摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Dr. Peter J. Sarre Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD, UK ~ Faraday Editorial Board Prof. M. N. R. Ashfold (Bristol) (Chairman) Dr. J. A. Beswick (Paris) Prof. A. R. Hillman (Leicester) Dr. D. C. Clary (Cambridge) Prof. J. Holzwarth (Berlin) Dr. L. R. Fisher (Bristol) Dr. D. Langevin (Paris) Dr. B. E. Hayden (Southampton) Dr. P. J. Sarre (Nottingham) Prof. J. S. Higgins (London) Dr. R. K. Thomas (Oxford) Editorial Manager and Secretary to Faraday Editorial Board Dr. Robert J. Parker The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF, UK Staff Editor: Dr.R. A. Whitelock Senior Assistant Editor: Mrs. S. Shah Assistant Editors: Dr. L. Milne, Mrs. C. J. Seeley Editorial Secretary: Mrs. J. E. Gibbs International Advisory Editorial Board R. S. Berry (Chicago) Y. Marcus (Jerusalem) A. M. Bradshaw (Berlin) B. J. Orr (North Ryde) A. Carrington (Southampton) R. H. Ottewill (Bristol) M. Che (Paris) R. Parsons (Southampton) M. S. Child (Oxford) S. L. Price (London) B. E. Conway (Ottawa) F. Rondelez (Paris) G. R. Fleming (Chicago) D. K. Russell (Auckland) R. Freeman (Cambridge) J. P. Simons (Oxford) H. L. Friedman (Stony Brook) S. Stolte (Amsterdam) H. H. J. Girault (Lausanne) J. Troe (Gottingen) H. lnokuchi (Okazaki) J. Wolfe (Kensington, NSW) J. N. lsraelachvili (Santa Barbara) C.Zannoni (Bologna) M. L. Klein (Philadelphia) R. N. Zare (Stanford) A. C. Legon (Exeter) A. Zecchina (Turin) R. A. Marcus (Pasadena) C. Zhang (Dalian) Journal of the Chemical Society, Faraday Transactions (ISSN 0956-5000) is published twice monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd., Black- horse Road, Letchworth, Herts. SG6 1 HN, UK. NB Turpin Distribution Services Ltd., dis- tributors, is wholly owned by the Royal Society of Chemistry. 1994 Annual subscription rate EC f744.00, Rest of World f800.00, USA $1400.00, Canada €840 (excl.GST). Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Second class postage is paid at Rahway, NJ. Airfreight and mailing in the USA by Mercury Airfreight International Ltd. Inc., 2323 Randolph Avenue, Avenel, NJ 07001, USA and at additional mailing offices. USA Postmaster: send address changes to Journal of the Chemical Society, Faraday Trans- actions, c/o Mercury Airfreight International Ltd. Inc., 2323 Randolph Avenue, Avenel, NJ 07001. All despatches outside the UK by consolidated Airfreight. PRINTED IN THE UK. @ The Royal Society of Chemistry, 1994. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, photographic, recording, or otherwise, without the prior permission of the publishers.Advertisement sales: tel. +44(0)71-287-3091; fax. +44(0)71-494-1134. INFORMATION FOR AUTHORS The Royal Society of Chemistry welcomes submission of manuscripts intended for pub- lication in two forms, Research papers and Faraday Communications. These should describe original work of high quality in the sciences lying between chemistry, physics and biology, and particularly in the areas of physical chemistry, biophysical chemistry and chemical physics. Research Papers Full papers contain original scientific work which has not been published previously. However, work which has appeared in print in a short form such as a Faraday Communi- cation is normally acceptable.Four copies including a top copy with figures etc. should be sent to The Editor, Faraday Transactions, at the Editorial Office in Cambridge. Authors may, if they wish, suggest the names (with addresses) of up to three possible referees. Faraday Communications Faraday Communications contain novel scientific work in short form and of such importance that rapid publication is war-ranted. The total length is rigorously restricted to two pages of the double-column A4 format. For a Communication consisting entirely of text and ten references, with no figures, equations or tables, this cor- responds to approximately 1600 words plus an abstract of up to 40 words.Submission of a Faraday Communication can be made either to The Editor, Faraday Transactions, at the Editorial Office in Cam- bridge or via a member of the International Advisory Editorial Board, who will arrange for the manuscript to be reviewed. In the latter case, the top copy of the manuscript including any figures etc., together with the name of the person through whom the Com- munication is being submitted, should be sent simultaneously to the Editor at the Cambridge address. Proofs of Communications are not normally sent to authors unless this is specifically requested. Faraday Research Articles Faraday Research Articles are occasional invited articles which are published follow- ing review. They are designed to be topical articles of interest to a wide range of research scientists in the areas of Physical Chemistry, Biophysical Chemistry and Chemical Physics. Full details of the form of manuscripts for Articles and Faraday Communications, con-ditions for acceptance etc. are given in issue number one of Faraday Transactions, published in January of each year, or may be obtained from the Editorial Manager. There is no page charge for papers published in Faraday Transactions. Fifty reprints are supplied free of charge. Dr. P. J. Sarre, Scientific Editor. Tel. : Nottingham (0602) 51 3465 (24 hours) E- Mai I(JAN ET) :PCZ PS F@ UK.AC. N On.VAX Fax: (0602) 513466 Telex: 37346 UNINOT G Dr. R. J. Parker, Editorial Manager. Tel. : Cambridge (0223) 420066 E-Mail (INTERNET): RSCl @RSC.ORG (For access from JANET use RSC1% RSC.0 RG(4UK.AC.NSF NET-R ELAY) Fax: (0223) 423623 or 420247 Telex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX049

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

International Reviews In Physical Chemistry D C Clary, Department of Chemisty, University of Cambridge, Cambridge, CB2 1E W,UK This journal publishes review articles describing frontier research areas in physicalchemistry. Internationally renowned scientists describe their own research in the wider context of the field. The articles are of interest not only to specialists but also to those intent on reading general and authoritative accounts of recent developments in physical chemistry, chemicaf physics and theoretical chemistry: the journal appeals to research workers, lecturers and research students alike. READERSHIP Specialists and generalists in research, industry and teaching who wish to keep abreast of recent advances in physical chemistry and chemical physics.SUBSCRIPTION INFORMATION Volume 13 (1994) 2 per annum ISSN 0144-235X Institutional: US320 / €190 I 0 Please send me a FREE sample copy of I INTERNATIONAL REVIEWS IN PHYSICAL CHEMISTRY II 0 Please send a complete Journals Catalogue* / Books Catalogue* 1 I (*Pleasedelete as appropriate) I1 1 [J Please add nic to your mailing list I IName1 Address I 1 I I I I Taylor & Francis Ltd Taylor & Francis Inc I I Rankine Road, Basingstokc 1900 Frost Road, Suite 101 I I Hampshire RG24 8PR, UK Bristol, PA 19007-1598,USA I I Tel: +44 (01256 840366 Taylor&Francis Tel: 1 800 821 8312 I I Fax: +44 (0)256 479438 Publishem since 17YU Fax: 215 785 5515 I HAZARDS IN THE CHEMICAL LABORATORY 5th Edition ‘.. . easy to read, an excellent reference text, and a worthwhile investment.’ Journal of the American Chemical Society reviewing the 4th Edition The new edition of this essential laboratory handbook is the ‘key’ requirement for all research, development, production, analytical and teaching laboratories worldwide. The 5th Edition provides: New features include: expanded ‘Yellow Pages’ section on 0 a quick guide to the hazardous properties of 1339 substances (over 800 more than were hazardous substances, providing immediate covered in the previous edition) information on hazardous properties, 0 details of the latest UK and EC regulations recommended control procedures and safety measures 0 an extremely useful emergency action check complete guide to labelling requirements to list -users can fill in their own key contacts for hospitals, fire etc.comply with EC directives and UK legislation, including the risk and safety phrases that must 0 handy tables, symbols and statistics for ease appearof reference 0 chapter on electrical hazards 0 a description of the American scene, including 0US legislation and safety practices -index to Yellow Pages’ section, with highlighting differences between the UWEC synonyms of compounds index to CAS Registry Numbers and USA PVC Protective Binding xx + 676 pages ISBN 0 85186 229 2 (1992) Price €45.00 If you have not yet ordered your copy of the NEW edition, do so now! Why take chances? Be informed and safe. To order, please contact: KOYAL Royal Society of Chemistry, Turpin Distribution Services Ltd, Blackhorse Road, Letchworth, Herts SG6 lHN, United Kingdom. Service\ Telephone: +44 (0)462 672555 Fax: +44 (0)462 486947. IIII

ISSN:0956-5000    

DOI:10.1039/FT99490BX051

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(I 3) 1811-2002 (1994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 1811 Ab initio quantum chemistry study of the gas-phase reaction of ClO with HO, D. Buttar and D. M. Hirst 1819 Mechanism of atmospheric oxidation of 1,1,1,2-tetrafluoroethane(HFC 134a) 0. V. Rattigan, D. M. Rowley, 0. Wild, R. L. Jones and R. A. Cox 1831 Transition vector symmetry and the internal pseudo-rotation and inversion paths of ClF,' R. M. Minyaev and D. J. Wales 1839 Gradient-line reaction paths for 1,2 H shift reactions in phosphinonitrene and formaldehyde, and H, elimination from formaldehyde R. M. Minyaev and D. J. Wales 1849 Non-linear optical properties of organic molecules. Part 13.-Calculation of the structure and frequency-dependent hyperpolarisability of a blue azothiophene dye J.0. Morley 1853 Non-linear optical properties of organic molecules. Part 14.--Calculations of the structure, electronic properties and hyperpolarisabilities of cyclopentadienylpyridines J. 0. Morley 1857 Photoinduced electron transfer in a-helical poly(L-lysine) carrying randomly distributed donor-acceptor pairs. A kinetic and conformational statistics investigation B. Pispisa, M. Venanzi and A. Palleschi 1865 Irreversible thermodynamic coupling between heat and matter fluxes across a gas/liquid interface S. C. Doney 1875 Thermodynamic properties of (r6 mol kg-' aqueous sulfuric acid from 273.15 to 328.15 K S. L. Clegg, J. A. Rard and K. S. Pitzer 1895 Pitzer model parameters for sparingly soluble salts from solubility measurements : Thallium(1) chloride in aqueous solutions of ammonium chloride, rubidium chloride and caesium chloride at 298.15 K K.H. Khoo, K. R. Fernando and L-H. Lim 1899 Ionic partial molar volumes in non-aqueous solvents Y. Marcus, G. Hefter and T-S. Pang 1905 Differential scanning microcalorimetric study of sodium di-n-dodecylphosphate vesicles in aqueous solution M. J. Blandamer, B. Briggs, P. M. Cullis, J. B. F. N. Engberts and D. Hoekstra 1909 Very large thermal separations for polyelectrolytes in salt solutions D. G. Leaist and L. Hao 1913 Redox properties of ubiquinone (UQ,,) adsorbed on a mercury electrode G. J. Gordillo and D. J. Schiffrin 1923 Studies of silver electronucleation onto carbon microelectrodes H.Sousa, S. Pons and M. Fleischmann 1931 Self-diffusion and viscoelasticity of dense hard-sphere colloids D. M. Heyes and P. J. Mitchell 1941 Hydration of polar interfaces. A generalised mean-field model S. Kirchner and G. Cevc 1953 Kinetics of self-replicating micelles J. Billingham and P. V. Coveney 1961 Micellisation and gelation of triblock copolymer of ethylene oxide and E-caprolactone, CL,E,CL, , in aqueous solution L. Martini, D. Attwood, J. H. Collett, C. V. Nicholas, S. Tanodekaew, N-J. Deng, F. Heatley and C. Booth 1967 EPR/ENDOR characterization of radicals produced in the photopolymerization of a dimethacrylate monomer E.Selli, C. Oliva and G. Termignone 1973 Dissolution of amorphous aluminosilicate zeolite precursors in alkaline solutions.Part 2.-Mechanism of the dissolution T. Antonic, A. Ciimek and B. Subotic 1979 Effects of hydrogen and deuterium concentration on measurements of the solubility and diffusivity of hydrogen iso-topes in yttrium T. Maeda, S. Naito, M. Yamamoto and M. Mabuchi 1983 Heterogeneous catalysis in solution. Part 27.-Reaction between titanium(rr1) and triiodide ions catalysed by platinum S. Xiao and M. Spiro 1987 X-Ray photoelectron spectroscopy, temperature-programmed desorption and temperature-programmed reduction study of LaNiO, and La2Ni0,+, catalysts for methanol oxidation J. Choisnet, N. Abadzhieva, P. Stefanov, D. Klissurski, J. M. Bassat, V. Rives and L. Minchev 1993 Addition of manganese to iron catalysts supported on silicalite-1 and its effect on CO hydrogenation G.Ravichandran, D. Das and D. K. Chakrabarty FARADAY COMMUNICATIONS 1999 Microwave synthesis of the colloidal poly(N4sopropylacrylamide) microgel system M. Murray, D. Charlesworth, L. Swires, P. Riby, J. Cook, B. Z. Chowdhry and M. J. Snowden ~ 2001 Corrigendum to Sorption of organic solvents into dense silicone membranes. Parts 1 and 2 E. Favre, Q. T. Nguyen, P. Schaetel, R. Clement and J. Nee1 Corrigendum to Fluorescence anisotropy decays and viscous behaviour of 2-methyltetrahydrofuran B. Brocklehurst and R. N. Young Corrigendum to Small-angle neutron scattering investigations of the structure of thixotropic dispersions of smectite clay colloids J. D. F. Ramsay, P.Lindner, A. Matsumoto and S. W. Swanton 2002 Corrigendum to Primitive model electrolytes in modified Poisson-Boltzmann theory C. W. Outhwaite, M. Molero and L. B. Bhuiyan Note: Where an asterisk appears against the name of one or more of the authors, it is included with the authors’ approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles. For further details 1 contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN, UK Tel: +44 (0)71-437 8656 Fax: +44 (0)71-287 9798 Telecom Gold 84: BUR210 Electronic l Mailbox (Internet) LIBRARY@RSC.ORG. If the material is not available from the Society’s Library, the staff will be pleased to advise on its availability from other sources.Please note that copies are not available from the RSC at Thomas Graham House, Cambridge. Faraday Transactions Faraday Editorial Board M.N.R. Ashfold (Chairman), School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS J.A. Beswick, Universite de Paris II, Lure, F-91405 Orsay, France D.C. Clary, University Chemical Laboratory, Cambridge University, Lensfield Road, Cambridge, UK CB2 1EW L.R. Fisher, H.H. Wills Physics Laboratory, Royal Fort, Tyndall Avenue, Bristol, UK BS8 1TL B.E. Hayden, Department of Chemistry, The University, Southampton, UK SO9 5NH J.S. Higgins, Department of Chemical Engineering, Imperial College, London, UK SW7 2AY A.R. Hillman, Department of Physical Chemistry, Leicester University, University Road, Leicester, UK LE1 7RH Prof.J. Holzwarth, Fritz-Haber-lnstitut der Max Planck-Gesellschaft, Faradayweg 4-6, D-1 000 Berlin 33, Germany D. Langevin, Laboratoire de Physique Statistique, Ecole Normale Superieure, 24 Rue Lhomond 75231, Paris Cedex 05, France P.J. Sarre, Department of Chemistry, University of Nottingham, University Park, Nottingham, UK NG7 2RD R.K. Thomas, Physical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QX International Advisory Editorial Board R.S. Berry, Department of Chemistry and The James Franck Institute, The University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637, USA A.M.Bradshaw, Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D- 1000, Berlin 33, Germany A. Carrington, Department of Chemistry, University of Southampton, Highfield, Southampton, UK SO9 5NH M. Che, Laboratoire de Reactivite de Surface et Structure, URA 1106, CNRS, Universite P. et M. Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France M.S. Child, Theoretical Chemistry Department, University of Oxford, 5 South Parks Road, Oxford, UK OX1 3UB B.E. Conway, Chemistry Department, University of Ottawa, 32 George Glimski Street, Ottawa K1N 6N5, Ontario, Canada G.R. Fleming, Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637, USA R. Freeman, Department of Chemistry, Cambridge University, Lensfield Road, Cambridge, UK CB2 1EP H.L.Friedman, Department of Chemistry, State University of New York, Stony Brook, NY 11794-3400, USA H.H.J. Girault, Laboratoire d’Electrochimie, Ecole Potytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland H. Inokuchi, Institute for Molecular Science, Myodaiji, Okazaki 444, Japan J.N. Israelachvili, Department of Chemical and Nuclear Engineering, University of California, Santa Barbara, CA 93106, USA M.L. Klein, Department of Chemistry and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 191 04-6323, USA A.C. Legon, Department of Chemistry, University of Exeter, Stocker Road, Exeter, Devon, UK EX4 4QD R.A.Marcus, Noyes Laboratory of Chemical Physics, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA Y. Marcus, Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel B.J. Orr, School of Chemistry, Macquarie University, North Ryde, NSW 21 11, Australia R.H. Ottewill, School of Chemistry, University of Bristol, Cantock‘s Close, Bristol, UK BS8 1TS R. Parsons, Department of Chemistry, The University, Southampton, UK SO9 5NH S.L. Price, Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H OAJ F. Rondelez, Directeur de Recherche Physicochimie des Surfaces et Interfaces, lnstitut Curie, Section Physique et Chimie, 11 rue P.et M. Curie, 75231 Paris Cedex 05, France D.K. Russell, Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand J.P. Simons, Physical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QX S. Stoke, Department of Theoretical and Physical Chemistry, Vrije Universiteit, de Boelelaan 1083, 10181 HV, Amsterdam, The Netherlands J. Troe, lnstitut fur Physikalische Chemie, Universitat Gottingen, Tammannstrasse 6, D-3400 Gottingen, FRG J. Wolfe, School of Physics, University of New South Wales, PO Box 1, Kensington, NSW 2033, Australia C. Zannoni, Dipartimento di Chimica Fisica, Universita di Bologna, Viale Risorgimento 4, 401 36 Bologna, Italy R.N.Zare, Department of Chemistry, Stanford University, CA 94305, USA A. Zecchina, Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica Materiali dell’Universita di Torino, Via P. Giuria 7, 10125 Turin, Italy C. Zhang, Director, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, Dalian, PR China Message from the Chairman of Faraday Editorial Board This summer heralds a number of changes in the ‘management’ of Faraday Transactions. Professor I. W. M. Smith retires as Chairman of Faraday Editorial Board after six years of exemplary service. Ian has overseen major (and very positive!) changes in the fortunes, the appearance and the perception of the Journal. Members of the Editorial Board had an opportunity to thank Ian personally for all his hard work after the last Board meeting; I know that I also speak for a great many others in the scientific community when I say, again, thank you Ian for all your efforts on behalf of Faraday Transactions over the past six years.It is my pleasure to have been appointed to succeed Ian as Chairman of Faraday Editorial Board. Be assured, I shall be thrilled if I can help introduce so much ‘added value’ during my tenure as Chairman! Professor H. M. Frey, another stalwart not just of Faraday Editorial Board but also of the Journals Management Committee of the Royal Society of Chemistry, also stands down this summer. Monty, too, has given generously, both of his time and his wisdom, and will be a very hard act to follow.Every cloud has a silver lining; however, and the retirements of Ian and Monty, my ‘elevation’, and the appointment of Professor A. R. Hillman (from 1 October) in succession to Dr. P. J. Sarre as Scientific Editor of Faraday Transactions-more on this later-opens the way for another injection of ‘new blood’ onto the Editorial Board of Faraday Transactions. As a result, Professors Julia Higgins and Alberto Beswick, and Drs Steven Scott and Domi- nique Langevin were all invited to serve on Faraday Editorial Board; I am delighted to be able to announce that all four were happy to accept our invitation. Their appointments will help ensure that the Board and, hopefully, through it the Transactions themselves, remain dynamic in, and responsive to, all of the major areas of physical chemistry, chemical physics and biophysical chemistry.I would like to highlight the appointments of Alberto and Dominique, two new Board members from Mainland Europe. Their appointment, following closely on the heels of that of Josef Holzwarth, should be seen as a very clear signal of the Board’s intention that Faraday Transactions should become firmly recognised as the major European journal in the areas of physical chemistry, chemical physics and biophysical chemistry. I have already alluded to Rob Hillman’s appointment as the next Scientific Editor of Faraday Transactions. He will succeed Peter Sarre, who retires from office on 30th September next after five outstanding years in this role.Peter, too, will be a very hard act to follow! This is neither the time nor the place for a full appreciation of Peter’s contributions on behalf of Faraday Transactions. Suffice to say at this stage that his boundless energy, his enthusiasm, his thoroughness and his attention to detail have been absolutely crucial to the striking progress made by the Transactions during his period as Scientific Editor. Readers will not be surprised to know that more papers were received by, and published in, Faraday Transactions in 1993 than ever before, that the average time between acceptance and publication of full papers is well below 4 months (an all-time low) and that the responses to the questionnaires circulated to authors whose work has recently appeared in the Transactions are revealing a gratifyingly high level of ‘consumer satisfaction’.Credit for this very fine state of affairs must rest in large part with Peter, but he will be the first to point out that none of it would have been achieved without the sterling efforts of Dr. Bob Parker and his colleagues in the Editorial Office in Cambridge. So, what of the future? Clearly, the Transactions once again have a momentum of their own, and thus we view the future with considerable optimism. The Faraday Research Articles published to date have been very well received; this innovation is one which we will certainly continue. Similarly, we will continue to publish occasional ‘Special issues’ of the Transactions devoted to subject areas in which we would like to encourage new authors.Topics so identified for the near future include colloid chemistry and surface science. Finally, I should flag one change that is in the pipeline. With passing time the distinction between Faraday Discussions and Faraday Symposia has become increasingly blurred, to the extent that Faraday Council has taken the welcome and logical decision that all such future meetings will be Faraday Discussions (three per year). Discussions have always been published as a separate volume, whilst recent Symposia have appeared as Special Issues of the Transactions. Clearly, with the distinction removed this latter input to the Transactions will cease and all Discussions will be reported via Faraday Dis- cussions volumes. Whether you be readers, authors, or just interested observers, I and all other members of Faraday Editorial Board would be keen to receive your views on Faraday Transactions, and any suggestions you may have as to how we might further improve the service provided by the Journal. I hope to hear from you! MIKEASHFOLD July 1994

ISSN:0956-5000    

DOI:10.1039/FT99490FP134

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aas,N., 1015 Abadzhieva, N., 1987 Abbott, A. P., 1533 Afanasiev,P., 193 Agren, H., 1479 Aikawa, M., 91I Aitken, C. G., 935 Akanuma, K., 1171 Akolekar, D. B., 1041 Albery, W. J., 1115 Aldaz, A., 609 Alfimov, M. V., 109 Al-Ghefaili, K. M., 383, Ali, V., 579, 583 Aliev, A. E., 1323 Allegrini, P., 333 Allen, N. S., 83 A1 Rawi, J. M. A., 1047 845 Blower, C., 919,931 Bocherel, P., 1473 Boddenberg, B., 1345 Boggis, S. A., 17 Booth, C., 1961 Borden, W. T., 1606,1614, 1616,1671, 1673, 1675, 1689,1733, 1734, 1735, 1743, 1744, 1802, 1807 Borge, G., 1227 Borisenko, V. N., 109 Bottoni, A., 1617 Boutonnet-Kizling, M., Bowker, M., 1015 Bozon-Verduraz, F., 653 Bradley, C. D., 239 Bradshaw, A. M., 403 Braun, B. M., 849 1023 Clegg, S. L., 1875 Clement, R., 2001 Clirnent, M.A., 609 Coates, J. H., 739 Coitiiio, E. L., 1745 Collett, J. H., 1961 Colmenares, C. A., 1285 Cook, J., 1999 Cooper, D. L., 1643 Cordischi, D., 207 Corma, A., 213 Cormier, G., 755 Corradini, F., 859, 1089 Corrales, T., 83 Cosa, J. J., 69 Costas, M., 1513 Cottier, D., 1003 Coudurier, G., 193 Courcot, D., 895 Fornes, V., 213 Fracheboud, J-M., 1197, Franci, M. M., 1605, 1740, Franck, R., 667,675 Freeman, N. J., 751 Frity, R., 773 Frey, J. G., 17, 817 Frostemark, F., 559 Fujiwara, Y., 1183 Galantini, L., 1523 Gandolfi, R., 1077 Gans, P., 315 Gao,Y., 803 Garcia, R., 339 Garcia Fierro, J-L., 1455 Garcia-Paiieda, E., 575 Gautam, P., 697 1205 1744 Heyes, D. M., 1133,1931 Higgins, S., 459 Hillier, I. H., 1575 Hillman, A. R., 1533 Hindermann, J-P., 501 Hirst, D.M., 517, 1811 Hiyane, I., 973 Hoekstra, D., 727, 1905 Hoffmann, R., 1507 Holmberg, B., 559 Holz, M., 849 Hoshino, H., 479 Hosoi, K., 349 Houk, K. N., 1599,1605, 1614, 1615, 1616, 1672, 1678, 1680,1810 Hrovat, D. A., 1689 Hu, W. P., 1715 HSU,J-P., 1435 Amorim da Costa, A. M., Amoskov, V. M., 889 Ando, M., 1011 Andres, J., 1703 689 Breysse, M., 193 Briggs, B., 727, 1905 Brocklehurst, B., 271, 2001 Brogan, M. S., 1461 Brown, N. M. D., 1357 Coveney, P. V., 1953 Cox, R. A., 1819 Cracknell, R. F., 1487 Craig, S. L., 1663 Cramer, C. J., 1802 Gavuzzo, E., 1523 Geantet, C., 193 Gengembre, L., 895 Gerratt, J., 1643, 1672, 1673, 1801 Hungerbiihler, H., 1391 Hutchings, G. J., 203 Hutton, R. S., 345 Iizuka, Y., 1301, 1307 Ikawa, S-i., 103 Andrews, S. J., 1003 Anson, C.E., 1449 Antonic, T., 1973 Aragno, A., 787 Arai, S., 1307 Aramaki, K., 321 Aravindakumar, C. T., 597 Asai, Y., 797 Ashfold, M. N. R., 1357 Asmus, K-D., 1391 Assfield, X., 1743 Attwood, D., 1961 Avila, V., 69 Baba,T., 187 Badia, A., 1501 Badri, A., 1023 Bagatti, M., 1077 Balaji, V., 1653 Ball, M. C., 997 Ball, S. M., 523, 1467 Bally, T., 1615, 1674, 1733, Ban, M. I., 1610 Baonza, V. G., 553 Baonza, V. G., 1217 Barbaux, Y., 895 Barker, S. A,, 1689 Barnes, J. A., 1709 Barthomeuf, D., 667,675 Basini, L., 787 Bassat, J. M., 1987 Bassoli, M., 363 Battaglini, F., 987 Bauer, C., 517 Bell, A. J., 17, 817 Belton, P. S., 1099 Bender, B. R., 1449 Bendig, J., 287 Bengtsson, L. A.. 559 Benko, J., 855 Benniston, A. C., 953 Beno, B., 1599 Bensalem, A., 653 Berces, T., 4 11 Bergeret, G., 773 1808 Brown, R.G., 59 Brown, S.E., 739 Bruna, P. J., 683 Brzezinski, B., 843, 1095 Buckley, A. M., 1003 Buemi, G., 1211 Burdisso, M., 1077 Busca, G., 1161, 1293 Buschmann, H-J., 1507 Butler, L. J., 1581, 1612, 1613,1614,1671, 1677, 1809 Butt, M. D., 727 Buttar, D., 1811 Byatt-Smith, J. G., 493 Cabaleiro, M. C., 845 Caceres, M., 1217 Caceres Alonso, M., 553 Cairns, J. A., 1461 Calado, J. C. G., 649 Caldararu, H., 213 Calvente, J. J., 575 Calvo, E. J., 987 Camacho, J. J., 23 Cameron, B. R., 935 Campa, M. C., 207 Campos, A,, 339 Canosa-Mas, C. E., 1197, Capobianco, J. A., 755 Caragheorgheopol, A., 213 Carlile, C. J., 1149 Carlsen, L., 941 Carvill, B. T., 233 Castaiio, R., 1227 Castro, S., 1217 Catalina, F., 83 Cataliotti, R. 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A., 1615, 1616, Wikander, G., 305 Wohlers, M., 403 Yano,H., 869 Zundel, G., 843, 1095 1678,1807 Wild, O., 1819 Wolthuizen, J. P., 1033 Yasuda, H., 1183 Wang, C. F., 605 Wilde, C. P., 1233 Wormald, C. J., 445 Yeh, C-t., 11 57 ... 111 Journal of the Chemical Society, Faraday Transactions Information for Authors Furaday Transactions is an international journal for the publi- cation of original research papers and communications con- cerned with the sciences lying between chemistry, physics and biology, and particularly in the areas of physical chemistry, biophysical chemistry and chemical physics.The purnal is published fortnightly. There is no page charge for papers published in Fmadq Transactiom. Research Papers Full papers contain original scientifk work that has not been published previously. However, work that has appeared in print in a short form such as a Faraday Communication or Chemical Communication is normally acceptable. 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Abstracts of about 300 words should be submitted by 31 May 1994 to: Professor I.W. M. Smith OR Dr R. J. Sodeau School of Chemistry School of Chemical Sciences University of Birmingham University of East Anglia Edgbaston, Birmingham B15 2TT, UK Nonvich NR4 7TJ, UK Full papers for publication in the Discussion volume will be required by December 1994. THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 101 Gels Paris, France, 6-8 September 1995 Organising Committee: Dr J. W. Goodwin (Chairman) Dr R. Audebert Dr R. Buscall Professor M. Djabourov Dr A. M. Howe Professor J. Livage Professor J. Lyklema Professor S. B. Ross-Murphy During the last few years there has been an increase in both theoretical and experimental work on gels as new techniques have been applied to a wide range of gelling systems.Typical of these are gels formed frum polymers by both physical and chemical interactions as well as gels formed by inorganic and surfactant systems. 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Superoxide Anion Radical Decay on Palladium Promoted Yttria Subsequent Peripheral Cyclopropanation as a Synthetic Approach to Cyclic and Cyclo-s ubstituted Tria ng ula nes Total Synthesis of the Archaebacterial C,,-Diol and its Enantiomer Based on (R)-5-Acetoxy4-methylpentanoic Acid An Efficient Synthesis of (S,Z)-Dodec-3-en-ll-olide(Ferrulactone I/) using 2-Ca rboxyeth yltriphen ylphosp honium Bromide Reaction of Trichloromethylarenes with Pyridine: A Novel Synthesis of N-(4-Pyridyopyridinium Salts and Aromatic Aldehydes Photochromic Behaviour of Non-transition Metal Chelate Complexes of Salicylaldimines Synthesis of Chromophores Based on Porphyrins and Open-chain Polypyrroles 3,4-Dinitrofuroxan-the First Example of a Pernitro Heterocycle New Intercalation Compounds of Molybdenum Disulfide with Transition metals Lower Oxidation States of Protactinium Photoionisation and Photoluminescence of Luminol in Aqueous Solution A Novel Mononuclear Tungsten(v1) Complex with 1-Hydroxyethylenediphosphonic Acid, Exhibiting a WO, Core Pressure Tensor and Local Density Profiles of Computer-simula ted Water Clusters Registration Method for Metastable Decomposition of Benzene-Ar" and Toluene-Arn Cluster Ions in RETOF Instruments Unusual Isotope Effect Induced by Photolysis of Uranyl Salts in Micelles Synthesis of Fluorinated 4H-1,4-Benzothiazine-2-carboxylic Acid 1,1-Dioxides-Thionated Analogues of Pe floxacin Natural Abundance Solid State 2H NMR Studies of Phase Transitions in Rotator Phase Solids Giant Pd Clusters observed by High Resolution Electron Microscopy xii

ISSN:0956-5000    

DOI:10.1039/FT99490BP138

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(13), 1811-1817 181 1 Ab lnitio Quantum Chemistry Study of the Gas-phase Reaction of CIO with HO, David Buttar and David M. Hirst* Department of Chemistry, University of Warwick, Coventry, UK CV4 7AL The reaction of CIO with HO, has been studied using ab initio molecular orbital theory with electron correlation being taken into account by Msller-Plesset (MP) perturbation theory. Saddle-point geometries, energies and harmonic vibrationai wavenumbers have been calculated at the MP2/6-31 G** level and barrier heights at the MP4/6-311G** level. The study shows that the products and mechanism depend on which potential-energy surface the reaction occurs. If the reaction proceeds along the triplet reaction surface, ground-state HOCl ('A') and ground-state 0, (3Xg-)are formed.If the reaction takes place on the singlet reaction surface the same products are formed, but in this case the molecular oxygen is formed in a singlet state. The CIO + HO, reaction proceeds via a multi-step reaction mechanism on the singlet surface and via a direct hydrogen-abstraction mechanism on the triplet surface. The reaction of chlorine monoxide (C10) and the hydro- peroxy radical (HO,) has been studied '-*extensively because of its importance in atmospheric chemistry and its unusual rate behaviour. The reaction can proceed through a number of possible reaction channels C10 + HO, +ClOO + OH OClO + OH HOCl + 0, HClO + 0, HCl + 0, ArH (298 K)9-11 channel /kJ mol-' 1 ca.12 2 ca. 25 3 CU. -191 4 ca. 71 5 CU. -62 leading to a wide range of products. However, the reaction enthalpies indicate that channels (3) and (5) should be the principal reaction channels. The formation of ozone through channel (5) has not been detected in experimental studies of this system and this pathway is therefore thought to be of minor importance. The principal reaction channel is that leading to the formation of HOCl and 0,.This is supported by experimental observation of HOCl as a product of the C10 + HO, The formation of ClOO + OH through channel (1) is only slightly endothermic and may be a minor reaction pathway under suitable conditions. The remaining channels are too endothermic to be of importance to the C10 + HO, reaction. The C10 + HO, reaction is believed to be involved in the destruction of stratospheric ozone.The destruction cycle is a consequence of the photolysis of HOCl and is outlined below, C10 + HO, +HOCl + 0, HOCl + hv +Cl + OH Cl + 0,+ClO + 0, OH + 0, +HO, + 0, net 2 0, +3 0, This reaction scheme is thought to be particularly important in the middle and lower stratosphere. Experimental studies'-6 of the C10 + 0,H reaction have also found that the reaction displays a complicated tem-perature dependence, which is inconsistent with a simple H-atom metathesis reaction. The rate of the reaction is very fast and the experimentally measured rate constant at 298 K is about (4.5-6.5) x cm3 molecule-' s-l, and has a negative temperature dependence.The experimental results suggest that the reaction proceeds by a multi-step mech- anism, possibly via a reaction intermediate. This postulate is supported by the Arrhenius A factor determined from the experimental studies. This is found to be 4.6 x 10-l2 cm3 molecule-' s-'. The range in magnitudes of the rate constant arises from the different experimental techniques that have been employed to study this system. The concept of a multi- step reaction mechanism has been postulated by a number of author^,^^^.^-* but to date there is no experimental evidence for the existence of reaction intermediates. The study of short-lived reaction intermediates using experimental techniques is extremely difficult. However, theo- retical techniques can be used to study the possibility of the existence of such intermediates.There have been two theo- retical studies of the ClO + HO, system. Mozurkevich' has studied the reaction using RRKM theory and achieved good agreement with experimental rate parameters by assuming that the reaction proceeds by a multi-step mechanism involv- ing a weakly bound intermediate. The structures and proper- ties of the intermediates were determined from established thermochemical methods. The only ab initio theoretical study, to date, that has performed a full characterisation of reaction intermediates is the work of Toohey and Anderson.8 In this study various ab initio techniques were used to study the triplet reaction surface, under the constraint that any reaction intermediates would be formed with C, symmetry.The authors report the existence of a single saddle point that appears to connect the reactants to the products HOCl and 0,. The Arrhenius A factor derived from this work is found to be in the range 8.7 x 10-l3-5.4 x lo-', cm3 molecule-' s-', in reasonable agreement with experiment. Here we report a complete characterisation of the C10 + HO, reaction surface using second-order Mlaller-Plesset perturbation theory. In this work we have studied the singlet and triplet potential surfaces and report the structures, ener- gies and harmonic vibrational wavenumbers of various inter- mediates and transition states for the C10 + HO, reaction on both surfaces. In the calculations reported here no sym- metry constraint has been imposed.The stationary points located on each surface have been further studied using higher-order perturbation theory. ComputationalDetails Searches for minima and saddle points at the self-consistent field (SCF) level employed the restricted and unrestricted Hartree-Fock methods (RHF, UHF)l2,l3with the 6-31G** basis of Pople and co-worker~.~~*~~ To study the effect of electron correlation on the singlet and triplet potential- energy surfaces of the C10 + HO, reaction, minima and saddle-point calculations were also made using second-order Msller-Plesset perturbation theory (MP2).16 All stationary points located on the singlet and triplet surfaces were charac- terized by harmonic vibrational frequency calculations.The highest level of calculation reported is fourth-order Msller-Plesset (MP4)’ 7*1 calculations using the larger 6- 311G** basis set.” The MP4 calculations include all single, double, triple and quadruple excitations. Barrier heights for the C10 + HO, reaction are reported at all levels of theory. At the highest level, MP4/6-3 1lG**, basis-set superposition error calculations have been performed using the Boys- Bernardi Although the UHF wavefunctions are not true eigen-functions of the (S2> operator through contamination by higher spin states, the largest values of (S2) for the doublet and triplet states considered here were 0.77 and 2.07, respec- tively. There is only a small deviation from the expected values of 0.75 and 2.0, indicating only minor spin con-tamination.The study of the singlet reaction surface required the struc- ture and energy of the oxygen molecule in the ‘8, state to be considered. This calculation was performed using complex orbitals at the RHF and MP2 levels with the 6-31G** basis set. The optimum bond length for this molecule was deter- mined using a non-gradient optimization routine. The results are in good agreement with previously reported results.22 Intrinsic reaction coordinate (IRC)23724 calculations were performed at the MP2/6-31G** level to confirm that the transition states on each reaction surface connect with stable intermediates, reactants or products. The IRC calculations follow the minimum-energy pathway from the transition states.The initial search direction is determined from the imaginary frequency computed at the transition-state geometry. Searches were performed on both the forward and reverse sides of the potential surface with a stepsize of 0.1-0.3 a, u1I2. All calculations were performed using the GAUSSIAN 9225 and GAMESS-UK26,27 software packages on the Intel iPSC/860 and the Convex 220 at the SERC, Daresbury Laboratory, and a HP9000/735 workstation. Results The ClO + HO, reaction can proceed through two com-peting reaction pathways. The first involves the direct attack on the ooHbond of HO, by the singly occupied n* (ClO) orbital. The ground-state configuration for the HO, molecule 5a’),( la“)2(6a’)2(7a’)2(2a’’)1.is (1 a’)2(2a’)2(3a’)2(4a’)2( The oOH orbital of HO, corresponds to the doubly occupied, in plane, 7a’ orbital.The interaction of these orbitals results in the for- mation of a planar, slightly bent, triplet intermediate. The transition state formed from this interaction decomposes to form the ground-state products HOCl + O,, as studied pre- viously by Toohey and Anderson.8 The second reaction pathway arises from the interaction of the singly occupied 2a“ orbital of HO,, which is predominantly a n*(02) orbital, with the singly occupied n*(ClO) orbital. This pathway results in the formation of a singlet intermediate, which is believed to be entropically unfavourable.8 Decomposition of the singlet intermediate leads to the formation of HOCl + 0,.However, due to spin conservation rules, one of the products is formed in an excited state. Fig. 1 shows the orbital interactions for both reactions considered here. Here we present a comprehensive ab initio study of the singlet and triplet potential surfaces for the reaction of C10 with HO,. The results discussed are concerned with the for- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 CIO + H02 -HOCl + 02 Fig. 1 Orbital diagram for the C10 + HO, reaction mation of hydrochlorous acid and molecular oxygen through reaction channel (3). Note that all possible reaction channels discussed previously were considered, but at the level of theory employed here only reaction channel (3) was found to be significant. There was no evidence, in this computational study, of the formation of any other products through the other possible reaction channels.Triplet Surface The calculated geometries, energies and harmonic vibrational wavenumbers of the reactants and products considered in this work are presented in Table 1. This table lists the results from calculations performed at the UHF/6-31G** and MP2/ 6-31G** levels. Also reported are results of single-point MPqSDTQ)/6-311G** calculations at the MP2 optimized geometries. The highest level of calculation reported for the singlet oxygen species is a MP2/6-3 1G** optimization. MP4 calculations on this species were not possible due to the SCF calculation requiring the use of complex orbitals. The MP4 results will be used later to compute ab initio barrier heights for the C10 + HO, reaction.The enthalpy of reaction for the ClO + HO, -,HOCl + 0, reaction under consideration is calculated, at the MP4/6-311G** level, to be ArH (0 K) = -241 kJ mol-I. This value is 50 kJ mol-’ larger than the experimental value of A,H (298 K) = -191 kJ mol-’. The computed heat of reaction could be improved by using higher levels of theory. Comparison of the experimental and calculated heat of reaction indicates that the C10 + HO, reaction surface is reasonably described at the MP4/6-311G** level. Table 2 summarises the results of the study of the triplet potential surface at the UHF/6-31G** and MP2/6- 3 1G** levels. The harmonic vibrational wavenumbers used to characterize the stationary points at the MP2/6-31G** level are tabulated in Table 3.This table also reports the har- monic vibrational wavenumbers for stationary points located on the singlet reaction surface. These results will be discussed in the next section. As discussed by Toohey and Anderson,* for the C10 + HO, reaction the UHF/6-31G** results provide a poor representation of the true MP2 saddle points. The results in Table 2 show that at the MP2 level the OH bond of the HO, species in transition state 1 is shorter than the bond length reported at the UHF level. Similarly the HO(3) bond length in the transition state is longer at the MP2 level than at the UHF level, indicating the transition state, at the MP2 level, occurs earlier in the reaction channel. The UHF transition state is found to occur later in the reaction channel, as a con- sequence of the UHF calculation underestimating the reac- tion exothermicity. These results follow the Hammond postulate2’ that relates the position of the transition state to the enthalpic change of the reaction.These results show the importance of using a correlated method to determine the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1813 Table 1 Reactants and products of the C10 + HO, reaction UH F/6- 3 1 G **O RClO RHO Roo LHOO LHOCl E/Eh 1 c10 1.620 -534.232 27 2 HO, 0.949 1.308 105.9 -150.17663 3 HOCl 1.669 0.947 105.2 -534.848 30 4 02(3%-) 1.168 -149.61791 5 O,(W 1.166 -149.557 49 MP2/6-31G** species &lo RHO Roo LHOO LHOCl E/Eh 6 CIO 1.607 (1.546)d -534.518 65 -534.668 49 7 HO, 0.975 (0.977) 1.325 (1.334) 104.4 (104.1) -150.513 23 -150.650 14 8 HOCl 1.713 (1.689) 0.970 (0.960) 102.4 (100.8) -535.18078 -535.331 43 9 0,(3q) 1.246 (1.207) -149.954 32 -150.079 13 10 O2(lA) 1.264 -149.917 50 harmonic vibrational wavenumbers'/cm -~~ 1 v1 832 6 v1 854 (vl 866)d 2 v1 1251, v, 1601, v3 4074 7 v1 1235, v2 1458, v3 3720 (vl 1098, v, 1392, v3 3436)3 v1 853, v, 1396, v3 4149 8 v1 756, V, 1298, v3 3823 (vl 739, v2 1242, v3 3626)4 v1 1996 9 v1 1411 (vl 1580) " All bond lengths in 8, and angles in degrees.MP4/6-311G** at MP2/6-3 1G**-optimized geometries. Harmonic vibrational wavenumbers calculated at the UHF/6-3 1G** and MP2/6-31G** levels. Experimental values in par en these^.^,^',^**'^*^'-^^. position of the saddle points accurately on the C10 + HO, ducts HOCl + 0,.The HO and C10 bond lengths of HOCl reaction surface. are computed to be 0.966 and 1.711 A, respectively, at the The triplet surface is found to be relatively simple with final point of the IRC calculation. The molecular oxygen only two stationary points being located at the MP2 level. bond length computed from the IRC calculation is 1.220 A. The first corresponds to the transition state discussed above, These results are in good agreement with the ground-state and was reported previously by Toohey and Anderson.* The parameters given in Table 1. In the reverse direction of the second stationary point, a loosely bound local minimum, was IRC calculation, stationary point 2 was located.The internal found as a result of performing an intrinsic reaction coordi- coordinates for this intermediate species are given in Table 2 nate calculation, using point 1 as a starting point. In the and the harmonic vibrational wavenumbers given in Table 3 forward direction the saddle point breaks down into the pro- indicate that this point corresponds to a local minimum. The Table 2 Triplet reaction surface" UHF/6-3 1G ** 1 0(1)0(2)H0(3)C1 1.245 1.110 1.265 1.627 109.7 172.6 107.6 0.0 0.0 -684.367 79 MP2/6-31G** 1 0(1)0(2)H0(3)CI 1.287 1.054 1.414 1.568 105.5 163.1 104.7 0.0 0.0 -685.029 59 -685.316 19 2 0(1)0(2)H0(3)C1 1.323 0.976 2.073 1.591 104.0 144.7 132.4 0.0 0.0 -685.041 24 -685.32435 ~~ ~~ a All bond lengths in A and angles in degrees. MP4/6-311G** results at MP2/6-31G**-optimized geometries.Table 3 Harmonic vibrational wavenumbers computed at the MP2/6-31G** level triplet surface/cm -stationary point V1 V2 v3 v4 v5 '6 v7 '8 v9 1 -1487 154 160 350 814 972 1496 1765 5853 2 46 63 88 150 343 858 1251 1479 3695 singlet surface/cm-' -65 262 395 493 714 797 962 1439 3679 -2205 104 120 28 1 515 782 1424 1531 2075 -152 106 194 367 459 84 1 1374 1759 3696 -291 155 368 517 640 833 854 1366 3791 -467 153 344 512 615 815 836 1401 3787 -281 138 274 361 459 860 1219 1334 3813 -1588 182 247 422 709 876 977 1207 2633 198 239 426 532 685 835 1300 1449 3626 171 182 274 392 683 88 1 1151 1332 3627 -148 120 257 329 520 836 1227 1323 3648 -1670 240 274 473 706 846 895 1467 1913 1814 mechanism for the C10 + HO, reaction on the triplet surface is shown in Scheme 1.$3) O(3j”-CI CI’ 2 1 Scheme 1 A gradient search was performed in an attempt to locate a saddle point that connects minimum 2 to the reactants. At the MP2 level of theory employed here no stationary point could be located on this region of the triplet surface. This could be a consequence of the intermediate species being formed with a negligible activation barrier. This abinitio study of the triplet reaction surface, supports and reproduces the results of Toohey and Anderson.* These results are reported here in order to facilitate comparison with the singlet reaction surface discussed in the next section.In both studies the C10 + HO, reaction on the triplet poten- tial surface is found to proceed through direct attack of the C10 radical on the OH bond of the HO, radical. This reac- tion leads to the formation of ground-state HOCl and 0, through a direct hydrogen-abstraction mechanism. Singlet Surface Here we report the first abinitio study of the singlet potential surface for the C10 + HO, reaction. The calculated geom- etries and energies of the stationary points located on the singlet surface are tabulated in Table 4. The harmonic vibra- tional wavenumbers computed at the MP2/6-3 1G** level to characterize the stationary points on the singlet surface are listed in Table 3. The MP2 singlet surface is more complex than the RHF/6- 31G** surface and the corresponding MP2 triplet surface.Unlike the triplet surface, where the MP2 stationary points were found to be formed earlier in the reaction channel than the corresponding UHF stationary points, the MP2 station-ary points on the singlet potential surface appear to occur later in the reaction channel than the corresponding RHF stationary points. This result and the large number of station- ary points reported in Table 4 indicate that the C10 + HO, reaction on the singlet surface occurs via a different mech- anism from that described for the triplet reaction surface. Eleven stationary points have been located and characterized on the singlet surface. However, IRC calculations indicate that stationary points 1, 4, 5 and 6 are not involved in the C10 + HO, reaction.These stationary points correspond to transition states arising from internal rotation in the reaction intermediates. Stationary point 1 corresponds to a saddle point arising from rotation about the central ClO-00H bond. The barrier to internal rotation about this bond is computed, at the MP2 level, to be ca. 20 kJ mol-l. The remaining rotational isomers, points 4, 5 and 6, correspond to saddle points that arise from rotation about the C100-OH bond. The barrier to internal rotation about this bond is similar in magnitude to the barrier about the C10-00H bond. Fig. 2 plots the relative energies of the stationary points, with respect to the reactants, against the reaction coordinate. The stationary points corresponding to rotational saddle points have been omitted for reasons of clarity.The figure also indicates the stationary points that are found to be con- 50/ J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 products ---, I I I reaction coordinate Fig. 2 Singlet potential surface (MP2/6-31G**). 0,0,see text. nected from IRC calculations. The figure shows that the ClO + HO, reaction on the singlet surface involves the formation of stable reaction intermediates. The reaction path shown in Fig. 3 also indicates that the reaction occurs via a multi-step reaction mechanism. The principal reaction channel on the singlet surface con- nects the local minima, points 8 and 9, to the saddle points 10,7 and 11.In Fig. 2, stationary point 10 does not appear to be a saddle point as it is lower in energy than the reactants to which it is connected. However, the harmonic vibrational wavenumbers given in Table 3 characterize stationary point 10 as a saddle point and the IRC calculation that connects point 10 to point 9 on the singlet reaction surface also shows that point 10 breaks down, in the reverse direction, to form the reactants C10 + HO,. This discrepancy in the energies probably arises from the potential surface not being corrected for the effects of zero-point vibrational energy and basis-set superposition error. The reaction path shown in Fig. 2 is shown schematically in Fig. 3. This reaction scheme shows that the reaction initially involves the formation of a loosely bound species, with the reactants separated by 2.7 A.The remaining stages involve a shortening of the 0(2)0(3) bond length and a migration of the H atom from the HO, species to the C10 radical. The Cl-0(1) distance in stationary point 11 is 2.4 A, whereas the corresponding distance in the triplet &l0(3) 3.3A hO(3)3.0 A 640(3) 2.5 A 10 9 7 H hO(3) .2 A RH0(3) 2.1 A 11 8 Fig. 3 Singlet surface reaction mechanism UHF/6-3 1G* * 1 C10( 1)0(2)0(3)H 1.512 2.615 1.404 0.949 112.1 107.7 100.2 105.2 159.6 -684.30604 2 ClO(1)0(2)0(3)H 1.683 1.343 1.396 0.949 110.0 105.4 100.6 86.0 172.9 -684.363 73 3 ClO( 1)0(2)0(3)H 1.659 1.409 1.356 0.949 105.7 103.2 103.8 184.8 274.2 -684.363 13 4 C10( 1)0(2)0(3)H 1.68 1 1.360 1.368 0.949 109.8 107.8 103.7 -86.1 96.4 -684.369 93 MP2/6-31G** 1 C10(1)0(2)0(3)H 1.702 1.528 1.402 0.972 103.0 101.9 101.4 159.8 86.9 -685.073 51 -685.13704 2 C10(1)0(2)0(3)H 1.671 2.856 1.378 1.087 131.2 43.6 107.2 -9.8 -30.8 -685.039 82 -685.313 64 3 C10(1)0(2)0(3)H 1.646 3.317 1.428 0.980 70.6 40.1 100.9 172.6 75.0 -685.072 96 -685.346 48 4 C10(1)0(2)0(3)H 1.757 1.382 1.487 0.973 109.2 103.7 96.9 -83.3 -169.0 -685.073 57 -685.35030 5 C10(1)0(2)0(3)H 1.768 1.393 1.478 0.974 109.2 107.6 100.3 -88.6 12.6 -685.071 63 -685.34808 6 C10( 1)0(2)0(3)H 1.508 2.807 1.489 0.971 116.8 98.7 96.6 102.4 160.8 -685.035 65 -685.339 03 7 0(1)0(2)H0(3)Cl 1.476 1.221 2.584 1.659 49.3 62.3 73.4 265.2 122.7 -685.02771 -685.291 93 8 0(1)0(2)HO(3)Cl 1.390 0.983 2.157 1.636 102.9 75.0 99.1 259.7 75.7 -685.081 48 -685.340 19 9 0(1)0(2)H0(3)Cl 1.373 1.857 3.054 1.613 31.1 48.8 61.2 268.9 139.9 -685.06624 -685.33692 10 0(1)0(2)HO(3)Cl 1.408 1.864 3.281 1.628 31.1 56.1 60.1 267.9 138.6 -685.064 71 -685.342 33 11 0(1)0(2)HO(3)Cl 1.395 1.164 1.175 1.639 104.5 124.6 105.4 -83.1 75.1 -685.059 65 -685.324 43 a All bond lengths in 8, and angles in degrees.MP4/6-311G** energies at MP2/6-31G**-optimized geometries. stationary point is 3.1 A. This shortening of the C1-0(1) dis-tance indicates that the Cl atom of C10 is involved in this reaction mechanism. The reaction scheme shown in Fig. 3 is a four-centre addition of HO, to C10, followed by hydrogen- atom migration leading to the formation of HOCl + 0,.Two additional stationary points have been located on the singlet surface. Point 3 corresponds to a loosely bound saddle point and point 2 corresponds to a tightly bound transition state. Scheme 2 shows that these stationary points arise from a two-centre addition of C10 to HO,. The Cl-0(2) distance in structures 2 and 3 is 4.1 and 3.2 A, respectively, indicating that in this case the C1 atom of C10 is not involved in the formation of a reaction transition state. From IRC calcu- lations saddle point 2 was found to be connected to the pro- ducts HOCl + 0, in the forward direction and to intermediate 8 in the reverse direction. At the level of theory employed here no stationary point could be located in the reverse direction from saddle point 3.However, point 3 was found to be connected to intermediate 8 in the forward direc- tion. It is believed that saddle points 2 and 3 are involved in an H-atom migration mechanism as discussed above. 3 2 Scheme 2 The singlet surface presented here shows that there are two possible reaction pathways leading to the formation of HOCl + 0,. JRC calculations were performed using saddle points 2 and 11 as starting points. These saddle points were found to decompose to form HOCl + 0,. At the final point of the IRC calculation the products are found to be separated by ca. 3 A and the OH and C10 bond lengths of HOCl were found to be about 0.97 and 1.71 A, respectively. The molecular oxygen species was found to have an internuclear distance of about 1.27 A.Comparison of these values with those given in Table 1 indicate that HOCl is formed in the ground state and that molecular oxygen is formed as a singlet. This conclusion is supported by the results of the IRC calculation on the triplet reaction surface, where the internuclear distance of the 0, product was found to be considerably shorter indicating the formation of the more stable, ground-state, triplet 0,. Discussion The results reported in the previous sections show that the C10 + HO, reaction can take place on the triplet and singlet reaction surfaces. The reaction energies, at the MP4/6-311G** level, for the triplet and singlet surfaces are -241 and -174 kJ mol-', respectively. The reaction energy for the singlet surface is an estimate obtained by assuming that the electron correlation recovered for singlet 0, on going from the MP2 to the MP4 levels is similar to that recovered for triplet 0,.The C10 + HO, reaction on both the triplet and singlet reaction surfaces is computed to be exothermic and energetically favourable. The energies reported in Tables 2 and 4 show that the energies of the two surfaces are very similar. The lowest energy, intermediate species are formed on the singlet surface. This indicates that on the singlet surface there is the possi- bility that the C10 + HO, reaction involves the formation of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 relatively long-lived intermediates. The barrier heights, with respect to the reactants, for the stationary points located on each surface are given in Table 5.The barrier heights are reported at the MP2/6-31G** and MP4/6-311G** levels. The MP4 results have been corrected for zero-point vibrational and basis-set superposition error effects. The barrier heights at this level of theory tend to overestimate the true barrier height^.^'.^' Brown and Truhlar have shown that even large- scale configuration interaction calculations can overestimate barrier heights by several kcal mol-'.3' For the singlet reac- tion surface the corrected barrier heights reported in Table 5 are slightly higher, less negative, than those shown in Fig. 2. However, the structure of the reaction surface is not dramati- cally altered from that shown in Fig.2 and these results support the reaction mechanism discussed in the previous section. The barrier height between stationary points 9 and 7 on the singlet surface is 112 kJ mol-', indicating that inter- mediate species 9 may be relatively long-lived. The corrected barrier heights for the triplet surface are slightly higher than those computed at the MP2 level. The MP4 results indicate that on the triplet reaction surface the CIO + HO, reaction proceeds with a negligible or small activation energy. These results indicate that the observed negative tem-perature dependence of the ClO + HO, rate constant cannot be explained by considering only the direct reaction mech- anism that takes place on the triplet reaction surface. Stimp- fle et al.' found that the experimental rate constant could be represented by the rate expression, k, = 3.3 x exp(-850/T) + 4.5 x lo-" (T/300)-3.7 cm3 molecule-' s-'.This expression shows that the Arrhenius plot of the C10 + HO, reaction is strongly curved and that at high tem- peratures the activation energy tends to zero. This result sug- gests that at high temperatures the triplet reaction surface is dominant, leading to the formation of ground-state HOCl + 0,. At low temperatures the Arrhenius plot displays a strong negative temperature dependence, indicating the influ- ence of a multi-step reaction mechanism. Therefore at low temperatures the singlet reaction surface must play an impor- tant role in the C10 + HO, reaction.Dynamics calculations will determine whether these initial conclusions are valid for the C10 + HO, reaction. However, to perform such calcu- lations accurately the energies of the stationary points pre- sented here will have to be recalculated at a higher order of theory. The singlet reaction mechanism results in a decrease in the formation of triplet molecular oxygen and a corresponding increase in the formation of singlet molecular oxygen. The role of singlet molecular oxygen, a relatively long-lived excited state of O,, in the atmosphere has been explored by Table 5 Barrier heights (kJ mol-') corrected stationary barrier point AEa AEb BSSE ZPVE' /kJ mol-' singlet surface 2 -20.3 13.1 25.3 -9.0 29.4 3 -107.3 -73.2 14.6 2.8 -55.8 7 11.5 70.1 19.8 -6.4 83.5 8 -129.6 -56.6 20.3 5.7 -30.6 9 -89.6 -48.0 17.0 2.2 -28.8 10 -85.6 -62.3 12.5 -0.4 -50.2 11 -72.3 -15.2 37.9 -9.1 13.6 triplet surface 1 -6.8 6.3 27.3 19.3 52.9 2 -23.9 -15.0 7.9 -2.1 -9.2 a MP2/6-31G** barrier heights.MP4/6-311G** barrier heights.'Determined at the MP2/6-31G** level. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1817 Handwerk and Zellner.,, Singlet oxygen was believed to be 10 JANAF Thermochemical Tables, ed. D. R. Stull, H. Prophet, involved in the production of ‘odd’ oxygen through the for- mation of ClO, but the recent work by Rauk et aL3, has shown that this is unlikely. The HOCl produced from the C10 + HO, reaction has an important role in stratospheric chemistry. If the hydrochlorous acid formed has a long life- 11 12 13 14 National Bureau of Standards, New York, 1979. G.Hirsch, P. J. Bruna, S. D. Peyerimhoff and R. J. Buenker, Chem. Phys. Lett., 1977, 52, 442. C. C. J. Roothaan, Rev. Mod. Phys., 1951,23,69. J. A. Pople and R. K. Nesbet, J. Chem. Phys., 1954,22,571. W. J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys., 1972, time it can be considered to act as a sink for ‘active’ chlorine, 56,2257. therefore decreasing the effects of stratospheric ozone destruction. However Guo~~has shown that HOCl can absorb a UV photon and dissociate to form radical pro- ducts., 15 16 17 P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28, 213. C. Msller and M. S. Plesset, Phys. Rev., 1934, 46, 618. R. Krishnan and J.A. Pople, Int. J. Quantum Chem., 1978, 14, 91. HOCl + hv -+ OH + C1 18 R. Krishnan, M. J. Frisch and J. A. Pople, J. Chem. Phys., 1980, 72, 4244. OH and C1 are the principal products of photodissociation of HOCl whereas the quantum yield of alternative products such as 0 + HC1 has been shown to be negligible.36 The pro- ducts formed from the photodissociation of HOCl would 19 20 21 R. Krishnan, J. S. Binkley, R. Seeger and J. A. Pople, J. Chem. Phys., 1980, 72,650. J. H. van Lenthe, J. G. C. M. van Duijneveldt-van de Rijdt and F. B. van Duijneveldt, Adv. Chem. Phys., 1987,69, 567. S. F. Boys and F. Bernardi, Mol. Phys., 1970,19, 553. result in accelerated ozone destruction as discussed in the 22 K. Nahm, Y. Li, J. D. Evanseck, K. N. Houk and C.S. Foote, J. introduction. The importance of the ClO + HO, reaction to stratospheric chemistry is therefore dependent on the rate of photolysis of HOCl under stratospheric conditions. In this work we have reported the results of a com-prehensive ab initio study of the C10 + HO, reaction. It is 23 24 25 Am. Chem. Soc., 1993,115,4879. C. Gonzalez and H. B. Schlegel, J. Phys. Chem., 1989,90,2154. C. Gonzalez and H. B. Schlegel, J. Phys. Chem., 1990,94,5523. GAUSSIAN 92, Revision A, M. J. Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. W. Wong, J. B. Foresman, B. G. Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R. Gom- found that the reaction can proceed on both the triplet and singlet reaction surfaces and that the unusual temperature dependence of the rate constant can be explained only through the consideration of the multi-step singlet reaction mechanism.It has also been shown that the products of the HO, + C10 reaction could have a detrimental effect on 26 27 perts, J. L. Andres, K. Raghavachari, J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. DeFrees, J. Barker, J. J. P. Stewart and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1992. M. F. Guest, P. Sherwood, GAMESS-UK and User’s Guide and Reference Manual, SERC Daresbury Laboratory, 1992. M. F. Guest, R. J. Harrison, J. H. van Lenthe and L. C. H. van Corler, Theor. Chim. Acta, 1987,71, 117. stratospheric ozone concentrations. 28 29 G. S. Hammond, J. Am. Chem. Soc., 1955,77, 334. W. J. Hehre, L. Radom, P.vR. Schleyer and J. A. Pople, Ab The authors gratefully acknowledge the SERC for generous grants of computer time at the Rutherford Appleton and Daresbury laboratories, for a grant to purchase a HP735 30 31 Initio Molecular Orbital Theory, Wiley, New York, 1986.A. A. Nanayakkara, G. G. Balint-Kurti and I. H. Williams, J. Phys. Chem., 1992,%, 3662. F. B. Brown and D. G. Truhlar, Chem. Phys. Lett., 1985, 117, workstation and funding for DB. 32 307. V. Handwerk and R. Zellner, Ber. Bunsenges. Phys. Chem., 1986, References 33 10,92. A. Rauk, E. T. Tschuikow-Roux, Y. Chen, M. P. McGrath and B. Reimann and F. Kaufman, J. Chem. Phys., 1978,69,2925. R. M. Stimpfle, R. A. Perry and C. J. Howard, J. Chem. Phys., 1979,71,5183. M-T. Leu, Geophys. Res. Lett., 1980, 7, 173. T. J. Leck, J-E. L. Cook and J. W. Birks, J. Chem. 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Paper 3/07015H; Received 25th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001811

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(13), 1819-1829 Mechanism of Atmospheric Oxidation of 1,I ,I ,2=Tetrafluoroethane (HFC 134a) Oliver V. Rattigan,* David M. Rowley, Oliver Wild and Roderic L. Jones Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Lensfieid Road, Cambridge,UK CB2 IEW R. Anthony Cox NERC, Polaris House, North Star Avenue, Swindon, UK SN2 IEU The chlorine-initiated photooxidation of hydrofluorocarbon 134a (CF,CH,F) has been studied in the temperature range 235-318 K and at 1 atm total pressure using UV absorption. Trifluoroacetyl fluoride [CF,C(O)F] and formyl fluoride [HC(O)F] were observed as the major products. IR analysis of the reaction mixture also showed car- bony1 fluoride [C(O)F,] as a product. By measurement of the yields of HC(0)F from the photooxidation as a function of [O,] and temperature, the rate of the unimolecular decomposition of the oxy radical, CF,CHFO, reaction (5),was determined relative to its reaction with O,, reaction (4): CF,CHFO + 0, +CF,C(O)F + HO, (4) CF3CHF0+CF3 + HC(0)F (5) The results were treated using both an arithmetic derivation and numerical integration with a detailed reaction scheme, Inclusion of other recently published kinetic data leads to the following recommended rate expression for reaction (5)at 1 atm k, = 7.4 x 10" exp[( -4720 f220)/77 s-' The errors are la.The observation of enhanced product yields in the present work is attributed to the reaction of the CF,O radical with HFC 134a leading to further peroxy radical formation.The results have been incorporated into a 20 atmospheric model to assess the environmental implications of HFC 134a release in the troposphere. It is now widely accepted that chlorofluorocarbons (CFCs) are directly responsible for the increasing levels of strato- spheric chlorine observed over the past decade.' This has in turn led to large losses of stratospheric ozone, particularly over the polar regions (see, e.g. ref. 2). Hydro-chlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have been proposed as replacements for CFCs. These compounds contain one or more carbon-hydrogen bonds and are therefore susceptible to attack by the hydroxy radical in the troposphere leading to shorter atmospheric lifetime^.^ These shorter lifetimes, coupled with reduced chlorine substi- tution, lead to a lower release of chlorine in the stratosphere and hence a lower ozone-depletion potential.In order to assess the environmental impact of HCFC and HFC release it is necessary to quantify the nature and yield of the products from atmospheric oxidation. Although the reaction of the hydroxy radical with these compounds has been the subject of several the detailed mechanis- tic and kinetic pathways are somewhat uncertain. HFC 134a (CF,CH,F) is a proposed replacement for dichlorodifluoromethane (CFC-12) which is widely used as a refrigerant and in air-conditioning systems. In the tropo- sphere OH-initiated oxidation of HFC 134a leads to the for- mation of the peroxy radical CF,CHFO, via the following reactions CF,CH,F + OH -,CF,CHF + H,O (1) CF3CHF+ 0, + M +CF,CHFO, + M (2) The reaction of CF3CHF0, with NO [reaction (3)] has been found to be and is expected to be a major loss reac- tion for the peroxy radical for most situations in the tropo- sphere CF,CHFO, + NO -+ CF,CHFO + NO, (3) Subsequently, CF,CHFO radicals formed in reaction (3) may either react with 0, [reaction (4)] or undergo unimolecular decomposition [reaction (5)] CF,CHFO + 0, -+ CF,C(O)F + HO, (4) CF,CHFO +CF, + HC(0)F (5) The primary product distribution from the oxidation of HFC 134a therefore depends on the relative rates of reactions (4) and (5).We have recently measured the UV spectra and absorption cross-sections, 0, for CF,C(O)F and HC(0)F9-" and in the present work we have used this information to determine the yields of these products in the chlorine photosensitised oxida- tion of HFC 134a by UV spectroscopy.Measurement of the yields as a function of [O,] and temperature gave informa- tion on the reaction mechanism and allowed a determination of the rate constant ratio k,/k, in the temperature range 235- 318 K. During the course of this work a number of other studies of the photooxidation of HFC 134a have been carried out and the combination of these results has greatly aided in the understanding of the complex reaction mechanism and determination of the kinetic parameters. Wallington et a1.l' studied the chlorine-initiated photooxi- dation of HFC 134a over the temperature range 261-353 K and at pressures of 15-5650 Torr using product analysis with FTIR.An expression for k,/k, of 1.58 x exp(3600/T) cm3 molecule-' s-' was reported at 2 atm and a strong pres- sure dependence below 1 atm was found due to fall-off in the 1820 rate of the unimolecular reaction (5).Tuazon and Atkinson', have also studied the photooxidation using FTIR and their branching ratio of k,/k, = 3.2 x exp(3510/T) over the range 273-320 K and 1 atm pressure is in excellent agree- ment. Edney and Driscoll'3 have reported a similar study at room temperature with results that are broadly consistent. Maricq and Szente', have recently reported a flash pho- tolysis UV absorption kinetics study of HFC 134a photooxi- dation.Diode array spectroscopy was used to obtain time-resolved spectroscopic information of the radicals CF,CHFO and CF3CHF0,. They determined the rate of the unimolecular decomposition of the CF,CHFO radical, k, = 3.7 x lo7 exp(-2200/T) s-' over the temperature range 210-372 K and at 250 Torr, from the time dependence of growth and decay of an absorption assigned to CF,CHFO. Zellner et ~1.'~studied the pulsed laser photolysis of HFC 1 34a-C1,-NO-02 mixtures with time-resolved measurement of [OH] and [NO,] using optical methods. By simultaneous fitting to the NO, and OH profiles, they were able to deter- mine values for k,, k, and k, of (1.7 f0.6) x lo-', cm3 molecule-' s-', (2.7 f0.6) x lo-', cm3 molecule-' s-' and (1.8 f0.4) x lo4 s-', respectively, at 295 K and 38 Torr total pressure.An absolute uncertainty of a factor of two was reported for the individual values of k, and k, , but the ratio k,/k, was well determined. There is considerable uncertainty about the fate of the CF, radical produced in reaction (5) and its effect on the kinetics and products in the system. Under the conditions employed in this work, CF, is likely to form CF302 radicals by the addition of 0,. The self- and cross-reactions of this peroxy radical lead to CF30.16317Good evidence has also been reported" for the formation of trioxide species such as CF,O,CF,, formed in the reaction of CF30 with CF,O,, although these substances are unlikely to be formed in the atmosphere in view of the low radical concentrations.Sehes- ted and Wallington found evidence that CF,O reacts with CF,CHFO, to form a trioxide which is less stable than CF,O,CF, ." Recent studies have also shown that the rate constant for the reaction of the CF,O radical in H-atom abstraction from hydrocarbons is rapid, with room-temperature rate coeffi- cients similar to those for OH Sehested and Wallington found that CF,O reacts with HFC 134a to form CF,OI-I which decomposes to give COF, and HF." It seems likely that CF,OH formation is the fate of CF,O in the tro- posphere. In the present work the relative importance of the thermal decomposition of CF,CHFO compared to its reaction with 0, has been computed using a 2D atmospheric model2, The fate of the product CF,C(O)F in the atmosphere is also dis- cussed.Experimental The dual-beam diode array spectroscopy system used in this study has been described in detail previo~sly.~~ A 1 m long jacketed quartz cell connected to a standard greaseless vacuum system was used as the reaction vessel. The cell was thermostatted at 235-318 K using flowing ethanol, and evac- uated dual-window assemblies prevented frosting of the optical faces at low temperatures. Four interchangeable pho- tolysis lamps were mounted adjacent and parallel to the cell. In the present study Philips TL/09 lamps of spectral output in the range 300-400 nm were used. The source of UV for absorption measurements was a deu- terium lamp (30W, Hamamatsu L 1636) collimated output from which was passed through a beam splitter (Oriel Scien- tific, model 78150) producing two beams (reference and J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 sample) which were collected in optical fibre couplers (Oriel Scientific model 77800) either directly (reference) or after passage longitudinally through the cell (sample). These two beams were then resolved and imaged by a 275 mm Czerny- Turner spectrograph, at a spectral resolution of 1.2 nm, separately onto two 512 channel unintensified silicon diode arrays (Reticon). The spectrograph and detector were con- trolled by a microcomputer (Dell 316SX) and software pack- ages (Spectroscopy Instruments Ltd.) were used for linearisation of the wavelength scale, for background subtrac- tion and averaging of the data and for calculation of the absorbance using the reference spectrum to correct for changes in the source lamp intensity.Despite this correction, however, the main limitation in absorbance measurements was baseline drift, attributed to inhomogeneities in the source output affecting the reference and sample beams differently. This limited the precision of measurements to f0.0005 absorbance units. CF,CH,F (99.8%) was obtained from ICI Chemicals and Polymers Ltd. and CF,C(O)F (97%) was obtained from Flu- orochem. Both samples were purified by trap-to-trap distilla- tion prior to use. Research grade samples of C1, (5% in N,), 0, and N, obtained from BOC were used without further purification. Formyl fluoride was prepared by the reaction of formic acid with benzoyl chloride using dry KHF, at 333 K.,, The formyl fluoride was first purified by passing through a trap at 255 K to remove benzoyl chloride vapours and then trapped at 78 K.Several successive distillations of the formyl fluoride were carried out. Results Reference UV absorption spectra of pure samples of CF,C(O)F and HC(0)F were recorded over a range of condi-tions, prior to the photooxidation study.'*'' Example spectra are shown in Fig. 1. The absorption cross-section for CF,C(O)F was 13.8 x lop2' cm2 molecule-' at the maximum, 214 nm, and was found to have a small tem- perature dependence.' For HC(0)F the cross-section at 230 nm was 6.85 x lo-,' cm2 molecule-', using a resolution of 1.2 nm (FWHM), in good agreement with the results of Gid- dings and Innes.26 The absorption cross-section was found to be independent of temperature in the range 233-318 K.IR spectra of the two main products were also recorded, in order to confirm their purity. For the photooxidation study, mixtures of CF,CH,F (8-12 Torr), Cl,, (1 Torr, 5% in N,) and 0, (20-730 Torr) were made up to a total pressure of 760 Torr using N, (17-731 Torr) and pre-mixed in the dark for several hours. Irradiation in the wavelength range 300-400 nm was then used to drive the C1-atom-initiated photooxidation of the hydro-fluorocarbon and at regular intervals prior to and during this photolysis, UV spectra of the reaction mixture were recorded.Fig. 2 shows three sequential spectra taken following 11 1, 171 and 243 s of irradiation. As can be seen from these spectra, there is a build-up of absorption around 230 nm, attributable to product formation and a reduction of absorption around 300 nm, attributable to chlorine consumption, with increas- ing irradiation time. Spectral stripping routines were applied to the sequential spectra for the identification and quantification of products formed during the photooxidation. Reference spectra re-ported above were used, allowing for the temperature depen- dence of the CF,C(O)F spectrum. However, owing to the very weak UV absorption of HFC 134a in the wavelength range used in this study, consumption of the hydro-fluorocarbon could not be monitored. Yields of the two main J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10.0 I I A 'I 4.0 I Gu-../0.01 . , , . , . . ,--, II 1 ,I1 I I. I I I I I I I I I 225.0 232.5 240.0 247.5 255.0 262.5 wavelength/nm 200 220 240 260 280 wavelength/nm Fig. 1 Reference UV absorption spectra of, A, HC(0)F and, B, CF,C(O)F at (a)238 and (b)293 K products were therefore expressed in terms of the molecular chlorine consumed, given that the experimental conditions were arranged such that chlorine atoms were converted stoi- chiometrically into CF,CHFO, radicals. Chlorine cross-sections were taken from the current NASA evaluation.' A graph of product formation against chlorine consumption gave the 'chlorine based' yield as the gradient, and the small non-zero intercept, where present, was attributed to differen-tial baseline shifting of the sample and reference beams and neglected.Errors on individual yields were taken from these graphs and combined with a +_5% uncertainty in the cross- sections. Typically, this resulted in a total error in the yield of 0.12 1 1 1 A 0.0 ' . ' wavelength/nm Fig. 2 Sequential UV absorption spectra of a photolysed HFC 134a-C12-0,-N, mixture at (a) 111, (b)171 and (c) 243 s f8%. Following spectral stripping of Cl,, HC(0)F and CF,C(O)F, the baseline of UV absorption showed no addi- tional significant absorbance. The photooxidation study was carried out at four experi- mental temperatures in the range 235-318 K, and at a range of oxygen partial pressures. This range of oxygen pressures used at each experimental temperature was chosen such that comparable and therefore relatively easily measurable con- centrations of the two main products were obtained.Chlorine based yields of HC(0)F and CF,C(O)F obtained are shown in Table 1. Not all spectra were analysed for the CF,C(O)F yield, however, since this quantity proved to be both difficult to ascertain and of limited use in extracting branching ratios. The problem is discussed further below. In addition to the UV yield analysis, qualitative experi- ments to determine the composition of the reaction mixture following photolysis were carried out using IR spectroscopy. A spectrum of a typical photolysed mixture of HFC 134a, Cl,, 0,and N, is shown in Fig.3. In addition to the major products, absorptions attributable to COF, , HF and SiF, were observed. Discussion In the photooxidation study, the C1-atom-initiated formation of peroxy radicals, CF,CHFO,, in the absence of NO, leads to the self-reaction CF,CHFO, + CF,CHFO, +2CF,CHFO + 0, (6a) -,CF,CHFOH + CF,C(O)F + 0, (6b) This reaction provides a convenient way of studying the reac- tion of the oxy radical in the absence of chain processes initi-ated by the presence of NO in the system. Oxy radicals formed in the non-terminating step (6a) then either react further with 0, [reaction (4)] or unimolecularly decompose [reaction (5)J The final product distribution reflects this branching.The basic mechanism for the photooxidation, in the absence of NO, is illustrated diagrammatically in Fig. 4. Table 2 shows the full mechanism, which is similar to that Table 1 Yields of products from the photooxidation of HFC 134a yield" T/K [0,]/10'8 molecule cm-, HC(0)F CF,C(O)F 293 5.98 2.48 1.35 9.89 1.96 1.87 19.3 1.35 1.65 24.0 1.15 1.85 235 0.822 0.98 b- 2.06 0.75 1.42 3.25 0.60 1.26 4.11 0.59 1.34 4.77 0.60 b- 6.1 1 0.36 1.57 273 5.71 1.54 2.13 10.59 1.07 1.67 14.27 0.725 1.82 17.73 0.754 1.75 318 9.78 3.78 b- 12.20 4.50 2.26 17.80 3.12 b- 22.20 3.00 1.44 Product yields are expressed in terms of the amount of C1, con-sumed. Not analysed, see text for details.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 t 1 I I I I 1 IllIlII1 11....1..-1...1---1...1--.1-I...I.. * -''.I.-. I ---1 . -.I. -'I --. 1-4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 wavenumber/cm-' Fig. 3 IR spectrum of a photolysed HFC 134a-Cl,-O,-N, mixture presented by Wallington and co-workers.' '919 Analysis of the depends on oxygen concentration measured product yields to obtain the CF,CHFO rate con- stant ratio k,/k, was carried out using two separate tech- B = k4Co,lM~4Co,l+ k,) niques. These methods are discussed individually below. Both of these branching ratios affect the observed product distribution. A preliminary analysis of the product data can Arithmetic Derivation of kJk, be undertaken by arithmetically expressing the yields.Thus, for a nominal photodissociation of x chlorine molecules in The branching ratio for the non-terminating channel of the the presence of excess of HFC and oxygen, 2x CF,CHFO,CF,CHFO, radical self-reaction is defined as 01 = k6d(k6a are produced.+ k6&. Similarly, the branching ratio for the CF,CHFO radical reaction with oxygen is defined as B. B therefore xC1, +2xC1+ 2xHCl+ 2xCF,CHFO, Table 2 Full mechanism for the photooxidation of HFC 134a in the absence of NO" reaction A EIR reaction c1, + hv +2c1 --fitted C1+ CF3CH,F +HCl + CF3CHF 1 x lo', 1958 CF3CHF + 0, +CF3CHF0, (2) 2CF3CHF0, -+ 2CF3CHF0 + 0, a6.7 x -700 (64 -+ CF,CHFOH + CF,COF + 0, (1 -a)6.7 x -700 (6b) CF3CHF0+ 0, -+ CF3COF+ HO, 6.0x 10-14 925 (4)CF,CHFO -+ CF, + HC(0)F fitted HO, + CF3CHF0, -+ CF,CHFO,H + 0, 5.7 x 10-13 -700 (7)HO, + HO, +H,O, + 0, (9)CF, + 0, +CF30, (10) CF,O, + CF30, +2CF,O + 0, 1.2 x 10-13 -800 (11) CF30, + HO, +CF30,H + 0, 5.7 x 10-13 -700 (12) CF,O, + CF3CHF0, -+ CF30 + CF3CHF0 + 0, a7.6 x lo-', -700 (13) -+ CF,OH + CF3COF + 0, (1 -a)7.6 x -700 (14) CF,O + HC(0)F --+ CF,OH + C(0)F 1.2 x lo-', 2030 (15)CF,O + CF30, +CF,O,CF, (16)CF,O + CF,CHFO, --+ CF3CHF03CF3 fitted CF,O + CF3CH,F -+ CF30H + CF3CHF 1.2 x lo-', 2030 (8) a Reactions of HO, with peroxy radicals are assumed to lead to stable hydroperoxides, with rate coefficients equal to that for the CH30, + HO, reaction.26 Temperature dependence of CF,CHFO, branching ratio, (1 -a)= 3.0 x loT3exp(1200/T).J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 CF3CH2Fexcess 2CI + 202I CF30 CF,C(O)F + CFSCHFOH L---cF302 1I CF3CH F02H 1 CF3CHF02 f CF30H ___t C(0)FZ + HF + CF3C(O)F Fig. 4 Photooxidation mechanism for HFC 134a in the absence of NO Similarly, in terms of a and p, the reaction of the peroxy radical can be expressed 2xCF3CHF0, +2xaCF3CHF0 (64 -+ 2x(1- a){CF,CHFOH + CF,C(O)F} (6b) For the oxy radical: 2xaCF,CHFO -+ 2xafiCF,C(O)F + 2xafiH0, (4) -+ 2xa(l -p)HC(O)F + 2x41 -/?)CF, (5) Considering the secondary chemistry, HO, radicals are known to react rapidly with RO, radical^,'^ usually some- what faster than the R02 self-reaction, and the most prob- able fate of HO, generated in reaction (4) is therefore the cross-reaction with the 'parent' RO, species.Thus 2xapH0, + 2xaflCF,CHFO, +2xaBCF,CHFOOH( +0,) (7) In the scheme described above, the formation of HO, can therefore be assumed to give a further consumption of xap photodissociated chlorine molecules. For CF, radicals, the chemistry is somewhat more complex. The radicals are likely to combine rapidly with oxygen to form CF302 radicals, which then undergo self- and cross-reactions with all other RO, type radicals present. A number of these reactions are expected to give rise to CF,O and, as has been recently these species react ana- logously to C1 atoms in hydrogen abstraction from hydrocar- bons. Thus, in the excess of HFC 134a used in these experiments, the CF,O radicals regenerate CF,CHFO, rad-icals via reactions (8) and (2), in this case without any con- comitant C1, consumption.The chemistry of the CF, radical is therefore simplified for the purposes of this model. It is assumed that, apart from the regeneration of CF,CHFO,, no other effect on the measured product distribution arises: 2xa(l -fi)CF, O2 2x41 -P)CF,O, CF302IR02 t 2x41 -B)OCF,O 2xa(l -p)6CF3O CF3CHzF excess 2x41 -P)BCF,OH+ 2xa(l -B)BCF,CHF (8) 2x41 -/?)6CF,CHF 02excess * 2xa( 1 -B)6CF3CHF0, (2) A further term, 0, is therefore defined here as the fractional efficiency of production of CF,O radicals from CF, . Sehested and Wallingt~n'~ have shown that CF,O radicals react with HFC 134a in this photooxidation and assign an IR absorption feature to CF,OH.The CF,OH is observed to decay slowly in the dark, giving C(0)F2 and HF. These pro- ducts were also identified in the IR spectrum of the pho- tolysed mixture from this study see Fig. 3. Furthermore, the observation of SiF, in this study presumably arises from the hetergeneous reaction of HF on the quartz cell walls. A chain reaction is therefore taking place following the ini- tiation of reaction by dissociation of chlorine, whereby both products CF,C(O)F and HC(0)F are formed and CF,CHFO, is regenerated. However, since only a fractional regeneration of the peroxy radical takes place, the chain length of this process is limited. The total amount of CF,CHFO, reacting, t(RO,), following initiation by disso- ciation of a nominal number, n, of C1, molecules can there- fore be expressed as the sum of a geometric series of the form a, ar, ar2, ar3, ..., where r is the fractional regeneration of CF,CHFO, and convergence is defined by r c 1.This sum is given by t(R0,) = a/(1 -r). The total yield of a given product, y(prod), can then be expressed as a fraction,f, of this total RO, reacting, divided by the nominal chlorine concen- tration considered to initiate reaction. Thus Y(PW =fCt(ROz)l/n wherefand t(R0,) are defined in terms of a and B. Solving in this way for the yield of the HC(0)F leads to HC(0)F yield = [2a(l -B)]/[1 -a8 -q?(1 + O)] Substituting for p in terms of k,/k, then gives the following linear relationship l/YCHW)FI = C(1 + a)/2QIC021(k4/k5)+ "1 -4/2a1 Plots of l/y[HC(O)F] us.LO,] for data at each experimen- tal temperature are presented in Fig. 5, and show reasonable linearity. Fitted parameters for these plots are shown in Table 3. Using values of a from the work of Wallington et a/.' ' values of 8 and k,/k, have been calculated and are also shown in the table. k,/k, is shown in Arrhenius form in Fig. 6. There is an indication of curvature in the Arrhenius plot, but an unweighted fit to the k,/k5 data gives k,/k, = 1.18 x exp(2860 380)/T cm3 molecule-' The errors are la. Table 3 Parameters obtained from plots of [HC(O)F]-' us. [O,] T/K intercept gradient" ab e k4k" 235 0.697 28.3 0.50 0.606 18.0 273 0.312 6.30 0.76 0.692 5.44 29 3 0.252 2.56 0.82 0.716 2.31 318 0.165 0.778 0.87 0.819 0.724 Units cm3 molecule-'. Cakulated from Wallington et al." J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.51 0.oLL 0 5 10 15 20 25 [02]/10'8molecule ~m-~ 0.1j 0 5 10 15 20 0l8molecule CM-~[0,]/10'8molecule ~m-~ [02]/1 Fig. 5 HC(0)Fyield-' us. [O,] at (a)293, (b) 235, (c) 273 and (d)318 K 8 increases almost linearly with temperature. The efficiency of regeneration of RO, radicals from CF, is therefore enhanced at higher temperatures. Thus, given that an excess of HFC 134a and oxygen was always present in these experi- ments, this can be attributed to an increased conversion of CF, into CF,O radicals at higher temperatures, presumably resulting from both an increased rate of the CF302 self- reaction relative to CF302 +RO, and a higher branching ratio for CF,O formation in the CF302+RO, cross-reactions.A similar approach was taken to try to quantify the CF,C(O)F yield in terms of a and fi. However, unlike HC(O)F, this compound is formed in small amounts in a number of different channels in the photooxidation of HFC 134a. The sensitivity of the yield to changes in a and fi is consequently reduced. Furthermore, because the apparent experimental yields of CF,C(O)F could be erroneous because of the presence of similar absorbing species, as discussed below, no meaningful estimates of branching ratios could be produced from this yield. lo-'*kj 10-21""1"1"'1'1 I'II"'I 1"""" 3.c 3.5 4.0 4.5 lo3 KIT Fig.6 kJk, deduced from arithmetic analysis (0)and computer simulation (0)of the product yields expressed in Arrhenius form In conclusion, the preliminary analysis of yields to give estimates of the branching ratio for the CF,CHFO radical reaction provides a useful indication of the likely chemistry involved in the HFC 134a photooxidation. The observed yields of HC(0)F confirm the presence of a chain reaction regenerating CF,CHFO, without chlorine consumption. Kinetic Modelling In order to provide more- exact- kinetic parameters for the chlorine-initiated photooxidation of HFC 134a, in particular the branching ratio for the reactions of the CF,CHFO radical, a computer simulation of the experimental system was carried out using numerical integration.The kinetic model used was based on the reaction mechanism shown in Table 2, together with the source references for the rate coeffi- cients. In order to define the efficiency of regeneration of CF,CHFO, radicals, 8, the degradation of the CF, radical produced in reaction (4) is elaborated in some detail. The degradation chemistry is in accordance with the product analysis work of Sehested and Wallingt~n'~ and the dis- covery in the present work of enhanced product yields attrib- uted to secondary attack on HFC 134a by the CF,O radical formed from CF302. The rate coefficients for the self- and cross-reactions of the peroxy radicals CF,O, and CF,CHFO, are taken from the recent direct kinetic studies of Maricq and Szente16 and from Nielsen et aI.17 The tem- perature dependence of the branching ratio for the formation of radical (CF,CHFO) and molecular products [CF,C(O)F +CF,CFHOH] from the CF,CHFO, self-reaction is taken from the work of Wallington et al." The same branching ratio was assumed for the cross-reaction of CF,O, and CF,CHFO, .The model also contained the newly discovered reaction of CF,O with peroxy to form stable trioxides, CF,O,R (R =CF, and CF,CHF). Rate coefficients for the formation and decomposition of the trioxides were taken from Sehested and Wallington." The temperature depen- dence of the decomposition of CF,O,CHFCF, was based on J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 the 295 K value of k = 2.7 x s-' and an assumed A factor of lo', s-'; this gave an activation energy of 99 kJ mol-'. Preliminary results from Sidebottom2' and Chen et a!.'' using a relative rate technique, and Zellner et using a flash photolysis-LIF method, have shown that the reactivity of CF30 in H-abstraction from hydrocarbons is comparable to that for OH radicals. Sehested and Wallington'' have shown that the primary product of the reaction of CF30 with HFC 134a is CF30H which decomposes heterogeneously to C(O)F, and HF. The rate coefficient for the reaction of CF,O with HFC 134a is based on the 295 K value of k =(1.1 -t 0.7) x lo-', cm3 molecule-' s-' determined by these workers, and an assumed A factor of 1 x lo-', cm3 molecule-'s-'. The rate coefficient for the reaction of C1 atoms with HFC 134a at 298 K, k = 1.4 x lo-'' cm3 molecule-' s-', is well establi~hed.~~~~~The temperature dependence is based on an assumed A factor of 1.2 x lo-', cm3 molecule-' s-'.The secondary removal of HC(0)F by reaction with C1 and CF30 was also included in the reaction scheme. The rate coefficients for the reaction of CF30 with HC(0)F were assumed to be the same as for CF30 with HFC 134a. The rate coefficient for C1 + HC(0)F at 295 K has been determined by Wall- ington et d.," k = 2.0 x lo-'' cm3 molecule-' s-'. The temperature dependence is based on an assumed A factor of 1.0 x lo-', cm3 molecule-' s-'. Using these rate coeffi- cients, the calculated secondary loss of HC(0)F was very small at the extent of reaction of HFC 134a used in the experiments. This was in accordance with the observations.The expression for the temperature dependence of the rate coefficient for the reaction of CF3CHF0 with 0, is based on the room-temperature value, determined by Zellner et a/.,' k, = 2.7 x lo-'' em3 molecule-' s-', and an A factor of 6.0 x lo-', cm3 molecule-' s-', equal to that recommended for the C,H,O + 0, reaction6 -a The unknown rate coefficient, k, (unimolecular decomposi- tion of CF,CHFO) and RCI2(the rate of Cl, photolysis) were obtained by fitting computer-generated concentration-time data for Cl,, HC(O)F, CF,C(O)F and other products, to experimental data obtained in 16 experiments covering a range of temperatures and 0, partial pressures.The kinetic equations were integrated using the FACSIMILE pr~gram,~' which contained an optimization routine for fitting unknown parameters to experimental data, using a non-linear least- squares criterion. In exploratory experiments it was found that the computed yields of CF,C(O)F were always substantially smaller than the experimentally determined yields, when values of k, and RC12were adjusted to fit the HC(0)F and C1, concentrations. This observation was consistent with results from the arith- metic analysis of yields and was therefore attributed to an experimental overestimation of the CF,C(O)F yield rather than a problem with the analyses. Such an erroneous obser- vation could arise as a result of further UV absorptions underlying the smooth CF,C(O)F spectrum.A number of candidates for these products exist, of which hydroperoxides (CF,CHFOOH, CF300H, H,O,) showing broad-band absorption in the UV region and trioxides (CF303CF,, CF,O,CHFCF,), for which no UV spectra have been re-ported, are strong possibilities. Interestingly, adding the mod- elled hydroperoxide products to the modelled CF,C(O)F yield significantly improved the fit to the experimental data, as shown in Fig. 7. However, because of this uncertainty, only HC(0)F and C1, experimental data were used to determine Rc12and k, . The effect of varying the branching ratio for the cross-reaction of CF,O, and CF,CHF02 was also tested since the amount of the molecular channel in this reaction influences the fraction of CF, radicals forming CF30, and hence the regeneration of CF3CHF0, ,i.e.the regeneration efficiency 6. As expected it was found that the value of k, needed to fit the I0 -0 100 200 300 400 500 -,111II. 0 100 200 300 400 500 600 700 800 50 100 150 200 250 300 350 Fig. 7 Computed (-) and experimental (0,HC(0)F; .,CF,C(O)F and +,Cl,) concentration-time profiles from HFC 134a photooxida- tion. Computed CF,C(O)F (---), (a) 235 K, 100 Torr; (b) 293 K, 589 Torr; (c) 273 K, 299 Torr; (d)318 K, 729 Torr. Computed CF,C(O)F + hydroperoxides (see text for details) (. . .). Table 4 Fitted values of RCl2,k, and fraction, 8, of CF, radicals recycled in the photooxidation of HFC 134a" 235 148.6 6.69 3.01 0.37 100.0 5.33 2.46 0.39 79.0 5.41 2.41 0.40 50.0 4.64 2.45 0.42 5.52 f0.85' 0.40 f0.02' 273 761.3 24.0 4.16 0.56 50 1.O 23.7 4.3 1 0.50 403.0 22.8 3.82 0.5 1 299.1 20.5 4.45 0.52 22.8 f1.6' 0.52 f0.02' 293 726.5 96.1 5.34 0.61 584.1 102 5.26 0.62 300.0 125 4.76 0.66 181.3 165 4.52 0.67 122 f31.3' 0.64 k0.03' 318 729.0 340 7.66 0.72 586.0 28 1 7.72 0.72 400.0 679 7.57 0.73 322.0 342 7.40 0.74 411 181' 0.73 f0.01* The errors are lo.Average values. HC(0)F yields increased as the branching ratio for formation of molecular products from this reaction increased. 8 and hence k, were also dependent on the relative values of the rate constants for the CF302 and the CF30 reactions, e.g.hydroperoxide and trioxide formation. Fig. 7 illustrates some of the computed and experimental concentration-time curves. Good fits to the experimental data for HC(0)F and C1, were obtained in all experiments and the values of RCll and k, were well and independently determined, except at 318 K and low 0, where the corre- lation coefficient for the fitted parameters reached 0.88. Table 4 summarizes the fitted values obtained at four temperatures for RC,2and k,, together with the values of 8, obtained from the ratio of the calculated yields of CF,OH from reactions (8), (14) and (15) and HC(0)F from reaction (5). The uncer- tainties are la values obtained from averaging four experi- ments at each temperature.The values of k,/k, obtained from the simulation are plotted in Fig. 6, together with the data from the arithmetic analysis. The agreement between the values of the ratios obtained from the two methods is good, and both data sets indicate possible curvature in the Arrhenius plot. The unweighted fit to the ratio k,/k, from the simulation gives k,/k, = 9.37 x exp(2960 & 620/T) cm3 molecule-' and a fit to results obtained from both analyses gives k,/k, = 1.05 x exp(2910 & 310/T) cm3 molecule-' The errors are la. Discussion of Kinetic Parameters Expressions for the temperature dependence of the relative rate of the decomposition of CF,CHFO compared to the reaction with 0, have been reported by Wallington et d." and by Tuazon and Atkinson', from data in the range 261-353 K and 273-320 K, respectively.In order to compare results for the unimolecular decomposition, k, was calculated in each case using the expression k, = 6.0 x lo-', exp(-925/T) cm3 molecule- s-' as used for the reference reaction in this study. Fig. 8 shows an Arrhenius plot of the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 lU3.0 3.5 4.0 4.5 103 KIT Fig. 8 Arrhenius plot for the CF,CHFO decomposition reaction. 0, kinetic modelling. Fitted values delimited Arithmetic analysis; 0, by A,Wallington et al.;" Turazon and Atkinson;I2 +, Maricq.,and Szente.14 The dashed line is the recommended fit (see text for details). data from these sources, together with data from the present work using both the arithmetic approach and the computer simulations.Over the temperature range in which the data overlap, the value of k, from the present study appears to be significantly greater than that obtained by Wallington et al.," which is, in turn, larger than the value of Tuazon and Atkinson," both determined at pressures 2 1 atm. The determination of Tuazon and Atkinson', is based on measurements of CF,C(O)F formation, assuming that this product is produced only in reactions (4) and (64. Other potential sources of this product exist, however, e.g. the HO,+ CF3CHF0, reaction, which could lead to an underesti- mation of the value of k, relative to k,. Wallington et al." also used the CF,C(O)F yields to determine k,/k,: these results could be similarly affected.HC(0)F yields were also used and indeed a decrease of ca. 20% in the ratio k,/k, was reported in this case implying a higher value of k, . In both studies, a reasonably complete carbon balance among the measured products was reported, particularly at high [O,]. However, in neither study was the fate of HO, formed in reaction (4) considered, whereas in the present work, substantial yields of CF,CHFOOH and CF300H were predicted by the simulations using the full mechanism. Heterogeneous decomposition of the hydroperoxide CF,CHFOOH could provide another source of CF,C(O)F which is included in the carbon balance. In the present work, only the HC(0)F yields were used in the extraction of kinetic parameters.This product has a unique source in reaction (5) and analysis of the yields using two independent techniques was used to determine k,/k, . Nevertheless, an overestimation of k, cannot be ruled out in this work, in view of the sensitivity of k, to the regeneration factor, 8. In the temperature range over which these studies overlap, the temperature dependence of k, from the three relative rate studies is in good agreement. However, at the lowest temperature (235 K) in the present work, the value of k, obtained is a factor of three higher than that predicted by linear extrapolation of the Arrhenius expressions based on all data from T > 261 K. Furthermore, both Wallington et al." and Tuazon and Atkinson'* have shown that the ratio k,/k, is pressure dependent, presumably owing to unimolecular fall-off in reaction (5) at pressure below 1 atm at room tem- perature.Although the results of Wallington et al." would be consistent with a high-pressure limit value of k, up to a factor of 2 higher than the 1 atm pressure value, it seems unlikely that fall-off effects could account for the apparent J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 curvature in the Arrhenius plot implied by the 235 K results in the present study. An alternative explanation for this cur- vature is non-Arrhenius behaviour of the reference rate coeffi- cient, k,, a possibility which cannot be discounted since no determinations of the temperature dependence of CF,CHFO + 0, kinetics have been reported. Direct measurements of k, have been reported by Maricq and Szentel, and Zellner et Maricq and Szente14 used a flash photolysis time-resolved UV spectroscopic study of the CF3CFH0, self-reaction over the temperature range 211-372 K.The production and loss of the CF,CFHO radical was monitored directly from the time dependence of an absorption feature centred near 270 nm, assigned to CF,CFHO. Rate constants were derived by fitting a mecha- nism similar to that used in the present work, which is based on the work of Wallington et a!. l1 The expression derived for k, for a total pressure of 230 Torr was k, = (3.7 f0.7) x lo7 exp[-(2200 & 150)/T] s-'. These data are shown in Fig. 8. The pressure dependence obtained by Wallington et al.l1 indicates that the value of k, at 230 Torr would be about a factor of two lower than the high-pressure limit value.However, even allowing for this effect it can be seen that the room temperature value, corrected for the pressure depen- dence, is a factor of three lower than that from the relative rate determinations, and that the temperature dependence of k, from the direct determinations is much less pronounced. As pointed out by Maricq and Szente,14 their reaction condi- tions would be expected to lead to values of A, and E, lower than the high-pressure limit values. However, if fall-off effects are used to account for their reported values, they would have to be even larger than those needed to account for the results of the present study at 235 K, which already seem inconsistent with the pressure dependence observed by Wall- ington et al." Maricq and Szente', also noticed that their data for CF,CHFO decomposition were not consistent with the branching ratio for the CF,CHFO, self-reaction, Q, extrapolated to low temperatures, indicating some mechanis- tic complications at low temperature.Zellner et a1." obtained a value of k, at 298 K and 230 Torr of (1.8 & 0.4) x lo4 s-' from the time dependence of OH and NO, formation in the pulsed photolysis of C1,-HFC 1 Ma-NO-0, mixtures. Using the pressure dependence of Wallington et al." this corresponds to a value of ca. 8 x lo4 s-l at 760 Torr, in better agreement with the relative rate studies than that of Maricq and Szente.', However, at present, both the direct determinations and the relative rate studies show considerable uncertainty and the reasons for these discrepancies are not apparent.Table 5 shows the Arrhenius parameters for reaction (5) obtained from the various temperature-dependent studies. All values except those of Maricq and Szente14 are close to the high-pressure limit according to the data of Wallington et al. Nevertheless, the A factors are all significantly lower than the typical value of 1013-10'4 s-expected for a radical decomposition at the high-pressure limit.,' This suggests that Table 5 Arrhenius parameters for the reaction CF,CHFO -+ CF,+ HC(0)FO 16.7 4820 12.0 this work (arithmetic) 180 3.8 1.9 5570 4525 4435 9.98 7.5 5.1 this work (modelling) Wallington et al." Tuazon and AtkinsonI2 0.00037 7.4 2200 4720 2.0 7.5 f 2.4 Maricq and Szente14 recommended a All results, except those of Maricq and Szente, are calculated rela- tive to k, = 6.0 x exp(-925/T) cm3 molecule-' s-'.the reaction may not be as close to the high-pressure limit as the data of Wallington et al.' indicate. Further experimental investigation of the pressure dependence is clearly needed to establish if this is the case. For the purposes of calculating the rate of CF,CFHO decomposition at atmospheric pressure the expression in Table 5 is recommended. This is based on the mean of the experimental values at 293 K and the E/R values from Wall- ington et a/.," Tuazon and Atkinson', and the present work, excluding the data at 235 K.The temperature dependences obtained from the present work if the 235 K data are included, and from the lower pressure data of Maricq and Szente,14 both lead to unrealistically low A factors and are therefore not included in the evaluation. Errors Errors in individual Arrhenius expressions in this work have been quoted, at the la level solely for the temperature depen- dence (E/R)of the returned k,/k, or the k, value from the analysis. This is in accordance with other recent published on this reaction and reflects the fact that although the relative rate k,/k, is well established, the pre-exponential factor and hence the absolute values of k, and k, are not. Furthermore, because of the limited number of data points obtained in this study, a full statistical analysis of errors is inappropriate. The error given in the final recommendation for k, has been obtained using the procedure adopted by the NASA panel for chemical data evaluation.' This method uses the error in the ambient temperature value of k, [f(293)] and that in the temperature dependence (AEIR)to determine the approximate la errors over the entire temperature range.The errors are obtained by multiplying or dividing the value of k, at any given temperature, T, byf(T). Thus AE 1f(T) =f(293) exp 1 -R (-T -L,1293 Adopting this procedure, the percentage errors in k, for this study are f30% at ambient temperature, rising to f56% at 235 K and f38% at 318 K.The average errors in the recommended k, value are &40% across the entire tem- perature range. Atmospheric Modelling There is considerable interest in the relative rate of pro- duction of CF,C(O)F and HC(0)F in the atmospheric oxida- tion of HFC 134a. Neither of these compounds will be significantly photolysed in the tropo~phere~?~ and their major fate is likely to be physical removal in the aqueous phase. Hydrolysis of CF,C(O)F leads to CF,C(O)OH, tri-fluoroacetic acid (TFA), which is the subject of some environ- mental concern., TFA is also likely to be physically removed in rain water3, and the source region of CF,C(O)F will therefore determine the atmospheric distribution of CF,C(O)OH produced from HFC 134a photooxidation.A 2D model2, was used to calculate the latitude-height dis-tribution of the OH-induced oxidation of HFC 134a. Coupled with a knowledge of the temperature and pressure dependence of the relative rate of formation of CF,C(O)F and HC(O)F, the distribution of the source term of these two products in the atmosphere was computed. The model2, is a classical zonally averaged Eulerian model. It extends horizontally from 90" S to 90" N in 19 discrete latitude boxes and from ground level vertically up to 60 km in 17 levels with a resolution of 3.5 km. The model includes a representation of tropospheric photochemistry in order to produce a seasonally varying OH concentration 50 40 E Y,$ 30 c.-e-lu 20 I, I l/+9;:--fI I'"1, ,-75 -50 -25 0 25 50 75 latitude Fig.9 Modelled atmospheric abundance of HFC 134a in ppt after photochemical loss by the hydroxy radical field, In the model HFC 134a was released mainly between latitudes 33 and 62" N at ground level at a fixed rate of 29.5 kt year-' i.e. 10% by volume of the 1986 CFC 11 emission rates. The calculated tropospheric lifetime of HFC 134a in the model due to loss by the hydroxyl radical, using a value for k(OH + HFC 134a) of 8.4 x lo-', exp -153513, was 12 years. This is somewhat shorter than the value of 15 years calculated by Prather et al.,, the differences probably arising from the higher hydroxy group concentrations in the present model. Fig. 9 shows the distribution of HFC 134a in ppt following a 40 year integration with photochemical destruction by the hydroxy radical.The even global distribution reflects the relatively long tropospheric lifetime (12-15 years) of this hydrofluorocarbon. Fig. 10 shows the logarithm of the annual average destruction rate of HFC 134a in molecule cm-, s-l due to loss by the OH radical. The major loss (ca. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10.0-E5 7.5. Q1-0 c.-c-(Ll 5.0 2.5 -75 -50 -25 0 25 50 latitude Fig. 11 Modelled fraction of CF,C(O)F formed from the CF,CHFO radical in the atmosphere 1 molecule cm-' s-') occurs in the tropical lower tropo- sphere from 40" S to 40" N corresponding to both high OH fields and high concentration of the hydrofluorocarbon source.Following the destruction of HFC 134a the oxy radical, CF,CHFO, formed may either react with molecular oxygen to form CF,C(O)F and HO, ,reaction (4) or undergo unimolecular decomposition to CF, and HC(O)F, reaction (5) CF,CHFO + 0, +CF,C(O)F + HO, (4) CF,CHFO +CF, + HC(0)F (5) Fig. 11 shows the fraction of CF,C(O)F formed in the tropo- sphere from the degradation of the CF,CHFO radical using the recommended parameters for k, and k, from Table 5 and the pressure dependence of the ratio k,/k, from Wallington et 1 2.5I -75 -50 -25 0 25 50 75 -75 -50 -25 0 25 50 75 latitude latitude Fig. 10 Logarithm (base 10) of the modelled annual destruction rate of HFC 134a (molecule cm-,s-') average Fig.12 cm-, s-l) formed from the photooxidation of HFC 134a Annual average production rate of CF,C(O)F (molecule J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1829 al." assuming the same pressure dependence at low tem- peratures. The fraction of CF,C(O)F produced increases with altitude (lower temperatures and decreasing pressure both favour this channel, the temperature effect being dominant), from a value of 20% at ground level to ca. 80% at the tropo- pause. However, the maximum loss of HFC 134a occurs in 10 11 12 0.V. Rattigan, D. M. Rowley, 0. Wild, R. L. Jones and R. A. Cox, in Kinetics and Mechanisms for the Reactions of Haloge-nated Organic Compounds in the Troposphere, CEC-AFEAS Workshop, University College Publishers, Dublin, 1993, p.88. T. J. Wallington, M. D. Hurley, J. C. Ball and E. W. Kaiser, Environ. Sci. Technol., 1992, 26, 13 18. E. Tuazon and R. Atkinson, J. Atmos. Chem., 1993, 16,301. the tropical lower troposphere (Fig. 10) and the rate of CF,C(O)F formation is a maximum in this region because the destruction rate of HFC 134a falls off more rapidly with altitude. Fig. 12 shows the computed rate of CF,C(O)F pro- duction from HFC 134a degradation. Preliminary results by DeBruyn et suggest that efficient removal of CF,C(O)F 13 14 15 E. 0.Edney and D. J. Driscoll, Int. J. Chem. Kinet., 1992, 24, 1067. M. M. Maricq and J. J. Szente, J. Phys. Chem., 1992, %, 10862. R. Zellner, A. Hoffmann, D. Bingemann, V. Mors and J. P. Kohlmann, Kinetics and Mechanisms for the Reactions of Halo-genated Organic Compounds in the Troposphere, STEP-HALOCSIDE/AFEAS Workshop, Dublin, 1991.by hydrolysis in cloud water to form CF,C(O)OH occurs with a lifetime of ca. 1 month. Most of the hydrolysis will occur in the tropical lower troposphere and hence the hydro- lysis product, CF,C(O)OH, will be formed in this region. 16 17 18 M. M. Maricq and J. J. Szente, J. Phys. Chem., 1992,%, 4925. 0. J. Nielsen, T. Ellermann, J. Sehested, E. Bartkiewicz, T. J. Wallington and M. D. Hurley, lnt. J. Chem. Kinet., 1992, 24, 1009. T. J. Wallington, J. Sehested, M. A. Dearth and M. D. Hurley, J. Photochem. Photobiol. A, 1993, 70, 5. The authors wish to thank the Alternative Fluorocarbon 19 J. Sehested and T. J. Wallington, Environ. Sci. Tech., 1993, 27, Environmental Acceptability Study SPA-AFEAS, Inc. and the Department of the Environment, UK for financial support.Thanks are also due to T. J. Wallington for the com- munication of his work prior to publication. 20 21 22 146. H. Sidebottom, personal communication. J. Chen, T. Zhu, H. Niki and G. J. Mains, Geophys. Res. Lett., 1992,19,2215. H. Saathoff and R. Zellner, Chem. Phys. Lett., 1993,206, 349. 23 K. S. Law and J. A. Pyle, J. Geophys. Res., 1993,98, 18377. References 24 0.Rattigan, E. Lutman, R. L. Jones, R. A. Cox, K. Clemitshaw and J. Williams, J. Photochem. Photobiol. A, 1992,66, 31 3. Scientijic Assessment of Ozone Depletion: 1991, WMO Global Ozone Research and Monitoring Project, Report No. 25, 1992, Geneva, Switzerland. R. S. Stolarski, M. R. Schoeberl, P.A. Newman, R. D. McPeters 25 26 27 G. A. Olah and S. J. Kuhn, J. Am. Chem. Soc., 1960,82,2380. L. E. Giddings Jr. and K. K. Innes, J. Mol. Spectrosc., 1961, 6, 528. P. D. Lightfoot, B. Veyret and R. Lesclaux, J. Phys. Chem., 1990, and A. J. Krueger, Geophys. Res. Lett., 1990, 17, 1267. M. Prather and C. M. Spivakovsky, J. Geophys. Rex, 1990, 95, 18723. 28 94, 708. T. J. Wallington and M. D. Hurley, Chem. Phys. Lett., 1992, 189, 437. Scientific Assessment of Stratosphere Ozone, WMO Global Ozone Research and Monitoring Project, Report No. 20, 1989, vol. 2, AFEAS report. W. B. DeMore, S. P. Sander, D. M. Golden, R. F. Hampson, M. J. Kurylo, C. J. Howard, A. R. Ravishankara, C. E. Kolb and M. J. Molina, Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, NASA Evaluation No. 10, 1992, Jet Propulsion Laboratory Publication 92-20, Pasadena, CA, USA. R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson, J. A. Kerr and J. Troe, J. Phys. Chem. R@. Data, 1992,21, 1125. T. J. Wallington and 0.J. Nielsen, Chem. Phys. Lett., 1991, 187, 33. 29 30 31 32 33 J. P. Sawerysyn, A. Talhaoui, B. Meriaux and P. Devolder, Chem. Phys. Lett., 1992, 198, 197. A. R. Curtis and W. P. Sweetenham, FACSIMILEICHEKMAT Users Manual, 1988, AERE-R12805, Harwell, Laboratory, Oxfordshire. S. W. Benson, Thermochemical Kinetics, Wiley-Interscience, New York, 2nd edn., 1976. J. H. Hu, Jeffrey A. Shorter, P. Davidovits, D. R. Worsnop, M. S. Zahniser and C. E. Kolb, J. Phys. Chem., 1993,93, 11037. W. DeBruyn, J. A. Shorter, P. Davidovits, D. R. Worsnop, M. S. Zahniser and C. E. Kolb, Atmospheric Wet and Dry Deposition of Carbonyl and Haloacetyl Halides, AFEAS Workshop, 1992, J. Peeters and V. Pultau, CEC-AFEAS Workshop, September Brussels, AFEAS, New York, 1993, p. 12. 1992, Leuven, CEC Air Pollution Research Report 45. 0. V. Rattigan, 0. Wild, R. L. Jones and R. A. Cox, J. Photo- chem. Photobiol. A, 1993, 73, 1. Paper 3/07325D; Received 13 December, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001819

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(13), 1831-1837 Transition Vector Symmetry and the Internal Pseudo-rotation and Inversion Paths of CIF,' Ruslan M. Minyaev Institute of Physical and Organic Chemistry, Rostov State University, Stachka A ve., 19413, Rostov-on- Don 344 104 , Russia David J. Wales University Chemical Laboratories, Lensfield Road, Cambridge, UK CB2 IEW A new definition of the transition vector symmetry group is suggested. The utility of the definition is demon- strated by calculations of the pathways of the internal pseudo-rotation and inversion of CIF,' (DZP and DZP/MP2 levels). The lowest internal pseudo-rotation proceeds by a Berry mechanism with an activation barrier of 6.7 kcal mol-'. The lever mechanism has a higher barrier of 39.5 kcal mol-' and would lead to more extensive fluorine scrambling.The inversion reaction that leads to enantiomerization corresponds to a planar D,, tran-sition state with an activation barrier of 60 kcal mol-'. 1. Introduction Considerable effort has been devoted to the elucidation of the connection between the symmetry elements of a transition state, and those of reactant and product. Many selection rules have been proposed.'-' ' The most important principles are probably the conservation of orbital symmetry introduced by Woodward and Hoffmann,' and selection rules for transition states due to Stanton and McIver., From a geometrical point of view, the transition-state structure is the nuclear configu- ration corresponding to a saddle point with one negative curvature2 (Hessian index, A = 1)12 on the potential-energy surface (PES) of a molecule.Any Hessian eigenvector at this point may be classified according to an irreducible representation' of the appropriate point group. Hence, the symmetry properties of the Hessian eigenvector with unique imaginary frequency (the transition ~ector)~.~ are also those of some particular irreducible representation. At the same time the transition vector considered as a local displacement vector creates an infinitesimally perturbed molecular configu- ration whose point group must be a subgroup of that of the transition state. The purpose of this paper is to focus on the symmetry properties of the transition vector in a slightly dif- ferent way to McIver and Stanton3v4 which may have some advantages.We illustrate this idea by ab initio calculations (at the DZP and DZP/MP2 levels) of the internal pseudo-rotation and inversion pathways of the ClF,+ cation. We chose this cation for the following reasons: first, inter- halogens represent a very unusual class of hypervalent stereo- chemically non-rigid molecules with different mechanisms of internal rearrangement (Berry pseudo-rotation,', turnstile rotati~n,'~*'~Bartell mechanism' etc.) and have conse-quently been much However, ClF4+ has attracted rather less attention and the internal mechanism of rearrangement remains ambiguous. The temperature dependence of the 19F NMR spectra indicates27 that rapid fluorine ligand exchange in ClF,+ occurs in a solution of ClF,-HF-AsF,.However, whether this is inter- or intra-molecular ligand exchange has not been el~cidated.~ The first ab initio (STO-6G) calculations of Guest et ~1.'~predicted a stable square pyramidal structure for ClF4+ with F-C1-F angles of 154" which was in dis- agreement with experimental observation^.^^-^* Ungemach and Schaefer' pointed out the reason for this disagreement and demonstrated the crucial influence of chlorine d basis functions. Ab initio calculations without such orbitals predict that the square-pyramidal C,, structure of ClF4+ is a minimum even with rather large DZ basis sets.*' Only when chlorine d orbitals are included do calculations find the correct C,, ClF,+ minim~m'~,~'which is consistent with vibrational28 and 19FNMR27spectra as well as with VSEPR concepts.29 Pershin and B01dyrev~~ considered the relative stability of the C2,, C,, and D,, structures of ClF,+.However, they did not consider the Hessian matrix and did not study the reaction pathways. Therefore, we decided to calculate approximate pathways for the different rearrange- ments of ClF4+. The organisation of the paper is as follows. In the second section we describe the methods of calculation. The third section is devoted to new considerations of the transition vector symmetry and its conservation along the gradient lineal1,30-33 The Berry and lever pseudo-rotations are con- sidered in Sections 4 and 5, and the inversion pathways and global topological structure of the PES are considered in Section 6.2. Methods of Calculation All the stationary point optimisations and approximate gra- dient line calculations in this work were performed by eigenvector-following34 (EF) using analytic first and second derivatives at every step which were generated by the CADPAC program.35 The particular EF implementation has been developed and discussed in some detail in previous The method enables one to follow a particular Hessian eigenvector uphill, whilst simultaneously minimising the energy in conjugate directions. This procedure is used to find transition states, while minimisation in all directions is used to locate minima. All calculations were conducted in Cartesian coordinates using a projection operator to remove overall translation and r~tation.~' A maximum step length criterion was used to scale the steps, if necessary.Approximations to the gradient lines were calculated by displacing each transition-state geometry along the two direc- tions corresponding to the (non-mass-weighted) Hessian eigenvector associated with the unique imaginary frequency. Perturbations consisted of adding/subtracting & of the com- ponents of the normalised eigenvector in each case. Pathways were always followed downhill, i.e. by minimising the energy, and no symmetry restrictions were imposed. Calculations were considered to be converged when the maximum step size was <lop4 a, (bohr) and the rms gradient E, a, 't for two consecutive steps, which usually reduces the six 'zero' Hessian eigenvalues to the order of 0.3 cm-'.The basis sets employed were those supplied in the standard CADPAC librar~;~' the DZP basis is obtained by adding polarisation functions with exponents 1.0 (H), 0.8 (C) and 1.2 (F)to the Dunning double zeta basis.38 It is well known that pathways calculated with second- derivative-based algorithms are not quite the same as true steepest-descent or gradient-line paths ; in fact, for some pathological surfaces it is possible to converge to the wrong minimum.39 However, we have no reason to expect such dim- culties in the present case, and the algorithm adopted also means that all the Hessian eigenvectors are available at each step.Hence it was not difficult to follow the evolution of the eigenvalues and eigenvectors along the path using the overlap (dot product) of eigenvectors at successive steps to identify the correlation. We expect our calculations to remain valid for closer approximations to the gradient lines. 3. Transition Vector Symmetry A transition-state structure is a molecular configuration cor- responding to a saddle point with one imaginary frequency2 independent of the PES in neighbouring regions. Therefore, the transition-state structure is defined purely from a geo- metrical point of view, and the transition-state symmetry is given by the appropriate point group.13 All the Hessian eigenvectors must transform according to one of the irreduc- ible representations of this group.13 This is true for the eigen- vectors of both the mass-weighted and non-mass-weighted Hessian ; here we consider the latter matrix exclusively.The transition vector (Hessian eigenvector corresponding to the unique imaginary frequency) transforms according to a non- degenerate irreducible repre~entation.~.~Infinitesimal dis-placements of all atoms along the transition vector lead to a slightly distorted structure whose point group is a subgroup of the transition-state point group', which we will call the transition-vector symmetry group. This definition of the transition vector symmetry is slightly different from that introduced earlier by McIver and Stant~n.~'~ By our defini- tion the transition vector possesses a symmetry property as a real vector rather than as a vibrational m~de.~.~,~ There are several important points to note concerning the gradient-line reaction path, the transition-vector symmetry group, defined above, and the application of the Pearson6- Pechukas' (PP) theorems. First, minimum-energy pathways (MEPs) can deviate from steepest-descent paths at branching points.Since the PP results only apply to steepest-descent paths, they may not be obeyed by MEPs, which can lead to confusion. For example, the reaction-path degeneracy may not be apparent. The second point is that the PP theorems state that the molecular point group can change only at a critical point. In contrast, by our definition the transition- vector symmetry cannot change at all along a steepest-descent path.The proof of this follows in just the same way as for the PP theorem, except that it extends to the critical points because we only consider perturbed geometries. We may therefore apply the principle of conservation of transition-vector symmetry group to any gradient line ema- nating from a transition state until it reaches the next critical point (which may be also a transition state). To clarify this situation let us consider the inversion of ammonia (Scheme 1). t 1 a, (bohr) E 5.291 77 x lo-" m. 1 E, (hartree) z 4.35975 x lo-" J. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 29 D3h Scheme I The3 transition vector of TS 2 by our definition metry group C,, and this is conserved along the whole pathway from la to 16 through 2.In contrast, in the McIver- Stanton3 approach the transition vector is classified in the D,, point group which is not conserved along the pathway. Other examples are internal rotation about the C-C bond in ethane4' and internal pseudo-rotation in phos-ph~ranes;'~,~~,~'in each case the transition-vector symmetry group by our definition is conserved along the whole gradient line from one critical point to another. In the following sec- tions we will demonstrate the utility of this idea for the inter- nal pseudo-rotation and inversion pathways of ClF4+. 4. Berry Pseudo-rotation Ab initio calculations predict that ClF4+ in the ground singlet state has the stable C,, structure, 3 (see Scheme 2), with two different bonds: axial and equatorial (see Table 1 and Fig.1). These results are in agreement with previous cal- culation~,'~~~~~~~experimental data27,28 and with Gillespie's VSEPR theory.29 FL 49 c'tv Scheme 2 Calculated vibrational frequencies and assignments for 3 are given in Table 2 and compared with experimental data.28 The agreement between calculated and experimental fre-quencies is satisfactory. Significant differences are observed for o2and w6, both of which are related to the stretching of axial bonds. The disagreement seems to be due to the harmo- nic approximation, the number of basis functions and the effects of correlation. Note that the calculations predict that Table 1 Total energies (EJE,) relative energies (AElkcal mol- I), number of imaginary frequencies (I.) and values of the imaginary fre- quencies (o/cm-') calculated for structures 3-8 at the DZP/SCF level E, AE 2 37 c2v -856.472 85 0 0 4, c4v -856.462 10 6.7 1 146.8 5, c, -856.409 87 39.5 1 295.1 69 c3v -856.408 92 40.1 2 1 8 1.2( E) 7,Td -856.251 59 138.0 3 72.0(T2)87 D4h -856.378 04 59.5 1 866.3 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.578 (1.638) (1.620) jF (lg,hJlq 108.1 (108.3)i F 1.514 (1.570)F 3,c2v (1.597) F (1.635) 1354 1.565 ( 1.657) i 1.653) 5, c, F 7, Td Fig. 1 Geometrical parameters of structures 3-8 calculated at DZP and DZP/MP2 (numbers in parentheses) levels. Bond lengths are given in A, bond angles in degrees.Structure 3 corresponds to a minimum (i= 0), 4, 5 and 8 to transition states (A = l), and 6 and 7 to critical points with A = 2 and 3, respectively, at the DZP level. Structures 3, 6, 7 and 8 correspond to minima (A = 0), and 4 and 5 correspond to transition states (A = 1) at the DZP/MP2 level. the co, mode is a mixture of equatorial and axial ligand dis- placements, as shown in Scheme 2, which is consistent with the assumption28 that separate modes corresponding to dis- placement of only equatorial or axial ligands do not exist. Structure 4 (C4,) corresponds to a transition state on the ClF4+PES for the Berry pseudo-rotation', with a calculated activation barrier of 6.7 kcal mol-'. This mechanism was first propo~ed'~ to explain the facile interconversion of penta- coordinated phosphoranes.' The validity of the Berry 5316,41 process has been demonstrated by X-ray structure-correlation for a large variety of five-coordinated phos-phor~~~~ (pentacoordinatedand d8-metal compo~nds~~,~~ Table 2 Vibrational assignment for ClF,' in C,, structure 3 frequency/cm-symmetry assignment exp." DZP DZP/MP2 o1 v (sym ClF, eq) 800 925 802 w, w3 v (sym ClF, ax) 6 (sciss ClF, eq) 571 510 695 585 565 497 w4 6 (sciss ClF, ax) 237 215 154 A, o5 ClF, twist 475 554 469 B1 o, w6 ClF, rocking v (asym ClF, ax) 795 537 937 611 834 518 B, O8 wg (asym C1F2 eq)6 (sciss CIF, ax)' 829 385 938 454 834 361 a Ref.28. * Out of plane. species have structures which range from trigonal bipyrami- dal to square pyramidal along the expected Berry pseudo- rotation pathway) as well as by calculations of the minimum-energy reaction paths and TSs of the topomeriza- tion of PH5,45 SiH,- 46 and SF4.47-49a It is also analogous to Lipscomb's DSD for B5H,'-.The calculated activation barrier for the Berry pseudo- rotation is consistent with the experimental observationz7 of rapid ligand exchange. Unfortunately, Christe and Sa~odny~~were not able to decrease the temperature of the ClF5-HF-SbF, mixture sufficiently (because of the freezing point of HF) to observe the fine structure of the "F NMR spectra in the absence of ligand exchange and thus obtain a value for the activation barrier. However, the similarity of the NMR spectrum behaviour for ClF4+z7 and sulfurane SF, 59,51 suggests that their activation barriers are compara- ble.The activation barrier deduced for the ligand exchange in SF, ranges from 4.5 to 16 kcal mol-' from various experi- ment~."-~~ predict values of l2,* Ab initio calculations48~49 and 10.6 kcal mol- '.49 All of these numbers are close to the value obtained for ClF4+, namely 6.7 kcal moi-'. Thus it seems likely that the ligand exchange in ClF4+ occurs intra- molecularly by the Berry mechanism. The Berry pseudo-rotation 3a 4 36 occurs along a gra- dient line of TS 4 that is tangential to the transition vector and enters minimum 3 tangential to the Hessian eigenvector with the smallest eigenvalue, which has A, symmetry.The C,, symmetry group of the TS 4 transition vector is con- served along the whole Berry pseudorotation pathway 3a s 4 +36 (Scheme 2). 5. 'Lever' Mechanism of Pseudo-rotation The Berry pseudo-rotation (Scheme 2) does not lead to com- plete fluorine scrambling, whereas the 'lever' mechanism (Scheme 3) proposed55 for SF, would result in sequential axial-equatorial permutations, 3a e5 e3c. The degree of scrambling can actually be made more precise in terms of the effective molecular symmetry group, as we discuss at the end of this section. The 'lever' mechanism is similar to the 'turnstile' pseudo-rotation in phosphoranes' and pro- 5916 ceeds via transition state 5 which has C, symmetry. Previous extended Hiickel and CND0/2 calculation^^^ of the lever pathway for SF, suggest that this mechanism has a slightly higher (about 2-10 kcal mol-') activation barrier than the Berry process, and under some conditions it might compete with Berry pseudo-rotation.Of course, these are only qualit- ative calculations. As far as we know the C, structure, 5, for SF, is the true transition structure for the lever mechanism and has been .~~found by Schleyer et ~1Our calculations predict that struc- ture 5 (C,)also corresponds to a true transition state (A = 1) for ClF4+ but with a rather high activation barrier of 39.5 kcal mol-'. Thus it appears that the lever mechanism cannot compete with the Berry process in this system. However, the lever rearrangement has a very interesting system of gradient-line reaction pathways; Scheme 4 shows one of four possible F' 7F4 F4ai 7.F4 Lcl,\\'' F3 -F'-ClLF3 axI F2 F2 % c,v 5, C, 3c, c,, Scheme 3.Mirror plane through 1 and 4 1834 1 1 23 3 513, c, (h=1) ,3c, c, (h= 0) 2 2 3 I 3a, C2, (h= 0) 6, C2, (h= 2) 54 c, (h= 1) 2 2I ­-1 4-1 /”\ I 31 3 5c, c, (h= 1) 3d, c,, (h= 0) Scheme 4 lever pathways from minimum 3a. For convenience, struc- tures 3, 5 and 6 in Scheme 4 are depicted with the Cl-F4 equatorial bond perpendicular to the page. The system cir- cumnavigates a rather flat hill, the top of which corresponds to C,, structure 6 with A = 2 (see Fig. 1 and Table l), and can return to starting point 3a via three equivalent steps.The gradient-line pathways along Hessian eigenvectors corre-sponding to the imaginary frequencies of structure 6 lead either to minimum 3 (C,,) or to transition state 5 (C,). The complete scrambling scheme taking into account all four dis- tinct lever pathways for each minimum, 3, is represented by the octahedral reaction graph in Fig. 2. Each vertex of the octahedron denotes one of the six ver- sions of minimum 3 that can interconvert when the lever 1 I 4-3I 1 2 I 3a, C2“(1= 1) 4 (h= 0) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 rearrangement is the only ‘fea~ible”~ mechanism. The solid lines connecting the two vertices designate the lever path- ways. The crosses at the midpoint of these lines denote the 12 versions of TS 5 (C,)that are involved.The effective molecular symmetry (MS) group contains 24 operations for the lever mechanism, giving half the complete nuclear permutation inversion (CNPI) Hence the reaction graph in Fig. 2 contains half of the 12 distinct ver- sions of minimum 3 and half of the 24 distinct versions of TS SS8We have not attempted to represent any further connec- tions with the higher-index saddles 6 and 7in this graph. F 69, c,, (1= 2) 7,Td(1= 2) Although the Berry mechanism on its own connects only pairs of versions of the C,, minimum, if both the Berry and lever mechanism are feasible then the effective MS group becomes the CNPI group. In this case all 12 distinct C,, minima can interconvert. All of these MS group character- isations were readily carried out with a recently developed computer pr~gram.’~ 6.Inversion Mechanism According to our ab initio calculations planar structure 8 (D4h) is a true TS (A = 1) at the DZP level for the inversion process with a calculated activation barrier of 59.5 kcal mol-’. The corresponding barrier for SF, is calculated to be 108.8 kcal mol-1.49” The gradient line along which the inver- sion reaction occurs emanates from TS 8 tangential to the eigenvector corresponding to the transition vector (see Scheme 5) and enters transition state 4 tangential to the eigenvector corresponding to the fourth smallest positive eigenvalue. It is important to emphasise that the C,, sym-metry group of the transition vector of TS 8 is conserved along this pathway from 4a to 4b via 8 according to our defi- nition.This gradient line connects three successive TSs ; however, there is no contradiction between the proposed inversion path and the Fernandez-Sinanoglu theorem,60 which states that a gradient line passing through a minimum cannot cross two successive saddle points. In our case the whole reaction path consists of three different gradient lines, while the Fernandez-Sinanoglu theorem holds only for a reaction path consisting of one gradient line. The complete gradient-line reaction path of Scheme 6 at the DZP level consists of three different gradient lines. Two of them are equivalent (GL1 and GL1’) and correspond to Berry pseudo-rotations 3ae4ae36 and 3f= 4b s3g passing through transition states 4a and 4b, respectively.The 1) Fig. 2 Schematic representation of the reaction graph for the gradient-line reaction path for the lever mechanism ligand exchange in CIF,+. Filled circles and crosses denote minima 3a-g (C2,)and transition states 5 (Cs), respectively. The solid lines designate the lever reaction pathways. 8,D,, (A= 1) Scheme 5 4b, C,, (A= 1) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1835 F’ F‘ F3 transition vector F’ F3 third gradient line (GL2) connects one transition state 4a to another 46 via the third TS, 8. Taking EF24 steps with starting displacements along one of the directions defined by the Hessian eigenvector of TS 8 corresponding to the unique imaginary frequency, we calcu-lated the pathway and energy profile from TS 8 to TS 4a.The pathway is only an approximate gradient line36 since the EF steps are not strictly parallel to the gradient vector.39 However, it is probably close enough to GL2 for our pur-poses and has the same symmetry properties, as corroborated by the evolution of the Hessian eigenvectors corresponding to the lowest Hessian eigenvalues along the pathway (Fig.3). The evolution of the five smallest Hessian eigenvalues along this path is shown in Fig. 4. The TS 8 transition vector transforms smoothly into the Hessian eigenvector of transition state 4a with A, symmetry, which has the fourth largest positive eigenvalue. The TS 4u transition vector (for Berry pseudo-rotation, see Scheme 2) is perpendicular to it, so that the gradient-line pathway changes direction in configuration space. In other words, the gradient-line path changes from GL2 to GL1 (or GLl’). Furthermore, from point 4a (or 44, there are two equivalent paths along both parts of GL1 (or GL1’) to minima 3a and 3b (or 3fand 3g).Of course, the total energy of the system decreases along the whole of the gradient-line reaction path (see Fig. 3). Note that on GL2 (from both sides of TS 8) there are two points B and B‘ (see Fig. 3 and 4) where a Hessian eigenvalue changes sign from plus to minus. These are ‘branching’61 points where the minimum-energy reaction paths and the symmetry is broken,66 i.e. the four-fold axis changes to two-fold. However, such points are not sta-tionary points, because VE does not vanish.Consequently, the minimum-energy pathway67-70 might appear to violate Pearson’s6 and Pechukas’’ theorems, except that the latter should be applied only to steepest-descent paths. The system cannot change its direction of motion at the branching points and by our definition the gradient-line reaction path con-tinues to follow GL2 and hence cannot change the point-I \\-856 4---856.41.>c -856.44 -856.41 r------h+-856.42. ‘\Y \-856 43. -856 44-\ -856 45 -a-856461 , , ‘\ /4’ c4v{ *a, -856 47 0 2 4 6 8 101214 . steps transition vector Fig. 3 Schematic three-dimensional representation of the ClF,+ PES (bottom) (no attempt has been made to show the branching points), energy profile (top) and evolution of two Hessian eigen-vectors along the gradient line from TS 8 (D4h)to TS 4a (C4J On the left is the transition vector for TS 4 which correlates with the smallest positive eigenvalue of TS 8; on the right is the transition vector for TS 8 which correlates with the fourth smallest positive eigenvalue of TS 4.0.41 a-?I9 -0.2-/ bifurcation pointal .-0 Q, -0.4-C .-cn8 -0.6-,iI -1.0 ! , 0 2 4 6 8 10 12 14 16 steps Fig. 4 Evolution of the five lowest eigenvalues of the projected Hessian (non-mass-weighted) along the approximate DZP/SCF gra-dient line path from TS 8 (D4h) to TS 4a (C4v),as determined by overlap of the corresponding Hessian eigenvectors. The steps corre-spond to the energy profile in Fig.3. group symmetry. Although a displacement in the direction corresponding to the new negative curvature would lead to a further decrease in energy, such steps are not taken because the gradient has no component in this direction. Thus, at points B and B' the gradient-line path deviates from the bifurcating minimum-energy paths which follow the valley bottoms, rather than moving along the ridge,39 and avoid transition-state structures 4a and 46. On the pathway 4aeSe46 the reaction follows one gra- dient line (GL2) which conserves C,, symmetry. Then, at point 4a (or 46) corresponding to the other transition state (for Berry pseudo-rotation), the pathway follows another gra- dient line (GL1 or GL1') and descends to minimum 3a or 3b (3f or 3g).In this case, we should apply Pearson's and Pechukas' theorems separately to the three gradient lines, GL1, GL1' and GL2, whose symmetry properties are differ- ent, and thus no contradiction arises. The schematic three-dimensional shape of the ClF4+ PES is depicted in Fig. 3. The main feature of this PES is that GL2, which connects TS 4a to the second saddle point 46 via saddle point 8, does not lead to any of the minima 3a-g directly. These minima are interconnected by GL1 or GL1' passing through saddle points 4a or 46, respectively. There- fore, by our definition the complete gradient-line reaction pathway consists of the three gradient lines GL1, GLl', and GL2. During the course of the reaction in Scheme 6, the scheme first moves along GL1, then at point 4a changes direction to GL2, and moves along GL2 to TS 46 via TS 8.Finally, the system can descend either part of GL1' to minimum 3for 3g. The observed topology of the ClF,+ PES in the inversion region may be quite common in organic, inorganic and other reactions. For example, the rearrangement of cyclo-octatetraene seems to follow a reaction pathway similar to that in Scheme 2. The reason for this is very simple: the gra- dient line emanating from the D8h transition structure along the single imaginary mode, as reported by Hrovat and B~rden,~'has B,, symmetry and should lead to the planar D4h structure. The same is true also for the 1,3 H shift from nitrogen to oxygen in sulfamic acid in the rearrangement reaction from the neutral (H,N-S0,OH) to the zwitterionic (H,H+-SO,-) form.72 The high activation barrier of the ClF, inversion reaction + suggests that derivatives might be obtained in an optically active form similar to cyclic ~ulfuranes.~~ Furthermore, the synthesis of a stable 10-1-4 periodonium ion by Dess and Martin7, supports this assumption.7. DZP/MP2 Calculations We also performed DZP/MP2 calculations (see Table 3 and Fig. 1) for structures >S. The agreement between experimen- Table 3 Total energies (EJE,), relative energies (AElkcal mol- '), number of imaginary frequencies (I)and values of the imaginary fre- quencies (w/cm-') calculated for structures 3-8 at the DZP/MP2 level reference E, AE il o/i weight" (%) 3, C," -857.521 33 0 0 90 4, C4" -857.5 16 34 3.1 1 86.6 89 5, c, -857.498 55 14.3 1 84.7 88 6, c,, -857.39451 79.6 0 88 7,Td -857.481 54 25.0 0 84 8, D,, -857.54708 -16.2 O 72 The reference weight is the contribution of the ground-state con- figuration to the perturbed wavefuncti~n.~~~~~ J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 tal and theoretical frequencies for structure 3 is improved but at this level of theory structures 8 (D4h), 6 (D3h) and 7 (7'J correspond to minima. Furthermore, structure 8 becomes lower in energy than 3 which is not consistent with experi- mental data27.28 and VSEPR theory.,' These results testify that for structures >8 there are numerous low-lying elec- tronic states, and that application of configuration inter- action might be necessary to obtain the correct results.22 This must be a matter for future investigation. 8.Conclusions There are two different topomerization mechanisms in ClF,+: the Berry and lever pseudo-rotations. At the DZP level the Berry process is lower in energy, although only the lever mechanism leads to significant ligand scrambling. Taking into account the quite low activation barrier calcu- lated for the Berry pseudo-rotation in ClF4+ we expect that the ligand exchange in ClF,+ occurs intramolecularly by this mechanism. Of course, these calculations do not rule out intermolecular ligand exchange, and the surface changes sig-nificantly at the DZP/MP2 level. 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ISSN:0956-5000    

DOI:10.1039/FT9949001831

出版商:RSC    

年代:1994

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J. CHEM. SOC. FARADAY TRANS., 1994, 90(13), 1839-1847 Gradient-line Reaction Paths for 1,2 H Shift Reactions in Phosphinonitrene and Formaldehyde, and H, Elimination from Formaldehyde Ruslan M. Minyaev Institute of Physical and Organic Chemistry, Rostov State University, Stachka Ave., 19413,Rostov-on-Don 344 104, Russia David J. Wales University Chemical Laboratories, Lensfield Road, Cambridge, UK CB2 IEW The reaction pathway for the 1,2 H shift in H,PN may be described as two equivalent gradient lines (double-line reaction path) leading from phosphinonitrene to cis-phosphazene rather than to trans-phosphazene. The gradient-line reaction path does not contain branching points and coincides with the minimum-energy pathway. Two different reactions, 1,2 H shift and H, elimination for formaldehyde, occur along two different gradient lines which both enter the minimum parallel to the softest Hessian eigenvector. 1.Introduction The notion of a transition state and the pathwaylP4 linking it on the Born-Oppenheimer potential-energy surface’ (PES) to both minima, reactant and product, plays a fundamental role in chemistry and biochemistry. Reaction rate calculations6-’ are based upon these concepts, along with our understanding of the reaction mechanism,’-’ the behaviour of non-rigid molecular systems,’ 3-1 ’ and the interpretation of vibrational spectra.16*17 Each of these concepts requires a detailed know- ledge of the PES topology along the whole reaction path from one minimum to another.Understanding enzyme cata- lytic action at the molecular level requires us to characterise the transition state and how it Usually one con- siders the minimum-energy reaction path (MERP)’ 1-26 con-necting reactant and product via the transition state. However, the MERP may bifurcate away from the line of steepest In this case, point group symmetry need not be conserved along the MERP and can change at the branching points.32 This does not contradict pear son'^^^ and Pe~hukas’~~theorems because the latter should be applied only to steepest descent paths. However, symmetry breaking along an MERP creates difficulties in understand- ing the dependence of the symmetry properties of the product on those of the reactant and transition state.Such behaviour also hampers construction of the true topology of the PES. Some unusual properties of reaction pathways are often attributed to bifurcation of the reaction path. For example according to ab initio calculation^^^-^^ the 1,2 H shift reac- tion (Scheme 1) occurs through a non-symmetrical transition state, 2. H H Scheme 1 The appearance of such a transition state is often attributed to the presence of bifurcation points on the PES. The aim of the present paper is to show that there are actually no branching points between minimum 1 and tran- sition state 2 for the reaction (Scheme 1) on the gradient-line reaction path, which is in fact described by Scheme 2. The reaction pathway of Scheme 2 differs significantly from that of Scheme 1.First, Scheme 2 leads to cis-phosphazene H \ P=N ,P=N, H’ HH Scheme 2 (4), rather than to trans-phosphazene. Secondly, the reaction pathway does not contain any bifurcation points and prin- cipally consists of two enantiomeric gradient lines passing through two transition states, 2a and 2b. In fact, there is a gradient line linking minimum 1 to minimum 3 which passes through a critical point of index two (2. = 2). ’It has also been suggested that all reaction pathways in mass-weighted coordinates (or ‘kinematic space’390) enter a minimum tangential to the softest normal mode.24*40*4’ Indeed, as is shown in Section 3, an infinite set of gradient lines do enter a minimum in the direction of the ‘softest’ Hessian eigenvector (eigenvector corresponding to the small- est eigen~alue)~’~ which may not correspond to the softest normal mode.The second aim of the present paper is to illus- trate two gradient-line reaction pathways for formaldehyde that enter the minimum parallel to the softest Hessian eigen- vector which has different symmetry from the softest normal mode. These results testify that the behaviour of the gradient line and intrinsic reaction pathway^^^,^' on the PES is differ- ent. Our considerations are all based upon the gradient-line reaction path (GLRP) which we have introduced in previous and have shown to have some useful properties. The GLRP is defined as the set of gradient lines linking two successive minima.43,44 Gradient lines cannot bifurcate or coalesce at any non-stationary points on the PES, and hence the transition-vector symmetry group4* is conserved along a gradient line.Such methodology has now been applied to Scheme 2. The methods are described in Section 2. Two reac- tion pathways, 1,2 H shift and H, elimination for formalde- hyde, are considered in Section 4. 2. Methods All the stationary-point optimisations and approximate gradient-line calculations in this work were performed by eigenvector-following45 (EF) using analytic first and second derivatives at every step generated by the CADPAC program.46 The particular EF implementation has been developed and discussed in previous ~ork.~~,~* The method enables one to follow a particular Hessian eigenvector uphill, whilst simultaneously minimising the energy in all the conju- gate directions.This procedure is used to find transition states, while minimisation in all directions is used to locate minima. All calculations were conducted in Cartesian coordi- nates using a projection operator to remove overall trans- lation and rotation4* A maximum step length criterion was used to scale the steps, if necessary. Approximations to the gradient lines were found by dis- placing each transition-state (or saddle-point) geometry along the two directions corresponding to the (non-mass-weighted) Hessian eigenvector associated with the (usually) unique ima- ginary frequency. Perturbations consisted of adding/ subtracting of the components of the normalised eigenvector in each case. Except for one case, the pathways were always followed downhill, i.e.by minimising the energy, and no symmetry restrictions were imposed. Calculations were considered converged when the maximum step size was -= a, and the rms gradient was < E, a, 't for two consecutive steps, which usually reduces the 'zero' Hessian eigenvalues to the order of 0.3 cm-'. The basis sets employed were those supplied in the standard CADPAC library;46 the DZP basis is obtained by adding polarisation functions with exponents 1.0 (H), 0.8 (C) and 1.2 (F) to the Dunning double zeta bask4' It is well known that pathways calculated with second- derivative-based algorithms are not quite the same as true steepest-descent or gradient-line paths; in fact, for some pathological surfaces it is possible to converge to the wrong minimum.50 However, we had no reason to expect such diffi- culties in the present case, and the algorithm adopted also means that all the Hessian eigenvectors are available at each step.Hence it was not difficult to follow the evolution of the eigenvalues and eigenvectors along the path using the overlap (dot product) of eigenvectors at successive steps to identify the correlation. However, since some of our results were unexpected we checked several of the paths by simply taking small steps along the negative gradient vector. The eigenvector-following pathways were found to be essentially correct in each case. 3. Behaviour of Gradient Lines in the Local Region of a Minimum The equations which define gradient lines in 3N-6 dimension-al configuration space (where N is the number of nuclei in the molecule) can be written in the form24,27,39b*41 where Qk(k = 1, 2, .. . ,3N-6) are curvilinear coordinates and gkJis a metric tensor.51 The PES E = E(Q)in the local region of a minimum can always be expressed as a Taylor expansion in diagonal form:52,53 1 3N-6( a2E ) @')' E(Q) = 2 (aQ32 min t 1 a, (bohr) ~5.29177x lo-" m. 1 E, (hartree) ~4.35975x lo-'* J. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Such a representation means that in the region of a minimum (or any stationary point) the Euclidian = 6kj (3) can be introduced. Here akj is the Kronecker symbol defined as (4) This local expansion is also called the Hessian eigenvalue rep- re~entation,~~'and corresponds to an orthogonal transform- ation of basis in terms of the Hessian eigenvectors.Taking into account relations (2) and (3),eqn. (1)can be rewritten as a system of ordinary differential equations with constant coefficients: -=-akQk, k = 1, 2, ..., 3N-6 (5)dt where ak = -((:22)min Eqn. (5) has two types of solution for each k: trivial39b Qk= 0 (6) and non-trivia12 9 9b27349 Qk = (Qk)o exP(-ak 1) (7) where (Qk))ois an initial point corresponding to t = 0 and ak is the kth Hessian eigenvalue at the minimum. Solutions with all Qk = 0 except for one particular Qj define the 3N-6 axis lines or directions of principal curvature at the minimum.Solutions with only two Qk non-zero, say (Ii and Qj, are parabolas in the Qi-Qj plane which enter the minimum parallel to whichever of the two vectors has the smallest eigen~alue,~~~ so long as the eigenvalues are non- degenerate (see Fig. 1). Thus, all the gradient lines (GLs) entering the minimum have components satisfying eqn. (6)or (7) in the local region of the minimum. The gradient lines coinciding with coordinate axes (principal directions of curvature) we will call principal gradient lines (pGLs); the rest we will call non-principal GL (npGL). Obviously, there are only 2(3N-6) pGLs (if we consider positive and negative directions for each axis39b) whereas there exists an infinite set of npGLs. In fact, every non-stationary point on the PES lies E 4 h=O Fig.1 Behaviour of principal (solid lines) and non-principal (thin lines) gradient lines in the local region of a minimum J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 on a unique gradient 1ine.34.52-54 Naturally, transition states may lie on either sort of GL, principal or non-principal. If two transition states lie on two different pGLs then the two gradient-line reaction path^^^-^^ enter the minimum perpen- dicular to one another. Transition states may also lie on an npGL. An interesting situation arises when the eigenvector q1 corresponding to the smallest eigenvalue (a1)is invariant under a reflection or two- fold rotation given by operation R. In this case there may be two equivalent gradient lines, npGL and R(npGL), leading to two (possibly enantiomeric) equivalent transition states, TS and R(TS) as shown in Scheme 3. min Scheme 3 The two npGLs, npGL and R(npGL), can lead either to the same minimum or to two different versions of the same minimum.The first case is observed for the 1,2 H shift reac- tion in phosphinonitrene which is considered in Section 4. 4. 1,2 H Shift in Phosphinonitrene According to our ab initio (DZP) calculations, H,PN in the ground electronic state has three different minima 1, 3 and 4, while structure 2 is a true transition state (A = 1)for the 1,2 H shift from P to N and structure 5 is a second index (A = 2) critical point (see Table 1 and Fig. 2). H ,P-N/\ H 5, c, ',l,390 (1.407) 126.3 (128.8) (1.515) H' ld2.2 (98.3) t,c, (h= 0) 3,cs(h= 0) H' (1.437) 2a,C1(h=1) H 5,cs(h= 2) 6,Cs(h= 1) Fig.2 Geometrical parameters of minima 1, 3 and 4, and transition states 2 and 6 for H,PN calculated at the DZP and MP2/6-31G** (numbers in parentheseP) levels. Structure 5 corresponds to a second index critical point. Bond lengths are given in A, bond angles in degrees. The total energies and geometrical parameters for structures 1-5 calculated in the present work at the DZP/SCF level agree with those obtained in previous ab initio calcu- lation~.'~-~*The latter work has shown that the positions of critical points in nuclear configuration space for this molecule do not change significantly when the computational effort is increased from SCF/DZP to MP4/6-311 + +G(df, p)35 (see Table 1).Hence the topology of the H,PN PES is insensitive to the level of the calculation and we therefore employed the DZP/SCF approach throughout. Within the Born-Oppenheimer approximation,' and from a classical point of the reaction path is a line belonging to the PES. In the absence of any external fields, the gradient lines of the PES define the local resultant force at any point. Scheme 4 is, in fact, composed of such gradient lines. H' H 26,C,(A= 1) Scheme 4 The complete GLRP (Scheme 4) consists of two equivalent lines [GLl and R(GLl)] corresponding to the 1,2 H shifts 1+2a e4 and 1 26 e4 passing through transition states 2a and 26, respectively.The gradient line (GL2) connecting minima 1 and 3 passes through second index critical point 5 where it correlates with the Hessian eigenvector with the largest-magnitude negative eigenvalue. Furthermore, minimum 3, trans-phosphazene, is linked to cis-phosphazene, minimum 4, by gradient line GL3 passing through structure 6,which corresponds to the transition state for inversion at nitrogen. Taking EF45 steps with starting displacements along both of the directions defined by the Hessian eigen- vector of TS 2 corresponding to the unique negative eigen- value, we calculated the pathways and energy profiles from TS 2 to minima 1 and 4. The pathway is only an approximate gradient line45v47 since the EF steps are not truly parallel to the gradient vector.50 However, this path is probably close enough to GL1 and R(GL1) for our purposes and has the same symmetry properties, as corroborated by the corre-lation of the Hessian eigenvectors corresponding to the lowest Hessian eigenvalues along the pathway.We also checked these pathways employing a steepest-descent method and obtained the same results: the steepest descent from 2 in one direction leads to minimum 1 and in the other direction to 4 (Fig. 3). The evolution of the five smallest Hessian eigen- values along these paths is shown in Fig. 4. Comparing Fig. 4(a) and (b) we see that the pathway obtained by EF gives a reasonable impression of how the eigenvectors and eigenvalues are correlated. However, since these paths maintain C,symmetry throughout there should be no crossings in Fig.4, barring accidental degeneracies. The Table 1 Total energies (EJ,relative energies (AE), number of imaginary frequencies (A) and values of the imaginary normal mode frequencies (io)calculated for structures 1-6 at different levels of theory EJEh AEjkcal mol -I, io/cm -MP4 MP4 MP4 MP4 structure symmetry DZP" 6-31 ++GLb /6-311 +G(df, p)' DZP" 6-31 ++G*b /6-311 ++G(df, p)' DZP" 6-31 ++G*' /6-311 ++G(df, p)' DZP" 6-31 ++G*b /6-311 ++G(df, p)' 1 c,, -396.22700 -396.213 62 -396.63647 0 0 0 0 0 4 C,(cis) -396.298 77 -396.281 48 --45.0 -42.6 -0 0 3 C,(trans) -396.30001 -396.283 33 -396.70084 -45.8 -43.7 -40.0 0 0 5 c, -396.11862 -396.102 10 -396.56056 68.0 70.0 47.6 2 2 2 c1 -396.181 38 -396.16909 -396.564 04 28.6 27.9 45.4 1 1 6 cs -396.277 60 ---31.7 --1 -~ ,,Present work.'Ref. 36.Ref. 35. Calculations with 3-21G* basis sets.36 Table 2 Total energies (EJ, relative energies (AE), number of imaginary frequencies (A) and values of the imaginary normal mode frequencies (io) calculated for structures 7-10 at different levels of theory EJEh AElkcal mol-' 1 iw/cm -' MP4(SDTQ)/6-3 1 1G** MPqSDTQ)/6-311G** MP4(SDTQ)6-311G** MPqSDTQ)/6-3116** structure symmetry DZP" 6-3 1G*' f/MP2/6-3 1G*' DZP" 6-31G*b //MP2/6-31G*' DZP" 6-31G*b //MP2/6-31G1' DZP" 6-31G*b //MP2/6-31G* c,_____~ ~~ 7 C," -113.895 51 -113.869 74 -1 14.262 63 0 0 0 0 0 0 ---9 C,(tr~n~) -113.81923 -113.79149 -1 14.177 82 47.9 49.1 53.1 0 0 ----~8 c, -113.736 31 -113.709 3 1 -114.177 82 100.0 104.6 81.4 1 1 1 2655.3 2710.0' 10 cs -1 13.726 76 -113.702 41 -114.12595 105.9 105.0 79.6 1 1 1 2307.2 2184.0d -"Present work.'Ref.63(a). Ref. 63(b). 4 wP J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 4 transition vector -396.23L-1 0 10 20 30 40 50 60 70 \ steps -396.221 h -396.28 -396.3 0 10 20 30 40 50 60 70 steps N Fig. 3 Energy profile and evolution of the transition vector along the gradient line (steepest-descent path) from TS 2 (C,) to minima 1 (C2J (a) and 4 (C,) (b).On the left is the transition vector for TS 2 which correlates with the smallest eigenvalue for both minima. On the right is the eigenvector corresponding to the smallest positive eigenvalue of TS 2 which correlates with the second smallest positive eigenvalue of both minima.avoided crossings are clear for the steepest-descent paths. Because of the mixing which must occur it is difficult to associate particular eigenvectors of the two minima with the reaction vector. Furthermore, in an EF search for a transition state with a reasonable step size the algorithm might take a 'forbidden' step in the vicinity of an avoided crossing, and cross over to the softest mode. We also note that the EF path has a region with two negative eigenvalues, indicating that it has strayed somewhat from the steepest-descent path. In order to elucidate the topology of the H,PN PES we calculated the gradient lines emanating from 5 along the Hessian eigenvector corresponding to the negative eigenvalue of smaller magnitude.In this case the path was not calculated by simple minimisation but instead EF steps were employed following the Hessian eigenvector corresponding to the most negative eigenvalue uphill. These gradient lines link structure 5 with transition states 2u and 2b as corroborated by the correlation of the Hessian eigenvectors corresponding to the lowest Hessian eigenvalues along the pathway and the evolu- tion of the five smallest Hessian eigenvalues along this path 1843 min 1 TS2 min 4 1.071 (a) 0.8 1 % X X X 2 00 ................':-0.2.~ -0.4 10 ( steps TS 2 min 4~~0.9'Imin 1 0.7 52 0.2-1 00 ..................... -0.1 { -0.2 J 1 ...............................7+---+%-*-q 1 ....... I -60 -40 -20 0 20 40 60 steps Fig.4 (a) Evolution of the five lowest eigenvalues of the (non-mass- weighted) projected Hessian along the approximate DZP gradient-line path from TS 2 to minima 1 and 4, as determined by overlap of the corresponding Hessian eigenvectors. (b) Same as (a), but for steepest descent rather than EF paths. The steps correspond to those in Fig. 3. (see Fig. 5 and 6).The H,PN PES has a topology which can be schematically represented by the two-dimensional surface in Fig. 7. Thus, the GLRP of Scheme 4 consists of two equiv- alent (enantiomeric) npGLs and the reaction is equally likely to proceed along either of them.33b,s5 Thus, we have a 'double-line' reaction pathway which consists of so-called narcissistic reactions as considered by Salems6 and others.29 Note that there are no branching points on GL1 and R(GL1): none of the initially positive Hessian eigenvalues change their signs along the pathways and the GLRP in this case coincides with the MERP.The pGL entering minimum 1 along the Hessian eigenvector corresponding to the small- est Hessian eigenvalue links it to the second index critical point 5. The observed double-line reaction pathway may be quite common in organic, inorganic and other reactions. For example, the 1,2 H shift in ethenes7 and H2PP,3s and inter- nal rotation in dimethyl ether,s7-60 propane,s9 acetones9 and other molecules61-62 containing two equivalent rotors all occur along double-line reaction pathways.1844 (0 c .-& 0.0-Q) C I-% -0.2-aI -0.4-5 TS 2-0.6 0 2 4 6 8 1012141 i I t t/ I t I Fig. 5 Evolution of the five lowest Hessian eigenvalues (top) and two eigenvectors (bottom) along the gradient line from structure 5 (Cs,A = 2) to TS 2 (Cl). The TS 2 transition vector correlates with the Hessian eigenvector of 5 with the most negative eigenvalue. The Hessian eigenvector of TS 2 corresponding to the smallest positive eigenvalue correlates with the Hessian eigenvector of 5 with the nega- tive eigenvalue of smaller magnitude. I .v 1 O.* 1 0.4 i -0.44 min 3 -0.6-, ,0 2 4 6 8 10 12 14 16 18 t t \ Fig.6 Top: Correlation of the five lowest eigenvalues of the (non- mass-weighted) projected Hessian along the gradient line from struc- ture 5 to minimum 3. Bottom: Evolution of the Hessian eigenvector corresponding to the imaginary frequency of greatest magnitude for the same path. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 7 Schematic three-dimensional representation of the H,PN PES 5. 1,2 H Shift and H, Elimination from Formaldehyde According to our ab initio (DZP) calculations, formaldehyde in the ground electronic state is stable in C,, form 7 with geometrical parameters, given in Fig. 8, which are in agree- ment with previous ab initio calculation^^^*^^-^^ and experi- mental data66,67 (see Table 2 and Fig. 8) The calculated frequencies together with experimental values66 (in parentheses) and the forms of the normal modes of this molecule are given below in Scheme 5.In Scheme 6 are shown the non-mass-weighted Hessian eigenvectors and eigenvalues. As can be seen from comparison of Schemes 5 and 6, the softest eigenvector, el, does not coincide with the softest normal mode and has different symmetry. H, H (1.208) 116.4\ '*I8' (116.5) -1.094(1.116)H 7,cs(h= 0) 9,cs(h= 0) (1.356) H (1.727)1.205 /\ 1.585 H 8,Cs(h= I) l0,Cs (h= 1) Fig. 8 Geometrical parameters for the stationary points of H,CO calculated at the DZP level. Bond lengths are given in A, bond angles in degrees. Values in parentheses for structure 7 are experi- mental data66 and for 8-10 are MP2,/6-31G* results.63 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 J"\ \/c--01(b1)=1337 cm-' 02(b2)=1369 cm-' %(a,)=1659 cm-' (1167) (1251) (1501) 04(al)=2~9cm-' os(a,)=3149 cm-' w6(b2)=3226 cm" (1746) (2766) (2843) Scheme 5 t e,(a1)=0.156 a.u. e2(b2)=0.196 a.u. e3(b,)=0.287 a.u. 't HB A=-/ r' e4(a,)=0.464 a.u. es(b2)=0.977 a.u. e6(a,)=2.135 a.u. Scheme 6 Formaldehyde can undergo a 1,2 H shift from carbon to oxygen (Scheme 7) with formation of a trans-hydroxymethylene 9 and can also undergo an H, elimination reaction (Scheme 8). /\ /H,,c-0 -,c-0 H -c-0 H H' H/ 10, c, Scheme 8 The reaction (Scheme 7) passes through planar transition state 8 and becomes parallel to el in the limit at minimum 7 (see Scheme 6).Taking EF45 steps with starting displace- ments along both of the directions defined by the Hessian eigenvector of TS 8, corresponding to the unique negative eigenvalue, we calculated the pathways and energy profiles from TS 8 to minima 7 and 9. We have also considered the evolution of the Hessian eigenvalues and the transition vector along this GL which are given in Fig. 9 and 10. Repeating these procedures for TS 10 we obtained the pathways and energy profiles from TS 10 to minimum 7 and the dissociated system H, + CO; the evolution of the Hessian eigenvalues and transition vector along this GL are given in Fig. 11 and 12. As can be seen from Fig. 9-12, the reactions of Schemes 7 and 8 evolve along GLs entering minimum 7 in the limit tangentially to el which does not correspond to the softest normal mode.f -0.2 -0.4 TS8 1 min 7-0.6:' r 1 1 -l 1 2 3 4 5 6 7 8 9 10 steps Fig. 9 Evolution of the five lowest eigenvalues of the (non-mass- weighted) projected Hessian along the approximate DZP gradient-line path from TS 8 to minimum 7 for H,CO, as determined by overlap of the corresponding Hessian eigenvectors. 4 c TS 8, C, min 7,CZv Fig. 10 Evolution of two Hessian eigenvectors along the gradient line from TS 8 to minimum 7 for H,CO. Below is the transition vector for TS 8 which correlates with the Hessian eigenvector corre- sponding to the smallest positive eigenvalue of 7. Above is the Hessian eigenvector corresponding to the second smallest positive eigenvalue of 8 which correlates with the second smallest positive eigenvalue of 7.The steps correspond to points 1, 4, 6 and 10 in Fig.9. _-*. . . ---1.o ,----0.84 A' 0.6--S 0.4-lu--. - 4- -. -* -_--. X X M I1 lu P O O D I1 0.231 r r w = + + a [:I ( > a o 0.0- + + + u)3 -0.2- TSlO / min 7 TS 10. C, min 7,Cpv transition vector Fig. 12 Evolution of two Hessian eigenvectors along the gradient line from TS 10 to minimum 7. Below is the transition vector for TS 10 which correlates with the smallest eigenvalue of 7, above is the Hessian eigenvector with the smallest positive eigenvalue for TS 10 which correlates with the second smallest positive eigenvalue of minimum 7.The steps correspond to points 1,4,6 and 10 in Fig. 11. 6. Conclusions If a reaction evolves along a non-principal gradient line then the path enters the minimum tangentially to the softest Hessian eigenvector for which it has a non-zero component. This may not correspond to the softest normal mode, as illus- trated by two different reactions, the 1,2 H shift and H, elimi- nation for formaldehyde. When the softest Hessian eigenvector has at least one element of symmetry the gradient-line reaction path may consist of two equivalent gradient lines, forming so-called narcissistic reaction paths. An example is the pathway for the 1,2 H shift in H,PN which may be described as two equiva- lent gradient lines (double-line reaction path) leading from phosphinonitrene to cis-phosphazene rather than to trans-phosphazene.The GLRP at the SCF/DZP level of theory does not contain branching points and coincides with the minimum-energy pathway. 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Phys. Chem., 1970, 74, 4161; A. Tachibana and K. Fukui, Theor. Chim. Acta, 1978,49,321. 42 R. M. Minyaev and D. J. Wales, J. Chem. Soc., Faraday Trans., 1994,90, 1831. 43 R. M. Minyaev and D. J. Wales, Chem. Phys. Lett., 1994, 218, 413. 44 R. M. Minyaev and D.J. Wales, J. Am. Chem. SOC.,submitted. 45 C. J. Cerjan and W. H. Miller, J. Chem. Phys., 1981, 75, 2800; J. Simmons, P. Jnrrgenson, H. Taylor and J. Ozment, J. Phys. Chem., 1983,87, 2745; D. O’Neal, H. Taylor and J. Simmons, J. Phys. Chem., 1984,88, 1510; A. Banerjee, N. Adams, J. Simmons and R. Shepard, J. Phys. Chem., 1985, 89, 52; J. Baker, J. Comput. Chem., 1986,7, 385; J. Baker, J. Comput. Chem., 1987,8, 563. 46 R. D. Amos and J. E. Rice, CADPAC: The Cambridge Analytic Derivatives Package, Issue 4.0, Cambridge, 1987. 47 D. J. Wales, J. Chem. SOC., Faraday Trans., 1990, 86, 3505; Mol. Phys., 1991,74,1; J. Chem. SOC.,Faraday Trans., 1992,88,653. 48 D. J. Wales, J. Chem. SOC., Faraday Trans., 1993,89, 1305. 49 T. H. Dunning Jr., J.Chem. Phys., 1970,53,2823. 50 J-Q. Sun and K. Ruedenberg, J. Chem. Phys., 1993, 99, 5257; 5269; J-Q. Sun, K. Ruedenberg and G. J. Atchity, J. Chem. Phys., 1993,99,5276. 51 L. P. Eisenhart, Riemannian Geometry, Princeton, New York, 1926. 52 B. A. Dubrovin, S. P. Novikov and A. T. Fomenko, Contempo-rary Geometry, Nauka, Moscow, 1986, ch. 3. 53 J. M. Bruce and P. G. Giblin, Curves and Singularities, Cam-bridge University Press, London, 1984. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1847 54 W. E. Williams, Partial Diflerential Equations, Clarendon Press, 63 (a)L. B. Harding, H. B. Schlegel, R. Krishnan and J. A. Pople, J. Oxford, 1980, ch. 3. Phys. Chem., 1980, 84, 3394; (b) M. J. Frisch, R. Krishnan and 55 R. C. Tolman, Phys.Rev., 1924,23,699. J. A. Pople, J. Phys. Chem., 1981,85, 1467. 56 L. Salem, Acc. Chem. Res., 1971,4, 322. 64 M. Dupuis, W. A. Lester Jr., B. H. Lengsfield and B. Liu, J. 57 W. J. Hehre, L. Radom, P. v. R. Schleyer and J. A. Pople, Ab Phys. Chem., 1983,79,6167. initio Molecular Orbital Theory, Wiley, New York, 1986, and 65 P. B. Karadakov, J. Gerratt, D. L. Cooper and M. Raimondi, J. references therein. Chem. Phys., 1992,97,76; 37. 58 A. G. Ozkabak and L. Goodman, Chem. Phys. Lett., 1991, 176, 66 W. M. Gelbart, M. L. Elert and D. F. Heller, Chem. Rev., 1980, 19. 80,403. 59 H. Guo and M. Karplus, J. Chem. Phys., 1989,91, 1719. 67 G. Herzberg, Molecular Spectra and Molecular Structure. ZII. 60 J. R. van Wazer, V. Kello, B. A. Hess Jr. and C. S. Ewig, J. Electronic Spectra and Electronic Structure of Polyatomic Mol- Chem. Phys., 1990,94,5694. ecules, Van Nostrand Reinhold, New York, 1966, p. 612. 61 H. S. Gutowsky, J. Chen, P. J. Hajduk, J. D. Keen, C. Chuang 68 T. Taketsugu and T. Hirano, J. Chem. Phys., 1993,99,9806. and T. Edmilsson, J. Am. Chem. SOC., 1991,113,4747. 62 H. Mayr, W. Forner and P. v. R. Schleyer, J. Am. Chem. SOC., Paper 4/00231H ;Received 14th January, 19941979,101,6032.

ISSN:0956-5000    

DOI:10.1039/FT9949001839

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(13), 1849-1852 Non-linear Optical Properties of Organic Molecules Part 13.~-Calculation of the Structure and Frequency-dependent Hyperpolarisability of a Blue Azothiophene Dye John 0. Morley Chemistry Department, University College of Swansea, Singleton Park, Swansea, UK SA2 8PP The st ruct u re of a co m m erc iaI dye, 2-(2-aceta m ido-4-dieth y lam ino p henylazo)-3,5-d in it rothiop hen e, with potent ia I application in non-linear optics has been calculated at the ab initio STO-3G level. The molecule is predicted to be essentially planar between donor and acceptor groups with a strong intramolecular hydrogen bond between the acetamido group and one nitrogen of the azo linkage. The hyperpolarisability of this structure calculated using a semi-empirical molecular orbital sum-over-states method is predicted to be large and arises mainly from the transition of an electron from the ground state to the first excited state.The value obtained is highly dependent on the frequency of the applied field and very substantial resonance enhancement effects are calcu- lated near the transition energy, particularly for linear electro-optic modulation. A considerable number of organic molecules have now been identified as effective materials for applications in non-linear optics, particularly in the area of second harmonic generation (SHG) or linear electro-optic modulation (LEO) where infor- mation is coded directly onto a low-energy carrier wave by application of a dc electric field.'-3 Most active molecules for these applications possess a large molecular hyperpolar-isability and contain a donor and acceptor group situated at either end of a suitable conjugation path ; 2-methyl-4-nitro-aniline is a typical e~ample.~ Most theoretical studies on the non-linear properties of conjugated organic molecules and their frequency-dependent effects have been carried out by calculating their molecular hyperpolarisabilities using molecular orbital theory coupled with sum-over-states (SOS) based on a per- turbation formalism derived by Ward.' ' The appropriate formula for the hyperpolarisability relating to the LEO effect and r'(rn) is the ith component of the position vector of elec- tron rn (of N), rnn= (n I riI n), Qng is the eigenvalue of +,, rela-tive to the ground state +g (the electronic transition energy), e is the magnitude of the electronic charge, and R is the fre- quency of the applied radiation field.This indicates n and n' t Part 12: ref. 18. may be restricted to run over excited states in increasing energy provided (gl I Ig) = 0, which holds only in the elec- tronic charge centroid system as discussed previ~usly.~ A similar expression has been derived by Ward'' for the SHG effect, again from perturbation theory where the appro- priate formula for the hyperpolarisability tensor (&j) is given by: where the terms have the same definition as in eqn. (1). All 27 components of the SHG tensor are calculated using these expressions but the most appropriate quantity is the vector component, 8, theoretically defined as7 where B is aligned to lie along the direction of the molecular dipole moment and is therefore directly related to the non- linear coefficients derived both from electric field induced second harmonic generation in solution and in poled polymer films where molecules are oriented along the direc- tion of their dipole moments by a strong dc field.The effect of a variation of the applied field on the experi- mental and calculated hyperpolarisability of both 4-nitro- and 2-methyl-4-nitro-aniline has been systematically explored for second harmonic generation where there are large reson- ance enhancement effects12 arising from terms such as Q,, -2R in the denominator of eqn.(2). In contrast, few studies have explored the likely resonance approaches the transmission frequency R, hyperpolarisability becomes. The present studies have been carried out to probe theoretically the magnitude of this effect in 2-(2-acetamido-4-diethylaminophenylazo)-3,5-dinitrothio-phene (I), a commercial dye with a strong absorption in the red region of the spe~trum'~ and a potential candidate for LEO applications in poled polymer films, although its struc- ture is unknown. I Method of Calculation The structure and electronic properties of the azothiophene I were calculated from an empirical structure at the ab initio STO-3G levelI4 using the GAMESS program15 with full optimisation of all bond lengths, angles and torsion angles. The derived structure was then used to calculate the molecu- lar hyperpolarisability using the sum-over-states procedure (SOS)which has been specifically parametrised for both SHG7 and LEO applications." calculations, the applied field was varied from 0 to 3.5 eV and Table 1 bond lengthsb S(l)-C(2) 1.741 S(1)-C(5) 1.743 C(2)-C(3) 1.339 C(3)- C(4) 1.439 C(4)-C(5) 1.360 C( 5)- N(6) 1.447 N(6)-N(7) 1.291 N(7)-C(8) 1.437 w-C(9) 1.423 C( 8)-C( 13) 1.406 C( 9) -C( 1 0) 1.397 C( 10)-C( 11) 1.396 C(ll)-C(12) 1.419 C(12)-C( 13) 1.364 C(2)- N( 14) 1.488 N( 14)-O( 15) 1.280 N( 14)-O( 16) 1.280 C(4)-N( 17) 1.487 N(17)-O( 18) 1.279 N(17)-O(19) 1.278 C(9)- N( 20) 1.412 N(20)-C(21) 1.431 C(21)-C(23) 1.542 C(21)-O(22) 1.220 C(ll)--N(24) 1.410 N(24)-C(25) 1.476 C( 25)- C( 26) 1.547 N(24)- C( 27) 1.476 C(27)-C(28) 1.547 See Scheme 1for key to atom positions.In A. ' enhancements for the LEO effect, particularly on relevant materials such as azo dyes where the magnitude of the hyperpolarisability is partly dependent on the reciprocal of terms such as ring -R [see eqn. (l)], i.e. the closer the electronic transition energy Rng the larger the CNDOVSB meth~d,~ a In the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the hyperpolarisability evaluated both in the Cartesian frame and along the direction of the molecular dipole moment using eqn. (3). Discussion Structure and Electronic Properties The results from the geometry optimisation using the num- bering convention shown below (Scheme 1) show the mol- ecule to be essentially planar with the exception of the pendant ethyl groups where the terminal carbons are almost perpendicular to the ring plane (Table 1).The bond lengths and angles in the thiophene ring appear to be fully consistent with those found for related structures16 with an approx-imate right angle at C(2)-S(l)-CC(5). The azo N(6)-N(7) bond at 1.291 A is significantly longer, however, than that found in the simple azobenzenes at around 1.270 which probably reflects the presence of the large acetamido group at the 2-position of the phenyl ring. Electronically, the sulfur atom is calculated to have a positive charge as a consequence of the presence of the two powerful electron-attracting nitro 'N(20)-H(29) '4-3 /28-27 \ N(24)-11 / \ 26-25 12-13 Scheme 1 Numbering system adopted for the azothiophene I Geometry and atomic charges of I calculated at the STO-3G level" bond angles' Mulliken charges S(1)-C(2)-C( 3) C(2)-S(l)-C(5) C(2)- C( 3)- C(4) C( 3)- C(4)- C( 5) C(4)-C( 5)-S(1) C(4)- C( 5)- N( 6) C(5)-N(6)- N(7) N( 6)- N( 7)- C( 8) N( 7)- C( 8)-C(9) C(8)-C( 9)- C( 10) C(9)-C( 10)-C( 11) C( 10)-C( 11)-C(12) C( 1 1)-C( 12)-C( 13) C(8)-C( 13)- C( 12) S(1)-C( 2) -N( 14) C(2)- N( 14)-O( 15) O(15)-N( 14)-O( 16) C( 3)- C(4)-N( 17) C(4)-N(17)-0( 18) O(18)- N( 17)- O(19) C(8)-C(9)-N( 20) C(9)-N(20)-C(2 1) N( 20)- C( 2 1)- O(22) N(20)-C(2 1)-C( 23) C(lO)-C(11)-"(24) C( 1 l)-N(24)-C(25) N( 24)-C( 25)- C( 26) N(24)-C(27)- C( 28) C(25)-N(24)-C(27) In degrees. 114.9 0.355 89.5 -0.036 109.9 -0.050 114.2 0.053 111.5 0.000 123.0 -0.166 111.8 -0.098 114.9 0.010 128.0 0.152 118.4 -0.138 122.7 0.151 118.1 -0.113 119.7 -0.027 122.8 0.133 121.0 -0.172 117.3 -0.182 125.0 0.137 120.8 -0.192 118.9 -0.175 123.7 -0.374 121.4 0.313 126.6 -0.272 124.3 -0.215 113.6 -0.283 121.2 0.003 120.7 -0.185 114.8 0.003 114.9 -0.184 116.6 0.26 1 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 groups and the phenylazo group in the thiophene ring. The molecule is held in a rigid conformation by a strong intra- molecular hydrogen bond between H(29) and the azo N(6) atom, as shown by the interatomic distance of 1.902 A and the relatively larger charges of 0.261 and -0.166, respectively (Table 1).Frequency-depeodentHyperpolarisabilities The calculated hyperpolarisability of the azothiophene I is dominated by the change in electron distribution in moving from the ground state to the first excited state. In the ground state the molecule is mainly polarised from the left-hand donor ring to the right-hand acceptor along the x coordinate, with a smaller component running from top to bottom along the y coordinate, as shown by the components of the dipole moment (Table 2). On excitation to the first excited state there is a large increase in the x component of the dipole moment towards the acceptor group and a substantial tran- sition moment which produces a large hyperpolarisability in the x direction.Both the dipole moment and hyperpolar- isability are negative. The inclusion of a further nine excited states results in little change to the hyperpolarisability, but after 25 excited states are included there is a fall of around 13% in its value (Table 2). Thereafter, the value remains rea- sonably constant up to a total of 80 included states and although the dipole moments of some of these are large, the corresponding transition moments are very small. Although the x component of the hyperpolarisability is very large and negative, in practice, only the vector com- ponent along the direction of the dipole moment is likely to be useful in applications such as poled polymer films.A re-orientation of the tensor components by the appropriate transformation then gives three components including the important vector component, B, which is now positive and somewhat smaller in magnitude than the x component because it lies between and x and y directions (p = 494 and -508, respectively, Tables 2 and 3). For the LEO effect, the frequency-dependent values of the vector component of the hyperpolarisability increase with increasing field strength and become extremely large as the applied field, R, approaches the transition energy, Rng.The predicted value at R = 2.08 eV, which is very close to the transition energy, an,,at 2.082 eV, is ca. lo1 (Table 3) which clearly demonstrates the importance of the R -R,, recipro-cal term in eqn.(1). In contrast, large values of ca. 106 are calculated for the SHG effect both at the second harmonic frequency, where R,, approaches 2R, and at the fundamental frequency, where Rngapproaches R, which shows the almost equal importance of the Rng-2R and R,, -R reciprocal terms in eqn. (2). Table 2 Dipole moments, transition moments and effect of the number of included states on the components of the hyperpolarisability calculated for I in the Cartesian frame using the CNDOVSB method dipole moment" hyperpolarisabilit y'excited state c"Y P* rgnPX PX BY Pz ground -5.24 -1.80 0.04 1 -15.88 -2.82 -0.47 4.02 -595.0 -39.7 -28.6 10 -7.34 -2.68 -0.14 0.37 -598.6 -45.6 -31.4 25 -10.41 -0.85 -0.16 1.57 -517.5 -58.9 -27.8 50 -6.96 -8.05 -0.25 -0.82 -5 14.4 -48.4 -26.8 80 -34.70 10.66 -0.97 -0.04 -508.0 -42.6 -26.5 In D for a given ground or excited state (1 D z 3.33564 x lop3' C m).* x component of the transition moment. Calculated for the LEO effect in units of lop3' cm5 esu-' (3.71 x C-' m3 F2)using from 1 to 80 excited states. Table 3 Calculated vector components of the frequency-dependent hyperpolarisability of I using the CNDOVSB methodu LEO effect applied field (Q)/nmb P" 0 (0)1890 (0.66) 2.08 1.43 4.94 x lo2 5.95 x lo2 2.08 0.78 1300 (0.95) 1.14 7.50 x 10' 0.19 1186 (1.04) 1.04 8.31 x 10' 0.002 1060 (1.17) 0.92 9.77 x 10' -0.52 800 (1.55) 0.54 2.15 x 103 -1.01 700 (1.77) 0.32 5.21 x 103 -1.45 650 (1.91) 600(2.07) 0.18 0.02 1-49 x lo4 1.18 x 107 -1.73 -2.05 593 (2.08) 0.002 3.47 x lo1' -2.08 550 (2.25) -0.16 1.41 x lo4 -2.4 1 500 (2.48) -0.39 2.23 x 103 -2.87 450 (2.76) -0.67 7.79 x 103 -3.43 354 (3.50) -1.41 4.42 x 103 -4.91 fl" and f12" are the calculated hyperpolarisabilities for the LEO and SHG effects, respectively, in units of (Qng -2R) values have been rounded to two decimal places except where stated otherwise.SHG effect 2.34 x lo2 9.71 x lo2 -1.81 x lo6 -7.90 x 10' 1.24 x 104 9.85 x 103 6.50 x 10' 1.71 x 103 5.12 x lo6 3.02 x 103 2.05 x 10' -1.33 x 103 -4.94 x 10' an5esu-'. The(Q,, -Q) and 1852 Conclusions The calculations demonstrate that the azothiophene I has a substantial hyperpolarisability which arises mainly from the transition of an electron from the ground state to the first excited state.The value obtained is highly dependent on the frequency of the applied field and substantial resonance enhancement is possible near the transition energy, particu- larly for the LEO effect. References 1 Nonlinear Optical Effects in Molecules and Polymers, ed. P. N. Prasad and D. J. Williams, Wiley, New York, 1991. 2 Nonlinear Optical Properties of Organic Molecules and Crystals, ed. D. S. Chemla and J. Zyss, Academic Press, New York, 1987. 3 Nonlinear Optics and Organics and Semiconductors, ed. K. Kobayashi, Springer-Verlag, Tokyo, 1989.4 G. F. Lipscomb, A. F. Garito and R. S. Narang, Appl. Phys. Lett., 1981,38,663. 5 J. A. Morrell and A. C. Albrecht, Chem. Phys. Lett., 1979,64,46. 6 S. J. Lalama and A. F. Garito, Phys. Rev. A, 1979,20, 1179. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 7 V. J. Docherty, D. Pugh and J. 0. Morley, J. Chem. SOC., Faraday Trans. 2, 1985,81,1179. 8 C. W. Dirk, R. J. Tweig and G. Wagniere, J. Am. Chem. SOC., 1986,108,5387. 9 DeQuan Li, M. A. Ratner and T. J. Marks, J. Am. Chem. SOC., 1988,110,1707. 10 J. 0. Morley and D. Pugh, J. Chem. SOC., Faraday Trans. 2, 1991,87,3021. 11 J. Ward, Rev. Mod. Phys., 1965,37, 1. 12 S. J. Lalama and A. F. Garito, Phys. Rev. B, 1983,28,6766. 13 0.Annen, R. Egli, R. Hasler, B. Henzi, H. Jakob and P. Mat-zinger, Rev. Prog. Coloration, 1987, 17,72. 14 W. J. Hehre, L. Radom, P. v. R.Schleyer and J. A. Pople, Ab Initio Molecular Orbital Theory, Wiley, New York, 1986. 15 M. F. Guest and P. Sherwood, GAMESS, An ab initio Program, The Daresbury Laboratory, Warrington, UK. 16 M. H. Charlton, R. Docherty, D. J. McGeein and J. 0.Morley, J. Chem. SOC.,Faraday Trans., 1993,89,1671. 17 Cambridge Structural Database, Cambridge Crystallographic Data Centre, Cambridge, UK. 18 J. 0.Morley, Int. J. Quantum Chem., 1993,46, 19. Paper 4/00680A; Received 4th February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001849

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(13), 1853-1855 1853 Non-linear Optical Properties of Organic Molecules Part 14.~-Calculations of the Structure, Electronic Properties and Hyperpolarisabilities of Cyclopentadienylpyridines John 0.Morley Chemistry Department, University College of Swansea, Singleton Park, Swansea, UK SA2 8PP The structures of a series of cyclopentadienylpyridines have been calculated using the AM1 method and reason- able correlations have been found with crystallographic data where available. Subsequent calculations have been carried out on these structures using the CNDOVSB method to obtain dipole moments, transition energies and hyperpolarisabilities. The two methods are found to give widely different values for the ground-state dipole moments.The calculated hyperpolarisabilities suggest that these molecules have considerably superior non-linear optical properties to conventional donor-acceptor aromatics. Poled polymer films containing an active non-linear com-ponent have attracted considerable interest as potential devices for electro-optic modulation (EOM) and second har- monic generation (SHG).'-' These materials are generally prepared by copolymerisation of a suitable monomer, such as methylmethacrylate, with an active organic such as a donor- acceptor aromatic containing an unsaturated linkage at the donor group. The resulting polymer is subjected to a strong electric field near the glass-transition temperature to orientate the active component along the direction of the molecular dipole moment, and then cooled in the presence of the same field to yield an orientated non-linear material.Typical aro- matics include derivatives of N-alkyl-4-nitroaniline and 4- alkoxy-4'-nitroazobenzene containing at least one acrylate group positioned at the end of the alkyl chain.'-3 The degree of orientation achieved in the poling process and the non-linear activity of the resulting polymer, however, are a function of a number of factors which include the strength of the electric field, the size of the active molecule, the magnitude of the molecular dipole moment and the molecular hyperpolarisability. Although existing materials have reasonable dipole moments, for example 6.3 D for 4- nitroaniline and 6.5 D for 4-methoxy-4-nitroazobenzene,4 molecules with larger values would be expected to show enhanced behaviour provided they also possess substantial molecular hyperpolarisabilities. The present studies have been initiated to explore the potential of the highly polar pyridinium cyclopentadienylide I, the related cyclo-pentadienylides 1,Cdihydropyridine I1 and the azolylidene 1,4-dihydropyridines 111 and IV, a class of molecule with large dipole moments but with unknown non-linear proper- ties.43 I M e-I1 t Part 13: ref. 16. \-/ "9 I11 M e-(,,)=(:DN IV Method of Calculation Molecular orbital calculations were carried out on empirical structures for the cyclopentadienylpyridines I-IV using the AM1 method5 with full optimisation of all bond lengths, angles and torsion angles.The derived structures were then used to calculate the molecular hyperpolarisability using the CNDOVSB method,6 a sum-over-states procedure (SOS) which has been specifically parametrised for SHG applica-tions. A similar SOS approach has been reported by several other a~thors.~-'' As in previous work, all 27 components of the tensor are calculated by the CNDOVSB method, though the most appropriate quantity is the vector component, P, theoretically defined as9 P = Pppp + 5 1(Ppii + Biip) ifp where /3 is aligned to lie along the direction of the molecular dipole moment (p). In addition to the frequency-dependent value, a static value is also calculated in the absence of the applied frequency to give the quantity B0, which is a measure of the intrinsic hyperpolarisability of a given molecular system and which has been used to compare the relative em- cacy of polyphenyls us.polyenes and other conjugated systems.' ' Discussion Structure Optimisation Initial calculations at the AM1 level' with full optimisation of all variables on 4-cyclopentadienylidene-1-methyl-1,4-dihy-dropyridine (11) with the numbering system shown below (Scheme 1) gives a planar structure which shows a reasonably J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Calculated geometries, heats of formation and dipole moments obtained for the cyclopentadienylpyridines I-IV using the AM 1 method" x, Y,z ~~ C(l)-X(2) X(2)-C(3)C(3)--C(4) C(4)-Y(5) Y(5)-W C(6)-Z(7) q7)-C(8) C(8)-C(9)C( 1)- X(2)-C( 3) X(2)-C( 3)-C(4) C(3)-C(4)-- Y(5) C(3)-X(2)-C(12) C( 4)- Y (5)-C( 6) C(4)-Y(5)-C(ll)Y(5)-C( 6)- Z(7) C(6)-Z(7)-C(8) q7)-c(8)-C(9) 1.395 1.394 1.386 1.366 1.463 1.380 1.449 120.4 122.0 118.1 121.4 117.2 126.7 107.8 108.9 1.435 1.390 1.365 1.449 1.362 1.471 1.371 1.466 120.8 121.9 121.9 118.2 122.9 114.2 127.1 108.5 108.7 1.487 1.364 1.354 1.428 1.388 1.442 1.364 1.413 121.1 122.2 122.2 117.8 123.3 113.4 127.2 108.2 109.0 1.438 1.386 1.369 1.441 1.383 1.442 1.324 1.492 120.8 122.0 120.9 118.5 122.1 115.7 124.4 104.8 109.6 1.440 1.383 1.373 1.435 1.400 1.429 1.343 1.499 120.7 121.9 120.8 118.5 122.0 116.0 123.5 104.3 109.2 1.478 1.339 1.366 1.387 1.446 1.359 1.384 1.408 119.9 120.8 120.5 120.3 121.4 117.2 122.1 103.3 108.8 heat of formation 104.04 93.96 115.23 142.54 dipole momentd 4.63 (13.3)" 6.37 (10.3)f 6.82 9.37 (9.03)' ~~ ~~ ~~ " Bond lengths are given in A, angles in degrees, heats of formation in kcal mol-' and dipole moments in D.Average data from the crystal structure of the N-(2,6-dichlorobenzyl) derivative (ref. 12). 'Ref. 13. Experimental results are given in parentheses. Ref. 4. Data for the N-benzyl derivative from ref. 4. Scbeme 1 Numbering system adopted for the cyclo-pentadienylpyridines (I-IV) good correlation with available crystallographic data on the closely related 1-(2',6'-dichlorobenzyl) derivative' (Table 1). The calculated results give C(3)-C(4), C(5)-C(6) and C(7)-C(8) bond lengths of 1,3865, 1.362 and 1.371 A, corre-sponding to double bonds, compared with values of 1.354, 1.388 and 1.364 A, found in the related crystal structure12 (Table 2).Furthermore, the nominal single bonds at C(4)-C(5) and C(6)-C(7) have calculated values of 1.449 and 1.471 8, compared with experimental values of 1.428 and 1.442 A. However, the geometric correlation is less satisfac- tory for 4-( benzimidazol-2-y1idene)-1 -methyl- 1,4-dihydropyri- dine (IV), where the central C(5)-C(6) bond is predicted to be shorter at 1.400 A than that found in the crystal structure at 1.446 A,13 although the double bond at C(3)-C(4) is well reproduced. Electronic Properties Previous calculations have shown that the CNDOVSB method gives a good correlation between the calculated and experimental transition energies for most conjugated organic systems6 and the result obtained for the pyridinium cyclo- pentadienylide I at 515 nm (Table 2) compares favourably with the experimental value of 511 nm determined in chloro- form.14 The approximations adopted to achieve a good spec- troscopic fit,6 however, result in an overestimation of the ground-state dipole moment and an underestimation of the excited-state dipole moment.6 In the examples explored here, the ground-state dipole moments are considerably larger than those obtained with the AM1 method (Tables 1 and 2).Table 2 Calculated hyperpolarisabilities and excited-state proper- ties of the cyclopentadienylpyridines I-IV obtained with the CNDOVSB method" I 13.3' 8.19 0.18 515 0.73 -55.8 -188.5 I1 10.3' 11.54 8.03 431 1.19 -23.3 -48.2 I11 9.92 6.22 471 1.17 -32.6 -81.7 IV 9.03d 14.14 6.32 521 1.35 -93.9 -344.5 a peXp,pgand pe are the experimental dipole moment and calculated ground- and excited-state dipole moments, respectively (in D); 1 is the transition energy or absorption maximum (in nm); f is the oscil- lator strength; Po and are the molecular hyperpolarisabilities at applied field strengths of zero and 0.95 eV, respectively [units of cm5 esu-' (3.71 x C-' m3 F2)].Ref. 4. Data for the N-benzyl derivative from ref. 4. Ref. 13. Experimental evidence suggests that both methods underesti- mate the dipole moment of the pyridinium cyclo-pentadienylide I at 13.3 Dt by a substantial margin,4 with the CNDOVSB method providing the best result at 8.19 D us.4.63 D for AM1. Furthermore, the CNDOVSB value for 4-cyclopentadienylidene-1-methyl- 1,4-dihydropyridine (11) at 11.54 D is much closer to the experimental value for the closely related N-benzyl derivative4 at 10.3 D than the AM1 value of 6.37 D. However, the AM1 dipole moment of 9.37 D for 4-(benzimidazol-2-ylidene)-1-methyl- 1,4-dihydropyridine (IV) is a better fit with the experimental value13 of 9.03 D than the CNDOVSB result which is too large (Table 2). The excited-state results (Table 2) show that there is a sub- stantial reduction in the value of the dipole moment in moving from the ground state to the first excited state (which is the major contributor to the value of the hyperpolarisability), particularly for the cyclo-pentadienylpyridines I and IV.This is a reflection of the t 1 D z 3.33564 x 10-30~. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 change of direction of charge transfer which takes place in each molecule upon excitation. In the ground state of the pyridinium cyclopentadienylide I the structure is polarised with electron donation from the left-hand pyridinium ring to the right-hand cyclopentadienylide ring with a strongly posi- tive nitrogen atom and with overall charges of k0.56, respec-tively. Upon excitation, there is a reversal of the direction of charge transfer, C(6) becomes strongly positive and the rings are now broadly neutral with values of k0.031 (Fig.1). Calculated Hyperpolarisabilities All of the molecules explored in the present studies gave hyperpolarisabilities which are considerably larger than expected with the negative values reflecting the reversal of the direction of charge transfer upon excitation. The values obtained for I and IV are much larger than those for I1 and I11 and mirror the large changes of around 8 D in their dipole moments upon excitation. Furthermore, the cyclo- pentadienylides I and IV show a larger frequency-dependent value, than the cyclopentadienylpyridines I1 and 111. This arises because the SHG expression for the hyperpolarisability l5 is partly dependent on the reciprocal of terms such as Qeg -2R, where Re, is the transition energy between ground (g) and excited state (e) and R is the magni- tude of the applied field (in this case 0.95 eV or 1300 nm).The closer the transition energy approaches the second harmonic of 650 nm the greater the expected resonance enhancement in p .022t029 .019 b .020 b .020 Fig. 1 Ground- (top) and excited-state (bottom) charge distribu- tions for pyridinium cyclopentadienylide (I) line with the calculated results. The approximate three-fold resonance enhancement for the pyridinium cyclo-pentadienylide I is matched by that for 4-(benzimidazol-2- y1idene)-l-methyl-l,4-dihydropyridine(IV) as the transition energies of both molecules at 515 and 521 nm, respectively, are very similar (Table 2). The overall values predicted for the cyclo-pentadienylpyridines I-IV are considerably larger than those for donor-acceptor aromatics such as 4-nitroaniline with Do = 7.02 and comparable to those of much larger systems such as 4-dimethylamino-/?-nitrostyrene with Po = 37.2 and 4-amino-4’-nitrostilbene with Po = 25.6.6 Conclusions The AM1 method appears to give a reasonable account of the geometry of the cyclopentadienylpyridines I-IV, but the calculated dipole moments differ from those obtained using the CNDOVSB method.The calculated hyperpolarisabilities are large and negative and considerably superior to those of conventional donor-acceptor aromatics. With appropriate functionalisation, the materials offer considerable potential for exploitation in non-linear devices. References 1 Nonlinear Optical Eflects in Molecules and Polymers, ed.P. N. and D. J. Williams, Wiley, New York, 1991. 2 Nonlinear Optics of Organics and Semiconductors, ed. K. Kobay-ashi, Springer-Verlag, Tokyo, 1989. 3 Nonlinear Optical Properties of Organic Molecules and Crystals, ed. D. S. Chemla and J. Zyss, Academic Press, New York, 1987. 4 A. L. McClellan, Tables of Experimental Dipole Moments, W. H. Freeman, San Francisco, 1963, vol. I; Rahara Enterprises, San Francisco, 1974, vol. 11. 5 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. Stewart, J. Am. Chem. SOC., 1985,107,3902. 6 V. J. Docherty, D. Pugh and J. 0. Morley, J. Chem. SOC., Faraday Trans. 2, 1985,81,1179. 7 J. A. Morrell and A. C. Albrecht, Chem.Phys. Lett., 1979,64,46. 8 S. J. Lalama and A. F. Garito, Phys. Rev. A, 1979,208, 1179. 9 C. W. Dirk, R. J. Tweig and G. Wagniere, J. Am. Chem. SOC., 1986,108,5387. 10 DeQuan Li, M. A. Ratner and T. J. Marks, J. Am. Chem. SOC., 1988,110, 1707. 11 J. 0.Morley, V. J. Docherty and D. Pugh, J. Chem. SOC., Perkin Trans. 2, 1987, 1351; J. 0. Morley, J. Chem. SOC., Faraday Trans. 2, 1991, 87, 3009; J. 0. Morley and D. Pugh,J. Chem. Soc., Faraday Trans. 2, 1991, 87, 3021; J. 0. Morley, J. Am. Chem. SOC., 1988, 110, 7660; J. 0. Morley, Znt. J. Quantum Chem., 1993,46, 19. 12 H. L. Ammon and G. L. Wheeler, J. Am. Chem. SOC., 1975, W, 2326. 13 E. Alcalde, I. Dinares, J. Frigola, J. Rius and C. Miravitlles, J. Chem. SOC.,Chem. Commun., 1989,1086. 14 J. P. Phillips, L. D. Freedman and J. C. Craig, Organic Elec- tronic Spectral Data, Interscience, New York, 1963, vol. I-X. 15 J. Ward, Rev. Mod. Phys., 1965,37, 1. 16 J. 0.Morley, J. Chem. SOC.,Faraday Trans., 1994,90, 1849. Paper 410068 1J ;Received 4th February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001853

出版商:RSC    

年代:1994

数据来源: RSC