggest ca. 95% localisation on carbon, similar to the value for ethyl radicals (CH3-cH2).The difference between these results and those reported in ref. 1 is that hyperfine coupling data probe the wavefunction of the SOMO on phosphorus unambiguously. Since the SOMO for the radical cations must be comparable with the HOMO for the parent molecules, the results can safely be used to describe the HOMO of the R3PX molecules. In the case of oxygen and sulfur derivatives some distortion or solvent interaction is required to lift the orbital degeneracy of the pn or pnb orbitals. The large shifts in grr values, however, shows that these are still similar in energy. It could be argued that the filled orbital constitutes the double bond, whilst the half-filled orbital is indeed non-bonding. It is difficult to envisage this in terms of any theoretical models that have been recently described.8 Also for the H2e- PR, radicals there is only one potential orbital that can comprise a n bond, and that is the SOMO for the radical.The EPR results show that this orbital is not n-delocalised, and is no more delocalised than that in the ethyl radical. Note that for the carbonyl radical cations, R2C=O'+, there is a clear distinction between the 2pn orbital on oxygen and the in-plane 2p non-bonding orbital. This distinction is well defined in the EPR results for the radical cations, for which the SOMO is unambiguously the formally non-bonding 0rbita1.~ These orbitals are now well separated, as shown by the small g-shifts. Hyperconjugation The EPR spectra for ethyl and related radicals have been interpreted in terms of hyperconjugation, or Q--7t delocal-isation.' This involves partial electron donation from the o-frame to give bonding and anti-bonding orbitals, the latter being the SOMO.This type of delocalisation largely involves electron donation from the B(C-H) Q orbitals and hence is of major importance for carbocations, of significance for rad- icals, but is not important for carbanions. The alternative, sometimes called reverse hyperconjugation, involves donation from the n unit into the Q* orbitals. This is clearly what is required in the present case, since the formal bonding involves moving from R3P+-X- to R,P=X with partial electron transfer from X to the R3P unit. If, as is now under- stood from theoretical calculations,'.* p(n)-dn bonding is not important, then the alternative of reverse hyperconjugation needs to be involved, in order to justify the double-bond for- malism.For normal hyperconjugation there is a nodal surface close to the 'central' atom. Thus in radicals such as ethyl, the CH, carbon acquires very little spin density, whilst the CH, protons gain positive spin density. However, for reverse hyperconjugation, there is a nodal surface between the atoms of the Q bonds [e.g. carbon and phosphorus in (H,C),P-OJ so that in the radical [(H,C),P-O'+] positive spin will be acquired by both, and hence the ,'P contribution should be positive. Hence, for (CH3)3P-O'+ and related species, spin polarisation of the P-0 CJ electrons will put negative spin density onto phosphorus, whilst the pn-o* contribution will be positive.There is strong evidence that the net contribution is, in fact, negative.' ' J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 One possible orbital is that of e symmetry shown in struc- ture II.’ X /3 I 0 This shows clearly that both the ligands and phosphorus should gain spin-density, but that for phosphorus this is in the 3p, orbital. This should give rise to a significant aniso- tropic term in the hyperfine coupling. In fact, the coupling is almost isotropic in those cases where both parallel and per- pendicular splittings are well defined. Also, the maximum values lie along the z axis rather than along the x axis. The conclusion must be that delocalisation via this process is small.Another result that gives an interesting insight into struc- ture is that for the parent radical cation of dimethyl phos- phonate (structure 111).” H .+\ In H&O-P-O /H3CO If either hyperconjugation process were involved, the spin density on hydrogen should reflect this (see structure 11). We accept that if the p orbital on oxygen were to adopt an x axis position, which placed the P-H bond in the y-z plane, this argument would be invalid, but there seems to be no reason for such a specific preference. In fact, no ‘H hyperfine coup- ling was detected, which set an upper limit of ca. 5 G, corre-sponding to < 1% delocalisation. It is significant that this hydrogen readily migrated onto oxygen to give (MeO),POH + radicals,” which would be unlikely to occur if the hydrogen were strongly confined to the nodal plane. Yet another result that establishes ca.unit spin density on one ligand is the ‘H and 14N hyperfine coupling for (CH,),NP[N(CH,),],O radicals form from hexamethyl phosphoramide.” Data are shown in Table 1. The 14N and ’H data are almost identical with those for (CH,),NH+ rad- icals. For these radicals, delocalisation is not expected to extend to the N-H bond except via spin polarisation, which puts negative spin density on H. The inference that this also applies to the HMPA radical is very strong indeed. Electron Addition An important experimental result that rules out the extensive use of 3d orbitals on phosphorus comes from the EPR results for electron adducts of the species under consideration.’ The contrast with comparable transition-metal complexes is clear; e.g.electron addition to Cr0,’- is into a pure 3dn orbital, with some delocalisation onto oxygen, and the EPR results are in full agreement.15 In marked contrast, electron addition to P043-,16,17 or (RO),PO-lts in a distor- tion of structure such that one 0-P-0 bond angle increases towards 180”, thus reducing the antibonding effect of the extra electron. This is in a formally non-bonding s-p hybrid orbital on phosphorus, with about equal 3s and 3p character (estimated from the ‘P hyperfine coupling).’ The o* character is retained in the form of some localisation, but only onto the two ‘axial’ ligands.Alternatively, in some cases a c* radical is formed of the type R,P-X- in which one specific P-X bond has elon- gated and hence trapped the electron in a localised c* orbital. 20~2’ These bending and stretching distortions, which increase the electron affinities of the parent molecules, do 265 1 not occur in the structures of the parents or their electron- loss analogues. The contrast between electron gain and loss can be judged by comparing the isotropic hyperfine splitting for a typical electron adduct ca. 700 G with that of an electron-loss centre from the same molecule (ca. 25 G). Also, the extent of anisotropy for the former (2B z 100 G) is far greater than those found for the latter (0-4 G).Conclusions These results confirm our previous conclusions, that n-type delocalisation is not important for (R,P-X’)’ systems. Are we justified in extrapolating back to the closed-shell mol- ecules, R,P-X (or R,P=X)? In view of the symmetry of the parent molecules, this extrapolation seems to be fully justi- fied. The original double-bond formulation, almost universal- ly used by chemists when X is oxygen, etc., originated when pn-dn ‘back-bonding’ was very much in vogue. It is, of course, of great importance for transition-metal complexes, and the use of a double bond for (L,V=O) systems, for example, is quite reasonable. However, current theoretical calculations now seem to rule out the involvement of 3d orbitak8 Hence, one would have expected that the double- bond concept would also be abandoned.Since it is still widely in use, attempts have been made to justify this in terms of the concept of c-n bonding, as discussed above. However, this is a universal phenomenon, so, if the double bond is intended to represent this, its use should be extended. One example is that of the N-oxides, R,N-0, which should, on this theory, be written as R,N=O. Alternatively, if R3N-O is accepted, then R,P-O should also be accepted. We advocate this in preference to the more cumbersome R,P+-O-. We stress that if the compound (R,P-OH)’ has a single bond (as claimed) then there is no case for inventing a double bond on proton loss. Again, it should be used for both or neither.Since we are all happy with positive charges on phosphorus, e.g. R4P+ ions, then why not with R,P+-O-? Clearly the o-electron densities will re-adjust to neutralise the charges extensively and no-one would expect to find unit charges experimentally: hence if it is wished to indi- 6+ 6-cate formal charges perhaps R,P-0 is best. Does it matter?. We believe it does, especially at the teaching level. Students have learned all about the o + n representation of bonds in, for example, carbonyl compounds, so when they see P=O they expect to be able to apply the same criteria, espe- cially with respect to reactivity of the double bond. However, such comparisons are most unsatisfactory. Finally, we should mention the formation of solvent adducts referred to above.These centres are members of the very important class of adducts that we have labelled o* rad-icals. These are sometimes said to have a ‘three-electron bond’. Their formation in freon systems is related to the fact that the solute radical cations have high electron affinities, so if their structures permit the formation of localised bonds, the problem is reduced by electron donation from a chlorine ligand of a solvent molecule into the o bond, leaving the unpaired electron in the corresponding o* orbital, e.g. struc-ture IV.7 \Q This reduces the spin density on oxygen and hence the 31P hyperfine coupling, but such reductions (always <50%), make no difference to the conclusions drawn herein. 2652 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 We thank the Commission of the European Communities for 12 R. James and M. C. R. Symons, J. Chem. SOC.,Faraday Trans. 2, financial assistance. 1990,86,2173. 13 I. S. Ginns, S. P. Misha and M. C. R. Symons, J. Chem. SOC., Dalton Trans. 2, 1973,2509. References 14 P. J. Krusic, W. Mahler and J. K. Kochi, J. Am. Chem. SOC., 1 2 3 4 5 6 7 8 9 D. G. Gilheany, in The Chemistry of Organophosphorus Com- pounds, ed. F. R. Hartley, Wiley, New York, 1992, vol. I1 p. 1. A. R. Lyons, G. Neilson and M. C. R. Symons, J. Chem. SOC., Faraday Trans. 2, 1972,68,1063. S. Subramanian, M. C. R. Symons and W. H. Wardale, J. Chem. Soc. A, 1970, 1239. S. Subramanian and M. C. R. Symons, J. Chem. SOC. A, 1970, 2367.M. C. R. Symons, Chem. SOC. Rev., 1984,12,393. M. Shiotani, Magn. Reson. Rev., 1987, 12, 333. T. Clark, A. Hasegawa and M. C. R. Symons, Chem. Phys. Lett., 1985, 116,79. See, for example, E. Magnusson, J. Am. Chem. SOC., 1990, 112, 7940. P. J. Boon, M. C. R. Symons, K. Ushida and T. Shida, J. Chem. 15 16 17 18 19 20 21 1972,94,6033. N. Bailey and M. C. R. Symons, J. Chem. SOC., 1957,203. H. Lozykowski, R. G. Wilson and F. Holuj, J. Chem. Phys., 1969,51,2303. M. C. R. Symons, J. Chem. Phys., 1970,53,857. D. Nelson and M. C. R. Symons, J. Chem. SOC., Perkin Trans. 2, 1977,286. M. C. R. Symons, in Chemical and Biochemical Aspects of Elec-tron Spin Resonance Spectroscopy, Van Nostrand Reinhold, Wokingham, 1978, p. 30. A. R. Lyons and M. C. R. Symons, J. Chem. SOC., Faraday Trans. 2,1972,68,1589. A. Abu-Ragabah and M. C. R. Symons, J. Chem. SOC., Faraday Trans. 2, 1990,86,3293; M. C. R. Symons and R. L. Peterson, J. Chem. SOC.,Faraday Trans., 1,1979,75,216. Soc., Perkin Trans. 2, 1984, 1213. 10 M. C. R. Symons, J. Chem. SOC., 1959,277. 11 A. Begum, A. R. Lyons and M. C. R. Symons, J. Chem. SOC., Faraday Trans. 2, 1967,1770. Paper 4102250H; Received 14th April, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002649

出版商:RSC    

年代:1994

数据来源: RSC

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J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2653-2662 31P and IH Powder ENDOR and Molecular Orbital Study of a Cog3- -Ion in X-Irradiated Carbonate containing Hydroxyapatitest Peter D. Moens," Freddy J. Callens1 and Paul F. Matthys Laboratory of Crystallography and Study of the Solid State, Krijgslaan 28161, B-9000 Gent, Belgium Ronald M. Verbeeck Laboratory for Analytical Chemistry, Krijgslaan 281612,B-9000 Gent, Belgium An X-irradiated synthetic carbonate-containing apatite powder is examined with EPR and ENDOR. At low micro- wave powers, the room-temperature EPR spectrum contains a major contribution of a signal with g values: g, = 2.0045, gy= 2.0034 and g, = 2.0014. In a related l3C-enriched sample, the radical was shown to exhibit a hyperfine interaction with one carbon nucleus.The l3C hyperfine tensor values are: A, = 263MHz, A,, = 263 MHz and A, = 423 MHz. The radical is assigned to a co33-molecular ion. It is demonstrated by means of CNDO/II and INDO calculations that by lowering the symmetry of the Co,,- ion from C,, to C,, an orthorhombic g tensor can be obtained. However, the deviation from axial symmetry for the 13C hyperfine tensor is so small that it is not measurable on a powder specimen. The thus-calculated spin-Hamiltonian parameters are in very good qualitative and quantitative agreement with the experimental ones, adding strong evidence for the assign- ment of the observed signal to a Co,,-radical. At low temperatures, both 31P and 'H ENDOR spectra are recorded for different settings of the magnetic field (i.e.when the magnetic field is swept through the EPR CO,,- spectrum). By a careful analysis of the ENDOR powder spectra using computer simulations based on the 'orientation-selection ' principle, a detailed model for the C033- ion could be proposed. In this way, it is established unambiguously that the co,3-ion substitutes for a phosphate group in the hydroxyapatite lattice, with a vacancy on the nearest hydroxy-group site. In addition, some deductions can be made about the substitution mechanism according to which the precursor of the COS3- radical (i.e.a carbonate ion) is incorporated into the apatitic lattice. Hydroxyapatite, Ca,o(PO,),(OH),, forms the basic mineral of calcified tissues such as bone, dental enamel and renal stones.These biological apatites, however, contain some impuiity ions among which carbonate is the most important. As tnere exists a correlation between the amount and the location of the carbonate ions in the hydroxyapatite lattice, and the demineralization process of these calcified tissues,'*2 a study of the incorporation of C032-into hydroxyapatite is of value from a medical point of view. Two valuable tools for studying the magnetic properties of carbonate ions located in the apatite lattice are electron para- magnetic resonance (EPR) and electron nuclear double res- onance (ENDOR). However, as both techniques are able to detect only paramagnetic radicals and as the carbonate ion itself is not paramagnetic, the apatite samples have to be irra- diated in order to create paramagnetic species.In the past decades, biological as well as synthetically pre- pared carbonated apatites have been studied intensively with EPR.3-18 After X-or y-irradiation, the EPR spectra observed in the biological apatites consist of an asymmetric signal around g = 2.00. In synthetic apatite powder, not enriched in the isotopes 13C and/or 170, a very similar EPR signal was observed, indicating that related species were formed upon irradiation. Already from the earliest measurements, it became clear that the observed EPR signal was composite, i.e. it consisted of several contributions arising from radicals differing in their molecular structure (0-,03-,CO,-, co33-, ...) and location [hydroxy group (A site), phosphate (B site) and surface site allocation].As the EPR signals of the different radicals overlap considerably, the overall EPR spec- trum is very complex. Deducing the spin-Hamiltonian parameters of the different contributing signals from such ~~ t This paper was presented at the 27th International ESR Con-ference at the University of Wales, Cardiff, 21st-25th March, 1994. ++ Senior Research Associate of the NFSR (Belgium). complex spectra is not a trivial task. It was only very recently that a multivariate statistical method was proposed for ade- quate decomposition of the complex EPR powder spectra observed in X-irradiated carbonated hydroxyapatites. Moreover, the synthetically prepared hydroxyapatite speci- mens offer the possibility of enriching the samples in the iso- topes I3C and/or 170.In this way, valuable information about the identity and the electronic structure of the different paramagnetic species can be obtained. Once the spin-Hamiltonian parameters of the distinct rad- icals are determined, it is in principle possible to deduce the nature of the radicals, e.g. by comparing the experimentally obtained spin-Hamil tonian parameters with those resulting from the literature or from theoretical calculations. That it is not a straightforward task to identify the radicals unam-biguously is illustrated by the contradictory conclusions drawn by several research groups concerning the radical responsible for the most intense and stable signal in both bio- logical and synthetic hydroxyapatites.This radical is charac- terized by the following g values: g, = 2.0030, g, = 1.9970, g, = 2.0015, while the values for the 13C hyperfine tensor are A, = 459 MHz, A, = 445 MHz and A, = 557 MHz. Some authors assigned it to a C0,- radi~al,".'~-'~ while others assigned it to a co33-This ambiguity is radi~al.'*~-~*l~~'~*~~ mainly due to the large isotropic 13C hyperfine interaction, typical for both types of radical. In the present study, a radical with spin-Hamiltonian parameters g, = 2.0045, g, = 2.0034, gz = 2.0014 and I3C hyperfine tensor values A, = 263 MHz, A, = 263 MHz, A, = 423 MHz, is observed in an X-irradiated carbonate- containing synthetic hydroxyapatite specimen.By means of theoretical calculations using two semi-empirical self-consistent field Hartree-Fock methods viz. the CNDO/II and the INDO method,21 the radical will be unambiguously iden- tified as co33-.In order to explain the orthorhombic char- acter of the g tensor, it has to be assumed that the molecular symmetry is lowered from C,, to C,, probably owing to the surrounding hydroxyapatite lattice. As a result of the theo- retical calculations, it will be proven that the stable and intense EPR signal observed in most apatite specimens cannot be identified as a Co,,-radical and hence has to be ascribed to a CO, -radical. Often, the nature of the different radicals is not of prime interest, but rather the location of the carbonate groups in the hydroxyapatite lattice.A way of obtaining such informa- tion is via the location of the paramagnetic centres derived from the carbonate ions upon irradiation. If the correspond- ing EPR signals have appropriate saturation characteristics, the interaction of the radicals with the surrounding nuclei can be studied quite profitably with ENDOR. With this tech- nique, it is, in principle and within certain approximations, possible to determine the type and the location of the inter- acting nucleus in the g tensor axes frame. Hence, detailed information about the environment of the paramagnetic centre can be obtained and thus the location of the species under study can be deduced. Up till now, there have been only a few reported cases where ENDOR was used to probe the location of the carbonate-derived radicals in irradiated hydroxyapatites.Sato7 recorded a structureless proton ENDOR signal in X-irradiated powdered human tooth enamel while monitoring the anisotropic C0,-EPR signal. Van Willigen et al.’ per-formed ENDOR measurements on the CO, -signal observed in human tooth enamel blocks. These authors detected a broad 31Psinglet and a ‘H doublet from which they deduced that the 31P and the ‘H nuclei had to be separated from the paramagnetic centre by at least 0.6 and 0.9 nm, respectively. From this they concluded that the C0,-radical had to be located at the surface of the apatite crystallites. Note that both authors erroneously ascribed the paramagnetic species to a Co,,-radical instead of to the C0,-ion.In two very recent publication^,^^,^^ the well known isotropic signal at g = 2.0007, attributed to CO,-, was investigated with ENDOR in synthetic apatite, synthetic monohydrocalcite and natural calcite (coral). From the ENDOR experiments, it could be deduced that the C0,-radical had to be located in the ‘occluded water’, i.e. a remnant of the aqueous solution from which the samples were precipitated, trapped between the crystallites. In conclusion, only radicals at the surface of the apatite crystallites or in the occluded water have been located with ENDOR so far. No firm evidence has yet been presented for radicals located in the bulk of the apatite crys- tallites, i.e. radicals located at a hydroxy-group site (A site) or at a phosphate site (B site).In this paper, ENDOR evidence will be presented for a CO,,-radical located at a phosphate site, with a vacancy on the nearest hydroxy-group site. Furthermore, we will be able to draw some conclusions about the substitution mechanism according to which the precursor of the Co,,-radical is incorporated into the hydroxyapatite lattice. To our know- ledge, this is the first time that the location of a carbonate-derived radical in the hydroxyapatite lattice has been determined unequivocally. Orientation-selection Principle in Powder ENDOR General Theory The different aspects of EPR powder spectra are well estab- li~hed.~~-,~In powder systems a large number of crystallites are randomly distributed, each crystallite still behaving like a monocrystal. As a result, the main axes of the crystallites (and hence also the molecular axes of the paramagnetic defects) can have any arbitrary orientation with respect to the mag- netic field vector.Although the magnetic field has a constant 3. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 direction in experimental practice, it is more convenient to assume that the magnetic field vector can have any arbitrary orientation in a certain molecular axes frame, the latter remaining fixed. For the cases to be considered in this paper, the electronic Zeeman interaction is the dominant term in the spin Hamiltonian and thus the coordinate system in which the g tensor is diagonal is an appropriate reference system.This is illustrated in Fig. 1. The EPR spectra of polycrystalline materials reflect a ‘powder’ average of all molecular orientations (each molecu- lar orientation being defined by a set of 8 and 4 values) with respect to the applied magnetic field. Every single molecular orientation has one or more contribution to the EPR powder spectrum, depending on the values of MI and M;. This can readily be seen from the following equation which holds for systems with S = 4and is correct to first order: Here, B is the Bohr magneton, A(8,4) is the hyperfine coup- ling of the unpaired electron with a nucleus belonging to the radical and A”(&4) is the hyperfine coupling with the nucleus not part of the paramagnetic species (superhyperfine coupling).g is the effective g value, vEPR is the microwave frequency and h is Planck’s constant. The g and A values are calculated as:27 (3) with a similar expression for the superhyperfine tensor. In eqn. (2) and (3), hi (i = 1-3) denote the direction cosines of the magnetic field vector in the g tensor axes frame. Eqn. (1) together with (2) and (3) indeed reveal that every molecular orientation has several contributions to the EPR powder spectrum. For ENDOR, the situation is different, as during the ENDOR experiment the magnetic field is kept fixed to a certain value. One has to perform the inverse calculation, i.e. to find the molecular orientations that are in resonance for a lgz7I /I n * I 0“ \ i -I I I \ Fig. 1 Definition of the different polar angles describing the mag- netic field vector and the direction of the interacting nucleus in the g tensor axes frame J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 given value of B,,,. The sets of (8, 4) values corresponding to for the superhyperfine tensors), the field direction [h,, h, , h3] a B,,, value can be calculated using a computer. In this way, and the field magnitude B,, as well as on the position of the it becomes clear that the applied magnetic field makes a8, (I,tensor axes frame gnucleus in the and t#)N, see Fig. 1). selection in the molecular orientations contributing to the ENDOR spectrum. The 'orientation-selection' principle was first suggested by Rist and Hyde28 and forms the basis of the interpretation of powder ENDOR spectra.Once the molecular orientations, selected by a given mag- netic field value, are determined, the nuclear resonance fre- quencies can be calculated. To first order, one has:27 ~VNM, = K(Ms) (4) with and PN and gN denote the nuclear magneton and the nuclear g factor (assumed to be isotropic), respectively. B, is the reson- ance field value and Ms the electronic quantum number. In powder ENDOR, usually the superhyperfine interaction is of interest. Hence, one has to use the tensor A" in eqn. (5). The matrix A" is the sum of the isotropic (A:,,) and the aniso- tropic superhyperfine interactions (A:). Assuming that the anisotropic interaction between the electron and nuclear spins is governed by a pure dipole-dipole interaction, one can write : where = & 47r (y) is a constant for each type of nucleus (3'P, 16.02 MHz; 'H, 39.78 MHz). ri (i = 1-3) denote the direction cosines describ- ing the direction of the interacting nucleus in the g tensor axes from (rl = cos 4Nsin ON, r2 = sin 4Nsin ON, r3 = cos ON), see also Fig.1. The distance between the nuclear and electron spins is r. When the g tensor anisotropy is small compared with the average g value, the electron and nuclear spins are quantized nearly along the same direction (the direction of the applied field) and hence eqn. (7) simplifies to: SUNA"(y) = -(3 cos2 y -1) + AYSOr3 y being the angle between the direction of the applied mag- netic field and the direction connecting the electron and nuclear spins.The final expression for the ENDOR frequencies is obtained by substituting eqn. (5) and (6)in eqn. (4).By doing so, one obtains in frequency units: with bN gN BOv, = -h Eqn. (9) shows that the ENDOR frequencies depend on the electronic g values, the type of nucleus (aNin the expressions As both B, and [h,, h,, h3] vary when the magnetic field is swept through the EPR powder spectrum, the ENDOR fre- quencies will also vary with the magnetic field. As the ENDOR frequencies depend on the position and type of the interacting nucleus, they give information not only on the electronic but also on the geometrical structure of the species studied. This method is called 'ENDOR crystallography' and is based entirely on the assumption that the anisotropic hyperfine coupling is purely dipolar.,' In many cases, the interacting nuclei are located far away from the paramagnetic centre and hence the dipolar inter- action becomes small. When it becomes smaller than the ENDOR linewidth, the ENDOR spectrum will consist of one or more isotropic resonances centred around the nuclear Zeeman frequency, depending on whether an isotropic hyper- fine splitting is present or not.This phenomenon is called distant ENDOR.30 A more profound outline of the theory of powder ENDOR can be found in the articles of Hoffman et ul.,31,32Hurst et ~l.,~~Henderson et and Greiner et u1.35,36as well as in the recent review article of Huttermann.37 From this point, we will consider only the cases relevant for this paper: systems with very small g tensor anisotropy [(gl -g3)/gavd O.OOl] exhibiting only superhyperfine interaction (and no hyperfine interaction).The ENDOR powder spectra observed in such systems were discussed in some detail by Moen~.~~ In the following two paragraphs, a short outline will be given of the methodology for analysing ENDOR powder spectra of species with very small g tensor anisotropy. The computer program for simulating ENDOR powder spectra will be discussed below. Systems with an Axial g Tensor When dealing with systems with an axial g tensor, the direc- tion of the magnetic field vector in the g tensor axes frame is determined only by the polar angle 8.Thus, when the mag- netic field has a value B(8), only those crystallites for which the angle between the direction of the applied magnetic field and the gllaxis is equal to 8, will be in resonance. For some field positions, one obtains 'single crystal-like' ENDOR powder spectra, comparable to those of an oriented single crystal. The angle 8 can easily be calculated using the follow- ing equation : with where B, is the magnitude of the applied magnetic field. When the g tensor anisotropy is small compared with the average g value, the difference between eqn. (7) and (8) is neg- ligible and hence, eqn. (8) can be applied to obtain good initial estimates for Aiso, r and 8,. Indeed, from eqn. (8), it follows that the largest splitting is found when the magnetic field vector is parallel to the axis connecting the electron and nucleus (y = 0O)assuming A:,~z O or A:,, < O and geaN/r3z I 2AYsOI.In this way, an estimate for 8, can be obtained using eqn. (11): the field value for which the largest splitting is found in the ENDOR powder spectrum gives the value for 8, (as 8 = 8, when the magnetic field is parallel to the axis con- necting the interacting nucleus and the paramagnetic radical). From eqn. (8), it follows that: gaNy = 0"-+ Av1 = AFs0+ 2 -r3 (13) gaNy = 90" +Av, = AEo --r3 or Avl + 2Av,A? 3 (1 5)1so = where Avl is the splitting measured in the ENDOR powder spectrum corresponding to y = o",whereas Av2 is the split- ting corresponding to y = 90".The two unknown variables Arso and r can be readily obtained from eqn. (15) and (16). The signs of the splittings, however, are not a priori known: they have to be determined using simulation^.^^ When A:& < 0 and g, aN/r3< I2ArS01, the largest splitting is observed for y = 90" corresponding to the situation in which the largest splitting is measured along the direction perpendicular to the axis connecting the electron and the nucleus. However, cases with such large negative Aim values have not occurred so far in the literature. The values for the parameters A&,, r and 8, obtained using the method described above are used as initial estimates in the simulation of the ENDOR powder spectra, using a self- written computer program (see below).By varying the three parameters independently and comparing the simulated spectra with the experimental ones, the set of parameters giving the best agreement between experimental and simu- lated spectra, for all field settings, are retained as the real ones. Systems with an Orthorhombic g Tensor In systems with an orthorhombic g tensor, the 4 and 4N angles are also relevant. In cases with a small g tensor anisot- ropy, however, it has been shown that one can still determine rough initial estimates for the parameters Aiso, r and 8, by assuming an axial g tensor.38 The thus-obtained values, together with the 4, parameter then have to be optimized by an iterative procedure. In the simulations, of course, an orthorhombic g tensor is used.Powder ENDOR Simulation (PENSI) Program In order to simulate ENDOR powder spectra for systems with S = 4,I = 0 and I" = 4, a program was developed using the formulae given above. The input parameters include the frequencies, v1 and v,, between which the ENDOR powder spectra will be calculated, the spectral resolution, Av, the principal g tensor values, the position and the type of the interacting nucleus in the g tensor axis frame (r, ON, &), the isotropic superhyperfine splitting, the homogeneous EPR linewidth, rEpR, the ENDOR linewidth, rEND, and the lineshape (Lorentzian or Gaussian). Then, eqn. (1) is calculated for 0 < 8 < 71 and 0 d # < 271. All the thus-calculated B values (two for each molecular orientation) are stored in the matrices BREsland BREs2 of which the column and the row numbers correspond to the values of 8 and 4, respectively. The field value, B,,,, for which the ENDOR powder spec- trum has to be calculated, is used as an input parameter, assuming that vE~R= 9.47 GHz. Using the homogeneous EPR linewidth as the FWHM, a Lorentzian is constructed J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 with mean B,,,. All the elements of the matrices BREs1 and BRES2 are weighted with this Lorentzian and are given a cor- responding amplitude. Only the sets of orientations, (8, 4),, for which the amplitude is greater than or equal to 0.05, are retained. For the sets of molecular orientations (8, 4), thus selected, the ENDOR frequencies are calculated using eqn.(9), making use of eqn. (7) for the superhyperfine coupling (thus allowing for orthorhombic g tensors). Centred on these resonance fre- quencies, a lineshape (Lorentzian or Gaussian) is placed with linewidth rEND. The final ENDOR powder spectrum is obtained by summing up all of the calculated ENDOR spectra for the selected molecular orientations with their cor- responding weight factor. Finally, the ENDOR powder spec- trum is displayed and can be stored on disk. If applicable, another ENDOR powder spectrum can be calculated for another field value with the same input parameters. In this way, the matrices BREs1 and BREs2 need not to be calculated afresh. The PENSI program was run on an HP-Apollo 423 work- station. The calculation of one ENDOR powder spectrum for typically 200 abscissa points takes ca.5 min. Materials and Methods Materials The sample studied in this paper was prepared according to .~the method originally proposed by Legeros et ~ 1 According ~ to this method lg of commercial Mallinckrodt AR grade monetite, CaHPO,, was hydrolysed in 1 1 of Na,CO, solu-tion (0.250 mol 1-') at 95"C.15-17 The suspension was con- tinuously and thoroughly stirred for 5 h, and precautions were taken to prevent CO, contamination from the atmo- sphere. The precipitate was then filtered off and washed thor- oughly with hot distilled water. After drying at 400 "C under vacuum until constant weight, the solid was analysed for its Ca, P and CO, content.Calcium was determined by com- plexometric titration with EDTA after separation of the phosphate. Phosphorus was analysed as phosphate using a slight modification of the spectrophotometric method of Brabson et aL4' Carbonate was determined by coulometric titration of the CO, evolved from an acidified aqueous solu- tion of the apatite. The sample contained 35.64 (kO.07) wt.% Ca, 12.38 (k0.02) wt.% P, 4.11 (k0.02) wt.% Na and 21.0 (&0.4) wt.% CO, . The X-ray diffraction pattern and the IR spectrum show sharp and well resolved peaks, characteristic of crystalline solids. The X-ray pattern is typical of an apatite, and no extraneous peaks other than for apatitic could be found. In the IR spectrum, absorptions around 872 (k0.8),1417 (k2.5) and 1470 (f4.7) cm-' indicate that the CO,,- ions occupy Po4,-lattice sites (B-type CO,),.Methods The EPR spectra were recorded using a Bruker ESP300 X-band spectrometer, with a maximum power of 200 mW. The magnetic field was modulated at 100 kHz with a peak-to-peak amplitude of 0.5 x lop4 T. All of the EPR spectra were normalized to the same frequency, i.e. 9.47 GHz, and hence can be directly compared. The magnetic field was measured using a Bruker NMR035M Gaussmeter. With this equipment it is possible to measure accurately the relative positions of the EPR signals present. Small shifts in the magnetic field positions down to 0.1 x T can be detected. For absolute g value determi- nation, a calibration using the g standard DPPH at 0.1 mW (g = 2.0036) was performed.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ENDOR spectra were recorded on the same spectrometer equipped with a Bruker ESP353E ENDOR/TRIPLE exten- sion (EN374 RF amplifier with a maximum power of 200 W, EN525 Schomandl synthesizer and an ER033M field-frequency lock unit). The best ENDOR signals were obtained with a microwave power of 0.2mW (27 dB) and 200 W radio frequency (RF) power (0 dB). The modulation depth was set to l00kHz and ten scans of 81 s each were run for each ENDOR spectrum. The irradiation of the sample was performed using a tung- sten anti-cathode Philips X-ray tube, operated at 60 kV and 40 mA, for 10 min, which corresponds to a dose of 13.2 kGy.Experimental Results EPR Results The sample studied in this paper was originally part of a series of ten samples, each having a different carbonate content (ranging from 8.12 to 21.0 wt.%). After X-irradiation, the samples exhibit strong resonances in the region around g = 2.00.'5-17 At low microwave powers (P < 0.1 mW), it has been shown that the observed EPR peaks exhibit contri- butions of several ~igna1s.l~ The two most important ones were labelled Z1 and A2'. The contribution to the overall 'low-power" spectra of the Z1 and A2' signals increases with increasing carbonate content of the samples as does the 21 : A2' ratio. As the sample studied in this paper has an extremely high carbonate content (21.0 wt.%), the low-power EPR spectrum exhibits mainly a contribution of the Z1 signal, with a small contamination of the A2' signal.Fig. 2 shows the EPR spectrum recorded at 0.01 mW, in which the Z1 and A2' signals are indicated. For the purpose of this paper, only the Z1 signal is of inter- est and hence we will restrict ourselves to this signal. The g values of the Z1 signal are obtained from a spectral decom- position of the observed 12C EPR spectra.17 The 13C hyper- fine tensor is obtained from a least-squares fit of the signal observed in a 13C-enriched ~amp1e.l~ In this way, one obtains: g, = 2.0045, gy = 2.0034, g, = 2.0014, A, = 263 MHz, A = 263 MHz and A, = 423 MHz. The computer fits of the 15C and 13C lineshapes are shown in Fig. 3 and 4, respectively.The Z1 signal remains visible down to 4 K, whereas the A2' signal is visible only at temperatures >50 K. In this way, the Z1 signal is found to be quite isolated at low temperatures. ". \\ 3370 3390 t3/10-4 T Fig 3 Computer fitting of the "C lineshape of the Z1 signal" ENDOR Results In order to obtain sufficient microwave saturation, the speci- men has to be cooled. The ENDOR resonances are visible from 4 K up to 80 K, with an optimum detection tem-perature of 8 K. Fortunately, at 8 K, the EPR powder spec- trum is not composite, i.e. only the Z1 signal contributes to the spectrum. In this way, it is ensured that the ENDOR res- onances are due to only one paramagnetic species. A typical ENDOR spectrum is shown in Fig. 5.As can be seen from this figure, the ENDOR spectrum consists of three peaks, centred around the nuclear Zeeman frequency of 3260 3510 q10-4 T Fig. 4 Computer fitting of the 3C lineshape of the Z1 signal" A2' iI,J , , , 0 3 0 1 20 t3/10-~T v/MHz 0.01 mW EPR spectrum recorded at room temperature. The Fig. 5 Typical ENDOR powder spectrum recorded at 8 K. For the different components are indicated in the figure. experimental conditions, see text. 23Na,31Pand 'H, respectively. ENDOR powder spectra are recorded for nine different magnetic field settings within the EPR powder envelope. It transpired that the 23Na hyperfine interaction is too small for discussion. Hence, only the angular variations of the 31P and the 'H interactions are studied in detail.' P Interaction Fig. 6 shows the angular variation of the 31Pinteraction. The most prominent feature is the broad and intense line centred on the nuclear Zeeman frequency of 31Pand which is assign- ed to the distant ENDOR signal. Fortunately, two small doublets (indicated by 1 and 2 in the figure) are also observed. We will confine our attention to these two doub- lets. The resonances of doublet 1 exhibit the largest splitting for g = 2.0043 (xgx). The splitting decreases only slightly with increasing g value and the smallest splitting is observed for g = 2.0018 (x9,). Following the procedure described above, Av, can be estimated from the ENDOR powder spec- trum at g = 2.0043 (largest splitting corresponding to y = 0), whereas Av, is obtained from the spectrum at g = 2.0018.Using an axial approximation for the g tensor (gl= 2.0045, g,, = 2.0014), one obtains 8,= 90", r = 0.40 nm and A:, = J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 g= 2.0024 5 7 v/M Hz Fig. 7 Theoretical angular variation for the resonances of doublet 1. For the values of the different parameters used in the simulation, see text. MHz, r = 0.49 (0.02) nm, 8, = 25" (k20").The value for 4, has almost no effect on the simulated spectra. The values of the other parameters are the same as those used in the simu- -0.23 MHz. The 4, value cannot be estimated from the lation of the resonances of doublet 1. The simulated spectra are shown in Fig. 9.experimental spectra.Using these values as input parameters in the PENS1 program, the following results were obtained The rather broad ENDOR lines of both doublets 1 and 2 suggest that several, not completely equivalent nuclei contrib- ute to the ENDOR spectrum. after some iterations: Ai, = -0.22 (L0.02) MHz, r = 0.39 + 90" ( fSo) and 4, = 30" (& lo"),rEPR= 0.5( f0.01)nm,8, x lov4T and rEND = 60 kHz. The theoretical angular varia- tion thus obtained, is shown in Fig. 7. From this figure, it can 'H Interactionbe seen that more than two resonances are present. The two outer (and strongest) resonances have the same dependence In contrast to the ENDOR spectra of the 31Pinteraction, the 'H resonances do not exhibit any resolvable anisotropic on the selected field value as the resonances of doublet 1; the other resonances are most probably obscured by the matrix ENDOR signal.Fig. 8 shows the experimental ENDOR powder spectra together with the simulated ones for three different field set- tings. The agreement between theory and experiment i.e. the reproduction of the resonance positions and intensities is satisfactory. The resonances of doublet 2 allow only a qualitative description as they are largely hidden under the matrix ENDOR signal. They become visible at g x 2.0034. The largest splitting is observed for g x 2.0025, corresponding to 8, x 30" (in an axial approximation for the g tensor). The fact that the angular variation cannot be followed completely results in an only roughly determined 8,.The value for 4N cannot be estimated at all. The spectra for doublet 2 are best simulated using the following parameters: Aiso= -0.1 (k0.1) 5 7 v/MHz v/M Hz Fig. 8 Comparison between the experimental and simulated 31P Fig. 6 Angular variation of the 31Phyperfine interaction. The ENDOR powder spectra for the resonances of doublet 1, for three doublets 1 and 2 are indicated in the figure. different magnetic-field settings J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 CT 0 n zw v/M Hz Fig. 9 Theoretical angular variation for the resonances of doublet 2. For the values of the different parameters used in the simulation, see text. hyperfine interaction. This is depicted in Fig. 10, where the two ENDOR spectra recorded for different magnetic field values are shown.The linewidth of the 'H ENDOR reson- ances is 140 kHz. As no anisotropic hyperfine splitting is observed (although carefully searched for), it has to be con- cluded that the splitting is smaller than the observed ENDOR linewidth. In this way, the distance between the paramagnetic centre and the nearest proton can be estimated to be at least 0.8 nm. Discussion The two most important features to be discussed are the nature and the site allocation of the Z1 radical. They will be treated separately. I%I i Y Y.LL 1 13 16 v/M Hz Fig. 10 'H interaction measured for the Z1 signal for two different magnetic-field settings at 8 K Nature of the Observed Species The EPR results reveal that the Z1 signal exhibits hyperfine interaction with one carbon nucleus and hence, the radical responsible for the observed signal must contain one carbon atom.Possible candidates are CO-, CO,-, CO,-and CO,, -molecular ions. The large isotropic 13C hyperfine interaction (Aiso= 316.3 MHz) excludes the CO,- ion, as this ion is characterized by a very small (and negative) 13C iso- tropic hyperfine value (Ais,,z -40 MHz).~'-~, On the same basis, the CO- ion can be excluded.44 Both the C0,- and Co3,-ions have large isotropic 13C hyperfine coupling con- stants and thus can be assigned to the Z1 radical. The free Co,,-ion has a pyramidal structure with C,, symmetry exhibiting axial spin-Hamiltonian parameter^.^^.^' Theoreti-cally, both the gL and the gll values should be larger than ge, with gL > glr,whereas for the 13C hyperfine tensor All > A,.However the C0,- ion has C,, symmetry giving rise to orthorhombic g and A tensor^.^'-^* The C0,- ion has very characteristic principal g tensor values, with gy z 1.997 (corresponding with the 0-0 axis). Because all g tensor values of the Z1 radical are quite dif- ferent from 1.997, this radical cannot be attributed to a C0,- ion. Thus, only the Co,,- ion is left as a possible candidate. The axial I3C hyperfine tensor strengthens this hypothesis. However, the Z1 radical exhibits an orthorhombic g tensor with one g tensor value somewhat lower than g,, in contrast with what would be expected. In order to elucidate this matter, theoretical calculations using two semi-empirical self-consistent field Hartree-Fock methods, viz.the CNDO/II and the INDO methods, were performed for the co33-molecular ion. The CNINDO program of Pople and Beveridge,' was adapted in order to calculate the g and A tensors. Only the I3C hyperfine tensor is reproduced as there are no experimental data available for the I7O hyperfine tensor. The g values are calculated using Stone's formula.49 In the theoretical calculations, three struc- tural parameters are varied independently: (1) the bond dis- tance, d, between the central carbon atom and the three oxygen atoms; (2) the distance, dl, between the carbon atom and the plane defined by the three oxygen atoms; (3) the bond angle, a,between two of the oxygen atoms [O(l) and 0(2)]. By varying a, the symmetry of the Co,,- ion is lowered from C,, to C,.The convention of the different molecular axes for the co,3-ion is as follows: the z axis is perpendicular to the plane of the three oxygen atoms, the y axis connects the two equivalent oxygen atoms (making an angle a), the x axis is perpendicular to both the y and the z axes. The results of the theoretical calculations are sum-marized in Table 1. Note that lowering the symmetry of the Co,,-ion from C,, to C, changes the g and A tensors from axial to orthorhombic. One principal g tensor value is around 2.0045, while the gy can vary between 2.0032 and 2.0043. However, the g, is somewhat lower than ge. On the other hand, the deviations from axial symmetry for the 13C Table 1 Results of the CNDO/II and INDO calculations for the C0,3- ion CNDO/II INDO 2.0045 (2.0041, 2.0046) 2.0044 2.0041 (2.0043, 2.0035) 2.0037 2.0022 (2.0023, 2.0020) 2.0020 286.9 (215, 485) 332.0 -52.0 (-44,-55) -56.5 -52.2 (-44,-55) -56.7 104.2 (88, 110) 113.2 Z1 radical (2.0042, 2.0046) 2.0045 (2.0042, 2.0032) 2.0034 (2.0023, 2.0016) 2.00 14 (216, 566) 316.0 (-60, -48) -53.0 (-60, -48) -53.0 (120, 92) 106.0 ~~ ~~ ~~ ~~~~ The geometry used in the calculations is: d = 0.1 19 nm, d, = 0.006 nm and a = 112" for CNDO/II; d = 0.1 14 nm, d, = 0.005 nm and a = 112" for INDO.The values in parenthese denote the limits between which the spin-Hamiltonian parameters vary when the molecular geometry is varied within the following regions: d, 0.110-0.121 nm; d,, 0.003-0.007 nm; c1, 110"-118" for CNDO/II; d, 0.110-0.1 15 nm; d,, 0.004-0.01 nm; a, 110"-118" for INDO.The hyperfine parameters are in MHz. 2660 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 \-*--@ \ \ @\ \ @--+--@ Q \ Q ances of doublet 1. The two NN 31P nuclei are situated in the mirror plane above and the mirror plane below the mirror plane in which the carbon nucleus of the co33-radical is situated (further indicated by the Co,,- mirror plane). The plane of the three oxygen atoms of the Co,,- radical (further denoted as the molecular plane) is oriented parallel with the hexagonal c axis of the hydroxyapatite crystallite, such that the two NN 31Pare lying in the molecular plane.The gx axis is assumed to be parallel with the hexagonal c 8, axis of the hydroxyapatite. In this way, the NN 31P nuclei is 90°, whereas the 4, angles for these two angle for the \ equivalent nuclei are ca. 30". All these data agree very well with the experimental ENDOR results for doublet 1. This model is depicted in Fig. 11. The eight next nearest neighbour (NNN) 31P nuclei are all situated at a distance of ca. 0.5nm:" four are situated in the co,3-mirror plane, two are situated in the mirror plane below and two in the mirror plane above the CO,,- mirror plane. The angles between the axis connecting the paramagnetic species and the distinct sur- rounding nuclei, and the gz axis of the co33-ion, vary between 10" and 50" for the different NNN nuclei when the above model for the co,3-is considered (see Fig.11). These data agree well with the data of doublet 2. The NN and the NNN 'P nuclei are indicated in Fig. 11 with indices 1 and 2, respectively. In view of the model described above, the NN 'H nucleus would have to be located at a distance of 0.35 nm, in contra- diction with the experimental ENDOR data. Moreover, the NNN protons, located at 0.50 nm, also seem to be removed. The question remains whether the hydroxy-group sites are occupied by vacancies or by carbonate ions (A-type carbonate). However, as no IR evidence is present for A-type carbonate ions in this ample,^' the hydroxy-group sites are probably occupied by vacancies.The 'H ENDOR data will be discussed in more detail in the next section. The ,'P ENDOR data strongly suggest a B-site allocation for the C0,3-ion. An A-site allocation for the radical can be ruled out as, in this case, the NN ,'P should be located at 0.35 nm," too small compared with the distance of 0.39 nm determined experimentally. In the case of an A-type ion, the NN 'H nuclei should be located at 0.34 nm. In addition, a strong correlation between the intensity of the EPR Z1 signal and the integrated IR absorption at 873 cm-' (typical for B-type carbonate), was observed in a series of related samples." Hence, the Z1 Co3,-radical is located at a B site, substituting for a phosphate group. The positions of the NN protons, however, still remain to be elucidated.Substitution Mechanism of the Precursor of the Coj3-Radical Returning to the basic question, viz. the location of the car- bonate ions and how the latter are incorporated in the hydroxyapatite lattice, one has to try to establish a corre-lation between a certain suggested substitution mechanism for the carbonate ions and the signal height of a certain EPR signal in a series of related samples. In our case, the Z1 signal is attributed to a B-type radical, and hence, we have to look for substitution mechanisms accounting for B-type substitut- ions. A possible mechanism is the one originally suggested by Labarthe et al. :52 Ca2+ +PO4,-+OH--V, +C03'-+V,, V, denotes a vacancy on an x site.According to this mecha- nism, one Po4,-ion is substituted by one C0,2- ion, with the loss of a Ca2+ and an OH- from the immediate vicinity. This situation is depicted in Fig. 11, i.e. a B-type radical with a vacancy on a Ca2+-OH- site. Furthermore, as the sample \ t3 Q\ o @OH c OP(Z=-~/~) 1/4, Z=3/4)~P(z= Fig. 11 Model of the C033-radical in the hydroxyapatite lattice. The nuclei with z = t are situated in the C033-mirror plane, whereas the nuclei with z = -iand z = $ are lying in the mirror planes below and above the C0,3-mirror plane, respectively. hyperfine tensor are so small that they are not measurable for powder systems. All these observations are in very good agreement with the experimental data, qualitatively and even quantitatively.Hence there can be no doubt about the nature of the Z1 radical, i.e. it is a co,3-radical. The distortion from C,, to C, symmetry most probably arises from the sur- rounding hydroxyapatite lattice. Consequently it is evident that the intense and stable EPR signal observed in X-irradiated human tooth enamel (with one g value of 1.997)cannot possibly be ascribed to a co33-radical, but has to be identified with a C0,- ion. However, many authors have made the wrong assignment. Site Allocation of the Paramagnetic Species Whereas the nature of the paramagnetic radical results from the EPR measurements, the location of the co,3-radical can be deduced from the ENDOR data. A detailed descrip- tion of the crystal structure of hydroxyapatite is given by Kay et al.O In our opinion, the ENDOR data can be explained only when the Co,,-ion is located at a B site, i.e. substituting for a phosphate group, since the distance from a B site radical to the nearest neighbour, (NN) phosphorus nuclei is 0.40 nm, as was derived from neutron diffraction experiments," com-pared with 0.39 nm determined experimentally for the reson- 1.E Y J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 studied in this paper has a large amount of carbonate ions, relative to the number of phosphate groups (for this sample, the ratio is ca. 1 : lS1),it is highly probable that one or more of the six phosphate ions situated around the hexagonal c axis in the mirror planes above and below the CO,,-mirror plane are also substituted by a carbonate ion with the accom- panying creation of a vacancy on a hydroxy-group site.In this way, the nearest proton can be located far from the para- magnetic centre (r > 0.84nm), in agreement with the experi- mental data. In addition, a detailed physical and chemical analysis of a series of related samples revealed that the B-type carbonate ions were mainly incorporated by means of the above-mentioned substitution mechani~m.~ Fig. 12 shows the contribution of the above-mentioned mechanism (as determined from the chemical analysis) plotted against the amplitude of the Zl EPR signal for a series of related samples, showing a positive correlation. Hence, it has to be concluded that the CO,,-ion is located on a B site with a vacancy on the NN hydroxy-group site.Conclusion The EPR signal observed in an X-irradiated synthetic car- bonate containing hydroxyapatite powder is assigned to a CO,,-molecular ion. This defect is characterized by the fol- lowing g tensor, g, = 2.0045, g,, = 2.0034, g, = 2.0014, and 13C hyperfine coupling tensor, A, = 263 MHz, A, = 263 MHz, A, = 423 MHz. The co,3-ion normally exhibits C,, symmetry and hence should have axially symmetric spin- Hamiltonian parameters. However, by lowering the sym- metry of the Co,,-radical from C,, to C,, an orthorhombic g tensor can be obtained, whereas the deviation from axial symmetry for the hyperfine tensor is negligible. The spin- Hamiltonian parameters of a deformed Co,,-ion with low symmetry were calculated using both the CNDO/II and the INDO methods, yielding results in very good qualitative and even quantitative agreement with the experimental findings.The lowering of the symmetry of the molecular ion most probably results from the incorporation of the radical in the hydroxyapatite lattice. In order to establish the site allocation of the Co,,-ion in the hydroxyapatite lattice, ENDOR studies were performed. At low temperatures, ENDOR resonances due to interactions with 23Na, ,'P and 'H nuclei were observed. The experimen- tal ,'P powder ENDOR spectra could be interpreted in terms of two sets of nuclei; the interactions with the NN as well as with the NNN phosphorus nuclei were resolved. On the other hand, the 'H ENDOR spectra revealed no aniso- tropic interactions, indicating that the distance of the nearest proton is at least 0.8nm.The 23Na interactions were too small to be discussed. By means of the ENDOR data, a precise model for the co,3-ion could be presented. In this way, it was established unambiguously that the radical has to be located on a B-site, i.e. substituting for a phosphate group, with a vacancy on the nearest hydroxy-group site. In addition, some conclusions could be drawn about the substitution mechanism by which the precursor of the radical (i.e. a carbonate ion) was incorp- orated into the hydroxyapatite lattice. The authors thank the Interuniversitair Instituut voor Kern- Wetenschappen (IIKW) and the Executieve van de Vlaamse GemeenschapDepartement Onderwijs for financial support.References G. Cevc, P. Cevc, M. Schara and U. Skaleric, Nature (London), 1980,286,425. 266 1 2 F. C. M. Driessens and R. M. H. Verbeeck, Biominerals, CRC Press, Boca Raton, FL, 1990. 3 J. D. Termine, I. Pullman and A. S. Posner, Arch. Biochem. Biophys., 1967, 122, 318. 4 P. Cevc and M. Schara, Radiat Res., 1972,51, 581. K. Ostrowski, A. Dziedzic-Glocawska, W. Stachowicz and J. Michalik, Clin. Orthop., 1973,97, 213. 6 R. A. Peckauskas and I. Pullman, Calcif: Tissue Res., 1978, 25, 37. 7 R. Sato, Calcif: Tissue Int., 1979,29,95. 8 H. J. Tochon-Danguy, M. Geoffroy and C. A. Baud, Arch. Oral Biol., 1980, 25, 357. 9 H. Van Willigen, A. H. Roufosse and M.J. Glimcher, Calcif. Tissue Int., 1980,31, 70. G. Bacquet, Vo Quang Truong, M. Vignoles, J. C. Trombe and G. Bonel, Calcif. Tissue Int., 1981, 33, 105. 11 Y. Doi, T. Aoba, M. Okazaki, J. Takahashi and Y. Moriwaki, Calcif: Tissue Int., 1982,33, 81. 12 Y. Doi, Y. Moriwaki, T. Aoba, M. Okazaki, J. Takahashi and K. Joshin, J. Dent Res., 1982,61,429. 13 M. Geoffroy and H. J. Tochon-Danguy, Calcif. Tissue Int., 1982, 34, s99. 14 F. J. Callens, R. M. H. Verbeeck, P. F. A. 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Posner, Nature{London), 1964 46 47 48 D. W. Ovenall and D. H. Whiffen, Mol. Phys., 1961,4, 135. S. A. Marshall, A. R. Reinberg, R. A. Serway and J. A. Hodges, Mol. Phys., 1964,8, 225. P. Meriaudeau, J. C. Vedrine, Y. Ben Taarit and C. Naccache, J. 51 52 D. Naessens, Ph.D. Thesis, University of Gent, 1992. J. C. Larbarthe, M. Therasse, G. Bone1 and G. Montel, hebd. Skanc. Acad. Sci. Paris, C, 1973,276, 1175. 49 Chem. Soc., Faraday Trans. 2, 1974,71,736. A. J. Stone, Proc. R. Soc. London, A, 1963,271,424. Paper 4/02224F; Received 14th April, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002653

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2663-2670 EPR and NMR Studies of Amorphous Aluminium Boratest Simion Simon, Andre van der Pol, Edward J. Reijerse, Arno P. M. Kentgens, Geert J. van Moorsel and Engbert de Boer Department of Molecular Spectroscopy, Faculty of Science, University of Nijmegen , Toernooiveld 6525 ED Nijmegen, The Netherlands Amorphous aluminium borates, Al,,, -x,B,,O, with 0 d x d 0.5,prepared from mixtures of aluminium nitrate, boric acid and glycerol, have been studied by EPR and MASNMR as a function of composition and heat- treatment temperature (Tt < 860 "C). EPR studies showed the presence of physisorbed NO,, NO and 0, mol-ecules, produced by decomposition reactions during the thermal treatment. The 0, molecules in the gaseous state were observed in a narrow temperature interval around 60 K and in the condensed phase at low tem- perature (t20K).The D value for condensed 0, amounts to 109 GHz, significantly lower than the value for 'free' O,, which is 119 GHz. Above 20 K the NO, molecules in all samples rotate rapidly (2lo7 Hz) about an axis parallel to the interatomic oxygen-xygen direction ; this mobility decreases with increasing heat-treatment temperature. Some EPR lines were tentatively ascribed to pairs or clusters of the abovementioned paramagne- tic molecules. ,'At MASNMR studies showed the presence of six-, five- and four-coordinate Al atoms, their relative concen- trations being strongly dependent on the thermal history and composition of the samples. The fractions of tetra-and penta-coordinated Al atoms were maximum at heat-treatment temperatures between 300 and 600 "Cand decreased considerably after the samples were exposed to air.Therefore the low coordinated Al atoms are predominantly located at the surface. The decreased mobility of NO, molecules, at high treatment temperatures, indicates that NO, interacts strong- ly with the pore surfacewhen it contains a large fraction of four- and five-coordinate Al ions. Aluminas are extensively used as the supporting material in catalytic reactions. They are acid-base catalysts with a high surface area.' Successful efforts have been made to prepare amorphous aluminas that exhibit a zeolite-type porosity. The pore configuration and dimensions depend on composition, preparation method and heat treatment.'., The addition of typical glass-forming components such as SiO,, P,O, and B,O,, increases the stability of aluminas and leads also to new proper tie^.^-^ The incorporation of transition-metal ions or rare-earth metal elements gives these materials interesting optical, magnetic and catalytic proper- ties.'-' ' Amorphous aluminium borates with high surface areas can be prepared by a sol-gel method from solutions of aluminium salts and boric acid using ammonium hydroxide or methanol as precipitant.Recently a new method has been developed which also results in an amorphous material having a high surface area. This method involves the low- temperature thermal decomposition of aluminium nitrate and boric acid sustained by the simultaneous oxidation of a suit- able organic agent. Materials prepared this way were studied by thermal analysis methods, X-ray diffraction and FTIR spectro~copy.~~~In this paper we report EPR and NMR studies of aluminium borates prepared according to this method.EPR studies of aluminium borates revealed the pres- ence of physisorbed NO,, NO and 0, molecules which are produced by decomposition reactions during the thermal treatment. The mobility of the NO, molecules, as reflected in the EPR spectra, was strongly dependent on the measure- ment temperature, the heat-treatment temperature and the sample composition. At low temperatures, EPR spectra of gaseous 0, as well as for 0, in the condensed phase were observed. 27Al MASNMR studies revealed three signals at 6, 30 and 60 ppm, corresponding to six-, five- and four-coordinate Al, respectively.Their relative intensities were strongly depen- dent on the composition and the thermal history of the samples. The structural information obtained from 27Al ?This paper was presented at the 27th International ESR Con-ference at the University of Wales, Cardiff, 21st-25th March, 1994. MASNMR is used to explain the different strength of the interactions between the identified paramagnetic gaseous species and the active sites of the pore surfaces, developed during the thermal treatment. Experimental Aluminium borate samples were prepared with composition Al,~l~x~B,x03with x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5.To a stoi- chiometric mixture of Al(NO,), * 9H,O and H,BO,, glycerol was added as an organic reducing agent (10 wt.% in all samples) and a small amount of distilled water. After the components had dissolved a single liquid phase was formed at room temperature. The clear solutions were heated to 95°C and after ca. 2 h spongy, bulky solid samples were obtained. At the end of the heating procedure decomposition reactions took place as apparent from emission of gaseous products. The conversion that takes place during the synthe- sis can be summarized as follows : 2[Al(NO,), .9H20] -+Al,O, + 6N02 + $0,+ l8H,O (1) 6NO, +6N0 + 30, (2) 2H3B03+B2034-3H20 (3) glycerol oxidation (4) EPR and NMR measurements were carried out on samples treated for 30 min at various temperatures (see Fig.1). For this procedure the solid material was crushed and placed in a cylindrical furnace for heating in the open air. Immediately after they had been heated the samples were sealed in quartz tubes for EPR measurements or placed in air-tight glass bottles for NMR measurements. Just before the beginning of the NMR measurements the samples were rapidly transferred to the spinners in order to keep hydration effects to a minimum. The solid samples were white for 100 d 17;l"C < 150, yellow-green for 150 < TJ"C < 200, yellow-brown for 200 d TJC < 300, and white for ?; =-300°C. The samples are denoted ABx-T,, where x refers to the boron content and y to the treatment temperature (y = TJlOO).2664 800 600 Y 400 2 4 6 8 tlh Fig. 1 Heat treatment diagram. The heating temperatures are denoted T,, where y = TJ100. EPR spectra were recorded on powdered samples on a Bruker ESP-380 X-band spectrometer at 5.8-300 K and at a static field between 0.05 and 13 kG. The average microwave frequency of the experiments was 9.3 GHz. MASNMR measurements were carried out at room tem- perature on a Bruker AM-500 spectrometer equipped with a solid-state accessory, using a home-built probe head equipped with a Jakobsen 5 mm MAS assembly. Usually 1 ps pulse excitations were applied and spinning speeds up to 14 kHz were employed. Spectra are referenced with respect to an external AI(NO3)3 solution [AI(H,0),3 '1.Results EPR on Aluminium Borate EPR spectra representative for various stages of the synthesis process are shown in Fig. 2-7. All samples exhibited a rela- tively small EPR line at ca. 1550 G with g = 4.23 originating from Fe3+ impurities present in Al(NO,), . This signal can be used as an internal standard for estimating the relative inten- sities of the other EPR signals. NO2 A characteristic feature present in the EPR spectra of all samples is a number of lines in the g x2.0 region extending over ca. 150 G. The highest intensity and best resolution was attained at low measurement temperatures. Fig. 3 and 4 illus-trate the EPR spectrum of this signal as a function of tem-perature on a more expanded scale.For samples with y -c 1.5 the lines became practically undetectable at measurement T, > 150 K (Fig. 3), but for samples with y 23 they were clearly observed even at room temperature (Fig. 4). These spectra can be ascribed to NO, and are well described in the In Fig. 3 the nitrogen hyperfine EPR lines are labelled with x, y or z, where y runs parallel to the inter- atomic oxygen-oxygen direction. The spectra show a clear temperature dependence. At low temperatures a powder-like spectrum is observed. Going to higher temperatures the x and z components merge, while the y components keep their positions. From this behaviour it can be concluded that the NO, molecules above 20 K rotate rapidly around an axis J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 62K 58 K 1 A* 12 3 4 5 6 7 8 9101112 ~/103G Fig. 2 EPR spectra of AB,.,-T, at: 12 K, 55 K, 58 K,62 K parallel to the y axis (2lo7 Hz), as has been observed pre- viously.'s~'8~'9For the sample AB,.,-T, this leads eventually to an isotropic spectrum (T 2 125 K), whereas for the sample AB,.,-T, even at 300 K the rotation is still anisotropic. Thus the mobility of NO, molecules depends on the thermal history of the samples. In the Discussion we will further elaborate on this. In Table 1 the magnetic parameters are listed together with those obtained for NO, adsorbed on similar systems and for NO, in the gas phase. From the simi- larity of the values in Table 1 it can be concluded that the adsorbed NO, is not greatly distorted by the aluminium borate matrix.-125K 3250 3300 3350 3400 HIG Fig.3 Temperature dependence of the NO, EPR spectrum f AB,,,-TI. The assignment of hyperfine lines is shown. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 n 3250 3300 3350 3400 HIG Fig. 4 Temperature dependence of the NO, EPR spectrum of ABo.,-T, In a relatively small temperature interval (50-65 K) a beauti- ful multiline spectrum was observed between 5 and 12.5 kG (Fig. 2 and 7). Comparing this multiline signal with that observed for 0,in the gaseous state20v21 it can be inferred that this signal originates from 0, molecules in gaseous form probably present inside the pores of the sample. Above ca. 65 K this spectrum broadens beyond detection (see Fig. 2).At low temperatures it disappears and is replaced by a new strong signal at cu. 11.7 kG (Fig. 2 and 7). Apparently decreasing the temperature causes the O2 molecules to con- dense on the surfaces of the pores and then gives rise to the well known signal at ca. 11.7 kG,22-25characteristic of 0,in the condensed phase. That the signal is due to 0, molecules produced by the decomposition reactions (1) and (2) is proven by the experiment illustrated in Fig. 5. In Fig. 5(a)the EPR spectrum is shown for the as-prepared sample AB,.,- T1.5.A very strong signal is observed at 11.7 kG. The quartz tube was then opened and connected to a vacuum system. After evacuation of the sample at room temperature the sample tube was sealed and subsequently the EPR spectrum was measured.A dramatic decrease of the signal intensity at 11.7 kGwas observed [see Fig. 5(b)].A further short heat r 1 4 I I 200 G II 123456789101112 ~~103G Fig. 5 EPR spectra of AB,,,-T,,5 at 14 K: (a) before evacuation, (b) after evacuation at room temperature and (c) after a new short (<1 min) heat treatment at 150"C treatment of the sample in the closed tube (<1 min) at the same temperature (150 "C) resulted in an enhanced signal [Fig. 5(c)]. This enhancement must be due to 0, molecules produced by the decomposition reactions that occur during the short heat treatment. The EPR spectrum of condensed triplet 0, has been analysed by using the following spin Hamiltonian H,= D[S,2 -fs(s+ l)] + E(S; -S;) + BB, -g s and with the aid of the EPR simulation program MAGRES.28 An excellent fit was obtained using the follow- ing set of parameters: gx = g,, = 2.02, gz = 0.673, D = 109.3 GHz, E = 0.075 GHz, Lorentzian linewidth = 200 G.The dotted line in Fig. 7 shows the simulation. In passing we note that a perfect fit for the signal at 11.7 kG could be obtained only by taking a non-zero asymmetry parameter E Table 1 EPR parameters of NO, molecules adsorbed on surface of various oxide matrices oxide matrix 9, g: 9, Ad MHz A:l MHz AZl MHz Ad MHz ref. - MgO (93 K) ZnO (77 K) 2.005 2.007 1.9915 1.994 2.002 2.003 1.9995 2.001 148 146.1 137 132 189 181.1 158 153.1 16 19 silica gel (77 K) 2.004 1.9907 2.004 Vycor (4.8 K) 2.005 1.9913 2.0017 zeolites (77 K) 2.0043 1.9922 2.0017 aluminium borates 2.00 15 1.9985 2.009 (20 K)NO, (gas) (293 K) 2.0062 1.9910 2.00 19 x = y is the axis parallel with the interatomic oxygen-oxygen direction.1.9996 165.7 137 165.7 156.1 18 1.9994 140.3 128.2 183.5 150.7 19 1.9993 143.3 137.2 189.1 156.5 17 2.0066 144.5 133.0 185 154.1 -a 1.9997 128.0 126.0 184.9 146.5 13 Present work. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 300 K 4 5 6 7 8 9101112 1 ~~~ ~ 4 5 6 7 8 9 1011 12 H/103 G Fig. 6 EPR spectra for ABO.3-T,,, (a)and AB,.,-T,., (b)at 14 K and the feature at ca. 5 kG could be reproduced only using an axial g tensor. In Table 2 the values of the parameters are listed together with those obtained in other matrices.The D values for trapped 0, molecules are significantly smaller than that for 'free' 0, , 119 GHz.,' NO The presence of NO in the samples is also revealed in the EPR spectra. The EPR spectra are shown for ABo.,-T'.5 [Fig. qa)] and AB0.5-TI.5 [Fig. 6(b)] measured at 14 K. The latter spectrum reveals an asymmetrical signal with a large tail at high field, characteristic of NO, at g x 2.0 without con- Table 2 EPR parameters of 0, molecules trapped in different matrices ~~ ~ AH1 Dl El matrix 9x g,, g: G GHz GHz ref. N, solid 2.02 2.02 0.7 40 108 -23 (<20 K) 2.0 2.0 2.0 25 108.3 -24 2.0 2.0 2.0 -107.1 -25 CO solid 2.02 2.02 0.7 80 108 -23 (<20 K) 2.0 2.0 2.0 400 108.5 -24 KBrO, 1.999 2.015 1.996 -108.9 1.3 26 crystals (26 K)NaClO, 2.004 2.006 2.003 -112.2 0.135 27 crystals (4.2 K)aluminium 2.02 borates ((20 K) * Present work.x = 2.02 0.673 320 109.3 0.075 -= z is the molecular axis. 111l1011111 123456789101112 H/103 G Fig. 7 Temperature dependence of the EPR spectra of AB,.,-T,. The dotted line is the simulated signal for trapped 0, molecules using parameters listed in Table 2. tamination from other signals. In Fig. 6(a) the NO signal is superimposed on the NOz signal, as is the case in Fig. 5(c) (see insert). Close inspection of the spectra of other samples always showed a contribution from NO. In Table 3 the mag- netic parameters are tabulated together with those of physi- sorbed NO molecules on y-al~mina,,~ ~ilica-magnesia,~~ Mg0I6 and zeolite^.'^'^^ Our values, estimated directly from the EPR spectra [Fig.5(c) and 61, are in accordance with those measured in similar systems. Other Paramagnetic Species In the EPR spectra of samples heat treated between 150 and 200°C we observed EPR signals that we could not identify. As an illustration of this we refer to Fig. 7, where spectra are shown for the sample AB,.,-T,. Broad lines are observed below ca. 5 kG. As can be seen from the figure, some lines shift to lower magnetic field values with decreasing tem- perature. It is suggested that these signals arise from pairs or clusters formed from paramagnetic species with S = 1/2. In Fig. 5(a) there are some features at 5 and 8 kG which might also be due to paramagnetic dimers or clusters.In some cases (at high NO concentrations) a well defined line at g = 4 was observed [see peak in Fig. qb), indicated by an arrow], which might be a half-field signal from paramagnetic NO dimers. Peaks due to dimers of NO, could be discerned in the full- field region of the NO, spectrum, especially at high NOz concentration, as was observed by Schaafsma and Komman- deur." Finally, in the spectra of all samples in the region around g x 2.0, especially at low temperature, weak signals were J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 EPR parameters of NO molecules adsorbed on surface of various matrices (materials) 41AXl AYI oxide matrix 9, s: MHz MHz MHz ref.y-alumina (77 K) 1.996 1.996 1.96 -95 -29 silica-magnesia 1.996 1.996 1.91-1.95 -85 -29 (77 K)MgO (77 K) 1.995 1.995 1.91 -93 -16 zeolites 1.986-2.00 1.978-1.998 1.83-1.93 -85-95 -30, 31 (77 K) --aaluminium borates 1.985 1.985 1.81-1.91 -85 (14 K) 'Present work. x = z is the molecular axis. observed superimposed on the strong signals of NOz and NO. They are possibly related to some crystal defects or to organic and/or inorganic radicals produced during the syn- thesis. NMR on Aluminium Borates It was expected that changes in coordination of aluminium would be manifest clearly in 27Al MASNMR spectra. This was indeed the case. The effect of heat-treatment temperature on the shape of the 27Al MASNMR spectra is shown in Fig.8 for samples without boron (spinning side bands are marked by asterisks). The dependence of the spectra on sample com- position is illustrated in Fig. 9 for samples with different n T8.6 T6 T4 T2 Tl .5 250 200 150 100 50 0 -50 -100 -150 6 Fig. 8 27Al MASNMR spectra of AB, samples at room tem-perature showing the effect of heat-treatment temperature. The spin- ning side bands are marked with asterisks and are outside the range of chemical shifts of the three A1 resonances. boron contents and the same heating temperature (T = 400°C). In all spectra three resonance peaks can be discerned with different intensities. The resonances at 0-8 and 58-64 ppm are unanimously accepted to originate from octa- hedrally and tetrahedrally coordinated Al, re~pectively.~'-~~ The third resonance at ca. 30 ppm is consistent with the chemical shift of penta-coordinated Al, as previously observed in NMR studies of crystalline materials with well defined penta-coordinated A133,3 or in studies of disordered matrices, as gels and glasses, containing A1 Fig.8 shows that mainly between 200 and 300°C penta-coordinated and tetra-coordinated A1 are formed at the expense of octahedrally coordinated Al. Above 800 "C the spectrum corresponds to the NMR spectrum of y-alumina, consisting of only tetrahedrally and octahedrally coordinated Al. Fig. 9 shows that the amount of penta-coordinated A1 increases, whereas the fraction of tetra-coordinated A1 decreases with increasing boron concentration.0.1 0 I I I ! I 1 I I I I 250 200 150 100 50 0 -50 -100 -150 -200 6 Fig. 9 27Al MASNMR spectra of AB,-T, samples showing the effect of the boron conten;; the value of^x k shown on the spectra. The spinning side bands are marked with asterisks and are outside the range of chemical shifts of the three A1 resonances. I\\ * * I I I I I 250 200 150 100 50 0 -50 -100 -150 6 Fig. 10 *'A1 MASNMR spectra for the AB,-T, sample: (a) as pre- pared, (b)after 1 month and (c) after 2 months storage in air, and (d) after a new heat treatment at the same temperature (400°C). The spinning side bands are marked with asterisks and are outside the range of chemical shifts of the three A1 resonances. For samples with y d 7.5 the 27Al NMR spectra changed in time when these samples were exposed to air.This effect, also dependent on the boron content, is illustrated in Fig. 10 for the sample AB,-T,. When, after the first measurement [Fig. lqa)], the sample is exposed to air, changes are observed caused by absorption of H20 molecules from the air [Fig. lqb), (c)]. The peak intensities of tetra- and penta- coordinated A1 decrease, whereas the intensity of the peak due to octahedrally coordinated A1 increases. After renewed heat treatment at the same temperature (400°C) a similar 27Al NMR spectrum was obtained to that measured for the as-prepared sample [Fig. lqa)]. The same effect was observed recently for other aluminium oxide matrices.32 Discussion NMR During sol-gel synthesis the materials pass through several stages.'* In the starting solutions at room temperature the principal process is the hydrolysis reaction, during which the majority of the present cations become coordinated to hydroxy groups and water molecules.By increasing the tem- perature, condensation reactions evolve with the formation of M-0-M bonds and the production of water. These con- densation reactions proceed during the drying of gels by suc- cessive heat treatments. At the end of the synthetic process amorphous porous xerogels are obtained. Our experiments shed more light on what happens specifi- cally during the synthesis of the xerogels. Up to a heat-treatment temperature of 200 "C decomposi-tion reactions occur and glycerol is partially oxidized.The 27Al MASNMR spectra recorded for samples taken at heat- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 treatment temperatures of 150 and 200°C hardly differ (see Fig. 8). A dramatic change in the local structure of the xerogels takes place between 200 and 300"C,as is nicely illustrated by the 27Al MASNMR spectra in Fig. 8. From these spectra it may be concluded that in the heat-treatment temperature range 300-600°C four- and five-coordinate A1 are formed at the expense of six-coordinated Al. This is more clearly demonstrated in Fig. 11 where the change in the relative peak areas is plotted against the heat-treatment temperature. The relative intensities of the NMR peaks were determined by deconvolution of the spectra, assuming that the line shapes can be fitted with Gaussians.Since the electric field gradient is different at each of the three sites, the peaks have different and asymmetrical line shapes. These effects were taken into account by allowing the lines for each site to be a linear com- bination of one, two or three Gaussians with different widths and positions. In this way good fits were obtained. Fig. 9 shows that four- and five-coordinate A1 are also present in the boron-containing samples. The possibility that the resonance peak at ca. 30 ppm is due to aluminium coordi- nated to boron atoms in the second coordination sphere6 can be ruled out because this resonance was also observed in pure alumina samples. Moreover, it was observed in 29Si NMR of boron-containing MFI zeolites that boron present in the second coordination sphere of Si had no effect on the Si chemical shift.39 Therefore, we do not expect any effect on the A1 chemical shifts either.From Fig. 9 it can furthermore be inferred that the boron ions prefer four- instead of six-coordination (see spectrum of sample AB,,,). The decrease in the number of aluminium atoms in the four- and five-coordinate ion sites in samples exposed to air (so-called air-equilibrated gels), suggests that low-coordinate A1 atoms are predominantly located at the surface of the xerogels. It is known that these four- and five-coordinate A1 atoms at the surface are associated with the catalytically active sites.'*40 The partial rehydroxylation that takes place when the sample is exposed to air is the principal factor involved in the different results reported in the literature for similar material^.^.^ The difference in preparation procedure seems to be of minor importance.Not only is the local structure changed during the synthe- sis of xerogels but also the internal porosity. By increasing 0.8 \ 0.6 c4-0 Y s z* 0.4 0.2 :\ 0.0 100 300 500 700 900 TJC Fig. 11 Fraction of (V) hexa-, (0)penta- and (0)tetra-coordinated A1 ions as a function of heat treatment temperature (TJ.The dotted lines are guides to the eye. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the heat-treatment temperature the dimensions of the pores diminish, the surface area decreases and the density of the samples increases (skeletal densification).After 3 h heat treat- ment at 860°Cthe NMR spectra of the samples (Fig. 8) are identical to that of y-alumina, proving that the amor- phous alumina xerogels had been transformed into poly- crystalline alumina. EPR The EPR results offer us new, important information about the evolution of catalytically active sites. The gaseous para- magnetic species that are produced by the decomposition reactions interact with these active sites before they change their initial configuration by structural relaxation or as a result of interactions with other non-paramagnetic molecules such as water. As indicated above, the mobility of physisorbed NO, mol-ecules depends on the heat-treatment temperature. Their mobility, determined by the strength of their interactions with the surfaces of the xerogels, is revealed by the tem- perature dependence of the EPR spectra shown in Fig.3 and Fig. 4. NO, molecules in samples treated at low temperatures have a higher mobility than in samples treated at higher tem- peratures. For instance in the sample AB,.,-T, the NO, mol- ecules at 125 K rotate isotropically, whereas in the sample AB,.,-T, at the same temperature the NO, molecules rotate anisotropically around an axis parallel to the interatomic axis between the oxygen atoms (denoted as the y axis). For samples with y Q 1.5 the mobility of the NO, molecules is comparable to the mobility observed for NO, adsorbed on Vycor glassIg or on zeolite^,^' but for samples with y > 2 the mobility of NO, is lower.At high treatment temperatures the samples become to a great extent dehydroxylated and accordingly the interaction between the NO, molecules and the surface becomes stronger, thus restricting their mobility. For the sample AB,.,-T, the NO, molecules move aniso- tropically even at room temperature (see Fig. 4). As shown in Table 1, the magnetic parameters of NO, in aluminium borate xerogels are comparable to the values observed for similar systems. Hence the adsorbed NO, mol-ecules are not greatly distorted by the aluminium borate matrix. Pietrzak and concluded from this that the majority of the NO, molecules dimerize at low temperature and are surrounded by several N204 molecules which have condensed on the walls of the zeolites studied by them, thus shielding the NO, molecule from strong cation interactions.The same conclusion may be drawn for the system investi- gated by us. Fig. 6 shows that on increasing the boron content the 0, EPR signal decreases. This can be rational- ized as follows. When the boron content increases less 0, will be produced by the decomposition of Al(NO,), because its concentration decreases as the boron content increases. The 0, molecules produced will be partly used in the oxidation of glycerol, the concentration of which is the same in all samples. Thus the 0, signal intensity observed will decrease as the boron content increases. This is not contradicted by the experiment described in Fig.5, where we see that the 0, EPR signal increases after a renewed heating procedure. We must remember that in this experiment the sample has been placed in a closed evacuated tube, so that the decomposition products can escape into the empty space above the sample, especially the non-polar 0, molecules. The polar molecules such as NO are preferentially fixed on coordinatively unsaturated active sites (A13+,B3+), developed during the heating process. Thus the 0,molecules that have escaped cannot be consumed in the pyrolysis of glycerol. After cooling the sample to 14 K the 0, molecules will condense on the surfaces, and will give rise to an enhanced EPR signal. At heating temperatures below 200°C we observed in the EPR spectra signals that we believe to arise from species with S 2 1.From the 27Al NMR spectra it can be concluded that below 200°C only a small number of low-coordinate A1 species are present (see Fig. 8). At the beginning they are not uniformly distributed over the surface area of the pores, but will appear in zones in which the hydroxylation process was most effective. From structural studies it was found that one oxygen anion vacancy can create as many as three five- coordinate A1 sites.37 The polar paramagnetic species, espe- cially the very reactive NO molecules, will react with these active sites and form paramagnetic clusters with S 2 1. The formation of these clusters is also favoured by the high gas pressure in the pores, which are closed at this temperature.The sharp signal at g = 4, indicated by an arrow in Fig. 6, may arise from NO dimers. Pairs of NO, molecules can also be formed, especially when the concentration of NO, is high.15 We found evidence for this in our spectra, measured below 20 K. Note that we found no evidence in our experiments of the presence of 0,-.When 0, is adsorbed on 'clean' surfaces of activated oxide materials, usually an electron is transferred to 02.41In our experiments the surfaces will be preferentially covered by polar molecules such as NO, N,O,, NO,, OH-or H,O, so that the 0, molecules are physisorbed on top of them and no electron transfer to 0, takes place. The appearance of the multiline spectrum of 0, was a surprising result.The multiline spectrum arises through the coupling of the rotational angular momentum, which is quenched in the liquid or solid phase, with the electronic spin and orbital angular momentum.21 This spectrum could be observed only over a narrow temperature interval of 15"C.If the 0, concentration is high, collisional broadening will occur. On cooling the sample, the concentration of 0, decreases by condensation of 0, molecules on the surfaces of the pores. A point is then reached where the concentration of 0, is large enough and the relaxation times long enough to make detection of the EPR spectrum of gaseous 0, possible. Further lowering of the temperature leads to total conden- sation and to the disappearance of the gaseous EPR spec-trum and to the appearance of the EPR spectrum of 0, in the condensed phase.The zero-field splitting (D) of 0, in the gaseous state is 119 GHz.,' For 0, in our system the D value amounts to 109 GHz. The reduction of the value of D has been attributed to torsional oscillation of the 0, molecules in a potential well provided by the matrix.42 Table 2 shows that the same effect has been observed for 0, trapped in other matrices. Conclusion Our EPR studies on aluminium borate materials show that the paramagnetic products of decomposition reactions in sol-gel processing of amorphous xerogels can be used as EPR probes for the study of the active sites developed during the synthesis on the surface of the pores.The strength of the interactions between the active sites and the paramagnetic products are reflected in the shape of the EPR spectra of the adsorbed molecules. The 27Al MASNMR spectra reflect directly the changes in A1 coordination during the heating procedure. In the tem- perature range from 200 to 300°C four- and five-coordinate A1 species are formed, which, on exposure to air, are partially transformed again to six-coordinate A1 by absorption of water present in the air. This observation stresses the impor- tance of working under well defined conditions in cases where 2670 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the materials studied have high surface area. Some conflicting results in the literature may be ascribed to air-equilibration of the samples.Finally, the simultaneous use of NMR and EPR techniques is shown to be a powerful tool for the study of the active sites 15 16 17 18 19 T. J. Schaafsma and J. Kommandeur, Mol. Phys., 1968,14,517. J. H. Lunsford, J. Colloid Interface Sci., 1968,26, 355. T. M. Pietrzak and D. E. Wood, J. Chem. Phys., 1970,53,2454. M. Iwaizumi, S. Kubota and T. Isobe, Bull. Chem. SOC. Jpn., 1971,44,3227. M. Shiotani and J. H. Freed, J. Phys. Chem., 1981,85,3873. in amorphous and crystalline materials. 20 21 R. Beringer and J. G. Castle, Phys. Rev., 1951,81, 82. M. Tinkham and M. W. P. Standberg, Phys. Rev., 1955,97,951. The authors thank Mr. A. A. K. Klaassen, Mr. G. E. Janssen and Mrs. G. H. Nachtegaal for their skilful experimental assistance.22 23 G. M. Graham, J. S. M. Harvey and H. Kiefte, J. Chem. Phys., 1969,52,2235. R. Simoneau, J. S. M. Harvey and G. M. Graham, J. Chem. Phys., 1971,54,4819. 24 H. Kon, J. Am. Chem. SOC., 1973,%, 1045. References 25 26 S. Hirokawa, J. Phys. SOC. Jpn., 1974,37, 897. J. R. Byberg, Chem. Phys. Lett., 1978,57, 579. 1 2 3 4 5 6 H. Knozinger, in Catalysis by Acids and Buses, ed. B. Imelik, C. Naccache, Y. Condurier, B. Tarit and J. C. Vedrine, Elsevier, Amsterdam, 1985, p. 11 1. G. Pajonk, M. Repellin and S. J. Techner, Bull. SOC. Chim. Fr., 1976, 1333. G. Tournier, M. Lacroix-Repellin, G. M. Pajonk and S. T. Teichner, Preparation of Catalysts IV, ed. B. Delmon, P. Grange, P. A. Jacobs and G. Poncelet, Elsevier, Amsterdam, 1987, p. 333.S. F. Mitchel, G. Marcelin and J. G. Goodwin, J. Catul., 1987, 105,521. D. L. Cocke, E. D. Johnson and R. P. Merrill, Catal. Rev. Sci. Eng., 1984,26, 163. K. P. Peil, L. G. Galya and G. Marcelin, J. Catal., 1989, 115, 441. 27 28 29 30 31 32 33 34 35 36 N. Bjerre, J. Chem. Phys., 1982,76, 3347. C. P. Keijzers, E. J. Reijerse, P. Stam, M. F. Dumont and M. C. M. Gribau, J. Chem. SOC., Faraday Trans. 1,1987,83,3469. J. H. Lunsford, J. Catal., 1969, 14, 379. J. H. Lunsford, J. Phys. Chem., 1970,71, 1519. P. H. Kasai and R. J. Bishop Jr., J. Am. Chem. SOC., 1972, 94, 5560. M. E. Smith, Appl. Magn. Reson., 1993,4, l., L. B. Alemany and G. W. Kirker, J. Am. Chem. SOC., 1986, 108, 6158. D. Massiot, A. Kahn-Harari, D. Michel, D. Muller and F. Tau-lelle, Magn. Reson. Chem., 1990,28, S82.M. E. Smith and S. Steuernagel, Solid State NMR, 1992,l 175. A. D. Irwin, J. S. Holmgren and J. Jonas, J. Muter. Sci., 1988,23, 2908. 7 8 9 10 11 F. Abbattista, A. Delmastro, G. Gozzelino, D. Mazza, M. Vallino, G. Busca and V. Lorenzelli, J. Chem. SOC., Faraday Trans., 1990,86,3653. A. Delmastro, G. Gozzelino, D. Mazza, M. Vallino, G. Busca and V. Lorenzelli, J. Chem. SOC., Faraday Trans., 1992,88,2065. S. Tanabe, K. Hirao, N. Soga and T. Hanada, J. Solid State Chem., 1992,97,481. R. Bechara, A. Aboukais and J. P. Bonnelle, J. Chem. SOC., Faraday Trans., 1993,89, 1257. J. G. Darab and R. K. MacCrone, Phys. Chem. Glasses, 1991,32, 191. 37 38 39 40 41 T. E. Wood, A. R. Siedle, J. R. Hill, R. P. Skarjune, C. J. Good- brake, Muter. Res. SOC. Symp. Proc., 1990,180,97. C. J. Brinker, in Glass: Science and Technology, ed. D. R. Uhlmann and N. J. Kreidl, Academic Press, New York, 1990, vol. 4A, p. 169. R. de Ruiter, A. P. M. Kentgens, J. Grootendorst, J. C. Jansen and H. van Bekkum, Zeolites, 1993,13, 128. H. Yong, B. Coster, F. R. Chen, J. G. Davis and J. J. Fripiat, New Frontiers in Catalysis, ed. L. Guczi, F. Solymosi and P. Tktenyi, Elsevier, Amsterdam, 1993, p. 1159. M. Che and A. J. Tench, Adv. Catal., 1983,32, 1. 12 S. Ikoma, K. Kawakita and H. Yokoi, J. Non. Cryst. Solids, 1990,122,183. 42 H. Meyer, M. C. M. OBrien and J. H. van Vleck, Proc. R. SOC. London, Ser. A, 1957,243,414. 13 G. R. Bird, J. C. Bird, A, W. Jache, J. A. Hodgson, R. F. Curl, A. C. Kunkle, J. W. Bransford, J. Rastrup-Anderson and J. Rosenthal, J. Chem. Phys., 1964,40,3378. 14 T. J. Schaafsma, G. A. van der Velde and J. Kommandeur, Mol. Phys., 1968, 14, 501. Paper 4/02417F; Received 25th April, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002663

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2671-2675 Effects encountered in EPR Spectroscopy and Imaging at Small Magnetic Fields Duncan G. Gillies, Leslie H. Sutcliffe and Mark R. Symms-f Chemistry Department, University of Surrey, Guildford, Surrey, UK GU2 5XH In vivo EPR studies of biological systems and other ‘wet’ systems are often performed at radiofrequencies (low static magnetic fields) in order to minimize conductive losses incurred with relatively large samples. Here, attention is drawn to several consequences which can arise from carrying out EPR spectroscopy and imaging (EMRI) at low fields. The first causes a distortion of the gradient when the applied gradient is not a small fraction of the external magnetic field. These ‘concomitant gradients ’ could pose problems if high-resolution imaging experiments are attempted. Other phenomena stem from the Breit-Rabi effect which causes spectral distortions and has implications for data acquisition and image processing. A third effect is the near removal of g factor anisotropy resulting from using low magnetic fields.Thus the powder spectrum for a typical aminoxyl radical is completely different from that observed at X-band: this has consequences for both imaging and for the measure- ment of rotational diffusion constants. It is shown that there are disadvantages in using 15N-labelled spin probes/labels for EPR measurements at radiofrequencies. As in NMR spectroscopy, sensitivity increases with frequency for EPR spectroscopy. At the typical microwave frequency of 9 GHz (X-band), the effective sample volume is about 0.3 cm3, but conductive losses encountered with ‘wet’ samples can reduce this volume to about 0.03 cm3.Thus, in order to study large ‘lossy’ samples by EPR spectroscopy it is neces- sary to use irradiating frequencies 51 GHz and, for many purposes, frequencies of about 200 MHz are more appropri- ate.’ The typical nitrogen hyperfine interaction for an amino- xyl spin probe is ca. 40 MHz, which is not small compared with the irradiating frequency at these low frequencies (fields). In this paper, we present three consequences arising from carrying out EPR experiments at radiofrequencies : (i) con- comitant gradient effects in imaging, (ii) Breit-Rabi effects in spectroscopy and imaging, (iii) the effect of the near elimi- nation of g anisotropy on ‘powder’ spectra and motional studies.Concomitant Gradient Effects in Radiofrequency (RF) Imaging Electron magnetic resonance imaging (EMRI) has proved to be a very useful technique for studying the spatial distribu- tion of free radicals in a sample.’ The basic principle behind the method is very similar to that of the ubiquitous NMR equivalent, namely MRI, but continuous wave (CW) rather than pulse techniques are almost always used owing to the short electron T2.A magnetic resonance line is broadened inhomogeneously by applying a linear magnetic field gra- dient in order to ‘frequency-encode’ (or more exactly, ‘gradient-encode’) the spins in space.Major differences are (i) the applied magnetic field is only 7 mT for EMRI at 200 MHz, (ii) the short electron spin-spin relaxation time and relatively large electron magnetogyric ratio make pulsed EMRI very difficult to implement, (iii) the large linewidth requires magnetic field gradients as large as 1000 mT m-I. The maximum gradient applied in practice is quite often determined by (a)the power supply available, (b)gradient coil heating problems, (c) the acceptable loss of signal : noise ratio: RF EMRI systems suffer particularly from low signal : noise ratios. However, as larger field gradients are used in the quest for increased resolution, low-field EMRI This paper was presented at the 27th International ESR con- ference at the University of Wales, Cardiff, 21st-25th March, 1994.may start to violate a restriction noted in MRI, namely that the gradient field must be a small perturbation of the main magnetic field.2 Interestingly, an alternative EMRI technique has been proposed that does not require field gradient^.^ Gradient Requirements for NMR and EPR As is well known, to obtain a resolution of 6x for a sample of linewidth proportional to 1/T2, the minimum gradient needed, G, is determined by the imaging version of the Ray- leigh criterion :2 where y is defined in a manner compatible with NMR, that is, o = yB. Thus for NMR and EPR experiments of the same resolution : Hence, for T2(NMR) = 0.1 s and T2(EPR) = 350 ns: (3) Concomitant Gradients Maxwell’s equations for a magnetic field in free space forbid the existence of a single pure linear magnetic field gradient. However, from the theoretical basis of MR12 it can be seen that the application of a gradient field is most accurately described as a tensor, and only for large static main fields and small gradients can the latter be expressed as a vector pertur- bation.For small static fields, higher-order terms will appear giving concomitant gradients that can cause various image distortion^.^ The latter are observed in MRI in the phase- encoding direction in several experiments, as this direction usually has a much smaller frequency per pixel than the frequency-encoding direction and is therefore more suscep- tible to distortion from flow artifacts, chemical shift or con- comitant gradients.However, apart from one interesting exception,’ CW imaging relies exclusively upon frequency- encoding gradients. Nevertheless, EMRI is potentially more vulnerable than MRI to concomitant effects, so we will now use the analysis of Norris and Hutchison4 to predict the nature and extent of the effects in frequency encoding. If we have a static field along the z axis: B, = B,k (4) and a gradient field: B(x)= xGk (5) then in order to satisfy Maxwell’s equation for free space: curl B = 0 (6) an extra (concomitant) gradient term is created such that : B(x, z) = xGk + zGi (7) The resulting applied field is: [B(x, 2)l2= (B, + xG)~+ (zG)’ (8) Expanding this to second order gives : B(x, Z) = Bo + xG + (zG)’/2BO (9) In CW EMRI, a linear magnetic field gradient G = dB,/dx is applied to the sample then the static field is swept slowly through resonance tracing a broadened spectrum. An advan- tageous feature in certain applications is that the sample is swept from one side of the gradient-labelled space to the other.Thus for a given value of sweep, B,, the signal arises from the shaded region shown in Fig. 1: there is a yz plane of spins proportional in width to 1/T2,such that: B, + GI + B,= o/y, (10) where o is the angular resonant frequency and GI is defined in Fig. 1. The gradient-broadened spectrum can be con-sidered to be the projection of the spin density of the sample taken in the direction of the applied linear gradient.A typical projection-reconstruction imaging experiment would con-tinue by incrementing the direction of the gradient through all possible angles in three-dimensional (3D)space.’ The con- comitant effect causes the volume of spins now contributing to the signal at a particular value of the sweep field to be curved. This curvature is the first consequence of the con- comitant gradient. Depending on the magnitudes of the main fields and the gradients used, it could cause problems when using frequency-encoding gradients in EMRI ; it could also cause extra dephasing of the signal during the selection pro- cedure in a pulse FT EMRI experiment. Size of Concomitant Distort ions At present, the signal :noise ratios of most RF EPR spec- trometers are too low to allow images to be taken with high enough resolution for the concomitant effect to be noticeable.However, the size of the effect can be estimated using a typical set of parameters, namely: T2= 350 ns (a typical value for an in uiuo aminoxyl radical); dx = 5 mm (a typical pixel size); B, = 7 mT (corresponding to a RF of 200 MHz); r = 0.1 m (sample radius, 40 pixels per projection permitted). From eqn. (I), G 3 mT m-’, thus each pixel has a band- 1-4 Fig. 1 Plane of spins of width proportional to 1/T, (shaded area) arising from the application of a linear magnetic field gradient G = dB,/dx in the CW EMRI experiment. The adjacent curved area is the region actually sampled due to the concomitant gradient effect.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 width of 1/T2 = 2.8 MHz. The concomitant distortion, E = Y(G~)~/~B,,is 1.3 MHz or about half a pixel at worst. A more ambitious set of parameters would give worse distortions. The relative distortion, d, or distortion per pixel bandwidth is given by: d = ET2 = T, Y(G~)~/~B, (11) Thus it may be seen that the concomitant distortion is made worse by use of (a)a low-field spectrometer, (b) large samples, (c)spin probes with large linewidths. Effect of Field Sweep The considerations given apply most accurately to a frequency-swept CW experiment. In a field-swept experiment, the full extent of the sweep field can no longer be regarded as a small perturbation of the static field since the nitrogen hyperfine splitting in an aminoxyl radical is ca.1.5 mT (=40 MHz). In this situation, the concomitant distortion is given by: B(x, 2) = B, + c(zG)2/2(B, + B,)] (12) It can be seen that the concomitant curvature will be more pronounced at the low-field end of the sweep. For example, using a total sweep of 5 mT, centred on 7 mT (=200 MHz), eqn. (12) gives a concomitant distortion that is about twice as bad at the low-field as at the high-field end of the sweep. Of course, the effect will be absent for some of the recently reported single-line spin probe^.^,^ Comparison of Concomitant Effects in EMRI and MRI From eqn. (11) and the optimum gradient strength G = l/(Tz ydx) and r = n6x (where n is the number of pixels in a projection) we obtain : d = n2/(20T2) (14) In order to compare EMRI with MRI, the ideal approach would be to take two systems of similar signal : noise ratios.However, this would require an extensive analytical compari- son of CW EPR with FT NMR which is beyond the scope of this paper. Instead, we shall compare the two magnetic reson- ances at (a)equal main field and (b) equal Larmor frequency. (a)Equal main field ~NMR= ~EP,Y, TXEPR)/T~(NMR)Y~ (15) thus The second case probably provides the better comparison between the two modalities. The equations suggest that con- comitant gradients may have a part to play in limiting the ultimate resolution achievable in low-field EMRI. Concomitant Effects in Overhauser Imaging Overhauser imaging (variously called DNP imaging’ or PEDR19) uses EPR to enhance the NMR (usually proton) signal. Apart from the opportunities afforded by cycling the main field, these techniques are similarly limited in sample size by conductive losses.Thus it is to be expected that conco- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 mitant gradient effects will occur. The original paper by Norris and Hutchison4 contains results obtained with a low- field MRI system which illustrate some of the phase-encoded distortions that are likely to be encountered in Overhauser imaging. Breit-Rabi Effects Second-order effects are encountered in EPR spectra when hyperfine interactions represent a significant fraction of the main magnetic field, B,. For solution spectra at X-band fre- quencies (7.8-12.4 GHz), when there are two or more equiva- lent magnetic nuclei present, extra lines may be observed (owing to the lifting of degeneracy) if the isotropic hyperfine interaction, a, is larger than ca.4 mT. Even when only one magnetic nucleus is present with I > 1/2, there are inequal- ities in the observed splittings (separations increasing with magnetic field); hence the magnitude of the hyperfine inter- action cannot be read directly from the spectra. In addition, the centre of the spectrum does not necessarily correspond to the g factor. However, even for the smaller a values, ca. 1.5 mT, typical of the aminoxyl nitrogen isotropic hyperfine interaction, precision work at X-band frequencies requires corrections to be made for both a (ca.0.4%)and the g factor. As noted above, in RF EPR spectroscopy and imaging the applied magnetic field is only of the order of 10 mT and this can lead to very large second-order effects for aminoxyls: it is the ratio of the applied magnetic field to the hyperfine inter- action which produces the effects. The mathematics were worked out by Breit and Rabi”.” from Stern-Gerlach experiments, long before magnetic resonance was discovered. When the high-field approximation (the Paschen-Back effect in atomic spectra) is not valid, the complete decoupling of the electron and nuclear spins, S and I, cannot be assumed and the quantum numbers F and MF are now ‘good’ quantum numbers.” For I = 1, F has values of 1/2 and 3/2, thus when Bola is relatively small, there are the energy states E(F, MF), which are explicitly : E( + 1/2, + 1/2); E( + 1/2, -1/2); E( +3/2, +3/2); E( +3/2, + 1/2);E( +3/2, -1/2); E( +3/2, -3/2).Since the selection rules are AMF = 0 for F = 0 states and AM, = 0, & 1 for other F states, there are a total of six allowed transitions” as zero field is approached (see Fig. 2). Four of the six transitions are of the n type (AMF = fl), being polarized in the x or y directions, while two of the tran- sitions (AMF = 0) are of the a type, being polarized in the z direction. This situation is to be compared with the three transitions observed at high fields for I = 1. Experiments were carried out by Decorps and Fric13 who obtained EPR spectra from aqueous solutions of Fremy’s salt (potassium peroxylamine disulfonate) at 80 and 210 MHz. At the higher frequency, the applied magnetic field is sufficiently large to almost exclude the a transitions, but at 80 MHz the random- ly oriented permanent magnetic field from the 14N nucleus is competing strongly with the main magnetic field in the z direction, thus both a and n transitions were observed. Recently, Guiberteau and Grucker14 have demonstrated that dynamic nuclear polarization with a-transition EPR irradia- tion is allowed at low magnetic fields and that with fields lower than 1.7 mT, saturation of a transitions is more effi- cient than that for saturation of the n transitions.Decorps and FricI3 also recorded the EPR spectrum of DPPH (a,a’-diphenyl-/3-picrylhydrazyl) in benzene and found that the spectrum at 40 MHz is compressed compared with spectra obtained at higher frequencies, that is, it is part way to becoming a single line.Note that at zero field, linewidths are F = 312 F= 112 I I I I 1 1-5 1 I 01234567 Sola Fig. 2 Energy levels of an I = 1 spin system in terms of B,/u, the ratio of the main magnetic field to the hyperfine interaction. The vertical lines represent the transitions for u = 1.5 mT and B, = 7 mT. narrow (ca. 0.5 MHz) and only one line is observed having an energy of a(Z + 1/2): a definitive review of zero-field EPR spectroscopy has been published by Bramley and Strach.” If third-order corrections are neglected, the hyperfine coupling constant can be determined for integral nuclear spins from the separation (=21a) of the extreme hyperfine lines, and for half-integral spins from the separation (=a) of the MI = f1/2 lines.An excellent paper by Boate et all2 describes the effect of very large isotropic hyperfine interactions (up to ca. 1900 mT) at X-band frequencies, which cause the recorded EPR spectra to bear no resemblance to the hyperfine patterns predicted by first-order theory. When a very large hyperfine interaction is present, only one EPR line is accessible corresponding to MI = -I (for positive a values): other mixed transitions may appear having intensities comparable to the EPR lines. They showed that the remaining EPR lines are observable only at microwave frequencies in excess of (I + 1/2)gBa.Currently, there is an interest in low-field EPR spectros- copy because of the wealth of biological problems that can be tackled with ‘lossy’ samples. Some published RF spectra of aqueous aminoxyl solutions show uneven line spacings, but none of the authors concerned have drawn attention to this fact: a selection of such data is shown in Table 1.The ratios of the splittings observed experimentally correspond to those predicted by diagonalization of the energy matrix. Thus these Table 1 Splittings observed for aminoxyl radicals at low fields ratio of spectrometer splitting/mT split tings frequency radical /MHz +1,0 0, -1 obs.” calc.’ ref. -CTPO 790 1.615 1.692 0.954 0.951 16 4-oxo-TEMPO 300 1.615 1.800 0.897 0.870 17 Fremy’s salt 280 1.247 1.443 0.864 0.861 18 Fremy’s salt 2 10 1.241 1.459 0.851 0.821 13 tert-Bu,NO 200 1.558 2.013 0.774 0.795 19 Fremy’s salt 80 0.983 1.761 0.558 0.593 13 ~ ~~ a Measured from published spectra.’Calculated using the program QPOWA. I. a.1 I L . . . I , . .......I 2.1 7. B/mT 12.1 Fig. 3 9.2 GHz (a)and 200 MHz (b)(computed) rigid limit spectra of the 1,1,3,3-tetra[2H,]methylisoindolin-2-yloxyl radical ratios can be used to check the linearity of the field sweep of low-field EPR spectrometers. Note, for the aminoxyl radical data given in Table 1, for frequencies less than ca. 100 MHz, that the centre line becomes relatively less intense than the outer lines, and the low-field line is more intense than that at high fields.Above 200 MHz, a may be determined reliably as half the separation of the outer lines. Implications for Low-field EMRI The intrinsic dependence of the line position on the externally applied magnetic field caused by the Breit-Rabi effect sug- gests that a large imaging gradient might apply additional 329 BImT 334 B/mT 12 Fig. 4 9.2 GHz (a) and 200 MHz (b)(computed) rigid limit spectra of the [''N]-1,1,3,3-tetra[2H,]methylisoindolin-2-yloxyl radical J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 n Fig. 5 Computed 9.2 GHz (a) and 200 MHz (b)spectra of 1,1,3,3- tetra[2H,]methylisoindolin-2-yloxyl radical for rotational diffusion constants of (i) lo9 s-l, (ii) 2 x 10' s-', (iii) 3 x 10' s-', (iv) 5 x 106 s-'.A linewidth of 0.04 mT was used in the calculations. The field sweep is 10 mT. Note that the total intensities of the spectra are not to scale. undesirable position-dependent dispersion to the broadened spectrum. In fact, it seems that for the most common EMRI experiment, namely field-swept CW EMRI, each part of the resonance is brought on to resonance at a constant frequency by the field sweep, hence no distortion is introduced. For Fourier transform or (as yet, hypothetical) frequency-swept EMRI, the Breit-Rabi effect will cause the frequency of spins to be non-linearly related to their position, thus producing image distortions. In principle, as the non-linearity is known, it should be possible to apply a post-processing correction during image reconstruction at the expense of incurring vari- able resolution across the image.Near Removal of g Factor Anisotropy The nature of a powder spectrum is determined by the anisotropy of the g factors and of the hyperfine interactions. The presence of these anisotropies is utilised in spin probe/ label experiments that are normally carried out at X-band frequencies. However, EPR spectra obtained at radiofre-quencies can have a completely different appearance since the g factor anisotropy essentially disappears (the hyperfine anisotropy remains), the consequences of which are discussed below. Imaging A powder X-band spectrum of a typical aminoxyl radical (l,l, 3,3-tetra[2H,]methylisoindolin-2-yloxyl,20 C2H,JTMIO) is shown in Fig.3(a). Owing to its complexity and large line- widths this spectrum would not be suitable for solid-state imaging. However, using the simulation program QPOWA J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 6 Computed 9.2GHz (a)and 200 MHz (b)spectra of ["Nl-l, 1,3,3-tetra[2H,]methylisoindolin-2-yloxylradical for rotational diffu- sion constants of (i) lo9 s-l, (ii) 10' s-', (iii) 10' s-l, (iv) 5 x lo6 s-'. A linewidth of 0.04mT was used in the calculations. The field sweep is 10 mT. Note that the total intensities of the spectra are not to scale. the RF EPR spectrum of the same radical [see Fig. 3(b)] is essentially a single narrow line and thus has imaging poten- tial. By contrast, it may be seen from Fig.4 that the reverse is true for the QPOWA-simulated spectrum of the 15N-substituted radical. The sharpness of the rigid limit central line of the 14N-containing radical may be seen by comparing the fast and slow rotational spectra shown in Fig. 5. Rotational Diffusion Using the computer program devised by Schneider and Freed,21 we have simulated sets of spectra for C2H1,]TMI0 and its "N analogue for a series of rotational diffusion con- stants. This has been done for spectra at X-band and at 200 MHz. Since the program requires the high-field approx- imation to hold we have obtained the latter by converting the g factors to their equivalents at X-band. Thus, the actual values" of gxx= 2.00820, gyy= 2.00523 and g,, = 2.00147 become 2.00504, 2.00498 and 2.00490, respectively : the corre- sponding hyperfine interactions remain unchanged at 3.382, 0.439 and 0.500mT for 14N.Fig. 5 shows the results obtained for [*H12]TMI0 at 9.2 GHz and at 200 MHz. The former show the familiar pattern as the rotational diffusion constant is decreased: in the 'fast' regime this constant can be calcu- lated in the usual wayz2 from the asymmetric line broading. In the slow region which occurs (for example) in solid poly- mers, motion is measured from the extrema of the powder- like ~pectra.'~ The spectra simulated for the "N- labelled radical are shown in Fig. 6. Again, rotational diffusion con- stants could be derived for the 'fast' region from the asym- metric line broadening of the X-band spectra.For the RF spectra, band-shape analysis would have to be employed to obtain the constants for the fast motional regime. Note that because the high-field approximation has been used to calculate the RF spectra, they do not show the Breit- Rabi effect as they should. To summarise this section, 'N-labelling is advantageous for Overhauser enhancement at radiofrequencies, but it is dis- advantageous for both radiofrequency spectroscopy and imaging. M.R.S. is indebted to SERC for financial support. We also wish to thank Dr. Gorazd Planinsic and Dr. K. F. Preston for helpful discussions, Profs. R. L. Belford and M. J. Nilges for allowing us to use their QPOWA computer program, and Dr. S. A. Fairhurst for her assistance.References 1 G. R. Eaton, S. S. Eaton and K. Ohno, EPR Imaging and In Vivo EPR, CRC Press, Boca Raton, Florida, 1991. 2 P. Mansfield and P. G. Morris, NMR Imaging in Medicine, Academic Press, New York, 1982. 3 E. Szczepaniak and J. P. Hornak, J. Magn. Reson., 1993, A104, 315. 4 D. G. Norris and J. M. S. Hutchison, Magn. Reson. Imaging, 1990,8,33. 5 R. K. Woods, G. G. Bacic, P. C. Lauterbur and H. M. Swartz, J. Magn. Reson., 1989,84, 247. 6 P. Turek, J-J. Andre, M. Moussavi and G. Fillion, Mol. Cryst. Liq. Cryst., 1989, 176, 535. 7 G. Bacic, K. J. liu, J. A. O'Hara, R. D. Harris, K. Szybinski, F. Goda and H. M. Swartz, Magn. Reson. Med., 1993,30,568. 8 D. Grucker, Magn. Reson. Med., 1990,14, 140. 9 D. J. Lurie, D. M. Bussell, L.H. Bell and J. R. Mallard, J. Magn. Reson., 1988, 76, 366. 10 G.Breit and I. I. Rabi, Phys. Rev., 1931,8,2082L. 11 J. E. Nafe and E. B. Nelson, Phys. Rev., 1948,73,718. 12 A. R. Boate, J. R. Morton and K. F. Preston, J. Magn. Reson., 1976,24,259. 13 M. Decorps and C. Fric, J. Phys. E: Sci. Instrum., 1972,5,337. 14 T. Guiberteau and D. Grucker, J. Magn. Reson., 1993, A105,98. 15 R.Bramley and S. J. Strach, Chem. Rev., 1983,83,49. 16 S. Ishida, H. Kumashiro, N. Tsuchihashi, T. Ogata, M. Ono, H. Kamada and E. Yoshida, Phys. Med. Biol., 1989,34, 1317. 17 J. A. Brivati, A. D. Stevens and M. C. R. Symons, J. Magn. Reson., 1991,92,480. 18 M. Alecci, S.D. Penn, A. Sotgiu, L. Testa and I. Vannucci, Rev. Sci. Instrum., 1992,63,4263. 19 J. P. Hornak, M. Spacher and R. G. Bryant, Meas. Sci. Technol., 1991, 2, 520. 20 R. Bolton, D. G. Gillies, L. H. Sutcliffe and X. Wu, J. Chem. SOC.,Perkin Trans. 2, 1993, 2049. 21 D. J. Schneider and J. H. Freed, in Biological Magnetic Reson- ance, ed. L. J. Berliner, Plenum, New York, 1989,vol. 8, p. 1. 22 P. L. Nordio, in Spin Labeling, Theory and Applications, ed. L. J. Berliner, Academic Press, New York, 1976. 23 P. Tormala, G. Weber and J. J. Lindberg, in Molecular Motion in Polymers by ESR, ed. R. F. Boyer and S. E. Keinath, Harwood, New York, 1980. Paper 4/02219J; Received 14th April, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002671

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2677-2681 Excess Molar Enthalpies of Nitrous Oxide-Toluene in the Liquid and Supercritical Regions R. Cesar Castells,? Carlos Menduifia, Concepcion Pando and Juan A. R. Renuncio Departamento de Quimica Fisica I, Universidad Complutense . E-28040 Madrid, Spain The excess molar enthalpies HEof [xN,O + (1 -x)C,H,CH,] have been measured in the liquid and supercritical regions over the whole concentration range. Mixtures at a temperature of 308.15 K and pressures of 12.27 and 15.00 MPa show moderate endothermic and exothermic mixing in the toluene-rich region and nitrous oxide-rich region, respectively. Mixtures at 308.15 K and 9.49 MPa and at 323.15 K and 12.27 and 15.00 MPa show moderate exothermic mixing. Mixtures at 323.15 K and 7.64 and 9.49 MPa show significant exothermic mixing.The changes observed in the excess enthalpy with temperature and pressure have been discussed in terms of liquid-vapour equilibrium and critical constants for nitrous oxidetoluene. The experimental values of HE at 308.15 and 323.15 K and those previously reported at 313.15 K have been analysed using the Peng-Robinson equation of state. In the last dFade, thermodynamic properties such as the excess molar ebthalpies of carbon dioxide-hydrocarbon mix-tures have been extensively studied in the near critical and supercritical regions because of their anomalous behaviour in the vicinity of tRe critical locus and because of their practical importance in high-pressure technology.'*2 Carbon dioxide has been employed commonly in supercritical fluid extraction because of its low toxicity, safety and low critical tem-perature.It has been reported that N,O is a better solvent than CO, for certain molecule^.^.^ The nitrous oxide and carbon dioxide molecules are isoelectronic and have the same molar mass and very close critical points. For CO, T, is 304.21 K and p, is 7.38 MPa,' while for N,O T, is 309.6 K and p, is 7.24 MPa.6 N20 has a weak dipole moment of 0.166 D,S while CO, has none. This fact seems to be related to the higher affinity shown by N,O to certain molecules. We reported previously measurements of the excess molar enthalpies HEfor carbon dioxide-toluene at temperatures of 308.15, 358.15, and 413.15 K and pressures up to 12.67 MPa' and for nitrous oxide-toluene at 313.15 K and pressures up to 15.00 MPa.' The results for carbon dioxide-toluene could be fitted to the Peng-Robinson equation of state."," This equation is a cubic equation of state of the form RT 4T)p=--v -b V(V + b) + b(u -b) For pure components, a and b are expressed in terms of the critical properties and the acentric factor 0: a( T) = 0.45724a(TXR2T:/p,) (2) b = 0.07780(RTc/p,) (3) a(T)= (1 + ic[l -(T/T,)''2])2 (4) IC = 0.37464 + 1.542260 -0.269920~ (5) For mixtures, a and b are given by a = 11xi xi1 -dij)(Ui Uj)1'Z (6)ij b = 1xibi (7) 1 where dij = dji is the binary interaction parameter which is usually determined from experimental binary data.t Permanent address : Universidad de La Plata-CIDEPINT, Argentina.1 1 D z 3.33564 x C m.Flow calorimetric measurements of HEcovering the whole concentration range for nitrous oxide-toluene at 308.15 and 323.15 K and 7.64, 9.49, 12.27, and 15.00 MPa are reported here. HE for these mixtures are calculated using the cubic equation of state mentioned above and the resulting values are compared with experimental values. The pressure and temperature conditions of the measurements were chosen in order to compare results for nitrous oxide-toluene with those previously obtained for carbon dioxide-toluene.' Experimental The measurements were made using the flow mixing appar- atus and the experimental procedure described previo~sly.~ The chemicals were pumped into the calorimeter by two ther- mostatted ISCO pumps (model LC2600).The calorimeter cell was thermostatted in a silicon oil bath (k0.0005 K) and the pressure was controlled by a back-pressure regulator. A manually controlled piston acts as a fine adjustment of the nitrogen pressure over the back-pressure regulator. Oscil- lations in pressure were smaller than kO.01 MPa. The materials employed were nitrous oxide (SEO 99.99 mol% pure) and toluene (Merck 99.5 mol% pure) previously dehydrated with sodium. All runs were made in the steady-state fixed composition mode. Flow rates were selected to cover the whole mole frac- tion range. In most cases, the measurements were carried out at a total flow rate of 0.010 cm3 s-'. A few measurements were carried out at a total flow rate of 0.005 cm3 s-'.Flow rates range from 5.0 x lo-' to 2.0 x lop4 mol s-'. The reproducibility of the results was & 1%. The flow rates mea- sured in cm3 s-' were converted to mol s-' and to mole fractions using the densities of the two materials estimated as follows. The densities of N20 at the temperature of the pump and at pressures of 7.64, 9.49, 12.27, and 15.00 MPa were calculated by interpolation of the pressure-volume isotherms of the liquid nitrous oxide measured by Couch et al." The densities of toluene at the temperature of the pump and at pressures of 7.64, 9.49, 12.27 and 15.00 MPa were calculated from the densities and isothermal compressibilities of Garba- josa et ~1.'~and Aicart et ~1.'~ Results Excess molar enthalpies were determined for [xN,O + (1 -x)C,H,CH,] over the entire composition range at 308.15 K and 9.49, 12.27 and 15.00 MPa and at 323.15 K and 7.64, J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Experimental and calculated excess enthalpies HEfor [xN,O + (1 -x)C,H,CH,] HE/(Jmol -') HE/(J mol-') HE/(J mol-') X expt. calc. X expt. calc. X expt. calc. 308.15 K, 9.49 MPa 0.050 1.6 -3.8 0.505 -165 -172 0.822 -425 -430 0.101 -1.4 -7.6 0.551 -223 -211 0.874 -426 -427 0.151 -18 -13 0.601 -271 -257 0.898 -418 -410 0.202 -24 -21 0.649 -298 -301 0.927 -373 -366 0.302 -60 -49 0.701 -359 -350 0.949 -307 -307 0.401 -85 -98 0.749 -386 -389 0.980 -168 -162 (145 1 -120 -131 0.802 -405 -422 308.15 K, 12.27 MPa 0.052 14 17 0.457 16 17 0.851 -200 -200 0.102 32 31 0.504 -5.4 -2.1 0.876 -202 -201 0.153 49 42 0.551 -21 -26 0.900 -194 -195 0.203 44 49 0.606 -54 -58 0.907 -190 -191 0.257 52 52 0.653 -86 -88 0.929 -176 -173 0.308 50 50 0.754 -161 -156 0.950 -142 -144 0.353 46 45 0.761 -163 -160 0.98 1 -69 -72 0.407 32 33 0.805 -184 -185 308.15 K, 15.00 MPa 0.053 32 32 0.463 112 111 0.854 -69 -73 0.104 56 57 0.510 100 101 0.879 -79 -80 0.156 82 79 0.558 86 86 0.890 -81 -81 0.207 92 95 0.612 63 63 0.902 -83 -82 0.262 108 108 0.665 37 35 0.909 -80 -81 0.358 120 119 0.719 -0.1 2.8 0.93 1 -78 -76 0.386 120 119 0.766 -29 -27 0.952 -63 -64 0.413 118 118 0.809 -51 -52 0.98 1 -34 -32 0.438 114 115 323.15 K, 7.64 MPa 0.050 -223 -249 0.496 -2587 -2673 0.798 -4291 -4359 0.099 -580 -535 0.546 -2953 -2960 0.848 -4482 -4561 0.151 -792 -833 0.549 -3016 -2979 0.892 -4435 -4307 0.204 -1142 -1123 0.603 -3354 -3285 0.953 -2099 -2 108 0.252 -1320 -1364 0.649 -347 1 -3534 0.970 -1255 -1258 0.300 -1635 -1604 0.698 -3833 -3798 0.975 -968 -996 0.352 -1872 -1872 0.750 -4125 -4086 0.990 -155 -351 0.399 -2155 -2119 323.15 K, 9.49 MPa 0.05 1 -113 -100 0.405 -850 -884 0.707 -1598 -1595 0.103 -176 -205 0.406 -922 -885 0.754 -1694 -1687 0.154 -338 -313 0.478 -1062 -1059 0.806 -1796 -1764 0.205 -419 -423 0.510 -1126 -1137 0.876 -1718 -1780 0.258 -546 -540 0.547 -1207 -1225 0.929 -1620 -1616 0.306 -668 -651 0.557 -1265 -1251 0.950 -1443 -1433 0.347 -729 -745 0.654 -1474 -1479 0.97 1 -1139 -1114 0.348 -749 -747 323.15 K, 12.27 MPa 0.054 -12 -18 0.464 -305 -300 0.722 -553 -543 0.106 -32 -40 0.468 -300 -304 0.759 -581 -569 0.211 -102 -97 0.504 -358 -338 0.762 -563 -571 0.213 -104 -98 0.515 -340 -349 0.812 -576 -591 0.266 -131 -134 0.516 -341 -350 0.829 -573 -592 0.267 -127 -135 0.562 -396 -395 0.892 -559 -554 0.27 1 -141 -138 0.568 -403 -401 0.900 -561 -543 0.318 -170 -173 0.597 -422 -430 0.949 -444 -405 0.357 -198 -205 0.613 -450 -446 0.952 -385 -388 0.366 -210 -212 0.617 -463 -450 0.982 -195 -198 0.391 -250 -234 0.670 -498 -499 0.990 -118 -117 0.416 -267 -256 0.690 -513 -517 0.99 1 -104 -111 0.418 -249 -257 0.715 -529 -538 323.15 K, 15.00 MPa 0.054 0.3 1.6 0.418 -69 -77 0.857 -316 -320 0.105 1.6 0.8 0.440 -87 -88 0.881 -314 -310 0.106 2.5 0.7 0.444 -85 -90 0.89 1 -302 -303 0.159 -2.8 -3.2 0.465 -110 -102 0.904 -299 -291 0.21 1 -11 -10 0.515 -127 -131 0.932 -256 -250 0.264 -29 -21 0.559 -163 -160 0.935 -240 -243 0.294 -28 -29 0.583 -186 -177 0.936 -236 -241 0.362 -49 -53 0.667 -238 -236 0.972 -142 -138 0.363 -61 -53 0.7 14 -277 -269 0.982 -95 -98 0.387 -62 -63 0.720 -263 -273 0.982 -87 -98 0.390 -66 -65 0.767 -300 -300 0.998 -4.9 -12 0.415 -71 -76 0.812 -312 -318 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I-E 30 z -200 -400 0 0.2 0.4 0.6 0.8 1.0 X Fig. 1 Plot of HE against x for [xN,O + (1 -x)C6H,CH3] at 308.15 K as a function of pressure: 0,9.49; A, 12.27;0,15.00MPa; (-) calculated from eqn. (8); (--) calculated from eqn.(1) 9.49, 12.27, and 15.00 MPa. The results are given in Table 1. Values for HEat each temperature and pressure studied were fitted to the equation: 1C,(2x -1)" HE/J mol-I = x(1 -x) 1 +,=' Bk(2x -(8) k= 1 The coefficients C, and Bk are given in Table 2 together with the standard deviations, s, between experimental and calcu- lated HEvalues. Fig. 1 and 2 are plots of HE against x for the isobars studied at 308.15 and 323.15 K, respectively. Fig. 3 is a plot of p against T for [xN,O + (1 -X)C6H,CH3] showing the vapour-pressure equilibrium curve of nitrous oxide,12 the critical locus in the vicinity of the N20 critical point, and the points at which experimental measurements of HE have been made. For toluene T, is 591.8 K and p, is 4.10 MPa.6 Vapour-liquid equilibrium data or critical locus data are not available for [xN,O + (1 -x)C6H,CH3].The critical locus shown in Fig. 3 has been estimated using the procedure developed by Heidemann and Khalil15 and the Peng-Robinson equation of state." The densities of N,O and toluene at the temperatures and pressures of the experiments are listed in Table 3. Values for N,O densities were taken from Couch et a1.' and from Langenfeld et a1.16 Values for toluene densities were calcu- lated from the coefficients of the Tait equation given by Takagi." The toluene enters the calorimeter as a liquid because the temperature is lower than the critical tem-perature of this component and the pressures are always 0 -1 000 I-0 E -20003 z -3000 -4000 0 0.2 0.4 0.6 0.8 1.0 X Fig. 2 Plot of HE against x for [xN20 + (1 -X)C6H,CH3] at 323.15 K as a function of pressure: 0,7.60; 0,9.49; A, 12.27; 0, 15.00 MPa; (-) calculated from eqn. (8); (--) calculated from eqn.(1) higher than its critical pressure. The values of the density of toluene shown in Table 3 are typical of a liquid and change very little with pressure. At 308.15 K the nitrous oxide also enters the calorimeter as a liquid because the temperature is lower than the critical temperature of this component and the pressures are always higher than its critical pressure. The values of the density of nitrous oxide at 308.15 K shown in I ' I ' I ' I ' Ii 00 13 00 I? 11T Q 9 7 0 rlI#I8I,I 300 310 320 330 TIK Fig.3 Plot of p against T for [xN,O + (1 -x)C6H,CH3] showing the vapour-liquid equilibrium curve (---) and critical point (A) of nitrous oxide, the critical locus (-) from x = 1.00 to 0.97, and (T, p) coordinates (0)where experimental measurements were made Table 2 Coefficients and standard deviation, s, for least-squares representation of HE/J mol-' for [xN,O + (1 -x)C,H,CH,] by eqn. (8) T = 308.15 K T = 323.15 K coefficients 9.49" 12.27" 15.00" 7.64" 9.49" 12.27" 15.00" ~~ -672.50 -1.0886 414.66 -10776.7 -4449.1 -1340.7 -487.70 1036.5 895.99 728.85 ,-752.96 820.66 851.63 -199.83 -271.33 -350.32 ----285.19 -329.81 -223.46 ----0.84701 0.73508 0.55148 1.0787 0.92391 0.83657 0.73433 ---0.13777 -------0.43665 -------0.98008 -----I 0.43 122 ------1.24735 ---10 3.2 2.0 83 25 11 5.8 " p/MPa.2680 Table 3 Densities of N,O and toluene under the temperature and pressure conditions of the experiments N*O toluene pjMPa 308.15 K 323.15 K 308.15 K 323.15 K 7.64 -222 -846 9.49 740 437 86 1 848 12.27 790 664 863 850 15.00 819 727 865 852 Table 3 are those typical of a liquid and change very little with pressure. At 323.15 K the nitrous oxide enters the calo- rimeter as a supercritical fluid because the temperature and pressures studied are always greater than those defining its critical point. However, this NzO fluid may be a low-density (gas-like) fluid or a high-density (liquid-like) fluid depending on the pressure.The values of the density of nitrous oxide at 323.15 K change as the pressure increases from those typical of a gas at 7.64 and 9.49 MPa to those typical of a liquid at 12.27 and 15.00 MPa. The resulting [xN,O + (1 -x) C,H,CH,] may be liquid, gas-like fluid, or liquid-like fluid, depending on the critical temperature and pressure of the particular mixture. The phase diagram for [N,O+ C,H,CH3] belongs to class I in the classification of van Konynenburg and Scott.18 The critical locus predicted by the Peng-Robinson equation" for [N,O + C6H5CH,] is similar to the critical locus for the [CO, + C,H,CH,].* The critical line goes through a maximum at p II 17.5 MPa and T 21 435 K and returns to the toluene critical point (T,= 591.8 K, p, = 4.10 MPa).Since the temperatures and pressures studied are far from those defining the critical point of toluene, [xN,O + (1 -X)C,H,CH,] is a liquid from x = 0 to x N 0.95, and is a fluid only in a narrow composition range in the N,O-rich region. Owing to the transition from a liquid to a fluid mixture, there is also a two-phase region between the liquid and the fluid mixture regions. This is detected by a high-slope linear section in the N,O-rich region of the isobars shown in Fig. 1 and 2. Unfortunately, the vapour and liquid equilibrium-phase compositions cannot be determined from these plots. When the states and densities of the pure components and the mixture are similar (liquid or liquid-like fluid nitrous oxide and liquid toluene forming a liquid or liquid-like fluid mixture), the values of HE are negative or slightly positive.This is so at 308.15 K for isobars at 9.49, 12.27 and 15.00 MPa and at 323.15 K for isobars at 12.27 and 15.00 MPa when the density values are similar for N,O and toluene. When the states and densities of the pure components differ (gas-like fluid nitrous oxide and liquid toluene), and the resulting mixture is a liquid, large negative values of HE are observed. This is so at 323.15 K for the isobars at 7.60 and 9.49 MPa when the density of N,O is much lower than that of toluene. The shape of the isobars in Fig. 1 and 2 denotes behav- iour similar to that previously reported for [xCO, + (1 -x)C,H,CH,].* The critical point of carbon dioxide is very close to that of nitrous oxide and the [xCO, + (1 -X)C6H5CH3]and [xN,O + (1 -X)C6H5CH,]critical loci are very similar.The temperature and pressures at which experimental measurements have been made for [XCO, + (1 -X)C,H,CH,] and [xNZO + (1 -x)C,H,CH,] have a similar position with respect to the critical locus in the p against T plot. This may be seen by comparing the plot of Fig. 2 with a similar one shown for [XCO, + (1 -x)C,H,CH3] in Fig. 3 of ref. 8. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Calculationof the Excess Molar Enthalpy The excess molar enthalpy of a binary mixture is given by HE= [H -H*lmiX-1xi[H -H*li (9) I where [H -H*lmiXis the residual molar enthalpy of the mixture and [H -H*li is that of components 1 and 2, respectively. The residual molar enthalpy is given by H -H* = RT(z -1) + JI{T[z]"-p} dv (10) where z is the compressibility factor.For a fluid which follows the Peng-Robinson equation of state, eqn. (10) becomes -da7'--a H-H*=RT(z-l)+ dT In [z + 2.414Bj (11)2312b z -0.414B where B is given by B=--b RT Comparison of the Peng-Robinson Equation of State with Experiment Excess enthalpies for the nitrous oxide-toluene system were calculated at 308.15, 313.15 and 323.15 K from 7.60 to 15.00 MPa using the expressions given in the previous section. The experimental values of HE at 313.15 K were reported in a previous paper and are shown in Fig. 4. The values used for the critical constants are those already given.Values for the acentric factor were taken from Reid et aL6 The binary inter- action parameter was adjusted to give the best fit to the experimental HE values at 308.15, 313.15 and 323.15 K. A value of 0.1018 was obtained for a,, . Since the vapour and liquid equilibrium-phase compositions are unknown, HE values were calculated only in the one-phase region. The curves of long dashes shown in Fig. 1, 2 and 4 are HEvalues calculated using this value for a,, . Although better results could be obtained if a,, was adjusted to give the best fit at each temperature, we prefer to make comparison with experi- ment using a single value for the interaction parameter. For the 9.49, 12.27 and 15.00 MPa isobars at 308.15, 313.15 and 323.15 K the mean deviation of the calculated values of HE from experiment is 80 J mol-'.For the 7.64 MPa isobar at 323.15 K the mean deviation is 100 4 mol-'. This is in good agreement if we take into account the fact that this isobar loo0 -1 000 I-0 E -2000 0002'-3000 -4000 0 0.2 0.4 0.6 0.8 1.0 X Fig. 4 Plot of HE against x for [xN,O + (1 -x)C,H,CH,] at 313.15 K as a function of pressure: 0,7.60; 0,9.49; A, 12.27; 0, 15.00 MPa; (--) calculated from eqn. (1) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 268 1 exhibits a minimum of -4500 J mol-'. The agreement is not ninger, M. Radosz, M. A. McHugh and V. J. Krukonis, Elsevier, good for the 7.60 MPa isobar at 313.15 K which exhibits a minimum of -2300 J mol-' (the mean deviation is 600 J mol -').This discrepancy between experimental measure-ments and the calculated curve seems to arise from the failure of the Peng-Robinson equation of state to give the correct value for the compressibility factor of nitrous oxide under these 3 4 5 6 Amsterdam, 1985. K. Sakaki, J. Chem. Eng. Data, 1992,37,249. N. Alexandrou, M. J. Lawrence and J. Pawliszyn, Anal. Chem, 1992,64,30 1. Carbon Dioxide, IUPAC Thermodynamic Tables of the Fluid State, Pergamon, Oxford, 1976. R. C. Reid, J. M. Prausnitz and B. E. Poling, The Properties of conditions of temperature and pressure. Couch et aI.12 reported experimental values for the nitrous oxide compress- ibility factor for a wide range of temperature and pressure. The agreement between these values for z and those calcu- lated by means of the Peng-Robinson equation or by means of the Kubic equation" is good except at 7.60 MPa and 7 8 9 10 Gases and Liquids, McGraw-Hill, Singapore, 1988.D. E. Stogryn and A. P. Stogryn, Mol. Phys., 1966,11,371. C. Pando, J. A. R. Renuncio, R. S. Schofield, R. M. Izatt and J. J. Christensen, J. Chem. Thermodyn., 1983, 15, 747. R. C. Castells, C. Menduiiia, C. Pando and J. A. R. Renuncio, J. Chem. Thermodyn., 1994,26,641. D-Y. Peng and D. B. Robinson, Ind. Eng. Chem. Fundam., 1976, 313.15 K when we are in the vicinity of the nitrous oxide critical point. Wormald and c~-workers~~-~~ have sucessfully used the Kubic equation of state to fit excess enthalpies of several binary mixtures at high pressures.HE data for the [N,O-C,H,CH, J system were also analysed using this equa- 11 12 13 15, 59. A. G. Casielles, C. Pando and J. A. R. Renuncio, Thermochim. Acta, 1989, 154, 57. E. J. Couch, L. J. Hirth and K. A. Kobe, J. Chem. Eng. Data, 1962,6, 229. G. Garbajosa, G. Tardajos, E. Aicart and M. Diaz Peiia, J. Chem. Thermodyn., 1982,14,671. tion. Results from this calculation are not reported in this paper because values for the deviation between experimental and calculated HE are higher than those obtained for the Peng-Robinson equation of state. 14 15 16 E. Aicart, G. Tardajos and M. Dim Peiia, J. Solution Chem., 1982, 11, 557. R. A. Heidemann and A. M. Khalil, AIChE J., 1980,26,769. J. J. Langenfeld, S. B. Hawthorne and D. J. Miller, Anal. Chem., 1992,64,2263.This work was funded by the Spanish Ministry of Education 17 18 T. Takagi, Rev. Phys. Chem. Jpn., 1978,48, 17. P. H. Van Koynenberg and R. L. Scott, Phil. Trans. R. SOC., (DGICYT) Research Project PB-9 1-0392. We appreciate the aid given to us by Dr. R. A. Heidemann in estimating the critical locus. R.C.C. acknowledges the Universidad Com- plutense of Madrid for a visiting research professorship at the Department of Physical Chemistry. 19 20 21 1980,298,495. W. L. Kubic, Fluid Phase Equilibria, 1982,9, 79. C. N. Colling, N. M. Lancaster, M. J. Lloyd, M. Masucci and C. J. Wormald, J. Chem. SOC., Faraday Trans., 1993,89,77. M. Masucci and C. J. Wormald, J. Chem. SOC.,Faraday Trans., 1993,89,1345; 3375. 22 M. Masucci, C. J. Wormald and L. Yan, J. Chem. SOC.,Faraday References 23 Trans., 1993,89,4193. C. J. Wormald and C. N. Colling, J. Chem. Thermodyn., 1993, 1 Supercritical Fluid Technology, ed. T. J. Bruno and J. F. Ely, CRC Press, Boca Raton, FL, 1991. 2 G. Morrison, J. M. H. Levelt Sengers, R. F. Chand and J. J. 24 25, 599. C. J. Wormald and M. J. Lloyd, J. Chem. Thermodyn., 1993, 26, 101. Christensen, Supercritical Fluid Technology, ed. J. M. L. Pen- Paper 4/0206 1 H ; Received 6th April, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002677

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2683-2689 Thermodynamic Study on the Transfer of the Tin@), Lead(l1) and Alkaline-earth-metal Ions from Water to Methanol, Dimethyl Sulfoxide, Acetonitrile, Pyridine and N,N-Dimethylthioformamide Monika Chaudhry, Yoshiaki Kinjot and lngmar Persson* Department of Chemistry, Swedish University of Agricultural Sciences, P.O.Box 7015,S-75007 Uppsala, Sweden The thermodynamic functions for the transfer reactions of the tin(ri), lead@) and alkaline-earth-metal ions from water to methanol, acetonitrile, dimethyl sulfoxide and pyridine, and of the tin(ir) and lead(ii) ions to N,N-dimethylthioformamide, are reported. The Gibbs energies of transfer, AtG", have been calculated from the standard electrode potentials of the Sn2+/Sn(s) and Pb2+/Pb(s) couples, which have been determined poten- tiometrically with the Ag(s)/Ag+ electrode as reference in the title solvents, and the Gibbs energies of transfer of the silver ion.The Gibbs energies of transfer of the alkaline-earth-metal ions were calculated from electrode or halfwave potentials of the M2+/M(am) couples in the title solvents and the difference in standard electrode potential between the M2+/M(am) and M2'/M(s) couples reported in the literature. The enthalpies of transfer, A,H*, have been obtained from calorimetrically determined enthalpies of solution of the anhydrous metal tri- fluoromethylsulfonates. The entropies of transfer, A$, have been calculated from the experimentally deter- mined AtG" and AtHe values. All measurements have been carried out at 25°C.The extrathermodynamic tetraphenylarsonium tetraphenylborate (TATB) assumption has been applied in order to calculate the contribu- tions from the single ions. The tin(it) and lead@) ions are solvated more strongly in dimethyl sulfoxide and N,N-dimethylthioformamide than in water, while they are solvated more weakly in methanol and acetonitrile. The small tin(ii) ion is solvated more weakly and the larger lead(ti) ion is solvated more strongly in pyridine than in water. The alkaline-earth-metal ions are solvated more strongly in dimethyl sulfoxide than in water, while meth- anol and the nitrogen donor solvents acetonitrile and pyridine solvate these ions more weakly than water. The enthalpies of transfer for these ions to the solvents studied are exothermic except for the tin(ii), calcium and strontium ions to acetonitrile.The entropies of transfer to all solvents are markedly negative except for N,N-dimethylthioformamide where the TAtS" values are close to zero. The difference in solvation of an ion between two solvents is mainly dependent on the bonding character of the ion-solvate bonds and on the way in which the ion affects the solvent bulk. The transfer thermodynamic functions for the tin(@, lead(@ and alkaline-earth-metal ions from water to methanol, ace- tonitrile, dimethyl sulfoxide and pyridine and for the tin@) and lead@) ions to N,N-dimethylthioformamide, are reported in this paper as part of a series of transfer thermodynamic studies of single cations and anions.'-5 The extra-thermodynamic tetraphenylarsonium tetraphenylborate (TATB) assumption, which states that the AS(C6H5)4+ and B(C6H5),- ions are equally solvated in every solvent,6-8 has been applied in this study in order to calculate the contribu- tions from the individual ions. The choice of the TATB assumption has been discussed previously.1,3 The solvation of metal ions depends mainly on the bonding characteristics of the solvent and the intermolecular forces in the solvent b~lk.~.~ Water and methanol are protic oxygen donor solvents which have mainly electrostatic inter- actions with metal ions, thus they solvate hard electron-pair acceptors well. Both hydrogens in the water molecule can participate in hydrogen bonding while only the hydrogen on the oxygen atom in methanol is able to form hydrogen bonds.This suggests that the aqueous bulk structure is sub- stantially more rigid and well ordered than the methanol one. Acetonitrile is an aprotic nitrogen donor solvent with weak and irregular solvating properties. Acetonitrile solvates the monovalent coinage metal ions well, 1-3*9* O while the copper(@, the alkali and the divalent d" metal ions are very weakly solvated in acet~nitrile.~-' Acetonitrile forms dimers and higher aggregates and the bulk is therefore fairly well f Present address : Chemical Laboratory, College of Education, University of the Ryukyus, Okinawa 903-1, Japan. ordered.' Dimethyl sulfoxide is an aprotic solvent with the possibility to coordinate either through its oxygen or sulfur atom.Coordination through the sulfur atom takes place only to the most soft electron-pair acceptors, e.g. palladium(I1) and platinum(n).' Dimethyl sulfoxide, as an oxygen donor, solvates both hard and soft metal ions ~e11,~-'*'~9'and is a markedly stronger electron-pair donor, D, = 27, than water and methanol with D, values of 18 and 17, respectively.'6*1 Pyridine is an aprotic solvent coordi- nating through its nitrogen atom. Pyridine has strong and typically soft electron-pair donor properties ; the D, value of pyridine is 38.16,17 Pyridine has no well ordered bulk structure18 as the intermolecular forces in pyridine are quite weak. The low relative permittivity of pyridine, E, = 12.3, indicate that long-range coulombic interactions are not effec- tively reduced in pyridine, and that the dipoles of the pyri- dine molecules are ordered around the ions in order to reduce the electric field as effectively as po~sible.~N,N-dimethylthioformamide is a sulfur donor solvent with typi- cally soft electron-pair donor properties, Ds = 52.16p1 N,N-dimethylthioformamide has a high relative permittivity, E, = 47.5,3and many salts are therefore very soluble and completely dissociated.A recent LAXS study has indicated intermolecular sulfur-sulfur interactions in the N,N-dimethylthioformamide bulk at about 3.1 Pyridine and N,N-dimethylthioformamide solvate soft electron-pair accep- tors strongly while their ability to solvate hard ones is poor.3,4 Gritzner et al.have previously reported Gibbs energies of transfer for the lead(I1) ion from acetonitrile to many solvents including the soft electron-pair donor solvents pyridine, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 tetrahydrothiophene and N,N-dimethylthiof~rmamide.~~-~~The standard electrode and halfwave potentials of the They used the bis(biphenyl)chromium(O)/(r) (BCr) extra-thermodynamic assumption2' to calculate the contribution from the individual ions. The hydration thermodynamics of the alkaline-earth-metal ions have been studied extensively.26-28 The divalent alkaline-earth-metal ions are significantly more weakly hydrated than the divalent transition metal ions. This is because the alkaline-earth-metal ions are larger and the elec- trostatic interactions are therefore weaker, and the ability of the alkaline-earth-metal ions, with filled electron shells, to form covalent interactions is weaker than that of the tran- sition metal ions.This suggests that the alkaline-earth-metal ions are solvated strongly by solvents able to form strong electrostatic interactions, i.e. typical hard donor solvents, while they are expected to be solvated weakly by typical soft donor solvents. The low solubility of the alkaline-earth-metal trifluoromethylsulfonates, most probably due to weak solva- tion, has prevented us from performing calorimetric studies in N,N-dimethylthioformamide. The amount of thermodynamic data available for the alkaline-earth-metal ions in non-aqueous solvents is limited14,''because electrochemical studies on the alkaline- earth-metal couples are difficult to perform.Gibbs energies of transfer of the barium ion have previously been reported from acetonitrile to a large number of solvents using polaro- graphic methods and the BCr extrathermodynamic assump- and enthalpies of transfer of the barium ion using TATB assumption are reported from water to methanol, dimethyl sulfoxide and a~etonitrile.~'.~' Various techniques have been developed to prepare alkaline-earth-metal amalgams as they cannot be prepared by mixing the metal^.^^^^^ The magnesium electrode is known to behave irregularly in aprotic solvent^,^'-^^ which complicates direct measurements.The basic problem with most of the alkaline-earth-metal amalgams is rapid corrosion and passi- vation both in water and non-aqueous solvents.37 The elec- trode and halfwave potentials for the magnesium, calcium and strontium couples in methanol, acetonitrile, dimethyl sul- foxide, pyridine and liquid ammonia, referred to the saturat- ed calomel electrode (SCE) or the silver electrode, have been determined mainly by means of polarographic methods.29v31v38-42 Several authors have stressed the impor- tance of using acetonitrile as reference solvent rather than water as more well defined polarographic waves are obtained. In the case of water, the potentials measured by polaro- graphic methods tend to decrease with time.43 All electrode potentials reported in this study have been recalculated through the Gibbs energies of transfer of the silver ion so they are referred to the NHE in water.Halfwave potentials from polarographic measurements were used as they are a function of the standard potential of M2+/M(am) couples, the solubility of the metal in mercury and its Gibbs energy of amalgamati~n.~~ As E" of a metal amalgam electrode in a solution containing the correspond- ing metal ions and the polarographic halfwave potentials El,, is given by E" = -(RT/nF)ln([(DR/Do)'/'(fo/fR)],where D, and Do are the diffusion coefficients of the reduced and oxidized forms, respectively, and fR and fo are the activity coefficients of the reduced and oxidized forms.45 As the ratios of the diffusion and activity coefficients in the equation above are nearly unity in almost all cases, is usually a very good approximation to E" for a reversible couple.45 The E,/, values were taken to be equal to E", and the Gibbs energes of transfer were calculated from these values. The Gibbs ener- gies of transfer of the tin(@ and lead@) ions have been obtained from potentiometrically determined standard elec- trode potentials of the Sn2 +/Sn(am) and Pb2 +/Pb(am) couples in the studied solvents.alkaline-earth-metal couples are indeed very difficult to measure and the accuracy is sometimes low. Consequently, the accuracy of the Gibbs energies of transfer of the alkaline- earth-metal ions is much lower than for the previously reported ions in this series.'-' The enthalpies of transfer, AtHe, for the metal ions from water to methanol, acetonitrile, dimethyl sulfoxide, pyridine and N,N-dimethylthioformamide were obtained from calori- metrically determined enthalpies of soluion of the anhydrous metal trifluoromethylsulfonates. The entropies of transfer have been calculated from the experimentally determined AtG" and AtH* values.All measurements have been per- formed at 25 "C. Calculations The basis for the TATB assumption is given elsewhere., The Gibbs energies of transfer are calculated from standard elec- trode potentials, and the enthalpies of transfer are calculated from enthalpies of solution, as described elsewhere.' Experimental Materials Solvents Methanol was distilled over calcium hydride.' Acetonitrile (Fluka, analytical grade) and dimethyl sulfoxide (Merck) were purified as described else~here.~~,~~Pyridine (Riedel-deHaen, analytical grade) was kept water-free by storage over molecular sieves of 3 8, pore size, and it was used without further purification.N,N-dimethylthioformamide was prepared by reacting N,N-dimethylformamide (Fluka) and phosphorus pentasul- fide (Merck) in benzene as described previou~ly.~?~~ Preparation of Salts Lead@) trifluoromethylsulfonate was prepared as described el~ewhere~~~~~and tin@) trifluoromethylsulfonate was pre- pared in a similar way. The salts were stored in a desiccator under reduced pressure at 120"C. Elemental analysis of Sn(CF,SO,), gave: Found: Sn, 27.5%; C, 5.7%; S, 15.0%; Calc.: Sn, 28.5%; C, 5.7%; S, 15.4%. Elemental analysis of Pb(CF,SO,), gave: Found: Pb, 40.4%; C, 4.8%; S, 12.5%; Calc.: Pb, 41.0%; C, 4.8%; S, 12.7%.Barium trifluoromethylsulfonate was prepared as described elsewhere,32 and magnesium, calcium and strontium tri-fluoromethylsulfonate were prepared according to the same procedure. The metal content of the salts was checked by EDTA titration.'' The prepared salts were stored in a desic- cator under reduced pressure at 120 "C. Amalgams Tin amalgam containing 0.6 wt.% Sn and lead amalgam con- taining 1.476 wt.% Pb were prepared as described else- here.'^.^^ The tin and lead amalgams were washed with dilute perchloric acid and water to remove impurities.The amalgams were stored under dry nitrogen. Freshly prepared amalgams were used for each experiment as the surface of these amalgams is easily oxidized. Potentiometric Measurements The apparatus and procedure used have been described pre- viou~ly.~Tetrabutylammonium perchlorate, (C,H,),NCIO, , 0.1 mol dm-', was used as ionic medium. The emfs of the cells Ag(s) I Ag+ (10 mmol dm-,) 11 M2+ (1-30 mmol dm-3) I M(am), M = Sn and Pb, were determined in all solvents. The J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 w M AN DMSO Py DMTF \0.8 \ \ \ \0.6 0,4 > 0.2 /0 -0.2 -0.4 -0.6 -0.8 Fig. 1 Diagram of standard electrode potentials of the couples Sn2+/Sn(s)(-----) and PbZ+/Pb(s) (-) in water (W), methanol (M), acetonitrile (AN), dimethyl sulfoxide (DMSO), pyridine (Py) and N,N-dimethylthioformamide(DMTF) at 25 "C.The standard elec- trode potentials for the Ag+/Ag(s) couple (---) are given for com- parison. resistance of the experimental cell was less than 1 MQ. The emfs were measured by a digital voltmeter, Keithley 197, autoranging microvolt DMM, with an internal resistance of 2 GQ. The difference in potential between Pb(s) and Pb(am), 5.7 mV,s4 was corrected for, while there is no potential differ- ence between Sn(s) and Sn(am). The electrodes obeyed Nernst's law within 0.5 mV. The experiments were performed at 25.0 "C by passing thermostatted water through the outer jackets of Ingold vessels. Calorimetric Measurements The enthalpies of solution, AsolHe, were measured with an ampoule calorimeter described previously. s,56 A gold vessel containing 80 ml of solvent without supporting electrolyte was used in the experiments.Two or three ampoules were smashed without changing the solvent. The salt concentra- tion never exceeded 1.5 mmol dm-3. The ampoules were sealed off with an oxygen-propane flame and they were cooled during the sealing to avoid decomposition of the salt. Six experiments in agreement were performed for each salt and solvent. The measurements were performed at 25.000 & 0.002 "C. Results The electrode potentials of the M2'/M(am), M = Sn and Pb, couples have been determined experimentally vs. the reference Ag+/Ag(s) couple (Fig.1). By combining the expe$mental electrode potentials vs. the silver couple and theAtG values of the silver ion, all standard electrode potentials of these couples have been calculated relative to the normal hydrogen electrode in water, see Table 1. The Gibbs energies of transfer have then been calculated from these standard electrode potentials, see Table 2; the AtG" value for the lead@) ion to liquid ammonia is included for comparison. The electrode potentials of the alkaline-earth-metal couples, M2+/M(am), taken from the literature, have been used to calculate the electrode potentials of the M2+/M(s) couples by adding the difference in potential between the M2+/M(am) and M2'/M(s) couples which is 0.400 V, 0.872 V, 1.10 V and 1.34 V, for magnesium,60 calcium,38 strontiums4 and barium,54 respectively, see Table 3.The Gibbs energies of transfer calculated from the standard elec- trode potentials in water and the solvent under study are summarized in Table 2. The enthalpies of solution, Aso,He, of anhydrous tin(@, lead@), magnesium, calcium, strontium and barium tri-fluoromethylsulfonate in water, methanol, dimethyl sulfoxide, acetonitrile, pyridine and N,N-dimethylthioformamide and the enthalpies of transfer of the trifluoromethylsulfonate ion from water to the other solvents3 are given in Table 4.The complete transfer thermodynamics for the tin(rI), lead@) and alkaline-earth-metal ions are summarized in Table 2 and Fig. 2. The Gibbs energies of transfer for the lead@) and barium ions have been reported previously by Gritzner et ~1.~~9~ who applied the BCr assumption to polarographic data. These data are in some cases in good agreement with the values obtained in this study, using the TATB assumption, but in other instances large deviations are observed, see Table 5.These large differences do not seem to be caused by the extrathermodynamic assumptions used.3 Discussion The transfer thermodynamic functions are affected by several processes taking place at the solvation of an ion. These pro- cesses have been discussed previously and include as main Table 1 (a) Experimentally determined cell potentials (in V), where the silver electrode has been used as the reference cell, in methanol (M), acetonitrile (AN), dimethyl sulfoxide (DMSO), pyridine (Py), N,N-dimethylthioformamide (DMTF) at 25 "C and in 0.1 mol dm- tetra-butylammonium perchlorate ionic medium, the given values have been extrapolated to 1 mol dmP3 concentrations of all species in the cell reaction; (b) electrode potentials based on the Gibbs energies of transfer of the silver ion, E"/V,of the silver, lead@) and tin@) couples in water (W), M, AN, Py and DMTF at 25 "C (a) cell M AN DMSO Py DMTF -0.8960 -0.3975 -0.7626 -0.1950 -0.2615 -0.8870 -0.4697 -0.7032 -0.4401 -0.3702 (b) system W M AN DMSO PY DMTF Ag +/Ag(s)"Sn2+/Sn(s) Pb2 +/Pb(s) +0.7997 -0.1375 -0.1205 +0.876 -0.020 -0.017 +0.568 +0.171 +0.093 +0.445 -0.318 -0.264 +0.190 -0.005 -0.256 -0.224 -0.486 -0.600 Ref.3. 2686 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Transfer thermodynamics for the magnesium, calcium, strontium, barium, lead@) and tin(@ ions from water to methanol (M), acetoni- trile (AN), dimethyl sulfoxide (DMSO), pyridine (Py) N,N-dimethylthioformamide(DMTF) and liquid ammonia mH,(l)] ~ ~~ M AN DMSO A,G~ A,H~ TA$ A,G~ A,Hg TA,~ A,G~ A,H~ TA$ +Sn2 +22.8 -15.6 -38.4 +59.6 +5.1 -54.5 -34.7 -83.4 -48.7 Pb2+ +20.1 -15.9 -36.0 +41.3 -5.2 -46.5 -27.6 -85.3 -57.7 Mg2+ -43.4 +llO -13 +Ca2 +36 -52.9 -89 +78 + 18.7 -59 -69 +Sr2 -53.8 +52 +7.9 -44 -14 -85.7 -72 BaZ+ + 18.4 -59.2 -77.6 +37 -6.2 -48 -25.1 -82.8 -57.5 ~ ~~~~ +Sn2 +25.5 -109.6 -135.6 -67.2 -75.4 -8.2 Pb2+ -26.1 -75.4 -49.3 -92.4 -85.5 +6.9 -114" -162' -48 Mg2+ +Ca2 -21.3 -58" -1 13' -46 +Sr2 -18.1 -79" -1 16' -37 Ba2+ +71 -26.0 -97 -58" -113b -55 ~ ~ ~ ~~~~ " Calculated from the standard electrode potentials reported in ref.57, and the Gibbs energy of transfer for the proton from water to liquid ammonia, -96 kJ mol-', ref. 14. Average value from ref. 58 (based on the enthalpy of formation of the ammoniated electron), and ref. 59 (based on the enthalpy of transfer of the sodium ion). contributions the bonding characteristics of the solvate bonds the endergonic Gibbs energies of transfer, see Table 2. The and the effect of the solvent bulk struct~re.~~~ enthalpies of transfer for these ions are exothermic, and the entropies of transfer are negative, which is especially pro- Methanol nounced for the alkaline-earth-metal ions, see Table 2. This The tin(II), lead(I1) and alkaline-earth-metal ions are all sol- transfer thermodynamic pattern with positive At G" and nega- vated more weakly in methanol than in water as shown by tive &He and T&Sg Values iS also found for the previously Table 3 Standard electrode or halfwave potentials based on the Gibbs energies of transfer of the silver ions, E"/V,of the magnesium, calcium, strontium, barium couples in water (W), methanol (M), acetonitrile (AN), dimethyl sulfoxide (DMSO), pyridine (Py) and liquid ammonia NH,(l) at 25 "C us.NHE in aqueous solution system W M AN DMSO Py NH 30)" Mg2+/Mds) -2.372' -1.W -2.44' -2.74 Ca2 +/Ca(s) -2.868' -2.68/ -2.46' -2.93* -3.17 Sr2 +/Sr(s) -2.89' -2.62' -2.96' -3.3 Ba2 +/Ba(s) -2.912b -2.817' -2.62; -2.72' -3.042' -2.54h -3.2 " Ref.42, based on the difference in standard electrode potential of the hydrogen electrode between water and liquid ammonia being 1.00 V; A,Ge = -96 kJ mol-', ref. 14. Ref. 38, 0.006-0.10 mol dm-, CaCl,. 'Ref. 40, 0.1 mol dm-3 (C,H9),NC10,. Ref. 41, 0.05 rnol dm-, (C,H,),NCF,SO, . Ref. 37,O-0.06 mol dm-3 CaCI, . Ref. 39, ionic medium not reported. Ref. 31,O.l mol dm-3 (C2H5),NC10,. Ref. 29, 0.1 mol dmP3 (C,H,)NClO, . Table 4 Enthalpies of solution, A,,,He/kJ mol -of magnesium, calcium, strontium, barium, lead(I1) and tin@) trifluoromethylsulfonate in water (W), methanol (M), acetonitrile (AN), pyridine (Py) and N,N-dimethylthioformamide(DMTF), the enthalpies of transfer, A,H*/kJ mol-', for the trifluoromethylsulfonate ion from water to other solvents are given as they are used to calculate the enthalpies of transfer for the metal ions, no supporting electrolyte has been used W M AN DMSO Py DMTF +Sn2 -53.1 f1.6 -58.7 1.0 -43.8 f 0.5 -132.9 f1.1 -171.3 f3.3 -106.9 f 1.4 Pb2+ -29.6 & 0.9 -35.5 f1.4 -30.9 f0.9 -111.3 k3.0 -113.6 f 0.8 -93.5 f1.2 Mg2+ -53.6 f 0.4 -87.0 f0.1 ins." ins." ins." ins." Ca2+ -33.6 f0.4 -76.5 f0.8 -9.7 f0.6 -111.3 f1.5 -63.5 f0.6 ins." +Sr2 -22.4 f0.6 -66.2 f1.4 -10.3 f0.3 -104.5 _+ 0.3 -49.1 f1.4 ins." Ba2+ -8.1 f0.9 ins." -10.1 & 0.3 -87.3 f0.3 -42.7 f0.4 ins." CF,SO,-+5.0 +2.1 + 1.8 -4.3 + 10.8 " The compound is not soluble enough for calorimetric measurements. Table 5 Comparison of the AIGe values of the lead@) and barium ions from acetonitrile (AN) to methanol (M), dimethyl sulfoxide (DMSO), pyridine (Py) and N,N-dimethylthioformamide (DMTF) obtained by the TATB and BCr extrathermodynamic assumptions M DMSO Py DMTF TATB BCr TATB BCr TATB BCr TATB BCr Pb2+ -21.2 -42.3" -68.9 -97.9" -67.4 -67.4' -133.7 -77.8" Ba2+ -19 -34.2' -62 -92.1" +34 -29.5' -36.5' " Ref.20. 'Ref. 21. 'Ref. 29. 2687J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 120 more order when the ion leaves than the methanol bulk loses 100 at the introduction of the same ion. This is expected as the aqueous bulk is more ordered than the methanol one. It can 80 be assumed that the energy in the ion-water and ion-methanol bonds is of the same order of magnitude as the bonding characteristics of water and methanol are similar.The weaker solvation of metal ions in methanol than in water is thus more an effect of solvent bulk perturbation than dif- ference in strength of the ion-solvent bond. Acetonitrile-".(.il20 0 The tin@), lead@) and alkaline-earth-metal ions are much -20 more weakly solvated by acetonitrile than by water. The -40 small and hard magnesium ion is solvated very poorly by acetonitrile, A,G" = +110 kJ mol- ',but the endergonicity -60 -80100-1decreases with increasing size and polarizability of the alkaline-earth-metal ions, see Table 2. The enthalpies of transfer of the magnesium ion could not be determined due to the low solubility of the anhydrous magnesium tri-fluoromethylsulfonate and perchlorate salts.The enthalpies of transfer of the other alkaline-earth-metal ions and the tin(I1) and lead(@ ions change from endothermic to exother- mic with increasing size of the ion, see Table 2. The entropies of transfer of the studied divalent metal ions to acetonitrile are slightly more negative than the corresponding values for the copper(II), zinc, cadmium and mercury(I1) ion^.^.^ The weak solvation of the magnesium ion in acetonitrile is also reflected in the Mg-0 and Mg-N bond distances in the hydrate and the acetonitrile solvate. The Mg-0 bond 20 0 -20 -40 -60 -80 -1 00 -1 20 -1 40 60 40 20 0 Fig. 2 The changes of Gibbs energy, -AtGe, (black), enthalpy, -AtHe, (white) and entropy, TAtSe, (hatched) in kJ mol-' for the transfer of the tin(r1) and lead@) ions from water to (a)methanol, (b) acetonitrile, (c) dimethyl sulfoxide, (d)pyridine and (e)N,N-dimethyl-thioformamide at 25 "C studied metal ions in The entropy of transfer term indicates that the difference in the effect of the methanol and aqueous bulks at the introduction of alkaline-earth- metal ions is about the same as for the other divalent metal ions studied, while this difference seems to be smaller for the tin@) and lead@) At the introduction of an ion into a solution solvent molecules are released from the solvent bulk structure for the solvation, and the internal bulk structure is affected when the solvated ions enter.These processes are endothermic and the absolute values increase with increasing strength of the intermolecular forces in the solvent. The nega- tive entropies of transfer show that the aqueous bulk gains distance in the hydrated magnesium ion, Mg(OH2)62+, is 2.11 A,61*62and the average Mg-N bond distance in the Mg(NCCH&,+ ion is 2.15 A.63 The M-N bond distance in the acetonitrile solvate is normally shorter than the M-0 bond distance in the corresponding h~drate.~~-~~ The only case where the M-0 and M--N bond distances are equal is the COPP~X(II) ion.64 The copper(r1) ion is substantially more weakly solvated in acetonitrile than in water, A,G" = +50 kJ mol-'.' The observation of longer Mg-NCCH, than Mg-OH, bond distances strongly supports the very weak solvation of the magnesium ion in acetonitrile.Dimethyl Sulfoxide As expected, dimethyl sulfoxide solvates, the tin(n), lead(I1) and alkaline-earth-metal ions well. The Gibbs energies of transfer of the magnesium, calcium and strontium ions are very similar, -13 kJ mol-', while the AtGe values for the barium, tin(r1) and lead(@ ions are more negative, -25, -35 and -26 kJ mol-', respectively. The enthalpies of transfer of the tin@), lead(@ and alkaline-earth-metal ions are markedly exothermic, about -85 kJ mol-'. This means that the entropies of transfer are markedly negative, -50 to -70 kJ mol-'. These values are much more negative than those found for the zinc, cadmium, mercury(@ and copper(I1) ions (-14.5, -13.8, -14.5 and -6.3 kJ mol- ', re~pectively).~,~ One reason for the large entropies of transfer of the alkaline- earth-metal ions may be that the hydrated and dimethyl sul- foxide solvated strontium and barium ions, and maybe also the calcium ion, have different coordination numbers.The hydrated strontium and barium ions are eight-coordinated in a square antiprismatic fashion, while the dimethyl sulfoxide solvated strontium and barium ions are ~ctahedral.~~ The coordination number of the dimethyl sulfoxide solvated calcium ion is most probably six, while it has been difficult to establish if the hydrated calcium ion is six-or eight-~oordinated.~~The structure of the hydrated tin(r1) ion has been reported to have two short and two long Sn-0 bond distances at 2.3 and 2.8 A, re~pectively.~~.~~The structure of the dimethyl sulfoxide solvated tin(@ ion has a similar struc- ture to that of the hydrate with Sn-0 bond distances at 2.18 and 2.7 A.'" The hydrated and dimethyl sulfoxide solvated lead@) ions are octahedral in solution.70 Pyridine The At G" value to pyridine is positive for the tin@) ion, while it is negative for the lead@) ion.The enthalpies of transfer are very exothermic for both ions, and the entropy of transfer is very negative for the tin@) ion, while the TAtS" value for the lead@) ion is less negative than found for other divalent metal ion^^,^ and in the same order of magnitude as the mono- valent metal ions,3 see Table 2.This may be explained by the fact that the lead@) ion is large and its charge density is lower than for previously studied divalent metal ions. The pyridine-solvated lead@) ion is octahedral in pyridine solu- ti~n.~'The Gibbs energy of transfer of the barium ion is markedly positive, showing that the barium ion is, as expected, weakly solvated in pyridine. The enthalpies of transfer of the alkaline-earth-metal ions to pyridine are weakly exothermic, see Table 2. This indicates that the AtG" values are largely positive also for the other alkaline-earth- metal ions. The obtained entropy of transfer of the barium ion is very close to the values found for the copper(I1) and divalent d" metal The pyridine-solvated strontium and barium ions are octahedral in pyridine ~olution.~' The very negative entropies of transfer to pyridine are most probably because (1) a change in coordination number has taken place, the pyridine-solvated strontium and barium ions are ~ctahedral,~' and (2) pyridine molecules in a fairly large volume around the ion are oriented towards the ions, as dis- cussed el~ewhere.~,~ A large-angle X-ray scattering study on a pyridine solution of lead@) trifluoromethylsulfonate shows two large peaks at 5.5 and 10 A in the radial distribution function (RDF),70 while these peaks are much less pro- nounced in the RDFs of pyridine solutions of uncharged and 18971*72monovalent species. The observation that pyridine solutions of ions with a fairly high charge density have a higher bulk order than those of neutral complexes evidences that large negative entropies of transfer indicate increasing bulk order. N,N-Dimeth ylthioformamide The Gibbs energies of transfer to N,N-dimethyl-thioformamide are markedly exergonic and the enthalpies of transfer are very exothermic showing that the tin@) and lead(I1) ions are strongly solvated by the soft electron-pair donor solvent N,N-dime thy1 thioformamide.The entropies of transfer are small with a positive value for the tin@) ion and a negative value for the lead@) ion. The Gibbs energy of transfer of the silver ion,3 -98 kJ mol-', and of the tin(I1) and lead(@ ions, -70 and -94 kJ mol-', respectively, are of the same order of magnitude, while the enthalpies of transfer differ significantly, -140, -75 and -85 kJ mol-', respec-tively. This indicates that the silver-N,N-dimethyl-thioformamide solvate bond has a more covalent character than the corresponding tin@)- and lead(@-N,N-dimethyl-thioformamide bonds.The entropies of transfer of the mono- valent metal ions are negative, about -40 kJ mol-', while the divalent ions studied previously are close to zero, see Table 2 and ref. 3 and 4. The N,N-dimethylthioformamide solvated tin@) ion is most probably a flattened square- pyramid with the tin(@ ion at the top; the Sn-S bond dis- tance is 2.67 A,'' while the N,N-dimethylthioformamide solvated lead@) ion is most probably five-coordinated, a J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 smaller coordinated number than with the solvates of the other solvents in this study. Liquid Ammonia The large -AtG" and -AtH" values, though not accurate, show that the lead@) and alkaline-earth-metal ions are strongly solvated by liquid ammonia. This shows that liquid ammonia solvates hard and borderline electron-pair accep- tors very well. The entropies of transfer are slightly negative as has been found previously for the mercury(i1) Conclusions These results show that the entropies of transfer of the studied divalent metal ions are very similar to values obtained for other divalent metal ions, see ref. 4 and 5 and Table 2. This supports the view that the entropy of transfer term reflects the effect on the solvent and aqueous bulk at the introduction of an ion and that ions with similar charge density and size affect the solvent bulk to about the same extent.The enthalpies of transfer give a strong indication of the bond character of the ion-solvent bond, where an increasing degree of covalency in the interaction results in an increasingly exothermic enthalpy of transfer. The difference in solvation of an ion between two solvents is thus mainly dependent on the bonding character of the ion-solvent bonds and the way in which the ion affects the solvent bulk. This study further supports the assertion that the entropies of transfer indicate perturbation of the solvent bulk at the intro- duction of a metal ion and that this perturbation is mainly dependent on the charge density of the ion.This study also supports the previous observation that the soft electron-pair donor solvents pyridine and N,N-dimethylthioformamidedis-criminate significantly between hard and soft electron-pair acceptors at solvation, see Table 2, while hard electron-pair donor solvents solvate more equally hard and soft electron- pair acceptors, see Table 6 in ref. 3 and Table 3 in ref. 4. The Swedish Natural Research Council is acknowledged for their financial support of this project. References 1 M. Johnsson and 1. Persson, Znorg. Chim. Acta, 1987, 127, 15. 2 M. Johnsson and I. Persson, Znorg. Chim. Acta, 1987, 127, 25. 3 H. D. Inerowicz, W. Li and I. Persson, J. Chem, SOC., Faraday Trans., 1994,90,2223, and references therein.4 M. Chaudhry, K. C. Dash, E. Kamienska-Piotrowicz, Y. Kinjo and I. Persson, J. Chem. SOC.,Faraday Trans., 1994,90,2235. 5 M. Chaudhry and I. Persson, J. Chem. SOC., Faraday Trans., 1994,90,2243. 6 E. Grunwald, G. Baughman and C. Kohnstam, J. Am. Chem. SOC.,1960, 82, 5801. 7 E. M. Arnett and D. R. McKelvey, J. Am. Chem. SOC.,1966, 88, 2598. 8 B. G. Cox and A. J. Parker, J. Am. Chem. SOC., 1973,95402. 9 M. Johnsson and I. Persson, Znorg. Chim. Acta, 1987, 127,43. 10 S. Ahrland and S.-I. Ishiguro, Znorg. Chim. Acta, 1988,142, 277. 11 J. Sadlej, Spectrochim. Acta, Part A, 1979,35, 681. 12 K. Nilsson and I. Persson, Acta Chem. Scand., Ser A, 1987, 41, 139. 13 J. Selbin, W. E. Bull and L. H. Holmes Jr., J. Znorg.Nucl. Chem., 1961, 16, 219. 14 Y. Marcus, Pure Appl. Chem., 1983, 55, 977, and references therein. 15 Y. Marcus, Pure Appl. Chem., 1985, 57, 1103, and references therein. 16 I. Persson, M. Sandstrom and P. L. Goggin, Inorg. Chim. Acta, 1987,129, 183. 17 M. Sandstrom, I. Persson and P. Persson, Acta Chem. Scand., 1990,40,653. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2689 18 19 A. Iverfeldt and I. Persson, Znorg. Chim. Acta, 1986,111, 171. I. Persson, M. Sandstrom, C. T. Stllhandske, C. M. V. 47 S. Ahrland and N.-0. Bjork, Acta Chem Scand., Ser A, 1974,28, 823. 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Stdlhandske and E. Kamienska-Piotrowicz, unpublished results. G. Gritzner, J. Chem. SOC.,Faruday Trans. 1, 1988,84, 1047, and references therein.G. Gritzner and S. Sperker, J. Solution Chem., 1990,19,543. G. Gritzner, 2. Phys. Chem. N. F., 1988,158,99. G. Gritzner and S. Sperker, J. Solution Chem., 1990, 17, 1133. F.Horzenberger and G. Gritzner, J. Chem. SOC., Faruday Trans., 1992,88,695. G. Gritzner, Inorg. Chim. Acta, 1977,24,5. R. M. Noyes, J. Am. Chem. SOC.,1962,84,513. D. R. Rosseinsky, Chem. Reo., 1965,65,467. B. G. Cox and A. J. Parker, J. Am. Chem. SOC., 1973,95,6879. G. Gritzner and G. Kraml, Znorg. Chem. Acta, 1989,156,227. F. Horzenberger and G. Gritzner, J. Chem. SOC.,Faraday Trans., 1992,88,3013. G. R. Hedwig, D. A. Owensby and A. J. Parker, J. Am. Chem. SOC., 1975,97, 3888. G. R. Hedwig and A. J. Parker, J. Am. Chem. SOC., 1974,%, 6589. K. Brauer and H. Strehlow, 2.Phys. Chem., N. F., 1958,17,336. 0.R. Brown and R. McIntyre, Electrochim. Acta, 1984,29,995. W. E. Elliott, J. R. Huff, R. W. Adler and W. L. Towle, Pro-ceedings of 20th Annual Power Sources Conference, 1966,p. 67. J. Farrer, R. Keller and M. M. Nicholson, High Energy Battery Systems, Final Report, No DA-28-043 AM (3-01394E), 1963, Rocketdyne, North American Aviation Inc. J. N. Butler, J. Electroanal. Chem., 1968, 17, 309. 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 V. Gutmann, K. Danksagmiiller and 0. Duschek, Z. Phys. Chem., N. F., 1974,92,199.T.Fujinaya and I. Sakamoto, J. Electroanal. Chem., 1976,73,21. G. Gritzner, J. Electroanal. Chem., 1983,144,259. G. Schwarzenbach and H. Flaschka, Die Komplexometrische Titration, F. Enke, Stuttgart, 1965.J. Gouy, J. Phys., 1895,4,320. H. E. Thompson, J. Phys. Chem., 1935,39,655. Handbook of Chemistry and Physics, CRC Press, Inc. Boca Raton, 73rd edn., 1992-1993,pp. 8-19. S.Sunner and I. Wadso, Sci. Tools, 1966, 13, 1. L.Kullberg, Acta Chem Scand., Ser. A, 1974,28,979. W. E.Dasent, Inorganic Energetics, Cambridge University Press, Cambridge, 2nd edn., 1982,p. 165. W. L. Jolly, Prog. Inorg. Chem., 1959,1,235. N. M. Senozan, J. Inorg. Nucl. Chem., 1973,35,727. 0.R. Brown and R. McIntyre, Electrochim. Acta, 1984, 29, 995, and references therein. R. Caminiti, G. Licheri, G. Piccalugo and G. Pinna, J. Appl. Cryst., 1979,12, 34. R. Caminiti, G. Licheri, G. Piccalugo and G. Pinna, Chem. Phys. Lett., 1979,61,45. M. G. B.Drew, P. P. K. Claire and G. R. Willey, J. Chem. SOC., Dalton Trans., 1988,215. I. Persson, J. E.Penner-Hahn and K. 0.Hodgson, Znorg. Chem., 1993,32,2497. K. Nilsson and I. Persson, Acta Chem. Scand., Ser. A, 1987, 41, 38 39 40 41 42 43 44 45 46 T. Mussini and A. Pagella, Chem. Eng. Data, 1971,16,49. E.Constantinescu, Rev. Roum. Chim., 1972,17,1819. T. Fujinaga and I. Sakamoto, J. Electroanal. Chem., 1976, 73, 235. V. Gutmann and G. Schober, 2. Anal. Chem., 1959,171,339. S. G. Bratsch and J. J. Lagowski, J. Solution Chem., 1987,16, 583. A. D.Covington and A. K. Covington, J. Chem. SOC.,Faraday Trans. I, 1975,71, 831. I. M. Kolthoff and J. F. Coetzee, J. Am. Chem. SOC., 1957, 79, 870. A. J. Bard and L. R. Faulkner, Electrochemical Methods, Wiley, New York, 1980,p. 160. S. Ahrland, K. Nilsson and B. Tagesson, Acta Chem. Scand., Ser A, 1983,37, 193. 66 67 68 69 70 71 72 139. T. Yamaguchi, G.Johansson, B. Holmberg, M. Maeda and H. Ohtaki, Acta Chem. Scand., Ser. A, 1984,38,437. M. Sandstrom, I. Persson, H. Yokoyama and M. Chaudhry, Znorg. Chem., submitted. G. Johansson and H. Ohtaki, Acta Chem. Scand., 1973,27,643. T. Yamaguchi, 0.Lindqvist, T. Claeson and J. B. Boyce, Chem. Phys. Lett., 1982, 93, 528. I. Persson, M. Chaudhry and M. Sandstrom, to be published. M. Sandstrom and I. Persson, J. Chem. SOC., Dalton Trans., 1985, 1597. F. HultCn and I. Persson, Acta Chem. Scand, Ser. A, 1987,41,87. Paper 3/06676B ; Received 8thNovember, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002683

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2691-2695 2691 Enthalpies of Transfer of Tetrabutylammonium Bromide from Water to highly Aqueous Water-Methanol, -Ethanol, -Propan4 -01and -Acetonitrile Mixtures at 298 K :Consideration of the Extended Coordination Model Solvation Parameters Patrick Hogan, Ita McStravick, James Mullally and W. Earle Waghorne* Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland Previous studies have established that the extended coordination model of solvation can satisfactorily account for the variation in the transfer enthalpies of solutes in mixed-solvent systems. However, the model parameter relating to the solute-induced disruption of the solvent structure shows a marked dependence on the nature of the mixed solvent.This result is not consistent with the underlying model of the solvation process. In the present paper we report the transfer enthalpies of tetrabutylammonium bromide, for which this depen-dence is large, into a series of highly aqueous mixed solvents. Analysis of these in terms of the extended coordination model confirms both the model's ability to account for the experimental data, and the variability of the structural disruption parameter. The possible origins of this latter result are considered in detail. We have recently reported the enthalpies of transfer, At He, of several solutes from water to aqueous organic solvent mix- tures.'-' These data were considered in terms of the extended coordination model of ~olvation.~.' These studies have revealed the existence of a transition in the solvating proper- ties of the aqueous systems at some critical composition, x;, which depends on the organic cosolvent.They also showed that the extent to which the solutes disrupt the solvent struc- ture, as measured by the model parameter (an+ BN),varied with the organic component. This second result was inter- preted as indicating that the organic components had the effect of rigidifying the water structure, with the extent of rigi- dification increasing in the order 1,4-dioxane, methanol < ethanol < 2-methylpropan-2-01 (tert-butyl alcohol, TBA), propan-1-01.~ However, it is clear that this explanation, while plausible, cannot be strictly correct and, rather, poses a theoretical problem.To understand the theoretical point we consider the model equation6 for the transfer enthalpy of a solute from some solvent, A, to mixtures of A with some second solvent, B. This contains three model parameters: p, which is an index of preferential solvation, AAH;, , the difference between the enthalpies of interaction of the solute with the two pure sol- vents A and B, and (an+ BN), which measures the extent to which the solute disrupts the solvent-solvent interactions through cavity formation and reorganisation of the solvent around the cavity. The remaining quantities in eqn. (1) are known: thus, xA, xB, LA, L, are the mole fractions and rela- tive partial molar enthalpies of the component solvents in the mixtures and AAHo* is the difference between the enthalpies of solvent-solvent interactions in the two pure solvents (calculated as the difference between the molar enthalpies of condensation).The three model parameters are recovered from the experimental A, H" values by fitting them to eqn. (1). It is implicit in this procedure, and in the derivation of eqn. (1),6 that the three model parameters are constant over the range of solvent compositions of interest. Since p and AAH(112reflect the differences in the interaction of the solute with solvents A and B they will depend on the cosolvent. In contrast, since water is the limit of the aqueous domains (i.e. those with cosolvent concentrations below x:) the values of (an+ BN) recovered could be expected to be equal, and equal to the value for pure water, in marked con- trast to the experimental results.One possible explanation for the dependence of (an+ BN) on the cosolvent is that it varies at low cosolvent concentra- tions, the values converging to that for pure water in the limit of low cosolvent composition. The data reported pre-viously'-' were measured across the entire range of solvent compositions and, with the exception of the aqueous TBA system, the density of data points at low cosolvent concentra- tions was too low to allow such a variation to be detected. The earlier studies concentrated largely on aqueous alcohol solvent systems. The complexity of these systems, at low alcohol concentrations, could account for such variations in (an+ PN). Thus there is the possibility that introduction of the alcohol at very low concentrations rigidifies the three- dimensional water structure, while at higher (but still very low) alcohol concentrations there is clear evidence for aggre- gation of the alcohol molecules.8 At still higher alcohol con- centrations there is another transition from a water-like structure to one more closely resembling that of the alcohol; this last transition has been identified with that occurring at x* and results in marked changes in the model parameters, in particular (an+ BN) which decreases significantly.' The (an+ BN) values for tetrabutylammonium bromide, TBABr, are larger than those of most of the solutes ~tudied,'.~and corresponding show large variations with changes in the cosolvent, increasing from ca.40 for aqueous methanol and 1,4-dioxan to ca. 70 for aqueous propan-1-01. It was also found that the (an+ BN) of a series of amides were significantly higher in aqueous acetonitrile mixtures than in the aqueous alcohol systems.' Thus we have measured the transfer enthalpies, At H", of TBABr from water to highly aqueous mixtures of methanol, ethanol, propan-1-01 and acetonitrile, and have analysed these in terms of eqn. (1). Experimental The purifications of all chemicals were as described pre- vi~usly.'*~.', The enthalpies of transfer of TBABr were calculated3 from the enthalpies of dilution of a concentrated aqueous TBABr solution, ADHe, and the relative partial J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Enthalpies of transfer of tetrabutylammonium bromide from water to aqueous methanol, ethanol, propan-1-01 and acetonitrile mix- tures at 298 K" methanol-water ethanol-water propan-1-01-water acetonitrile-water ~~~ A, H" XEIOH A, H" XPrOH A, H* XMeCN A, H" ~~ O.oo00 0.0200 0.0250 0.0750 0.0900' 0.1900' 0.2700'' 0.3600' 0.4m 0.57006 0.6922' 0.0 2.1 3.3 12.7 13.5 25.5 29.2 30.9 29.2 25.9 24.2 O.oo00 0.0200 0.0376' 0.04oO 0.0600 0.0668' 0.0800 0.0929' 0.1354' 0.2000 0.261 1' 0.3739' 0.5256' 0.0 4.4 10.8 13.6 18.6 19.6 27.2 26.5 36.1 44.1 38.8 36.8 34.6 O.oo00 0.0200 0.0322' 0.0600 0.0696' 0.0800 0.1000 0.1138' 0.2305' 0.4 114' 0.5447' 0.0 10.3 17.3 33.2 32.8 35.9 38.0 40.5 39.1 35.2 27.5 O.oo00 0.0200 0.0300 0.0500 0.0800 0.1000 0.1500 0.2000 0.2500 0.3000 0.4OOo 0.5000 0.6OOo 0.7000 0.0 8.2 11.6 17.1 22.2 25.4 26.4 26.5 25.5 24.9 23.8 21.5 20.6 18.3 ~ ~~ ~~~ Units are kJ mol-', precisions are +0.5 kJ mol-I or better.'Data from ref. 3. 'Data from ref. 1. molar enthalpies of water, Lw ,in the solvent mixtures as: A, H" = AD@'(mixture) -ADH"(water) -n~, (2) where n is the number of moles of water associated with one mole of TBABr in the concentrated solution (cu. 14-19 in the systems studied here). The experimental uncertainty in A, H" values determined this way depends on those of both of ADHe and L,. In the systems studied here both of these are significant. The value of A,H"(water) is cu.-25 kJ mol-' (varying slightly with the concentration of the concentrated solution) with an esti- mated precision of 1% or better, the precisions of the other ADHe values are of the same order. The L, values for the aqueous methanol,'-' ethanol' and a~etonitrile'~ systems were recovered from excess enthalpy data, and should have precisions of 10-20 J mol-', corresponding to k0.4 kJ rnol-' or less in A, He. The excess enthalpy data for the aqueous propan-1-01 system are less c~nsistent'~*'~ and, for this system, the rela- Table 2 Relative partial molar enthalpies of water and propan-1-01 and molar excess enthalpies of aqueous propan-1-01 mixtures at 298 K" O.oo00 0 -9590 0 0.0110 -4 -9040 -103 0.0150 -20 -8700 -150 0.0200 -27 -8250 -191 0.0250 -54 -7550 -241 0.0300 -64 -6740 -264 0.0350 -83 -6000 -290 0.04oO -110 -4900 -302 0.0450 -170 -3850 -336 0.0500 -215 -2890 -349 0.0550 -293 -2090 -392 0.0600 -305 -1310 -365 0.0650 -320 -750 -348 0.0700 -360 -260 -353 0.0800 -430 320 -370 0.1000 -475 750 -353 0.1200 -484 820 -328 0.1400 -490 880 -298 0.1600 -480 860 -266 0.1800 -465 850 -228 0.2000 -470 800 -216 " Units are J mol-'; precisions of Liare the larger of k 1% or f15 J mol-'.tive partial molar enthalpies of water and propanol were measured directly. The estimated precisions of the L, values in this system are again f20 J mol-'. The calorimetric measurements were made using the automated calorimeter system described el~ewhere.~ Results The A, Hevalues for TBABr for the systems studied are listed in Table 1.Analysis of the A,H" data in terms of eqn. (1) requires the relative partial molar enthalpies of the com- ponents of the mixed solvents. These were recovered from the excess enthalpies for the aqueous ehtanol,' TBA'7*'8 and a~etonitrile'~ systems. Since the agreement between the reported excess enthalpies for the aqueous propan-1-01 was significantly poorer, we undertook the direct 4500 1 I I I I 1 I I 0.00 0.05 0.10 0.15 0.20 0.25 XPrOH Fig. 1 Comparison of the relative partial molar enthalpies of water in aqueous propan-1-01 mixtures; (0)directly measured values, (-), calculated values from the relative molar enthalpies of propan-1-01 uia eqn.(4) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0 -1 00 .--I 0 -2002 j5 II -30C 0 I I I I 0.25I-4Ot I 0.00 0.05 0.10 0.15 0.20 XprO H Fig. 2 Molar excess enthalpies of water-propan-1-01 mixtures at 298 K; (-) those reported here, (a)and (0)values from ref. 15 and 16, respectively measurement of the Livalues, as the molar enthalpies of solu-tion of water and propan-1-01 in their mixtures. In this case the excess enthalpy, AHE,is calculable as : AH^ = x&, + X,L, (3) In part this approach is dictated by the available calorimeter system. However, it has one considerable advantage over the direct measurement of AHE in that the Livalues are related by the Gibbs-Duhem relationship, making it possible to check the consistency of the experimental data.Thus values of L, may be calculated from the experimental L, data as: (4) which is easily solved by representing L, as a polynomial expansion in x '. The experimental Li values for the propan-1-01 water system are listed in Table 2, along with the calculated excess enthalpies. Fig. 1 shows a comparison of the experimental values for water with those calculated from the values for propanol-1-01 uia eqn. (4). The agreement between the two sets of Livalues shown in Fig. 1 is excellent, and provides considerable support for the data listed in Table 2. Fig. 2 compares the values of AHE with those taken from the liter- ature.' 5*16 The agreement is excellent at low propan- l-ol concentrations, but the data reported here indicate a slightly deeper minimum (by about 25 J mol-') in the excess enth- alPY.Discussion As discussed previously, there are transitions in the solvating properties of these mixed aqueous solvent systems. Thus, two sets of model parameters are required to reproduce the experimental A, He data, the transition occurring relatively abruptly at some critical composition x:. The parameters reported here refer to the water-rich compositions. Table 3 Solvation parameters for tetrabutylammonium bromide in aqueous methanol, ehtanol, propan-1-01, TBA, acetonitrile and 1,4-dioxane P (an + #?N) AAH7,fkJ mol-' methanol 0.5, f 0.05 39 f 12 235 f150 ethanol 0.5 f 0.1 60f 20 230 f500 propanol-1-01 0.6, k 0.1 67 & 20 -50 +_ 200 TBA 0.6 f0.1 60 f 8 118 f160 acetonit rile 0.5 f0.1 148 f 30 2600 & 2500 1,4-Dioxana 1.0 f0.2 34 k 5 350 & 40 Data from ref.3. The parameters recovered from these analyses are listed in Table 3, and the experimental and calculated A, Hevalues are compared in Fig. 3. The precisions indicated for the model parameters warrant some comment. As discussed above, the experimental errors in the A, He are significant, reflecting those in both the ADHOand the L, values. In addition, there are significant uncertainties in the calculated At He values, reflecting the precisions of the Lidata. These factors dictate significant uncertainties in the reported model parameters.However, since the central point of the present paper was to investigate the observed variations in the (an+ PN) values, the uncertainties quoted for the model parameters are very much upper limits, so as to ensure that these differences are not artefacts. The curves shown in Fig. 3 were calculated using the best-fit parameters listed in Table 3. The parameters for the aqueous methanol, ethanol and TBA systems are the same as those reported previ~usly,~ but those for the aqueous propan- l-ol system differ somewhat from the earlier values. The difference in the latter case results principally from the greater precision in the shape of the A, He against solvent composition profile provided by the I I 1 0.0 0.2 0:4 ( 6 Xorg Fig.3 Comp$rison of the calculated and experimental enthalpies of transfer, A, H , of tetrabutylammonium bromide from water to aqueous methanol (0,---), ethanol (a,-), propan-1-01 (A, ---), TBA (A,-) and acetonitrile (+,-) additional At He data; there is also, however, some contribu- tion from the revised Livalues used. It can be seen from Fig. 3 that the agreement between the experimental At H" values and those calculated using eqn. (1) and the parameters from Table 3 is excellent in all cases and, significantly, in each case, this agreement extends from pure water to x; (with clear deviations at higher concentrations of the organic component). The agreement between the calcu- lated and experimental values at low concentrations of the organic cosolvent indicates that any contributions to the transfer enthalpies from structural changes or solvent aggre- gation are accounted for by the variations in the L, values (which will reflect these changes).Operationally this confirms that the parameters listed in Table 3 will reproduce the experimental At He data over the water-rich composition domains of each of these solvent systems. That is, eqn. (1) can be used to predict the A,H" data for these, and by implica- tion, those for the other systems studied previously with con- siderable precision. The theoretical problem, however, remains. The (an+QN) values in the aqueous alcohol systems are broadly in agree- ment, within the rather wide limits listed in Table 3 (and these precisions are very much upper limits); however, the value recovered for the aqueous acetonitrile system is clearly larger, by far more than the sum of the estimated precisions.Clearly not all of these values can be that for pure water. This result is entirely consistent with those of our previous studies. Thus the (an+QN)values for a series of amides in mixtures of methanol with dimethyl s~lfoxide'~ or acetonitrile" differs markedly from each other and from those obtained for the same solutes in the organic-rich aqueous acetonitrile5 and aqueous methanol4 systems. In effect then, the (an+QN)values recovered from the analyses of transfer enthalpies using eqn. (1) depend on both of the components of the mixed solvent. That is, they appear to be properties of solvation by the mixed solvent rather than by the individual components of the mixture.This is not truly consistent with the derivation of eqn. (l),which incorporates the approximations that the values of a and /? are the same for each of the componepts of the mixed solvent and that both of these and n (= nA +nB) and N (= N, +NB)are con- stant over the range of solvent compositions where eqn. (1) applies. This problem cannot easily be resolved. One possible approach is to make the approximation that a and Q are con- stant, but different for the component solvents; i.e. aA # aB and PA # QB.This leads to a tractable equation with four model parameters: p, AAH;,, (an+QN)A and (an+#IN)B; however, analysis of the transfer enthalpies in this way leads to no significant improvement in the fits to the data.More- over, the values recovered for (an+/?N)Aand (an+BN)Bare essentially equal and also equal to those recovered using eqn. (1). A second approach is to assume that n (=nA +nB) and N (NA+NB)are not constant but vary to reflect the differences in the volumes of the cosolvent molecules. This simply involves recasting eqn. (1) in terms of the volume fractions, rather than the mole fractions, of the component solvents and leads to an equation entirely analogous to eqn. (1). For the systems studied here this leads to only marginal changes in the (an+QN)values recovered, leaving the variations essen- tially unchanged, and again gives no improvement in the fits to the experimental data.In effect then, the problem appears not to lie in the form of eqn. (1). This points to the possibility that the parameters used to account for the contributions from changes in solvent-solvent interactions are unsatisfactory. The form of eqn. (1) dictates that the values for (an+PN) are recovered from the term involving the Livalues, since J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 those involving AAH:, and AAHo* having the same mathe- matical form. Thus we must question the direct use of the Li values when applying eqn. (1). The use of a common value of (an+PN) effectively assumes that all of the solvent-solvent interactions are equally perturbed by the introduction of the solute or, that all such interactions are equal.Clearly this would be the case for solvent systems such as mixtures of rare gases, where the solvent-solvent interactions would be symmetrical. However, in the systems considered here it need not be. In effect, the Li values contain contributions from each of the various solvent-solvent interactions. In the case of an aqueous alcohol system for example, these include, at the very least, the water-water, water-alcohol and alcohol-alcohol hydro- gen bonds and also the interactions of the water or alcohol molecules with the alkyl residue of the alcohol; each of these will differ in strength. In a situation wherein the tetra-alkylammonium ions are caged within the hydrogen-bonded network of the solvent system, leaving the alkyl residues of the cosolvent molecules relatively unaffected, changes in the strengths of the solvent-solvent hydrogen bonding would contribute significantly to At H*, but those involving the alkyl residues would not.In effect then, the common (an+QN) would apply only to those contributions of Li,which result from the hydrogen-bonding interactions. Thus, inclusion of contributions from the alkyl group interactions (which are reflected in the raw Livalues) would lead to (an+QN)values which varied with the contribution from the alkyl group interactions; that is, in the order: methanol < ethanol <propan-1-01, TBA, as observed. Of course, one could reverse the argument, taking the primary solute-solvent interactions to be with the solvent alkyl residues, but this leads to a similar conclusion.If this explanation of the variations in (an+BN) is correct then it has implications for the other model parameters recovered from eqn. (1). Most clearly, the values of AAH;, would not accurately reflect the differences in the direct solute-solvent interactions in the pure solvents, since their calculation involves the use of (an+PN); although this would be offset by the use of the enthalpies of condensation, which, like the Livalues, contain contributions from all solvent-solvent interactions. The values of p may be some- what compromised, but it is not possible, a priori, to assess the extent of this. One should not overstate the negative aspects of these points. Clearly caution is indicated in the use of their absol- ute values ;however, comparisons of parameters, particularly for similar solutes in a mixed-solvent system, are likely to remain valid.Moreover, the marked breaks in solvating properties of aqueous systems (at xt) are observed for far too wide a range of solutesolvent systems to be artefacts. In this context it is worth noting that, in some cases at least, there is strong evidence that the values of p and AAH;, are physically reasonable. Thus, in the acetonitrile-methanol solvent system the values of the model parameters for the silver halides can be determined independently of the A,H" data in the mixed solvents, and these predict accurately the variation in these with solvent composition.2 1*22 Moreover, the values of AAHy2 recovered for the silver halides in this system, and for a series of alkali-metal halides in aqueous methanol systems,6 are substantially equal to their transfer Gibbs energies between the pure solvents.Again, this result is predicted theoretically, in this case by application of Ben Naim's compensation prin~iple.,~ These results indicate either that the values of (an+BN), for these systems at least, are reasonable or, possibly, that the error induced by the use of the raw Livalues is compensated by the use of the enthalpies of condensation in calculating J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2695 AAHo* as indicated above. The ultimate resolution of this 4 G. Carthy, D. Feakins, C. O’Duinn and W.E. Waghorne, J. situation will require dissection of the Li and AAHo* values into their various contributions and a corresponding adjust- ment of eqn. (1). Conclusions 5 6 7 Chem. SOC.,Faraday Trans., 1991,87,2447. D. Feakins, P. Hogan, C. O’Duinn and W. E. Waghorne, J. Chem. SOC.,Faraday Trans., 1992,88,423. D. Feakins, E. de Valera and W. E. Waghorne, J. Chem. SOC., Faraday Trans. I, 1983,79, 1061. D. Feakins, E. de Valera and W. E. Waghorne, J. Chem. SOC., Faraday Trans. I, 1985,81,2703. Operationally it has been confirmed that the extended coor- dination model, via eqn. (l), will satisfactorily reproduce the transfer enthalpies of solutes in these complex aqueous mixed solvents and, by implication, those of other solute-mixed solvent systems.However, the exact physical significance of the model parameters recovered from the transfer enthalpies remains an open question and these must be treated with some caution. In particular the values of the model structural disruption parameter (an+ PN) for some of the systems con- sidered here are probably too large, with the value of 35-40 obtained from the aqueous methanol and 1,4-dioxan systems being a likely upper limit for tetrabutylammonium bromide 8 9 10 11 12 13 14 15 16 F. Franks and J. Desnoyers, Water Sci. Rev., 1985, 1, 170. M. K. Dutta-Choudry and H. B. Mathur, J. Chem. Eng. Data, 1974,19,2321. V. P. Belousov and M. Yu. Panov, Vestn. Leningrad Univ. Fiz. Khim., 1976, 2, 149. S. Murakami, R. Tanaka and R. Fujishiro, J. Solution Chem., 1974,3, 71.R. F. Lama and B. C. Y. Lu, J. Chem. Eng. Data, 1965,10,216. M. J. Costigan, L. J. Hodges, K. N. Marsh, R.H. Stokes and C. W. Tuxford, Aust. J. Chem., 1980,33,2103. R. H. Stokes, J. Chem. Thermodyn., 1977,19,977. V. P. Belousov, Vestn. Leningrad Univ., Fiz. Khim., 1961, 16, 144. V. P. Belousov, N. L. Makarova and M. Yu. Panov, Vestn. Leningrad Univ., Fiz. Khim., 1971, 113. in water. In turn, the values of AAHy2 must also be treated carefully. 17 18 H. Arm, Helv. Chim. Acta, 1962,45, 1803. J. Kenttamaa, E. Tommilia and M. Martti, Ann. Acad. Sci. Fenn., Ser. A, 1959,93, 1. We gratefully acknowledge support from the European Union. 19 20 D. Feakins, C. C. O’Duinn and W. E. Waghorne, J. Solution Chem., 1987,16,907. A. Costigan, D. Feakins, I. McStravick, C. O’Duinn, J. Ryan and W. E. Waghorne, J. Chem. SOC.,Faraday Trans., 1991, 87, References 21 2443. B. G. Cox and W. E. Waghorne, J. Chem. SOC.,Faraday Trans. 1 G. Carthy, D. Feakins and W. E. Waghorne, J. Chem. SOC., Faraday Trans. I, 1987,83,2585. 2 D. Feakins, J. Mullally and W. E. Waghorne, J. Solution Chem., 22 23 I, 1984,80,1267. W. E. Waghorne, Chem. SOC.Rev., 1993,22,285. A. Ben-Naim, J. Chem. Phys., 1978,82,874. 1990, 19,401. 3 D. Feakins, J. Mullally and W. E. Waghorne, J. Chem. SOC., Faraday Trans., 1991,87,87. Paper 41005495; Received 28th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002691

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2697-2701 ATR-FTIR Studies of Ion-Solvent and Ion-Ion Interactions in Divalent-metal Perchlorate-Acetonitrile Solutions W. Ronald Fawcett," Guojun Liut and Alla A. Kloss Department of Chemistry, University of California Davis CA 95616,USA ~ ~ ~~~~ The effect of divalent-metal ions on the IR spectrum of acetonitrile has been examined by studying the depen- dence of the spectral features on concentration for seven divalent-metal-ion perchlorates. Data are reported for both the frequency shifts and molar absorption coefficients of the major bands in acetonitrile which are affected by the presence of a cation in solution. It is shown that ion association plays a major role in determining the magnitude of the frequency shift.Other features discussed include the effect of the combination band in proto-nated acetonitrile, and the role of metal-ion softness in cation-acetonitrile interactions. Vibrational spectroscopy is a powerful tool for examining the solvation of electrolytes in acetonitrile.'-' By examining changes in the frequency and intensity of important solvent bands, such as the CEN stretching frequency (v,), one can assess the strength of cation-solvent interaction and deter- mine solvation numbers and the extent of ion pairing. An important goal of our work has been to assess changes in cation-solvent interaction with changes in cation size and charge. These can be determined, to a first approximation, by measuring the shift in the v, band when the electrolyte is added to the s~lvent.~*~ However, in protonated acetonitrile, this shift is affected by the nearby combination band (v3 + v,) with which the v, band is in Fermi In order to avoid this interaction, studies have been carried out in deu- teriated acetonitrile in which the shifts in the CEN band give a true measure of the strength of the interaction of cations with the electronegative end of the solvent molecule without the complication of a nearby combination In order to compare values of Av, for different cations one must also know how the extent of ion pairing changes with cation nature.Ion pairing is conveniently studied by examining the changes in intensity of bands resulting from cation-solvent interaction with cation c~ncentration.~ The solvation numbers for divalent cations in acetonitrile have been assumed to equal six, on the basis of previous work.' In a recent paper,3 a detailed examination of FTIR spectra obtained for Mg(ClO,), solutions in acetonitrile showed that the average solvation number for Mg2 + is much less, i.e.3.4. Evidence was presented that the predominant cationic species in solution were the Mg2+ cation solvated by four acetonitrile molecules, and a species surrounded by three acetonitrile molecules and one perchlorate anion. We have extended this study to six other divalent ions, namely, the alkaline-earth-metal cations Ca2+, Sr2 + and Ba2+, the transition-metal ions ZnZ+ and Cd2+, and the post-transition-metal ion, Pb2 +.Estimates of the average solva- tion numbers are presented and the roles of ion size, ion pairing, and metal-ion softness assessed in determining the strength of cation-solvent interactions.Experimental Mid-IR spectra were obtained using an IBM Instruments IR-98 FTIR spectrometer equipped with a liquid-nitrogen cooled HgCdTe detector and a SpectraTech variable-angle ATR attachment. Details of the method of data acquisition t Present address : Center of Advanced Technology Development, Institute of Physical Research and Technology, Iowa State Uni-versity, Ames, IA 50011, USA. were given earlier4 except that resolution was increased to 2 cm-'. Band peak maxima were found by fitting a Lorentzian curve to the band using Spectro Calc software (Galactic Industries). Band positions were reproducible to better than 1 cm-'.Acetonitrile (Fisher, HPLC grade) and deuteriated acetoni- trile (Aldrich, 99.5% deuteriated) were used as received. The perchlorate salts were obtained as hydrates either from Johnson-Matthey [Ca(ClO,), and Cd(CIO,),] or from Strem [Sr(ClO,), , Ba(C10,), , Zn(ClO,), and Pb(ClO,),]. They were purified by recrystallizing them twice from nanopure water. Considerable effort was made to obtain well dried salts. This was achieved by heating them under vacuum in the presence of P205in a drying pistol. The temperature of the drying operation was controlled by surrounding the pistol by a refluxing organic solvent. Salts such as Ca(ClO,),, Cd(C10,), and Sr(C10,), which have low melting points were dried in three steps: (a) under vacuum for two days at room temperature; (b)heated at 56 "C (acetone vapour) for 1 day and (c) heated for several days at 140°C (xylene vapour) for Ca(ClO,), and Sr(ClO,), , or at 80 "C (benzene vapour) for Cd(C10,), .Zn(C10,), was dried at 80 "C for five days and then for a week at 110°C (toluene vapour). Pb(ClO,), was dried at 56°C for 2 days, and then at 80°C for 4 days. During the drying operation, the P,05 was replaced when it showed signs of being wet. Drying was stopped when there was no visible sign of wetness on the P,05. A test solution of the dried salt was then made and monitored by IR spectros- copy in the region of the water bands. The salt was only con- sidered dry when no evidence of water contamination was obtained in this manner.All solutions were prepared in a dry environment, exposure of salts and solvent to the open atmo- sphere being kept to a minimum. Results IR spectra were collected in protonated acetonitrile solutions containing the six different metal-ion perchlorates in the con- centration range 0.02 to 1.0 moll-'. The difference spectra of CH3CN in the presence of Sr(C10,), are shown as a typical example in Fig. 1 and 2. The intensities of the CH, symmetric stretching band (vl, 2943 cm-') and the asymmetric stretch- ing band (v5, 3003 cm- ') increase significantly with electro- lyte concentration [Fig. l(a)], but their positions are essentially the same as in the pure solvent.These changes are attributed to interaction between the methyl group at the positive end of the molecular solvent dipole and the perchlor- ate anion., Three bands are seen in the CSN stretching region [Fig. l(b)], one of them increasing in the negative direction. This latter band is due to the C=N stretching fre- I 3080 2900 2i80 2 0 wavenumber/cm-' Fig. 1 IR difference spectra for acetonitrile solutions containing various concentrations of Sr(ClO,), in (a)the CH, stretching region and (b)the C'N stretching region. The concentration of Sr(ClO,), increases from bottom to top. quency in unassociated acetonitrile (v2, 2253 cm-') whose concentration decreases with increase in electrolyte concen- tration. The band at 2288 cm-' is attributed to the CEN stretching frequency for the solvent when it is associated with the Sr2+ cation, whereas that at 2316 cm-' is due to the combination band (v3 + v4).Bands observed at lower fre- quencies are shown in Fig. 2. There are two bands which increase in the negative direction corresponding to the free C-C stretching mode (v4, 918 cm-') and the overtone due to the C-CEN deformation mode (2v8, 746 cm-l) for unas- sociated solvent molecules. The two bands increasing in the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Frequency shifts (cm-')of the major IR bands of CH,CN in the presence of divalent-metal cations band metal cation v1 v2 v3+4 v4 2% ~~ ~~ ~ Mg2+ -1 36 24 -21 Ca2+ -2 22 13 12 28 Sr2+ -2 36 24 21 46 Ba2+ -1 11 7 6 17 Zn2+ -2 34 23 19 39 Cd2+ -3 28 18 16 29 Pb2+ -2 12 6 6 17 pure solvent' 0 (2945) 0 (2253) 0 (2293) 0 (918) 0 (748) 'The frequency of the band in the pure solvent is given in brackets.positive direction at 939 and 795 cm-' are clearly attribut- able to CH3CN molecules associated with the electrolyte. The band at 746 cm-' is due to the 2v, mode. The absorp- tion intensity observed in the region from 914 to 939 cm-' is partially due to the C-C stretching mode in the associated molecule, and also to the symmetrical stretching modes of the perchlorate anion which fall in this range. The frequency of the v2 band varies significantly with the nature of the cation when acetonitrile is associated with a cation.Significant frequency shifts are also seen for the v4, 2v8 and the v3+4 combination band. These frequency shifts for the six cations in the present study, together with those for Mg2+ obtained ear lie^,^ are summarized in Table 1. As one would expect the largest frequency shift is observed for Mg2+ which is the smallest cation and therefore expected to have the largest polarizing effect on the electron density at the electronegative end of the acetonitrile molecule. At the same time, the smallest shift is seen for the largest ions, Ba2+ and Pb2+. However, the shift for each of the modes con- sidered does not follow the change in cationic size in a regular manner. This is most clearly seen in the alkaline- earth-metal series where the frequency shifts in v2, v4 and the v3+4 combination band are the same for Mg2+ and Sr2+.Thus, some other factor must play a role in determining the extent of interaction between the cation and the solvent mol- ecule. This question is examined in more detail below. The concentration dependence of the integrated band intensities for the major solvent bands was examined in order to determine molar absorption coefficients and to assess the extent of ion pairing. The data obtained in the case of Sr(ClO,), are shown in Fig. 3. Very good linear relations between integrated intensity and electrolyte concentration are 15.0 1000 900 800 700 .oo 0.32 0.64 wavenum ber/cm -[Sr (CIO4) 2] /mol 1 -' Fig. 2 IR difference spectra for acetonitrile solutions containing various concentrations of Sr(ClO,), in the C-C stretching and Fig.3 Integrated intensity us. concentrationof Sr(ClO,), for 0,the C-C=N deformation regions. The concentration of Sr(ClO,), v2 band in free acetonitrile and for the A, v,; A, v3+,; +,v1 and 0, increases from bottom to top. v5 bands in coordinated acetonitrile J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Molar absorption coefficients (intensity units 1 mol-') for the major IR bands of CH,CN in the presence of divalent-metal cations band ~ ~ ~~ ~~ metal cation v1 v2 (coord) v2 (free) v3+4 v5 Mg2+ 3.75 10.2 -2.41 9.6 2.48 Ca2+ 2.7 1 13.5 -3.52 5.4 2.01 +Sr2 4.1 1 11.7 -4.25 11.1 2.87 Ba2 2.50 14.6 -3.49 4.3 2.17+ Zn2+ 4.50 11.7 -4.11 13.8 3.35 Cd2+ 3.99 11.9 -3.88 8.4 3.03 Pb2+ 1.68 12.5 -3.61 3.5 1.64 obtained for the v2, v~+~,v1 and v5bands for coordinated acetonitrile.The intensity data for the v2 band of free aceto- nitrile are also linear in electrolyte concentration, the inten- sity at zero concentration corresponding to that for pure acetonitrile. Similar data were obtained for the other five electrolytes studied here. A summary of the molar absorption coefficients for these bands is given in Table 2. These data may be used to calculate an average coordination number, S, for acetonitrile with a given metal ion. The analysis used here makes use of the molar absorption coefficient for acetonitrile measured in carbon tetrachloride solutions, namely, 0.718 intensity units per mole., The intensity of the v2 band in pure acetonitrile is 13.3 intensity units corresponding to a molar concentration of 18.5 mol 1-'.On the basis of the density of acetonitrile (0.776 g ml-'), the molar concentration of ace- tonitrile in the pure liquid is 18.9 mol I-'. Thus, the experi- mental results obtained in the CCl, solutions can be used to determine an approximate coordination number, S, in the electrolyte solutions. The molar absorption coefficient for the v2 band of free acetonitrile measured with respect to electro- lyte concentration is related to the number of acetonitrile molecules coordinated to the electrolyte on the basis of its molar concentration. By dividing this number by the molar absorption coefficient for acetonitrile in CCl, one obtaips the average coordination number of acetonitrile with the metal cation of the electrolyte.In the case of Sr2+, the result is 5.92. The result is approximate because the value of S is expected to vary with electrolyte concentration owing to changes in solution density and the extent of ion pairing. However, this variation is not expected to be large as can be seen from the quality of the linear fit for the v2 band for free acetonitrile seen in Fig. 4. Moreover, the precision of the present data is not sufficiently high to warrant a more detailed analysis. Similar analyses were carried out for the other cations con- sidered in this study. The average coordination number, S, varies from a low of 3.35 for Mg2+ to a high of 5.92 for Sr2+ (see Table 3).The fact that good linear relationships were obtained between the integrated intensity and electrolyte con- centration (r > 0.96) demonstrates that S is approximately constant over the concentration range used in the present 50 40 r' 305-..520 10 0 0 5 10 15 20 25 z, r-'/nm -' Fig. 4 Frequency shift of the v2 band in deuteriated acetonitrile us. the effective field ze/r for seven divalent-metal cations. The straight line was drawn using a one-parameter least-squares fit. studies. The large variation in the coordination number with the nature of the cation is due to the effects of ionic size and extent of ion association. The larger cations would clearly have a coordination number of six in the absence of ion association.The extent of ion association also reflects the strength of the metal-solvent bond which is clearly stronger for a transition-metal ion such as Cd2+ than for a main- group element such as Ca2+, in spite of the fact that these ions are approximately the same size. When one examines the data for the alkaline-earth-metal ions only, the value of S does not change smoothly with increase in ionic radius. Thus, other factors come into play and they are considered in more detail below. On the basis of the data summarized in Table 2, it is clear that the v2 and v,+~bands are considerably enhanced when the acetonitrile molecule is coordinated to a metal cation.Quite large variations in this enhancement are seen for the combination band v,+4. The observed enhancement is attrib- uted to the fact that the dipole moment change in the coordi- nated species is more sensitive to the CSN stretching than in the isolated molecule. Molar absorption coefficients for the CH, stretching bands v1 and v5 also depend on the nature of the cation. The observed enhancement is attributed to interaction of per- chlorate anions with the positive end of the molecular dipole which is located at this group. The variation with the nature of the cation is because the concentration of free perchlorate anions depends on the cation owing to varying degrees of ion association. On the basis of these results, ion association is strongest in the presence of Pb2+ and very strong in the presence of Cat+ and Ba2+.The least amount of ion associ- ation occurs in solutions containing the Zn2+ ion. Discussion One of the most interesting features of the present study is that the frequency shift for the signature band in acetonitrile, namely, the CEN stretching frequency, does not follow Table 3 Other relevant parameters for divalent-metal-ion perchlorates in acetonitrile Pauling radius, effective solvent coordination effective cationic frequency shift for the v2 band Marcus softness ion r/Pm number, S charge, z, in CD3CN, AvJcm-' parameter, Mg2+ 65 3.35 1.35 42 -0.37 Ca2+ 99 4.90 0.90 24 -0.67 Sr2+ 113 5.92 1.92 42 -0.59 Ba2+ 135 4.85 0.85 12 -0.60 Zn2+ 74 5.72 1.72 41 0.37 Cd2+ 97 5.40 1.60 32 0.59 Pb2+ 121 5.03 0.97 11 0.58 2700 changes in ionic size.This can be partly attributed to the effects of ion association which lower the effective charge on the cation. If a maximum of one anion is associated with a given cation, then the fraction,f; which are paired is given by f=N-S (1) where N is the maximum number of solvent ligands around a given cation. For the present group of ions, N is assumed to be six for all cations except Mg2+ for which it is four. The average effective charge on the cation is then Z, = 2 -N + S (2) The simplest explanation of the variation in Av, with cation nature is that is follows the variation in the field due to cation charge, ze/r, where r is the cation radius taken as the Pauling value (see Table 3).In order to test this proposal, values of Av, determined for CD,CN were chosen since these are not affected by the proximity of a combination band. A reasonable linear correlation is found between Av, and ze/r, the resulting relationship on the basis of a one-parameter fit being Av, = 2.0ze/r (3) The correlation coefficient, r, is equal to 0.937 indicating that approximately 88% of the observed variation in Av, can be attributed to the change in the effective field of the coordi- nating cation. The outlying points involve the hard cations + +Ca2+ and Sr2 and the soft cations Pb2 and Zn2 + . Some improvement in the understanding of the factors affecting Av2 can be made if one considers the role of cation softness.A parameter quantifying this factor was developed by Marcus8 in order to assess the tendency of cations to undergo covalent bonding. The softness parameter, 0, is determined from the difference between the ionization poten- tial of the gaseous atom to form the cation and the enthalpy of hydration of the latter. From the values listed in Table 3, o varies from -0.67 for hard cations such as Ca2+ to 0.59 for soft cations such as Cd2+. When this is included in the description of the variation of Av2 with cation, the resulting relationship from a two parameter fit is Av, = 2.Oze/r-6.20 (4) with a correlation coefficient equal to 0.973. As a result, 95% of the observed variation in Av, is explained, with 79% of the explained variation being attributed to the effective cation field through changes in ze/r, and 21% to changes in cation softness.The resulting fit is illustrated in Fig. 5. The values of the coordination number, S, determined from the change in intensity of the v2 band for free acetonitrile 50 I 1 -40 r -' 30 zj--. 20-jot 7Ba2+ I OO" 10 20 30 40 50 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3' h* +? m\? 2. > v \ a 1-0' I -2 0 2 4 6 8 A(Av2)/cm-' Fig. 6 Ratio of molar absorption coefficients for the v2 and v3+4 bands. RI(v2/v3+J, us. the difference in frequency shifts for the v2 band in deuteriated and protonated acetonitrile, A(Av2).The straight line was fitted by least squares. vary significantly with the nature of the cation. In the case of Mg2+, for which the maximum value of S is assumed to be 4, the result obtained indicates that CQ. 65% of the Mg2+ ions are paired with a C104 anion., In the case of Sr2+ and Zn2+, the extent of ion pairing is small, amounting to 8% and 28% of the ions, respectively. The solvation numbers for Ca2+ and Ba2+ are less than 5. These results are taken as evidence that ion triplets are formed in these solutions. Supporting evi- dence for this conclusion is obtained by analysing the per- chlorate bands observed in acetonitrile solutions containing these electrolytes.' Finally, it should be noted that the degree of ionic association reflected in the value of S is consistent with the observed changes in intensity of the CH, stretching modes, v1 and v5.As pointed out above, the intensity and position of the v2 band for the coordinated acetonitrile is effected by the fact that it is in Fermi resonance with the nearby combination band v3+4 for the same species. This effect was assessed by comparing the ratio of the molar absorption coefficients for these bands RI(v2/v3+J with the difference in the CzN stretching frequency shifts between the deuteriated molecule and the protonated molecule, A(Av,). The latter difference is ca. 0 for the Ba2+ and Pb2+ ions and increases to 7 cm-' for the Zn2+ ion (see Tables 1-3). A plot of RI(~2/~3+4)against A(Av,) shown in Fig.6 demonstrates that there is a linear relationship between these quantities. This result can be interpreted on the basis of the following argument. The pres- ence of Fermi coupling between the v2 and v3+4 bands in acetonitrile results in their frequencies being shifted apart, the v2 band moving in the red direction and the v3+4 band in the blue direction. When a cation is added to the system both of these bands are blue-shifted for a coordinated acetonitrile molecule, the shift for the combination band being about two thirds of that for the v2 band. Thus, the frequency separation between this pair of bands is smaller for the coordinated mol- ecule, and Fermi coupling is stronger. As a result the net blue shift of the v2 band is reduced in magnitude by a red shift due to Fermi coupling.Since Fermi resonance is not present in the CD,CN spectrum in this region, the shift in the v, band measured in this solvent gives a measure of the influence of the cation alone. Thus, the quantity A(Av2) gives a measure of the influence of Fermi resonance on the position of the v2 band in the protonated solvent. Another effect of Fermi coupling is that the combination band borrows intensity 2.02,r-l -6.20 from the v2 band to an extent which depends on their fre- quency separation. Thus, the ratio RI(v,/v, +4) also reflects Fig. 5 Frequency shift of the v2 band in deuteriated acetonitrile us. the function 2.02, r--6.20. The constants were determined by a the strength of Fermi coupling in the presence of a given two-parameter least-squares fit, and the straight line drawn with a cation. It follows that there is a linear relationship between slope of unity.RI(v2/v3+4) and J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 One other feature of the spectra merits comment. Because must be considered in order to understand the frequency of Fermi resonance, the shift of the combination band Av~+~shifts and intensity changes observed in the spectra. By con- should be greater than the sum of the shifts Av3 and Av4. The shift Av, for the symmetric deformation band is zero for all systems studied. Examination of that data in Table 1 reveals that Av, +4 is always greater than Av4. However, the differ- ence between these frequency shifts should be approximately equal to the net shifts in the v2 band due to Fermi resonance, namely, A(Av2). However, A(Av2) is clearly greater, that is A(Av2) > Av3+4 -Av, -Av, This observation is attributed to the effects of vibrational anharmonicity when the acetonitrile molecule is coordinated to a metal ion.In pure protonated acetonitrile, the com-bination band is at 2293 cm-' which is the exact sum of the frequencies of the v3 band (1375 cm-') and the v4 band (918 cm-I). This signifies that the blue shift in the combination band due to Fermi coupling is offset by a red shift of equal magnitude due to the anharmonicity of the vibration. When acetonitrile is coordinated to a cation, its anharmonicity con- stant changes. Thus, one must consider three factors in rationalizing the frequency shifts of the combination band, namely, the effect of the cation reflected in the large blue shift of the v4 band, the small blue shift due to Fermi resonance, and the small red shift due to the anharmonicity effect.In conclusion, the effects of divalent cations on the IR spectrum of acetonitrile cannot be understood simply on the basis of the size of the cation and its polarizing effect on the electronegative end of the acetonitrile molecule. Ion associ- ation plays an important role in these systems, and its effects sidering a wide range of cations from both main group and transition elements one can assess the role of covalent bonding in the metal-acetonitrile interactions. Further infor- mation about the nature of the ion association can be obtained by examining the perchlorate region of the spectra. These results will be presented in a subsequent paper. The financial support of the Officeof Naval Research, Wash- ington, is gratefully acknowledged. References 1 D. E. Irish and M. H. Brooker, Adv. Znfiared Raman Spectrosc., 1976,2, 212. 2 I. S. Perelygin, in Ionic Soluation, ed. G. A. Krestov, Nauka, Moscow, 1987, ch. 3. 3 W. R. Fawcett and G. Liu, J. Phys. Chem., 1992, %, 4231. 4 W. R. Fawcett, G. Liu, P. W. Faguy, C. A. Foss Jr. and A. J. Motheo, J. Chem. Soc., Faraday Trans., 1993,89,811. 5 P. Gans, J. B. Gill and P. J. Longdon, J. Chem. Soc., Faruday Trans., 1989,85, 1835. 6 P. Gans, J. B. Gill and P. J. Longdon, J. Chem. Soc., Faruday Trans., 1994,90, 315. 7 Y. Marcus, Ion Solvation, Wiley-Interscience, New York, 1985, ch. 4. 8 Y. Marcus, Zon Solvation, Wiley-Interscience, New York, 1985, ch. 3. 9 Guojun Liu, Doctoral Dissertation, UC Davis, 1993. Paper 3/06779C; Received 12th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002697

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2703-2708 Phase Transitions in the Bilayers of Vesicles formed from Binary Mixtures of Symmetric Di+Alkylphosphates in Aqueous Solutions Michael J. Blandamer," Barbara Briggs and Paul M. Cullis Department of Chemistry, The University, Leicester, UK LEI 7RH Jan B. F. N. Engberts, Anno Wagenaar and Elly Smits Department of Organic & Molecular Inorganic Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Dick Hoekstra Department of Physiological Chemistry, University of Groningen , Bloemsingel 10,9712 KZ Groningen, The Netherlands Anna Kacperska Department of Physical Chemistry, University of Lodz, Pomorska 18,91416 Lodz, Poland Vesicles in aqueous solutions were prepared from binary equimolar mixtures of di-n-alkyl-phosphates (sodium and potassium), (R'0)2P02-M+ and (R20)2P02-M+.When the number of carbon atoms in R' and R2 differs by two and when R' or R2 = C12H25, C,,H2, , C1&& and C,,H,, the membranes undergo well defined gel to liquid crystal transitions at characteristic temperatures T, .The recorded T,s are intermediate between the melting temperatures for vesicles prepared from the respective single di-n-alkylphosphates. Furthermore, the extrema recorded by differential scanning microcalorimetry show that the vesicle membrane is made up of domains that differ in composition. For those vesicles produced from di-n-alkylphosphates where the number of carbon atoms in R' and R2 differs by more than two the plots recorded by the scanning microcalorimeter are complex.The scans show many extrema, suggesting that the bilayers are formed from many domains having different compositions. In all cases, the scan patterns are essentially repeated through several heat-cool- heat.. .cycles. The temperatures T, are increased relative to those of the component surfactants when K+ and Na+ salts are mixed, showing that the counter cations play an important role in determining the thermotropic properties of the vesicles reflecting the importance of electrical interactions in determining the packing within the bi layers. In aqueous solution vesicles' formed by sodium di-n-dodecylphosphate (DDP) undergo a gel to liquid crystal tran- sition near 34.8 "C. This transition produces a sharp well defined feature in the scan recorded by a differential scanning microcalorimeter2 (DS).Reproducible and, for the most part, reversible scans were obtained2 when DDP vesicles were pre- pared using a carefully defined protocol. The recorded extre- mum in the scan is characterised by a standard enthalpy of melting A,H" equal to 3.9 kcal (mol monomer)-' and a patch number, n, of 168. The latter describes the number of surfactant monomers which form domains in which coopera- tive melting takes place. In other words, the minimum fluc- tuation in the bilayers necessary to trigger the main phase transition involves 168 DDP monomers. With increase in alkyl chain length, T, increases, reaching 77.1 "C for the case where R' = R2 = C,,H3,. In the study reported here we measured the DSC scans for vesicles prepared using equi- molar mixtures of two symmetric di-n-alkylphosphates where the alkyl groups differ in chain length; e.g.mixtures of (R'O),PO,-Na+(aq) and (R20),P02-Na+, where R' and R2 are C,2H25, Cl4H29, C16H33 and C18H3,. Our interest in these mixed-vesicle systems was prompted by the recogni- tion that although synthetic amphiphiles4 such as DDP form bilayer structures similar to lipids,' naturally occurring lipid systems usually comprise mixtures of amphiphiles.6 We show that for a mixed-vesicle system formed from two di-n-alkyl- phosphates where R' and R2 differ by C2H4, the scan shows a single extremum at a temperature intermediate between the T,S characteristic of vesicles prepared from equimolar solu- tions containing single di-n-alkylphosphates. Where R' and R2 differ by more than C2H4, the scans are complicated showing several extrema, although the patterns are repro- duced through several heat-cool-heat...cycles. Experimental Materials The di-n-alkylphosphates were prepared as described pre- viously. Differential Scanning Microcalorimetry A MicroCal (USA) differential scanning microcalorimeter was used as previously described.2 The reference cell was filled with water. Scans were normally recorded between 15 and 90°C. After the first scan had been recorded, the solution was allowed to cool slowly to 15°C and the scan again recorded from 15 to 90 "C. This protocol was repeated several times, the aim being to determine if the same scan pattern was recorded over several heat-cool cycles.The scan data were recorded on 3.5 in7 discs and later analysed using the Origin (MicroCal) software. Preparation of Vesicles Previously we ~howed~?~ the importance of developing a pro- tocol for vesicle preparation which leads to DSC scans which are reversible and repeatable. The excellent sensitivity of the scanning microcalorimeter in these studies2 confirmed that this same consideration applied to solutions prepared using binary mixtures of surfactants. For the purpose of compari-son, we report here the properties of vesicle solutions con- taining fixed total concentration of surfactant, 8.42 x mol dm-3.The appropriate masses of the two surfactants t 1 in x 2.54 cm. were weighed to produce a suspension (volume, 2.2 cm3) con- taining (R10)2P02-M,+ (aq; ca. 4.21 x mol dm-3) and (R20)2P02-M1+ (as; 4.21 x mol dm-3). The aqueous suspension was heated to a temperature T* (see below) and held at that temperature for 1 h. The resulting solution was cooled and carefully placed in the sample cell of the calorimeter, ensuring that no air bubbles were trapped in the cell. The solution was cooled in the sample cell to 15 "C, which was the starting temperature for the scans. In detail, the temperature T* was as follows; for (i) R' = C12,H25, R2 = C14H29, M1+ = M2+ = Na', (ii) R' = R2 = C12H25, M,+ = Na+, M2+ = K+, and (iii) R' = R2 = C14H29, M1+ = Na+, M2+ = K+, T* x 60°C; for (iv) R' = C12H25, R2 = C16H33, M1+ = M2+ = Na+, (v) R' = C14H29, R2 = C16H33, M1+ = M,' = Na', (vi) R' = R2 = C16H33, M1+ =Na+, M2+ =K+, T*=70"C; for (vii) R' = C12H25, R2 = C1BH37, M1+ = M2+ = Na', (viii) R' = C14H29, R2 = C18H37, M1+ = M2+ = Na+, (ix) R' = C16H33, R2 = C18H37, M1+ = M2+ = Na+ and (x) R' = C16H33, R2 = trans-CH3(CH2),CH=CH(CH,), , M1+ = M2+ = Na+, T* x 75°C.Results The scans recorded for aqueous solutions containing vesicles formed from mixtures of di-n-alkylphosphates, (R'0)2P02-Ml + and (R20)2P02 -M, were strikingly dif- + ferent, on the one hand for vesicles where R' and R2 differ by two CH, groups and, on the other hand, where R' and R2 differ by more than two CH, groups.Where the difference was two CH, groups, the scans showed extrema intermediate between those characteristic of solutions containing the single di-n-alkylphosphate at the same overall concentration; Fig. 1A. This pattern is illus-trated by vesicles containing sodium di-n-alkylphosphates, where R' = C12H25 and R2 = C14H29. Solutions containing a single di-n-alkylphosphate have extrema at 34.8 & 0.2 "C (R = C12H25) and 52.2 f0.1 "C (R = C14H29) where the quoted means of estimates for T, (Table 1) are calculated using at least four successive scans on the same solution; Fig. 1A. The equimolar mixture produced a broad extremum at 40.8 & 0.1 "C; Fig. 1A. The same broad pattern was produced by the mixture in five repeat scans taken over a period of 17 h, showing that the processes involved are reversible equi- librium properties of the mixed system having short time constants; Fig.2. With increasing alkyl chain length but holding the differ- ence between R' and R2 at two methylene groups, the tem- perature T, of the pure solutions moves to higher temperatures; Fig. 1B and Table 1. Furthermore, the tem- perature difference between T,s decrease; e.g. for R = C16H33,T, = 66.3 f0.1 "c.The scans for equimolar mix- tures, R, = C14H29 and R2 = C16H33, show an extremum between the two T,s at 57.6 f0.1 "C; Fig. 1B. An important finding was that with increase in chain length the scan recorded for the mixture broadens and then separates into two components. The latter pattern is particularly clear-cut for the mixture where R' = C16H33 and R2 = C18H37;Fig.1C. For solutions containing the single surfactant R = C18H37, T, = 77.1 f0.1 "C. For these mixtures, the pattern shown by the scans was broadly similar when recorded several times over a period of 6 h. In the case of the mixture Fig. 1 Dependences on temperature of the differential heat capa- cities (reference = water) for aqueous solutions containing vesicles prepared from the sodium salts of di-n-alkylphosphates, (RO) PO,-Na+ [total concentration = 8.4 x (mol monomer) dm-33 and from equimolar [ca. 4.2 x lo-' (mol monomer) dm-3] of two sodium di-n-alkylphosphates, (R'O),PO, -Na+ and J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.06LA I I I I 20 40 60 TPC 1 I I 1 40 60 TrC 0.06 0.04 0.02 I I I I 60 80 T/OC (R20),P0,-Na+.A, Scan (a) for R = C,,H,, where T, = 343°C; scan (b) for R' = C,,H,, and R2 = C14H29 where T, = 40.8"C; scan (c) for R = C,,H,, where T, = 52.2"C. B, Scan (a) for R = C,,H,, where T, = 52.2"C; scan (b) for R = C16H33 where T, = 66.3"C; scan (c) for R' = C14H,, and R2 = C16H33 where T, = 57.6"C. C, Scan (a)for R = C,,H,, where T, = 66.3 "C; scan (b) for R = C,,H,, where T, = 77.1 "C; scan (c) for R' = C,,H,, and R2 = C18H37where T, = 67.6 and 757°C. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I* 1 lB 40 60 60 80 TpC Fig. 2 Dependences on temperature of the differential heat capa- cities for equimolar mixtures [total concentration 8.4 x low3(mol monomer) dm -3] of two sodium di-n-alkylphosphates, (R'O),PO,-Na+ and (R20),P02-Na+.A, R' = C12H25 and R2 = C14H29 where the recorded scans (a)-(d) were recorded consecutively and scan (e) was recorded after the solution had stood at 15°C for 8 h; B, R' = C16Hj3and R2 = C18H3,where the scans were recorded as in A except that scan (e) was recorded after the solution had stood at 15 "Cfor 6 h. where R' = C16H3, and R2 = C18H37, the scans were char- acterised by two well resolved extrema at 67.2 and 77.4"C. After each repeat scan the extremum originally at 77.4"C moved to lower temperatures. After 6 h the extremum at the higher temperature had moved to 75.7"C, the extremum at the lower temperatures being slightly shifted to 67.6 "C; Fig. 2.The patterns emerging from the scans are much more com- plicated when the number of CH, groups in the alkyl chains differs by more than two. A typical example is shown in Fig. 3A, which records the DSC scans for vesicles prepared using, in one case (Cl,H2,0),P02-Na+ and, in the other case, (C16H3J0)2P02-Na+.These scans are compared with that recorded for vesicles in an aqueous solution containing equi- molar amounts of the two di-n-alkylphosphates. The contrast is striking in that the new scan shows a broad almost feature- less plot. Interestingly, the same types of pattern were recorded in successive scans for solutions containing + +(C ,H2 ,O),PO, -Na and (C ,H 70)2PO, -Na [Fig. 3(b)], showing that within a time period of ca.8 h the changes contributing to the differential heat capacities are reversible. Nevertheless, over a longer time period, changes had taken place producing new extrema near 40°C. For mixtures con- taining (C14H,50)2P02-Na+ and (C18H370)2P02-Na+ a broad extremum with partially resolved maxima at 55.7 and 67.4"C was recorded; Table 1. Repeat scans of the equimolar mixtures (cf. Fig. 3B) containing (Cl,H2,0)2P0,-Na+ and (C16H330)2P02-Na+ showed a similar pattern with one slight difference. In the first scan, an extremum was observed near 38.5 "C,but this feature disappeared over seven suc- cessive scans, reappearing on an eighth scan after the solution had stood for 3 h at 15°C. Otherwise, all remaining features in the complicated pattern were repeated over eight recorded scans.In the examples quoted above, the counter cation was fixed at Na+, whereas the length of the alkyl chains was varied. In the next series of experiments the alkyl chain was fixed but the mixtures were prepared using equimolar amounts of sodium and potassium salts. The T,s of the mixtures were generally higher than for those solutions containing either sodium or potassium salts. For the (Cl,H,,0),P02 -system, T,(K+) = 52.0 & 0.1 "C, T,(Na+) = 52.2 & 0.1 "C but T, for the Na+ + K+ mixture = 53.2 & 0.1 "C; Fig. 4A. A similar pattern was recorded for the solution containing (C16H,,0),P02-where the recorded T,s are 66.3 & 0.1 (Na+), 65.2 & 0.1 (K') and 66.5 & 0.1 (K+ + Na+)"C; Fig.4B. The processes responsible for the extrema are reversible, as confirmed by the scans in Fig. 4C. Table 1 Derived parameters characterising the gel to liquid crystal transitions for di-n-alkylphosphate vesicles formed from (R'O),PO,-M + and (R20)PO -M + AmH"/kcal patch number (mol monomer) -n 34.8 f0.2 3.9 168 52.2 f0.1 5.6 140 66.3 f0.1 7.9 91 77.1 f0.1 9.1 45 Q 40.8 f0.1 3.7 158 f1 57.6 f0.1 5.1 137 k 7 67.6 0.1; 75.7 f0.1 8.4b 149 f15 C d e e 33.3 f0.1 4.4 153 52.0 f0.1 6.3 90 65.2 f0.1 8.3 84 35.0 f0.1 3.3 435 13 53.2 f0.1 5.6 308 f9 66.5 f0.1 7.8 277 & 7 Four important maxima in the DSC scan at 25.2 f 0.1, 42.3, 47.4 0.1, 50.9 f0.1.Two transitions. ' Many weak extrema observed of which those at 32.8, 54.0 and 57.8 "C are the most striking in the DSC scan. Three important extrema in the DSC scans at 33.6 f0.1, 63.1 0.1 and 66.9 f0.1 "C. Enthalpy of transition refers to a mean molar mass for the mixtures of dialkylphosphates. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.06 c I -0.04 m0 \ $ 0.02 OL I I 20 40 TPC 60 80 I I I I-0.04' 20 40 60 80 TfT Fig. 3 Dependences on temperature of the differential heat capa- cities (reference = water) for aqueous solutions containing vesicles prepared from the sodium salts of di-n-alkylphosphates, (R0)2P02-Na+ [total concentration = 8.4 x (mol monomer) dm-3] and from equimolar [ca.4.2 x (mol monomer) dm-3] mixture of two sodium di-n-alkylphosphates, (R'O),PO,-Na+ and (R20),P02-Na+. A, R = C12H,, where T, = 343°C; (b) mixture where R' = C1,H25 and R2' = C16H33; (c) R = CI6H33 where T, = 663°C. B, Differential scans of the mixture, R' = C12H2, and R2 = CI8H3, recorded successively [scans (a)-(d)]; scans recorded after standing at 15"C for 6 h [scan (e)] and for 12 h [scan (f)]. Fig. 4 Dependences on temperature of the differential heat capa- cities (reference = water) for aqueous solutions containing vesicles prepared from sodium and potassium di-n-alkylphosphates, [total concentration = 8.4 x (mol monomer) dm-3] and from equi- molar [ca. 4.2 x (mol monomer) dm-'] mixtures of the sodium and potassium salts. A, (a) (C,4H,,0)2P0,-K+ [aq; 8.4 x lop3 (mol monomer) dm-3]; (b) (C,,H,,O),PO,-Na+ [aq; 8.4 x (mol monomer) dm-3] and (c) equimolar mixture where the concen- tration of both salts is ca.4.2 x lo-' (mol monomer) dm-j. B, (a) (C16H330)2PO;K+ [aq; 8.4 x (mol monomer) dm-3]; (b) (Cl,H330)2P02-Na+ [as; 8.4 x (mol monomer) dm-3] and (c) equimolar mixture where the concentration of both salts is ca. 4.2 x (mol monomer) dm-3. C, Repeat scans for the mixture described in B where successive scans were recorded in curves (a)-(d) and where scan (e) was recorded after standing at 15 "C for 5 h in the sample cell. 50 60 TPC B 0.06 0.04 0.02 0 C 0.04 0.02 0 -0.02 u70 6o TPC J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 A slightly more subtle experiment involved recording the scans for aqueous mixtures of the sodium salts of (Cl,H370)2P02- and [trans-CH,(CH,),CH= CH(CH2),0],P02 -;Fig. 5. As previously rep~rted,~ the pattern produced by the unsatu- rated di-n-alkylphosphate [aq; 8.4 x (mol monomer) dm-3] showed several extrema, those at (i) 25.2 f0.1, (ii) 42.3 & 0.1, (iii) 47.4f0.1 and (iv) 50.9& 0.1 "Cbeing particu- larly intense. Vesicles formed from (C,8H370)2P02-Na + [as; 8.4 x lo-, (mol monomer) dm-3] produce a single sharp extremum near 77.1 & 0.1 "C; Table 1. Interestingly, five features are apparent in the scan for the mixture with an almost one-to-one matching with the features recorded for the separate solutions. In common with the systems described above, five successive scans showed broadly similar patterns indicating that many of the processes responsible for the extrema in the differential scans are reversible.Analysis The dependences of differential heat capacity 6Cp on tem- perature were analysed in three stage^.^ First, a previously recorded water-water baseline was subtracted from the recorded scan. Secondly, a chemical baseline was subtracted from the derived scan. In contrast to the analysis of scans for micellar systems7 this subtraction was straightforward. The traces showed no overall changes in heat capacities over a range of temperatures below and above the extrema centred on T,. In the final stage of the analysis the dependence of calculated molar isobaric heat capacity C,,on temperature was fitted to eqn. (1) using three variables; (i) the patch number (cf.concentration of patches), (ii) the temperature T, and (iii) calorimetric enthalpy of reaction, the last being brought into agreement with the calculated van't Hoff en- thalpy of reaction, Av,Ho using the patch number, where K is the equilibrium constant at temperature T. In some cases the shape of the recorded envelope did not conform to the simple bell-shape required by eqn. (1).In the cases discussed below the envelope could be fitted assuming that one or more independent processes of the form X(aq)e Y(aq) contributed to the recorded scan. In these cases the 0.04 .-I Y-m0 \ (;ip 0.02 TrC Fig. 5 Dependences on temperature of the differential heat capa- cities (reference = water) for aqueous solutions containing vesicles prepared from sodium salts of saturated and unsaturated C,,-di-n- alkylphosphates; (a)[trans-CH,(CH,), = (CH,),O],PO,-Na+ [aq; 8.4 x lo-' (mol monomer) drn-,]; (b) (C,,H,,O),PO,-Na+ [aq; 8.4 x lo-, (mol monomer) dm-'1; (c) an equimolar mixture of these di-n-alkylphosphates [aq; 4.2 x lo-' (mol monomer) dm-'1.shape of the recorded envelope could be accounted for in terms of the sum of the separate independent transitions. Discussion In principle, the recorded extrema associated with the gel to liquid crystal transition yield three pieces of information, patch number n, the melting temperature T, and the enthalpy changes, van't Hoff and calorimetric.The latter is given in the first instance by the area under the scan envelope. When these parameters are calculated from the scans recorded for aqueous solution containing (i) (RlO),PO2-M+, (ii) -(R20),P02-M+ and (iii) (R10)2P02M i-+ (R20),PO2-M+, interesting patterns emerge particularly when R' and R2 differ by two CH, groups; Table 1. The recorded T, for mixtures where R' = C12H2, and R2 = C14H29, 40.8"C, is intermediate between the T,s for solutions containing single surfactants, 34.8 (C12H24 and 52.2 (C14H29)oC.The patch number n, 158 & 1 is also inter- mediate between the estimates of n for the pure system, 168 and 140,respectively. However, the enthalpy change, 3.7 kcal (mol monomer)-' is lower than the enthalpy changes re- corded for the two pure surfactants, 3.9 and 5.6 kcal (mol monomer)-', respectively.For both simple and mixed systems a higher patch number is linked with a more co- operative melting process. Further analysis of the scans for the mixed systems shows that the envelope is satisfactorily accounted for in terms of three independent two-state trans- itions; Fig. 6.We assign the lowest calculated T, at 39.4"C to a melting of domains which are richer in the surfactant with the shorter hydrocarbon tail, (C,,H,,O),PO,-Na+. Simi-larly, we assign the extremum at the highest calculated tem- perature T,, 41.9"C, to the melting of domains richer in the surfactant, (Cl,H290)2P02-Na+. In other words, in a single vesicle we envisage three different types of patches which undergo gel to liquid crystal transitions at slightly different temperatures.A similar pattern emerges for the (C14H290)2P02 -Na+ and (C,,H330)2PO2-Na+ surfactants, the T, at 57.6"C for the mixture being intermediate between that for the single surfactants for the mixture, whereas the patch number 120 000 80 000 r I Y-m--. 2 40 000 I0 30 40 50 Fig. 6 Dependences on temperature of the molar heat capacity of an aqueous solution containing vesicles formed from equimolar 14.2 x lo-, (mol monomer) dm-3] of (C,,H,,O),PO,-Na+ and (C,,H2,0),P02-Na+; (a)full line is obtained from the scan in Fig. 1A; (b)dotted lines show calculated contributions (Origin software, MicroCal Ltd.) from three independent two-state transitions where T, = 39.4 & 0.1, 39.6 & 0.01 and 41.9 k0.1 "C.2708 137 & 7 is close to that for the C,, surfactant, 140 rather than that for the c16 surfactant, 91. The overall enthalpy change is lower at 5.1 kcal (mol monomer)-' relative to 5.6 and 7.9 kcal (mol monomer)- ', respectively for the pure sur- factant solutions. However, the shape of the scan envelope is accounted for in terms of two independent transitions. Two resolved extrema are, in fact, recorded for aqueous solutions containing an equimolar mixture of (C ,H,,O),PO, -Na + and (C,,H,,O),PO,-Na' surfactants; Fig. 1C. Thus with increasing chain length, the T,s of solutions containing single surfactants move to higher temperature^'.^ but their differ- ence decreases when the difference in chain length is C,H,. At the same time, the scan pattern for equimolar mixtures evolves into two extrema; Fig.1C and 2. The scan envelope for this mixture is satisfactorily accounted for in terms of three independent transitions of the form, X(aq) eY(aq) each transition having the same patch number, 149 & 15 which is larger than the patch number of the solutions containing single surfactants, 91 and 45, respectively; Fig. 7. The calori- metric (integrated) enthalpy change over the whole envelope is 8.4 kcal (mol monomer)- which is between that calculated for the two pure systems, 7.9 and 9.1 kcal (mol monomer)-', respectively. The patterns can be accounted for in terms of vesicles in which there are three types of patches which are in turn rich in C,,-surfactant, rich in C,,-surfactant and con- taining both c16- and C,,-surfactants.It is noteworthy that all patch numbers for the mixed system are ca. 150 irrespective of chain length of the sur- factant chains, again with the proviso that they differ by only two CH, units. The similarity in C,,,, at the maximum also suggests that the range of compositions in the mixed patches is small, a conclusion again supported by the similarity in the patch numbers for the mixed systems. 200 000 c I Y-0" 100 000--. 0" Q 0 T/"C Fig. 7 Dependences on temperature of the molar heat capacity of an aqueous solution containing vesicles formed from equimolar C4.2 x (mol monomer) dm-'] of (C,,H,,O),PO,-Naf and (C,BH,,0)2P02-Na+; (a) recorded scan; (b) dotted lines show cal- culated contributions (Origin software, MicroCal Ltd.) from three independent two-state transitions where T, = 67.7 f 0.1, 74.9 f 0.3 and 76.7 & 0.2"C.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The complicated envelopes produced by mixtures of sur- factants where the number of alkyl groups differed by more than two CH, units meant that the shapes could only be reproduced using equations based on eqn. (1) but involving many such contributions. Indeed, when we had recorded the first scan the complexity prompted us to conclude that each subsequent scan would have slightly different shapes on the grounds that having melted the chains would, on recooling, adopt different conformations.We attributed the complex- ities in the scan to the problem of repacking chains of differ-ent lengths in the vesicles. This conclusion is supported by the complexity in the scans recorded for mixtures of saturated and unsaturated surfactants. There the scan pattern for the mixture is close to that predicted by combining the scans recorded for the solutions containing the single surfactants ; Fig. 5. Nevertheless, the reproducibility of many key features in successive scans shows that the domains have intrinsically well defined structure which maintain their integrity over many heat-cool cycles. For the scans produced by mixing K+ and Na' salts of the same di-n-alkylphosphates, the increase in T,, on going to the mixed system is indicative of an increase in chain packing of the gel phase although the enthalpy change associated with the melting decreases; 3.3 kcal (mol monomer)-' for the equimolar mixture of (C12H250)2P02-Na+ and (Cl,H,,O),PO,-Kf compared to 3.9 and 4.4 kcal (mol monomer)-', respectively, for the solutions containing the two pure surfactants.Nevertheless, this change is a striking demonstration of the importance of the counter cations in determining the properties of vesicular bilayers in aqueous solutions. The patch number for the mixture, 438 f13, is much larger than the patch number for the separate solu- tions. Therefore, the melting is much more cooperative in the mixed vesicles, suggesting that the mixed vesicles are smaller and that the accompanying change in entropy is important in determining T,, . We thank the SERC for their support through the Molecular Recognition Initiative and the British Council for an award to A.K. References T. Kunitake, Angew. Chem., Int. Ed. Engl., 1992, 13, 709. M. J. Blandamer, B. Briggs, P. M. Cullis, J. B. F. N. Engberts, A. Wagenaar, E. Smits, D. Hoekstra and A. Kacperska, J. Chem. SOC., Faraday Trans., 1994,90,2709. A. Wagenaar, L. A. M. Rupert, J. B. F. N. Engberts and D. Hoekstra, J. Org. Chem., 1983,54,2638. T. Kunitake and Y. Okahata, J. Am. Chem. SOC., 1977,99,3860. J. H. Fendler, Membrane Mimetic Chemistry, Wiley, New York, 1982. T. A. A. Fonteijn, J. B. F. N. Engberts and D. Hoekstra, Cell and Model Membrane Interaction, ed. S. Ohki, Plenum Press, New York, 1991. M. J. Blandamer, B. Briggs, J. Burgess, P. M. Cullis and G. Eaton, J. Chem. SOC., Faraday Trans., 1992,88,2874. D. Marsh, Biochim, Biophys. Acta, 1991, 1062, 1. H-N- Lin, Z-Q. Wang and C-H. Huang, Biochim. Biophys. Acta, 1991, 1067, 17. Paper 4/01 93 1H ;Received 30th March, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002703

出版商:RSC    

年代:1994

数据来源: RSC