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THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Dr. Peter J. Sarre Department of Chemistry University of Notting ham 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. 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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(a UK.AC.NOlT.VAX Fax: (0602) 51 3466 Telex: 37346 UNINOT G Dr. R. J. Parker, Editorial Manager. Tel.: Cambridge (0223) 420066 E- Mai I ( INTER N ET) : RSCl (a RSC.ORG (For access from JANET use R SC1%RS C.0RG(aUK.AC.NS F NET-RELAY) Fax : (0223) 423623 or 420247 Telex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX005

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

HAZARDS IN THE CHEMlCAL LABORATORY 5th Edition ‘. . . easy to read, an excellent reference text, and a worthwhile investment.’ Journal of the American Chemical Society reviewing the 4th Edition. The new edition of this essential laboratory handbook is the ‘key’ requirement for all research, development, production, analytical and teaching laboratories worldwide. The 5th Edition provides: New features include: expanded ‘Yellow Pages’ section on 0 a quick guide to the hazardous properties of 1339 substances (over 800 more than were hazardous substances, providing immediate covered in the previous edition) information on hazardous properties, recommended control procedures and safety 0 details of the latest UK and EC regulations measures 0 an extremely useful emergency action check complete guide to labelling requirements to list -users can fill in their own key contacts for hospitals, fire etc.comply with EC directives and UK legislation, including the risk and safety phrases that must handy tables, symbols and statistics for ease appearof reference chapter on electrical hazards 0 a description of the American scene, including index to ‘Yellow Pages’ section, with US legislation and safety practices -synonyms of compounds highlighting differences between the UWEC index to CAS Registry Numbers and USA PVC Protective Binding xx + 676pages lSBN 085186229 2 (1992) Price f45.00 If you have not yet ordered your copy of the NEW edition, do so now! Why take chances? Be informed and safe. To order, please contact: ROYAL Royal Society of Chemistry, Turpin Distribution Services Ltd, Blackhorse Road, Letchworth, CHEMISTRY Intormat ionHerts SG6 IHN, United Kingdom. Services11111 Ill Telephone: +44 (0)462 672555 Fax: +44 (0)462 486947.

ISSN:0956-5000    

DOI:10.1039/FT99490BX007

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(2) 239-393 (1994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 239 Collisional activation of large ions. Energy losses and an impulsive collision theory of energy transfer C. D. Bradley, J. M. Curtis, P.J. Derrick and M. M. Sheil 249 Dynamical studies of the reaction Be + HF(v, J) +BeF(tl', J') + H on a new ab initio potential-energy surface X. Liu 253 Orientational ordering in the solid fullerene oxide: C,,O A. Cheng and M. L. Klein 263 New families of triply periodic minimal surfaces A. Fogden and M. Haeberlein 27 1 Fluorescence anisotropy decays and viscous behaviour of 2-methyltetrahydrofuran B. Brocklehurst and R. N. Young 279 Effects of protolytic interactions on the photophysics of phenyl pyridyl ketones F.Elisei, G. Favaro and F. Ortica 287 Electron transfer from aromatic compounds to phenyliodinium and diphenylsulfinium radical cations Y. Yagci, W. Schnabel, A. Wilpert and J. Bendig 293 Raman spectroscopic monitoring of oxygen clathrate hydrate formation from microporous amorphous solid water A. Hallbrucker 297 IR and NMR studies of hydrogen bonding in hexan-1-01-tetrabutylammonium iodide solutions in the temperature range 28-145°C and in tetrachloromethane 0. N. Kalugin, D. A. Nerukh, I. N. Vyunnik, E. G. Otlejkina, Y. N. Surov and N. S. Pivnenko 305 Effect of glycerol on the translational and rotational motions in lipid bilayers studied by pulsed-field gradient 'H NMR, EPR and time-resolved fluorescence spectroscopy G.Oradd, G. Wikander, G. Lindblom and L. RA. Johansson 311 Reactivity of some square-planar palladium(I1) complexes in aqueous solution and in heptane-AOT-water microemulsions F. P. Cavasino, C. Sbriziolo, M. L. Turco Liveri and V. T. Liveri 315 Spectrochemistry of solutions. Part 26.-Alkali-metal and alkaline-earth-metal thiocyanates in dimethylformamide and acetonitrile solutions : Hot bands, stability constants and thermicity for the formation of inner- and outer-sphere ion pairs P.Gans, J. B. Gill and P. J. Longdon 321 Electrochemical synthesis of soluble poly(9-hexylfluorene) and poly( 1 -hexylindene) J. Matsuda, K. Aramaki and H. Nishihara 327 Structural studies on paracyanogen and paraisocyanogen L. W. Jenneskens, J.W. G. Mahy, E.J. Vlietstra, S. J. Goede and F. Bickelhaupt 333 Photochromism, thermochromism and solvatochromism of some spiro[indolinoxazine)-photomerocyanine systems: Effects of structure and solvent G. Favaro, F. Masetti, U. Mazzucato, G. Ottavi, P. Allegrini and V. Malatesta 339 Modified interpenetration function accounting for the excluded-volume effects in ternary polymer systems R. Garcia, C. M. Gomez, I. Porcar, V. Soria and A. Campos 345 Photothermal imaging of electrochemical reaction dynamics R. S. Hutton and D. E.Williams 349 Stepwise growth of size-confined CdS in the two-dimensional hydrophilic interlayers of Langmuir-Blodgett films by the repeated sulfidation-intercalation technique I. Moriguichi, K. Hosoi, H. Nagaoka, I. Tanaka, Y.Teraoka and S. Kagawa 355 Thermogravimetry-FTIR study of the surface formate decomposition on Cu, CuCl, Cu,O and CuO. Correlations between reaction selectivity and structural properties J. Lin, K. G. Neoh and W. K. Teo 363 Heat of water chemisorption on cx-Al,O, at 200-400 "C P.F. Rossi, G. Oliveri and M. Bassoli 369 Frequency response analysis for multicomponent diffusion in adsorbents L. M. Sun, G. M. Zhong, P. G. Gray and F. Meunier 377 Photocatalytic decomposition of dinitrogen oxide on Cu-containing ZSM-5 catalyst K. Ebitani, M. Morokuma, J-H. Kim and A. Morikawa 383 Brransted acid strength in US-Y:FTIR study of CO adsorption M. A. Makarova, K. M. Al-Ghefaili and J. Dwyer 387 Influence of framework substitution of A13+ by Fe3+ on the sorption characteristics of /? zeolite E. M. Joseph and V. P. Shiralkar P. N. Joshi, Note: Where an asterisk appears against the name of one or more of the authors, it is included with the authors' approval to indicate that correspondence may be addressed to this person.

ISSN:0956-5000    

DOI:10.1039/FT99490FP019

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Afanasiev, P., 193 Alfimov, M. V., 109 Ebitani, K., 377 Elisei, F., 279 Kalugin, 0.N., 297 Katsumura, Y., 93 Neoh, K. G., 355 Nerukh, D. A., 297 Salmon, G. A., 75 Sbriziolo, C., 31 1 Al-Ghefaili, K. M., 383 Eustaquio-Rincon, R., 113 Kawashima, T., 127 Nicholson, D., 181 Schnabel, W., 287 Allegrini, P., 333 Allen, N. S., 83 Favaro, G., 279,333 Filimonov, I. N., 219, 227 Kida, I., 103 Kim, J-H., 377 Ninomiya, J., 103 Nishihara, H., 321 Shaw, N., 17 Sheil, M. M., 239 Aramaki, K., 321 Avila, V., 69 Baba,T., 187 Bassoli, M., 363 Fogden, A., 263 Fornes, V., 2 13 Frey, J. G., 17 Gans, P., 315 King, F., 203 Klein, M. L., 253 Kondo, Y ., 121 Kuwamoto, T., 121 Nonaka, O., 121 Nyholm, L., 149 Occhiuzzi, M., 207 Ohtsu, K., 127 Shiralkar, V.P., 387 Silva, C. J., 143 Silva, F., 143 Soria, V., 339 Bell, A. J., 17 Bendig, J., 287 Garcia, R., 339 Geantet, C., 193 Langan, J. R., 75 Leaist, D. G., 133 Oliveri, G., 363 Ono, Y., 187 Sun, L. M., 369 Surov, Y. N., 297 Bickelhaupt, F., 327 Gill, J. B., 315 Lei, G-D., 233 Oradd, G., 305 Tabrizchi, M., 17 Boggis, S. A., 17 Borisenko, V. N., 109 Goede, S. J., 327 Gomez, C. M., 339 Lerner, B. A., 233 Li, J., 39 Ortica, F., 279 Ota, K-i., 155 Takagi, T., 121 Takahashi, K., 155 Bradley, C. D., 239 Breysse, M., 193 Gray, P. G., 369 Green, W. A., 83 Lin, J., 355 Lindblom, G., 305 Otlejkina, E. G., 297 Ottavi, G., 333 Tanaka, I., 349 Teo, W. K., 355 Brocklehurst, B., 271 Grimshaw, J., 75 Liu,C-W., 39 Ozutsumi, K., 127 Teraoka, Y., 349 Brown, R.G., 59 Haeberlein, M., 263 Liu, X., 249 Padley, M. B., 203 Timms, A. W., 83 Caldararu, H., 213 Camacho, J. J., 23 Campa, M. C., 207 Hall, G., 1 Hallbrucker, A., 293 Handa, H., 187 Loginov, A. Yu., 219,227 Longdon, P. J., 315 Lu, J-X., 39 Paradisi, C., 137 Pardo,A., 23 Parsons, B. J., 83 Trejo, A., 113 Turco Liveri, M. L., 311 Turco Liveri, V., 311 Campos, A., 339 Hao, L., 133 Lunelli, B., 137 Pedulli, G. F., 137 Vedrine, J. C., 193 Caragheorgheopol, A., 213 Carvill, B. T., 233 Harrison, N. J., 55 Helmer, M., 31 Mahy, J. W. G., 327 Makarova, M. A., 383 Pereira, C. M., 143 Peter, L. M., 149 Villamagna, F., 47 Villemin, D., 97 Catalina, F., 83 Hosoi, K., 349 Malatesta, V., 333 Petrov, N. Kh., 109 Vlietstra, E.J., 327 Cavasino, F. P., 311 Hutchings, G. J., 203 Mallon, D., 83 Pivnenko, N. S., 297 Vollmer, F., 59 Cheng, A., 253 Hutton, R. S., 345 Mandal, A. B., 161 Plane, J. M. C., 31 Vyunnik, I. N., 297 Cherqaoui, D., 97 Ikawa, S-i., 103 Martins, A., 143 Porcar, I., 339 Whitaker, 9. J., 1 Chesta, C. A., 69 Ikonnikov, I. A., 219 Masetti, F., 333 Potter, C. A. S., 59 Whitehead, M. A., 47 Cho,T., 103 Cordischi, D., 207 Indovina, V., 207 Ishigure, K., 93 MatijeviC, E., 167 Matsuda, J., 321 Poyato, J. M. L., 23 Previtali, C. M., 69 Wikander, G., 305 Williams, D. E., 345 Corma, A., 213 Iwasaki, K., 121 Mazzucato, U., 333 Rettig, W., 59 Wilpert, A., 287 Corrales, T., 83 Jayakumar, R., 161 Meunier, F., 369 Rey,F., 213 Yagci, Y., 287 Cosa, J. J., 69 Jenneskens, L.W., 327 Moriguichi, I., 349 Richter, R., 17 Yoshitake, H., 155 Coudurier, G., 193 Curtis, J. M., 239 Derrick, P. J., 239 Dickinson, E., 173 Dwyer, J., 383 Dyke, J. M., 17 Jennings, 9. J., 55 Jiang, P. Y., 93 Johansson, L. B-A., 305 Joseph, E. M., 387 Joshi, P. N., 387 Kagawa, S., 349 Morikawa, A., 377 Morokuma, M., 377 Nagaishi, R., 93 Nagaoka, H., 349 Navaratnam, S., 83 Rocha, M., 143 Rochester, C. H., 203 Rofia, S., 137 Rossi, P. F., 363 Ryde,N., 167 Sachtler, W. M. H., 233 Yotsuyanagi, T., 93 Young, R. N., 271 Zholobenko, V. L., 233 Zhong, G. M., 369 The following papers were accepted for publication between 1st and 30th November, 1993: Limiting partial molar volumes of electrolytes in dimethylformamide-water mixtures at 298.15 K A.Maestre, E.Garcia-Paiieda, C. Yanes and J. J. Calvente Photocatalytic decomposition of dinitrogen oxide on Cu-containing ZSM-5 catalyst A. Morikawa, K. Ebitani, M. Morokuma and J-H. Kim Infrared studies of cerium dioxide: Influence of impurities and defects F. Bozon-Verduraz and A. Bensalem Bilayer phases in aqueous mixtures of dodecylpentaoxyethylene glycol monoether (C,2E5) and sodium decyl sulfonate (C,,SO,Na) C. B. Douglas and E. W. Kaler Effect of preferential solvation on reactivity of a free radical in binary solvent mixtures 0. It0 and H. Watanabe Formation of oxygenates of C, on rhodium-containing catalysts during CO + H, reactions. An FTIR study of acetaldehyde adsorption J-P. Hindermann, D.Demri, C. Diagne and A. Kiennemann Laser flash photolysis studies on hydrogen-atom transfer from the triplet hydroxynaphthylammonium ion to benzophenone via a triplet exciplex. Which group is more reactive for hydrogen atom transfer, -OH or -NH+,? H. Shizuka, M. Yamaji and K-I. Tamura Al-Pillared saponites. Part 1.-IR studies J-F. Lambert, S. Chevalier, R. Franck, H. Suquet and D. Bart homeuf Al-Pillared saponites. Part 2.-NMR studies J-F. Lambert, S. Chevalier, R. Franck, H. Suquet and D. Barthomeuf Kinetics of thermal decomposition of the diazines: Shock tube pyrolysis of pyrimidine J.C. Mackie and A. Doughty Computer modelling of phosphate biominerals: Transfer of parameters for interatomic potentials for different polymorphs of divalent metal diphosphates M.G. Taylor, K. Simkiss and M. Leslie Reaction of molecular oxygen with C6,: Spectroscopic studies A. M. Bradshaw, H. Werner, Th. Schedel- Niedrig, D. Herein, M. Keil, B. Henog and R.Schlog Voltammetric and SNIFTIRS study of the adsorption and oxidation of L(+)-ascorbic acid on Pt electrodes in acid medium. Effect of Bi adatoms A. Aldaz, M.A. Climent, A. Rodes, M.J. Valls, J.M. Perez and J.M. Feliu Synthesis, structures and electrical properties of the charge-transfer salts of 4,5-ethylenedithio-4’-5’-(2-oxatrimethy1enedithio)diselenadithiafulvalene (EOST) with linear anions (I3-, lBr,-, ICl;, 12Br-, AuBr;, Au(CN),) T. Naito, A. Tateno, T. Udagawa, H. Kobayashi, R.Kato, A. Kobayashi and T. Nogami Pulse radiolytic one-electron reduction of 2-hydroxy- and 2,6-dihydroxy-9,1O-anthraquinones H.Pal, T. Mukherjee and J. P. Mittal Characterization of transients formed in aqueous solutions of substituted alkyl sulfides: A pulse radiolysis study J. P. Mittal, D. K. Maity and H. Mohan U1 trasonic velocities and isentropic compressibilities of some tetraalkylammonium and copper(1) salts in acetonitrile and benzonitrile D. S. Gil, J. Singh, T. Kaur and V. Ali Transport and compressibility studies of some copper(1) perchlorates in binary mixtures of benzonitrile and acetonitrile D. S. Gill, R. Singh, V. Ali, J. Singh and S. K. Rehani Primary yields of water radiolysis in concentrated nitric acid solutions Y. Katsumura, R.Nagaishi, P-Y. Jiang and K. Ishigure Oxygen exchange between magnesium oxide surface and carbon dioxide H.Hattori, H. Tsuji, T. Shishido, A. Okamura, Y. Gao and H. Kita Response kinetics of polymer coated bulk acoustic wave devices on exposure to gases and vapours N. J. Freeman, I. P. May and D. J. Weir Photocatalysts with tunnel structures for decomposition of water. Part l.-BaTi,O, having a pentagonal prism tunnel structure and its combination with various promoters Y.Inoue, Y. Asai and K. Sat0 Study of the hydrogen reduction of cerias with different textures and their reoxidation by oxygen V. IPerrichon, A. Laachir, 0. Touret, G. Bergeret, R.Frety and L. Tournayan 11 Theoretical and experimental study of the flow of condensed molecular monolayers on a Langmuir trough J. G. Byatt-Smith and B.R. Malcolm Water-induced structural changes within the L, phase of DDAB-cyclohexane-water systems J. Eastoe and R. K. Heenan Volumetric properties of steam-carbon dioxide mixtures derived from excess molar enthalpy measurements C. J. Wormald and M. Massucci Kinetic model for serum albumin adsorption: Experimental verification J. J. Ramsden, R. Kurrat and J. E. Prenosil Infrared and submillimetre-wave spectra of doped poly (p-phenylene vinylene) K. Davidson and S. El-Atawy Catalytic reactions of o-xylene and rn-xylene with deuterium on metal films C. Kemball and R. J. Harper Application of simple expressions for the high-pressure volumetric behaviour of liquid mesitylene V. G. Baonza, M. C. Alonso and J. N.Delgado Structures and vibrational spectra of CH,OCH,CH,OH: The hydrogen-bonded conformers J.J. C. Teixeira- Dias, F. P. S. C. Gil, A. M. Amorim da Costa and R. Fausto Theoretical potential-energy functions and the rovibronic spectrum of the SiH,+ ion D. M. Hirst, C. Bauer, D. I. Hall, P. J. Sarre and P. Rosmus Electrochemical study of the heterogeneously catalysed reaction between NJV-dimethyl-p-phenylenediamine and Co"'(NH,),C12+ at monometallic and bimetallic surfaces of silver and gold U.Nickel, Y-H. Chen and M. Spiro DRIFT and mass spectrometric experiments on the chemistry and the catalytic propeties of small Ir clusters at the surfaces of polycrystalline a-Al,O, L. Basini and A. Aragno Reactions of N(2 ,D) and N(2 2P) with 0, H. Umemoto, Y. Shihira, T. Suzuki, S-i. Unayama and S.Tsunashima Hydrogen evolution reaction on electrodes coated with conducting-polymer films K. Doblhofer and K. Maksymiuk Metallocyclodextrins of 6A-(3-aminopropylamino)-6A-deoxy-~-cyclodextrin:Their formation and enantioselective complexation of (R)-and (3-tryptophan anions in aqueous solution S. F. Lincoln, S. E. Brown, J. H. Coates and C. J. Easton Internal rotation in auramine 0 A. Harriman and P. Gautam Adsorption of binary mixtures of heptane and alkanols by activated carbon A. M. Gongalves da Silva, V. A. M. Soares and J. C. Calado NMR study of E-caprolactam in various solvents: Graphical determination of monomer shift, dimer shift and dimerization constant from the dilution shift data J-S. Chen Dual-cylinder microelectrodes. Part 2.-Steady-state generator and collector electrode currents B.J. Seddon, C. F. Wang, P. Li, W. Peng and X.Zhang Stability of thin polar films on non-wettable substrates A. Sharma and A. T. Jameel ... 111 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 69 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 WC1H OAJ iv 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 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 solifliquid and liquid/liquid 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 W1 V OBN.V 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. vi MENDELEEV COMMUNICATIONS Mendeleev Communications is a prestigious new primary journal, produced as a collaborative venture between The Russian Academy of Sciences and The Royal Society of Chemistry. It publishes original papers directly in English, giving the international chemical community rapid access to important new research from Russia and the other states of the former Soviet Union in the shortest possible time. The journal contains preliminary accounts of novel and significant results of wide general appeal or exceptional specialist interest and covers all branches of chemistry.In format and range of subject matter it closely resembles its 'sister' publication, the popular RSC journal Chemical Communications. A selection of recent papers:Transformation of Androsta4,9-diene-3,17-dione into 16a,l7a-Epoxycorticosterone Alevtina M. Turuta,include: Aleksei V. Kamernitskii, Ta t'yana M. Fadeeva and Luu Duc Huy* Primary publication in English of original A Novel Approach to Carbacycline Genrikh A. Tolstikov, Rinat R.Akhmetvaleev, Vadim M. Zhurbachemistry from Russia and other states of the and Mansur S. Miftakhov former USSR Oxidation of Alkanes by Dioxygen in the Presence of +-Two stages of rigorous refereeing -once in an Iron Complex immobilized on Modified Silica.Chemical Model of Methane Monooxygenase Moscow and once in the UK -to maintain the Vera S. Belova, Alexander M. Khenkin, Victor N. highest possible standards Editorial Boards in both Moscow and the UK Postnov, Valerii E. Prusakov, Alexander E. Shilov and Marina 1. Stepanova composed of eminent scientists who will Cyclic Oligophosphonic Anhydrides /van S. Alfer'ev advise on refereeing policy Rapid publication -and Sergey Yu. Bobkov appearance of papers Cyclopropanation of Unsaturated Compounds with Diazomethane Generated in situ: A New Efficient within 12 weeks of receipt in the UK and Practical Route to Cyclopropane Derivatives Oleg M. Nefedov, Yurii V. Tomilov, Andrei 8. * High quality production and editing Kostitsyn, Usein M.Dzhemilev and Vladimir A. * News section containing details of Ookitchev forthcoming international conferences and Direct Evidence for Bidentate Character of Potentially information about The Russian Academy of Tridentate N,S-Ligands. Molecular Structures of Bis~N-(2-pyridyl)thiosalicylidene-~S-amino-~~-Sciences nickel(it) and Bis ( 1-isopropyl-3-methyl4-[N-(2-* Essential reading to keep up-to-date with pyridyl)imino-~N-methyl]pyrazole-5-thiolato-~S}-copper(ii) Alexander E. Mistryukov, @or5. current chemical research Mendeleev Communications is NOT a Vasil'chenko, Vladimir 5.Sergienko, Alexander 1. Nivorozhkin, Stanislav G. Kochin, Mikhail A. Porai-Koshits, Leonid E. Nivorozhkin and Alexander 0.translation journal Garnovs kii ROYAL Position Organisation Address Inlormation Srrvlc es Please return to: Sales and Promotion Department, Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, United Kingdom.Tel: + 44(0)223 420066 Fax: + 44(0)223 423623 Telex: 818293 ROYAL. vii Take a look at the following contents page of a recent issue and discover what you have been missing: Volume 21 Issue I Pages 1-84 March 1992 Chemistry of Potentially Prebiological Natural Products By Alben €schenmoser and Eli Loewenrhal (pp.l-16) The demonstration that biologically relevant molecules can be generated non-enzymicully from precursors as simple as hydrogen cyanide, and found in interstellar space, identifies possible pathways for the molecular mechanisms of the origin of life.The reason why Nature uses pentoses in nucleic acids and not hexoses can be understood by comparing the properties of 'homo-DNA' oligonucleotides derived from e.g. 2',3'-dideoxyallose with those of DNA oligonucleotides from deoxyribose. The Theory of Atomic and Molecular Collisions By John N. Murrell and S. Danko Bosanac (pp.17-28) The inter atom/molecule potential governs molecular behaviour over a vast range from physical properties in bulk phase to intrinsic chemical reactivity. Scattering data from collisions in crossed beam experiments provide the most important probes of such potentials. The classical, semiclassical, and quantum mechanical theories available to analyse the data from elastic, inelastic, and reactive scattering processes are reviewed.Cyclopentadienyl Molybdenum and Tungsten Dihalides By Malcolm L. H. Green and Philip Mounfford (pp.29-38) Two alternative structures observed for binuclear cyclopentadienyl molybdenum or tungsten dihalides. one with a metal-metal single bond, the other with a triple bond, illustrate 'a delicate balanct between mctal-metal and metal-ligand bonding'. The 'rich and diverse reaction chemistry' of these complexes is illustrated with referena to the structural tjpe and nature of the halide. Lariat Ethers: From Simple Sidearms to Supramolecular Systems By George W. Gokel (PP.3947) This review constitutes a highly personal account by Professor Gokel of crown ethers having side-arms -or lariat ethers, as the author has imaginatively dubbed them.He describes how the first systems. built up of single arms containing donor groups, have evolved into much more extensive molecukr arrays capable of exhibiting self-assembly at a supramolecular level under appropriate conditions. LUDWIG MOND LECTURE. Taking Stock: The Astonishing Development of Boron Hydride Cluster Chemistry By Norman N. 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The theory IShere summarized in the light afthese refinements. The Nature of the Hydrogen Bond to Water in the Gas Phase By A.C.Legon and D. J. Millen (pp.71-78) The most fascinating and important weak intermolecular interaction is the hydrogen bond in water. It holds the key to the idiosyncratic behavrour of liquid water and dominates the chemistry of living systems. Recent advances in high resolution spectroscopy in the gas phase have enabled the intrinsic properties of this bond to be obtained by detailed study of a range of water-containing dimers (H,O.HX).The results of such studies are assessed and, in particular, questions relating to the proton donor/acceptor behaviour of water are discussed as are the factors governing dimer structure and the tendency for water to form protonated complexes. 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ISSN:0956-5000    

DOI:10.1039/FT99490BP021

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 239-247 Collisional Activation of Large Ions Energy Losses and an Impulsive Collision Theory of Energy Transfer Caroline D. Bradley, Jonathan M. Curtis and Peter J. Derrick* Institute of Mass Spectrometry and Department of Chemistry, University of Warwick, Coventry, UK CV4 7AL Margaret M. Sheil Department of Chemistry, University of Wollongong, Wollongong,NSW,Australia The translational energy losses, A€,experienced by keV beams of singly charged ions of high masses (> 1000 u) in collisions with either an inert-gas atom or hydrogen molecule depend on the mass of the target gas employed. There is no evidence of a dependence on the ionisation energy of the target gas. A€s are similar with He and D, targets, and both are larger than those with Ar.This behaviour is found with organic ions composed of light atoms and with inorganic cluster ions composed of heavy atoms. The measured Us are consistent with internal energy uptake, Q, occurring via direct momentum transfer in an impulsive collision. It is concluded that the Q taken up by an ion is dependent upon the masses of its constituent atoms. In collision with He, a light-atom ion of a given molecular mass takes up more Q than does a heavy-atom ion of similar molecular mass under the same experimental conditions, but the A€ can be similar in the two cases. The development over the past 20 years of techniques for the ion energy and under comparable conditions, the trans-formation of beams of molecule ions and cluster ions' has lational energy losses for ions of different masses showed a opened up possibilities for investigations of the dynamics of direct relationship to incident-ion mass, i.e.were larger for collisions involving very large molecules.2 Collisions involv- heavier ions. Taken as a whole, these findings point over-ing large ions hold intrinsic interest as regards energy transfer whelmingly to a momentum mechanism, rather than elec- and disposal, and understanding such events is directly rele- tronic excitation with minimal momentum transfer. vant to the analytical technique of tandem mass spectrometry Bricker and Russell6 examined more closely the variation which is assuming increasing importance for the determi- in energy losses when using different inert gases as targets, nation of the molecular structure of biomolecules and syn- and reported a linear relationship between the ionisation thetic polymer^.^ In the context of tandem mass energy of the target and the energy loss.On this basis, they spectrometry, collision between incident ions in a beam and argued in favour of excitation of the target gas as a major gas molecules serves to induce dissociation of the ions, so-process during collision. They studied the formation of a called collision-activated dissociation (CAD).3 Using He as fragment ion (m/z 614.2) from protonated chlorophyll-a, m/z the collision gas, there is evidence to suggest that the efi- 893.5. The linear relationship was not universally valid, ciency of the CAD becomes low, if the mass of an incident because with other ions D, behaved as a target gas more like ion mi+ is greater than ca.1500 u given an incident-ion trans- He than Ar (D2has the same mass as He and approximately lational energy of 8 eV (or The original explanation the same ionisation energy as Ar)." The better relationship put foward was that the large ions did not fragment within seems to be between the mass of the target and the energy the time-frame (lo-' s) of the experiment. The long lifetimes loss. That the formation of m/z 614.2 from protonated were attributed to the large numbers of internal degrees of chlorophyll-a is not an exceptional case in any regard has freedom over which excitation energy can be distributed. On been confirmed unequivocally in a careful series of experi- the other hand, it has been suggested that the inefficiency of ments.12 CAD of large ions is more a consequence of weak excitation That the relationship between ionisation energy and energy in the collision.6 loss for the inert gases may be no more than a consequence For many years, the accepted view of CAD of small of ionisation energy of an inert gas being related to mass does organic ions (mass -= 100 u) was that the ion was elec-not in itself disprove a proposal that energy uptake is small tronically excited in the collision, that the translational in collisions of multiatomic ions with light target gases.In energy lost by the ion in the collision was negligible and that which case, the observed dissociation of large ions following there was no direct momentum tran~fer.~ Electronic excita- collisional activation would be a consequence of multiple col- tion was assumed to be followed by internal conversion, and lisions.That is to say, an ion would undergo a number of the ion fragmented by vibrational predissociation.' The collisions and accumulate suficient internal energy for disso- observation that CAD of large ions (the term 'multiatomic' ciation. There is evidence from Fourier-transform ion cyclo- will be employed for the purpose of distinction from polyato- tron resonance spectrometry, to support a mechanism of this mic ions) was typically accompanied by translational energy type in the case of the organic ion valinomycin and low- losses of the incident ions as large as 10-lo2 eV was a clear energy collisions, where the extent of fragmentation was indication of momentum transfer in the c~llision.~~~~' increased on raising the time available for collision and hence The cross-sections for CAD of multiatomic ions were small, and the number of collision^.'^ The alternative view, namely that were consistent with energy uptake by the ion oia direct fragmentations of large ions can be induced by single (keV) momentum The translational energy losses were collisions, has, however, been given support by experiments large for He collision gas, and smaller for heavier target gases at different collision-gas pressures.14*' 'In these experiments, such as Ar.' These trends were observed for biological, the measured energy losses AE associated with certain frag- polymer and inorganic cluster ions.A further significant ment ions were found to be independent of pressure, suggest- trend observed for organic ions was that at a given incident- ing that a single collision induced fragmentation. Assuming that direct momentum transfer occurs, two extreme theoretical treatments can be distinguished. In the one case, the ion can be regarded as a single entity so that the collision becomes quasi-diatomic. Simply conserving energy and momentum then provides the relationship (1) among AE, Q and scattering angle 8 for a single collision. + 2(mi/mg)cos0[1 -(AE/Ei)]1/2 Ei is the translational energy of the incident ion prior to colli- sion and mi and m, the masses of the incident ion and target gas.The earliest AE, for multiatomic peptide ions were related to Q on this basis, making the assumption that scatter was negligible4.59'6 (i.e. 8 = 0). More recently this quasi- diatomic treatment of what is inelastic scattering (Q # 0) has been described as the 'limiting elastic model'.12 The other extreme is where the interaction of the target with the ion is considered to involve only one atom in the ion. The inter- action of this one atom with the other atoms in the ion is considered as a separate subsequent event. On this basis, the following relationships have been derived. ' The constants E and p are given by expressions (3) and (4).'* ma is the mass of the atom in the ion considered to be involved with an atomic target m,.These equations, which will be referred to as the 'impulsive c'ollision transfer (ICT)' model, have been used in considering collision of cluster ion^.'^-^^ (3) (4) 1 In the case of a small target such as He, which of the two extremes is a more plausible model hinges upon the relative strengths of, on the one hand, the interactions between the target and the atom, and, on the other hand, the interactions between atoms within the ion. With an ion composed mainly of hydrogen, carbon, nitrogen and oxygen, and considering a keV collision in which the incident ion loses 10-lo2 eV of kinetic energy, the interactions within the ion are likely to be weaker than the seemingly very strong interaction in the col- lision.This being the case, the impulsive collision in which one atom in the ion is considered to be involved in the colli- sion might be expected to be the more realistic of the two possibilities considered. Molecular dynarnic~~.'~ calculations have supported the impulsive collision model. In the present paper, we report AEs measured for keV collisions of valinomycin molecule ions, containing predomi- nantly hydrogen, carbon, nitrogen and oxygen, and for cluster ions of caesium iodide. AEs have been determined from all sufficiently intense fragment ions in all cases. Our previous studie~~.~,' have, for the most part, presented mean energy losses, i.e. some sort of average over a set of fragment ions from a given incident.Other earlier studies have focussed attention on just one selected fragment ion for a particular incident ion.6.' Comparison between two ions of approximately the same mass, where one consists of a small number of heavy atoms and the other of a large number of light atoms, ought to provide an indication as to which of the two extreme models provides the better description. Valinomycin has a relative molecular mass, M,, of 1110.6 and 498 internal degrees of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 freedom, whereas (CsI),Cs has an M, of 1172.1 and 21 inter-nal degrees of freedom. Thus in collision, the relative velo- cities and centre-of-mass energies for molecule ions of valinomycin and [(CsI),Cs]+ will be similar, given the same incident-ion energy in each case (vide infra).The global con- straints of conservation of energy and momentum will be approximately the same in the two cases. The dynamics of energy transfer are predicted to be approximately the same if the quasi-diatomic model applies, whereas the ICT model predicts that Q should differ significantly. Whether or not the Qs are similar should be evidenced by the degrees of fragmen- tation. According to Rice-Ramsperger-Kassel-Marcus theory, the species with the smaller number of atoms should exhibit a stronger dependence of rate of fragmentation upon internal energy,23 if critical energies are similar. Experimental All measurements have been made using an unusually large research mass spectrometer (see Fig.l), which has been described previ~usly.~~ Incident ions were selected using a magnetic sector (nominal radius of ion optical axis 800 mm), the resolution of which was sufficient to separate the pure I2C[M + K]+ valinomycin ion from the species containing one 13Catom. The collision cell (Fig. 1) was 10 mm in length, and the surrounding area was differentially pumped. The ion gauges in the vicinity of the collision cell were calibrated for different gases ; conductances and pumping speeds associated with the components of the differential pumping system were determined in separate experiments. The translational ener-gies of undissociated incident ions and of fragment ions were measured using a cylindrical electric sector (radius of ion optical axis lo00 mm).The acceptance angle of the electric sector was &lo(in the plane of deflection) in terms of the direction of travel of ions out of the collision cell. Electric- sector potentials were varied under computer control from a few V to their maximum values (&340 V for ion energy 10.5 keV) in steps of 120 mV. Electric-sector potentials were calibrated against a digital voltmeter of known accuracy. Metastable peaks (i.e.peaks due to fragment ions from spon- taneous unimolecular decomposition in the field-free region between the sectors) of test compounds appeared at their expected positions (to better than 0.1 V). All spectra were recorded using ion counting and signal averaging. The particle-multiplier detector and counting electronics 'floated' at negative potentials of up to 40 kV, in order that ions would be accelerated into the multiplier and detected effi- ciently. Signals were transferred from the counting electronics to the computer via optical fibres.Field desorption (FD) was employed as the ionisation method for valinomycin. The FD emitters were 25 pm tung- sten wires covered in carbon microneedles and had been acti- vated in an atomosphere of ben~onitrile.~' Samples were loaded by repeatedly dipping the emitter into a concentrated sample solution. The emitter was positioned 2 mm from a grounded counter electrode and a positive potential of 10.5 kV applied. A small heating current was passed through the emitter wire to promote desorption.Caesum iodide cluster ions were formed by bombardment of a sample-coated target with 8 keV xenon atoms [fast atom bom bardmen t (FAB)]. In all collision experiments, the incident-ion energies were 10.5 keV. The collision conditions were such that the incident-ion beam was reduced to 3540% of its original intensity when the target gas was admitted into the collision cell. The AEs were obtained from the measured electric-sector potentials corresponding to peak maxima of the fragment ions. Let the electric-sector potential required for transmis- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 24 1 second field-free region rst field-free region I ?-protective cage II I I I L----I '----J Fig. 1 Schematic of the ion-beam spectrometer sion of the incident ion in the absence of gas be q.If the incident ion mi+ were to decompose to a fragment ion mf+ with no loss of translational energy, the fragment ion would be transmitted at an electric-sector potential V, [eqn. (4)]. V, = (mf/mi)b (4) Suppose that the incident ion mi+ loses translational energy, AE, as a result of collision and subsequently decomposes to the particular fragment ion m,,. This is the so-called 'two-step model'.' A smaller electric-sector potential, called V, , will be required to transmit the fragment ion mf+ formed from an incident ion which has lost translational energy AE. Eqn. (5)shows the relationship of AE to V,. AE = (mJm,)(EJK)(Q-V,) (5) Ei is the translational energy of mi+ prior to collision.If the incident ion mi+ loses energy AE in the collision but does not decompose, the energy-deficient mi+ ion will appear at a lower electric-sector potential, 5.Eqn. (6) shows the relation-ship of AE to 5. AE = [(V, -5)/q]Ei (6) In general, the shift in electric-sector potential (V, -Vd is dif-ferent for different fragment ions, and under a given set of conditions is a reproducible characteristic of any particular fragment. The precision achieved in the determination of AEs depends upon the mass, m,, of the fragment ion measured, relative to the mass, mi, of its parent ion. Provided a peak was smooth and symmetrical, its position, V,, in terms of electric-sector potential could be determined to k0.016 V.This uncertainty arose very largely in determining the cen-troid of a peak. The consequent uncertainties in AE range from f0.5 eV at a mass ratio mf/mi of 0.9 to f5 eV at 0.1. Taking the results for [(CsI),Cs] + with He, the uncertainties arising in this way are 21.0 0.5 eV [(CsI),Cs]', 49.0 0.7 eV [(CsI),Cs]+, 66.7 f0.8 eV [(CsI),Cs]+, 53.9 & 1.0 eV [(CsI),Cs]+, 61.6 f1.4 eV [(CsI),Cs]+ and 57.9 & 2.4 eV [(CsI)Cs]+. The values of the AEs determined (Tables 1-3) are dependent upon experimental conditions, in particular collision conditions and ionization conditions in the source. The values in the tables were, as far as possible, based on measurements made under identical conditions, so that com-parisons can be made among values in the tables for the pur-poses of elucidating the effects of constituent-atom mass upon AE.Results AEs obtained on the basis of expression (5) from measure-ment of the fragment ions of the [M + K]' m/z 1149.7 of valinomycin are shown in Table 1. The molecular structure of valinomycin is shown in Fig. 2. The mass assignments for the fragment ions are based on the four-sector fast atom bom-bardment mass spectrum of the [M + K]+ of valinomycin, which is shown in Fig. 3. Energy spectra [these spectra will be referred to as mass-analysed ion kinetic energy (MIKE) spectra] are shown in Fig. 4. AEs obtained from fragment ions of [(CsI),Cs]+ m/z 1172.1 and also of [(CsI),Cs]' m/z 1951.7 are shown in Tables 2 and 3, respectively.The MIKE spectra of [(CsI),Cs]+ and [(CsI),Cs]' are shown in Fig. 5 and 6, respectively. The contributions to the fragment-ion peaks from metastable decompositions were found to be neg-ligible in the valinomycin case and do not affect the energy loss values. Metastable decompositions contributed to a small number (<20%)of the cluster-ion fragment peaks. With both the [M + K] valinomycin ion and the caesium+ iodide cluster ions, AEs tend to be similar for a given frag-ment ion with either He or D, (Tables 1-3). These AEs tend to be greater than the corresponding vaues for Ar (Tables 1-3). The Qs (Tables 1-3) have been calculated from the cor-responding AEs through eqn. (2)-(4) in the cases of He and Ar. These equations cannot be applied directly, if the incident ion undergoes more than one collision prior to fragmentation.Further, if the incident ion collides and breaks down to a fragment ion and that fragment ion collides and decomposes to give the observed fragment ion, the energy shift from the observed fragment ion will yield an exaggerated AE.26 The large AEs for m/z 132.9 from the caesium iodide cluster are possibly influenced by this latter effect. A factor affecting the interpretation of observed energy losses is the extent of scat-tering as a result of collision. An ion is able to sustain a larger AE while being deviated through a small angle when J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 AE and Q (according to ICT theory) (in eV) for valinomycin [M + K]+ m/z 1149.7 colliding with He, D, and Ar fragment ion m,+/u AE(He) Q(W UD,) AE(Ar) QtAr) loss of CH, 1133.7 40.3 14.8 40.9 6.7 5.7 loss of H,CO 11 19.7 58.5 21.6 43.5 19.0 16.2 loss of C3H, 1106.7 38.2 14.1 38.5 18.0 15.4 loss of CH, and C3H7 1091.6 64.8 23.9 67.9 15.4 13.2 [(HVLV),VHV + K]+ -2H 1075.7 39.7 14.6 44.8 27.3 23.3 [(HVLV),VHV + K]+ -CH, + H 1063.7 63.4 23.4 67.2 19.5 16.7 [(HVLV),VLV + K]+ -2H 1047.7 59.2 21.8 76.9 11.4 9.7 [(HVLV),VLV + K]+ -CH3 -H 1033.6 57.5 21.2 33.9 [(HVLV),HV + K]+ 978.6 63.4 23.4 68.3 35.0 29.9 [(HVLV),HV + K]+ -CH3 + H 964.6 47.5 17.5 69.5 19.4 16.6 [(HVLV),VL + K]+ + 2H 952.6 65.7 24.0 72.2 33.6 28.7 [(HVLV),VL + K]' -CH3 + H 936.6 66.2 24.4 71.3 27.3 23.3 [(HVLV),V + K]+ -CH3 + 2H 865.6 82.9 30.6 67.4 25.8 22.0 [(HVLV),L + K]' + H 852.0 104.5 38.6 80.3 45.7 39.0 [(HVLV),L + K]+ -CH3 -C,H7 + 2H 795.5 55.4 20.4 62.3 [(HVLV), + K]+ -CH3 + H 765.5 56.2 20.7 68.6 [(HVLV), + K]+ -CO + H 753.5 76.3 28.2 86.3 [(HVLV)VHV + K]+ -CH3 + 2H 694.5 56.8 21.0 88.4 [(HVLV)HVL + K]+ -CH3 + H 666.4 68.2 25.2 [(HVLV)HVL + K]+ -CO + H 653.5 72.4 26.7 [(HVLV)HV + K]+ -CH3 + H 594.4 65.4 24.1 ~ H : hydroxyisovaleric acid, L: lactic acid, V: valine.Table 2 AE and Q (according to ICT theory) (in eV) for [(CsI),Cs] m/z 1172.1 colliding with He, D, and Ar + C(CSU3CSI+ 912.34 19.2 0.1 20.8 2.3 0.5 C(CsI),Cs!+ 652.53 55.8 0.4 48.9 3.7 0.8 C(CSI)CSl 392.72 69.2 0.5 55.2 4.8 1.oDl+ 132.91 189.5 1.3 35.6 10.4 2.2 Table 3 AE and Q (according to the ICT theory) (in eV) for [(CsI),Cs]+ m/z 1951.7 colliding with He, D, and Ar [(csI),csI+ 1691.76 21.0 0.6 20.8 0.3 0.1 ~(CSI),CSl+ 143 1.95 49.0 1.4 43.1 6.5 1.5 C(C~~)'aCSl+ 1172.15 66.7 1.9 49.0 7.5 1.7 [(CSI) 3CSI + 9 12.34 53.9 1.5 45.5 13.4 3.0 C(CsI),Cs!+ 652.53 61.6 1.7 44.3 24.1 5.4 C(CSI)CSl 392.72 57.9 1.6 40.9 31.2 7.0 CCSI + 132.91 65.8 14.7 Val Lac Va I HYV VaI Lac -CH-NH-C-CH-o -C -CH -NH -C-CH -? 0-CH-C-NH-CH-C-0-CH-C-NH-CH-C-0-CH-C-NH-CH-6 I I1 I I1 I II I II I I1 I 11CH 0 CH 0 CH, 0 CH 0 O Oti&/ 'CH, H,C /\ CH, H,C/ 'CH, H,C CH, H,C CH, Hyv Val Lac Val HYV Val Fig.2 Structure of the cyclic depsipeptide valinomycin (M, = 11 10.6). Hyv = hydroxyisovaleric acid, Lac = lactic acid. J. CHEM. SOC.FARADAY TRANS., 1994, VOL. 90 100-n s W >-4-.-v) al .-E 50-0, .--U 0-Fig. 3 Four-sector tandem mass spectrum of valinomycin [M + K]' ions formed by fast atom bombardment less massive target gases are employed.'2*'7 When the range of observable scattering angles is fixed, as in our experiments, the range of observable AE will be somewhat smaller in the case of Ar than with He or D2.Under the experimental con- ditions used, incident ions of valinomycin, [(CsI),Cs] and+ [(CsI),Cs]+ would not have been lost in the ion optical plane as a result of a single collision with either He or D2. The variations in AE from one fragment ion to another for a given target (Tables 1-3) reflect the dynamics of the com- peting fragmentation channels. The collision of a multiatomic ion with the target will give rise to a distribution of internal energy states of the ion.Different states and total energies apparent m/z , . . , I, . . , . , .-.-qw.. , . , . , , . , , . . . , , 200 300 ESA potentialp n apparent m/z -.-'-----' '".I ' ' ' ' '.' ' ' I ' '.' ' ' ' 200 300 ESA potentialp favour different fragmentation pathways, because the ener- getics of different fragmentation pathways differ from each other. Consequently, it is to be expected that the Qs associ-ated with formation of individual fragment ions and hence the AEs manifested will differ from one fragment ion to another. The AE observed represents the sum of the recoil energy of the target-gas atom and the internal energy uptake Q of the ion.The recoil energy of a light target such as He (or D2)can be greater than that of the heavier target Ar. The Qs could be similar in all three cases, even though the observed AEs are greater for the lighter targets. For the valinomycin ion, the fragmentation patterns observed in the spectra (Fig. 4) are broadly similar for all three target gases. There are ---'I "' 'I ' ' ', I ' ' p600 800 1000differences in the qualities of the spectra due to a lower frag- mentation efficiency observed for Ar, i.e. the number of frag- ment ions measured compared to the total number of apparent m/z -,----7-------r-----, . , Y., 1 . . . . . , . , . . . . , 200 300 incident ions entering the collision cells was 5% as compared with 10% for He and D,.The lower fragmentation efi- ciencies with Ar may be due solely to scattering but there could be other contributing factors such as charge exchange. For [(CsI),Cs] +,the intensities of the lighter fragment ions relative to [(CsI),Cs]+ (Fig. 5) are higher in the case of Ar than with He or D,. The difference is more pronounced for [(CsI),Cs] (Fig. 6). Again overall fragmentation efficiencies + are greater with He than with Ar, which may be attributed to scattering effects. The ICT theory has been used to calculate Qs associated with different fragment ions, using expressions (2)-(4) and the measured AEs (Tables 1-3). The calculations have been per- formed for the atomic targets.Use of the expressions requires a decision as to the mass of the interacting atom within the ion which is involved in the collision. In the case of valinomy- cin, the values of Q in Table 1 have been calculated with a hypothetical atom of mass 6.803 u (average of masses of ESA potentialp Fig. 4 Field desorption MIKE spectra of valinomycin [M + KJ+ ions with (a) helium, (b)deuterium and (c) argon as target gases atoms in valinomycin). For [(CsI),Cs] and [(CsI),Cs) +,+ again the averages of masses of the atoms were considered. The calculations show the Qs for an ion composed of pre- dominantly light atoms (valinomycin) to be large in most cases (tens of eV) and to be similar overall whether the target be He or Ar. The means of the Qs in Table 1 are 23 eV and 20 eV for He and Ar, respectively. The Qs for the 'heavy atom' cluster ions (Tables 2 and 3) are much smaller, although the AEs are of similar magnitude to the valinomy- cin case. With one or two exceptions, the calculated Qs for fragment ions from valinomycin exceed 10 eV and the major- ity exceed 20 eV (Table 1).With one exception, the Qs from caesium iodide clusters are < 10 eV and the majority are only J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 n -1000 300 ESA potentialp 500 1000 apparent m/z 300 ESA potentialp (c1 ,-500 1000 apparent m/z r-~--r-vr.-,-, , . . , , . , . , , . , . , . . , . . , . . , -200 300 ESA potentialp Fig. 5 Fast atom bombardment MIKE spectra of [(CsI),Cs]+ ions with: (a)helium, (b)deuterium and (c) argon as target gases ca.1 or 2 eV (Tables 2 and 3). For the clusters, the calculated Qs are also lower on the whole with He than with Ar. The means of the Qs for [(CsI),Cs] + in Table 2 are 0.6 eV and 1.1 eV for He and Ar, respectively; the means of the Q[(CsI),Cs]+ in Table 3 are 1.5 eV and 4.8 eV for He and Ar, respectively. The quasi-diatomic model makes predictions significantly different from those of ICT theory. The quasi-diatomic model considers that, for a given AE, Q depends on the mass of an ion, but not on the mass of its individual constituent atoms. A consequence of this is that for two incident ions of the same mass, where one is composed of heavy atoms and the other of lights atoms, the predicted Qs will be identical for the apparent m/z 19.2 (b)In s W ESA potentialp .--:I ? 1000 1500 i 1 apparent m/z -7.1.... .1.. .I 200 300 ESA potentialp 32.4 (c)1 L1 500 1000 1500 apparent m/z 7 .'...'!..I..--.--....-1 .'I..', I....'.'. t 1 200 300 ESA potentialp Fig. 6 Fast atom bombardment MIKE spectra of [(CsI),Cs]+ ions with: (a) helium, (b)argon and (c) hydrogen as target gases same AE. In Tables 4 and 5, Qs are given for valinomycin and the caesium iodide cluster ions, calculated with 8 = 0. The quasi-diatomic model indicates large internal Qs in most cases for both types of parent ion and further indicates that the Qs are in most cases larger for a given fragment ion with He than with Ar.Discussion The similarity among the fragmentation patterns of the valin- omycin [M + K]' ion found with He, Ar and D, as target gases (Fig. 4) indicates that the internal energies of the parent ions decomposing to the observed fragments are similar to J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 AE and Q calculated using the quasi-diatomic model with 8 = 0" for valinomycin [M + K] m/z 1149.7+ fragment ion loss of CH, loss of H,CO loss of C3H7 loss of CH, and C3H, [(HVLV),VHV + K]' -2H [(HVLV),VHV + K]+ -CH3 + H [(HVLV),VLV + K]+ -2H [(HVLV),VLV + K]+ -CH3 -H[(HVLV),HV + K]+ [(HVLV),HV + K]+ -CH3 + H [(HVLV),VL + K]' + 2H [(HVLV),VL + K]+ -CH3 + H [(HVLV),V + K]+ -CH, + 2H [(HVLV),L + K]+ + H [(HVLV),L + K]+ -CH, -C3H7 + 2H [(HVLV), + K]' -CH3 + H [(HVLV), + K] -CO + H [(HVLV)VHV + K]+ -CH3 + 2H [(HVLV)HVL + K]+ -CH3 + H [(HVLV)HVL + K]' -CO + H [(HVLV)HV + K]' -CH, + H m,+b AE(He) QW) WD2) Q(D2) AE(Ar) Q(Ar) 1133.7 40.3 30.0 40.9 29.3 6.7 6.7 1119.7 58.5 35.1 48.5 30.6 19.0 18.7 1106.7 38.2 28.1 38.5 28.1 18.0 17.8 1091.6 64.8 36.3 67.9 35.7 15.4 15.3 1075.7 39.7 28.7 44.8 31.2 27.3 26.8 1063.7 63.4 35.7 67.2 35.7 19.5 19.2 1047.7 59.2 35.1 76.9 35.7 11.4 11.3 1033.6 57.5 34.4 33.9 25.5 978.6 63.4 35.1 68.3 37.0 35.0 34.2 964.6 47.5 3 1.9 69.5 36.3 19.4 19.1 952.6 65.7 35.7 72.2 36.3 33.6 32.8 936.6 66.2 35.7 71.3 35.7 27.3 26.8 865.6 82.9 35.1 67.4 36.3 25.8 25.3 852.0 104.5 29.3 80.3 35.7 45.7 44.3 795.5 55.4 34.4 62.3 35.7 765.5 56.2 34.4 68.6 37.0 753.5 76.3 35.7 86.3 35.1 694.5 56.8 35.1 88.4 35.1 666.4 68.2 35.7 653.5 72.4 35.7 594.4 65.4 36.3 Table 5 AE and Q calculated using the quasi-diatomic model with 8 = 0"derived for fragment ions of [(CsI),Cs] m/z 1951.7+ fragment ion m,+ IU AE(He) Q(W [(CsI),csI + r(csI),csl+ C(CsI),CsI+ C(CSU3CSI 1 C(CSI),cS~ C(CSI)CSl 1172.15 169 1.76 1431.95 912.34 652.53 392.72 66.7 21.0 49.0 53.9 61.6 57.9 14.7 15.9 20.4 20.4 16.6 18.5 CCSI + 132.91 each other.This conclusion is strengthened by the recent dis- covery that fragmentation of cationised valinomycin follow- ing collisional activation can vary dramatically, depending upon the method of formation of the parent The centre-of-mass collision energies for valinomycin [M + K] + colliding with He, Ar and D, are 36, 353 and 36 eV, respec- tively.The AEs vary greatly from one fragment ion to another, but for He and D, most fall in the range 35-60 eV and for Ar many fall below 20 eV. It is pointed out that the AEs falling outside these ranges tend to be for the more minor fragmentations. The strongest fragmentations are loss of C,H;, which has AE of 38.2, 38.5 and 18.0 eV for He, D, and Ar, respectively, and loss of 16 u (presumably CH,), which has AE of 40.3, 40.9 and 6.7 eV for He, D, and Ar, respectively. If both He and Ar deposit the same internal energy, this represents a greater proportion of the centre-of- mass collision energy in the case of He.ICT reproduces these trends satisfactorily. The greater efficiency of internal energy uptake for He is predicted, arising from the better mass-match between the light atoms in the organic ion and He (compared with Ar).'7,28 Considering loss of C,H;, the Q with He is 14.5 eV and with Ar is 15.6 eV according to ICT (Table 1). If for loss of C,H; and He AE is 38.2 eV and Q is 14.5 eV, the recoil energy of He is the difference between, them, i.e. 23.7 eV. The recoil energy for Ar in the case of decomposition by loss of C,H; is 2.4 eV. The recoil energy for He is an order of magnitude greater than that for Ar, reflecting the direct momentum transfer supposed by ICT.The critical energies for the decompositions of the valinomy- cin are not accurately known, but most will be several eVs. The compound is cyclic, and many of the observed fragmen- tations necessitate rupture of two, or more, covalent bonds. WD,) QPz) AE(Ar) Q(W 20.8 15.9 0.3 0.4 43.1 21.0 6.5 6.4 48.9 21.0 7.5 7.4 45.5 21.7 13.4 13.2 44.4 22.3 24.1 23.4 40.9 21.0 31.2 30.1 65.8 60.7 Thus, Qs of 10-30 eV are not unreasonable given the com- pound's high number of internal degrees of freedom. The quasi-diatomic model explains the valinomycin results in Table 1 well enough. The calculated Qs are, on the whole, larger for He than for Ar. Considering loss of C,H;, the He figure is 28.1 eV and the Ar figure is 17.8 eV.The calculated Q would fall if non-zero scattering angles were considered, and the dependence of the calculated Q on scattering angle 8 would be stronger in the case of He [expression (l)]. The fragmentation patterns (Fig. 5 and 6) of the caesium iodide cluster ions are not identical. In the case of [(CsI),Cs] , intensities of the lower-mass fragments relative + to that of [(CsI),Cs]+ are higher with Ar than with He. This suggests that Qs are higher with Ar than with He, as loss of CsI to form [(CsI),Cs)+ is the lowest-energy fragmentation pathway and formation of other fragment ions requires higher internal energies. The fragmentation pattern with H, as the target gas (Fig.6) is consistent with this view, in that the relative intensities of the lower-mass fragments are still lower than with He. The centre-of-mass collision energy with H, is half that with He (11 eV as compared with 21 ev), and the mass-match (see below) is less favourable for H, than with He. Thus, it is reasonable to suppose that H,, while perhaps behaving similarly to He, would lead to smaller Qs than would He. The same trend for lower-mass fragments to have lower intensities relative to the CsI-loss fragment [(CsI),Cs]+ with He, compared to Ar, is evident from the spectra of [(CsI),Cs]+ (Fig. 5). Again, the implication is that less Q is taken up in collision with He than with Ar. According to ICT, caesium iodide clusters ought to take up internal energy more efficiently in collision with argon than with helium, because the mass-match of caesium (133 u) is J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 6 AE and Q calculated using the quasi-diatomic model with 8 = 0" for fragment ions of [(CsI),Cs] m/z 1172.14+ ~~ ~ II(CsWsl+ 912.34 19.2 16.6 C(CSI)2CS!+ 652.53 55.8 33.8 392.72 69.2 35.7[(CsWICCSl + 132.91 189.5 65.0 better with argon (40 u) than with helium (4 u).17i28The mea- sured fragment ions (Fig. 5 and 6) are associated with AEs generally comparable in size to those observed with valin- omycin [M + K]' in the case of He (Tables 1-3); with Ar the AEs are smaller with the salt clusters than with valinomy- cin. The Qs derived from these AEs by means of ICT are smaller for the cluster ions compared to the organic ion, by something like an order of magnitude for both He and Ar (Tables 1-3).The Qs calculated by ICT for the salt clusters are for the most part less than 1 or 2 eV, which is consistent with the observed fragmentations. The caesium iodide clus- ters are small systems [(CsI),Cs] + and [(CsI),Cs] have 39+ and 21 internal degrees of freedom, respectively, if judged by the standard of valinomycin [Sol degrees of freedom of (M + IS)+]. The binding energies of the clusters are not known as they must depend on cluster structure, but they could be tenths of an eV for loss of CsI are are unlikely to exceed 1.5 eV (for loss of CsI) even in the case of the most stable str~ctures.~~,~~ The cluster ions also undergo sponta- neous unimolecular fragmentation (i.e.in the absence of colli- sion gas). Deposition of a few eV of internal energy into such systems could be expected to induce a significant degree of fragmentation. In the case of [(CsI),Cs]+, ICT indicates that the Qs associated with different fragment ions are larger with Ar than with He for the smallest fragment ions, much the same for Ar and He for [(CsI),Cs]+ and [(CsI),Cs]+ and larger with He in the case of the major fragment [(CsI),Cs]'. These values are not inconsistent with the fragmentation patterns (Fig. 6). In the case of [(CsI),Cs]+, ICT indicates the Qs (derived from measured AE) to be larger with Ar for all frag- ment ions (Table 2), which is consistent with the fragmenta- tion patterns (Fig.5). The quasi-diatomic model does not easily account for the experimental results with the caesium iodide cluster ions. The calculated Qs (Tables 5 and 6) are unacceptably high in most cases. Setting the scattering angle 0 to some finite value (rather than zero) has the effect of reducing the calculated Qs, but the reduction occurs for both the cluster ions and the organic ion. When the magnitudes of Q for the clusters are reasonable, those for the organic ion become unacceptably low. Conclusion AEs measured for the organic valinomycin [M + K]+ ion and for the [(CsI),Cs] ion of similar mass (1 172 u as com- + pared with 1150 u for the organic ion) are of comparable magnitudes in the two cases, both with He and with Ar as the collision gas.AEs measured for the [(CsI),Cs]+ ion are still closer in value to those of the organic ion. AEs are in general greater with He than with Ar, both with the organic ion and the cluster ion. From consideration of the measured fragmen- tation patterns, there is evidence that He and Ar impart similar internal energies Q to the organic ion, but that Ar imparts somewhat more internal energy than He to the cluster ions. These experimental findings are reproduced satisfactorily by ICT. According to ICT calculations, the internal energies 20.8 17.9 2.3 2.3 48.9 31.9 3.6 3.6 55.1 34.4 4.8 4.8 35.6 27.4 10.4 10.3 taken up by the organic ion are similar with He and Ar.The larger AEs with He are accounted for by the predicted larger recoil energies of He. According to ICT calculations, the cluster ions' Qs are smaller by an order of magnitude than those acquired by the organic ion, both with He and Ar. The recoil energies are greater in the case of the cluster ions, hence AEs are as high as with the organic ion. The quasi- diatomic model does not distinguish between an organic ion of mass 1150 u and a caesium iodide cluster ion of similar mass, because only the total mass of the ion is considered in the model. To explain the spectral differences between the organic ion composed of over 150 atoms and the cluster ion of similar total mass composed of nine atoms necessitates that consideration be given to the masses of the constituent atoms in each case.We are pleased to acknowledge the financial support of the Australian Research Council, Kratos Analytical and the SERC. References 1 ion Formation from Organic Solids,ed. A. Hedin, B. U. R. Sun- dqvist and A. Benninghoven, Wiley, Chichester, 1990. 2 S. C. Davis, P. J. Derrick and Ch. Ottinger, Z. Naturforsch. A, 1990,45, 1151. 3 Tandem Mass Spectrometry, ed. F. W. McLafferty, Wiley, New York, 1983. 4 G. M. Neumann and P. J. Derrick, Org. Mass Spectrom., 1984, 19, 145. 5 G. M. Neumann, M. M. Sheil and P. J. Derrick, 2. Naturforsch. A, 1984,39,584. 6 D. L. Bricker and D. H. Russell, J. Am. Chem. SOC., 1986, 108, 6174. 7 MetastabZe ions, ed. R. G. Cooks, J. H. Beynon, R.M. Caprioli and G. R. Lester, Elsevier, Amsterdam, 1973. 8 J. Durup, Recent Developments in Mass Spectrometry, ed. K. Ogato and T. Hayakawa, University Park Press, Baltimore, 1970, p. 92 1. 9 R. G. Gilbert, M. M. Sheil and P. J. Derrick, Org. Mass Spec- trom., 1985,20,43 1. 10 M. M. Sheil, R. G. Gilbert and P. J. Derrick, Advances in Mass Spectrometry, 1985, ed. J. F. J. Todd, Wiley, Chichester, 1986, p. 1161. 11 M. M. Sheil, Ph.D. Thesis, University of New South Wales, Aus- tralia, 1987. 12 A. L. Alexander, P. Thibault and R. K. Boyd, J. Am. Chem. Soc., 1990,112,2484. 13 M. Guilhaus, M. M. Sheil and P. J. Derrick, Org. Mass Spec-trom., 1990, 25, 671. 14 C. D. Bradley and P. J. Derrick, Org. Mass Spectrom., 1991, 26, 395. 15 C.D. Bradley and P. J. Derrick, Org. Mass Specrrom., 1993, 28, 390. 16 M. S. Kim, Org. Mass Spectrom., 1991,26, 565. 17 E. Uggerud and P. J. Derrick, J.Phys. Chem., 1991,95,1430. 18 H. J. Cooper, P. J. Derrick, H. D. B. Jenkins and E. Uggerud, J. Phys. Chem., 1993,97,5443. 19 M. F. Jarrold and J. E. Bower, J. Chem. Phys., 1992, %, 9180. 20 R. C. Mowrey, M. M. Ross and J. H. Callahan, J. Phys. Chem., 1992,%, 4755. 21 M. F. Jarrold and E. C. Honea, J. Am. Chem. SOC., 1992, 114, 459. 22 M. F. Jarrold and E. C. Honea, J. Phys. Chem., 1991,95,9181. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 247 23 24 W. Forst, Unimolecular Reaction Theory, Academic Press, New York, 1973. (a) D. E. Rogers, Ph.D. Thesis, La Trobe University, Australia, 1980; (b) D. E. Rogers, P. G. Cullis, G. M. Neumann and P. J. Derrick, Ado. Mass Spectrom., 1980, 8, 1729; (c) P. G. Cullis, Ph.D. Thesis, University of New South Wales, Australia, 1988 ; (d) M. G. Darcy, D. E. Rogers and P. J. Derrick, int. J. Mass Spectrom. Ion Phys., 1978,27,335. 27 28 29 30 J. M. Curtis, C. D. Bradley, P. J. Derrick and M. M. Sheil, Org. Mass Spectrom., 1992,21, 502. E. Uggerud and P. J. Derrick, 2. Natuiforsch. A, 1989,44,245. H. J. Hwang, D. K. Sensharma and M. A. El-Sayed, Chem. Phys. Lett., 1989, 160, 243. H. J. Hwang, D. K. Sensharma and M. A. EI-Sayad, Phys. Reo. Lett., 1990,64, 808. 25 26 T-W. D. Chan, A. W. Colburn, D. S. Alderdice and P. J. Derrick, Int. J. Mass Spectrom. Ion Proc., 1992,107,491.M.M. Sheil and P. J. Derrick, Org. Mass Spectrom., 1988, 23, Paper 3/05394F; Received 8th September, 1993 429.

ISSN:0956-5000    

DOI:10.1039/FT9949000239

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 249-252 Dynamical Studies of the Reaction Be + HF(v, J) + BeF(v', J') + H on a New at,initio Potential-energy Surface Xinhou Liu Institute of Photographic Chemistry, Academia Sinica , Beijing 100101,People's Republic of China A new ab initio potential-energy surface for BeHF has been Fused for classical trajectory calculations on Be + HF(v,J) + BeF(v', J') + H. A reactive cross-section of 0.656 A2 is obtained for a collision energy of 83.58kJ mol-' on the ro-vibrational state of HF (0,0). This cross-section is increased by a factor of ca. five for the (1, 0) and ca. twelve for the (0, 30). It has been found that many trajectories pass through a deep well for the collinear complex FBeH. The role of the complex and the rotational excitation of HF is discussed.The energy dispersal in the products is also discussed. The simple atom-exchange reactions of the Group 2 atoms with hydrogen halides to form the metal monohalides have served over the past ten years to illustrate several conceptual principles in reaction dynamics. The reactions of Ba, Sr, and Ca with HX (X = F, C1, Br, I) have been studied in detail, experimentally and the~retically.'-~ Recently, trajectory calculations on the Ca + HF reaction have been reported by Jaffe." This study showed that a deep H-Ca-F potential-energy well dominates the collision dynamics of the reaction. For example, at a relative initial kinetic energy of 21 kcal mol-' more than 98% of the trajec- tories sample this well.Thus the Ca + HF reaction consti- tutes a type of neutral three-atom chemical insertion reaction which proceeds through a long-lived complex. A recent study16 of Ca + DF(u = 2, J) supported this reaction mecha- nism since the internal state distributions of the CaF are also well represented by statistical distributions. In particular, it has been found that the nearly isoenergetic reactions Ca + HF(v = 1, J = 7) and Ca + DF(v = 2, J = 1) give iden- tical product state distributions. This suggests that the reac- tion retains no 'memory' of the initial form of the reactant energy and the excess energy of the reaction is dispersed sta- tistically into all possible modes. It is clear from the experimental evidence that reactions of Group 2 atoms with hydrogen halides may proceed via the formation of a complex.This mechanism in the reactive encounters is important, particularly for the lighter alkaline- earth-metal atoms. Thus for the Ba + HX system, the reac- tion appears to be a direct abstraction. At high collision energies, Sr + HX behaves similarly, but at low collision energies complex reactions become important. However, Ca + HX reactions proceed mainly through complexes. Probably it may be speculated that for the lightest member of this family, Be, complexes should also play an important role. However, it is still not clear why the reactions show a forward-scattering behaviour. Generally speaking, for a reac- tion which proceeds via a complex that is long-lived com- pared with its rotational period, a forward-backward sym-metry angular distribution of the product should be obtained because the long-lived complex will lose the 'memory' of its initial translational vector so that the product will be scat- tered in all directions.Further experimental studies on this point are, therefore, still needed. In this paper, classical trajectory calculations on the reaction: Be + HF(v, J) + BeF(v', J') + H using a new ab initio potential-energy surface are given. Classical Trajectory Calculations Recently, we reported a new ab initio potential-energy surface using a large polarization basis set (6-311G**) for the BeHF system.32 Electronic correlation energies were given by fourth-order Moller-Plesset perturbation theory (MP4). Then the ab initio points were fitted to an analytical function using the many-body expansion method (PES2).The main features of this new function (PES2) will be mentioned briefly in this section. The reaction, Be + HF + BeF + H, is exothermic by 25 kJ mol-' on this surface. Like other surfaces for BeHF'7*'8,22 there is a very deep potential well corresponding to the stable, linear structure, F-Be-H, 373 kJ mol-' below the reactants. The transition state is very bent, at an angle of 70°, which is very different to those and the barrier height is only 79.99 kJ mol-'. The transition state on PES2 is slightly displaced towards the exit channel. Therefore, it belongs to the so-called 'late barrier'. It is very interesting that the transition state on this new surface is quite similar to that of the CaHF system reported by Jaffe et a1." In the CaHF system, the transition state is located in the product channel at an angle of 75" and the barrier is 68 kJ mol-' above the reactants.It can therefore be predicted that there may be some common dynamical features between the BeHF and CaHF systems. Classical trajectory calculations have been also performed on PES2.32 Relative translational energies ET of 84 kJ mol-', 105 kJ mol-' and 126 kJ mol-' were chosen. The initial ro-vibrational states of HF were taken as (v = 0, J = 0, 10, 20, 30) and (v = 1, J = 0) for each value of ET in order to investi- gate the effect of the reactant rotation on the reactive dynamics.More than 500 trajectories were run for each initial condition. The potential was monitored throughout the collisions in order to probe the reactive complex. Since the potential well on PES2 is deep, the energy threshold for the complex formation was chosen as 65 kJ mol- ' below that of the reactants. If the potential fell to this value and its con- figuration was near the linear complex, then the trajectory was deemed to have entered the region of the potential well. The calculated results are given in Table 1. The program, POT3DND,33 was used in the calculations. The values of b,,, obtained by carrying out a small number of trajectories, were found to vary between 1.2 A at the lowest collision energy and 2.4 8, at the highest collision energy and at the highest ro-vibrational state of HF.These b,,, values are much larger than those obtained for PES1.22 Similar b,,, values have been found for the Ca + HF reac- tion." Opacity functions P(b) for selected energies are given in Fig. 1. The decrease of P(b)with increasing b values is typical of reactions occurring on potential-energy surfaces having a barrier to reaction. For rotationally cold HF, increasing the translational J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Summary of dynamical calculations on the MP4/6-311G** surface 83.58 0, 0 1.2 53 0.656 83.58 14 21 65 104.50 0, 0 1.4 55 1.016 104.50 19 23 58 83.58 0, 10 1.5 65 2.721 108.98 15 20 65 125.40 0, 0 1.4 52 1.034 125.40 23 26 51 83.58 1, 0 1.6 59 3.224 127.48 16 21 65 104.50 0, 10 1.8 69 2.723 129.9 21 22 57 104.50 190 1.8 63 4.478 148.40 18 21 61 125.40 0, 10 1.9 72 2.722 150.8 28 22 50 125.0 1, 0 1.8 67 4.784 169.3 19 22 59 83.58 0, 20 2.2 74 6.3 10 180.97 19 23 58 104.50 0, 20 2.2 75 6.858 201.89 22 25 53 125.40 0, 20 2.5 79 9.07 1 222.79 25 28 47 83.58 0, 30 2.4 78 8.487 298.58 22 27 53 104.50 0, 30 2.4 81 9.029 319.50 23 19 48 125.40 0, 30 2.6 84 11.63 340.40 25 30 45 energy results in little overall change in either b,,, or P(b)so approach unity for small b values.This is different from the that cross-sections are little changed, but increasing vibra- result obtained using PES1.22 This implies that there is a tional energy increases both b,,, and P(b)and hence the reac- steric preference for the reaction even for J = 10.However, as tion cross-sections. The increase of 0.49eV (47kJ mol-') in the rotational quantum number of the reactant increases for the total energy will also be influential in increasing the any initial vibrational or translational energy P(b) cross-sections. approaches unity for small b values. This seems to show that The effect of increased rotational excitation of the HF on the reaction proceeds for high rotational states even when the the opacity functions is also shown in Fig. 1. From J =0-10, Be atom initially encounters the H end of the HF.However, a the overall change in P(b)is very slight, but from J = 10-20, factor which should be considered is the relative timescales of there is more than a three-fold increase in P(b).This is prob- the reactant. The rotational periods of HF for J = 10 and 20 ably because the total energy has been increased. The rota- are 86 x s and 43 x s, whereas the average tional excitation energies of the HF for J = 10, 20 and 30 are direct trajectory time is 40 x s on PES2. Thus an 25 kJ mol-', 97.5 kJ mol-' and 216.0 kJ mol-', respectively. asymptotic approach from the H end of the molecule with Therefore, it seems that at a given translational energy both J =20 may still be directed towards the F in the collision vibrational and rotational excitations enhance the reactivity region.For an even higher rotational state, say J = 30, there with the vibrational effects being more pronounced. A similar is a greater opportunity to reach the favoured approach result has been obtained using Chapman's MCSCF surface. '* angle of 70" during the trajectory so that for some b values, Such behaviour is expected, when the barrier is not displaced P(b)can reach unity. Thus the reaction on PES2 at low rota- early in the entrance channel. tional states of the reactants has a steric preference. Actually, Another dynamical feature shown by Fig. 1 is that for rota- that is not surprising because contour plots for Be moving tionally and vibrationally cold HF, opacity functions do not around the HF shows an angular preference to insert into the HF molecule.32 Therefore, PES2 shows a larger anisotropy and a much stronger energetic preference for the non-collinear reaction pathway. Fig.2 gives a qualitative view of the dependence of upon the reactant energies. It is clear that for cold initial states of i, I the HF, increasing translational energy increases the reacti- 0.25----I 0.251 000' " ' -t 1 .I.I,,.,.I I I . -t-,, 1 -1-L I o 05 1.0 1.5 20 25 o 0.5 1.0 1.5 2.0 2.5 o 0.5 i.0 1.5 2.0 2.5 impact parameter/A EJkJ mol-' Fig. 1 Opacity functions [P(b)].E, = A, 84; B, 105 and C, 126 kJ Fig. 2 Cross-sections. tr, J = (+) 0, 30; (+) 0, 20; ( X ) 40; (A) 0, 10 mol-'; u, J = (a)0,O; (b) 1,O; (c) 0, 10; (d)0,20 and (e)0.30 and (0)0,O. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 vity very slightly, even for J = 10, but for higher ro-vibrational states of the HF as the translational energies rise, the reactivity will increase, particularly for J = 20 and 30. A fact to be noted is that the state (u = 1, J = 0) at ET = 84 kJ mol-has the same total energy as the state (u = 0, J = 0) at E, = 125 kJ mol-', and the state (v = 0, J = 10)at ET = 104 kJ mol-', but the order of the reactivity is (u = 1, J = 0) > (u = 0, J = 10) > (u = 0, 3 = 0). Thus the effect of increasing reactant energies on reactivity appears to follow the order, vibration > rotation > translation. Histograms showing product energy distributions are given in Fig. 3-5. Each panel is normalized to unit area.Fig. 3 presents the BeF product vibrational state distributions p(u'). For the same translational energy, the product vibrational 0.2500.125t i 0.3751 i0.125 (d' I0.125 0.0 0 5 10 15 20 5 10 15 20 5 10 15 20 25 vibrational state, N, at BeF Fig. 3 Distributions of product vibration states. A-C and (a)-(e)as in Fig. 1. 0.40 0.30 0.20 0.10 0.30 0.20 0.10 > c. *-0.30-.-a 0.2c2 g 0.10 n 0.30 0.20 0.10 0.30 0.2c tf +-+L 10.1c 0.300 0.2250.150 t 251 I I I (b) i I t n BeF scattering angles/degrees Fig. 5 Distributions of product scattering angles. AX and (a)-(e) as in Fig. 1. distributions become broader on increasing the reactant vibrational excitation.Increasing the translational energy also broadens the distribution somewhat. Small rotational excitation causes little change while high excitation leads to a much broader distribution. In part this may be a simple reflection of added total energy. Product rotational distribu- tions are shown in Fig. 4. A similar trend is obtained: increas- ing energy broadens the distributions. Angular distributions of the BeF product are shown in Fig. 5. As shown in Fig. 5(a) and (b)(where results for J = 0 are reported), the shape exhi bits approximate forward-backward symmetry peaks when the reactant is rotationally cold. As the values of J increase it slowly evolves to a forward scattering pattern [Fig. 5(c)-(e)]. In addition, as the translational energy is increased and the vibrational state of the reactant is excited from v = 0 to u = 1, forward scattering is also increased.Thus at higher total energies, the reaction exhibits a forward- scattering behaviour. Such behaviour can be attributed to the long-lived complexes whose number decreases with increas- ing energy. Table 1 shows that there are many trajectories which pass over the potential well on this surface. It should be expected that isotropic scattering would give a flat dis- tribution. Table 1 also gives the partitioning of the total energy into product translation, rotation and vibration; (A) is the frac- tion of energy in the ith mode. The reaction on the MP4/6- 31 1G**(PES2) surface strongly favours product translation, more than 50% of the available energy being channelled in this mode.This contradicts experimental observations on such reactions.' Product rotation is least sensitive to the reactant energy distribution. At the lowest total energy studied (Etota,= 84 kJ mol-') which is near the reaction threshold, less than 15% of the available energy goes into vibration but the fraction rises to ca. 20-30% at higher ener- gies. As the translational energy is increased the BeF vibra- tion takes an increasing share of the total energy. Rotational excitation has little effect on product energy distribution. LI,,,; LA,,L.i30 60 90 120 30 60 90 120 30 60 90 120 150 Therefore, to use the notation of P~lanyi,~~ near the thresh- rotational state, N, at BeF old,Fig.4 Distributions of product rotational states. A-C and (a)-(e) as in Fig. 1. AT AT' -k AR' (1) while at higher energies, AT +AT' -+ AR' 4-AV' (2) As mentioned before,22 it is known that the presence of a potential-energy well is not sufficient to produce long-lived complexes. Table 1 gives the percentage of reactive trajec- tories passing through the potential well. It can be seen that more than 50% of reactive trajectories sample the region of the well and usually they spend a long time in this region. The number of non-reactive trajectories which pass through the potential well was found to be very small. Thus, it can be concluded that encounters leading to the formation of the complex will enhance the reaction.Increasing translational or vibrational energies of the reactant do not favour the forma- tion of the complex. However, as the rotational quantum number of the reactant is increased, many more trajectories sample the well. A plausible explanation for this result is that at constant E,, a fairly slow moving heavy Be atom approaching a rapidly rotating HF molecule passes through the favourable 70" < Be-F-H configuration and thus is more likely to enter the well. In addition, it seems that a more bent transition state leads to easier insertion of Be into HF so that the opportunity to form the complex increases. Conclusions At present there have been no experimental results for BeHF reported in the literature. One reason for this is that the barrier to this reaction was thought to be much higher than for other reactions of this series, but our calculations using the large basis set and MP4 correlation give a barrier which is only half the height of earlier calculations.Another reason for the reaction not having been studied experimentally is the high toxicity of beryllium. The studies reported in this paper predict the following important features of Be + HF scattering: (1) At an initial translational energy, the reaction cross-section increases with increasing values of the initial HF vibrational, rotational and translational energies. Vibrational excitation of HF is more effective in promoting BeF forma- tion than in placing a corresponding amount of energy in initial translation or HF rotation.The order of promoting reaction is: vibration > rotation > translation. (2) The percentage of trajectories which sample the H-Be-F potential well is increased by increasing the HF rotational energy. (3) The complex, F-Be-H, plays an important role in the process of the reaction. A large number of reactive trajec- tories (> 50%) pass through this potential well. The author wishes to thank Prof. J. N. Murrell, Dr. P. J. Knowles and Dr. S. Carter for helpful discussions. The J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 author gratefully acknowledges the support of K. C. Wong Education Foundation, Hong Kong. References 1 H. W. Cruse, P. J. Dagdigian and R. N. Zare, Faraday Discuss., Chem. SOC.,1973,55,277. 2 J.G. Pruett and R. N. Zare, J. Chem. Phys., 1976,64,1774. 3 Z. Karny and R. N. Zare, J. Chem. Phys., 1978,68,3360. 4 Z. Karny and R. N. Zare, J. Chem. Phys., 1978,69,5199. 5 R. N. Zare, Faraday Discuss., Chem. SOC.,1979,67,7. 6 A. Siege1 and A. Schultz, J. Chem. Phys., 1980,72, 6227. 7 A. Gupta, D. S. Perry and R. N. Zare, J. Chem. Phys., 1980, 72, 6237. 8 A. Torres-Filho and J. G. Pruett, J. Chem. Phys., 1980,72,6736. 9 A. Gupta, D. S. Derry and R. N. Zare, J. Chem. Phys., 1980, 72, 6250. 10 Chin-Kwan Man and R. C. Estler, J. Chem. Phys., 1981, 75, 2779. 11 A. Torres-Filho and J. G. Pruett, J. Chem. Phys., 1982, 77,740. 12 F. Engelke and K. H. Weiwes-Broer, Chem. Phys. Lett., 1983, 108, 132. 13 C. Noda, J. S. McKillop, M.A. Johnson, J. R. Waldeck and R. N. Zare, J. Chem. Phys., 1986,85,856. 14 R. Altkorn, F. E. Bartoszek, J. Dehaven, G. Hancock, D. S. Perry and R. N. Zare, Chem. Phys. Lett., 1983,98,212. 15 P. Chaguin, J. Phys. Chern., 1987,91, 1440. 16 R. Zhang, D. J. Rakestraw, K. G. McKrendrick and R. N. Zare, J. Chem. Phys., 1988,89,6283. 17 H. Schor, S. Chapman, S. Green and R. N. Zare, J. Chem. Phys., 1978,69,3790. 18 S. Chapman, J. Chem. Phys., 1984,81,262. 19 R. L. Jaffe, M. D. Puttengill, F. G. Mascarello and R. N. Zare, J. Chem. Phys., 1987,86,6150. 20 J. M. Alvarino and A. Lagana, Chem. Phys. Lett., 1988,144,558. 21 J. M. Alvarino, M. L. Hernandez, J. Margarido and A. Lagana, J. Chem. Phys., 1990,93, 1082. 22 Xinhou Liu and J. N. Murrell, J. Chem. SOC., Faraday Trans., 1991, 87, 435. 23 J. C. Polanyi, Acc. Chem. Res., 1972, 5, 161. 24 C. A. Mims, Shen-Maw Lin and R. R. Hem, J. Chem. Phys., 1972,57,3099. 25 M. H. Mok and J. C. Polanyi, J. Chem. Phys., 1969,51, 1451. 26 J. C. Polanyi and W. G. Wong, J. Chem. Phys., 1969,51, 1439. 27 P. S. Perry, J. C. Polanyi and C. W. Wilson Jr., Chem. Phys., 1974, 3, 31 7. 28 S. B. Jaffer and J. B. Anderson, J. Chem. Phys., 1968, 49, 2859; 1969,51, 1057. 29 K. G. Anlanf, D. H. Maylotte, J. C. Polanyi and R. B. Bernstein, J. Chem. Phys., 1969,51,5717. 30 D. S. Perry, J. C. Polanyi and C. W. Wilson Jr., Chem. Phys. Lett., 1974, 24, 484. 31 D. S. Perry and J. C. Polanyi, Chem. Phys., 1976,12,419. 32 Xinhou Liu, J. Chem. SOC.,Faraday Trans., 1993,89,2969. 33 POT3DND program, Dr. S. Carter, Department of Chemistry, University of Reading and Dr. W. Craven, University of Sussex. Paper 3/04234K; Received 20th July, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000249

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 253-261 Orientational Ordering in the Solid Fullerene Oxide: C,,O Ailan Cheng and Michael L. Klein Department of Chemistry and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA, 19104-6323,USA Constant-pressure molecular dynamics simulations have been employed to investigate the orientational order- ing in the solid phases of fullerene oxide, C6,0. A pairwise additive atom-atom intermolecular potential model developed for solid c60 is modified slightly to reflect the functionalized character of the C6,0 molecule. The simulation results indicate that at low temperature the carbon cages are frozen into a Pa3-like structure, as in the pure c60 solid. Most oxygen atoms point randomly to one of the neighbouring octahedral interstitial sites (i.e:(100) direction) but about 20% point to the smaller tetrahedral sites ((111) direction).Above the transition temperature, estimated to be around 210 K compared with the measured value of 278 & 2 K, C600 molecules rotate about the centre-of-mass-oxygen axis. The bridging oxygen atoms tend to wobble in their interstitial sites rather freely but they cannot move from one pocket to another. However, at very high temperature (ca. 800 K), the oxygen atoms are able to hop between different interstitial sites on the molecular dynamics timescale. Fullerene derivatives have recently attracted considerable attention, and already, many new molecules have been reported.'-'' One of the simplest among them is the epoxide C600.5,6In this molecule, the attached oxygen atom breaks the icosahedral symmetry of the c60 molecule and hence it should affect the solid-state properties and both the intra- and inter-molecular dynamics.For example, the far infrared spectrum should be particularly rich. The orientations and motion of the oxygen atoms in the solid are of particular interest. Ab initio calculations have already been carried out to explore the structure of an isolated C600molecule,' '-14 and we recently reported the results of a preliminary study15 of the orientational ordering in the bulk solid. Our earlier inves- tigation used the molecular dynamics (MD) simulation tech- nique and Lennard-Jones atom-atom potential model. The MD results suggested that the solid should undergo a phase transition at a temperature close to that of pure c60.However, even above the transition temperature C6,0 mol-ecules rotated anisotropically ; the oxygen atoms tending to point preferentially to the octahedral interstitial sites that lie along the crystal (100) directions. A subsequent detailed X-ray diffraction study16 found that solid C6,0 indeed exhibits a phase transition at 278 & 2 K (compared with 260 K in pure C60).17-20 At low temperature, the cages of the C6,0 molecules are frozen in a Pa3-like structure analogous to pristine solid c60, but about two-thirds of the oxygen atoms preferentially occupy the tetrahedral sites (( 11 1) direction) and one-third the octahedral sites (( 100) direction).However, in the high-temperature phase the reverse situation obtains, namely, the octahedral sites seemed to be favoured.I6 In this study, we have employed a more refined pairwise additive atom-atom potential model based on one developed for solid c60.2'The latter has been shown to reproduce the experimentally observed Pa3 structure and also correlates with a wide range of properties such as the transition tem- perature, jump in the lattice constant at the transition, phonon density of states, reorientational relaxation time, bulk compressibility and far-infrared spectrum for solidc6,.18.20,22-26 The present simulation results confirm the anisotropic rotation of C6,0 molecules at room temperature.The carbon cages are predicted to rotate most easily about the centre-of-mass-oxygen axis ;the motion of oxygen atoms is mostly confined within the interstitial sites. At high tem- perature, the two-fold axis begins to reorient occasionally. The estimated residence time for an oxygen atom at 796 K in a particular site is of the order of 10-20 ps, whereas the transit time from site to site is usually less than 1 ps. Upon cooling below room temperature the rotation about the two- fold axis freezes-out and the carbon cages orient the same way as in the Pa3 structure of pure c60; a result which agrees with the X-ray data.16 However, we find that most oxygen atoms (ca. 80%) point randomly to one of the neighbouring octahedral sites. Only about ca.15% of the 0 atoms freeze into tetrahedral sites. The disagreement with experiment con- cerning the relative occupancy of octahedral us. tetrahedral sites16 suggests that the intermolecular potential model needs to be refined further. Details of Calculation Structure of C,,O Molecule In the current simulation C6,0 molecules are treated as rigid since the coupling between the intramolecular vibrations and the molecular translational and rotational motions is negligi- ble in the temperature range of interest. The NMR, FTIR and UV-VIS spectra reveal that the C6,0 molecule retains the essential structural and electronic character of c60 .' The same study also found that C6,0 has the epoxide structure (C2J with an oxygen atom bridging across the double bond that fuses two hexagons in the parent C,, molecule.Ab initio calculations" found that the isomer (C,) with a bridging oxygen atom across the bond connecting a pentagon and a hexagon is more stable. However, a more recent Car-Parrinello density functional theory calculation l ' indicates that the C2v epoxide structure has a slightly lower energy than the C, isomer. As in our previous study, the epoxide structure is assumed in the current MD calculations. Since the remaining cage carbon atoms are only slightly disturbed by the presence of the oxygen atom we assume they have the same structure as in a c60 molecule. The distance between two carbon atoms next to the 0 atom obtained from the various theoretical cal- culations is, 1.54," 1.5613 and 1.56 whereas the C-0 bond length is, 1.45," 1.4313 and 1.47 A,14 respectively.The best fit to the X-ray data yields 1.6 0.3 %, for the C-C bond and 1.43 f0.06 A for the C-0 bond.16 In the current study, C-C and C-0 bond lengths are chosen to be 1.532 and 1.47 A, respectively. We will see later that this choice of C-C bond length was almost certainly too small. Potential Model Previously, we have developed a pairwise additive atom- atom potential model which gives a good account of a variety of solid-state properties of pure c60.21 In this model, carbon atoms are considered to interact with each other via a Lennard-Jones (12-6) potential, and extra repulsive sites are placed at the centre of each double bond.In addition, electro- static interactions are modelled by fractional charge, qc = 0.175 e, assigned to carbon atoms, with a neutralizing charge, 4,, = -0.35 e, placed at the centre of double bonds. In the current calculation, the same basic scheme has been adopted with minor modification to the sites in the imme- diate vicinity of the 0 atom. The bond bridged by the oxygen atom is no longer an interaction site. The two carbon atoms adjacent to 0 still interact via a Lennard-Jones (12-6) poten-tial with the same parameters as other carbon atoms. The oxygen atoms are also treated as Lennard-Jones interaction sites with parameters ~(0-0)= 78 K and o(0-0) = 3.17 A, typical of many organic molecules.27 The van der Waals interactions between the unlike pairs are obtained by using the combining rules, which yield ~(0-C) = 46 K and a(0-C) = 3.28 A.Each oxygen atom also carries a charge (40= -0.5 e),27 which is balanced by the charges (4c = 0.25 e) on the two C atoms bonded to the 0 atom. With this charge distribution, the net dipole moment is 3.06 D,? which is larger than the ab initio calculation value, 1.27 D.13 Details of the Simulations The Parrinello-Rahman constant-pressure MD simulation technique has been employed throughout this work. The Newtonian equations of motion for translational motion of the centres-of-mass of the C6,0 molecules are solved by using the third-order Gear predictorxorrector algorithm. The rotational degrees of freedom, represented by quatern- ions, are integrated by a fourth-order algorithm.Owing to the large number of interaction sites (90 per molecule), most simulations are carried out for a 2 x 2 x 2 face-centred-cubic (fcc) lattice, which is replicated using periodic boundary con- ditions to mimic an infinite lattice. Typically, at a given tem- perature the system is allowed to equilibrate for about 25 ps. Then, trajectories (positions, velocities, angular velocities) covering the next 25-50 ps are collected in order to compute the various quantities of interest. Simulation Results Phase Transition and Low-temperature Structure The constant-pressure MD simulations were performed at zero pressure for temperatures ranging from 20 to 800 K. The simulation started at high temperature after which the system was cooled gradually.Various quantities were monitored to characterize the structural and orientational ordering tran- sition. At high temperature the C6,0 molecules are rotating but retain the imposed fcc structure. During the MD cooling run the lattice remained cubic as evidenced by the lengths of the cubic edges of the simulation box [see Fig. 1 (top)]. An abrupt change of configurational energy and average lattice constant (or volume) sets in at around 210 K; behaviour that. signals a structural transformation. The calculated lattice constant is almost identical with that from a previous simula- tion of solid C60.21However, the calculated lattice constant is significantly smaller than that obtained from X-ray scat- -f 1 D z 3.33564 x C m.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 14.3 r I I I 1 4 .t ,I I I I 'I II14.2 t ,I I I 1I I Ic .d i -200 @*.@"4 I-210 ' 0 200 400 600 800 TIK Fig. 1 The three edge lengths of the simulation box (top), average lattice constant (middle) and configurational energy (bottom) as a function of temperature. In the last two panels, the solid circles are for the cooling run and open circles for the heating run. Also, values of the configurational energy for the run started at low temperature in the P2,3 structure are represented by the squares (open symbols for heating and solid squares for cooling). tering experiments. For example, the present simulation yields a = 14.10 A at 300 K, whereas the experimental value is a = 14.185 A.The discrepancy here is likely to be due to the fact that a smaller C-C bond length, and hence c60 radius (R = 3.518 A), is used in the MD simulation than is suggested by the X-ray lattice constant data which are better fitted by R = 3.54 A.16 The recent ab initio calculations also suggest a slight expansion of the molecule when functional- ized by an 0 atom.12 The isobaric thermal volume expansion coefficient, E", can be estimated from the data in Fig. 1. The current simulation gives average values, a, = 6.7 x lo-' K-' and 5.4 x lo-' K-' for temperatures above and far below the transition point. The X-ray experimentI6 obtained the corresponding values, a, = 7.3 x lo-' K-' and 10.9 x lo-' K-'.However, the latter value is for the temperature range just below the transition, and is likely to be much larger than that far below the transition temperature, as is the case in pure C,, where a, = 6.2 x lo-' K-' for T < 245 K and T > 260 K, and 21 x lo-' K-' for 245 K < T/K < 260. Our previous MD study of pure c60 solid2' yielded a, = 7.0 x lo-' K-' above transition and 5.7 x lo-' K-' far below. These values agree with the experimental results rather well. The orientational behaviour of C6,0 molecules can be characterized in a fashion similar to that used in the study of pure c60 solid.28 For this purpose, one defines the order parameters as the ensemble average of the appropriate com- binations of spherical harmonic functions, YIm.If we assume that the 60 carbon atoms to be averaged over maintain icosa- hedral symmetry, the lowest non-zero order parameters are then those for I = 6.These 13 independent parameters indi- cate that the change in the lattice constant and the configu- rational energy shown in Fig. 1 is accompanied by an orientational ordering transition. More accurately, at low J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 temperature, the carbon cages orient the same way as in the low-temperature Pa3 structure of C,,; an observation which suggests that the interactions between cages are dominant. Locations of Oxygen Atoms Fig. 2 shows an instantaneous configuration taken from the simulation at T = 145 K.The snapshot reveals that most of the oxygen atoms occupy randomly the available octahedral sites in the fcc lattice.The orientation of a C,,O molecule can be specified by the Euler angles (p, 6,$) using the crystal axes as a Cartesian frame. Here, we have adopted Goldstein's notation for Euler angles.29 However, the motion of an oxygen atom can be monitored more easily by following the unit vector along the direction connecting the centre-of-mass of C,,O and the atom in question. This unit vector (i)can be characterized by the polar angle, 8, and azimuthal angle, a. These two angles are related to the usual Euler angles p and 4, where the polar angle 8 = B and the azimuthal angle a = 4 -90". The probability distributions of the two Euler angles, p(B) and p(4), are shown in Fig.3. At 796 K (top panel), though the &distribution is roughly uniform over all directions, the B-distribution display broad peaks at 0",90" and 180" which confirm that in the present simulation the oxygen atoms slightly prefer octahedral sites, which lie in the (100) directions. At lower temperature but still well above the freezing temperature, the probability distributions (central panel) suggest that oxygen atoms have well defined orienta-tions with most oxygen atoms favouring the octahedral sites. However, some point to the tetrahedral sites indicated by the satellite peaks at /.? M 50" and 120", even though the tetra-hedral sites lying in the (100) directions are smaller. Below the transition temperature when C,,O molecules stop rotating, the octahedral-site occupancy is still higher than the tetrahedral one. Fig.3 (bottom panel) shows p(B) at T = 126 K,the splitting of peak at 90" is related to the fact that oxygen atoms now point to the edge rather than the centre of the octahedral voids. The deviation from an ideal (001) direction is related to the Pa3 structure preferred by the carbon cages. The average probability for an 0 atom to point to the octahedral and tetrahedral sites was evaluated and the result is shown in Fig. 4. Here, the probability is defined in terms of Fig. 2 Snapshot of C,,O fcc lattice at T = 145 K. The white balls represent the oxygen atoms. Oxygen atoms prefer to occupy octa-hedral interstitial sites but the 0 atom occupying the tetrahedral site (middlerieht of fieure) should be noted.255 0.08 0.042 0.06 0.03 v Q-0.04 0.02 0.02 0.01 0.12 0.08 0.08 0.040.04 0 0 45 90 135 -90 0 90 180 /3/degrees +/degrees Fig. 3 Probability distributions, p(#?) and p(+), for various tem-peratures: 796 K (top),281 K (middle) and 126 K (bottom) the percentage of molecules pointing to a particular direction. At high temperature ca. 60% of 0 atoms prefer the (100) direction and ca. 25% point to tetrahedral voids, which agrees well with the X-ray diffraction data.16 As the system is cooled, the octahedral occupancy increases slightly and reaches 80% at the orientational freezing temperature. However, analysis of the X-ray data suggests that at low tem-perature the occupancy of octahedral and tetrahedral sites becomes reversed at 36 and 64%,respectively.' In an attempt to understand this discrepancy we carried out a second series of MD simulations this time starting on purpose with a P2,3 structure as initial configuration.In the P2,3 structure, all oxygen atoms point to tetrahedral sites. This structure did not seem to be stable. At 100 K about 10% of the 0atoms jumped spontaneously to octahedral sites after only 25 ps of simulation. However, the temperature 100 K is too low to facilitate rapid reorientation and hence find the 'true' ground state in a reasonable time. The system was therefore heated gradually. Between 300 and 400 K,only 25% of the 0 atoms remain in the tetrahedral pockets.Above the transition temperature the occupancy of the two types of 0.8 0.6 >- E 0.4 0 200 (00 600 800 TIK Fig. 4 Average probability for occupancies of octahedral sites (sauares). tetrahedral sites (hexagons) and other orientations (circles) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (4 Fig. 5 Trajectory plots of C,,O molecules in an fcc lattice at 381 K (a)and 145 K (b).The white traces are associated with rotational diffusion about the two-fold axis. The localized black trajectories represent the confined oxygen atoms. sites agreed with that of the previous MD run. We then son of the two sets of simulations suggests that the run quenched the high-temperature phase first to 200 K and then started with all tetrahedral occupancy has higher configu- further to 100 K.The tetrahedral occupancy increased by rational energy (see Fig. 1). Thus, the low-temperature only 5% relative to the previous MD simulations. Compari- 0-atom orientations obtained from first MD runs is likely to J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 be reasonably close to the equilibrium value for the assumed potential model. Reorientation of C,oO Below the transition temperature, the C,,O molecules are completely frozen on the MD simulation timescale. Essen- tially, molecules only librate about preferred orientations. Around room temperature, the carbon cages rotate about the two-fold i axis rather freely. The spinning of molecules at 381 K is obvious in the trajectory plots of C,,O molecules shown in Fig. 5.However, the rotation is rather anisotropic. Basi- cally, oxygen atoms are still trapped for extended periods in the interstitial sites; they can wobble in their pockets but jumping between sites is relatively rare. The trajectories of oxygen atoms are shown in Fig. 6. Below the freezing temperature they are pinned in either the octahedral or tetrahedral sites. Even above the transition temperature, when the cage undergoes hindered rotation about the two-fold axis, the i axis still cannot reorient easily. As the temperature increases, hopping between different orientations sets in. Though some 0 atoms still stay in one pocket at 796 K, most of them make at least one hop during the 50 ps simulation.Fig. 7 shows the variation of the Euler angles, fl and 4, as a function of time. This picture gives an idea of the residence time in a particular pocket and the transit time between pockets for several molecules. For example, for the molecule labelled A [Fig. 7(a)],the 0 atom stays in an octahedral site for ca. 10 ps, and then flips 180" to another octahedral pocket within 0.5 ps. Molecule B [Fig. 7(b)]experiences two such events: first from a tetrahedral (T) to an octahedral site (0), then, to another octahedral site. The typical residence time is of the order of 10-20 ps and the transit time is usually ca. 0.5 ps with occasional slower events. Several hopping patterns such as 0 -+ T, 0 -P 0,T -+ T and T + 0 are exhibited.Mol- ecules C and D [Fig. 7(c), (41follow the hopping patterns, 0 -+ T, -,T, , and 0 -+ T, respectively. The reorientational relaxation time was examined at various temperatures. In particular, the autocorrelation func- tion, Cl(f),defined in ref. 15 for unit vectors along the three molecular axes 1,9 and f were estimated. Here i is along the centre-of-mass-0 axis, 1 and 3 are perpendicular to i, and 1 is the order of the Legendre polynomial. If we assume that the reorientation is uia rotational diffusion3' one can fit the correlation function to an exponential form, and extract an estimate of the reorientational relaxation time, T~.Only the calculated values for unit vectors 3 and i for I = 1 are summarized in Table 1, since that for the unit vector iis similar to that for 3.There are clearly two timescales. The relaxation time obtained for motion of the unit vector 3 is associated with the timescale of the spinning of carbon cage about the two-fold axis. This timescale is expected to be Table 1 Reorientational relaxation time for the molecular axes of C600 182 103 810 220 38 329 234 65 258 272 52 218 28 1 34 126 374 20 166 381 23 100 572 6 50 659 6 18 796 9 13 Fig. 6 Trajectory plots of oxygen atoms for various temperatures. At 126 K (a) oxygen atoms are locked in the interstitial sites. At 281 K (b), they can move in a particular pocket freely but hop-ping between different sites is rare. This motion is more prevalent at 796 K (c).J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 90 90 0 -90 80 90 0 -90 0 A An A 9010i 1 w -901-4 )!I1 ' -180' ' I 1 0 5 10 15 20 25 0 5 10 15 20 25 tlPS tlPS Fig. 7 Euler angles, B and 4, which describe the motion of an oxygen atom, as a function of time. From top to bottom the curves are for molecules A, B, C and D, respectively. The oxygen atom of molecule A jumps from one octahedral site to another, and that of molecule B hops first from a tetrahedral (T) to octahedral (0),then, to another octahedral site. Molecules C and D follow the hopping patterns 0 -,T, -+ T, ,and 0 -,T, respectively. similar to that of pure c60 solid.28 The unit vector t^ reflects the motion of oxygen atoms.The time taken for an oxygen atom to flip over is much longer. Below 200 K, the reorien- tational relaxation time z is much longer than the current simulation time~cale,~~ and the simulation simply shows a complete frozen phase similar to an orientational glass.32 Assuming that the value of zl at high temperature can be described by an Arrhenius law, z = zo exp(EJk, T), one can fit zi and zf by this form. For I = 1, this fit gives the pre- factors, rg = 2.9 x s and zt = 6.0 x s, and acti- vation energies, Ei = 690 f50 K and E,' = 960 f50 K. The estimated activation energy of 690 K for molecular spinning motion about the t^ axis is in reasonable agreement with experimental values, 480,24 69525 and 400 K,22 reported for pure c60 solid.The activation energy, 960 K, for the reorien- tation of the two-fold i axis is much higher. Even though the C600molecule is only slightly distorted relative to a pristine c60 molecule, the small oxygen bump causes a dramatic increase of rotational anisotropy. As shown above the reorientation time below the tran- sition temperature is much longer than the simulation time- scale. It is simply impractical to observe such motions within a reasonable computing time. However, the dynamics in this temperature range are of great interest. For example, it is known that pure C,o undergoes a 'ratchet' reorientation below the transition temperat~re.~~.~~.~~Recent NMR experiments suggest that C6,0 molecules display a similar type of motion, i.e.infrequent hopping between equivalent orientation^.^' The key question concerning this type of acti- vated motion relates to the barrier height of the associated pathway. To obtain some semi-quantitative information, we have taken one of the configurations generated in the MD simulation at 100 K, and arbitrarily chosen one molecule as a test particle. The static configurational energy is then calcu- lated as the test molecule is rotated while artificially freezing the translational and rotational motion of the remaining mol- ecules. The initial orientation of the test molecule was chosen so that the three molecular axes are aligned with the crystal axes, x, y and z. In this configuration the oxygen atom on the 60008ooo i 200 -11 '3" 3000 t I) 0 90 180 270 360 0 90 180 270 360 yldegrees yldegrees Fig.8 Configurational energy relative to the initial orientation of a test molecu!e as a function of rotation angle about (a) (100), (b) (OlO), (c) (1 10) and (d) (1 11) axes, respectively (see text for details). The configuration is taken from the simulation at 100 K. test molecule points to the (001) direction. The configu- rational energy relative to that of the initial orientation is then calculated while the test molecule is rotated about several axes. Fig. 8 shows the relative configurational energy values as a function of the angle rotated about the axes (loo), (OlO), (il0) and (lll), respectively. The energy profiles show strong angle dependence.When rotating about the (100) and (010) axes, the oxygen atom will bump into the four nearest neighbours along the path (recall that the low-temperature phase has the Pa3 structure). When the protrud- ing oxygen atom approaches the nearest-neighbour molecules lying in the (1 10) directions, the configurational energy increases dramatically. The peaks at 3 15" when rotating about the (100) axis and at 45" about the (010) axis corre- spond to configurations where the oxygen atom on the test molecule hits the 0 atom of the neighbouring molecules. The four wells at 0", 90", 180" and 270" correspond to the four octahedral sites along the (100) directions. The rather flat- bottomed potential wells reflect the large size of the octa- hedral sites, which is consistent with the above finding that the oxygen atoms can wobble rather freely in this type of site (recall Fig.5). When the test molecule is rotated about the (!lo) axis its oxygen atom will pass two types of interstitial site. On flip- ping the oxygen atom by 180", it first moves away from the initial octahedral site to an adjacent tetrahedral site. Then it passes a nearest neighbour to find another tetrahedral site, and eventually it falls into the final octahedral vacancy. The energy barrier between the octahedral and tetrahedral sites is much less than that needed to pass the nearest neighbour. Rotation about the (111) axis reflects the three-fold sym- metry of the lattice. Here, the three prominent peaks also indicate that the energy barriers are lower than that of passing nearest neighbours but higher than that between an octahedral and a tetrahedral site.From this calculation it seems that the energy barrier is lowest when rotating about the (il0) direction, which corre- sponds to jumping from a tetrahedral to an octahedral site or vice versa. As mentioned earlier, we have set up a system, on purpose, in a P2,3 structure and allowed it to relax to its equilibrium state. At 150 K one molecule hopped from a tetrahedral site to an octahedral site during the first 25 ps. The time dependence of the Euler angles, /3 and 4, for this molecule shown in Fig. 9 confirms that the molecule indeed J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0-T I1 t 1 180 I I I I I t 4 got-&t i a OI-LiYd!dd-1 800 5 10 15 20 25 tlPS Fig.9 Euler angles, /? and 4, as a function of time for a molecule taken from the simulation started with the P2,3 structure. This plot suggests that the molecule hops from a tetrahedral site to an octa- hedral site through the valley between the two nearest-neighbour molecules. passes the valley between the two nearest neighbours during its reorientation. In fact, the trajectory plots in Fig. 6 also reveal that even at high temperature most jumps are along the diagonal in the picture, which corresponds precisely to the path along the valley mentioned above. The actual values of the barrier heights vary slightly with configuration and the test molecule chosen. In the real situ- ation, all the surrounding molecules are also moving (librating and vibrating); therefore, the reorientational barrier will be somewhat time dependent.However, the main fea- tures of the energy profiles are likely to be the same. The path through the unfilled space between the two nearest neigh- bours is likely still to be the optimal route. This energy calcu- lation also indicates that the lowest barrier, ca. 3000 K, estimated from Fig. 8, is sufficiently high that thermally acti- vated hopping is unlikely to occur on the simulation time- scale at low temperatures. The energy profile also indicates that for the present model the system has slightly lower energy when the oxygen atom points to the octahedral voids rather than the tetrahedral sites.Again, while this is consis- tent with the higher octahedral occupancy (ca. 80%)relative to the tetrahedral occupancy (ca. 20%) observed in the simu- lation, it is not in accord with low-temperature X-ray data.' At higher temperatures the energy barriers are reduced somewhat owing to the rotation of the molecules and the expansion of the lattice. The elevated thermal motion also facilitates reorientation. From an examination of molecular reorientation in a configuration taken from a simulation at 796 K it is clear that the energy barriers are reduced. However, the (110) pathway still has the lowest barrier. Typically, the energy needed for an oxygen atom to jump from a tetrahedral to an octahedral location is ca.10 kJ mol-' and ca. 20 kJ mol-' for the opposite process. This energy scale is roughly consistent with the average activation energy, 960 K, obtained from an analysis of the reorientation time. Because the oxygen atom must necessarily follow the lowest-energy barrier path during the molecular reorientation the pathway for the C6,0 cage might be different from that of pure c60.Consider an oxygen atom pointing into an octa- hedral site, if it moves so that it is close to the unfilled space between the two nearest neighbour molecules, it can then librate about one of the (I 10) axes and jump to a tetrahedral site. After the hopping of this oxygen atom the cage can rotate about the i axis to rest_ore the original Pa3 orientation.For a c60 molecule in a Pa3 structure one can always find one of the 30 double bonds pointing to one of the neighbour- ing voids (either tetrahedral or octahedJal), so it is always possible for the carbon cage to find a Pa3 orientation for any of the orientations of the oxygen atom pointing to an octa- hedral or tetrahedral site. However, during the reorientation the carbon cage may be temporarily away from the ideal Pa5 configuration. The present MD simulation also suggests that an oxygen atom is likely to go through a tetrahedral site on the way from one octahedral to another octahedral location. This type of pathway is also observed in the simulation at high temperature (see trajectory in Fig. 6). Phonon and Libron Densities of States The densities of states (DOS) for the librational and trans- lational motions have been calculated.The crystal dynamics are of importance because they can provide a stringent test for the intermolecular potential model. Velocity and angular velocity autocorrelation functions (ACF) have been evalu- ated. One simply obtains the density of states by taking the Fourier transform of the ACFs. Fig. 10 gives both the libra- tional and translational DOS at several temperatures together with that for pure c60. In comparison with the c60 data at T = 106 K, both the translational and librational bands of C6,0 are shifted about 5 cm-' to higher frequency and are slightly broader. The gap between the translational and librational bands is also narrower in C,,O relative to the c60 solid.Also, there seems to be a weak coupling between the rotational and translational motion of C,,O molecules. As temperature increases both bands shift to lower frequency. At 374 K, when molecules are rotating, the coupling is enhanced slightly. It is likely that when oxygen atoms try to jump from one interstitial site to another, the nearby mol- ecules may recoil to open up more space and hence facilitate the hopping event. Since the rotation is very anisotropic for C,,O, the angular velocity ACFs about three molecular axes, x, y and z, have 3 2 1 0 2 1 h v c,o 1 0 2 1 0 20 40 60 v/cm -' Fig. 10 Density of states for librational (dotted lines) and trans- lational (solid lines) motions.The curves are for (a) pure C,, at 106 K. and C,,O at 100 (b)374 (c) and 796 K (d). 260 4 3 2 1 0 h 52 u 1 0 4 L 1,, -12 0 10 20 30 40 v/cm -' Fig. 11 Fourier transforms of the angular velocity ACFs, represent- ing the rotation about the three molecular axes, f (solid lines), 3 (dotted lines) and i (dash lines): (a) 100, (b) 374, (c) 796 K been determined separately. The Fourier transform of these three ACFs are shown in Fig. 11 for T = 100, 374 and 796 K, respectively. The libration about the i axis has the lowest frequency, while rotation about the f and 3 axis, which re- flects the tumbling of oxygen atoms, has a much higher frequency. As the temperature increases, the C600 molecule begins to spin about the two-fold axis and a central peak appears.However, even at 796 K, the frequency correspond- ing to the wobbling of oxygen atoms is still peaked at 5 cm-',confirming the previous finding that oxygen atoms are not rotating freely. Note that both the translational and rota- tional DOS show a lot of structure. The small oscillations near the ends of the bands are due to the truncation artifacts 10 5---0-4 L I I I I I I I J I I I 1 J 0 200 400 600 800 TIK Fig. 12 Average librational frequencies corresponding to the rota- tion about the three molecular axes, f (triangles), j (squares) and i (circles). Plateaus at high temperature for f and j components suggest that oxygen atoms are wobbling with confined amplitude in the interstitial sites.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 of the Fourier transform. However, the prominent peaks are independent of the truncation in the calculation. In fact, the choice of trucation only changes the resolution of the spectra but not the overall features. The average frequency and width of the librational band have been estimated as these values can be obtained from inelastic neutron scattering experiments.22 The calculated values are shown in Fig. 12. The abrupt change in the average frequency correlates with the freezing of the rota- tions. The plateau above the transition temperature for f and 9 components is consistent with the picture that C6,0 mol- ecules undergo anisotropic rotation even at high temperature.Conclusion Molecular dynamics studies have been performed for solid C6,0 using a revised pairwise additive atom-atom potential. The present simulation confirms the presence of a phase tran- sition from an anisotropic rotator phase to an orientational glass (ratchet) phase at a temperature close to that found in the pure c60 solid.21 Below the transit_ion temperature, the carbon cages are found to adopt a Pa3-like structure, as in pure c60. Evidently, the effect of the attached oxygen atom was not strong enough to alter the packing of the carbon cages. Oxygen atoms are locked randomly into one of the neighbouring interstitial sites. In the simulation, octahedral occupancy (ca. 80%) is favoured over the tetrahedral one (15- 20%).Just above the transition temperature, C6,0 molecules rotate anisotropically : molecules spin about their i axis almost freely, but on the MD timescale the 0 atoms are mostly restricted to wobbling around in a particular intersti- tial site, i.e. libration about a fixed orientation. As the temperature increases further, hopping of oxygen atoms become possible. However, the residence time is still much longer than the transit time. The reorientation time and activation energy for the spinning about the two-fold axis are consistent with those of c6,. The reorientation of the molecular i axis is much slower and the activation energy is estimated to be ca. 1000 K. Below the transition temperature the lowest-energy barrier for several paths examined in this study is ca.3000 K. The densities of states for the librations and centre-of-mass vibrations (phonons) have been calculated and our results remain to be tested by experiment. The simulation results disagree with the X-ray data16 con- cerning the octahedral and tetrahedral site occupancies at low temperature. The calculated values of ca. 80% and ca. 20%, respectively, are different from the experimental findings of 36% and 64%. This may be due to an unrealistic choice of the oxygen charge and size, and the C-C and C-0 bond lengths, since the site occupancy of the smaller tetrahedral void is likely to be very sensitive to modest variation of these parameters. The estimated transition temperature (210 K) is lower than the experimental value of 278 K.16 This most likely arises because the parameters for the c60 cage were chosen to be the same as in the pure c60.2' The transition temperature and the interstitial-site occupancies appear to provide a stringent test of the potential parameters.Although additional work is needed to refine the intermolecular poten- tial, the present results should provide a useful starting point for this endeavour. We would like to thank Don Cox, Paul Heiney, Amos Smith and Gavin Vaughan for stimulating discussions and sharing their experimental results prior to publication. We are grate- ful to Steve Erwin and Kari Laasonen for supplying details of their numerical work. This research was supported by NSF under grants DMR 91-20668 and CHE 92-23546 and bene- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 26 1 fitted from facilities provided by the Laboratory for Research on the Structure of Matter. References 1 2 3 4 J. M. Hawkins, A. Meyer, T. A. Lewis, S. D. Loren and F. J. Hollander, Science, 1991, 252, 312; J. M. Hawkins, S. D. Loren, A. Meyer and R. Nunlist, J. Am. Chem. SOC.,1991,113,7770. P. J. Fagan, J. C. Calabrese and B. Malone, Science, 1991, 252, 1160. P-M. Allemand, K. C. Khemani, A. Koch, F. Wudl, K. Holczer, S. Dovovan, G. Griiner and J. Thompson, Science, 1991, 253, 301. A. L. Balch, V. J. Catalano, J. W. Lee, M. M. Olmstead and S. R. 17 18 19 20 21 P. A. Heiney, J. E. Fischer, A. R. McGhie, W. J. Romanow, A. M. Denenstein, J.P. McCauley Jr., A. B. Smith 111 and D. E. Cox, Phys. Rev. Lett., 1991, 66, 291 1 ; R. Sachidanandum and A. B. Harris, Phys. Rev. Lett. Comm., 1991,67, 1467. P. A. Heiney, G. B. M. Vaughan, J. E. Fischer, N. Coustel, D. E. Cox, J. R. D. Copley, D. A. Neumann, W. A. Kamitakahara, K. M. Creegan, D. M. Cox, J. P. McCauley Jr. and A. B. Smith 111, Phys. Rev. B, 1992,45,4544. W. I. F. David, R. M. Ibberson J. C. Mathewman, K. Prassides, T. J. S. Dennis, J. P. Hare, H. W. Kroto, R. Taylor and D. R. M. Walton, Nature (London), 1991,353, 147. W. I. F. David, R. M. Ibberson, T. J. S. Dennis, J. P. Hare and K. Prassides, Europhys. Lett., 1992, 18, 219. M. Sprik, A. Cheng and M. L. Klein, J. Phys. Chew., 1992, 96, 2027. 5 6 7 8 9 10 11 12 13 14 15 Parkin, J. Am. Chem.SOC.,1991,113,8953. K. M. Creegan, J. L. Robins, W. K. Robins, J. M. Millar, R. D. Sherwood, P. J. Tindall, D. M. Cox, A. B. Smith, 111, J. P. McCauley Jr., D. R. Jones and R. T. Gallagher, J. Am. Chem. SOC., 1992, 114, 1103. Y. Elmes, S. K. Silverman, C. Sheu, M. Hao, C. S. Foote, M. M. Alvarez and R. L. Whetten, Angew. Chem., Int. Ed. Engl., 1992, 31, 351. A. Penicaud, J. Hsu, C. A. Reed, A. Koch, K. C. Khemani, P-M. Allemand and F. Wudl, J.Am. Chem. SOC., 1991,113,698. N. M. Dimitrijevik, P. V. Kamat and R. W. Fessenden, J. Phys. Chem., 1993,97,615. K. Chu and D. Mauzerall, Nature (London), 1993,361, 138. P. J. Krusic, E. Wasserman, P. N. Keizer, J. R. Morton and K. F. Preston, Science, 1991, 254, 1183; P. J. Krusic, D. C. Roe, E. Johnston, J. R. Morton and K.F. Preston, J. Phys. Chem., 1983, 97, 1736. K. Raghavachari, Chem. Phys. Lett., 1992, 195,221. M. Menon and K. R. Subbaswamy, Chew. Phys. Lett., 1993,201, 321. W. Andreoni and K. Laasonen, 1993, Personal communication. M. R. Pederson and S. C. Erwin, 1993, Personal communication. A. Cheng and M. L. Klein, J. Chem. SOC.,Faraday Trans., 1992, 88, 1949. 22 23 24 25 26 27 28 29 30 31 32 D. N. Neumann, J. R. D. Copley, R. L. Cappelletti, W.A. Kami- takahara, R. M. Lindstrom, K. M. Creegan, D. M. Cox, W. J. Romanow, N. Coustel, J. P.McCauley Jr., N. C. Maliszewsky, J. E. Fischer and A. B. Smith 111, Phys. Rev. Lett., 1991,67,3808. C. S. Yannoni, R. D. Johnson, G. Meijer, D. S. Bethune and J. R. Salem, J. Phys. Chew., 1991,95,9. R. Tycko, G. Dabbagh, R. M. Fleming, R. C. Haddon, A. V. Makhija and S. M. Zahurak, Phys. Rev. Lett., 1991,67, 1886. R. D. Johnson, C. S. Yannoni, H. C. Dorn, J. R. Salem and D. S. Bethune, Science, 1992,255, 1235. J. E. Fischer, P. A. Heiney, A. R. McGhie, W. J. Romanow, A. M. Denenstein, J. P. McCauley Jr., A. B. Smith 111 and D. E. Cox, Science, 1991, 252, 1288. W. L. Jorgenson, J. Phys. Chem., 1986,90, 1276. A. Cheng and M. L. Klein, J. Phys. Chem., 1991,95, 6750; Phys. Rev. B, 1992,45, 1889. H. Goldstein, Classsical Mechanics, Addison-Wesley, New York, 1980. W. B. Moniz, W. A. Steele and J. A. Dixon, J. Chew. Phys., 1963, 38,2418; W. A. Steele, personal communication. J. Millar, D. M. Cox, A. B. Smith 111, P. A. Heiney and G. Vaughan, personal communication. A. Cheng, M. L. Klein and L. J. Lewis, Phys. Rev. Lett., 1991,66, 624. 16 G. B. M. Vaughan, P. A. Heiney, D. E. Cox, A. R. MaGhie, D. R. Jones, R. M. Strongin, M. A. Cichy and A. B. Smith 111, Chew. Phys., 1992,168, 185. Paper 3f04968J; Received 16th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000253

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 263-270 New Families of Triply Periodic Minimal Surfaces Andrew Fogden* and Markus Haeberleint Inorganic Chemistry 2,Chemical Center, P.O. Box 124,S-22100 Lund, Sweden The established results regarding balanced minimal surfaces of orthorhombic symmetry are analysed and sup- plemented here. We commence by reviewing the simplest examples, possessing a genus value (per unit cell) of 3. From this foundation we construct a generalised approach, and apply it to the next simplest cases. In particu- lar, we derive two related families of surfaces possessing genera of 5. These two families contain, in total, three previously unrealised examples. For all 11 surfaces considered the exact mathematical equations specifying them are given, along with their space group-subgroup specifications, and illustrations of their elements are provided.Increasing awareness of the geometrical intricacy of atomic or molecular assemblies accessible to nature has elevated the importance of surfaces as a means for describing structures. The description provides an understanding of the way in which the unification of local affinities (on the surface) builds a scheme of global discrimination (either side of the surface). In recent years these developments have been most strikingly exemplified by the observation of bicontinuous phases in a variety of systems, and the modelling of their smooth par- titioning using minimal (zero mean curvature) surfaces. 1-3 For ordered phases the observed space group is often consis- tent with that of known infinite periodic minimal surfaces (IPMS).The interpenetrating pair of tunnel systems defined by such a surface then suggests a possible picture of the topo- logical manner of bicontinuity. Within this known set of IPMS topological options common to the space group, we choose that particular surface which most closely matches any additional experimental information. Typically the infor- mation can distinguish at most the gross topological aspect, namely the value of the genus (per unit cell).4 In the absence of a solid basis for distinction it is usually assumed that the lowest genus is favoured. The restriction to a specific geometrical condition (zero mean curvature) may appear unnecessary since a continuum of bicontinuous partitions fits the same space group. However, the limitation is beneficial for working purposes by the very fact that its admission of only a strictly countable set of pos- sibilities is compatible with the order of typical experimental resolution.Further, the continuum naturally falls into classes sharing the same topology as a member of the IPMS set, rendering this special case an adequate representative of its class and the obvious departure point for perturbation once more information comes to hand. This matching procedure implicitly relies upon a know-ledge of all IPMS, or more specifically, all those without self- intersections. An exhaustive listing is clearly unattainable since there exists a (countable) infinity of examples; instead one should aim to establish a catalogue containing, within all viable space groups (at least of the higher symmetry types), all IPMS possessing relatively simple topologies.To bias the task still further towards the weight of experimental evidence, most attention should be directed at the ‘balanced’ IPMS, for which the two, interpenetrating tunnel systems are sym- metrically interchangeable (by two-fold rotation axes and/or roto-inversion centres lying on the surface). Using real-space methods, based on construction of surfaces spanning crystal- lographic nets, Fischer and Koch have collated a list totalling 43 balanced IPMS of symmetry no lower than orthorhombic 7 Permanent address; Department of Inorganic Chemistry, The Royal Institute of Technology, S-100 44 Stockholm, Sweden. Foror rh~mbohedral.~~~each case the particular space group is provided, together with its (black-white) subgroup derived by abolishing the balancing invariances.Similarly, the genera are identified, displaying a variation through the cases ranging from 3 (the minimum conceivable value) up to 37. Significantly, their list includes three instances of sub- sets of surfaces (R3, MC2, MC3, MC4 and R2, MC6 and HS3, ST1) which possess not only common space groupsub- groups [P6/rncc--Pci/m and 14/mcrn--P4/rnbrn and P6,22-P6,22(2c), respectively] but also identical genera (13 and 9 and 7, respectively). Such coincidences illustrate a general shortcoming of the matching procedure, since the members of the subsets possess different topologies but cannot be differentiated from the gross standpoint of genus.However, in these three instances the genera are sufficiently high to exclude their manifestation in the simpler physical systems. Any such surface derivation relying upon real models in three-dimensional space carries with it a drawback. Although an analysis of space-group scenarios compatible with bal- anced IPMS establishes a rigorous foundation, the options for spanning a given framework of symmetry operations is limited by human imagination, which tends to underestimate the full topological versatility of minimal surfaces. Conse- quently, a number of options may be realised while others closely related to (but topologically distinct from) them may be missed.One of the present authors has recently employed an alter- native approach*-’ ’ based upon the general Weierstrass rep- resentation of a minimal surface in terms of its two-parameter space on the unit sphere, or equivalently, the complex plane. By transfer to its natural mathematical domain the essence of the IPMS problem is reduced to a sequence of necessary conditions, in which space groups are circumvented by spherical groups and topologies by counting schemes imposed on them. This clarified approach facilitates derivation of the Weierstrass function yielding the parametri- sation of the IPMS, and simultaneously embraces all related surface options.In a previous study” this algorithm was used to generate a family of balanced IPMS of trigonal symmetry and genus 7 which includes, in addition to the C(H) and MC1 surfaces listed by Fischer and K~ch,~.~ three previously unidentified relatives termed the MC(H), H2 and H3 surfaces. Here we apply it in a similar manner to the derivation of two such families possessing orthorhombic symmetry and genus 5. It is found that two members of one of these families possess iden- tical space groupsubgroup specifications, representing another source of ambiguity in the IPMS matching pro- cedure. However, compared with the instances mentioned above, the reduction to orthorhombic symmetry admits ambiguities at this significantly lower genus.Accordingly the situation now borders on relevance to bicontinuous phases in nature, since these two families represent the next-to-simplest IPMS of orthorhombic symmetry. To illustrate the approach we commence with a brief review of the simplest such sur- faces, those of genus 3. Orthorhombic IPMS of Genus 3 Any discussion of IPMS benefits from a familiarity with the situation for genus 3 which, while encapsulating all essential properties of IPMS in general, possesses certain simplifying features. Most importantly, each of the degenerate positions, termed ‘flat’ points, on an IPMS of genus 3 is of first order only, and moreover, necessarily acts as an inversion centre. Consequently, the surfaces are balanced. Further, the collec- tion of surfaces with symmetry no lower than orthorhombic or rhombohedral is free from ambiguities, as each is specified by a distinct space groupsubgroup.This collection is based on five examples: the cubic D, P and G surface^'^,'^ and the tetragonal CLP and hexagonal H surface^.'^-'^ All other members may be obtained by symmetry degradations of these. For the purposes of this study we limit attention to the subtype of these degradations with orthorhombic symmetry containing two-fold axes on the surface in the x, y and z axis directions and/or mirror planes slicing the surface parallel to the yz, zx and xy planes, respectively [i.e. for each of the three coordinate directions either a two-fold axis or a mirror plane (or both) must exist].The members of this subtype are derived from the D, P and CLP surfaces. (The other two examples, H and G, also give rise to degradations in this col- lection, as discovered by recent applications of the Weierstrass parametrisation algorithm. In particular, the H surface was found to admit orthorhombic dist~rtion,~ and distortion of the G surface was exhibited by construction of its rhombohedral variants.’ ’However, no such degradations meet these restrictions on the specific orthorhombic subtype considered here.) The tetragonal versions of the D, P and CLP surfaces give rise to pairs of orthorhombic distortions of this type, denoted oDa, oPa; oDb, oPb and oCLP, oCLP’, re~pectively.~ In Fig. l(a)-(f) we display representatives of the respective surface elements for these six cases, i.e.the smallest subunits bounded entirely by the two-fold axis lines and/or mirror plane curves sketched there. The surface elements in each of Fig. l(c)-(f) possess (first-order) flat points at their_ 2/m vertices, while the elements in Fig. l(a), (b)both exhibit 1 symmetry about their internal flat point. Note that, as opposed to the situation at the mm2 or 222 vertices, the normal-vector directions at each 2/m and i position have one and two degrees of freedom, respectively. Thus all six surface elements carry, in total, two degrees of freedom, naturally giving rise to their two-variable scope for orthorhombic distortion (ignoring the uniform dila- tation factor). The translational units (of genus 3), obtained by repeated two-fold rotation and/or mirror reflection, com- prise 16 such elements for Fig.l(c)-(f) and eight for Fig. l(a), (b).In this way the units for all six cases contain eight flat points. With this information the parametrisations of the six cases may be readily derived uia the abovementioned algorithm.” The method is outlined here in a sufficiently broad context to motivate the generalisations in the subsequent sections. To a minimal surface we apply the stereographically projected Gauss map, taking a point (x, y, z) on it to be the complex number o = u+iu which is the projection into the (complex) plane of the point on the unit sphere marking the direction of the normal vector to the surface there.The properties of this J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 1 1 Fig. 1 Sketches of the bounding circuits, comprising straight edges along two-fold axes and/or curved edges in mirror planes, of the surface elements for the orthorhombic IPMS, (a) oDa, (b) oPa, (c) oDb, (6)oPb, (e)oCLP, (f)oCLP’. The numerals 1 mark the sites of first-order flat points. The coordinate axes x, y and z are oriented to the right, into the page and vertically upwards, respectively. (These two conventions will be maintained throughout all subsequent surface-element depictions.) map dictate that its inverse, parametrising the surface, may be expressed in the form’* (x, y, z) = Re [1 -d2,i( 1 + o”),2w’]R(co’)do’ (1)r for some complex-analytic function, R(o).The algorithm thus relies upon interpretation of properties of the IPMS through this Weierstrass representation as necessary conditions on its particular R(w), the so-called Weierstrass function that we seek.In the six cases the Gauss maps of their generic surface elements are pairwise identical. The stereographic projections of the maps of Fig. l(a), (b)and (c), (d)and (e),(f)are imaged in Fig. 2(a),(b)and (c), respectively. The bounding segments of the surface elements, Gauss-mapped to arcs of three mutually perpendicular great circles, appear in the projection as follows. A two-fold line in the y axis direction and/or a mirror curve parallel to the zx plane is imaged along the real axis; similarly, those related to the x axis md/or yz plane and to the z axis and/or xy plane lie along tb imaginary axis and along the unit circle, respectively.The boundary image thus defines a single circuit delimiting one octant in Fig. 2(b)and (c) and a (continuous) double circuit delimiting two octants in Fig. 2(a). Note that the dihedral angles at the (first-order) flat points are increased by a factor of two at their image sites, i.e. Fig. 2 Projected Gauss-map image of the surface elements, (a) oDa and oPa, (b) oDb and oPb, (c) oCLP and oCLP’, in the complex o-plane (Re and Im denote the real and imaginary axes). The double- hatching in (a), and the single-hatching in (b) and (c), represent double and single coverings of the octant, respectively. The numerals 1 indicate the image sites of the first-order flat points, i.e.sites of first-order branch points of the corresponding Weierstrass function (again a convention maintained throughout all subsequent image depictions). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 from n/2 to 7t in Fig. 2(b),(c) and from 2n to 4n in Fig. 2(a)(in which the site pins the two distinct sheets). Extension of the surface element over a bounding segment via its two-fold rotation or mirror-reflection operation induces a reflection of its Gauss map (on the unit sphere) across this boundary arc. This corresponds, in projection, to application of the operations w -+ 0,co -+ -6 and w -P l/G for the three types of boundary images listed above. Contin- uing this process, the attainment of the translational unit amounts to a uniform covering of two copies of the unit sphere, or complex plane.In this way the sites of the eight flat-point images are symmetric with respect to all reflections in the octant tiling, i.e. the two ‘edge’ sites in Fig 2(b),(c) both possess four equivalents and the ‘face’ site in Fig. 2(a) possesses eight equivalents. Further, each of these sites now pins the two sheets (so the angle of 4n is fulfilled there). These double-sheeted coverings thus define the complete domains of the Weierstrass functions in eqn. (1) The fact that the translational units of genus g = 3 correspond to domains comprising two sheets (s = 2) is consistent with the general IPMS relation’ g=s+l (2) Each of the sites mentioned above are first-order branch points of their Weierstrass function.The existence of eight such sites in each case, so their total branch-point order W = 8, is again consistent with the general IPMS rule’ w= 4s (3) Establishment of the Weierstrass functional forms match- ing the domain structures is a straightforward matter. These forms are composed from polynomials (in w), the root sets of which must all be symmetric with respect to the reflection scheme of the octant tiling underlying Fig. 2. Consider the single octant in Fig. 3. Any polynomial with simple roots sta- tioned at the four reflected equivalents of a single edge site on this octant may be expressed as p,(o) = (aZ+ 1)2 -yew2 (4) for some real ye value.The three kinds of edge are distin- guished by the subdivided ye ranges borne by them in Fig. 3. Similarly, any polynomial with simple roots at the eight I 0 -co >Re w Fig. 3 The labelling of the three kinds of octant edges indicates the (monotonic) ranges of the parameter ye corresponding to them in eqn. (4) equivalents of a single face site on the octant may be written as Pf(4 = C(W2 + -yfo2][(oZ + -YfW2] (5) for some complex number yf in (say) the upper half-plane. The functional forms, thus comprised, pertaining to the cases here are then fixed by the requirement that the Weierstrass function be non-zero throughout (since the Gaussian curvature is finite everywhere on the surface), and that its pair of values (on the two sheets) for each w be equal and opposite (since the corresponding pair of surface points are related by inversion).In particular, the form relevant to Fig. 2(b) and (c)is then ‘(a) = +exp(i8)Cpe,(w)pe,(w)I-lj2 (6) where pe,(w)and pe,(o)are given by eqn. (4) with ye = ye, and ye = ye,, respectively, and that for Fig. 2(a)is ‘(a)= +exp(i8)pf(w)-1/2 (7) The zeros of the eighth-degree polynomials in these forms are the eight branch points. The situation in Fig. 2(b) and (c), differing in the kinds of edges bearing the two branch points, are thus distinguished in eqn. (6) by the ranges of their real parameters ye, and ye, as per Fig. 3. Hence we take ye, > 4 and 4 > ye, > 0 for Fig. 2(b)and 4 > ye,, ye, > 0 for Fig. 2(c).In eqn. (7) the parameter yf, specifying the site of the face branch point in Fig. 2(a),is complex as mentioned above. The Weierstrass functional forms in eqn. (6)and (7) contain three degrees of freedom [ignoring a constant, real factor which merely effects uniform dilatation of the surfaces uia eqn. (l)]. In the context of the surface cases considered here, the real variable B(mod n) is restricted to particular values (particular members of the Bonnet associates it defines), thus reducing the forms to two-variable freedom, as required. Spe- cifically, surfaces containing two-fold axes and/or mirror planes correspond to the value 8 = 0 or 8 = 42. The two choices, termed the surface-adjoint surface pair, are related by interchanging the identification of two-fold axes and mirror planes on the same Gauss map.It is precisely these two choices which restore the six surface cases in Fig. 1 from their pairwise identical images in Fig. 2. The pair oDb, oPb and the pair oCLP, oCLP’ correspond to eqn. (6) (for the abovementioned variable ranges pertinent to Fig. 2(b)and (c), respectively) with 8 = n/2, 8 = 0 for both. Similarly, the pair oDa, oPa [imaged to Fig. 2(a)] is in turn given by eqn. (7) with f3 = 42, 8 = 0. The totality of surface information regarding these six cases is summarised, for reference, in the first six rows of Table 1. Generalisationto a Family of Genus 5 The basis for all IPMS of genus 3, namely their exhibition of flat points of lowest (i.e. first) order, also suffices for ortho- rhornbic IPMS of any genus.By this we mean that non-degenerate or simple degenerate positions (flat points of order 0 or 1, respectively) on a minimal surface can exhibit all point-group symmetries admissible to an orthorhombic space group. In particular, non-degenerate positions can access the symmetries mm2, 222, m, 2 (either lying on the surface or per- pendicular to it) and 1, while simple degeneracits can display symmetries 2/m, m, 2 (necessarily on surface), 1 and 1. Flat points of higher orders may possibly occur as a coalescence of simple degeneracies, for example of second order at mm2 or 222, and of third order at 2/m, positions due to merging of a pair of first-order positions on an m or 2. However, these are merely special occurrences; any analysis of orthorhombic IPMS must originate from the assumption that flat points of, at most, first order are involved.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Summary of all information relating to the 11 orthorhombic, balanced IPMS studied here (listed in column 1); the information relating to their real-space manifestation is listed in columns 2-4, while that specifying their parametrisation is listed in columns 5-8 minimal space group surface subgroup genus oPa Immm-Pmmm 3 oDa Pnnn-Fddd 3 oPb Fmmm-Cmmm 3 oDb Cmma-Imma 3 oCLP‘ Cmmm-Pmmm 3 OCL P Pccm-Cccm 3 PT Fmmm-Cmmm 5 lll Fmmm-Cmmm 5 Cmmm-Pmmm 5 VAL Cmma-Cmmaa 5 PV Pccm-Pccmb 5 ~~ a (c’ = 2c). (a’ = 2aj or (b‘ = 2b). surface element variable ranges element image Weierstrass function (Fig.) (Fig.) (eqn.1 Bonnet angle, 0 ye, Ye2 l(b) l(4l(d) 1(c)Uf1 l(4 6(4 2(4 2(42(b) 2(b) 2(c)2(c) 5(4 (7) (7)(6) (6) (6)(6) (9) subject to (10) (minus sign) and (1 1) 0 4 0 42 0 XI2 0 if Yel + ye2 -~ee> 0 XI’ if Ye, + Ye2 -Yee 0 (4, a) (4, a) (0, 4) (0, 4) (-a,Oj (094) (0, 4) (0,4) (0, 4) (4, a) 6(c) 5(c) (9) subject to (10) (plus or minus sign) and (12) 42 (4, a) (YeeT 4) 6(b) 5(b) (9) subject to (10) (minus sign) and (12) 42 (4, a) (4, Yet) 8(b) 7(b) (1 3) subject to (14) (plus or minus sign) and (15) 0 (4, a) (Ye,, 4) 8(4 7(4 (minus sign) and (15) (13) subject to (14) 0 (4, a) (4, Yel) In this section we again address only that subtype of orthorhombic IPMS possessing surface elements bounded by (on-surface) two-fold axes in the x, y and z axis directions and/or mirror planes parallel to the yz, zx and xy planes, respectively.To generalise the results of the preceding section we now reverse the sequence traced there, establishing our primary considerations within the parametric space on the unit sphere, or complex plane. The projected image of any such surface element must define some region over the complex plane bounded by segments lying along the real axis, the imaginary axis and the unit circle (all three kinds of edge of the underlying octant tiling). The image of a (first-order) flat point at a 2/m vertex lies on an edge of the region, and hence an edge site ‘e’ on an octant.The image of a flat point on an rn or 2 boundary again resides on an octant-edge site, which we now denote ‘ee’ since it is a double-edge site of the region (i.e. the boundary jmage wraps one full turn around it). For a flat point at a 1 or 1 position in the interior, the region winds twice around the image, which lies on an octant-face site ‘f’. By (projected) reflection over its boundary segments, the region must cover some number, s, of copies of the plane such that the superposed, complete distribution of flat-point image sites (totalling W)is symmetric with respect to the reflection scheme of the octant tiling, and the full angle of 4n is subtended at each of the flat-point images. Whatever the values of W and s may be, we require that their ratio matches eqn.(3). Moreover, the accumulated order of the flat points residing on a single element must amount to four times the area fraction of its image region, i.e. one-half of the number, N, of octants that it occupies. Distinguishing the flat-point sites according to the three varieties defined above, and denoting the numbers of each on the image region by n,, neeand n,, its accumulated order must then obey the relation an, + $nee + n, = 3N The first such situation is then a single-octant region, N = 1. In this scenario, image sites of the last two types, ee and f, are inadmissible, so nee = n, = 0 and the only solution forthcoming from eqn. (8) is n, = 2. Since all three octant edges are originally equivalent, there are only two distinct possibilities for placement of the two edge sites, represented by the images displayed above in Fig.2(b)and (c). Extending to the situation of a double-octant region, N = 2, one solution of eqn. (8) is then n, = nee = 0, n, = 1. The solution pertains to a scenario of two overlain octants on distinct sheets pinned at this single face site, represented by the image given above in Fig. 2(a). This scenario is more naturally grouped with the first situation since the site dis- tribution is identical on the two octants (although it cannot be symmetrically subdivided by octant segments). Hence these simplest scenarios correspond to precisely those three images in Fig. 2. The subsequent derivation of the Weierstrass functional forms replicates that in the preceding section, and the consequent reconstruction of the surface- adjoint surface pairs defined by them returns us to the orig- inal point of departure, the six cases of genus 3 in Fig.1. The situation N = 2 also admits a second scenario, that of two adjoining octants comprising, say, the first quadrant of the complex plane. Face sites are thus rendered inadmissible, and the solutions of eqn. (8) (with n, = 0) are then n, = 4 or n, = 2, nee= 1 or nee = 2. We must ignore solutions pos- sessing no bounding segment(s) along the unit circle (since they relate to IPMS of symmetry other than the orthorhom- bic subtype considered here) and those that do, but are inter- nally reflection-related across it (since they reduce to the N = 1 situation).By these criteria the solutions n, = 4 and nee = 2 are both discarded, leaving n, = 2, nee = 1 as the only viable candidate. The basic image region associated with this solution is rep- resented in Fig. 4.The boundary arc following the unit circle wraps around the site ee. Internal symmetry across this arc is broken by the distribution of the two edge sites designated el and e,, which may each be located on any of the six seg- ments a, b, c, a‘, b’, c’. Irrespective of the locations of el and e, ,exhaustion of all boundary reflections generates a set of 16 regions covering four sheets. In this way the four sheets are pinned pairwise by the two flat-point images stationed at each of the four reflec- tion equivalents of the site ee in the octant tiling.Further, two of the four sheets are pinned by the single flat-point image residing at the four reflection equivalents of the sites e, and e2, while the other two sheets are unpinned there by virtue of the absence of internal symmetry (i.e. a, b, c are distinct from a’, b’, c’ in the region). The determination of the Weierstrass functional form matching this four sheeted domain structure is a natural gen- eralisation of the derivation in the preceding section. Again the form is composed from polynomials, the roots of each necessarily displaying reflection equivalence with respect to this same, underlying octant tiling. In particular, the poly- nomials possessing simple zeros at the four equivalents of the J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Im o II Fig. 4 The image template associated with the solution n, = 2, nee = 1, occupying (a single copy of) one quadrant. Its boundary comprises six segments, a and a’ along the real axis, joined by the unit-circle ‘tongue’ formed by b and b’ wrapping around the double- edge site ee, together with c and c‘ joined continuously along the imaginary axis. The two additional edge sites e, and e2 are not indi- cated since their locations are not yet fixed, we may assign them to any of these six segments. edge sites ee, el and e, ,denoted pee,pel and p,, , are given by eqn. (4) with ye = ye,, ye = ye, and ye = ye,, respectively. Recalling Fig.3, since ee lies on the unit circle, we require 4 > ye, > 0, while the distinction of six segments a, b, c, a’, b‘, c’ on which el and e, may reside reduces to the choice of three ranges for ye, and ye,, since the segments a, a’ both correspond to ye > 4, the segments b, b’ both to 4 > ye > ye,, and c, c’ to 0 > ye. The Weierstrass functional form, with its four branches pinned at first-order branch points as described above, and meeting all other requirements, is then given by R(o)= el fexp(i0) Here pe, is an extra polynomial, similarly given by eqn. (4), with ye = ye,. This variable ye, is, in turn, expressed in terms of the three branch point variables ye,, ye, ,ye, by Note that the right-hand side is real, as required, since the product ?,(ye -ye,) is non-negative for all three ranges of ye cited above.Further, it has two alternative values [the fsigns in eqn. (10) are independent of those two defining the four branches of R(o)in eqn. (9)]. These two alternatives resurrect the distinction between the six segments from the three ranges of the edge variable. A choice of values of ye, and ye, (at fixed ye,) each admits two locations, say el on segments q1 or q; and e, on segments q, or q;, and when taken simul- taneously, presents the two possible combinations (el, e,) on (ql, q2) or (ql, 9;) [the other two combinations (qi, 9;) or (qi, q,) are symmetry-related to these by unit-circle reflection]. These two possibilities are precisely those given by the fsigns in eqn.(10). In total, the three ranges for ye, and ye, together yield six choices (since the designations e, and e, are arbitrary): ye, > 267 Ye, > 4; Ye, > 4 and 4 > ye, > Yee; 0 > ye, and ye2 > 4; 4 > Ye, > Ye, > Yee; 0 > Ye, and 4 > Ye2 > Ye,; 0 > ye, > ye,. This then doubles into the 12 possibilities for (el, e,) on: (a, a), (a, a’); (a, b), (a, b’); (c, a’), (c, a); (b, b), (b, b’); (c, b’), (c, b); (c, c), (c, c’); respectively. In each of these six pairs the first men- tioned possibility is given by the minus sign in eqn. (lo), and the second by the plus sign. The imposition of all necessary conditions pertaining to this scenario in image space thus results in the Weierstrass functional form of eqn.(9) [in conjunction with eqn. (lo)], which embraces this list of 12 possibilities, each having four degrees of freedom 6, ye,, yel, ye,. We must now return to real space via eqn. (1) to assess the materialisation of these possibilities. As stated in the analysis for genus 3, the requirement that the image boundary is manifested as a surface-element boundary comprising two-fold axes and/or mirror planes dic- tates that the variable 6(mod IL)takes either the value 6 = 0 or 0 = 42. So each image possibility gives rise to two such cases, a surface-adjoint surface pair, further doubling the possibilities to a family of 24 surfaces. However, as opposed to the genus 3 cases, each of the present surfaces now con- tains three variables y,,, y,,, ye,, corresponding to the edge sites borne by the image region. These singly variable normal vectors of the flat points on the element boundary provide the surface with three degrees of freedom (in addition to uniform dilatation), i.e.one degree in excess of the scope for orthorhombic distortion. This excess arises naturally in each of the 24 surface ele- ments, as is made apparent by a ‘qualitative’ reconstruction of the cases. For the 12 possibilities listed above, we sketch the boundary circuit of the generic surface element possessing this image circuit, in the two cases related by interchanging the identification of two-fold axes and mirror planes on it (in the same way that Fig. 1 may be reconstructed from Fig.2). Each of these cases is found to require one supplementary (real) constraint forcing the relative arrangement of these point-group symmetries to be commensurate in the y axis direction. Although a (countable) infinity of ratios are acces- sible, we will always address the simplest representative, since all others are guaranteed to generate self-intersections of the surface. Surface elements given by alternatives (ql, b) and (ql, b’) are continuously related (by merely pushing a flat point through a 2/m vertex) thus it serves to unify the pairs of pos- sibilities (a, b), (a, b’); (b, b), (b, b’); (c, b), (c, b’) as (a, bb’); (b, bb’); (c, bb’), respectively, thus spanning both sign alterna- tives in eqn. (10). Further, the minimal surfaces must adopt the highest symmetry compatible with the fixed boundary conditions.Accordingly, for the (ql, 9;) possibilities (a, a’), (b, bb’), (c, c’) the supplementary constraint will lead to halving of the surface elements, via two-fold symmetry in the z-axis direction, or mirror symmetry parallel to the xy plane in the adjoints. So these cases will reduce to the genus 3 surfaces oDb, oCLP, oPb, respectively, and their adjoints to oPb, oCLP’, oDb, respectively. [Equivalently, the image region becomes unit-circle symmetric, thus with ye, = ye,, the plus sign in eqn. (10) yields ye, = ye, = ye,, collapsing the Weierstrass function in eqn. (9) to the genus 3 form in eqn. (6).1This reduction leaves only 12 separate, non-trivial cases, namely the surface-adjoint surface for the six possibilities (a, a), (a, bb’), (c, a), (c, a’), (c, bb’), (c, c).Of these, it is found that nine cases still give rise to self-intersections on rotation and/or reflection of their surface element ;the three remaining cases are, however, true (i.e.self-intersection-free) IPMS. Spe- cifically, these three cases are the surface for (c, a’) and the adjoint surfaces for (a, a) and (a, bb’). The three image regions J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 are represented in Fig. 5(a),(b),(c), respectively, with the per- tinent surface case discerned from its adjoint surface in each by requirement of the affixed labelling scheme p and 1 (indicating boundary sections corresponding to mirror planes and two-fold lines, respectively) rather than its opposite, 1 and p.[In the last case the possibility (a, b) has been dis- played, that for (a, b‘) is obtained effectively by sliding ee down the unit circle to below e,, carrying the labelling along with it]. The boundary circuits delimiting surface elements reconstructed from Fig. 5(a), (b),(c)are sketched in Fig. qa), (b), (c), respectively. [The possibility (a, b‘) is similarly Imo Re o \ I / Im o \ I 1 el 1 el Fig. 6 Sketches of the bounding circuits delimiting the surface ele- ments for the IPMS, (a) PT, (b) ll2, (c) lll, obtained as the simplest commensurate reconstructions of the images in Fig. 5(4, (b), (c), respectively IlTi( 1 +o”)R(o’)do’ I= 1[:i( 1 +d2)R(o’)do’ (11) Fig.5 Particular possibilities of Fig. 4, given by (el, ez) on (a)(c, a’), (b) (a, a), (c) (a, bb’), The IPMS cases corresponding to these three possibilities (and conforming to the labelling scheme of p and I symbols explained in the text) are referred to as the (a) PT, (b) ll2, (c) Ill, surfaces. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (directed along the y-axis) of its central point. The (projected) image of any such element is bounded by segments lying along only the real axis and the unit circle, i.e. two of the three kinds of octant-tiling edge (over which it is continued uia the edge-reflection operations o -P 0 and o -,l/&, respectively). In addition, the image displays invariance under the internal operation w +P -l/w.Note that the quadrant scenario illustrated above in Fig. 4 gives rise to two families, since there are two means for con- tinuation over the imaginary axis segment (i.e. the third kind of octant-tiling edge). Assignment of the reflection operation o + -6 corresponds to the previous subtype, for which the family was derived in the preceding section. On the other hand, doubling of the region over this segment uia the oper- ation o + -l/wcorresponds to this new subtype, the family of which will be analysed here. Since it is so closely related to its predecessor, much of the analysis is common to both. To evoke the similarity we employ an identical notation. In par- ticular, we denote the double-edge site by ee and the extra two edge sites by el and e2.The location of these latter two edge sites is now restricted to the segments a, b, a’, b’, since the other two segments c, c’ in Fig. 4no longer act as reflec- tion edges. Again 16 regions (eight double regions) cover four copies of the complex plane such that the overall distribution, and occupancy number, of these sites is completely analogous to that of the previous case. The Weierstrass functional form associated with the domain structure is now given by R(w)= & exp(i8) in which As before, the variable ye, carries two independent alterna- tives, which are guaranteed to be real-valued since the quan- tity ye -ye, is non-negative for any edge site e on segments a, b (or a‘, b’). The distinction of the possibilities embraced by eqn.(13) and (14)is identical to that in the preceding section, save for this eradication of the third edge type. To repeat, the two ranges for ye, and ye, provide three simultaneous choices: Ye1 > Ye, > 4; Ye, > 4 and 4> ye2 > Yee; 4> Ye1 > Ye, > Yee, then developing pairwise to the six possibilities (a, a), (a, a’); (a, b), (a, b’); (b, b), (b, b’); the first (second) member of each pair is given by the minus (plus) sign alternative in eqn. (14). Inserting eqn. (13)into eqn. (l),the region boundary abb’a’ of reflection segments corresponds to a surface element bound- ary of two-fold axes and/or mirror planes only for the values 8 = 0 and 8 = n/2 [although the operation co -+ -l/w is manifested as a two-fold axis along the normal (y) direction for any 81.Hence again, each image-space possibility above doubles into two such real-space cases (its surface and adjoint surface) containing the three free variables yee, ye,, ye?, giving a total of 12 of these cases to consider. Qualitative recon-struction of these three-variable surface elements verifies that each is in need on one supplementary constraint to ensure commensurability, now in the z-direction. As previously, we apply the simplest constraint to each case, yielding its two- variable orthorhombic version which minimises any self- intersection. Merging of those possibilities which are continuously related in real space reduces the distinction to (a, a), (a, a’), (a, bb’), (b, bb’).Of these, (a, a’) and (b, bb’) are symmetrically subdivided under the action of their supplementary con-straints (specifically, the surfaces revert to oDb and oCLP and their adjoint surfaces to oPb and oCLP’). Only the sur- faces derived from the two possibilities (a, a) and (a, bb’) are true IPMS (their adjoint surfaces both generate self-intersections). Their corresponding regions are represented in Fig. 7(a) and (b),respectively, retaining the p and 1 labelling convention. In both cases we display the double region (with internal symmetry co c,-l/o)to provide a closed boundary of reflection segments. The boundary circuits of two-fold lines and mirror-plane curves delimiting the surface elements (possessing this internal, rotational invariance) are recon-structed, in their simplest commensurate state, in Fig.8(a) and (b),respectively. The first case constitutes a new IPMS, which we call the PV surface. As in the preceding section, we have displayed only the possibility (a, b) for the second case [its merging partner (a, b’) is readily obtained by sliding ee below e2 in Fig. 7(b),and hence pushing ee through e2 to form an inflection point on the top- and bottom-face curves in Fig. 8(b)].In the limit bridging these two subcases, the flat point ee, and its mirror equivalent, coincide at the flat point Im wIk \1u I Re o Fig. 7 Particular possibilities of the image template associated with the second subtype (in which the two quadrants are symmetry-related about o = i by composition of reflections in the imaginary axis and unit circle), given by (e,, ez) on (a) (a, a), (b) (a, bb’).The corresponding IPMS cases (again complying with the imposed p and 1 schemes) are referred to as the (a) PV, (b) VAL surfaces. fb 1 Fig. 8 Sketches of the bounding circuits of the surface elements for the IPMS, (a) PV, (b) VAL, reconstructed, in the simplest com- mensurate state, from their images in Fig. 7(a), (b), respectively. The dashed line (in the y direction) denotes the internal two-fold axis. e2 to form a third-order flat point there. The IPMS in this special limit was discovered previously and called the VAL surface.lo The surface was correctly parametrised there (although note that its image in displayed erroneously in Fig.5 of that study, the branch point at o = 1/A should instead reside at o = -l/A). However, it was not appreciated that the extra degree of freedom present in the generic case, which we will continue to refer to as the VAL surface here, would be required to fit a supplementary constraint. For both the PV and VAL surfaces this supplementary constraint demands that the plane containing the pair of two- fold lines (and the internal two-fold axis) in the y direction must be parallel to the xy plane. Hence, along the imaginary axis segment of the images we must impose that Re sd2o’R(o’)do’ = 0 (15) where the integration may be taken on any of the four branches of their Weierstrass functions.The abovementioned facts relating to the parametrisation of these two cases are listed in rows 10 and 11 of Table 1, together with the genus value [again 5 from eqn. (2)] and space groupsubgroup of their translational units. Conclusions In this study we have addressed the general algorithm for parametrisation of IPMS in the context of these two ortho- rhombic symmetry subtypes. Any such surface is categorised according to the number, N, of spherical octants occupied by the image of its basic element. This number is directly related to the Euler characteristic per surface element: xE = -N/4. The IPMS translational unit, comprising A4 such surface ele- ments, then possesses Euler characteristic xT = -MN/4 and hence genus g = 1 -xr/2 = 1 + MN/8.For the simplest situ- ations N = 1 and N = 2 we have exhaustively derived all possibilities, resulting in a total of 11 IPMS (all of which are balanced). The information pertaining to each is summarised in Table 1. The study may readily be extended to situations N > 2 of more complicated surface-element topology. In all of these J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 situations the product MN must be at least 32, so the genus can be no lower than 5. As an illustration, for N = 3 one solution of eqn. (8)is n, = 4, nee = 1, n, = 0. This solution, for which M = 16, thus corresponds to non-trivial families of balanced IPMS with genus 7. The families warrant further investigation, since no examples of such surfaces are presently known.Of all the previously established IPMS, there is only one additional example with orthorhombic symmetry and genus 5. This surface, which is likewise balanced and of the first symmetry subtype, was discovered by Koch and Fischers*6*20and named oMC5, since it is an orthorhombic distortion of their tetragonal MC5 surface, It derives from the situation N = 8, and specifically, the solution n, = 0, nee = 8, n, = 0 of eqn. (8), for which M = 4. In summary, the two families with genus 5 analysed here are clearly the simplest orthorhombic IPMS which cannot be obtained via crystallographic degradation of surfaces of higher symmetry (i.e.cubic, tetragonal or hexagonal). For this reason, we would anticipate the two families to represent plausible options for interfacial shapes in self-assembling bicontinuous phases.One of the authors (A.F.) was supported by the Swedish Natural Research Council (NFR). References 1 L. E. Scriven, Nature (London), 1976,263, 123. 2 E. L. Thomas, D. M. Anderson, C. S. Henkee and D. Hoffmann, Nature (London), 1988,334, 598. 3 J. Prost and F. Rondelez, Nature (London) (Suppl.) 1991,350, 11. 4 P. Barois, S. T. Hyde, B. W. Ninham and T. Dowling, Langmuir, 1990,6, 1 136. 5 W. Fischer and E. Koch, Acta Crystallogr., Sect. A, 1989,45, 726. 6 E. Koch and W. Fischer, Acta Crystallogr., Sect. A, 1990,46, 33. 7 E. Koch and W. Fischer, Acta Crystallogr., Sect. A, 1993,49, 209. 8 A. Fogden and S. T. Hyde, Acta Crystallogr., Sect. A, 1992, 48, 442. 9 A. Fogden and S. T. Hyde, Acta Crystallogr., Sect. A, 1992, 48, 575. 10 A. Fogden, Acta Crystallogr., Sect. A, 1993,49, in the press. 11 A. Fogden, J. Phys. I, France, 1992,2,233. 12 A. Fogden, Z. Kristallogr., in the press. 13 H. A. Schwarz, Gesammelte Mathematische Abhandlungen, Vol. I, Springer, Berlin, 1890. 14 A. H. Schoen, Infinite Periodic Minimal Surfaces without Self- intersections, NASA Techn. Rep. D-5541,1970. 15 S. Lidin and S. T. Hyde, J. Phys. France, 1987,48, 1585. 16 S. Lidin, J. Phys. France, 1988,49,421. 17 A. Fogden, M. Haeberlein, S. Lidin, J. Phys. I France 1993, in the press. 18 K. Weierstrass, Untersuchungen iiber die Flachen, deren mittlere Kriimmung iiberall gleich Null ist, Monatsber. d. Berliner Akad. 1866, p. 612. 19 W. Fischer and E. Koch, J. Phys. Fr. C7 Colloq., 1990,51, 131. 20 E. Koch and W. Fischer, Acta Crystallogr., Sect. A, 1989, 45, 169. Paper 3/03579D; Received 22nd June, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000263

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 271-278 27 1 Fluorescence Anisotropy Decays and Viscous Behaviour of 2-Methyltetrahydrofuran Brian Brocklehurst" and Ronald N. Young Chemistry Department, The University of Sheffield, Sheffield, UK S3 7HF The decay of the fluorescence anisotropy has been measured for solutions in 2-methyltetrahydrofuran (MTHF) between -50 and -150°C.Solutes include both neutral species (perylene, tetracene, carbazole) and ion pairs (of carbazolyl-, 1,&diphenylallyl-and its vinylogues). The results lie between values calculated for ellipsoids using the 'stick' and 'slip' boundary conditions. In some cases, notably perylene, two-exponential decay is predicted but not observed, suggesting that the solvent-solute interaction is comparable with solventsolvent forces, contrasting with the hydrogen-bonded solvents used by other workers.The data are used to discuss cis-trans photo-isomerisation of diphenylallyl ions and to estimate the viscosity of MTHF, for which measure- ments cover eleven orders of magnitude. The Williams-Landel-Ferry (WLF) equation is obeyed well over the whole range with To = 81 K. MTHF forms a glass on cooling. The glass transition tem- perature is near 90 K so that it can form a rigid clear glass at the temperature of liquid nitrogen; this has been widely used to trap reactive species, especially radical anions, for spectro- scopic studies.'S2 The normal boiling point of MTHF is 80°C: its large liquid range makes it very useful for a variety of kinetic studies in which reaction may be initiated in the liquid state3 or by irradiation of glassy solutions followed by ~arming.~Since reactions of radicals etc.are generally diffusion-controlled, knowledge of the viscosity is important ; measurements have been made at high ( -75-+25 OC)' and low temperatures (92-108 K),6 ranging over some eleven orders of magnitude. As in the case of other glass-formers, the activation energy for viscous flow is not constant but increases with falling tem- perature; however, the WLF equation, provides a good fit over a very wide range; the characteristic temperature, To, lies a few degrees below the glass tran- ~ition.~ We have studied the temperature dependence of the fluo- rescence of 1,3-diphenylallyl anions (DPA) and related species' in MTHF.The excited-state behaviour of these species closely resembles that of 1,2-diphenylethene (stilbene);' cis-trans isomerisation competes with fluores-cence from the first excited singlet. As in the case of stilbene, isomerisation of excited trans,trans-DPA involves crossing an intrinsic energy barrier (ca. 20 kJ mol-'), but there appears to be no barrier in the case of the cispans form, probably as a result of steric repulsion. The fluorescence of cispans-DPA has not been observed. Methyl substitution on the central carbon of the ally1 group stabilises the ground state of the cis,trans form: fluorescence is then observed but only at very low temperatures. As in the case of cis-stilbeneg and other sterically crowded species,''-' ' isomerisation appears to be dependent on the viscosity of the solvent rather than the tem- perature.The use of the macroscopic (shear) viscosity to describe microscopic processes is debatable.' 3-15 Following the example of others,I5 we have used the decay of fluorescence anisotropy16 to measure the rate of overall rotation of a number of species in MTHF for comparison with the rate of internal rotation of DPA etc. However, with suitable scaling it is possible to calculate the viscosity approximately and for our data to fill the gap between previous measurements. Species studied are shown in Fig. 1. Measurement of both internal and overall rotation in a single system, such as cis,trans-2-methylDPA, is not feasible because of the rapid decay of fluorescence in the region of interest.The behaviour of trans,trans-DPA is complicated at higher temperatures by photo-isomerisation and changes in ion-pairing. However, at temperatures below -70 "C where fluorescence anisotropy becomes measurable, only loose ion-pairs are present for lithium and sodium salts in MTHF and isomerisation is rela- tively slow. Results are presented for Na+ DPA- and for the lithium salts of 1,5-diphenylpentadienyl anions (DPPD) and 1,7-diphenylheptatrienylanions (DPHT). For comparison with the photo-labile anions we have studied fluorescence of the rigid anion, carbazolyl,' and three neutral species : carbazole, for comparison with car-bazolyl, and perylene and tetracene which have been the sub- jects of a number of previous mainly in alcohols.Experimental MTHF was stirred over a sodium-potassium alloy under high vacuum in the presence of a little benzophenone. The development of a permanent blue colour due to the forma- tion of the anions of benzophenone indicated the absence of water or other protogenic impurities. The mixture was allowed to stand for 24 h before MTHF was distilled from it to ensure that all the benzophenone had been reduced to the involatile ions. 1 2 4 5 Fig. 1 Solutes used: 1, Carbazole; 2, carbazolyl lithium; 3, perylene; 4, n = 1, 1,3-diphenylallyl anion (DPA), n = 2, 1,Sdiphenyl-pentadienyl anion (DPPD), n = 3, 1,7-diphenylheptatrienylanion (DPHT); 5, tetracene.NB arrows denote the orientation of the tran- sition axis. Carbazole, perylene and tetracene were used as received. Preparation of DPA and its vinylogues has been described previ~usly.~*’~An early study concluded that DPPD is prob- ably entirely in the all-trans conformation;24 recent work has confirmed this and shown that DPHT is also all-tr~ns.’~ Lithium carbazolyl was prepared indirectly since the reaction between lithium and carbazole in MTHF gives a product contaminated by several by-products. A solution of the lithium salt of the dimeric dianion of a-methylstyrene was titrated into a solution of carbazole under high vacuum until equivalence was reached. The approach of this condition was indicated by the decreasing speed with which the red colour of the styryl ion was discharged; the precise end point was indicated by the disappearance of the carbazole absorption band at 338 nm.X-Ray crystallography has shown that, in the solid state, lithium carbazolyl is complexed with two mol- ecules of THF and is dimeric.26 Several related species have similarly been shown to be dimerised and in some cases NMR spectorscopy in solution has also revealed dimer- i~ation.~’Preliminary studies in this laboratory have shown that the electronic absorption spectrum of lithium carbazolyl in MTHF is virtually independent of concentration in the range lop4 to 5 x lop6 mol dmp3, suggesting that aggre- gation probably does not occur at this di1uti0n.I~ The cryostat’ and the use of visible and near-ultraviolet radiation from the Synchrotron Radiation Source (SRS) at SERC’s Darebury Laboratory (DL) to study of the decay of fluorescence and fluoresence anisotropy28 have been described previously.Temperatures quoted are believed accu- rate to 0.2”C. Our present apparatus limits us to excitation wavelengths above ca. 320 nm. The exciting light passes through a Spex 0.75 m vacuum monochromator and a pol- ariser. Fluoresence at right angles to the incident beam passes through a second, rotatable polariser and a filter (Table 1) before falling on a Mullard XP2020 or similar photomulti- plier. Conventional single-photon counting is used to record fluorescence polarised vertically (parallel) and horizontally (perpendicular) and also the ‘prompt ’, the instrument reponse function, measured by scattering light at the fluorescence wavelength from a dilute suspension of Ludox.Wavelengths used are listed in Table 1. Count rates were kept below 30 kHz. The timescale was adjusted to cover some 5-10 decay times; counting continued until the peak channel (of 1024) contained 20-50 OOO counts. Data were analysed on the DL Convex C220 computer using the programe ‘srd fluor’. The parallel and perpendicu- lar decays are first deconvolved from the instrument response using a series of eight exponentials to fit them. The anisot- ropy, r(t),defined as is calculated and then fitted to one or more exponentials. Since no emission monochromator is used, there should be J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 no inherent bias in the detection system towards one or other polarisation, i.e. the limiting value r(m) should be zero. To check for effects of errors of alignment, this quantity was treated as a variable. Values of 0-0.01 were obtained in most cases. The pulse width of the synchrotron is ca. 160 ps and the measured FWHM of the instrument reponse was 600-700 ps. Synchrotron radiation has the advantage over flash-lamps that the pulse shape is independent of wavelength, so decon-volution should give access to times < 100 ps. However, diffi- culties were sometimes experienced at short times, probably the result of small amounts of internal reflection in the appar- atus. Measurement of long times, much greater than the fluo- rescence decay time, is also difficult, or course.The denominator of eqn. (2) is the total emission, indepen- dent of polarisation. This quantity was analysed in terms of one or two exponential decays using non-linear squares deconvolution. Apart from tetracene which showed evidence of a small amount of an impurity with a longer lifetime, one exponential sufficed for all these systems in the temperature range of interest. Values of decay times are listed in Table 1. Theory Molecular Rotation The foundations of the subject were laid by Einstein29 and Perrin3* who used the hydrodynamic theory of Stokes to describe Brownian rotation of particles in suspension. For a sphere, the rotational correlation time, 7R,is given by 1 (3) where, the rotational diffusion constant, D,, is kTD,= -(4)6rlV In the absence of viscous drag, zR would equal zI, which cor- responds to purely inertial rotation and is given by3* For the molecules considered here T~ is a maximum of 1-2 ps, and can be neglected. For less symmetrical species, three dif- fusion coefficients, D,, D,, D,, are needed to describe the rotation.The axis perpendicular to the molecular plane is denoted by x throughout this paper: the transition moment defines the z direction (see Scheme 1). In the most general case, five exponentials are needed to describe the decay of fluorescence ani~otropy,~~ but, if the transition moments for absorption and emission are in the same direction (as in all the measurements reported here), the time-dependence of r Table 1 Experimental and molecular parameters lifetime/ns excitation filter dimensions/nm at -150°C wavelength/nm cut-off/nm 22 2Y tetracene 4.9 440 470” 1.420 0.730 perylene 4.64 430 470 1.133 0.888 carbazole 13.2 338 365” 0.684 1.133 Li carbazolyl-26.2 370 400 1.133 0.684 Na DPA 4.6 555 590 1.408 0.704 Li DPPD 2.75 550 610 1.649 0.704 Li DPHT 2.34 630 665 1.890 0.704 ~~ a 10 nm bandpass interference filter.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 simplifies to r(t) = (0.2 + a)exp( -(6D + 2A)t) + (0.2 -a) x exp{ -(6D -2A)t) (6) where u = 0.3 D = (D,+ D,+ D,)/3 A = J(D: + 0,’+ DI -D, D,-D, D, -D,D,) Eqn.(4) represents the ‘stick’ limiting condition. The solvent is treated as a continuum and it is assumed that a thin layer of solvent moves with the solute. This approach is found to be satisfactory in many cases and especially for very large molecule^.^ However, molecular rotation in liquids is generally faster than predicted, especially when the rotor is comparable in size with the solvent molecule^.^^^^^ Such behaviour is described as ‘slip’: in the limiting case, there is no frictional drag and D depends only on the need to displace solvent: a sphere can rotate freely (rR= 2,) and so can a spheroid rotating round its symmetry axis. The consequences for rotation about other axes have been treated theoretically for spheroids36 and ellipsoid^.^ Rotation times also depend on the solvent: e.g.trans-stilbene rotates more r pidly in alcohols than in alkanes of the same vis~osity.~’ In some cases,22.3 9 -42 ‘saturation’ of the rotation time is observed at high viscosities; the effect can be very large, a factor of 50 in the polyhydric alcohol^.^' Such behaviour is described as ‘sub-slip’ and has been interpreted in terms of the molecular structure of the solvent:43 such large effects must reflect the strong solvent-solvent interaction, which can leave the solute in a cavity larger than its molecular dimensions. These results demonstrate the danger of using the macroscopic viscosity to describe microscopic motion. Perrin3’ extended eqn. (4) to ellipsoids, bodies with ellip- tical cross-sections and unequal hemi-axes (x, y, z), for the stick condition.Youngren and Acrivos3’ used numerical methods to calculate friction coefficients for ellipsoids in the slip case. We have used their results to calculate values of D,, D, and D, for the molecules used in this work. The anions were treated as bare ions though in fact they are loose (solvent-separated) ion pairs; it was assumed that they were in the all-trans conformation (see Discussion). All the molecules were treated as rectangular blocks of dimensions 2x x 2y x 22; molecular dimensions used are listed in Table 1. The molecular thickness was taken to be 340 pm; 2y and 22 were put equal to the maximum length and width. The dimensions were then scaled to give those of an ellipsoid of the same volume, i.e.8xyz = (4x/3)x’y’z’.Values of the com- ponents of D were substituted in eqn. (6)to give eqn. (7): r(t)= r,(O)exp -+ r,(O)exp -(7)L:l {::I[r,(O)+ r2(0)= 0.41. These calculated rR values are probably too small for propeller-shaped molecules like DPA etc. which will displace large volumes of solvent than ellipsoids of the same volume. (For this reason, the maximum rather than the average width was used in the calculations.) Results of the calculations are presented in Table 2. Also listed are rR values calculated for a sphere of the same volume. The viscosity of the liquid has been measured down to -75 OC5 which overlaps the range accessible to us, so this temperature was chosen for comparison of theory and experi- ment.Results A typical anisotropy decay is shown in Fig. 2. In all cases, the decay could be fitted satisfactorily (0.9 < x2 < 1.1) by a single exponential; the measured lifetime is equated with the rota- tional correlation time, T~.Values of r(0)were in the ranges: 0.2-0.24 (tetracene), 0.32-0.35 (perylene), 0.155-0.18 (carbazole), 0.13-0.16 (carbazolyl), 0.33 (DPA; only two measurements), 0.33-0.34 (DPPD), 0.30-0.33 (DPHT). Experimental rR values at -75 “C are listed in Table 2 for comparison with theoretical calculations. Though the decay times are small, they are consistent with the more accurate measurements at lower temperatures: this can be seen in Fig. 3-5 which show the temperature dependences of the quantity ‘R a Table 2 Rotational correlation times/ns for MTHF at -75 “C calculated ellipsoid experimental sphere ‘stick’ ‘slip’ ‘stick ’ r(0) TR tR r(0) 511 r(0) TR tetracene 0.23 0.124 0.265 0.305 0.328 0.382 0.099 0.095 0.630 0.018 0.215 perylene 0.33 0.116 0.258 0.003 0.397 0.354 0.461 0.291 0.109 0.043 0.329 carbazole” 0.16 0.135 0.198 0.299 0.233 0.400 0.075 0.101 0.376 Li carbazolyl- 0.13 - 0.439 - -0.198’ 0.400’ - 0.376’ - -0.1006 -0.075’ 0.3006 0.1506 Na DPA 0.33 0.278 0.254 0.400 0.605 0.007’ 0.061‘ 0.393’ 0.199‘ Li DPPD 0.33 0.432 0.297 0.400 0.860 0.006 0.081 0.394 0.257 Li DPHT 0.31 0.686 0.341 0.400 1.178 0.002 0.077 0.398 0.309 ~ ~ a Transition taken to be short-axis polarised.Predictions for long-axis polarised transition (see text). Extrapolated value (see text). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -1 I 0 7 4 h rlns Fig. 2 Decay of fluorescence anisotropy of perylene in MTHF at -120°C; (-), data; (-), fitted single exponential; (---), (-. -), cal-culated curves using 'stick ' and 'slip' assumptions, respectively Fitting the previous viscosity mea~urements~~~ to eqn. (1) gives : 162 log(q/kg m- s-') = -3.986 + T/K -81 (8) The data are shown in Fig. 6 together with some of our data for perylene which have been converted to viscosities using eqn. (3) and (4) and scaled by multiplication by a factor of 2.5. + 1','0 I 1 I 4 7 h 8 9 103K/T Fig.3 Temperature dependence of the decay of fluorescence anisot- ropy: experimental results and curves calculated from eqn. (8), scaled to fit at -100°C: +, x ,(-), perylene (two series of experiments); 0,(---), tetracene t</ , I '11 4 7 6 8 9 103KjT Fig. 5 As for Fig. 3: +, (-), Li+DPPD; 0,(---), LifDPHT;0,Na'DPA For comparison with the zRT data, interpolated values of the viscosity, (scaled to fit the data at -100°C) are plotted in Fig. 3-5 ;these quantities should be proportional according to eqn. (4). The fit is generally good except at the lowest tem- peratures. This discrepancy appears to be real, but, unfor- tunately, measurements in this region are difficult: zRis much larger than zF and the solvent is liable to crystallise suddenly and violently, usually breaking the cell.zF values for DPA, DPPD and DPHT increase as the tem- perature is reduced because the rate of skeletal twisting 7723decreases.**' For tetracene, perylene and carbazole at 20"C, lifetimes of 5.04, 5.10 and 13.9 ns, respectively, were obtained; the perylene value lies within the range 4.8 to 6.9 ns reported for a variety of the tetracene value is lower than the 6.4 ns quoted for cyclohexane. zF for car- bazolyl ion pairs showed a small increase on cooling (23.5 ns at +2O"C, 25.1 at -51 "C, 26.1 at -140"C), presumably the result of a temperature-dependent internal conversion process, but perylene and neutral carbazole both show a small decrease in zF.Non-radiative processes commonly decrease in rate with falling temperature, so this effect must be due to changes in the radiative rate, A. In the case of perylene, this can be ascribed to the dependence of A on the refractive inde~,~~-~~ but for carbazole the effect is too large, suggesting a change in the solvent perturbation with tem-perat ure (below). 10 -,' + ,,,+ + ,'+8-'+ -+ +,+' h I 6-+ ,' v) + ,,' c I E m Y+7-e-v m-4' 0 0' 0' 0 20 u) ho 80 4 , h 8 103K/T 1O~K/(T-81 Fig. 4 As for Fig. 3: +, (-), neutral carbazole; 0,(---), Li+ Fig. 6 Temperature dependence of the viscosity of MTHF; 0,this carbazolyl-work (perylene; scaled-see text); x ,ref. 5; +, ref. 6; (---), eqn. (8) J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Discussion Transitions The fluorescence transition of perylene is strongly allowed but that of carbazole is considerably weaker as is shown by their purely radiative lifetimes, 6.5 and 44.2 ns at 20°C. The first excited state of carbazole has been assigned as 1Lb;47 transitions from such states in alternant hydrocarbons are 'accidentally forbidden' and become allowed partly by inter- action with a symmetrical vibrations and partly by solvent perturbations, the Ham The former are temperature independent but the latter may well increase with falling tem- perat ure. The values of r(0) will be discussed further below, but it is clear from Table 2 that they are ca. 0.3 with exceptions such as carbazole and its ion.The polarisation of the fluorescence transition of carbazole is predominantly short-axis (i.e. through the nitrogen atom), but contains long-axis and out-of-plane components The transition in the anion has not been assigned, apparently, but it is likely to be similar in nature, since deprotonation does not directly involve the 7t electrons. Theoretical calculations for both long-axis and short-axis components are included in Table 2: in principle, anisotropy decay measurements could distinguish the two cases, but this is not possible in MTHF, given the generally poor agreement between experiment and theory. The observed low values of r(0)in both ion and neutral molecule are probably due to this mixed polarisation.Similar results for tetracene can be explained in the same way.22 Rotational Difhsion Anisotropy decay times and r(0) values calculated from eqn. (6) and the parameters listed in Table 1 are compared with experimental results in Table 2: the approximation of molec- ular shapes to ellipsoids must be borne in mind. It is imme- diately clear that the experimental zR for the neutral species are much less than the 'stick' values calculated for either ellipsoids or spheres. Perylene In Fig. 2 calculated curves are compared with experiment at -120.2 "C.While the stick curve is essentially single expo- nential, the decay is too slow by a factor of 4; the slip curve is closer in position but has the wrong shape. A number of trials were made with two exponentials, essentially varying the D,D,, ratio.We estimate that this ratio must be less than two to fit our data; though a small proportion of a slow decay would be hard to detect, it is noteworthy that one exponential sufficed at all temperatures except possibly the lowest. Weber first suggested that molecules such as perylene could rotate much faster in plane than out of plane, on the basis of careful steady-state measurements and studies of the effect of wavelength and of substituents." This was con-firmed by time-resolved measurements of the anisotropy, which showed the presence of two decays: values of the ratio D, D,, (out of plane in plane) of 28 in propylene glycol,' 10 & 1 in paraffinlg and glycerolZo and 6.5 & 0.3 in glycerol- water mixtures2' were obtained.It is surprising therefore that only one exponential is observed for MTHF. It appears that perylene rotates like a sphere; this implies that some solvent rotates with the large solute molecule (weakly bound, above and below the molecular plane), i.e. friction between solvent molecules is comparable with that between solute and solvent. This is not true for the hydrogen-bonded solvents used elsewhere or for a paraffin with chains much longer than the solute. Perylene gives r(0) values in agreement with those in the literature;'8-2' DPA and its vinylogues which also have strongly allowed transitions (no significant vibronic interaction) give similar values, significantly less than the theoretical value of 0.4.Zinslilg found that this difference was temperature dependent for perylene in paraffin as did Viovy for dimethylanthracene in glyceryl tripr~pionate.~~ Both authors ascribe the effect to libration but Christensen et aLZ1 have cast doubt on this interpretation, putting forward alter- native explanations. We observe no consistent variation with temperature. Tetracene Wirth and ChouZ2 observed 'sub-slip' behaviour and double exponential decay for tetracene in long-chain alcohols; the ratio of the D values was found to be temperature dependent. Our data fit a single exponential over the whole range. Stick behaviour rather than slip is predicted to give two exponen- tials (Table 2), but the values are close enough that dis- tinguishing them would be difficult. The difference between experiment and theory is less in this case.Carbazole The behaviour of the neutral molecule parallels that of the hydrocarbons, single exponential decay at a rate between stick and slip, where theory predicts two decays, though the decay times are close in the stick case. As expected the rota- tion time is considerably longer for the carbazolyl ion pair. On could regard this as a switch from slip to stick behaviour, but in the light of the above, it is simpler to suppose that the effective size of the ion is larger due to the solvation. The counter-ion has not been included in the calculations in Table 2: it is noteworthy that in this case, molecular rotation must involve considerable movement of the lithium ion and its solvation shell: it is believed to lie in the molecular plane,50 where it is some distance from the rotation axes involved ; this contrasts with DPA etc.where the counter-ion is at the centre of the rotor. DPA, DPPD and DPHT For these long molecules, with transitions aligned along the long axis, single-exponential decay is expected (Table 2). Unlike carbazolyl, their rotation rates are faster than the stick values. The ratio is not constant, i.e. DPHT rotates more slowly than DPA even after allowing for its size. This is surprising insofar as interaction with the solvent might be expected to decrease with molecular size, the charge being more delocalised. However, it must be recalled that the departure from ellipsoidal shape is large for these molecules.Viscosity Fig. 6 shows collected data for the viscosity of MTHF. The value of To = 81 K is largely determined by the low temperature6 data, of course. The scatter shows that there is considerable uncertainty in To. The high-temperature data are more precise, but their range is not adequate for accurate determination of To;taken alone, they give a best fit to eqn. (1) with To = 40 K. The temperature dependence of our own data (Fig. 3-5) agrees well with the value of 81 K though the fit is not exact and differs slightly between solutes. Dainton and Salmon4 quote a value of 75 K for To for MTHF, presumably obtained by fitting their reaction rate data. The glass transition temperature, 7, lies higher than it is a kinetic rather than an equilibrium phenomenon, the temperature at which flow becomes very slow on the laboratory timescale.This is commonly taken to be 10l2 kg ,-1 s-l P). Eqn. (8) gives < = 91 K. Kato et aL3 extrapolated the low temperature viscosity data6 to obtain < = 88 K. The empirical equation, eqn. (l), has been interpreted in terms of free volume theory;" the activation step is supposed to involve creation of a space into which movement can take J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 temperatures (T9 To)the discrepancy is small, but in the light of the results obtained here, it is more appropriate to combine eqn. (1)and (9) and differentiate to give place. More sophisticated theories have follo~ed;~~*~~-~~ in E' is the activation energy obtained by applying the Arrhe- particular, Adam and GibbsS2 discussed dielectic relaxation behaviour near the glass transition in terms of the entropy involved in solvent movements, pointing out that the number of molecules involved increases rapidly as the glassy state is approached; Kivelson and Kivel~on~~ have described the two types of relaxation behaviour (fast movements within a fixed potential and slower changes in the potential itself) which are observed experimentally.The apparent saturation behaviour at low temperatures (Fig. 3, 5) is interesting in this context: possibly a change in the solvent structure is occurring, leaving a larger cavity for solute motion.In common with most other worker^,'^-^ '*33-35 we find that the rotational correlation time follows the viscosity quite closely at high temperatures, even though the proportionality constants vary from system to system. The fit is not exact (Fig. 3-9, imply-ing that the free volume for molecular rotation is larger than for viscous flow. GoldsteinS3 has warned against the difficulties of inter- preting measurements near the glass transition, but, given the wide range of viscosity data now available for MTHF, the plots in Fig. 3-5 must be regarded as satisfactory, vindicating the use of eqn. (1). To can be regarded as the temperature at which motion of the second kind stops completely. Photo-isomerisat ion In fluid solutions at normal temperatures, singlet excited aryl-alkenes decay rapidly by twisting from the planar to the perpendicular form which then decays back to the ground state of the original species or its cis or trans isomer.Mea- surements of fluorescence lifetimes, zF,give the rate constant of this process as (l/zF -l/zo) where zo here is the lifetime at low temperatures or in a solid where rotation cannot occur. The behaviour of stilbene has been studied in great detail in recent years.g The role of solvent viscosity has received par- ticular attention',' and in some cases the intrinsic '-14718*54 and solvent barriers have been distinguished by comparing rates in different solvents at the same viscosity. The same picture is appropriate to DPA and related compounds. In the trans,trans form, the barriers are ca.15-30 kJ mol-': we have now studied several s~lvents'~'~*~~ with the aim of eluci-dating the role of the nature of the ion pairings; the role of the viscosity cannot yet be separated. Isomerisation from cis to trans is simpler, in some respects: there is no intrinsic barrier, so that twisting is probably a simple one-dimensional process : for trans-stilbene simultaneous rotation round the central bond and round the carbon-phenyl bond has been suggested.5 In the case of a reaction with an intrinsic energy barrier, Ei, a convenient simple relation between the rate constant and the viscosity can be derived from free volume theory:' 1,54 A( -$exp( $)k = (9) where a can be regarded as the ratio of the free volumes required for reaction and for viscous flow : observed values are typically in the range 0.1.13 For most work in this field, the temperature dependence of viscosity has been expressed by the Andrade equation, i.e.T, is put equal to zero. At high nius equation. For MTHF, the viscosity parameters give con- tributions from the second term in eqn. (10) of 6.8, 8.8, 14.7 and 86 kJ mol-' at 250, 200, 150 and 100 K, respectively (a = 1). When the transition-state region is a broad maximum on the potential-energy surface, crossing it can be regarded as a diffusive process and a will be large; when it is a sharp peak, a will be small.' Since MTHF is a 'good' solvating solvent, DPA and its derivatives generally form loose ion pairs, especially at reduced temperature^.'^ Usually the decay of the excited state gives linear Arrhenius plots with activation energies of ca.20-25 kJ mol-' and the rate becomes negligible below 150 K.' For stilbene, solvent variation, e.g. among the n-alkanes,'TS4 has made possible the separation of viscosity and intrinsic barriers. Studies of carbanions are limited by the sol- vation requirement; viscosity is likely to contribute to E' but the linear Arrhenius plots' suggest that its role is fairly small. Steeper, sometimes curved, plots which can be studied at lower temperatures are given by tight ion pairs;' they give linear 1/(T -To)plots but it is necessary to use other sol- vents for which viscosities are not available.In MTHF, 2-aza derivatives of DPA also give curved Arrhenius but their behaviour is complicated: the curved portion lies at higher temperatures than is predicted by eqn. (10). In this case, isomerisation or deactivation may involve inversion at the nitrogen rather than twisting. The solvent displacement will then be small and viscosity less important. Fluorescence from cispans DPA has not been observed. Experimental measurements even on cispans 2-methyl DPA are difficult; though substitution in the central position sta- bilises the cis,trans form relative to trans,trans, the fluores- cence of the latter predominates until very low temperatures are reached: some preliminary for the range -120 to -155"C are shown in Fig.7. The rate constant for isom- erisation is ca. 1 ns-' at -150 "C;for comparison, the rate of overall rotation (for DPA) has the same value at -104 "C. The linear relationship between the twisting rate of cispans 2-methylDPA and 1/(T-81) shows that Ei is negligi- ble in this case, paralleling the behaviour of cis-stilbene. The slope corresponds to 1.56 kJ mol-', i.e. a = 0.50, a typical 103K/(7-81) ' ' Fig. 7 Temperature dependence of the rate (ns-') of skeletal twist- ing of Li' 2-methylDPA in MTHF J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 result for rotational deactivation processes. [The use of eqn. (1) should be recalled; use of an Arrhenius fit in this region would give a smaller value of a.] Calculation of the absolute value of the rate of these pro- cesses is very dificult.13 A simple formula has been given by RappS8 l/k = 4nxyzq/f (1 1) x, y and z are here the hemi-axes of the rotating group.Taking the rotational force-constant, f, to be 40 kJ mol-' rad -',a value taken from semi-empirical calculation^,'^ gives a rate constant, k, of 4.3 x 10' s-l at 150 K, compared to the experimental value of 4.7 x lo9 s-'. Fitting the rate in this way would require an implausibly large f value. An alterna- tive approach is to regard A in eqn. (9) as the limiting rate in the gas phase. The value of 1.9 x 1013 s-l is considerably larger than the 3 x lo'* s-l observed for ~is-stilbene,'~ con-firming that the driving force for twisting is very large. Tight ion pairs in dimethyltetrahydrofuran' and some loose ion pairs in amine solvents23 give very large pre-exponential factors: in all these cases, it appears necessary to postulate a positive entropy of activation due to changes in solvation.The rates of bimolecular reactions have been related to vis- cosity in much the same way. A square-root relation is often .~found,60 i.e. 01 = 0.5. Kato et ~1 measured the rate of ion recombination in MTHF solutions after radiolysis over nine orders of magnitude. Replotting their results against 1/( T-81) gives a straight line, but the slope corresponds to a value of o! of 1.26 which is puzzling. It disagrees with the value of 0.5 found for similar measurements over a smaller range in parafin. Conclusion The synchrotron operated in single bunch mode is a very convenient source for the study of fluorescence and fluores- cence anisotropy decays down to 100 ps.Access time avail- able is very limited which has prevented us from accumulating the large numbers of counts required to better characterise the decays better. Nonetheless it appears that at temperatures above -130-140°C all our data fit well to single-exponential decays. The disagreement with theory sug- gests that the conventional hydrodynamic approach is not appropriate to MTHF. Very tentatively, we suggest that there is some very weak complex formation, resulting in a rotor which is more nearly spherical and which can 'slip' relative to the rest of the solvent, However, it is also clear, that if appropriately scaled, the macroscopic viscosity is a good guide to the temperature dependence of both overall and intramolecular rotation rates.At temperatures below -140"C, the anisotropy decays are faster than expected; in this region there is some evidence for two exponentials, but the results are more uncertain because the decay of anisot- ropy is slower than the fluorescence decays. It is interesting that in this region intramolecular rotation can be treated in terms of the viscosity while molecular rotation cannot: this must be due to the large driving force in the former case, much greater than the random forces of thermal motion. Our main aim has been to characterise the dynamic pro- cesses occurring in MTHF. It should be noted that we are dealing with a racemic mixture; possibly this leads to local structure at low temperatures.Previous workers have usually used solvents with an intrinsic structure, insofar as they consist of long chains or they are hydrogen bonded. Further work on ethereal solutions appears to be worthwhile. We thank the Science and Engineering Research Council for providing facilities at Daresbury and Dr. C.E. Oliver, and Dr. D.A. Shaw, Dr. M. Behan-Martin and other members of the Daresbury Laboratory staff for their assistance. References 1 M. R. Ronayne, J. P. Guarino and W. H. Hamill, J. Am. Chem. SOC.,1962,84,4230. 2 W. H. Hamill, in Radical Ions, ed. E. T. Kaiser and L. Kevan, Wiley-Interscience, New York, 1968; T. Shida, The Electronic Absorption Spectra of Radical Ions, Elsevier : Amsterdam, 1988.3 N. Kato, T. Miyazaki, K. Fueki, S. Miyata and Y. Kawai, J. Phys. Chem., 1984,88, 1445. 4 F. S. Dainton and G. A. Salmon, Proc. R. SOC.,A, 1965,285, 319. 5 D. Nicholls, C. Sutphen and M. Szwarc, J. Phys. Chem., 1968, 72, 1021. 6 A. C. Ling and J. E. Willard, J. Phys. Chem., 1968, 72, 1918. 7 A. J. Barlow, J. Lamb and A. J. Matheson, Proc. R. SOC., A, 1966,292,322. 8 S. S. Parmar, B. Brocklehurst and R. N. Young, J. Photochem., 1987,40, 121. B. Brocklehurst, R. N. Young and S. S. Parmar, J. Photochem. Photobiol., A, 1988,41, 167. 9 D. H. Waldeck, Chem. Rev., 1991,91,415; D. C. Todd and G. R. Fleming, J. Phys. Chem., 1993,97,269. 10 H. Stegemeyer, Ber.Bunsernges. Phys. Chem., 1968,72, 335. 11 D. Gegiou, S. Sharafy and K. A. Muszkat, J. Am. Chem. SOC., 1968, 90, 12; S. Sharafy and K. A. Muszkat, J. Am. Chem. SOC., 1971,93,4119. 12 G. Fischer, G. Seger, K. A. Muszkat and E. Fischer, J. Chem. SOC., Perkin Trans. 2, 1975, 1569. 13 B. Bagchi, Int. Rev. Phys. Chem., 1987,6, 1. 14 J. Saltiel and Y-P. Sun, J. Phys. Chem., 1989, 93, 6246; J. Saltiel, A. S. Waller, D. F. Sears and C. 2. Garrett, J. Phys. Chem., 1993, 97, 25 16. 15 Y-P. Sun, J. 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Paper 3/04982E; Received 17th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000271

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 279-285 Effects of Protolytic Interactions on the Photophysics of Phenyl Pyridyl Ketones Fausto Elisei, Gianna Favaro" and Fausto Ortica Dipartimento di Chimica Universita di Perugia , I46123 Perugia Italy The photophysics of phenyl 3-pyridyl ketone (3-PPK) and phenyl 4-pyridyl ketone (CPPK) were investigated in aqueous solution as a function of pH (0-10) by steady-state and pulsed emission spectroscopy and by nanosec- ond laser flash photolysis. From phosphorimetric and triplet-triplet absorption acid-base titrations of the excited state, two protonation steps were evidenced for 3-PPK, which involved both the carbonyl oxygen and nitrogen atoms. The triplets of the neutral molecule and of the mono- and di-cation were identified from the shape of the transient absorption spectra, analysis of the kinetic profiles and effect of charged quenchers.For 4-PPK, the titration curves showed only one inflection point. Phosphorescence emission and T, +T,, absorption spectra, over the whole pH range explored, showed only the triplet of the neutral molecule. Indirect evidence of protonation was obtained from the weaker emission and absorption intensities and shorter lifetimes with increasing acidity. The comparison with ground-state pKs showed that the basicity of these molecules increases greatly upon electronic excitation. Interest in pyridyl ketones as potential water-soluble photo- sensitizers, which could possibly be used also in acidic solu- tions, stimulated this study of the effects of pH on their pho tophysics.Previous studies on the photo~hemistry'-~ and photo physic^^*^ of phenyl pyridyl ketones (n-PPKs) gave information on the nature and reactivity of their excited states. These molecules were found to possess lowest singlet and triplet states of n,n* character with excitation energies slightly lower than that of benzophenone owing to the electron- withdrawing effect of the heterocyclic nitrogen. In aqueous solutions, only two of the three isomers, the 3- and 4-PPKs, exhibited a triplet behaviour which was similar to that of benzophenone (B). In contrast, the 2-isomer, which still showed similar absorption spectra and low-temperature n,n* triplet emi~sion,~ underwent photochemical rearrangement with formation of coloured photo product^,^ when irradiated in a water solution.In such a medium, this isomer did not show either room temperature phosphorescene, or transients which could be securely attributed to the n,n* carbonyl triplet, at least with nanosecond time resolution. This paper is concerned with the 3- and 4-PPKs, which were found to be photostable enough to be used as triplet photosensitizers in benzene, as well as in water solution. The protolytic interaction of the 3-PPK in the excited state was previously investigated by phosphorescence quenching and sensitization measurements. The results showed inter- esting pH effects on phosphorescence emission intensity and triplet lifetime. It seems worthwhile to re-examine these mol- ecules by using nanosecond laser flash photolysis and a high- sensitivity spectrofluorimeter in order to gain more insight into the protolytic interaction in the excited state and the transients produced under laser excitation. The ionization constant of an electronically excited mol- ecule may differ from that of the ground state by several orders of magnitude due to the different electronic distribu- tion in the excited state.In PPKs, it is difficult to establish the sequence of the excited-state protonations because of the presence of two basic centres (on the carbonyl oxygen and on the heterocyclic nitrogen). By combining kinetic and steady- state phosphorescence measurements with laser flash pho- tolysis measurements on transient absorption intensity and decay, a reliable scheme is proposed for protolytic interaction of triplet excited 3- and 4-PPK in the 0-10 pH range.Experimental Materials The 3- and 4-PPKs were purchased commercially (Aldrich). The 3-PPK (mp 38°C) was recrystallized from ethyl ether at -25 "C and 4-PPK (mp 75 "C) from water-ethanol. Britton buffer solutions were used from pH 10 to pH 2, at constant ionic strength (p = 0.01); HClO, solutions were used in the higher acidity range, down to pH = 0. The pH values of the solutions were determined with an Orion digital pH-meter SA-520 equipped with an Orion 9103 semimicro electrode. Equipment The absorption spectra were recorded on a Perkin-Elmer Lambda 5 (double beam) spectrophotometer.The phosphorescence measurements were performed on a Spex Fluorolog-2 FL112 spectrofluorimeter controlled by the Spex DM3000F spectroscopy computer. A home-made apparatus, previously described,6 was used for the phosphorescence decay time measurements in fluid solution (order of magnitude: 1-300 p).The decay curves were mono-exponential over at least two or three half-lives. The reproducibility of the mean lifetimes was within 20%. For laser flash photolysis measurements, the 308 nm line from an Xe-HCl excimer laser (Lambda Physik LPX 105) or the 347 nm line from a ruby laser (J.K., second harmonics) were used. The laser energy was less than 10 mJ per pulse. Details of the equipment and data processing methods have been described elsewhere.' The transient spectra were obtained by a point-to-point technique over the spectral range 300-700 nm.Phosphorescence decay kinetics were also recorded with the same apparatus to compare the decay rate constants of absorption and emission under the same experi- mental conditions. Measurement Conditions Phosphorescence quantum yields were determined in dilute (c < 3 x lo-, mol dm-3), low absorbance (A d 0.07) solu-tions, using quinine bisulfate in 0.5 mol dmP3 H,S04 as a standard. Intersystem crossing yields were obtained from measurements of sensitized biacetyl phosphorescence, using benzophenone ($Isc = 1) as a reference molecule. For the phosphorimetric titrations, the solutions were excited at an isosbestic point between the neutral and nitrogen-protonated form.For the triplet-triplet absorption titrations the absorbance of the solutions was adjusted on a constant value (A = 0.25) at the exciting wavelength : the transient absorptions were monitored at the visible maximum wavelength, 50 ns after the laser pulse. Transient lifetimes were measured as a function of pH keeping the sample concentration constant (c = 7 x mol dm-3, with 308 nm excitation, and 2 x mol dm-3, with 347 nm excitation). The expected accuracy in lifetime was within 20%. Quenching experiments with the inorganic anion Cr(CN); -were carried out adjusting the quencher concentra- tion in the range 1.5 x 10-2-1.5x mol dm-3, depend- ing on the lifetime of the ketone.Fresh solutions of salt were used in order to avoid thermal aquation. At the exciting wavelength (347 nm) photoaquation was negligible owing to the low absorbance of the anion. All measurements were performed at room temperature. The solutions were de-aerated with oxygen-free nitrogen or argon, unless otherwise indicated. Results Photophysical Parameters based on Room-temperature Phosphorescence Emission The 3-and 4-PPKs were previously found to exhibit phos- phorescence emission in aqueous neutral solutions, like ben- zophenone. The typical vibrational structure, showing the carbonylic progression, and rather long lifetime (in the micro- second domain) indicated that the lowest triplets are n,n* in ~haracter.~ Some photophysical parameters of 3-and 4-PPK in aqueous solution are compared with those of benzophenone in Table 1.Triplet energies were taken from the 0-0 transition of the low-temperature (77 K) phosphorescence ~pectra.~ Phosphor-escence yields and lifetimes were extrapolated at infinite dilu- tion in order to compare their values independently from concentration. The self-quenching rate parameters of PPKs, k,, were obtained as the slope of a plot of the inverse of the experimental phosphorescence lifetime, l/~~,~,against con- centration (2 x 10-'-2 x mol dm-3), keeping the pH of the solution constant. The intercept of the plot T-' us. con-centration gives the sum of the radiative, k,, and non-radiative, k,, , rate parameters. The radiative constant was determined through the relationship: k, = (&c 7)-'4,. The values obtained are in reasonable agreement with those cal- Table l Triplet-state parameters of 3-PPK and CPPK obtained from phosphorescence measurements in alkaline aqueous solution at room temperature, compared with those of benzophenone (E, refers to 0-0 transition from the low-temperature phosphorescence spectra) 3-PPK 4-PP benzophenone E,"/kJ mol-' 4P 'TIPs 278 1.3 x lo-' 70' 273 2.6 x 15 280 4.4 x 200 lo-' k,/dm34Isc mol-'s-' 6 x 0.7 4.4 x 0.5 107 1.7 x 1 loac kp/s-k,,ls - 2.6 x 10' 1.4x lo4 3.5 x 10' 6.6 x lo4 2.2 x 10' 4.8 x lo3 Data from ref.4; from ref. 5; from ref. 18. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 culated from the phosphorescence lifetime measured in a glassy solution at 77 K (kp = l/t = 2.8 x lo2 sC1 and 5.9 x lo2 s-l, for 3-and 4-PPK, respectively).* Both phosphorescence intensity and lifetime decreased, as the acidity of the solution increased, down to undetectable values at pH < 0 for 3-PPK and pH < 4 for 4-PPK.In a previous paper,' where we studied the effect of pH on the phosphorescence of 3-PPK, no change in the spectral dis- tribution of the emission could be detected in the pH range 2-10: this was probably due to the weakness of the emission and insufficient instrumental sensitivity. Here, with improved sensitivity, a clear difference was observed in the spectral shape between the emission in an alkaline solution and that in an acidic one (Fig.1). It can be seen that the spectrum in an acidic solution is very broad and shifted to the red com- pared with that in an alkaline solution; the emission yield in acidic solution is reduced by about an order of magnitude. For the 4-PPK, the spectral shape of the emission was maintained within the pH range where it could be detected (PH = 4-10). Transients detected by Laser Flash Photolysis The transient absorptions, observed immediately after laser excitation of alkaline aqueous solutions of 3-and 4-PPKs, were securely assigned to triplets, based on comparison with literature values for these moleculesg and benzophenone'0-'2 and by matching of the lifetime values with those obtained by phosphorescence measurements. Also, the rate constants for quenching by 0, [k,, = (1-2) x lo9 dm3 mol-' s-'1 were of the order of magnitude expected for carbonyl tri~1ets.I~ As the acidity increased, spectral changes were accompa- nied by a decrease in transient lifetimes.No notable change in spectral distribution was observed between de-aerated and air-equilibrated solutions over the whole pH range explored. The spectral and kinetic characteristics of 3-and 4-PPK are reported in Table 2. 3-PPK The transient absorption spectra of 3-PPK at three typical pH values in de-aerated solutions are shown in Fig. 2. As can be seen, an intense absorption was always present in the 320-330 nm region, while the spectrum changed at longer wave- lengths. In the visible region, a maximum was observed at 530 nm in neutral and basic solutions.At pH = 2.5-3, the absorption in the visible consisted of two bands of compar-able intensity (A,,, = 505 and 760 nm). With increasing acidity, the spectrum evolved to a maximum at 480 nm, in 1' I I I I I I II 0.0 400 500 600 700 A/nm Fig. 1 Normalised phosphorescence spectra of 3-PPK in aqueous solution at room temperature. 1, pH = 8; 2, pH = 2.7. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 28 1 Table 2 Spectral and kinetic characteristics of the transients obtained in aqueous solutions of 3-PPK and 4-PPK excited with 347 nm laser excitation 3-PPK 4-PPK 2.7 0.5PH 7.4 11 6.9 5.5 4.1 4naJnm 530 505; 760 480 510 520 515 dps 6.45 2.6; 2.1 1.25 k,,/109 dm3 mol-ls-' 1.9 0.8 2.1 0.29 4.9; 4.7 0.013k: lo9 dm3 mol-' s-l 4.35 1.6 0.36 3.3 0.5 1.8 0.36 0.34 0.25 0.38 a Kinetic rate constants for the quenching by the inorganic anion Cr(CN):- addition to the 325 nm transition.The kinetics of transient disappearance, evaluated at several wavelengths, are shown in Fig. 3. The triplet lifetime (first-order kinetics), measured in basic and neutral solutions (z = 6.45 ps, under the experimen- tal conditions used, c = 1.2 x mol dm-3) decreased as 0.10 the acidity increased, starting from pH 5. In the pH range 3.5-2.5 the lifetime remained constant (t= 2.6 p),then it decreased further to 1.2 ps at pH 0.5. In addition to the short-lived triplet, a long-lived transient, absorbing in the UV region was revealed by time-resolved spectroscopy; this appeared more evident in acidic solutions [Fig.2(6) and (c)]. The absorption maxima (320-330, 360- 0.00 370 and 480-490 nm) and their relative intensity changed a little as the pH changed. Because of the weakness of the absorption signals and partial overlapping with the triplet spectra, the kinetic and spectral analyses were not easily feas- ible. However, in an acidic Ar-purged solution of 3-PPK, the 0.04 AA 10 pst 1i:' .. I 360 nm signal remained constant over a 2 ms timescale, while 0.05 -OCQ -0.00 -!I I , I , I , , I-o.lo-~o' I " ' I ' ' I-kk{ 0.15 0.10 &lo 0.02 0.00 I 0.08 0.04 I I I n, . 1ow .I \' I IIk AA ...-:.. 'W 0.05 ..:.. .. .. ...,.'.?..-..::..:..-..-.._. * .....I. 1 I .::. time 0.00 0.10 0.05 lo% 300 400 500 600 700 800 A/n m Fig. 2 Transient absorption spectra of 3-PPK in aqueous solution. (a) pH = 7.4; (0)0.8 and (A) 9 ps after the flash; (b)pH = 2.7; (0) 0.2 and (A) 3 ps after the flash; (c) pH = 0.5; (0)0.2 and (A) 2 ps after the flash. Fig. 3 Decay profiles of the transients obtained from 3-PPK in aqueous solution. (a) pH = 7.4; 1, 325; 2, 390 and 3, 530 nm; (6) pH = 2.7; 1, 360; 2, 510 and 3, 700 nm; (c) pH = 0.5; 1, 325; 2, 360 and 3,480 nm. in the presence of air it decayed with a first-order rate con- stant k = 1.3 x lo3 s-'. Since no accumulation of these transient(s) could be observed during the triplet decay, we believe that the triplet is not a prescursor. Noting that inter- system crossing yields are less than unity (Table l), these transient(s) could possibly come from the singlet. Photochem- istry was confirmed as a minor decay root by analysis of the So +S, absorption spectra obtained after many shots.4-PPK The absorption spectra of the transients obtained on flashing the 4-PPK at three pH values are shown in Fig. 4 and the 282 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.15 I 0 0.1 0 0.05 00 0.08 p' 0.00 0.10 II I I I I I I I I II I I I- 0.00 I .., . .. .. ... . I I 0 I I AA 0.05 0 0.08 AA 0.00 0.04 0.10 .. _.0 I.. , . .... . ..*:... ...-.... 00 0.05 0.00 300 400 500 600 700 A/n m Fig. 4 Transient absorption spectra of CPPK in aqueous solution.(a) pH = 10; (0)0.8 and (A) 6 ps after the flash; (b)pH = 7.4; (0) 0.3 and (A) 4 ps after the flash; (c) pH = 4.1; (0)0.02 and (A) 0.2 ps after the flash. corresponding kinetic profiles are illustrated in Fig. 5. At pH = 10, the absorption spectrum is very similar to that found in an inert solvent, perfluoromethyl-cyclohexane,2with a high-intensity band at 325 nm and a lower-intensity band centred at 510 nm which consisted of two peaks (505 and 520 nm). The lifetime was 4.35 ps (c = 7 x mol dm-j). At pH = 7 these two peaks coalesced into one at 520 nm and the lifetime decreased to 3.3 ps. At pH = 4, the spectrum showed only a slight change (A,,, = 330 and 515 nm), but the lifetime decreased to 0.25 ps. When the pH was less than 4, the triplet absorption was below our detection limits, due to further decrease in lifetime.Even in this case, photochemistry was observed as a minor process. Excited-state Acid-Base Titrations In order to obtain information about the excited-state basi- city of these molecules, phosphorescence intensity, triplet life- time and triplet-triplet absorption were measured as a function of pH in the range 0-10. The results of the excited- state titrations, from both phosphorescence intensity and T-T absorption (delay time: 50 ns), are compared with ground-state spectrophotometric titrations in Fig. 6 and 7 for 3-and 4-PPKs, respectively. In both titration plots of excited-state 3-PPK, two inflec- tion points were observed, while, for excited-state 4-PPK, the decrease in phosphorescence intensity and T-T absorption was spread over a large pH range.A shift to lower pH values of the T-T absorption titration curve, with respect to the phosphorescence titration curve, was observed for both mol- ecules. For 3-PPK, two inflection points are present, while, 0.00 I I I I I I 0.08 0.04 0.00 time Fig. 5 Decay profiles of the transients obtained from CPPK in aqueous solution (a) pH = 10; 1, 325; 2, 350 and 3, 515 nm; (b) pH = 7.4; 1, 330; 2, 350 and 3, 510 nm; (c) pH = 4.1; 1, 330; 2, 360 and 3,510 nm. for 4-PPK, T-T absorption and phosphorescence emission decrease continuously over a large pH range. For both mol- ecules, the difference between excited-state and ground-state titration indicates a change in basicity upon excitation.-8 -4 0 4 8 Ho PH -+--+ Fig. 6 Phosphorescence (0)and T, +T, (A) titration curves of 3-PPK compared with ground-state spectrophotometric titrations (0).(Ho: acidity function, from M. A. Paul and F. A. Long, Chem. Reu., 1957,57, 1.) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I‘ 7 h .-v)4-5 1.0 I I -a -4 0 4 a Ho + PH + Fig. 7 Phosphorescence (0)and T, -+ T, (A) titration curves of CPPK compared with ground-state spectrophotometric titrations (0).(H,,: acidity function as Fig. 6.) Triplet lifetimes, measured as a function of pH, followed the same trend as phosphorescence titrations. Discussion Excited-state pK*s The ground-state dissociation constants of the pyridinium cations derived from PPKs were previously determined (pK3-PPK= 3.0 and pK,_,,, = 3.1).4 Those for the carbonyl protonated molecules (pK < -6) were smaller than that for benzophenone (pK = -4.74;12 -6.1;13 -5.714) because of the electrostatic repulsion by the protonated nitrogen.Relative phosphorescence intensity us. pH plots are expected to yield sigmoid curves. The inflection point pH cor- responds to pK* only when the equilibrium is reached and the lifetimes of the conjugate acid-base pair are identi-cal 15.16 pK* = pH + log zacid/zbase (1) The relative emission intensities, Zo/Z, should fall with increasing acidity according to the equation :‘ = 1 + k,Tb[H+]/(1 + kzt,) (2) where k, and k, refer to the bimolecular interaction of the basic species with the proton and dissociation of the proto- nated species, respectively, and zb and z, are the correspond- ing lifetimes.Linearity of the Zo/Z us. pH plots was observed only within limited pH ranges: 7-5.5, for 3-PPK, and 8.5-7, for 4-PPK. The slopes, divided by zb (the lifetime measured in alkaline solution), gave kJ(1 + k,z,) values (3 x lo9 dm3 mol-’ for 3-PPK5 and 3 x 10l2 dm3 mol-’ for 4-PPK) which were larger than that for benzophenone (4 x lo8 dm3 mo1-I). Owing to the relatively long lifetime of the triplet state, which generally allows acid-base equilibrium in the excited state to be attained, the inflection point in the triplet-triplet absorption titration should give the pK* directly.” Since the molecules absorb at the wavelength dictated by the laser, the relative amounts of acid and base initially excited, in the pH regions where the ground-state equilibria are established, are different from the ground-state acid :base ratio, because of the difference in their absorption coefficients.However, mea- surements with 308 nm laser excitation = 2.1, for 3-PPK, and 2.3, for 4-PPK) and 347 nm laser excitation 283 (Eacid/&base = 1.1, for 3-PPK, and 1.5, for 4-PPK) gave similar results. This, generally, denotes equilibrium attainment during the excited-state lifetime. If this is the case, the inflec- tion points in the T, -+ T, titrations (for 3-PPK, at pH = 3.7 and 1 .O; for 4-PPK, at pH = 4.1) could be taken as a measure of the pK*s.The occurrence of the inflection points at higher pH values in the phosphorescence titration curves than in the triplet- triplet absorption titrations indicates that the protonated forms are shorter-lived species than the basic ones, in the excited state.” From the shift between the phosphorescence titration curve and the T, -,T, titration curve, the lifetime ratio (z,,id/zbase) can be obtained on the basis of eqn. (1). This ratio is less than 0.02 for the 3-PPK (first inflection point, pK* = 3.7 & 0.2 from the laser measurements) and less than 0.001 for the 4-PPK. For the second protonation step of 3-PPK (pK* = 1.0 f0.5, from the T-T absorption titration), the inflection point was determined with less precision in both phosphorescence and triplet-triplet titrations, because of the weakness of the signals and, therefore, the small difference between the two titrations (0.5 pH units) can only be con-sidered as indicative of similar lifetime of the equilibrating species.Lifetime data did not fit the Ware’* and/or Wyatt” equa- tions which would be an alternative way to determine the excited state pK*. This may indicate either the occurrence of multiple acid-base equilibria or non-attainment of the equi- librium in the excited state. Transient Assignments To assign the absorptions, observed under laser excitation, to the triplet species which were involved in the acid-base equi- libria, the principal information was derived from the com- parison of phosphorescence emission with laser data and from charged-ion quenching experiments.The first point is that room-temperature phosphorescence will come exclusively from triplet n,n* excitation localized on the carbonyl. When the carbonyl protonation occurs, the n,x* emission is expected to disappear, as is also observed for benz~phenone.’.’~ In addition, two emitting species, the neutral molecule and the pyridinium cation, are expected because of the presence of the nitrogen atom. Thus, for the 3-PPK, the two phosphorescence emissions, shown in Fig. 1, are assigned to the carbonyl triplet (n,n*) of the neutral molecule (in alkaline solution) and to the triplet (n,n*) of the pyridinium cation (recorded at pH = 3).Analo-gously, the T, +Tn absorption spectra, detected in these pH regions [spectra (a) and (b) in Fig. 21, are assigned to the neutral triplet and to the nitrogen-protonated triplet, respec- tively. These species are characterized by triplet lifetimes in the ratio z,/q, x 0.4, which greatly exceeds that found from the relative shifts of the emission and absorption titration curves (z,/zb < 0.02). Consequently, the excited-state pK* determined from the titration curves does not refer to equi- librium establishment between the triplet molecule and the triplet pyridinium cation. The triplet molecules equilibrate with a shorter-lived species (not revealed by absorption or emission spectroscopy), which we assign to the carbonyl protonated cation.When the pyridinium cation is directly excited (ground-state pK = 3), the positive charge on the nitrogen atom opposes carbonyl protonation. Thus the absorption and emission signals from this species are con- stant within a limited pH range (2-3). Further decrease of pH induces carbonyl protonation and the last absorption spec- trum in Fig. 2 (pH = 0.5) corresponds to the dication, which does not show room-temperature phosphorescence emission. 284 In order to ascertain the cationic nature of the transients observed in acidic solution and to decide whether the two bands observed around pH x 3 for the 3-PPK belong to the same species, quenching experiments were performed using a negatively charged ion, Cr(CN);-, as a quencher.The triplet energy of this anion (ET= 160 kJ mol-') is lower than that of triplet ketones (see Table 1) so as to ensure diffusional energy transfer. Based on the Debye equation [eqn. (3)],19 k:irf = (BNk, T/3000q)a[exp(a) -13-' (3) a = ZDZQe2/r&kB (34T when both the donor (D) and quencher (Q) are ionic species, the diffusional constant depends on the product of their charge, ZDZ,. N and k, are the Avogadro's and Boltzmann's constants, e is the electron charge, q and E are the viscosity and relative permittivity of the medium and r is the encoun- ter distance. The influence of the ionic strength, p, on the rate constant can be evaluated by the Debye-Brmsted equation (4) log kdiff = log k,Oiff + 1*02Z,z, JPI(1 + JP) (4) Quenching measurements of the triplet lifetime of 3-PPK (D) by Cr(CN):- (Q) were carried out at the three pH values which corresponded to the neutral, mono- and di-cationic species, on the basis of the above assignment.Quencher con- centrations were adjusted to have at least 50% of quenching at each pH value. Linear Stern-Volmer plots of T-' us. [Q] were obtained. From the data reported in Table 2, it can be seen that, for the 3-PPK, the quenching rate constant increases on going from neutral solution (k = 2.9 x lo8 dm3 mol-' s-') to pH = 2.7 (k = 4.8 x lo9 dm' mol-' s-') and then decreases again when pH 0 is approached (k, = 1.3 x lo7dm3 mol- ' s-'). It is worthwhile noting that the same rate parameter was found for the 505 and 760 nm bands at pH = 2.7.These results are consistent with the prevalent presence, depending on the acidity of the solution, of a neutral species (pH = 7), a mono-cationic species (pH x 3) and a di-cationic species (pH x 0.5). The quenching rate parameter of the neutral triplet transient is somewhat lower than expected from a diffusion-controlled process (6.6 x lo9 dm3 mol-' s-') but it is of the same order of magnitude as those found with other organic molecules as donors.20 Non- diffusional k, values were attributed to shielding of the ionic quencher by polar solvent molecules.20 The value measured at pH = 3 is higher because of the attractive electrostatic force between PPKH' and the inorganic anion and is in good agreement with the calculation from eqn.(3) and (4) (kdiff = 5.9 x 109 dm3 mol-' s-', at the experimental p = 0.011), when an encounter distance of 15 A is assumed. Even though the electrostatic interaction increases with the di-cation, the rate constant decreases because of the ionic strength effect in the strongly acidic medium (p= 0.79). The calculation for ZDZQ = -6 leads to kdiff = 2.6 x i07 dm3 mol-' s-'. The agreement with the experimental value is satisfactory, considering the approximations inherent in the calculation and the large difference with the calculated value of the mono-cation interaction in this medium (ZDZQ= -3, kdiff = 4.5 x 108 dm3 mol-' s-I). For 4-PPK, the phosphorescence measurements indicate the presence of a unique emitting species, which we identify as the triplet of the neutral molecule.Quenching experiments, carried out at four pH values with Cr(CN);-as a quencher, did not show any particular evi- dence of the presence of cationic species for 4-PPK. The quenching rate parameter measured [k, = (3.6 f0.2) x lo* dm3 mol-' s-'1, which is very close to that found for the neutral triplet of the 3-PPK, is constant, at constant ionic J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 strength (p= 0.011), in the pH range 4-10. Thus, it can be concluded that it always refers to the unprotonated triplet molecule. This does not mean that protonation does not occur, but, owing to the very short lifetime of the protonated form(s), only the neutral species is quenched, even if it is present as a very small fraction at the lowest pHs explored.Therefore, the T, + T, absorption spectra, which could be observed within the same pH limits, also belong to the uncharged triplet species. The negligible differences observed in the spectral distribution (Fig. 4) are within the measure- ment uncertainty. Triplet lifetime and absorption intensity are reduced because of interaction with the proton as the acidity increases. If we recall that the lifetime difference between neutral and protonated forms, evaluated from the shift between the phosphorescence and triplet-triplet absorp- tion titration curves, is larger than three orders of magnitude, T,,id should be less than 0.004 ps Thus, the triplet of the protonated form cannot be revealed either from quenching measurements or from its absorption spectrum with our experimental set-up.The evidence of only one distinct emis- sion spectrum, which became undetectable below pH = 4, not only denotes that protonation occurs first on the car- bony1 in the excited state, but also that it is immediately fol- lowed by nitrogen protonation, since no pH value could be found where the pyridinium mono-cation could be detected. Double protonation should be easier for 4-PPK than for 3-PPK, due to the larger distance between the two proto- nation sites. Thus, owing to the closeness of the two pK* values, it was impossible to separate the two protonation steps. Therefore, the inflection point in the titration curve, which is due to two overlapping protonation steps, probably does not correspond to a real pK*. The comparison with benzophenone on the basis of eqn.(2) points to a marked difference in lifetime of the acidic forms (TB > T3-ppK % Tq-ppK). Concluding Remarks The results of this work demonstrate further that the excited triplet carbonyl is a much more basic species than the ground state. For both these molecules, the nitrogen effect is that of enhancing the basicity of the triplet carbonyl compared with benzophenone. This result was unexpected since an electron- withdrawing group, such as pyridine, should decrease the pK*. However, an analogous behaviour was found for the dissociation constants of the ketyl radicals derived from PPKs with respect to the diphenylketyl radical.21 The increased basicity of benzophenone upon excitation was attributed to the intramolecular charge-transfer character of the T, state in polar media.22 In the present cases, charge transfer from the pyridyl ring to the carbonyl should be more efficient than that from the phenyl ring as previously pro- posed for the di-pyridyl ketone^.^ The stabilization of n,n* states on introducing the nitrogen atom in pyridyl ketones compared with benzophenone, that can be deduced from the bathochromic shift of the n,n* transitions, supports this inter- pretation.These two molecules exhibit very similar photophysical properties in organic solvents as well as in alkaline aqueous solutions. In contrast, they are well differentiated in acidic media where the protonated species could be spectrally dis- criminated for the 3-isomer only.The very short lifetime pre- vents any spectral evidence of the 4-PPK acidic form from being obtained by either emission and absorption spectros- copy (nanosecond time resolution) and, probably, makes it impossible to obtain acid-base quilibrium in the excited state. We thank Prof. Riccieri for the sample of K,Cr(CN),. The financial support of the Italian Consiglio Nazionale delle J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Ricerche and Minister0 per l'Universita e la Ricerca Scienti- fica is gratefully acknowledged. References 1 P. Traynard and J. P. Blanchi, Mol. Photochem., 1972,4,223. 2 J. P.Blanchi and A. R. Watkins, Mol. Photochem., 1974,6, 133. 3 C. R. Hurt and N. Filipescu, J. Am. Chem. SOC., 1972,94,3649. 4 G. Favaro, J. Chem. SOC., Perkin Trans. 2, 1976, 869. 5 G. Favaro and F. Masetti, J. Phys. Chem., 1978,82, 1213. 6 G. Favaro, F. Masetti and A. Romani, Spectrochim. Acta, Part A, 1989,45339. 7 A. Romani, F. Elisei, F. Masetti and G. Favaro, J. Chem. Soc., Faraday Trans., 1992,88,2147. 8 G. Favaro, J. Photochem., 1986,33,261. 9 S. Monti, N. Camaioni and P. Bortolus, Photochem. Phorobiol., 1991,54,325. 10 M. B. Ledger and G. Porter, J. Chem. Soc., Faraday Trans. I, 1972,683,539. 11 R. V. Bensasson and J. C. Gramain, J. Chem. Soc., Faraday Trans. I, 1980,77, 1801. 285 12 D. M. Rayner and P. A. H. Wyatt, J. Chem. Soc., Faraday Trans. I, 1974,70,945. 13 A. Fischer, B. A. Grigor, J. Packer and J. Vaughan, J. Am. Chem. Soc., 1961, 83, 4208. 14 T. G. Bonner and J. Phillips, J. Chem. SOC. B, 1966,650. 15 J. F. Ireland and P. A. H. Wyatt, Ado. Phys. Org. Chem., 1976, 12, 1931. 16 A. Samanta, N. Chattopadhyay, D. Nath, T. Kundu and M. Chowdhry, Chem. Phys. Ltt.,-1985,121, 507. 17 G. Favaro and G. Bufalini, J. Phys. Chem., 1975,80,800. 18 W. R. Ware, D. Watt and J. D. Holmes, J. Am. Chem. Soc., 1974, 96,7853. 19 P. Debye, Trans. Electrochem. Soc., 1942,82,265. 20 H. F. Wasgestian and G. S. Hammond, Theor. Chim. Acta, 1971, 20, 186. 21 D. A. Nelson and E. Hayon, J. Phys. Chem., 1972,76,3200. 22 H. Shizuka and E. Kimura, Can. J. Chem., 1984,62,2041. 23 H. Shizuka and E. Kimura, Can. J. Chem., 1984,62,2041. Paper 3/03114D;Received 1st June, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000279

出版商:RSC    

年代:1994

数据来源: RSC