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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 Nottingham University Park Nottingham NG7 2RD, UK Faraday Editorial Board Prof. I. W. M. Smith (Birmingham) (Chairman) Prof. M. N. R. Ashfold (Bristol) Dr. B. E. Hayden (Southampton) 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 Senior Assistant Editors: Mrs.S. Shah, Dr. R. A. Whitelock Assistant Editor: Mrs. C. J. Seeley Editorial Secretary: Mrs. J. E. Gibbs International Advisory Editorial Board R. S. Berry (Chicago) Y. Marcus (Jerusalem) A. M. Bradshaw (Berlin) 6. J. Orr (North Ryde) A. 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Full details of the form of manuscripts for Articles and Faraday Communications, con- ditions for acceptance etc. are given in issue number one of Faraday Transactions, published in January of each year, or may be obtained from the Editorial Manager. There is no page charge for papers published in Faraday Transactions. Fifty reprints are supplied free of charge. Dr. P. J. Sarre, Scientific Editor. Tel. : Nottingham (0602) 51 3465 (24 hours) E-Mail (JANET): PCZPSF@UK.AC.NOTT.VAX Fax: (0602) 513466 Telex : 37346 U N INOT G Dr. R. J. Parker, Editorial Manager. Tel.: Cambridge (0223) 420066 €-Mail (INTERNET): RSC1@RSC.ORG (For access from JANET use RSCl %RSC.ORG@UK.AC.NSF NET- RELAY) Fax: (0223) 423623 or 420247 Telex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX013

出版商:RSC    

年代:1994

数据来源: RSC

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5th Edition ‘. easy to read, an excellent reference text, and a worthwhile invest men t.’ Journal of the American Chemical Society reviewing the 4th Edition. The new edition of this essential laboratory handbook is the ‘key’ requirement for all research, development, production, analytical and teaching laboratories worldwide. The 5th Edition provides: New features include: expanded ’Yellow Pages’ section on 0 a quick guide to the hazardous properties of 1339 substances (over 800 more than were hazardous substances, providing immediate covered in the previous edition) information on hazardous properties, 0 details of the latest UK and EC regulations recommended control procedures and safety measures 0 an extremely useful emergency action check complete guide to labelling requirements to list -users can fill in their own key contacts for hospitals, fire etc.comply with EC directives and UK legislation, including the risk and safety phrases that must handy tables, symbols and statistics for ease appearof reference chapter on electrical hazards 0 a description of the American scene, including index to ‘Yellow Pages’ section, with US legislation and safety practices -synonyms of compounds highlighting differences between the UWEC index to CAS Registry Numbers and USA PVC Protective Binding xx + 676 pages ISBN 0 85186 229 2 (1992) Price €45.00 If you have not yet ordered your copy of the NEW edition, do so now! Why take chances? Be informed and safe. To order, please contact: ROYAL Royal Society of Chemistry, Turpin Distribution # SOCIETYOF Services Ltd,- Blackhorse Road,’ Letchworth, CHEMISTRY InformationHerts SG6 1HN, United Kingdom. Services Telephone: +44 (0)462 672555 Fax: +44 (0)462 486947. II I 0956-5000(199434:1-4

ISSN:0956-5000    

DOI:10.1039/FT99490BX015

出版商:RSC    

年代:1994

数据来源: RSC

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ISSN 0956-5000 JCFTEV(4) 517-682 (1 994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 517 Theoretical potential-energy functions and the rovibronic spectrum of the SiHi ion C. Bauer, D. M. Hint, D. I. Hall, P. J. Sarre and P. Rosmus 523 Production of CH(X211) from the multiphoton dissociation of CH,CO at wavelengths of 279.3 and 308 nm S. M. Ball, G. Hancock and M. R. Heal 533 Laser flash photolysis studies on hydrogen-atom transfer from the triplet hydroxynaphthylammonium ion to benzo- phenone via a triplet exciplex. Which group is more reactive for hydrogen atom transfer, -OH or -NHl? M. Yamaji, K-i. Tamura and H. Shizuka 541 Kinetics of thermal decomposition of the diazines: Shock-tube pyrolysis of pyrimidine A.Doughty and J. C. Mackie 549 Reactions of N(2 2D)and N(2 2P) with 0, Y.Shihira, T. Suzuki, S-i. Unayama, H. Umemoto and S. Tsunashima 553 Application of simple expressions for the high-pressure volumetric behaviour of liquid mesitylene V. Garcia Baonza, M. Caceres Alonso and J. NGez Delgado 559 Speciation, structural characteristics and proton dynamics in the systems NH,N03 1.5H20 and NH,N03 . 1.5H20-(NH03, NH,F, NH,)-H,O at 50 "C L. A. Bengtsson, F. Frostemark and B. Holmberg 571 Effect of preferential solvation on reactivity of a free radical in binary solvent mixtures 0. Ito and H. Watanabe 575 Limiting partial molar volumes of electrolytes in dimethylformamide-water mixtures at 298.15 K E. Garcia-Pafieda, C. Yanes, J.J. Calvente and A. Maestre 579 Ultrasonic velocities and isentropic compressibilities of some tetraalkylammonium and copper@ salts in acetonitrile and benzonitrile J. Singh, T. Kaur, V. Ali and D. S. Gill 583 Transport and compressibility studies of some copper@) perchlorates in binary mixtures of benzonitrile and acetonitrile D. S. Gill, R. Singh, V. Ali, J. Singh and S. K. Rehani 587 Kinetic model for serum albumin adsorption: Experimental verification R. Kurrat, J. J. Ramsden and J. E.Prenosil 591 Primary yields of water radiolysis in concentrated nitric acid solutions R. Nagaishi, P-Y. Jiang, Y. Katsumura and K. Ishigure 597 Pulse radiolysis study of the reactions of SO;-with some substituted benzenes in aqueous solution G. Merga, C. T. Aravindakumar, B.S. M. Rao, H. Mohan and J. P. Mittal 605 Dual-cylinder microelectrodes. Part 2.-Steady-state generator and collector electrode currents B. J. Seddon, C. F. Wang, P. Li, W. Peng and X.Zhang 609 Voltammetric and subtractively normalized interfacial FTIR study of the adsorption and oxidation of L(+)-ascorbic acid on Pt electrodes in acid medium: Effect of Bi adatoms M. A. Climent, A. Rodes, M. J. Valls, J. M. Perez, J. M. Feliu and A. Aldaz 617 Electrochemical study of the heterogeneously catalysed reaction between N,N-dimethyl-p-phenylenediamine and Co'WH3),Cl2+ at monometallic and bimetallic surfaces of silver and gold Y-H. Chen, U. Nickel and M. Spiro 625 Stability of thin polar films on non-wettable substrates A. Sharma and A. T.Jameel 629 Influence of the cetyltrimethylammonium chloride micellar pseudophase on the protolytic equilibria of oxyxanthene dyes at high bulk phase ionic strength N.0. Mchedlov-Petrossyan and V. N. Kleshchevnikova 641 Computer modelling of phosphate biominerals. Transfer of parameters for interatomic potentials for different poly- morphs of divalent metal diphosphates M. G. Taylor, K. Simkiss and M. Leslie 649 Adsorption of binary mixtures of heptane and alkanols by activated carbon A. M. Gonplves da Silva, V. A. M. Soares and J. C. G. Calado 653 IR studies of cerium dioxide: Influence of impurities and defects F. Bozon-Verduraz and A. Bensalem 659 Catalytic reactions of o-xylene and rn-xylene with deuterium on metal films R.J. Harper and C. Kemball 667 Al-Pillared saponites. Part 1.-IR studies S. Chevalier, R. Franck, H. Suquet, J-F. Lambert and D. Barthomeuf 675 Al-Pillared saponites. Part 2.-NMR studies J-F. Lambert, S. Chevalier, R. Franck, H. Suquet and D. Barthomeuf Note: Where an asterisk appears against the name of one or more of the authors, it is included with the authors’ approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles. For further details 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/FT99490FP037

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Afanasiev, P., 193 Aldaz,A., 609 Alfimov, M. V., 109 Al-Ghefaili, K. M., 383 Ali, V., 579, 583 Allegrini, P., 333 Allen, N. S., 83 Aramaki, K., 321 Aravindakumar, C. T., 597 Avila, V., 69 Baba,T., 187 Ball, S. M., 523 Barthomeuf, D., 667,675 Bassoli, M., 363 Bauer, C., 5 17 Bell, A. J., 17 Bendig, J., 287 Bengtsson, L. A., 559 Bensalem, A., 653 Bkrces,T., 411 Bickelhaupt, F., 327 Biczok, L., 41 1 Boggis, S. A., 17 Borisenko, V. N., 109 Bozon-Verduraz, F., 653 Bradley, C. D., 239 Bradshaw, A. M., 403 Breysse, M., 193 Brocklehurst, B., 271 Brown, R. G., 59 Byatt-Smith, J. G., 493 Caceres Alonso, M., 553 Calado, J. C. G., 649 Caldararu, H., 213 Calvente, J. J., 575 Camacho, J.J., 23 Campa, M. C., 207 Campos, A., 339 Caragheorgheopol, A., 2 13 Carvill, B. T., 233 Catalina, F.. 83 Cavasino, F. P., 311 Chen, J-S., 429 Chen, Y-H., 617 Cheng, A., 253 Cherqaoui, D., 97 Chesta, C. A., 69 Chevalier, S., 667, 675 Cho,T., 103 Christensen, P., 459 Climent, M. A., 609 Cordischi, D., 207 Corma,A., 213 Corrales, T., 83 Cosa, J. J., 69 Coudurier, G., 193 Curtis, J. M., 239 Demeter, A., 41 1 Demri, D., 501 Derrick, P. J., 239 Diagne, C., 501 Dickinson, E., 173 Doughty, A., 541 Douglas, C. B., 471 Dwyer, J., 383 Dyke, J. M., 17 Eastoe. J., 487 Joseph, E. M., 387 Joshi, P. N., 387 Nagaoka, H., 349 Navaratnam, S., 83 Shaw,N., 17 Sheil, M. M., 239 Ebitani, K., 377 Elisei, F., 279 Eustaquio-Rincon, R., 113 Fantola Lazzarini, A.L., Favaro, G., 279,333 Feliu, J. M., 609 Filimonov, I. N., 219, 227 Fogden, A., 263 Fornes, V., 213 Franck, R., 667,675 Frey, J. G., 17 Frostemark, F., 559 Gans, P., 315 423 Kagawa, S., 349 Kaler, E. W., 471 Kalugin, 0.N., 297 Katsumura, Y., 93, 591 Kaur, T., 579 Kawashima, T., 127 Keil, M., 403 Kemball, C., 659 Kida, I., 103 Kiennemann, A., 501 Kim, J-H., 377 King, F., 203 Kirschner, J., 403 Klein, M. L., 253 Neoh, K. G., 355 Nerukh, D. A., 297 Nicholson, D., 181 Nickel, U., 617 Ninomiya, J., 103 Nishihara, H., 321 Nonaka, O., 121 Nuiiez Delgado, J., 553 Nyholm, L., 149 Occhiuzzi, M., 207 Ohtsu, K., 127 Oliveri, G., 363 Ono,Y., 187 Oradd, G., 305 Sheppard, N., 507, 513 Shiao, J-C., 429 Shihira, Y., 549 Shiralkar, V.P., 387 Shizuka, H., 533 Silva, C. J., 143 Silva, F., 143 Simkiss, K., 641 Singh, J., 579, 583 Singh, R., 583 Soares, V. A. M., 649 Soria, V., 339 Spiro, M., 617 Sun, L. M., 369 Garcia, R., 339 Garcia Baonza, V., 553 Garcia-Pafieda, E., 575 Geantet, C., 193 Gill, D. S., 579, 583 Gill, J. B., 315 Goede, S. J., 327 Gomez, C. M., 339 GonGalves da Silva, A. M., 649 Kleshchevnikova, V. N., Kondo, Y., 121 Kossanyi, J., 41 1 Kurrat, R., 587 Kuwamoto, T., 121 Lambert, J-F., 667, 675 Langan, J. R., 75 Lazzarini, E., 423 Leaist, D. G., 133 629 Ortica, F., 279 Ota, K-i., 155 Otlejkina, E. G., 297 Otsuka, K., 451 Ottavi, G., 333 Ozutsumi, K., 127 Padley, M. B., 203 Palleschi, A., 435 Paradisi, C., 137 Pardo,A., 23 Suquet, H., 667,675 Surov, Y. N., 297 Suzuki, T., 549 Tabrizchi, M., 17 Takagi, T., 121 Takahashi, K., 155 Tamura, K-i., 533 Tanaka, I., 349 Taylor, M. G., 641 Teo, W.K., 355 Gray, P. G., 369 Green, W. A., 83 Grimshaw, J., 75 Haeberlein, M., 263 Hall, D. I., 517 Hall, G., 1 Hallbrucker, A., 293 Hamnett, A., 459 Hancock, G., 523 Handa,H., 187 Hao, L., 133 Harper, R. J., 659 Harrison, N. J., 55 Heal, M. R., 523 Heenan, R. K., 487 Helmer, M., 31,395 Herein, D., 403 Herzog, B., 403 Higgins, S., 459 Hindermann, J-P., 501 Hirst, D. M., 517 Holmberg, B., 559 Hoshino, H., 479 Hosoi, K., 349 Hutchings, G. J., 203 Hutton, R. S., 345 Ikawa, S-i., 103 Ikonnikov, I. A., 219 Indovina, V., 207 Ishigure, K., 93,591 Ito,O., 571 Iwasaki, K., 121 Jameel, A. T., 625 Jayakumar, R., 161 Jenneskens, L. W., 327 Jennings, B. J., 55 Jiang, P-Y., 591 Jiang, P.Y., 93 Johansson, L. B.-A., 305 Lei,G-D., 233 Lerner, B. A., 233 Leslie, M., 641 Li, J., 39 Li, P., 605 Lin, J., 355 Lindblom, G., 305 Liu,C-W., 39 Liu, X., 249 Loginov, A. Yu., 219,227 Longdon, P. J., 315 Lunelli, B., 137 Mackie, J. C., 541 Maestre, A., 575 Mahy, J. W. G., 327 Makarova, M. A,, 383 Malatesta, V., 333 Malcolm, B. R., 493 Mallon, D., 83 Mandal, A. B., 161 Mariani, M., 423 Martins, A., 143 Masetti, F., 333 Massucci, M., 445 MatijeviC, E., 167 Matsuda, J., 321 Mazzucato, U., 333 Mchedlov-Petrossyan, N. O., Merga, G., 597 Meunier, F., 369 Mittal, J. P., 597 Mohan, H., 597 Moriguichi, I., 349 Morikawa, A., 377 Morokuma, M., 377 Muir, A. V. G., 459 Nagaishi, R., 93, 591 Lu, J-X., 39 629 Parsons, B. J., 83 Pedulli, G. F., 137 Peng, W., 605 Pereira, C.M., 143 Perez, J. M., 609 Peter, L. M., 149 Petrov, N. Kh., 109 Pispisa, B., 435 Pivnenko, N. S., 297 Plane, J. M. C., Porcar, I., 339 Potter, C. A. S., 59 Poyato, J. M. L., 23 Prenosil, J. E., 587 Previtali, C. M., 69 Ramsden, J. J., 587 Rao, B. S. M., 597 Rehani, S. K., 583 Rettig, W., 59 Rey,F., 213 Richter, R., 17 Rocha, M., 143 Rochester, C. H., 203 Rodes, A., 609 Roffa, S., 137 Rosmus, P., 5 17 Rossi, P. F., 363 Ryde,N., 167 Sachtler, W. M. H., 233 Saitoh, T., 479 Salmon, G. A., 75 Sarre, P. J., 517 Sbriziolo, C., 311 Schedel-Niedrig, Th., 403 Schlogl, R., 403 Schnabel, W., 287 Seddon, B.J., 605 Shahid, G., 507,513 Sharma,A., 625 31, 395 Teraoka, Y., 349 Timms, A. W., 83 Timney, J. A., 459 Trejo, A., 113 Tsunashima, S., 549 Turco Liveri, M.L., Turco Liveri, V., 311 Umemoto, H., 549 Unayama, S-i., 519 Valat, P., 411 Valls, M. J., 609 Vedrine, J. C., 193 Venanzi, M., 435 Villamagna, F., 47 Villemin, D., 97 Vlietstra, E. J., 327 Vollmer, F., 59 Vyunnik, I. N., 297 Wang, C. F., 605 Watanabe, H., 571 Werner, H., 403 Whitaker, B. J., 1 Whitehead, M. A., 47 Wikander, G., 305 Williams, D. E., 345 Wilpert, A., 287 Wintgens, V., 411 Wohlers, M., 403 Wormald, C. J., 445 Y agci, Y ., 287 Yamaji, M., 533 Yamanaka, I., 451 Yanes, C., 575 Yoshitake, H., 155 Yotsuyanagi, T., 93,479 Young, R. N., 271 Zhang, X., 605 Zholobenko, V. L., 233 Zhong, G. M., 369 3 11 1 The following papers were accepted between 1st and 31st December 1993: FTIR study of the interaction of hydrogen cyanide with alkali-metal ion, silver(1) and nickel(I1) ion-exchanged near-Faujasite zeolites T.D. Smith and C. J. Blower FTIR spectroscopic study of the zeolitic adsorption of hydrogen cyanide on acidic sites T. D. Smith and C. J. Blower Enthalpies of mixing a non-ionic surfactant with water at 303.15 K studied by calorimetry K. Weckstrom, K. Hann and J. B. Rosenholm Triplet of cyclooctatetraene: Its reactivity and properties T. N. Das and K. I. Priyadarsini Theory of the monolayers of non-Gaussian polymer chains grafted to a surface. Part 1.-General theory V. A. Pryamitsyn and V. M. Amoskov Brensted acid sites in zeolites: FTIR study of molecular hydrogen as a probe for acidity testing J. Dwyer, M. A. Makarova, V. L. Zholobenko, K. M. Al-Ghefaili, N. E. Thompson and J.Dewing Thermodynamic properties of 0-6 mol kg-' aqueous sulfuric acid from 273.15 to 328.15 K S. L. Clegg, J. A. Rard and K. S. Pitzer Pulse radiolysis of iodate in aqueous solution S. P. Mezyk and A. J. Elliot Influence of structure on the optical spectra of Eu3+ in Pb(PO,), glass: Molecular dynamics simulation and crystal-field theory G. Cormier, J.A. Capobianco and C. A. Morrison Chemiluminescent reaction of oxygen atom with dimethyl disulfide and dimethyl sulfide K. V. S. Rama Rao, U. B. Pavanaja, H. P. Upadhyaya, A. V. Sapre and J. P. Mittal Enthalpies of interaction between dimethyldioctadecylammonium bromide vesicles in aqueous solution and either dipicolinate or sulfate anions M. J. Blandamer, B. Briggs, M. D. Butt, P. M. Cullis, M.Waters, J. B. F. N. Engberts and D. Hoekstra Photolysis of HOBr and DOBr at 266 nm: OH and OD product-state distributions J. G. Frey, N. Shaw, A. J. Bell and M. J. Crawford Mechanism of branched carbon-chain formation from CO and H, over oxide catalysts. Part 1 .-Adsorbed species on ZrO, and CeO, during CO hydrogenation K-I. Maruya, A. Takasawa, M. Aikawa, T. Haraoka, K. Domen and T. Onishi Surface characterisation and catalytic activity of C0~g,++4120, solid solutions: Oxidation of carbon monoxide by oxygen F. Pepe and M. Occhiuzzi CoAPO molecular sieve acidity investigated by adsorption calorimetry and IR spectroscopy J. Janchen, M. P. J. Peeters, J. H. M. C. Van Wolput, J. P. Wolthuizen, J. H. C. Van Hooff and U. Lohse Cationic micellar effect on the kinetics of the protolysis of aromatic carboxylic acids studied by the ultrasonic absorption method H.Yano, T. Yamashita, M. Yamasaki, T. Sano and S. Harada Far-IR study of the hydrogen-bond vibration of intramolecular bonds in substituted 2-diethy laminomethylphenol N-oxides as a function of the pK, of the phenolic group G. Zundel, B. Bmezinski and A. Rabold Effect of potassium on the surface potential of titania D. Courcot, L. Genegembre, M. Guelton, Y. Barbaux and B. Grzybowska Preferential solvation of a p-sensitive dye in binary mixtures of a non-protic and a hydroxylic solvent M. C. Rezende, M. Scremin, S. P. Zanotto and V. G. Machado Forces of inertia acting on the aqueous pore fluid of anionic polyelectrolyte gels D.Woermann and N-T. Dang Morphology and polymorphism in molecular crystals: Terephthalic acid R. J. Davey, S. J. Maginn, S. J. Andrews, S. N. Black, A. M. Buckley, D. Cottier, P. Dempsey, R. Plowman, J. E. Rout, D. R. Stanley and A. Taylor The adsorption and decomposition of methanol on TiO,, SrTiO, and SrO M. Bowker, N.Aas and T. J. Pringle Protonated carbamic acid, collisional activation and unimolecular dissociation of CH,NO,' H. Egsgaard and L. Carlsen 11 Temperature dependence of the rate constant for the reaction e,-+ OH in water up to 150 "C A. J. Elliott and D. C. Ouellette Spin trapping of radicals upon irradiation of organobromide compounds with low-energy X-rays A. Halpern and V. E. Zubarev Enzyme catalysis at hydrogel-modified electrodes with soluble redox mediator E.J. Calvo and F. Battaglini Water adsorption in active carbons described by the Dubinin-Astakhov equation F. Stoeckli, T. Jakubov and A. Lavanchy Transfer Gibbs energies for ClO,', BrO;, IO,, C10,- and 10, anions for water-acetonitrile and water-tert-butyl alcohol mixtures J. Benko and 0.Vollarova High-temperature diffusion of hydrogen and deuterium in palladium S. Naito, T. Maeda, M. Yamamoto, M. Mabuchi and T. Hashino Configuration interaction studies on the S, surface of H,CO 2 'A' (0,x* x, n;") as perturber of 1 'B, (n,3s) M. Hachey, P. J. Bruna and F. Grein Catalytic studies with dealuminated Y zeolite. Part 2.-Disproportionation of toluene R. Rudham and N. P. Rhodes Study of the structure-breaking effect in aqueous CsCl solutions based on H,O-D,O isotope effects on transport coefficients and microdynamical properties A.Sacco, H. Weingartner, B. M. Braun and M. Holz ... 111 FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Annual Congress: The Reactive Interface in Electrochemistry and Catalysis To be held at the University of Liverpool on 12-15 April 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN Neutron Scattering Group Neutron Scattering Data Analysis To be held at the Rutherford Appleton Laboratory on 13-15 April 1994 Further information from Mrs S. Humphreys, The Rutherford Appleton Laboratory, Chilton, Didcot 0x11 ORA Colloid and Inter$ace Science Group Theoretical Modelling and Simulation in Colloid and Interface Science To be held at the University of Bristol on 18-20 April 1994 Further information from Dr R.Buscall, ICI Corporate Science Group, PO Box 1 I, The Heath, Runcorn WA7 4QE ~~ Division Autumn Meeting: Reactions and Mechanisms for Fine Chemicals To be held at the University of Glasgow on 6-9 September 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Gas Kinetics Group 13th International Symposium on Gas Kinetics To be held at University College, Dublin on 11-15 September 1994 Further information from Dr H. Sidebottom, Department of Chemistry, University College, Dublin Electrochemistry Group with the SCI ELECTROCHEM 94 To be held in Edinburgh on 12-1 6 September 1994 Further information from Professor D.E. Williams, Department of Chemistry, University College London, 20 Gordon Street, London WClH OAJ iv THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 97 Structure and Dynamics of Van der Waals Complexes University of Durham, 6-8 April 1994 Organising Committee: Dr B. J. Howard (Chairman) Dr P. Hamilton Dr J. M. Hutson Dr D. C. Clary Professor A. C. Legon Dr B. Soep Dr P. R. R. Langridge-Smith Since Faraday Discussion No. 73 on Van der Waals molecules, in 1982, the study of weakly bound molecular complexes has developed rapidly. Spectroscopic studies can now yield detailed information on intermolecular potential-energy surfaces in molecular systems.Studies of trimers, tetramers and higher clusters are giving insight into solvation effects and providing information on many-body forces, which are important in understanding the properties of condensed phases. Investigations of photodissociation and predissociation processes are helping us to understand the dynamics of fundamental chemical processes such as molecular rearrangement and energy transfer. In addition, Van der Waals complexes provide an opportunity to control the orientation of colliding molecules and the energies and impact parameters of reactive collisions, and have added significantly to our understanding of the pathways of simple chemical reactions. This discussion will bring together experimentalists and theoreticians who are involved in the study of Van der Waals molecules. The final programme and application form may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN.THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 98 Polymers at Surfaces and Interfaces University of Bristol, 12-14 September 1994 Organising Committee: Professor Sir Sam Edwards (Chairman) Dr R. Buscall Professor R. H. Ottewill Dr T. Cosgrove Professor J. S. Higgins Dr R. W. Richards Dr R. A. L. Jones New experimental methods and new theoretical and omputational i chniques have recently led to great progress in understanding the difficult but technologically important problems associated with the conformation of polymer molecules at surfaces and interfaces.The purpose of this Discussion is to bring together experimentalists and theoreticians working towards a molecular understanding of polymers at surfaces and interactions to survey the progress in the area to date and to indicate future directions of research. The meeting will attempt to bring a unified approach to the problem, encompassing problems of the structure of surfaces and interfaces in polymer melts, the conformation of polymers at solidfliquid and 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. V THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Organising Committee Professor L.D. Barron (Chairman) Dr A. F. Drake Dr D. L. Andrews Professor R. E. Hester Professor A. D. Buckingham Traditional optical activity measurements such as CD are confined to the visible and near-ultraviolet spectral regions where they provide stereochemical information on chiral molecules via polarized electronic transitions. Thanks to prompting from theory and new developments in instrumentation, optical measurements are now being made in the vibrational spectrum using both infrared and Raman methods.Studies over the past decade on a large range of chiral molecules, from small organics to biological macromolecules, have demonstrated that vibrational optical activity opens up a whole new world of fundamental studies and practical applications undreamt of in the realm of conventional electronic optical activity. The meeting seeks to bring together experimentalists and theoreticians to discuss the current and future experimental possibilities and the development of theories, including ab initio computational methods, which can relate the observations to stereochemical details. The increasing importance now being attached to molecular chirality and solution conformation in the life sciences should also encourage the partipation of biomolecular scientists.The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, London W1V OBH. v1 Are you looking for a source of international research results coveiring all branches of L.chemistry yet in a coricise and easy-to-read format? JOURNALOF CHEMICAL RESEARCH is the answer! Sponsored jointly by the Royal Society of Chemistry, the Gesellschaft Deutscher Chemiker and Societe Franqaise de Chimie, the journal of Chemical Research publishes papers from around the world on all developing areas of chemistry. Unique in format, it consists of two parts: Part S contains brief, browsable synopses and short papers. Part M reproduces the full texts of the above synopses for reference and is available in miniprint or microfiche versions.* Quick to scan* Follow-uptexts* Rapid publication * * * All topics covered Competitive subscription rates international research Don’t waste time in the library when you want to be in the lab -subscribe io ihe Journalof Chemical Research. For further information complete and return the attached enquiry form. 0Please send me further information on the Journalof Chemical Research Name: Position: ROYALAddress : S( KIETY OFC H E M IS1 R Y6&& Please return to: Sales and Promotion Department, Iniomi.itton St*nI( t~,Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge, CB4 4WF, UK.vii Joint Discussion Meeting 1994 "Self-Organization of Biopolymers" Jena 10-13 April Deutsche Bunsengesellschaftfiil. Physikulische Chemie together with: Associazione Italiana di Chimica Fisica, Divizione di Chimica Fisica Della Societd Chimica Italiuna, Faraday Division of the Royal Society of Chemistry, SocitW FranCaise de Chimie, Division de Chimie Physique Organizers: Manfred Eigen, Rainer Jaenicke, John McCaskill, Peter Schuster (Germany), Len Fisher (UK), Giovanni Giacometti (Italy), Gilbert Weill (France) Self-organization is a decisive process in biological organisms complementing the conservative transfer of information. It is characterised as the emergence of functional relationships between components subject to a comon dynamical process.The molecular level is of special interest in that it allows a physico-chemical description of biological organization. A paradigm of this functional self- organization is presented by the evolution of specifically encoded biopolymers from polymerization kinetics in solution. The interplay between chemical kinetics and the biopolymers it generates is the subject of this workshop. The following areas will be discussed by theoreticians and experimentalists: 0 Principles and Origins of Replication 0 The RNA World Principles and Origins of Coding Systems Protein Mediated Networks Spatial and Functional Organization Papers will be given by: C. Biebricher (MPI Gottingen), M. Eigen (MPI Gottingen), L. De Santis (Univ. Rome), A.Ellington (Univ. Indiana), A. Eschenmoser (ETH Zurich), W. Fontana (Santa Fe Inst.), M. Gebinoga (IMB Jena), A. Goldbeter (Univ. Bruxelles), P. Hogeweg (Univ. Utrecht), H. Jackle (MPI Gottingen), S. Kauffmann (Sante Fe Inst), G. von Kiedrowski (Univ. Freiburg), P.L. Luisi (ETH Zurich), J.S. McCaskill (IMBJena), J. Putz (IBMC Strasbourg), S. Rasmussen (Los Alamos Nat. Lab.), P. Schuster (IMB Jena), R. Thomas (Univ. Bruxelles), P. Willis (Univ. Auckland). Correspondence regarding participation and/or poster presentation should be sent to one of the organizers: Professor Dr J.S. McCaskill, Professor Dr Peter Schuster, Institut fur Molekulare Biotechnologie, Postf. 100813, 07708 Jena Professor Dr Manfred Eigen, Max-Planck-Institut fur Biophysikalische Chemie, Am Fafiberg, 37077 Gottingen Professor Dr Rainer Jaenicke, Institut fiir Biophysik und Physikalische Biochemie, Universitatsstr. 3 1, 93053 Regensburg Dr L.R. Fisher, Physics Department, University of Bristol, Tyndall Avenue, Bristol, BS8 lTL, UK Professor Giovanni Giacometti, Dept. di Chimica Fisica del Univ. di Padova, Via Loredan 2,1-35100 Padova Prof. Gilbert Weill, Institut Charles Sadron, 6 Rue Boussingaut, F-67083 Strasbourg-CCdex ... Vlll

ISSN:0956-5000    

DOI:10.1039/FT99490BP039

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(4), 517-521 Theoretical Potential-energy Functions and the Rovibronic Spectrum of the SiH: Ion Cornelia Bauer Fachbereich Chemie der Universitat, 040439 Frankfurt, Germany David All. Hirst* Department of Chemistry, University of Warwick, Coventry, UK CV4 7AL David 1. Hall and Peter J. Sarre Department of Chemistry, University of Nottingham, University Park, Nottingham, UK NG7 2RD Pave1 Rosmus Universite de Marne la Vallee, F-93160Noisy le Grand, France ~~ Three-dimensional potential-energy functions for the 2 2Al and A 2Bl states of SiHz have been derived from extensive ab initio multi-reference configuration interaction (MRCI) calculations. Spectroscopic constants have been derived from the potential-energy functions by second-order perturbation theory.The calculated values are in very good agreement with the experimental data and also provide a secure value for the A, rotational constant for the % 2Al state. Rovibronic levels were calculated by a variational method which takes into account Renner-Teller coupling. Theoretically derived transition wavenumbers are in good agreement with experimen- tal results. Over the past few years a substantial amount of spectro- scopic data for the molecular ion SiHl has been obtained in high-resolution laser photodissociation experiments'.' in which the A 'B, +x 'A, transition has been observed by monitoring dissociation to form Si' + H, and SiH+ + H. The ground-state X'A, is bent with a bond angle of ca.120" whereas the equilibrium structure of the A state is linear. The 8 and A states are the two components of a Renner-Teller pair which correlate with 'nu for linear geometries. The SiHl ion is an important member of the 'Renner-Teller' class of AH, molecules but is of particular interest for two main reasons. First, it represents a rare example where rota- tionally resolved competitive dissociation into chemically dis- tinct channels is observed.' Secondly, while some of the SiHl ions formed by electron impact ionization of silane are gener- ated in low-lying rovibrational levels, allowing laser-excited transitions involving levels with low rotational angular momentum to be observed,' a significant proportion of the ions are formed in high-K, states with values of K, up to 25, Transitions between high-K, states give rise to a sub-band spectral structure which is wholly different from that for low K, values., Moreover, an unusual dynamical behaviour is observed in which the predissociation rates are found to decrease with increasing K, value for K, between 15 and 23.' Apart from the intrinsic interest in ab initio calculations on a prototypical Renner-Teller AH, molecule, these special spec- troscopic and dynamical aspects provided a strong motiva- tion for undertaking this work.Moreover, the calculations were conducted in the expectation that, if there were found to be good agreement between theory and experimental data, the derived potential-energy functions would be of value for future spectroscopic assignments and dynamical calculations.In the earlier experimental work,' analysis of the spectrum in the region 600-650 nm gave well determined values for the rotational constants B", C" and B'. The rotational constant A" was estimated from B" and C" with the use of inertial defect theory and from the rotational constants the geometry of the electronic ground state was obtained. The later studies in the spectral ranges 540-660 nm and 700-850 nm have been analysed in terms of transitions involving high K, values.2 These spectra have been interpreted with the aid of Renner-Teller calculations of the energy levels using an empirical potential-energy function. A fuller report of this work will be given in a forthcoming paper.3 Methods for the variational calculation of rovibronic energy levels of Renner-Teller molecules are now well estab- li~hed.~Recent applications include H20 ,5 BH, ,6 CO; 7*8+ and CH, .9 These methods require accurate potential-energy surfaces for the two components of the Renner-Teller pair.There have been a number of theoretical treatments of the SiH; but none of these has yielded potential- energy surfaces of sufficient accuracy for the calculation of rovibronic levels for comparison with experiment and pos- sessing predictive value. In this paper we report ab initio potential-energy surfaces for the 8'A1 and A2B1 states of SiHi calculated with large basis sets and with extensive treatment of electron correlation.These data have been fitted to polynomial functions which have been used in variational calculations of rovibronic levels. Molecular parameters are derived and compared with experimental data. Ab initio Calculations The basis for Si consisted of the 17s12p Gaussian basis set of Partridge17 contracted to 1ls8p (with the outer ten s and seven p functions uncontracted) augmented with four sets of Cartesian d functions (with exponents 2.1, 0.9, 0.4 and O.l5/ag) and two sets of Cartesian f functions (with exponents 1.6 and 0.55/a;). The hydrogen basis was the 8s basis of van Duijneveldt18 contracted to 4s with the addition of three sets of p functions (with exponents 1.8, 0.6 and 0.2/a;) and one set of d functions with exponent 0.7/a;.This resulted in a total of 117 basis functions. Molecular orbitals for the R'A, and A2Bl states were obtained in separate CASSCF calculations for each state.For the 'A, state the active space consisted of the orbitals 4al-7al, 2b1, 2b2-4b, whereas for the 'B, state the orbitals 4al-6al, 2b1, 3b1, 2b2-4b, constituted the active space. The CASSCF orbitals were used in MRCI calculations in which all single and double excitations were generated with respect to a set of reference configurations. For the 'A, state the ref- erence set of configurations consisted of all configurations obtained by distributing five electrons amongst the orbitals 4a,-6al, 2b2-4b, resulting in 499 180 configurations. In the -289.70 *J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 the range 180"-100". The ab initio calculations were made with the GAMESS-UK suite of program^.'^ -289.72. Adiabatic Potential-energy Functions (APEF) -289.74. The MRCI total energies, in terms of the bond lengths T,, r2 and the included angle a, were fitted to polynomial expan- sions -289.76 I v(r1, r2 a) = 1cijk RiR', Ok (1)S ijk%& -289.78 in terms of the coordinates R,, R, and 0.R, and R, werez chosen as dimensionless Simon-Parr-Finlan coordinates,Q) Ri = (1 -rref/ri).For 0the displacement coordinate 0= (a-289.80 -aref)(in radians) was employed. For the electronic ground state we used polynomials up to sixth order in R,, R, and -289.82 seventh order in 0.The corresponding orders for the upper state were set to be 5 (Rl, R2)and 6 (0).The surfaces were expanded around the calculated minimum geometry ()7: ,A, -289.84 state) and a near-minimum geometry (A2B1,rref= 2.8a0, aref= 180').The fits included 84 geometries for the A2Bl state because each ab initio point corresponds to two geome- -289.86 ' I tries on the surface. For the ground state 101 calculated ener- 180.0 160.0 140.0 120.0 100.0 80.0 60.0 gies for the z2A, state were augmented by 28 linearaldegrees geometries calculated for the A state in order to ensure Fig. 1 Cut of the potentialenergy functions of the 8 'A, and a 2Bl degeneracy of the two surfaces for linear geometries. The states of SiH; along the bending coordinate for rl = r2= 1.482 A bending potential-energy curves for r, = r2 equal to 2.8~~ (1.482 A) are illustrated in Fig.1. With these APEFs, which are valid for bond lengths 1.3 < MRCI calculations for the ,B, state 386 335 configurations r/A < 2.1 and angles between 90" and 180°, we were able to resulted from a set of reference configurations in which five reproduce the ab initio energies within a range of 6 cm- '. electrons were distributed amongst the orbitals 4a,-6al, 2b,, However, variational Renner-Teller calculations of the 2b,, 3b,. Calculations were made at 65 geometries for the transition frequencies between the 8,A, and A 2B, states ground state for bond angles in the range 90"-160". For the implied an error in the barrier to linearity of at least 10 cm-I *B, state 55 geometries were included with bond angles in (see next section).In order to improve the agreement between Table 1 Expansion coefficients" of the three-dimensional modified MRCI near-equilibrium potential-energy functions of the 8 2A, and A 2Bl states of SiH; x2A, state coo,: 289.845 159 46 c100: O.OOOOOO00 coo1 : O.OOOOOO00 0.716918 62 '110: -0.024 675 56 c101: 0.007 986 88 0.059 158 80 c300: -0.217 446 30 c120: -0.035 965 22 0.005 224 35 c111: -0.024 643 63 c012: -0.028 226 75 -0.012 770 25 c400: -0.546 603 86 c310: -0.095 198 88 c220: 0.067 331 32 '301 : 0.036271 12 c121: -0.034 61 129 c202: -0.081 139 73 '112: 0.024 40401 c013: 0.022 605 17 c004: 0.OOO261 32 '500: -0.741 254 12 c410: -0.345 089 11 '320' 0.217 267 25 '401 0.123 604 57 c311: -0.146 16454 c221: -0.049 335 11 '302' 0.025 463 73 c122: 0.041 141 24 '203 0.026 846 04 '113: -0.052 436 24 c014: 0.017 865 66 coos : -0.001 456 29 c600: -0.003 14092 Cs10: 0.001 799 56 c420: -0.OOO268 21 '330: 0.002 220 26 Cso1: 0.404232 67 '411: 0.329 033 11 '321 : -0.205 719 29 c402: -0.017 771 55 0.224 433 64 c222: -0.158 569 94 '303 : -0.051 31258 -0.038 26 193 '204' 0.046 908 83 cl 14: 0.042 667 9 1 -0.010 185 80 c006: O.OO0 894 55 coo,: -0.008 815 15 A2Bl state coo,: 289.807 478 53 c100: 0.011 999 95 c200 0.721 704 10 -0.032 501 02 coo,: 0.026 23 1 73 c300: -0.210894 13 -0.07 1106 26 c012: -0.013 121 22 c400: -0.437 889 09 -0.160 562 60 c040: -0.437 889 09 c112: 0.020 8 19 95 -0.030 59 140 '004' 0.003 008 15 c212: 0.086 91 1 33 -0.00301005 csoo: -0.315 390 35 '114: 0.011 608 09 -0.08 584 18 0.00000167 c222: -2.697 129 1 1 " The coefficients C, and cjik are equal by symmetry.Each coefficient cijk is given in the table in hartree units E,. * The coordinates are Ri = 1 -rrcf/r, i = 1, 2 with rrcf= re = 1.482 A for the ground state and rref= 2.8 a, for the upper state and 0= a -arefwith aref= a, for both states. For the R and A states aref= 119" and 180°, respectively. R, and R, are dimensionless. 0is given in radians. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the calculated and experimental data, we have slightly modi- fied the original height of the barrier to linearity in the 2Al state. This was done by adding 10 cm-' to the Cooocoeffi-cient in the expansion of the APEF for the A2B, state.All energies for linear geometries of the Z2A, state were increased accordingly. In the fit for the electronic ground state no energies for angles 160 < a/degrees < 180 were con- sidered. For linear geometries the APEFs in Table 1 are degenerate to within 1 cm-' for bond lengths in the range 1.1 c r/A < 2.1. The expansion coefficients of the modified potentials are given in Table 1. The barrier to linearity for the 8 2Al state is now determined to be 8247.7 cm-' relative to the bottom of the potential-energy well. After determining the equilibrium bond length and angles of both states, the APEFs were used in the calculation of some spectroscopic constants. First, the quartic force field in internal coordinates was cal- culated. This was then transformed by /-tensor algebra22 to the force field in dimensionless normal coordinates.Using second-order perturbation theory the equilibrium rotational constants, harmonic vibrational frequencies and other con- stants were determined. At this stage the Renner-Teller coup- ling as well as spin-orbit interaction were neglected. The rotational constants for the individual vibrational levels were then obtained by using the equilibrium rotational constants and the set of a, values. Our constants, which are given in Table 2 together with some previously obtained data, are very close to the experimental results. This work provides a secure value for the A, constant in the 2 state for the first time. Variational Calculations The rovibronic levels were obtained using the approach of Carter and Handy,23*24 which accounts for the full dimen- sionality, anharmonicity, rotation-vibration, electronic angular momenta and electron spin coupling effects. Some additional approximations were made.First, the L,, L, operators and the geometry dependence of the L, , L: operators were disregarded. The expectation values of the latter operators were set to unity. As Brommer et aL7 showed, this simplification makes only a small contri- bution to the relative positions of the rovibronic levels. Furthermore, the spin-orbit coupling constant Aso was set to zero. A value of ca. 140 cm-'has been obtained from a fit of experimental results obtained for high K values, but no sufficiently resolved data are available for lower K values as these levels are too strongly predissociated.2 ' In the variational calculations 24 symmetry-adapted har- monic oscillator functions were employed for each stretching symmetry coordinate Si = (l/&)(Ar, fAr2) using the inter- nal force constantsf, +LRwithf, andf,, obtained from per- turbation theory (see above).The basis set for the bending mode comprised 48 associated Legendre functions. We calculated the rovibronic levels up to 17000 cm-'for N = 0, 1 (N = J -S) covering the K, values 0 and 1. The energies are converged up to 1 cm-'. In Table 3 we give the bending levels up to 18000 cm-'. As no stretching and com- bination levels are known experimentally, they are tabulated up to 8000 cm-' for the electronic ground state and some selected levels are given for the upper state.The symmetric and antisymmetric stretching frequencies for the electronic ground state are predicted to be 2097.5 cm-' and 2174.6 cm-', respectively (for N = K, = 0). For the 'pure' bending levels with higher quantum numbers it is not always possible to assign them in terms of two different electronic states and harmonic vibrational quantum numbers (see footnotes to Table 3), especially for K, 3 1, where effects due to electron-nuclear motion coup- ling occur. For instance, the wavefunctions of the (0, 2, 0) levels of the A state for N = K = 1 are strongly mixed with the j?: state bending levels (0, 11, 0) with N = K, = 1 (for the linear state u2 is given in linear notation and K = IA + 1 I), the bending quantum number for the electronic ground state is vbent.,cf.Table 3). The analysis of the two-dimensional bend contraction reveals that the (0, 2, 0) levels of the upper state in fact belong ca. 50% to the ground state. The same holds for the (0, 11, 0) levels, which cannot be unambiguously assigned to either electronic state. The K, reordering in the bending levels of the electronic ground state starts with ub;"* = 9, which we expected from the height of the barrier to linearity. For this bending quantum Table 2 Spectroscopic constants for SiH: SCF spectroscopic constants experiment ref. 1 ref. 12 ref. 11 ref. 10 UHF ref. 14 MRCI ref.13 MRCI this work ~ ~~ 8 2A, a Jdegrees rJA 1.49 & 0.01 119 f 0.5 1.465 120.09 1.490 119.20 1.4737 119.95 1.49 120 1.482 120 1.4819 119.77 A,/cm-' a 15.75 f 0.25 5.094 (2) 3.772 (4) 2268 966 10404 f81 7469 k2 A 'B, rJA 1.465 a Jdegrees B,,,/cm-' o,/cm -02/cm- 3.956 (1) 03/cm-' a Calculated from the full set of a-constants obtained from the potentials in Table 1. components as 'C+states. 'Results for SCF calculations. 2148.4' 834.6' 208 3.1 ' 7867.5 16.7723 5.0379 3.8022 2.8466 -0.0490 0.5158 0.0235 2189.7 912.6 2267.1 8247.7 1.472 1.468 180 2237.1' 21 16.5' 676.5" 1.4693 3.9434 180 2220.1 621.1 2346.8 These results were obtained treating both Renner-Teller J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 Table 3 Rovibronic levels for the A-f( system of SiHi (in cm-') (a) Bending levels, (ul = u3 = 0) 8 'A1' A 'Bib ,,lin c ubent 000 101 111 110 2 001 101 110 1122 18 1725 1 .3' 17260.e 17830.8' 17832.9' 15 17979.8 17988.0 17 16121.7 16130.6 16684.0 16686.2 14 17298.3' 1 7299.9' 16 14987.2 14995.9 14456.7 14458.8 13 16684.0 16692.2 16025.215 13771.7 13780.1* 13 150.9 13 152.9 12 16025.5 14 12575.2 12584.e 12283.8 12285.6 11 15394.1 15402.2 13 11657.8 1 1666.7 11324.4' 11326.2' 10 14740.4 14740.9 12 10490.7 10499.6' 101 3 1.9 10133.9 9 14111.4 141 19.4 11 9582.7 9591.6 9220.3" 9222.0" 8 1 3474.6' 13475.3' 10 8692.0 8700.9 8501.6 8503.1 7 12836.7 12844.4 9 7823.3 7832.2 7790.3 7792.0 6 12156.4 12157.0 8 6971.1 6980.0 7006.0 7007.7 5 11571.3 11579.2 7 6123.8 6132.7 6170.5 6172.2 4 10853.3 10853.9 6 5272.1 528 1.0 5314.9 5316.5 3 10316.4 10324.3 5 4412.0 4420.9 4448.8 4450.4 2 9605.3" 9605.7" 4 3543.3 3552.2 3575.1 3576.6 1 9073.4 908 1.2 3 2666.9 2675.8 2694.7 2696.2 2 1783.6 1792.6 1808.5 1809.9 1 894.6 903.5 917.1 918.4 0 0.0 8.96 20.6 21.9 (b) Stretching motion and combination levels in the ft state (in cm-')11 1 ~~ 0 0 2097.5 2106.3 2117.8 21 19.0 0 0 1 2174.6 2183.5 2194.8 2 196.0 1 1 0 2980.3 2989.1 3002.5 3003.8 0 1 1 3054.4 3063.2 3076.4 3077.7 1 2 0 3858.8 3867.6 3883.3 3884.7 0 2 1 3929.4 3938.2 3953.6 3955.0 2 0 0 4139.6 4148.3 4159.6 4160.8 1 0 1 4186.8 4195.5 4206.6 4207.8 0 0 2 43 15.4 4324.2 4335.2 4336.4 0 3 1 4799.1 4807.9 4826.1 4827.6 1 3 0 4732.3 4741.1 4759.8 4761.3 1 1 1 5054.5 5063.2 5076.2 5077.5 2 1 0 5010.3 5019.0 5032.1 5033.5 0 4 1 5662.7 567 1.5 5693.4 5695.0 0 1 2 5180.5 5189.2 5202.0 5203.3 1 2 1 59 18.7 5927.4 5942.6 5944.0 1 4 0 5599.8 5608.6 5631.3 5632.8 2 0 1 6132.2 6140.8 6151.7 6152.9 2 2 0 5877.9 5886.6 5902.1 5903.4 0 0 3 6411.0 6419.6 6430.4 6431.6 0 2 2 6041.5 6050.3 6065.2 6066.6 0 5 1 6519.3 6528.1 6554.8 6556.4 3 0 0 61 12.4 6121.0 6132.0 6133.2 1 3 1 6778.6 6787.3 6805.5 6806.8 1 0 2 6274.2 6282.8 6293.7 6294.9 2 1 1 6987.6 6996.2 7008.9 7010.2 1 5 0 6460.2 6469.0 6496.6 6498.2 2 3 0 674 1.4 6750.1 6768.6 6770.1 0 3 2 6898.0 6906.7 6924.4 6925.8 3 1 0 6970.0 6978.5 699 1.4 6992.7 (c) Stretchin g motio n and combination level s in the A s tate (in cm-')b u3 even u3 odd 2u1 u;" 03 001 101 110 112 u1 din 03 001 101 110 112 1 1 0 10316.4 11203.6 0 1 1 11312.6 11320.3 1 2 0 11771.9" 11773.3' 0 2 1 11821.9 11822.8 2 1 0 13269.1 13276.6 1 1 1 13360.1 13367.7 0 1 2 13510.6 13518.2 2 2 0 13822.6' 13824.0' 0 2 2 13947.5' 13948.2" ~~ ~ ~~ a The notation for the levels of the fi.'A, state is NKaK,. For the linear A 'B, state we used the notation NK,.u:" = 2uyn' + K + 1 with K = I A + I I. 'These levels are perturbed by anharmonic resonances; assignment ambiguous. 'There is a strong Renner-Teller interaction between the levels )7 (ul, 11, u3)and A (ul, 2, u3)for N = K = 1.number for the first time the K, = 1 levels lie below the cor- Conclusion responding K, = 0 level. The theoretically derived transition wavenumbers between The potential-energy functions for the 2 2A, and A 2B,states the two states are compared with the experimental data in of SiHl were used to calculate rovibronic levels up to 18OOO Table 4. After correcting the barrier by 10 cm-' as described cm-' for K = 0, 1. The transition wavenumbers between in the previous section our theoretically calculated data cor- both states were obtained and compared with experimental respond closely with the experimental results. data. In general the agreement between the theoretical and J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 521 Table 4 Comparison of transition wavenumbers obtained from experiment and variational calculations using both original and modified potentials (N = 1)" 0';v; 0;" 4 2 K U; exp. pot. Id pot. IIe level separations in the electronic ground state 'A, 0 1' 0 0 2' 0 00 0' 1' 0 0 890.3 889.6 896.5 891.4 level separations in the excited electronic state 2Bl 0 9' 0 0 7' 0 1261.0 1275.0 0 1 lo 0 0 9' 0 1280.2 1282.9 0 13' 0 0 11' 0 1296.7 1289.9 0 15' 0 0 13' 0 1300.7 1295.8 transition wavenumbers in the A-8 system 0 7' 0 0 0' 0 12831.8 12812.4 12823.8 0 9' 0 0 0' 0 14092.8 14087.1 14098.8 0 1 1' 0 0 0' 0 15373 15369.9 1538 1.7 0 13' 0 0 0' 0 16669.3 16659.8 1667 1.6 0 15' 0 0 0' 0 17970 17955.9 17967.4 0 13' 0 0 1' 0 15779 15759.1 15775.1 0 15' 0 0 1' 0 17076 17055.2 17070.9 u2 are either given in ubCn'for the electronic ground state or ulin for the excited electronic state with din= ubcn' notation, the superscript K means K,, for the linear 2B,state K ations.= PEFs with corrected barrier. experimental molecular parameters is good. Given that the experimental constants were obtained from spectra which are intrinsically low-resolution owing to lifetime broadening of the excited state, it may be that the accuracy of the rotational constants obtained from the theoretical potentials now exceeds that of the current experimental values.We may con- clude that the theoretical potentials and parameters now provide a sound basis on which further theoretical and experimental studies can be based. These include: (a)dynami-cal calculations directed towards an understanding of the predissociative decay mechanism(s) and lifetimes, interpreta- tion of the partitioning between the two dissociation channels from well defined rovibronic levels, and prediction of the internal quantum state distributions of the products, (b) further high-resolution laser-based photodissociation studies in both frequency and time domains, and (c) experimental searches for spectra of the SiH; ion in silane discharges and, conceivably, in interstellar clouds. C.B. and P.R. thank S. Carter and N.C. Handy for providing them with access to the variational Renner-Teller program. This work was supported by the Deutsche Forschungsge- meinschaft, Fonds der Chemischen Industrie and by the SERC. D.I.H. thanks BP for a research studentship. References M. C. Curtis, P. A. Jackson, P. J. Sarre and C. J. Whitham, Mol. Phys., 1985,56,485. D. I. Hall, A. P. Levick, P. J. Sarre, C. J. Whitham, A. Alijah and G. Duxbury, J. Chem. SOC.,Faraday Trans., 1993,89,177. D. I. Hall, A. P. Levick, P. J. Sarre and C. J. Whitham, in prep- aration. S. Carter, N. C. Handy, P. Rosmus and G. Chambaud, Mol. Phys., 1990, 71, 605. 2ubcn1+ K + 1. In the case of the = 1 A + 1 I. Upper state. 'Lower state. Original PEFs without alter- 5 M. Brommer, B.Weis, B. Follmeg, P. Rosmus, S. Carter, N. C. Handy, H-J. Werner and P. J. Knowles, J. Chem. Phys., 1993,98, 5222. 6 M. Brommer, P. Rosmus, S. Carter and N. C. Handy, Mol. Phys., 1992, 77, 549. 7 M. Brommer, G. Chambaud, E-A. Reinsch, P. Rosmus, A. Spiel-fiedel, N. Feautrier and H-J. Werner, J. Chem. Phys., 1991, 94, 8070. 8 G. Chambaud, W. Gabriel, P. Rosmus and J. Rostas, J. Phys. Chem., 1992,%, 3285. 9 W. H. Green, N. C. Handy, P. J. Knowles and S. Carter, J. Chem. Phys., 1991,94,118. 10 J. R. Ball and C. Thomson, Int. J. Quantum Chem., 1978, 14, 39. 11 M. S. Gordon, Chem. Phys. Lett., 1978,59,410. 12 J. M. Dyke, N. Jonathan, A. Morris, A. Ridha, M. J. Winter, Chem. Phys., 1983,81,481. 13 D. M. Hirst and M. F. Guest, Mol. Phys., 1986,59, 141. 14 M. Gonhlez, R. Sayos, F. Mota and A. Aguilar, Chem. Phys., 1987,113,417. 15 R. S. Grev and H. F. Schaefer, J. Chem. Phys., 1992,97,8389. 16 J-P. Gu, M-B. Huang, F. Kong and S-H. Liu, J. Mol. Struct. (Theochem),1992,253,217. 17 H. Partridge, NASA Technical Memorandum 89449, Ames Research Center, Moffett Field, CA, 1987. 18 F. B. van Duijneveldt, IBM Technical Research Report RJ-94.5, 1971. 19 M. F. Guest and P. Sherwood, GAMESS-UK, User's Guide and Reference Manual, 1992, SERC Daresbury Laboratory, UK. 20 S. Carter and N. C. Handy, J. Chem. Phys., 1987,87,4294. 21 P. J. Sarre, Faraday Discuss. Chem. SOC., 1991,91,414. 22 A. R. Hoy, I. M. Mills and G. Strey, Mol. Phys., 1972, 24, 1265. 23 S. Carter and N. C. Handy, Mol. Phys., 1984,52, 1367. 24 S. Carter, N. C. Handy, P. Rosmus and G. Chambaud, Mol. Phys., 1990, 71, 605. Paper 3/05836K; Received 28th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000517

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(4), 523-531 Production of CH(X 'II) from the Multiphoton Dissociation of CH,CO at Wavelengths of 279.3 and 308 nm Stephen M. Ball, Graham Hancock" and Mathew R. Healt Oxford Centre for Applied Kinetics, Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, UK OX 13QZ ~ ~~ CH(X 'n) has been observed by laser-induced fluorescence (LIF) spectroscopy as the product of the two-photon dissociation of ketene (CH,CO) at 279.3 and 308 nm. The nascent distribution of rotational levels is Gaussian in profile, consistent with a ' rotational reflection ' principle in the dissociation. Thermodynamic arguments imply a fragmentation pathway to CH + HCO following an initial one-photon absorption to the 'A" excited state of ketene, a second photon absorption, and dissociation following rearrangement via a formylmethylene isomer.Analysis of A doublets in the LIF spectra shows no orbital alignment of CH produced on photolysis at 279.3 nm, but some propensity for ll (A') symmetry alignment (TC orbital of CH parallel to the plane of rotation) for photolysis at 308 nm. The photochemistry of ketene (CH,CO) has been widely studied, but in contrast to the wealth of investigation into the single-photon dissociation in the near UV, comparatively little work has been expended on the multiphoton disso- ciation channels in a similar wavelength region. Initial work concentrated on the single-photon dissociation of ketene as a major source of the methylene radical, CH, .lP3 Subsequent investigations of photolysis in the wavelength region 260-370 nm demonstrated the production of both the ground triplet (x3B1) and first excited singlet (5 'Al) electronic states of methylene4 with the relative yield of the excited singlet product increasing with decreasing photolysis wavelength within this near-UV region.' The dynamics of the process have been studied by measurements of both the CH, (5 'A,) quantum state resolved photofragment yield spectra,6-8 and of the CO(X 'C') product state distribution^.^*' Deviation of the measured excited product state distributions" and excited-state lifetimes from those predicted by conventional phase space theory" have been successfully modelled by means of a variational form of the RRKM the~ry.'~.'~ Of direct interest to the present study are measurements of the excited-state lifetimes of 19 and 130 ps for photolysis at 279 and 308 nm, respectively.Examination of ketene photolysis further into the UV has been less extensive. A rotational temperature of 6700 K for the CO fragment following photolysis at 193 nm indicates that the dissociation must proceed through a non-linear pathway.' ' Vibrationally excited CO distributions from the channel yielding CH, and CO have been measured by Fuji- mot0 et all6 and Unfried et These latter workers have also reported the first direct evidence for the production of the ketenyl radical from photolysis of ketene at 193 nm, clearly demonstrating a branching in the photodissociation process at this wavelength.A recent resonance Raman spec- troscopic investigation by Liu et al." for excitation wave- lengths between 217 and 200 nm has confirmed the ab initio work of Allen and S~haeffer'~.~~ that the initial dynamics of the CH,CO 'B, dissociation proceed through an out-of-plane bend of the CCO skeleton. In contrast to the above, information on multiphoton pro- cesses is spar~e.~'-~' Although CH has been widely reported as a photofragment following irradiation of ketene in the near-UV between 285-330 nm and detection by resonance- enhanced multiphoton ionization (REMPI) with the same t Present address: School of Chemistry, University of Leeds, Leeds, UK LS2 9JT. laser radiation, the studies have been largely spectroscopic, with little information on the dissociation dynamics.Simple energetic considerations show that to produce CH from ketene at these wavelengths requires the absorption of at least two photons. Multiphoton dissociation of other polya- tomics (CHBr, at 266 nm,26 CH,Br, and CHClBr, at 248 nm 27 and CH,I at 193 nm has long been used as a source of CH and recently there has been an attempt to char- acterise the nature of the multiphoton fragmentation pathway to CH from CHBr, .29 In some similar dissociations the fluo- rescence emission from electronically excited states of CH, when dispersed, has shown a propensity for the Il (A') A-doublet and further unusual rotational state dependent behavi~ur.~~,~The equivalent fluorescence has recently been characterised for CH produced from multiphoton disso- ciation at 193 nm of ketene itself3' and shows similar results.Rotational levels with N' from 14 to 19 in the CH (A 'A) state showed higher intensity emission in the symmetric ll (A') A component than the antisymmetric, while the opposite was observed for levels with N' between 20 and 23. Following a preliminary report on the kinetic measure-ment of the removal rate constant for CH by CH2C0,33 the present work details an investigation into the pathways of ground state CH(X ,ll) production from the multiphoton dis- sociation of ketene at wavelengths of 279.3 and 308 nm, the radical being detected by LIF spectroscopy of the A 'A tX 'n transition near 430 nm.From observations of the population distribution into particular CH rotational levels and the appearance of the associated A doubling com- ponents, the possible pathways of photolysis and of align-ment in the dissociation process are discussed. Experimental A standard LIF detection apparatus was used for most of the work described in this paper. The CH radical was detected by on-resonance fluorescence following one-photon excitation within the Q and R branches of the CH A 'A tX *l7system with band origin at CQ. 431 nm. 34 Photolysis was carried out on a static gas sample contained within an all-stainless-steel cubic reaction cell. The photolysis and probe laser beams intersected orthogonally at the centre of the cell and fluores- cence was observed with an EM1 9813QKB photomultiplier tube (PMT) positioned mutually perpendicular above the intersection region.Pressures within the cell were measured using a Datametrics 0-10 Torr capacitance manometer. Pho- tolysis radiation was obtained either from the 308 nm unpo- larked output of a Questek 2240 excimer laser operating with 524 XeCl or, for plane polarised light at 279.3 nm, the frequency- doubled output of a QuantaRay 5200 series dye laser pumped by the Questek excimer laser. The probe laser beam was obtained variously from a Molectron Corp. UV 24 nitro-gen laser pumped Molectron DL 200 series dye laser (energies up to ca. 50 pJ pulse-') or a Lambda Physik excimer laser-pumped dye laser combination, EMG101/ FL2002, with a higher output energy.The bandwidths of these two systems were CQ. 1.0 an-' or ca. 0.4 cm-', respec-tively, and both beams were vertically plane polarised with respect to the laboratory frame. The laser linewidths were sufficient to separate the A doublet transitions within a spec- tral scan but insufficient in general to resolve fully the spin- orbit components. Typical laser pulse rate was 10 Hz. The probe laser beam was passed through long side arms which contained a series of baffle rings and Brewster-angled windows in order to minimise scattered light that might otherwise be viewed by the PMT. This was of particular importance in these experiments since fluorescence was always detected on-resonance with the exciting radiation.A short focal length quartz lens (f=22 cm) focussed the pho- tolysis radiation into the centre of the cell. In the case of the 279.3 nm radiation, for which the photolysis beam was also polarised, two distinct relative polarisation geometries of J' N' I 16 16.5 2As 15.5 15 2 15.5 N " e +- f 16 2 e 15 f +- n(A") 15.5 F1 manifold -J=_N+-i J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 photolysis-probe beam could be used; first, the electric vector of both beams parallel to each other and vertically orientated with respect to the laboratory frame and secondly, with the polarisation of the photolysis beam rotated through 90". Fluorescence was imaged onto the PMT with a further series of lenses and baffle rings and passed through an inter- ference filter.The PMT signal was fed either into a 20 MHz Thurlby DSA524 digital storage adaptor and thence to a PC, or was gated and integrated using a Brookdeal 9415 linear gate and 9425 scan delay generator combination. The output was taken to a chart recorder. Triggering of all electronic equipment and both lasers was effected by a home-built pulse-delay generator capable of delivering a sequence of four variable delay pulses. Experiments were performed at a ketene pressure of 930 mTorr with a delay between disso- ciation and probe lasers of 60.3 ps. The measurement of fluorescence lifetimes utilised the 50 input of a 20 MHz digitising board of the PC (Markenrich Corp. WAAG card) capable of averaging over several thousand laser shots.The ketene precursor (CH,CO) used in all experiments was prepared by the pyrolysis of acetone vapour passing over an electrically heated nichrome element at ca. 650°C in a stream of inert He carrier gas.35 The ketene was purified by distilla- tion at 195 K (trichloroethylene-dry ice slush bath), trap-to- Jf N' - 15.5 2A3 2 14.5 14.5 15 R22ee R22ff J" N" n(K')-15.5 n(A') 14.5 15 F1manifold -J=_N-$ Fig. 1 Rotational energy level diagram for CH illustrating the nomenclature of the energy levels and an example of the four allowed R-branch transitions for a given value of N" within the A 'A-X 'II band J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 trap distillations at 77 K and continuous pumping on the product at 113 K using an isopentane-liquid-nitrogen slush bath.Sample purity was confirmed on each occasion by mass spectrometric analysis and UV absorption between 200 and 400 nm with cross-section at a maximum of 2.0 x lo-,’ cm2.36 During experiments the ketene was stored at pressures less than 20 Torr in a darkened bulb to prevent polymeris- ation. CH Spectroscopy The ground state of CH(X211) is subject to both A doubling and spin-orbit splitting. The spin-orbit parameter is suffi- ciently small compared with the rotational constant (A = 28.14 cm-’,34 Be = 14.46 cm-’ 37) to ensure that it approaches Hund’s case (b) at relatively low values of the total angular momentum quantum number J”. Fig.1 illus-trates the energy levels involved. Spin-orbit splitting creates two manifolds of levels designated F, (J” = N“ + 4) and F, (J” = N” -j),where N” is the total angular momentum excluding spin. A doubling produces splittings in each of these J” levels, to give states of + and -symmetry and the labelling of these levels with respect to inversion of all coordi- nates follows the convention of Brown et aL3* For a Hund’s case (b) molecule such as CH(X ’IT) all lower levels of each A doublet pair in the F, spin-orbit manifold are designated by the notation f, whereas in the F, manifold the lower com- ponents all constitute a series of e levels. The levels can be additionally classified according to the behaviour of the elec- tronic wavefunction on reflection in the plane of rotation in the limit of high J.As the case (b) limit is approached the F,, and F,, wavefunctions of a ,ll state acquire symmetric char- acter with respect to the reflection while the F,, and F,, wavefunctions acquire antisymmetric character. Current notation3’ describes these levels as being of II (A’) or II (A”) symmetry, respectively, for all values of J. Observation of preferential population in one of the A doublets can yield important information on the dynamics of a dissociative or reactive process, provided there is complete alignment of the pn electron lobe parallel or perpendicular to the total angular momentum vector J at a given value of quantum number J. For the case of CH(X211), this degree of electron alignment rapidly approaches its limiting value of unity at relatively low values of J ( k4.5)40so that any propensity in A population can be ascribed directly to dynamical effects.The first excited state of CH (A2A) is also subject to a A-type doubling, but its effect, and that of spin-orbit coup- ling (A = -1.1 are both small in comparison with the ground-state behaviour. The selection rules3 * dictate that absorption transitions from the ground state to the first excited state are allowed from all four of the rotational levels taking the same lower quantum number N” and these form a closely spaced quartet in the spectrum, although at high values of N” the A splitting dominates considerably over the spin-orbit splitting which may not be fully resolved.Fig. 1 illustrates the four allowed R-type transitions for N” = 15 of the A-X spectrum. It is important to note, however, that both the closely spaced transitions of spin-orbit origin within one pair of the A doublet (for example Rllee and R,,,,) together probe the population of CH levels having II (A’) symmetry, while the closely spaced transitions R,,,, and Rzzeetogether probe population of CH having ll (A”) symmetry. The rela- tive populations of CH in these two symmetries are therefore determined directly from a limited LIF scan of one group of of N”, in contrast to R branch (or P branch) transitions arising from a single value similar studies for the OH (X2n) for a quadratic dependence on photolysis intensity.The single- photon absorption spectrum of ketene in the same wavelength region radi~al.~’ is shown as a dashed line for comparison. Results and Discussion The original aims of the present experiments were to try to devise a method of detection of ground-state methylene radical, CH,(g 3B1),by two-photon excitation of the upper predissociated 3A, state followed by LIF detection of the expected CH(X211) product. Ketene was photolysed at 308 nm to produce CH,(ii’A,), which was then quenched to the ground state by collisions with Ar. At a suitable delay the two-photon excitation radiation at either 279.3 nm or 282.9 nm was fired into the cell in an attempt to form the 3A, state, these wavelengths being those expected for excitation of the (0, 1, 0) or (0,0, 0) levels.In the event copious CH LIF signals were observed with a single UV wavelength (308, 279.3 or 282.9 nm) directed into the cell, and no additional formation of CH(X ,II) from the route 308CH2CO --‘CH, + CO ‘CH, + Ar -3CH, + Ar 3CH, -+ -+ CH;(3A,) -+ CH(X211)+ H could be detected. Furthermore, strong emission from elec- tronically excited CH was detected in the A2A-X211 and B 2C-X 217 bands near 430 and 390 nm following ketene pho- tolysis at all three wavelengths. As the A-X emission was at the same wavelength as the LIF used to detect ground-state CH, conditions were sought to minimise this contribution which appears as background in the LIF spectrum. Both the A state emission and the LIF signals exhibited the same quadratic dependence on photolysis laser intensity, and thus this parameter was ineffective in maximising the signal to background ratio: this point will be further considered when the mechanism for formation of the excited state of CH is discussed.However, the photolysis wavelength dependence of the signals differed markedly, as illustrated in Fig. 2. At 279.3 nm it can be seen that the signal to background ratio is near its maximum value, and hence this wavelength was chosen for the majority of the studies of the dissociation dynamics where short time delays were required between dissociation and probe laser pulses. Further studies at 308 nm were facili- tated by the better energy stability of the excimer laser output compared with that of the frequency doubled layer dye laser.1.0 1 I .. h u) j’4-.-C 3 c 270 275 280 285 290 295 wavelength/nm Fig. 2 The variation with photolysis wavelength of the LIF signal at the Q-branch head of the CH A ’A cX ’II band (0)and of the nascent CH emission recorded at the peak of the A ’A X ’II band-P [(.)and solid line]. Both fluorescence signals have been corrected J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Thermodynamics of Photodissociation The presence of ground-state CH was unequivocally identi- fied by assignment of the transitions within the LIF spectra with reference to the compilation of Moore and Broida4' and Bembernek et dJ4and spectra recorded for R-branch tran- sitions following photolysis at 279.3 nm and 308 nm are shown in Fig.3 and 4, respectively. The magnitude of the LIF signal was measured as a function of dissociation laser inten- sity at 279.3 nm, both at the Q branch head of the CH A'A t X'n band at 431.7 nm and at the unresolved spin- orbit shorter A wavelength component [probing total lT (A") symmetry] of the N" = 10 R-branch transition at 424.9 nm. In both instances a value of n = 1.9 & 0.1 for a dependence of the form I" indicates a dissociation process to yield CH(X 'n)that requires the absorption of at least two photons at a wavelength of 279.3 nm. Thermodynamic heats of formation data43*44 dictate that the following two fragmentations are both energetically accessible pathways for production of CH.CH,CO(X 'A,) +CH(X 'n) + HCO(8 'A'); AH: = 56 726 * 838 cm-' (l) CH,CO(X 'A,) +CH(X 'n) + H(*S) + CO(X 'C+); AH: = 61 664 184 cm-' (2) The total energy supplied by absorption of two photons at 279.3 or 308 nm is equal to 71 608 or 64935 cm-', respec-tively, which is sufficient in both cases to exceed the ther- modynamic thresholds of both the above fragmentations. It is reasonable to assume that the dissociation processes leading to formation of CH(X 'II) are the result of absorption of no more than two photons at the wavelengths used, by application of the following arguments. An observed quadra- tic dependence on laser intensity would necessitate that one absorption step be fully saturated for all laser fluences inves- tigated if the photodissociation required absorption of more than two photons.Well documented work as cited in the R(0, 0)N" 22 20 18 16 I1 1 I I I I I II II II introduction has established that absorption of one photon in the near-UV readily excites ketene to its 'A" surface during dissociation to 'CH, and CO products. Under our experi- mental conditions, the 380 nm excimer beam of 80 mJ pulse-' focussed to a minimum beam diameter of 3 mm supplies a maximum photon density of 1.7 x 10l8 cm-2. An absorption cross-section of 2 x lo-'' cm2 for CH,CO at this wavelength36 indicates that this transition is far from saturat- ed, and this was confirmed by measurements of a linear dependence for 'CH, formation, observed by LIF of the 'B, (0, 16, O)t 'A,(O, 0, 0) transition at 537.5 nm, on 308 nm laser energy.Photon densities at 279.3 nm were also calcu- lated to be below the ketene saturation limit. Direct population of this real molecular eigenstate will thus be the predominant absorption process of the first photon, but for subsequent absorption, photofragmentation of the 'A" state must now compete. At the photon intensities used (ca. 2 x photons cm-2 s-'), the second photon absorp- tion cross-section at 279.3 nm must be CQ. cm2 to compete with the (relatively slow) ketene dissociation rate of 5.3 x 10''s -'.''This high cross-section (considerably higher than that for absorption of the first photon) is probably not achieved for absorption from the 'A" state, and implies that a second absorption process is again not saturated.At 279.3 nm, two-photon absorption to Rydberg states (which might live long enough to absorb further) is not seen in REMPI studies of ketene25 (although a weak feature is observed near 308 nm) and thus non-saturated absorption probably leads to a dissociative level above the thermodynamic threshold to form CH. Further absorption cannot be ruled out, but must be saturated, which from a presumed completely dissociative state is an unlikely event; coherent two-photon absorption from the 'A" state is ruled out because of the observed quad- ratic energy dependence. As dissociation of the 'A" intermediate state almost cer- tainly predominates over further absorption, we need to con- sider the formation of CH from the dissociation of the 'CH, intermediate. This can only take place with the first absorp- 14 12 10 8 II II II II II II II n 418 420 422 424 426 excitation wavelength/nm Fig.3 LIF spectrum in the R-branch region of the A *A +X 211band for nascent CH(X %) produced in the two-photon dissociation of 23 mTorr ketene at 279.3 nm using the Molectron nitrogen laser pumped dye laser combination as probe at a delay of ~0.3p.Positions of the R-branch transitions in the (0,O)band are indicated as a function of N". J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 17 6 5 4 3 1 I I I 1 2-I A' F1 I I I 1 I I 1 A' F, I 1 1 I 1 1 I A" F1 I I I 1 I I I A" F2 I I I 1 1 427 420 429 430 43 1 I 15 I 14 I 13 12 I 11 I 10 1 9 I A'F, 7 I I I I 1 I I I A' F2 1 I 1 I I I I I A'FI- I I 1 I I 1 I A"F2F 422 423 424 425 426 excitation wavelength/nm Fig.4 LIF spectrum of the A *A cX 2fl band for nascent CH(X 211) produced in the two-photon dissociation of 30 mTorr ketene at 308 nm using the narrow band Lambda excimer laser pumped dye laser combination as probe at a delay of ~0.3ps. Positions of the R-branch transitions originating from the four levels associated with each N" value are indicated. tion step of 'CH, being to a real eigenstate: no absorption features at these UV wavelengths have been observed in the electronic spectra of 'CH2.4s The excess energy at 308 nm above the singlet threshold for dissociation into 'CH, and CO (measured spectroscopically as 30 116.2 cm-' ') is 2351 cm-'. Molecular beam studies of Hayden et al.' have demonstrated that an average fraction of 0.33 of this excess energy is channelled into product translation, (Etrans).When the average fractions of excess energy partitioned into CO rotation and vibration are included, 0.08 and 0.40, respec-tively," only 19% of available energy is apportioned to inter- nal modes of 'CH,.This amounts to 632 cm-' when the internal energy of ketene at room temperature (450 cr")'o is included in the dissociation. The enthalpy of dissociation for CH, (2 'A,) into CH(X 211) and H('S) is 31 348 & 36 cm-' and from this is derived an excess energy of only 920 f36 cm-' after absorption of a second photon at 308 nm, and thus a maximum of 1452 cm-' for the average energy available for disposal to the CH + H fragments.The observed peak of N" = 6 (E,,, z 580 cm-1)46 for the CH nascent rotational distribution would therefore imply a minimum value of 40% for the average fraction of the excess energy in the second photolysis step that would be given to CH rotation. This value is sufficiently unrealistic for such a process to be rejected. For comparison, in the photo-dissociation of H,O, a bent dihyride with a bond angle similar to 'CH, ,average energy fractions patitioned into OH rotation have been measured at only 2%4',47*48or 3Y04' for J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 dissociation at 157 nm or 193 nm, respectively.Similarly only 3% of the excess energy is associated with rotation of the NH fragment in the dissociation of HN, at 266 nm.” Analogous thermodynamic arguments further demonstrate that a two-photon dissociation to CH + H + CO products directly is rather less probable than to CH + HCO products again on the basis of observed populations of CH rotational levels, quite apart from the lower probability of a three-body fragmentation. The excess energy of ca. 3720 cm- ’ (including internal energy of bulk room-temperature ketene) for the former pathway at 308 nm just permits population of the N“ = 16 rotational levels with E,,, z 3740 cm-’ 46 to be an allowed product state. The observed LIF spectrum at 308 nm shows some evidence for population of the N” = 15 doublet above the baseline noise.Although this is not inconsistent with energetics of dissociation to CH + H + CO, it would require that for some proportion of the dissociation processes virtually the entire excess energy be channelled solely into CH rotation. This is intuitively unreasonable for a process which ultimately yields three co-fragments, all three of which are expected to possess translational energy and (in the case of CH and CO)vibrational and rotational energy. Consider- ation must also be given to whether population of such high rotational levels of CH is consistent with an overall conserva- tion of angular momentum, either through rotation of the CO co-fragment or through total orbital angular momentum of the products.Rotation of CO (Be= 1.93 cm-l ”) with a similar number of quanta necessitates additional energy of ca. 460 cm- ’ to be added to that associated with ‘CH, rotation. For dissociation at 279.3 nm the highest observed level is N” = 22 (Fig. 3) and in this case this corresponds to 70% of the available energy in the three-body fragmentation appear- ing as CH rotation, again considered to be unlikely. We may therefore conclude that dissociation of ketene at the wavelengths investigated must occur through a sequential two-photon absorption (within the photolysis pulse width of 10 ns) where absorption of a second photon from the inter- mediate electronic state competes effectively with unimolecular dissociation to ‘CH, + CO fragments.We note that the close similarity between the photodissociation yield spectrum of CH(X 211)and the ketene one-photon absorption cross-section is probably fortuitous, as over the wavelength range 279-290 nm the ‘A” intermediate-state lifetime increases by a factor of 2 l2 and this would imply a larger fraction of molecules absorbing in the second step. Reduction of the second absorption cross-section by a similar factor could counter this effect. The production of CH as a fragment invokes a rearrange- ment to a formylmethylene HCCHO isomer of CH2C0, a process which must be accomplished on a timescale compara- ble to internal conversion. The single-photon dissociation is relatively slow with measured lifetimes of 130 and 19 ps at 308 and 279 nm, respectively,’2 consistent with a significant intramolecular energy redistribution into the reaction coordi- nate of the polyatomic before fragmentation.’, Several experiments have demonstrated that ketene will undergo isomerisation reactions.Russell and Rowland’ photolysed ketene labelled at the methylene carbon and measured signifi- cant yields of 14C0, while in a separate study Montague and R~wland’~observed carbon exchange in the reaction of labelled singlet methylene, 14CH2 (5 ‘Al), with CO, also to yield 14C0 as a product. The symmetric O-bridged oxirene has been postulated as an intermediate. More recently a thor- ough investigation into the kinetics of intramolecular carbon exchange in ketene by Lovejoy et a1.” has concluded that the isomerisation of ketene does compete effectively with unimolecular dissociation.RRKM calculations using an oxirene transition state are in quantitative agreement with the experimental rate constants for most of the energy range studies (up to 4000 cm-’ above the singlet methylene disso- ciation threshold). In the context of the present work it is important to note that rearrangement of ketene to an oxirene species must proceed through a 1,2-hydrogen shift as a first step leading to formation of formylmethylene as an interme- diate species. Carbon-atom exchange by rearrangement through an oxiranylidene intermediate is energetically unfa- vourable because of the extremely high barrier (489 kJ mol- ’ above ground-state ketene) which lies along the reaction coordinate between these two isomer^.'^,'^ In contrast, the same ab initio calculation^'^^^^ show there to be an energy separation of only 310 kJ mol-’ between ketene and for- mylmethylene and only a very small energy barrier towards subsequent oxirene ring formation, although more recent high-level calculations by Vacek et ~1.’~have suggested that, at yet higher levels of theory, oxirene may be reduced to a transition state only.These latter workers were also unable to locate any transition state separating oxirene and for-mylmethylene and concluded that formylmethylene was unstable, in agreement with Tanake and Y~shimine.~~ Nascent CH(X *ll)Rotational Distributions The data of Fig.3 and 4 were converted into relative rota- tional populations as a function of N” using the standard procedure, taking account of the laser intensities (a linear variation of LIF signal with probe laser energy was established) and detection sensitivities, but neglecting any polarisation terms arising from fragment alignment. The latter effects are expected to be small and experiments to probe this, in which relative polarisations of probe and pho- tolysis lasers were varied, showed no observable spectral dif- ferences. Rotational line strengths were calculated from the formulae derived by M~lliken.’~ CH ’A radiative lifetimes have been shown to be invariant for u’ = 0 at N’ levels between 6 and 23,60 and thus populations derived by LIF in this work were not influenced by predissociation of the upper state.An experimental check on the radiative lifetimes at the Q branch head and on all four A doublet and spin-orbit components of the N’ = 16 level showed values of 550 40 ns, consistent with previously reported measurement^.^'-^^ For excitation at 279.3 nm the resultant populations shown in Fig. 5 are averages over the two spin-orbit states for a given N”, but are clearly separated into Il (A’) and ll (A”) components for the levels measured (N” between 8 and 22). 1.o 0. 0.5 -I I I I I I I I 0 5 10 15 20 25 N Fig. 5 Relative rotational populations as a function of N” of the nascent CH(X *lI) ground-state levels corresponding to the spectrum of Fig.3 for photolysis of 23 mTorr of ketene at 279.3 nm. (0)Levels of TI (A’) symmetry, (0)lI (A”) symmetry. The dashed line is the best-fit Gaussian to both A doublet components of the distribution. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The spectrum indicates that both A doublets show equal population for N” levels probed, and this was checked by careful slow scans at higher resolution over the selected spec- tral region. For N“ = 10, 15 and 18 the ratio l-I (A”) : l7 (A’) was found to be 1.05 & 0.12, 0.98 k0.10 and 1.0, with error bars here showing deviation from the mean of a number of experiments (a single run in the case of N” = 18). As can be seen in Fig. 4, the observations at lower N” for photolysis at 308 nm (carried out with the higher-resolution excimer pumped dye laser) allowed resolution, in some cases, of all four R branch lines.Fig. 6 shows the population of the A‘ F, levels over J”values from 1.5 to 12.5, these representing the levels which could be resolved from their neighbours over the widest range of J”. Clearly apparent from the spectrum in Fig. 4 is that now the A doublets are not of the same intensity (although care must be taken in interpreting peak heights in these cases as equivalent to populations). Again scans over the areas of the relevant peaks now show values of ll (A”): rI (A’) of 0.63 f0.04 for N” = 10 and 0.86 & 0.06 for N“ = 12. The distributions at the two wavelengths show Gaussian forms, peaking at N” = 12 and J = 7.5 for photolysis at 279.3 and 308 nm, respectively, corresponding to average fractions of 17% and 7% of total excess energy partitioned into rotation for the two-photon dissociation producing CH and HCO fragments.Further experimental results at 279.3 nm in which an excimer pumped dye laser was used (at higher resolution than the data of Fig. 3) as the LIF source, and for which different detector sensitivities applied, showed essentially the same Gaussian distribution (and confirmed the low population in low N“ levels). Fig. 3 also shows some evi- dence of weaker signals from u” = l in CH (for example the doublets between those illustrated for N” = 8 and 9 in the 0,O band) but low signal to noise levels precluded full analysis.It should however be noted that these are absent in the 308 nm case. As Franck-Condon factors and detection sensitivities for the 1,l and 0,O bands are similar, it can be concluded that these vibrational levels are certainly not inverted, and the fraction leading tc vibrational excitation in CH is low. A Gaussian-shaped distribution amongst rotational levels is indicative of a ‘rotational reflection principle’64 where there is a strong final-state interaction in the exit channel on the excited state potential V,, accessed immediately after the absorption process. According to the Franck-Condon (FC) principle, the absorption step to V,, defines an initial prob- ability P,(e, u, j) which is modified by the intermolecular forces induced by the excited state potential as the molecule dissociates.In a direct photodissociation where the final-state 5 10 15 J“ Fig. 6 Relative rotational populations as a function of J” of the CH A F, ground-state levels marked in the spectrum of Fig. 4 for pho-tolysis of 30 mTorr ketene at 308 nm. The dashed line is the best-fit Gaussian to the component state distribution. interaction is zero or very weak (the FC limit) the initially prepared distribution of angular momentum states j is not changed during fragmentation and the final rotational state distribution is a simple map of the initial parent wavefunc- tion. When a strong final-state interaction exists, the initially prepared distribution is more or less completely destroyed and the final-state distribution becomes a direct map of the anisotropy of V,, around the ground-state eq~ilibrium,~’ i.e.the final-state distribution is roughly a reflection of the ground-state bending wavefunction of the bond being broken with probability proportional to the square of the modulus of the bending angle. (Each final rotational state is essentially determined by one initial angle.) Since the dissociation of ketene into CH uia a two-photon absorption cannot be a direct process from a repulsive surface, the final-state dis- tribution of CH must be predominantly determined in the exit channel and all memory of the CH2C0 initial-state dis- tribution destroyed. This is the case for the dissociation of formaldehyde, H2C0, into H2 and CO in the near-UV66 or the photolysis of H202 at 193 nm.67 In both these examples, the energy associated with the peak centres of the Gaussian CO and OH rotational distributions corresponds to ca.15-20% of the total available excess energy and these are similar fractions to those observed in our CH distributions. However, in order to model the dynamical photodissociation mapping fully in this instance it is crucial to know the exact details of the excited-state potential-energy surface (PES), V,, , information which is unavailable at present for a two-photon absorption in ketene. The less clearly defined Gaussian- shaped rotational distribution following photolysis at 308 nm may be a consequence of the predicted energy dependence for these distributions in the strong coupling limit64 owing to access of different regions of anisotropy in the excited PES.Alternatively, it is possible that in this absorption region FC mapping as well as dynamical mapping may overlap or that the two-photon ketene dissociation system may be compli- cated by the involvement of more than one excited state as has been inferred in the photodissociation of OCS.68 Orbital Alignment of CH The nascent CH produced in two-photon phytolysis of ketene of 279.3 nm clearly shows that for high values of N” (2lo), where spin-orbit coupling is small, both symmetry components of each A doublet are equally populated to within a reasonable experimental error. The degree of elec- tron alignment for the CH radical indicates that alignment of the n orbital parallel or perpendicular to J occurs at relatively low values of J.Therefore any observed propensity in A doublet population or change in propensity with increasing J (or N) may be ascribed to a dynamical effect in the photo- dissociation process. No such effect is observed. Furthermore these ratios were invariant to a 90” change in the plane of polarisation of the linearly polarised probe laser beam rela- tive to the plane of polarisation of the photolysis laser beam in the laboratory frame. This lack of polarisation dependence does not provide any direct information on the stereo-chemical dynamics of the photodissociation. When the parent molecule is photoselected by the polarised laser beam, the optical transition moment p (linked to the molecular framework) is preferentially aligned along the E vector of the exciting radiation.This establishes a stereochemical relation- ship between the direction of the orbital correlating to the n orbital of the outgoing diatomic and the rotating impulsive imparted to it by the fragmentation. The photolysis of trans-HONO at 350 nm to produce OH with A doublets prefer- entially populated in n(A’)symmetry (n orbital parallel to plane of rotation)69 or the photolysis of methyl nitrite, CH,ONO, at 364 nm,70 or of cold water in its first absorp- tion band at 157 nm41 both yielding diatomics with pref- erential population in I1 (A”) symmetry, are all excellent examples of this stereoselectivity. However, when the frag- mentation process is slower than out-of-plane molecular rotation the transfer of preferential alignment from parent to products will be lost and consequently no dependence on relative photolysis-probe polarisation geometries will be observed.In the experiments described here a bulk sample only of ketene (at T z298 K) was probed. The rotational constants of ketene have been determined both from micro- wave and infrared spectro~copy~~ and ab initio calculations5 and there is excellent agreement. From a value of 9.41 cm-for the A rotational constant an average value of 0.4 ps for the fastest rotational period of ketene at room temperature can be derived. So unless the dissociation is extremely fast (less than ca.30 fs) significant rotation of the parent molecule between the initially photoselected plane and the plane at the instant of fragmentation will tend to smear out any non- equilibrium of A doublet population, if one were to exist for a non-rotating (cold) parent ketene molecule. The fastest real- time single-photon state-to-state dissociation lifetime mea- sured for ketene is 19 ps l, at a wavelength of 280 nm and this is many times the rotational period of a room-temperature sample of ketene. In contrast with the clearly equivalent A doublet popu- lations of CH in the 279.3 nm photolysis above, there is evi- dently a slight degree of orbital alignment in favour of II (A’) symmetry for rotational levels of CH produced in the two- photon dissociation at 308 nm (Fig.4). Although the degree of orbital alignment is not large (<0.2 for N” = 10 and N” = 12) it immediately poses the two questions of why there is alignment and why there is a discrepancy with the behav- iour observed for dissociation at 279.3 nm. The answers are not at all clear. Possibly there are different dissociation pro- cesses dependent on available energy or the existence of two different, but close-lying, electronic states of the postulated formylmethylene intermediate where the 308 nm pathway produces CH with a degree of alignment that out-of-plane rotation of the parent molecule does not completely remove even at room temperature. It should be noted that at 308 nm a weak feature appears in the REMPI spectrum of CH,CO 25 and may be indicative of a Rydberg state existing at this two-photon energy.There may be a dissociation process moving towards a completely unconstrained cleavage of the H-C-C bond, following predictions of the model of Bronikowski and Zare7, which has been developed to account for the 2 : 1 ratio of ll (A’) :ll (A”) A doublet pro- pensities in 211 products of bimolecular reactions A + BC + AB + C, but to extrapolate this to the photo- dissociation (rather than reaction) under consideration here would assume first, that the lone .n orbital on the CH arises from the breaking of the C-C bond and secondly, and unlikely for dissociation from a specifically prepared reagent, that fragmentation of this bond is equally likely from any angular orientation of H-C-C.We comment finally on the observations of spontaneous fluorescence from the CH (A and B) states, The minimum energy required to produce these excited fragments corre- sponds to the absorption of at least three-photons, and the observed quadratic dependence of the fluorescence upon laser intensity shows that for a three-photon process, one step must be saturated. Fig. 2 shows the markedly different wave- length dependence for the two processes, showing that they cannot originate from the same photodissociation process. Unequal A doublet populations in the N’ levels of the 2A state have been observed by Luque et (for a photolysis wavelength of 193 nm) and the ratio demonstrated to depend J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 upon N’. These authors suggest that multiphoton disso-ciation of the ‘CH, product could be responsible for the for- mation of CH(A2A); in the present experiments at longer photolysis wavelengths this cannot be so, as such a process, unsaturated in the first absorption step to the ‘A” state of ketene, could not yield a quadratic energy dependence if fol- lowed by a true two-photon absorption by singlet methylene. M. R. H. gratefully acknowledges the SERC for the award of a maintenance grant. References 1 R. G. W. Norrish, H. G. Crane and 0.Saltmarsh. J. Chem. SOC., 1933, 1533. 2 G. B. Porter, J. Am. Chem. SOC.,1957,79,827. 3 A. H. Laufer, J. Phys. Chem., 1969,73,959. 4 T.W. Eder and R, W. Carr, J. Phys. Chem., 1969,73,2074. 5 P. M. Kelley and W. L. Hase, Chem. Phys. Lett., 1975,35, 57. 6 H. Bitto, D. R. Guyer, W. F. Polik and C. B. Moore, Faraday Discuss.,Chem. SOC., 1986,82, 149. 7 I-C. Chen, W. H. Green, and C. B. Moore, J. Chem. Phys., 1988, 89, 314. 8 W. H. Green, I-C. Chen and C. B. Moore, Ber. Bunsenges. Phys. Chem., 1988,92,389. 9 C. C. Hayden, D. M. Neumark, K. Shobatake, R. K. Sparks and Y. T. Lee, J. Chem. Phys., 1982,76,3607. 10 D. J. Nesbitt, H. Petek, M. F. Foltz, S. V. Filseth, D. J. Bamford and C. B. Moore, J. Chem. Phys., 1985,83,223. 11 W. H. Green, A. J. Mahoney, Q-K. Zheng and C. B. Moore, J. Chem. Phys., 1991,94, 1961. 12 E. D. Potter, M. Gruebelle, L. R. Khundkar and A. H. Zewail, Chem.Phys. Lett., 1989,164,462. 13 S. J. Klippenstein and R. A. Marcus, J. Chem. Phys., 1989, 91, 2280. 14 S. J. Klippenstein and R. A. Marcus, J. Chem. Phys., 1990, 93, 2418. 15 B. I. Sonobe and R. N. Rosenfeld, J. Am. Chem. SOC., 1983, 105, 7528. 16 G. T. Fujimoto, M. E. Umstead and M. C. Lin, Chem. Phys., 1982,65, 197. 17 K. G. Unfried, G. P. Glass and R. F. Curl, Chem. Phys. Lett., 1991, 177, 33. 18 X. Liu, S. G., Westre, J. D. Getty and P. B. Kelly, Chem. Phys. Lett., 1992, 188, 42. 19 W. D. Allen and H. F. Schaeffer 111, J. Chem. Phys., 1986, 84, 2212. 20 W. D. Allen and H. F. Schaeffer 111, J. Chem. Phys., 1988, 89, 329. 21 P. Chen, J. B. Pallix, W. A. Chupka and S. D. Colson, J. Chem. Phys., 1986,84,527. 22 J. B. Pallix, P.Chen, W. A. Chupka and S. D. Colson, J. Chem. Phys., 1986,84,5208. 23 P. Chen, J. B. Pallix, W. A. Chupka and S. D. Colson, J. Chem. Phys., 1987,86, 516. 24 J. W. Hudgens, C. S. Dulcey, G. R. Long and D. J. Bogan, J. Chem. Phys., 1987,87,4546. 25 M. N. R. Ashfold, A. D. Couch, R. N. Dixon and B. Tutcher, J. Phys. Chem., 1988,92,5327. 26 M. R. Berman and M. C. Lin, J. Chem. Phys., 1984,81,5743. 27 K. H. Becker, B. Engelhardt, P. 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SOC., 1970, 92, 7508. 5. Paper 3/04863B; Received 1lth August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000523

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(4), 533-539 Laser Flash Photolysis Studies on Hydrogen-atom Transfer from the Triplet Hydroxynaphthylammonium Ion to Benzophenone via a Triplet Exciplext Which Group is More Reactive for Hydrogen-atom Transfer, -OH or -NHS ? Minoru Yamaji, Ken-ichi Tamura and Haruo Shizuka" Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan Laser flash photolysis studies on hydrogen-atom transfer (HT) from the triplet hydroxynaphthylammonium ion (the protonated form of Saminonaphth-1-01, HORNH;) to benzophenone (BP or >CO)in methanol-water (9: 1 v/v) at 295 K have been carried out in order to elucidate which hydrogen atom of the substituent groups is more reactive for HT, and what is the mechanism. It was found that HT from triplet HORNH; (3HORNH,+*), produced by triplet energy transfer from 3BP*, occurs to BP to yield the hydroxynaphthylamine radical cation (HORNH?) and the benzophenone ketyl radical (>COH) with efficiencies of 0.92 and 0.48 in the presence of 0.015and 0.5 mol dm-3 H,SO, , respectively.The decay rate of 3HORNH,+* (kobs) decreases with increasing acid concentration, approaching a constant value at higher acid concentrations. This behaviour of kobs at higher acid concentrations cannot be explained by the HT mechanism alone for the naphthylammonium ion (RNH;)-BP system previously reported (S. Kohno, M. Hoshino and H. Shizuka, J. Phys. Chern., 1991,86,1297).With an increase of [BPI, kobs increases showing a levelling off at higher [BPI, which indicates the formation of a triplet exciplex between 3HORNH3+* and BP.The HT mechanism for the HORNHi-BP system was interpreted as a composite mechanism. The best-fit kinetic parameters were found to be k, = 5.0 x lo5 s-', k,, = k,, = lo7 s-', K, = 5 x lo2 dm3 mot-', K; = 3 dm3 mol-' and ki/kb = 6 x lo2 dm3 mol-'. It is concluded that (i) the hydrogen atom of the NH,' group (which is more protic than that of OH) is the more reactive for the HT reaction in the present system and (ii) the proposed composite mechanism including two different conformations of the protonated triplet exci- plexes, 3(HORNH;. -.>&OH)* and 3(H;NROH. . . $OH)*, is involved in the present HT. Acid-base reactions in the excited states of aromatic com- pounds have been extensively studied since they are elemen- tary processes in both chemistry and biochemistry.'-'' In this decade, we have paid much attention to photochemical and photophysical properties of aromatic compounds in the presence of protons in terms of proton-transfer reac-tions."-'8 In general, proton transfer occurs mainly in the excited singlet state of aromatic compounds, whereas in the triplet state, hydrogen-atom transfer is predominant.'' It is well known that the triplet carbonyl compounds produce their ketyl radicals by hydrogen-atom transfer from a variety of hydrogen-atom donors, such as alcohols, hydro- carbons and amines. A large number of studies on hydrogen- atom transfer reactions of triplet carbonyls have been rep~rted,'~-~'and it has been revealed by nano- and pico- second laser flash photolyses that these reactions proceed by either hydrogen-atom transfer or electron transfer followed by proton transfer.Until recently, little attention has been paid to the photo- chemical and photophysical processes of triplet aromatic compounds produced by triplet sensitization from carbonyl compounds. In particular, the hydrogen-atom transfer and electron-transfer reactions between triplet naphthalene deriv- atives and benzophenone and the effects of protons on these reactions have been of great interest to us.32-38 In the case of the naphthol (R0H)-benzophenone (BP or )CO) system, it has been revealed by nanosecond laser photolysis that the hydrogen-atom transfer (HT) reaction occurs from triplet naphthol ('ROH*) to BP to yield the naphthoxy radical (ROO) and benzophenone ketyl radical (>COH) via the triplet exciplex, 3(ROH.. )CO)*, which has a weak charge- transfer intera~tion.~~,~~ In the presence of protons, the HT -f This work was presented at the 14th IUPAC Symposium on Photochemistry, Leuven, Belgium, July, 1992. rate of the ROH-BP system increases with an increase of proton c~ncentration.~~The mechanism of the proton-enhanced HT reaction of 3ROH* is interpreted as follows: protonation to 3(ROH- -)CO)* forms a protonated triplet exciplex, 3(ROH. * -)eOH)*, where intraexciplex electron transfer gives rise to a triplet radical pair, 3(ROH'+ + \eOH), which rapidly dissociates into RO', H+ and S(%H.'6 On the other hand, the proton effect on HT of the naphthylammonium ion (RNHl)-BP system differs from that of 3ROH*.It has been shown that HT from the triplet naphthylammonium ion (3RNH3f*) to BP proceeds via a triplet exciplex, 3(RNH,'. -. )CO)*, to produce the naphthyl amine radical cation (RNH;') and >cOH.35 The HT rate decreases markedly with an increase of proton concentration. This proton effect of suppressing the HT rate is interpreted by the mechanism in which protonation to 3(RNH,'... \FO)* forms a protonated triplet exciplex, 3(RNH,' -$OH)*, which rapidly decomposes into 3RNHl* + BP + H due to Coulombic repulsion.35 This demonstrates that + the effects of protons on the HT rate for the triplet naphtha- lene derivative-BP system depend on the substituent groups.On the basis of the above findings, the following questions arise. In the case of a triplet naphthalene derivative having both ammonium ion (-NH,') and hydroxy (-OH) groups, from which group does HT occur, and what is the reaction mechanism which includes proton effects on HT? In order to solve the above questions, the present study on HT from the triplet 5-hydroxy-1-naphthylammonium ion to benzophe-none was carried out by laser flash photolysis at 355 nm. Experimental 5-Aminonaphth- 1-01 (HORNH,) and benzophenone from Kanto Chemicals were purified twice by sublimation in uucuo. 534 Sulfuric acid (97%, Wako) was used without further purifi- cation. H2S04 was used as a proton source, since it is known that the counter-ion (SO:-) of H2S04 does not quench the triplet state of the molecules.39 Methanol (Spectrosol, Wako) was used as supplied.Deionized water was distilled. A meth-anol (Me0H)-water mixture (9: 1 v/v) was used as the solvent. The concentration of HORNH, was typically 8.0 x mol dm-3. Benzophenone was used as a triplet sensitizer in the concentration range 6.7 x 10-3-0.2 mol dm-3. The H2S04 concentrations used were in the range 0.015-1.0 mol dm-3. All samples were thoroughly degassed by means of freeze-pumpthaw cycles on a high-vacuum line in a quartz cell of 1 or 10 mm pathlength. The transient spectral data were obtained within a 10% error using fresh samples to avoid excessive exposure to laser pulses.Laser flash photoly- sis was carried out at 295 K. Absorption spectra were recorded on a Ubest-50 spectro- photometer from Jas. Co. A nanosecond Nd3+ : YAG laser system at 355 nm (JK Laser, HY-500;pulse width 8 ns, laser powder 70 mJ pulse-' at 355 nm) was used for sample excitation. The detection system has been reported else~here.~~ Results and Discussion Absorption Spectra in the Ground State The absorption spectra of HORNH, with [H2S04] = 0 and 1.5 x lo-, mol dm-3 in MeOH-water (9 :1 v/v) at 295 K are shown in Fig. l(a) and (b), respectively. Spectrum (b) is blue-shifted compared with spectrum (a),and shows no spec- tral change in the range 0.015 < [H,SO,J/mol dm-3 < 1.0. In the presence of protons, HORNH, is in the following equilibrium, HORNH, + H+eNORNHi (1) Therefore, spectrum (b) is assigned to be that of the 5-hydroxy-1 -naphthylammonium ion (HORNH;) in MeOH- water (9 :1 v/v) with [H,S04] 3 0.015 rnol dm-3.When benzophenone (BP) is added to a MeOH-water (9 :1 v/v) solution of HORNH;, the absorption spectrum of the HORNHi-BP system is identical to a superposition of those of HORNH; and BP at 295 K. Since the absorption spectrum of BP in MeOH-water (9: 1 v/v) exhibited no change in the range 0.015 < [H,SO,]/mol dm-3 < 1.0, it is concluded that no interaction between HORNH; and BP in the ground state occurs in MeOH-water (9 :1 v/v) under the concentrations of H,SO, employed at 295 K. 10 250 300 350 400wavelength/nm Fig.1 Absorption spectra of HORNH, (---) and HORNH; (-) in MeOH-water (9 : 1 v/v) at 295 K 1. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2.0 r I v) f-0 F2 1.0 I I 1A "0 1 2 3 [HORNH,+]/10-3rnol dm-3 Fig. 2 Plots of kobs us. [HORNH; J obtained by 355 nm laser pho- tolysis in the HORNHi-BP (6.7 x mol dm-3) systems with (a) 0.015 and (b) 0.5 mol dm-3 H2S0, in MeOH-water (9 :1 v/v) Since HORNH; has no absorbance at 355 nm, only BP is excited upon 355 nm laser excitation in the HORNHi-BP system. Triplet Energy Transfer from Triplet BP to HORNHf After the excited singlet state of BP ('BP*) is produced upon 355 nm laser excitation in the HORNHi-BP system, triplet benzophenone (3BP*) having triplet-triplet (T-T) absorption at 525 nm is formed via fast intersystem crossing (ca.10 ps)40.41 according to the El-Sayed rule.42 In the presence of HORNHi ,efficient triplet energy transfer (ET) occurs from 3BP* to HORNH; since the triplet energies of BP and HORNH; are known to be 69.243 and 58.3 kcal mol-',t respectively. In the present system, although hydrogen-atom abstraction (HA) of 3BP* from HORNH; is expected to occur in competition with ET, it is known that ET predomi-nates HA in polar media.44 The ET reaction from 3BP* to HORNH; was studied for the HORNH; (0-3.0 x mol dm-3)-BP (6.7 x mol dmh3) systems in MeOH-water (9 : 1 v/v) with 0.015 and 0.5 mol dm-3 H2S04. The first- order rates for the decay of 3BP* (k,Bbp,)observed at 525 nm are shown as a function of [HORNH;] for 0.015 and 0.5 mol dm-3 H2S04 in Fig.2(a) and (b), respectively. Since the plots ofk:!s us. [HORNH;] (f3.0 x mol dm-3) gave a straight line, quenching of 3BP* by HORNH; is demon- strated to follow a Stern-Volmer relationship. Therefore, kt:s is expressed as follows, kt:s= ktP+ kr[HORNHi] (1) where ktP and ky are the rate constants for the decay of 3BP* in the absence of HORNH; and the quenching of 3BP* by HORNH;, respectively. From the intercept and ~ t The triplet energy of HORNH; was determined from the phos- phorescence spectrum in a mixture of MeOH-water (9 :1 v/v) con-taining 0.015 mol dm-3 H,SO, at 77 K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I I1 1 0.10 Q) c (I)e s9 (I) 0.05 0' ' L I I I 400 600 800 wavelengthfnm Fig.3 Time-resolved transient absorption spectra obtained by 355 nm laser photolysis at: 1, 80; 2, 200; 3, 350; and 4, 900 ns in the HORNH; (8.0 x mol dm-3)-BP (1.2 x mol dm-3)-H2S04 (0.015 mol dm-7 system in MeOH-water (9 :1 v/v) slope of the line, kEP and kr were found to be 3.9 x lo6 s-' and 5.8 x lo9 dm3 mol-' s-' for 0.015 mol dm-3 H2S04 and 7.4 x lo6 s-' and 3.0 x lo9 dm3 mol-' s-' for 0.5 mol dm-3 H2S04, respectively. Both values of k? for 0.015 and 0.5 mol dmP3 H2S04 are close to those of diffusion-controllcd processes, whereas ky at the former concentration is 1.9 times greater than k:' at the latter. As for ktP, the value for 0.5 mol dm-3 H2S04 is 1.9 times greater than that for 0.015 mol dm- H2S04. These results can be interpreted by considering the protonated 3BP* (3BPH+*), which has a small lifetime (ca.13 ns) due to the pK,* value of 3BP* (0.18).45-46 HT from Triplet HORNHZ to BP The ET reaction from 3BP* to HORNH: produces triplet HORNH; ,(3HORNH;*) in the nanosecond time region. In order to elucidate the deactivation processes of 3HORNH: * produced by ET, the transient absorption spectra in the microsecond time region were analysed. Fig. 3 and 4 show the transient absorption spectra observed after 355 nm laser photolyses in the HORNH; (8.0 x mol dm-3)-BP (1.2 x lop3mol dm-3) systems with 0.015 and 0.5 mol dme3 H2S04, respectively, in MeOH-water (9:l v/v). The transient absorption band at 430 nm observed at 70 ns in Fig.3 or at 150 ns in Fig. 4 after z laser pulse was ascribed to the T-T absorption spectrum of HORNH;, since it resembled the T-T absorption spectrum of naphth-1 -0134*36and the transient absorption spectrum which was quenched by introduction of air obtained by 266 nm laser photolysis of HORNH; in MeOH-water (9 :1 v/v) with 3 mol dm-3 H2S04. While the 430 nm band for 3HORNH3+* in both Fig. 3 and 4 decreases with isosbestic points at 490 and 610 nm, new transient absorption bands appear at 455, 545 and $00nm with time. The 545 nm band is known to be the benzophenone ketyl radical ( >eOH, with molar absorption coefficient, E = 3220 dm3 mol-' cm-' at 545 nm4'). The transient absorption bands at 455 and 800 nm are ascribable to the 5-hydroxy- 1-naphthylamine radical cation (HORNH;+) since they are similar to the absorption spectrum of HORNH;+ obtained by y-radiolysis of HORNH, in a PVC film.? Fig. 5 and 6 show the time traces of the transient absorb- ance changes at 430 nm for 3HORNHi*, 545 nm for )COH and 760 nm for HORNH;' after laser pulsing in the 7 Unpublished data.535 0.10 Q10 (I) 2% 0.05 0 400 600 800 wavelengt h/nm Fig. 4 Time-resolved transition absorption spectra obtained by 355 nm laser photolysis at: 1, 150 ns; 2, 700 ns; 3, 1.5 ps; and 4, 3.5 ps in the HORNH; (8.0 x mol dm-3)-BP (1.2 x mol dm-3)-H2S0, (0.5 mol dm-3) system in MeOH-water (9 : 1 v/v) HORNH: (8.0 x mol dm-3)-BP (1.2 x mol dm-3) system with 0.015 and 0.5 mol dm-3 H2S04, respec-tively, in MeOH-water (9 :1 v/v).The first-order rate con- stant (kobs) for the decay of 3HORNH3+* at 430 nm is almost identical with those for the increase of )tOH at 545 nm and of HORNH;+ at 760 nm within a 10% experimental error (kobs = 4.1 x lo6 s-' for 0.015 mol dm-3 H2S04 and 8.2 x lo5 s-' for 0.5 mol dm-3 H2S04). These results show that HT proceeds from 3HORNHl* to BP, resulting in the formation of HORNH;' and )eOH : bbr 3HORNH3+*+ >CO -HORNH;' + )tOH (2) 0.150 10.01 I I I I, (c)-0.1 -1--0.010.150 t 1O.OO' 0.075 1 ~I time/ps Fig. 5 Time traces of the absorbance changes for the transient species observed at 430 nm (3HORNH3+*) (a), 545 nm (>COH) (b) and 760 nm (HORNH;+) (c) obtained by 355 nm laser photolysis in the HORNH; (8.0 x mol dm-3)-BP (1.2 x mol dm-3)-H2S0, (0.015 mol dm-3) system in MeOH-water (9 :1 v/v).kobs= 4.2 x lo6 s-l (a),4.1 x lo6 sP1 (b),(c). 536 0.1 0.01 0.001 0.1 0.0 0.1 02 0.01 al 0.001 C m e 0.12D I I I I 0.1 0.01 0.001 0.1o.21 0.01 I I I I 0 4 8 time/ps Fig. 6 Time traces of the absorbance changes for the transient species observed at 430 nm (3HORNHl*) (a), 545 nm (>COH) (b) and 760 nm (HORNH;') (c) obtained by 355 nm laser photolysis in the HORNH; (8.0 x lop3 mol dm-')-BP (1.2 x lo-' mol dm-3)-HZS04 (0.5 rnol dm-3) system in MeOH-water (9 :1 v/v). kobs = 8.0 x lo5s-' (a), 8.2 x 10' s-l (b),(c). Therefore, this indicates that HT of 3HORNHi* occurs not from the -OH group but from -NH,f .Proton Effects on the Efficiency and Rate of HT from 3HORNHi* to BP In order to estimate the efficiency for HT from 3HORNHl* to BP, it is necessary to determine the molar absorption coef- ficients of the transient species. Fig. 7 shows the reference spectra of 3HORNHi*, >COH and HORNH;+. The molar absorption coefficient of HORNH;' can be readily determined to be 2200 dm3 mol- ' cm-' at 455 nm by comparison with that of >COH (3220 dm3 mol-' cm-' at 545 nm47), since HORNH;+ and 6 430 nm mzz nL 0 400 600 800 wavel engt h/n m Fig. 7 Reference absorption spectra of 3HORNH:*, >eOH and HORNH;'. See text for details. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 >eOH are produced by HT from 3HORNHi* to BP in a 1 :1 ratio.The molar absorption coefficient of 3HORNH,f* was estimated to be 5600 dm3 mol-' cm-' at 430 nm in MeOH-water (9 :1 v/v) on the assumption that the oscillator strength of the T-T absorption of HORNH; is the same as that of naphth-1-01 (7300 dm3 mol-' cm-' at 430 nm in acetonitrile,,), since they have isoelectronic structures. With the use of the molar absorption coefficients of 3HORNH3+*, >eOH and HORNH;' shown in Fig. 7, the efficiency (4HT)for HT from 3HORNH3+* to BP was deter- mined as follows. &,* is defined by eqn. (11). 4HT = A[ >COH]/A[3HORNHi*] = A[HORNH;+]/A[3HORNH,f*] (11) where A[ )cOH], ACHORNH;'] and AC3HORNH,f*] are the concentration changes of )eOH, HORNH;' and 3HORNH:* induced by HT, respectively.On the other hand, the absorbance change, AA at the monitored wave-length, A (430, 545 or 760 nm) can be written as, AA = E( )eOH)A[ >eOH] + &(HORNH;+)A[HORNH;+] -E(~HORNH,~')A[~HORNH~*] (111) where E( >eOH), &(HORNH;+) and d3HORNH:*) are the molar absorption coefficients of >COH, HORNH;+ and 3HORNH,f* at the wavelength, A, respectively. From eqn. (11)and (111), we obtain AA = { ~HT[E( )COH) + &(HORNH;+)]-E(~HORNH:*)} x AC3H0RNH,f*] (IV) With the use of the observed AA in Fig. 3 and 4, the E values in Fig. 7 and eqn. (IV), we obtained $HT values of 0.92 and 0.48 for the HORNH; (8.0 x mol dmP3)-BP (1.2 x mol dm-3) systems with 0.015 and 0.5 mol dm-3 H2S04, respectively, in MeOH-water (9 : 1 v/v).These $HT values indicate that increased proton concentrations reduce the efficiency for HT in the HORNHl-BP system. As men-tioned above, the kobs value obtained for the system with 0.015 mol dm-3 H2S04 is five times greater than that obtained for the 0.5 mol dmP3 H2S0, system, which indi- cates that HT in the HORNHl-BP systems is obviously sup- pressed by protons. These proton effects of suppressing both the efficiency and rate for HT have been reported for the RNH,f-BP system in a previous paper.35 In order to elucidate the effect of protons on HT from 3HORNHl* to BP, kobs for the HORNHl-BP system was measured at various acid concentrations. Fig. 8 shows plots of kobs at 430 nm as a function of [H2S0,] obtained by 355 nm laser photolysis in the HORNHl (8.0 x mol dm-3)-BP (1.5 x mol dm-3) system in MeOH-water (9 : 1 v/v).Although kobs decreases markedly with increase of [H,SO,], it asymptotically approaches a constant value at higher [H,SO,] (>0.3 mol dm-3). This levelling off at higher [H2S0,] cannot be accounted for simply by the mechanism for HT in the RNHi-BP system.35 The solid curve in Fig. 8 was calculated according to a mechanism proposed for the HORNHl-BP system, as will be described in the following section. BP Concentration Effect on HT Kinetic studies on kobs for the HORNHl-BP system at various [BPI were performed. Fig. 9 shows the plots of kobs at 430 nm for the decay of 3HORNHl* as a function of [BPI (d0.2 mol dm-3) at 0.015, 0.1 and 0.5 mol dmP3 H,SO,, J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3-I si --% 82-& 1-nl I1 I L 1 I I I “0 0.3 0.6 0.9 [H 2S04]/mot dm-Fig. 8 Decay rate (kobJ of 3HORNH:* as a function of [H,SO,] observed at 430 nm, obtained by 355 nm laser photolysis in the HORNH: (8.0 x mol dm-’)-BP (1.5 x lo-’ mol dm-3) system in MeOH-water (9 :1 v/v). The solid curve was calculated by eqn. (V). See text for details. respectively, obtained by laser photolysis in the HORNH: (8.0 x mol dm-3)-BP system in MeOH-water (9: 1 v/v). kobs increases considerably with an increase of [BPI at all [H2S04] used, but not linearly. In particular, a levelling off at higher [BPI (>0.05 mol dm-3) is observed at 0.015 mol dmd3 H,S04.Such non-linear quenching behaviour is typical of reactions via triplet exciplexes formed between triplet naphthalene derivatives and BP.35-3 Therefore, HT in the HORNHi-BP system is expected to proceed via a triplet exciplex, 3(HORNH,’ -. . )CO)*, as reported for the RNHl-BP system.35 However, we were unsuccessful in fitting the experimental kobs values in Fig. 9 with the same mechanism as that for the RNHi-BP system.35 The solid curves in Fig. 9 were calculated according to the proposed mechanism, as will be described in the following section. ReactionMechanism for HT from 3HORNH3+* to BP In a previous paper on HT of the naphthylammonium ion (RNHl)-BP system, it has been shown that the HT rate decreases markedly with an increase of acid concentration owing to Coulombic repulsion in the protonated triplet exci- r I v) I z5 00 0.1 0.2 [BP]/mol dm-3 Fig.9 Decay rate (koh) of ’HORNH:* as a function of [BPI observed at 430 nm obtained by 355 nm laser photolysis in the HORNH: (8.0 x mol dm-3)-BP systems with 0.015 (O),0.1 (A) and 0.5 mol dm-3 HISO, (0)in MeOH-water (9 : 1 v/v). The solid curves were calculated by eqn. (V). See text for details. plex, 3(RNH,’. . * )60H)*.35 Similarly, in the present HORNHl-BP system, the HT rate decreases significantly with increase of acid concentration. We consider that the suppression of the HT rate is affected by the -NH; group in both cases. However, the behaviour of the HT rate at higher acid concentrations differs between the two naphthyl-ammonium ions.kobs in the HORNHl-BP system approaches a constant asymptotically, while that in the RNHl-BP system simply decrease^.^' We consider that the difference in behaviour of the HT rates at higher acid concen- trations arises from the effect of the other substituent group (-OH) of HORNH;. It was previously reported that in the case of the naphthol (R0H)-BP system, the HT rate increases linearly with an increase of acid c~ncentration.~~ Therefore, we anticipate that the levelling off of kobs at higher acid concentration is due to cancellation of the opposing effects of both substituent groups (-NH; and -OH) on the HT rate. In order to explain the experimental results, we pro- posed a composite mechanism for the HORNHl-BP system, which contained elements of those for the RNHl-BP and ROH-BP systems, as shown in Scheme 1.Here, NH;3(R< OH . . >CO)* represents the triplet exciplex formed between 3HORNH,f* and BP with an equilibrium constant K,(=k,/k,), which undergoes an intraexciplex HT reaction to produce HORNH;’ and )COH with a rate constant (kHT). 3(HORNH,’. . -)tOH)* and ’(HlNROH. -->&OH)* are protonated triplet exciplexes with different configurations. The former is produced by protonation with a rate of k,[H+], and has a configuration favourable for decomposi- tion into 3HORNHi* + >CO + H+ due to Coulombic repulsion, with rate constant kdis. kdis was considered to be greater than k,[H+], since dissociation was rapid.This mechanism of decomposition of the protonated triplex exci- plex due to Coulombic repulsion is similar to that for the RNHi-BP The latter protonated triplex exciplex formed with an equilibrium constant K, (=k,/k-,) has a configuration that is less prone to decomposition. Here intraexciplex electron transfer occurs with a rate constant k,, to yield a triplet radical pair, 3[(H;NROH)” + >eOH], by the spin conservation rule. The hydroxynaphthylammonium ion radical cation (Hl NROH)” readily decomposes into HORNH;’ and H+, since the pK, of (H;NROH)*+ is con-sidered to be very negative. This mechanism involving intraexciplex electron transfer is similar to that for the ROH-BP system.36 The rate constants, k,, KO and Kh are for the non-radiative decay processes of 3(HORNH;)*, 3(R( 7;* >CO)* and 3(H,”ROH-.* )6OH)*.Here, we denote k,, = kb + k,, and k, = kg + k,,. According to the proposed mechanism, the decay rate of kobs can be formulated by eqn. (v). -1 k, + K,[BP] (1 + k&[HzS04]) kb kbx {1 + Kl[BP]( 1 + 5[H2S04]>’ Since the activity of H2S0, in MeOH-water (9 : 1 v/v) was unknown, we used k,[H+] = kd[H,S04] and K2[H+] = J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 HORNH3' + >CO k 3HORNH3*' + >CO 2'(Re::;------->COj kb 3[(HORNH3') .* + >;OH] \ / HORNHi* + Scheme 1 K;[H,SO,] in eqn. (V). When [H,SO,] + 0 and co,kobs in eqn. (V) approaches k,, and k,, respectively, as [BPI -,co.As shown in Fig. 9, both plots of kobs us.[BPI at 0.015 and 0.5 mol dmd3 H2S04 approach the same constant value (lo7 s-') as [BPI + co. Therefore, we assumed k,, = k, = lo7 s-'. k, was found to be 5.0 x lo5 sC1 by 266 nm laser pho- tolysis in an MeOH-water (9 : 1 v/v) solution of HORNH; with 3 mol dm-3 H,SO,. With the use of these values of k,, k,, and k,, we employed the best-fit method to our kobs results shown in Fig. 8 and 9, and obtained K, = 5 x lo2, K; = 3 and kdkb = 6 x lo2 dm3 mol-'. Using the deter- mined values of k, , k,, , k,, K,,K; and kd/kb,the calculated values of kobs are expressed as the solid curves in Fig. 8 and 9. Note that the calculated values of kobsfollow the experimen- tal ones very well (Fig. 8 and 9). Therefore, we conclude that the proposed mechanism shown in Scheme 1 can be applic- able to HT for the HORNHZ-BP system.The efliciencies (+HT) for HT from 3HORNH;* to BP were experimentally determined as 0.92 and 0.48 for the HORNH; (8.0 x lob3 mol dm-3)-BP (1.12 x mol dm-3) systems in the presence of 0.015 and 0.5 mol dm-3 H2S04, respectively, as described above. On the other hand, $HT can be obtained according to Scheme 1 as follows: Using eqn. (VI) and the values of k,, k,, , k , K,,K; , kdkb and $HT, we obtained kH, w k,, and k,, x E,. These results indicate that once the triplet exciplex and the protonated triplex exciplex are established in equilibria, the intraexciplex HT and electron-transfer reactons occur to yield HORNH;' and )COH very efficiently. In Scheme 1, we proposed protonated triplex exciplexes with two configurations.The triplex exciplexes are considered to have a weak charge-transfer interaction since their absorp- tion spectra are similar to those of the normal triplet aro- andmatic c~mpounds,~~-~~ are revealed to have NH'sandwich-like str~ctures.~~ Since 3(R< od * )CO)* is also considered to be formed by a weak charge-transfer inter- action with a loose sandwich-like structure, it would be pos-sible in the present system for triplet exciplexes, such as 3(HORNH,'. -* )CO)* and 3(H,'NROH. -)CO)*, to alter their configurations in the loose sandwich-like structures. That is, in the former, the carbonyl group and -NH; may be situated at the same side, and in the latter, at the opposite side. Consequently, when protons attack \he configuration 3(HORNH,'.--)CO)*, 3(HORNH;. ->COH)* is pro-duced, resulting in rapid decomposition due to Coulombic repulsion. On the other hand, 3(H;NROH... )CO)* is attacked by protons to 5! roduce the protonated triplex exci- plex, 3(H;NROH... )COH)*, which gives rise to the intraexciplex electron-transfer reaction, as illustrated in Scheme 1. In the present study, it is revealed that the hydrogen atom of -NH; is more reactive than that of -OH for HT from 3HORNH,'* to BP. The difference in the HT reactivity between -NH; and -OH groups can be understood by considering the electronic structure of the triplex exciplex. The triplet exciplex has been reported to be formed by a weak charge-transfer intera~tion,~,-~~ which means that the BP site is comparatively electron-rich owing to a slight elec- tron migration from 3HORNH;* to BP.On the other hand, the hydrogen atom of -NH; is relatively more protic than that of -OH. Therefore, the protic hydrogen atom prefer- entially transfers to the electron-rich carbonyl group of BP due to Coulombic attraction. It can be said that the more protic hydrogen atom is the more reactive in HT via triplet exciplexes. Concluding Remarks It has been shown by 355 nm laser flash photolysis in the HORNHZ-BP system at 295 K that HT from 3HORNH3+* to BP occurs from the -NH; group to yield HORNH;' J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 539 and )COH. In other words, the more protic hydrogen atom is the more reactive in HT uia triplet exciplexes.A novel HT mechanism for the present HORNHi-BP system is proposed by combining those for the RNHi-BP and ROH-BP systems, as illustrated in Scheme 1, in order to account for the proton effect on the HT rate which signifi- 22 23 24 N. J. Turro, Modern Molecular Photochemistry, Benjamin/ Cummings, Menlo Park, 1978. S. G. Cohen, A. Parola and G. H. Parsons Jr., Chem. Rev., 1973, 73, 141. S. J. Formoshinho, J. Chem. SOC., Faraduy Trans. 2, 1976, 72, 1913; 1978,74, 1978; S. J. Formoshinho and L. G. Amout, Adu. Photochem., 1991,16,67. cantly decreases with increasing acid concentration, approaching a constant value at higher acid concentrations. 25 26 T. Okada, T. Karaki and N. Mataga, J. Am, Chem.SOC., 1982, 104, 7191; S. Arimitsu, H. Masuhara and H. Tsubomura, J. Phys. Chem., 1975,79, 1255. K. S. Peters, S. C. Freilich and C. G. Shaefer, J. Am. Chem. SOC., This work was partially supported by Grant-in-Aid from the Ministry of Education, Science and Culture of Japan. 27 1980, 102, 5701; J. D. Simon and K. S. Peters, J. Am. Chem. SOC., 1981,103,6403; 1982,104,6542; Acc. Chem. Res., 1984,17,277. M. Hoshino and H. Shizuka, J. Phys. Chem., 1987, 91, 714; M. Hoshino, H. Seki, M. Kaneko, K. Kinoshita and H. Shizuka, Chem. Phys. Lett., 1986,132,209. References 28 C. Devadoss and R. W. Fessenden, J. Phys. Chem., 1990, 94, 4540; 1991,95,7253. 1 2 Th. Forster, 2.Electrochem. Angew. Phys. Chem., 1950, 54, 42; 531. A. Weller, Ber. Bunsenges. Phys. Chem., 1952, 56, 662; 1956, 66, 29 H.Miyasaka, K. 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G. Schulman, in Fluorescence and Phosphorescence Spectros- copy, Pergamon, Oxford, 1977. J. F. Ireland P. A. H. Wyatt, Adu. Phys. Org. Chem., 1976, 12, 131. 33 34 35 598. H. Shizuka, H. Hagiwara, H. Satoh and M. Fukushima, J. Chem. SOC., Chem. Commun., 1985,1454. H. Shizuka, H. Hagiwara and M. Fukushima, J. Am. Chem. SOC., 1985,107,7816. S. Kohno, M. Hoshino and H. Shizuka, J. Phys. Chem., 1991,86, 10 11 12 13 14 15 W. Klopffer, Adu. Photochem., 1977,10,311. H. Shizuka, Acc.Chem. Res., 1985,18, 141. K. Tsutsumi and H. Shizuka, Chem. Phys. Lett., 1977,52,485; 2. Phys. Chem. (Wiesbaden), 1978,111,129. H. Shizuka and S. Tobita, J. Am. Chem. SOC., 1982,104,6919. H. Shizuka, M. Serizawa, H. Kobayashi, K. Kameta, H. Sugiy- ama, T. Matsuura and I. Saito, J. Am. Chem. SOC., 1988, 110, 1726. H. Shizuka, M. Serizawa, T. Shimo, 1. Saito and T. Matsuura, J. 36 37 38 39 40 1297. S. Kaneko, M. Yamaji, M. Hoshino and H. Shizuka, J. Phys. Chem., 1992,%, 8028. M. Yamaji, T. Sekiguchi, M. Hoshino and H. Shizuka, J. Phys. Chem., 1992,%, 9353. T. Sekiguchi, M. Yamaji, H. Tatemitsu, Y. Sakata and H. Shizuka, J. Phys. Chem., 1993,97,7003. H. Shizuka and H. Obuchi, J. Phys. Chem., 1982,86,1297. R. W. Anderson Jr., R. M. Hochstrasser, H. Lutz and G. W. Am. Chem. SOC., 1988,110,1930. Scott, J. Chem. Phys., 1974,61,2500; Chem. Phys. Lett., 1978,28, 16 17 18 19 20 21 H. Shizuka, K. Kameta and T. Shinozuka, J. Am. Chem. SOC., 1985,107,3956. H. Shizuka and M. Serizawa, J. Phys. Chem., 1986,90,4573. H. Shizuka, M. Serizawa, K. Okazaki and S. Shioya, J. Phys. Chem., 1986,90,4694. J. C. Scaiano, J. Photochem., 1973/1974,2,81. P. J. Wagner and G. S. Hammond, Adu. Photochem., 1968,5,21; P. J. Wagner, Acc. Chem. Res., 1971, 4, 168; Top. Curr. Chem., 1976,66, 1; P. J. Wagner, R. J. Truman, A. E. Puchalski and R. Wake, J. Am. Chem. SOC., 1986,108,7727. N. J. Turro, J. C. Dalton, K. Dawes, G. Farrington, R. Hautala, D. Morton, M. Niemczyk and N. Schore, Acc. Chem. Res., 1972, 5, 92; J. C. Dalton and N. J. Turro, Annu. Reu. Phys. Chem., 1970,21,499. 41 42 43 44 45 46 47 153. D. E. Damschen, C. D. Merritt, D. L. Perry, G. W. Scott and L. D. Telly, J. Phys. Chem., 1978, 82,2268. M. A. El-Sayed, J. Phys. Chem., 1962, 36, 573; 1963, 38, 2834; 1964,41,2462. S. L. Murov, Handbook of Photochemistry, Marcel Dekker, New York, 1973. M. Yamaji, T. Tanaka and H. Shizuka, Chem. Phys. Lett., 1993, 202, 191. H. Shizuka and E. Kimura, Can. J. Chem., 1984,62,2041. M. Hoshi and H. Shizuka, Bull. Chem. SOC. Jpn., 1986,59,2711. E. J. Land, Proc. R. SOC.London, Ser. A, 1968,305,457. Paper 3/04607I; Received 2nd August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000533

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS.,1994, 90(4), 541-548 54 1 Kinetics of Thermal Decomposition of the Diazines :Shock-tube Pyrolysis of Pyrimidine Alan Doughty and John C. Mackie" Department of Physical and Theoretical Chemistry, University of Sydney, NSW 2006,Australia The kinetics of pyrolysis of pyrimidine diluted in argon have been studied behind reflected shock waves over the temperature range 1200-1850 K, at uniform gas residence times of 85G1000ps and pressures of 13-15 atm. The major products of pyrimidine pyrolysis were found to be acetylene, HCN, acrylonitrile, cyanoacetylene and H, . Using both end-product analysis and real-time UV spectrometry the kinetics of pyrimidine disappearance were found to be first order with respect to reactant concentration over the concentration range of 0.07-0.3 mol%.The two techniques yielded a first-order rate constant (kdlS)for the disappearance of pyrimidine given by the expres-sion 1012.3(*0.4)exp[ -275( f13) kJ mol-'/RTl s-'. A detailed reaction model incorporating a free-radical mechanism for the decomposition of pyrimidine has been developed, and shown to predict the reactant and product concentrations between 1250 and 1600 K. Impor-tant radicals in the mechanism were found to be o-and p-pyrimidyl, with H atoms and CN radicals being radical chain carriers. Sensitivity and flux analysis of the kinetic model has shown the most important initiation pathway to be the loss of an H atom from pyrimidine to yield o-pyrimidyl. Optimisation of the Arrhenius parameters for this initiation reaction yields an activation energy consistent with a heat of formation of the o-pyrimidyl radical of 376(& 10) kJ mol- '. This study of the pyrolysis of pyrimidine is part of a wider study aimed at investigating the mechanism by which oxides of nitrogen (NO,) are formed from the combustion of coal and heavy fuels.Combustion of these materials, at least under fuel-rich conditions, is thought to proceed first by the thermal decomposition of the fuel, followed by reaction of the pyrolysis products with oxygen. Study of the pyrolysis of nitrogen-containing molecules present in coal should there- fore contribute to the understanding of the mechanism of NO, formation during coal combustion. Gaining insight into the NO, formation process by study- ing the pyrolysis of coal model compounds requires know- ledge of the functional form of nitrogen in the unperturbed coal matrix.The pyrolysis of pyridine, pyrrole and 2-picoline have all been studied by this group.'-4 These compounds were chosen as models on the basis of analysis of coal- derived liquids,' and on the results of X-ray photoelectron spectroscopy (XPS) st~dies~,~.~ on coal, which indicated that the pyridine and pyrrole rings were important forms of nitro- gen. It is possible, however, that the broadness of XPS peaks results in the significance of the diazines being underesti- mated. The DNA bases thymine and cytosine both are deriv- atives of pyrimidine, and their presence in plant and microbial DNA would be expected to lead to their presence in soils, and possibly coal.Pyrimidine has been found to be present in soilsg and coal analysis has found that compounds containing two or more nitrogens are significant sources of nitrogen in coal.' Pyrimidine could therefore be considered to be a useful model compound for the study of the evolution of NO, during coal combustion. An important part of this study has been the development of a detailed kinetic reaction mechanism which can model the temperature profiles for product formation and of depletion of the reactant. The kinetic model consists of 25 reactions and 18 species, and successfu!ly models the thermal decompo- sition of pyrimidine over the temperature range 1250 to 1600 K, at a pressure of 13-15 atm.Experimental Pyrolysis experiments were carried out using the single-pulse shock tube (SPST). On-line capillary and packed column gas chromatography (GC) were used to quantify the major and minor products. In addition to the analysis of end products, the kinetics of decomposition of pyrimidine was also probed using real-time UV spectrometry. Details of the shock tube,"." GC analysis' and WV spectrometry' have been given elsewhere. In this study the GC analysis differs slightly from that previously described3 in that all products, includ- ing nitrogen compounds, were quantified using a flame ionis- ation detector (FID). Pyrimidine used in this study was obtained from Aldrich, of stated purity >98%.The pyrimidine was used without further purification to prepare mixtures of the vapour dilute in argon at concentrations of 0.2-0.4 mol%. Experiments were also carried out at a lower concentration range of 0.06-0.08 mol%. Analysis of the reactant mixtures by GC yielded >99.5% of the nitrogen present as pyrimidine. Products were identified using gas chromatography-mass spectrometry (GC-MS) as described in ref. 3. Assignments were confirmed by comparison with commercial samples where available. FID responses for pyrimidine and its decomposition pro- ducts were measured using commercially available samples. In the case of cyanoacetylene the FID response was esti- mated assuming its response to be similar to acrylonitrile and ethylcyanide.The yield of cyanogen could not be quantified since a calibration sample was not available, and there is no other molecule which might be expected to possess a similar FID response. End-product analysis for the low-concentration series of experiments can be considered to be less accurate than the end-product analysis performed for the higher concentration series. This is a consequence of the considerably lower con- centrations of the species present in the product mixtures for the low-concentration series. This problem is exacerbated in the measurement of HCN due to the relatively low sensitivity of the FID to HCN. For example, the FID is more sensitive to pyrimidine compared to HCN by a factor of nine. This low sensitivity results in the measurement of HCN in the low- concentration series being less accurate than the measure-ments made for other products of pyrimidine decomposition, in the low-concentration series of experiments. Hydrogen was measured using a previously described GC method.' The limit of detection for hydrogen was found to be 0.05 mol%.A measurement of 0.05 mol% hydrogen from an initial concentration of pyrimidine of 0.3 mol% would corre- spond to a yield (based on the original pyrimidine) of hydro- gen of 17%. Therefore hydrogen measurements could only be made where the yield of hydrogen was greater than 17%. This was only observed at intermediate to high levels of decomposition. The relatively high limit of detection for hydrogen precluded the measurement of hydrogen for the low-concentration series.Real-time UV spectrometry was carried out at two wave- lengths, both corresponding to vibronic bands of the 'B, t 'A,(n* n) electronic transition of py~imidine.'~ The majority of the UV kinetic data was obtained by monitoring the absorption at a wavelength corresponding to the 0-0 vibron-ic band of the above-mentioned electronic transition, which occurs at ca. 320.8 nm (using a 1 nm bandpass). No signifi-cant difference was observed when comparing data obtained at this wavelength with data obtained when monitoring absorption changes at 290 nm (2 nm bandpass). The two techniques were used to obtain overall decompo- sition kinetics over the temperature range 1200-1850 K.End-product analysis allowed the decomposition products to be profiled over the temperature range 1250-1600 K, which cor- responded to pyrimidine decomposition between 0 and 90%. Temperatures and pressures in the reflected shock were calcu- lated from measured incident and reflected shock velocities. Residence times were in the range of 850-1000 ps, with reac- tion pressures of 13-15 atm. Results The major nitrogen-containing products for the decomposi- tion of pyrimidine were HCN, acrylonitrile and cyano-acetylene. Temperature profiles for these products, and for the disappearance of the reactant are given in Fig. 1-5. Cyanogen was observed at what appeared to be low levels. Traces of pyrazine were also present in the reaction products at low extents of decomposition.The only major hydrocar- bon product was found to be acetylene. At moderate to high extents of decomposition, methane, ethylene, vinylacetylene and diacetylene were minor hydrocarbon products. Hydro- gen was found to be a major product, at intermediate to high extents of decomposition. The high limit of detection for hydrogen prevented the detection of hydrogen at low extents of pyrimidine decomposition. Since much of the kinetic data in this study are in the form of temperature profiles of reactant and product concentra- tions, it is important to ascertain whether the concentrations 't .-k 20 Q 101 a 0'1200 1250 1300 1350 1400 1450 1500 1550 1600 I1650 temperaturejK Fig. 1 Temperature dependence of % pyrimidine remaining in the pyrolysis of pyrimidine, with initial concentrations of pyrimidine of 0.2-0.4% (a)and 0.06-0.08 mol% (0)in Ar.Model predictions for 0.3 and 0.07mol% pyrimidine in Ar, are represented by (-) and (---), respectively. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 g 100 Y 120 t .-> 80/ / 7.-1200 1250 1300 1350 1400 1450 1500 1550 1600 temperature/K Fig. 2 Temperature dependence of the yield of HCN from pyrolysis of pyrimidine. Symbols as in Fig. 1. 30 r / I' Y 251 9' .g 201 Q) Q) -> c 101 / temperature/K Fig. 3 Temperature dependence of the yield of cyanoacetylene from pyrolysis of pyrimidine. Symbols as in Fig. 1. l5 I h s Y 0-1200 1250 1300 1350 1400 1450 1500 1550 1600 temperature/K Fig.4 Temperature dependence of the yield of acrylonitrile from pyrolysis of pyrimidine. Symbols as in Fig. 1. -40 -35 h 5 30-9 .!? 25 -> 01200 1250 1300 1350 1400 1450 1500 1550 1600 temperature/K Fig. 5 Temperature dependence of the yield of acetylene from pyrolysis of pyrimidine. Symbols as in Fig. 1. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 of species measured in the product gas mixtures are consis- tent with the concentration of pyrimidine prior to reaction. To ensure this consistency, and thus the validity of the data, mass balance calculations were performed for each run con- ducted. In Fig. 6, the N recovered in the post shock mixture (i.e.the sum of N in the products, including unreacted pyrimidine) is plotted as a percentage of the measured con- centration of pyrimidine in the unreacted gas mixture. As can be seen from the plot, the bulk of the recovery data falls between 90 and 110%. The deviation at very high extents of pyrimidine decomposition is most likely due to limitations in the accuracy of the HCN calibration constant. The most important observation to make from the mass balance plot is that at low to moderate extents of decomposition there is no systematic loss of nitrogen from the system. This suggests that cyanogen could not be a major product of the decompo- sition of pyrimidine. When the results from the runs carried out with the higher initial concentration of pyrimidine (0.2-0.4 mol%) are com- pared with the runs carried out at lower initial concentration (0.06-0.08 mol%), no significant differences in the reactant decomposition profile can be observed.This suggests that the decomposition of pyrimidine is close to first order with respect to the reactant concentration, under the conditions of this study. The time-resolved UV signal intensities measured during the passage of the shock wave were used to calculate the absorbances (A)at the corresponding wavelength from the expression derived from the Beer-Lambert Law : A = -log(I/I()) where I is the photomultiplier output (V), and I, the photo- multiplier output (V) at 100°/otransmittance. A typical absorption trace as a function of time during the passage of a shock wave is included in Fig.7. The change in absorption due to the increase in density at the arrival of the incident and reflected shock fronts can be clearly seen, fol- lowed by a decay due to the decomposition of the pyrimidine. The major products of the decomposition of pyrimidine do not absorb light at either of the wavelengths probed, so the absorption decay trace should be entirely due to the disap- pearance of the reactant. This is supported by the observa- tion that the kinetics from measurements taken by monitoring the UV absorption at 290 nm showed no differ-ence to the kinetics obtained from absorption measurements conducted at 320.8 nm. In view of the low concentrations of pyrimidine used in this study, the decrease in temperature due to reaction would be expected to be less than 2 K.Since the absorbance in the reflected shock is proportional to the pyrimidine concentration, a first-order rate constant 120 c o.20 r RSF 0.05 I 0.00I -200 -100 0 100 200 300 400 500 600 700 800 time/ps Fig. 7 Real-time absorbance trace of light of wavelength 320 nm in a shock wave heated mixture of 0.23 mol% pyrimidine in Ar (-). Least-squares fit of the exponential decay of the absorbance is also shown (----). Times are shown from t = 0 as used in the exponential fit. Reflected shock gas temperature, 1682 K. ISF = incident shock front, RSF = reflected shock front. for the disappearance of pyrimidine can be calculated from the absorbance decay traces.Using the method of least squares, the absorbance decay was fitted to the expression: A = b eXp(-kdjs t) where t is the time from the arrival of the reflected shock front, b is a constant and kdis is the first-order rate constant. The resulting exponential function is plotted in Fig. 7. A first-order rate constant for pyrimidine disappearance can also be calculated from the SPST data. If the decomposi- tion of pyrimidine is taken as being first order with respect to the reactant concentration, kdis can be calculated from the following expression : where t,,, is the residence time of the reaction and y is the percentage of pyrimidine remaining in the product gases. Calculations of kdjs from the UV-absorption measurements and the SPST data are summarised in the Arrhenius plot in Fig.8. As can be seen from the plot, the two techniques for obtaining kdjs cover complementary temperature ranges. Values for kdis calculated from SPST data are taken from measurements at low to moderate extents of decomposition, whereas UV spectroscopic measurements of kdis are taken at temperatures where the pyrimidine has undergone moderate to complete decomposition at the end of the residence time. 1 0 :i_-I I 0 5 6 1047KIT 0 9 80 Fig. 8 Arrhenius plot for the rate constant k,, for overall disap- 1200 1250 1300 1350 1400 1450 1500 1550 1600 pearance of pyrimidine. (+) kdis obtained using UV spectrometry,temperature/K (---) regression fit to UV spectrometric kdis.(a) kdis from end- kdisFig. 6 Variation with temperature of the percentage of nitrogen product analysis, (-) regression fit to end-product kdis. (0) recovered values used in regression fit to end-product kdis. Table 1 Pseudo-Arrhenius parameters for the formation of the major products of pyrimidine decomposition product Als-' EJkJ mol - ~ cy anoacetylene HCN acr ylonitrile acetylene 11.4 ( f1.3) 12.6 (f1.0) 10.9 (f0.8) 11.6 (f0.8) 259 ( f29) 279 ( f22) 250 (f21) 266 (& 19) The linearity of the Arrhenius plots in Fig. 8 indicate that the Arrhenius pre-exponential factor for the decomposition of pyrimidine shows no discernible temperature dependence. Arrhenius parameters for the disappearance of pyrimidine have been calculated from the Arrhenius plots using linear regression analysis.For the SPST data, and Arrhenius pre- exponential factor of 1012*3(*0.s)s-' and an apparent activa- tion energy of 275( & 13)kJ mol-' were obtained. This is in excellent agreement with the values taken from the UV spec-trophotometric data, which yielded an Arrhenius pre-exponential factor of 1012*3(*0-4)s-' and an apparent activation energy of 275( 13)kJ mol- '. Pseudo-Arrhenius parameters for the formation of the major products have also been calculated from the SPST data. The pseudo-first-order rate constants for the formation of these products (kfor,J were calculated from the approx- imate expression : where c is the concentration of the product (in yield %) in the product gas.This approximate expression is only valid at low extents of decomposition, and thus only data for the first 20% of pyrimidine decomposition are used in these calcu- lations. Arrhenius parameters taken from the kfom data for each of the products are summarised in Table 1. Discussion The pyrolysis of pyrimidine yields four major products, all of which are present in concentrations which are not related to the concentrations of other products by any simple whole number ratio. This type of product distribution is consistent with a free-radical mechanism for the decomposition of pyrimidine. A free-radical mechanism is also supported by evidence of chain carriers, and of radical termination products present as products of reaction.The large concentrations of H, observed in the reaction products would be formed if H atoms were radical chain carriers. CN radicals are evident from the detection of cyanogen, which could be formed from either a termination reaction, or by the reaction of a CN radical with HCN14 or a nitrile product of rea~tion.'~ Either of the chain carriers would be expected to propagate through H-abstraction from pyrimidine to yield o-, p-or 5-pyrimidyl radicals (see Table 2 for structures of the pyrimidyl radicals). The most likely initiation process in the free-radical decomposition of pyrimidine would take place through C-H bond fission to yield a pyrimidyl radical. If the C-H bond energy in pyrimidine is assumed to be the same as that for the C-H bond in the o-position of pyridine'.3*'8 (i.e.420 kJ mol-'), an activation energy of ca.420 kJ mol-' is obtained. This is considerably higher than the activation energy of 275 (f13) kJ mol- ' observed for the decomposition of pyrim-idine. The relatively low overall activation energy observed for the decomposition of pyrimidine suggests that the rate of decomposition is strongly influenced by the rate of propaga- tion steps of the free-radical chain mechanism. In the case of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Thermochemical parameters for the pyrimidine system 4Ho,oo/, so,oo/structure name" kJ mol-J mol-' K-I 193b 282' p-PMDYL 405 O-PMDYLd 376 287 *Q -5-pyrimidyle 440 520 350 500 350 HC=N, ,CN NC,H,CN 520 350 HC=CH HC=N /CN HCNCN~J 450 230 ~~ Name as shown in Table 3.* Ref. 16. 'Calculated from data in review article ref. 17. Heat of formation optimised using kinetic modelling. Not included in kinetic model (see text). J Calculated assuming N,(CN) = CJHXCN) -C,(H)(C) + N,(C). pyridine,"" the overall rate of disappearance of reactant cor- responds to the rate of the initiation reaction yielding o-pyridyl and H radicals. It is an interesting question as to why the kinetics of reaction for the two systems would be so dif-ferent, when the rates of the initiation reactions would be expected to be similar. Explanations for the difference in the overall kinetics between pyridine and pyrimidine can only be made with some knowledge of the mechanism for decomposi- tion of pyrimidine.In the discussion to follow, a free-radical mechanism for the decomposition of pyrimidine will be pos-tulated. Using kinetic modelling, the validity of the reaction mechanism will be tested to ensure that the temperature pro- files of the major products predicted by the mechanism agree with the observed profiles. Reaction Mechanism The pyrimidyl radicals would be expected to be important radicals in the decomposition mechanism of pyrimidine. They would be formed in initiation reactions yielding pyrimidyl and H atoms, and from the abstraction of H from pyrimidine by H atoms or CN radicals to yield pyrimidyl and H, or HCN. The relative importmce of the three pyrimidyl radicals would be largely determined from their thermochemistry, and also from the rate at which the particular pyrimidyl radical can decompose to form stable products and regenerate the chain carriers H and CN.If there is no low-energy pathway for the decomposition of a pyrimidyl radical, it will tend to recombine with H atoms to reform pyrimidine. Some insight into the relative stability of the pyrimidyl radicals can be gleaned from studies on the decomposition of pyridine.'*'8 The heat of formation of o-pyridyl derived from these earlier studies suggests that the radical centre interacts with the nitrogen lone pair in the case of o-pyridyl. This J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 results in o-pyridyl being ca. 35 kJ mol-' more stable' than predicted by assuming the o-C-H bond strength in pyridine to be the same as the C-H bond strength in benzene.This conclusion is supported by ab initio molecular orbital calculations1 on o-pyridyl. These calculations indicated that there is significant interaction between the radical centre and the nitrogen lone pair in o-pyridyl. Estimation of the heats of formation of the pyrimidyl rad- icals is complicated by the possibility of the radical centre being ortho to no nitrogen atoms (the 5-pyrimidyl radical), one nitrogen atom (the p-pyrimidyl radical), or two nitrogen atoms (the o-pyrimidyl radical). The heat of formation of the 5-pyrimidyl radical should be estimable from the C-H bond strength of benzene, yielding a heat of formation (at 300 K) of 440 kJ mol-'. It would be anticipated that for the pyrimidyl radical, where the radical centre is ortho to a single N atom, the heat of formation should be estimable from the o-C-H bond strength in pyridine.This yields a heat of formation of 405 kJ mol-'. The heat of formation of the o-pyrimidyl radical is difficult to estimate since there is no previously studied radical which would be a useful analogue for this species. However, it would be logical to expect that the stabilisation of o-pyrimidyl would be no less than that observed for o-pyridyl. An upper limit for the stabilisation energy in o-pyrimidyl could be taken to be double the stabilisation energy of o-pyridyl. Using this upper and lower limit, the heat of forma- tion of o-pyrimidyl would be expected to fall within the range 370-405 kJ mol-'.Refinement of this estimate will be dis- cussed later, when the results of the kinetic modelling of the decomposition of pyrimidine are presented. On the basis of the estimated relative heats of formation of the pyrimidyl radicals, it is not possible to exclude the 5-pyrimidyl radical from being important in the decomposition mechanism for pyrimidine. The expected high heat of forma- tion of this species makes it unlikely to be formed in an important initiation pathway, however. Its estimated heat of formation would also suggest that its formation by H-abstraction by H atoms would be endothermic by CQ. 20 kJ mol-'. Nevertheless, because of the high heat of formation of the CN radical, abstraction of H from pyrimidine by CN to form the 5-radical would be exothermic.Therefore on the basis of thermochemistry alone, the 5-pyrimidyl radical cannot be excluded from playing a role in the decomposition of pyrimidine. Using the group additivity method outlined by Benson," the energetics of various pathways to products from the pyri- midyls have been explored. Both the o-and the p-pyrimidyl radical can undergo simple ring fission to yield cyano rad- icals. The p-pyrimidyl radical can also undergo fission to yield an acetylenic open-chain radical which is estimated to have a heat of formation 160 kJ mol-' higher than that of the cyano radical. Ring fission to yield the cyano radical would therefore be greatly favoured.Similar conclusions have been made previously concerning the stability of open chain radicals in the pyridine system.' The 5-pyrimidyl radical differs from the other pyrimidyls in that it cannot undergo fission to yield a cyano-radical. It is therefore unlikely that the 5-pyrimidyl radical could undergo decomposition to yield products. On this basis the 5-pyrimidyl radical can be considered to be a species of little importance in the decomposition of pyrimidine. The decomposition of the open-chain radicals formed from the fission of o-and p-pyrimidyls can be shown to lead to the observed products of pyrimidine decomposition. Importantly, these decomposition pathways also lead to the regeneration of the H atom or CN radical chain carriers.In Scheme 1, pathways for the decomposition of the para-+HCN iC=CH#CN A H + HCCCN Scheme 1 Reaction scheme for decomposition of the p-pyrimidyl radical. Numbers in brackets refer to reaction number in Table 3. radical are illustrated. The heats of formation of the various intermediates as estimated by group additivity are given in Table 2. It can be seen that p-pyrimidyl decomposes to form HCN and cyanovinyl radical. Cyanovinyl would principally undergo loss of an H atom to form cyanoacetylene, although H abstraction from pyrimidine by cyanovinyl could also be a minor route for the formation of acrylonitrile. The decomposition routes of the o-pyrimidyl radical are shown in Scheme 2. There are two thermochemically feasible pathways by which o-pyrimidyl can decompose. Through a series of simple fission steps, acetylene and HCN are the stable products formed, with CN radical being regenerated.Alternatively, the radical formed from the ring opening reac- tion of o-pyrimidyl could undergo 1,3-H transfer to yield a species which cleaves to form CN radicals and acrylonitrile. It should be noted that unlike p-pyrimidyl, the CN radical is regenerated in the decomposition of o-pyrimidyl, in both of the possible pathways. \(4 0 (12) H&=CH ,CN HC=CYHC=N ,CN -'C=N1.3.H transfer 0 + HCN + CN Scheme 2 Reaction scheme for decomposition of the o-pyrimidyl radical. Numbers in brackets refer to reaction number in Table 3. In the development of a kinetic model, rate constants must be estimated in addition to the thermochemistry of the par- ticipating species.Rate constants for the reactions illustrated in Schemes 1 and 2 have been estimated largely by analogy to previously studied systems. The relationship between the acti- vation energy and the enthalpy change for various types of reaction has been discussed by Benson.20 The range of pos- sible Arrhenius pre-exponential factors for the reaction types has also been discussed by Benson." By means of kinetic modelling these initial estimates of the Arrhenius parameters for the various reactions can be refined. Through the use of sensitivity and flux analysis, the effects of these parameters on the predictions of the model can be assessed.Kinetic Modelling Kinetic modelling was carried out using the CHEMKIN2' code, in addition to a shock tube code22 and the ordinary differential equation solver LSODE.23 The shock tube code J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 reactions 1 0-PMDYL + HePMD 2 p-PMDYL + H PMD 3 H + PMDeo-PMDYL + H, 4 CN + PMDeHCN + O-PMDYL 5 HCCHCN + PMD=H,CCHCN + 0-PMDYL 6 H + PMDep-PMDYL + H, 7 CN + PMD p-PMDYL + HCN 8 HCCHCN + PMDzp-PMDYL + H,CCHCN 9 C3H3NCN 0-PMDYL 10 C,H, + HCNCN eC3H,NCN 11 CN + HCN eHCNCN 12 C,H,NCN eBC,H,N, 13 H,CCHCN + CN=BC,H,N2 14 p-PMDYLeNC,H,CN 15 NC,H,CN=HCN + HCCHCN 16 HCCHCN + MeHC,N + H + M 17 H + HCCHCNeH,CCHCN 18 H + H,CCHCNsC,H, + HCN 19 H + H,CCHCNsH, + HCCHCN 20 H + C,H,(+M)eC,H,(+M) 21 2H+MeH,+M 22 CN + HCN=C,N, + H 23 CN + H,=HCN + H 24 C,N, + Me2CN + M 25 HCN+M=H+CN+M Reaction model for pyrimidine pyrolysis" forward reaction reverse reaction log A n E log A n E ref.13.56 0.00 0.0 15.09 0.00 397.8 S 13.58 0.00 0.0 15.42 0.00 427.1 est 12.70 0.00 33.4 11.77 0.00 72.2 S 13.30 0.00 25.1 12.89 0.00 139.6 S 11.30 0.00 41.8 11.64 0.00 86.2 est 12.78 0.00 41.8 11.55 0.00 51.3 S 13.30 0.00 37.6 12.58 0.00 122.9 S 11.48 0.00 46.0 11.51 0.00 61.2 est 11.00 0.00 20.9 15.08 0.00 175.2 est 12.48 0.00 16.7 14.43 0.00 151.0 S 13.00 0.00 8.4 13.10 0.00 115.8 S 12.85 0.00 102.4 12.84 0.00 123.3 S 13.60 0.00 20.9 13.99 0.00 114.8 est 14.90 0.00 154.7 11.13 0.00 29.7 est 14.00 0.00 41.8 12.86 0.00 38.9 est 15.90 0.00 179.7 16.67 0.00 16.8 1 13.30 0.00 0.0 15.18 0.00 442.2 est 13.00 0.00 29.3 1 1.06 0.00 29.3 S 13.00 0.00 33.4 11.73 0.00 27.8 est 13.00 0.00 11.3 12.73 0.00 180.1 25 18.00 -1.00 0.0 15.04 0.00 425.0 26 7.58 1.57 0.4 14.60 0.00 47.0 14 5.69 2.44 8.9 14.94 0.00 112.8 27 16.86 0.00 419.7 14.32 0.00 -121.1 28 15.66 0.00 440.2 14.54 0.00 -72.1 29 a Units for A are cm3 mol-' s-' or s-l as appropriate.Units for E are kJ mol-'. S indicates rate constant to which the model predictions are sensitive, est indicates rate constant estimate to which model predictions are not sensitive. was modified to take into account cooling by the reflected rarefaction wave and to allow for reaction to occur in the cooling wave.A detailed kinetic model for the pyrolysis of pyrimidine is included in Table 3. The model consists of pathways illus- trated in Schemes 1 and 2, in addition to radical-radical and radical abstraction reactions not shown in the schemes. An explanation of the symbols used in Table 3 is given in Table 2. Product and reactant concentration profiles predicted by the model are compared with experiment in Fig. 1-5. It can be seen that the model predictions are in good agreement with the experimental data. At low to moderate extents of decomposition, the model predictions are very sensitive to the estimated rates of reactions included in the model. The methods for estimation of the reaction rates, discussed above, result in rates which are strongly dependent on the proposed reaction pathway.Therefore the ability of the model to predict the reactant and product profiles at low to interme- diate extents of decomposition strongly supports the pro- posed mechanism. Included in Fig. 1-5 are model predictions for the low- concentration series of experiments. These predictions test whether the model is able to reproduce the observed order of the reaction. The model predicts an order of reaction of slightly greater than first order, with respect to reactant, for the rate of disappearance of reactant and the rate of forma- tion of products. This slightly non-first-order behaviour pre- dicted by the model is observed experimentally only for cyanoacetylene, acrylonitrile and acetylene.The model pre- dicts the formation of HCN to be slightly greater than first order, which is not observed experimentally. When compar- ing the model predictions for the low-concentration runs with the experimental data, the lower accuracy of the low concen- tration data must be considered. Difficulties in measuring low levels of HCN could easily account for the deviations of the model predictions from the experimental measurements. It was previously concluded on the basis of the experimen- tal data that the decomposition of pyrimidine proceeds at a rate which is close to first order with respect to the reactant concentration. The low-concentration series of experiments uses reactant concentrations which are a factor of approx- imately four lower than the high concentration series.The model, however, predicts only small changes in the rate of formation of products and disappearance of reactant for the two reactant concentrations studied. The model therefore supports the conclusion based on the experimental observa- tions that the rate of pyrimidine decomposition is close to first order with respect to reactant concentration. The model, however, predicts the order of the reaction to be slightly greater than 1. Considering the difficulty in measuring the products for the low-concentration series, this conclusion based on the modelling may indicate that the order of the reaction is non-integer, which is common for free-radical chain mechanisms of long kinetic chain length.Optimisation of the rate of the initiation reaction to yield o-pyrimidyl and H-atoms has allowed the initial estimate of the heat of formation of o-pyrimidyl to be refined. The final value for the heat of formation of o-pyrimidyl was found to be 376 ( f10)kJ mol- '. This falls within the earlier estimated upper and lower limits, based on the heat of formation of o-pyridyl. Sensitivity and Flux Analysis When interpreting the results of sensitivity analysis it should be remembered that these are calculations based on the given kinetic model. Their relevance to the decomposition of pyrimidine will depend therefore on the correctness of the model.Fig. 9-11 show variation in the sensitivity coeflicients with temperature for the most sensitive reactions for formation of HCN, cyanoacetylene and acrylonitrile. The reactions impor- tant for the formation of these products are also important J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 l*OI -4 0.41--0.41---' I 1250 1300 1350 1400 1450 1500 temperature/K Fig. 9 Variation with temperature of the sensitivity coefficients for HCN. Only the most sensitive reactions from Table 3 are shown. Numbers refer to reaction number in Table 3. for the formation of acetylene, and for the rate of decomposi- tion of the reactant. From the sensitivity plots for HCN and acrylonitrile, com- petition between the decomposition pathways involving o-pyrimidyl and p-pyrimidyl is evident.This is manifested in the form of negative sensitivity coefficients for the rate of H- abstraction from pyrimidine by CN to yield p-pyrimidyl. The negative sensitivity coefficient has been shown by flux analysis not to be due to the reaction moving in the reverse direction. HCN and acrylonitrile are both formed from the __---..---0.6 1 1 1250 1300 1350 1400 1450 1500 temperature/K Fig. 10 Variation with temperature of the sensitivity coefficients for acrylonitrile. Only the most sensitive reactions from Table 3 are shown. Numbers refer to reaction number in Table 3. 1.0, r c .-'5 0.4 c..-ln 0.2 0'01k50 ' 1300 1350 1400 1450 ' 1500 temperature/K Fig. 11 Variation with temperature of the sensitivity coefficients for cyanoacetylene.Numbers refer to reaction number in Table 3. Only the most senstive reactions from Table 3 are shown. o-pyrimidyl radical, meaning that any abstraction to yield the p-pyrimidyl radical will tend to decrease the quantity of these products formed. The plots of sensitivity coefficients illustrate the rate-determining nature of the H-abstraction reaction by CN rad- icals. The rate of H abstraction by CN and the rate of the initiation reaction yielding o-pyrimidyl and H atoms have a comparable effect on the concentrations of HCN and acrylo- ni trile. The importance of the rate of H-abstraction from pyrim- idine by CN can help to explain the low apparent activation energy of the overall rate of disappearance of pyrimidine.Previous modelling studies on the decomposition of pyri- dine,' 2-~icoline,~ and the but en en it rile^^^ have shown that for these systems the rates of formation of products are influ- enced mainly by the initiation reactions. In each of these cases the overall rate of decomposition of the reactant closely corresponded to the rate of the initiation reaction of the free- radical decomposition mechanism. This is typical of kinetic data obtained for pyrolysis reactions using the shock tube, and is a consequence of the short residence time of this type of experiment. The observation of an overall rate of disap- pearance of reactant being close to the initiation rate for the decomposition mechanism is in contrast to the observed rate parameters for pyrimidine pyrolysis.In the case of pyrim- idine, sensitivities to the rate constant for H-abstraction of pyrimidine by CN show comparable sensitivity to the initi- ation reactions over the entire range of temperatures studied, and thus a relatively low overall activation energy is observed. The concept of 'kinetic chain length' described by Benson2' can be used to rationalise the differences in the kinetics of pyrimidine and pyridine. In the case of pyridine, since the rate of the initiation step is close to the overall rate of pyridine decomposition, the kinetic chain length is ca. 1. This is very different from the pyrimidine case in which the rate of decomposition is much faster than the rate of initi- ation, yielding a chain length of ca.80 at 1350 K. This illus- trates the greater importance of propagation steps in the case of pyrimidine, and the low activation energy observed for overall kinetics reflects this importance. Having established that the low activation energy for the decomposition of pyrimidine is a result of increased influence of propagation steps, it is interesting to try to draw conclu- sions about which aspects of the pyrimidine pyrolysis mecha- nism give rise to this behaviour. The CN radical is an important propagating radical in the decomposition of pyrimidine, with the major products HCN and acrylonitrile being formed from CN chains. This is a major difference between the mechanism for pyrimidine decomposition, and the mechanism of decomposition of the other organo-nitrogen compounds studied.',2*4,24 For pyridine, at low to moderate extents of decomposition, the radical chain carriers are H-atoms.The radical chain carriers for the butenenitriles and 2-picoline are H-atoms and methyl radicals. It is possible that CN chains propagate much more rapidly than H or methyl chains with less efficient termination reactions. This would result in the large kinetic chain length observed for pyrimidine. The importance of CN radicals in the decomposition of pyrimidine can be attributed to the high heat of formation of pyrimidine compared with pyridine or 2-picoline. The open- chain C,H,NCN radical, formed from o-pyrimidyl, can undergo decomposition in two steps to yield acetylene, HCN and CN.The total endotherm for the formation of these pro- ducts from the open-chain radical is 280 kJ mol-'. In the case of pyridine, the open-chain C,H,CN radical formed from the ring opening of o-pyridyl' could decompose in an 548 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 analogous fashion to yield two acetylene molecules and CN. However, the total endotherm for the formation of these pro- ducts from the open-chain C,H,CN radical is 410 kJ mol-'. The considerably higher endotherm for generation of the CN radical in the pyridine system explains why CN radicals are of little importance in the pyrolysis of pyridine at low to intermediate extents of decomposition. 9 10 11 12 13 14 R.D. Hauck, ACS Fuel Chem., 1975,20,85. S. R. Palmer, E. J. Hippo, M. A. Kruge and J. C. Crelling, Proceedings of the International Conference on Coal Science, Butterworth-Heinemann, Oxford, 1991,p. 993. W. S. Cathro and J. C. Mackie, J. Chem. SOC., Faraday Trans., 1972,68,150. K. R. Doolan and J. C. Mackie, Combust. Flame 1983,49,221. K. K. Innes, H. D. McSwiney Jr., J. D. Simmons and S. G. Tilford, J. Mol. Spectrosc., 1969, 31, 76. A. Szekely, R. K. Hanson and C. T. Bowman, Int. J. Chem. Conclusion Kinet., 1983, 15, 1237. Thermal decomposition of pyrimidine dilute in argon takes 15 16 S. Zabarnick and M. C. Lin, Chem. Phys., 1989,134, 185. M. Nabavian, R. Sabbah, R. Chaste1 and M. Laffitte, J. Chim. place over the temperature range 1200-1600 K, at a residence time of 850 to lo00 ps, and pressure of 13-15 atm.A free-radical mechanism consisting of 25 reactions has been shown to predict successfully the concentrations of products formed and of pyrimidine remaining in the product mixtures. Impor- tant radicals in this mechanism are o-and p-pyrimidyl, with 17 18 19 Phys., 1977, 74, 115. K. K. Innes, I. G. Ross and W. R. Williams, J. Mol. Spectrosc., 1988,132,492. H. I. Leidreiter and H. G. Wagner, 2. Phys. Chem., Neue Folge, 1987, 153, 99. 0. Kikuchi, Y. Hondo, K. Morihashi and M. Nakayama, M. Bull. Chem., 1988,61, 291. H atoms and CN radicals being the radical chain carriers. The CN radical as a chain carrier in the decomposition of pyrimidine is a significant departure from the decomposition mechanisms of other organo-nitrogen compounds such as 2-picoline, pyridine or pyrrole.It is postulated that the rela- tively low overall activation energy for the rate of decomposition of pyrimidine is due to the efficiency of the CN chain. The feasibility of generation of CN from interme- diate radicals in the decomposition of pyrimidine is thought to be a consequence of the relatively high heat of formation of pyrimidine. 20 21 22 23 24 S. W. Benson, Thermochemical Kinetics, Wiley, New York, 1976. R. J. Kee, J. A. Millar and T. H. Jefferson, CHEMKIN; A General Purpose, Problem Independent, Transportable FORTRAN Chemical Kinetics Codes Package, Sandia National Laboratories Report SAN80-003, March, 1980. R. E. Mitchell and R. J. Kee, A General Purpose Kinetic Code for Predicting Chemical Kinetic Behaviour Behind incident and Reflected Shocks.Sandia National Laboratories, SAN82-8205, March, 1982. A. C. Hindmarsh, LSODE and LSODE; Two New Initial Value Differential Equation Solvers, ACM Signum Newsletters, 1980, 15. A. Doughty and J. C. Mackie, J. Phys. Chem., 1992, %, 272. The authors thank Jacqueline Palmer for assistance with the 25 26 W. A. Payne and L. J. Stief, J. Chem. Phys., 1976,64,1150. G. Dixon-Lewis, Philos. Trans. R. SOC. London, Ser. A, 1981, SPST experiments. The financial assistance of the Australian Research Council is gratefully acknowledged. 27 A303, 181. B. Atakan, A. Jacobs, M. Wahl, R. Weller and J. Wolfrum, Chem. Phys. Lett., 1989,154,449. 28 A. Szekely, R. K. Hanson and C. T. Bowman, J. Chem. Phys., References 1984,80,4982; M. B. Colket, Int. J. Chem. Kinet., 1984, 16, 353; J. C. Mackie, M. B. Colket and P. F. Nelson, J. Phys. Chem., 1990,94,4099. J. C. Mackie, M. B. Colket, P. F. Nelson and M. Esler, Int. J. Chem. Kinet., 1991, 23, 733. A. Terentis, A. Doughty and J. C. Mackie, J. Phys. Chem., 1992, %, 10334. A. Doughty and J. C. Mackie, J. Phys. Chem., 1992,%, 10339. L. R. Snyder, Anal. Chem., 1969,41,314. D. T. Clarke and R. Wilson, Fuel, 1983,62,1034. S. Wallace, K. D. Bartle and D. L. Perry, Fuel, 1989,68, 1450. P. F. Nelson, A. N. Buckley and M.D. Kelly, 24th International 29 T. Fueno, K. Tabayashi and 0.Kajimoto, J. Phys. Chem., 1973, 77, 575; K. Najarajan, K. Thielen, H. D. Hermanns and P. Roth, Ber. Bunsenges. Phys. Chem., 1986, 90, 533; M. W. Slack, E. S. Fishburne and A. R. Johnson, J. Chem. Phys., 1971,54,1652. P. Roth and Th. Just, Ber. Bunsenges. Phys. Chem., 1976, 80, 171; P. Roth, Forsh. Ingenieurwes, 1980,46,93; A. Szekely, R. K. Hanson and C. T. Bowman, Symp. Int. Shock Tubes Waves Proc., 1982, 13, 617; A. Szekely, R. K. Hanson and C. T. Bowman, J. Phys. Chem., 1984, 88, 666; P. Roth and Th. Just, Materials Research Symp. Proc., NBS Spec. Pub. 561, 1979, 10, 91; K. Thielen and P. Roth, Combust. Flame, 1987, 69, 141. Symposium on Combustion, The Combustion Institute, Sydney, 1992, p. 1259. Paper 31047236; Received 5th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000541

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(4), 549-552 Reactions of N(2 ’0) and N(2 ‘P) with 0, Yoshitaka Shihira Department of Energy Sciences, Tokyo Institute of Technology, Ookayama , Meguro-ku, Tokyo 152, Japan Teruaki Suzuki, Shin-ichi Unayama, Hironobu Umemoto* and Shigeru Tsunashima Department of Applied Physics, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan The rate constants for the reactions of N(2 ,D) + 0, and N(2 ’P) + 0, have been measured by employing a pulse radiolysis-resonance absorption technique at temperatures between 210 and 295 K. The rate constants were expressed by the following Arrhenius equations: kN(zD)+Oz= 9.4 x exp(-210/~) and kN(zP)+02= 3.1 x lo-’, exp(-6O/T) in units of cm3 s-’. The results for N(’D) + 0, were compared with the results of quasiclassical trajectory calculations on the basis of modified LEPS potential-energy surfaces.It is suggested that the main exit channel for the N(’D) + 0, reaction is the production of O(’D) under the present conditions. The reactions of metastable atomic nitrogen, N(’D) and They suggested that reaction (1) ought to follow a TI’’ tem-N(’P), with 0, are of great interest, from both an atmo-perature dependence, or possibly a somewhat stronger depen- spheric and reaction dynamics point of view. N(,D) and dence because of the presence of an energy barrier. On the N(’P) are important constituents in the upper atmosphere of other hand, the reaction of N(’P) may have a weaker or even the earth. The rate constants for the deactivation of these a negative temperature dependence, since the initial approach species at room temperature have been measured by many for insertion favours less 0, rotational motion and the investigator^.'-^ The contributions of these reactions to the increased lifetime of the collision complex at lower tem-formation of NO in the upper atmosphere have been dis- peratures raises the chance of a non-adiabatic transition.It is, cussed since a large amount of NO was observed in rocket therefore, necessary to measure the temperature dependence experiments.’ Thermodynamically possible reactive channels of the rate constants quantitatively and to check the model for N(,D) are as follows: proposed by Rawlins et al. Recently, we have succeeded in measuring the temperature N(,D) + 0, -+ NO(X ,lI)+ O(’D) + 175 kJ mol-’ (1) dependence of the rate constants for the deactivation of N(,D) + 0, --+ NO(X ’n) + O(3P)+ 365 kJ mol-’ (2) N(*D) and N(2P) with H, and D,.14 The rate constants could well be represented by Arrhenius equations.The Arrhe- Both these processes are symmetry-allowed. O(‘D) atoms nius parameters for N(2D)+ H, and D, could be reproduced formed in reaction (1) have been regarded as the major by transition-state theoretical calculations as well as quasi- source of the emission at 630 nm in the airglow and aurora.’ ’ classical trajectory calculations. Possible exit channels were Recent laboratory studies have provided important impli- also discussed. In the present work, the temperature depen- cations for the exit channels of these reaction^.'^.'^ Rawlins dence of the rate constants for the deactivation of N(’D) and et a!.’ measured the vibrational and rotational-state dis-N(,P) by 0, were measured.The experimental results were tributions of NO(X ,II) formed in the reactions of N(’D) and compared with those of quasiclassical trajectory calculations N(’P) with 0, near 100 K. They assumed that rotationally on the basis of modified LEPS potential-energy surfaces. excited NO results from reactions of N(’P) with 0,, while rotationally thermal NO results from reactions of N(’D). On Experimentalthe basis of this assumption, they concluded that N(’D) reacts with 0, uia a direct abstraction mechanism, along the The experimental apparatus and the procedure were the same ’A‘ surface which leads to O(’D) formation.They also con- as those described previou~ly.’~ A mixture of N, and 0, in a cluded that O(3P)is produced as result of a transition from stainless-steel vessel was irradiated by a pulsed electron beam the ,A’ surface to the ,A’ or ’A” surface which correlates with from a Febetron 706 apparatus (Hewlett Packard) to produce O(3P). It was also proposed that the reaction of N(,P) and metastable atomic nitrogen. The temporal variation of the 0, proceeds, in contrast, through a long-lived complex, uia concentration of N(,D) or N(,P) was traced by means of the an insertion mechanism on a highly attractive potential absorption of atomic lines of nitrogen. The wavelength used surface.for the detection of N(’D) was 149 nm corresponding to the On the other hand, quantitative experimental information 3,P+-2 ’D transition, while that for N(2P)was 174 nm cor- on the temperature dependences of the rate constants is not responding to the 3 ’P +2 ,P transition. These atomic lines available. Slanger et al. have reported the temperature depen- were derived from a cw microwave discharge in a flow of dence of the rate constant for the deactivation of N(,D) by N,-He. Transmitted light was detected with a photomulti- 02,1 but their result at room temperature has not been sup- plier tube (Hamamatsu, R976) through a VUV monochro-ported by recent measurements.’ Rawlins et al. predicted the mator (Shimadzu, SGV-50). The photomultiplier signal was trend of the temperature dependences of the rate constants.amplified and processed with a wave memory (NF Circuit 5 50 12 c ln m 0 4 0 0, pressure/Pa Fig. 1 Pseudo-first-order decay rates of N(’D) as a function of 0, pressure at 245 K Table 1 Rate constants obtained in the present work ~~ rate constant reaction temperature/K cm3 s-’ N(~D)+ 0, 295 4.57 f0.22 270 4.45 f0.16 245 3.72 f0.12 230 3.81 f0.20 210 3.48 0.15 N(,P) + 0, 300 2.53 f0.11 270 2.53 f0.09 245 2.09 f 0.07 227 2.46 f 0.13 212 2.34 f 0.10 T/K 250 200 1 0-133. 0 4, 0 5b0 103 KIT Fig. 2 Arrhenius plots for the N(,D) + 0, (0)and N(’P) + 0, (a)reactions Table 2 Arrhenius parameters for the N(,D) + 0, and N(’P) + 0, reactions reaction A/10-’2 an3s-l EJkJ mol-’ N(’D) + 0, N(’P) + 0, 9.4 & 1.5 3.1 f0.9 1.8 f0.3 0.5 f0.4 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Design Block, WM-852) and a computer (NEC, PC-9801). The temperature of the reaction system was controlled by introducing cold nitrogen gas from boiling liquid nitrogen to copper tubing surrounding the vessel. For the decay rate measurement of N(’P), the typical pressure of N, was kept at 100 kPa, while for the measurement of N(,D) the gas mixture was diluted with He because N(,D) is deactivated efficiently by N, .15*16 Typical partial pressures of 0, were 0-12 Pa. The sample gases were purified before being introduced into the reaction vessel. 0, (Toyo Sanso) was passed through a cold trap filled with glass beads. N, (Nihon Sanso) was passed through a column containing reduced copper chips at 590 K and a trap filled with glass beads at 77 K.He (Japan Helium Center) was passed through a column of copper chips heated at 590 K and a trap filled with molecular sieve 13X at 77 K. Experimental Results The time variation of the concentration of N(,D) and N(2P) showed an exponential decay. In Fig. 1, the pseudo-first- order decay rates of N(’D) at 245 K are plotted as a function of 0, pressure. Similar linear plots could be obtained at other temperatures and also for N(’P). The rate constants can be obtained from the slopes of such plots. Table 1 sum-marizes the rate constants obtained. The error limit is one standard deviation.The temperature dependence of the rate constants is shown in Fig. 2. The Arrhenius parameters, as determined by a non-linear least-squares method, are listed in Table 2. Both these reactions are characterized by small pre- exponential factors as well as small activation energies. Quasiclassical Trajectory Calculation Three-dimensional (3D) quasiclassical trajectory (QCT) cal- culations were carried out to evaluate the thermally averaged rate constants as well as the nascent vibrational-state dis- tribution of NO. The computational procedure is similar to those for N(,D) + H, and D, described previo~sly.’~ In order to carry out the QCT calculations, the exit channel must be specified. In the reactions of N(,D), both reactions (1) and (2) are energetically and symmetrically allowed. Then, we carried out the calculations by assuming that the reaction proceeds via reaction (1) or reaction (2).It may be considered that both channels are open and com- peting. However, the small pre-exponential factor observed in the present work strongly suggests that most of the exit chan- nels are closed and only one main channel is open. The potential-energy surfaces were calculated by using the modi- fied LEPS method employed in our previous work on N(,D) + H, and D,. In reaction (l),NO(X ,II) + O(’D) does not correlate with N(4S)+ O(3P)+ O(’D), but correlates with N(4S) + O(3P)+ O(3P). The reactant, N(’D) + 0, , correlates adiabatically with N(4S) + O(3P) + O(3P), not with N(,D) + O(3P)+ O(’P).Therefore both the diatomic potential curves for N-0 and 0-0 must be constructed in a manner similar to that employed previously for H--H.I4 For the N-0 curve, the equilibrium internuclear distance was assumed to be the same as that for a free NO molecule, 0.1 15 nm. The disso- ciation energy was calculated to be 448 kJ mol-’ by subtrac- ting the energy difference between O(’D) and O(3P)from the dissociation energy of NO. The value of be was calculated to be 32.7 nm-by using the modified D, and spectroscopically determined 0,.The potential parameters for 0-0, the dis- sociation energy, the equilibrium internuclear distance and the Morse parameter Be, were also determined in a similar J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 manner to be 0.121 nm, 273 kJ mol-' and 36.0 nm-', respec- tively. QCT calculations were performed for various sets of Sat0 parameters, ANOand Am. When AN0 and Aoo were chosen to be 0.161 and -0.020, respectively, the experimental results for the absolute values of the rate constants as well as the temperature dependence could be reproduced satisfactorily. Here, it was assumed that only one ,A' surface leads to the production of O('D). In other words, the apparent rate con- stants were divided by the ratio of the degeneracy factors, 15. This surface has a small, early barrier in the entrance channel. The barrier heights at various bearing angles are summarized in Table 3. The calculated results for the rate constants are shown in Fig.3 together with the experimental results. The vibrational-state distribution of NO was also cal- culated. The result is illustrated in Fig. 4 together with the estimation by Rawlins et al.' Canonical variational transition-state theoretical calcu-lations (CVTST) were also carried out by assuming collinear geometries. The procedure is the same as that described pre- vi~usly.'~In the case of N(2D)+ H, and D,, fair agreement was obtained between the QCT and CVTST results. In the present case, however, the rate constants obtained by the QCT calculation could not be reproduced by the CVTST cal- culation. For example, in reaction (l), the rate constant obtained by CVTST was around twice as large as that for QCT, when the above potential-energy surface (PES) was used.This discrepancy can be ascribed to the very low bending frequency at the transition state. The bending vibra- tional potential is too loose to be represented by a mixed harmonic-quartic potential.' ' Table 3 Saddle-point geometries and bearing angles barrier heights at various (?/degrees rltMlnm r&,/nm V/kJ mol-' 0 0.262 0.121 0.577 25 0.253 0.121 0.614 50 0.223 0.121 0.918 60 0.198 0.122 1.64 70 0.158 0.123 7.66 80 0.134 0.130 46.6 8,Bearing angle between the 0-0 axis and the vector connecting N and the centre of mass of 0,. riM,Distance from N to the centre of mass of 0,. r&,, ,0-0 internuclear distance. V, Barrier height at the saddle point.TIK ~ , ,30,0 , , , , 25; , , , , 27,0-11 -13.0' ' " . " ' " ' 10-13 3. 0 4. 0 5. 0 lo3 KIT Fig. 3 Comparison of the calculated and the experimental rate con- stants for N('D) +0,.(---) Results of QCT calculations. 55 1 0 10 I/' Fig. 4 Comparison of the calculated and the experimentally esti- mated vibrational-state distributions of NO(X *II)at 100 K for N('D) + 0,. 0,Experimental estimate by Rawlins et al.I3 a,QCT calculation for the O('D) channel. A,QCT calculation for the O(3P) channel It was also possible to reproduce the experimental results for the temperature dependence of the rate constants by assuming reaction (2). A satisfactory QCT result was obtained when AN0 and Am were chosen to be 0.180 and -0.385, respectively.However, reaction (2) seems to be less probable because the vibrational distribution of the product NO estimated by Rawlins et al. could not be reproduced at all, as is shown in Fig. 4. We also tried to reproduce the experimentally obtained rate constants for the reactions of N(2P)by assuming various exit channels. However, it was hard to find a LEPS surface which is consistent with the present experimental results. The calculated rate constants were found to be much larger than the experimental ones. The reaction of N('P) with 0, may involve a non-adiabatic process, as has been proposed by Rawlins et ~1.'~ Discussion Comparison with Previous Results Table 4 compares the present results for the rate constants obtained at room temperature with the literature values.'-' For both reactions of N('D) and N(2P),the present results Table 4 Comparison of the rate constants at room temperature reaction k/10-* cm3 s-' technique"*b ref.N('D) + 0, 4.57 f0.22 7.4 PR-RA FP-CL this work 1 5.2 & 0.4 FP-RA 2 5.3 k0.5 DF-RF 3 6.1 DF-EPR 4 6.6 k 1.0 DF-RA 6 4.6 f0.5 DF-RF 7 N(2P)+0, 2.53 f0.11 2.6 k0.2 PR-RA FP-RA this work 2 3.5 k0.13 DF-RF 3 2.51 0.14 1.8 *0.2 PR-RA DF-MPI 5 8 2.2 f0.4 DF-RF 9 Methods for production of metastable nitrogen atoms, PR, pulse radiolysis; FP, flash photolysis; DF, discharge flow. Methods for detection of metastable nitrogen atoms : RA, resonance absorption; CL, chemiluminescence; EPR, electron paramagnetic resonance; RF, resonance fluorescence; MPI, multiphoton ionization.agree well with those of the most recent and reliable measure- ments by Piper et aL7*' The present result for N(,P) also agrees well with our previous value.' Slanger et al.' measured the rate constants for the reaction of N(,D) with 0, over the temperature range 237-365 K. They concluded that the rate constants have a T112depen-dence. The Arrhenius fit to their results yields an activation energy of 0.23 kJ mol-'. This value is much smaller than the present result, 1.8 kJ mol-'. In their flash photolysis study, the relative number density of N(,D) was monitored by the emission of NO(B211-X211) resulting from the reaction of N(,D) with N,O.This measurement must have been affected by the lack of sensitivity. Note that their result at room tem- perature has not been reproduced by more recent and direct measurements. The NO(B 2rI-X 'll) emission mechanism may also be affected by the change in temperature. Exit Channel for the Reaction N('D) + 0, The main exit channel for the reaction N('D) + 0, has been disputed. Link and Swaminathan suggested in their review" that the O(3P)channel is favoured over the O('D) channel by at least 9 : 1 at thermospheric temperatures (300-1000 K). On the other hand, Rawlins et af. suggested that the adiabatic O('D) formation, reaction (l), is the major channel at 100 K by analysing the product state distributions.' The present results for the temperature dependence of the rate constants are consistent with either exit channel.However, judging from the result of the QCT calculations on the vibrational- state distribution of NO, the conclusion of Rawlins et al. is preferred. Rawlins et al. estimated the initial vibrational-state dis- tribution of NO formed in the reaction of N('D) with 0, by using a surprisal analysis. The distribution was fairly excited and peaked at u = 6. Fig. 4 shows a comparison of their result and that of the present QCT calculations. Fair agree- ment between the experimental and calculated results was obtained when reaction (1) was assumed. On the other hand, the calculated vibrational-state distribution is much hotter than the experimental one when reaction (2) was assumed.Both of the potential surfaces employed for reactions (1) and (2) had small, early barriers and their characters were similar except for the exothermicity. Therefore, the difference in the vibrational-state distributions for reactions (1) and (2) must reflect the difference in the exothermicity. Reaction (2) is more exothermic and more energy is partitioned into the product vibrational motion. The discrepancy between the present result and the propo- sal of Link and Swaminathan that the main product is O(3P) can be settled if we consider that the main exit channel J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 changes above 300 K. There must be a large activation energy for the production of O(3P) and this channel is closed at low temperatures.Since the activation energy obtained in the present work is small, it is reasonable to assume that the main exit channel is the formation of O('D) at the present temperatures, 210-295 K. It must be recalled, however, that the present assumption does not exclude the possibility of the production of O(3P)through a non-adiabatic transition after passing through the barrier. In conclusion, the present results are generally consistent with the model presented by Rawlins et al. that the adiabatic O('D) formation, reaction (l), is the major channel for the reaction N('D) + 0, and O(3P) formation, reaction (2), occurs only through a non-adiabatic transition in the 'exit' channel of reaction (1). The reaction of N('P) with 0, may involve a non-adiabatic process.For further discussion, tra- jectory calculations which take surface-hopping into con-sideration will be profitable. Rate constant measurements at high temperatures will also be helpful. References 1 T. G. Slanger, B. J. Wood and G. Black, J. Geophys. Res., 1971, 76, 8430. 2 D. Husain, S. K. Mitra and A. N. Young, J. Chem. SOC.,Faraday Trans. 2, 1974,70, 1721. 3 M. P. Iannuzzi and F. Kaufman, J. Chem. Phys., 1980,73,4701. 4 B. Fell, I. V. Rivas and D. L. McFadden, J. Phys. Chem., 1981, 85, 224. 5 H. Umemoto, K. Sugiyama, S. Tsunashima and S. Sato, Bull. Chem. SOC.Jpn., 1985,58, 3076. 6 P. D. Whitefield and F. E. Hovis, Chem. Phys. Lett., 1987, 135, 454. 7 L. G. Piper, M. E. Donahue and W. T. Rawlins, J. Phys. Chem., 1987,91,3883. 8 C. M. Phillips, J. I. Steinfeld and S. M. Miller J. Phys. Chem., 1987,91, 5001. 9 L. G. Piper, J. Chem. Phys., 1993,98,8560. 10 C. A. Barth, J. Geophys. Rex, 1964,69, 3301. 11 R. Link and P. K. Swaminathan, Planet. Space Sci., 1992, 40, 699. 12 J. P. Kennealy, F. P. Del Greco, G. E. Caledonia and B. D. Green, J. Chem. Phys., 1978, 69, 1574. 13 W. T. Rawlins, M. E. Fraser and S. M. Miller, J. Phys. Chem., 1989,93, 1097. 14 T. Suzuki, Y. Shihira, T. Sato, H. Umemoto and S. Tsunashima, J. Chem. SOC.,Faraday Trans., 1993,89,995. 15 K. Schofield,J. Phys. Chem. Ref. Data, 1979,8,723. 16 K. Sugawara, Y. Ishikawa and S. Sato, Bull. Chem. Soc. Jpn., 1980,53,3 159. 17 B. C. Garrett and D. G. Truhlar, J. Phys. Chem., 1979,83, 1915. Paper 3/05960J; Received 5th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000549

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FAKADAY TRANS., 1994, 90(4), 553-557 Application of Simple Expressions for the High-pressure Volumetric Behaviour of Liquid Mesitylene Valentin Garcia Baonza,* Mercedes Caceres Alonso and Javier Nuhez Delgado Departamento de Quimica Fisica Facultad de Ciencias Quimicas Universidad Complutense de Madrid 28040-Madrid Spain Experimental ppT measurements of mesitylene (1,3,5-trimethylbenzene) obtained with an expansion technique have been used to calculate the molar density p and the mechanical coefficients (isothermal compressibility) and a, (thermal expansion coefficient) as functions of pressure (up to 110 MPa or freezing pressure, when lower) and temperature (between 238 and 298 K). The whole data set has been used to confirm the ability of simple power functions to represent the experimental measurements as well as to test the possibility of using the functions for extrapolation.These expressions simultaneously relate the pressure dependence of the mecha- nical coefficients with the divergence pressure along the so-called pseudospinodal curve. The study of high-pressure thermodynamics of liquids has advanced greatly in recent years. However, the behaviour of ,quantities such as isothermal compressibility, K~ the thermal expansion coefficient, a,, or constant-pressure heat capacity C, with pressure is still poorly understood for most systems at very high pressures. Although physical chemists and chemical engineers have derived some correlation schemes for several groups of compounds, these are too inaccurate for most purposes and are of limited use for extrapolation.This is mainly due to the lack of precise measurements on impor- tant model liquids, and to the absence of a general and simple theory which can give a reliable foundation to these correlations. At present, it seems that the only way of obtain- ing accurate values of such quantities at very high pressures is by direct measurement. Recent experimental studies on simple liquids such as n- hexane,' carbon dioxide,2 n-butane2 or toluene3 suggest that the thermal expansion coefficient, ap, and isothermal com- ,pressibility, K~ follow simple power laws along isothermal paths up to extremely high pressures. Speedy: using assump- tions from the so-called stability-limit conjecture, gave a suc- cessful unified interpretation of the properties of water including anomalies such as the density maxima.However, since his expressions were obtained from the limiting behav- ~iour of a,, and IC near the spinodal curve,4 the divergence of both quantities was characterized by an exponent of -0.5, unlike the observations of Pruzan' who found that while a, also follows an exponent close to -0.5 in the stable liquid range, icT follows a power law characterized by an exponent close to --0.85 for liquids such as argon, carbon dioxide or n-hexane. We have recently confirmed this feature for other liquids such as krypton, tetrafluoromethane, trifluoro-methane, carbon disulfide, tetramethylsilane, cyclopentane and 2,3-dimeth~lbutane.~In addition, the expression for molar density, p, that results from the integration of the expression K~ has been used sucessfully to correlate the experimental high-pressure isotherms of liquids such as tetra- methylsilane, cyclopentane and 2,3-dimeth~lbutane.~ Finally, we have also confirmed that both quantities can be referred to approximately the same pseudospinodal curve (as that of Speedy in the case of water) only when exponents close to -0.5 and -0.85 are used to correlate experimental ,results of a, and K~ respectively.However, as far as we know, in spite of the surprisingly large range of validity of these expressions, they have not yet been used for extrapolation for any liquid. In this paper we establish that these expressions may be used to correlate our experimental measurements for mesitylene 1,3,5-tri-methylbenzene and show that both a, and K~ can be extrapo- lated far beyond the experimental range.The extrapolated K= results have been compared with direct measurements found in the literature7 for this compound. Experimental In our laboratory the equations of state of important model liquids from 198 to 298 K and at pressures up to 110 MPa are being studied. The experimental device is based on an expansion principle.* Both method and apparatus have been widely described in the literat~re.~' lo Because of the low vapour pressure of mesitylene at room temperature, the following changes were introduced in our experiments: (1) expansions of the compressed liquid were performed at 333.15 K in order to achieve a higher accuracy in the measurement of the expansion pressure, (2) the high- pressure cell (ten = 4.2 cm3) used in present work was the same as that used for but-2-yne,11 and (3) an expansion cell of about 300 cm3 was used in this work instead of that of 200 cm3 used in previous investigation^.'^*'^ The solid-liquid coexistence curve was also obtained, fol- lowing the procedure described in ref.11. Temperatures were measured with an accuracy of 0.01 K, using a Leeds and Northup calibrated platinum resistance thermometer, and were referred to the international tem- perature scale ITS-90. Temperatures in both high-pressure cell and expansion cell baths were controlled electronically to within kO.01 K.High pressures were measured with a Heise bourdon gauge with an absolute accuracy of 0.01 MPa together with a Sensotec TJE1108-20 transducer with an accuracy of about 0.02%. Both devices were calibrated with a Desgranges et Huot 5403 dead-weight gauge. The low pres- sures reached in the expansion system were measured with a Maywood P-102 transducer with an accuracy of 0.07%. An absolute reference density for mesitylene, ~(298.15 K, 0.1 MPa) = (7.1643 +_ 0.0002) mol dm-3 was taken from ref. 7. The second virial coefficient at 333.15 K of mesitylene in the vapour phase was estimated following a similar pro- cedure to that described in ref. 11 and 13 from experimental data taken from the compilation of Dymond and Smith.14 The accuracy of the densities reported here is always greater than 0.0012 mol dmP3 for high-pressure results and about 0.006 mol dm-' for those along the 0.1 MPa isobar.The a, values are accurate to within 0.02 kK-' for high- pressure results and to within 0.01 K-' for those at 0.1 MPa, whilst K~ values are accurate to within 0.01 GPa- or better depending on the pressure. 554 The mesitylene used in this work was Fluka 'puriss' which was dried over a molecular sieve prior to its introduction into the experimental device. Results and Discussion Freezing Pressures The melting curve of mesitylene was measured in the range 10-65 MPa, and the results are recorded in Table 1. The accuracy of the freezing pressures is about 5%.Freezing pres- sures of mesitylene have been previously reported in ref. 7 up to 345 MPa. The average deviation between both sets of measurements is 1.5 MPa in the range of temperatures covered in our experiments. The melting point obtained by extrapolation of the measurements at 0.1 MPa is (220.5 0.2) K. This value is in agreement with the values previously reported by Easteal and Woolf7 (221.1 K) and Hirschler and Faulconer' (221.3 K) for modification I11 of solid mesitylene. As Easteal and Woolf also observed, under the present conditions it seemed that modification I11 was always produced, possibly due to specific impurities in the liquid, although it is not the stable crystal f~rm.~,'~ Other transitions were not detected throughout our measurements. ppT Results The 294 experimental ppT points, 71 along two isobars and 223 along four isotherms, are recorded in Tables 2 and 3, respectively. The experiments have been carried out with relatively small temperature (along the isobars) or pressure Table 1 Experimental freezing pressures for mesitylene 238.7 63.4 236.5 55.8 233.6 45.7 231.8 39.2 230.4 34.4 228.3 27.4 226.8 21.9 224.9 15.8 223.3 9.7 Table 2 Molar density, p/mol dmP3, for liquid mesitylene at differ- ent temperatures, T/K, and pressures, p/MPa ~ ~ T P T P T P T P p = 0.10 238.17 7.5825 254.09 7.4707 269.07 7.3660 285.41 7.2524 239.72 7.57 16 255.49 '1.4605 270.79 7.3540 286.96 7.24 17 241.37 7.5600 256.80 7.45 17 272.42 7.3426 288.58 7.2304 243.09 7.5479 258.14 7.4423 274.06 7.3312 290.40 7.21 78 244.75 7.5362 259.63 7.43 19 275.87 7.3 186 291.91 7.2074 246.29 7.5254 26 1.49 7.4189 277.14 7.3098 293.97 7.1931 248.16 7.5123 262.7 1 7.4104 278.14 7.3028 295.52 7.1825 249.55 7.5025 264.05 7.4010 280.03 7.2897 297.54 7.1685 251.18 7.491 1 265.42 7.3914 282.03 7.2759 298.23 7.1637 252.65 7.4808 267.25 7.3787 283.78 7.2637 p = 44.82 238.I9 7.7544 253.15 7.6605 268.5 1 7.5658 283.72 7.4742 240.40 7.7404 254.81 7.6501 270.19 7.5555 285.63 7.4629 242.23 7.7289 256.83 7.6376 272.15 7.5436 288.63 7.4452 244.00 7.7 178 258.50 7.6272 273.98 7.5325 290.60 7.4337 245.75 7.7068 260.33 7.6160 275.59 7.5228 292.21 7.4243 248.04 7.6924 262.40 7.6032 277.67 7.5103 294.35 7.41 19 249.75 7.68 17 264.3 1 7.59 15 279.61 7.4987 296.5 1 7.3994 251.13 7.673 1 266.37 7.5788 281.64 7.4865 298.16 7.3899 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (along the isotherms) steps since it is of particular interest to give reliable results for the mechanical coefficients a, and xT. The differential expansion method' used in this work is par- ticularly useful since it gives direct information about pres- sure and temperature derivatives of molar density. Thermal Expansion Coefficient The molar densities recorded in Table 2 were used to evalu- ate ap by simple finite differences. Unlike the density isobars, which are almost linear, those of a, exhibit, in general, curvi- linear forms.In addition, the temperature dependence of a,, changes sign at intermediate pressures as shown in Fig. 1. The change in the sign of (aa,/aT), indicates that intersec- tions of the ap isotherms are always expected at pressures below 45 MPa. This characteristic behaviour has been pre- viously observed for most liquids in the same range of pres- sures. The near constancy of a, at the lowest temperatures along the 45 MPa isobar suggest that intersections are dis- placed somewhat with temperature, as already observed in t01uene.~ However, the precision of our data is also compat- ible with a common intersection at a single point of the (p, a,) plane (at about 25 MPa) in terms of the expression given by Alba et a/.' or a recent extension of it,6 so we cannot draw a definitive conclusion.The relation of this phenomenon to the behaviour of constant-pressure heat capacity, C, , at high pressures has been recently discussed by the present authors16 for some benzene derivatives. Selected values of ap for mesitylene are given in Table 4. Isothermal Compressibility As for a,, molar densities recorded in Table 3 were used to evaluate K~ by simple finite differences. Direct results are plotted in Fig. 2. The relatively small scatter of both the a,(T) (see Fig. 1) and x&) results gives an idea of the suitability of the differential expansion method to study volumetric behav- iour of real liquids. The oscillating scatter of xT in Fig. 2 can be attributed to the temperature-controlling procedure when a selected isotherm is measured since, although the variation is about 2-3 mK for a given temperature, this amounts to about 0.02 K for the whole experiment. Calculated xT values up to 110 MPa along the 298.15 K isotherm are compared with those of Easteal and Woolf7 up 1.00 1 I I I I I t 0.75 ' I I I I 230 250 270 290 310 T/K Fig.1 Thermal expansion coefficient, up,of mesitylene as a function of temperature calculated by finite differences from the molar density data of Table 2 along the 0.1 (a) and 45 MPa (b) isobars J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Molar density, p/mol dmP3, of liquid mesitylene at different pressures, p/MPa, and temperatures, T/K P P P P P P P P T = 238.15 47.50 7.7644 35.03 7.7 186 21.81 7.6685 8.04 7.6148 45.60 7.7578 33.41 7.7 128 19.69 7.6602 5.99 7.6079 44.32 7.7533 31.81 7.7069 18.09 7.6540 4.74 7.6020 43.00 7.7485 30.1 1 7.7002 16.43 7.6477 3.44 7.5964 41.58 7.7435 28.3 1 7.6932 14.80 7.6412 2.31 7.5918 39.72 7.7368 26.77 7.6873 13.11 7.6349 1.33 7.5876 38.3 1 7.7320 24.86 7.6800 11.31 7.6279 0.38 7.5837 37.00 7.7270 23.41 7.6746 9.48 7.6205 0.10 7.5825 T = 258.15 103.92 7.8203 73.66 7.7228 44.57 7.6225 16.83 7.5 154 102.27 7.8 150 71.71 7.7162 43.01 7.6169 15.09 7.5082 100.53 7.8092 69.62 7.7093 41.21 7.6104 13.18 7.5003 98.76 7.8035 67.5 1 7.702 1 39.43 7.6037 11.30 7.4923 97.22 7.7985 65.52 7.6953 37.69 7.5973 9.62 7.4851 95.27 7.7925 63.92 7.6899 36.07 7.5913 7.71 7.4768 92.03 7.7827 62.20 7.6841 34.25 7.5843 6.29 7.4706 89.76 7.7756 60.49 7.6783 32.56 7.5779 5.07 7.4652 88.23 7.7707 58.60 7.6719 30.85 7.5714 3.90 7.4599 85.98 7.7637 56.81 7.6657 29.10 7.5646 2.90 7.4553 84.00 7.7573 55.15 7.6599 27.33 7.5578 1.91 7.4507 82.30 7.7519 53.49 7.6542 25.59 7.5510 1.22 7.4474 80.43 7.7457 51.66 7.6476 23.90 7.5443 0.38 7.4436 78.62 7.7399 50.05 7.6419 2 1.92 7.5363 0.10 7.4423 77.01 7.7347 48.22 7.6354 20.15 7.5290 75.61 7.729 1 46.3 3 7.6284 18.70 7.5230 T = 278.15 108.21 7.7230 78.38 7.6250 49.32 7.5205 18.87 7.3932 106.38 7.7168 76.72 7.6193 46.54 7.5101 16.99 7.3848 103.63 7.7083 75.02 7.6135 44.94 7.5039 15.39 7.3772 102.13 7.7034 73.19 7.6073 43.21 7.4970 13.69 7.3692 100.50 7.6981 71.31 7.6006 41.51 7.4901 12.02 7.36 13 98.71 7.6922 69.53 7.5943 39.70 7.4828 10.66 7.3548 96.98 7.6867 67.55 7.5874 37.85 7.4748 9.41 7.3490 95.26 7.681 1 65.76 7.581 1 36.21 7.4680 7.98 7.3422 93.50 7.6755 63.22 7.5723 32.10 7.4507 6.85 7.3367 91.68 7.6696 61.50 7.5661 30.85 7.4455 5.56 7.3303 89.93 7.6638 59.80 7.5600 29.3 1 7.4389 4.22 7.3238 87.95 7.6573 58.07 7.5537 27.60 7.43 15 3.16 7.3185 86.17 7.65 13 56.15 7.5466 25.82 7.4237 2.12 7.3 133 84.22 7.6447 54.52 7.5405 24.10 7.41 62 1.14 7.3082 82.05 7.6376 52.83 7.5340 22.39 7.4086 0.37 7.3043 80.18 7.63 12 51.01 7.5270 20.41 7.4000 0.10 7.3029 T = 298.15 104.24 7.6065 76.34 7.5038 45.45 7.3838 15.48 7.2476 105.98 7.6124 74.16 7.4982 43.48 7.3765 14.01 7.2400 104.24 7.6065 72.44 7.49 18 41.59 7.3685 12.63 7.2329 102.27 7.5997 70.94 7.4860 39.80 7.3609 11.21 7.2255 100.72 7.5943 69.29 7.4796 38.12 7.3534 9.87 7.2184 99.02 7.5884 67.49 7.4727 36.19 7.3451 8.48 7.2108 97.35 7.5826 65.94 7.4666 34.49 7.3376 7.12 7.2035 95.36 7.5757 63.28 7.4565 32.73 7.3297 5.77 7.1959 93.21 7.5682 61.37 7.4491 31.00 7.3219 4.32 7.1878 91.34 7.5584 59.50 7.4417 29.24 7.3 139 3.12 7.1813 88.85 7.5528 57.60 7.4340 27.46 7.3058 2.26 7.1764 86.97 7.5450 55.75 7.4266 25.72 7.2974 1.35 7.1714 84.93 7.5402 54.24 7.4205 23.96 7.2890 0.40 7.1659 83.41 7.5325 52.34 7.4 127 22.33 7.28 13 0.10 7.1643 81.54 7.5265 50.60 7.4055 20.67 7.2733 79.9 1 7.5 172 48.75 7.3978 18.89 7.2646 77.70 7.5133 46.9 1 7.3900 17.14 7.2559 to 275 MPa in Fig.3. The agreement between the two sets of m,(P) = ao(P -Psp) -0*50 (1)data is quite good, differences being always within the com- (2)bined uncertainties. Selected values of K= for mesitylene are KA) = KO@ -psp)-0.85 given in Table 5. (3) Testing the Expressions for p, apand + at High Pressures where ao, x0, psp and psp are the parameters to be deter-mined.The parameters pspand pspreflect the divergence pres- The explicit forms of the expressions studied are the following sure and liquid density, respectively, along the along the isothermal paths: pseudospinodal curve. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Values of a#-' for mesitylene at round values of pres- sure, p, and temperature, T T/K 0.1 5 10 20 50 100 -238.15 0.931 0.915 0.899 0.870 -258.15 0.941 0.922 0.904 0.872 0.791 0.696 278.15 0.951 0.930 0.910 0.873 0.785 0.683 289.15 0.963 0.938 0.915 0.874 0.778 0.670 Although an important feature of these expressions is their simplicity, it is maybe even more important that a similar value of psp is obtained from the correlation of experimental a,,, K~ and p data.This provides an excellent thermodynamic test of consistency when data or measurements of several sources are examined. In addition, psp must be consistent with other estimations (i.e. from Fiirth's hole theory using surface tension measurements' '1. Unfortunately, in spite Of the fact that these kind of expressions have been proved to be 0 85 0.75 0.65 c I (II pY 0.55 0 45 0.35 0 20 40 60 80 100 120 PIM Pa Fig. 2 Isothermal compressibility, K~ of mesitylene calculated by , finite differences from the molar density data of Table 3 along the 278.15 (a),258.15 (0)isotherms 298.15 (O), and 238.15 K (A) Table 5 Values of (GPa-') for mesitylene at round values of pressure, p, and temperature, T T/K 0.1 5 10 20 50 100 -238.15 0.543 0.534 0.524 0.506 -258.15 0.596 0.581 0.566 0.538 0.470 0.390 278.15 0.697 0.673 0.650 0.608 0.512 0.408 298.15 0.781 0.752 0.724 0.675 0.563 0.444 0.7 Ro'8 qk 7 0.6 -2 y"? 0.5 --0.4 50 100 150 200 250 300 PI MPa Fig.3 Comparison of extrapolated values of K~ using eqn. (2) with the parameters of Table 6 along the 298.15 K isotherm with those given by Easteal and Woolf up to 275 MPa (Experimentalresults obtained in this work are represented by open circles) valid for a great variety of substances over wide ranges of pressure (as noted above), these functional forms have not yet received theoretical support. The parameters in eqn. (1)-(3) for mesitylene at the four temperatures studied are recorded in Table 6.Eqn.(1) and (2) represent a, and IC~to within 0.01 kK-' and 0.01 GPa-', respectively, while eqn. (3) represents the experimental den- sities to within 0.001 mol dm-3 or better. Since eqn. (3) can be directly obtained by integration of eqn. (2), the values of K~ and psp used in eqn. (3) were the same as those obtained ,from correlation of K~ so only psp was determined from experimental molar densities. The values of psp obtained from Furth's hole theory using surface tension results" are included in Table 6 for compari- son. Surface tension results compiled by Jasper" were used. Available surface tension results from 283 to 373 K were cor- related with the expression r~ (mN m-') = 55.69(1 -T/T,)'.''6,with T, = 637.3 K," which was used to calcu- late pspat different temperatures. As in the case of n-hexane' and 2,3-dimethylbutane,' psp obtained from a,, IC~and surface tension are quite close, which reinforces the ther- modynamic consistency of eqn.(1)-(3), These results together with those of ref. 5 suggest that the extensive use of eqn. (3) (a three-parameter equation of state) for density correlations offers some important advantages over other well known expressions e.g. that of Tait and its modifications thereof: (1) the three parameters have physical meaning, (2) the isothermal compressibility can be readily cal- culated from ico and pspusing eqn. (2), and (3) with a reason- able estimate of psp the minimum amount of data is required to correlate and predict other quantities such as or a, from eqn.(1) and (2). Table 6 Parameters in eqn. (1)-(3) for mesitylene at the four temperatures studied 238.15 10.93 41.23 4.210 -137.7 -159.8 -138.3 258.15 10.35 39.42 4.301 -120.9 -134.4 -127.8 278.15 9.82 39.47 4.269 -106.2 -115.9 -107.2 298.15 9.33 41.12 4.126 -93.7 -105.8 -94.1 ~~~ ~ Divergence pressures pspobtained through the Fiirth hole theory' using surface tension results'8 are included for comparison. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 7 Comparison of the extrapolated values of rcT (GPa-') for Mesitylene at 298.15 and 313.15 K with experimental data' T = 298.15 K T = 313.15 K p/MPa this worka ref. 7 this work ref. 7 ~ 0.1 0.781 0.81, 0.909 0.89, 100 0.444' 0.45, 0.487 0.47, 150 0.3 69 0.376 0.400 0.388 200 0.3 17 0.322 0.340 0.327 2 50 0.279 0.280 0.297 0.280 275 0.263 0.260 0.279 0.259 Direct extrapolation from eqn.(2) with parameters of Table 6. From integration of eqn. (4) (see text). Experimental data from Table 5, not extrapolated. Extrapolation of K~ to Higher Pressures and Temperatures Our experience reveals that when reliable experimental data are used parameters of eqn. (1) and (2) are not very much influenced by the range of pressures or the number of mea- surements considered. This, together with the large range of validity exhibited by these equations, suggest that they may be used for extimating apand KT at very high pressures. Experimental results of Kr for mesitylene were reported by Easteal and Woolf at 298.15 and 313.15 K up to 275 MPa.' Unfortunately, we have not found results for this up at high- pressure for substance and these cannot be derived from Easteal and Woolf's data since only two isotherms were reported.Comparison of experimental results at 298.15 K with those extrapolated from eqn. (2), using the parameters given in Table 6, are recorded in Table 7. Differences between estimated and experimental results are always less than 0.01 kK-', which is always within the estimated uncertainty. For estimating Kr at other temperatures, the following standard thermodynamic relation can be used : Assuming that both a. and psp remain constant in the tem- perature interval 298.15-3 13.15 K, a simple expression (~T(P3J3.15K) -KT(P~~~.~~,J} is obtained.We have found that differences of about 5-10% in these parameters only sig- nificantly effect the low-pressure results, where the ratio p/psp is small. In any case, differences in final calculations do not exceed 2-3% (even at low pressures), which correspond to the estimated experimental uncertainty in most cases. Compari- son of the estimated Kr results at 313.15 K up to 275 MPa is also recorded in Table 7. When similar calculations are performed with results of Tables 5 and 6, differences between calculated and experi- mental values are always found to be less than 0.02 GPa-', giving additional support to the assumed consistency of eqn.(1) and (2) with experimental apand Kr data. Conclusion Accurate molar density measurements obtained with an expansion technique using a differential method have been obtained for mesitylene. These measurements have been used to evaluate the mechanical coefficients apand Kr of this com- pound as a function of pressure and temperature. The overall data have been used to test simple expressions which relate p(p),a,@) and xT(P)with the divergence pressure, psp,along the pseudospinodal curve. The near coincidence of the divergence pressures obtained from the different expres- sions provides a good test of thermodynamic consistency for the correlation method suggested here. The possibility of using the expressions for extrapolation has been tested by comparing extrapolated data with direct measurements at very high pressures.The comparison sug- gests that these expressions can be used to accurately extrapolate density, thermal expansion coefficient and iso- thermal compressibility measurements of normal liquids obtained at relatively low pressures to high pressures. This work was supported by CICYT (M.E.C., Spain), Project NO.: PB92-0553. References 1 Ph. Pruzan, J. Phys. Lett., 1984,45, L-273. 2 C. Alba, L. Ter Minassian, A. Denis and A. Soulard, J. Chem. Phys., 1985, 82, 384. 3 L. Ter Minassian, K. Bouzar and C. Alba, J. Phys. Chem., 1988, 92, 487. 4 R. J. Speedy, J. Phys. Chem., 1982,86,3002. 5 V. G. Baonza, M. Caceres and J. Nuiiez, J.Phys. Chem., accepted. 6 V. G. Baonza, M. Caceres and J. Nuiiez, J. Phys. Chem., 1993, 97, 10813. 7 A. J. Easteal and L. A. Woolf, Int. J. Thermophys., 1985,6, 331. 8 W. B. Streett and L. A. K. Staveley, J. Chem. Phys., 1971, 55, 2495. 9 J. C. G. Calado, P. Clancy. A. Heintz and W. B. Streett, J. Chem. Eng. Data, 1982,27, 376. 10 V. G. Baonza, J. Nuiiez and M. Caceres, J. Chem. Thermodyn., 1989,21, 231. 11 V. G. Baonza, M. Caceres and J. Nuiiez, J. Phys. Chem., 1992, 96,1932. 12 V. G. Baonza, M. Caceres and J. Nuiiez, J. Phys. Chem., 1993, 97, 2002. 13 V. G. Baonza, M. Caceres and J. Nuiiez, Ber. Bunsenges. Phys. Chem., 1992,96, 1859. 14 J. H. Dymond and E. B. Smith, in The Virial Coeflcients ofpure Gases and Mixtures, Clarendon Press, Oxford, 1980. 15 A. E. Hirschler and W. B. M. Faulconer, J. Am. Chem. Soc., 1946,68,210. 16 M. Caceres, V. G. Baonza, J. M. Arsuaga and J. Nuiiez, 76th Canadian Society for Chemistry Conference and Exhibition, Sher-brooke, Canada, 1993. M. Caceres, V. G. Baonza, J. E. F. Rubio, J. M. Arsuaga and J. Nuiiez, Ber. Bunsenges. Phys. Chem., in the press. 17 V. P. Skripov, in Metastable Liquids, Wiley, New York, 1974, p. 226. 18 J. J. Jasper, J. Phys. Chem. Ref: Data, 1972, 1, 841. 19 K. H. Simmrock, R. Janowsky and A. Ohnsorge, in Critical Data for Pure Substances, Chemistry Data Series 11, DECHEMA, Frankfurt. 1986. Paper 3/057 14C; Receioed 2 1st September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000553

出版商:RSC    

年代:1994

数据来源: RSC