摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Editorial Manager Prof. A. Robert Hillman Dr. Robert J. Parker Department of Chemistry The Royal Society of Chemistry University of Leicester Thomas Graham House University Road Science Park Leicester LEI 7RH, UK Milton Road Cambridge CB4 4WF, UK Staff Editor: Dr. R. A. Whitelock Senior Assistant Editor: Mrs. S. Shah Assistant Editors: Dr. G. F. McCann, Miss J. C. Thorn Editorial Secretary: Mrs. J. E. Gibbs Faraday Editorial Board Prof, M. N. R. Ashfold (Bristol) (Chairman) Dr. J. A. Beswick (Paris) Prof. A. R. Hillman (Leicester) Dr. D. C. Clary (Cambridge) Prof. J. Holzwarth (Berlin) Dr. L. R. Fisher (Bristol) Dr. D. Langevin (Paris) Dr.B. E. Hayden (Southampton) Dr. S. K. Scott (Leeds) Prof. J. S. Higgins (London) Dr. R. K. Thomas (Oxford) Dr. R. J. Parker (RSC, Cambridge) (Secretary) International Advisory Editorial Board R. S. Berry (Chicago) R. A. Marcus (Pasadena) A. M. Bradshaw (Berlin) Y. Marcus (Jerusalem) A. Carrington (Southampton) B. J. Orr (North Ryde) G. Cevc (Munich) R. H. Ottewill (Bristol) M. Che (Paris) R. Parsons (Southampton) M. S. Child (Oxford) S. L. Price (London) B. E. Conway (Ottawa) F. Rondelez (Paris) G. R. Fleming (Chicago) D. K. Russell (Auckland) R. Freeman (Cambridge) J. P. Simons (Oxford) H. L. Friedman (Stony Brook) S. Stoke (Amsterdam) H. H. J. Girault (Lausanne) J. Troe (Gottingen) H. lnokuchi (Okazaki) J. Wolfe (Kensington, NSW) J.N. lsraelachvili (Santa Barbara) C. Zannoni (Bologna) M. L. Klein (Philadelphia) R. N. Zare (Stanford) A. C. Legon (Exeter) A. Zecchina (Turin) C. Zhang (Dalian) Journal of the Chemical Society, Faraday Transactions (ISSN 0956-5000) is published twice monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. 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They are designed to be topical articles of interest to a wide range of research scientists in the areas of Physical Chemistry, Biophysical Chemistry and Chemical Physics. Full details of the form of manuscripts for Articles and Faraday Communications, con- ditions for acceptance etc. are given in issue number one of Faraday Transactions, published in January of each year, or may be obtained from the Editorial Manager. There is no page charge for papers published in Faraday Transactions. Fifty reprints are supplied free of charge. Prof. A. R. Hillman, Scientific Editor. Tel. : Leicester (01 16) 2525226 (24 hours) E-Mail (JANET):ARH7@UK.AC.LElCESTER Fax: (01 16) 2525227 Dr. R. J. Parker, Editorial Manager. Tel.: Cambridge (0223) 420066 E-Mail (INTERNET): RSCI @RSC.ORG (For access from JANET use RSCI %RSC.ORG@UK.AC.NSF NET-R ELAY) Fax: (0223) 426017

ISSN:0956-5000    

DOI:10.1039/FT99490FX081

出版商:RSC    

年代:1994

数据来源: RSC

ISSN:0956-5000    

DOI:10.1039/FT99490BX083

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(21) 3205-3375 (1 994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 3205 Non-reactive interaction of ammonia and molecular chlorine : Rotational spectrum of the ‘charge-transfer ’ complex H,N. -C1, A. C. Legon, D. G. Lister and J. C. Thorn 3213 Gas-phase IR spectrum of 7-azaindole. Scaled quantum mechanical force field and complete spectrum assignment E. Cane, P.Palmieri, R. Tarroni and A. Trombetti 3221 Low-temperature kinetics of reactions between neutral free radicals. Rate constants for the reactions of OH radicals with N atoms (103 d T/K < 294) and with 0 atoms (158 < T/K d 294) I. W. M. Smith and D. W. A. Stewart 3229 Interaction potentials and fragmentation dynamics of the Nee -.Br2 complex in the ground and electronically excited states A.A. Buchachenko, A. Yu. Baisogolov and N. F. Stepanov 3237 Theoretical vibrational energies of [Cr0,12-, [Mn0J2-and [FeO,]’- R. J. Deeth and P. D. Sheen 3241 Theoretical study of the electroreduction of halogenated aromatic compounds. Part 3.-0-, rn-and p-dibromobenzenes studied by AM1 and PM3 methods R. Andreoli, G. Battistuzzi Gavioli, M. Borsari and C. Fontanesi 3245 New approach to sensitivity analysis of multiple equilibria in solutions I. Fishtik, I. Nagypal and I. Gutman 3253 Effect of complexation on the excited state of uranyl /3-diketonato complexes: Application of the energy gap law T. Yamamura, S. Iwata, S-i. Iwamaru and H. Tomiyasu 3261 EPR study of NaCl :C0,-and NaCl :SO2-P.D. Moens, S. E.Van Doorslaer, F. J. Callens, F. R. Mae, P. F. Matthys and J. M. D’heer 3267 Time-resolved EPR study of spin polarization and dynamics of F+ centres in additively coloured CaO crystals C. Corvaja, L. Franco, L. Pasimeni and A. Toffoletti 3273 EPR, ENDOR and TRIPLE resonance characterization of three paramagnetic redox stages of 5-methyl-5H-diben-zo[a,LJcycloheptene M. L. T. M. B. Franco and M. C. R. L. R.Lazana 328 1 Mean activity coefficients of NaCl in glucose-water and sucrose-water mixtures at 298.15 K J. Wang, W. Liu, J. Fan and J. Lu 3287 Preferential solvation in acetonitrile-water mixtures. Relationship between solvatochromic parameters and standard pH values J. Barb and V. Sanz-Nebot 3293 Effect of solvent on the reaction of coordination complexes.Part 19.-Base hydrolysis of (a/?S)-(o-methoxybenzoato)-(tetraethylenepentamine)cobalt(m)in aquo-organic solvent media A. N. Acharya and A. C. Dash 3301 Thermodynamics and kinetics of the reaction of copper@) and iron(m) with ultra-small colloidal chalcopyrite (CuFeS,) E. Silvester, F. Grieser, T. W. Healy, D. Meisel and J. C. Sullivan 3309 Reaction of peroxomonosulfate radical with manganese@) in acidic aqueous solution. A pulse radiolysis study J. Berglund, L. I. Elding, G. V. Buxton, S.McGowan and G. A. Salmon 3315 Time-resolved microwave conductivity. Part 1.-TiO, photoreactivity and size quantization S. T. Martin, H. Herrmann, W. Choi and M. R.Hoffmann 3323 Time-resolved microwave conductivity.Part 2.-Quantum-sized TiO, and the effect of adsorbates and light intensity on charge-carrier dynamics S. T. Martin, H. Herrmann and M. R. Hoffmann 3331 X-Ray investigations on liquid-crystalline copolysiloxanes for second-harmonic generation E.Wischerhoff, R. Zentel and H. Fischer 3335 Thermal behaviour and physico-chemical characterization of synthetic and natural iron hydroxyphosphates D. Rouzies, 3. Varloud and J. M. M. Millet 3341 Applications of EPR to a study of the hydrogenation of ethene and benzene over a supported Pd catalyst: Deection of free radicals on a catalyst surface A. F. Carley, H. A. Edwards, B. Mile, M. W. Roberts, C.C. Rowlands, F. E. Hancock and S. D. Jackson 3347 FTIR studies on the selective oxidation and combustion of light hydrocarbons at metal oxide surfaces.Propane and propene oxidation on MgCr,O, E. Finocchio, G. Bum, V. Lorenzelli and R. J. Willey 3357 Oxidative coupling of methane over La,O,. Influence of catalyst preparation on surface properties and steady and oscillating reaction behaviour V. R. Choudhary and V. H. Rane 3367 Comparative IR spectroscopic study of low-temperature H, and CO adsorption on Na zeolites S. Bordiga, E. Garrone, C. Lambed, A. Zecchina, C. 0.Arean, V. B. Kazansky and L. M. Kustov 3373 Book reviews: R. Parsons, M. C. Ball, G. Duxbury, J. C. Whitehead, V. E. Steiner, R. T. Bailey 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)71-2879798 Telecom Gold 84: BUR210 Electronic Mailbox (Intenet) 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/FT99490FP224

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aas,N., 1015 Baur, W. H., 2141 Bruna, P. J., 683 Chevalier, S., 667, 675 Diebler, H., 2359 Abadzhieva, N., 1987 Beagley, B., 2775 Bruque, S., 3103 Chi, Q., 2057 Dines, T. J., 1461 Abbott, A. P., 1533 Beer, P. D., 2931 Brzezinski, B., 843, 1095 Child, M. S., 1739 Doblhofer, K., 745 Abraham, R. J., 2775 Beeston, M. A., 3109 Buchachenko, A. A., 3229 Chiu, S. S-L., 1575 Domen, K., 91 1 Abramowicz, T., 2417 Bell, A. J., 17, 817 Buchner, R., 2475 Chmiel, G., 1 153 Doney, S. C., 1865 Acharya, A. N., 3293 Belton, P. S., 1099 Buckley, A. M., 1003 Cho,T., 103 Dong, S., 2057 Aeby, D., 3129 Benavente, J., 3103 Buemi, G., 121 1 Choi, W., 3315 Donnamaria, M. C., 2731 Afanasiev, P., 193 Bender, B. R., 1449 Bujan-Nhiiez, M. C., 2737 Choisnet, J., 1987 Dore, J.C., 2497 Agren, H., 1479 Bendig, J., 287 Biilow, M., 2585 Choudhary, V. R., 3357 Dory, M., 2319 Aikawa, M., 911 Benfer, S., 2969 Burdisso, M., 1077 Chowdhry, B. Z., 1999 Dossi, C., 1335 Aitken, C. G., 935 Bengtsson, L. A., 559, 2401, Burgard, C., 3077 Christensen, P., 459 Doughty, A., 541 Akanuma, K., 1171 253 1 Burgess, J., 3071 Chung, Y-L., 2547 Douglas, C. B., 471 Akolekar, D. B., 1041 Benko, J., 855 Burget, D., 2481 CiZmek, A., 1973 Downing, J. W., 1653 Alava, I., 2443 Benniston, A. C., 953, 2627 Busca, G., 1161, 1293,3181, Claridge, J. B., 2799 Duarte, M. L. T. S., 2953 Albert, I. D. L., 2617 Beno, B., 1599 3347 Clark, G. R., 3139 Duke, M. M., 2027 Albery, W. J., 1115 Bensalem, A., 653 Busch, T., 2611 Clark, T., 1669, 1678, 1783, Dunford, H.B., 3201 Alcober, C., 2395 Bensch, W., 2791 Buschmann, H-J., 1507 1807, 1808,1809, 1810 Dunmur, D. A., 1357 Aldaz,A., 609 Berbaran-Santos, M. N., Butler, L. J., 1581, 1612, Clegg, S. L., 1875 Dunstan, D. E., 1261 Alfimov, M. V., 109 2623 1613, 1614, 1671,1677, ClCment, R., 2001 Duplgtre, G., 1501 Al-Ghefaili, K. M., 383, Birces, T., 411,2635 1809 Climent, M. A., 609 Duxbury, G., 1357,3373 1047 Bergeret, G., 773 Butt, M. D., 727 Coates, J. H., 739 Dwyer, J., 383, 1047 Ali, V., 579, 583 Berglund, J., 3309 Buttar, D., 1811 Coitiiio, E. L., 1745 Dyke, J. M., 17 Aliev, A. E., 1323 Bernardi, F., 1617, 1669, Buxton, G. V., 3309 Collett, J. H., 1961 Dziembaj, R., 2099 Allegrini, P., 333 1671,1672 Byatt-Smith, J. G., 493 Colmenares, C.A., 1285 Eastoe, J., 487,2497, 3121 Allen, N. S., 83 Berndt, H., 2837 Cabaleiro, M. C., 845 Coluccia, S., 3167 Easton. C. J., 739 Almond, M. J., 3153 Bertran, J., 1679, 1757, Caceres, C., 2125, 3203 Cook, J., 1999 Ebitani, K., 377 Alonso, J. L., 2849 1800,1806 Caceres, M., 1217 Cooney, R. P., 2579 Eder, F., 2977 Alparone, A., 2873 Beutel, T., 1335 Caceres Alonso, M., 553 Cooper, D. L., 1643 Edwards, H. A., 3341 A1 Rawi, J. M. A., 845 Beyer, H. K., 1329 Cairns, J. A., 1461 Cordischi, D., 207 Eggen, B. R., 3029 Amorim da Costa, A. M., Bhuiyan, L. B., 2002 Calado, J. C. G., 649 Conna, A., 213 Eggins, B. R., 2249 689 Bickelhaupt, F., 327, 1363 Caldararu, H., 213,2643 Connier, G., 755 Egsgaard, H., 941 Amoskov, V. M., 889 Bickley, R. I., 2257 Callens, F.J., 2541, 2653, Corradini, F., 859, 1089 El-Atawy, S., 879 Ando, M., 1011 Biczok, L., 411,2635 3261 Corrales, T., 83 El Baghdadi, A., 1313 Andre, J-M., 2319 Bielanski, A., 2099 Calvaruso, G., 2505 Corvaja, C., 3267 El-Basil, S., 2201 Andreoli, R., 3241 Biggs, P., 1197, 1205 Calvente, J. J., 575 Cosa, J. J., 69 Elding, L. I., 3309 Andres, J., 1703,2365 Billingham,J., 1953 Calvo, E., 2395 Costas, M., 1513 Elisei, F., 279 Andrews, S. J., 1003 Bilmes, S. A., 2395 Calvo, E. J., 987 Cottier, D., 1003 Elliot, A. J., 831, 837 Anson, C. E., 1449 Binet, C., 1023 Camacho, J. J., 23 Coudurier, G., 193 Endregard, M., 2775 Antonit, T., 1973 Binks, B. P., 2743 Cameron, B. R., 935 Courcot, D., 895 Engberts, J. B. F. N., 727, Aragno, A., 787 Black, S. N., 1003 Caminati, W., 2183 Coveney, P.V., 1953 1905,2703,2709 Arai, S., 1307 Blackett, P. M., 845 Cammack, R., 2921 Cox, A. P., 2171 Enomoto, N., 1279 Aramaki, K., 321 Blake, J. F., 1727 Campa, M. C., 207 Cox, R. A., 1819 Escribano, V. S., 3181 Aravindakumar, C. T., 597 Blanco, M., 2125,3203 Campelo, J. M., 2265 Cracknell, R. F., 1487 Eustaquio-Rincon, R., 113, Arean, C. O., 3367 Blanco, S., 1365 Campos, A., 339 Craig, P. J., 3153 2913 Asai, Y., 797 Blandamer, M. J., 727, Cant, E., 3213 Craig, S. L., 1663 Ewins, C., 969 Ashfold, M. N. R., 1357 1905,2703,2709 Canosa-Mas, C. E., 1197, Cramer, C. J., 1802,3203 Fan, J., 3281 Asmus, K-D., 139 1 Blaszczak,Z., 2455 1205 Crawford, M. J., 817 Fantola Lazzarini, A. L., Assfield, X., 1743 Blower, C., 919,931 Capitan, M.J., 2783 Crisafulli, C., 2809 423 Attwood, D., 1961 Bocherel, P., 1473 Capobianco, J. A., 755 Crowther, D., 2155 Farhoud, M., 2455 Aveyard, R., 2743 Boddenberg, B., 1345 Caragheorgheopol, A., 213 Cruzeiro-Hansson, L., 1415 Fausto, R., 689,2953 Avila, V., 69 Boesman, E., 2541 Carley, A. F., 3341 Cullis, P. M., 727, 1905, Favaro, G., 279,333 Axford, S. D. T., 2085 Boggis, S. A., 17 Carlile, C. J., 1149 2703,2709 Favero, L. B., 2183 Baas, J. M. A,, 2881 Bohm, F., 2453 Carlsen, L., 941 Curtis, J. M., 239 Favero, P. G., 2183 Baba, T., 187 Booth, C., 1961 Carrizosa, I., 2783 DAlagni, M., 1523 Favre, E., 2001 Baba, Y., 2423 Borden, W. T., 1606, 1614, Carvill, B. T., 233 Damiani, D., 2183 Fawcett, W. R., 2697 Back, G-H., 2283 1616, 1671, 1673, 1675, Castaiio, F., 2443 Dang, N-T., 875 Feliu, J.M., 609 Badia, A., 1501 1689, 1733, 1734,1735, Castaiio, R., 1227 Danil de Namor, A. F., 845 Fenn, C., 1507 Badri, A., 1023 1743, 1744, 1802, 1807 Castellani, F., 2981 Das, D., 1993 Fernando, K. R., 1895 Bagatti, M., 1077 Bordiga, S., 2827, 3367 Castells, R. C., 2677 Das, T. N., 963 Fierro, J. L. G., 2125,3203 Bailey, R. T., 3373 Bordoni, S., 2981 Castro, S., 1217 Dasannacharya, B. A., 1149 Filimonov, I. N., 219,227 Baisogolov, A. Yu., 3229 Borello, E., 2827 Catalina, F., 83 Dash, A. C., 3293 Finger, G., 2141 Balaji, V., 1653 Borge, G., 1227 Cataliotti, R. S., 1397 Dash, K. C., 2235 Finocchio, E., 3347 Ball, M. C., 997, 3373 Borisenko, V. N., 109 Cavani, F., 2981 Datka, J., 2417 Fischer, H., 3331 Ball, S.M., 523, 1467 Borsari, M., 3241 Cavasino, F. P., 3 11,2505 Davey, R. J., 1003 Fisher, I., 2425 Bally, T., 1615, 1674, 1733, Bottoni, A., 1617 Ceccarani, M. L., 1397 David, G., 2611 Fishtik, I., 3245 1808 Boutonnet-Kizling, M., Cense, J-M., 2015 Davidson, K., 879 Flamigni, L., 2331 Ban, M. I., 1610 1023 Centeno, M. A., 2783 Davies, M. J., 2643 Fleischmann, M., 1923 Baonza, V. G., 553 Bowker, M., 1015 Cevc, G., 1941 De Benedetto, G. E., 1495 Fletcher, P. D. I., 2743 Baonza, V. G., 1217 Bowmaker, G. A., 2579 Chakrabarty, D. K., 1993 de Boer, E., 2663 Flint, C. D., 1357 Barbaux, Y., 895 Bozon-Verduraz, F., 653 Chang, T-h., 1157 de Castro, B., 3071 Fogden, A., 263 Barbero, C., 2061 Bradley, C. D., 239 Charlesworth, D., 1999 Deeth, R. J., 3237 Fontanesi, C., 2925,3241 Barbosa, J., 3287 Bradshaw, A.M., 403 Charlesworth, P., 1073 Defrance, A., 1473 Fornes, V., 213 Barczynski, P., 2489 Branton, P. J., 2965 Chaudhry, M., 2235,2243, Dejaegere, A., 1763 Fowler, P., 2865 Barker, S. A., 1689 Bratu, I., 2325 2683 de Leng, H. C., 2459 Fracheboud, J-M., 1197, Barnes, J. A,, 1709 Braun, B. M., 849 Che,M., 2277 Delhalle, J., 2319 1205 Barthel, J., 2475 Brei, V. V., 2961 Chen, J-S., 429,717 Demeter, A., 41 1,2635 Franci, M. M., 1605, 1740, Barthomeuf, D., 667,675 Breysse, M., 193 Chen, J. S., 2765 Dempsey, P., 1003 1744 Bartl, H., 2791 Briggs, B., 727, 1905, 2703, Chen, K., 3089 Demri, D., 501 Franck, R., 667,675 Bartlett, P. N., 2155 2709 Chen, L., 2467 Deng, N-J., 1961 Franco, L., 3267 Basini, L., 787 Brocklehurst, B., 271,2001, Chen, Y-H., 617 Deng, Z., 2009 Franco, M.L. T. M. B., Bassat, J. M., 1987 2897 Chen, Z., 2931 Denkov, N. D., 2077 3273 Bassoli, M., 363 Brogan, M. S., 1461 Cheng, A., 253 Derrick, P. J., 239 Frank, J., 3201 Battaglini, F., 987 Brown, N. M. D., 1357 Cheng, C. P., 1157 Dewing, J., 1047 Franke, O., 2821 Battistuui Gavioli, G., Brown, R. G., 59 Cheng, Y., 2517 Dheer, J. M., 3261 Freeman, N. J., 751 3241 Brown, S. E., 739 Cherqaoui, D., 97,2015 Diagne, C., 501 Frety, R., 773 Bauer, C., 517 Briickner, A., 3 159 Chesta, C. A., 69 Dickinson, E., 173,2737 Frey, J. G., 17, 817 1 Frostemark, F., 559, 2401, Fujiwara, Y., 1183 Funabiki, T., 2107 Galantini, L., 1523 Gale, J. D., 3175 Gale, P. A., 2931 Gallardo Amores, J. M., 253 1 3181 Harris, K.D. M., 1313, Harris, P. J. F., 2799 Harrison, N. J., 55 Haruta, M., 1011 Haselbach, E., 2481 Hashimoto, K., 1177 Hashino, T., 899 Hashitomi, O., 2423 1323 Iwamaru, S-i., 3253 Iwasaki, K., 121 Iwata, S., 3253 Jackson, S. D., 3341 Jacob, K-H., 2969 Jacobs, W. P. J. H., Jacques, P., 2481 Jain, S. K., 2065 Jakobsen, H. J., 2095 1191 Knozinger, E., 2969 Knozinger, H., 1335 Kobayashi, A., 763 Kobayashi, H., 763 Kobayashi, T., 1011 Koga, N., 1789 Kondo, Y., 121 Kong, Y. C., 2375 Kontturi, A-K., 2037 Ludemann, H-D., 2071 Lui, Y-P., 1735 Luna, D., 2265 Lunelli, B., 137 Luthjens, H., 2459 Ma, J., 1351 Mabuchi, M., 899,1979 MacFarlane, A. J. B., 2511 Machado, V. G., 865 Galvagno, S., 2803,2809 Gameiro, A. P., 3071 Gandolfi, R., 1077 Cans, P., 315,2351 Gao,Y., 803 Garcia, A., 2265 Garcia, B.E., 2913 Garcia, R., 339 Garcia Fierro, J-L., 1455 Garcia-Paiieda, E., 575 Garrone, E., 3367 Gautam, P., 697 Gavuzzo, E., 1523 Gazzano, M., 2981 Geantet, C., 193 Gengembre, L., 895 Gerratt, J., 1643, 1672, Gerry, M. C. L., 2601,302: Getty, S. J., 1689 Ghiggino, K. P., 2845 Giamello, E., 3167 Giglio, E., 1523 Gil, A. M., 1099 Gil, F. P. S. C., 689 Gilbert, B. C., 2643 Gilchrist, J., 1149 Gill, D. S., 579, 583 1673,1801 Hasik, M., 2099 Hatchikian, E. C., 2921 Hattori, H., 803 Hawkins, G. D., 1802,3203 Hayashi, H., 2133 Haymet, A. D. J., 1245 Heal, M. R., 523,1467 Healy, T. W., 1251, 3301 Heath, J. A,, 2931 Heatley, F., 1961 Heenan, R. K., 487,2497, Hefter, G. T., 1899,2475 Heinzle, M. G., 2337 Helmer, M., 31, 395 Hemptenmacher, P., 2753 Hensel, K.D., 3023 Henson, N. J., 3175 Herein, D., 403 Hernanz, A., 2325 Herod, A. A., 1357 Herrero, C. P., 2597 Herrington, T. M., 2085 Herrmann, H., 3315,3323 Herrmann, J-M., 1441 Herzog, B., 403 Heyes, D. M., 1133,1931, 3121 3039 Jakubov, T., 783 Jameel, A. T., 625 Jiinchen, J., 1033,2837 Jancke, K., 2141 Jayakumar, R., 161,2725 Jayasooriya, U. A., 1265 Jeevan, R. G., 2725 Jenneskens, L. W., 327, Jennings, B. J., 55 Jiang, D-z., 1351 Jiang, P-Y., 591 Jiang, P. Y., 93 Jiao, H., 1559 Jobic, H., 1191 Johansson, L. B.-A., 305 Johari, G. P., 883, 1143, John, S. A., 1241 Johnston, R. L., 3029 Joly, H. A., 3145 Jones, M. N., 2511 Jones, R. L., 1819 Jones, V. W., 2061 Jorgensen, W. L., 1727 Jorgenson, W. L., 1735, Joseph, E. 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C., 3089 Varloud, J., 3335 Watanabe, H., 571 Yamaji, M., 533 Tsuchiya, S., 21 19 Vauthey, E., 2481 Waters, M., 727 Yamamoto, K., 3117 iv FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Endowed Lecture Symposium: Simulation of Condensed Matter from First Principles To be held at Cambridge University on 23 November 1994 Further information from Mrs.Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Theoretical Chemistry Group with CCPl Electronic Structure: From Molecules to Enzymes To be held at University College London on 30 November 1994 Further information from Dr. P. J. Knowles, School of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ ~~~ Colloid and Interface Science Group Surface Forces and Probe Microscopy To be held at Imperial College, London on 19 December 1994 Further information from Dr.D. 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Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN V Division Joint Meeting with the Division de Chimie Physique de la Societe' Francaise de Chimie, Deutsche Bunsen Gesellschaft fur Physikalische Chemie and Associazione Italiana di Chimica Fisica Fast Elementary Processes in Molecular Systems To be held at the UniversitC de Lille, France on 16-30 June 1995 Further information from Dr. C. Troyanowsky, Division de Chimie Physique, Laboratoire de Chimie Physique, 11 rue Pierre et Marie Curie, 75005 Paris, France Biophysical Chemistry Group Dynamic Processes in Biophysics To be held at the University of East Anglia on 6-8 September 1995 Further information from Dr. D.C. Clark, Institute of Food Research, Norwich Laboratory, Norwich Research Park, Colney, Norwich NR4 7UA ~~~ ~ Polymer Physics Group Biennial Meeting To be held at the University of Leeds on 6-8 September 1995 Further information from Professor G. R. Davies, Department of Physics, University of Leeds, Leeds LS2 9JT British Carbon Group Carbon '96 To be held at the University of Newcastle upon Tyne on 7-12 July 1996 Further information from Dr. K. M. Thomas, Northern Carson Research Laboratories, The University, Newcastle upon Tyne NE1 7RU THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications Unii zsity 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 Trad tional optical activity measurements such as CD are confined to the visible and near-ultravio'zt 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 participation of biomolecular scientists. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, London W1V OBH.vi THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 100 Atmospheric Chemistry: Measurements, Mechanisms and Models University of East Anglia, Norwich, 19-21 April 1995 Organising Committee: Professor I. W. M. Smith and Dr J. R. Sodeau (Co-chairmen) Dr R. A. Cox Dr J. C. Plane Dr J. Pyle Professor F. Taylor The priority now given by national governments to the study of atmospheric science confirms that our understanding of global climate and compositional changes depends upon measurements in both the laboratory and the field. The data obtained by the experimentalists are then applied by modellers who provide the most significant input into legislative controls on pollution matters.However there have been few opportunities for laboratory and field workers along with the modelling community to attend an "interdisciplinary" discussion in which overall progress in our understanding of specific atmospheric problems is assessed. The object of this discussion is to bring together the researchers in the diverse disciplines that make up atmospheric chemistry so that their individual results and conclusions can be communicated to each other. Some of the key issues to be discussed will include: ozone balances in the atmosphere; heterogeneous processes; the interaction of chemistry and dynamics in determining atmospheric composition and change. Particular reference will be made to the input of data to global models from the use of satellite, airborne and ground-based instrumentation. Contributions are invited for consideration by the Organising Committee covering topics within the area of chemistry, dynamics and modelling in the lower and upper atmosphere.Abstracts of about 300 words should be submitted by 31 May 1994 to: Professor I. W. M.Smith OR Dr R. J, Sodeau School of Chemistry School of Chemical Sciences University of Birmingham University of East Anglia Edgbaston, Birmingham Nonvich B15 2TT, UK NR4 7TJ, UK Full papers for publication in the Discussion volume will be required by December 1994. THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 101 Gels Paris, France, 6-8 September 1995 Organising Committee: Dr J.W. Goodwin (Chairman) Dr R. Audebert Dr R. Buscall Professor M. Djabourov Dr A. M. Howe Professor J. Livage Professor J. Lyklema Professor S. B. Ross-Murphy During the last few years there has been an increase in both theoretical and experimental work on gels as new techniques have been applied to a wide range of gelling systems. Typical of these are gels formed from polymers by both physical and chemical ; interactions as well as gels formed by inorganic and surfactant systems. The meeting will deal with the structure and dynamics : of gels with the latter heading covering both swelling and rheological behaviour. Mixed systems such as polymer/surfactant and poIymer/particle gels will also be discussed. The Discussion will bring together experimentalists and theoreticians interested in different types of gelling systems and encourage them to interact and assess the current scene and provide a benchmark for future developments.Contributions are invited for consideration by the Organising Committee. Titles and abstracts of about 300 words should be submitted by 30 September 1994 to: Dr J. W. Goodwin, School of Chemistry, University of Bristol, Cuntock's Close, Bristol, BS8 ITS, UK Full papers for publication in the Faraday General Discussion 101 volume will be required by May 1995. vii THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 102 Unimolecular Reaction Dynamics Exeter College, Oxford, 19-21 December 1995 Organising Committee: Professor M.J. Pilling (Chairman) Professor J. P. Simons Professor M. S. Child Professor I. W. M. Smith Dr D. C. Clary Professor J. Troe Professor R. Walsh Unimolecular reactions, defined as processes occurring in the ground electronic state, depend on the interplay between microcanonical dissociation or isomerisation and energy transfer. Recent years have seen developments in both experimental and theoretical techniques for probing aspects of such reactions. The Discussion will highlight these latest developments and promote interaction between a potentially wide range of fields. Topics of inclusion are: . Theories of microcanonical reactions: quantum dynamics calculations, quantum resonances, developments beyond RRKM -Experimental studies including dissociation of ions, clusters and Van der Waals molecules; teal time and frequency domain studies; IVR; multi-channel reactions Relaxation and supercollisions .Unimolecular dynamics in condensed phases Contributions are invited for consideration by the Organising Committee. Titles and abstracts of about 300 words should be submitted by 15 January 1995 to: Professor M. J. Pilling, School of Chemistry, University of Leeds, Leeds LS2 9JT. Full papers for publication in the Faraday General Discussion 102 volume will be required by 31 August 1995. ... 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ISSN:0956-5000    

DOI:10.1039/FT99490BP226

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3205-3212 Non-reactive Interaction of Ammonia and Molecular Chlorine : Rotational Spectrum of the 'Charge-transfer' Complex H,N. GI, A. C. Legon," David G. Lister? and Joanna C. Thorn Department of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX4 4QD The ground-state rotational spectra of five isotopomers of the 'charge-transfer' complex H,N- -.CI, have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The complex has C,, symmetry with the nuclei in the order H,N- .CI-CI. A detailed analysis of the CI nuclear quadrupole hyperfine structure in tran- sitions of H31 5N- * -35C12, H315N. 37CI, and H3l5N. . *37CI35CIgave the rotational constant, B,, the centrifu- gal distortion constants D, and D,, , and the nuclear quadrupole coupling constants x(CI,) and x(CI,) (i = inner, o = outer) in each case.Theodistance r(N. * .CI,) was obtained by an r,-type method and an r,-type method and lies in the range 2.73 k 0.03 A. A detailed analysis that allowed for bond shrinkage on isotopic substitution in the 35CI, subunit of H315N..-35CI, gave the r,-type coordinates of Cli and CI, and hence the distance rs (CI-CI) = 2.00 A. This value is very close to that in free CI, and indicates only a slight perturbation of this subunit when the complex is formed. The relatively small intermolecular stretching force constant, k, = 12.71(3) N m-' determined from D, and the weak perturbation of x(Cli) and x(CI,) from the value in free CI,, reinforce this conclusion.The observed difference x(~~CI,) -~(~~cl,)= 13.99 MHz can be interpreted in terms of a trans- fer of 0.064efrom Cli to CI, on formation of H,'5N-..35C12. It seems likely that the molecular interaction is mainly electrostatic in origin and charge-transfer effects are small. The nature of the initial non-reactive interaction of a pair of extent of electric charge redistribution attending complex for- simple molecules that can readily undergo a subsequent mation. mutual chemical reaction is of special interest in chemistry. One way to characterise a complex such as H,N- * -C1,is Ammonia and chlorine are examples of reactive substances through its rotational spectrum.' The spectroscopic proper- whose non-reactive interaction has received attention.Mulli- ties thereby obtained refer to the molecule in isolation and ken, in his theory of electron donor-acceptor interactions,' are therefore more appropriate for comparison with the classified complexes H,N-..X, as being of the n.ao type, results of ab initio calculations. The changes in the C1 nuclear where n signifies dative electrons from a non-bonding pair on quadrupole coupling constant of C1, induced by bringing N while aa indicates that an antibonding o orbital of X, acts NH, up to its equilibrium position in H,N. * eC1, will be of as the electron acceptor. In discussing chemical reactions in particular interest because the coupling constants in the systems such as NH, + I,, he further distinguished between complex give, by simple proportionality, the electric field gra- weak 'outer' complexes H,N- '1, and strong 'inner' com-dients at the nuclei to which they refer.The technique of plexes [H,NI]+I-, the latter postulated as favoured in, for pulsed-nozzle, Fourier-transform microwave (FTMW) spec- example, polar solvents.' The extent of charge transfer in troscopy," with its combination of high sensitivity for com- molecular complexes thereby became a topic of central sig- plexes and high resolution, is particularly appropriate here, nificance. but its application to mixtures of NH, and C1, is made diffi- Originally, Mulliken used the name 'charge-transfer cult by the chemical reaction between the two components. complexes' to describe the very wide range of systems that The reaction between ammonia and chlorine has been fell within the various classes of complex he identified.' Fol- known for many years.It was used, for example, by lowing the demonstration by Hanna3p4 that electrostatic Hofmann' in his classic demonstration of the composition forces made important contributions in at least some cases, of ammonia but the reaction is clearly not simple. Noyes and Mulliken and Persons conceded in 1969 that the extent of Lyon" argued that although the stoichiometry is 8NH3 charge transfer may have been overestimated in weak corn- + 3C1, = N, + 6NH4Cl for the reaction when aqueous plexes such as those in the bnaa class (ie.those with a n-ammonia drops into excess chlorine gas, nitrogen trichloride bonding orbital acting as donor to a o-antibonding orbital as is produced as an intermediate but subsequently decomposes.acceptor) and that a more descriptive name is electron When the gases are mixed under carefully controlled condi- chloramine (NH,CI) is a sig- donor-acceptor complexes. Nevertheless, the charge-transfer tions of relative compo~ition'~ interaction was still viewed as significant in n.ao complex- nificant product while NCl, results if C1, is in excess. To es of the H,N-..X, type, especially when X = I. Some theo- observe the rotational spectrum of H,N.-.Cl, in an retical attempts to gauge the relative importance of the NH,-Cl, gas mixture, such reactions must be inhibited. various contributions (electrostatic, polarisation, exchange, In the work reported here, the ground-state rotational dispersion and charge-transfer) to the interaction energy fol- spectra of several isotopomers of H,N.-.Cl, have been con-observed by using a fast-mixing in our FTMW spec-lowed.6-8 For example, Morokuma and co-worker~~.~ cluded that H3N...C1, is a weak complex in which the trometer.This type of nozzle allows the components to electrostatic and charge-transfer terms make contributions of remain separate until the point of their simultaneous coaxial comparable importance. Others emphasised the importance supersonic expansion. Complexes H,N. --C1,produced in of electrostatics.8 It is clearly desirable to characterise this way achieve collisionless expansion in ca. 10 ps and their H,N. ..C1, experimentally and to seek evidence about the spectra can then be observed while progress along the reac- tion coordinate is temporarily frozen. Molecular properties t Permanent address : Dipartimento di Chimica Industriale, Uni- derived from the spectral analyses with the aid of suitable versita di Messina, Salita Sperone 31, Casella Postale 29, 98166 Sant' models of H,N- .-Cl, allow a reasonably detailed character- Agata di Messina, Italy. isation of this species which complements our identification 14 643.9 14 6U.1 14 644.3 frequency/M Hz Fig.1 Frequency-domain recording of three Cl nuclear quadrupole hyperfine components in the J = 4 t3, K = 0 t0 and K = 1 t1 transitions of H3’’N..-35C12.Points are spaced by 3.90625 kHz and have been joined by straight lines.The stick diagram indicates the relative intensities of the three components calculated by assuming an effective temperature of 300 K for the distribution between the K = 0 and K = 1 states. Observed relative intensities are unreliable because of the very high Q of the Fabry-Perot cavity. For assignment of quantum numbers, see Table 1. of this prototype system in the gas phase reported in a pre- liminary communication. ’ Experimental The ground-state rotational spectrum of H,N- * sC1, was observed with a pulsed-nozzle, FTMW spectrorneterl6 of the Balle-Flygare design’ operating in the conventional con- figuration with the axis of the nozzle perpendicular to the axis of the Fabry-Perot cavity. To avoid a reaction between NH, and Cl,, a fast-mixing nozzle14 was used.Chlorine gas (Aldrich) was flowed continuously through a glass capillary from a reservoir held at a pressure of ca. 200 Torr to give a nominal pressure of CQ. 1 x Torr in the evacuated Fabry-Perot cavity. The outlet of the capillary was coaxial and coterminal with an outer Teflon tube down which was pulsed a mixture of 2% ammonia (Argo International) in argon. The pulses were repeated at a rate of 2 Hz and the gas was held at a stagnation pressure of 3 atm. The two com- ponent gases were thus kept separate until they simulta- neously expanded into the cavity. Complexes H,N. .C1, formed at the interface between the concentric gas flows were then polarised by pulses of MW radiation in the usual way and the subsequent free-induction decay detected as described elsewhere.16 15NH3 (Aldrich) was used in order to J.CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 simplify the hyperfine structure in transitions and make the spectral analysis more straightforward. Three nuclear quadrupole components of the J = 4 +3 transition of H315N. -35C12 are shown in Fig. 1. This illus- trates that no Doppler doubling is observed when the fast- mixing nozzle is used and that individual hyperfine components had a half-width at half maximum of 16 kHz. Accordingly, frequency measurements had an estimated accu- racy of approximately 2 kHz. Results Spectroscopic Analysis The initial search for H,N. -Cl, with isotopically normal NH, and C1, led to the observation of a very rich, rather weak spectrum.The richness arose from the presence of hyperfine splitting generated by the presence of three quadru- polar nuclei (14N and the two Cl nuclei) coupled with the near coincidence in frequency of the hyperfine patterns of the isotopomers H314N. ”Cl 35Cl and H314N- --37Cl ,’Cl that results from the closeness of the inner chlorine nucleus to the dimer centre of mass. This spectrum was not analysed in detail. Instead, the estimated unperturbed centres of two con- secutive J + 1+J transitions were used to obtain B, and this was fitted with a model of CJv symmetry in which monomer geometries were assumed unchanged to give r(N. XlJ. The frequencies of the transitions of the spectro- scopically more tractable species H315N- --C1, were then pre- dicted with this r(N.-C1,) and the model.15 The ground-state rotational spectra of the isotopomers H315N.. .35C135Cl, H3’5N.. .37C135CI and H3’5N.. . ,’Cl ,’Cl were observed near to the predicted frequencies. Each was unmistakably that of a symmetric-top molecule carrying two Cl(I = 3) quadrupolar nuclei on the molecular symmetry axis. Frequencies of the observed nuclear quadru- pole hyperfine components in J + 1+J, K tK transitions having J = 1, 2 and 3 and K = 0 and 1 for the isotopomer H,”N. * -,’Cl are given in Table 1, while the same tran- sitions of the H315N. s. 35Cl 37Cl and H3I5N- -37Cl iso- topomers are collected in Table 2. Transitions having K > 1 were not observed, presumably as a result of collisional depopulation of higher K states in the supersonic expansion. No attempt was made to measure the weak transitions associated with the least abundant isotopomer (H,”N.--j7Cl wl) in the 15NH,-C1, mixture for reasons of cost. For each observed isotopomer, the hyperfine frequencies (as given in Tables 1 and 2) were fitted in an iterative least- squares analysis to give the rotational constant, B,, the cen- trifugal distortion constants, D, and DJK, and the nuclear quadrupole coupling constants, X(C1,) and ~(cl,), where i and o delineate the inner and outer chlorine nucleus, respectively. The Hamiltonian, H = H, -*Q(ClJ :VE(C1J -iQJC1,) :VE(C1,) (1) where H, is the familiar energy operator describing the rota- tion of a semi-rigid symmetric-top molecule, was constructed in the coupled basis Ii+ Z, = I; Z + J = F and diagonalised in blocks of F in the usual way.A convenient source of the matrix elements of the terms -&(Cl,) :VE(C1.J describing the interaction of the C1 nuclear electric quadrupole moment Q(C1,) with the electric field gradient VE(C1,) at nucleus x (x = i or 0)is given in the paper of Keenan et ~1.’~For a prolate symmetric-top molecule, only the component x(C1,) = -eQB2V/Bzzis observable. The residuals from the final cycle of the least-squares fits are given in Tables 1 and 2 while the corresponding sets of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Observed and calculated transition frequencies of H,l5N*..35Cl, 24-1 0 344-34 7287.1206 0.7 1 3 44-3 3 7293.0984 0.9 0 334-12 7308.4248 0.3 1 244-23 7321.0587 0.5 1 234-22 7321.1320 -0.3 0 234-22 7321.2282 -6.2 0 234-23 73 2 1.3703 9.4 0 244-23 7321.435 1 1.o 0 354-34 7326.072 1 0.9 1 354-34 7334.2598 1.8 3+-2 0 354-35 10948.1828 0.8 0 224-02 10953.3150 -1.9 0 234-23 10953.8482 -0.4 1 3 54-3 5 10959.9779 -1.1 0 324-31 10960.2965 -7.7b 1 234-23 10960.3 194 0.9 0 324-12 10966.5867 -0.3 0 334-32 10968.1439 0.8 1 344-34 10968.2712 -0.6 1 334-32 10970.901 1 0.0 1 3 44-3 3 10972.2265 -0.7 0 344-34 10972.4306 0.0 1 324-3 1 10974.6626 0.8 1 23+-22 10974.8323 0.2 0 314-31 10976.7090 0.1 1 124-1 1 10977.7386 -3.1 0 144-13 10978.3364 -1.1 1 354-34 10979.3309 -0.2 0 134-12 10979.3641 8.0b 0 344-33 10980.08 18 -0.7 1 144-13 10980.6039 -1.4 0 034-02 10980.9505 -0.5 1 224-21 10981.0643 -1.2 1 244-23 1098 1.2435 0.0 1 254-24 10981.4404 -0.9 0 244-23 10981.7753 0.2 0 254-24 10982.0335 -1.0 0 234-22 10982.9742 0.7 0 364-35 10984.5937 1.4 0 124-11 10984.6456 -6.1b 1 334-33 10986.6 193 0.3 0 354-34 10987.1324 -0.9 1 3 64-3 5 10987.2401 1.o 1 134-1 2 10987.2753 -1.0 1 034-02 10987.7027 -0.6 1 32t32 10993.5882 2.2 0 324-32 10998.3095 -1.0 0 334-33 10999.2569 0.8 0 22t22 11010.8823 -2.0 44-3 1 13t33 14635.3431 -1.1 1 344-33 1463 5.6670 -0.2 1 33t32 14636.9709 2.7 0 344-33 14637.0522 1.6 1 324-31 14637.0522 -1.7 1 35-34 14638.4578 -0.7 1 244-23 14639.1835 -0.7 1 134-1 2 14639.7238 -1.0 1 154-14 1464 1.0701 0.1 1 234-22 14641.405 1 4.6 1 254-24 14641.4779 0.7 1 264-25 14641.7414 1.3 0 144-13 1464 1.7936 -3.0 0 044-03 14642.2422 1.4 0 254-24 14642.2636 -0.6 0 264-25 1 4642.541 6 0.3 0 354-34 14642.5739 1.9 1 364-35 14642.6163 0.1 0 244-23 14642.8629 -1.0 1 144-1 3 14643.9967 1.4 0 374-36 14644.1 549 -0.3 1 044-03 14644.2074 -2.7 1 374-36 14644.9184 -0.1 0 364-35 14645.7608 0.2 a Av = vOb -vcalc.Omitted from fit, see text. spectroscopic constants are in Table 3. Certain transitions were omitted from the fits because of overlap with stronger transitions nearby which caused uncertainty in their fre- quency measurement. The standard deviations of the fits, included in Table 3, are of the same order as the estimated experimental error and are therefore satisfactory.Attempts to include terms describing the spin-rotation coupling of the C1 nuclei in the fit did not improve the residuals and led to inde- terminate coupling constants. Also included in Table 3 are the quantities x(~'C~,)(~~Q/"'Q) i.e. 37Cl nuclear quadrupole coupling constants scaled by the known ratio'' of the nuclear electric quadrupole moments of 35Cl and 37Cl. We note that for both H,15N. -* 37C1 35C1 and H315N-. .35C1 37C1 the scaled value agrees with that of the appropriate nucleus in the other (H,N, 35Cl 37Cl) isotopomer, indicating that the fitting procedure is in order. We note also that isotopic sub- stitution at one C1 nucleus in H,'5N-. .35Cl 35Cl leaves the coupling constant at the other C1 nucleus essentially unchanged. Rotational constants estimated from the approximate centre of the very rich hyperfine structure in two consecutive J + 1 tJ transitions of the isotopomers H314N..* ,'Cl, and DH214N-..35Cl, are also included in Table 3. A detailed analysis of the species DH214N- ,'Cl, was not made for the -a reasons discussed above in connection with H, 14N. * * 35Cl,. Molecular Geometry and Nature of the Interaction The fact that H,N. ..Cl, is a symmetric-top molecule, coupled with the observed changes in the rotational con-stants, B,, on isotopic substitution in H315N. -.,'Cl 35Cl, is consistent only with a geometry of C,, symmetry with the order of the nuclei indicated, as discussed in more detail in ref.15. There is definite evidence that the interaction between NH, and C1, is not strong and leads to only a weak perturbation of the C1, bond length from its free molecule value. It has been shown" for weakly bound complexes that, in the quad- ratic approximation and with the assumption of rigid sub- units, the centrifugal distortion constant, D,, of a symmetric-top molecule like H,N. --Cl, is related to the intermolecular stretching force constant, k, ,by k, = (16n2pB~/D,)(1-B0/BNH3-B,/BC'Z) The k, values determined from the D, and B, values for each of the three isotopomers, H3I5N--* 35Cl ,'Cl, H315N...37C1 35C1 and H315N. -35Cl 37Cl, when used in eqn. (2) are given in Table 4. The ground-state rotational con- stants, BtH3and Bg'z, of the free molecules NH, and Cl,, which are collected with various other monomer properties in Table 5, were used in the evaluation.20*2' The conclusion from the k, values of H,N.-.Cl, is that the strength of binding, as gauged by the intermolecular stretching force constant, is reduced by about one third from that2' of the hydrogen-bonded species H,N.-.HCl and is similar in mag- nitude to that23 of H,O. * -HCl. On this basis H,N. * eC1, is a rather weak complex and large perturbations of the C1, bond length, for example, on its formation do not seem likely. In principle, the availability of rotational constants for H315N.. -35C135C1, H315N.. -37C135Cl and H3'5N.. . 35Cl 37CI allows the T, coordinates of the chlorine nuclei and hence the rs bond length of Cl,, within the complex, to be determined. The simplest possible approach is Costain's methodz4 which uses ground-state moments of inertia in Kraitchman's equation, a; = AI:/pL, (3) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Observed and calculated transition frequencies of H3”N. .* 37Cl ”Cl and H3l5N*. .”Cl j7CI H315N.. .37C135Cl H315N.. .35c1J7C1 Av/kHz“ v,,,$MHz Av/kHz“J’ 4-J” K I’ F’ 4-I” F” VobJMHz 24-1 1 244-23 73 18.7272 -0.5 7 161.4682 2.0 0 244-23 73 19.1 154 0.7 7 161 S927 0.3 0 35-34 7 3 2 3.2 39 8 0.9 7165.4933 0.6 1 354-34 73 30.4540 -0.5 7172.8433 -1.5 3+-2 3 054-35 10948.5730 -0.2 0 334-32 10966.2462 2.7 10729.4673 -0.6 1 344-34 10966.2907 -2.0 --1 334-32 10968.633 1 -0.3 10731.8987 -1.0 1 344-33 10969.8020 1.5 10733.051 7 1.1 1 234-22 10972.1237 1.7 10735.2769 -0.9 1 124-11 10974.7003 -4.2 10737.9719 1.5 1 324-12 --10738.0843 -0.5 0 034-12 --10738.7826 0.3 0 144-13 10975.26 18 2.3 10739.3684 -0.4 1 354-34 10976.1294 -0.8 107 39.0629 -0.8 0 344-33 10976.8399 -1.3 10740.2699 -0.8 1 144-13 10977.2104 0.7 10741.2269 -1.0 0 134-02 --10741.4326 2.3 0 034-02 10977.5917 -2.5 1 24-23 10977.8440 1.3 10740.3 770 0.5 1 254-24 10977.9680 -1.4 10741.6354 -0.3 0 224-21 10978.3951 3.4 0 244-23 10978.3951 -2.7 10740.6378 -1.0 0 254-24 10978.5685 0.9 10742.0995 0.4 0 214-21 10978.5685 -0.6 0 23-22 10979.4122 1.o 10742.7 169 0.5 0 364-35 10980.8473 0.6 10744.1706 0.6 0 12-11 --10744.4163 -1.7 1 334-33 10746.1700 1.9 0 354-34 10983.1078 -4.5 10746.2270 0.6 1 364-35 10983.1292 3.3 10746.5197 -1.2 1 134-12 10746.5720 -0.7 1 034-02 10983.5564 -1.3 10746.8955 -0.2 0 334-33 10993.8434 1.6 10757.7508 0.0 44-3 1 354-34 14634.2203 -0.5 14318.6330 -0.6 1 244-23 14634.8666 -0.9 143 19.2376 1.6 1 134-12 14635.3431 0.9 1 154-14 14636.4857 -10.5” 1 234-22 14636.8986 2.8 1 254-24 14636.9434 -2.4 0 154-14 14636.7619 -0.2 1 264-25 14637.1232 0.6 14321 3221 -1.1 -1 254-14 -14321S374 -1.6 1 224-21 14637.0522 7.9 0 264-25 14637.9271 0.5 14322.5327 1.3 0 354-34 --14322.3970 -3.5 1 364-35 14637.9271 2.1 14322.0935 2.4 0 374-36 14639.3681 0.2 14323.7828 0.8 1 04co3 14639.3151 -1.3 14323.9225 -0.3 1 374-36 14639.9542 0.4 14324.4256 0.4 0 364-35 14640.8003 -1.9 14325.0259 1.o ~~~ ~~ ~ ~ ~~ a Av = vObs -v,,,~.” Omitted from fit, see text. Table 3 Ground-state spectroscopic constants of five isotopomers of the ammonia-chlorine dimer H315N-..35Cl, 1830.355 1( 1) 1.347(4) 101.13(7) -115.785(7) .-101.794(7) 1.9 ~,15N.. .35C137cl 1790.32625(8) 1.262( 3) 96.67(4) -115.810(6) -80.227(7) 1.3 -101.799* ~~15~...37C135cl 1829.7798( 1) 1.33 7(4) 101.12( 7) -9 1.27( 1) -101.8q1) 2.0 -115.812” H314N.*. 35Cl, 1889.6(3)’ H,D 14N. ..35C12 18 13.9(5)’ ~~ ~ ~~~~~ a The niare the standard deviations of the fits reported in Tables 1 and 2. ~(~~cl)multiplied by the ratio of 35Q/3’Q from ref. 18. Calculated from estimated v, values of the J = 3 4-2 and J = 4 +3 transitions, with D, held at 1.35 kHz.For the species H2D 14N. * .35C12 the reported quantity is ca. (B, + C,)/2. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Molecular properties of three isotopomers of the ammonia+hlorine dimer ~~~ ~ ~~ H3"N.. .35C12 3.789(3) 2.730(3) 12.7 1( 3) H3"N*. .35C1 37C1 3.812(3) 2.73 l(3) 12.74(3)H315N** * 37Cl "Cl 3.761(3) 2.73 l(3) 12.74(3) appropriate to substitution of an atom, i, on the symmetry axis, a, of a symmetric-top molecule to give a change, AIF, in the principal moment of inertia, 1,. In eqn. (3) ,us= AmM/ (Am + M) and is the 'reduced mass' for the substitution. The results for a(C1,) and a(Cl,) when the rotational constants B, (=h/8z21,)from Table 3 are employed in eqn. (3), are given in the second column of Table 6.Physical sense dictates that a(Cli) and a(C1,) must be of opposite sign. The C1, subunit in the zero-point state of H,N. * .C1, will be undergoing an angular oscillation, B, of the type defined in Fig. 2. Since /I,, = COS-~(COS~/I)"' = 7.5" is available (see next section), an estimate of the Cl-Cl bond length in the zero-point state can be obtained from r(C1--1) = (a(C1,) -a(Cl,)>/cos #I,,and this value is also included in Table 6. Given that the value of rs(Cl-Cl) in the free chlorine mol- ecule is 1.9920 A (see Table S), this naive approach leads to the conclusion that the formation of the complex barely changes the chlorine molecule bond length. However, eqn. (3) leads to errors for two reasons. First, it is not well suited to application to weakly bound complexes and in any case a(C1,) Table 6 CI-C1 bond length in H315N.. .35C12, calculated by three methods modified r, method coordinate modifiedb + bond or distance/A rs method" rs method shrinkage' ~~ ~~ ~~~~ ~ 4c13 -0.2109 -0.1899 -0.2050 1.7780 1.7757 1.7766 4cij -4c1j 1.9889 1.9656 1.9882 r(C1-Cl) 2.006 1.9826 2.005 Calculated using eqn.(3). 'Calculated using eqn. (4), with pa, = c~s-~(cos'/3)"' = 7.5" (see text for discussion). 'Calculated as in but allowing the bond to the C1 atom to shrink by 5 x 8, on 37C1substitution (see text for discussion). Fig. 2 Model used to discuss the geometry and internal dynamics of H3N**C1* is small and Costain's method is known to underestimate small coordinates significantly.A more sophisticated approach to r, coordinates of atoms on the symmetry axis of weakly bound complexes like H,N...Cl, takes into account the fact that the C1, subunit undergoes an angular oscillation of the type shown schemati- cally in Fig. 2 even in the zero-point state. The atoms Cli and C1, execute a circular motion in the plane perpendicular to the line between the two subunit mass centres and the mass of each atom is assumed to be spread evenly over the circle. It can then be shown26 in the case of a linear molecule B...Cl, that the Q coordinate of each atom is given by a Kraitchman-like expression a; = (AI:/p,) -(AZf(sin2 /I)/2pL,) (4) where AI: and AIT are changes in the zero-point moments of inertia of the dimer and monomer, respectively, that accom- pany the isotopic substitution.It is assumed in the derivation of eqn. (4) that (sin2 #?)is insignificantly changed by isotopic substitution and that a similar equation to eqn. (4) applies to isotopic substitution of an atom on the symmetry axis of a symmetric-top molecule. We show in the next section that a reasonable estimate of /I,, = COS-~(COS~#?)'j2 is 7.5". When this is used in eqn. (4), the values of a(Cli) and &lo) that result are those given in column 3 of Table 6. Although the change in a(C1,) from the value estimated using eqn. (3) is very small, the smaller coordinate, u(ClJ, becomes consider- ably reduced in magnitude. We note also from Table 6 that r(C1--1) estimated from these modified r, coordinates is now smaller than in free Cl,.If the complex H,N. eC1, is of the n.aa type, electron donation to a a-antibonding orbital should weaken, and therefore presumably lengthen the C1, bond. This unexpected shortening arises, however, from a serious underestimate of the coordinate a(C1,) when either eqn. (3) or (4) is used to estimate it. It is well known that substitution of an atom by a more massive isotope leads to an effective shrinkage of the bonds in which the substituted atom is involved. For example, iso- topic substitution of "C by 13C in CO, leads to a decrease27 in 1, in the zero-point state and hence to an imaginary rs coordinate of C. By assuming that each C-0 bond shrinks by 5 x lo-' A as a result of the isotopic substitution, the correct values AI, = 0 and a(C)= 0 are obtained.To investi- gate the effects of bond shrinkage in H,N--.Cl,, we assume that both r(N.aC1) and r(C1-Cl) shrink by 5 x lo-' A when 37Clreplaces ,%l. This value is the difference in ro between 3'C179Br and 37C179Br, i.e. the shrinkage in BrCl that attends the corresponding isotopic substitution." The reason for using BrCl is that the necessary rotational con- stants are better characterised for this molecule than for C1,. The procedure is to find a value of r(N.*.Cl) for H, "N-. -,'C12 that reproduces the observed B, when unchanged monomer geometries2'V2' (see Table 5) are assumed and use it to calculate B, for H315N.--37C135Cl. The bonds N.-.Cl and Cl-CI are then allowed to shrink and the new value, Bb, for H3"N. -,'Cl ,'Cl calculated.The correction AXF = 1; -1, is then added to Iibs and the corrected moment of inertia used in eqn. (4) to give a(C1,) Table 5 Molecular properties of the monomers NH, and C1, B,/MHz C,/MHz ro/A -= HNHldegrees XoIMHZ 35~135~1 7287.95(60)" -1.99 15( 1)" 1.992od --11 1.7904(38)' 35~137~1 7090.99(60)" -1.991 7( 1)" ---"NH3 297388.12' 187W 1.0156' -107.28b -I4NH3 298 11 5.37' 187W -" Ref. 21. * Ref. 20. 'Calculated from the geometry given in ref. 20. Estimated from the B, values using eqn. (3). Ref. 29. corrected for shrinkage effects. A similar procedure was applied to correct a(Cl,), except that only shrinkage of the Cl-Cl bond was assumed. The results are given in column 4 of Table 6.We note that r(C1--1) is now only very slightly in excess of the free molecule r,-bond length of chlorine. It has to be admitted that the assumption of the same shrinkage in the r(N...Cl) and r(Cli-Clo) distances on iso- topic substitution at Cli is somewhat arbitrary. Fortunately, however, the results given in column 4 of Table 6 are not very sensitive to the shrinkage assumed for r(N.-.Cli). For example, when this shrinkage is taken as zero while a shrink- age of 5 x A is assumed for r(Cli-Clo), the result is r(C1--1) = 1.9917 A. Thus, the arguments in the preceding paragraphs, when taken together, provide some evidence that the geometry of the Cl, molecule is only slightly perturbed when it is incorporated in the weak complex H,N.sC1,. Although the change AZb accompanying isotopic substitu- tion by 14N in H315N-. . 35Cl, is not well determined because of the crude method of estimation of B, for the species H314N.. .35Cl, (see above), it is still possible to use eqn. (4), but replacing 8 by a, to obtain a value for the rs coordinate of 15N in the principal inertial axis system of H,'5N.-.35Cl,. We assume a,, = sin-'(sin2 = 15", a reasonable choice that is justified below and to which the conclusions are rela- tively insensitive. The necessary rotational constants are in Tables 3 and 5. The result is then 4N) = -2.96(3) A, where the relatively large error follows from that in the Bo value of H314N.1 .j5CI,. This coordinate is sufficiently large that the correction for bond shrinkage is negligible compared with the other sources of error.The corresponding value of a(N) -a(C1,) is 2.75(3) 8, but this is the distance between the two rings described by the N and Cli atoms. An estimate of the equilibrium distance r,(N. .C1) (i.e. that corrected for the angular oscillations a and fi),is then r,(N. * .Cl) = Ia(N)I -r(1 -cos a,,) -Ia(C1Jl + r'(1 -cos Pa,) (5) where r and r' are the distances of the N and Cli nuclei from their respective subunit mass centres measured along the molecular symmetry axis and are available from the geomet- ries in Table 5. The value r,(N. -.C1) = 2.74 (3) A is obtained in this way. Once it can be assumed that the NH, and Cl, subunit geometries are not significantly perturbed by formation of H2N...C1,, it is possible to estimate r(N-..Cl) by another route.When rigid subunits are assumed to execute angular oscillations a and pivoted at their mass centres, as described in Fig. 2, and the separation rcmof the mass centres is assumed fixed, it is readily shown that IF of the dimer is given in good approximation by 1: (Ibb) = p(I,?m)1'2+ 31rH3(i+ (COS' a)) + +IrH3(sin2a) + 3ZF'2(1 + (cos2 8)) (6) where IFH3,ZFH3 and Ip2are principal moments of inertia of the monomers (available from the rotational constants20J1-f in Table 5), IF refers to the dimer (Table 3) and ,u = + mNH3). Using pa, = COS-~<COS~(mC12mNH3)/(mC12 /3)"2 = 7.5", the vdues of (r,?,,,)'/2shown in Table 4 are obtained for the three isotopomers of H3l5N.* .C1, investigated. A partial equilibrium distance r(N. * .ClJ (i.e. that corrected for the angular oscillations a and /3) is then given by r(N*. *Cli) = (r:m)1'2 -r' -r (7) t The rotational constant, C, for NH, isotopomers was calculated from the geometry given in ref. 20. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 where r and I' are as defined previously. The results for r(N. -.Cli)so obtained are included in Table 4 and are seen to agree, within experimental error, with the value rs(N. -ClJ = 2.74 (3) A estimated from eqn. (5). The justification of the choice fi,, = 7.5"is made in the next section. The value a*, = 15" is that2, appropriate to H,N...HCl . In fact, the results in Table 6 are not strongly dependent on ct,,, a change of 2" in this angle corresponding to a change of only 0.003 8, in r(N.-*Cli). C1 Nuclear Quadruple Coupling and the Nature of the Interaction The intermolecular interaction in H,N- * .C1, is relatively weak, as established in the previous section by consideration of the force constant, k,, and the small perturbation of the C1-C1 bond length. Further evidence about the nature of the interaction is available from the Cl nuclear quadrupole coup- ling constants. We note from Table 3 that x(Cli) and x(C1,) for H,'5N..-35C12 are changed from the value of the coupling constant in free 35C1, (Table 5), but not greatly.28 This is further evidence of a weak interaction. Several factors contribute to the change in x(C1) on complex formation.First, when NH, is brought up to its equilibrium position along the C1, molecular axis its electric charge distribution will cause x(C1,) to increase in magnitude but x(C1,) to decrease in magnitude from the free molecule value, xo(Cl). This change arises from the additional electric field gradients at the C1 nuclei along the z axis resulting from the response of the Cl, electronic distribution to the electric field and its gradients due to NH,. To model the changes, the various response tensors of Cl, and the electric charge distribution of NH, are required and are being calc~lated.~~ Secondly, the x(C1) will change if there is any charge transfer between H3N and C1, . Thirdly, in the absence of any electri- cal effects, both x(Cli) and x(C1,) in the zero-point state of the complex would be reduced from the free molecule value through the angular oscillation /3 of the C1, subunit (see Fig.2) alluded to in the previous section. The necessary expres- sion in that case would be x(C1,) = +x0(c1)(3 cos2 /3 -1) (x = i or 0) (8) A deconvolution of the various effects contributing to the difference x(ClJ -x(C1,) must await a satisfactory modelling of the response of the C1, electronic distribution to the NH, subunit but some progress is possible with the aid of some recent ab initio calculati~ns.~~~~~These have shown that in molecules such as H,N. -.35Cl, and 14N2. -.HX (X = CN, CSCH) the mean value of the two 35Cl or 14N nuclear quadrupole coupling constants of or 14N, in the equi- librium conformation of the complex differs by less than 1% from the coupling constant of the free molecule 35Cl, or 14N2, even though the inner and outer atom coupling con- stants are changed by several percent.In H,"N. * -,%12, the observed mean value of the zero-point coupling constants is $(x(Cli)+ x(C1,)) = -108.790(7) MHz. If all of the difference of this quantity from the free molecule value,29 x0(35Cl) = -111.7904(38) MHz, is then attributed to the zero-point angular oscillation /3 of the C1, subunit (see Fig. 2), eqn. (8) leads to pa, = COS-~(COS~/3)'/2 = 7.5", which constitutes the justification for the assumption of this value of /la,in the dis- cussions of molecular geometry in the previous section.Another important property can be derived under the assumption that ${x(Cl,) + x(C1,)) is unaffected by the electric charge distribution of the NH, and hence differs from xo(Cl) only through zero-point averaging. The property in question is the mean fractional change,f, in the electric field gradient J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (efg) of the inner and outer Cl nuclei as a result of bringing NH, up from infinity to its equilibrium position in the complex and is given by f = [Xe(Cli) -~e(C1o)l/[~e(C1J+ xe(C1o)I += Cx(Cli) -~(Clo)l/C~(Cli)~(Cl0)l (9) In eqn. (9), xe(C1,) = -eQa2V/di? is the equilibrium value of the C1 nuclear quadrupole coupling constant at the nucleus, x, and -a2V/dz2is the equilibrium efg along the dimer sym- metry axis z.The second equality in eqn. (9) holds in the approximation that the effects of zero-point averaging cancel between the terms in the numerator and denominator. The resulting value off for H,N. . ,'Cl, is 0.064 and indicates only a small electronic charge redistribution on formation of the complex. Discussion The ground-state rotational spectra of five isotopomers of the complex H,N..-Cl, have been observed by the pulsed-nozzle, FTMW technique. The symmetric-top nature of the spectrum and the changes in the rotational constant, B,, of the species H,I5N. --,'Cl, accompanying isotopic substitu- tion at each inequivalent atom in turn are consistent only with a geometry of C,, symmetry in which the atoms lie in the order indicated.The distance r(N...ClJ has been obtained by two different methods, one of the r, type and the other of the ro type, and lies in the range 2.73(3) A. Ab initio calculations lead to values ranging from 2.57 A to 2.93 A for this A detailed analysis that allowed for the effects of bond shrinkage on r,-type coordinates gave a value r,(Cl-Cl) = 2.00 A,establishing that the geometry of the Cl, subunit is only slightly perturbed when the complex is formed. This conclusion is reinforced by the relatively small value of the intermolecular stretching force constant, k, , and the weak perturbation of the C1 nuclear quadrupole coupling constants that attends complex formation. The observed dif- ference ~(Cli) -x(C1,) in fact corresponds to a mean frac- tional change of only f= 0.064 in the efgs at the inner and outer nuclei.This also suggests that the extent of any charge transfer between the NH, and C1, subunits is likely to be small. It has been indicated elsewhere31 that for all complexes B...Cl, so far investigated in detail by rotational spectros- copy, the angular geometry for a given B is identical with that of the corresponding member of the series B. * -HCl. In addi- tion there is a systematic relationship of the distances r(B. . .C1) and of the k, between the two series. This parallel behaviour has been interpreted31 as indicating that the empirical rules3' and the electrostatic model3, used suc-cessfully to describe the series B...HCl also apply to the series Be eC1,.Of course, the simple electrostatic model implies that polarisation of one molecule by the other is unimportant in determining, for example, angular geometry. In fact, the small value of f= 0.064 in the most strongly bound member of the series Be. .Cl, so far investigated (B = NH,) provides evidence of, at most, only small reorganisation of electric charge and hence further support for the electro- static model. The quantity,f, can be interpreted in terms of the extent of electric charge redistribution within the C1, subunit on the basis of the Townes-Dailey if charge transfer from NH, to C1, is assumed negligible. According to the Townes- Dailey model in its simplest form, a transfer of a 3p electron from Cli to C1, to give Cli+.-eC1,-would lead to C1 nuclear quadrupole coupling constants of ~(Cli) = -223 MHz and x(C1,) = 0 MHz. These values in eqn. (9) would then give 3211 0.07-0.06-0.05-0.04-0.03-0.02-I 1 2 4 6 8 10 12 14 kJN m-I Fig. 3 Variation of the fractional change,f, in efg between Cli and C1, on formation of B.. .C1, with intermolecular stretching force constant, for the n.an complexes where B = CO, HF, PH,, HCN and NH, f= 1 and it is obvious thatfis therefore also a direct measure of the fraction of electronic charge transferred from Cli to C1, when Be * aC1, is formed. The result,f= 0.064, for H,N. -.C1, indicates only minor electric charge reorganisation. Fig. 3 shows a plot off vs.k, for all Be -.C1, complexes of the n.ac type so far investigated7 in this way, namely B = CO, HF, PH,, HCN and NH,. We note that f is a monotonically increasing, nearly linear function of the strength of the inter- molecular interaction as measured by k, . Although f is very small in all cases, the polarisation of C1, by B increases as the interaction strength increases, as might be expected. We thank the SERC, MURST (60% funds) and the EC (contract no. ERBCHRX 9301 57) for research grants in support of this work and the Ruth King Trust of the Uni- versity of Exeter for a studentship (for J.C.T.). References 1 R. S. Mulliken and W. B. Person, Molecular Complexes, Wiley-Interscience, New York, 1969 and references therein.2 R.S. Mulliken, J. Phys. Chem., 1952,56,801. 3 M. W. Hanna, J.Am. Chem. SOC., 1968,90,285. 4 M. W. Hanna and D. E. Williams, J. Am. Chem. SOC., 1968,90, 5358. 5 R. S. Mulliken and W. B. Person, J. Am. Chem. SOC., 1969, 91, 3409. 6 K. Morokuma and K. Kitaura, in Molecular Interactions, ed. H. Ratajczak and W. J. Orville-Thomas, Wiley, New York, 1980, vol. 1, ch. 2. 7 H. Umeyama, K. Morokuma and S. Yamabe, J. Am. Chem. SOC., 1977,99,330. 8 I. Rnreggen and T. Dahl, J. Am. Chem. SOC., 1992,114,511. 9 See,for example, A. C. Legon, Chem. SOC.Rev., 1990,19, 197. 10 T. J. Balle and W. H. Flygare, Rev. Sci. Instrum., 1981,52, 33. 11 See, for example, T. M. Lowry and A. C. Cavell, Intermediate Chemistry, Macmillan, London, 1958, ch. 16, p.184. 12 W. A. Noyes and A. C. Lyon, J. Am. Chem. SOC., 1901,23,460. 13 P. W. Schenk, in Handbook of Preparative Inorganic Chemistry, ed. G. Brauer, Academic Press, New York, 1963, vol. 1, section 8, p. 477; H. H. Sisler, F. T. Neth, R.S. Drago and D. Yaney, J. Am. Chem. SOC., 1954,76,3906. 14 A. C. Legon and C. A. Rego, .I.Chem. SOC., Faraday Il'rans., 1990,86,1915. t For a convenient summary off and k, values for B.+ C1, , where B = CO, HF, PH,, HCN and NH,, see ref. 32 and references therein. 3212 15 A. C. Legon, D. G. Lister and J. C. Thorn,J. Chem. Soc., Chem. Commun., 1994,757. 16 A. C. Legon, in Atomic and Molecular Beam Methods, ed. G. Scoles, Oxford University Press, New York, 1993,vol. 2,ch. 9. 17 M.R. Keenan, D. B. Wozniak and W. H. Flygare, J. Chem. Phys., 1981,75,631. 18 A. C. Legon and J. C. Thorn,Chem. Phys. Lett., 1993,215,554. 19 D. J. Millen, Can. J. Chem., 1985,63, 1477. 20 P.Helminger, F. C. DeLucia and W. Gordy, J. Mol. Spectrosc., 1971,39,94. 21 H. G. M.Edwards, D. A. Long and H. R. Mansour, J. Chem. Soc., Faraday Trans. 2,1978,74,120. 22 N.W. Howard and A. C. Legon, J. Chem. Phys., 1988,88,4694. 23 A. C. Legon and D. J. Millen, J. Am. Chem. SOC., 1987,109,356. 24 C. C. Costain, J. Chem.Phys., 1958,29,864. 25 J. Kraitchman, Am. J. Phys., 1953,21, 17. 26 A. Haynes and A. C. Legon, J. Mol. Struct., 1988, lsS, 153. J. CHEM. SOC. FARADAY TRANS., 1994,VOL. 90 27 V. W. Laurie and D. R. Herschbach, J. Chem. Phys., 1962, 37, 1687. 28 Y. Xu,W. Jager, I. Ozier and M. C. L. Gerry, J. Chem. Phys., 1993,98,3726. 29 P. W. Fowler, A. C. Legon and S.A. Peebles, unpublished work. 30 A. C. Legon and P. W. Fowler, 2.Naturforsch., A, 1992,447,367. 31 H. I. Bloemink, K.Hinds, A. C. Legon and J. C. Thorn, Chem. Phys. Lett., 1994,223, 162. 32 A. C. Legon and D. J. Millen, Faraday Discuss. Chem. SOC.,1982, 73,71;Chem. SOC.Reu., 1987,16,467. 33 A. D. Buckingham and P. W. Fowler, Can. J. Chem., 1985, 63, 2018. 34 C. H. Townes and A. L. Schawlow, Microwave Spectroscopy, McGraw-Hill, New York, 1955,ch. 9,p. 234. Paper 4/04066J;Received 4th July, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003205

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3213-3219 Gas-phase IR Spectrum of 7-Azaindole Scaled Quantum Mechanical Force Field and Complete Spectrum Assignment Elisabetta Cane, Paolo Palmieri, Riccardo Tarroni and Agostino Trombetti" Dipartimento di Chimica Fisica ed lnorganica, Universita di Bologna, Wale Risorgimento 4,40136 Bologna, Italy The gas-phase IR spectrum of 7-azaindole has been recorded from 100 to 4000 cm-', using a multipass cell heated to ca. llO°C, and completely assigned using theoretical predictions based on the scaled quantum mechanical (SQM) method. The harmonic force field of 7-azaindole, evaluated at the HF-SCF level using 6-31G** orbitals, is corrected by scaling the force field over a convenient set of internal coordinates. Scaling factors were determined by least-squares fitting of the theoretical to the experimental frequencies of the two parent molecules, pyridine and pyrrole, and their perdeuteriated isotopomers.Our final prediction gives frequencies for 7-azaindole which on average differ from experiment by 18 cm-'. 7-Azaindole (7-A21), is an important bicyclic aza-aromatic molecule: it is iso-electronic to purine and has a close relationship with the nucleic bases adenine and guanine. Its planarity, in the gas phase, has been demonstrated by micro- wave spectroscopy;' additional spectral properties have been determined by electronic spectro~copy.~-~ By comparison, the information available on its vibrational properties is scarce, since only the Raman spectrum in methanol solution hGzeportedby Fuke et aL2 Using a multipass cell heated to ca.llO°C, we have recorded the gas-phase IR spectrum of this molecule, at low pressure, from 100 to 4000 cm-', and in the following sec- tions we assign all fundamentals. The main advantages of the IR gas-phase spectrum over the solution or solid-state spectra are that the symmetry of most vibrational coordi- nates is easily determined from the rotational profiles of the IR bands and that hydrogen-bond formation and other inter- molecular interactions are avoided. For the assignment of the IR bands, we compare the experimental and the theoretical IR spectrum evaluated by the SQM force field method of Pulay and co-workers.6-s We have implemented this method and have recently reported a similar investigation on the related molecule indazole,' of similar complexity ; a few applications to bicyclic molecules have also appeared in the literature."-12 Using Hartree-Fock theory and standard atomic orbital sets, the harmonic force field of the molecule has been first evaluated by ab initio methods.The main limitations of the theoretical description were next removed using empirical scaling factors for the force field, which were determined by fitting to the experiment theoretical IR spectra of simpler, closely related parent molecules evaluated at the same level of theory; pyridine and pyrrole were taken as the natural refer- ence molecules of 7-AZI. With the optimized scaling factors, we make our final prediction of the IR spectrum and, by comparison with the experimental spectrum, assign all funda- ment als.Experimental 7-AZI (mp 105-7°C) was purchased from Aldrich Chem. Co. (98%) and was purified by vacuum sublimation at 50 "C. The gas-phase IR spectrum was recorded on a Bomem DA8 Fourier-transform spectrometer from 100 to 4000 cm- ' using a multipass cell with a 4.8 m pathlength, heated to ca. 110 "C.The spectrum above 450 cm -was measured using a KBr beam splitter and a liquid-nitrogen-cooled MCT detec- tor, with a final resolution ranging from 0.060 to 0.180cm-'. The spectrum from 100 to 650 cm-'was recorded with 3 and 6 pm Mylar beam splitters and a DTGS detector; the sample was heated to ca. 110°C and the final resolution of the spectrum is 0.2 cm-'.At least 256 scans were accumu- lated for each spectrum. The instrumental wavenumber accu- racy was estimated to be 0.01 cm-', by calibration against standard water absorption lines. Using the same experimen- tal apparatus, we have also recorded the polycrystalline spec- trum of 7-A21 as a CsI pellet in the range 100-4OoO cm-'. 7-AZI is a planar asymmetric rotor and the vibrational bands can be classified as A, B, hybrid A/B or C types according to the direction of the vibrational transition moment with respect to the inertial frame. The experimental band-origin wavenumbers are listed in Table 1. The error limits are related only to the uncertainty in the location of the band origins and not to spectral resolution.SQM Vibrational Frequencies and Intensities The HF-SCF equilibrium geometries, the force fields and the dipole moment derivatives of 7-A21 and the parent molecules pyridine and pyrrole, have all been calculated using a stan- dard set of 6-31G**13 orbitals and the suite of computer pro- grams CADPAC, version 514 for quantum chemistry. We followed the procedure described in detail in ref. 9. The set of internal coordinates listed in Table 2, with the atoms labelled as in Fig. 1, was chosen for 7-AZI while the internal coordinates of the two parent molecules, pyridine and pyrrole, were taken from ref. 15 and 16. The scaling factors for the force fields of the parent mol- ecules were determined at the theoretical equilibrium geome- tries, by a Newton-Marquard non-linear fitting procedure : these are listed in Table 3.Compared with those of Xie et our scaling factors for pyrrole differ only marginally 12 I 13 15 Fig. 1 Numbering of atoms in 7-azaindole J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Experimental and computed vibrational spectrum of 7-azaindole vapour IR" calculated transition moment/a.u.' freq. band freq. int. mode /cm -int. shape /cm-' A B C /kmmol-' sym. PED' 216.5(5) m C 221.5 -0.655 18.1 A 63S3, + 22S3,'39 204(30)d 233.5 0.047 0.09 A" 61S3, + 19s38 4-18S3,'38 420.3(3) m B 419.2 -0.242 -0.428 10.2 A' as31 + 13s36 + 1os7'27 456.08(6) s C 461.5 -1.244 65.4 A" 62Sz, + 22s33 + 2os3,'3 7 468.8(1) s C 411.4 -0.984 40.9 A as,, + 36Sz8'36 552.ql)' 549.0 0.001 0.096 0.4 A 68S30'26 580.6(1) w C 567.7 -0.318 4.2 A" 37s34 + 37S32 + 24338'3 5 60511) vw ? 612.5 -0.042 0.1 A" 49s37 + 45s38'34 623.6(2) w A + B 622.1 -0.293 -0.165 4.8 A' 34S31 + 31s36 + 14S35 + 1osz'25 718.1(1) vs C 718.3 1.373 79.7 A" 59sz6 + 27Sz7'33 758.3( 1)' 755.2 0.211 -0.394 8.4 A' 17s30+ 15S, 4-14sz + 4-11s6+ llsl'24 774.29(5) s C 771.7 1.052 46.8 A" 52s2, + 15s~~'32 798.49(5) m C 803.0 -0.359 5.4 A" 49s3z -k 19s37'3 1 869.6(1) w B 888.0 -0.098 -0.229 2.6 A' 45szg + 15sg + 13s31'23 898.24(3) m A 896.1 0.615 0.107 16.5 A' 35s35 + 25s36 + 21s1'2 2 925.q1) m C 873.7 0.108 0.5 A" 72Sz7 + 39sz6'30 944.1(1) m C 948.2 -0.113 0.5 A 59S,, + 46Sz,'29 96035) w C 967.2 -0.115 0.5 A 45S2, + 40Sz, + 34Sz, -12S3,'28 1032/ 1039.0 0.025 0.183 1.4 A 49S, + 18S, + lOS,,'2 1 1063.9(2) w A + B 1058.9 -0.390 0.713 27.9 A' 5os10 + 24s~~+ 11sz0'20 1083.3(2) w A + B 1085.4 -0.032 0.566 13.6 A' 34Sz1 + BSzo + 15s8'19 1115.5(1) w A + B 1108.1 -0.440 -0.120 8.8 A' 28Sl8 + 18S, + 18S,,'18 1200.6(1) w A + B 1192.6 -0.336 -0.146 5.7 A' 2os7 + 19sz9 + 12s1, + 11sz0'17 1252# 1238.6 -0.097 -0.069 0.6 A' 31s3 + 18S17+ 12sz,'16 1284.3(1) s A 1287.5 -0.375 0.087 6.3 A' 25s19+ 16Sg '15 1308.5(5) m B 1269.9 0.451 0.910 43.6 A' 3osz 4-17s3-k loszz'14 1350.3(1) m ? 1358.1 -1.127 0.159 54.7 A 21S17+ 18S1 + 13Sz1'13 1413.3(1) s A + B 1423.5 -0.718 -0.347 26.9 A 22Szz + 21S17+ 20S,, + 14.9,'12 1425.3(1) s A + B 1440.8 -1.196 0.141 61.3 A' 18S,8 + 16Slg + 16S,'11 1495.q2) w A + B 1507.5 0.495 -0.534 22.4 A' 18S5+ 17s18 + 13Sg + 12S17'10 1509.5(5) m B 1528.2 0.092 -0.820 28.8 A' 42s8 + 13Szl + IIS35 + 10s~'9 1579.7(2) m B 1594.4 0.088 0.988 41.6 A 20s1 + us, + 10s6'8 1607.4(2) m A + B 1608.5 -1.073 0.044 48.8 A' 23s6 + 16Ss + 13S19 + 11Szv7 3042(1) w -3054.7 0.471 0.331 14.0 A' 77Sl1 -k l6Sl2'6 3066(2) m -3064.5 0.345 0.663 23.6 A' 79s13 + 14s11'5 3085(2) w -3088.0 0.751 -0.260 26.7 A 78S1, + 13S13'4 3100(2) w -3133.7 -0.138 -0.150 1.8 A 66S14 + 34S15'3 3142.1(3) w A + B 3153.6 0.409 0.137 7.9 A 66S15 + 33S1,'2 3517.5(5) s B 3531.3 0.677 -1.552 121.2 A 1ooS1,'1 " Numbers in parenthesis are uncertanties in units of the last digit.Components of the dipole transition moments in the molecular inertial frame.Only PED contributions 2 10% are quoted. Estimated from microwave spectra.' 'Experimental value from UV absorption spec-tr~m.~f Experimental value from SVLF spectrum.' # Experimental value from polycrystalline IR spectrum of 7-AZI as a CsI pellet. owing to the better quality of our atomic orbital set leading deformation of the pyridine ring. The frequencies evaluated to small modifications in the computed equilibrium from the scaled force field are listed in Table 1. The compari- geometry. With the optimized scaling factors, all experimen- son with the experimental spectrum is satisfactory and is dis- tal fundamental frequencies of pyridine and [2H5]pyridine,17 cussed in detail in the next sections.of pyrrole and [2H,]pyrrole'8 are reproduced with a root- mean-square deviation (RMSD) of 11.5 and 11.9 cm-', Infrared Spectrarespectively. After this work was completed, new IR data became avail- The gas-phase and solid-state spectra of 7-AZI from 450 to able for pyrrole" leading to better estimates of the band 3550 cm-' are compared in Fig. 2. Their intensities differ origins for all fundamentals. The new frequencies differ at mainly in the region from 550 to 650 cm-' and in the region most by 20 cm-' from the values used for the refinement of above 3000 cm-'.The rotational band contours are better the force field and confirm all previous assignmentsI8 of the shown in Fig. 3, where the gas-phase spectrum is plotted fundamentals.These frequencies modify our scaling factors using an expanded scale. The gas-phase and the solid-state only slightly with negligible effects on the calculated IR spec-spectra from 180 to 450cm-'are shown in Fig. 4. trum, so their inclusion was not felt important for the The rotational contours of most bands are easily recog- analysis of the IR spectrum of 7-AZI. nized, apart from a few cases that show strong overlapping or The scaling factors of the parent molecules were next trans- very low band intensity. Although the spectra have been mea- ferred to the corresponding internal coordinates of 7-AZI as sured in vacuum, several very sharp lines are observed in the listed in Table 2. The scaling factor for the C1-C6 stretching gas-phase spectra from 1300 to 2000 cm-' and from 180 to coordinate (see Fig.l), was taken to be equal to that of the 400 cm-', due to the absorption of the residual H20 in the C-C pyridine stretches and that for the butterfly motion of cell and in the interferometer. These are easily distinguished the two rings was assumed to be equal to that of the A, from the 7-AZI bands. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Internal coordinate definition for 7-azaindole and scaling factors applied in the prediction of the vibrational spectrum definition description scale factor 1 C-C stretch for bond common 0.8434 2 3 to both rings C-N stretches (pyridine ring) 0.7523 4 5 C-C stretches (pyridine ring) 0.8434 6 7 8 C-C stretches (pyrrole ring) 0.7825 9 10 C-N stretches (pyrrole ring) 0.8502 11 12 C-H stretches (pyridine ring) 0.8357 13 14 C-H stretches (pyrrole ring) 0.8428 15 16 N-H stretch 0.8053 17 18 C-H rocking (pyridine ring) 0.8310 19 20 C-H rocking (pyrrole ring) 0.8323 21 22 N-H rocking 0.7593 23 C-H wagging (pyridine ring) 0.7709 24 25 26 C-H wagging (pyrrole ring) 0.7521 27 28 N-H wagging 0.9098 29 trigonal deformation 0.8458 (pyridine ring) 30 asymmetric deformation 1 0.8289 (pyridine ring) 31 asymmetric deformation 2 32 33 34 35 t(3, 2, 1, 6) -t(2, 1, 6, 5) + t(1, 6, 5, 4) ~(3,2, 1, 6) -~(1,6, 5, 4) -t(6, 5, 4, 3) + t(5,4, 3, 2) -t(4, 3, 2, 1) +~(6,5, 4, 3) -~(4,3, 2, 1) +2t(5, 4, 3, 2) -t(6, 5, 4, 3) -~(4, 3, 2, 1) 2t(2, 1, 6, 5)-t(3, 2, 1, 6) -t(1, 6, 5, 4) 441, 9-81 + ~0~(1440)~449,8,7)+ 446, 1,911 +cos(72")[&8, 7, 6) + 447, 6, l)] puckering (pyridine ring) (pyridine ring) asymmetric torsion 1 (pyridine ring) asymmetric torsion 2 (pyridine ring) ring deformation 1 (pyrrole ring) 0.7872 0.7345 0.8458 36 37 38 +[I -~0s(144")1[&8, 7, 6) -d47, 6, I)] +cos(144")[z(9, 8, 7, 6) + t(7, 6, 1, 9)] +cos(72")[~(1, 9, 8, 7) + 46, 1, 9, 8)] [COS(144") -~0S(72")][&9, 8, 7) -&6, 1, 9)] t(8, 7, 6, 1) [~0s(144")-~0S(72")1[~(7,6, 1, 9) -t(9, 8, 7, 6)] ring deformation 2 (pyrrole ring) ring torsion 1 (pyrrole ring) ring torsion 2 (pyrrole ring) 0.8264 39 +[1 -~0S(144")][~(6,1, 9, 8) -~(1,9, 8, 7)] 45, 6, 1, 9) -47, 6, 1, 2) butterfly mode ortho-coupling (pyridine ring) meta-coupling (pyridine ring) para-coupling (pyridine ring) 0.8458 0.7642 0.5233 0.5002 ~~~~ ~ ~ n Here r(a,b) is the bond length between atoms a, b; &a, b, c) is the angle abc; a(a, 6, c, d) is the angle between the line passing through atoms a, b and the plane defined by b, c, d; r(a, b, c, 6)is the dihedral angle between planes defined by a, b, c and b, c, d, respectively.Many sequences of the transitions arising from the large amplitude motions are apparent in the strong bands at 216, 456.08,468.8, 718.29, 774.29 and 798.49 cm- '. Vibrational Assignments and Analysis The final comparison of all computed and experimental fre- quencies is shown in Table 1. The RMSD of the 39 funda- mentals is 18 cm-'.We included in the comparison five experimental frequencies obtained from the analysis of microwave' and electroni~~*~ spectra. The global RMSD reduces to 8 cm- if we exclude five fundamentals: v3, v14, vJO, vf6 and v38 whose deviation from the calculated wave- numbers is greater than 20 cm-'.From low to high wave- numbers the following comments are in order. Spectral Region around 220 cm-' Only one band with a C-type rotational contour is observed at 216. In the solid-state spectrum the band of medium inten- sity at 236 cm-' has a shoulder at 232 cm-'.From the inten- sity of the vibrational satellites in the microwave spectrum' and from the shifts of the second moment of inertia, the origin of the butterfly motion has been estimated at 193 k30 cm-' and the out-of-plane bending along the A axis at 204 30 cm-'.In the UV spectrum5 two long sequences, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Optimized scaling factors for pyridine and pyrrole pyridine ~~ pyrrole internal coordinate factor internal coordinate factor stretching C-H stretching C-C stretching C-N rocking C-H wagging C-H A, ring deformation B, ring deformation B, ring torsion A, ring torsion ortho-coupling meta-coupling para-coupling 0.8357 0.8434 0.7523 0.83 10 0.7709 0.8458 0.8289 0.7872 0.7346 0.7642 0.5232 0.5002 stretching N-H stretching C-H stretching C-C stretching C-N rocking N-H rocking C-H wagging N-H wagging C-H ring deformations ring torsions 0.8053 0.8428 0.7825 0.8502 0.7593 0.8323 0.9098 0.7521 0.8458 0.8264 generated by ground-state vibrations of about 200 cm-', have been observed.In conclusion, all evidence points to the presence, in this region, of two fundamentals of A" symmetry. With our scaled force field we predict in this region the out-of-plane butterfly motion at 221.5 cm-' and an asym- metric torsion of the six- and the five-membered rings at 233.5 cm-', with the latter vibration carrying zero intensity. Therefore, based on computed frequencies and intensities we assign the band at 216.5 cm- 'in the vapour spectrum to which corresponds to the butterfly motion. The experimental wavenumber for v38 comes from the microwave spectrum and our theoretical value is within the experimental uncer- tainty.Spectral Region around 450 cm-Here the gas-phase IR spectrum is not easy to interpret. One B-type and three C-type bands are observed at 420.3, 456.08, 468.8, and 475.2 cm- ',respectively. The solid-state spectrum also presents four bands, while the analysis of the single vibrational level fluorescence (SVLF) spectra' places an A' 16 14 12 10 8 6 4 722c. .-co =I 3550.0 2775.0 fundamental at 432 cm-'. The latter value differs by more than 10 cm-' from the wavenumber of the corresponding IR band. With SQM we predict three fundamentals in this region: two strong of A" symmetry, at 411.4 and 461.5 cm-', and an A' of medium intensity at 419.2 cm-'. The assign- ments reported in Table 1 are based on the comparison between computed and experimental intensities and band contours.Spectral Region around 600cm-' We predict four weak fundamentals in this region: three of them are assigned to weak features in the gas spectrum (see Fig. 3). The only plausible assignment for the A' fundamental corresponding to an asymmetric deformation of the six-membered ring and calculated at 549.0 cm-' with very low intensity, is the hot band at 552.0 cm-' observed in the UV absorption ~pectrum.~ Spectral Region 700-970 cm' ' The most intense fundamentals are observed in this part of the spectrum. The strong C-type bands at 718.1 and 774.29 cm-' are assigned to the C-H wagging vibrations estimated at 718.3 and 771.7 cm-', respectively. Three additional C-H wagging vibrations of A" symmetry are calculated at 873.7, 948.2 and 967.2 cm-' with weak intensity.We favour their assignments to the weak and the medium C-type bands at 925.0, 944.1 and 960.5 cm-', respectively. Owing to the large difference between the computed and the experimental fre- quency of the first band (ca. 50 cm-') the alternative assign- ment of this fundamental to the very weak band at 853.9 cm-' has been considered: this would give better agreement between computed and experimental frequencies but would leave unassigned the 925.0 cm- 'band, for which no plausible frequency combinations have been found. Moreover, the presence in the spectral region 1610-2000 cm-' of com-bination bands involving the 925.0 cm- frequency supports the assignment of this band as a fundamental.With this 2000.0 1225.0 450.0 3550.0 2775.0 2000.0 1225.0 450.0 wavenumber/cm-' Fig. 2 Comparison of two IR spectra of 7-azaindole from 450 to 3550 cm-': (a) polycrystalline phase, resolution 0.5 cm-';(b) vapour phase, resolution 0.18 an - J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3550.0 3356.3 3162.5 2968.8 2775.0 ~ ~ ~ ~~~~~~~ nv) C.-t 8-g 6-Y s 4-c..-v) C5 2-.-0 1 1 I 2000.0 1806.3 1612.5 1418.8 1225.0 Oi I I I I I 1225.0 1031.3 837.5 643.8 450.0 wavenumber/cm-Fig. 3 Expanded plot of the vapour-phase IR spectrum of 7-azaindole from 450 to 3550 cm-'(resolution 0.18 cm-') 5f I I I I 450.0 382.5 315.0 247.5 180.0 wavenurnber/cm -' Fig.4 Comparison of the two IR spectra of 7-azaindole from 180 to 450 cm-':(a) polycrystalline phase, resolution 0.5 cm-';(b) vapour phase, resolution 0.20cm-' 3218 assignment, the band at 853.9 cm-' is then assigned to the combination ~14-v37 (1308.5 -456.08 = 852.4 cm- '). The C-type band of medium intensity at 798.49 cm-' is assigned to the six-membered ring-puckering-motion coordinate, v3 ', which is calculated at 803.0 cm- '. Three additional A' funda-mentals, mainly deformation modes of both rings, are calcu- lated at 755.2, 888.0 and 896.1 cm-'. Only the highest frequency mode is clearly identified in the IR spectrum as due to the pure A-type profile of the band at 898.24 cm-', in agreement with the computed transition moment.The weak B-type band at 869.6 cm-' corresponds to the fundamental calculated at 888.0 cm -'. The low-frequency band, probably hidden under the intense C-type bands in this region, is not observed in the IR spectrum: the hot band at 758.5 cm-' in the UV absorption spectrum' is a likely candidate for this assignment. Spectral Region 1040-1210 cm" Five weak A' fundamentals are predicted in this interval, with mainly C-C stretching and C-H rocking character. Four of these fundamentals are easily identified from their intensities and rotational profiles. The missing fundamental, calculated at 1039.0 cm- ' with a very weak intensity, is identified with the band at 1032 cm- ' in the SVLF spectrum.' Spectral Region 1250-1610 cm" As in the preceding region, only totally symmetric fundamen- tals are observed here: three out of nine have strong, and the others medium, intensities, with main contributions from the C-C stretching, C-H rocking, C-N stretching and N-H rocking coordinates.In the theoretical spectrum five out of ten bands are computed to be strong and the others are of medium-strong intensity. Only for the weakest of these fun- damentals, calculated at 1238.6 cm- ',were we unable to find a correspondence in the IR spectrum but the spectrum of the solid reveals a plausible candidate for this fundamental at 1252 cm-'. For the assignment of the bands at 1284.3 and 1308.5 cm-'as v15 and ~14we note that only the rotational contours of these bands agree well with our prediction and that the total theoretical intensities and the frequency of v14 show larger deviations (38 cm-') from the experimental values.We are aware that, for a molecule of this complexity, an assignment based only on the observed contour could be incorrect since the rotational contour may be distorted by perturbations,20 and therefore we cannot entirely exclude the inverse ordering of the two bands. Spectral Region 1610-2000 cm-' In this part of the spectrum many combination and overtone bands are observed with weak intensity. In Table 4 we list the experimental wavenumbers with the proposed assignments. Spectral Region 3040-3520 cm-* Six fundamentals are observed in this region: five C-H stretching, three of which are from the six-membered ring and two from the five-membered ring and the N-H stretch-ing.The three fundamentals of the pyridine ring form a com- posite band, where discontinuities in the profile can be identified as Q-branches of A-type bands. Both the C-H stretching bands of the five-membered ring are weak and only that on the high-frequency side, observed at 3142 cm-' and calculated at 3154.7 cm-', is easily identified and as- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Experimental wavenumbers and assignments in the spec- tral region 1610-2000 an-' ~~~~~~~~ ~~~ frequency band /cm-' intensity shape assignment 1623-8(4) vw A + B ~30 (925.0 + 718.1 = 1643.1)+ ~33 1652.2(5) w A + B vZ9 + v~~ (944.1 + 718.1 = 1662.2) 1705.1(5) w ? vZ9 + v~~ (944.1 + 774.29 = 1718.1) 172q 1) vw ? ~28 (960.5 + 774.29 = 1734.8)+ ~32 1856.9(5) w A + B 2v3, (2 x 925.0 = 1850.0) v~~ + vZ9 (925.0 + 944.1 = 1879.1) 1886.5(5) w B 2v2, (2 x 944.1 = 1888.2) 19 19( 1) w A + B 2vZ8 (2 x 960.5 = 1921.0) signed as v2.The experimental wavenumber of the remaining C-H stretching fundamental, v3, is apparently observed as an inflexion at 3100 cm-', with 30 cm-' difference from the calculated value. Owing to the low intensity computed for this band we take this assignment as tentative, as all the other five X-H stretching fundamentals appear to be very well predicted by theory. The N-H stretching is one of the strongest fundamentals: it is of B type, observed at 3517 cm-' and calculated at 3531 cm-'.Conclusions The gas-phase IR spectrum of 7-azaindole has been recorded from lo0 to 4000cm-'. Spectral assignments are based on the SQM method: using a convenient set of internal coordi- nates, the scaling factors for the theoretical force field were determined by least-squares fitting of the theoretical to the experimental frequencies of the parent molecules pyridine and pyrrole and their perdeuteriated isotopomers. The final RMSD of the observed and calculated fundamen- tals is 18 cm-', better than that obtained for indazole' using the same approach. In particular the deviations of the C-H and N-H stretching frequencies are 10 and 14 cm-', respec-tively, while for indazole they were 30 and 50 cm-'; the dis- crepancies for the bands v36 and v37 corresponding to the N-H wagging motion are 60 and 5 cm-', while for indazole the deviation was greater than 100 cm-'.If we exclude from the comparison the vj , v14, ~30,v36 and v38 fundamentals, whose frequencies differ from the calculated values by more than 20 cm-', the RMSD reduces to 8 cm-'. These results confirm the validity of the SQM method for spectral IR assignments of complex molecules. Financial support from CNR within 'Progetto Nazionale di Informatica Chimica' and 'Progetto Calcolo Avanzato in Chimica', MURST, and from the EEC under the 'Human Capital Mobility Program' (contract N. ERBCHRXCT93- 01 57) is gratefully acknowledged. References W. Caminati, S. Di Bernard0 and A. Trombetti, J.Mol. Struct., 1990,223,415. K. Fuke, H. Yoshiuchi and K. Kaya, J. Phys. Chem., 1988, 88, 5840. K. H. Hassan and J. M. Hollas, J. Mol. Spectrosc, 1989, 138, 392. S. K. Kim, and E. R.Bernstein, J. Phys. Chem., 1990,94,3531. E. Canb, F. Giuliani and A. Trombetti, in XIII Colloquium on High Resolution Molecular Spectroscopy, ed. Cooperativa Uni- versitaria Studio Lavoro, Bologna, 1993, Poster communication H28. P. Pulay, G. Fogarasi, G. Pongor, J. E. Boggs and A. Vargha, J. Am. Chem.SOC.,1983,105,7037. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3219 7 8 9 10 G. Fogarasi and P. Pulay, in Vibrational Spectra and Structure, ed. J. R. Dung, Elsevier, New York, 1985, vol. 14, p. 125. G. Fogarasi and P. Pulay, J. Mol. Struct., 1986, 141, 145. E. Cane, P. Palmieri, R. Tarroni and A. Trombetti, J. Chem. Soc., Faraday Trans., 1993,89,4005. H.Sellers, P. Pulay and J. E. Boggs, J. Am. Chem. SOC., 1985, 15 16 17 18 (a) G. Pongor, P. Pulay, G. Fogarasi and J. E. Boggs, J. Am. Chem. Soc., 1984, 186, 2765; (b) G. Pongor, G. Fogarasi, J. E. Boggs and P. Pulay. J. Mol. Spectrosc., 1985, 114,445. Y. Xie, K. Fan and J. E. Boggs, Mol. Phys., 1986,38,401. K. N. Wong and S. D. Colson, J. Mol. Spectrosc., 1984,104, 129. R. Navarro and J. M. Orza, An. Quim., Ser. A, 1983, 79, 557; 11 12 13 107,6487. R. Liu, X. Zhou and P. Pulay, J. Phys. Chem., 1992,%, 3669. W. B. Collier, J. Chem. Phys., 1988,88, 7295. W. J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys., 1971, 54,724. 19 20 1983,79,571; 1984,80,59; 1985,81,5. T. D. Klots, R. D. Chirico and W. V. Steele, Spectrochim. Acta, Part A, 1994,50,765. K. N. Wong and S. D. Colson, J. Phys. Chem., 1983,87,2102. 14 R. D. Amos and J. E. Rice, The Cambridge Analytic Derivatives Package, Cambridge, 1987. Paper 4/04094E; Received 5th July, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003213

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3221-3227 3221 Low-temperature Kinetics of Reactions between Neutral Free RadicaIs Rate Constants for the Reactions of OH Radicals with N Atoms (103 < T/K < 294) and with 0 Atoms (158 < T/K < 294) Ian W. M. Smith* and David W. A. Stewart School of Chemistry, The University of Birmingham, Edgbaston, Birmingham, UK B 15 2TT Rate constants have been determined for the reaction between OH radicals and N atoms at temperatures down to 103 K and for the reaction between OH and 0 atoms down to 158 K. Discharge-flow methods were used to generate known steady-state concentrations of N and 0 atoms in a cryogenically cooled flow of gas. OH radicals were produced, in concentrations much smaller than those of the atomic radicals, by pulsed laser photolysis of a small concentration of HNO, introduced into this gas flow, and their first-order kinetic decays were observed using the time-resolved laser-induced fluorescence technique.The rate constants for both reactions increase monotonically down to the lowest temperatures achievable in the present experiments but do not exhibit any simple functional dependence on temperature. It is suggested that expressions of the form: k(T) = Felec(T) (T/298)"k'(298) are used to fit the data on these reactions at temperatures d515 K. Here Fele,(T) is the temperature-dependent ratio of the electronic degeneracy of the lowest potential surface, leading adiabatically from reagents to products, to the product of the electronic partition functions for the reagents, n has the value -0.17 for the N + OH reaction and -0.24 for 0 + OH, and the values of k'(298) are 2.0 x lo-'' cm3 molecule-' s-' for N + OH and 3.7 x lo-'' cm3 molecule-' s-' for 0 + OH, and correspond to rate constants for reaction on the lowest potential-energy surface at 298 K.Because the value of Felec(T)for both reactions is essentially constant for T < 50 K, it is suggested that expressions of the form k(T) =ATB are used in chemical models of interstellar clouds with A = 2.0 x lo-'' cm3 molecule-' s-' and B = -0.17 for N + OH and A = 3.7x lo-'' cm3 molecule-' s-' and B = -0.24 for 0 + OH. This paper reports new kinetic measurements on the reac- laboratory measurements of k, and k, over a similarly wide tions of OH radicals with N and with 0 atoms: temperature range, not least because the factors controlling the temperature dependence of the rate constants of radical- N + OH +NO + H; A,H0(298 K) = -203.8 kJ mol-' radical reactions, especially those between reagents exhibiting (1) considerable electronic degeneracy and near-degeneracy, are 0 + OH -'02+ H; A,H0(298 K) = -70.7 kJ mol-' complex and only partially under~tood.'~ Reactions (1) and (2) are examples of the simplest kind of (2) non-associative reaction between free radicals and therefore Previous direct experiments 1-3 have established the rate con- serve as valuable prototypes of radical-radical reactions in stants for these reactions at temperatures between 220 and general.For this reason and because of their importance in a 515 K.Based on measurements made between 250 and 515 wide variety of environments, they have received considerable K, Howard and Smith' suggested that the temperature attention from theoreticians. There have been numerous cal- dependence of the rate constants of these reactions could be culations of the potential-energy surfaces that are involved in expressed by the functions : both reaction (1)'5-20and reaction (2).21-25 In addition, the dynamics of both reactions have been examined by a variety k,(T)= 5.3 x 10-"(T/298)-0.25 cm3 molecule-' s-' of theoretical methods26 including quasiclassical trajec-7-30k,(T)= 3.8, x 10-"(T/298)-0.50 cm3 molecule-' s-' t~ries,~ quantum scattering calculation^,^' transition state theories,32 statistical adiabatic channel models,33 and In evaluating these results, and those reported by Lewis and adiabatic capture Watson2 and Brune et ~l.,~ In our own laboratory, the for the purposes of atmospheric dynamics of reaction (1) have been investigatedchemistry, Baulch and co-workers have proposed the expres- e~perimentally~~by determining the nascent distribution of sions: the NO product over its energetically accessible vibrational k,(T)= 3.8 x lo-'' exp(85/T) cm3 molecule-' s-' levels.Although the distribution is close to that predicted by statistical phase-space theorie~,~~.~~ doubted if Smith etk2(T) = 2.3 x lo-'' exp(llO/T) cm3 molecule-' s-' this was evidence for reaction via a long-lived complex, and for the temperature ranges 220-500 K for reaction (1) and this conclusion appears to be confirmed by very recent trajec- 250-500 K for reaction (2).The experiments reported in this tory calculations by Guadagnin and S~hatz.'~ Rather, Smith paper extend the kinetic data base on these reactions down to et al. suggested that the tight angular-momentum constraints 103 K in the case of reaction (1) and to 158 K for reaction (2). present in this system might necessarily lead to a distribution The reactions between OH and nitrogen and oxygen atoms close to the phase-space prediction. and the reverse of these two reactions, particularly that A major motivation for the present low-temperature mea- between hydrogen atoms and 0,,play an important role in a surements on reactions (1) and (2) has been our wish to number of complex environments including combustion improve the kinetic data base for neutral-neutral reactions systems,' the atmospheres of the earth and other planets,6 used by those attempting to model the chemistry leading to and interstellar cloud^.^-'^ These environments span a wide molecular synthesis in interstellar clouds.Until very recently, range of temperature and it is therefore important to have there were very few measurements of rate constants for such J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 reactions below 200 K and none below 80 K. Consequently, personal computer signal amplifier modellers have had to rely on rough estimates of the required . rate constants or the results of untested theoretical calcu- lations. Within the last two years, a start has been made towards filling this experimental ~oid.~'*~' Rate constants have been determined for several reactions of CN;' OH41 and CH42 radicals at temperatures as low as 13 K, using a CRESU (Cinetique de Reactions en Ecoulement Super- sonique Uniforme) apparatus in which an ultra-cold gas mixture is prepared by its expansion through a Lava1 nozzle.As in the present experiments, the reaction is then initiated by creating radicals by pulsed laser photolysis (PLP) of a suitable radical precursor and the kinetic decays of the rad- icals are observed by time-resolved laser-induced fluorescence (LIF). Despite the success of this method it is clear that it will be difficult to apply it to those reactions, like (1) and (2), between two unstable free radicals.It is therefore important to obtain rate data for these prototypical radical-radical reactions down to as low a temperature as possible using more conventional techniques. The experimental method adopted for the present measure- ments is based on that employed by Howard and Smith.' The atomic radicals are produced and their steady-state con- centrations established employing standard discharge-flow techniques. The actual kinetic measurements are however made in 'real time'. A small concentration of HNO, is intro- duced into the gas flow downstream from the discharge and is partially dissociated to produce OH radicals by PLP at 266 nm.Subsequent decays in the concentration of OH are then observed using time-resolved LIF. In the experiments reported here, the flowing gas mixture could be cooled, using a variety of cryogens, to temperatures as low as 103 K. Although the design of the cell differed from those used in earlier experiments in our laboratory:' the general principle in the present experiments was the same as before. Experimental Fig. 1 shows a schematic diagram of the apparatus which was employed. The apparatus incorporates the following capabil- ities: (i) The generation of steady-state concentrations of N or 0 atoms in a flowing gas mixture and their estimation by standard discharge-flow techniques. (ii) The creation of a small concentration of OH radicals ([OHIO 4 [Nl or [O]) by PLP of HNO, which was added to the gas flow down- stream from the microwave discharge cavity.(iii) The obser- vation, by time-resolved LIF, of the decay with time of relative concentrations of OH following their creation by PLP of HNOJ. (iv) The variation of the temperature at which experiments were performed from room temperature down to ca. 100 K using a variety of cryogens. The central part of the apparatus was a reaction cell which was constructed from Pyrex. In contrast to the jacketed reac- tion vessels used in other low-temperature measurements in our lab~ratory,~, it was designed in the form of a cross and was similar to the cell used by Smith et a1." in their measure- ments of the NO vibrational state distribution from reaction (1).The main tube, id 40 mm and ca. 50 cm long, carried the gas flow and was evacuated by a high-speed rotary pump (Leybold, Trivac D40B). This tube incorporated a hollow insert through which refrigerant could flow. This insert extended for about 10 cm before and after the observation ,zone at the centre of the cross formed by the main flow tube and the side-arms through which the laser beams entered and left the apparatus. There was a gap of CQ. 3 cm between the two sections of the cooling insert before and after the obser- vation zone, allowing the beams from the photolysis and probe lasers unimpeded access to the gas flow. I W delay generator :!& d flow Ik = 282.5 nm 12 NO Fig. 1 Schematic diagram of the apparatus The flow rate of the gas mixture could be adjusted by manipulating a large stopcock at the downstream end of the flow tube.A balance had to be struck between the require- ments: (i) to minimise atom loss (see below) and condensation of the HNO, on the cold surfaces of the inserts and (ii) to maximise the time for the gas to equilibrate at the tem- perature of the cryogen. In addition, in the present experi- ments where the beams from the photolysis and probe lasers are perpendicular to the gas flow, the linear flow could not be too fast, or transport of OH radicals past the region illumi- nated by the probe laser would contribute a major, non- exponential, loss term to the background decay of the LIF signals.This compromise led us to use linear flow speeds of ca. 2 m s-'. The temperatures which were achieved for par- ticular combinations of cryogen and gas flow at the centre of the cross, where kinetic observations were actually made, could be measured by inserting a thermocouple probe either down the length of the main tube, using the moveable injec- tor, or across the main tube, through one of the side-arms. These measurements indicated that the temperatures were constant to 1 K for & 1 cm about the central axis of the tube and for at least +_2 cm upstream and downstream of the point at which the laser beams crossed the gas flow. Using liquid nitrogen as the coolant, it was possible to reach 103 K, the lowest temperature in the present series of experiments. It is estimated that the temperatures which are cited below should be accurate to +4 K.All gas flows were controlled by mass flow controllers. Nitrogen atoms were generated by passing N2 through a microwave discharge applied in the side-arm leading to the main flow tube. A range of N atom concentrations was pro- duced by altering the power applied to the discharge in the J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 range 25-120 W. The flow, and hence the concentration, of atomic nitrogen was determined by titration with NO.44NO could be added either at a fixed port (shown as point 1in Fig. 1) on the side-arm upstream of the main reaction cell but downstream of the discharge cavity, or through the injector (point 2 in Fig.1) which can be moved along the central axis of the main flow tube. The extent of the rapid titration reac- tion: N + NO -,N, + O(,P) (3) was followed by observing the chemiluminescent emissions from N,, up to the titration end-point, and from NOz, beyond the end-point, using a photomultiplier tube (EM1 9659QAM), placed on the main flow tube downstream from the observation point at the centre of the cross formed by the main flow tube and its side-arms. At the end-point, the emis- sions were extinguished. Reaction (3) was used not only to determine the steady-state concentration of N atoms, but also to convert those N atoms quantitatively to atomic oxygen. Once a known steady-state concentration of N or 0 atoms had been established in the flowing gas, a small flow of HNO, diluted in argon was added just upstream of the first cooled section of the main flow tube.Since these genuine flow tube experiments used quite large gas flows, rather than pre- preparing an HN0,-Ar mixture, a steady flow of argon was bubbled through a 1 :2 mixture of concentrated nitric and sulfuric acids maintained at room temperature. The argon flow was kept as low as possible consistent with obtaining acceptable LIF signals from the OH radicals produced by PLP of the HNO, . A cruciform shape was adopted for the reaction cell for two reasons. First, it means that the time for which the radical precursor and the radical atoms are mixed can be minimised. Although this was unimportant in the present series of experiments, since any reaction between HNO, and N or 0 atoms seems to be very slow, it may become a real difficulty in other radical-radical systems which we plan to study.The second reason is pertinent to the present experi- ments. The present design of the reaction cell incorporates a moveable injector so that atom concentrations can be mea- sured at various points along the flow tube. In the usual con- figuration for our low-temperature experiments:, the beams from the photolysis and probe lasers counterpropagate along the axis of the flow so that a moveable injector cannot be included in the apparatus. Measuring the concentrations of N and 0 atoms close to the reaction zone was important since they were found to decrease down the cooled section of the flow tube.The results of these experiments and visual inspec- tion of afterglows in similar cross-shaped vessels suggests that there is negligible loss of atoms into the essentially static gas occupying the side-arms. These measurements, and the esti- mation of concentrations of atoms in the observation zone, are described separately in the next section of this paper. The photolysis and probe laser beams entered the reaction cell in counterpropagating directions, and across the direc- tion of the gas flow, uia the two side-arms forming the cross. These side-arms were 30 mm id and ca. 30 cm long, and they carried bames to reduce scattered laser light and fluorescence from the Spectrosil windows mounted, at Brewsters angle, at the end of each side-arm.Photolysis of HNO, to produce OH radicals was effected by the frequency-quadrupled output at 266 nm of a Nd: YAG laser (Spectron Lasers, model SL404) which operated at 10 Hz and gave ca. 15-20 mJ per pulse. The probe radiation was provided by a frequency-doubled dye laser (JK Lasers, system 2000) pumped by a frequency-doubled Nd :YAG laser (JK Lasers, DLPY4) which was tuned at ca. 282.5 nm to a rotational line in the (1,O) band of the A ,X+-X,IT system of OH. Fluorescence in the (1, 1) and (0, 0) bands was observed through two filters: a filter which eliminated radiation below ca. 295 nm (UHQ, WG295), and a narrow band interference filter with a bandpass of 10 nm centred at 310 nm (Ealing Electro-Optics, 35-8044).The methods for controlling the delays between the pulses from the two lasers, for gathering and storing the LIF signals from OH and for fitting the decay traces were identical to those described in early papers from this lab~ratory.~, Results Corrections for Atomic Recombination Preliminary experiments, both in the present series and earlier,45 showed that the concentrations of N and 0 atoms decreased as the gas mixture containing them flowed through the reaction cell. This effect increased as the tem- perature was lowered, was greater for 0 atoms than N atoms and could be affected by deposition of HNO, on the cold walls of the inserts carrying the cryogens. As has already been mentioned, to counter this last effect the flow of HNO, was kept to a minimum.Under our experimental conditions, there was no noticeable change in the rate constants for removal of OH which could be attributed to build-up of HNO, on the cold surfaces of the reactor. To illustrate the need to guard against, or allow for, atom losses, the results of two series of experiments exploring such losses between titration point 1 and the reaction zone are shown in Fig. 2. Fig. 2(a) compares the N and 0 atom con- centrations at these points at room temperature and 4 Torr total pressure, with a linear flow speed in the main flow tube of 1.1 m s-'. The greater loss of 0 atoms is clearly evident. Fig. 2(b)compares the loss of 0 atoms in experiments at dif- ferent temperatures using the same mass flow and total pres- sure in each case.The loss of atoms clearly increases as the temperature is lowered. The rate constants for the gas-phase recombinations of N and of 0 and their temperature depen- dence throughout most of the temperature range employed in our experiments, are well established, and hence the extent of atom loss by homogeneous, as distinct from heterogeneous, association could be estimated. Comparison with the mea- sured losses (see below) suggests that gas-phase recombi- nation was insignificant at room temperature, but that it could play a significant role at the lowest temperatures of our experiments. . . 7 E45-y-3,1I / r-----7 -0iii4 01234 [atom] at 1/10i4 molecule ~m-~ [OJat 1/1014 molecute ~m-~ Fig.2 Examples of curves showing the loss of N and 0 atoms between the first titration point (1 on Fig. 1) and the reaction zone. (a) Losses for (a)N atoms and (A) 0 atoms at 294 K and a total pressure of 4 Torr, with a linear velocity in the main flow tube of 1.1 m s-'. (b)Losses of 0 atoms at different temperatures: (a),294 (A), 227 (m) 190 and (V)158 K. The mass flow rate and the total pressure (2.5 Torr) was the same in each case. It should be emphasised that the experimental conditions leading to the results shown in Fig. 2 were chosen to illus- trate that atom losses occurred, not to minimise the effects of those losses. In the 'real' experiments, where OH decays.were measured, conditions were chosen to reduce the effects of atom losses between the discharge and the LIF observation zone below those shown in Fig.2. Thus, to minimise the time for homogeneous and heterogeneous reactions to occur, the linear flow rate was made as large as possible. Despite this precaution, atom losses were still significant. For N atoms, no correction for such loss was strictly necessary, as N + NO titrations could be performed by adding NO, via the move- able injector, directly into the zone where the side-arms cross the main flow tube, as well as at the fixed point 1, a few centimetres downstream from the discharge cavity. In both cases, the chemiluminescent emissions were observed using the photomultiplier tube near the downstream end of the main flow tube.In this way, a comparison could be made, for wide ranges of total pressure and temperature, between the concentrations of N atoms estimated by titration at point 1 and those actually present at the point where time-resolved experiments were carried out. Experiments like these were conducted with the moveable injector in the same position as in the LIF measurements on OH radical decays which are described in the next section of this paper. They enabled the required N atom concentrations to be determined from titra- tions carried out at the fixed titration point 1. The results for N + OH quoted in Table 1 include the results of experiments in which the concentrations of N atoms were estimated by both methods: 'direct' titration in the reaction zone, and titration at point 1 with subsequent correction for atom loss.In only four out of the 36 sets of experiments was the maximum correction for loss of N atoms greater than 16%. In the case of experiments at 103 K, where the losses of N atoms were quite extensive, their concentrations were assessed only by titration with NO added through the injector directly into the reaction zone. The situation in respect of 0 atoms was rather different, and less favourable, partly because the losses were greater and partly because it was less straightfoward to estimate their concentration in the reaction zone. The latter difficulty arose because, in order to ensure complete mixing and hence quan- titative conversion of N to 0 atoms by the reaction zone, it was necessary to add NO to the gas flow several centimetres upstream.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Therefore, in order to estimate the loss of 0 atoms between titration point 1 and the reaction zone, a method was adopted which was based on observing the NO, afterglow emission at the photomultiplier downstream from the reaction zone in two series of experiments. In each of the first set of measure- ments, the N atoms in the flowing gas were titrated against NO which was admitted to the reaction zone through the moveable injector. Then a known, small, excess of NO was added and the intensity of the NO, chemiluminescence was measured, By repeating this process for a number of atom concentrations and with the same excess [NO], a plot was obtained of the concentration of 0 atoms in the reaction zone against the intensity of the NO, emission.Experiments in the second series were similar but now NO was added at the fixed titration point 1, just downstream of the discharge cavity. The titration end-point gave [O] at point 1, whilst the intensity of the NO, afterglow with the same excess [NO] present as before, by reference to the earlier graph of intensity against concentration, gave the con- centration of 0 atoms in the reaction zone. The two sets of experiments enabled us to construct graphs of [O] at point 1 us. [O] in the reaction zone for the ranges of total pressure and temperature used in the kinetics experiments. Only in one case (at 158 K) did the maximum correction for 0 atom loss exceed 35%.At 103 K, the loss of atomic oxygen was too great to make an acceptably accurate correction. Kinetic Measurements on the Reactions of OH with N and 0 Atoms A typical example of the LIF signal decays obtained in the present experiments is shown in Fig. 3. These observed signals were fitted to an exponential decay using a non-linear least-squares fitting program. Inspection of the residuals and autocorrelations, also provided by the computer program, confirmed that the experimental signals were accurately matched by a single-exponential decay function. The pseudo-first-order rate constants (kist) obtained at a given temperature by fitting the experimental LIF signals in the manner just described were plotted against the concentra- tions of the radical atom making due allowance for atom loss. Examples of plots of klst us.[N] and kist us. [O] are given in Fig. 4. The gradients of these plots yield second- order rate constants for reactions (1) and (2). The number of runs and the conditions used for each combination of tem- Table 1 Summary of experimental conditions and rate constants for N + OH and 0 + OH reactions' no. of expts. PI or COl/loi4molecule p/Torr /lo-" k, or k, cm3 molecule-' s-I N+OH 294 14 0.5-3.5 2.5-6.0 5.2& 0.3' 227 5 0.5-2.4 2.0-2.5 6.0& 0.4 190 9 0.6-2.2 1.5-2.5 6.4f0.5 158 5 0.6-2.6 1.5-2.5 7.0 0.6 103 3 0.25-1.5 1 .o 8.0 & 0.8 50 10 -- -- -- (14.0) (10.2)" O+OH 294 11 0.5-2.9 2.5-4.0 4.2& 0.2 227 5 0.5-2.1 2.0-2.5 4.5& 0.3 190 9 0.5-2.0 1.5-2.5 5.2& 0.3 158 3 0.5-2.5 1.5-2.0 6.1 f0.6 103 50 10 --- --- --- (8.O)ll (11.1) (16.0) ~___~ a Values given in brackets are estimated, using the method described in the text.'Weighted average of the individual determinations with errors quoted as single standard deviations. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 01 I1 I I I I 0 60 120 180 240 300 360 time/lO-* s Fig. 3 Example of the decay of LIF signals from OH as the delay time between the pulses from the photolysis and probe lasers was increased. Temperature 103 K, total pressure 1.0 Torr and the con- centration of N atoms 1.2 x loi4molecule perature and radical atom are summarised in Table 1, which also lists the average values of k, and k, at each temperature.The average values of the rate constants at each temperature were obtained by taking a weighted average of the individual values of k,(T) and k,(T) determined from plots of k,,, us. 20 18-16 I-7 14-II,," 12-m 0 z17 10-I:r cc 8--Y.?u 2 , ,"v, ji-", 10.O.O 0.5 1.0 1.5 0 1 2 3 [N]/1014molecule cm-' [O]/lO'" molecule ~rn-~ Fig. 4 Examples of the plots from which values of the second-order rate constants k, and ki were determined. Each panel shows the variation of the pseudo-first-order rate constants for decay in the LIF signal from OH: (a) with the concentration of N atoms, T = 103 K and p = 1.0 Torr; (b) with the concentration of 0 atoms, T = 158 K and p = 1.5 Torr.60 708090100 200 300 400 5001600700 TIK Fig. 5 log-log plot showing the variation of the rate constants for reaction between OH and N atoms with temperature. (0)Experi-mental rate constants observed in the present work and (A)by Howard and Smith,' the line is their recommended fit. (H)Corre-spond to values of the hypothetical rate constants for reaction on the lowest potential-energy surface correlating with reagents and pro- ducts (see text). o&-------- J 100 200 300 400 500 600700 T/K Fig. 6 log-log plot showing the variation of the rate constants for reaction between OH and 0 atoms with temperature. (0)Experi-mental rate constants observed in the present work and (A)by Howard and Smith,' the line is their recommended fit.(m) Corre-spond to values of the hypothetical rate constants for reaction on the lowest potential-energy surface correlating with reagents and pro- ducts (see text). [N] or [O],like those shown in Fig. 4,the weighting factors being inversely proportional to the variances of the individ- ual values. The errors quoted in Table 1 correspond to single standard deviations calculated from the weighted variance of the individual results.48 The unweighted averages of the indi- vidual results and the standard deviations of these results about that mean value are very similar to the results sum- marked in Table 1. Errors in the corrections for atom loss were judged to be small relative to other errors.Moreover, they would show up in differences between the various values of the second-order rate constants determined at a particular temperature but under different flow conditions. Finally, Fig. 5 and 6 display the temperature dependence of the observed rate constants. Where the present results overlap those obtained by Howard and Smith,' the agree- ment is excellent. The dashed lines represent the functions derived by Howard and Smith to match their experimental rate constants and given in the Introduction of the present paper. It is evident that k, and k, continue to increase as the temperature is lowered through the range covered in the present experiments and that the rates of these increases appear to be greater than predicted by the expressions given by Howard and Smith.Discussion The variation with temperature of the rate constants for reac- tions (1) and (2) can be considered to result from two effects. One factor arises from the temperature dependence of the reagent populations in different spin-orbit states, which changes the fraction of collisions which occur on the poten- tial surface across which reaction takes place. In addition, there will be some temperature dependence resulting from the dynamics of the collisions on the reactive potential. Quantum-chemical calculations suggest that, for both (1) and (2), reaction takes place over the lowest potential-energy surface which correlates adiabatically with reagents and pro- ducts.In the case of reaction (1),24 the symmetry of the surface is 3A'';for reaction (2)," the lowest energy surface has 'A'' symmetry. In the absence of non-adiabatic effects, an assumption which appears to be supported by the detailed study of the O(3P)+ OH reaction by Graff and the effect of electronic degeneracy and near-degeneracy in the reagents can be calculated by elementary statistical mecha- nics and allowed for by a factor (FJ,which is simply the ratio of the electronic degeneracy of the surface over which J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 reaction occurs to the product of the electronic partition functions of the reagents; i.e. F,, = gz/ng,,, i. Consequently, the effect of changes in this factor as the temperature is changed can be allowed for by dividing the observed rate constants by the appropriate value of F,, .? This procedure yields rate constants (k; and k;) relating only to those collisions whose dynamics are controlled by the lowest potential-energy surface.We have calculated values of these rate constants and examined their temperature depen- dence as shown in Fig. 5 and 6. Within experimental error these log-log plots are linear so that one can derive the fol- lowing expressions for k, and k,: k,(T)= F,,(T)(T/298)-0.'72.0 x lo-'' cm3 molecule-' s-' k2(T)= F,,(T)(T/298)-0.243.7 x lo-'' cm3 molecule-' s-' Our present results confirm that the rate constants for the reactions between the OH radical and the atomic radicals N(4S) and O(3P) continue to increase as the temperature is lowered, to 103 K in the case of reaction (1) between N atoms and OH, and to 158 K for 0 +OH, and that this tem- perature dependence is not due entirely to variations in the electronic factors F,, .The 'negative temperature dependences' which are observed for k;(T)and k;(T)are con- sistent with rate constants or cross-sections for reaction which are determined by the ability of the intermolecular potential at long and medium range to 'capture' the reagents and bring them into close collisions. It seems likely that, in keeping with other radical-radical system^,^^.^' the crucial part of the intermolecular potential only becomes that arising from the dispersion forces and the non-symmetric charge dis- tributions on the reagents, at very low temperatures indeed. At higher temperatures, the crucial transition-state region moves to shorter inter-reagent separations, where chemical forces start to act and this change contributes to the decrease in the rate constants as the temperature is increased.A major motivation for the present work has been the wish to provide a basis on which reasonably accurate estimates could be made of the rate constants for reactions (1) and (2) under the conditions prevailing in interstellar clouds. We believe that the analysis given above, in which the tem-perature effects due to changes in the fine structure popu- lations are separated from those resulting from the dynamics on the potential-energy surface for reaction, provides such a basis.At Td 50 K, both O(3P)atoms and OH(X 'II) radicals will be predominantly (> 99% and >98%, respectively) in their lowest spin-orbit state, if the populations are thermally equilibrated, so that the factors F,, become temperature inde- pendent and equal to 0.375 for reaction (1) and 0.2 for reac- tion (2) under interstellar conditions (10 < T/K d 50). Therefore, for the purposes of modelling interstellar cloud chemistry, the rate constant can be expressed in the form: k(T)= AT', with A= 2.0 x lo-'' cm3 molecule-' s-', B = -0.17 for the reaction between N atoms and OH, and A= 3.7 x lo-'' cm3 molecule-' s-', B = -0.24 for the 0 + OH reaction. The effect that revised rate constants for neutral-neutral reactions can have on models of interstellar cloud chemistry can be dramatic. Until the mid-l980s, modellers used expres- sions for k,(T) and k,(T) with a positive (T'") temperature dependen~e,'.~which yielded rate constants at 10 K roughly 1/20th of the estimated values which are given in Table 1.~~ ~~ ~ -~ t For OH, g,, is calculated by summing the populations in the rotational levels of the two spin-orbit states and evaluating gel as 2(1 +f')where f' is the ratio of the populations in the upper and lower sets of levels. Since then, values of k,(T) and k2(T)based on the rate expressions given by Howard and Smith' have been incorp- orated into a number of model^.^-'^ Here, we cite three examples of the results of such revised calculations. First, based on Howard and Smith's rate constant for reac- tion (l), Tarafdar and Dalgarno' argue that the reaction between N atoms and OH is the principal source of NO in diffuse interstellar clouds, although they note that the con- centrations of NO estimated on this basis are in poor agree- ment with those inferred from observations on the Ophiuchi cloud.Secondly, Nercessian et ~1.'~have estimated that the number density of OH in a molecular cloud near the star HD-29647 drops by an order of magnitude when rate constants are used for reaction (1) based on the expression of Howard and Smith.' This change improves the agreement with observation. Finally, we point to a paper by Davidsson and Stenh~lm.~~ In addition to using classical trajectory methods, an extended Langevin model and Howard and Smith's experimental results in order to estimate values of k2(T) in the temperature range appropriate to interstellar clouds, they discussed the implications for interstellar chem- istry of larger low-temperature rates for the 0 + OH reac-tion, particularly problems associated with the non-detection of 0, in interstellar clouds.Although the present experimental results and the pro- posed method of extrapolation provide a reasonable basis for the estimated rate constants which are given in Table 1, there is, of course, no substitute for direct laboratory measure- ments at the ultra-low temperatures of interstellar clouds.At present, the best hope for such measurements would seem to lie with the CRESU rneth~d.~'However, application of this technique to reactions involving two unstable radical species is likely to prove very difficult, as it will be necessary to gen- erate one of the radicals in a large known concentration. Until such time as results from direct measurements at ultra- low temperatures become available, we suggest that the rate constants estimated in this paper should be employed in astrochemical models. Summary Using a combination of pulsed laser photolysis and discharge-flow techniques in a cryogenically cooled reaction vessel, it has proved possible to obtain rate constants for the reactions of OH radicals with N atoms down to 103 K and with 0 atoms down to 158 K.These experiments significantly increase the temperature range for which direct kinetic data for these reactions have been measured. The rate constants for both reactions increase monotonically as the temperature is lowered. By factorising out effects due to the temperature dependence of the reagent populations in different fine struc- ture states, the rate constants for both reactions obtained in the present work and in the experiments of Howard and Smith' can be fitted to a relatively simple expression. It is suggested that this fit can be used to make accurate estimates of the rate constants for reactions (1) and (2) at ultra-low temperatures and hence provide values of k,(T) and k,(T) which can be used in chemical models of molecular synthesis in interstellar clouds.We thank SERC and CEC, under its Science Plan pro- gramme, for support of our work on gas-phase kinetics at low temperature. D.W.A.S. is also grateful to SERC for the award of a research studentship. We acknowledge the assist- ance of Ken Boyle with some of the experimental measure- ments reported in this paper. We thank Professor George Schatz for preprints of his papers, ref. 20 and 27. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3227 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 (a) M. J. Howard and I. W. M. Smith, Chem. Phys. Lett., 1980, 69, 40;(b) M. J. Howard and I. W. M. Smith, J. Chem. SOC., Faraday 2, 1981,77,997.R. S. Lewis and R. T. Watson, J. Phys. Chem., 1980,84,3495. W. H. Brune, J. J. Schwab and J. G. Anderson, J. Phys. Chem., 1983,87,4503. (a) R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson Jr., J. A. Kerr and J. Troe, J. Phys. Chem. Ref:Data, 1989, 18, 881; (b) R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson Jr., J. A. Kerr and J. Troe, J. Phys. Chem. Ref. Data, 1992,21, 1125. J. A. Miller, R. J. Kee and C. Westbrook, Annu. Rev. Phys. Chem., 1990,41,345. R. P. Wayne, Chemistry of Atmospheres, Clarendon Press, Oxford, 2nd edn., 1991. S. Prasad and W. T. Huntress Jr., Astrophys. J. Suppl. Ser., 1980, 43, 1. T. E. Graedel, W. D. Langer and M. A. Frerking, Astrophys. J. Suppl. Ser., 1982,48, 321. Y. P. Viala, Astron. Astrophys. Suppl. Ser., 1986, 64, 391.G. Pineau des ForQts, E. Roueff and D. R. Flower, Mon. Not. Roy. Astron. Soc., 1990,244,668. S. P. Tarafdar and A. Dalgarno, Astron. Astrophys., 1990, 232, 239. E. Nercessian, J. J. Benayoun and V. P. Viala, Astron. Astro- phys., 1988, 195, 245. R. D. Brown and E. H. N. Rice, Mon. Not. Roy. Astron. SOC., 1986,223,405. I. W. M. Smith, J. Chem. SOC., Faraday Trans., 1991,87,2271. G. A. Gallup, Inorg. Chem., 1975,14,563. (a) P. J. Bruna and C. M. Marian, Chem. Phys. Lett., 1979, 67, 109; (b) P. J. Bruna, Chem. Phys., 1980,49,39. (a) S. P. Walch and C. M. Rohlfing, J. Chem. Phys., 1989, 91, 2939. T. J. Lee, J. Chem. Phys., 1993,99,9783. F. Pauzat, Y. Ellinger, G. Berthier, M. Gerin and Y. Viala, Chem. Phys., 1993,174,71. R. Guadagnin, G. C. Schatz and S. P. Walch, J.Chem. Phys., 1994, in the press. C. F. Melius and R. J. Blint, Chem. Phys. Lett., 1979,64, 183. S. R. Langhoff and R. L. Jaffe, J. Chem. Phys., 1979,71,1475. T. H. Dunning, Jr., S. P. Walch and M. M. Goodgame, J. Chem. Phys., 1981, 74, 3482. S. P. Walch, C. M. Rohlfing, C. F. Melius and C. W. Bauschli- cher Jr., J. Chem. Phys., 1988,88,6273. A. J. C. Varandas, J. Brandgo and L. A. M. Quintales, J. Phys. Chem., 1988,92,3732. D. C. Clary, Annu. Rev. Phys. Chem., 1990,41,61. R. Guadagnin and G. C. Schatz, J. Chem. Phys., 1994, in the 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 (a) N. Markovic, G. Nyman and S. Nordholm, Chem. Phys. Lett., 1989, 159, 435; (b) G. Nyman and J. Davidsson, J. Chem. Phys., 1990,92,2415. D. C. Clary and H-J. Werner, Chem.Phys. Lett., 1984,112,346. S. N. Rai and D. G. Truhlar, J. Chem. Phys., 1983,79,6046. J. Troe, J. Phys. Chem., 1986,90,3485. D. C.Clary, Mol. Phys., 1984,53, 3. (a) L. F. Phillips, Chem. Phys. Lett., 1990, 165, 545; (b) L. F. Phillips, J. Phys. Chem., 1990,94, 7482. J. Davidsson and L. G. Stenholm, Astron. Astrophys., 1990, 230, 504. I. W. M. Smith, R. P. Tuckett and C. J. Whitham, J. Chem. Phys., 1993,98,6267. M. Quack and J. Troe, Ber Bunsenges, Phys. Chem., 1976, 80, 1140. (a)P. Pechukas, J. C. Light and C. Rankin, J. Chem. Phys., 1966, 44,794; (b)J. C. Light, Discuss. Faraday SOC., 1967,44, 14. (a)I. R. Sims, J-L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, B. R. Rowe and I. W. M. Smith, J. Chem. Phys., 1992, 97, 8798; (b) I. R. Sims, J-L. Queffelec, A.Defrance, C. Rebrion-Rowe, D. Travers, P. Bocherel, B. R. Rowe and I. W. M. Smith, J. Chem. Phys., 1994, 100, 4229; (c) I. R. Sims, J-L. Queffelec, A. Defrance, D. Travers, B. R. Rowe, L. Herbert, J. Karthauser and I. W. M. Smith, Chem. Phys. Lett., 1993,211,461. (a) I. R. Sims,P. Bocherel, A. Defrance, D. Travers, B. R. Rowe and I. W. M. Smith, J. Chem. SOC., Faraday Trans., 1994, 90, 1473; (b) I. R. Sims, I. W. M. Smith, D. C. Clary, P. Bocherel and B. R. Rowe, J. Chem. Phys., 1994,101, 1748. B. R. Rowe, I. R. Sims, P. Bocherel and I. W. M. Smith, Am. Inst. Phys. Conf: Proc., 1994, in the press. (a)M. J. Frost, P. Sharkey and I. W. M. Smith, Faraday Discuss. Chern. SOC., 1991,91, 305; (b)P. Sharkey and I. W. M. Smith, J. Chem. SOC., Faraday Trans., 1993, 89, 631; (c) M. J. Frost, P. Sharkey and 1. W. M. Smith, J. Phys. Chem., 1993,9, 12254; (d) C. M. Moore, I. W. M. Smith and D. W. A. Stewart, Znt. J. Chem. Kinet., 1994, 26,813. M. A. A. Clyne in Reactive Intermediates in the Gas Phase, ed. D. W. Setser, Academic Press, 1979, ch. 1. M. J. Frost, P. Sharkey and I. W. M.Smith, unpublished results. W. Brennen and E. C. Shane, J. Phys. Chem., 1971,75,1552. I. M. Campbell and C. N. Gray, Chem. Phys. Lett., 1973,18,607. M. J. Pilling in Modern Gas Kinetics: Theory, Experiment and Applications, ed. M. J. Pilling and I. W. M. Smith, Blackwells, Oxford, 1987. M. M. Graff and A. F. Wagner, J. Chem. Phys., 1990,92,2423. S. J. Klippenstein and Y-W. Kim, J. Chem. Phys., 1993,99,5790. I. W. M. Smith in The Chemical Kinetics and Dynamics of Small Radicals, ed. K. Liu and A. Wagner, World Scientific, Singapore, 1994, in the press. 28 press. (a)J. A. Miller, J. Chem. Phys., 1981,74, 5120; 1981, 75, 5349; (b) J. A. Miller, J. Chem. Phys., 1986,84,6170. 29 (a) L. A. M. Quintales, A. J. C. Varandas and J. M. Alvarino, J. Phys. Chem., 1988, 92, 4552; (b) A. J. C. Varandas, J. Brandao and M. R. Pastrana, J. Chem. Phys., 1992, 96,5137; (c)A. J. C. Varandas, J. Chem. Phys., 1992,97,4050. Paper 4/04127E; Received 6th July, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003221

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3229-3236 Interaction Potentials and Fragmentation Dynamics of the Ne-Br, Complex in the Ground and Electronically Excited States Alexei A. Buchachenko, Alexei Yu. Baisogolov and Nikolai F. Stepanov Department of Chemistry, Moscow State University, Moscow I 19899,Russia Quantum calculations have been performed on the structure and vibrational predissociation dynamics of Ne. -.Br, van der Waals complexes in the ground and electronically excited states associated with the X'Z:,+(O:) and B 311,(0:)terms of Br, using simple empirical potential-energy surfaces. A reasonable approximation has been found for interactions in the ground state and the best potential for the excited state is chosen from three trial ones. Some results of classical dynamics calculations are also presented and discussed. The structure and dynamics of weakly bound systems (van der Waals molecules, hydrogen-bonded complexes, clusters, etc.)have attracted increasing attention by theoreticians over the last two decades (e.g.ref. 1 and 2). Developments in high- resolution spectroscopic techniques have made accurate data available on the bound and metastable states of the com- plexes and have allowed one to probe the intermolecular potential in the vicinity of an attractive well, which is hardly accessible in collisional measurements. Theory provides the essential step in the inversion procedure for converting the observed spectroscopic data into the interaction potential parameters. The ground-state potential-energy surfaces (PES) are usually determined from reliable microwave or IR spectra,' whereas the information on bound states of electronically excited complexes is much poorer as a rule.In this case one can gain additional information by analysing the dynamics of various decay (predissociation) processes. The rare-gas atom-excited halogen molecule van der Waals (vdW) complexes are excellent examples for studying the vibrational predissociation (VP), or decay, of a metastable state uia vibrational energy transfer, Rg. .-Hal,(u, n,, nb)+Rg + Hal,(o', j') (1) where u and u' designate the initial and final vibrational states of the halogen fragment, n, and nb are the (approximate) initial quantum numbers of the vdW stretching and bending modes, and j' is the diatom rotational momentum.Among the Rg. ..Hal, (B 'nu)complexes, the chlorine complexes were the subject of detailed high-resolution fluorescence studies4 and accurate quantum calculations5 based on realis-tic empirical PESS.~ Extensive experimental investigations, including time-resolved measurements, were carried out for Rg--.I,(B) systems as well.7 These complexes traditionally serve as a test for various approximate theoretical approaches.' Less theoretical attention has been paid to bromine com- plexes despite the substantial body of experimental informa- tion Vibrational predissociation widths of the Ne. --Br2 (B 'nu)complex for a wide range of initial vibra- tional excitations (u = 11-30) were estimated from low-resolution spectra by Janda and co-workers.'' Subsequent measurements with higher resolution performed by the same group allowed one to establish the T-shaped equilibrium con- figuration, determine the vibrationally averaged geometric parameters of this complex" and improve the data on decay widths for several values of u which fall within the range" of 10-20. Limited information on the product vibrational and rotational state distributions was obtained in dispersed fluo- rescence studies.' 2,1 ' In addition, direct lifetime measure- ments were carried out for the ground-state Ne. -.Br, (X'El, u = 1) ~omplex.'~ Theoretical studies of Ne...Br, are exhausted by the implementation of Ewing's momentum gap law" and our classical and quantum calculations within a restricted two- dimensional (2D) T-shaped model.'' These approaches are useful for qualitative analysis of VP dynamics, but they do not provide a quantitative test of the PES involved. This is the subject of the three-dimensional (3D) study of the structure, spectra and fragmentation dynamics of the Ne. * .Br, complex discussed here. The important point of the present investigations is the simultaneous consideration of ground and electronically excited states of this system, which allows us to treat the measured frequencies of the BtX transition directly. Together with the calculations on lifetimes and VP product state distributions, this analysis provides an almost complete spectroscopic characterization of the PESs.The main theoretical approach implemented here is the decoupled Fermi golden rule approximation. At the same time, we also involve in the discussion some results of quasi-classical trajectory (QCT) calculations on Ne. -.Br2(B) VP dynamics. Computational Procedures The total Hamiltonian of the triatomic complex is Jacobi coordinates has the form5 p2 p2 j2H = -+ -+ -+ -l2 + U(r,R, 6) (1)2m 2p 2mr2 2pR2 where r is the Br, internuclear distance, R is the Ne...Br, centre of mass separation, and 6 is the angle between the r and R vectors, p and P are the classical linear momenta con- jugated to r and R, respectively, or their quantum counter- parts, j and I are the angular momentum variables (or corresponding quantum mechanical operators) representing the rotational momentum of the diatom and the orbital momentum of the Ne atom; m and p are the reduced masses, U(r,R, 6) denotes the PES.QuasiclassicalTrajectory Method The procedure used here is similar to that applied by Wozny and Gray for the Ne. -C12(b) fragmentation dynamics.' In brief, initial conditions are selected quasiclassically using the action-angle form of the separable Hamiltonian function HO= h,(p, r) + h,(~,R) + h~, (11)e) evaluated by the Taylor expansion of the PES nearby equi- librium point r = F, R = 2, 8 = n/2. For each initial excita- tion of the diatom u specified as v + 1/2 action of the h, oscillator, the ground state of the vdW subsystem (n,= 0, nb = 0) is considered by setting the corresponding actions to 1/2.The Hamiltonian equation of motion with the total Hamil- tonian function [eqn. (I)] for zero total angular momentum, are integrated up to a preselected time limit, and the trajec- tories reaching some large critical interfragment separation R are regarded as dissociative. An exponential fit to the survival probability obtained from an ensemble of lo00 trajectories defines the lifetime of a complex. Internal state distributions of the Br, fragment are computed uia the standard histogram method by making assignments of the final classical action of the h, oscillator and angular momentum j to their nearest quasiclassical (half-integer and even) values.' Fermi Golden Rule Approximation In quantum formalism, the vibrational predissociation dynamics may be treated within the Fermi golden rule (FGR) approximation' which implies a splitting of the total Hamil- tonian into two terms, H=H,+V (111) where intra- and inter-molecular motions of a complex are decoupled in the unperturbed part H,, i.e.H, = h,, + hvdW, by picking up all coupling terms in the perturbation operator V. Hence, to the zeroth order, the metastable states of H cor-respond to the bound of H, , and their decay widths associated with process (1)are evalu- ated in first order: where Yu,j, is the continuum eigenstate of the total Hamilto- nian H with the eigenvalue exactly equal to Eungnb. However, we introduce an additional simplification by assuming sepa- rability of intra- and inter-molecular motions for the final wavefunction (or, in other words, we neglect the coupling between the different scattering channels u').Hence, Yu#j, is replaced in eqn. (V) by Y:rj(,the unperturbed continuum solu- tion for H, at the same energy. We therefore omit the null superscript at all wavefunction designations and, by dealing only with the ground vdW level n, = 0, nb = 0, we drop these indices too. The computational scheme implemented here for FGR integrals relies upon further approximations which resemble J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 where the radial function $(') depends parametrically on 8. For the initial discrete state it results in a series of 1D radial Schrodinger equations at fixed 8 values whose eigenvalues, n,, form the effective angular potential which determines F(") for the nb bending level. For the continuum states eqn.(VIII) is equivalent to the well known rotational infinite order sudden approximation,22 which substitutes angular momen- tum operators j2 and P by the effective expectation values iG+ 1) and (r+ l), respectively. In contrast to the inelastic scattering, the 'half-collision-' problem allows one to make a consistent choices of 3 and 1 parameters (treating the former as a true angular momentum quantum number rather than a measure of average rotational energy, i.e. for the case of the zero total angular momentum considered here, 1=j.Among them the most accurate is 1 =j'.,' The function F in eqn.(VIII) is simply the spherical function with angular momen- tum j' and zero body-fixed frame projection. Provided that only potential terms are responsible for the interaction between intra- and inter-molecular degrees of freedom, the expression for the partial width, eqn. (V), acquires the final form where E is the translational energy release which is equal to the vibrational energy loss of the diatomic fragment, E, -E,, minus the final rotational energy, BUtj'(j'+ l), and the initial vdW vibrational energy, EZw,defined by eqn. (VIII). The total predissociation width is evaluated by summing partial ones for all significant v' andj' channels. The above procedure which we term the vibrationally dia- batic decoupling rotational infinite order approximation (VDD/RIOSA) has been shown to provide a sufficiently accu- rate description of the direct (one-quantum) VP processes of the triatomic vdW complexes.2' However, it is necessary to indicate its most vulnerable place : vibrational decoupling in the continuum wavefunction which leads to the single-channel scattering picture disabling the account for asymp- totic state interactions.Strictly speaking, this approximation is valid only if the decay via the one-quantum, Au = u' -u = -1, vibrational channel strongly dominates over the consecutive transfer of several vibrational quanta. A second less important inaccuracy is connected with the fixed- angle approximation for the bound-state problem which introduces an error in the resonance position which, in turn, implicitly affects the resonance width.Potential-energy Surfaces The PES U(r,R, 0) consists of two terms describing intra- and inter-molecular interactions, the early approach of Delgado-Barrio and co-w~rkers,'~~~~ but formulated at a more quantitative level." Namely, intra- and inter-molecular variables are decoupled diabati~ally,~ so that the 'slow' intermolecular motion is described by the Hamiltonian averaged over 'fast ' vibrations of the diatom, aw = (XU(d I H I XUW> (VI) where xuis the eigenfunction of the hmoloperator correspond- ing to an isolated non-rotated diatomic fragment. The eigen- value problem for the 2D h3* operator e) = ~g~p)(~, (VII)h%w p(~, e) is eliminated by the diabatic decoupling approximation :19v20 +(u)(R,e) z $(u)(R; (VIII)e)F(V)(e) Potential curves of isolated Br, molecules constituting the intramolecular part of the global PES Umolare represented by the RKR turning points for both ground and excited states.23 Being suitable for quantum calculations, this choice is not optimal for classical dynamics.For this reason, in QCT calculations on the excited-state complex we use the Morse function with dissociation energy D, = 3788 cm-',inverse-length parameter a, = 2.045 A-', and the equilibrium dis- tance f = 2.667 A, which provides a good fit to the RKR potential for u = 10-30. Test quantum calculations with this Morse curve do not exhibit considerable deviation from those carried out with the RKR potential.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The intermolecular potential, UvdW, is taken as a sum of pairwise Morse potentials UVdW(r,R, 0) = U,(9,) + U,(9,) (XI) U,(@) = D{exp[ -2a(9 -@)I-2 exp[ -a(B -@)I> (XI0 Here 9,and 9,denote the two Ne-Br distances, related to Jacobi coordinates by 9:,,= R2 + r2/4 & rR cos 0 (XIII) and the bar indicates an equilibrium separation. This simple form of UvdWwas found to provide quite a reasonable description of the VP dynamics of rare gas-halogen complex- es in many previous applications (see, e.g. ref. 17, 20 and 24). The separable Hamiltonian, Ho, appropriate for quasi- classical quantization consists of two Morse oscillators, h, and h,.Parameters of the former obviously coincide with those of the isolated Br, oscillator, whereas those of the latter are related to the constants of pairwise interaction by the fol- lowing formulae:25 DR = 20; aR = aR/@; R2 = B2-r2/4 The angular term h, is approximated by the harmonic oscil- lator Hamilt onian ' where Parameters of the interaction potential, eqn. (XII), are deter- mined using the Ho Hamiltonian which allows us to express observables Do and R,, the bond energy and vibrationally averaged intermolecular separation, via the pairwise potential parameters analytically. Assuming that the range parameter o! is 1.72 A-' (the value for the NeKr interaction26) we iter- atively adjust D and I? to reproduce Do = 70.5 cm-' and R, = 3.67 A determined experimentally for the ground state."*'2 The final set of parameters of the Ne-Br,(X) inter-action are given in Table 1.Three different PESs were involved in the study of the elec- tronically excited complex. The first, PES I, was obtained by a similar 'inversion' procedure, but neglecting the bending contribution he to the energy.I6 Since it provides a good description of the Ne. -.Br ,(B) fragmentation dynamics within the restricted 2D model,I6 this surface may be con- sidered to be a reasonable initial guess for 3D studies. The second, PES 11,is constructed on the basis of the 3D QCT simulations discussed below. Finally, the third set of param- eters PES I11 have been suggested by Walter and Stephenson for an Ne- * -IBr(A) complex.27 Actually, this potential pri- marily reflects the interactions in rare-gas dimers.Table 1 Parameters and constants of the Ne-Br potentials 323 1 -1 00 4.1 4.6 5.1 5.6 6.1 6.6 7.1 RIA loo! . *.. -1 00 InI I I1 I I I, I I I! I 14 r 1 I I1 111 I1 I I I I I 4 1 t I I1 I rnl I ' 11 I "~r'-' '"'1 " 2.7 3.2 3.7 4.2 4.7 5.2 5.7 RIA Fig. 1 Sections of Ne. .-Br, interaction potentials at (a)r = i,8 = 0 a)and (b) r = i,8 = 42. (. *. Ground-state PES; (---) PES I; (-- - -) PES II;(-) PES 111. Parameters of all these PESs are listed in Table 1. Also given are crude estimations of Do and R, constants derived for the Ho Hamiltonian. Fig. 1 shows the sections of inter- action potentials at r =? for linear and T-shaped configu- rations of the complex.Results Ground-state Complex The energy of the ground rovibrational state of the Ne. -.Br, complex is found to be -71.25 cm-' [relative to the Nee --Br,(X) dissociation limit] and the vibrationally aver-aged distance, R,, is 3.64 A. These figures agree with the experimental estimations described above (Do = 70.5 f2.0 cm-' and R, = 3.67 & 0.01 A) as should be expected since the latter have been used to parametrize the PES. An independent test of the interaction potential quality is provided by VDD/RIOSA calculations of the vibrational pre- dissociation lifetime for the first metastable state Ne. --Br,(X, ground state 46.0 1.72 3.7 92.0 3.52 70.2 3.68 38.52 1.72 3.58 77.04 3.32 56.84 3.50 43.0 1.75 3.84 86.0 3.60 64.39 3.77 42.0 1.67 3.9 84.0 3.66 63.63 3.83 a Estimated with the Ho Hamiltonian. u = 1).The theoretical result, 5.9 ps, falls within the error of the experimental data, 8 & 3 VS.'~ Hence, our ground-state interaction potential provides a good reference point for estimations of the band origins of the B tX transition spectrum of the complex. Excited State: QCT Results Let us first consider results from the QCT approach which we thought to be a useful guideline for the preliminary PES characterization. Table 2 compares computed predissociation widths with the low-resolution data from ref. 10 and 12. Obviously, the 3D classical decay rates for PES I strongly underestimate the experimental ones.Calculated vibrational product state distributions appear to be colder and narrower than those measured. For example, the branching ratios for Au = u'-u = -1, -2, -3 and -4 channels at u = 27 are estimated as 1 :0.32 :0.07 :0.007 in contrast to 0.7 : 1 :0.6 :0.3 deduced from dispersed fluorescence studies.I2 In agreement with experimental findings,12*13 the QCT method indicates the insignificant role of the rotational energy transfer in VP dynamics (energy transferred into the product's rotation amounts to less than 10% of the total available energy). To eliminate the shortcomings of the above results and to gain more insight into the applicability of the classical approach we perform QCT calculations for several trial inter- action potentials of the form given in eqn.(XII) with par_am- eters D, a and 9varying within 40-45 cm-', 1.65-1.80 A-' and 3.83-3.85 A ranges, respectively, which seems most prob- able in terms of the Hamiltonian function Ho. Among these potentials we choose one providing the closest agreement Table 2 Predissociative widths of the Nee ..Br,(B) complex (cm-I): QCT calculations and experimental values ~~~ ~ quasiclassical trajectories' experimental 0 PES I PES I1 valuesb 25 0.290 f0.001 - 1.09 f0.2 26 0.344 f0.002 - 2.38 f0.4 27 0.421 f0.002 0.65 & 0.02 3.12 f0.4 28 0.310 f0.001 0.647 k0.001 2.50 f0.4 29 0.537 f0.001 0.926 f0.002 2.88 f0.4 30 0.665 f 0.003 1.100 f0.003 1.88 f0.4 'Indicated errors are the standard deviations of the exponential fits and do not account for statistical uncertainty.Experimental values are taken from ref. 10; revised error bars are taken from ref. 12. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 with the measured decay width at u = 27 and repeat trajec- tory calculations on the Ne. .Br2(B) predissociation dynamics for u = 27-30. This 'improved' potential is exactly PES I1 as described above. Although the decay rates for PES I1 are twice as large as those for PES I, they still fall far below the experimental data, see Table 2. Markedly better agreement is, however, attained for the vibrational branching ratios which are 1 :0.98 :0.20 :0.04 for u = 27. Rotational product state distributions become also somewhat hotter than those for PES I.Excited State: Quantum Results Energies and Structural Parameters The positions of the lowest Nee .-Br,(B, u) resonances -Dt) computed by the VDD/RIOSA) method with all three PESs (Reported in Table 3) may be compared with the experimen- tal estimate Do= 61.2 & 2.0 cm-' (established from the vibrational energy distributions,I2 this value should be referred to u = 27, 28). PES I underestimates the dissociation energy, whereas PESs I1 and I11 give values within the experi- mental error limits, but closer to the upper bound. Note that Do values derived from the oversimplified Hamiltonian Ho (Table 1) reproduce the results of the more rigorous calcu- lations (Table 3) with a maximum deviation of <2 cm-'.The analysis of the B tX transition frequency shifts in the complex relative to those in the free halogen molecule is more informative, since it is the only directly measurable character- istic of the resonance positions. With the dissociation energy of the ground-state complex D,(X) = -71.25 cm-' (see above), we may easily estimate the frequency shift as (XVII) Calculated and measured lo*'' frequency shifts are presented in Table 3 and Fig. 2. The results for PES I1 are in excellent agreement with the band origins of low-resolution excitation spectrum over the whole studied range of u, whereas PES I11 reproduces them within a 1 cm-error. In contrast, the shifts obtained with PES I are nearly twice as large as the mea- sured ones. This discrepancy is much larger than the uncer- tainty of the experimental estimations for the dissociation energies and, therefore, cannot be attributed to the inaccurate determination of ground-state PES.The calculations with PESs I, I1 and I11 yield for the vibra- tionally averaged intermolecular separation, R, (at u = 10) values of 3.43, 3.70 and 3.77 A, respectively, whereas the Table 3 Energies of the lowest metastable levels, -Dt)/cm-', of the Ne. .Br,(B) complex and shifts of B cX transition frequencies, Am/cm-':quantum calculations and experimental values 10 -57.49 13.76 -65.04 6.21 -64.23 7.02 6.045' 14 -57.23 14.02 -64.77 6.48 -63.99 7.26 6.59 16 -57.06 14.19 -64.60 6.65 -63.84 7.41 6.66 17 -56.98 14.27 -64.51 6.74 -63.76 7.49 6.82 20 -56.66 14.59 -64.19 7.06 -63.48 7.77 - 22 -56.41 14.84 -63.93 7.32 -63.25 7.99 7.50 25 -55.94 15.31 -63.45 7.80 -62.83 8.41 7.38 26 --55.76 15.49 -63.26 7.99 -62.66 8.58 8.01 27 -55.55 15.70 -63.05 8.20 -62.48 8.77 8.80 -28 -55.32 15.93 -62.81 8.44 -62.28 8.97 9.10 29 35 --55.07 -54.80 16.10 16.45 -- -- -- - 9.59 9.62 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0. c 9.0-I 3.0U 4 1TE 71 I TJ:< 8.0i 1'Ilv) * r r >. s 7.0: c 2.0 .-a Gi!: .i 1 x6.0i 5.05 1 8 12 16 20 24 28 32 V :I -\, I,,Fig. 2 Frequency shift of the B tX transition in the Ne. * .Br, complex relative to free Br, molecules. (a * -) (-) 8 10 12 14 16 18 20 22 24 26 28 30 32 calculated with Nee ..Br,(B) PES 11; (---) calculated with PES 111. V Fig. 3 Predissociative widths of the Nee .Br,(B) complex. (0)and ( x ) Data of high-I2 and low"-resolution experiments, respectively; analysis of the excitation spectrum lineshapes estimates Ro as calculated with PES I(----); I1 (---); I11 (-)3.65 f0.01 A." The PES I result is again the worst. Predissociat ion Widths mates the decay rates by two. None of the PESs, however, are Table 4 and Fig. 3 summarize the VDD/RIOSA results for able to reproduce correctly the critical excitation at which the predissociation widths of the Ne. * SBr complex for three predissociation width falls off: PES I shifts it to u= 30, excited-state PESs. They are compared with two sets of whereas for PES I1 the Au = -1 channel appears to be com-experimental data available from high-resolution excitation pletely closed at u= 27, but the two lowest rotational states, spectroscopy for u= 10-20,12 and from low-resolution j' = 0, 2, belonging to this channel are energetically accessible measurements" (with improved error bars presented in ref.at u= 26. PES I11 again provides the most acceptable12) for u= 10-30. Only the former provides a good reference picture, placing the complete locking of the one-quantum point for elucidation of the PES quality because the reliablity decay channel at the correct value of u= 28. However, the of the latter seems questionable. Indeed, at low excitation maximum of the decay rate is shifted to u= 26 owing to its energies (u< 20) low-resolution data not only quantitatively partial locking associated with thej' > 10 asymptotic states.deviate from high-resolution linewidth estimations, but also The decrease in the fragmentation rate above the critical exhibit qualitatively incorrect behaviour. Consequently, low- excitation energy is too sharp for all potentials. This reflects resolution results should be adopted with caution at high uas the inaccuracy of the computational scheme, described above, well. However, the predissociation rate fall-off at the critical which completely decouples scattering channels, disabling excitation u= 28 indeed occurs, being caused by the locking population transfer among them.Being unreliable for the two of the dominant decay channel Au = -1, as is confirmed by quantum decay processes above, this approximation should measurements on the vibrational product state distribu-also disturb the energy dependence of the predissociation tions.12 The nature of the second observed rate maximum at width just below it. u= 29 seems unclear although the analogous effect arising from resonant interactions among open and closed asymp- Product State Distributions totic channels has been theoretically predicted by Roncero et Suppressing the asymptotic state interaction strongly affects al. for the Ne- * -12(B) vdW complex.28 the vibrational energy distributions of the product molecule. The agreement with high-resolution data is best for PES As a consequence, they appear to be very cold for all PESs, 111.PES I yields slightly worse results and PES I1 overesti-accumulating 97-100% of the population in the dominant Table 4 Predissociative widths of the Nee * .Br,(B) complex (cm-I); quantum calculations and experimental values theory experiment 0 PES I PES I1 PES I11 low resolution" high resolutionb 10 0.019 0.028 0.017 -0.015 f0.005 14 0.058 0.080 0.052 0.16 0.051 f0.005 16 0.101 0.138 0.089 0.09 0.057 & 0.006 17 0.130 0.178 0.1 16 0.06 0.081 f0.008 20 0.285 0.368 0.253 -0.151 & 0.015 -22 0.453 0.570 0.403 0.75 f0.2 -25 0.948 1.073 0.755 1.09 f0.2 -26 1.180 1.079 0.994 2.38 k0.4 -27 1.397 0.073 0.780 3.12 f0.4 28 1.491 0.057 0.035 2.50 +_ 0.4 --29 1.408 --2.88 f 0.4 30 0.105 -1.88 f 0.4 From ref.10. From ref. 12. 3234 Table 5 Rotational temperatures of the Br,(B, u -1) predissocia-tion products of the Ne-.-Br,(B, u) complex, K: quantum calcu- lations and experimental values theory U PES I PES I1 PES I11 experiment' 10 4.1 4.4 4.4 6b 14 3.9 4.2 4.7 -17 3.6 3.9 4.0 -20 3.4 3.5 3.5 -22 3.1 3.2 3.3 5 25 2.9 2.2 1.9 -26 2.5 1.1 1.9 -27 2.0 0.2 0.5 <1 C C C28 1.1 -C C C-29 0.5 --From ref. 12. The value of 6.9 K is given in ref. 11. The u' = u -1 channel is closed. exit channel. Only the qualitative trend in the product vibra- tional temperature T,,(PES I) < T,,(PES 11)x T,,(PES 111) is therefore of value. In contrast to the vibrational decoupling, the rotational infinite order sudden approximation does not influence rota- tional product state distributions so dramatically, although it somewhat overcools them.,' All rotational distributions peak at j' = 2 and cannot be fitted by a Boltzmann distribution.To estimate the Br,(B, u') rotational temperature in the domi- nant vibrational decay channel v', the simple relation (Erot) x ikTotmay be used, where (Erot) denotes the first moment of rotational distribution with the Br,(B) rotational constant, B,, and k is the Boltzmann constant. Computed and meas~red'~.'~ T,,values are presented in Table 5. One can again see the superiority of PES 111. Discussion Excited-state Interaction Potential The possibility of deriving a definite conclusion on the reli- ability of the empirical potential surface depends on the quality of the experimental data and on the accuracy of their theoretical evaluation.Uncertainties in both quantities are pertinent to the present study of the Nee -.Br, complex so we find it instructive to give here a brief account of the main issues of comparative analysis of the B-state Ne-Br, inter-action potential. In principle, the most stringent test for the attractive well region of the PES is provided by energy-level calculations. However, for vibronic transitions it implies that one of the relevant PESs is known with definite accuracy. So far, our guess for the ground-state Ne- -Br, potential satisfactorily fits all the available spectroscopic information, it seems rea- sonable to estimate its uncertainty to be that of the experi- mental data used for parametrization.Taking the corre-sponding error as 2-3 cm-l, one can discard PES I but cannot distinguish PESs I1 and I11 on the basis of the line- shift analysis (Table 3). If the VDD/RIOSA approach is able to find resonance positions with an accuracy of at least several times smaller than the experimental uncertainty irrespective of the excita- tion energy, the situation with regard to the predissociation width is more delicate. With increasing u, the decay channel interference effect, which increases the contribution of the multiquantum channels, prevents our method from yielding a uniformly accurate energy dependence of the VP rate.At the same time, as discussed above, the measured lifetimes also have different error limits. Hopefully, our approach appears J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 to perform sufficiently well just in the excitation energy domain, where the high-resolution experiments have been carried out (v = 10-20), since the Au = -1 decay channel correctly treated by the VDD-RIOSA method, strongly dominates therein. Analysis of the linewidths calculated for this range unambiguously shows that PES I11 is superior over the two others (see above). To extend the comparison for higher excitations it is instructive to use the well known energy gap law relating the predissociation width to the vibrational quantum number of the diatomic fragment (see, for example, ref.9): (XVIII) where EA, = Eu+Au -E, is the vibrational energy release of the diatom and A and B are adjustable parameters. Being derived by Beswick and J~rtner,~ within a 2D FGR model, this relationship also neglects final-state interactions. There- fore, the extrapolation of high-resolution data for T, (in which the dominant Av = -1 decay channel amounts to at least 90% of the population'2) to higher u allows one to pick up the single-channel contribution to the decay rate. Fig. 4 compares the energy gap law extrapolation with the partial widths for the Av = -1 channel. Evidently, PES I11 behaves better than the others at higher excitation energies too. This qualitative analysis provokes us to estimate the total 'high-resolution linewidths' for high energy taking the results of the energy gap law extrapolation as a Av = -1 partial width and using experimentally measured vibrational product-state distributions.12 The total decay widths for v = 22 and v = 26 so obtained are 0.4 and 2.0 cm-', respec-tively, values remarkably smaller than the low-resolution ones, 0.75 and 3.12 cm-'.lo Being rather approximate, these speculations nevertheless may be treated as pointing to an overestimation of actual Ne..Br,(B) fragmentation rates in low-resolution experiments. The same conclusion follows from common consideration of the deconvolution of a line profile, whose width is much smaller than the bandwidth of a pumping laser." As concerns the critical quantum number u* at which the Av = -1 channel closes, its value may be seriously affected by the inaccuracy of the computational scheme, which cannot predict the resonance positions exactly.Even very small errors in this quantity, which may be neglected in the lineshift calculations, may result in the open or closing of several rota- V Fig. 4 Predissociative widths of the Ne. -.Br,(B) complex. (0)Data of high-resolution experiments;I2 ( x ) their extrapolation by the energy gap law, eqn. (XVIII); calculated with PES I (-- - -) ;I1 (---); I11 (-). J. CHEM. SOC. FARADAY TRANS,, 1994, VOL. 90 tional exit channels,j’, owing to the small Br, rotational con- stant and, therefore, significantly disturb the behaviour of the predissociation rate near the critical energy.However, on the whole, PES I11 reproduces the critical behaviour better than the other potentials. To quantify these qualitative considerations, more rigorous theoretical approaches, which account for the close coupling between the vibrational decay channels, should be imple- mented (see, for example, ref. 5, 6 and 24). In particular, they should reliably treat the product state distributions and eluci- date the nature of the second predissociation rate maximum observed in low-resolution measurements. Classical and Quantum Dynamics Correspondence A somewhat unexpected conjecture of the present study is the failure of the quasiclassical trajectory method to reproduce the decay rates of Nee * .Br,(B) complexes, which contrasts with experience from classical dynamics calculations on the Ne.-C12(B),17 He. -.Br,(B),30 He. -.12(B)31932 and Nee -.12(B)33complexes. The most reasonable explanation for this is the inaccuracy of the selection procedure for the initial conditions which may overestimate the bending contribution to the initial vdW zero-point vibrational energy and, there- fore, lead to an underestimation of the fragmentation rate. At the same time, note that QCT calculations represent the vibrational product state distributions reasonably well, working much better than the decoupled FGR approx-imation. Besides, classical dynamics exhibit the correct sensi- tivity to the parameters of the PES: the differences in predissociation widths and product distributions computed classically for PESs I and I1 follow the same trends as those derived from quantum calculations.Another remarkable feature of the QCT data is the irregu- lar dependence of classical VP rate on the excitation energy. Interestingly, the fall-off in the classical rate occurs at the same energy as the actual retardation observed experimen- tally (Table 2). This coincidence, also found for I,(B) com-plexes with helium3’ and neon,33 seems surprising because the real irregularity originates from a purely quantum effect. Indeed, there are strong indications for its accidental nature, e.g. the maxima of QCT rates are the same for both PESs being sufficiently far from the critical excitations established for PESs I and I1 in quantum calculations. Actually, the clas- sical irregularity should be attributed to the onset of strong non-linear resonances, a reason entirely distinct from the exit-channel energy balance controlling the quantum critical behaviour.Comparison of Two-and Three-dimensional Dynamics The data obtained in the present 3D calculations for PES I may be compared with those derived within a restricted T-shaped model for the same potential and very similar theo- retical approaches.I6 Fig. 5 depicts the ratio of 2D and 3D predissociation widths as a function of excitation energy for QCT and FGR methods. These ratios are essentially constant over the whole range (except for the points u = 28, 29 near the critical excitations which are, of course, different in 2D and 3D cases).This finding is consistent with the observation of the negligible effect of rotational energy transfer in VP dynamics and implies that the variation in the predissocia- tion rates originates almost completely from the energy shift by the zero-point bending contribution. Indeed, appealing to the decay width-squared shift ~orrelation,’~ one can find that the average ratio of two- and three-dimensional widths for the quantum approach (1.19) is very close to the average squared ratio of the corresponding dissociation energies Do (1.23). 4.0I -0 3.0; 25o^ 2.0:z?. L I .-**---.-------Cb 0.0 8 12 16 20 24 28 32 V Fig. 5 Ratios of Ne. . .Br,(B) predissociative widths calculated for PES I within and three-dimensional models of the complex by (0)FGR and (0)QCT approaches Conclusions Implementing an approximate quantum-mechanical approach, we performed detailed tests on three model PESs describing vibrational predissociation of the Ne- .Br ,(B) vdW complexes.An empirical potential constructed within a 2D model (PES I) was discarded on the basis of B +-X tran-sition frequency shift calculations, whereas the potential func- tion originating from the adjustment of quasiclassical trajectory data (PES 11) yielded too high fragmentation rates. The most acceptable agreement for all available spectro- scopic and dynamical information for the Ne. * .Br,(B) complex was attained with the Walter-Stephenson PES I11 deduced from interactions in rare-gas dimers.A reasonable approximation for the ground-state Ne-Br,(X) PES was also suggested. The results of the quantum calculations allowed us to discuss qualitatively the reliability of the experimental data, the validity of the quasiclassical trajectory approach and the correspondence of two- and three-dimensional dynamics. Financial support from the Russian Fund of Fundamental Research under project 93-03-4329 is gratefully acknow- ledged. References Structure and Dynamics of Weakly Bound Complexes, ed. A. Weber, Reidel, Dordrecht, 1987. Dynamics of Polyatomic Van der Waals Complexes, ed. N. Hal-berstadt and K. C. Janda, Plenum, New York, 1990. A. C. Legon and D.J. Millen, Chem. Rev., 1986, 86, 635; D. J. Nesbitt, Chem. Rev., 1988,88, 843. K. C. Janda, Adv. Chem. Phys., 1985, 60,201; J. I. Cline, B. P. Reid, D. D. Evard, N. Sivakumar, N. Halberstadt and K. C. Janda, J. Chem. Phys., 1988, 89, 3535; J. I. Cline, N. Sivakumar, D. D. Evard and K. C. Janda, J. Chem. Phys., 1987,86, 1636; D. D. Evard, C. R. Bieler, J. I. Cline, N. Siavkumar and K. C. Janda, J. Chem. 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Thompson, J. Chem. Phys., 1982, 77, 3939; T. R. Horn, R. B. Greber and M. A. Ratner, J.Chem. Phys., 1989,91, 1813; S. Das and D. J. Tannor, J. Chem. Phys., 1990, 92, 3403; S. K. Gray and S. A. Rice, Faraday Discuss. Chem. SOC., 1986, 82, 307; R. E. Gillilan and G. S. Ezra, 22 23 24 D. J. Kouri, in Atom Molecule Collision Theory, ed. R. B. Bern- stein, Plenum Press, New York, 1979, p. 301. R. F. Barrow, T. C. Clarck, J. A. Coxon, and K. K. Yee,J. Mol. Spectrosc., 1974,51,428. 0.Roncero, J. A. Beswick, N. Halberstadt, P. Villarreal and G. Delgado-Barrio, J. Chem. Phys., 1990, 92, 3348; R. Waterland, M. I. Lester and N. Halberstadt, J. Chem. Phys., 1990,92,4261; S. K. Gray and C. E. Wozny, J. Chem. Phys., 1991,94,2817; D. 9 10 11 12 J. Chem. Phys., 1991,94,2648. L. J. van de Burgt, J-P. Nicolai and M. C. Heaven, J. Phys. Chem., 1984, 81, 5514; D.G. Jahn, S. G. Clement and K. C. Janda, J. Chem. Phys, 1994,101,283. B. A. Swartz, D. E. Brinza, C. M. Western and K. C. Janda, J. Phys. Chem., 1984,88,6272. F. Thommen, D. D. Evard and K. C. Janda, J. Chem. Phys., 1985,82,5295. J. I. Cline, D. D. Evard, B. P. Reid, N. Sivakumar, F. Thommen 25 26 27 H. Zhang and J. Z. H. Zhang, J. Chem. Phys., 1991,95,6449; S. K. Gray, Chem. Phys. Lett., 1992,197,86; G.Delgado-Barrio, J. Compos-Martinez, S. Miret-Artes and P. Villarreal, in Dynamics of Polyatomic Van der Waals Complexes, ed. N. Halberstadt and K. C. Janda, Plenum Press, New York, 1990,409. S. K. Gray, S. A. Rice and D. W. Noid, J. Chem. Phys., 1986,84, 3475. M. V. Bobetic and J. A. Barker, J. Chem. Phys., 1976,64,2367. S. A. Walter and T. A. Stephenson, J. Chem.Phys., 1992, 96, 13 14 15 16 17 and K. C. Janda, in Structure and Dynamics of Weakly Bound Complexes, ed. A. Weber, Reidel, Dordrecht, 1987, p. 533. N. Sivakumar, J. I. Cline, C. R. Bieler and K. C. Janda, Chem. Phys. Lett., 1988, 147, 561. N. Sivakumar, D. D. Evard, J. I. Cline, C. R. Bieler and K. C. Janda, Chem. Phys. Lett., 1987, 137,403. G. E. Ewing, J. Phys. Chem., 1987,91,4662. A. A. Buchachenko, A. Yu. Baisogolov and N. F. Stepanov, Russ. J. Phys. Chem., 1994,68,660; 844. C. E. Wozny and S. K. Gray, Ber. Bunsenges. Phys. Chem., 1988, 28 29 30 31 32 3536. 0 Roncero, J. Campos-Martinez, A. M. Cortina, P. Villarreal and G. Delgado-Barrio, Chem. Phys. Lett., 1988,148,62. J. A. Beswick and J. Jortner, J. Chem. Phys., 1978,68,2277. A. A. Buchachenko, A. Yu. Baisogolov and N. F. Stepanov, to be published. S. K. Gray, J. Chem. Phys., 1987,87,2051. J. A. Beswick, G. Delgado-Barrio, P. Villarreal and P. Mareca, Faraday Discuss. Chem. SOC., 1982, 73, 406; G. Delgado-Barrio,P. Villarreal, P. Mareca and G. Albelda, J. Chem. Phys., 1983, 92,236. 78,280. 18 19 20 21 L. M. Ruff and D. L. Thompson, in Theory of Chemical Reaction Dynamics, ed. M. Baer, CRC, Boca Raton, 1985, vol. 111,p. 1. J. A. Beswick and G. Delgado-Barrio, J. Chem. Phys., 1980, 73, 3653; M. Aguado, P. Villarreal and G. Delgado-Barrio, Chem. Phys. Lett., 1983, 102, 227. P. Villarreal, G. Delgado-Barrio, J. Campos-Martinez and 0. Roncero, J. Mol. Struct. (THEOCHEM), 1988, 166, 325. A. A. Buchachenko, A. Yu. Baisogolov and N. F. Stepanov, to be published. 33 34 G. Delgado-Barrio, P. Villarreal, P. Mareca and J. A. Beswick, J. Comput. Chem., 1984,5,322. R. J. LeRoy, M. R. Davies and M. E. Lam, J. Phys. Chem., 1991, 95.2167. Paper 4/02714K; Received 9th May, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003229

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3237-3240 Theoretical Vibrational Energies of [CrO,]’-[MnO,J2-and [Fe0,l2-Robert J. Deeth* and Paul D. Sheen Inorganic Computational Chemistry Group, School of Chemistry, University of Bath, Claverton Down, Bath, UK BA2 7A Y Density functional theory (DFT) calculations of the vibrational energies of the tetrahedral complexes [M0,l2-(M = Cr, Mn, Fe) are reported using both local and non-local functionals. The computed results vary slight with choice of basis set but are less sensitive to the choice of functional. The computed data tend to be slightly too large for the high-energy stretching modes and slightly too small for the low-energy bending/torsion modes. The average deviation between theory and experiment is about 4%, with a maximum average error of only 7% depending on which experimental data are used in the comparison.Terminal metal-oxygen bonds are a common feature of the high oxidation state chemistry of the transition elements.’ They often participate in the important process of 0x0 trans- fer.’ X + AOsXO + A Among the metals of the left half of the transition series a variety of metal-oxo subunits exist, ranging from simple mono-oxo to tetra-oxo species. When two or more terminal oxygen ligands are present, several geometric arrangements become possible and examples of virtually every isomeric configuration can be found, e.g. cis and trans O=M=O units, trigonal planar and trigonal pyramidal MO, units, tetrahedral and ‘see-saw’ MO, units.Among the latter, a range of discrete, tetrahedral tetra-oxo complexes are found for each element of the vanadium through to iron triads. These species represent some of the simplest metal-oxo mol- ecules.’ The richness of metal-oxo chemistry has stimulated a good deal of theoretical interest. Studies in this laboratory have examined (i) the electronic transition energies of a range of d’ complexes of Cr03+ VO” and Moo3+, (ii) the opti- mised structures and vibrational energies of Mo’X, species (M = Cr, Mo; X = F, C1)6 (iii) the optimised bond lengths and electronic structures of [MO,]’-compounds (M = Cr, Mn, Fe).7 In each case, a theoretical model based on density functional theory (DFT) was used and found to give good agreement with experimental data.This contrasts with the results of Hartree-Fock treatments of the M-0 bonding in, for example, tetrahedral oxyanions which are qualitatively and quantitatively po~r.~,~ The present paper extends our earlier DFT treatment of the [MO,]’-species7 to the esti- mation of their vibrational energies. Once again, the DFT approach yields accurate results. Subject to a minor revision of the assignment of the spectrum of [FeO,]’-, the experi- mental bands are reproduced to within 7% or better. Computational Details All DFT calculations were based on the Amsterdam density functional (ADF) program system due to Baerends et al.” with STO basis sets of double-c plus polarisation (DZP) and triple-c plus polarisation (TZP) quality.’ ‘,12 The method has been described in detail el~ewhere.~.’ The uniform electron gas local density appr~ximation’~ (LDA) was used in con- junction with analytical energy gradients’ for all geometry optimisations which were constrained to & symmetry.The LDA correlation energy was computed according to Vosko et UI.’S’~ parametrisation of electron gas data and includes Stoll et d’s’ 7,18 correction for self-interaction. Non-local gradient corrections (GC)to the LDA exchange and corre- lation terms employed the formulations of Becke” and Perdew,20.21 respectively. The lower core shells on the atoms (1s on 0 and up to 2p on the metal atoms) were treated by the frozen-core approximation.22 The total molecular elec- tron density was fitted in each SCF cycle by auxiliary s, p, d, f and g STO functions.23 Harmonic vibrational energies were estimated via finite dif- ference of the analytical first derivatives. The calculations were run without symmetry constraints (i.e.with all nine degrees of freedom), with two additional points used to esti- mate each second derivative. The resulting nine computed vibrational energies were then averaged over symmetry-equivalent components assigned by reference to the com-puted eigenvectors. Notionally equivalent vibrational com-ponents never differed by more than ca. 15 cm-’,which indicates the approximate level of numerical ‘noise’ inherent in this procedure. Our previous DFT study’ on these complexes gave opti- mised M=O distances within 0.02 A of the experimental values.Test calculations using the experimentally observed bond lengths and the DFT-optimised bond lengths gave computed vibrational-energy differences of around 10 cm-’. This error is of the same order as the numerical ‘noise’ described above and hence the averaged experimental bond lengths were used throughout, uiz. 1.658 A,’, 1.659 A’’ and 1.656 for Cr-0, Mn-0 and Fe-0, respectively. The formally d’ [MnO,]’- ion possess a 2E Jahn-Teller active ground state. In principle, a regular & geometry would not correspond to a minimum on the potential-energy surface and would yield an imaginary vibrational frequency. However, there is no experimental evidence for a marked Jahn-Teller di~tortion.~’To circumvent this problem and maintain the high symmetry for comparison with the other two molecules, the unpaired electron was distributed equally between the two components of the 2e partially occupied MO (molecular orbital).The DFT formalism remains consis- tent even with non-integral MO occupation^.'^ This pro- cedure then generates a ’A, state which now corresponds to a (constrained) energy minimum. The fact that this state is not the global minimum does not appear to affect adversely the computed vibrational energies (vide infra). Results and Discussion Accurate second derivatives are required for characterising potential-energy surface features. For example, a minimum gives all real eigenvalues while a saddle point corresponding to a transition state will have exactly one imaginary eigen- value.At present, computation is the onZy way to obtain detailed transition-state geometries. In addition, vibrational energies derived from the second derivatives can be used to compute zero-point energies and, using statistical mechanics, thermodynamic quantities like enthalpy and the entropy.'* A completely theoretical estimation of reaction energetics requires the accurate estimation of the second derivatives and molecular vibrational energies. The calculated and observed data -for the three complexes are collected in Table 1 along with error estimates in terms of percentage differences. Generally, there is a marginal improvement using the larger basis but the difference between local and non-local functionals is slight (uide infra).Considering the order of the vibrational bands; in each case the theoretical assignment, labelled (1) below, yields energies which increase in the order : E < F, < A, < F, The experimental assignment for [CrO,]' -has been reported by Weinstock et aL2' while Grifith and Gonzalez- Vichez have given assignments for the other two complexes on two separate occasion^.^^^^^ Assignment (1) is suggested for both [MnOJ2- and [FeO,]'- in the earlier However, in the later work,3' an alternative assignment, (2), was proposed for [Fe0,I2- based on the polarisation of lines in the Raman spectrum. F, < E < F2< A, This conflicts with the present DFT results.However, given the similarity of the species and the otherwise good numerical agreement, we suggest a revision back to the original assign- ment (1). The most favourable agreement between computed and observed energies uses the earlier data for [Mn0,12- with J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the more recent band maxima for [Fe0,I2 -reassigned. Under these conditions, the average deviation among the TZP results is between 3 and 5%, with the largest percentage error on any one band being 7.6% (for the lower F, vibration of [MnO4l2- and with the largest energy difference of 55 cm-' for the A, band of [FeOJ2- at the GC TZP level. The largest percentage deviation for any TZP result is 12% corre-sponding to the highest F, band for [FeO,]'- which, at the GC TZP level is 95 cm-higher than experiment, provided one is prepared to accept assignment (2).If assignment (1) is considered but with any of the reported experimental data, the largest error is 7% corresponding to a deviation of 55 cm-' between observed and calculated (GC TZP) A, vibra-tions for [Fe0,]2- The difference between best and worst cases is, therefore, not too large. The best case comparison with respect to the experimental data is illustrated in Fig. 1. At a detailed level, theory tends to overestimate slightly the energies of the highest two bands, which correspond to M-0 stretching vibration^.^' The LDA is noted to lead to overbinding" which could lead to steeper potential wells and hence to computed energies that are too high.However, the non-local GC results, which should correct the overbinding, always give a slightly increased energy relative to the compa- rable LDA TZP data. Evidently, the depth of the potential well is improved using GC functionals but its shape is vir- tually unchanged. Therefore, the GC results for the two high- energy bands, tend to be in slightly worse agreement with the experimental results. In contrast, the LDA calculations for the lower two bands tend to underestimate slightly the experimental values such that the increase found for the GC results brings the latter into slightly better agreement with the experimental. In summary, the LDA results are slightly better than the GC for the two higher-energy bands and slightly worse for the two lower-energy bands.However, it is clear that the effects of Table 1 Observed and calculated vibrational energies (an-'),percentage deviations [A (%)I and the average magnitude of A (%)(I A (%)I) for tetrahedral [MO,]'-complexes; where multiple sets of experimental data are quoted, corresponding sets of A (%) and I A (%) I are also given' vibrational energylcm -' complex method E A (%) F, A(%) A, A(%) F2(%) A(%) lA(%)[ [CrO,l2 - exp.bLDA, DZP LDA, TZP GC, TZP 349 332 323 330 -7.7 -7.5 -5.4 378 356 356 380 -5.8 -5.8 +0.5 846 836 866 870 -1.2 +2.4+2.8 890 894 912 926 +0.5+2.5+4.0 3.8 4.6 3.2 [MnO,]'- exp.'exp.d LDA, DZP LDA, TXP GC, TZP 325 325 307 311 321 -5.5 -5.5 -4.3 -4.3 -1.2 -1.2 332 328 330 336 353 -0.6 +0.6 + 1.2 +2.4 +6.3 -7.6 812 816 856 844 853 +6.4+4.9 +3.9+3.4+5.1 +4.5 820 862 909 876 889 10.9+5.5+6.8+ 1.6+8.9 +3.1 5.6 4.1 4.1 2.9 5.3 4.1 [FeO,]'- exp.dexp.'Vd exp.f LDA, DZP LDA, TZP GC, TZP 320 340 322 292 304 31 1 -8.8 -14.1 -9.3 -5.0 -10.6 -5.6 -2.8 -8.5 -3.4 340 322 340 3 14 328 341 -7.7 -2.5 -7.7 -3.5 + 1.9 -3.5 +0.3 +5.9 +0.3 778 832 790 810 833 845 +4.1 -2.6 +2.5 +7.1 +0.1 +5.4+8.6+1.6+7.0 800 790 832 864 867 885 +8.0 +9.4 3.9+8.4+9.8+4.2 10.6 12.0+6.4 7.2 7.2 5.9 6.0 5.6 4.7 5.6 7.0 4.3 ~~~ See text for explanation of methods and assignments.Ref. 29. 'Ref. 31. Ref. 30. 'Based on published assignment (2). f Based on assign-ment (1). J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 1000 r--------- I10001 I 800 800 600 - 600 400 - 400 200 - 200 0 07-800 0 obs.m 600 -. calc.(LDA, DZP) calc.(LDA, TZP) --m400 calc.(GC, TZP) 200 --0 ?--Fig. 1 Graphical comparison of the observed and calculated vibrational energies (cm-')for [MO,]'-complexes listed in Table 1. M: (a) Cr, (b)Mn, (c)Fe. 400380 i" 360 (a' 360t -340 320 300 1 Cr Mn Fe Cr Mn Fe 920 920 900 900 - 880 880 - 860 860 --;(d) 840 840 820 820 800 800 780 780 Cr Mn Fe Fig. 2 Systematic deviations between observed (m) vibrational energies (cm-')and those calculated at the LDA TZP level (0)for vibrational bands (a) E, (b)F,, (c)A, and (6)F, 3240 changing basis set and functional are all relatively small and of a similar order to the numerical ‘noise’ of around 10-15 cm-’. This range can be considered a rough guide to the theoretical error bars and essentially implies that the LDA DZP, LDA TZP and gradient corrected TZP results are fairly comparable.As found for the dioxodihalides of Cr and Mo,~ harmonic vibrational energies from DFT reproduce the experimental data which are intrinsically anharmonic. Evidently, these systems either obey the harmonic oscillator approximation quite well or there is a fortuitous cancellation of errors in the DFT calculations. Either way, the DFT method appears to give consistently reliable results. Assuming assignment (1) throughout, then, one sees that each band tends to decrease in energy in the order [CrOi-> [Mn0,12-> [FeO,]’-.The comparison between theory (LDA TZP) and experiment is displayed in Fig. 2. The only discrepancy seems to be that the lower- energy F, band observed in [MnO,]’- is anomalously low. Otherwise, there appears to be a relatively systematic discrep- ancy between experiment and the DFT results. However, as noted above, a special procedure was adopted for the Mn anion in order to avoid a ground-state orbital degeneracy. It is possible that this procedure is responsible for the apparent anomaly but, given the agreement found for the other three bands, it seems possible that the experimental data may be suspect. The two low-energy bands of [MnO,]’- are report- ed to be only 3-7 cm-’ apart and presumably overlap.It may be that the exact positions of the band maxima are subject to a larger uncertainty. An increase of only 20 cm-l for the lower F, band would establish a consistent trend. However, in the absence of the actual spectra, the reliability of this assertion cannot be judged. Conclusions Density functional theory calculations of the vibrational energies of [CrO,]’-, [MnO,]’-and [FeO,]’-using double and triple-[ STO basis sets give accurate agreement with experimental results subject to a revision of the assign- ment of [FeO4l2- back to the original proposal. The average error for the larger basis sets using both local and non-local functions is only about 4%, with a maximum average devi- ation of about 7% depending on which experimental data one compares with.Theoretical estimates for the two high- energy stretching modes tend to be slightly too small. These errors are of the same order as the numerical noise, which is around 10-15 cm-’. This study, together with the previous work on these complexes,’ demonstrates that DFT provides a very satisfactory description of the potential-energy surface of these molecules in the vicinity of the ground state. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The authors acknowledge the support of the Engineering and Physical Sciences Research Council for the provision of a stu- dentship (to P. D. S.) and computer hardware (through the Computational Science Initiative and the Science and Materials Computing Committee).References 1 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley-Interscience, New York, 5th edn., 1988. 2 R. H. Holm, Chem. Rev., 1987,87,1401. 3 R. J. Deeth, J. Chem. SOC., Dalton Trans., 1990 365. 4 R. J. Deeth, J. Chem. SOC., Dalton Trans., 1991, 1467. 5 R. J. Deeth, J. Chem. SOC., Dalton Trans., 1991, 1895. 6 R. J. Deeth, J. Phys. Chem., 1993,97, 11625. 7 R. J. Deeth, J. Chem. SOC., Faraday Trans., 1993,89,3745. 8 P. W. Fowler and E. Steiner, J. Chem. Soc., Faraday Trans., 1993,89,3745. 9 M. A. Buijse and E. J. Baerends, Theor. Chim. Acta, 1991, 79, 389. 10 E. J. Baerends, D. E. Ellis and P. Ros, Chem. Phys., 1973,2,41. 11 J. G. Snijders, P. Vernoojks and E. J. Baerends, At. Data Nucl. Data Tables, 1981,26,483.12 P. Vernooijs, G. P. Snijders and E. J. Baerends, Slater Type Basis Functions for the Whole Periodic System, Internal Report, Free University, Amsterdam, 1981. 13 T. Ziegler, Chem. Rev., 1991,91,651. 14 J. C. Slater, Adv. Quantum Chem., 1972,6, 1. 15 L. Versluis and T. Ziegler, J. Chem. Phys., 1988,88, 322. 16 S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 1980, 58, 1200. 17 H. Stoll, E. Golka and H. Preus, Theor. Chim. Acta, 1980,55,29. 18 H.Stoll, C. M. E. Pavlidou and H. Preuss, Theor. Chim. Acta, 1978,49, 143. 19 A. J. Becke, Chem. Phys., 1986,84,4524. 20 J. P. Perdew, Phys. Rev. B, 1986,33,8822. 21 J. P. Perdew, Phys. Rev., B, l987,34,7406(erratum). 22 E. J. Baerends, D. E. Ellis and P. Ros, Theor. Chim. Acta, 1972, 27, 339. 23 J. Krijn and E. J. Baerends, Fit Functions for the HFS Method, Internal Report, Free University, Amsterdam, 1984. 24 J. S. Stephens and D. W. J. Cruckshank, Acta Crystallogr., Sect. B, 1970,26,437. 25 Comprehensive Coordination Chemistry, ed. G. Wilkinson, Perga- mon, Oxford, 1987, vol. 4, p. 109. 26 R. J. Audette, J. W. Quail, W. H. Black and B. E. Robertson, J. Solid State Chem., 1973,8,43. 27 J. C. Slater, Quantum Theory of Molecules and Solids, McGraw-Hill, New York, 1974, vol. 4. 28 S. W. Benson, Thermochemical Kinetics, Wiley, New York, 1976. 29 N. Weinstock, H. Schulze and A. Muller, J. Chem. Phys., 1973, 59, 5063. 30 W. P. Grifith, J. Chem. SOC.A, 1966, 1467. 31 F. Gonzalez-Vichez and W. P. Griffith, J. Chem. SOC., Dalton Trans., 1972, 1416. 32 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley-Interscience, New York, 3rd edn., 1978. Paper 4/041251; Received 6th July, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003237

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3241-3244 Theoretical Study of the Electroreduction of Halogenated Aromatic Compounds Part 3.”f-, m-and p-Dibromobenzenes studied by AM1 and PM3 Methods Roberto Andreoli, Giovanna Battistuzzi Gavioli, Marco Borsari and Claudio Fontanesi* Universita degli Studi di Modena , Dipartimento di Chimica, Via Campi 183,4I 100Modena , Italy ~~~ The electroreductive potential values of the o-, rn-and p-dibromobenzenes (DBBs) follow an unusual pattern in that, unlike the structurally related dichlorobenzene (DCB) derivatives, these three isomers exhibit a strong ‘ortho effect’ and are accompanied by a large difference in the €,/2 values. Any interference with the mecha- nism of reduction on the part of the chemical enivironment can be safely ruled out, given the consistency of the €,/* values obtained in four different solvents.The use of theoretical indices calculated by the PM3 method enables the experimental behaviour of the DBBs to be rationalised on the basis of the electronic structure of the neutral isolated molecule. Remarkably, PM3 indicates the formation of a o-type radical anion in the case of the reduction of the bromo benzene derivatives. The theoretical rationalisation of the electroreductive mecha- nism of aromatic compounds bearing one or more halogens bound to the aromatic moiety has long been a matter of debate.’-’’ The influence of a broad set of parameters on the electrochemical reduction of this class of compounds (e.g. solution acidity and the number and type of substituents bound to the aromatic ring) has been extensively examined.The experimental evidence suggests that, as is generally accepted, reduction involves the cleavage of the C-X bond (which turns out to be a multiple cascade cleavage process when more than one halogen is present’ 2,1 3, and is an overall irreversible diffusion-controlled two-electron process. The first step of the reduction mechanism involves a reversible one-electron transfer which produces a radical anion. This latter species, following the cleavage of the C-X bond, yields the aryl radical. A second electron transfer or a dispro-portionation reaction (or, more rarely, other types of interme- diate stepsI4), both followed by protonation, leads to the final aromatic product.”-’ Mo reover, great efforts have been made to rationalise these results theoretically in terms of the molecular structure of the compounds undergoing reduction. In the present study, all the mono-and di-substituted chloro-and bromo-benzene derivatives are investigated under the same experimental conditions in four solvents fea- turing quite different physical and chemical characteristics : dimethyl sulfoxide (DMSO), dimethylformamide (DMF), ace- tonitrile (ACN) and ethanol (EtOH). In particular, the electroreductive behaviour of the dichloro- and dibromo-substituted benzenes is closely com- pared, since the ordering of the reduction potential is reversed when the experimental Eliz values of the ortho, meta and para derivatives of the two series are considered.The dibromobenzenes, unlike the dichlorobenzenes, exhibit an ‘ortho effect’ (i.e. the reduction potential of the ortho isomer is patently more positive than that of the meta and paru The experimental findings were subjected to a process of theoretical rationalisation using the AM1 l9 and the quite recent PM320 parametrisations, both of which belong to the family of NDDO-based calculations. AM1 was particularly effective in the study of the electro- reduction of a series of compounds featuring both bromo- and chloro-substituted aromatics, whereas the original MNDO parametrisation t Part 2: J. Chem. SOC.Faraday Trans., 1993,89, 3931. The PM3 method was selected by virtue of the fact that simultaneous optimisation of the parameters relating to H, C, C1 and Br atoms has been reported.” In the case of the orig- inal MND02’ and AMl19 parametrisation, the H and C atom parameters are determined simultaneously, while the C1 and Br atom parameters are obtained in two successive steps.19.2 1-24 Calculations All molecular orbital data are calculated at the single-determinant level and are of RHF or ROHF type.Open-shell calculations are made by employing the half-electron method2’ as implemented in the AMPAC 2.1 and MOPAC 6 suite of pr~grarns.~~,~~ Geometries of the neutral molecules are fully optimised. Experimental Chloro- and bromo-benzene, and dichloro- and dibromo- benzenes were purchased from Carlo Erba (R.P.E.) and used without any further purification.Polarographic and voltam- metric measurements in the depolariser concentration range (0.5-5) x mol dm-3 were carried out by a PAR 273A Potentiostat/Galvanostat, using DMF (Fluka, <0.01% H,O), DMSO (Fluka, <0.01% H20), ACN (Fluka, <0.01% H,O) and EtOH (Carlo Erba, <0.1% H20) as solvents. The working electrodes were a DME and an HDME. The reference electrodes were: Ag/AgCl, NaCl,,,. in DMF, Ag/ AgC1, KCl,,,. in DMSO, Ag/AgN03 (0.1 mol dmP3) ACN and Hg/Hg2C12, KCl,,,. in EtOH. A Pt sheet was used as the counter electrode. The Eli! values were calibrated against the cobaltocene/ cobaltocenium couple and an aqueous saturated calomel electrode (SCE), with an accuracy of ca. f.10 mV, was used as reference.The ionic strength of all the solutions was kept constant (I = 0.1 mol dmP3), using Bu,NClO, (Fluka) as the supporting electrolyte, as was the cell temperature (T = 298 & 0.1 K). Results and Discussion Electrochemical Behaviour The half-wave potentials of the eight halogenobenzene derivatives in DMF, DMSO, ACN and EtOH, are reported in Table 1. Chloro- and bromo-benzene present only one bielectronic, diffusion-controlled reduction wave, no matter which solvent J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Polarographic reduction potential values in DMSO, DMF, ACN and EtOH, dropping time 1 s no. compound chlorobenzene bromobenzene 1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene 1,Zdichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene DMSO DMF ACN EtOH wave I wave I1 wave I wave I1 wave I wave I1 wave I wave I1 2.740 - 2,750 - - - 2.820 - 2.610 - 2.630 - 2.700 - 2.680 - 2.505 2.735 2.525 2.755 2.635 - 2.610 2.810 2.475 2.740 2.485 2.750 2.620 - 2.570 2.810 2.470 2.735 2.490 2.750 2.595 - 2.565 2.820 2.027 - 2.045 - 2.060 - 2.005 2.685 2.155 2.615 2.180 2.635 2.215 2.690 2.140 2.685 2.330 2.615 2.355 2.630 2.385 2.695 2.365 2.680 is used.The semilogarithmic plot indicates that the electron transfer is irreversible, while the a, value is ca. 0.65 in aprotic solvents (DMF, DMSO, ACN) and 0.69 in EtOH. In the case of the dihalogenobenzenes, and with the excep- tion of 1,2-dibromobenzene, which exhibits a single four- electron wave, two bielectronic, diffusion-controlled waves are observed.Again, the electron transfer is invariably irre- versible and the a, values are similar to those obtained for the monohalogenobenzenes, except in the case of 1,2-dibro- mobenzene, for which an = 0.72 and 0.75 in aprotic and protic media, respectively. The product id q1/2/n (where id is the diffusion current, q the solvent viscosity and n the number of electrons involved in the reduction process determined by coulometric measurements) is practically constant for all the polarographic waves in all four solvents. The irreversibility of the overall reduction process was also confirmed by cyclic voltammetry measurements for scan rates up to 1OOovs-'. In particular, for DCBs and DBBs, the value of the first reduction wave depends on the type and position of the halogen atom on the benzene ring, while the ElIz value of the second wave (save for o-DBB) is constant and corre-sponds to the EIl2 value of the monohalogenosubstituted compounds.In general, the observed polarographic behav- iour is in agreement with previously published studies on the electroreduction of chlorobenzenes. '-'vl 2-14918 According to the evidence obtained in the case of the reduction of other halogenoaromatic compounds, and to the results of the UV spectra analysis performed on the products of the coulometry, the overall electrochemical process under- lying the first wave of the dihalogeno-derivatives can be sum- marised as follows : PhX, + 2e-+ H+ -+ XPhH + X-(1) and XPhH is subsequently reduced to PhH by the same mechanism.For o-DBB the same result is obtained in a single polarographic wave. Fig. 1 shows plots of in DMSO, ACN and EtOH us. in DMF, to be linear. The same sign of the slope for all the solvents indicates that the compounds always undergo reduction in the neutral molecular form. In particular, the linearity and the slope of unity suggest that structural effects play the same role in the determination of the values in the different solvents. These findings also indicate the absence of any significant interference by the solvent on the reduction mechanism illustrated above (e.g.rearrangement in the solva- tion shell or direct solvent intervention in the redox process). Note that there is close agreement between the electro- chemical behaviour described here and the data reported both in ref. 18 (concerning compounds 1, 2, 6, 7, 8: El.,; 18 =ref -0.823 + 1.02E;y; work, r = 0.996; besides the good linearity the two series of data are monotonically ordered, i.e. without any inversion) and in ref. 12 (again, a monotonic ordering of the reduction potentials of chlorobenzene and of the DCB isomers is observed). Theoretical Indices In this study the chlorinated aromatic derivatives serve essen- tially as reference compounds, as their electrochemical behav- iour has been satisfactorily described in ref.2 and 3. Attention is drawn rather to the o-, rn-and p-DBB com- pounds, which follow a rather peculiar reductive pattern (Table 1).In particular, there is a strong 'ortho effect', charac- terised by considerable divergence of the El/2 values of the individual isomers, whereas the half-wave potentials of the DCBs are fairly close. Attempts have been made to rationalise the anomalous behaviour of the o-, rn-and p-DBBs by invoking certain par- ticular " In the case of the reduction of the ortho isomer the forma- tion of benzyne has been hypothesised.18 This could account for the single reduction wave shown by the ortho-isomer, but the difficulties encountered in attempting to rationalise the behaviour of the rneta and para isomers are left unresolved (difficulties shown both by the MND06 and AM1 methods.Fig. 2 and 3). Also, although the multivariate fit of the ElIz values using CT*energies and the net charge of the leaving group = a + bE@ + cqleavingX)is able to predict correctly the ortho-rneta ordering of the DBBs, it fails completely to predict that of the DCBS;~ for, in DMF the calculated potentials are -1.740 and -1.88 V for the o-and rn-DCB isomers, respec- 3.0 I I , I 02.8 v 0" 2.6 v) 2 $2.4 . 7 G I d'2*2ii, 12.0 -, , 2.0 2.2 2.4 2.6 2.8 3.0 -Ei/,(DMF)/V VS. SCE Fig. 1 El,, measured in DMF us. El/, measured in DMSO (V),ACN (0)and EtOH (0) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -0.1 0 I 4o03 h2 -0.2 05 -0.3106 07 t -0.4 - (38 - -0.5." " ' " ' 1 , l I I I , tively,6 while experimentally they were reported as under- going reduction at the same value of potential (-1.81 V).6 Note also that this latter experimental result is inconsistent both with the data presented in ref. 12 and with our findings. (Table 1, column 2). These two sets of experimental data (ours and that of ref. 12) indicate that the qualitative predic- tion of the ordering of the reduction of the DCBs6 is also wrong. This failure is probably due to the inability of the original MNDO parametrisation to account for variation in the molecular electronic structure when dealing simulta- neously with aromatic compounds bearing Cl, Br and I sub-stituents, at least in the field of electrored~ction.~*~~ Effects induced by the chemical environment (e.g. prefer-ential solvation) can be excluded, as indicated by the absence of any inversion, in the series of the EIl2 values, when the solvent is changed (Table 1, Fig.1) despite the fact that the four different solvents employed are characterised by signifi- cant variations in both physical (relative permittivity, vis- cosity, etc.) and chemical (acidity, DN, AN, etc.) proper tie^.^^ Thus they create reaction environments which are substan- tially different. Against this background, the overall electro- reductive behaviour of DBBs (notably, evidence of the 'ortho effect') still remains unclear. The lowest unoccupied molecular orbital (LUMO) energy (and type) and vertical electron affinity, A, = E,,,, URA 0.0 -0.2 -0.4 2 4--.400 3 -0.6 05 07 06-0.8 08 -1 .o 2.0 2.2 2.4 2.6 2.8 3.0 -EII,(DMF)/V vs. SCE Fig. 3 Calculated electron affinity, A,, us. El/* measured in DMF; AM1 method 3243 -E,,,, ,f are the theoretical indices relating to the char- acterisation of the electrochemical behaviour of the com-pounds studied, they are calculated using AM1 and PM3 methods. The molecular descriptors obtained by the AM1 method give the wrong reduction ordering (Fig. 2 and 3), mainly for the DBB isomers. The use of the AM1 method in the study of the electroreduction of organic compounds appeared very prom- ising, owing to its ability to rationalise, in a single corre- lation, the electroreductive behaviour of organic compounds bearing a Br or a C1 substituent, as well as affording a correct prediction of the reduction product^.^ This failure was there- fore particularly frustrating.Alternatively, the values calculated by means of the PM3 method afford a monotonically ordered and satisfactorily linear pattern when plotting both LUMO energies and verti- cal electron affinity us. El12.(Fig. 4 and 5). This finding sug- gests that the electroreductive behaviour of both the chloro- and bromo-benzene derivatives is governed by the electronic -0.4":, 2.0 2.2 2.4 2.6 2.8 3.0 -EII,(DMF)/V VS. SCE Fig. 4 Calculated LUMO energies us. El/2 measured in DMF; PM3 method -0.2 1 -0.4 2--. q! -0.6 -0.8 -1 .o 21.0 2.2 2.4 2.6 2.8 3.0 -E,/,(DMF)/V vs.SCE Fig. 5 Calculated electron affinity, A,, us. E,/, measured in DMF; PM3 method t URA stands for unrelaxed radical anion; unrelaxed means, in this context, that the spatial nuclear coordinates of the radical coin- cide with those of the neutral parent molecule. 3244 features of the isolated neutral molecule. This is in agreement with results obtained in previous studies. 1-3 In particular, in the case of the o-DBB, the formation of benzyne as a reaction intermediate cannot be completely excluded. For, its LUMO energy is -0.902 eV (PM3), which is lower than the value determined for the o-DBB. Actually, if benzyne were formed following the reduction of the o-DBB, it would undergo sudden reduction.On the other hand, o-DBB fits satisfactorily the correlations in Fig. 4 and 5, thus if we accept the hypothesis of benzyne formation, we must con-clude that it has a negligible effect on the reduction potential value. Note that, beyond the purely structural nature of the EL,,, us. E 1/2 correlation, which enables a clear structure- activity relationship to be established between the com-pounds in question and their observed experimental behaviour, the linear trend also found when plotting A, us. ElI2 (Fig. 5) indicates that the first electron transfer deter- mines the value of El,,; moreover, approximation of the slope to unity in Fig. 5 implies that our El/, values are directly proportional to the E' values (i.e.E' = a + E1/2).3917 In fact, a qualitative systematic difference between the values obtained by the two theoretical methods, appears to constitute the underlying cause of the significantly different outcome.For, the LUMOs calculated by the AM1 method are always of the n-type, and the same result is found not only when dealing with a large series of halogenated com- pound~,~ Thisbut also when using the MNDO meth~d.~.~ would appear to be a general tendency of these NDDO- based methods. However, the LUMOs of all 12 possible chlo- rinated benzene derivatives calculated by CND0/2 have been reported as being a mixture of CT and n virtual molecular orbitals. When calculated by PM3, the LUMOs were found to be of a n nature of the chlorinated compounds and a for the bromo- and dibromo-benzenes. Note that the a-type LUMO is localised on the C-Br bond, so the bromine is predicted to be the leaving group, as is found experimentally.Note that in the calculation of A,, both the LUMO of the neutral parent compound (no. 2, 6, 7, 8) and the SOMO (singly occupied molecular orbital) of the radical anion is of the a-type. This suggests that, in the case of the bromine derivatives, a r~ radical is formed, which is at variance with the findings for the chloro-substituted aromatics (compounds no. 1, 3, 4, 5). The parameters used in the MNDO, AM1 and PM3 methods reveal that the most important change is due to a variation in bromine parametrisation. In particular, the &/&, ratio changes progressively: 0.9, MNDO; 2.2, AM1; 4.6, PM3; where s and p refer to the s and p atomic orbitals, respectively.Indeed, b, and j?, are the two parameters used in the calculation of the Fock matrix elements connecting orbitals on different atoms.,' Consequently, this seems to be the origin of the r~ nature of the LUMO in the case of the bromo-substituted benzenes when calculations are carried out by means of the PM3 method. In any event, the finding that bromo-substituted benzenes featuring a-type LUMO (and SOMO) electronic configu- rations are the most stable neutral closed-shell (and radical anion) species (with respect to the n ones) should be viewed with caution, and verified before any definitive physical meaning is attached to it.This particular point is currently being investigated using ab initio methods. Conclusion In the study of the electroreduction (or oxidation) of organic compounds the use of molecular descriptors/half-wave poten- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 tial relationships as an aid to the interpretion of the redox mechanism at a molecular level, is widespread. The 'quality' of the theoretical indices employed in such relationships has been improved from Hammett constants to quantum-mechanical molecular descriptors, which can be cal- culated at various levels of sophistication. l~~ On the basis of the present findings the PM3 method appears to be able to rationalise correctly the peculiar redox behaviour of DBBs in terms of the electronic structure of the neutral isolated molecule.Thus, its use is suggested when theoretical indices are employed in the modelling of the redox behaviour of bromo- and chloro-substituted derivatives. This work was supported by a grant from the Consiglio Nazionale delle Ricerche (CNR), Roma, and from the Minis- tero dell' Universita e della Ricerca Scientifica e Tecnologica, Roma, MURST 40%. Calculations were performed at the CICAIA, Universita di Modena. 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ISSN:0956-5000    

DOI:10.1039/FT9949003241

出版商:RSC    

年代:1994

数据来源: RSC