摘要:

Corrigenda Vol 8 No 4 1979 “Non-isoprenoid Long Chain Phenols” by J. H. P. Tyman. Page 509. Formula (21c). For C13H31-n read C17H35-n. Page 512. Ref. 24 (b). Far 1965 read 1968. Page 514. Formula (34). Far C15H37 read C15H31. Page 525. Scheme 2. In the structure to the right of formula (42) the single-bonded C-0 should be double bonded. Vol 9 No 3 1980 We regret that in the lists of contents on the title page and back cover of this issue the name of one of the authors, W. E. Waghorne, was accidentally omitted from the last review article listed. This entry should have read: Thermodynamics of Ion-Solvent Interactions By B. G Cox and W. E. Waghorne

ISSN:0306-0012    

DOI:10.1039/CS98110FX001

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Chemical Society Reviews Vol 10 No 1 1981 Page TILDEN LECTURE ~~-Cyclopentadienyland q6-Arene as Protecting Ligands towards Platinum Metal Complexes By P. M. Maitlis 1 Electrochemistry of the Viologens By C. L. Bird and A. T. Kuhn 49 TILDEN LECTURE Some Uses of Silicon Compounds in Organic Synthesis By Ian Fleming 83 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds By K. D. Bartle, M. L. Lee, and S. A. Wise 113 The Royal Society of Chemistry London

ISSN:0306-0012    

DOI:10.1039/CS98110BX003

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Electrochemistry of the Viologens By C. L. Bird IBM UK LABORATORIES LTD., HURSLEY PARK, WINCHESTER, HANTS. SO21 2JN A. T. Kuhn DEPARTMENT OF BIO-MATERIALS SCIENCE, INSTITUTE OF DENTAL SURGERY, 256 GRAY’S INN ROAD, LONDON WClX 8LD 1 Introduction It is nearly half a century since Michaelisl first reported on the electrochemical behaviour of a class of compounds which he christened the ‘viologens’, formally known as 1 ,l’-disubstituted 4,4’-bipyridinium ions (1). Since that time there have been successive waves of interest in this class of compounds, each wave producing its corresponding literature and yet with little or no links between them, and much information has been lost as a result. The present review is an attempt to link together the literature from the various sources.(1) The viologens were originally investigated as redox indicators in biological studies,l and they retain their importance here as possessing one of the lowest (most cathodic) redox potentials of any organic system showing a significant degree of reversibility. Subsequently they were the parent compounds for one of the most exciting new types of herbicide discovered for many vears, the ‘paraquat’ family. Herbicidal activity was found to be linked to the redox potential.2-3 More recently there has been an attempt to construct electrochemical display devices as substitutes for LEDs and LCDs, and the viologens have been one of the most strongly favoured candidate compounds in this, with their electrochemically reversible behaviour and the marked colour change between the two oxidation states. Later still they have been adopted as ‘mediating compounds’ in a range of biological studies, which are discussed in the section on applications.It has been found that compounds that are not themselves electroactive can still be reduced by electrochemically reducing the viologen, which then chemically reduces the other compound. Further evidence of their reversibility lies in their having been seriously considered as one component of secondary batteries. L. Michaelis and E. S. Hill, J. Gen. Physiol., 1933, 16, 859. a J. Volke, Collect. Czech. Chem. Commun., 1968, 33, 3044. a L. Pospisil,J. Kuta, and J. Volke, J.Electroanal. Chern. Interfacial Electrochem., 1975,58, 217. Electrochemistry of the Violagens Finally, as their cation radicals have a strong optical absorption band in the visible region of the spectrum, the viologens are, de fucto, candidates for the investigation of photoelectrochemical processes.However, the effect of light irradiation on viologens falls outside the scope of this review. Two other reviews should be mentioned here, which, although much broader in scope than this exercise, are highly relevant in that they provide the general context of the chemistry discussed here. Bard, Ledwith, and Shine4 have written about ‘For- mation Properties and Reactions of Cation Radicals in Solution’. Kosowers has published ‘Pyridinyl Radicals in Biology’.* The various contributions reviewed below are derived from many of these interests.Although we have tried where- ever possible to assign the relevant portion of each contribution into the appro- priate sections of this review, this separation has not always been possible. In addition a major collation of polarographic data, hitherto unpublished, has been included; because of its importance it was decided to allow it to stand alone. 2 General Considerations A. Simple Chemistry and Electrochemistry.-The viologens exist in three main oxidation states, namely V2+ + V+* + VO. The first reduction step is highly reversible and can be cycled many times without significant side reaction. The further reduction to the fully reduced state is less reversible, not least because the latter is frequently an insoluble species as well as being an uncharged one.The compounds are also very stable chemically, although in more alkaline solutions they will dealkylate according to equation (1) as reported by Farrington, Ledwith, and Stam.6 + OH-4 N\m / e + MeOH (1) Because the methanol resulting from the dealkylation can be a reducing agent, solutions of methyl viologen in alkali can spontaneously be reduced and will then turn blue as the monocation radical is formed. The process can be catalysed, it is reported, by platinized platinum in weakly alkaline solution^.^ It is further suggested, on the basis of plots of open-circuit potential versus time, that the reduction in the presence of platinum is accompanied by oxidation of the dication.This will be pursued in a later section. The viologens may also be incorporated into polymers, the resulting materials retaining to a large extent the chemical and electrochemical properties of the monomeric species. Simon and Moore8 have reported the chemical and electro- *It is also worth reading ‘The Bipyridinium Herbicides’, L. A. Summers, Academic Press, London and New York, 1980. A. J. Bard, A. Ledwith, and H. J. Shine, Adv.Phys. Org. Chem., 1976, 13, 155. E. M. Kosower in ‘Free Radicals in Biology’, ed. W. A. Pryor, Academic Press, 1976, Vol. 2. J. A. Farrington, A. Ledwith, and M. F. Stam, J. Chem. SOC.,Chem. Commun., 1969,259.’I. V. Shelepin and 0. A. Ushakov, Zh. Fiz. Khim., 1975, 49, 1736. M.S. Simon and P. T. Moore, J.Polym. Sci., Polym. Chem. Ed., 1975, 13, 1. Bird and Kuhn chemical behaviour of a range of these, also showing u.v.-visible spectra. Subsequent studies in this area have been reported by Factor and Heinsohng and recently by Sat0 and Tamamura.lo The latter authors stress the small potential gap between first and second viologen reductions and the problems this causes, particularly in electrochromic displays, on account of the relative irreversibility of the second reduction. By using polyviologens they claim that the first reduction occurs at a more anodic, the second at a more cathodic potential. They make much of the fact that their polymers have a much narrower molecular-weight range than those of Factor et al. or Noguchi,ll but they do not refer to Simon and Moore.In this paper they show how Ere"(both for first and second reduction) changes as a function of chain length. They also show u.v., visible, and near-i.1. spectra of monomers, dimers, and oligomers of stated chain length. The synthesis of polymerizable viologens with a terminal vinyl group is reported by Kamogawa and Mizuno.12 Redox-potential data as well as u.v.-visible spectra are included. Yamana and Kawatal3 have also studied viologen polymers, their interest being in the electrochromic characteristics. It might be noted that cathodic-reduction studies of the unquaternized 4, 4-bipyridyls have also been reported, with details of product ana1y~is.l~ B. Synthesis.-Michaelis and Hill1 describe the syntheses of the methyl, ethyl, and benzyl viologens and the betaine derived from chloracetic acid.Other authors who discuss preparative details include Evans and Evansls (diethyl, dipropyl, dibutyl, dibenzyl) and Van Dam and Ponjee,lG who refer to the Menschutkin reaction in which an amine is quaternized with an alkyl halide. Hunig and Schenk17 have published on the synthesis of bipyridinium salts (as distinct from details mentioned in passing). Bruininkls has reported synthesis details for both the 1,l '-diheptyl bipyridinium dibromide and the tetramethylene bis[4-( 1-ben- zylpyridine-4'-yl)pyridinium] perchlorate as well as the tetrafluoroborate analogue of the same species. N.m.r. and other analytically useful information is included. When investigating alternative anions most workers synthesize or purchase the bromide and prepare other salts by ion exchange.C.Effect of the 1,l'-Substituents.-The observation that the 1,l '-substituents affect both the solubility of the cation radical and the reversible potential of both reduction steps has influenced the choice of viologens for herbicidal applications and for electrochromic displays. A. Factor and G. E. Heinsohn, Polym. Lett., 1971, 9, 289. lo H. Sat0 and T. Tamamura, J. Appl. Polym. Sci., 1979, 24, 2075. l1 H. Noguchi, Proc. 35th Nat. Meet. Chem. SOC. Jpn., 1976. l2 H. Kamogawa and H. Mizuno, J. Polym. Sci., 1979, 17, 3149. l3 M. Yamana and T. Kawata, Nippon Kagaku Kaishi, 1977, 7, 941. l4 Yu. N. Forestyan, Sov. Electrochem., 1971, 7, 691.l5 A. G. Evans, J. C. Evans, and M. W. Baker, J. Chem. SOC., Perkin 2, 1977, 1787. l6 H. T. Van Dam and .I.J. Ponjee, J. Electrochem. SOC., 1974, 121, 1555. l7 S. Hunig and W. Schenk, Liebigs Ann. Chem., 1979, 727. la J. Bruinink, C. G. A. Kregting, and J. J. Ponjee, J. Electrochem. SOC.,1977, 124, 1854. Electrochemistry of the Viologens Inevitably much effort has gone into the optimum choice of substituent, usually in conjunction with the choice of anion. However, when considering the implica- tions of the various findings for the general electrochemistry of the viologens it is important at each stage to draw the distinction between those systems having soluble cation radicals and those in which a precipitate is formed. In illustration of the latter case Table 1, taken from reference 16, shows a range of substituents forming a permanent film (precipitate) in the presence of bromide ion.Table 1 Electrochemical reduction of viologens in aqueous solutions of dibromides for preparation of permanent f;lml6 R Efective length of R in Permanentfilm CH2 units Me 1 Et 2 Pr 3 Bu 4 C5Hll 5 Cf3H13 6 C7Hl5 7 CBH17 8 iso-CsX11 4 Ph-CH2 A5 C1 \ CH-OZH2 4 /Me Me-CH =CH-CH2 4 CH~ECH-C~H~ 4-5 NC-GHs ”5 Variations of the 1,l’-substituents have also appeared in the patent literature relating to electrochromic di~p1ays.l~ In this case the basic substituent is an aryl group ;further substitution of this influences the colour and other characteristics of the radical-cation film.Barltrop and co-workers have also carried out some studies on the effects of varying the 1,l’-substituents.20 D. Effect of the Anion.-It is apparent that when the viologen cation radical forms a precipitate the anion not only affects the precipitation process but also the nature of the aggregate. lS Ger. Pat. 25 27 638 (equiv. Br. Pat. 1 514 466). ao J. A. Barltrop, personal communication. Bird and Kuhn Motivated by display considerations, Van Dam and Ponjeels selected the combination of heptyl viologen with the bromide anion. Chloride was also found to induce precipitation. With a similar interest in electrochromic displays Jasinski21 investigated a number of anions with heptyl viologen and mentioned that he was not successful in isolating solid salts of the dication other than with bromide ion.Anions found by him to be compatible with the viologen dication and its reduction products were Br-, HzP04-, S(h2-, F-, formate, and acetate. Van Dam et a1.16 have studied C1-, BF4-, and c104-. The following ions were not found useful by Jasinski21 (for various reasons that he covers) : HC03-, BH4-, CN-, PFs-, Sbh-, AsFs-, and SCN-. The precipitation of dodecyl viologen with CN-ion has been reported by Winters.22 The MeS04- ion is referred to in references 23 and 24, and the HzPOZ- ion in reference 25. It is suggested by Jasinski26 that divalent mono-hydrogen phosphate ions suppress formation of the monocation film on Au and degrade the adherence of the film that was formed.In reference 21 a number of anion effects are mentioned. Thus COO--based films on Au and Ag are said to recrystallize rapidly at open circuit.21 The same was true when formate ion was added to phosphate solution that had hitherto not shown the same effect. Bicarbonate was said to form ‘well behaved’ films free from recrystallization effects. However, the minimum pH possible was 7.5, and slow hydrolysis of the viologen was apparent after a few days. The same ion is discussed in reference 26. Acetate gave a stable film if the concentration exceeded 0.5 mol dm-3, but at lower concentration film loss due to blue-haze formation took place. 3 Thermodynamic Aspects A. Reversible Electrode Potentials.-Taking the reactions (2), we may measure the reversible potentials for each step for a given viologen.We may also formulate here the so-called ‘conproflortionation’ and ‘disproportionation’ reactions [equations (3) and (4)]. 2V+*-+V2+ + Vo (disproportionation) (3) Vo + V2+ --f 2V+-(conproportionation) (4) These reactions can take place both in solution and at the surface of an electrode. In the latter case the process is presumably a combination of the homogeneous reaction and coupled anodic and cathodic reactions on the common metal [equations (5)-(7)]. R. J. Jasinski, J. Electrochem. SOC.,1977, 124, 637. 22 L. Winters and A. Borror, Tetrahedron Lett., 1976, 24, 2313. as R. Fielden and L. A. Summers, Experientia, 1974, 30, 843.24 A. L. Black and L. A. Summers, J. Heterocycl. Chem., 1971, 8, 29. 25 European Published Patent Application 0001 912. R. Jasinski, Abstr. 205 ECS Meeting, Las Vegas, Oct. 1976. 53 Electrochemistry oj' the Viologens VO -e = V+* (anodic) (5) V2+ + e = V+* (cathodic) (6) ~~ ~ Vo + V2+ = 2V+* (overall) (7) The conproportionation constants for a number of the viologens have been obtained from the two redox potentials by using equation (8), and the values obtained are quite large, suggesting that little disproportionation occurs. kcon, for the 1,l'-dimethyl compound has been variously quoted as 6 x 106 in ace- tonitrile;27 in reference 28 the value is given as 1 x lo7in both acetonitrile and DMF. Shelepin and Ushakov7 give K as 4 x lo6 but do not indicate whether the solvent is DMF or water.Hunig and Sa~er~~ have quoted the expression (9), where Ksemis equivalent to kconp. % semiquinone form = Ksem* x 100 (9)Ksem* + 2 B. Values for Eland &.-A range of values of the redox potentials may be found in the literature. This is summarized in Table 2. Some authors, such as Hunig, obviously feel that reversible potentials can only be obtained in organic solvents in which all the reactants and products are highly soluble. Others would not agree with this. Redox potentials have been obtained either by static, i.e. potentio-metric, methods or by using cyclic voltammetric data, where El is taken as the mean value between the oxidation (outward scan) and reduction (homeward scan) peaks. Other authors have used E+ values from polarographic methods.In theory such approaches are only valid if the reaction is truly reversible, but in practice acceptable values for the redox potential can be obtained even when a reaction is somewhat less than fully reversible. Finally, where one of the com- ponents of an electrochemical equilibrium is a solid it would be expected that the free energy of formation of this species, and therefore the reversible potential, would depend on the morphology of the solid state. It will be seen that such differences have indeed been observed in respect of the heptyl viologen radical cation. Ito and Kuwana30 quote the potential of the first ieduction for methyl viologen as -0.446 vs.NHE, citing the work of Elofsen and Edsberg,33 Osa and Kuwana,S4 Michaelis and Hil1,l and Volke.2 They state that the value is pH-independent in the range pH 5-1 3, while Jasinski21 states that though the Br- salt of the heptyl 27 S. Hunig, B. J. Garner, G. Ruider, and W. Schenk, Liebigs Ann. Chem., 1973, 1036. as S. Hunig, J. Gross, and W. Schenk, Liebigs Ann. Chem., 1973, 324. 29 S. Hunig and G. Sauer, Liebigs Ann. Chem., 1971, 748, 189. 30 M. Ito and T. Kuwana, J. Electroanal. Chem. Interfacial Electrochem., 1971, 32, 415. 31 T. Kawata and M. Yamamoto, Jpn. J. Appl. Phys., 1975, 14, 725. 3a W. R. Boon, Chem. Ind., 1965, 782. 33 R. M. Elofson and R. L. Edsberg, Can. J. Chem., 1957, 35, 646. 34 T. Osa and T. Kuwana, J.Electroanal. Chem. Interfacial Electrochem., 1969, 22, 389. 54 Bird and Kuhn homologue is pH-independent the phosphate potential does depend on pH. Bard4 quotes authorities to suggest that there is slight pH dependence for the second reduction of methyl viologen, but no dependence for the first stage. Kuwana’s quoted value is the mean of the four workers they cite, & 0.005 V. The second reduction potential is given as -0.88 V. In Table 2 are listed published values for a range of viologens, such values being dependent on solvent, substituent radical, and anion as well as temperature. A correlation with Hammett values was first attempted in 197035 and showed a close fit between these and E+ (or Erev)values. More recently the exercise was repeated by Hunig36 using his own data for viologens in acetonitrile.It is note-worthy that even though the aqueous data are not ‘reversible’ in the polarographic sense the Ere, values are real and reproducible, unlike many others found in organic electrochemistry. Thus the values may be used with Hammett functions in that they do represent genuine thermodynamic measurements. Table 2 Published reversible potentials in aqueous media Compound Anion Ei Ez d E/dpH Ref. Methyl Cl-c -0.446a -0.88 -30 Methyl Cl-c -0.446a -0 1 Methyl MeS04--0.36a -0.70 0 24 Ethyl CI-C -0.449a -0 1 PrOPYl Br--0.690’ -0.965 -31 Hexyl Br--0.7 10’ -0.930 -31 Heptyl Br--0.600’ -0.800 -31 Octyl Br--0.705’ -0.945 -31 Benzyl Cl-c -0.359a -0 1 Benzyl c1--0.350a --32 Benzyl Br--0.570’ -0.790 -31 Benzyl c1--0.573’ -0.790 -31 Benzyl I--0.568’ -0.775 -31 Betaine Cl-c -0.444a -0 1 n-Propyl I--0.43Sa --32 n-Hexyl Br--0.439a --32 Ethoxycarbonyl-methyl C1- -0.422a --32 2-H y drox yethyl Br--0.408a --32 2-Ethoxyet hyl I--0.386a --32 2-Carboxyethyl c1--0.431a --32 2,2’-Dicyano-l,1’-dimethyl MeS04-+ 0.09a -0 23 (suggest other values avail to -0.7 V) NHE.bv~.SCE. eIn phosphate buffers. 36 B. G. White, Proc. Br. Weed Control Conf: (loth), 1970, 997 36 S. Hunig and W. Schenk, Liebigs Ann. Chem., 1979, 1523. Electrochemistry of the Viologens For the n-heptyl viologen Jasinski21 quotes Table 3 for first and second reduction potentials, showing the effect of both anion and electrode material.The least negative number relates to the first reduction, the more negative number to the full reduction. (Erevfrom the hydrogen evolution reaction at pH 5.5 was found to be -520 mV with the same reference.) Table 3 First and second reduction potentials for the n-heptyl viologen Potentials (mV vs. Perma Probe) (+520 mV vs. H2/H+ at pH M21) Anion Au Pt Ag Br- (0.3M) HzP04- (2M) -420 -390 -730 -770 -430 -390 (<-600)-430 (<-600)-390 -700 - Formate (0.4M) -570 -650 -550 -670 HC03-(1M) -490 -680 -500 -670 Acetate (0.5M) -550 -650 F-(lM)* -540 -600 -590 -530 no colour -540 -650 -570* -620* -520 -640 no colour *Millimolar viologen. Unfortunately we must resist the temptation to seek a correlation of the data in Tables 2 and 3.The variation of conditions is wide, and the available infor- mation is insufficient to correct for these differences. It is notable in this respect that the buffer or supporting electrolyte used is rarely quoted. In view of the fact that there can be no indifferent electrolyte with the viologen system, this is a serious omission. There is an obvious need for a systematic approach to corre- lating reversible potential data over as wide a range of viologens as possible, and such an approach was in fact made by White and co-workers over ten years ago. These results are reported below for the first time. C. Further Data on the Effects of Substituent Variation on Potentials.-At ICI’s Jealott’s Hill Laboratories (Plant Protection Ltd.) Dr.B. G. White and col- leagues have over the years studied some 400compounds of the 4,4’-bipyridinium type with a view to examination of their herbicidal activity. A part of this effort was devoted to polarographic studies on these compounds. The majority of these results have never previously been published, and the authors are extremely grateful to Dr. White and ICI Ltd. for permission to do so. The compounds tested fell into the following categories: (i) asymmetric quaternaries in which one substituent was always the methyl group, (ii) symmetric quaternaries, Bird and Kuhn Table 4 Asymmetric quaternaries R1 E+/mV(NHE) Anion PH +O -490 CH3S04-6.8 Me -443 c1-6.8 CHzCH=CH2 -426 I-6.8 CHzCHzOH -422 Cl-6.8 CHzPh -408 Br-6.8 CHzCH =CHCOzMe -381 I-6.8 CHzCOMe -380 I-1.5 Me J7 -374 Cl-6.8cH2coYMe CHzCONHz -371 I-6.8 ACHzCSN -371 I-6.8up CHzCH =CHCN -369 Br-6.8 CHzC02E t -362 I-6.8 CHzCSNHPh -357 I-6.8 CH =C(SMe)2 -330 I-6.8 CHzCN -287 c1-6.8 CH(CN)COzEt -287 I-1.5 Table 5 Symmetric quaternaries (R, R) R1 EJmV (NHE) Anion PH OMe -651 I-6.8 NH2 complex behaviour I-varied H -485 c1-1.5 Ph -288 c1-6.8 (iii) symmetric quaternaries where the substituent was -CH2R1, (iv) symmetric quaternaries where the substituent was -(CH2)2R1, (v) symmetric quaternaries of type -(CH2)3R1, and (vi) miscellaneous compounds.Results obtained are given in Tables 4-12. Tables 10-12 show a more recent series of measurements completed by White and his co-workers.These tables also show CPvalues and compare observed with calculated E, values. Electrochemistry of the Viologens Table 6 Symmetric compounds of type [R1(CHe), -(CHz)R1] RQ E+/mV (NHE) Anion PH Me -443 c1-6.8 CHzCOO--410 c1-5.O CHzPh -340 Br-6.8 CH2n -325 c1-6.8 "d CHzSEt -308 c1-6.8 CHzCOMe -305 Br-1.5 CHzCONMez -302 c1-6.8 CHzCONHz -296 c1-6.8 CHzCOOEt -267 Br-6.8 CH(Ph)COOEt -188 Br-6.8 CHzCN -150 I-6.8 CH(Ph)CN -73 Br-1.5, 7.0 Table 7 Symmetric compounds of type [R1(CHz)z, -(CH2)2R1] RQ E+/mV (NHE) Anion PH CHzCHzOH -399 c1-6.8 CHzCHzCONMez -385 c1-6.8 CHzCHzOEt -386 I-6.8 CHzCHzCOOEt -376 Br-6.8 CHzCH(0Et)z -373 c1-6.8 CHzCHzCl -335 Cl-6.8 CHzCHzkMe3 -280 Br-6.8 Experimentally, quaternary salts were usually 10-3 mol dm-3, but occasionally this was reduced to avoid absorption peaks.Solutions were buffered at 6.8, but for systems with strong electron-attracting groups more acidic solutions were used to prevent interaction with the solvent and betaine formation. Buffer solutions were either phosphate mol dm-3, KCl 10-1 rnol dm-3, 1 :1 water-methanol with borax buffer (pH = 9.8) 4.2 x low2 mol dm-3, or KCI-NaOH 5.1 x mol dm-3, with added polyethyleneglycol-100 at 0.01 % as a suppressor. White and co-workers fully recognized these effects, which are described by us elsewhere in the text. However, they normally operated in a re- gion where the half-wave potentials were concentration independent.It was recognized by these authors3 long before others who published subsequently that a body of data such as the above could be used to determine a* values that would otherwise be extremely difficult to obtain. They also found that under all conditions used by them the E data did conform to the Bird and Kuhn Table 8 Symmetrical quaternaries of type [R1(CH2)3, -(CH2)3R11 RQ E+/mV (NHE) Anion PH (CH2)3COOEt -433 Br-6.8 (CH2)3CONMe2 -399 c1-6.8 (CH2)3CN -362 c1-6.8 (CH2)3Nt Me3 -331 Br-6.8 Table 9 Miscellaneous quaternaries RQ E+/mV (NHE) Anion PH (CH2)4NMe3 -367* Br-8.0-10.0 CHzCH =N-OH -325 Br-6.8 CHzC=CH -266 c1-6.8 II Cl Cl *Value obtained potentiometrically. linear free-energy relationship.The equation E+(l) = 0.241(0*) -0.443 (vs. NHE) held. They obtained the further relationship E,(2) = 0.6[E+(1) + 0.11, as did H~nig,3~ and concluded this phase of their work with calculations of the semi- quinone formation constants from the relationship E,(1) -E,(2) = 0.0591ogK. They were able to test all this by comparing predicted and actual E+ values, using literature (J* values for the former purpose; to within 1-5 mV agreement was good. D. pH Dependence.-From the stoicheiometry of the redox equations it would appear that the reversible potentials should be pH-independent, and this is indeed found in almost all cases where the test has been made. However, Jasinski21 states that the di-n-heptyl viologen phosphate system does show pH dependence.The trends were in agreement with the mechanism of Van Dam and Ponjee discussed below, based on the sole involvement of the monophosphate anion in controlling the concentration of V+*. This is perhaps less surprising when one considers that phosphate buffers contain more than one anion, in an equilibrium which itself determines the pH. Fitting the potentials of ten viologen dihydrogen phosphate solutions (pH 5.5-7.5), Jasinski obtained Nernstian behaviour with an EO-0.147 vs. NHE f. 6mV (the value of pK1,2 for the H2P04--HP042- equilibrium was taken as 6.71). The potential of Eo2was -0.540 to -0.690 vs. NHE, in- dependent of pH, viologen concentration, and total phosphate concentration. E. Anion Dependence.-Van Dam and Ponjee16 have made a study of the effect of anion concentration on half-wave potential and the reversible potential of the m0 Table 10 Asymmetric compounds of type [R-(CH2), --(CH2)R'] (aqueous) X Y (T* cr* co* X Y mV vs.NHE calculated %Et CHzCH=CH2 -0.100 +0.233 0.133 428 733 427 3(CH2)20H C5Hll +0.197 -0.145 0.042 438 758 438 3CH2CH=CH2 (CH2)3CN +0.233 +0.165 0.398 383 678 395 n-Pr (CH2)2C02Me -0.115 +0.255 0.140 408 743 41 6 $-(CH2)3F CHgCH=CHMe +0.141 +0.130 0.27 1 41 8 663 410 2 CHzCH =CH2 (CH2)2CONEt2 +0.233 +0.240 0.473 383 691 386 Et (CH2)zSEt -0.100 +0.200 0.100 428 688 43 1 (CH2)sCN (CH2)2CH =CH2 +0.165 +0.083 0.248 394 677 41 3 CH2-C=CH2 CHzCONHz ? +0.600 0.820a 336 652 I Me CHzCH=CH2 CH2COO-+0.233 ? 0.370a 390 755 CH2-C=CH2 (CH2)20Me ? +0.185 0.325Q 396 671 I Et CH2CH=CMe (CH2)20Me ? +0.185 0.3504 393 658 I c1 aValues derived from the plot of Et.(l) vs.Za+. Bird and Kuhn Table 11 Second asymmetric series (as before but in water-methanol mixtures) Y -E* (1) -4(2) mV vs. NHE Me Me 361 780 Et CHzCH =CH2 333 668,808 (CH2)20H n-CsH11 --CHzCH =CH2 (CH2)3CN 286 n-Pr (CH2)2C02Me --(CHhF CHzCH=CHMe 320 718 CHzCH =CH2 (CH2)zCONEtz 293 71 8 Et (CHd2SEt 325 71 8 (CH2)sCN (CN2)zCH =CH2 298 710 CH2C(Me) =CH2 CHzCONHz 257 678 CH2CH=CH2 CH2CO2-328a 758 CH2C(Et) =CH2 (CH2)20Me 293 723 CHzCH =C(Cl)Me (CH2)20Me --UAlmost certainly this compound is displaced through differential ionization Table 12Symmetric types (R-CH2, CH2-R) (aqueous, bufered) -E* (1) -E* (2)mV vs.NHE Me 443 793 (CH2)20H 399 749 CHzCOzEt 270 564 (CH2)2NH2 280 556 (CH2)2Cl 335 605 CH2C02-410 764 CHzCN 150 408 CH( CN)Ph 73 21 8 CH2CH=CH2 408 689 CHzCONHz 295 628 CHzCH=NOH 325 629 Me 305 670cH2coYMe 435 850 45 1 848 70 240 41 5 695 61 Electrochemistry of the Viologens n-heptyl compound. A plot of half-wave potential versus log anion concentration is linear over some three decades of concentration, for both the first and the second reduction step, while a similar plot for the e.m.f. of the Ptlviologen solutionIAgC1IAg cell is likewise linear for chloride and bromide salts of the n-heptyl viologen. Van Dam and Ponjee analyse their results in terms of the Nernst equation, writing L = [V+*] [X-1, where L is the solubility product, X- the anion in question, E(l) = Eo(1) + 0.05910g ["+I lx-] first step ~E(2) = Eo(2) + O.05910g [X-I L wO1 second step but rightly pointing out that half-wave potentials are not to be equated with reversible potentials.Using these equations they find L values of 8.5 x 10-6 for heptyl viologen chloride and 3.9 x lO-7,for the corresponding bromide. F. Non-aqueousMedia.-There are numerous reports of E, for various viologens in non-aqueous media. The major collation of values in acetonitrile is due to Hunig and Schenk,36 who list El and EZvalues for some 17 4,4'-bipyridyls as well as a further eight that are only stable in the reduced form. They then plot the first and second reduction potentials against cr or cr* values, obtaining an excellent straight -line plot in each case.They also show -as the former fact implies -that a straight-line relationship is given between El and Ez. With this range of substituents one might suppose that Hunig has virtually exhausted this area. Surprisingly, the collation of eight compounds reported by Van Dam and Ponjeel6 reproduced in Table 13 contains only one substituent -ethyl -in common with those studied by Hunig and Schenk, although their compounds are all long-chain aliphatics. It must be stated that the reported values for ethyl viologen, both apparently measured versus AgClIAg, do not appear to agree at all well. Table 13 Half-wave potentials for viologen tetrafluoroborates in acetonitrile16 R -EdV -EdV 1st.reduction step 2nd. reduction step vs. AglAgCl vs. AglAgCl Ethyl 0.48 0.89 n-Propyl 0.47 0.90 Ally1 0.44 0.85 n-Butyl 0.45 0.98 n- Amy1 0.46 0.89 iso- Amy 1 0.46 0.90 Hexyl 0.49 0.93 Heptyl 0.41 0.88 62 Bird and Kuhn Shelepin and Ushakov’ quote half-wave potentials for methyl viologen in DMF with NaC104 or TEAP as supporting electrolyte, and certain other data are also quoted for these systems, such as Epeak. H~nig~~quotes polarographic data for the same compound also in DMF. G. Calorimetric Data.-Gundry38 gives the standard free enthalpy of the 1,l’-dimethyl cation as 183.2 kJ mol-l, and other thermodynamic information for the same species is also quoted. 4 Kinetics of Reduction The cathodic reduction of the viologens has been studied both by classical dropping mercury electrode polarography and also by other methods of electrode kinetic study.As is so often the case it is unfortunate that very little tiein between the two bodies of work can be made. In both cases we must differentiate between work done in dilute (ca. 10-3 mol dm-3) solutions and that done in much stronger (ca. 1 mol dm-3) supporting electrolytes. A. Solid-electrode Kinetic Studies.-Results have been reported using Ag, Pt, Au, and SnO2 or In203 electrodes, both stationary and, in one or two cases, rotating. Where the cation radical is completely soluble the kinetics are straightforward and usually diffusion controlled since almost all workers appear to have used millimolar solutions.An exception is LeesP9 who shows an exponentially shaped current-voltage curve for 0.01 mol dm-3 viologen. Cyclic voltammetry and polarography on the dropping mercury electrode both show traces in which not less than two peaks (first and second reductions) are seen and frequently others too, which have been explained in terms of adsorption. These will be discussed in the section on ‘Polarography’ (Section 4D). The mechanism becomes more complex when the cation radical is a solid, the solubility of which is controlled, as we have seen, by the nature of the substituents in the 1,l’-positions and also by the anions used. Here it is mainly the electrochromic display oriented studies that are relevant, since the need for a solid deposit forms the basis of most of such devices. B.Cyclic Voltammetric Studies.-This technique has been the most widely used in the study of the viologens, predominantly in work on the heptyl homologue for electrochromic display purposes. Analysis of data can be subdivided as follows: (i) qualitative inspection of scans, and effect of scan rates and scan limits, (ii) quantitative measurements, e.g. testing ip as a function of Scan rate, and comparison of Qaand Qc, 37 S. Hunig and J. Gross, Tetrahedron Lett., 1968, 21, 2599. 38 H. A. Gundry, D. Harrop, and A. J. Head, J. Chem. Thermodyn., 1978, 10, 203. 39 R. E. Leest, J. Electroanal. Chem. Interfacial Electrochem., 1973, 43, 251.Electrochemistry of the Violagens (iii) association of cyclic voltammetric data with another technique. Bruinink18 shows cyclic voltammograms on tin dioxide electrodes as a function of sweep speed and sweep limits. With a fast scan (100 mV sec-1) up to four peaks are seen on the anodic sweep, three (including a shoulder) on the cathodic. The authors suggest that the peak due to Vo formation is not seen on the anodic side at slow scan rate because of the conproportionation reaction, but nevertheless there are more peaks than the simple stoicheiometry of the reaction will account for, and the authors suggest that oxidation of charge-transfer complexes between Vo and V+* might explain these. Interesting results are also obtained by arrest of the scan in the cathodic region for 1 min.The subsequent anodic peak is displaced to a more anodic potential in the case of ‘unstable’ species such as the heptyl viologen cation radical bromide, and the authors ascribe this to oxidation of ‘re-oriented’ parts of the cation radical film, which can also, it is stated, be seen microscopically. Turning now to the Qa:Qc ratios, Bruinink and Kregting find that this ratio is unity so long as the cathodic limit is not too extreme. When it is, that ratio drops. The critical cathodic potential depends on the viologen used and is less cathodic for the ‘unstable’ heptyl viologen bromide. It is accepted that these findings contradict those of Belinko40 who explored the effect of progressively more extreme cathodic excursions and plotted the results as Qa:Qc.He also shows relative transmission on an optically transparent electrode (OTE) as it is associated with these potential sweeps. As the cathodic limit is increased beyond cu. -1.O V vs. SCE the transmission, which drops as the cation radical is formed, fails to increase again as the scan returns to the anodic region, so indicating the breakdown of reversibility. One can offer explanations involving Vo, hydrogen evolution, V+* dissolution, and impurities. A comprehensive explanation of Qa:Q ratios would however require consideration not only of the behaviour of the deposit but also of the erase mechanism itself. Gavrilov and Ushakov41 studied methyl viologen perchlorate and chloride in both water and DMF and mixtures of the two.The contrast between the solvents is interesting. In the organic solvent with perchlorate anion the system is rever- sible (defined in that the anodic and cathodic peaks in the cyclic voltammogram are only slightly displaced); much less so in water, with results in the mixed solvent having intermediate values. Open-circuit decay curves are plotted as E vs. log time. Whereas in aqueous media the potential is known to be steady until virtually all the surface film has been oxidized, this was decreasingly true as the proportion of DMF increased, and the authors explain this in terms of the Nernstian response of concentration with time. The authors explain the difference in reversibility in terms of the low solubility of the reduced form in aqueous media containing perchlorate.They describe the film as grey in colour, turning black with time. Shelepin’ shows that ip cc (scan rate)* for millimolar viologen solutions. He also compares scans in sulphuric acid with and without 6 mmol 40 K. Belinko, Appl. Phys. Lett., 1976, 29, 363. 41 V. I. Gavrilov, 0.A. Ushakov, and I. V. Shelepin, Sov. Electrochem. (Engl. Trunsl.), 1978, 14,958. 64 Bird and Kuhn dm-3 methyl viologen and suggests that the latter is adsorbed because the hydrogen peaks are suppressed. However in the anodic region there is no sign of the V2+ oxidation propounded elsewhere. The work of Ito and Kuwana30 tells us little about mechanism, partly because a submillimolar methyl viologen (MV) solution is used, although the cyclic voltammograms and potential step (coupled with optical transmission recording) conform closely to theory for diffusion-controlled processes.However, they do quote a value for the rate of the conproportionation reaction MV2+ + MVO = 2MV+*, with Keq = 4 x 107 and kt = 3 x 109 (quoting Winograd and Kuwana42). Steckhan and Kuwana43 continue the work in equally dilute solutions but with both benzyl and methyl viologens and quote Keq(MV) > 103. Cyclic volt- ammograms on SnQ2 OTE are shown, and chronopotentiometric data are reported. They summarize their studies on methyl and benzyl viologens in Table 14. Table 14 Various parameters for methyl and benzyl viologens Parameter Methyl viologen Benzyl viologen AEpeek/mVa 52 55 EoIVb -0.449 -0.358 D/cm2 sec-lc 0.86 k 0.02 0.43 k 0.02 aFrom cyclic voltammetry (0.033-16.5 V s-l). *Formal potential vs.NHE; solutions are ca. 1 mmol dm-3 and all data are on tin oxide electrodes. CFrom chronopotentiometry, using i YS. tf.plot slope ( x lo5). It is stated, although not precisely how, that all values are corrected for iR drop. EO = formal potential vs. NHE and is obtained from the cyclic volt- ammogram at 0.85 of ip (cathodic). The peak-separation data are obtained by plotting AEp and E0.85 vs. square root of scan rate and extrapolating to zero. The plots were linear over a 20-fold range. Ratios of ip (anodic:cathodic) are stated to be unity except for benzyl viologen where ip (c:a) = 0.76; ip vs.(scan rate)0a5 was linear over a wide range. Chronoabsorptiometric experiments were also reported. In this paper the authors dwell on surface-adsorption phenomena. Analysis of A-t curves (absorp- tion vs. time) shows three cases, namely the ideal case (benzyl viologen on SnOz), adsorption of product (methyl viologen on In203), and adsorption of both reactant and product (benzyl viologen on Pt). Dication concentrations were 0.2 mmol dm-3; raising this to 0.4 for the benzyl case caused cation-radical adsorp- tion on SnO2 to become evident. Differences in pzc values are invoked to explain the dependence on the nature of the electrode. Vargalyuk44 investigated the effect of electrode material on kinetics of heptyl viologen bromide reduction in DMF.DO was found to be 1.24 x cm2 sec-l 42 N. Winograd and T. Kuwana, J. Am. Chem. SOC.,1970,92,224. 49 E. Steckhan and T. Kuwana, Ber. Bunsenges. Phys. Chem., 1974, 78, 253. 44 V. F. Vargalyuk, T. I. Starokozheva, Yu. M. Losharev, and E. A. Nechaev, Sov. Elecfro-chem., 1979, 15, 108. 65 Electrochemistry of the Violagens by chronoamperometric methods where itO.5 was constant. The electrode material exerted a considerable effect, the reaction becoming irreversible with PbOs but quasi-reversible on Pt and vitreous carbon. Pre-treatment of PbO2 with BaCh led to a 6 x rate increase (explained in terms of 5 potentials and Br- adsorption), and it is suggested that similar effects might occur on SnO2. Jasinski21 uses chronopotentiometry as well as cyclic voltammetry to study heptyl viologen.The chronopotentiogram shows plateaux of unequal length on the anodic and cathodic sides, explained in terms of charge loss due to the con- proportionation reaction. In his cyclic voltammetric data the charge recovery is explored under a variety of conditions. With phosphate electrolytes he found the cathodic charge recoverable. This was not so in bromide solutions. The emphasis of Jasinski’s paper is largely on side reactions and the occurrence of ‘spotting’. Presumably this is another manifestation of the ‘recrystallized’ cation radical as discussed by Bruinink.18 The anion, potential limits, and electrode material are all stated by Jasinski to affect this phenomenon, as they do also the question of adherence of the solid film to the electrode surface.Thus Au and Ag electrodes at pH 7.5 show a ‘blue haze’ instead of a coherent film, and this is explained in terms of micellization phenomena. Other products reported include a ‘yellow oily film obtained by pulsing to +600 mV which could be wiped off though it was electrochemically inactive ’ (after anodic treatment) and (after pulsing on Pt to + 1.2 V) a green-yellow material. These results are apparently with phosphate ion. With bromide ion the yellow tribromide salt is known to be formed.45 The most detailed cyclic voltammetric study is due to Bruinink and Kregtir~g,~~ who compare the behaviour of the heptyl viologen with analogues based on metal deposition and other models and conclude that the deposition of the radical does indeed involve a nucleation process. At lower concentrations they, like Shele~in,~ obtain a linear ip vs.(scan rate)* plot up to mol dm-3 and from this obtain a value for D of 7.4 x 10-6 cm2 sec-l. Their conclusion that the mechanism was indeed a nucleation process rather than solution precipitation is the starting point of an equally important paper by Barrada~.~7 He shows how an overall sequence of charge transfer-nucleation-hemispherical diffusion-linear diffusion occurs and points out that loss of V+*from electrode surface to solution is a significant process. This paper is also interesting because it is shown that ‘Tafel slope’ data can be obtained (using minute current densities) for the reduction of n-heptyl viologen, thus also electrochemical reaction order plots.C. Potential-step Transients.-This well known technique is reported by Bruinink and Van Zanten.48 After the initial double-layer charging, the logi-logt plots show a plateau followed by a current decay, which gives a slope of -8 and 45 US Pat. 3912368. 46 J. Bruinink and C. G. A. Kregting, J. Electrochem. SOC.,1978, 125, 1397. 47 S. Fletcher, L. Duff, and R. G. Barradas, J. Electroanal. Chem. Interfacial ‘Electrochem., 1979, 100, 759. 48 J. Bruinink and P. Van Zanten, J. Electrochem. SOC.,1977, 124, 1232. Bird and Kuhn suggests diffusion control; the plateau suggests an initially reversible process. The greater the overvoltage, the shorter is the plateau, and it seems possible (though the authors do not attempt this) that Qplateau might be constant in all cases shown by them.The work is done on SnO2 with mol dm-3 diheptyl viologen bromide, for which the diffusion coefficient is found to be 7 x cm2 S-1. The potentiostatic (or galvanostatic) step function is used in electrochromic displays. Kawatal3 has shown the ‘write-time’ (an arbitrarily defined absorption density) versus applied potential for heptyl viologen and a polymeric viologen. In a plot of optical density versustime a curious and large inflection is seen at one voltage. There is a linear relationship between log(write-time) and log(concentra- tion) from 10-3-10-1 mol dm-3 viologen. The ‘erase’ process is also considered, and erase-time is proportional to applied potential.Potential-step data are also given in reference 49, where step length and applied potential are plotted versus reflectance, the latter probably referring to A,,, (545 nm). This explains the assertion by the authors that 2 mC cm-2 can give a contrast ratio (CR) of 5:1, a higher value for this charge density than might be expected. It is also interesting to re-plot their data so that change of CR with time, at a series of potentials, is shown. The authors also quote data for Q (applied charge density) and reflec- tance R,simultaneously, over a complete cycle of write, hold, erase, i.e. cathodic pulse, open-circuit and anodic pulse. D. Polarographic Studies.-Many of these studies are aimed simply at establish- ing a quantitative analytical technique.Only very few are mechanistically oriented. Hunig and Gross37 explore the effect of structure on some twenty-three examples of Weitz-type salts, using polarographic techniques in DMF and acetonitrile. For methyl viologen the following data are quoted for E vs. AgC1( Ag reference [slope = i/(ilh. -i)]: (a) in water E(l) = -0.640 (slope = 60 mV), E+ z -0.680 vs. SCE (from ref. 33) (6) in DMF E(1) = -0.38 (slope 59 mV; semiquinone formation constant = lo’), E(2) = -0.80 (system reversibility good) (c) in MeCN E(l) = -0.40 (slope 62 mV ;semiquinone formation constant = lo7), E(2) = -0.82 (slope 61 mV; good reversibility). In a further paper Hunig, Gross, and Schenk28 use both a.c.and d.c. polaro- graphy to study the methyl viologen and several further related Weitz-type molecules. Using the ‘K’value first introduced by Michaelis,50 i.e. the formation constant for the semi-quinone, VO + V2+ = 2V+-, obtained from the equation E2 -El = 0.0591ogK (at 25 “C), they show, using calculated models, that as K ID C. J. Schoot, P. T. Bolwijn, H. T. Van Dam, R. A. Van Doom, and J. J. Ponjee, Appl. Phys. Lett., 1973, 23, 64. L. Michaelis, Chem. Rev., 1935, 16, 243. Electrochemistry of the Viologens decreases in magnitude it becomes increasingly difficult to resolve polarographi- cally. While a.c. polarography allows values as low as K = 50 to be resolved, the limit for the d.c. method is closer to 100 or 150.Half-wave potentials (saturated KCl in DMF) obtained by them are seen to be in good agreement with the literature, from which they quote Boon32 and other authors.51 In aqueous media it is normally assumed that only the first reduction step is reversible, whereas in organic solvents both values are accessible, probably because the fully reduced compound is soluble in the medium. The study of Kawata31 has previously been considered and is mainly of interest in respect of the E+ values quoted earlier. Pospisil and Volke3 have studied the adsorption coupled kinetics of reduction of the 1,l’-dibenzyl viologen and the dimethyl analogue. Admittance data, differential capacitance values, and d.c. polarography all on the DME are reported, all at very low concentrations (ca.mol dm-3). ElsewhereS2 they report the interaction of dipyridinium ions with adsorbed halides. More recently two Russian papers along similar lines have appeared. Grachev and Zhdanov53 show the considerable effect of the anion on the polarograms. Using both a.c. and d.c. polarography, they show that both the concentration of the anion and its nature can drastically alter the polaro- grams. They correlate the molecular weight of the anion with first, second, and third diffusion wave potentials. Grachev, Zhdanov, and Supin54 earlier used both a,c. and d.c. methods to study in depth the NN-dimethyl-4,4’-bipyridiniumbis-(00-dimethyl phosphate). Leduc, Thevenot, and Buvet55 present perhaps the most thorough polarographic study of benzyl viologen in the millimolar con- centration range.They show a.c. and d.c. polarography on the DME, differential capacitance, and work on platinum and gold RDEs. The d.c. polarogram shows five peaks at 0.5 mmol dm-3 buffered at pH = 9.8. These peaks correspond to five in-phase a.c. peaks and two out-of-phase peaks. Two additional ax. peaks are also seen both in and out of phase and yet another out of phase. At lower benzyl viologen concentrations some of these disappear. Cyclic voltammetry on the DME shows two reversible steps and one irreversible. The effect of benzyl viologen concentration on the polarographic peak heights is reported fully. On this basis the peaks are assigned in terms of adsorption pre- waves or diffusion-controlled peaks.It is noteworthy that on the RDE (where Au and Pt are stated to give the same results) the general pattern of results is said to be more complex and harder to interpret. Volke and Volkova have studied not only the bipyridinium ion (reference 56 and references quoted) but also the uncharged species itself.57 Others papers of a purely analytical nature, either for s1 L. Michaelis, J. Am. Chem. Soc., 1933, 55, 1481; R. F. Homer and T. E. Tomlinson, J. Chem. Soc., 1960, 2498; L. A. Summers, Nature, 1967, 214, 381; R. F. Homer, T. E. Tomlinson, and G. C. Mees, J. Sci. Food Agric., 1960, 11, 309. L. Pospisil and J. Kuta, J. Electroanal. Chem. Interfacial Electrochem., 1978, 90, 231. b9 V. N. Grachev and S. 1. Zhdanov, Sov.Electrochem., 1979, 15, 1154. s4 V. N. Grachev, S. I. Zhdanov, and G. S. Supin, Sov.Electrochem., 1978, 14, 1353. 6b P. Leduc, D. Thevenot, and R. Buvet, Bioelectrochem. Bioenerg., 1976, 3, 491. 66 J. Volke and V. Volkova, Collect. Czech. Chem. Commun., 1969, 34, 2037. 67 J. Volke and V. Volkova, Collect. Czech. Chem. Commun., 1972, 37, 3686. 68 Bird and Kuhn quantitative estimation of the 4,4’-bipyridinium salt or the simultaneous esti- mation of this and the 2,2’-compound, are given in references 58 and 59. Schwarz60 has studied the polarographic behaviour of the diethyl viologen and interpreted the results, both reductive and oxidative, in terms of the monomer-dimer equilibrium. An interested observation relates to the way in which the blue cation radical is attracted to the Hg electrode, and as the latter (a pool) is with- drawn the blue colour follows it.Bearing in mind what is visible to the eye, this shows not only the adsorptive forces, metal-viologen, but also the solution association. The two recent papers53954 by Grachev and co-workers on dropping mercury electrode polarography of the viologens, both d.c. and a.c., show the various diffusion- and adsorption-controlled peaks. The second of these is the more important in that it investigates the role of the anion. Suffice to say here that three different types of polarogram are found, differing in respect of the number of diffusion-controlled and adsorption-controlled waves, depending on the anions used.The authors relate this behaviour to the solubility of the cation- radical film, as has been discussed earlier in this review for solid-electrode studies, and plot half-wave potential versus molecular weight of anion, obtaining two straight lines, one for simple inorganic ions and the other for organic anions, mainly aromatic. The relationship so found recalls that of ionic radius reported by Van Dam and Ponjee.l6 Grachev et al. go on to suggest that ohmic resistance of the cation-radical film causes some of the observed phenomena, and they draw an analogy with anodic dissolution of Hg drops in halide media, where without doubt such a resistive film does occur. The resistivity of heptyl viologen films is a subject which occurs elsewhere in this review.5 Reactions and Properties A. Dimerization, Oligomerization, Micellization, and Clustering.-It has been shown, for any of the three oxidation states of theviologens, that association occurs, or that there are good grounds for expecting that it would do so. Where the 1,l’-substituents are long-chain alkyl groups, the dication solution would be expected to show micellization at higher concentrations, as discussed, for example, by Fendler.G1 Unpublished work using a variety of the normal techniques has shown the existence of a c.m.c. for heptyl viologen.610 The electro- lytic conductance of the heptyl viologen dibromides has been measured in water and methanol by Van Dam62 but not up to sufficient concentrations to reveal the c.m.c.In water, up to the highest concentration quoted (93 x 10-4 equiv. 1-l), it E.* Yu. F. Balyabin and A. V. Kotova, Zavod. Lab., 1967, 33, 24. 69 N. G. Sheremet and G. S. Supin, Zh. Anal. Khim.,1973, 28, 1422. O0 W. Schwarz, Ph.D. Thesis, University of Wisconsin, 1961. J. H. Fendler and E. J. Fendler, ‘Catalysis in Micellar and Macromolecular Systems’, Academic Press, New York, 1975. *la A. C. Lowe, personal communication. H. T. Van Dam, J. Electrochem. SOC.,1976,123, 1181. 69 Electrochemistry af the Violagens is reported that the dicationic species was the only form present. Measurement of D, the diffusion coefficient for the dication, provides one method for monitoring the onset of association. Table 15 gives data from JasinskP3 and others.In addition, Jasinski plots a graph of D vs. concentration for phosphate and bromide over a concentration range from 0.1 mmol dm-3 to 0.1 mol dm-3. Table 15 Diflusion coeficients for violagen salutions Molarity Anion D( x 106 cm2 s-1) Species Ref. 1 (PH 4) 2 (PH 4) 1 (PH 5) 2 (PH 5) 1 (PH 7) 10-3 0.2 1.4 0.15 0.2 3.1 7.4 heptyl heptyl heptyl heptyl heptyl heptyl 63 63 63 63 63 46 0 (inf. dilution) 6 heptyl 63 0.01 (0.1 M-KBr) heptyl 64 water water (40%)+ DMF(60%) 4.10 2.91 DMF 1.24 Vargalyuk,65 in common with many other authors, discusses association of the cation radical and also points out that the Vo species should be considered a candidate for dimerization. His open-circuit decay studies are intended to sup- port this hypothesis.B. Association of the Cation Radical.-There is a great deal of evidence for some form of association of the cation radical. In their review on ‘Cation Radicals’ Bard, Ledwith, and Shine4 point out that, as a class, one expects dimerization of these compounds. However, such dimers may be either monocation dimers or dication dimers, M+-+ M = Mz+-or 2M+* = M22+, although Vargalyuk rejects the second of these on grounds of electrostatic repulsion forces. Other authors have discussed ideas such as ‘pi-dimerization’s or clustering.66 E.s.r. or u.v.-visible spectroscopy is the basis for most of the evidence for radical-cation dimerization or association. Thus Ivanov et al.67 have shown that heptyl viologen radical cations dimerize and that such dimers do not give e.s.r.signals. R. Jasinski, J. Electrochem. SOC.(Accelerated Commun.), 1979, 126, 167. O4 V. F. Vargalyuk, T. I. Starokozheva, and Yu. M. Loshkarve, Sov. Electrochem., 1979,15, 1337. 86 V. F. Vargalyuk, T. I. Starokozheva, Yu. M. Loshkarev, and N. Yu. Savel’eva, Sov. Electrochem., 1979, 15, 200. 66 M. J. Blandamer, J. A. Brivati, and M. C. R. Symons, Trans. Furuduy SOC.,1967,63, 1850; M. J. Blandamer and M. C. R. Symons, J. Chem. Soc., Chem. Commun., 1965, 629. V. F. Ivanov, A. D. Grishnia, and B.I. Shapiro, Izv. Akad. Nauk SSSR, Ser. Khim., 1976, 1383. 70 Bird and Kuhn Mel’nikov, Novikov, and Khaskin68 suggest a quinhydrone structure for the dimer. These ‘dipyridyl violets’ are known to exist in solution in equilibrium with cation radicals (2).I I4 ’ /x-I (2) Van Dam and PonjeelG propose a sandwich-type ion-pair structure for the heptyl viologen cation-radical dimer. A rather different hypothesis is advanced by Vargqlyuk, Starokozheva, and Loshkarev.64 They suggest that cation-radical association with the anion takes place at the electrode surface and base their finding on a chronoamperometric study of the di-N-heptyl bromide at three different electrodes, Pt, vitreous carbon, and SnO2, where they find very different chronoamperometric data. Sat0 and Tamamura,lo though their work deals ostensibly with viologen-based polymers, also report u.v., visible, and near4.r. spectra of monomers. From these they draw the important conclusions that in the case of the monomeric viologen cation radicals there exists a strong intermolecular interaction and the ‘deteriora- tion’ of films of such radicals is caused by this and can be depicted as a trans- formation from the ion-pair state (3) to the ion-bonding state (4).L L J X-(3) (4) Recently Furue and Nozakura69 have obtained spectroscopic evidence that l’,1“-trimethylenebis-(1-methyl-4,4‘-bipyridinium) perchlorate can be photo-reduced to form exclusively the intramolecular radical-cation dimer. An interesting general conclusion running through these studies is that water promotes association far more than any other solvent. Thus Kosower and Cotter,’O drawing largely on the work of Schwarz,60 show how a decrease in temperature can favour dimer formation.Spectral data from Schwarz’s work clearly show formation of a new peak at 8700A and a shift in a* N. N. Mel’nikov, E. G. Novikov, and B. I. Khaskin, ‘Chemistry and Biological Activity of Bipyridyls and Their Derivatives’, Gosimdat, Moscow, 1975, p. 35. an M. Furue and S.-I.Nozakura, Chem. Lett., 1980, 821. 70 E. M. Kosower and J. L. Cotter, J. Am. Chem. SOC.,1964,86,5524. Electrochemistry of the Viologens both U.V. and visible regions to shorter wavelengths. They postulate the equi- librium (V+*)2+2V+*,with a dissociation constant of 2.6 x lov3at 1 mol dm-3 salt concentration. That the observed colour changes are seen only in water and not in acetonitrile is explained in terms of ion-pair formation in the organic solvent, precluding dimerization.Kosower also shows energy-level diagrams for all the species under discussion. He dismisses the idea that the dimer might be a complex of the oxidized and reduced forms of the cation radical. However, it is interesting to note that a solid complex consisting of two MV+* and the dication (iodide as anion) has been isolated and analysed and is stable in dry air.71 Dimerization has been studied by Evans and Evansl5 largely using e.s.r., who show the process to be favoured at low temperatures. Full thermodynamic data for a number of bipyridyls are quoted by these authors, and the equilibrium constant for the monomer t-)dimer process gave good straight-line plots against l/Tfor several of these compounds in methanolic solution.Two points emerge from this work. Firstly, a second undetermined but paramagnetic species is detected at low temperatures. Secondly, while dimerization is clearly only important at low temperatures in the methanol solutions used in this work, there is a strong argument to suggest that when an 'insoluble' cation-radical film is formed on an electrode the existence of this as a condensed phase would favour dimerization or higher-order aggregation. A similar study by Ivanov and Gri~hina'~ used methyl and heptyl viologens in a PVA matrix. U.v.-visible spectra show a hypsochromic shift, and e.s.r. measurements were used to determine the dimer concentration. Optical densities for the monomer and dimer are plotted, as are the dimer dissociation constants as a function of temperature.The removal of water is seen to favour dimerization, again an observation with apparent implications for electrochemically formed deposits. However, Gavrilov and U~hakov,~~in contrast to other authors, suggest that methyl viologen can associate in aprotic solvents, such as DMF. In the same paper they use spectro- scopic data to show that in concentrated solutions association is almost com- plete, and the spectrum resembles closely that of an evaporated methyl viologen film formed in vacuum. They also go on to suggest, quoting the work of Kosower and Cotter,70 that dimerization may be a precursor to more extensive aggregation. The dissociation constant of 2.6 x 10-3, quoted above, corresponds to a dG of -14.7 kJ mol-l.This value is consistent with those quoted by Evans15 for various viologens: AGO ranges from -10.6 to -16.37 kJ mol-l, the latter figure being the methyl viologen value. When the dimerization -or higher-order aggregation -is accompanied by precipitation, further complications can ensue. Thus Bruinink18 refers to the 'instability' of heptyl viologen bromide films and discusses this in terms of reorientation or recrystallization of the film. The point is illustrated by an open- circuit potential decay plot, showing a shift of some 35 mV in the reversible potential. This value, corresponding to -3.4 kJ mol-l, is obviously indicative of 'l B. Emmert and H. Haffner, Chem. Ber., 1924, 57, 1792.'Is V. F. Ivanov and A. D. Grishina, Izv. Akad. Nauk SSR, Ser. Khim., 1977, 8, 1873. Bird and Kuhn a more subtle process than the dimerization itself. The dGvalue is comparable with conformational free-energy changes. C.Mechanistic Considerations.-It has been seen that the cation radical may either be soluble or form a film at the electrode surface, and this will be a function of the solvent, the alkyl substituent, and the anion. If the radical is soluble then the kinetics are simple and appear to be reversible. However, if a film is formed then the kinetics of both its formation and reoxidation are complex. It appears that its formation involves a nucleation-type process. To date most mechanistic investigations have employed dilute solutions.In order to form a more complete picture of the processes operating, particularly when the cation radical undergoes deposition, a greater volume of data is required. These data must cover the concentration dependencies, particularly in view of the use of stronger solutions in display applications. Once the film is formed one sees abundant evidence that changes take place, and this is presumably some form of aggregation. Colour and texture (both features of ‘spotting’) form part of this evidence. Potential changes at open circuit and changes in the reoxidation kinetics as shown by potential-step data afford other evidence. Whether one can explain all these effects in ‘change-of- state’ terms or whether one must invoke chemical explanations is unresolved.The longer the film is allowed to exist on the electrode surface the greater will be such effects. However, one must recall that dissolved oxygen can never be wholly absent from these systems, and, if present, there is evidence that it can react irreversibly with the cation radical.73 Jasinski74 formulates a mechanism for the electrochemical process. The film is formed, either in one or two steps, from bulk solution. (Whether there is a first step being the formation of a soluble radical cation, followed by precipitation of the salt, is debatable.) The film so formed is non-porous since undercutting and peeling of the film are not found on anodic oxidation. Finally there appears to be an equilibrium between the radical cation in the film and the same species in bulk solution.Jasinski returns in reference 63 to the growth processes of heptyl viologen dihydrogenphosphate films on gold electrodes. Some comparison measurements were made with viologen bromide. Looking at the time interval from 60 ms to 40 s after application of the potential sweep (during which time the film grew from 0.5 mC cm-2 to more than 40 mC cm-2) his main findings were as follows. Firstly, ipeak vs. (sweep rate)Oa5 gave hear plots over a large range, that range being greatest in bromide, or in phosphate of pH 4, but somewhat more restricted in phosphate of pH 5. Jasinski calculates diffusion coefficients in a number of ways and finds that values obtained from cyclic voltammetry and chronocoulometry agree well.In a figure he shows how the value of D is the same for phosphate and bromide at low concentrations and how the value drifts slowly down with increasing concentration over the range 0.2 mmol dm-3 to 0.01 mol dm-3. At 73 R. N. F. Thorneley, Biochim. Biophys. Acta, 1974, 333, 487. 74 R. J. Jasinski, J. Electrochem. SOC.,1978, 125, 1619. Electrochemistry of the Viologens this point, however, while the bromide continues the slow decrease phosphate plunges rapidly down, decreasing by a full order of magnitude in the concentra- tion range 0.01--0.1 mol dm-3. At infinite dilution D = 6 x 10-6 cm2 s-1 for both anions. On the basis of the magnitude of D and its dependence on solution properties Jasinski argues that the diffusion control must be from solution and not diffusion in the film as it is formed.Plots of charge, Q, vs. t0.5 are also linear, again supporting the diffusional process. However, the intersection at Q = 0 occurs at a finite time, varying from 10 to 120 ms. In his conclusion Jasinski poses a key question. Since film growth is diffusion limited the film itself must be either electronically conducting or porous. In the latter case further deposition could only result from diffusion of the dications through the pores to the electrode surface, followed by diffusion out of the radical cation, presumably to nucleate onto the existing deposit. Furthermore, oxidation, too, would have to occur at the metal surface, so detaching the outer layers of radical cation from the electrode surface, and this has not been seen in practice.Jasinski therefore opts for the electronically conducting model and points out that charge flow through it need only be as fast as the inward diffusion of dications. From the spectrophotometric data of Schwarz60 and Chang75 he estimates film thickness as 90A mC-l cmz. A simple calculation reveals this thickness to be equivalent to a density of 5.Og ~m-~, avalue whichseemssurprisingly high. However, from this thickness and the measured diffusion coefficient he uses the Cottrell equation to obtain a resistivity of lo4 cm for the film grown in phosphate at pH 4 and the higher value of lo5 in phosphate at pH 5.5. Cyclic voltammetric i-v* plots show a fall-off at the higher pH, which Jasinski attributes to higher ohmic resistance, but it must be noted that a film of the thickness reported should not produce a large ohmic drop.The film formed by reduction of the di-n-heptyl cation is stated46 to have negligible resistances even at a charge density of 2 mC cm-2. Nevertheless it appears to block further deposition of cations either from concentrated viologen solutions or when recovery time is allowed to replenish the diffusion layer. Murano and Kam~ra7~~ have studied the electrical and spectroscopic properties of an evaporated methyl viologen film in vacuum. They obtained resistance values of 105-106 C2 for a thickness of 103-104 A. Their deduction was that con- ductivity is electronic not ionic.Barna75b has studied the open-circuit reorientation of deposited heptyl viologen films, finding that the film reorients to a phase with a higher degree of ordering and hence a greater optical anisotropy. The same birefringence measure- ments showed that higher deposition overpotentials resulted in a less ordered film and that the number of active sites available for film nucleation was potential dependent, thereby confirming previous arguments.46~74 D. Oxidation of the Cation Radical.-Once formed, the kinetics of the cation- 76 I. F. Chang, B. L.Gilbert, and T. I. Sun,J. Electrochem. SOC.,1975, 122,955. ?s@ K. Murano and Y.Kamura, Bull. Chem. SOC.Jpn., 1976,49,2407. 76b G. G. Barna, J. Electrochem. SOC., 1980, 127, 1317. 74 Bird and Kuhn radical reoxidation have been studied mostly with cyclic voltammetry.However, Jasinski74 has applied a potential step to a previously formed film. His con- clusions can be summarized: that, firstly, the greater the overvoltage the faster the reoxidation and, secondly, the more material on the electrode surface as a film the longer the oxidation requires. He also observes a definite ‘tail’ and suggests that this corresponds to the 0.03 mC cm-2 of charge associated with the monolayer immediately adjacent to the electrode surface, which is expected to behave differently. Finally he shows at some length that in the case of SnO2 the history of the surface has a profound effect on these results, and washing or chemical pre-treatment are all examined.However, there are no indications here that similar effects would apply on metallic surfaces, assuming these were reasonably clean. E. Reaction with Oxygen.-The viologen cation radical reacts rapidly with oxygen. The reaction has been proposed as a means for determination of oxygen dissolved in water in low concentrations. S~eetser~~ uses proflavine and EDTA to generate the MV+* photochemically. LeesP9 generates the radical electrochemically. He formulates the overall reaction as 2MV+* + 402 = 2MVZ++ 20H-which might be followed by de-alkylation (4.v.). It is now recognized that the reaction is not quite so simple. It appears that in a fast first step oxygen is reduced to hydrogen peroxide or its ion, followed by a slower step in which the hydroxyl ion is formed with the 0-0 bond being broken.Workers who have used excess oxygen observe only the first of these two steps, by monitoring the rate of disappearance of the monocation radical. So we ’have the data of Farrington,77 who quotes rate constants of 7.7 x lo* dm3 mol-l for the reaction of PQ+- with 02 and 6.5 x lo8dm3 mol-1 for reaction with 02-, and Rauwe178 quotes a rate ‘in excess of 108 dm3 mol-l s-l’, citing Farrington in support of this.79 Pulse-radiolysis techniques were used for the studies by Farrington, a ring-disc electrode by Rauwel. White,35 quoting unpublished work of Farrington, shows how the first step (reaction with molecular oxygen) has a pH-independent half-life of less than 1 microsecond, while the second step has a half-life from ca.200 s at pH 4 to 0.2 s at pH 8. This analysis is supported and extended by the work of Th~rneley,~~ who quotes data for rate constants of the second (slow) step over the pH range 7.5-9. The latter author, however, raises an interesting question that remains unans- wered. There appears to be controversy as to whether the oxidation of the cation radical is reversible or not. On the one hand it is suggested that the oxidation by air is reversible in the sense that as much cation radical can be regained by re- reduction as was originally present. On the other hand there seems to be evidence from both Thorneley himself and other workers he cites that oxidation of the 76 P. B. Sweetser, Anal. Chem., 1967,39, 979.77 J. A. Farrington, Biochim. Biophys. Acta, 1973, 314, 372. F. Rauwel, J. Electroanal. Chem. Interfacial Electrochem., 1977, 75, 579. 7B J. A. Farrington and M. Ebert, Biochim. Biophys. Acta, 1973, 314, 372. Electrochemistry af the Viologens cation radical by dissolved H202 does lead to an irrecoverable loss of some 15% or so that cannot be re-reduced. All the foregoing relates to homogeneous reactions. In the presence of an electrochemically active metal surface, such as Ag, there is every reason to think that a pair of coupled reactions can take place in which oxygen is reduced and the cation radical is oxidized. F. Reaction with Molecular Hydrogen.-Molecular hydrogen can, when cata- lysed, react with viologens. Considering this as a pair of redox reactions, one of which has a pH-dependent value (H2 oxidation) and the other of which is pH- independent (viologen reduction), a pH can be selected where the reaction MV2++ 4H2 $ MV+ + H+*may go either forwards or backwards, and indeed whatever the bulk pH it will be seen that the reaction itself causes a pH change that will act locally.This reaction was studied by Beresin et aZ.80 using methyl viologen at pH 7.2-7.8 at a concentration of 4 x 10-4 mol dm-3 up to 4 x 10-3 mol dm-3 and using enzymes to catalyse it in both the forward and backward direction. A later study by Okura,sl who does not cite Beresin, is very similar and uses near-u.v. light to form hydrogen by reduction of water with oxidation of the MV+* previously formed.Interestingly colloidal Pt is also shown to act as a catalyst for this reaction. Beresin obtains the expression (10) as an equilibrium constant for the reaction. Other work is quoted in reference 82. Kp (Hs= 1 atm) = [MV+’l[H+l = 8 & 2 x 10-Qmol dm-3 (10)w2+1 G. Oxidationof the Viologen Dication.-Air oxidation of V2+ has been discussed by Shelepin and Ushakov.7 Apart from the observation that the products can be detected by their green fluorescence in U.V. light, little more is known. According to them methyl viologen is stable in acid and neutral solution. The rate of oxi- dation depends on concentrations of dissolved oxygen, OH-ions, and viologen. A 2 mmol dm-3 solution in 0.1 mol dm-3 alkali is stable in air for three days. Trebling the viologen concentration results in a yellow coloration in one day. Ja~inski~~states that de-aerated solutions which were stored in air “developed a pungent odour within a few days unless contained in ‘actinic’ glass”. Shelepin’s work does, however, suggest that the rate of anodic oxidation even on platinized platinum electrodes is extremely small.The wavelength of the radiation used by Shelepin and Ushakov is not stated, but they describe how illumination of 1.5 mmol dm-3 MV2+-0.13 mol dm-3 KH2p0.1 (pH 4.7) gives the cation radical as well as oxidation products. Open- circuit potential measurements of a platinized Pt electrode again indicated that I. V. Beresin, Dokl. Akad. Nauk SSSR, Ser. Khim.,1975, 225, 105. I. Okura and N.Kim-Thuan, J. Mol. Catal., 1979, 6, 449. J. Kiwi and M. Graetzel, Angew. Chem., int. Ed. Engl., 1979, 18, 624. 76 Bird and Kuhn oxidation and reduction were occurring simultaneously. However, it is not quite clear whether air was present in these experiments. The effect of light irradiation on viologens falls outside this review. It has been extensively documented by Calderbank,*3184 and in considering this effect the presence or absence of air is crucial but not always clear in the primary literature. An interesting comment by Shele~in~~ is that the photo-oxidation of methyl viologen by air is increased by phosphate anions, but for an unknown reason. His paper reviews the degradation products of illumination, as do those of Cal der ban k84 and Shchegoleva.H. Effect of the Substrate.-This section is of particular relevance to electro- chromic displays. Previous references have been made to the effect of the substrate on the appearance of a heptyl viologen film21 and to the effects that cleaning and pretreatment have on the behaviour of the deposited monocation. There are other references in the literature to substrate effects. For example in reference 26 Jasinski suggests that on Au, with F-or so42-, pale yellow films are formed, presumably the fully reduced species (VO). To what extent this observa- tion reflects the colour of the metal itself is not certain. He also states that in the same fluoride solutions that gave the yellow films on Au the cation radical (presumably blue) was formed on Pt.Jasinski quotes Ko~ower,~~ andV~lke,~~ Schwarzeo in support of the idea that pre-adsorption of the radical cation or the dication might explain these effects. He also suggests that the substrate might affect the reversibility of the electrochemical oxidation-reduction in that on Pt the reduction to Vo followed by its reoxidation to the dication could be repeatedly done without leaving a residue on the electrode surface, a reversibility not found on Ag or Au electrodes with the solutions used. On these metals polariza- tion into the VO region altered the shape of the cyclic voltammograms and also gave rise to black discoloration of the purple film. It is hard to explain such differences, and certainly with other anions the effects reported by Jasinski are not seen.On the active noble metals such as Pt, where hydrogen evolution would be most favoured, one might expect the greatest resulting change in pH with the damaging consequences of alkalization. But of the other side reactions that can be envisaged it is hard to see how Au, a singularly inactive metal in the catalytic or electrocatalytic sense, might exert the effects suggested above. 6 Colour and Spectrum A. Visual Appearance of Solid Films.-The solid cation film deposited on the cathode appears violet, and, at extreme thicknesses, black and can in this form be shiny and crystalline. However, more unusual departures from these norms have been reported. Jasinski21 reports a red/violet film on Pt, using heptyl 83 A.Calderbank, Adv.Pest Control Res., 1968, 8, 127. 8p A. Calderbank and J. A. Farrington, J. Chem. Soc., Perkin Trans. 1, 1972, 138. 86 1. V. Shelepin, V. A. Barachevskii, and N. I. Kunavin, Zh. Fiz. Khim., 1975, 49, 1731. I. S. Shchegoleva, Khim. Vys. Energ., 1976, 10, 398. 77 Electrochemistry of the Viologens viologen fluoride, and also on Pt and Ag in sulphates. However, neither anion, he states, produced a coloured film on Au cathodes, although electrochemical data confirmed that reduction was taking place. Phosphate/formate anions do give the same colour on Au. Addition of F-suppresses it. In addition it is suggested that the combination of an Ag cathode with F-anion gives a less stable or more soluble film.Reduction of NN’-di(pcyanophenyl)-4,4‘-dipyri-dinium dichloride gives a green film (Brit. Pat. 1314049). The second phenomenon that can be visually observed is ‘spotting’, the formation of small islands of a lighter colour and having a crystalline appearance. These have been reported by Bruininklg and other authors. When an attempt is made to reoxidize the cation radical back to the V2+ state these ‘spots’ are harder to oxidize, taking longer or requiring a more anodic potential to do so. Gavrilov again reports similar effects.41 Bruinink et al. have added further observations on this ‘spotting’. It is reported to be sensitive to factors such as (a) the amount of Vo formed during the reduc- tion (i.e. cathodic limit), (b) electrode material, (c) electrolyte, (d) electrode texture, and (e) viologen structure.Thus the 1,l ’-diheptyl viologen bromide showed spotting after 10 seconds or so, while a tetramethylene bis[4-(l- benzylpyridine-4’-yl)pyridinium]tetrafluoroborate showed similar effects only after several minutes; the perchlorate analogue is also stable. As discussed in the preceding section on dimerization, the ‘spotting’ has been linked with changes in open-circuit potential, and Bruininklg shows how the onset of spotting in the less stable heptyl viologen bromide film is associated with an anodic potential shift of ca. 35 mV after 30 seconds, which is not shown by the more stable viologens such as those listed above. (There is also a much smaller and slower anodic shift seen in all cases, which is because of the restoration of the V2+ concentration at the interface after its depletion due to the reduction.) Point (b) above was largely confirmed by Ja~inski,~~ who showed that pre- treatment of the electrode surface (mainly SnO2) was critically important and that properly treated (degreased, etc.) surfaces showed much less charge loss after a minute or so at open circuit.B. Colour and Spectrum of Monocation Radicals.-As has been seen, some of the monocation radicals are soluble in aqueous media while others form a pre- cipitated film at the electrode surface. Almost all, whether dissolved or solid, are blue to reddish-purple in colour. The questions with which we may concern ourselves relate to the colour, the spectrum in the u.v.-visible region, and the extinction coefficient. The literature has many examples of recorded spectra of the viologens. Michaelisl shows that of the methyl, ethyl, benzyl, and betaine viologen as well as the 4,4’-dipyridyl the first two are similar to one another.In the benzyl a secondary peak at 550 nm is more intense than the primary one at 600 nm. Van Dam and Ponjee show16 transmission spectra of ethyl viologen chloride in water and that of the n-heptyl as the tetrafluoroborate in acetonitrile and as the bromide on SnO2. There are many other examples in papers cited elsewhere in Bird and Kzhn this review. A factor which besets the interpretation of all these spectra is the dimerization, which has been discussed elsewhere.Kosower and Cotter,70 also quoting Schwarz,so show how dimerization can cause the appearance of a separate shoulder in the visible region of the spectrum. Kawata31 has recorded similar observations. Warming and cooling the solution70 also reveals the mono- mer-dimer transition, but only in water and not in acetonitrile. Thorneley73 extends the point by showing that the apparent extinction coefficient of methyl viologen cation radical decreased as concentration increased (in the millimolar range). Though many other values of E are quoted for methyl viologen, only he appears to have taken this factor into account, and this must cast doubt on the others. The same question of dimerization has been used by Kawata to explain colour changes of both methyl viologen (quoting the work of Kosower) and benzyl vi~logen.~~ This author studied benzyl viologen bromide in water and in methanolic solution, as well as the iodides and chlorides and the cation radical in glycerin.Other spectra recorded in various solvents including water, acetonitrile, and ethanol are reported by Hunig87 and Kosower and Cotter70 as well as Guerin- Ouler88 and Winograd and Kuwana89.42. Thanks to the data of Kawata31 on the one hand and of Hunig and S~henk~~ on the other some order can be brought to the field. Kawata shows a relation- ship between peak wavelength and solution polarity, using propanol, methanol, and DMSO, which have dielectric constants from 20 to 45 to show that a wave- length shift occurs from 605 nm to 614 nm.Hunig, on the other hand, shows that peak wavelength may be correlated with E+ and so with Hammett functions. All these data, both spectral and electrochemical, relate to acetonitrile as a solvent. As a result of the work of Hunig and also Kawata a measure of prediction can be brought to bear in respect of systems as yet unstudied. In respect of cation radicals which form solid films there are again some data, all obtained using SnO2. Kawata31 reports that peaks are shifted to shorter wavelengths when solid films are formed. It is well known that the visible spectrum of organic dyes is a function of their aggregation state, and such observations are not therefore surprising.90 Viologen can be used as the basis of a copolymer, and the spectrum of this, both infrared and u.v.-visible, is reported by Simon and Moore8 and more recently by Sat0 and Tamamura.10 Gavrilov and Ushakov41 use an In203 electrode to record spectra for methyl viologen with both C1- and c104-anions.Malpas and Bardg1 have recently published a report on the use of a new technique, 'photoacoustic spectroscopy', to measure heptyl viologen bromide 87 S. Hunig and D. Scheutzow, J. Chem. Phys., 1971,75, 335. D. Guerin-Ouler and C. Nicollin, Can. J. Spectrosc., 1974, 19, 69. 8B N. Winograd and T. Kuwana, J. Am. Chem. SOC., 1964, 86, 5524; N. Winograd and T. Kuwana, J. Elecrroanal. Chem. Interfacial Electrochem., 1969, 23, 33. S. F. Mason in 'The Chemistry of Synthetic Dyes', ed. K.Venkataraman, Academic Press, 1970, Vol. 3. 91 R. E. Malpas and A. J. Bard, Anal. Chem., 1980, 52, 109. 79 Electrochemistry of the Viologens spectra on metal films. They state that their data agree with classical spectro- photometric measurements on SnOz quoted by Chang et al.75 C.Extinction Coefficients.-These have been quoted for methyl viologen by Beresin,so Bard,4 H~nig,~~ Steckhan and Kuwana (benzyl and Downe~,~~and Krumh0ltz.~3 Other authors are Winograd and K~wana,4~ Schoot for the n-heptyl solid film,49 Barclay,g4 and Eisenstein and Way.95 However, the most impressive reference, precisely because values are quoted as a function of concentration and because errors in previous estimates are discussed, is due to Th0rneley,~3 and his value for methyl viologen, extrapolated to zero concentration, is 1.3 x 104 at 600 nm.However, Kawata13 has plotted absorbance versuscharge density. He obtains a straight line (absorbance = 0.8 for 5 mC cm-2 at 545 nm) for heptyl viologen with just over half this value for the polymeric viologen at 595 nm. In respect of solid viologen films the whole concept of extinction coefficient is a hazy one, and it would seem that much depends on the state of aggregation of the solid and quite possibly, bearing in mind dimerization, the 'age' of the film.No careful measurements appear to have been reported here. D. Other Spectroscopic Data,-E.s.r. data are reported by Shchegoleva,*G Guerin-O~ler,~~~~~and Johnson and Gutowski97 as well as Kosower70 and Blandamer.66 In many cases the data are used to support evidence of oligomeriza-tion (q.~.).Semi-empirical MO calculations of some viologens and their corre- lation with experimental spectra are reported by Hunigs7 and G~erin-Ouler.~~ 7 Newer Aspects A. Viologen-modified Electrodes.-Hawkridge and co-workersQ~~g9 have pub- lished papers in which a gold or platinum electrode is subjected to an extended cathodic step in viologen solution.As a result of this, it is claimed, a modified form of viologen, possibly akin to the polymers reported by Simon and Moore,8 is formedat the electrode surface. They also claim that this polymeric form of the viologen is stable in air for some days but at the same time is electro-active in that it can act as a mediator.In the latter role it can heterogeneously reduce spinach ferrodoxin many times faster than an untreated gold electrode does in the classical electrochemical mode. The authors recognize the three oxidation states of viologens but maintain that once a given potential (e.g. -0.950) has been exceeded on the cathodic side the subsequent regeneration of the higher oxidation sa J. E. Downes, J. Chem. SOC.(C),1967, 1491. Da P. Krumholtz, J. Am. Chem. Soc., 1951, 73, 3487. 94 D. J. Barclay, C. L. Bird, D. H. Martin, J. Electron. Mater., 1979, 8, 311. D6 K. K. Eisenstein and J. Way, J. Biol. Chem., 1969,244, 1720. D. Guerin-Ouler and C. Nicollin, Mol. Phys., 1977, 34, 161. s7 C. S. Johnson and H. S. Gutowski, J. Chem. Phys., 1963, 39, 58.D8 H. L. Landrum, R. T. Salmon, and F. M. Hawkridge, J. Am. Chem. SOC.,1977,99,3154. gD J. F. Stargardt, F. M. Hawkridge, and H. L. Landrum, Anal. Chem., 1978,50, 930. Bird and Kuhn states is no longer possible, or at least not readily so. SEM studies reveal an amorphous deposit on the gold electrodes. It is pointed out that a similar product could not be reproduced on an Hg electrode -a very different, brown species was formed which rapidly turned blue in air. Another remarkable feature of these deposits is their ability to store ‘reducing power’ even in air. Thus after such exposure they are still capable of reducing the methyl viologen dication to produce a blue colour in solution. This inertness to air is in very striking contrast to the reactivity of the cation radical to oxygen, discussed elsewhere.In their second paper99 benzyl and methyl viologens were compared in this role. They behaved very differently and gave different deposits on electrodes as shown by the SEM. However, the earlier behaviour was confirmed. A viologen-modified electrode -but in a very different sense -is also described by Cieslinski and Armstrong.100 It is now well known that all manner of mole- cules can be bonded to glass surfaces (and others) by silyl linkages. These authors so bonded heptyl viologens onto a SnO2 or indium-tin oxide optically transparent electrode (OTE) and showed the cyclic voltammograms and also spectral intensity at fixed wavelength during reduction and reoxidation.Little difference is seen in the case of the cyclic voltammetric data. However, there are clear indications that silylated surfaces show greater intensity of absorption, and react faster, than their untreated analogues. The converse is also true in that reoxidation appears to be slower. B. Technological and other Applications of Viologen Electrochemistry.-It was indicated at the beginning of this review that the electrochemical behaviour of the viologens had been found useful in a wide range of practical applications. The relationship between electrochemical and herbicidal properties has been noted, as has the use of the viologens for estimation of dissolved oxygen.78 Still in the realm of analytical chemistry, their use as redox indicators was recognized very early on by Michaelis and Hill.1 There is a growing emphasis on physico-chemical studies of biological systems.It is frequently found that a given compound cannot be directly reduced electrochemically, although application of electrochemical techniques, with their many advantages, would be highly desirable. In such a situation the use of a ‘mediating compound’ has become widespread, and the viologens, with their ready reversibility, are among the most widely used mediators. The mediating compound is reduced electrochemically, and then chemically reduces the compound to be studied. Because the process is 100%efficient, one can thus use the idea to apply coulometric analyses or spectroelectrochemical methods. The method is now so widely used that one can only quote a few examples.Fielden and Summers list many of these,23 and the work of Hawkridge9*fg9 applied to spinach ferrodoxin or sperm-whale myoglobin has been quoted earlier. Stom- baughlol studies proteins in this way, while Beresinso has also used methyl viologen as a mediator in the oxidation of hydrogen via enzyme catalysis. loo R. C. Cieslinski and N. R. Armstrong, J. Electrochem. Soc., 1980, 127, 2603. lol N. A. Stombaugh, Biochemistry, 1976, 15, 2633. Electrochemistry of the Viologens RauweP8 has studied cytochrome c, as have Mackey and Kuwana, who also studied cytochrome oxidase.lO2 Andersonl03 used pulse radiolysis to study electron-transfer rates between various heterocyclic compounds including viologen and biologically important species.The reversibility of viologens has led to their consideration as candidates for battery half cells.5o Photochemical effects fall outside the scope of the present review, but references 104 and 105 indicate the role which could be played by methyl viologen and its homologues in the direct conversion of sunlight to electrical energy. Last but not least is the interest in the use of viologens to create electro- chromic displays. These (used, for example, in the seven-segment display mode) could be used to display time in watches or wherever a passive display is called for. A more detailed discussion of such devices is to be found elsewhere, but the majority of scientific literature relating to them has been quoted in this review.More applied publications, such as that of Shapiro et aZ.,l06 compare the heptyl, decyl, and benzyl viologens in terms of cycle life and contrast ratio found. Heptyl and decyl gave 104-105 cycles, and the contrast ratio was marginally superior for the heptyl. However, too little is known about the test conditions to evaluate these data. It is interesting to note, in conclusion, that in respect of electrochromic displays the first patents involving heptyl viologen were filed in 1970. The authors wish to thank Dr. D. J. Barclay for his advice and assistance. lo' L. N. Mackey and T. Kuwana, papers presented at 4th Bioelectrochem. Symp., Julich, West Germany, 27th Oct., 1975. loS R. F. Anderson, 2.Phys. Chem., 1976,80,969. lo*A. B. Bocarsley, D. C. Bookbinder, et al., J. Am. Chem. Soc., 1980, 102, 368. lo6 A. J. Bard, A. B. Bocarsley, et al., J. Am. Chem. SOC.,1980, 102, 3671. lo' B. I. Shapiro and I. N. Savkina, Prib. Sist. Upr., 1977, 2,45.

ISSN:0306-0012    

DOI:10.1039/CS9811000049

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

TILDEN LECTURE* Some Uses of Silicon Compounds in Organic Synthesis By Ian Fleming UNIVERSITY CHEMICAL LABORATORY, LENSFIELD ROAD, CAMBRIDGE CB2 1EW 1 Introduction My task in a TiIden lecture is to deal with the progress made in some branch of chemistry. I have, of course, chosen to discuss the contributions made by organosilicon chemistry to the art of organic synthesis. Let us go back to 1968, a date which marks the turning point in the applications of organosiliconchemistry to organic synthesis, a field which has blossomed amazingly in the past few years, and which can be expected now to bear substantial fruit. To be sure, there were significant contributions before 1968,l but, until that date, synthetic organic chemists as a whole were slow to appreciate the potential which the vast amount of known silicon chemistry held for them.Even at that date, silicon chemistry, although second only to carbon chemistry in the number of papers published every year,2 was the preserve largely of dedicated silicon chemists. In 1968, several key publications alerted us all to this untapped resource, and I want briefly to pay tribute to these formative influences. Pierce’s books showed us how easy it was to put silyl groups onto hydroxy-groups and to-ROH AROSiMe, 100% Reagents: i, Me,SiCI, (MeSSi),NH; ii, MeOH Scheme 1 OSiMe, + EtOSiMe, OSiMe, 76 % Reagent: i, Na, Me,SiCl Scheme 2 *Deliveredin various versions starting on the 15th October, 1980 in the University of Birmingham.C. Eaborn, ‘Organosilicon Chemistry’, Butterworths, London, 1960; J.F. Klebe, Adv. Org.Chem., 1972, 8, 97. a 1. Haiduc, J. Chem. Documentation, 1972, 12, 175. A. E. Pierce, ‘Silylation of Organic Compounds’, Pierce Chemical Company, Rockford, Illinois, 1968. Same Uses of Silicon Campounds in Organic Synthesis Ph SiMe,, LiOSiMe,[:vSMePh );. i, ] -1lhSMe+ O(SiMe,),Jii 56 % SiMe, Reagents: i,/ Li<sMe ; ii, H,O Scheme 3 0 0-OSiMe,, Reagents: i, base; ii, Me,SiCl; iii, MeLi Scheme 4 SiEt, J ii -CHO ?# I1 4OH SiEt, 65 % a~ 2,4-DNP Reagents: i, MCPBA, r.t., CH,Cl,, 12h; ii, H,O+ Scheme 5 Fleming L 0:p-LiO SIR,, -p0 0 SIR, J 11 polymer 0p Reagents: i, -78 "C-r.t., 30 min; ii, NaOMe, MeOH, reflux 3h Scheme 6 Me,Si wc,-& Me,SiF -k f KCI ROH & ROSiMe,But 111 Reagents: i, KF, DMSO; ii, ButMe,SiC1, imidazole, DMF, 35 "C, 10h; ii, Bu,N+F-, THF, 25 "C, 40 min Scheme 7 take them off again (Scheme l), one area which was perhaps already appreciated; Bloomfield published4 his discovery that Riihlmann's reaction5 was applicable to the acyloin condensation of succinate esters (Scheme 2); Peterson published6 his work (Scheme 3) on a silicon-equivalent of the Wittig reaction, a reaction which now bears his name; and Stork and Hudrlik described7 some key features of the preparation and reactions of silyl enol ethers (Scheme 4).In this last paper, one of the most prominent and influential synthetic chemists set his imprimatur on the subject, and we all took notice.Carbon-bound silicon chemistry also received Professor Stork's imprimatur with the publication* three years later of the J. J. Bloomfield, Tetrahedron Lett., 1968, 587 K. Riihlmann, Synthesis, 1971, 236. D. J. Peterson, J. Org. Chem., 1968, 33, 780. G. Stork and P. F. Hudrlik, J. Am. Chem. SOC.,1968, 90, 4462, 4464; see also H. 0. House, L. J. Czuba, M. Gall, and H. D. Olmstead, J. Org. Chem., 1969, 34, 2324. G. Stork and E. Colvin, J. Am. Chem. SOC.,1971,93,2080;G. Stork and M. E. Jung, ibid., 1974,96,3682; P. F. Hudrlik, J. P. Arcoleo, R. H. Schwartz, R. N. Misra, and R. J. Rona, Tetrahedron Lett., 1977, 591. Some Uses of Silicon Compounds in Organic Synthesis Stork-Colvin reaction (Scheme 5) and then, two years later,g of the Stork- Ganem modification (Scheme 6) of Robinson annelation.Furthermore, in 1972, both Cunico and DexheimerlO and another of the most prominent synthetic chemists, E. J. Corey,ll independently introduced the use of fluoride ion as a powerful and specific method for removing silyl groups (Scheme 7). Thus the stage was set for what has proved to be a rapidly growing and CH,CI, Scheme 8 00 0 OSi Me, NMe, < =&Me2 no catalyst cz,fJ%J80Xr.t. 0 CH,CI, Scheme 9 G. Stork and B. Ganem, J. Am. Chem. SOC.,1973,95,6152; R. K. Boeckman, ibid., 1974, 96, 6179; G. Stork and J. Singh, ibid., 1974, 96, 6181. lo R. F. Cunico and E. M. Dexheimer, J. Am. Chem. SOC.,1972,94,2868.l1 E. J. Corey and B. B. Snider, J. Am. Chem. SOC.,1972, 94, 2549; E. J. Corey and A. Venkateswarlu, ibid., 1972, 94, 6190. Fleming OSiMea 178% Ph %SPh PhSCI Ph3 Ph r.t. scheme 10 Me0,C -C02But MeO,CACO,SiMe, OSiMe, P 87 %up*87 % Reagents : i, Me,SiOSO,CF,, EtSN, dioxan-reflux, 20 min ; Me0 OMe ii, J, ,5 % Me,SiOSO,CP,,-78 "C, 1Oh; iii, Ph-O-O-Ph , 5 % Me,SiOSO,CF,, 18"C, 12h %heme 11 successful line of research entered into by many groups around the world.12 In this lecture, I shall try to bring out some of the major features of organosilicon chemistry, and illustrate them largely with examples from our own work. 2 Silicon-Oxygen Chemistry One line of work in which we and others have been engaged is based on the P.F. Hudrlik, J. Organomet. Chem. Library, 1976, 1, 127; E. W. Colvin, Chem. SOC.Rev., 1978, 7, 15; I. Fleming in 'Comprehensive Organic Chemistry', ed. D. H. R. Barton and W. D. Ollis, Pergamon, Oxford, 1979, Vol. 111, p. 541 ; T. H. Chan and I. Fleming, Synfhesis,1979, 761 ; I. Fleming, Chimia, 1980, 34,265 and P. Magnus, AZdrichimicaAcfu, 1980, 13, 43. 87 Some Uses of Silicon Compounds in Organic Synthesis chemistry of compounds having a silicon-oxygen bond. Thus silyl groups, especially the t-butyldimethylsilyl group,ll are extensively used to protect hydroxy-groups (Scheme 7). Similarly, silyl enol ethers act as the most easily made derivatives of enols; they can be used either, as Stork and Hudrlik have shown, as the precursors of enolates (Scheme 4), or, in their own right, as Mukaiyama and his co-workers and others have shown, as versatile carbon nucleophiles (Schemes 8,13 9,14 and 1015), duplicating and extending much of enolate chemistry.In all this work on oxygen-silicon compounds, the silyl group functions as a substitute for the proton of an alcohol or enol. Like a proton, a silyl group is easy to put on and take off, yet the silicon-oxygen bond is slower to break than the corresponding hydrogen-oxygen bond. In silicon-oxygen chemistry, therefore, the silyl group is like a rather feeble proton. An intriguing aspect of this idea is that trimethylsilyl triflate is a Lewis acid (Scheme 11)16 which is compatible with the presence of amine bases. Reagents: i, t3utC1, CH,CI,, TiCl, scbeme 12 l3 Clockwise from top-left: T.Mukaiyama, K. Banno, and K. Narasaka, J. Am. Chem. SOC., 1974,96, 7503; T. Mukaiyama and M. Hayashi, Chem. Lett., 1974, 15; K. Narasaka, K. Soai, Y. Aikawa, and T. Mukaiyama, Bull. Chem. SOC.Jpn., 1976, 49, 779; M. Miyashita,T. Yanami, and A. Yoshikoshi, J. Am. Chem. SOC.,1976,98,4769. l4 Clockwise from top-right: ref. 17 and M. T. Reetz and W. F. Maier, Angew. Chem. Znt. Ed. Engl., 1978, 17, 48, M. T. Reetz, W. F. Maier, H. Heimbach, A. Giannis, and G. Anastassious, Chem. Ber., 1980, 113, 3734 and M. T. Reetz, W. F. Maier, I. Chatziisifidis, A. Giannis, H. Heimbach, and U. Lowe, ibid., 1980,113,3741, and T. Sasaki, A. Usuki, and M. Ohno, Tetrahedron Lett., 1978, 4925; T.Shono, I. Nishigushi, T. Komamura, and M. Sasaki, J.Am. Chem. SOC.,1979,101,984; S. Danishefsky, T.Kitahara, R. McKee, and P. F Schuda, ibid., 1976,98,6715;%-I. Inaba, I. Ojima, K. Yoshida, and M. Nagai,J. Organomet.Chem., 1979,164, 123. l6 Clockwise from top-right: I. Paterson, Tetrahedron Lett., 1979, 1519 and M. T. Reetz, S. Huttenhain, P. Walz, and U. Lowe, ibid., 4971 ;R. H. Reuss and A. Hassner, J. Org. Chem., 1974,39, 1785; G. M. Rubottom, J. M. Gruber, R. K. Boeckman, M. Ramaiah, and J. B. Redwind, Tetrahedron Lett., 1978, 4603 and L. Blanco, P. Amice, and J. M. Conia, Synthesis, 1976, 194; S.Murai, Y. Yuroki, K. Hasegawa, and S. Tsutsumi, J, Chem. SOC., Chem. Commun., 1972, 946; J. K. Rasmussen and A. Hassner, J. Org. Chem., 1974, 39, 2558.la J. Borgulya and K. Bernauer, Synthesis, 1980, 545; S. Murata, M. Suzuki, and R. Noyori, J. Am. Chem. SOC.,1980,102, 3248 and Tetrahedron Lett., 1980,2527. Fleming A. Phenylthioalkylationof Silyl En01Ethers.-Our first contribution in this area was made by Ian Paterson as an extension of his work1' with Professor T. H. Chan on the t-alkylation of silyl enol ethers (Scheme 12). He invented the Lewis acid-catalysed phenylthioalkylation of silyl enol ethers,l8 and showed that it was a high-yielding and regiospecific method for the introduction of primary alkyl or alkylidene groups a to the carbonyl group of qdehydes, ketones, and esters 91 % --+ /M/ CHO i, iidCHO 97 % 86 % SPh Reagents: i, Et,N, 62% Me,SiCl; ii, Ci Y , TiC14-Ti(OPri)4, CH,Cl,, -78 "C, 1h SPh Scheme 13 (Schemes 13, 14, and 15).Then he and Jon Goldhill showedl9 that phenyl- thioalkylation could often be used to introduce primary alkyl or alkylidene groups in the y-position of silyl dienol etheis (Scheme 16). Although the extent of y-relative to a-attack was not always complete (Scheme 17), this reaction was strikingly different from the 'corresponding reaction of alkyl halides on lithium dienolates, where the attack is almost always at the a-position (e.g. Scheme 18).20 More recently, Tom Lee has continued this work; he has found that phenyl- thiomethyl chloride is the least y-selective of a range of carbon electrophiles (Scheme 19), and that triphenylsilyl dienol ethers are significantly more y-selective than the corresponding trimethylsilyl ethers (Scheme 20),21 giving -a high pro- portion (84:16) of y-attack even with the least y-selective carbon electrophile.3 Silicon-Carbon Chemistry In much of the chemistry of compounds having a silicon-carbon bond, the silyl group again functions as a substitute for a proton. This time, however, the l7 T. H. Chan, I. Paterson, and J. Pinsonnault, Tetrahedron Lett., 1977, 4183. la 1. Paterson and I. Fleming, Tetrahedron Lett., 1979, 993, 995, 2179. lS I. Fleming, J. Goldhill, and I. Paterson, Tetrahedron Lett., 1979, 3205, 3209. *O N. W. Atwater, J. Am. Chem. SOC.,1960,82,2847; C. N. Lam, J. M. Mellor, P. Picard, M. F. Rawlins, and J. H. A. Stibbard, Tetrahedron Lett., 1978, 4103.I. Fleming and T. V. Lee, Tetrahedron Lett., 1981, 705. Some Uses of Silicon Compounds in Organic Synthesis Me,SiO 0 SPhb-8-i 71 %, 94 % Me,SiO ... t+111 95 "/o 0 93 % SPh Reagents: i, CiL ,ZnBr,(0.02 eq.), CH,CI,, r.t., lh; ii, Ni; iii,CI iv, NaI04, MeOH, r.t., 42h; v, CCI,, 60°C, 60h Scheme 14 siliconxarbon bond usually breaks more easily than the corresponding hydrogen- carbon bond (Scheme 21). Thus, Chan and his co-workers22 have used the /?-elimination of haloalkylsilanes to give allenes and allene oxides (Scheme 22), and Hudrlik and his co-workers have shown23 that the /%eliminations are stereospecific (Scheme 23). T. 6.Chan and W. Mychajlowskij, Tetrahedron Lett., 1974, 171; T. H. Chan, M. P. Li, W.Mychajlowskij, and D. N. Harpp, ibid., 1974, 3511. Is P. F. Hudrlik, D. Peterson, and R. J. Rona, J. Org. Chem., 1975, 40, 2263. Fleming iii, iv, vOSiMe, i-k k A 98 % 92 % 89 % Reagents: i, LDA; ii, Me,SiCI; iii,CInSPh, SPh, ZnBr,; iv, NaIOo; v, heat; vi,a V' SPh ZnBr,; vii, [HI;viii, [O] Scheme 15 However, all the applications of this idea stem from another of the publica- tions of that key year, 1968. I have left it until last, because it is so important. In that year, Eaborn and Bott published a long review24 of the chemistry of compounds containing the silicon-carbon bond, and from this review come many of the insights which have helped to fuel the rapid progress of recent years. Eaborn and Bott made three generalizations of great predictive value.(i) As mentioned above, a silyl group is usually displaced from carbon more easily than a proton is displaced from the corresponding carbon (Scheme 21). (One must now add that it is likely to be true only when the nucleophile is an oxygen or halogen nucleophile, when the carbon is not digonal, and when there are no steric restraints to inhibit the participation of the silyl group.) On carbon, therefore, a silyl group, far from being like a rather feeble proton, as it is on oxygen, is better thought of as a sort of super-proton. (ii) A silicon-carbon bond stabilizes a p-carbocation more than a hydrogen-carbon or carbon-carbon bond does (Scheme 24). This is another example of the silyl group as a super-proton: the stabilization stems from the electropositive (i.e.metallic) character of silicon, which leads the orbitals of the silicon-carbon bond to be favourably polarized 94 C. Eaborn and R. W. Bott in 'Organometallic Compounds of the Group IV Elements', Vol. 1, Part I, ed. A. G. MacDiarmid, Dekker, New York, 1968. 91 Some Uses of Silicon Compounds in Organic Synthesis OSiMe, 77 % 57 ”/, SPh19 Or Reagents: i, LDA; ii, Me,SiCI; iii, ,ZnBr,(0.02 eq.); ivy [HI; v, [OICI Scheme 16 PhS * 84 % 33 :67 PhS-92 % 16:84 SPh Reagents: i, ,ZnBr, c1 Scheme 17 Fleming starting + material 44 % 9% 33 % Reagents: i, KOBut, ButOH, MeI; ii, Me1 Scheme 18 Me,SiO Ph PhE b + PhL' E+ a:y yield 1% PhS-CI, ZnBr, PhS 55:45 65 MeO-CI.ZnBr, Me0 41 :53 53 (EtO),CHMe. ZnBr, EtO +/' 4o:m 78 Pr" PhSACI, ZnBr, 85 (EtO),CMe,, TiCI, EtO < (0:loo) 78 (MeO),CH. TiCI, Me0 < (0:loo) 52 OMe Scheme 19 and energetically well-matched for effective overlap with the empty p-orbital. (iii) In spite of these useful properties, and the high reactivity that comes from them when the silyl group is close to suitable functionality, a silyl group remote from functionality can be relied upon to survive most of the reaction conditions used in modern organic synthesis. Only strongly nucleophilic (Scheme 25)25 and C. C.Price and J. R. Sowa, J. Org. Chem., 1967, 32,4126. 93 Some Uses of Silicon Compounds in Organic Synthesis R, a:y yield/ % Me, Me, Me 55 :45 65 Et, Et, Et 62: 38 75 Me, Me, But 77:23 62 Me, Me, Ph 39:61 91 Me, Ph, Ph 32 :68 74 Ph, Ph, Ph 14:86 93 Scheme 20 For oxygen and halogen nucleophiles, Nu--k. Si -Cp/ is usually faster than the / \ P/corresponding reaction : Nu--H -C\ Scheme 21 ph%l +r66 % 0 Ph Reagents: i, KF, DMSO, r.t., 10h; ii, KF, MeCN, r.t., 53h Scheme 22 strongly electrophilic conditions (Scheme 26)26 are unsafe, and these can usually be avoided.It is this last property which is essentially unique to silicon. Whereas other ‘weak metals’, like boron, aluminium, tin, selenium, phosphorus, and arsenic, might be substitutes for, and even superior to silicon in some reactions, only a C. Eaborn, J. Chern. Soc., 1949, 2755; L.H.Sommer, R. P. Pioch, N. S. Marans, G. M. Goldberg, J. Rockett, and J. Kerlin,J. Am. Chem. Soc., 1953,75,2932. Fleming SiMe, H->99.5:0.5 98 OH+I-'H+' 98:2 100% H->99.5:0.5 93 %'H+' 99.5:0.5 98 "/,2 \ iii) I bH Reagents: i, KH, THF,r.t., Ih; ii, BF,:OEt,, CH,Cl,, O"C, Ih Scheme 23 \ + is stabilized relative to c-c' 0' \ , -pc 15Me23 Si-C +.,. c-c if--.. -3 111 C-C-Cf or H-C .+'or H-c-c+ the coefficients the energy match Scheme 24 f--X ButO-Me,SiT?"e -ButOSiMe, + Me-H t Reagent: i, DMSO,25 "C,secs Scheme 25-ISiEt,Et,Si-Q I-* All, + EtI fI-0 Me,Si O(-Si'd.), + Me-Hf f Me, Reagents: i, 153"C,1.5h; ii, H,SO,, 10"C, 1.5h Scheme 26 95 4 Some Uses of Silicon Compounds in Organic Synthesis silyl group possesses the stability which enables one to carry it with confidence through many different kinds of reaction, before performing the key step which actually uses the silyl group.This unique capability of silicon has been the basis of much of our own work. We have been intent on demonstrating how the three generalizations above can be put together to give us control of otherwise quite well-known organic reactions. In essence, all our efforts, like those of several other groups, have been based on the proposition that a carbonium ion (1) or an alkyl halide (2) can be relied upon to lose the silyl group, rather than to do anything else, and so lead to a single olefin (3) in which the double bond is placed at a specific site (Scheme 27).(2) Scheme 27 Perhaps the most dramatic, simple example of this idea came when Andrew Pearce showed27 that the acetal (4) reacted with Lewis acid to give a single product (3,whereas the corresponding compound without the silyl group was known2* to give a mixture of products (Scheme 28). This result has recently, and most gratifyingly, been extended by Professor J0hnson2~ to a polyolefin cycli- zation (Scheme 29). The idea summarized in Scheme 27 is therefore a powerful one. It is a single idea with a wide variety of applications: all that changes is the route by which one arrives at the structure (1) or (2). A. Viny1silanes.-Combining the first two generalizations above led us to predict that a vinylsilane, other things being equal, would undergo electrophilic substi- tution at the carbon atom carrying the silyl group (Scheme 30).Andrew Pearce showed30 that this extension of an idea, already proved for aryl~ilanes,~~ worked for vinylsilanes (Scheme 31). The idea has been shown to be fairly general32 and I. Fleming, A. Pearce, and R. L. Snowden, J. Chem. SOC.,Chem. Commun., 1976, 182; 1. Fleming and A. Pearce, J. Chem. SOC.,Perkin Trans. I, 1981, 251. 1E A. van der Gen, K. Wiedhaup, J. J. Swoboda, H. C. Dunathan, and W. S. Johnson, J. Am. Chem. SOC.,1973,95,2656. L. R. Hughes, R. Schmid, and W. S. Johnson, Bioorg. Chem., 1979, 8, 513. 8o I. Fleming and A. Pearce, J. Chem. SOC.,Chem. Commun., 1975, 633; J. Chem. SOC., Perkin Trans. I, 1980, 2485.O1 D. HIbich and F. Effenberger, Synthesis, 1979, 841. T. H. Chan and I. Fleming, Synthesis, 1979, 761. SiMe, SiMe, SiMe, -OiMe3i;l' = q? (# Fleming Me0 OMe Me0 OMe MeO+ MeO H (4) Me0 OMc M eOT o/ MeO Me0Po 0" Scheme 28 6-Nu SiMe, I -H+ f-progesteroneReagent: i, TFA, CH,CI,, -35 "C Scheme 29 97 Some Uses of Silicon Compounds in Organic Synthesis vinyl silanes +--Nu Scheme 30 Reagents: i, AcCl, AICl,, CHzClz, O'C, 15 min Scheme 31 SiMe, mB0"rSiMe, \I) Reagents: i, AcCl, AlC1,; ii, Br,, CH,Clz Scheme 32 Fleming 20 min, 0“C.THF -,dSiMc.,Ph W/// (PhMe,Si),Cu Li R+ 1 R 2 PhMe,SiLi + CuCN SiMe,Ph R=H I Me Scheme 33 the reaction is also highly ~tereospecific3~~3~ (Scheme 32).Our only other contrib~tion~~to vinylsilane chemistry has been Felix Roessler’s new synthesis of vinylsilanes from acetylenes (Scheme 33). B. Allylsi1anes.-The same generalizations suggested that allylsilanes would react with electrophiles with allylic rearrangement (Scheme 34),35 as many allyl- metal compounds are known to react.36 This had already been shown to be the allyl silanes Scheme 34 33 T. H. Chan, P. W. K. Lau, and W. Mychajlowskij, Tetrahedron Lett., 1977, 3317. 34 I. Fleming and F. Roessler, J. Chem. SOC.,Chem. Commun., 1980, 276; I. Fleming, T. W. Newton, and F. Roessler, J. Chem. Sac., Perkin Trans. 1, 1981, in press. 96 L. H. Sommer, L. J. Tyler, and F. C. Whitmore, J. Am. Chem. SOC.,1948, 70, 2872.30 W. G. Young and J. D. Roberts, J. Am. Chem. SOC.,1946,68, 1472; K. W. Wilson, J. D. Roberts, and W. G. Young, ibid., 1950, 72, 215. Some Uses of Silicon Compounds in Organic Synthesis artemesia ketone uc,Reagents: i, AcOH, reflux; ii, ,AICI,, -78 "C Scheme 35 SiMe, 500" SiMe, Scheme 36 case by FrainneP and by Calas and Dunoguks and their co-workers3* (Scheme 35). The unique property of silicon in this context was its comparative immunity from [1,3]-sigmatropic rearrangement (Scheme 36), which takes place only at high temperat~res,3~ in contrast to the great ease of such processes with all other metals. Once an allylsilane has been prepared, it can almost always be relied upon not to rearrange in this sense, and hence to behave itself in the manner of Scheme 34.The key to any use of allylsilanes in synthesis is, therefore, to have a method for making them regioselectively. In some very unsymmetrical cases (Scheme 37)38 this is easy. In others it is an inherent result of the synthetic design (Scheme38).*0 Our work has been on: (i) Ian Paterson's use of allylsilanes de- rived from a Wittig reaction (Scheme 39)41 and Decio Marchi's multi-step substitute for the Wittig reaction (Scheme 40),*2 useful for those ketones, like s7 E. Frainnet and R. Calas, C.R. Hebd. Seances Acad. Sci., 1955, 240, 203; E. Frainnet, R. Collongues, and J. Thery, Bull. SOC.Chim. Fr., 1959, 1441. J. P. Pillot, J. Dunoguks, and R. Calas, Tetrahedron Letr., 1976, 1871; and for a review, see R.Calas, J. Organomet. Chem., 1980, 200, 11. J. E. Nordlander, W. G. Young, and J. D. Roberts, J. Am. Chem. SOC.,1961, 83, 494; J. Slutsky and H. Kwart, ibid., 1973, 95, 8678. 'O D. J. Coughlin and R. G. Salomon, J. Org. Chem., 1979,44, 3784. I. Fleming and I. Paterson, Synthesis, 1979, 446. D. Marchi, Synthesis, in press. FZeming Roagent: i, Me,SiCl scheme 37 SiMe, SiMe, terpinoline 60% 100 % 80 % Reagents: i, Mg, Me,SiCl, THF, reflux, 2h; ii, Li, NHs, EtOH; iii, HCl, HpO, MeOH, THF, r.t., 24h Scheme 38 SiMe, -SiMe, Reagents: i, Ph3P’ ;ii, E+ Scheme 39 SiMe,Ph 90% Reagents: i, (PhMe,Si),CuLi; ii, BF,AcOH Scheme 40 cyclopentanone, particularly susceptible to enolization; (ii) the Diels-Alder reactions of 1-silylbutadienes, where Martin Carter found useful reactivity (Scheme 41),43 and Alan Percival found high regioselectivity only when other 43 M..J. Carter and I. Fleming, J. Chem. SOC.,Chem. Commun., 1976, 679 and ref. 45. 101 Some Uses of Silican Compounds in Organic Synthesis Ho<La Goo\ ii 0Me,% Me,SiIH 0 76 % 74 % ...oeOCOzH 111 ,HOQ,R"~~ * \ , 'C0,H *CO,H Me&I 66 % ,CO,Me PhS ,CO,Me iv O:-C02MF Q'* C02Me Me,SiI 80 % Reagents: i, I 0, 1OO"C, 2h; ii, TsOH, C,H,, reflux, 2h; iii, Ac02H, Et,O, r.t., 24h;c0 iv, PhS+BF,-, MeNO, Scheme 41 g j + qm,Q-C0,Me Me& Me,Si Me,Si LQ C0,MeC0,Me Me,Si Me,Si 86 % 51 % Reagents: i, fcozMe, 150°C, 24h; ii, TsOH, C,H,, reflux, 3h Scheme 42 Fleming Me,Si -i (7) J.ix -xii loganin Reagents: i, CHCl,COCI, Et,N, C,HI,,O"C, 5h; ii,Zn, AcOH, H,O, r.t., 24h; iii, MeCH:N,, MeOH, EtOH, O"C, 5h; iv, Zn, AcOH, H,O, r.t., 60h; v, NaOMe, MeOH, r.t., 7h; vi, NaBH,, MeOH, O'C, 2h; vii, MsCI, Py, rat., 18h; viii, Et,N+OAc-, Me,CO, reflux, 2.5h; ix, O:C:NSO,Cl, CCI,, r.t., 2.5h; x, NaNO,, AcOH, Me,CO, Ac,O, O'C, 16h; xi, NaOAc, H,O, r.t., 3h; xii, CH,N,, Et,O, r.t., 0.5h; xiii, O,, CH,Cl,,-65 "C, 2h; xiv, Me$, CH,CI,, r.t., lh; xv, Buchi, 1970 Scheme 43 substituents were present on the diene (Scheme 42);44 and (iii) keten cyclo- additions to 5-trimethylsilylcyclopentadieneand the subsequent reactions of the adduct (6).45 This last example gave Boon-Wai Au-Yeung the opportunity to synthesize loganin (Scheme 43).46 He was easily able to preserve the allylsilane group through seven steps (6) + (7) before the key step (7) -+ (8), which took advantage of the allylsilane group itself.He was able, therefore, to demonstrate in the most vivid way the unique value of the silyl group as a very special sort of metal. C.Silicon p to Carbonyl Groups.-The comparative stability of silicon-carbon bonds is also the basis of another major part of our work. A silyl group /3 to a carbonyl group (10) is stable to most of the conditions used in the enolate chemistry of the carbonyl group. Indeed, when a silicon-carbon bond in (9) is forced to break, it is one of the methyl-carbon bonds which is attacked (Scheme 26),26and not the /%carbon-silicon bond. Thus a substantial amount of building could go on, in the sense (9) -+ (lo), without disturbing the silyl group.However, one reaction does bring the silyl group into play, namely bromination (10) -+ (1 l), 44 I. Fleming and A. Percival, J. Chem. Soc., Chem. Commun., 1976, 681 and 1978, 178, and ref. 45; M. E. Jung and B. Gaede, Tetrahedron, 1979, 35, 621. 45 M. J. Carter, I. Fleming, and A. Percival, J. Chem. SOC.,Perkin Trans. I, 1981, in press. 4B B. W. Au-Yeung and I. Fleming, J. Chem. SOC.,Chem. Commun., 1977, 79, 81 ; Tetra-hedron, in press. 103 Some Uses of Silicon Compounds in Organic Synthesis SiMe, SiMe, SiMe, scheme 44 75-80 % Br Reagents: i, Br3, CCl,; ii, HBr; iii, NaHCO, Scheme 45 0 OLi lii 0 0 72% 66 % 99% 64% Reagents: i, (PhMe,Si),CuLi; ii, MeI: iii, CuBr, Scheme 46 vi vii SiMe,Ph dihydrojasmone 96 ?,; Reagents: i, (PhMesSi),CuLi; ii, Me,SiC1; iii,OH'-, TiCI,; iv, TsOH; v, BF,, AcOH, CCI,, reflux, th; vi, H,, Pd-C, r.t., 2h; vii, PhhMe, Bi3, THF, O'C, 2 min Scheme 47 which will set up the conditions (11; arrows) for desilylbromination, and the formation of a double bond at a site determined by where the silyl group was originally placed (Scheme 44).This idea was shown to be sound by Jon G0ldhi11,~~ who also proved (Scheme 45) that the initial site of bromination need not matter: equilibration (11) + (12) could be used to set up the necessary conditions for desilylbromination (1 1 ; arrows), and the final double bond could be put back into the position determined by where the silyl group had originally been placed.David Ager showed48 that the p-silyl group could be introduced by conjugate addition of a silyl-cuprate reagent, and subsequently removed by desilyl- bromination (Scheme 46). Shailesh Patel has recently used this reaction to synthesize dihydrojasmone (Scheme 47)48 from the readily available but rarely used precursor (13). Alan Per~ival~~ used the siIyl enol ether (14) as a substitute for the Danishefsky diene (Scheme 48), with the potential advantage in some situations that development of the C=C double bond in the final product (15) could be delayed while other synthetic steps are carried out.Recently, David Perry has used the p-silylenone (16),the precursor of Percival's diene, in another 47 I. Fleming and J. Goldhill, J. Chem. SOC.,Chem. Commun., 1978, 176; J. Chem. SOC., Perkin Trans., 1980, 1493. 48 D. J. Ager and I. Fleming, J. Chem. SOC.,Chem. Commun., 1978, 176; D. J. Ager, I. Fleming, and S. K. Patel, J. Chem. SOC.,Perkin Trans. I, 1981, in press. 105 Same Uses of Silicon Compounds in Organic Synthesis Me,Si Me,Si 0 iii , C0,Me Me,SiO Me,SiO 95 % (15) 71% 82% Reagents: i, LDA; ii, Me,SiCI; iii, ,130°C,64h; iv, NBS, THF, r.t., 15 min; YOzMe v, H20, HCl, THF, r,t., 0.5h; vi, DMSO, r.t., 5 min; vii, Br,; viii, DMSO Scheme 48 sense (Scheme 49).49 Conjugate addition to the enone, phenylthioalkylation (17) +(18) of the intermediate silyl enol ether, reduction, and desilylbromination gave, overall, the enone (19).The starting enone is therefore a synthon of the general type (20). D. Silicon-controlled Carbonium Ion Rearrangements.-Since suitably placed silyl groups control the formation of a double bond from carbocations (l), it seemed possible that the homologous arrangement (21) might lead to cyclo-propanes (22) (Scheme 50). Alternatively, rearrangement (21) -+ (23) might be encouraged, because the silyl group would stabilize the intermediate (23), and the outcome would be controlled by the loss of the silyl group (23) -+ (24). Bath pathways have been observed by others (Scheme 51).5O Our early work, carried out by Roger Snowden, Andrew Pearce, and Ian Paterson in collaboration with my colleague Stuart Warren, was concerned first with the controlled migration of the diphenylphosphinoyl group (Scheme 52),S1 where the position of the double bond in the product (25) was determined by the placing of the silyl group and a rearrangement (26) 4(27) was forced on an otherwise reluctant system by the 49 D.A. Perry, unpublished results. 50 L. H. Sommer, R. E. Van Strien, and F. C. Whitmore, J. Am. Chem. SOC.,1949,71, 3056; H. Sakurai, T. Imai, and A. Hosomi, Tetrahedron Lett., 1977, 4045. 51 A. H. Davidson, I. Fleming. J. I. Grayson, R. L. Snowden, and S. Warren, J. Chem. SOC., Perkin Trans. I, 1977, 550. 106 Fteming vi9vii .1 viii, ix, xvi, vii I 1 Reagents: i, Me,CuLi; ii, Me,SiCI; iii, PhSCI; iv, Phsy,ZnBr,; v, Ni;Vi, MCPBA; CI -vii, CCl,, reflux; viii, PhNfMe, Br,; ix, HBr, CCl,, 65°C; x, DBU; xi, BF3AcOH.only E -isomer reacts after 72h, r.t. Scheme 49 Scheme 50 107 Some Uses of Silicon Compounds in Organic Synthesis -Me,SiwBr v 92% Reagents: i, AlCl,, r.t.; c,& ,TiCl,, -78 "C, CH,Cl,, 3h Scheme 51 Ph Me,Si F!Ph2 kh:" 95 "/,II0 Reagents: i, TFA, r.t., 1.5h Scheme 52 Me,Si OH SPh 58% SPh geraniol/nerol 56 % or linalool 62 % Reagents: i, SOCl,, EtSN, LiCI, HMPA, r.t., 20 min Scheme 53 108 Fleming Nu- C0,Me C02Me 98% Reagents: i, Ag+, MeOH Scheme 54 100 % (n. m. r.) (28) Reagents: i, BF,, AcOH, CCI,, r.t.. 15 s; ii, BF,, AcOH, 70°C, 1 min; iii, BF,, AcOH, CH2C12, O'C, 5 min Scheme 55 Ph Ph Reagents: i, BF,, AcOH, CH2CI,, O'C, 5 min Scheme 56 presence of a silyl group, and secondly with driving the rearrangement, effectively uphill, of a phenylthio group (Scheme 53).52 Later, Jo Michael showed53 that a rearrangement of a carbon migrating group could be encouraged in the norbornyl series (Scheme 54).Recently, Shailesh PateF4 has investigated open-chain systems 6a P. Brownbridge, I. Fleming, A. Pearce, and S. Warren, J. Chem. Soc., Chem. Commun., 1976, 751 ;I. Fleming, I. Paterson, and A. Pearce, J. Chem. SOC.,Perkin Trans. I, 1981,256. 63 I. Fleming and J. P. Michael, J. Chem. SOC.,Chem. Commun., 1978,245; J. ChPm. Soc.. Perkin Trans. I, 1981, 1549.64 S. K. Patel, unpublished results. 109 Some Uses of Silicon Compounds in Organic Synthesis Reagents: i, BF,, AcOH, CH,Cl,, OT, 1 min; ii, BF,, AcOH, CCI,, r.t., 15 s; iii, BF,, AcOH, CH,Cl,, O"C, 1 min Scheme 57 -Me,SiMe,Si --i-SiMe, -ii, iii92 % iv Me,Si Me,Si Reagents: i, MeCOCl, AlCl,, CS,; ii, LiAlH,; iii, (PyH)+,Cr,O,'-; ivy Ph,CuLi; v, MeLi Scheme 58 with tertiary migration termini. In every case he gets rearrangement and no cyclopropanes. He has examples of hydrogen shift (Scheme 55), phenyl shift (Scheme 56), and alkyl shift (Scheme 57), all under mild conditions and in high yield. This level of control in carbonium ion rearrangements is remarkable. The y-silyl alcohols used in this work were easily prepared by a variety of routes, including one which takes advantage again of the versatile chemistry of the @-silylenone(16) (Scheme 58) and another which uses phenylthioalkylation to introduce the silyl group (Scheme 59).These routes show the capacity of a silyl group to be carried through several steps before it is cleanly dispensed with in the rearrangement step, a reaction tamed by the presence of the silyl group. Fleming Ph Ph PhS A4e3SigOH & Me,Si iv Me,% $0 (29) 95 % 69 xi 97 % PhS Reagents: i, Me,CuLi; ii, CiSiMe,; iii,Me3SiACl, ZnBr,; iv, Raney Ni; v, MeLi Scheme59 4 Conclusion Silicon is no longer an exotic element in synthesis. Its organic chemistry is easily understood, first because of the close relationship of silicon to carbon, the element we know most about, and secondly because oversimplified but vivid concepts, such as those that I have presented in this lecture-that a silyl group is a sort of feeble or super proton, depending upon whether it is bonded to oxygen or carbon-help us to appreciate its special, weakly metallic properties.It is getting easier now for us all to work out new ways in which silicon can be used to solve the endless problems we meet in organic synthesis. I hope that I have helped, both by explication and example, to spread the good news. I warmly thank all my co-workers, both those identified by name in the text above and those others, Chris Floyd, Kathy Flynn, Federico Gianni, Jenny Langley, Patrick Lau, Trevor Newton, Ian Wallace, and Richard Williams, who have done invaluable work, for which there was no room here.

ISSN:0306-0012    

DOI:10.1039/CS9811000083

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds By K. D. Bartle DEPARTMENT OF PHYSICAL CHEMISTRY, UNIVERSITY OF LEEDS, LEEDS LS2 9JT M. L.Lee DEPARTMENT OF CHEMISTRY, BRIGHAM YOUNG UNIVERSITY, PROVO, UTAH 84602, U.S.A. and S. A. Wise ORGANIC ANALYTICAL RESEARCH DIVISION, CENTER FOR ANALYTICAL CHEMISTRY, NATIONAL BUREAU OF STANDARDS, WASHINGTON, D.C. 20234, U.S.A. 1 Introduction Polycyclic aromatic compounds (PAC) are ubiquitous environmental pollutants, and although they are formed from both natural and anthropogenic sources, the latter are by far the major c0ntributors.l Natural sources include forest and prairie fires,2 volcanoes,3~4 and in situ synthesis from degraded biological material, which has led to the formation of these compounds in various sediments,5-13 fossils,l4J5 and fossil fuels.16-27 Several rare PAC minerals, R.E.Laflamme and R. A. Hites, Geochim. Cosmochim. Acta, 1978,42,289. M. Blumer and W. W. Youngblood, Science, 1975, 188, 53. A. P. Ilnitsky, G A. Belitsky, and L. M. Shabad, Cancer Lett., 1976, 1,291. A. P. Ilnitsky, V. S. Mischenko, and L. M. Shabad, Cancer Lett., 1977, 3, 227. W. L. Orr and J. R. Grady, Geochim. Cosmochim. Acta, 1967, 31, 1201. F. S. Brown, M. J. Baedecker, A. Nissenbaum, and I. R. Kaplan, Geochim. Cosmochim. Acra, 1972, 36, 1185. Z. Aizenshtat, Geochim. Cosmochim. Acra, 1973, 37, 559.* R. Ishiwatari and T. Hanya, Proc. Jpn. Acad., 1975, 51, 436. * S. G. Wakeham, Environ. Sci. Technol., 1977, 11, 272. lo B. R.T. Simoneit, Geochim. Cosmochim. Acta, 1977,41,463. l1 C. Spyckerelle, A. C. Greiner, P. Albrecht, and G. Ourisson, J. Chem. Res., 1977, 330. It R. A. Hites, R. E. Laflamme, J. G. Windsor, jun., J. W. Farrington, and W. G. Deuser, Geochim. Cosmochim. Acta, 1980,44, 873. I3 S. G. Wakeham, G. Schaffner, and W. Giger, Geochim. Cosmochim. Acta, 1980,44,415. I4 M. Blumer, Science, 1965, 149, 722. l5 A. C. Sigleo, Geochim. Cosmochim. Acta, 1978, 42, 1397. W. Carruthers and D. A. M. Watkins, Chem. Ind. (London), 1963, 1433. B. J. Mair and A. G. Douglas, Geochim. Cosmochim. Acta, 1964, 28, 1303. V. L. Berkofer and W. Pauly, Brennst. Chem., 1969, 50, 376. ‘Organic Geochemistry’, ed. G. Eglinton and M.T. J. Murphy, Springer-Verlag, New York, 1969.A. Van Dorsselaer, A. Ensminger, C. Spyckerelle, M.Dastillong, 0. Sieskind, P. Arpino, P. Albrecht, G. Ourisson, P.W. Brooks, S. J. Gaskell, B. J. Kimble, R. P. Philp, J. R. Maxwell, and G. Eglinton, Tetrahedron Lett., 1974, 14, 1349. s1 B. J. Kimble, J. R. Maxwell, R. P. Philp, G. Eglinton, P. Albrecht, A. Ensminger, P. Arpino, and G. Ourisson, Geochim. Cosmochim. Acra, 1974,38, 1165. Modern Analytical Methods far Environmental Polycyclic Aromatic Campounds pendletonite,28JQ curti~ite,~0.30 and idrialite,SQ have also been characterized. Some reviews are available that cover in detail the anthropogenic sources of PAC.31-34 Major sources include the burning of coal refuse banks, coke pro-duction, residential fireplaces, coal-fired residential furnaces, automobiles, commercial incinerators, oil-fired commercial boilers, and rubber tyre wear.A minor source in terms of total PAC production, but of considerable importance with respect to human health, is tobacco smoking. Until the beginning of this century there existed a natural balance between the production and natural degradation of PAC, which kept the background concentration low and fixed.35 However, with increasing industrial development throughout the world, the natural balance has been disturbed and the production and accumulation rates of PAC are constantly rising. 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Klimeck, J. Assoc. OH.Anal. Chem., 1968,51, 122. s1 J. Borneff, F.Selenka, H. Kunte, and A. Maximus, Environ. Res., 1968, 2, 22. A. J. Malanoski, E. L. Greenfield, J. M. Worthington, and F. L. Joe, J. Assoc. Of.Anal. Chem., 1968,51, 114.O3 G. Grimmer, Dtsch. Apoth.-Ztg., 1968, 108, 529. O4 D. J. Tilgner and H. Daun, Residue Rev., 1969, 27, 19. O6 J. W. Howard and T. Fazio, J. Agric. Food Chem., 1969, 17, 527. s6 J. W. Howard and T. Fazio, Ind. Med. Surg., 1970, 39, 435. O7 T. Saito, Kaguku to Seibutsu, 1970, 8, 178. K. S. Rhee and L. J. Bratzler, J. Food Sci., 1970, 35, 146. G. Grimmer and D. Duevel, 2.Nuturforsch., Teil B, 1970, 25, 1171. looM. Giaccio, Quad. Merceol., 1971, 10, 21. lol R. H. White, .I.W. Howard, and C. J. Barnes, J. Agric. Food Chem., 1971, 19, 143. lo8G. Grimmer and A. Hildebrandt, J. Assoc. Of.Anal. Chem., 1972, 55, 631. lo3I. A. Kalinina, Vopr. Onkol., 1972, 18, 112. 115 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds water,35*79-82 sediment^,^ 95-13962.83-89 f~od~,~~-~~~and tobacco sm0ke.~O~-~~5 These efforts stem from the demonstrated carcinogenic and/or mutagenic activity of many PAC.116PAC are usually present in environmental samples as Table 1 Classi$cation of environmental PAC Name Class Refs.Acridine Polycylic aromatic nitrogen heterocycles Carbazole (PANH) 56,117-1 39 2-Aminoanthracene Amino polycyclic aromaticmNH* hydrocarbons (APAH) 139-141 2-Nitroanthracene Nitro polycyclic aromatic hydrocarbons (NPAH) 142-145 2-Cyanoanthracene Cyano polycyclic aromatic hydrocarbons (CPAH) 146, 147 Dibenzothiophen Polycyclic aromatic sulphur heterocycles (PASH) 148-153 Dibenzofuran Polycyclic aromatic oxygen heterocycles (PAOH) Anthraquinone Pol ycyclic aromatic quinones PFluorenone (PAQ) 154-1 59 2-H ydroxyanthracene Hydroxy polycyclic aromatic hydro- carbons (HPAH) 160 2-Carboxyan thracene Carboxy polycyclic aroma tic hydro- wcooHcarbons (CXPAH) 56 lo' G.Grimmer and H. Bohnke, J. Assoc. 08.Anal. Chem., 1975, 58, 725. lobK. Fretheim, J. Agric. Food Chem., 1976, 24, 976. loo J. S. Warner, Anal. Chem., 1976,48, 578. lo' R. Hamm, Pure Appl. Chem., 1977, 49, 1655. lo* M.-T.Lo and E. Sandi, Residue Rev., 1978, 69, 35. loS R. L. Stedman, Chem. Rev., 1968, 68, 153. 110 D. Hoffmann and G. Rathkamp, Anal. Chem., 1972,44, 899. Bartle, Lee, and Wise ll1 R. F. Severson, M. E. Snook, R. F. Arrendale, and 0.T.Chortyk, Anal. Chem., 1976,48, 1866. I1¶ R. F. Severson, M. E. Snook, H. C. Higman, 0.T. Chortyk, and F.J. Akin, in ‘Carcino- genesis -A Comprehensive Survey : Polynuclear Aromatic Hydrocarbons’, ed. R. I. Freudenthal and P. W. Jones, Raven Press, New York, 1976, Vol. 1, p. 253. n3M. L. Lee, M. Novotny, and K. D. Bartle, Anal. Chem., 1976, 48, 405. 111 M. E. Snook, R. F. Severson, H. C. Higman, R. F. Arrendale, and 0.T. Chortyk, Beitr. Tabakforsch., 1976, 8, 250. 116 M. E.Snook, R.F. Severson, R. F. Arrendale, H. C. Higman, and 0. T. Chortyk, Beitr. Tabakforsch., 1977, 9, 79. 11( A. Dipple, in ‘Chemical Carcinogens’, ed. C. E. Searle, American Chemical Society, Washington, D.C., ACS Monograph, Vol. 173, 1976, p. 258. 117 E.Sawicki, S.P.McPherson, T. W. Stanley, J. Meeker, and W. C. Elbert,Znt. J. Air Water Pollution, 1965,9,515. 118 E.Sawicki, J. E. Meeker, and M. J. Morgan, Int. J, Air Water Pollution, 1965, 9, 291. E. Sawicki, J. E. Meeker, and M. J. Morgan, Arch. Environ. Health, 1965, 11, 773. G. Alberini, V. Cantuti, and G. P. Cartoni, ‘Gas Chromatography’, Int. Symp., Anal. Instrum. Div., Instrum. SOC. Amer., 1966, 6, 258. E. Sawicki, Arch. Environ. Health, 1967, 14, 46. K. Rothwell and J. K. Whitehead, Chem. Znd.(London), 1969, 1628. la*D. Hoffmann, G. Rathkamp, and S. Nesnow, Anal. Chem., 1969,41, 1257. u4L. R. Snyder, Anal. Chem., 1969, 41, 314. D. Brocco, A. Cimmino, and M. Possanzini, J. Chromatogr., 1973,84, 371. R.W. Frei, K. Beall, and J. Cassidy, Mikrochim.Acta, 1974, 12, 859. la’ J. F.McKay, T. E. Cogswell, J. H. Weber, and D. R. Latham, Fuel, 1975, 54, 50. la8 J. F. McKay, J. H. Weber, and D. R. Latham, Anal. Chem., 1976, 48, 891. lagH. J. Klimisch and A. Beiss, J. Chromatogr., 1976, 128, 117. lso M. Dong, D. Locke, and D. Hoffmann, Environ. Sci. Technol., 1977, 11, 612. lS1 J. E.Schiller, Anal. Chem., 1977, 49, 2292. M. Blumer, T. Dorsey, and J. Sass, Science, 1977, 195, 283. lS3 I. Schmeltz and D. Hoffmann, Chem. Rev., 1977, 77, 295. M. Dong, 1. Schmeltz, E. LaVoie, and D. Hoffmann, in ref. 63, p. 97. IS6 M. E. Snook, in ref. 63, p. 203. lSe M. E.Snook, R. F. Arrendale, H. C. Higman, and 0.T. Chortyk, Anal. Chem., 1978,50, 88. la’ S. G. Wakeham, Environ. Sci. Technol., 1979, 13, 1118.118 M.Novotny, R. Kump, F. Merli, and L. J. Todd, Anal. Chem., 1980,52,401. B. W. Wilson and R. A. Pelroy, Fuel, in the press. 140 W. W. Paudler and M. Chaplen, Fuel, 1979, 58, 775. 141 B. W. Wilson, R. A. Pelroy, and J. T. Cresto, Mutation Res., 1980, 79, 193. 14’ J. Jager, J. Chromatogr.,1978,152,575. J. N. Pitts, K. A. Van Cauwenberghe, D. Grosjean, J. P. Schmid, D. R. Fitz, W. L. Belser, G. B. Knudson, and P. M. Hund, Science, 1978, 202, 515. 144 J. N. Pitts, Phil. Trans. R. SOC.Lond., Ser. A., 1979, 290, 551. 146 C. Y. Wang, Chemosphere, 1980, 9, 83. 14# G. R. Dubay and R. A. Hites, Environ. Sci. Technol., 1978, 12, 965. S. Krishnan, D. A. Kaden, W. G. Thilly, and R. A. Hites, Environ. Sci. Technol., 1979, 13, 1532. H. V. Drushel and A.L. Sommers, Anal. Chem., 1967, 39, 1819. E. R. Adlard, L. F. Creaser, and P. H. D. Matthews, Anal. Chem., 1972, 44, 64. IS0 M. L. Lee and R. A. Hites, Anal. Chem., 1976,48, 1890. B. Wenzel and R. L.Aiken, J. Chromatogr. Sci., 1979, 17, 503. M. L. Lee, C. Willey, R. N. Castle, and C. M. White, in ‘Polynuclear Aromatic Hydro- carbons: Chemistry and Biological Effects’, ed. A. Bj~rrseth and A. J. Dennis, Battelle Press, Columbus, Ohio, 1980, p. 59. 15s C. Willey, M. Iwao, R.N. Castle, and M. L. Lee, Anal. Chem., 1981, 53, 400. IS4 S. S. Epstein, N. Mantel, and T. W. Stanley, Environ. Sci. Technol., 1968, 2, 132. T. W. Stanley, M. J. Morgan, and J. E. Meeker, Environ. Sci. Technol., 1969, 3, 1 198. A. Gold, Anal. Chem., 1975, 47, 1469. lC7 R.C. Pierce and M. Katz, Environ. Sci. Technol., 1976, 10, 45. lS8 K. Winters, J. C. Batterton, and C. Van Baalen, Environ. Sci. Technol., 1977, 11, 270. I. Schmeltz, J. Tosk, G. Jacobs, and D. Hoffmann, Anal. Chem., 1977, 49, 1924. 117 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds complex mixtures that vary greatly in the relative concentrations of individual components. Table 1 gives structures of representative compounds in the main classes of PAC that have been found in environmental samples in addition to the PAH. Compound structure, position of substitution, and presence of hetero- atoms in the ring have all been found to affect the carcinogenic and/or mutagenic properties of these aromatic compounds, and therefore exact structural eluci- dation of individual constituents of mixtures is necessary in order to deter- mine the full carcinogenic potential of the sample.As is evident from Table 1,PAC mixtures can be extremely complex, since they contain numerous isomeric compounds. The complexity increases for samples that contain alkylated, multi-substituted, and partially hydrogenated compounds in addition to the parent PAC. The success of the chemical analysis, whether it be quantitative or qualitative (of even a single-mixture component), hinges on the resolving power of the analytical method used. In the following sections an attempt is made to review the most important analytical techniques for the analysis of PAC. It is virtually impossible to cite all papers that have been published on this subject.A bibliography of over lo00 references on the occurrence and analysis of PAC has recently been prepared,161 several monographs covering various aspects of environmental PAC are avail- able,162-165 and the proceedings of the first four International Symposia on Polynuclear Aromatic Hydrocarbons have been publi~hed.16~-16~ In this review, emphasis will be placed on more recent studies involving advanced analytical techniques. 2 Sample Preparation PAH are soluble in many organic solvents and there have been several recom- mendation~l7~-~~~for the best solvent for Soxhlet extraction of solid environ- mental samples, especially air particulates. Among these, acetone, benzene, and looW.S. Scholtzhauer, D. B. Walters, M.E. Snook, and H. C. Higman, J. Agric. Food Chem., 1978, 26, 1277. lol A. Martin and M.Blumer, ‘Polycyclic Aromatic Hydrocarbons: Occurrence and Analysis -A Partial Bibliography’, Woods Hole Oceanographic Institution, Woods Hole, MA, Rept. WH01-75-22, 1975. Is* ‘Polycyclic Hydrocarbons and Cancer’, ed. H. V.Gelboin and P. 0. P. Ts’o, Academic Press, New York, 1978, Vols. 1 and 2. Io3 J. M. Neff, ‘Polynuclear Aromatic Hydrocarbons in the Aquatic Environment’, Applied Science, London, 1979. 164 ‘Hydrocarbons and Halogenated Hydrocarbons in the Aquatic Environment’, ed. B. K. Afghan and D. Mackay, Plenum Press, New York, 1980. lo6M. L. Lee, M. Novotny, and K. D. Bartle, ‘Analytical Chemistry of Polycyclic Aromatic Compounds’, Academic Press, New York, 1981.160 ‘Carcinogenesis -A Comprehensive Survey: Polynuclear Aromatic Hydrocarbons’, ed. R. Freudenthal and P. W. Jones, Raven Press, New York, 1976, Vol. 1. Is’ ‘Carcinogenesis -A Comprehensive Survey : Polynuclear Aromatic Hydrocarbons’, ed. P. W. Jones and R. Freudenthal, Raven Press, New York, 1978, Vol. 3. loo‘Polynuclear Aromatic Hydrocarbons’, ed. P.W. Jones and P. Leber, Ann Arbor Science, Ann Arbor, 1979. loD ‘Polynuclear Aromatic Hydrocarbons :Chemistry and Biological Effects’, ed. A. Bjarseth and A. J. Dennis, Battelle Press, Columbus, Ohio, 1980. lT0F. S. C. Lee, T. J. Prater, and F. Ferris, in ref. 168, p. 83. 118 Bartle, Lee, and Wise cyclohexane are all said to be nearly 100% efficient in the extraction of benzo[a]pyrene.171 Extraction curves for five PAH from glass-fibre filters showed that extraction with benzene was complete after 6 h.51 Yields of major PAH were found to be >99 % after only 2 h or 20 Soxhlet cycles with benzene in another study.173 However, since cyclohexane extracts fewer uncharacterized materials than does ben~ene,l~~J~~ and is less hazardous, its use has been endorsed by official bodies such as the World Health Organisation (WHO).176 Methanol has also been recommended as an efficient solvent for the extraction of PAH,177 but has also been found to extract more inorganic material178 and organic materia158J79 other than PAH. Ultrasonic vibration at room temperature is an alternative to Soxhlet extrac- tion.Benzo[a]pyrene and 'total PAH' were completely extracted after only 30 min.172 The procedure has been refined by sonicating in the presence of silica powder to adsorb polar materiaLl8O Recovery of PAH between 95 and 98 % was concluded, and both extraction efficiency and reproducibility were superior when compared with Soxhlet extraction, which may result in losses of volatile com- pounds;18O this method has now been adopted by the National Institute of Occupational Safety and Health.la1 Extraction of PAC may be difficult from materials on which they are strongly adsorbed, such as carbon blackl82 or fly ash;l83 low ((30%) recoveries of [14C]benzo[a]pyrene from spiked fly ash were found even by ultrasonic extraction.l83 Higher recoveries were found for two- and three-ring PAH, but larger PAH are likely to be incompletely extracted.183 Many solvents have been suggested for the liquid-liquid partition of PAC from water (e.g., C5-cB alkanes, chloroform, carbon tetrachloride, and dichloro- methane);81J84 while benzene is recommended by WHO, this may soon be replaced by cyclohexane.l85 An alternative to liquid extraction is preconcentra- 171 T.W. Stanley, J. E. Meeker, and M. J. Morgan, Environ. Sci. Technol., 1967, 1, 927. 17* G. Chalot, M. Castegnaro, J. L. Roche, R. Fontagnons, and P. Obatan, Anal. Chim. Ada, 1971, 53, 259. 178 G. Broddin, L. Van Vaeck, and K. Van Cauwenberghe, Afmos. Environ., 1977, 11, 1061. l" B. S. Das and G. H. Thomas, Anal. Chem., 1978, 50, 967.176 M. Dong, D. C. Locke, and E. Ferrand, Anal. Chem., 1976, 48, 368. WHO -Working Group on Air Standardization of Sampling and Analytical Procedure for Estimation of Polynuclear Aromatic Hydrocarbons in the Environment, Geneva, December 1969. 17? D.Grosjean, Anal. Chem., 1975, 47, 797. Io8 H. H. Hill, jun., K. W. Chan, and R. W. Karasek, J. Chromatogr., 1977, 131, 245. 17s W. Cautreels and K. Van Cauwenberghe, J. Chromatogr., 1977, 131, 253. lSo C. Golden and E. Sawicki, Int. J. Environ. Anal. Chem., 1975, 4, 9. lS1 National Institute of Occupational Safety and Health, in 'NIOSH Manual of AnalyticalMethods', HEW Publication No. (NIOSH 77-1 57), 1977. la' W. L. Fitch and D. H. Smith, Environ. Sci. Technol., 1979, 13, 341. lBs W.H. Griest, J. E. Caton, M. R. Guerin, L. B. Yeatts, jun., and C. E. Higgins, in ref. 169, p. 819; Anal. Chem., 1980,52, 199. le4 R. A. Hites, Adv. Chromatogr., 1977, 15, 69. 185 M. Fielding and B. Crathorne, Proc. Anal. Div. Chem. SOC.,1978, 15, 155. 119 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds tion on plugs of active carbon,l84 Amberlite XAD resin,l84J86 Tenax GC,l87 ion-exchange resins,l88 and open-pore polyurethane.188Jag Removal of PAH is then accomplished by stripping with a solvent, or by thermal desorption. Pre- concentration on HPLC columns is an attractive alternative1s7JB0 (see below). Benzene/methanol mixtures are preferred for the extraction of PAC from soils and sediments;85 for oils and fats that are themselves soluble in a non-polar solvent, cyclohexane is adequate.104 For insoluble fats, and protein-rich foods and biotic samples such as meat and fish, extraction with saturated hydrocarbons is incomplete unless prior saponification is carried out, generally with alcoholic KOH.104.191 Extracts of environmental samples inevitably contain other materials in addition to PAC, and concentration and clean-up procedures usually must be applied to yield a PAC fraction sufficiently free from extraneous materials for analysis.A typical scheme is outlined in Figure 1, and is generally applicable, e.g., to extracts of air-pollution filters,48955 cigarette-smoke condensates,l13 and synthetic fuels.l53 The organic material is dissolved in dichloromethane and washed successively with dilute alkali and acid to remove, respectively, acidic (e.g.phenols) and basic constituents. These steps may be omitted for air- particulate samples that contain predominantly neutral materials. Hydrophilic and polar compounds are next removed by partitioning between cyclohexane and either methanol/waterl92 or acetone/water.45 Further enrichment of the PAH is then possible by extraction from cylohexane into nitromethane,lg2 dimethyl- sulphoxide (DMS0),193 or dimethylformamide (DMF).45J04 Five partitions with nitromethane are necessary to achieve >99 % extraction because many of the partition coefficients are less than 2. The nitromethane is then evaporated to concentrate the PAH. DMSO and DMF have often been preferred to nitromethane because the PAH may be recovered from these by dilution with water and back extraction into an alkane solvent, which may be evaporated at a lower temperature.High recoveries of PAH have been reported using DMSO,l94 and a simple scheme based on three extractions from n-pentane into DMSO followed by back extractions from DMSO/water into n-pentane has been shown to separate PAH from alkanes, alcohols, acids, and phenols.195 Unfortunately, partition coefficients between DMSO and alkanes are less favourable for methyl derivatives of PAH,lg3Jg5 and losses of such compounds may be significant. In a recent and detailed study of the la' G. A. Junk, J. J. Richard, M. D. Grieser, D. Witiak, J. L. Witiak, M.D. Arguello, R. Vick, H. J. Svec, J. S. Fritz, and G. V. Calder, J. Chromatogr., 1974, 99, 745. W. E. May, S. N. Chesler, S. P. Cram, B. H. Gump, H. S. Hertz, D. P. Enagonio, and S. M. Dyszel, J. Chromatogr. Sci., 1975, 13, 535. 18a J. D. Navratil, R. E. Sievers, and H. F. Walton, Anal. Chem., 1977, 49, 2260. lagD. K. Dasu and J. Saxena, Environ. Sci. Technol., 1978, 12, 791. lSo K. Ogan, E. Katz, and W. Slavin, J. Chromatogr. Sci., 1978, 16, 517. lgl J. W. Howard, R.T. Teague, R. H. White, and B. E. Fry, jun., J. Assoc. Off.Awl. Chem., 1966,49, 595. lg* D. Hoffmann and E. Wynder, Anal. Chem., 1960, 32,295. lD3J. W. Howard and E. 0.Haenni, J. Assoc. Of.Agric. Chem., 1963, 46, 933. M. A. Acheson, R. M. Harrison, R. Perry, and R. A. Wellings, Water Res., 1976,10,207.lg6 D. F. S. Natusch and B. A. Tomkins, Anal. Chem., 1978,50, 1429. 120 1. Dissolved in 250 ml CH2ClZ ;------I----,,+CH2C12 layer Washed with x 100 ml CH2C12 , NaOH iayer , Washed with 2 x 100 ml H20 NaOH Layer CH2C12 Layer , H20 l,ayer , ,;-;--------+CH2C12L, , Washed with 900 ml cyclohexane Washed with 250 ml 0.W HCl H20 layer Cyclohexane' Layer 1. Evaporated to dryness ,2. Dissolved in CHZC12 HCl L,ayer 4H2Cl 2Layer ,;-------Washed with 300 ml CH2ClZ Washed with 2 x 100 ml H20 HC1 Layer CH2C12Layer ,I-H20 Layer ;---~ ---~CH2Clp Layer ~ 1 Washed with'5 x 100 ml CH2C12 1. Evaporated to dryness I Layer 2. Added 200 ml cyclohexaneH20 Layer CH2C12 1' 3. Washed with 3 x 200 ml 4:l CH30H:HZ0 I I CH 0H:H 0 Layer +Cyclohexajle Layer , '1 m1 I'ashe~y~~~$a~~oo 1.Evaporated to 200 ml 2. Washed with 6 x 100 ml OMS0 CH30H:H20 Layer Cyclohexk Layer r 1 Cyclohcxanc Layer OMSO LbyerI 1. Diluted 2:l H O.OMSO 2. Back-extractei with 2 x 100 ml: a. Cvclohexane H~O:DMSO Layer Combined Cyclohexane, Hexane. and CH C1 Layers2f 1. Evaporated to dryness 2. Adsorbed to 29 silicic acid Introduced to two 2g Silicic Acid Columns I Collected 400 ml hexane AROPdTIC FRACTION Figure 1Fractionation scheme for concentration of PAH from environmental samples Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds general extraction characteristics of PAH based on the variation of partition coefficients for a variety of solvents, it has been shown how the selectivity of extraction conditions may be tailored to the separation problem.196 Column chromatography is used in two ways in the preparation of PAC samples for analysis. First, clean-up on silica gel,197 or latterly silicic acid or a lipophilic gel such as Sephadex LH-20,83 can be used to remove any residual compounds that remain after the various partition steps.Secondly, the PAC fraction may be separated further into sub-fractions containing compounds of, for example, similar molecular weight. The latter is more readily achieved using alumina rather than silica, but other adsorbents such as FlorisiP8 and cellulose acetate199 have also been recommended. Classical adsorbents suffer the disadvantages of high adsorptivity, which can result in losses and in peak tailing with consequent less sharp fractionation, and in poor reproducibility because of adsorption of traces of water.More recently the trend has been to use lipophilic gels, e.g., Sephadex LH-20 or the Bio-Beads series in adsorption/partition mode. With benzene as eluent, Bio- Beads SX-12 (porous styrene divinylbenzene co-polymer) yields fractions containing compounds in sequence of increasing ring number111 and also separates multi-alkylated PAH from parent plus mono-alkylated PAH.200 Elution of PAH with isopropanol from Sephadex LH-20 is also in sequence of increasing ring number.55 9 48p Several approaches have been made to the isolation of the nitrogen heterocyclic fraction of PAC, and its separation into further fractions according to the nitrogen chemical type.Early methods depended on extraction with aqueous acid, but subsequently it was shown that neutral nitrogen compounds are important consituents of, for example, tobacco-smoke condensate.136 Column chromatography on alumina of coal liquids yields a total-nitrogen fraction.201 A fraction containing indoles and carbazoles, and another in which aza-arenes, aromatic amines, and nitriles were concentrated, were obtained by chromato- graphy on silicic acid of tobacco-smoke ~0ndensate.l~~ Purification of these materials was achieved by gel filtration on Bio-Beads SX-12. Chromatography on Sephadex LH-20 has also formed part of other schemes for the sepaiation of nitrogen heter0cyc1es.l~~ Anion-exchange chromatography can be used to remove acidic compounds before separation of the nitrogen fra~tion.~28~~02~~0~ A cation-exchange resin such as Amberlyst-15 then retains bases, and may yield fractions containing different nitrogen type~.~~*Jo3 Integrated schemes employing lS6 W.K. Robbins, in ref. 169, p. 841. lS7 A. A. Rosen and F. M. Middleton, Anal. Chem., 1955, 27, 790. lS8 D. Hoffman and E. L. Wynder, in ‘Identification and Measurement of Environmental Pollutants’, ed. B. Westley, Symposium, Ottawa, Canada, June 1971, p. 9. lSs W. H. Griest, H. Kubota, and M. R. Guerin, Anal. Lett., 1975, 8, 949. aoo M. E. Snook, Anal. Chim.Ada, 1976, 81, 423. aol J. E. Schiller and D.R. Mathiason, Anal. Chem., 1977, 49, 1225 and 2292. L. R. Snyder and B. E. Buell, Anal. Chem., 1968, 40,1295. *OS D. M. Jewel], J. H. Weber, J. W. Bunger, H. Plancher, and D. R. Latham, Anal. Chem., 1972,44, 1391. Bade, Lee, and Wise combinations of ion-exchange and column chromatography have been described.202 The application to coal-derived products of a procedure for the isolation of a sulphur compound fraction from petroleum148 involves oxidation of the PAC concentrate with H202 in acetic a~id.~~~J~~ Sulphur heterocycles are oxidized to sulphones, which can be separated from PAH, etc., by chromatography on silicic acid. Sulphones are then reduced back t3 sulphides with LiAlH4. 3 ChromatographicMethods A. High-performance Liqdid Chromatography.-Since its inception in the early 1970's, high-performance liquid chromatography (HPLC) has been used for the separation of PAH.At present, HPLC does not approach the high separation efficiency of capillary gas chromatography (GC). However, HPLC does offer several advantages for the determination of PAH. First, HPLC offers a variety of stationary phases capable of providing unique selectivity for the separation of PAH isomers that are often difficult to separate by GC. Selectivity in HPLC is achieved because of interactions of the solute with both the stationary phase and the mobile phase rather than with only the stationary phase as in GC. Secondly, ultraviolet (u.v.) absorption and fluorescence spectroscopy provide extremely sensitive and, more important, selective detection for PAH in HPLC.Finally, HPLC provides a useful fractionation technique for the isolation of PAH for subsequent analysis by other chromatographic and spectroscopic techniques. Because of these characteristics, HPLC has been employed extensively for the determination of PAH in ~ater,187JQ~l~~~ marine biota,207 sediment~,~~5*~~~ air particulates,208-211 automobile exha~st,~l~*~l~ and petroleum and related fuels.214-217 Chemically-bonded Stationary Phases for PAH. Reverse-phase HPLC on chemi- cally-bonded CIS (octadecyl) stationary phases is by far the most popular liquid *04 F. Eisenbeiss, H. Hein, R. Joester, and G. Naundorf, Chem.-Tech.,1977,6, 227 (Englishtranslation in Chromatogr.Newslett., 1978, 6, 8). S. A. Wise, S. N. Chesler, H. S. Hertz, L. R. Hilpert, and W. E. May, Anal. Chem., 1977, 49,2306. J. J. Black, P. P. Dymerski, and W. F. Zapisek, Bull. Environ. Contam. Toxicol., 1979, 22, 278. *07 B. Dunn, Chromatogr. Newslett., 1980, 8, 10. *O* S. A. Wise, W. J. Bonnett, and W. E. May, in ref. 169, p. 791. *OD E. P. Lankmayr and K. Muller, J. Chromatogr., 1979, 170, 139. *loA. M. Krstulovic, D. M. Rosie, and P. R. Brown, Anal. Chem., 1976,48, 1383. *11 D. Fechner and B. Seifert, Fresenius' Z. Anal. Chem., 1978, 292, 199. *l*T. Nielsen, J. Chromatogr., 1979, 170, 147. *la P. Roumeliotis, K. K. Unger, G. Tesarek, and E. Muhlberg, Fresenius' Z. Anal. Chem., 1979,298,241. *14 J. P. Durand and N. Petroff, J.Chromatogr., 1980, 190, 85. *15 H. S. Hertz, J. M. Brown, S. N. Chesler, F. R. Guenther, L. R. Hilpert, W. E. May, R. M. Parris, and S. A. Wise, Anal. Chem., 1980, 52, 1650. ¶loJ. M. Brown, S. A. Wise, and W. E. May, J. Environ. Sci. Health, 1980, AIS, 613. *17 W. A. Dark, W. H. McFadden, and D. L. Bradford, J. Chromatogr. Sci., 1977, 15, 454. Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds chromatographic mode for the separation of PAH. Reverse-phase HPLC provides unique selectivity for the separation of PAH isomers and particularly alkyl-substituted PAH. In addition, the compatibility of reverse-phase HPLC with gradient-elution techniques and the rapid equilibration of these columns to new mobile phase compositions make reverse-phase HPLC a convenient separation technique.The reverse-phase separations of some PAH on 10pm Cl8 columns are shown in Figure 2. A g C: a L Sc t: bLi;Figure 2 Comparison of reverse-phase HPLC separation of PAH on two diferent C,, columns; A, Zorbax ODs, monomeric, B, Vydac 201TP, polymeric. Chromatographic conditions: mobile phase linear gradient 80-100 % acetonitrile in water at 1% min-l and 1cms min-l; U.V. detection at 254 nm Several recent reportszo5,208,212,218~2l9have included retention data for some PAH on specific CLa columns. A comparison of these retention data and recent studies by Wise et a1.,208 Ogan and Katz,220 and Colmsjo and MacDonaldZ2l indicate that c18columns from various manufacturers provide not only different separation efficiencies, but different selectivities and retention characteristics for PAH.The different selectivities for two c18 columns, one with a polymeric and the other a monomeric c18 layer, are illustrated in Figure 2. (Compare the separation of benz[a]anthracene and chrysene and the elution order reversals of *la G. P. Blumer and M. Zander, Z. Anal. Chem., 1977, 288, 277. *lo J. Chmielowiec and A. E. George, Anal. Chem., 1980, 52, 1154. *loK. Ogan and E. Katz, J. Chromutogr., 1980, 188, 115. **l A. L. Colmsjo and J. C. MacDonald, Chromafogruphiu,1980, 13, 350. Bartle, Lee, and Wise benzo[ghi]perylene, indeno[l,2,3-cd]pyrene, and benzo[b]chrysene on the polymeric vs. the monomeric columns.) The retention characteristics of over 90 PAH on a monomeric and a polymeric CIS column have been reported.208 Polyphenyl arenes exhibit significantly different retention characteristics on monomeric vs.polymeric CIS stationary phases. These compounds are generally retained longer, relative to the fused ring PAH, on the monomeric than on the polymeric phases.208e222 Some of the selectivity differences observed on various CIS columns can be attributed to the surface coverage of the cl8 layer.223 Selectivity differences for columns from the same manufacturer, but from different production lots, have also been des~ribed.~~Oe~~* The mechanism of retention of PAH on chemically bonded CIS phases has not been established and is a topic of much discussion and research.225 Sleight,226 and more recently Blumer and Zander,218 correlated the retention of PAH on c18 phases with the number of carbon atoms in the solute. Sleight226 and Lo~ke~~’ suggested that the retention was dependent to some degree on their solubilities in the polar mobile phase.Recently, Wise et aL2z2described a relationship between the shape of PAH solutes, particularly the length-to-breadth ratio, and the reverse-phase retention. In this study the length-to-breadth ratios for 84 unsub-stituted and methyl-substituted PAH were compared with the HPLC retention characteristics. In nearly all cases, this ratio was successful in predicting the elution order of isomeric PAH, i.e. the retention increases with increasing length-to-breadth ratio.These ratios were found to be particularly useful in predicting the unique selectivity of reverse-phase HPLC for methyl-substituted PAH. The linear correlation for retention data vs. length-to-breadth ratios for 23 methyl-substituted isomeric benzo[c]phenanthrenes, benz[a]anthracenes, and chrysenes is shown in Figure 3. Parameters such as composition of mobile phase and temperature can be used to achieve changes in the elution order of PAH in reverse-phase HPLC. Katz and Ogan228 studied the effect of the mobile-phase composition on the selectivity factors for PAH on different CIS columns. The use of temperature to achieve changes in selectivity was demonstrated by Chmielowiec and Sa~atzky.~~~ Polar chemically bonded stationary phases, used in conjunction with nonpolar mobile phases (normal phase HPLC), have also been employed for the separation of PAH.Several polar phases are available containing such functional groups as amine (NH2), diamine [R(NH2)2], nitrile (CN), diol [R(OH)2], ether (ROR), and nitrophenyl (NO$ bonded to the silica particles. In the normal-phase mode on these polar columns, the PAH separations achieved are similar to those obtained on the classical adsorbents such as silica and alumina. S. A. Wise, W. J. Bonnett, F. R. Guenther, and W. E. May, J. Chromatogr. Sci., submitted for publication. *a3 S. A. Wise, unpublished data. aa4 K. Ogan and E. Katz, J. Liq. Chromatogr., 1980, 3, 1151. H. Colin and G. Guiochon, J. Chromatogr., 1977, 141, 289.**‘ R.B. Sleight, J. Chromatogr., 1973, 83, 31. D. C. Locke, J. Chromatogr., 1974, 12, 433. aa8 E. Katz and K. Ogan, Chromatogr. Newslett., 1980, 8, 20. J. Chmielowiec and H. Sawatzky, 1.Chromatogr. Sci., 1979, 17, 245. 125 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds 4.4 .' X I4.2-a 0 J 4.0 -8 3.8 -Lengt h-to- breadt h Ratio Figure 3 Linear correlation of reverse-phase LC retention (log I) vs. length-to-breadthratio for methylbenzo[c]phenanthrenes(m), methylbenz[a]anthracenes( x ), and methyl- chrysenes (@) (From ref. 222) Wise et aL208have described the retention characteristics of over 90 unsub-stituted and alkyl-substituted PAH on a polar amine phase. On this column the retention increases with increasing number of condensed aromatic rings (or number of aromatic carbon atoms).In contrast to reverse-phase HPLC, the presence of an alkyl group on the PAH has only a slight effect on retention on the amine phase. This characteristic is often advantageous in pre-fractionation techniques for the analysis of complex mixtures. The use of normal-phase HPLC on polar-bonded phases also eliminates one of the major difficulties encountered with classical adsorbents, i.e., non-reproducible retention owing to small changes in the moisture content of the eluent. Finally, the use of volatile nonpolar mobile phases facilitates the concentration of collected fractions by evaporation. Blumer and Zander218 studied the retention of a number of PAH, biaryls, and PANH on silica modified with polar nitrophenyl groups.A linear relationship between log retention and carbon number was observed for these three groups of compounds. In addition, a class separation of the PAH and PANH could be achieved on this phase due to the more polar nature of the N-containing com- pounds. Blumer et ul.230 separated more than 100 components of coal tar using this polar stationary phase. Lankmayr and Miiller209 compared nitrophenyl, amine, and Cl8 columns for the separation of 17 PAH commonly found in dust samples, and found that the nitrophenyl phase provided the best separation for those PAH evaluated. An extensive characterization and discussion of the s30 G.-P. Blumer, R. Thorns,and M. Zander, Erdoel, Kohle, Erdgas,Petrochem.Brennst.-Chem., 1978,31, 197. Bartle, Lee, and Wise retention of PAH and alkyl-substituted PAH on the nitrophenyl phase has not yet appeared in the literature. Chmielowiec and George219 recently investigated the performance of several bonded phases (i.e. amine, nitrile, diol, ether, diamine, and quaternary ammonium) for normal-phase separations of 10 PAH and advocated the use of the diamine column. Several other polar-bonded phases have been described for PAH separations such as n itr ofluorenimine, p h t ha1 imidoprop y1,232 3-(2,4-dinitroanilino)propyl,233 and picramidopropyl.234 Spectroscopic Detection in HPLC. A major advantage of HPLC for the deter- mination of PAH is the availability of extremely sensitive and selective detectors.U.V.absorption and fluorescence detectors are ideally suited for the detection of PAH. U.V. and fluorescence detectors are generally used in series; the U.V. detector is universal for PAH, whereas the fluorescence detector provides high sensitivity and specificity. Several workers210~211~235 have described the use of variable-wavelength U.V. detection to achieve some degree of selectivity. Thoms and Zander236 used complete U.V. absorption spectra of PAH for their identification in HPLC effluents. Others210y237 have employed absorbance ratios at several wavelengths for qualitative analysis. Fluorescence detection nrovides selectivity for individual PAH and the possibility of identification crf specific compounds in complex mixtures.Several reports have described the application of both filter fluorimeters and spectro- fluorimeters for the determination of PAH in diesel exhaust,238 cigarette smoke,239 shale 0ip5 0iI,21~92~6and air particulates.211 Recently, Ogan et al.240 compared the use of a cut-off filter and a monochromator in the fluorescence detection of PAH in environmental samples. The cut-off filter provided an improvement of 3-5 times in sensitivity over the monochromator; however, these gains were often offset by spectral interferences from other compounds in the sample. Christensen and May241 compared the sensitivities of several filter fluorimeters, a spectro- fluorimeter, and U.V. detectors for PAH determinations. Selective fluorescence quenching of certain PAH in the presence of nitro- methane has been investigated as a selective detection system for HPLC.242p243 Sawicki et al.244and later Dreeskamp et al.245reported that in the presence of 931 C.H. Lochmuller, R. R. Rydall, and C. W. Amoss, J. Chromatogr., 1979, 178, 298. D. C. Hunt, P. J. Wild, and N. T. Crosby, J. Chromurogr., 1977, 130, 320. pa3 L. Nondek, M. Minarik, and J. MBlek, J. Chromatogr., 1979, 178, 427. a34 S. A. Martin, W. J. Lough, and D. G. Bryan, HRC & CC, 1980, 3, 33. g31 R. D. Smillie, D. T. Wang, and 0. Meresz, J. Environ. Sci. Health, 1978, A13, 47. 436 R. Thoms and M. Zander, Fresenius’ 2. Anal. Chem., 1976, 282, 443. 937 R. K. Sorrel1 and R. Reding, J. Chromatogr., 1979, 185, 655. g36 D. E.Seizinger, Trends in Fluorescence, 1978, 1, 9. g39 N. M. Sinclair and B. E. Frost, Analyst (London), 1978, 103, 1199. *40 K. Ogan, E. Katz, and T. J. Porro, J. Chromatogr. Sci., 1979, 17, 597. g41 R. G. Christensen and W. E. May, J. Liq. Chromatogr., 1978, 1, 385. a41 G.-P. Blumer and M. Zander, Fresenius’ 2. Anal. Chem., 1979, 296, 409. a43 P. L. Konash, W. E. May, and S. A. Wise, submitted for publication in J. Liq. Chromatogr. E. Sawicki, T. W. Stanley, and W. C. Elbert, Talanta, 1964, 11, 1433. 245 H. Dreeskamp, E. Koch, and M. Zander, 2.Narurforsh., Teil A, 1975, 30, 1311. 127 5 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds nitromethane, the fluorescence emission of six-membered ring PAH was quenched to a much greater degree than was that of those containing a fluoranthenic structure. Blumer and Zander242 briefly described the application of this selective quenching as an HPLC detector.Recently, Konash et al.2*3 investigated further the potential of this phenomenon for the selective detection of the benzofluor- anthene isomers in the presence of the benzopyrene isomers and perylene. As illustrated in Figure 4, the addition of 0.5% nitromethane to the mobile phase significantly quenches all the non-fluoranthenic PAH (except a small percentage of anthracene) resulting in a selective chromatogram of only fluoranthenic PAH. This technique was found to be particularly useful using an inexpensive broad- Anthracene 1 AAnthracene Phenanthrene Fluarene Fluoranthene 4 8 12 16 20 24 Ti= (min.) Figure 4 Reverse-phase HPLC separation of coal-tar extract with: A, detection at 254 nm;B, fluorescence detection with filter fluorimeter (hez = 250-380 nm and hem = >380 nm); C, fluorescence detection with 0.5 % nitromethane in the mobile phase.Chromato- graphic conditions: Vydac 201TP column; mobile phase linear gradient 50-1 00% acetonitrile in water at 2 %min-l and 2 cmamin-' (From ref. 243) Bartle, Lee, and Wise band filter fluorimeter to achieve selectivity for fluoranthenic PAH as a group. This procedure has been used for quantifying benzo[k]fluoranthene in a shale oil sample.243 A recent development in spectroscopic detection of PAH in chromatographic effluents is the use of multi-channel rapid scanning spectrometers.These detectors permit the recording of fluorescence spectra ‘on-the-fly’, thereby eliminating stop flow or valving to trap the chromatographic peak in the flow cell. Jadamec et aZ.246described the use of such a system for the characterization of petroleum fractions in the determination of the source of oil spills. More recently, Warner and ~o-workers2~7 reported a two-dimensional ‘video fluorimeter’ as an LC detector. They recorded the fluorescence intensity as a function of emitting and exciting wavelengths every 25 s to produce an emission-excitation matrix for PAH standards as they eluted from the LC. These systems, when combined with computers and floppy disks for data storage, will allow the rapid acquisition of an enormous amount of data during one chromatographic run.These data can then be evaluated to identify and quantify the PAH in thecomplex chromatograms. The potential of the combination of liquid chromatography-mass spectrometry (LC-MS) for the separation and identification of organic compounds has generated considerable research in the past few years. The inherent problems of introducing a liquid stream (-1 cm3 min-1) into the high-vacuum system of the mass spectrometer have led to several approaches. Arpino and G~iochon~~~ and Games249 recently reviewed the various LC-MS interfaces and described the construction, operating principles, and performance of each approach. Another review of the LC-MS techniques by McFaddenz50 emphasized the current applications of this technique.At present, two LC-MS approaches are commercially available, i.e. direct liquid injection and depositing the effluent onto a moving belt from which the mobile phase is evaporated before introduction into the mass spectrometer. Both of these approaches transfer only a portion of the LC solute into the MS. Christensen et ~1.25~ recently described a new LC-MS interface that combines several of the advantages of both the direct liquid-injection and moving-belt techniques. The LC effluent is concentrated by evaporation as it flows down an electrically heated wire. The concentrated effluent then flows through a small needle valve and is sprayed into the MS ion source.The application of this approach to the analysis of PAH standards resulted in a 20-fold enrichment of the analyte in the mobile phase before it entered the MS. Dark et a1.217 using the moving-belt method demonstrated the LC fractionation of coal liquid samples with structural characterization by LC-MS. From the LC retention data on two different columns and the molecular weight data, they J. R. Jadamec, W. A. Saner, and Y.Talmi, Anal. Chem., 1977,49, 316. u7D. C.Shelly, W. A. Ilger, M. P. Fogarty, and I. M. Warner, Altex Chromatogram, 1979,3,4. 14( P.J. Arpino and G. Guiochon, Anal. Chem., 1979,51, 682A. 14* D.E. Games, Proc. Anal. Div. Chem. SOC.,1980, 17,110. W. H. McFadden, J. Chromatogr. Sci., 1980,18,97. *‘I R.G. Christensen, H. S.Hertz, S.Meiselman, and E. White V, Anal. Chem., 1981,53, 171. 129 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds identified a number of aromatic compounds (molecular weight up to -250) in the coal liquid. Other applications of LC-MS involving PAH have been rep~rted.~~~,~~~In the future, LC-MS will find greater application in the deter- mination of higher molecular weight PAH that are not amenable to GC analysis. Multi-dimensionalHPLC Techniques.As described earlier, both reverse-phase and normal-phase HPLC are useful for the separation of PAH. However, even the most selective and efficient HPLC columns available are unsuccessful in resolving all the PAH in complex mixtures. The determination of individual PAH in these mixtures generally requires the use of several chromatographic modes [e.g., thin-layer chromatography (TLC), GC, and various LC modes] and/or selective detection for the determination of individual PAH.HPLC is extremely useful for both prefractionation/isolationtechniques and for analytical determinations. Traditionally in PAH analyses, sample preparation has consisted of separation on silica or alumina by column chromatography, TLC, or more recently HPLC. After isolation of the PAH fraction, GC or spectroscopic techniques have generally been employed for the final analytical measurement. However, multi- dimensional or sequential chromatographic analysis using two different HPLC modes has been found to be useful for the determination of PAH in complex mixtures.In their early work with a chemically bonded cl8 phase, Schmit et recognized the potential of reverse-phase LC separation of PAH after an initial separation step involving gel permeation chromatography for the analysis of automobile-exhaust condensate. More recently, Wise et aZ.2059208 have advocated the use of normal-phase HPLC on a polar chemically bonded amine phase for the separation of PAH based on the number of condensed aromatic rings, followed by separation of the various PAH isomers and/or alkyl-substituted PAH by reverse-phase HPLC. This sequential combination of normal- and reverse-phase HPLC has been employed for the quantitation of benzoralpyrene in some virgin, waste, and recycled oils,21s and for the quantitation of several PAH in shale In the sample of shale oil the amounts of pyrene, fluoranthene, benzo[e]pyrene, and benzo[a]pyrene determined using this technique were similar to the values obtained using either ‘classical’ extraction methods followed by GC-MS quantitation or direct sample injection GC-MS with single-ion monitoring for ~electivity.2~~ have also used similar sequential techniques with Dark et 01.~~~9~55 silica or an amine column followed by reverse-phase separation with mass- spectrometric detection for the characterization of coal liquids.Unique Applications of HPLC for PAH Determinations. The low levels of PAH normally found in water samples necessitates the use of a sample-enrichment step T. Takeuchi, Y. Hirata, and Y.Ohumura, Anal. Chem., 1978, 50, 660. W. H. McFadden, J. Chromatogr. Sci., 1979, 17, 2. a04 J. A. Schmit, R. A. Henry, R. C. Williams, and J. F. Dieckman, J. Chromatogr. Sci., 1971, 9,645. W. A. Dark and W. H. McFadden, J, Chromatogr. Sci., 1978, 16, 289. 130 Bartle, Lee, and Wise and sensitive detection methods. HPLC satisfies both of these criteria. Traditional enrichment procedures involve concentration of an organic solvent extract by evaporation. However, PAH enrichment directly on a reverse-phase LC column has been des~ribed.187J90~2~~ In 1975, May et ~1.187 described the use of a ‘coupled column’ procedure for the enrichment and separation of trace-level PAH in water. In this procedure a large water sample (1-2 dm3) is passed through a short LC column containing 27-50 pm pellicular CIS material to concentrate the nonpolar compounds.After the enrichment step, the short column is connected at the head of an analytical column containing 10pm totally porous microparticulate C18 material. A mobile- phase gradient of 30-100% methanol was used to desorb the PAH from the pre- column (enrichment column) and to separate them on the analytical column. At the initial mobile-phase composition of 30% methanol, the PAH are rapidly desorbed from the pellicular packing, but are retained as a narrow band at the inlet of the analytical column, thereby resulting in negligible peak broadening during the analytical separation. Recently, Eisenbeiss et and Ogan et ~1. described similar approaches for the concentration and determination of PAH in aqueous sampies.In these two reports an LC injection valve was used to place the enrichment column in-line with the analytical column after the enrichment step. Most environmental PAH mixtures can be analysed using capillary GC; however, low volatility limits the application of GC for the determination of high-molecular-weight PAH (molecular weight >300). At present, HPLC is the most advantageous technique for the separation of these compounds. Coal-tar pitch contains a relatively high proportion of high-molecular-weight PAH and PANH, and recently, Blumer et al.230reported that approximately 70% of the pitch could be eluted from a nitrophenyl phase column compared to only 30 % by GC.Separations of high-molecular-weight PAH in both normal- and reversephase systems require stronger mobile-phase compositions than are generally employed in order to achieve sufficient solubility of the sample. Blumer and Zander256 achieved normal-phase separation of PAH of molecular weight 400-600 on a nitrophenyl phase with 60-80 % chloroform in hexane. In a reverse-phase system using a c8 column, Felscher and Stein257 used a gradient of 40-100% acetonitrile/DMF (75 :25) in water to separate coal tar constituents. Numerous high-molecular-weight PAH (up to molecular weight 448) were separated and identified in a carbon-black extract using a CIS column with a gradient from 50 % waterlacetonitrile to 100% ethyl acetate and finally to 100 % methylene chloride (see Figure 5).2s8 HPLC Separations of PolycycIic Aromatic Nitrogen-, Sulphur-, and Oxygen- heterocycles.Normal- and reverse-phase HPLC are useful in the separation of a60 G.-P. Blumer and M. Zander, Compendium 78/79 Supplement to Erdoel, Kohle, Erdgas, Petrochem., 1978, p. 1472. D. Felscher and J. Stein, Z. Chem., 1979, 19, 303. *59 P. Peaden, M. L. Lee, Y. Hirata, and M.Novotny, Anal. Chem., 1980,52, 2268. 131 Modern Analytical Methods for Enviranmental Polycyclic Aromatic Compounds I 424 I I I I I 0 1 2 3 4 5 HOURS Figure 5 Non-aqueous reverse-phase HPLC separation of carbon-black extract. Chromato- graphic conditions: Vydac 201 TP column; mobile phase flow rate 0.7 cms min-l; solvent programme isocratic for the first 15 min with 50 % acetonitrile in water, non-linear pro- gramme (Programme 5 on Waters Model 660 solvent programmer) to 100 % acetonitrile for the next 70 min, linear programme to 100% ethyl acetate for the next 130 min, and linear programme to 100% methylkne chloride for the last 60 min; fluorescence detection with he, = 335 nm and hem = 480 nm with 20 nm slit widths (Reproduced by permission from Anal.Chem., 1980, 52, 2268) PANH, but only a few applications have been reported.57~126,218~259-263D0% et al.57 described the retention characteristics of 16 PANH on both a cl8 phase and on p-silica. Blumer and Zander218 studied the retention of 12 similar PANH on chemically bonded cl8 and nitrophenyl phases. On chemically bonded CISphases the retention characteristics of PANH are similar to that of PAH, i.e.retention is governed by the number of aromatic carbon atoms, but less organic solvent in the mobile phase is required owing to the increased polarity of the PANH. Isomeric PANH can often be resolved in reverse-phase HPLC, but retention data are available for only a few of these compounds. The normal-phase HPLC separations of PANH on p-silica,57 amine,259 and nitrophenylZ18 columns appear to depend on the steric availability of the ass P. L. Konash and S. A. Wise, unpublished data. W. E. May, J. M. Brown, L. R. Hilpert, and S. A. Wise, ‘Proceedings, 5th International Symposium on Polynuclear Aromatic Hydrocarbons’, Columbus, Ohio, October, 1980. e61 R. Vivilecchia, M.Thitbaud, and R. W. Frei, J, Chromatogr. Sci., 1972, 10, 411. 863 S. Ray and R. W. Frei, J. Chromatogr., 1972, 71, 451. Chemex Scientific Ltd., Product Literature, Ottawa, Ontario, Canada. 132 Bartle, Lee, and Wise nitrogen lone pair. Compounds such as benz[c]acridine, benzo[h]quinoline, and dibenz[a,h]acridine are only weakly retained on the silica or amine column, when compared to the isomers benz[a]acridine, benzo[f]quinoline, and di benz[a, j]-acridine, owing to the steric hindrance of the lone pair. May et a1.260 quantified benzo[f]quinoline in shale oil using a multi-dimensional HPLC procedure on an amine column followed by analysis on a ClS column. Complex formation of PANH on silver-impregnated adsorbents has been described by Vivilecchia et a1.2e1 and Frei et a1.126,262 More recently, a similar column, based on chemically bonded mercury salts (phenylmercury acetate), has become commercially a~ailable.~6~ These columns exhibit excellent thermal and chemical stability and may be used in either normal- or reverse-phase modes.Separations are insensitive to the degree of alkyl substitution and are therefore capable of giving class separations. Examples of separations on this organo- mercury phase have been illustrated for nitrogen-, sulphur-, and oxygen-heterocyclic compounds as well as PAH.262 The use of HPLC for the deter- mination of heterocyclic compounds will certainly increase in the future. B. Gas Chromatography.-The extreme complexity of PAC mixtures demands the greatest resolution possible in their analysis, and in this respect gas chromato- graphy (GC) with packed c0lumns~6~ even as long as 20 m has fallen short of the capabilities of glass-capillary column GC.265 First used in 1964, for the analysis of PAH,36 the latter technique was significantly refi11ed2~6J6~ when it was found that acid-leaching of Lewis acids from the glass from which i9 it _-I I I I I I I 1 40 75 100 125 150 175 200 225 250 TEMPERATURE(C) Figure 6 Capillary column gas chromatogram of coal-tar PAH.Chromatographic condi- tions: 20 m x 0.30 mm i.d. glass column coated with 0.10 pm firm SE-52; temperatureprogrammed from 40 to 80 "C at 10"C min-l and then from 80 to 250 "C at 2 "Cmin-l; He carrier gas velocity 50 cm s-l.Peak (1) naphthalene, (2)phenanthrene, (3) anthracene, (4)fluoranthene, (5) pyrene, (6) benz[a]anthracene, (7) chrysene, (8) benzo[e]pyrene, (9)benzo[a]pyrene,(1 0)perylene 264 G. Grimmer, H. Bohnke, and A. Hildebrandt, Fresenius' Z. Anal. Chem., 1976, 279, 139. M. L. Lee and B. W. Wright, J. Chrumatogr. Sci., 1980, 18, 345. M. L. Lee, Thesis, Indiana University, 1975. 267 M. L. Lee, K. D. Bartle, and M. V. Novotny, Anal. Chem., 1975, 47, 540. 133 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds capillaries were made greatly improved the deactivation and efficiency of columns. Figure 6 shows a chromatogram of coal-tar PAH obtained with an acid-leached column, subsequently coated with SE-52.The performance of this column, as measured by the resolution between the isomer pairs phenanthrene-anthracene, benz[a]anthracene-chrysene, and benzo[e]pyrene-benzo[a]pyrene, represents the best resolution currently available. Glass,268 and most recently fused-~ilica,26~ columns are now used universally, and if surface activity is controlled by acid leaching and other treatments,268 particularly ~ilanization,27~ trace compounds that may be adsorbed on high- surface area supports are eluted as sharp peaks.265 A variety of stationary phases has been employed in capillary-column separa- tions of PAH,265but because of their moderate selectivities and high temperature stabilities, SE-52 (methylsilicone gum with 5 % phenyl groups) and SE-54 (methylsilicone gum with 1% vinyl and 5 % phenyl groups) have gained wide acceptance; only minoi further selectivity effects are observed for more polar pha~es.~?l-~?~For capillary-column GC of the nitrogen heterocycles, more polar phases such as XE-60,274 Versamid 900,125and SP 2340138have been used in addition to SE-52.49 The retention of a planar PAH on a non-polar or slightly polar phase is related only to the logarithm of vapour pressure, and therefore to the boiling point and molecular weight.275 On a nematic liquid-crystal phase, however, a variation in activity coefficient, expressed in terms of a length-to-breadth ratio shape factor, also influences the retention of PAH.276p277Thus, liquid crystal phases such as the NN’-bis(p-alkoxybenzylidene)-or,~’-bi-p-toluidinesshow pro- nounced selectivity for PAH in packed column GC277-279 and allow resolution of a variety of isomer pairs such as anthracene and phenanthrene, and benzo[a]- and benzo[e]pyrenes. Elution of c24 hydrocarbons is possible on the most stable liquid-crystal phase and there have been applications to PAH analysis.280-282 Aza-arenes may also be separated on a nematic liquid Attempts to coat these phases onto glass capillaries have generally resulted in inefficient columns, M.L. Lee and B. W. Wright, J. Chromatogr., 1980, 184, 235. R. D. Dandereau and E. H. Zerenner, HRC & CC, 1979, 2, 351. *70 B. W. Wright, M. L. Lee, S. W. Graham, L. V. Phillips, and D. M. Hercules, J. Chromatogr., 1980, 199, 355.e71 L. S. Lysyuk and A. N. Korol, Chromatographia, 1977, 10, 712. 271 H. Borwitzky and G. Schomburg, J. Chromatogr., 1979, 170, 99. *73 M. P. Maskarinec and G. Olerich, Anal. Chem., 1980, 52, 588. p74 G. Alberini, V. Cantuti, and G. P. Cartoni, in ‘Gas Chromatography 1966’, ed. A. B. Littlewood, Institute of Petroleum, London, 1967, p. 258. p7s K. D. Bartle, M. L. Lee, and S. A. Wise, Chromatogruphiu, 1981, 14, 69. 278 A. Radecki, H. Lamparczyk, and R. Kaliszan, Chromatographia, 1979, 12, 595. 277 M. Janini, K. Johnston, and W. L. Zielinski, Anal. Chem., 1975, 47, 670. 278 W. L. Zielinski and G. M. Janini, J. Chromatogr., 1979, 186, 237. G. M. Janini, G. M. Muschik, and W. L. Zielinski, Anal. Chem., 1976, 48, 1879. G. M. Janini, B.Shaikh, and W. L. Zielinski, J. Chromatogr., 1977, 132, 136. S. Wasik and S. Cheder, J. Chromatogr., 1976, 122, 451. IanA. Radecki, H. Lamparczyk, J. Grzybowski, and J. Halkiewicz, J. Chromatogr., 1978,150, 527. M. Pailer and V. Hlozek, 1. Chromatogr., 1976, 128, 163. Bartle, Lee, and Wise but recent work has shown how mixed stationary phases composed of blends of a nematic liquid crystal with SE-52 result in retention of both efficiency and selectivity for PAH separation by capillary GC.284 For most PAC analytical work, columns need be no longer than 10-25 m with internal diameter 0.2-0.3 mm and film thicknesses near 0.3 pm.265 A graph of column resolution (measured by the separation number) against square root of column length has shown that loss in resolution between 15 and 30 m is minima1.285 Useful separations can even be achieved285 on columns as short as 4 m.Smaller column internal diameters lead to increased retention and slightly greater efficiency, although larger i.d. columns have greater capacity, as do columns with thicker film~.~~5 There is however a decrease in efficiency with increase in film thickness because of greater resistance to mass transfer.265 Analysis times can be shortenedzss by the use of hydrogen as carrier gas (which results in lower elution temperatures) at linear velocities up to 100 cm s-1.265.285 Glass-capillary-column GC was used in many of the representative applications referred to in Section 1. Chromatograms generally extend up to cor~nene~~~ (molecular weight 300).Some attempts have been made recently to extend the temperature range of capillary columns in order to chromatograph compounds of higher molecular weight. GrobZ8' was able to chromatograph compounds from coronene (C24H12) to rubrene (C42H28) on a short (5.5 m x 0.32 mm i.d.) glass capillary coated with OV-101, with temperature programming from 200 to 260°C. Figuie 7 shows a chromatogram recently obtained of an extract of carbon black.265 The column was a 15 m x 0.27 rnm i.d. Pyrex capillary that was Figure 7 High-temperature capillary-column gas chromatogram of the PAH extracted from a carbon black. Chromatographic conditions: 15 m x 0.21 mm i.d. glass column coated with SE-52; temperature programmed from 40 to 110 "C at 10 "C min-I and then from 110 to 350°C at 2°C min-l (Reproduced by permission from J.Chromatogr. Sci., 1980, 18, 345) lapR. J. Laub, W. L. Roberts, and C. A. Smith, HRC & CC, 1980, 3, 355. aa6 B. W. Wright and M. L. Lee HRC & CC, 1980, 3, 352. G. Schomburg, R. Dielmann, H. Borwitzky, aud H. Husmann, J. Chromatogr., 1978, 167, 337. ls7 K. Grob, Chromatographia, 1974, 7, 94. 135 Modern Analytical Methods for Environmental Polycyclic Aromatic Campounds extremely well deactivated270 before being coated with SE-52. The oven was programmed from 110 to 350°C and hydrogen was used as the carrier gas. The capillary gas chromatograph is an excellent separation tool, but is less effective for identification.Some information can be gained concerning the general composition of PAC mixtures by using selective detectors as discussed below. In addition, the chromatographic system can provide retention data that can yield complementary information for the positive identification of resolved components. Some studies have reported1049288 Kovats retention indices for PAC, and it was found that the retention indices of the PAC are influenced by the stationary-phase film thickness, the length of the column, the temperature programming rate, the carrier-gas flow rate, and the injection system. For this reason, a new and more reliable index system was defined by Lee et al.289 based on the set of standards -naphthalene, phenanthrene, chrysene, and picene -and the retention indices of over 200 PAC were determined.The average 95% confidence limits for four measurements on each PAC were kO.25 index unit. The performance of a sample-introduction device is critical when mixtures of compounds differing widely in volatility, such as PAC, are chromatographed. A comparison of the various sample-introduction techniques with regard to discrimination and decomposition effects has been publi~hed.2~~ In general, the splitless techniques are much more desirable than those involving splitting. The development of the on-column injector29l has essentially eliminated discrimination caused by differences in volatility, polarity, or concentration. The sample must, however, be relatively free of non-volatile material to avoid reduction of column life if on-column injection is used. The most widely used gas-chromatographic detector for PAC is the flame ionization detector (FID).This is a result of its universally accepted charac- teristics of excellent response linearity, sensitivity, and reliability. Lao et uI.~~ have reported response factors for a large number of PAC using an FID. As expected, response increases with molecular weight and response factors are similar for most structural isomersr Since discrimination can occur when chromatographing a wide range of PAC as a result of injection technique, degree of column deactivation, and detector design, it is advisable to measure response factors for each chromatographic system if accurate quantitative results are desired.As early as 1965, Cantuti et a1.379274showed that the response of the electron capture detector (ECD) for PAH was dependent on the structure of the com- pound, and that the detector could be selective for PAH in hydrocarbon mix- tures. Bjarrseth and Eklund67 measured the ECD/FID response ratios for 29 PAH and found that many isomers could be differentiated by measurements of these ratios. In a recent study, Grimsrud et al.292 found that adding oxygen to the a*8 H. Ekernaert, J. Chromatogr., 1979, 173, 109. M. L. Lee, D. L. Vassilaros, C. M. White, and M. Novotny, Anal. Chem., 1979,51,768. ID0 G. Schomburg, H. Behlau, R. Dielmann, H. Husmann, and F. Weeke, J. Chromatogr., 1977,142, 87. ¶DlK. Grob and K. Grob, jun.,J. Chromatogr., 1978, 151, 311.pea E. P. Grimsrud, D. A. Miller, R. G. Stebbins, and S. H. Kim, J. Chromatogr.,1!%0,197,51. Bartle, Lee, and Wise carrier gas greatly enhanced the signal obtained from a constant-current ECD for certain PAH. This response enhancement was found to be dependent on the structural detail of the PAH. It was suggested that this structure-dependent response enhancement could assist in the identification of resolved PAH isomers. A nitrogen-selective thermionic detector has been used for the selective detection of nitrogen heterocycles in air particulate matter49 and in lake sedi- ment~.~~~The selectivity factor for nitrogen over hydrocarbons has been shown to be 104:l. The selectivity and sensitivity of this detector for various nitrogen compounds has been described.49 Recent improvements in the Hall electrolytic conductivity detector (HECD) have provided some potential for the use of this detector for PAC.The HECD can selectively detect both nitrogen- and sulphur-containing PAC. The selectivity factor for nitrogen over hydrocarbons is 106 :1, and for sulphur over hydro-carbons is 105:1.293 In recent studies, a flame photometric detector (FPD) has been employed for the analysis of sulphur heterocycles in petroleum,15l various coal-derived pr0d~~t~,2~J5~~~~~and shale 0ils.~94 Care must be taken when using this detector because of its often non-linear response and observed response quenching due to other co-eluting compounds.151 The selectivity factor of sulphur over hydro- carbons can be as large as lo4:1.The photoionization detector (PID) can be used selectively for PAC. The use of U.V. lamps with different wavelengths allows some selectivity in detection because only radiation with energy greater than or equal to the ionization potential of the species will produce a signal. By using both PID (with 10.2 eV lamp) and FID, one can differentiate between aromatic and aliphatic hydro- carb0ns.~95 The relative molar response for the PID increases with unsaturation, and when the PID and FID responses are normalized to any alkane and the normalized PID/FID ratios calculated, those with a ratio of 5-10 are due to aromatics, 24 to alkenes, and t2 to alkanes. Although this detector has been optimized for use with capillary columns, a slight loss in resolution is observed owing to the volume of the detector.296 Resonance enhanced two-photon photoionization has recently been applied to the detection of PAH in gas-chromatographic eHuents.297 A small-volume proportional counter cell and a low-power laser were used.This method has several advantages with respect to the conventional one-photon photoionization detection previously discussed. One can expect increased spectral selectivity, improved ionization efficiency, and decreased detection volume in applications involving capillary columns. Detection limits of the order of 10 pg were obtained for a number of PAH using this detector.297 Gas-phase spectroscopic detectors for PAC include both U.V.and fluorescence. V. F. Cox and R. J. Anderson, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Abstract No. 208, Atlantic City, N.J., March, 1980. Ia4 C. Willey, M. Iwao, R. N. Castle, and M. L. Lee, Anal. Chem., 1981, 53, 400. J. N. Driscoll, J. Ford, L. F. Jaramillo, and E, T. Gruber, J. Chromarogr., 1978,158, 171. I@' L. F. Jaramillo and J. N. Driscoll, HRC & CC, 1979, 2, 536. Io7 C. M. Klimcak and J. E. Wessel, Anal. Chem., 1980, 52, 1233. Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds While earlier st~dies~~8~~~~ of U.V. detection were limited by rather poor sensiti- vities (pg levels), Novotny et d3Oo have successfully coupled glass-capillary columns to a variable-wavelength U.V.detector with a 50 pl cell volume. Detec- tion limits similar to those obtained with an FID were observed. Spectrofluorimetric detectors offer more promise than do U.V. detectors because of their inherent higher sensitivity and greater selectivity. Some studies involving the use of gas-phase fluorescence detection of gas-chromatographic effluents have been rep0rted,301-~~~ and many PAC have been detected in sub-nanogram amounts.304J07 The most powerful approach available today for the analysis of complex PAC mixtures is capillary column gas chromatography/mass spectrometry (GC-MS). The maximum resolution of' mixture components before mass-spectral analysis is of the utmost importance in providing unambiguous identifications of individual compounds.This is especially true in the case of PAC because the conventional mass spectra of many isomers are identical (see below). The gas-chromatographic peaks obtained from capillary columns are often extremely narrow, sometimes only a few seconds. This means that the scan times of the mass spectrometer must be short in order to obtain several spectra per peak and without having too much distortion of the mass spectra as a result of the changing concentrations along the peak elution profile. Modern GC-MS- computer systems have been built that easily handle four mass spectra per second. With the short cycle times for obtaining the mass spectra, the number of spectra taken during the gas-chromatographic run can easily run into thousands.At four mass spectra per second, the number of mass spectra in 45 min is already more than lo4.These figures emphasize the need for computerized data acquisi- tion and reduction. The power of the combined GC-MS system for the analysis of PAC is demonstrated by the numerous applications found in the literature. Most of the identifications of PAC in the applications listed in Section 1 were made using GC-MS techniques. 4 Mass Spectrometry In the last decade, mass spectrometry (MS) has gained wide acceptance for the analysis of PAC. New rapid-scan techniques coupled with high-resolution chromatography as discussed in the previous section have greatly surpassed any W. Kaye, Anal. Chem., 1962, 34, 287. tDD J. Merritt, F. Comendant, S.T. Agrams, and V. N. Smith, Anal. Chem., 1963, 35, 1461. 300 M. Novotny, F. J. Schwende, M. J. Hartigan, and J. E. Purcell, Anal. Chem., 1980, 52, 736. 301 M. C. Bowman and M. Beroza, Anal. Chem., 1968, 40,535. 30p H. P. Burchfield, R. J. Wheeler, and J. B. Bernos, Anal. Chem., 1971, 43, 1976. 308 H. P. Burchfield, E. E. Green, R. J. Wheeler, and S.M. Billedeau, J. Chrumatugr., 1974, 99,697. D. J. Freed and L. R. Faulkner, Anal. Chem., 1972, 44, 1194. 3os J. W. Robinson and J. P. Goodbread, Anal. Chim. Actu, 1973, 66, 239. R. P. Cooney, T. Vo-Dinh, and J. D. Winefordner, Anal. Chim. Acta, 1977, 89, 9. 307 R. P. Cooney and J. D. Winefordner, Anal. Chem., 1977,49, 1057. 138 Bartle, Lee, and Wise other method or combination of methods used for PAC analysis.Furthermore, new ionization methods that differentiate between isomeric PAC promise increased diagnostic power in the future. Figure 8 Electron-impact mass spectra ofi A, triphenylene; B, chrysene; C, benz[a]anthra-cene; D, naphthacene; E,benzo[c]phenanthrene m/e 228 m/e 226 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds A. Electron Impact.-The electron-impact mass spectra of PAC are well charac- terized as being quite simple, mainly consisting of an intense molecular ion and lower intensity ions due to the loss of one to four hydrogen atoms. The (M + 1)+ ion is always present and is due mainly to the 13C isotope. Doubly charged molecular ions are quite common and are usually near 20 % of the abundance of the molecular ion.Ions due to the expulsion of C2H2 are present, but at very low intensities. Alkylated PAH demonstrate the normal (M -15)+,(M -29)+, etc., fragmentation pattern tor alkyl chains even in the case of methyl-substituted compounds. The (M -15)+ ion is lower in abundance for methyl-substituted compounds than it is for PAH with longer alkyl side chains because of the favourable loss of a proton followed by ring expansion to form the tropylium ion. In most cases, differentiation of PAH isomers by electron-impact mass spectra alone cannot be achieved. Even in the cases of isomers with very different structures, such as fluoranthene and pyrene, the mass spectra are most often indistinguishable. Figure 8 compares the mass spectra of the four-ring isomers, triphenylene, chrysene, benz[a]anthracene, naphthacene, and benzo[c]phenan- threne.All the mass spectra are essentially identical except for benzo[c]phenan- threne. In this case, steric interaction between the protons on the 1 and 12 carbons facilitates the loss of these two protons with the subsequent formation of the benzo[ghi]fluoranthene ion [equation (l)]. Similarly, steric interactions are probably responsible for the increased intensities of the (M -l)+, (M -2)+, and (M -3)+ ions for 4-methylphenanthrene, 1-methyltriphenylene, 12-methylbenz- [alanthracene, 4-methylchrysene, and 5-methylchrysene as compared with their respective isomers. The mass spectra for hydro-aromatic compounds such as fluorene and ace- naphthene show the ease of removal of protons from saturated carbons under electron impact to give abundant (M -1)’ ions for both compounds and an abundant (M -2)+ ion for acenaphthene.The substitution of a heteroatom in the ring makes little difference to the appearance of the mass spectra. The position of nitrogen substitution in the ring makes essentially no difference to the observed spectra. The same results are found in the mass spectra of sulphur-containing PAH except for the presence of the 34S isotope peaks. In comparing the mass spectra for dinaphtho[2,1 -b:1 ’,2’-dlthiophen and dinaphtho[1,2-b:1 ’,2’-d]- thiophen (Figure 9) large abundances of the (M -1)+ and (M -2)+ ions are seen in the former. This is due probably to the formation of the perylo[l,l2- bcdlthiophen ion by the same mechanism that was observed previously for benzo[c]phenanthrene [equation (2)].A large number of electron-impact mass spectra for PAC have been compiled and can be found in several reference S. Safe and 0. Hutzinger, ‘Mass Spectrometry of Pesticides and Pollutants’, CRC Press, Cleveland, 1973, p. 77. 309 E. Stenhagen, S. Abrahamsson, and F. W. McLafferty, ‘Registry of Mass Spectral Data’, John Wiley and Sons, New York, 1974, Vol. 1-4. 310 S. R. Heller and G. W. A. Milne, ‘EPA/NIH Mass Spectral Data Base’, U.S. Government Printing Office, Washington, D.C. 1978, Vol. 1-4. ‘Eight Peak Index of Mass Spectra’, Mass Spectrometry Data Centre, Aldermaston, Reading, United Kingdom, 1974, Vol. 1-4.Bade, Lee, and Wise A ‘s’ loo, 1 80* 40-20 -loo] I B 40 20 rl 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 80 -60 -40 -20 -0-, , , , , , , I,I, Figure 9 Electron-impact mass spectra ofi A, dinaphtho[2,1-b : 1 ’,2’-dlthiophen; B,dinaphtho[1,2-b:1’,2’-dJthiophen Modern Analytical MethodsforEnvironmental Polycyclic Aromatic Compounds l+ -2H.+B-8 ‘s’ m/e 284 m/e 282 B. Chemical Ionization.-Conventional chemical ionization (CI) mass spectro- metry, using methane as the reagent gas, produces mass spectra of PAC that also appear quite simple. The most abundant ion is the (M + I)+ or protonated molecular ion, and the second most abundant ion is the (M + 29)+ ion.Because of the proton affinities of PAC, the major ionization process is protonation of the aromatic compound by the reagent gas, which produces the (M + 1)+ ion. Of second importance is the addition of an ethyl group to the aromatic system to produce the (M + 29)+ion. Munson and Field312 have discussed the CI of some alkylbenzenes and two alkylnaphthalenes, but little has been published on larger ring systems. Recent studies,65JO6~3~3 however, do indicate that the mass spectra of different isomers appear in most cases to be identical. The use of a mixed charge exchange-chemical ionization reagent gas for MS of PAH that differentiates between isomers has recently been reported.s5~3139314 The theory behind the use of the mixed reagent gas was discussed by Arsenault3l5 and later by Begg~.~l~ The relative rates of the charge-exchange and proton- exchange reactions, and hence the ratio of the abundance of the (M + 1)+ion to the abundance of the M+ ion will vary according to the proton affinity and/or ionization potential of each PAH.Since the ionization potentials of PAH isomers are dependent on the specific structure of the molecule, the argon-methane reagent can produce quite different spectra for different isomers. This is seen by comparison of the argon-methane CI spectra for anthracene and phenanthrene (Figure 10). Hunt et al.317--320 have recently described the use of oxygen as reagent in both positive and negative CI mass spectrometry of PAH. Electron bombardment of oxygen at 1 mm Hg pressure yields 02+ and O+ in the positive ion mode, and 02-and 0-in the negative ion mode.The reaction of 02+ with aromatic systems M. S. B. Munson and F. H. Field, J. Am. Chem. SOC.,1967,89, 1047. 313 M. L. Lee and R. A. Hites, J. Am. Chem. SOC.,1977,99,2008. 314 R. A. Hites and G. R. Dubay, in ref. 63, p. 85. 316 G. P. Arsenault, J. Am. Chem. SOC.,1972, 94, 8241. 316 D. P. Beggs, Hewlett-Packard Applications Note No. 176-19. 317 D. F. Hunt, C. N. McEwen, and T. M. Harvey, Anal. Chem., 1975, 47, 1730. 318 D. F. Hunt, G. C. Stafford, jun., F. W. Crow, and J. W. Russell, Anal. Chem., 1976,48, 2098. 31s D. F. Hunt and S. K. Sethi, in ‘High Performance Mass Spectrometry: Chemical Appli- cations’, ed. M.L. Gross, Am. Chem. SOC.Symp. Ser., 70, American Chemical Society, Washington, D.C., 1978, p. 150. 340 D. F. Hunt, P. J. Gale, and S. K. Sethi, 26th Annual Conference on Mass Spectrometry and Allied Topics, St. Louis, Missouri, May, 1978. 142 Bartle, Lee, and Wise l00-l 80-(M+29)+ 60-(a) 40-\\ .I cn 20-C Q)CI t II / 100-80-M+\ (M+29)+60-(b) 40 -20-m/e Figure 10 Argon-methane chemical ionization mass spectra ofi A, anthracene; B ,phenanthrene such as pyrene yields spectra containing a single ion, M+. In the negative ion mode, CI (02/H2) spectra show an abundant (M +15)-ion formed by either capture of a thermal electron followed by reaction with neutral oxygen, or by direct reaction with 02-.Simultaneous recording of positive and negative ion CI mass spectra can be accomplished by pulsing the polarity of the ion source potential and the focusing lens p0tential.3~~ An example of the differentiation between isomeric PAH using pulsed positive ion negative ion CI can be seen by comparing the spectra obtained for benzo[ghi]perylene and indeno[l,2,3-~d]pyrene3~~(Figure 1 1). Both isomers exhibit a single ion corresponding to M+ in the positive ion mode, but in the negative mode, benzo[ghi]perylene shows ions corresponding to M-and (M +15)- in a 1/1 ratio while indeno[l,2,3-cd]pyrene has a much more abun- dant M-ion. The presence of the five-membered ring in the indenopyrene facilitates formation of the stable negative ion. Hunt et aZ.320 have also shown that careful selection of reagent gases based on proton affinities can provide a means for differentiating PAH isomers. For example, the use of CZHBODas a reagent gas can produce different spectra for phenanthrene and anthracene.C. Other Ionization Techniques.-Field-ionization (FI) and field-desorption (FD) mass spectrometry produce virtually fragment-ion-free mass spectra and should be ideally suited to the group-type quantitative analysis of PAH mixtures. 143 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds A B 103 N/ P = 22.2 POS 276 50 M+ 276 WV M+ z 4 n z3 Q m J -1 I 291 W K I 29 ;M+151- 50 NEG indeno(l,2,3-cd) pyrene 276 ben z o( ghi 1 per y Ie ne mw 276 mw 276 M’ 100 Figure 11 Pulsed positive ionlnegative ion chemical ionization (OJH,) mass spectra of: A, benzu[ghi]perylene; Byindeno[ lY2,3-cd]pyrene, The intensities of reagent ions are 50-100 times greater than as shown [Reproduced by permission from High Performance Mass Spectrometry: Chemical Appli- cations, ed.M. L.Gross, Am. Chem. SOC. Symp. Ser., No. 70, 1978, p. 150 (ref.319)l Both techniques provide virtually single molecular-ion spectra for compounds like PAH, which makes them attractive techniques for the determination of molecular-weight distributions in mixtures. Although these techniques have not enjoyed very widespread use, the recent determination of FI relative sensitivities for a number of PAH321 and improvements in quantitative aspects of FD322p323 have helped.The applications of FI and FD mass spectrometry in the analysis of the aromatic fractions of fossil fuels have been described in several recent st~dies.32~-327 Photoionization (PI) techniques yield pure molecular-ion spectra for aromatics, but the low-voltage FI technique is preferable because of the experimental difficulties of PI. A description of the method for analysis of mixtures including the analysis of aromatic hydrocarbons is given by Se~erin.~~~ 321 S. E. Scheppele, P. L. Grizzle, G. J. Greenwood, T. D. Marriott, and N. B. Perreira, And. Chem., 1976, 48,2105. 3aa D. F. Barofsky, E. Barofsky, and R. Held-Aigner, Adv.Mass Spectrom., 1978, 7, 109. 323 S. Pfeifer, H.D. Beckey, and H.-R. Schulten, Fresenius’ Z. Anal. Chem., 1977, 284, 193. 324 A. M. Hogg and J. D. Payzant, Int. J. Mass Spectrom. Ion Phys., 1978, 27, 291. 825 S. E. Scheppele, G.J. Greenwood, P. L. Grizzle, T. D. Marriott, C. S. Hsu, N. B. Perreira, P. A. Benzon, K. N. Detwiler, and G. M. Stewart, Prepr. Div. Pet. Chem., Am. Chem. SOC., 1977,22, 665. 326 M. Anbar and G. A. St. John, Fuel, 1978, 57, 105. 327 G. A. St. John, S. E. Buttrill, and M. Anbar, Am. Chem. SOC. Symp. Ser., 71, 1978, p. 223. 328 D. Severin, Compend. -Dtsch. Ges. Mineraloelwiss. Kohlechem., 1976, 76-77, 949. 144 Bade, Lee, and Wise D. Group-type Analysis.-In as early as 1951, Brown329 showed that compound types could be reliably determined in complex mixtures by selecting the appro- priate masses that are characteristic of the compound type, calculating response factors, and formulating a series of linear simultaneous equations for taking into consideration the contributions of different components in the mixture to the analytical mass-spectral peaks.Although the early group type analyses were made using conventional 70 eV electron-impact mass spectrometry, the use of low-ionizing-voltage mass spectrometry has become widespread for the analysis of complex samples. The original description of the method and many of its advantages and difficulties was given by Field and Ha~tings~~O and later by L~rnpkin.~~~ Although group-type mass-spectral analysis has been used most extensively in the analysis of petr0le~m,~~~J~~~~~~-3~~the techniques have recently been extended to the study of coal and coal-derived produ~ts3~~-352 and the analysis of environmental po1lutants.l 2v 60s83--85 132I 353-359 The typical analytical approach is to introduce a portion of the PAC sample into the mass spectrometer through the direct introduction probe system and to vaporize the sample slowly with 3a9 R.A. Brown, Anal. Chem., 1951, 23, 430. 330 F. H. Field and S. H. Hastings, Anal. Chem., 1956, 28, 1248. 331 H. E. Lumpkin, Anal. Chem., 1958, 30, 321. 33p H. E. Lumpkin and B. H. Johnson, Anal. Chem., 1954, 26, 1719. 333 S. H. Hastings, B. H. Johnson, and H. E. Lumpkin, Anal. Chem., 1956, 28, 1243. 334 K. S. Quisenberry, T. T. Scolman, and A. 0.Nier, Phys.Rev., 1956, 102, 1071. 336 H. E. Lumpkin and T. Aczel, Anal. Chem., 1964, 36, 181. 330 H. E. Lumpkin, Anal. Chem., 1964, 36, 2399. 337 E. J. Gallegos, J. W. Green, L. P. Lindeman, R. L. LeTourneau, and R. M. Teeter, Anal. Chem., 1967, 39, 1833. 33E B. H. Johnson and T. Aczel, Anal. Chem., 1967,39, 682. 33E L. R. Snyder, B. E. Buell, and H. E. Howard, Anal. Chem., 1968, 40, 1303. 340 L. R. Snyder, Anal. Chem., 1969, 41, 1084. 341 C. J. Robinson and G. L. Cook, Anal. Chem., 1969, 41, 1548. 34a T. Aczel, D. E. Allan, J. H. Harding, and E. A. Knipp, Anal. Chem., 1970, 42, 341. C. J. Robinson, And. Chem., 1971, 43, 1425. 344 J. L. Schultz, A. G. Sharkey, jun., and R. A. Brown, Anal. Chem., 1972,44, 1486. 346 J. E. Schiller, Anal. Chem., 1977, 49, 1260.346 H. W. Holden and J. C. Robb, Fuel, 1960, 39, 39. 347 A. G. Sharkey, jun., J. L. Shultz, and R. A. Friedel, Fuel, 1962, 41, 359. 348 J. L. Shultz, R. A. Friedel, and A. G. Sharkey, jun., ‘Mass Spectrometric Analyses of Coal-Tar Distillates and Residues’, Washington, U.S. Dept. of the Interior, Bureau of Mines, 1967. 349 T. Kessler, R. Raymond, and A. G Sharkey, jun., Fuel, 1969, 48, 179. 350 J. T. Swansiger, F. E. Dickson, and H. T. Best, Anal. Chem., 1974, 46, 730. 361 A. G. Sharkey, jun., in ref. 112, p. 341. 36a H. E. Lumpkin and T. Aczel, in ref. 319, p. 261. 353 A. G. Sharkey, jun., J. L. Shultz, T. Kessler, and R. A. Friedel, Res. Dev., 1969, 20, 30. s54 A. G. Sharkey, jun., J. L. Shultz, T. Kessler, and R. A. Friedel, in ‘Proceedings of the Second International Clean Air Congress’, ed.H. M. England and W. T. Berry, Academic Press, New York, 1971, p. 539. 3bb D. Schuetzle, A. L. Crittenden, and R. J. Charlson, J. Air Pollut. Control ASSOC., 1973, 23, 704. 356 J. L. Shultz, A. G. Sharkey, jun., and R. A. Friedel, Biomed. Mass Spectrom., 1974, 1, 137. 357 D. Schuetzle, Biomed. Mass Spectrom., 1975, 2, 288. 35E A. Hase and R. A. Hites, Geochim. Cosmochim. Actu, 1976, 40, 1141. 369 J. L. Stauffer, P. L. Levins, and J. E. Oberholtzer, in ref. 63, p. 89. 145 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds increasing temperature while mass-spectral data are being collected. Plots can then be constructed that display the relative abundance of each parent compound and its alkyl-derivatives as a function of carbon number.These are referred to as ‘alkyl homologue’ plots. E. Metastable Ions and Collision Spectroscopy.-A relatively new technique that is growing rapidly and is related to the processes involved in the formation of metastable ions is collision spectroscopy.360 By introducing a collision gas in the field-free region, intermolecular processes such as charge-exchange and collision-induced fragmentations can be studied. One reported study of the fragmentation processes of aromatic hydrocarbons in the first field-free region employed a mass spectrometer with fields arranged in the order accelerating voltage (V)/electric sector voltage (E)/and magnetic field (B), and used a detector located behind the energy-resolving slit located between the sectors.361 Scanning of the electric sector voltage gave a fingerprint of all fragmentations without prior mass separation. This is called an ion-kinetic-energy (IKE) scan because the ions are separated on the basis of their kinetic energies.Subsequent mass analysis of selected peaks in the IKE spectrum can be made. Both the IKE spectra and mass spectra can be used to give detailed fingerprints of organic compounds. PAH that have been studied using this technique include naphthalene, 2-methylnaphthalene, biphenyl, anthracene, dimethylnaphthalene, benzo[a]pyrene, and benzo[k]fluoranthene Shushan et aI.364 recently described a method in which both B and E are scan- ned so that (B/E) is held at the constant value required to transmit stable parent ions of pre-selected (m/e) ratio; in this way fragment ions formed in the first field-free region, from the pre-selected parent ions, are successively transmitted.The daughter-ion spectra of the four isomers, chrysene, triphenylene, benz[a]- anthracene, and naphthacene, showed considerable differences in the relative abundances of the M+, (M -H)+, and (M -2H)+ ions, the latter two being the products of fragmentation of M+ ions in the first field-free region. Plots of the (M/M -H) and (M -H/M -ZH)intensity ratios against the number of benzo interactions per molecule facilitated the structural assignments of these isomers. Another widely used configuration is that in which the second stage of the instrument is an energy rather than a mass analyser.With the arrangement of fields V/B/E, fraqentations between the sectors can be observed. The initial magnetic field is set to select the ml+ ions, and the fragmentation products of these ions can then be plotted in a single spectrum by varying the electric sector voltage. This is called a MIKE scan for ‘mass-analysed ion kinetic energy’. An example of the use of MIKE for the analysis of coal liquids was reported by 360 ‘Collision Spectroscopy’, ed. R. G. Cooks, Plenum Press, New York, 1978. 361 J. H. Beynon, R. M. Caprioli, W. E. Baitinger, and J. W. Amy, Org. Mass Spectrom., 1970, 3,455. 36a T. Ast, J. H. Beynon, and R. G. Cooks, Org. Mass Spectrom., 1972, 6, 719.363 R. C. Lao, R. S. Thomas, and J. L. Monkman, Adv. Mass Spectrom., 1974, 6, 129. 364 B. Shushan, S. H. Safe, and R. K. Boyd, Anal. Chem., 1979, 51, 156. Bartle, Lee, and Wise Zakett et aL3"5 Identifications of specific nitrogen heterocycles were made with- out prior treatment or fractionation of the sample. The mass spectroscopic method chosen for the analysis of PAH is dependent on the type of results desired. For group-type analysis, low- and high-voltage electron impact, field ionization, field desorption, photoionization and sometimes chemical ionization may be used. By far the most universal approach is low- voltage electron impact. The availability of instrumentation, sensitivity data, and reproducibility are the main reasons for this.On the other hand, if structural identification of individual components of mixtures is desired, GC-MS is the method of choice as discussed earlier. The relatively new techniques of chemical ionization, metastable ion, and collision-induced mass spectrometry have the potential of being able to solve many of these problems without prior sample fractionation. 5 Spectroscopic Methods A. Luminescence.-The limitations of relatively low sensitivity and the poor specificity of spectra containing broad lines (although derivative spectroscopy and computer resolution can increase the information a~ailable)3~6 now severely restrict the applications of U.V. absorption spectroscopy in the analysis of PAC. Luminescence methods frequently afford 10-103 times the sensitivity of absorption methods and are much more specific.Thus, while benzo[a]pyrene and benzo[ghi]perylene have similar U.V. absorption spectra, and therefore similar fluorescence excitation spectra, they are readily distinguished by their fluorescence emission spectra.367 Many methods for the determination by fluorescence techniques of single compounds in mixtures containing interfering components are available,36s and many of these refer to benzo[a]pyrene.369 Commonly used analytical procedures for 'marker' PAH, such as those specified for drinking water by the WHO,370 employ fluorimetry preceded either by coluinn chromatography on alumina or by two-dimensional TLC. In the latter method, each PAC is identified on the plate from its position and fluorescence colour under U.V.light. Quantitation is then by densitometry, with either visual comparison with reference plates, 01 by use of a chromatogram ~canner.3~1 Alternatively, the spots may be microsublimed from the plate, or more usually scraped off and e~tracted,~~ before recording of the fluorescence excitation and emission spectra. Care may be necessary in inter-preting the results of combined TLC/fluorimetric procedures, since tests with 365 D. Zakett, V. M. Shaddok, and R. G. Cooks, Anal. Chem., 1979, 51, 1849. 366 G. E. Barker and M. F. Fox, Chem. SOC.Rev., 1980, 9, 143. 367 T. J. Porro, J. Assoc."Ofl;Anal. Chem., 1973, 56, 607. 36B 0.Hutzinger, S. Safe, and M. Zander, Analabs Res. Notes, 1973, 13(3), 13. 36s L.Dubois, A. Zdrojewski, and J. L. Monkman, Mikrochim. Acta, 1967, 903. 370 J. Borneff, 'Fate of Pollutants in the Air and Water Environment', ed. I. H. Suffet, Wiley, New York, 1977, Part 2, p. 393. 371 R. Tomingas, G. Voltmer, and R. Bednarik, Sci. Total Environ., 1977, 7, 267. Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds 14C-labelled compounds have shown poor reproducibility for quantities below 1 pg.372 The selectivity of phosphorimetry is greater than that of fluorimetry, although .~ its range of application is not as great; fewer compounds have measurable phosphorescence than fluorescence, but the spectra, especially of PAC, may be more characteristic.373 The limits of detection of PAC in phosphorimetry and fluorimetry are of the same order of magnitude.Because of non-radiative inter- actions in solution, phosphorescence was, up until recently, generally observed at low temperature in frozen-solution glasses ;the technique is usually less sensitive than is fluorimetry. However, if concentrations are available at which both techniques give analytically useful signals, it is convenient to measure both fluorescence and phosphorescence : by combining the two techniques, an analytical range over four decades results.374 Phosphorimetry is most applicable where the mixture contains strongly fluorescent but weakly phosphorescent interfering species. For example, perylene interferes with the determination of benzo[rst]pentaphene by fluorimetry but gives negligible interference in phosphorimetry.374 Applications of phosphori- metry in air pollution studies were pioneered by Sawicki,375 who detected PAH on thin-layer chromatograms by their phosphorescence in liquid nitrogen.Phosphorescence emission may also be observed at room temperature from salts of ionic compounds adsorbed on silica gel, alumina, and even filter paper.376 The effect, ascribed to increased molecular rigidity through adsorption, which is presumed to reduce collisional deactivation, has been developed as a sensitive spectroscopic method of analysis.377-379 Winefordner378 showed that sodium iodide and silver nitrate induce room-temperature phosphorescence (RTP) from a number of PAH and allow detection at the ng level for phenanthrene and pyrene.Strong RTP emission can be induced from nitrogen heterocycles adsorbed on silica gel chromatoplates containing a polymeric binder.379 These approaches may increase the application of phosphorescence in the analysis of PAC. The luminescence characteristics of PAC are markedly dependent on the environment. Thus, reduction of luminescence intensity, or quenching, occurs if an electronically excited molecule gives up its energy to a solvent or other molecule. This property may be used to increase the selectivity of luminescenc: in various ways, collectively known as ‘quenchofluorimetry’. A commonly applied approach uses the property of a substituted atom or group such as chlorine, bromine, or nitro in the solvent to enhance phosphor- 37a F.dewiest, D. Rondia, and H. Della Fiorentina, J. Chromatogr., 1975, 104, 399. 373 M. Zander, ‘Phosphorimetry’, Academic Press, New York, 1968. 374 L. V. S. Hood and J. D. Winefordner, Anal. Chim. Acta, 1968, 42, 199. 375 E. Sawicki and J. D. Pfaff, Anal. Chim. Acta, 1964, 32, 521. 376 E. M. Schulman and C. Walling, Science, 1972, 178, 53. 377 S. L. Wellons, R. A. Paynter, and J. D. Winefordner, Spectrochim. Acta, Part A, 1974,30, 2133; Anal. Chem., 1974, 46, 736. 378 T. Vo-Dinh, F. Lue Yen, and J. D. Winefordner, Anal. Chem., 1976, 48, 1186; Talanta, 1977, 24, 146. 379 0.D. Ford and R. J. Hurtubise, Anal. Chem., 1980, 52, 656. Barile, Lee, and Wise escence at the expense of fluorescence; intersystem crossing from excited-singlet to phosphorescent-triplet states is made easier through an increase in spin-orbit coupling (the ‘heavy-atom’ effect).380 Zander Z81 found that the addition of 10% by volume of iodomethane to a standard solvent system of diethyl ether, iso- pentane, and ethanol (55:2 by volume) gave significantly improved detection limits, e.g., from 2 x 10-6 to 5 x 10-7 g cm-3 for the determination of fluoranthene by phosphorescence.The fluorescence of this compound is cor- respondingly quenched, as is that of numerous other PAH, e.g., anthracene, chrysene, naphthacene, and benzo[a]pyrene.3s1 However, certain PAH are exceptions ; dibenzo[b,def]chrysene shows only slight quenching in the presence of iodomethane,3g1 and perylene, 2-methylpery- lene, and dibenzo[a,f]perylene are not quenched.382 It is thus possible to deter- mine small concentrations (0.1-2 %) of perylene in the presence of large excesses of benzo[e]pyrene and naphthacene by fluorimetry.This application may be important in view of the suggested use of perylene as a geochemical marker.lp9 Other heavy-atom perturbers of fluorescence that have been investigated include iodoethane in ethanol solutions of PAH, for which two kinds of be- haviour are observed :383+3S4(a) fluorescence is quenched and phosphorescence enhanced relative to ethanol alone; (b) fluorescence is enhanced and phos- phorescence quenched according to perturber concentration. The first effect is typical of benzo[a]- and benzo[b]fluorene, benz[a]anthracae, benzo[a]pyrene, and dibenz[a,c]anthracene, and increases with increasing iodoethane concentra- tion.However, the limits of detection of these compounds decrease accordingly. Naphthalene, anthracene, phenanthrene, and triphenylene exhibit the second effect. Zander has shown how silver nitrate selectively enhances the phosphorescence of aza-aromatics relative to that of PAH because of the strong electron-donor properties of the former compounds.385 Another useful application of the heavy- atom effect in the observation of RTP of PAC in presence of silver and sodium nitrates has already been noted. Selective quenching of the fluorescence of PAH containing only six-membered rings by electron acceptors such as nitromethane was noted by Sa~icki;~~~ the fluorescence of PAH containing the fluoranthene skeleton was found not to be quenched.Dreeskamp et ~1.2~5have shown a few exceptions to this ‘rule’; benzo[b]- and benzo[k]fluoranthene showed significant quenching. However, this phenomenon is still general enough386 to allow the identification of non-380 W. J. McCarthy, in ‘Spectrochemical Methods of Analysis’, ed. J. D. Winefordner, Wiley-Interscience, New York, 1971, Chapter 8. 381 M. Zander, Fresenius’ Z. Anal. Chem., 1967, 226, 251; 1973, 263, 19; Erdoel. Kohle, 1969, 22, 81. 382 M. Zander, Fresenius’ Z. Anal. Chem., 1967, 229, 352. 383 L. V. S. Hood and J. D. Winefordner, Anal. Chem., 1968, 38, 1922. 3n4 M. Zander, Int. J. Environ. Anal.Chem., 1973, 3, 29. 385 M. Zander, Z. Naturforsch., Teil A, 1978, 33, 998. 386 M. Zander, U. Breymann, H. Dreeskamp, and E. Koch, 2.Naturforsch., Teil A, 1977,32, 1561. 149 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds 600 500 -hl nml B 3 I --+ 17 20 I 700 600 500 f--hlnml Figure 12 A, Fluorescence spectra in acetonitrile with excitation at 383 nm OR(1) a mixture of mainly alternaltt PAH; (2) same mixture in acetonitrile containing 50% v/v nitro-methane; (2a) pure acenaphtho [ 1,2-j]fluoranthene.B, Fluoresence spectra in acetonitrile with excitation at 430 nm of: (1) a mixture of mainly non-alternant PAH; (2) same mixture in acetonitrile containing 40% v/v 1,2,4-trimethoxybenzene;(2a) pure dibenzo[a, 1Ipentacene (Reproduced by permission from Fresenius' 2.Anal.Chem., 1978,293, 208) 150 Bartle, Lee, and Wise alternant PAH after separation, e.g., on thin-layer plates, in HPLC elutes (as reported above), and in mixtures with alternant PAH [Figure 12(A)]. Comple- mentary quenchofluorimetry with electron donors such as 1,2,4-trimethoxyben- zene, moreoever, suppresses the fluorescence of non-alternants and allows recognition of alternants in mixtures387 [Figure 12(B)]. The broadened peaks of the fluorescence spectra of PAC recorded in liquid solution may inhibit analysis, and the usefulness of low-temperature lumi- nescence is well known, particularly in the enhancement of sensitivity that results from the increase in fluorescent intensity and the narrowing of lines from PAH in frozen-solution gla~ses.~~*~~~* For example, laser excitation of PAH in glycerol/water glasses at 4 K yields sharp-line spectra, and the problems of solubility, concentration gradients, and light scattering are minirnized.38QJ90 Sub part-per-trillion (1 x 10-l2 g cmW3) detection of contaminant PAC in water is then possible with a tunable dye-laser source.390 However, exceedingly line-rich Shpol’skii spectra are produced if PAC molecules are separated by large distances and embedded in a crystalline solvent lattice like an oriented gas molecule.391 Vibrational and rotational energy are reduced, interaction between the solute molecules is prevented, and compounds having liquid-solution spectra with half-bandwidths of several nm have frozen- solution half-bandwidths of one or two nm, further split by the Shpol’skii effect to extremely sharp lines with half-bandwidths of the order of a few hundredths of a nm.Further sharpening of lines is then possible with laser excitation. For Shpol’skii luminescence to be observed, the dimensions of’ the solute molecules must be approximately equal to those of the solvent; n-alkanes of suitable molecular dimensions are the most commonly used solvents, although spectra are also observed in tetrahydrofuran.392-3g* The sharpness of the Shpol’skii spectra also depends on the temperature; at 77 K, the spectrum of pyrene in n-hexane contains about 60 lines, whereas at 4 K the same solution shows approximately 220 lines.395 Although the majority of the many quasilinear luminescence emission spectra of PAH so far reported have been obtained near 77 K, even a temperature reduction to 63 K with a cell cooled by nitrogen at its freezing temperature allows a marked improvement in resolution396 (Figure 13).Causey et aZ.397 have compared some excitation sources (including a helium- cadmium laser), sample cells, and various detection systems and concluded that a 387 U. Breymann, H. Dreeskamp, E. Koch, and M. Zander, Fresenius’Z. Anal. Chem., 1978, 293,208. S. P. McGlyn, B. T. Neely, and C. Neely, Anal. Chim. Acta, 1963, 28, 472. 38g J. C. Brown, M. Edelson, and G. J. Small, Anal. Chem., 1978, 50, 1394. 390 J. C. Brown, J. A. Duncanson, jun., and G.J. Small, Anal. Chem., 1980, 52, 1711. 3g1 E. V. Shpol‘skii, Soviet Phys. Usp., 1959,2, 378; 1960,3, 372; 1962,5, 522; 1963,6,252, 411. 38a G. F, Kirkbright and C. G. delima, Analyst (London), 1974, 99, 338. 393 G. F. Kirkbright and C. G. delima, Chem. Phys. Lert., 1976, 37, 165. 394 E. V. Shpol’skii and T. N. Bolotnikova, Pure Appl. Chem., 1974, 37, 183. 396 L. A. Klimova, Opt. Spectrosc., 1963, 15, 185. A. Colmsjo and U. Stenberg, Chem. Scr., 1977, 11, 220. 397 B. S. Causey, G. F. Kirkbright, and C. G. delima, Analyst (London), 1976, 101, 367. Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds b d ' 9OK 1 1, Figure 13 Fluorescence spectra of dibenz [a,h Ianthracene in n-heptane at diferent temperatures (Reproduced by permission from Chem.Scr., 1977, 11, 220) xenon arc or mercury lamp with a specially designed copper cryostat containing a Dewar tube as cell and d.c. integration of the luminescence signals constitute a simple and reliable system. Detection limits between 0.1 ng CM-~(benzo[a]pyrene) and 300 ng cm-3 (indeno[l,2,3-~d]pyrene)have been quoted.392 Shpol'skii spectra can provide very useful fingerprints of individual compounds, and a variety of qualitative analyses have been reported. Thus, line-rich spectra were obtained by Colmsjo and Stenberg398 for PAH from a number of environ- mental sources such as automobile-exhaust gas. Drake et ~1.39~ showed how such spectra allowed twelve PAC containing between three and ten rings to be identified in extracts of coal and coal-tar pitch, including ovalene, previously only postulated as a constituent of coal.In these mixtures only species resistant to deactivation and quenching are detected because of the formation of inhomo-geneous micellar aggregates. This effect favours the detection of benzo[a]pyrene at levels down to 0.1 p.p.m., and many applications of the quasi-linear lumi- nescence method have been reported for this compound. However, benzo[a]- pyrene quenches the spectrum of benzo[e]pyrme,398 so they must be separated before Shpol'skii analysis. Thus, benzo[a]pyrene has been determined in atmospheric particulates by direct measurement of the intensity of the emission at 79 Dikun401 used luminescence to determine benzo[a]pyrene via the Shpol'skii effect using benzo- [ghilperylene as internal standard, and a similar method was reported by per so no^,^^^ using coronene for the same purpose.The standard addition procedure to minimize the effect of quenching was preferred by Muel and 398 A. Colmsjo and U. Stenberg, Anal. Chem. 1979, 51, 145 and in 'Polynuclear Aromatic Hydrocarbons', ed. P. W. Jones and P. Leber, Ann Arbor Science, Ann Arbor, 1979, p. 121. 399 J. A. G. Drake, D. W. Jones, B. S. Causey, and G. F. Kirkbright, Fuel, 1978, 57, 663. 400 M. J. Eichhoff and N. Kohler, Fresenius' 2. Anal. Chem., 1963, 197, 212. 401 P. 0.Dikun, Vop. Onkol., 1961, 7, 42 (Chem. Abstr., 1961, 57, 658). 403 R. 1. Personov, Zh. Anal. Khirn., 1962, 17, 506.152 Bartle, Lee, and Wise Lacroix403 for the determination of benzo[a]pyrene in cigarette smoke, water, and various alcoholic drinks from the 403.0 nm emission line in n-octane at 83 K; a relative precision of 10%was claimed. In spite of numerous applications of the Shpol’skii effect to the analysis of PAC in a variety of mixtures, the practical utility of the method may be limited and few examples have quoted reproducibility, accuracy, or precision. For these reasons, two detailed studies have been made of the applicability of Shpol’skii luminescence in the analysis of PAH. Gaevaya and Khesina404 investigated the conditions necessary for the determination of fifteen PAH in multi-component solutions by using the effect. A combination of addition and internal-standard methods was necessary, and standard deviations of 2-10% were found for sensitivities of 10-8-10-10 g cm-3. Kirkbright and deLima392 first demon- strated that unambiguous qualitative identification of PAH at very low con- centration is possible.A careful investigation of the conditions necessary for the quantitative analysis of a mixture of dibenzopyrenes also showed that a combined standard addition plus internal-standard calibration procedure was necessary. Although internal standardization improves precision by decreasing random errors, an ‘inner filter’ effect may occur because of overlap of the excitation spectrum with that of the analyte so that there is a reduction in emission intensity and increase in detection limit.392 The limitations of Shpol’skii luminescence led Wehry et aL405to investigate matrix-isolation (MI) fluorescence for the analysis of PAH.Hydrocarbon vapours from a Knudsen cell are mixed in an evacuable cryostat head with a large excess of nitrogen and condensed on a cold (11-15 K) sapphire surface. Detection limits of the order of g and linear quantitative working curves over five decades were observed for the resulting matrices. MI fluorimetry is substantially free from interference by intramolecular energy transfer or inner-filter effects even for samples containing high (i.e. pg) concentrations of several closely related PAH.405 Thus, the working curve for chrysene in a four-component mixture is virtually the same as that of pure chrysene, while linear calibration curves were also obtained for 4-methylchrysene in the presence of a fifty-fold excess of three of its isomers.406 Band-widths in nitrogen and argon matrices were generally greater than in Shpol’skii matrices, although MI experiments with n-alkanes as the matrix gave quasilinear spectra similar to those in conventional Shpol’skii fluorescence.407 Selective dye-laser excitation of individual compounds in complex mixtures is then possible both for vapour-dep~sited~~~ and conven- tiona1409 Shpol’skii n-alkane matrices.X-Ray excited optical luminescence of PAC in frozen solutions may result in 403 B. Muel and G. Lacroix, Bull. SOC.Chim. Fr., 1960, 2139. 404 T. Ya. Gaevaya and A. Ya. Khesina, Zh.Anal. Khim., 1974, 29, 2225. 405 E. L. Wehry and G. Mamantov, Anal. Chem., 1979,51, 643A. Io8P. Tokousbalides, E. R. Hinton, R. B. Dickinson, jun., P. V. Bilotta, E. R. Wehry, and G. Mamantov, Anal. Chem., 1978,50, 1189. 407 P. Tokousbalides, E. L. Wehry, and G. Mamantov, J. Phys. Chem., 1977, 81, 1769. 408 J. R. Maple, E. L. Wehry, and G. Mamantov, Anal. Chem., 1980, 52, 920. 40D Y.Yang, A. P. D’Silva, V. A. Fassel, and M. Iles, Anal. Chem., 1980, 52, 1350. Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds the population of electronic levels not accessible to U.V. excitation and thus eliminate optical interaction between exciting and emitted radiati~n.~lO-~~~ This method also results in the observation of substantial phosphores~ence,~~~ and may augment analyses for PAC already made by Shpol’skii pho~phorimetry.~~~g~~~ Recent approaches to the luminescence analysis of PAC have been aimed at increasing the sensitivity and selectivity of the methods through the use of new excitation sources and dispersive systems, more sophisticated detectors, and data-handling systems based on computer meth~ds.~~~-~l~ Laser light sources allow selective and sensitive detection in luminescence (e.g., allowing detection limits in the sub part-per-trillion range and a linear dependence of fluorescence intensity on concentration extending over six decades)417 and, if pulsed, yield time resolution.417 For example, while benzo[a]pyrene and benzo[k Jfluoranthene cannot easily be distinguished from each other by steady-state MI fluorimetry, their fluorescence decay times in N2 matrices are sufficiently different (78 ns for BaP and 13 ns for BkF) to produce excellent temporal resolution of emission spectra for these compounds.418 Correspondingly, pulsed-source time-resolved phosphorimetry shows improved sensitivity and selectivity compared with continuously operated source methods and can be applied to PAC.419 Time resolution is also possible by computer methods,419 and computation also allows the first- or second-derivative luminescence spectrum to be obtained, which contains much more information than the ‘zeroth’ order ~pectrurn.~~~,~~~ If the spectra of two fluorophors overlap, selective modulation fluorescence excitation and/or emission spectra can be recorded for either component; the procedure involves wavelength modulating one monochromator, scanning the other, and detecting with a lock-in amplifier.422 Modulation fluorescence has been used in a demonstration that the compound eluted with benzo[e]pyrene in the HPLC of air-particulate PAH is benz~[k]fluoranthene.~~~ In synchronous luminescence spectroscopy, both excitation and emission wavelength ale scanned while keeping a constant interval between them, usually selected to be close to the Stokes ~hift.~24?$25 The synchronous spectrum of each 410 A.P. D’Silva, G. J. Oestreich, and V. E. Fassel, Anal. Chem., 1976, 48, 915. 411 C. P. Woo, A. P. D’Silva, V. E. Fassel, and G. Oestreich, Enviran.Sci. Technol., 1978,12, 173. 419 C. S. Woo, A. P. D’Silva, and V. E. Fassel, Anal. Chem., 1980, 52, 159. 41s V. D. Tuan and U. P. Wild, J. Lumin., 1973, 6, 296. 414 K. R. Naqvi, A. R. Holzwarth, and U. P. Wild, Appl. Spectrosc. Rev., 1977, 12, 131. 416 M. Soutif, Pure Appl. Chem., 1976, 48, 99. 4r6 ‘Modern Fluorescence Spectroscopy’, ed. E. L. Wehry, Plenum Press, New York, 1976, Vol. 1 and 2. *17 J. H. Richardson and M. E. Ando, Anal. Chem., 1977, 49, 955; Proceedings Ninth Materials Research Symposium on Trace Organic Analysis, Gaithersburg, Maryland, April, 1978, Natl. Bur. Stand. (U.S.)Spec. Publ., 519, 1979, p. 723. R. B. Dickinson, jun., and E. L. Wehry, Anal. Chem., 1979,51, 776. 41s G. D. Boutillier and J. D. Winefordner, Anal.Chem., 1979, 51, 1384. 480 G. L. Green and T. C. O’Haver, Anal. Chem., 1974, 46,2191. 4a1 G. Talsky, L. Mayring, and H. Kreuzer, Angew. Chem. Int. Ed. Engl., 1978, 17, 785. 4aa T. C. O’Haver and W. M. Parks, Anal. Chem., 1974,46, 1886. 4a3 M. A. Fox and S. W. Staley, Anal. Chem., 1976, 48, 992. 494 T. Vo-Dinh, Anal. Chem., 1978, 50, 396. lP6 J. B. F. Lloyd, Nature (London), 1971, 231, 64. Bade, Lee, and Wise PERYLENE - ANT HRACENE - NAPH T HALENE I I, TETRACENC 300 350 400 4 50 500 WAVELENGTH lnm) Figure 14 A, Conventional fluorescence spectrum of a mixture of naphthalene, phen- anthrene, anthracene, perylene, and naphthacene. B, Synchronous spectrum of the mixture (Reproduced by permission from Anal. Chem., 1978, 50, 396) PAH now consists of a single peak (Figure 14).Caution may be necessary in applying the method to the quantitative analysis of multi-component mixtures, since the relative concentrations of species present would need to be fortuitous if errors from spectral overlap are to be av0ided.~~6 However, in spite of' these reservations, there have been valuable applications such as the characterization of naphthalene derivatives in waste water,427 the determination of perylene in 4*8 H. W. Latz, A. H. Ullman, and J. D. Winefordner, Anal. Chem., 1978,50,2149; 1980,52, 191. 4a7 T. Vo-Dinh, R. B. Gammage, A. R. Hawthorne, and J. H. Thorngate, Environ. Sci. Technol., 1978, 12, 1297. Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds environmental sample^,^ and, in a development of RTP, the analysis of complex mixtures such as Synthoil on filter paper.428 Lloyd has also shown how syn- chronous luminescence can provide ‘fingerprints’ in forensic science,429-431 and can be used simultaneously with heavy-atom quenching.430 Video fluorimetry432 involves the gathering of the emission-excitation matrix, M, the elements of which, Mij, represent the fluorescence intensity measured at wavelength hj for excitation at Xi.A row of M represents the fluorescence- emission spectrum, and a column an excitation spectrum. The matrix is rapidly compiled by a computer-controlled fl~orimeter,~~3 and computational pro- cedures may be applied to derive the individual spectra of up to six components in a mixture.Spectra may be displayed either as contours or as isometric projection~~3~(Figure 15). Anthracene \ Figure 15 Total luminescence spectrum of anthracene and ovalene (Reproduced by permission from Anal. Chem., l978,50,936A) The use of parallel optoelectronic image detectors435 (OID) allows the spectral matrix to be rapidly acquired so that transient spectrofluorimetry of photochemically labile compounds, and HPLC and GC detection are possible. 42* T. Vo-Dinh, R. B. Gammage, and A. R. Hawthorne, in ref. 168, p. 111. 4aQ J. B. F. Lloyd, J. Forensic Sci.SOC.,1971, 11, 153 and 235. 430 J. B. F. Lloyd, Analyst (London), 1974, 99, 729. 431 J. B. F. Lloyd, Anal. Chem., 1980, 52, 189. 438 1. M. Warner, G.D. Christian, E. R. Davidson, and J. B. Callis, Anal. Chem., 1977, 49, 564. 433 I. M. Warner, J. B. Callis, E. R. Davidson, M. Gouterrnan, and G. D. Christian, Anal. Lett., 1975, 9, 665. 434 C.-N. Ho, G. D. Christian, and E. R. Davidson, Anal. Chem., 1980, 52, 1071. 436 Y.Talmi, D. C. Baker, J. R. Jadamec, and W. A. Saner, Anal. Chem., l978,50,936A. Bartle, Lee, and Wise Spectra obtained with OID may be smoothed, source compensated, differentiated, etc., in a computer. Laser-induced phosphorescence decay times may also be measured, and OID with diagonal scans across the spectral matrix, can yield information identical to that obtained by synchronous luminescence. B. Nuclear Magnetic Resonance.-Conventional single-scan n.m.r.may confirm the presence of a given class of compounds, e.g., alkyl-substituted derivatives, and may also be applied to the identification of specific individual compounds. Thus, Keefer et ul.436carefully investigated the methods of identification of methyl-substituted PAH from environmental samples by continuous-wave lH n.m.r. The use of the chemical shifts of CHs (and their different concentration dependences), and of the benzylic coupling between CH3 and ring protons were studied. The identities of 4-methylpyrene (1.1 Hz doublet) and 2-methylpyrene (0.7 Hz triplet) in the monomethyl pyrene fraction of petrolatum were confirmed from their chemical shifts; the dimethylpyrene fraction from the same material showed no resonances in regions characteristic of -CH2CH3 groups so that ethyl derivatives could be excluded, a conclusion not possible from mass-spectrometric data.A time-averaging method and use of a microcell as sample tube also allowed Keefer et ~11.~~~to analyse quantitatively mixtures with only 30-40 pg of individual components. Pulse Fourier-transform (FT) lH n.m.r. can be applied with much more .~~~facility to small samples. Thus, Bartle et ~ 1 were able to identify by FT 1H n.m.r. as little as 20 pg of single PAH in mixtures generally totalling less than 1 mg separated from atmospheric dust and from the condensates of tobacco and marijuana smoke; pulse FT spectra at 90 MHz allowed identification of both parent PAH and their methyl derivatives. For example, the spectra of all the fluoranthene-pyrene fractions showed a number of other peaks near 2.5-3.0 p.p.m., in addition to those of CH3 of the three methylpyrenes, indicating the presence of all five methylfluoranthenes (Figure 16).I3Cn.m.r. spectroscopy is a promising technique when sufficient sample is available. Signals from two sets of four-proton-bearing carbons and thiee quaternary carbons in the spectrum of a sulphur-containing PAC with molecular formula C14HeS isolated from com- mercial pyrene suggested it to be phenanthr0[4,5-bcd]thiophen.~~~ C.Infrared.-The chief disadvantages in the conventional infrared (i.r.) spectro- scopy of PAC from the environment are the absence of unique features in the spectra and the lack of proportionality between band strengths and concentra- tion, and until the introduction of matrix isolation FT-i.r.there had been few applications. Because the interactions of solute molecules with each other and with the solvent are minimized, the i.r. spectra of matrix-isolated species at cryogenic temperatures consist of very sharp vibrational lines.439 It is hence 436 L. K. Keefer, L. Wallcave, J. Loo, and R. S. Peterson, Anal. Chern., 1971, 43, 1411. 437 K. D. Bartle, M. L. Lee, and M. Novotny, Analyst (London), 1977, 102, 731. 438 W. Karcher, R. Depaus, J. van Eijk, and J. Jacob, in ref. 168, p. 341. 439 I. R. Dunkin, Chem. SOC.Rev., 1980, 9, 1. 157 Modern Analytical Methods for Environmental Polycyclic Aromatic Compounds 4-PY 1 8-F \ 1 I I I I I I 3.2 3.0 2.8 2.6 2.4 p.p.m. Figure 16 Methyl region of FT lH n.m.r.90 MHz spectrum of fluoranthenelpyrene fraction (0.9 mg) of tobacco-smoke condensate (Reproduced by permission from Analyst (London), 1977,102,731) possible to record highly resolved fingerprint spectra of PAC and to construct Beer’s law plots as long as FT4.r. is used to overcome the sensitivity limitations that apply for the now dilute analyte. Wehry et a1.405.406-4409441 have conducted detailed investigations of the applicability of MI FT4.r. in the qualitative and quantitative analysis of PAH. Samples in nitrogen matrices on a CsI surface at 15 K were produced by mix- ing the vapours of PAH effusing from a Knudsen cell with a large excess of nitrogen gas. Analyses of moderately complex mixtures (e.g., the methyl- chrysenes406) are made possible by the line-rich spectra obtainable at 2 cm-l resolution. The recording of FT-i.r. spectra in the gas phase of compounds eluted from a GC column allows a ‘chemigrani’ to be determined by plotting absorbance in different i.r. spectral windows, corresponding to different functional group frequences, against time.442 440 G. Mamantov, E. L. Wehry, R. R. Kemmerer, and E. R. Hinton, Anal. Chem., 1977,49, 86. 441 G. Mamantov, E. L. Wehry, R. R. Kemmerer, R. C. Stroupe, E. R. Hinton, and G. Goldstein, Adv. Chem. Ser., 1978, 170, 99. 44a M. D. Erickson, D. L. Newton, E. D. Pellizzari, and K. B. Tomer, J. Chromatogr. Sci., 1979,17,449.

ISSN:0306-0012    

DOI:10.1039/CS9811000113

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

INDEXES Volume 10, 1981 The indexes in this issue cover Volumes 1-10 (Figures in bold type refer to the volume number) Index INDEX OF AUTHORS Aarons, L. J., 5, 359 Ahluwalia, J. C., 2, 203 Allen, N. S., 4, 533 Arnbroz, H. B., 8, 353 Angyal, S. J., 9, 415 Atkinson, D., 8,475 Baker, A. D., 1, 355 Barker, B. E., 9, 143 Bartle, K. D., 10, 113 Bartlett, P. D., 5, 149 Baxendale, J. H., 7, 235 Beattie, 1. R., 4, 107 Bell, R. P., 3, 513 Bentley, P. H., 2, 29 Berkoff, C. E., 3, 273 Bird, C. L., 10, 49 Bird, C. W., 3, 309 Blandamer, M. J., 4, 55 Blundell, T. L., 6, 139 Boelens, H., 7, 167 Bradshaw, T. K., 6,43 Braterman, P. S., 2, 271 Breslow, R., 1,553 Brown, D. H., 9, 217 Brown, I. D., 7, 359 Brown, K.S., jun., 4, 263 Brundle, C. R., 1, 355 Buchanan, G. L., 3,41 Burdett, J. K., 3, 293; 7, 507 Burgess, J., 4, 55 Burrows, H. D., 3, 139 Burtles, S. M., 7, 201 Butterworth, K. R., 7,I85 Cadogan, J. I. G., 3, 87 Carabine, M. D., 1, 411 Cardin, D. J., 2, 99 Carless, H. A. J., 1, 465 Casellato, U., 8, 199 Cetinkaya, B., 2, 99 Chamberlain, J., 4, 569 Chatt, J., 1, 121 Chesters, J. K., 10, 270 Chivers, T., 2, 233 Clark, G. M., 5, 269 Collins, C. J., 4, 251 Colvin, E. W., 7, 15 Connor, J. N. L., 5, 125 Corfield, G. C., 1, 523 Cornforth, J. W., 2, 1 530 Cotton, F. A., 4, 27 Coulson, E. H., 1, 495 Cowan, J. M., 8, 419 Cox, B. G., 9, 381 Coyle, J. D., 1, 465; 3, 329; 4, 523 Cragg, G.M. L., 6, 393 Cramer, R. D., 3, 273 Crammer, B., 6, 431 Cross, R. J., 2, 271; 9, 185 Curthoys, G., 8,475 Dack, M. R. J., 4, 211 Dainton, F. S., 4, 323 Dalton, H., 8, 297 Davies, D. I., 8, 171 de Rijke, D., 7, 167 de Silva, A. P., 10, 181 de Valois, P. J., 7, 167 Dobson, J. C., 5, 79 Dowle, M. D., 8, 171 Doyle, M. J., 2, 99 Drummond, I., 2, 233 Dunkin, I. R., 9, 1 Elliott, M., 7, 473 Emsley, J., 9, 91 Eschenmoser, A., 5, 377 Evans, D. A., 2, 75 Evans, J., 10, 159 Fenby, D. V., 3, 193 Fenton, D. E., 6, 325; 8, 199 Ferguson, L. N., 4, 289 Fisher, L. R., 6, 25 Fleming, I., 10, 83 Flygare, W. H., 6, 109 Forage, A. J., 8, 309 Fox, M. F., 9, 143 Fry, A., 1, 163 Funk, R.L., 9, 41 Garson, M. J., 8, 539 Georghiou, P. E., 6, 83 Gheorghiu, M. D., 10, 289 Gibson, K. H., 6, 489 Gilbert, J., 10, 255 Gillespie, R. J., 8, 315 Goodings, E. P., 5, 95 Gorman, A. A., 10, 205 Gray, B. F., 5, 359 Green, C. L., 2, 75 Greenhill, J. V., 6, 277 Greenwood, N. N., 3, 231 Grey Morgan, C., 8, 367 Griffiths, J., 1, 481 Grimshaw, J., 10, 181 Grossert, J. S., 1, 1 Groves, J. K., 1, 73 Guilford, H., 2, 249 Gutteridge, N. J. A., 1, 381 Haines, R. J., 4, 155 Hall, G. G., 2, 21 Hall, L. D., 4, 401 Hall, T. W., 5, 431 Halliwell, H. F., 3, 373 Hamdan, I. Y., 8, 143 Hamer, G., 8, 143 Harmony, M. D., 1, 211 Harris, K. R., 5, 215 Harris, R. K., 5, 1 Harrison, L.G., 10, 491 Hartley, F. R., 2, 163 Hartshorn, S. R., 3, 167 Hathway, D. E., 9, 63, 24 I Henderson, J. W., 2, 397 Hepler, L. G., 3, 193 Hilburn, M. E., 8, 63 Hinchliffe, A., 5, 79 Holland, H. L., 10, 435 Holm, R. H., 10,455 Hore, P. J., 8, 29 Horton, E. W., 4, 589 Hudson, M. F., 4, 363 Huntress, W. T., Jr., 6,295Hutchinson, D. W., 6, 43 Ikan, R., 6,431 Isaacs, N. S., 5, 181 Isbell, H. S., 3, I Jaffe, H. H., 5, 165 James, A. M., 8, 389 Jamieson, A. M., 2, 325 Janes, N. F., 7,473 Jencks, W. P., 10, 345 Jenkins, J. A., 6, 139 Johnson, A. W., 4, 1; 9, 125 Johnson, S. P., 5, 441 Johnstone, A. H., 7, 317; 9, 365 Jones, J. R., 10, 329 Josh, C. G., 8, 29 Jotham, R.W., 2,457 Kalyanasundaram, K., 7,453 Keenan, A. G., 8,259 Kemp, T. J., 3, 139; 8, 353 Kennedy, J. F., 2, 355; 8,221 Kennewell, P. D., 4, 189; 9,477 Kenny, A. W., 4,90 King, G. A. M., 7,297 Kirby, G. W., 6, 1 Kitaigorodsky, A. I., 7, 133 Koch, K. R., 6, 393 Kolar, G. F., 9, 241 Kresge, A. J., 2, 475 Krishnaji, 7, 219 Kuhn, A. T., 10,49 Lappert, M. F., 2, 99 Lee, M. L., 10, 11 3 Lee-Ruff, E., 6, 195 Leigh, G. J., 1, 121; 4, 155 Lemieux, R. U., 7,423 Leznoff, C. C., 3, 65 Lindberg, B., 10,409 Lindsay, D. G., 10, 233 Lindoy, L. F., 4, 42€ Linford, R. G., 1,445 Lipscomb, W. N., 1, 319 Lynch, J. M., 3, 309 Lythgoe, B., 9, 449 McKean, D. C., 7, 399 McKellar, J.F., 4, 533 McKervey, M. A., 3, 479 Mackie', R. K., 3, 87 McLauchlan, K. A., 8, 29 McNab, H., 7, 345 Maitland, G. C., 2, 181 Maitlis, P. M., 10, 1 Manning, P. G., 5, 233 Maret, A. R., 2, 325 Maslowsky, E., 9, 25 Mason, R., 1,431 Mayo, B. C., 2, 49 Meadowcroft, A. E., 4, 99 Menger, H. W., 2, 415 Midgley, D., 4, 549 Millen, D. J., 5, 253 Mills, R., 5, 215 Moore, H. W., 2,415; 10,289 Morley, R., 5, 269 Morris, J. H., 6, 173 Mulheirn, L. F., 1, 259 Munn, A., 4, 87 Newman, J. F., 4, 77 Nightingale, W. H., 7, 195 Norman, R. 0. C., 8, 1 North, A. M., 1, 49 Oakenfull, D. G., 6, 25 Overton, K. H., 8, 447 Page, M. I., 2, 295 Perkins, P. G., 6, 173 Pickford, C. J., 10, 245 Pletcher, D., 4, 471 Poliakoff, M., 3, 293;7, 527 Prakash, V., 7, 219 Pratt, A.C., 6, 63 Ramm, P. J., 1, 259 Rao, C. N. R., 5,297 Ratledge, C., 8, 283 Rattee, I. D., 1, 145 Redl, G., 3, 273 Richards, D. H., 6, 235 Ritch, J. B., jun., 5, 452 Roberts, M. W., 6, 373 Robinson, F. A;, 5, 317 Roche, M., 5, 165 Rodgers, M. A. J., 7, 235; 10, 205 Rose, A. E. A., 6, 173 Rouvray, D. H., 3, 355 Rowlinson, J. S., 7, 329 Sanders, J. K. M., 6, 467 Sarma, T. S., 2, 203 Satchell, D. P. N., 4, 231; 6, 345 Satchell, R. S., 4, 231 Schlegel, W., 7, 177 Self, R., 10, 255 Senthilnathan, V. P., 5, 297 Shorter, J., 7, 1 Simpson, T. J., 4, 497 Singh, S., 5, 297 Slorach, S. A. 10, 280 Smith, E.B., 2, 181 Smith, K., 3, 443 Smith, K. M., 4, 363 Index Smith, W. E., 6, 173; 9,217 Snell, K. D., 8, 259 Stacey, M., 2, 145 Staunton, J., 8, 539 Stevens, M. F. G., 7, 377 Stoddardt, J. Fraser., 8, 85 Suckling, C. J., 3, 387 Suckling, K. E., 3, 387 Sutherland, J. K., 9, 265 Sutherland, R. G., 1,241 Sutton, D., 4, 443 Swan, J. S., 7, 201 Swindells, R., 7,212 Symons, M. C. R., 5, 337 Takken, H. J., 7, 167 Taylor, J. B., 4, 189; 9,477Taylor, S. E., 10, 329 Theobald, D. W., 5, 203 Thomas, T. W., 1, 99 Thompson, M., 1, 355 Thornber, C. W., 8, 563 Tincknell, R. C., 5, 46 Toennies, J. P., 3, 407 Tolman, C. A., 1, 337 Truax, D. R., 5,411 Twitchett, H. J., 3, 209 Tyman, J. H.P., 8, 499 Underhill, A. E., 1, 99; 9,429 van Dort, J. M., 7, 167 van der Linde, L. M., 7, 167 Varvoglis, A., 10, 377 Vaughan, K., 7, 377 Vidali, M., 8, 199 Vigato, P. A., 8, 199 Vollhardt, K. P. C., 9, 41 Wain, R. L., 6, 261 Walker, E. R. H., 5, 23 Walker, I. C., 3, 467 Waltz, W. L., 1, 241 Ward, I. M., 3, 231 Watkins, D. M., 9, 429 White, A. J., 3, 17 Whitfield, R. C., 1, 27 Wieser, H., 5, 411 Wiesner, K., 6, 413 Williams, G., 7, 89 53 1 Index Williams, R.J. P., 9, 281, 325 Wilson, A. D., 7,265 Wise, S. A,, 10, 113 Yoffe, A. D., 5, 51 5 32 Index INDEX OF TITLES Absorption bands in the spectra of stars, a crystal field approach, 5,233 Acidity of solid surfaces, 8,475Across the living barrier, 6, 325 Acylation by ketens and isocyanates, a mechanistic comparison, 4, 231 Acylation, Friedel-Crafts, of alkenes 1, 73 Adamantane rearrangements, 3, 379 Affinity chromatography, chemical aspects of 3, 249 Alcohols and amines, conformational analysis of, 5, 411 Alkali-metal complexes in aqueous solution, 4, 549 Alkaloids, aconite, synthesis of, 6, 413 Alkenes, the Friedel-Crafts acylation of, 1, 73 Aluminium phosphates, the chemistry and binding properties of, 6, 173 Amines and alcohols, conformational analysis of, 5,411Analysis of trace constituents of the diet, organic and inorganic, 10, 245, 255 Analytical methods, modern, for en- vironmen tal polycyclic aromatic compounds, 10, 113 Aphids and scale insects, their chemistry, 4,263Application of electrochemical tech-niques to the study of homogenous chemical reactions, 4,471Applications of e.s.r.spectroscopy to kinetics and mechanism in organic chemistry, 8, 1Application of research findings to the development of commercial flavour- ings, 7, 177 Aqueous mixtures, kinetics of reac-tions in, 4, 55 Aqueous solution, micelles in, 6, 25 Aryl cations-new light on old inter- mediates, s, 353 --halides, photochemistry and pho tocyclization of, 10, 181 Arylidiazonium cations, co-ordination chemistry of, 4,443Aryliodine(m) dicarboxylates 10, 377 Atmosphere, interactions in, of drop-lets and gases, 1,411 Autocatalysis, 7, 297 Azidoquinones and related com-pounds, chemistry of, 2, 415 Azobenzene and its derivatives, photo- chemistry of, 1,481 Bile pigments, 4, 363 Binding of heavy metals to proteins, 6, 139 Binding properties and chemistry of aluminium phosphates, 6, 173 Biological surfaces, molecular aspects of, a, 389 Biomimetic chemistry, 1, 553 Biosynthesis of sterols, 1, 259 Biosynthetic products from ara-chidonic acid, 6, 489 --studies, carbon-1 3 nuclear mag- netic resonance in, 4,497 Blood groups, human, and carbo-hydrate chemistry, 7,423Bond strengths, CH, in simple organic compounds: effects of conformation and substitution, 7, 399 --valences-a simple structural model for inorganic chemistry, 7, 359 Bredt’s rule, 3, 41 Bransted relation-recent develop-ments, 2,475Butadiene, polymerization and copoly- merization of, 6, 235 Calciferols, hormonal : chemistry of ‘vitamin’ D 6, 83 Calorimetric investigations of hydro- gen bond and charge transfer complexes, 3, 193 Cancer and chemicals, 4,289 Carbohydrate chemistry and human blood groups, 7,423 Carbohydrate-directed macromole-cules, transition-metal oxide chelates of, 8, 221 Carbohydrate-protein complexes, gly- coproteins, and proteoglycans, of human tissues, chemical aspects of, 2,355 Carbohydrates to enzyme analogues, 8, 85 Carbon-13 nuclear magnetic resonance in biosynthetic studies 4,497Carbonium ions, carbanions, and radicals, chirality in, 2, 397 Index Carbonyl clusters, metal, relationship with supported metal catalysts, 10, 159 _-compounds, photochemistry of, 1,465Carcinogens, chemical, mechanisms of reaction with nucleic acid, 9, 241 Catalysis and surface chemistry, new perspectives, 6, 373 Catalysis, homogenous, and organo- metallic chemistry, the 16 and 18 electron rule in, 1,337 -of the olefin metathesis reaction, 4, 155 Catalysts, supported metal, relation-ship with metal carbonyl clusters, 10, 159 CENTENARY LECTURE.Biomimetic chemistry, 1, 553 CENTENARY Light scattering LECTURE. in pure liquids, and solutions, 6, 109 CENTENARY Metal Clusters LECTURE, in biology, 10,455 CENTENARYLECTURE.Quadruplebonds and other multiple metal to metal bonds, 4, 27 CENTENARYLECTURE. Rotationally and vibrationally inelastic scattering of molecules, 3,407CENTENARYLECTURE. Systematic development of strategy in the synthesis of polycyclic polysubsti- tuted natural products: the aconite alkaloids, 6,413 CENTENARYLECTURE.Three-dimen- sional structures and chemical mechanisms of enzymes, 1, 319 Charge transfer and hydrogen bond complexes, calorimetric investiga-tions of, 3, 193 Chemical applications of advances in Fourier transform spectroscopy 4, 569 --aspects of affinity chromato-graphy 9 2, 249 --of glycoproteins, proteo- glycans, and carbohydrate-protein complexes of human tissues, 2, 355 --education research: facts, find- ings, and consequences, 9, 365 --interpretations of molecular wavefunctions, 5, 79 Chemically-induced dynamic electron polarization (CIDEP), role in chem- istry, 8, 29 Chemicals in rodent control, 1, 381 -which control plant growth, 6, 261 Chemistry and binding properties of aluminium phosphates, 6, 173 CHEMISTRY AND FLAVOUR I Molecular Structure and Organo- leptic Quality, 7, 167 I1 Application of Research Find-ings to the Development of Commercial Flavourings, 7, 177 I11 Safety Evaluation of Natural and Synthetic Flavourings, 7, 185 IV The Influence of Legislation on Research in Flavour Chemistry, 7, 195 V The Development of Flavour in Potable Spirits, 7,201 VI The Influence of Flavour Chem- istry on Consumer Acceptance, 7,212 and the new industrial revolution, 5, 317 --,-a topological subject, 2, 457 of aphids and scale insects, 4, 263 --of azidoquinones and related compounds, 2,415 of dental cements, 7, 265 of dyeing, 1, 145 -of the gold drugs used in the treatment, of rheumatoid arthritis, 9,217 --of homonuclear sulphur species, 2,233 --of long-chain phenols of non-isoprenoid origin, 8,499 --of the production of organicisocyanates, 3, 209 --of transition-metal carbene com- plexes and their role as reaction intermediates, 2, 99 --of ‘vitamin’ D: the hormonal calciferols, 6, 83 , some considerations on the philosophy of, 5, 203 Chirality in carbonium ions, carban- ions, and radicals, 2, 397 Chlorophyll chemistry, n.m.r. spectral change as a probe, 6,467Chromatography, affinity, chemical aspects of, 2, 249 Cis-and trans-effects of ligands, 2, 163 Clathrates and molecular inclusion phenomena, 7, 65 Collisional transfer of rotational energy and spectral lineshapes, 7,219 Compartmental ligands: routes to homo- and hetero-dinuclear com- plexes, 8, 199 Complex formation between sugarsand metal cations, 9,415 -hydride reducing agents, the functional group selectivity of, 5, 23 Complexes, alkali-metal, in aqueous solution, 4, 549 --homo-and hetero-dinuclear, routes via compartmental ligands, 8, 199 --, 1-D metallic, 9,429 Complexes, square-planar, isomerisa- tion mechanisms of, 9, 185 Computer resolution of overlappingelectronic absorption bands, 9, 143 Conductivity and superconductivityin polymers, 5, 95 Conformation and substitution, effects of, on individual CH bond strengths in simple organic compounds, 7, 399 --of rings and neighbouring group effects, development of Hawort h’s concepts of, 3, 1 Conformational analysis of some alcohols and amines: a comparison of molecular orbital theory, rota- tional and vibrational spectroscopy, 5, 411 --studies on small molecules, 1, 293 Contribution of ion-pairing to ‘memory effects’, 4, 251 Contributions of pulse radiolysis to chemistry, 7,235 Conversion of ammonium cyanate into urea-a saga in reaction mechanisms 7?1 Co-ordination chemistry of aryldia-zonium cations : aryldiazenato (ary- lazo) complexes of transition metals, and the aryldiazenato-nitrosyl ana-logy7 4,443 Corrin synthesis, p0st-B~ problems in, 5, 377 Crystal field approach to absorptionbands in the spectra of stars, 5, 233 Crystals and molecules, organic, non- bonded interactions of atoms in, 7, 133 Cyanoketenes: synthesis and cyclo-addit ions, 10,289 Index Cyclization, initiation of, using 3-methylcyclohex-2-enone derivatives, 9, 265 Cyclopol ymerizat ion, 1, 523 Dental cements, chemistry of, 7,265 Development of flavour in potablespirits, 7, 201 Dielectric relaxation in polymer solu- tions, 1, 49 Diffusion in liquids, the effect of isotopic substitution on, 5, 215 Difluoroamino-radical, gas-phasekinetics.of, 3, 17 Droplets and gases, interactions in the atmosphere of, 1, 411 Drug design, isosterism and molecular modification in, 8, 563 --, quantitative, 3, 273 Dyeing, chemistry of, 1, 145 Echinoderms, 1, 1 Education, chemical, a reassessment of research in, 1, 27 --__ , review of research and development in the U.K., 1972-1976, 7, 317 Effect of isotopic substitution on diffusion in liquids 5,215 Electrochemical techniques, applica-tion of to study of homogenouschemical reactions, 4,471 Electron as a chemical entity, 4, 323 --scattering spectroscopy, thres-hold, 3,461 --spectroscopy , 1,355Electronic absorption bands, over-lapping, computer resolution of, 9, 143 Electronic properties of some chain and layer compounds, 5, 51 -_ transitions, vibrational intensi- ties in, 5, 165 Electrons, solvated, in solutions of metals, 5, 337 Electrophilic aromatic substitutions, non-conventional, and related reac- tions, 3, 167 --C-nitroso-compounds, 6, 1 Elimination reactions, isotope effect studies of, 1, 163 Enaminones, 6,277 Energetics of neighbouring groupparticipation, 2, 295 Index Enumeration methods for isomers, 3,355 Environmental lead in perspective, 8, 63 --poIycyclic aromatic compounds, modern analytical methods for, 10, 113 --protection in the distribution of hazardous chemicals, 4, 99 -regulation: an international view, 5,431Enzyme analogues from carbohy-drates, 8, 85 Enzymes, immo bi Iized, 6, 215 -in organic synthesis, 3, 387 --, the logic of working with, 2, 1 --, three-dimensional structures and chemical mechanisms of, 1, 319 Enzymic reactions, stereocheniical choice in, 8,447 E.s.r.spectroscopy, applications to kinetics and mechanism in organic chemistry, 8, 1 Experimental studies on the structure of aqueous solutions of hydro-phobic solutes, 2, 203 FARADAY The electron as a LECTURE. chemical entity, 4, 323 Fats grown from wastes, 8, 283 Fe(Co)4, 7, 527 5-Substituted pyrimidine nucleosides and nucleotides, 6, 43 Fixation, of nitrogen, 1,121 Forces between simple molecules, 2,181 Foreign compounds in mammals, importance of non-enzymic chemi- cal reaction processes to the rate of, 9, 63 Formation of hydrocarbons by micro- organisms, 3, 309 Fourier transform spectroscopy,chemical applications of advances in, 4, 569 Four-membered rings and reaction mechanisms, 5, 149 Friedel-Crafts acylation of alkenes, 1, 73 Functional group selectivity of com-plex hydride reducing agents, 5, 23 Gas-phase kinetics of the difluoro-amino-radical, 3, 17 Gases, and droplets, interactions in the atmosphere of, 1, 411 Glycoproteins, proteoglycans, and carbohydrate-protein complexes of human tissues, chemical aspects of, 2,355Gold drugs used in the treatment of rheumatoid arthritis, chemistry of, 9,217Growth of computational quantum chemistry from 1950 to 1971, 2, 21 Handling toxic chemicals-environ-mental considerations, 4, 77 HAWORTHMEMORIAL TheLECTURE.consequences of some projectsinitiated by Sir Norman Haworth.2, 145 HAWORTHMEMORIAL TheLECTURE. Haworth-Hudson controversy and the development of Haworth's con- cepts of ring conformation and of neighbouring group effects, 3, 1 HAWORTH MEMORIAL LECTURE. Human blood groups and carbo- hydrate chemistry, 7, 507 HAWORTH MEMORIAL LECTURE. Structural studies of polysac-charides, 10,409Hazards in the chemical industry-risk management and insurance, 8, 419 Health hazards to workers from industrial chemicals, 4, 82 Homogenous catalysis, and organo-metallic chemistry, the 16 and 18 electron rule in, 1, 337 Homogenous chemical reactions, application of electrochemical tech- niques to the study of, 4, 471 Human blood groups and carbo-hydrate chemistry, 7,423 Hydrocarbon formation by micro-organisms, 3, 309 Hydrogen bond and charge transfer complexes, calorimetric invest iga- tions of, 3, 193 --bonding, very strong, 9, 91 -_ is0 tope effects, kinetic, recent advances in the study of, 3, 513 Hydrophobic solutes, experimentalstudies on the structure of aqueous solutions of, 2, 203 Imines, photochemistry of, 6, 63 Immobilized enzymes, 6, 215 Importance of (non-enzymic) chemical reaction processes to the fate of 5 36 foreign compounds in mammals, 9, 63 Importance of solvent internal pressure and cohesion to solution phenomena 4,211Inclusion phenomena, molecular, and clathrates, 7, 65 Individual CH bond strengths in simple organic compounds: effects of con- formation and substitution, 7, 399 Industry, chemical, hazards in : risk management and insurance, 8,419 Influence of flavour chemistry on consumer acceptance, 7,212 Influence of legislation on research in flavour chemistry, 7, 195 Infrared and Raman vibrational spec- troscopy in inorganic chemistry, 4, 107 INGOLDLECTURE.Four-membered rings and reaction mechanisms, 5, 149 INGOLD How does a reaction LECTURE. choose its mechanism, 10,345Initiation of cyclization using 3-methylcyclohex-2-enone derivatives, 9, 265 Inorganic chemistry, bond valences, a simple structural model for, 7, 359 Inorganic pyro-compounds Mal(X207)bJ, 5, 269 Insect attractants of natural origin, 2, 75 Insecticides, a new group of: syntheticpyrethroids, 7, 473 Interactions in the atmosphere of droplets and gases, 1,411 --, ion-solvent, thermodynamics of, 9, 381 _-, metal-metal, in transition-metal complexes containing infinite chains of metal atoms, 1,99 --, non-bonded, of atoms in organiccrystals and molecules, 7,133 Introducing a new agricultural chemi- cal 4, 77 Ion-molecule reactions in the evolu- tion of simple organic molecules in interstellar clouds and planetaryatmospheres, 6, 295 Ion-pairing, contribution to ‘memory effects’, 4, 251 Ion-solven t interactions, thermody-namics of, 9, 381 Isocyanates and ketens, a mechanistic Index comparison of acylation by, 4, 231 -, organic, chemistry of the produc- tion of, 3, 209 Isomer enumeration methods, 3, 355 Isomerisation mechanisms of square- planar complexes, 9, 185 Isosterism and molecular modifica-tion in drug design, 8, 563 Isotope effect studies of elimination reactions, 1, 163 Isotopic hydrogen exchange in purines : mechanisms and applications, 10, 329 -substitution effects on diffusion in liquids, 5,215 JOHN JEYES Chemicals which LECTURE.control plant growth, 6, 261 KELVINLECTURE. Across the livingbarrier, 6, 325 Ketens and isocyanates, a mechanistic comparison of acylation by, 4, 231 Kinetics and mechanism in organicchemistry, applications of e.s.r. spectroscopy to, 8, 1 --, gas-phase, of the difluoro-aminoradical, 3, 17 -of reactions in aqueous mixtures, 4, 55 p-Lactams, synthetic routes to, 5, 181 Lanthanide shift reagents in nuclear magnetic resonance spectroscopy, 2, 49 Laser light scattering, quasielastic, 2.325 Laser spectroscopy of ultra-trace quantities, 8, 367 Lasers, tunable, 3,293 Lead, environmental, in perspective,8, 63 Ligands, cis-and trans-effects of,2, 163 --, compartmental: routes to homo- and hetero-dinuclear complexes, 8, 199 Liquid, surface of, 7, 329 LIVERSIDCE On first looking LECTURE. into nature’s chemistry : I The role of small molecules and ions: the transport of elements, 9,281 I1 The role of large molecules, especially proteins, 9, 325 Index LIVERSIDGE Recent advances LECTURE.in the study of kinetic hydrogenisotope effects, 3, 513 LIVERSIDGE The surface of aLECTURE. liquid, 7, 329 Macrocyclic ligands, synthetic, transi- tion metal complexes of, 4, 421 Main-group elements, ring, cage, and cluster compounds of, 8, 315 Matrix isolation technique and its application to organic chemistry, 9,1 Mechanisms, chemical, and three-dimensional structures of enzymes, 1, 319 --, isomerisation, of square-planar complexes, 9, 185 --, or reaction between ultimate chemical carcinogens and nucleic acid, 9, 241 MELDOLA ChemicalMEDAL LECTURE. aspects of glycoproteins, proteo-glycans, and carbohydrate-protein complexes of human tissues, 2, 355 MEDALLECTURE.MELDOLA Fe(C0)47, 527 MEDALLECTURE.MELDOLA Molecular collisions and the semiclassical approximat ion, 5, 125 MEDALLECTURE.MELDOLA Molecular Shapes, 7, 507 MELDOLA MEDAL LECTURE.N.m.r. spectral change as a probe of chlorophyll chemistry, 6,467MELDOLA The rela- MEDAL LECTURE. tionship between metal carbonylclusters and supported metal cata- lysts, 10, 159 Meldrum’s acid, 7, 345 Metal carbonyl clusters, relationship with supported metal catalysts, 10,159 --clusters in biology, 10,455 Metal-metal bonding and metallo-boranes, 3, 231 Metal-ion-promoted reactions of organo-sulphur compounds, 6, 345 1 -D Metallic complexes, 9, 429 Metalloboranes and metal-metal bonding, 3, 231 _-bonds, multiple (especially quad- ruple), 4, 27 --interactions in transition-metal complexes containing infinite chains of metal atoms, 1, 99 Metals, binding to proteins, 6, 139 Methyl group removal in steroid biosynt hesis, 10,435 Micelle-forming surfactant solutions, photophysics of molecules in, 7, 453 Micelles in aqueous solution, 6, 25 Microbes, use in the petrochemicalindustry, 8, 297 Micro-organisms, protein production by7 8, 143 Molecular aspects of biological sur-faces, 8, 389 -collisions and the semiclassical approxi mat ion, 5, 125 -orbital theory, comparison with rotational and vibrational spectro- scopy in conformational analysisof alcohols and amines, 5, 41 1 --shapes, 7,507 -_ structure and organolepticquality, 7, 167 --wavefunctions, chemical inter-pretations of, 5, 79 Monoalkyl triazenes, 7, 377 Morphogenesis, biological, the physi- cal chemistry of, 10, 491 Motion, molecular, and time-corre-lation functions, 7, 89 Multistability in open chemical reac- tion systems, 5, 359 Natural products from echinoderms, 1, 1 ----, pol ycycl ic pol ysu bs t it uted, systematic development of strategy in, 6,413 Neighbouring-group effects and ring conformation, development of Haworth’s concepts of, 3, 1 --participation, enegetics of, 2, 295 New perspectives in surface chemistry and catalysis, 6, 373 Nitrogen fixation, 1, 121 C-nitroso-compounds, electrophilic, 6,.1 Non-bonded interactions of atoms in organic crystals and molecules, 7, 133 Non-conventional electrophilic aro-matic substitutions and related reactions, 3, 167 Nuclear magnetic resonance and the periodic table, 5, 1 -__ --,carbon-13, in bio-synthetic studies, 4,497 ----methods (new) for tracing the future of hydrogen in biosyn t hesis, 8, 539 ----spectral change as a probe of chlorophyll chemistry, 6, 467 ~ __ --spectroscopy, lan-thanide shift reagents in, 2, 49 -----: spin-latticerelaxation, 4,401 Nucleic acid, mechanisms of reaction with ultimate chemical carcinogens, 9, 241 Nucleosides and nucleotides, pyrimi- dine, kubstituted, 6, 43 Nutritional chemistry of inorganictrace constituents of the diet, 10, 270 NYHOLM LECTURE.MEMORIAL Chemi-cal education research : facts, find- ings, and consequences, 9, 365 NYHOLM LECTURE.For-MEMORIAL ward from Nyholm’s Marchon Lecture, 3, 373 NYHOLM LECTURE.MEMORIAL Growth, change, challenge, 5,253NYHOLM MEMORIALLECTURE.Ring,cage, and cluster compounds of the main group elements, 8, 315 Olefin metathesis and its catalysts, 4, 155 Olefinic compounds, photochemistry Of, 3, 329 On first looking into nature’s chemi- stry : I The r6le of small molecules and ions: the transport of theelements, 9,281I1 The rble of large molecules, especially proteins, 9, 325 Organic chemistry of superoxide, 6, 195 Prganoboranes as reagents for organic synthesis, preparation of, 3, 443 Organoborates in organic synthesis : the use of alkenyl-, alkynyl-, and cyano-borates as synthetic inter-mediates , 6, 393 Organometallic chemistry and homo- genous catalysis, the 16 and 18 electron rule in, 1, 337 Organomethyl compounds, synthesis, Index structure, and vibrational spectra, 9, 25 Organosulphur compounds, metal-ion-promoted reactions of, 6, 345 Organo-transition-metal complexes:stability, reactivity, and orbital cor- relations, 2,271 Oxygen, singlet molecular, 10, 205 PEDLER LECTURE.Porphyrins and related ring systems, 4, *1 Phase boundaries, reactivity of organic molecules at, 1,229 Phenols, long-chain, of non-isoprenoidorigin, 8,499Philosophy of chemistry, some con-siderations, 5,203Phosphates, aluminium, the chemistry and binding properties of, 6, 173 Phosphorus compounds, tervalent, in organic synthesis, 3, 87 Photochemistry of azobenzene and its derivatives, 1,481 -of carbonyl compounds, 1,465 -of imines, 6, 63 -of olefinic compounds, 3, 329 -of organic sulphur compounds, 4, 523 -of the uranyl ion, 3, 139 --of transition-metal co-ordination compounds-a survey, 1, 241 Photocyclization and photochemistry of aryl halides, 10, 181 Pho todegradation and stabilization of commercial polyolefins, 4, 533 Photophysics of molecules in micelle- forming surfactant solutions, 7,453 Plant growth, control by chemicals, 6,261Platinum metal complexes, +cycle-pentadienyl and +arene as protect- ing ligands towards, 10, 1 Polymer solutions, dielectric relaxa- tion in, 1, 49 -supports, insoluble, use in organic chemical synthesis, 3, 65 Polymerization and copolymerization of butadiene, 6,235 Polymers, conductivity and super-conductivity in, 5, 95 Polyolefins, commercial, photodegra- dation and stabilisation of, 4, 533 Pol ysaccharides, structural studies of, 10,409 Index Porphyrins and related ring systems, 4, 1 Post-Blz problems in corrin synthesis, 5,377 Preparation of organoboranes: re-agents for organic synthesis, 3,443PRESIDENTIALADDRESS1976.Chemi-stry and the new industrial revolu- tion, 5, 317 Properties and syntheses of sweetening agents, 6,431 Prostaglandins, tomorrow's drugs 4, 589 --, thromboxanes, PGX: biosyn-thetic products from arachidonic acid, 6,489Prostanoids, total syntheses of, 2, 29 Protecting ligands, 95-cyclopentadienyl and q6-arene towards platinummetal complexes, 10, 1 Protein production by micro-organ- isms, 8, 143 Proteins, binding of heavy metals to, 6, 139 Proteins, r61e of in nature's chemistry, 9, 325 Pulse radiolysis, contributions to chemistry, 7, 235 Purines, isotopic hydrogen exchange in, mechanisms and applications, 10, 329 Pyrimidine nucleosides and nucleo-tides, 5-su bst it uted, 6, 43 Pyro-compounds, inorganic, Ma[(X207)b], 5,269 Quadruple bonds and other multiple metal to metal bonds, 4, 27 Quantitative drug design, 3,273 Quantum chemistry, computational, growth of from 1950 to 1971, 2, 21 --mechanical tunnelling chemi-StrY, 1,211 Quasielastic laser light scattering, 2, 325 Radioactive and toxic wastes: a com- parison of their control and disposal, 4, 90 Radiolysis, pulse, contributions to chemistry, 7, 235 Raman and infrared vibrational spec- troscopy in inorganic chemistry,- 4.107 Reaction mechanisms, four-memdered rings and, 5, 149 ----, the conversion of ammo- nium cyanate into urea, 7, 1 Reactivity of organic molecules at phase boundaries, 1,229Recent advances in the study of kinetic hydrogen isotope effects, 3, 513 Recent syntheses in the Vitamin D field, 9, 449 Research in chemical education: a reassessment, 1, 27 RESOURCES CONSERVATION BY NOVEL BIOLOGICAL PROCESSES I Grow Fats from Wastes, 8, 283 I1 The Use of Microbes in the Petro- chemical Industry, 8,297I11 Utilization of Agricultural and Food Processing Wastes con-taining Carbohydrates, 8, 309 Review of chemical education research and development in the U.K., 1972-1 976, 7, 317 Ring, cage, and cluster compounds of the main group elements, 8, 315 ROBERT ROBINSON Post-Bl2LECTURE.problems in corrin synthesis, 5, 377 ROBERT LECTURE.ROBINSON The logic of working with enzymes, 2,. 1 ROBERT ROBINSON LECTURE.Vitamin BIZ.Retrospect and Prospects, 9, 125 Rodent control, chemicals in, 1, 381 Role of chemically-induced dynamic electron polarization (CIDEP) in chemistry, 8, 29 Rotationally and vibrationally inelastic scattering of molecules, 3,407 Safety evaluation of natural and synthetic flavourings, 7, 185 Scale insects and aphids, chemistry of, 4,263Silicon compounds in organic syn-thesis, some uses of, 10,83 -in organic synthesis, 7, 15 16 and 18 Electron rule in organo- metallic chemistry and homogenous catalysis, 1,337 Small molecules, conformation studies 0.", 1,293 Solids, surface energy of, 1,445 Solute-solvent interactions, spectro-scopic studies of, 5,297 Solution phenomena, the importance of solvent internal pressure and cohesion, 4, 211 Solutions of metals: solvated electrons, 5, 337 Solvent internal pressuie and co-hesion, importance to solution phe- nomena, 4,211Some considerations on the philosophy of chemistry, 5, 203 Some recent developments in chemistry teaching in schools, 1,495Spectra of stars, absorption bands in, a crystal field approach, 5, 233 Spectral lineshapes, collisional trans-fer of rotational energy with, 7, 219 Spectroscopic studies of solute-solvent interactions, 5, 297 Spectroscopy , electron, 1,355 -, Fourier transform, chemical ap- plications of advances in, 4, 569 --, laser, of ultra-trace quantities, 8, 367 --, rotational and vibrational, com- parison with molecular orbital theory in confirmational analysis of alcohols and amines, 5, 411 __ , threshold electron scattering, 3, 467 Spin-lattice relaxation: a fourth dimension for proton n.m.r.spectro- SCOPY, 4,401Square-planar complexes, isomerisa-tion mechanisms of, 9, 185 Stability, reactivity, and orbital cor-relations of organo-transition-metal complexes, 2, 271 Stereochemical choice in enzymic reactions, 8, 447 Steroid biosynthesis, methyl group removal in, 10,435 --, routes to by intramolecular Diels-Alder reactions of o-xyly-lenes, 9, 41 Sterols, biosynthesis of, 1, 259 Structure of aqueous solutions of hydrophobic solutes, experimental studies on, 2, 203 Substitution and conformation, effects of, on individual CH bond strengths in simple organic compounds, 7,399 Sugars, complex formation with cations, 9, 415 Sulphoximides 4, 189 Index Sulphoximides-an update, 9, 477 Sulphur compounds, organic photo- chemistry of, 4, 523 -_ , organic compounds of, metal- ion-promoted reactions of, 6, 345 -_ species, homonuclear, chemistry of, 2, 233 Superconductivity and conductivity in polymers, 5, 95 Superoxide, organic chemistry of, 6, 195 Surface chemistry and catalysis, new perspectives, 6, 373 --energy of solids, 1,445 -_ modified electrodes, 8, 259 -_ of a liquid, 7, 329 Surfaces, biological, molecular aspects of 8, 389 --, solid, their acidity, 8, 475 Sweetening agents, properties and synthesis of, 6, 431 Syntheses and properties of sweetening agents, 6,431 --, recent, in the Vitamin D field, 9,449 _-, total, of prostanoids, 2, 29 Synthesis and cycloadditions, cyano- ketenes, 10, 289 __ and synthetic utility of halo-lactones, 8, 171 --, of corrins, post-B~z problems in, 5,377 --of polycyclic polysubstitutednatural products, systematic de-velopment of strategy in, 6, 413 ___, organic, enzymes in, 3, 387 __ , organic, preparation of organo- boranes as reagents for, 3, 443 __ , organic, silicon in, 7, 15 __ , organic, some uses of silicon compounds, 10, 83 --, organic, tervalent phosphorus compounds in, 3, 87 --, organic, use of inorganic poly- mer supports in, 3, 65 --, organic, the use of organo-borates as synthetic intermediates, 6, 393 --, structure, and vibrational spec- tra of organomethyl compounds, 9, 25 Synthetic pyrethroids, a new group of insecticides, 7,473 --routes to p-lactams, 5,181 Index Systematic development of strategy in the synthesis of polycyclic poly- substituted natural products: the aconite alkaloids, 6, 413 TATE AND LYLE LECTURE.From carbohydrates to enzyme analogues, 8, 85 TATEAND LYLE LECTURE. Spin-latticerelaxation: a fourth dimension for proton n.m.r. spectroscopy, 4, 401 TATEAND LYLE LECTURE. Transition-metal oxide chelates of carbo-hydrate-directed macromolecules, 8, 221 Teaching of chemistry in schools, some recent developments in, 1,495 Tervalent phosphorus compounds in organic synthesis, 3, 87 Thermal, photochemical, and transi- tion-metal mediated routes to steroids by intramolecular Diels-Alder reactions of o-xylylenes (u-quinodimethanes), 9, 41 Thermodynamics of ion-solvent inter- actions, 9, 381 Three-dimensional structures and chemical mechanisms of enzymes,1, 319 Threshold electron scattering spectro- SCOPY, 3,467 Thromboxanes, prostaglandins, PGX : biosynthetic products of arachidonic acid, 6,489 TILDENLECTURE.Applications of e.s.r. spectroscopy to kinetics and mechanism in organic chemistry, 8, 1 TILDENLECTURE. Concerning stereo- chemical choice in enzymic reac-tions, 8, 447 TILDEN +CyclopentadienylLECTURE. and +arene as protecting ligands towards platinum metal complexes, 10, 1 TILDENLECTURE. Electrophilic C-nitroso-compounds, 6, 1 TILDENLECTURE. New perspectivesin surface chemistry and catalysis, 6, 373 TILDENLECTURE. The initiation of cyclization using 3-methylcyclohex- 2-enone derivatives, 9, 265 TILDEN Some uses of silicon LECTURE. compounds in organic synthesis, 10,83TILDEN Valence in transition-LECTURE.metal complexes, 1,431Time-correlation functions and mole- cular motion, 7, 89 Topological subject-chemistry, 2,457 Trace constituents of the diet, chemi- cal aspects, 10, 233 __ organic constituents of the diet, sources and biogenesis, 10, 280 Transition-metal carbene complexes, chemistry and role as reaction intermediates, 2, 99 -_ complexes, containing intinite chains of metal atoms, metal-metal interactions in, 1999 -_ complexes of synthetic macrocy- clic ligands, 4,421 --complexes, valence in, 1,431 --co-ordination compounds, photo- chemistry of, 1, 241 --oxide chelates of carbohydrate-directed macromolecules, 8, 221 Tunable lasers, 3, 293 Uranyl ion, photochemistry of, 3, 139 Use of insoluble polymer supports in organic chemical synthesis, 3, 65 Utilization of agricultural and food processing wastes containing carbo- hydrates, 8, 309 Valence in transition-metal complexes, 1,431Valences, bond, a simple structural model for inorganic chemistry, 7, 359 Very strong hydrogen bonding, 9, 91 Vibrational infrared and Raman spec- troscopy in inorganic chemistry, 4, 107 --intensities in electronic transi- tions, 5,165 -spectra, synthesis, and structure of organomethyl compounds, 9, 25 Vibrationally and rotationally in-elastic scattering of molecules, 3,407Viologens, electrochemistry of, 10, 49 Vitamin B12, retrospects and prospects, 9, 125 ‘Vitamin’ D, chemistry of: the hor- monal calciferols, 6, 83 Vitamin D, recent syntheses in, 9,449

ISSN:0306-0012    

DOI:10.1039/CS9811000529

出版商:RSC    

年代:1981

数据来源: RSC