摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Prof. Peter J. Sarre Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD, UK 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) Prof. P. J. Sarre (Nottingham) Prof. J. S. Higgins (London) Dr. R. K. Thomas (Oxford) Editorial Manager and Secretary to Faraday Editorial Board Dr. Robert J. Parker The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF, UK Staff Editor: Dr.R. A. Whitelock Senior Assistant Editor: Mrs. S. Shah Assistant Editors: Dr. L. Milne, Mrs. C. J. Seeley Editorial Secretary: Mrs. J. E. Gibbs International Advisory Editorial Board R. S. Berry (Chicago) R. A. Marcus (Pasadena) A. M. 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They are designed to be topical articles of interest to a wide range of ,esearch scientists in the areas of Physical :hemistry, Biophysical Chemistry and Zhemical Physics. %I1 details of the form of manuscripts for 4rticles and Faraday Communications, con- jitions for acceptance etc. are given in issue lumber one of faraday Transactions, iublished in January of each year, or may be )btained from the Editorial Manager. rhere is no page charge for papers published n faraday Transactions. Fifty reprints are upplied free of charge. 'rof. P. J. Sarre, Scientific Editor. -el.: Nottingham (0602) 51 3465 (24 hours) i-Mail (JANET): PCZPSF@U K.AC. NOlT.VAX :ax: (0602) 51 3466 -elex: 37346 UNINOT G lr. R. J. Parker, Editorial Manager. -el.: Cambridge (0223) 420066 :-Mail (INTERNET): 3SC1 @RSC.ORG (For access from JANET ise RSCl% RSC.0 RG @ U K.AC. NS F IET-R ELAY) :ax: (0223) 423623 or 420247 -elex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX069

出版商:RSC    

年代:1994

数据来源: RSC

ISSN:0956-5000    

DOI:10.1039/FT99490BX071

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(18) 2601 -2847 (1994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 2601 Microwave spectrum of sulfuryl chloride fluoride, S0,ClF : Structure, hyperfine constants and harmonic force field H. S. P. Miiller and M. C. L. Gerry 261 1 Spectroscopic investigations on iminophosphanes and methylenephosphanes G. David, V. von der Gonna, E.Niecke, T. Busch, W. W. Schoeller and P. Rademacher 2617 Further studies on the polarizabilities and hyperpolarizabilities of the substituted polyenes and polyphenyls I. D. L. Albert, D. Pugh and J. 0.Morley 2623 Intrinsically unpolarized fluorescence of c6, M. N. Berbaran-Santos and B. Valeur 2627 Photoinduced and thermal isomerization processes for bis-oxonols : Rotor volume, stereochemical and viscosity effects A.C. Benniston and A. Harriman 2635 Spectroscopic properties of aromatic dicarboximides. Part 2.-Substituent effect on the photophysical properties of N-phenyl-l,2-naphthalimide A. Demeter, T. Berces, L. Biczok, V. Wintgens, P. Valat and J. Kossanyi 2643 EPR spin-labelling and spin-trapping study of proteins in reverse micelles G. S. Timmins, M. J. Davies, B. C. Gilbert and H. Caldararu 2649 EPR data do not support the P=O representation for triakyl phosphates and phosphine oxides or sulfides U. S. Rai and M. C. R. Symons 2653 31P and 'H powder ENDOR and molecular orbital study of a co33-ion in X-irradiated carbonate containing hydroxyapatites P.D. Moens, F. J. Callens, P.F. Matthys and R. M. Verbeeck 2663 EPR and NMR studies of amorphous aluminium borates S. Simon, A. van der Pol, E. J. Reijerse, A. P. M. Kentgens, G. J. van Moorsel and E. de Boer 267 1 Effects encountered in EPR spectroscopy and imaging at small magnetic fields D. G. Gillies, L. H. Sutcliffe and M. R.Symhm 2677 Excess molar enthalpies of nitrous oxide-toluene in the liquid and supercritical regions R. C. Castells, C.Menduiiia, C. Pando and J. A. R.Renuncio 2683 Thermodynamic study of the transfer of the tin@), lead(@ and alkaline-earth-metal ions from water to methanol, dimethyl sulfoxide, acetonitrile, pyridine and N,N-dimethylthioformamide M. Chaudhry, Y. Kinjo and I. Persson 269 1 Enthalpies of transfer of tetrabutylammonium bromide from water to highly aqueous water-methanol, -ethanol, -propan-1-01 and -acetonitrile mixtures at 298 K : Consideration of the extended coordination model solvation parameters P.Hogan, I. McStravick, J. Mullally and W. E.Waghorne 2697 ATR-FTIR studies of ion-solvent and ion-ion interactions in divalent-metal perchlorate-acetonitrile solutions W. R. Fawcett, G. Liu and A. A. Kloss 2703 Phase transitions in the bilayers in vesicles formed from binary mixtures of symmetric di-n-alkylphosphates in aqueous solutions M. J. Blandamer, B. Briggs, P.M. Cullis, J. B. F. N.Engberts, A. Wagenaar, E. Smits, D. Hoekstra and A. Kacperska 2709 Gel to liquid crystal transitions for vesicles in aqueous solutions prepared using mixtures of sodium dialkylphosphates (R'O)(R20)P02-Na+ and (R30)2P02-Na+, where R1 = C1,H2,, R2 = C,,H,, or C'BH37 and R3 = C12H2S, C14H29, C16H33 or C,,H3, M.J. Blandamer, B. Briggs, P. M. Cullis, J. B. F. N. Engberts, A. Wagenaar, E. Smits, D. Hoekstra and A. Kacperska 2717 Effects of surfactant charge and structure on excited-state protolytic dissociation of 1-naphthol in vesicles Y. V. Il'ichev, K. M. Solntsev, M. G. Kuzmin and H. Lemmetyinen 2725 Aggregation, hydrogen bonding and thermodynamic studies on Boc-Val-Val-Ile-OMe tripeptide micelles in chloroform R. Jayakumar, R. G. Jeevan, A. B. Mandal and P.T. Manoharan 273 1 Interaction of water with a,a-trehalose in solution : Molecular dynamics simulation approach M. C. Donnamaria, E. I. Howard and J. R. Grigera 2737 Brownian dynamics simulation of a multi-subunit deformable particle in simple shear flow M.C. Bujan-Nuiiez and E. Dickinson 2743 Surface chemistry and microemulsion formation in systems containing dialkylphthalate esters as oils R. Aveyard, B. P. Binks, P. D. I. Fletcher, P. A. Kingston and A. R. Pitt 2753 Solutions of aluminium in liquid lithium: Electrical resistivity of liquid alloys R. J. Pulham, P. Hubberstey and P. Hemptenmacher 2757 Electrical properties of an ethanol-dodecane mixture near the upper critical solution point K. Orzechowski 2765 Directional symmetry of the time lag for downstream absorptive permeation studied by the matrix method J. S. Chen 2769 Molecular conformation of n-alkyloligo(oxyethy1ene)sin the solid state studied by Raman spectroscopy.Effect of the end group S. Masatoki and H. Matsuura 2775 EXAFS studies of molecular geometries of some CO" and Co"' porphyrins M. Endregard, D. G. Nicholson, R. J. Abraham, I. Marsden and B. Beagley 2783 EXAFS data analysis for lanthanide sesquioxides P. Malet, M. J. Capitan, M. A. Centeno, J. A. Odriozola and I. Carrizosa 2791 Single-crystal structure of C,, at 300 K. Evidence for the presence of oxygen in a statically disordered model W. Bensch, H. Werner, H. Bart1 and R. Schlogl 2799 High-resolution electron microscopy studies of a microporous carbon produced by arc-evaporation P. J. F. Harris, S. C. Tsang, J. B. Claridge and M. L. H. Green 2803 Characterization of carbon-supported ruthenium-tin catalysts by high-resolution electron microscopy G.Neri, R. Pietropaolo,S. Galvagno, C. Milone and J. Schwank 2809 Ru-Cu/SiO, catalysts: Characterization by FTIR spectroscopy C. Crisafulli, R. Maggiore, S. Scire and S. Galvagno 2815 FTIR study of the influence of sulfate species on the adsorption of NO, CO and NH, on CuO/Al,O, catalysts M. Waqif, M. Lakhdar, 0.Saur and J-C. Lavalley 2821 Adsorption of MCM-14 mesoporous molecular sieves. Part 1 .-Nitrogen isotherms and parameters of the porous structure J. Rathousky, A. Zukal, 0.Franke and G. Schulz-Ekloff 2827 IR study of ethene and propene oligomerization on H-ZSM-5 : Hydrogen-bonded precursor formation, initiation and propagation mechanisms and structure of the entrapped oligomers G. Spoto, S. Bordiga, G. Ricchiardi, D.Scarano, A. Zecchina and E. Borello 2837 Acid properties of a ZSM-20-type zeolite H. Kosslick, H. Berndt, H. D. Lanh, A. Martin, H. Miessner, V. A. Tuan and J. Janchen FARADAY COMMUNICATIONS 2845 Intramolecular excimer formation in 175-di(9-anthryl)-n-pentane T. A. Smith, G. D. Scholes, G. 0.Turner and K. P. Ghiggino Note: Where an asterisk appears against the name of one or more of the authors, it is included with the authors' approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles. For further details contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN, UK Tel: 44 (0)71-437 8656 Fax: 44 (0)71-287 9798 Telecom Gold 84: BUR210 Electronic Mailbox (Internet) LIBRARY@RSC.ORG. If the material is not available from the Society's Library, the staff will be pleased to advise on its availability from other sources. Please note that copies are not available from the RSC at Thomas Graham House, Cambridge.

ISSN:0956-5000    

DOI:10.1039/FT99490FP192

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aas, N., 1015 Abadzhieva, N., 1987 Abbott, A. P., 1533 Abraham, R. J., 2775 Abramowicz, T., 2417 Afanasiev, P., 193 Agren, H., 1479 Aikawa, M., 91 1 Aitken, C. G., 935 Akanuma, K., 1171 Akolekar, D. B., 1041 Alava, I., 2443 Albert, I. D. L., 2617 Albery, W. J., 11 15 Alcober, C., 2395 Aldaz, A,, 609 Alfimov, M. V., 109 Al-Ghefaili, K. M., 383, Ali, V., 579, 583 Aliev, A. E., 1323 Allegrini, P., 333 Allen, N. S., 83 Al Rawi, J. M. A., Amorim da Costa, A. M., Amoskov, V. M., 889 Ando, M., 1011 Andre, J-M., 2319 AndrCs, J., 1703, 2365 Andrews, S. J., 1003 Anson, C. E., 1449 AntoniC, T., 1973 Aragno, A., 787 Arai, S., 1307 Aramaki, K., 321 Aravindakumar, C. T., 597 Asai, Y., 797 Ashfold, M. N. R., 1357 Asmus, K-D., 1391 Assfield, X., 1743 Attwood, D., 1961 Aveyard, R., 2743 Avila, V., 69 Axford, S.D. T., 2085 Baba, T., 187 Baba, Y., 2423 Back, G-H., 2283 Badia, A., 1501 Badri, A., 1023 Bagatti, M., 1077 Balaji, V., 1653 Ball, M. C., 997 Ball, S. M., 523, 1467 Bally, T., 1615, 1674, 1733, Ban, M. I., 1610 Baonza, V. G., 553 Baonza, V. G., 1217 Barbaux, Y., 895 Barbero, C., 2061 Barczynski, P., 2489 Barker, S. A., 1689 Barnes, J. A., 1709 Barthel, J., 2475 Barthomeuf, D., 667,675 Bartl, H., 2791 Bartlett, P. N., 2155 Basini, L., 787 Bassat, J. M., 1987 Bassoli, M., 363 Battaglini, F., 987 Bauer, C., 517 Baur, W. H., 2141 Beagley, B., 2775 Bell, A. J., 17, 817 Belton, P. S., 1099 Bender, B. R., 1449 Bendig, J., 287 Bengtsson, L. A., 559, 2401, Benko, J., 855 Benniston, A. C., 953, 2627 Beno, B., 1599 Bensalem, A., 653 Bensch, W., 2791 1047 845 689 1808 2531 Berbaran-Santos, M.N., Btrces, T., 41 1,2635 Bergeret, G., 773 Bernardi, F., 1617, 1669, Berndt, H., 2837 Bertran, J., 1679, 1757, Beutel, T., 1335 Beyer, H. K., 1329 Bhuiyan, L. B., 2002 Bickelhaupt, F., 327, 1363 Bickley, R. I., 2257 Biczok, L., 41 1,2635 Bielanski, A., 2099 Biggs, P., 1197, 1205 Billingham, J., 1953 Bilmes, S. A., 2395 Binet, C., 1023 Binks, B. P., 2743 Black, S. N., 1003 Blackett, P. M., 845 Blake, J. F., 1727 Blanco, M., 2125 Blanco, S., 1365 Blandamer, M. J., 727, 1905,2703,2709 Blaszczak, Z., 2455 Blower, C., 919,931 Bocherel, P., 1473 Boddenberg, B., 1345 Boesman, E., 2541 Boggis, S. A,, 17 Bohm, F., 2453 Booth, C., 1961 Borden, W. T., 1606, 1614, 1616,1671, 1673,1675, 1689,1733, 1734,1735, 1743,1744,1802, 1807 2623 1671, 1672 1800,1806 Bordiga, S., 2827 Borello, E., 2827 Borge, G., 1227 Borisenko, V.N., 109 Bottoni, A., 1617 Boutonnet-Kizling, M., Bowker, M., 1015 Bowmaker, G. A., 2579 Bozon-Verduraz, F., 653 Bradley, C. D., 239 Bradshaw, A. M., 403 Bratu, I., 2325 Braun, B. M., 849 Breysse, M., 193 Briggs, B., 727, 1905, 2703, Brocklehurst, B., 271, 2001 Brogan, M. S., 1461 Brown, N. M. D., 1357 Brown, R. G., 59 Brown, S. E., 739 Bruna, P. J., 683 Brzezinski, B., 843, 1095 Buchner, R., 2475 Buckley, A. M., 1003 Buemi, G., 121 1 Bujan-Nuiiez, M. C., 2737 Biilow, M., 2585 Burdisso, M., 1077 Burget, D., 2481 Busca, G., 1161,1293 Busch, T., 2611 Buschmann, H-J., 1507 Butler, L. J., 1581, 1612, 1613, 1614, 1671, 1677, 1809 1023 2709 Butt, M.D., 727 Buttar, D., 1811 Byatt-Smith, J. G., 493 Cabaleiro, M. C., 845 Caceres, C., 2125 Caceres, M., 1217 Caceres Alonso, M., 553 Cairns, J. A., 1461 Calado, J. C. G., 649 Caldararu, H., 213,2643 Callens, F. J., 2541, 2653 Calvaruso, G., 2505 Calvente, J. J., 575 Calvo, E., 2395 Calvo, E. J., 987 Camacho, J. J., 23 Cameron, B. R., 935 Caminati, W., 2183 Campa, M. C., 207 Campelo, J. M., 2265 Campos, A., 339 Canosa-Mas, C. E., 1197, Capitan, M. J., 2783 Capobianco, J. A., 755 Caragheorgheopol, A,, Carlile, C. J., 1149 Carlsen, L., 941 Carrizosa, I., 2783 Carvill, B. T., 233 Castaiio, F., 2443 Castaiio, R., 1227 Castells, R. C., 2677 Castro, S., 1217 Catalina, F., 83 Cataliotti, R. S., 1397 Cavasino, F. P., 31 1,2505 Ceccarani, M. L., 1397 Cense, J-M., 2015 Centeno, M.A., 2783 Cevc, G., 1941 Chakrabarty, D. K., 1993 Chang, T-h., 1157 Charlesworth, D., 1999 Charlesworth, P., 1073 Chaudhry, M., 2235,2243, Che, M., 2277 Chen, J-S., 429, 717 Chen, J. S., 2765 Chen, L., 2467 Chen, Y-H., 617 Cheng, A., 253 Cheng, C. P., 1157 Cheng, Y., 2517 Cherqaoui, D., 97,2015 Chesta, C. A., 69 Chevalier, S., 667, 675 Chi,Q., 2057 Child, M. S., 1739 Chiu, S. S-L., 1575 Chmiel, G., 11 53 Cho,T., 103 Choisnet, J., 1987 Chowdhry, B. Z., 1999 Christensen, P., 459 Chung, Y-L., 2547 CiZmek, A., 1973 Claridge, J. B., 2799 Clark, T., 1669, 1678, 1783, Clegg, S. L., 1875 Clement, R., 2001 Climent, M. A., 609 Coates, J. H., 739 Coitiiio, E. L., 1745 Collett, J. H., 1961 Colmenares, C. A., 1285 Cook, J., 1999 Cooney, R. P., 2579 Cooper, D. L., 1643 Cordischi, D., 207 Corma,A., 213 Cormier, G., 755 Corradini, F., 859, 1089 Corrales, T., 83 Cosa, J.J., 69 Costas, M., 1513 Cottier, D., 1003 Coudurier, G., 193 Courcot, D., 895 Coveney, P. V., 1953 Cox, A. P., 2171 Cox, R. A., 1819 1205 213 2683 1807, 1808, 1809, 1810 Cracknell, R.F., 1487 Craig, S. L., 1663 Cramer, C. J., 1802 Crawford, M. J., 817 Crisafulli, C., 2809 Crowther, D., 2155 Cruzeiro-Hansson, L., 1415 Cullis, P. M., 727, 1905, Curtis, J. M., 239 DAlagni, M., 1523 Damiani, D., 2183 Dan& N-T., 875 Danil de Namor, A. F., Das,D., 1993 Das, T. N., 963 Dasannacharya, B. A., 1149 Dash, K. C., 2235 Datka, J., 2417 Davey, R. J., 1003 David, G., 261 1 Davidson, K., 879 Davies, M. J., 2643 De Benedetto, G. E., 1495 de Boer, E., 2663 Defrance, A., 1473 Dejaegere, A., 1763 de Leng, H.C., 2459 Delhalle, J., 2319 Demeter, A., 41 1, 2635 Dempsey, P., 1003 Demri, D., 501 Deng, N-J., 1961 Deng, Z., 2009 Denkov, N. D., 2077 Derrick, P. J., 239 Dewing, J., 1047 Diagne, C., 501 Dickinson, E., 173,2737 Diebler, H., 2359 Dines, T. J., 1461 Doblhofer, K., 745 Domen, K., 911 Doney, S. C., 1865 Dong, S., 2057 Donnamaria, M. C., 2731 Dore, J. C., 2497 Dory, M., 2319 Dossi, C., 1335 Doughty, A., 541 Douglas, C. B., 471 Downing, J. W., 1653 Duke, M. M., 2027 Dunmur, D. A., 1357 Dunstan, D. E., 1261 Duplbtre, G., 1501 Duxbury, G., 1357 Dwyer, J., 383, 1047 Dyke, J. M., 17 Dziembaj, R., 2099 Eastoe, J., 487, 2497 Easton, C. J., 739 Ebitani, K., 377 Eggins, B. R., 2249 Egsgaard, H., 941 El-Atawy, S., 879 El Baghdadi, A., 13 13 El-Basil, S., 2201 Elisei, F., 279 Elliot, A.J., 831, 837 Endregard, M., 2775 Engberts, J. B. F. N., 1905,2703,2709 Enomoto, N., 1279 Eustaquio-Rincon, R., 113 Ewins, C., 969 Fantola Lazzarini, A. L., Farhoud, M., 2455 Fausto, R., 689 Favaro, G., 279,333 Favero, L. B., 2183 Favero, P. G., 2183 Favre, E., 2001 Fawcett, W. R., 2697 Feliu, J. M., 609 2703,2709 845 727, 423 Fenn, C., 1507 Fernando, K. R., 1895 Fierro, J. L. G., 2125 Filimonov, I. N., 219, Finger, G., 2141 Fisher, I., 2425 Flamigni, L., 2331 Fleischmann, M., 1923 Fletcher, P. D. I., 2743 Flint, C. D., 1357 Fogden, A., 263 Fornts, V., 213 Fracheboud, J-M., 1197, Franci, M. M., 1605, 1740, Franck, R., 667,675 Franke, O., 2821 Freeman, N. J., 751 Frety, R., 773 Frey, J. G., 17, 817 Frostemark, F., 559, 2401, Fujiwara, Y., 1183 Funabiki, T., 2107 Galantini, L., 1523 Galvagno, S., 2803,2809 Gandolfi, R., 1077 Gans, P., 315,2351 Gao,Y., 803 Garcia, A., 2265 Garcia, R., 339 Garcia Fierro, J-L., 1455 Garcia-Paiieda, E., 575 Gautam, P., 697 Gavuzzo, E., 1523 Geantet, C., 193 Gengembre, L., 895 Gerratt, J., 1643, 1672, Gerry, M.C. L., 2601 Getty, S. J., 1689 Ghiggino, K. P., 2845 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 Gillies, D. G., 2345, 2547, Goede, S. J., 327, 1363 Gomez, C. M., 339 Gonplves da Silva, A. M., Gonzalez-Carrefio, T., 2257 Gonzalez-Elipk, A. R., 2257 Goodfellow, J. M., 1415 Gordillo, G. J., 1913 Gouder, T. H., 1285 Goworek, T., 1501 Gray, P. G., 369 Gready, J.E., 2047 Green, M. L. H., 2799 Green, W. A., 83 Grein, F., 683 Grieser, F., 1251 Grifith, W. P., 1105 Grigera, J. R., 2731 Grimshaw, J., 75 Grzybowska, B., 895 Guelton, M., 895 Guilhaume, N., 1541 Guillaume, F., 13 13 Guldi, D. M., 1391 Gulliya, K. S., 953 Gunning, A. P., 2551 Hachey, M., 683 Hadjiivanov, K., 2277 Haeberlein, M., 263 Hakin, A. W., 2027 Hall, C., 2095 Hall, D. I., 5 17 Hall, G., 1 227 1205 1744 253 1 -1673, 1801 267 1 649 i Hallbrucker, A., 293 Halpern, A., 721 Hamnett, A., 459 Hancock, G., 523,1467 Handa, H., 187 Hann,K., 733 Hao, L., 133, 1223, 1909 Harada, S., 869 Haraoka, T., 911 Hardy, J. A., 2171 Harland, P. W., 935 Harper, R.J., 659 Harriman, A,, 697,953, Hams, K. D. M., 1313, Harris, P. J. F., 2799 Harrison, N. J., 55 2627 1323 Jacobs, W.P. J. H., Jacques, P., 2481 Jain, S. K., 2065 Jakobsen, H. J., 2095 Jakubov, T., 783 Jameel, A. T., 625 Janchen, J., 1033,2837 Jancke, K., 2141 Jayakumar, R., 161,2725 Jayasooriya, U. 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D., 1689 Yagci, Y., 287 Yamabe, S., 2561 Yamaji, M., 533 Yamamoto, M., 899,1355, Yamamoto, S., 2021 Yamanaka, I., 451 Yamasaki, M., 869 Yamasu, H., 1537 Yamauchi, N., 1307 Yanagisawa, Y., 2561 Yanes, C., 575 Yang, Z-Q., 947 Yano,H., 869 Yasuda, H., 1183 Yasukawa, A., 2567 Yeh, C-t., 1157 Yoshida, H., 2107 Yoshida, S., 2107 Yoshitake, H., 155 Yotsuyanagi, T., 93,479 Young, R. N., 271,2001 Zamaraev, K.I., 2147 Zambonin, C. G., 1495 Zanotto, S. P., 865 Zecchina, A., 2827 Zhang, J., 2057 Zhang, M., 1233 Zhang,X., 605 Zhang, Z. C., 1335 Zhao, Z., 2467 Zholobenko, V. L., 233, Zhong, G. M., 369 Zielesny, A., 2215 Ziolek, M., 1029 Zubarev, V.E., 721 Zukal, A., 2821 Zundel, G., 843,1095 1839 1678,1807 1581 2453 1680 1979,2423 Yu, J-S., 2283 1047 111 The following papers were accepted for publication between 1st and 31st July 1994: Luminescence from irradiated butadiene solutions: Fluorescence of ally1 radicals? B. Brocklehurst and D. N. Tawn Determination of some electrical parameters for UO,(O,PC,H,) films deposited on a porous support J. Benavente, J. R. Ramos-Barrado, S. Bruque and M. Martinez Complex formation between naphthalene and dimethyl-P-cyclodextrin by sealed heating. Part 1 K. Yamamoto, H. Kawashima, E. Yonemochi and T. Oguchi Preferential solvation in acetonitrile-water mixtures: Relationship between solvatochromic parameters and standard pH values J.Barbosa and V. Sam-Nebot Transition-state theory treatment of capture collisions between ions and symmetric top dipolar molecules J. Niedzielski, J. Turulski and B. Pezler Oxidative coupling of methane over La,O,: Influence of catalyst preparation surface properties and steadyhnsteady reaction behaviour V. R.Choudhary and V. H. Rane Physisorption of argon, nitrogen and oxygen by MCM-41, a model mesoporous adsorbent P. J. Branton, P. G. Hall, S. W. Sing, H. Riechert, F. Schuth and K. K. Unger Reaction of peroxornonosulfate radical with manganese(@ in acidic aqueous solution. A pulse radiolysis study J. Berglund, G. V. Buxton, L. I. Elding, S. McGowan and G. A. Salmon Mean activity coefficients of NaCl in glucose-water and sucrose-water mixtures at 298.15 K J.Wang, W. Liu, J. Fan and J. Lu Conformational properties of buta- 1,3-diene- 1,4-diones (bisketenes). Computational and photoelectron spectroscopic studies T. J. Tidwell, N. H. Werstiuk, J. Ma, M. A. McAllister and D. Zhao Effect of solvent on the reactions of coordination complexes. Part 19. -The base hydrolysis of (ap,Y)-(o-methoxy benzoato)(tetraethylenepentamine)cobalt(~~~)in aquo-organic solvent media A. C. Dash and A. N. Acharya EPR studies of caesium-doped V,O,-Fe,O, catalysts: New evidence for active centres A. Bruckner, G. U. Wolf, M. Meisel, R. Stosser and H. Mehner Ligand-controlled oxidation state ambivalence in copper-quinone complexes. Replacement of N-donor by S-donor ligands favours the copper(1)-semiquinone over the copper(II)-catecholate form K.Wolfgang and J. Rall Characterization of the intermediates formed in the reaction of A1 atoms with H20, H,S and H2Se by EPR spectroscopy H. A. Joly, J. A. Howard, M. Tomietto and J. S. Tse Applications of EPR to study the hydrogenation of ethene and benzene over a supported Pd catalyst. Detection of free radicals on a catalyst surface A. F. Carley, H. A. Edwards, F. E. Hancock, S. D. Jackson, B. Mile, M. W. Roberts and C. C. Rowlands Time-resolved microwave conductivity. Part 1. -TiO, photoreactivity and size-quantization M. R.Hoffman, S. T. Martin, H. Herman and W. Choi Time-resolved microwave conductivity. Part 2. -Quantum-sized TiO, and the effect of adsorbates and light intensity on charge-carrier dynamics M.R. Hoffmannn, S. T. Martin and H. Herrmann Structure and linear dichroism of Langmuir-Blodgett films of uracilic amphiphiles P. Suppan and D. Aeby Co-deposited Pt-WO, electrodes. Part 1.-Methanol oxidation and in situ FTIR studies A. C. C. Tseung, K. Chen and P. Shen Leapfrog transformation and polyhedra of Clar type P. W. Fowler and T. Pisanski Non-isothermal crystallization from liquid solutions: Effect of heat of crystallization on growth rate J. Hsu and B. Liu Derivation of interatomic potentials for microporous aluminophosphates from the structure and properties of berlinite J. Gale and N. J. Henson iv Surface and subsurface acidity of faujasite-type zeolites in relation to their composition: Coupled study of XPS and TPD of ammonia C.Guimon, A. Zouiten, A. Boreave, G. Pfister-Guillouzo, P. Schulz, F. Fitoussi and C. Quet Effects of dry-air calcination the physico-chemical and catalytic properties of HZSM-5 zeolite R. Rudham and A. W. Winstanley Chemisorption of H, and H,-U, on polymorphic zirconia E. Knozinger, K-H. Jacob and S. Benfer Physical properties of liquid water by molecular dynamics simulations D. M. Heyes Investigation of the structure of the columnar liquid-crystalline phase of copper(I1) carboxylates. An FTIR spectroscopic study R. Fausto, M. F. Ramos Moita and M. T. S. Duarte Pressure-induced structural changes in water-in-propane microemulsions J. Eastoe, D. C. Steytler, B. H. Robinson and R. K. Heenan Comparative IR spectroscopy study of low-temperature H, and CO adsorption on Na zeolites A.Zecchina, S. Bordiga, E. Garrone, C. Lamberti, C. 0. Arean, V. B. Kazansky and L. M. Kustov Microwave spectrum of tetrolyl fluoride M. C. L. Gerry and K. D. Hensel Explosive decomposition of gaseous chlorine dioxide J. E. Sicre, A. A. Croice and M. I. Lopez Entropy change in the two-dimensional phase transition of adenine adsorbed at the Hg electrode/aqueous solution interface C. Fontanesi Thermal behaviour and physico-chemical characterization of synthetic and natural iron hydroxyphosphates D. Rouzies, J. Varloud and J. M. M. Millet Intramolecular hydrogen bond and molecular conformation. Part 3.-Effect of temperature and pressure on IR spectra of o-halophenols in dilute solutions S-i.Ikawa and M. Okuyama Rotating-disc electrode voltammetry as a probe of adsorption rates on solid particles in liquids. Applications to Zn(I1) adsorption at the hydroxyapatite/aqueous interface P. R. Unwin, R. D. Martin, M. A. Beeston and M. E. Laing Effects of fluorine substitution on the solvatochromic behaviour of an iron(II)+yanide Schiff base complex J. Burgess, R. C. Lane, K. Singh, B. De Castro and A. P. Gameiro Dos Santos Multiple hard-ellipsoid model for rotationally inelastic collisions A. J. Marks Preparation, structure and vibrational spectrum of the dimethylmethyleniminium ion, including the role of cationic polymers in its formation M. J. Taylor, G. R. Clark, G. L. Shaw, P. W. J. Surman and D. Steele X-Ray investigations on liquid crystalline copolysiloxanes for second-harmonic generation H.Fischer, E. Wischerhoff and R. Zentel The reaction of H,S with OH and a study of the HSO and SOH isomers. High-level ab initio calculations D. M. Hirst and C. Wilson Complexing effects on the excited state of uranyl P-diketonato complexes. Application of the energy gap law H. Tomiyasu, T. Yamamura, S. Iwata and S. Iwamaru Hydrogenation of carbon monoxide and ethene over Ni-doped supported Pt catalysts J. A. Lercher and F. Eder Non-reactive interaction of ammonia and molecular chlorine: Rotational spectrum of the charge-transfer complex H,N ...Cl, A. C. Legon, D. G. Lister and J. C. Thorn Low-temperature kinetics of reactions between neutral free radicals. Rate constants for the reactions of OH radicals with N atoms (103dT/Kd294) and with 0 atoms (1585T/K5294) I.W. M. Smith and D. W. A. Stewart Thermodynamics and kinetics of the reaction of copper(I1) and iron(Ir1) with ultra-small colloidal chalcopyrite (CuFeS,) F. Grieser, E. J. Silvester, 1 !isel, T. W. Healy and J. C. Sullivan V ~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Autumn Meeting: Reactions and Mechanisms for Fine Chemicals To be held at the University of Glasgow on 6-9 September 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Gas Kinetics Group 13th International Symposium on Gas Kinetics To be held at University College, Dublin on 11-15 September 1994 Further information from Dr H.Sidebottom, Department of Chemistry, University College, Dublin Electrochemistry Group with the SCI ELECTROCHEM 94 To be held in Edinburgh on 12-16 September 1994 Further information from Professor D. E. Williams, Department of Chemistry, University College London, 20 Gordon Street, London WClH OM Biophysical Chemistry Group with the Industrial Division Biotechnology Group Peptide + Water = Protein To be held at University College, London on 19 September 1994 Further information frcm Professor J. L. Finney, Department of Physics and Astronomy, University College London, Gower Street, London WClE 6BT British Carbon Group Applications of Microporous Carbons To be held at the University of Leeds on 28 and 29 September 1994 Further information from Professor B. Rand, Department of Chemistry, The University, Leeds LS2 9JT Theoretical Chemistry Group with CCPI Electronic Structure: From Molecules to Enzymes To be held at University College London on 30 November 1994 Further information from Dr P. J. Knowles, School of Chemistry, University of Sussex, Falmer, Brighton BN19QJ 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 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 Universit6 De Lille, France on 16-30 June 1995 Further information from Dr C. Troyanowsky, Division de Chimie Physique, Laboratoire de Chimie Physique, 11 rue Pierre et Marie Curie, 75005 Paris, France British Carbon Group Carbon '96 To be held at the University of Newcastle upon Tyne on 7-12 July 1996 Further information from Dr K. M. Thomas, Northern Carson Research Laboratories, The University, Newcastle upon Tyne NE1 7RU

ISSN:0956-5000    

DOI:10.1039/FT99490BP194

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC.FARADAY TRANS., 1994, 90(18), 2601-2610 2601 Microwave Spectrum of Sulfuryl Chloride Fluoride, S0,CIF : Structure, 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, B.C., Canada V6T 1Zl ~~ The pure rotational spectrum of sulfuryl chloride fluoride, SO,CIF, has been investigated in the frequency range 5.6-24.0 GHz using a pulsed molecular beam microwave Fourier-transform spectrometer. Between 48 and 170 lines of 8 to 26 rotational transitions have been observed for the six isotopomers SO,CIF, SO, 37CIF, 34S0,CIF, 34S0, 37CIF, SO'80CIF and SO "0 37CIF (unlabelled atoms indicate l60,"F, 32S and 35CI) in natural isotopic abundance.The rotational and quartic centrifugal distortion constants have been determined. ro TAP and r,-type I structural parameters have been evaluated. A harmonic force field has been calculated to derive ground-state average and estimated equilibrium geometries. The description of the normal modes in terms of internal coordi- nates is discussed. Chlorine and fluorine hyperfine structures have been resolved, allowing quadrupole coup- ling (including zbcfor the isotopomers containing "0) and spin-rotation constants to be determined. Variations in the chlorine quadrupole coupling constants with different isotopomers have allowed the CI quadrupole tensor to be diagonalized and indicate that its z-axis coincides with the SCI bond. Although sulfuryl chloride fluoride, SO,ClF, has been known for more than 50 years,' there is only one study, by Holt and Gerry, reporting its structure in the gas phase., Using a con- ventional Stark modulated microwave spectrometer, they determined rotational constants, chlorine quadrupole coup- ling constants and an effective distortion constant for each of the two most abundant isotopomers, S0,ClF and SO, 37ClF (unlabelled atoms indicate l60,19F, 32S and 35Cl). By assuming values for r(S0)and L(C1SF) from a comparison with the related molecules sulfuryl fluoride, SO,F,, and sul- fury1 chloride, S02C12, they were able to derive the remain- ing geometrical parameters.Recently Gombler investigated 32/34Sisotopic shifts of the "F NMR frequency of sulfur- and fluorine-containing com- pound~.~From these results he suggested that the SF bond in S02F2 should be longer than that in SO,ClF, in contrast to the conclusions from the microwave studies.More recently a single-crystal X-ray diffraction study of S0,XY (X, Y = C1, F) at low temperatures has yielded the precise geometrical parameters of these compounds in the solid state.4 Raman and IR spectra have also been re~orded.'.~ In addi- tion, Pfeiffer obtained a force field of S0,ClF by transferring force constants and structural parameters from S02C1, and S02F2.7 These force constants reproduced most of the observed vibrational wavenumbers moderately well. Better agreement was obtained when the assignments of two pairs of fundamentals in ref.5 were exchanged. In addition, the earlier assignment of the v6 band' was rejected and it was assumed that this fundamental is overlapped by v9. The assignments of Pfeiffer have been confirmed in a recent reinvestigation of the Raman and IR spectra.' At room temperature the microwave spectrum of S0,ClF is very dense, with many transitions between rotational levels at high J.However, it has recently been shown that, at the low rotational temperatures achievable in a supersonic jet (ca. 1 K), most of these rotational transitions have a very low inten~ity,~making the lines of the rarer isotopomers measur- able. The basic aim of the present work has thus been to measure transitions of several rare isotopic species (containing ''0 and 34S), using a cavity pulsed microwave Fourier-transform (MWFT) spectrometer," into which samples are injected as pulsed supersonic jets.From the resulting rotational constants the complete geometry has been derived. In addition, because of the high resolution available with the MWFT spectrometer, the hyperfine con- stants have been substantially improved. Finally, the harmo- nic force field has been refined using the centrifugal distortion constants. Experimental The spectra were measured in the frequency range 4-25 GHz using a cavity pulsed MWFT spectrometer. Samples were injected as pulsed jets of gas consisting of ca. 0.5-1% S0,ClF in neon at 1500-3000 mbar total pressure; rota- tional temperatures achieved were 51 K.Linewidths as low as 7 kHz full width at half maximum were obtained. It was possible to resolve (at least partially) lines ca. 5 kHz apart. The precision and accuracy of measurements for strong, well resolved lines are believed to be about f0.5 and f1 kHz, respectively. In order to minimize distortions due to overlap effects in power spectra, the frequencies of closely spaced lines were determined by fits to the time-domain ('decay') signals. The sample of S0,ClF was kindly provided by Prof. Dr. H. Willner, Universitat Hanover, Germany. It had been obtained by the combined synthesis of SO,F, and S0,ClF from S02C12 and NaF.13 Results Observed Spectra and Analysis S0,ClF is an asymmetric rotor close to the prolate sym- metric limit (K = -0.988).It has C, symmetry, with dipole components along the a and b inertial axes., The main fea- tures of its spectrum are extensive series of 6-type Q-branches Of the types JK,J-K-JK-l,J-K+l and JK,J-K+l -JK-1, -+ ,. These branches are partially overlapped at low J and K.Further overlap occurs because of the presence of the two abundant C1 isotopes (35/37Cl). In addition, there are several a-and b-type R-branches with sufficient intensity to be easily observed. All transitions show C1 nuclear quadru- pole hyperfine structure, which was partially resolved in the previous microwave study.' At the resolution available with the MWFT spectrometer the "F magnetic hyperfine struc- ture was expected to be observable. Initially, measurements were focused on the region 4-12 GHz.The lo,-Ooo and 1,'-Ooo transitions of SO'ClF and SO237ClF were measured first, in order to refine the chlorine nuclear quadrupole coupling constants. The anticipated "F hyperhe structure was also observed. It was concluded from the optimized microwave pulse length (n/2condition) of these transitions that the a-component of the dipole moment is about twice as large as the b-component. Further transitions involving J < 2 were predicted using the rotational constants of ref. 2,and were located within ca. 250 kHz of the predic- tions. Most of the lines were observable with one experimen- tal cycle. Since the hyperfine splittings due to the C1 nucleus are much larger than those due to the F nucleus, the assign- ments have been made using quantum numbers correspond- ing to the coupling scheme J + Zcl= F1,F1+ IF= F.In preliminary analyses the observed frequencies were fit to the hyperfine constants and the hypothetical unsplit line fre- quencies. The latter were then used to calculate rotational and some approximate centrifugal distortion constants (DJ, DJK,DK),from which further transitions were predicted and measured. Simultaneous fitting of these measured frequencies to the rotational, centrifugal distortion and hyperfine con- stants, using the program SPFIT,14 further reduced the uncertainties of these constants, but the two remaining quartic distortion constants, dl and d2, in Watson's S-reduction, were barely determined, and the precision of the Cl spin-r ot at ion coupling constants was only moderate.Uns pli t lines were given an uncertainty of 0.5 kHz (1.0 when only one Doppler component was observed). Unresolved groups of lines were given an uncertainty of ca. half the calculated split- ting, and each component was weighted according to its pre- dicted intensity; when the. latter was less than 0.65 of the strongest component, it was not taken into account. To improve the constants further, the frequency range of the experiments was extended to 24 GHz, with most mea- surements being of selected transitions between 18 and 24 GHz. Owing to the low rotational temperature of the jet, the line intensities rapidly decreased with J and K,, and only lines with J < 4 and K,< 3 were measured (a total of 155 and 170lines, of 25 and 26 rotational transitions of S02ClF and SOz3'C1F, respectively; a selection is given in Table 1 and a complete list is available from the authors).Inclusion of these transitions in the fits produced well determined values for the rotational, hyperfine, and most of the centrifugal dis- tortion constants. Reasonable values were obtained for dl and d2 despite their small magnitudes. F-Cl spin-spin coup ling constants were barely determined, and their values affected all other constants at most in their last quoted figures; consequently they were omitted in the final fits. The final spectroscopic constants of S02ClF and SO237ClF are presented in Table 2. The standard deviations of the fits, which are also in Table 2, are within experimental error.The constants are essentially uncorrelated, with the absolute value of only a few of the correlation coefficients being larger than 0.5.The largest correlation occurs between A and DK (CU. -0.82). Rotational transitions of the rarer isotopomers 34S02C1F, 34S0237ClF,SO 180ClF and SO l8037ClF, have been mea- sured for the first time in this work. Those of the first two were the dasiest to observe and assign, because 34S is more abundant than l80. Furthermore, because of the proximity of the S atom to the centre of mass, the shifts of the rotation- al constants on substituting 34S for 32S are relatively small. Using the structural parameters in ref. 2,lines of the lll-Ooo transition of both 34S02C1F and 34S0237ClF were found to be within 300 kHz of the predicted values.On the other hand, four sets of candidates were found in the regions pre- dicted for lo,-Ooo.Owing to the small changes in the rota- tional constants, it was expected that the set closest to the predicted frequencies was due to the 34S isotopomers. These J. CHEM. SOC. FARADAY TRANS.,1994, VOL. 90 Table 1 Observed frequency (MHz) of rotational transitions (selection) of S0,ClF and SO237ClF and residuals (kHz) ~~ S0,ClF SO, 37ClF transition" observed o-cu observed 04 101-000 1.5, 2-1.5, 2 5797.4754 -1.46 5645.2228 -1.55 2.5, 2-1.5, 1 5816.6061 -0.45 5660.2999 -0.29 2.5, 3-1.5, 2 58 16.6 135 0.09 5660.3070 -0.07 0.5, 1-1.5, 2 5831.9240 -0.81 5672.3702 -0.95 111-000 0.5, 1-1.5, 2 7971.4398 0.94 7895.9430 0.33 0.5, 0-1.5, 1 7971.4468 1.12 7895.9499 0.43 2.5, 2-1.5, 1 7978.5874 -0.02 7901 S739 0.15 2.5, 3-1.5, 2 7978.5994 -0.28 7901.5857 -0.29 1.5, 1-1.5, 1 7987.5148 -1.33 7908.6 1 12 0.26 1.5, 2-1.5, 2 7987.5233 1.73 7908.6 165 0.12 211-110 2.5, 2-1.5, 1 11315.3372 -1.60 2.5; 3-1.5; 2 11315.3458 0.47 0.5, 0-1.5, 1 11639.2436 0.71 11327.4094 1.40 0.5, 1-1.5, 2 11639.2494 -0.52 11327.4142 -0.99 1.5, 2-2.5, 3 11640.5091 0.23 1.5, 1-2.5, 2 -0.15 220-1 11 0.5, 1-0.5, 1 1 8068.17 19 0.53 3.5, 3-2.5, 2 181 50.5532 1.29 18073.2941 1.61 3.5, 4-3.5, 3 1 8 1 50.5 628 0.15 18073.3033 -0.18 2.5, 2-1.5, 1 18160.7709 0.38 18081.3449 0.7 1 2.5, 3-1.5, 2 18160.7838 -0.24 1808 1.3 576 -0.31 221-110 3.5, 3-2.5, 2 18137.8267 -0.66 18061.1949 -0.75 3.5; 4-2.5, 3 18 137.8400 -1.07 18061.2082 -1.18 2.5, 2-1.5, 1 18146.7658 -0.57 18068.2374 -0.62 2.5, 3-1.5, 2 18146.7838 -1.03 18068.25 19 -1.06 313-220 3.5, 3-2.5, 2 10853.5452 -0.44 101 60.8301 -0.95 3.5, 4-2.5, 3 10853.551 1 1.59 10160.8342 -0.34 2.5, 2-1.5, 1 10874.2296 0.25 10177.1290 0.29 2.5, 3-1.5, 2 10874.2343 -0.8 1 10177.1339 -0.24 4.5, 4-3.5, 3 10887.6855 0.981.39 10187.73684.5, 5-3.5, 4 -0.58 -0.56 1.5, 1-05, 1 10908.3267 0.01 1.5, 2-0.5, 1 10908.3501 -0.57 10204.0230 -0.29 404-313 5.5, 5-4.5, 4 21 116.8318 -0.38 20414.7762 0.18 5.5, 6-4.5, 5 -0.48 -0.19 3.5, 3-3.5, 3 21119.3816 0.14 0.63 3.5, 4-3.5, 4 -0.39 20416.7861 -0.13 413-312 3.5, 3-2.5, 2 23274.9520 -0.38 22652.0250 -0.34 3.5, 4-2.5, 3 23274.9571 -0.19 22652.0304 0.04 4.5, 4-3.5, 3 23275.8735 -0.34 22652.75 10 -0.55 4.5, 5-3.5, 4 23275.8784 -0.06 22652.7564 0.13 2.5, 2-2.5, 2 23290.5785 -0.11 22664.3384 -0.32 2.5, 3-2.5, 3 23290.5846 0.02 22664.345 1 0.27 -J&,,k,,/F'l,F' -F:, F; observed minuscalculated.assignments were confirmed with the resulting values of the C1 quadrupole coupling constants x,, , perpendicular to the C, plane, which, neglecting vibrational effects, should be the same as those of the corresponding 32S isotopomers; the values agreed to four significant figures. The remaining sets of lines were much further away and mostly showed values of xce significantly different from those of the 32S isotopomer.These lines were probably vibrational satellites of the "S iso-topomer; their assignment was beyond the aim of this study. Table 2 Spectroscopic constants' of isotopomers of sulfuryl chloride fluoride, S0,CIF rotational constants/MHz centrifugal distortion constants/kHz 5P A B C DJ DJK DK dl 4 4 pS0,ClF 5080.455 582 (90) 2912.846885 (40) 2899.926 403 (38) 0.47931 (103) 2.0282 (57) -1.5653 (147) 0.00336 (84) 0.00593 (76) so, 37~1~ 5080.472 921 (90) 2834.764 222 (37) 2822.517 563 (36) 0.46064(101) 1.9541 (52) -1.4510 (147) 0.00313 (83) 0.00573 (64) 8 34S02C1F 5078.573 758 (189) 2907.049 659 (63) 2893.493 326 (50) 0.47184 (135) 2.0414 (95) -1.499 (36) 0.00410 (174) 0.00545 (124) 34s0237~1~ 5078.590 843 (224) 2828.571 272 (80) 2815.727 184 (44) 0.4532 (33) 1.9575 (164) -1.410 (48) 0.00288* 0.00573' SO 180C1F 4893.158 903 (172) 2878.723 789 (80) 2823.393 752 (78) 0.4512 (32) 1.8893 (116) -1.430 (42) -0.0229 (27) -0.0063 (19) so 180 37~1~ 4893.084 694 (105) 2801.081 782 (62) 2748.638 373 (59) 0.4334' 1.806' -1.35' -0.0214' -0.00606 chlorine nuclear quadrupole coupling constants/MHz spin-ro ta tion coupling cons tan t s/kHz xaa x-lxal xbb L' M,(Cl) M,(Cl) Mm(C1) M,(F) Mbb(F) Mm(F) standard deviationkHz S0,ClF -76.551 60 (29) -5.10491 (61) 35.723 34 (45) 40.82826 (45) 0.330 (42) 1.860 (46) 1.740 (54) 12.247 (121) 3.252 (134) 8.176 (130) 0.72 SO, 37CIF -60.329 18 (28) -4.028 97 (65) 28.15011 (46) 32.17907 (46) 0.262 (43) 1.497 (44) 1.328 (51) 12.380 (112) 3.447 (117) 8.013 (123) 0.66 34S02C1F -76.574 70 (39) -5.087 38 (78) 35.743 66 (59) 40.831 04 (59) 0.451 (65) 1.920 (68) 1.609 (61) 12.29 (20) 2.81 (22) 8.06 (18) 0.84 34S0237ClF -60.34729 (52) -4.01407 (103) 28.16661 (78) 32.18068 (78) 0.118 (113) 1.501 (88) 1.495 (113) 12.54 (29) 3.48 (31) 7.13 (30) 0.65 SO "OClF -76.60849 (51) 2.17549 (110) 2.223 (32) 39.391 99 (80) 37.21650 (80) 0.051 (129) 1.812 (97) 1.698 (95) 12.24 (23) 6.53 (21) 5.01 (20) 0.88 SO "037ClF -60.37657 (54) 1.71742 (120) 1.768 (40) 31.04700 (87) 29.32957 (87) 0.183 (139) 1.557 (121) 1.360 (118) 12.43 (38) 5.80 (27) 5.19 (25) 0.75 Uncertainties reflect 16.'Fixed at values derived from the force field and other isotopomers, see text.Derived constants. Further transitions of the 34S isotopomers were measured and assigned by boot-strapping, following a similar pro- cedure to that used for the 32S species.Owing to the rela- tively low abundance of 34S, fewer lines were measured (for 34S02C1F and 34S0237ClF, a total of 111 and 71 lines, respectively, of 20 and 12 rotational transitions with J < 4 and K,< 2, of which a selection is given in Table 3). Their spectroscopic constants are in Table 2. For 34S0237ClF it was necessary to fix the constants dl and d, to values derived from the force field, scaled according to ratios of other isotopomers (Table 4). The rms deviations of the fits were again within experimental error. Measurements of the spectrum of SO I80C1F presented a particular challenge for several reasons.First, this isotopomer is less abundant by a factor of ca. 250 than SO,ClF, and a much larger number of cycles was necessary to obtain a satis- factory signal-to-noise ratio. Secondly, its transitions were much more difficult to predict than those of the 34S iso-topomers. This was mainly because the similarities of the masses of 0 and F atoms and the respective bond lengths to the S atom cause a large rotation (ca. 125", Fig. 1) of the b-and c-inertial axes around the a-axis on l80isotopic substi- tution, and small changes in the geometrical parameters had a large effect on this angle of rotation. Thirdly, this iso- topomer should thus exhibit a-, b-and c-type transitions, and it was not obvious a priori which of the latter two should be stronger.Furthermore, the uncertainty of the angle of rota- tion produced uncertainties in the predictions of B and C, and of x-. Fortunately, x-of S0,ClF is small; this is thus also the case for x-of SO '80C1, irrespective of the angle of rotation. Fourthly, because of the dense spectrum, accidental interference by lines of more abundant isotopomers may render the assignment of lines of SO l80C1F more difficult. Nevertheless, a successful assignment has been made. The a-type transition lol-O0, was found within 5 MHz of a pre- diction using the structure in ref. 2. It yielded an estimate of xu. Of the 2-1 a-type transitions 21,-1 was expected to be least affected by the presence of S0237ClF lines. The two strongest C1 hyperfine components, only slightly affected by x-, were found near 11 464 and 11 445 MHz, within 10 MHz of the prediction.With refined B and C values,.additional strong 2-1 a-type lines were easily found; these resulted in a reasonable estimate of x-, so that all strong components of these transitions could be observed. Finally, using distortion constants estimated from a simple force field, detection of 3-2 a-type as well as some b-and c-type transitions was straight- forward. In total, 95 hyperfine components of 19 rotational transitions with J < 3 and K,< 2 were observed; a selection is given in Table 5. From estimates of the optimized micro- \ c' \ \ Ib \\ F / / b ;/ / , Fig. 1 Projection of the S0,F moiety into the bc plane.The prin- cipal axes of S0,ClF (solid lines) and SO '*OCIF (dashed lines) as well as the substituted 0 atom are indicated. (The C1 atom is almost at the origin, below the plane of the paper.) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Observed frequencies (MHz) of rotational transitions (selection)of 34S02C1F and 34S0, "ClF and residuals (kHz) 3734S0,CIF 34~~, ~1~ transition" observed O-Ca observed 04 101-000 1.5, 2-1.5, 2 5785.2406 -1.39 5632.2360 -1.29 2.5, 2-1.5, 1 5804.3 774 -0.20 5647.3179 -0.34 2.5,3-1.5, 2 5804.3845 0.38 5647.3248 0.19 0.5,1-1.5, 2 5819.6993 -1.21 5659.3918 -0.48 1 11-000 0.5, 1-1.5, 2 7963.1 194 0.57 7887.2664 0.31 0.5, 0-1.5, 1 7963.1259 0.28 7887.2737 1.04 2.5, 2-1.5, 1 7970.2712 -0.26 7892.9006 -0.12 2.5, 3-1.5, 2 7970.2834 -0.27 7892.9 127 0.17 1.5, 1-1.5, 1 7979.2044 -0.85 7899.9416 -0.28 1.5, 2-1.5, 2 7979.21 15 0.83 7899.9474 0.28 202-1011.5, 1-0.5,0 11581.8784 2.47 11273.4474* 1.92 1.5, 2-0.5, 1 11581.8897 1.43 11273.4584 0.88 0.5, 0-0.5, 1 11600.9982 -2.27 11288.5210 0.84 0.5, 1-0.5, 1 11601.0110 -0.36 11288.5307 -0.09 2.5, 2-1.5, 1 1 1602.6431 0.42 11289.8 158 0.22 2.5, 3-1.5, 2 2.16 0.2311289.8230 -0.093.5, 3-2.5, 2 11602.6522* -0.63 3.5, 4-2.5, 3 1 1602.6592 0.45 11289.8291 0.24 1.5, 1-1.5,1 11616.3409 -0.07 1.5, 2-1.5, 2 11616.3475 0.71 0.5, 0-1.5, 1 11635.4627 0.81 0.5,1-1.5, 2 11635.4709 1.02 211-110 2.5, 2-1.5, 1 11600.0932 -0.84 11289.9669 0.25 2.5,3-1.5, 2 11600.1008 0.85 11289.9728 -0.14 1.5, 1-1.5, 1 11294.9973 -0.55 1.5, 2-1.5, 2 11295.0027 0.14 2.5, 2-2.5, 2 11610.2997 0.51 2.5, 3-2.5, 3 -0.35 0.5,0-1.5, 1 11615.4226* -0.71 1 1302.0438 0.83 0,5,1-1.5, 2 11616.4308* 0.97 1 1302.0496 -0.31 1.5, 2-2.5, 3 1 1616.6845 0.30 -0.34 1 1303.0420 0.24 1.5, 1-2.5, 2 -0.41 3.5, 3-2.5, 2 1 l305.O475* -1.22 1.96 3.5, 4-2.5, 3 0.5, 1-0.5, 1 11316.5362 -0.07 212-111 2.5, 2-1.5, 1 11573.0880 -0.42 11264.3625 -0.65 2.5, 3-1.5, 2 11573.0975 -0.37 11264.3713 -0.61 1.5, 1-1.5, 1 11580.3828 1.20 11270.1 109* 0.57 1.5, 2-1.5, 2 11580.3876 -1.22 11270.1 149* -2.09 2.5, 2-2.5, 2 2.99 2.5, 3-2.5, 3 11582.0252* 0.33 1.5, 1-2.5, 2 1 1589.3 165 1.12 0.69 11277,1518 0.31 1.5, 2-2.5, 3 0.22 0.5, 0-1.5, 1 11590.5966 -0.05 0.5,1-1.5, 2 1 1590.6063 -0.72 3.5, 3-2.5, 2 11592.2176 -0.05 11279.4420 -0.33 3.5, 4-2.5, 3 1 1592.2241 0.64 11279.4487 1.21 1.5, 1-0.5,0 11596.4601 -1.12 11282.7788 -0.75 1.5, 1-0.5, 1 11596.4675 -0.51 11282.7857 -0.42 1.5, 2-0.5, 1 11596.4800 -0.66 11282.7983 0.28 " See Table 1.wave pulse lengths of the 1-0 transitions (42 condition) it was estimated that pb zpc zpJ2. Using an rI.e structure (uide infu) determined from these five isotopomers, rotational constants of SO l8 037CIF were predicted. All other constants were estimated from the force field and other isotopic data. The high accuracy of these con- stants allowed some transitions to be observed, despite the very low isotopic abundance.The number of cycles required ranged from 400 (ca. 7 min averaging time, Fig. 2), to a J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Comparison of measured vibrational wavenumbers" (cm- ') and centrifugal distortion constantsb (kHz) of S0,ClF with those calcu- lated from the force field SOzCIF so, 37~1~ 34S0,C1F 34~0~37~1~ SO '*OClF SO "0CIF obs. calc. obs. calc. obs. calc. obs. calc. obs. calc. obs. calc. 1230.1 1230.2 1230.1 1218.5 1218.5 1205.6 1205.5 826.5 826.3 826.3 818.3 818.3 822.9 822.8 632.1 632.1 630.7 622.0 620.5 626.5 625.0 503.0 503.0 502.5 500.1 499.6 495.9 495.4 422.0 422.8 415.0 416.0 422.8 415.9 419.5 412.7 295 294.4 292.8 294.0 292.4 290.4 288.8 1469.4 1469.3 1469.2 1448.4 1448.4 1449.8 1449.8 475.9 476.4 476.0 473.8 473.4 469.0 468.6 303 302.0 300.5 301.7 300.2 297.4 295.9 0.47931 0.48 107 0.46064 0.46038 0.47 184 0.47900 0.4532 0.4583 0.4512 0.4583 0.4334' 0.4385 2.0282 2.0199 1.9541 1.9591 2.0414 2.0203 1.9575 1.9591 1.8893 1.9052 1.806' 1.8327 -1.5653 -1.4865 -1.4510 -1.4054 -1.499 -1.4854 -1.410 -1.404 -1.430 -1.390 -1.35' -1.317 0.00363 0.00355 0.00313 0.00313 0.00410 0.00343 0.00288' 0.00288 -0.0229 -0.0123 -0.0214' -0.0114 0.00593 0.00675 0.00573 0.00604 0.00545 0.00681 0.00573' 0.00610 -0.0063 -0.00523 -0.00W -0.00467 * Ref.8. This work. Derived from the force field and scaled according to ratios of other isotopomers.maximum of 15000 (ca. 250 min). The deviation between tion, all other constants were affected to a certain extent experimental and predicted rotational constants was very (Q0.7 kHz), with values closer to those expected, especially in small: 27.9, 2.0 and 1.6 kHz for A, B and C, respectively. the case of the centrifugal distortion and the chlorine spin- Because only a few rotational transitions (eight, with J, K,< rotation coupling constants. Because of the limited input 2 and a total of 48 hyperfine components) were observed, data, the correlations were slightly higher than for the other centrifugal distortion constants were fixed at values derived isotopomers (the highest for SO "0ClF is -0.88 between A from the force field, scaled according to ratios of other iso- and D, ;for SO '*O37ClF -0.68 between B and C, because topomers (CJ: Table 4). A selection of the assigned transitions no distortion constants were fitted). The rms deviations is in Table 5 and the derived constants are in Table 2. became comparable with those of the other isotopomers.The initial fits to the l80isotopomers assumed that effects The contributions of xbc in the K, = 1 stack of SO "OCIF of the off-diagonal C1 quadrupole coupling constants could are for J = 1, ca. 5.51, 3.61 and 0.22 kHz for F, = 0.5, 1.5 and be neglected; however, the rms deviations were ca. 2 kHz, 2.5, respectively; they decrease rapidly for higher J. Although somewhat greater than the measurement uncertainties, and the levels J,, J-,and JK,J-K+ are much closer for K 2 2, also larger than the corresponding values of the other iso- essentially no perturbation was present in these levels topomers (< 1 kHz).In addition, very large deviations (of up (observed and calculated for K < 3 and J Q 4). Inspection of to 7 kHz) occurred for some of the 1 ,,-Oo0 and 1lo-Ooo lines. the matrix elements in the symmetric rotor limit showed that Since the levels l,, and l,, are relatively close, it was for every level connecting certain J,, -,with J,, -,+ , thought that the off-diagonal quadrupole coupling constant through &,& there was a corresponding level connected xbc might be contributing significantly to the frequencies, in through 4zt$d which cancelled the effect. spite of an anticipated small (absolute) value of 52.5 MHz.The only non-zero off-diagonal quadrupole coupling con- Inclusion of xbc in the fits yielded moderately well determined stant for isotopomers with two l60atoms is xab.Its effects values for both isotopomers (CJ: Table 2), of the right order of in the observed spectrum of S02ClF were <1.2 kHz, even magnitude and, as expected from the structure, approx- smaller than those of xbc for the isotopomers with "0. The imately in the ratio of the C1 quadrupole moments. In addi- resulting value, Ixabl= 5.77 (39) MHz, must be viewed cau- tiously, even though it is reasonably close to the expected 3.5,4-2.5, 3 value of 7.43 MHz (see Discussion on hyperfine constants). The standard deviation of the fit is reduced from 0.72 to 0.63 kHz, and all other constants are affected to within less thanA-2.5,2-105, ' twice their uncertainties.Similar results are obtained for the other isotopomers: Ixabl= 4.13 (57), 5.04 (48), 4.61 (72) and 4.73 (63) MHz for S0237ClF, 34S02C1F, 34S0237ClF and SO l80C1F, with the standard deviations reduced to 0.64, 0.79, 0.58 and 0.85 kHz, respectively. Although the absolute values of the correlation coefficients involving xab are small (<0.55), this constant has been omitted from the final fits because of its small contributions. Structural Parameters I I 1 Although precise isotopic data are now available, the deter- I 11 099.8 mination of the geometry of S02ClF was not straightfor- 11 099.7 frequency/MHz ward.The difficulties arose from several sources. Owing to the proximity of the S atom to the centre of mass, and also Fig*2 of the 202-101 transitionOf '*O37C1Fin natural because of the very small b-coordinate of the C1 atom, vibra- isotopic abundance, showing "F and "Cl hyperfine structure, dis- They are shown in com-played as a power spectrum. 400 averaging cycles were used. A stick tional effects are diagram indicates the calculated relative intensities and the line posi- parisons of the A rotational constant of SO2CIF and tions from the decay fit. The lines are Doppler-split by CQ. 57 kHz. S0237C1F, both with 32S and with 34S, where 2606 Table 5 Observed frequencies (MHz) of rotational transitions (selection)of SO "OClF and SO l8O37ClF and residuals (kHz) SO '80C1F so 180 37~1~ transition' observed o-c' observed O-C' 101-000 1.5, 2-1.5, 2 5686.8088 -1.45 5537.6527 -0.78 2.5, 2-1.5, 1 5705.9540 0.16 5552.7418 0.41 2.5, 3-1.5, 2 5705.96 1 1 0.33 5552.7485 0.5 1 0.5, 1-1.5, 2 5721.2835 -0.45 5564.8221 0.42 11 1-000 0.5, 1-1.5, 2 7706.6891 -0.42 7633.9492 0.13 2.5, 2-1.5, 1 7714.5768 0.60 7640.1643 1.28 2.5, 3-1.5, 2 77 14.5869 0.34 7640.1729 -0.70 1.5, 1-1.5, 1 7724.4 178 -1.39 7647.9213 -0.86 1.5, 2-1.5, 2 7 724.4249 1.12 7647.9271 0.25 116000 0.5, 1-1.5, 2 7762.5724 -0.21 7686.8272 -0.66 0.5, 0-1.5, 1 7686.8366* 2.65 2.5, 2-1.5, 1 7770.0144 -0.02 7692.6919 -0.20 2.5, 3-1.5, 2 7770.025 1 -0.58 7692.7022 -0.85 1.5, 1-1.5, 1 7779.3200 0.16 7700.0247 -1.18 1.5, 2-1.5, 2 7779.3257 0.86 7700.03 15 0.76 202-10 1 1.5, 1-0.5, 0 11383.9576 1.34 11083.3633 -0.05 1.5, 2-0.5, 1 11383.9700 0.66 11083.3765 0.69 25, 2-2.5, 2 2.33 2.5, 3-2.5, 3 11385.5992* -1.71 0.5, 0-0.5, 1 11403.0735 0.97 11098.4339 0.21 0.5, 1-0.5, 1 11403.0841 0.04 11098.4441 -0.58 3.5, 3-2.5, 2 11404.7383 0.68 11099.7465 1 .00 3.5, 4-2.5, 3 11404.7436 -0.27 11099.7503 -1.16 2.5, 3-1.5, 2 11404.7519 0.47 11099.7585 1.67 1.5, 2-1.5, 2 11418.4425 -0.54 211-110 2.5, 2-1.5, 1 11445.0859 0.80 11 140.4638 0.27 2.5, 3-1.5, 2 11445.0928 -0.94 11 140.471 1 -0.41 1.5, 1-1.5, 1 11452.1206 0.05 11 146.0083 0.58 1.5, 2-1.5, 2 11452.1264 -0.74 11 146.0136 -0.17 2.5, 2-2.5, 2 1.07 2.5, 3-2.5, 3 11454.3916* -1.29 3.5, 3-2.5, 2 11464.2253 0.09 11155.5506 -0.72 3.5, 4-2.5, 3 11464.2299 -0.57 11 155.5562 0.27 1.5, 1-0.5, 0 11 159.1988 -0.85 1.5, 2-0.5, 1 11468.8792 -0.17 11 159.2170 0.35 0.5,O-0.5, 1 1 1 166.9729 0.84 0.5, 1-0.5, 0 1 1166.9789 -0.69 0.5, 1-0.5, 1 11478.7404 0.58 11 166.9854 -0.28 212-1 11 2.5, 2-1.5, 1 11334.3863 0.82 11035.5450 0.16 2.5, 3-1.5, 2 11334.3921 -0.99 11035.5517 -0.70 1.5, 1-1.5, 1 1 1341.0344 -1.12 11040.7848 0.27 1.5, 2-1.5, 2 11341 .M26 1.31 11O40.7901 -0.15 2.5, 2-2.5, 2 1.03 2.5, 3-2.5, 3 11344.2295* -0.82 3.5, 3-2.5, 2 11353.5195 -0.77 11050.6287 -0.36 3.5, 4-2.5, 3 11353.5247 0.00 11050.6336 0.26 1.5, 1-0.5, 0 11358.7589 -0.53 11054.7514 -0.33 1.5, 2-0.5, 1 11358.7760 0.44 11054.7680 -0.03 0.5, 0-0.5, 1 1 1368.0748 -0.39 1 1062.09 15 -0.61 0.5, 1-0.5, 0 11062.1002 0.96 0.5, 1-0.5, 1 1 1368.0873 -0.84 1 1062.105 1 -0.03 a See Table 1.A(37Cl)> @$Cl), in contrast to the rigid rotor model. In addition, since F has only one stable isotope, it cannot be isotopically substituted. Several approaches have been taken to derive geometrical parameters. These have produced ro and r,-based param-eters,15*16 r,-based a ground-state average (rJ structure" and an estimate of the equilibrium (re)struc-ture.17 For the r,, r,-based and r, structures, least-squares J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 fits were carried out for the six parameters needed to describe the geometry completely. These were chosen to be r(SO), r(SCl), r(SF), 1/2L(OSO),and the angles between the bisector of ~(0S0)and the SCl and SF bonds, respectively.From these were derived the internal coordinates L(OSO), L(ClSO), L(FSO), and L(C1SF) along with their uncer-tainties. The program RU111J'6*'8 was used for the ro and ro-based structures. Initially, ro parameters were obtained by a least-squares fit of the moments of inertia derived from the effective rotational constants of Table 2, all given equal weight. The standard deviation for this fit was 0.0028 ut A', in agreement with cu. 0.003 u A2,expected under these cir- cumstances. In order to give the data more appropriate weight, the rotational constants were weighted according to the inverse squares of their experimental uncertainties (Table 2) for all subsequent structure calculations.The exception was SO ''0 37ClF, where the error bars were increased by a factor of 1.5 because spectral fits of the centrifugal distortion constants were not made. This procedure seems reasonable, because the relative uncertainties are of the same order of magnitude and reflect (roughly) the quality of the data. For the ro and r,-based structures the six parameters mentioned above were fitted to the principal planar moments PA, P, and P,, derived from the rotational constants. Because the C, symmetry of the molecule was a constraint, the values of P, for substitutions in the symmetry plane were omitted from the fit." The standard deviation of the fit is improved slightly (0.0019 u A2) over that obtained earlier.The geometrical parameters are given as the ro structure in Table 6,in com- parison with the approximate ro structure of Holt and Gerry.' There is agreement within the uncertainties of the latter. If vibrational effects could be reduced it should be possible to obtain more reasonable structural parameters and to reduce the standard deviation of the fit. Several r,-based pro- cedures are available to do this. They are based on the equa- tions Po= + E'U and where P and I refer to the principal planar and inertial moments, respectively, and g = a, b or c, to the principal axes of inertia. Although corresponding E'@ and (if9) are different for different isotopomers, they do not in general vary enor- mously. As a result, Rudolph has proposed a first-order approach by introducing isotopically invariant E'~ or E''~ as independent parameters in the fits.'5*'' The resulting struc- tures, called rp,eand r1,&,are identical when the same data are used in the fits: the different names indicate the fitted quantities.l5 An alternative approach is to fit the structural parameters to isotopic differences in principal inertial or planar moments.20 The resulting so-called 'pseudo-Kraitchman' structures, designated rAI or rApas appropriate, are the same as the rIv0and rp,estructures, although the methodology is different.' There are advantages in making a particular choice, however.Since the rI, and rp, fits produce values for E they are well suited for predictive purposes (e.g.to estimate rotational constants of hitherto unobserved isotopomers). Alternatively, the rA1 and rAp fits permit correction for 'Costain's (in which uncertainties in coordinates of substituted atoms are inversely proportional to their absolute t 1 u x 1.661 x lO-*'kg. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 6 Structural parameters" (pm, degrees) of S0,ClF r(w 140.8 (6)/ 140.711 (81) 140.877 (138) 140.876 (171) 140.877 (81) 140.721 (31) 140.543 r(SC1) 198.5 (15) 198.460 (130) 198.556 (125) 198.57 (29) 198.556 (81) 198.957 (52) 198.571 r(SF) 155.0 (20) 155.46 (23) 154.02 (6 1) 154.02 (46) 154.02 (37) 155.231 (95) 154.895 L(OS0) 123.7 (10) 123.941 (125) 123.30 (37) 123.31 (38) 123.30 (35) 124.039 (47) 124.039 L(ClS0) 107.5 (25) 109.177 (69) 108.974 (81) 108.99 (38) 108.974 (51) 108.979 (30) 108.979 L(FSO) 107.5 (20) 106.719 (121) 107.27 (21) 107.27 (21) 107.27 (21) 106.961 (52) 106.961 L(C1SF) 99.0 (30)' 97.906 (90) 98.09 (22) 98.04 (111) 98.090 (137) 97.673 (36) 97.673 See text for further details.Ref. 2. 'Coordinates and uncertainties of F fixed to values of rhp. With non-trivial first and second moment conditions. 'Estimates of equilibrium bond lengths, derived from r,, see text. f Assumed, see text. magnitudes). The method is described in greater detail in ref. 16; examples of these structure calculations were presented previously.' 'e2' Fits were made using the tr,e, rp,cand rap methods. The rr,e fit made to the first five isotopomers measured gave very good predictions of the rotational constants of SO l8037ClF, as indicated in the previous section.The fit to all six iso- topomers using the rp,e method reduced the standard devi- ation to 0.00006 u A' and yielded values for the &IArovibrational parameters of = 0.3577 (107), dB= 0.3017 (64) and dc = 0.3059 (64) u 8'. The error bars obtained for both the structural and the rovibrational parameters are probably artificially small, for the data set with the inclusion of SO'*03'ClF produced larger error bars than the set without it. In the ru fit, following the suggestion of Rudolph,16 Costain's error was accounted for using 6(Pg-P)= 0.003 u A'; where 6 denotes the difference between calculated and measured quantities, P denotes the planar moments of SO,ClF, P'g the corresponding moments for substituted iso- topomers. The resulting structure is presented in Table 6.It is preferred to the rr,e or rp,e structures, because the derived uncertainties are more realistic: in this case inclusion of SO ''0"ClF decreased the uncertainties slightly. The values of the r, and rp, parameters are essentially the same. The r,-based parameters were obtained using Rudolph's least-squares fitting program RU238J.I6 Initially, Typke used Kraitchman's equations to develop a least-squares pro- ~edure.~~ to take The method was extended by R~dolph,'~ symmetry considerations into account.'' In this program planar moments of inertia are fitted to the Cartesian coordi- nates of those atoms which have been substituted.To obtain a complete r,-type structure, positions of unsubstituted atoms, or of atoms close to inertial axes, can be fixed at values determined from alternative methods. In applying the method to S0,ClF several potential diffi- culties must be taken into account. The first two have been alluded to in the first paragraph of this section. In addition, the rotation of the b-and c-axes around the a-axis is ca. 125" on the substitution of one l6O atom by "0.In r,-type fits, vibrational effects on the structure are not compensated for when the rotation angle is so large.25 These effects have been accounted for by introducing the eg obtained from the rp,efit as constants in the r, fit (along with their uncertainties and correlation coefficients). Several calculations of r,-based parameters were carried out, of which the two most reasonable are described in detail.In both cases the coordinates of the F atom were fixed at the rAp values: a(F) = 73.81 (17) and b(F) = 136.57 (24) pm. This assumption is justified because r, structural parameters deter- mined by least-squares fitting of a large isotopic set converge to values from an rp,e or rap Costain's error was employed as described for the rap fit. The resulting geometri- cal parameters are given in Table 6 as r, values: their uncer- tainties are fairly large, mainly because the b(C1) coordinate is very small, thus incorporating a large uncertainty [-0.89 (365) pm]. The advantage of the least-squares method can be demonstrated best for this structure, when the error bars of a fit with five isotopomers (without S018037ClF) are com- pared with those from a fit with six.In the latter case the uncertainties of b(C1) are reduced by a factor of three, those of b(0)and 40)by 30%, and even b(S)is well determined. In the second r,-based fit the first and second moment con- ditions for the a-and b-axis and for the C, plane were also included. These had the effects of giving a well determined value of b(Cl), which agreed with the value from the rdp fit. The resulting geometrical parameters are presented in Table 6 as rl values. They agree with the values from both the rM and r, fits, though they have considerably smaller uncer- tainties. When the first and second moment conditions were taken into consideration to determine the F coordinates, these coordinates did not change with respect to the rAp values, again justifying the assumption made above.Note that accounting for the E# values was important in obtaining consistent r,-type structures: without their use deviations occurred between the r, and rl structures [e.g. 0.25 pm for <SF)]. Deviations were also obtained when the first and second moment conditions were used to locate the F atoms [e.g. 1 pm for r(SF)]. To obtain a ground-state average structure (rhof the main isotopic species, SO,ClF, a harmonic force field was evalu- ated as described in the following section. The harmonic con- tributions to the a-constants were subtracted off the effective rotational constants in Table 2, and the resulting BE were fitted to the geometrical parameters using Typke's program MWSTR.Isotopic variations in the bond lengths were accounted for using the equation17 6r, = 3/2a6(u2) -6K where (u') and K are, respectively, the zero-point mean- square amplitude of a given bond and its perpendicular amplitude correction, both obtained from the force field. The constants a are Morse anharmonicity parameters, which were approximated by values from the respective diatomics. Values of 2.072, 1.646 and 1.892 A-' for SO,26SCl (derived from ref. 27), and SF (derived from ref. 28), respectively, were used. Centrifugal distortion and electronic mass contribu-tions to the a-constant~~~ were neglected.The BB,values were weighted similarly to the BB, values. The resulting rz param-eters are in Table 6. Finally, equilibrium bond lengths were estimated from the r, structure according to ref. 17 rz = re + 3/2a(u2) -K (2) Changes in the bond angles between r, and re were neglected. The result is presented in Table 6 as re. When structural fits to the data of five (without SO l8O37ClF) and six isotopomers are compared, essentially no changes occurred for rAp and r:. ro and r, showed a greater dependence on the isotopic set (but within 1 a). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 High correlation coefficients ( IcijI 2 0.9) were obtained for reproduced moderately well. For the refinement of the force all structure models.The most severe occurred for the ro and field the input data were weighted according to their experi- rz structures [-0.9997 for r(S0) and L(OS0)and -0.98 for mental uncertainties (0.1 or 1 cm-' for the vibrational wave- r(SF) and L(FSO)]. The value of most Icij I decreased for the numbers (cf: ref. 8) and three times the values of Table 2 for rp,c structure; however, E~ and E' are highly correlated the distortion constants). No attempt was made to estimate (0.9993),indicating that some of the high correlations are due anharmonic corrections of the input data.jO 13 force con- to vibrational effects. All the larger correlations are further stants were fitted, chosen because of their greatest sensitivity reduced for rhp, the largest occurring for r(SF) and L(OSO) to the input data with the largest deviations between mea- (0.96) and L(FSO)(-0.96)' respectively.For the r, structure, sured and calculated values (v8 and the distortion constants), inclusion of the data of S018037ClF reduced the corre- as indicated in Table 7. In the iterative procedure employed, lations substantially, but three lcijl were still larger than 0.9 a direct fit was first made, then the values offso, fSF andf,,,,, C0.996 for L(ClSO)/L(ClSF) 3. The inclusion of the first and were slightly adjusted to reproduce vl, v2 and vg better, and a second moment conditions reduced most of the correlations; new fit was carried out. All the resulting non-zero force con- the highest occurred between L(OS0)and L(FSO) (-0.96)/ stants are given in Table 7, along with the potential-energy L(C1SF)(-0.88) with all others smaller than 0.8.distribution. A comparison of the observed spectroscopic constants with those calculated from the final force field is Harmonic Force Field given in Table 4. This force field calculation confirms the assignments made A harmonic force field was calculated for S02ClF in order to in ref. 7 and is thus incompatible with that in ref. 5. However,determine the ground-state average structure r, (uide supra). the description of the vibrational modes in terms of internal Since the force field in the literature7 was obtained by taking coordinates is rather complex because of strong mixing, average force constants of S02C1, and S02F2, we have whereas that given in ref.7 is a simple one, based on the attempted to refine it by fitting the force constants to the potential-energy distribution in terms of symmetry coordi- vibrational wavenumbers of ref. 8 and to the distortion con- nates. vl, v2 and v7 are essentially unmixed and well described stants of this study. The refined force field should permit a as v,(SO,), V(SF) and v,(S02), respectively. Taking also into further confirmation of the assignment proposed by Pfeiffer. account the Cartesian displacement of the normal coordi- The refinement was carried out in terms of internal coordi- nates, v3 and vs can be rationalized as asymmetric and sym- nates, using Christen's programs NORCOR and NCA.29 metric combinations, respectively, of the S02F umbrella Of the ten internal coordinates seven are different [r(SO), deformation and the SCl stretching vibration.This can be r(SCI), r(SF), L(OSO), L(ClSO), L(FSO) and L(CISF)], symbolized as [d,,,(SO,F) + v(SCl)],, and [v(SCl)which result in 34 separate internal force constants (Table 7); + dUmb(SO2F)],, respectively. The remaining vibrations are this number is reduced to 27 when redundancy is taken into rather difficult to describe briefly in terms of internal or sym- account. Owing to the limited input data (see Table 4) not all metry coordinates. of the force constants could be determined independently. The r: structure was used to desribe the molecular Discussion geometry. Initial estimates of the force constants were taken Structure and Force Field from ref, 7; deformation force constants were normalized to 100 pm bond length, whereas in ref.7 the SO bond length Despite differences in approach and the inherent problems of was taken as the reference. With these force constants the calculating structural parameters of S02ClF the structures vibrational wavenumbers and distortion constants were presented in Table 6 are remarkably similar, especially when Table 7 Harmonic force constants" (100 N m-I) and potential-energy distributionb (PED)of S02ClF PED 10.85' 0.88 0.08 0.05 0.94 0.04 3.028' 0.40 0.09 0.54 5.03' 0.05 0.90 0.03 0.04 1.745 0.04 0.03 0.31 0.38 0.04 0.27 1.606' 0.23 0.03 0.12 0.5 1 0.16 1.34 1.788' 0.06 0.06 0.55 0.10 0.03 0.98 0.43 2.254' 0.16 0.04 1.03 -0.16' 0.25 0.028' 0.386 -0.15 0.13 -0.03 -0.056' 0.246' -0.05 0.379 -0.08 0.05 0.47 -0.21 0.09 -0.05 -0.30 0.379' 0.08 -0.28 0.04 -0.05 0.047' 0.489 0.07 0.04 0.15 -0.05 -0.41 0.404' -0.06 -0.06 -0.05 0.06 0.19 -0.36 0.129' -0.06 0.12 0.61 1' 0.18 0.06 -0.66 0.483' 0.15 -0.27 -0.12 0.415 -0.06 -0.04 -0.10 ~SO/OSO,~SO/CISO, ,~SO/FSO,fkp~Deformation force constants normalized to 100 pm bond length ;~SO/SCI,~SO/SF, ~~,CISO ,fSO/CSF &F/OSO &F/CISO fixed to zero, see text.Only contributions 20.03 are given. 'Fitted, see text. Adjusted, see text. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the uncertainties are taken into account.Therefore, any of the structural methods may be regarded as a reasonable approx- imation to the equilibrium structure re. As may be expected,l63l8 the rAp and rk structural parameters are identi- cal within their quoted values. The additional assumptions for r: (fixing the F atom at the rAp coordinates and using the first and second moment conditions) are mutually compat- ible, showing that even for S0,ClF a reliable substitution- type structure is obtainable, despite the problems occurring for each atom. On the other hand, the ro and rz geometries (and therefore re as well) may be less favourable than the others because of the high correlations and a stronger depen- dence of the structural parameters on the isotopic set. and 34S0,37C1F, using the changes in x, and xbb.For iso- topomers containing 37Cl the constants were scaled by the ratio of the xcc values of the appropriate 35/37C1 isotopic pair. According to the r: structure, the a-axis is rotated by -0.023", 0.090' and 0.067", respectively, from that of S0,ClF. Using the method described in ref. 35, the angle between the z-quadrupolar axis and the a-inertial axis is then found to be 4.33 (53)", 3.55 (14)" and 3.53 (26)", respectively, in S0,ClF. The error bars reflect only the uncertainties in xaa and xbb, which are assumed to be much greater than those of the axis rotation in this structural model. These data strongly indicate that the z-axis coincides with the SCl bond, which is at an angle of 3.78 (11)" to the a-axis.This value agrees quite In spite of the limited data available in the previous micro- well with 8,,= 2.93 (20)" derived from xab of S0,ClF. The wave study,, the geometry was very reasonable. The present results are all within the quoted uncertainties in ref. 2. Note, however, that the angle of rotation of the b-and c-axes on the substitution of one l60atom by "0, which is very sensi- tive to the structure, is 123.0', while that from the structure in ref. 2 is 130.1". The bond lengths and stretching force constants of S0,ClF fit well in the series of related sulfuryl compounds (Table 8) and are as expected from the vibrational spe~tra.~,~,~~ Com-paring the 32/34Sisotopic shifts on the "F NMR frequency of a variety of sulfur fluorine compounds, Gombler suggested the SF bond in S0,ClF to be shorter than in S0,F,;3 this does not agree with both the previou~~*~,"*~~ and the present results.The bond angles follow mostly the expected trends in the series of S0,XY (X, Y = C1, F) compounds. Whilef,,, of S0,ClF was fixed between the values of SO,Cl, and SO,F,, the situation is less obvious for the other bending force con- stants (cf: Table 8). This is mainly due to the fact that for S0,ClF a greater number of interaction force constants has been fitted. Although some changes in the structural param- eters occur by going from the gas phase to the solid phase, data of S0,XY (X, Y = C1, F) from both phases agree well (see Table 8 and ref. 4). Hyperfine Constants The principal values of the C1 quadrupole coupling tensor have been evaluated.For isotopomers with C, symmetry xcc = xyy (representation 1'). Thus xcc should be the same for the isotopic pair S02ClF/34S0,ClF, as well as for the pair S0237C1F/34S0,37C1F. In both cases small deviations are within twice the combined uncertainties. Furthermore, for both pairs of isotopomers, S0,ClF/S0,37ClF and 34S02C1F/34S0,37ClF, the ratios of xcc , 1.268 783 (32) and 1.268 806 (49), respectively, agree very well with the expected value of 1.2688773 (15).34 To determine I,, and xxx the pro- cedure set out in ref. 35 was applied to the species retaining C, symmetry upon substitution, namely S0,37C1F, 34S0,C1F Table 8 Structural parameters (pm, degrees) and diagonal force con- stants" (100 N m-') of selected sulfuryl halides S02F2 SO,ClF, SO,Cl, r(S0)lfSo 139.7p1.22 140.9/ 10.85 141.8/10.53 r(SCl)If,C, 198.6/3.03 20 1.2/2.24" r(SF)IfSF 153.0/5.24 154.0/5.03 L(OSO)~~,~~122.6/1.90 123.3/1.75 1 23.5/ 1.60 L(ClS0)If,,,O L( FSo)/!fFSO L(XSY)If,s, 108.6/1.94 96.7/2.17 109.O/ 1.6 1 107.3/1.79 98.1/2.25 108.0/1.76' 100.3/1.90 Deformation force constants normalized to 100 pm bond length.r, structural parameters, ref. 33; force constants, ref. 7. This work, I: structural parameters. rgstructural parameters, ref. 32; force con- stants, ref. 7. e Exchange of wavenumber assignments of v, and vg leads to values off,,, = 2.45 andfc,,o = 1.55; see ref. 39. principal values of the quadrupole tensor are thus x,, = -77.045 (30), xxx = 36.217 (30) and xyy = 40.828 (2) MHz.The asymmetry parameter q = 0.0599 indicates a moderately cylindrically symmetrical SCl bond. The value for xzr agrees well with those derived from solid-state NQR measurements (78.7 MHz).~~Similar values were derived for other sulfur- chlorine compounds: x,, = -74.4 MHz and q = 0.085 for S0,Cl,,37 xzz= -71.5 MHz and q = -0.132 for S0,C1NC0,38 x,, = -63.8 MHz and q = -0.014 for SOC1, ,39 and x,, = -79.7 MHz, q = 0.200 for SCl, ?' The variations in x,, reflect the differences in the bond length^,^' those in q the different amounts of 7c bonding.42 The values of xii (i = a, b, c) for SO'80ClF and SO'8037ClF can be evaluated from the principal values.For the former, using the rk parameters, the results are xaa = -76.646, Xbb = 39.132 and xcc = 37.514 MHz. Assuming the SCl bond to be the z-axis, one gets xzz= -60.723 and xxx= 28.544 MHz for S037ClF, so that the results for the latter are xaa = -60.401, xbb = 30.829 and xcc = 29.572 MHz. For both isotopomers the agreement with the experimental values in Table 2 is reasonable. The values of x-and xbc of SO 180C1F and so 37ClF have been used to calculate the angle ex, in these iso- topomers. The values of 121.96 (17)" and 122.05 (27)" for SO 180C1F and SO l8037ClF, respectively, are very close to the angle of rotation around the a-axis on substitution of one l60atom by l80: 123.23' and 123.30", respectively, as might be expected from the quadrupole analysis given above.Although the magnitudes of the nuclear spin-rotation con-stants are much smaller than those of the C1 quadrupole coupling constants, they are in general well determined. The only exceptions are the values of M,,(Cl) for some of the rare isotopomers. To a first-order approximation, these constants are proportional to gN Bi, where gN is the nuclear g-factor of the respective nucleus and Bi the corresponding rotational con~tant.~'Since gN(F) is CQ. 5.257 and gN(3sc1) is ca. 0.548,42 it is reasonable that the values of M,AF) are considerably larger than their corresponding Mi,(Cl) values. However, while for M, of S0,ClF the ratio is ca. 4.7, M,,(Cl) and Mbb(F) are much smaller than expected from this simple cal- culation.This approach should hold strictly for the ratios of Mii (i = a, b, c) of S0,ClF and S0,37C1F for both nuclei (F and Cl). The experimental ratios of 1.26 (44),1.24 (7) and 1.31 (9) for Ma,, Mbb and M,, , respectively, of chlorine and 0.989 (19), 0.94 (7) and 1.020 (26) for those of fluorine are in moderate-to-good agreement with the expected values of 1.201, 1.234, 1.234, 1.000, 1.0275 and 1.0274, re~pectively.~' Similar agreements are obtained for the 34S isotopomers. Given the large rotation of the principal axis system around the a-axis upon l80substitution, the larger deviations of Mbb(F) and M,,(F) are not unexpected. Flygare has shown that the spin-rotation constants can be accounted for by two terms.43 One, a nuclear term, depends 2610 Table 9 Nuclear and electronic contributions (kHz) to the experi- mental spin-rotation constants (kHz) of S0,ClF nuclear electronic experimental contribution contribution ~~ M,(CO 0.330 -0.101 0.413 Mbb(C1) 1.860 -0.390 2.250 M,, (C1) 1.740 -0.391 2.131 M,(F) 12.247 -7.520 19.767 Mbb(F) 3.252 -1.636 4.888 M, (F) 8.176 -5.062 13.238 only on the molecular geometry; this has been evaluated for all components using the ri structure, and has been sub- tracted from the experimental values to give the electronic contributions (Table 9).Compared with the electronic contri- butions, the nuclear ones are quite large for the F nucleus and somewhat smaller for Cl. The ratios M, : M,, :Mcc for both contributions and for both nuclei are roughly those of the experimental spin-rotation constants.Similarly, the "F spin-rotation constants have been taken to calculate the average paramagnetic shedding cf) of the F atom.,, With the rk structure cf) = -589.8 ppm is obtained. The experimental chemical shift of S0,ClF is -99.1 ppm relative to CC1,F.43 With the absolute shielding of 188.7 ppm for F in CC13F,45 this gives CT:;) = 89.6 ppm. The diamagnetic shielding is calculated by ahF)= 0:;) -t~$ = 679.4 ppm. This value is comparable to that of SiF, (655 ppm) and the calcu- lated values for SF, (665 and 681 ppm for Fa, and Feq, respectively), but is smaller than that of SF, (747 ppm), and larger than that of NaF (524 ppm) or KF (553 ppm)?6 By analogy to "F, a value gP= -889 ppm has been derived for 35Cl in SO,ClF, similar to oP= -989 ppm in SOCl, .39 We are not aware of any 35Cl NMR measurement of S0,ClF which can be used for comparison with this value.We are indebted to Prof. Dr. H. D. Rudolph, Ulm, for pro- viding his structure programs, for many suggestions on their use, and for a preprint of ref. 16. We would also like to thank Dr. W. Jager for help during the experiments, Dr. D. Chris- ten, Tubingen, for help in using his force field programs, and Prof. Dr. H. Willner, Hanover, for providing the sample. Funding by the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. References 1 H. S. Booth and C. V. Hermann, J. Am. Chem. SOC.,1936,58,63.2 C. W. Holt and M. C. L. Gerry, Chem. Phys. Lett., 1971,9,621. 3 W. Gombler, 2.Naturforsch. B Chem. Sci., 1985,40,782. 4 D. Mootz and A. Merschenz-Quack, Acta Crystallogr., Sect. C 1988,44,924. 5 R. J. Gillespie and E. A. Robinson, Spectrochim. Acta, 1962, 18, 1473. 6 T. Birchall and R. J. Gillespie, Spectrochim. Acta, 1966,22,681. 7 M. Pfeiffer, 2. Phys. Chem. (Leipzig), 1969, 240, 380, and refer- ences therein. 8 N. C. Craig and K. Futamura, Spectrochim. Acta, Part A, 1989, 45, 507. 9 N. Heineking, W. Jager and M. C. L. Gerry, J. Mol. Spectrosc., 1993,158,69. 10 T. J. Balle and W. H. Flygare, Rev. Sci. Instrum., 1981,52,33. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 11 Y. Xu, W. Jager and M. C. L. Gerry, J. Mol. Spectrosc., 1992, 155,403.12 J. Haekel and H. Mader, Z. Naturforsch., A. Phys. Sci., 43,203. 13 Ed. G. Brauer, Handbuch der Praparativen Anorganischen Chemie, Enke Verlag, Stuttgart, 1975, 3rd edn., Bd. 1. 14 H. M. Pickett, J. Mol. Spectrosc., 1991, 148, 371. 15 H. D. Rudolph, Struct. Chem., 1991,2,581. 16 H. D. Rudolph, in Advances in Molecular Structure Research, ed. I. Hargittai and M.Hargittai, JAI Press, Greenwich (USA), vol. 1, to be published. 17 (a)K. Kuchitsu, J. Chem. Phys., 1968, 49,4456; (b)K.Kuchitsu, T. Fukuyama and Y. Morino, J. Mol. Struct., 1968, 1, 463; (c) 1969,4,41. 18 K. Epple and H. D. Rudolph, J. Mol. Spectrosc., 1992,152, 355. 19 H. D. Rudolph, J. Mol. Spectrosc., 1981, 89, 460 and references therein. 20 R. H. Schwendeman, in Critical Evaluation of Chemical and Physical Structural Information, ed.D. R. Lide and M. A. Paul, Academic Press, New York, 1976, pp. 1-67. 21 (a)C. C. Costain, Trans. Am. Crystallogr. ASSOC., 1966, 2, 157; (b) B. P. van Eijck, J. Mol. Spectrosc., 1982,91,348. 22 (a) M. LeGuennec, G. Wlodarczak, W. D. Chen, R. Bocquet and J. Demaison, J. Mol. Spectrosc., 1992, 153, 117; (b) M. LeGuen- nec, G. Wlodarczak, J. Demaison, H. Burger, M. Litz and H. Willner, J. Mol. Spectrosc., 1993, 157,419. 23 V. Typke, J. Mol. Spectrosc., 1978,69, 173. 24 H. D. Rudolph, J. Mol. Spectrosc., 1981,89,430. 25 L. Nemes, in Vibrational Spectra and Structure, ed. J. R. Durig, Elsevier, Amsterdam, 1984, vol. 13. 26 K. Kuchitsu and Y. Morino, Bull. Chem. SOC.Jpn., 1965,38,805. 27 E. Tiemann, H. Kanamori and E. Hirota, J. Mol. Spectrosc., 1989,137,278. 28 Y. Endo, K. Nagai, Ch. Yamada and E. Hirota, J. Mol. Spec-trosc., 1983,97,213. 29 D. Christen, J. Mol. Struct., 1978,48, 101. 30 (a) H. J. Becher, Fortschr. Chem. Forsch., 1968, 10, 156; (b) H. S. P. Muller and H. Willner, J. Phys. Chem., 1993,97,10589. 31 E. A. Robinson, Can. J. Chem., 1963,41,3021. 32 M. Hargittai and I. Hargittai, J. Mol. Struct., 1981, 73,253. 33 K. Hagen, V. R. Cross and K. Hedberg, J. Mol. Struct., 1978,44, 187. 34 J. H. Holloway, Ph.D. Thesis, Harvard University, 1956. 35 N. Heineking and H. Dreizler, Z. Naturforsch., A Phys. Sci., 1992,47, 51 1. 36 (a)R. M. Hart and M. A. Whitehead, Trans. Faraday SOC., 1971, 67, 3451; (b) I. P. Biryukov and A. Ya. Deich, Zh. Fiz. Khim., 1972,46,2385; Russ. J. Phys. Chem., 1972,46,1362. 37 I. Merke and €3. Dreizler, Z. Natuforsch., A Phys. Sci., 1992,47, 1153. 38 0.L. Jo, J. D. Graybeal, F. J. Lovas and R. D. Suenram, J. Mol. Spectrosc., 1992,152,261. 39 H. S. P. Muller and M. C. L. Gerry, to be submitted. 40 I. Merke and H. Dreizler, Z. Naturforsch., A Phys. Sci., 1992,47, 1141. 41 V. E. Bel'skii, V. A. Naumov and I. A. Nuretdinov, Dokl. Akad. Nauk SSSR, 1974, 215, 355; Dokl. Phys. Chem. Proc. Acad. Sci. USSR, 1974,215,260. 42 W. Gordy and R. L. Cook, Microwave Molecular Spectra, Wiley, New York, 3rd edn., 1984. 43 W. Flygare, J. Chem. Phys., 1964,41,793. 44 P. A. W. Dean and R. J. Gillespie, J. Am. Chem. SOC., 1969, 91, 7260. 45 D. K. Hindermann and C. D. Cornwell, J. Chem. Phys., 1968,48, 4148. 46 (a)J. Mason, J. Chem. Soc., Dalton Trans., 1975, 1426, and refer- ences therein; (b) Adv. Inorg. Chem. Radiochem., 1976, 18, 197; (c) Adv. Inorg. Chem. Radiochem., 1979,22, 199. Paper 4/01862A; Received 29th March, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002601

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2611-2616 261 1 Spectroscopic Investigations on lminophosphanes and Methylenephosphanes G. David, V. von der Gonna and E. Niecke" Anorganisch-Chemisches lnstitut der Universitat Bonn, Gerhard-DomagkStr. I, 53l21-BonnI Germany T. Busch and W. W. Schoeller* Fakultat fur Chemie der Universitat Bielefeld, Postfach 10 01 3 I, 33615 Bielefeld, Germany P. Rademacher lnstitut fur Organische Chemie der Universitat Essen HGS, Universitatsstrasse 57,4514 I Essen I, Germany According to qualitative theoretical considerations, P'" double-bonded systems possess two energetically closely spaced frontier orbitals. This concept, which explains the reactivity of these species, has been investi- gated experimentally via a systematic study of the photoelectron spectra of a selected variety of substituted iminophosphanes.The assignment of the bands is assisted by UV spectroscopic investigations and quantum chemical calculations at MNDO and a6 initio levels of theory. The latter are of double-[ quality and were per- formed at SCF and MCSCF levels of sophistication. Since the pioneering studies of Dimroth and Hoffmann on phosphamethane cyanines,' PIr1 double-bonded systems (Scheme 1) have been investigated in detail as documented in a pertinent review.2 While the preparative aspects of this new class of compounds are now becoming well understood, reports on spectroscopic details are rare.3 Qualitative theo- retical arguments reveal two energetically closely spaced fron- tier orbitals in the PI1' double-bonded compounds.This is depicted schematically in Scheme 2 for the case of the parent iminophosphane (Scheme 1, X = NH). For this system the highest occupied molecular orbital (HOMO4) is a orbital constituted mainly by the lone pair at the phosphorus. The molecular orbital lower in energy is the A orbital of the PN double bond. In accordance with qualitative theory' such an orbital pattern dictates carbenic behaviour to the imin-ophosphanes. The predictions are supported by a variety of experiment a1 investigations. 3a The fact that a simple frontier orbital theory on the reacti- vity of PIr1double-bonded systems exists while the factors that influence the levels of the two closely spaced frontier orbitals are only poorly understood prompted us to investi- \ /P=X x=C ,N\ \ Scheme 1 H @/ /P=NH x.x* x* -4-b-Scheme 2 gate this aspect in more detail by various spectroscopic tech- niques. In the present work we report photoelectron spectroscopic and UV spectroscopic investigations on a selected variety of imino- and methylene-phosphanes. In particular, we will be concerned with a detailed investigation of the substituent effects on the spectroscopic properties of these compounds. The experimental findings will be supplemented by quantum- chemical calculations at ab initio and semiempirical levels of sophistication. On this basis, a more clearcut interpretation of the spectroscopic properties is possible. Theoretical Procedures The ab initio calculations were carried out at double-[ level.The corresponding basis sets were constructed from Huzin- aga primitive functions6 At times they were augmented with polarization functions at the atoms. For the ab initio calcu-lations the following basis sets were used. Basis I: P, Si (1 Is, 7p) in the contraction [5, 6 x 1/4, 3 x 13 + Id ([d = 0.5; 0.4); N, C (8s, 4p) in the contraction [5, 3 x 1/3, 13 + Id (rd = 0.95; 0.8); H (4s) [3, 13. Basis 11: P (lls, 7p) [5, 6 x 1/4, 3 X 13 4-2d ([d(l, 2) = 0.96; 0.32); N (9s, 5p) [5, 4 X 1/3, 2 X 13 + 2d ([d(l, 2) = 1.654; 0.469); c (9S, 5p) [5, 4 X 1/3, 2 x 13 + 2d ([d(l, 2) = 1.097; 0.318); H (4s) in the contraction [3, 13 + 2p([d(1, 2) = 1.20;0.30). The SCF calculations were performed with the Karlsruhe version of the Columbus set of programs.' The energy opti- mizations of the structures were directed with the Murtagh- Sargent algorithm.8 All energy minima were obtained with an accuracy of 0.1 pm (0.05') and subjected to vibrational analysis within the harmonic approximation.On this basis it was ascertained that the closed species are true energy minima on the electronic hypersurface. At the SCF geome- tries, additional correlation calculations were performed including all single and double excitations from the ground state (SDCI). MCSCF'" calculations were carried out at the parent compounds methylene- and imino-phosphane. Com- plete active space (CAS-SCF) was included in the energy optimizations at the MCSCF level.Finally, the results were refined by multi-reference CI calculations (MRC19b*'). All correlation calculations were performed with the program system MOLPRO.'? For the larger molecules ab initio calcu-lations of this accuracy were not possible. For these cases, i.e. the heavily substituted n-systems, the assignment of bands in the photoelectron spectroscopic investigations was assisted with energy-optimized MNDO calculations. lo Experimental All of the various substituted methylene-and imino-phosphanes were synthesized according to literature pro- cedures.2*'' UV Spectra The UV spectra were recorded at room temperature with a Kontron UVIKON spectrometer. The compounds dissolved in heptane were placed in quartz cuvettes (d =0.1 cm) and their spectra were taken at intervals and standardized against that of pure solvent.The concentration of the samples varied from lop3 to mol 1-'. The samples were prepared under an atmosphere of nitrogen, since the compounds are sensitive to moisture and oxidation. The solvents were, in addition, dried by refluxing over lithium aluminium hydride for several days. After purification, the solvents were stored over molecular sieves (4 A). Before use, the solvents were degassed via the frozen pump technique. The maxima of the bands were determined with an accu- racy of &1 nm. The error in the molar absorption coefficients is within &15%. Photoelectron Spectra The photoelectron spectra were recorded on a Leybold-Heraeus UPG 200 spectrometer with He I excitation (21.21 eV).Calibration of the spectra was performed before and after each measurement with an Ar-Xe gas mixture. The determined vertical ionization energies are the average of several measurements. The accuracy of determination was f50 mV. Results and Discussion Vertical Ionization in Iminophosphanes At the beginning of our study we report theoretical aspects of the ionization energies of the parent compounds imino- and methylene-phosphane. For this purpose we employed high- quality ab initio calculations. From a quantum-chemical viewpoint, ionization energies can be computed at various levels of sophistication: (a) on the basis of Koopmans' theorem12 and (b) by separate evaluation of the energies of the ground states and the corresponding radical cations; after correction for electron corrleation, the energy differences then refer to the vertical ionization energies of the molecules under consideration.Procedure (b) is, in general, more accurate for .the quantum-chemical evaluation of ionization energies. We have probed in detail the assignment of bands for the parent compounds with both procedures. The energy-optimized structures obtained at the MCSCF level (basis 11, CAS-SCF) for the parent compounds methylene- and imino-phosphane (cis and trans) are sum- marized in Fig. 1. The geometrical parameters of these compounds have alreddy been determined by previous investigators at the SCF J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 119.2 H 169.3 &/l09.8 109.6 H 159.7 &/103.0 P=N 142.3/4 H 99.0 158.7 P=N 144.0/# %\102.6 H 105.6 117.1 H Fig. 1 Ab initio MCSCF geometries of methylene- and imino- phosphane; bond lengths (angles) in pm (degrees) level,13 and their values agree with ours. However, as expected, the n bonds at the MCSCF level are slightly longer than those obtained at the SCF level. Based on the energy- optimized geometries, ionization energies were computed at various levels of sophistication. The results are collected in Table 1. The photoelectron spectrum of the parent methy-lenephosphane has been reported. l4 In the experiment, the first two ionization energies (lowest in energy) appear at 10.30 and 10.70 eV.This is in good accord with our best results at the MCSCF/MRCI level (see Table 1). Our calculations place the 7c orbital slightly below the c orbital in energy. In con- trast, we predict for the parent iminophosphane a sizeable energy between the two lowest ionization energies. In con- trast to methylenephosphane, the first ionization is due to electron loss from the c orbital and the second to that from the 7c-orbital. This is in accord with previous qualitative pre- dictions.' Ionization Energies in Substituted Imino- and Methylene- phosphanes Irninophosphanes,R'P=NR A representative photoelectron spectrum of a substituted iminophosphane is shown in Fig. 2. The first two lowest- energy ionization bands are strongly separated [appearing at 7.88 eV (Ei, and 9.58 eV (Ei,2)], which agrees with the results of the quantum-chemical calculations.A variety of substituted iminophosphanes have been measured and the resulting ionization energies are collected in Table 2. Quantum-chemical theory predicts ionization either from the 71 orbital (of the PN double bond) or from the c orbital (n) at phosphorus. As will be shown (vide infra)assignment of the orbitals is possible by utilization of a linear relationship between UV and photoelectron (PE) data. The UV spectra of the iminophosphanes show two well separated bands in the short-wave region, one of high inten- Table 1 Computed vertical ionization energies (in eV) of the parent methylene- and imino-phosphanes compound Ei(a) Ed.) method HPNH (trans) 9.184 (10a') 11.386 (2a") SDCI" 8.835 10.726 8.562 10.472 HPNH (cis) 9.407 10.753 MCSCFb MCSCF/MRCIb MCSCFb MCSCF/M RCI MCSCF~ MCSCF/MRCIb ~ ~ ~ ____~~ 9.687 11.071 -kMOLPRO-89 is a package of ab initio programs written by H. J.HPCH, 9.427 9.262 'tyerner aiid P. J. Knowles, with contributions from J. Almlof, R. 10.193 9.935 .knos, S. Elbert, W. Meyer, E. A. Reinsch, K. Pitzer and A. Stone.' 3.E .arevim of the various methods see ref. 9(d). 'Basis I level; Basis I1 level. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Vertical ionization energies (in eV) of iminophosphanes, RP-NR' ~ CEt, But 7.88 (n) 9.58 (n) this work But CEt , 7.94 (n) 9.59 (n) 10.00 this work But Bu 8.10 (n) 9.70 (n) 3(4N(SiMe,), SiMe, 8.10 (n) 8.75 (n) 3(4N(SiMe,), But 8.05 (n) 8.48 (n) 3(4 TmP SiMe, 7.85 (n) 8.25 (n) this work N(Bu')SiMe, Bu' 7.89 (n) 8.06 (n) 9.76 (n) 3(4NPri Bu' 7.90 (n) 7.99 (n) 3(4Bu' TmP 7.23 (n) 7.65 (n) this work But NMe, 7.66 (n) 8.32 (n) this work TmP N(SiMe,), 7.32 (n) 8.08 (n) 8.89 (n) this work CP* But 7.44 (na) 8.13 9.33 this work CP* NMe, 7.03 7.68 8.10 9.25 this work CP* N(SiMe,), 7.11 7.96 9.07 this work R :Tmp, tetramethylpiperidyl ;Cp*, pentamethylcyclopentadienyl. >C-P=N-Cf >C-P=N-Cf 0 >C*N-PP= NC~ >C-P=N-NC2f .,--,A1O-1 ' ' ' ' ' I ' ' -1.0 -0.5 0.0 0.5 1.0 1.5 -Ei(n)l/eV I ,8 10 12 14 16 Fig.3 Relationship between ionization energy differences and the EJeV corresponding electron excitations in the iminophosphanes, RP=NRFig.2 Photoelectron spectrum of the iminophosphane, Et,C-P-N-Bu' sets of quantities are linearly related.I5 A corresponding plot sity and the other of low intensity (Table 3), referring to n-n*, for the case of the iminophosphanes is shown in Fig. 3. Hence o-n* transitions, respectively. The latter has a low molar on the basis of the linear correlation, the first two bands in absorption coefficient. It is clear that the the UV data refer to the PE spectrum can be assigned unequivocally; the corre- energy differences between ground and excited states of the sponding results are collected in Table 2. neutral molecule, while the photoelectron spectroscopic mea- The trend in the ionization energies is obvious.An amino surements refer to energy differences between ground states of substituent affects the n orbital by conjugation, lowering the the corresponding ionized forms. It has been shown that both Table 3 UV data of iminophosphanes, RP-NR' 4naJnm (E,,,,J~o-~1 mol-' cm-') R R' n-n* n-n* ref. CEt, Bu' 230 (1 1.7) 440 (0.25) this work But CEt, 235 (10.8) 448 (0.21) this work B u' Bu' 233 (12.0) 431 (0.25) 3(4N(SiMe,) SiMe, 262 (6.4) 354 (0.27) 3(4N(SiMe,), But 267 (5.6) 348 (0.20) 3(4 '4 TmP SiMe, 276 (5.4) 354 (0.26) this work N(Bu')SiMe, Bu' 278 (4.9) 345 (0.21) 3(4NPri Bu' 315 (4.6) 340 (s) 3(9 But ButBut TMP 318 (3.0) 340 (0.39) this work \ \ Pr;N \ (Me3si)p\But NMe, 310 (4.7) 345 (0.50) this work P= N P= N P=N P= N TmP N(SiMe,), 304 (6.0) 398 (0.72) this work \ \ \ \ CP* NMe, 264 (2.7) 346 (0.50) this work me2 But But But CP* N(SiMe,), 271 (2.6) 341 (0.50) this work Scheme 3 ionization, which is stronger in magnitude at nitrogen than at phosphorus.This, in fact, is due to the larger coefficient of the n orbital at N compared with that at P. Hence conjugation with an attached amino group exerts stronger destabilization (energy lowering of the n orbital) at N than at P. Note the different action of the disilylamino group compared with the dialkylamino substituent; this is illustrated in Scheme 3. The Si,N substituent is a less effective n donor than the C,N sub- stituent, owing to the tendency of a silyl group to delocalize the lone-pair orbital at the amino nitrogen atom.As a conse- quence, the Si,N group is less effective for n donation into the PN double bond. In order to confirm these assignments of the PE bands we computed the electronic properties of various model substi- tuted iminophosphanes R’P = NR2 (basis I level), in their trans conformations and with a conjugating (planar) amino group. The geometrical parameters obtained are collected in Table 4. Again we have determined the two vertical ioniza- tion energies from separate computations of the various ionized states [method (b)].The results of these investigations are listed in Table 5. The calculated data confirm the experi- mental assignments. Interestingly, for the case of dial-kylamino (disilylamino) groups at nitrogen, the theory always predicts the a orbital to be below the n orbital in energy, which is in accord with the stronger n-donation ability of an amino group at nitrogen than at phosphorus.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Pentamethylcyclopentadienyl exhibits a characteristic PE spectrum. The first ionization band of RPNR (R= CP*, R’ = Bu‘) appears at almost the same energy as that of cyclo- pentadiene, Ei, = 7.48 (n,),Ei,, = 9.54 eV (n1).l6Hence it is assigned to ionization of the n2 orbital of the diene unit. According to MNDO calculations, the n orbitals of the cyclopentadiene system mix strongly with the central x bond in all of the pertinent Cp compounds.Consequently, further assignment of this band to local contributions is not possible. Methylenephosphanes RP=CR’R” We have also recorded PE spectra of a variety of methylene- phosphanes. The results are listed in Table 6. The assignment of bands was again performed via a linear correlation of PE and UV data. The amino compound exhibits a markedly lowered first ionization energy. Again silyl groups tend to lower the n ionization energies relative to those of the parent amino group. Iminophosphanes, RP=NMes* For these cases (see Table 7) the first ionization band in the photoelectron spectrum appears at almost the same energy, affected by the substituents only to a minor extent, except for the amino compounds which have a slightly lowered ioniza- tion energy.Similarly, the positions of the second ionization bands are almost constant at 8.1-8.2 eV; we assign these two bands (Ei.,) Ei,and as ionizations from the aryl group, which is confirmed by the MNDO calculations on these Table 4 Geometrical parameters of planar amino-substituted imino- systems. According to the calculations, the third ionization phosphanes; bond lengths (angles) in pm (degrees), at SCF level band refers to the n orbital mainly located at phosphorus. R’ R2 <R’PN <R2NP PN NN’ Note that a-electron-withdrawing substituents, such as the NR, or OR groups tend to lower the n-orbital energy, a con- H H 99.9 112.3 154.8 sequence of the increasing s character of the lone-pair orbital NH2 H 104.7 113.6 153.6 at phosphorus with increasing electronegativity of the neigh- N(CH3)2 105.7 113.0 154.0 bouring substituents.As for the iminophosphanes discussed N(SiH3), H 102.8 116.6 153.5 H NH2 96.0 123.9 159.2 129.7 H N(CH3)2 95.3 125.1 160.3 128.9 H N(SiH,), 96.6 124.1 157.6 135.9 Table 7 Vertical ionization energies (in eV) of iminophosphanes, R-P-N-Mes* R Table 5 Theoretically derived ionization energies (in eV) of model substituted iminophosphanes (trans conformation) But Pri 7.32 7.25 8.27 8.17 8.62 8.79 9.40 9.40 CEt, 7.22 7.20 8.44 9.27 NH2 H 8.860 (13a’) 9.325 (3a”) CH( SiMe ,) PBu; 7.58 7.20 8.14 8.09 8.21 8.59 9.57 9.45 N(CH3)2 N(SiH,), H H H H NH, N(CH,), N(SiH,), 8.534 9.120 8.918 8.457 8.702 (19a’) (25a’) (13a’) (19a’) (25a’) 8.294 9.661 8.121 7.366 8.491 (5a”) (7a”) (3a”) (5a”) (7a”) N(SiMe,), N( Bu‘)SiMe, OBu‘ Ph Mes* 6.95 6.95 7.26 7.22 7.20 8.25 8.10 8.12 8.17 8.20 8.76 8.71 9.02 9.03 9.64 9.55 9.48 9.39 9.70 For R’ = R2 = H see Table 1.Mes*, 2,4,7-tri-tert-butylphenyl. Table 6 Vertical ionization energies (in eV) of methylenephosphanes, RP-CR‘R” R R‘ R“ Ei, 2 Ei, 3 ref. But SiMe, SiMe, 8.10 (n) 8.85 (n) 9.63 3(b) TmP H SiMe, 7.48 (n) 8.82 (n) 9.26 (n) this work TmP SiMe, SiMe, 7.26 (n) 8.56 (n) 8.97 (n) 3(b)c1 SiMe, Ph 8.60 (n) 9.27 (n) 9.98 this work Cl SiMe, SiMe, 9.22 (n) 9.94 3(b)SiMe, OSiMe, Pr 7.88 (n) 8.22 (n) 9.80 this work But NMe, H 7.17 (n) 8.62 (n) 10.09 (A) this work But But H 8.74 (n) 8.95 (n) 3(4 J. CHEM. SOC.FARADAY TRANS., 1994, VOL. 90 earlier, Ei, appears at the same level as the PN double bond. Hence, tentatively this band may be assigned to ionization of the PN n bond. Methylenephosphanes, Mes*-P=CR'R" In the case of the compounds Mes*-P=CR'R", assignment of the bands could no longer be achieved by comparison with the corresponding UV data. The UV spectra of these com- pounds are extremely complex, indicating only minor changes due to substituent effects. An assignment of bands in the PE spectra could be achieved only with the aid of MNDO calculations. The results obtained are listed in Table 8. The first band is assigned to ionization from the aromatic ring system, as has already been observed for the correspond- ing iminophosphane compounds. The second band refers to ionization from the PC n bond.This is also in accord with the above discussed results on other methylenephosphanes. Finally, Ei, is assigned to ionization from the n orbital. As for the iminophosphanes, halogens increase the ionization energy Ei, 3 (n). 31P NMR Shifts and Electron Excitation energies of RP=NMes* We now discuss the relationship between 31PNMR shifts in substituted iminophosphanes of the type RP=NMes* and corresponding UV data. Within this class of compounds, a large variation in the n-n* transitions is' observed, e.g. that for Bu:P-P-NMes* appears at 580 nm. This indicates that the LUMO in these compounds is strongly affected by sub- stituents. The same tendency is also observed for the 31P NMR shifts (Table 9).A plot of both quantities (Fig. 4) reveals a satisfactory linear relationship; a regression analysis yields the following Table 8 Vertical ionization enephosphanes, Mes*P= CR'R" energies (in eV) of methyl- R' R" H H 8.17 8.81 9.33 H F 8.24 8.50 9.64 H c1 8.20 8.45 9.47 H H SiMe, NMe, 7.83 6.83" 8.50 7.76" 8.90 8.09 Br Br 8.09 8.37 9.42 Br c1 8.12 8.38 9.42 c1 c1 8.19 8.44 9.52 Ph Si Me 7.67 8.12 8.90 Table 9 31P NMR shifts and UV n -,n* transitions for imino- phosphanes,R-P=NMes* R d(31P) A(n-n*)/nm PBu', 580 570 CEt, 520 544 Pr 49 1 533 Bu 490 525 CH(SiMe,), 476 532 Ph 415 576 Mes* 396 568 N(SiMe,), 327 416 N(SiMe3)Bu' 313 40 1 OBu' 179 340 OMe 156 335 2615 600-500- --.* n I *5 400- I a/ 3001 /O-0- I I I I 100 200 300 400 500 600 700 S(3'P) Fig.4 Relationship between 31PNMR chemical shifts and electron excitations (n-n*) in the iminophosphanes, RP=NMes* equation c~(~IP)= 1.6lnP,p-359.94 (1) with a correlation coefficient r2 = 0.99. The result in Fig. 3 is, therefore, not surprising, since it indicates that the paramag- netic contribution to the NMR chemical shifts is dominant. Conclusion In the present publication we report detailed investigations on the spectroscopic properties (UV and PE data) of a selec- ted variety of imino- and rnethylene-phosphanes. Our find- ings can be summarized as follows. (1) According to quantum-chemical calculations, Prrr double-bonded systems possess two closely spaced frontier orbitals, a n and a c orbital.The latter is constituted by a combination of lone-pair orbitals at phosphorus and nitro- gen. In the case of the parent iminophosphane, the o orbital in the HOMO is well separated from the n orbital (HOMO1). For the parent compounds the vertical ionization energies were calculated at an ab initio MCSCF level of soph-istication. (2) Experimentally, the assignment of ionization bands in the PE spectra is performed by using the linear relationship between PE and UV data. The effects of substituents are in agreement with expected trends; a n-donating amino group lowers the ionization energy of the PN double bond, in other words, the orbital energy of the PN double bond is decreased.Dialkylamino groups are more effective 7t donors than disilylamino groups, owing to the competing accepting ability of the silyl substituent. In addition, n donors are more effective at nitrogen than at phosphorus. This tendency is even more pronounced for an analogous substitution pattern (P us. C substitution on the resulting n-orbital energies) in methylenephosphanes. (3) The 31P NMR parameters of the investigated imin- ophosphanes are strongly influenced by the UV properties. In fact, a linear correlation is observed which emphasizes the major role of the paramagnetic term in the.NMR properties of this new class of Prrrdouble-bonded systems. This work has been supported by the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft.The quantum-chemical calculations were performed on the Convex C240 at the University of Bielefeld and on the Cray- YMP at KFA, Jiilich. References 1 (a) K. Dimroth and P. Hoffmann, Angew. Chem., 1964, 76, 433; Angew. Chem., Int. Ed. Engl., 1964, 5, 846; (b) G. Markl, Angew. Chem., 1966,78,907; Angew. Chem., Znt. Ed. Engl., 1966,5,846. 2616 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2 3 4 5 6 7 M. Regitz and 0.J. Scherer, Multiple Bonds and Low Coordi- nation In Phosphorus Chemistry, Thieme Verlag, Stuttgart, 1990. (a) E. Niecke, D. Gudat, W. W. Schoeller and P. Rademacher, J. Chem. SOC., Chem. Commun., 1985, 1050; (b) D. Gudat, E. Niecke, W. Sachs and P.Rademacher, Z. Anorg. Allg. Chem., 1987, 545, 7. K. Fukui, Theory of Orientation and Stereoselection, Springer-Verlag, Berlin, 1975. W. W. Schoeller and E. Niecke, J. Chem. SOC., Chem. Commun., 1982,569. S. Huzinaga, Approximate Atomic Functions 11, Technical Report, University of Alberta, 1971. R. Ahlrichs, H. J. Bohm, C. Ehrhardt, P. Scharf and H. Schiffer, 10 11 12 13 14 M. J. S. Dewar, M. L. McKee and H. S. Rzepa, J. Am. Chem. SOC.,1978,100,3607. (a) D. Barrion, Dissertation, University of Bonn, 1990; (b) H. Schiffner, Dissertation, University of Bonn, 1991. T. Koopmans, Physica, 1934,1,104. (a) D. Gonbeau, G. Pfister-Guillouzo and J. Barrans, Can. J. Chem., 1983, 61, 1371; (b) P. Bruna, 0.Krumbach and S. Pey-erimhoff, Can. J. Chem., 1985, 63, 1594; (c) M. W.Schmidt and M. S. Gordon, Inorg. Chem., 1986, 25, 248; M. W.Schmidt, P. N. Truong and M. S. Gordon, J. Am. Chem. SOC., 1987, 109, 5217. S. Lacombe, D. Gonbeau, J-L. Cabioch, B. Pellerin, J-M. Denis and G. Pfister-Guillouzo, J. Am. Chem. SOC., 1988, 110, 6964; 8 9 J. Comput. Chem., 1985,6,200. T. Busch, unpublished results. (a) H. J. Werner and W. Meyer, J. Chem. Phys., 1980, 73, 2342; H. J. Werner and P. J. Knowles, J. Chem. Phys., 1985, 82, 5053; Chem. Phys. Lett., 1985, 115, 259; (b) R. J. Adamitz and R. Ahl- richs, Chem. Phys. Lett., 1988, 143, 413; (c) H. J. Werner and P. J. Knowles, J. Chem. Phys., 1988,89, 5802; P. J. Knowles and H. J. Werner, Chem. Phys. Lett., 1988, 145, 514; (d) H. J. Werner, Ado. Chem. Phys., 1987,69, 1. 15 16 H.Bock and M. Bankmann, Angew. Chem., 1986, 98, 281; Angew. Chem., Int. Ed. Engl., 1986,25,265. E. Haselbach and A. Schmelzer, Helv. Chim. Acta, 1971, 54, 1575. D. Gudat, Dissertation, University of Bielefeld, 1987. Paper 4/03664F; Received 30th March, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002611

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2617-2622 Further Studies on the Polarizabilities and Hyperpolarirabilities of the Substituted Polyenes and Polyphenyls Israel D. L. Albert and David Pugh Department of Pure and Applied Chemistry, University of Strathclyde,295 Cathedral Street Glasgow, UK GI IXL John 0. Morley Department of Chemistry, University College of Swansea, Singleton Park, Swansea UK SA2 8PP The polarizabilities and first and second hyperpolarizabilities of the all-trans donor-acceptor substituted poly- enes and polyphenyls, (CH,),N-(CH=CH-CH=CH),-NO, and (CH,),N-(C,H,),-NO, have been calculated for values of n = 1 to 9 at a frequency corresponding to 0.65 eV, using a modified CNDOVSB method. A basis set including the 325 singly and doubly excited n-electron configurations obtained from a group of six occupied and four unoccupied Hartree-Fock n orbitals has been used and the polarizabilities and hyperpolarizabilities calcu- lated by the correction vector method.The results are compared with earlier work based on an expansion in terms of a large set of singly excited configurations only. In the case of n = 3 for the polyenes and n = 2 for the polyphenyls calculations have been carried out with the complete set of n-n* configurations for each molecule, using both the correction vector method and the sum-over-states expansion. The results confirm the assessment of the quadratic non-linear optical potential of these compounds made in earlier work, although the absolute values of the first hyperpolarizabilities are somewhat reduced.Theoretical studies of the polarizabilities and hyperpolar- izabilities of conjugated organic molecules have attracted considerable attention in recent years, since there can be very large macroscopic optical non-linearities associated with their exceptionally large hyperpolarizabilities. 'y2 The study of the polarizability and the hyperpolarizabilities is also important as a probe of the electronic structure of the molecules. The frequency-dependent response to an applied electric field in the optical region is determined by the distribution of verti- cally excited electronic states which is often strongly affected by electron ~orrelation.~-' A model that can successfully explain the frequency-dependent hyperpolarizabilities must necessarily be capable of giving a reasonable account of the lower excited electronic states and the analysis of the hyper- polarizabilities may lead to the refinement of such models.It is well known, for example, that Hiickel models (even with a full configuration interaction) and Hartree-Fock calculations without adequate configuration interaction, lead to second hyperpolarizabilities of the wrong The accurate analysis of the distortion of the electronic charge distribution under perturbations is also closely connected with features of primary chemical interest such as reactivity and changes in bonding under substitution. In an earlier paper' we examined the first hyperpolar- izabilities of the donor-acceptor substituted polyenes (I) and polyphenyls (11) using a level of semi-empirical quantum I II theory which has usually been felt to be adequate for the study of electronic spectra and the first hyperpolarizability.In this work the sum-over-states (SOS) method was used, the ground state was represented by a CNDO Hartree-Fock determinant and the excited states by a configuration inter- action amongst a fairly large set of singly excited determi- nants. However, there has, over the last decade, been a considerable development of more sophisticated semi-empirical approaches to excited-state calculations based on much larger sets of configurations. Notable amongst these have been model exact PPP studies7 employing the diagram- matic valence bond method and singly and doubly excited configuration interaction (SDCI) using CNDO' or IND09 hamiltonians.Model exact here means that all configurations that can be constructed from the atomic n orbitals are included. When using very large numbers of configurations it is not possible to compute a correspondingly large set of excited-state eigenfunctions explicitly, as is required for the SOS method. An alternative method, the correction vector meth~d,~,'~."is used to express the wavefunctions of the perturbed molecule directly in terms of the configurations. Much of this work has been directed towards the study of the second hyperpolarizability, y, which is finite in all mol- ecules whether or not they have a centre of symmetry.In view of the importance attached to exploitation of large second-order effects in device applications it is also necessary to investigate how the understanding of the first hyperpolar- izability in non-centrosymmetric systems is modified when more sophisticated theories are used. In this paper we re-examine the donor-acceptor polyenes and polyphenyls using a method which incorporates some of the newer developments. The method uses a basis set of singly and doubly excited n-n* configurations constructed by promoting electrons from occupied to virtual Hartree-Fock molecular orbitals in a systematic way. The computational procedure for the calculation of the polarizability and hyper- polarizabilities employs the correction vector method.Fuller accounts of the theoretical and computational methods used can be found in ref. 6 and 11. The level of electron correlation introduced through the doubly excited configurations is suffi- cient to obtain a first approximation to the second hyperpo- larizability (y) and we also re-assess the results for the excitation energies and the first- and second-order polariza- bilities using the new level of approximation. While the general trends predicted for the quadratic effects in ref. 8 are confirmed, there are some significant quantitative differences. Theory When an electric field of frequency (427~)acts on a molecule an induced dipole moment, is produced. The subscripts here refer to coordinate direc- tions, E is the electric field acting on the molecule, a, and y are, respectively, the polarizability and the first and second hyperpolarizability tensors.The directional dependence of the response of the molecule is accounted for fully by the differ- ent components of the tensors specified by the subscripts. In this paper we shall be concerned only with the components relating to the case where all fields and the induced dipole moment are directed along the long axis of the molecule. The overall structure of the frequency-dependent hyperpo- larizability tensors and their relation to the spectroscopic properties of the molecule are best exhibited by expressing them in terms of the SOS expansion over the ground state and all the excited states of the unperturbed molecule.These constitute a complete set of functions in terms of which the wavefunction of the molecule perturbed by the field may be exactly expressed. The SOS formulae for the polarizability and first- and second-hyperpolarizabilities can be expressed in the following schematic form which is adequate as a basis for the discussion of certain qualitative features :-(3) -4 In these formulae ol, o2and o3are the frequencies of the applied fields and mu is the output frequency (which is the sum of the input frequencies). For example in a frequency- doubling experiment the inputs would be o,=o2=co and the output frequency, o,=o1+o2=20 and the effect would involve the first hyperpolarizability for second harmo- nic generation, P(-2o; o,0).The indices, rn, n, I refer to excited electronic states, g, to the ground state and the prime on the summations indicates that the ground state is excluded.The quantities, rim etc. are the dipole matrix ele- ments between the states n and rn: (5) where xi is the ith component of the position vector. The second term on the right arises since the reference state, from which the molecule is perturbed by the fields, is the ground state. It can be seen that diagonal matrix elements are there- fore essentially equal to the dipole moment of the excited state minus the ground-state dipole moment. These quantities are zero in centrosymmetric molecules and large second- order effects in organic molecules have always been associ- ated with the presence of a charge-transfer state, a low-lying excited state with appreciable oscillator strength that has a dipole moment substantially different from that of the ground state.The excitation energies, from g to n are equal to hang J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and the factors, LU,1,2 etc. indicate that terms resulting from all possible permutations of the frequencies in the denomina- tors must be added. More explicit versions of these formulae are to be found in references.6.’ l-l While one of the important qualitative features that emerges from the SOS approach, the relation between charge-transfer states and large values, has already been mentioned, an examination of the formulae reveals several other points which will be of assistance in interpreting the calculated results.First, the presence of the resonance denominators shows that enhanced effects are to be expected when the input or output frequencies approach the transition frequencies. Secondly, it will be noted that there is no prime on the intermediate summation for the third-order effects. Each term on the right-hand side of the formulae contains a chain of matrix elements that begins and ends on the ground state, rgn rng , rgmrmnrngand rgmrmnr,, rlg,respectively, in the first, second and third orders. Since, according to eqn. (5) the matrix element rgg is always zero it is not possible for the ground state to occur in intermediate positions in the first and second orders, though it may do so in the third order giving a sequence of the type, rgmrmgrglrZg.The intermediate factors in the denominator, ong-ol, etc., associated with such sequences are negative, since ong=ogg=0, and these terms make a negative constribution to y.The third order formula also differs from the others in that states inaccessible from the ground state can occur in the intermediate position. This means that a great number of two-photon states which, in the absence of interaction between the excited configu- rations would be represented by doubly excited determinants, may be involved. These produce a net positive contribution and must be included if a reasonable approximation to y is to be obtained. It follows that, while the first- and second-order effects may be adequately represented by single excitations from the ground state, we must include double excitations in calculations of y. These aspects of the SOS method have been explored quantitatively in calculations on smaller aromatic and linear conjugated system^.^*^.^,'^*^ 6-1 * Here we employ a method that can be applied to rather more extended chains.The SOS formulae contain excitation energies and matrix elements between the many-electron molecular eigenfunctions and a large number of these must therefore be calculated before the formulae can be applied. The diagonalization of the large CI matrices to obtain both eigenvalues and eigen- vectors for all states is the most time-consuming part of the computation and becomes impossible when the number of configurations is very large.The correction vector method enables the computation of the hyperpolarizabilities to be made directly in terms of the configurations themselves without the necessity of performing the CI calculation to obtain the excited states. The ground-state eigenfunction and energy, which are necessary in the correction vector method, can be obtained, using the Davidson algorithm,” without performing a complete diagonalization. Provided the same set of configurations (including the ground state) is used, the results of the SOS and correction vector calculations should be identical. The full interpretation of the results in terms of the contributions of individual excited states, analysis of nearly resonant terms etc.is not explicitly available from the correction-vector calculation, although it is possible, again avoiding a complete diagonalization, to obtain a few of the lower eigenvalues even with very large basis sets. The correc- tion vector method is computationally very much more efi- cient than the SOS method and has been used routinely to obtain the results reported. In a few cases an analysis of the evolution of the hyperpolarizability values, as excited states are added to the series in eqn. (2)-(4), has been followed through the explicit form of the SOS method. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Results and Discussion The molecular geometries of the donor-acceptor polyenes (I) and polyphenyls (11) are discussed in ref.8 and the atomic coordinates empolyed here are identical with those used in that reference. The Hartree-Fock molecular orbital calculations, which provide the basis for the CI expansions, have been made with the CNDOVSB program described in earlier pub-lication~.~*~~*~~This is a modified version of the CNDO/S2' and CNDO/UV2' methods which has been devised with the object of reproducing the optical excitation energies of polar conjugated systems. Although the CNDOVSB method includes all valence electrons, the excited configurations described here have been constructed by promoting one or two electrons from the n molecular orbitals only, since the major non-linearities have been linked both theoretically and experimentally to the response of the n-electron system.For smaller molecules it is possible to use a many-electron basis set that includes all singly and doubly excited configurations between n molecular orbitals, but this becomes impracticable for larger homologues. Here, an alternative strategy is employed which is analogous to the complete active subspace self-consistent field (CASSCF)22 approach (often used in ab initio calculations of excited states), where the set of molecu- lar orbitals selected includes those of appropriate symmetry that lie nearest the energy gap between the occupied and unoccupied states. For the larger molecules studied here this set has been restricted to include ten molecular n orbitals which (because of the extra n electrons donated by the N atoms in the NMe, and NO, groups) are occupied by 12 electrons, so that six of the set are below the energy gap (HF occupied orbitals) and four above (HF unoccupied orbitals).All single and double excitations between these orbitals have been included, to give a total of 325 configurations. The polyene (I, n = 3) and the polyphenyl (11, n = 2) have the same number of n electrons and results of the full n-electron SDCT (with 2080 configurations) are also included in these cases for the sake of comparison. Such a calculation for the higher homologues is impossible. The analysis (Fig. 2 and 3, later) of the restricted versus full SDCI treatment, for both the polyene (I, n = 3) and the polyphenyl (11, n = 2), shows that most of the qualitative features are reproduced in the active space formed from the 10 n orbitals and that the intro- duction of a larger active space only has the effect of produc- ing an overall reduction in the hyperpolarizabilities.This is to be expected since the electron4ectron correlation, which is more completely accounted for when the basis set is extended, always tends to reduce the hyperp~larizabilities.~~ The results of the SDCI calculations are also compared with those obtained using the SCI method (Table 1) which are similar to, but not identical with, the results described in ref. 8. The differences arise because only those singly excited configurations derived from promoting electrons within the set of 10 n orbitals defined above have been used in the present studies, whereas in the earlier studies a larger set was selected, in order of increasing energy, including excitations between at least 10 occupied and 10 virtual orbitals.The ground-state dipole moments and the A,,, values cal- culated with the SCI and SDCl treatments for the polyenes and polyphenyls show a number of differences (Table 1). For the polyphenyls, the change in Amax between the SCI and SDCI values is marginal, ca. 2 nm except for the first member of the series where it is 10 nm. The A,,, values from the SDCI calculation are always larger than those from the SCI calcu- lation. In the case (2* in the table) where all the n-orbital configurations are included the SDCI value of A,,, is reduced by ca.10 nm. In the substituted polyenes the differences Table 1 Dipole moments and excitation energies polyene pol yphen y 1 'max 'max n pg SCI SDCI n pg SCI SDCI ~ ~~ ~~ ~ ~~ ~ 1 11.93 381.4 392.5 1 9.03 323.5 333.4 2 15.47 503.6 508.5 2 9.72 368.1 369.5 2* 9.72 369.5 359.8 3 17.95 600.5 588.3 3 9.96 380.9 382.8 3* 17.95 601.3 547.2 4 19.78 677.2 658.6 10.13 382.8 384.0 5 21.14 737.7 712.5 10.19 387.2 390.2 6 22.55 789.9 759.3 10.22 388.3 390.0 7 23.88 827.8 790.9 10.24 389.9 390.1 8 24.03 857.3 816.6 10.33 375.9 377.0 9 25.58 909.5 - 10.34 39 1.4 391.7 10 24.97 920.1 11 25.27 929.8 12 25.52 936.7 The values of n are to be interpreted in terms of structures I and 11.pg is the ground-state dipole moment in Debye; Amax is the wave- length, in nm, of the first allowed optical transition. (SCI, SDCI: see text.) The asterisk * indicates that the full z-n* SDCI has been used. between the SCI and SDCI A,,, values are much greater with the SDCI values being lower except in the first two homo- logues. The general trend with n is similar to that found in the unsubstituted polyenes, although, in the latter, the SDCI &,,, is greater than the SCI value. The larger discrepancies between the SCI and SDCI results in the unsubstituted poly- enes are related to the strong correlation effects that lead to the appearance of a low-lying 'A, excited singlet below the lowest dipole-allowed 'B, state.In the polar polyenes the lowest excited state, as given in the table, can be regarded as derived from a mixture of these two states. As such it would be expected to show some of the sensitivity to the inclusion of electron correlation found in the non-polar molecules. It is also clear that the A,,, values for the polyphenyls approach a steady value, despite some irregularity, much more rapidly with increasing values of n than do those of the polyenes. Tables 2 and 3 give the polarizability and hyperpolar- izability results for the polyenes and polyphenyls at an input photon energy of 0.65 eV (equivalent to 1.907 pm, a conve- nient experimental wavelength for tripling experiments which avoids the electronic re-absorption of the third harmonic, at least in the smaller compounds).The subscript, x, in the quantities x,,, P,, yxxx, refers to the coordinate along the long axis of the molecule, which coincides exactly with the direction of the molecular dipole in the poyphenyls and approximately in the polyenes. P,, the component of the vector part of the fi tensor along the molecular dipole, given by3 Px = Pxxx + (Pxyy + Pxzz + 28yxy + 2PzxJ can be measured using the electric-field-induced second har- monic generation technique (EFISH).24 The major contribu- tion to /3, comes from the diagonal component, Pxx, which gives the quadratic response when all fields are in the x direc-tion. For y the component y,,,, for frequency tripling is given.This is by far the largest component of the tensor. In the case of the polyenes, the change from SCI to SDCI has the effect of reducing the magnitude of the linear and quadratic response. The divergence between the two sets of results increases with increasing chain length. For the longest chain (n = 8) the ct and /3 values calculated with SDCI are, respectively, reduced by ca. 5% and one third compared with the SCI values. A measurement of 0for the all-trans mol- ecule, H2N-(CH=CH),-NO, has been reported2' as J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 n ascl 1 2.505 (2) 2 8.131 (2) 3 1.682 (3) -3* 4 2.785 (3) 5 4.066 (3) 6 5.591 (3) 7 7.108 (3) 8 8.660 (3) 9 1.071 (4) 10 1.217 (4) 11 1.363 (4) 12 1.504 (4) Table 2 Polarizability and hyperpolarizability data at 0.65 eV for polyenes aSDCI PCC' BSX1 YSC1 2.334 (2) 0.181 (2) 0.181 (2) -0.014 (1) 6.993 (2) 1.270 (2) 1.052 (2) -0.174 (1) 1.431 (3) 1.279 (3) 4.735 (2) - 3.269 (2) 2.743 (2) 0.176 (1) - 2.418 (3) 1.377 (3) 8.743 (2) -2.503 (2) 3.642 (3) 3.089 (3) 1.970 (3) -4.170 (2) 5.133 (3) 6.239 (3) 4.072 (3) -7.887 (2) 6.643 (3) 8.274 (3) 1.1 14 (4) 1.803 (4) 7.290 (3) 1.204 (4) -1.474 (3) -2.536 (3) - 5.090 (4) - - - 7.645 (4) - - - 1.145 (5) - - - 1.688 (5) - - YSX* 0.039 (1) 0.103 (2) 1.398 (2) 0.659 (2) -1.145 (3) -1.039 (3) -1.868 (3) -5.059 (3) 9.041 (3) ----esu; for /I,1The a values are expressed in au, /? in esu and y in lo5 au.(au = atomic unit; for a, 1 au = 1.4817 x au = 8.6374 x esu and for y, 1 au = 5.0366 x esu). The asterisk * indicates that the full n-n* SDCI has been used. The numbers in parentheses indicate the power of 10 that the value should be multiplied by [2.505(2) = 2.505 x lo2,etc.]. (86 x esu which is consistent with the results in Table 2 for n = 1 and n = 2. The effect of the introduction of doubly excited configu- rations in the polyphenyls is less marked and, for the higher homologues, the divergence is in the opposite direction, amounting to an increase of a few per cent in the a values and of ca. 10% in the p s, although, as in the previous work on this series' some irregularities are found. The general trends are similar in the SCI and SDCI results and the much less rapid increase of /3 with n in the poly- phenyl series, as contrasted with the polyenes, is confirmed.The frequency-tripling y values in the polyenes calculated using SCI are negative for all cases except n = 3, where a small positive value is found. The SDCI values are positive up to n = 3 and then become negative up to n = 8. The data for the higher polyenes are incomplete owing to convergence difficulties near the resonance condition. The change from positive to negative values at n = 4 is because the tripled photon energy (30 = 1.95 eV) exceeds the excitation energy to the charge-transfer state (on*= 1.85 eV). This changes the sign of some of the major terms in the SOS expansion which contain the factor (ong3o)-'.The value of y in the SDCI-approximation is always found to be positive for cases where input and output frequencies are below the lowest optical excitation frequency.This is in accordance with other and the generally accepted interpretation of experi- mental results. The interpret at ion of the h yperpolarizabili ties in terms of the virtual excitations of the SOS expansion are illustrated in Fig. 1 to 4 for the polyene (I, n = 3) and poly- Oo01 3 750 0I.l I I I I I I 0 10 20 30 40 50 60 70 80 90 100 number of excited states Fig. 1 Development of /3, as excited states are added in order of increasing energy in the SOS expansion for the polyene with n = 3. 0,Basis set of 325 configurations, see text; +,full n-n* SDCI calcu- lation with 2080 configurations.There is no appreciable further variation beyond 100 states. 5 601 220 phenyl (11, n = 0 0 40; 2). In each case the figure shows the develop- 40 excited 7010 20 30number of50 60 states 80 90 100 ment of the final value of the hyperpolarizability as more excited states are added in the SOS expansions of eqn. (3) and (4). Table 3 Polarizability and hyperpolarizability data at 0.65 eV for pol yphen yls n $5DCIax' 1 156.4 138.1 14.48 11.40 0.14 0.20 2 304.8 275.0 42.30 34.85 0.94 1.07 2* 276.5 28.32 1.01 3 410.8 394.8 70.89 64.99 2.18 2.96 4 472.4 473.7 96.55 95.73 3.68 5.04 5 580.4 598.1 98.16 106.72 3.99 7.61 6 681.1 701.9 107.69 117.09 3.78 9.20 7 767.6 791.1 127.28 139.97 4.72 12.04 8 737.7 752.8 182.70 191.37 15.96 21.78 9 1023.3 1060.0 131.48 152.29 1.88 16.40 The a values are expressed in au, in lo-" cm5 esu and y in lo5au.The asterisk * indicates that the full n-n* SDCI has been used. Fig. 2 Development of p, in the SOS expansion for the polyphenyl with n = 2. Symbols as in Fig. 1. I I 1 1 I I I0-10 20 30 40 50 60 70 80 90 100 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.6 1 lb1.2 0.8 0.0 I4 I 1 I 1 I 1 I I I 1 0 10 20 30 40 50 60 70 80 90 100 number of excited states Fig. 4 Development of y,,,, for the polyphenyl with n = 2. Symbols as in Fig. 1. Fig. 1 shows the development of /?, for the polyene (I, n = 3) for the case where the excited states have been calcu- lated using (a) the 325 singly and doubly excited configu- rations that arise from the standard set of 10 n orbitals and 12 electrons described above and (b)the complete set of 2080 n-n* configurations for the molecule, which is the largest set for which we are able to carry out the explicit diagonalization required for the SOS method, because of memory restrictions on the SG workstation used for the calculation (128 Mbytes maximum).The number of terms in the perturbation theory sums of eqn. (3) has been restricted to 100, but it can be seen from the diagrams that convergence has been achieved well before this cut-off is reached. The two calculations produce plots of /?, us.number of configurations which are almost identical in shape. In each case there is a large positive con- tribution from the first state with non-zero oscillator strength, the charge-transfer state, but the final value is sub- stantially reduced through the diagonal and non-diagonal terms that occur as more states are added. The quadratic hyperpolarizability is often treated in terms of the two-state model where only the charge-transfer state and the ground state are included. While it is usually true that the charge- transfer contributions in different molecules give a reasonable indication of the relative sizes of the quadratic effect, more extensive CI calculations often show, as in this case, that the absolute value predicted is greater than the true value.The approximate nature of the internal field factors, which must always be used when fl values are extracted from solu- tion measurements, prohibits a direct comparison between theory and experiment at a level of accuracy sufficient to dis- criminate between the SCI and SDCI results. The shapes of the plots are similar to many results obtained using SCI calculations,8 which confirms that the general qualitative ideas derived from the SCI calculations are not altered by the inclusion of the doubly excited configu- rations. Fig. 2 shows the analagous results for the polyphenyl (11, n = 2). The comments of the previous paragraph also apply to this case, although there are significant contributions from higher states which are absent in the substituted polyenes. Fig.3 shows the development of y,,,, for the polyene (I, n = 3), again for the two cases (a)and (b)above. The first few contributions are negative and are associated with terms with intermediate ground state in eqn. (4). These terms are rela- tively much more important in the non-polar polyenes. In the polar molecules the essentially positive nature of the charge- transfer type contributions biases the results more decisively towards positive values. Nevertheless, in the case of y, it is not the direct diagonal term from the charge-transfer state that dominates, but an indirect term which also involves a higher state as an intermediate. This pattern of behaviour has .also been observed in previous work.Mazumder et ~1 have ~ described a theory of the two-photon absorption in polyenes 262 1 which emphasises the importance of a limited number of active states such as those which can be identified from these plots. Finally, Fig. 4 shows the equivalent plots for the poly- phenyl (11, n = 3) y values. There is here a more complicated variation which reflects the more complicated structure of the repeat unit of the chain. The negative effect that was observed in the polyenes is not present, the initial rise being attribut- able to the effect of the charge-transfer state as in the case of p. There is also a greater difference in the development of y in the full and truncated SDCI calculations as compared with the polyene calculations.Conclusion The polarizability and hyperpolarizability of the trans-polyenes (I) and polyphenyls (XI) substituted on opposite ter- minal carbons with nitro and dimethylamino acceptor and donor groups have been calculated at a photon energy of 0.65 eV. By using a computationally efficient method based on the correction vector procedure it has been possible to perform CNDO/SCF/CI calculations using a set of all singly and doubly excited n-n* configurations constructed from the six occupied and four unoccupied n orbitals nearest to the energy gap. The results show trends similar to those of an earlier study based on singly excited configurations only. The linear polarizabilities calculated by the SCI and SDCI methods are in very close agreement as are the first hyperpo- larizabilities for the polyphenyls. The values of the polyenes are reduced by up to one third in the SDCI calculations com- pared with the SCI results.The more drastic effect of using a more correlated wavefunction on the polyene hyperpolar- izabilities, as opposed to those of the polyphenyls, is not unexpected since the excited-state spectrum of the polyenes is well known to be qualitatively altered by electron correlation effects. It is interesting to note that the evolution of yxxxx as states of increasing energy are added, is very similar to that found in the non-polar polyenes. The more general conclu- sion, that the polyenes are much more effective as second order non-linear materials than the polyphenyls because of the nature of the variation with chain length in the two classes of compound, is essentially unchanged from that in ref. 8.The method has been checked against the full n-n* SDCI calculation for one of the smaller molecules in each class and, although there is a further appreciable reduction in /? for the polyenes the interpretation of the effects in terms of the SOS expansion is almost identical for the full and truncated SDCIs. The analysis of the ‘hyperpolarizability density’ and its variation with chain length, which was the basis of the com- parison of the efficacy of the two classes given in ref. 8 is not changed in any substantial way when the results of the SDCI calculations are substituted for the earlier SCI results.Calculations of the second hyperpolarizability for fre-quency tripling from an input photon energy of 0.65 eV have also been carried out using the same basis sets. For the case of the polyene (I, n = 3) the truncated SDCI value of y is about twice that for the full SDCI indicating the larger con- tribution of the correlated parts of the wavefunction to the higher hyperpolarizability. Beyond n = 3 the polyene ys enter the near-resonant region and become negative. The poly- phenyl y values are considerably smaller and the agreement between the full and truncated SDCI results for (11, n = 2) is an indication that the importance of highly correlated behav- iour in the wavefunction is again less than in the polyenes.~The authors would like to thank the SERC and Zeneca, plc for financial support for this work. 2622 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 References 1 Nonlinear Optical Properties of Organic Molecules and Crystals, 14 D. Pugh and J. 0. Morley, in Nonlinear Optical Properties of Organic Molecules and Crystals, ed. D. S. Chemla and J. Zyss, Academic Press, New York, 1987. 2 ed. D. S. Chemla and J. Zyss, Academic Press, New York, 1987. P. N. Prasad and D. J. Williams, Nonlinear Optical Efects in Molecules and Polymers, Wiley-Interscience, New York, 1991. 15 16 C. Flytzanis, in Quantum Electronics: a Treatise, ed. H. Rabin and C. L. Tang, Academic Press, New York, 1975, vol. 1A. S. J. Lalama and A. J. Garito, Phys.Rev. A, 1979,20,1179. 3 Int. J. Quantum Chem., 1992,43. 17 D. Li, T. J. Marks and R. A. Ratner, J. Am. Chem. SOC., 1988, 4 B. S. Hudson, B. E. Kohler and K. Schulten, in Excited States, 110,1707. 5 ed. E. C.Lim, Academic Press, New York, vol. 6. J. R. Heflin, K. Y. Wong, 0.Zhamani-Khamari and A. J. Garito 18 V. J. Docherty, D. Pugh and J. 0. Morley, J. Chem. SOC., Faraday Trans. 2, 1985,81,1179. 6 7 Phys. Rev. B, 1988,38, 1573. J. 0.Morley, P. Pavlides and D. Pugh, Int. J. Quantum Chem., 1992,43, 7. S. Ramasesha and Z. G. Soos, Chem. Phys. Lett., 1988, 158, 171; 19 20 21 E. R. Davidson, J. Comput. Phys., 1975, 17, 87. J. Del Bene and H. H. Jaffe, J. Chem. Phys., 1968,48, 1807. P. Francois, P. Carles and M. Rajzmann, J. Chim. Phys., 1977, 74,606; 1979,76,328. 8 9 10 11 Z. G. Soos and S. Ramasesha, J. Chem. Phys., 1989,90, 1067; S. Ramasesha and I. D. L. Albert, Phys. Reo. B, 1990,42, 8587. J. 0.Morley, V. J. Docherty and D. Pugh, J. Chem. SOC., Perkin Trans. 2, 1987, 1352. B. M. Pierce, J. Chem. Phys., 1989,91, 791. H. F. Hameka and E. N. Svendsen, Int. J. Quantum Chem. Symp., 1984, 80, 525; E. N. Svendsen, C. S. Willand and A. C. Albrecht, J. Chem. Phys., 1985,83,5760. I. D. L. Albert, J. 0.Morley and D. Pugh, J. Chem. Phys., 1993, 22 23 24 25 26 B. 0.Roos, in Ab Initio Methods in Quantum Chemistry-11, ed. K. P. Lawley, Wiley, New York, 1987,399. I. D. L. Albert, J. 0.Morley and D. Pugh, to be published. L-T. Cheng, in Organic Molecules for Nonlinear Optics and Pho- tonics, Nato AS1 Series, 1991, vol. 194, pp. 121-136. D. S. Donald, in New Aspects of Organic Chemistry 11, ed. Z. Yoshida and Y. Ohshiro, Kudamska, Tokyo, 1990, p. 40. S. Mazumdar, D. Guo and S. N. Dixit, J. Chem. Phys., 1992,96, 686. 99,5197. 12 13 B. J. Orr and J. F. Ward, Mol. Phys., 1971, 20, 513. J. F. Ward, Rev. Mod. Phys., 1965,37, 1. Paper 4/01 828A; Received 28th March, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002617

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2623-2625 Intrinsically Unpolarized Fluorescence of C,, Mario N. Berberan-Santos" Centro de Quimica-Fisica Molecular, lnstituto Superior Tecnico, 1096 Lisboa Codex, Portugal Bernard Valeur Laboratoire de Chimie Generale (CNRS ER 77),Conservatoire National des Arts et Metiers, 292 Rue Saint-Martin, 75003Paris, France The fluorescence spectrum and the fluorescence steady-state anisotropy of C,, in a toluene+?thanol 10 : 1 (v/v) mixture have been obtained at room temperature and at 140 K. At 140 K, where molecular rotation is negligible during the excited-state lifetime, essentially zero fluorescence anisotropy was measured, irrespective of the excitation wavelength. It is concluded, in accordance with theoretical expectations, that the fluorescence of c60 is intrinsically unpolarized. c60 is the first organic molecule to show this behaviour.Numerous papers have been devoted to the fullerenes in recent years, with special attention to the photophysical properties of c60, see e.g. ref. 1 and references therein. The weak fluorescence of this compound has been observed and chara~terized,~~~no discussion of its polarization hasbut appeared so far. It is the aim of the present work to show that the fluorescence emitted by c60 is intrinsically unpo- larized, as shown by experiments performed at low tem-perature in a rigid solvent and as expected from theoretical considerations. The polarization of the fluorescence emitted at right angles to the excitation direction is conveniently measured by the quantity anisotropy, r,* where Illis the intensity of the fluorescence with vertical polarization and I, is the intensity of the fluorescence with horizontal polarization, the excitation being made with verti- cally polarized light.For a ground-state isotropic distribution of molecules, the fluorescence anisotropy is a direct measure of the angular correlation between the (one-photon) absorption and the emission transition dipoles' 3(cos2 a)(t) -1 r = 0.4 2 (2) where a is the angle between absorption and emission tran- sition dipoles and (.-.) denotes the ensemble average which is in general a function of time. If rotation and energy migration do not occur within the excited-state lifetime, the anisotropy in response to excitation by a d(t) pulse is constant in time and identical to that obtained for steady-state excitation.In that case, both reduce to the fundamental anisotropy ro ,9 3(cos2 a) -1 ro = 0.4 2 (3) where (. ..) is now an average over the angular distribution within the molecular framework. Upper and lower bounds for the fundamental anisotropy are 0.4 (collinear absorption and emission) and -0.2 (orthogonal absorption and emission). If three mutually perpendicular axes are defined with respect to the molecular framework (molecular frame), these three axes are frequently non-equivalent from the symmetry point of view. In that case, the angle a is unique for a given pair of excitation and emission wavelengths; in particular it is zero for excitation at the 0-0 band of S,, provided the emit- ting s, retains the Franck-Condon geometry.Hence, the fun- damental anisotropy takes its maximum value, 0.4, when exciting at the S, 0-0 band. However, if two of the axes of the molecular frame are equivalent, x and y say, and if the absorption and the (several equivalent) emission transition moments occur in the xy plane, then the fundamental anisotropy will have as its maximum value only O.l.'o*' This was conclusively shown to be the case for benzene (ground-state symmetry point group D6h) and triphenylene (ground-state point group D3h). O-' For the even more symmetrical molecules belong- ing to the tetrahedral, octahedral and icosahedral point groups, where the x, y and z axes are equivalent, the possi- bility of intrinsically unpolarized fluorescence arises.An interesting candidate is the c60 molecule, which belongs to the icosahedral (I,) point group, as convincingly shown by its 13C NMR,13,14 IR ab~orption'~,'~ and vibra- tional Raman16 spectra. The discovery of the weak fluores- cence of C602-5 prompted us to investigate its fundamental polarization. It was found that the fluorescence of c60 had negligible polarization when measured at 140 K in a rigid solvent. It is thus concluded that the fluorescence is intrinsi- cally unpolarized, as expected on theoretical grounds. c60 is the first organic molecule to display intrinsically unpolarized fluorescence. Experimental Experimental procedures for the determination of low-temperature fluorescence polarization spectra have been described previously.' Briefly, fluorescence measurements were performed with an SLM 8000 C spectrofluorometer equipped with single-grating monochromators and an Oxford DN1704 cryostat. In order to suppress the second- order excitation, a Schott OG550 cut-off filter (1 > 600 nm) was used in the emission. Owing to the extremely weak signals, the sample photomultiplier was set close to the maximum value and 32 nm bandpasses were used when mea- suring the polarized components.? For each wavelength recorded, the integration time was 10 s. Electronic thermal noise was recorded under identical conditions and subtracted from the polarized components.The uncertainty in the anisotropy was estimated to be f0.005. Solvent fluorescence was not observed under the conditions used. Intensities were measured in the ratio mode, using Rhodamine 101 as the t Identical but noisier results were obtained with 16 nm band- passes. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 >. c.-v)CW cC.-Q, C W5:L -3 w-I I I 600 650 700 750 800 850 A/n m Fig. 1 Fluorescence emission spectrum (16 nm bandpasses) of c6, in a toluene-thanol (10 : 1 v/v) mixture at (a)room temperature and (b) 140 K. Excitation wavelength = 390 nm. quantum counter. Solid c60 (Kaesdorf, Munich, >99.9%) was dissolved in a toluene (Aldrich, HPLC grade)+thanol (Merck p.a.) 10 : 1 v/v mixture to yield 2 x lop4 mol dmP3 solutions.The presence of ethanol, while slightly reducing the solubility of c60,was necessary for an optically clear glass to be obtained at 140 K. Results and Discussion Fluorescence spectra of c60, recorded at room temperature and at 140 K are shown in Fig. 1 : one shoulder at about 663 nm and two peaks at 697 and 736 nm are observed. From a published spectrum4 obtained in 77 K cyclohexane with better resolution, Negri et aL6 made a detailed assignment of the fluorescence vibronic structure. Apart from the normal narrowing of the vibronic bands upon cooling, the integrated intensity changes negligibly, implying an essentially temperature-independent fluorescence quantum yield, re-ported to be 2.2 x in room-temperature t01uene.~ This is in agreement with the dominant non-radiative path from S, being a non-activated intersystem crossing to the T, state.‘‘7 l9 Previously, excitation dependence of the fluores- cence quantum yield of c60 had been reported, raising the possibility of the existence of photoprocesses from higher excited state^.^ However, this wavelength dependence was latter shown to be non-existent.’ The fluorescence anisotropy at 140 K is shown in Fig.2 together with the absorption spectrum at room temperature.$ As regards the anisotropy, four distinct regions can be defined (A-D in Fig. 2). In regions A (380-410 nm) and C (460-540 nm) the anisotropy is essentially zero, while it is slightly higher than zero in B (410-460 nm) and progressively 1From the excitation spectrum recorded at 140 K with 2 nm bandwidths, the absorption shows little change when the sample is cooled.Q, Clu e2B 0.4 2 0.3 0 Lc .-2 0.2 Cm 8 0.1 C W $0 :L -0.1 -0.2 380 480 580 680 A/nm Fig. 2 Absorption spectrum (room temperature) and steady-state fluorescence anisotropy (140 K) of C6, in a toluene-thanol (10 : 1 v/v) mixture. The fluorescence anisotropy is recorded as a function of the excitation wavelength, the emission wavelength being fixed at 740 nm. departs from zero in D (>540 nm). The non-zero value in B is attributed to residual stray light (whose polarization is mainly vertical) which becomes of concern when little exciting light is absorbed, as occurs around the absorption minimum at 438 nm (absorbance =0.05):this has the effect of increas- ing the amount of stray light and at the same time the fluo- rescence becomes weaker.In region D, the progressive increase in the anisotropy is due to the combined effect of stray light, increasingly important as the emission wavelength is approached, and polarized Raman scattering by the solvent (ca. 3100 cm-’,prominent toluene fundamental), the magni- tude of which was found to be comparable to the fluores- cence of c60. Artefacts observed in regions B and D could be reduced by increasing the concentration of the solute; however, 2 x mol dm-3 is already close to the solubility limit, and higher concentrations were not feasible.The elimination of the arte- facts could in principle be achieved by temporal discrimi- nation, as Rayleigh and Raman scattering are essentially instantaneous, while fluorescence is not. It seems, however, that the zero anisotropy repeatedly obtained in the wide and disconnected intervals A and C already provides convincing evidence of the unpolarized character of the fluorescence of c60 -We now address another possible reason for an unpo-larized emission, other than the high symmetry of the mol- ecule, viz. molecular rotation in the excited state. At 140 K, the toluene+thanol mixture used forms a very viscous (q 9lo3 cP, as observed) glass. At lower tem-peratures, the glass ‘cracks’ within minutes, but it is stable for several hours at 140 K.From the Perrin equation,” which relates the observed anisotropy r with the rotational correlation time z,, where z is the fluorescence lifetime, one sees that, in order to have r c 0.005, as observed, either the fundamental anisot- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ropy r, is close to zero or T/T~% 1. Since the maximum pos- sible value for r, is 0.4, it follows that z/z,> 79, or, with 7 = 1 ns,5719 7, < 13 ps. This value could be observed only for essentially free rotation. Almost free rotation was indeed observed for pure solid c60, but only above the phase- transition temperature, ca. 250 K.21 Immediately below this temperature it is 500 ps21 and at 100 K it is 50 ps.22Factors making free rotation highly unlikely are, besides the low tem- perature, the strong interaction between toluene and C6,23,24 and the much smaller size of the solvent.25 Also, the rotation- al correlation time of C,, in a non-viscous solvent (1,1,2,2- tetrachloroethane) at room temperature is 16 PS.~' The high macroscopic viscosity of the toluene-ethanol mixture at 140 K is thus expected to yield a rotational correlation time of at least hundreds of ns, i.e.much higher than the fluorescence lifetime. It is therefore concluded that it is the fundamental anisotropy that is zero, as expected for icosahedral symmetry. However, the symmetry of the excited state may be differ- ent from that of the ground state. It is important to study the effect of a possible symmetry reduction on the polarization.Analysis of the MCD spectrum of C6026 shows that the S, state belongs to the irreducible representation Tlg. The one- photon transition between the ground state ('Ag) and the orbitally triple degenerate S, state is symmetry-forbidden.'.6 Accordingly, the radiative lifetime computed from the absorption is quite high, ca. 1 ps.7,27The radiative lifetime obtained from combined fluorescence lifetime and fluores- cence quantum yield measurements is of the same order of magnit~de,~ indicating that no significant symmetry reduction occurs in the emitting S, state. However, since it is orbitally degenerate in icosahedral symmetry, it must undergo some molecular framework distortion, according to the Jahn-Teller theorem.28 The question then arises as to whether the instability of the I,, symmetry in the excited state will lead to an effective symmetry reduction (static Jahn- Teller effect) or merely to a dynamic equilibrium between the distorted configurations (dynamic Jahn-Teller effect), thus preserving the initial (icosahedral) symmetry.28 In view of the large size of the molecule, the excitation of a single electron from So to S, is not expected to cause an important modifi- cation of the potential-energy surface, in agreement with quantum-chemical calculations showing little activity of Jahn-Teller-active vibrational modes.6 A weak, dynamic Jahn-Teller effect is therefore expected.EPR studies of the triplet T, (3T2g)29,30 give evidence for a dynamic effect for that state.This effect is equivalent to a pseudo-rotation, owing to the interconversion of the degenerate Jahn-Teller conformer^.^^ The rate of interconversion is temperature dependent, but interconversion is still operative at 8 K, albeit slow, as measured by Closs et Terazima et al., on the other hand, reported static distortions at 3 K, but possibly dynamic ones at 77 K.30 From the observed anisotropy of the time-resolved EPR signal, it was also proposed that the precursor S, state (from which the T, sublevels are populated by intersystem crossing) was also in a distorted ~ymmetry.~' The existence of Jahn-Teller distortion in the exciting state, even if it reduces the symmetry from the initial I,, does not change the unpolarized nature of the fluorescence, at least in isotropic media, where distortion is equally likely along any three orthogonal axes. While each excited molecule may or may not still belong to I,, depending on whether the effect is dynamic or static, the macroscopic ensemble will still have unpolarized fluorescence, because of the original ground-state 1, symmetry.It is therefore concluded that the fluorescence of c60 is intrinsically unpolarized, owing to the high symmetry of the ground state. This work was supported in part by JNICT (Portugal) and FEDER (EU) through research project STRDA/C/CEN/421/ 92. References 1 S. Leach, M. Vervloet, A. Despres, E. Breheret, J. P. Hare, T.J. Dennis, H. W. Kroto, R. Taylor and D. R. M. Walton, Chem. Phys., 1992,160,451. 2 C. Reber, L. Yee, J. McKiernan, J. I. Zink, R. S. Williams, W. M. Tong, D. A. A. Ohlberg, R. L. Whetten and F. Diederich, J. Phys. Chem., 1991,95,2127. 3 S. P. Sibley, S. M. Argentine and A. H. Francis, Chem. Phys. Lett., 1992, 188, 187. 4 Y. Wang, J. Phys. Chem., 1992,96, 764. 5 D. Kim, M. Lee, Y. D. Suh and S. K. Kim, J. Am. Chem. SOC., 1992,114,4429. 6 F. Negri, G. Orlandi and F. Zerbetto, J. Chem. Phys., 1992, 97, 6496. 7 Y. Sun, P. Wang and N. B. Hamilton, J. Am. Chem. SOC., 1993, 115,6378. 8 A. Jablonski, Bull. Acad. Polon. Sci., Ser. Math. Astr. Phys., 1960, 8, 259. 9 F. Perrin, Ann. Phys. (Paris), 1929, 12, 169; A. Jablonski, Z. Naturforsch., Teil A, 1961, 16, 1.10 P. P. Feofilov, The Physical Basis of Polarized Emission, Consul-tants Bureau, New York, 1961. 11 F. Dorr, Angew. Chem., Int. Ed. Engl., 1966,5478. 12 R. D. Hall, B. Valeur and G. Weber, Chem. Phys. Lett., 1985, 116, 202. 13 R. Taylor, J. P. Hare, A. K. Abdul-Sada and H. W. Kroto, J. Chem. SOC., Chem. Commun., 1990, 1423. 14 H. Ajie, M. M. Alvarez, S. J. Anz, R. D. Beck, F. Diederich, K. Fostiropoulos, D. R. Huffman, W. Kraetschmer, Y. Rubin, K. E. Schriver, D. Sensharma and R. L. Whetten, J. Phys. Chem., 1990, 94, 8630. 15 W. Kratschmer, L. D. Lamb, K. Fostiropoulos and D. R. Huffman, Nature (London), 1990,347, 354. 16 D. S. Bethune, G. Meijer, W. C. Tang, H. J. Rosen, W. G. Golden, H. Seki, C. A. Brown and M.S. de Vries, Chem. Phys. Lett., 1991, 179, 181. 17 M. N. Berberan-Santos, J. Canceill, J. C. Brochon, L. Jullien, J. M. Lehn, J. Pouget, P. Tauc and B. Valeur, J. Am. Chem. SOC., 1992,114,6427. 18 J. W. Arbogast, A. P. Darmanyan, C. S. Foote, Y. Rubin, F. N. Diederich, M. M. Alvarez, S. J. Anz and R. L. Whetten, J. Phys. Chem., 1991,95,11. 19 D. K. Palit, A. V. Sapre, J. P. Mittal and C. N. R. Rao, Chem. Phys. Lett., 1992, 195, 1. 20 F. Perrin, J. Phys. Radium, 1926,7, 390. 21 R. D. Johnson, D. S. Bethune and C. S. Yannoni, Acc. Chem. Res., 1992, 25, 169. 22 R. Tycho, R. C. Haddon, G. Dabbagh, S. H. Glarum, D. C. Douglass and A. M. Mujsce, J. Phys. Chem., 1991,95, 518. 23 H. Levanon, V. Meiklyar, A. Michaeli, S. Michaeli and A. Regev, J. Phys. Chem., 1992, %, 6128. 24 R. Seshadri, C. N. R. Rao, H. Pal, T. Mukherjee and J. P. Mittal, Chem. Phys. Lett., 1993, 205, 395. 25 J. L. Dote, D. Kivelson and R. N. Schwartz, J. Phys. Chem., 1981,85,2169. 26 Z. Gasyna, P. N. Schatz, J. P. Hare, T. J. Dennis, H. W. Kroto, R. Taylor and D. R. M. Walton, Chem. Phys. Lett., 1991, 183, 283. 27 R. J. Sension, C. M. Phillips, A. Z. Szarka, W. J. Romanow, A. R. McGhie, J. P. McCauley, A. B. Smith I11 and R. M. Hochs- trasser, J. Phys. Chem., 1991,95, 6075. 28 G. Herzberg, Molecular Spectra and Molecular Structure, Elec- tronic Spectra and Electronic Structure of Polyatomic Molecules, Van Nostrand, New York, 1966. 29 G. L. Closs, P. Gautam, D. Zhang, P. J. Krusic, S. A. Hill and E. Wasserman, J. Phys. Chem., 1992,96, 5228. 30 M. Terazima, N. Hirota, H. Shinohara and Y. Saito, Chem. Phys. Lett., 1992, 195, 333. Paper 4/01981D; Received 5th April, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002623

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC.FARADAY TRANS., 1994, 90(18), 2627-2634 Photoinduced and Thermal lsomerization Processes for Bis-Oxonols:Rotor Volume, Stereochemical and Viscosity Effects Andrew C. Benniston and Anthony Harriman" Center for Fast Kinetics Research, The University of Texas at Austin, Austin, TX 78712,USA Four bis-oxonols have been synthesized which possess different alkyl substituents appended to the thiobarbitur-ate subunit. The nature of the alkyl substituent affects the photophysical properties of the dye in solution since it modulates the rate of rotation of the thiobarbiturate subunit around one of the double bonds in the connecting trimethine bridge. Rates of light-induced (trans to cis) and thermal (cis to trans) isomerization processes have been measured for one of the dyes in protic (i.e.alkanols) and aprotic solvents at various temperatures.These rates, together with activation energies derived from Arrhenius plots, are discussed in terms of the hydrogen bonding and stereochemical properties of the solvent. The thermal step is very sensitive to the stereochemistry of the solvent while the light-induced process is controlled mostly by the size of the rotor and the solvent viscosity. The overall effects may be explained, at least in a qualitative sense, in terms of the medium-enhanced barrier model. Relative changes in fluorescence intensity are commonly used in biophysics to probe local environments and to monitor concentrations of intracellular species. Fluorescent dyes that undergo facile photoisomerization are of particular interest as molecular probes because the fluorescence properties can be extremely sensitive to the environment.' Indeed, such dyes are often employed to elicit information regarding membrane fluidity, viscosity, polarity, potential and phase-transition temperature2 while related dyes have been used to determine intracellular pH and specific ion concentrations3 and to stain microbes, antibodies, proteins et~.~The design of new bio- physical probes is aided considerably if the mechanism of the isomerization process is well understood and if the nature of any interaction between dye and adjacent solvent molecules can be identified.However, the mechanisms for light-induced isomerization reactions can be surprisingly ~omplex,~ espe-cially for polar molecules,6 and there is still conSiderable debate over how best to describe friction between the twisting groups and adjacent solvent molecule^.^ The photoisomeriza- tion of stilbene, for example, has been studied continuously for the past 30 years and is still under debate.8 Here, we consider the dynamics of light-induced and thermal isomerization reactions for some bis-oxonols in polar organic solvents.Such compounds have long been used in the colourimetric analysis of lipoperoxides formed by in situ oxi-dation of lipid membranesg and for the fluorimetric deter- mination of membrane potentials." Indeed, it has been reported that there is a 20-fold increase in fluorescence yield upon binding to membranes or proteins, without an apparent change in absorption or fluorescence maxima.' ' More recent studies12 have shown that bis-oxonols undergo photoisomer- ization from the first excited singlet state on the timescale of a few hundred picoseconds.The resultant isomer reverts to the initial configuration over some tens of microseconds. We have now examined these isomerization steps in more detail using a small series of bis-oxonols, more correctly named as 5,5'-(prop-1-en- 1 -ylidiene(bis(N,N'-dialkyl-2-thiobarbituric acid) derivatives. The objective of this work was to identify the major parameters that influence the rates of isomerization in these (supposedly) simple compounds in polar solvents. Experimental Starting materials were purchased from Aldrich or Eastman- Kodak and were used as received.Synthesis of compounds R :N4s IN<R 0-'HN(CH2CH3)3 lI3 and 312 (in its protonated form, not as a tri-ethylammonium salt) has been described previously while compounds 2 and 4 were prepared by simple modification of the method used to prepare 3. Reaction involved the triethylamine-catalysed condensation of a thiobarbituric acid with the corresponding 5-(3-methoxyprop-l-en-2-ylene)bar-bituric acid and was followed by absorption spectroscopy.' Each dye was purified by extensive chromatography on silica using ethyl acetate as eluent and was analysed by 'H NMR, TLC and FAB mass spectrometry. Compound 3 was further analysed by exact mass spectrometry, elemental analysis, NMR and FTIR. Intermediate reagents were routinely analysed by TLC and 'H NMR.Preparation of reagents needed for synthesis of the hexyl derivative 4, the most exotic of the new compounds, is described below while the corres- ponding reagents for synthesis of 2 were prepared by the same route starting with propylamine. Preparation of Hexyl Isothiocyanate Hexylamine (20 g, 200 mmol) was added dropwise to a stirred solution of CS, (15 g, 200 mmol) and NaOH (8 g, 200 mmol) in water (50 ml) maintained at 10-15°C. The mixture was subsequently heated to 100°C for 2 h. After the mixture had been cooled to 35-40°C, ethyl chlorocarbonate (21.6 g, 200 mmol) was added and the mixture was stirred for 30 min. The organic layer was separated, dried over Na,SO, and distilled to give a colourless liquid.Yield: 21 g (75%). 'H NMR (CDCI,): 6 0.85-0.89 (t, 3 H, J6.7 Hz); 1.25-1.44 (m, 6H); 1.61-1.69 (m, 2 H); 3.46-3.51 (t, 2 H, J6.6 Hz). Preparation of N,N'-Dihexylthiourea Hexylisothiocyanate (10 g, 70 mmol) was added over a period of 1 h to a stirred solution of hexylamine (10.5 g, 104 mmol) in water (50 ml). The mixture was stirred overnight and the organic layer was extracted with CH,Cl, (100 ml). After it had been dried with MgSO,, the organic layer was evapor- ated under vacuum and the residual oil was freeze-dried over- night. The solid residue was recrystallized from hexane as a while solid. Yield: 10 g (59%). 'H NMR (CDCl,): S = 0.86-0.89 (t, 6 H, J 6.7 Hz); 1.30-1.57 (m, 12 H); 1.60 (m, 4 H); 3.39 (t, 4 H, J 6.5 Hz); 5.7 (broad, 2 H, N-H).Preparation of N,N'-Dihexyl-2-thiobarbituric Acid N,N'-Dihexylthiourea (5 g, 20.5 mmol) was added to a freshly prepared solution of sodium ethoxide [l g Na in absolute ethanol (50 ml)] and diethylmalonate (6.6 g, 41 mmol). The mixture was heated under reflux with strong agitation and a constant flow of N, for 3 days. After the mixture had been cooled to room temperature, water (50 ml) was added slowly before the ethanol was removed by distillation. The residue was poured into water (100 ml), cooled, and filtered. The fil- trate was acidified with dilute hydrochloric acid and the aqueous solution was subsequently extracted with CH,Cl, (3 x 100 ml).The combined organic layers were dried with MgS04 and evaporated to dryness. The resultant red oil was purified by gravitational chromatography on silica gel with CH,Cl,-hexane 1 : 1 as eluent to give a yellow oil which solidified upon standing. Yield: 2.6 g (41%). 'H NMR (CDCl,): 6 0.85-0.90 (t, 6 H, J 6.7 Hz); 1.29-1.44 (m, 12 H); 1.56-1.66 (m, 4 H); 3.69 (s, 2 H); 4.27-4.33 (t, 4 H, J 7.7 Hz). Preparation of N,N'-Dihexyl-2thio-5-(3-methoxyprop-l-en-2-y1ene)barbituric Acid To a vigorously stirred solution of 1,3,3-trimethoxyprop-l- ene (5.5 g, 42 mmol) in methanol (20 ml) was added rapidly N,N'-dihexylthiobarbituric acid (2.6 g, 8.3 mmol). The pre- cipitate was quickly removed by filtration and dried under vacuum. Yield: 2.2 g (69%).'H NMR (CDCl,): 6 0.79-0.85 (t, 6 H, J 6.5 Hz); 1.27 (m, 12 H); 1.62 (m, 4 H); 3.89 (s, 3 H); 4.31-4.38 (9, 4 H, J 7.6 Hz); 7.38-7.52 (4, 2 H, J 12.7 Hz); 8.02-8.06 (d, 1 H, J 11.6 Hz). 'H NMR spectra were recorded with a Bruker WH250 FT-NMR instrument with TMS as internal standard. Absorption spectra were recorded with a Hitachi U3210 spectrophotometer and fluorescence spectra were recorded with a fully corrected Perkin-Elmer LS5 spectrofluorimeter. Solutions for fluorescence studies were adjusted to possess an absorbance of (0.05 at the excitation wavelength of 510 nm. All temperature dependence studies were made with the sample cuvette housed in a thermostatted metal block having a thermocouple maintained in direct contact with the solu- tion.Singlet excited-state lifetimes were measured with a syn- chronous streak-camera following excitation at 532 nm with a 30 ps laser pulse. Approximately 100 individual traces were averaged and analysed by non-linear, least-squares computer iteration, after deconvolution of the instrumental response function. The time resolution of this instrument was ca. 40 ps. Solutions were adjusted to possess an absorption of ca. 0.15 at 532 nm and were deoxygenated by purging with N, . Fluo-rescence was isolated from scattered laser light with a 532 nm notch filter used in conjunction with narrow band-pass filters selected to isolate a 10 nm wide spectral window at 600 nm. Flash photolysis studies were made with a frequency-doubled Quantel YG481 Nd :YAG laser (pulse width 10 ns; pulse energy 40 mJ).Solutions were adjusted to possess an absorbance of ca. 0.2 at 532 nm and were purged with N,, 0,or air according to the needs of the experiment. Transient differential absorption spectra were recorded point-by-point with five individual laser shots being averaged at each wave- length. Kinetic studies were made at fixed wavelength with 50 individual laser shots being averaged and analysed by com- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 puter non-linear, least-squares iterative procedures. Where necessary, the laser intensity was attenuated with crossed- polarizers. Transient differential absorption coefficients were measured by the complete bleaching method and the laser intensity was calibrated using zinc meso-tetraphenyl-porphyrin in benzene as ~tandard.'~.' Results Structure of the Ground State The authenticity of 3 was established by 'H and 13C NMR in CDC1, , exact mass spectrometry and elemental analysis.Furthermore, the presence of the N,N,N-triethylammonium cation was clearly apparent in the 'H NMR spectrum. From examination of space-filling molecular models,' it was con- cluded that a trans arrangement of the protons in the tri- methine bridge was much preferred over the cis isomer due to the reduced degree of steric crowding. This finding was fully supported by high-resolution 'H NMR spectroscopy carried out in CDC1, solution. Close inspection of the coupling con- stants observed for the trimethine protons, which are easily identified from their particular chemical shifts,' allows com- plete assignment of the ground-state structure.Thus, the 'H NMR spectrum shows a doublet (corresponding to two protons) and a triplet (corresponding to one proton) at 8.14 and 8.54 ppm, respectively, with identical coupling constants of 13.8 Hz (Fig. 1). Such a pattern is consistent with either an all-cis or an all-trans orientation of the trimethine protons. Since an all-cis protonic configuration cannot be accommod- ated, the ground state must adopt an all-trans proton arrangement. The other bis-oxonols showed identical 'H NMR spectral patterns for the trimethine bridge and, there- fore, they are also believed to possess the trans geometry in the ground state.Furthermore, the two terminal methine protons are equiv- alent, suggesting complete delocalization of the negative charge over all four oxygen atoms. This finding appears con- sistent with FTIR spectra recorded in CHCl, since only a single C=O stretching band was observed (at 1635 cm-'), together with C=C (at 1604 cm-') and CIS (at 1510 cm-') stretching vibrational bands. The C-N amide bonds should also be considered as possessing partial double-bond charac- ter, hence bringing the terminal C-S bond into partial con- jugation with the trimethine bridge. There is, therefore, -pH(, ,'H(3'0- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 extended n-electron conjugation throughout the entire mol-ecule and numerous resonance forms can be drawn; two such forms are shown below.Finally, the bond order of the C atoms in the trimethine bridge must be ca. 1.5. S S R.NAN.R R.NAN.R o+o- o+o- Further examination of space-filling molecular models indicates that twisting of the central trimethine bridge, to form the corresponding cis isomer, involves considerable steric strain. Indeed, there is marked steric crowding between H(2) and the 0 atoms on the rotor. The resultant cis isomer appears to be slightly non-planar, suggesting that conjuga-tion between the terminal oxonol groups will be less pro-nounced than in the trans isomer. The N-alkyl groups are not responsible for this steric crowding and, therefore, the activa-tion barrier for isomerization might be expected to be only weakly dependent on the size of these substituents.One of many likely resonance forms for the cis isomer is shown below: S 'iNyAN A Photophysical Properties in Ethanol Absorption and fluorescence spectral profiles recorded for 3 in dilute ethanol solution are shown in Fig. 2. The fluores-cence excitation spectrum gave a good match to the absorp-tion spectrum while the Stokes' shift, measured in dilute ethanol solution, was 620 k30 cm-' which indicates the 2629 absence of substantial geometric changes upon promotion to the first excited singlet state. The fluorescence quantum yield (Of), measured relative to Rhodamine 101 in ethanol,I7 was found to be 0.084f0.008 and the radiative lifetime, calcu-lated from the Strickler-Berg expression," was derived as 2.15 f0.15 ns.The fluorescence lifetime (TJ was found to be 180 k 10 ps by laser-induced fluorescence spectroscopy. Laser flash photolysis studies carried out in N,-saturated ethanol solution have been described previously' and indic-ate that the triplet state is formed in extremely low quantum yield (i.e. <0.001). A second transient, which absorbs strongly at 565 nm (Fig. 3) and which does not react with oxygen, is believed to be a geometric isomer formed by rotation around one of the central double bonds.'* No other transient species were observed in the decay records and, following excitation with a 30 ps laser pulse at 532 nm, it was confirmed that the isomer was produced directly from the first excited singlet state.By completely bleaching the dye with a saturating laser pulse at 532 nm in the presence of oxygen, the differential molar absorption coefficient for the isomer at its differential absorption maximum of 565 nm was found to be (1.5 f0.2) x lo4 1 mo1-' cm-'. Using this value, the quantum yield for formation of the cis isomer (ai)was derived to be 0.65 0.07. By correcting the differential tran-sient absorption spectrum (Fig. 3) for depletion of the ground state (Fig. 2), the corrected absorption spectrum for the cis isomer was compiled and is compared with that of the trans form in Fig. 4.It is clear that the cis isomer absorbs some-what to the red of the ground state with a significantly lower maximum absorption coefficient.Such spectral changes are consistent with decreased overlap of the n-electronic orbital~'~caused by the deviation from planarity that accom-panies conversion from trans to cis isomers. This effect is associated with stereochemical inhibition of resonance. Photophysical measurements made with the other dyes in ethanol solution showed very similar behaviour. The nature of the substituent did not affect the absorption or fluores-cence spectral profiles nor the molar absorption coefficient measured at the absorption peak of 540 nm. The derived life-times and quantum yields are collected in Table 1 and it is seen that the nature of the alkyl substituent appended to the thiobarbiturate subunit exerts only a modest effect on the photophysical properties.In particular, increasing the length of the alkyl substituent causes a progressive increase in both af and t,, without affecting the magnitude of the Stokes' shift. There was a corresponding increase in the yield of the unstable isomer with increasing length of the alkyl substitu-20 1 11 I 1 1 I I I -30 ! I1I 1I II 1I tI 1I I 460 480 500 520 540 560 580 600 620 A/n m I/nm Fig. 3 Differential absorption spectrum recorded 1 ps after excita-Fig. 2 Absorption and fluorescence spectra recorded for bis-oxonol tion of bis-oxonol 3 in 0,-saturated ethanol solution with a 10 ns 3, in dilute ethanol solution. The excitation wavelength used for the laser pulse at 532 nm. The transient species is believed to be the cis fluorescence spectrum was 510 nm.isomer. 2630 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 200 I I I forces with adjacent solvent molecules. In order to assess the r 160 and/or polarity. Radiative lifetimes (q)were calculated from magnitude of such interactions, the photophysical properties of 3 were recorded in a series of solvents of differing viscosity the Strickler-Berg expression'* for each solvent (l/rf = 2.5n2 x lo8 s-',where n is the solvent refractive index at 550 nm). It was observed that the absorption and fluorescence spectral profiles and peak maxima were only slightly affected by changes in solvent and that the Stokes' shift remained essen- tially independent of solvent polarity (Table 2).These find- ings can be used to infer that there is no significant change in dipole moment upon excitation to the first excited singlet state.6 Both the fluorescence quantum yield and excited \ I singlet state lifetime were found to depend on the nature of Photophysical Properties of 3 in Different Solvents Discussion The principal route for deactivation of the first excited singlet Efiwt of Changingthe Volume of the Rotor state of the bis-oxonols involves isomerization of one of the bonds in the trimethine bridge. This process is expected to The molar volumes of the rotating groups (V,) for the various require large-scale torsional motion and to involve frictional bis-oxonols were calculated'' and the derived values are col- Table 1 Photophysical properties measured for the various bis-oxonols in dilute ethanol solution 1 538 5.28 0.058 125 0.68 9.6 100 2 540 5.27 0.066 140 0.66 10.6 120 3 540 5.28 0.084 180 0.65 11.8 140 4 540 5.22 0.100 210 0.62 13.3 180 Absorption maximum, f1 nm.'Mola r absorption coefficient at the absorption maximum, f6%. f10%. f10 ps. f0.2 ps. Molar volume. Table 2 Photophysical properties of bis-oxonol3 in various solvents at 22 "C methanol 0.632 538 700 0.055 125 0.7 1 9.8 ethanol 1.132 540 665 0.084 180 0.65 11.8 propan- l-ol 1.970 540 690 0.112 240 0.63 13.3 butan-1-01 2.607 54 1 620 0.131 250 0.63 14.5 pentan-1-01 3.441 541 620 0.140 300 0.60 15.4 hexan- l-ol 4.547 542 650 0.157 340 0.58 16.7 decan- l-ol 11.542 543 660 0.200 430 0.55 20.8 propan-2-01 3.421 540 670 0.135 290 0.61 19.4 butan-2-01 3.042 54 1 680 0.125 270 0.62 22.3 isobutyl alcoholB 3.446 542 690 0.130 295 0.61 12.2 tert-butyl alcoholh 4.420 541 665 0.180 380 0.57 33.2 pentan-2-01 3.390 542 655 0.138 300 0.60 22.6 pentan-3-01 4.024 542 670 0.170 365 0.57 26.5 isopentyl alcohol' 3.786 543 665 0.135 290 0.61 12.7 2-methylbutan- l-ol 4.460 541 670 0.150 320 0.61 13.7 cyclohexanol 56.5 543 680 0.375 800 0.17 22.9 glycerol 141.2 544 700 0.495 1040 0.02 -acetonitrile 0.345 540 700 0.090 195 0.66 47.8 DMSO 1.996 545 650 0.180 3 80 0.52 57.8 acetone 0.304 539 635 0.105 230 0.61 36.4 diethyl ether 0.242 542 655 0.080 195 0.70 35.2 ethyl acetate 0.455 54 1 630 0.1 30 275 0.61 36.5 propylene carbonate 2.53 540 670 0.200 420 0.51 49.0 1P-dioxane 1.439 540 650 0.160 345 0.54 46.3 nitromethane 0.608 542 660 0.125 270 0.62 38.5 DMF 0.924 540 670 0.150 315 0.65 45.2 Absorption maximum, f2 nm.' Stokes' shift, f25 cm-'.' f7%. f10 ps. f 10%. k0.2 ps. 2-Methylpropan-1-01. 2-Methylpropan-2-01. * 3-Methylbutan-1-01. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 lected in Table 1. Replacing the ethyl substituents with hexyl groups almost doubles the molar volume of the rotor and is accompanied by modest decreases in the rates of isomer-ization. In fact, the rates of both light-induced (k, = l/z, -l/q) and thermal (k, = l/zi) isomerization processes correl-ate reasonably well with the inverse of the molar volume throughout this small series of bis-oxonols (Fig.5) according to where kisom relates to k, or k, and A is the rate of isomer-ization for a (hypothetical) infinitely large rotor. This finding is consistent with isomerization involving large-scale tor-sional motion and suggests that the rate of isomerization should depend on frictional forces with surrounding solvent molecules.' From the intercepts to Fig. 5, rates of isomer-ization with infinitely bulky substituents are extrapolated to be ca. 2 x lo9 and ca. 5 x lo4 s-', respectively, for light-induced and thermal isomerizations in ethanol at 22°C. The slopes to Fig. 5 are quite disparate, that for light-induced isomerization (i.e. 57 mol ml-') being almost double that for the thermal process (i.e. 32 mol ml-').Such behaviour indi-cates that photoisomerization is more sensitive towards fric-tional forces with adjacent solvent molecules than is the corresponding dark reaction. Comparable behaviour was observed in ethanol and acetonitrile solutions, although the slopes of plots to eqn. (1) were significantly smaller (ie. 40 and 17 mol ml-'.respectively, for light-induced and thermal steps) in the aprotic solvent. This latter effect may arise from closer packing of a hydrogen-bonding solvent around the bis-oxonol molecule. 9.9 -. h r 9.8-(J, -9.7-9.6--1 I I9.5 4IL I1I I I I 5.0 6.0 7.0 8.0 9.0 10.0 11.0 (l/V,)/mol ml-' -75.151 15.05 T 4.75 ! I I I I I I I 5.0 6.0 7.0 8.0 9.0 10.0 11.0 (1/V,)/mol ml-' Fig.5 Correlation between rate of isomerization and the inverse of the molar volume of the rotor for (a) light-induced and (b) thermal isomerization processes for the various bis-oxonols in ethanol 2631 Photoisomerization in Alkanol Solvents The dynamics for isomerization of polar molecules may depend on viscosity21 and/or polarity22of the solvent. Many studies have shown that shear viscosity (q) often provides a poor description of the extent of friction between a rotor and adjacent solvent molecules and the concept of microviscosity has been introduced as an alternati~e.~~Similarly, solvent polarity can be presented by the E,(30) parameter,24 although this may not be entirely satisfactory in all cases.Within these limitations, the rate of isomerization (kisom)can be expressed in terms of the following general equation: The /3 term describes the dependence of the barrier height EA on solvent polarity,22 whereas the latter parameter corres-ponds to the inherent (polarity-independent) activation energy. The o! term allows for a power dependence in the effect of viscosity on the rate of isomerization2' while the pre-exponential factor B corresponds to the activationless rate of isomerization in a non-polar solvent having unit viscosity. Steady-state absorption and fluorescence spectra recorded for the bis-oxonols appear independent of solvent polarity (Table 2) suggesting that, at least to a first approximation, /3 x 0.We consider, therefore, that the dynamics of light-induced and thermal isomerization steps are essentially independent of solvent polarity. Fitting the derived rates of light-induced isomerization (k,) as obtained at 22°C in alkanols of differing viscosity25 to eqn. (2) with /3 = 0 gave a linear plot over a very wide vis-cosity range (Fig. 6). Therefore, within the quality and quant-ity of the available kinetic data, shear viscosity gives an appropriate measure of microfriction for this system. For the light-induced isomerization process, the power coefficient c1 has a value of 0.51 f0.01. The intercept to this plot corres-ponds to ln(B -EdRT) and has a value of 22.45. It is inter-esting to note that kinetic data collected in most of the alcohols, including cyclohexanol, glycerol and several branched alcohols, fit on a single line (Fig.6). The most serious exceptions to this general correlation are 2-methylpropan-2-01 and pentan-3-01. These solvents are branched at the hydroxy group and, if hydrogen bonding is important, there might be important steric factors associated with organizing the solvent cage around the bis-oxonol. This becomes an even bigger problem for isomerization of the more sterically strained cis isomer. Arrhenius plots were made for light-induced isomerization in several alkanols and were found to be linear in each case. Since viscosity can be described in terms of the following 22.5 - 22.0 - 21.5- 21 .o- 20.5- 20.0-r 19.5 ! I I I lI I I I -1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 In(tllcP) Fig.6 Effect of solvent viscosity on the rates of light-induced isom-erization for bis-oxonol3 in a series of alkanol solvents at 22 "C Table 3 Activation energies and inherent rates of twisting derived from Arrhenius plots made for bis-oxonol3, in various solvents light-induced thermal E A B/1Ol2 EA B/lO" solvent /kJmol-' s-l /kJmol-' s-l ethanol 15.4 2.7 38 5.7 butan-1-01 15.6 2.9 38 6.1 hexan- l-ol 15.5 2.6 39 6.1 decan- l-ol 15.6 2.9 39 6.2 cyclohexanol 15.3 2.8 34 1.7 tert-bu t yl 16.2 3.2 42 12.5 alcohol isobutyl 15.3 2.7 35 1.7 alcohol acetonit rile 17.3 3.5 45 14.7 empirical equation25 where qo is the limiting viscosity at 0 K and E, refers to the activation energy for viscous flow, the intercepts and slopes obtained from these Arrhenius plots can be used to derive estimates for B and E,.The resultant values are collected in Table 3 and it is seen that both the activation energy and the inherent (activationless) rate of twisting of the rotor are only weakly dependent on the identity of the solvent. Again, alka- nols that are sterically crowded at the hydroxy group might be considered as exceptions to the general behaviour. For 2- methylpropan-2-01, for example, both the activation energy and the inherent rate of twisting are significantly higher than for linear alkanols and approach those found for aprotic sol- vents (Table 3). Such behaviour seems consistent with changes in the structure (or density) of the solvent cage and is indicative of the important role played by hydrogen bonding in controlling the mechanics of isomerization.Photoisomerization in Aprotic Solvents The photophysical properties of bis-oxonol 3 were measured in a series of polar organic solvents at 22°C (Table 2). There are no obvious effects of solvent polarity, as measured in terms of the E,(30) parameter or the static relative permit- tivity, on either spectral parameters or rates of isomerization. There are, however, large variations in rates of photoisomeri- zation among the different solvents that correlate reasonably well with changes in solvent viscosity (Fig. 7). From this correlation, both the power coefficient (a = 0.37 & 0.03) and 22.4 1 1 I I I l L 21.4"4 -1.60 -1.20 -0.80 -0.40 0.00 0.40 0.80 1.20 In(rl/cP) Fig.7 Effect of solvent viscosity on the rates of light-induced isom- erization for bis-oxonol 3, in a series of aprotic solvents (given in Table 2) at 22 "C J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the intercept to Fig. 7 [i.e. ln(B -EJRT) = 21.701 were found to be significantly smaller than those observed with protic solvents. It should also be recalled that the effects of rotor volume on the rates of photoisomerization are less pro-nounced in acetonitrile than in ethanol. These findings indic- ate to us that protic solvents promote photoisomerization and experience more pronounced microfriction, presumably due to hydrogen bonding to the bis-oxonol.A linear Arrhenius plot was observed for light-induced isomerization in acetonitrile solution, allowing determination of E, and B (Table 3). Relative to most alkanol solvents, both parameters are higher. Again, this finding appears to be indicative of hydrogen bonding lowering the barrier to pho- toisomerization but increasing frictional forces between rotor and adjacent solvent molecules. The latter may have the (apparent) effect of increasing the volume of the rotor while the former may be associated with slight differences in elec- tron density (or bond order) at the isomerizing bond. Thermal Isomerization in Alkanols Similar studies were made for the thermal isomerization process, but it was observed that the rate of isomerization was not a simple function of solvent viscosity (Fig.8). However, for the linear alkanols there was good correspond- ence between rate and viscosity. For such solvents, the power coefficient a was found to be 0.26 f0.02 whilst the intercept to Fig. 8 was 11.4 f0.08. In these solvents, it appears that the thermal process is less sensitive to changes in solvent vis-cosity (or microfriction) than is the corresponding photoiso- merization. This finding appears consistent with the observed effects of the rotor volume on the rates of forward and reverse isomerizations. Rates of thermal isomerization mea- sured in the branched alkanols and cyclohexanol (the cis isomer is not formed in glycerol) differ markedly from those found in linear alkanols, after allowing for changes in vis- cosity (Fig.8). Solvents for which branching occurs at the hydroxy group (i.e. propan-2-01, butan-2-01, pentan-2-01, pentan-3-01 and 2-methylpropan-2-01) exhibit slower rates than expected by comparing their viscosity with that of the linear alkanols. This effect is attributed to steric blocking of the hydroxy group, an explanation which demands that hydrogen bonding promotes cis to trans isomerization. This hypothesis is supported by the observation that rates of thermal isomerization are much slower in aprotic than protic solvents of comparable viscosity (Table 2). The deviation 11.6 I I 1 1 h.-b 11.0s 25 S 10.8 Ic rn 10.64 rn 10.4 --rn I 1 1 1 I10.2 I 1 1 1 I J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 from expected rate (D) can be (crudely) expressed as D = Ck,lobs/(B/?")exP(-E,/R T) (4) where [kJobsis the observed rate constant, a = 0.26, and the parameter [B exp(-EJJRT)] has a value of 9.1 x lo4 s-'. As can be seen from Fig. 9, this factor correlates reasonably well with the steric coefficient for alkoxy substituents (v) as formu- lated by Charlton.26 This correlation is consistent with hydrogen bonding between solvent and the cis isomer pro- moting isomerization to the stable trans form. Indeed, the rate of thermal isomerization observed for 2-methylpropan-2- 01 (k, x 3 x lo4 s-'), the most sterically blocked alkanol, still exceeds that expected in an aprotic solvent of equal viscosity (k x 1 x lo4 SKI;see below).Interestingly, alkanols having a branch site remote from the hydroxy group (2-methylpropan- 1-01, 2-methylbutan- l-ol and 3-methylbutan-1-01) exhibit a positive deviation from expected rate (i.e. D > 1). This cannot be explained in terms of steric hindrance at the hydrogen-bonding site but suggests that these alkanols present a lower density of solvent mol- ecules around the rotor. This is a more subtle stereochemical effect and, possibly, provides for a larger cavity in which isomerization can occur. Cyclohexanol also gives a higher than expected rate of isomerization which might be associ- ated with the manner in which the solvent packs around the rotor. Linear Arrhenius plots were obtained for thermal isomer- ization in several alkanol solvents and the derived B and E, values are compiled in Table 3. Inspection of these param- eters shows that, relative to photoisomerization, the thermal isomerization step is characterized by a significantly higher activation energy and a slower inherent rate of twisting in a particular solvent.The differences noted between light-induced and thermal processes must relate to different elec- tron densities (or bond order) at the isomerizing bond and to differing degrees of stereochemical strain in the reactants. For the thermal isomerizations, activation energies and inherent rates of twisting are essentially constant throughout the series of linear alkanols, but discrepancies occur for the other alka- nols.For alkanols giving rise to anomolously fast rates of thermal isomerization (i.e.cyclohexanol and 2-methylpropan- l-ol) the activation energies are lower by ca. 5 kJ mol-' while the inherent rates of twisting are only ca. 25% of those found in linear alkanols. In contrast, the sterically crowded 2-methylpropan-2-01 exhibits an activation energy and an 0.90 I 1 1 1 I 0.80t Th 1 L\ I 0.70 D 0.50 0.~70 0.80 0.90 1.00 i.io i.io i.io steric factor, w Fig. 9 Correlation between the deviation of observed rate of thermal isomerization for bis-oxonol 3 from the expected rate for a linear alkanol of the same viscosity and the steric factor for the solvent. The data were collected in (from left to right) propan-2-01, butan-2-01, pentan-2-01, pentan-3-01 and 2-methylpropan-2-01 at 22 "C.2633 inherent rate of twisting that appear somewhat higher than expected for the corresponding linear alkanol. These proper- ties are attributed to changes in the extent of interaction between solute and solvent. Thermal Isomerization in Polar Aprotic Solvents The lifetime of the unstable cis isomer was measured in a series of aprotic solvents at 22°C (Table 2). The rates of thermal cis to trans isomerization gave a reasonable corre- lation with solvent viscosity in these solvents, despite con- siderable scatter (Fig. 10). From the plot, the power coefficient a = 0.16 & 0.03 and the intercept [ln(B -EJJRT)] has a value of 10.00 0.07. As noted above, the thermal isomerization process is less sensitive to microfriction with adjacent solvents than is the corresponding light-induced step and, again, there is a marked distinction between protic and aprotic solvents.The activation energy and inherent rate of twisting measured in acetonitrile (Table 3) are both signifi- cantly higher than the corresponding values found in protic solvents. In particular, the activation energy is ca. 6 kJ rno1-l higher and this difference must be a consequence of hydrogen bonding to the rotor in alkanol solvents. The increased inher- ent rate of twisting is indicative of the actual rotor having a smaller apparent volume and suggests less microfriction with adjacent solvent molecules. Mechanics of Isomerization According to transition-state theory, the non-adiabaticity factor for thermal isomerization must be ca.four-fold lower than that for the corresponding photoisomerization process to account for the derived B values. This seems surprising in view of the steric strain and non-planarity associated with the cis isomer since the thermal reaction might be expected to exhibit 'steric acceleration'. The results can be considered, however, in terms of the medium-enhanced barrier model" where it is assumed that the arrangement of solvent mol- ecules around the solute is random with respect to the pre- ferred geometric requirements for isomerization. As such, a certain proportion of solute molecules exist in solvent cages that provide unfavourable geometries for isomerization while the remaining solute is in solvent cavities that do not restrict rotation.Interconversion of the two conformations is con- sidered to be rapid with respect to isomerization so that B = kot Km (5) where k,,, is the rate of rotation within the solvent cavity and K, is the equilibrium constant for interconversion between 10.3 '" -1.60 -1.20 -0.80 -0.40 0.00 0.40 0.80 1.20 In (rl/c P) Fig. 10 Effect of solvent viscosity on the rates of thermal isomer- ization for bis-oxonol 3 in a series of aprotic solvents (given in Table 2) at 22 "C J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the two types of solvent cage, provided K, is small. Applica- tion of this model to the bis-oxonol case requires that K, is smaller for the cis isomer than for the corresponding trans isomer. A further consequence of this model is that the power coefficient a can be equated to the fraction of the activation energy for viscous flow that is imposed by the solvent as an additional barrier to twisting of the rotor.* Throughout this study, several key points prevail: (1) thermal isomerization is less sensitive towards microfriction with adjacent solvent molecules than is the corresponding light-induced step in any given class of solvents; (2) micro-friction is less pronounced in aprotic than in protic solvents; 2 3 4 5 6 7 8 9 D.L. Taylor, A. S. Waggoner, R. F. Murphy, F. Lanni and R. R. Birge, Applications of Fluorescence in the Biomedical Sciences, Alan R. Liss, New York, 1986.Methods in Cell Biology, ed. D. L. Taylor and Y-L. Wang, Aca- demic Press, New York, 1989, vol. 30. 1. A. Hemmila, Applications of Fluorescence in Zmmunoassays, Wiley-Interscience, New York, 1991. Y-P. Sun and J. Saltiel, J. Phys. Chem., 1989,93, 8310. A. Harriman, J. Photochem. Photobiol. A: Chem., 1992,65,79. J. E. I. Korppi-Tommola, A. Hakkarainen, T. Hukka and J. Subbi, J. Phys. Chem., 1991,958482. Y-P. Sun, J. Saltiel, N. S. Park, E. A. Hoburg and D. H. Waldeck, J. Phys. Chem., 1991,95, 10336. K. Nakashima, T. Ando, T. Nakamizo and S. Akiyama, Chem. (3) inherent rates of twisting are faster in aprotic than in protic solvents; (4) hydrogen bonding lowers the activation energies, despite probable increases in volume of the rotor; and (5) the viscosity and stereochemistry of the solvent influ- ence the rates of isomerization at ambient temperature. Each of these findings relates to the extent of interaction between solvent and solute within the solvent cage.Hydrogen 10 11 12 13 14 Pharm. Bull., 1985,33, 5380. F. C. Mohr and C. Fewtrell, J. Zmmun., 1987,138, 1564. T. J. Rink, C. Montecucco, T. R. Hesketh and R. Y. Tsien, Biochim. Biophys. Acta, 1980,595, 15. A. C. Benniston, A. Harriman and K. S. Gulliya, J. Chem. Soc., Faraday Trans., 1994,90,953. R. G. Shepherd, J. Chem. SOC., 1964,4410. L. Pekkarinen and H. Linschitz, J. Am. Chem. SOC., 1960, 82, bonding to the bis-oxonol, which can occur at S, N or 0 atoms, increases the apparent volume of the rotor and pro- vides for increased friction with the solvent reservoir.In order to explain the observed trend in activation energies and inherent rates of twisting, the extent of hydrogen bonding 15 16 2407. J. K. Hurley, N. Sinai and H. Linschitz, Photochem. Photobiol., 1983,38,9. A. J. Gordon and R. A. Ford, The Chemist’s Companion. A Handbook of Practical Data, Techniques, and References, Wiley-Interscience, New York, 1972. must increase in the order of proximally branched alka-nols < linear alkanols < remotely branched alkanols. These effects are more pronounced for the sterically strained cis isomer where conjugation between the terminal oxonol sub- units is less significant. We may surmise, therefore, that hydrogen bonding increases electron delocalization through- 17 18 19 20 21 S. R. Meech and D. Phillips, J. Photochem., 1983,23, 193. S. J. Strickler and R. A. Berg, J. Chem. Phys., 1962,37,814. N. S. Isaacs, Physical Organic Chemistry, Longman Group, Harlow, 1987. A. Bondi, J. Phys. Chem., 1964,68,441. S. P. Velsko, D. H. Waldeck and G. R. Fleming, J. Chem. Phys., 1983,78,249. out the molecule and, thereby, reduces the bond order at the isomerizing bond. This work was supported by the National Institutes of Health (CA 53619). 22 23 24 25 J. M. Hicks, M. T. Vanderall, E. V. Sitzmann and K. B. Eisen-thal, Chem. Phys. Lett., 1987, 135,413. A. Spernol and K. Wirtz, 2.Naturforsch., Teil A, 1953,8, 522. J. D. Simon and S-G. Su, J. Phys. Chem., 1990,94,3656. Landolt-Bornstein, Zahlenwerte und Functionen, Sechste AuJlage, Band ZZ, Teil5, Springer-Verlag, Berlin, 1969. References 26 27 M. Charlton, J. Org. Chem., 1978,43, 3995. J. Saltiel and J. T. DAostino, J. Am. Chem. SOC., 1972,94, 6445. 1 J. Davila, A. Harriman and K. S. Gulliya, Photochem. Photobiol., 1991,53, 1. Paper 4/02348J; Received 20th April, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002627

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2635-2641 Spectroscopic Properties of Aromatic Dicarboximides Part 2.y-Substituent Effect on the Photophysical Properties of N-Phenyl-l,2-naphthalimide Attila Demeter, Tibor Berces and Laszlo Biczok Central Research Institute for Chemistry, Hungarian Academy of Sciences, H-1025 Budapest, Pusztaszeri u.59-67,Hungary Veronique Wintgens, Pierre Valat and Jean Kossanyi Laboratoire des Materiaux Moleculaires, U.P.R. 241 du CNRS, 2-8 rue H. Dunant, 94320 Thiais, France Absorption and fluorescence spectra, fluorescence decay times, fluorescence quantum yields and triplet yields have been determined for N-phenyl-l,2-naphthalimide and its phenyl-substituted methyl derivatives in different solvents. N-Phenyl-l,2-naphthalimide emits long-wavelength fluorescence in hexane (IF= 550 nm) which is red shifted by methyl substitution at the meta and para positions of the phenyl ring and by using a solvent of higher polarity.The fluorescence decays on the sub-nanosecond timescale. When the N-phenyl-l,2-naphthali- mide has o-methyl substituents in the phenyl group, they emit dual fluorescence. The location of the short- wavelength component is constant while that of the long-wavelength component is blue shifted as a result of o-methyl substitution. Ortho substitution also increases the decay time of the long-wavelength fluorescence. The results are explained in terms of vibronic interaction between the S, (m*)and S,(nn*)excited states (pseudo- Jahn-Teller effect) which is enhanced by solvent relaxation and twisting of the phenyl ring towards a coplanar geometry.Nitrogen-heterocyclic and aromatic carbonyl compounds often possess low-lying neighbouring nn* and nn* states. The close proximity of these states appears to have a significant effect on the photophysical properties. 1,2 Luminescence and non-radiative decay processes depend strongly on substitut- ion, solvent and temperature. Enhancement of the rate of radiationless transition from the lowest excited state is belie~ed~.~often to be the result of vibronic interaction between the close-lying nn* and nn* states by virtue of the potential-energy distortion (frequency change) and displace- ment (geometry change) along the vibronically active out-of- plane bending mode (pseudo-Jahn-Teller effe~t).~.~.~ As a result of the vibronic interaction between the close- lying states, the force constant of the vibronically active out-of-plane bending mode in the upper state increases while that in the lower state decrease~,~,~ thus, a red-shifted emis- sion may occur.Zachariasse et d7suggested that the dual fluorescence observed for cyano-substituted anilines, which are considered as typical 'twisted intromolecular charge-transfer (TICT) is brought about by a solvent-induced pseudo-Jahn-Teller coupling between S and the charge-transfer S, states, arguing that the N-inversion of the amino group acts as a promoting mode. The purpose of this work is to study the nature of dual luminescence and to reveal the factors responsible for the effi- cient S, +So internal conversion observed for N-phenyl-1,2- naphthalimide and some of its methyl derivatives.Therefore, substituent effects and the solvent dependence of the absorp- tion and emission spectra, the quantum yields of fluorescence and triplet formation and fluorescence decay times have been investigated by steady-state spectroscopic and time-resolved techniques. Experimental N-Phenyl- 1,2-naphthalimide (N-Ph- 1,2-NI) and its N-phenyl substituted derivatives (designated in a similar way as N-Ph- 1,2-NI) were prepared from 1,2-naphthalic anhydride (Chemsyn Science Lab.) and the appropriate substituted aniline (Aldrich) and were purified by a procedure described t Part 1: ref.11. for N-methyl-l,2-naphthalimide(N-Me-1,2-NI) previously.' All compounds were further purified by TLC (Merck PLC silicagel) using chloroform and benzene eluents and finally by recrystallization from hexane. This careful purification was essential because of the weak luminescence emitted by the compounds. The melting points of the purified compounds were as follows : N-Ph-1,2-NI, 165 "C ; N-(2-Me-Ph)-1,2-NI, 145.5 "C; N-(3-Me-Ph)-1,2-N1, 119 "C; N-(4-Me-Ph)-1,2-NI, 190"C; N-(2,4-di-Me-Ph)-l,2-N1, 137 "C ;N-(2,6-di-Me-Ph)- 1, 2-NI, 161.5 "C; N-(2,5-di-But-Ph)-l,2-NI, 209 "C. High-grade solvents were used, as described previously.' ' The UV-VIS spectra were obtained on an HP 8452a spec- trometer. Fluorescence spectra were recorded on a home-built photon-counting spectral fluorimeter, equipped with a Princeton 1140 A/B detection system, using 366 nm excita- tion.For long-wavelength fluorescence measurements, a red- sensitive grid was used in the monochromator and a Hama- matsu R929 photomultiplier was applied, which extended the spectral range for the fluorimeter up to 850 nm. Nanosecond fluorescence decay measurements were made with a time-correlated single-photon-counting technique using an Applied Photophysics SP-3 instrument.12 Pico- second lifetime determinations were carried out in Prof. F. C. De Schryver's laboratory in Leuven, Belgium, with the single- photon-timing technique, using a mode-locked argon-ion pumped dye laser which has a time resolution of 35 ps.' Triplet yields in hexane solutions were determined in laser flash photolysis experiments at 308 nm using the triplet- triplet energy transfer method with 9,lO-dibromantracene as acceptor.' In other solvents, triplet yields were obtained from transient absorption measurements, at the absorption maximum around 505 nm,' against the same naphthalimide in hexane solution which had the same absorbance at the excitation wavelength.Results Absorption Spectra UV absorption spectra of N-Ph-1,2-NI and its methyl deriv- atives were recorded in solvents of differing polarity. Charac- terization of the long-wavelength bands was based on the observed solvent shifts, on comparison with the N-Me-1,2-NI spectrum and on Huckel cal~ulations.'~ The results of Huckel calculations for N-Ph-1,ZNI are shown in Fig.1. The absorption spectra of N-Me-1,2-NI, N-Ph-1,Z-NI and N-(2-Me-Ph)-1,2-NI in hexane are shown in Fig. 2. The maxima of the absorption bands in the range 23 000-50 OOO cm-' are as follows: N-Me-1,2-NI: 29650 cm-' (337 nm), 35300 cm-' (283 nm), 39900 cm-' (251 nm), 46800 cm-' (214 nm); N-Ph-1,2-NI: 29080 cm-' (344 nm), 34030 cm-' (294 nm), 38 880 cm-'(257 nm), 46 950 cm- ' (213 nm); N-(2- Me-Ph)-192-NI: 29 500 cm-'(339 nm), 34 200 cm- ' (292 nm), 39 400 cm- '(254 nm), 46 900 cm- '(213 nm). The two short-wavelength bands are the strongest bands in all three spectra. These are structureless and show a slight red shift of ca. 100-150 cm-' if hexane is replaced by acetonitrile as the solvent.Therefore, we conclude that the 251 nm band of N-Me-1,2-N1, the 257 nm band of N-Ph-1,ZNI and the 254 nm band of N-(2-Me-Ph)-1,2-N1 correspond to the same type of .n -+ n* transition. This is also the case for the three shortest-wavelength bands. The oscillator strengths for the two short-wavelength bands of the N-phenyl compounds are greater than those of the corresponding N-Me-1,2-NI bands ; however, the intensity appears to decrease with ortho methyl substitution [as seen from the comparison of spectra (b),(c) and that of N-(2,6-di-Me-Ph)-1,2-N1,which is not shown in the figure]. The lowest-energy singlet-singlet transitions have vibra- tional fine structures of variable resolution in all solvents. A small red shift (100-200 cm-') is observed with increasing molecular orbitals energy@ LUMO 0 -0.027 0.655 HOMO -1 4P 0.821 HOMO -2 Q 1.ooo Fig.1 Hiickel n molecular ~rbitals'~ for N-Ph-1,2-NI. The size of the circles is proportional to the value of the atomic orbital coefi- cients, full and open circles correspond to positive and negative charges, respectively. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 64000 48000 32000 I 16000 -E 0m E 9 c5 48000.-0 i$ 8 32000 C 0.= 16000E sa 0 L ([I-0 48000 32000 16000 0 23000 32000 41000 50000 wavenumber/cm-' Fig. 2 Absorption spectra of (a)N-Me-1,2-NI, (b) N-Ph-1,ZNI and (c)N-(2-Me-Ph)-1,2-N1 in hexane solvent polarity which indicates that the lowest-energy tran- sitions are n -,n* transitions.The excited state formed in this transition is denoted as the S,(nn*) state of N-Ph-1,2-N1 and its derivatives. Characterization of the second singlet-singlet transition requires careful consideration of the experimental observa- tions. The weak band of N-Me-1,2-NI, with a maximum around 35 300 cm-' in hexane, is shifted to 37 300 cm-' in acetonitrile, confirming the n -+ n* character of this tran-sition. There are some indications in the spectra of the N-phenyl- 1,Znaphthalimides in non-polar solvents that this n --+ n* transition also occurs for the N-phenyl compounds. However, since the strong nn* absorption band is red-shifted when the N-methyl group is replaced by an N-phenyl group, it overlaps with the weak nn* band which is hardly observa- ble in hexane and disappears completely under the strong m*band in acetonitrile.In the following we refer to this band as the S,(nn*) band of N-Ph-1,2-NI and its derivatives (which corresponds to the S, band of the N-methyl compounds). A relatively stronger new band is observed around 300 nm for the N-phenyl compounds investigated. The band has some vibrational structure and a maximum at 34030 cm-' for N-Ph-1,2-NI in hexane. It is blue-shifted by 870 cm-' in acetonitrile. The observed solvent shift as well as the results of Huckel calculations suggest that the band corresponds to a n +n* transition, forming an S,(nn*) state which has some charge-transfer character.The results of Huckel calculations indicate (Fig. 1) that the main characteristic of the formation of this state is the electron transfer from the aniline moiety to the n* orbital of the carbonyl group, which reverses the direc- tion of the dipole moment in the S2state compared to that in the So (see the electron distribution calculated for HOMO-1 and LUMO). This is not the case, however, according to the results of the calculations for the So + S, transition (compare the electron distribution given for the HOMO and LUMO). !? J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.o h v)4-.-C 3 $ v a 0.5 t 0 3-Y-0.0 14000 18000 22000 26000 wavenumber/cm-' Fig. 3 Fluorescence spectra of N-Me-1,2-NI (O),N-Ph-1,2-NI (A) and N-(2-Me-Ph)-1,2-N1 (0)in diethyl ether.(Curves are normal-ized; Qf for N-Ph-1,2-NI, N-(2-Me-Ph)-1,2-N1 and N-Me-1,2-NI are lop4,4.6 x lop3and 0.46, respectively.) 0.52y \0.26 ... *. 0.00 0.26 0.13 0.000.00 -' 0.10 0.30 0.20 0.00 14000 18000 22000 26000 wavenumber/cm-' 2637 Fig. 1 also shows that the N-Ph bond is much stronger in the S,(nn*) state than in the S,(m*) state. The absorption spectra change with methyl substitution in the N-phenyl ring. Substitution in the meta or para position causes a red shift of ca. 1100 cm-of the S,(nn*) band. It is, however, particularly the ortho substitution of the N-phenyl ring which has a remarkable effect: for each ortho methyl substitution, a 1200-1500 cm-' blue shift is observed and a change by a factor of ca.5 occurs in thef(S,)/f(S,) ratio of the oscillator strengths corresponding to bands S, and S,. For instance, this ratio in hexane is 1.0, 0.25 and <0.05 for N-Ph-1,2-NI, N-(2-Me-Ph)-1,2-NI and N-(2,6-di-Me-Ph)-1,2-NI, respectively. Fluoroescence Spectra, Fluorescence Decay Times and Quantum Yields Fluorescence spectra of N-Ph-1,2-NI and N-(2-Me-Ph)-1,2-NI in diethyl ether are compared with each other and with that of N-Me-1,2-NI in Fig. 3. The fluorescence of N-Ph-1,2-NI is structureless with a red-shifted maximum at 17200 cm-' (567 nm) in diethyl ether. On the other hand, N-phenyl-1,2-naphthalimides with methyl substituents in the ortho position of the N-phenyl ring emit shorter-wavelength fluorescence.Methyl substitution in the phenyl ring of N-Ph-1,2-NI has a considerable effect on the fluorescence spectrum and on the photophysical properties, as seen for hexane in Table 1. Compared to N-Me-1,2-NI, N-Ph-1,2-NI shows red-shifted fluorescence with a maximum at 550 nm in hexane. This long-wavelength emission is even more red-shifted for the N-(3-Me-Ph) and N-(4-Me-Ph) compounds. Together with this shift, a decrease in the fluorescence quantum yield and in the triplet yield also occurs. Characteristic changes take place also with increasing solvent polarity : the replacement of hexane by acetonitrile causes a 1670 cm-' red shift in the fluorescence maximum of N-Ph-1,2-NI, a considerable decrease in the fluorescence yield and a smaller decrease in the triplet yield.Similar changes were observed for the 3-Me-Ph and 4-Me-Ph com-pounds. Fluorescence decay of N-Ph-1,2-NI, N-(3-Me-Ph)-1,2-NI and N-(4-Me-Ph)-1,2-NI follows first-order kinetics in non-polar and in polar solvents and the decay parameter is inde-pendent of the fluorescence wavelengths. Most probably, this indicates that emission originates from a single electronic state. An important characteristic of the fluorescence of these N-phenyl compounds is their very short lifetime. The lifetime of N-Ph-1,2-NI in hexane is 450 ps, and the fluorescence Fig. 4 Fluorescence spectra of N-(2-Me-Ph)-1,2-NI in (a)hexane, (b) decay times of N-(3-Me-Ph)-1,2-NI and N-(4-Me-Ph)-1,2-N1 ethyl acetate and (c) acetonitrile.The gaussian curves used in the are even shorter (400 and 160 ps, respectively), which are to deconvolution of the SW and LW fluorescence components are indi-be compared with the 34 ns lifetime of the N-methyl com-cated on the spectra. pound.' ' The short fluorescence decay time together with the Table 1 Photophysical properties of N-phenyl-1,2-naphthalimideand its methyl derivatives in hexane subst ituents AY/nm mf x lo4 mISc x lo2 A,"ax/nm mf x lo4 QlSc x 10' IY/nm Qf x lo4 QIsc x 10' R = H, R' = H 550 3.0 2.1 561 2.3 2.2 582 0.85 1.3 R=CH,,R'=H 506 12.0 7.1 511" 2.8" 2.7" 539 3.5 3.3 R = CH,, R'= CH, 441 110 35 " Data for N-(2,5-di-But-Ph)-l,2-NI. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 LW fluorescence maxima (Ay&,/nm) and contribution of the LW state to fluorescence (r) hexane diethyl ether acetonitrile hexane diethyl ether acetonitrile hexane diethyl ether acetonitrile substituents (d 2;l'L;y (4 (4 (4 A""" 1. LW (4 4Yw (r)1. LW (r) Gyw (4 4yw (4 Amsx R=H, R'=H 550(1.00) 567 (1.00) 605 (1.00) 561 (1.00) 582 (1.00) 605 582 (1.00) 610 (1.00) 620 R = CH,, R' = H 506 (1.00) 510 (0.95) 551 (0.38) 520" (0.85) 512" (0.52) 505" (0.44) 539 (1.00) 546 (0.84) 561 (0.59) R = CH,, R' = CH, 482 (0.49) 513 (0.26) 517 (0.18) a Data for N-(2,5-di-Bu1-Ph)-l,2-NI. very small fluorescence and triplet yields indicate the compounds had a maximum around 430-460 nm and was occurrence of very efficient internal conversion from the practically independent of further methyl substitution.The lowest excited singlet state. location of the maximum of the long-wavelength fluores- Data given in Table 1 clearly show that methyl substitut- cence, however, changed with the structure of the naphthali- ion in the ortho position of the N-phenyl group leads to con- mide and with the solvent. This is shown in Table 2, where siderable blue shift of the fluorescence maximum compared the maxima of the long-wavelength fluorescence component to the unsubstituted N-phenyl derivative. This is accompa- are given in three solvents. A significant blue shift occurs with nied by a significant increase of the fluorescence quantum ortho methyl substitution in the N-phenyl ring.Also these yield and triplet yield. These results indicate a decrease in the compounds show a relatively small solvatochromic shift. efficiency of the radiationless transition (internal conversion) Another important property given in Table 2 (in brackets) from the excited singlet state to the ground state as a result of is the contribution of the long-wavelength fluorescence com- the ortho methyl substitution. ponent to the total measured luminescence, i.e. r = aPFW/(@fw A more detailed examination of the fluorescence of the + a:"). This contribution decreases markedly with 2-methyl o-Me-Ph-naphthalimides shows that these compounds emit substitution in the N-phenyl ring and with increasing solvent dual luminescence in medium-polar solvents.Dual emission polarity. Therefore, the experimentally measured photo-appears as a partially resolved spectrum which can be physical properties, including those presented in Table 1, are decomposed into a short-wavelength and a long-wavelength of composite nature with various contributions from the component by deconvolution using gaussian curves for fitting short-wavelength and long-wavelength fluorescence-emitting (Fig. 4).The width at half-height of each gaussian, obtained states. as a result of fitting, was ca. 5000 cm-'. The short-The contribution of more than one excited state to the wavelength and long-wavelength fluorescence components fluorescence and other photophysical properties of N-phenyl- are designated as SW and LW luminescences, respectiv.ely.1,2-naphthalimides with ortho methyl substituents is clearly The SW component of the fluorescence of all 2-Me-Ph- demonstrated by the double-exponential character of the Table 3 Fluorescence yields, triplet yields and decay parameters for the short-lived fluorescence component in different solvents OR R R' R" R R R" R R R" R R' R HHH CH, H H CH, CH, H Bu' H Bu' hexane af x lo4 3.0 12 110 2.8 -ZlbS 0.45 1.3 6.6 OISCx lo2 2.1 7.5 35 2.7 benzene q x 104 0.8 7.0 61 6.2 -T,/ns 0.24 0.8 3.8 qscx lo2 1.2 3.0 16 2.2 diethyl ether mf x 104 1 .o 5.0 46 5.0 z,/ns 0.2 0.52 2.9 0.23 qscx lo2 1.7 3.3 15 2.2 ethyl acetate afx lo4 -4.0 37 3.7 z,/ns -0.2 1.9 0.12 qscx lo2 1.4 3.0 14 2.8 dichloromethane #f x lo4 0.8 6.0 104 2.9 -Tllns -0.18 3.1qScx lo2 1.4 2.5 12 1.4 acetonitrile q x 104 0.3 3.3 94 3.6 -T,/ns <0.1 0.21 2.2 xqsc lo2 0.9 2.1 10 1.7 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 fluorescence decay observed for N-(2-Me-Ph)-1,2-NI and N-(2,5-di-But-Ph)- 1,2-NI. A short-lived component (zl)with a large pre-exponential factor (cl) and a long-lived component (7,) with a small pre- exponential factor (c,) were identified. The c1/c2 ratio of the pre-exponential factors was of the order of 10' and the decay parameter of the longer-lived component (7,) was ca. 10-16 ns. z,varied with structure and solvent polarity. It increased with methyl substitution in the ortho position of the N-phenyl ring. Thus, the z1decay time in hexane was 0.45, 1.3 and 6.6 ns for N-Ph-1,2-N1, N-(2-Me-Ph)-1,2-N1 and N-(2,6- di-Me-Ph)- 1,2-NI, respectively.z1 also decreased with increasing solvent polarity. Thus, z1for N-(2-Me-Ph)-1,2-NI was 1.3, 0.52 and 0.21 ns in hexane, diethyl ether and acetoni- trile, respectively. Moreover, a similar dependence of z1 on solvent polarity was obtained for N-(2,6-di-Me-Ph)-l,2-N1. The z1parameters together with the triplet yields for various solvents are summarized in Table 3. Discussion Energy Scheme We previously estimated" the energy of the lowest singlet excited state of N-Me-1,2-NI to be 73 kcal mol-'. Assuming that the vibrational structure in the longer-wavelength absorption bands of N-Ph-1,2-NI and of its methyl deriv- atives is similar to the vibrational structure of the N-Me-1,2- NI bands (Fig.2), the S,(nn*) and S,(nn*) state energies of N-phenyl compounds are obtained. The energy diagram for hexane derived in this way is shown in Fig. 5. The energy gap between the S,(nn*) and S,(nn*) states is smallest for the p-Me and rn-Me substituted compounds (ca. 13 kcal mol-'), it has an intermediate value for N-Ph-1,2-NI and increases by 3 kcal mol-' for each further ortho methyl substitution up to 22 kcal mol-', which is attained for N-(2, 6-di-Me-Ph)-1,2-N1. Similar results are obtained in acetoni- trile, except that the S,(nn*) energies are lower by 1.2-2.2 kcal mol-' and the S,-S, energy gaps are larger by 3-4 kcal mol-'. Model of Fluorescent States The energies given in Fig.5 correspond to those of the Franck-Condon (FC) states. The differences between the nx 90 * * nx 86 nn 07 energies of the S, and S, states are rather large which is at least partly due to the reversal of the direction of the dipole moment in the formation of the S, state which results, imme- diately after formation, in a solvent environment correspond- ing to a much higher excited-state energy than the equilibrium energy. If relaxation did not change the S,-S, energy gap considerably, the photophysics of the N-Ph- 1,2- NIs would be controlled solely by the lowest, S,(nn*) elec-tronic state. This is, however, not in accordance with a number of observations. Therefore, a model ought to be developed which can explain the observed dependence of the fluorescence spectra, the decay kinetics and singlet lifetimes, as well as the quantum yields of radiative and non-radiative processes on the structure of the N-Ph-1,2-NIs and on the solvent.In agreement with the kinetic scheme previously ~uggested'~to explain the dual fluorescence observed in the case of N-Ph-2,3-NI, we assume that the FC state, formed as the result of absorption of light, undergoes two competitive relaxation processes, yielding two different excited states separated by an energy barrier that is high enough to prevent mutual transformation. These singlet excited states are desig- nated as SW and LW states, indicating that they are responsible for the emission of the short-wavelength and long-wavelength fluorescence, respectively, emitted by some of the N-Ph-1,2-NIs studied.The SW state is assumed to be similar to the luminescent state of the N-alkyl-1,2-NIs which has a geometry quite similar to that of the FC state. (Small changes in some inter- atomic distances and pyramidalization' of the originally planar nitrogen atom17 may occur.) Therefore, the energy of the SW state is expected not to be far below the energy corre- sponding to the FC state geometry, the energy difference between FC and SW being mainly determined by the solvent- cage relaxation occurring alongside the fast vibrational relax- ation. On the other hand, in addition to vibrational and solvent- cage relaxation, an intramolecular geometrical relaxation is assumed to occur, in the formation of the LW from the FC state.The intramolecular geometrical relaxation consists of the partial twisting of the phenyl group, changing the angle between the phenyl and the phthalimide planes from ca. 49" (obtained by X-ray measurements17) in the FC state towards nx* 97 * nx 94 ** * xx * xn * 74.4 lclc 74.6 xlc xx 73.2 73.473.1 N-(4-Me-Ph)-l,2-NI N-(3-Me-Ph)-l,2-N1 N-Ph-l,2-NI N-(2-Me-Ph)-l,2-N1 N-(2,6-diMe-Ph)-l,2-N1 Fig. 5 Energy diagram of the non-relaxed S,(m*) and S,(nn*) states of N-Ph-1,ZNI and its methyl derivatives in hexane a coplanar geometry. The stabilization of the coplanar struc- ture is explained in terms of the extended conjugation spread- ing over the phenyl and the naphthalimide moieties.The energy of the ‘excited state with extended conjugation’ (ESEC) is expected to lie well below the energy of the FC state. An additional factor which stabilizes the LW state is the charge-transfer character of this state. Although the structure developed in the excited state is more favourable for phenyl rotation than the structure of the ground state (because of the elongated bonds in the excited state), it is probable that, even in the case of the unsubstituted N-Ph- 1,2-NI, steric hindrance prevents sufficient rotation of the phenyl group for a co-planar structure to be attained in the LW state. Therefore, the stabilization energy associated with the development of the ESEC is only partly realized.In addition, the phenyl rotation is hindered to a greater extent for or tho-substitu ted derivatives than for unsu bs ti tuted com- pounds. Radiative and Non-radiative Processes The effect of naphthalimide structure and the influence of the solvent on the rate of radiative and non-radiative processes depend on (i) how these factors change the branching ratios of SW and LW state formation from FC and (ii) how they influence the rates of photophysical processes setting out from the two excited states. The energy change corresponding to the structural and solvent-cage relaxation during the formation of the SW state is relatively small compared to the S,-S, energy gap in the FC state. Therefore, the energy gap will be similar in the SW to that in the FC state and the photophysical processes occurring from SW are determined by the characteristics of the S,(nn*) state for ortho-substituted naphthalimides.In accordance with the significant S,-S2 energy gap, the decay time of the SW luminescence was relatively long (10-16 ns) and the luminescence was quenchable by oxygen. Moreover, the location of the SW luminescence was practically indepen- dent of methyl substitution (Fig. 4).Therefore, we expect that the rate coeficient of the FC +SW transition is practically constant and independent of structure. However, for LW state formation, the relaxed S,(nn*) state energy may approach the S,(nn*) energy, i.e. the S,-S, energy gap may become small, if a nearly coplanar geometry is attained by phenyl group rotation.This is expected because (i) the energy of the S,(nn*) state is decreased, due to the structural relaxation, much more than that of the S,(nn*) state, as verified by the Huckel calculations, and (ii) stabiliza- tion by solvent relaxation is more significant for the S,(nn*) state (consider the dipole moments and electron distributions in Fig. 1). The small S,-S, energy gap may cause strong vib- ronic interaction between the close-lying S,(nn*) and S1(nn*) states. This induces a potential-energy distortion in the lowest excited state and a displacement along the vibronically active mode (which may be an out-of-plane bending or, even more likely, a phenyl torsion mode), i.e. a pseudo-Jahn-Teller effect3v5g6 occurs.Vibronic interaction between the two nearby states increases the force constant of the vibronically active mode in the upper state and decreases it in the lower The proximity effect has significant consequences on the photophysics: (i) it further decreases the lowest singlet- state energy and increases the red shift of the fluorescence; a dominant FC + LW transition occurs because the formation of the LW state from the FC state becomes energetically more favourable than that of the SW state; (ii) a rapid radi- ationless transition takes place from the LW state to the ground state. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The conditions for the rotation of the phenyl group and the formation of a nearly coplanar geometry are most favour- able in case of the naphthalimides with no ortho-methyl sub- stituents, i.e.for N-Ph-1,2-N1, N-(3-Me-Ph)-1,2-NI and N-(4- Me-Ph)-1,2-NI. Therefore, only LW excited states are formed from the FC state of these compounds, as indicated by the 100% contribution of LW to fluorescence (see Table 2). In accordance with the significant pseudo-Jahn-Teller effect, the LW fluorescence decay times are short (<500 ps) and as a consequence, the fluorescence yields and triplet yields are small. The 3-Me and 4-Me substitution (which increases the dipole moment in the S, excited state) and the increasing solvent polarity (which stabilizes the S, state) further decrease the lifetime of the LW state and the quantum yields of the fluorescence and ISC processes of the LW state.Twisting of the phenyl group towards a coplanar geometry is hindered by the presence of an ortho methyl group in the N-phenyl ring. Therefore, for such compounds, the formation of the SW state from FC is expected to compete with the LW state formation. (The bulky ortho methyl group reinforces pyramidalization of the N atom in the SW state, which pre- vents any SW + LW conversion that may have occurred in the unsu bs ti tu ted N-Ph- 1,2-NI.) Owing to the ortho substitution, a larger angle exists between the rings of the phenyl and naphthalimide moieties in the LW states of the o-methylphenylnaphthalimidesthan in those of the unsubstituted compounds. At this larger angle, the LW excited-state energy will be higher, while the corre- sponding ground-state energy will be lower than in case of the unsubstituted compound.Consequently, the energy differ- ence between the LW and its FC ground state will be larger for the ortho methyl derivatives than for the compounds with no ortho substituents, therefore their fluorescence will be blue-shifted (Table 1). Since the twisting angle of the phenyl group (ie. the deviation from coplanarity) appears to be the dominant factor in determining the location of the LW fluo- rescence maximum, it is not surprising that Arpwfor a given ortho-substituted derivative is practically independent of the solvent. The most remarkable effect, caused by the increase of the phenyl twisting angle in the LW state due to ortho methyl substitution, is the increased S,-S, energy gap, which implies decreased vibronic interaction between the S,(nn*) and S,(nn*) states and as a consequence a weakening- of the pseudo-Jahn-Teller effect.The result of the decreased vibro- nic interaction is the significant increase of the LW decay times, which amounts to more than an order of magnitude for N-(2,6-di-Me-Ph)-1,2-NI compared with N-Ph-1,2-NI (Table 3). It has already been mentioned that the energy difference between the unrelaxed S, and S, excited states was larger by 3-4 kcal mol- in polar than in non-polar solvents. This increase in the energy gap is, however, overcompensated by the larger energy change due to the solvent-cage relaxation of the charge-transfer-type S,(nn*) excited state.Finally, this results in a stronger vibronic interaction between the S, and S, states in polar solvents. The increased pseudo-Jahn-Teller effect shortens the lifetime of the LW state in acetonitrile compared to hexane, as found in the experiment (Table 3). The variation of the fluorescence and intersystem crossing quantum yields with ortho methyl substitution and with the solvent more or less follows the variation of the LW decay times. For triplet yields there is close correlation with l/zFw, indicating that the triplet state is formed mainly from the LW state. The values show somewhat smaller changes with ortho methyl substitution and solvent polarity than the l/z,Lw values, in accordance with the fact that both SW and LW states contribute to fluorescence.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Conclusion Dual fluorescence was observed for some of the naphthali- mides investigated in this study. The SW emission was found to exhibit similar luminescence properties to those of the N-alkyl derivatives, while the LW fluorescence appeared to be characteristic of the N-phenyl compounds. It was sug- gested that the formation of the LW state from the FC state was induced by twisting of the phenyl group towards a copla- nar geometry, which was stabilized by extended conjugation covering the whole molecule. This extended conjugation together with solvent-cage relaxation decreased the energy gap sufficiently between the charge-transfer S,(nn*) and the lowest excited S,(nn*) states to cause vibronic coupling between the two states, i.e.a pseudo-Jahn-Teller effect. The model described of ‘excited state with extended conjugation’ (ESEC) differs essentially from the TICT model proposed originally by Grabowski and co-workers.8 The major stabilizing effect in the ESEC system is the extension of conjugation to the whole, nearly coplanar molecule, which may be coupled with smaller or larger charge separation and with vibronic interaction of the two lowest excited states. On the other hand, the most important feature of the TICT state is the transfer of exactly one electronic charge from the donor to the acceptor part of the molecule, accompanied simulta- neously by a twist which makes the donor orthogonal to the acceptor moiety. In this TICT model, the proximity effect does not play any role, although it has been suggested recently’ that a solvent-induced pseudo-Jahn-Teller coupling between the S, and S, states may be a requirement for the occurrence of intramolecular charge transfer and dual fluo- rescence in excited aminobenzonitrile molecules.This work was supported by the Hungarian Science Founda- tion (OTKA, Project no. T 014079) and by an exchange pro- gramme between CNRS and the Hungarian Academy of Sciences. We thank Dr. F. C. De Schryver for allowing us to use his single-photon-timing equipment. References 1 E. C. Lim, in Excited States, ed. E. C. Lim, Academic Press, New York, 1977, vol. 3., p.305. 2 E. C. Lim, J. Phys. Chem., 1986,90,6770, and references therein. 3 R. M. Hochstrasser and C. A. Marzacco, in Molecular Lumines- cence, ed. E. C. Lim, Benjamin Inc., New York, 1969, p. 631. 4 E. C. Lim, in Molecular Luminescence, ed. E. C. Lim, Benjamin Inc., New York, 1969, p. 469. 5 W. Mofitt and A. D. Liehr, Phys. Rev., 1957, 106, 1195; A. D. Liehr, 2.Naturforsch., 1961, 16,641. 6 A. J. Duben, L. Goodman and M. Koyanagi, in Excited States, ed. E. C. Lim, Academic Press, New York, 1974, vol. 1, p. 295. 7 K. A. Zachariasse, T. von der Haar, A. Hebecker, U. Leinhos and W. Kuhnle, Pure Appl. Chem., 1993,65, 1745. 8 K. Rotkiewicz, K. H. Grellmann and Z. R. Grabowski, Chem. Phys. Lett., 1973, 19, 315; see also erratum 1973, 21, 212; K. Rotkiewicz, K. H. Grellmann and Z. R. Grabowski, Chem. Phys. Lett., 1973, 19, 315; Z. R. Grabowski, K. Rotkiewitz, A. Sie- miarczuk, D. J. Cowley and W. Baumann, Now. J. Chim., 1975, 3, 443. 9 W. Rettig, Angew. Chem., Int. Ed. Engl., 1986,25,971. 10 K. Bhattacharyya and M. Chowdhury, Chem. Rev., 1993, 93, 507. 11 V. Wintgens, P. Valat, J. Kossanyi, L. Biczok, A Demeter and T. Berces, J. Chem. Soc., Faraday Trans., 1994,90,411. 12 L. Biczok and T. Berces, J. Phys. Chem., 1988,92, 3842. 13 M. M. H. Khalil, N. Boens, M. Van der Auweraer, M. Ameloot, R. Andriessen, J. Hofkens and F. C. De Schryver, J. Phys. Chem., 1991,95,9375. 14 A. Streitwieser Jr., Molecular Orbital Theory for Organic Chem- ists, John Wiley, New York, 1961. 15 P. Valat, V. Wintgens, J. Kossanyi, L. Biczok, A Demeter and T. Berces, J. Am. Chem. SOC.,1992,114,946. 16 J. M. Lehn and J. Wagner, Tetrahedron, 1970,26,4227. 17 Y. Dromzee, J. Kossanyi, V. Wintgens, P. Valat, L. Biczok, A Demeter and T. Berces, 2.Kristallogr., submitted. Paper 4/02168A; Received 12th April, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002635

出版商:RSC    

年代:1994

数据来源: RSC