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. I. W. M. Smith (Birmingham) (Chairman) Prof. M. N. R. Ashfold (Bristol) Dr. B. E. Hayden (Southampton) Prof. D. C. Clary (Cambridge) Prof. A. R. Hillman (Leicester) Dr. L. R. Fisher (Bristol) Prof. J. Holzwarth (Berlin) Prof. H. M. Frey (Reading) Dr. P. J. Sarre (Nottingham) 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 Senior Assistant Editors: Mrs.S. Shah, Dr. R. A. Whitelock Assistant Editor: Mrs. C. J. Seeley Editorial Secretary: Miss. J. E. Chapman 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) J. P. Simons (Oxford) R. Freeman (Cambridge) S. Stolte (Amsterdam) H. L. Friedman (Stony Brook) J. Troe (Gottingen) H. lnokuchi (Okazaki) J. Wolfe (Kensington, NSW) J. N. lsraelachvili (Santa Barbara) C. Zannoni (Bologna) M. L. Klein (Philadelphia) 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 lHN, UK. NB Turpin Distribution Services Ltd., dis- tributors, is wholly owned by the Royal Society of Chemistry. 1994 Annual subscription rate EC €744.00, Rest of World f800.00, USA $1400.00, Canada f840 (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, mec ha n ica I, photographic, record ing, 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 C hem ica I 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- Mail (JANET): PCZPSF($U K.AC. NOTT.VAX Fax: (0602) 513466 Telex: 37346 UNINOT G Dr. R. J. Parker, Editorial Manager. Tel.: Cambridge (0223) 420066 E-Mail ( INTER N ET): RSCl (qRSC.ORG (For access from JANET use RSC1% RSC.ORG @ UK.AC.NSF NET-RELAY) Fax: (0223) 423623 or 420247 Telex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX001

出版商:RSC    

年代:1994

数据来源: RSC

ISSN:0956-5000    

DOI:10.1039/FT99490BX003

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Afanasiev, P., 193 Alhov, M. V., 109 Allen, N. S., 83 Avila, V., 69 Baba,T., 187 Bell, A. J., 17 Boggis, S. A., 17 Borisenko, V. N., 109 Breysse, M., 193 Brown, R. G., 59 Caldararu, H., 213 Camacho, J. J., 23 Campa, M. C., 207 Caragheorgheopol,A., Carvill, B. T., 233 Catalina, F., 83 Cherqaoui, D., 97 Chesta, C. A., 69 Cho,T., 103 Cordischi, D., 207 Corma,A., 213 Corrales, T., 83 Cosa, J. J., 69 213 Coudurier, G., 193 Dickinson, E., 173 Dyke, J. M., 17 Eustaquio-Rincon, R., 113 Filimonov, 1. N., 219, 227 Fornks, V., 213 Jiang, P. Y., 93 Katsumura, Y., 93 Kawashima, T., 127 Kida, I., 103 King, F., 203 Kondo, Y., 121 Ninomiya, J., 103 Nonaka, O., 121 Nyholm, L., 149 Occhiuzzi, M., 207 Ohtsu, K., 127 Ono,Y., 187 Rocha, M., 143 Rochester, C.H., 203 Roffia, S., 137 Ryde,N., 167 Sachtler, W. M. H., 233 Salmon, G. A., 75 Frey, J. G., 17 Geantet, C., 193 Kuwamoto, T., 121 Langan, J. R., 75 Ota, K-i., 155 Ozutsumi, K., 127 Shaw,N., 17 Silva, C. J., 143 Green, W. A., 83 Grimshaw, J., 75 Hall, G., 1 Handa,H., 187 Leaist, D. G., 133 Lei,G-D., 233 Lerner, B. A., 233 Li, J., 39 Padley, M. B., 203 Paradisi, C., 137 Pardo, A., 23 Parsons, B. J., 83 Silva, F., 143 Tabrizchi, M., 17 Takagi, T., 121 Takahashi, K., 155 Hao,L., 133 Liu,C-W., 39 Pedulli, G. F., 137 Timms, A. W., 83 Harrison, N. J., 55 Helmer, M., 31 Loginov, A. Yu., 219,227 Lu, J-X., 39 Pereira, C. M., 143 Peter, L. M., 149 Trejo, A., 113 Vedrine, J. C., 193 Hutchings, G. J., 203 Ikawa, S-i., 103 Lunelli, B., 137 Mallon, D., 83 Petrov, N.Kh., 109 Plane, J. M. C., 31 Villamagna, F., 47 Villemin, D., 97 Ikonnikov, I. A., 219 Mandal, A. B., 161 Potter, C. A. S., 59 Vollmer, F., 59 Indovina, V., 207 Ishigure, K., 93 Martins, A., 143 MatijeviC, E., 167 Poyato, J. M. L., 23 Previtali, C. M., 69 Whitaker, B. J., 1 Whitehead, M. A., 47 Iwasaki, K., 121 Nagaishi, R., 93 Rettig, W., 59 Yoshitake, H., 155 Jayakumar, R., 161 Jennings, B. J., 55 Navaratnam, S., 83 Nicholson, D., 181 Rey,F., 213 Richter, R., 17 Yotsuyanagi, T., 93 Zholobemko, V. L., 233 1 Journal of the Chemical Society, Faraday Transactions Information for Authors F~adayTransactions is an international pumd 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 pumal is published fortnightly. There is no page charge for papers published in Fw& Transactionr. Research Papers Full papers contain original scientific work that has not been published previously. However, work that has appeared in pht in a short form such as a Faraday Communication or Chemical Communication is normally acoeptable. Papers should be typewritten in double spacing on one side only of the paper. Four copies of text, illustrations, tables and any other matter should be sent to: The Editor, Far* Transactionr, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK.Authors may, if they wish, suggest the names (With ad-dresses) of up to three possible referees. Faraday Research Articles Faraday Research Articles are!occasional invited articles which are published following review. They are designed to be topi-cal articles of interest to a wide range of research scientists in the areas of Physical Chemistry, Biophysical Chemistry and Chemical Physics. Faraday Communications Faraday Communications contain novel scientifk work in short form and of such importance that rapid publication is war-ranted. The total length is rigorously restricted to two printed A4 pages. The manuscript will be returned for reduction if this length is exceeded.For a Communication consisting of text and ten references, with no figures, equations or tables, this cone-sponds to approximately 1600 words plus an abstract of up to 40words. Submission of a Faraday Communication can be made either to The Editor, Far-Trmactions, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK,or via a member of the Inter-national Advisory Editorial Board. In the latter case, the top capy of the manuscript including any figures etc., together with the name of the person to whom the communication is being submitted, should be sent simultaneously to The Editor at the Cambridge address. In order to avoid delay in publication, proofs of communi-cations are not sent to authors unless this is specifically re- quested.Administration Receipt of a paper will be acknowledged, and the paper will be given a reference number which authors are asked to quote on all their subsequent currespondence. If no such acknow- ledgment has been received after a reasonable period of time authors should check with the Editorial Office as to whether the paper or the acknowledgement has gone astray. Editorial Policy. Every paper (except communications) will be submitted to at least two referees, by whose advice Faraday Editorial Board will be guided as to its acceptability. Papers that are accepted must not be published elsewhere except by permission of the Royal Society of chemistry.Submission of a manuscript will be regarded as an undertaking that the same material is not being considered for publication by mother journal.Conditions governing acceptance are printed in Issue 1 each year and are available from the Editorial Manager. Copyright. The whole of the literary matter (including tables, figures, diagrams and photographs) in Faraduy Transactionsis subject to copyright and may not be reproduced without per-mission from The Royal Society of Chemistry and such other own= of the copyright as may be indicated. Reprints. Fifty repints of each paper are supplied free of charge on request. Additional reprints can be purchased if ordered at the time of publication. Details are sent to authors with the poofs of articles and with the letter of acceptance of COlnIllUIliCatiOnS.Free Copies. Any author who is publishing m Far-Trm-actions is entitled to a free copy of the issue in which hisher Paper appears. Notes on the Preparation of Papers 1 Manuscripts must be typed in double-line spacing, single sided on A4 paper, with margins at top, bottom and left-hand side of at least 4 cm. 2 The first page should be set out as follows: (i) Name and address of the author to whom proofs and correspondence should be sent. (ii) Title of the paper, with capitals for the first letter of each noun and adjective only. (iii) Authors’ names, including one forename for each author. (iv) The address where the work was carried out; if this is different from the current address a footnote indicating the present address of this author should be included.Resent addresses of other authors are not normally given. (v) Abstract, preceded and followed by a horizontal line, and typed in double-line spacing. 3 Suitable headings and sub-headings should be used in the main text as appropriate (exoept for communications in which no headings are used). References should be numbered serially in the text by means of superscript arabic numerals. 4 Bibliographic references (not footnotes) should follow the main text and should have the following format: 1 C. Jarque and A. D. Buckingham, J. Chem. SOC., Faraday Trans., 1992,88, 1353. 2 R. M. Bamx and R. J. B. Craven, m New klopments in Zdite Science and T&bgy, ed.Y.M-A. Iijima and J.W.Ward, Kodansha, Tokyo, 1986, p. 521. Journal titles should be abbreviated according to the Chemical Abstracts Senice Source Index (CASSI). 5 Tables should be typed on separate sheets at the end of the manuscript. 6 Diagrams should be accompanied by a separately typed set of captions. Extensive identifying lettering should be placed in the caption rather than on lines on graphs, etc. Original art-work should be supplied wherever possible. Colour photo-graphs will be accepted only when a black-and-white photograph fails to show some vital feature. 11 7 Bulk information (such as primary kinetic data, computer programs and output etc.) which accampanies papers published in Faraday Transactions may be deposited, free of charge, with the Society’s Supplementary Publications Scheme, either at the request of the author and with the approval of the referees or on the recommendation of the referees with the approval of the author.Details are available from the Editorial office. 8 Molecular modelling studies should be subject to the same rigorous scientific standard required of other types of experi- ment, such that objective evaluation by independent investiga- tors is possible. Authors are therefore strongly enmuraged to provide sufficient details of any computationally assisted mod-elling results they report that might assist in any such an evalu- ation. This information should include: (a) A precise description of any computer software used, in-cluding any version or revision numbers, the type of computer used and a reference to a source for the program or a published definition of the algorithm used.(b) A concise indication in a ‘Computational Details Section’ or a footnote of standard options involved such as basis sets, SCF methods, electronic states, parameter sets, charge distribu- tion schemes, symmetry, geometry optimisation methods, con- vergence criteria, cut-offs, time constants, etc. More explicit details of any non-standard use of e.g. basis sets, force-field parameters, algorithmic options, etc. should be particularly pro-vided (c) Key stationary points in a potential surface which are es-sential to conclusions discussed in the text should be accurately characterised by reparting e.g.the calculated energy and im-portant geometrical parameters. Authors are encouraged to provide more complete information such as atom types,mole-cular coordinates and connectivity data if available for these points in the form of supplementary tables, or preferably in computer-readable form as e.g. program input data sets or archive files. Further details of proposed guidelines in molecular modelling are to be found in P. Gun4 D. C.Bany, J. M.Blaney and N. C. Cohen, J. Med. Chem., 1988,31,2230. Nomenclature Current TUPAC nomenclature and symbolism should be used. Attention is drawn to the following Publications in which the rules thmlves and guidance on their use are given: Nomenchatwe @Organic Chemistry, Sections A, B, C, D, E, F and H,Pergamon Press, Oxford, 1979 edn.Nomenclatrve of Inorgm‘c Chemktry, Blackwell Scien- tific Publications, Oxford, 1990. Biochemical Nmnclature and Related Docuner~rs,The Biochemical Society, London, 1978. Compendium of Chemical Terminology: IUPAC Recom-mendutions, Blackwell Scientific Publications, Oxford, 1987. Units and Symbols The recommendations of IUPAC should be followed. Their basis is the ‘Systihne Internationale d’Unit6s’ (SI). A detailed treatment is given in the so-called Green Book Q~iries, Unitsand Symbols in Physical Chemistry,Blackwell ScienWic Publications, Oxford, 1988 edn. iii JOURNALS OF THE ROYAL SOCIETY OF CHEMISTRY Refereeing Procedure and Policy (I 994) 1.0 Contributions to Dalton, Perkin and Faraday Transactions, J.Mater. Chem., The Analyst, J. Anal. At. Spectrom. and J. Chem. Research 1.1 Introduction This document summarises the procedure used for assessing papers submitted to the four Transactions, J. Mater. Chem., The Analyst, J. Anal. At. Spectrom., and J. Chem. Research, and provides guidelines for referees engaged in this assessment. 1.2 Subject Matter Papers are submitted to the various journals according to subject matter. If it is felt that a paper would be published more appropriately in an RSC journal other than the one suggested by the author, the referee should inform the Editor. The topics covered by the various journals are as follows. Dalton Transactions (Inorganic Chemistry).All aspects of the chemistry of inorganic and organometallic compounds, including bioinorganic chemistry and solid-state inorganic chemistry; the applications of physicochemical techniques to the study of their structures, properties and reactions, including kinetics and mechanism; new or improved experimental techniques and syntheses. Faraday Transactions (Physical Chemistry and Chemical Physics). Gas-phase kinetics and dynamics; molecular beam kinetics and spectroscopy, photochemistry and photophysics; energy transfer and relaxation processes: laser-induced chemistry; spectroscopies of molecules, molecular and gas- phase complexes: quantum chemistry and molecular structure, statistical mechanics of gaseous molecules and complexes; spectroscopies, statistical mechanics and quantum theory of the condensed phase, computational chemistry and molecular dynamics; colloid and interface science, surface science, physisorption and chromatographic science, chemisorption and heterogeneous catalysis, zeolites and non-exchange phenomena; electrode processes, liquids and solutions; solid-state chemistry (microstructures and dynamics); reactions in condensed phases; physical chemistry of macromolecules and polymers; materials science; thermodynamics; biophysical chemistry and radiation chemistry.Perkin Transactions I (Organic Chemistry). All aspects of organic and bio-organic chemistry. These include synthetic organic chemistry of all types, organometallic chemistry, chemistry and biosynthesis of natural products, the relationship between molecular structure and biological activity, the chemistry of polymers and biological macromolecules, and medicinal and agricultural chemistry where there is originality in the science. Perkin Transactions 2 (Physical Organic Chemistry).Physicochemical aspects of organic, organometallic, and bio- organic chemistry, including kinetic, mechanistic, structural, spectroscopic and theoretical studies. Such topics include structure-activity relationships and physical aspects of biological processes and of the study of polymers and biological macromolecules. Journal of Materials Chemistry. The chemistry of materials, particularly those associated with advanced technology; modelling of materials; synthesis and structural characteris- ation; physicochemical aspects of fabrication; chemical, structural, electrical, magnetic and optical properties; applic- ations. The Analyst (Analytical Science).Theory and practice of all aspects of analytical chemistry, fundamental and applied, inorganic and organic, including chemical, physical and biological methods. Journal of Analytical Atomic Spectrometry. The development and analytical application of atomic spectrometric techniques, including ICP MS. Journal of Chemical Research. All areas of chemistry. The format of this journal (one- or two-page printed synopsis in Part S, plus microform version of authors’ full text typescript in Part M) makes it particularly suitable for papers containing lengthy experimental sections or extensive data tabulations.1.3 Procedure Each manuscript is considered independently by two referees. The referees’ reports constitute recommendations to the appropriate Editorial Board, which is empowered to take final action on manuscripts submitted. The Editor, acting for the Editorial Board, is responsible for all administrative and executive actions, and is empowered to accept or reject papers. It is the Editor’s duty to see that, as far as possible, agreement is reached between authors and referees; although the referees may need to be consulted again concerning an author’s reply to comments, further refereeing will be avoided as far as possible. 1.3.1 Adjudication of disagreements.If there is a notable discrepancy between the reports of the two referees, or if the difference between authors and referees cannot be resolved readily, a third referee may be appointed as adjudicator. In extreme cases, differences may be reported to the appropriate Editorial Board for resolution. When a paper is recommended for rejection by referees, the Editor will inform the authors and return the top copy of the manuscript. Authors have the right to appeal to the Editorial Board if they regard a decision to reject as unfair. The Editor may refer to the Editorial Boards any papers which have been recommended for acceptance by the referees, but about which the Editor is doubtful. 1.3.2 Anonymity. The anonymity of referees is strictly preserved, and reports should be couched in terms which do not disclose the identity of the writer.A referee should never communicate directly with an author, unless and until such action has been sanctioned by the Society, through the Editor. 1.3.3 Confidentiality. A referee should treat a paper received for assessment as confidential material. Information acquired by a referee from such a paper is not available for citation until the paper is published. iV REFEREEING PROCEDURE AND POLICY (1994) 1.4 Policy The primary criterion for acceptance of a contribution for publication is that it should advance scientific knowledge significantly. Papers that do not contain new experimental results may be considered for publication only if they either reinterpret or summarise known facts or results in a manner presenting an advance in chemical knowledge. Papers in interdisciplinary areas are acceptable if the chemical content is considered satisfactory.Papers reporting results regarded as routine or trivial are not acceptable in the absence of other, desirable attributes. Although short papers are acceptable, the Society strongly discourages the fragmentation of a substantial body of work into a number of short publications; such fragmentation is likely to be grounds for rejection. The length of an article should be commensurate with its scientific content; however, authors are allowed every latitude (consistent with reasonable brevity) in the form in which their work is presented.Figures and flow-charts can often save space as well as clarify complicated arguments, and should not be excised unless they are unhelpful or really extrava- gant. If a paper as a whole is judged suitable for the Journal, minor criticisms should not be unduly emphasised. It is the responsibility of the Editor to ensure the use of reasonably brief phraseology, and to assist the author to present his work in the most appropriate format. However, referees should not hesitate to recommend rejection of papers which appear incurably badly com-posed. It should be clearly understood that referees’ reports are made in confidence to the Editor, at whose discretion comments will be transmitted to the author.To assist the Editor, referees are requested to indicate which comments are designed only for consideration, as distinct from those which, in the referee’s view, require specific action or an adequate answer before the paper is accepted. Referees may ask for sight of supporting data not submitted for publication, or for sight of a previous paper which has been submitted but not yet published. Such requests must be made to the Editor, not directly to the author. 1.4.1 Authentication of new compounds. Referees are asked to assess, as a whole, the evidence in support of the homogeneity and structure of all new compounds. No hard and fast rules can be laid down to cover all types of compounds, but the Society’s policy is that evidence for the unequivocal identification of new compounds should wherever possible include good elemental analytical data; for example, an accurate mass measurement of a molecular ion does not provide evidence of purity of a compound and must be accompanied by independent evidence of homogeneity.Low-resolution mass spectrometry must be treated with even more reserve in the absence of firm evidence to distinguish between alternative molecular formulae. Where elemental analytical data are not available, appropriate evidence which is convincing to an expert in the field may be acceptable. Spectroscopic information necessary to the assignment of structure should normally be given. Just how complete this information should be must depend upon the circumstances; the structure of a compound obtained from an unusual reaction or isolated from a natural source needs much stronger supporting evidence than one derived by a standard reaction from a precursor of undisputed structure.Referees are reminded of the need to be exacting in their standards but at the same time flexible in their admission of evidence. It remains the Society’s policy to accept work only of high quality and to permit no lowering of standards. 1.5 Titles and Summaries Referees should comment on titles and summaries with the following points in mind. Titles of papers are used out of context by several organizations for current awareness purposes. To enable such systems to serve chemists adequately, titles must be written around a sufficient number of scientific words carefully chosen to cover the important aspects of the paper.Summaries should preferably be self-contained, so that they can be understood without reference to the main text. 1.6 Speed of Refereeing The Editorial Boards are anxious to maintain and to reduce further if possible the publication times now being achieved. In this connection, referees should submit their reports with the minimum of delay, or return manuscripts immediately to the Editor if long delay seems inevitable. 1.7 Suggestions of Alternative Referees The Editor welcomes suggestions of alternative referees competent to deal with particular subject areas. Such suggestions are particularly helpful in cases where referees consider themselves ill-equipped (in terms of specialist knowledge) to deal with a specific paper, and in highly specialized or new areas of research where only a limited number of experts may be available.If, in such a case, the alternative and the original referee work in the same institution, the manuscript may be passed on directly after informing the Editor. 1.8 Short Papers and Letters ‘Short Papers’ are published in J. Chem. Research. They are intended for the description of essentially complete pieces of work which can be described in two printed pages or less. They are NOT preliminary communications, nor in any way an alternative to Chemical Communications, for which there are additional criteria of novelty and urgency.The quality of material contained in a short paper should be the same as that in a full paper. Investigations arising out of some larger project but not prosecuted to the same degree are particularly appropriate for this format. A short paper should not normally exceed in length about 8 pages of typescript, including figures, tables, etc. It should comprise a one-sentence abstract and discussion, but adequate experimental details are required. As a consequence of its length, it appears in full in Part S with no microform version in Part M. ‘Letters’, published only in Dalton Transactions, are a medium for the expression of scientific opinions and views normally concerning material published in that journal; it is intended that contributions in this format should be published rapidly. The letters section is for scientific discussion, and is not intended to compete with media for the publication of more general matters such as Chemistry in Britain.Only rarely should a Letter exceed one printed column in length (about 1-2 pages of typescript). Where a letter is polemical in nature, and if it is accepted, a reply will be solicited from other parties implicated, for consideration for publication alongside the original letter. 1.9 Relationship with Communications Journals In cases where a preliminary report of the work described has appeared (for example in Chemical Communications), referees should alert the editor to any excessive and unnecessary repetition of material; this can arise in connection with communications journals in which the restrictions on length V REFEREEING PROCEDURE AND POLICY (1 994) and the reporting of experimental data are less severe than those of Chemical Communications.Furthermore, the acceptability of the full paper must be judged on the basis of the significance of the additional information provided, as well as on the criteria outlined in the foregoing sections. 2.0 Contributions to Chemical Communic-ations Chemical Communications is intended as a forum for preliminary accounts of original and significant work, in any area of chemistry that is likely to prove of wide general appeal or exceptional specialist interest. Such preliminary reports should be followed up in most cases by full papers in other journals, providing detailed accounts of the work.It is Society policy that only a fraction of research work warrants publication in Chemical Communications, and strict refereeing standards should be applied. The benefit to the reader from the rapid publication of a particular piece of work before it appears as a full paper must be balanced against the desirability of avoiding duplicate publication. The needs of the reader, not the author, must be considered, and priority in publication should not be allowed to determine acceptability. Acceptance should be recommended only if, in the opinion of the referee, the content of the paper is of such urgency that rapid publication will be advantageous to the progress of chemical research.The length of Communications is strictly limited; only in exceptional circumstances should it exceed one printed page (two-and-a-half to three A4 pages of typescript) and referees should be particularly critical of manuscripts longer than this. Communications do not contain extensive spectroscopic or other experimental data, but referees may ask for sight of such data before reaching a decision. The refereeing procedure for Communications is the same as that for full papers, except that rapidity of reporting is crucial in order to maintain rapid publication. 3.0 Communications submitted to Analytical Proceedings and J. Anal. At. Spectrom. Criteria for acceptance of communications submitted to Analytical Proceedings and J.Anal. At. Spectrom. are similar to those for contributions to Chemical Communications, except that they should be concerned specifically with analytical chemistry. A decision whether or not to publish rests with the Editor, who will obtain advice from at least one referee. 4.0 Communications submitted to Perkin, Dalton or Faraday Transactions or J. Mater. Chem. Criteria for acceptance of Communications submitted to Perkin, Dalton or Faraday Transactions or J. Mater. Chem. are similar to those for contributions to Chemical Communications, except that the work will be of more specialist interest. For Perkin and Dalton Communications inclusion of key experi- mental data is expected. Assessment is carried out by a small nucleus of referees, consisting largely of members of the appropriate Editorial Boards. 5.0 Contributions to Mendeleev Communic- ations Mendeleev Communications, published jointly by the Royal Society of Chemistry and the Russian Academy of Sciences, is a sister publication to Chemical Communications, containing preliminary reports of the same type, in any area of chemistry.The majority of contributions are from Russian authors. Assessment involves two stages of refereeing. Manuscripts submitted to the Moscow Editorial Office are refereed initially by a Russian scientist. If found acceptable they are then reviewed by Western scientists chosen by the Royal Society of Chemistry. Manuscripts submitted to the UK Editorial Office undergo this two-stage refereeing process in reverse.6.0 X-Ray Crystallographic Work 6.1 All papers containing crystallographic determinations will be refereed by two referees, one a structural chemist. If the editor considers it advisable, the paper may also be sent to a specialist crystallographer for comment. Referees will not normally be expected to check values of structural parameters for publication (e.g.bond lengths and angles against atomic co- ordinates; this will be done after publication by the appropriate crystallographic data centre), but should still pay attention to the quality of the experimental crystallographic work. However their primary concern should be such new chemistry as is involved in the structure. 6.2 Papers will often contain the information in their titles that an X-ray structure determination has been carried out.However, this is not obligatory, especially if the X-ray determination forms only a minor part. Summaries should normally contain this information. 6.3 A structure referred to in a Communication will normally be fully refined. The Communication can then be considered to fulfil the archival function, and the structure determination may not require further detailed refereeing when presented as part of a full paper. In the full paper, the author’s purpose will then be served by a simple reference back to the original communication. However, if the crystallography is discussed again at any length in the full paper, the data should be re-presented to the referees in full, and re-published if considered necessary.6.4 There may be other cases when an author wishes to publish a full paper in which the result of a crystal structure determination is discussed, but in which details or extensive discussion are considered unnecessary. The crystallographer may even be omitted as a co-author (for example when the determination is carried out by a commercial company). If the author is able to show the referees that this procedure is appropriate, it will be allowed provided that it does not lead to unnecessary fragmentation. However, the author must provide, as supplementary information, sufficient data relating to the crystal structure determination to allow a referee to make sure that the point made is correct, and co-ordinates etc.will be deposited. The brief published description of the determination should be supplemented by appropriate reference to ‘unpub- lis hed work’ . vi INSTRUCTIONSFOR AUTHORS (1994) APPENDIX IUPAC Publications on Nomenclature and Symbolism 1.O Compilations 1.1 Nomenclature of Organic Chemistry, a 550-page hardcover volume published in 1979, available from Pergamon, Oxford. Section A: Hydrocarbons Section B: Fundamental heterocyclic systems Section C: Characteristic groups containing carbon, hy-drogen, oxygen, nitrogen, halogen, sulfur, selenium and tellurium Section D: Organic compounds containing elements not exclusively those referred to in the title of Section C Section E: Stereochemistry Section F: General principles for the naming of natural products and related compounds Section H: Isotopically modified compounds 1.2 A Guide to IUPAC Nomenclature of Organic Compounds, a 182-page hardcover volume published in 1993, available from Blackwell Scientific Publications, Oxford, to be used in conjunction with item 1.1.1.3 Nomenclature of Inorganic Chemistry, a 278-page hardcover volume published in 1990, available from Blackwell Scientific Publications, Oxford. Chapter 1: General aims, functions and methods Chapter 2: Grammar Chapter 3: Elements, atoms and groups Chapter 4: Formulae Chapter 5: Names based on stoichiometry Chapter 6: Neutral molecular compounds Chapter 7: Names for ions, substituent groups and radicals, and salts Chapter 8: Oxoacids and derived anions Chapter 9: Co-ordination compounds Chapter 10: Boron hydrides and related compounds 1.4 Biochemical Nomenclature and Related Documents, a 348-page softcover manual published in 1992 by Portland Press Ltd.for IUBMB, and available from the publisher (59 Portland Place, London W 1N 3AJ, UK). The contents are as follows: Nomenclature of organic chemistry. Section E: Stereo-chemistry (1 974) Nomenclature of organic chemistry. Section F: Natural products and related compounds (1976) Isotopically modified compounds Recommendations for the presentation of thermodynamic and related data in biology (1985) Citation of bibliographic references in biochemical journals (197 1) Nomenclature and symbolism for amino acids and peptides.( 1983) Abbreviated nomenclature of synthetic polypeptides or polymerized amino acids (1 971) Abbreviations and symbols for the description of the conformation of polypeptide chains (1 969) Nomenclature of peptide hormones (1974) Nomenclature of glycoproteins, glycopeptides and peptidoglycans (1 985) Nomenclature of initiation, elongation and termination factors for translation in eukaryotes (I 988) Nomenclature of multiple forms of enzymes (1976) Symbolism and terminology in enzyme kinetics (198 1) Nomenclature for multienzymes (1 989) Abbreviations and symbols for nucleic acids, poly-nucleotides and their constituents (1 970) Abbreviations and symbols for the description of the conformations of polynucleotide chains (1 982) Nomenclature for incompletely specified bases in nucleic acid sequences (1 984) Carbohydrate nomenclature.Part I (1 969) Nomenclature of cyclitols (1 973) Numbering of atoms in myo-inositol(l988) Conformational nomenclature for five- and six-membered ring forms of monosaccharides and their derivatives (1 980) Nomenclature of unsaturated monosaccharides (1 980) Nomenclature of branched-chain monosaccharides ( 1980) Abbreviated terminology of oligosaccharide chains (1980) Polysaccharide nomenclature (1980) Symbols for specifying the conformation of polysaccharide chains (1 98 1) Nomenclature of lipids (1 976) Nomenclature of steroids (1 989) Nomenclature of quinones with isoprenoid side chains (1 973) Nomenclature of carotenoids ( 1970) and amendments (1 974) Nomenclature of tocopherols and related compounds (1981) Nomenclature of vitamin D (1981) Nomenclature of retinoids (1 98 1) Prenol nomenclature (1 986) Nomenclature of phosphorus-containing compounds of biochemical importance (1976) Nomenclature and symbols for folic acids and related compounds (1986) Nomenclature for vitamins B-6 and related compounds (1 973) Nomenclature of corrinoids (1 973) Nomenclature of tetrapyrroles (1986) 1.5 Compendium of Analytical Nomenclature, a 280-page hardcover volume published in 1987, available from Blackwell Scientific Publications, Oxford.The contents are as follows: Presentation of the Results of Chemical Analysis Solution Thermodynamics (activity coefficients, equilibria, PW Recommendations for Terminology to be used with Precision Balances Recommendations for Nomenclature of Thermal Analysis Recommendations for Nomenclature of Titrimetric Analysis Electrochemical Analysis Analytical Separation Processes (precipitation, liquid- liquid distribution, zone melting and fractional crystallis- ation, chromatography, ion exchange) Spectrochemical Analysis (radiation sources, general atomic emission spectroscopy, flame spectroscopy, X-ray emission spectroscopy, molecular methods) Recommendations for Nomenclature of Mass Spec-trometry Recommendations for Nomenclature of Radiochemical Methods Surface Analysis (including photoelectron spectroscopy) vii 1.6 Compendium of Macromolecular Nomenclature, a 172-page hardcover volume published in 1991, available from Blackwell Scientific Publications, Oxford.The contents are as follows: Basic Definitions of Terms Relating to Polymers Stereochemical Definitions and Notations Relating to Polymers Definitions of Terms Relating to Individual Macromolecules, their Assemblies, and Dilute Polymer Solutions Definitions of Terms Relating to Crystalline Polymers Nomenclature of Regular Single-strand Organic Polymers Nomenclature for Regular Single-strand and Quasi-single- strand Inorganic and Coordination Polymers Source-based Nomenclature for Copolymers A Classification of Linear Single-strand Polymers Use of Abbreviations for Names of Polymeric Substances 1.7 Compendium of Chemical Terminology: IUPAC Recommendations, a 456-page volume published in 1987, available in hardcover and softcover from Blackwell Scientific Publications, Oxford. 1.8 Quantities, Units and Symbols in Physical Chemistry, a 166-page softcover volume published in 1993 by Blackwell Scientific Publications, Oxford. 2.0 Documents not included in the compil- ations 2.1 Nomenclature of Elements and Compounds Boron Compounds Nomenclature of inorganic boron compounds (Pure Appl.Chem., 1972,30,681). Delta Convention Nomenclature for cyclic organic compounds with contiguous formal double bonds (Pure Appl. Chem., 1988,60, 1395). Elements Recommendations for the names of elements of atomic number greater than 100 (Pure Appl.Chem., 1979,51,381). Enzymes Enzyme Nomenclature (1992), published by Academic Press in hardcover and softcover editions. Heterocyclic Compounds Revision of the extended Hantzsch-Widman system of nomenclature for heteromonocycles (Pure Appl. Chem.., 1983, 55,409). Hydrogen Names for hydrogen atoms, ions and groups, and for reactions involving them (Pure Appl. Chem., 1988,60, 1115). Isotopically ModiJied Compounds Nomenclature of inorganic chemistry. Part 11. 1. Isotopically modified compounds (Pure Appl. Chem., 1981,53, 1887). Lambda Convention Treatment of variable valence in organic nomenclature (Pure Appl. Chem., 1984, 56, 769). Nitrogen Hydrides Nomenclature of hydrides of nitrogen and derived cations, anions and ligands (Pure Appl.Chem., 1982,54,2545). Numerical Terms Extension of Rules A-1.1 and A-2.5 concerning numerical terms used in organic chemical nomenclature (Pure Appl. Chem., 1986,58, 1693). Polyanions Nomenclature of polyanions (Pure Appl. Chem., 1987,59,1529). Polymers Nomenclature of regular double-strand (ladder and spiro) organic polymers (Pure Appl. Chem., 1993,65, 1561). INSTRUCTIONS FOR AUTHORS (1994) Radicals and Ions Revised nomenclature for radicals, ions, radical ions and related species (Pure Appl. Chem., 1993,65, 1357). Zeolites Chemical nomenclature and formulation of compositions of synthetic and natural zeolites (Pure Appl. Chem., 1979, 51, 1091). 2.2 Terminology, Symbols and Units, and Presentation of Results General Glossary of terms used in physical organic chemistry (Pure Appl.Chem., 1983,55, 1281). Glossary of atmospheric chemistry terms (Pure Appl. Chern., 1990, 62, 2167). English-derived abbreviations for experimental techniques in surface science and chemical spectroscopy (Pure Appl. Chem., 1991, 63, 887). Analytical Recommendations for publication of papers on a new analytical method based on ion exchange or ion-exchange chromatography (Pure Appl. Chem., 1980,52,2555). Recommendations for presentation of data on compleximetric indicators, 1. General (Pure Appl. Chem., 1979,51, 1357). Recommendations for publishing manuscripts on ion-selective electrodes (Pure Appl. Chem., 1981,53, 1907).Recommendations on use of the term amplification reactions (Pure Appl. Chem., 1982,542553). Recommendations for the usage of selective, selectivity and related terms in analytical chemistry (Pure Appl. Chem., 1983, 55, 553). Nomenclature for automated and mechanised analysis (Pure Appl. Chem., 1989,61, 1657). Nomenclature for sampling in analytical chemistry (Pure Appl. Chem., 1990,62, 1193). Nomenclature for chromatography (Pure Appl. Chem., 1993, 65, 819). Biotechnology Glossary for chemists of terms used in biotechnology (Pure Appl. Chem., 1992,64, 143). Selection of terms, symbols and units related to microbial processes (Pure Appl. Chem., 1992,64, 1047). Clinical Physicochemical quantities and units in clinical chemistry with special emphasis on activities and activity coefficients (Pure Appl.Chem., 1984,56, 567). Quantities and units in clinical chemistry (Pure Appl. Chem., 1979,51,2451). Quantities and units in clinical chemistry: nebulizer and flame properties in flame emission and absorption spectrometry (Pure Appl. Chem., 1986,58, 1737). List of quantities in clinical chemistry (Pure Appl. Chem., 1979, 51,2481). Proposals for the description and measurement of carry-over effects in clinical chemistry (Pure Appl. Chem., 1991,63, 301). Quantities and units for metabolic processes as a function of time (Pure Appl. Chem., 1992,64, 1569). Glossary for chemists of terms used in toxicology (Pure Appl. Chem., 1993,65,2003). Colloids and Surface Chemistry Definitions, terminology and symbols in colloid and surface chemistry.I (Pure Appl. Chem., 1972, 31, 577). 11, Hetero-geneous catalysis (Pure Appl. Chem., 1976, 46, 71). Part 1.14: Light scattering (provisional) (Pure Appl. Chem., 1983, 55, 93 1). Reporting experimental pressure-area data with film balances (Pure Appl. Chem., 1985,57,621). ... Vlll INSTRUCTIONSFOR AUTHORS (1994) Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Pure Appl. Chem., 1985,57,603). Reporting data on adsorption from solution at the solid/ solution interface (Pure Appl. Chem., 1986,58,967). Manual on catalyst characterization (Pure Appl. Chem., 1991, 63, 1227).Electrochemistry Nomenclature for transfer phenomena in electrolytic systems (Pure Appl. Chem., 1981,53, 1827). Electrode reaction orders, transfer coefficients and rate constants-amplification of definitions and recommendations for publication of parameters (PureAppl. Chem., 1980,52,233). Classification and nomenclature of electroanalytical techniques (Pure Appl. Chem., 1976,45,81). Recommendations for sign conventions and plotting of electrochemical data (Pure Appl. Chem., 1976, 45, 131). Electrochemical nomenclature (PureAppl. Chem., 1974,37,499). Recommendations on reporting electrode potentials in non- aqueous solvents (Pure Appl. Chem., 1984,56,461). Definition of pH scales, standard reference values, measurement of pH and related terminology (Pure Appl.Chem., 1985, 57, 53 1). Interphases in systems of conducting phases (Pure Appl. Chem., 1986,58,437). The absolute electrode potential: an explanatory note (Pure Appl. Chem., 1986,58,955). Electrochemical corrosion nomenclature (Pure Appl. Chem., 1989,61, 19). Terminology in semiconductor electrochemistry and photo- electrochemical energy conversion (Pure Appl. Chem., 199 1,63, 569). Nomenclature, symbols, definitions and measurements for electrified interfaces in aqueous dispersions of solids (PureAppl. Chem., 1991,63, 895). Nomenclature, symbols and definitions in electrochemical engineering (Pure Appl. Chem., 1993, 65, 1009). Kinetics Symbolism and terminology in chemical kinetics (provisional) (Pure Appl.Chem., 1981,53,753). Photochemistrj, Recommended standards for reporting photochemical data (Pure Appl. Chem., 1984,56,939). Glossary of terms used in photochemistry (Pure Appl. Chem., 1988,60, 1055). Quantum Chemistry Expression of results in quantum chemistry (Pure Appl. Chem., 1978, 50, 75). Reactions Nomenclature for organic chemical transformations (Pure Appl. Chem., 1989,61, 725). System for symbolic representation of reaction mechanisms (Pure Appl. Chem., 1989,61,23). Detailed linear representation of reaction mechanisms (Pure Appl. Chem., 1989,61, 57). Rheological Properties Selected definitions, terminology and symbols for rheological properties (Pure Appl. Chem., 1979,51, 1215). Spectroscopy Recommendations for publication of papers on methods of molecular absorption spectrophotometry in solution (Pure Appl.Chem., 1978,50, 237). Recommendations for the presentation of infrared absorption spectra in data collections. A, Condensed phases (Pure Appl. Chem., 1978,50,231). Definition and symbolism of molecular force constants (Pure Appl. Chem., 1978,50, 1709). Nomenclature and conventions for reporting Mossbauer spectroscopic data (Pure Appl. Chem., 1976,45,211). Recommendations for the presentation of NMR data for publication in chemical journals. A, Proton spectra (Pure Appl. Chem., 1972,29,625). B, Spectra from nuclei other than protons (Pure Appl. Chem., 1976,45,217). Presentation of Raman spectra in data collections (Pure Appl.Chem., 1981,53, 1879). Names, symbols, definitions and units of quantities in optical spectroscopy (Pure Appl. Chem., 1985,57, 105). A descriptive classification of the electron spectroscopies (Pure Appl. Chem., 1987,59, 1343). Presentation of molecular parameter values for IR and Raman intensity (Pure Appl. Chern., 1988,60, 1385). Recommendations for EPR/ESR nomenclature and conven- tions for presenting experimental data in publications (Pure Appl. Chem., 1989,61,2195). Nomenclature, symbols, units and their usage in spectro-chemical analysis. VII. Molecular absorption spectroscopy, UV and visible (Pure Appl. Chem., 1988, 60, 1449); VIII. Nomenclature system for X-ray spectroscopy (Pure Appl. Chem., 199 1,63,735); X. Preparation of materials for analytical atomic spectroscopy (Pure Appl.Chem., 1988, 60, 1461); XII. Terms related to electrothermal atomization (Pure Appl. Chem., 1992, 64, 253); XIIT. Terms related to chemical vapour generation (Pure Appl. Chem., 1992,64, 261). Recommendations for nomenclature and symbolism for mass spectroscopy (Pure Appl. Chem., 1991,63, 1541). Thermodynamics A guide to procedures for the publication of thermodynamic data (Pure Appl. Chem., 1972,39, 395). Assignment and presentation of uncertainties of the numerical results of thermodynamic measurements (Pure Appl. Chem., 1981, 53, 1805). Notation for states and processes; significance of the word ‘standard’ in chemical thermodynamics and remarks on commonly tabulated forms of thermodynamic functions (Pure Appl.Chem., 1982,54, 1239). ix Minutes of the 21st Annual General Meeting of the Faraday Division The 21st Annual General Meeting of the Faraday Division of The Royal Society of Chemistry was held at 09.00 on Thursday, 15 April 1993 at Strathclyde University with Professor R. Parsons in the Chair. 1. Minutes The minutes of the 20th Annual General Meeting, which were tabled, had been printed in Faraday Transactions and were approved. 2. The 1992 Annual Report of the Faraday Division Faraday Discussion 93 The Structure and Activity of Enzymes was held at the University of Cambridge on 1-3 April 1992. There were 165 participants of whom 39 were from outside the UK representing 12 countries.Professor A. D. Buckingham was Chairman of the Organising Committee. The meeting was opened by an Introductory Lecture by Dr M. Perutz. The Faraday Division AGM was held on 2 April. Discussion 94 was held at the University of Newcastle upon Tyne on 'The Liquid Solid Interface at High Resolution on 7-9 September 1992. There was an attendance of 90 including 39 from outside the UK representing 15 countries. The Introductory Lecture was given by Professor A. J. Bard and concluding remarks were made by Professor A. Fujishima. The Chairman of the Organising Committee was Professor A. Hamnett. The Symposium in 1992 was on 'Chemistry in Interstellar Medium and was held on 16-18 December 1992 at Birmingham University. There were 115 participants of whom 67 were from overseas representing 12 countries.The meeting was opened by the 24th Spiers Memorial Lecture delivered by Professor A. Dalgarno. Professor I. W. M. Smith was Chairman of the Organising Committee. The annual joint meeting with the Associazione Italiana di Chimica Fisica, Deutsche Bunsen Gesellschaft fir Physikalische Chemie and Division de Chimie Physique de la Societk Francaise de Chimie was on 'Molecular Electronics and was held in Padua, Italy, on 24-28 August 1992. Professor A. E. Underhill represented the Faraday Division on the Organising Committee. The 1992 Annual Congress was held at UMIST, Manchester on 13-16 April when the Division mounted a symposium on Characterisation of Solids and Surfaces' convened by Professor J.0. Williams. During the Symposium a half-day session was held in honour of Professor Sir John Meurig Thomas on the occasion of his 60th birthday. At the Autumn Meeting in Dublin on 16-18 September 1992 the Division held a joint symposium with the Industrial Division on Light on Polymers -Photochemistry for Advanced Materials . The coveners were Professor J. M. Kelly, Dr C. McArdle and Mr M. J. de Faubert Maunder. The 1992 Bourke Lecturer was Professor R. Saykally of the University of California, Berkeley but his lecture tour was postponed until early 1993. Professor B.H. Robinson of the University of East Anglia, was awarded the R A Robinson Memorial Lectureship and delivered lectures in Kuala Lumpur, Penang, Bangkok, Melbourne and Auc kland.There were three half-day symposia featuring endowed lectures of the Society in 1992, one of which took place in Cambridge following a decision to hold some of these symposia outside London: 26 March 1992 at The Royal Institution, London 'Chirality and the Origin of Life including the Centenary Lecture by Professor V.I. Goldanskii (Academy of Sciences Moscow). 14 May 1992 at the Scientific Societies' Lecture Theatre, London Molecular Beams in the Chemical Era including the Faraday Lecture by Professor Y.T. Lee (University of California, Berkeley). 3 December 1992 at the University of Cambridge 'New Applications of Quantum Chemistry ' including the Centenary Lecture by Professor H.F. Schaefer (University of Georgia, Athens). The Marlow Medal was not awarded in 1992. Meetings organised by the 13 Subject Groups affiliated to the Division in 1992 were: X Reactions on Complex Potential Energy Surfaces (Gas Kinetics Group) Fuels and Feedstocks: The Next Generation of Catalysts and Processes (Surface Reactivity and Catalysis Group with the Institute of Chemical Engineers and the RSC Process Technology Group) Analytical Applications of Chemically Modified Electrodes (Electrochemistry Group with Electroanalytical Group) Spring Meeting 1992 (Theoretical Chemistry Group) Polymer Colloids (Colloid and Interface Science Group) Understanding Catalysts: Catalysis and Surface Characterisation (Surface Reactivity and Catalysis Group) Environment Aspects of Coal Utilisation and Carbon Science (Carbon Group with SCI Carbon and Graphite Group) Quasielastic Neutron Scattering Workshop (Neutron Scattering Group) Scattering and Interfaces (Polymer Physics Group) Photons, Beams and Chemical Dynamics (Molecular Beams and Dynamics Group) Statistical Mechanics of Industrially Important Materials and Processes (Statistical Mechanics and Thermodynamics Group with MacroGroup UK and SERC CCPS) Fullerenes: The New Carbon Materials (Carbon Group) Solid State Chemistry and Superconductivity (Polar Solids Group) Graduate Students’ Meeting (Electrochemistry Group) XIIth International Symposium on Gas Kinetics (Gas Kinetics Group) Polymer Modelling (Polymer Physics Group with Polygen) Colloids in the Aquatic Environment (Colloid and Interface Science Group with SCI Colloid and Surface Chemistry Group with SCI Water and Environment Group) Molecular Organic Magnets (Polymer Physics Group with Organic Reaction Mechanisms Group, Applied Solid State Chemistry Group, IOP Magnetism Group, and IEEE Magnetism Group) Colloids in External Fields (Colloid and Interface Science Group) Condensed Matter and Materials Physics Conference (Neutron Scattering Group) Neutrons and Metallic Magnetism (Neutron Scattering Group with IOP Magnetism Group) Fracture of Polymers: Fundamentals and Applications (Polymer Physics Group) Annual Christmas Meeting (Polar Solids Group) Polymers in Motion (Polymer Physics Group) Gas Kinetics under Extreme Conditions (Gas Kinetics Group) Colloidal Aspects of Aerosols (Colloid and Interface Science Group) High Resolution Spectroscopy (High Resolution Spectroscopy Group) During the year moves were initiated with a view to forming two new Subject Groups in the areas of biophysical chemistry and astrophysical chemistry.The year was a successful one for Faraday Transactions with further growth in submissions, new authors and subject coverage, coupled with a decrease in the rejection rate. Invited topical articles entitled ’ Faraday Research Articles ’ were introduced into the Journal. The papers and discussion arising from two Faraday Discussions and one Faraday Symposium were published.A new style Newsletter was circulated to members in January. The Division had 4013 members in 1992, a small increase on 1991 reversing the downward trend of recent years. 3. Treasurer’s Report The Treasurer reported that the Division had performed within budget in 1992. There had been a satisfactory financial outcome to the 1992 conferences and the Discussion on The Structure and Activity of Enzymes had been particularly successful. The practice of offering low conference fees for students was to be continued. 4. Elections to Council Members of Council elected to take office from the Society’s Annual General Meeting in 1993 were as follows: xi POSITION NAME TO 5. Future Activities RETIRE President Professor J.P. Simons 1995 Vice Professor P. Gray Presidents Professor N. Sheppard who have Professor A. D. Buckingham served as Professor R. H. Ottewill President Professor R. Parsons Vice Professor M. J. Pilling 1994 Presidents Professor I. W. M. Smith 1994 Professor Sir John Meurig Thomas 1994 Professor R. N. Dixon 1995 Professor A. Carrington 1996 Professor M. A. Chesters 1996 Professor F. S. Stone 1996 Ordinary Professor M. N. R. Ashfold 1994 Members Professor R. J. Donovan 1994 Professor H. M. Frey 1994 Professor A. Hamnett 1995 Professor J. Lyklema 1995 Dr W. Mackrodt 1995 Professor D. A. Parkes 1995 Dr D.W. Fowler 1996 Dr S. L. Price 1996 Dr S. K. Scott 1996 Chairman Faraday Editorial Board: (Professor I.W.M.Smith) 1994 Chairman Standing Committee on Conferences: (Professor M.A. Chesters) Honorary Secretary (Professor M. J. Pilling) Honorary Treasurer (Professor F. S. Stone) Representatives on RSC Council: Professor J. P. Simons (ex ofsicio) Professor M. J. Pilling 1996 Secretary Mrs. Y. A. Fish The President thanked Professor Whiffen, the retiring Vice-president who had served as President, for his service to the Division over many years. He also thanked Dr Clary and Professor Phillips who were retiring as Ordinary Members. The programme of future meetings had been tabled. The President informed the meeting of the decision to replace the annual Symposium with a General Discussion. Starting in 1994 there would be three General Discussions each year and the Symposium series would cease.xii ~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Annual Congress: The Reactive Interface in Electrochemistry and Catalysis To be held at the University of Liverpool on 12-15 April 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN Neutron Scattering Group Neutron Scattering Data Analysis To be held at the Rutherford Appleton Laboratory on 13- 15 April 1994 Further information from Mrs S. Humphreys, The Rutherford Appleton Laboratory, Chilton, Didcot 0x11 ORA Colloid and Inte$ace Science Group Theoretical Modelling and Simulation in Colloid and Interface Science To be held at the University of Bristol on 18-20 April 1994 Further information from Dr R.Buscall, ICI Corporate Science Group, PO Box 11, The Heath, Runcorn WA7 4QE ~~~~ Division Autumn Meeting: Reactions and Mechanisms for Fine Chemicals To be held at the University of Glasgow on 6-9 September 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Gas Kinetics Group 13th International Symposium on Gas Kinetics To be held at University College, Dublin on 11-15 September 1994 Further information from Dr H. Sidebottom, Department of Chemistry, University College, Dublin ~~ ~ Electrochemistry Group with the SCI ELECTROCHEM 94 To be held in Edinburgh on 12-16 September 1994 Further information from Professor D.E. Williams, Department of Chemistry, University College London, 20 Gordon Street, London WClH OAJ THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 98 Polymers at Surfaces and Interfaces University of Bristol, 12-14 September 1994 Organising Committee: Professor Sir Sam Edwards (Chairman) Dr R. Buscall Professor R. H. Ottewill Dr T. Cosgrove Professor J. S. Higgins Dr R. W. Richards Dr R. A. L. Jones New experimental methods and new theoretical and computational techniques have recently led to great progress in understanding the difficult but technologically important problems associated with the conformation of polymer molecules at surfaces and interfaces. The purpose of this Discussion is to bring together experimentalists and theoreticians working towards a molecular understanding of polymers at surfaces and interactions to survey the progress in the area to date and to indicate future directions of research.The meeting will attempt to bring a unified approach to the problem, encompassing problems of the structure of surfaces and interfaces in polymer melts, the conformation of polymers at solidfliquid and 1iquidAiquid interfaces, and extensions towards more complicated biological systems. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly, London W 1V OBN. ... Xlll THE ROYAL, SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 97 Structure and Dynamics of Van der Waals Complexes University of Durham, 6-8 April 1994 Organising Committee: Dr B.J. Howard (Chairman) Dr P. Hamilton Dr J. M. Hutson Dr D. C. Clary Professor A. C. Legon Dr B. Soep Dr P. R. R. Langridge-Smith Since Faraday Discussion No. 73 on Van der Waals molecules, in 1982, the study of weakly bound molecular complexes has developed rapidly. Spectroscopic studies can now yield detailed information on intermolecular potential-energy surfaces in molecular systems. Studies of trimers, tetramers and higher clusters are giving insight into solvation effects and providing information on many-body forces, which are important in understanding the properties of condensed phases. Investigations of photodissociation and predissociation processes are helping us to understand the dynamics of fundamental chemical processes such as molecular rearrangement and energy transfer.In addition, Van der Waals complexes provide an opportunity to control the orientation of colliding molecules and the energies and impact parameters of reactive collisions, and have added significantly to our understanding of the pathways of simple chemical reactions. This discussion will bring together experimentalists and theoreticians who are involved in the study of Van der Waals molecules. The final programme and application form may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN. THE ROYAL, SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Organising Committee Professor L. D. Barron (Chairman) Dr A. F. Drake Dr D. L. Andrews Professor R. E. Hester Professor A. D. Buckingham Traditional optical activity measurements such as CD are confined to the visible and near-ultraviolet spectral regions where they provide stereochemical information on chiral molecules via polarized electronic transitions. Thanks to prompting from theory and new developments in instrumentation, optical measurements are now being made in the vibrational spectrum using both infrared and Raman methods. Studies over the past decade on a large range of chiral molecules, from small organics to biological macromolecules, have demonstrated that vibrational optical activity opens up a whole new world of fundamental studies and practical applications undreamt of in the realm of conventional electronic optical activity. The meeting seeks to bring together experimentalists and theoreticians to discuss the current and future experimental possibilities and the development of theories, including ab initio computational methods, which can relate the observations to stereochemical details. The increasing importance now being attached to molecular chirality and solution conformation in the life sciences should also encourage the partipation of biomolecular scientists. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, London W1V OBH. xiv

ISSN:0956-5000    

DOI:10.1039/FT99490BP005

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, Wl), 17-21 Ultraviolet Photolysis of HOCl :REMPI Measurement of the Relative Population of Chlorine Atom ('P) Spin-Orbit States A. J. Bell, S. A. Boggis, J. M. Dyke, J. G. Frey,* R. Richter, N. Shaw and M. Tabrizchi Department of Chemistry, The University, Southampton, UK SO95NH The nascent chlorine atom (2P) relative spin-orbit state population resulting from UV photolysis of HOCl has been measured by resonance-enhanced multiphoton ionization (REMPI) in single laser experiments. The ratio of (0.30 f0.07) for CI* : CI has been obtained, indicating a stronger preference for production of ground-state C1(2P,,,) than expected from statistical weights. Also, the CI+ arrival-time spectra show that most of the energy available in the photolysis is converted into kinetic energy of the photofragments. HOCl has been implicated in ozone depletion reactions in the stratosphere as a possible C1 and OH reservoir, and in the antarctic stratosphere, because of reactions at ice surfaces which generate Cl, .In both cases the photochemical stability of HOCl will determine the atmospheric lifetime of HOC1.'V2 In recent studies by Molina and Molina3 and Mishalanie et aL4 using long-pathlength absorption cells (2 and 20 m, respectively) for enhanced sensitivity, absorptions were observed at 250 and 290 nm, the latter band being approx- imately half as intense as in the earlier observations of Fer- gusson et aL5 Two recent high-level ab initio calculation^,^.^ using the MIDI4** basis set (split valence with polarization), while pre- dicting a similar pattern of HOCl excited states to the pre- vious calculation^,^^^ computed the excited electronic states to be at significantly lower energy than the earlier work.Ver- tical transitions were computed at ca. 320 nm (to an A" state) and ca. 250 nm (to an A' state). This is in close agreement with the UV spectra observed by Molina et aL3 and Mish- alanie et aL4 Also, ab initio studies of the interconversion of the HOCl/HC10 isomers and the photodissociation products of these isomers have been performed by Bruna et a1." Using standard thermodynamic data,' ' photolysis of HOCl in the near-UV could proceed by one of four accessible channels :' Ami Jnm AHlkJ mol- HOCl + hv --+ O(3P)+ HCl(X 'Z+) 481 248.6 (la) -,C1(2P)+ OH(X 2n) 473 252.9 (lb) -P H(2S)i-CIO(X 211) 291 411.0 (lc) --* O('D) + HCI(X lZC) 272 439.7 (16) On the basis of the results of the ab initio calculations, some general conclusions can be drawn about these possible dissociation pathways: (a) The ground electronic state of the HClO isomer (1 A') is predicted to be 2.8 eV (270 kJ mol-') less stable than that of the HOCl isomer at their respective equilibrium geometries and there is only a small activation barrier (<0.1 eV = 9.6 kJ mol-l) for the HClO -,HOCl interconversion.(b) All energetically accessible (hv < 6 eV) excited electronic states of HOCl are highly repulsive in the 0-C1 coordinate and correlate with the C1 + OH products; (c) Photolysis via the H + C10 pathway is expected to be unfavourable as all accessible excited electronic states are bound in the O-H coordinate. (6)Oxygen removal by process (la) is also unlikely, as the only excited electronic states of HOCl which correlate with O(3P) + HCl(X 'C') are the triplet states 13A' and 2 3A" with computed vertical exci- tation energies of 4.7 and 3.5 eV, respectively." A number of experimental studies have been performed on the photolysis of HOCl and its dissociation products.Butler and Phillips' investigated photodissociation of HOCl at 308 nm using an XeCl excimer laser and subsequently attempted to detect 0 and H atoms by means of an atomic resonance lamp. No statistically significant signals were observed, sug- gesting that processes (la), (lc) and (16) were of minor impor- tance.This conclusion is consistent with the work of Vogt and Schindler" who used a filtered mercury arc lamp as a photodissociation source. Radical scavengers were used to convert primary photofragments into stable species that could be detected by mass spectrometry. The branching ratio of process (lb) was established as being at least 0.95. A study by Molina et ~1.'~used a single laser to dissociate HOCl in a gas cell and also to detect the OH photofragments by fluores- cence excitation, thus confirming the importance of channel (lb) in HOCl photolysis. All the previous state-specific photolysis experiments have concentrated on the OH fragment.Since both the OH and C1 fragments are degenerate in their respective ground states, there are four possible channels associated with the OH + C1 dissociation pathway. It is clearly important to know the relative populations of the two Cl spin-orbit states, 'PIL2and 2P3,2, and the two OH spin-orbit states, 211112and ll312, produced on photolysis. This work measures the C1 2P1/2: 'P3,, population ratio by MPI spectroscopy. Experimental As the MPI ion-current spectrometer used in this present work has been described previously,15 only the main features and changes introduced to the ion-detection geometry neces- sary for the present study are described. An aqueous solution of HOCl was prepared by bubbling C1, (99.9% Union Carbide) through distilled water in the presence of red mercury(II1) oxide (99% BDH Chemicals Ltd.).16 HOCl vapour from this solution was introduced into the ionization chamber of the spectrometer without further purification.To minimize HOCl decomposition on surfaces, an inlet system composed of Teflon and Pyrex was used. The pressure at the photolysis point was kept at ca. mbar. The effusive vapour beam containing HOCl was inter-cepted at right angles by the focused output of an XeCl excimer (Lumonics EXSOO) pumped-dye laser (Lumonics Hyperdye 300) operating with the dye Coumarin 480. The linearly polarized output of the dye laser was frequency doubled using a BBO crystal giving the wavelength range required (234-239 nm), The laser wavelength scale (fundamental laser output) was calibrated optogalvanically using a neon hollow cathode lamp.''Typical pulse energies of the frequency-doubled radiation were about 1 mJ per pulse. When necessary, the power of the laser radiation was reduced in a controlled way by passing the beam through a solution containing a known concentration of acetone in water. Relative power changes were monitored using a fast J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 quadrupole mass spectrometer + to pump quadrupole mass spectrometer to pump Fig. 1 (a)Schematic geometry of the electrostatic ion lens. E indi-cates the direction of the laser polarization vector. (b)Second experi- mental arrangement. The distance from the focal point (x) to the detector was ca.1.5 cm. photodiode. Spectra were recorded by stepping the dye laser fundamental wavelength at, typically, 0.002 nm intervals, at laser repetition rates of between 10 and 30 Hz. The same laser pulse was used both for photolysis and ionization of the pho t ofragments. Two geometries were used to collect the CI+ ions pro- duced in the laser focus. One [Fig. l(a)] provided a flight path of ca. 18 cm and could be used with a quadrupole mass spectrometer to mass analyse the ions. Ion-trajectory simula- tions (see below) have shown that the collection efficiency of this system depends strongly on the ion initial energy dis- tribution, being most efficient for low-energy ions. For ions of higher energies (>0.5 eV) the acceptance angle of this ion lens was found to be lower, at <20°.In the second arrange- ment [Fig. l(b)] the detector was placed very close to the ionization region ensuring an almost 100% collection effi- ciency. Owing to the short flight path no mass selection of the ions was possible in this experimental arrangement. The wavelengths chosen to monitor the C1 2P112: 2P3/z population ratio were 235.26 nm for the ’P3/2 state and 237.73 nm for the state, corresponding to the 4p 2D3/:-ZP;b2 and 4p 2D3/2-2Pl/2 two-photon transitions, respectively, in which a C1 3p electron is excited to a 4p Rydberg state. Both transitions are (2 + 1) multiphoton ion- izations and have been used previously for the detection of the two C1 atom (’P) spin-orbit states.”-2’ Ma ss-resolved experiments on the photolysis of HOCl showed that the two Cl isotopes account for the majority of the ion current at these wavelengths.A minor peak at 18 u was also observed. With changing wavelength the intensity of this feature closely followed the Cl’ signal. This indicates that ion-molecule reactions of C1+ were the most likely cause of this signal. To check for contributions to the observed C1+ signals from other chlorine-containing species control experiments have been performed by introducing either pure C1, or an HCl-Ne mixture into the ionization chamber. Under similar experimental conditions (total pressure and range of laser powers), the C1+ signals recorded at laser wavelengths corre- sponding to both atomic chlorine transitions were negligible.Results Power Dependence and Population Ratio As mentioned earlier, the absorption spectrum of HOCl has been the subject of a number of e~perimental~.~and the~retical~’~.~.’~.~’studies. The results show that there is little difference between the absorption at the two wave-lengths used in this study, both lying near the maximum of the (A ‘A’-X ‘A’) band. In the results presented below no cor- rection was made for this difference in HOCl absorption. Fig. 2 shows power dependences (i.e. observed signal us. laser intensity plotted on logarithmic axes) recorded for the two C1 atom transitions using the experimental set up shown in Fig. l(b). The intensities used in these plots correspond to integrated areas of the resonance lines.Both curves indicate a second-order process over a wide power range, consistent with the C1 two-photon resonant step in the overall (2 + 1) process being the rate-determining factor. As shown in Fig. 2(b), the 4p 2D3/2-2P3,2transition reaches saturation at lower power densities. In previous studies of the two transitions’9920an effective ratio of the two-photon transition moments of (2.5 k0.1)19 and (2.7 k0.3)20has been measured by comparing MPI intensities with VUV fluorescence results” or calibrating recorded REMPI signals against a known fine structure state distribution.” This difference in transition moments is thought to be the main reason for the fact that the 2D3/2-2P3/2transition saturates at lower power than the 2D3/2-2P!,2 transition.For the determination of the relative populations of the 2P3/2and 2P1/2states of chlorine atoms, only data from the linear parts of the curves shown in Fig. 2 were used. After correcting for laser power differences and using the factor of 2.5 l9 for the relative transition probabilities, a ratio of (0.30 & 0.07) for C1* :C1 was obtained. Arrival-time Spectra Single-photon photolysis of HOCl at 235.26 nm (the 4p ’D3/2-’P3/2 two-photon transition wavelength) results in 2.648 eV total energy being made available to the OH(X 211) and 35c1(2P312)photofragments. Calculations by Guo,’~ and also experimental data of the OH internal energy distribution at similar photolysis wavelengths (248 23 and 266 nm 24), indi-lo6, / 0 1o31 1o4 1o5 1o6 laser power (arb.units) Fig. 2 Power dependences recorded for the two C1 atom tran- sitions: (a) ’D3/2-’P3/2 ;(b)2D3/2-2P,,2. The slope of the two lines is (a) 2.05 f0.20 and (b)2.0 k0.15. The highest recorded values on the laser power scale correspond to ca. 700 pJ per pulse. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 cate that most of this energy is converted to kinetic energy of the fragments. Energy and momentum conservation imply that the maximum kinetic energy available to 35c1(2P3,2)is 0.866 eV. Because of the relatively large linewidth of the laser used (ca. 0.20 cm-’ at 470 nm) and the multiphoton nature of the probe, it is not possible to obtain accurate values for the C1 atom velocity from the observed lineshapes.Some infor- mation, however, can be gained by recording the C1’ arrival-time spectrum over the 18 cm flight path [geometry Fig. l(a)]. An example recorded at 235.26 nm and at relatively low laser power (ca. 200 pJ per pulse) is shown in Fig. 3. The quadrupole mass spectrometer was operated in the total ion mode, so the two C1 isotopes at 35 u and 37 u both contrib- ute to the signal shown in this figure. The shape of the arrival envelope obtained indicates the presence of two groups of C1+ ions. The approximately equal intensity of the two main features means that they cannot be attributed to the 35Clf and 37Cl+ ions. To interpret this arrival-time envelope, simu- lations of ion trajectories were performed.The electric field produced by the ion lens was calculated using Simi~n.~~ Trajectory simulations on this potential were carried out using a routine which was specifically written to allow for systematic changes in the starting parameters (position, initial energy and starting angle). Because of the large number of evaluated trajectories (ca. lo6), the calcu- lations were carried out on a Convex C3800 supercomputer. All calculations were performed on a two-dimensional slice through the cylindrical geometry. The results showed that the structure in the arrival-time spectrum is due to two groups of chlorine atoms moving initially towards and away from the detector. This could be confirmed experimentally by record- ing a signal on both detectors simultaneously [geometry Fig.l(b)] with zero voltages on the plates. As the size and shape of the laser focus could not be measured, some assumptions 1.5 1.2 h v)4-.-5 0.9+ v > c..-v)5 0.6 .-C 0.3 0 7.8 8.0 8.2 8.4 8.6 8.8 9.0 arrival time/p Fig. 3 Experimental (a)and simulated (b)ion arrival-time spectra. The field strength in the focal regon was ca. 115 V cm-’. In the simulated spectrum the starting parameters are chosen using a random function weighted with the geometrical parameters of the laser focus and chlorine atom initial energy distribution (see text for details). The ratio of the two chlorine isotopes was set to 3 : 1. The simulated spectrum is a result of lo6 ion trajectory calculations.The assumed detector resolution is 5 ns. of these characteristics were necessary to model the arrival- time spectrum. The minimum beam waist can be calculated for an idealized Gaussian beam26 to be ca. 15 pm for the lens used (focal length of 15 cm), assuming a diffraction-limited focus size. The shape of the focal volume was assumed conical with the cone angle determined by the minimum beam waist and the diameter of the beam at the focusing lens (ca. 1.5 mm). The power density change was set proportional to the square of the cone radius. In the simulation the initial chlorine atom distribution was assumed to be proportional to the square of the power density within the conical focal volume, consistent with the experimentally recorded power dependence.A further unknown factor in the model is the C1 atom initial energy and angular distribution. As all simulations were performed in the plane perpendicular to the laser polar- ization, the angular distribution was assumed to be uniform in that plane. The double-peak structure observed for both isotopes in the calculated arrival-time spectra (Fig. 3) thus results from the angular selectivity of the detection geometry, which discriminates against ions moving in off-axis direc- tions. For the simulation shown in Fig. 3 the chlorine atoms were also assumed to carry the maximum kinetic energy available from the photolysis process (0.866 and 0.834 eV for 35Cl and 37Cl isotopes, respectively), broadened by a Gauss- ian distribution with a FWHM of 0.03 eV.Owing to factors like the initial rotational and vibrational excitation of HOCl and final OH rotational and spin-orbit population distribu- tion, the experimental width of the chlorine atom energy dis- tribution will almost certainly be broader than this and not symmetric. On comparison of the experimental and calculated arrival- time spectra, it can be seen that the model is capable of reproducing the main features of the experiment. There are, however, significant differences which preclude an accurate kinetic energy estimation from the arrival spectra. An exten-sion of the calculations to three dimensions should improve the model. Also, in the experiment, space-charge effects are thought to be the resolution-limiting fa~tor.~~-~~ This could be demonstrated by recording the arrival-time spectra with higher laser power, which resulted in significantly broadened peaks.An inclusion of this effect in the model, although in principle possible, would lead to a huge increase in the required computing time. Trial calculations for a small number of ions distributed in a tight laser focus (ca. 10% of the volume estimated above) showed a clear broadening of the peaks in the arrival-time spectrum. Clearly, a more accu- rate description of the laser focus conditions and initial chlo- rine atom velocity and energy distributions are needed to obtain more than qualitative results. Discussion The ground electronic state of HOCl (X 1 ‘A’) has the elec- tronic configuration : .. .(9a’)2( 10a’)2(3a’’)2( 1 la’)’. The charac- ter of the loaf, 3a” and lla’ orbitals can be described as non-bonding in-plane chlorine 3p (3pc,), OC1 n antibonding (E&-~) and OCI a antibonding (a&), respectively. Calculations by Nanbu and Iwata6g7 indicate that there are three acces- sible excited electronic states in the UV region (200-400 nm). These overlap giving a UV absorption spectrum with two maxima at 245 and 337 nm, corresponding to the experimen- tally observed absorption bands at 250 and 290 nm.3*4 The states of interest are the 1 ‘A”, 2 ‘A’ and 2 ‘A” states which arise from the following electronic excitations : 1 1A”.. (n&) (a&-l); 3a” -+ lla’ 2 ‘A’: (3p,,) -+ (a&); 10a‘ -+ lla’ 2 ‘A”: (n&)’ + (3a”)2 --+ (1la’)2 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 All three transitions are symmetry allowed in the C, point group. At 235 nm the absorption cross-section of the 2'A state is predicted to be the only significant transition. Absorption to the 1 'A" state reaches its maximum at around 330 nm while the 2'A" state becomes significant at shorter wavelengths (<210 nm). The transitions to the triplet states in this region are predicted to have very low transition moments. The other product resulting from HOCl photolysis, OH, has not been studied experimentally at the wavelengths used here. However, two investigations have examined the rota- tional, vibrational, spin-orbit, and lambda-doublet OH populations at photolysis wavelengths of 266 24 and 248 nm.23 The OH rotational distribution has also been com- puted at wavelengths of 240 nm22 in a time-dependent quantum mechanical study using ab initio potential-energy surfaces.In all three cases absorption to the 2'A state has been recognized as the dominant dissociation pathway. This assignment of the upper state accessed at these wavelengths is also supported by recent ab initio cal~iilations.~~~Thus it can be assumed that the photolysis of HOCl at 235.3 and 237.7 nm also proceeds via the same mechanism. A two-photon photolysis process via a higher state (or states) of HOC1, fol- lowed by a (2 +1) ionization of atomic chlorine could also contribute to the observed signals, but owing to the fast pho- tolysis of the 2 'A' state this route is thought to be of minor importance in the laser power range used in the present study.The quality of the experimental data does not permit an accurate determination of the C1 atom velocity. From the results of the simulations it can be seen that the kinetic energy of the chlorine atoms is in the range of 0.8 eV. The maximum rotational energy imparted to the OH fragment produced in the 248 nm photolysis of HOCl is ca. 10% of the available energy.23 The majority of the energy is released into relative translational motion of the fragments with only a small torque being imparted to the fragments. This is consis- tent with a rapid dissociation along a steeply repulsive potential-energy surface.The C1 fragment would therefore also be expected to have a fast and narrow energy distribu- tion, as is indeed observed in the REMPI results. Simulations of the arrival-time spectra show that the experimental arrangement used detects mainly photolysis products with their initial velocities aligned in the plane per- pendicular to the photolysis laser polarization, with little contribution from velocities aligned at angles >lo" to this plane. As the laser output is only ca. 95% linearly polarized in the vertical plane, some contribution to the signal from other polarizations cannot be excluded. Ab initio calculations place the transition moment for the 2 'A'-X 'A transition in the molecular plane, making an angle of ca.80" with the C10 bond,7 and calculations by Guo~~predict a recoil of chlorine atoms perpendicular to the OH axis during the photolysis. The equilibrium structure and normal-mode frequencies of HOCl have been obtained from micro~ave~~*~' and high- resolution infrared The structure was found to be non-linear with C, symmetry. As the ground-state equi- librium angle is known to be 102.4°,30 a significant fraction of chlorine atoms should be produced having their velocity vectors aligned perpendicular to the photolysis laser polariza- tion direction, consistent with the experimental observation of the present work. The measured population ratio of the C1 spin-orbit states 2P1,2:2P3/2.of (0.30 & 0.07) indicates a stronger preference for production of the lower 2P3/2state, than expected from statistical weights of the states involved. The OH rotational-state distribution measured at 248 nm 23 gives a ratio for the populations of the OH (2113/2) :OH(21T1,2)at ca. 1.4; at 266 nm 24 the spin-orbit ratio (now given for each lambda- doublet component) is given below : state average ratio :*nLj2 OH II (A) OH ll (A") 1.8 2.7 0.2 0.2 It is not clear how to interpret these ratios and the change with wavelength. At all the photolysis wavelengths investi- gated the population of the lower energy OH(211,i2)predomi-nates. Photolysis at both 248 nm and in the 236 nm region should be similar. At the longer wavelengths the influence of the 1A" state may be being felt.The correlation rules for photodissociation of HOCl via either the 2 'A' or 1 'A" states would imply the production of only OH(211,i2) together with C1(2P31?).35 Although a minor fraction of the observed C1* and C1 signals could also arise from photolysis via higher excited states, the present result, together with the OH 2113/2 :211 population ratios, indicates that non-adiabatic coupling of the potential-energy surfaces correlating with dif- ferent spin-orbit states of the products, is important in deter- mining the final state distribution in the photolysis process via the 2 'A' state. Conclusions The present observation of the C1 atom fragment from pho- tolysis of HOCl is consistent with previous experimental work on the photodissociation of HOCl at 248 nm and the ab initio calculations on the photolysis of HOC1.Photolysis in the 236 nm region occurs via the steeply repulsive 2'A' excited-state surface resulting in fast C1 atoms with a rela- tively narrow energy distribution. The C1 spin-orbit popu-lation ratio is not statistical and it cannot be explained on the basis of a simple correlation diagram; in this respect it is similar to the OH fragment spin-orbit population ratio. In order to explain the spin-orbit populations, more detailed calculations on the asymptotic parts of the potential-energy curves and the interactions between them will need to be per- formed. The authors are grateful for the financial support from the SERC and the European Community.References 1 T. J. Leck, J. E. Cook and J. W. Birks, J. Chem. Phys., 1980,72, 2364. 2 R. M. Stimpfle, R. A. Perry and C. J. Howard, J. Chem. Phys., 1979,71,5183. 3 L. T. Molina and M. J. Molina, J. Phys. Chem., 1978,82,2410. 4 E. A. Mishalanie, C. J. Rutkowski, R. S. Hutte and J. W. Birks, J. Phys. Chem., 1986,90,5578. 5 W. C. Fergusson, I. Slotin and D. W. G. Style, Trans. Faruday SOC., 1936,32,956. 6 S. Nanbu, K. Nakata and S. Iwata, Chem. Phys., 1989,135,75. 7 S. Nanbu and S. Iwata, J.Phys. Chem., 1992,%, 2103. 8 R. L. Jaffe and S. R. Langhoff, J. Chem. Phys., 197868, 1638. 9 G. Hirsch, P. J. Bruna, S. D. Peyerimhoff and R. G. Buenker, Chem. Phys. Lett., 1977,52,442. 10 P. J. Bruna, G.Hirsch, S. D. Peyerimhoff and R. J. Buenker, Can. J. Chem., 1979,57, 1839. 11 JANAF Thermochemical Tables, NSRDS-NBS 37, National Bureau of Standards, Washington DC, 2nd edn., 1971. 12 R. Vogt and R. N. Schindler, J. Photochem. Photobiol. A: Chem., 1992,66,133. 13 P. J. D. Butler and L. F. Phillips, J. Phys. Chem., 1983,87, 183. 14 M. J. Molina, T. Ishiwata and L. T. Molina, J. Phys. Chem., 1980,84,821. 15 M. Barnes, J. Baker, J. M. Dyke, M. Feher and A. Morris, Mol. Phys., 1991,74, 689. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 21 16 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 26 D. C. Hanna, M. A. Yuratich and D. Cotter, Nonlinear Optics of 17 18 19 Wiley Interscience, New York, 4th edn., 1980.S. H. Ashworth and J. M. Brown, An Atlas of Optogalvanic Transitions in Ne, SERC, 1991. L. J. Radziemski Jr. and V. Kaufman, J. Opt. SOC.Am., 1969,59, 424. K. Tonokura, Y. Matsumi, M. Kawasaki, S. Tasakaki and R. 27 28 Free Atoms and Molecules, Springer Series in Optical Sciences, Springer-Verlag, Berlin, 1979. M. N. R. Ashfold, I. R. Lambert, D. H. Mordaunt, G. P. Morley and C. M. Westem, J. Phys. Chem., 1992, %, 2938. X. Xing, D. Charalambidis and C. Fotakis, Opt. Commun., 1990, 79, 181. 20 21 Bersohn, J. Chem. Phys., 1992,97,8210. V. J. Barclay, B. A. Collings, J. C. Polanyi and J. H. Wang, J. Phys. Chem., 1991,95,2921. S. Arepalli, N. Presser, D. Robie and R. J. Gordon, Chem. Phys. Lett., 1985, 118, 88. 29 30 31 M. Crance, J. Phys. B, 1986,19, L267; L671. A. M. Mirri, F. Scappini and G. Cazzoli, J. Mol. Spectrosc., 1971,38,218. W. D. Anderson, M. C. L. Gerry and R. W. Davis, J. Mol. Spec- trosc., 1971, 38, 218. 22 23 24 H. Guo, J. Phys. Chem., 1993,97,2602. A. J. Bell, P. R. Pardon, C. G. Hickman and J. G. Frey, J. Chem. Soc., Faraday Trans., 1990,86,3831. C. G. Hickman, N. Shaw, M. J. Crawford, A. J. Bell and J. G. 32 33 34 35 C. M. Deeley, J. Mol. Spectrosc., 1987, 122,418. R. A. Ashby, J. Mol. Spectrosc., 1967,23, 439. C. M. Deeley and I. M. Mills, J. Mol. Spectrosc., 1985, 114, 368. Y. Matsumi, K. Tonkura, M. Kawasaki, T. Tsuji and K. Obi, J. 25 Frey, J. Chem. Soc., Faraday Trans., 1993,89, 1623. D. A. Dahl and J. E. Delmore, A Simion PC/AT User’s Manual Chem. Phys., 1993,98,8330. Version 2.0, Idaho National Engineering Laboratory, EG&G Idaho Inc., 1986; D. A. Dahl, J. E. Delmore and A. D. Appel- haus, Rev. Sci.Instrum., 1990,61, 607. Paper 3/0460OA; Received 2nd August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000017

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, Wl), 23-30 Power-series Expansions as Fitting Functions of Potential-energy Curves J. J. Carnacho,* A. Pardo and J. M. L. Poyato Departamento de Quimica-Fisica Aplicada , Facultad de Ciencias, C-XIV, Universidad Autonoma de Madrid, Cantoblanco, 28049, Madrid, Spain A comparative study of the most important power-series expansions, Dunham, Simons-Parr-Finland, Ogilvie Tipping, Thakkar, Engelke, Mattera, Surkus and Huffaker, as fitting functions of potential-energy curves is report- ed. A study of the leading terms and intervals of convergence is also shown. As an example, the calculation of the interval of convergence for an Engelke's series is given. The method is applied to the molecules: CO (X'C+), H, (X'C,') and 'LiH (X'C+ and A'C+).An analysis of the variation for the leading term of the power- series expansions with two non-linear parameters is presented for CO (X 'C+). The optimum non-linear param- eters are obtained when the left-hand side of the interval of convergence is very near and below the first point of the input potential. Moreover, we observed that a good fit through the leading term of a power-series expansion is obtained for Engelke, Mattera or Surkus functions with two non-linear parameters. For fitting power-series expansions with an intermediate number of basis functions it is better to use a Thakkar or Huffaker type function with only one non-linear parameter. A model potential-energy function should have a simple analytic form. However, there is no expression capable of approximating accurately the experimental potential-energy curve.The best potentials are obtained by direct inversion of spectral data by using numerical methods such as RKR,lP3 IPA4*5 or hybrid functions (for example PMO-RKR long-range potentials6). Nevertheless, these experimental potentials are presented in the form of tables of numbers, that normally show the vibrational and rotational energies for the experi- mental vibrational levels together with the corresponding classical turning points. It is necessary to interpolate to deter- mine the potential energy at any intermediate point. The interpolation between the given points is very critical and indeed appears to be the major source of error in the eigen- value problem.Moreover, it is difficult to choose the inter- polation method. For very accurate potentials it is observed that an interpolation by spline is usually better than a Lagrangian interpolation. Nevertheless, a Lagrangian inter- polation is less sensitive to numerical noise in cases in which the potential is not very accurate. On the other hand, the most accurate potential-energy curves, obtained from experi- mental data, have as a basis the RKR semiclassical pro- cedure. This method is an inversion of the first-order WKB approximation7 or Bohr-Sommerfeld quantization condition. This approximation is inadequate in zone of the minimum of the potential-energy curve and in the neighbourhood of the dissociation. Thus numerical noise is produced in the poten- tial which can be detected when the successive derivatives of the potential are obtained.Increasing attention has been given to the problem of the most convenient and accurate representation of the potential- energy curves through some type of analytic function: (i) empirical functions, (ii) Pade approximants or (iii) power-series expansions. Empirical functions as Morse,' Lennard-Jones,' Rydberg," Rosen-Mores,' Poschl-Teller,' Helmann,' Hulbert-Hir~chfelder,'~ Lippincott,' Frost-Musulin,'6 Varshni,' Linnett " and Schubert-Certain" are simple and easy to use but the assumption of single function- al form produces important deviations of the 'true' potential-energy function. Pade approximants2' have less convergence problems than power series but we must use many coefficients to fit a potential defined in a discrete set of points2' Power-series expansions can be used to give a very accurate potential-energy curve close to the minimum zone and, if the series converges, it is possible to represent the potential energy for large values of the internuclear distance, with an intermediate number of parameters.In some cases, it is possible that although there was a large convergence zone, the convergence could be slow and the corresponding approximation was not adequate. In recent years, power- series expansions for molecular potentials with improved convergence properties relative to the first Dunham expan- sion have been introduced by different authors.The most important power-series expansions have been reported by Dunham," Sim~ns-Parr-Finlan~~(SPF), Ogil~ie-Tipping,~~ Thakkar," Engelke,26 Mattera,27 Surkus2' and H~ffaker.~' In this work, we present a comparative study of these power- series expansions for fitting potential-energy curves. We review the definition of these power-series expansions and present a study of the corresponding leading terms. Then we present the results of our calculations of the interval of con- vergence and dissociation limits for the above-mentioned power-series expansions. As an example we also show the cal- culation of the interval of convergence for an Engelke type power-series development with p > 0 and p < 0. The results and discussion of the different power series expansions as fitting functions for the ground state of 12Cl60, 'LiH and H, and the A 'Cf state of 7LiH are presented in the final section.Here we also present a study on the variation of the standard deviation for the leading term of power-series expansions with two non-linear parameters for the ground state of CO. Power-series Expansions In principle any function can be expanded in a complete set of basis functions. In practice, this set is chosen according to the physical problem to be studied, so that the series con- verges after a small number of terms. So any potential-energy function V(r) can be expressed as a linear combination of a set of basis functions [fi(r)]i=o,1, ..,,,,: where the coefficients co, cl, .. . are real numbers independent of the internuclear distance r. To get V(r,)= 0 and (dV/dr),=,e= 0, co = c1 = 0 is taken. The basis functionsJ;(r) can be represented as a power of degree i of certain funda- mental functionsf(r). Thus eqn. (1)can be written as V(r)= dof2(r) 1 + difi( (2) “i= 1 The most important fundamental functions used in the approximation of potentials are shown in column 2 of Table 1. These functionsf(r) have different functional forms of r and they can be described by any (Dunham, SPF and Ogilvie), one (Thakkar and Huffaker) or two (Engelke, Mattera and Surkus) non-linear parameters. Some of these functions are special cases of another. In Table 2 relations between them are shown.Prior to a study of the power-series expansions it is very instructive to see the behaviour of the leading term Vo(r)= dof2(r) for the different expansions. Except for Dunham’s series, this leading term has an anharmonic form with an inflection point for r > re. They also have a finite asymptote as r -+ 00 corresponding to the dissociation limit, except for the Thakkar, Engelke and Mattera potential for p < 0. Owing to this fact, we have only considered these last potential functions with p > 0. Table 3 shows the calculated limits of Vo(r)when r +0 and r + co and the inflection points together with their energies. We note that although the Thakkar, Engelke and Mattera potentials have different values of the internuclear distance for the inflection point their corresponding energies are equal.We see that all these simple potential functions, except the Dunham’s function, represent the conventional form of a potential-energy curve with a single minimum and a smooth asymptotic rise to the dissociation limit. Although the potential-energy curves should become infinite at r = 0 as happens for an SPF, Thakkar (p > 0),Engelke (p > 0, B> 0),Matter (p > 0, y = p) and Surkus (p > 0, p = 0),this condition is not very strict because the results are practically the same if V becomes very J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Relations between different power-series expansions Engelke Mattera Surkus (P, P) Y) (P, P) Intervals of Convergence A fundamental requirement to approximate any potential- energy curve in terms of a power-series expansion is to deter- mine its interval of convergence. The radius of convergence of a power-series expansion can be obtained from the singu- larity of the potential, which appears at r = 0, closest to the equilibrium internuclear distance re in the complex r plane.Thus the nearest singularity on the real axis gives an upper limit to the radius of convergence. The values of the singu- larities for the different power-series expansions considered here are shown in column 3 of Table 1. For obtaining the interval of convergence of a power-series expansion in terms of its fundamental functionf(r) we used the ratio test; if then the series is absolutely convergent for L -= 1 and diver- gent for L > 1, and the test fails for L = 1.This interval of convergence can be simply obtained by determining the values of r for which If@) I < 1. The intervals of convergence obtained by us for different f(r)functions are summarized in Table 1. As an example of the method employed here for determin- large at r = 0, as for the leading term of Huffaker’s which ing the intervals of convergence we present its application to corresponds to a Morse potential. an Engelke’s potential. For an Engelke’s potential with p > 0, Table 1 Summary of singularities, intervals of convergence and dissociation limits calculated for the most important power-series expansions used for describing potential-energy curves of bound states of diatomic molecules dissociation function ref.f (r) singularity interval of convergence limit Dunham 22 -1 m SPF 23 -m T Ogilvie 24 -1 f Thakkar 25 -00 f -1 m Engelke 26 T m Mattera 27 T 0O Surkus 28 [(+)l‘prc, m] T [.(z)”’p’re] Q1-P Huffaker 29 1 -exp[-a(r -re)];(a > 0) 1 -exp(ar,) (re -fy,m) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Limits as r +0 and inflection points for the leading term Vo(r)= a,, f2(r)of the different power-series expansions 3rcSPF co -2 Ogilvie Thakkar (p > 0) Engelke (p > 0) -..[(y>”q2 Mattera (p > 0) ao[(9_)”P-Y -11’ Surkus (p > 0) a0- P2 Huffaker a,[exp(ar,) -112 a0 P2 [P2(2P -1)2 + 2p(2p2+ 1) + (2p + 1)2 + p(2p (2P + 1) if (4) Inequality (4) can be expressed as follows The argument of the sgn function is positive for r > re, and, in this case l/(r + Br,)’ > 0, which is true for all r.Otherwise for r < re {[ey--1) > -1 By solving inequality (6) we obtain (7) As all power-series expansions of the type of eqn. (2) are con- vergent at r = re, the interval of convergence for an Engelke’s series expansion with p > 0 is [re(B+ 1)/2’/” -#Ire, 003. An Engelke’s power-series expansion with p < 0 is con-vergent for Inequality (8) is equivalent to The argument of the sgn function is positive if r -c re. In this case the inequality (9) leads to which is true for every r. Thus a first interval of convergence is (0, re).For r > re, the argument of the sgn function of a *1 In 2-+re -1) + 2p + 13 . Expression too large. inequality (9)is negative. So + pr,[mi< and hence r < 21/lPl + 111 -Bre (121 A second interval of convergence is {re, 21’1p1[re(/3+ l)] -Sre}. As all power-series expansions (Table 1) are con-vergent for the equilibrium internuclear distance, the interval of convergence for an Engelke’s series expansion with p < 0 is (0,2’/’”“re(P+ l)] -&}. Numerical Results and Discussion For the above-mentioned series expansions we have deter- mined the limits when r goes to infinity. These results are shown in the last column of Table 1. We have taken p for Thakkar’s series; p and B for Engelke’s series; p and y for Mattera’s series; p and ,u for Surkus’s series and a for Huffa- ker’s series as adjustable non-linear parameters.We see that for a potential of type Thakkar, Engelke or Mattera it is better to choose the parameter p positive for obtaining a finite asymptote as r -+00 corresponding to the dissociation energy. Moreover, these power series with p < 0 are prefer- able to those with p < 0 because their interval of convergence is extended to infinity. The optimum non-linear parameters are obtained by con- sidering the best least-squares fit of the input potential. Con- sequently, it is necessary to determine the absolute minimum on the graph which shows the variation of the standard devi- ation of the fit as a function of the non-linear parameters.For each power-series expansion, by substituting the optimum non-linear parameters and the equilibrium internuclear dis- tance into the fourth column of Table 1 we get the corre- sponding range of convergence. We have carried out a comparative study of the different power-series expansions shown in Table 1 as fitting functions for approximating rotationless RKR potentials. These poten- tials consist of pairs of (I, VRKR)points which correspond to the classical turning points for the experimental vibrational levels. The output of the least-squares fits includes: the fitted parameters in the best fit; F-ratio tests for comparing the models specified in the function, as well as the degrees of freedom and sums of squares used to obtain F-ratios; table of confidence intervals for the best-fit parameters; the estimated covariance and correlation matrix of the fit coefficients; the variance or its square root which corresponds to the standard deviation; the mean deviation and the multiple correlation coefficient.In the results we present only the standard and mean deviations, possibly the best single statistical measures of the success of the chosen truncated series in fitting the data. First, we have considered the ground state of the "C l60 molecule which is the most stable diatomic molecule. The available spectroscopic information for different isotopic species of carbon monoxide is very extensive and accurate. Detailed references can be found in ref.30. Although there is accurate spectroscopic information up to the vibrational level u = 41 with a vibrational energy G(41)= 68 042.5 cm-I, this vibrational level is nevertheless far from the estimated disso- ciation limit D,= 90674.4 cm-'. 31 The RKR CO (X 'C+) potential curve up to u = 37 has been constructed by different by employing different techniques to remove the singularity that appears at the upper limit of the integration when one calculates thefand g integrals related to the clas- sical turning points (see, e.g. ref. 32). All these potentials have been checked by solving numerically the radial wave equa- tion. The self-consistency between the calculated eigenvalues with the radial wave equation. The self-consistency between the calculated eigenvalues with the experimental energy values is ca.0.4 cm-'. The self-consistency between the calcu- lated eigenvalues with the experimental energy values is ca. 0.4 cm-'. This fact supposes that this potential is employed as a test case for analysing the method for constructing potential-energy curves. We have determined the RKR potential for the electronic ground state of 12Cl60 up to u = 41 from the spectroscopic information given in ref. 30. The results are shown in Table 4 together with the corre- sponding eigenvalues E, of the rotationless potential U,(J = 0) obtained by numerical solution of the radial Schrodinger equation For the RKR potential we determined the RMS error (a= ~~=l(dE,)2/n)where n is the number of terms and 6E, is the difference between the experimental vibrational energy G(u)+ Yo, and the quantum mechanical energy eigenvalue E,.For the RKR potential defined in Table 4, CT = 0.605 cm-', whereas for the potential given in ref. 32, CT = 0.263 cm-'. As the self-consistency of this last potential is better, we have considered the potential presented in ref. 32 for comparing the ability of different power-series expansions indicated above as fitting functions. So, we have carried out a least- squares fit of the tabulated RKR potential of the X 'Z+ state of C032by using eqn. (2) and considering the different funda- mental functionsf(r) defined in Table 1. We suppose that the optimum fit is obtained when the standard and mean devi- ations are minima for a given number of linear parameters.We also followed the residual method.38 If the residuals (algebraic deviations) plotted against r are scattered uni- formly on either side of the zero line, the fit can be considered to be good. For example, in Fig. 1 we show a fit of the ground state of CO by using an SPF power-series expansion with N = 16. However, if the residuals occur in clusters, the fitting function has some problems. In this case we observed a behaviour such as that shown in Fig. 2 which corresponds J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 RKR potential-energy curve and eigenvalues for the X 'X+ state of '2C "0 0 1081.776 1.083 269 6 1.178 7250 1081.820 1 3 225.047 1.0534107 1.219 597 7 3 225.149 2 5 341.838 1.034 222 1 1.249 885 5 5 341.975 3 7 432.21 5 1.019 399 6 1.275916 1 7 432.375 4 9 496.245 1.007 121 5 1.299 525 0 9 496.41 8 5 11 533.998 0.996 555 54 1.321 547 8 11 534.175 6 13 545.545 0.987 241 46 1.342 450 8 13 545.720 7 15 530.957 0.978 890 9 1 1.362 523 4 15 531.129 8 1 7 490.3 1 1 0.97131147 1.381 962 1 17 490.48 1 9 19 423.680 0.964 366 48 1.400 907 1 19423.852 10 21 331.144 0.957 954 66 1.419 462 7 21 331.322 11 23 212.781 0.951 998 04 1.437 708 8 23 2 12.969 12 25 068.671 0.946 436 73 1.455 709 6 25 068.871 13 26 898.896 0.941 222 69 1.473 517 5 26 899.109 14 28 703.538 0.936 3 16 39 1.491 1754 28 703.764 15 30 482.68 1 0.93 1 683 79 1.508 718 2 30 482.9 18 16 32 236.409 0.927 299 2 1 1.526 178 6 32 236.655 17 33 964.807 0.923 138 74 1.543 583 1 33 965.060 18 35 667.960 0.919 18256 1.560955 7 35 668.219 19 37 345.951 0.9 15 41 3 95 1.578 318 2 37 346.218 20 38 998.867 0.911 817 32 1.595 689 4 38 999.142 21 40 626.790 0.908 380 08 1.613087 5 40 627.078 22 42 229.805 0.905 090 22 1.630 528 5 42 230.109 23 43 807.99 1 0.901 938 60 1.648 029 2 43 808.316 24 45 361.430 0.898 9 15 34 1.665 603 2 45 361.779 25 46 890.198 0.89601242 1.683 265 0 46 890.576 26 48 394.372 0.893 222 80 1.701 028 3 48 394.780 27 49 874.022 0.890 540 01 1.7 18 906 7 49 874.462 28 51 329.218 0.887 957 34 1.736912 6 51 329.690 29 52 760.024 0.885 470 63 1.775 060 4 52 760.523 30 54 166.499 0.883 073 39 1.773 361 6 54 167.037 31 55 548.700 0.880 762 14 1.791 830 7 55 549 273 32 56 906.673 0.878 532 43 1.810481 0 56 907.287 33 58 240.462 0.876 380 85 1.829 326 9 58 241.126 34 59 550.102 0.874 303 63 1.848 382 6 59 550.829 35 60 835.620 0.872 297 28 1.867663 1 60 836.427 36 62 097.035 0.870 359 39 1.887 1847 62 097.943 37 63 334.356 0.868 486 3 1 1.906 063 6 63 335.388 38 64 547.58 1 0.866 676 71 1.927018 6 64 548.763 39 65 736.698 0.864 947 30 1.947 387 9 65 738.053 40 66 901.68 1 0.863 282 31 1.968 078 5 66 903.227 41 68 042.49 1 0.861 675 12 1.989 107 9 68 044.236 re = 1.128 3232 A.to a fit of the X 'Z' state of CO by a Dunham power-series development with N = 6.We also note the difference of scale for the residual in both figures. Some results for different fits of the ground state of CO potential are shown in Table 5. As the input potential is defined between r-(u = 37) = 0.868 176 7 8, and r+(u = 37) = 1.906655 5 A, in principle, any f(r) contains the full potential inside its interval of con-vergence. We see that the interval of convergence depends only on re and the non-linear parameters. From the results in Table 5 we observe that the worse fit corresponds to an .0.2 I : -..* ...'. .. ._.a--___-1. --rJA1.2 * '1.4 . '1.6 * -1.8 .. .. a. Fig. 1 Residuals for the ground state of the CO molecule using as fitting function an SPF power-series expansion with N = 16 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I * .a*. . .. 7 -? 50 3.45I Fig. 2 Residuals for the ground state of the CO molecule using as fitting function a Dunham power-series expansion with N = 6 Ogilvie function which has the greater interval of con-vergence. This is because the convergence of an Ogilvie's potential is very slow and its limit when r -+ 0 gives a finite small number (see Table 3). Moreover we see that the optimum non-linear parameters for different power-series expansions and for all the potentials studied had values such that the lower limit of the interval of convergence is some- what smaller than the first internuclear distance of the input potential.For example, for the ground state of 'LiH the best fit is obtained for a Thakkar's potential with p = 1.48 corre-sponding to an interval of convergence of (0.998898 A, co) whereas the inner first point of the input potential39 is r-(u = 22) = 1.004707 A. For the potential with two non-linear parameters (p > 0) an interval of convergence (0, co)is obtained for p = { 1/[2("P) -1)) defined in the range 0 < r < m, then one would think these values were optima. The optimum parameters are obtained when the left-hand side of the interval of convergence is very near to or less than the first point of the input potential. On the other hand, when we fit potentials defined very near to the dissociation limit (for example, the ground state of 'LiH and H2) we must see expressions for the potential whose upper extreme of the interval of convergence is infinity.For a potential defined distant to the dissociation limit (for example the ground state of CO) it is not important to use functions with an interval of convergence defined up to infin- ity. For definitions off(r) whose right extreme in the interval of convergence is not infinity we have an asymptotic behav- iour of the potential going to infinity. However, if the func- tions have a right extreme in the interval of convergence of infinity, the potential tends toward a finite limit correspond- ing to the dissociation. On the other hand, we have observed that it is almost equivalent to fit any V(r)by N + 1 basis functions to eqn.(2) and determine the optimum non-linear parameters of f(r)by fitting with only one basis function (the leading term) and from these last non-linear parameters to fit using eqn. (2). This procedure allows us to economize on the time of calcu- lation when we are looking for various non-linear param- 3.08; 2.70. 2.32 - 2.32 P 1.95 1.95 1.57 1.57 1.19' 1.19 0.82JJ L' 10.82 eters. Evidently the coefficients di are different but the accuracy of the fits is the same. For example, when we fit the ground state of CO by a Thakkar's potential to eqn. (2) at N = 19, with an optimum value of p = 1.90, a standard devi- ation of 0.0126cm-' is obtained whereas by fitting using only the leading term, the optimum value of p is 1.81, and if we employed this last optimum p value and now fitted to eqn.(2) a standard deviation of 0.0127 cm-' is obtained. For Engelke, Mattera and Surkus type functions we must con- struct a grid for determining their two non-linear parameters. As we have described above it is sufficient to use the leading term of the series for obtaining the optimum non-linear parameters and after that to fit to eqn. (2). Asan example, in Fig. 3 we have shown the contour lines of the standard devi- ation of fits of the ground state of CO by using the leading term of an Engelke's potential [t = (r -re)/re] as a function of the non-linear parameters for 3 < p < 12 and 0.44 6 /3 d 3.83. In this case, the optimum parameters are p = 10.1 and p = 3.1 1, with a fitted linear parameter of do = 101 184.019 cm-'.This minimum corresponds to a standard deviation of 20.71 cm-' and a mean deviation of the residuals of 15.85 cm-'. Thus, we see that with only one term in the power-series expansion it is possible to fit the starting CO potential in a manner which is sufficiently accurate up to an energy of 63334.1 cm-' The results shown in Table 6 were obtained by fitting with these last p and optimum parameters by eqn. (2), varying the number of basis functions. The optimum number of linear parameters for an Engelke's potential as a fitting function of the ground state of CO is at N = 18. The introduction of more linear parameters does not Table 5 Statistical values for the least-squares fit of the X 'E' state of I2C I6O defined between r-(u = 37) = 0.868 176 A and r+(u = 37) = 1.906 655 5 A for different truncated power series expansions interval of standard mean ' function N optimum parameters convergence/A deviationlcm - deviation/cm - Dunham 20 (0, 2.256 646) 0.697 0.477 SPF 20 (0.564 161 5, 00) 0.018 0.012 Ogilvie Thakkar Engelke Mattera Surkus 20 19 18 18 18 p = 1.81 p = 10.1; B = 3.11 p = 10.8; y = 2.47 p = 2.8; p = 0.234 (0, m)(0.769 340 3, 00) (0.820 740 1, 00) (0.821 632 7, 00) (0.800 895 5, 00) 1.38 0.013 0.013 0.012 0.013 0.96 0.008 1 0.008 0 0.008 0 0.008 1 Huffaker 19 a = 2.35 (0.833 366 7, oc) 0.013 0.008 1 re = 1.128 323 3 A.Table 6 Standard mean deviations of least-squares fits to different number of fundamental basis Engelke and Mattera type functions for the X 'Xfstate of CO Engelke (p = 10.1, /I= 3.11) Mattera (p = 10.8, 7 = 2.4) standard mean standard mean N deviation /cm - deviation /cm-' deviation /cm - deviation /cm-' 2 20.7 15.8 30.2 24.7 3 20.6 15.9 30.1 24.6 4 19.7 14.2 19.8 13.0 5 19.8 14.1 18.6 12.7 6 11.9 8.30 12.2 8.5 1 7 9.93 7.33 9.79 7.26 8 5.45 3.86 5.47 3.91 9 4.2 1 3.2 1 4.20 3.19 10 1.99 1.45 2.01 1.45 11 1.39 1.06 1.39 1.06 12 0.60 0.44 0.60 0.45 13 0.39 0.29 0.38 0.29 14 0.15 0.1 1 0.15 0.11 15 0.090 0.066 0.088 0.068 16 0.032 0.023 0.033 0.024 17 0.020 0.014 0.020 0.015 18 0.013 0.0080 0.013 0.0081 19 0.013 0.008 1 0.013 0.0080 20 0.0 13 0.0080 0.013 0.0079 21 0.0 13 0.008 1 0.013 0.0080 32 0.0 13 0.0079 0.013 0.0080 give any significant improvement.For any power-series expansion studied here this last result can be extrapolated. From Table 6 we see that the fits are not improved by increasing the number of basis functions from 2 to 5. However, the linear basis terms with N = 3,4 and 5 must be considered in the fits because their omission produces impor- tant deviations. For example, if the basis terms with N = 2, 6 and 7 are considered, the fit is equivalent in accuracy to that considering only the term with N = 2 with a standard devi- ation of 19.84 cm-' (mean deviation 15.19 cm-l). Similarly other power-series expansions with two non-linear param- eters were studied.In Fig. 4 the contour map of the standard deviation of fits of CO (ground state) by the leading term of a Mattera's potential Vo(r)= do[ 1 -(1 + :)--J as a function of its non-linear parameters 4 d p < 13 and 2.20 d y < 2.50 is shown. By comparing Fig. 4 with Fig. 3, P 4.67 6.00 7.33 8.67 10.00 11.33 12.672.50 2.50 2.47 2.47 2.44 2.44 2.41 2.41 2.38 2.38 y 2.35 2.35 2.32 2.32 2.29 2.29 2.26 2.26 2.23 2.23 720_._-2.20 4.00 5.33 6.67 8.00 9.33 10.67 12.00 P Fig. 4 Contour lines of the standard deviation of fits of the ground state of CO molecule using as fitting function the leading term of a Mattera's potential J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the zone of the minimum is now very well defined with respect to the second parameter y. The standard deviation of the minimum is now 30.2 cm- ' (mean deviation 24.7 cm-') corresponding to the optimum parameters: p = 10.8 and y = 2.47. From Table 6 we can see that both Engelke and Mattera potentials are equivalent for N 2 4 as fitting func- tions. In Fig. 5 we show a three-dimensional representation of the variation of the standard deviation of the fit of the X'C' state of CO by using the leading term of a Surkus potential as a function of its non-linear parameters for 2.70 < p < 2.90 and 0.20 < p < 0.26. The minimum is at 118 cm-' for p = 2.80 and p = 0.234. We see that the leading terms of an Engelke or Mattera potential function are prefer- able to a Surkus's function, although for an intermediate number of basis functions (N zz 8) all these functions are equivalent.We have also considered potential-energy functions defined very near to the dissociation of the ground state of 7LiH 39 and HZ4O molecules. The statistical parameters of the least- squares fits for these potentials are shown in Tables 7 for H, , and 8 for7LiH. In 'general, we see that the accuracy of these fits is worse than for CO, with a standard deviation ca. 0.7 cm-' for H, and cu. 9 cm-' for 7LiH. In the fits we have only considered power-series expansions with an interval of convergence up to infinity except for the Dunham function which as we saw presents serious convergence problems.For H, we see that the fits by different power-series expansions using the same number of parameters are similar. For 'LiH there is little oscillation between the fits; the better fit is obtained for a Thakkar's potential with p = 1.48. The expla- nation of the difference between the fits for H, and 7LiH may be that the 7LiH potential is a mixture of two ionic and cova- lent potential^.^' This mixture of states is most clear when considering the first excited state A 'Z+, 39 with an unusual form of the potential curve and anomalous negative values of the anharmonicity constants wex, and OL,The statistical . parameters of the least-squares fits of this potential are shown in Table 9.The standard deviation is cu. 4.8 cm-' when using 11-13 basis functions. We see that the methods discussed here are also useful for abnormal potential-energy curves such as the A'X+ state of alkali hydrides. Conclusions Eight different power-series expansions are compared both by consideration of convergence properties and by fitting experimental potential-energy curves. The quality of fits r2.90 Fig. 5 Three-dimensional representation of the variation of the standard deviation of fits of the ground state of the CO molecule using as fitting function the leading term of a Surkus's potential J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 7 Statistical values for the least-squares fit of the X 'El state of H, defined between (0.268 50 A, 5.652 15 A), using different truncated power-series expansions function N optimum parameters interval of convergence/A standard deviation/cm -' mean deviation/cm -' Dunham SPF Ogilvie Thakkar Engel ke Mattera Surkus Huffaker 20 20 20 19 18 18 18 19 p = 0.5 p= 1;p=O.9 p=5;y= 1 a=lp=2;p=l (0, 1.483) (0.37075, co) (0,(0.185 375, oc) (0.037 075, co) (0.261 566, 00) (0.048352 5, 00) (0, a) 13 540 0.72 0.76 0.74 0.77 0.63 0.6 1 0.70 8 567 0.49 0.43 0.48 0.45 0.38 0.4 1 0.40 ~~ re = 0.741 499 A.Table 8 Statistical values for the least-squares fit of the X 'Z+ state of 7Li 'H defined in the interval (1.00470 A, 5.204851 A) for different truncated power-series expansions optimum interval of standard mean function N parameters convergence/A deviation/cm -' deviation/cm -' Dunham 20 (0, 3.191 168) 12901 5 672 SPF 20 (0.797 792, m) 11.18 5.76 Oglvie 20 (0, 03) 14.81 7.82 Thakkar 19 p = 1.48 (0.998 897 7, oc) 8.07 4.79 Engelke 18 p=4;p= 1 (1.087 857, 00) 11.21 7.22 Mattera 18 p=4;7=2 (1.087 857, 00) 12.60 7.23 Surkus 18 p = 0.5; p = -0.5 (0.897 515, m) 11.5 7.74 Huffaker 19 a= 1.36 (1.085916, oc) 9.79 5.41 re = 1.595 584 A.Table 9 Statistical values for the least-squares fit of the A 'Z+ state of 7Li 'H defined in the interval (1.4162 A, 4.9538 A) for different truncated power-series expressions optimum interval of standard mean function N parameters convergence/( A) deviation/cm-' deviation/rm -~ Dunham 13 (0, 5.1926) 175.3 139.8 SPF 13 (1.2982, 00) 4.95 3.95 Ogilvie 13 (0, 0O) 5.35 4.27 Thakkar 12 p = 0.85 (1.1487, 00) 4.83 3.95 Engelke 11 p = 2.2; p = 1 (1.1929, oc) 4.72 3.94 Mattera 11 p = 2.3; y = 1.1 (1.1838, oc) 4.72 3.95 Surkus 11 p = 1.7; p = 0.6 (1.074, oc) 4.75 3.97 Huffaker 12 Q = 1.0 (1.9032, 00) 4.79 3.9 1 re = 2.5963 A.which can be achieved for accurately known RKR potentials starting potential with an accuracy of ca. tenths of cm- '.For is shown for 3 range of examples. potentials defined near the dissociation, by using power-series We observed that a good fit with only the leading term of a expansions with an intermediate number of linear param- power-series expansion may be obtained more accurately eters, it is possible to fit the starting potential with an accu- from an Engelke, Mattera or Surkus function with two non- racy ca.tenths of cm-'. For potentials defined up to the linear parameters. For fitting power-series expansions with dissociation, it is possible to fit with an accuracy of a few an intermediate number of fundamental basis functions it is cm-'. better to use a Thakkar or Huffaker type function with only one non-linear parameter because the effort in calculating the second optimum non-linear parameter of an Engelke, References 1 R. Rydberg, 2.Phys., 1932,73,376.Mattera or Surkus function does not compensate the preci- sion of the fits (see Tables 5-9). In conclusion, we feel that the 2 0.Klein, 2. Phys., 1932,76, 226. 3 A. L. G. Rees, Proc. Phys. SOC., London, 1942,59,998.power-series expansions can be used, with sufficient accuracy, 4 W.M. Kosman and J. Hinze, J. Mol. Spectrosc., 1975,56,93.as fitting functions of potential-energy curves whose defini- 5 C. R. Vidal and W. C. Stwalley, J. Chem. Phys., 1982,77,883. tion rank is within the interval of convergence of the series. 6 A. Pardo, J. J. Camacho and J. M. L. Poyato, Chem. Phys., 1986, Moreover, in the zone of short distances it is preferable that 108, 15. the interval of convergence includes the first right point of the 7 N. Froman and P. 0.Froman, JWKB Approximation Contribu-input potential and its lower extreme is in the neighbourhood tion to the Theory, North-Holland, Amsterdam, 1965. of that point. For potentials defined far away from the disso- 8 P. M. Morse, Phys. Rev., 1929,34, 57.9 J. E. Lennard-Jones, Proc. R. SOC.London, A., 1924, 106,463. ciation, by using power-series expansions with an interme- 10 R. Rydberg, 2.Phys., 1933,73,21. diate number of linear parameters, it is possible to fit the 11 N. Rosen and P. M. Morse, Phys. Rev., 1932,42,210. 30 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 12 G. Poschl and E. Teller, 2. Phys., 1933,83, 143. 29 J. N. Huffaker, J. Chem. Phys., 1976, 64, 3175, 4564; 1981, 74, 13 H. Helimann and W. C. Stwalley, J. Chem. Phys., 1982,77,883. 1217. 14 H. M. Hulburt and J. 0. Hirschfelder, J. Chem. Phys., 1941, 9, 30 R. Farrenq, G. Guelachvili, A. J. Sauval, N. Grevesse and G. B. 61. Farmer, J. Mol. Spectrosc., 1991, 149, 375. 15 E. R. Lippincott, J. Chem. Phys., 1953,21,2070; 1955,23,603.31 J. A. Coxon and P. G. Hajigeorgou, Can. J. Phys., 1992,70,40. 16 A. A. Frost and B. Musulim, J. Am. Chem. SOC., 1954,76,2045. 32 H. Telle and U. Telle, J. Mol. Spectrosc., 1981, 85, 248. 17 Y. P. Varshni, Rev. Mod. Phys., 1957,29,664; 1959,31,839. 33 A. W. Mantz, J. K. G. Watson, K. Narhari Rao, D. L. Albritton, 18 J. W. Linnett, Trans. Faraday SOC., 1940,36,1123; 1942,38, 1. A. L. Schmeltekopf and R. N. Zare, J. Mol. Spectrosc., 1971, 39, 19 I. N. Levine, J. Chem. Phys., 1966,45, 827. 180. 20 G. A. Baker and J. L. Gammel, The Padt Approximant in Theo- 34 J. N. Huffaker, J. Mol. Spectrosc., 1977,65, 1. retical Physics, Academic Press, New York, 1970. 35 A. S. Dickinson, J. Mol. Spectrosc., 1972,44, 183. 21 A. Pardo, J. J. Camacho and J. M. L. Poyato, Chem. Phys. Lett., 36 H. E. Fleming and K. Narahari Rao, J. Mol. Spectrosc., 1972, 1986,131,490. 44, 189. 22 J. L. Dunham, Phys. Rev., 1932,41,721. 37 S. M. Kirschner and J. K. G. Watson, J. Mol. Spectrosc., 1973, 23 G. Simons, R. G. Pam and J. M. Finland, J. Chem. Phys., 1973, 47, 234. 59, 3229. 38 C. Daniels and F. S. Wood, Fitting Equations to Data, Wiley, 24 25 J. F. Ogilvie, Proc. R. SOC.London, A, 1981,378, 287. A. J. Thakkar, J. Chem. Phys., 1975,62, 1693. 39 New York, 1980. Y. C. Chan, D. R.Harding, W. C. Stwalley and C. R. Vidal, J. 26 R. Engelke, J. Chem. Phys., 1978,68,3514; 1979,70,3745. Chem. Phys., 1986,85,2436. 27 L. Mattera, C. Salvo, S. Terreni and F. Tommasini, J. Chem. Phys., 1980,72,6815. 40 A. Pardo, J. J. Camacho and J. M. L. Poyato, Specrrochim. Acta, Part A, 1991,47,377. 28 A. A. Surkus, R. J. Rakauskas and A. B. Bolotin, Chem. Phys. 41 R. S. Mulliken, Phys. Reu., 1936,50, 1017. Lett., 1984, 105, 291. Paper 3/03278G; Received 8th June, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000023

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, Wl), 31-37 Kinetic Study of the Reaction between Fe and 0, under Mesospheric Conditions M. Helmer and J. M. C. Plane* School of Environmental Sciences University of East Anglia Norwich, UK NR4 7TJ Detailed observations of the layer of atomic iron in the mesosphere have been made in the last four years. However, there is almost no information about the gas-phase chemistry of neutral iron which is necessary to understand the characteristic features of the layer. In this paper, a study of the reaction Fe + 0,+ FeO + 0, is reported. The reaction was investigated by the pulsed photodissociation at 193.3 nm of ferrocene vapour to produce Fe atoms in an excess of 0, and N, bath gas, followed either by time-resolved laser-induced fluores- cence spectroscopy of atomic Fe at 248.3 nm [Fe(x 5Fg-a 5D4)], or by time-resolved chemiluminescence from the 'orange bands' of FeO at A = 590 f5 nm [Fe0(5A,-X5Ai)].The rate coefficient is given in the Arrhenius form by k(189 < T/K < 359) = (3.44 f0.76) x lo-'' exp[-(1210 f420)J mol-'/RTl cm3 molecule-' s-', where the quoted uncertainties are 2a. This result is compared with the predictions of long-range capture theory, and contrasted with the analogous reactions of other metal atoms with 0,. The title reaction is then demonstrated to be the most rapid oxidation process of atomic iron in the atmosphere between 65 and 100 km. Furthermore, it is shown that this reaction may make a significant contribution to the night-time production of O,(b 'Z:) in the upper atmosphere below 90 km.The layer of atomic Fe in the mesosphere was first observed in 1976 by photometric measurement of the twilight emission at 386.0 nm [Fe(z'DZ-a5D4)].' In the last four years, lidar (laser radar) systems operating on the atomic Fe transition at 372.0 nm [Fe(z 'Fg-a 'D4)] have revealed the characteristics of this layer in Compared with the much more studied Na layer, the column abundance of the iron layer is larger by a factor of ca. 2. This was unexpected because the major source of both metals is believed to be meteoric abla- ti~n,~and the relative abundance of Fe to Na in meteoritic minerals is ca. 8.6 The peak height of the iron layer is also ca. 5 km lower at 85-88 km, the layer is ca.25% narrower, and it has a smaller scale-height on the underside, of only about 1 km. In order to understand these differences between Fe and Na in the upper atmosphere, an obvious starting point is to contrast the pertinent gas-phase chemistries of these metals. Laboratory studies over the past decade have revealed a sub- stantial amount about the chemistry of sodium species, enabling models to be developed which are now in very good agreement with the observed features of the Na layer.'.' However, with the exception of an upper limit for the rate constant of the reaction between Fe and 0, at 300 K,* there appear to have been no studies of the likely atmospheric reactions of iron species at low temperatures. Thus, we will report here an investigation of the reaction Fe + 0, -+ FeO + 0, (1) followed by a study of the recombination reaction Fe+O,+N,+FeO,+N, (2) in a later paper.g By analogy with the chemistry of Na,5 reac- tion (1) is likely to be the most rapid reaction undergone by freshly ablated Fe atoms in the mesosphere.FeO(X5A4) has a large bond energy of 402 f8 kJ mol-'. lo Thus, reaction (1) is sufficiently exothermic [AH;(l) = -301 f8 kJ mol-'1 to populate several excited electronic states of FeO. In fact, emission from the infrared, orange and blue systems of FeO has been observed in Fe-0, diffusion flames.' ' West and Broida' ' determined a photon yield for the orange bands (500-700 nm) of ca. 2% in a diffu- sion flame at 700 K, and at a total pressure of 1.2 Torr. These workers' ' assigned a system of Fe0(5Z+/51-I-X 'X+) tran-sitions to account for the orange bands.However, subsequent work employing laser photoelectron spectrometry of FeO-, and laser-induced fluorescence studies of the orange system combined with observation of the infrared emission from an FeO discharge' have demonstrated that the ground state of FeO is in fact a 'A state. The nature of the excited electronic states associated with the orange bands is not as well established.14 These states appear to comprise a severely perturbed 'A state," and an extensive number of additional states, probably arising from a 'C+ state.16 West and Broida" also determined that the radiative lifetimes of the excited electronic state(s) giving rise to the orange system are 450 f100 ns.Thus, time-resolved molecular chemilumin- escence should be a suitable tool for studying the kinetics of reaction (1). In the present paper we will report the determination of k,(T) over a significant temperature range, including tem- peratures applicable to the upper atmosphere, so that the atmospheric importance of reaction (1) can be examined. This study will also provide a test of several theoretical formalisms for calculating the rate coefficients of fast reactions governed by long-range attractive forces between neutral species. Experimental Reaction (1) was investigated by the pulsed photolysis offer- rocene [dicyclopentadienyl iron, Fe(C,H,),] to produce atomic Fe in an excess of 0, and N, bath gas.The reaction was then monitored either by time-resolved laser-induced fluorescence (LIF) spectroscopy of the atomic Fe, or by time- resolved chemiluminescence from the product FeO. Fig. 1 is a schematic diagram of the experimental system employed. The reaction was initiated and then monitored spectroscopically in the central cylindrical chamber of the stainless-steel reactor, which has been described in detail elsewhere." Four horizontal side-arms attached to this chamber provided the optical coupling for the lasers into and out of the central chamber. These side-arms also carried the flows of ferrocene and 0,, diluted in N,, into the chamber, and served as an exit for the gas flows to the vacuum pump.A fifth, vertical, side-arm provided the optical coupling to the photomultiplier tube which monitored the LIF or chemiluminescence signals. I ins I laser HV power Fig. 1 Block diagram of the pulsed laser photolysis system with time-resolved monitoring of the reaction by laser-induced fluores- cence. Components are identified as follows: B, baffle to exclude scat- tered laser light ; BBO, /3-barium borate frequency-doubling crystal; E, Suprasil end-window at the Brewster angle; F, furnace around the central chamber of the reactor; f,, flow of pure N, (5 sccm); f2, flow of 0, and N, (195 scan); H, low-pressure mercury lamp; L,, Supra-sil lens, (f=50 cm);L,, Suprasil lens (f=30 cm); L, = Suprasil lens (f=10 cm);L,, Suprasil lens (f=30 cm);L,, Suprasil lens (f=5 cm);MC, monochromator; P, photomultiplier tube; T, thermocouple in the central chamber of the reactor. Powdered ferrocene was placed in a glass trap which was usually immersed in an ice-bath.The vapour pressure offer- rocene at 273 K is 1.6 x lo', molecules cm-,. '' This vapour was entrained in a flow of N, and carried into the central chamber, where it mixed with the flow of 0, in N, .It was therefore diluted to a concentration of ca. 4.0 x 10" molecules ern-,, before being photolysed with an ArF excimer laser at 193.3 nm (Questek, Model 2110, pulse energy 5 mJ, pulse rate 4 Hz). The excimer beam was shaped by a system of lenses and a pin-hole, and then loosely focused through the chamber. For the time-resolved LIF experiments, the resulting Fe atoms were probed at 248.3 nm [Fe(x5Fg-a5D,)] using a nitrogen-pumped dye laser (Laser Photonics, Models LN1000/LN107, laser dye Coumarin 500, pulse energy = 240 CLJ; bandwidth = 0.03 nm), frequency-doubled with a BBO crystal (Inrad Corp.) to give a pulse energy at 248.3 nm of ca.10 pJ. The diameter of the dye laser was set to be ca. 80% of that of the excimer, and the two lasers crossed orthogonally in the centre of the chamber. The time-resolved LIF signal was measured by a photomultiplier tube (Thorn EM1 Gencom Inc., Model 9816QB) after passing through an inter- ference filter centred at 250 nm (Microcoatings, Model ML3- 250, f.w.h.m. = 10 nm), and was then recorded using a gated integrator (Stanford Research Systems, Model SR250) inter- faced to a microcomputer. For the time-resolved chemiluminescence experiments, the dye laser was switched off and the gated integrator replaced by a fast photon-counting system comprising an emitter-follower (Thorn EMI, Model Aped-11) and a multi-channel J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 scaler (EG&G Ortec, Model ACE-MCS) interfaced to a microcomputer. The chemiluminescence signal from the orange bands [FeO('Ai-X 'Ai)] was then measured by time- resolved photon counting after passing through an inter-ference filter at 2 = 590 & 5 nm (Oriel Corp.). Reaction (1) was studied at low total pressures (5-18 Torr) in order to minimise the turnover time of the gas mixture in the central chamber (0.5-1.6 s), thus preventing excessive loss of ferrocene vapour and 0, on the chamber walls.The 0, concentration was monitored down-stream of the reactor by optical absorption of the 253.7 nm line from a low-pressure mercury lamp. This was measured in a 1 m long absorption cell with a 0.9 nm resolution monochromator (Optometrics Corp., Model MC1-02) coupled to a photomultiplier tube. The ozone absorption cross-section at 298 K was taken to be 1.143 x cm' from an average of several recent determi- nations." Although ferrocene absorbs relatively strongly at 254 nm,20 the gas-phase concentrations of ferrocene employed were too small to compete with the absorption due to 0,. Iron pentacarbonyl, Fe(CO), , was employed initially as a photolytic precursor of atomic Fe.However, the higher concentrations of this precursor that were required to produce a satisfactory yield of Fe atoms interfered with the 0, absorption measurements. When high levels of either Fe(CO), or ferrocene were added to the reactor, there was evidence from the absorption measurements that a reaction occurred with 0,, although this appeared to be limited by the short turnover times of the gas mixture in the reactor and was much less rapid in the case of ferrocene. The rapid reac- tion between Fe(CO), and some gas-phase oxidants has been noted previously.' Materials N,, 99.9999% purity (Air Products) was used without further purification. 0, was made by passing 0, , 99.999% pure (Air Products), through a commercial ozoniser (Clearwater Tech, model M-1500).The resulting 5-8% 0,-0, mixture was col- lected on silica gel at 156 K, and the 0, then pumped off at 195 K until a >50% 0,-0, mixture was degassed from the gel. Ferrocene (Aldrich, 98%) was purified by pumping on the sample in the glass trap at 77 K for several hours prior to kinetic experiments. Results The dissociation energy of ferrocene to produce atomic Fe(a 'D,) is 749.7 kJ mol- ',21 so that the photolysis of fer-rocene to yield atomic Fe requires at least two photons at 193.3 nm. In fact, the absorption of three or four photons at 193.3 nm has been shown to yield predominantly ions such as Fe(C,H,)+ and Fe'. 21 We investigated the yield of Fe atoms, measured by LIF at 248.3 nm [Fe(x 'F;-a 'D,)], as a function of the photolysis laser pulse energy at 193.3 nm.The laser fluence was varied over the range (7-42) x 10'' photons cm-' pulse-', where the pulse width is ca. 7 ns. The atomic Fe yield was observed to be only slightly dependent on the laser fluence over this range. This probably indicates that the absorption of the first two photons is nearly saturated at these laser fluences. The relative atomic Fe yield may also be reduced by the increasing yield of ionic over neutral photo- lysis products.21 Since the two-photon photolysis of ferrocene to give atomic Fe leaves the products with 488 kJ mol-' of excess energy,21 a significant fraction of the atomic Fe was probably produced in excited states, by analogy with the photolysis of Fe(CO), .22723 However, the low-lying excited states of Fe, and even the higher spin-orbit multiplets of the ground-state J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Fe(a5Di), are quenched rather rapidly by N2.22,23 For example, at the lowest bath-gas pressure employed in the present study (5 Torr), excited states such as Fe(a 5F,, a ,F, and a 5P3)have a lifetime, with respect to quenching by N,, of less than 1 Also, the ground-state multiplet Fe(a 5D,) has a lifetime of 7 ps with respect to collisional quenching to Fe(a 5D,),22,23so that a Boltzmann equilibrium between the ground-state multiplets would have been established rapidly,, on the timescale of the chemical reaction between Fe and 0, (see below).Thus, the reactions of the individual multiplets could not be observed independently in this system. The separation of the i = 3 and 4 levels of Fe(a 5Di) is only 416 cm-',whereas the i = 2, 1 and 0 multiplets are 704, 888 and 978 cm-above the lowest i = 4level, re~pectively:~~ Hence, over the temperature range of 190 to 357 K at which k,(T) was determined, the Boltzmann fraction of Fe(a 'Di) in i = 3 varied from 3.2 to 12.7'/0, with essentially the rest in i = 4. The excimer laser energy was limited in order to keep the photolysis of 0, to less than l%." One reason for this is because the reaction FeO + 0 -+ Fe + 0, (3) probably proceeds at close to the collision number. This can be seen by consideration of the reverse reaction, i.e.Fe + 0, , which has been studied at temperatures above lo00 K. Fontijn and Kurziu~,~ obtained k-, (1600 K) = (3.6 f1.4) x 10-l3 cm' molecule-' s-', in very good accord with the Arrhenius expression k-, (lo00 < T/ K < 2500) = 2.1 x 10-lo exp[ -(84.6 f10.2 kJ mol-')/RT] .~~cm' molecule- ' s-' from Akhmadov et ~1 It appears that this activation energy must be nearly equal to AH:( -3), since this would imply that D,(Fe-0) = 408 f10 kJ mol-' which is very close to the current literature value" of 402 f8 kJ mol-' for the bond energy of FeO. If we assume that these energies are indeed equal, and take the molecular parameters for FeO from ref. 13, then detailed balancing of reaction (3) indicates that k, (300 K) = 1.8 x lo-'' cm' molecule-' s-'.Thus, if the laser photolysis of 0, in the present experiments had been significant so that the resulting ratio of [O] : [O,] was large enough, reaction (3) would probably have caused significant regeneration of atomic Fe and k, would then have been underestimated. Another source of error in studies of the reactions between metal atoms and 0, can arise from the recycling of the metal oxide by a reaction such as (4b): FeO + 0,4 FeO, + 0, (44 -+ Fe + 20, (46) The analogous reaction of NaO does indeed give rise to a significant artifact when studying the reaction between Na and 0,. 27p29 However, in the case of FeO reaction (46) appears to be endothermic, AH:(46) = 11 f8 kJ mol-', lo and so this reaction should not be fast enough at the low temperatures of the present study to affect the measurement of k,.It should be noted that if we take our recent ab initio estimate for D,(Fe-0,) x 180 kJ mol-', then AHE(4a) -170 kJ mol- ' and so this channel of reaction (4) is prob- ably the major removal process for FeO in these experiments. Under the conditions of the present study, where the con- centration of 0, was always well in excess of the concentra- tion of Fe atoms resulting from the pulsed photolysis of ferrocene vapour, the loss of Fe atoms should be described by the pseudo-first-order decay coefficient, k', where The term kdiff,Fedescribes diffusion of the Fe atoms out of the volume defined by the dye laser beam and within the field of view of the photomultiplier tube.17 The third term accounts for the loss of Fe by reaction (2).In fact, the ratio of [O,] : [O,] in the central chamber varied from 1 : 1 at reactor temperatures below 320 K, down to 1 :2.5 at the highest temperature of 357 K. We have recently shown' that k,(T)is very slow at these low temperatures, so that this term contributed less than ca. 0.005% to k' over the range of bath-gas pressures employed in the present study, and could there- fore be neglected. The observed decays of the LIF signal were of a simple exponential form and were well fitted to the expression A exp(-k't), as shown in Fig. 2(a). In the experiments where chemiluminescence was moni- tored, the emission from excited Fe0(5Ai) not only provides evidence of the reaction product, but can also be used as a time-resolved spectroscopic marker of the atomic Fe concen- tration.This is because the 0, concentration is in a large excess over the Fe, so that the intensity of the chemilumine- scence is only proportional to the Fe atom concentration. In addition, the radiative lifetime of the orange bands of FeO is 450 & 100 ns," which is much faster than the chemical time- constants. A typical decay of the chemiluminescence signal, I I I I 50 100 150 200 time/ps 0 0 100 200 300 400 500 ti me/p Fig. 2 (a) Time-resolved profile of the laser-induced fluorescence signal from atomic Fe at 248.3 nm [Fe(x 'F2-a 'D4)], following the pulsed photolysis at 193.3 nm of ferrocene vapour in the presence of 0, and N,; [O,] = 1.26 x 1014 molecule an-,,m2]= 2.60 x 10'' molecule cm-,, T = 192 K, excimer laser fluence at 193.3 nm is ca.1 x 1OI6 photons cm-2 pulse-'. The solid line is a fit to the form A exp( -k't). (b)Time-resolved profile of chemiluminescence from the orange bands of FeO at II = 590 f5 nm [FeO('Ai-X 'Ai)]; [O,] =J =1.27 x 1014 molecule cm-,, p2]2.84 x 10'' molecule cm-, T = 196 K. The solid line is a fit to the form A exp(-k't) + B. including a fit to the form A exp(-k') + B, is illustrated in Fig. 2(6). Note that both the decays in Fig. 2 were measured at the lowest temperatures of the present study, where the signal-to-noise tended to be somewhat reduced because of the significant loss of ferrocene vapour to the cold chamber walls.It should be noted that if k,, were small, and the degree of photolysis of 0, were large enough to make reaction (3) sig-nificant, then the Fe atom concentration would have reached a steady state after an initially rapid decay, and this steady- state concentration would have slowly decreased at longer times owing to diffusion out of the volume defined by the excimer laser. None of the time-resolved decays in the present study, such as those shown in Fig. 2, exhibited evidence of such a steady state, and this is further evidence that recycling of FeO to Fe was not significant. Plots of k' versus [O,] are illustrated in Fig. 3 for a selec- tion of the temperatures at which reaction (1) was studied. Two sets of plots, one derived from time-resolved LIF and the other from time-resolved chemiluminescence measure- ments, are illustrated in Fig.3(a) and (6). Both sets of plots exhibit a clear linear dependence of k' on [O,]. The inter- cepts of these plots yield values of kdiff,Fewhich are in sens- ible accord17 with the diffusional loss of Fe atoms from the volume of the excimer laser beam. The slopes of these plots thus yield k, as a function of temperature, and the results are listed in Table 1 with the pressure at which each measure- ment of k,(T) was made. The upper temperature limit at which reaction (1) could be studied was constrained by the 0.0 0.5 1.o 1.5 2.0 2.5 [03]/10'4molecules c~r-~ 4, // 0.0 0.5 1.o 1.5 2.0 [O,]/l 014 molecules ~ m -~ Fig.3 Selected plots of k' against [O,] over the experimental tem- perature range for the reaction Fe + 0,-,FeO + 0,: (a) time-resolved LIF measurements of atomic Fe at 248.3 nm [Fe(x 5F:-a 5D4)]; V, 192, V,293, 0, 358 K. (b) Time-327 and .,resolved molecular chemiluminescence measurements of the orange bands of FeO at 1 = 590 +_ 5 nm [Fe0(5Ai-X 5Ai)]; 0,196, a, 293 and 0,357 K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Experimental determination of kl(Fe + 0, + FeO + 0,) as a function of temperature T/K kl/lO-lo cm3 molecule-' s-technique" pressure/Torr 190 1.58 & 0.16 L 14.9 192 1.64 f0.16 L 5.2 196 1.71 f0.04 C 5.8 218 1.55 f 0.06 C 12.7 238 1.89 f0.02 C 9.5 262 2.35 f0.10 L 11.5 263 1.82 f0.10 C 5.5 27 1 2.20 f 0.16 C 10.5 326 2.35 f 0.28 L 17.2 293 1.92 f0.12 L 16.1 293 1.85 f0.06 C 16.5 293 2.00 f0.16 C 10.5 3 20 2.16 f0.18 C 7.0 327 2.12 f0.22 L 13.8 328 2.13 f0.04 C 13.8 357 2.24 k 0.08 C 12.5 358 2.52 f0.24 L 12.3 Quoted uncertainty is 2a.L = laser-induced fluorescence of atomic Fe at 248.3 nm [Fe(x 'F;-a 5D4)];C = chemiluminescence from the orange bands of FeO at i= 590 ? 5 nm [FeO('Aj-X 5Ai)]. decomposition of 0,in the central chamber becoming too rapid, compared with the residence time of the gas mixture in the reactor. The lower limit was set by the use of solid CO, as a refrigerant. An Arrhenius plot for reaction (1) is shown in Fig. 4. A fit of the data to the Arrhenius form, by means of a linear regression, yields k,(189 < T/K < 359) = (3.44 0.76) x x exp[-(1210 & 420) J mol-'/RT] cm3 molecule-' s-' (11) where the uncertainties are at the 2a level.The small tem- perature dependence of k, may be emphasised by employing the common alternative expression k1(189< T/K < 359) = (2.11 & 0.43) 10-10(T/298 ~)(0.56*0.20) cm3 molecule-' s-' (11) We consider that systematic errors, particularly those associ- ated with determining the concentration of 0,, are contained T/K 0 300 250 200 -_--__---__-I 2.5 3.0 3.5 4.0 4.5 5.0 5 5 lo3 KIT Fig. 4 Arrhenius plots for the reaction Fe + 0, FeO + 0,,com- paring the results from the present experimental study with (. . .) the predictions of the harpoon mechanism (ref.33) and (---) the modi- fied harpoon mechanism (ref. 37). The solid line through the experi- mental points is a linear regression fit. A, Chemiluminescence, FeO('Ai-X 5Aj); 0,LIF, Fe(x 5F:-a 'D,). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 within the quoted limits. It is clear from inspection of Fig. 4 (or Table 1) that the values of k,(T) obtained by time-resolved LIF and time-resolved chemiluminescence are in excellent agreement with each other. Discussion Reaction (1) has thus been studied both by monitoring the disappearance of Fe atoms, and by observing chemilumine- scence from the product FeO. Although the chemilumine- scence from FeO(’Ai-X5Ai) confirms that FeO is formed by reaction (l), the diffusion flame study of West and Broida’ ’ found that production of excited FeO (in emitting states) only accounted for ca.2% of the consumption of Fe. The remain- ing products could have been dark excited states of FeO, or ground-state FeO(X ’Ai). In addition, there are two other possible reaction channels: Fe + 0, FeO, + 0 (5)-P Fe + 0, + N, +FeO, + N, (6) We have recently estimated that the Fe0,(7A,) ground state has a bond energy D,(Fe-0,) of ca. 180 kJ mol-’. Thus, reaction (5) is exothermic by only ca. 80 kJ mol- ’,consider-ably less than AHg(1) = -301 & 8 kJ mol-’ for production of FeO(X 5Ai).10 Furthermore, it is known that the products of the reactions Na + 0, and Ca + 0, 30*31 are the metal monoxides, rather than the superoxides.Hence, we assume that reaction (5) is a minor removal process for atomic Fe compared with reaction (1). Reaction (6) would require a recombination rate coefficient greater than cm6 molecule-2 s-’, and would also have to be at the high- pressure limit, in order to contribute significantly to the removal of atomic Fe in this study. These conditions seem highly unlikely to be the case. In any event, inspection of Table 1 reveals no evidence of a pressure dependence in kl(T).Eqn. (111) demonstrates that reaction (1) proceeds essen- tially at the collision number, with a very small T depen-dence. In Fig. 5, the Arrhenius plots of reaction (1) and the analogous reactions of Na and Ca are compared. The three reactions have very similar T dependences and the rate coef- ficients decrease in the order Na > Ca > Fe.This is in the reverse order of the ionization energies of these metal atoms, in accord with the original proposal by Kolb and Elginj’ that the reaction Na + 0, proceeds via the harpoon or electron-jump mechanism. The rate coefficient for the simple 4 10 c I ‘“7 r I Q,55 Q, 0 ,E ‘35 0 r I z2 1 Fig. 5 Arrhenius plots A, for the reaction Fe + 0, -+ FeO + 0, from the present study, and the analogous reactions of 0,Na (ref. 27) and A, Ca (ref. 30). The solid lines illustrate linear regression fits through each data set. 35 harpoon mechanism is given by3, k(T)= &(8k, T/~P)’/~ (IV) where p is the reduced mass of the reactants, k, is the Boltz- mann constant, and R, is the charge-transfer distance deter- mined from the simple relationship,, R, = e2/[4m,(Ei -EJ] (V) where e is the electronic charge, Ei is the ionisation energy of atomic Fe, and E,, is the electron affinity of 0,.If we adopt the adiabatic electron affinity of 0, (since this has been accu- rately measured34) and the ionisation energy of Fe34*35 that are both listed in Table 2, then R, = 2.49 A.This value of R, is probably an upper limit since the (smaller) vertical electron affinity should really be applied in this calculation. This is because the charge transfer occurs on a much shorter time- scale than the nuclear motions of the approaching reac- tan ts. 36 It should also be noted, however, that atomic Fe has a moderately large polarizablity (Table 2) and so there is a sig- nificant attractive dispersion force between Fe and 0, at longer range than R,.In this case, the maximum impact parameter may be largely determined by the orbiting cri- terion on the effective potential surface,33 and the modified harpoon mechanism of Gi~lason,~ is then more appropriate. The long-range interaction is governed by the C,/R6 poten-tia1,38 where c -cd,isp + Ctd (VI)6-The London formula may be conveniently used to estimate the dispersion coefficient,38 where a and Ei are the polarizability and ionisation energy of each collision partner. Using the data listed in Table 2 yields CdsiSp= 3.1 x lo-’’ J A6 molecule-’. The dipoleinduced dipole coefficient,, Ctd = (p1)2a2/(4~~o)2 (VIII) is only 2.4 x lo-’’ J molecule-’ because of the small dipole moment, ,ul, of 0,,and thus hardly contributes to the overall c6 coefficient.In the modified harpoon mechanism,,’ if 2C6/R,6 > k, T [which is the case for reaction (l)], then the rate coefficient is essentially given by the orbiting criterion:,, k( T) = n(2C,/k, T)”,(8k, T/np)’/’r(2/3) (Ix) Eqn. (IX) should actually determine the upper limit to the rate coefficient, since it assumes that every collision that sur- mounts the centrifugal barrier on the effective potential will then orbit until an electron transfer occurs at closer range, thereby ensuring a successful reaction. The rate coefficients predicted by eqn. (IV) and (IX) for reaction (1) are compared with the experimental results in Fig.4. This shows that the modified harpoon mechanism [eqn. (IX)] does indeed provide an upper limit to the rate constant, overpredicting k,(T) by about a factor of 2. Although the T”2 dependence of the simple harpoon model Table 2 Molecular parameters for the harpoon model calculations ~~ parameter Fe 03 -0.53‘ 8.4’ 3.2’ 7.87‘ 12.4‘ -2.1OC 1 D (Debye) x 3.33564 x lop3’ C m. Ref. 35. Ref. 34. 36 [eqn. (IV)] is in good agreement with the experimental result of ~(0.56f0.20) [eqn. (111)], this model underpredicts the experimental rate coefficient by about a factor of 2. The likely reason for this is that R, is underestimated. The calculation of R, from eqn. (V) assumes that the charge transfer occurs at a sufficiently large distance that the reactants and resulting ions can be approximated by point charges. However, the ionization energy of atomic Fe is relatively large, so that the electron jump at R, = 2.49 8, is really a close-range charge transfer to form the Fe+...O, i~n-pair.~~ The 0; then pre- sumably undergoes field-induced dissociation to 0-and 0, in the very strong electric field of the Fe+ ion.39 The physical size of the 0, , in particular, and the close-range orbital inter- actions between the reactants, indicate that the use of eqn.(V) to estimate the cross-section is unrealistic. In fact, we have shown previo~sly~~ that the electron transferred from the metal atom to the 0, most probably goes into a b, orbital where most of the electron density is on the terminal oxygen atoms.This implies that during a collision the interaction between the Fe atom and either of the terminal oxygens should allow the charge transfer to take place, thus enhanc- ing the total reaction cross-section. Atmospheric Implications The importance of reaction (1) in the upper atmosphere can most easily be explored by comparing the first-order rates of loss of atomic Fe by this reaction and three other pathways. These are recombination with 0, [reaction (2)], and charge exchange with the most abundant positive ions in the meso- sphere,' *40 Fe + NO' -+ Fe+ + NO (7) Fe + 0; +Fe+ + 0, (8) The rates of reactions (l), (2), (7) and (8) are plotted in Fig.6 as a function of altitude between 65 and 110 km, for night- time conditions. In the daytime, the rates of reactions (7) and (8) will increase by about a factor of 10 because of the enhancement in the concentrations of NO+ and 0; due to photo-ioni~ation.~,~~Indeed, atomic Fe itself will be photo- ionised in the mesosphere with a coefficient estimated by Swider4' to be ca. 5 x lop7s-'. Inspection of Fig. 6 indi-100--E5 90---0 4-.-c 80-\ \ \ 70 log(first-order loss of Fe/s -) Fig. 6 Vertical profile in the earth's atmosphere between 65 and 110 km, depicting the first-order rates of loss of Fe atoms due to reaction with (-) 0, (this study), (---) 0, (ref. 9), and charge exchange with (----) 0; and (.. . .) NO+ (ref. 40).The atmospheric concentra- tions of the trace species 0,,NO+ and 0; are taken from a recent model (ref. 5 and 7), and the dependence of the pressure and tem- perature with altitude are from ref. 42. The conditions are for Decem- ber, 40" N. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 cates that this is not a competitive process. Reaction (1) is clearly the most rapid process for removal of atomic Fe between 65 and 105 km, i.e. in the mesosphere and lower thermosphere. However, Fig. 6 also shows that reaction (1) is most competitive compared to these other processes at ca. 87 km, which is where the peak of the Fe layer tends to We have demonstrated above that reaction (3) probably proceeds at close to the collision number, and this reaction recycles FeO back to Fe.Thus, to a first approx- imation the appearance of atomic Fe in the upper atmo-sphere should be controlled by the ratio of 0 to 0,. Indeed, at the peak of the Fe layer the ratio [O] : [O,] is more than 500 : 1,43 and the rate of reaction (1) is more than 10' times faster than the other removal processes in Fig. 6. However, even though the [O] :[O,] ratio is even larger at higher alti- tudes, the formation of Fe+ by reactions (7) and (8) becomes much more competitive as a removal process for atomic Fe compared with reaction (1). Indeed, between 87 and 100 km the relative rates of these respective processes change from ca. 2 x lo-' to and so formation of Fe+ most probably controls the topside of the layer.The underside of the Fe layer often exhibits a sharp gradient (or small negative scale- height) just below 80 km,,v4 which is where the atomic 0 concentration also decreases markedly so that at 75 km the ratio [O] : [O,] is less than : l.,, Thus, more than 99% of atomic Fe will be converted to FeO below 80 km. Of course, the atmospheric chemistry of iron is almost certainly more complex. For instance, by analogy with our recent model of the mesospheric sodium layer,7 it is likely that FeO then recombines with trace atmospheric species such as H20 and CO, to form the more stable iron-containing molecules Fe(OH), and FeCO, . Finally, we consider the possible role of reaction (1) as a source of excited 0, in the upper atmosphere at night.If FeO is formed in the ground state (X'AJ, or in one of its low-lying quintuplet and the spin multiplicity of reaction (1)is preserved, then excited singlet 0, will be produced: Fe('DJ + 03('Ai) -,FeO(X 'AJ + O,(a 'Ag or b 'Cg) (la) There are two ways in which the formation of singlet 0, could be avoided. The first would be the production of a state of FeO of different spin multiplicity, since this might allow formation of O,(X 'Xi) as an adiabatic product. However, although there is some the~retical~~ and experimental4' evi- dence for a low-lying 7C+state of FeO, there does not appear to be evidence that FeO(7C+) is formed in reaction (l),and so we assume in the following discussion that it is a minor product.Secondly, the reaction could be significantly non- adiabatic. However, although reactions between metal atoms and oxidants such as N,O sometimes involve non-adiabatic transitions with respect to spin, these transitions tend to have a low probability and do not form the major reaction chan- nel~.~~We assume that this is also the case here. The peak atmospheric concentration of atomic Fe, at a height of ca. 87 km, is typically 1.5 x lo4 atom cm-,. ,s4 Taking k1(200 K) = 1.7 x lo-'' cm3 molecule-' s-' from eqn. (111), and [O,] = 5 x lo8 ern-,, 43 then the upper limit to the production of singlet 0, is ca. 2 x lo3 molecule cm-, s-'. Emission from 02(a 'Ag -+ X 'Z;) gives rise to the infra- red atmospheric bands at CQ. 1270 nm.40947*48At 87 km, the volume emission from these bands at night-time is typically 4 x lo4 photons cmP3 s-l, 48 which imp lie^^'.^^ that the concentration of O,(a 'Ag) is ca.2 x lo8 molecule cm-,. Since spontaneous emission and physical quenching appear to be the dominant removal processes of 02(a1Ag) in the upper atm~sphere,~~.~~ it then follows that the rate of night- time production of O2(a'Ag) is ca. 5 x lo4 molecule cm-, s-at 87 km. Thus, if reaction (la) produces O,(a 'Ag) exclu- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 37 sively, this would account for only up to 4% of the total pro- duction, and does not appear to be significant. A similar calculation can be performed for emission from 02(b'Z, +X 3C-), which produces the atmospheric bands at 761.9 nm.40*47*49 The night-time emission of these bands is ca.500 photons cm-3 s-' at 87 km,48 which implies a night- 19 20 W. B. DeMore, S. P. Sander, M. J. Molina, R. F. Hampson, M. J. Kurylo, C. J. Howard and A. R. Ravishankara, Chemical Kinetics and Photochemical Data for use in Stratospheric Model- ing, JPL Publication 90-1, Jet Propulsion Laboratory, Pasa- dena, 1990. L. Kaplan, W. L. Kester and J. J. Katz, J. Am. Chem. SOC., 1952, 74, 5531. time 02(b 'Z,) concentration of ca. 6000 molecule cm-3, and a production rate of 2000 molecule cmP3 s-'. Thus, if reac-tion (la) only produces O2(b1Zg), this would account for a major fraction of the 0, (b 'C,) formed at this altitude. Indeed, profiles of the 0, (b'Z,+X3Zg-) nightglow emis- sion, made by rocket-borne photometers, often reveal a small 21 22 23 24 J.Optiz, D. Bruch and J. von Bunau, Org. Mass Spectrom., 1993, 28, 405. A. B. Callear and R. J. Oldman, Trans. Faraday SOC., 1967, 63, 2888. S. A. Mitchell and P. A. Hackett, J. Chem. Phys., 1990,93, 7813. J. R. Fuhr, G. A. Martin, W. L. Wiese and S. M. Younger, J. Phys. Chem. Re$ Data, 1981,10, 305. secondary peak between 85 and 90 km that is distinct from the primary emission peak at ca. 95 km.48 The calculations presented here, therefore, indicate that reactions between metal atoms of meteoric origin and 0, may be a source of O,(b 'C,) that is significant in the night-time mesosphere below 90 km. However, it should be stressed that this possi- 25 26 27 28 A. Fontijn and S. C. Kurzius, Chem.Phys. Lett., 1972, 13, 507. U. S. Akhmadov, I. S. Zaslonko and V. N. Smirnov, Kinet. Catal., 1988,29, 251. J. M. C. Plane, C.-F. Nien, M. R. Allen and M. Helmer, J. Phys. Chem., 1993,97,4459. J. W. Ager 111, C. L. Talcott and C. J. Howard, J. Chem. Phys., 1986,85, 5584. bility requires that the yield of 0, (b 'C,) from these types of reactions is high. Further laboratory experiments will be required to determine this. 29 30 D. R. Worsnop, M. S. Zahniser and C. E. Kolb, J. Phys. Chem., 1991,95,3960. H. Helmer, J. M. C. Plane and M. R. Allen, J. Chem. SOC., Faraday Trans., 1993,89,763. We thank the SERC for a research award, the NERC and the Royal Society for equipment grants, and the School of 31 32 B. S. Ault, and L. Andrews, J. Chem. Phys., 1975, 62, 2312; L.Andrews and B. S. Ault, J. Mol. Spectrosc., 1977, 68, 114. C. E. Kolb and J. B. Elgin, Nature (London), 1976,263,488. 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ISSN:0956-5000    

DOI:10.1039/FT9949000031

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(1), 39-45 Ab inifio Studies of Electronic Structures and Quasi-aromaticity in M,S,-.O;+ (M = Mo, W; n = 0-4) Clusters Jun Li,* Chun-Wan Liu and Jia-Xi Lu Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences and the State Key Laboratory of Structural Chemistry of China, Fuzhou, Fujian 350002, China Electronic structure calculations have been carried out on the incomplete cubane-type transition-metal cluster cores M, S4-"O;+ (M = Mo, W; n = 0-4,of which the cluster core Mo,S:+ has been previously shown to exhibit what has been referred to as quasi-aromaticity. All the clusters are theoretically viewed to exhibit so-called quasi-aromaticity, i.e. having benzene-like behaviour. The calculations were accomplished at the level of Hartree-Fock self-consistent field theory by using an ab initio quantum chemical method with relativistic effec- tive core potentials.The nature of the quasi-aromaticity is elucidated by analysing the calculated canonical molecular orbitals and localised molecular orbitals by the Foster-Boys localisation technique. On the basis of canonical molecular orbital and localised molecular orbital calculations, as well as Mulliken population analysis and natural population analysis, the d-px bonding scheme in these clusters is established. The concept of aromaticity, extended strikingly in the last those with water ligands, have been synthesized, though those two decades,'-, has become one of the most fundamental of the types (V) and (VII) with capping oxygen atoms are concepts in modern chemistry. In 1987, the new concept of relatively rare.The X-ray structure determinations, particu- quasi-aromaticity of some Mo,S:+ cluster compounds, larly those by the Cotton and Shibahara groups, and the Mo,(p,-S)(p-S),(~-dtp),(p-dtp).L (Fig. l), with the incom- kinetic studies by Sykes and co-workers have provided plete cubane-type cluster core M,(p,-S)(p-S);+, was pro-invaluable information in characterizing such complexes.9*' posed in our lab~ratory~'~ the basis of a series of Some interesting reviews of relevant structures have recently on analogies of the cluster cores with benzene in aspects of struc- appeared.' '-" This offers us a complete series of trinuclear ture, reactivity, as well as magnetic and spectroscopic proper- molybdenum and tungsten cluster compounds with oxygen ties.The quasi-aromaticity discussed here refers to and/or sulfur ligands and made it possible to ascertain the benzene-like behaviour with regard to the chemical reacti- effects of ligand substitution on electronic structure and vities and molecular structures of the cluster compounds. quasi-aromaticity. Experimental evidence on quasi-aromaticity of this kind of The bonding and electronic structures of these clusters, cluster has been summarized in two reviews6 and preliminary especially the metal-metal (i.e. M-M) bonding of some clus- quantum chemical studies on some six-membered ring mol- ters of this series, have been thoroughly studied by Cotton et ecules and the MO,S:+ cluster core have been made using a 1 8-2 5 To explore the possible quasi-aromaticity of CND0/2 calculation with the localised molecular orbital [M,S4-,0,(H20)9]4+ (M = Mo, W; n = 0-4) clusters, we (LMO) technique of Edrni~ton-Ruedenberg.~.~In order to report in this paper relativistic effective core potential ab shed light on the nature of the quasi-aromaticity of initio calculations on the electronic structures of the cluster transition-metal clusters, more reliable theoretical research is cores, Mo3(~3-SHp-s):+ (11, Mo~(~~-OHP-S)~+(2), Mo~(cL~-essential.SXP-O)!' (3), Mo~(P~-OXCL-O)~+ (5h(4), W~(CL~-SHP-S)~+The incomplete cubane-type clusters of mixed 0x0-thio W3(~3-0)( + +PL-S)':+ ((9, W~(P~-SHP-O)':(7h W3(~3-0X~-0); cores M,S,-,Off+ (M = Mo, W; n = 0-4) may exist in eight (8), as well as Mo,(p,-S)(p-S)(p-O)~+ (9) and W3(p3-S)(p-kinds of structural forms as depicted in Fig. 2.So far a S)(p-O):'(lO). On account of the structural similarity and the number of compounds possessing cluster cores of symmetry of 9 and 10, which is lowered only from C,, to C,, M3S4-,,Off+(n= 0-4) type, in particular aqueous ions or the calculated results will not be presented for brevity. The bonding, electronic structures and nature of the quasi-0 I IV VI 111 V VII VIII n=O n= 1 n=2 n=3 n=4 Fig. 1 Molecular configuration of M~,(p,-S,)(p-S,),(~-dtp)~(p-Fig. 2 Eight isomers of M,S,-,O:+ (M = Mo, W; n = 0-4) cluster dtp).L (x denotes the chelating ligand, dtp denotes (EtO),PS;) cores aromaticity of these M,(p3-Xc)(p-Xb)3 clusters (where X, and x, denote the capping and edge-bridging atoms, i.e.the triple- and double-bridging atoms, respectively) are investi- gated in this paper. Theoretical Details Since we are principally concerned with the effects of varia- tions of O/S and Mo/W, no terminal water ligands were taken into account in the calculations, to save time. The cal- culations on the [M,S4 -,0,(H20),]4' cluster indicate that, as a preliminary approximation, neglect of the weakly bound terminal water ligands in the calculations is reasonable. Coordination of the nine terminal water ligands has a very small effect on the electronic properties of the cluster cores. The electronic structures of cluster cores 1-10, models of [M3S4-,0,(H20)9]4+, were calculated by use of ab initio methods within the restricted Hartree-Fock formalism, thereby generating the delocalised or canonical molecular orbitals (CMO) in the level of self-consistent field (SCF) theory.The atomic cores of molybdenum and tungsten atoms were replaced by the effective core potentials (ECPs) pro- posed by Hay and Wadt,26 in which both the mass-velocity and the Darwin relativistic effects were included, while those of the sulfur and oxygen atoms were substituted by the average relativistic effective potentials (AREP) suggested by Pacios and Chri~tiansen.~~The valence basis sets corre-sponding to these ECPs were adopted and contracted in minimal basis sets.The LMOs were derived from the Foster-Boys localisation scheme.28 The populations and the atomic charges were given in the framework of Mulliken population analysis29 as well as the natural population analysis" based on the natural bond orbital theory put forward by Weinhold and co-~orkers.~~-~~The CMO and LMO percentage composi- tions were given from the Mulliken population scheme. The geometry of M3(p3-Xc)(p-Xb)3, as well as the puckered six- membered ring of M,(p-X,),, are shown in Fig. 3. All cluster cores were idealized to C,, symmetry except for 9 and 10, where C, symmetry was adopted. The geometrical parameters J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 /M\ Xb Xb Xb xb' I M MM (a) (b) Fig. 3 Skeletal configuration of M3(p3-Xc)(p-Xb), and the puckered six-membered ring of M,(p-X,), ;(a) top view, (b)side view ings between the metal atoms and the bridging atoms.Owing to the impossibility of 0-n separation, the d-pn bonding dis- cussed here refers to overlapping that is n-like, rather than n-type symmetry. Both the bonding and antibonding d-pn energy levels, as well as the LUMO-HOMO energy gaps (AEHL) and the energy difference between the lowest unoccupied n orbitals and the highest occupied n orbitals (A&), for 1-8 are listed in Table 1 and the correlations between the d-pn orbitals are depicted in Fig. 4. However, the difference between their electronic structures is manifest. The larger electronegativity, as well as the lower orbital energies of the oxygen atoms compared with those of the sulfur atoms, gives rise to the result that, in a cluster with more oxygen atoms, the energy levels are generally lowered.Consequently, the energy levels of the HOMO and the LUMO, as well as the d-pn bonding MOs in 1-8, follow analogously the subsequent order: 1 > 2 >3 > 4 and The calculations were accomplished using a DEC VAX-88 10 computer and the Gaussian 90 system of programs.37 Results and Discussion CMO Analysis The CMO calculations at the SCF level show that the eigen- value spectra are rather akin to each other. The similarity between these cluster cores is due to the fact that, as far as the main compositions are concerned, the four MOs with lowest energy contain 2s/3s lone pairs at the capping and bridging atoms; the 15 bonding orbitals with M-X,, M-X, and M-M interactions are next lowest in energy, where bonding or antibonding orbitals involving either M-M or M-X, are present in the HOMO or LUMO regions.By analysis of the bonding in these MOs, it is found that in addil tion to metal-metal bonding, there are apparent d-pn bond- t A number of crystal structures of Mo,S,-,,O~+ (n = 0-4) clus-ters were reported in the literature [see, e.g. ref. lqb)]. Some of those employed in the calculations are given in ref. 35: 1,35' 2, 35b, 9,,'" 3,35d4.35eW,S,-,,O~+ (n =0-4) clusters with various peripheral ligands were characterized by many chemists [see, e.g. ref. 13 of ref. 35(a)]. The structures used in this paper are as follows: 5,36" 7,36c8.36dThe structural data for 6 (W-W = 0.2650 nm, W-0, = 0.210 nm, W-S, = 0.2250 nm, LO,WS, = 104.0") were taken from the analogous cluster^^^^.^ owing to the absence of this structure.5 > 6 > 7 > 8, and the LUMO-HOMO energy gaps increase were determined from X-ray diffraction experiment~.~~*~~tin the reverse order. This may be utilized to account for the fact that, with increase of the number of oxygen atoms in M3S4-,,0:+ (M = Mo, W) clusters, the UV-VIS absorption peaks gradually shift toward lower wavelength^.^^',^^ For -0.7 -0.8 -0.9 -1 .a -1.1 -1.2 a1 ---1.3 -1.4 -1.5 Fig. 4 Energy-level correlation diagram of bonding and anti-bonding d-pn CMOSin 1-8 J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 d-pn energy levels, LUMO-HOMO energy gaps (AEHL)and n energy difference (AE,) for 1-8 (in Eb) 1 2 3 4 5 6 7 8 a1 -0.765 -0.788 -0.818 -0.850 -0.753 -0.772 -0.808 -0.830 e -0.81 1 -0.83 1 -0.876 -0.905 -0.791 -0.809 -0.846 -0.863 e -1.120 -1.138 -1.321 -1.357 -1.114 -1.164 -1.296 -1.311 a1 -1.251 -1.302 -1.448 -1.500 -1.259 -1.317 -1.416 -1.466 *E"L 0.309 0.307 0.351 0.373 0.322 0.325 0.363 0.377 0.309 0.307 0.445 0.452 0.323 0.355 0.450 0.448 Table 2 Mulliken atomic net charges (M, X,, Xb) and overlap populations (M-Xc , M-Xb) of 1-8 system M XC xb M-X, M-Xb 1.137 -0.015 0.202 0.323 0.536 1.261 -0.529 0.249 0.285 0.545 1.638 0.105 -0.340 0.322 0.453 1.824 -0.497 -0.325 0.278 0.461 1.103 0.006 0.228 0.366 0.599 1.244 -0.491 0.253 0.310 0.621 1.646 0.070 -0.336 0.369 0.491 1.813 -0.488 -0.317 0.303 0.502 example, the MO,S:+ and Mo,OS:+ clusters are green, Mo,O,Si+ is grey, while Mo,0,S4+ and Mo,Oz+ are red, corresponding to wavelengths of 602, 588, 572, 512 and 505 nm,loc for the absorption peaks of the first excitations. The Mulliken net charges on the atoms (M, X,, Xb) and overlap populations between M and the ligands (M-X,, M-X,) of 1-8 are listed in Table 2.The natural electronic Table 3 Natural configurations and natural net charges in 1-8 cluster atom net charge configuration ~ ~~~ 1.33 d4.62s0.02 0 02Mo3S:+ (I) Mo P' SC -0.19 s1.96 4 23P' s1.97 P3.910.06'b 1.45 d4 5lS0.02 0 02Mo,OS:+ (2) Mo P' 0, -0.68 s1.94 4 14 P' 0.1 1 s1.96 3 93 'b P' d4.12s0.02 0 03Mo3SO:+ (3) Mo 1.84 P.-0.10 s1.96 4 14 P'SC Ob -0.47 s1.95 4 52P' 2.02 d3.95s0.02 0 02Mo30:+ (4) Mo P' 0, -0.66 s1.94 4 72P. sl.95 P'Ob -0.46 4 51 1.45 d4.46s0.06 0 03w3s:+ (5) W P' SC -0.28 s1.95 4 34 P' s1.96P'407-0.02'b d4.3SsO.05 0 03W30S:+ (6) W 1.57 P' 0, -0.69 s1.94 4 16P' -0.003 s1.95 405 'b P' 1.95 d3.96s0.05 0.04 W3SO:+ (7) w P s1.95 P4.32-0.27SC Ob -0.53 s1.95 4 58P' d3.83s0.05 0 03W,Ot+ (8) W 2.09 P' 0, -0.73 s1.94 4 79P' Ob -0.5 1 s'.94 4 57P' configurations and natural net charges, more reasonable than Mulliken's populations, are listed in Table 3 for comparison.Although the Mulliken charge differs from the natural charge, in general they tend to coincide with each other. For example, in the cluster core with more oxygen atoms the positive charge on M is larger and the charge density of the bridging atom is always much lower than that of the capping atom. The natural net charge of the metal atom is more posi- tive than the Mulliken net charge. By comparison of the net charges of Mo-containing clusters with their W-containing counterparts, Mulliken net charges of the two series of clus-ters are not so regular as the natural net charges. The eigenvalue spectra and the results of population analysis of the Mo-containing cluster and its W-containing counterpart indicate that: (i) The energy levels of the HOMO and the LUMO for Mo-containing clusters are lower than those for the W-containing clusters.(ii) The natural net charge of Mo is lower than that of W. (iii) The overlap popu- lation of Mo-X, is smaller than that of W-X,. (iv) The sequence of d-pn bonding strength, measured by overlap populations between M and xb in the 7r orbitals, is Mo-S, < W-S, but MeO, > w-0,. These facts are in agreement with the higher d-orbital energy of W than Mo. Of particular significance is the extensive MeX, d-pz bonding in these cluster cores, which results from the d-pn conjugation of the lone pair on xb with the vacant d-orbitals of M. The d-pn CMOs in 1-8, which are analogous with each other, have been illustrated in Fig. 5, where the bonding patterns of M,(p-X,), and C& are compared.Comparison of the energy levels of the M3S4-nO:f cluster with those of benzene is quite interesting. Owing to the similarities in the topologies of c, and M,(i~-x,)~, it is easy to see that the 7c bonding pictures of M3S4-,,O;+ and C6H, are akin to each other. In the M3S,-,0,4+ cluster, six electrons are used in the metal-metal bonding, while another six electrons from the fi e Fig. 5 Comparison of the bonding d-pn CMOs between M3(p-Xb), and C,H,. (a) C,H,, top view; (b)M3(p-Xb),top view. lone pairs of three bridging atoms fill the three d-pn bonding orbitals, i.e. (a,)2(e)4, similar to the ppn bonding in benzene,39 i.e. (a2,)2(e,,)4. Owing to the full occupation, by six electrons, of the d-pn bonding orbitals and the full vacancy of the antibonding orbitals, the delocalised MO picture of M,X:+ with strong d-pn bonding will be similar to that of benzene and the cluster will be relatively stable.The overlap population of M-X, may serve as a rough measure of the d-pn conjugation effect. The similarity between M3S,-,0t+ and C6H6 explains the unusual struc- ture, reactivity and stability of the former. Therefore, as in the case of certain Mo3S:+ cluster compounds,6 by taking account of their structure and reactivity these M3S4-nOt+ (M = Mo, W; n = 0-4)cluster compounds may also be con- sidered to be quasi-aromatic systems. This viewpoint of d-pn bonding will be expanded further in the subsequent popu- lation analysis and LMO results.In respect of the population analyses, although the capping ligands are triply bridged to three M atoms, whereas the bridging atoms are doubly bridged to two M atoms, the charge density in the capping ligand is larger than those in the bridging ligands when xb and X, are the same elements. That is, electron transfer from the ligands to the M centre is larger in M-Xb bonds than in M-X, bonds. The natural electronic configurations of X, and xb show that, though the populations in the s orbitals of x, and xb are practically identical, the population of the p orbital in xb, mainly the p, orbital, is markedly smaller than that of the p orbital in X, Therefore, Mulliken population analysis and the natural population analysis both point to the existence of d-pn bonding in M,S,-,Ot+ (M = Mo, W; n = 0-4) clusters, which provokes additional transfer of the electron density from xb to M, and hence enhances and shortens the M-X, bonds.Note that, when comparing with the calculation on the [M3X4(H20)J4+ cluster,40 the electronic structures of the cluster cores are relatively less affected by water ligands. For example, the energy-level sequences of the core MOs are almost the same in the coordinated and uncoordinated cores. These results accord with the fact that the chemical shift of the 95Mo NMR in the Mo301+ cluster is found experimen- tally to lie in a narrow range,,, showing that the cluster core is less influenced by the coordination surroundings.LMO Results The LMO technique may supply us with one kind of intuitive chemical bonding The degree of delocalisation of the d-pn bonds in the systems may serve as a good measure of the aromatic character.,,-,* The LMO compositions (%) and the assignments of the bonding are shown in Table 4, in which A, Q and n denote the lone pair, and the Q-and n- bonds, respectively, and the total number of each kind of LMO is included. The bonding pictures in the cluster cores 1-8 are analo- gous. In addition to four lone pairs at the capping and bridg- ing atoms, there exist strictly three M-X, Q bonds, six M-X, Q bonds, three M-M c bonds and three M-X,-M type three-centred two-electron, i.e. 3c2e (d-pd) n bonds. Note that, in addition to the M-M bonding thoroughly studied by Cotton and Haas,18 the 3c2e (d-pd) n bonding is remarkable and cannot be neglected in the clusters studied. For example, the 3c2e Mo-sb-Mo LMO in 1 possesses 35.2% d-orbital character from the molybdenum atoms and 63.4% p-orbital character from the sulfur atom, which arises from delocalisation of the lone pair of the bridging S to the empty d orbital of Mo.On the other hand, LMO composi- tions show that the M-S-M n bond is stronger than the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 LMO percentage compositions (%) and bonding assign- ment of LMOs in 1-8 molecule composition assignment of LMOs Mo,S,+ (1) S(95.1) I (S,) x 1 S(97.5) I (SJ x 3 Mo(38.9) + S(63.7) u (Mo-S,) x 3 Mo(42.3) + S(60.0) u (Mo-S,) x 6 Mo(45.2) + Mo(45.2) u (Mo-Mo) x 3 Mo(17.6) + S(63.4) + Mo(17.6) 71 (Mo-S,-Mo) x 3 Mo,OS:+ (2) O(96.5) l(0,)x 1 S(9 8 .O) Mo(27.0) + O(74.6) u (Mo-0,) x 3 Mo(39.8) + S(62.6) u (Mo-S,) x 6 Mo(46.8)+ Mo(46.8) u (Mo-Mo) x 3 Mo(20.3) + S(57.6) + Mo(20.3) 71 (Mo--S,-Mo) x 3 Mo,SO:+ (3) S(96.5) I (S,) x 1 O(98.2) I(0,) x 3 Mo(40.6) + S(62.4) u (Mo-S,) x 3 Mo(34.7) + O(67.3) u (Mo-0,) x 6 Mo(46.7)+ Mo(46.7) u (Mo-Mo) x 3 Mo(10.9) + O(78.0) + Mo(10.9) 71 (Mo-0,-Mo) x 3 Mo,OZ+ (4) O(97.6) I(0,) x 1 O(98.9) I(0,) x 3 Mo(27.8) + O(74.2) u (Mo-0,) x 3 Mo(34.0) + O(68.2) u (Mo-0,) x 6 Mo(47.8) + Mo(47.8) u (Mo-Mo) x 3 Mo(12.0) + O(75.7) + Mo(12.0) 71 (Mo-0,-Mo) x 3 W,St+ (5) S(95.7) I (S,) x 1 S(99.1) I (SJ x 3 W(37.9) + S(65.0) u (W-SJ x 3 W(40.7) + S(62.5) u (W-S,) x 6 W(47.5) + W(47.5) u (W-W) x 3 W(19.7) + S(59.8) + W(19.7) n (W-s,-W) x 3 W,OS:+ (6) O(97.5) 1(0,)x 1 S(99.5) I (SJ x 3 W(26.8) + O(74.3) u (W-0,) x 3 W(41.5) + S(62.3) u (W-S,) x 6 W(47.8) + W(47.8) u (W-W) x 3 W(20.4) + S(58.5) + W(20.4) n (W-S,-W) x 3 w,so:+ (7) S(96.8) 1 (S,) x 1 O(97.7) (Ob)W(39.6) + S(64.0) u (W-S,) x 3 W(33.9) + O(67.7) u (W-0,) x 6 W(48.4) + W(48.4) u (W-W) x 3 W(9.8) + O(80.1) + W(9.8) 7I (W-0,-W) x 3 WPt+ (8) O(98.4) 3(0,)x 1 O(99.0) A (Ob)W(27.7) + O(74.3) u (W-0,) x 3 W(33.7) + O(68.7) u (W-0,) x 6 W(48.7) + W(48.7) u (W-W) x 3 W(11.6) + O(76.6) + W(11.6) n(W-O,-W) x 3 M-0-M n bond, which is expected from the differences in the electronegativities and atomic radii.Table 5 lists the combination coefficients of the d-type hybridized orbital of the Mo, atom (lying the y axis of the coordinate system used) in two adjacent 3c2e (d-p-d) n LMOs, where d,' denotes the hybridized d orbital overlap in the clockwise direct and d, the hybridized d orbital overlap in the anticlockwise direction. It is obvious that the bonding (d-p-d) n LMOs of M,(p-X,), and the bonding (ppp) n LMOs of C,H6 are analogous (Fig. 6), therefore further con- firming the quasi-aromaticity hypothesis. Just as the aromati- city of the planar six-membered ring molecules arises from three continuous and closed 3c2e (ppp) n bonds with strong interaction^,^^ the quasi-aromaticity in these puckered M3X3 six-membered ring clusters is similarly dependent upon the d-pn system, composed of three continuous and closed 3c2e (d-pd) n bonds with strong interactions.From the LMO coefficients, it is very easy to demonstrate these proper- ties of the (d-p-d) n bonds. First of all, except for the coeffi- cients of the d,, and d,, orbitals (their percentage composition is small and may be omitted), the d,' and d; J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 5 d-Orbital coefficients of Mo, in adjacent (d-pd) n LMOs of 1-8 Mo,SZ+ (1) d: 0.21 0.07 0.16 -0.20 0.08 d; 0.21 -0.07 0.16 -0.20 -0.08 Mo,OS':+ (2) d: 0.24 0.09 0.17 -0.21 0.08 d, 0.24 -0.09 0.17 -0.21 -0.08 Mo,SO:+ (3) d: 0.15 0.02 0.12 -0.17 0.04 d; 0.15 -0.02 0.12 -0.17 -0.04 Mo,O:+ (4) d: 0.17 0.03 0.13 -0.16 0.04 d, 0.17 -0.03 0.13 -0.16 -0.04 W,St+ (5) d: d; 0.22 0.22 0.09 -0.09 0.16 0.16 -0.20 -0.20 0.11 -0.11 W,OS:+ (6) d: 0.23 0.09 0.16 -0.20 0.10 d; 0.23 -0.09 0.16 -0.20 -0.10 W,SO:+ (7) d: 0.15 0.04 0.12 -0.14 0.07 d; 0.15 -0.04 0.12 -0.14 -0.07 W,Ot+ (8) d: d; 0.16 0.16 0.01 -0.01 0.11 0.11 -0.18 -0.18 0.05 -0.05 hybridized orbitals are mainly made up of d,, , d,, and dx2-,, orbitals and thus two adjacent (d-p-d) n bonds may be con- sidered to share the same hybridized d orbital of the M atoms (see Table 5 and Fig. 6), thereby indicating the contin- uity of the n electron cloud.Secondly, owing to this contin- uity, the three (d-p-d) n bonds are not mutually isolated (as in the case of the 'Dewar islands' of the out-of-plane (d-p-d) n bond in P,N,Cl, molecule^^^), but are closed. Finally, the percentage character of the d orbital of the M atoms in the (d-pd) n bonds is rather large, thus causing strong inter- actions between each couple of two (d-pd) n bonds. It is these strong interactions of the 3c2e (d-p-d) n bonds that are responsible for the unique structures, stability and benzene- like reactivities of these clusters. From another viewpoint, the Q (c) Fig. 6 Comparison of the bonding (d-p-d)z LMOs between M3(p Xb), and C,H,. (a) C,H,, top view; (b)M,(V-X~)~,top view; (c) side view of one (d-p-d) K bond in M,(p-X,),. 3c2e (d-p-d) n bonds that we are mainly concerned with here, merely originate from the strong delocalisation of the lone-pair electrons in the bridging atoms over the empty d orbitals of the M atoms.The greater delocalisation of the lone-pair electrons is certain to give rise to a greater percent- age composition of the M atom in the M-X,-M n bonds, and thus a stronger interaction between the LMOs. So the greater the M percentage in the (d-p-d) n bonds, the stronger the d-pn conjugation, and thus the more aromatic the system should be. The W,S:+ system is therefore more aromatic and more stable than the Mo3Stf system. As far as the qualitative bonding types are concerned, the LMO pictures of these systems obtained here have confirmed in general those previously derived from CNDO calculations with an Edmiston-Ruedenberg ~cheme.~.~However, it seems that the LMO interaction energy in the CNDO/spd scheme is unduly large, and no four-centred bonds (M-S,-M-S,) are found here.These disparities between the CNDO and ab initio results, as well as the population analysis, indicate the overemphasis by the previous CNDO calculation on the d orbitals of the sulfur atoms in the Mo3S:+ clusters. Inspec- tion of the role played by the d orbitals of the sulfur atoms in the Mo3S:+ cluster compound will be given in the Appendix. In summary, the CMO and LMO calculations of these tri- nuclear Mo and W clusters show that their d-pn electronic structures and bonding are very similar to the ppn ones of benzene.Although the quasi-aromaticity idea has been put forward on the basis of experiments, the present theoretical investigation of their electronic structures and bonding indi- cates that quasi-aromatic clusters should possess the corre- sponding electronic features, large overlap populations and a stable d-pn configuration (CMO calculations), and contin- uous, closed 3c2e (d-pd) n bonds with strong interactions (LMO calculations). Note that many experimental observa- tions on the clusters under consideration, e.g. the formation of cubane-like clusters of Mo,S4-,0,4+ and Fe, Co, Ni, CU,~~~*~~may be interpreted by virtue of the electronic char- acteristics and the quasi-ar~maticity.~' A further discussion will be given elsewhere.Moreover, in addition to the clusters of Mo-W and S-0 studied in this paper, several analogous Se-or Te-containing clusters of and Mo-W mixed clusters with O/S ligand~~~.~~have been synthesized and characterized recently. A further study of such clusters is now in progress. Conclusion The CMOS calculated using ab initio methods with rela- tivistic effective core potentials indicate that there exists extensively delocalised d-pn bonding in the M,(p-X,), puck-ered rings of the incomplete cubane-type clusters, M,S4-,0,4+ (M = Mo, W; n=0-4), akin to the p-pn bonding in the planar ring of benzene. This d-pn bonding is further demonstrated not only by population analysis but also by the LMO technique.The LMO calculations have generated one set of 3c2e (d-pd) n LMOs, which is shown to be analogous to the LMOs of aromatic benzene molecule. It is shown that the (d-p) n bonding in the puckered six-membered ring of the M,(p-X), core arises from a closed continuous ring of three adjacent localised (d-pd) n bonds with strong interactions. It is these strong interactions of the 3c2e (d-pd) n bonds that account for the unusual stability and quasi-aromaticity of these clusters. Based on the theoreti- cal analysis and their properties, all the clusters studied are viewed to exhibit quasi-aromaticity. We are grateful to the National Natural Science Foundation of China and the Foundation of the State Key Laboratory of 44 Structural Chemistry of China for grants in support of this work.We would like to thank Prof. Qianer Zhang for helpful discussion and the Computation Centre of Fujian Province for favourable service. Appendix The calculation on the Mo3S:+ cluster was carried out by employing the more accurate ECP and valence basis sets of Wadt and Hay,58 in which the d-type orbitals scaled by 0.5972 for the S atom were added and the outermost core atomic orbitals of Mo were retained, so as to make clear the effect of the core-valence correlation (effect of core orbitals on the valence ones) and the role played by the d orbitals in the bonding of sulfur atoms. Therefore the [Ar] core for Mo was adopted and a strict ab initio calculation was performed on the (4s4p4d5s) subshells.The basis sets used for Mo and S atoms were, respectively, (lOs5p4d)/[2slpld] and (3s3pld)/ [1s1p 1d]. The eigenvalues of the bonding and antibonding d-p7c CMOs are as follows (in E, t), a,( -1.244)e(-1.106)e (-0.797)a1(-0.754), and the 3c2e (d-pd) n LMO is Mo( 17.2) + S(62.6)+ (17.2). The Mulliken overlap popu-lations of Mo-S, and Mo-S, are 0.226 and 0.367, whereas the natural net charges of S, and S, are -0.15 and 0.09, corresponding to the configurations s1.94p4. 6d0.05 and s1.95 391p . d0 .04, respectively. These results show that for the Mo3S2+ cluster two kinds of ECP and basis set lead to very similar consequences, indicating that, in this case, the core- valence correlation of Mo is insignificant for describing the bonding, and the d orbital of the S atom plays a negligible part in this kind of cluster, which is in accord with the experi- ence of chemists. However, no four-centred two-electron LMOs, presented in the literature given by the CND0/2 cal- culation with Edmiston-Ruedenberg localisation pro-are~edure,~.~ found here.That is the reason why the relatively less laborious ECPs and basis sets which do not include d orbitals for S atoms are adopted in the calculations in this paper. The irreducible representations and eigenvalues of the occupied and several unoccupied CMOs and the Mulliken percentage compositions (%) of M, X, and xb in 1-8 are listed in additional Tables S1, S2, S3 and S4 of the Supple- mentary material.$ The results for the Mo,SZ+ cluster core calculated using the (lOs5p4d)/[2slpld] basis set for Mo and the (3s3pld)/[lslpld] basis set for S are presented in Table s5.References 1 M. V. Gorelik, Russ. Chem. Rev., 1990, 59, 116. 2 P. J. Garratt, Aromaticity, McGraw-Hill, London, 1971; P. J. Garratt, Aromaticity, Wiley, New York, 1986. 3 P. J. Garratt, Comprehensive Organic Chemistry, Pergamon Press, Oxford, 1979, vol. 1, p. 215. 4 J. Q. Huang, J. L. Huang, M. Y. Shang, S. F. Lu, X. T. Lin, Y. H. Lin, M. D. Huang, H. H. Zhuang and J. X. Lu, Pure Appl. Chem., 1988,60, 1185. 5 J. Q. Huang, S. F. Lu, M. Y. Shang, X. T. Lin, M. D. Huang, Y. H. Lin, D. M. Wu, H. H. Zhuang, J. L. Huang and J. X. Lu, J. Struct. Chem., 1987,6, 219. 6 (a)J. X.Lu, J. Struct. Chem., 1989,8, 327; (b)J. X. Lu and Z. D. Chen, Annu. Rev. Phys. Chem., in the press. 7 Z. D. Chen, J. Li, W. D. Chen, J. Q. Huang, C. W. Liu, Q. E. Zhang and J. X. Lu, Chinese Sci. Bull., 1990,35, 1698. t 1 E, 2 4.359 75 x lo-" J. 1 Supplementary publication no. SUP 56974, 3 pp. Details avail- able from the Editorial Ofice. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 8 Z. D. Chen, J. X. Lu, C. W. Liu and Q. E. Zhang, Polyhedron, 1991,10,2799. 9 (a)F. A. Cotton, Z. Dori, R. Llusar and W. J. Schwotzer, J. Am. Chem. SOC., 1985, 107, 6734; (b) T. Shibahara, H. Kuroya, K. Matsumoto and S. Ooi, Inorg. Chim. Acta, 1987, 116, L25; (c) T. Shibahara, H. Akashi, S. Nagahata, H. Hattori and H. Kuroya, Inorg. Chem., 1989,28, 362, and references cited therein.10 (a) B-L. Ooi and A. G. Sykes, Inorg. Chem., 1989, 28, 3799; (b) Y-J. Li, C. A. Routledge and A. G. Sykes, Inorg. Chem., 1991,30, 5043; (c) M. Martinez, B-L. Ooi and A. G. Sykes, J. Am. Chem. SOC., 1987, 109, 4615; (d) M. A. Harmer, D. T. Richens, A. B. Soares, A. T. Thornton and A. G. Sykes, Inorg. Chem., 1981, 20, 4155. 11 F. A. Cotton, Polyhedron, 1986,5, 3. 12 I. G. Dance, Polyhedron, 1986,5, 1037. 13 P. Zanello, Coord. Chem. Rev., 1988,83, 199. 14 S. Harris, Polyhedron, 1989,8, 2834. 15 S. C. Lee and R. H. Holm, Angew. Chem. Int. Ed. Engl., 1990,29, 840. 16 T. Shibahara, Adv. Inorg. Chem., 1990,37, 143. 17 R. H. Holm, Adu. Inorg. Chem., 1992,38, 1. 18 (a) F. A. Cotton and T. E. Haas, Inorg. Chem., 1964, 3, 10; (b) F.A. Cotton, Inorg. Chem., 1964,3, 1217. 19 F. A. Cotton, Rev. Chem. SOC.,1966,20,389. 20 B. E. Bursten, F. A. Cotton, M. B. Hall and R. C. Najar, Inorg. Chem., 1982,21,302. 21 (a)J. Q. Li and W. D. Cheng, J. Mol. Struct., Theochem., 1987, 151, 19; (b) J. Q. Li, J. Mol. Struct., Theochem., 1988, 181, 185. 22 (a) W. D. Cheng, Q. E. Zhang, J. S. Huang and J. X. Lu, Poly-hedron, 1989,8,2785; (b)W. D. Cheng, Q. E. Zhang, J. S. Huang and J. X. Lu, Polyhedron, 1990,9, 1625. 23 J. Li, C. W. Liu and J. X. Lu, Book of Abstracts of 7th Interna- tional Congress of Quantum Chemistry, Menton, 1991, p. 249. 24 F. A. Cotton and X. Feng, Inorg. Chem., 1991,30,3666. 25 Z. R. Li, Q. S. Li, J. K. Feng, Z. R. Zheng and J. Zheng, Chinese Chem.Lett., 1992,3,43. 26 (a) P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270; (b) W. R. Wadt and P. J. Hay, J. Chem. Phys., 1985,82,284. 27 L. F. Pacios and P. A. Christiansen, J. Chem. Phys., 1985, 82, 2664. 28 (a) S. F. Boys, Rev. Mod. Phys., 1960, 32, 296; (b) J. M. Foster and S. F. Boys, Rev. Mod. Phys., 1960,32,300. 29 R. S. Mulliken, J. Chem. Phys., 1955,23, 1833. 30 A. E. Reed, R. B. Weistock and F. Weinhold, J. Chem. Phys., 1985,83, 735. 31 J. P. Foster and F. Weinhold, J. Am. Chem. Soc., 1980, 102, 721 1. 32 A. E. Reed and F. Weinhold, J. Chem. Phys., 1985,83, 1736. 33 A. E. Reed, F. Weinhold, L. A. Curtiss and D. J. Pochatko, J. Chem. Phys., 1986,84,5697. 34 A. Reed, L. A. Curtiss and F. Weinhold, Chem. Rev., 1988, 88, 899.35 (a)H. Akashi, T. Shibahara and H. Kuroya, Polyhedron, 1990,9, 1671; (b) S. F. Lu, M. Y. Shang, J. Q. Huang, J. L. Huang and J. X. Lu, Sci. Sin. B, 1988, 31, 147; (c) 1.Shibahara, T. Yamada and H. Kuroya, Inorg. Chim. Acta, 1986, 113, L19; (d) 1.Shiba-hara, H. Hattori and H. Kuroya, J. Am. Chem. SOC., 1984, 106, 2710; (e)D. T. Richens, L. Helm. P-A. Pittet, A-E. Merbach, F. Nicolo and G. Chapuis, Inorg. Chem., 1989,28, 1394. 36 (a) T. Shibahara, M. Yamasaki, G. Sakane, K. Minami, T. Yabuki and A. Ichimura, Inorg. Chem., 1992, 31, 640; (b)T. Shi-bahara, A. Takeuchi, T. Kunimoto, H. Kuroya, Chem. Lett., 1987, 867; (c) Z. Dori, F. A. Cotton, R. Llusar and W. Schwot- zer, Polyhedron, 1986, 5, 907; (d)V. R. Mattes and K. Menne- mann, 2.Anorg.Allg. Chem., 1977,437, 175. 37 M. J. Frisch, M. Head-Gordon, G. W. Trucks, J. B. Foresman, H. B. Schlegel, K. Raghavachari, M. Robb, J. S. Binkley, C. Gonzalez, D. J. DeFrees, D. J. Fox, R. A. Whiteside, R. Seeger, C. F. Melius, J. Baker, R. L. Martin, L. R. Kahn, J. J. P. Stewart, S. Topiol and J. A. Pople, Gaussian 90, Gaussian Inc., Pitts- burgh, PA, 1990. 38 Q. T. Liu, J. X. Lu and A. G. Sykes, Inorg. Chim. Acta, 1992, 198, 623. 39 F. A. Cotton, Group Theory and Its Applications, Wiley-Interscience, New York, 2nd edn., 1971. 40 J. Li, C. W. Liu and J. X. Lu, unpublished results. 41 S. F. Gheller, T. W. Hambley, R. T. C. Brownlee, M. J. O'Connor, M. R. Snow and A. G. Wedd, J. Am. Chem. SOC., 1983,105, 1527. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 45 42 43 0. Chalvet and R. Daudel, Localization and Delocalization in Quantum Chemistry, Reidel, New York, 1975, vol. 1 and 2. G. Naray-Szabo, P. R. Surjan and J. G. Angyan, Applied Quantum Chemistry, Reidel, New York, 1987. 52 53 J. Li, Ph.D. Dissertation, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 1992, ch. 7. P. Kathirgamanathan, M. Martinez and A. G. Sykes, Poly-hedron, 1986,5, 505. 44 W. England, K. Ruedenberg, Theor. Chim. Acta, 1971,22, 196. 54 V. P. Fedin, M. N. Sokolov, A. V. Virovets, N. V. Podberezs- 45 A. A. Bhattacharyya, A. Bhattacharyya, R. R. Akins and A. G. kaya and V. Ye. Fedorov, Polyhedron, 1992,11,2973. 46 Turner, J. Am. Chem. SOC.,1981,103,7458. D. A. Dixion, D. A. Kleier and W. N. Lipscomb, J. Am. Chem. 55 M. Nasreldin, G. Henkel, G. Kampmann, B. Krebs, G. L. Lamp-recht, C. A. Routledge and A. G. Sykes, J. Chem. SOC., Dalton Soc., 1978,100,5680. Trans., 1993, 737. 47 R. C. Haddon, J. Org. Chem., 1979,44,3608. 56 A. Pate1 and D. T. Richens, J. Chem. SOC.,Chem. Commun., 1990, 48 G. Naray-Szabo and K. Horvath, Croat. Chem. Acta, 1977, 49, 274. 49 461. (a) C. W. Liu, J. Li and J. X.Lu, J. Mol. Sci., 1992, 8, 76; (b) J. Li, C. W. Liu and J. X. Lu, Chinese J. Chem., 1993,12,481. 57 58 T. Shibahara and M. Yamasaki, fnorg. Chem., 1991,30, 1687. P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985,82,299. 50 M. J. S. Dewar, E. A. C. Lucken and M. A. Whitehead, J. Chem. SOC., 1960, 2423. 51 T. Shibahara,. M. Yamasaki, H. Akashi and T. Katayama, Inorg. Chem., 1991,30,2693, and references cited therein. Paper 3/04020H; Received 12th July, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000039

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, Wl), 47-54 Comparison of Complete Conformational Searching and the Energy- optimized Tree Branch Method in Molecular Mechanics Calculations F. Villamagnat and M. A. Whitehead* Theoretical Chemistry Group, Department of Chemistry, McGill University, 80 1 Sherbrooke Street West, Montreal, Quebec, Canada, H3A 2K6 An efficient method, the energy-optimized tree branch method to obtain the global, energy-minimized, molecular structure for a molecule by molecular modelling, is described. Molecular me~hanicsl-~ is well accepted to model both struc- ture and The method uses the classical equations of motion summed to create the potential function to rep- resent a molecule. The function is minimized to give the minimum energy configuration.It is known that the potential-energy functions used in molecular mechanics cal- culations give a large number of minima in addition to the true global minimum on the potential-energy surface, and it was impossible to identify the true global minimum from a single minimization. Global minima are easily obtained when minimizing highly rigid molecules with few degrees of freedom.'-' For molecules with many degrees of freedom, experimental evidence and intuition are used to avoid the multiple-minima problem. Successful manual searches for the global minimum are achieved by preselecting the most prob- able structure, and tweaking the atomic positions through a narrow range of values. The simplifications used in manual searches are insufficient for molecules with more than five or six internal degrees of freedom.Many other methods have been developed, all relatively complex. A conceptually simpler approach is exemplified by Scherega," who used Monte Carlo methods to select conformers randomly from the potential-energy surface. This is a stochastic method, in which average properties such as energy are obtained from weighted averages. For the statistics to yield accurate results several million configurations must be generated.' Monte Carlo methods do not find an energy minimum, but sample a large ensemble of states, progressively mapping the potential surface. This approach works well for relatively small mol- ecules, but for large molecules yields too many conformers. Biased sampling quickly evaluates regions of interest,I2 with the method being referred to as umbrella sampling.Like all potential surface search methods it is a sophisticated but brute force approach to the problem, and requires large amounts of CPU time. Molecular dynamics calculations have been proposed to circumvent the multiple-minima problem. If the dynamics simulation begins with a high-energy struc- ture, random. motion of the atoms may eventually produce a molecule with a configuration corresponding to a lower energy minimum. This requires a large number of steps in the simulations for even relatively small changes in configuration. Simulated annealing14 has been proposed to circumvent the multiple-minima problem.The analogy between annealing and potential-energy minimization is not straightforward, since there is no reason that the potential-energy surface at high temperatures should be directly related to the surface at the lower temperatures. The calculations have been shown to t Present address: ICI Explosives USA Inc., North American Research and Technology, Tamaqua Site, P.O. Box 577, Tamaqua, PA 18252 produce nearly degenerate 'high-temperature' minimum-energy rather than a single minimum.I4 These states are randomly occupied, and may or may not be present in the annealed system. l4 Though successful minimizations have been reported, the technique requires many conformers to be evaluated when the starting geometry is far from the minimum-energy configuration and requires large amounts of CPU time.Distance geometry methods16 do not attempt to reduce the number of conformers to be evaluated, but reduce the computational time per minimization, by removing the slow convergence torsional and bending energy terms in the potential function, and treating all non-bonded interactions as two centre-bonded interactions over long distances. This significantly reduces the time per minimization, allowing more structures to be evaluated. The method provides some improvements, but large numbers of minimizations are still required for any large molecule. The grid search method13 and the tree branch met hod 7-' represent different means to overcome the multiple-minima problem. The methods parallel the way in which chemists create structures using molecular modelling kits.The skeletal configuration is first drawn from a geometry-optimised structure using standard bond lengths and angles, followed by evaluation of torsional energy differ- ences across all rotatable bonds. No minimizations are required at this point, and only structures whose energies meet certain criteria are retained. As a result the calculations are relatively fast. The energies are calculated for each rota- tion condition using fixed torsional angles, and are repeated for increments of usually 30 to 60 degrees. Once the low- energy skeletal conformers have been identified, the complete molecules are individually minimized using the full potential. While this preselection of skeletal structure represents an improvement over the brute force methods, it still requires lengthy computational times.This paper extends the grid search to a direct energy mini- mization tree. Extensive computational times are avoided by using minimum-energy conditions to eliminate large portions of the tree, and to decrease the number of structures evalu- ated. The method minimizes the energy of a small precursor of the target molecule; the small precursor is identified as any functional group in the molecule. An energy minimum is first defined for one of the precursors, and the molecule is grown by one heavy atom (non-hydrogen) at a time. Each new heavy atom (X,J is entered at the position of one of the hydrogens on the precursor, (X0,J.The appropriate number of hydrogens are added to the added atom, and the energy of the new structure minimized. The procedure is repeated for all other hydrogen positions on Xold, and only the lowest- energy structure from X,,, is retained to continue the growth. For an sp3 hybridized atom such as the terminal 1 i Fig. I An sp3 hybrid energy tree. When all the structures are identi- fied there are 3N structures to be minimized, where N is the number of heavy (carbon) atoms. With ten sp3 heavy atoms this gives 3'' = 59049 minimizations, which is an acceptable number. With twenty heavy atoms this becomes 3'' = 3.49 x lo9 conformers. carbon in R-CH,, three minimizations are needed for each new substituted atom.By selecting the lowest-energy con-former, a very specific path is followed along the energy tree. When all the structures identified on the tree have to be mini- mized, molecules with twenty heavy atoms have 3" or lo9 conformers (Fig. l), whereas following the minimum-energy path of the tree branch method reduces the number of con- formers to be minimized to 3N -2 namely 58. The proposed method identifies a configuration close to the overall global minimum in both structure and energy. Repeated mini-mizations will give the global energy minimum. However, it identifies one of the lower-energy conformers for the target molecule and most likely the lowest-energy conformer. The configuration of this conformer will be approximately the same as that of any other low-energy conformer and structur- al information, such as bond lengths, bond angles, volume, area and cross-sectional diameter, will also be the same.Hydrogen bonding is specifically included in the potential function and will occur in all the lower-energy conformers. For most industrial applications it is these average molecular properties that are important, as opposed to the exact struc- ture of the global minimum. Any desired conformer can be calculated by following a path other than the lowest-energy path and the shape and volume of these conformers calcu- lated. If the conformer is required to fit a known molecular shape it can be constructed to do so and the total energy of the two molecules then calculated.Calculations Energy minimizations for the optimized tree branch method were performed on a personal computer using PCMODEL from Serena Software,2o which uses Allinger's MM2 model force field." ' Complete conformational searches were per- formed on a Microvax I1 using Sybyl with the MM2 model force field from Tripos Associates.21 Minimum energy refers to the lowest calculated energy based on the method used: this will be close to but not necessarily the exact global energy minimum. Construction of oleic acid exemplifies the method. A detailed tree of the calculation is in Fig. 2. Analysis of oleic acid began by evaluating the minimum- energy configuration of formic acid using PCMODEL. The structure of formic acid was compared to the one obtained from the semi-empirical calculation using the AM 1 Hamilto-nian.22 For such small precursors the PCMODEL and AM1 structures were in excellent agreement, showing that molecu- lar mechanics calculations in PCMODEL do give good structures for small molecules (Table 1).J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Structural data for formic acid from PCMODEL and AM 1 bond no. atom type bond length/8, angle/degrees connectivity PCMODEL 1 0 O.ooO0 2 C 1.2079 1 0 0 3 0 1.3375 123.9259 2 1 0 4 H 0.9720 116.8968 3 2 1 5 H 1.1Ooo 123.4997 2 3 0 AM 1 1 0 O.oo00 2 C 1.2104 1 0 0 3 0 1.3400 123.9054 2 1 0 4 H 0.9718 116.6980 3 2 1 5 H 1.1098 123.5012 2 3 0 For the first substitution to methanoic acid, a carbon atom was introduced at the position of a hydrogen.The carbon atom was hydrogenated to give acetic acid, and the resulting structure, energy minimized. The methyl group on acetic acid has three hydrogen atoms, each of which was individually substituted with a carbon atom. In each case the carbon was hydrogenated to give propionic acid, and the structure energy minimized to give minimum energies of the resultant propi- onic acids -0.65, -0.22, and -0.22 kcal mol-'. The more stable conformer corresponding to -0.65 kcal mol-' was used to continue the growth. The terminal methyl group of this molecule also had three available hydrogen positions, whose individual substitution with a carbon atom followed by hydrogenation, resulted in the formation of three butanoic acid conformers with energies 0.02,0.64, and 0.64 kcal mol- '.The lowest-energy, most stable, structure, the trans configu-ration, was used for the next addition. Analysis of the contri- butions to the energy showed that the repulsion of the axial methyl groups caused the higher energy of the other con- formers. The procedure was repeated up to the C, chain, using only the lowest-energy conformer at each step. The double bond was formed by removing two hydrogen atoms from the terminal single bond on the C, chain. Substitution of the double bond with a carbon atom to continue the growth, represents one of the limitations of the tree branch method, because the lowest-energy path would be to substi- tute the double bond trans.However, the known structure of oleic acid has the double bond substituted cis,23and the only means of achieving this structure is to force the cis substitut-ion. The use of similar constraints is not restricted to the tree branch method, but is common to all molecular modelling techniques. A similar intervention would be required when growing highly strained systems, or cyclic structures. Such intervention does not detract from the usefulness of the method. Once the double bond is formed and substituted, the growth is continued as before, until all 18 carbon atoms are introduced. Because of the steric interaction between the chains on either side of the double bond, substitution at C,, and C,, quickly results in distortions, and reorientation of the two chains.The molecule no longer has the bent structure depicted for oleic acid in stick diagrams, and the two chains rotate at about a 90 degree angle relative to one another, giving a clearer insight into molecular shape. To ensure the precision of the method, the growth of oleic acid was repeat- ed using the energy-optimized tree branch method starting with different functional groups. In addition to the formic acid initiated growth, the molecule was also grown from ethene and ethane. In all cases the double bond was substi- tuted cis. The calculated minimum energies for oleic acid J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 49 H402H E= -1.73I H( l)H(2)H(3)CO,H in plane of the carbon backbone out-of plane (axial) of the carbon backbone / CH34H2402H CH34H2402H CH34H2402H €= -0.65 1 E = -0.24 €=-0.24 CH,-(CH2),4O,H E=0.02 CH3-(CH2)2402H E = 0.64 CH,-(CH&402H E= 0.64 1 CH3-(C H,),--CO,HE=0.66 CHs-(CH2)3402HE= 1.56 CH3-(CH2)3402H E= 1.56 1 I CH3-(CH2)4--C02H €=1.30 CHj-(CH2)4402HE=2.19 CHs-(CH2)4402H E= 2.19 CH3-(CH2)5402HE =1.95 CH3-(CH2)5402HE= 2.83 CH3-(CH2)5402H E= 2.83 1 I CH3-(CH,)G--CO,H E =2.58 CH,-(CH2),402H E= 3.46 CH3-(CH2),402H E= 3.46 CH3-(CH2 )+02H CH3-(CH2)402H E=-1.92 E =3.23 E=4.101 CH3=CH3-(CH2)402H E= 3.45 introductionof double bond overcomes out-of-plane (axial) interactions -R = -CH2=CH,-(CH,)r CH3--R--COZH E=4.37: CH34H2-R-CO2H CH34 H2--R4 O2H CH34H2--R--C02H E = 8.90 E=5.56 E= 5.26 double bond effect common to all conformers CHj-(CH2)2-R-C02H CH3-( C H,),--R4O,H CH3-(CH2)2-R402H E = 5.85 E = 6.42 €= 6.08: CH3-(CH2)3--R-C02H CH,--(CH,)3--R--COzH CH3-( CH2)3-R402HE = 7.30 E= 6.43 E= 7.27 C H 3-( C H2)4-R-C02H CH3-(CH2)4--R--C02HCH,-( C H2),-R--C02H E = 7.07 E=7.92 E = 7.93I CH3-(CH2)5--R-C02H CHj-(CH2)5--R--CO2H CH,-(CH2)5--R--CO,HE = 7.69 E = 8.59E = 8.59 CH,-(CH2),--R-C02H CH3-(CH2),--R--CO2H CH3-(CH2),--R--C02HE= 8.34 E = 9.20 E= 9.20I CH3-(CH2)rR--C02H CH3-(CH2)rH--COzH CH3-(CH2)rR--C02HE= 8.97 E = 9.85 E = 9.85 Fig.2 Energy tree for oleic acid. The higher conformers are caused by the repulsion between axially substituted (out-of-plane) 1,3 groups.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 AI p' P 5 1 initiated structure C Q) lacid initiated structure I 0 4 a 12 16 number of carbon atoms Fig. 3 Energy plots of the three calculations of the energy-optimized tree branch method for oleic acid starting from the double bond and the acid group. The final energies are very close (9.10,9.02, 8.97 Kcal mol -). from the ethene and ethane paths were 9.10 and 9.02 kcal mol-' respectively, which compares very well to the energy of 8.97 kcal mol-' from the acid-initiated growth (Fig. 3). The structural data for the acid region in the three con-formers is compared in Table 2. The actual energies during growth of course differ all along the build up, but the final structures are almost identical and the energies very close, confirming the precision of the method.Molecular surface areas and cross-sectional diameters were determined using van der Waals radii around each atom, and read directly from PCMODEL. The average molecular diameter along the length of the oleyl tail for the acid, double bond, and ethane- initiated fatty acids were 6.82, 6.79, and 6.83 A, respectively. Oleyl tail lengths for the same three conformers were 18.80, 18.83, and 18.81 A,respectively. Repulsion and strain around the double bond region of the oleyl chain causes the segments on either side of the double bond to be at right angles relative to one another. This gives the molecule a slight helical appearance.Oleic acid was also analysed through a complete conformational search using the Sybyl with the TRIPOS force field,21 run on a Digital Corporation microvax I1 oper-ating at 2 MIPS. Calculated energies of 8.24-10.7 kcal mol- were obtained for the 100 lowest-energy structures, which compared well to values obtained from the energy optimized tree branch method which also uses the MM2 force field which was parametrized for hydrocarbons. TRIPOS is a general potential function which attempts to use fitting con- stants to describe the interactions of heteroatom functional groups, as well as hydrocarbons. The two potential functions were in very good agreement confirming the structural properties of oleic acid.A total of 2.5 million low-energy con- formers were identified, requiring about 4.5 days. The tree branch method provides a lowest-energy structure very close to the complete conformational search, confirming the main hypothesis of the technique. All the Sybyl structures with energy between 8.4 and 10.7 kcal mol-' also had the slight helical twist around the double bond that was also found in the energy-optimized tree branch growth. Another interesting result was that the overall lowest-energy structure from Sybyl, corresponding to 8.24 kcal mol- ',was the bent struc- ture in Fig. 4. The calculated global minimum from complete conformational searches may not have a configuration resem- bling molecules present in a bulk sample, because it only rep- resents conditions obtained by summing a group of interactions. The average structure of the lower-energy con- formers provides a better representation of the molecular surface area and cross-sectional diameter. This further vali- dates the energy-optimized tree branch method: it is not necessary to obtain the global minimum.This conclusion is intuitively correct, since in a bulk sample at room tem-perature, thousands of configurations may be present simul- taneously.-To demonstrate that the method was also applicable to branched molecules, 2,2,5,5-tetramethylhexaneand 2-methy- 5-ethylhexane, were grown using the same procedure. 2,2,5,5- tetramethylhexane was grown from ethane going from one side to the other, introducing the branches as the structure progressed.The minimum energy for the structure for this sequence was 9.15 kcal mol- '. The molecule was also grown following other paths in the energy tree, with insignificant changes to the total energy or final geometry. The good agreement between the structures can be attributed to the small size of the molecule. Further analysis of the branched systems used 2-methyl-Sethylhexane. The molecule was first grown from ethane as indicated by the energy tree in Fig. 5, inserting the branches as the structure progressed. In some steps equal-energy structures were obtained and the molecule with which to continue the growth was randomly selected. The minimum energy of the final structure was 8.03 kcal mol-', which was in good agreement with the value of 8.10 kcal mol-I obtained by growing hexane, and then substitut- ing the methyl and ethyl branches.A listing of the structural data is in Table 3. Both structures and energies were in good agreement with the value of 8.25 kcal mol-', obtained for the global minimum generated from a complete conformational search using the ESOP force field.24 Despite the ESOP parameters being different to those employed in the com- mercial packages, such as PCMODEL, the similarity of the energy is remarkable. Remember that all these force fields were parametrized using similar molecular data. The struc- tural data from the ESOP calculation is in Table 4. In this example the tree branch method provided a structure identi- cal to the global minimum from the complete conformational search, a function of the small size of the molecule.Selecting a path other than the one in Fig. 4 resulted in the configu- ration in Table 5. This conformer had a calculated minimum energy of 8.15 kcal mol-', and has the same probability of being present as any other low-energy conformer. Once again, the method has shown that relying on a single path does not necessarily produce the global minimum, or even all the lowest-energy minima, however, the structure is one of the possible lowest-energy conformers, and its structural properties such as bond lengths, angles, volume, cross-sectional diameter and molecular surface area, are the same as for the other low-energy conformers.Polyisobutylene with a molecular weight of 600 g rno1-I was also constructed using the energy-optimized tree branch method. The most obvious growth sequence for polyisobutyl- Fig. 4 Minimum-energy structure of oleic acid from Sybyl J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Comparison of oleic acid calculated from different starting groups atom bond bond angle torsional tYPe length/A /degrees angle/degrees connectivity ethane 0 0 C 1.2089 1 0 0 0 1.3402 122.1137 2 1 0 C 1.5186 110.8128 179.9604 2 3 1 C 1.5365 1 13.2504 177.8234 4 2 3 C 1.5379 112.1259 179.6342 5 4 2 C 1.5375 1 1 1.6270 179.3274 6 5 4 C 1.5374 112.3939 179.5047 7 6 5 C 1.5378 11 1.5461 178.8991 8 7 6 C 1 S356 1 12.2807 179.2737 9 8 7 C 1 SO78 110.6588 178.5212 10 9 8 C 1.3426 126.2329 100.2658 11 10 9 C 1SO64 125.9936 0.3876 12 11 10 C 1.5356 110.7653 96.5628 13 12 11 C 1.5368 1 1 1.6877 179.8506 14 13 12 C 1.5370 112.1 168 179.7753 15 14 13 C 1.5375 11 1.5907 179.8972 16 15 14 C 1.5369 112.1299 179.903 1 17 16 15 C 1.5372 11 1.7874 179.9441 18 17 16 H 0.97 19 116.6471 O.oo00 3 2 1 C 1.5346 11 1.8221 180.oooO 19 18 17 acid 0 0 0 0 C 1.2089 1 0 0 0 1.3402 122.1 353 2 1 0 C 1.5185 110.81 54 179.9604 2 3 1 C 1 S365 113.2664 177.8356 4 2 3 C 1S379 1 12.0993 179.5922 5 4 2 C 1.5357 11 1.6729 179.3779 6 5 4 C 1.5374 112.3298 179.4398 7 6 5 C 1.5378 11 1.6298 178.9474 8 7 6 C 1,5357 112.2148 179.2209 9 8 7 C 1.5078 110.7261 178.6558 10 9 8 C 1.3526 126.1960 10.191 1 11 10 9 C 1.5065 125.9616 0.4293 12 11 10 C 1.5359 110.8218 96.2108 13 12 11 C 1.5369 11 1.6741 179.7521 14 13 12 C 1.5371 112.1354 179.823 1 15 14 13 C 1.5377 11 1.6027 179.8064 16 15 14 C 1.5369 112.1300 179.9093 17 16 15 C 1.5371 11 1.7938 180.oooO 18 17 16 H 0.97 19 116.6472 0.oooO 3 2 1 C 1.5345 1 1 1.8208 180.oooO 19 18 17 ethene 0 0 C 1.2089 1 0 0 1.3402 122.1350 2 1 0 C 1.5186 110.8149 179.9605 2 3 1 C 1.5365 1 13.2558 177.8305 4 2 3 C 1.5378 112.0982 179.5902 5 4 3 C 1.5375 1 1 1.6523 179.381 4 6 5 4 C 1.5375 112.3215 179.4736 7 6 5 C 1.5378 11 1.6192 178.9503 8 7 6 C 1.5357 112.2150 179.2259 9 8 7 C 1.5078 110.7281 1 78.66 1 0 10 9 8 C 1.3426 126.2123 100.2 101 11 10 9 C 1 SO65 125.9642 0.3852 12 11 10 C 1.5358 110.8176 96.452 1 13 12 11 C 1.5369 1 1 1.6259 179.6082 14 13 12 C 1.5371 112.1350 179.4562 15 14 13 C 1.5376 11 1.6128 179.8856 16 15 14 C 1.5370 112.1255 179.9225 17 16 15 C 1.5371 1 1 1.6998 180.oO20 18 17 16 H 0.97 19 1 16.647 1 0.oooO 3 2 1 C 1.5345 111.8215 179.9986 19 18 17 ene was to add the methyl groups as the structure progressed.mated from Langmuir trough studies of polyisobutylene sur- The calculated diameter and length based on van der Waals factants at the air/water interfa~e.~’ radii around each atom for the 660 molecular weight struc- The energy-optimized tree branch method was also applied ture were 6.60 and 33.1 8, respectively. The cross-sectional to structures containing heteroatoms, with the amino ester in diameter compared very favourably to values of 6-7 %i esti-Fig.6. The molecule was grown from formic acid, first adding J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 CH3CH3 E= 0.82 CH34H2XH3 -CH34H24H3 -CH34H24H3 E= 1.40 E= 1.40 E= 1.40 I I CH3-CH(1)H(2)-CH3 CH3-CH-CH3 I CH3 E= 1.94 out of plane of carbon backbone I CH2CH3 E = 3.63 of carbon backbone in plane I CH2CH3 E = 3.63 CH,-CH--CH,I CHzCH3 E= 4.36 1 CH3-CH-CH3 I CH24H2-CH3 CH2-CH,-C H3 kH2-CH2-CH3 E= 4.30 E= 6.87 E = 5.03 CH,-CH--CH, CH CH--CH, CH CH-CH,1 '-I 3-I CH24H2-CHZ-CH3 CH24H24HZ-CH3 CH2-CH2--CH2--CH3 E= 4.92 E= 5.94 E= 5.71 CH2-CH2-CH-CH3 CH2-CH2-CH--CH3 I I CH3 CH3 E= 6.52 E= 6.24 CH3-CH-CH3 CH3-CH4H3 I I6H2-CH,-CH-CH 3 &H2PH2--CH-CH3 6H2-CH,-C H-CH I I I 6~24~3 CH~-CH~ E = 8.34 E= 9.53 Fig.5 Energy tree for 2-methyl-5-ethylhexane. The higher energy conformers are caused by the repulsion between axially (out-of-plane) 1,3 substituted groups. the terminal methyl group to give acetic acid, and then method. This is an undesirable but necessary approach, since forming the ester and subsequent amine portions. The no means of overcoming the problem through a lowest-minimum energy obtained from the tree branch method was energy path exists. Configurations for standard cyclic or 10.92 kcal mol-', which compared well to the minimum strained structures, such as six-, five-, and four-member rings energies of 10.43 to 19.15 kcal mol-' obtained from a com-are conveniently stored in memory in all molecular modelling plete conformational search using ESOP.A comparison of packages, and added to the growth sequence as required for the tree branch structure to the 10.43 kcal mol-' one from ESOP is in Fig. 6. The hybridization on the oxygen atom in the ester, and the nitrogen atom, do not effect the tree branch method. Strained structures can be directly analysed by the tree branch method using paths other than the lowest energy path to create the molecules. The most obvious example is ring closure, where two terminal groups must be brought close together before being bonded. Repulsion will prevent the molecule from ever assuming such a configuration, unless constrained to do so.If the molecule contains rings which are joined by acyclic segments, the complete rings can be intro-duced as a single preformed unit of known energy and con-Fig. 6 Comparison of ESOP and tree branch structure of amino nected by acyclic structures obtained from the tree branch ester J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Comparison of structures 1 and 2 of 2-methyl-5-ethyl hexane bond bond angle torsional atom type length/A /degrees anglejdegrees connectivity structure 1 C C 1.5396 C 1.5429 110.1234 C 1.5383 1 13.9036 174.5269 C 1.5454 1 14.5020 169.6240 C 1.5403 109.2033 173.0486 C 1.5427 11 1.8547 62.873 1 C 1.5353 1 14.278 1 179.944 1 C 1.5379 112.3129 122.6583 structure 2 C C 1.5395 C 1.5430 110.1585 C 1.5385 1 13.8844 173.99 16 C 1.5455 114.4665 169.1028 C 1.5405 109.2397 172.2038 C 1.5426 11 1.8746 63.7338 C 1.5353 114.2387 178.921 7 C 1.5378 1 12.2839 122.6894 this very reason.Remember, however, that ring opening can formers similar to the lowest-energy structures generated by be modelled and the resulting structures calculated by the complete conformational searches. It does not attempt to tree branch method. evaluate more than a few minima, but is essentially the global Conclusion minimum. However, from the Sybyl example where the lowest-energy structure for oleic acid was the bent structure The energy-optimized tree branch method provides a simple which has a relatively low probability of being present in the and reliable means of predicting structural features such as bulk phase, knowledge of the global minimum is not impor- molecular surface area, volume, length, and diameter for tant for most studies of interest.Unlike other tree branch small to medium size organic molecules. The calculations can methods which have been limited by the time required in the be performed on a personal computer in a few hours, making energy minimization phase of the analysis, l4 increased com- the technique assessable to industrial groups who may not putational speed will not provide any benefit. The tree have access to dedicated work stations or mainframes. The branch method can be used to generate any required struc- method has been shown to provide energy-minimized con- ture.Table 4 Structural data from ESOP complete conformational search of 2-methyl-5-ethyl hexane bond bond torsional atom type length/A anglejdegrees anglejdegrees connectivity 1.5399 1.5417 1 10.0709 1.5397 113.8977 175.3067 1 S439 114.3066 168.3503 1.5377 10 1.7702 173.0486 1.5446 1 1 1.8875 57.6454 1.5338 114.2685 180.oooO 1.5367 112.1453 122.6732 Table 5 Degenerate path for 2-methyl-5-ethylhexane bond bond ~~~~~ torsional atom type length/A angle/degrees angle/degrees ~~ connectivity 0 1.5401 0 1 1.5432 110.075 1 0 2 1 1.5385 114.061 1 17 1.9479 1 3 2 1 1.5461 114.55 15 179.68 10 1 4 3 2 1 S453 109.5887 169.7861 1 5 4 3 1.5388 1 1 1.7400 66.4639 1 5 4 3 1.5352 114.3971 17 3.594 1 1 6 5 4 1.5283 112.683 1 122.5498 1 2 3 1 54 This research was partially supported by the NSERC (Canada).The authors acknowledge the help of Professor M. J. S. Dewar and Dr. E. Healy for the copy of AMPAC (AMl), Dr. B. Wilkes of the Clinical Research Institute for the use of Sybyl, and Dr. B. Massek of ICI Pharmaceuticals for the use of ESOP. References 1 F. H. Westheimer and J. E. Mayer, J. Chem. Phys., 1946,14,733. 2 F. H. Westheimer, J. Chem. Phys., 1947, 15,252. 3 M. Reiger and F. H. Westheimer, J. Am. Chem. SOC., 1950, 72, 19. 4 N. L. Allinger, Adu. Phys. Org. Chem., 1976,13, 1. 5 U. Burkett and N. L. Allinger, Molecular Mechanics, ACS Monograph 177, Am. Chem. SOC., 1982. 6 E. M. Engler, J.D. Andose and P. V. R. Schleyer, J. Am. Chem. SOC.,1973, 95, 8005. 7 S. Fitzwater and L. S. Bartell, J. Am. Chem. SOC., 1976,98,5107. 8 N. L. Allinger, J. Am. Chem. Soc., 1977,99, 8127. 9 N. L. Allinger, Y. H. Yuh and J. H. Lii, J. Am. Chem. SOC., 1989, 111,8576. 10 J. H. Lii and N. L. Allinger, J. Am. Chem. SOC., 1989, 112,765. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 11 H. A. Scheraga, The Multiple Minima Problem in Conformational Energy Calculations on Polypeptides and Proteins, UCLA Symp. Mol. Cell Biol., 1989. 12 M. Lipton and C. W. Still, J. Comp. Chem., 1988,9,4. 13 M. Saunders and R. M. Jarret, J. Comp. Chem., 1986,7,4. 14 J. H. R. Clarke, Chemistry in Britain, 1990, 26, 349. 15 S. Kirkpatrick, C. D. Gelatt and M. P.Vecchi, Science, 1983,220, 4598. 16 W. C. Still and I. Galynker, Tetrahedron, 1981,37, 3981. 17 R. Bruccoleri and M. Karplus, Biopolymers, 1987,26, 137 18 P. J. DeClerq, Tetrahedron, 1984,40,3717. 19 Computer Aided Drug Design Methods and Applications, ed.T. J. Perun and C. L. Propst, Marcel Dekker Inc. 1989. 20 K. Gilbert, Serena Software, Box 3076, Bloomington, IN. 21 B. Wilkes, Clinical Research Institute, Montreal, Canada. per- sonal communication, Sybyl is available from Tripos Associates. 22 M. J. S. Dewar, E. G. Zoebisch, E. H. Healy and J. J. P. Stewart, J. Am. Chem. SOC., 1985,107, 13 (1985). 23 The Merck Index, Merck & Co. Inc., 9th edn. 1976. 24 Energy and Structure Optimization Program. ICI Americas Inc. Pharmaceuticals Structural Chemistry Group. 25 L. Ghaicha, R. M. Leblanc and A. K. Chattopadhyay, J. Phys. Chem., 1992,96, 10948. Paper 3/03612J, Received 23rd June, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000047

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(1), 55-57 Optically Induced Kerr Constants for a Homologous Series of Alk-I-enes and Three Alkadienes Neil J. Harrison and Barry R. Jenningst Optics Group, J. J. Thomson Physical Laboratory, Reading University, Reading, UK Measurements are reported of the laser-induced Kerr effect for the homologous series of alk-1-enes up to the icosene member. The effect was induced with the electric vector of an Nd : YAG laser beam, pulsed in the nanosecond timescale and operating at either 1064 nm or 532 nm wavelength. On average, values for 532 nm wavelength are some 50% higher than results for 1064 nm. The first recorded measurements on three alka- dienes are also reported using an inducing wavelength of 1064 nm. These data indicate that separation of the unsaturated bonds could be an important factor in the magnitude of the effect.The anisotropic properties of molecular pure liquids have been the subject of study for many years.'-12 The non-linear optical properties, particularly in organic materials, have been a major area of research. One of the principal methods of investigation has been the time-resolved optically induced Kerr effect (OKE). This is where the electric field in an intense laser beam induces transient birefringence in a medium through interaction with the anisotropic electrical polarisability of the component molecules. A second weak laser beam is usually used to probe the resulting birefringence of the medium. On the femtosecond and picosecond time- scales, OKE gives information about intra-and inter-molecular processes. O-" On the nanosecond timescale, resolution of the different fast molecular phenomena is extremely difficult with the exception of some re-orientational effects in liquid crystalline systems.' However, effects pro- duced by nanosecond duration laser pulses can give useful information about the contributions to the total macroscopic response from various molecular bond types and functional groups in the molecule.An important area of current research is the development of novel organic materials that offer enhanced second-order and third-order non-linear optical properties over the common crystalline materials in use today. A great deal of effort has been expended in the investigation of unsaturated molecules with multiple 71-bonds.Compounds such as trans-retinol,14 /3-carotene," polyenes and polynes (for a com-prehensive review of theoretical and experimental results see ref. 16) have all been studied extensively using non-linear optical techniques. Little work has been done, however, on molecules with a single unsaturated bond. We have pre- viously shown 17*18 that for several different molecular species, the contribution from the unsaturated bond domi- nates any contribution from an unsaturated alkyl chain. In this work we have extended some preliminary published results" for short alk-1-enes to higher chain members in the search for any observable effects from the alkyl chain. We also report the first OKE results for the alk-1-enes at a wave- length of 532 nm in an attempt to find any dependence on the inducing laser wavelength.Calculations of the charge distribution in butadiene have shown that the central single bond in the molecule has a certain degree of double bond characteristic^'^ due to an apparent smearing of the two adjacent 71-bonds in the mol- t Also at ECC International Ltd., John Keay House, St. Austell, Cornwall, UK PL25 4DJ. ecule. We have therefore made OKE measurements on the three higher alkadienes, hexadiene, octadiene and decadiene, to see if the increasing separation between the two double bonds with these structures, influences non-linear optical properties. Background Theory The induced birefringence response of a pure liquid to an intense laser pulse can be defined by an optical Kerr con- stant, B,, comparable to the more widely known electrically invoked Kerr constant of a pure liquid.The optical Kerr con- stant is defined by: AnBo = -Ap E2 where E2 is the mean-squared value of the applied electric field associated with the inducing laser beam of wavelength Ii.The parameter An is the birefringence induced in the sample and Ap is the wavelength of the probe beam. Alterna- tively the optical Kerr constant can be related to the third- order susceptibility tensor xi:il. For an isotropic pure liquid the requisite components of the tensor are given in the expression :20 Of paramount importance to any attempt to quantify OKE data is an accurate assessment of the effective electric field in the inducing laser pulse.As previous authors have often failed to give details of the formulae used to estimate the rms effective field strength, E, the equation used by the present authors in this and former studies is derived. Three parameters of the inducing laser pulse can be mea- sured, namely, the total pulse energy J, the pulse duration T and the spatial radius w of the beam. Assuming the spatial intensity to be Gaussian, this is the half-width where the intensity is (e-') of the peak value. For the probe beam, an equivalent half-width is R. The Poynting vector of energy flow rate per unit area along the direction of light propagation defined by unit vector G is s= E x H= IEllHlR (3) where H is the rms effective magnetic vector in the beam.From the definition of the intrinsic impedance of a beam in J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 free space, z = E/H = ,/(,u,/E,) s= J(+PO (4) But the intensity I of a beam is the rate of energy flow per unit area so that E = (~,/E,)~/~Z~’~ Amongst the precautions specific to the experiments were (a) that single mode TEM,, inducing beams were used to ensure an effective unidirectional electric vector and (b) that the probe beam was of smaller diameter than the inducing beam and passed coaxially with it through the sample. In the nano- second pulse regime, the induced birefringence closely fol- lowed the inducing laser temporal profile, so that the maximum birefringence amplitude corresponded to the peak of the inducing laser intensity profile.For Gaussian profiles, the intensity in the beam at a dis- tance r from the centre and at any instant t, is given by I(r, t) = I(O, t)exp(-r2/w2) (6) and the total power in the beam at this instant is P(t)= I(r, t)2m dr (7)r Hence, the fraction of the total inducing beam power which is associated with the common beam area contained in the circle of effective radius R is [Z(r, t)2m dr o3F= = [I -exp(-R2/w2)] (8) I(r, t)21rrdr We may rewrite the average intensity associated with the common region as Zav(t), with (9) and at time t = 0, Z,,(O) equates with I,,, . Assuming that the temporal pulse profile of a laser is also Gaussian, then and the fraction of pulse energy transmitted through the cir- cular centre of radius R is (JF) = 21rR~1,,, rexp( -t2/r2)dt (1 1) Integrating and substituting field strength values for inten- sities via eqn.(5) gives Finally, allowance is made for the energy loss from the indu- cing beam through absorption in the sample. Using the average energy of the inducing pulse as J,, = (1 -;)J where qS is the fraction of the total inducing beam energy absorbed over the sample length. The final equation for the effective field in the inducing laser beam responsible for the observed birefringence becomes (1 -@/2)J[ 1 -exp(-R2/w2)] (14)R22 Since the electric field in the laser beam is difficult to quantify and the equations used to derive a value vary wildly between authors, an optical Kerr constant relative to benzene has commonly been defined. In this paper both B, and Brelare given.Experimental A Q-switched Nd :YAG laser (Lumonics HY400) was used as the source of the high-power, birefringence-inducing, laser pulses. To ensure that the inducing laser pulses had a uni- directional electric vector across the spatial profile, an intra- cavity aperture was used to select the TEM,, mode, which was Gaussian in profile. The Nd : YAG operated at a funda- mental wavelength of 1064 nm and could optionally, with the inclusion of an extra-cavity frequency-doubling crystal, operate at the second-harmonic wavelength of 532 nm. The probe beam was selected to be anharmonic to both the indu- cing laser beam frequencies and was provided by a CW Helium-Cadmium laser (Omnichrome model 456X) oper- ating at 442 nm in the TEM,, mode.The probe beam was linearly polarised at 45” azimuth to the inducing beam pol- arisation direction. The sample was held in a glass cell placed between two crossed polarisers. Following the application of the inducing laser pulse, the induced birefringence was transient in nature and recorded by a fast photomultiplier which was connected to a 1 Giga- sample per second digitising oscilloscope. This was also used to record simultaneously the laser pulse profile which was recorded in transmission using a fast response energy meter. Simultaneous measurement of both the pulse energy and temporal profile by the energy meter enabled correct estima- tion of the inducing field amplitude within the applied laser pulse. The recorded signal transients were then transferred to a computer which acted as an instrument controller and also performed automatic signal evaluation and analysis.The experimental apparatus has been described in detail else- where.” Results and Discussion The OKE constants for the series of alkenes with chemical formula CH,=CHC,H,,+,, are tabulated in Table 1 and plotted in Fig. 1. Results for the long alkyl chain alkenes, dodec-1-ene to icos-1-ene at Aj = 1064 nm continue the approximately constant trend as previously observed for the lower members of the family with only a slight increase in value for hexadecane and above.For molecules consisting solely of an alkyl chain and a single double or triple bond, B, appears to be dominated by the contribution from the Ir-bonds in the system. Any effect due to the alkyl chain is negligible, at least up to the eighteenth chain length member of the series. Results for the alk-1-enes at an inducing wavelength of 532 nm are more variable and interesting. First, the overall mag- nitude of the effect is substantially higher for this green indu- cing wavelength than for the infra-red Izi = 1064 nm values. This is in agreement with the results for the alkanoic acids” and for benzene.21 Secondly, data recorded with an inducing wavelength of 532 nm appear to demonstrate an unusual J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Optical Kerr constants for the alk-l-enes 're I B,/i0-14 m V-' pure liquids 532 nm 1064 nm 532 nm 1064 nm pent- l-ene - 0.332 - 0.171 hex-1-ene 0.3 1 0.287 0.242 0.148 hept- l-ene 0.36 0.332 0.283 0.171 oct-1-ene 0.43 0.290 0.344 0.149 non-l-ene 0.37 0.333 0.292 0.171 dec-1-ene 0.33 0.323 0.257 0.166 undec- l-ene 0.30 0.324 0.239 0.167 dodec-1-ene 0.28 0.348 0.219 0.179 tridec-1-ene 0.32 - 0.252 - tetradec- 1-ene 0.39 0.33 1 0.307 0.170 hexadec-1-ene - 0.39 1 - 0.201 octadec-1-ene - 0.400 - 0.206 icos-1-ene - 0.367 - 0.189 Wavelengths Ai = 532 nm and li= 1064 nm. Ap = 442 nm for all measurements. Values carry an experimental uncertainty within 5% at 1064 nm and 10% at 532 nm wavelengths.response with increasing chain length for the alk-l-enes, which does not appear to correlate with any facet of the infrared induced results. The variability is very reminiscent of the 'odd-even' effect reported for OKE experiments on liquid crystals, of which the data on p-p'-di-n-alkoxyazoxy- benzene are typical.' In the present study, however, it is not a true function of the odd and even number of carbon atoms in the molecular backbone. For the series members studied, the data might fit a cubic function, or relate to four or five unit segment collective behaviour. The wavelength variability of the data suggests that they could well relate to selective absorption properties of the molecules.If true, this could be exploited in optoelectronic devices. Further experiments are needed to understand this phenomenon for these liquids. The results for the alkadienes, of chemical formulae CH,=CH(CH,),CH=CH,, are listed in Table 2. Clearly for these liquids there is a significant reduction in the size of the effect with the Ir-bond separation. One tentative explanation of the results is feasible by analogy with the charge distribu- tion studies on butadiene. l9 For comparatively small n-bond 0.4 I 1 T 0.01 ' '. '. 1 ' I" " " '. " 2 8 14 20 alkyl chain length (n) Fig. 1 Variations of the optically-induced Kerr constant (B,) for the alk-l-enes. Parameter n indicates the series member in the formula CH,=CHC,H(,,+ ,,. Data measured at 442 nm wavelength with inducing laser wavelength of 1064 nm (full line) and 532 nm (broken line).57 Table 2 Optical Kerr effect values for alkadienes pure liquid 're, Bo/1Wl4 m V-* 1,Shexadiene 0.572 0.294 1,7-0ctadiene 0.542 0.279 1,9-decadiene 0.374 0.192 Measured at Ai = 1064 nm and i,= 442 nm. Experimental uncer- tainty of 5%. separations, there is an effective smearing of the two Ir-bond electrons along the molecule so that the alkadiene then acts as a single, larger, polarisable unit in which the electron cloud can easily be distorted over the length of the molecule. As the bond separation increases this effect is reduced and the two Ir-bonds act as independent polarisable units reducing the overall effects.More experimental data are required before such a simplistic explanation is entertained. N.J.H. would like to thank the SERC for a studentship during which work was carried out. We are also grateful to the Ministry of Defence for an equipment grant to B.R.J. References 1 Z. Blaszczak and P. Gauden, J. Chem. SOC., Faraday Trans. 2, 1988, 8, 239. 2 K. C. Rustagi and J. Ducing, Opt. Commun., 1974, 10, 258. 3 R. A. Huijts and G. J. L. Hesselink, Chem. Phys. Lett., 1988, 156, 209. 4 B. F. Levine and C. G. Bethea, Appl. Phys. Lett., 1974,24,445. 5 J. L. Oudar and D. S. Chemla, Opt. Commun., 1975, 13, 164. 6 A. F. Garito, C. C. Teng, K. Y. Wong and 0.Zamani-Khamiri, Mol. Cryst. Liq. Cryst., 1984, 106, 219. 7 L-Tak Cheng, W.Tam, S. H. Stevenson, G. R. Meredith, G. Rikken and S. R. Marder, J. Phys. Chem., 1991,95, 10631. 8 J. R. Helfin, K. Y. Wong, 0.Zamari-Khamiri and A. F. Garito, Phys. Rev. B, 1988,38, 1573. 9 J. C. Altman, P. J. Elizondo, G. F. Lipscomb and R. Lytel, Mol. Cryst. Liq. Cryst. Znc. Nonlin. Opt., 1988, 157, 515. 10 D. McMorrow, W. T. Lotshaw and G. A. Kelly-Wallace, ZEEE J. Quantum Elec., 1988,24,443. 11 B. A. Garetz and J. M. Khosrofian, Chem. Phys. Lett., 1983, 94, 494. 12 M. G. Kuzyk, R. A. Norwood, J. W. Wu and A. F. Garito, J. Opt. SOC. Am. B., 1989,6, 154. 13 E. G. Hanson, Y. R. Shen and G. K. L. Wong, Phys. Reu. A, 1976,14, 1281. 14 J. P. Hermann, D. Ricard and J. Ducing, Appl. Phys. Lett., 1973, 23, 178. 15 J. Etchepare, G. Grillon, A. Migus, J. L. Martin and G. Harmo- niaux, Appl. Phys. Lett., 1983,43,406. 16 Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, Orlando, FL, ed. D. S. Chemla and J. Zyss, 1987, vol. 1 and 2. 17 N. J. Harrison and B. R. Jennings, Chem. Phys. Lett., 1993, 210, 393. 18 N. J. Harrison and B. R. Jennings, J. Phys. Chem., 1993, W, 151 1. 19 J. N. Murrell, S. F. A. Kettle and J. M. Tedder, The Chemical Bond, John Wiley and Sons, 2nd edn., 1990. 20 N. J. Harrison and B. R. Jennings, Meas. Sci. Technol., 1992, 3, 120. 21 N. J. Harrison and B. R. Jennings, J. Appl. Phys., 1993,73,8076. Paper 3/05324E ;Received 6th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000055

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM.SOC. FARADAY TRANS., 1994, Wl), 59-67 Role of Twisted Intramolecular Charge-transfer States in the Decay of 2-(2'-Hydroxyphenyl)benzothiazole following Excited-state Intramolecular Proton Transfer Charles A. S. Potter and Robert G. Brown* Chemistry Department, University of Central Lancashire, Preston, Lancashire, UK PR 1 2HE Friedrich Vollmer and Wolfgang Rettigt lwan-NStranski Institut, Technical University of Berlin, Strasse des 17 Juni 112, D-10623 Berlin 12, Germany The photophysics of 2-(2'-hydroxyphenyl)benzothiazole in non-polar and alcoholic solution is reported for tem- peratures in the range 96-298 K. In all solvents a rise in both fluorescence quantum yield and lifetime is observed as the temperature is decreased. It is proposed that a viscosity-dependent non-radiative process leading to a non-emissive, twisted excited state accounts for these observations.Results of quantum chemical calculations are consistent with this interpretation. 242'-Hydroxyphenyl)benzothiazole(HBT) is an example of a molecular system which undergoes excited-state intramolecu- lar proton transfer (ESIPT) to yield an excited keto form of the original enol. The system has been studied by a number of different workers'-8 and it is clear that depending on the medium in which the HBT is situated, there are three ground-state species which may be important in terms of the absorption properties of the molecule; the neutral enol, anion and cation. All three can also contribute to the excited-state properties of HBT, together with the excited keto form resulting from ESIPT.Scheme 1 shows the various species which can potentially contribute to the photophysics of HBT together with some of the related spectral and pK data. In the solid state at room temperature, only fluorescence from the excited keto species is ~bserved.~.~ This is also the case at low temperature in argon at 12 K6and in methyl- cyclohexane at 77 K.7 Phosphorescence from the keto excited triplet state peaking at 648 nm was also reported in the latter t Present address: W. Nernst Institut fur Phys. und Theor. Chem., Humboldt University of Berlin, Bunsenstr. 1, D-10117 Berlin, Germany. study. In solution at room temperature, the keto fluorescence band is usually observed independent of the medium.Fluo- rescence from the excited anion, enol or cation requires a strongly hydrogen-bonding solvent to disrupt the intramole- cular hydrogen bond so that other processes such as inter- molecular proton transfer and the normal radiative and non-radiative decay processes are able to compete with ESIPT.'*5,8 Under conditions where ESIPT is not disrupted, its kinetics have been found to be extremely rapid with only a lower estimate for the rate constant usually being calculable. In tetrachloroethylene, Laermer et al. have determined a rate constant of cu. 6 x 10l2 for the ESIPT process from the 170 fs risetime of the keto fluores~ence.~ It is a reasonable assumption that similar rate constants for ESIPT will be the case in other non-hydrogen-bonding solvents.It has been suggested that the observed fluorescence from the keto tauto- mer consists of contributions from both the cis and trans con-formations.2,'0 Further evidence to support this proposal are reports of two ground-state transient species with micro- second lifetimes. '**' ' We have been studying the HBT system for a number of c lh ; 510nm I 7 -a;*-0r-p-0;q -0 0 cis -keto trans -keto -9 Scheme 1 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 perature dependence of HBT fluorescence in a variety of solvent media which we have undertaken in order to explore the possible contribution of twisted intramolecular charge transfer (TICT) states12-14 in the photophysics of HBT.The literature reports of two excited- and ground-state keto tau- tomers suggest that twisting about the 2-1’ carbon-carbon bond may play a significant role. We also report the results of some molecular orbital calculations which model the excited- state energetics of the various species which may contribute to the photophysics in this system. Experimental HBT was prepared and purified as described previously.2v5 All the solvents used here were of spectroscopic grade (mostly Merck uvasol) and were used as supplied. Absorption spectra were measured on Perkin-Elmer Lambda 3 or Cary 14 spec-trophotomers and corrected fluorescence spectra on Perkin- Elmer LS5 or 650/60 spectrofluorimeters. Fluorescence quantum yields were determined relative to quinine sulfate in 0.05 mol dmP3 sulfuric acid (& = 0.5515).The error in the quantum yield values is estimated at f10%. Fluorescence lifetime measurements were undertaken at the BESSY synchrotron radiation source using low-temperature equipment described previously.l6 The time- correlated, single-photon counting technique’ ’ is used to accumulate the data using the single bunch mode of oper- ation of the synchrotron. Excitation wavelengths in the 340- 360 nm region were employed. The decay profiles were analysed by iterative reconvolution and the ‘goodness of fit’ judged on the basis of x2 values and the distribution of residuals. With an apparatus response function of 0.5-0.7 ns, decays could be analysed down to 0.05-0.1 (f0.05) ns.The quantum chemical calculations for the ground state were conducted using the GAUSSIAN88 package” for ab initio calculations on the STO-3G level and for AM1. Energy differences between the ground state and the various excited states were derived by the CNDO/S-CI method according to Del Bene and Jaffe. 9t Results and Discussion The photophysical properties of HBT vary considerably with the medium in which the HBT is situated. In this study we have measured the fluorescence properties of HBT as a func- tion of temperature between 96 and 298 K in a non-polar solvent mixture (methylcyclohexane-2-methylbutane:MCH-2-MB) and in polar, hydrogen-bonding solvents (ethanol, propan-1-01 and mixtures of the two). Although the absorp- tion spectra of HBT in these solvents are very similar (see Fig.1A) and are in agreement with literature parameters:.’ the fluorescence spectra differ considerably between the two classes of solvents. As may be seen from Fig. lB, HBT in the alcohols exhibits two emission bands which can be attributed to the enol (E, short-wavelength band) and the keto species (K, long-wavelength band). Only the K band is observed in the aprotic non-polar solvent MCH-2-MB. We have pre- viously reported three emission bands for HBT in aqueous solution depending on the PH.~ The additional fluorescence band in water originates from the HBT anion and, in wave- length terms, lies between the two bands observed in the alcohols. In other polar, non-hydrogen-bonding solvents such ~ t Program QCPE no. 333 from Quantum Chemistry Programme Exchange, Bloomington, Indiana, was used with the original param- eters, 50 singly excited configurations were used for configuration interaction.0.8 4 wavelengt h/nm 100 B 300 350 400 450 500 550 600 E wavelength/nm Fig. 1 A, Absorption and B, fluorescence spectra of HBT in MCH- 2-MB (a)and ethanol (b) (concentration 2.0 x mol dm-’) as acetonitrile, a single, long-wavelength fluorescence band attributable to K is observed as in non-polar Solvents. The origins of the three fluorescence bands observable in the various solvents are summarised in Scheme 1. The intensity of HBT fluorescence at room temperature in the solvent used here is weak.As Tables 1 and 2 show, fluo- rescence quantum yields in the range 0.01-0.05 are found. As the temperature is decreased, the fluorescence quantum yields uniformly increase such that at least an order of magnitude increase over room temperature is observed at 96 K. In the alcohol solvents it is noticeable that, as the temperature is lowered, there is a greater increase in the amount of keto tautomer fluorescence relative to the enol such that the ratio ~enoJ4ke,ofalls from a value in the region of 10 at room tem- perature to a value closer to unity at 96 K. These quantum yield changes are depicted in Fig. 2 for the pure ethanol solvent which typifies the quantum yield behaviour in alco- hols as a function of temperature. At room temperature, the fluorescence from both the enol and keto species is weak and causes considerable scatter in the values of the ratio 4enoJ4keto(Table 2).We are therefore not convinced that there is a real solvent effect on this ratio at room tem-perature. At reduced temperature, where the quantum yield values are higher, the variation of 4enoJ4kctowith the com- position of the alcohol solvent may be significant. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Fluorescence quantum yield and decay data for HBT in MCH-2-MB as a function of temperature i/nm x2 4f 1.Ooo 0.11 --1.15 0.1 1 0.095 8.9 298 {500 0.01 1 540 1.Ooo 0.08 --1.26 0.08 0.141 13.2 223 -------0.062 --0.210 0.98 0.790 2.51 1.02 2.19 0.089 0.37 173 {500 0.200 520 1.Ooo 2.60 --1.29 2.60 0.075 0.3 1 0.278 1.16 0.722 3.74 0.92 3.03 0.095 0.24 0.290 1.Ooo 5.01 --1.29 5.01 0.057 0.14 1.Ooo 5.30 --0.95 5.30 0.060 0.13 0.320 96 1.Ooo 5.16 --1.14 5.16 0.06 1 0.13 We have also measured fluorescence decay profiles for calculated by eqn.(1): HBT in these solvents over the same temperature range as the fluorescence spectra and quantum yields. The data (7) = xui 7Jxai (1) obtained are presented in Tables 1, 3 and 4 and some typical Note that all of the fluorescence decay profiles exhibit a decay profiles are shown in Fig. 3. Corresponding emission prompt rise. There is no evidence of a risetime slower than cu. spectra are shown in Fig. 4. For the most part the decays are 50 ps in any of the profiles in the different solvents or at the not monoexponential except for some of the keto fluores- different temperatures.These observations are clearly in cence profiles at the lowest temperature used. Here the decay accord with the rapid ESIPT step obtained by Laermer et time is very similar to that found for solid HBT at room ~1 as far as the keto fluorescence is concerned. They also .~ temperature3 although the keto fluorescence yield is still only suggest that the enol fluorescence observed in the alcohols is a fraction of the fluorescence quantum yield for the solid. either excited directly from the ground state (i.e.excitation of The majority of the fluorescence decays require a sum of an HBT molecule intermolecularly hydrogen bonded to two exponential components to fit the data adequately.In a solvent molecules) or is produced from the intramolecularly few cases, three exponential components are required to give hydrogen-bonded enol tautomer of HBT which is the precur- a x2 value < 1.2 and a random distribution of residuals. The sor of the keto tautomer. As the excitation spectra for enol use of three exponential components in the analysis of the and keto fluorescence are identical, there is no obvious spec- data does not tend to alter the mean lifetime (T) which is tral evidence for the presence of both intra- and inter-Table 2 Fluorescence quantum yields for the enol(4,J and keto (4K)forms of HBT in alcohol solvents at various temperatures solvent temperature/K 4cotal 4E 4K 4J~K 100% EtOH 298 0.021 0.019 0.002 8.8 223 0.048 0.042 0.006 7.6 173 0.102 0.090 0.012 7.7 123 0.148 0.125 0.023 5.3 96 0.161 0.128 0.033 3.9 75% EtOH-25% PrOH 298 0.02 5 0.022 0.003 8.3 223 0.067 0.06 1 0.006 10.2 173 0.150 0.136 0.014 10.0 123 0.21 3 0.180 0.033 5.5 96 0.230 0.177 0.053 3.4 50% EtOH-50% PrOH 298 0.044 0.034 0.010 3.4 223 0.078 0.065 0.013 5.1 173 0.136 0.109 0.027 4.0 123 0.227 0.172 0.055 3.1 -96 25% EtOH-75% PrOH 298 0.012 0.010 0.002 6.1 223 0.068 0.055 0.013 4.3 173 0.154 0.120 0.034 3.6 123 0.257 0.180 0.077 2.4 96 0.280 0.173 0.107 1.6 100% PrOH 298 0.010 0.008 0.002 4.7 223 0.029 0.024 0.005 4.5 173 0.064 0.050 0.014 3.4 123 0.088 0.059 0.029 2.0 96 0.093 0.056 0.037 1.5 50 100 150 200 250 300 temperature/K Fig.2 Variation of fluorescence quantum yields and lifetimes for the enol and keto forms of HBT in 100% ethanol as a function of temperature:(a)enol lifetime, (0)keto lifetime; (H)enol yield, (A)keto yield 10 1 10000 g10 000 3 2 1000 2 1000 v) Q)c. 100 .E 100 Q) c a 10 5: 10 E -2 ’c Fig. 3 Fluorescence decay profiles for the enol (a) and keto (b)forms of HBT in ethanol as a function of temperature J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 molecular hydrogen-bonded ground-state forms of HBT in alcohols. However, the rapidity of ESIPT does raise the ques- tion as to whether breaking of the intramolecular hydrogen bond to yield the intermolecularly hydrogen-bonded species (and hence enol fluorescence) can compete efficiently with ESIPT.The data are more compatible with a scheme involv- ing ground-state solvates (Scheme 2) which differ only mar- ginally in their absorption properties. Eointer solvation EOinra -KO Scheme 2 Although both the enol and keto fluorescence decay pro- files are multiexponential under most of the conditions studied, there is no evidence from the analysis of the decays for a kinetic relationship between the two excited species once they are formed. There appears to be no relationship between the decay parameters for the enol and keto fluores- cence from HBT in a given alcohol solvent at a given tem- perature. Reverse ESIPT in the excited keto tautomer is therefore not competitive with its radiative and non-radiative decay routes.Neither is reformation of the intramolecular hydrogen bond in the HBT enol with intermolecular hydro- gen bonds to solvent molecules competitive with fluorescence and non-radiative decay in this species. The two fluorescent species appear to be kinetically isolated from one another. The origin of the multi-exponential decay profiles is there-fore not immediately apparent. In the absence of any obvious risetime components and with the two fluorescent species kinetically isolated, the multi-exponential decays must be an inherent property of the two species. In the alcohol solvents where a range of solvated species could be anticipated to exist, the observation of multi-exponential decays which reflect a range of excited species with a distribution of life- times is not unreasonable and echoes conclusions some of us have drawn elsewhere.*’ A possible kinetic mechanism accounting for the data observed in alcohols is given in Scheme 2.Here, the indices inter and intra refer to differently solvated HBT species, with Eintrabeing the intramolecularly hydrogen-bonded enol tautomer prepared for the ESIPT process, and Einterthe HBT species intermolecularly hydro- gen bonded to other alcohol molecules. This can happen in a variety of fashions, hence a distribution of rate constants k: and k:, (see below) is expected if excited-state equilibration is not fast enough. Given that the fluorescence quantum yield will present an ‘averaged’ view of the emission efficiencies of this range of excited species and is effectively the integrated intensity under the decay curve, we feel that it is reasonable to use the (z) values from eqn. (1) in our later analysis.The biexponential decays, even though many are only slightly biexponential, observed in MCH-ZMB are not as easy to explain in these non-interacting solvents. Once again (7) values will be used in our further analysis of the data. Combination of the measured yields with the (z) values gives radiative (k,) and non-radiative (k,,,) rate constants for the excited enol (k:, k:,) and the keto (kr, kf,) tautomers. These values are shown in Tables 1 and 5. In MCH-2-MB, kr is independent of temperature whereas kfr decreases sharply with decreasing temperature.A temperature-dependent non-radiative process is indicated which appears to be almost completely ‘frozen out’ at 96 K. We can there- fore calculate the temperature-dependent part of the non- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Fluorescence decay parameters for the enol and keto forms of HBT in alcohol solvents at various temperatures solvent Alnm temperature/K a1 r,/ns a2 t,/ns X2 (t>/ns 100% EtOH 380 298 0.998 0.39 0.002 4.50 1.22 0.40 223 0.771 0.47 0.229 0.77 1.65 0.54 173 0.908 0.64 0.092 1.21 1.30 0.69 123 0.877 0.64 0.123 1.32 1.26 0.72 96 0.860 0.73 0.140 1.59 1.08 0.85 510 298 0.988 0.14 0.012 3.00 1.71 0.18 223 0.982 0.24 0.018 1.86 1.63 0.27 173 0.67 1 0.73 0.329 1.65 1.14 1.03 123 1.Ooo 5.13 - - 1.14 5.13 96 1.Ooo 5.71 - - 1.01 5.71 75% EtOH-25Oh PrOH 380 298 0.999 0.40 0.00 1 3.65 1.43 0.40 223 0.968 0.55 0.032 1.38 1.67 0.58 173 0.935 0.64 0.065 1.35 1.05 0.68 123 0.889 0.64 0.111 1.36 1.35 0.72 96 0.882 0.67 0.118 1.39 1.17 0.76 510 298 0.990 0.16 0.010 2.97 2.25 0.19 223 0.98 1 0.29 0.019 2.04 1.32 0.32 173 0.600 0.96 0.400 2.04 1.15 1.40 123 0.143 2.97 0.857 5.66 1.01 5.27 96 1.Ooo 5.70 - - 1.17 5.70 50% EtOH-50% PrOH 380 29 8 0.999 0.40 0.001 2.68 1.29 0.40 223 0.890 0.50 0.1 10 0.92 1.34 0.55 173 0.819 0.56 0.181 1.03 1.20 0.66 123 0.868 0.60 0.132 1.23 1.17 0.68 96 0.854 0.63 0.146 1.27 1.10 0.72 510 29 8 0.994 0.13 0.006 3.1 1 1.32 0.15 223 0.986 0.34 0.014 1.98 1.24 0.36 173 0.602 1.25 0.398 2.40 0.9 1 1.71 123 1.Ooo 5.39 - - 1.22 5.39 96 1.Ooo 5.77 - - 1.14 5.77 25% EtOH-75% PrOH 380 298 0.999 0.39 0.00 1 3.98 1.36 0.40 223 0.969 0.53 0.03 1 1.32 1.48 0.56 173 0.986 0.57 0.014 1.14 1.23 0.64 123 0.884 0.6 1 0.116 1.29 1.15 0.69 96 0.886 0.65 0.1 14 1.37 1.04 0.74 510 298 0.990 0.14 0.010 3.49 1.32 0.17 223 0.984 0.42 0.016 2.04 1.42 0.44 173 0.49 1 1.56 0.509 2.75 0.93 2.17 96 1.Ooo 5.76 - - 1.18 5.76 100% PrOH 380 298 1.OOo 0.38 - - 0.63 0.38 173 0.735 0.47 0.265 0.94 1.20 0.60 96 0.796 0.57 0.204 1.23 1.29 0.70 510 298 0.990 0.11 0.010 2.78 1.25 0.13 173 0.75 1 2.29 0.249 4.04 1.30 2.72 123 0.182 0.53 0.818 5.50 1.17 4.60 96 0.070 1.89 0.930 5.83 2.47 5.55 radiative rate [k;,(T)]from eqn.(2): the range (9.2-11.7) f2.0 kJ mol-'. However, given that the lifetime values at room temperature can be considered as 1 1 only an upper limit owing to the instrumental responsekf,(T)= ---profile, the true activation energy values will be somewhat r(T) ~(96K) higher than these and will be similar to the activation energy An Arrhenius plot of log kf,(T)0s. 1/T yields an activation in MCH-2-MB. The corresponding rate constants for the energy of 12.2 f2.0 kJ mol-'.enol fluorescence in alcohols exhibit much smaller changes In the alcohol solvents the same observations as in MCH-with temperature. 2-MB may be made with respect to k: and k;,. The values of Most interestingly, there is a reasonably clear trend for k:, kr are more scattered than those calculated for the non-polar which increases by a factor of ca. 5 between 298 and 96 K, solvent but there is no obvious trend to the values. We attrib- whereas kfi, decreases by a factor of only ca. 3 between these ute the scatter to the low fluorescence quantum yield of the two temperatures (Table 5). The temperature dependence of keto species at all temperatures in the alcohols coupled with k: as well as the non-exponential nature of the decay, sup- short fluorescence lifetimes at the higher temperatures.These ports the above view of a number of emitting states, Eintcr properties, together with the presence of the two ground-state (incapable of ESIPT owing to their intermolecular solvation), enols, introduce sufficient uncertainty into the kr values that with different individual k: and k:, values, which may depend we have to conclude that kr is probably invariant with tem- differently on temperature, The temperature dependence of perature in all the solvents studied. The slopes of the Arrhe- the /cEr values transforms into changes of the relative popu- nius plots are all quite similar and give activation energies in lations of the different Einte, species and therefore into a 64 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Fluorescence decay parameters for the HBT keto form in alcohol solvents at various temperatures and emission wavelengths solvent temperature/K I/nm a1 '51/ns a2 Tz/ns xz ('5>/ns 100% EtOH 298 510 0.988 0.14 0.012 3.00 1.71 0.18 520 0.993 0.13 0.007 3.00 1.60 0.14 530 0.995 0.1 1 0.005 3.01 1.55 0.12 173 510 0.67 1 0.73 0.329 1.65 1.14 1.03 520 0.652 0.73 0.348 1.60 1.22 1.03 530 0.727 0.79 0.273 1.69 1.23 1.04 96 510 1.Ooo 5.7 1 1.01 5.71 520 1.Ooo 5.72 1.22 5.72 530 1.ooo 5.73 0.94 5.73 75% EtOH-25% PrOH 298 510 0.990 0.16 0.010 2.97 2.25 0.19 520 0.993 0.14 0.007 2.86 1.62 0.16 530 0.99 1 0.19 0.009 2.95 1.37 0.21 50% EtOH-50% PrOH 298 510 0.994 0.13 0.006 3.1 1 1.32 0.15 520 0.997 0.1 1 0.003 3.14 1.39 0.12 530 0.998 0.010 0.002 3.27 1.34 0.1 1 100% PrOH 96 510 0.070 1.89 0.930 5.83 2.47 5.55 520 1.Ooo 5.58 3.00 5.58 530 1.Ooo 5.59 3.16 5.59 change of the effective kfi,, which is an amalgam of the state and in the non-polar solvent at 96 K.This assumption weighted contributions of the individual Einterspecies. is borne out by the fluorescence lifetimes of 5.0-6.0 ns mea- Although the data are insufficient to draw any detailed con- sured at 96 K in the various alcohol solvents. The reduced & clusions on these distributions the above observations clearly values (Table 2) in these solvents may be attributed to the rule out the possibility that there is a single emitting Einter absorbed light being distributed between Einlcrand Eintra species in alcohols, whereas in hydrocarbon solvents the clas- species such that only a portion actually leads to the excited sical two-state scheme (Scheme 2 without the contribution of keto species.Since the calculation of &assumes that all of Einler)seems to be appropriate. the absorbed light contributes to the formation of the keto Note that, with the exception of the keto fluorescence yield species, these quantum yield values will automatically be in alcohols at 96 K, the photophysical properties of the HBT reduced from their true values which would be expected to lie keto form in frozen solution at 96 K and as a solid at room in the region of 0.3. temperature are virtually identical. It is therefore reasonable This allows us to suggest that ca.10% of the HBT mol- to assume that in the alcohol solvents the excited keto species ecules are intramolecularly hydrogen bonded in 100% will have similar properties to those observed in the solid ethanol [&(observed) :&(expected) =0.033 :0.31 and 100% Table 5 Radiative and non-radiative rate constants for the enol and keto forms of HBT as a function of temperature in ethanol-propanol mixtures derived from fluorescence quantum yields and mean lifetimes enol tautomer keto tautomer solvent T/K & rJns k,E/10-9 s-l k310-9 s-1 $J~ 7Jns kr/10-9 s-' kfr/10-9 s-l 100% EtOH 298 0.019 0.40 0.049 2.48 0.002 0.18 0.013 5.70 223 0.042 0.54 0.079 1.77 0.006 0.27 0.02 1 3.70 173 0.090 0.69 0.130 1.31 0.012 1.03 0.01 1 0.96 123 0.125 0.72 0.173 1.22 0.023 5.13 0.005 0.19 96 0.128 0.85 0.151 1.03 0.033 5.71 0.006 0.17 75% EtOH-250/, PrOH 298 223 0.022 0.06 1 0.40 0.58 0.056 0.106 2.43 1.62 0.003 0.006 0.19 0.32 0.014 0.019 5.3 1 3.11 173 0.136 0.68 0.199 1.26 0.014 1.40 0.010 0.71 123 0.180 0.72 0.252 1.15 0.033 5.27 0.006 0.18 96 0.177 0.76 0.235 1.09 0.053 5.70 0.009 0.17 50% EtOH-50% PrOH 298 0.034 0.40 0.086 2.42 0.010 0.15 0.069 6.69 223 0.065 0.55 0.1 19 1.70 0.013 0.36 0.036 2.72 173 0.109 0.65 0.169 1.38 0.027 1.71 0.016 0.57 123 0.172 0.68 0.252 1.22 0.055 5.39 0.010 0.18 96 0.72 5.77 25% EtOH-75% PrOH 298 0.010 0.40 0.026 2.49 0.002 0.17 0.010 5.87 223 0.055 0.56 0.100 1.70 0.013 0.44 0.029 2.24 173 0.120 0.64 0.188 1.37 0.034 2.17 0.01 5 0.45 123 0.180 0.69 0.262 1.19 0.077 - - - 96 0.173 0.74 0.235 1.12 0.107 5.76 0.019 0.16 10Oo/;, PrOH 298 0.008 0.38 0.022 2.61 0.002 0.13 0.014 7.62 223 0.024 - - - 0.005 - - - 173 0.050 0.60 0.083 1.59 0.014 2.72 0.005 0.36 123 0.059 - - - 0.029 4.60 0.006 0.2 1 96 0.056 0.57 0.099 1.66 0.037 5.55 0.007 0.17 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 4 Fluorescence spectra for the HBT keto form as a function of temperature in (a) MCH-ZMB and (b)ethanol. The spectra are all normalised to the same value to allow the spectral shapes to be com- pared propanol [&(observed) : &(expected) = 0.037 : 0.31, whereas the proportion is apparently greater in the mixed ethanol- propanol solvents reaching a maximum of some 35% in 25% ethanol-75% propanol (quantum yield ratio 0.107 : 0.3).For the keto fluorescence in alcohols, the radiative rate, k:, depends much less on temperature (Table 5), roughly consis- tent with emission from a single intramolecularly hydrogen- bonded species. However, in view of the observed slight non-exponentialities of the decays, different Kintrasolvates may be present here too, involving some intermolecular hydrogen bonding in addition to the intramolecular one of main importance. On the other hand, the non-radiative process (kfr) competing with the keto fluorescence depends much more strongly on temperature than kr or the rate con- stants of the enol species.The temperature dependence of ktr could be due to a viscosity-dependent non-radiative process available to the excited keto form. This may involve twisting of the 2-1’ bond connecting the aromatic systems as pro- posed for 3-hydroxy-2,2’-bipyridyI2’to lead to a non-emissive, lower-energy TICT state. The proposal of a viscosity-dependent non-radiative decay process for K is further substantiated by the results presented in Table 5, where different mixtures of ethanol and propanol are com- pared at the same temperature. Fig. 5 depicts the data for 173 K where the lifetime changes are most clear cut and signifi- cant.For the keto tautomer, the lifetime lengthens by a factor of more than two when changing from the less viscous ethanol to the more viscous propanol, and the derived kfr decreases. For the enol tautomer, these effects are marginal and in the opposite sense. The activation energy associated with the temperature dependence of k:r in ethanol is some- what lower than that for the solvent viscosity, consistent with the Kramers theory that the non-radiative process is intrinsi- cally barrierless as typified by other TICT molecules.22 Quantum chemical calculations can help to elucidate the question regarding the nature of the viscosity-dependent non- radiative decay channel of K. As mentioned above, one possi- bility is the involvement of a TICT-like specie^'^-'^ which is non-emissive in nature.As a formal double bond is involved in the twisting of the K species (cis-keto in Scheme l), this process can be described within the general theory of biradicaloid which, for example, can interpret the simultaneous twisting of the double and single bonds in sub- stituted ~tilbenes.~’,~~Such a twisting process towards a biradicaloid state A* with twisted geometry is possible in the excited state if the twisted product is less energetic than the planar precursor. The same argument holds for the ESIPT process in that the observation of K fluorescence tells us immediately that the energy of the K* state has to be less than that of the E* precursor. To decide whether a possible TICT state A* could be lower in energy than the other states E* and K* with planar geometry, a combination of quantum chemical methods for ground (ab initio STO-3G and AM1) and excited states (CNDO/S-CI) was used. To start with, the ESIPT reaction was modelled.In a first step, the ground-state geometry of E and K was optimized to two levels, starting with the X-ray structure of E determined by Sten~on.~’ At the first levels (a), the six most relevant bonds and bond angles which change in the ESIPT process were optimized, and at the second level (b),seven more geo- metric parameters were optimized, as in the formula in Table 6. In order to obtain a stable energetic minimum for K, the N-H bond length, r3, had to be fixed to an assumed value of 100 pm.If r3 was left free, the molecule relaxed to the E structure in the ab initio calculation. This behaviour indicates 3 1.6 ‘K1.4 -1.2 0 25 50 75 100 propan-2-01(Oh) Fig. 5 Variation of mean lifetime and derived non-radiative rate constant of the enol and keto forms of HBT as a function of viscosity in ethanol-propanol mixtures at 173 K; (0)enol lifetime, (0)keto lifetime; (B)kfi, ,(A)kfr J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 6 Structural parameters for the optimized geometries of the enol and keto forms of HBTin the ground state' (a) Enol form HI3 0 -+r2 ~~ X-ray structureb 148.13 130.48 -128.07 140.42 137.20 175.73 123.08 124.09 -110.74 115.39 109.21 {::I1;;{ 149.28 136.71 99.68 ----121.52 124.95 104.85 ---149.13 136.65 99.71 130.55 143.18 140.51 174.60 121.20 125.08 105.00 110.08 114.19 110.18 146.89 135.92 97.23 ----127.24 129.13 110.81 --146.07 135.87 97.27 132.02 141.38 142.35 168.44 127.55 129.10 110.86 110.66 114.03 1&59 (b) Keto form STO-3G (a) (b) 142.56 140.98 128.97 128.03 100.00 (fixed) 100.00 -135.26 -141.24 -140.62 -176.33 115.55 116.71 119.88 121.10 138.21 136.40 -113.99 -111.72 -111.20 (fixed) AM1 (a) (b){ 142.57 140.58 126.69 126.38 (fixed) 100.00 100.00 -137.12 -140.71 -142.01 -169.47 127.17 128.06 123.90 124.05 123.97 123.97 -113.38 -112.52 -110.13 (fixed) Bond lengths in pm, angles in degrees.Ref. 28. that on the ground-state hypersurface, there is no energetic barrier between the E and K structures.The geometric and energetic results are collected in Table 6 and Fig. 6 and 7. The results for the optimized E structure closely corre-spond to the experimental X-ray determination, with some difference only for the C-0 bond length, r2. In the opti- mized K structure, the main geometric changes are the short- ening of the inter-ring C-C bond, the shortening of the C-0 bond, and a shrinkage of the angle wl, which leads to an approach of the N and 0 atoms. The optimized structures for E and K are shown in Fig. 6. Owing to the approach of N and 0 in the structure for the keto species, the hydrogen atom is not only close to N but also close to the oxygen atom, thus facilitating ESIPT.The ESIPT process can be viewed as initiated and accompanied by an in-plane molecu- lar vibration modifying the hydrogen-tunnelling distance. A similar conclusion was recently reached regarding the related molecule 2-(2'-hydroxyphenyl)benzoxazole (HB0).28 In a second step, the energy differences between ground and excited states were calculated for the various optimized ab initio (STO-3G) ENOL _______ KETO structures by CNDO/S-CI (Fig. 7). With the geometries for ab initio STO-3G, the first excited state is of nn* nature, and the first allowed nn* state is situated some 0.6 eV above. This is clearly in contradiction to the experimental results with the rather high fluorescence quantum yields at low temperature indicating that S, is an allowed state.Furthermore, the energy of K* is calculated as being greater than that of E*, which is also contrary to the experimental observation of K fluorescence, indicating an exothermic E* +K* reaction. This is mainly due to a rather high energy for reaching K in the ground state which diminishes considerably within the AM1 framework so that the excited-state reaction E* +K* xx*nn* T nx* . . . . - - - - . . . . . . . . to.2 Fig. 7 Ground-and excited-state energies (eV) for the ESIPT Fig. 6 Structural representation of the optimized geometries of the process in HBT as derived from ab initio (left) and AM1 (right) calcu- enol (-) and keto (---) tautomers in the ground state according lations at optimization levels (a) and (b) (see text for details), com- to the ab initio (STO-3G)calculations [optimization level (a)] bined with CNDO/S-CI calculations J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 67 E* K* A' support and BESSY for access to beamtime via the EC 10.1 Large-scale Installations Programme (GE 1-00 18-DCB). I I4.1 3.2 K A Fig. 8 Ground- and excited-state energies (eV) of precursor and products on the excited-state hypersurface of HBT. The results are derived from a combination of the methods AM1 and CNDO/S and involve partial geometrical optimization. Whereas the products are highly endothermic on the ground-state surface, they are exothermic on the excited-state surface, and the photoreaction E* -,K* -+ A* can take place becomes slightly exothermic.Moreover, with the AM1 opti- mized geometries, the nz* state for K* is not situated below but approximately isoenergetic with the lowest m* state. In summary, the results in Fig. 7 indicate that the E* +K* reaction can be modelled qualitatively to be consistent with experiment by a combination of AM1 and CNDO/S. Ab initio ground-state calculations at the optimization levels and with the basis set STO-3G used here seem to overestimate strongly the energy difference between E and K. In a third step, the TICT reaction was modelled by twist- ing around the inter-ring bond using the K* structure from AM 1, optimization level (a). The ground state increases further in energy, but the energy difference between ground state and S, (of TICT nature, with electron transfer from the benzothiazole to the ketonic moiety) becomes rather small, less than half that in E*, so that the reaction from K* to the TICT state A* becomes exothermic (Fig.8). These results support the involvement of a TICT state responsible for the viscosity-dependent non-radiative decay channel of HBT. The fluorescence quenching process of HBT can thus be viewed as the result of a two-step photochemical reaction E* -+K*+A* with the first step being in the femtosecond range and the second step ranging from nanoseconds at low temperature to some tens of picoseconds at room tem-perature and being strongly viscosity dependent owing to the involvement of a large-amplitude molecular relaxation (intramolecular twisting process).We thank DAAD, the British Council, the Ciba-Geigy Trust, NATO and the BMFT (project 05 414 FAB1) for financial References 1 M. 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ISSN:0956-5000    

DOI:10.1039/FT9949000059

出版商:RSC    

年代:1994

数据来源: RSC