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 RSCl %RSC.ORG@UK.AC.NSF NET- RELAY) Fax: (0223) 42601 7

ISSN:0956-5000    

DOI:10.1039/FT99490FX089

出版商:RSC    

年代:1994

数据来源: RSC

ISSN:0956-5000    

DOI:10.1039/FT99490BX091

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(23) 3473-3590 (1 994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 3473 Microwave spectroscopic investigation of thionyl chloride, SOCl, : Hyperfine constants and harmonic force field H. S. P. Miiller and M. C. L. Gerry 3483 Vibrational spectroscopic study of N-acylglycine monomeric and dimeric salts: The long acyl chain effect on the NH (ND) stretch, amide I, amide I1 and CH,-wag modes K. Ohshima, H. Okabayashi and T. Yoshida 349 1 Molecular structures of cis-and trans-S-ethyl thiocrotonate. A combined vibrational spectroscopic and ab initio SCF-MO study R. Fausto, P. J. Tonge and P. R. Carey 3505 Classical and non-classical silicon radical cations: H,SiX'+ species (X = N, 0,F, P, S and C1) M.Sana, M. Decrem, G. Leroy, M. T. Nguyen and L. G. Vanquickenborne 3513 Aromatic character of graphite intercalation compounds J-i. Aihara and T. Tamaribuchi 3517 Light stability of a B-cyclodextrin inclusion complex of a cyanine dye Y. Matsuzawa, S-i. Tamura, N. Matsuzawa and M. Ata 3521 Determination of the NMR monomer shift and dimerization constant in a self-associating system by direct application of the least-squares method H. K. S. Tan 3527 Equation of state of liquid o-xylene at low temperatures and high pressures M. Taravillo, S. Castro, V. Garcia Baonza, M. Caceres and J. Nuiiez 3533 Theoretical investigation of steroidal inhibitors of glucose-6-phosphate dehydrogenase M. H. Charlton and C. Thomson 3539 Alkaline-earth-metal@) complexes with hydroxide and fluoride in molten NH4N0, * 1.5H,O at 50 "C F.Frosternark, P. Malmquist, L. A. Bengtsson and B. Holmberg 3545 Adsorption of dialkyl derivatives of thiourea from an ethanol solution at a mercury electrode B. Marczewska 3549 Comparison of the cation environment in polymer electrolytes based on poly(ethy1ene oxide) and transition-metal bromides H. M. N. Bandara, W. S. Schlindwein, R. J. Latham and R. G. Linford 3555 Self-diffusion, thermal effects and viscosity of a monodisperse associative polymer : Self-association and interaction with surfactants K. Persson, G. Wang and G. Olofsson 3563 Characterisation behaviour of the complexation of copper@) with polymer-bound vinylimidazole ligands Y. Kurimura, T.Abe, Y. Usui, E. Tsuchida, H. Nishide and G. Challa 3567 Time-resolved fluorescence probing of oil-in-water microemulsions stabilised by non-ionic surfactants P. D. I. Fletcher and R. Johannsson 3573 Characterization of acidity of pillared clays by proton affinity distribution and DRIFT spectroscopy T. J. Bandosz, J. Jagiello, K. Putyera and J. A. Schwarz 3579 Neutron scattering study of protonic species in ammonium pentamolybdate, (NH,)M0,.,,H30,, R. C. T. Slade, G. P. Hall, A. Ramanan and J. M. Nicol 3585 Room-temperature catalytic fluorination of C, and C, chlorocarbons and chlorohydrocarbons on fluorinated Fe,O, and c0304 J. Thomson 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 W 1V OBN, UK Tel: 44 (0)71-437 8656 Fax: +44 (0)71-287 9798 Telecom Gold 84:BUR210 Electronic Mailbox (Internet) LIBRARY @RSC.ORG. If the material is not available from the Society’s Library, the staff will be pleased to advise on its availability from other sources. Please note that copies are not available from the RSC at Thomas Graham House, Cambridge.

ISSN:0956-5000    

DOI:10.1039/FT99490FP252

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aas,N., 1015 Bassat, J. M., 1987 Briggs, B., 727, 1905, 2703, Che,M., 2277 Defrance, A., 1473 Abadzhieva, N., 1987 Bassoli, M., 363 2709 Chen, J-S., 429, 717 Dejaegere, A., 1763 Abbott, A. P., 1533 Battaglini, F., 987 Brocklehurst, B., 271,2001, Chen, J., 3455 de Leng, H. C., 2459 Abe,T., 3563 Battistuzzi Gavioli, G., 2897 Chen, J. S., 2765 Delhalle, J., 2319 Abraham, R. J., 2775 3241 Brogan, M. S., 1461 Chen, K., 3089 Demeter, A., 41 1, 2635 Abramowicz, T., 2417 Bauer, C., 517 Brown, N. M. D., 1357 Chen, L., 2467 Dempsey, P., 1003 Acharya, A. N., 3293 Baur, W. H., 2141 Brown, R. G., 59 Chen, Y-H., 617 Demri, D., 501 Aeby, D., 3129 Beagley, B., 2775 Brown, S. E., 739 Chen, Z., 2931 Deng, N-J., 1961 Afanasiev, P., 193 Beer, P.D., 2931 Bruckner, A., 3159 Cheng, A., 253 Deng, Z., 2009 Agren, H., 1479 Beeston, M. A., 3109 Bruna, P. J., 683 Cheng, C. P., 11 57 Denkov, N. D., 2077 Aihara, J-i., 3513 Bell, A. J., 17, 817 Bruque, S., 3103 Cheng, Y., 2517 Derrick, P. J., 239 Aikawa, M., 911 Belton, P. S., 1099 Brzezinski, B., 843, 1095 Cherqaoui, D., 97, 2015 Dewing, J., 1047 Aitken, C. G., 935 Benavente, J., 3103 Buchachenko, A. A., 3229 Chesta, C. A., 69 Dheer, J. M., 3261 Akanuma, K., 1171 Bender, B. R., 1449 Buchner, R., 2475 Chevalier, S., 667,675 Diagne, C., 501 Akolekar, D. B., 1041 Bendig, J., 287 Buckley, A. M., 1003 Chi,Q., 2057 Dickinson, E., 173, 2737 Alava, I., 2443 Benfer, S., 2969 Buemi, G., 1211 Child, M. S., 1739 Diebler, H., 2359 Albert, I. D. L., 2617 Bengtsson, L.A., 559, 2401, Bujan-Nuiiez, M. C., 2737 Chiu, S. S-L., 1575 Dines, T. J., 1461 Albery, W. J., 1115 2531,3539 Bulow, M., 2585 Chmiel, G., 1153 Doblhofer, K., 745 Alcober, C., 2395 Benko, J., 855 Burdisso, M., 1077 Cho,T., 103 Domen, K., 911 Aldaz, A., 609 Benniston, A. C., 953,2627 Burgard, C., 3077 Choi, W., 3315 Doney, S. C., 1865 Alfimov, M. V., 109 Beno, B., 1599 Burgess, J., 3071 Choisnet, J., 1987 Dong, S., 2057 Al-Ghefaili, K. M., 383, Bensalem, A., 653 Burget, D., 2481 Choudhary, V. R., 3357 Donnamaria, M. C., 2731 1047 Bensch, W., 2791 Busca, G., 1161,1293,3181, Chowdhry, B. Z., 1999 Dore, J. C., 2497 Ah, V., 579, 583 Berbaran-Santos, M. N., 3347 Christensen, P., 459 Dory, M., 2319 Aliev, A. E., 1323 2623 Busch, T., 261 1 Chung, Y-L., 2547 Dossi, C., 1335 Allegrini, P., 333 Berces, T., 41 1, 2635 Buschmann, H-J., 1507 Cihek, A., 1973 Doughty, A., 541 Allen, N.S., 83 Bergeret, G., 773 Butler, L. J., 1581, 1612, Claridge, J. B., 2799 Douglas, C. B., 471 Almond, M. J., 3153 Berglund, J., 3309 1613,1614, 1671,1677, Clark, G. R., 3139 Downing, J. W., 1653 Alonso, J. L., 2849 Bernardi, F., 1617, 1669, 1809 Clark, T., 1669, 1678, 1783, Duarte, M. L. T. S., 2953 Alparone, A., 2873 1671, 1672 Butt, M. D., 727 1807,1808, 1809,1810 Duke, M. M., 2027 A1 Rawi, J. M. A., 845 Berndt, H., 2837 Buttar, D., 1811 Clegg, S. L., 1875 Dunford, H. B., 3201 Amorim da Costa, A. M., BertrPn, J., 1679, 1757, Buxton, G. V., 3309 CICment, R., 2001 Dunmur, D. A., 1357 689 1800,1806 Byatt-Smith, J.G., 493 Climent, M. A., 609 Dunstan, D. E., 1261 Amoskov, V. M., 889 Beutel, T., 1335 Cabaleiro, M. C., 845 Coates, J. H., 739 Duplltre, G., 1501 Ando, M., 1011 Beyer, H. K., 1329 Caceres, C., 2125,3203 Coitiiio, E. L., 1745 Duxbury, G., 1357,3373 Andre, J-M., 2319 Bhuiyan, L. B., 2002 Chceres, M., 1217, 3527 Collett, J. H., 1961 Dwyer, J., 383, 1047 Andreoli, R., 3241 Bickelhaupt, F., 327, 1363 Caceres Alonso, M., 553 Colmenares, C. A., 1285 Dyke, J. M., 17 Andrts, J., 1703,2365 Bickley, R. I., 2257 Cairns, J. A., 1461 Coluccia, S., 3167 Dziembaj, R., 2099 Andrews, S. J., 1003 Biczok, L., 411,2635 Calado, J. C. G., 649 Cook, J., 1999 Eastoe, J., 487,2497,3121 Anson, C. E., 1449 Bielanski, A., 2099 Caldararu, H., 213,2643 Cooney, R. P., 2579 Easton, C.J., 739 AntoniC, T., 1973 Biggs, P., 1197, 1205 Callens, F. J., 2541,2653, Cooper, D. L., 1643 Ebitani, K., 377 Aragno, A., 787 Biggs, S., 3415 3261 Cordischi, D., 207 Eder, F., 2977 Arai, S., 1307,3449 Billingham, J., 1953 Calvaruso, G., 2505 Corma, A., 213 Edwards, H. A., 3341 Aramaki, K., 321 Bilmes, S. A., 2395 Calvente, J. J., 575 Cormier, G., 755 Eggen, B. R., 3029 Aravindakumar, C. T., 597 Binet, C., 1023 Calvo, E., 2395 Corradini, F., 859, 1089 Eggins, B. R., 2249 Arean, C. O., 3367 Binks, B. P., 2743 Calvo, E. J., 987 Corrales, T., 83 Egsgaard, H., 941 Asai, Y., 797 Black, S. N., 1003 Camacho, J. J., 23 Corvaja, C., 3267 El-Atawy, S., 879 Ashfold, M. N. R., 1357 Blackett, P. M., 845 Cameron, B. R., 935 Cosa, J. J., 69 El Baghdadi, A., 1313 Asmus, K-D., 1391 Blake, J.F., 1727 Caminati, W., 2183 Costas, M., 1513 El-Basil, S., 2201 Assfield, X., 1743 Blanco, M., 2125,3203 Cammack, R., 2921,3409 Cottier, D., 1003 Elding, L. I., 3309 Ata, M., 3517 Blanco, S., 1365 Campa, M. C., 207 Coudurier, G., 193 Elisei, F., 279 Atrill, S. R., 3469 Blandamer, M. J., 727, Campelo, J. M., 2265 Courcot, D., 895 Elliot, A. J., 831, 837 Attwood, D., 1961 1905,2703,2709 Campos, A., 339 Coveney, P. V., 1953 Endregard, M., 2775 Aveyard, R., 2743 Blaszczak, Z., 2455 Cant, E., 3213 Cox, A. P., 2171 Engberts, J. B. F. N., 727, Avila, V., 69 Blower, C., 919,931 Canosa-Mas, C. E., 1197, Cox, R. A., 1819 1905,2703,2709 Axford, S. D. T., 2085 Bocherel, P., 1473 1205 Cracknell, R. F., 1487 Enomoto, N., 1279 Baas, J.M. A., 2881 Boddenberg, B., 1345 Capitan, M. J., 2783 Craig, P. J., 3153 Escribano, V. S., 3181 Baba,T., 187 Boesman, E., 2541 Capobianco, J. A., 755 Craig, S. L., 1663 Eustaquio-Rincon, R., 113, Baba,Y., 2423 Boeyens, J. C. A., 3377 Caragheorgheopol, A., 213 Cramer, C. J., 1802,3203 2913 Back, G-H., 2283 Boggis, S. A., 17 Carey, P. R., 3491 Crawford, M. J., 817 Ewins, C., 969 Badia, A., 1501 Bohm, F., 2453 Carley, A. F., 3341 Crisafulli, C., 2809 Fan, J., 3281 Badri, A., 1023 Booth, C., 1961 Carlile, C. J., 1149 Croce, A. E., 3391 Fantola Lazzarini, A. L., Bagatti, M., 1077 Borden, W. T., 1606, 1614, Carlsen, L., 941 Crowther, D., 2155 423 Bailey, R. T., 3373 1616,1671, 1673, 1675, Carrizosa, I., 2783 Cruzeiro-Hansson, L., 1415 Farhoud, M., 2455 Baisogolov, A.Yu., 3229 1689,1733,1734, 1735, Carvill, B. T., 233 Cullis, P.M., 727, 1905, Fausto, R., 689,2953,3491 Balaji, V., 1653 1743,1744, 1802,1807 Castaiio, F., 2443 2703,2709 Favaro, G., 279,333 Ball, M. C., 997, 3373 Bordiga, S., 2827, 3367 Castaiio, R., 1227 Curtis, J. M., 239 Favero, L. B., 2183 Ball, S. M., 523, 1467 Bordoni, S.. 2981 Castellani, F., 2981 DAlagni, M., 1523 Favero, P.G., 2183 Bally, T., 1615, 1674, 1733, Boreave, A., 3461 Castells, R. C., 2677 Damiani, D., 2183 Favre, E., 2001 1808 Borello, E., 2827 Castro, S., 1217, 3527 Dang, N-T., 875 Fawcett, W. R., 2697 Bh, M. I., 1610 Borge, G., 1227 Catalina, F., 83 Danil de Namor, A. F., 845 Feliu, J. M., 609 Bandara, H. M. N., 3549 Borisenko, V. N., 109 Cataliotti, R.S., 1397 Das,D., 1993 Fenn, C., 1507 Bandosz, T. J., 3573 Borsari, M., 3241 Cavani, F., 2981 Das, T. N., 963 Fernando, K. R., 1895 Baonza, V. G., 553,1217, Bottoni, A., 1617 Cavasino, F. P., 31 1,2505 Dasannacharya, B. A., 1149 Fierro, J. L. G., 2125,3203 3527 Boutonnet-Kizling, M., Ceccarani, M. L., 1397 Dash, A. C., 3293 Filimonov, I. N., 219,227 Barbaux, Y., 895 1023 Cense, J-M., 2015 Dash, K. C., 2235 Finger, G., 2141 Barbero, C., 2061 Bowker, M., 1015 Centeno, M. A., 2783 Datka, J., 2417 Finocchio, E., 3347 Barbosa, J., 3287 Bowmaker, G. A., 2579 Cevc, G., 1941 Davey, R. J., 1003 Fischer, H., 3331 Barczynski, P., 2489 Bozon-Verduraz, F., 653 Chakrabarty, D. K., 1993 David, G., 2611 Fisher, I., 2425 Barker, S. A., 1689 Bradley, C. D., 239 Challa, G., 3563 Davidson, K., 879 Fishtik, I., 3245 Barnes, J. A., 1709 Bradshaw, A.M., 403 Chang, T-h., 1157 Davies, M. J., 2643 Fitoussi, F., 3461 Barthel, J., 2475 Branton, P. J., 2965 Charlesworth, D., 1999 De Benedetto, G. E., 1495 Flamigni, L., 2331 Barthomeuf, D., 667,675 Bratu, I., 2325 Charlesworth, P., 1073 de Boer, E., 2663 Fleischmann, M., 1923 Bartl, H., 2791 Braun, B. M., 849 Charlton, M. H., 3533 de Castro, B., 3071 Fletcher, P. D. I., 2743, Bartlett, P. N., 2155 Brei, V. V., 2961 Chaudhry, M., 2235,2243, Decrem, M., 3505 3567 Basini, L., 787 Breysse, M., 193 2683 Deeth, R.J., 3237 Flint, C. D., 1357 1 Fogden, A., 263,3423 Fontanesi, C., 2925,3241 Fornes, V., 213 Fowler, P., 2865 Fracheboud, J-M., 1197, Franci, M. M., 1605, 1740, Franck, R., 667,675 Franco, L., 3267 Franco, M.L. T. M. B., 1205 1744 3273 Gutman, I., 3245 Hachey, M., 683 Hadjiivanov, K., 2277 Haeberlein, M., 263 Hagemeyer, A., 3433 Hakin, A. W., 2027 Hall, C., 2095 Hall, D. I., 517 Hall, G., 1 Hall, G. P., 3579 Hall, P. G., 2965 Hallbrucker, A., 293 Howard, E. I., 2731 Howard, J. A., 3145 Hrovat, D. A., 1689 Hu, W. P., 1715 Hubberstey, P., 2753 Hummel, A., 2459 Hungerbiihler, H., 1391 Hutchings, G. J., 203 Hiittermann, J., 3077 Hutton, R. S., 345 Ichikawa, T., 2901 HSU, J-P., 1435,2945 Kaur,T., 579 Kawashima, H., 3 117 Kawashima, T., 127 Kazansky, V. B., 3367 Keil, M., 403 Keller, J. M., 2071 Kemball, C., 659 Kentgens, A. P. M., 2663 Kessel, D., 1073 Kevan, L., 2283 Khan,H., 2413 Khoo, K. H., 1895 Leroy, G., 3505 Leslie, M., 641 Li, J., 39 Li, P., 605 Li, W., 2223 Li, X., 1429,2939 Li, Y., 947,1599 Liang, X., 1763 Liang, Y., 1271 Lidin, S., 3423 Lillerud, K.P., 1547 Lim,D., 1727 Frank, J., 3201 Franke, O., 2821 Freeman, N. J., 751 Frity, R., 773 Frey, J. G., 17, 817 Frostemark, F., 559, 2401, Fujiwara, Y., 1183 Funabiki, T., 2 107 Galantini, L., 1523 Gale, J. D., 3175 Gale, P. A., 2931 Gallardo Amores, J. M., 2531,3539 3181 Hallen, D., 3397 Halpern, A., 721 Hamnett, A., 459 Hancock, F. E., 3341 Hancock, G., 523,1467 Handa, H., 187 Hann,K., 733 Hao, L., 133,1223,1909 Harada, S., 869 Haraoka, T., 91 1 Hardy, J. A., 2171 Harland, P. W., 935 Harper, R. J., 659 Harriman, A., 697,953, Igawa, K., 2119 Iizuka, Y., 1301, 1307, 3449 Ikawa, S-i., 103, 3065 Ikonnikov, I. A., 219 Ilczyszyn, M., 141 1 Il'ichev, Y.V., 2717 Ilyas, M., 2413 Imamura, H., 2119 Indovina, V., 207 Inerowicz, H. D., 2223 Inoue, Y., 797,815 Ishiga, F., 979 Ishigure, K., 93, 591 Ishikawa, T., 2567 Kida, I., 103 Kiennemann, A., 501 Kim, J-H., 377 Kimura, M., 1355,2423 King, F., 203 Kingston, P. A., 2743 Kinjo, Y., 2235,2683 Kirby, A. R., 2551 Kirchner, S., 1941 Kirschner, J., 403 Kita, H., 803 Kitchen, D. C., 1581 Klein, M. L., 253, 2009 Kleshchevnikova, V. N., Lim, L-H., 1895 Lin, J., 355 Lincoln, S. F., 739 Lindblom, G., 305 Lindner, P., 2001 Lindner, R., 2425 Linford, R. G., 3549 Linstead, D., 3409 Lister, D. G., 2849, 3205 Liu, B-T., 1435,2945 Liu, C-W., 39 Liu, G., 2697 Liu, W., 3281 Liu, X., 249 Galvagno, S., 2803,2809 Gameiro, A. P., 3071 Gandolfi, R., 1077 Gans, 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 Gebicka, L., 341 1 Gebicki, J. L., 341 1 Gengembre, L., 895 Gerratt, J., 1643, 1672, Gerry, M. C. L., 2601,3023, Getty, S. J., 1689 Ghiggino, K. P., 2845 Giamello, E., 3 167 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 Gill, J. B., 315, 2351 1673,1801 3473 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 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. 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E., 721 Zukal, A., 2821 Zundel, G., 843,1095 iv ~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS 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. Clark, Institute of Food Research, Norwich Laboratory, Norwich Research Park, Colney, Norwich NR4 7UA Division Endowed Lecture Symposium: Recent Advances in the Study and Preparation of Novel Surfaces To be held at The Royal Institution on 26 January 1995 Further information from Mrs. Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Division Endowed Lecture Symposium: Spectroscopy and Dynamics of Electronically Excited States To be held at University of Manchester on 29 March 1995 Further information from Mrs.Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Colloid and Interface Science Group Concentrated Dispersions To be held at the University of Bristol on 29-31 March 1995 Further information from Dr. A. Lips, Unilever Research, Colworth Laboratory, Colworth House, Sharnbrook, Bedford MK44 1LQ Statistical Mechanics and Thermodynamics Group with the Experimental Thermodynamics Society Experimental Thermodynamics Conference To be held at the University of Reading on 5-7 April 1995 Further information from Dr.J. Henderson, School of Chemistry, University of Leeds, Leeds LS2 9JT Division Annual Congress: Lasers in Chemistry To be held at Heriot Watt University, Edinburgh on 10-13 April 1995 Further information from Dr. J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Division Endowed Lecture Symposium: Molecular Complexes and Interactions To be held at the University of Bristol on 4 May 1995 Further information from Mrs. Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN ~ ~~ Division Joint Meeting with the Division de Chimie Physique de la Societe' Francaise de Chimie, Deutsche Bunsen Gesellschafl 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 Biop h ys ica 1 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 V 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 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 resonaqces, developments beyond RRKM -Experimental studies including dissociation of ions, clukters and Van der Waals rnolxules; real 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, School1 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. 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 Norwich BI.5 2U, 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, 64 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 polymer/particle gels will also be discussed.The Discussion will bring together experimentalists and theoreticians interested in different types of gelling systems and encourage them to interact and assess the current scene and provide a benchmark for future developments. Contributions are invited for consideration by the Organising Committee. Titles and abstracts of about 300 words should be submitted by 30 September 1994 to: Dr J. W. Goodwin, School of Chemistry, University of Bristol, Cuntock’s Close, Bristol, BS8 1TS, UK Full papers for publication in the Faraday General Discussion 101 volume will be required by May 1995. vii EUROPACAT-I1 is the second European Congress on Catalysis and will be organised by the European Federation of Catalysis Societies (EFCATS) under the co-auspices of the Royal Netherlands Chemical Society (KNCV).EUROPACAT-I1 will be held in the Maastricht Exhibition and Congress Centre in Maastricht, The Netherlands on September 3 -8, 1995. This congress will provide an opportunity to discuss new developments in the area of both heterogeneous and homogeneous catalysis to academic and industrial researchers who are professionally active in the broad field of catalysis. Presentations will be organized in several symposia. An exhibition will be organized alongside the EUROPACAT-11 Congress. For information, please contact the Congress Secretariat: Congress Organization Services Van Namen & Westerlaken PO Box 1558 NL-6501 BN Nijmegen The Netherlands Tel: + 31 (0) 80 23 44 71 Fax: + 31 (0) 80 60 1159 THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Organising Committee Professor L.D. Barron (Chairman) Dr A. F. Drake Dr D. L. Andrews Professor R. E. Hester Professor A. D. Buckingham Traditional optical activity measurements such as CD are confined to the visible and near-ultraviolet spectral regions where they provide stereochemical information on chiral molecules via polarized electronic transitions. Thanks to prompting from theory and new developments in instrumentation, optical measurements are now being made in the vibrational spectrum using both infrared and Raman methods. Studies over the past decade on a large range of chiral molecules, from small organics to biological macromolecules, have demonstrated that vibrational optical activity opens up a whole new world of fundamental studies and practical applications undreamt of in the realm of conventional electronic optical activity. The meeting seeks to bring together experimentalists and theoreticians to discuss the current and future experimental possibilities and the development of theories, including ab initio computational methods, which can relate the observations to stereochemical details. The increasing importance now being attached to molecular chirality and solution conformation in the life sciences should also encourage the participation of biomolecular scientists. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, London W1V OBH. ... Vlll

ISSN:0956-5000    

DOI:10.1039/FT99490BP254

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(23), 3473-3481 3413 Microwave Spectroscopic Investigation of Thionyl Chloride, SOCI, : Hyperfine Constants and Harmonic Force Field Holger S. P. Muller and Michael C. L. Gerry Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T IZI The rotational spectrum of the most abundant isotopomer of thionyl chloride, 32S’6O35CI,, has been recorded in the frequency range 7.8-24.7 GHz using a pulsed molecular-beam microwave spectrometer. 262 lines of 27 rotational transitions (b-and c-type) have been used to determine rotational and quartic centrifugal distortion constants and the complete 35CI nuclear quadrupole coupling tensor, as well as spin-rotation coupling con- stants. Whereas the diagonal quadrupole coupling constants in the inertial axis system agree with those from previous microwave studies, disagreement is observed for the principal values xzz (-63.8 MHz) and q (-0.0143).However, the value of xzz is in good agreement with those derived from solid-state NQR measure-ments. The centrifugal distortion constants together with vibrational wavenumbers and amplitudes have been used to refine the harmonic force field. The principal quadrupole coupling constants and the force constants are compared with those of related molecules, and the latter have been used to propose a reassignment of two vibrational bands of the related molecule, SO,CI, . Thionyl chloride, SOCl,, has been the subject of several so that their precise determination requires the high studies to determine its chemical and physical properties.’ Its resolution obtainable with microwave Fourier-transform gas-phase structure and vibrational amplitudes were obtained (MWFT) spectrometers. Recently such measurements were from an electron diffraction study,, and were used together performed on the 32Sl6035Cl 37Cl isotopomer.28 Reason- with vibrational wavenumbers from the liquid phase to deter- able values were obtained for the diagonal constants of both mine a harmonic force field.3 Later all fundamentals were C1 nuclei.However, the only off-diagonal constant included measured for the gas phase, and the results were used to re- in the fit was xac; it was not particularly well determined, and evaluate the force field.4 Recently the structure of solid the standard deviation of the fit was considerably larger than SOC1, was determined by X-ray diffra~tion.~ the measurement uncertainties.The microwave spectrum of thionyl chloride has been the This article reports a reinvestigation of the rotational spec- subject of several investigations. The early reports included trum of the most abundant isotopomer, 32Sl6035c1,, using 6--9 and a cavity-pulsed-MWFT ~pectrometer.’~ assignments of spectra of 32S16035~1,, Because of the high 32Sl6035Cl 37Cl, lo*ll and values for their rotational and resolution even small perturbations could be detected, so that centrifugal distortion constants, as well as approximate 35Cl all three off-diagonal coupling constants have been deter- nuclear quadrupole coupling constants, were deri~ed.~ More mined (to our knowledge, for the first time for a molecule precise coupling constants were obtained by Wenger et a1.12 with 35Cl and with two quadrupolar nuclei).The 35Cl More extensive measurements for both these isotopomers nuclear quadrupole tensor could thus be diagonalized, have been obtained recently by Suzuki et all3 and by Mata revealing a rather normal S-Cl bond. In the process, precise and car ball^,'^,' who also observed transitions of rotational, centrifugal distortion and 5Cl nuclear spin-32Sl6O37C12. Geometric parameters were derived from both rotation constants have also been obtained. The distortion these groups. constants have been used to improve the harmonic force field The 35Cl nuclear quadrupole coupling constants given in of SOC1, ,and to reassign two of the vibrational modes of the ref.7 and 12-14 were limited to the diagonal values in the related molecule S0,C12 . inertial axis system. They were used to evaluate the principal values of the quadrupole tensor by assuming the S-C1 bond to be a principal axis and by performing the appropriate Experimental linear transformations. In all three cases xzz was found to be The spectrum has been measured in the frequency range 7.8-ca. -96 MHz, and the S-Cl bond rather asymmetric. These 24.7 MHz using a Balle-Flygare-type30 cavity-pulsed-values, which for SOCl, are very sensitive to the choice of MWFT spectrometer, which is described in greater detail structural parameters,12 disagree strongly with the values of elsewhere.29 Samples were injected as pulsed jets of gas con- ca.-64 MHz derived from nuclear quadrupole resonance sisting of ca. 0.5% SOCl, in neon at 1500-3000 mbar total measurements? on solid SOCl, . 16-18 Fu rthermore, the pressure. Rotational temperatures achieved were 5 1 K. Line-apparent asymmetry of the S-C1 bond implies considerable widths of ca. 7 kHz were obtained, and lines 5 kHz apart .n bond character, which is inconsistent with data from could be resolved. The precision and accuracy of measure- related SCl compounds. 9-2 ments for strong, well resolved lines are ca. 0.5 and 1.0 kHz, The discrepancies can be resolved only by evaluating the respectively. The transition frequencies, particularly of closely full C1 quadrupole tensor.SOCl, has C, symmetry, with the spaced lines, were determined from fits to the time-domain S-CI bond lying in none of the principal inertial planes. As a (‘decay’) signals.31 result, all three off-diagonal coupling constants, zab,x,, and xbc, are non-zero, and must be evaluated. These constants Results produce rather small perturbations of the hyperfine structure, Observed Spectrum and Analysis t x = -2vNqR(1 + q2/3)-1”; the sign of xzzis not determined by The symmetry plane of SOCl, is the bc-inertial plane, and NQR? both b-and c-type lines are of comparable intensity. Because 3474 the molecule is rather asymmetric (K = -0.4484 for the investigated isotopomer), with relatively small rotational con- stants, the rotational spectrum is rich even at low J.For example, there are 59 transitions of 32Sl6O35C12 involving levels with J < 4 and AK, = 1 in the frequency range of our spectrometer. Because of the low rotational temperatures the intensities of lines decreased rapidly with J and K, ,and over- lapping of lines of different isotopomers or rotational tran- sitions rarely occurred. On the other hand, as can be seen from Fig. 1, the low rotational constants produced enough rotational near-degeneracies so that the determination of all three off-diagonal quadrupole coupling constants became feasible. The transitions 423-31, and 3,,-2, were investigated first, because the levels involved were relatively isolated (c$ Fig.l), and effects of the off-diagonal quadrupole coupling constants were expected to be small. Lines of the two transitions were predicted with the program SPCAT3, using the spectroscopic constants of ref. 15. A variety of lines was found, mostly within 2 MHz of the prediction. The lines were assigned and fitted to the spectroscopic constants using the program SPFIT.32 All lines were weighted using equal uncertainties (0.5 kHz), except where weak lines were close to stronger ones, or when only one Doppler component was observed, in which case the uncertainty was doubled. Although a fit with A, B, xaa and x-yielded constants quite close to their initial values, an unsatisfactory standard deviation of the fit (ca. 8 kHz) was obtained.When the constant lab was included, a moderately well determined value was obtained (quite close to the value of the final fit); the standard deviation was reduced to 3.3 kHz. Measurement of hyperfine components of 321-212 and 3,,-212 permitted c, xbc, xaC, and the spin-rotation con-stants, Mii, to be included in the fit. Except for C,the contri- butions of these constants were rather small, so that their values were somewhat different from those of the final fit. -523 LI 80 -4:;70 60 -3:':I" 50 ?s P S 40 30 20 11 -212 -202 10 =1;':-101 0 -000 Fig. 1 Part of the rotational energy level diagram of 32Sl6O35Cl, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 To obtain the final values of all constants, including the quartic centrifugal distortion constants, a total of 271 lines from 27 rotational transitions (14 b-type and 13 c-type) was recorded.Because of their low rotational temperatures, J 6 4 and J + K,< 6; see Fig. 2 for an example. The measured lines were mainly either strong lines or those with relatively large effects of the off-diagonal coupling constants. A selec-tion of measured frequencies, along with their assignments in the symmetric coupling scheme I, + I2 = I, Z + J = F,is pre-sented in Table 1. A list of investigated transitions is in Table 2; a complete listing of their measured hyperfine components is available from the authors. Note that if only the diagonal elements of the quadrupole tensor were significant the selec- tion rules would have been AF = 0, +_ 1 and a nominal A1 = 0, +2 (I is not a good quantum number).With the inclusion of the off-diagonal constants xab and x,, transitions with nominal A1 = _+ 1 became allowed and gained consider- able intensity.33 Table 1 also includes calculated contribu- tions from the individual off-diagonal quadrupole and the spin-rotation coupling constants. Nine measured lines were omitted from the final fit because the frequencies could not be determined reliably. The spectroscopic constants from the final fit are in Table 3. All the constants are well determined, and essentially uncorrelated : whereas the correlation coefficient between A, and AJKis -0.82, the absolute values of all others are <0.70. Table 3 also includes the principal 35Cl quadrupole con-stants.From the orientation of the s-Cl bond in the prin- cipal inertial axis system (see Fig. 3) it follows that &,(I) = --&(2); XaC(1) = -X,(2); &(I) = xbc(2), where the numbers in parentheses refer to the two different C1 nuclei. For a given nucleus the sign of the product of the off-diagonal coupling constants is negative; a positive sign gives unreasonable prin- cipal values. The signs of the spin-rotation constants follow the usual spectroscopic conventions and are mostly deter- mined by the sign of the magnetic moment of the given nucleus. The standard deviation of 0.39 kHz agrees well with the measurement uncertainties, implying that all relevant effects have been accounted for.For example, effects of spin- spin coupling, and centrifugal distortion contributions to the quadrupole coupling constants, were negligible. 1,3 1.2 -I I I 1 1 I I I I I I 13545.0 13545.2 frequency/M Hz Fig. 2 Detail of the 2,,-lO1 transition of 32Sl60 3s Cl,, showing 35C1 quadrupole splitting, displayed as a power spectrum. 100 averaging cycles were used. A stick diagram indicates the calculated relative intensities and the line positions from the decay fit. The lines are Doppler-split by ca. 68 kHz. The quantum numbers I and F of the lower state are 3 and 2, respectively; for the upper state the values are indicated. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3475 Table 1 Observed frequencies of rotational transitions of 32Sl6035C12(selection), residues" and contributions of selected hyperfine constants frequencies contributionsb/kHz J' K.K:-JK,K~ I', F-I, F' observed/MHz o -c*/kHz xab Xac Xbc C Xij C Mii 21 1-10, 3,l-3,2 13 545.0668 -0.02 37.83 7.80 10.30 55.84 -7.33 1,2-3,2 13 545.0826 0.32 28.14 7.49 7.43 42.74 -0.31 1,3-3,2 13 545.1397 0.22 18.45 7.65 1.45 27.58 3.92 3,2-3,2 13 545.1862' -0.52 38.69 12.70 0.5 1 52.26 -2.05 3,5-3,4 13 551.7350 0.1 1 -1.29 0.69 2.09 1.49 4.42 3,4-3,4 13 551.8427 0.47 -13.45 1.30 4.95 -7.17 -2.56 2,4-2,3 13 554.3097 0.21 12.03 6.94 0.09 19.0 1 2.94 2,3-2,3 13 554.3233' 0.45 28.09 9.76 -0.42 37.35 -2.63 2,2-2,2 13 554.3300' -0.08 103.42 17.58 6.38 127.41 -1.20 2,3-2,2 13 554.4058 0.06 96.86 19.49 0.54 116.94 1.32 1,2-3,3 13 561.7733' 0.52 19.75 6.15 7.65 33.20 -2.69 1,3-3,3 13 561.8301 0.12 10.06 6.3 1 1.67 18.04 1.54 3,2-3,3 13 561.8782 0.98 30.30 11.36 0.72 42.72 -4.43 3,4-3,3 13 561.9322 -0.05 15.83 5.77 4.73 26.38 2.72 3,3-3,3 13 561.9689 0.44 35.87 5.71 6.66 48.24 -0.30 220-1 1 1 3,l-3,2 18 275.7857 -0.33 -35.27 10.80 32.10 10.89 -7.28 1,2-3,2 18 277.5452 0.27 -40.14 9.11 3 1.46 3.65 2.10 1,2-1,1 18 277.5617 0.00 -54.28 4.56 12.75 -37.39 -2.64 2,2441 18 278.0803 -0.29 -29.43 9.71 24.7 1 2.99 0.65 2,2-1,2 18 278.1009 0.02 -147.83 6.96 1.68 -136.73 -0.92 3,5-3,4 18 281.6617 -0.02 0.08 0.44 2.06 2.57 5.34 1,3-2,2 18 283.701 7' -0.42 -3.82 -1.82 9.58 -2.23 -1.08 1,3-3,3 18 283.7736 -0.59 -49.80 0.82 20.95 -27.53 2.00 2,3-1,2 18 285.6192 0.79 -161.56 4.33 -0.80 -155.62 1.66 2,4-2,3 18 285.6930 -0.33 -26.60 6.17 0.07 -20.34 3.56 3,4-3,3 18 291.8609 -0.12 -24.05 5.83 21.93 4.18 3.38 0,2-2,1 18 293.363 1 0.07 -21.11 5.39 24.37 10.66 -0.83 3,3-2,2 18 293.8016 -0.23 24.87 4.49 9.75 32.91 -3.48 3,3-3,3 18 293.8733 -0.60 -21.10 7.13 21.12 7.61 -0.40 303-2i 2 3,4-3,4 11 99 1.6972 -0.12 21.27 6.43 0.89 28.65 -2.10 3,2-3,l 1 1 99 1.7950 -0.20 -1.01 -1.17 1 1.47 9.22 -0.22 3,3-3,2 11 991.8171 -0.07 -23.25 0.70 0.55 -21.98 -0.54 3,4-3,3 11 993.5223 -0.19 2 1.72 2.87 4.53 29.15 1.09 2,3-2,2 11 994.4861 -0.03 14.21 3.06 4.28 21.62 0.15 1,4-1,3 1 1 995.28 16 -0.11 -3.30 1.87 0.39 -1.05 1.71 1,2-1,l 11 995.3746 -0.03 8.63 3.8 1 2.36 14.83 -1.42 3,6-3,5 11 997.6324 0.11 0.87 0.49 1.05 2.39 3.48 3,2-3,2 12 000.6164 0.15 -29.91 -0.41 2.98 -27.36 -5.23 321-211 3,O-3,1 22 088.8908 0.47 12.82 0.30 -5.02 8.24 -6.89 3,l-3,2 22 089.4600' 0.13 10.24 -3.83 3.25 9.33 -10.67 2,3-2,3 22 089.7732' 0.02 19.50 -1.53 2.01 19.98 -2.24 1,3-1,3 22 089.7809' 1.32 26.37 -0.05 1.20 27.53 -2.89 1,3-1,2 22 089.8375 0.72 16.69 0.11 -4.78 12.36 1.34 2,3-2,2 22 089.8482 -0.63 12.95 0.38 -3.82 9.52 0.29 3,6-3,5 22 090.4377 0.36 3.60 -0.27 -1.20 2.13 5.07 1,4-3,4 22 09 1.209 1 1.11 7.51 -1.21 -3.89 2.37 0.80 1,4-1,3 22091.3103 0.04 13.26 -1.76 -0.84 10.70 1.97 2,5-2,4 22091.5537 -0.85 21.07 -0.60 0.04 20.5 1 3.37 3,3-3,3 22 092.3 102 0.52 0.86 1.54 -5.93 -3.53 -6.16 3,3-3,2 22 092.3998 -1.12 6.42 -4.12 0.00 1.99 -2.03 3,5-3,4 22 092.6224 -0.33 16.81 0.10 -2.57 14.31 3.08 1,2-1,1 22 092.7 15 1 -0.54 18.82 -0.69 -2.84 15.13 -2.11 0,3-0,2 22 093.2233 -0.64 8.90 0.16 -3.62 5.45 -0.57 0,3-2,3 22 093.3258 0.00 19.53 -1.53 2.01 20.00 -2.23 3,4-3,3 22 093.5257 0.26 2.18 0.29 -3.52 -1.04 0.91 3,4-3,4 22 093.5628 1.14 22.22 0.24 -1.59 20.83 -2.10 322-212 3,2-1,l 23 732.8930 -0.05 -10.70 3.79 3.08 -3.19 -3.36 2,2-1,l 23 732.91 52 0.44 2.59 4.73 2.8 1 9.47 -7.65 1,4-3,4 23 733.1 181 -0.96 -25.72 2.03 3.75 -19.65 -1.82 3,5-3,4 23 733.1450 -0.10 -2.36 3.06 3.19 3.91 3.14 330-322 3,6-3,6 13 616.6670 -0.33 4.19 -0.31 -1.12 2.76 2.28 2,4-2,4 13 622.0967 0.10 7.44 -0.39 -1.12 5.93 0.25 2,5-2,5 13 622.1 107 -0.24 17.66 1.01 -0.05 18.63 1.52 3476 J.CHEM.SOC. FARADAY TRANS., 1994,VOL. 90 Table l-continued frequencies contributionsb/kHz J'K;K;-J K,K, I', F'-I, F' observed/MHz o -c"/kHz xab xac xbc C xij C Mii 33-33 13627.5285 0.39 11.18 1.31 -0.16 12.31 0.77 3,4-3,4 13631.9727 -0.36 -0.45 1.39 -0.05 1.14 -2.74 3,4-3,3 13631.9861 0.36 10.42 1.26 -0.45 10.75 14.64 3,1-~zi1,3-1,3 12 634.1216 0.33 7.61 1.16 -1.08 7.69 -0.51 2,3-2,3 12 634.1916 -0.48 6.21 1.01 -0.91 6.31 -0.43 3,6-3,5 12 634.2198 0.34 -2.95 -0.73 -0.95 -4.65 11.57 3,6-3,6 12 636.5121 -0.09 -1.91 0.26 0.54 -1.13 2.61 2,4-2,4 12 640.7419 -0.27 1.55 -0.14 1.22 2.65 0.29 2,5-2,5 12 640.7535 0.39 9.07 1.65 -0.08 10.62 1.74 3,5-33 12 644.9786 0.23 11.28 1.29 -0.67 11.91 0.87 3,5-3,6 12 647.2712 0.10 12.32 2.28 0.82 15.43 -8.09 0,3-0,3 12 647.2953 0.11 6.16 1.00 -0.91 6.26 -0.44 3,4-3,4 12 648.4405 0.19 5.41 1.36 -1.45 5.32 -0.14 404-3i 3 3,5-3,4 16 602.4479 -0.17 -14.67 -1.81 2.18 -14.29 1.32 1,5-1,4 16 602.9596 -0.04 -1.95 0.83 0.10 -1.01 1.37 1,4-1,3 16 603.8855 0.55 -9.67 -0.89 1.63 -8.94 0.26 3,6-3,5 16 603.9154 -0.16 -0.13 0.78 1.29 1.97 2.62 0,4-0,3 16 603.9460 -0.21 -9.75 -0.89 1.63 -9.04 0.06 3,7-3,6 16 604.3521 0.19 0.76 0.28 0.56 1.59 3.78 3,2-3,2 16 606.1263 0.31 5.39 3.01 0.22 8.58 -5.58 3,4-3,4 16 606.8685 0.07 -16.85 -4.54 3.18 -18.21 -6.66 414-303 33-3,4 18 019.9389 0.17 -3.30 0.28 0.04 -3.01 1.31 1,5-1,4 18 020.8483 -0.22 3.22 -0.19 -0.39 2.64 1.50 3,6-3,5 18 021.5838 -0.12 3.73 0.69 -0.87 3.54 2.87 3,7-3,6 18 022.4156 -0.21 1.40 0.02 -0.07 1.36 4.46 423-31 22,4-2,4 24613.5075 0.22 17.55 -0.19 0.31 17.68 -3.52 3,6-3,6 24 613.5859 -0.42 4.44 0.24 -0.44 4.23 -4.91 33-33 24614.1853 0.41 25.15 2.11 -1.09 26.17 -3.37 33-3,4 24615.1313 0.63 -4.24 0.30 -3.00 -6.93 1.33 3,4-3,3 24 6 15.2347 0.03 7.67 -1.37 -0.18 6.10 -0.17 2,4-2,3 24615.3273 -0.44 0.41 -0.02 -2.19 -1.80 -0.14 1,3-1,2 24 615.5741 -0.31 4.08 -0.90 -0.96 2.22 -2.50 3,6-3,5 24 615.9667 -0.03 8.42 0.15 -1.11 7.44 3.19 3,3-3,2 24 6 16.0298 0.17 14.62 -0.80 -1.08 12.75 -1.67 1,5-1,4 24616.2385 1.67 5.93 -1.05 0.19 5.09 1.66 2,5-2,4 24616.2432 -0.81 4.19 -1.40 0.35 3.15 1.54 2,6-2,5 24 616.2530 -0.41 9.12 -0.14 0.00 8.98 3.48 3,7-3,6 24 617.0188 -0.37 0.16 -0.04 -0.59 -0.48 5.22 1,4-1,3 24617.1382 0.97 0.64 0.04 -2.29 -1.62 0.03 0,4-0,3 24617.1472 -0.55 0.40 -0.02 -2.20 -1.82 -0.15 Observed~alculated.Calculated frequency minus calculated frequency without respective parameter (this also holds for xij, whichtherefore does not always equal the sum of the individual contributions). The precision is given by the program and is in keeping with the precision of the derived constants. ' I, + I,= Z, Z + J = F, Reduced weight, see text. Harmonic Force Field The harmonic force field of thionyl chloride has been refined in order to provide an improved description of the bonding, and of the normal coordinates in terms of internal coordi- nates. In previous calculations only vibrational wave-numbers4 (plus amplitudes in one case)3 of one isotopomer were used. Inclusion of the distortion constants of Table 3 in0 (b the fit allowed us to refine the force field.The computation was carried out using Christen's program NCA.34The coor- dinates used were the valence coordinates of Fig. 1 of ref. 4. The pseudo-substitution structure of ref. 15 was used to0x / describe the molecular geometry. 61 / CI The input data, centrifugal distortion constants, vibrational Fig. 3 Geometry of SOCI, and the principal axes of the "Cl amplitudes2 and wavenumbers? were weighted inversely to nuclear quadrupole tensor projected (a)onto the SCI, plane and (b) the squares of the experimental uncertainties; 1 cm-' was along the SCI bond assumed for the uncertainties of the wavenumbers. During J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 Table 2 Investigated rotational transitions and number of observed hyperfine components' b-type c-type rotational no. observed transitions hyperfine comp. 212-10 1 18 12 220-1 11 14 15 221-1 10 19 22 5 13 9 3 8 18 15 7 5 4 6 6 8 10 3 6 4 7 15 6 4 ~~ a Measured frequencies not given in Table 1 are available from the authors on request. the calculations it turned out that the weights of the distor- tion constants were too high to obtain a balanced fit; the weights were decreased by a factor of 400. No anharmonic correction was undertaken. The initial force field was taken from ref. 4; it included some interaction force constants which were not considered in their discussion (see Table 2 of ref.4). According to a more recent convention, deformation force constants were normal- ized to 100 pm bond length. These constants reproduced the input data quite well; the largest deviation (ca. 20%)occurred for AK.In a first step fs, Jsand fas were released, because they had the largest effects on AK. In a second step onlyf,, Table 3 Spectroscopic constants" of thionyl chloride, SOCl, rotational constants/MHz A 5086.748209 (47) B 2822.530125 (49) C 1960.313918 (41) centrifugal distortion constants/kHz AJ 1.12652 (149) AJK -2.2336 (52) A, 6.9923(44)6, 0.39460 (67) 6, 1.2526(80) nuclear quadrupole coupling constants/MHz Xaa -25.13085 (19) X--25.77995 (30) I Xab(') Ib 42.978 (29) IXac(1)Ib 19.309 (73) IXbc(') Ib 13.949 (30) Xbb: -0.32455 (18) xcc 25.45540 (18) Xz,c -63.814 (40) xxx; 32.362 (59) XYY 31.452 (38) ,pd -0.0143 (11) spin-rotation coupling constants/kHz Maa 1.943 (26) Mbb 1.265 (27) Mcc 1.374 (25) standard deviation/kHz CT 0.39 a Representation 1'.Numbers in parentheses are one standard devi- ation in units of the last significant figures. &(I) = -xab(2), Xac(1)= -Xac(?), = Xd2h sig{Xab(i)Xac(l)Xa(i)}= -1. Derived constants. Dimensionless. Table 4 Comparison of measured vibrational wavenumbers" (cm-l), centrifugal distortion constantsb (kHz) and mean-square vibrational amplitudes' (A) with those calculated from the force field constant observed calculated 1261 1261.0 500 500.1 354 353.9 194 193.4 464 464.0 283 283.0 1.127 1.126 -2.234 -2.229 6.992 6.992 0.3946 0.3945 1.253 1.206 0.031 0.036 0.051 0.054 0.101 0.095 0.076 0.079 Ref.4. This work. Ref. 2. fRr,fRor and fRB were held fixed, because derivatives of the input data to these constants were small. Finally, fR was adjusted. These force constants reproduced all input data except 6, within the measurement uncertainties (see Table 4). Inclusion OffRa andfRB in the fit suggested both constants to be small and positive, but their values were not well deter- mined; the fit was not improved significantly. Forf,, even the sign has to remain uncertain. The final force field, along with the initial one and the potential-energy distribution are given in Table 5.The present force field indicates slightly stronger mixing of the valence coordinates than that suggested in ref. 4. Discussion Hyperfine Constants The primary objective of this work, to obtain well determined values of all three off-diagonal 35Cl quadrupole coupling constants, has been achieved, in spite of the small contribu- tions of these constants to the transition frequencies (see Table 1). The contributions of Xab are largest, up to 162 kHz, though most are much less than 100 kHz. Those of zacand Xbc are all less than 20 and 37 kHz, respectively; in general, these contributions decrease with increasing J and K, (see Table 1). This success is a result of the high resolution and precision obtained with the MWFT spectrometer, combined with the large number of measured lines and a fitting program which takes hyperfine splitting properly and exactly into account. Note that the degree of near-degeneracy of rotational levels is not an adequate measure with which to estimate the effects of the off-diagonal quadrupole coupling constants in SOCl, : for example the level J,, gets closer to J,, with increasing J (see Fig.l), but the contributions of Xab decrease. Similarly, the level JK,,-becomes closer to JK,,-+ ,with increasing K,, but the contributions of Xbc decrease. The latter situation was found to be more pronounced in 32Sl60 '*O35/37C1F, where perturbations by xbc were negligible in those levels for K, 2 2.,' On the other hand, significant perturbations were observed in transitions like 4,,-3,, and 3,,-2,, (see Table l), although these levels are relatively isolated (see Fig.1). The diagonal coupling constants in the inertial axis system agree well with those from previous studies (summarized in ref. 14), with the best agreement being with those of Suzuki et aZ.,13 for which the deviations are 0.07-0.19 MHz, about twice their 2.50 uncertainties. However, the precision of the present constants is greater. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 5 Harmonic force constants" (100 N m-') and potential-energy distributionb of SOC1, experimental PED parameter ref. 4 this work V1 V2 v3 v4 V5 '6 fR f, fa fa fR* fRa 9.89 2.23 1.198 1.121 0 0.058 0 9.900 1.975 1.297 1.186 0 0.058 0 0.99 0.56 0.2 1 0.08 0.3 1 0.52 0.12 1.01 0.89 0.39 0.28 0.95 0.32 0.276 0.08 0.04 -0.12 -0.04 0.115 0.042 -0.03 -0.029 -.0.030 0.06 1 0.138 -0.05 0.270 0.327 0.05 0.13 -0.10 -0.24 0 0.255 0.07 --0.20 " Deformation force constants normalized to 100 pm bond length.* Only contributions 2 0.03 are given. Table 6 Comparison of nuclear quadrupole coupling constants (MHz) and bond lengths (pm) of some SCl compounds gas phase solid phase r(SC1) Xzr rl" r(SC1) VNQRb Xzz" SCI, 201.4' -79.7d 0.2Ood 200.6, 201.5, 41.53, 39.69, 39.17, -82.51, -78.86, -77.82, 201.7, 201.8" 38.74, 38.44 -76.97, -76.37 SO,Cl, 201.21 -74.48 0.08Y 198.0" 37.80, 37.59 -75.50, -75,09 S0,ClF 198.6' -77.05' 0.060' 196.4h 39.36 -78.68soc1, 207.4' -63.81' -0.014' 206.5, 207.5"' 32.09, 31.89 -64.17, -63.77 SZC1, 205.5" 206.7, 207.6" 35.98, 35.59 -71.96, -71.17 " q = (xXx -x,,,)/x,,; q from gas phase measurements.Ref. 18; see also ref. 17. 'Ref. 22. Ref. 19. Ref. 23. Ref. 24. Ref. 20. Ref. 25. Ref. 21. Ir Ref. 15. ' This work. Ref. 4. " Ref. 26. Ref. 27. The major difference between the present and earlier z-axes of the two nuclei slightly larger than L(CISCI) (see results is found for the principal values of the 35Cl quadru- Fig. 3). The x-axis is well outside the ClSCl plane, by 20.1-pole tensor, which have been obtained here directly through 20.8", and it is better described with respect to the S-0 diagonalization, rather than through assuming the S-Cl bond, to which it is almost perpendicular (within 0.4-1.4").bond to be a principal axis. The value of xzr in Tables 3 and 6 The diagonal quadrupole coupling constants of (-63.814 MHz) differs substantially from ca. -96 MHz, 32Sl6O35Cl 37Cl of ref. 28 seem to reflect the changes in Oij obtained but is now consistent with the values (i = a, b, c;j = x, y, z) of both nuclei in comparison to those derived from NQR measurements? (-63.77 and -64.17 of 32 S 1603Tl,. However, the absolute values of their xac MHZ).l6 -l The two different NQR values can be are unreasonably high. related'7*35 to the two different S-Cl bond lengths in solid When the x,, value of SOCl, is compared with those of the longer bond is almost as other SCl compounds, such as SCl, and S02C1, (Table 6),SOCl,, 207.45 and 206.48 ~m;~ long as that in gaseous SOCl, (see Table 7).It can also be these values may seem irregular at first sight. However, it has seen from Table 3 that the charge distribution around the been shown previously that these values depend largely on S-Cl bond is not asymmetric, as suggested in the earlier changes in the hybridization of the sulfur o-orbital to chlo- work~,'~-'~but is rather cylindrically symmetrical, as one rine, the inductive and conjugative effects of the substituents would have expected. and the degree of d,-bonding of the S at~m.'~?'~.~~Electro-The disagreement with results of previous microwave negative substituents increase the absolute value of xz, ,while measurements becomes comprehensible when one considers electropositive ones decrease it (compare S0,ClF and the large angles between the principal quadrupole axis system SO,Cl, in Table 6); the elimination of an electronegative and the principal inertial axis system.Small uncertainties in substituent decreases the absolute value of xz, (compare the structural parameters can then have large effects when SOCl, and SO,Cl,). According to ref. 17 and 35, x,, and principal values are derived by axis rotation as in ref. 12-14. r(SC1) are approximately in a linear relationship; some recent In ref. 12 values of xzz = -106.0 MHz and q = -0.417 were results are shown in Table 6. Despite some scattering of the initially derived from the ra structure [L(ClSC1) = 96.1 data, the trend is apparent.The scattering is significantly (7)"];, increasing the angle by 0.8" resulted in x,, = -95.1 reduced, when one focuses on RSOCl and RS0,Cl alone, or MHz and q = -0.348.', Clearly the calculation is very sensi- on RSC1.'7*35 tive to the angle, so that if either the angle is incorrect, or the A simple model is described in ref. 42 to calculate the ionic z-principal axis and the bond axis do not coincide, incorrect and z-bonding contributions from the principal quadrupole principal values will be obtained. From the present results coupling constants. In the series SCl,, S02C1,, SOCl, , the the z-axis is within 2.1-2.6" of the S-Cl bond, according to .n-bonding contribution decreases (0.10, 0.04, <0.01), while the structural models of ref.15, with the angle between the the ionic contribution increases (0.23,0.30,0.42). Although the effects of nuclear spin-rotation coupling are about three orders of magnitude smaller than those of -f As footnote on p. 4. nuclear quadrupole coupling, the respective constants have Table 7 Comparison of selected structural parameters and force constants of some sulfur chlorine compounds SCI" 197.5 3.3113.21 S,C12b 205.5 2.19sc1,' 201.4 102.6 26712.77 0.25 f0.34 1.06/ 1.03 SOCl2d 207.41207.6 142.51 144.3 96.8196.1 108.0/106.3 1.98 0.28 9.90 1.19 1.30 0.33 S02C12e 201.2 141.8 100.3 108.0 2.24/2.45 0.5210.32 10.53 1.90 1.76/1.55 0.39 S0,CIF' 198.6 140.9 109.0 3.03 10.85 1.61 CISNg 216.1 1.58 a Geometry from ref.36; force constants derived from ref. 37 (two models). Geometry from ref. 26; force constants from ref. 38. Geometry from ref. 22; the two sets of force constants are from ref. 22 and ref. 39. Two sets of structural parameters from ref. 15 and ref. 3; force constants from this work. Geometry from ref. 24. The first set of force constants is from ref. 40;the second one has been derived from a reassignment of v, and v9;see text. Ref. 21. Ref. 41. w tW J. CHEM. SOC. FARADAY TRANS., 1994, VOI,. 90 Table 8 LfR fa OSClZb 9.900 1.975 1.297 OSF,' 11.222 4.070 1.654 SSF,d 5.319 3.464 1.346 ClClO,' 1.246 8.167 1.175 FClO,f 2.412 9.370 1.564 Comparison of harmonic force fields" of pyramidal XEY compounds fo fRr fRa A, La fla Lo fma fa@ 1.186 O.Ob 0.05gb O.Ob 0.276 0.042 -0.030 0.138 0.327 0.255 1.804 l.Ob O.Ob O.Ob 0.352 0.002 -0.098 0.135 0.388 0.411 1.616 0.385 0.187 O.Ob 0.275 0.029 0.009 0.216 0.280 0.276 1.702 -0.135 -0.011 -0.042 -0.263 0.034 -0.101 0.057 0.412 0.102 2.047 -0.293 0.013 -0.147 -0.137 0.130 0.038 0.212 0.484 0.484 R = r(XE), r = r(EY), ct = L(XEY), fl= L(YEY); x = 0,S, C1, F; E = S, C1; Y = C1, F, 0;deformation force constants normalized to 100 pm bond length.This work. Ref. 47. Ref. 48. Ref. 49. Ref. 5qa);structure and centrifugal distortion constants are taken from ref. 5qb); vibrational wavenumbers are taken from ref. 5qc). been determined precisely. According to F!~gare~~ each can be separated into an electronic and a nuclear contribution (the latter in brackets): 2.197 (-0.254), 1.525 (-0.260) and 1.603 (-0.229) kHz for Ma,, Mbb, and M,, , respectively.Compared with the magnitude of the spin-rotation constants, the nuclear contributions of SOCl, are slightly smaller than those of SO,ClF," but larger by a factor of ca. 5 than those of ClF or HCl (derived from ref. 44 and 45). The spin-rotation coupling constants permit the calcu- lation of the average paramagnetic shielding of the 35Cl nucleus (OF))in SOCl, .43 A value of -989 ppm has been obtained, similar to the value of -889 ppm in S0,C1F,2' but rather different from -1611 and -189 ppm of C1F and HC1, respectively (derived from ref. 44 and 45). The 35Cl NMR of SOCl, has been measured (+ 650 ppm relative to a saturated solution of NaCl in water),46 but to our knowledge there is no absolute shielding scale available for the 35Cl nucleus, so that a value for the average diamagnetic shielding cannot be calculated.Furthermore, there are only a few compounds for which both 35Cl spin-rotation coupling constants and the 35Cl NMR frequency have been measured, so that only a limited ensemble of data would be available for a comparison of values of oic'). Force Field In the present study it has been possible to determine the quartic centrifugal distortion constants quite precisely (see Table 3), in spite of the low values observed for J and K,. The results agree very well with those of ref.12, but give a slightly worse agreement with those of ref. 8; the constants of ref. 15 differ by rather larger amounts. Although the present force field is in reasonable agreement with the initial one (see Table 5) it reproduces all available data better and fits better into the series of related SCl and pyramidal XEY ,compounds (Tables 7 and 8, respectively). In general, as might be expected, the SCl stretching force con- stants decrease with increasing S-Cl bond lengths in the former series. An exception occurs with SO,Cl, , whose pre- ferred force field includes a rather small value forf,,,, but a rather large value for fscl,sc,.40 However, note that for SO,Cl, the assignments of the lowest three vibrational modes ~5(a,), v,(b,) and v,(b,) are not obvious.Exchange of the wavenumbers of v7 and v9 (currently at 362 and 388 cm-', respectively) would produce values for fscl and fscllSc, of 245 and 32 N m-', respectively:' now in good agreement with the general trends. Furthermore, a value of 155 N m-' would result for fclso 40 slightly smaller than in S0,ClF (see Table 7), as one would have expected from the trends in the stretching force constant^.^ 1,40 A trend similar to that off,,, is also observed for fso (see Table 7). Similarly, fclso in SOCl, is smaller than that in SO,Cl,. If the S-Cl bond in SOCl, were as short as those of SCl, and SO,Cl,, one would have expected a slightly larger value for L(ClSC1) (ca. 101') and forfclsCl (ca. 145 N m-', see Table 7), but because the S-Cl bond of SOCl, is much longer than that of SC1, and SO,Cl, ,fclscl of SOCl, is smaller and much closer to that of SC1, than that of SO,Cl, .Although the pyramidal molecules in Table 8 are quite dif- ferent (compare for example the diagonal force constants), the interaction force constants reveal some similarities :f,,,As,fa, and fas are moderately large, while the remainder are smaller or not very well determined. For the molecules with larger and smaller values off,,f,is negative and positive, respec- tively. From comparisons with the other pyramidal mol- ecules, fRa and fRBof SOCl, are probably small, while little can be concluded forfRr. Conclusion For the first time, to our knowledge, all three off-diagonal quadrupole coupling constants have been determined for a molecule with 35Cl and with two quadrupolar nuclei.It has been shown that this determination has been crucial to obtain the principal 'Cl nuclear quadrupole coupling con- stants of thionyl chloride, SOCl,. The S-Cl bond has been found to be cylindrically symmetrical, in keeping with other SCl molecules. The seemingly peculiar situation, obtained earlier, of an asymmetric bond was caused by the neglect of the off-diagonal quadrupole coupling constants in previous studies.' An improved harmonic force field has been obtained by including the distortion constants in the determination. Com- parison of the force constants with those of related SC1 com- pounds has allowed a vibrational reassignment for the related molecule SO,Cl, to be proposed.The rotational constants may in future studies permit improvement in the structural parameters of SOC1,. Financial support by the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowl- edged. References Gmelin Handbuch der Anorganischen Chemie. Schwefel, Erg. Bd. I: Thionylhalogenide, ed. M. Becke-Goehring, Springer, Berlin, 8th edn., 1978. I. Hargittai, Acta Chim. (Budapest), 1969,60, 231. I. Hargittai and S. J. Cyvin, Acta Chim. (Budapest), 1969,61, 51. G. Hopf and R. Paetzold, 2. Phys. Chem. (Leipzig), 1972, 251, 273. D. Mootz and A. Merschenz-Quack, Acta Crystallogr., Sect. C, 1988,44,926. J. Burie, J-L. Destombes, A. Dubrulle and G. Journel, Compt. Rend.Acad. Sci. (Paris), Ser. B, 1968,267,48. G. Journel, P11.D. Thesis, University of Lille, France, 1969. G. Journel, A. Dubrulle, J-L. Destombes and C. Marliere, Compt. Rend. Acad. Sci. (Paris), Ser. B, 1970, 271, 331. J-L. Destombes, Ph.D. Thesis, University of Lille, France, 1970. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 348 1 10 11 A. Dubrulle, Ph.D. Thesis, University of Lille, France, 1972. A. Dubrulle and D. Boucher, Compt. Rend. Acad. Sci. (Paris), 31 J. Haekel and H. Mader, Z. Naturforsch., A: Phys. Sci., 1988,43, 203. 12 Ser. B, 1972,274, 1426. H. U. Wenger, A. Bauder and Hs. H. Giinthard, Chem. Phys., 1973, 1, 441. 32 33 34 H. M. Pickett, J. Mol. Spectrosc, 1991, 148, 371. W. H. Flygare and W. D. Gwynn, J.Chem. Phys., 1962,36,787. D. Christen, J. Mol. Struct., 1978,48, 101. 13 S. Suzuki, M. Yamaguchi, M. Onda, T. Sakaizumi, 0. Ohashi 35 V. E. Bel’skii, V. A. Naumov and I. A. Nuretdinov, Dokl. Akad. 14 15 16 and I. Yamaguchi, J. Mol. Struct., 1981,73, 41. F. Mata and N. Carballo, Z. Naturforsch., A: Phys. Sci., 1983, 38, 769. F. Mata and N. Carballo, J. Mol. Struct., 1983, 101, 233. R. Livingston, Phys. Rev., 1951, 82, 289; J. Phys. Chem., 1953, 57, 496. 36 37 Nauk SSSR, 1974, 215, 355; Dokl. Phys. Chem., Proc. Acad. Sci. USSR, 1974,215,260. Ch. Yamada, J. E. Butler, K. Kawaguchi, H. Kanamori and E. Hirota, J. Mol. Spectrosc., 1986, 116, 108. E. Tiemann, H. Kanamori and E. Hirota, J. Mol. Spectrosc., 1989,137,278. 17 R. M. Hart and M. A. Whitehead, Trans.Faraday SOC.,1971,67, 345 1. 38 R. F. Filgueira, L. L. Fournier and C. E. Blom, J. Mol. Struct., 1988,175,105. 18 19 20 I. P. Biryukov and A. Ya. Deich, Zh. Fiz. Khim., 1972,46, 2385; Russ. J. Phys. Chem., 1972,46, 1362. I. Merke and H. Dreizler, 2.Naturforsch., A: Phys. Chem., 1992, 47, 1141. I. Merke and H. Dreizler, Z. Naturforsch., A: Phys. Chem., 1992, 39 40 41 D. Bielefeldt and H. Willner, Spectrochim. Acta, Part A, 1980,36, 989. M. Pfeiffer, 2.Phys. Chem. (Leipzig), 1969,240, 380. T. Beppu, E. Hirota and Y. Morino, J. Mol. Spectrosc., 1977,65, 455. 21 47, 1153. H. S. P. Muller and M. C. L. Gerry, J. Chem. SOC., Faraday 42 W. Gordy and R. L. Cook, Microwave Molecular Spectra, Wiley, New York,3rd edn., 1984. 22 23 Trans., 1994,90,2601.R. W. Davis and M. C. L. Gerry, J. Mol. Spectrosc., 1977, 65, 455. R. Kniep, L. Korte and D. Mootz, 2. Naturforsch., B: Chem. 43 44 W. Flygare, J. Chem. Phys., 1964,41,793. (a) R. E. Davies and J. S. Muenter, J. Chem. Phys., 1972, 57, 2836; (b) R. E. Willis Jr. and W. W. Clark 111, J. Chem. Phys., 1980,72,4946. 24 Sci., 1984,39, 305. M. Hargittai and I. Hargittai, J. Mol. Struct., 1981, 73, 253. 45 E. W. Kaiser, J. Chem. Phys., 1970, 53, 1686, and references therein. 25 D. Mootz and A. Merschenz-Quack, Acta Crystallogr., Sec. C, 1988,44,924. 46 K. Barlos, J. Kroner, H. Noth and B. Wrackmeyer, Chem. Ber., 1980,113,3716. 26 27 28 29 C. J. Marsden, R. D. Brown and P. D. Godfrey, J. Chem. SOC., Chem. Commun., 1979,399. R. Kniep, L. Korte and D. Mootz, 2. Naturforsch., B: Chem. SOC.,1983, 38, 1. I. Merke and H. Dreizler, 2. Naturforsch., A: Phys. Sci., 1992, 47, 1150. Y. Xu, W. Jager and M. C. L. Gerry, J. Mol. Spectrosc., 1992, 47 48 49 50 N. J. D. Lucas and J. G. Smith, J. Mol. Spectrosc., 1972, 43, 327. R. W. Davis, J. Mol. Spectrosc., 1986, 116, 371. H. S. P. Muller and H. Willner, Inorg. Chem., 1992,31,2527. (a)H. S. P. Muller, unpublished results; (b) A. G. Robiette, C. R. Parent and M. C. L. Gerry, J. Mol. Spectrosc., 1981, 86, 455; (c)H. S. P. Muller, Ph.D. Thesis, University of Hanover, Germany, 1992. 30 155,403. T. J. Balle and W. H. Flygare, Rev. Sci. Instrum., 1981,52, 33. Paper 4/04712E; Received 1st August, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003473

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(23), 3483-3490 Vibrational Spectroscopic Study of N-Acylglycine Monomeric and Dimeric Salts: The Long Acyl Chain Effect on the NH (ND) Stretch, Amide I, Amide II and CH,-Wag Modes Kunihiro Ohshima, Hirofumi Okabayashi" and Tadayoshi Yoshida Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466,Japan The vibrational spectra of N-acylglycine monomeric and dimeric salts and their N-deuteriated derivatives have been investigated in order to elucidate the long N-acyl chain effect, which promotes the polyglycine II (PGII)-like structure of N-acylglycine oligomers (n = 3-5). The results are summarized as follows. For these very simple oligomers, the long acyl chain induces a large upward shift of the NH-stretch mode and a downward shift of the amide I and II modes.This long acyl chain effect is pronounced for the monomeric salts, indicating that it occurs predominantly at the peptide linkage adjacent to the acyl terminal. For the monomeric and dimeric salts, the Raman spectral patterns of the glycine-CH, characteristic modes, which are similar to those for the PGII-like structure of N-acylglycine oligomers (n = 3-5) and to those for PGII, are promoted by the long acyl chain. We have previously'P2 described investigations of the confor- mations of the N-acylglycine oligomer acid types (trimer, tetramer and pentamer) and their potassium salts, using vibrational spectroscopy and X-ray diffraction powder pat- terns. The results have shown that the long acyl chains induce a further polyglycine I1 (PGI1)-like structure in the solid state.However, the reason why such a PGII-like struc- ture is preferentially stabilized for the long acyl chain deriv- atives of the oligomers still remains unresolved. In order to understand the role of the long hydrocarbon terminal in the preferential stabilization of the PGII-type structure in N-acylglycine oligomers, we have synthesized potassium or sodium salts of N-acylglycine monomers and dimers having varying acyl chain lengths, and their N-deuteriated salts, and have investigated their vibrational spectra in detail. In particular, the long acyl chain effect on the NH (ND) stretch, amide I, amide I1 and the glycine-CH, characteristic modes has been discussed.A systematic study of the many structural problems associ- ated with the interaction between lipid and membrane pro- teins is of great significance for the elucidation of structure-function relationships of membrane proteins. Recently, the structure of a bacterial photosynthetic reaction centre crystallized in the form of a protein-surfactant been shown that surfactant molecules promote the ordered packing of protein-surfactant molecule^.^.^ This ordered packing may then induce conformational changes of the protein molecule in a protein-surfactant complex. However, very little is known about the role of the close-packing- induced conformational change in structure-function relationships. For a detailed study of such a conformational change, further studies of model systems may also be required. Experimental N-Acylglycine oligomer acid types (residue number, n = 1-5) with varying acyl chains were synthesized by the procedure previously1v2 described.The potassium or sodium salts of the monomers and dimers were then prepared from the corre- sponding acid types and potassium or sodium hydroxide in methanol. N-Deuteriated salts of the monomers and dimers were prepared by proton-deuterium exchange in D,O solu-tion. The abbreviations for the compounds used are listed in Table 1. Samples were identified by elemental analysis and the agreement between the calculated and observed values was within 0.5%. Raman spectra below 4000 cm-' were obtained with a JEOL JRS-400D Raman spectrometer using the excitation complex has been acetyl (rn = 0) propanoyl (rn = 1) butanoyl (m = 2) pentanoyl (m = 3) hexanoyl (rn = 4) octanoyl (m = 6) Furthermore, it has Table 1 Abbreviations" for the N-acylglycine oligomeric saltsb monomer (n = 1) dimer (n = 2) trimer (n = 3) tetramer (n = 4) pentamer (n = 5) AcGlK AcG2K AcG3K AcG4K AcG5K A& 1K(ND) PrGlK AcG2K(ND) PrG2K AcG 3K(ND) PrG3K AcG4K(ND) PrG4K AcGSK(ND) PrG5K PrGlK(ND) BuGlK PrG2K(ND) BuG2K PrG3K(ND) BuG3K PrG4K(ND) BuG4K PrGSK(ND) BuG5K BUG1 K(ND)PeG 1K BuG2K(ND) PeG2K BuG3K(ND) PeG3K BuG4K(ND) PeG4K BuGSK(ND) PeGSK PeGlK(ND) HeGlNa PeG2K(ND) HeG2Na PeG3K(ND) HeG3Na PeG4K(ND) HeG4Na PeG 5K( ND) HeGSNa HeG 1 Na(ND) OcGlNa HeG2Na(ND) OcG2Na HeG3Na(ND) OcG3Na HeG4Na(ND) OcG4Na HeGSNa(ND) OcG5Na OcGlNa(ND) QcG2Na(ND) OcG 3Na( ND) OcG4Na(ND) OcGSNa(ND) " K, Na and ND denote the potassium salt, sodium salt and N-deuteriated sample, respectively. The numbers (n = 1-5) in the abbreviations indicate the residue number. CH,(CH2),CO(NH-CH2-CO),-O-Mf (M+:K+ or Na+).3484 wavelength of 514.5 nm of an argon-ion laser (NEC, GLC-3200, 2W) at room temperature. IR spectra were recorded on a Perkin-Elmer 1700 Fourier-transform IR spectrometer (4000-400 cm-') with the sample dispersed in KBr discs at room temperature. The following abbreviations are used for the assignment of the vibrational spectra of these oligomers: amide I, mainly C-0 stretching vibration; amide 11, NH in-plane bending vibration coupled with amide CN stretching; amide 111, mainly amide CN-stretching vibration coupled with N- H in-plane bend and C"-H bend modes; amide IV, C-0 in-plane deformation vibration; amide V, N-H out-of-plane bending vibration.' Results and Discussion The Raman and IR spectra of N-acylglycine monomeric and dimeric salts and their N-deuteriated derivatives were measured and compared with those of simple peptide compoundsg in the solid.The Raman spectra of AcGlK and AcGlK(ND) are shown in Fig. 1. The observed band fre- quencies for AcGlK and AcG2K are listed in Tables 2 and 3 with a tentative assignment. For AcGlK, the weak and broad Raman band at 3259 cm-' and the IR band at 3260 cm-' are assigned to the NH-stretch modes, perturbed by Fermi resonance with the first overtone of the amide I1 mode.'*^" The Raman bands at 1611,1644 and 1678 cm-' and IR bands at 1613, 1642 and 1678 cm-' are definitely assigned to amide I modes. The Raman band at 1611 cm-' and IR band at 1613 cm-' may be overlapped with the bands coming from the asymmetric stretch mode of CO,-.The Raman band at 1532 cm-' and the IR band at 1531 cm-' are due to amide I1 modes. Upon N-deuteriation the Raman band at 1399 cm-' decreases in intensity and the bands at 1259 and 1320 cm-' disappear, as is seen in Fig. l(b). Therefore, these bands evi- dently contain a contribution from the amide characteristic modes. The vibrational spectra of AcG2K and AcG2K(ND) were also compared with those of simple amide compoundsg and ti 1700 ' 1600'1500'1400'1300'1200' IlOO'lOOO' 900 Raman shift/cm-' Fig.1 Raman spectra of (a) AcGlK and (b) AcGlK(ND) in the solid state in the 900-1700 cm-' region J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Observed band frequencies" of AcGlK and their tentative assignment frequency/cm-Raman IR assignmentb 3259 w, br 3260 s, br amide A 3067 vw amide B 1678 w 1678 m } amide I1644 m 1642 s 1610 vw, b 1613 vs VaKO 2 -)1532 vw, b 1531 m, b amide I1 1453 s 1449 sh das(CH3)1423 m 1425 sh CH, sci 1399 vs 1397 vs, b v,(co,-),d,(CH,)1320 m 1310 s amide I11 +CH, wag 1310 sh CH, wag 1259 s 1257 w amide I11 +CH, tw 1225 m 1223 w CH, tw 1164 vw 1128 w 1127 m ] CH, rock1049 vw 1050 m 1003 vs 1002 s skel.str. 979 vs 979 m 914 w 912 m skel. str. +CH, rock 884 vw 878 vw 751 vw, b 754 m, b (amide IV) 693 m 697 m, vb 616 m 616 s } z::580 w 581 w, b rock 527 m 528 s " Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoul- der; b, broad. Amide A, N-H stretching; amide B, the overtone mode of amide 11; va, and v,, asymmetric and symmetric stretching vibrational modes, respectively; 6, deformational vibration; wag, wagging; tw, twisting; sci, scissors; rock, rocking; skel. str., skeletal stretching. N-acylglycine oligopeptides (n= 3-5).'.2 The observed band frequencies are listed in Table 3 with their assignments. For longer acyl chain derivatives, as well as the N-acetylglycine monomeric and dimeric salts, the bands arising from the NH Table 3 Observed band frequencies" of AcG2K and their tentative assignment frequency/cm-Raman IR assignment" 3290 m, b 3297 vs amide A 3113 vw amide B 1680 m 1692 w 1660 sh 1666 vs amide I 1634 m 1 1590 w, b 1603 vs, b 1579 s 1540 vw, vb 1544 s 1461 w 1461 m 1453 vw 1439 m 1423 s 1421 m CH, sci 1390 vs 1386 s v,(co,-),d,(CH3)1319 s 1317 m amide I11 +CH, wag 1290 s 1292 w CH, wag +amide I11 1267 vs 1264 s amide I11 + CH2 tw 1223 s 1224 sh CH, tw 1143 m 1144 m 1079 w, b 1077 w C-CH, rock 1034 vs 1036 s skel.str. 1011m 1013 m 952 s 952 vw 916 vs 914m } skel. str. + CH, rock 889 m 889 w " Abbreviations as in Table 2.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (ND) stretch, amide I, amide I1 and CH, characteristic modes, coupled with the amide characteristic modes, could be assigned. In particular, the spectral features of these vibra- tional modes have been found to depend on acyl chain length. In this present discussion, we focus upon the effect of the long acyl chain on these modes. NH- and ND-stretching Modes Fig. 2A shows the N-acyl chain-length dependence of the Raman spectral features in the NH-stretch region for the monomeric salts. The spectral features depend strongly upon the acyl chain length. For PrGlK, two broad Raman bands are observed at 3315 and 3290 cm-', showing that the NH groups participate in two different hydrogen-bonding environments.For BuGlK, the broad NH-stretch band appears at 3285 cm-'. However, for HeGlNa and OcGlNa, the spectral patterns in the NH-stretch mode region are quite different from those of AcGlK, PrGlK and BuGlK. With further increase in acyl chain length, the NH-stretch band evidently becomes sharper and stronger in intensity than those of the shorter acyl chain derivatives. Furthermore, the NH-stretch bands of longer acyl chain derivatives shift mark- edly to a higher frequency, compared with those of BuGlK. For N-deuteriated derivatives of the monomeric salts (Fig. 2C), and for the dimeric salts and their N-deuteriated deriv- atives (Fig. 2B and 2D), similar observations for the spectral features in the NH (ND) stretch region may be made.We have already reported that, for the NH-stretch Raman bands of the N-acylglycine oligomers (n = 3-5), the Raman intensity of the amide A band relative to that of amide B increases with an increase in acyl chain length.' This observa- tion has been ascribed to the Fermi resonance between the amide A and amide B (first overtone of amide 11) modes. Fur- thermore, we have confirmed that the Fermi resonance between the amide A and amide B modes brings about an increase in Raman intensity and a change in band frequency. A B h YIcs(rl h -For the N-deuteriated longer-chain oligomers, it has also been reported that the spectral pattern variation in the ND- stretch region, induced by the long acyl chain, derives from Fermi resonance between the ND-stretch modes and com- bination bands.' Accordingly, for the long N-acyl chain monomeric and dimeric salts, we may ascribe the band frequency shift and the intensity increase to the same mechanism as for the longer-chain oligomers.The Fermi-resonance-corrected frequencies (unperturbed frequency, vi) for the monomeric and dimeric salts are listed in Table 4. Unperturbed frequencies of the amide A band depend markedly upon the acyl chain length. We may use the Avi value [Avi = He~i(unperturbed NH-stretch band fre- quency for HeGlNa)-'"v; (unperturbed NH-stretch band frequency for BuGlK)] as a measure of the long acyl chain effect. For the monomeric and dimeric salts, the Avi values are 68 and 20 cm-', respectively, while for the longer N-acylglycine oligomers (n = 3-5) they are 10-15 cm- '.'These data emphasize our conclusion that the long acyl chain induces formation of the weak hydrogen-bonding environ- ment for the peptide NH group adjacent to the acyl chain terminal. Thus, for the shorter acyl chain derivatives of the mono- meric and dimeric salts, typical N-Ha * .O-C hydrogen bonds are probably formed between intermolecular chains. However, for the longer acyl chain derivatives, such hydrogen bonds may be broken, and the NH groups may participate in very weak hydrogen-bond systems. Amide I and I1 Modes For the N-acylglycine oligomers (n = 3-9, the PGII-like structure induced by the long acyl chain is markedly reflected in the amide I and I1 regions.'y2 However, for the peptide linkage adjacent to the N-acyl terminal, the long acyl chain effect on the amide I and I1 modes still remains obscure.C D -I I I 1 1 3400 3300 3200 3100 3400 3300 3200 3100 3000 2500 2400Raman shift/cm-' Fig. 2 Effect of acyl chain length on the NH (ND) stretch Raman bands for the N-acylglycine monomeric (A) and dimeric salts (B) and their N-deuteriated derivatives (C and D). A, (a) AcGlK, (b) PrGlK, (c) BuGlK, (d) HeGlNa and (e) OcGlNa; B, (a) AcGK (b) PrGZK, (4 BuG2K, (d)HeG2Na and (e)OcG2Na; C, (a)BuGlK(ND) and (b) HeGlNa(ND); D, (a) BuG2K(ND) and (b) OcG2NGND). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Acyl chain length dependence of observed and unperturbed frequencies" (cm-') of NH-stretch modes for the N-acylglycine mono- meric and dimeric salts monomer dimer observed calculated observed calculated chain VA VB 1, v;; v; VA VB 1, vi v; Ac 3259 3067 0.25 3220 3106 3290 3113 0.22 3258 3145 Bu 3285 3084 0.20 3252 3118 3318 3092 0.18 3284 3127 He 3346 3085 0.11 3320 3111 3327 3093 0.11 3304 3116 oc 3346 3085 0.11 3320 3111 3327 3090 0.12 3302 3116 v,, and vi,B: observed frequencies (cm-')and unperturbed frequencies (cm-'), respectively, for amide A and amide B modes.These unper- ~turbed frequencies were calculated by using the equations: v ~ = ,$[(vi + v:) s], where s = vA-vB and I, = I,JIA = (s -6/s + a), where 6 = v; -v; and I, and I, denote the band intensities of amide A and amide B modes, respectively.Fig. 3A shows the Raman spectra of the monomeric salts in the region 1500-1700 cm-'. For the Raman spectrum of BuGlK, the bands corresponding to the 1678 cm-' band for AcGlK and to the 1656 cm-' band for PrGlK disappear, and the two Raman bands due to the amide I modes are observed at 1605 and 1635 cm-'. For HeGlNa and OcGlNa, with longer acyl chains, strong Raman bands coming from the amide I modes are observed at 1624-1625 cm-'. For the N-acylglycine oligomer salts (n = 3-5), it has been pointed out' that four vibrational bands at 1620, 1639-1640, 1655 and 1680 cm-' are observed in the amide I region and that the three bands at 1620, 1639-1640 and 1680 cm-' arise from the peptide linkages adjacent to the N-acyl terminal group.B d. Therefore, we may assume that the amide I bands observed for the monomeric and dimeric salts come from the peptide linkages adjacent to both N-acyl and carboxylate terminals. When we compare the amide I band frequency for BuGlK with those for HeGlNa and OcGlNa, it can be seen that the band frequency shifts markedly to a lower frequency, as the acyl chain length increases. This shift implies that the peptide C-0 group adjacent to the acyl terminal is strongly influ- enced by the long acyl chain. Thus, the splitting feature of the amide I mode for the monomeric salts is generally found to depend on the acyl chain length (Table 5). For the amide I1 modes of the monomeric salts, a long acyl chain also induces a downward shift (from 1553 to 1542 cm-').For the N-deuteriated monomeric salts, this down- ward shift for the amide 11' modes is also found [from 1473 to 1466 cm-', Fig. 5(a')and (b')]. Fig. 3B shows the Raman spectral patterns of the dimeric salts in the amide I and I1 regions. The long acyl chain also affects the Raman spectral pattern of the dimer in this region. However, for the dimeric salts, it should be emphasized that the downward shift of the amide I and I1 modes, induced by the long acyl chains, is not so marked. The Raman bands at 1587-1588 cm-', observed for HeG2Na and OcG2Na, are assigned to the asymmetric stretching mode of CO,-, since these bands do not shift downward on N-deuteriation.For the IR spectra of the monomeric and dimeric salts, it is also found that long acyl chains induce a marked amide I splitting, as listed in Table 5. This splitting should be due to a transition dipole-dipole interaction. lo,' ' Both the intra- and inter-molecular geometry of the simple oligomers is reflected in the splitting patterns of the amide I For the N-acyl-L-glutamic acid oligomers (n = 3 and 4) having varying acyl chain length, we have previously shown17 that these oligomers take up a sheet structure similar to the /12-type structure of poly(L-glutamic acid) and the intersheet spacing tends to decrease as the acyl chain length increases. For the N-acylglycine monomeric and dimeric salts, we may assume that the variation in the intra- and inter-molecular geometry is induced by the long acyl chain, resulting in such an amide I splitting.To test this assumption, we calculated, using Chirgadze and Nevskaya's theoretical treatment,14-' the resonance interactions of amide I modes for a single infinite antiparallel-chain pleated sheet, as well as 1700 1600 1500 1700 1600 Raman shift/cm-' Fig. 3 Effect of acyl chain length on the Raman bands in the 1500-1700 cm -region for the N-acylglycine monomeric (A) and dimeric salts (B). A, (a) AcGlK, (b)PrGlK, (c) BuGlK, (d)HeGlNa and (e) OcGlNa; B, (a) AcG2K, (b)PrG2K, (c) BuG2K, (d)HeG2Na and (e) OcG2Na. for various kinds of its finite fragments. We have confirmed that the reduction in the intersheet spacing and the geometri- cal variation bring about a marked change in the frequency- splitting pattern: for example, for the amide I modes of an infinite antiparallel-chain pleated sheet, when we assume that the intersheet spacing decreases from 9.5 to 8.5 A, the fre- quency splitting increases by ca.21 cm-'. Thus, the close packing of the long acyl chain in the solid state probably J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 5 Observed band frequencies" (cm-') of amide I and I1 modes for N-acylglycine monomeric and dimeric salts monomeric salts dimeric salts amide I amide I1 amide I amide I1 chain Raman IE Raman IR Raman IR Raman IR 1675 s 1575 vw, b 1572 m 1656 m 1660 vs 1552 s Pr 1643 s 1542 vw 1545 m 1645 vs 1647 m 1636 s 1635 s 1613 m 1601 vs, b 1605 vs 1677 vw 1677 m 1577 m 1553 vw 1550 m 1660 vw 1654 vs 1567 vw, b 1555 w Bu 1635 s 1642 s 1641 s 1662 vw 1629 vs 1605 w 1603 vs, b 1606 s (1670 sh) 1635 vs 1552 m 1645 vs 1642 vs 1553 w 1556 s He 1624 vs 1613 sh 1605 vs 1542 vw 1601 s 1540 sh 1680 sh 1679 vw 1560 s 1552 m 1670 sh 1669 vw 1555 w 1544 vw oc 1635 s 1625 vs 1611 sh 1604 s 'Abbreviations as in Table 2.results in a decrease in the inter-peptide linkage spacing and its configurational variation. This decrease must, in turn, bring about the marked splitting of the amide I and I1 modes. CH, Characteristic Modes of the Glycine Residue For polyglycine, the vibrational modes characteristic of the glycine-CH, group are markedly dependent on the confor- mation in the band frequency."*'* When polyglycine I (PGI)" takes up the antiparallel-chain rippled-sheet struc- ture, the CH, bend, twist and wag modes are easily dis- tinguished from the amide characteristic modes.l4 Conversely, for polyglycine I1 (PGII)' 'having a helical struc- ture, the spectral patterns in the CH, characteristic modes are quite different from those for PGI.The CH,-wag modes are heavily mixed with the NH in-plane bend and C"C- stretch modes, and appear in the 1330-1390 cm-' region." The large downward shift of the CH,-wag mode at 1380 cm-' to lower frequency (ca. 1350 cm-') on deuteriation has been well predicted by normal-mode analysis of PGII.' ' We now confirm that the Raman bands of N-deuteriated PGII at 1383 cm-' disappear on N-deuteriation and a band at 1347 cm-' is observed.Thus, we may emphasize that the large downward shift of the CH,-wag mode at ca. 1380 cm-' (to ca. 1350 cm-') is characteristic of the CH, group partici- pating in the C"H"-..O=C hydrogen bonds in the PGII-like structure. Furthermore, the Raman band at 1244 cm-' for PGII has been assigned to the CH,-wag modes characteristic of PGII coupled with the CN stretch, NC" stretch and NH in-plane bend modes. ' ' The CH,-twist bands characteristic of PGII, coupled with the CH, wag, NH in-plane bend and CN-stretch modes, are predicted to appear in the 1250-1300 cm-' region." These coupled modes are attributable to the existence of C"H". -.O=C hydrogen bonds.For the N-acylglycine oligomers (n= 3-5),, a detailed dis- cussion on the CH, characteristic modes has not yet been made. We discuss below the relationship between the vibra- 1659 sh 1537 vw 1645 vs 1644 vs 1524 vw 1636 sh 1620 sh 1623 vs 1609 m tional modes of the longer oligomers and the CH, environ- ment in the PGII-like structure. Fig. 4 shows the acyl chain length dependence of the Raman spectral patterns for the tetramer salts and their N-deuteriated derivatives in the region of 1100-1500 cm-'. The Raman bands at 1384-1388 cm-' are observed in common for AcG4K, PrG4K, BuG4K and PeG4K, and for longer acyl chain derivatives (HeG4Na and OcG4Na), the 1381-1382 cm-' bands also appear. These bands correspond well with the 1384 cm-' band of PGII and may be assigned to the CH,-wag modes coupled with the amide characteristic modes." For the series AcG4K(ND)-PeG4K(ND), the intensities of the Raman bands at 1390-1391 cm-' markedly decrease as the acyl chain length increases until they finally disappear for HeG4Na(ND) and OcG4Na(ND).This decrease implies that the contribution of the amide characteristic modes to the band at 1384-1388 cm-' increases with an increase in acyl chain length. Furthermore, for these N-deuteriated deriv-atives, the Raman bands at 1350-1354 cm-' are observed in common and correspond well with the 1350 cm-' band for PGII(ND). These bands may be assigned to the CH,-wag mode shifted to lower frequencies upon N-deuteriation, and can be regarded as bands characteristic of a PGII-like struc- ture.The Raman bands at 1291-1294 cm- 'for AcG4K, PrG4K and BuG4K disappear on deuteriation of the NH group, showing that these bands come from the CH, characteristic modes coupled with the amide I11 modes (CN stretching)." For PeG4K(ND), HeG4Na(ND) and OcG4Na(ND), however, the bands at 1296-1301 cm-' correspond well with the 1291-1294 cm- bands and tend to increase in intensity with an increase in the acyl chain length. These bands may be due to the CH,-twist modes of the acyl chain. Therefore, for the longer acyl chain tetramer salts, the bands arising from the glycine-CH, twist-amide I11 coupled modes may be over-lapped with the twist modes of the CH, groups belonging to the N-acyl chain in the 1292-1300 cm-' region.I I II 1 I I I 1500 1400 1300 1200 1500 1400 1300 1200 Raman shift/cm-' Fig. 4 Effect of acyl chain length on the Raman spectral patterns for N-acylglycine tetramer salts and their N-deuteriated derivatives in the 1100-1500 cm-' region. The peak heights are normalized. The asterisked peaks are the focus of the present paper. (a)AcG4K; (a') AcG4K(ND); (b) PrG4K; (b') PrG4K(ND); (c) BuG4K; (c') BuG4K(ND); (d) PeG4K; (d') PeG4K(ND); (e) HeG4Na; (e') HeG4Na(ND);(f)OcG4Na; (f')OcG4Na(ND). For the series AcG~K-OCG~N~, very strong and broad Raman bands at 1255-1262 cm-' are observed in common and these come mainly from the CH, twist-wag coupled modes of the glycine residues.However, deuteriation of the NH groups brings about a variation in frequency and inten- sity for these bands. In particular, the bands at 1255-1256 cm-' disappear on deuteriation, indicating that these bands are heavily mixed with the amide characteristic modes. In the Raman spectra of N-acylglycine trimer and penta- mer salts, for the CH, characteristic modes coupled with the amide characteristic modes, similar observations were made (spectra not shown). Thus, for the N-acylglycine oligomers (n = 3-9, the Raman spectral patterns of the CH, characteristic modes mixed with the amide characteristic modes are very similar to that for PGII. Moreover, note that the long acyl chains induce a further PGII-like spectral pattern in the region of the CH, characteristic modes.The PGII-like conformation of these longer oligomers has already been confirmed by X-ray diffraction powder patterns for the N-acylglycine oligomers (acid types, n = 3-5)," and their salts.2b Therefore, the pat- terns of the PGII characteristic CH, modes observed for these longer oligomer salts indicate that the long acyl chain promotes the formation of the C"H". * .O-C hydrogen bond. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 A detailed investigation was also made of the glycine CH, characteristic bands of the N-acylglycine monomeric and dimeric salts and their N-deuteriated derivatives. Fig. 5 shows the representative acyl chain length dependence of the spectral patterns for the dimeric salts in the region of 1100- 1500 cm-'.The Raman band at 1391 cm- ' for BuG2K closely corre- sponds to those at 1384-1388 cm-' for the series AcG4K- PeG4K. This band may also be assigned to the CH,-wag modes coupled with the amide characteristic modes, since the 1391 cm-' band decreases in intensity and the band at 1355 cm-'appears upon N-deuteriation. The Raman bands at 1243 and 1296 cm-' for BuG2K decrease in intensity on deuteriation. Therefore, these bands evidently come from the CH, twist and wag modes mixed with the amide I11 modes. The Raman bands at 1378, 1293-1295 and 1249-1250 cm-',observed in common for HeG2Na and OcG2Na, cor- respond well with the bands at 1381-1382, 1292-1294 and 1255-1256 cm-', respectively, for the tetramer salts and dis- appear upon N-deuteriation.In particular, the 1378 cm- ' band is found to shift to lower frequencies (by 9-11 cm-'), compared with the corresponding bands (1384-1388 cm-') for the shorter acyl chain derivatives. Moreover, this band frequency shift induced by the long acyl chain is larger in extent, compared with that (6-7 cm- ') for the tetramer salts. As is seen in Fig. 6, for the monomeric salts and their N- deuteriated derivatives, the Raman spectral patterns in the region of the CH,-characteristic modes are very similar to those seen for the dimeric and longer oligomer salts. Con- versely, for N-acetylglycine monomeric salts and their N- deuteriated derivatives, the spectral patterns for the CH, characteristic modes are different from those for the longer acyl chain derivatives, since no Raman bands corresponding to the 1380 cm-' band for PGII and to the 1350 cm-' band m 2. c 4-.-c c k a L 1500 ' 1400 ' 1300 ' 1200 ' 1500 ' 1400 ' 1300 ' 1207 Raman shift/cm-' Fig.5 Raman spectra of (a)BuG2K, (a') BuG2K(ND),(b)HeG2Na and (h')HeG2Na(ND) in the 1100-1500 cm-'region J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 cu\A 5 .-v) +-P, .-C a3d 71l1'1'1 lll~l~ 1500 1400 1300 1200 1500 1400 1300 1200 Raman shift/cm-' Fig. 6 Raman spectra of (a)BuGlK, (a')BuGlK(ND), (b)HeGlNa and (b')HeGlNa(ND) in the 1100-1500 cm- region for PGII(ND) are observed. Thus, for both the monomeric salt and the dimeric salt, we may assume that the PGII-like environment of glycine CH, groups are promoted by the long acyl chain.For the monomeric and dimeric salts, we have no evidence from X-ray diffraction powder pattern analysis for the exis- tence of a PGII-like structure. Therefore, it is difficult to con- clude the formation of C"H". * .O=C hydrogen bonds in Fig. 7 Schematic representation of C"H". ..O=C hydrogen bonds between different molecular chains for OcGlNa (a)and OcG2Na (b) these very simple oligomers. However, from a comparison of the Raman spectral patterns in the CH, modes of the mono- meric and dimeric salts with those of longer oligomers (n = 3-5), we may speculate the formation of such hydrogen bonds even in the monomeric and dimeric salts. The CH,-stretch modes reflect the PGII-like structure.' For N-acylglycine monomeric and dimeric salts, the CH, characteristic bands of the glycine residues are overlapped with the CH-stretch bands for the CH, and CH, groups of the N-acyl chains.A Fourier self-deconvolution te~hniquel~-~~may prove to be useful for separating the glycine CH, modes from the spectrum in this vibrational region. Conclusions For the N-acylglycine monomeric and dimeric salts and their N-deuteriated derivatives having varying acyl chain length, the Raman bands arising from the NH (ND)stretch, amide I, amide I1 and CH, characteristic modes have been investi- gated, in order to understand the role of the long acyl chains in promoting the PGII-like structure of the N-acylglycine oli- gomers (n = 3-5). The results are summarized as follows.The long acyl chain induces a large upward shift in the NH (ND) stretch bands and a downward shift of the amide I and I1 bands. This effect is particularly pronounced for the monomeric salts, indicat- ,, ing that the long acyl chain effect occurs predominantly at the peptide linkage adjacent to the acyl terminal. The large upward shift of the NH (ND) stretch band for the monomeric salts indicates that the long acyl chain induces the formation of a weak hydrogen-bonding environ- ment for the peptide NH group adjacent to the acyl terminal. The downward shift of the amide I and I1 bands arises from the reduction in the intermolecular spacing and the geo- metrical variation induced by the long acyl chain, resulting in a marked variation of the frequency-splitting pattern.For the monomeric and dimeric salts, the Raman spectral patterns of the glycine CH, characteristic modes [similar to those for the PGII-like structure of the N-acylglycine oligo- mers (n = 3-5) and for PGII] are promoted by the long acyl chain; Ca-Ha. .O=C hydrogen-bond formation in the monomeric and dimeric salts is thus seen to be a possibility, as shown in Fig. 7. We thank Prof. Charmian J. O'Connor, Department of Chemistry, The University of Auckland, New Zealand for reading the manuscript prior to publication and making sug- gestions for its revision. We thank Mr Hiroto Hirata for his help in the calculation of the dipole-dipole interaction and Mr.Kazuya Watanabe for his help in the preparation of the samples and in measuring the IR spectra. References 1 H. Okabayashi, K. Ohshima, H. Etori, K. Taga, T. Yoshida and E. Nishio, J. Phys. Chem., 1989,93, 6638. 2 H. Okabayashi, K. Ohshima, H. Etori, R. Debnath, K. Taga, T. Yoshida and E. Nishio, J. Chem. Soc., Faraday Trans., 1990, 86, 1561. 3 H. Michel and D. Oesterhelt, Proc. Natl. Acad. Sci. USA, 1980, 77, 1283. 4 M. Garavito and J. P. Rosenbusch, J. Cell. Biol., 1980,86, 327. 5 H. Michel, J. Mol. Biol., 1982, 158, 567. 6 H. Michel, Trends Biochem. Sci., 1983,8, 56. 7 R. M. Garavito, Z. Markovic-Housely and J. A. Jenkins, J. Cryst. Growth, 1986, 76, 701. 8 M. Roth, A. Lewit-Bentley, H. Michel, J. Deisenhofer, R.Huber and D. Oesterhelt, Nature (London), 1989,340,659. 9 Y. Koyama and T. Shimanouchi, Biopolymers, 1968,6, 1037. 3490 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10 W. H. Moore and S. Krimm, Biopolymers, 1976,15,2439. 19 J. K. Kauppinen, D. J. Moffatt, H. H. Mantsch and D. G. 11 12 A. M. Dwivedi and S. Krimm, Biopolymers, 1982,21,2377. S. Krimm and Y. Abe, Proc. Natl. Acad. Sci. USA, 1972, 69, 20 Cameron, Anal. Chem., 1981,53, 145. J. K. Kauppinen, D. J. Moffatt, H. H. Mantsch and D. G. 13 2788. W. H. Moore and S. Krimm, Proc. Natl. Acad. Sci. USA, 1975, 21 Cameron, Appl. Spectrosc., 198 1,35, 27 1. J. K. Kauppinen, D. J. Moffatt, H. H. Mantsch and D. G. 14 72,4933. Yu. N. Chirgadze and N. A. Nevskaya, Biopolymers, 1976, 15, 22 Cameron, Appl. Opt., 1981, 20, 1866. P. R. Grifiths and W-J. Yang, Comput. Enhanced Spectrosc., 607. 1984, 1, 157. 15 Yu. A. Chirgadze and N. A. Nevskaya, Biopolymers, 1976, 15, 23 W-J. Yang, P. R. Grifiths, D. M. Byler and H. Susi, Appl. Spec- 627. trosc., 1985,39, 282. 16 N. A. Nevskaya and Yu. N. Chirgadze, Biopolymers, 1976, 15, 637. 17 A. M. Dwivedi and S. Krimm, Macromolecules, 1982,15, 177. 18 T. Uehara, H. Okabayashi, K. Taga, T. Yoshida, H. Kojima and E. Nishio, Bull, Chem. SOC.Jpn., 1993,66, 2210. Paper 4/03894K; Received 27th June, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003483

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(23), 3491-3503 3491 Molecular Structures of cis-and frans.S-Ethyl Thiocrotonate A Combined Vibrational Spectroscopic and Ab lnitio SCF-MO Study Rui Fausto* Departamento de Quimica, Universidade de Coimbra, P-3049 Coimbra, Portugal Peter J. Tonge The Picower institute for Medical Research, Manhasset, NY 11030, USA Paul R. Carey Department of Biochemistry, University of Ottawa , Ottawa, Ontario, Canada K IH8M5 Ab initio 6-31G*SCF-MO calculations have been carried out for cis-and transS-ethyl thiocrotonate [cis-and trans-CH,-CH=CH-C(=O)SCH,CHJ. Fully optimized geometries, relative stabilities, dipole moments and harmonic force fields for several conformers of these molecules have been determined and the results com- pared with those of similar molecules.Combined with FTIR spectroscopic data, the theoretical results demon- strate that transS-ethyl thiocrotonate exists in two different conformations about the C,-C bond (the scis and s-trans forms, with C=C-C=O dihedral angles equal to 0" and 180°, respectively), the scis conformation being more stable than the s-trans form by ca. 7 kJ mol-' for the isolated molecule situation, while the ciss-ethyl thiocrotonate molecule adopts only the scis conformation about this bond. A comparison of the experimental and theoretical vibrational spectra also shows that the existence of the less stable s-trans isomer of an a$-unsaturated thioester can be successfully monitored by the IR band at ca. 1165 cm-', ascribed to the C,-C stretching mode of this form.Additional conformationally sensitive bands are also identified, but these are difficult to use in the IR spectrum because of overlap with other features. Resonance Raman spectroscopic studies of enzyme-substrate complexes of the form X-CH=CH-C(=O)-O-enzyme, where X is an unsaturated ring moiety, have provided exqui- site structural and mechanistic detail on key catalytic groups in the active site.'Y2 These studies involve the serine protease group of enzymes where the alkoxy oxygen, -0-, forming a transient covalent link between the enzyme and the acyl group is from an active site serine. The a$-unsaturated acyl group provides an ideal chromophoric label by which to generate the resonance Raman spectrum.It is important to extend these studies to the cysteine protease family of enzymes, via intermediates of the type X-CH=CH-C(=O)-S-enzyme, to investigate common mechanistic feature^.^ Moreover, technical innovations now permit us to obtain high-quality Raman data for a,p-unsatu- rated esters bound to proteins for derivatives of coenzyme-A, e.g. CH3-CH=CH-CH=CH-C(=O)-S-CoA bound to crotona~e.~ A factor which limits our interpretation of the Raman or resonance Raman spectra of the a$-unsaturated thioesters, free in solution of bound to their target enzyme, is the paucity of information in the literature concerning the con- formational and spectroscopic properties of these com-pounds. The present paper is the first in a series whose prupose is to fill that void.Despite their relevance, simple a$-unsaturated carbonyl thioesters have not yet been the subject of any detailed structural and vibrational experimen- tal study, at least in part because these compounds are con- siderably unstable and difficult to handle. In addition, to the best of our knowledge, no theoretical studies on this family of compounds have been undertaken until now. Thus, in the present study, the conformational preferences and vibrational properties of cis-and trans-s-ethyl thiocrotonate (abbreviated c-ETC and t-ETC) were studied by a combined ab initio SCF-MO calculation and vibrational spectroscopic approach, as a first step to the general understanding of the conformational and vibrational properties of a,&unsaturated thioes ters.Experimental and Computational Methods Synthesis and Equipment 0.74 ml (0.01 mol) ethanethiol and 1.39 ml (0.01 mol) tri- ethylamine were combined in 50 ml dry CH, Cl,. The solu- tion was stirred on an ice-water bath under nitrogen and 10 ml of dry CH,C12 containing 1 ml (0.01 mol) crotonyl chlo- ride was added dropwise over 10 min. The solution was stirred for a further 15 min at room temperature and then extracted with 2 x 30 ml H,O, 2 x 30 ml 0.1 moll-' sodium acetate pH 4, and 2 x 30 ml saturated aqueous NaCl. After drying the organic layer with dry Na,CO,, solvent was removed under vacuum. A sample of the remaining brown liquid was dissolved in hexane and purified by high-performance liquid chromatography (HPLC) on a silica column using hexane as eluent (3 ml min-').Peaks were detected using 220-260 nm absorbance and further analysed by UV, NMR, Raman and FTIR spectroscopies. For the NMR experiment peaks were combined, the hexane removed under vacuum, and samples redissolved in CD,CN. The UV, Raman and FTIR studies were performed directly on the HPLC fractions in hexane or on the redissolved compounds in CD,CN. The HPLC column used for sample purification was a Supercosil LC-SI semiprep. 25.0 cm x 10 mm 5 pm pore silica column. UV spectra were obtained with a Cary 3 spec- trometer (t-ETC: 217 and 263 nm, ratio of peak heights, 217/ 263 = 2.7; c-ETC: 221 and 266 nm, ratio of peak heights, 221/266 = 2.0). 'H NMR spectra were obtained using a Varian Unity 400 NMR spectrometer operating at 400 MHz (t-ETC: 6 1.86 {dd, 1, J 2, 7 Hz}; 6.91 {dq, 1, J 7, 18 Hz}; 6.19 {dq, 1, J 2, 15 Hz}; 2.91 {q, 1, J 7 Hz}).FTIR spectra were obtained using a Digilab FTS 40A spectrometer equipped with a DTGS detector. Data collection was per- formed using a demountable liquid cell equipped with KBr windows. For each spectrum 64 scans were obtained and co- added. Raman spectra were obtained using a single mono- chromator, equipped with a CCD detector and s super notch filter. Data collection was performed using 90" sampling geometry with 300 mW 647.1 nm laser excitation. For each spectrum 20 scans each 10 s were acquired and co-added. Computational Details The ab initio SCF-MO calculations were carried out with the 6-31G* basis set6 using the GAUSSIAN 92 program system7 running on a VAX9000 computer.Molecular geometries were fully optimized by the force gradient method using Berny's algorithm.8 The largest residual internal coordinate forces were always less than 3 x E, a, '(1 Eh = 2625.5001 kJ mol-'; 1 a, = 5.29177 x lo-" m) or Eh rad-', for bond stretches and angle bends, respectively. The stopping cri- terion for the SCF iterative process required a density matrix convergence of less than The force constants (symmetry internal coordinates) to be used in the normal coordinate analysis were obtained from the ab initio Cartesian harmonic force constants using the program TRANSFORMER.' This program was also used to prepare the input data for the normal coordinate analysis programs used in this study (BUILD-G and VIBRAT").The calculated force fields were scaled down by using a simple linear regression in order to adjust the calculated frequencies to the observed ones. Fre- quencies corresponding to unobserved or doubtfully assigned vibrations were then calculated from the ab initio force fields by interpolation using the straight line obtained previously. While very simple, this scaling procedure has the advantage, over more elaborate force-field scaling procedures which use several scale factors, of preserving the potential-energy dis- tributions (PEDs) as they emerge from the ab initio calcu- lations. Results and Discussion Geometries and Energies Table 1 shows the 6-31G* energies and optimized geometries for the molecules studied.The atom numbering is presented J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 in Fig. 1. As shown in our previous studies on thioesters and thioacids,"-'4 molecules having an s-trans conformation about the O=C-Y-R (Y = 0 or S; R = H or alkyl) axis have a much higher energy than those adopting an s-cis con- formation about this axis (AEs-trans(c-y)-(c-y) > 13 kJ mol-') and thus were not considered in this study. From the results presented in Table 1, the following conclusions can be drawn: (i) The energy of c-ETC is ca. 12 kJ mol-' higher than that of t-ETC. This increase in energy in going from t-ETC to c-ETC can be ascribed to a reduced methyl-carbonyl dis- tance in the latter molecule, which leads to steric strain.Indeed, this conclusion is reinforced by the relative values of the C-C-0, C=C-C(1), C(8)-C=C and C-C-H(16) bond angles in the two molecules, which are all larger in c-ETC owing to the methyl-carbonyl steric repulsion. Apart from the changes observed in the above bond angles (and in those which must adjust to compensate these changes), the structural differences between t-ETC and c-ETC are not sig- nificant. (ii) The internal rotation about the C,-C bond in t-ETC originates two different conformers : the s-cis and s-trans forms, with the C=C-C=O dihedral angle equal to 0 and 180", respectively. For the isolated molecule, the s-cis form is ca. 7 kJ mol-I more stable than the s-trans form.In c-ETC, only the s-cis conformation corresponds to a minimum in the potential-energy profile for internal rotation about the C,-C bond, whilst the s-trans, conformation corresponds to a con- formational transition state (i.e. to a saddle point having an energy of ca. 18 kJ mol-I above the energy minimum). This different conformational behaviour found for t-ETC and c-ETC can be easily understood considering the relative importance of the steric interactions involving the 1-carbon substituent cis to the C(=O)S fragment in the two molecules. When compared with the H atom [the 1-carbon substituent cis to the C(=O)S fragment in t-ETC], the larger methyl sub- stituent present in c-ETC leads to stronger steric repulsions with either the oxygen (in s-cis forms) or sulfur (in s-trans Ca CQ 0-\H(15) H(14) Ta Fig.1 Numbering of atoms for the relevant conformers on (a)c-ETC and (b) t-ETC J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3493 Table 1 6-31G* calculated optimized geometries, energies and electric dipole moments for the relevant forms of c-ETC and t-ETC C-ETC t-ETC parameter cg Ca cg Ca Tg Ta c-0 119.20 119.18 119.20 119.18 119.14 119.13 C( 1)- s 178.62 178.49 178.24 178.08 178.95 178.75 C(l)-C(4) 148.47 148.50 148.42 148.45 148.43 148.44 c-c 132.80 132.79 132.36 132.35 132.29 132.27 C-H(6) 107.54 107.55 107.58 107.59 107.62 107.62 C-H(7) 107.88 107.89 107.67 107.67 107.71 107.73 C-C(8) 149.96 149.96 149.76 149.76 149.92 149.92 C-H( 16) 107.63 107.63 108.33 108.33 108.31 108.31 C-H(17) 108.71 108.71 108.64 108.65 108.63 108.64 C-H( 18) 108.71 108.71 108.64 108.65 108.63 108.64 s-C(9) 181.90 181.96 181.95 181.99 182.01 182.04 C-H( 11) 108.02 108.04 108.02 108.04 107.98 108.01 C-H(12) 108.30 108.04 108.30 108.04 108.31 108.01 C(9)-C( 10) 152.46 152.56 152.45 152.56 152.42 152.54 C-H( 13) 108.63 108.49 108.63 108.49 108.63 108.49 C-H( 14) 108.42 108.45 108.42 108.45 108.42 108.45 C-108.22 108.45 108.22 108.45 108.21 108.450-c-sH( 15) 121.79 121.45 122.42 122.08 121.49 121.18 c-c-0 126.00 126.10 124.25 124.33 120.90 121.00 c-c-c 126.00 126.01 120.44 120.46 126.12 126.10 H( 6)- C- C 115.36 115.34 117.72 117.72 112.52 112.50 H(7)-C-C 115.77 115.79 117.88 117.85 119.85 119.82 C(8)-C=C 129.74 129.68 125.15 125.17 124.48 124.53 C-C(8)-H( 16) 112.77 112.75 111.93 111.93 111.73 11 1.72 C-C(8)-H( 17) 109.29 109.31 110.34 11 1.34 110.36 110.40 C-C( 8)- H( 1 8) 109.30 109.31 110.33 110.34 110.40 110.40 c-s-c 101.14 100.56 101.13 100.57 100.56 99.88 C( 10)-c-s 113.96 109.94 1 13.94 109.95 114.05 106.70 H(ll)-C-S 107.53 108.25 107.48 108.22 107.61 108.3 1 H( 12)-C-S 104.83 108.25 104.84 108.22 104.51 108.31 C-C-H(13) 109.69 109.51 109.68 109.50 109.64 109.41 C-C-H( 14) 11 1.20 11 1.48 11 1.20 109.48 11 1.20 111.51 C-C-H( 15) 110.83 11 1.48 110.81 111.48 110.86 111.51 c-c-c-c 0.04 0.00 180.00 180.00 179.66 180.00 H-C-C-180.03 180.00 0.01 0.00 -0.14 0.00 c-c-c-0 C( 1) -0.34 0.00 -0.11 0.00 -175.81 180.00 H-C-C-0 179.73 180.00 179.92 180.00 3.58 0.00 c-C(-0)-s 180.37 180.00 -179.65 180.00 -179.33 180.00 0-c-s-c 0.99 0.00 0.93 0.00 2.71 0.00 C( 1)- s-c-c 80.42 180.00 80.25 180.00 79.75 180.00 H(l1)-C-S-C -43.17 58.00 -43.27 58.30 -44.05 58.38 H( 12)- C- S-C -158.07 -58.00 -158.18 -58.30 -158.89 -58.38 H(13)-C-C-S 178.39 180.00 178.51 180.00 178.19 180.00 H( 14)- C- C-S 58.71 60.34 58.83 60.54 58.54 60.58 H( 15)-C-C-S -62.25 -60.34 -62.15 -60.55 -62.49 -60.58 H( 16)-C-C=C -0.32 0.00 0.02 0.00 0.11 0.00 H( 17)-C-C=C 121.39 121.75 121.02 1 20.99 121.00 120.90 H( 18)-C-C=C -122.12 -121.75 -120.97 -120.99 -120.81 -120.90 AE" -1.394 -1.501 7.181 8.502 (11.56) (12.95) IPI 1.140 1.034 1.459 1.370 2.209 2.226 Bond lengths in pm, angles in degrees, energies in kJ mol-', dipole moments in Debye (1 D = 3.33564 x C m); see Fig.1 for atom numbering. " Energies relative to the most stable conformer; for c-ETC, the values presented in parentheses are energies relative to the most stable conformer of t-ETC. The total energy for the most stable form of t-ETC is -705.397 963 896 E, . conformations) atoms. This increase of energy due to the with the s-cis -+ s-trans isomerization. Thus, in t-ETC, the H +CH, substitution is, however, particularly critical for the C-C-S, C=C-C and H(7)-C=C angles increase con-s-trans conformation, where the steric interaction involves the siderably (ca. 4.2, 5.5 and 2.0°, respectively), whereas the sterically more important sulfur atom. Indeed, this inter- C-C=O, O=C-S, H(6)-C-C, H(6)-C=C, C(8)-C=C action is strong enough to make planar s-trans conformation and C(8)-C-H(7) angles decrease to compensate those in c-ETC less stable than the non-planar structures about the changes.In turn, no significant changes are observed in the C,-C bond, despite the fact that a planar C=C-C=O axis bond lengths, which usually are determined mainly by elec- leads to a more efficient overlap of the p(n) orbitals partici- tronic effects."-14 In c-ETC, the structural differences pating in the mesomerism associated with the C=C-C=O between the stable s-cis conformation and the s-trans fragment. C=C-C-0 axis (saddle point) follow identical patterns of (iii) The fundamental importance of the above-mentioned variation [note that C(8)H, is now the B-carbon substituent steric repulsions to the conformational preferences of the cis to the C(=O)S fragment instead of H(7)]: the C-C-S, C=C-C=C axis in the studied molecules is reinforced by C=C-C and C(8)-C=C angles increase by ca.8, 7 and 3", the calculated changes in the structural parameters associated these changes being compensated by those of C-C=O, 0-C-S, H(6)-C-C, H(6)-C=C, H(7)-C=C and C(8)-C-H(7); in addition, the C-C(8)-H(16) angle increases by 2". (iv) Besides the forms resulting from internal rotation about the C,-C axis, both t-ETC and c-ETC may exist in two different-by-symmetry stable conformations differing in the relative orientation of the terminal methyl group of the S-CH,CH, fragment.The most stable of these conforma- tions is a doubly degenerate conformational state (point group Cl), corresponding to C(1)-S-C-C axes of ca. +80" (gauche forms). The second form is the C, symmetric anti form [C(l)-S-C-C dihedral angle equals 180"], which is ca. 1.5 kJ mol-' less stable than the gauche conformation in both c-ETC and t-ETC. In this latter molecule, the anti-gauche (C-S-C-C axis) relative energy does not depend on the relative orientation of the C=C-C=O axis). The fact that the conformation of the C-S-C-C axis is not signifi- cantly affected by the conformation of the acyl fragment is consonant with our previous studies on ethyl dithioacetate and ethyl dithiopropionate,' 5916 where the conformational preferences of this axis, in particular its trend to adopt gauche conformations, have been analysed in detail.The main struc- tural changes associated with the anti -+ gauche rotameriza- tion occur in the C(l0)-C-S, H(ll)-C-S and H(12)-C-S angles (i.e. those angles involving the atoms directly affected by the internal rotation), which increases by ca. 4" and decrease by ca. 1 and 4,respectively, and follow the general trends reported in our previous studies on ethyl dithioesters.' 5,16 (v) The calculated electric dipole moments of the various conformers of t-ETC are larger than those of c-ETC. This result can be understood considering the closer proximity of the positively charged p-carbon methyl substituent of the negatively charged carbonyl oxygen in c-ETC.Indeed, this interpretation is reinforced by the considerably larger electric dipole moments found for the s-trans conformers of t-ETC, where these groups are optimally displaced from each other. On the other hand, the electric dipole moments associated with an anti or gauche C-S-C-C axis are very similar. Thus, it can be anticipated that the relative population about the s-cisls-trans C=C-C=O axis must be sensitive to the polarity of the solvent, while the gauchelanti C-S-C-C axis population ratio will not vary significantly upon chang- ing solvent polarity. In summary, c-ETC exists as two different conformers, which has the C=C-C=O axis in the s-cis conformation and the C-S-C-C axis either in the anti or gauche confor- mations (Ca and Cg forms, see Fig.1). In turn, t-ETC exists in four different conformational states differing by the relative orientation of the C=C-C=O and C-S-C-C axes; besides the Ca and Cg forms similar to those found for c-ETC, two different forms having the C-C-C-0 axis in the s-trans conformation also participate in the conforma- tional equilibrium (the Ta and Tg conformers, see Fig. l). The different conformational behaviour between the two compounds is essentially determined by the presence of the strong repulsive steric interactions between the P-carbon methyl substituent and the sulfur atom in the s-trans C=C-C=O conformation of c-ETC, which considerably increase the energy of this conformation. Steric repulsions (this time involving the P-carbon methyl substituent and the carbonyl oxygen atom) are also the main factor which deter- mine the relative energies of the most stable forms of c-ETC and t-ETC (AE(c-ETC)-(t-ETC)= 12 kJ mol-').All these steric effects are reflected quite clearly in the structural parameters of the different molecules considered. Finally, considering the relative values of the dipole moments of the various con- formers, the calculations indicate that solvent variation may J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 be used successfully to change the C=C-C-0 s-cisls-trans relative populations, but does not provide a useful way to modify significantly the untilgauche C-S-C-C conforma-tional equilibrium. Vibrational Studies Both c-ETC and t-ETC molecules have 48 fundamental vibrations.In the case of the symmetric (C,) conformers (Ca, Ta),the normal modes will span the irreducible representa- tions 30A' + 18A", while those of the non-symmetric gauche forms (Cg and Tg, C, point group) belong to the A sym-metry species. Hence, all vibrations are active in the IR. Table 2 presents the definition of the internal symmetry coor- dinates used in this study. As mentioned in Experimental and Computational Methods, calculated spectra were obtained from the 6-31G* ab initio wavefunctions at the equilibrium geometries of the various conformers considered, and then adjusted to the observed frequencies by linear regression. In this adjustment, data related to the most stable conformers (Cg) of both c-ETC and t-ETC were used simultaneously (a total of 60 frequency values were used). As the calculated us.experimen-tal frequencies yielded a straight line (vex = O.8996vc,,, -14.4) with a high correlation coefficient (R' = 0.999279), the calculated frequencies of these conformers were appropri- ately scaled (the mean and maximum errors in this scaling were 1.3% and 5.49%, respectively). The same frequency scaling was assumed for all the remaining conformers. Table 3-10 summarize the vibrational results of this study. The experimental and calculated spectra are presented in Fig. 2-4. C-ETC The FTIR spectrum of c-ETC in hexane solution (Fig. 2) is dominated by the intense bands at 1680 and 1014 cm-'. These two bands are easily assigned to the v(C=O) and v(C,-C) stretching modes, in agreement with the theoretical results, which also predict that these modes will be the most intense bands in the IR spectrum of this compound (Table 3).It should be noted, however, that the calculations overesti- mate the intensity of the v(C,-C) band and that, inter- estingly, the predicted intensity of the band due to the v(C=C) stretching mode at ca. 1630 cm-' is also consider- al c .E 0.6 1 $ 300.-250.. E*200->.-5 150.-C;100-.-50 -. iOL -L. 550 750 950 1150 1350 1550 1750 wavenumber/cm -' Fig. 2 FTIR spectrum (550-1750 an-' region) of c-ETC in hexane solution at room temperature (a) and 6-31C* calculated (Cg;scaled frequencies)IR spectrum (b) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 coordinate symmetryb Sl A’ s2 A’ s3 A’ s4 A’ A’s5 A’’6s7 A’ S8 A’ s9 A’ SlO A’ SI 1 A” A’s12 A”’13 A’’14 As15 Sl, A“ A’’17 Definition of the internal symmetry coordinates used in the normal coordinate analysis” V(C=O) vEC( 1)-Sl vCC(8)-CIV(C=C) vCC--C(8)14s-c( 1011 ~(9)-c(1011 vcc-H(6)I vcc-~(7)12v[C-H(16)] -v[C-H(17)] V[C-H(17)] -v[C-H(18)] v[C-H(16)] + v[C-H(17)] ~[C-H(ll)l -v[C--H(12)] v[C-H(11)] + v[C-H(12)] 2v[C-H(13)] -v[C-H(14)] v[C-H(14)] -v[C-H(~S)] v[C-H(13) + v[C-H(14)] definition -S[C-C-H(11)] -S[C-C-H(12)] -v[C-H(18)] + v[C-H(lS)] -v[C-H(~S)] + v[C-H(lS)] -S[C(4)-C-S]Sl8 A’ 2S(O-C-S) -S[C-c-0) SI 9 A’ S(C-c-0) -S[C(4)-C-S] s20 A’ S(C-s-C) s2 1 A‘ 4S[S-C-C(lO)] -G[S-C-H(11)] -S[S-C-H(lZ)]A 26[C=C-C(l)] -S[C=C-H(16)] -S[C(l)-C-H(6)]s22 ’23 A’ &C=C-H(16)] -6[C(l)-C-H(6)] ’24 A 26[C(8)-C=C] -S[C(8)--C-H(7)] -6[C=C-H(7)] A’ S[C-C-H( 7)] -S[C(8)-C-H( 711‘2 5 ‘26 A’ 26[H(17)-C-H(18)] -6[H(16)-C-H(17)] -S[H(16)-C-H(18)] A” S[H( 16)-C-H( 17)] -6[H( 16)-C-H( 18)]s27 ’28 A’ 6[H(17)-C-H(18)] + S[H(16)--C-H(17)] + S[H(16)-C-H(18)] ’29 A’ 26[H(14)-C-H(15)] -S[H(13)-C-H( 14)] -S[H(13)-C-H(15)] A” S[H(13)-C-H(14)] -6[H( 13)-C-H(15)] ’30 A’ S[H(14)--C-H(15)] + S[H(13)-C-H(14)] + S[H(13)-C-H(15)]’3 1 -S[C-C-H(16)] -6[C-C-H(17)] -b[C-C-H(18)] -S[C-C-H(13)] -S[C-C-H(14)] -S[C-C-H(15)] A’ 26[C-C-H(16)] -6[C-C-H(17)]’36 A” S[C-C-H( 17)]-6[C-C-H( 18)]s37 S38 A 26[C-C-H(13)] -S[C--C-H(14)] A” S[C-C-H(14)] -S[C-C-H(lS)]s39 A” Q[C-C(-O)-C1’40 ’4 1 A” Q{C-C[-H(6)]-C} A” Q{C-C[-H(7)lEC}’42 A” 7(C-C=C-C)s43s44 A” T(c=c-c=o) s45 A” 7(O=C -s-C) A” 7[C-S-C-C(10)]S46 A” T[C=C-C-H( 16)] + r[C-C-C-H(s47 ’32 A’ 56[H(ll)-C-H(12)] -S[S-C-C(lO)] -S[S-C-H(ll)] -S[S-C-H(12)] -S[C-C-H(ll)] -S[C--C-H(12)]A S[S-C-H(ll)] + 6[S-C-H(12)] -S[C-C-H(ll)] -S(C-C-H(12)]s33 A” S[S-C-H(l l)] -S[S--C-H(12)] -6[C-C-H(ll)] + S[C-C-H(12)]s34 s35 A” S[S-C-H(ll)] -6[S-C-H(12)] + G[C--C-H(l l)] -6[C-C-H(12)] -6[C-C-H(18)] -h[C-C-H(15)] 17)] + r[C=C-C-H( 18)] A” r[S-C-C-H(13)] + r[S-C-C-H(14)] + r[S-C-C-H(15)]S48 Normalization constants are not given here; they are chosen as N = (ZC~)-’’~,where ci are the coefficients of the individual valence coordinates; v, bond stretching; 6, bending, y, rocking, o, wagging, tw, twisting, T, torsion, R, out-of-plane deformation.See Fig. 1 for atom numbering. Symmetry of coordinates taken as reference the symmetric (C,) conformers. In the case of the non-symmetric C, forms, all coordinates belong to the A representation. 401 (b)-35.. 5 30.-1 ,I I: . 25.-E< 20.. r.% 15.-5 10.. c .r 5 -. 1 73.0, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 50 .--550 750 950 1150 1350 1550 1750 wavenumber/cm -' Fig. 4 FTIR spectrum (550-1750 cm-' region) of t-ETC in hexane solution at room temperature (a) and 6-31G* calculated (Cg and Tg; scaled frequencies) IR spectrum (b).M, s-cis; 0,s-trans. The calculated intensities of the bands due to the s-trans conformer are multiplied by the factor 1/2.25 (see text). Table 3 Experimental frequencies and calculated frequencies and intensities for c-ETC" calculated approximate cg Ca experimentalb description ab initio scaled intensity ab initio scaled intensity FTIR (in hexane solution) 3377 3024 4.6 3377 3024 4.4 3369 3016 6.1 3368 3016 6.8 3325 2977 18.1 3328 2979 16.1 3325 2977 17.8 3324 2976 22.1 3302 2956 17.0 3282 2938 45.3 327 1 2928 33.7 3279 2938 21.6 3254 2912 15.7 3268 2926 0.4 3242 2902 26.6 3242 2902 27.0 3210 2873 39.8 3212 2875 34.5 3200 2864 30.0 3200 2864 30.9 1951 1741 214.0 1952 1741 235.5 1680 1855 1654 170.5 1856 1655 167.8 1627 1647 1467 3.5 1646 1466 2.0 1470 I641 1462 7.7 1642 1643 7.8 1457 1636 1457 5.4 1640 1461 6.5 1449 1621 1444 35.7 1621 1444 33.9 1436 1612 1587 1436 1413 4.6 9.4 1629 1587 1451 1413 8.8 9.0 1417 (Cg), 1436 (Ca) 1411 1562 1391 3.4 1567 1395 1.6 1375 1547 1377 5.8 1547 1377 5.9 1365 1452 1292 28.6 1445 1286 31.3 1267 1408 1402 1252 1247 0.2 4.7 1384 1402 1231 1247 0.0 4.6 1248 (Cg), 1239 (Ca) 1248 1226 1088 0.8 1225 1088 0.7 1072 1203 1068 0.1 1203 1068 0.1 1056 1180 1047 24.9 1176 1043 41.0 1050 1171 1039 3.4 1164 1033 0.1 1043 I125 998 319.7 1127 999 3 17.7 1014 1110 984 2.9 1110 984 2.8 - 1059 938 7.8 1064 943 8.1 974 973 861 74.5 973 861 74.6 911 903 798 79.9 903 798 84.4 822 859 758 49.9 860 759 41.2 750 828 730 4.9 849 749 12.2 - 730 697 642 613 4.1 0.1 755 696 665 612 1.6 0.0 655 (Cg),680 (Ca) 620 58 1 508 1.6 602 527 1.6 49 1 427 3.6 484 42 1 3.9 385 332 2.4 362 311 5.2 357 307 1.4 361 310 1.4 307 262 7.0 264 223 0.2 297 253 4.4 298 253 3.6 244 205 3.0 249 210 5.1 163 132 1.3 120 93 1.9 117 91 0.0 120 93 0.5 93 69 0.9 56 36 0.0 77 55 3.5 97 73 4.2 33 15 0.9 31 13 1.2 Frequencies in cm-'.Intensities in km mol-', v, bond stretching; 6,bending, y rocking, a,wagging, tw, twisting, z, torsion, see Table 2 for definition of symmetry coordinates.Frequencies in italic were those used for force filed scaling. Except in the cases explicitly indicated in the Table, all observed bands are considered to have contributions from both Cg and Ca conformers. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 approximate description Normal coordinate analysis for c-ETC (form Cg)" calculated frequency 3377 3369 3325 3325 3302 327 1 3254 3242 3210 3200 1951 1855 1647 1641 1636 1621 1612 1587 1562 1547 1452 1408 1402 1226 1203 1180 1171 1125 1110 1059 973 903 859 828 730 697 58 1 49 1 385 357 307 297 244 163 117 93 77 33 PEDb Frequencies in cm-'.v, bond stretching; 6, bending, y, rocking, o,wagging, tw, twisting, z, torsion, see Table 2 for definition of symmetry coordinate. Only PED values greater than 10% are given. ably overestimated by the calculations. Since the C-C and C,-C bonds are interacting by mesomerism and the exten- sion of this effect certainly contributes significantly to the relative polarity of these two bonds'7-19 these results may be related, and they may indicate that the basis set used in the calculations may have some limitations in describing preci- sely the electron distribution in this fragment. The difference between the experimental and calculated intensities of the v(C=C) and v(C,-C) bands could also have been ascribed to solute-solvent interactions, but these interactions should then also affect significantly the intensity of v(C=O), and this is not in agreement with the observations.Apart from the fore- going point, the general agreement between the calculated and experimental spectrum of c-ETC (both frequencies and intensities) is remarkable (see Fig. 2 and Table 3). As the energy difference between the two conformers of this com- pound (Cg and Ca)is predicted to be small (ca. 1.4 kJ mol- '), both conformers are present in significant amounts at room temperature and they must both contribute to the observed spectrum. However, since besides being more stable the Cg form is doubly degenerate, it must provide the main contri- bution to the bands observed.The following additional points, resulting either from the comparison between the experimental and calculated spectra or from the normal coor- dinate analysis data (Tables 4 and 5) are of note: (i) Above 600 cm-', the frequencies calculated for the Ca and Cg forms are, in general, very similar. Thus, few band splittings can be seen in the spectrum. However, the 6(CH,), tw(CH,), y(CH,) and v(S=C) modes are calculated to have considerably different frequencies in the two forms and should, in principle, give rise to observable doublets. Unfor- tunately, only four bands can be ascribed to a single con- former, and all of these are either very weak or partially overlapped bands: 655 cm-' [v(S-C) Cg], 680 cm-' [v(S-C) Ca],1239 cm-' [tw(CH,) Ca] and 1417 cm-' [G(CH,) Cg]; the remaining bands are superimposed with bands due to other vibrations [G(CH,) Ca contributes to the band at 1436 cm-', assigned to 6(CyHJas; tw(CH,) Cg con- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 approximate description Table 5 Normal coordinate analysis for c-ETC (form Cay calculated frequency A' 3377 A' 3368 A 3328 A' 3324 A' 3282 A'' 3279 A' 3268 A" 3242 A' 3212 A' 3200 A' 1952 A' 1856 A 1646 A" 1642 A" 1640 A 1629 A' 1621 A' 1587 A' 1567 A' 1547 A' 1445 A' 1402 A" 1384 A' 1225 A" 1203 A' 1164 A" 1176 A' 1127 A" 1110 A' 1064 A' 973 A' 903 A" 860 A" 849 A' 755 A" 696 A 602 A 484 A' 362 A 361 A' 298 A" 264 A' 249 A' 120 A" 120 A'' 97 A" 56 A 31 PEDb Frequencies in cm-'.v, bond stretching; 6, bending, y, rocking, w, wagging, tw, twisting, T, torsion, see Table 2 for definition of symmetry coordinates.Only PED values greater than 10% are given. tributes to the band at 1248 cm-',assigned to 6(C,-H), y(CH,) Ca and Cg are probably underneath the intense band at 750 cm-I, due to the y(C,-H) vibration]. (ii) The experimental frequencies of v(C=C) and, in partic- ular, of v(C=O) are significantly smaller than the calculated (scaled) values. These results are consonant with a slightly increased polarity of these bonds (less double-bond character) due to weak interactions with the solvent.(iii) The S-C stretching coordinate does not mix very much with any other coordinate, thus giving rise to essen- tially pure vibrations in both Ca and Cg forms. On the other hand, both C,-C and C-S coordinates are considerably mixed with each other and with other oscillators: v(C,-C) mixes mainly with v(C-S) and 6(O=C-S); v(C-S) with v(C,-C) and a series of bending coordinates [S(C=C-C), d(C-C=C), S(C-C-O)]. Indeed, though v(C-S) is here assigned to the intense band at 822 cm-' (calculated, 798 cm-'), the normal mode analysis yields greater PED values for this coordinate associated with the calculated bands at 508 cm-' (Cg) and 527 cm-' (Ca), here ascribed to 6(C-CcC).While it can be concluded that in this kind of molecules v(C-S) is definitely not a well localized vibration, the assignment of the 822 cm-' band to v(C-S) is justified for two main reasons: first, its relatively high intensity [v(C-S) is generally an intense band in the IR];15,'6 sec-ondly, because it can be expected that, owing to an increased partial double-bond character of the C-S bond when com- pared with the S-C bond, due to mesomerism involving the C(=O)S fragment, the frequency of the stretching vibration associated with this bond will be higher than that of v(S-C). t-ETC In the case of t-ETC, contributions to the vibrational spectra are expected from all four conformers (Cg, Ca, Tg, Ta). As for c-ETC, the calculated spectra of forms differing in the relative orientation of the C-S-C-C axis do not differ very much, and thus only a few bands may be ascribed to individual conformers differing in the relative orientation of J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 6 Experimental frequencies and calculated frequencies and intensities for t-ETC" calculated experimental (FTIR)bGl Ca 7-9 Ta approximate (in hexane (in D,CN description a6 initio scaled intensity ab initio scaled intensity ab inrtro scaled intensity ab initio scaled intensity solution) solution) 3371 3018 6.8 3370 3017 6.7 3358 3006 8.3 3356 3005 9.8 3037' 3368 3016 1.9 3367 3015 2.5 3370 3017 11.2 3369 3016 11.4 301Y 3326 2978 13.7 3328 2979 15.6 3329 2980 10.9 3332 2983 14.7 2991' 3303 2957 16.4 32aa 2944 21.6 3304 2958 17.7 3283 2939 45.2 2976' (Cg), 2961' (Ca) 3288 2944 21.4 3282 2938 45.4 3290 2945 18.1 3290 2945 17.7 2961' 32 71 2928 37.1 3279 2935 22.0 3271 2928 36.6 3279 2935 22.8 2937' 3255 2914 15.9 3268 2925 0.4 3255 2914 15.9 3270 2927 0.5 2924' 3253 2912 23.5 3253 2912 33.7 3254 2913 23.8 3254 2913 24.1 2924' 3210 2873 39.7 3212 2875 34.1 3210 2873 39.0 3212 2875 32.0 2878' 3202 2866 29.2 3200 2864 29.6 3202 2866 29.8 3202 2866 29.6 2858' I949 1739 206.3 1951 1741 226.6 1951 1741 352.0 1953 1742 367.1 1683, 1675.1672, 1666. 1864 1662 186.4 1865 1663 180.2 1877 1674 23.8 1878 1675 23.5 1664 1636 1647 1467 3.5 1646 1466 1.9 1647 1467 3.5 1646 1466 1.8 1462 1556 1636 1457 5.9 1640 1461 6.4 1636 1457 6.6 1640 1461 6.5 1462 1456 1628 1450 23.8 I628 1450 29.1 1629 1451 12.2 1629 1451 11.9 1445 1447 1623 1446 6.6 1623 1446 6.6 1622 1445 7.0 1622 1445 7.0 1430 1438 1612 1436 4.9 I629 1451 3.3 1612 1436 5.9 1629 1451 7.8 1416 (Cg), 1430 (Ca) 1419 (Cg), 1438 (Co)1564 1393 4.1 1564 1393 4.1 1565 1393 1.7 1565 1393 2.0 1375 1378 1562 1391 3.5 1467 1305 2.1 1562 1391 3.8 1467 1305 1.8 1375 (Cg), 1300 (Ca) 1378 (Cg), 1304 (Ca) 1464 1303 8.4 1465 1304 5.0 1423 1266 1.6 1422 1265 0.4 1300 1304 1452 1292 19.6 1445 1286 18.8 1452 1292 23.3 1446 1286 21.9 1267' 126Y 1437 1278 62.2 1437 1278 74.7 1460 1299 2.9 1461 1300 2.3 1286 1287 1408 1252 0.4 1384 1231 0.0 1408 1252 0.6 1384 1231 0.0 -I231 1093 38.1 1231 1093 40.9 1194 1060 11.0 1193 1059 2.6 1116 1120 1201 1066 3.0 1201 1066 3.0 1193 1059 23.4 1191 1057 58.3 1089' 1092' 1187 1053 71.0 1182 1049 110.8 1172 1040 10.0 1170 1038 5.0 1054' 1W 1172 1040 5.3 1164 1033 0.2 1169 1037 26.6 1164 1033 0.1 1054' 1W 1152 1022 171.9 1155 1025 114.2 I298 1153 241.8 1299 1154 262.0 1035, 1163* 1039, 1167.1102 977 35.1 1102 977 35.1 1107 98 1 39.4 1107 98 1 39.4 962,974' 1059 938 7.2 1064 943 6.4 1059 938 7.2 1064 943 6.7 934d 9% 882 71.4 996 882 70.9 1024 907 27.4 1024 907 27.6 911' 915 809 10.3 916 810 10.2 923 816 7.0 923 816 6.5 822 901 796 106.0 903 798 109.5 6% 612 39.1 708 622 57.1 81Qf 829 731 2.8 850 750 2.0 831 733 4.3 851 75 1 2.2 751 728 640 3.8 755 665 0.9 739 650 35.4 763 672 16.4 635 (Cg)#, 661 (CO)~ 698 614 13.3 697 613 14.3 673 59 1 12.4 674 592 12.6 620' 557 487 0.2 575 503 0.4 609 533 2.5 610 534 4.4 439 38 1 7.1 436 378 6.0 330 282 6.0 326 279 0.7 392 338 2.4 313 267 1.4 387 334 0.4 363 312 5.8 333 285 8.1 378 326 9.2 150 120 0.8 210 174 6.3 2% 251 2.1 265 224 0.3 297 253 3.7 265 224 0.3 226 189 1.5 225 188 0.3 197 163 0.1 I97 163 0.1 221 184 1.6 219 183 3.8 214 178 2.4 115 89 1.5 211 175 0.6 212 176 0.8 220 184 3.0 218 182 1.2 145 I16 1.2 109 84 1.8 491 427 3.4 502 43 7 2.5 105 80 0.1 79 57 2.6 106 81 1.2 33 15 1.3 89 66 0.9 43 24 1.o 94 70 0.4 60 40 0.1 56 36 3.1 110 85 0.7 29 11 1.7 113 87 1.9 " Frequencies in cm-'.Intensities in km mol-'. v, bond stretching, 6, bending, y, rocking, o,wagging, tw, twisting, z, torsion, see Table 2 for definition of symmetry coordinates.Frequencies in italic were those use for force field scaling. Bands marked with * belong to the s-trans conformers; the intensity of all the remaining bands are almost exclusively due to the s-cis conformers. Except in the cases explicitly indicated in the table, all observed bands are considered to have contributions either from Cg and Ca conformers or from Tg and Ta forms. This band has also a small but significant contribution from the corresponding vibration of the s-trans forms. v(C,-C) vibration of s-trans conformers also contributes to the total intensity of this band. The v(C,-C) mode in s-trans forms contributes to the total intensity of the band at 934 an-'. The v(C-S) mode in s-trans forms contributes to the total intensity of the band at 620 cm-'.The v(S-C) vibration of the Tg conformer also contributes to the totai intensity of this band. The $3-C) vibration of the Ta conformer also contributes to the total intensity of this band. The v(S-C) vibration of s-trans conformers also contributes to the total intensity of this band. this axis. In addition, the considerably lower energy of the 1165 cm- 'is of note, as it corresponds to a very intense band s-cis forms about the C-C-C=O axis when compared with appearing in a 'clean' spectral region (see Fig. 3 and 4). Since the s-trans forms leads to a prevalence in the observed it is not expected that the v(C,-C) stretching mode will spectra of features due to s-cis conformers. The analysis of change much in frequency in other a,P-unsaturated thioesters the results presented in Tables 6-10 and Fig.3 and 4 lead to (for instance this mode gives rise to a band at ca. 1170 cm-' the following conclusions : in S-ethyl thioacrylate,20 this band may be used as a probe of (i) Both in hexane and deuteriated acetonitrile (CD,CN) the presence of s-trans conformers of a,@-unsaturated thioes- solution t-ETC exists as a mixture of four different con- ters in more complex situations (for example, in enzyme formers, in agreement with the theoretical data for the iso- active sites). lated molecule. In the more polar CD,CN solvent, those (ii) The experimental spectra obtained in hexane and bands ascribable only to the more polar s-trans conformers CD,CN solutions are well fitted by the calculations when the increase in relative intensity [bands at 1675 cm-' and 1163 calculated spectra of s-cis and s-trans conformers are added cm-', assigned respectively to the v(C-0) and v(C,-C) with the s-trans spectra multiplied by factors of 1/2.25 and stretching vibrations of the s-trans conformers].In addition 1/1.70, respectively. As expected, the relative magnitude of to these bands, several other bands have substantial intensity these weighting factors agrees with a stabilization of the contributions from the s-trans forms, and have also their rela- s-trans conformers in the more polar solvent. In addition, tive intensities increased in the spectrum of the compound assuming a Boltzmann population distribution, the (s-trans) dissolved in the more polar solvent (e.9.the bands at ca. 1090 -(s-cis)energy difference may be estimated to be ca. 2.0 and and 1060 cm-'). The s-trans v(C,-C) stretching band at CQ. ca. 1.5 kJ mol-',respectively, in hexane and in CD,CN solu- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 7 approximate description Normal coordinate analysis for t-ETC (form Cg)" calculated frequency 3371 3368 3326 3303 3288 3271 3255 3253 3210 3202 1949 1864 1647 1636 1628 1623 1612 1564 1562 1464 1452 1437 1408 1231 1201 1187 1172 1152 1102 1059 996 915 90 1 829 728 698 557 439 392 333 295 226 22 1 21 1 145 105 89 56 PEDb Frequencies in cm-'. v, bond stretching, 6, bending, y, rocking, o,wagging, tw, twisting, z, torsion, see Table 2 for definition of symmetry coordinates.Only PED values greater than 10% are given. tion. Although this estimate may have a substantial error, considering that intrinsic band intensities may be signifi- cantly different in the isolated molecule situation and for the solutions phases, the magnitude of the differences in energy found is large enough to enable us to conclude that the experimental data point to a considerably smaller energy dif- ference between the two stable conformations about the C=C-C=O axis for t-ETC in solution than for the isolated molecule (ca. 7 kJ mol-I). (iii) Similarly to the case of c-ETC, it is not possible to identify, in the IR spectra of t-ETC, many bands ascribable to a single conformer differing in the relative position of the C-S-C-C axis.In fact, with the exception of the 6(CH,) bending vibration of the Cg form, ascribed to the band at 1416 cm-', all modes which are predicted to have signifi- cantly different frequencies in the gauche and anti C-S-C-C forms have frequencies nearly coincident with those of different modes, and have been ascribed to bands which also have contributions from these latter vibrations [for example, the 6(CH3) symmetric bending modes of Cg and Ca contribute to the total intensities of the bands at 1375 and 1300 cm-', also assigned to the 6(C,H3) symmetric and 6(C,-H) bending vibrations, respectively (see Table 6)]. The bands observed at 635 cm-' and 661 cm-' [v(S-C)] are a slight exception to the above rule: each one contains signifi- cant contributions from the two conformers having the same conformation of the C-S-C-S axis but different confor- mation around the C=C-C-0 axis.In fact, the calcu- lations clearly indicate that the intensity of the v(S-C) vibration is much higher in the s-trans than in the corre-sponding s-cis conformers, while the frequencies depend only on the relative orientation of the C-S-C-C axis, being larger for the anti conformation. Thus, the 635 cm- band is here assigned to v(S-C) in both Cg and Tg conformers, while the 661 cm-' band is ascribed to the same mode in the two symmetric forms (Ca and Ta). (iv) Also as was found for c-ETC, the experimental fre- quencies for v(C=C) and v(C=O) in t-ETC are significantly J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 approximated description Table 8 Normal coordinate analysis for t-ETC (form Cay ~~~ ~~ calculated frequency A 3370 A 3367 A" 3328 A' 3288 A' 3282 A" 3279 A' 3268 AI' 3253 A 3212 A 3200 A 1951 A' 1865 A' 1646 A" 1640 A' 1629 A 1628 A" 1623 A 1567 A' 1464 A 1465 A' 1445 A' 1437 A" 1384 A' 1231 A 'I 1201 A 1182 A" 1164 A' 1155 A" 1102 A' 1064 A 996 A 916 A' 903 A" 850 A' 755 A 697 A' 575 A' 436 A' 378 A' 313 A" 265 A" 225 A' 219 A" 212 A" 110 A 109 A" 79 A" 43 ~~ PED~ Frequencies in cm-'.v, bond stretching; 6, bending, y, rocking, w, wagging, tw, twisting, r, torsion, see Table 2 for definition of symmetry coordinates! Only PED values greater than 10% are given. smaller than the calculated (scaled) values, pointing to an increased polarity of these bonds (less double-bond character) due to intermolecular interactions with the solvent. As expected, the frequency red shift is considerably more pro- nounced when the solvent is CD,CN. In addition, the red shifts are also more relevant for v(C=O) than for v(C=C) and, for the same mode and solvent, for the more polar s-trans conformers (Table 6). (v) Finally, also for this molecule v(S-C) is an essentially pure mode whichever conformer is considered [Table 7-10, in Tg this coordinate mixes somewhat with the v(C-S) coordinate], while both v(C,-C) and v(C-S) are consider- ably mixed modes.Particularly relevant is the fact that v(C-S) reduces its frequency by ca. 190 cm-' in going from the s-cis to the s-trans C-C-C-0 axis conformation. Indeed, in both Cy and Ca forms, this mode gives rise to a band nearly at the same position as in c-ETC (810 us. 820 cm-'), while v(C-S) in the Tg and Ta conformers contrib- utes to the total intensity of the band at ca. 620 cm-', also ascribed to the y(C=O) out-of-plane bending vibration. An interesting conclusion that can be derived from these data and that may be important to the analysis of the a,&unsatu- rated thiolacylenzyme vibrational data is that assuming the s-trans conformation about the C,-C axis leads to a weakening of the catalytically relevant C-S bond.The importance of this result is underlined by the fact that the structural data do not reflect such a weakening (the C-S bond lengths are nearly equal in the s-cis and s-trans forms, Table l), because the changes with conformation of the rele- vant intramolecular interactions, which determine the bond lengths in the two forms, compensate each other. R.F. acknowledges financial support from Junta Nacional de Investigaciio Cientifica e Tecnologica, J.N.C.I.T., Lisboa. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 9 Normal coordinate analysis for t-ETC (form Tg)” approximate description calculated frequency PEDb 3370 3358 3329 3304 3290 327 1 3255 3254 3210 3202 1951 1877 1647 1636 1629 1622 1612 1565 1562 1460 1452 1423 1408 1298 1194 1193 1172 1169 1107 1059 1024 923 83 1 739 696 673 609 49 1 387 3 30 297 220 214 197 150 106 94 29 Frequencies in a-’.v, bond stretching, 6, bending, y rocking, 0,wagging, tw, twisting, T, torsion, see Table 2 for definition of symmetry coordinates.* Only PED values greater than 10% are given. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 10 Normal coordinate analysis for t-ETC (form Ta)" approximate description calculated frequency PED~ A 3369 A' 3356 A" 3332 A' 3290 A 3283 A" 3279 A' 3270 A" 3254 A 3212 A' 3202 A' 1953 A 1878 A' 1646 A" 1640 A' 1629 A 1629 A 1622 A' 1567 A' 1465 A' 1461 A' 1446 A' 1422 A" 1384 A 1299 A 1193 A' 1191 A' 1170 A" 1164 A" 1107 A 1064 A 1024 A 923 A" 851 A' 763 A' 708 A" 674 A 610 A 502 A' 363 A' 326 A" 265 A" 218 A 210 A 197 A' 115 A" 113 A" 60 A" 33 a Frequencies in cm-'. v, bond stretching, 6, bending, y, rocking, tw, twisting, z, torsion, see Table 2 for definition of symmetry coordinates.Only PED values greater than 10% are &en. References GMAT and FPERT, H. Fuher, V. B. Kartha, K. G. Kidd, P. J. Krueger and H. H.Mantsch, Natl. Res. Council Can.Bull., 1976,1 P. R. Carey and P. J. Tonge, Chem. SOC.Rev., 1990,19,293. 15, 1). 2 P. J. Tonge and P. R. Carey, Biochemistry, 1992,31, 9122. 11 R. Fausto and J. J. C. Teixeira-Dias, J. Mol. Struct. (Theochem.), 3 P. J. Tonge and P. R. Carey, J. Mol. Liq., 1989,42, 195. 1987,150,381.4 M. Kim, H. Owen and P. R. Carey, Appl. Spectrosc., 1993, 47, 12 R. Fausto, L. A. E. Batista de Carvalho, J. J. C. Teixeira-Dias 1780. and M. N. Ramos, J. Chem. SOC., Faruday Trans. 2, 1989, 85,5 V. E. Anderson, P. R. Carey and P. J. Tonge, unpublished work. 1945.6 W. J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys., 1972, 13 R. Fausto, L. A. E. Batista de Carvalho and J. J. C. Teixeira- 56,2257. Dias, J. Mol. Struct. (Theochem.), 1990,207,67.7 M. J.Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. 14 R. Fausto, J. Mol. Struct. (Theochem.), in the press. W. Wong, J. B. Foresman, B. J. Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R. Gomperts, J. L. Andres, K. Raghava-15 R. Fausto, A. G. Martins, J. J. C. Teixeira-Dias, P. J. Tonge and P. R. Carey, J. Mol. Struct., 1994,59,323.chari, J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. 16 R. Fausto, A. G. Martins, J. J. C. Teixeira-Dias, P. J. Tonge and Defrees, J. Baker, J. J. P. Stewart and J. A. Pople, GAUSSIAN P. R. Carey, J. Phys. Chem., 1994,98,3592.92 (Revision C). Gaussian Inc., Pittsburgh PA, 1992. 8 H. B. Schlegel, Ph.D. Thesis, Queen's University, Kingston, 17 M. D. G. Faria, J. J. C. Teixeira-Dias and R. Fausto, Vibrat. Ontario, 1975. Spectrosc., 1991, 1,43. 9 M. D. G. Faria and R. Fausto, TRANSFORMER (oersion Z.O), 18 M. D. G. Faria, J. J. C. Teixeira-Dias and R. Fausto, Vibrat. Spectrosc., 1991, 2, 107.Departamento de Quimica, Universidade de Coimbra, Portugal, 19 M. D. G. Faria, J. J. C. Teixeira-Dias and R. Fausto, J. Ramn1990). 10 M. D. G. Faria and R. Fausto, BUILD-G and VIBRAT, Depar- Spectrosc., 1991, 22, 519. tamento de Quimica, Universidade de Coimbra, Portugal, 1990 20 P. J. Tonge, P. R. Carey and R.Fausto, to be published. (These programs incorporate several routines from programs Paper 4104662E; Received 29th July, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003491

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(23), 3505-3511 Classical and Non-classical Silicon Radical Cations :H Six'+ Species (X = N, 0,F, P, S and CI) n Michel Sana, Michel Decrem and Georges Leroy Laboratoire de Chimie Quantique, Universite Catholique de Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium Minh Tho Nguyen" and Luc G. Vanquickenborne Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B3001 Leuven, Belgium A theoretical analysis of the classical and non-classical silicon radical cations, H,SiX'+ with X = N, 0, F, P, S and CI has been carried out. Heats of formation of the ions considered have been determined using a6 initio molecular orbital (MO) calculations at the UMP4/6-31 + G(2df, p) level with UMP2/6-31G(d, p) optimized geome- tries plus vibrational and thermal corrections.No classical halogen radical cations (H,Si-X'+ with X = F and CI) could be found. In other cases, non-classical ions (H,Si-XH,-,'+) are consistently more stable than their classical counterparts (H,Si-XH,-,'+). The difference in stability between both isomers depends on the nature of X: it is large with N and 0, but rather small with P and S. lnterconversion between both isomeric forms has also been investigated. When X is a second-row atom, the energy of the transition structure for the 1,2-hydrogen shift is close to that of the fragments H,-,SiH+ + H. When X is a third-row atom, the barrier height increases, but remains smaller than the dissociation energy. The potential-energy surface of the H,SiN'+ ions is reported in detail.Overall it has been shown that silicon-containing non-classical radical cations do not have a formal separation of both charge and radical centres as is the case in carbon analogues. Therefore they cannot prop- erly be described as ' distonic radical cations' under the currently accepted definition. In recent years, the growing awareness of the stability and general appearance of the distonic radical cations' and ion- molecule complexes2 has opened up new avenues in the field of gas-phase ion chemistry. Such species do not generally have stable neutral counterparts and can only be generated via rearrangement and/or fragmentation of the primary ions. Therefore, quantitative information obtained from reliable ab initio MO calculations has proved to be essential in this field for the interpretation of experimental results as well as for the discovery of new species.Distonic radical cations formally arise from ionization of biradicals or zwitterions. The simplest distonic species thus correspond to the ylidions, >C-X--'+, in which X is a het- eroatomic centre.' While the structures and stabilities of the carbon-centred ylidions have been well studied experimen- tally and theoretically,, little is known about the silicon ana- logues. In a preliminary theoretical study, Pius and Chandra~ekhar~compared energies of the H2Si-X'+ ions, with X = NH,, OH, and FH, relative to their corresponding conventional isomers. Nevertheless only Hartree-Fock wave-functions with the small 3-21G basis set have been employed.In addition, the kinetic stability of the isomers with respect to unimolecular rearrangements and fragmentations has not been examined. In view of the scarcity of quantitative information on the silicon radical cations, we have carried out ab initio MO cal- culations on a coherent series of H,SiX'+ species in which X are the most electronegative elements of the second-row (N, 0 and F) and third-row (P, S and Cl) atoms. In each of the systems considered, the molecular and electronic structures, enthalpies of formation and energy barriers for 1,2-H shift and H-loss processes have been investigated. Calculations Ab initio MO calculations have been carried out with the aid of the GAUSSIAN 90 series of programs.' Both vibrational and electronic population analyses have been performed at the Hartree-Fock level with the dp-polarized 6-31G(d, p) basis set at the corresponding optimized geometries.Improved energies have been obtained from calculations at the full fourth-order Merller-Plesse t perturbation theory (MP4SDTQ) level using the larger 6-31 + G(2df, p) basis set and geometrical parameters optimized at the second-order perturbation theory MP2/6-3 1G(d, p) level.? The unrestricted formalism (UHF, UMP) has been employed for open-shell species.In general, the spin contamination present in UHF refer- ences of equilibrium structures are not particularly large, the (S2) expectation values being 0.80-0.90.Because spin projec- tion has not been applied, a slow and unbalanced con-vergence of the UMP series could occur. At this level of theory we could expect an error of &3 kcal mol-' for the heats of formation. Results and Discussion Molecular and Electronic Structures Molecular geometries of the H,SiX'+ radical cations as well as those of the corresponding neutral molecules (H,SiX) and unsaturated cations (H,- ,Six +) have been determined at both HF and MP2 levels using the 6-31G(d, p) basis set. Fig. 1 displays only selected MP2/6-31G(d, p) parameters. The unsaturated closed-shell cation exhibits an Si-X bond dis- tance that is in general shorter than its counterpart in the parent neutral molecule. This shortening, due to the double- bond character in planar structures, is, however, less than 0.1 A.The phosphorus derivative H2Si-PH2+ is not planar. Such a distorted conformation around a partial double bond has already been noticed for several other multiply bonded third-row derivatives, e.g. H2Si=SiH2. In contrast, both clas- sical and non-classical forms of each radical cation show a larger Si-X bond distance than the corresponding neutral molecule. The increase in length upon ionization is quite sig- nificant with N, 0 or S (0.2 A), but becomes less important with P (0.05 A). The Si-X distances are rather similar in t Unless otherwise noted, the relative energies quoted in the text refer to the values derived from MP4SDTQ/6-31 + G(2df, p)//MP2/ 6-31G(d, p) + ZPE calculations.3506 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 H Hl'\ 1.648 / H H la lb lc Id l+ H\ 1.667 H H H 'H'H 2a 2b 2c 2d H HSH HA. 2.139 l+ H. 2252 v ,,,Si -P Si-P- ....,,, H- HI H 'HH 4a 4b 4c 4d H\ 2138 H,,..p-S H \ H H. 2.284 Hbs.jsi-sH H\ 1O+ H\ 2.296H-si-sc,,H H 1'+ '\ 2.027 H/si-s 'H l+ 5a 5b 5c 5d lo+ H l+H\ 1.616 \si1.560 ,,,Si--F H/H 3a 3b 3b' 3c 3d H l+H\ 2.056 \ -1.947 Hs*?si-c' H</s'-c'H 6a 6b 6b' 6c 6d Fig. 1 Selected (U)MP2/6-3 1G(d, p) optimized geometries of the silicon-containing neutral H,SiX molecules, classical and non-classical H,SiX'+ radical cations and unsaturated H,-,Six+ cations. Bond lengths are given in 8, and bond angles in degrees.both classical and non-classical forms. Sometimes the former exhibits a slightly shorter Si-X bond than the latter, some- times an opposite situation occurs. The halogen-containing species presents a more regular behaviour except that we have not been able to locate any classical structures. Instead, two ion-molecule complexes have been found for each system. These complexes are also described in Fig. 1. The first complex 3b corresponds to that of the hydrogen atom and the fluorosilyl cation (H,FSi+). The optimized geometry as well as the Mulliken population analysis of the spin popu- lation confirm this viewpoint. The second complex 3b corre-sponds to that between the molecular hydrogen and the fluorosilylene radical cation (HFSi' ').A similar picture has also been found for the chlorine derivative with two com- plexes 6b and 6b (Fig. 1). The equilibrium structures shown in Fig. 1 are the lowest-energy minima; other conformers and transition structures for internal rotation can also be deter- mined. For example, H3Si-SH'+ has an s-trans C,confor-mation; the gauche conformer is 0.54 kcal mol-' higher in energy. The equilibrium structure of H,Si-OH'+ has a con- formation between the s-trans and gauche. However, the Si-X rotational barriers are consistently very small with UHF/6-3lG(d, p) values (in kcal mol-' at 0 K and without ZPE corrections): H5SiN'+ H,SiO'+ H,SiF" H,SiP ' H,SiS" H,SiCr+ classical ion 0 0.02 - 0.4 0.5 - nonclassical ion 0.7 0.6 0 0.9 0.8 0.8 We now turn to the electronic structure of the radical cations considered.For a description we have employed three different tools. First, the Mulliken population analysis indi- cates the electronic charge distribution on the nuclear skele- ton. In this case, the electrostatic potential does not lead to any meaningful picture because it is dominated by the effect of the positive charge. Secondly, the spin density map pro- vides a picture of the spin distribution on the atoms. Thirdly, the Boys localization of the MOs gives access to the elec- tronic distribution in terms of bond, lone pair and unpaired electron. For the sake of simplicity, we only detail the analysis in the case of H5SiN+. The other systems show similar behaviour.The charges on the molecular fragments obtained from UHF/6-31G(d, p) wavefunctions show that the silicon group carries approximately two thirds of the positive charge: q(SiH,) = 0.66 in the classical form lb and q(SiH,) = 0.63 in the non-classical form lc. The spin-density maps in the H-Si-N plane are displayed in Fig. 2(a)and (b) for the clas- sical and non-classical cations,, respectively. While in the former, the nitrogen carries the unpaired electron almost entirely, in the latter, the silicon has an excess of a-spin elec- tron. Taken together, we can write the following conventional picture : for the classical ion lb: H,Si+ -NH2 (1) and for the non-classical ion lc: J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 (b ) 0.000001 0.01' Si N Fig. 2 Spin-density maps in the H-Si-N plane for (a)H,Si-NH2'+ and (b) H2Si-NH,'+. (-) ,Positive spin densitycorresponding to excess of alpha over beta spin; (. .) negative spin density. In (11) we describe a dative type of bond between Si and N. This is a rather conventional manner in which the nitrogen shares the electron pair with silicon, but is purely a formal representation. The term dative does not imply a longer or weaker bond. As discussed above, both types of bond have similar lengths. The Boys localized orbitals illustrated in Fig. 3 confirm the schematic descriptions (I) and (11). One unpaired electron with alpha spin is centred on Si in H2Si--NH3'+, while the excess of alpha spin (designated by +) over beta spin (designated by x) appears around the nitrogen in H3Si--NH2".Distonic radical cations are often defined as the species in which both charge and radical sites are formally separated. In This is true for species such as carbon-centred ylidions,' but apparently it does not hold for the silicon radical cations studied in this work. As seen in description (11),both charge and radical centres are formally localized on the silicon atom of the non-classical structure. An opposite situation is observed for the more classical species (I). This result, no doubt due to the difference in electronega- tivity between carbon and silicon, indicates that the H,Si-XH,-," ions are not distonic species in the sense of their currently accepted definition.A more general definition encompassing not only carbon and silicon compounds, but also other heteroatom analogues is clearly desirable. For sid iJ OO" b Fig. 3 Centroids of charge of localized orbitals for (a)H3Si-NH,'+ and (b)H2Si-NH,". The sign (+) corresponds to an alpha electron while the sign ( x ) stands for a beta electron. such purposes, a more systematic study should be carried out. Energies and Unimolecular Rearrangements Table 1 lists the electronic energies computed at two levels of theory along with zero-point energies (ZPE) and thermal cor- rections at 298.15 K (TC) of the optimized structures shown in Fig. 1. The TC values have been obtained from statistical thermodynamics assuming perfect gas, rigid rotor and har- monic vibration approximations.Harmonic vibrational wavenumbers needed for determining vibrational contribu- tions are calculated at the (U)HF/6-31G(d, p) level and scaled down by the relationship: v(theoretica1)= -46 + 0.92227v(HF), in cm-', in order to account for the systematic overestimation.6 In all the systems examined, the more stable isomer corre- sponds to the non-classical arrangement of the nuclei. The energy difference between both forms could be large, e.g. 21 and 37 kcal mol-' for the N and 0 species, respectively, or very small, ca. 2 kcal mol-' when P and S are involved. Another important point to be assessed is their kinetic stabil- ity. For this purpose we have located the transition structures for 1,2-hydrogen shifts.Fig. 4 displays their UMP2/6-3 1G(d, p)-optimized geometries while their energies are also included in Table 1. Fig. 5 shows the potential-energy surface of the H,SiN species. Each of the structures le, 2e and 5e shows a three-membered cycle in which the migrating hydrogen atom is bridged to the single Si-X bond. The electron spin is almost evenly distributed over both heavy atoms. The migrating hydrogen carries no formal charge. The energy barrier separating the classical ion from the non-classical is calcu- lated to be higher for the nitrogen or sulfur system (22.9 and 18.2 kcal mol-' via le and 5e, respectively) than for the oxygen one (1.5 kcal mol-' via 2e). For the phosphorus species, a transition structure has been located at the UHF/6- 31G(d, p) level with a barrier height of 42 kcal mol-', but we were not able to confirm it at the correlated UMP2 level.Note that barrier heights obtained from UMP4 calculations are usually overestimated owing to the higher spin contami- nation in UHF references of transition structures. We can now compare the barriers for 1,2-hydrogen shifts with the dissociation energies of the classical ions into 3 508 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Total energies (E),zero-point energies (ZPE) and thermal corrections (TC) of the species considered structure" sym. E[MP2/6-31G(d, p)]/E: E[MP4/6-31 + G(2df, p)]/E: ZPE'/kcal mol- TCd/kcal mol- ' la SiH,NH, -346.588 07 -346.680 74 30.07 33.53 lb SiH 3NH2'+ -346.252 94 -346.333 65 29.46 32.78 lc SiH,NH,'+ -346.298 68 -346.372 87 33.30 36.65 Id SiH ,NH ,'+ -345.721 02 -345.795 99 25.93 28.81 le [lb + lc]' -346.21 1 56 -346.296 15 28.84 31.89 If H...SiH2NH2-+ -346.220 77 -346.296 37 26.31 30.62 1g [lb --+ If]' -346.220 24 -346.298 93 26.66 30.09 2a SiH,OH -366.441 20 -366.546 90 23.1 1 26.24 2b SiH,OH'+ -366.030 05 -366.125 64 21.47 24.80 2c SiH,OH,'+ -366.101 52 -366.188 65 24.20 27.64 2d SiH,OH'+ -365.545 50 -365.634 11 18.13 20.97 2e [2b + 2c)' -366.025 15 -366.122 50 2 1.04 23.98 3a SiH,F -390.438 38 -390.553 62 16.46 19.11 3b H. . .SiH,F'+ -390.015 09 -390.11984 12.27 15.75 3b H,. . .SiHF'+ -390.033 82 -390.139 93 13.69 17.43 3c SiH,FH'+ -390.036 44 -390.13633 15.83 19.14 3d SiH,F'+ -389.5 10 7 1 -389.608 86 11.16 13.75 4a SiH,PH, -632.781 49 -632.878 81 25.25 28.84 4b SiH PH ,' -632.469 23 -632.552 61 24.83 28.56+ 4c SiH ,PH ,'+ -632.474 25 -632.557 39 26.02 29.78 4d SiH,PH,' -63 1.883 48 -631.96695 20.1 1 23.55+ 4e [4b -+4cl' --23.42 27.05 5a SiH,SH -689.025 28 -689.131 93 20.39 23.74 5b SiH ,SH'+ -688.683 07 -688.779 11 24.25 27.77 5c SiH ,SH,'+ -688.684 20 -688.778 91 20.66 24.24 5d SiH2SH'+ -688.129 27 -688.176 60 15.58 18.57 5e [Sb -+ 51' -688.639 10 -688.740 88 18.42 21.63 6a SiH ,C1 -750.434 13 -750.545 32 15.70 18.50 6b H.* .SiH ,C1'+ -750.020 83 -750.128 12 11.21 15.02 6b H,. * .SiHCl'+ -750.035 33 -750.137 50 11.74 16.36 6c SiH ,ClH'+ -750.043 24 -750.143 84 14.29 17.67 6d SiH ,C1'+ -749.512 50 -749.612 13 10.41 13.10 Based on (U)MP2/6-31G(d, p) geometries given in Fig.1 and 4. 1 E, (hartree) = 1 au. ' Obtained from (U)HF/6-31G(d, p) wavenumbers and appropriately scaled (see text). Thermal corrections at 298.15 K including both zero-point vibrational and thermal contributions. unsaturated cations plus a hydrogen atom : 0 case seems to require a much lower activation energy than H3Si-NH2'+ -+ H,Si=NH2+ + H; the cases involving P and S. For the nitrogen species, we have refined the two-step endothermic by 22.7 kcal mol-I rearrangement path. There exist two additional stationary points lying between the reactant H,Si-NH2'+ and the H,Si-OH'' -,H2Si=OH+ + H; product H2Si-NH2+ + H: they correspond to the atom-ion exothermic by 6.1 kcal mol- 1 H,Si-PH,*+ -+ H,Si=PH2+ + H; endothermic by 51.6 kcal mol-' 2 H,Si-SH'+ -,H,Si=SH+ + H; 0 -??.endothermic by 58.2 kcal mol- L3 These values suggest that while dissociation can compete with the hydrogen shift in the first two cases (N and 0),the 4 # .t -60 -40 -20 0 20 40 60 eldegrees : 6 *' H46.8 ,,,......Si-N..HH/ 1.740 K'H !A,*' 10 H r E -lg 50 Fig. 5 Two- and three-dimensional energy maps of the 1,Zhydro- Fig. 4 Selected UMP2/6-31G(d, p) optimized geometries of the gen shift connecting H3Si-NH2'+ and-H2Si--NH3'+[UHF/6-transition structures for 1,2-H shift and dissociation (A and degrees) 31G(d, P)1 J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 complex If and the transition structure lg.The dissociation path can be written as follows: H3Si-NH2'+ + [He --H,Si=NH,+]* + lb 1g H.. .H,Si=NH2+ If +H,Si=NH2+ + H Id (111) The relevant energy profile is schematically illustrated in Fig. 6.The energy variation between If and (ld + H) is negligible. The geometry of the H2Si=NH2+ cation in both structures If and lg is very little perturbed by the hydrogen. The large Si. * .H distance points towards a very weak complex. In view of this result, we have not searched for the H-cation com-plexes in the remaining systems. In both halogen-containing systems, only non-classical forms have been found. With fluorine, the ion 3c is 4.4 kcal mol-' less stable than the H,...HFSi'+complex 3b. With chlorine, the energy ordering is different, the ion H2Si-ClH'+ 6c being more stable than the H,.-.HClSi'+ complex 6b.In fact, 6c lies 1.4 and 6.8 kcal mol-' below 6b and 6b, respectively. The electronic picture of the H,SiN'+ ions seen in Fig. 2 and 3 indicates that the octet rule is only satisfied for silicon in the classical ion H3Si-NH2'+ and for nitrogen in the non-classical ion H2Si-NH3'+. The other heavy atom of each ion that carries the spin is electron deficient and does not obey the octet rule. It is apparent that the more stable isomer of the two radical cations is the one in which the most electronegative atom has its valence shell completed. Fig. 7 summarizes the energy differences in the six systems exam- ined. The gain in stability of the classical ion relative to the non-classical counterpart (G) increases when the difference in electronegativity (Ax) between both heavy atoms (Si and X) decreases.Thus a correlation between Ax and G may be drawn: G = 10 -27 Ax (1) with a correlation coefficient of 0.95. On the other hand, the barrier height for the 1,2-hydrogen shift depends on the exo- thermicity of the reaction. When the latter becomes very large, the transition barrier vanishes. This is presumably what happens with the halogenated species. Enthalpies of Formation MO theory remains approximate in evaluating total energies of chemical species even when advanced methods are employed. In contrast, enthalpies of formation can be better evaluated using a semi-empirical approach.It is possible to ,SiH3++ NH2 -60ri7 65.4 2-40-05' >. P 20-C -(0.-2 O-SiH3 N H2+c a 20-lb 1 find some reactions in which the reactants and the products are sufficiently similar in terms of electronic structure. In such reactions, the errors in total energies due to the incomplete basis set and/or the lack of electron correlation tend to cancel to a large extent so that the calculated reaction energies could reach a chemical accuracy (+2 kcal mol-I). Let us choose a reference working reaction in which only one heat of formation is unknown. If the enthalpy of this reaction (A,H) could be accurately calculated, we would then be able to derive the unknown enthalpy of formation [A,H =xikiNiAf H(i)] making use of the calculated A, H and experi- mental heats of formation.Apart from the total energies, zero-point vibrational contributions and thermal corrections (from 0 to 298.15 K) should be evaluated. Previous work6 showed that for H,XY compounds, the hydrogenation reaction constitutes a good choice as long as electronic energies are calculated at least at the MP4 level with large basis sets. Table 2 records heats of formation obtained using this scheme. In Table 3 some reference data that have been used for Table 2 are listed. This refinement in terms of enthalpies of formation provides more confident esti- mates for relative energies. Although only a few comparisons with experimental results are possible, it seems that as long as heats of formation of reference compounds are well estab- lished, our theoretical values should be reasonably accurate.As seen in Table 2, the enthalpies of formation of the H,SiX'+ ions depend on the accuracy of experimental data for H3Si, H,Si and H,X'+. To verify the agreement between theory and experiment, we also report in Table 3 the total energies of these species computed at the MP4SDTQ/6- 311 + + G(3df, 2p)//MP2/6 -31G(d, p) level. It has been shown that the use of isogyric reactions7 at this level of theory can give quite accurate enthalpies of formation for open-shell species.* Thus, most of the theoretical values recorded in Table 3 are within & 2.5 kcal mol- ' of the well established experimental values.From the data given in the two last columns of Table 3, a linear correlation could be drawn: Af H,,,(eXptl) = 0.34 + 1.0024Af H2,,(the0) (2) The independent term (0.34 kcal mol-') indicates that the results are almost free from systematic errors; the slope being close to unity. Nevertheless, three values, theoretical or experimental, appear to be questionable : we find a difference of 6.4 kcal mol-' on H2Si'+, 5.1 kcal mol-' on H,S'+ and 3.9 kcal mol-' on H3P'+. Considering the generally good behaviour of the theory, we believe that the theoretical values are more accurate. This leads us to reconsider some of the enthalpies of formation reported in Table 2. SiH2++NH3 52.4 I 20.2 H- -SiH2NH2+ If -20.8 Fig. 6 Schematic potential-energy profiles showing unimolecular rearrangements of the H,SiN'+ ions.Energies obtained from UMP4/6-31 + G(2df, p) + ZPE calculations, in kcal mol-'. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 H2---SiH2PH2+T-r 30., 20 -10. 0--10-7 -30 Si H 2NH3' -301 -201 -40 1 H..,.,. -' +'30-30-H2Sil-----'SH H2---SiH2SH' 20 * .HI ,\,' + ' 10-H2Si---'OH * SiH30H+ -10-SiH3SH+ Si H2SH 2t -20 --30-201-30 -Si H20H2+v '1-40 -40 J 30 -30--SiH2CI+-t. H 20--SiH2F++ H 20 -10 --H---SiH2F+ 10--H ---SiH2CI+v V0 -Hz---SiHCI'-o: H2---SiH2F+-SiH2FH+ SiH2CIH+ -10 --10 --20 --20 --30 --30 --40 -40 -Fig. 7 Relative energies 1 p) + ZPE calculations. Table 2 Enthalpies of formation at room temperature (A, H~,,/kcal mol-') of the compounds of interest compound bond hydrogenation reaction AfH other A, H la SiH,NH, SiH,NH, + H, -+ SiH, + NH, -10.1 -11.74" lb SiH ,NH ,'+ SiH3NH2'++ H, +SiH, + NH," 213.1 lc SiH,NH ,'+ SiH,NH3'+ + H, +SiH,' + NH4+ 193.1 Id SiH2NH2+ SiH2NH2++ H, -+ SiH,' + NH, 184.9 If H..SiH ,NH2'+ He -.SiH2NH2'++ H, -+ SiH, + NH," 234.3 2a SiH,OH SiH,OH + H, +SiH, + OH, -64.6 -66.73" 2b SiH ,OH'+ SiH,OH'+ + H, -+ SiH, + OH2'+ 201.0 2€ SiH ,OH2*+ SiH20H2'++ H, +SiH,' + OH,+ 164.9 SiH,OH+ SiH20H++ H, +SiH3+ + OH, 145.3 3a SiH,F SiH,F + H, -+ SiH, + FH -82.1 -83.80" -84.9b 3b H-* -SiH ,Fa+ H. * .SiH,F'+ + H, +SiH, + FH'+ 190.1 3b H,-* .SiHF'+ H,. .-SiHF'+ + H, +SiH, + FH" 179.2 3c SiH2FH'+ SiH2FH'+ + H, +SiH, + FH'+ 183.2 3d SiH2F+ SiH2F++ H,+SiH,' + FH 149.9 4a SiH,PH, SiH,PH, + H, +SiH, + PH, 8.1 4b SiH,PH,' SiH3PH2'+ + H, +SiH, + PH," 217.3 (221.2)' + 4c SiH,PH ,*+ SiH2PH3'+ + H, -+ SiH, + PH3'+ 215.5 (219.4)' 4d SiH2PH2+ SiH2PH2++ H, +SiH,' + PH, 219.4 (223.3)' 5a SiH,SH SiH,SH + H, -+ SiH, + SH, -6.5 -6.11" 5b SiH3SH'+ SiH,SH'+ + H, -+ SiH, + SH2'+ 223.5 (228.6)' 5c SiH ,SH ,*+ SiH2SH2'++ H, -+ SiH, + SH2'+ 220.1 (225.2)' sd SiH,SH+ SiH,SH+ + H, -+ SiH3++ SH, 232.0 (2317.2)' 6a SiH,CI SiH,Cl + H, -+ SiH, + C1H -32.9 -32.5' 6b H..-SiH ,Cl'+ He 3 .SiH ,C1'+ + H, +SiH, + ClH'+ 230.2 6b H,. .-SiHCl'+ H,. .*SiHCl'++ H, -+ SiH, + ClH" 225.6 6c SiH2ClH'+ SiH,ClH'+ + H, +SiH, + ClH'+ 222.9 6d SiH2Cl+ SiH,Cl+ + H, +SiH,+ + C1H 191.6 " Theoretical result at MP4SDTQ/6-311 + + G(3df, 2p)//MP2/6-31G(d, p) level, G.Leroy, M. Sana, C. Wilante and D. RSi Temsamani, J. Mol. Struct. (Theochem),1991,259, 369. 'R. Walsh, in The Chemistry of Organosilicon Compounds, ed. Patai S. and Z. Rappoport, Wiley New York, 1988, ch. 5. 'Using theoretical heat of formation of PH,+ and SH,+ (Table 3), see discussion. 'R. Walsh, J. Chem. Soc., Farday Trans. 1, 1983, 79,2233. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 351 1 Table 3 Some reference data compound E(MP4)"/Eh E(MP4)b/E, TC/kcal mol -Af H(exp.)/kcal mol -A, H(th.)'/kcal mol -H' -0.498 23 -0.499 82 1.48' 52.10' _--N -54.493 68 -54.5 1 1 73 1 .48' 112.97' 0 -74.939 89 -74.970 93 1.61' 59.53' -_-F -99.566 92 -99.617 3 1 1.56' 18.97' -Si -288.906 16 -288.926 30 1.56' 107.55' -P -340.81223 1 .48' 75.62' -S -397.61 3 76 -397.643 97 1.58' 66.2odc1 -459.624 37 -459.661 33 l.5od 28.99' -H2 -1.16457 -1.17023 8.32' 0.0 NH,' -55.765 86 -55.790 93 13.65' 45.5' 45.9" NH3 -56.443 18 -56.471 38 23.94' -10.97' -9.02/NH," -56.074 17 -56.098 62 22.25 225.8 227.15 NH,+ -56.781 69 -56.808 37 32.46 1 52.4h 153.74 OH2 -76.294 84 -76.333 01 15.24' -57.8od -58.03/ OH2'+ -75.835 20 -75.869 77 16.57' 234.7' 237.93 OH3+ -76.567 37 -76.605 19 23.42' 143.h 145.05 FH -100.285 13 -100.339 68 7.9 1' -65.14' -66.15/ FH" -99.698 28 -99.749 84 6.34 304.9h 306.28 SiH," -289.806 19 -289.829 75 9.87 282.6* 276.18 SiH,' -290.748 64 -290.776 34 1 5.24k 47.9' 48.90" SiH,+ -290.456 03 -290.48 1 68 15.90 237.lh 235.06 SiH, -291.396 55 -29 1.426 43 2 1.73& 8.2"' 8.14/ PH3 -342.647 12 -342.680 52 17.03' 1.3547' 4.05 +PH 3' -342.291 90 -342.322 18 17.07 229.h 232.89 SH2 -398.886 32 -398.924 59 11.56' -4.9d -5.40 SH2'+ -398.509 47 -398.540 10 11.38 236.h 241.10 HC1 -460.284 61 -460.326 86 6.1 6' -22.06' -22.22 HCl" -459.823 29 -459.863 05 5.84 27 1.9"' 272.44 Electronic energies at the MP4/6-31 + G(2df, p)//MP2/6-31G(d, p) level. Electronic energies at the MP4/6-311 + + G(3df, 2p)//MP2/6-31G(d, p) level. Values at the MP4/6-311 + + G(3df, 2p)//MP2/6-3lG(d7p) level using isogyric reactions.' 'M.W. Chase Jr., C. A. Davies, J. R. Downey Jr., D. J. Frurip, R. A. McDonald and A. N. Syverud, JANAF Thermochemical Tables, 3rd edn., J.Phys. Chem. Ref: Data, 1985,14, suppl. " M. Sana, M. T. Nguyen, Chem. Phys. Lett., 1992,196,390. M. Sana and G. Leroy, J. Mol. Struct. (Theochem), 1991,226, 307. H. M. Rosenstock, K. Draxl, B. W. Steiner and J. T. Herron, J. Phys. Chem. Ref: Data, 1977,6, suppl. 1. S. G. Lias, J. E. Bartness, J. F. Leibman, J. L. Holmes, R. D. Levin and W. G. Mallard, J. Phys. Chem. Ref: Data, 1988, 17, suppl. 1. ' See M. Sana, G. Leroy, M. Hilali, M. T. Nguyen and L. G. Vanquickenborne, J. Chem. Phys. Lett., 1991, 190, 551. 'See Table 1 of M. Sana, G. Leroy, M. Hilali, M. T. Nguyen, and L. G. Vanquickenborne, J. Chem. Phys. Lett., 1991, 190, 551. M. Sana, G. Leroy, C. Wilante and D. Temsamani, J. Mol. Struct. (Theochem), 1992, 259, 369. J. A. Seetula, Y. Feng, D. Gutman, P.W. Seakins and M. J. Pilling, J. Phys. Chem., 1991,95, 1658. "' A. M. Doncaster and R. Walsh, J. Chem. SOC.,Faraday Trans. 2,1986,82,707. S. R. Gunn and L. G. Green, J. Phys. Chem., 1961,65,779. Thus, using the theoretical heat of formation for H,P'+, The Louvain-la-Neuve group thanks the FNRS for a we find Af H298(H3SiPH2'+)= 221.2 kcal mol-', research grant in the field of supercomputing technologies. ArH,,,(H2SiPH3'+) = 219.4 kcal mol-' and The Leuven group is indebted to the Belgian Government Af H,,,(H,SiPH,') = 223.3 kcal mol-l. Use of the (DPWB-DWTC) and NFWO for continuing support. theoretical heat of formation for H,S'+ yields the following results: Af H,,8(H3SiSH'f) = 228.6 kcal mol-', Af H,,,(H,SiSH,'+) = 225.2 kcal mo1-' and Af H298(H2SiSH+)= 237.2 kcal mol- '.The adiabatic ioniza- References tion energies of H,SiX species can easily be derived from (a)B. F. Yates, W. J. Bouma and L. Radom, J. Am. Chem. Soc.,heats of formation. 1984, 106, 5805; (b) S. Hammerum, Mass Spectrom. Rev., 1988, 7,123; (c)G. Bouchoux, Mass Spectrom. Reo., 1988,7, 1; 203. Conclusion R. D. Bouwen, Acc. Chem. Res., 1991,24,364. In this paper we have shown that the silicon-containing non- (a) B. F. Yates, W. J. Bouma and L. Radom, J. Am. Chem. Soc., classical radical cations, H,Si-XH,- ,*+, are consistently 1987,109,2250; (b)J. W. Gauld and L. Radom, J. Phys. Chem., 1994, 98, 777; (c) M. Sana, G. Leroy, M. Hilali, M. T. Nguyen more stable than the corresponding classical counterparts, and L. G.Vanquickenborne, Chem. Phys. Lett., 1992,109,551.H3Si-XH,-3*+. When silicon is connected to a halogen, K. Pius and J. Chandrasekhar, Int. J. Mass Spectrom. Ion Pro- only non-classical forms have been found. Analysis of the cesses, 1989,87, R15. electronic structure shows that the most stable isomer of the GAUSSIAN 90, M. J. Frisch, M. Head-Gordon, G. W. Trucks, two possible radical cations corresponds to the one in which J. B. Foresman, H. B. Schlegel, K. Raghavachari, M. A. Robb, the more electronegative atom satisfies the Lewis octet rule. J. S. Binkley, C. Gonzalez, D. J. DeFrees, D. J. Fox, R. A. In this case, the unshared electron is carried by the silicon. As Whiteside, R. Seeger, C. F. Melius, J. Baker, R. L. Martin, L. R. Kahn, J. J. P. Stewart, S. Topiol and J. A. Pople, Gaussian Inc. a consequence, non-classical silicon radical cations do not Pittsburg, PA, 1990. have a formal separation of both charge and radical sites as is M. Sana and G. Leroy, Ann. SOC. Sci. Bruxelles, Ser. 1, 1991, the case in carbon analogues. Hence, either they cannot be 105, 67. classified as 'distonic species', or this term should be J. A. Pople, B. T. Luke, M. J. Frisch and J. S. Binkley, J. Phys. redefined to encompass the observed heteroatom behaviour. Chem., 1985,89,2198. (a) M. Sana and G. Leroy, J. Mol. Struct. (Theochem), 1991,The conversion paths linking classical to non-classical 226, 307; (b) M. Sana and M. T. Nguyen, Chem. Phys. Lett., isomers contain significant energy barriers except for the 1992,196,390.oxygen species. However, the possibility of observing the classical ion is questionable in some cases owing to the ease of fragmentation. Paper 4/02578D; Received 3rd May, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003505

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(23), 3513-3516 351 3 Aromatic Character of Graphite Intercalation Compounds Jun-ichi Aihara Department of Chemistry, Faculty of Science, Shizuoka University, Oya , Shizuoka 422, Japan Tsugu h iro Tamar ibuchi Department of Physics, Faculty of Science, Shizuoka University, Oya , Shizuoka 422,Japan A semi-topological method is presented for estimating the degree of aromaticity in graphite intercalation com- pounds (GICs). Aromaticity in a graphite sheet has been found to decrease as it accumulates positive or nega- tive charge. Charged graphite sheets in C,Li are predicted to be essentially non-aromatic. However, most GlCs are charged to a lesser extent, so that they must be more or less aromatic in nature. There are no antiaromatic GICs.Graphite intercalation compounds (GICs) are formed by insertion of various atoms, molecules or ions between the layers of graphite.’-’ For a given guest species, many stoi- chiometric compositions are obtainable, each corresponding to a ‘stage’. As illustrated in Fig. 1, the stage of a GIC is defined as the ratio of host layers :guest layers. For a stage rn compound, every mth interlayer void contains a guest species, so that the highest concentration of guest occurs in the stage 1 compound. Electronic and geometric structures of grap hi te-me tal intercalation compounds have been studied extensively. Metal atoms and graphite layers in these GICs are charged positively and negatively, respectively. Among other guests in the GICs are Cl,, Br, , and various halides, oxides and sul- fides of metals. These electron acceptors are present as anions, so that the graphite host is cationic.Thus, graphite sheets in GICs are charged to varying extents. The inherent properties of a cyclic n-electron molecule have been referred to collectively as aromaticity., Topologi- cal resonance energy (EJ is used to evaluate the degree of aromatic The percentage topological reson- ance energy, %E,,, of a given molecule is defined as 100 x E,,, divided by the total n-binding energy of the polyene reference.’-’’ This quantity is useful when aromaticities of different molecules are compared. However, the E,, method in its original form cannot be applied to a graphite sheet (1) since its conjugated system is too large.We previously devised a method for estimating the degree of aromaticity in a graphite sheet and carbon nanotubes.” A graphite sheet was then found to be highly aromatic with a large %E,,. However, aromaticity in graphite must vary if it bears positive or negative electric charge. It is the purpose of 1 000000 000000 000000 000000 000000 000000 000000 000000 000000 000000 000000 000000 stage 1 stage 2 stage 3 Fig. 1 Graphite intercalation compounds of different stages this paper to acquire some insight into the aromatic charac- ter of charged graphite sheets. A semi-topological method, based on the reasoning applied to graphite,” is presented for estimating the %E,, of charged graphite layers in different GIG.Theory It has been accepted that even in concentrated GICs, such as KC, and LiC, ,hybridization between metal s and graphite n orbitals is not strong throughout the Brillouin zone, so we can assume that transport properties are mainly determined by graphite-like bands. ‘s3l4 In particular, all interlayer inter- actions are negligible in stage 1 acceptor compound^.^ On this basis, every charged graphite sheet in GICs is treated below as an isolated n-electron system. All possible inter- actions between host and guest are neglected for simplicity. Hiickel theory is used in its simplest form, which is identical to a simple tight-binding approximation in solid-state physics. 9 The x-band energy of a single graphite sheet can be expressed as a function of a two-dimensional (2D) wavenum- ber vector k:13*’4 + 4 cos’(y)]l’z Here, a, /? and d are the Coulomb integrals for carbon 2p, orbitals, the resonance integral for CC nbonds and ,/3 times the CC bond length, respectively.There are two C atoms in a unit cell of graphite. The above expression was utilized to obtain the unit n-binding energy (&b) or the total n-binding energies per C atom of a graphite sheet. This Eu,turned out to be 1.57460 \/?I.”*’“ E,b of a charged graphite sheet can be obtained numerically using the so-called triangle method, a L I number of n electrons per carbon atom (1 +q) Fig. 2 Unit reference energy of 1 as a function of the number of n electrons per carbon atom two-dimensional (2D) variant of the tetrahedron method for zone integration.' 7*1 For computational details, see the Appendix.A polyene-like reference structure is required to estimate El, of any m~lecule.~~~ We previously pointed out that the polyene reference of the hypothetical 54-carbon molecule (1) can be used as a substitute for the polyene reference of graph- ite.12 All straight and curved lines in 1 represent CC n bonds. There are 81 n bonds in this carbon molecule. All C atoms in 1 are identical and bonded to three other C atoms." Inter-estingly, this molecule consists of an edgeless hexagonal network like an infinite graphite sheet. Therefore, 1 can be regarded as a graphite molecule of a finite size.These struc- tural aspects of this carbon molecule support our view that its polyene reference might be used as a substitute for the polyene reference of graphite.' Zeros of the reference (or matching) polynomial for 1 rep-resent the energies of 54 n orbitals in the polyene reference of 1.12 E,, for this polyene reference was calculated to be 1.52784 and will be referred to as the unit reference energy (E,,). This value was found to be very close to that estimated by Hess and Schaad (1.5216 IfiI),20,21 which is an approximate E,, of graphite obtained on their own theoreti- cal grounds. Therefore, it is reasonable to interpret the differ- ence between the E,, of graphite and the E,, of 1 as an approximate El, of graphite. The %E,, is given as 100 x E,, per C atom, divided by E,, (1). %El,of graphite is 3.061.12 The E,, of a charged graphite sheet can be estimated again from the polyene reference of 1.The E,, of 1 with n addi-tional R electrons is obtained by summing up the energies of all occupied n orbitals and then dividing the sum by 54. This must be very close to E,, of a graphite sheet with n/54 addi- tional R electrons per C atom because both materials have very similar structures. As shown in Fig. 2, a smooth natural spline is obtained by fitting a set of cubic polynomials to the plot of the E,, us. 454 R electrons. The number of additional n electons borne by every C atom in a charged graphite sheet is denoted by q. In prin- ciple, the absolute value of q is smaller than unity.'-' We can estimate from Fig.2 the E,, of a graphite sheet with any amount of charge per C atom. The %E,, of a charged graph- ite sheet then is given as 100 times the difference between the E,, of the sheet and the corresponding E,,, divided by the same E,,. A graphite sheet with q =c has the same %E,, as that with q = -c since it is a kind of alternative hydrocar- bon. Here, c is an arbitrary number. Results and Discussion A single graphite sheet can be viewed as a very large poly- cyclic benzenoid hydrocarbon. Table 1 indicates that an uncharged graphite sheet is comparable in aromaticity to a J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 %E,, values of uncharged and charged benzenoid hydro- carbons species %E,,(q =0) %E,,(q = f1/6) benzene (2) 3.528 -2.910 triphenylene (3) 3.012 -1.043 dibenzo[ fg,op]naph thacene (4) 2.963 -0.609 hexabenzo[bc,ef,hi,kl,no,qr]coronene (5) 2.917 -0.333 dodecabenzocoronene (6) 2.696 0.304 graphite 3.061 -0.321 group of highly aromatic hydrocarbons, such as benzene (2), triphenylene (3), dibenzo[fg,op]naphthacene (4) and hexabenzo[bc,ef,hi,kl,no,qr]coronene (5).These molecules are classified as fully benzenoid hydrocarbons since there are no formal double bonds in the Clar structures.22 As shown in Fig. 3, there are also no formal double bonds in the Clar structure of graphite. The %E,, values of all these fully benze- noid species are larger than 2.90. Therefore, graphite can also be classified as an infinite fully benzenoid hydrocarbon.4 5 A 6 Fig. 3 n-Electron system of graphite (A) and the Clar structure (B) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 %E,, values of charged graphite sheets 4 E"ball B I E"rb/lP I %,E,, f112 1.0932 1.1193 -2.327 f113 1.3075 1.3422 -2.588+ 114 1.3952 1.4213 -1.8367115 1.4438 1.4584 -1.004 f116 1.4739 1.4787 -0.321 -+ 118 1.5083 1.4990 0.621 & 1/12 1.5381 1.5140 1.588 f1/16 1.5507 1.5195 2.054 1/24 1.5615 1.5237 2.485 0 1.5746 1.5278 3.06 1 * Unit n-binding energy. Unit reference energy. The %E,, values of differently charged graphite sheets are listed in Table 2. Aromaticity in graphite rapidly decreases as 141 increases. A graphite sheet can bear electric charge up to 4 = _+ 1/6 by forming GICs in vacuum or in dry air.'-5 Graphite layers of the highest charge density (q = -1/6) are found in the stage 1 compound C,Li, in which lithium atoms are fully ionized.Therefore, these graphite layers are the least aromatic ones. As a matter of fact, graphite sheets with 4 = +1/6 are substantially non-aromatic with a very small negative %E,,. The %E,, rapidly decreases as 141 goes beyond 1/6. Even a graphite sheet with q = klj5 is moder- ately antiaromatic. Such a trend in aromaticity suggests that it must be very difficult to obtain graphite sheets with 141 > 1/6. This is consistent with the fact that appreciably antiaromatic GICs are never formed under ordinary condi- tions. Graphite sheets in most GICs are less charged than those in C6Li.Therefore, most GICs are less aromatic than graph- ite itself although they are still aromatic in nature. This must be partly responsible for the kinetic instability or chemical reactivity of GICS.~ True GICs are chemically reactive and frequently not stable in the absence of free intercalant.* Thus, the %E,, proved to be useful as a practical index for estimat- ing the degree of chemical or kinetic stability of charged graphite sheets. We can safely say that, as in the case of cyclic conjugated molecules, the E,, model can be applied to various infinite 2D conjugated systems. Note that no theories in solid-state physics can be used to predict the chemical or kinetic instability of graphite layers. In this context, one should remember that guest atoms or molecules in GICs are not always fully ionized.'-' For example, measurements on the C,Cs system suggest a 50% ionization for the first stage (x = 8) and a 100% ionization for higher stages (x 3 12).29s The stage 1 compound C,Cs consists of graphite layers with q = -1/16 and 50% ionized caesium atoms.The present simple model must be applied with caution to such GICs with incompletely ionized guest species, since atomic or molecular orbitals of guest species must participate in the valence and conduction bands. However, there are no other methods for estimating the %E,, values of these materials. We see from Table 2 that an iso- lated graphite sheet with q = f1/16 is moderately aromatic.In higher-stage compounds, in general, most transferred charge resides on the two graphite sheets adjacent to the intercalant layer^.^.^ Non-adjacent graphite layers, if any, bear much less charge. Therefore, the unit structure of the stage 3 compound c&s can be roughly assumed to consist of two graphite layers with q = -1/24 and one graphite layer with 4 = 0. It follows that graphite layers adjacent to guest species are again moderately aromatic, but that non-adjacent ones are as highly aromatic as an uncharged graphite sheet. The %E,, of benzenoid hydrocarbon ions with q = _+ 1/6 are listed in Table 1. Charged fully benzenoid hydrocarbons 2-5 with q = k1/6 are all more or less antiaromatic. Anti- aromaticity decreases as the size of the molecule increases. Note that sufficiently large fully benzenoid hydrocarbons with q = k1/6 are substantially non-aromatic like a graphite sheet with q = +1/6.The hepta-anion of 5 (4 = 1/6) is comparable in aromaticity to the charged graphite sheets in C6Li. This again supports the view that graphite can be seen as a kind of fully benzenoid hydrocarbon. PM3 molecular orbital (MO) calculations revealed that even the antiaromatic monoanion of 2 (4 = 1/6) is planar in geometry. Therefore, it is very likely that slightly anti- aromatic or non-aromatic n-electron systems remain planar. However, non-aromatic systems still have large conjugation energies to such an extent that the molecular structures are kept planar. This is consistent with the crystal structure of C,Li, in which non-aromatic graphite sheets are planar.Among the hydrocarbons studied, dodecabenzocoronene (6) is not a fully benzenoid hydrocarbon, so is slightly less aro- matic than 2-5. This molecule is as large in size as 5, but its nanoanion (q = lj6)has a very small positive %E,, . 6 Conclusion This study was the first attempt to estimate the degrees of aromaticity in charged graphite sheets. Using the polyene- like reference structure of the hypothetical 54-carbon mol- ecule (1) as a substitute for that of graphite, we succeeded in doing so. In this sense, this approach is semi-topological in nature. The obtained results are fairly consistent with the chemical behaviour of the GICs. It should be stressed that the concept of aromaticity is very useful for assessing kinetic stability of graphite.There is no such concept in solid-state physics. The same approach can, in principle, be applied to many other infinitely large 2D conjugated systems. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education Science and Culture, Japan. Appendix Triangle Method for Calculating the a-Binding Energy per Carbon Atom of a Charged Graphite Sheet In order to obtain the n-binding energy per unit structure for a periodic system, such as a single graphite sheet, we must perform integration in k-space. In the present study, integra- tion in 2D k-space was made by using a so-called triangle method. In this method, a 2D Brillouin zone is divided into many small triangles.Any integrand [e.g. the n-band energy of a graphite sheet, ~(k),given in eqn. (l)]is accurately evalu- ated only at the vertices of the triangles. Then, assuming that the gradient of the integrand with respect to k is a constant vector within each triangle, the contribution of the triangle to the integral is estimated by interpolation. 3516 The area of the ith triangle with vertices at k,, k, and k3 in k-space, Si, is given by S.=ITI(k 2-kl)x(k3-kl)l (Al)I The area of the entire Brillouin zone, S, is equal to the sum of all trianglar areas : s=csi I The number of n electrons per C atom in a graphite sheet, N, can be formulated as dkXCNi (A3) &(k)<EF I Here, Ni is the approximate contribution of the ith triangle to N, and the summation is made over all triangles.The Fermi energy, cF, is determined so as to give a desired value of N. The number of additional 7r electrons per C atom in a charged graphite sheet, q, is q=N-l (A4) The n-binding energy per C atom of a graphite sheet is denoted by E, which is obtained by integrating &(k) over the k-space : E =1 &(k) dk x Ei (A5)s E(k)%F I where Ei is the approximate contribution of the ith triangle to the integral, and the summation is again made over all triangles. 7r-Band energies at three vertices k,, k, and k3 of a given ,triangle are denoted by E,~E~ and E~ respectively. Here, we assume that E, 6 E, 6 c3 without loss of generality.Approxi- mate formulae derived for Eiand Ni are expressed as follows: (1) For triangles in which E~ <E~, (2) For triangles in which E~ 6 EF 6 ~3, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (3) For triangles in which E, 6 cF <E~, (4) For triangles in which E~ <cl, Ei =0 (A121 Ni =0 (A 13) References 1 J. E. Fischer, in Intercalated Layered Materials, ed. F. LCvy, Reidel, Dordrecht, 1979, pp. 481-532. 2 H. Selig and L. B. Ebert, Adv. Znorg. Chem. Radiochem., 1980,23, 281. J. E. Fischer, Physica B, 1980,99, 383. M. S. Dresselhaus and G. Dresselhaus, Adv. Phys., 1981,30, 139. S. Tanuma, Kagaku Sosetsu, 1983,42, 185. J. Aihara, Sci. Am., 1992,266(3), 62. J. Aihara, J. Am. Chem. SOC., 1976,98,2750.I. Gutman, M. Milun and N. Trinajstie, J. Am. Chem. SOC., 1977, 99,1692. 9 J. Aihara, J. Am. Chem. SOC.,1992,114,865. 10 J. Aihara and S. Takata, J. Chem. SOC.,Perkin Trans. 2, 1994,65. 11 J. Aihara, J. Mol. Struct. (Theochem), 1994,311, 1. 12 J. Aihara, T. Yamabe and H. Hosoya, Synth. Met., 1994,64,309. 13 P. R. Wallace, Phys. Rev., 1947,71,622. 14 R. Saito, M. Fujita, G. Dresselhaus and M. S. Dresselhaus, Phys. Rev. B, 1992,46,1804. 15 S. E. Stein and R. L. Brown, J. Am. Chem. SOC.,1987,109,3721. 16 T. G. Schmalz, W. A. Seitz, D. J. Klein and G. E. Hite, J. Am. Chem. SOC., 1988,110,1113. 17 J. Rath and A. J. Freeman, Phys. Rev. B, 1975,11,2109. 18 G.Gilat and N. R. Bharatiya, Phys. Rev. B, 1975, 12, 3479. 19 H. Hosoya, Y. Tsukano, M. Ohuchi and K. Nakada, in Computer-Aided Innovation of New Materials 11, ed. M. Doyama, J. Kihara, M. Tanaka and R. Yamamoto, Elsevier, Amsterdam, 1993, pp. 155-158. 20 B. A. Hess Jr. and L. J. Schaad, J. Am. Chem. SOC.,1971,93,305. 21 B. A. Hess Jr. and L. J. Schaad, J. Org. Chem., 1986,51,3902. 22 E. Clar, The Aromatic Sextet, Wiley, London, 1972, ch. 6. Paper 4/03200D; Received 31st May, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003513

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(23), 3517-3520 Light Stability of a P-Cyclodextrint Inclusion Complex of a Cyanine Dye Yoko MatsuzawaJ Shin-ichiro Tamura," Nobuyuki Matsuzawa* and Masafumi Ata SONY Corporation Research Center, 174 Fujitsuka-cho, Hodogaya-ku, Yokohama 240, Japan The stability to light of 1,3,3,1',3',3'-hexamethyl-2,2'-indotricarbocyanine iodide (HITC) included in p-cyclodextrin (/I-CyDx) has been examined. The quantum yield of the photodegradation of HlTC was found to decrease with increasing formation of the inclusion complex, showing that the photostability of the dye can be improved by the formation of the complex. Emission from singlet oxygen ('Ag-+3X;, 1268 nm and 1580 nm) for systems of HlTC with or without p-CyDx in aqueous solution was also measured.It was found that the amount of singlet oxygen generated by flash irradiation is reduced by the inclusion complex formation, and that the lifetime of singlet oxygen increased with increasing concentration of p-CyDx, suggesting that the photodegradation of HlTC for the system of HITCIB-CyDx is reduced by two factors; first, the inhibited formation of singlet oxygen, and secondly, the inhibited attack of singlet oxygen on the dye. Although cyanine dyes have been of significant interest because of their applications to dyeing textiles, silver halide photography and heat-mode optical recording, most of the dyes do not possess sufficient light-fastness for these applica- tions. Consequently, the photooxidation or photodegradation of the dyes has been intensively studied.'-4 In general, the singlet oxygen molecule ('Ag) produced by energy transfer from an excited-state dye molecule to ground-state oxygen has been shown to play an important role in the light- bleaching of dyes, especially for cyanines which contain exo- cyclic C-C double bonds3 The singlet oxygen so formed could cause the 1,4-dipole addition reaction to cis-diene to form a dioxene structure, or the 1,2-addition to alkene to form a 1,2-dioxetane leading to the light-bleaching of dyes. For optical recording applications, several approaches have been initiated in order to improve the stability of the dyes.' One of these is the use of singlet oxygen quenchers, and for this purpose, compounds possessing a low ionization energy are preferred as the q~encher.~ Another approach is the introduction of bulky groups to the exocyclic C-C bond, which, for example, leads to squarilium derivatives.' Recently, Kasatani and co-workers have reported that cyanine dyes can be included in the cavity of cyclo-de~trin.~-" C yclodextrins (CyDx) are composed of or-1,4- linkages of a number of D( +)-glucopyranose units, and desig- nated by a Greek letter; a-, p-, y-and 6-CyDx for 6, 7, 8 and 9 units of glucopyranose residues, respectively.They are able to include a wide variety of organic molecules in their It has also been shown that organic n-radicals can even be stabilized in the cavity by the steric hindrance of CyDx." In the articles by Kasatani and co-workers, changes in absorption spectra suggested the formation of inclusion complexes of cyanine dyes, which was further supported by molecular mechanics cal~ulations.~-~ ' The calculations indi- cate that the central polymethine chain composed of the exo- cyclic C-C bonds is included in the cavity of CyDx. Thus, it may be expected that the formation of the complex leads to an improvement in the light stability of the dye due to the steric hindrance of CyDx, which prevents the charge transfer between the dye and singlet oxygen which initiates photo- degradation.We have therefore investigated this effect and t Cyclomaltoheptaose.1Present address : Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Y oko- hama 227, Japan.demonstrated a new method of improving the light stability of cyanine dyes. Experimental The cyanine dye 1,3,3,1',3',3'-hexamethyl-2,2'-indotricarbocy-anine iodide (HITC) obtained from The Japanese Research Institute for Photosensitizing Dyes Co., Ltd. was used in these experiments. The structure of HITC is shown in Fig. 1. p-Cyclodextrin (p-CyDx) obtained from Tokyo Kasei was used as a host molecule, after being recrystallized twice from water and dried in vacuo at 80 "C. For measurements of the light stability of HITC with or without P-CyDx, the 365 nm line from a super-high-pressure mercury lamp (USH-SOOD, Ushio) was irradiated through IRA-25s and UV-D35 filters (Toshiba). Deionized water, purified using a Milli-Q SP system (Millipore), was used as the solvent.Absorption spectra were measured using a Hitachi U-3210 spectrophotometer. Emission from singlet oxygen in deuteriated water (D,O, Aldrich Chemical) ('Ag-+3Zg, 1268 nm and 1580 nm16317) was measured using an ordinary flash photolysis apparatus. A flash lamp (Xenon Corp. N-851D) equipped with filters IRA-25s and UV-D35 was used for the excitation of the cyanine dye. An InGaAs photodiode (Hamamatsu Photonics, G3476) with an applied voltage of 5 V was used as the detec- tor with an Si wafer used as a filter (<900 nm). Signals were amplified using an oscilloscope (Tektronix 7603) and a tran- sient digitizer (Tektronix 7912AD) which were equipped with Tektronix 7A26 and 7A13 amplifiers, respectively. The total gain of the amplifiers was 50, and the response time of the system was 400 ns and 2 ps for rise up and down, respec- tively.Measurements were repeated 25 times for each sample, and the data obtained were accumulated in order to raise the signal-to-noise ratio. The data were analysed using a personal computer (NEC PC-9801VX). ML Me 1-Fig. 1 Structure of HITC Results As noted above, CyDxs include cyanines to form a cyanine dimer, cyanine,/CyDx, and in the case of HITC, the CyDxs which have the ability to include the dyes are P-and y-CyDx, depending on the cavity size.'-" In our study, P-CyDx was used in order to reduce the possibility of the migration of oxygen molecules into the complex.Kasatani et al. observed the formation of cyanine,/P-CyDx at a concentration of [cyanine] = 2 x lo-' mol dm-3 and [Q-CyDx] = 5 x mol dm-3, 'so we prepared samples with similar concentra- tions. Note that in case of the DODC-Q-CyDx system (DODC, is 3,3'-diethyloxadicarbocyanine iodide), ca. two-thirds of the cyanine molecules should form the complex according to equilibrium constants reported,' ' although the constants are expected to become smaller in the case of HITC because of its larger steric bulk compared with that of DODC. In the range of concentrations used, our 'H NMR mea- surements (using a JEOL GSX-270 FT-NMR spectrometer operating at 270 MHz with D,O solvent) showed a 6-8% decrease in the relaxation time (Tl) of CyDx hydrogens that are located in the cavity, and that chemical shifts of hydro- gens connected to the polymethine chain of HITC exhibited changes to higher magnetic fields.This actually confirmed the results of the molecular mechanics calculations,'~'O where the polymethine chain is included in the cavity. Light Stability of HITC-fl-CyDx Systems The quantum yield of photodegradation for the dye was mea- sured in order to investigate the effects of inclusion complex formation on light stability. Samples were prepared with several molar ratios of [HITC]/[%CyDx] with [HITC] = 7 x mol dm-3. The quantum yield of photo- degradation was determined by applying eqn. (l).'8,'g 1n(exp[2-303A736(t)E36 5/&3761 -1 -1n(exp[2*303A736(o)E,,,/E,,,1 -'1 = -lm+degrd &3652.30310t (1) where ,4736 is the absorbance of the mixed solution of the cyanine dye and B-CyDx at 736 nm (the absorption peak of HITC in aqueous solution) after the irradiation time, t, A736(0) is the initial absorbance of the solution at 736 nm before irradiation, I, is the intensity of the irradiated light which was 6.51 x mol cm-' s-', &365 is the molar absorption coefficient at the wavelength irradiated (365 nm), &736 is the molar absorption coefficient at 736 nm, and +degrd is the quantum yield of photodegradation for H1TC.t The 365 nm line was used as the irradiation wavelength for the measurements because we wanted to improve the long- term light stability of cyanines in an ambient environment for the purpose of obtaining, for example, optical recording media with a sufficiently high lifetime.In such an environ- ment, it is expected that UV light plays a more important role in the bleaching of the dyes than that of visible or IR light. A plot of the quantum yield us. [P-CyDx]/[HITC] is shown in Fig. 2. The quantum yield decreases with increasing t In our experiment, the solutions measured can be regarded as optically dilute, since the concentration of HITC itself is quite low (7 x mol dm-3), and the absorbance of the samples at the irra- diated wavelength of 355 nm is typically in the region of 0.1-0.2. Note that in the article by Niizuma and Koizumi,lg in which the photoreduction of acridine was investigated, the absorbance of the samples at the irradiated wavelength was ca.0.5-1.0 (the concentra- tion of acridine was ca. 1 x mol dmb3). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0 071.0 0 0 .-0 E 0.8 0 4-0 0.1 1 1a1 100 1000 [P-CyDxl/[H ITCI Fig. 2 Plot of the quantum yield of the photodegradation of HITC against [P-CyDx]/[HITC]. Note that the value for HITC without fi-CyDx is shown on the vertical axis which is at [fi-CyDx]/[HITC] = 0.1. concentration of P-CyDx, although the data are slightly scat- tered. In the fractional region of 5.0 x lo-' < [fl-CyDx] /[HITC] < 1.0 x lo2, the quantum yield is almost constant at ca. 0.9 x which is ca. 30% smaller compared with that for HITC without P-CyDx, which is 1.3 x A further decrease in the quantum yield is observed in the region where [fi-CyDx]/[HITC] > 1.0 x lo2, and for the system of [%CyDx]/[HITC] = 6.0 x lo2, the quantum yield (0.45 x is only ca.35% that for HITC without P-CyDx. Thus, it is concluded that the inclusion complex for- mation does lead to a significant improvement in the light stability of the dye. The quantum yield of photodegradation of HITC has been measured using a xenon lamp by Lepaja et aL4 Values of 0.5 x 1.5 x 2.7 x lop4 and 8 x have been reported for methanol, acetone, DMSO and chloroform solu- tions, respectively, with a dye concentration of ca. lo-' mol dm-3. Note that our value in D,O solution is an order of magnitude larger than these values. Byers et al. have also measured the quantum yields of photodegradation of some cyanine dyes using a super-pressure mercury lamp.3 For 3,3'- diethyl-4,5,4',5'-dibenzothiacyanine,the value in methanol solution with a concentration of 10-5-10-6 mol dm-3 was less than lop6, whereas in 98 : 2 (v/v) water-methanol solu- tion (3.5 x lo-' mol dm-3), the value became 3.4 x which is quite similar to the value reported in this paper for the system of HITC without P-CyDx.They attributed this enhanced photodegradation in the water-met hanol solution to the formation of H- aggregate^,^ as they observed a corre- sponding aggregation band, which, on the other hand, was not observed in our experiments, nor in the results by Kasa- tani et al.' ' using HITC. Emission from Singlet Oxygen Molecule Emission from singlet oxygen molecules in the systems of HITC with or without 8-CyDx was measured in order to obtain an insight into the mechanism of the photo-degradation of the dye.Three samples with different concen- trations of HITC and p-CyDx in D,O (HITC without P-CyDx, and [P-CyDx]/[HITC] = 4.3 x lo1 and 2.8 x lo2) were prepared with [HITC] being constant (1.1 x lo-' mol dm-3). D20 was used as the solvent instead of H,O, because the lifetime of singlet oxygen in H20 (4 ps 20) is expected to be too short for our apparatus to measure accurately, whereas that in D20 (68.1 i1.5 p ,') is much longer. In order to eliminate scattered light from a flash lamp which emits over the wavelength range of the emission from '0, ('Ag), the decay in the emission was obtained as a difference between signals obtained from systems before and after nitro- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 20 40 60 80 100 time/ps Fig. 3 Decays of emission from excited singlet oxygen for the systems of HITC without p-CyDx (a), and [p-CyDx]/[HITC] = 4.3 x 10' (b) and 2.8 x 10, (c) gen bubbling. The decays in the emission are shown in Fig. 3. The lifetime of singlet oxygen (T), which was determined by fitting the curves in Fig. 3 using a single exponential function, is shown in Table 1. The quantum yield of the reaction between HITC and singlet oxygen (+rxn) is also shown in Table 1. The yield was determined by applying eqn. (2). 4rxn = kRIHITCI/(kO + kRIHITC1) (2) where k,' corresponds to the lifetime of singlet oxygen in D20 without HITC (68.1 f1.5 ps 21), and kR is the rate con- stant of the reaction, and can be obtained from eqn.(3). 1/~= k, + k,[HITC] (3) The lifetime of the singlet oxygen (z) is increased by increasing the concentration of Q-CyDx; z for the system of [Q-CyDx]/[HITC] = 2.8 x lo2 is 2.3 times longer than that for the system of HITC without Q-CyDx. In the system of [B-CyDx]/[HITC] = 2.8 x lo2, the value becomes 57 ps. This value is in reasonable agreement with previously reported values of 58 ps 22 and 68.1 f1.5 ps,21but is significantly smaller than a value of 100 ps.23If we assume that the former values are more reliable than the latter, the agreement vali- dates our experimental technique, and since values of ca.60 ps can be regarded as an intrinsic lifetime of singlet oxygen in D20,21.24 it can be concluded that quenching of singlet oxygen by HITC is not dominantly present in the Q-CyDx- HITC system where [Q-CyDx]/[HITC] = 2.8 x lo2. (Note that 4,,, is actually 0.16.)7 Discussion It has been reported that excited single oxygen (lo2)can be formed by energy transfer from an excited sensitizer molecule Table 1 Lifetime of singlet oxygen for the HITC-B-CyDx systems 0 25 0.63 4.3 x 10' 28 0.59 2.8 x lo2 57 0.16 t This non-zero value for 4,,, suggests the occurrence of reactions between singlet oxygen and the rings of the indole residue in HITC which may not be included in the cavity of cyclodextrin.Such reac- tions explain the slight difference between the lifetime of '0, obtained by Ogilby and Foote (68.1 k1.5 ps and our value (57 PSI-(i.e. HITC).25,26 Possible mechanisms of the formation are shown in eqn. (4) and (5).2' 'So + hv -+ 'S, -+ 'S, 3S1+ 302(3C,) -+ 'So + 'O2('Ag or 'C;) (4) 'So + hv -,'S, 'S, -+ 302(3CC,)-,3S1 + '02(lAg or 'El) (5) The intensity of the emission shown in Fig. 3 is clearly in the order of that for HITC without Q-CyDx > [p-CyDx] /[HITC] = 4.3 x 10, > [B-CyDx]/[HITC] = 2.8 x lo2. This indicates that the formation of singlet oxygen shown in eqn. (4) and (5) is inhibited by the inclusion complex formation. Since B-CyDx includes the polymethine chain of HITC, the inclusion should reduce close interactions between the poly- methine chain and oxygen in the ground state.MNDO/PM- 3 calculations, which will be described below, predicted that the LUMO of HITC is localized on the polymethine chain, and thus, the Dexter-type energy transfer28 from HITC to oxygen should effectively be inhibited. Of course, the Forster- type energy transfer2' can occur even if HITC is included, and also the two molecules can interact closely at the indole rings in HITC, which could lead to the formation of singlet oxygen even for the system of [/3-CyDx]/[HITC] = 2.8 x lo2. Note that changes in the geometry parameters of HITC which could be caused by the inclusion, or the dimerization of HITC in the cavity, may also affect the for- mation of singlet oxygen.Aggregation of cyanines, especially for H-aggregated cyanines, is known to raise triplet yields and enhance the ph~todegradation.~' Disruption of the aggregates caused by the formation of the inclusion complex may also lead to the lowering of the quantum yield of photo- degradation of HITC, although we note again that no aggre- gation band was observed in our system. The quantum yield of the reaction between HITC and Q-CyDx also decreases with an increase in [p-CyDx], and that for the system of [Q-CyDx]/[HITC] = 2.8 x lo2 is only 25% of that for HITC alone. Thus, the improved light stabil- ity of HITC upon the addition of Q-CyDx can be attributed to two factors. One is the decreased formation of excited singlet oxygen, and the other is the decreased reaction between singlet oxygen and HITC.The size of the cavity of Q-CyDx is 7.9-8.0 8, in height,12-14 whereas the size of the polymethine chain of HITC is estimated to be 7.36 8, {d[C(2)-C(2')]) or 9.66 8, {d[C(l)-C(l')]) in length as calcu- lated by the MNDO/PM-3 meth~d~l-~~ using the program MOPAC.34$ Thus, the C(2) atoms are expected to be located in the cavity, whereas the C(l) atoms are outside the cavity. It has been reported that in the photo-oxidation process of cyanine dyes, oxygen reacts with the edge C-C double bond in the polymethine chain [i.e. C(l)-C(2) and C(l')=C(2') bond^].^ Thus, the geometry considerations suggest that for the HITC-Q-CyDx system, the bonds which are most likely to react with oxygen are at least partially protected by the presence of the steric bulk of CyDx in terms of its cavity size.This supports the conclusion that CyDx blocks the attack of singlet oxygen on the dye. Conclusions We examined the photostability of the Q-CyDx inclusion complex of a cyanine dye, HITC. The quantum yield of the photogradation of HITC decreases with the increase in for- mation of the inclusion complex. The yield for the system [Q-1An alliant FX-2800 computer was used for the calculation with the SCF convergence criterion of kcal mol-'. 3520 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 CyDx]/[HITC] = 6.0 x lo2 is 0.45 x which is only ca. 35% of that for the system without /I-CyDx. This clearly demonstrates that the formation of the cyclodextrin inclusion complex does improve the photostability of the dyes. We also measured the emission from singlet oxygen for the 9 10 11 K.Kasatani, M. Ohashi, M. Kawasaki and H. Sato, Chem. Lett., 1633,1987. M. Ohashi, K. Kasatani, H. Shinohara and H. Sato, J. Am. Chem. SOC., 1990,112,5824. M. Kasatani, M. Ohashi and H. Sato, Carbohydr. Res., 1989, 192, 197. systems of HITC with or without b-CyDx. It was found that the amount of singlet oxygen formed can be reduced by the formation of the complex, and that the lifetime of singlet oxygen increases with increasing concentration of P-CyDx. The lifetime of the system [P-CyDx]/[HITC] = 2.8 x lo2 is 57 ps, which is quite close to that in D20 without HITC or 12 13 14 15 16 M. L. Bender and M. Komiyama, Cyclodextrin Chemistry, Springer, New York, 1977.V. Ramamurthy and D. F. Eaton, Acc. Chem. Res., 1988,21,300. W.Saenger, Angew. Chem., Int. Ed. Engl., 1980,19,344. M. Aoyagi, M. Ata, Y. Gondo, Y. Kubozono and Y. Suzuki, Supramol. Chem., 1993,2,277. A. U. Khan and M. Kasha, Proc. Natl. Acad. Sci. USA, 1979,76, any solute (68.1 & 1.5 ps 21). The quantum yield of the reac- tion between HITC and singlet oxygen is 0.16 for the system of [P-CyDx]/[HITC] = 2.8 x lo2, and this value is signifi- cantly smaller than that without /3-CyDx (0.63). These results show that the photodegradation of HITC is inhibited by two 17 18 19 6049. A. U. Khan, Chem. Phys. Lett., 1980 72, 112. S. Kato, S. Minagawa and M. Koizumi, Bull. Chem. SOC.Jpn., 1961,34,1026. S. Niizuma and M.Koizumi, Bull. Chem. SOC. Jpn., 1963, 36, 1629. factors; one is the inhibited formation of singlet oxygen, and the other is the inhibited attack of singlet oxygen on the dye. The authors would like to acknowledge useful discussions with Dr. N. Kishii and Dr. A. J. Hudson of SONY Research Center. Many thanks are due to Dr. J. Seto (Deputy Director, SONY Research Center) for giving the authors the opportunity to perform this investigation. 20 21 22 23 24 25 26 M. A. J. Rodgers and P. T. Snowden, J. Am. Chem. SOC., 1982, 104,5541. P. R. Ogilby and C. S. Foote, J. Am. Chem. SOC., 1983,105,3423. J. R. Hurst, J. D. McDonald and G. B. Schuster, J. Am. Chem. SOC.,1982,104,2065. A. A. Krasnovskii, Chem. Phys. Lett., 1981,81,443. P. R. Ogilby, J.Phys. Chem., 1989,93,4691.K. Kawaoka, A. U. Khan and D. R. Kearns, J. Chem. Phys., 1967,46,1942. K. Kawaoka, A. U. Khan and 11. R. Kearns, J. Chem. Phys., References 27 1967,47, 1883. Singlet 0,. Volume I: Physical--Chemical Process, ed. A. A. 1 2 3 4 5 6 7 C. Foote, Science, 1968,162,963. A. Timbers and E. Lingafelter, J.Am. Chem. SOC., 1949,71,4155. G. W. Byers, S. Gross and P. M. Henrichs, Photochem. Photo- biol., 1976,23, 37. S. Lepaja, H. Strub and D-J. Lougnot, 2. Naturforsch., Teil A, 1983,38, 56. R. W. Denny and A. Nickon, Org. React., 1973,20, 133. A. A. Frimer, Chem. Rev., 1979,79,359. N. J. Turro, Modern Molecular Photochemistry, Banjamin/ 28 29 30 31 32 33 34 Frimer, CRC Press, Boca Raton, FL, 1985. D. L. Dexter, J. Phys. Chem., 1953,21, 836. Ref. 7, p. 302, and references cited therein. D. Pant, C. Bhagchandani, N. Pant and S. Verma, Chem. Phys. Lett., 1971,9, 546. J. J. P. Stewart, J. Comput. Chem., 1989, 10, 209. J. J. P. Stewart, J. Comput. Chem., 1989,10,221. J. J. P. Stewart, J. Cornput.-Aided Mol. Design, 1990,4, 1. J. J. P. Stewart, QCPE Program 455, 1983, version 5.01, 1989. Cummings, Menlo Park CA, 1978. 8 M. Emmelius, G. Pawlowski and H. W. Vollman, Angew. Chem., Int. Ed. Engl., 1989,28, 1445. Paper 4/03978E; Received 30th June, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003517

出版商:RSC    

年代:1994

数据来源: RSC