ISSN:0306-0012    

DOI:10.1039/CS99019FX013

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年代:1990

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ISSN 0306-001 2 CSRVBR 19(4) 355-491 (1990) Chemical Society Reviews Vol19 No 4 1990 Page Coordination Compounds of C-Nitroso-compounds By M. Cameron, B. G. Gowenlock, and G. Vasapollo 355 Two-Dimensional Nuclear Magnetic Resonance By James Keeler 38 1 Infrared Laser Powered Homogenous Pyrolysis By Douglas K. Russell 407 Dynamic Decay Properties of Excited Electronic States of Polyatomic Molecular Ions Studied with Synchrotron Radiation By Richard P. Tuckett 439 1990 Indexes 47 1 The Royal Society of Chemistry Cambridge Chemical Society Reviews EDITORIAL BOARD Dr. M. J. Blandamer Professor H. W. Kroto F.R.S. (Chairman) Dr. A. R. Butler Professor J. A. McCleverty Professor B. T. Golding Professor S. M. Roberts Professor M.Green Professor B. H. Robinson Editor: Mr. K. J. Wilkinson Chemical Society Reviews (ISSN 0306-0012) is published quarterly and comprises approximately 20 articles (ca. 500pp) per annum. Articles of three types appear: (a) personalized accounts of their own contributions by recognized authorities; (b) in-depth articles covering the state of the art of the subject under review; (c) introductory reviews of new topics, suitable for non-specialist readers. The texts of the lectures given by the Society’s named lecturers are also published in Chemical Society Reviews. Although the majority of articles are intended to be specially commissioned, the Society is always prepared to consider offers of articles for publication. In such cases a short synopsis, rather than the completed article, should be submitted to the Managing Editor, Books and Reviews Section, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF.Members of the Royal Society of Chemistry may subscribe to Chemical Society Reviews at E22.00 per annum; they should place their orders on the Annual Subscription renewal forms in the usual way. All other orders accompanied with payment should be sent directly to The Royal Society of Chemistry, The Distribution Centre, Blackhorse Road, Letch- worth, Herts. SG6 1HN England. 1990 annual subscription rate U.K. f64.00, E.E.C. (x U.K.) f71, Rest of World f74.00, U.S.A. $144.00. Air freight and mailing in the U.S.A. by Publications Expediting Inc., 200 Meacham Avenue, Elmont, New York 11003.U.S.A. Postmaster: Send address changes to Chemical Society Reviews, Publications Expediting Inc., 200 Meacham Avenue, Elmont, New York 11003. Second class postage is paid at Jamaica, New York 11431. All other despatches outside the U.K. by Bulk Airmail within Europe, Accelerated Surface Post outside Europe. 0The Royal Society of Chemistry, 1991 All Rights Reserved No part of this book may be reproduced or transmitted in any form or by any means -graphic, electronic, including photocopying, recording, taping, or information storage and retrieval systems -without written permission from The Royal Society of Chemistry Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF Printed in England by Clays Ltd, St Ives plc

ISSN:0306-0012    

DOI:10.1039/CS99019FP015

出版商:RSC    

年代:1990

数据来源: RSC

ISSN:0306-0012    

DOI:10.1039/CS99019BP017

出版商:RSC    

年代:1990

数据来源: RSC

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Chem. SOC.Rev. 1990,19,355-379 Coordination Chemistry of C-Nitroso-compounds By M. Cameron and B. G. Gowenlock * DEPARTMENT OF CHEMISTRY, HERIOT-WATT UNIVERSITY, RICCARTON,EDINBURGH EH14 4AS, SCOTLAND G. Vasapollo CENTRO MIS0 DEL CNR, DIPARTIMENTO DI CHIMICA, UNIVERSITA DI BARI, VIA AMENDOLA 173, 70126 BARI, ITALY 1 Introduction The chemistry of C-nitroso-compounds commenced in 1874 with the synthesis of p-nitrosodimethylaniline (NODMA) and of nitrosobenzene (NOB). It was not until the 1960s, however, that the coordination chemistry of C-nitroso-compounds began to be explored in detail, and relatively few coordination compounds were reported before 1950. The chemistry of C-nitroso-compounds has been actively investigated from the time of the first synthesis and has been successively reviewed from 1903 onwards 3--8 without attention being directed to the potential of nitroso-compounds for coordinating to metals.A few early references to coordination compounds of NOB and NODMA are to be found in the literature, beginning with Pickard and Kenyon’ in 1907 who claimed the preparation of (NOB)5.Cd12 and (NODMA)2.ZnC12 by direct addition of alcoholic solutions of the components. Baudisch ‘Ovl’ interacted NOB and the hexacyanoferrate(r1) anion to form (NOB)Fe(CN); -,which was present in solution. Other NOB complexes with titanium and tin tetrachlorides (MC14.NOB) were prepared in 192712 and a series of uranyl complexes with NODMA and p-nitrosodiethylaniqne (NODEA) containing the ions UO; + (NODMA), and UO;’ (NODEA), (n = 1 or 2) were first prepared in 1933.13 In 1965, Gustorf and Jun l4 drew attention to the formation of an unanalysed solid * To whom correspondence should be addressed.A. Baeyer and H. Caro, Ber., 1874,7,809. A. Baeyer, Ber., 1874,7, 1638. J. Schmidt, ‘Sammlung chemischer und chemisch-technischer Vortrage’, Vol. 8, Chapter 11, ed. F. B. Ahrens, Enke, Stuttgart, 1903. N. V. Sidgwick, ‘The Organic Chemistry of Nitrogen’, Clarendon Press, Oxford, 1937, p. 204. B. G. Gowenlock and W. Liittke, Quart. Rev., 1958,12, 321. P. A. S. Smith, ‘The Chemistry of Open-Chain Organic Nitrogen Compounds’, Vol. 2, Benjamin, New York, 1966, p. 355. ’‘The Chemistry of the Nitro and Nitroso Groups, Part 1’,ed.H. Feuer, Interscience, New York, 1969. ‘Houben Weyl, Methoden der organischen Chemie’, Band Xjl, Stickstoff-Verbindungen, ed. E. Miiller, Thieme, Stuttgart, 1971. R. H. Pickard and J. Kenyon, J. Chem. Soc., 1907,91,896. lo 0.Baudisch, Ber., 1921,54,413. 0.Baudisch, Ber., 1929,62, 2706. ‘’ H. Reihlen and A. Hake, Justus Liebigs Ann. Chem., 1927,452,47. l3 R. Rascanu, Ann. Sci.Univ. Jassy, 1933, 10, 130. l4 E. K. Gustorf and M.-J. Jun, Z. NaturSorsch. B., 1965,20, 521. Coordination Chemistrj of C-Nitroso-compounds 4OM -N, R M I II 111 IV R R R\ N=O+ M /\MM V VI VII Scheme 1 resulting from the exposure to light of a solution of iron pentacarbonyl in nitrobenzene They suggested that, by comparison with their own detailed studies, it was probable that Dewar and Jones had prepared [NOBFe(C0)3]2 and that this was therefore the first example of a C-nitroso-compound coordin- ated to a metal 2 Synthesis of Complexes of C-Nitroso-compounds It is necessary to anticipate the structural studies of the coordination chemistry by making use of the classification l6 l7 of the seven different bonding types exhibited by monomeric C-nitroso-compounds as reported in Scheme 1 Such a classification assists our understanding of the synthetic routes available A.Preparation of Complexes of Types I, 111, and VII by Direct Addition of the Partners and without Ligand Displacement.-Most of the early preparations listed aboveg l3 employed this simple preparative technique It has been frequently employed for preparation of NODMA complexes [equation 1, where M =Co,'* N1," Pd," Cd," Zn,23 and (CH3)2Sn24] The donor strength of the nitroso-group is markedly increased by substitution in the para position of the benzene ring of a strongly electron-donating group l5 J Dewar and H 0 Jones Proc Roj Soc A 1905 76,558 l6 R S Pilato C McGettigan G L Geoffroy A L Rheingold and S J Geib Organometallrcs 1990 9 312 I' M Cameron B G Gowenlock and G Vasapollo J Organomet Chem 1991 in press I'D M SamsandR J Doedens Inorg Chem 1979 18 153 I9 C J Popp and R 0 Ragsdale Inorg Chem 1968 7 1845 lo I Batten and K E Johnson Can J Chem 1969 47 3075 21 A S Pilipenko L L Shevchenko and A P Pope1 Zh Prrklad Spectrosk 1976 24 365 12P 0 IkekwereandK E Johnson Synth React Inorg Met Org Chem 1985 15 883 23 P 0 Ikekwere Sqnth React Inorg Met Org Chem 1988 18 629 l4 G Matsubayashi and K Nakatsu Inorg Chrm ALta 1982 64 L163 356 Cameron, Gowenlock, and Vasapollo MCl2 + 2NODMA -MClz(N0DMA)z (1) Other p-nitrosoaniline derivatives of Ni, Co, and Cu have been similarly prepared.25A range of uranyl complexes with NODMA are also made by direct addition,26 as are the corresponding lanthanide complexes MX3(NODMA)3 (M = La, Ce, Pr, Nd, Sm, Ho, Er, Tm; X = C1, N03).27 A further example of coordination by direct addition is provided by the reaction of a range of substituted nitrosobenzenes with ferrohaemoglobin under anaerobic conditions.** In these type I coordinations it is noteworthy that coordination is completely inhibited by an ortho t-butyl group but not by smaller ortho alkyl groups, also by a pair of ortho methyl groups, chlorine or bromine atoms, but not by a pair of the same substituents elsewhere on the benzene ring.Such effects are clearly compatible with steric hindrance preventing o-N (type I) coordination. In a recent paper28" it has been reported that NOB inserts into [Cp*Rh- (p-C1)]2 (Cp* = CSMeS), the Rh" dimer having a reactive Rh-Rh bond and a type 111 complex results. B. Preparation of Complexes of Types I, 11, I11 by Direct Addition of the Nitroso- compound to a Coordination Compound accompanied by Ligand Displacement.- In contrast to method A which is primarily applicable to NODMA complexes, the ligand displacement syntheses are primarily used for NOB, substituted NOB, 2-methyl-2-nitrosopropane, and trifluoronitrosomethane complexes.The first use of this technique is provided by the Baudisch reaction lo between the hexacyanofer- rate@) ion and nitrosobenzene which follows the mechanism 29 shown in reactions 2 and 3. H20 + Fe(CN):-Z [Fe(CN)5H20]3-+ CN-(2) NOB + [Fe(CN)5H20]3-G==+ [Fe(CN)5NOB]3-+ H20 (3) The method has been used for a variety of type I complexes as illustrated in equations 4-15. 2NOB + PdC12(PhCN)2-PdC12(NOB)2 + 2PhCN (ref. 30) (4) NOB + FePc(Bu"NH2)2-(NOB)FePc(Bu"NHz) + Bu"NH2 (where Pc = phthalocyanine) (ref. 31) (5) "C. J. Popp and R. 0.Ragsdale, J. Chem. SOC. A, 1970, 1822. 26 G. Condorelli, I.Fragala, and S. Giuffrida, J. Inorg. Nucl. Chem., 1975,37, 1177. 27 A. Serninara, Boll. Sedute Accad. Gioenia Sci. Nature Catania, 1969 [4], 10, 147. Chem. Abstr., 1971, 74,71 050. K. Hirota and H. A. Itano, J. Biol. Chem., 1978,253,3477. 28a P. R. Sharp, D. W. Hoard, and C. L. Barnes, J. Am. Chem. Soc., 1990,112,2024. 29 D. Pavlovic, I. Murati, and S. Asperger, J. Chem. Soc., Dalton Trans., 1973,602. 30 A. L. Balch and D. Petridis, Inorg. Chem., 1969,8 2247. 31 J. J. Watkins and A. L. Balch, Inorg. Chem., 1975, 14, 2720. 357 Coordination Chemistry of C-Nitroso-compounds PtC1:- + RNO- PtCly(RN0) + C1- (ref 32) (6) PtC1: + 2RN0 __* PtC12(RNO)2 + 2C1 (ref 32) (7) (where R = Bu' or Ph) CPM(C0)3 CpM(C0)tTHF + CO 9CpM(C0)2NOB + THF (where Cp = C5H5, M = Mn, Re) (ref 33) (8) 2RN0 + [MCl(diene)I2 -2MCl(diene)RNO (ref 34) (9) [where R = C6H5, p-BrCsH4, M = Rh, Ir, diene = cyclooctadiene (COD) or norborna- diene (NBD)] An extension 35 of reaction 9 leads to further complexes such as RuC12(COD)-(NOB)z, RuClz(DMS0)2(NOB)2, PdC12L2 (where L = para-substituted NOB), PtClz(p-Me C6H4 NO)2, and RhCl(C0)2(NOB) Reaction 8 has been extended by Link 36 to some tungsten complexes (reaction 10) and manganese complexes (reaction 11) CpMn(C0)s --$& CPM~(CO)~THF CpMn(C0)2(RNO) + THF (11) [where R = t-C4H9, neo-C5H1 ICMe2, cyclo-C3HS (reaction 19 only), 1-adamantyl] These reactions have been further extended [Cp(CO)(PPh,)Fe(THF)]+ + RNO -[Cp(CO)(PPh3)Fe(RNO)]+ + THF (13) + AgBF4/-AgC' ,[Cp(PPh3)Fe(RN0)]+[BF4]-(14)Cp(PPh3)Ru-C1 2 + RNO/CHzClZ 32 D Mansuy, M Dr&me, J C Clottard, and J Guilhem, J Organomet Chem, 1978,161,207 33 V N Setkina, S P Dolgova, D V Zagorevskii, V F Sizoi, and D N Kursanov, Bull Acad Scr USSR Chem Ser , 1982,1239 34 G Vasapollo, P Giannocaro, C F Nobile, and F Allegretta,J Organomet Chem ,1984 270 109 35 (a) G Vasapollo, C F Nobile, P Giannocaro, and F Allegretta, J Organomel Chem 1984 277 417 (b)H Alper and G Vasapollo, Tetrahedron Lett, 1987,28,6411 36 M Link, Doctoral Thesis, University of Hamburg, 1988 358 Cameron, Gowenlock, and Vasapollo Vasapollo's reaction (9) has been extended3' to a NODMA complex where M = Rh,diene = COD.The range of this preparative method is exemplified by the preparation of type I1 complexes (reactions 16-20) and type 111 complexes (reactions 21-22).RNO + Ni(Bu'NC), -N~(RNO)(BU'NC)~+ (n -2)Bu'NC (where n = 2 or 4 and R = p-X.C6H4,X = Me2N, MeO, Me, H, C1, Br) (ref. 38) (16) NOB + M(C2H4)(PPh3)2 --+ M(NOB)(PPh3)2 + CzH4 (where M = Ni, Pd, Pt) (ref. 38) (17) Pt(PPh&(C2H4) + Bu'NO &Pt(PPh3)2(ButNO) + C2H4 (ref. 39) (18) M(PPh3)3 + CF3NO =M(PPh3)2(CF3NO) + PPh3 (where M = Pt, Pd) (ref. 39) (19) OsCl(NO)(PPh3)2L + NOB ---+ OsCI(N0) (PPh3)2(NOB) + L (where L = C2H4 or PPh3) (ref. 40) (20) 3PdLz + 3NOB --+ 3L + [Pd(N0B)Ll3 (refs. 41'38) (21) (where L = PBuS, PPhBul) ~[CPCO(CZH~)Z]+ 2NOB --+ [CpCo(NOB)]2 + 4CzH4 (ref. 42) (22) C. Preparations of Complexes of Types I, 111, IV, V, VI using Nitro-compounds as the Source of the RNO Group.-(i) Without Irradiation.The only example known of a type VI complex is prepared from the reaction of tri-iron dodecacarbonyl with nitroethane 43 to give a nitrene-nitroso-complex of formula Fe4(CO)1 1-(NEt)( ONE t). Nickel(o) phosphine complexes react with a variety of aliphatic and aromatic nitro-compounds according to the overall equation 23. 37 G. Matsubayashi and T. Tanaka, J. Chem. SOC.,Dalton Trans., 1990,437. 38 S. Otsuka, Y. Aotani, Y. Tatsuno, and T. Yoshida, Inorg. Chem., 1976, 15,656. 39 M. Pizzotti, F. Porta, S. Cenini, F. Demartin, and N. Masciocchi, J. Organomet. Chem., 1987, 330, 265. 40 M. Herberhold and A. F. Hill, J. Organomet. Chem., 1989,363, 371. 41 M.Calligaris, T. Yoshida, and S. Otsuka, horg. Chim. Acta, 1974, 11, L15. 42 S. Stella, C. Floriani, A. Chiesa-Villa, and C. Guastini, J. Chem. SOC.,Dalton Trans., 1988,545. 43 G. Gervasio, R. Rossetti, and P. L. Stanghellini, J. Chem. SOC.,Chem. Commun., 1977, 387; J. Chem. Rex (M), 1979, 3943. Coordination Chemistry of C-Nitroso-compounds RNOz + NIL4-(RNO)NiLz + L + LO (ref. 44) (23) (where R = Me, Et, Pr‘,Bu‘, Ph,p-XC6H4 (X = F, C1, Me, MeCO) and L = PEt3 or PPhJ) The mechanism below (reactions 24-6) has reaction 25 as the rate determining step: These nickel complexes have a type I1 structure. Nitrobenzene reacts rapidly 45 with the molybdenum complex Mo(CO)~- (S2CNEt2)t in dichloroethane to give MoO(NOB)(S2CNEt& This complex can be reduced by triphenyl phosphine to give the type I1 complex Mo(N0B)- (S2CNEt212.Further molybdenum complexes, Mo02(RNO), where R = Ph, Me, o-MeC6H4 can be obtained from refluxing molybdenum hexacarbonyl and the nitro-compound under nitrogen.46 An intriguing NOB complex was prepared47 in an attempt to reduce nitrobenzene to aniline with a highly reactive catalyst, PtH2(PMe3)2. The resultant complex, Pt2(NOB)3(PMe3)3, is unusual in that it displays three different coordination modes, namely types I, IV, and V. (ii) With Irradiation. A type 111 complex is prepared from irradiation of iron pentacarbonyl in nitrobenzene l4 or substituted nitrobenzene4* using either light or y-rays according to reaction 27. 2Fe(COh + 2PhN02h”,[NOB.Fe(C0)3]2 + 2C02 + 2CO (27) D.Preparation from 1mido-complexes.-This synthetic route has been applied solely to rhenium complexes and the direct oxygenation route is illustrated in equation 2K4’ An alternative oxidation route for the second of these imido complexes is available using p-nitrotoluene at 8&9OoC as the oxidant, the other product being p-toluidine. The nitroso-complexes formed are presumed to be type I. 44 R S Berman and J K Kochi, Inorg Chem , 1980,19,248 45 E A Maatta and R A D Wentworth, Inorg Chem, 1980,19,2597 46 A R Middleton and G Wilkinson, J Chem Soc , Dalton Trans, 1981, 1898 47 D L Packett, W C Trogler, and A L Rheingold, Inorg Chem , 1987, 26,4309 48 E K Gustorf, M C Henry, R E Sacher, and C DI Petro, Z Naturforsch B, 1966,21, 1152 G La Monica and S Cenini, Inorg Chim Acta, 1978.29, 183, J Chem Soc , Dalton Trans, 1980, 1145 360 Cameron, Gowenlock, and Vasapollo ReC13(RNO)(OPPh3) + ReC14(NR)(OPPh3) + OPPh3 + by-products (R = p-X.C6H4where X = H, MeO, Me) It would be of considerable interest to extend this reaction to other imido com- plexes.E. Preparation from Hydroxylamines-Type I1 complexes of molybdenum 5035’ and tungsten 51 have been prepared by reaction of metal 0x0-containing compounds (M=O) with aryl 50,51 and N-t-butyl hydroxylamine according to the general reaction (29). M=O + + H20RNHOH -N-R (29) and Scheme 2 illustrates the products obtained. Several other preparations have been reported. The essential feature is the oxidation of the hydroxylamine followed by coordination of the nitroso-compound.Waters 52 has extended the range of Baudisch complexes [Fe11(CN)5.RN0)]3-by utilising the reaction of aqueous sodium aqua-pentacyanoferrate (3 -) with alkyl and aryl hydroxylamines at controlled pH ranges. The reaction (30) represents the overall process. [Fe(CN)5.H10]3-+ 2RNHOH -[Fe(CN)5.RNO]3-+ RNHz + HzO (30) It has long been known that nitrosoarenes bind to haemoglobin53*54 and Mansuy and co-workers 55,56 have shown that nitrosoalkane complexes of myoglobin, haemoglobin, and cytochrome P-450are formed during the oxidation of N-alkylhydroxylamines in the presence of these haemoproteins, and have extended the method to porphyrins 57 whilst Lindeke 58 has suggested a mechanism (reaction 31) to account for the observations.L. S. Liebeskind, K. B. Sharpless, R. D. Wilson, and J. A. Ibers, J. Am. Chem. Soc., 1978, 100, 7061. 51 D. A. Muccigrosso, S. E. Jacobson, P. A. Apgar, and F. Mares, J. Am. Chem. Soc., 1978,100,7063. 52 W. A. Waters, J. Chem. Soc., Perkin Trans. 2, 1976, 732. 53 F. Jung, Biochem. Z., 1940,305,248. 54 D. Keilin and E. F. Hartree, Nature, 1943,151,390. 55 D. Mansuy, P. Beaune, J. C. Chottard, J. F. Bartoli, and P. Gans, Biochem. Pharrnacol., 1976,25, 609. 56 D. Mansuy, J. C. Chottard, J. F. Bartoli, and P. Gans, Eur. J. Biochem., 1977,76,607. ”D. Mansuy. J. C. Chottard and M. Lange, J. Am. Chem Soc., 1977.99.6441, 58 B. Lindeke, Drug Melubolism Reviews, 1982, 13, 71.Coordination Chemistry of C-Nitroso-compounds R'NHOH Scheme 2 F. Nitrosyl Migration Reactions.-An overall reaction scheme for the varied range of such migration reactions is given in reaction 32. Cameron, Gowenlock, and Vasapollo OH 0 It P-450Fe(1II) NHI NaDPH + 02 --P-450Fe(1I) c N I IR R (31) 0 0It +L II -M-N-RON-M-R ___t L-M-N-R (32) There are a variety of syntheses reported in the literature of which the systems using nitric oxide as a reagent deserve first mention. Klein and Karsch59 prepared the pentacoordinated d 7-complex CO(CH~>~(P(CH~>~)~which reacted with nitric oxide according to equation 33. Co(CH3)zL3 + NO -L + Co(CH3)z.NO.Lz --%[CoCH3(CH3NOJLz]z (33) (L = (CH313P) where the resultant dinuclear complex exhibits type I11 coordination. Middleton and Wilkinson 6o presented evidence for a type I1 nitrosomethane complex as an intermediate in the reaction sequence shown in equation 34.NOCpzNb(CH3)z yozCpzNb(CH3)zNO -CpzNbCH3.CH3NO -CpzNbO(CH3) (34) They extended the range of complexes produced46 from reaction of nitric oxide with transition metal methyls obtaining both type I1 complexes of rhenium, chromium, and molybdenum, and type 111complexes of cobalt and molybdenum. In addition a rhenium type I1 complex of the previously unknown nitroso- trimethylsilylmethane was obtained. In 1973, Brunner and Loskot6’ prepared the first complexes of di-nitroso- compounds from reaction of cyclopentadienyl cobalt carbonyls or nitrosyls with nitric oxide and alkenes of the norbornene type. These syntheses of type I complexes were further extended by Bergman et al.They showed that unstrained alkenes could form complexes 62 of dinitrosoalkanes (reaction 35). 59 H.-F. Klein and H. H. Karsch, Chem. Ber., 1976, 109, 1453. 6o A. R. Middleton and G. Wilkinson, J. Chem. SOC.,Dalton Truns., 1980, 1888. 6’ H. Brunner and S. Loskot, J. Organomet. Chem., 1973,61,401. ”P. N. Becker and R. G. Bergman, J. Am. Chem. SOC.,1983,105,2985;Organometallics, 1983,2,787 Coordination Chemistry of C-Nitroso-compounds 0 0 Scheme 3 Further studies 63 of the migratory insertion of coordinated nitric oxide into Co-C bonds use the reaction sequence (36) to form type I nitrosoalkane complexes [where L = PPh3 and R = CH3, CHJCHZ, (CH3)2CH, and p-CHZC~H~CH~]. Na/Hg [CpCoNOIz -Na* [@Co-NO] 2@C{ NO Et20 R lL 0 11 The accompanying kinetic investigations by the authors establish the inter- mediacy of the nitrosyl compound CpCo(N0)R and the sequence given in reaction 37.A similar reaction system occurs 64 when cyclopentadienylnitrosyl iron dimer b3 W. P. Weiner and R. G. Bergman, J. Am. Cltem. Soc., 1983,105,3922. 64 M. D. Seidler and R. G. Bergman, Organometallics, 1983,2, 1897. Cameron, Gowenlock,and Vusupollo 0 0 N ~/No I1 -CpCo’ I1 CPCO, N ‘R R CpCd ‘R ‘L (37) is reduced with sodium in dimethoxyethane and the resulting radical anion salt is doubly methylated. Rearrangement of the product leads to a monomeric cyclopentadienyl dimethyl nitrosyl iron which undergoes migratory insertion of NO in the presence of trimethyl phosphine (Scheme 4).0 II N II 0 1 0 II CP, /”\ ,CH3 Cp -Fe,-Me/No -45 c 1 + Fe-Fe-112 1 ‘/ ‘CpCH/Me II 0 0 II /N-MeCp -Fe -Me ‘Me Scheme 4 A similar migratory insertion of NO into the metalkarbon bond in the presence of trimethylphosphine has been reported 65 for pentamethyl cyclo- pentadienyl dimethyl nitrosyl iron. T~~-C~M~~F~(CH~)~NO (38)I~’-C~M~~F~.CH~.P(CH~)~.CH~NO 65 B N Diel, J Organornet Chern, 1985,284,257 365 Coordination Chemistry of C-Nitroso-compounds Extension to ruthenium complexes has been established 66 (reaction 40). q5-C5Me5RuPhEtN0PPhMez q5-C5Me5Ru.Ph PPhMe2 EtNO (39)85 "C The insertion of the nitrosonium ion into a Cr-CH3 bond (reaction 40) to give a formaldoxime complex 67 is presumed to involve the type I nitrosomethane complex as an intermediate.q5-CpCr(N0)2Me+ NOPF6 [q5-CpCr(N0)2{N(CH2)OH}]+PFi(40) G. Electrophilic Substitution in Aromatic Systems by Co-ordinated Nitrosy1.-The coordinated nitrosyl group in R~(bipy)~(N0)X~ + can function as an electrophile in aromatic substitution reactions (41) with activated arenes such as N-methyl and N,N-dimethylaniline leading to type I complexes 68 of the p-substituted nitrosoarenes. (x= CI or N02,R=Hor Me) (41) H. Radiation Syntheses.-Hoffman has utilised one-electron reduction of de- oxygenated aqueous solutions of metal-nitrosyl coordinated compounds of ruthenium 69,71 and iron 70,71 in the presence of organic compounds (RH) using both continuous and pulse radiolysis.The radiolysis of the neutral aqueous solutions produces e,, *OH, and He and the two latter species produce the organic radical Re by H-abstraction from RH. The general synthetic route [reaction 42, where M" = Fe(CN)g-, Ru(NH3):+, and Re = *CH2C(CH3)20H, *CH2C(CH3)2NH3+, *CH2C(CH3)2COY, *CH*C(CH3)(NH;)COY, CH2C(CH3)(0H)C0;, *CH2(CH3)NC(O)CH3] takes place giving type I com-plexes. [M" -NO'] [M" -NO] 2[M" -N(O)R] 66 J Chang, M D Seidler, and R G Bergman, J Am Chem Soc ,1989,111,3258 67 P Legzdins, B Wassink, F W B Einstein, and A C Willis, J Am Chem Soc , 1986,108,317 68 W L Bowden, W F Little, and T J Meyer, J Am Chem Soc , 1974,96,5605, 1976,98,444 69 J N Armor, R Furman, and M Z Hoffman, J Am Chem SOC,1975,97,1737 70 R P Cheney, M G Simic, M Z Hoffman, I A Taub, and K -D Asmus, Inorg Chem, 1977, 16, 2187 71 R P Cheney, S D Pel], and M Z Hoffman, J Inorg Nucl Chem ,1979,41,489 Cameron, Gowenlock, and Vasapollo The ruthenium complexes were sufficiently stable to be isolated, as the tetraphenyl borate salts.Similarly the sodium salt, N~~[F~(CN)SN(O)CH~C- (CH3)20H] could also be isolated. I. Other Synthetic Routes to Short-lived Complexes.-Waters ’’ has drawn attention to the Legal complexes7’ arising from reaction of the carbanion of acetone and also from the carbanions of other aliphatic aldehydes and ketones (reaction 43).These complexes are relatively short-lived and liberate the isomeric oximino-compound.73 [Fe(CN)~hol’-+ CHKOMe -[Fe(CN)SN(0)CH2COMe]3-(43) The reaction of benzyl bromide with various nitrosyl ruthenium complexes gives benzaldoxime as a final product74 and initial formation of phenyl-nitrosomethane complexes by reaction 44is suggested to occur prior to isomeriza- tion to the oxime complex. (44)Ru(N0)2(PPh3)2 + PhCH2Br -RUB~(NO){N(=O)CH~P~}(PP~~)~ J. Complexes of Dimeric Nitroso-Compounds.-There are very few reports of the formation of such compounds but three examples are of interest. Gaseous dinitrogen tetroxide when bubbled into an ethereal solution of tetramethyl lead 75 or into a cooled ethyl acetate solution of tetramethyl tin 76 produces a 1: 1 complex of the cis-dimer of nitrosomethane with dimethyl metal dinitrate (45). 2N204 + Me4M -Me2M(N03)2-(MeN0)2 (45) M = Pb,Zn Reduction of 1,2,3-trimethoxy-5-nitrobenzenewith aqueous ethanolic am-monium chloride and zinc powder and a controlled amount of nitrous acid produced a white solid claimed to be a 1 : 1 complex of dimeric 1,2,3-tri-methoxy- 5-nitrosobenzene and zinc nitrite.77 3 Reactions of Coordinated Nitroso-compounds It is well known5-’ that C-nitroso-compounds exhibit a wide range of reactions and it is of interest to note whether coordination to metals results in any change in the patterns of these reactions and whether there are reactions that are characteristic of the coordination mode.Wherever K-NO coordination is participating (types 11, 111, IV, VI) it is to be expected that the weakening of the 72 N.V. Sidgwick, ‘Chemical Elements and their Compounds’, Clarendon, Oxford, 1950, Vol. 2, p. 1345 73 L. Cambi, A. Cagnasso, and T. Ricci, Chem. Abstr., 1931,25,2383. 74 J. A. McCleverty, C. W. Ninnes, and I. Wolochowicz, J. Chem. SOC.,Dalton Trans., 1986,743. 75 K. C. Williams and D. W. Imhoff, J. Organomet. Chem., 1972,42, 107. 76 K. C. Williams and D. W. Imhoff, Inorg. Nucl. Chem. Left., 1973,9,227. 77 H. I. Bolker and F. L. Kung, J. Chem. Soc., 1969,2298. 367 Coordination Chemistry of C-Nitroso-compounds bonding between N and 0 may lead to new reaction pathways. The following survey of reactions is necessarily selective and we have classified the reactions into nine major areas.A. Displacement of RNO Ligand.-It is to be expected that nitroso-compounds may be displaced from a complex by another molecule that can be bonded more strongly to the metal centre. Examples include the displacement of NOB from a Baudisch complex by cyanide ion 78 (reaction 46). [Fe(CN)5NOB13-+ CN-e[Fe(cN)6l4-+ NOB (46) which has been shown to proceed by a complex mechanism with an initial SN1 dissociation step (reaction 47), [Fe(CN)5NOBI3---+Fe(CN)S3-+ NOB (47) and the displacement of NOB from Rh(COD)(NOB) by triphenylph~sphine.~~ Many complexes are highly stable in the presence of other ligands, e.g. it has been shown 62 that cobalt dinitrosoalkane complexes are inert to carbon monoxide and to trimethylphosphine.On the other hand some type TI nickel complexes can undergo ligand displacement relatively rapidly.44 (Bu'NO)Ni(PEt3)z + 2L -Bu'NO + Ni(PEt3)2L2 (48) where L = PEt3 or CO The complex W(C0)5NOB undergoes slow ligand displacement when dissolved in a~etonitrile.~~ The complex W(CO)5ButN0 is stable to carbon monoxide, triphenylphos- phine, and t-butyl-isocyanide in methylene chloride although the same complex undergoes rapid ligand displacement by solvent in acetone and THF solution. l6 B. Reversible Exchange with A1kenes.-Becker and Bergman 62 have shown that thermal exchange of alkenes in cyclopentadienyl cobalt dinitrosoalkane com-plexes can occur and that this is dependent upon the dissociation reaction 49.0 C. Thermal Decomposition of Complexes-There are wide variations in the thermal stability of the complexes. Watkins and Balch3' showed that the rate of ''D. Pavlovic, I. Murati, and S. Asperger, J. Chem. SOC.,Dalron Trans., 1973,602 Cameron, Gowenlock, and Vusapollo exchange between free and complexed nitrosotoluene in a ferrous phthalocyanine complex (CH3CsH4NO)FePc(n-C4H9NH2)occurred by a dissociative reaction of AHS = 20.1 kcal mol-'. This is the only kinetic value for the enthalpy of activation of a dissociation reaction of a type I complex. DTA, DTG, and TG measurements have been made79 on a NODMA complex of Pd", the material being thermally stable up to about 200 "C. D. Reduction and Deoxygenation Reactions.-Ready reduction of the dinitroso- alkane ligand in cyclopentadienyl cobalt complexes is achieved by reaction with lithium aluminium hydride at -78 to -50 "C, the diamine being the product.62 It appears that such reductions have not been attempted for most nitroso-compound complexes.There is, however, an unusual self-reduction reaction that has been reported for the nitrosobenzene tungsten pentacarbonyl complex which is stable in THF solution at -70 OC and which on warming to 20 "C yields aniline tungsten penta~arbonyl.~~ Deoxygenation of coordinated NOB occurs 38 when the type TI complex N~(NOB)(BU'NC)~is boiled with t-butyl isocyanide in benzene, t-butylisocyanate being produced. Other electron donating agents which lead to N-0 cleavage of the coordinated NOB with presumed phenyl nitrene formation are triphenyl- phosphine and nitrosobenzene.A further deoxygenation reaction by a tertiary phosphine to give a nitrene complex has been studied by La Monica and Cenini.49 Unsuccessful attempts at deoxygenation of four complexes of 2-methyl-2- nitrosopropane using triphenylphosphine and t-butylisocyanide have recently been reported.I6 Some type I nitrosoarene complexes of rhodium, iridium, ruthenium, palladium, and platinum react with carbon mon~xide,~~,~~ most probably by an initial oxygen abstraction giving a nitrene complex (equation 5 1). Ph-N=O + CO +Ph-N + C02 I IM M In the presence of free nitrosobenzene further reaction occurs (reaction 52) giving azoxybenzene. Ph-N + PhNO +Ph-N=N(O)Ph + M IM (52) In ethanol, aniline is formed in equivalent quantities to the carbon dioxide presumably by a reaction of the coordinated nitrene with the protic solvent.l9D. N. Todor, A. Tanase, V. David, and G. E. Baiulescu, Rev. Roum. Chem., 1989,34.877. 369 Coordination Chemistry of C-Nitroso-compounds These complexes may also be used to catalyse3'" the conversion of ni- trosobenzene and carbon monoxide into azoxybenzene and phenylisocyanate (benzene solution) and to urethane (ethanol solution). Similar complexes may be involved in the carbonylation of nitrosobenzene in the presence of palladium acetate and cupric acetate.35b E. Isomerization of Nitrosoalkane Ligands.-The isomerization reaction of primary and secondary nitrosoalkanes to give the corresponding oxime has a long history with frequent, incorrect, attributions of the inability of the nitroso-compounds to be synthesized.The lack of kinetic investigations of reaction 53 makes it difficult to decide whether coordination of the nitroso-group to a R'R'CHNO -R'RZC=NOH (53) metal catalyses or inhibits the isomerization. The Legal complexes [Fe(CN)sN(0)CH~COR]3- have lifetimes of only a few minutes " and pre- sumably form oximes, RCOCH=NOH. A reaction sequence invoking a nitroso- propene intermediate has been proposed *' for rhodium and iridium q3-propene complexes (reaction 54). A B L2co M(C0)3L2* + CH,=CHCH=NOH -(54) A further proposal of an isomerization to oxime has been proposed67 to account for the formation of a formaldoxime chromium complex uia insertion of the nitrosonium ion into a chromium-methyl bond (Scheme 5).M. W. Schoonover, E. C. Baker, and R.Eisenberg,J. Am. Chem. SOC.,1979,101, 1880. Cameron, Gowenlock, and Vasapollo Cr NO’/&I, ’NO* 1+ Scheme 5 Detailed investigation of nitroso --+oxime reactions in other nitrosoalkane complexes is required in order to understand the mechanism of these isomeriza- tions. F. Reaction with Bases.-A systematic examination has been made66 of the reactions of some nitrosoethane ruthenium compounds with bases in the presence of trisubstituted phosphines or t-butyl isocyanide. The overall reaction leads to an oximate complex (reaction 55). From detailed kinetic studies the initial step is proposed to be as shown in reac- tion 56, i.e.a bimolecular step that is first order both in the complex and in the 37 1 Coordination Chemistry of C-Nitroso-compounds NaOSiMe, -CH3pJCH+ L m PPhMe, L = PPhMe2, PMe3, tBuNC r" + BH' Bronsted base. At present these results are unparalleled for other nitrosoalkane complexes. G. Reaction with Acids.-An interesting protonation of type I1 coordinated NOB in OsCl(NO)NOB(PPh3)2 occurs 40 when this nucleophilic ligand of an osmium complex is interacted with aqueous hydrochloric acid. The protonated product is probably a divalent hydroxylaminato-osmium complex [OsC12(N(OH)( Ph)- (NO)(PP~S)~)].Reaction of a nitro-group with acid in a type I NODMA complex has been reported.68 Ru(bpy)2(NODMA)NO2+ + 2H' --+ Ru(bpy)2(N0DMA)NO3++ H2O (57) There are very few reports of reactions of complexes of nitroso-compounds with acids. H.Reaction with Oxygen.-The majority of the complexes reported in the literature are stable to air. Some, however, can only be prepared with total exclusion of oxygen. The nickel(0) complexes (RNO)NiL2 are examples. It is reported that the products obtained from bubbling oxygen through a benzene solution of one such complex (R = But) are a precipitate (probably nickel hydroxide) and approximately equimolar quantities of 2-methyl-2-nitrosopropane and 2-methyl-2-nitr0propane.~~Another irreversible oxidation in benzene solu- tion is found 81 for the Fe(porphyrin)(Pr'NO)(L) complexes, the stability of the complexes depending upon the magnitude of the binding constant of the ligand. It *' D.Mansuy, P. Battioni, J.-C. Chottard, C. Riche, and A. Chiaroni, J. Am. Chem. Soc., 1983, 105,455 Cameron, Gowenlock, and Vusupollo is therefore probable that the oxidation process is due to the interaction of oxygen with the pentacoordinate species Fe(porphyrin)(Pr’NO). As an example of the resistance of many complexes to oxidation the failure to observe any reaction34 for a number of rhodium and iridium type I complexes on boiling for twelve hours in aerated benzene is particularly noteworthy. I. Reactions with NO and NO +.-Uncoordinated nitroso-compounds can react with nitric oxide to form N-nitrosohydroxyl amine nitrites which rearrange to diazonium nitrates (58), R-N=O + 2N0 +R-N-0-N=O -RNpNO3-I N=O (58) and it is of interest to investigate how the coordination mode of the nitroso- compound can modify this reactivity.The reaction of nitric oxide with some transition metal alkyls leads to the formation of N-alkyl-N-nitrosohydroxylamin-ate compounds and reactions (59 and 60) have been proposed6’ to account for this invoking a paramagnetic nitrosoalkane intermediate. M-R + NO __* M-O-N-R (59) It is, however, possible that the postulated intermediate is a type VII complex. A similar reaction product results 82 when type IT platinum complexes Pt(RNO)(PPh,)2 react with the nitrosyl cation to give the complex (2). Reaction of the type I1 complex with nitric oxide, on the other hand, leads to the formation of a nitro-complex [Pt(N02)2(PPh3)2].J. Insertion Reactions into the Metal-Nitrogen Bond.-An important class of reactions of type I1 platinum complexes Pt(RNO)(PPh3)2 is provided by insertion of another molecule into the metal-nitrogen bond. This class of reactions has been the subject of more sustained investigation than any other type of reaction of nitroso-compound complexes. The other reactant contains a double (or triple) 82 C. J. Jones, J. A. McCleverty, and A. S. Rothin, J. Chem. Soc., Dalton Trans., 1985,401. 373 Coordination Chemistry of C-Nitroso-compounds bonded function which can be incorporated into the Pt-N bond. The reactants carbon disulphide, phenyl isocyanate, phenyl isothiocyanate, and p,p-di-cyanostyrene can insert into the Pt-N bond for the three cases R = Ph, Bu', and CF3 39,83-86 (Scheme 6).R PPh, \/ N/ cs2 PPh3 0-N /R \IPt \c=syl -PPh4 0 PPh[ 's' R /PhN=C=O PPh3 .,p-N\c_____t =0 PPhf 'N' I Ph R/CN PPh3, ,0-N H t PI PPh/ 'Ph'c' 'C' CN' 'CN Scheme 6 Other insertion reactions occur with tetracyanoethene (R = Ph,84 CF339)and for dimethyl acetylenedicarboxylate (R = Ph,83 Bu' 39). It is apparent that alkenes require substituent electron withdrawing CN groups for insertion to occur. Fumaronitrile undergoes insertion34 (R = Ph) but this is also ac-companied by displacement of the NOB to give the product (Ph3P)zPt(NCCH=CHCN). The reaction with carbon dioxide shows the greatest differences between the different nitroso-ligands.When R = Ph insertion of carbon dioxide into the Pt-N bond takes place as for carbon di~ulphide,'~ but the C02 reaction is rever~ible.~~ When R = But there are two products,39 the insertion compound (3) and a carbonate complex (4); when R = CF3 only the carbonate complex is formed.39 83 P L Bellon, S Cenini, F Demartin, M Pizzotti, and F Porta, J Chem Soc ,Chem Commun ,1982, "S Cenlni, F Porta, M Pizzotti, and G La Monica, J Chem SOC,Dalton Trans, 1984,355 85 S Ceninl, F Porta, M Pizzotti, and C Crotti, J Chem SOC,Dalton Trans, 1985, 163 g6 F Demartin, M Pizzotti, F Porta, and S Cenmi, J Chem Soc ,Dalton Trans, 1987,605 374 Cameron, Gowenlock, and Vasapollo,But PPh, 0PPh3, 0-N \ "< )=oP/, ,c=o PPh( 0 PPh/ 0 (3) (4) A further example of carbon dioxide insertion is provided by the rhodium complex 87 Rh(4-MeC6H4)CButP(CH2CH2CH2PPh2)2]PhNO.Infrared evidence suggests that this is a type I complex and 31PNMR implies that the insertion compound is present as two of the four possible stereoisomers in which the tolyl ligand can be coordinated either cis or trans to Rh-OC(0) or syn or anti with respect to the t-butyl substituent.We consider that it is more likely that the NOB complex dissociates to give a 16-electron Rh' species which then reacts with carbon dioxide and subsequently with NOB rather than that the carbon dioxide inserts into a type I RhtN bond followed by subsequent rearrangement to produce a metallacyclic compound.4 Spectroscopy and Structure The classification of coordinated nitroso-compounds into seven distinct classes 16*17 is a consequence of X-ray crystallographic studies and a sufficient number of studies has been made to establish the generality of these classes. The NO bond lengths that result show that in types 11, 111, IV, V, and VI the NO bond is much closer to the length characteristic of a single bond whereas in types I and VII the double bond character of the NO group is maintained. X-Ray crystallographic studies are lacking for many coordination compounds and correlations have been attempted between infra-red spectroscopy and coordination mode. It has been suggested that as the NO stretching frequency is altered upon coordination, the change in frequency can be correlated with coordination mode.Coordination by types I1 and 111 leads to a large drop in vNO, of about 400 cm-', which is expected on the basis of the bond lengthening and x electron donation and in these cases the correlation is relatively straightforward. We have shown elsewhere that there is a direct interdepend- ence of VNO with NO bond length in C-nitroso-compounds. Type I coordination usually leads to a slight extension of the NO bond and thus it is to be expected that vNO will decrease by a small amount on coordination. The major problems arise for type VII coordination which is apparently confined to p-substituted nitrosoanilines such as NODMA coordinated to d9 or d" metals. For some time incorrect attributions of vNO in NODMA have been employed88 and thus the correlation of type VII coordination with a small rise in VNO is insecure.This is further emphasized by the fact that in the two examples of crystallographic study of type VII NODMA complexes the NO bond lengths are 1.218 A24and 1.305 A89 and therefore it is to be expected that in the first of these the vNO value will rise whereas in the second it will fall. L. Dahlenburg and C. Prengel, Inorg. Chzm. Acta, 1986, 122, 55. M. Cameron, B. G. Gowenlock, and G. Vasapollo, J. Organomet. Chem., 1989,378,493. 89 S. Hu, D. M. Thompson, P 0. Ikekwere, R. J. Barton, K. E. Johnson, and B. E. Robertson, Inorg. Chem., 1989,28,4552 Coordination Chemistry of C-Nitroso-compounds Other spectroscopic techniques have been used to confirm structures of coordinated nitroso-compounds.Frank and Bunbury 90 used Mossbauer spectro- scopy to show that five nitroso-aromatic iron tricarbonyls had the same structure independent of the character of the substituents in the aromatic ring. Combination of this evidence with the X-ray crystallographic evidence9' for one of the compounds emphasized the fact that type I11 coordination was operative in all cases and that the varying association character implied by molecular weight determinations48 was not in evidence in the solid state which illustrated dimeric structure. Recently it has been shown9' that the Mossbauer spectrum for triphenyl-phosphine nitrosobenzene iron dicarbonyl l4 has the same features that characterize the above tricarbonyls and that therefore this material is also dimeric in the solid state.The use of X-ray photoelectron spectroscopy (XPS) in distinguishing between monomeric and dimeric C-nitroso-compounds 93 suggested to us that it could be applied to discriminating between the structural types of coordination com-pounds. In the case of uncoordinated nitroso-compounds it has been shown93 that the difference between the 0 1s peak and the N 1s peak AEB(0-N) is about 133-134 eV for monomers and 128-129 eV for either cis or trans dimers. In coordination compounds of type I the value of AEB(0-N) drops94 by about 1.5-2.5 eV compared with that of the uncomplexed monomer, whereas there is no significant change in A&(O-N) for type I1 and type VII coordination.It seems likely that this technique could be further used to distinguish between type I and type VII coordination by NODMA. NMR spectroscopy may be used as a structural tool for coordinated nitroso- compounds. In Table 1 the available 'H and I3C data for complexes of 2-methyl- 2-nitrosopropane are presented. It should be noted that the C-NO value for the free monomer is at a higher frequency than for other t-butyl compounds thereby demonstrating the high electron withdrawing character of the -NO group. It is of particular interest therefore to note that type I coordination appears to increase this electron-withdrawing character. The preparation of further complexes of this ligand is obviously necessary in order to extend such generalizations and to relate them to the structure of the complexes.It may be noted that the zpso-C resonance in nitrosobenzene occurs at a higher frequency than for almost every other substituent, again demonstrating the high electron-withdrawing character of the -NO group and that in the coordination compound Kf (NOBOPtC1,)-this resonance increases 95 by 6.2 ppm. Even when nitrosobenzene participates in type 111coordination in (NOB=Fe(C0)3)2 the resonance of the ipso-carbon 96 at 90 E Frank and D St P Bunbury, J Organomet Chem , 1970,23,229 91 M J Barrow and 0 S Mills, J Chem Soc A, 1971,864 92 M Cameron, B G Gowenlock, and R V Parish, unpublished results 93 C D Batich and D S Donald, J Am Chem Soc , 1984,106,2758 94 G Vasapollo, C F Nobile, A Sacco, B G Gowenlock, L Sabbatini, C Malitesta, and P Zambonin, J Organomet Chem ,1989,378,239 95 A S F Boyd, G Browne, B G Gowenlock, and P McKenna, J Organomet Chem , 1988,345,217 96 M Cameron and B G Gowenlock, unpublished results 97 B M Al-Tahou and B G Gowenlock, Rec Trac.Chim , 1986.105,353 Cameron, Gowenlock, and Vasapollo Compound Me3C-N0 Me3C-NO 'H (6) Reference Bu'NO 96.4 23.1 1.20 95,96 ( Bu'NO)2 76.5 25.1 1.51 95,96 (ButN0)2PtC12 105.6 27.4 1.80 95,32 (Bu'NOPtC13)-K + 105.4 27.5 1.70 95,32 =,(CO)WBu'NO 101 28.8 1.45 16,36 Bu'NOCpFe(CO)(PPh3)BF4 107.2 28.8 1.40 16 ButNOCpRu(PPh3)2BF4 100.3 29.7 1.12 16 Bu'NOCpMn(C0)2 1.31/1.32 16,36 Table 1 NMR data for Bu'NO and its coordination compounds (all values in ppm) 164.38 ppm is only slightly lower than in NOB it~elf,~' suggesting that the electron-withdrawing characteristics of the -N=O group are retained despite the donation from both the N atom and from the 7c-NO bond.The use of 'H-NMR spectroscopy for structural correlations is best exemplified by the complexes of 2-methyl-2-nitrosopropane because all nine protons of the t- butyl group are equivalent. For the small number of type I complexes studied it is apparent that as rNo decreases both VNO and 6 increase. The unusually low value of 6 in the ruthenium compound suggests that a crystal structure value for TNO and N" labelling to pinpoint VNO would be important pieces of information necessary to sustain such correlations.The predicted, relatively long N=O bond (about 1.32-1.35 A) could well lead to enhanced reactivity of the nitroso-ligand. The theory of bonding in the most common, type I, complexes has received some attention following the earliest proposals 68 of metal ligand charge-transfer interactions leading to more detailed molecular orbital calculations. The6736 nitroso-ligand acts as a o-donor via the overlap of the occupied HOMO, which is localized on the N atom with the metal dz2orbital. In addition the nitroso-ligand acts as a n-acceptor, the occupied metal d,, orbital donating electron density into the unoccupied LUMO orbital 7c* (x). The different degrees of n-donation from the metal to the ligand 7c* orbital lead to the varying vNO values of the ligand.Weak n-x* donation results in very little change in VNO and rNO from the values of the uncomplexed ligand as was first pointed out by Bowden6' and by Man~uy.~'It is important therefore to note that if vNO can be unambiguously identified in type I complexes information is provided on the extent of metal to ligand n-n* donation. Further to this treatment the electronic spectra of the complexes can be understood in terms of a lowest energy electronic transition from the non-bonding d,,, dxyorbitals to the dxz-7c*(x) LUMO, a transition that has metal-to-nitroso charge-transfer character. Applications to the quantitative analysis of many transition metals are well established, see e.g.ref. 98. Studies of the photoelectron spectra of some type I tungsten and manganese complexes have also been reported 36 and it is of obvious importance for further studies to be attempted covering other types of complexes. We have noted above that type VII complexes are confined to NODMA and 98 R B. Wilson and W D Jacobs, Anal Chem, 1961,33, 1652. Coordination Chemistry of C-Nitroso-compounds similar ligands in which there is a contribution from a dipolar quinonoid structure (5) to the monomeric nitrosoaniline (6). "'\NON&* Me' (5) (6) X-Ray crystallographic investigations show that NODMA acts as a type I ligand to transition metals and this is in contrast to its action as a type VII ligand to d" metals such as tin24 and zinc.89 Type I1 coordination for NODMA has been reported for nickel 38 and for platinum 94complexes.If type VII coordination is assisted by the partial negative charge on the oxygen in NODMA it is reasonable to assume that coordination by cis-dimeric nitrosomethane to tin 76 and lead'' occurs similarly by the dimer molecule acting as a bidentate ligand bearing partial negative charges on both oxygen atoms (7). Such a conclusion suggests the possibility of synthesizing other d"-complexes containing cis-dimeric nitroso-compounds. The most versatile nitroso-ligand is NOB, for which type I, 11, 111, IV, and V coordination is clearly established. Although no theoretical studies of type I1 coordination have appeared, it seems probable that it is similar to q2-alkene metal bonds in that there is o-donation from the filled NO n:-orbital into a vacant metal hybrid orbital accompanied by n-back-donation from a filled metal d orbital into the low lying vacant n:* NO orbital.The resultant lengthening of the NO bond to values in the range 1.41- 1.45 A shows that the bond is approaching single bond character and the hybridization of N changes up to sp3. A further characteristic feature of n: coordination by NOB is that the phenyl ring lies at an angle to the equatorial plane of the metal ON ring. In the three type I11 complexes whose structures have been determined the lengthening of the NO bond is slightly less (1.35-1.40 A) whereas the hybridization change and twisting of the phenyl group from co- planarity with the NO bond is repeated.In both of these types of n-bonded complexes there is a similarity to the alkene complexes of transition metals in the theoretical basis of bonding although the bond lengthening and lowering of vibrational frequency is much greater for NO than for CC. It has been noted that there are relatively few alkene complexes of the early transition metals and this is true for all types of C-nitroso-compound complexes of these metals. In Table 2 the existing position is displayed for nitroso-compound complexes of metals of groups 3-14 inclusive. In our opinion, Cameron, Gowenlock, and Vasapollo Table 2 Metal complexes of C-nitroso-compounds arranged according to structural type and the periodic classlfication 3 4 5 6 7 8 9 10 11 1.2 13 14 Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge -aorh 1a 1a.c.d 1a.c.d ~b,e b - - VIIb I1 111" IIIa*d VIIb -h VI Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn IId I" 11a,c.d Ia,b,c,d 1a.b 111" 1a.b 11" -aorh VII VII -h I11a La b- Hf Ta W 104 Re I" 0s II Ir 1a.b.8 Pt 1a.c Au Hg VII T1 Pb h- 11" IId 11" I1a,b I11"?) IVa V" a = NOB or substituted NOB; b = NODMA or substituted NODMA; c = Bu'NO; d = other alkyl NO; e = dinitrosoalkane;f = HNO; g = nitrosoalkene; h = dimeric nitroso-compound.Coordinated type I-VII as in text. If type is unknown, indicated by -. the coordination chemistry of nitroso-compounds, which is already both diverse and fascinating, is likely to develop rapidly.Acknowledgements. M. C. thanks the SERC for a maintenance grant. We thank Professor S. Elbel and Dr. M. Link for the opportunity to use unpublished material.

ISSN:0306-0012    

DOI:10.1039/CS9901900355

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Chem. Soc Rev., 1990,19,381406 Two-Dimensional Nuclear Magnetic Resonance Spectroscopy By James Keeler DEPARTMENT OF CHEMISTRY, UNIVERSITY OF CAMBRIDGE, LENSFIELD ROAD, CAMBRIDGE, CB2 1EW 1 Introduction It is hardly possible to open an issue of any current chemical journal without being presented with several two-dimensional NMR spectra. The ubiquity of such spectra shows that the technique has now moved from the realm of a few specialist users to being a widely available and appreciated tool for structural studies. Two-dimensional spectra are even beginning to make an appearance on University examination papers, a further sign that the technique is now firmly established. The purpose of this review is to describe firstly how two-dimensional NMR spectra are used in structural studies and secondly the principles behind the experiments used to record such spectra.The approach adopted will be non- mathematical and it will be assumed that the reader has a basic knowledge of conventional proton and carbon-1 3 NMR spectroscopy. A very great number of different two-dimensional NMR experiments have been devised and it would be both impossible and unprofitable to describe even a fraction of these in the present work. However, there are only a few basic principles involved and these can be illustrated by reference to a small number of experiments. Fortunately, it turns out that these basic experiments are also the most widely used ones. For further information on these techniques and their applications there is a large number of specialized reviews and books available.Some approach the topic from the point of view of the chemical information obtainable from such technique^,'-^ others adopt a more physiochemical and finally there are advanced texts which adopt a strict quantum mechanical approach to the ~ubject.~,~ 2 Two-Dimensional NMR in Action A. Assignment of Spectra.-To be of any use to us an NMR spectrum must first be assigned, which means working out which peaks are associated with which ’J. K. M. Sanders and B. K. Hunter, ‘Modern NMR Spectroscopy’, OUP, 1987. A. E. Derome, ‘Modern NMR Techniques for Chemical Research’, Pergamon Press, 1987. R. Benn and H. Giinther, Angew. Cliem., lnt. Ed. Engl., 1983,22, 350. G. A. Morris, Mag.Reson. Chem., 1986,24, 371. H. Kessler, M. Gehrke, and C. Griesinger, Angew. Chem., In[.Ed. Engl., 1988,27,490. N. Chandrakumar and S. Subramanian, ‘Modern Techniques in High-Resolution FT-NMR’, Springer- Verlag, 1987. ’A. Bax, ‘Two-Dimensional Nuclear Magnetic Resonance in Liquids’, Delft University Press, 1982. R. R. Ernst, G. Bodenhausen, and A. Wokaun, ‘Principles of Nuclear Magnetic Resonance in One and Two Dimensions’, OUP, 1987. M. Goidman, ‘Quantum Description of High-Resolution NMR in Liquids’, OUP, 1988. 38 1 Two-Dimensional Nuclear Magnetic Resonance Spectroscopy I I I I ppm 4.0 3.0 2.0 1.o N N I I I I I ppm 5.0 4.0 3.0 I I I I I ppm 8.0 6.0 4.0 2.0 00 Figure 1 Proton NMR spectra of three molecules of dgfering complexity (a) the spectrum of ethanol which can be assigned by simple chemical shft arguments, (6) the spectrum of 1-0-methyl a-D-glucopyranoside (1) (recorded at 400MHz) which can be partially assigned by simple arguments, (c) the spectrum (recorded at 500MHz) of two 'zincfingers'from SWIS, a transcription activator protein of molecular weight ca 7ooO isolated from yeast The spectrum of even such a relatively small protein can only be assigned with the aid of a whole armoury of two-dimensional NMR techniques(SWIS spectrum kindly provided by David Neuhaus, MRC, LMB, Cambridge) hydrogens in the structure. Once this has been done, more information can be extracted, such as the values of couplings which give conformational information (for example via the Karplus equation).An assignment is also crucial in interpreting the results of other NMR experiments, such as nuclear Overhauser effect (NOE) or relaxation time studies. If the structure is unknown, the spectroscopist attempts to fit the spectrum to a series of trial structures until a consistent assignment is found. Figure 1 shows the proton spectra of three molecules of greatly differing Keeler OHI Ho* 3 'OH j1 complexity. The first, Figure 1 (a), is the spectrum of ethanol which can be assigned simply on the basis of arguing that, due to the presence of the electronegative oxygen substituent, the methylene protons should appear at lower field than the methyl protons. The spectrum from 1-0-methyl WD-glucopyranoside, compound (l), shown in Figure 1 (b) is rather more complex.Certain resonances can be assigned on the basis of shift arguments, for example the anomeric proton is easily identified, as is the characteristically sharp methyl resonance. A complete assignment could not be made from this spectrum alone, however it will be shown below that such an assignment can be made with the aid of one two-dimensional experiment. Further, a second two-dimensional experiment will enable us to assign the carbon-13 spectrum as well. Finally, in Figure 1 (c) is shown the proton spectrum of a 'zinc finger' protein. The spectrum is complex because it contains very many overlapping lines and the assignment of such a spectrum is a challenging and complex procedure even with the aid of two-dimensional NMR.B. COSY.-The spectrum of (1) can be assigned with the aid of a two-dimensional COSY spectrum. COSY stands for Correlation SpectroscopY.". ' ' At this point attention will be focused on the appearance and interpretation of such a spectrum, later the way in which the experiment actually works will be discussed. Conventional NMR spectra (one-dimensional spectra) are plots of intensity us. frequency; in two-dimensional spectroscopy intensity is plotted as a function of two frequency axes usually called F1 and F2. There are various ways of representing such a spectrum on paper, but the one most usually used is to make a contour plot in which the intensity of the peaks is represented by contour lines drawn at suitable intervals, in the same way as a topographical map.The position of each peak is specified by two frequency co-ordinates corresponding to FI and F2. Two-dimensional NMR spectra are always arranged so that the F2 co-ordinates of the peaks correspond to those found in the normal one-dimensional spectrum, and this relation is often emphasized by plotting the one-dimensional spectrum alongside the F2 axis. Figure 2 shows a schematic COSY spectrum of a hypothetical molecule lo W P Aue, E Bartholdi, and R. R Ernst, J Chem. Phys., 1976,64,2229.'' A. Bax and R. Freeman, J. Magn. Reson., 1981,44542. Two-Dimensional Nuclear Magnetic Resonance Spectroscopy F2 Figure 2 Schematic COSY spectrum of a two coupled spins denoted A and X For convenience the normal one-dimensional spectrum is plotted alongside the F1 and F2 axes and the diagonal (F, = F2) is indicated by a dashed line This spectrum shows two types of multiplets those centred at the same F1 and F2 frequencies called diagonal-peak multiplets and those centred at dgferent frequencies in the two dimensions called cross-peak multiplets Each multiplet has four component peaks The appearance of a cross-peak multiplet centred at F1 = i?iA, Fl = 6~ indicates that the proton with shft 6A is coupled to the proton with shft 6~ This observation is all that is required to interpret a COSY spectrum containingjust two protons, A and X, which are coupled together with a scalar coupling of JAXHz The one-dimensional spectrum is plotted alongside the F2 axis, and consists of the familiar pair of doublets centred on the chemical shifts of A and X, ??A and 6x respectively In the COSY spectrum the F1 co-ordinates of the peaks in the two-dimensional spectrum also correspond to those found in the normal one-dimensional spectrum and to emphasize this point the one-dimen- sional spectrum has been plotted alongside the F1 axis It is immediately clear that this COSY spectrum has some symmetry about the diagonal F1 = F2 which has been indicated with a dashed line In a one-dimensional spectrum scalar couplings give rise to multiplets in the spectrum, which are described by the familiar terms doublet, triplet, doublet of doublets etc In two-dimensional spectra the idea of a multiplet has to be expanded somewhat so that in such spectra a multiplet consists of an array of individual peaks often giving the impression of a square or rectangular outline Several such arrays of peaks can be seen in the schematic COSY spectrum of Figure 2 These two-dimensional multiplets come in two distinct types dzagonal-peak multiplets which are centred around the same F1 and F2 frequency co- ordinates and cross-peak multiplets which are centred around different F1and F2 co-ordinates Thus in the schematic COSY spectrum there are two diagonal-peak Keeler multiplets centred at F1 = F2 = SA and F1 = F2 = Sx one cross-peak multiplets centred at F1 = SA, F2 = SX and a second cross-peak multiplet centred at FI = 6x, F2 = SA.The appearance in a COSY spectrum of a cross-peak multiplet FI = SA, F2 = SX indicates that the two protons at shifts SA and SX have a scalar coupling between them.This statement is all that is required for the analysis of a COSY spectrum, and it is this simplicity which is the key to the great utility of such spectra. From a single COSY spectrum it is possible to trace out the whole coupling network in the molecule. This important point is best illustrated by moving to a hypothetical molecule which has three coupled protons, A, M, and X. Figure 3 shows schematic COSY spectra of such a three spin system for two cases: in (a) the spectrum is shown for the case of A coupled to M, and M coupled to X but with no coupling between A and X.In Figure 3 (b) the spectrum is shown for the case where all three protons are coupled to one another. As in the case of the two-spin system, both spectra show the uninformative diagonal-peak multiplets, as well as several cross-peak multiplets. In Figure 3 (a) there are cross-peak multiplets at F1 = SA, F2 = SMand F1 = SM,F2 = 6x indicating that proton A is coupled to M, and M is coupled to X. However, there is no cross-peak multiplet at F1 = SA, F2 = Sx indicating that protons A and X are not coupled. In contrast, the spectrum of Figure 3 (b) shows cross-peak multiplets between all three spins. Thus, from the COSY spectrum it is possible to deduce the topology of the coupling network between the protons. The spectrum of Figure 3 (a) indicates a linear arrangement of spins, whereas in (b) the topology is a triangular arrangement. Since couplings are transmitted through bonds, tracing out the topology is very closely related to tracing out the molecular framework.Diagonal-peak multiplets do not give any extra informa- tion, but their presence in the spectrum is useful in that they make for an eye- catching symmetry which allows correlations to be identified readily. For more complex molecules the COSY spectrum can be interpreted in just the same way as the simple systems described above; the rule is simply that a cross-peak multiplet indicates a coupling. If there is no cross-peak multiplet there is no coupling. Figure 4 shows the COSY spectrum of 1-O-methyl X-D-glucopyranoside, (1).It is immediately clear that there are several cross-peak multiplets present, as well as an easily recognizable series of diagonal-peak multiplets. Parts of the spectrum look rather different to the schematic COSY spectra of Figure 3. The reason for this is that the resolution in the experimental COSY spectrum is not always sufficient to separate clearly all the individual peaks which form a cross- or diagonal-peak multiplet. As they merge together the two-dimensional multiplets take on the appearance of a square or rectangular blob. This loss of resolution does not result in the loss of any information, as a cross-peak multiplet, even if just a blob, still indicates the presence of a coupling. The spectrum of (I) contains some features that we can identify at once. The sharp resonance at 3.3 ppm is clearly the OMe group, and the lowest field multiplet, H, is easily identified as the anomeric proton, H1, on the basis of its A-M-X \/X Figure 3 Schematic COSY spectra for a three spin system AMX with two dfferent coupling topologies Spectrum (a> is appropriate for the case where spin A is coupled to A4 A4 is coupled to X but where there is no coupling between A and X The presence of cross-peakmultiplets at F1 = F2 =6~ and F1 =6M F2 =6x together with the absence of a cross- peak multiplet centred at F1 =t5A F2 =6~ confirms this linear coupling topology Spectrum (b) is appropriate for the case where all three spins are coupled to one another as there are cross-peaks indicating each of these couplings A triangular topology of couplings IS indicated as shown under the spectrum Keeler 3.2 D AE 0 F CH CE 3.4 DG DF AD FP0 0 3.6 3.8 4.6 BIB CH IL 4.8 4.8 4.6 3.8 3.6 3.4 3.2 Fi Figure 4 COSY spectrum of 1-0-methyl cdbglucopyranoside, (l), recorded at 400MHz. For convenience the conventional proton spectra have been plotted alongside the FI and Fz axes and a blank part of the spectrum has not been plotted: the tops of intense peaks have been truncated.Each proton multiplet in the conventional spectrum is assigned a letter and the cross-peak multiplets are labelled with two letters to indicate which two proton multiplets are responsible for the cross-peak. The logic behind the assignment is described in the text.Note that the F1 axis is horizontal in this spectrum, in contrast to Figures 2 and 3 in which the F2 axis is horizontal characteristic chemical shift. With the aid of the COSY spectrum of Figure 4it is possible to assign this spectrum by using the anomeric proton as a starting point and following the series of three-bond couplings round the molecule. Multiplet H shows a clear cross-peak to C, identifying C as HZ. Multiplet C shows a cross-peak to E, identifying the latter as H3. Multiplet E shows a cross-peak to the highest field multiplet, A, which is partly obscured by the OMe resonance. We thus identify multiplet A as being from H4. Multiplet A shows a cross-peak to D, although this is somewhat obscured by the stronger cross-peak between A and E; D is therefore identified as H5.Finally, D shows cross-peaks with both F and G, while F and G also show a cross-peak between them, thus F and G are the 387 Two-Dimensional Nuclear Magnetic Resonance Spectroscopy two methylene protons 6 and 6 The only slight difficulty with this assignment is in recognizing which of the multiplets E and D a particular cross-peak correlates with Careful inspection of the spectrum will show that the cross-peaks marked AE and CE align with one another and not with the cross-peaks marked DG and DF, dotted lines have been added to aid in this distinction Thus, the COSY spectrum has enabled us to follow the chain of couplings round the molecule and find a complete assignment The description and use of a COSY spectrum for assigning an NMR spectrum has revealed many of the features which make two-dimensional NMR such a powerful tool In particular, the information in such a spectrum is presented in d readily appreciable and unambiguous manner A second feature, which has gone unremarked so far, is that the multiplets in the two-dimensional spectrum are dispersed into a much wider area of frequency space than they are in a one- dimensional spectrum This gives two-dimensional spectra a much greater effective resolution than their one-dimensional counterparts The COSY spectrum of Figure 4 is an illustration of this point multiplet A is overlapped with the OMe resonance in the one-dimensional spectrum, but in the COSY the triplet structure of the A multiplet is easily discerned from the cross-peak multiplet AE Likewise the overlapping multiplets E and D give rise to distinct separated cross- peaks This increase in effective dispersion obtained in two-dimensional spectra is much larger than can be achieved by going to even the highest magnetic field strengths 3 The Mechanics of Two-Dimensional Experiments A.Pulse Fourier Transform NMR.-In conventional optical spectroscopy light is passed through a sample and its absorption recorded as the wavelength of the light is scanned through the region of interest Originally NMR spectra were recorded in much the same way by sweeping a radio-frequency transmitter through the spectrum However a far better way of recording such spectra is to * use a version of the experiment known as pulse Fourier transform NMR In a pulsed NMR experiment the sample is subjected to a very intense pulse of radio-frequency power, typically only lasting a few microseconds This pulse causes the nuclear spins to ring much in the same way that a bell rings when it is struck The ringing signal, called a free induction decay, is recorded as a function of time, a typical example of such a signal is shown in Figure 5 (a) The free induction decay is not useful directly to the spectroscopist as it is a plot of intensity as a function of time, rather than as a function of frequency If there is only one line in the spectrum the free induction decay looks rather simple as it is just a cosine wave which gradually dies away If there are two lines in the spectrum, the free induction decay is the sum of two decaying cosines of different frequencies which gives rise to a beat pattern However, a spectrum consisting of perhaps hundreds of lines gives a free induction decay that is a hopeless jumble Luckily there is a mathematical operation, called Fourier transformation, which R K Harris Nuclear Magnetic Resonance Spectroscopy Longman 1983 Keeler frequency Figure 5 A typical free induction decay (a) obtained when a sample is excited with a short pulse of radio-frequency power, and the corresponding spectrum (b) obtained by Fourier transformation of (a).The free induction decay is a function of time and consists of a superposition of many cosine waves of different frequencies and amplitudes. Fourier transformation takes this data and converts it into the frequency domain in which the x and yco-ordinates of a point are the frequency and amplitude of a particular decaying cosine wave in the free induction decay will turn the free induction decay into the familiar spectrum.' 3*14Essentially, the Fourier transform picks out the intensities of all the frequency components that are present in the free induction decay and reconstructs the information in the form of a spectrum.On modern computers this Fourier transformation is a trivial calculation taking just a few seconds. The result of transforming the time domain signal in Figure 5(a) is shown in (b): it is the familiar spectrum.There are many reasons why pulsed Fourier transform NMR is superior to swept frequency NMR. The most important of these is that the whole spectrum is examined at once so that the technique is more time-efficient than sweeping through the spectrum; this results in higher sensitivity. In addition, more than one pulse can be applied to the sample so as to manipulate the nuclear spins in rather subtle ways. It will be seen in the following sections that such 'multiple pulse' NMR experiments are used to give two-dimensional spectra. The free induction decay is transferred into the computer memory using an analog to digital converter (ADC). This device measures the amplitude of the l3 R. N. Bracewell, 'The Fourier Transform and its Applications', Plenum, 1978.l4 'Transform Techniques in Chemistry', ed. R. Griffiths, Plenum, 1978. 389 Two-Dimensional Nuclear Magnetic Resonance Spectroscopy EVOLUTION -PREPARATION MIXING rl c, tl f2 Figure 6 The general scheme for two-dimensional NMR experiments The NMR signal in the form of a free induction decay is recorded during the time t2 This process is repeated for a series of values of tl During the preparation period one or more pulses are applied so as to generate a signal which will evolve during the time t 1 The mixing period consists of one or more pulses and it is the nature of the mixing process that determines how the evolution during tl affects the signal observed during t2 NMR signal at a particular instant and converts this value into a digital format that the computer can store The free induction decay is measured and digitized at regularly spaced intervals, the time between each sampling being the same If this time between samples is A, then the resulting spectrum covers a range of 0 to 1/(2A) Hz, this range is known as the spectral width For example, a typical proton spectrum spans 10 ppm which, on a 250MHz spectrometer, corresponds to a frequency range of 10 x 10 x 250 x lo6 = 2500Hz The sampling interval would be set to 1/(2 x 2500) or 200 ps The sampling process is carried on until the signal has decayed away, which is typically after a second or two B.General Scheme for Two-Dimensional NMR.-In one-dimensional pulsed Fourier transform NMR the signal is recorded as a function of one time variable and then Fourier transformed to give a spectrum which is a function of one frequency variable In two-dimensional NMR the signal is recorded as a function of two time variables, tl and t2, and the resulting data Fourier transformed twice to yield a spectrum which is a function of two frequency variables Figure 6 shows a general scheme for two-dimensional spectroscopy In the first period, called the preparation time, the sample is excited by one or more pulses The resulting signal is allowed to evolve for the first time period, tl Then another period follows, called the mixing time, which consists of a further pulse or pulses After the mixing period the signal is recorded as a function of the second time variable, t2 This sequence of events is called a pulse sequence and the exact nature of the preparation and mixing periods determines the information found in the spectrum It is important to realize that the signal is not recorded during the time tl, but only during the time t2 at the end of the sequence Just as was described above for one-dimensional spectroscopy, the data are recorded at regularly spaced intervals in both 22 and tl The two-dimensional signal is recorded in the following way First, tl is set to zero, the pulse sequence is executed and the resulting free induction decay recorded Then the nuclear spins are allowed to return to equilibrium, typically taking a few seconds to achieve this, and tl is then set to Al, the sampling interval in tl The sequence is repeated and a free induction decay is recorded and stored separately from the first Again the spins are allowed to equilibrate, tl is set to Keeler M tl t2 tl 12 Fi 20 T.Figure 1 Schematic two-dimensional spectra showing the interpretation of the co-ordinates of a peak in terms of the evolution during tl and t2. In spectrum (a) a peak appears at F1 = 20Hz, F2 = 80Hz. This can be interpreted as being the result of a signal which evolved during tl at 20Hz and which was then transferred, by some mixing process, to a signal that evolved at 80Hz during t2. In spectrum (b) the peak has the same co-ordinates in each dimension and so comes from a signal whose frequency was unaffected by the mixing process.Finally, in spectrum (c) two peaks are seen with the same F1 frequency of 20H2, but with Fz frequencies of 20 and 80 Hz. Such an arrangement of peaks is the result of the mixing process partly transferring the signal to one which evolves at 80Hz during t2 and leaving part of it to continue evolving at 20 Hz during t2 2A1, the pulse sequence repeated and a free induction decay recorded and stored. The whole process is repeated again for tl = 3A1, 461 and so on until sufficient data are recorded, typically 50 to 500 increments of tl. Thus recording a two- dimensional data set involves repeating a pulse sequence for increasing values of tl and recording a free induction decay as a function of t2 for each value of 11.C. Interpretation of Peaks in a Two-Dimensional Spectrum.-Within the general framework outlined in the previous section it is now possible to interpret the appearance of a peak in a two-dimensional spectrum at particular frequency co- ordinates. Suppose that in some unspecified two-dimensional spectrum a peak appears at F1 = 20Hz, F2 = 80Hz. The interpretation of this peak is that a signal was present during tl which evolved with a frequency of 20Hz. During the mixing time this same signal was transferred in some way to another signal which evolved at 80Hz during t2. This is illustrated in Figure 7 (a). Likewise, if there is a peak at F1 = 20Hz, F2 = 20Hz the interpretation is that there was a signal evolving at 20Hz during tl which was unaffected by the mixing period and continued to evolve at 20Hz during t2, as illustrated in Figure 7 (b).The processes by which these signals are transferred will be discussed in the following sections. Finally, consider the spectrum shown in Figure 7 (c). Here there are two peaks, one at F1 = 20H2, F2 = 8OHz and one at F1 = 20Hz, F2 = 20Hz. The interpretation of this is that some signal was present during tl which evolved at 391 Two-Dimensional Nuclear Magnetic Resonance Spectroscopy 0 20Hz and that during the mixing period part of it was transferred into another signal which evolved at 80 Hz during t2 The other part remained unaffected and continued to evolve at 20 Hz On the basis of the previous discussion of COSY spectra, the part that changes frequency during the mixing time is recognized as leading to a cross-peak and the part that does not change frequency leads to a diagonal-peak This kind of interpretation is a very useful way of thinking about the meaning of peaks in a two-dimensional spectrum It is clear from the discussion in this section that the mixing time plays a crucial role in forming the two-dimensional spectrum In the absence of a mixing time, the frequencies that evolve during tl and t2 would be the same and only diagonal-peaks would appear in the spectrum To obtain an interesting and useful spectrum it is essential to arrange for some process during the mixing time to transfer signals from one spin to another 4 EXCSY Spectra EXCSY stands for Exchange SpectroscopY l5 It is a two-dimensional NMR experiment which gives a spectrum in which the cross-peaks indicate which spins are undergoing mutual chemical exchange This experiment will be illustrated using the exchange processes that take place in dimedone (2), whose one-dimensional spectrum and structure are shown in Figure 8 Figure 9 (a) shows the pulse sequence used to record EXCSY spectra and parts of the resulting two- dimensional spectrum of dimedone are shown in (b) and (c) Dimedone undergoes a reversible keto-enol tautomerism in the conditions under which the spectrum was recorded, and this exchange is slow enough on the NMR timescale that distinct resonances are seen for both species The assignment and a simple reaction scheme for the exchange process is shown in Figure 8 In the EXCSY spectrum there is a cross-peak at an F1 = shift of proton K1 and F2 = shift of proton El, indicating that these two protons are exchanging In addition, there are cross-peaks between the methyl resonances E3 and K3, and between the ring methylene resonances E2 and K2, further confirming the nature of the exchange process Turning now to the H20/0H peak, this is seen to have a cross-peak with the keto proton, K1 but no cross-peak with El This observation implies that K1 exchanges directly with the water, but El does not This spectrum thus provides, in a very simple way, a great deal of information about the exchange processes that are taking place in this molecule In thls J Jeener B H Meier P Bachmann and R R Ernst J Chem Phps 1979 71 4546 Keeler K2 K3 E3 K1 E2 I,,XL I I I I I ppm 8.0 7.0 6.0 5.0 4.0 Figure 8 Proton spectrum of dimedone (2) recorded at 250MHz.Under the conditions in which this spectrum was recorded, dimedone undergoes a slow interconversion between a keto and an enol form, (2K) and (2E) respectively, as indicated by the reaction scheme. The assignment of resonances to individualprotons in the two species is indicated on the spectrum simple case, it simply confirms our existing chemical intuition, but in more complex cases involving fluxional molecules or complex reactions, EXCSY spectra provide invaluable insight into the nature of the chemical processes taking place. In the same way that COSY provides an almost visual map of the coupling network, EXCSY provides a similar map of the exchange processes taking place.The EXCSY experiment has been chosen at this point as it is possible to explain the detailed form of the two-dimensional experiment without recourse to quantum mechanics. The experiment can be described with reference to a simple vector picture of pulsed NMR. It is not possible to describe fully or justify this simple picture within this review (details can be found in other places),'2-'6 but for the present purposes some rules for the manipulation of these vectors can be stated: (i) Each separate spin in the molecule gives rise to a magnetization vector. (ii) At equilibrium these magnetization vectors lie along the direction of the applied magnetic field, conventionally taken as z.l6 R. Freeman, 'A Handbook of Nuclear Magnetic Resonance', Longman, 1988. 393 38 F1 40 K2 PPmI40 38 ppm F2 -50 -60 FiI El -70 -80 PPm Figure 9 (a) Pulse sequence uped to record two-dimensional EXCSY spcctra (h) and (c) parts of the EXCSY spectrum of dimedone (2) For convenience the conventional proton spectrum has also been plotted alongside the F1 and FZuxes and the trto regions have been plotted on dfferent scales The presence of a cross-peak in the EXCSY spectrurn at F1 = 8A Fz = tiB indicates that the spins resonating at SA and Sg are undergoing mutuul chemitul exchange In the EXCSY spectrum ~e see a set of cross-peaks II hith ure torisi~tent~iththe reaction scheme of Figure 8 see text for ftrrther diwussion chemicaliexchange Figure 10 Vector precession diagrams illustrating the effect of the EXCSY pulse sequence, Figure 9 (a).The experiment starts with magnetization of spin A, indicated as a bold arrow, aligned along the z axis, (a). A 90" pulse rotates the magnetization vector onto the -y axis, (b) and the vector then precesses in the xy plane for a time tl. At the end of tl the vector makes an angle 0 = hfAt1 with the -y axis, (c). A second 90' pulse rotates the -y component of the vector (proportional to cos0) onto the -z axis, (d). During the mixing time, T,,,,~, the molecule may undergo a conformational change which converts spin A into spin B.During such a change the spin 'takes its magnetization with it', as indicated in (f). The jinal pulse rotates the vector onto the y axis, (g), where it is detected and gives rise to a signal at fB, (i). Ifno conformational change takes place during the mixing time the vector retains its identity as A and the jnal pulse gives rise to a signal that is detected at fA, (e) and (h). Most importantly, the size of the signal detected at either fA or 4 is proportional to cos 0 (iii) A radio-frequency pulse can be applied to rotate these vectors about the x axis. The angle through which the vector is rotated is called the flip angle. For example, a pulse with a flip angle of 90" rotates the vector from the z axis to -Y* (iv) Once in the xy plane, each vector precesses around the z axis at a frequency which depends on its chemical shift.(v) Only magnetization vectors in the xy plane give rise to a free induction signal. Imagine a molecule which has just one kind of proton which is undergoing an interconversion, slow on the NMR timescale, between two conformers A and B. In the conventional spectrum two peaks are observed, one for each of the conformers, at shifts fA andfB. Figure 10 shows the action of the pulse sequence of Figure 9 (a) using vector diagrams and for simplicity only the vector from conformer A is considered. The initial 90" pulse rotates the magnetization vector about the x axis moving it from z to -y, (a) and (b). During tl the vector Two-Dimensional Nuclear Magnetic Resonance Spectroscopji precesses in the xy plane so that after tl the vector makes an angle of 2nfAtl with the -y axis, Figure 10 (c) The second 90" pulse is applied and causes a further rotation about the x axis This pulse will have no effect on the Y component of the magnetization vector, as this is aligned along the axis about which the pulse acts In contrast, the component along the -y axis is rotated to the -z axis If the original vector is of length M, then the component which is rotated to the --z axis is of size M COS(2nfAtl), Figure 10 (d) The component left along the Y axis is of no further interest in the current experiment and its effect can be ignored for the moment The next event in the pulse sequence is the mixing time, T,,,,~ This time is made long enough so that there is a significant chance of the molecule undergoing a conformational change from A to B Suppose that the molecule under considera- tion in the previous section does indeed undergo such a change, then the magnetization vector aligned along the -2 axis of size M cos(2nfAtl) remains unaltered by the chemical process, but it is now associated with a spin on a molecule of type B, Figure 10 (f) The final pulse of the sequence rotates this magnetization from the --z axis to the y axis, Figure 10 (e) and (8) The free induction decays from the precessing magnetizations are observed and after Fourier transformation give rise to spectra (h) and (I) For the magnetization which has been transferred from being on molecule A to molecule B during the mixing time the spectrum will show a peak at fB, (i), whereas the magnetization which remains on molecule A gives a peak at fA, (h) Note, however, that the magnetization that gives rise to both of these signals precessed at fA during tl The final result of this analysis is to predict the appearance of a cross-peak at F1 =fA, F2 =f~,and a diagonal peak at F1 =fA, F2 =fA The cross-peak arises from magnetization that was transferred by chemical exchange during the mixing time, and the diagonal-peak from magnetiza- tion that was not transferred It is appropriate at this point to look in more detail at how the evolution during tl effects the signal observed during t2 From the above analysis it was seen that the y magnetization present after the last pulse has a size proportional to cos(2nfAtl) in other words the size of the signal observed during t2 depends on the evolution during tl For the first increment of tl (tl = 0), the signal will be a maximum, the second increment will have size proportional to cos(27cf~A1), the third proportional to COS(~Z~A~A~), and so onthe fourth to cos(27cf~36~) This modulation of the amplitude of the observed signal by the tl evolution is illustrated in Figure 11 In the Figure the first column shows a series of free inductions decays that would be recorded for increasing values of fl and the second column shows the Fourier transforms of these signals The spectra in this second column clearly show how the amplitude of the line in the spectrum varies with ti The final step in constructing the two-dimensional spectrum is to Fourier transform the data along the tl dimension This process is also illustrated in Figure 11 Each of the spectra shown in the second column are represented as a series of data points, where each point corresponds to a different F2 frequency Keeler f2 F2 tl FI Figure 11 Illustration of how the modulation of afree induction decay by evolution during tl gives rise to a peak in the two-dimensional spectrum.In the left-most column is shown a series of free induction decays that would be recorded for successive values of tl; tl increases down the page. Note how the amplitude of these free induction decays varies with t 1, something that becomes even plainer when the time domain signals are Fourier transformed, as shown in the second column.In practice, each of these F2spectra in column two consist of a series of data points. The data point at the same frequency in each of these spectra is extracted and assembled into an interferogram, in which the horizontal axis is the time tl. Several such interferograms, labelled a to g, are shown in the third column. Note that as there were eight F2 spectra in column two corresponding to drfferent tl values there are eight points in each interferogram. The Fz frequencies at which the interferograms are taken are indicated on the lower spectrum of the second column. Finally, a second Fourier transformation of these interferograms gives a series of Fl spectra shown in the right hand column.Note that in this column F2 increases down the page, whereas in the jirst column tl increase down the page. Thejnal result is a two-dimensional spectrum containing a single peak Two-Dimensional Nuclear Magnetic Resonance Spectroscopy The data point corresponding to a particular F2frequency is selected from the spectra for tl = 0, tl = A1, tl = 2A1 and so on for all the tl values. Such a process results in a function, called an interferogram, which has tl as the running variable. Several interferograms, labelled a-g, computed for different F2 frequencies are shown in the third column of Figure 11. The particular F2 frequency that each interferogram corresponds to is indicated in the bottom spectrum of the second column.The amplitude of the signal in each interferogram is different, but in this case the modulation frequency is the same. The final stage in the processing is to Fourier transform these interferograms to give the series of spectra which are shown in the right most column of Figure 11. These spectra have F1 running horizontally and F2 running down the page. The modulation of the time domain signal has been transformed into a single two-dimensional peak. Note that the peak appears on several traces corresponding to different F2 frequencies because of the width of the line in F2. In summary, cross-peaks in an EXCSY spectrum arise by magnetization, which has been labelled with evolution at one frequency, being transferred during the mixing time to another spin where it is observed at a different frequency. The information about the frequencies with which the magnetization has evolved during tl is encoded into the amplitude of the signals that are observed during t2.This encoding is unravelled into a two-dimensional spectrum by double Fourier transformation. All other two-dimensional experiments share this method of modulating the observed signals by the evolution during tl. 5 NOESY Spectra This experiment utilizes the same pulse sequence as the EXCSY technique and is essentially based on the same idea. The only difference is that the phenomenon responsible for transfer of zmagnetization between spins during the mixing time is that of cross relaxation rather than chemical exchange.17 Cross relaxation is a special type of interaction that occurs between two spins that share a through space dipole-dipole coupling.It is the phenomenon responsible for the nuclear Overhauser effect (NOE). 2, **l A NOESY spectrum has cross-peaks which show which spins are undergoing mutual cross relaxation and are therefore close in space to one another. Such information is clearly very useful in establishing the three-dimensional shapes of molecules. For technical reasons small and medium sized molecules do not yield good quality NOESY spectra. However, large molecules, such as proteins and DNA fragments, give excellent NOESY spectra which are very widely used in studying the structures of such molecules.20 l7 S.Macura and R. R. Ernst, Mol. Phys., 1980,41,95. J. H. Noggle and R. E. Schirmer, ‘The Nuclear Overhauser Effect’, Academic Press, 1971. l9 D. Neuhaus and M. P. Williamson, ‘The Nuclear Overhauser Effect in Structural and Conformational Studies’, VCH, 1989. K. Wuthrich, ‘NMR of Proteins dnd Nucleic Acids’, Wiley, 1986. Keeler 6 COSY Spectra The pulse sequence for recording COSY spectra, such as those already discussed, is shown in Figure 12. It is a particularly simple sequence consisting of just two pulses, but despite this it is not possible to make an adequate description of how cross-peaks arise in a COSY spectrum without recourse to some quantum mechanical calculations. The results of such a calculation will be presented here; a detailed description can be found in several other places.5~63s39~'6~21 A partial explanation of how a COSY spectrum arises can be made using the vector picture introduced in the previous section.For simplicity consider a system of two coupled spins, A and X, and concentrate on the A multiplet which consists of two lines at frequenciesfA JJAX. These two lines can be represented as two vectors and after the first 90" pulse they are both aligned along the -y axis, Figure 12 (a). The two vectors precess at different frequencies so that at the end of the time tl they make angles 27c(fA + $JAX)tl and k(fA -$JAX)fl with the -y axis, Figure 12 (b). Normally the x and y components of these two vectors would be considered, but the quantum mechanical calculations indicate that a different decomposition is appropriate.The vectors are decomposed into four components which have the following arrangements: (1) both vectors aligned along the -y axis, (2) both vectors aligned along the x axis, (3) one vector aligned along y and one along -y, (4) one vector aligned along x and one along -x. The reason for this choice should become clearer as the explanation proceeds. The components numbered (1) and (2) are called in-phase magnetization along y and x respectively; components (3) and (4) are called anti-phase magnetization along y and x respectively. This decomposition is indicated in Figure 12 (c). As with the EXCSY sequence, the size of these different components depends on the angles through which the two vectors have precessed and hence on the frequencies of the two lines.A full quantum mechanical calculation reveals that the second 90" pulse has a different effect on each of the four components. In- phase magnetization along x is unaffected and evolves during t2 as a signal giving rise to the A spin doublet. In-phase magnetization along y is rotated to the z axis and is therefore no longer observable. Anti-phase magnetization along x is transferred into a special state called multiple yuanfurn coherence which is not observable. Finally, and most importantly, anti-phase magnetization along JI which started out on the A spin is transferred into anti-phase magnetization associated with the X spin. During 22 this gives a signal which results in an X spin doublet in the spectrum. The fate of the various components are shown in Figure 12 (d).This last process, known as coherence transfer, is responsible for generation of cross-peaks in the COSY spectrum. As its name implies, coherence transfer results in magnetization on the A spin being transferred to the X spin. As in the case of EXCSY, the size of the signal transferred from A to X depends on the evolution '' 0.W. Smensen, G. W. Eich, M. H. Levitt, G. Bodenhausen, and R. R. Ernst, Prog. NMR Specrrosc., 1983, 16, 163. 399 Two-Dimensional Nuclear Magnetic Resonance Spectroscopy X J J J J z magnetization multiple quantum (d) diagonal-peak cross-peak Figure 12 At the top is shown the pulse sequence used to record two-dimensional COSY spectra and below are vector precession diagrams indicating how the experiment proceeds The A spin doublet of an AX spin system is considered After the Jirst 90" pulse the two vectors corresponding to the two lines of the doublet are both aligned along the -y axis (a) After time tl they have precessed at dfferent speeds and hence are separated in the xy plane(6) A full quantum mechanical calculation reveals that the two vectors should be resolved into four components as indicated in (c) Components (1) and (2) have both vectors aligned along the -y and x axes respectively and are called in-phase states Component (3) has the two vectors aligned with one along +y and one along -y and likewise (4)has them aligned along -+ x and -x These are called anti-phase states The effect of the last pulse can only be deduced from a full quantum mechanical calculation This shows that componert (1) is turned to the z axis and component (4)becomes multiple quantum coherence both of which are unobservable Component (2) is unaffected by the pulse and remains associated with the A spin giving rise to a diagonal-peak multiplet Component (3) undergoes coherence transfer to the coupled spin X and gives rise to a cross-peak multiplet in the COSY spectrum during tl and it is in this way that the observed signal IS modulated by the evolution during tl Keeler The details of this calculation reveal some further features of the COSY spectrum which have not been discussed so far.The diagonal peaks result from magnetization that was in-phase just before the last pulse, and remains in-phase after it. This results in all four components in the diagonal peak having the same sign. In contrast, the cross-peaks arise from the transfer of anti-phase magnetiza- tion from one spin to another and this results in the cross-peak multiplet having two positive and two negative components arranged on the diagonals of the square. The spectra presented in this review have all been processed and plotted in the absolute ualue or magnitude mode in which the modulus of the (complex) spectral data is displayed.16 Although this masks the different signs of individual peaks in the spectrum it turns out for a variety of technical reasons to be a convenient way of plotting spectra.Recent developments have now made it more common to process and plot spectra in such a way that this sign information is not lost. Further discussion of this important point is beyond the scope of this review, but is described in detail in reference 22. 7 Heteronuclear Correlation Spectra So far all the experiments described have involved only the proton spectrum, however, it is equally possible to devise two-dimensional experiments which involve other nuclei, for example carbon-13. In this section one such experi- ment, usually known as ‘shift correlation’ will be de~cribed.~~.~~ This experi- ment gives rise to a spectrum in which the FI coordinate of a cross-peak is the proton chemical shift and the F2 coordinate is the carbon-13 chemical shift.Using such a spectrum the proton assignment of (1) can be used to assign all the carbon- 13 resonances. As with COSY spectra, the method of finding which carbon-13 atoms are joined to which protons is to devise an experiment in which there is coherence transfer from protons to carbon-13. Figure 13 (a) shows one possible pulse sequence that could be used to achieve this. This sequence is just the direct analogue of the COSY experiment, modified so that the pulses affect both the carbon-13 and proton spectra. The reason for needing separate pulses for each type of nucleus is that their resonance frequencies are so widely separated that a single pulse cannot affect both types of nucleus.With virtually all NMR spectrometers it is only possible to record signals from one type of nucleus at a time and it is most usual in these shift correlation experiments to record the signals from carbon-13. Since the aim is to present a spectrum in which the F1 coordinates of the cross-peaks are proton shifts, the first pulse is applied only to protons. Hence, only proton magnetization evolves during tl. The one-bond carbon-13 proton coupling is typically in the range 120 to 200Hz and is much larger than any of the long-range couplings between the two nuclei. This fact can be used to make a modification of the pulse sequence of 22 J. Keeler and D. Neuhaus, J. Mugn. Reson., 1985,63,454. 23 G. A. Morris and R. Freeman, J. Chrm.Soc., Chem. Commun., 1978,684. 24 A. A. Maudsley, A. Kumar, and R. R.Ernst, J. Mugn. Reson., 1977,28,463. 401 Two-Dimensional Nuclear Magnetic Resonance Spectroscopy carbon 13 7 tl t2 I proton I decouple I I(b) I-I carbon 13 I A tl A1 A2 f2 Figure 13 (a) Shows a simple pulse sequence for recording two-dimensional rhlft correlation spectra in which the cross-peaks indicate the presence of a coupling between a proton and a carbon-13 The sequence is very similar to that for COSY except that only the carbon-13 signal is observed This sequence can be mod$ed so that splittings in both FZand Ft due to carbon-13 proton couplings are removed from the spectrum This is achieved in the F2 dimension by using broadband proton decoupling during data acquisition and in Ft byinserting aproton 180°pulse midway in tt in theJigure the 180" pulse is represented by an open box For reasons described in the text tMo delays At and At also have to be included in the sequence (b) These delays should be set to approximately 1/(2Jc~) where JCH is a typical value of the carbon-13 proton coupling through which correlations are desired By setting these delays for a coupling of around 130Hz only cross-peaks between direcfly bonded carbon-13 proton pairs will be seen in the spectrum Such a one-to-one map of directly bound proton vs carbon-13 shlfts IJ very useful in assignment Figure 13 (a) which results in a spectrum which shows only correlations between directly bonded carbon-13 proton pairs In addition the resulting spectrum is simpler in appearance and has higher sensitivity There are two modifications made to the pulse sequence The first is to add broadband decoupling of the protons during the acquisition time, t2 Such decoupling is the normal practice for recording one-dimensional carbon- 13 spectra for the reason that it collapses the often complex carbon-1 3 multiplet to a single sharp line, thereby greatly increasing both sensitivity and resolution However, in analogy with COSY spectra, the magnetization that has been transferred from protons to carbon-13 by the last two pulses in the sequence is in anti-phase Applying broadband decoupling to such a state causes the coupling to collapse to zero and thus the two anti-phase lines will cancel one another and no signal will be observed This problem is circumvented by leaving a suitable delay, A2, prior to switching on the decoupler The simple vector picture can be used to predict the correct value of this delay the two vectors which represent the lines of Keeler the doublet start out aligned along the y and -y axes respectively, and during the delay they evolve through angles 2n(& + &JcH)Az and 2n(& -&JcH)Az respectively.Thus the angle between the vectors increases by ~JCHAZduring the delay, and it is clear that if A2 is set to 1/(2JCH) this angle will be n at the end of the delay. Thus, after such a delay the vectors have evolved from being in anti- phase (180" apart) to being in-phase.The decoupler may now be switched on safely. The second modification is to place a carbon-13 180" pulse in the middle of the tl period. It can be shown that such a pulse has the effect of preventing any of the carbon-13 proton couplings causing splittings in the F1 dimension of the spectrum. However, in order to have any coherence transfer from protons to carbon- 13 it is necessary that an anti-phase state is created, and this is achieved by adding a further delay, AI, of 1/(2JcH)just prior to the last two pulses. The final pulse sequence is shown in Figure 13 (b). Figure 14 shows the carbon-13 proton shift correlation spectrum of (1) recorded using the pulse sequence of Figure 13 (b). Although the experiment is based on the same ideas as COSY, that of coherence transfer, it looks very different indeed to a COSY spectrum.There are two reasons for this difference: first, the carbon-13 proton couplings have been removed from each dimension so that each cross-peak is a single peak; secondly, the spectrum consists entirely of cross-peaks, the diagonal-peaks have not been recorded because only the carbon- 13 signal was observed. In principle, proton-proton couplings are still present in the P1dimension, but the resolution is often not adequate to reveal them. The shift-correlation spectrum of (1) enables the proton assignment previously determined to be used to assign the carbon-13 spectrum, since each cross-peak indicates a directly bonded carbon-13 proton pair. In more complex cases where the proton assignment is only partially known, the correlation spectrum can be used along with the COSY to aid in the assignment of both the carbon-13 and proton spectra.It is also possible to devise modified carbon-13 proton shift correlation spectra that show cross-peaks between long-range coupled pairs of nuclei; such spectra are of great utility in assigning the quaternary carbons in a molecule. Recently, it has been shown that there are considerable advantages in recording these carbon- 13 proton shift correlation spectra with observation of the proton signal rather than the ~arbon-13.~~-~' Such experiments are generally known as inverse experiments as they are recorded in the opposite way from what was the normal practice. These inverse experiments benefit from the fact that, due to the higher resonance frequency of protons than carbon-13, a given number of protons give rise to a much larger signal than the same number of carbon-13 atoms.Thus the sensitivity of the inverse experiment is much higher, an advantage which enables such experiments to be carried out successfully on larger molecules than was previously possible. 25 L. Miiller, J. Am. Chem. Soc., 1979,101,4481. 26 A. Bax, R. H. Griffey, and B. L. Hawkins, J. Map. Reson., 1983,55,301. "J. Cavanagh, C. A. Hunter, D. N. M. Jones, J. Keeler, and J. K. M. Sanders, Mngn. Reson. Chem., 1988, 26, 867. Two-Dimensional Nuclear Magnetic Resonance Spectroscopy c5 c2Lilll 3.0I 3.5 4.0 FI7I 4.5 5.0 ppm 100 90 80 70 60 F2 Figure 14 Carbon-13 proton shift correlation spectrum of (1) obtained using the pulse sequence of Figure 13(b).The delays At and A2 have been set so that on1.v correlations through one-bond carbon-proton couplings are seen. The conventional carbon-13 and proton spectra are plotted along the F2 and Ft axes respectively. Using the assignment of the proton spectrum found from rhe COSY spectrum, a complete assignment of the protonated carbons is a trivial task from this spectrum 8 General Features of Two-Dimensional Spectroscopy It was noted in the introduction that a very large number of two-dimensional NMR experiments have been devised, but it turns out that in day-to-day use the COSY, EXCSY (NOESY) and shift correlation experiments account for well over two-thirds of all spectra recorded. The other experiments are generally somewhat more complex and have been devised to solve particular complex problems of assignment.It is often supposed that two-dimensional spectroscopy is a fundamentally insensitive technique. This erroneous point of view arises from the observation that recording a two-dimensional experiment involves recording a one-dimen- sional spectrum for each value of tl. Inspection of Figure 11 reveals why this point of view is incorrect. After the first Fourier transformation with respect to t2 the modulated signal appears in each spectrum which was recorded for each tl value. However, after the second Fourier transformation with respect to tl this signal is concentrated into one peak in the two-dimensional spectrum, rather than being spread out as it was before transformation.This concentration of the signal achieved by the double Fourier transformation shows that the signal-to- noise ratio in the two-dimensional spectrum is much better than the signal-to- 404 Keelev noise ratio acheived for a spectrum recorded at a particular tl value. In fact, the final signal-to-noise ratio of the two-dimensional spectrum is closely related to the signal-to-noise ratio that would be achieved in an equivalent one-dimensional experiment which lasted for the same total duration as the whole two-dimensional experiment. 8*29 Many two-dimensional experiments are recorded using pulse sequences that contain several radio-frequency pulses.As described in the previous sections, such pulses are used to achieve coherence transfer or to rotate magnetization between the z axis and the xy plane. It turns out that radio-frequency pulses tend to be rather indiscriminate in the way they affect the spins, and in addition to achieving the desired transformation they may also move magnetization to unwanted places or generate unwanted multiple quantum coherences. These processes can give rise to peaks in the two-dimensional spectrum that may cause confusion or may obscure other wanted peaks. In addition, imperfections in the spectrometer can also give rise to further peaks. Often, it is possible to select just the desired processes, and hence the desired peaks in the spectrum, by a technique known as phase cy~ling.~~3~'This involves repeating the pulse sequence (for a fixed value of tl) with a systematic variation of the phases of the pulses in the sequence.The way in which phase cycles are devised is beyond the scope of this review, but suffice it to say that such cycles are essential for many two-dimensional experiments. The techniques described in this work have been illustrated by reference to proton and carbon-1 3 spectra but the experiments are equally applicable to other nuclei such as nitrogen-15, boron-1 1, phosphorus-31, and fluorine-19. Two- dimensional NMR has been widely used to study the structures of molecules containing these less common nuclei.It has not been possible in this review to discuss two very important applications of two-dimensional spectroscopy. The first has been the study of multiple quantum spectra.32 These spectra arise from transitions which are not allowed by the normal selection rules for NMR, but as has been alluded to above it is possible to generate multiple quantum coherence with a suitable pulse sequence. Such multiple quantum coherences cannot be observed directly but their evolution can be followed using two-dimensional spectroscopy. A large number of applications have been found for multiple quantum spectra, ranging from spectral simplification and editing to relaxation studies. The second area that has not been mentioned is the application of two- dimensional NMR to the study of solid materials; such materials have radically different spectra to those encountered in liquid samples but despite this two- dimensional spectroscopy is used in a similar way to aid in assignment and interpretati~n.~~ 28 W.P. Aue, P. Bachmann, A. Wokaun, and R. R. Ernst, J. Mugn. Reson., 1978,29, 523. 29 M. H. Levitt, G. Bodenhausen, and R. R. Emst, J. Magn. Reson., 1984,58,462. 30 G. Bodenhausen, H. Kogler, and R. K.Ernst, J. Mugn. Reson., 1984,58,370. 31 J. Keeler in 'Multinuclear Magnetic Resonance in Liquids and Solids--Chemical Applications', ed. P. Granger and R. K. Harris, NATO AS1 Series C: Vol. 322, Kluwer (Dordrecht), 1990. 32 G. Bodenhausen, Prog. NMR Spectrosc., 1981, 14, 137. 33 9.Bliimich and H. W. Spiess, AngeKi.Chem.,Int. Ed. Engl., 1988,27, 1655. Two-Dimensional Nuclear Magnetic Resonance Spectroscopy Acknowledgements. I am grateful to Adrian Davis and Julia Richardson for carefully reading the manuscript and making numerous suggestions for improve- ments, and to Dr. Jeremy Sanders for encouragement and invaluable advice on the selection of example compounds.

ISSN:0306-0012    

DOI:10.1039/CS9901900381

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Chem. SOC. Rev., 1990,19,407-437 Infrared Laser Powered Homogeneous Pyrolysis By Douglas K. Russell DEPARTMENT OF CHEMISTRY, UNIVERSITY OF LEICESTER, LEICESTER LE1 7RH 1 Introduction Infrared molecular lasers, capable of very high output power, were discovered over a quarter of a century ago. Almost immediately, the potential of these lasers in the initiation and study of chemical reactions, particularly in the gas phase, was recognized. Early experiments were motivated by the possibility of the promotion of selective reaction arising from the very specific nature of the excitation process. It was also hoped that the narrow band excitation provided by laser sources could be used in the efficient separation of isotopes. While these aims have in part been realized, it soon became clear that infrared lasers had the potential for considerably wider application in kinetic and mechanistic investiga- tions.In particular, the need for a chance coincidence between laser output and absorption in the target molecule could be eliminated by use of a chemically inert IR absorber. In this method, energy is absorbed in a vibrational mode of the photosensitizer, and rapidly converted into heat (translational energy) via efficient relaxation processes. Energy is then transferred to the reagent molecule via collisions in much the same manner as in conventional pyrolysis, and this process is therefore known as Infrared Laser Powered Homogeneous Pyrolysis, or IR LPHP. The major advantage conferred by this technique, in comparison with conventional methods, is implied in the term ‘homogeneous’. Since energy is conveyed directly into the gas, then initiation of reaction is unambiguously homogeneous.In practice, most experiments are carried out under conditions where cell walls remain cool, and thus secondary wall processes are also largely eliminated. This can lead to a great simplification in disentangling the contribu- tions of homogeneous and heterogeneous processes in complex systems; this is especially so in cases where deposition occurs, and the surface thus produced acts as an auto-catalyst for primary or subsequent processes. On the other hand, the generation of an inherently non-uniform temperature brings its own problems, especially in the extraction of meaningful kinetic parameters from observed rates.In the next section of this Review I shall trace the development of the IR LPHP technique from the early, essentially qualitative, studies, to the sophisti- cated pulsed methods now used in the determination of reaction parameters. There follow descriptions of the major experimental components, and of some pertinent validation investigations and theoretical considerations. The final section consists of three case studies of areas where the IR LPHP technique has found useful application, including some drawn from my own laboratory. It is Infrared Laser Powered Homogeneous Pyrolysis intended that this Review should be introductory rather than in-depth in its approach, and illustrative rather than comprehensive in its coverage.If I have under-represented some aspects of the topic I can therefore only offer my apologies, and assure readers that no judgement of relative significance is thereby implied. A. Note on Nomenclature and Scope.-The interaction of infrared laser radiation with molecules in the gas phase is a vast subject, and has been the subject of an enormous number of investigations over the past 25 years. Clearly, there have been many objectives behind these efforts, and it is therefore appropriate at this stage to delineate the scope of this Review, and to introduce some terminology. The investigation of the absorption of IR laser radiation by molecules forms, of course, part of the subject of IR Spectroscopy.The study of the fate of absorbed energy, initially deposited into a single vibrational (and also sometimes a single rotational) level, at powers below that required for any chemical action we can class as IR Laser Photophysics. Such studies would include, for example, the rates and selection rules for energy transfer between vibrational, rotational, and translational degrees of freedom. If the energy deposited is sufficient to cause specific chemical action within the absorbing molecule, we may consider this to be IR Laser Photochemistry: because several IR photons are usually required to overcome the activation barrier in a single molecule, this may also be known as IR Laser Multiple Photon Dissociation, or IR LMPD. The eventual fate of the absorbed IR energy is its distribution over all degrees of freedom in the system, including translation (heat).Under the right circumstances, the temperature rise thus produced may be sufficient to induce thermal chemical reaction, either in the absorber or in added reagents. This process, known as IR Laser Powered Pyrolysis (IR LPP) or IR Laser Induced Pyrolysis (IR LIP), must be carefully distinguished (both conceptually and experimentally) from the direct photo- chemical effect described above. The situation is slightly confused by the com- mon usage of the word ‘photosensitizer’ to describe the IR absorbing species; although ‘energy transfer agent’ or even ‘energy conversion agent’ would perhaps be more accurate, I shall use the term sanctioned by convention.Finally, we add the term ‘homogeneous’ to yield IR Laser Powered Homogeneous Pyrolysis (IR LPHP) to distinguish this process from those where gas phase reaction is induced through laser heating of a surface, as is used in the process of laser writing (or pantography). It is the features and advantages peculiar to the LR LPHP technique which I shall discuss here, although the neighbouring areas described above clearly provide us with essential information. 2 Historical Background The first molecular laser operating in the infrared region of the electromagnetic spectrum was the carbon dioxide laser, discovered in 1964 by Patel and co- workers.’ This was rapidly followed by developments which led to the operation ’ C K N Patel, W L Faust, and R A McFarlane, Bull Am Phyr Soc ,1964,9, 500 Russell of the laser at high power.2 This laser operates on transitions between the u3 = 1 vibrational level and the u1 = 1 and the u2 = 2 vibrational levels of C02 (i.e., the 36 1p and 36 2q transitions).Population inversion of these levels is efficiently achieved in an electric discharge by near-resonant energy transfer from vibra- tionally excited nitrogen molecules to the u3 = 1 antisymmetric stretching vibration of C02. Laser emission may be realized over a range of vibration- rotation components of these vibrational transitions near 10.6 pm amd 9.6 pm. These components are conventionally described in the form ‘lOP(22)’ for the J = 22 component of the P-branch of the 10.6 pm band.The first use of IR lasers in inducing chemical reaction seems to have been that of Borde and co-~orkers.~ These workers observed that ammonia at pressures of a few torr (1 torr = 1 mmHg = 133.3 N m-2) emitted a yellow luminescence on irradiation with the lOP(20) line of the C02 laser at powers above 40W. Spectroscopic analysis of this luminescence showed that the emitter was the NH; free radical. By using a pulsed C02 laser, they were able to measure a decay rate for the light emitted by the radical. These observations they attributed to the near resonance of rotational components of the 2; vibrational transition in NH3 with the C02 laser output, and IR LMPD of the NH2-H bond. At the pressures used, it is more likely that it was IR LPHP that was actually observed.This work was followed in 1967 by similar observations on a range of hydrocarbon^.^ In this case, various aromatic hydrocarbons were produced from ethene, propene, or allene. Although the above work represents the first experimental observations, a number of workers, principally Russian, had suggested the possibility earlier, and had considered some theoretical aspect^.^ As mentioned above, one of the motivations behind early work on IR laser induced reactions arose from the hope that the excitation of specific vibrational modes in molecules might lead to control of the course of reaction, and in particular might open the way to efficient separation of isotopes. The first demonstration of this latter phenomenon was reported by Mayer et aL6 These workers subjected a mixture of CH30H, CD30D, and Br2 to the output of a HF chemical laser, and observed depletion of the CH30H at the expense of the CDJOD.This was attributed to selective excitation of the OH bond, relative to the OD bond. Doubt has since been cast on the interpretation of these results, which may have been due to a simple kinetic isotope effect. The first genuine laser isotope separation was reported in 1974 by the group of Let~khov,~ who ’ G. Moeller and J. D. Rigden, Appl. Phys. Lett., 1965, 7, 274; C. K. N. Patel, P. K. Tien, and J. H. McFee, ibid.,290. C. Borde, A. Henry, and L. Henry, Compt. Rend. Acad. Sci. Paris, Ser. B, 1966,262, 1389. C. Cohen, C. Borde, and L.Henry, Compt. Rend. Acad. Sci.Paris, Ser. B, 1967,265,267. F. V. Bunkhin, R. V. Karapetyan, and A. M. Prokhorov, Sou. Phys. JETP, 1965, 20, 145; G. A. Askar’yan, ibid., 1965,21,439. S. W. Mayer, M. A. Kwok, R. W. F. Gross, and D. J. Spencer, Appl. Phys. Lett., 1970,17,516. R. V. Ambartzumian, V. S. Letokhov, E. A. Ryabov, and N. V. Chekalin, Sou. Phys. JETP Lett., 1974, 20, 273; R. V. Ambartzumian, Yu. A. Gorokhov, V. S. Letokhov, and G. N. Makarov, ibid., 1975, 21, 171. 409 Infrared Laser Powered Homogeneous Pyrolysis achieved a 1.6-fold enrichment of “B: loB by exposing mixtures of BCI3 and D2S to the lOR(30) line of a pulsed C02 laser. We now turn to the photosensitized IR LPHP process, which forms the central theme of this Review.In 1972, Tardieu de Maleissye suggested that the pyrolysis of saturated hydrocarbons or alkynes (neither of which absorbs C02 laser radiation directly) could be achieved by exposing a mixture of the gas and SF6 to laser radiation; shortly afterwards, he and co-workers demonstrated this ex- perimentally for both ethyne and ethane.g The end products in this case were a fine powder of carbon (which rapidly occluded the entrance window, and prevented further reaction) and presumably hydrogen. This technique was developed to a state of considerable sophistication in an extensive investigation reported in 1975 by Shaub and Bauer.” Among other achievements, these workers demonstrated the range of temperatures achievable and established suitable experimental regimes of pressure, laser power, and time of exposure; in addition, they demonstrated the possibility of using a reference ‘chemical thermometer’ technique for the measurement of the kinetic parameters of a series of reagents. This work established the credentials of IR LPHP as a viable and quantitative technique of considerable versatility, and it is highly recommended to the interested reader as an introduction to this Review.Since 1975, work using IR LPHP on both the development and application fronts has been extensive. As will become apparent shortly, one of the principal impediments to accurate quantitative work arises from the generation of a non- uniform temperature profile in the laser-induced pyrolysis. This has been elegantly circumvented in a number of developments using pulsed laser sources, principally by the group at SRI in California.l1 Of other workers who have made significant contributions, we must mention the very extensive work of the group of Pola in Prague12 on the application front, and the investigative studies of Zitter and Koster at Southern Illinois University.13 The topic has formed a more or less significant part of a number of reviews since 1970, and some of these are given as references 14-20. The interested reader is also referred to the proceedings of two Conferences held in Czechoslovakia in 1986 and 1989, which were wholly or partially devoted to the topic of IR LPHP.2’,22 * J Tardieu de Maleissye, Compr Rend Aiad Sci Paris, Ser C, 1972,275,989 J Tardieu de Maleissye, F Lempereur, and C Marsal, Compf Rend Acad Sci Paris, Ser C, 1972,275, 1 153, F Lempereur, C Marsal, and J Tardieu de Maleissye, ibid, 1974, 279,433 lo W M Shaub and S H Bauer, Int J Chem Kinet ,1975,7,509 G P Smith, P W Fairchild, J B Jeffries, and D R Crosley, J Phys Chem ,1985,89, 1269 l2 J Pola, Spectrochim Acta, Part A, 1990,46,607 l3 R N Zitter, Spectrochim Acta, Part A, 1987,43, 245 l4 C B Moore, Annu Rev Phys Chem, 1971,22,387 l5 N K Karlov, Appl Optics, 1974,13,301 J T Knudtson and E M Eyring, Annu Rev Phys Chem ,1974,25,255 l7 S Kimel and S Speiser, Chem Rev, 1977,77,437 V S Letokhov, Annu Rev Phys Chem, 1977,28,133 l9 W C Danen and J C Jang, in ‘Laser Induced Chemical Processes’, ed J I Stelnfeld, Plenum, New York, 1981 2o K L Kompa and J Wanner, ‘Laser Applications in Chemistry’, Plenum, New York, 1984 ”‘Chemistry by IR Lasers’, Spectrochim Acta, Parf A, 1987,43, 129-300 22 ‘Laser Induced Chemistry’, Spectrochirn Acta, Purr A, 1990,46,441--669 Russell Mechanical Power meter or wavelength monitor Figure 1 Schematic diagram of the basic elements of the IR LPHP technique.Not shown are arrangements for monitoring cell contents 3 Experimental Considerations In this section, I shall consider the requirements of the equipment used in IR LPHP experiments. Of course, precise experimental arrangements have been very varied, depending on the nature of the investigation. However, some elements are common to all studies, and I shall describe variations and sophistications within the framework of the very simple setup illustrated in Figure 1.In this arrangement, the reagent gas or gases, mixed with a photosensitizer if required, are contained in a static cell. The cell is fitted with suitable ports for filling and sampling as required, and one or two windows of appropriate material. The cell is exposed to the output of an IR laser, whose characteristics are monitored in an appropriate manner. Changes in the chemical composition of the cell contents are monitored by one of a number of methods. Other parameters of the cell and its contents (pressure, some measure of temperature, etc.) may also be monitored. I shall now consider some of the components of Figure 1 in more detail.A. Pyrolysis Cells.-The most popular design for static pyrolysis cells is the Pyrex or metal cylinder, fitted with windows for axial irradiation, as indicated in Figure 1. For investigations related to the importance of convective motion in the cell, it may be mounted either horizontally or vertically. Typically, such cells are quite small-in our own experiments, for example, the cell has a length of 7-10 cm and an outside diameter of 3.8 cm, giving an overall gas volume of 5&l00cm3; many are much smaller. The advantage of this is that at the pressures used (a few torr), very small quantities of material are used (a few pmol), and this permits the study of scarce or expensive compounds. Moreover, the small dimensions aid the rapid establishment of a steady state temperature distribution in the cell, and also the rapid redistribution of reactants and products (see below).The cell is fitted with ports as appropriate-at least a filling port, and access for such techniques as gas chromatography, nuclear magnetic resonance spectroscopy, or mass spec- trometry. For other purposes, specialized cells have been designed. Some workers have added a buffer volume to maintain pressures at or near the room temperature 411 Infrared Laser Powered Homogeneous Pyrolysis value In the pulsed experiments of Smith et a1 ,11 23 a shorter and wider toroidal cell is employed to ensure a more uniform absorption of laser radiation in the irradiated reaction zone than in that described above, this is further aided by replacing the rear window by a mirror In addition, in these investigations the gas mixture flows slowly through the cell In their investigations of the effects of convection on reaction rates, Shaub and Bauer used a multi-compartment cell which could be irradiated either horizontally or vertically lo For large scale mechanistic and synthetic studies, Bristow et a1 have used a mechanically pumped circulating cell, which proved capable of preparations on the 100 mg scale 24 For temperature studies, cells fitted with thermocouples have been designed 25 26 Krasa et aI have used a NaCl box for IR thermoluminescence studies of temperature distributions 27 B.Window Materials.-The primary requirements for a suitable window material are that it be highly transparent to the laser radiation (usually C02 laser radiation near 10 pm), that it possess adequate mechanical strength and thermal stability, and that it be chemically inert These factors, allied to economic considerations, have naturally established NaCl and KCl as favourite materials However, these do suffer from some disadvantages The generation of ‘hot spots’, caused either by uneven multimode laser power profiles or absorption by solids deposited on windows, can easily lead to thermal stress and unpredictable fracture along cleavage planes 24 Another problem of particular importance in our own work lies in the hygroscopic nature of the alkali halides, even minute traces of adsorbed water can quite radically alter the course of reaction in moisture-sensitive organometallic compounds Both of these problems are avoided by use of ZnSe windows, which has lower absorption at 10 pm, lower thermal expansion, and higher thermal conductivity, 28 on the other hand, its high refractive index does require that such windows be provided with anti-reflection coatings Despite its higher cost, its greater durability leads to a considerably enhanced useful life C.Infrared Lasers.-As described in the Introduction above, the COZ laser is almost universally used in IR LPHP experiments Apart from its high power capabilities, it is relatively cheap, robust, and simple to operate In its most familiar low pressure mode of operation, this laser is capable of continuous wave output powers of up to several kW in beams of a few cm2 area at about 200 discrete wavenumbers ranging from 900 to 1100 cm-’, 29 typically, however, more 23 D F McMillen, K E Lewis, G P Smith, and D M Golden, J Phys Chem, 1982,86, 709, also K E Lewis, D F McMillen, and D M Golden, J Phys Chem , 1980,84,226 24 N J Bristow, B D Moore, M Pohakoff, G J Ryott, and J J Turner, J Organomet Chem, 1984, 260,181 25 P Kubat and J Pola, Collect Czech Chem Comm , 1948,49,1354 26 W P Horn, M S Sheldon, and P C T de Boer, J Phys Chem, 1986,90,2541’’J Krasa, P Engst, and M Horak, Spectrochzm Acta Part A, 1990,46,559 28 Manufacturers, Catalogue, Specac Ltd , 1989 29 K Narahan Rao and A W Mantz, in ‘Molecular Spectroscopy Modern Research’, ed K N Rao and C W Mathews, Academic Press, 1972, New York Russell modest powers of up to 1OOW are normally used in LPHP applications.For reproducible quantitative work, it is advisable to use a laser provided with both frequency and power stabilization. By pressure broadening the laser emissions in a transversely excited atmospheric pressure (TEA) COZ laser, frequency coverage over the working range can be made quasi-continu~us.~~Low frequency modulation of the laser putput may be accomplished either by a mechanical chopper (up to a few kHz) or by electric modulation of the resonant laser cavity length using a mirror mounted on a piezoelectric stack. For faster time-resolved applications, it is necessary to produce short, powerful, and reproducible pulses.By using the technique of Q-switching, it is possible to produce pulses of a few J in times of a few ns (i.e. instantaneous powers in the GW range). A review of recent technical developments and requirements in this area of relevance to IR LPHP has been provided by Quack et aL3' In all applications, it is sometimes necessary to focus the laser beam into the pyrolysis cell; this is usually accomplished using NaCl or Ge optics, or front reflecting mirrors at very high powers. The wavelength and power characteristics of the laser output are readily monitored in continuous wave experiments using commercial monochromators and thermal power meters. Where necessary, the power profile of the laser beam can be visualized using standard thermoluminescent plates, or measured using techniques such as traversing a slit or pinhole across the beam profile.32 For pulsed measurements, the fast rise times of photon drag or pyroelectric detectors are required, together with the appropriate electronic signal processing.Of other lasers in the mid-IR region, chemical lasers based on vibration-rotation transitions in HF and HC1 have found some application, especially in the study of the dynamics of these species themselves. Since they both have output in the 3 pm region, they have both also been used in the direct excitation of 0-H bonds6 D. Monitoring of Reaction.-Once again, the choice of method for monitoring the composition of the cell contents is largely dictated by the aims of the particular investigation.For gross compositional changes on a long (>1 second) time scale, any standard analytical technique may be used. In their original work, Shaub and Bauer used gas chromatography with flame-ionization detection," and variants of this method have also been used in subsequent st~dies.~~,~~ On the other hand, the majority of current workers have favoured non-invasive spectroscopic techniques, notably IR ~pectroscopy.~ This is particularly 5-3 30 A. J. Beaulieu, Appf. Phys. Lett., 1970, 16, 504; A. K. Laflamme, Rev. Sci. Instrum., 1970,41, 1578. 31 M. Quack, C. Ruede, and G. Seyfang, Spectrochim. Acta, Part A, 1990,46,523. 32 R. N. Zitter, D. F. Koster, A. Cantoni, and J. Pleil, Chem. Phys., 1980,46, 107.33 H. Pazendeh, C. Marsal, F. Lempereur, and J. Tardieu de Maleissye, Int. J. Chem. Kinet., 1979, 11, 595. 34 K. A. Holbrook, G. A. Oldershaw, and M. Matthews, Int. J. Chem. Kinet., 1985,17, 1275. 3s K. Dathe, P. Engst, J. Pola, and M. Horak, Collect. Czech. Chem. Comm., 1980,45, 1910. 36 W. Fuss, G. Mengxiong, K. L. Kompa, and Z. Linyang, Spectrochim. Acta, Part A, 1987,43, 193. 37A. Watanabe, Y. Koga, K. Sugawara, H. Takeo, K. Fukuda, C. Matsumura, and P. M. Keehn, Spectrochim. Acta, Part A, 1990,46,463. 413 Infrared Laser Po wered Homogeneous Pyrolysis * * h I I" I" 0I2 v II !? 3000 2600 2200 1800 1400 Wavenumbers Figure 2 FTIR spectra of a mixture of SFs and CH3CHO before (above) and after (below) exposure to COZ laser radiation Features not identlfed on the Figure are due to SF6 (m) and CH3CHO (*c> convenient, since one may examine the cell contents via either the irradiation windows or an additional pair mounted crosswise for in sztu monitoring This is the method used in our own work,38 the well-known additional advantages conferred by Fourier transform IR spectroscopy, such as the speed of data acquisition and the ease of spectrum manipulation, are even more evident in this application Figure 2 shows 'before' and 'after' FTIR spectra in a typical IR LPHP experiment Other workers have used mass spectrometry 39 For monitoring on a shorter time scale, particularly in pulsed experiments, faster methods are obviously needed For example, in their study of the reaction of OH radicals with hydrocarbons, Smith et al monitored OH concentrations using Laser Induced Fluorescence," and this method is also the most widely used in IR LMPD studies Other workers have exploited the rapid scan and high sensitivity and resolution capabilities of tunable lead salt semi-conductor diode IR lasers to monitor changes in rotational and vibrational levels,40 and others G A Atiya, A S Grady, S A Jackson, N Parker, and D K Russell, J Organomet Chem 1989 378 307 39 F W Larnpe and J Biedzyckl, Spectrochim Acta, Part A, 1990,46,631''K Sugawara, T Nakanaga, H Takeo, and C Matsumura, Chem Phys 1989 135 301 414 Russell have used techniques such as simple or resonance enhanced multiple-photon ionizati~n.~~.~~ It is pertinent under this heading to raise an issue to which we shall return in more detail below.As already pointed out, the IR LPHP technique generates a non-uniform temperature profile in the pyrolysis cell, with a maximum near the cell axis, and the cell walls remaining at close to room temperature. Under these conditions, of course, reaction rates are highly non-uniform, with the bulk of reaction effectively occurring in a relatively small volume around the temperature maximum; some workers have called this the 'active' volume. Unless sophistica- tions are introduced to account for this (as in many of the pulsed experi- ments "yZ3), monitoring techniques such as those described above can measure only bulk (ie., cell-averaged) reaction rates, and the extraction of meaningful kinetic parameters is apparently not possible.However, as shown below, this may not actually be the case: indeed, the observation of cell-averaged concentra- tions may well be the most straightforward way of obtaining such parameters. To this end, we have investigated the possibility of using the acoustic resonance frequency of the cell and its contents as an in situ method for monitoring the average concentration in the cell during pyrolysis.43 This is also described more fully below. E. Phot0sensitizers.-The ideal photosensitizer is characterized by the following features: (i) very strong absorption of radiation at wavelengths emitted by the laser; (ii) very rapid intra-molecular vibration-vibration and vibration-rota-tion/translation energy conversion, and very efficient intermolecular energy transfer; (iii) high thermal stability; (iv) chemical inertness; (v) low thermal conductivity. Just as the most widely used laser is the C02 laser, by far the most popular photosensitizer is SF6.This molecule has an extremely high absorption for a number of lines in the COZ laser spectrum, reaching a maximum of 1.8 x lo7 mol-I cm2 (at room temperature) for the lOP(16) line near 948 ~m-'.~~ Neglecting the effects of bleaching and temperature dependence, this leads to an absorption of >99.5% of incident laser light over a path of 5 mm at the normal working pressure of 10 torr. Partly because of this strong absorption, and partly because of its importance in IR LPHP, SF6 has been the subject of many studies of state-to-state energy transfer, both in power regimes where dissociation occurs 41 D.M. Rayner and P. A. Hackett, J. Chem. Phys., 1983,79,5414. 42 G.P. Smith, personal communication, 43 G. A. Atiya and D. K. Russell, unpublished work; G. A. Atiya, Ph.D. Thesis, University of Leicester, 1990. 44 A. V. Nowak and J. L. Lyman, J. Quant. Spectrosc. Radiat. Transfer, 1975,15,945. 415 Infrared Laser Powered Homogeneous Pyrolysis and below this threshold.45 Recent measurements using a pulsed TEA COz laser and infrared diode laser measurements of CO/SF6 mixtures have shown that the vibrational energy absorbed by the SF6 is transferred to translation and rotation with a relaxation time of 10 ps t~rr.~'Thus, at the usual working pressures, IR laser radiation absorbed by SF6 appears as heat in the system within 1 ps.The chemical stability of SF6 is very familiar as an elementary example of the relative influence of kinetic and thermodynamic factors: the SF5-F bond dissociation enthalpy (at 389 kJ mol-') is actually lower than that of some competing photosensitizers (e.g.SiF4 at 594 kJ mol-') but reactions involving SF6 are rarely observed under the normal working conditions. Exceptions to this comes from our own work on organoaluminium species,38 and that of Pola et al. on CH3SiC13;46 these observations are quite consistent with the known exceptional strengths of the AI-F and Si-F Thermally, SF6 is stable well above 1200K.48 The final point concerning thermal conductivity has received less attention than the others described above.SF6 is a notoriously poor thermal conductor: indeed, it is widely used as a gaseous insulator.49 This permits the generation of highly inhomogeneous temperature profiles at comparatively modest laser power input. Indeed, in some studies inert gases are added precisely to flatten out this profile (e.g., ref. 50). However, as we shall see below, this may actually detract from one of the major advantages of the JR LPHP technique. Of other photosensitizers, only SiF4 has found widespread use; 5' one example where it has particular value is in the study of PH3, with which SFs reacts.52 Although it is thermally as stable as SF6, it is a somewhat poorer IR absorber and is fairly reactive towards, for example, water.Other workers have used C6F6 53 or NH3.54 4 Investigative and Validation Experiments and Theoretical Considerations Not surprisingly, the experimental complexity of the IR LPHP technique has engendered a considerable number of theoretical investigations of aspects such as the absorption of the IR laser radiation, the rate of appearance of this energy as heat, the temperature, diffusion, and convection characteristics of the heated gas mixture, the mechanistic interpretation of observed reactions, and the deduction of reaction rate parameters. Many of these considerations have been coupled with experimental investigations designed to test the validity of the conclusions drawn; in this section I shall follow this lead.45 M Dubs, D Harradine, E Schweitzer, J I Steinfeld, and C Patterson, J Chem Phys, 1982,77,3824 46 J Pola, J M Bellama, and V Chvalovsky, Collect Czech Chem Comm ,1981,46,3088 47 Handbook of Chemistry and Physics (60th Edition), ed R C Weast, Chemical Rubber Co Press, Boca Raton, 1980 4R J L Lyman, J Chem Phys, 1977,67,1868 49 'Physical Properties of Inorganic Compounds', ed A L Horvath, 1975, Arnold, London J Pola, M Farkacova, and P Kubat, J Chem Soc ,Faraday Trans I, 1984,80, 1499 51 W Tsang, J A Walker, and W Braun, J Phys Chem, 1982,86,719 52 J Blazejowski and F W Lampe, Spectrochim Acta, Part A, 1990,46,627 53 N Selamoglu and C Steel, J Phys Chem ,1983,87, 1133 54 C Steel, V Starov, R Leo, P John, and R G Harrison, Chem Phys Lett, 1979,62, 121 Russell A.Photosensitizer Absorption Spectroscopy.-Partly as a consequence of the popularity of the C02 laser as a source, and SF6 as a photosensitizer, the absorption spectrum of SF6 in the neighbourhood of its u3 (triply degenerate S-F stretching) vibration at 948 cm-' has been the subject of intense scrutiny in the past few years.55 The rotation-vibration structure is very dense and complex. Its experimental resolution requires sophisticated laser spectroscopic techniques, and its interpretation presents a considerable theoretical challenge; only recently has significant progress been made.56 Of course, for the process of multiple photon absorption, which almost certainly occurs to some extent in IR LPHP, excited state spectroscopy is also necessary.This work is also stimulated by the use of SF6 as a target molecule for 32S:34Sisotope ~eparation.~~ Perhaps of more immediate significance here are the measurements of Nowak and Lyman,44 who reported absorptions by SF6 of a number of commonly used CO2 laser lines over temperatures ranging from 300 to 1800K. The absorption is highly temperature dependent: for the lOP(20) line at 944 cm-', for example, it falls from 1.0 x lo7 mol-' cm2 at 300K to 1.3 x 1OSmol-' cm2 at 1700K. These measurements have a considerable bearing on estimates of power absorption in the IR LPHP arrangement, of course. In some early estimates of temperatures produced in the pyrolysis cell, this temperature variation was neglected, and this led to very considerable errors." The same is true for some early experiments involving the interpretation of changes in reaction rates as the exciting laser frequency is varied. 58 B.Photosensitizer Photophysics.-The next step to be considered is the fate of the absorbed IR energy. Work on energy disposal in the laser irradiated SF6 has, if anything, been even more abundant than that on its absorption spectrum. This work has considered both singly excited (k,u3 = 1) SFs, and the more challenging problem of multiple photon e~citation.~~The details of such processes are of fundamental significance in IR LMPD, of course, but as far as simple continuous or pulsed IR LPHP is concerned, the central feature of interest is the rate at which the absorbed IR energy appears as translational energy in the reaction mixture.This is of particular importance in pulsed applications, where pyrolysis effectively takes place within a time scale of a few A recent study by Sugawara et has reported the time development of vibrational, rotational, and translational effective temperatures in C02-laser pulse irradiated SF6. Since direct measurement using the IR spectrum of SF6 is precluded by its dense structure, measurements were carried out using tunable 55 C. Borde and C. J. Borde, Chem. Phys., 1982,71,417; 1984,84, 159. B. Bobin, C. J. Borde, J. Bordt, and C. Brtant, J. Mol. Spectrosc., 1987, 121,91. 57 V.Yu. Baranov, E. P. Velikhov, S. A. Kazakov, Yu. R. Kolomiiskii, V. S. Letokhov, V. D. Pis'mennyi, E. A. Ryabov, A. I. Starodubtsev, Sou. J. Quantum Electron (English translation), 1979,9,486. 58 R. N. Zitter and D. F. Koster, J. Am. Chem. Soc., 1976,98, 1613; ihid., 1977,99, 5491. 59 e.g., M. Lenzi, E. Molinari, G. Piciacchia, V. Sessa, and M. L. Terranova, Spectrochim. Actu, Part A, 1987,43, 137; Chem. Phys., 1990, 142,463 and 473; D. W. Lupo and M. Quack, Chem. Rev., 1987,87, 181; J. I. Steinfeld, Spectrochim. Acta, Part A, 1987,43, 129; and references therein. 417 Infrared Laser Powered Homogeneous Pyrolysis diode IR laser spectroscopy of an added trace of CO, which has a much simpler vibration-rotation spectrum. Estimates of the vibrational temperature were deduced from measurements of the intensity of the u = 2 -1 hot band; those of the rotational temperature were obtained from the intensity distribution within rotational components of the u = 1-0 fundamental band; and the translational temperature was measured using the Doppler width of these components.Sugawara et al. found that, at total pressures of 1torr, the rise time of the rotational and translational temperature was 30 ps, whereas that of the vibrational temperature was about 1 ms. These workers were also able to measure the rate of establishment of temperature equilibrium in the cell contents by monitoring the concentration of CO in the same way. From these results, it can be concluded that under normal working conditions the IR radiation appears as heat in times much less than 1 ps, and that a thermal steady state is established over a period of a few ms.Self-consistency in these measurements was established by a close agreement for temperature rises observed and calculated using a simple equipartition of the absorbed energy. Similar investigations have been carried out by Smith and Laine6' and McMillen et al.23as part of their validation experiments using pulsed IR LPHP. These workers monitored the rise of IR fluorescence from a trace of CO or CH4 added to the IR LPHP gas mixture (SF6 with either SO2 or C02 added as a bath gas) following exposure to short (ps) C02 laser pulses. They showed that the onset of fluorescence follows on very rapidly (much less than the time resolution of the equipment used) after the laser pulse, followed by a drop in a time period of a few ps as the heated wave expands outwards.Similar conclusions have been drawn from studies by Dai et ~1.~'and Tsang and co- worker~.~1*62 C. Temperature Profiles in the Laser-heated Gas.-As already indicated, the central feature of the IR LPHP technique is the generation of a non-uniform temperature profile within the pyrolysis cell. This is the source of the major advantages of the technique, since it permits the study of reactions under unambiguously wall-less conditions, and the isolation of otherwise unobservable intermediate^.^^ Perversely, it is also the root of the major problem in quantitative kinetic studies, since reaction proceeds at very different rates at locations throughout the cell. For this reason, considerable efforts have gone into both the theoretical characterization and experimental measurement of the temperature profiles generated within the cell.The calculation of the temperature distribution is a complex problem, involving a large number of factors. A complete calculation must include the 6o G P Smith and R M Lame, J Phys Chem, 1981,85,1620 61 H -L Dai, E Specht, M R Berman, and C B Moore, J Chem Phys, 1982, 77, 4494, also M R Berman, P B Comita, C B Moore, and R G Bergman, J Am Chem Soc, 1980,102,5694 62 W Braun and W Tsang, Chem Phys Lett, 1976, 44, 354, D Gutman, W Braun, and W Tsang, J Chem Phys, 1977,67,4291 63 A S Grady, A L Mapplebeck, D K Russell, and M G Taylorson, J Chem SOL,Chem Commun, 1990,929 Russell extent of absorption of laser radiation by the photosensitizer, which is, of course, power, temperature, and density dependent.44 Furthermore, heat transport within the cell by the processes of both conduction and convection is also temperature and pressure dependent.The detailed profile and wavelength of the laser beam, and the effect of heat released in any chemical reaction must also be considered. Neither is the verification of calculation straightforward. Precisely located, but invasive, techniques such as the introduction of microthermocouples can produce quite significant perturbations, particularly if they are placed directly in the path of the laser beam. On the other hand, any external method such as the use of standard reactions of well-known kinetic parameters (the 'chemical thermometer',' O) can measure only spatially-averaged temperatures, and are therefore of only limited value.In describing some of the efforts to surmount these problems reported in the literature, I shall chart the approxi- mately chronological development in the sophistication of both calculation and measurement. Once again, we may take as a starting point the work of Shaub and Bauer." These workers attempted to calculate the temperature distribution within the cell, allowing for thermal conduction as the only mode of heat transport. They calculated temperatures which were clearly at odds with those deduced from observed chemical conversion rates, being far too high.Although they attributed these errors to the neglect of convection, it is more likely that the neglect of temperature dependence of both the absorption of the IR radiation by the photosensitizer and thermal conductivity was the major problem. In an effort to demonstrate the significance of convection, they carried out experiments on the rate of the well-known unimolecular conversion of cyclopropane into propene in a three-compartment cell irradiated both vertically (from below or above) and horizontally. Considerable differences in the extent of conversion in the three compartments were found, which were considered to indicate the importance of convection; however, the experimental conditions employed (short irradiation times and narrow cells) were precisely those which emphasize the effects of convection, and the applicability of these results under more commonly employed conditions is not clear.Shaub and Bauer also pioneered the 'chemical thermometer' technique in this work. In this method, the reaction under study is compared with one of known kinetic characteristics, either in the same pyrolysis cell, or in an identical second cell. Shaub and Bauer compared a number of well-characterized unimolecular reactions in this way, and obtained self-consistent results (to within a few percent). While this method can be useful (and indeed has been widely used), it does have a number of drawbacks. The first of these is the difficulty in matching cells precisely-a very slight mismatch in, for example, window transmission or photosensitizer partial pressure can easily modify the effective pyrolysis tempera- ture by 10 or 20K.Ideally, one would place both the unknown and standard reactions in the same cell, but this is only possible in the case of non-interfering reactions of the type studied by Shaub and Bauer." Of more serious consequence is the requirement that the unknown and standard reactions have very similar Infrared Laser Po wered Homogeneous Pyrolysis kinetic characteristics. Such a standard can, of course, be found in some cases, but for more complex radical or unknown mechanisms it may simply not be possible to find a sufficiently closely matching reaction.Zitter et a1.32,64*65 refined the calculations of Shaub and Bauer by accounting for the temperature dependence of conductivity k in an empirical form which fits fairly well to known data, and leads to a soluble form for the heat conduction equation. The exponent rn was taken as a variable parameter. They solved the equation of conduction in the pyrolysis cell, taking into account radial (but not axial) heat conduction, and neglecting convection. This latter they justified on the basis of observing no difference in conversion rates in horizontally or vertically irradiated cells. Apart from the neglect of axial heat flow (which is not justified), they apparently neglected the very strong temperature dependence of absorption.These workers compared the results of their calculations with measurements by interferometry. In this, the local density decrease associated with the local rise in temperature of the laser-heated gas is measured using the change in refractive index; the latter is conveniently measured by counting the number of intensity cycles (fringes) produced in an interferometer with a helium- neon laser as a source. This method can only measure a temperature average along the sight of path of the He-Ne laser beam, which was chosen to coincide with that of the COZ laser in the experiments of Zitter et al. They found fairly good agreement (1%) between observed and calculated temperatures; an inci- dental, but significant, result from their work was the confirmation that the steady state temperature profile in the cell was established within about 50 ms of initiation of radiation.In 1984, Kubat and Pola reported measurements using microthermocouples introduced directly into the cell,25 an idea presented earlier by Schuster and Li in theses and conference proceedings.66 Their measurements suggested that convec- tion within the cell was important. This conclusion they based on the observation that while a cylindrically symmetrical temperature distribution was observed when the cell was irradiated vertically, in a horizontally heated cell the temperature maximum lay above the incoming laser beam centre. However, in their work no mention is made of correcting for the perturbing influence of the thermocouples, the most important of which is conduction of heat through the leads; thus the significance of these results is not clear.They supported their measurements by calculations based on the same assumptions as Zitter et a[.3 2.64.6 5 Perhaps the most reliable calculations performed to date, neglecting convection, "R N Zitter, D F Koster, A Cantoni, and A Ringwelski, High Temp &I, 1980,12,209 6s R N Zitter, D F Koster, A Cantoni, and A Ringwelski, Chem Phys, 1981, 57, 11''J P Schuster, M Sc Thesis, Cornell University, Ithaca, NY, 1979, W 0 Li, M Sc Thesis, Cornell University, Ithaca, NY, 1980, J P Schuster, W 0 Li, and W J McLean, 15th AIAA Thermolysis Conference, Snowmass, CO, 1980 Russell are those of Zhu and Ye~ng.~~ These workers solved the conduction equation within the cell taking into account both the spatial intensity distribution of the laser beam, and the temperature variation of density, conductivity, and IR absorption of the cell contents.The conduction equation was solved numerically by dividing the cell into thin slices along its axial length, and iterating the radial heat flow within each slice; the absorption was then calculated to provide the power incident on the next slice. This necessitated the artificial device of ‘buffer zones’ at the front and rear windows, and for this reason their axial temperature distributions are probably unreliable. However, the maximum temperatures calculated agree reasonably well with those deduced from kinetic studies, and by adjusting the thickness of the buffer zones, they were able to match calculated and observed transmissions of laser radiation through the cell. The most detailed and sophisticated calculations and measurements are those of Horn et a1.26 These workers included temperature variations of thermal and absorption parameters, and also included convection in their calculations. They also measured temperature distributions using thermocouples, but they did consider the corrections to be made for the effects of conduction and radiation losses introduced thereby.Their calculated and measured temperature profiles were in very good agreement. As an example of their calculations and measurements, in a vertically oriented cell of length 9.6 cm and internal diameter 3.5 cm, filled with SF6 (2.25 torr) and Ar (50 torr) and irradiated with 1OW IR laser power in a Gaussian beam of radius 0.7 cm, they calculated maximum temperatures of 900-1000K.They also found a maximum convective gas velocity of about 20 cm s-I. In our own laboratory, we have made measurements of the temperature distribution by adding a small partial pressure (<0.5 torr) of unreactive CO to the reaction mixture, and observing the intensities of vibration-rotation compo-nents using tunable diode laser spectroscopy.68 In order to get a complete as possible picture of the distribution, we have used a cell fitted with an additional pair of CaF2 windows running the length of the cell walls for perpendicular viewing (see Figure 3).Non-uniformity of the temperature along the path of the probing diode laser beam (of diameter <1 mmj is indicated by a deviation from the Boltzmann distribution in the vibration-rotation component intensities. Although a complete picture of the distribution has yet to be obtained, two conclusions can already be drawn: (i) the maximum of the temperature distribution is not at the front window, but some 3 cm along the axis, even with horizontal irradiation. This emphasizes the importance of including axial heat conduction in the calculation of profiles. (ii) we find no significant deviation from cylindrical symmetry in the J. Zhu and E. S. Yeung, J. Phys. Chem., 1988,92,2184.‘* G. A. Atiya, D. A. Pape, and D. K. Russell, unpublished results; G.A. Atiya, Ph.D. Thesis, University of Leicester, 1990. 421 Infrared Laser Powered Homogeneous Pyrolysis IR diode laser CaF, probe beam window I C02 laser beam ZnSe CaF2 window window t Figure 3 Arrangement for the measurement of temperature projles in IR LPHP using TDL spectroscopy The dotted line represents a schematic isotherm in the laser-heated gas mixture, compare Figure 5 temperature distribution, even with horizontal irradiation. This suggests that convection is of little importance in our particular configuration. Otherwise, we find results similar to those of other workers. A plot of mean temperature along the probe beam path through the axis of the cell versus laser power for two positions along the cell is presented in Figure 4.In the same diagram we present a plot of the mean temperature in the cell derived from measurements using ethyl ethanoate as a chemical thermometer. lo The difference between the two sets of results emphasizes the necessity for finding a reaction closely matching the unknown in activation energy. In the results presented in Figure 4,the spectroscopic measurements effectively measure the mean tempera- ture using a low energy (1&-20 kJ mol-') probe, whereas the pyrolysis measurements use a high energy (200 kJ mol-') probe, and the difference in the two measurements is very significant. We have also investigated means of visualizing the temperature distribution by inducing chemiluminescent reactions in the Figure 5 shows a photograph of the luminescence produced on irradiating a mixture of I2 vapour and SF6 with 30W of COz laser power: this power is very much higher than that normally used in our experiments, since it is sufficient to initiate pyrolysis of the SFs itself.We have not investigated the origin of the luminescence, but it probably arises from reaction of F atoms with 11, or perhaps simply from excited 13 produced by recombining I atoms. In any event, it is very apparent from Figure 5 that the temperature maximum is some 2 cm from the entrance window, and that the temperature distribution is indeed cylindrically symmetrical (the photograph represents a sideways view during horizontal irradiation). It will be apparent from the above discussion that the question of the importance of convection as a mode of heat transport is a contentious and unsettled one, with investigators coming down more or less equally on opposite sides. It is likely that the importance varies with the precise conditions.As pointed out by Zitter et ~l.,~~it is less likely to be of importance in situations 69 G A Atiya, A S Grady, and D K Russell, unpublished results, G A Atiya, Ph D Thesis, University of Leicester, 1990 Russell 0 0 0 700 o o 0 0 600 X Y 0 d. X 0 XE 500 08 0 front 400 X x back0 chemical st n. 0 4 8 12 Laser Power (watts) Figure 4 Mean temperatures in IR LPHP measured by TDL spectroscopy at the cell entrance window (a)and 3.8 cm from the window ( x).For comparison, temperatures estimated using CH3COOC2H5as a chemical thermometer are also presented (0) where the heat source in the cell is very narrow compared with the cell diameter, and where steep temperature gradients are generated. This is the situation in our own experiments, where no buffer gas is added to the SF6-reagent mixture. On the other hand, Krasa et al. have found asymmetric temperature distributions (using IR thermovision monitoring) under very similar conditions,* so that the question remains open. It may also be that the lower absorption of the laser radiation by the ZnSe window material in our cell reduces surface heating effects in comparison with NaCl. In any event, almost all measurements have indicated that the temperature and any velocity distribution usually become steady in a time (<1 s) short compared with reaction times, although some workers have observed long scale oscillations in temperat~re.~' For this reason, the question is of less practical consequence than might appear at first sight, as discussed below.The problems attendant on measurements of temperature distributions in pulsed systems are rather different. It might appear that one has simply compounded the difficulties described above by adding the extra dimension of temporal variation. However, if experimental conditions are chosen correctly, the 'OK. Ernst and J. J. Hoffmann, Phys Lett. A, 1981, 87, 133; R. T. Bailey, J. Chem. Phys., 1982, 77, 3453; S. Ruschin and S.H. Bauer, J. Phys. Chem., 1984,88,5042. Infrared Laser Powered Homogeneous Pyrolysis Figure 5 Photograph of chemiluminescence during IR LPHP of SF6 and 12. The pyrolysis cell is not visible in this photograph; the front cell window is approximately 1 crn to the right of the onset of the luminescence, which extends almost to the back window of the 10 cm longcell and occupies the axial region of the cell use of pulsed sources can actually bring about considerable simplifications in the interpretation of data. This is discussed in detail in two papers by the SRI gro~p.~~,~'The essential feature of the method is that a short pulse (< 1 ps) produces rapid heating within a well-defined volume of the reaction cell, wherein reaction takes place for 5-10 ps until the gas is cooled by expansion.It is apparent from the validation experiments carried out by McMillen et that the experimental configuration may be designed so that the temperature rise produced in the reaction volume is quite homogeneous, and that reaction may be effectively spatially and temporally defined. This still leaves the problem of actually measuring the temperature produced; this has been achieved by monitoring the IR fluorescence of a small amount of CO or CH4 added to the sy~tem,~~.~'or by studying the Boltzmann distribution in the laser-induced fluorescence of OH radicals." The temperatures measured in this way (IO& 1600K) agree very well with those predicted using a simple calorimetric approach from the energy absorbed.In other pulsed studies of energy transfer, techniques such as diode laser spectroscopy have also been used.40 These studies have all confirmed the rapid conversion of the IR radiation absorbed into translational energy under normal IR LPHP conditions. D. Measurement of Kinetic and Thermodynamic Parameters.-Apart from its use in mechanistic studies, the principal application of the IR LPHP technique is in the measurement of reaction rates under rather clearly defined conditions. Because of the homogeneous nature of the heating process, reactions are truly Russell ‘wall-less’, i.e., they cannot be initiated by surface processes. Moreover, much higher temperatures than in conventional pyrolysis may be routinely used, and attainment of these high temperatures is very rapid, with very little ‘dead’ or lead- in time.Finally, the rapid diffusion of the products of the primary steps of the reaction sequence out of the hot zone into cool regions of the cell can lead to a simplification of the overall process; it can also lead to the synthesis of otherwise labile products.63 However, as we have already stated a number of times, the determination of activation energies and other kinetic parameters is far from straightforward. Since this problem is approached rather differently in the case of continuous and pulsed excitation, we shall consider these separately below. (i) Continuous Excitation. If we consider a reaction with a typical activation enthalpy of 250 kJ mol-’, in an IR LPHP cell with a temperature of 1OOOK at the maximum and the walls at 300K the rate of the reaction will vary by a factor of lo3’ over the cell.Thus as the reaction progresses, reactant and products will rapidly acquire very different concentrations (or, more accurately, partial pressures) throughout the cell. These partial pressure gradients will be attenuated by the processes of diffusion and convection, but it is clear that the overall time and space dependence of partial pressure will be very complex. These factors have led a number of workers to conclude that direct measurement of reaction rates in the continuous IR LPHP process is of little use, and this has led to the development of comparative methods. However, as we have seen above, even this method requires some theoretical justification, and is not without its drawbacks.Recently, we have shown that, under correctly chosen conditions, the measure- ment of spatially averaged concentrations (which is automatically realized by monitoring the cell contents in the absence of the laser radiation) can lead to useful kinetic data, provided details of the temperature distribution are known.’ Details of this will be given elsewhere,” but the conclusions are as follows. For a first order reaction, the rate of change of the average concentration cavis given by a first order relation where k,, is the spatial average of the position-dependent rate constant k(r) over the cell. This equation holds true provided the diffusion coefficient D and the average rate constant k,, obey the relation for the axial length I (or for a box-shaped cell), or 3.8322D/a2 > ka, (4) for the radius a of a cylindrical cell.These conditions generally hold true for the D. K. Russell, unpublished work; G. A. Atiya, Ph.D. Thesis, University of Leicester, 1990. Infrared Laser Powered Homogeneous Pyrolysis 1.0 0 0.8 0 0 X 0 0.6 i 08 V. 7 0 20 40 60 80 Time (sec.) Figure 6 Normalized FTIR signal of CH3CHO subjected to IR LPHP for a total of 60 seconds and a number of exposure times + = 60 s, 0= 30 s, + = 20 s, x = 10 s, 0= 5s typical IR LPHP cell, and for reactions whose half life is longer than a few seconds Similar conditions hold for non-first-order reactions 71 This condition represents the most stringent case, totally neglecting the effects of convection (which will undoubtedly relax this requirement) Our conclusion is that measure- ment of cav can therefore yield useful kinetic data, even if the instantaneous concentration in the cell shows very large variations This conclusion IS easily subjected to experimental verification, as shown in Figure 6 This Figure shows the time development of the concentration of ethanal (which, incidentally, is known to undergo pyrolysis according to 3/2 order kinetics) subjected to IR LPHP for a total of 60 seconds, but with irradiation intervals of 5, 10, 15, 20, 30, or 60 seconds The absence of any significant deviations confirms the validity of equation 2 above Similar observations have been made by other workers (eg , ref 72), but the effect has been hitherto attributed to the effects of rapid convection, our analysis has shown that it is not necessary to invoke convection as the major mode of redistribution Having demonstrated the validity of timed measurements of cell-averaged concentrations in the determination of reaction rates, one might enquire if there "J Pola Int J Chem Kinet 1983 15 1119 Russell are methods by which this quantity might be measured in situ as the pyrolysis actually proceeds.This would be of considerable benefit, since even the relatively rapid FTIR method is quite time-consuming. What is required is a non-invasive and rapid method of monitoring the entire cell contents-methods such as spectroscopy are not usually applicable, since they generally monitor only a portion of the cell.We have investigated the possibility of using an acoustic resonance frequency of the cell and its contents during irradiation and reaction.43 A suitable resonance frequency is easily measured by modulating the output power of the laser (by applying an AC voltage to a piezo-electric stack onto which one of laser cavity mirrors is mounted) and observing the resulting photoacoustic wave generated in the cell by means of a small microphone mounted in the wall. Very small (<1%) modulation depths are required, and if a high freqency (a few kHz) resonant mode is chosen, the modulation has little effect on the temperature of the cell.The resonance frequency of the cell is a complex function of the temperature distribution, and also of the mean relative molecular mass of its contents. Thus changes in molecularity are very easily monitored. In Figure 7 we show the results of measurements of the pyrolysis of ethyl ethanoate monitored using both FTIR spectroscopy in the usual way, and the new photoacoustic method. The very close agreement between the two methods is very encouraging, and it seems that the photoacoustic technique may indeed provide accurate rate measurements. (ii) Pulsed Excitation. The difficulties in straightforward measurement of rate parameters described above for continuous excitation have led some workers to investigate the possibilities of pulsed e~citation.~~,~'.~~ The principle of this variation is that a short IR laser pulse produces a very rapid rise in temperature within a precisely defined spatial region of the pyrolysis cell. Pyrolysis then occurs within this region; this proceeds until the heated gas is cooled by expansion into the remainder of the cell.Temperature rises within the 'active' zone can in principle be calculated by simple calorimetry, if the absorbed energy is measured. Clearly, a number of approximations have been made in this simple analysis. The principal of these are that the reaction zone is precisely limited, that the reaction temperature is uniform within this zone, and that a reaction time can be defined. The credit for the invention of this method probably lies with Tsang and co- workers,62 and other early investigations were carried out by the groups of Moore61 and The most detailed description of the method, including a very careful consideration of the approximations involved and a considerable number of investigative experiments, has been provided by McMillen et and this work is highly recommended as a particularly readable account of this application of the techinique. In addition to verifying the validity of the approximations described above for unimolecular reactions, these workers have shown that the technique can provide unequivocal information about radical reactions, such as the pyrolysis of azo compounds.A number of other results are discussed in the case studies below.Infrared Laser Pou7ered Homogeneous Pyrolysis Xt 0 X 0.4 0 0 40 80 120 Time (sec.) Figure7 Progress of the IR LPHP o~CH~COOC~H~monitored using FTIR spectroscopy ( X) and the acoustic cell resonance frequency (0 and 0)The tMo wts of resonance freyuencj results arise from the use of different approvimutions in the calculations 5 Three Case Studies In this final section, I shall describe three areas of research in which the IR LPHP technique has been utilized to good effect Inevitably, this represents something of a personal selection, and there is of course some degree of overlap among these three topics Nonetheless, the examples chosen are representative of investigations currently exploiting the unique characteristics of the method A.Studies of Halogenated Organic Pyrolysis Mechanisms.-The pyrolysis of organic molecules, especially those containing halogens, has been a happy hunting ground for the IR LPHP experimenter A selection of these is presented in Table 1, by and large, studies of IR LMPD have been omitted, since the motivation of this type of work is usually somewhat different A considerable number of the reactions listed in Table 1 are effectively dehydrohalogenations An excellent summary of some of this work has been presented by P~la,'~ and the examples used in this section draw heavily on this author's work In many cases, the principal differences between mechanisms in conventional (hot-wall) pyrolysis and in IR LPHP may be ascribed to the complete suppression of wall-initiated reactions, in particular radical chain Russell processes.A good example is that of ally1 chloride.73 In conventional pyrolysis, a wide range of polymeric products is whereas under IR LPHP only propyne, propadiene, benzene, and HCl are formed in significant quantities. These observations are readily rationalized in the scheme below: CH2=CH-CH2Cl* CH2=CH-CHCl-4C6H6 + HCl (6) On the hot walls of conventional pyrolysers, these steps are followed by further reaction of the propyne and propadiene. A second illustration comes from the investigation of polychlorinated ethanes by Pola and co-workers 46,61,73 and by Holbrook et In these systems, the IR LPHP process at low laser power (i.e., temperature) results in a purely molecular elimination of HCI, as shown by the lack of any effect on addition of well-known C1 and alkyl radical traps.On the other hand, in hot-wall pyrolysis (and also IR LPHP at higher laser power 34), more complex radical chain reactions lead to a much wider range of observed products, and a sensitivity to radical traps. Two other investigations are worthy of particular note. The first is a comparative study of the IR LPHP of CH3CC!3, CH3SiC13, and CH3GeC13.46 The first of these is pyrolysed via a molecular elimination, as described above. This pathway is not favoured for the latter two, since Si=C and Ge=C bonds are not stable. In these cases, the reaction therefore proceeds oia a radical mechanism.Interestingly, the case of CH3SiC13 provides one of the few instances where the SF6 plays a part in the reaction; the abstraction of F from SF6 by Si- centred radicals is amply attested in other Finally, IR LPHP has proved its worth in industrial applications. Commercially available germanium tetrachloride contains up to 1000 ppm of organic chlorides and bromides, but its use as a precursor for Ge-doping in fibre optics requires a level of these contaminants below 50 ppm. Pola demonstrated that these impurities are readily and selectively removed by using IR LPHP.76 The method has also been used in the purification of other precursors for the semi-conductor and other industries, notably SiH4,78 and BC13.79 B. Organometallic Pyrolysis Mechanisms.-A second area where the IR LPHP has proved of particular value is in the study of the pyrolysis mechanisms of l3 J.Pola, J. Chem. Soc., Perkin Trans. 2, 1983, 231. l4 S. Kunichika, Y. Sakakibara, and M. Taniuchi, Bull. Chem. SOC.Jpn., 1969,42, 1082. l5 I. M. T. Davidson and C. E. Dean, Organometallics, 1987,6,966. 76 J. Pola, M. Farkacova, and V. Chvalovsky, J. An. App. Pyr., 1985,7,351. 77 R. V. Ambartzumian, Yu. A. Gorokhov, S. L. Grigorovich, V. S. Letokhov, G. G. Makarov, Yu. A. Malinin, A. A. Puretskii, E. P. Filippov, and N. P. Furzikov, Kvanovaya Elektron. (Moscow), 1977, 4, 171. 7R S. M. Freund and W.C. Danen, Inorg. Nucl. Chem. Lett., 1979, 15,45. ''J. A. Merritt and L. C. Robertson, J. Chem. Pkys., 1977,67, 3545. 429 Table 1 Representative halogenated organic compounds studied by IR LPHP Compound CF2C12 C2H2Fz C2HzClz CH3CHCICH3 CF2Cl2 CHF3 CHClF2 CYCIO-C~F~ (CH3)3CC1 CHClF2 + NzO CHBr3 CH2Br2 CZF6 CFzClCFzCl CF2ClCFzCl CH3CF2CI CH3CFzCl CH3CF2Cl cis-3,4-dichlorocyclobutene CH31,CD31 CF3COOH (CFJCO)Z~ CH 3CC13 CH3 SIC13 CH 3GeC13 (COC1)Z CH 3CH~CHBrCH 3 CH 2ClCHCICH 3 Reaction products and comments Ref C2F4CI2+ C12, variation with laser wavelength observed a cis -trans, and trans -cis b CIS --+ trans, and trans -cis b Deh ydrochlorination b Various products b Various products b Various products b CzF4, C3F6 b Deh ydrochlorination b CF20, HCI b Brominated alkenes b Brominated alkenes b CzF4 b Various, mainly C2F4 and CFzC12 C Various, as above d Dehydrochlorination to CH2CF2, enthalpy limit deduced e Activation enthalpy, CH3COOC2H5 used as ‘chemical thermometer’ .f Reaction kinetics investigated g Dichlorobutadienes, Woodward-Hoffmann rules obeyed h CH4, C2H6, C2H4, C2H2, 12, isotope effects, wavelength dependent 1 CFzO, CO, CF3CF0 (and minor products) J CF20, CO, CF3CF0 (and minor products) k Dehydrochlorination to CH2=CC12 I Reaction of silyl radicals with SF6, SlF4 observed 1 HCl, CH4, GeCI3H, no reaction with SF6 observed 1 COCl2, CO ( + Clz), no radicals trapped rn Pulsed, dehydrobromination, used as a reference reaction n Many channels yielding isomeric chloropropenes 0 Table 1-continued Compound Photosensitizer Reaction products and comments Ref: CzH5CI Dehydrochlorination to C2H4 used as a standard P CH3CC13 Dehydrochlorination to CH2==CC12 P CHzClCHCl2 Dehydrochlorination to cis or trans CHCICHCI; no CHzCCl2 P CHZ=CHCH2CI HC1, CH3CCH, CH2CCH2, C6H6; no polymeric products 4 CH 3CC13 Molecular dehydrochlorination; conventional pyrolysis radical chain r CH3CC13 Dehydrochlorination; kinetic studies S C2H5Br Molecular dehydrobromination; conventional pyrolysis radical chain t C3H7Br Molecular dehydrobromination t Various chloroethanes Mechanistic study of dehydrochlorination reactions U RCl and RBr Used to remove halocarbon impurities from GeC14 U C2F4 Reaction with 02; dioxetane intermediates proposed w,x, Y CsFio Reaction with 02;dioxetane as intermediate Y Various chlorocarbons SF6 Mechanistic investigation using radical traps z CF30CF2CF21 SF6 Radical reaction mechanism aa CHBrCICF3 SF6 Many products bb CF3CFCFCF3 SF6 Reaction with O2 proceeds via a dioxetane CC (RfC0)20 SF6 Rr = CF3, GFs, C3F7 dd R.N. Zitter, R. A. Lau, and K. S. Wills, J. Am. Chem. Soc., 1975,97, 2578. Ref. 10. Ref. 58. Ref. 58. R. N. Zitter and D. F. Koster, J. Am. Chem. SOC., 1978, 100, 2265. Ref. 31. Ref. 65. W. C. Danen, D. F. Koster, and R. N. Zitter, J. Am. Chem. Soc., 1979, 101, 4281. ' Ref. 35; G. A. Atiya, R. Maw, D. K. Russell, S. Salfity, and M. Ward, unpublished results. J. Pola, Collect. Czech Chem. Commun., 1981, 46, 2854. Ir J. Pola, Collect. Czech. Chem. Commun., 1981, 46, 2860. ' Ref.46. J. Pola, Collect. Czech. Chem. Commun., 1982, 47, 3258. Ref. 23. O Ref. 51. Ref. 61. 4 Ref. 73. Ref. 72. P. Kubat and J. Pola, Collect. Czech. Chem. Commun., 1985, 50, 1537. ' P. Kubat and J. Pola, Collect. Czech. Chem. Comrnun., 1985, 50, 1548. Ref. 34. " Ref. 75. J. Pola and J. Ludvik, Spectrochim. Actu, Part A, 1987, 43, 297. li J. Pola and J. Ludvik, J. Chem. Soc., Perkin Trans. 2, 1987, 1727. P. K. Chowdhury, J. Pola, K. V. S. Rama Rao, and J. P. Mittal, Chem. Phys. Lett., 1987, 142, 252. P. Kubat and J. Pola, Z. Phys. Chem. (Leipzig), 1987, 268, 849. J. Pola, J. An. App. Pyr., 1988, 13, 151. bb J. Pola and Z. Chvatal, J. Fluorine Chem., 1989, 42, 233. cc J. Pola and J. Vitek, Collect. Czech. Chem. Commun., 1989,543083. dd P. Kubat and J. Pola, Collect.Czech. Chem. Commun., 1990, in press Infrared Laser Powered Homogeneous Pyrolysis organometallic compounds. Simple organometallic compounds of modest volati- lity have leapt to prominence recently as highly practical precursors to the deposition of metals and metal oxides for applications such as semi-conductors, solar cells, and fibre-optics.80 A typical example is trimethyl gallium in the metal- organic chemical vapour deposition (MOCVD) of gallium arsenide: s’ Ga(CH3)3 + AsH3 -GaAs + 3CH4 (7) Clearly, the design of custom-made precursors for new materials requires an understanding of the mechanisms of pyrolysis of such compounds, and here fundamental information is lacking. The reason for this is very simple; many such compounds are notoriously air and moisture sensitive, and attempts to study their pyrolysis have often been frustrated by surface reactions.A typical example comes from the oxidation of tetramethyl tin in the production of Sn02; the oxide layer thus produced is highly auto-catalytic (as shown by observed zero-order kinetics), and thus it is almost impossible to study the homogeneous pyrolysis by conventional methods.82 The technique of IR LPHP is ideally suited to this kind of problem, and indeed a number of studies have been conducted. A selection of these is listed in Table 2. I have excluded from consideration compounds of B, Si, and Ge,83*84 although a considerable quantity of work has been carried out on such systems. I have, however, also included carbonyls, since many of the above remarks also apply to these compounds.As an example of the utility of the IR LPHP technique in the study of organometallic reaction mechanisms, I shall consider work carried out in our own laboratory over the past two For the most part, this has been concentrated on the mechanisms of pyrolysis of the simple alkyls of Group I11 metals and their derivatives. Much of this work has been directed towards an understanding of the characteristics required of suitable precursors for the deposition of Ga (in GaAs) and A1 (in AlGaAs). The most widely used organoaluminium precursor is trimethyl aluminium, P D Dapkus, Annu Rev Muter Sci , 1982,12,243 ‘Gallium Arsenide and Related Compounds 1980’, ed H W Thim, Institute of Physics, London, 1981 82 A P Ashworth, E N Clark, and P G Harrison, J Chem Soc, Chem Commun , 1987,782 83 eg, A M Barriola, C Manzanares, and J de Jesus, lnorg Chim Acta, 1985, 98, L43, A M Barriola and L A Valcarcel, Spectrochrm Acta, Part A, 1990,46,449 84 H M Frey, A Kashoulis, L N Ling, I M Pidgeon, and R Walsh.J Chem Soc, Chem Commun, 1981, 915, J Pola, V Chvalovsky, E A Volnina, and L E Guselnikov, J Organomet Chem, 1988, 341, C13, J Pola, E A Volnina, and L E Guselnikov, J Organornet Chem ,in press’’A S Grady, M P Coogan, M D Robertson, and D K Russell, J CryTt Growth, to be submitted A S Grady, R D Markwell, and D K Russell, J Chem SOL,Chem Commun ,submitted for publica- tion ”A S Grady, S G Puntambekar, and D K Russell, Spectrochim Acta, Part A, to be published, J Organomet Chem ,to be submitted A S Grady, R D Markwell, and D K Russell, unpublished results 89 A S Grady, R D Markwell, D K Russell, and A C Jones, J Cryst Growth, 1990,106,239 90 A S Grady, R D Markwell, D K Russell, and A C Jones, J Cryst Growth, to be published 91 G A Atiya and D K Russell, unpublished results, G A Atiya, Ph D Thesis, University of Leicester, 1990 Table 2 Representative organometallic compounds studied by IR LPHP Compound Photosensitizer Reaction product and comments Ref: Fe(CO)5 SF6 Fe(C0)s + 02 SF6 Fe(C0)5 SF6 Fe('2C0)5 + 13C0 SF6 Fe(CO)5 + PF3 SF6 Cr(C0)6 SF6 MO(CO)~ SF6 W(CO)6 SF6 Me20s(C0)4+ CO SF6 H20s(CO)d + CO SF6 oS('2co)5 + l3c0 SF6 HMn(C0)s + CO SF6 MR.SF6 Ge(OCH3)4 None Zr(OBu')4 None U(OCH3)6 None AKCH313 SF6 Ga(CH3h SF6 Ga(C&)3 SF6 AI(CH 3)2H SF6 AIH3*N(CH3)3 SF6 AIH~.N(CH~)J SF6 Sn(CH3)4 SF6 Sn(CH3)4 + 02 SF6 Ti(OCH(CH3M4 C2H4 (CH3)Mn(C0)5 SF6 (CH3)Mn(CO)5+ (CH3)3SiH SF6 Fe + CO a Iron oxides + CO a Pulsed; bond enthalpy determination b, c CO exchange d CO-PF3 exchange d Pulsed; bond enthalpy determination c Pulsed; bond enthalpy determination c Pulsed; bond enthalpy determination c os(Co), d oS(co)5 d CO exchange d H-CO exchange d Various metals and metal alloys from alkyls e Isotope separation f Isotope separation f Isotope separation; molecule 'tailored' to IR LPHP g A1(CH3)2 radical trapped with SF6 or CC14 h CH3 radical trapped with D2 1 Ga(C2H5)2H and Ga(C2H5)H2 isolated j,k Alternative A1 MOCVD precursor to A1(CH3)3 1 Alternative clean A1 precursor m Mixtures with Ga(CH& and Ga(C2H5)3 studied n, 0 Precursor for Sn in MOCVD P Precursor for Sn02; auto-catalysed in conventional pyrolysis P Precursor for Ti02 4 CH4, CO, and Mn; precursor for Mn MOCVD r (CH3)3SiMn(C0)5; precursor for MnSi deposition r a Ref.10. Ref. 60. K. E. Lewis, D. M. Golden, and G. P. Smith, J. Am. Chem. SOC., 1984, 106, 3905. Ref. 24. S. H. Bauer, Spectrochim. Acta, Part A, 43, 227. Y. Okada, S. Kato, S. Satoaka, and K. Takeuchi, Spectrochim. Acta, Part A, 1990, 46, 643. S. S. Miller, D. D. DeFord, T. J. Marks, and E. Weitz, J. Am. Chem. Soc., 1979,101,1036.'Ref. 38; G. A. Atiya, A.S. Grady, D. K. Russell, and T. A. Claxton, Spectrochim. Acfa, Part A, to be published. ' Ref. 85. Ref. 63. 'Ref. 87. Ref. 88. " Ref. 89. Ref. 90. P Ref. 91. * G. W. Rice and R. L. Woodin, Spectrochim. Acfa, Part A, 1987, 43, 299. 'A. S. Grady, I. M. T. Davidson, M. Pennington, and D. K.p Russell, unpublished results Infrared Laser Powered Homogeneous Pyrolysis (CH3)3AI However, the pyrolysis of this precursor leads to the incorporation of unwanted carbon in the deposited metal, in direct contrast with the corresponding gallium case 92 Conventional pyrolysis has failed to provide a convincing explanation for this phen~menon,~~ largely because organometallic intermediates produced in the gas phase are very rapidly destroyed on hot walls,94 and are not therefore detected However, in the IR LPHP of (CH3)3Al, it is possible to trap the (CH3)zAl’ radical formed as either the fluoride, (CH3)zAlF, or chloride, (CH3)2AlC138 Furthermore, we have also shown that it is the monomer (trimethyl aluminium is largely dimeric at room temperature) that undergoes pyrolysis 38 These observations, coupled with the observation of CH; radicals in matrix isolation and laser spectroscopic experiment^,^^ unambiguously confirm the usual assumption that Al-CH3 bond homolysis is the primary step The CH; radical can readily abstract H atoms from the parent molecule, leading to the formation of a >AI-CHZ bond, ab znztzo calculations96 have shown this to have considerable double bond character, and therefore increased stability It therefore seems feasible that these processes may be involved in the incorporation of C into deposited A1 On the other hand, although the first step in the pyrolysis of (CH3)3Ga is also Ga-CH3 bond hornoly~is,~~ the slightly stronger C-H bond in the latter 89 reduces the occurrence of H-abstraction from the parent molecule, and this pathway is no longer significant This has been shown by the IR LPHP of mixtures of (CH3)3Ga and D2 In this system, considerable quantities of CH3D are produced, whereas in mixtures of (CH3)3A1 and Dz, no CH3D is observeds5 Thus in the competition for methyl radicals between D2 and the parent (CH3)3M, the D2 wins when M = Ga, but abstraction from the parent is more rapid when M = A1 Armed with this insight, we have investigated the pyrolysis mechanisms of some promising alternative A1 precursors, such as (CH3)2AlH 87 and AIH3-N(CH3)3 88 90 A second system of considerable interest, both industrially and from a fundamental point of view, is triethyl gallium Compounds of A1 and Ga with alkyl groups containing p-hydrogens have long been believed to undergo p-elimination of alkenes on pyrolysis, eg However, attempts to isolate the dialkylgallanes thus produced have failed, partly because of the competing reverse reaction, and partly because of subsequent catalysed polymerization of the alkene 97 The IR LPHP technique neatly bypasses these problems, since the relatively involatile diethylgallane 92 T F Kuech, E Veuhoff, T S Kuan, V Deline and R Potenski, J Cryst Growth, 1986,77,257 93 M Suzuki and M Sato, J Electrochem Soc Solid Slate SCI Tech, 1985,132, 1684 y4N Suzuki, C Anayama, K Masu, T Tsubouchi, and N Mikoshiba, Jup J Appl Phys 1986 25, 1236 ys J B Raynor personal communication D W Squire, C S Dulcey, and M C Lin, J Vuc Scr Technol, B, 1985,3, 1513 96 T A Claxton, personal communication ”J J Eisch, J Am Chem Soc, 1962,84,3830 Russell produced is condensed onto the cold walls of the reaction cell, where it can be examined at leisure.63 Not only that, we have also shown that, under the correct conditions, it is possible to carry out a second p-elimination step, and also isolate the monoethylgallane produced: 86 (C~HS)~G~H-(CzH5)GaHz + C2H4 (9) This compound has never before been isolated, indeed it is the first demon- strated example of a free monoalkylgallane prepared by any method.As has been hinted by a number of other workers in the field of organometallic pyrolysis,24 the IR LPHP method opens up reaction pathways not reached by conventional techniques. C. Quantitative Kinetic Studies.-As a final example of applications of the IR LPHP technique, I shall consider some of the measurements of kinetic parameters (principally activation energies) which have been reported. A selection is given in Table 3; a number of less quantitative results have also been reported. The selection of Table 3 covers most of the variations described in the paragraphs above: continuous wave excitation, using either direct measurement of temperature or the chemical standard technique, and pulsed excitation studies of both unimolecular and bimolecular reactions. Several aspects of the studies described in Table 3 have already been discussed in the sections above, and I shall restrict my comments at this stage to the results of chemical interest.Of the many applications of the 'chemical thermometer' technique since its invention by Shaub and Bauer," perhaps a typical investiga- tion is that of Lavrushenko er al. on the pyrolysis of SiC13H.98 In this study, SiC13H was pyrolysed in the presence of SiF4 as a photosensitizer. The reaction produces HC1 cleanly, and the Sic12 simply results in a polymeric deposit on the cold cell walls: SiC13H-HC1 + SiC12 (10) The reaction was monitored by IR spectroscopy and mass spectrometry of the starting silane and the product HC1.The chemical standard used in this case was 1,1,2,2-tetrafluorocyclobutane,which dissociates through one of two pathways with an activation enthalpy of 292 kJ m01-I.~~ The activation enthalpy determined for the silane decomposition was 295 kJ m~l-'.~~ This investigation highlights the simplifying nature of the IR LPHP technique, since the reaction is complicated in conventional pyrolysis by the effects of the silene SiC12; loo it also illustrates the necessity to utilise a standard reaction of very similar kinetic charac- teristics. 98 B. B. Lavrushenko, A. V. Bakianov, and V. P. Strunin, Spectrochim. Acta, Part A, 1990,46,479.99 R. T. Conlin and H. M. Frey, J. Chem. SOC., Faraday Trans. 1, 1980,76,322. loo Y.N. Tang, in 'Reactive Intermediates', ed. R. A. Abramovitch, 1982, 2, Plenum Press, New York, 297. p Table 3 Representative kinetic studies using IR LPHP w m Compound cis -trans 2-butene trans -cis 2-butene cyclopropane -propene CC13CH3-CClaCHz CHClzCH2Cl-CHClCHCl Cyclobutanone CH3CH2CHBrCH3 CH3COzCH(CH3)2 CH3C02C2HS 2,2’-azoisopropane CH3CHN02CH3-various SlC13H -Sic12 + HCl a Ref 10 * Ref 61 Ref 54 Ref 23 Photosensitizer NH3 SF6 SiF4 Mode and comments Continuous wave extensive series of studies demonstrating the ‘chemical a thermometer’ technique for unimolecular reactions with activation 2b energies ranging from 180 to 280 kJ mol 2 $2 =L $ Pulsed, temperatures of 1100-1400K, Ea, A measured C2H5Cl used as a b $! standard 2 n Pulsed, two reaction paths with different Ea C E *a Pulsed, first three used as test reactions, unimolecular elimination d 5 reactions, new results on azo compounds gk Concerted mechanism suggested Pulsed, first example of bimolecular reaction OH generated from e H202, detected by LIF, temperatures 8&1400K, difficult to obtain otherwise Continuous wave, based on calculated temperatures, Ea found s Continuous wave, CH3C02C2HS as standard g Continuous wave, 1,1,2,2-tetrafluorocyclobutaneas standard h Ref 11 Ref 32 J Pola, M Farkacova, P Kubat, and A Trka, J Chem Soc, Faraday Trans 1, 1984,80, 1499 Ref 98 Russell Of reports using pulsed methods that of Smith et al.on the reaction of OH radicals with a number of hydrocarbons is perhaps of special significance as representing the first application of IR LPHP to a bimolecular reaction.'' In this study, OH radicals were generated by the dissociation of H202 in the IR laser irradiated region. Here they reacted with the substrate (CH4 as a diagnostic case, and also propane and propene), the decay in OH concentration being monitored by Laser Induced Fluorescence using a dye laser. At the same time, effective temperatures were determined by monitoring the relative populations of several different rotational levels in the OH radical. Smith et a!. were able to measure bimolecular rate constants with a typical uncertainty of 15-25% at temperatures up to 1400K, very much higher than in previous studies.These workers have also reported a number of further studies of high temperature reactions of OH radicals, amply demonstrating the usefulness of this particular development."' Acknowledgements. I would like to thank all my co-workers who have collabor- ated in the studies described above, particularly postgraduate students Ghalib Atiya, David Pape, and Andrew Grady, and post-doctoral research associates Shakher Puntambekar, Ross Markwell, and Mark Pennington. The work has been funded through grants from the DTI and the SERC, and has received active encouragement and support from the MOD and Epichem Ltd; the latter have also generously donated chemicals in these studies. I would also like to express my thanks to Dr. G. P. Smith and Professor J. Pola for helpful correspondence about their own work. lo' J. B. Jeffries and G. P. Smith, J. Phys. Chem., 1986, 90, 487; G. P. Smith, Int. J. Chem. Kinet., 1987, 19,269.

ISSN:0306-0012    

DOI:10.1039/CS9901900407

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

Chem. Soc. Rev., 1990,19,439-470 Dynamic Decay Properties of Excited Electronic States of Polyatomic Molecular Ions Studied with Synchrotron Radiation By Richard P. Tuckett SCHOOL OF CHEMISTRY, THE UNIVERSITY OF BIRMINGHAM, EDGBASTON, BIRMINGHAM B15 2TT 1 Introduction This review is concerned with a family of polyatomic molecular ions whose excited valence electronic states show interesting and unexpected decay properties. The species of interest are the halogenated ions of group IV of the periodic table, and we are now extending these studies to group I11 cations. They have valence states which lie 10-30 eV above the ground state of the neutral molecule, and this article will describe experiments using tunable vacuum ultra-violet (VUV) radiation from a synchrotron as a photoionization source to measure the different decay properties of these states.Molecular ions are transient, free radicals whose importance in comets, inter- stellar space, and flame chemistry (to name but three examples) is now widely appreciated, and the past 15 years has seen an enormous growth in techniques to study both their spectroscopic and dynamic properties. The different spectro- scopic techniques which can now cover a huge range of the electromagnetic spectrum was the subject of a Royal Society Discussion meeting in 1987’ and advancements in electronic spectroscopy were described by Maier in a recent edition of the Chemical Society Reviewx2 In any spectroscopic experiment the primary aim is to produce as high a concentration of the molecular ion (in this case) of interest as possible, and the presence of unwanted species is not important so long as they do not mask the spectral features (i.e.line positions and intensities) of the ion. This goes a long way to explaining why electron impact and many different forms of electrical discharge4 have been popular methods for producing ions for spectral characterization. Electron impact ionization is an example of what I will refer to as a ‘non-resonant’ technique, in that the energy of the ionizing electron is often very much greater than that needed to populate the ground state of the ion (for absorption experiments) or an excited state of the ion (for emission or fragmentation experiments). The subsequent fragmentation that can occur in a polyatomic ion (to both neutral and ionic fragments) is compensated by the large cross-sections for electron impact ionization. Therefore, so long as spectral masking does not occur, this Royal Society Discussion Meeting on the Spectroscopy of Molecular Ions, f/dOF.Trans. R. Soc. London, A 1988, vol. 324. J. P. Maier, Chem. Soc. REV.,1988, 17, 45. J. P. Maier, Chimia, 1980,34, 219. G. Herzberg, Qurirt. Rev. Chem. SOC.,1971, 25, 201. Decay Properties of Excited Electronic States of Polyatomic Molecular Ions method is excellent at producing a high concentration of ions for spectral characterization. The absolute value of the concentration is not important. A dynamics experiment has different requirements.To study the decay of an isolated state of a gas phase species, the aim now is to produce only that state of the ion to be studied and ideally with a known concentration or flux. A non-resonant excitation source such as electrons can no longer be used and techniques using photons are usually employed. In the study of singly-charged molecular ions with electronic states of energy less than 21.2 eV above the ground state of the parent neutral molecule, photoionization using the He-I line at 58481 (or 21.2 eV) from a helium discharge lamp is often used: AB, is a generalized, polyatomic molecule and the asterisk represents an excited electronic state. Photoionization of this kind is a non-resonant process because the electron can carry away the energy difference between the He-I photon and the energy of (AB,)+(*) as excess kinetic energy.The concentration of ion produced is then determined by the partial ionization cross-section of that electronic state for an incident photon energy of 21.2 eV. An electron energy analyser is often used to define the kinetic energy (KE) of the ejected electron and by this way the state of the ion (AB,)+ to be studied is defined uniquely. A range of coincidence techniques have been developed to measure dynamic decay properties of such isolated states of molecular ions. These include the photoion-fluorescence photon (PIFCO) te~hnique,~ the photoelectron-photoion (PEPICO) technique,6 and the photoelectron-fluorescence photon (PEFCO) te~hnique.~ The decay properties measured have included fluorescence quantum yields, radiative life- times, and non-radiative unimolecular decay rates of isolated electronic states of polyatomic ions.Such measurements are now being extended to doubly-charged molecular ions produced by non-resonant photoionization using the He-I1 line at 40.8 eV.8 Such techniques suffer two disadvantages. Firstly, in the case of He-I photoionization, the ionic states that can be studied are limited to those with energy less than 21.2 eV. Secondly, the resolution of the experiment (i.e. the spectral width of the ionic state whose decay properties we wish to investigate) is limited by that of the electron energy analyser, and this often means that only widely spaced vibrational features within an electronic state manifold can be distinguished.In theory both problems can be circumvented using tunable VUV radiation from a synchrotron as a ‘resonant’ photoionization source: S Leach, J Mol Struct ,1984, 141,43 T Baer, Adv Chem Phys ,1986,64,111’J P Maier and F Thommen, In ‘Gas-Phase Ion Chemistry’, Vol 3, ‘Ions and Light’, ed M T Bowers, Academic Press, London, 1984, p 357 * D M Curtis and J H D Eland, Int J Mass Spectrom Ion Phys ,1985,63,241 Tuckett This review describes such experiments. Neutral, polyatomic molecules are photoionized by radiation from the Science and Engineering Research Council Daresbury synchrotron source, and the competing radiative and non-radiative decay channels of excited electronic states of (ABn)+ are investigated by two complementary techniques: or (AB,)+(*)~fragment ions (3) The competition between these pathways lies at the heart of our understanding of molecular dynamics, and this review describes these processes for a range of polyatomic systems, namely the cations of the pure and mixed halides of group IV of the periodic table.The radiative decay of (ABn)+(*) (occurring at a rate krad) is measured in a relatively simple crossed gas spray-tunable VUV beam apparatus with un-dispersed fluorescence dete~tion.~Non-radiative fragmentation of (AB,) +(*) (occurring at a rate knon-rad) is measured in a more complicated energy scanning PEPICO apparatus,' which is in effect a novel form of photoionization mass spectrometry.In the radiative decay experiment, thresholds for fluorescence are determined, and in favourable cases an estimate of the fluorescence quantum yield (Dkv or (more accurately) radiative probability of the excited electronic state can be made. Using the synchrotron in its pulsed mode (see Section 2) radiative lifetimes z can be measured very accurately. Since (Dhv = krad/(krad + knon-rad) and T-1 = (krad -I-knon-rad) changes in ahvand T with excitation energy are especially interesting, since they can usually be attributed to a change in knon-rad, i.e. the onset of a competing, non-radiative channel. However the fragment ions produced by such a process are not probed directly. These measurements are made in the energy scanning PEPICO apparatus, again using tunable VUV radiation from a synchrotron source as the means of producing the polyatomic ions.The Daresbury synchrotron radiation source and the experimental details are described in Section 2. Section 3 reviews earlier 'spectroscopically-based' experi- ments made in Birmingham on the cations of the halides of group IV, i.e. the family of ions built upon CF;. These experiments preceded those at Daresbury, and indicated the value of using a tunable vacuum UV photon source to create ions for dynamics experiments. Section 4 describes the results from the radiative decay experiment, Section 5 from the non-radiative decay experiment, and some general conclusions are made in Section 6. I. R. Lambert, S. M. Mason, R.P. Tuckett, and A. Hopkirk, J. Chem. Phys., 1988,89,2683. lo J. C. Creasey, I. R. Lambert, R. P. Tuckett, K. Codling, L. J. Frasinski, P. A. Hatherly, M. Stankie-wicz, and D. M. P. Holland, J. Chem. Phys., 1990,93,3295. 441 Decay Properties of Excited Electronic States of Polyatomic Molecular Ions 2 Experimental Several books have been written about the properties of synchrotron sources and the reader is referred to such texts for an understanding of the fundamental physics of these sources. A review of the characteristics of the United Kingdom Synchrotron Radiation Source (SRS) at Daresbury in Cheshire was given by Holland in 1986," and although the number of beam lines and extent of the experiments has increased substantially in the last four years this review still remains an excellent reference to the properties of this source.Briefly, the Daresbury SRS is a 2 GeV electron storage ring which is injected from a 600 MeV synchrotron booster. The ring has a radius of ca. 15 m, the electron flight time is ca. 320 ns and the ring can accommodate 160 electron bunches in what is generally referred to as the 'multi-bunch' mode. The electron pulse width is ca. 200 ps. Tangential to the ring to collect the associated electromagnetic radiation are 13 ports, one of which is dedicated to vacuum UV/soft X-ray gas phase molecular physics. Three monochromators serve this port, a normal incidence 1 m Seya, a normal incidence 5 m MacPherson, and a 3 m toroidal grating grazing incidence instrument.The first two operate in the energy range 5-35 eV, the third 20-150 eV. All the experiments described in this review used the Seya monochromator which has a quoted photon flux of 1 x 10" s-' A-' at 600A. The properties of the SRS that are exploited are directionality, easy energy tunability, and excellent timing properties when only one electron bunch is in the ring (the 'single-bunch' mode which is in effect a pulsed source of electro- magnetic radiation, pulse width 200 ps with a repetition rate of cu. 3 MHz). The photon beam also has a high degree of linear polarization. Up to now we have not exploited this properly in our work, but future experiments to measure the degree of polarization of fluorescence from polyatomic molecular ions are planned.Two different experiments are used to investigate radiative and non-radiative decay of electronic states of polyatomic molecular ions and full details can be found el~ewhere.~.' Common to both experiments, however, is the orthogonal crossing of a gas spray of the neutral molecule with tunable vacuum UV radiation from the 1 m Seya monochromator at the Daresbury ring. The majority of the experiments use photon wavelengths well below the lithium fluoride cut-off of 106 nm, so a windowless, differentially pumped system has to be employed. Thus the radiation is focused out of the exit slit of the Seya into a 2 mm internal diameter 1-5 cm long glass capillary which ends ca. 5-10 mm from the crossing region with the gas spray.(The lengths and distances used depend on which experiment is being performed.) The capillary channels the radiation to the interaction region and ensures poor conductance between monochromator and experiment, so only one stage of differential pumping between the Seya and the experimental chamber is needed to maintain the necessary, low pressure of the monochromator. The absolute synchrotron flux in the experimental chamber is measured either by a A1203 photocathode (in the '' D M P Holland, Phy~Scrip, 1987,36, 22 Tuckett 52 Gas in VUV from Seya ’‘5can1Clock p % r I0rCAMAC1 2-3 L Quad Count -1-radiative experiment) or by a sodium salicylate coated window plus photo- multiplier tube used in the photon counting mode (in the non-radiative experiment).The Seya has two gratings (both with 1200 lines per mm) mounted back-to-back on a single kinematic mount. They are blazed at ca. 600 and 900A, providing photon flux between 350-850A (‘high energy’ grating) and 700-1200 A (‘low energy’ grating) respectively. Second order radiation is only problematic with the high energy grating for h > 800& with the low energy grating for h > 1150A. The ultimate resolution of the Seya is ca. OSA, although the majority of our spectra were recorded at a resolution of 2A. In the radiative decay experiment (Figure 1) undispersed fluorescence from the interaction region is focused by a f/1.3 75mm focal length concave mirror through a MgFz window and a 50 mm square filter onto a photomultiplier tube.Two such tube + filter combinations can be used so that fluorescence in two different spectral regions can simultaneously be detected. The photomultiplier tubes are used in the photon counting mode and the signals are processed by CAMAC electronics and a dedicated LSI 11 mini-computer. Data are transferred to the Daresbury Mainframe Computer (Convex C220) for analysis. Two types of experiment can be performed. In the ‘multi-bunch’ (or quasi c.w.) mode, the photon wavelength (or energy) is scanned, total fluorescence collected, and fluorescence excitation spectra are recorded. Such experiments measure thresholds for fluorescence, and in favourable cases an estimate of the fluorescence quantum yield of the electronic state can be made.In the ‘single- Decay Properties of Excited Electronic States of Polyatomic Molecular Ions Figure 2 Diagram of the non-radiative fragmentation apparatus. G1-G8 are grids on the pair of identical TOF mass spectrometers. MCP = microchannel plates, PMT = photo-multiplier tube, TDC = time to digital converter (Reproduced by permission from J. Chem. Phys., 1990,93,3296) bunch' (or pulsed) mode, radiative lifetime measurements are made (see Section 2), and decays can be measured at different photon excitation energies. In the non-radiative experiment (Figure 2), the interaction region between gas spray and photon beam occurs at the centre of a pair of identical 10 cm long time-of-flight (TOF) mass spectrometers.Electrons are accelerated out of the interaction region to one set of microchannel plates, parent and fragment ions to the other set, the two signals are counted in delayed coincidence by the PEPICO technique, and a TOF mass spectrum is obtained. The photon energy is changed and the experiment repeated. The apparatus is again controlled by CAMAC electronics interfaced to two interacting computers, an IBM PC/AT with colour graphics and an LSI 11/23. The former sets the experimental conditions, displays the data in real time and stores it on disc. Data accumulate as a three-dimensional histogram of photon energy versus ion TOF versus coincidence count rate, with the last variable represented by colour. The LSI 11 steps the Seya monochromator, stores auxiliary measurements (including the total electron count), and accesses the Daresbury mainframe computer.TOF spectra accumu- late normalized to the synchrotron flux (which has to be in the multi-bunch Tuckett mode), as described elsewhere." Data can be displayed either as the original 3-D histogram, or by taking cross-sectional cuts through it in one of two ways. Either intensity uersus ion TOF is displayed at a fixed photon energy, or the intensity of a particular fragment ion is displayed as a function of photon energy (i.e. an ion yield curve). If the signal-to-noise ratio is high enough, the latter method is more revealing because it gives directly thresholds for ion fragmentation. The former method is used with weak fragmentation channels (by comparing TOF spectra above and below a certain energy), but the threshold energy is not defined uniquely.This technique has many similarities to photoionization mass spectrometry (PIMS),12 but with three main advantages. First, the use of a synchrotron source means that in our experiment wavelengths down to 350A can be accessed, whereas most PIMS experiments use a helium continuum and vacuum UV monochromator as the photon source with wavelengths limited to ca. h > 600A. Second, the use of TOF detection means that all the fragment ions are collected simultaneously, and quantitative comparisons can be made between different ions collected in a single experiment. Third, time correlation of the ejected photoelectron with a fragment ion in the TOF mass spectrometer means that the KE release in a fragmentation channel can be measured." The apparatus does however have one principle limitation, in that there is no energy analysis of the photoelectrons; in fact electrons with (r-8 eV kinetic energy are all estimated to be collected with 100%efficiency.13 Therefore, the electrons which provide the 'start' pulses in the PEPICO experiment can originate from photoionization of AB, to several different electronic states of (AB,)+ up to the energy available from the incident VUV photon.Ideally the detected photoelectron would have zero kinetic energy (i.e. a threshold electron), then the internal energy state of (AB,)' would be uniquely defined (see equation 2). This modification to the apparatus is in progress.3 Spectroscopic Experiments on Group IV Tetrahalide Molecular Ions Initial experiments on the spectroscopic properties of excited electronic states of the group IV tetrahalide molecular ions MX: (M = C,Si,Ge; X = F,Cl,Br) were made at Birmingham in a crossed supersonic beam-electron beam apparatus with dispersed fluorescence detection. The supersonic beam provides substantial rotational, and to a lesser extent vibrational, cooling of the polyatomic parent neutral molecule. The process of non-resonant electron impact ionization involves little transfer of rotational angular momentum, so whereas the vibra- tional levels within an electronic state of the parent ion of MX4 are produced with a Franck-Condon distribution, each vibronic state has a very low rotational temperature (typically c 30K).If this state is bound and decays radiatively by l2 J. Berkowitz, 'Photoabsorption, Photoionisation, and Photoelectron Spectroscopy', Academic Press, 1979, p. 410; Radiat.Phys. Chern., 1988,32,23. l3 M. Stankiewicz, P. A. Hatherly, L. J. Frasinski, K. Codling, and D. M. P. Holland, J. Phys. B, 1989, 22,21. l4 A. Carrington and R. P. Tuckett, Chem. Phys. Lett., 1980,74, 19. Decay Properties of Excited Electronic States of Polyatomic Molecular Ions 0,o 0,I 0,2 0,3 084 (V,I, V,r') I 1 I I I I I I I I 1 I 360 380 400 620 h /nm Figure 3 Spectrum of CF; 2A1--C 2T2resulting from electron impact ionization of CF4 at a low rotational temperature The vibrational assignment (vl', v'i) is shown (Reproduced by permission from Mol Phys ,1987,60,763) photon emission to a lower-lying bound state (a 'bound-bound' transition), the emission spectrum will be condensed into a few rotational components This spectral simplification, compared to the relative complexity of a room temperature source, has been essential for detailed spectroscopic analysis of these polyatomic ions On the other hand, if the lower electronic state to which photon emission occurs is repulsive, fluorescence spectra will only be observed as a broad band 'bound-free' spectrum The spectral width is then a reflection of the repulsive nature of the curve in the Franck-Condon region below the fluorescing bound state, and the rotational simplification afforded by the molecular beam is not utilised We have observed both bound-bound and bound-free spectra in this study of electronic spectra of MXZ A.Bound-Bound Discrete Spectra in MX:.-Bound-bound spectra are observed in the fluorides of MX:, i e CF:, SiF:, and Ge: and the transition is between the fourth and third excited electronic states of these ions The states have symmetry O2A1 and e2T2 respectively, are connected by an allowed electric dipole tran-sition, and lie ca 18-25 eV above the ground state of the parent neutral mol-ecule The main spectroscopic interest has centred on the triply-degenerate 'T2 state This state shows first-order Coriolis splitting, spin- orbit splitting, and has the correct symmetry to distort from tetrahedral geometry '' J F M Aarts, S M Mason, and R P Tuckett, Mol Phys ,1987,60,761 S M Mason and R P Tuckett, Mol Phys, 1987,60,771 l7 S M Mason and R P Tuckett, Mol Phys, 1987,62,979 I I I 1 I I I I 30 20 10 0 30 20 10 0 +Ti' $/c m-' Figure 4 (a) The 1; band of CF; 0-c at 381 nm recorded ut u resolution of 1.5 cm-'.(b)The best simulated spectrum; the six branches are marked (Reproduced by permission from Mol. Phys., 1987,62,184) by the Jahn-Teller effect. The vibrational structure of the B-c transition in these three fluorides indicates immediately that Jahn-Teller distortion is occurring in SiF; and GeF: c2T2 but not in CF4f c2T2. Figure 3 shows the spectrum of CF; B-c between 36-20 mm recorded at ca.25K in the molecular beam apparatus. Only vibrational bands involving the u1 totally symmetric C-F stretching mode are observed, and the absence of bands involving the other three modes u2, u3, and u4 (of e, t2, and t2 symmetry respectively) shows that the e state has tetrahedral symmetry and is not exhibiting Jahn-Teller distortion. __+Figure 4(a) shows the spectrum of the u\ = 0 u; = 2 band at 381 nm recorded under much higher resolution. Figure 4(b) is a computer simulation of the rotational structure.'* The agreement is excellent and confirms that this band is indeed due to a vibronic component of a 2A1-2T2 transition in a tetrahedral molecule observed at a low rotational temperature. Several spectroscopic parameters (e.g. rotational constants and spin-orbit splitting constants) were deduced from this work.Recently, these bands in both CF2 '' and SiF: 2o have been photographed at Doppler-limited resolution (ca. 0.05 cm-'), and when the analyses are complete these constants will be improved. S. M. Mason and R. P. Tuckett, Mol. Phys., 1987, 62, 175; S. M. Mason, Ph.D. Thesis, University of Cambridge, 1988. l9 J. F. M. Aarts, personal communication. J. L. Chotin, S. Leach, I. R. Lambert, and R. P. Tuckett, unpublished data. Decay Properties of Excited Electronic States of Polyatomic Molecular Ions I I I I I I 1 350 4 00 4 50 500 550 600 650 A/nm Figure 5 Spectrum resulting from electron impact ionization of a He-seeded supersonic beam of SiC14.The two broad continuous bands are due to emission from SiCl,' c 2Tz (Reproduced by permission from J. Chem. Phys., 1988,89,2678) B. Bound-Free Continuous Spectra in MX:.-Bound-free transitions are observed in CF,' and the tetrachloride and tetrabromide ions of silicon and ger- mani~m.~~.~~The 2T2 state of CF,' decays radiatively by photon emission to the ground (8'TI) and first excited (A 2T2) staLes, both of which are repulsive states in the Franck-Condon region. The e-A and c--8 are electric dipole allowed transitions, so the spectrum appears as two (overlapping) structureless bands with peaks at 290 and 230 respectively. The four ions SiCl,', GeCl,', SiBr4f ,and GeBr,' all show radiative decay from the same e2T2 state to repulsive A and 8 states, and therefore two broad bands are observed in each emission spectrum; Figure 5 shows the spectrum of SiCl,' c-8, A resulting from electron impact ionization of a molecular beam of SiC1,.The ions CCl,' and CBr4f do not fluoresce. Spectroscopically, continuous spectra of this kind are of limited value. They do indicate that the upper electronic state is bound, and therefore can decay on the relatively slow timescale of photon emission (ca. 105-1010 s-'). Also, because emission from the parent ion is proposed, it has to be shown that the wave- lengths of the observed transitions are consistent with the separation of the electronic states of MX,' as revealed by photoelectron spectroscopy. However, because detailed vibrational and rotational structure is not present, it is not 21 J.F. M. Aarts, Chem. Phys. Lett., 1985, 114, 114. 22 I. R. Lambert, S. M. Mason, R. P. Tuckett, and A. Hopkirk, J. Chem. Phys., 1988,89,2675. 23 J. C. Creasey, I. R. Lambert, R. P. Tuckett, and A. Hopkirk, J. Chem. Soc., Faraday Trans., 1990,86, 202 1. Tuckett possible to assign an electronic spectrum on the basis of its high resolution spectroscopic features, as was possible with the CF4f 2A1-C 2T2 spectrum.15 Thus assignments of bound-free spectra are always more uncertain. C. Spectroscopic/Dynamics Experiments on MX; .-These spectroscopic experi- ments served to highlight a number of new problems to be solved. Perhaps the first point to understand is why such fluorescence spectra are observed at all.Photon emission is being observed from excited electronic states of MX; which lie up to 10 eV above many dissociation channels (see Section 4).Whereas this behaviour would not be surprising for diatomic cations, with such sized five- atom polyatomics non-radiative processes would be expected to dominate. The observation of radiative decay is therefore very surprising. Some of the other questions to be answered are: (a) For these states of MX4f that decay by photon emission, what are the fluorescence quantum yields? Do they change when different parts of the potential energy surface are accessed? (b) What are the radiative lifetimes of the fluorescing states, and again do they change with excitation energy? (c) For a given central atom M, why do the fluorides behave differently from the chlorides and bromides, and for a given halide why does the carbon species behave differently from the silicon or germanium species? (d) If the excited electronic state of MX: does not decay radiatively but (as expected) by a non-radiative process, what are the fragment ions produced and at what rate do these non-radiative processes occur? (e) When the parent neutral molecule in the supersonic beam is excited with 200 eV non-resonant electrons, does fragmentation of MX4 compete with ionization? Do the relative cross-sections for these processes change with energy? (f) Perhaps most important of all, what if any is the relation between the spectroscopic and dynamic decay properties of these valence electronic states of MX;? To take one example, CF4f z' 2T2 does not show Jahn-Teller distortion from tetrahedral ~ymmetry,'~ and this state decays radiatively.SiF; and GeF; c do distort from Td geometry,16.17 but both states decay non-radiatively by loss of a fluorine atom to SiF3f/GeF3f + F (Section 9.'' Does this imply that all excited electronic states of polyatomic ions that show Jahn-Teller distortion do not decay by photon emission, or is this a particular property of highly symmetrical tetrahedral species? The pre-requisite to be able to address these problems is the ability to create a known flux of a particular electronic state of MX;, ideally at vibrational resolution, in the absence of other species. This is most easily achieved with tunable vacuum UV photons from a synchrotron source and explains why we started experiments at Daresbury in 1987.4 Experiments to Probe Radiative Decay of Excited Electronic States of Polyatomic Molecular Ions We have used the apparatus described in Section 2 to probe radiative decay of Decay Properties of Excited Electronic States of Polyatomic Molecular Ions 21 22 23 24 25 26 27 28 29 30 EXC I TAT I ON ENERGY (EV) Figure 6 Undispec~edpuorescerzceuf CF4 excited by VUVphotons in the range 21-31 eV. In (A)Jluorescence in the range 25&390 nm is collected, in (B) 12&200 nm only is collected. The signals have been normalized to the synchrotron flux lo,and the scale of this uxis is different in the two spectra (Reproduced by permission from J.Chern. Phys., 1988,89,2685) excited electronic states of polyatomic molecular ions, especially the group IV tetrahalide ions. This section reviews our results. It is divided into ‘multi-bunch’ fluorescence excitation experiments and ‘single-bunch’ lifetime measurements. A. Multi-bunch (Quasi c.w.) Experiments.-(i) The fluorides of Group IV.9 In these experiments we have used the tunable vacuum UV photon source to confirm what we already knew from our spectroscopic studies in Birmingham, that the singly-degenerate B 2AI state fluoresces in all three fluorides CF:, SiF:, and GeF: whereas only the triply-degenerate c2Tz state in CF: decays radiatively. Figure 6 shows the fluorescence excitation spectrum of CF4 when excited by photons in the range 21-31 eV.In (A) an EM1 9883 QB photomultiplier tube is filtered so that only photons in the range 250-390 nm are collected; this encompasses most of the bound-free e-f;: and c--A transitions in CF:. In (B) an EM1 CsI 9413 solar blind tube (collecting 120 < h < 200 nm) is used; this encompasses most of the bound-free 0-A and B-g transitions in CF:. Thresholds are observed at 21.7 0.1 and 25.0 & 0.1 eV, in excellent agreement with the adiabatic ionization potentials (IP) of the c and b states of CF2 (Table 1). These spectra show two important characteristics of a photoionization process. Firstly, the steepness of the ‘turn-on’ is governed by ionization Franck-Condon factors; Figure 6(B) shows a steep rise at threshold Tuckett Table 1 Energetics of dissociation channels of CF:, SiFi, CCl,', und SiCl,' in eV Neutral/ Parent Dissociation Dissociation Adiabatic Ion Channel EnergyleV IPIeV CF; B2A1 25.1 CF2 + Fg 23.2 CF3 + Ff 22.9 CF' + F + Fz 22.1 c 2T2 21.7 CFt + F + F 20.8 CF: + F2 19.2 B 2E 18.3 A 'T2 17.1 CF: XzTi 15.3 CF: + F 14.7 CF4 X'A1 0 SiF2 + F; 26.3 SiF' + F + F2 24.6 SiF3 + F+ 24.3 SiFi + F + F 23.0 SiF: B2A1 21.5 SiF: + F2 21.4 19.3 18.0 17.3 SiF: + F 16.2 SiF: X2T1 16.1 SiF4 X'Al 0 19.3 16.3 CCI+ + c1 + Cl2 16.3 CClJ + c1+ 16.0 cc1: + c1 + c1 15.8 CClZ + c1: 15.0 B 2E 13.4 cc1: + Cl2 13.3 12.3 cc1: + c1 11.8 11.5 0 SiC1: O2A1 18.1 Sic13 + C1+ 17.8 Sic11 + CI + C1 17.7 Sic]+ + C1 + Clz 16.8 Sic12 + Cl: 16.6 SiCl: + Clz 15.2 451 Decay Properties of Excited Electronic States of Polyatomic Molecular Ions Table 1 Continued NeutrallParent Dissociation Dissociation Adiabatic Ion Channel EnergyleV IPIeV C2T* 15 1 B 2E 13 5 A 'T2 12 8 s1c1: + c1 12.7 I1 8 0 because ionization to D2A1 is a vertical process with very little change in molecular geometry, whereas the rise in Figure 6(A) is shallower because ionization to c 'T2 involves a substantial change (0.08A) in the C-F bond distance." Secondly, the fluorescence signal remains non-zero for photon energies well in excess of threshold.This is because photoionization is a non- resonant process since the electron can carry away the excess energy.The intensity of the emission is then determined primarily by the variation of the partial ionization cross-section for the ionic state in question with energy. The EM1 9883 QB photomultiplier tube which detects CF: c-R,A emission can also detect Nzf B-X (0,O) emission at 391 nm, and it has been possible to calibrate the photon collection efficiency of the apparatus with N:. Since the partial ionization cross-section into CF; c is known as a function of energy from angle-resolved photoelectron spectro~copy,~~ it is possible to estimate the fluorescence quantum yield of CFZ c.The procedure is described el~ewhere.~ We obtain (Dhv = 0.5 & 0.4, suggesting that a competing non-radiative channel is operative (see Section 5).Measurements of fluorescence quantum yields by this method are rare, since partial ionization cross-sections are only known for relatively few of the polyatomic molecules we have studied, and the photo- multiplier tube used has to be sensitive at 391 nm to calibrate with N2+ B-X. For this latter reason it was not possible to estimate Ohvfor the D state of CF:. (ii) The Chlorides and Bromides of Group IV.22,23In these experiments we used the tunable vacuum UV radiation to show that the bound-free broad bands seen in the electron impact spectra (Section 3.B) are related to the initial formation of the e2T2 state of the parent molecular ion. Figure 7 shows the fluorescence excitation spectrum of SiC14 excited with photons in the range 14-35 eV.Only fluorescence in the range 32-70 nm was collected, corresponding to the lower wavelength band in Figure 5. An identical excitation spectrum was obtained when fluorescence in the range 505-705 nm was collected, corresponding to the higher wavelength band in Figure 5. Therefore the two emission bands have the same upper state. The two characteristics of a photoionization process described in Section 4.A(i) are observed, and the threshold appears at 15.1 0.1 eV. This corresponds to the adiabatic IP of the c 2T2 state of SiCl,' 25 and therefore the 24TA Carlson, A Fahlman, W A Svensson, M 0 Krause, T A Whitley, F A Gnmm, M N Piancastelh, and J W Taylor, J Chem Phys ,1984,81,3828 25 P J Bassett and D R Lloyd, J Chem SOCA, 1971,641 Tuckett 14 16 18 20 22 24 26 28 30 32 34 36 EXCITATION ENERGY (EV) Figure 7 Undispersed Jluorescence of SiC14 excited by VUV photons in the range 14-35 eV.Only photons in the range 32-70 nm are collected, and the signal has been normalized to the synchrotronflux lo (Reproduced by permission from J. Chenz. Phys., 1988,89,2679) emission is related to the initial formation of SiClt c. The excellent agreement of the wavelengths of the emissions with predictions from photoelectron data for SiC14 22 suggests very strongly that the two bands are due to Sic12 e--8 and C-A. Partial ionization cross-section data are available for SiC14,26 and we used them to estimate the fluorescence quantum yield of SiCl,‘ c to be 0.2-0.4.The lifetime data (Section 4.B), however, suggest that radiative decay is the only channel operative and therefore (Dhv should be unity. Very similar fluorescence excitation spectra were obtained with GeCL, SiBr4, and GeBr4. In all cases the two fluorescence bands observed in the electron impact spectrum can be assigned to parent ion emission from (the bound) e2T2 state to lower-lying (repulsive) -8 2T1and A2T2 states. Partial ionization cross- section data are not available, so it has not been possible to estimate Ohv for these states. However, like SiCl; c they are believed to have the maximum value of unity. (iii) The Mixed Halides of Group In order to establish whether the observation of radiative decay from excited states of the tetrahedral ions MX: was due to their high degree of molecular symmetry, we studied halides of group 26 T.A. Carlson, A. Fahlman, M. 0.Krause, T. A. Whitley, F. A. Grimm, M. W. Piancastelli, and J. W. Taylor, J. Chem. Phys., 1986,84,641. ”J. C. Creasey, I. R. Lambert, R. P. Tuckett, and A. Hopkirk, Mol. Phys.; two papers In press. Decay Properties of Excited Electronic States of Polyatomic Molecular Ions 14 16 i8 20 22 21 26 28 30 52 34 36 Exc I tat i on Energy (e\') Figure 8 Undispersed juorescence of CF3H excited by VUV photons in the range 13.5-35.5 eV. Only photons in the range 23-00 nm are collected, and the signal has been normalized to the synchrotron pux I0 IV of lower symmetry. Four molecules were studied, CFJH, CF3C1, and CFC13 (of C3,, symmetry) and CF2C12 (of CZ,, symmetry), first in the electron beam apparatus at Birmingham and then in the fluorescence excitation apparatus at Daresbury.Parent ion emission is observed in CF3H' and CF3CI' but not in the other two molecules. Figure 8 shows the fluorescence excitation spectrum of CF3H excited with vacuum UV photons in the range 13.5-35.5 eV, fluorescence being collected in the range 23Ck-400 nm. Two quite different features are observed. The peaks between 14 and 18 eV are resonant peaks and are due to CFZ &-g fluorescence following excitation of dissociative Rydberg states of CF3H. The shape of this feature is characteristic of a neutral excitation process where the photon energy scans through the Franck-Condon region of the dissociative Rydberg states.Thus the excitation spectrum shows a slow increase from threshold, reaches a peak, and recedes to the baseline. Because this is a resonant process, these states cannot be populated by higher energy photons. The much weaker feature with threshold at 20 eV, however, is due to a primary photoionization process [see Section 4.A(i)] and the fluorescing state of CF3H + is the (bound) B2A1 state with a vertical ionization potential of 20.5 eV. Emission is to lower electronic states of CF3H' which may be bound or re- pulsive. This spectrum highlights particularly well one of the advantages of using photons over electrons as a method for ionization. Using 200 eV electrons in the molecular beam apparatus, the dispersed fluorescence spectrum from CF3H 454 Tuckett between 230 and 400 nm is dominated by CF2 A--8 emission,27 since the much weaker bands due to CF3H+ in this region are swamped.The second advantage is that thresholds for both resonant dissociative and non-resonant ionization processes are much more accurately determined with photons than with electrons. For an ionization process, this is because with photon excitation the cross-section usually shows a step function at threshold, whereas with electrons the cross-section increases only slowly and linearly with the excess energy above threshold.28 Establishing the threshold in the latter case can therefore be difficult, and there are many inaccurate values in the literature over the past twenty years just because this point has not been appreciated.This has often led to over- interpretation and incorrect assignments. The fluorescence excitation spectra of CF3C1, CF2C12, and CFC13 all show resonant features in the 10-19 eV range, fluorescence being due to excited states of the fragments CF3, CF2, and CF. Full details can be found elsewhere.27 Like CF3H, CF3CI shows parent ion emission from a bound state at higher photon energy, the fluorescing state being the E2A1 state of CF3CI’ with a vertical ionization potential of 20.2 eV. Parent ion emission could not be detected from any valence ionic state of CF2C1; or CFCI;. (iv) The Halides of Group III. We have just initiated a study of fluorescence processes following vacuum UV photoexcitation or electron excitation of the halides of group 111 (see ref.35). We wish to determine whether the (surprising) observation of radiative decay from highly excited valence states of the group IV halides is a property of group TV species, or whether it is a general property of similar-sized molecules. In this paper we report preliminary results on BF3 and BC13, two halides of group 111 of D3h symmetry. In the tetrahedral ions MX;, the states from which radiative decay is observed (c2T2 and 02A1)arise from electron removal from o-bonding molecular orbitals in neutral MX4, whereas the lower electronic states of the ion (R2Tl,A2T2, and B2E) arise from electron removal from halogen pn: non-bonding orbitals.29 In both BF3 and BC13, simple MNDO calculations show that the same pattern of molecular orbitals is observed.The first four HOMOS (electron removal from which give the X’A;,A 2E’, B 2E”, and c 2A’; states of the ion) are predominantly halogen non-bond- ing orbitals, whereas the next two orbitals (electron removal from which give B2E’and g2A;) are o-bonding between boron 2s or 2p and the halogen npo atomic orbitals. If the same pattern is observed, it is therefore from the 0 or states of BF? and BCl? that we might expect to observe radiative decay. These states have adiabatic energies of 621 and 576A in BF3f,30.31 809 and 699A in BC13f,30so are suitable for study using the high energy grating of the Seya mono- chromator. Both molecules show resonant dissociative peaks at high photon wavelengths 28 G.H. Wannier, Phys. Rev., 1953,90,817. 29 R. N. Dixon and R. P. Tuckett, Chem. Phys. Lett., 1987, 140, 553. 30 P. J. Bassett and D. R. Lloyd, J. Ch~ni.SOL‘.,A, 1971, 1551. 31 L. Asbrink, A. Svensson, W. von Niessen, and G. Bieri, J. Elec. Spectrosc., 1981, 24, 293. Decay Properties of Excited Electronic States of Polyatomic Molecular Ions I 1 I I I 16 18 20 22 24 Energy/eV Figure 9 Undispersedjuorescence of BC13 excited by VUVphotons in the range 15-25 eV. Only photons in the range 300-550 nm are collected, and the signal has been normalized to the synchrotronjux I,-, (h > 850A) and non-resonant ionization peaks at lower wavelengths. BCl3 has been studied previously by Lee et al.32 both above and below the lithium fluoride cut-off. Resonant peaks between 1050and 1250A were assigned to BCl2 emission, at 920A to BCl A-X emission, and we observe these same features.Lee et al. assign a sharp threshold at 8lOA in the fluorescence excitation spectrum to emission from the B2E’ state of BCl;, and they also believe the c2Al;state fluoresces weakly. Whereas our spectra are similar in the region of the resonant dissociative peaks, our excitation spectra at lower wavelengths are rather different. Figure 9 shows our spectrum of BC13 excited in the range 15-25 eV, fluorescence being observed between 300 and 550 nm. Steep rises are observed at 15.3 eV (8lOA) and 17.6 eV (705 A), and we assign these to emission from the fi 2E’ and E2A; states of BC1; respectively.Similar spectra are obtained when the filter is replaced by a different cut-on filter transmitting h > 420 nm, and by a UV filter transmitting 23-00 nm. Thus both 0 and state fluorescence IS occurring over a wide range of the UV/visible, and this is compatible with the wavelengths of allowed electronic transitions in BC1; predicted from photoelec- tron spectroscopy. We see no evidence for emission from the c 2Al; state. VUV photoexcitation of BF3 has not been studied before. Figure 10 shows the fluorescence excitation spectrum between 11.3 and 17.7 eV with fluorescence 32 M Suto, C Ye, J C Han, and L C Lee, J Chem Phys, 1988,89, 6653, L C Lee, J C Han, and M Suto, J Chem Phys, 1989,91,2036 Tuckett 12 13 14 15 16 17 Energy /eV Figure 10 UndispersedJluorescence of BFJ excited by VUVphotons in the range 11.3-17.7 eV.Only photons in the range 230-400 nm are collected, and the signal has been normalized to the synchrotronflux I0 collected between 230 and 400 nm. The two strong resonant peaks at 13.1 and 14.0 eV show very different relative intensities when filters isolating different parts of the 23WOO nm region are used. They also have different radiative lifetimes (Section 4.B). By analogy with BC13, we assign these peaks to BF2 emission, fluorescence emanating from two close-lying excited electronic states of this radical. Emission is probably to its ground state. Surprisingly, there are no experimental or theoretical data on this 17-electron triatomic free radical, and we hope this work will stimulate ab initio potential energy surface calculations.We comment that BF2 is isoelectronic with FCO, and although a gas phase electronic spectrum of FCO has not categorically been observed, Jacox33 has observed two close-lying UV electronic transitions of FCO in an argon matrix. The bands we observe in BF2 may be analogous to these two transitions. Figure 11 shows the excitation spectrum of BF3 in the range 18-28 eV. The steep rise at 21.5 eV (57681) is assigned to emission from the 'A; state of BF;. The use of different filters shows that emission is predominantly in the UV below 300 nm, and this is compatible with predictions from photoelectron spectroscopy. There is no evidence for emission from the fi or states of BF: at 20.0 eV (621 A) and 18.9 eV (655 A) respectively.(v) Non-fluorescing Halides of Groups ZII-VZ. Of all the halides of groups 111, 33 M. E. Jacox,J. Mol. Spectrosc., 1980,80,257. Decay Properties of Excited Electronic States of Polyatomic Molecular Ions I I 1 120 22 24 26 Energy/eV Figure 11 Undispersedpuorescence of BF3 excited by VUV photons in the range 18 -28 eV Photons in the range 23WOO nm are collected and the signal har been normalized to the synchrotronjux 10 IV, and VI which have been studied, radiative decay has not been observed from excited electronic states of CClt, CBrt, and SF6+ We estimate that, given a favourable partial ionization cross-section, it should be possible to detect radiative decay from an electronic state with fluorescence quantum yield greater than ca in our apparatus Thls means that for these states of CCli, CBrl, and SF6+ non-radiative decay (eg by fragmentation) is occurring at least one thousand times faster than any radiative decay process B.Single-bunch (Pulsed) Experiments.-Radiative lifetimes were measured using the single-bunch mode of the synchrotron source The pulse width is ca 200 ps, the flight time around the ring 320 ns, and this is a superb source for measuring radiative lifetimes in the range 1-100 ns with a high degree of precision Lifetimes outside this range (especially smaller values) can be measured, but careful deconvolution techniques for the excitation pulse need to be employed Data accumulate rapidly due to the high repetition rate (3 MHz) of the source, so these experlments can be performed very quickly Results are shown in Tables 2 and 3 Table 2 gives the lifetimes of fluorescing states of parent molecular ions, whereas Table 3 gives values for neutral fragments formed by resonant photodissociation of a Rydberg state of the parent neutral molecule [Sections 4 A(iii) and 4 A(iv)] For strong fluorescing states of parent ions, lifetimes z were measured at Tuckett Table 2 Radiative lifetimes of jluorescing states of polyatomic molecular ions studied in this work Electronic Adiabatic Ion State IPIeV Tins Rej CF4f b 2A1 25.1 2.1 34 C2T2 21.7 9.7-8.6 9 SiF4f b 2A1 21.5 9.3 9 GeF2 b 'Al 21.3 3.1 and 6.3 9 SiCl,' C 2T2 15.1 38.4 9 GeC14f C 2T2 14.6 65.4 9 SiBr4f C 'T2 13.8 47.6 23 GeBr4f C 2T2 13.4 67.1 23 CF3H' b 'Al 20.0 82 27 CFjCl' f? 2A1 19.9 27 27 BF: f? 'A; 21.5 11.8 35 BClS 2A; 17.7 17.3 35 BCI: b 2E' 15.3 13.8 35 Table 3 Radiative lifetime ofjluorescing states of neutral fragments studied in this work Fluorescence Precursor Excitation Collection Molecu le EnergyleV Regionlnm T/ns Assignment Ref: CF3H 11.7 200-400 17.3 CF3 UV band 27 11.7 400-560 17.5 CF3 visible band 27 15.7 23U00 51.8 CF2 W 'B1 27 CFjCl 15.9 230-400 46.3 CF2 'B1 27 CF2C12 13.0 230-400 47.7 CF2 A 'B1 27 19.4 230-400 26.8 CF A 'C+ 27 BF3 13.1 230-400 47.5 BF2 UV band 1 35 14.0 230--400 16.1 BF2 UV band 2 35 BC13 10.7 23CL-400 26.2 BCl2 35 10.7 42@--560 24.0 BCl2 35 several excitation energies above threshold.Since T-' = krad + knon-rad, these experiments were performed because a change in z would indicate the presence or even onset of a competing non-radiative decay channel of that electronic state. Thus lifetimes were measured as a function of energy for CFZ c,SiF: B, 34 J. E. Hesser and K. Dressler, J. Chem. Phys., 1967,47, 3443. 35 J. C. Creasey, I. R. Lambert and R. P. Tuckett, in preparation. 459 Decay Properties of Excited Electronic States of Polyatomic Molecular Ions SiCl,' c,GeC1; c, and SiBr,' c. In the other cases the weakness of the signal and/or the limited time available at the SRS for single-bunch experiments precluded these measurements. Surprisingly and rather disappointingly, only in the case of CF; c were we able to observe a change of T with energy,g and even then the change is small (Table 2); the lifetime decreases from 9.7 ns just above threshold to a minimum of 8.6 ns at ca.4 eV above threshold. This result suggests that a non-radiative channel is competing weakly with radiative decay, and this is compatible with the non-unity fluorescence quantum yield for this state of 0.5 0.4 [Section 4.A(i)]. In fact this channel is fragmentation to CFZ + F2 or (F + F) (Section 5), and it is occurring at a relatively slow rate so that radiative decay can be observed with reasonable sensitivity. For the other four ionic states mentioned above, no variation of r with energy is observed. This strongly implies that no non-radiative channel is competing with fluor- escence, and therefore <Dh, has its maximum value of unity in all four cases.The large values of the lifetimes (especially for the chlorides and bromides) appear to confirm this. There are several extra comments to make about the data in Tables 2 and 3: (a) For the other parent ions, lifetimes were measured at the peak of the excitation spectrum [e.g. that of BF; was measured at 23.2 eV (Figure 1 l)]. (b) We now believe that the bi-exponential decay observedg in GeF,' is an artefact of how the electronics were set up for that particular measurement. In no other cases are anything other than single exponential decays observed. (c) There is some controversy about the electronic spectroscopy of CF3.Suto and Lee36 believe that the two emission bands in the UV and visible emanate from different electronic states, whereas our lifetime data seem to suggest that only one state is involved in radiative decay. (d) The lifetime of CF2 A has been measured by several workers in totally different experiments, and our value of ca. 50 ns is in excellent agreement. (e) Our value for the CF A2X+ state lifetime agrees well with that of a recent measurement by Booth and Han~ock,~' and disagrees with the earlier accepted value of ca. 19 ns. (f) The very different lifetimes of the two strong resonant peaks in BF3 suggest that two fluorescing states of the fragment are being formed. BF2 seems the most likely candidate, and ab initio calculations of the energies of the ground and excited electronic states, electronic transition moments, and radiative lifetimes would be most timely.5 Experiments to Probe Non-radiative Decay of Excited Electronic States of Polyatomic Molecular Ions We have used the coincidence apparatus described in Section 2 to probe the non- radiative decay channels of electronic states of the group IV tetrahalide ions. We are especially interested in those excited states which do not decay by photon 36 M. Suto and L. C. Lee, J. Chem. Phys., 1983,79, 1127. 37 J. P.Booth and G. Hancock, Chem. Phys. Lett., 1988,150,457. 0 2 It 6 8 TOF/ys 2 8 10 Tuckett emission or have a fluorescence quantum yield which is less than unity, since a non-radiative channel must then be operative.To date, we have made an extensive study of fragmentation of CF,', SiF:, CCl,', SiCl,' and GeC1,' (ref. lo), SiBr,' and GeBr; (ref. 38), and SF; (ref. 39). Data have not been obtained for GeF,' and CBr:, in the former case because GeF4 was found to poison the microchannel plates, in the latter case because CBr4 has too low a vapour pressure. We have unanalysed data for CF3H+, but have not yet studied the three chlorofluoro- carbon ions CF3C1+, CF2Clz, and CFC13f because they have been studied in a more conventional PIMS apparatus el~ewhere.~' Since this work involves the formation of fragment ions, we need to know the energies of the different ionic fragmentation channels.Values are given in Table 1 for the possible channels from CF;, SiF,', CCl,', and SiCl,' up to ca. 25 eV. These thermodynamic energies are calculated from heats of formation of the neutrals and ionization potentials of the fragments. Full details and the sources of the data are given el~ewhere.~~~~ Two points should be made. Firstly, it should be noted that several dissociation channels are energetically open to the c and n states of MX,', so observation of radiative decay from these states is a surprising phenomenon. For example, CF2 c 2T2 lies above channels dissociating to CF,f and CF;, yet this state fluoresces admittedly with a quantum yield which is less than unity [Section 4.A(i)]. Its B2A1 state at 25.1 eV has six different ionic channels open to it, yet it fluoresces strongly almost certainly with a quantum yield of unity.Secondly, the fluorides and chlorides behave very differently. For example, the B2A1 state of CClt has the same equivalent six dissociation channels open that are open to CF,' o2A1, yet it decays as one might expect non-radiatively to one or more of these channels. Thermodynamic data for the two bromides studied and SF; can be found in the respective paper^.^^.^^ A. Fragmentation of the Fluorides of Group 1V.l'-Non-radiative decay of CF4 and SiF4 excited by vacuum UV radiation was studied from below the energy of the 22T1 ionic state to above that of the highest valence ionic state B 2A1. Thus for CF4, the range 48k82081 was covered in six wavelength scans of ca.70& each with a small overlap between scans. The colour three-dimensional map incorporating all the scans 'spliced' together is shown in Figure 12 (see opposite p. 460). Only two ion peaks are observed, CF: for h < 572A and CF: for h < 785A, and the ion yield curves as a function of photon wavelength are shown in Figures 13 and 14. Two thresholds are apparent for CF: production at 785A (15.8 eV) and 720A (17.2 eV), and these correspond to the adiabatic energies of the 2and A states of CF:. CFZ does not turn on at its thermodynamic energy of 14.7 eV or 843A. Similarly a threshold for CF; production is observed at 572A (21.7 eV) which is the adiabatic IP of the C2T2 state of CFZ. Again CF2f does 38 J. C. Creasey, I. R.Lambert, R. P. Tuckett, K. Codling, L. J. Frasinski, P. A. Hatherly, and M. Stankie-wicz, unpublished data. 39 J. C. Creasey, I. R. Lambert, R. P. Tuckett, K. Codling, L. J. Frasinski, P. A. Hatherly, M. Stankiewicz, and D. M. P. Holland, J. Chem. Soc., Faraday Trans.,submitted. 40 H. W. Jochims, W. Lohr, and H. Baumgartel, Ber. Bunsenges. Phys. Chem., 1976,80,130. 461 Decay Properties of Excited Electronic States of Polyatomic Molecular Ions Energy /eV 18 17 16 ri I 1 ..... , . :.,:. ............... ...................................... ..... L-760 760 800 ...................I .... 4 0-..........*'--.,.&~\.'.......~,~.*..L,.A,..-...... 0 4 Figure 13 Photoionization yield curve of CFS from CF4 in the range 690-820A.The two thresholdsfor CFS production are marked (Reproduced by permission from J. Chem. Phys., 1990,93,3299.) not turn on at the lowest thermodynamic channel forming CFZ of 19.2 eV or 645A.It is important to appreciate that this is the opposite of what is usually observed with polyatomic molecular ions (e.g. CH; 41), where the fragment ions appear at their thermodynamic threshold. This is almost certainly due to the totally repulsive nature of the ground state of CF2. This state dissociates promptly to CF; +F, and dissociation of high vibrational levels of CFi is not a route for fragment ion production. This work shows that the non-radiative decay channel of CF; c which competes with radiative decay is fragmentation to CFZ.It is not easy to say whether the other product is F2 or F +F. In principle this should be possible because the former channel involves a KE release of 2.5 eV at threshold, the latter only 0.9 eV (Table I), and the width of the CF; ion peak in the TOF spectrum is related to the KE release. In practice it is not so simple" because many different vibrational levels of CF4f c contribute to the KE release when the photon energy exceeds threshold, and the available energy may also be channelled into vibration and rotation of the molecular fragments. The fact that fragmentation is slow enough that fluorescence can compete does perhaps suggest that one of the products involves the formation of a new chemical bond 41 0.Dutuit, M.Alt-Kaci, J. Lemaire, and M. Richard-Viard,Phys. Scr., 1990, T31,223: Ref. 12, p. 274. 462 Tuckett I I 1 1 I I .....;.., ,;... # ,.I,.... ;:...................... * -.... ........,::\*. 8. . ......,” ....:.:::*....... .-. In c a+ .-c +N 1u, i..,*.,U .... ..........r ........... ..“.*.-,,> ....‘..d./,..‘~.~-.....:_.~-~_.~hucc.0-. 0 I I I I I I 1 I I Figure 14 Photoionization yield of CF: from CF4 in the range 480-640A. The threshold for CF: production is marked (Reproduced by permission from f. Chem. Phys., 1990,93,3299) (i.e. CF2f + Fz). As mentioned in Section 2, accurate interpretation of KE releases in this form of experiment needs the internal energy of the parent ion to be defined uniquely, and this is most easily accomplished using a threshold electron detector.No new thresholds are observed for CFZ, CFZ, or any other ion at the CF; state adiabatic energy of 493A (25.1 eV), and this is confirming evidence that this state decays only by photon emission. With SiF4 only SiF: and SiF: are observed in the photon range studied (550-775A). Despite being energetically accessible SiFzf is not observed. The SiF; ion yield curve shows a threshold at the adiabatic energy of the % 2T1 state (770A or 16.1 eV), as expected. The SiF? curve (Figure 15) shows thresholds at 763A (16.2 eV) and 718A (17.3 eV). The former is the thermodynamic energy of the SiF; + F channel, the latter the adiabatic energy of SiF; A2T2. This result can be explained as follows.Unlike CFZ, the ground state of SiFz does have a minimum in some part of its multi-dimensional potential energy surface, and therefore production of SiF: via dissociation of excited vibrational levels of SiF: x is the lowest energy route. SiFZ A however is totally repulsive, dissociating promptly by loss of a fluorine atom to SiF: + F, and therefore a second threshold is observed at the A state adiabatic energy. No increase in SiF: production is observable at the SiFZ 2T2 state energy of 642A (19.3 eV). This is disappointing because this state has an unmeasurably low fluorescence quantum yield l6 [Section 4.A(i)], so non-radiative decay must dominate. 463 Decay Properties of Excited Electronic States of Polyatomic Molecular Ions I I I .':.."..:. ::, . ........... .. , .. ,......... , ............. . .'....... )r .* . c.-Y, c Q) ........... 1 .c .-t ...............' . :,. + . ... ......... ,..', , ., .*. I .* ...... . m L.- v) .'._I...* ......... 0- .............,.....;.. ..- 0 1 1 I Figure 15 Photoionization yield curve of SiF: from SiF4 in the range 655-775A. The two thresholds for SiF: production are marked (Reproduced by permission from J. Chem. Phys., 1990,93,3299) Energetically SiF3f + F is the only accessible channel (Table l), and therefore an increase in SiF; signal would be expected at this energy. This highlights particularly well one limitation of having no electron energy analysis in our PEPICO apparatus.At any photon energy where several electronic states of SiF; are accessible, the SiF3f signal has contributions from all those states which fragment to SiF3f + F. Since the sum of the partial ionization cross-sections into SiF; x, A,and P is much greater than into around 20 eV,42 we are attempting to observe a very small increase in SiF: signal against a very much larger signal already present. No new thresholds are observed at the energy of SiF: fi (576A or 21.5 eV) because this state decays radiatively with unity fluorescence quantum yield. B.Fragmentation of the Chlorides of Group IV."-The results for fragmentation of CC14 when excited by vacuum UV photons are perhaps the most interesting of the group IV tetrahalide ions studied.Figure 16 (see opposite p. 461) shows that three ions are present over the photon range studied (56&-790A), CCI' for B. W. Yates, K. H. Tan, G. M. Bancroft, L. L. Coatsworth, and J. S. Tse, J. Chem. Phys., 1985, 83, 4906. Tuckett h < 645A (19.2 eV), CC1; for h < 760A (16.3 eV), and CCl,' over the complete wavelength range, and it should be noted that the adiabatic energies of CCl,' b and c are 19.3 and 16.3 eV respectively (Table 1). Thus CCl,' turns on at the e state adiabatic energy, CC1' at the b state, and these non-radiative fragmentation rates are presumably fast enough to dominate any much slower radiative decay. Thus the fluorescence quantum yields of these two states are unmeasurably low [Section 4.A(ii)].The question immediately arises as to whether these decay channels are unique, e.g. does CCli b only fragment to CCl+? This is difficult to answer with total confidence because any competing fragmentation pathway of either state to, say, CCl,' + C1 would be difficult to observe in the CCl,' ion yield curve because of the lack of energy analysis in the electron TOF spectro- meter. However, if the channels are unique, and assuming all three ions are collected with equal efficiency, then at any photon energy above the CCl,' b state the ratio of the CC1+ to CCl,' to CCl,' intensities should equal the ratio of OD to (TC to (OX + (TA + oB), where (~iis the partial ionization cross-section into electronic state i. The ratio of the ion yields is easily obtained by computer integration of the three peaks in the TOF spectra.Unfortunately cross-section data are very limited in this photon range,43 and as a general statement this work has highlighted the need for much more experimental data on partial ionization cross-sections of valence states of polyatomic ions in the vacuum UV. The comparison cannot therefore be made. However, we comment that this is one advantage of using a TOF mass spectrometer rather than a quadrupole as a means of ion detection in a photofragment experiment, since in the former case quantitative comparisons can be made accurately between the intensities of the different fragment ions. The results for SiCl,' and GeC1,' fragmentation are not so informative. Photofragmentation was studied from below the energy of the c state to above that of the b state, and in both cases only two ions are clearly present, the parent ion MC1i and the trichloride ion MCl:.The e state of both parent ions fluoresce with a long radiative lifetime (Section 4.B), so no new fragmentation channels are to be expected at the c state adiabatic energies; this is indeed the case. Unlike the fluorides, the d states of MC1,' do not fluoresce, so a non-radiative fragmentation pathway is to be expected. No new channel is immediately apparent from the colour maps. However by careful comparison of the TOF spectra above and below the b state adiabatic energies (Section 2), a threshold to SiCl,' is observed for energies above SiCl,' d, and two thresholds to GeCl+ and GeC1,' for energies above GeC14f b.The channels are weak presumably because the partial ionization cross-sections of both Sic14 and GeC14 into their b ionic states are very low. C. Fragmentation of the Bromides of Group IV.38-Fragmentation of both SiBr4 and GeBr4 has been studied from below the energy of the 2ionic state to above that of the 0 state. Both gratings of the Seya were needed to cover this large 43 T. A. Carlson, M. 0.Krause, F. A. Grimm, P. Keller, and J. W. Taylor, J. Chem. Phys., 1982,77, 5340. 465 Decay Properties of Excited Electronic States of Polyatomic Molecular Ions range (ca 50Cr1200A) The data were recorded only very recently at Daresbury, and they have not yet been properly analysed, however the broad features are apparent from the colour maps Not surprisingly SiBrZ and GeBr2 appear to show similar fragmentation pathways Like the chlorides two main peaks are observed, MBr2 and MBr; The parent ion peak turns on at the 8 ionic state adiabatic energy, whereas the tribromide ion turns on at the A state energy The monobromide ion MBr’ turns on at the adiabatic energy with the dibromide ion appearing at even higher energy As expected, no new channel is observed at the energy of the state, since both SiBr: and GeBr4f c decay radiatively The MBr; results are the most interesting since they suggest that the A states of MBr2 are repulsive, dissociating promptly by loss of one bromine atom to MBr; + Br In other words the thermodynamic dissociation energy for MBr: + Br lies close to or more probably below the MBr2 A adiabatic energy, but above the 2state energy This is exactly the situation with CCli, SiC12, and GeC14f lo (Table 1) The energetics of the dissociation channels of SiBr2 and GeBrZ were given in one of our earlier papers,23 and the surprising point to emerge then was that the best available thermodynamics placed the SiBr; + Br channel ca 5 eV higher than that to be expected We commented at the time that this was a difficult observation to believe, and that an energy scanning PEPICO experiment was needed to determine the true dissociation energy of SiBr4 into SiBr: + Br The preliminary analysis of this experiment does indeed confirm that the SiBr3f + Br channel lies between 10 9 and 11 5 eV, and not at 16 2 eV as suggested earlier 23 Full details will appear later 38 There is no obvious discrepancy between the preliminary analysis of GeBr2 photodissociation and the thermo- dynamics in reference 23 D.Fragmentation of SF6+.jg--We have made an extensive study of photofrag- mentation of SFs excited with VUV photons from 440-810A The wavelength range extends from below the energy of the ground state of SFZ to above that of the highest valence electronic state SF6 and CF4 belong to closely related molecular point groups (Td is a sub-group of Oh),their molecular orbitals and photoelectron spectra have many similarities, and both have repulsive ionic ground states dissociating promptly by loss of a fluorine atom to SF: and CF: respectively 44 Therefore as with CF2, we hoped to observe radiative decay from high lying valence states of SF: As mentioned in Section 4A(v) no photon emission could be detected in our fluorescence excitation apparatus, therefore all valence states of SFZ must decay non-radiatively by fragmentation Berkowitz4’ has used conventional PIMS to study photofragmentation of SF6, but was limited to photon wavelengths > 600A due to the nature of the ionization source (the helium continuum) Thus the products from dissociation of the higher energy valence states of SF: were uncertain Our detailed results can 441 G Simm C J Danby J H D Eland and P I Mansell J Chem Soc Faradai Trans 1976 72 45 Ref 12 p 325 Tuckett be found el~ewhere,~' and only a brief summary is given here.Thresholds for SFf production are observed at the adiabatic energies of the x 2T1g and first two excited states A/B 2Tlu/2T2uof SF;. These states lie 1.5 and 2.7 eV above the SFC + F thermodynamic dissociation channel. The ion yield curve for SF: turns on at the adiabatic energy of the third excited state of SF:, C2Eg; this lies 1.2 eV above the lowest channel forming SF:. SF; thresholds are observed at the adiabatic energies of both the fourth (02T2,) and fifth (B 2T1u) excited states of SF:, whilst SF; turns on at the adiabatic energy of the highest valence ionic state of SFL (F2Al,). Thus in all cases SF: (n = 2-5) shows a threshold at an adiabatic energy of an electronic state of SF:, and not at the lowest thermo- dynamic dissociation channel forming that ion.This is exactly what we observe with CF:, and arises because of the totally repulsive nature of the ground electronic state. Dissociation via high vibrational levels of this state is therefore not a route for fragment ion production. As with CCli (Section 5.B) the question arises whether these fragmentation channels are unique; for example, does SF6+ c only fragment to SF: or is there a competing channel to SFf ? Partial ionization cross-sections or photoelectron branching ratios are available for all the valence states of SF6f,46 so a comparison can now be made between the relative yields of SF:, SF:, SF; with that expected from the cross-section data.The latter results overestimate the yields of SF; and SFZ to SFSf, and this may be evidence that the c,0,and states of SF; do fragment to SF: in competition with dissociation to SF: and SF:. From the widths of the peaks in the TOF spectra, KE releases have been measured in SF:, SF:, and SF:. Interpretation of the values for SFf and SF: are consistent with dissociation of SF: + A/B and c to (SF; + F) and (SF:+ F2) respectively. The SF: value cannot be used to extract a unique total KE release because dissociation of SF; to SF; is necessarily many bodied, involving more than two products. 6 Conclusions I have not attempted to give a full review of the applications of tunable vacuum UV photons from a synchrotron source to gas phase molecular photophysics.Instead I have concentrated on its application to study a particular area of chemical physics into which I came from a spectroscopic background. Much work has been done on fragmentation of both singly- and double-charged ions using a synchrotron as a photon source, and the full range of coincidence techniques has been applied to a large number of diatomics and polyatomics. However, rather to our surprise, no other group has combined the powerful combination of the PEPICO coincidence technique with the step-by-step tunability of the photon source as described in this review. Fluorescence excitation spectroscopy using synchrotrons has generally been limited to wavelengths above the lithium 46 B. M. Addison-Jones, K. H.Tan, B. W. Yates, J. N. Cutler, G. M. Bancroft, and J. S. Tse, J. Electron Specfrosc. Relat. Phenom., 1989,48, 155. "L. C. Lee, J. C. Han, C. Ye, and M. Suto, J. Chem. Phys., 1990,92,133. Decay Properties of Excited Electronic States of Polyatomic Molecular Ions fluoride cut-off (h> 106 nm), precluding the need for differential pumping, and only one other group (L C Lee and co-workers, refs 32, 47) is working in the vacuum UV down to 50 nm I believe that the main conclusions to come out of this work and future areas of investigation are as follows (1) The observation of radiative decay from high lying valence electronic states of polyatomic ions is not confined to the highly symmetrical tetrahedral species like CF; Fluorescence has now been observed from excited states of BFZ and BCl: (of D3h symmetry)35 and CF3H' and CF3Clf (of C3" symmetry),27 and we are confident that this effect will be observed in other species (2) Referring specifically to the group IV tetrahalides MX;, it would be interesting to understand why for a given X the carbon species behaves differently from silicon and germanium, and for given M the fluoride behaves differently from the chloride and bromide The different decay properties of the D2A1 state of the chloride/bromide ion from that of the fluoride could be a consequence of spin-orbit coupling effects in the heavier species creating non- radiative decay channels via doubletquartet coupling These problems can only be addressed if detailed positions of the potential curves of both doublet and quartet valence states of MX; are known Ab initio calculations have recently been reported on the energies of the five valence doublet electronic states of MF4+,48 but there have been no calculations yet on the quartet states or on any of the states in MC1; and MBr: (3) All the molecular ions studied have repulsive or scarcely bound ground and low-lying excited electronic states These states arise from electron removal from halogen non-bonding p7c orbitals (where the halogen p orbitals are orthogonal to the M-halogen bonds), and tend to have structureless photoelectron bands By contrast the states from which radiative decay has been observed are formed by electron removal from M-halogen o-bonding orbitals, and often have vibra- tionally resolved structured photoelectron bands This is surprising, because it is not at all obvious why the latter ionic states tend to have deep minima in their potential curves and hence are more likely to decay radiatively by the relatively slow process of photon emission than the former states (4)It is difficult to estimate accurately fluorescence quantum yields of radiating electronic states by the method described in Section 4A(i), and data for partial ionization cross-sections need to be known @hv is best estimated by a coincidence technique, eg the PIFCO meth~d,~and we are building an apparatus to perform these measurements Perhaps the strongest indication of a fluorescence quantum yield of unity is the absence of variation of T with excitation energy (Section 4 B), but it can only be an indication (5) The combination of a continuously tunable photon source and TOF detection for fragment ions is a powerful tool for investigating non-radiative fragmentation of electronic states of polyatomic ions We believe our technique has several advantages over conventional PIMS * (which were described in 48R A Bearda H R R Wiersinga J F M Aarts andJ J C Mulder Chem Phys 1989 137 157 Tuckett Section 2), which help to countenance the difficulties of working within limited time schedules at a synchrotron source.Fragment thresholds are observed to 5A in most of our work. This resolution is determined by the slit width of the Seya monochromator, and in principle thresholds could be measured much more accurately if unlimited beam time was available. (6) The most interesting point to come out of the fragmentation work is that when the ionic ground state is repulsive (e.g.in CF; and SF:44) fragment ions have appearance thresholds at the adiabatic energies of electronic states of the parent ion, and not at the lowest thermodynamic threshold forming that ion. This is not the expected pattern for large polyatomic ions, but rather that which is observed with diatomic ions. For polyatomics the conventional view is that the initial ionization event (whether by electrons or photons) serves only to create the molecular ion in a range of electronic states, but equilibration takes place so rapidly that one need only consider the density of vibrational states of the electronic ground state of the ion in determining the fragmentation pattern.This pattern can be determined by statistical methods [e.g. quasi-equilibrium theory (QET)] where one calculates the probability for dissociation of a bond to form a fragment ion at a certain total energy. In QET the fragment ions have thresholds at their thermodynamic energy, and any excess energy appears mainly as internal energy of the fragments, not as translational energy. However with CF: and SF:, in effect because the ionization potentials of CF3 and SF5 are small compared to CF4 and SFs, the state of these ions lies above the lowest dissociation channel to CFZ or SFC + F. These states are therefore repulsive.QET cannot apply to the fragmentation of CF: or SF:, and one can only form the smaller fragment ions (e.g.CF;, SF:) by non-radiative decay of excited states of CF: or SF; which lie above their thermochemical energies. The excess energy is (partially) converted into KE release in the fragments, and in the ions this manifests as a broadening in the width of the TOF peak. As mentioned several times in this review, accurate determination of these KE releases is not possible in our apparatus because the internal state of the parent ion initially populated by the vacuum UV photon is not defined uniquely. This is most easily accomplished with a threshold electron detector, and this modification is being incorporated into the new apparatus we are currently building.Acknowledgements. It is a pleasure to acknowledge the help of my research group (J. C. Creasey, Dr. P. A. Hatherly, I. R. Lambert, and Dr. S. M. Mason) in this work. I am especially grateful to Professor K. Codling and Dr. L. Frasinski of Reading University for a fruitful collaboration, since the fragmentation work uses a modified form of one of their apparatus, and to Dr. M. Stankiewicz of Krakow University who wrote much of the software for this experiment. I would like to thank Dr. A. Hopkirk of the Daresbury Laboratory who was involved in the design and construction of the fluorescence excitation apparatus, and others at Daresbury (especially Drs. M. Hayes, D. M. P. Holland, D. A. Shaw, and J. B. West) for help and encouragement. The financial support of the Science and Engineering Research Council is acknowledged.Decay Properties of Excited Electronic States of Polyatomic Molecular Ions Note added in prooJ Fluorescence following vacuum UV photoexcitation of BF3 has very recently been reported by Lee et al. (Phys. Reu. A, 1990,42,424). They see the same ‘BF2’ bands that we observe (Figure 10) at low photon energies, but do not observe emission from excited states of the parent ion BF3’ (unlike our results of Figure 11). They measure lifetimes for the two BF2 bands which are microsecond range. These are indirect measurements, and are much greater than our values (Table 3) of 16 and 47 ns. The reasons for these discrepancies are as yet unclear.

ISSN:0306-0012    

DOI:10.1039/CS9901900439

出版商:RSC    

年代:1990

数据来源: RSC

摘要:

INDEXES Volume 19,1990 The indexes in this issue cover Volumes 1-19 (Figures in bold type refer to the volume number) INDEX OF AUTHORS Aarons, L. J., 5,359 Ackroyd, J., 11,321Ager, D. J., 11,493 Ahlberg, P., 18,209 Ahluwalia, J. C., 2,203 Allen, N. S., 4, 533; 15, Angyal, S. J., 9,415 Ambroz, H. B., 8,353 Atkinson, D., 8,475 Attygalle, A. B., 13, 245 Baker, A. D., 1,355 Bamfield, P., 13,443 Barker, B. E., 9, 143 Barron, L. D., 15,261 Bartle, K. D., 10, 113 Bartlett, P. D., 5, 149 Baxendale, J. H., 7, 235 Beattie, I. R., 4, 107 Beddell, C. R., 13,279 Beer, P., 18,409 Bell, R. P., 3, 513 Belson, D. J., 11, 41 Bender, C. J., 15,475; 17,317 Bentley, P. H., 2, 29 Berkoff, C. E., 3,273 Billington, D.C., 14, 93; 18,83 Bird, C. L., 10,49 Bird, C. W., 3, 309 Blackburn, B. K., 17,147 Blandamer, M. J., 4, 55; 14,137 Blundell, T. L., 6, 139 Boelens, H., 7, 167 Bolton, R., 15,261 Bowman, W. R., 17,283 Bradshaw, T. K., 6,43 Braterman, P. S., 2, 271 Brennan, J., 17, 1 Bresciani Pahor, N., 18, 225 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 Bryce-Smith, D., 15, 93 Brycki, B. E., 19,83 Buchanan, G. L., 3,41; 17,91 Bulman Page, 19, 147 Burdett, J. K., 3, 293; 7, 507 Burgess, J., 4, 55; 14, 137 Burnett, M. G., 12, 267 Burrows, H. D., 3,139 Burtles, S. M., 7, 201 Butler, A. R., 16, 361 Butterworth, K. R., 7, 185 Cadogan, J.I. G., 3,87 Cameron, M., 19,355 Carabine, M. D., 1,411 Cardin, D. J., 2,99 Carey, P. R., 19,293 Carless, H. A. J., 1,465 Casellato, U., 8, 199 Cetinkaya, B., 2, 99 Chamberlain, J., 4, 569 Chandrasekhar, S., 16, 313 Chatt, J., 1, 121 Chesters, J. K., 10,270 Child, M. S., 17, 31 Chisholm, M. H., 14,69 Chivers, T., 2,233 Clark, G. M., 5, 269 Clark, R. J. H., 13, 219; 19,107 Collins, C. J., 4, 251 Colvin, E. W., 7, 15 Connelly, N. G., 18, 153 Connor, J. N. L., 5, 125 Corey, E. J., 17, 111 Corfield, G. C., 1,523 Cornforth, J. W., 2, 1 Cotton, F. A., 4,27; 12, 35 Coulson, E. H., 1,495 Covington, A. K., 14, 265 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 Craig, D., 16, 187 Cramer, R. D., 3,273Crammer, B., 6,43 1; 17, 229 Cross, R. J., 2, 271; 9, 185; 14,197 Curthoys, G., 8,475 Dack, M. R. J., 4,211 Dainton, F. S., 4, 323 Dalton, H., 8,297 Davies, D. I., 8, 171 Davies, S. G., 17, 147 de Rijke, D., 7, 167 de Silva, A. P., 10, 181 de Valois, P. J., 7, 167 Dickinson, E., 14,421 Dickinson, L. C., 12, 387 Dobson, J. C., 5, 79 Dowle, M. D., 8, 171 Doyle, M. J., 2, 99 Drummond, I., 2,233 Duffield, J. R., 15,291 Dunkin, I. R., 9, 1 Durant, G. J., 14,375 Duxbury, G., 12,453 Dymond, J. H., 14,317 Elliott, M., 7, 473 Emsley, J., 9, 91 Engberts, J. B. F. N., 14,237 Eschenmoser, A., 5,377 Evans, D. A., 2,75 Evans, J., 10, 159 Fenby, D.V., 3,193 Fensham, P. J., 13, 199 Fenton, D. E., 6, 325; 8, 199; 17,69 Ferguson, L. N., 4,289 Fisher, L. R., 6, 25 Fleming, I., 10, 83 Flygare, W. H., 6, 109 Forage, A. J., 8, 309 Garson, M. J., 8, 539 Georghiou, P. E., 6,83 Gheorghiu, M. D., 10, 289 Gibson, K. H., 6,489 Gilbert, J., 10, 255 Gilchrist, T. L., 12, 53 Gillespie, R. J., 8, 315 Glidewell, C., 16, 361 Goldschmidt, Z., 17, 229 Goodings, E. P., 5,95Goodrich, J. A., 14, 225 Gordon, I. M., 18, 123 Gordon, P. F., 13,443 Gorman, A. A., 10,205 Gosney, I., 16, 45 Gowenlock, B. G., 19, 355 Gray, B. F., 5,359 Gray, H. B., 15, 17 Grebenik, P., 17,453Green, C. L., 2,75 Greenhill, J. V., 6, 277 Greenwood, N.N., 3, 231; 13,353 Grey Morgan, C., 8,367 Grice, R., 11, 1 Griffiths, J., 1,481 Grigg, R., 16,89 Grimshaw, J., 10, 181 Grinter, R., 17,453 Grossert, J. S., 1, 1 Groves, J. K., 1, 73 Guilford, H., 2, 249 Gutteridge, N. J. A., 1, 38 1 Haines, R. J., 4, 155 Hall, G. G., 2, 21 Hall, L. D., 4,401 Hall, T. W., 5,431Halliwell, H. F., 3, 373 Hamdan, I. Y., 8, 143 Hamer, G., 8, 143 Harmony, M. D., 1,211 Harris, K. R., 5, 2 15 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 1 Hayward, R. C., 12,285 Heaven, M. C., 15,405 Heelis, P. F., 11, 15 Henderson, J. W., 2, 397 Hepler, L. G., 3, 193 Hilburn, M.E., 8,63 Hinchliffe, A., 5, 79 Hoffman, D. M., 14,69 Holbrook, K. A., 12, 163 Holland, H. L., 10,435; 11,37Holm, R. H., 10,455 Hooper, M., 16,437 Hore, P. J., 8, 29 Horton, E. W., 4, 589 Hough, L., 14, 357 Hounsell, E. F., 16, 161 Hudson, M. F., 4,363 Huffman, J. C., 14,69 Huntress, W. T.,Jun.,6, 295 Hutchins, G. J., 18,251 Hutchinson, D. W., 6, 43 Ibers, J. A., 11,57 Ikan, R., 6,431Isaacs, N. S., 5, 181 Isbell, H. S., 3, 1 Jaffe, H. H., 5, 165 James, A. M., 8, 389 Jameson, R. F., 18,477 Jamieson, A. M., 2,325 Janes, N. F., 7,473 Jencks, W. P., 10,345Jenkins, 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 Jones, P.G., 13, 157 Josh, C. G., 8, 29 Jotham, R. W., 2,457 Kalyanasundaram, K., 7,453 Katritzky, A. R., 13,47; 19,83Keeler, J., 19,381 Keenan, A. G., 8,259 Kemball, C., 13, 375 Kemp, T. J., 3, 139;8, 353 Kennedy, J. F., 2, 355; 8,221Kennewell, P. D., 4, 189; 9,477 Kenny, A. W., 4,90Kerridge, D. H., 17, 181 King, G. A. M., 7,297 Kirby, G. W., 6, 1 Kitaigorodsky, A. I., 7, 133 Klair, S. S., 19, 147 Koch, K. R., 6,393 Kochetkov, 19,29 Kolar, G. F., 9,241Korpela, T., 12, 309 Kresge, A. J., 2,475 Krishnaji, 7, 2 19 Index Kroto, H. W., 11,435 Kriiger, H., 11,227 Kuhn, A. T., 10,49 Lappert, M. F., 2,99 Lee, I, 19, 133,317 Lee, M. L., 10, 113 Lee-Ruff, E., 6, 195 Legon, A.C., 16,467; 19,197 Leigh, G. J., 1, 121; 4, 155 Lemieux, R. U., 7,423; 18,347 Leznoff, C. C., 3,65Lilley, D. M. J., 18, 53 Lindberg, B., 10,409 Lindley, J., 16, 275 Lindsay, D. G., 10,233 Lindoy, L. F.,4,421 Linert, W., 18,477 Linford, R. G., 1,445 Lipscomb, W. N., 1, 319 Liu, M. T. H., 11, 127 Lloyd, D., 16,45 Lorimer, J. P., 16, 239 Lynch, J. M., 3, 309 Lythgoe, B., 9,449Makela, M. J., 12, 309 McCleverty, J. A., 12, 33 1 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 Maier, J. P., 17,45 Maitland, G. C., 2, 18 1 Maitlis, P. M., 10, 1 Mann, B. E., 15, 167 Mann, J., 16,381 Manning, P. G., 5, 233 Maret, A.R., 2, 325 Markov, P., 13,69Marzilli, L. E., 18, 225 Maskill, H., 18, 123 Maslowsky, E., 9,25Mason, R., 1,431 Mason, S. F., 17,347Mason, T. J., 16, 239, 275 Mayo, B. C., 2,49 Meadowcroft, A. E., 4, 99 Index Menger, H. W., 2,415 Meyer, A. Y., 15,449 Midgley, D., 4, 549 Millen, D. J., 5, 253; 16, 467 Mills, A., 18, 285 Mills, R., 5, 215 Mingos, D. M. P., 15, 31 Mitchell, J. C., 14, 399 Moore, D. S., 12,415 Moore, H. W., 2,415; 10,289 Morgan, E. D., 13,245 Morley, R., 5, 269 Morris, D. G., 11, 397 Morris, J. H., 6, 173 Morris, J. L., 15, 1 Muller, J., 16, 75 Muetterties, E. L., 11, 283 Mulheirn, L. F., 1,259 Munn, A., 4,87 Murphy, P. J., 17, 1 Murphy, W.S., 12,213 Musumarra, G., 13,47 Newman, J. F., 4,77 Nightingale, W. H., 7, 195 Norman, N. C., 17,269 Norman, R. 0.C., 8, 1 North, A. M., 1,49 Noyori, R., 18, 187 Oakenfull, D. G., 6,25 O’Donnell, T. A., 16, 1 Ormiston, R. A., 16,45 Overton, K. H., 8,447 Page, M. I., 2,295 Paleos, C. M., 14,45 Papaconstantinou, E., 18, 1 Parker, D., 19,271 Paton, R. M., 18,33 Pattenden, G., 17,361 Paulsen, H., 13, 15 Pelter, A., 11, 191 Perkins, P. G., 6, 173 Perutz, R. N., 17,453 Pickford, C. J., 10, 245 Pindur, U., 16,75Pletcher, D., 4,471 Poliakoff, M., 3,293; 7, 527 Prakash, V., 7,219 Pratt, A. C., 6, 63 Pratt, J. M., 14, 161 Puddephatt, R. J., 12,99 Ramm, P. J., 1,259 Ramsay, J. D.F., 15, 335 Randaccio, L., 18,225 Rao, C. N. R., 5,297; 12,361 Ratledge, C., 8,283Rattee, I. D., 1, 145 Redl, G., 3,273 Redpath, J., 12, 75 Reid, G., 19,239 Rees, C. W., 15, 1 Richards, D. H., 6,235 Ritch, J. B., jun., 5,452 Roberts, M. W., 6, 373; 18,451 Robins, D. J., 18, 375 Robinson, F. A., 5, 3 17 Robinson, J. A., 17,383 Robinson, S. D., 12,415 Roche, M., 5,165 Rodgers, M. A. J., 7, 235 Roesky, H. W., 15,309 Rose, A. E. A., 6, 173 Rosenthal, S., 19, 147 Rouvray, D. H., 3,355 Rowlinson, J. S., 7,329; 12,251 Ruasse, M.-F., 18, 123 Russell, D. K., 19,407 Sanders, J. K. M., 6, 467 Sarma, T. S., 2, 203 Satchell, D. P. N., 4, 231; 6, 345; 19, 55 Satchell, R. S.,4,231; 19,55 Scheinmann, F., 11, 321 Schlegel,W., 7, 177 Schroder, M., 19,239 Scriven, E.F. V., 12, 129 Scurrell, M. S., 18, 251 Self, R., 10, 255 Senthilnathan, V. P., 5, 297 Sermon, P. A., 16,339 Sherman, L. R., 14,225 Sherwood, P. M. A., 14, 1 Shorter, J., 7, 1 Simonetta, M., 13, 1 Simpkins, N. S., 19, 335 Simpson, T. J., 4,497; 16,123 Singh, S., 5,297 Slorach, S. A., 10,280 Smith, E. B., 2, 181, 15, 503 Smith, I. W. M., 14, 141 Smith, J. A. S., 15,225 Smith, K., 3,443 Smith, K. M., 4, 363 Smith, W. E., 6, 173; 9, 217 Snell, K. D., 8, 259 Somorjai, G. A., 13, 321 Spiro, M., 15, 141 Stacey, M., 2, 145 Staunton, J., 8, 539 Staveley, L. A. K., 13, 173 Stevens, M. F. G., 7, 377 Stoddart, J.F., 8, 85 Stokes, R. H., 11,257 Strachan, A. N., 11,41 Suckling, C. J., 3, 387; 13,97 Suckling, K. E., 3,387 Sutherland, I. O., 15,63 Sutherland, J. K., 9,265 Sutherland, R. G., 1, 24 1 Sutton, D., 4,443 Sutton, K. H., 17, 147 Swan, J. S., 7,201 Swindells, R., 7, 212 Sykes, A. G., 14,283 Symons, M. C. R., 5, 337; 12, 1, 387; 13, 393 Takken, H. J., 7,167 Taylor, J. B., 4, 189; 9, 477 Taylor, S. E., 10, 329 Thea, S., 15, 125 Theobald, D. W., 5,203 Thibblin, A., 18,209 Thomas, T. W., 1,99Thompson, M., 1,355 Thornber, C. W., 8,563 Tincknell, R. C., 5,463Toennies, J. P., 3,407 Tolman, C. A,, 1,337 Tonge, P. J., 19,293 Trost, B. M., 11, 141 Truax, D. R., 5,411 Tuckett, R. P., 19,439 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 Vasapollo, G., 19,355 Vaughan, K., 7,377 Vidali, M., 8, 199 Vigato, P. A., 8, 199; 17, Volkov, S. V., 19, 21 Vollhardt, K. P. C., 9,41 Wain, R. L., 6,261Walker, E. R. H., 5,23 Walker, 1. C., 3,467 Waltz, W. L., 1, 241 Ward, 1. M., 3,231 Ward, R. S., 11, 75; 19, 1 Watkins, D. M., 9,429 Wattanasin, S., 12,213 Westwood, N. P. C., 18, 317 White, A. J., 3, 17 Whitfield, R. C., 1,27 Whittaker, M., 17, 147 Widom, B., 14, 121 Wieser, H., 5,411 Index Wiesner, K., 6,413 Williams, A., 15, 125 Williams, D. H., 13, 131 Williams, D.L. H., 14, 171 Williams, D. R., 15, 291 Williams, G., 7,89 Williams, G. H., 15, 261 Williams, R. J. P., 9, 281,325 Wilson, A. D., 7, 265 Wise, S. A., 10, 113 Witzel, H., 16, 75 Woodhouse, J. R., 18, 25 1 Yoffe, A. D., 5, 51 Zangrando, E., 18,225 Zeelen, F. J., 12,75 INDEX OF TITLES Abiotic receptors, Absorption bands in stars, a crystal 12,285 the spectra of field approach,5,233 Applications of e s r spectroscopy to kinetics and mechanism in organic chemistry, 8, 1 Applications of multinuclear NMR to structural and biosynthetic studies of polyketide microbial metabolites, 16,123 Application of research findings to the development of commercial flavour- ings, 7,177 Aqueous carbonate solutions, po-tentiometric titrations of, 14, 265 Aqueous mixtures, kinetics of reactions 1% 4, 55 Aqueous solution, micelles in, 6,25 Arsonium ylides (with some mention also of arsinimines, stibonium and bismuthonium ylides), 16,45 Arylation, homolytic, of aromatic and polyfluoroaromatic compounds,15,261 Aryl cations-new light on old inter- mediates, 8,353 -halides, photochemistry and pho- tocyclization of, 10,181 Aryldiazonium cations, co-ordination chemistry of, 4,443 Aryliodine(m) dicarboxylates, 10, 377 Atmosphere, interactions in, of droplets and gases, 1,411 Autocatalysis, 7,297 Azidoquinones and related compounds, chemistry of, 2,415 Azobenzene and its derivatives, photo- chemistry of, 1,481 B 2-Dependent isomerase enzymeshow the protein controls the active site, 14,161 B 2 Models, structural properties of organocobalt coenzyme, 18,225 Benzene compounds, substituted, syn- thesis from acyclic compounds,13,441 Bile pigments, 4,363 Binding of heavy metals to proteins, 6,139 Binding properties and chemistry of aluminium phosphates, 6,173 Bio-active molecules, structural studies 0n, 13,131 Biological surfaces, molecular aspects of, 8,389 Acetamide and acetamide complexes, the chemistry, 17,181 Acidity of solid surfaces, 8,475 Across the living barrier, 6,325 Activation parameters for chemical reactions in solution, 14,237 Acylation and alkylation catalysts,4-dialkylaminopyridines, super,12, 129 -by ketens and isocyanates, a mechanistic comparison, 4, 23 1 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 Aliphatic nucleophilic subs ti tu tion reac- tions, new insights into, from the use of pyridines as leaving groups, 13,47 Alkali-metal complexes in aqueoussolution, 4,549 Alkaloids, aconite, synthesis of, 6,413 Alkenes, the Friedel-Crafts acylationof, 1, 73 K-Allylnickel halides as selective rea- gents in organic synthesis, 14,93 Aluminium phosphates, the chemistry and binding properties of, 6, 173 Amines and alcohols, conformational analysis of, 5,411 Analysis of trace constituents of the diet, organic and inorganic, 10,245, 255 Analytical methods, modern, for en-vironmental polycyclic aromatic compounds, 10,113 Angular geometries of hydrogen-bonded dimers a simple electrostatic interpretation of the success of the electron pair model, 16,467 Anionic cyclization of phenols, 12, 213 Ants, chemicals from the glands of, 13,245 Aphids and scale insects, their chem- istry, 4,263 Application of electrochemical techni- ques to the study of homogeneous chemical reactions, 4,471 Biomime tic chemistry, 1,553 Biomolecular homochirality, 17, 347 Biosynthesis of pyrrolizidine alkaloids, 18,375 Biosynthesis of sterols, 1,259 Biosynthetic products from arachidonic acid, 6,489 -, studies, carbon-1 3 nuclear mag- netic resonance in, 4,497 _____ of polyketide microbial meta- bolites, applications of multinuclear n.m.r.to structural and, 16, 123 Bis( dipheny1phosphino)met hane, chemistry of, 12,99 Blood groups, human, and carbohy- drate chemistry, 7,423 Bonding in molecular clusters and their relationship to bulk metals, 15, 31 Bond strengths, CH, in simple organic compounds: effects of conformation and substitution, 7,399 valences-a simple structural model for inorganic chemistry, 7, 359 Boron reagents, carbon-carbon bond formation involving, 11,191 Bredt’s rule, 3,41 Br0nsted relation-recent develop-ments, 2,475 Brownian dynamics with hydrody-namic interactions: the application to protein diffusional problems, 14,421 Butadiene, polymerization and copoly- merization of, 6,235 Calciferols, hormonal: chemistry of ‘Vitamin’D, 6, 83 Calorimetric investigations of hydrogen bond and charge-transfer complexes, 3,193 Cancer and chemicals, 4,289 Carbohydrate chemistry and human blood groups, 7,423 Carbohydrate differentiation antigens, structural and conformational char- acterization of, 16,161 Carbohydrate-directed macro-molecules, transition-metal oxide chelates of, 8,221 Carbohydrate-proteincomplexes,glyco-proteins, andproteoglycans, ofhuman tissues, chemical aspects of, 2, 355 Carbohydrates to enzyme analogues, 8,85 Carbon-carbon bond formation in-volving boron reagents, 11, 191 Index Carbon-13 nuclear magnetic resonance in biosynthetic studies, 4,497 Carbonium ions, carbanions, and radi- cals, chirality in, 2,397 Carbonyl clusters, metal, relationship with supported metal catalysts,10,159 compounds, photochemistry of, 1,465 equivalents, silicon-containing,11,493 -group transpositions, 11,397 Carcinogens, chemical, mechanisms of reaction with nucleic acid, 9,241 Catalysis and coordination compounds involving electron-rich main group elements, 15,309 Catalysis and surface chemistry, new perspectives, 6,373 Catalysis, homogeneous, and organo- metallic chemistry, the 16 and 18 electron rule in, 1,337 ~ of the olefin metathesis reaction, 4, 155 Catalysts for oxygen and chlorine evo- lution, heterogeneous redox, 18,225 Catalysts, supported metal, relationship with metal carbonyl clusters, 10, 159 Cationic species in protonic superacids and acidic melts, stabilization of un-usual, 16, 1 CENTENARY LECTURE.Biomimetic chemistry, 1,553 CENTENARYLECTURE. Catalysis and coordination compounds involving electron-rich main group elements, 15,309 CENTENARY Chemical multi- LECTURE. plication of chirality: science and applications 18,187 CENTENARY Cyclopentanoids:LECTURE. a challenge for new methodology11, 141 CENTENARYLECTURE. Hydrocarbon reactions at metal centres, 11,283 CENTENARY Light scattering LECTURE. in pure liquids and solutions, 6, 109 CENTENARY LECTURE. Long-range electron-transfer in blue copper pro- teins, 15,17 CENTENARYLECTURE.Metal Clusters in biology, 10,455 CENTENARYLECTURE.Molecular In-gredients of heterogeneouscatalysis, 13,321 Index CENTENARYLECTUREOrganic reaction paths a theoretical approach, 13, 1 CENTENARYLECTURE Phase equili-brium and interfacial structure, 14,121 CENTENARY LECTURE Quadruplebonds and other multiple metal to metal bonds, 4,27 CENTENARYLECTURE Reactivities of carbon disulphide, carbon dioxide, and carbonyl sulphide towards some transition-metal systems, 11, 57 CENTENARYLECTURERotationally and vibrationally inelastic scattering of molecules, 3,407 CENTENARY LECTURE Systematicdevelopment of strategy in the synthesis of polycyclic polysubsti- tuted natural products the aconite alkaloids, 6,413 CENTENARYLECTURE Three-dimen- sional structures and chemical mech- anisms of enzymes, 1,319 Charge transfer and hydrogen bond complexes, calorimetric investiga-tions of, 3,193 Charge-transfer complexes, theoretical models of, 15,475 Chemical applications of advances in Fourier transform spectroscopy,4,569 -aspects of affinity chromato-graphy 2,249 --of glycoproteins, proteogly- cans, and carbohydrate-protein com-plexes of human tissues, 2,355 Chemical education, conceptions,misconceptions, and alternative frameworks in, 13,199 --and chemistry teacher educa- tion worldwide, new trends in, 17, 135 --research facts, findings, and consequences, 9,365 -interpretations of molecular wavefunctions, 5, 79 -models of enzymic transimin-ation, 12,309 processes on heterogeneouscatalysis, 13,375 reactions in molten salts and their reactions, 19,21 Chemically-induced dynamic electron polarization (CIDEP), role in chemistry, 8,29 Chemicals from the glands of ants, 13,245 in rodent control, 1,381 ~ which control plant growth, 6,261 Chemisorption and reaction pathways at metal surfaces the role of surface oxygen, 18,451 Chemistry and binding properties of aluminium phosphates, 6,173 CHEMISTRY AND FLAVOUR I Molecular Structure and Or-ganoleptic Quality, 7,167 I1 Application of Research Findings to the Development of Com-mercial Flavourings, 7, 177 111 Safety Evaluation of Natural and Synthetic Flavourings, 7, 185 IVThe Influence of Legislation on Re- search 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, Chemistry and the new industrial re- volution, 5,317 -, a topological subject, 2, 457 ~ of aphids and scale insects, 4,263 -of azidoquinones and related com- pounds, 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 molten acetamide and acetam- ide complexes, 17, 181 -of the production of organicisoc yanates, 3,209 ~ of peroxonium ions and dioxygen ylides, 14,399 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 philo- sophy of, 5,203 Chirality, chemical multiplications of, science and applications, 18, 187 Chirality in carbonium ions, car-banions, and radicals, 2,397 Chirality, symmetry, and molecular, 15,189 Chlorophyll chemistry, n.m.r.spectral change as a probe, 6,467 Chromatography, affinity, chemical as- pects of, 2,249 cis-and trans-Effects of ligands, 2, 163 Clathrates and molecular inclusion phenomena, 7, 65 Cobalt-mediated radical reactions in organic synthesis, 17,361 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 aqueoussolution, 4,549 -homo-and hetero-dinuclear, routes via compartmental ligands, 8, 199 -, l-D metallic, 9,429 Complexes, square-planar, isomeriza- tion mechanisms of, 9,185 Computer resolution of overlappingelectronic absorption bands, 9, 143 Conductivity and superconductivity in 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 Haworth’s concepts of, 3, 1 Conformational analysis of some al-cohols and amines: a comparison of molecular orbital theory, rotational and vibrational spectroscopy, 5,411 --of transition metal q ‘-acylcomplexes: steric interactions and stereoelectronic effects, 17, 147 -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 mech-anisms, 7, 1 Co-ordination chemistry of aryl-Index diazonium cations: aryldiazenato(arylazo) complexes of transition metals, and the aryldiazenato-nitrosyl analogy, 4,443 Co-ordination chemistry of C-nitroso- compounds, 19,355 Corrin synthesis, post-B12 problems in, 5,377 Crystal field approach to absorption bands in the spectra of stars, 5,233 Crystal structure determination: a criti- cal view, 13, 157 Crystals and molecules, organic, non- bonded interactions of atoms in, 7,133 Current aspects of unimolecular reactions, 12,163 Cyanocobalt(II1) complexes, the syn-thesis of mononuclear, 12,267 Cyanoketenes: synthesis and cyclo-additions, 10,289 Cyclization, initiation of, using 3-methylcyclohex-2-enone derivatives, 9,265 -of phenols, anionic, 12,213 Cyclopentanoids: a challenge for new methodology, 11,141 Cyclopolymerization, 1,523 Dakin-West reaction, 17,91 Dental cements, chemistry of, 7, 265 Designing drugs to fit a macro-molecular receptor, 13,279 Development of flavour in potablespirits, 7,201 4-Dialkylaminopyridines: super acyl-ation and alkylation catalysts,12, 129 Diazirines, the thermolysis and photoly- sis of, 11,127 P-Dicarbonyl compounds, light-induced tautomerism of, 13,69 Dielectric relaxation in polymer solu- tions, 1,49 Diels-Alder reaction, stereochemical as- pects of the intramolecular, 16, 187 Diffusion in liquids, the effect of iso- topic substitution on, 5,215 Diffusional problems, Brownian dyna- mics with hydrodynamic interactions: the application to protein, 14,421 Difluoroamino-radical, gas-phasekinetics of, 3, 17 Droplets and gases, interactions in the atmosphere of, 1,411 Index Drug design, isosterism and molecular modification in, 8,563 --, quantitative 3,273 Dyeing, chemistry of, 1,145 Dynamic decay properties of excited electronic states of polyatomic mole- cular ions studied with synchrotron radiation, 19,439 Echinoderms, 1, 1 Education, chemical, a reassessment of research in, 1, 27 Y , review of research and development in the U.K., 1972-1976, 7,317 Effect of isotopic substitution on dif- fusion in liquids, 5,215 Electrochemical techniques, applica-tion of to study of homogeneouschemical reactions, 4,471 Electrode and related structures, photoelectron spectroscopic struc-tures of, 14, 1 Electron as a chemical entity, 4, 323 pair model, angular geometries of hydrogen-bonded dimers: a simpleelectrostatic interpretation of the suc- cess of the, 16,467 -scattering spectroscopy, thresh- old, 3,467 spectroscopy, 1,355 Electronic absorption bands, overlap- ping, computer resolution of, 9, 143 Electronic properties of some chain and layer compounds, 5, 51 ~ transitions, vibrational intensities in, 5,165 Electrons, solvated, in solutions of metals, 5,337 Electron spin resonance of haemo-globin and myoglobin, 12,387 Electron-transfer, long range, in blue copper proteins, 15,17 Electrophilic aromatic substitutions, non-conventional, and related reac-tions, 3,167 -C-nitroso-compounds, 6, 1 Electrophoresis, historical develop-ment of sodium dodecyl sulphate- polyacrylamide gel, 14,225 Elimination reactions, isotope effect studies of, 1, 163 Enaminones, 6,277 Energetics of neighbouring group par- ticipa tion, 2,295 Enumeration methods for isomers, 3,355 Environmental chemical influences on behaviour and mentation, 15,93 -lead in perspective, 8, 63 -polycyclic aromatic compounds, modern analytical methods for, 10,113 -protection in the distribution of hazardous chemicals, 4,99 -regulation: an international view, 5,431 Enzyme analogues from carbohy-drates, 8, 85 Enzymes, immobilized, 6,215 -in organic synthesis, 3,387 -, the logic of working with, 2, 1 -of secondary metabolism in microorganisms, 17,383 -, three-dimensional structures and chemical mechanisms of, 1, 319 Enzyme-catalysed reactions, reactive intermediates in, 13,97 Enzymic reactions, stereochemical 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 hydrophobic solutes, 2,203 FARADAY The electron as a LECTURE.chemical entity, 4,323 FARADAY LECTURE. The molecular theory of small systems, 12,251 Fast reactions, techniques for the kine- tic study of, 11,227 Fats grown from wastes, 8,283 Fe(C014, 7,527 5-Substituted pyrimidine nucleosides and nucleotides, 6,43 Fixation, of nitrogen, 1,121 Flavins (isoalloxazines), the photo-physical and photochemical proper- ties of, 11,15 Fluorine, modern methods for introduc- tion into organic molecules: an approach to compounds with altered chemical and biological activities, 16,381 Fluorescence decay dynamics of the halogens and interhalogens, 15,405 Fluxionality of polyene and polyenyl metal complexes, 15, 167 Forces between simple molecules, 2,181 Foreign compounds in mammals, im- portance of non-enzymic chemical 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 Glass transition: salient facts and theo- retical models, 12,361 Glycoproteins, proteoglycans, and carbohydrate-protein complexes of human tissues, chemical aspectsof, 2,355 Glycoproteins, synthesis of complexoligosaccharide chains of, 13, 15 Gold drugs used in the treatment of rheumatoid arthritis, chemistry of, 9,217 Growth of computational quantumchemistry from 1950 to 1971, 2,21 Guanidine derivatives acting at his-taminergic receptors 14,375 Haemoglobin and myoglobin, electron spin resonance of, 12,387 Halogens and interhalogens, fluores- cence decay dynamics of, 15,405 Handling toxic chemicals-environ-mental considerations, 4, 77 Hard-sphere theories of transport pro- perties, 14,317 HAWORTH MEMORIAL LECTURE. The consequences of some projectsinitiated by Sir Norman Haworth, 2,145 HAWORTH MEMORIAL LECTURE.The Haworth-H udson controversy and the development of Haworth’s concepts of ring conformation and of neighbouring group effects, 3, 1 Index HAWORTH MEMORIAL LECTURE. The sweeter side of chemistry, 14, 357 HAWORTH MEMORIAL LECTURE. Human blood groups and carbohy- drate chemistry, 7,423 HAWORTH LECTURE.MEMORIAL Micro-bial polysaccharides: new ap-proac hes, 19,29 HAWORTH LECTURE.MEMORIAL Struc-tural studies of polysaccharides,10,409 HAWORTHMEMORIALLECTURE.Syn-thesis of complex oligosaccharidechains of glycoproteins, 13,15 Hazards in the chemical industry- risk management and insurance, 8,419 Health hazards to workers from in- dustrial chemicals, 4, 82 Heterocyclic compounds, prototropic routes to 1,3-and 1,5-dipoles, and ylides for the synthesis of, 16,89 Heterogeneous catalysis, chemical pro- cesses on, 13,375 High resolution laser spectroscopy,12.453 Histaminergic receptors, guanidinederivatives acting at, 14,375 Historical development of sodium dodecyl sulphate-polyacrylamide gel electrophoresis, 14,225 Homogeneous catalysis, and or-ganometallic chemistry, the 16 and 18 electron rule in, 1,337 Homogeneous chemical reactions, application of electrochemical techni- ques to the study of, 4,471 Homogenous pyrolysis, infrared laser powered, 19,407 Homolytic arylation of aromatic and polyfluoroaromatic compounds,15,261 Human blood groups and carbo-hydrate chemistry, 7,423 Hydrocarbon formation by micro-organisms, 3,309 -reactions at metal centres, 11, 283 Hydrogen bond and charge-transfercomplexes, calorimetric investiga-tions of, 3,193 ~ bonded liquids, thermodynamics of, 11,257 __ bonding, very strong, 9,91 ~ isotope effects, kinetic, recent advances in the study of, 3, 513 48 1 Index 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 foreign compounds in mammals, 9,63 Importance of solvent internal pressure and cohesion to solution phenom- ena, 4,211 Inclusion phenomena, molecular, and clathrates, 7, 65 Individual CH bond strengths in simple organic compounds effects of conformation and substitution, 7,399 Industry, chemical, hazards in risk management and insurance, 8,419 Influence of flavour chemistry on con- sumer acceptance, 7,212 Influence of legislation on research in flavour chemistry, 7, 195 Infrared and Raman vibrational spec- troscopy in inorganic chemistry,4,107 Infrared laser powered homogenouspyrolysis, 19,407 INGOLDLECTUREFour-membered rings and reaction mechanisms, 5, 149 INGOLDLECTUREHow does a reaction choose its mechanism? 10,345 Initiation of cyclization using 3-methyl-cyclohex-2-enone derivatives, 9,265 Inorganic chemistry, bond valences, a simple structural model for, 7, 359 Inorganic pyro-compoundsMa[(X207)bl, 5,269 myo-Inositol phosphates, recent de-velopments in the synthesis of, 18,83 Insect attractants of natural origin,2, 75 Insecticides, a new group of synthetic pyrethroids, 7,473 Interactions in the atmosphere of droplets and gases, 1,411 -, ion-solvent, thermodynamics 0f, 9,381 -, metal-metal, in transition-metal complexes containing infinite chains of metal atoms, 1, 99 -, non-bonded, of atoms in organic crystals and molecules, 7, 133 Introducing a new agricultural chem- ical, 4, 77 Ion-molecule reactions in the evolu- tion of simple organic molecules in interstellar clouds and planetary atmospheres, 6,295Ion-pairing, contribution to ‘memory effects’, 4,251 Ion-solvent interactions, thermo-dynamics of, 9,381 Ion transfer across model biological membranes, voltammetric studies of, 17,319 Isocyanates and ketens, a mechanistic comparison of acylation by, 4,231 --, organic, chemistry of the produc- tion of, 3,209 Isocyanic acid, preparation and proper- ties of, 11,41 Isokinetic relationship, 18,477 Isomer enumeration methods, 3, 355 Isomerization mechanisms of square- planar complexes, 9, 185 Isosterism and molecular modification in drug design, 8,563 Isotope effect studies of elimination reactions, 1, 163 Isotopic hydrogen exchange in purines mechanisms and applica-tions, 10,329 ~ substitution effects on diffusion in liquids, 5,215 JOHN JEYES LECTUREChemicals which control plant growth, 6,261 JOHN JEYES LECTUREEnvironmental chemical influences on behaviour and mentation, 15,93 JOHN JEYES LECTUREThe environ-mental chemistry of radioactive waste disposal, 15,291 KELVINLECTUREAcross 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 spec-troscopy to, 8, 1 -, gas-phase, of the difluoroamino- radical, 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 Lanthanides and actinides, macro-cyclic Schiff base complexes of, 17,69 Laser light scattering, quasielastic,2,325 Laser spectroscopy of ultra-trace quantities, 8,367 Lasers, tunable, 3,293 Lead, environmental, in perspective,8, 63 LENNARD-JONESLECTURE.Recent experimental and theoretical work on molecularly simple liquidmixtures, 13, 173 Leukotrienes; a new class of biologi- cally active compounds including SRS-A, the synthesis of, 11,321 Ligand substitution reactions of square-planar molecules, 14, 197 Ligands, cis-and trans-effects of, 2,163 -, compartmental: routes to homo- and hetero-dinuclear complexes,8, 199 Light-induced tautomerism of P-dicar- bony1 compounds, 13,69 Lignans and neolignans, the synthesis 0f, 11,75 Liquid mixtures, recent experimentaland theoretical work on molecularly simple, 13,173 Liquid, surface of a, 7,329 LIVERSIDGE 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 LIVERSIDGE Recent advances LECTURE. in the study of kinetic hydrogen isotope effects, 3,513 LIVERSIDGE The surface of a LECTURE. liquid, 7,329 Macrocyclic ligands, synthetic,transition-metal complexes of, 4,421 __ Schiff base complexes of lan-thanides and actinides, 17,69 Macromolecular receptor, designingdrugs to fit a, 13,279 Index Main-group elements, ring, cage, and cluster compounds of, 8,315 Matrix isolation technique and its application to organic chemistry, 9, 1 Measurement of effective charge in an organic reaction in solution,15,125 Mechanisms, chemical, and three-dimensional structures of enzymes, 1,319 --, isomerization, of square-planar complexes, 9, 185 -of hydrolysis of thioacetals, 19, 55 ___ of the microbial hydroxylation of steroids, 11,371 ~ of reaction between ultimate chemical carcinogens and nucleic acid, 9,241 Mechanisms of nucleophilic substitu- tion in aliphatic compounds, 19,83 Medicinal chemistry of anti-leprosydrugs, 16,437 MELDOLA ChemicalMEDAL LECTURE.aspects of glycoproteins, proteogly- cans, and carbohydrate-protein com-plexes of human tissues, 2,355 MELDOLA Fe(C0)4,MEDAL LECTURE. 7,527 MEDALLECTURE.MELDOLA Molecular collisions and the semiclassical ap- proximat ion, 5,125 MELDOLA MolecularMEDAL LECTURE.shapes, 7,507 MELDOLA MEDAL LECTURE. N.m.r. spectral change as a probe of chlorophyll chemistry, 6,467 MELDOLA MEDAL LECTURE. Organo- transition complexes incorporating bismuth, 17,271 MELDOLA Redox re- MEDAL LECTURE. sponsive macrocylic receptor molecu- les containing transition metal redox centres, 18,409 MELDOLA The rela- MEDAL LECTURE. tionship between metal carbonylclusters and supported metal catalysts, 10, 159 Meldrum’s acid, 7,345 Metal carbonyl clusters, relationship with supported metal catalysts,10,159 __ centres, hydrocarbon reactions at, 11,283 __ clusters in biology, 10,455 Index Metal-metal bonding and metallobor- anes, 3,231 ~ bonds of various orders, synergic interplay of experiment and theory in studying, 12,35 Metal-ion-promoted reactions of organo-sulphur compounds, 6,345 1 -D Metallic complexes, 9,429 Metalloboranes and metal-metal bond- 1% 3,231 __ bonds, multiple (especially quad- ruple), 4, 27 ~ interactions in transition-metal complexes containing infinite chains of metal atoms, 1, 99 Metallocenes as reaction inter-mediates, 17,453 Metals, binding to proteins, 6, 139 Methyl group removal in steroid biosynthesis, 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 by, 8,143 Mixed-valence complexes, the chemistry and spectroscopy of, 13,219 Modern methods for the introduction of fluorine into organic molecules an approach to compounds with altered chemical and biologicalactivities, 16,381 Molecular aspects of biologicalsurfaces, 8,389 -beam reactive scattering, 11, 1 __ collisions and the semiclassical approximation, 5,125 orbital theory, comparison with rotational and vibrational spectro-scopy in conformational analysis of alcohols and amines 5,411 -recognition by synthetic re-ceptors, 15,63 ___ shapes, 7,507 tectonics, the construction of poly- hedral clusters, 13,353 ~ structure and organoleptic qual- ItY, 7,167 theory of small systems, 12, 251 -wavefunctions, chemical inter-pretations of, 5, 79 Molecules, the size of, 15,449 Molybdenum and tungsten, alkoxy,amido, hydrazido, and related com- pounds of, 12,331 Monoalkyltriazenes, 7,377 Morphogenesis, biological, the physical chemistry of, 10,491 Motion, molecular, and time-correla- tion functions, 7, 89 Multistability in open chemical reac- tion systems, 5,359 Myoglobin and haemoglobin, electron spin resonance of, 12,387 Natural products from echinoderms, 1, 1 _____ , polycyclic polysubstituted, systematic development of strategy in, 6,413 Neighbouring-group effects and ring conformation, development of Haworth’s concepts of, 3, 1 __ participation, energetics of, 2,295 Neutron scattering techniques, small angle, recent developments in the characterization of oxide sols, 15,335 New insights into aliphatic nucleo-Dhilic substitution reactions from the he of pyridines as leaving groups, 13,47 New perspectives in surface chemistry and catalysis, 6,373 Nitrile sulphides, the chemistry of, 18,33 Nitrogen fixation, 1,121 S-Nitrosation and the reactions of S-nitroso compounds, 14, 171 Nitroso-alkenes and nitroso-alkynes, 12,53 C-Nitroso-compounds, electrophilic, 6, 1 ~~~ , co-ordination chem- istry of, 19,355 N m r and vibrational spectroscopicstudies, structure in solvents and solutions, 12, 1 Non-bonded interactions of atoms in organic crystals and molecules, 7, 133 Non-conventional electrophilic arom-atic substitutions and related reac-tions, 3, 167 Non-enzymatic transformations in-volving symmetrical bifunctional compounds, 19, 1 Nuclear magnetic resonance and the periodic table, 5, 1 -__-,carbon-13, in biosyn- thetic studies, 4,497 ---methods (new) for tracing the future of hydrogen in biosynthesis, 8,539 _____-spectral change as a probe of chlorophyll chemistry,6,467 ~_____spectroscopy, lan-thanide shift reagents in, 2,49 __---: spin-lattice re-laxat ion, 4,401 ~~~ , two-dimensional,19,381 ___ quadrupole interactions in solids, 15,225 Nucleic acid, mechanisms of reaction with ultimate chemical carcinogens, 9,241 Nucleophiles, reactivity of substituted aliphatic nitro-compounds with, 17,285 Nucleosides and nucleotides, pyrim- idine, 5-substituted, 6,43 Nutritional chemistry of inorganictrace constituents of the diet, 10,270 NYHOLM LECTURE.MEMORIAL Chemical education research: facts, findings, and consequences, 9,365 NYHOLM LECTURE.MEMORIAL Concep-tions, misconceptions, and alterna- tive frameworks in chemical educa- tion, 13,199 NYHOLM LECTURE.MEMORIAL Forward from Nyholm’s March On Lecture, 3,373 NYHOLM LECTURE.MEMORIAL Growth, change, challenge, 5,253 NYHOLM MEMORIAL LECTURE.New trends in chemical education and chemistry teacher education world- wide, 17,135 NYHOLM MEMORIAL LECTURE.Ring,cage, and cluster compounds of the main group elements, 8,315 NYHOLM LECTURE.MEMORIAL Solvingchemical problems, 11, 171 NYHOLM LECTURE.MEMORIAL Synergicinterplay of experiment and theory in studying metal-metal bonds of various orders, 12,35 NYHOLM MEMORIAL LECTURE.Syn- thesis, structure, and spectroscopy of Index metal-metal dimers, linear chains, and dimer chains, 19,107 Olefin metathesis and its catalysis, 4,155 Olefinic compounds, photochemistry of, 3,329 On first looking into nature’s chem- istry:I The role of small molecules and ions: the transport of the elements, 9,281 IIThe role of large molecules, especially proteins, 9,325 Organic chemistry of superoxide,6,195 Organic reaction paths: a theoretical approach, 13,1 Organoboranes 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 hom-ogeneous catalysis, the 16 and 18 electron rule in, 1,337 Organomethyl compounds, synthesis, structure, and vibrational spectra,9, 25 Organosulphur compounds, metal-ion-promoted reactions of, 6,345 Organotransition complexes in-corporating bismuth, 17,271 Organotransition-metal redox reac-tions, synthetic applications of,18,153 Organotransition-metal complexes:stability, reactivity, and orbital correlations, 2,271 Orthoesters and dialkoxycarbeniumions: reactivity, stability, structure, and new synthetic applications,16,75 Overtone spectroscopy and uni-molecular reactions, 17,31 Oxide catalysts, oxidative coupling of methane using, 18,251 Oxygen, singlet molecular, 10,205 PEDLERLECTURE.Organic poly-(sulphur-nitrogen) chemistry, 15,1 PEDLERLECTURE. Porphyrins and related ring systems, 4,1 Peroxonium ions and dioxygen ylides, the chemistry of, 14,399 485 Index Phase boundaries, reactivity of organic molecules at, 1,229 Phase equilibrium and interfacial struc- ture, 14,121 Phenols, anionic cyclization of, 12,213 , long-chain, of non-isoprenoidorigin, 8,499 Philosophy of chemistry, some con-siderations, 5,203 Phosphates, 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 polyoxometallates of molyb- denum and tungsten and/orvanadium, 18, 1 -of the uranyl ion, 3, 139 of transition-metal co-ordination compounds-a survey, 1,241 Photocyclization and photochemistry of aryl halides, 10,181 Photodegradation and stabilization of commercial polyolefins, 4,533 Photoelectron spectroscopic studies of electrode and related structures, 14, 1 Photophysical and photochemical pro- perties of flavins (isoalloxazines),11,15 Photophysics of molecules in micelle- forming surfactant solutions, 7,453 Plant growth, control by chemicals, 6,261 Plastocyanin, structure and electron- transfer reactivity of the blue copper protein, 14,283 Platinum metal complexes, q 5-cycIo-pentadienyl and q6-arene as protect- ing ligands towards, 10, 1 Polyatomic cations, spectroscopicstructure of open-shell, 17,45 Polyatomic molecular ions studied with synchrotron radiation, dynamic decay properties of electronic excited states, 19,439 Polyelectrodes the behaviour and ap- plications of mixed redox systems, 15, 141 Polyhedral clusters, the construction of, 13,353 Polymer solutions, dielectric relaxation in, 1,49 -supports, insoluble, use in organic chemical synthesis, 3, 65 Polymerization and copolymerization of butadiene, 6,235 Polymerization in organized systems, 14,45 Polymers, conductivity and supercon- ductivity in, 5,95 Polymers, recent advances in the photo-oxidation and stabilization of, 15,373 Polyolefins, commercial, photodegrada- tion and stabilization of, 4, 533 Polysaccharides, structural studies of, 10,409 Poly(su1phur-nitrogen) chemistry, or- ganic, 15, 1 Porphyrins and related ring systems, 4, 1 Post-BIZ problems in corrin synthesis, 5,377 Potentiometric titrations of aqueouscarbonate solutions, 14,265 Preparation of organoboranes rea-gents for organic synthesis, 3,443 __ and properties of isocyanic acid, 11,41 PRESIDENTIALADDRESS 1976 Chem-istry and the new industrial revolu- tion, 5,317PRIESTLEY ‘On the science of LECTURE deep-sea diving-observations on the respiration of different kinds of air’, 15,503 Product stability in kinetically-con-trolled organic reactions, 16, 3 13 Properties and syntheses of sweetening agents, 6,431 Prostaglandins, tomorrow’s drugs,4,589 -, thromboxanes, PGX biosyn-thetic products from arachidonic acid, 6,489 Prostanoids, total syntheses of, 2,29 Protecting ligands, q 5-cyclopentadienyland q6-arene towards platinummetal complexes, 10, 1 Protein production by micro-organ- isms, 8, 143 Proteins, binding of heavy metals to, 6, 139 Proteins, role of in nature’s chemistry, 9,325 Prototropic routes to 1,3-and 1,5-dipoles, and 1,2-ylides: applications to the synthesis of heterocyclic com- pounds, 16,89 Pulse radiolysis, contributions to chem- istry, 7,235 Purines, isotopic hydrogen exchange in, mechanisms and applications,10,329 Pyridines as leaving groups, new in-sights into aliphatic nucleophilicsubstitution from the use of, 13,47 Pyrimidine nucleosides and nucleotides, 5-substituted7 6,43 Pyro-compounds, inorganic,Ma[(X2?7)bl, 5,269 Pyrrolizidine alkaloids, biosynthesis of, 18,375 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 in chem- istry, 1,211 Quasielastic laser light scattering,2, 325 Radical cations in condensed phases, 13,393 -reactions, cobalt-mediated, in or- ganic synthesis, 17,361 Radioactive and toxic wastes: a com- parison of their control and disposal, 4,90 Radioactive waste disposal, the environ- mental chemistry of, 15,291 Radiolysis, pulse, contributions to chemistry, 7,235 Raman and infrared vibrational spec- troscopy in inorganic chemistry,4,107 R.A. ROBINSON LECTURE.MEMORIAL Potentiometric titrations of aqueous carbonate solutions, 14,265 R. A. ROBINSON LECTURE.MEMORIAL Thermodynamics of hydrogen-bonded liquids, 11,257 Reaction branching and extreme kin- etic isotope effects in the study of reaction mechanisms, 18,209 Reaction mechanisms, four-membered rings and, 5, 149 Index -, the conversion of ammonium cyaiiate into urea, 7,.1 Reactions involving the triple bond In dimolybdenum and ditungsten hexa- alkoxides and C-C, C-N, C-0 triplebonds, 14,69 Reactive intermediates in enzyme-catalysed reactions, 13,97 Reactivities of carbon disulphide,carbon dioxide, and carbonyl sul-phide towards some transition-metal systems, 11,57 Reactivity of organic molecules at phase boundaries, 1,229 --substituted aliphatic nitro- compounds with nucleophiles,17,285 Recent advances in the photo-oxida- tion and stabilization of polymers, 15,373 Recent advances in the study of kinetic hydrogen isotope effects, 3, 513 Recent chemical studies of sodium nitroprusside relevant to its hypotensive action, 16,361 Recent developmentsin thecharacteriza- tion of oxide sols using small angle neutron scattering techniques, 15,335 Recent syntheses in the Vitamin D field, 9,449 Redox responsive macrocylic receptor molecules containing transition metal redox centres, 18,409 Redox systems, polyelectrodes: the be- haviour and applications of mixed, 15,141 Research in chemical education: a reas- sessment, 1, 27 Resonant descriptions of bonding and reactivity of group VIII-IB metals in the solid state, 16,339 RESOURCES CONSERVATION BY NOVEL BIOLOGICAL PROCES- SES I Grow Fats from Wastes, 8, 283 I1 The Use of Microbes in the Petrochemical Industry, 8,297 I11 Utilization of Agricultural and Food Processing Wastes contain- ing Carbohydrates, 8,309 Retrosynthetic thinking-Essentialsand examples, 17, 111 Review of chemical education research and development in the U.K.,1972-1976, 7,317 Index RH6NE-POULENC LECTURE The origin of the specificity in the recognition of oligosaccharides by proteins,18,347 Ring, cage, and cluster compoundsof the main group elements, 8,315 ROBERTROBINSONLECTURE Post-B problems in corrin synthesis, 5,377 ROBERT ROBINSON LECTURE Re- trosynthetic thinking-Essentialsand examples, 17, 111 ROBERT ROBINSON LECTUREThe 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 syn- thetic flavourings, 7,185 Scale insects and aphids, chemistry of, 4,263 Semistable molecules in the laboratory and in space, 11,435 Silicon compounds in organic syn-thesis, some uses of, 10,83 -containing carbonyl equivalents, 11,493 in organic synthesis, 7, 15 SIMONSENLECTURE Cobalt-mediated radical reactions in organic syn-thesis, 17,361 16 and 18 Electron rule in organometal- lic chemistry and homogeneouscatalysis, 1,337 Size of molecules, 15,449 Small molecules, conformation studies on, 1,293 Sodium nitroprusside, recent chemical studies of, relevant to its hypotensive action, 16,361 Solids, surface energy of, 1,445 Sols, oxide, recent developments in the characterization of, using small angle neutronscatteringtechniques, 15,335 Solute-solvent interactions, spectro-scopic studies of, 5,297 Solution phenomena, the importance of solvent internal pressure and cohe- sion, 4,211 Solutions of metals solvated electrons, 5,337 Solvent internal pressure and cohesion, importance to solution phenomena, 4,211 Solving chemical problems, 11, 171 Some considerations on the philosophy of chemistry, 5,203 Some recent developments in chemistry teaching in schools, 1,495 Sonochemistry Part 1-the physical aspects, 16,239 Sonochemistry Part 2-the syntheticapplications, 16,275 Spectra of stars, absorption bands in, a crystal field approach,5,233 Spectral lineshapes, collisional transfer of rotational energy with, 7,219 Spectroscopic structure of open-shell polyatomic cations, 17,45 ~ studies of solute-solvent interac- tions, 5,297 Spectroscopy and chemistry of mixed- valence complexes, 13,219 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 conformational analysis of alcohols and amines, 5,411 -, threshold electron scattering,3,467 Spin-lattice relaxation a fourth dimen- sion for proton n m r spectroscopy,4,401 Square-planar complexes, isomeriza-tion mechanisms of, 9, 185 Square-planar molecules, ligand substi- tution reactions of, 14,197 SRS-A, the synthesis of leukotrienes a new class of biologically active com- pounds including, 11,321 Stability, reactivity, and orbital correla- tions of organo-transition-metal com- plexes, 2,271 Stabilization of unusual cationic species in protonic superacids and acidic melts, 16, 1 Stereochemical and conformational control of metal redox processes the coordination chemistry of the mixed 488 N-and S-donor macrocyclic crowns [18]aneN2S4 and Me2[18]aneN& 19,239 Stereochemical aspects of the intramol- ecular Diels-Alder reaction, 16, 187 Stereochemical choice in enzymic reac- tions, 8,447 Stereoelectronic origins of the intrinsic barrier to &2 reactions, 19, 133 Stereoselective synthesis of steroid side-chains, 12,75 Steric interactions and stereoelectronic effects, a conformational analysis of transition metal q '-acyl complexes, 17,147 Steroid biosynthesis, methyl group re- moval in, 10,435 , the mechanism of the microbial hydroxylation of, 11,371 -, routes to by intramolecular Diels- Alder reactions of o-xylylenes, 9,41 side-chains, stereoselective syn- thesis of, 12,75 Sterols, biosynthesis of, 1,259 Structural isomerization in DNA: the formation of cruciform structures in supercoiled DNA molecules, 18, 53 Structure and electron-transfer reac-tivity of the blue copper proteinplas tocyanin, 14,283 Structure in solvents and solutions- N.m.r.and vibrational spectroscopic studies, 12, 1 -of aqueous solutions of hydro- phobic 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 Sulphonyl transfer reactions, 18, 123 Sulphoximides, 4,189 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 Index 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 syn- theses of, 6,431 Sweeter side of chemistry, 14,357 Symmetry and molecular chirality,15,189 Syntheses and properties of sweetening agents, 6,431 of mononuclear cyanocobalt(n1) complexes, 12,267 -, recent, in the Vitamin D field, 9,449 -, total, of prostanoids, 2,29 Synthesis and chemistry of acyl silanes, 19,147 Synthesis and cycloadditions, cyano- ketenes, 10,289 -, organic, cobalt-mediated radical reactions in, 17,361 -and synthetic utility of halolac- tones, 8,171 , of corrins, post-BIZ problems in 5,377 -of complex oligosaccharide chains of glycoproteins, 13,15 -, of heterocyclic compounds, ap- plications ofprototropic routes to 1,3- and 1,5-dipoles, and 1,2-ylides, 16, 89 -of leukotrienes: a new class of biologically active compounds in- cluding SRS-A, 11,321 -of lignans and neolignans, 11,75 -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 phosphoruscompounds in, 3, 87 , organic, use of inorganic polymer supports in, 3,65 -, organic, the use of organoboratesas synthetic intermediates, 6,393 , structure, and vibrational spectra of organomethyl compounds, 9,25 of substituted benzene com- Index pounds from acyclic precursors,13,441 Synthetic pyrethroids.A new group of insecticides, 7,473 routes to p-lactams, 5,181 Systematic development of strategy in the synthesis of polycyclic poly- substituted natural products: the aconite alkaloids, 6,413 TATEAND LYLE LECTURE.From car-bohydrates to enzyme analogues,8,85 TATEAND LYLE LECTURE. Spin-latticerelaxation: a fourth dimension for proton n.m.r. spectroscopy, 4,401 TATEAND LYLE LECTURE. Structural and conformational characterization of carbohydrate differentiation an-tigens, 16,161 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 Techniques for the kinetic study of fast reactions in solution, 11,227 Tervalent phosphorus compounds in organic synthesis, 3, 87 Theoretical models of charge-transfer complexes, 15,475 Thermal, photochemical, and transition-metal mediated routes to steroids by intramolecular Diels-Alder reactions of o-xylylenes (o-quinodimethanes), 9,41 Thermodynamics of ion-solvent inter-actions, 9,381 Thermolysis and photolysis of diazir- ines, 11, 127 Three-dimensional structures and chem- ical mechanisms of enzymes, 1,319 Threshold electron scattering spectro- SCOPY, 3,467 Thromboxanes, prostaglandins, PGX: biosynthetic products of arachidonic acid, 6,489 TILDENLECTURE.Alkoxy, amido, hydrazido, and related compounds of molybdenum and tungsten,12,331 TILDENLECTURE.Applications of e.s.r. spectroscopy to kinetics and mech- anism in organic chemistry, 8, 1 TILDEN LECTURE.Carbon-carbon bond formation involving boron rea- gents, 11, 191 TILDEN LECTURE. Chemistry and spectroscopy of mixed-complexes,13.219 TILDEN LECTURE. Concerning stereo-chemical choice in enzymic reac-tions, 8,447TILDEN q 5-CyclopentadienylLECTURE. and q6-arene as protecting ligands towards platinum metal complexes 10,1 TILDEN LECTURE. Electrophilic C-nitroso-compounds, 6, 1 TILDENLECTURE.Initiation of cycliza- tion using 3-methylcyclohex-2-enonederivatives, 9,265 TILDEN Molecular beam reac- LECTURE. tive scattering, 11, 1 TILDEN LECTURE. Molecular recogni-tion by synthetic receptors, 15,63 TILDEN New perspectives in LECTURE. surface chemistry and catalysis,6,373 TILDEN LECTURE.Overtone spectro-scopy and unimolecular reactions, 17,31 TILDEN Semistable molecules LECTURE. in the laboratory and in space,11,435 TILDENLECTURE.Some uses of silicon compounds in organic synthesis,10,83 TILDEN Structural studies on LECTURE. bio-active molecules, 13, 131 TILDEN LECTURE. Structure and electron-transfer reactivity of the blue copper protein plastocynanin, 14,283 TILDEN The collision dyna- LECTURE. mics of vibrationally excited mole- cules, 14,141 TILDEN LECTURE. The properties of hydrogen-bonded dimers from rota- tional spectroscopy, 19, 197 TILDEN LECTURE. Valence in transition-metal complexes, 1,431 Time-correlation functions and molecu- lar motion, 7, 89 Topological subject--chemistry, 2,457 Trace constituents of the diet, chemical aspects, 10,233 -organic constituents of the diet, sources and biogenesis, 10,280 Index Transimination, chemical models of enzymic, 12,309 Transition-metal carbene complexes,chemistry and r81e as reaction inter- mediates, 2,99 complexes, containing infinite chains of metal atoms, metal-metal interactions in, 1,99 -complexes of synthetic macro-cyclic ligands, 4,421 complexes, valence in, 1,43 1 co-ordination compounds,photochemistry of, 1,241 -hydride complexes, 12,415 -systems, reactivities of carbon disulphide, carbon dioxide, and car- bonyl sulphide systems towards, 11,57 -oxide chelates of carbohydrate-directed macromolecules, 8,22 1 Transport properties, hard-spheretheories of, 14,317 Triple bond in dimolybdenum and ditungsten hexa-alkoxides and C-C, C-N, C-0 triple bonds, reactions involving the, 14,69 Tunable lasers, 3,293 Two-dimensional nuclear magnetic resonance spectroscopy, 19, 381 Ultraviolet photoelectron studies of unstable molecules with relevance to synthesis, quantum chemistry, and spectroscopy, 18,317 Unimolecular reactions, current aspects of, 12,163 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, 1,431 Valences, bond, a simple structural model for inorganic chemistry, 7,359 Very strong hydrogen bonding, 9,91 Vibrational and n.m.r.spectroscopicstudies, structure in solvents and solutions, 12, 1 --, infrared, and Raman spectro-~copyin inorganic chemistry, 4, 107 -intensities in electronic transi-tions, 5,165 spectra, synthesis, and structure of organomethyl compounds, 9,25 Vibrationally and rotationally inelastic scattering of molecules, 3,407 Vibrationally excited molecules, the col- lision dynamics of. 14. 141 Vinylcyclopropane rearrangements,’ 17.231 Viologens, electrochemistry of, 10,49 Vitamin B 2, retrospects and prospects, 9,125 Vitamin D, chemistry of: the hormonal calciferols, 6, 83 Vitamin D, recent syntheses in, 9,449 Voltammetric studies of ion trans-fer across biological membranes, 17,319 Wittig olefination reaction with car-bonyl compounds other than al-dehydes and ketones, 17, l Ylides, arsonium, (with some mention also of arsinimines, stibonium and bismuthonium ylides), 16,45 Ylides, the chemistry of peroxonium ions and dioxygen, 14,399 491

ISSN:0306-0012    

DOI:10.1039/CS9901900471

出版商:RSC    

年代:1990

数据来源: RSC