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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 Notti ng 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) Dr. 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 Staff Editor: Dr.R. A. Whitelock Senior Assistant Editor: Mrs. S. Shah Assistant Editors: Dr. L. Milne, Mrs. C. J. Seeley Editorial Secretary: Mrs. J. E. Gibbs Inter nat iona I Advisory Editorial Board R. S. Berry (Chicago) Y. Marcus (Jerusalem) A. M. Bradshaw (Berlin) B. J. 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ISSN:0956-5000    

DOI:10.1039/FT99490FX041

出版商:RSC    

年代:1994

数据来源: RSC

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Edited by P.K. Datta, University of Northumbria at Newcastle J.S. Gray, University of Northumbria at Newcastle This title is published in three highly illustrated volumes and covers the latest developments in the subject. It provides a unique combination of the science of coatings and surfaces, the technologies of deposition, surface modification and analysis, and practical applications. The three volumes provide a useful and comprehensive blend of reviews and state-of-the-art papers, written by international experts, and reflect the current status of and likely future advances in surface engineering. Surface Engineering Fundam en tal s of CoatingsVolume 1: This volume considers principles of coating/substrate design in aqueous and high temperature corrosion, and wear properties, scanning the coatings spectrum from organic, through metallic to ceramic.The emphasis in this volume is on the sciencand design of coatings and substrate systems rather than on technology. Hardcover ISBN 0 85186 665 4 (1993) xvi + 370 pages Price f52.50 Surface 11:Volume Engineering Engineering Applications Volume II is dedicated to topics concerning the performance of coatings and surface treatments embracing four main areas: the inhibition of wear and fatigue; corrosion control; application of coatings in heat engines and machining; and qualities and properties of coatings. Hardcover ISBN 0 85186 675 1 (1993) xvi + 342 pages Price f52.50 Surface Engineering Process Technology and Surface AnalysisVolume 111: Volume Ill has two thrusts as indicated by its title: process technology and surface analysis.Both areas are central to surface engineering and each holds particular promise, not only for improvement in existing types of coatings performance, but also in the design, development and evaluation of totally new coating/substrate systems. Hardcover ISBN 0 851 86 685 9 (1 993) xvi + 312 pages Price f52.50 Special Package Price (Volumes 1-111) €1 40.00 -save over 1o%! Make sure you order without delay To order, please contact: Turpin Distribution Services Ltd., Blackhorse Road, Letchworth, Herts SC6 7 HN, United Kingdom Tel: +44 (0)462 672555 Fax: +44 (0)462 480947 ROYAL RSC members should obtain members’ prices and order from: Membership Administration ISTRY Royal Society of Chemistry Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF,information United Kingdom Services Tel: +44 (0) 223 420066 Fax: +44 (0)223 423623 Editors: R E Hester, University of York, UK R M Harrison, University of Birmingham, UK A new series fo tackle imporfanf environmental issues.. . In response to the rapid rowth of interest in the environment and tae acute need for concise, authoritative and up-to-date reviews of topical issues, the Royal Society of Chemistry is launching Issues in Environmental Science and Technology. This new series will publish review articles on to ics of global concern, written b worlsexperts in their specialized fie1 cys. It will present a multidisciplinary approach to pollution and environmental science and in addition to covering the chemistry of environmental processes, will focus on broader issues such as economic, legal and poIit ica I considerations.Issues in Environmental Science and Technology will review the effects on human and non-human biota of man-made substances and will provide assessments of the possible practical solutions to perceived environmental problems, including the worldwide efforts currently underway to establish ‘Clean Technologies’ . Who will be reading Issues in Environm6hfaI Science and Technology? ‘Issues’ will prove invaluable for scientists and engineers in industry, public service, consuItancy and academic institu tions, who wish to keep up-to-date on topical subjects in this emotive field.It will also be essential reading for students taking specialized courses in environmental chemistry and will provide excellent supplementary reference material for general science courses. Each issue will address a different theme and will contain approximately six articles, most of which will be specially commissioned by the editors. The first two issues will cover: Waste Incineration 2 and the Environment lSBN 0 85404 200 8 April 7994 lSBN 0 85404 205 9 July 1994 Price f15.00 Price f75.00 Also Available ISSN 1350-7583 Published twice yearly from 1994 on subscription. .. EC f25.00 USA $47.00 Canada f28.00+ GST Rest of World f27.00 ROYAL To order please contact: SOCIETY OF Turpin Distribution Services Ltd, Blackhorse Road, Letchworth I-CHEMISTRY Herts SG6 1HN, United Kingdom Tel: +44 (0) 462 672555. Fa;: +44 (0)462 480947 RSC Members should order from: Membership Administration, Royal Socie of ChemistryThomas Graham House, Science Park, Mkm Road Information Cambrid e CB4 4WF, United Kingdom 0956-5000C1994311:1-3 Services Tel: +44 70)223 420066. Fax: +44 (0)223 423623

ISSN:0956-5000    

DOI:10.1039/FT99490BX043

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(11) 1467-1557 (1994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 1467 Collisional behaviour with Ar of the A doublets of CH(X 'JI) N" = 15 produced in the two-photon dissociation of CH,CO at 279.3 nm S. M. Ball, G. Hancock and M. R. Heal 1473 Ultra-low temperature kinetics of neutral-neutral reactions : Rate constants for the reactions of OH radicals with butenes between 295 and 23 K I. R. Sims, I. W. M. Smith, P. Bocherel, A. Defrance, D. Travers and B. R. Rowe 1479 Configuration interaction study of the 0,-C,H, exciplex: Collision-induced probabilities of spin-forbidden radiative and non-radiative transitions B. F. Minaev, V. V. Kukueva and H. Agren 1487 Grand canonical Monte Carlo study of Lennard-Jones mixtures in slit pores.Part 3.-Mixtures of two molecular fluids: Ethane and propane R. F. Cracknell and D. Nicholson 1495 Electroanalytical/X-ray photoelectron spectroscopy investigation on glucose oxidase adsorbed on platinum G. E. De Benedetto, C. Malitesta and C. G, Zambonin 1501 Normal and anomalous positronium states in ionic and molecular solids investigated cia magnetic field effects T. Goworek, A. Badia and G. Duplatre 1507 Host-guest complexes of cucurbituril with the 4-methylbenzylammonium ion; alkali-metal cations and NH,+ R. Hofhnann, W. Knoche, C. Fenn and H-J. Buschmann 1513 General thermodynamic analysis of the dissolution of non-polar molecules into water. Origin of hydrophobicity M. Costas, B. Kronberg and R.Silveston 1523 Micellar aggregates of sodium glycocholate and sodium taurocholate and their interaction complexes with bilirubin- 1x3. Structural models and crystal structure M. D'Alagni, L. Galantini, E. Giglio, E. Gavuzzo and L. Scararnuzza 1533 Quartz crystal microbalance study of the adsorption of ions onto gold from non-aqueous solvents A. P. Abbott, D. C. Loveday and A. R. Hillman 1537 Glass transition of liquid-crystalline 4-alkoxyphenyl and 4-cyanophenyl 4-(2,4-dialkoxybenzoyloxy) benzoates S. Takenake and H. Yamasu i54i Catalytic combustion of methane: Copper oxide supported on high-specific-area spinels synthesized by a sol-gel process N. Guilhaume and M. Prirnet 1547 Two members of the ABC-D6R family of zeolites: Zeolite phi and Linde D K.P. Lillerud, R. Szostak and A. Long 1553 Book reviews: P. C. Jurs; H. A. C. McKay; J. Penfold; J. Lee; J. Vigue; J. Maher Note: Where an asterisk appears against the name of one or more of the authors, it is included with the authors' approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles. For hrther details contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN, UK Tel: +44 (0)71-437 8656 Fax: +44 (0)71-287 9798 Telecom Gold 84: BUR210 Electronic Mailbox (Internet) LIBRARY@RSC.ORG. If the material is not available from the Society’s Library, the staff will be pleased to advise on its availability from other sources. Please note that copies are not available from the RSC at Thomas Graham House, Cambridge.

ISSN:0956-5000    

DOI:10.1039/FT99490FP103

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aas,N., 1015 Abbott, A. P., 1533 Afanasiev, P., 193 bigren, H., 1479 Aikawa, M., 911 Aitken, C. G., 935 Akanuma, K., 1171 Akolekar, D. B., 1041 Albery, W. J., 1115 Aldaz,A., 609 Alfimov, M. V., 109 Al-Ghefaili, K. M., 383, 1047 Bruna, P. J., 683 Brzezinski, B., 843, 1095 Buckley, A. M., 1003 Buemi, G., 1211 Burdisso, M., 1077 Busca, G., 1161,1293 Buschmann, H-J., 1507 Butt, M. D., 727 Byatt-Smith, J. G., 493 Cabaleiro, M. C., 845 Caceres, M., 1217 Caceres Alonso, M., 553 Cairns, J. A., 1461 Dunmur, D. A., 1357 Dunstan, D. E., 1261 Duplstre, G., 1501 Duxbury, G., 1357 Dwyer, J., 383, 1047 Dyke, J. M., 17 Eastoe, J., 487 Easton, C. J., 739 Ebitani, K., 377 Egsgaard, H., 941 El-Atawy, S., 879 El Baghdadi, A., 1313 Elisei, F., 279 Harper, R.J., 659 Harriman, A,, 697,953 Harris, K. D. M., 1313, Harrison, N. J., 55 Haruta, M., 1011 Hashimoto, K., 1177 Hashino, T., 899 Hattori, H., 803 Haymet, A. D. J., 1245 Heal, M. R., 523, 1467 Healy, T. W., 1251 Heenan, R. K., 487 1323 Knoche, W., 1507 Knozinger, H., 1335 Kobayashi, A,, 763 Kobayashi, H., 763 Kobayashi, T., 1011 Kondo, Y., 121 Kossanyi, J., 41 1 Kronberg, B., 1513 Kukueva, V. V., 1479 Kurrat, R., 587 Kusalik, P. G., 1405 Kuwamoto, T., 121 Laachir, A,, 773 Ali, V., 579, 583 Aliev, A. E., 1323 Allegrini, P., 333 Allen, N. S., 83 A1 Rawi, J. M. A,, Amorim da Costa, A. M., Amoskov, V. M., 889 Ando, M., 1011 845 689 Calado, J. C. G., 649 Caldararu, H., 213 Calvente, J. J., 575 Calvo, E. J., 987 Camacho, J. J., 23 Cameron, B. R., 935 Campa, M. C., 207 Campos, A., 339 Canosa-Mas, C.E., 1197, Elliot, A. J., 831, 837 Engberts, J. B. F. N., Enomoto, N., 1279 Eustaquio-Rincon, R., 113 Ewins, C., 969 Fantola Lazzarini, A. L., Fausto, R., 689 Favaro, G., 279,333 727 423 Helmer, M., 31, 395 Herein, D., 403 Herod, A. A., 1357 Herrmann, J-M., 1441 Herzog, B., 403 Heyes, D. M., 1133 Higgins, S., 459 Hillman, A. R., 1533 Hindermann, J-P., 501 Lajtar, L., 1153 Lambert, J-F., 667, 675 Lamotte, J., 1029 Langan, J. R., 75 Lavalley, J-C., 1023, 1029 Lavanchy, A,, 783 Lazar, K., 1329 Lazzarini, E., 423 Leaist, D. G., 133, 1223 Andrews, S. J., 1003 1205 Feliu, J. M., 609 Hirst, D. M., 517 Lee, J., 1553 Anson, C. E., 1449 Aragno, A., 787 Arai, S., 1307 Capobianco, J. A., 755 Caragheorgheopol, A., 213 Carlile, C.J., 1149 Fenn, C., 1507 Filimonov, I. N., 219, 227 Flint, C. D., 1357 Hiyane, I., 973 Hoekstra, D., 727 Hoffmann, R., 1507 Legon, A. C., 1365 Lei, G-D., 233 Lerner, B. A., 233 Aramaki, K., 321 Aravindakumar, C. T., 597 Carlsen, L., 941 Carvill, B. T., 233 Fogden, A., Fornes, V., 263 213 Holmberg, B., 559 Holz, M., 849 Leslie, M., Li, J., 39 641 Asai, Y., 797 Castaiio, R., 1227 Fracheboud, J-M., 1197, Hoshino, H., 479 Li, P., 605 Ashfold, M. N. R., 1357 Castro, S., 1217 1205 Hosoi, K., 349 Li, X., 1429 Asmus, K-D., 1391 Catalina, F., 83 Franck, R., 667,675 HSU, J-P., 1435 Li, Y., 947 Avila, V., 69 Baba,T., 187 Badia, A., 1501 Badri, A., 1023 Bagatti, M., 1077 Ball, M. C., 997 Cataliotti, R. S., 1397 Cavasino, F.P., 3 11 Ceccarani, M. L., 1397 Chang, T-h., 11 57 Charlesworth, P., 1073 Chen, J-S., 429, 717 Freeman, N. J., 751 Frety, R., 773 Frey, J. G., 17, 817 Frostemark, F., 559 Fujiwara, Y., 1183 Galantini, L., 1523 Hungerbuhler, H., 1391 Hutchings, G. J., 203 Hutton, R. S., 345 Iizuka, Y., 1301, 1307 Ikawa, S-i., 103 Ikonnikov, I. A., 219 Liang, Y., 1271 Lillerud, K. P., 1547 Lin, J., 355 Lincoln, S. F., 739 Lindblom, G., 305 Liu, B-T., 1435 Ball, S. M., 523, 1467 Baonza, V. G., 553 Baonza, V. G., 1217 Barbaux, Y., 895 Barthomeuf, D., 667,675 Basini, L., 787 Bassoli, M., 363 Chen, Y-H., 617 Cheng, A., 253 Cheng, C. P., 1157 Cherqaoui, D., 97 Chesta, C. A., 69 Chevalier, S., 667, 675 Chmiel, G., 11 53 Gandolfi, R., 1077 Gans, P., 315 Gao,Y., 803 Garcia, R., 339 Garcia Fierro, J-L., 1455 Garcia-Paiieda, E., 575 Gautam, P., 697 Ilczyszyn, M., 1411 Indovina, V., 207 Inoue, Y., 797,815 Ishiga, F., 979 Ishigure, K., 93, 591 Isoda, T., 869 Ito, O., 571 Liu,C-W., 39 Liu,X., 249 Loginov, A.Yu., 219,227 Lohse, U., 1033 Long, A., 1547 Longdon, P. J., 315 Lorenzelli, V., 1293 Battaglini, F., 987 Bauer, C., 517 Cho, T., 103 Christensen, P., 459 Gavuzzo, E., Geantet, C., 1523 193 Iwasaki, K., 121 Jacobs, W. P. J. H., 1191 Loveday, D. C., Lu, J-X., 39 1533 Bell, A. J., 17,817 Belton, P. S., 1099 Bender, B. R., 1449 Climent, M. A., 609 Coates, J. H., 739 Colmenares, C. A., 1285 Gengembre, L., 895 Giglio, E., 1523 Gil, A. M., 1099 Jakubov, T., 783 Jameel, A. T., 625 Janchen, J., 1033 Lunelli, B., 137 Ma, J., 1351 Mabuchi, M., 899 Bendig, J., 287 Bengtsson, L.A., Benko, J., 855 559 Cordischi, D., 207 Coma, A., 213 Cormier, G., 755 Gil, F. P. S. C., 689 Gilchrist, J., 1149 Gill, D. S., 579, 583 Jayakumar, R., 161 Jayasooriya, U. A., 1265 Jenneskens, L. W., 327, Machado, V. G., 865 Mackie, J. C., 541 Mackintosh, J. G., 1121 Benniston, A. C., 953 Bensalem, A,, 653 Birces, T., 41 1 Bergeret, G., 773 Beutel, T., 1335 Beyer, H. K., 1329 Bickelhaupt, F., 327, 1363 Biczok, L., 411 Biggs, P., 1197, 1205 Binet, C., 1023 Black, S. N., 1003 Corradini, F., 859, 1089 Corrales, T., 83 Cosa, J. J., 69 Costas, M., 1513 Cottier, D., 1003 Coudurier, G., 193 Courcot, D., 895 Cracknell, R. F., 1487 Crawford, M. J., 817 Cruzeiro-Hansson, L., 1415 Cullis, P. M., 727 Gill, J.B., 315 Goede, S. J., 327, 1363 Gomez, C. M., 339 GonGalves da Silva, A. M., Goodfellow, J. M., 1415 Gouder, T. H., 1285 Goworek, T., 1501 Gray, P. G., 369 Green, W. A., 83 Grein, F., 683 649 Jennings, B. J., 55 Jiang, D-z., 1351 Jiang, P-Y., 591 Jiang, P. Y., 93 Jobic, H., 1191 Johansson, L. B.-bi., 305 Johari, G. P., 883, 1143 John, S. A., 1241 Joseph, E. M., 387 Joshi, P. N., 387 1363 Macpherson, A. N., 1065 Madariaga, J. M., 1227 Maeda,T., 899 Maestre, A., 575 Maginn, S. J., 1003 Maher, J., 1553 Mahy, J. W. G., 327,1363 Maity, D. K., 703 Makarova, M. A., 383, Maksymiuk, K., 745 1047 Blackett, P. M., 845 Blanco, S., 1365 Blandamer, M. J., 727 Blower, C., 919,931 Bocherel, P., 1473 Boddenberg, B., 1345 Boggis, S. A,, 17 Borge, G., 1227 Borisenko, V. N., 109 Boutonnet-Kizling, M., 1023 Curtis, J.M., 239 DAlagni, M., 1523 Dang, N-T., 875 Danil de Namor, A. F., 845 Das, T. N., 963 Dasannacharya, B. A., 1149 Davey, R. J., 1003 Davidson, K., 879 De Benedetto, G. E., 1495 Defrance, A,, 1473 Demeter, A., 41 1 Grieser, F., 1251 Grifith, W. P., 1105 Grimshaw, J., 75 Grzybowska, B., 895 Guelton, M., 895 Guilhaume, N., 1541 Guillaume, F., 13 13 Guldi, D. M., 1391 Gulliya, K. S., 953 Hachey, M., 683 Haeberlein, M., 263 Jurs, P. C., 1553 Kagawa, S., 349 Kakuta, N., 1279 Kaler, E. W., 471 Kalugin, 0.N., 297 Karge, H. G., 1329 Kato, R., 763 Katsumura, Y., 93,591 Kaur,T., 579 Kawashima, T., 127 Keil, M., 403 Malatesta, V., 333 Malcolm, B. R., 493 Malitesta, C., 1495 Mallon, D., 83 Mandal, A. B., 161 Marcheselli, L., 859 Marchetti, A., 859, 1089 Mariani, M., 423 Martins, A,, 143 Maruya, K-i., 9 11 Masetti, F., 333 Bowker, M., 1015 Bozon-Verduraz, F., 653 Dempsey, P., 1003 Demri, D., 501 Hall, D.I., 517 Hall, G., 1 Kemball, C., 659 Kessel, D., 1073 Mason, R. S., Massucci, M., 1373 445 Bradley, C. D., 239 Bradshaw, A. M., 403 Braun, B. M., 849 Breysse, M., 193 Briggs, B., 727 Brocklehurst, B., 271 Brogan, M. S., 1461 Brown, N. M. D., 1357 Derrick, P. J., 239 Dewing, J., 1047 Diagne, C., 501 Dickinson, E., 173 Dines, T. J., 1461 Doblhofer, K., 745 Domen, K., 911 Dossi, C., 1335 Hallbrucker, A., 293 Halpern, A., 721 Hamnett, A., 459 Hancock, G., 523, 1467 Handa, H., 187 Hann,K., 733 Hao, L., 133, 1223 Harada, S., 869 Kida, I., 103 Kiennemann, A., 501 Kim, J-H., 377 Kimura, M., 1355 King, F., 203 Kirschner, J., 403 Kita, H., 803 Klein, M.L., 253 MatijeviC, E., 167 Matsuda, J., 321 Matsumura, Y., 1177 May, I. P., 751 Mauucato, U., 333 McGilvery, D., 1055 Mchedlov-Petrossyan, N. O., 629 Brown, R. G., Brown, S. E., 59 739 Doughty, A., 541 Douglas, C. B., 471 Haraoka, T., 911 Harland, P. W., 935 Kleshchevnikova, V. N., 629 McKay, H. A. C., McNaughton, D., 1553 1055 1 Meadows, G., 1429 Medforth, C. J., 1073 Medina, F., 1455 Melrose, J. R., 1133 Merga, G., 597 Meunier, F., 369 Ottavi, G., 333 Ouellette, D. C., 837 Owari, T., 979 Ozutsumi, K., 127 Padley, M. B., 203 Pais, A. A. C. C., 1381 Rosseinsky, D. R., 1127 Rossi, P. F., 363 Rout, J. E., 1003 Rouvet, F., 1441 Rowe, B. R., 1473 Rudham, R., 809 Sueiras, J-E., 1455 Sun, L.M., 369 Sun,T., 1351 Suquet, H., 667,675 Surov, Y. N., 297 Suzuki, T., 549 Varandas, A. J. C., 1381 Vedrine, J. C., 193 Venanzi, M., 435 Vigue, J., 1553 Villamagna, F., 47 Villemin, D., 97 Mezyk, S. P., 831 Mills, A., 1429 Milton, D. M. P., 1373 Pal, H., 711 Pal-Borbkly, G., 1329 Palleschi, A., 435 Ryde,N., 167 Sacco, A., 849 Sachtler, W. M. H., 233, Svishchev, I. M., 1405 Szostak, R., 1547 Tabata, M., 1171 Visscher, P. B., Vlietstra, E. J., Vollarova, O., 1133 327, 1363 855 Min, E-z., 1351 Paradisi, C., 137 1335 Tabrizchi, M., 17 Vollmer, F., 59 Minaev, B. F., 1479 Misono, M., 1183 Mitchell, P. J., 1133 Mittal, J. P., 597, 703,711, Miyake, Y., 979 Mizuno, N., 1183 Mizushima, T., 1279 Moffat, J. B., 1177 Mohan, H., 597,703 Monk, P.M. S., 1127 825 Pardo,A., 23 Parry, A. J., 1373 Parsons, B. J., 83 Patel, S. G., 1083 Pathmanathan, K., 1143 Patrykiejew, A,, 1153 Paul, D. K., 1271 Pavanaja, U. B., 825 Pedulli, G. F., 137 Peters, M. P. J., 1033 Penfold, J., 1553 Saitoh, T., 479 Salagre, P., 1455 Salmon, G. A., 75 Sam,D. S. H., 1161 Sanada, M., 1307 Sano,T., 869 Sapre, A. V., 825 Sarre, P. J., 517 Sassi, P., 1397 Sato, K., 797 Saur, O., 1029 Tagliazucchi, M., 859, 1089 Takagi, T., 121 Takahashi, K., 155 Takasawa, A., 911 Takenake, S., 1537 Tamaura, Y., 1171 Tamura, K-i., 533 Tanaka, I., 349 Tanigaki, H., 1307 Taravillo, M., 1217 Tassi, L., 859, 1089 Volta, J-C., 1161, 1441 Vyunnik, I. N., 297 Wales, D. J., 1061 Wang, C. F., 605 Wang, J., 1245 Watanabe, H., 571 Waters, M., 727 Wayne, R.P., 1197,1205 Weckstrom, K., 733 Weingartner, H., 849 Weir, D. J., 751 Mordi, R. C., 1323 Moriguichi, I., 349 Morikawa, A., 377 Morioka, Y., 1279 Morokuma, M., 377 Morrison, C. A., 755 Mount,A. R., 1115,1121 Peng, W., 605 Pepe, F., 905 Pereira, C. M., 143 Ptrez, J. M., 609 Perrichon, V., 773 Peter, L. M., 149 Petrov, N. Kh., 109 Sbriziolo, C., 311 Scaramuzza, L., 1523 Schedel-Niedrig, Th., 403 Schlogl, R., 403 Schnabel, W., 287 Scremin, M., 865 Seddon, B. J., 605 Tateno, A., 763 Tatham, A., 1099 Taylor, A., 1003 Taylor, M. G., 641 Teixeira-Dias, J. J. C., Teo, W. K., 355 Teramoto, M., 979 689 Werner, H., 403 Whitaker, B. J., 1 White, L. R., 1251 Whitehead, M. A., 47 Wikander, G., 305 Wilde, C. P., 1233 Wilhelm, M., 1391 Muir, A. V. G., 459 Mukherjee, T., 711 Mukhopadhyay, R., 1149 Nagaishi, R., 93, 591 Nagaoka, H., 349 Naito, S., 899, 1355 Naito, T., 763 Nalewajski, R.F., 1381 Navaratnam, S., 83 Neoh, K. G., 355 Nerukh, D. A,, 297 Nicholson, D., 181, 1487 Nickel, U., 617 Ninomiya, J., 103 Nishihara, H., 321 Nogami, T., 763 Nonaka, O., 121 Norton, J. R., 1449 Nuiiez, J., 1217 Nuiiez Delgado, J., 553 Nyholm, L., 149 Occhiuzzi, M., 207,905 Ohji, N., 1279 Ohtsu, K., 127 Okamura, A., 803 Olazabal, M. A., 1227 Olejnik, J., 1095 Oliveri, G., 363 Onishi, T., 911 Ono,Y., 187 Oradd, G., 305 Ortica, F., 279 Oswal, S. L., 1083 Ota, K-i., 155 Otlejkina, E. G., 297 Otsuka, K., 451 Pispisa, B., 435 Pivnenko, N. S., 297 Plane, J. M. C., Plowman, R., 1003 Porcar, I., 339 Potter, C. A. S., 59 Powell, D. B., 1449 Poyato, J.M. L., 23 Prenosil, J. E., 587 Previtali, C. M., 69 Primet, M., 1541 Pringle, T. J., 1015 Priyadarsini, K. I., 963 Pryamitsyn, V. A., 889 Psaro, R., 1335 Rabold, A., 843 Ramaraj, R., 1241 Rama Rao, K. V. S., Ramis, G., 1293 Ramsden, J. J., 587 Rao, B. S. M., 597 Rastelli, A., 1077 Rehani, S. K., 583 Rettig, W., 59 Rey,F., 213 Rezende, M. C., 865 Rhodes, N. P., 809 Ricchiardi, G., 1161 Richter, R., 17 Robertson, E. G., 1055 Rocha, M., 143 Rochester, C. H., 203 Rodes,A., 609 Rofiia, S., 137 Rosenholm, J. B., 733 Rosmus, P., 517 31, 395 825 Seidel, A., 1345 Sellen, D. B., 1357 Shahid, G., 507, 513 Shallcross, D. E., 1197, Sharma, A., 625 Shaw, N., 17,817 Sheil, M. M., 239 Shen, J-p., 1351 Sheppard, N., 507, 513, Sherwood, P. M. A., 1271 Shiao, J-C., 429 Shihara, Y., 549 Shiralkar, V. P., 387 Shishido, T., 803 Shizuka, H., 533 Siders, P., 973 Silva, C.J., 143 Silva, F., 143 Silveston, R., 1513 Simkiss, K., 641 Sims, I. R., 1473 Singh, J., 579,583 Singh, R., 583 Smart, S. P., 1313 Smith, I. W. M., 1473 Smith, K. M., 1073 Smith, T. D., 919,931 Soares, V. A. M., 649 Sokolowski, S., 1153 Soria,V., 339 Spiro, M., 617, 1105 Stanley, D. R., 1003 Stewart, B., 969 Stoeckli, F., 783 1205 1449 Teraoka, Y., 349 Thompson, K. M., 1105 Thompson, N. E., 1047 Thorn, J. C., 1365 Timms,A. W., 83 Timney, J. A., 459 Togawa, T., 1171 Tomkinson, J., 1149 Tosi, G., 859, 1089 Touret, O., 773 Tournayan, L., 773 Trau, M., 1251 Travers, D., 1473 Trejo, A., 113 Trevifio, H., 1335 Truscott, T. G., 1065,1073 Tsuchiyama, T., 1355 Tsuji, H., 803 Tsuji, M., 1171 Tsunashima, S., 549 Tsunetoshi, J., 1307 Tung, C-H., 947 Turco Liveri, M.L., Turco Liveri, V., 311 Turner, P. H., 1065 Udagawa, T., 763 Ueno, A., 1279 Ugo, R., 1335 Umemoto, H., 549 Unayama, S-i., 549 Upadhyaya, H. P., 825 Valat, P., 411 Valls, M. J., 609 van Hooff, J. H. C., van Santen, R. A., 1191 van Wolput, J. H. M. C., 1033 311 1033 Williams, D. E., 345 Wilpert, A,, 287 Wintgens, V., 41 1 Woermann, D., 875 Wohlers, M., 403 Wolthuizen, J. P., 1033 Wormald, C. J., 445 Xin, Q., 973 Yagci, Y., 287 Yamaji, M., 533 Yamamoto, M., 899,1355 Yamanaka, I., 451 Yamasaki, M., 869 Yamasu, H., 1537 Yamauchi, N., 1307 Yanes, C., 575 Yang, Z-Q., 947 Yano,H., 869 Yasuda, H., 1183 Yeh, C-t., 1157 Yoshitake, H., 155 Yotsuyanagi, T., 93,479 Young, R. N., 271 Zambonin, C.G., 1495 Zanotto, S. P., 865 Zhang, M., 1233 Zhang, X., 605 Zhang, Z. C., 1335 Zholobenko, V. L., 233, Zhong, G. M., 369 Ziolek, M., 1029 Zubarev, V. E., 721 Zundel, G., 843,1095 1047 11 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 solid/liquid 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 W 1V OBN. THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Urganising 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 W 1V OBH.... 111 THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 100 Atmospheric Chemistry: Measurements, Mechanisms and Models University of East Anglia, Norwich, 19-21 April 1995 Organising Committee: Professor I. W. M. Smith and Dr J. R. Sodeau (Co-chairmen) Dr R. A. Cox Dr J. C. Plane Dr J. Pyle Professor F. Taylor The priority now given by national governments to the study of atmospheric science confirms that our understanding of global climate and compositional changes depends upon measurements in both the laboratory and the field. The data obtained by the experimentalists are then applied by modellers who provide the most significant input into legislative controls on pollution matters.However there have been few opportunities for laboratory and field workers along with the modelling community to attend an "interdisciplinary" discussion in which overall progress in our understanding of specific atmospheric problems is assessed. The object of this discussion is to bring together the researchers in the diverse disciplines that make up atmospheric chemistry so that their individual results and conclusions can be communicated to each other. Some of the key issues to be discussed will include: ozone balances in the atmosphere; heterogeneous processes; the interaction of chemistry and dynamics in determining atmospheric composition and change. Particular reference will be made to the input of data to global models from the use of satellite, airborne and ground-based instrumentation.Contributions are invited for consideration by the Organising Committee covering topics within the area of chemistry, dynamics and modelling in the lower and upper atmosphere. Abstracts of about 300 words should be submitted by 31 May 1994 to: Professor I. W. M. Smith OR Dr R. J. Sodeau School of Chemistry School of Chemical Sciences University of Birmingham University of East Anglia Edgbaston, Birmingham Norwich BlS 21T. UK NR4 7TJ, UK Full papers for publication in the Discussion volume will be required by December 1994. THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 101 Gels Paris, France, 6-8 September 1995 Organising Committee: Dr J.W. Goodwin (Chairman) Dr R. Audebert Dr R. Buscall Professor M. Djabourov Dr A. M. Howe Professor J. Livage Professor J. Lyklema Professor S. B. Ross-Murphy During the last few years there has been an increase in both theoretical and experimental work on gels as new techniques have been applied to a wide range of gelling systems. Typical of these are gels formed from polymers by both physical and chemical interactions as well as gels formed by inorganic and surfactant systems. The meeting will deal with the structure and dynamics of gels with the latter heading covering both swelling and rheological behaviour. Mixed systems such as polymer/suifactant and polymer/particle gels will also be discussed.The Discussion will bring together experimentalists and theoreticians interested in different types of gelling systems and encourage them to interact and assess the current scene and provide a benchmark for future developments. Contributions are invited for consideration by the Organising Committee. Titles and abstracts of about 300 words should be submitted by 30 September 1994 to: Dr J. W. Goodwin, School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 ITS, UK Full papers for publication in the Faraday General Discussion 101 volume will be required by May 1995. iv FARADAY DIVISION INFORMAL AND GROUP MEETINGS Statistical Mechanics and Thermodynamics Group Cellular Automata and their Applications to Molecular Fluids To be held at the University of Manchester on 19 and 20 July 1994 Further information from Dr A.Masters, Department of Chemistry, University of Manchester, Manchester M13 9PL Division Autumn Meeting: Reactions and Mechanisms for Fine Chemicals To be held at the University of Glasgow on 6-9 September 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Gas Kinetics Group 13th International Symposium on Gas Kinetics To be held at University College, Dublin on 11-15 September 1994 Further information from Dr H. Sidebottom, Department of Chemistry, University College, Dublin Electrochemistry Group with the SCI ELECTROCHEM 94 To be held in Edinburgh on 12-16 September 1994 Further information from Professor D.E. Williams, Department of Chemistry, University College London, 20 Gordon Street, London WClH OAJ Biophysical Chemistry Group with the Industrial Division Biotechnology Group Peptide + Water = Protein To be held at University College, London on 19 September 1994 Further information from Professor J. L. Finney, Department of Physics and Astronomy, University College London, Gower Street, London WC 1E 6BT British Carbon Group Applications of Microporous Carbons To be held at the University of Leeds on 28 and 29 September 1994 Further information from Professor B. Rand, Department of Chemistry, The University, Leeds LS2 9JT Theoretical Chemistry Group with CCPl Electronic Structure: From Molecules to Enzymes To be held at University College London on 30 November 1994 Further information from Dr P.J. Knowles, School of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ Division Annual Congress: Lasers in Chemistry To be held at Heriot Watt University, Edinburgh on 1G13 April 1995 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Division Joint Meeting with the Division de Chimie Physique de la Societe' Francaise de Chimie, Deutsche Bunsen Gesellschaft fur Physikalische Chemie and Associazione Italiana di Chimica Fisica Fast Elementary Processes in Molecular Systems To be held at the UniversitC De Lille, France on 16-30 June 1995 Further information from Dr C.Troyanowsky, Division de Chimie Physique, Laboratoire de Chimie Physique, 11 rue Pierre et Marie Curie, 75005 Paris, France British Carbon Group Carbon '96 To be held at the University of Newcastle upon Tyne on 7-12 July 1996 Further information from Dr K. M. Thomas, Northern Carson Research Laboratories, The University, Newcastle upon Tyne NE1 7RU V REVIEWS high-quality translation Russian Chemical Reviews is an authoritative, specialist publication, providing easy access to reviews of important new work from Russia and the other countries of the former USSR. 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ISSN:0956-5000    

DOI:10.1039/FT99490BP105

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1467-1471 1467 Collisional Behaviour with Ar of the A Doublets of CH(X 'n)N" = 15 produced in the Two-photon Dissociation of CH2C0 at 279.3 nm Stephen M. Ball, Graham Hancock* and Mathew R. Heal? Oxford Centre for Applied Kinetics, Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford,UK OX1 3QZ Relaxation of the N" = 15 A doublets of CH(X211) produced in the two-photon dissociation of ketene at 279.3 nm has been observed in the presence of Ar. An initially equilibrated nascent A doublet population is seen to be removed in such a way that the component of A" symmetry (n orbital perpendicular to the plane of rotation) dominates. The application of recent theory describing collisions of 'll diatomics with closed shell atoms is brief I y discussed.The CH radical is one of the most ubiquitous and reactive of all free radicals and plays a key role in the chemistry of com-bustion and atmospheric systems. In addition to its kinetic importance in elementary reaction steps, CH is of fundamen- tal interest to theoreticians and experimentalists studying the dynamics of collisional quenching and energy transfer pro- cesses of small radical species. In particular, state-resolved observations of the population of the A doublets of the radical yields valuable information on the stereochemical behaviour of the singly occupied 71 electron orbital with respect to the rotating molecular framework. An initial bias towards population in specific A doublets has been noted in the dispersed emission of nascent rota- tional levels in the first excited electronic state of CH('A), a commonly observed product from the multiphoton disso- ciation of CH containing polyatomics. For example, Stuhl and co-workers' have observed a propensity for population of CH('A) in rotational levels of II(A') symmetry (pn orbital parallel to the diatomic rotating plane) following the 193 nm photolysis of acetone.This is in partial agreement with Nagata et aL2 for the same system who report a switching to ll(A") propensity for rotational levels greater than N' x 20. Similar experiments with ketene, CH,CO, as a precursor at a photolysis wavelength of 193 nm, also report a higher inten- sity emission in the symmetric A component for rotational levels with N' = 14-19, but the opposite for levels N' = 20-The present work focuses on the A doublet populations of the ground electronic state.We have recently reported mea- surements on the nascent rotational populations for ground- state CH(X 'll) produced from the two-photon photolysis of ketene at wavelengths of 279.3 and 308 nm.4 At 279.3 nm the nascent rotational distribution probed by LIF exhibits equal population in the A doublets, while at 308 nm there was a slight degree of orbital alignment in favour of II(A') sym- metry for the higher rotational levels populated. The likely dissociation pathways leading to these results were discussed. Here we report on the subsequent evolution of the CH(X 211) rotational populations in collisions with Ar.We observe a significant inequality in the initial behaviour of the popu- lations of the two A symmetries with respect to both time and pressure of Ar. These observations must indicate a pref- erential rotational energy transfer mechanism, dependent on 71 orbital symmetry, that operates for certain rotational levels of CH in collisions with the inert gas. The results are dis- cussed in terms of the extensive theoretical treatment for inelastic scattering of 'II diatomics which has been developed by Alexander and co-~orkers.~-' t Present address: School of Chemistry, University of Leeds, Leeds, UK LS2 9JT. Experimental In these experiments a standard LIF detection apparatus was used which has been described in more detail The CH(X 'II) radical was generated by the two-photon pho- tolysis of ketene, CH,CO, at 279.3 nm,4 and detected by on- resonance LIF within the Q and R branches of the CH A'A-X'll system at wavelengths between ca.418 nm and 432 nm." Plane polarised photolysis light at 279.3 nm was produced using the frequency-doubled output of a Quanta Ray 5200 dye laser pumped by a XeCl Questek 2240 excimer. The probe laser radiation was obtained either from a Molec- tron Corp. UV 24 nitrogen laser pumped Molectron DL 200 dye laser (energies up to ca. 50 pJ pulse- ',bandwidth ca. 1.0 cm-') or a Lambda Physik excimer laser pumped dye laser, EMG101/FL2002 combination (output up to 10 mJ pulse- and bandwidth of ca.0.4 cm-I). The LIF was observed per- pendicular to the orthogonal intersection of the horizontal photolysis and probe laser beams using an EM1 9813QKB photomultiplier tube and an appropriate interference filter. Data were acquired either by gating and integrating the signal using a Brookdeal 9415/9425 boxcar combination and a chart recorder or by digitisation via a 20 MHz Thurlby DSA524 Digital Storage Adaptor and a PC. Ketene precursor was prepared as described previously by the pyrolysis of acetone vapour in He passed over an electri- cally heated nichrome element at ca. 650°C.'' Purity was always checked by mass spectrometry and UV absorption between 200 and 400 nm. When not frozen the ketene was stored at pressures less than 20 Torr in a darkened bulb to prevent polymerisation. Argon (Ar) diluent was obtained from BOC with a stated purity of 99.995% and used as received.Experiments were conducted using a static gas sample within the stainless steel cell of ca. 30 mTorr CH,CO and variable partial pressures of Ar bath gas. Each rotational level within the CH(X 211) and CH(A 2A) manifolds is split by spin-orbit coupling [Hund's case (b)] and by A doubling, although the magnitude of the splitting is considerably greater in both instances for the ground state. The selection rules allow transitions within the A-X system from all four levels associated with a given lower state rota- tional quantum number N".' ,The fine-structure populations can therefore be obtained directly from a LIF scan within a single rotational branch.For CH(X 21'1) with its relatively large rotational constant (Be = 14.46 cm-' 13) but weak spin-orbit coupling constant (A = 28.14 cm-' lo), the A splitting rapidly dominates the spin-orbit splitting as rota- tional energy increases. The bandwidth of the Molectron laser as probe was sufficient to resolve the A doublets but not the spin-orbit components contained within each. In order to standardise the notation associated with the rotational levels J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 R(0,O)N" I I I I I 418 420 42 2 424 426 excitation wavelength/nm Fig. 1 (a)LIF spectrum in the R branch region of the A 2A-X211transition for nascent CH(X 2n)produced in the two-photon dissociation of ketene at 279.3 nm using the Molectron nitrogen laser pumped dye laser combination as probe.Positions of the R branch transitions in the (0,O) band are indicated as a function of N". For each N" doublet the transition at lower wavelength probes the levels of n(A)symmetry, whilst that at higher wavelength probes the levels of n(A)symmetry. Partial pressure of ketene 30 mTorr. (b) As for Fig. l(a), but under collisional conditions of 5 Torr pressure of Ar and a photolysis-probe delay of 0.3 ps (equivalent to ca. 15 gas kinetic collisions). The preferential population of n(A") levels under collisional conditions is clearly seen for N levels 3 12. of such systems Brown et al.12 and Alexander et all4 have These correspond to the singly occupied x orbital of the classified the fine-structure levels according to the behaviour radical being aligned parallel or perpendicular to the plane of of the electronic wavefunction on reflection in the plane of rotation, respectively.For the N" = 15 levels of CH(X211), rotation of the diatomic in the limit of high J. As the case (b) the subject of the present study, the R branch transition con- limit is approached the F,, and F,, wavefunctions of a ,ll sists of a doublet, the higher wavelength component of which state acquire symmetric character with respect to this reflec- corresponds to transitions from the unresolved F,, and F,, tion and are denoted ll(A') while the F,, and F,, wavefunc-levels, each of ll(A') symmetry, with the lower wavelength tions acquire antisymmetric character and are denoted ll(A"). component being from F,, and F,, levels, both of ll(A'l) sym- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 01 I I I I I I ' I I ' I 01 2345678910 probe delay/ps Fig. 2 Semi-logarithmic plot of the population af both components of the N" = 15 A doublet of CH(X 'n) as a function of increasing time delay following photolysis at 279.3 nm. Partial pressures of CH,CO and Ar were 30 mTorr and 1.0 Torr, respectively. The squares correspond to CH rotational levels of n(A") symmetry and the circles to levels of n(A') symmetry. metry. This part of the spectrum thus gives a convenient probe of the A doublet behaviour. Two distinct pumpprobe beam relative polarisation geometries could be investigated, either both electric vectors parallel to each other and vertically orientated with respect to the laboratory frame or alternatively with the polarisation of the photolysis beam rotated through 90". At short pump- probe delays copious amounts of nascent 'background' emis- sion from excited CH(A2A) had to be subtracted from the LIF signal of interest and significantly impaired sensitivity.Results Previous work in this laboratory has shown that the nascent population of CH prepared by the two-photon photolysis of ketene at 279.3 nm clearly has a 1 : 1 ratio in A doublet population for all N" levels probed in the R branch region between 418 and 426 nm.4 The LIF spectrum is reproduced here in Fig. l(a) for comparison. The corresponding spectrum of CH under collisional conditions (5 Torr Ar at a delay of 0.3 ps, corresponding to ca.15 gas kinetic collisions) is illus- trated in Fig. l(b). Here there are obvious unequal A doublet intensities for all rotational levels N" 2 12. 0.51 I I I0 5 10 15 probe delay/ps Fig. 3 The variation in ratio of population of n(A)and n(A') com- ponents of the N" = 15 A doublet of CH (photolysis at 279.3 nm) as a function of probe delay at a fixed Ar pressure of 0.5 Torr. The circles correspond to both photolysis and probe laser beams verti- cally plane polarised with respect to the laboratory frame, and the squares to a 90" relative polarisation geometry. The n(A):n(A')ratio rises to a limiting value in a way that is independent of polari- sation geometry.The increasing error bars with time reflect the reduction in the magnitudes of the absolute LIF signals as CH is removed. For this effect to be attributed to unequal A doublet popu- lations in the ground 211 state, the influence of different exci- tation and fluorescence rates needs to be considered. First, lifetimes T of the 'A N' = 16 levels accessed in the transition were found to be represented by 7 = 560 40ns for Ar pres- sures between 0 and 5 Torr, in agreement with previous mea~urements'~-'*and eliminating the effect of quenching of the upper 'A level on the fluorescence intensities. Secondly, the A doublet ratio was found to be independent of relative polarisation of the pump and probe beams, showing that alignment effects were negligible.Finally, the ratio remained constant over a ten-fold change in probe laser intensity, eliminating saturation effects in the transition. The propen- sity is thus a phenomenon occurring within the ground state of CH, with the propensity for the shorter wavelength com- ponent of the transition for a given N" indicating a greater population for CH of lI(A") symmetry. From Fig. 1 it can be seen that although disequilibrium between the A doublet levels has been established in a time corresponding to ca. 15 gas kinetic collisions, the overall rotational state population is not markedly affected, i.e. the two rotational distributions for Fig. l(a) and (b) peak at approximately the same value of N".It should also be noted that the disequilibrium effect is most apparent at high N". Rotational levels with N" < 12 appear to be unaffected under the collisional conditions. Fig. 2 shows the time dependence of the two A doublets in the presence of Ar. Both components decrease monotonically with time, but it can be seen that a disequilibrium between the two is established rapidly (< 1 ps for a pressure of 1 Torr Ar as shown in Fig. 2) and is maintained over a timescale of several lower state lifetimes. Fig. 3 shows the evolution of the A doublet ratio as a function of time at a pressure of 0.5 Torr Ar and 30 mTorr ketene, reaching a limiting value of ca. 1.7 in a timescale corresponding to ca. 20 Ar collisions. Under these conditions, CH(X 'n) is removed predominantly by reaction with ketene,9 but the effect was shown to be depen- dent on the presence of Ar by experiments in which the A doublet ratio was found to be invariant (1.1 & 0.1) over the same timescale with ketene alone, and to show a similar increase to a limiting value when measured as a function of Ar pressure at constant time delay and ketene pressure.Our conclusions about the anomalous behaviour are as follows: (1) The relaxation of a 'hot' distribution of CH rota- tional states initially in A doublet equilibrium in collisions with Ar (when chemical removal by reaction with ketene also occurs) takes place through a marked A doublet disequi- librium. (2) The effect is not solely due to preferential chemi- cal removal of a single A doublet; Ar collisions are necessary.(3) The persistence of the disequilibrium after the photolysis pulse is over shows that the effect is not dominated by col- lisional processes affecting the initial A doublet population. In previous studies it was proposed that the ketene disso- ciation process involved single-photon absorption to a pre- dissociative state followed by a second absorption step.4 Any collisional relaxation of the intermediate to a lower-energy longer-lived state might affect the subsequent energy distribu- tion (and possibly the A doublet ratio). A collisional effect on the nascent CH quantum yield was seen, but as Fig. 2 and 3 show, the disequilibrium effect persists at timescales orders of magnitude greater than the duration of the photolysis pulse.Discussion We first note that the ketene system is not the first in which such anomalous A doublet ratios have been seen. In an experiment very similar to this, Stuhl and Heinrich' have observed an almost identical effect on the N” = 15 A doub-lets of CH found in the multiphoton dissociation of CH, Br, at 193 nm. An initially equal ratio of A doublets increased to a n(A”): II(A’) ratio value of cu. 2.5 with Ar collisions, i.e. with both time and pressure of inert diluent. The collisional behaviour described in the present work is a quenching for CH incorporating a preference for A doublets of II(A”) sym-metry, regardless of the nascent population of A doublets that has been produced on photolysis.This bias is for the electron density of the d electron to lie parallel to the total angular momentum vector J, and perpendicular to the plane of rotation of the radical, in the limit of high angular momen- tum.I4 Macdonald and Liulg have used a cross molecular beam apparatus to collide CH N = 1 radicals with He atoms at collision energies up to 12 kJ mol-’. An initially equal dis- tribution of all four fine-structure states of N = 1 resulted in preferential rotational energy transfer of population to the If and 2e states [of ll(A”) symmetry] in levels of N > 1, as com-pared to population in the le and 2f states of the same final levels, Towards the upper end of collision energies, the ratio of the population of the two symmetries of CH in the excited rotational levels was typically n(A”) : II(A’) = 2.5 : 1.The ratio increased exponentially with decreasing energy as the threshold for the excitation process was approached. The probe-laser resolution available to us in the present work was insufficient for such full characterisation of all fine-structure states. We are also concerned wth rotational quenching of the CH radical in downward transitions, rather than the mea- surement of differential cross-sections for net inelastic energy transfer to the radical in upward rotational transitions. Theoretical work on a general formalism for inelastic scat- tering of a diatomic radical with a structureless collision partner has been extensively developed by Alexander and co- worker~.~*~**~~~In these open-shell systems the nearly degen- erate potential-energy surfaces at long range may interfere as a collision partner approaches, and prevent treatment of the collision as an event taking place on a single surface.Com- plications arise also from different coupling systems of the various momenta. Calculations have been applied to the spe- cific example of inelastic scattering and resultant orbital alignment of CH in collisions with He,8 and general agree- ment found with the molecular beam result^.'^ The non- statistical population of the final state A doublet levels arises directly from an interference between the scattering ampli- tudes on the two potential-energy surfaces upon which inelas- tic collisions occur.Approach of a spherical scattering partner to the ’ll radical lifts the electronic degeneracy of the 211state giving a total wavefunction for the system that is either symmetric A’ or antisymmetric A” with respect to reflection of the electronic spatial coordinates in the triatomic plane. Collisions are described using average and difference linear combinations of these surfaces, i(VA.+ VAj,)and i(VArr -VA!),respectively. In a Hund’s case (b) diatomic, where neither L nor S is strongly coupled to the internuclear axis, both the average and difference potentials can contribute to both fine-structure conserving collisions (influence of the average potential) and fine-structure changing collisions (influence of the difference potential).Interference between the scattering amplitudes of the different paths which lead to the same final spin-orbit state imposes a bias in differential cross-section and a propensity in A doublet population. The preferred final symmetry is dependent on the relative signs and magnitudes of the average and difference potentials. A single theoretical calculation for loss of rotational energy in the CH + He system, J = 3,F, +J = 4, F,, predicts a dif- ferential cross-section ratio of cA,,/cA~ = 0.53,’ i.e. preferential de-excitation into states of II(A’) symmetry. This prediction J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 has not been tested experimentally and is opposite to our findings presented here.The reason for the discrepancy pre- sumably arises from the different functional form of the potential-energy surfaces required to describe collisions of CH with a much higher degree of rotational excitation and with a heavier Ar scattering partner as investigated in the present work. A change in relative signs of the average and difference potentials will affect the symmetry of the states [to II(A) for de-excitation collisions] of a particular level into which CH is preferentially quenched. The long-range disper- sion interaction in the collision is proportional to the polar- isability of the spherical scattering partner and will be strongest when the 71 electron of CH lies in the orbital within the triatomic plane defined by the collision.Since the polar- isability of an Ar atom is almost an order of magnitude greater than that of He, the signs of the dispersion interaction for the two symmetries may play an influential role in the overall balance of scattering amplitudes. A further consideration is that the entire scattering process may be a J (or N) dependent quantity. This would account for the observed discrepancy that A doublets with N” > 12 in the LIF spectrum under collisional conditions for CH pro- duced at 279.3 nm photolysis show a collisional propensity while levels of lower rotation continue to exhibit the nascent equilibrium population [Fig. l(b)]. Alternatively, this obser- vation may be a manifestation of the system striving towards the equilibrium that would be eventually attained in all ther- mally populated levels at ambient conditions were it not for the population of CH being removed by chemical reaction before an equilibrium can be established.Evidently, there is a requirement for further detailed studies on the behaviour of many more A doublets and spe- cific quenching rates through individual rotational levels. In particular, fully time-resolved data for decay rates of fine structure components at fixed pressures of Ar and a thorough investigation at additional photolysis wavelengths are required to unravel completely the mechanisms underlying the dissociation pathways to, and collisional quenching of, the resulting CH fragment. The award of a studentship to M.R.H. by the SERC is grate- fully acknowledged. References 1 P.Heinrich and F. Stuhl, 1990, unpublished results. 2 T. Nagata, M. Suzuki, K. Suzuki, T. Kondow and K. Kuchitsu, Chem. Phys., 1984,88, 163. 3 J. Luque, J. Ruiz and M. Martin, Chem. Phys. Lett., 1993, 202, 179. 4 S. M. Ball, G. Hancock and M. R. Heal, J. Chem. SOC., Faraday Trans., 1994,90, 523. 5 M. H. Alexander, J. Chem. Phys., 1982,76,5974. 6 M. H. Alexander, Chem. Phys., 1985,92,337. 7 G. C. Corey and M. H. Alexander, J. Chem. Phys., 1986, 85, 5652. 8 P. J. Dagdigian, M. H. Alexander and K. Liu, J. Chem. Phys., 1989,91, 839. 9 G. Hancock and M. R. Heal, J. Chem. SOC., Faraday Trans., 1992,88,2121. 10 Z. Bembernek, R. Kepa, A. Para, M. Rytel, M. Zachwieja, G. J. Janjic and E.Marx, J. Mol. Spectrosc., 1990, 139, 1. 11 J. W. Williams and C. D. Hurd, J. Org. Chem., 1940, 5, 122. 12 J. M. Brown, J. T. Hougen, K. P. Huber, J. W. C. Johns, I. Kopp, H. Lefebrve-Brion, A. J. Merer, D. A. Ramsay, J. Rostas and R. N. Zare, J. Mol. Spectrosc., 1975,55, 500. 13 P. Bernath, J. Chem. Phys., 1987,86,4838. 14 M. H. Alexander, P. Andresen, R. Bacis, R. Bersohn, F. J. Comes, P. J. Dagdigian, R. N. Dixon, R. W. Field, G. W. Flynn, K. H. Gericke, E. R. Grant, B. J. Howard, J. R. Huber, D. S. King, J. L. Kinsey, K. Kleinermans, K. Kuchitsu, A. C. Luntz, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1471 15 A. J. McCaffery, B. Pouilly, H. Reisler, S. Rosenwaks, E. W. Rothe, M. Shapiro, J. P. Simons, R. Vasudev, J. R. Wiesenfeld, C. Wittig and R. N. Zare, J. Chem. Phys., 1988,89,1749. J. Brzozowski, P. Bunker, N. Elander and P. Erman, Astrophys. J,, 1976,207,414. 18 19 20 5652. W. Bauer, B. Engelhardt, P. Wiesen and K. H. Becker, Chem. Phys. Lett., 1989, 158,321. R. G. Macdonald and K. Liu, J. Chem. Phys., 1989,91,821. G. C. Corey and M. H. Alexander, J. Chern. Phys., 1988, 85, 16 K. H. Becker, H. H. Brenig and T. Tatarczyk, Chem.Phys. Lett., 17 1980,71,242. M. Ortiz and J. Campos, Physica C., 1982,114,135. Paper 4/00718B; Received 7th February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001467

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1473-1478 Ultra-low Temperature Kinetics of Neutral-Neutral Reactions :Rate Constants for the Reactions of OH Radicals with Butenes between 295 and 23 K Ian R. Sims* and Ian W. M. Smith School of Chemistry, The University of Birmingham Edgbaston ,Birmingham, UK B 152TT Pascal Bocherel, Andre Defrance, Daniel Travers and Bertrand R. Rowe Departement de Physique Atomique et Moleculaire, U.A. 1203 du C.N.R.S., Campus de Beaulieu, Universite de Rennes I, 35042 Rennes Cedex, France The first experiments on the kinetics of reactions of the OH radical at temperatures below 80 K are reported. They have been carried out by applying the pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique for studying the kinetics of free radical reactions in the ultracold environment provided by the gas flow in a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus.This method has yielded rate constants for the reactions of OH with but-1-ene, (Z)-but-2-ene and (€)-but-2-ene at temperatures down to 23 K. The rate constants for all three reactions increase monotonically as the temperature is lowered and this dependence of the rate constants on temperature can be fitted to an empirical expression of the form (k/tO-” cm3 molecule-’ s-’) =a exp[b(T/298K)] with a and b equal to 5.2 and -2.8 for but-1-ene, 4.7 and -2.1 for (Z)-but-2-ene and 5.4 and -2.1 for (€)-but-2-ene. Until very recently, the kinetic data base for elementary reac- tions between electrically neutral species at temperatures below ca.200 K was restricted to a study of the recombi- nation of hydrogen atoms by Ham et al.’ In recent years, the increasing use of cryogenically cooled reaction cells has yielded rate constants, down to ca. 80 K in some cases, for the combination of 0 atoms with O,,, and for a number of reactions of the CN3 and OH4 radicals. In the latter experi- ments, by Smith and co-workers, pulsed laser photolysis (PLP) has been used to generate the free radicals, and the laser-induced fluorescence technique (LIF) has been employed to observe their kinetic decay. Despite the success of the above experiments, methods based on the use of cryogenically cooled cells are limited. First of all, it is difficult to operate below the temperature of liquid N, , although Simpson and co-workers5 have measured the rates of energy transfer processes involving stable neutral species such as H,, D, and 4He in the gas phase down to 35 K, using cells cooled by He or N, vapour.Condensation on the walls of cooled cells is rapid and conse- quently the partial pressure of any species, whose concentra- tion must be accurately known in order to extract precise rate constants, must be significantly below its partial pressure at the temperature of the cell walls. In the past two years, we have successfully devised a method of studying the kinetics of elementary gas-phase reac- tions between neutral species at much lower temperatures than hitherto which avoids the limitations inherent in the use of cryogenically cooled cells.These experiments employ a CRESU apparatus which was designed and constructed in Rennes with kinetic measurements on reactions between neutral species specifically in mind. The CRESU technique was originally devised by Bowe and cozworkers6 in order to determine rate constants for ion-molecule reactions at ultra- low temperatures. The method takes advantage of the flow properties of gaseous expansions from convergent-divergent Laval nodes into a low-pressure chamber. In the original experiments on ion-molecule chemistry in the CRESU apparatus, reactions were initiated by creating ions just beyond the exit of the nozzle using an electron beam and the rates and mechanisms of subsequent reactions were observed by sampling a portion of the flow downstream with a quad- rupole mass spectrometer.More recently, a selected ion source has been incorporated into the CRESU apparatus’ and, by cooling the gas reservoir in liquid N,, rate measure- ments have been made at temperatures as low as 8 K6,’The results obtained have made a substantial contribution to the understanding of molecular synthesis in interstellar clouds.6-8 The CRESU technique provides, via the isentropic expan- sion of gas through the Laval nozzle, a ‘collimated’ flow of ultra-cold gas which is uniform in temperature, density and velocity. The expansion and subsequent cooling are rapid enough that heavily supersaturated conditions may prevail, avoiding the major problem of condensation associated with the use of cryogenically cooled cells. However, in contrast to free jet expansions/molecular beams where the concept of temperature is not really valid, the relatively high gas density (10’6-10’7 molecule cm-3) of the supersonic flow ensures that frequent collisions take place during the expansion and subsequent flow, maintaining thermal equilibrium at all times.The environment is an excellent one in which to measure thermal rate constants for reactions between neutral species by the well established PLP-LIF meth~d.~.~ In our first experiments applying the PLP-LIF method in a CRESU apparatus, we determined rate constants for reactions of the CN radical with O2down to 13 K and with NH, down to 26 K,’ and with the hydrocarbons C&, C2H4 and CZH2 down to 26 K.” A full and detailed description of this new experi- mental method has been given.” The results which have been obtained have already attracted the attention of theoreticians” and astrochemists12 who seek to model the complex chemistry responsible for molecular synthesis in interstellar clouds.In this paper, we report results from the first experiments at ultra-low temperatures on reactions of the OH radical. One limitation of our new experimental technique is that the reaction under investigation must remove the radical species which is being observed with a pseudo-first-order rate con- stant which exceeds ca. lo3 s-’. This requirement arises because the gas downstream from the exit of the Laval nozzle moves at ca.600 m s-’ and good flow conditions only survive for ca. 30 cm. Consequently, the sample of radicals which is generated by the photolysis laser propagating along the axis of the gas flow moves rapidly past the point, located at the downstream end of the uniform flow, at which the rela- tive radical concentrations are observed by LIF. At pulse- probe time delays greater than CQ. 1 ms, there will still be some radicals in the gas in the observation zone, but they will have been created by photolysis of the radical precursor in the nozzle or the gas reservoir. Coupled with the fact that the concentration of the second reagent cannot exceed a few per cent of the total gas density without destroying the integrity of the flow, the requirement that the kinetic decay constant exceeds ca. lo3 s-' means that it is difficult to measure second-order rate constants which are less than ca.lo-', cm3 molecule -'s -'. The above limitations were considered when deciding which reactions to target for our first experiments on the kinetics of the OH radical at ultra-low temperatures. Many reactions of OH have been extensively studied above 200 K, because of their importance in atmospheric1 and comb~stion'~chemistry. In general, the reactions of OH rad- icals with saturated molecules are slower than the corre-sponding reactions of CN, although there are exceptions.15 Furthermore, the reactions of OH with unsaturated hydro- carbons occurs by addition (and the rates for small alkenes and alkynes therefore depend on total pressure) rather than by the pressure-independent replacement of an H atom as in the reactions with CN." For these reasons, we chose to study the reactions of OH with a number of butenes which seemed likely to be rapid and in their high-pressure limit under the conditions generated in the CRESU apparatus.The kinetics of these reactions have been examined in a number of investigations at room temperature.16-22 These studies demonstrate that the rate is independent of pressure down to 1 Torr at room temperature and that the predomi- nant reaction channel is addition to form a hydroxybutyl radical, although there is apparently some disagreement2'S2' as to the level of the small contribution of H-atom abstraction to the total rate.Atkinson and Pitts17 have measured rate constants for the reaction of OH with the four butenes, but-1-ene, isobutene, (Z)-but-2-ene and (E)-but-2- ene, between 295 and 425 K. At 295 K, their values for the rate constants are: 3.5 x lo-", 5.1 x lo-", 5.4 x lo-" and 7.0 x lo-'' cm3 molecule-' s-', respectively, and they found the rates to decrease as the temperature was raised. In the light of these previous results, the reactions of OH with butenes seemed promising candidates for our first experiments on reactions of OH radicals in the CRESU apparatus. Here, we report rate constants for the reactions of OH with but-1-ene, (q-but-2-ene and (E)-but-2-ene at tem- peratures between 295 and 23 K.OH radicals were generated by pulsed laser photolysis of H,O, at 266 nm and their kinetic decays observed by exciting LIF in lines of the (1, 0) band of the OH(A ,E+-X'll) system at ca. 282 nm.4 Experimental The apparatus and procedures used in the present series of experiments have been described in detail elsewhere.9b In par- ticular, interested readers are referred to the schematic of the apparatus given as Fig. 1 of ref. 9(b). Here, we shall give a relatively brief description of the method, emphasising those aspects of the experiments which are peculiar to measure- ments on reactions of the OH radical. The heart of the CRESU apparatus is an axisymmetric Laval nozzle which is mounted on a reservoir fitted with a perforated Teflon disc to ensure laminar flow and good mixing of the gas streams entering the reservoir.Although the gas reservoir is jacketed, permitting cryogenic cooling, this was not made use of in the present experiments. All the tem- peratures in the gas flows were achieved by isentropic expan- sion of the gas mixture prepared in the reservoir through the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 nozzle and into the main chamber. This expansion generates a supersonic flow of gas in which the Mach number, the tem- perature, the density of the gas, and the mole fraction of the reagent in excess (here, a butene) are constant along the axis of the flow. Several nozzles were employed, each providing a particular temperature and density for the selected carrier gas.The integrity of the gas flow, and therefore the design of a particular nozzle, could be checked in two ways. All of the nozzles were characterised by impact-pressure measurements. In addition, the temperatures provided by some nozzles were determined by performing spectroscopic measurements on the (0, 0) band of the (B2E+-X2E+) system of CN.9b The temperature determined from the relative intensities of the rotational lines confirmed the values inferred from the impact-pressure measurements. OH radicals were generated by the photolysis of H,O, at 266 nm using the pulsed output from a frequency-quadrupled Nd: YAG laser (Spectron Lasers). 85% H,O, (Solvay Interox) was placed in a glass/stainless-steel vessel and H,O, vapour entrained into the main carrier gas flow, by bubbling a controlled flow of He through the liquid H202 and intro- ducing this small precursor flow directly into the reservoir by means of PTFE tubing.Detection of OH radicals was achieved by LIF, exciting the (1,O) band of the OH (A ,Z+-X,lI)system at ca. 282 nm, and detecting off-resonance fluorescence from the (1, 1) band, and any in the (0, 0) band resulting from vibrational relax- ation in the electronically excited state, at ca. 310 nm. Probe laser radiation was provided by a Nd: YAG-pumped dye laser coupled to an autotracking frequency doubler unit (Spectra Physics). Kinetic data were gathered using the strongest available rotational line, usually the Ql(l) line at 282 nm.The photolysis and probe laser beams were combined on a dichroic mirror and directed along the axis of the supersonic flow. They entered the CRESU apparatus through a Brewster angle window, passed through another such window mounted on the back of the reservoir and co-propagated out through the throat of the Laval nozzle and along the axis of the flow, before leaving the vacuum chamber via a third Brewster angle window. LIF was gathered at a known dis- tance downstream of the Laval nozzle (usually 10-30 cm)by a UV-enhanced, optically fast telescope-mirror combination mounted inside the main vacuum chamber, focused through a slit to reduce scattered light and directed onto the photo- cathode of a UV-sensitive photomultiplier tube (EMI) after passing through a narrowband interference filter centred at 310 nm (bandpass 10 nm FWHM; Corion).The signals were accumulated, processed and analysed by the same procedures as before." The flows of reagent butene gases [Union Carbide: but-1- ene 99% ;(Z)-but-2-ene 95% ;(E)-but-Zene 95%] and carrier gases (He, Ar or N,; Air Liquide, U-grade) were taken directly from the cylinders and regulated by means of mass flow controllers (Tylan). Knowledge of the total gas density from aerodynamic (Pitot) measurements and the individual gas flows enabled the calculation of the butene concentration in the supersonic flow, essential for the kinetic measurements. Results and Discussion In our experiments on reactions of CN,'*l' the radicals were produced by photolysis of NCNO at 583 nm, a wavelength only just below the threshold for photodissociation of this molecule to CN + NO.23 Consequently, the CN radicals were produced exclusively in the lowest vibrational level, tr = 0, and with very little rotational excitation: any relax- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ation necessary to bring the radicals into thermal equilibrium with the bath gases would certainly be very rapid. In contrast to the situation just described, in the photolysis of H202 at 266 nm, 242 kJ mol-' of energy is available for redistribution among the relative translational and internal motions of the two OH radicals which are produced. Studies of the photodissociation dynamics at this wavelength24 show that almost all of the large excess energy appears as trans- lational recoil energy of the OH fragments, with the remain- der (ca. 10%) going into the rotations of the two fragments.Production of OH in vibrational levels above u = 0 could not be detected. Although the fractional yield of rotational excitation is quite low, the initial rotational energy is nevertheless well in excess of its thermal value, even that at room temperature, and the OH radicals are distributed quite widely over the rotational energy levels. An initial rotational temperature of 1500-1700 K has been determined.24 Furthermore, because the rotational energy levels of OH are quite widely spaced, rotational relaxation is comparatively slow.Fig. 1 displays two LIF spectra of OH recorded under identical conditions in the CRESU apparatus, but with differ- ent time delays between the photolysis and probe laser pulses. The first spectrum reflects the 'hot' rotational dis- tribution resulting from the photolysis of H202, the second the thermal distribution after time has been allowed for com- plete rotational relaxation. The second spectrum confirms that, at the lowest temperatures attainable in our experi-ments, about 90% of OH radicals occupy the lowest rovib- ronic energy level. In kinetics experiments, it was essential to allow time for complete rotational relaxation before fitting the LIF signals to a single exponential decay curve in order to find a pseudo-first-order rate constant (kist) for reaction under the conditions of that experiment, An example of a trace of the LIF signal decay from OH of the kind used to extract values of klsI is displayed in Fig.2(a) (a) I 281.2 281.4 281.6 281.8 282.0 282.2 282.4 excitation wavelength/nrn 281.2 281.4 281.6 281.8 282.0 282.2 282.4 excitation wavelength/nrn Fig. 1 (a) LIF spectrum of OH recorded at 23 K in He with a delay of 100 ns between the pulses from the photolysis and probe lasers. (b)LIF spectrum from the same gas mixture at the same temperature but with a delay of 40 ps between the pulses from the photolysis and probe lasers. 1.o it 0.5$ 0.0 E -0.5 -1.o I I I I I 7 h 8 I ' I I I I I I cv).- 56 4 -z4 C CJ,'Z 2 !A J 0 0 20 40 60 80 100 120 delay t ime/ps 18 16 r I4 12 "0 10F 28 6 4 2 t 1 0 0 1013 2x1013 3x1013 [(E)-but-2-ene] /molecu les cm -Fig.2 (a) First-order decay of LIF signal from OH in the presence of 2.6 x lOI3 molecules of (E)-but-Zene at 23 K in He, fitted to a single exponential decay, with residuals shown above. (b) First-order decay constants for OH at 23 K in He plotted against the concentration of (E)-but-2-ene. and its quality is typical of those obtained in the present experiments. These curves were fitted to a single exponential decay function using a non-linear least-squares fitting program employing the Levenburg-Marquardt algorithm. Examination of the residuals provided by the computer program confirmed that the decays were truly exponential.The values of kist obtained at a particlar temperature and for different concentrations of a given butene were plotted against the concentration of the butene as shown in Fig. 2(b). The gradients of these plots yielded the second-order rate constants which are listed in Tables 1-3. As the errors quoted for the second-order rate constants comprise the standard error resulting from an unweighted least-squares analysis of the klsI us. [butene] plots, multiplied by the Student's t-factor appropriate for the 95% confidence interval and the number of degrees of freedom. In the CRESU apparatus rate constants can be determined at ambient temperature (295 K) by increasing the pressure in the main chamber, causing a shock front to form, thus ensur- ing the recovery of the original reservoir temperature within the subsequent flow.The rate constants at room temperature obtained in this manner are compared in Table 4 with those for the reactions of OH with but-1-ene, (Z)-but-2-ene and (15)-but-2-ene measured in previous direct experiments.' 6-' The agreement is entirely satisfactory. The rate constants for all three reactions which we have studied increase monotonically as the temperature is lowered from 295 to 23 K. Although there is no theoretical justifica- tion, the temperature dependence of the rate constant, k, can, in each case but especially for the reactions with (2)-but-2- 1476 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Rate constants for the reaction of OH with but-1-ene obtained in the CREW apparatus at temperatures between 295 and 23 K carrier number of total density but-1-ene] rate constant T/K gas measurements molecules cm-3 /loi4 molecules /lo-" cm3 molecules-' s-' ~ 23 He 8 4.68 0.2-0.7 42.7 f5.6" 44 Ar 7 2.87 0.2-1.0 31.5 f4.0 75 N2 7 1.67 0.6-1.4 27.3 f1.6 170 N* 7 0.57 0.3-1.3 7.71 f1.06 295 Ar 7 45.6 5.9-23.4 3.49 f0.11 295 Ar 7 76.2 4.2-29.2 3.30 2 0.12 ~~ " Errors quoted are fta statistical error where t is the appropriate value of the Student's t-distribution for the 95% point. Table 2 Rate constants for the reaction of OH with (Z)-but-2-ene obtained in the CRESU apparatus at temperatures between 295 and 23 K carrier number of total density [(Z)-but-Zene] rate constant T/K gas measurements molecules cm-3 /loi4 molecules cm-3 /lo-" cm3 molecules-' s-I 23 He 9 4.73 0.04-0.3 38.9 f2.3" 44 Ar 6 2.90 0.1-0.3 32.8 f3.3 75 N2 7 1.67 0.4-1.3 30.2 f 1.4 170 N2 7 0.57 0.3-1.1 13.0 _+ 1.3 295 Ar 6 45.6 5.7-22.7 6.18 f0.57 " Errors quoted are fta statistical error where t is the appropriate value of the Student's t-distribution for the 95% point.Table 3 Rate constants for the reaction of OH with (E)-but-Zene obtained in the CRESU apparatus at temperatures between 295 and 23 K ~~~~ carrier number of total density [( E)-bu t-2-enel rate constant TIK gas measurements molecules /loi4 molecules cm-3 /lo-" cm3 molecules-' s-' 23 He 9 4.73 0.03-0.3 45.2 f3.2" 44 Ar 8 2.90 0.2-0.8 40.3 f4.4 75 N2 11 1.67 0.9-2.9 31.7 f2.4 170 N2 6 0.57 0.3-1.1 16.9 _+ 0.83 295 Ar 7 45.6 12.3-48.9 6.83 & 0.22 " Errors quoted are _+ ta statistical error where t is the appropriate value of the Student's t-distribution for the 95% point, ene and (E)-but-2-ene, be fitted to expressions of the form: stants do increase as the temperature is lowered and second that the rate constants for all three reactions appear to(k/10-" cm3 molecule-' s-l) = a exp[b(T/298K)] approach a common limiting value at the lowest tem-The values of a and b are: peratures which we have studied.a = 5.2 k0.4; b = -2.80 & 0.15 for but-1-ene, Both these features of the kinetics are also observed in the a = 4.7 f0.2; b = -2.06 f0.18 for (Z)-but-2-ene, and reactions of CN with C,H, and C,H," and the values of the a = 5.4 & 0.1; b = -2.12 k0.06 for (E)-but-2-ene, where the errors quoted correspond to a single standard devi- ation as estimated by the non-linear least-squares fitting pro- cedure.To facilitate comparison with the temperature dependence reportedg." for the rate constants for other reactions at ultra-low temperature, the rate constants for the reactions of OH with but-1-ene, (Z)-but-2-ene and (E)-but-Zene are dis- played in Fig. 3-5 on log-log plots. These diagrams illustrate two aspects of the results: first, the fact that the rate con- Table 4 Comparison of rate constants (k/lO-'' an3 molecule-' s-') for the reactions of OH radicals with butenes at 295 K I I IIIIII 1 I I L IIIref.but-1-ene isobutene (a-but-2-ene (E)-but-2-ene 10-1' 1 I0 100 1000this work 3.30 & 0.12 -6.2 f0.6 6.8 _+ 0.2 16 4.1 6.5 6.1 7.1 T/K 17 3.5 & 0.4 5.1 & 0.5 5.4 f 0.5 7.0 f0.7 Fig. 3 Rate constants for the reaction of OH with but-1-ene at dif- 18 2.96 & 0.19" -4.32 f0.41" -ferent temperatures. The filled circles show the results of the present 18 2.94 & 0.14' -4.26 _+ 0.25b -measurements in the CRESU apparatus and the line represents the 19 3.3 & 0.25 ---exponential fit described in the text, while the open circles show the results of Atkinson and Pitts" at temperatures between 298 and 424 'In 3 Torr He. In 20 Torr He. K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 i I0 100 1000 T/KFig. 4 Rate constants for the reaction of OH with (Z)-but-2-ene at different temperatures. The filled circles show the results of the present measurements in the CRESU apparatus and the line rep- resents the exponential fit described in the text, while the open circles show the results of Atkinson and PittsI7 at temperatures between 298 and 425 K. rate constants for those reactions at the lowest temperatures studied (26 K) are very similar to the ones we find here at 23 K for the reactions of OH with but-1-ene, (Z)-but-2-ene and (E)-but-Zene, although the latter reactions are appreciably slower at room temperature. The magnitude of these low- temperature rate constants, and the negative dependence of the rate constants on temperature, provide strong evidence for the absence of any maximum of electronic potential energy along the minimum-energy path leading from separat- ed reagents to the radical adduct.At the lowest temperatures, it appears as if the only factor limiting the rate of reaction is the ability of the long-range attractive potential to bring the reagents together. ‘Capture’ theories which treat reactions occurring over attractive potentials have been developed by Clary and co-workers’ for ion-molecule and radical- radical reactions. It appears that the reactions of unsaturated hydrocarbons with strongly electronegative free radicals like OH and CN comprise another case where capture theories could be applied. Two factors may cause the rate constants of these reactions to fall below simple capture values at higher energies.First, 1 I I I 1 Ill 1 I I I I I11lo-” ’ 1 I0 100 1000 T/K Fig. 5 Rate constants for the reaction of OH with (E)-but-Zene at different temperatures. The filled circles show the results of the present measurements in the CRESU apparatus and the line rep- resents the exponential fit described in the text, while the open circles show the results of Atkinson and Pitts17 at temperatures between 298 and 424 K. at higher reagent energies adiabatic potentials develop maxima at shorter reagent separations where the chemical contribution to the intermolecular potential can no longer be neglected. Based on this idea, Klippenstein and Kimllb have provided a satisfactory explanation of the negative tem- perature dependence of the rate constants for the reaction between CN radicals and O2 above 50 K.Alternatively, at low temperatures, reaction may be aided by the formation of weakly bound, energised, van der Waals complexes which survive sufficiently long for the system to explore the potential-energy surface and find the correct configuration for the reaction. At higher temperatures, on the other hand, kinetic energies increase, collisions become direct, and reac- tion only occurs for those collisions with a favourable orien- tation at impact. Summary This paper reports the first kinetic measurements on elemen- tary reactions of the OH radical at ultra-low temperatures (<80 K).The experiments use the PLP-LIF method in the ultra-cold environment provided by expansion through a Lava1 nozzle in a CRESU apparatus. Rate constants are reported for the reactions of OH with but-1-ene, (Z)-but-2- ene and (E)-but-2-ene at temperatures down to 23 K. The rate constants for all three reactions increase monotonically as the temperature is lowered reaching very similar values at 23 K. It is suggested that, at the lowest temperatures of these studies, reaction occurs whenever the long-range attractive potential between the reagents brings them together, possibly because the formation of an energised van der Waals complex allows the reagents to find a favourable orientation for creation of the hydroxyalkyl radical which is the product of these reactions.We acknowledge funding from the CEC under the Science Plan (Contract No. SC*CT89-0261), as well as from the GDRs ‘Physicochimie des Molecules Interstellaires ’ and ‘Dynamique des Reactions Moleculaires’ programmes. The lasers were borrowed from the SERC Laser Loan Pool at the Rutherford-Appleton Laboratory, for which we express thanks. We are grateful to Solvay Interox of Germany for donating the 85% H20, used in these experiments. References D. 0. Ham, D. W. Trainor and F. Kaufman, J. Chem. Phys., 1970,53,4395. (a) W. T. Rawlins, G. E. Calendonia and R. A. Armstrong, J. Chem. Phys., 1987, 87, 5209; (b) H. Hippler, J. Rahn and J. Troe, J. Chem. Phys., 1990,93,6560. (a) I. R. Sims and I. W. M.Smith, Chem. Phys. Lett., 1988, 151, 481; (b) I. R. Sims and I. W. M. Smith, J. Chem. SOC., Farday Trans., 1993,89, 1. (a)M. J. Frost, P. Sharkey and 1. W. M. Smith, Faraduy Discuss. Chem. SOC., 1991,91, 305; (b)P. Sharkey and I. W. M. Smith, J. Chem. SOC., Faraday Trans., 1993, 89, 631; (c) M. J. Frost, P. Sharkey and I. W. M. Smith, J. Phys. Chem., 1993,89,12254. (a) J. J. Andrew, D. C. McDermott, S. P. Mills and C. J. S. M. Simpson, Chem. Phys., 1991, 153, 247; (b) G. J. Wilson, M. L. Turnidge, A. S. Solodukhin and C. J. S. M. Simpson, Chem. Phys. Lett., 1993,207, 521. (a)B. R. Rowe, G. Dupeyrat, J. B. Marquette and P. Gaucherel, J. Chem. Phys., 1984, 80, 4915; (b) B. R. Rowe and J. B. Mar-quette, Int. J. Mass Spectrom. Ion Processes, 1987,80,239.C. Rebrion, J. B. Marquette and B. R. Rowe, J. Chem.Phys., 1989,91,6142. (a) B. R. Rowe, J. B. Marquette and C. Rebrion, J. Chem. Soc., Faraday Trans. 2, 1989,85, 1631; (b) B. R. Rowe in Rate Coefl- cients in Astrochemistry, ed. T. J. Millar and D. A. Williams, Kluwer, Dordrecht, 1988, p. 135. 1478 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 9 (a) I. R. Sims, J-L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, B. R. Rowe and I. W. M. Smith, J. Chem. Phys., 1992, 97, 8798; (b) I. R. Sims, J-L. Queffelec, A. Defrance, C. Rebrion- Rowe, D. Travers, P. Bocherel, B. R. Rowe and I. W. M. Smith, 18 19 A. R. Ravishankara, S. Wagner, S. Fischer, G. Smith, R. Schiff, R. T. Watson, G. Tesi and D. D. Davis, Int. J.Chem. Kinet., 1978, 10, 783. W. S. Nip and G. Paraskevopoulos, J. Chem. Phys., 1979, 71, 10 11 12 J. Chem. Phys., 1994,100,4229. 1. R. Sims, J-L. Queffelec, D. Travers, B. R. Rowe, L. B. Herbert, J. Karthauser and I. W. M. Smith, Chem. Phys. Lett., 1993, 211, 461. (a) D. C. Clary, T. S. Stoecklin and A. G. Wickham, J. Chem. Soc., Faruday Trans., 1993, 89, 2185; (b) S. J. Klippenstein and Y-W. Kim, J. Chem. Phys., 1993,99,5790. E. Herbst, H-H. Lin, D. A. Howe and T. J. Millar, Mon. Not. R. 20 21 22 23 2 170. H. W.Biermann, G. W. Harris and J. N. Pitts Jr., J. Phys. Chem., 1982,86,2958. K. Hoyermann and R. Sievert, Ber. Bunsenges. Phys. Chem., 1983,87,1027. R. Atkinson, Chem. Rev.,1986,86,69. I. Nadler, M. Noble, H.Reisler and C. Wittig, J. Chem. Phys., 1985,82,2608. 13 14 15 Astron. Sue., 1994, in the press. R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson Jr., J. A. Kerr and J. Troe, J. Phys. Chem. Ref. Data, 1992,21, 1125. D. L. Baulch, C. J. Cobos, R. A. Cox, C. Esser, P. Frank, Th. Just, J. A. Kerr, M. J. Pilling, J. Troe, R. W. Walker and J. Warnatz, J.Phys. Chem. Ref. Data, 1992,21,411. I. R. Sims and I. W. M. Smith, J. Chem. Soc., Faraday Trans. 2, 24 25 (a)S. Klee, K-H. Gericke and F. J. Comes, J. Chem. Phys., 1986, 85,40; (b)K-H. Gericke, S. Klee, F. J. Comes and R. N. Dixon, J. Chem. Phys., 1986, 85, 4463; (c) G. J. Germann and J. J. Valentini, Chem. Phys. Lett., 1989,157, 51. (a) D. C. Clary in Rate Coeflcients in Astrochemistry, ed. T. J. Millar and D. A. Williams, Kluwer, Dordrecht, 1988, p. 1; (b) D. C. Clary, Annu. Rev. Phys. Chem., 1990,41,61. 1989,85,915. 16 17 E. D. Morris, Jr. and H. Niki, J. Phys. Chem., 1971,75,3640. R. Atkinson and J. N. Pitts Jr., J. Chem. Phys., 1975,63,3591. Paper 4/00806E; Received 9th February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001473

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1479-1486 Configuration Interaction Study of the O,-C,H, Exciplex : Collision-induced Probabilities of Spin-forbidden Radiative and Non-radiative Transitions Boris F. Minaev'? and Vitalina V. Kukueva Cherkassy Engineering and Technological Institute, Cherkassy, 257006,Ukraine Hans Agren Department of Physics and Measurement Technology, Linkoping University, S-58183Linkoping, Sweden The collision complex between molecular oxygen and ethene has been studied by configuration interaction calculations, scanning the intermolecular distance in C,, collision symmetry. The calculations give a good explanation for the enhancement of the ethene T, tSo and the oxygen A'3A, t X3X; transition probabilities observed by Evans at high oxygen pressure.A number of other oxygen and cooperative transitions are explained and predicted, considering the studied system as a general model for unsaturated hydrocarbon-0, interactions. The intermolecular potentials show negligible complex formation energies for the lower X 3C;, a 'As and b 'Xl states, but exciplex character for higher states. The ethene (m*)excited S, state has a fairly deep minimum and a very strong transition moment to the T, state, which also accounts for the non-radiative quenching of the hydrocarbon fluorescence by molecular oxygen. Introduction The interaction of molecular oxygen with unsaturated hydro- carbons induces many interesting spectroscopic phenomena. It is well known that the 0, molecule is an efficient fluores- cence and phosphorescence quencher in its ground triplet state (X3Eg-).The quenching has been considered to occur through formation of exciplexes in the encounter complexes. However, these exciplexes have been elusive species, and they have still not been theoretically explored to any appreciable extent. A lot of other photophysical and photochemical effects induced by oxygen are connected with these key species, but also remain to be interpreted. Theoretically interesting phenomena have been derived from observations of weak transitions in gas mixtures at high O2 press~re.~?'40 years ago Evans showed that dissolved oxygen strongly induces singlet-triplet (S-T) absorption of aromatic molecules and other unsaturated hydrocarbons (UHC).3 This was the first direct indication of a spin-catalysis phenomenon: the rate of the process was greatly increased by the oxygen catalyst, which enabled the transition to become spin-allowed.The nature of this enhancement has attracted great attention, but has been the subject of controversy for a long time.4-8 The perturbing effect of the triplet ground-state oxygen molecule O2(X3Z;) on the T, tSo absorption spectra of unsaturated hydrocarbons was originally attrib- uted to a spin-orbit coupling perturbation of the UHC triplet state by the inhomogeneous field of the paramagnetic 02(X 'Xi) species.' This magnetic field perturbation was, however, later taken to be negligibly small in order to explain the observed enhancement of the T, tSo tran~ition.~An ordinary intermolecular interaction was found to be much more important.By introducing the two unpaired electron spins to the collision complex, the 02(X 'Xi) molecule effec- tively enables the T, +So transition inside the UHC moiety to become spin-allowed, because both states in the complex become triplets; the united state of the complex (X'X-, T,) provides singlet, triplet and quintet multiplicities.'-' The nature of the T, So transition enhancement is thus deter- ? Also at: Department of Physics and Measurement Technology, Linkoping University, Sweden. mined by small admixtures to the united triplet component of the other triplet states of the complex, namely the (%;, S,) state6 and the chargetransfer (CT) ~tate.~.~ The former, so called exchange mechanism, explains the T, cSo transition enhancement as a borrowing of intensity from the very strong S,-So tran~ition,~.'and the latter accounts for the borrowing of the CT band inten~ity.~.~ Both mechanisms have received much attention in qualitative discussions, but it has been dif- ficult to determine which is the most important, because in a general approach they have the same dependence on inter- molecular overlap.In addition to the enhancement of the T, eSo transition in UHC molecules, dissolved oxygen induces a number of other spectroscopic and photophysical effects. The appear- ance of a new absorption at shorter wavelengths has been assigned to charge tran~fer~'~ and cooperative transitions (simultaneous TI cSo excitation of the UHC molecule and a IAg t X 'Xg- excitation of oxygen by one q~antum).~ For ethene and other UHC molecules with large ionization potentials a new shorter wavelength absorption (280 nm) has been tentatively assigned to the enhanced Herzberg I tran- sition A 'E; tX 3Cg-,2 The presence of molecular oxygen in solution influences drastically the quenching of the S, and TI states by enhancing the intersystem crossing (non-radiative transitions).' The singlet oxygen emission a 'Ag +X 'Eg- which is sensitized by dyes and enhanced by solvents is another type of spectral phenomenon determined by the intermolecular exchange interaction between O2 and UHC molecules.'O~'' The nature of all these effects can be con- sidered by configuration interaction (CI) approaches.The first CI calculations of the oxygen-UHC complexes were carried out at a semiempirical leve1.12-16 They were sur- prisingly successful and explained an observed intensity enhancement of T-S transitions in 0, and in UHC moieties' at intermolecular distances that were qualitatively consistent with kinetic radii. The INDO/S rnethodl4 cannot reproduce any potential curves and even the MIND0/3 CI approximation gives repulsive potentials for the 0, + C,H, intermolecular interaction.' ' In order to study the oxygen intermolecular spectroscopic effects in more detail, including the proper distance dependence, ab initio CI calculations are presented in this paper.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -0.681. -0.51 6 -0.682 I -0.683 S c 9% >> P -0.517 CC Q) -0.684 -0.685 -0.686 -0.51 8 3 4 4 ) R/A R/A Fig. 1 Intermolecular potential for the X3Xg-, So state of the Fig. 3 Intermolecular potential for the '(X 3Xi, T,)state of the . .,C2H,-0, system; the 3B, symmetry in the C,, group. -227 E, must C2H442 system; the 'A2 symmetry in the C,, group. -227 E, must be added. [l E, (hartree) w 4.359 75 x lo-'' 5.1 be added. -0.467 1--0.472 S 5 -0.461 Q) Q) -0.465 4 -0.486 ! 2 3 4RIA Fig. 2 Intermolecular potential for the C'X;, So state of the R/A C2H4-02 system; the 'A, symmetry in the C,, group. -227 E, must Fig. 4 Intermolecular potential for the a 'Ag, T, states of the be added. C2H4-02 system; (D)3A2 and (+) 3B,.-227 Eh must be added. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -0.44 0.05 0.04 -0.45 0.03 C 09.> 4th i:a 2 0.02 -0.46 0.01 -0.47 0.00 3 4 RIA Fig. 5 Intermolecular potential for the A' 'Au So and A 'Z:, So Fig. 7 Collision-induced S-S transition dipole moments (in ea,) in A 'A,,, So ('A2); (+) A' 'Au, So the system C,H,4,; (0)states of the C2H4-02 system; (0) b 'Z:, So ('A,)-a 'Ag, So ('Al); (+) c 'Xi, ('B,); (W) A 3Z:y So ('B,). -227 E, must be added. So ('A2)-a '4.So ('B2); (a)'(X 'Xi, T,) ('A2)- 'AgySo (lB2) -0.30 0.08 --0.31 0.06-0 3 r'S9. !-> 0.04-Q, -0.32 0.02 -0.002.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 -0.32 3 4 RIA Fig. 6 Intermolecular potential for the X'Z;, S, state of the C,H,-O, system; the 'A, symmetry in the C,, group.-227 E, must be added. Computation From previous semiempirical calculations' ',,'it is known that the C,, symmetry of the ethene-oxygen collision complex is sufficient for the purposes of the present investiga- tion. We have used the same type of collision complex as has been studied before:,' C-C and 0-0 bonds form a regular trapezium (zx plane) where the intermolecular z-axis is per- pendicular to the plane of the ethene molecule (yx). The restricted open-shell Hartree-Fock (ROHF) method, has been applied to the triplet ground-state complex with the standard 6-31G* basis set (valence double-zeta plus a set of d-type polarization functions).18*' CI calculations account- ing for all single, double, triple and quadruple excitations have been performed in an active space of six occupied and six unoccupied MOs for singlet states. This active space includes lb3, and lb,, (n) ground-state doubly occupied orbitals and lb,, (n*), 3b,,, 3a,, 2b,, and 2b,, empty orbitals of the ethene ground state; molecular oxygen is rep- resented by the ground-state configuration (~,)~(n,)'and by the 30, unoccupied MO. Such a CI is taken into account because it is impossible to reproduce the degeneracy of the 'A, state without all quadruply excited configurations; it gives a wavefunction with ca. 25 OOO configuration state func- tions (CSF).For the triplet states all single and double excita- tions are included together with triple and quadruple excitations for the ngelectrons; this corresponds to wavefunc- tions with about 6200 CSFs. Singlet and triplet states of dif- ferent symmetry corresponding to various combinations of locally excited states inside the ethene and oxygen moieties have been calculated, scanning the intermolecular distance from 5 to 2.4 A. A few charge-transfer states were also studied. The computations were carried out with fixed intra- fragment geometrical parameters for 0, and ethene : 40-0)= 1.168 A, r(C=C) = 1.333 A, r(C-H) = 1.085 A, &HCC) = 121.7'. The choice of the CI wavefunction is based on the fulfilment of the following criteria.(1) Approximate size consistency; our restricted active space wavefunction, with high excitations in a limited set of orbitals, describes the valence correlation well, which is important for open-shell complexes like C2H4 + O,, but neglects a large part of the dynamic correlation. However, only differences of correlation energy at different intermolecu- lar distances are of relevance, and the valence space corre- lation fulfils the size consistency criterion quite well, as shown here. (2) Reproduction of the degeneracy of the alAg state at large intermolecular distance (R) in the complex. (3) Reproduction of other properties of the separated adducts such as the Mulliken atomic charges, spin density and the sum of total energies at large R.(4) Good accordance with experiment2'V2l and with large MCSCF CI calculations22 for the vertical excitation energies of the lower-lying valence excited states of the adducts. The orbitals used in the extensive CI calculations are the ROHF ground-state orbitals. They have not been reopti- mized for each excited state. We suspect that the effects of orbital reoptimization would be quite small. MCSCF CI cal- culations for the ground triplet and singlet states of the complex at the equilibrium distances show that the popu- lation analysis does not change much. Although the main purpose of this paper is connected with collision-induced transition intensities, we also present the calculated intermo- lecular potentials for different states. The qualitative features of all potentials are reproduced quite well for different sets of CI expansions and in different atomic basis sets (3-21G, 4-31G).23 The results of the above described calculations (6- 31G*) are shown in Fig.1-8. All calculations have been carried out using the GAMESS programme package.lg J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Results and Discussion Intermolecular Potentials The potential for the intermolecular interaction in the ground state of the complex X3Zg-, So is shown in Fig. 1. It has a very shallow minimum at intermolecular distance, Re = 3.8A. The depth of the well, estimated as a difference E(5 A) -E(R,) = D,, is only 0.42 kJ mol-'. The potential curves for the low-lying singlet states of the complex (a 'Ag and b oxygen states in collision with the ground-state ethene) are quite similar to the curve represented in Fig.1. The disso- ciation energies are somewhat higher; D, = 0.48 and 0.49 kJ mol-for the a 'A, and b 'El states, respectively. The equi- librium intermolecular distances are only 0.005 A shorter. The complex is extremely weak in all these states and cannot exist at room temperature, so it is better to consider it as a collision complex. This is in qualitative agreement with obser- vations., The next excited singlet state of the complex is pro- duced by the oxygen transition to the clX; state (Fig. 2), which is responsible for the Herzberg I1 band in the near-UV region. The minimum is deeper (D, = 1.42 kJ mol-') and shifted to a shorter distance, Re = 3.5 A.Consequently, this state could be considered as a very weak exciplex (note that the energy scales are different in the figures). The ethene T, excited state (,B,,) interacts with the triplet round-state oxygen without any barrier at a distance of 2.4 1, the short- est found in this work. The singlet component of this 'double-triplet' state is shown in Fig. 3; approximately the same behaviour is exhibited by the triplet counterpart of this term. The energy diminishes drastically at small distances, hotochemical activity of the X3Xg-, T, states (at showingR = 2.4 1the energy is lowed by 0.5 eV). Because vertical transition geometry is used for the T, state of the ethene moiety and no geometry optimization has been performed for the photochemical reaction coordinate, these results have only qualitative meaning.In accordance with the previous MIND0/224 and MIND0/3 CISD it is found that the lV3(X,T,) channels lead to funnels produced by avoided crossings with the ground-states channels (X 'Xi, So and a 'Ag, So) in the transition-state region for the biradical H,C'-CH2-0,' formation reactions.26 This can explain the 'quasichemical' mechanism of the physical T, state quen- ching, including the energy transfer to the singlet a'Ag oxygen (spin ~atalysis).~ 3926 It follows that the singlet funnel appears at longer distances (R w 1.8 A) than the triplet funnel (R w 1.7 and it could mean that the a 'Ag quantum yield is higher than that of complete quenching. The lowest potential-energy surfaces of both multiplicities (X'Xi, So and alAg, So) go to the T and S biradicals (Rc4 = 1.4 A) through the activation barriers, which are formed by avoided crossings with the upper states 'v3(X, T,).The funnels appear on the upper potential-energy surfaces and the T, state quen- ching can proceed through these funnels.26 This could be considered as a general mechanism for many hydrocarbons. The singlet reaction path (a 'A,, So) for C2H4 + 0, addition has been studied recently at the MCSCF level,27 supporting the previous semiempirical results about the biradical forma- tion reaction path.'3*24*25 The chemical path that leads to dioxetan is connected with the other component of the a 'A, state, and follows the 'quasibiradical' reaction mechanism.For more details of the 0,reactivity in S and T states see ref. 13. Ab initio calculations on these funnels are now in progress. The triplet excited ethene, T,, in collision with the singlet oxygen alAg state is described by split potentials, shown in Fig. 4. Both states have shallow minima at large R; the upper potential has a minimum at R, = 4.05 A (D, = 0.18 kJ mol-') and the lower one has a metastable well at Re = 3.8 A J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (D, = 0.22 kJ mol-'), which goes through the activation barrier (5.32 kJ mol- ') to the photochemical reaction zone. Quite interesting features are exhibited by the potentials of the A'A, and A3Z: states, which determine the Herzberg I11 and I bands, respectively (Fig.5, with a larger scale). The A 'A, state is split by intermolecular interaction with the ground state of ethene. The lowest component ('A,) is more stable, D, = 3.38 kJ mol-', having a short intermolecular equilibrium distance, Re = 3.0 A; the potential of the other component is quite similar to that of the A3Zi state, D, = 1.5 and 1.28 kJ mol-l, respectively, as well as to the pre- viously described potential of the c 'Xi state. Singlet excited ethene, S, (n,z*),forms a very stable exci- plex with the ground, triplet-state oxygen. The intermolecular potential (Fig. 6) exhibits a deep minimum, D, = 61 kJ mol-', at a short distance, Re = 2.7 A. This state, X 'Z;, S,, has a large charge-transfer (CT) configuration admixture of the type O,-C,H4+, which also contributes to the X 'Zg-, T, state, although to a lesser extent.All these states as well as the lower components of the A' 'Au, So and a 'Ag, T, states, have 3A, symmetry in the C,, point group of the complex. The CT contributions to these latter states are not so sensi-tive to the intermolecular distance. We have not studied the intermolecular potential of the Schumann-Runge state (B 'Xi) at this level of calculation because it has a much too large vertical excitation energy (> 12 eV). Preliminary CI cal- culations with a small active space (ng,II and n* MOs) in the STO-3G basis set have shown that the. B 'ZC,, So state poten- tial also exhibits a very strong exciplex character, but we cannot discuss its properties because of the crude character of such an approach.The calculated transition energies for the separated mol- ecules and for the complex are discussed el~ewhere.,~ In summary, the sum of the total energies of the separated mol- ecules and the energy of the complex at a large distance (R = 5 A is considered as a dissociation limit) coincide quite well for many states, the differences are <0.001 eV. The exceptions are given by the doubly excited states in both mol- ecules (a ,Ao and b 'X; oxygen states in contact with the T, ethene and with the triplet component of the X 'Ze-, T, state). The difference with the sum of the energies of the free mol- ecules is of the order of 0.01 eV at large distances.The reason for this discrepancy is obvious; all quadrupole excitations are not taken into account for the triplet states of the complex, while all single and double excitations are included in the same restricted active space for the separated molecules. The calculated vertical transition energies are in reasonable agree- ment with the and theoretical" data. For example, for the T, state of ethene we obtain 4.61 eV, in good accordance with experiment (4.36 ev)." For the alAg and b 'El states we have obtained 0.84 (0.98) and 1.56 (1.63) eV, respectively (where the experimental values are given in parentheses). The energies of the vertical Herzberg transitions are in very good agreement with the best ab initioZ2 results: 5.91 (5.87), 6.07 (6.11) and 6.24 (6.25) eV for the c 'Z;, A' 3Au and A 3Z: states, respectively (where the vertical transition energiesz3 are given in parentheses).The only notable excep- tion is the singlet excited '(n,n*)state of ethene, S,. It is well known that it is necessary to take into account the Rydberg atomic orbitals (AOs) for such but for all other states the 6-31G* basis set employed is quite good for quanti- tative reproduction of the vertical transition energies. All transitions between states in consideration, except the S,-So transition in the ethene moiety, are forbidden in the system of two noninteracting molecules. Intermolecular inter- action mixes the locally excited, CT and cooperative states, producing non-zero electric dipole probability for all of the formerly forbidden transitions.The collision-induced singlet- singlet (S-S) transition moments are represented in Fig. 7 and those for the triplet-triplet (T-T) transitions are shown in Fig. 8. SingletSinglet Transitions in the C2H,-0, System At large distances (R 9 Re)the most intensive among the S-S transitions turns out to be the c'C;-a 'A, transition in molecular oxygen induced by collision with the So ground-state ethene. This is really one of the most prominent cases of strong emission from oxygen immersed into condensed media.28-30 c'Ci-a 'Ag Transition The c 'CL-a 'Ag transition in the complex with So ethene cor- responds to the 'A,-'B, transition in the C,, group and is polarized along the 0-0 bond (x axis), so it can borrow intensity from the Schumann-Runge system.The a 'Ap state is split into a lower component, 'A,, which is the antiphase combination of the two n,' closed shells, and an upper com- ponent, 'B,, which has an orbital part of the configuration wavefunction (zg,z,ng,J that is very similar to the triplet ground-state orbital f~nction.'~ The upper component, lB,, is active in the c 'Ci-a 'Ag transition, while the lower com- ponent, 'A,, is active in forming the blZl-alAe Noxon band. The c-a transition has been observed in absorption and emission;21 its oscillator strength in free 0, is of the order of ca. 13v21 The short-lived c -,a emission from 0,diluted in Ar crystals has been studied for different iso- topes and irradiation condition^.^^.^^ The radiative lifetime of the c 'C; state of 16-1802 in matrices is supposed to be ca.1 The oxygen complex with ground-state ethene roughly simulates 0, in an Ar matrix, because the nature of the perturbations which induce the transition probability is qualitatively similar. The calculated oscillator strength for the clZ;-alAg transition at the equilibrium distance of the upper state (Re= 3.5 A) is equal to 2.3 x which corre- sponds to the radiative lifetime, z, = 39 ps. At R = 3.8 A the oscillator strength for the c + a absorption is 7.8 x and z, = 0.11 ms. The value z, = 1.1 ms (f= 7.9 x lo-') appears only at a very large distance, R = 4.6 A. The three to four orders of magnitude enhancement of the c-a transition prob- ability induced by intermolecular interaction with a dia-magnetic species could easily be understood by this calculation. That the observed z, value in Ar matrix is so large can thus be explained as follows.The Ar atoms situated on the opposite sides of the 0, molecule induce the c-a tran-sition moments in opposite directions. In a symmetric static crystal all contributions would cancel each other. The final c 'Ei-a ,Ag transition probability results from non-symmetrical distortion of the crystal and constitutes only a difference of the non-equal contributions from the opposite Ar atoms. Because the distances shorter than Re normally are available for the upper state, the largest contribution to the c 'Zi-a 'Ag transition probability (R = 3.2 A, the left side of the dissociation threshold in Fig.2) could be used for estima- tion of the lower limit for the z, value in the studied C2H4 + 0, complex; it gives z, = 10 ps. b 'Z; + a 'Ag Transition The b 'Z: a 'Ag transition, which was firstly observed by 4 Noxon,', is of pure quadrupole nature3,," in free 0, and has a transition probability, A = 0.0017 s-'. 21 In the colli- sion complex the b -,a transition is enhanced even at a large distance; at R = 4.8 A the Einstein coefficient, A = 0.13 s-', is much larger than in the free molecule. Because the equi- librium value (A = 4.6 s-') is of importance, the distances accessible at normal conditions, R = 3.6-3.5 A, would provide transition probabilities in the region A = 11.6-18.3 s-',respectively.So the four-order enhancement is easily pre- dicted for this transition probability. Only one component of the a 'A, state ('A,) is active for the b 'X: -+ a 'Ag transition and provides z polarization. Transitions to the other com- ponents of the alAg state ('B,) are not forbidden (y polarization), but have much smaller probability. The tran- sition moments at R = 3.6 and 3.2 A are equal to 0.0014 and 0.0021 ea,,t respectively; the ratios of the transition prob- abilities for the (B,/A,) components at these distances 0.07 and 0.03, respectively. So we can neglect splitting of the a 'Ag state and consider the b 'El +a 'A, transition intensity as being determined only by emission to the 'A, component. Three to six orders of magnitude enhancement of the oxygen b +a transition probability has been observed in the gas phase,33 in rare-gas matrices34 and in organic solu- tion~.~~Ethene has not been studied as a perturber in these experiments; however, the calculated A, values are in rea- sonable qualitative agreement with observations for other gases.Cooperatiue '(X 'Xg-, T,)-(a 'A,, So) Transition Quite unexpected behaviour is predicted for this transition probability. If this transition could be observable in emission it would correspond to cooperative phosphorescence, which is 7900 cm- red shifted in comparison with a 'normal' phos- phorescence of the hydrocarbon molecule. The transition moment is negligible at large distances (R > 4.6 A)and is less than that of the b3C: +a 'A, transition up to R = 3.5 8, (Fig.7), but at smaller distances it rises rapidly; at R = 2.8 the cooperative transition moment is the largest among all S-S transitions in the collision complex. The radiative life- time at this point is 6 x lo-* s. Because the upper (X 'Xi, T,) state is unstable upon the initiation of the first (not final) step of photochemical oxygen addition to the C-C double bond, as follows from our preliminary discussion (Fig. 3), we cannot obtain any definite conclusion about the possibility of observation of cooperative phosphorescence. The observation is impossible probably because of the very strong com-petition with non-radiative quenching.We should remember the model character of the system with respect to real dyes with T,(nz*) phosphorescent states. From the present calcu- lations and from earlier semiempirical CI calculations' it follows that the '(X "Xi, T,) state potential is attractive up to the transition state of an intermediate biradical formation reaction (Rca x 1.7 A), where an adiabatic transition to the repulsive (a 'A,, So) state occurs. So the physical quenching of the T,(m*) phosphorescent state by molecular oxygen, which is usually accomplished by the intermediate a 'A, state prod~ction,'~could proceed through such a 'quasichemical' path." Another mechanism of T,(nn*) state quenching is given by long-range (R x 3-4 A) dipole-dipole energy trans- fer from the interaction of collision-induced transition dipole moments [of the b 3X:-a 'Ag and '(X 'Xi, T,)-(a 'Ag, So) transitions], which is accomplished by b state gener- ation.13*" This process is very effective because of large values of these collision-induced transition moments (ca.twice as much in comparison with the previouscalculation^'^" '). On the other hand, observation of the absorption '(X 'Xi, T,) t(a 'A,, So) seems to be easily realized in flash photoly- sis experiments on oxygen-saturation solutions of dyes or even in the gas mixtures studied here. The oscillator strengths for this absorption at the normal accessible distances R = 3.6, 3.4 and 3.2 A, are equal to 2.3 x 7.8 x and t 1 ea, z 8.478 36 x C m.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2.5 x lo-', respectively, so it should be possible to detect such weak absorptions by modern laser techniques achieving large concentrations of a'A, oxygen. In our studied system the transition energy of 3.7 eV is well separated from other absorptions in this region. For naphthalene, the transition energy would be 1.6 eV and this region is transparent for such observations in the absence of large concentrations of the T,(nn*) state. The blX: tX3X, transition is not enhanced by collision and has negligible absorbance. l3 Triplet-Triplet Transitions in the C,H,-O, System The transitions in the triplet manifold (Fig. 8) of the collision complex can be divided into two groups, transitions from the ground state and transitions from the T,(nz*) ethene excited state which is in contact with the ground-state oxygen.The first group has intensity comparable with that of the S-S transitions, which connect the a 'A, oxygen state with other singlet excitations (note that the scale is different in Fig. 7 and 8). The second group has much more enhanced tran- sition probabilities. Transition between Excited S, and T, States Special attention must be paid to the collision-induced (X 3Zi, S,) t(X 'Xi, T,) transition. Although the problem of competition between the absorption and non-radiative quen- ching rates cannot be solved now, we stress the huge rise in this transition probability. At the equilibrium distance of the upper exciplex state (Re= 2.7 A) the oscillator strength is 0.08, which is comparable with symmetry-allowed S-S tran-sitions.The (X 'Xi, s,)-(X 'Xg-, T,) transition moment rises so rapidly (0.1 and 0.41 ea, at R = 3.7 and 3.2 A,respectively) that it is impossible to show it on the same scale with other collision-induced transitions; only a part of it is shown in Fig. 8. Even at large distances (R = 4.2 A), where the energy perturbation of both states is negligible, the oscillator strength of the S,-T, transition is as large as 2.5 x lo-', which should be sufficient for observation. The upper and lower states are both of 'A, symmetry and therefore the tran- sition is z polarized (intermolecular direction). Its intensity cannot borrow from the locally excited z-z* transitions but is determined by charge-transfer perturbations.The possi- bility of observing this transition in emission from the ethene molecule is also questionable, because of rotation of the CH, groups, but it should be possible to find such an exciplex emission in aromatic molecules perturbed by oxygen, Picose- cond laser flash photolysis studies of an oxygen-saturated solution of na~hthalene~~ indicate an exciplex state in which the S, naphthalene combines with an O,(X 'Xi) molecule. Its very fast relaxation to the T, state36 could be understood in terms of our prediction of the unexpectedly strong electron dipole (X'X;, S,)-(X3Ci, T,) transition moment and its dipole-dipole interaction with intramolecular vibrations." This model of electronic relaxation as a dipoldipole energy transfer to intramolecular vibrations has been shown to be quite useful for analysis of non-radiative transitions in oxygen-containing systems.' 5*32 (a'b,, T,) 4 (X 'C;, T,) Emission The next intensive transition from the second group is the oxygen a-X transition induced by collision with the triplet- excited ethene.Its intensity increases rapidly and becomes larger than those of the first group at R > 3.8 A (Fig. 8). The transition has an oscillator strength equal to 6.8 x lo-' at an accessible distance, R = 3.2 A, which is large enough for detection. Because the upper state ('A,) is quite stable upon passing through the photochemical funnel (Fig. 4), we should J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 expect the possibility of observing such a transition in emis- sion. The large collision-induced electric dipole moment of this transition could explain the very effective quenching of the a 'A, state by triplet-excited UHC molecules.15 The tran- sition has 'A,-'A, symmetry, it is z polarized and is of CT nature. Absorption Observed by Evans Among the first group, the transition (X 'Xg-, T,) c(X 'Z;, So) is of great importance and determines the enhancement of the T, tSo absorption by the 0, perturbation., The oscil- lator strengths for this absorption are calculated to be 4.5 x 7 x and 2.7 x at the normally acces- sible distances of 3.6, 3.4 and 3.2 A,respectively. The latter distance corresponds to a rather high pressure of oxygen in the 0,+ C2H4 gas mixture and determines the more than four order of magnitude enhancement of the vertical T, tSo absorption of pure ethene.Spin-orbit coupling calculations predict the oscillator strength for this transition to be of the order of lo-'. 37,38 Such enhancement is in qualitative agree- ment with the observations of Evans.2 The collision-induced absorption in the short wavelength region of the T, +So band, observed by Dijkgraff and Hoijtink7 and attributed to the cooperative (a ,Ag, T,) t (X 'Z;, So) transition, is slightly less intense (Fig. 8). This result is also in accordance with ob~ervation.~ Finally, we have a relatively large enhancement of the Herzberg I11 band (A' 'A,, So) + (X 'Xi, So) to account for.This is a 'A, + 'B, transition, polarized along the x axis. It is of a similar nature to the collision-induced c'C;-a'A, band, but the transition moment is higher (see Fig. 7 and 8). Evans observed this absorption in the 0,-C2H4 gas mixture and assigned it to the Herzberg I transition A 'C,f-X 'Xi. * Our calculations, however, do not support his assignment. The last transition is forbidden in the studied geometry of the complex. For other geometries of collision the Herzberg I transition probability is negligible in comparison with the Herzberg 111 collision-induced transition.' 3*14 This is in accordance with many findings of the UV-induced emission of oxygen in mat rice^^^-'^ and with the enhanced absorption A' 'Au tX 'X,-in pure oxygen at high pressure.2' Evans stressed that ethene enhances this absorption much more than oxygen itself.The oscillator strength of the Herzberg 111 band, (A' 3Au, So) t(X 'Xi, So), is very large in our calcu- lation. At the accessible distances in the ground state, 3.4 and 3.2 A, it is equal to 5 x and respectively. The calculated radiative lifetime of the lower component ('A2) of the (A"A,, So) state of the complex at the equilibrium dis- tance (Re= 3 A) is equal to 3.5 ps. This is shorter than the value observed in an Ar matrix, z x 80 ps.28The reason for this discrepancy is the same as discussed above for the c 'Xi state. The upper component of the (Ar3A,, So) state of the complex (3B,) cannot emit to the ground state (3B2); in this collision geometry the transition is dipole forbidden.The splitting between the components of the (A' 'A,, So) state of the complex is 523 cm-' at Re = 3 A; this is larger than the spin-orbit coupling (SOC) induced irregular splitting in the free oxygen molecule (149 cm-').13*39So in the complex (and perhaps in the matrices) the orbital angular momentum of the A' 'A,, state is effectively quenched. The zero-field splitting (ZFS) of the orbitally non-degenerate 3A2 (A'3A,) state is estimated by the second-order SOC perturbation to be D = -63 cm-', E x 0, where the 0-0 bond direction (x-axis) is the main axis of the ZFS tensor. This means that the t, spin-sublevel is 42 cm-' higher in energy than the t, and t, spin-sublevels.In pure oxygen the lowest R = 3 component of the A' 'Au state is less active in the Herzberg I11 A 'Au t though the whole band is extremely weak (f< In matrices, the emission from the lowest t, and t, spin-sublevels must be observed, though all three spin-sublevels have equal transition probabilities (they are induced by the M,-independent intermolecular interaction, not by SOC). We can also compare the transition energy shifts on going from the free molecule to the complex or to the Ar matrix. The peak positions of the c 'Ci-a 'A, transition in the Ar crystal shift only slightly to the red of the gas-phase values. The difference between the matrix and gas-phase peak posi- tion for the 5-0 transition of the c + a emission is 49 cm-', in cf.108 cm-' for the 0-5 transition of the A' 3Au-+ X 'Xi emission.28 For the C2H4-02 complex the calculated red shift for the vertical c +-a absorption (which could be com- pared with the quoted value) is 66 cm-'. For the correspond- ing A' 3Au + X transition, the calculated red shift, Av = v(R = 5 A) -v(R = 3 A), is 157 cm-', which is of the correct order of magnitude. The intensity of the S,-So tran-sition in ethene in contact with the 02(X3Xg-) molecule is notably diminished. A ca. 10% lower oscillator strength has been obtained at R = 3.4 A.23 This is in accordance with the experimental and semiempirical data. l4 Conclusions CI calculations on the complex C,H,-O, give a good expla- nation for the strong T, + So and A' 3AutX 'Z-transition probability enhancements observed by Evans.' We have changed the assignment of the last UV band (260 nm) from the tentative A'X; tX'Z; assignment of Evans.Co-operative absorptions of different types are predicted. The intermolecular potentials show negligible complex formation energies for the lower states, X 3Xg, a 'Ag and b 'c: of the system. The ethene excited 3(nn*)triplet state, T,, reacts with the triplet X oxygen state without barrier. In accordance with previous semiempirical studies," this means that the ','(X, T,) channels lead to a funnel produced by avoided crossings with the ground-state channels (X 'Xi, So and a 'A,, So) in the transition-state region for the biradical H,C'-CH,-O,' formation reaction.This can explain the 'quasichemical' mechanism of the T,state quenching, includ- ing energy transfer to the singlet oxygen (spin-catalysis).' ' A stable exciplex formed by interaction of the singlet excited '(nn*)ethene with the triplet ground-state oxygen is predicted. The exciplex is stabilized because of the large charge-transfer admixture. By the same reasoning, the singlet-triplet transition '(71n*)-'(zn*)in planar ethene in contact with 02(X 'Zg-) has an enormous probability, compa- rable with that of an allowed singlet-singlet transition. This fact can explain the very fast S, -,T, intersystem crossing in unsaturated hydrocarbons induced by molecular oxygen. (The non-radiative transition is treated by intramolecular energy transfer on vibrations through a collision-induced dipoleaipole interaction.") These predictions are in accord- ance with recent subnanosecond studies of the naphthalene- oxygen e~ciplex.~~ Using the studied system as a crude model for the inter- action of oxygen excited states with diamagnetic closed-shell species, the emission of the 0,-containing inert gas matrices can be explained qualitatively in terms of the b 'X; -,a 'Ag, c 'Xi + a lAg and A' 3Au-,X 'Zg-oxygen transitions. The enhancement can be described as a charge-transfer pertur-bation. References 1 J. B. Birks, Photophysics of Aromatic Molecules, Wiley, New York, 1970, pp. 301-371,492. X3Z, band than the upper s2 = 1, 2 cornp~nents,'~*~~2 D. F. Evans, J. Chem. SOC.,1960,1735.1486 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3 4 D. F. Evans, J. Chem. SOC.,1953,345; 1957,1351. H. Tsubomura and R. S. Mulliken, J. Am. Chem. SOC., 1960, 82, 5966. 23 24 25 B. F. Minaev, Zh. Fiz. Khim., in the press. B. F. Minaev, Zh. Strukt. Khim., 1982,23, 7. B. F. Minaev and V. A. Tikhomirov, Zh. Fiz. Khim., 1984, 58, 5 6 7 8 9 J. N. Murrell, Mol. Phys., 1960, 3, 319. G. J. Hoijtink, Mol. Phys., 1960,3,60. C. Dijkgraaf and G. J. Hoijtink, Tetrahedron, 1963, 19, 179, S. P. McGlynn, T. Azumi and M. Kinosita, Molecular Spectros- copy ofthe Triplet State, Prentice-Hall, Englewood Cliffs, 1969. 0. L. J. Gijzeman, F. Kaufman and G. Porter, J. Chem. SOC., suppl. 2. 26 27 28 646. B. F. Minaev, The Terenin’s Jubilee Conference on Lumines- cence.Theses, Leningrad, Leningrad University Press, 1982, p. 17. G. Tonachini, H. B. Schlegel, F. Bernardi and M. A. Robb, J. Am. Chem. SOC., 1990,112,483. F. Okada, H. Kajihara and S. Koda, Chem. Phys. Lett., 1992, 10 11 12 13 Faraday Trans. 2, 1973,69,708. A. A. Krasnovsky Jr., Chem. Phys. Lett., 1981,81,443. A. P. Darmanyan, Chem. Phys. Lett., 1993,215,477. B. F. Minaev and V. S. Cherkasov, Opt. Spektrosk., 1978, 45, 264. B. F. Minaev, Dissertation Dr Sc., N. N. Semenov Institute of 29 30 31 32 192, 357. R. Rosetti and L. E. Brus, J. Chem. Phys., 1979,71,3963. A. J. Matich, M. G. Bekker, D. Lennon, T. I. Quickenden and C. G. Freeman, J. Phys. Chem., 1993,97,10539. J. F. Noxon, Can. J.Phys., 1961,39, 1110. E. B. Sveshnikova and B. F. Minaev, Opt. Spektrosk., 1983, 54, 14 Chemical Physics, Moscow, 1983. V. K. Mikhalko, G. M. Zhidomirov and 0.L. Lebedev, Zh. Fiz. Khim., 1984,58, 1857. 33 542. E. H. Fink, K. D. Sester, J. Wildt, D. A. Ramsay and M.Verv-loet, Int. J. Quantum Chem., 1991,39, 287. 15 B. F. Minaev, Zh. Prikl. Spektrosk., 1985,42,766. 34 A. C. Becker, U. Schurath, H. Dubost and J. P. Galaup, Chem. 16 17 18 B. F. Minaev, Opt. Spektrosk., 1985,58,761. C. C. J. Roothaan, Rev. Mod. Phys., 1962,32,179. P. C. Hariharan and J. A. Pople, Chem. Phys. Lett., 1972, 66, 35 36 Phys., 1988, 125, 321. P. T. Chou and H. Frei, Chem. Phys. Lett., 1985,122,87. S. L. Logunov and M. A. J. Rodgers, J. Phys. Chem., 1992, %, 217. 2915. 19 20 21 M. W. Schmidt, K. K. Baldridge, J. A. Boatz, J. H. Jensen, S. Koseki, M. S. Gordon, K. A. Nguyen, T. L. Windus and S. T. Elbert, QCPE Bulletin, 1990,10, 52. E. H. van Veen, Chem. Phys. Lett., 1976,41,540. K. P. Huber and G. Herzberg, Constants ofDiatomic Molecules, Van Nostrand, New York, 1979. 37 38 39 I. S. Irgibaeva, B. F. Minaev, Z. M.Muldahmetov and D. M. Kizhner, Zh. Prikl. Spektrosk., 1980,32,66. B. F. Minaev, S. Knuts and H.Agren, Chem. Phys., in the press. P. C. 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ISSN:0956-5000    

DOI:10.1039/FT9949001479

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1487-1493 Grand Canonical Monte Carlo Study of Lennard-Jones Mixtures in Slit Pores Part 3.t-Mixtures of Two Molecular Fluids: Ethane and Propane Roger F. Cracknell and David Nicholson* Department of Chemistry, Imperial College of Science, Technology and Medicine, London, UK SW72AY Mixtures of ethane and propane in a slit micropore at ambient temperatures have been simulated in the grand ensemble. Ethane was modelled as two Lennard-Jones sites and propane as three Lennard-Jones sites. A new type of selectivity-pressure profile, outside the Tan-Gubbins classification, has been noted. The mixed adsorp- tion of the two gases was found to be thermodynamically ideal. Molecular shape and pore size were found to play an important role in determining the extent of adsorptive separation.Much of the observed behaviour is shown to be entropy driven. For adsorptive separation of particular gaseous mixtures to be a viable industrial process, one component must be selec- tively adsorbed on a suitable microporous adsorbent relative to the other. Selectivity is controlled by a number of factors: the adsorbate-adsorbate interactions for like and unlike mol- ecules including the strength of adsorbate-adsorbent inter-actions, as well as geometric considerations such as micropore width and the size and shape of the adsorptive species. The techniques of molecular simulation are ideally suited to discriminate between the relative importance of these factors since model micropores and adsorbents can be defined in an unambiguous fashion on the computer.Molec- ular dynamics' and Monte Carlo have been used to model physisorbed mixtures. Other workers have used mean field density functional theory for the same Density functional theories have the advantage of being computationally faster than full molecular simula- tions; however, studies are at present limited to spherical par- ticles and effects due to molecule shape cannot be probed. On the basis of extensive DFT calculations, Tan and Gubbins' have introduced a classification scheme for selectivity iso- therms; in their class I, for cases where the temperature is significantly greater than the capillary critical temperature of the mixture, the selectivity isotherm shows a single maximum.Class I1 selectivity isotherms occur where the temperature is just above the capillary critical temperature and have double maxima. Class I11 and IV selectivity isotherms occur at tem- peratures below the capillary critical temperatures and are characterised by discontinuities corresponding to capillary condensation and layering transitions. This is the third of a series of papers from an ongoing study of various fluid mixtures in slit-shaped micropores. In the first paper,g we presented results for a grand canonical study of Lennard-Jones mixtures of methane and ethane in graphitic slit pores using spherical models for both com-ponents in order to facilitate comparison with the density functional theory calculations of Tan and Gubbim8 In that paper we established a methodology for performing GCMC simulations of mixtures in an efficient manner by using par- ticle interchange trials in addition to the familiar move, cre- ation and deletion trials.Comparison was also made between our simulation results and predictions made from the ideal adsorbed solution theory (IAST)' derived from single-component isotherms of the model ethane and methane; it t Part 1 :ref. 9. Part 2: ref. 11. was found that the IAST worked well for the system under study. In the second paper' ' we used a two-centre Lennard-Jones model for ethane and a spherical Lennard-Jones model for methane. Ethane molecules in the first adsorbed layer tended to lie with their axes either parallel or perpendicular to the wall, the relative distribution of orientations of molecules being strongly dependent on pore size.This was consistent with Sokolowski's observations for a two-centre Lennard- Jones model of oxygen in slit pores.12 For a full pore, a plot of ethane-methane selectivity us. pore size showed an oscil- latory behaviour as was also noted for the spherical ethane and methane. However, when elongated models of ethane were used, there were significant differences in the positions of maxima and minima. A logical extension of previous work is to study a mixture of two non-spherical particles; in this paper we present results for an adsorbed mixture of ethane, modelled as before by a two-centre Lennard-Jones particle, and propane, model- led as a non-linear three-centre Lennard-Jones particle.Method Potentials The Lennard-Jones (12-6) potential between sites i and j on two molecules is given by uij = -4Eij[($ -(?2)12] The interactions were cut (but not shifted) at 1.756 nm (five times the ethane CT parameter). The parameters used in the simulation are given in Table 1; E, CT are the usual Lennard- Jones parameters defined in eqn. (l),1 is the bond length and 8 is the bond angle. The model of ethane is due to Fischer et ~1.'~and that of propane is due to L~stig.'~ Both models have been shown to reproduce bulk thermodynamic properties accurately,' although we note that for propane other models exist with different geometries and energy parameters, which also accord with experimental thermodynamic properties;" the Lustig potential, however, has the advantage of having three identical Lennard-Jones sites, making it simpler to use from a computational standpoint.The graphitic surface was treated as stacked planes of Lennard-Jones atoms. The interaction energy between a fluid particle and a single graphite surface is given by the 10-4-3 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 potential of Steelel' 4 u,Xz) = 2xpsESf a:f A{ 1(3)"-(2r-52 3A(0.616 + z)~ (2) where A is the separation between graphite layers (0.335 nm) and ps is the number density of carbon atoms in graphite (1 14 &sfnm-3). osf and are the solid-fluid Lennard-Jones param- eters which were calculated by combining the graphite parameters in Table 1 with the appropriate fluid parameters using the Lorentz-Berthelot rules.The external field for a single Lennard-Jones site, dl),in a slit pore of width, H, is the sum of the interaction with both graphitic surfaces and can be expressed mathematically as U(l) = usf(z)+ u,(H -z) (3) (H is the C centre-(: centre separation across the pore). Since our immediate interest here is in equilibrium adsorption at ambient temperatures, we have not accounted for the surface structure of the graphite planes on the pore walls. It should be stressed that a slit pore bounded by stacked parallel layers of graphite represents only a model of a porous carbon, and it is not clear that this is necessarily the best representation; for example, some workers have considered a pore of triangu- lar cross-section to be a more appropriate representation of porous carbon~.'~ Comparison with Experimental Values of & To test the potentials for the simulation, we calculated values for the isosteric heat of adsorption at zero coverage for both propane and ethane on a single graphitic surface using the relationship j$ry w)exp[:-Pu(r, @I dt- do 4&= RT -L (4) exPC-P%(rY 41 dr do In eqn.(4), us is the energy of interaction between the pore and a molecule located at position r with orientation o, j9 = l/kT and L is Avogadro's constant. We used a Monte Carlo integration method with 1 x lo7 trial insertions in order to evaluate eqn.(4). In the limit of large H, there is no pore enhancement effect and the potential represents two separate isolated graphitic surfaces; it was necessary to evalu- ate 4& for single surfaces in order to facilitate unambiguous comparison with experimental data. Our results are sum- marised in Table 2. La1 and Spencer2' have collected various literature values for isosteric heats of adsorption of hydrocarbons on non- porous carbons; our calculated values for both ethane and propane fall within the scatter of the various experiments which gives us confidence in the potentials we are using. GCMC Technique The GCMC method is ideally suited to adsorption problems because the chemical potential of each adsorbed species is specified in advance.The chemical potential can then be Table 1 Potential parameters used in the simulations pair o/nm (E&) 0 I/nm ref. C,H,-C,H, 0.3527 119.57 90 0.216 14 C2H6--c2H,(2CLJ) 0.3512 139.81 -0.2352 13 C(graphite)-C(graphite) 0.340 28.0 --18 Table 2 4& values for adsorption on non-porous carbons; compari- son of potentials used in this work us. experiment adsorbate T/K theoretical experimental" C3H8 300 24.9 24.8-27.3 C2H6 300 18.6 16.0-19.7 a From ref. 20. related to the external pressure by use of an equation of state or by running GCMC simulation of bulk homogeneous adsorbate. We note that the isothermal-isobaric (NPT) Monte Carlo method has been successfully applied to the study of mixtures on a single s~rface;~ however, extension to problems involving pores is problematic because the pressure tensor normal to the walls cannot be equated to the bulk pressure.As in both of our previous papersg*" we used four types of trial, attempts to move particles, attempts to delete particles, attempts to create particles and attempts to swap particle identities. A detailed discussion of the method is given in ref. 9 and 11. The z dimension of the simulation box was equal to the pore width. Periodic boundary conditions were applied in the x and y directions; for a given simulation the x and y dimensions of the system were chosen so that ca. 100-250 molecules were present in the simulation. The minimum x and y box size was 3.527 nm, which is the minimum image distance for the potential used.No significant system size effects were observed. The simulations were generally run for 5 x lo6 configurations on Intel i860 processors in a Trans- tech 'parastation' with a PC front end acting as host. Sta- tistics were not collected over the first 2 x lo7configurations. The simulations took between 2 and 8 h for a single point depending on the number of particles in the system. The acceptance rate was particularly low for the smaller pore sizes studied (H < 0.762 nm) and 1 x lo7configurations were required for adequate convergence of the simulation (error bars are shown for the selectivities in Fig. 1). It is clear that studies of more complex mixtures in pores will require a biasing technique' to be computationally viable.II40 S 30 20 nL-0.0 0.1 0.2 0.3 0.4 0.5-10 0' I I 0 5 10 P/bar Fig. 1 Separation us. pressure profiles for a propaneethane mixture of equal bulk mole fraction at pore sizes H/nm = 0.763 (O),0.9525 (V), 1.143 (0)and 3.048 (A). The inset shows the low-pressure region in greater detail. T = 296.2 K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Selectivity at Zero-pressure Limit The selectivity in the limit of zero pressure can be determined from extrapolation of low-pressure GCMC results. Alterna- tively, it can be deduced from the ratio of the one particle configurational integrals. Thus $ exp[ -/?u~RoPANE(r,o)]dr du, so = $riJva)]dr do (5) exp[ --fiuFTHANE(r, The integrals were calculated by Monte Carlo numerical inte- gration using 1 x 10’ trial insertions. Results and Discussion Selectivity Isotherms We use the conventional definition of selectivity22 as the ratio of the mole fractions in the pore to the ratio of the mole fractions in the bulk, thus the selectivity of propane over ethane is defined as where x refers to a pore mole fraction and y to a bulk mole fraction.Fig. 1 shows the selectivity us. pressure profile for various pore sizes at 296.2 K with equal bulk absolute activities [the absolute activity, z = exp(/3p)/(A3A,), with A the thermal de Broglie wavelength, and Ar the reciprocal of the rotational molecular partition function]. In the low-pressure limit, this is equivalent to equal bulk mole fractions (ye = y, = 0.5).At higher pressures there is some densification of propane in the bulk relative to ethane; thus, for example, at a pressure of 8.7 bar, y, = 0.54 with equal bulk absolute activities. Density functional calculations* show weak dependence of selectivity on mole fraction and the small difference in bulk mole frac- tions would not be expected to have a significant effect on selectivity. The simulated propane-ethane selectivity isotherms for the three largest pores in Fig. 1 would all be classified as type I in the classification scheme of Tan and Gubbins,” with the selectivity going through a maximum before decaying with additional pressure. The small pore (H = 0.762 nm) does not show the maximum and does not conform to any isotherm in the Tan-Gubbins classification.Experimentally, however, it is quite common: for example, Fig. 10 of the original paper on the ideal solution theory” (see below) shows a selectivity isotherm for a C02-C2H, mixture on an activated carbon at ambient temperature which has just this shape. In our paper on mixtures of methane and a two-centre Lennard-Jones model of ethane,’ we investigated the effect of arbitrarily elongating the ethane molecule when simulating mixture adsorption in a pore of a given width. We observed that the selectivity of ethane over methane decreased mark- edly with ethane molecular length at all pressures and the maximum in the selectivity isotherm also disappeared. As the molecular length increases, the number of orientations which do not cause repulsive overlaps in the system become fewer. Consequently the number of configurations which make a positive contribution to the configurational integral become fewer, or in other words the entropy decreases.The reason for maxima in selectivity isotherms is that the more strongly adsorbed component undergoes an element of cooperative filling which increases the selectivity above that observed at the zero-pressure limit for enthalpic reasons. For molecular fluids, however, a higher pore density increases the risk of one molecule ‘interfering’ with the rotation of another thus there is an entropic penalty associated with higher pore density. For sufficiently small pores (for example, the H = 0.762 nm plot in Fig.1) the entropic effect swamps the enthalpic effect and the maximum ceases to exist. Tan and Gubbins used spherical molecules in their density functional calculations and consequently could not possibly observe an entropic effect of the type we describe. Comparison with Results from Single-component Isotherms Experimentally, multicomponent isotherms are relatively dif- ficult to measure as compared to single-component data. It is useful therefore to be able to use single-component data to be able to predict multicomponent adsorption. For a pore of width H = 0.9525 nm we used GCMC to generate single- component isotherms for ethane and propane (Fig. 2). The adsorption is plotted as a density with the pore volume in that calculation including all the space between the two opposing planes of carbon centres.The isosteric heat of adsorption, qsT, was calculated, assuming the gas phase to be ideal, using the relation where 0 is the configurational energy per adsorbed particle. The calculated values are shown in Fig. 3. The differential (derivative) molar entropy of adsorption, As, can be defined in different ways, first as the difference between the differen- tial entropy of the adsorbed phase, s,, and the molar gas entropy, s, , at a particular temperature and pressure, in which case As1 = S, -S, = -qsT/T (8) where s refers to differential entropy. An alternative definition is the difference between the differential entropy of the adsorbed phase and the molar gas entropy at a particular reference state, S; (in this work we have taken the reference state to be a pressure of 1 atm) thus (9) The differential entropies of adsorption corresponding to the two definitions are shown in Fig.4. The definition of eqn. (9) 0 0 1 2 3 Plbar Fig. 2 Single-component isotherms for propane (V) and ethane (0) in a pore of width H = 0.9525 nm. The physical width, H (the dis-tance between carbon centres in adjacent graphitic planes) is used to calculate the volume of pore for the density shown in the figure. T = 296.2 K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 related according to 25 1n(P0)= 7rno(p) d In p It is possible for example to calculate xi for a given P and yi by first solving for Po(n)in the equation This equation follows from eqn.(10) since the sum of the mole fractions in the pore must equal unity. To implement the IAST, it is necessary to choose a suitable fitting function for the single-component isotherms. We used two alternative fitting functions, the first a Langmuir-Freundlich (LF) isotherm of the form n(P)= x (KP)” 1 +(KPp where n is the uptake for a given pressure P and X, K and a are adjustable parameters. The advantage of using an LF fit is that the integral in eqn. (11)can be carried out analytically. A disadvantage is that it does not reduce to Henry’s law in the limit of low pressure. While for certain purposes this may not be a problem, it is the loading divided by the pressure which is integrated with respect to pressure in eqn. (1 1) and the low-pressure region makes a substantial contribution to the integral.23 An alternative function for fitting single- component data to the Langmuir uniform distribution (LUD) equation, which is the Langmuir isotherm modified for a patchwise heterogeneous surface, with m, C and s adjustable parameters is: 1 This does reduce to Henry’s law at low pressure but the inte- gral to determine the spreading pressure cannot be done ana- lytically, making it more cumbersome.We adopted essentially the algorithm suggested by Myers in the Appendix to ref. 23 in order to use the LUD equation in the IAST. As shown by Fig. 5, the IAST was found to work well for the pore size we tested it on (H = 0.9525 nm), with little dif- ference observed between the LF and LUD fitted single- 35 -5 0123456 ~/nrn-~ Fig.3 Isosteric heat of adsorption (single component) for propane (V) and ethane (0)us. density in a pore of width H = 0.9525 nm. T = 296.2 K. (which is shown with filled symbols) provides the more useful comparison between ethane and propane, since both sets of data refer to the differential entropy of the adsorbed phase minus the same constant factor. The differential entropy of adsorption for propane does decrease more sharply with pore fdling than is the case for ethane. This would appear to bear out the arguments used in the proceeding section concerning an entropic effect hindering selective adsorption of propane.The extent to which single-component isotherms can give information about mixed adsorption depends on the ideality of the mixing process: The ideal adsorbed solution theory (IAST)’’ is in essence Raoult’s law applied to adsorbed mix- tures ; comparison of IAST predictions from single-component data with mixture isotherms should give a good indication of the ideality or otherwise of the mixture. For a given component i, we can write Py, = Po(n)xi (10) where yi and xi are the bulk and pore mole fractions of i, respectively, P is the total bulk pressure and Po(7t) is the bulk pressure corresponding to spreading pressure 7t in the single- component isotherm of component i, for which Po and 7t are I I I I I I I I I I II-25 I 0123456 ~/nrn-~ Fig.4 Differential entropy of adsorption (single component) of V)and ethane (0,propane (0, 0)us. density in a pore of width H = 0.9525 nm. T = 296.2 K. The hollow points are As calculated by eqn. (8); the filled points by eqn. (9) taking 1 atm of bulk gas pressure as the reference state. I I 0 5 10 P/bar Fig. 5 Separation us. pressure profiles for a propane-ethane mixture of equal bulk mole fraction at a pore size of 0.9525 nm. Comparison of results from mixture GCMC simulation (0)with ideal adsorbed solution theory calculations from the single-component isotherms of Fig. 2 which were fitted by Langmuir-Freundlich (a) and Langmuir uniform distribution (b)fitting functions. T = 296.2 K. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 component isotherms in Fig. 2. The IAST appears to underpredict systematically the selectivity by ca. 10-1 5%, indicating a limited degree of non-ideality. It would be unwise, however, to attempt to draw too many conclusions from the discrepancy since the mixture GCMC results are subject to a certain amount of statistical error because of low acceptance rates. In our previous studies of methane and ethane, the IAST was also found to give good results; however, in that study the LUD variant gave superior results to calculations based on fits to the LF equation. Effect of Pore Size on Selectivity The zero-pressure selectivity was calculated using eqn. (5) for various pore widths at 296.6 K.Selectivities (for equal absol- ute activities) at 8.7 bar were obtained from GCMC and results for both pressures are displayed in Fig. 6. The zero- pressure plot shows a single maximum. The selectivity decreases to the right of the maximum because the depth of the potential wells become less. The selectivity decreases to the left of the maximum because of a molecular sieving effect, with propane unable to enter the smallest pores. The molecu- lar sieving effect is also observed for the higher pressure; however, the selectivity does not decrease monotonically to the right of the first maximum. To attempt to understand the 1o2 10' 1oc lo-' 0.0 0.5 1.o 1.5 2.0 8 6 \ (04 2 9 0.991 0 ,o0 L I I I 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Hlnm Fig. 6 Separation us.pore size at zero pressure (line) and 8.7 bar (0)with equal bulk mole fraction of propane and ethane. (a)Plot showing results at both pressures, (b)plot showing 8.7 bar simulation results only. The particular pore widths marked are used in the density profiles. Fig. 7 Diagrammatic representation of centre of mass and vectorial direction for (a)propane and (b)ethane. The density profiles in Fig. 8 and 10 refer to the positions of the centre of mass. The orientations in Fig. 9 and 11 show the orientation of the vectorial direction shown in this figure relative to the line normal to the pore walls. molecular features underlying the variation of selectivity with pore size, we show distribution functions and snapshots for four pore widths [those pore widths are labelled on Fig.qb)]. Two types of distribution function are plotted, the density distributions refer to the density of centres of mass of par- ticles, for a given distance, r, from the centre of the pore. The angular distributions refer to the square of the angle between a particular reference vector on each molecule and the normal to the pore walls. The particular vectors are for propane the bisector of the CCC angle and for ethane, the C-C bond. The positions of centres of mass and the refer- ence vectors are shown in Fig. 7 for propane and ethane. Fig. 8 and 9 show the density and angle profiles for ethane at 8.7 bar. In the smallest pore (H = 0.762 nm) there is a narrow density peak in the centre of the pore and the angle distribution suggests that the ethanes are constrained to lie flat against the wall.For the next size of pore in the distribu- tions (H = 0.857 nm), there is again a sharp peak in the centre of the pore but the angle profile indicates that the ethanes are orienting themselves so as to have a methyl group in the energy minimum of opposite walls. The density profile shows a shoulder, which corresponds to some ethanes lying parallel to the wall. At H = 0.991 nm, the density in the centre has diminished considerably although the molecules are oriented so as to be almost perpendicular to the wall; there is a sharp density peak corresponding to ethanes lying flat against the wall. At H = 1.238 nm, there are virtually no ethane molecule centres in the pore centre (the noise in the angle profile in the centre is a consequence of this).There is a broad peak, corresponding to molecules lying flat against the wall, and a shoulder, corresponding to ethanes perpendicular I I mI E 35 +-h 0 0.0 0.1 0.2 0.3 rfnm Fig. 8 Density profiles for ethane in pores of widths (a)0.762, (b) 0.857, (c) 0.991 and (d) 1.238 nm. P = 8.7 bar. T = 296.6 K. 0.9 1 ' 1 -.-. L " ..I 0.00.0 0.10.1 0.20.2 0.30.3 r/nm Fig. 9 Angle profiles ((cos' 0)) for ethane in pores (widths as given in Fig. 8). P = 8.7 bar. T = 296.6 K. 0 is the angle between the normal to the pore wall and the C-C bond in ethane (see Fig. 7). to the wall, but the pore is now too wide for an ethane to span it and have methyl groups in the potential minima of both walls.Fig. 10 and 11 show the density and angle profiles for propane. The propane densities are higher because propane is more strongly adsorbed than ethane giving rise to the selec-tivity. In the pore of width 0.762 nm, the propane molecules are sterically constrained to lie flat. In the pore of width H = 0.857 nm, the minimum energy occurs when all three methyl groups lie in the potential minimum of a wall. One way to achieve this is for the molecule to lie flat against one wall; there is an entropic penalty to this and few propanes do it (although ethanes will lie flat, as evidenced by the shoulder in the H = 0.857 nm density profile).The other minimum-energy positions occur when one bond is parallel to the wall and one perpendicular (that the bonds can be exactly parallel and perpendicular is a consequence of the 90" bond angle in the propane model used here). From geometrical consider-ations it can be deduced that this situation corresponds to the maximum in the density profile and the shoulder at (cos' 0) = 0.5 in the angle profile. The other minimum-energy situation is to have the middle methyl group on one wall and the other two on the wall opposite, such is likely to be the case for particles with centre of mass in the pore centre. For H = 0.991, there is a small broad peak corre-sponding to propanes lying flat and a very broad central 80 70 60 0 50 E c 40-' r-Q 30 20 10 0 0.0 0.1 0.2 0.3 rjnm Fig.10 Density profiles for propane in pores (widths as given in Fig. 8). P = 8.7 bar. T = 296.6 K. The densities refer to the centres of mass of the propane molecules (see Fig. 7). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.9 I I I 0.8 -0.7 -o.6 <(W NG0.5 -1 ,.-. ...8 0.4 -\ ;' '. . ,' '..(d) .a. I. ,I' ' ' ...../0.3 -,. .,,.,; .,.. .... .;. ; . .. .. Fig. 11 given in Fig.-8).P = 8.7 bar. T = 296.6 K. 8 is the angle between the normal to the pore wall and the bisector of the C-C-C angle in propane (see Fig. 7). peak of greater maximum density than that for the propane lying flat (in sharp contrast to the behaviour of ethane). The H = 1.238 nm density profile can be interpreted in terms of some propanes lying flat against a wall and a larger number with one or two methyl groups in the potential well of the wall.It is possible to provide an interpretation of the minima in Fig. 6(b)in terms of the structure of the adsorbed phase. At H = 0.762 nm both propane and ethane are constrained to lie flat; however, it is more entropically unfavourable for propane to lie flat than for ethane. At H =0.991 nm, it is energetically favourable for the adsorbate to pack into two layers flat against the walls, again it is much more entropi-cally unfavourable for propane to do so than ethane and another minimum in selectivity is found. Fig. 12 shows snapshots for ethane-propane mixtures in the pores of width 0.762 nm (a), 0.857 nm (b), 0.991 nm (c), 1.238 nm (4. Only the bonds are shown; the bonds for propane are black, those for ethane are grey.The planes of carbon centres in the graphite are also shown in black. Snap-shots only represent a particular configuration and do not necessarily give any indication of the configurationally aver-aged adsorbate structure; nevertheless, they can be used to help visualise the profiles in Fig. 8-11. For example, one can see that the adsorbate molecules are constrained to lie rela-tively flat in Fig. 12(a). In Fig. 12(b) a number of propane molecules have one bond parallel to the wall and one perpen-dicular, as suggested by the density profiles. In Fig. 12(c) we see the propanes distributed over a wide single layer whilst in Fig.12(4 we see two distinct layers emerging. Note the lack of recognisable ordering in an individual configuration, the ordering being revealed only by statistical averages (i.e. profiles). While this may not be too surprising at the tem-perature of the simulation, it does help us to understand why entropic effects are so important in determining selectivity. Conclusions We have presented results for selective adsorption of a two-centre Lennard-Jones model of ethane and a three-centre Lennard-Jones model of propane in model microporous carbons. We have shown that adsorbed molecular fluids can give a more diverse range of selectivity isotherms than adsorbed atomic fluids, We have found a new class of selec-tivity isotherm, frequently observed experimentally, but not hitherto seen by simulation or theory.This class of isotherm appears when the entropy considerations outweigh energy considerations, leading to a monotonic decrease of selectivity J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 12 Snapshots of ethane and propane in pores of width 0.762 (a),0.857 (b),0.991 (c) and 1.238 nm (d). P = 8.7 bar. T = 296.6 K. Equal bulk mole fractions. Only the bonds are shown, with C-C bonds shown as black in propane and grey for ethane. The planes of C centres in the pore walls are also shown as black. with adsorption. It is clear that the Tan and Gubbins classi- fication scheme,' although a major advance in the under- 6 7 E.Kierlik and M. L. Rosinberg, Phys. Rev. A, 1991,44,5025. 2. Tan, U. M. B. Marconi, F. van Swol and K. E. Gubbins, J. standing of adsorptive separation, is incomplete. The ideal adsorbed solution theory (IAST) can provide a quite accurate description of propane-ethane mixture adsorption, suggesting a reasonable degree of ideality of the adsorbed phase for this case. 8 9 10 11 Chem. Phys., 1989,90,3704. 2. Tan and K. E. Gubbins, J. Phys. Chem., 1992,%, 845. R. F. Cracknell, D. Nicholson and N. Quirke, Mol. Phys., 1993, 90,885. A. L. Myers and J. M. Prausnitz, AIChE J., 1965,11, 121. R. F. Cracknell, D. Nicholson and N. Quirke, Mol. Sim., 1994, in The behaviour of selectivity with pore size has also been investigated. At zero pressure, a single maximum in selectivity is observed.At higher pressures, several minima have been observed which we have ascribed to entropic effects. 12 13 14 15 the press. S. Sokolowski, Mol. Phys., 1992,75,999. J. Fischer, R. Lustig, H. Breitenfelder-Manske and W. Lemming, Mol. Phys., 1984,52,485. R. Lustig, Mol. Phys., 1986, 59, 173. R. Lustig, A. Torro-Labbe and W. A. Steele, Fluid Phase This project was funded under BRITE EURAM CON-TRACT BREU-CT92-0568. The authors thank Dr. N. G. 16 17 Equilib., 1989,48, 1. R. Lustig and W. A. Steele, Mol. Phys., 1988,65,475. S. Toxvaerd, J. Chem. Phys., 1989,91,3716. Parsonage of Imperial College, Prof, N. Quirke of ECCSAT, and Mr. S. Tennison of BP for their helpful comments and encouragement. 18 19 W. A. Steele, The Interaction of Gases with Solid Surfaces, Perga-mon, Oxford, 1974. M. Bojan and W. A. Steele, Ber. Bunsenges. Phys. Chem., 1990, 94,300. 20 M. La1 and D. Spencer, J. Chem. SOC., Faruday Trans. 2, 1973, 21 70, 910. R. F. Cracknell, D. Nicholson, N. G. Parsonage and H.Evans, References 1 S. Sokolowski and J. Fischer, Mol. Phys., 1990,71, 393. 2 F. Karavias and A. L. Myers, Mol. Sim., 1970,8,51. 3 D. M. Razmus and C. K. Hall, AIChE J., 1991,37,5. 4 J. E. Finn and P. A. Monson, Mol. Phys., 1992,72,661. 5 E. Kierlik, M. Rosinberg, J. E. Finn and P. A. Monson, Mol. Phys., 1992, 75, 1435. 22 23 Mol. Phys., 1990,71,931. D. M. Ruthven, Principles of Adsorption and Adsorption Pro- cesses, Wiley, New York, 1984. A, L. Myers, in Fundamentals of Adsorption, ed. A. L. Myers and G. Belfort, Engineering Foundation, New York, 1984. Paper 4/00546E; Receiued 28th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001487

出版商:RSC    

年代:1994

数据来源: RSC

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J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1495-1499 Electroanalytical/X-Ray Photoelectron Spectroscopy Investigation on Glucose Oxidase adsorbed on Platinum G. E. De Benedetto, C. Malitesta" and C. G. Zambonin Laboratorio di Chimica Analitica, Dipartimento di Chimica, Via Orabona, 4, 70726 Bari, Italy Glucose oxidase has been adsorbed on Pt from a solution of the enzyme (0.5-15 pmol dm-3) in phosphate or acetate buffer (pH 3.9-8.0). The amperometric response to glucose of the modified electrodes so prepared has been evaluated. A spectroscopic (XPS) investigation of the system surface indicates that the enzyme molecules are stacked on the Pt surface, some directly interacting with the metal and the remaining part adsorbed on them. At the surface, the enzyme has a conformation in which its carbohydrate part tends to cover the proteic portion.Biosensors represent a major field of investigation, owing to their widespread potential applications. Among them, immo- bilized enzyme biosensors represent an important class. The immobilization of enzymes at electrode surfaces, however, is still a critical step in amperometric biosensor assembling. Such an operation has classically been performed by methods which produce unstable devices (e.g. adsorption) or which consist of complex procedures (e.g. chemical binding). Elec- trochemical immobilization in an electrosynthesized polymer matrix has recently emerged as a strategy to overcome both of these drawbacks in a one-step operation. In addition, a proper choice of entrapping polymers1-6 can produce, at the same time, a significant improvement in interferent rejection, which is classically obtained by use of additional membranes.Adsorption of the enzyme at the electrode surface before the electrochemical polymerization has an important role' in determining the enzyme loading and so the final response of the sensor. So, a study of the adsorbed biomolecule at the surface could be of paramount importance in designing these kinds of biosensors. In this respect, X-ray photoelectron spec- troscopy (XPS), which is a surface technique able to give chemical information, appears to offer interesting pos-sibilities. In fact, amino acids and proteins have already been investigated'.' by this approach, as well as the adsorption"-12 of proteins on a solid surface.In spite of this, only two13*14 attempts, to our knowledge, have been made in using XPS for investigating immobilized enzyme biosensors. In particular, both works investigated only the electrode sur- faces at which the enzyme had to be bound, giving informa- tion on chemical surface modifications following some pretreatments, a base from which to attempt the interpreta- tion of some properties of the electrode+xzyme system sub- sequently assembled. A glucose electrode based on glucose oxidase (GOx) bound at a Pt electrode surface is, by far, the most studied biosensor. Nonetheless, the adsorption of GOx on Pt is only classified as irreversible and no spectroscopic investigation has been undertaken.Consequently, an XPS study has been carried out in this laboratory and relevant results are reported. An electrochemical characterization of active enzyme on the surface was also performed and the results critically com- pared with the spectroscopic data. Experimental Chemicals GOx (VII type from Aspergillus niger) and 8-D-glucose were obtained from Sigma. 0.5 mol dm-3 glucose solution (freshly prepared in phosphate buffer, pH 7.0, every week) was allowed to mutarotate at 4°C overnight before use. Triply distilled water was used in all experiments. All of the other chemicals were of analytical grade and were used without further purification. GOx solutions were prepared by dilution of 15 pmol dm -stock solutions in the selected buffers.Apparatus All electrochemical experiments were carried out using a PAR 174A polarographic analyser (EG&G Princeton Applied Research) coupled to a Hewlett Packard model 1070 XYt recorder (HP, Palo Alto, CA). The conventional three- electrode cell contained an Ag I AgCl reference electrode and a Pt foil counter electrode. When necessary the solutions were magnetically stirred. XPS was performed using a Leybold LSHlO spectrometer. The spectrometer energy scale was calibrated for Cu 2p,,, = 932.6 eV and Au 4f,,, = 84.0 eV. Preparation of Samples with Adsorbed Enzyme Unless otherwise specified, GOx was adsorbed on a Pt(Au) disc (1 mm diameter) sealed in glass. The electrode surface was polished with emery paper and alumina powder (0.3 pm), then sonicated.Finally, the electrode (Pt) was etched for a few minutes by hot HNO, or (for Au) cycled15 in phosphate buffer (pH 6.0) between -0.9 and 1.0 V us. Ag IAgCl. Enzyme adsorption was performed, immediately before use, in a selec-ted buffer containing GOx (0.5-15.0 pmol dm-3, typical con- centrations in biosensor preparation). The enzyme electrode was then thoroughly washed with triply distilled water for removing the loosely bound enzyme. XPS samples were simi- larly prepared by employing Pt sheets (20 mm x 10 mm) which were fully immersed in the relevant solution. The excess water was eliminated by a flow of nitrogen. Drying was then completed in the spectrometer vacuum. Response to Glucose of the Adsorbed Enzyme The enzymatic activity present on each enzyme electrode was estimated amperometrically in selected buffers : acetate (I = 0.1, pH = 3.9, 5.2) and phosphate (I = 0.1, pH = 6.0, 6.5, 7.3, 8.0).For this purpose, the Pt I GOx electrode was main- tained at +0.7 V us. Ag I AgCl in a stirred buffer up to a low, nearly constant current value. A known amount of glucose solution was then added and the i us. t curve for the oxida- tion, at a given potential, of the H20,produced according to the known enzymatic process glucose + 0, -+ gluconolactone + H202 (1) was recorded. As each experiment was carried out on a new (because of the cleaning procedure) electrode surface, a nor- malization procedure for the glucose response was designed, consisting of the addition of a known amount of H,O, after glucose injection.The resulting anodic current increment was recorded. Results are presented as 'normalized glucose response' [i,(glucose)/i,(H20,)]. In this manner the standard deviation of the response improves by about 40%.The nature of the buffer does not influence the response, as verified by performing measurements at a selected pH (5.63),realized by either acetate or phosphate buffer. XPS Experiments The first few experiments were performed by cooling samples to 100 K in order to avoid degradation. In the following we observed that some degradation occurs (see Appendix 1) only if traces of the buffers are left on the samples. So, samples described in the paper have been thoroughly washed with water before XPS analysis at room temperature.For all samples wide-scan (FRR mode) and high-resolution 0 Is, Pt 4f, N Is, C 1s (FAT mode) spectra were acquired. Analysis of the spectra were performed as reported.16 The spectrum energy scale was corrected by setting the hydrocarbon con- taminant, C 1s = 284.8 eV. The occurrence of a local charg- ing effect different from site to site was excluded on the basis of the shape of the N 1s signal which was always nearly sym- metric and had the proper full width at half maximum (FWHM). Results and Discussion Following immersion of Pt in GOx solution a rapid adsorption l7 occurs. Preliminary results from EQCM (electrochemical quartz crystal microbalance) measure-ments" have been obtained recently. They show qualitatively that adsorption consists of a first rapid step followed by a further slower one.The system produced in this laboratory by enzyme adsorp- tion has been electrochemically and spectroscopically charac- terized. All of the reported results are relevant to a 15 pmol dm-enzyme solution for enzyme electrode preparation. The data obtained by employing different enzyme concentrations, in the range 0.5-15 pmol dm-3, were qualitatively similar. Electrochemical Results A typical response to glucose of the enzyme adsorbed on platinum is shown in Fig. 1. The response time to glucose is shorter than that measured for other devices with the enzyme differently immobilised (see e.g.ref. 19). This is probably due to the absence of any kind of membrane or film that could slow down the response. The effect of the pH of the buffers in which the measurements were carried out on the activity of the adsorbed enzyme was studied, measuring the response of the sensor to a 5 mmol dm-3 glucose solution. The plot of normalized response us. pH, shown in Fig. 2 for two different times of adsorption, appears interesting: it seems to exhibit two maxima, which are not very marked, particularly at short times of adsorption, but which are reproducible. In contrast, only one peak is shown by the activity of GOx in solution2' or when immobilized2' on a solid such as a porous glass. Tentatively, since the enzyme in solution has its maximum of activity at around pH 5.5, it seems reasonable to attribute the low pH curve maximum to enzyme molecules slightly modi- fied by adsorption. The second component, with a maximum around pH 7, could be due to the enzyme directly and J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 j*O"A 10 s c c g 30 it glucose time Fig. 1 Typical current-time response to glucose and to H,O, for a Pt I GOx electrode obtained by 10 min adsorption (15 pmol dm-3 GOx solution in phosphate buffer, pH = 7.3, I = 0.1). E = +0.7 V vs. Ag I AgC1. open-circuit potential ( + 0.8 V us. Ag I AgCl), the Pt electrode is likely to be covered by negative species (electrode anions, or enzyme molecules if pH > 4.2 which is the enzyme isoelectric" point) and a shift of the maximum activity region to higher pH is expected under these conditions.22 Two find- ings seem to be in agreement with the above view: (1) an increase of the low-pH peak (larger than that of the high-pH peak) at longer adsorption times, and (2) a shift of the high-pH peak towards higher pH at longer adsorption times (larger amount of negatively charged molecules on the electrode).A similar model was proposed by Wilson and co- worker~,~~who postulated for the adsorption of GOx on a different electrode material, that first a layer of directly bound molecules builds up, on which other enzyme layers can later be adsorbed. For the sake of comparison with the immobilized enzyme biosensors, the stability of Pt I GOx was studied by measuring the response to glucose at different times from its prep- aration.The electrodes when not in use were stored in acetate buffer. The response, nearly constant in the first 2 h, shows a decrease of up to 50% (Fig. 3) after 5 h. Later, the anodic n~ I I 0 3 4 5 6 7 8 1 I PH Fig. 2 pH-activity profile of Pt I GOx electrodes prepared for differ- ent adsorption times: 10 s(.)and 10 min (@). Each point represents strongly adsorbed onto the electrode surface. In effect, at the mean value on at least seven replicates performed over 15 days. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1401 0 0 0:'i 0 0 20- current measured upon adding the glucose was stable up to about 100 h after preparation. This is consistent with the relatively rapid loss of the weakly bound enzyme species which are responsible for the low-pH peak in Fig.2. A comparison between the active enzyme loadings on Pt and Au was also performed by means of normalized anodic response to glucose. The results for Au were 40% higher than those for Pt. This suggests that the enzyme adsorbed on the gold electrode is either in greater abundance or simply more active than that adsorbed on platinum. On the other hand, the absolute response to glucose (and to H202) for an Au electrode was definitely lower than for a Pt electrode of the same area. This fact indicates that many fewer centres active for H202oxidation are present on the gold surface. Finally, a comparison can be made between the present sensor and the one-step amperometric sensor based' on GOx immobilized in an electropolymerized PPD film.The anodic current measured with the former was 65-70% of that obtained with the latter device. The importance of adsorption in the building of sensors such as the Pt I PPD IGOx sensor is confirmed. The remaining 30-35% of the current is likely to be due to entrapping of additional enzyme in the membrane, even if it is only 100 A thick. Spectroscopic Investigations Fig. 4 shows a widescan spectrum for a sample of GOx adsorbed on Pt. The presence of an N Is signal, coupled to intense 0 1s and C 1s peaks, suggests that, even after the 1497 Table 1 Binding energies of XPS signals recorded on GOx adsorbed on Pt from a 15 pmol dm-3 solution of GOx in phosphate buffer (pH = 6.0, I = 0.1);possible errors up to 0.3 eV signal E,IeV Pt 4f,,, 70.7 0 1s 531.4 N 1s 399.7 284.8 286.2 288.4 washing procedure (see Experimental), a certain amount of enzyme remains strongly adsorbed on the Pt surface.At the same time, the presence of the Pt 4f signal in the spectrum indicates that the enzyme layer is either discontinuous or, if continuous, is formed according to the smallest dimension of the enzyme molecule (vide infra). As far as the high-resolution spectra are concerned, only the C 1s signal showed an evident multicomponent profile and was studied in detail. The binding energies (EJ of all photoelectronic signals are reported in Table 1.N and 0 signals showed E, values in good agreement with those already reported8Pi2 for amino acids, peptides and proteins as powders and adsorbates. Fig. 5 reports a typical C 1s spectrum for a sample pre- pared by 10 s adsorption. Three components are present: C-H, C-0 and C=O (see Table 1). C-H represents the hydrocarbon component. The second contribution due to the C-0 component belongs to the carbohydrate chain and to the protein present in a pr~bable~~-~~ ratio of 78 :22. The C=O component is due to contributions of the peptidic (ca. 90%)and the carbohydrate (ca. 10%)chains. On the basis of the relevant signal areas obtained with 10 s samples, atomic ratios of C(C-0) : N = 0.93(k0.08):1 and C(C-0) : C(C-0) = 1.3q40.13): 1 were calculated.While the former agrees with its theoretical value24-26 (1.0 :1) the latter is very24-26 different (0.6 : 1). Theoretical values were calculated by considering a homogeneous distribution of the functional groups in the GOx molecule. Actually, a large part of the carbohydrate moiety is deeply buried in the polypep- tide chain,25 so that the theoretical value for the C(C-0) : C(C-0) ratio is likely to be lower. The discrep- ancy relevant to C(C-0) :C(C-0) could indicate that in the conformation of the adsorbed GOx the exterior part (which gives the largest contribution to the XPS signal) con- sists of the carbohydrate chain. In other words, this chain should cover the protein portion which is obviously the nearest part to the Pt surface and is likely to be involved in the bonding responsible for adsorption.The agreement rele- vant to C(C-0) :N is not surprising considering that the C-0 present in GOx belongs to amidic groups for about 500 1000 1500 kinetic energy/eV 1 I I 1 I Fig. 4 Typical overall spectrum for GOx adsorbed on a Pt foil (10 s 290 288 286 284 adsorption in 15 pmol dmP3 GOx solution in acetate buffer kinetic energy/eV pH = 5.2, I = 0.1). The sample was washed with water after the adsorption step. Fig. 5 High-resolution C 1s XP spectrum for the sample of Fig. 4 Table 2 N : Pt ratios for the Pt IGOx,,, sample adsorption time N :Pt coverage 10 s 0.39 :1 0.82 10 min 0.67 :1 0.88 The 10 s sample was obtained by 10 s adsorption from a 15 pmol dm-3 GOx solution in phosphate buffer (pH = 6.0, I = 0.1).After XPS measurements it was reimmersed in the enzyme solution for a total of 10 min, thus obtaining the 10 rnin sample. 83%; in any case, the proximity of the two species guarantees that in the most part C-0 and N are sampled simulta- neously, producing equivalent signals. Qualitatively similar results were obtained with samples prepared by a 10 rnin immersion in the enzyme solution. Information on the coverage and arrangement of ellipsoidal' enzyme molecules on the platinum surface was inferred from N :Pt ratios: some of them are collected in Table 2. These results are the product of an experiment designed to investigate accurately the correlation between enzyme loading and adsorption time.A sample was prepared by 10 s adsorption, analysed and then again exposed to enzyme solution for a total period of 10 min. The sample was then reanalysed. Similar results were obtained for 10 s and 10 min samples prepared in different experiments. N: Pt ratio can be correlated to the coverage, 0 (see Appendix 2), employing the approach suggested by Dilks.,' To this end, the enzyme was first considered to be distributed in one monolayer, consisting of molecules oriented perpen- dicular or parallel to the surfa~e.'~ More complex arrange- ments were not considered from a quantitative point of view. The N :Pt ratio for a full coverage of the electrode surface by horizontally oriented molecules is 0.26 :1 (the maximum theoretical value), well below any measured ratio (see e.g. Table 2).On this basis, it was discarded. In contrast, when molecules are in a standing position N :Pt can span from 0 to co.Table 2 also reports the coverages resulting from the latter model and calculated from the relevant N :Pt ratios. The devised simple picture cannot explain the higher current response (+ 30%) found (vide ante) for 10min in com- parison to 10 s adsorption: in fact, the coverage correspond- ing to the relevant N :Pt values increases by only 6% (Table 2). An increase of the activity of the same amount of enzyme for longer times of contact with Pt has to be excluded because the opposite effect is often rep~rted.'~,~~ A compari-son between electrochemical and XPS data is not always pos- sible since, in general, modifications could occur in the sample under XPS measurements (e.g.drying). Nonetheless, in this case the comparison is meaningful. Since, N is homogeneously distributed in the enzyme molecule, the thickness (t) of the monolayer is the main factor influencing the N :Pt ratio. t could, at most, be reduced (for example in the drying step). This would produce a limiting value for R, (see Appendix 2) less than 0.26, and cause the 8, values in Table 2 to become even closer, confirming the above observa- tions. It was therefore necessary to adjust the model since the enzyme molecules were partly vertically and partly horizon- tally oriented for both adsorption times. A possible explana- tion could be that the first molecules are adsorbed in a horizontal position (maximising the contact surface area) and then some of them stand up, leaving surface available as further molecules approach the electrode.In this manner, a much greater number of molecules can be accommodated on the surface even if the coverage changes slightly. An alterna- tive picture is possible, i.e. enzyme molecules arranged as multilayer islands over the electrode surface. These islands could perhaps consist of a layer of horizontally oriented mol- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ecules directly adsorbed onto the electrode over which other enzyme layers may be adsorbed. The fact that GOx prefers to grow in multilayers instead of exhaustively covering the Pt surface, could simply be the result of a pretreatment method that is unable to distribute uniformly active sites on the surface.In this case, the uncovered Pt should simply form the part free of active sites. The island model appears to be more consistent with the electrochemical data and agreed7 with some literature reports. This view could also explain the apparent lack of any change in the measured Ebvalues fol- lowing interactions (adsorption) between Pt and GOx. A definitive choice between these models requires further inves- tigation and a more suitable method of sample preparation able to produce monolayer fraction coverage, In particular, a study by atomic force microscopy (AFM) is planned for the near future.Appendix 1 The reader interested in performing XPS experiments of the kind described in the present work must be advised that it is mandatory to wash out thoroughly any trace of the buffer employed in the adsorption step before executing the spectro- scopic analysis. We found that in the presence of acetate or phosphate, C 1s spectra exhibited a fourth component at higher binding energy (5.2-6.2 eV higher than the hydrocar- bon peak, often detectable from the beginning of the experiment), probably attributable to degradation products. While this effect is particularly evident (Fig. Al) when the acetate buffer was used, the component was present even with phosphate. This constant presence seems to indicate that the above-mentioned component at high is also related to an enzyme degradation.Nonetheless, the absence of the effect in thoroughly washed samples suggests that the buffer has a role in the degradation phenomena. On the basis of the E, value, the possibility that the component is due to either carbon monoxide or carbon dioxide adsorbed over Pt seems to be excluded.28 These C species, at present unknown, seem to be organic carbonates. 29*3 Appendix 2 In obtaining the relationship between coverage and N :Pt ratio for a GOx monolayer two different arrangement^'^ of I I I I II 293 291 289 287 285 Fig. A1 C 1s spectrum for a sample of GOx adsorbed on a Pt foil (10 s adsorption in 15 pmol dm-3 GOx solution in acetate buffer, pH = 5.2, I = 0.1). The sample was washed with the buffer after the adsorption step.The carbonyl component contains in this case the signal due to acetate as well. The selected spectrum is relative to an experiment in which the degradation component (X) was particularly evident. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. A2 Arrangements of the GOx molecule on the Pt surface: (a) parallel, (b) perpendicular to the surface the ellipsoidal enzyme molecule on the Pt surface were con- sidered: (a) parallel and (b) perpendicular to the Pt surface (Fig. A2). Calculations were performed employing the approach sug- gested by Dilks.,’ For this purpose, the shape of the mol- ecule was approximated to a cylinder (base, ellipse with axes 140 and 50 A; height, 50 A for the horizontal molecule; base, circle of radius 25 A; height, 140 8, for the vertical molecule).The intensity of the N 1s signal for a homogeneous N dis- tribution and the direction for electron collection normal to the surface can be expressedz7 as: 1, = FkN UN AN 8S[i -eXp(-t/&)]cN (All where F is the X-ray flux, k, is an instrumental parameter also depending on the kinetic energy of the photoelectrons, 0, is the cross-section for the photoemission, iN is the inelas- tic mean free path for N 1s photoelectrons, S is the sampled surface, 8 is the coverage (i.e. the fraction of S covered by the enzyme), t is the thickness of the ‘N’ layer and C, is its N concentration. The term in parentheses spans from 0 (t = 0) to 1 (t = a).The last condition is verified in practice when t 2 31.1 values in the enzyme for N 1s (32.1 A) and Pt 4f (37.8 A)electrons (excited by Mg-Kcr) were calculated by the equation reported3 for organic materials taking the density of the enzyme as ca. 1 g cmd3 [estimated from molar volume (cylindrical shape) and weight (186 OOO g mol-‘)I.The intensity of the Pt 4f signal contains two contribu- tions: Ipt= Fk, optAbt 6Scptexp(-t/Apt) + Fkpt gpt Ah(1 -6)SCpt (A2) The first term of eqn. (A2) represents the Pt lying under the enzyme and whose signal is revealed after attenuation through the enzyme layer. The second represents the contri- bution of uncovered Pt. The apex in Abt means that the value is relevant to the metallic platinum medium. From2’ kPt Opt &dkN gN AN = SPt/SN (A31 where si is the relative sensitivity factor of the element ‘i’, it is possible to define the quantity, R : = (lN/SN)/(lPt/SPt) (A41 which can be easily correlated to 8.The relevant equations for the two arrangements of the enzyme molecules [(a) and (b)in Fig. A2, respectively] are: 6, = RJ0.069 + 0.73Rl) (A5) 6,+O*R,-+O (A5’) 6, + 1 R, + 0.26 (A57 6, = Rz/(0.087 + R,) (A61 8, + O= R, + 0 (A67 6, + 1 * R, -+ 00 (A6) The reason for a finite limit in the case of horizontal mol- ecules can be easily rationalised if one considers that even at full coverage Pt 4f electrons can be detected after travelling through the enzyme layer since 3RPt > 50 A. Francesco Palmisano (University of Bari) and Robert Hill- mann (University of Leicester) are gratefully acknowledged for helpful discussions.The work was carried out with the financial support of Minister0 dell’universita e della Ricerca Scientifica (MURST) and Consiglio Nazionale delle Ricerche (CNR). References 1 C. Malitesta, F. Palmisano, L. Torsi and P. G. Zambonin, Anal. Chem., 1990,62,2735. 2 D. Centonze, A. Guerrieri, C. Malitesta, F. Palmisano and P. G. Zambonin, Fresenius J. Anal. Chem., 1992,342,729. 3 D. Centonze, A. Guerrieri, C. Malitesta, F. Palmisano and P. G. Zambonin, Ann. Chim. (Rome), 1992,82,219. 4 D. Centonze, C. Malitesta, F. Palmisano and P. G. Zambonin, Electroanalysis, 1994, in the press. 5 F. Palmisano and P. G. Zambonin, Anal.Chem., 1993,65,36. 6 F. Palmisano, D. Centonze, A. Guerrieri and P. G. Zambonin, Biosensors, 1993,8, 393. 7 P. N. Bartlett and R. G. Whitaker, J. Electroanal. Chem., 1987, 224,37. 8 D. T. Clark, J. Peeling and L. Colling, Biochem. Biophys. Acta, 1976,453,533. 9 K. D. Bomben and S. B. Dev, Anal. Chem., 1988,60, 1393. 10 B. D. Ratner, T. A. Horbett, H. R. Thomas and D. Shuttleworth, J. Colloid Interface Sci., 1981,83, 630. 11 B. D. Ratner, T. A. Horbett, H. R. Thomas and R. W. Paynter, J. Colloid Interface Sci., 1984, 101, 233. 12 H. Fitzpatrick, P. F. Luckham, S. Eriksen and K. Hammond, J. Colloid Interface Sci., 1992, 149, 1. 13 A. Proctor, J. F. Castner, L. B. Wingard Jr. and D. M. Hercules, Anal. Chem., 1985,57, 1644. 14 F. A. Armstrong, P.A. Cox, H. A. 0. Hill and V. J. Lowe, J. Electroanal. Chem., 1987,217, 331. 15 A. Szucs, G. D. Hitchens, and J. O’M. Bockris, J. Electrochem. SOC.,1989,136,3748. 16 C. Malitesta, L. Sabbatini, P. G. Zambonin, L. Peraldo Bicelli and S. Mafi, J. Chem. SOC.,Faruday Trans. I, 1989,85, 1685. 17 Chem. Abstr., 1990,112, no. 51261. 18 A. R. Hillman and M. J. Swann, personal communication. 19 H. Gunasingham and C. B. Tan, Analyst (London), 1989, 114, 695. 20 D. Keilin and E. F. Hartree, Biochemistry, 1948,42, 221. 21 K. B. Ramachandran and D. D. Perlmutter, Biotechnol. Bioeng., 1976, 18,669. 22 L. Goldstein, in Methods in Enzymology, ed. K. Mosbach, Aca- demic Press, London, 1976, vol. XLIV, ch. 29. 23 B. S. Hill, C. A. Scolari and G. S. Wilson, J. Electroanal. Chem., 1988,252,125. 24 J. H. Pazur, K. Kleppe and A. Cepure, Arch. Biochem. Biophys., 1965,111,351. 25 S. Nakamura and S. Hayashi, FEBS Lett., 1974,41,327. 26 H. J. Hecht, H. M. Kalisz, J. Hendle, R. D. Schmid and D. Scomburg, J. Mol. Biol., 1993,229, 153. 27 A. Dilks, in Electron Spectroscopy, ed. C. R. Brundle and A. D. Baker, Academic Press, London, 1981, vol. 4, ch. 5. 28 P. R. Norton, Surf: Sci., 1474,44, 624. 29 U. Gelius, P. F. Heden, J. Hedman, B. J. Lindberg, R. Manne, R. Nordberg, C. Nordling and K. Siegbahn, Phys. Scr., 1970,2,70. 30 G. Beamson and D. Briggs, High Resolution XPS of Organic Polymers, Wiley, Chichester, 1992, App. 1. 31 M. P. Seah, in Practical Surface Analysis, ed. D. Briggs and M. P. Seah, Wiley, Chichester, 2nd edn., 1990, vol. 1, p. 209. Paper 4/00555D; Received 28th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001495

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1501-1506 Normal and Anomalous Positronium States in Ionic and Molecular Solids investigated via Magnetic Field Effects T. Goworek,? A. Badia and G. Duplatre" Laboratoire de Chimie Nucleaire, CNRS/lN2P3 and Universite Louis Pasteur, Centre de Recherches Nucleaires, B.P.20,67037Strasbourg Cedex 2,France ~ ~~~ An external magnetic field (B) is applied in positron lifetime spectroscopy and Doppler broadening (DBARL) experiments to derive information on the positronium (Ps) state in solid matrices. The solids investigated are thorium phosphate and a molecular matrix, p-terphenyl, doped with either 0.5% anthracene or 1.5% chrysene to produce extrinsic defects of sufficient concentration and size to allow Ps formation. In thorium phosphate at zero field, the triplet Ps (0-Ps) pick-off lifetime is short (700 ps), reflecting annihilation in rather small (0.105 nm) free volumes of the lattice.This lifetime rises to 1160 and 1440 ps in p-terphenyl doped with pyrene and anthracene, respectively, reflecting annihilation in microvoids of 0.19-0.23 nm radius, which corresponds roughly to the size of a naphthalene molecule. The variations of the parameter R and of the 0-Ps (rn = 0) magnetic substrate lifetime with B in thorium phosphate are 'anomalous', requiring the use of two fitting parameters. The first parameter is the contact density affecting the hyperfine splitting, with a low value (q = 0.22) denoting an expand- ed Ps wavefunction. For the second fitting parameter, two possibilities exist: either a contact density affecting the Ps singlet state (p-Ps) lifetime, q' = 0.36, distinct from q, or a p-Ps pick-off lifetime (390ps) different from that of 0-Ps (700 ps).Analysis of the DBARL data shows that the second hypothesis is unlikely. By contrast, the magnetic field effects in doped p-terphenyl are 'normal', with a single fitting parameter, q = 0.80and 0.83,for anthracene and pyrene as dopants, respectively. After penetrating matter, the positron (e') can form a bound state with one of the electrons (e-) it has released by ionis- ation of the medium at the end of its track.' This bound state, positronium (Ps), possesses two spin states, triplet (ortho-positronium, 0-Ps) and singlet (para-positronium, p-Ps) . In matter, the positron in the latter state is annihilated essentially with its bound electron, in an intrinsic mode, while in the case of 0-Ps it is annihilated mainly in an extrinsic mode, termed 'pick-off ',with an electron from the surround- ing molecules.Positronium has been increasingly used during the past decade as a probe for determining various physico- chemical properties of matter.' In solids, and more specifi- cally in molecular solids, Ps appears to be a unique tool for investigating defects, whether intrinsic or not. Therefore, a quantitative expression has been established,'*2 correlating the 0-Ps lifetime with the size of those microvoids of the solid matrix in which 0-Ps is annihilated. The relative amount, or intensity, of 0-Ps formed is most generally correlated to the concentration of defects: no Ps is formed in solids where no free spaces are present, or if these are too small. Fruitful applications of Ps as a probe demand a good knowledge of its properties.These are most commonly assessed by using positron annihilation lifetime spectroscopy (LS), which delivers the lifetimes and intensities of the various positron states. To complement this, angular correlation or Doppler broadening of the annihilation radiation lineshape (DBARL) techniques give access to the intensities of the posi- tron states and to the momentum distributions of the annihi- lating e+/e- pairs, which also depend on the immediate surroundings of e+ and Ps. Combining these two classes of independent techniques (e.g.LS and DBARL) is very impor- tant to characterize the various processes in Ps chemistry, particularly regarding the reactions of solutes involved either at the moment of Ps formation or thereafter.3 However, the information derived from these techniques may not be suffi- cient. Therefore, many properties of Ps have remained poorly known, such as the triplet to singlet formation ratio, the p-Ps t On leave of absence from Instytut Fizyki UMCS, Lublin, Poland. pick-off process and the existence of excited or distorted Ps states. A convenient way to gain information on the Ps wave-function, and thereby shed some light on the above unknowns, is to apply an external magnetic field (B).3The mixing of the rn = 0 substates of Ps induced by B provokes changes in the LS spectra that can be analysed and quantified through equations involving the contact density parameter (q)which expresses the electron density, I $(0)12,at the posi- tron in matter (subscript m) with respect to that in vacuum (subscript v; q = 1 in vacuo): This density influences both the value of the hyperfine split- ting, AE, and the decay rate constants of p-Ps (A:) and o-Ps (A:), in the absence of magnetic field.However, if excited states of Ps are present (L # 0), with non-spherical wavefunc- tions, the contact density parameter affecting AE can be dif- ferent from that affecting the As (denoted v'):~ (3) A; = q2, + Ap3 % Ap3 (4) where As = 8 and At = 0.007 ns-' are the intrinsic p-Ps and 0-Ps decay rate constants, respectively; Apl and Lp3 are the pick-off decay rate constants for p-Ps and 0-Ps, respectively which are usually considered to be the same.In the pure liquids studied up to now, whether polar or non-polar, Ps appears to behave normally, with 0.6 < q < 1, which corresponds to a Ps atom that is slightly swollen com- pared with its size in vacu~m.~?~ In solids, in contrast, the results are complex. In many mat rice^,^-^ ranging from ionic KC1, with a rather short 0-Ps lifetime (ri = 0.68 ns),' to molecular octadecane (T: = 1.5 n~)~and polymeric Teflon, with a long 0-Ps lifetime (r: = 4.15 ns),8 the contact density parameter has values similar to those in liquids. However, in a variety of solids, strongly anomalous variations in the LS parameters are observed, particularly at low B, which have been quantitatively described and explained in various ways.Therefore, in a series of polymers, the data are interpreted by supposing that there exist two types of Ps atoms in the solid, presumably corresponding to two different sites or regions of the lattice^:^" the 'normal' type, with q > 0.6, gives a contri- bution ranging from about 100% in Teflon' down to 65% in terfane;8 while the 'anomalous' species is strongly affected by B, with q as low as 0.1 and even 0.05, in isotactic poly- pr~pylene.~In naphthalene, fitting of the data requires the use of two distinct, rather low contact density parameters, affecting the hyperfine splitting and the intrinsic decay rate constant, respectively.lo Finally, whereas quite normal q values are found in some polycrystalline organic scintil- lators," much stronger magnetic field effects are observed in others,12 which are not quantified and are tentatively explained by the interaction of the nascent Ps atoms with the magnetic moments of radiolytic species produced in the posi- tron spur. Note that the role of the contact density parameters, q and q', is not restricted to magnetic field effects.Together with A,,, they intervene in the expression of the p-Ps decay rate constant in matter, eqn. (3), and therefore in the expressions for the various intensities ; consequently, they should affect the experimental results of all positron annihilation tech- niques : LS, DBARL, three-quantum annihilation, etc.Note that the increase in p-Ps lifetime implicit in eqn. (3) for q' < 1 was never observed experimentally. Considering that the factors governing the above parameters are far from under- stood, and the complete lack of an experimental estimate of Apl, the aim of this paper is to gain more information of Ps behaviour, via magnetic field effects, in a variety of solid matrices. These include an ionic compound in which the pick-off lifetime is expected to be short, and molecular com- pounds, previously studied in the absence of field,l3?l4 doped with smaller molecules in order to produce extrinsic defects of sufficient concentration and size to allow Ps formation and reasonably long pick-off lifetimes ; thorium phosphate, for which preliminary results have been presented at the last Positron and Positronium Chemistry Workshop (PPC4)l and p-terphenyl doped with either 0.5 wt.% anthracene or 1.5 wt.% chrysene.To derive more detailed information, both LS and DBARL were used. Experimental Materials Thorium phosphate was polycrystalline, prepared at high temperature' and compressed into self-supporting pellets of suficient thickness for complete absorption of the positrons. Briefly, synthesis of this compound was achieved by dis- solving thorium nitrate (from Rhbne Poulenc) in concen-trated phosphoric acid (from Merck), and then evaporating to dryness. The volatile residues were eliminated by heating the solid in an oven, at a rate of 1 K h-', and then keeping the temperature constant for 1 h, successively at 573 and 873 K.Thereafter, the samples were heated again, at the same rate, and kept for 6 h, successively at 1223 and 1573 K, with continual grinding. As established by various methods, including proton-induced X-ray emission (PIXE) and X-ray photoelectron spectroscopy (XPS), the stoichiometry of the final salt is Th,(PO,), . The molecular matrix, p-terphenyl, usually employed to prepare liquid scintillators, was from Nuclear Enterprises and was used as received. Preliminary LS measurements showed no detectable long-lived component in this compound. The dopants, pure grade from Aldrich, were further purified by J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 recrystallization. The preparation of the doped matrices has been described previ0us1y.l~ In the case of anthracene, the doped samples were in the form of single crystals grown from the vapour phase17 and cut in the primary cleavage plane, while with chrysene, they were polycrystalline, moulded directly from the melt in the form of pellets. Previous experi- ments have shown that no significant difference exists in the LS parameters between such polycrystalline samples and single crystals. l4 In either case, doping introduces defects in the p-terphenyl lattice, allowing for Ps formation, with the 0-Ps intensity increasing with dopant concentration, up to a saturation val~e.'~*'~The amounts of dopants, 0.5 wt.% anthracene and 1.5 wt.% chrysene, were thus chosen to corre- spond to the onset of saturation.Techniques The positron source consisted of ca. 1 MBq ',Na embedded between two thin Kapton foils, giving a source correction of 8%, and was sandwiched between two pieces of samples for LS and DBARL countings. The LS spectrometer, equipped with plastic scintillators and light guides, was specially designed for shielding the photomultipliers from the magnetic field.' The time resolution, as given by the full width at half maximum (FWHM) of the 6oCoprompt curve, was 380 ps. At B = 0, complementary measurements were made using spectrometers with resolutions of 310 ps (plastic scintillators) or 215 ps (BaF, scintillators).When the field is off, the LS spectra include three com- ponents of lifetimes zo (or decay rate constants, A! = l/zo) and intensities I?, where i = 1, 2 and 3, referring to p-Ps, free e+ and 0-Ps, respectively. At B # 0, the m = 0 substates of Ps are mixed, resulting in the appearance of four components: 0-Ps (rn = f1) remains unaffected, with a lifetime (z3 = 7:) and an intensity (I3 = 21:/3) independent of B, while p-Ps and 0-Ps (rn = 0), the latter with subscript i = 4, have con- stant intensities (Il = I, = 1:/3) but decay rate constants varying with B according to the following equations:19 A1 = (y'l: + A?)/(l + y2) (5) Ah = (y2A?+ Ai)/(l + y2) (6) with y = [(l + x')~', -1]/~; x = 0.027 56B(T)/q (7) In the presence of B, owing to the limited resolution of the spectrometer, the LS spectra cannot be analysed in terms of four components without fixing some parameters.Therefore, the z, values were derived by fixing some of the parameters as measured at zero field: T:, I, = 21:/3 and I, = Z:/3.576 To avoid such constraints, the LS data can, alternatively, be pre-sented in the form of parameter R, which is the ratio of the normalized areas (f) of the spectra in a specified time window (t,,tb),when the field is on and off: t, is chosen such that the contribution of the short-lifetime components is very small, and t,, at the limit of statistically significant counts.5s6 The experimental errors were within 40 ns for z4, and 0.013 for R. The DBARL data were collected using a hyperpure Ge detector with a resolution of 1.38 keV at 514 keV (85Sr photopeak). The data will be reported in the form of the FWHM of the experimental spectra: the error on FWHM was within 0.01 keV.The spectra can be deconvoluted using the resolution function, and resolved into Gaussian com-ponents, corresponding to the various positron states, with intensities ID and FWHMs Ti, where i denotes the same states as for the LS parameters.,' Correlations exist and can J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 be established quantitatively between the LS and DBARL intensities. All experiments were carried out at 294 K. Results and Discussion Thorium Phosphate Table 1 displays the LS parameters derived at B = 0 when using the BaF, scintillators.Within experimental error, these agree with those found using the two other spectrometers. It may be seen that z: is rather short, indicating that Ps is trapped in a small void of the lattice. Quantitatively, an expression has been derived correlating the 0-Ps decay rate constant, A:, to the radius (R,) of the presumably spherical microvoids in which Ps is annihilated:'*2 A:/ns-' = 211 -RJR, + (1/2n)sin(2nRv/R,)] (8) where R,/nm = R, + 0.166. In the present case, with 2: = 1/0.7 ns-', the radius of the largest free space in the thorium phosphate lattice is thus expected to be R, = 0.105 nm. As no crystallographic data are yet available for thorium phos- phate, it is not possible to compare R, with effective sizes of the free spaces in this lattice.The decrease with B of both R and z4, as shown in Fig. 1, is as expected from eqn. (5)-(7) : increasing B progressively decreases z4 and removes 1/3 of the integral of counts from the time window (la, tb). Several hypotheses (noted h) were considered to fit the data, based on eqn. (3) and (4): (hl) a 1.0 0.9 R 0.8 > 2.74 9 2 2.73 z 2.72 Fig. 1 Variations of parameter R (a),0-Ps (rn = 0) lifetime, zJns (O),and FWHM/keV of the DBARL line (a)with magnetic field intensity, B/T, in thorium phosphate. The broken lines are calculated on the hypothesis of a single fitting parameter [(hl) in Table 11, and the solid lines, on the hypothesis of two fitting parameters C(h2) or (h3) in Table 11.Error bars are given on the figure. single fitted parameter is used, with q = q', and the pick-off lifetimes for o-Ps and p-Ps are supposed to be the same, zpl = zp3 = z:; (h2) and (h3) two fitting parameters are involved, with either q # q' and rpl = zY3= z: or q = q' and zpl # zp3.Note that as far as the variations with B of R and z4 are concerned, (h2) and (h3) correspond to the same fitting, as parameters q' and zpl appear as a sum in eqn. (3). The resulting values are given in Table 1, together with the ensuing standard deviations, cR and cZ4.With the simplest hypothesis (hl), usually valid in most cases reported, the stan- dard deviations, c,, = 40 ps and uR = 0.021, compare reason- ably well with the experimental errors, 40 ps and 0.013, respectively.However, the fitting is such that for both R and z4 the calculated variations with B (broken lines in Fig. 1) appear systematically below the experimental plots at low field intensity and above at high B. Furthermore, trial calcu- lations show that q x 0.35 is the optimum value to give the lowest average slopes of the variations. For these reasons, (hl) does not appear to be realistic and the better standard deviations obtained with either (h2) or (h3), which lead to excellent agreement with the shapes of the experimental plots (solid lines in Fig. l), are more acceptable. However, as stressed above, which of these two hypotheses is correct cannot be decided from the LS data alone. As is usual in all liquids previously studied,*' a small broad component, with intensity (4 +_ 0.3)% and r = (7.1 & 0.2) keV, was present in the deconvoluted DBARL spectra at all values of B.This component is poss- ibly attributable to the annihilation of energetic positrons or, alternatively, may correspond to some contribution of core electrons in the annihilation of the various positron states; it is implicitly taken into account in all treatments of the data that follow. Owing to the limited resolution of the DBARL apparatus and to the mediocre value of I: in thorium phosphate (Table l),it appears difficult to derive significant results when resolv- ing the deconvoluted spectra into four components (including the broad component) without fixing some parameters.Therefore, the 15 spectra obtained at B = 0 were first analysed by fixing the free positron intensity, such as I: = (100 -41:/3). This led to a very reproducible value of r2= (2.74 +_ 0.02) keV. Other possibilities were explored which all led to very similar results, thus giving confidence in the above value. Therefore, the average values found after each fitting run for the remaining parameters were fixed, successively for rl and I?. At each run, the DBARL parameters were very similar to those obtained at the last run, with all parameters fixed except r3,the values of which are given in Table 1 (the intensities are corrected for the broad component). The variation of FWHM with B calculated using the data in Table 1 is in excellent agreement with the experimental plot (solid line in Fig.1). The values of r resemble those usually obtained in liquids." In particular, rl is much higher than 0.23 keV, the value expected for thermalized free Ps atoms at 294 K,6 indi-cating that Ps is effectively trapped in a microvoid of the lattice. Note that the use of a single parameter corresponding to the pick-off process, r3,implies that the momentum dis- tributions from pick-off annihilation of both o-Ps and p-Ps Table 1 Experimental LS and DBARL parameters for thorium phosphate at B = 0, and parameters derived from the fitting of the variations of both R and z4 with B [q, q' and zpl, according to hypotheses (hl), (h2) and (h3)] together with the resulting standard deviations (a); time windows, t, = 1.6 ns, t, = 5 ns r;/PS 4/PS 1; (%) 1: ("0) 1: (Yo) F1/keV r,/kev r,/kev 322 f 10 700+ 17 22.7 f1.2 4.5 -t 0.5 25.8 k 1.3 0.88 f0.1 2.74 t-0.02 1.95 f0.03 (hl) = 0.32 & 0.02; oT4= 40PS;oR= 0.021.(h2), (h3) q = 0.22 _+ 0.06; h(2) q' = 0.36 & 0.06;(h3) zPl = 390 f50 PS;or4= 17 PS; oR= 0.010. are the same; the limited resolution of DBARL does not allow one to seek differences between these distributions. The intensity for intrinsic annihilation, I?, is significantly lower than @3, showing that a large portion, ca. 40%, of p-Ps is annihilated via pick-off: this is expected on the basis of eqn. (3), which predicts that if q’ [or q, on hypothesis (hl)] is low, the contribution from pick-off can become important. Whereas the LS data do not allow (h2) and (h3) to be distin- guished, the DBARL results should provide the information by considering the following equations : for (h2) I? = (W3)~’&h’4 + Ap3) (9) for (h3) I? = (m3)q&/(v&+ Jpl) (10) From these expressions, and the data in Table 1, the expected values for I? are 4.8% and 2.9% for (h2) and (h3), respec- tively.By comparison with the value of 4.5% derived from the experimental spectra, it appears that (h2), which implies a similar value for the pick-off lifetimes of p-Ps and 0-Ps and distinct values for q and q’, is more likely than (h3). p-Terphenyl as a Matrix Table 2 collects the LS parameters obtained for the doped p-terphenyl samples. It may be seen that these are very similar with either dopant.The 0-Ps intensities, at about 20%, both represent the values obtained at saturation, when the dopant concentration is increased. The 0-Ps lifetimes and intensities in both mixtures are in excellent agreement with previous determinations. l4*I8 In accordance with the increase of I! with dopant concentration, the lifetimes are long enough to indicate that Ps is effectively trapped in those defects introduced by the dopant molecules. Thus, from eqn. (8), a spherical void of the size of a whole benzene molecule, with a radius of 0.14 nm, would lead to a lifetime of 855 ps only. In the present case, it is expected that anthracene and chrysene should introduce defects of roughly the size of a naphthalene molecule.The latter is not spherical, but two estimates of the size of such a defect can be obtained by con- sidering the distances from the centre of symmetry of the molecule to the farthest (0.252 nm) or second-farthest (0.185 nm) carbon atoms: from eqn. (8), the corresponding 0-Ps life- times are 1660 ps and 1120 ns, respectively. The values in Table 2 are effectively within these limits, as are several others for a variety of dopants in p-ter~henyl,’~ which were found to range between 1110 ps with a-benzopyrene to 1620 ps with phenanthrene. The linear shape of anthracene appears to allow a better insertion of this molecule into the p-terphenyl lattice, with 7; closer to the value corresponding to the largest possible void, than chrysene which produces defects of a smaller size reflecting some geometrical hin- drance.When applying B, with either anthracene or chrysene as dopants, both R and z4 decrease (Fig. 2 and 3) in a much smoother way than in thorium phosphate. Quantitatively, these variations are described very well using a single fitting J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 R -0.8 I 1 1 I I 0 05 1.0 1.5 B/T Fig. 2 Variations of parameter R (0)and 0-Ps (rn = 0) lifetime, zJns (O),with magnetic field intensity, B/”, in solid p-terphenyl doped with 0.5% anthracene. The broken and solid lines are calcu- lated on the hypothesis of a single parameter, q = 1 and q = 0.80 (Table 2), respectively. Error bars are given in the figure. parameter, q.The derived values are given in Table 2, together with the standard deviations, which compare well with the experimental errors; consequently, the resulting cal- culated curves (solid lines in Fig. 2 and 3) show excellent agreement with the experimental plots. By contrast with thorium phosphate and numerous liquids previously studied, it was not possible to resolve the DBARL spectra into components. A reason for this failure is that the momentum distributions of the free annihilation (r,)and -1.0 -0.9 R -0.8 I I 0 0.5 1.o 1.5 B/T Fig. 3 Variations of parameter R (a)and 0-Ps (rn = 0) lifetime, rJns (O), with magnetic field intensity, BIT, in solid p-terphenyl doped with 1.5%chrysene. The broken and solid lines are calculated on the hypothesis of a single parameter, q = 1 and q = 0.83 (Table 2), respectively.Error bars are given in the figure. Table 2 Experimental LS parameters for p-terphenyl doped with either anthracene (0.5 wt.%)or chrysene (1.5 wt.%), and fitting parameter (q) for the variations of both R and r4 with B together with the resulting standard deviations (a), time windows, t, = 2.4 ns, t, = 9 ns for anthracene and t, = 2.4 ns, t, = 7 ns for chrysene dopant z:/PS r;lPs 1; (%) q %JPS OR anthracene 313 -t 4 1438 _+ 15 19.9 -t 0.2 0.80 f 0.02 22 0.009 chry sene 317 f 3 1160+ 13 22.5 0.3 0.83 f 0.02 22 0.008 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 pick-off (r,)components appear to be very close and thus cannot be resolved owing to the limited resolution of the DBARL apparatus.Furthermore, even when fixing rl to 0.97 keV (the value found in p-terphenyl doped with anthracene by angular correlation measurements18) the intensity of the narrow component, I:, is found to be slightly negative. This may reflect the fact that Gaussian functions are not adequate to describe the momentum distributions in this doped matrix. As in thorium phosphate (Fig. l), the variations of FWHM with B (not reported) show a smooth decrease, confirming the increase in the proportion of the Ps narrow component, with annihilation occurring in an intrinsic way owing to the mixing of states. Origin of the Anomalous States of Ps in Solids From Tables 1 and 2, it appears that doped p-terphenyl belongs to the class of solids in which Ps behaves normally, while in thorium phosphate it behaves anomalously.Except for those compounds in which the existence of two distinct Ps trapping sites is s~spected,~*' anomalous magnetic quenching effects have been found only in naphthalene" and in some organic polycrystalline scintillators. l2 In the latter case, the authors emphasize that the magnetic quenching is regular, when comparison is possible, in amorphous samples com- pared with that of the crystalline phases. They conclude that crystallinity is a necessary, although not sufficient, condition to observe anomalous effects, a conclusion which would hold for the present results. However, the explanation proposed,' based on possible magnetic interactions between Ps and radiolytic products in the positron spur, does not appear likely.In particular, it is difficult to see how the nature of the triplet states formed at the end of the positron track could differ (for long enough to affect Ps via magnetic interactions) in amorphous and polycrystalline samples of a specified com- pound. In the following, two different possibilities are exam- ined and discussed. As suggested by those cases where more than one type of Ps seems to be present,'.' a possible alternative to the above hypothesis is the existence of a distribution of the contact density parameter. In the case of naphthalene for instance, it has been shown, by comparing the activation energy of Ps formation in this matrix and the sublimation energy, that those sites at which 0-Ps is annihilated do not correspond to intrinsic vacancies, in contrast to what is found in a number of plastic molecular solids.2' As is the case in solid poly- mers,' Ps is possibly formed and decays in the natural free volumes of the naphthalene lattice.At a specified tem-perature, these free volumes are not of a constant size and thermal movements of the molecules in the lattice can give rise to a distribution of void sizes and, consequently, of Ps lifetimes and contact densities. Such lifetime distributions have been claimed to be found in a variety of However, the corresponding experimental LS spectra are well fitted using a single 0-Ps lifetime; this implies that most prob- ably only a very broad distribution of q will make it neces- sary to distinguish between q and u' in eqn.(2) and (3). To assess the validity of the present hypothesis, LS spectra involving a contact density distribution were simulated for various field strengths, up to 1.7 T, and subsequently analysed in the same way as the experimental spectra. The spectra were calculated using LS parameters similar to those obtained with thorium phosphate. The distributions were normalized Gaussians of specified maximum (11') and FWHM, truncated on the lower side, at q 2 0.01. As illus- trated in Table 3, the resulting analyses show that even with an extremely broad distribution, the simulated spectra are quite well described by a single fitting parameter, q.The Table 3 Fitting parameter (q) and resulting standard deviations (uR,orjfor the variations of both R and r4 with B from simulated spectra with a Gaussian distribution (qo, FWHM) of the contact density parameter FWHM ~~~~~ 0.6 0.2 0.60 O.OO0 0 0.6 0.62 0.003 4 1.4 0.79 0.006 7 0.3 0.1 0.29 0.001 1 0.3 0.25 0.004 5 1.o 0.58 0.011 12 1.4 0.69 0.009 11 necessity for two fitting parameters as encountered in the case of thorium phosphate cannot therefore be taken as indicative of the presence of a distribution of the contact density parameter due to some distribution in the size of the lattice sites. Although theory predicts that q should be different from 21' in the case of an excited state of Ps,~it is likely that the corre- sponding formalism, expressed through eqn. (2), (3) and (5)-(7), would give satisfactory results when describing any Ps wavefunction suficiently distorted compared with the spher- ical ground-state function.Distortion of the Ps atoms can be provoked by local conditions within the solid matrices. From the wealth of data accumulated, two situations are found: (i) presence of local fields with non-spherical geometries and (ii) existence of free spaces of sufficient volume to allow Ps for-mation and survival, but of shapes significantly different from spherical. The first case relates to ionic crystals. It is amazing that Ps does exist in such tightly packed lattices as that of Kcl.'~~~In this matrix, the largest space available corre- sponds to the tetrahedral sites circumscribed by four C1- anions.With a radius of 0.0845 nm, these sites would give an 0-Ps lifetime of 627 ps [eqn. (S)], which agrees well with the values of 7: = 628 ps24 or 680 ps' found experimentally, thus confirming that Ps in pure KCl is not annihilated in intrinsic vacancies. As T! in thorium phosphate is very close to the latter value, a similar conclusion holds. The difference in the liability to magnetic field effects in the two solids is therefore ascribable to a non-spherical electric field present in the free sites in thorium phosphate, because of the asymmetry in the balance of charges in the Th4+ and PO:-ions, in contrast to KCl which presents a regular arrangement of alternating monopositive and mononegative charges.In the case of naphthalenelo and other molecular com- pounds,I2 the existence of distorted Ps atoms should find explanation in the shape of the free spaces in which Ps is annihilated : most probably, if the voids available are of sufi-cient volume to allow Ps formation but of a shape signifi- cantly different from spherical (like rods), the latter may impose a distorted wavefunction on Ps. Conclusion The present results confirm that the Ps states in solids can be diverse, resulting in either normal or anomalous effects of an external magnetic field. In the 'normal' matrices studied here (p-terphenyl doped with either anthracene or chrysene molecules) Ps is created and is annihilated in extrinsic defects introduced by the dopants : approximating these defects to spheres with a radius similar to that of a naphthalene mol- ecule leads to a quantitatively good agreement with the experimental pick-off lifetime of 0-Ps, z: .Anomalous mag- netic field effects are found in ionic thorium phosphate, although the value for 7: is close to that found in another ionic lattice, KC1, in which normal magnetic effects have been J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 observed. This difference of behaviour is attributed to a dif- ference in the symmetry of the static electric field in those voids where Ps is annihilated in the lattices, which would be more spherical in the case of KCl than of thorium phosphate.It is suggested that, more generally, distortion of the Ps wave-function from spherical will occur, and therefore result in anomalous field effects, whenever local conditions are favour- able: such conditions can result not only from the electrical field, in the case of ionic crystals, but also from specific geo- metrical constraints, in the case of molecular solids. It would be rewarding to verify these hypotheses by examining a larger variety of solids, particularly molecular compounds, to gain information on Ps and thereby enlarge its applicability as a probe of the physico-chemical properties of matter. The authors thank C. Merigou and M. Genet, of Laboratoire de Radiochimie at Institut de Physique Nucleaire d’Orsay (France), for kindly providing the thorium phosphate samples, and C.Rybka for preparation of the doped samples. References Positron and Positronium Chemistry, ed. D. M. Schrader and Y. C. Jean, Elsevier, Amsterdam, 1988. M. Eldrup, D. Lightbody and J. N. Sherwood, Chem. Phys., 1981, 63, 51. G. Duplftre, in Positron and Positronium Chemistry, ed. Y. C. Jean, World Scientific, Singapore, 1990,p. 329. A.P.Mills Jr., J. Chem. Phys., 1975,62, 2646. I. Billard, J. Ch. Abbe and G. Duplftre, Chem. Phys., 1988, 127, 273. 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 F. Didierjean, I. Billard, W. F. Magalhaes and G. Duplitre, Chem. Phys., 1993,174,31. G. Consolati and F. Quasso, Appl. Phys., 1991,52,295.A. Bisi, G. Consolati, G. Gambarini and L. Zappa, Nuov. Cim. D, 1985,6, 183. G. Consolati and F. Quasso, J. Phys. C, 1988,21,4143.A. Bisi, G.Consolati and L. Zappa, Hype$ Inter., 1987,36,29. G. Consolati, D. Gerola and F. Quasso, 2. Phys. B, 1992,88, 131. G. Consolati, N.Gambara and F. Quasso, 2.Phys. D, 1991,21, 259. T.Goworek, C.Rybka and J. Wawryszczuk, Phys. Status Solidi B, 1977,444, K49. T.Goworek, C. Rybka and J. Wawryszczuk, Phys. Status Solidi B, 1978,89,253. T. Goworek, A. Badia and G. Duplftre, J. Phys. ZV, 1993, 3, 217. C. Merigou, N. Ouillon, T. Chopin and M. Genet, to be published. M. Radomska, R. Radomski and K. Pigon, Mol. Cryst. Liq. Cryst., 1972,18,75. T.Goworek, in ref. 3, p. 533. 0.Halpern, Phys. Rev., 1954,94,904. G. Duplitre, J. Ch. Abbe, J. Talamoni, J. C. Machado and A. Haessler, J. Chem. Phys., 1981,57, 175. D. Lightbody, J. N. Sherwood and M. Eldrup, Chem. Phys., 1985, 94, 475. Q. Deng and Y. C. Jean, J. Polym. Sci.,Part B, 1992,30, 1359. Q. Deng and Y. C. Jean, Macromolecules, 1993, 26, 30. C.Bussolati, A. Dupasquier and L. Zappa, Nuov. Cim. B, 1967, 52, 529. Paper 4/OO413B ;Received 24th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001501

出版商:RSC    

年代:1994

数据来源: RSC