摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Dr. Peter J. Sarre Department of Chemistry University of Notti ng ham University Park Nottingham NG7 2RD, UK Faraday Editorial Board Prof. I. W. M. Smith (Birmingham) (Chairman) Prof. M. N. R. Ashfold (Bristol) Dr. B. E. Hayden (Southampton) Dr. D. C. Clary (Cambridge) Prof. A. R. Hillman (Leicester) Dr. L. R. Fisher (Bristol) Prof. J. Holzwarth (Berlin) Prof. H. M. Frey (Reading) Dr. P. J. Sarre (Nottingham) Dr. R. K. Thomas (Oxford) Editorial Manager and Secretary to Faraday Editorial Board Dr. Robert J. Parker The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF, UK Senior Assistant Editors: Mrs.S. Shah, Dr. R. A. Whitelock Assistant Editor: Mrs. C. J. Seeley Editorial Secretary: Mrs. J. E. Gibbs International Advisory Editorial Board R. S. Berry (Chicago) Y. Marcus (Jerusalem) A. M. Bradshaw (Berlin) B. J. Orr (North Ryde) A. Carrington (Southampton) R. H. OttewiII (Bristol) M. Che (Paris) R. Parsons (Southampton) M. S. Child (Oxford) S. L. Price (London) B. E. Conway (Ottawa) F. Rondelez (Paris) G. R. Fleming (Chicago) J. P. Simons (Oxford) R. Freeman (Cambridge) S. Stolte (Amsterdam) H. L. Friedman (Stony Brook) J. Troe (Gottingen) H. lnokuchi (Okazaki) J. Wolfe (Kensington, NSW) J. N. lsraelachvili (Santa Barbara) C. Zannoni (Bologna) M. L. Klein (Philadelphia) A. Zecchina (Turin) R. A. Marcus (Pasadena) C. Zhang (Dalian) urnal of the Chemical Society, Faraday Transactions (ISSN 0956-5000) is published ice monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, dton Road, Cambridge CB4 4WF, UK.All orders accompanied with payment should be nt directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd., Black- rse Road, Letchworth, Herts. SG6 lHN, UK. NB Turpin Distribution Services Ltd., dis- 3utors. is wholly owned by the Royal Society of Chemistry. 1994 Annual subscription rate : f744.00, Rest of World f800.00, USA $1400.00, Canada f840 (excl. GST). Customers ould make payments by cheque in sterling payable on a UK clearing bank or in US dollars yable on a US clearing bank. Second class postage is paid at Rahway, NJ.Airfreight and ding in the USA by Mercury Airfreight International Ltd. Inc., 2323 Randolph Avenue, renel, NJ 07001, USA and at additional mailing offices. ;A Postmaster: send address changes to Journal of the Chemical Society, Faraday Trans- !ions, c/o Mercury Airfreight International Ltd. Inc., 2323 Randolph Avenue, Avenel, NJ 001. All despatches outside the UK by consolidated Airfreight. PRINTED IN THE UK. The Royal Society of Chemistry, 1994. All rights reserved. No part of this publication may reproduced, stored in a retrieval system, or transmitted in any form, or by any means, ztronic, mechanical, photographic, recording, or otherwise, without the prior permission the publishers. lvertisement sales: tel. +44(0)71-287-3091 ;fax.+44(0)71-494-1134. INFORMATION FOR AUTHORS The Royal Society of Chemistry welcomes submission of manuscripts intended for pub- lication in two forms, Research papers and Faraday Communications. These should describe original work of high quality in the sciences lying between chemistry, physics and biology, and particularly in the areas of physical chemistry, biophysical chemistry and chemical physics. Research Papers Full papers contain original scientific work which has not been published previously. However, work which has appeared in print in a short form such as a Faraday Communi- cation is normally acceptable. Four copies including a top copy with figures etc. should be sent to The Editor, Faraday Transactions, at the Editorial Office in Cambridge.Authors may, if they wish, suggest the names (with addresses) of up to three possible referees. Faraday Communications Faraday Communications contain novel scientific work in short form and of such importance that rapid publication is war-ranted. The total length is rigorously restricted to two pages of the double-column A4 format. For a Communication consisting entirely of text and ten references, with no figures, equations or tables, this cor- responds to approximately 1600 words plus an abstract of up to 40 words. Submission of a Faraday Communication can be made either to The Editor, Faraday Transactions, at the Editorial Office in Cam- bridge or via a member of the International Advisory Editorial Board, who will arrange for the manuscript to be reviewed.In the latter case, the top copy of the manuscript including any figures etc., together with the name of the person through whom the Com- munication is being submitted, should be sent simultaneously to the Editor at the Cambridge address. Proofs of Communications are not normally sent to authors unless this is specifically requested. Faraday Research Articles Faraday Research Articles are occasional invited articles which are published follow- ing review. They are designed to be topical articles of interest to a wide range of research scientists in the areas of Physical Chemistry, Biophysical Chemistry and Chemical Physics. Full details of the form of manuscripts for Articles and Faraday Communications, con- ditions for acceptance etc. are given in issue number one of Faraday Transactions, published in January of each year, or may be obtained from the Editorial Manager. There is no page charge for papers published in Faraday Transactions. Fifty reprints are supplied free of charge. Dr. P. J. Sarre, Scientific Editor. Tel. : Nottingham (0602) 51 3465 (24 hours) E- Mail (JANET) :PCZPSF@U K.AC. NOlT.VAX Fax: (0602) 51 3466 Telex: 37346 UNINOT G Dr. R. J. Parker, Editorial Manager. Tel. : Cambridge (0223) 420066 E-Mail (INTERNET): RSCl @RSC.ORG (For access from JANET use RSCl %RSC.ORG@UK.AC.NSF NET-R ELAY) Fax: (0223) 423623 or 420247 Telex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX017

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

HAZARDS IN THE CHEMICAL LABORATORY 5th Edition '. . . easy to read, an excellent reference text, and a worthwhile investment.' Journal of the American Chemical Society reviewing the 4th Edition The new edition of this essentia laboratory handbook is the 'key' requirement for all research, development, production, analytical and teach ing Iaboratories wo rldw ide. The 5th Edition provides: New features include: expanded 'Yellow Pages' section on 0 a quick guide to the hazardous properties of 1339 substances (over 800 more than were hazardous substances , providing im med Iat e covered in the previous edition) information on hazardous properties. 0 details of the latest UK and EC regulations recommended control procedures and safety measures 0 an extremely useful emergency action check complete guide to labelling requirements tolist -users can fill in their own key contacts for hospitals, fire etc.comply with EC directives and UK legislation, including the risk and safety phrases that must 0 handy tables, symbols and statistics for ease appearof reference chapter on electrical hazards 0 a description of the American scene, including 0US legislation and safety practices -index to 'Yellow Pages' section, with highlighting differences between the UWEC synonyms of compounds index to CAS Registry Numbers and USA PVC Protective Binding xx + 676pages ISBN 085186 229 2 (1992) Price €45.00 If you have not yet ordered your copy of the NEW edition, do so now! Why take chances? Be informed and safe. To order, please contact: Roval Societv of Chemistrv. TurDin Distribution ROYAL Services Ltd, Blackhorse Road,' Letchworth, CHEMISTRYHerts SG6 lHN, United Kingdom. Information Telephone: +44 (0)462 672555 Fax: +44 (0)462 486947. Services 0956 5000(1'3'3[+ "1 ! -3

ISSN:0956-5000    

DOI:10.1039/FT99490BX019

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(5) 683-816 (1994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 683 Configuration interaction studies on the S, surface of H,CO: 2 'A'(a, z*/z, z*) as perturber of 1 'B,(n, 3s) M. Hachey, P.J. Bruna and F. Grein 689 Structures and vibrational spectra of CH,OCH,CH,OH : The hydrogen-bonded conformers F. P. S. C. Gil, R. Fausto, A. M. Amorim da Costa and J. J. C. Teixeira-Dias 697 Internal rotation in Auramine 0 P. Gautam and A. Harriman 703 Characterization of transients formed in aqueous solutions of substituted alkyl sulfides : A pulse radiolysis study D. K. Maity, H. Mohan and J. P.Mittal 711 Pulse radiolytic one-electron reduction of 2-hydroxy- and 2,6-dihydroxy-9,1O-anthraquinonesH. Pal, T.Mukherjee and J. P.Mittal 717 NMR study of E-caprolactam in various solvents. Graphical determination of monomer shift, dimer shift and dimer- ization constant from the dilution shift data J-S. Chen 721 Spin trapping of radicals formed upon irradiation of organobromine compounds with low-energy X-rays V. E.Zubarev and A. Halpern 727 Enthalpies of interaction between dimethyldioctadecylammonium bromide vesicles in aqueous solution and either dipicolinate or sulfate anions M. J. Blandamer, B. Briggs, M. D. Butt, P.M. Cullis, M. Waters, J. B. F. N.Engberts and D. Hoekstra 733 Enthalpies of mixing a non-ionic surfactant with water at 303.15 K studied by calorimetry K. Weckstrom, K. Hann and J. B. Rosenholm 739 Metallocyclodextrins of 6A-(3-aminopropylamino)-6A-deoxy-~-cyclodextrin:Their formation and enantioselective com- plexation of (R)-and (S)-tryptophan anions in aqueous solution s.E.Brown, J. H. Coates, C. J. Easton and S. F. Lincoln 745 Hydrogen evolution reaction on electrodes coated with conducting-polymer films K. Maksymiuk and K. Doblhofer 751 Response kinetics of polymer-coated bulk acoustic wave devices on exposure to gases and vapours N.J. Freeman, I. P.May and D. J. Weir 755 Influence of structure on the optical spectra of Eu3+ in Pb(PO,), glass: Molecular dynamics simulation and crystal- field theory G. Corrnier, J. A. Capobianco and C. A. Morrison 763 Synthesis, structures and electrical properties of the charge-transfer salts of 4,5-ethylenedithio-4'-5'-(2-oxatrimethylene-dithio)diselenadithiafulvalene (EOST) with linear anions [I;, IBr;, ICl,, 12Br-, AuBr;, Au(CN),] T.Naito, A. Tateno, T. Udagawa, H. Kobayashi, R. Kato, A. Kobayashi and T. Nogami 773 Reduction of cerias with different textures by hydrogen and their reoxidation by oxygen V. Perrichon, A. Laachir, G. Bergeret, R. Frety, L. Tournayan and 0.Touret 783 Water adsorption in active carbons described by the Dubinin-Astakhov equation F. Stoeckli, T. Jakubov and A. Lavanchy 787 DRIFT and mass spectrometric experiments on the chemistry and catalytic properties of small Ir clusters at the surfaces of polycrystalline a-Al,O, L. Basini and A. Aragno 797 Photocatalysts with tunnel structures for decomposition of water. Part 1.-BaTi,O, and its combination with various promoters Y.Inoue, Y. Asai and K. Sat0 803 Oxygen exchange between magnesium oxide surface and carbon dioxide H. Tsuji, T. Shishido, A. Okamura, Y. Gao, H. Hattori and H. Kita 809 Catalytic studies with dealuminated Y zeolite. Part 2.-Disproportionation of toluene N.P. Rhodes and R. Rudham FARADAY COMMUNICATIONS 815 Acoustic effects on catalytic activities of Cu and Pd thin films combined with piezoelectric lead strontium zirconium titanate activated by low-frequency voltage Y. Inoue 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/FT99490FP047

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Afanasiev, P., 193 Aldaz,A., 609 Alfimov, M. V., 109 Al-Ghefaili, K. M., 383 Douglas, C. B., 471 Dwyer, J., 383 Dyke, J. M., 17 Eastoe, J., 487 Jenneskens, L. W., 327 Jennings, B. J., 55 Jiang, P-Y., 591 Jiang, P. Y., 93 Muir, A. V. G., 459 Mukherjee, T., 71 1 Nagaishi, R., 93, 591 Nagaoka, H., 349 Sheppard, N., 507,513 Shiao, J-C., 429 Shihara, Y., 549 Shiralkar, V. P., 387 Ali, V., 579, 583 Easton, C. J., 739 Johansson, L. B.-A., 305 Naito, T., 763 Shishido, T., 803 Allegrini, P., 333 Ebitani, K., 377 Joseph, E. M., 387 Navaratnam, S., 83 Shizuka, H., 533 Allen, N. S., 83 Elisei, F., 279 Joshi, P. N., 387 Neoh, K. G., 355 Silva, C. J., 143 Amorim da Costa, A. M., Engberts, J. B. F. N., 727 Kagawa, S., 349 Nerukh, D.A., 297 Silva, F., 143 689 Eustaquio-Rincon, R., 113 Kaler, E. W., 471 Nicholson, D., 181 Simkiss, K., 641 Aragno, A., 787 Fantola Lazzarini, A. L., Kalugin, 0.N., 297 Nickel, U., 617 Singh, J., 579, 583 Aramaki, K., 321 423 Kato, R., 763 Ninomiya, J., 103 Singh, R., 583 Aravindakumar, C. T., 597 Fausto, R., 689 Katsumura, Y., 93,591 Nishihara, H., 321 Soares, V. A. M., 649 Asai, Y., 797 Favaro, G., 279,333 Kaur, T., 579 Nogami, T., 763 Soria, V., 339 Avila, V., 69 Feliu, J. M., 609 Kawashima, T., 127 Nonaka, O., 121 Spiro, M., 617 Baba, T., 187 Filimonov, I. N., 219,227 Keil, M., 403 Nuiiez Delgado, J., 553 Stoeckli, F., 783 Ball, S. M., 523 Fogden, A., 263 Kemball, C., 659 Nyholm, L., 149 Sun, L. M., 369 Barthomeuf, D., 667,675 Fornes, V., 213 Kida, I., 103 Occhiuzzi, M., 207 Suquet, H., 667,675 Basini, L., 787 Franck, R., 667,675 Kiennemann, A., 501 Ohtsu, K., 127 Surov, Y.N., 297 Bassoli, M., 363 Freeman, N. J., 751 Kim, J-H., 377 Okamura, A., 803 Suzuki, T., 549 Bauer, C., 517 Frety, R., 773 King, F., 203 Oliveri, G., 363 Tabrizchi, M., 17 Bell, A. J., 17 Frey, J. G., 17 Kirschner, J., 403 Ono,Y., 187 Takagi, T., 121 Bendig, J., 287 Frostemark, F., 559 Kita, H., 803 Oradd, G., 305 Takahashi, K., 155 Bengtsson, L. A., 559 Gans,P., 315 Klein, M. L., 253 Ortica, F., 279 Tamura, K-i., 533 Bensalem, A,, 653 Gao, Y., 803 Kleshchevnikova, V. N., Ota, K-i., 155 Tanaka, I., 349 Berces, T., 411 Garcia, R., 339 629 Otlejkina, E.G., 297 Tateno, A., 763 Bergeret, G., 773 Garcia Baonza, V., 553 Kobayashi, A., 763 Otsuka, K., 451 Taylor, M. G., 641 Bickelhaupt, F., 327 Garcia-Paiieda, E., 575 Kobayashi, H., 763 Ottavi, G., 333 Teixeira-Dias, J. J. C., 689 Biczok, L., 411 Gautam, P., 697 Kondo, Y., 121 Ozutsumi, K., 127 Teo, W. K., 355 Blandamer, M. J., 727 Geantet, C., 193 Kossanyi, J., 411 Padley, M. B., 203 Teraoka, Y., 349 Boggis, S. A,, 17 Gil, F. P. S. C., 689 Kurrat, R., 587 Pal, H., 711 Timms, A. W., 83 Borisenko, V. N., 109 Gill, D. S., 579, 583 Kuwamoto, T., 121 Palleschi, A., 435 Timney, J. A., 459 Bozon-Verduraz, F., 653 Gill, J. B., 315 Laachir, A., 773 Paradisi, C., 137 Touret, O., 773 Bradley, C. D., 239 Goede, S. J., 327 Lambert, J-F., 667,675 Pardo, A,, 23 Tournayan, L., 773 Bradshaw, A.M., 403 Gomez, C. M., 339 Langan, J. R., 75 Parsons, 9. J., 83 Trejo, A., 113 Breysse, M., 193 Gonqalves da Silva, A. M., Lavanchy, A., 783 Pedulli, G. F., 137 Tsuji, H., 803 Briggs, B., 727 649 Lazzarini, E., 423 Peng, W., 605 Tsunashima, S., 549 Brocklehurst, B., 271 Gray, P. G., 369 Leaist, D. G., 133 Pereira, C. M., 143 Turco Liveri, M. L., 311 Brown, R. G., 59 Green, W. A., 83 Lei,G-D., 233 Perez, J. M., 609 Turco Liveri, V., 3 11 Brown, S. E., 739 Grein, F., 683 Lerner, B. A., 233 Perrichon, V., 773 Udagawa, T., 763 Bruna, P. J., 683 Grimshaw, J., 75 Leslie, M., 641 Peter, L. M., 149 Umemoto, H., 549 Butt, M. D., 727 Hachey, M., 683 Li, J., 39 Petrov, N.Kh., 109 Unayama, S-i., 549 Byatt-Smith, J. G., 493 Haeberlein, M., 263 Li, P., 605 Pispisa, B., 435 Valat, P., 41 1 Caceres Alonso, M., 553 Hall, D. I., 517 Lin, J., 355 Pivnenko, N. S., 297 Valls, M. J., 609 Calado, J. C. G., 649 Hall, G., 1 Lincoln, S. F., 739 Plane, J. M. C., 31, 395 Vedrine, J. C., 193 Caldararu, H., 213 Hallbrucker, A,, 293 Lindblom, G., 305 Porcar, I., 339 Venanzi, M., 435 Calvente, J. J., 575 Halpern, A., 721 Liu,C-W., 39 Potter, C. A. S., 59 Villamagna, F., 47 Camacho, J. J., 23 Hamnett, A., 459 Liu,X., 249 Poyato, J. M. L., 23 Villemin, D., 97 Campa, M. C., 207 Hancock, G., 523 Loginov, A. Yu., 219,227 Prenosil, J. E., 587 Vlietstra, E. J., 327 Campos, A., 339 Handa, H., 187 Longdon, P.J., 315 Previtali, C. M., 69 Vollmer, F., 59 Capobianco, J. A., 755 Hann, K., 733 Lu, J-X., 39 Ramsden, J. J., 587 Vyunnik, I. N., 297 Caragheorgheopol, A,, 213 Hao, L., 133 Lunelli, B., 137 Rao, B. S. M., 597 Wang, C. F., 605 Carvill, B. T., 233 Harper, R. J., 659 Mackie, J. C., 541 Rehani, S. K., 583 Watanabe, H., 571 Catalina, F., 83 Harriman, A., 697 Maestre, A,, 575 Rettig, W., 59 Waters, M., 727 Cavasino, F. P., 311 Harrison, N. J., 55 Mahy, J. W. G., 327 Rey,F., 213 Weckstrom, K., 733 Chen, J-S., 429, 717 Hattori, H., 803 Maity, D. K., 703 Rhodes, N. P., 809 Weir, D. J., 751 Chen, Y-H., 617 Heal, M. R., 523 Makarova, M. A., 383 Richter, R., 17 Werner, H., 403 Cheng, A., 253 Heenan, R.K., 487 Maksymiuk, K., 745 Rocha, M., 143 Whitaker, B. J., 1 Cherqaoui, D., 97 Helmer, M., 31, 395 Malatesta, V., 333 Rochester, C. H., 203 Whitehead, M. A., 47 Chesta, C. A., 69 Herein, D., 403 Malcolm, B. R., 493 Rodes, A., 609 Wikander, G., 305 Chevalier, S., 667, 675 Herzog, B., 403 Mallon, D., 83 Roffia, S., 137 Williams, D. E., 345 Cho,T., 103 Higgins, S., 459 Mandal, A. B., 161 Rosenholm, J. B., 733 Wilpert, A., 287 Christensen, P., 459 Hindermann, J-P., 501 Mariani, M., 423 Rosmus, P., 517 Wintgens, V., 41 1 Climent, M. A., 609 Hirst, D. M., 517 Martins, A., 143 Rossi, P. F., 363 Wohlers, M., 403 Coates, J. H., 739 Hoekstra, D., 727 Masetti, F., 333 Rudham, R., 809 Wormald, C. J., 445 Cordischi, D., 207 Holmberg, B., 559 Massucci, M., 445 Ryde,N., 167 Yagci, Y., 287 Corma,A., 213 Hoshino, H., 479 MatijeviC, E., 167 Sachtler, W.M. H., 233 Yamaji, M., 533 Cormier, G., 755 Hosoi, K., 349 Matsuda, J., 321 Saitoh, T., 479 Yamanaka, I., 451 Corrales, T., 83 Hutchings, G. J., 203 May, I. P., 751 Salmon, G. A,, 75 Yanes, C., 575 Cosa, J. J., 69 Hutton, R. S., 345 Mazzucato, U., 333 Sarre, P. J., 517 Yoshitake, H., 155 Coudurier, G., 193 Ikawa, S-i., 103 Mchedlov-Petrossyan, N. O., Sato,K., 797 Yotsuyanagi, T., 93,479 Cullis, P. M., 727 Ikonnikov, I. A., 219 629 Sbriziolo, C., 311 Young, R. N., 271 Curtis, J. M., 239 Indovina, V., 207 Merga, G., 597 Schedel-Niedrig, Th., 403 Zhang, X., 605 Demeter, A,, 41 1 Inoue, Y., 797,815 Meunier, F., 369 Schlogl, R., 403 Zholobenko, V.L., 233 Demri, D., 501 Ishigure, K., 93,591 Mittal, J. P., 597,703, 71 1 Schnabel, W., 287 Zhong, G. M., 369 Derrick, P. J., 239 Ito,O., 571 Mohan, H., 597,703 Seddon, B. J., 605 Zubarev, V. E., 721 Diagne, C., 501 Iwasaki, K., 121 Moriguichi, I., 349 Shahid, G., 507, 513 Dickinson, E., 173 Jakubov, T., 783 Morikawa, A., 377 Sharma, A,, 625 Doblhofer, K., 745 Jameel, A. T., 625 Morokuma, M., 377 Shaw,N., 17 Doughty, A., 541 Jayakumar, R., 161 Morrison, C. A., 755 Sheil, M. M., 239 i FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Annual Congress: The Reactive Interface in Electrochemistry and Catalysis To be held at the University of Livepol on 12-15 April 1994 Further information from Dr J.F. Gibson, The Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN Neutron Scattering Group Neutron Scattering Data Analysis To be held at the Rutherford Appleton Laboratory on 13-15 April 1994 Further information from Mrs S. Humphreys, The Rutherford Appleton Laboratory, Chilton, Didcot 0x11 ORA ~~ Colloid and Inteqace Science Group Theoretical Modelling and Simulation in Colloid and Interface Science To be held at the University of Bristol on 18-20 April 1994 Further information from Dr R. Buscall, ICI Corporate Science Group, PO Box 11, The Heath, Runcorn WA7 4QE Division Autumn Meeting: Reactions and Mechanisms for Fine Chemicals To be held at the University of Glasgow on 6-9 September 1994 Further information from Dr J.F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN ~~~ ~ ~ ~ ~ ~~ Gas Kinetics Group 13th International Symposium on Gas Kinetics To be held at University College, Dublin on 11-15 September 1994 Further information from Dr H. Sidebottom, Department of Chemistry, University College, Dublin Electrochemistry Group with the SCI ELECTROCHEM 94 To be held in Edinburgh on 12-16 September 1994 Further information from Professor D. E. Williams, Department of Chemistry, University College London, 20 Gordon Street, London WClH OAJ 11 THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 97 Structure and Dynamics of Van der Waals Complexes University of Durham, 6-8 April 1994 Organising Committee: Dr B.J. Howard (Chairman) Dr P. Hamilton Dr J. M. Hutson Dr D. C. Clary Professor A. C. Legon Dr B. Soep Dr P. R. R. Langridge-Smith Since Faraday Discussion No. 73 on Van der Waals molecules, in 1982, the study of weakly bound molecular complexes has developed rapidly. Spectroscopic studies can now yield detailed information on intermolecular potential-energy surfaces in molecular systems. Studies of trimers, tetramers and higher clusters are giving insight into solvation effects and providing information on many-body forces, which are important in understanding the properties of condensed phases. Investigations of photodissociation and predissociation processes are helping us to understand the dynamics of fundamental chemical processes such as molecular rearrangement and energy transfer.In addition, Van der Waals complexes provide an opportunity to control the orientation of colliding molecules and the energies and impact parameters of reactive collisions, and have added significantly to our understanding of the pathways of simple chemical reactions. This discussion will bring together experimentalists and theoreticians who are involved in the study of Van der Waals molecules. The final programme and application form may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly, London W 1V OBN. THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 98 Polymers at Surfaces and Interfaces University of Bristol, 12-14 September 1994 Organising Committee: Professor Sir Sam Edwards (Chairman) Dr R.Buscall Professor R. H. Ottewill Dr T. Cosgrove Professor J. S. Higgins Dr R. W. Richards Dr R. A. L. Jones Nev experimental methods and new theoretical and computational techniques have recently 1 d to great progress in understanding the difficult but technologically important problems associated with the conformation of polymer molecules at surfaces and interfaces. The purpose of this Discussion is to bring together experimentalists and theoreticians working towards a molecular understanding of polymers at surfaces and interactions to survey the progress in the area to date and to indicate future directions of research.The meeting will attempt to bring a unified approach to the problem, encompassing problems of the structure of surfaces and interfaces in polymer melts, the conformation of polymers at solifliquid and liquid/liquid interfaces, and extensions towards more complicated biological systems. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly, London W 1V OBN. ... 111 THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL, DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Organising Committee Professor L. D. Barron (Chairman) Dr A.F. Drake Dr D. L. Andrews Professor R. E. Hester Professor A. D. Buckingham Traditional optical activity measurements such as CD are confined to the visible and near-ultraviolet spectral regions where they provide stereochemical information on chiral molecules via polarized electronic transitions. Thanks to prompting from theory and new developments in instrumentation, optical measurements are now being made in the vibrational spectrum using both infrared and Raman methods. Studies over the past decade on a large range of chiral molecules, from small organics to biological macromolecules, have demonstrated that vibrational optical activity opens up a whole new world of fundamental studies and practical applications undreamt of in the realm of conventional electronic optical activity.The meeting seeks to bring together experimentalists and theoreticians to discuss the current and future experimental possibilities and the development of theories, including ab initio computational methods, which can relate the observations to stereochemical details. The increasing importance now being attached to molecular chirality and solution conformation in the life sciences should also encourage the partipation of biomolecular scientists. 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Tel: + 44(0)223 420066 Fax: + 44(0)223 423623 Telex: 818293 ROYAL. vii Joint Discussion Meeting 1994 "Self-Organization of Biopolymers" Jena 10-13 April Deutsche Bunsengesellschaftfur Physikalische Chemie together with: Associazione Italiana di Chimica Fisica, Divizione di Chimica Fisica Della Societd Chimica Italiana, Faraday Division of the Royal Society of Chemistry, Soci6t6 FranFaise de Chimie, Division de Chimie Physique Organizers: Manfred Eigen, Rainer Jaenicke, John McCaskill, Peter Schuster (Germany), Len Fisher (UK), Giovanni Giacometti (Italy), Gilbert Weill (France) Self-organization is a decisive process in biological organisms complementing the conservative transfer of information.It is characterised as the emergence of functional relationships between components subject to a common dynamical process. The molecular level is of special interest in that it allows a physico-chemical description of biological organization. A paradigm of this functional self- organization is presented by the evolution of specifically encoded biopolymers from polymerization kinetics in solution. The interplay between chemical kinetics and the biopolymers it generates is the subject of this workshop.The following areas will be discussed by theoreticians and experimentalists: 0 Principles and Origins of Replication 0 The RNA World 0 Principles and Origins of Coding Systems 0 Protein Mediated Networks Spatial and Functional Organization Papers will be given by: C. Biebricher (MPI Gottingen), M. Eigen (MPI Gottingen), L. De Santis (Univ. Rome), A. Ellington (Univ. Indiana), A. Eschenmoser (ETH Ziirich), W. Fontana (Santa Fe Inst.), M. Gebinoga (IMB Jena), A. Goldbeter (Univ. Bruxelles), P. Hogeweg (Univ. Utrecht), H. Jackle (MPI Gottingen), S. Kauffmann (Sante Fe Inst), G. von Kiedrowski (Univ. Freiburg), P.L. Luisi (ETH Zurich), J.S. McCaskill (IMB Jena), J. Piitz (IBMC Strasbourg), S.Rasmussen (Los Alamos Nat. Lab.), P. Schuster (IMBJena), R. Thomas (Univ. Bruxelles), P. Willis (Univ. Auckland). Correspondence regarding participation and/or poster presentation should be sent to one of the organizers: Professor Dr J. S. McCaskill, Professor Dr Peter Schuster, Institut fur Molekulare Biotechnologie, Postf. 100813, 07708 Jena Professor Dr Manfred Eigen, Max-Planck-Institut fur Biophysikalische Chemie, Am Fdberg, 37077 Gottingen Professor Dr Rainer Jaenicke, Institut fir Biophysik und Physikalische Biochemie, Universitatsstr. 3 1, 93053 Regensburg Dr L.R. Fisher, Physics Department, University of Bristol, Tyndall Avenue, Bristol, BS8 lTL, UK Professor Giovanni Giacometti, Dept. di Chimica Fisica del Univ. di Padova, Via Loredan 2,1-35100 Padova Prof. Gilbert Weill, Institut Charles Sadron, 6 Rue Boussingaut, F-67083 Strasbourg-Ciidex ... Vlll

ISSN:0956-5000    

DOI:10.1039/FT99490BP049

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 683-688 683 Configuration Interaction Studies on the S, Surface of H,CO: 2 'A'@, n*/z,n*)as Perturber of 1 lB2(n, 3s) Michel Hachey, Pablo J. Bruna and Friedrich Grein* Department of Chemistry, University of New Bruns wick, Fredericton, New Bruns wick, Canada €38 6E2 MRD-CI calculations reveal that the third singlet surface S, of H,CO exhibits two energetically close-lying minima. The more stable state at 7.09 eV is the experimentally known 1 'B2(n, 3s).The second minimum at 7.53 eV corresponds to the pyramidal state 2'A', of mixed CT, n* and IT, n* character near equilibrium. This S, valence minimum, which apparently has not been experimentally observed, is suggested to cause the blue shift of the S,+-So system of H,CO in condensed media (as well as in related aldehydes and ketones), ruling out n +CT& ('9,) as responsible for this blue shift.The 2'A' state is non-adiabatically coupled with the ground state, explaining the observed predissociation of 1 '9, into the CH, + 0 continuum. The planar (C2J and pyrami- dal (C,) minima can be connected only through a C,intermediary, requiring participation of the antisymmetric CH-stretch and CH, bend vibrations, besides the CO-stretch and out-of-plane modes. A dipole moment p = +3.45 D (1 D x 3.33564x C m) with polarity (H2C)-0' is predicted for 1 'B,(n, 3s)at equilibrium, at variance with an experimental ,u of +0.33 D. An experimental route for studying the valence region of S, is proposed. The photochemical decomposition of H,CO into H, + CO and H + HCO via the 1'A, +-% 'A, (S, tSo) transition at energies below 4.0 eV has been studied extensively by experimental' and theoretical group^.^ By contrast, almost nothing is known about the photochemical fragmentation into CH,(8 3B,) + O(3PJ.Since these products lie at 7.64 eV (experimental),,? rupture of the CO bond can only be affected via primary absorption into electronic states lying above 7 eV. In CZVsymmetry, the dissociation channel CH2(8 3B,) + O(3P,) correlates with 'v3,'[A,, A,, B,] states of H,CO. Focusing on the singlet manifold, the surfaces So, S, and S, (the lowest singlet surfaces irrespective of symmetry) correlate with the ground-state products CH, + 0.The lower states So (8'A,) and S, (1 'A,) can be ruled out as precursors of the CH, + 0 products since rupture of the CO bond would require a large amount of vibrational energy.Instead, the vibrationally highly excited states of So and S, decompose preferentially into H, + CO or H + HCO, as these are more stable than CH, + 0 by ca. 7.6 and 4 eV, respectively.'q2 In the vertical region, the S, surface corresponds to 1 'B,(n, 3s), a Rydberg state placed experimentally at 7.09 eV,4-9 only 0.55 eV below CH, + 0. The process 1 'B, +-2'A, (S, cSo) thus constitutes a good candidate for investi- gating the dissociation H,CO CH, + 0. Interestingly, in 1935 Price" assumed that the diffuse character of the S, tSo system was caused by predissociation into CH, + 0 products.A careful analysis of the experimental literature leads to the conclusion that the S, tSo transition is more complex than expected for an n+ 3s excitation of the type non-bonding -+ non-bonding. Besides the predissociation effects mentioned above, three additional experimental peculiarities are of interest, namely (i) the blue shift in rare-gas fluids, (ii) the anomalous electric dichroism and (iii) the irregular vibra- tional structure. On the first point, the S, tSo spectrum has been recorded several times by gradually increasing the Ar density." In these experiments, the maximum at 7.09 eV (relatively narrow t Af H"data (in eV) used: -1.085 (H,CO), 4.001 (CH, ,8'B,) and 2.558 (0,3P,). band) in the Ar-free spectrum is shifted to ca.7.40 eV (rather broad band) in the Ar-dense spectrum. Since Rydberg bands should not be observable in condensed media,', their appar- ent persistence in the spectrum is an indication of the pres- ence of a valence state lying close to the s, Rydberg minimum. Note that a similar coalescence into a 'valence' band in the condensed media has been reported for the corre- sponding 'B,(n, 3s) Rydberg state of CH,HCO and (CH,),C0.12 Although there is no direct proof to substan- tiate such an assignment, the experimental literature favours n +gzo (lB2) as the underlying valence ~tate.**'~.~~ On the second point, the relative change in dipole moment for the S, + So excitation was investigated by recording the modulated electric field spectrum (electrochromism).l4 These measurements show that the transition moment is strongly affected by the applied field, a feature also pointing out that 1'B,(n, 3s) is perturbed by a nearby valence state. Finally, the vibrational structure of the vertical transition S, tSo in both the optica16*8 and electron-impact7*'' spectra looks more complex than that of the photoionization spec- trum H2CO+(% ,B,) tH,CO (% Mental1 et aL6 con-cluded that the vibrational structure extending from 7.09 to 7.37 eV involves all three a, modes v,, v2, v3, (CH- and CO- stretch, and CH,-bending, respectively), as well as the b, mode v4 (out-of-plane). Later, the high-resolution spectrum from 6.89 to 7.62 eV was interpreted by Lessard and Moule' as arising essentially from excitations of all totally symmetric modes, though the participation of the vibrations ~4(b1) and Vs(b2, CH,-rocking) was not conclusively ruled out.At this point we can assume that the anomalies reported for the vertical transition S, tSo (1 'B, 8 'A,) are caused t by a common, up-to-date unidentified state. In general terms, the perturbing state should be of valence character, lie close (geometrically and energetically) to the planar 1 'B, Rydberg minimum, and be somehow connected with CH, + 0 pro-ducts. The search for a relatively low-lying singlet excited state of valence character [other than 1 'A,(n, IT*)] can also be interpreted as asking for the location and relative stabil- ities of three valence excitations: n -+ czo ('B,), CT +n* ('B,) and n + IT*('A').None of these excitations has as yet been characterized e~perimentally.'~~-'~~''~~ In this work, an ab initio investigation of the lowest Rydberg state 1 'B,(n, 3s) and close-lying singlet valence states is carried out. The CO-stretch potential curves for both planar (C,,) and pyramidal (C,)conformations are presented for states relevant to this work. Higher-lying singlet, triplet and quintet states are discussed elsewhere.18 As shown in the next sections, the valence state perturbing (in fact, mixing with) the Rydberg portion of the S, surface of H,CO can be identified as a bent 'A' species of mixed (r, n* and n, n* com-position. Contrary to expectations, the excitation n + azo does not interact with n + 3s near equilibrium.Technical Details For carbon and oxygen the lOs6p/5s4p basis sets from Huzinaga-Dunning were used. Additional d-polarization functions were added, with exponents of 0.75 for C2' and of 0.74 and 1.52 for O.,, A CO bond function of s-type was also included with a, = 1.15.,, Rydberg AOs of s, p and d type were added to each heavy atom. In that order, the exponents are 0.023, 0.021 and 0.015 for C, and 0.032, 0.028 and 0.015 for O.,' This basis set, containing 85 contracted AOs, will be called basis I. It has been used for all MRD-CI calculations on planar H,CO. A second A0 basis set (called basis 11) was derived from basis I by replacing the two d-polarization functions on the 0 atom by a single function with an exponent of 0.85; the d-Rydberg function on 0 was deleted.This basis set, contain- ing 73 contracted AOs, was employed to study the non- planar conformations. The majority of the CI calculations reported here were carried out with 1 'A,(n, n*)SCF MOs. The standard frozen- core approximation has been used throughout. The CI results were obtained with the multireference MRD-CI method developed by Buenker, Peyerimhoff and co-worker~.~~ The total and relative energies correspond to the so-called esti- mated full CI values, which are obtained by a generalized Langhoff-Davidson c~rrection.,~ Depending on the particular symmetry representation under study, the number of reference configurations varied from 40 to 60. In general, the selected CI spaces have dimen- sions from 20000 to 32000, out of a total generated space of the order of 3 x lo6.The bulk of the calculations was carried out by selecting simultaneously the six lowest roots (states) of each symmetry; the selection threshold used was 10 pE,. The geometry optimizations of the 1 'B, (planar) and 2'A' (pyramidal) states were carried out by using a smaller selec- tion threshold T = 5 pE, and by selecting only one ('B,) or two ('A)roots. The standard C,, orientation is used throughout. The z axis (CO bond) transforms like A,; the y component lies in the molecular plane and behaves like the B, representation; the x axis transforms like B,. Starting from planar H,CO, two different C,geometries can be generated where either the molecular or perpendicular plane is maintained. These planar Table 2 Adiabatic excitation energies (in ev) with respect to C-0 methods" 1 'B,(n, 3~)~ 2 1A(o,n*/n, n*) 2 'Alp; 3PJ (left minimum) 7.08 7.53 7.98 (ca.7.06) 8.40 -7.99 9.197.93 (7.97) 8.51; 7.70 8.14; 8.49 -; (8.47) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and pyramidal conformations are labelled as C,(yz) and C,(xz), respectively. Results A summary of the equilibrium geometries predicted for the 1 'B, and 2 'A' states of H,CO is given in Table 1. Literature data on the ground states of H,CO and H,CO+ as well as the 1 'A" state of H,CO are also given in Table 1 for com- parison. Adiabatic excitation energies (with respect to C-0) for the H,CO states 1 'B,, 2 'A1, 1 'B, (C2J and 2 'A' (C,, xz) are collected in Table 2.The corresponding CO-stretch potential curves are shown in Fig. 1 ;the energetic location of the products CH, + 0 is also indicated. CO-stretch Potential Curves for Planar H2C0 W2" Symmetry) 1 'B, State This state has n -+ 3s Rydberg character at equilibrium. As shown in Table 1, the equilibrium geometry of 1 'B, as pre- dicted in this work differs by <0.02 a, for the CO and CH bonds and 3" for <HCH from the experimental ground-state values.25 Our results for 1 'B, agree closely with those pre- dicted by Feller and Davidson26 for the jf2B, state of HzCO+, in particular R(C0) and <HCH. A prior GVB in~estigation,~on the 1 'B, state of H,CO obtained slightly larger values of R(C0) and <HCH.By contrast, two recent CIS studie~~~~~~ have reported opposite changes: shorter R(CO), larger R(CH) and smaller < HCH. Table 1 Ab initio equilibrium geometries of the 1 'B, and 2 'A states of H,CO and literature data as the 8 'A, and 1 'A" states of H,CO (experimental) and on the 8,B, state of H,CO+ (theoretical) ~ R(CO)/a, R(CH)/a, <HCHIdegrees eldegrees ref. H2C0, C2,, 1 'B,(n, 3s)2.288 2.090 119.0 - this work 2.324 2.060 124.0 - 27 2.123 2.138 2.218 2.220 100.2 100.5 -- 29 28 2.274 2.080 H2C0, CZv,fl: 'A,116.2 - 25 2.287 2.079 H2CO+,C,, , x ,B,118.0 - 26 2.888 H,CO, C,, 2 'A(on*, nn*) 2.040 111.3 ca. 44 this work 2.813 2.034 122.5 49.9 28 2.8 13 2.038 121.9 46.5 29 2.500 2.075 H,CO, C,, 1 'A(n, n*) 118.4 34.0 4 of selected low-lying singlet states of H,CO as predicted by ab initio ~ 2 'A,(n, IC*) (right minimum) 1 'B,(o, n*)c ref.7.95 8.27 this workd 8.50 (7.99) (< 8.32) 30' 8.61 -28f 8.65; 8.60 8.35; 8.79 29g a Planar C,, geometries except for 2 'A' in C,(xz) symmetry. * To, = 7.09 eV, experimental (ref. 1, 4-9, 11, 12, 15, 17). 'Saddle point in C,,. Estimated full-C1 results from MRD-CI treatment. See also ref. 18. 'Langhoff-Davidson correction in brackets. CIS results including zero-point corrections. CIS (first entry) and CIS-MP2 results (second entry). Value in brackets corresponds to the vertical transition energy. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 9.5 8.5 > 7.5 G \Wd 6.5 5.5 4.5 1.8 2.3 2.8 3.3 3.8 R(CO1/a0 Fig. 1 CO-stretching potential curves for selected states of H,CO (full line, C,, symmetry; dotted lines, C, symmetry). Composition of the 2lA, state: n, 3p, (left minimum) and n, n* (right minimum). Composition of 2 'A' near equilibrium: 0, n* (45%) and n, n* (30%). The dissociation products CH,(8 3B,) + O(3P,)are also shown. The 1 'B,(n, 3s) minimum lies at 7.08 eV, compared with an experimental To, value of 7.09 eV.1*4-971 5,1 By keeping all other geometrical parameters fixed at the ground-state values, the corresponding R(C0) potential curve shows a maximum at approximately 3.05 a, and 8.9 eV (Fig. 1). Close to this maximum, the 1 'B, state undergoes a change in struc- ture, from n, 3s (bound, left side) to n, CT& (repulsive, right). Energetically, this C,, dissociation barrier lies ca.1.8 eV above the left minimum, or 1.25 eV above CH, + 0. Since this change in structure takes place geometrically and energetically far away from the n, 3s minimum, the exci- tation n -,CT:~can be ruled out as causing the S, (Rydberg) band anomalies. Moreover, it can be inferred that disso- ciation of H,CO into CH, + 0 via the vertical absorption S, tSo should involve intermediate valence configurations on the S, surface of lower than C,, symmetry (i.e. more accessible energetically). One is thus compelled to look for other low-lying valence states which may interact with the Rydberg portion of the S, surface. A closer look at the C,, potential curves in Fig.1 points out that 1 'B,(n, 3s) is crossed by 2 'A, and 1 'B, at R(C0) shorter (2.6-2.7 a,) and energies lower (8.1-8.3 eV) than cor- responding values of the 1 'B, maximum. It is suggested that either 2 'A, or 1 'B, (or both) are involved in the process So S, -+ CH, + 0 rather than the higher-lying excita- tion n -+O~~('B,). 2 'A, State As shown in Fig. 1, our study corroborates earlier predictions by Schaefer and co-~orkers~~ about the double-minimum character of the 2 'A, potential. This feature results from an avoided crossing near R(C0) = 2.55 a, between the non-adiabatic states n, 3py (Rydberg, left side) and n, n*/n2 (valence, right). The present MRD-CI data place the inner and outer minima at essentially the same energy, i.e.7.98 and 7.95 eV, respectively. In the valence region of 2 'A1, with increasing R(C0) the contribution of n2 to n, n* increases. For example, at R = 2.65 a,, one finds 50% n, n* and 25% n2; at R = 3.0 a, it is 45% n, n* and 40% n2; and at R = 4.0 a, 10% n, n* and 685 50% n2. The valence configuration CT,gz0 starts to contribute for R > 3.5 a,; at R = 4.0 a,, for instance, the 2 'A, state has ~~30% u, CT character. In other words, slightly above R = 2.5 a, the 2'A1 state is mainly of R, n* type, but at larger R it acquires a predominantly 'closed-shell' . . n2 character. The existence of such a heavy mixing between both configurations was first noticed by Allen and S~haefer.~" Related to this is the change in electronic structure under- gone by the ground state.Near equilibrium, xlAl has ca. 70% 'closed-sheT1' * * . n2 character, but the relative weight of this configuration decreases almost linearly with R(CO), to ca. 45% at R = 3.0 a,, and to ca. 10% at R = 4.0 a,. Com-plementarily, the 'open-shell' n, n* configuration increases its contribution to RIAl from 15 to 45 to 65% at the same R(C0)distances. In conclusion, for R(C0) >2.5 a, the R'A, and 2'A1 states are strongly coupled (non-adiabatically). The highest degree of mixing between -. . n2 and nn* occurs near 3.0 a,, close to the 2 'A, (pseudo) minimum. At that geometry both states are energetically separated by ca. 5.4 eV, corresponding to a region in which the ground state is vibrationally excited by ca.2.6 eV in the v,(CO) mode (roughly nv2 z 15). Since both minima of 2 'A, lie energetically slightly above the CH,+ 0 channel, a non-radiative decay of this excited state into the ground state may consequently cause the rupture of the CO bond. Other electronic states interacting with the n, n* configuration are also indirectly coupled with the ground state, so that the phenomenon of predissociation into CH, + 0" might affect several states of H,CO lying above 7.5 eV*l5,17 1'B, State The 1 'B,(o, n*) minimum lies at 8.27 eV [only 0.32 eV above that of 2 'A,(n, n*)] with an equilibrium R(C0) of 2.68 a,, ca. 0.4 a, longer than for the ground state but ca.0.24 a, shorter than for 2 'A1(n, R*). Other theoretical studies have estimated a C,, transition energy T,(1 'B,) also around 8.30 eV (Table 2). Earlier calculations by Buenker and Peyerimhoff 31 demon-strated that the planar minimum of 1 'B, actually corre-sponds to a saddle point as the configuration u,n* stabilizes into a pyramidal equilibrium structure through the out-of- plane bending mode v4. Moreover, a concomitant stabilizing effect with important photochemical consequences takes place by such a symmetry lowering. The C,, states 2 'A, and 1 'B, can mix since both species correlate with 'A' of C,(xz). A great deal of stabilization is expected to occur relative to the C,, potentials since both configurations o,n* (B,) and n, R* (A') favour non-planar conformations, and in addition their energetic separation is relatively small.CO-stretchPotential Curves for Pyramidal H,CO <c,Symmetry) 2 'A' State The optimized geometry at the MRD-CI level (Table 1) com-pares fairly well with that obtained by Foresman et aL2' and .~~Hadad et ~1 The only significant discrepancy between our results and the other studies lies in the HCH angle (ca. 10" deviation). When compared with the ground state, the major changes in geometry correspond to R(C0) and out-of-plane angle 0, with AR(C0) = 0.6 a, and A0 = 44".Since 2 'A' cor-relates with the 1 'B,(o, n*) and 2 'A1(n, n*) states [with R(C0) minima at ca. 2.7 and 3.0 a,, respectively], a relatively large R(C0) of ca. 2.9 a, predicted for 2 'A' is understand- able.Interestingly, the 2 'A'(S,) equilibrium geometry is not too different from that of the S, minimum, 1 'A"(n, n*).The data in Table 1 indicate that the major structural difference between them lies in R(C0) with AR(C0) x 0.39 a,. As shown in Fig. 1 (dotted potential curve), the 2'A' state lies well below 1 'B, and 2 'A, [for R(C0) > 2.4 a,]. Our data place the 2IA' minimum at 7.53 eV relative to the ground state, i.e. ca. 0.45 eV above 1 'B,(n, 3s) or ca. 0.10 eV below CH, + 0. Recent CIS (single-excitation) and MP2 studies by Fore- .~~ .~~sman et ~1 and Hadad et ~1 assigned to 2 'A' pure a, n* character and higher excitation energies (7.99 and 8.49 eV, Table 2). Both studies place the Rydberg states up to 1.4 eV too high, rendering the energetics useless for quantitative comparisons.The CO-stretching potential of 2'A' exhibits a rather broad minimum, a feature reflecting the mixed valence char- acter of this state. For instance, 2 'A' at equilibrium consists of 45% a, n*, 30% n, n* and 10% * -n2. At R(C0) = 3.6 a,, this state is mainly described by 8,n*, with some n'. At R(C0) < 2.4 a,, the 2 'A' state assumes n, 3py character; since this Rydberg configuration favours a planar conforma- tion, it is understandable why the 2 'A' potential lies above 2 'A, for R < 2.4 a, (Fig. 1). The relatively high contribution of n, n* to the 2 'A' state is of spectroscopic importance. Although the 2 'A' minimum lies practically at the same energy as the CH, + 0 products, both regions are separated by a huge energy barrier (Fig.1). In other words, despite the fact that the 2 'A' S, minimum is connected adiabatically with ground-state CH, + 0, a large amount of vibrational energy is needed to reach dissociation. Nevertheless, rupture of the CO bond can proceed without any additional vibrational excitation of 2'A' by the non- radiative decay S, -+ So, a process in which the energy-rich state S, (2 'A') releases its electronic energy into nuclear motion (CO-stretching in particular) of the ground state. Dipole Moments The ground state has an experimental dipole moment p = -2.33 D,32 with a polarity (H,C)+O-. The present MRD-CI study slightly overestimates this quantity, giving p = -2.56 D with 8 'A, SCF MOs and p = -2.42 D with natural orbitals.Other dipole moments studied here should similarly be accurate to ca. f10%. For 1 'B,(n, 3s) at equilibrium, we obtain p = +3.45 D, corresponding to a reversed polarity (H,C)-O+. This rela- tively high p value confirms an earlier GVB-CI result of +3.068 D.33Both theoretical estimates are at variance with an experimental result of +0.33 D obtained by electro- chromism determinations. l4 However, all studies agree in that the n -+ 3s excitation is accompanied by a shift of elec- tron density from oxygen to the CH, group. Quite interestingly, measurements of the Stark effect in H,CS led to dipole moments of -1.649 D for X'A, and +2.2 D for 1 'B,(n, 4s).4b These experiments indicate that a ground-state polarity (H,C)+S- is reversed to (H2C)- S+ in the 1'B, Rydberg state, a similar behaviour as in H,CO.Moreover, the relative change Ap/p(R 'A,) is of comparable magnitude in both molecules. For H,CS it is 2.33 (e~perimental),~'and for H,CO it is 2.43 (this work), 2.30 (ref. 33) and 1.14 (e~perimental'~). This discrepancy between theory and experiment in H,CO could be settled by carrying out on H,CO the same type of Stark-broadening measurements as have been done on H,CS.46,34 According to Li~tay,~' the major difference between dipole moments derived via Stark effect or electro- chromism resides in the procedure used to evaluate the experimental data (in both techniques the absorption spec- trum is analysed under the influence of an external electric J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 field). While the Stark effect focuses on the broadening and splitting of certain lines, electrochromism measures the field dependence of the molar absorption coefficient (i.e. band intensity). In other words, the variation of the transition moment with the applied field constitutes the key quantity for electrochromism, but is irrelevant for the Stark effect. The electrochromism experiment^'^ detected a pertur-bation on 1'B,(n, 3s) caused by a nearby state (assumed to be of 'B, symmetry, too) as the field strength was varied. In fact, 2 'B,(n, 3pz) lies only 0.9 eV above 1 'B,, so that 3s-3pZ mixing can occur. The dipole moment of 2 'B, has been pre- dicted to be ca. -2.2 D1*and -1.83 D,33 i.e.slightly smaller than for the ground state but having the same polarity (H,C)+O-. Assuming that the applied field induces signifi- cant 3s-3pz mixing, the dipole moment derived for 1'B, via electrochromism should not be as strongly (H,C)-O+ pol- arized as theory predicts because of the large contribution of the (H2C)+O- structure from 2 'B,. At its pyramidal minimum, the predicted dipole moment of 2 'A' is -3.47 D, with a polarity (H,C)+O- like the ground state or the planar 2 'A, state.30 The vector p has an out-of- plane angle 8 = 34", almost parallel to the CO bond (e = 440). This result points out the extent of the electronic charge reorganization taking place between the 1'B, (Rydberg) and 2 'A' (valence) states. Since we obtained p(1 'B,) = +3.45 D, it follows that in going from 1 'B, to 2 'A' the polarity is reversed, corresponding to a total change of ca. 6.9 D.This is understandable since for 1 'B,(n, 3s) one has a H,CO+ core plus a spatially extended 3s electron, whereas for 2 'A' (a, n*; n, n*) the electron cloud is obviously more compact. It is worth emphasizing that this drastic polarity reversal is caused primarily by a shift of the CO electron density from C-0' to C'O- for an increment AR(C0) of only 0.6 a,. Other electric properties of S, (quadrupole moment, pol- arizability etc.) are also expected to change significantly along the reaction path connecting the minima of 1'B, and 2 'A'.The same behaviour is expected for the transition moment of S, + So,so that the intensity distribution of this system is no longer governed by the Franck-Condon approximation. Discussion Earlier speculations about the existence of a nearby state per- turbing 1'B,(n, 3s) at 7.09 eV are confirmed by the present MRD-CI study. At equilibrium, the valence state 2 'A' (mixed a, n* and n, K*) lies only 0.45 eV higher than 1'B,. The n, a&( 'B,) state, thought by experimentalists as low-lying and perhaps responsible for some of the S, t So irregularities is seen to cross 1'B,(n, 3s) at energies and geometries far removed from the 1'B, minimum, and does therefore not qualify as perturber. Since the excitation spectrum of H,CO has been recorded several time~,'*~-~*l it appears somewhat astonishing that 'a1 none of these experiments claims the existence of a valence state (ie.2 'A') lying adiabatically near 7.53 eV (60 740 cm-'). However, this apparent failure is in part understandable since direct absorption 2 'A t2 'A, should consist of a long pro- gression of low-intensity peaks. Regardless of the magnitude of the transition moment, our statement is justified by the rather small Franck-Condon factors (FCFs) expected for a transition having such large changes in geometry. Note that the valence portion of the S, surface (i.e. 2'A') might have been detected indirectly during the Ar high- pressure experiments carried out by Messing et al." via verti-cal absorption into the Rydberg portion of S, .According to these authors, the n -+ 3s transition at 57 192 cm- ' reveals a vibrational structure which persists up to the liquid density; J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 moreover, the band becomes rather broad (half width ~3000 cm-') and the absorption maximum is blue shifted (ca. 2500 cm-') as the Ar pressure is increased. A broad band extend- ing from roughly 58000 to 61 250 cm-' (7.19 to 7.59 eV), with a maximum at 59345 cm-' (7.36 eV) compares quite well with the present theoretical estimate of T, = 60 740 cm-' (7.53 eV) for the 2 'A' tj?: 'A, excitation. The broadness of the band can also be correlated with the different equilibrium conformations of the upper and lower states. The discussion above sheds light on what is actually taking place in these high-pressure experiments : the non-verticality of the 2 'A + 'A, transition (poor FCFs) is partly compen- sated for by the vertical Rydberg excitation 1 'B, tj?: 'A, (favourable FCFs).This initial Rydberg population might evolve into valence population through internal conversion. The population flow from 1 'B, to 2 'A' is favoured with increasing Ar pressure by at least two effects: First, the S, surface gradually loses its Rydberg character (which ener- getically implies higher absorption energies) and secondly, the collision frequency increases, thereby facilitating intermode vibrational couplings. At this point, an alternative experimental route for directly studying the valence region of S, can be envisaged, namely by optical absorption from the S, (1'A", nn*) s~rface.~ As shown in Table 1, the states 2 'A and 1 'A have comparable equilibrium geometries and consequently their Franck-Condon factors are more favourable than those between 2 'A' and 8 'A,.The feasibility of experimentally observing the excitation 2 'A t 1'A (S, tS,) is additionally supported by a relatively long radiative lifetime of ca. 10 ps assigned to 1 'A" (S,).' However, a dificulty may arise from the concur- rent decomposition process s, + H, + CO,'., which could significantly reduce the 1'A population available for absorption. Fortunately, formaldehyde isolated in solid rare- gas matrices does not decompose into H, + CO after S, +-So absorptionL6 (as the coupling between S, and the disso- ciation continuum is essentially eliminated).Since the matrix cage at the same time suppresses the Rydberg states, the most practical experimental route to detect 2 'A' appears to be by 2 'A't 1 'A" absorption in a matrix. On the basis of an experimental T,(1 'A) = 3.50 eV,' the valence-valence tran- sition s, ts, is expected near 4.0 eV (ca.32 OOO cm-'). As the stationary minima of 1'B, and 2 'A' both lie on S, , the question arises about the vibrational mechanism connect- ing these two regions. The main dilemma to be solved is the following. By lowering the symmetry from C,, to C,(xz) via the v4 mode (out-of-plane), the 'B, planar state correlates with 'A" but not with 'A.The only possibility to obtain an intermediate conformation having a spatial symmetry which correlates with both 1'B, (C2J and 2 'A' (C,, xz) is to work within the C, point group. Such an intermediate C, structure can be visualized, for instance, by distorting pyramidal H,CO (C,, xz) through the two antisymmetric a" modes, i.e. CH-stretch and CH,-rocking. Similarly, starting from planar H2C0 (C,,), the C, complex can be obtained by the simultaneous activation of the antisymmetric vibrations b, (v4,) and b, (v5 or v6, or both). Moreover, since the CO equilibrium distances are quite different for 1 'B, and 2'A', the v2(C0) stretching motion should also play an important role along the path c,,-b c, --* C,(xz). In short, internal conversion between both S, minima through a C, intermediary requires the simultaneous activa- tion of several vibrational modes : CO-stretch, out-of-plane bend and at least one of the CH- and CH,-antisymmetric distortions.The existence of a strong coupling between various modes justifies the complex structure of the Rydberg portion of the S, tSo spectrum and the difficulties found by experimentalists in obtaining a satisfactory vibrational a~signment.~,~,~Obviously, the double-minimum topology of the S, surface of H,CO does not exactly match with the single minimum of the ground-state surface of H2CO+,18 a feature also explaining why the experimental band profile of the S, tSo transition is much broader and more complex than that of the photoionization spectrum H,CO+ tH,CO. Assuming that the 2 'A' basin of S, is populated indirectly from 1'B, via the vertical absorption 1'B, +-ft 'A1, several decay processes may act upon 2 'A'.For instance, radiative emissions such as 2'A'+ 1 'A" (AEoo~4.0eV) and 2 'A' -+ 'A, (AllOOx 7.40 eV) are possible. The former process is expected to generate vibrationally excited 1 'A" molecules, which in turn may decompose into H + HCO or H, + CO. On the other hand, the non-verticality of the emis- sion 2 'A' -+ j?: 'A, would generate So molecules vibrationally excited up to roughly 3 eV. Since such internal energy is smaller than the barrier for the thermal decomposition So +H, + CO,' this emission process would allow the study of vibrational relaxation within the So surface.Parallel to these radiative emissions, the 2'A' state may decay non-radiatively into 8 'A,. This process will lead directly to CH, + 0 fragments. Summary and Conclusion The observed spectrum for the transition 1 'B,(n, 3s) t% of H,CO shows diffuseness and a complex vibrational structure. Previously, the perturbing state was assumed to be n, a&. However, the CI calculations presented here find n, a& to be very high in energy. Instead, we propose the perturbing state to be the non-planar 2'A', consisting of a, n* mixed with n, n*.Its minimum is calculated to lie only 0.45 eV above the n, 3s minimum, shifted towards a larger CO distance by 0.6 a,. Since in pyramidal C, symmetry 'B, transforms into 'A", the internal conversion 1'B,(n, 3s) -+ 2 'A'(a, n*/n, n*) can only take place by further lowering the symmetry to C,.It is shown that in C, symmetry, 1 'B,(n, 3s) and 2'A'(a, 7c*/7c, n*) lie on the same surface, the third singlet surface commonly designated by S,. On that surface, a low barrier, estimated to be <0.7 eV above the 1 'B, minimum, exists between the two minima. This has to be overcome by vibra- tional excitation. Since the internal conversion 1 'B, + 2 'A' can only take place in C, symmetry, participation of anti-symmetric vibrational modes v5 and v6 (b,)and the out-of- plane motion v4 (b,) are expected, as well as the C-0 stretch motion v2 due to the required change in C-0 distance, and most likely also v1 and v3 (all a').Here the C,, notations are used. This explains the complex vibrational structure of the transition in question, and the observation of many, if not all, vibrational modes of H,CO, After internal conversion from 1 'B,(n, 3s) to 2 'A' has taken place, coupling with high vibra- tional levels of the ground-state %'A, may lead to disso- ciation into the products CH, + 0, which lie energetically slightly above the minimum of 2 'A'. Since both the ground state and the 2'A' state contain part of the n, n* configu-ration, the two states can couple non-adiabatically. The blue shift observed for the 1'B, tj?: 'A, system under higher pressure [also observed for CH3HC0 and (CH,),CO] can similarly be explained by internal conversion from the Rydberg state 1 'B, to the valence state 2 'A'.An experimental route to studying the valence region of S, ,by optical absorption from 1'A"(n, n*),is proposed. The authors thank NSERC (Canada) for financial support and Computing Services of the University of New Brunswick for the allocation of computer time. 688 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 H. Okabe, Photochemistry of Small Molecules, Wiley, New York, 1978; C. B. Moore and J. C. Weisshaar, Annu. Rev. Phys. Chem., 1983,34,525. J. D. Goddard and H. F. Schaefer, J. Chem. Phys., 1979, 70, 5117; M. Dupuis, W. A. Lester, B. H. Lengsfield and B. Liu, J. Chem. Phys., 1983, 79, 6167; G. E. Scuseria and H. F. Schaefer, J. Chem. Phys., 1989,90, 3629, and references therein.D. L. Baulch, R. A. Cox, R. F. Hampson, J. A. Kerr, J. Troe and R. T. Watson, J. Phys. Chem. Ref. Data, 1984,13,1259. (a) D. C. Moule and A. D. Walsh, Chem. Reo., 1975, 75, 67; (b) D. J. Clouthier and D. A. Ramsay, Annu. Rev. Phys. Chem., 1983, 34, 31; (c)D. J. Clouthier and D. C. Moule, in Top. Curr. Chem., Springer Verlag, Berlin, 1989, vol. 150, pp. 167. G. Fleming, M. M. Anderson, A. J. Harrison and L.W. Pickett, J. Chem. Phys., 1959,30,351. J. E. Mentall, E. P. Gentieu, M. Krauss and D. Neumann, J. Chem. Phys., 1971,555471. M. T. Weiss, C. E. Kuyatt and S. Mielczarek, J. Chem. Phys., 1971,54,4147. C. R. Lessard and D. C. Moule, J. Chem. Phys., 1977,66,3908. P. Brint, J. P. Connerade, Ch. Mayhew and K. Sommer, J. Chem. SOC.,Faraday Trans.2,1985,81,1643. W. C. Price, J. Chem. Phys., 1935,3,256. I. Messing, B. Raz and J. Jortner, Chem. Phys., 1977, 23, 351; 1977,25, 55. M. B. Robin, Higher Excited States of Polyatomic Molecules, Academic Press, New York, 1974 and 1985, vol. 1-111. A. D. Walsh, Proc. R. SOC. London, Ser A, 1946, 185, 176; G. Lucazeau and C. Sandorfy, J. Mol Spectrosc., 1970,33,274. G. C. Causley and B. R. Russell, J. Chem. Phys., 1978, 68, 3797; J. Am. Chem. SOC., 1979,101,5573. S. Taylor, D. G. Wilden and J. Comer, Chem. Phys., 1982, 70, 291. B. Niu, D. A. Shirley and Y. Bai, J. Chem. Phys., 1993,98,4377, and references therein. 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 M. Hachey, P. J. Bruna and F. Grein, to be published. T. H. Dunning, J. Chem. Phys., 1971,55,716. T. H, Dunning and P. J. Hay, in Modern Theoretical Chemistry, ed. H. F. Schaefer, Plenum Press, New York, 1976, vol. 3. B. Roos and P. Siegbahn, Theor. Chim. Acta, 1970,17,199. K. H. Thunemann, S. D. Peyerimhoff and R. J. Buenker, J. Mol. Spectrosc., 1978,70,432. R. J. Buenker, S. D. Peyerimhoff and W. Butscher, Mol. Phys., 1978, 35, 771, and references therein; D. B. Knowles, J. R. Alvarez-Collado, G. Hirsch and R. J. Buenker, J. Chem. Phys., 1990,92,585. R. J. Buenker, S. D. Peyerimhoff and P. J. Bruna, in Computa-tional Theoretical Organic Chemistry, ed. 1. G. Csizmadia and R. Daudel, NATO AS1 C67, Reidel, Dordrecht, 1981. J. L.Duncan, Mol. Phys., 1974,28, 1177. D. Feller and E. R.Davidson, J. Chem. Phys., 1984,80,1006. S. Bell and J. S. Crighton, J. Chem. SOC., Faraday Trans. 2,1985, 81, 1813. J. B. Foresman, M. Head-Gordon, J. A. Pople and M. J. Frisch, J. Phys. Chem., 1992,%, 35. Ch. M. Hadad, J. B. Foresman and K. B. Wiberg, J. Phys. Chem., 1993,97,4293. (a) G. Fitzgerald and H. F. Schaefer, J. Chem. Phys., 1985, 83, 1162; (b)W. D. Allen and H. F. Schaefer, J. Chem. Phys., 1987, 87, 7076. R. J. Buenker and S. D. Peyerimhoff, J. Chem.Phys., 1970, 53, 1368. B. Fabricant, D. Krieger and J. S. Muenter, J. Chem. Phys., 1977,67,1576. L.B. Harding and W. A. Goddard, J. Am. Chem. SOC., 1975,97, 6293. W. Goetz, D. C. Moule and D. A. Ramsay, Can. J. Chem., 1981, 59, 1635. W. Liptay, in Excited States, ed. E. C. Lim, Academic Press, New York, 1974, vol. I. 17 M. Suto, X.Wang and L. C. Lee, J. Chem. Phys., 1986,85,4228. Paper 3/06639H; Received 5th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000683

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 689-695 Structures and Vibrational Spectra of CH,OCH,CH,OH :The Hydrogen-bonded Conformers Francisco P. S. C. Gil,t R. Fausto, A. M. Amorim da Costa and J. J. C. Teixeira-Dias* Department of Chemistry, University of Coimbra ,Pi3049 Coirnbra, Portugal ~ ~~~ Ab initio calculations at the MP2/6-31G* and MP2/6-31G*//6-31G* levels have been carried out for the monomer of 2-methoxyethanol (CH,OCH,CH,OH). The MP2/6-31G* results indicate that the two more stable conformers (tGg’ and gGg‘) display intramolecular hydrogen bonds directed from the hydroxy H atom to one of the lone pairs of the ether 0 atom, and that the tGg’ conformer is 6.3 kJ mol-’ more stable than the gGg’ conformer. As the MP2/6-31G* and MP2/6-3lG*//6-31G* calculations do not yield results differing by more than a few tenths of a kJ mol-‘, it is concluded that the structure-sensitive and the dynamic correlation corrections are far from being additive.While the optimization of geometry for the correlated wavefunction generally leads to increase of bond lengths and reduction of bond angles, on the whole the geometrical parameters undergo similar changes in different conformers. Ab initio harmonic 6-31G* derived force fields were used to perform normal mode analyses for the more stable conformers. The calculated v(CH) frequencies are found to correlate linearly with some of the ab initio calculated CH bond lengths. An interpretation of the FTIR and Raman spectra for the liquid phase consonant with the structural and vibrational ab initio results is presented.Two spectral features observed both in Raman and in FTIR spectra and associated with v(0H) in monomeric species are ascribed to conformers, in accord with the theoretical and experimental results. On the whole, both the structural and the vibrational results presented point to a distinction between the hydrogen-bonded G-type conformers (tGg’ and gGg‘) and the higher energy T-type conformers (tTg and tTt). Compounds with the general formula Normal-coordinate analyses were performed’ using the ab C,H,, + l(OCH,CH,),OH, abbreviated C,E, ,display a wide initio derived force fields. While correlated wave functions at range of interesting molecular properties and aggregation the MP2/6-31G* level were used for the determination of patterns in solution ’-’ with important pharmaceutical and structures and energies, the less expensive and more tractable industrial applications.On the whole, these properties result 6-31G* basis set was used for the evaluation of the force from a subtle interplay between the conformational degrees fields at reference geometries obtained with the same basis of freedom, the possibility of intramolecular hydrogen set.21 bonding, and the relative importance of the polar and non- Local C, symmetry internal coordinates were used polar fragments in the extent of intermolecular interactions throughout. As the calculated frequencies vs. observed fre- especially of the hydrogen-bonding type. While C,E, com-quencies yielded a straight line with a high correlation coefi- pounds have been widely studied by different spectroscopic cient (0.998 87), the calculated frequencies of the most stable = 0.9123and thermodynamic techniques, an assessment of the relative conformer were appropriately scaled [v(~~~~~~) -importance of conformational effects and hydrogen-bond x v(,,~,)72.5).On the whole, 26 frequency values were interactions at the unimolecular level is still lacking, though it used. The mean error in this scaling was 0.5%, and the largest is of fundamental importance for the understanding of the error which occurred in some low-frequency modes did not properties of these compounds at the oligomer level and of exceed 17%. Modes which were doubtfully assigned or could their aggregation patterns in solution.not be observed were excluded from the linear regression. In this work, the structures and relevant conformations of The same frequency scaling was assumed for the less stable CIEl (CH,OCH,CH,OH) are determined by ab initio MO conformers. calculations at the MP2/6-3 lG* level. Ab initio harmonic Fig. 1 represents schematically the four more relevant con- 6-31G* derived force fields are used to perform normal mode formations of CIE,, numbers the atoms, and identifies the analyses for the more stable conformers. The structural and conformations. For the identification of the atoms in struc- vibrational ab initio results are discussed in the light of tural parameters and whenever ambiguity does not occur, the Raman and FTIR spectra for the liquid phase, and of concen- numbers of the atoms are omitted and a left-to-right order of istration and temperature variation spectroscopic studies for the underlined atomic symbols in ~(H,)~(H,)C(H,)OJ the v(0H) region.adopted. The optimized geometries of C,E, are identified by a three-letter acronym specifying the CO-CC (lower case), OC-CO (upper case) and CC-OH (lower case) axes as Computational and Experimental Methods trans (t, T), +gauche (9, G) or -gauche (g’, G’)arrangements. Ab initio calculations at the MP2/6-31G* and MP2/6-31G*// 2-Methoxyethanol was obtained from Aldrich. Samples of 6-31G* levels were carried out with the GAUSSIAN 92 the pure compound and dilute solutions in CC1, were pre- program system” adapted to VAX computers.The absolute pared. IR spectra were recorded on a Nicolet FTIR 740 spectro- errors in bond lengths and bond angles with respect to the meter, equipped for the 4000-400 cm-’ region with a germa- equilibrium geometrical parameters are less than 1 pm and nium on CsI beam splitter and with a DTGS detector with 0.1”, respectively, and the stopping criterion for the SCF iter- CsI windows. Temperature variation was carried out using a ative process required a density matrix convergence of less variable-temperature AgCl cell (accuracy to f1 K).than loe8. Raman spectra were recorded on a triple monochromator Jobin-Yvon T MOO0 Raman system (focal distance 0.640 m, t Department of Physics. aperturefl7.5). The pre-monochromator stage was used in the tTt Fig. 1 Numbering of atoms for the four more stable conformations of CIEl (schematic) subtractive mode.The system is equipped with holographic gratings corrected for aberration (1800 grooves mm- '), a thermoelectrically cooled photomultiplier and a non-intensified CCD, for optional mono- and multi-channel detec- tion, respectively. The 514.5 nm line of an argon laser (Coherent, model Innova 300-05) was used as excitation radi- ation. Under the experimental conditions used, the estimated frequency errors were approximately 1 em-'. Results and Discussion Energies and Geometries Table 1 presents the relative conformational energies and dipole moments, and the most relevant optimized structural parameters for the four more stable forms, calculated at the MP2/6-31G* level.The order of conformational energies is the following: tGg' < gGg' < tTg -c tTt. As can be seen, tGg' and gGg' are the two more stable conformers and tGg' is J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the most stable conformer, in agreement with previous calcu- lations.lo Consideration of the short non-bonded atom O...O(H) distances (Table 1) suggests the and O...H(O) occurrence of intramolecular hydrogen bonds in the tGg' and gGg' conformers, between the ether 0 atom and the hydroxy H atom. These intramolecular interactions seem to be deci- sive to explain the greater stability of these conformers, since the non-hydrogen bonded G-type conformers g'Gt, tGt, and tGg are less stable than the T-type conformers tTg and tTt.Model comparison of the tGg' and gGg' structures suggests that the repulsive interaction between one of the methyl H atoms and one of the H atoms in the methylene group bonded to the alcohol 0 atom should be mainly responsible for the higher energy of the gGg' conformer. One important consequence of this repulsive interaction is the opening of the OCC angle in the gGg' conformer by 4.3" with respect to its value in the most stable conformer (Table 1). The stabilization effect of correlation increases with the number of gauche arrangements along the series tTt < tTg < tGg' < gGg', being largest for gGg' (Table 1). This order seems to indicate that the effect of electrons avoid- ing each other is less important for trans arrangements which tend to keep the lone pairs of distinct 0 atoms and the H atoms bonded to different atoms further apart.Considering the various trans +gauche transitions involving the studied conformers, the stabilization energy (kJ mol- ') increases in the following order: CO-CC (tGg' +gGg' = 3.3) < OC-CO (tTg + tGg' = 5.3) < CC-OH (tTt -+ tTg = 8.7). These correlation corrections are of the same magnitude, or in some cases larger than, conformational energy differences and change appreciably with the conformational transition considered. This stresses the importance of including corre- lation whenever conformational energy differences in this type of molecules are evaluated.In addition, since these correlation corrections reach their largest value for the trans -+gauche transition in the CC-OH axis, it is concluded that correlation is also of great importance to account ade- quately for the energetics of hydrogen-bond formation. Comparison of the MP2/6-3 lG* and MP2/6-31G*//6-31G* results enables us to assess the extent of the structure- sensitive correlation correction, as the first calculation includes geometry optimization at the MP2 level and the second uses the 6-31G* optimized geometries. As far as con- formational energy differences are concerned, these two cal- Table l conformer CO-CC; OC-CO; CC-OH/degrees EIE,"AE~ AEcorrelation b,c P/Ddbond lengths/pm' c-00-c c-cc-0 O-H bond angles/degrees' 0-c-cc-c-0 C-0-H non-bonded atom distances/pm 0...O(-H) 0-*H(-O)* Relevant MP2/6-31G* results for the more stable conformers of C,E, tGg' gGg' tTg tTt -173;60; -50 82; 54; -42 -178; 180; 72 180; 180; 180 -268.700 41 1 2 -.268.698 024 1 --268.694 894 1 -268.694 856 9 0.0 6.3 14.5 14.6 0.0 -3.3 5.3 14.0 2.86 2.87 2.35 0.32 142.0 142.4 141.8 141.7 142.4 143.1 141.7 141.8 151.3 151.9 151.8 151.3 142.1 142.1 142.7 142.7 97.4 97.5 97.2 97.1 105.8 110.1 107.4 107.1 110.4 109.8 11 1.1 106.3 104.6 104.6 107.3 107.6 275.1 277.6 363.8 358.4 225.1 221.8 392.1 429.4 E, = 2625.5 kJ mol-'.Energies in kJ mol-'; AE = E(conformer)-E(tGg').For tGg', Eforrelation= -1966.2 kJ mol-'. D = 3.33564 x C m. 'For identification of the atoms, a left-to-right order in C(H,)OC(H,)C(H,)OH is followed. Ta le 2 Vibrational spectra and PED, for relevant conformations of CIEl" tGg' gGg' intensity intensity description R (liq) IR (liq) freq. n-sc. (sc.) Raman IR PED (%)b freq. n-sc. (sc.) Raman IR PED (%)b 1 v(0H) 3608" 360T 4097 (3665) 43 57 1 :1w 4090 (3659) 42 56 1 [lo01 2 v(CH, as.A) 2988 2980 3307 (2944) 89 46 2 391 3308 (2945) 93 43 2 [89] 3 vCC(9)H as.] 2952 -3278 (2918) 88 61 3 :76] + 6[22] 3269 (2910) 93 44 3 [57] + 6[29] + 5[12] 4 v(CH, as.A) 2935 2931 3232 (2876) 61 99 4 1991 3241 (2884) 55 79 4 [98] 5 v[C(6)H as.] 2897 2895 3221 (2866) 23 103 5 1731 + 6[21] 3286 (2925) 68 71 5 [64] + 8[21] + 3[ll] 6 vCC(9)H s.] 2884 2882 3199 (2846) 168 11 6 1521 + 3[21] + 5[20] 3182 (2830) 29 13 6 [49] + 7[26] + 3[22] 7 v(CH, s.) 2834 2828 3186 (2834) 127 84 7 :65] + 8[22] 3186 (2834) 42 60 7 [46] + 8[26] + 6[12] + 5[10] 8 v[C(6)H s.] 2780 -3172 (2821) 9 28 8 170) + 7[25] 3205 (2851) 177 89 8 [52] + 7[19] + 5[12] 9 "(6)H21 1477 1472 1674 (1455) 10 1 9 1601 + 13[18] + ll[12] 1652 (1435) 16 4 9 [64] + ll[18] + 13[ll] 10 W(9)H,I 1457 1456 1660 (1442) 14 2 10 1891 + ll[ll] 1670 (1451) 8 2 10 [78] + 13[12] 1 1 S(CH, as.A') --1654 (1436) 5 8 11 1662 (1444) 6 5 11 [59] + 13[12] + 10[12] 12 S(CH, as.A") 1413 1408 1645 (1428) 18 5 12 c1011 1648 (1431) 18 3 12 r86i 13 6(CH3 s.) 1376 1369 1634 (1418) 11 5 13 [77] + 9[15] 1630 (1415) 11 14 ~[c(~)H,I -1326 1596 (1384) 4 42 14 [45] + 15[29] + 25[15] 1587 (1375) 3 15 oCC(6)HJ 1287 -1551 (1342) 2 65 15 1542 (1334) 2 16 6(COH) 1243 I233 1515 (1310) 11 14 16 I 1505 (1301) 12 17 tCC(6)H21 I199 1194 1398 (1203) 9 27 17 I1593 1445 (1 246) 7 18 y(CH, A) 1160 I157 1383 (1189) 5 37 18 I1311 + 17[21] + 24[16] + 19[15] 1359 (1 167) <1 87 18 c52 I + 24[26] 19 tCCP)H,I I129 1124 1322 (1134) 4 101 19 I1281 + 16[16] + 21[16] + 18[16) + 24[12] 1332 (1143) 6 28 19 [42 I + 16[27] 20 y(CH3 A") 1095 -1300 (1113) 6 5 20 [351 1298 (1112) 4 8 20 [85 21 v[C(1)-0(2)] 1072 1066 1284 (1099) 7 93 21 I1461 + l8[2l] + 24[15] 1279 (1094) 4 56 21 [21 + 23[20] + 24[12] + 22[ll] 22 YCC(6)H,I 1048 -1242 (1061) 3 15 22 [:37] + 23[36] + 26[13] 1214 (1035) 4 4 22 [27 + 18[17] + 26[14] + 24[12] + 25[ll] 23 v[C(9)-0( 12)] 1022 1017 1200 (1022) 2 127 23 [:38] + 25[18] + 26[14] + 16[13] 1206 (1028) 4 153 23 [60 ++ 26[24]16[14] + 24[20]24 v[O(2)-C(6)] 97 1 963 1129 (957) 5 17 24 I:28] + 21[22] + 26[12] 1097 (928) 8 6 21 [33 25 v(C-C) 895 891 991 (832) 6 12 25 i1311 + 22[27] + 23[22] 969 (812) 7 11 25 [35 + 22[27] + 23[20] 26 YCC(~)H,I 839 833 925 (771) 9 25 26 I1391 + 24[28] + 25[13] 911 (759) 9 36 24 [33 + 26[24] + 25[18] + 21[12] 27 S(CC0) 545 539 590 (466) 2 10 27 I1381 + 30[22] + 22[ll] + 29[10] + 26[10] 571 (448) 1 17 30 [43 + 27[39] + 26[17] + 22[13] 28 z(CC0H) --448 (336) 3 166 28 I:961 439 (328) 2 130 28 [93 29 S(C0C) --395 (288) 1 10 29 I1451 + 27[32] 468 (354) tl 8 29 [65 30 S(0CC) --296 (1 98) <1 5 30 I:39] + 29[30] + 32[27] 320 (219) <1 12 27 [38 + 30[36] + 29[12] 31 z(HC0C) --239 (146) <1 7 31 I1761 + 30[10] 217 (125) 6 7 31 [45 + 32[23] + 33[16] 32 z(OCC0) --158 (72) <1 5 32 I1811 + 30[13] + 31[17] + 28[ll] 151 (65) <1 5 32 [66 + 31[37] + 30[21] + 28[ll] 33 z(C0CC) --99 (18) tl 2 33 I391 79 (73) <l 1 33 [ll '3 + 32[28] + 31[26] + 28[19] " Frequencies in cm-'; n-sc.= non-scaled; sc. = scaled; for the frequency scaling see 'Experimental and Computational Methods'; v, stretching; 6, bending;w, wagging;y, rocking;t, twisting;5, torsion; s, symmetric; as, asymmetric. PED values smaller than 10% are not shown. Values obtained in diluted solutions in CCl, . culations do not yield results differing by more than a few tenths of a kJ mol-'.This leads us to conclude that the correlation correction at the MP2/6-3 lG*//6-31G* level accounts also for the energy change which results from geometry optimization at the MP2/'6-31G* level, i.e. the structure-sensitive and the dynamic correlation corrections are far from being additive. In addition, from the small changes of the conformational energy differences obtained by these two types of calculations, it can be concluded that the geometry optimization carried out with the correlated wave function tends to stabilize the intramolecular hydrogen bond, since it increases slightly the energy separation between the hydrogen bonded G-type conformers (tGg' and gGg') and the T-type conformers (tTg and tTt). Considering now the geometrical parameters in both calcu- lations, it is concluded that the optimization of geometry for the correlated wave function leads, in general, to an increase of bond lengths and a reduction of bond angles, following a previously mentioned trend which improves general agree- ment of the absolute values with experimental data.22.23 However, on the whole, the geometrical parameters undergo similar changes in different conformers. This observation is of great practical importance, as it indicates that the more time- consuming and expensive geometry optimization carried out at the MP2 level is not strictly necessary to evaluate changes in the geometrical parameters which are conformationally induced.Based on this conclusion we will obtain geometrical parameters profiles along different dihedral coordinates and force fields for different conformers using, in both cases, 6-3 lG* optimized geometries.At room temperature, approximately 92% of the molecular population should have the tGg' conformation, leading us to expect that this is the most dominant form in the gaseous phase at sufficiently low pressures or in very dilute solutions in inert solvents, when both oligomer formation and aggre- gation in general are not likely to occur. By contrast, the single symmetric form among those studied herein, i.e. the all-trans conformer, besides having an energy much higher than that of the tGg' conformer (14.6 kJmol-'), presents also a very small dipole moment (0.32 D), in fact, the smallest dipole moment among the considered conformers (Table 1).Hence, dipole-dipole non-specific intermolecular interactions like those which are expected to occur in diluted solutions of polar aprotic solvents should not contribute much for the stabilization of the symmetric all-trans conformer in solution. In order to characterize the most important intramolecular interactions in the various conformers, the conformational dependence of some relevant structural parameters (Table 1) is now considered and discussed. Starting with the more stable form (tGg'), the most pro- nounced variations, dihedral angles and 0.* .O(-H) and 0...H(-O) distances apart, occur for the CCO angle, which opens by ca. 4,and for the COH angle, which closes by 3", with respect to the tTt form.These variations are consistent with the formation of an intramolecular hydrogen bond (0.h.H-0) in the tGg' conformer, as they point clearly towards a more linear O...H-O axis and a shorter 0-n.H distance (see Table 1). It is interesting to point out that similar changes are observed in the same structural param- eters for the gGg' conformer, in particular, the CCO angle increases by 3.6" and the COH angle diminishes by 3". However, if these two conformers (tGg' and gGg') are com- pared (Fig. 1) and the tGg'+gGg' transition is considered, the OCC angle opens by ca. 4" in the gGg' form in order to reduce the steric repulsive interaction in this conformer between one of the methyl H atoms and one of the H atoms in the methylene group bonded to the alcohol 0 atoms (see Fig. 1).This repulsive interaction in the gGg' conformer may J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 explain the greater stability of the tGg' conformer. The tGg' +gGg' transition is concomitant with an increase of 2.5 pm in the O...O(-H) distance and a reduction of 3.3 pm in the 0.. .H(-O) distance (Table 1). The latter variation seems to result from the fact that the ether 0 atom becomes more in line with the H-0 bond in the gGg' conformer than in the tGg' conformer, as the C-0 and 0-C bonds increase slightly in the gGg' conformer, Finally, for the tTg form, the most pronounced geometrical change with respect to the tTt conformer occurs for the CCO angle which is ca.5" larger in the tTg form in order to reduce a repulsive steric interaction between the hydroxy H atom and one of the H atoms in the methylene group bonded to the ether 0 atom (see Fig. 1). . In conclusion, the OCC, CCO, and COH bond angles are very sensitive to different conformational changes due to internal rotations about the CO-CC, CC-OH and OC-CO axes, respectively. Vibrational Frequencies In this section, the 6-31G* derived force field is used to calcu- late the frequencies, the potential-energy distribution (PED), and the IR and Raman intensities of the more relevant con- formers. Emphasis is given to the modes which exhibit large sensitivities, in frequency and/or composition, to conforma- tional changes. Table 2 presents the vibrational calculated results for the tGg' and gGg' conformers.The occurrence of two 'windows' at ca. 2700-1500 and 800-600 cm-', both in the calculated and experimental Raman and IR spectra of C,E,, immediately suggests the consideration of three spectral regions for the discussion of the vibrational spectra: 3700-2700 cm-', 1500-800 cm-', and below 600 cm-'. However, since the OH stretching is particularly sensitive to aggregation which, in turn, should not be involved in the discussion of the conformational degrees of freedom of the monomer for clarity of this text, the v(0H) region will be separately considered at the end of this section in a 'spectral region' of its own. For sufficiently diluted solutions in CCl, , the occurrence of two IR spectral features associated with v(0H) in monomeric species is now well e~tablished.'~~*~~~~*~~~-~~With the excep- tion of this spectral region, the number of intense (vs, s, and m) and well defined bands in the spectra of the liquid at room temperature does not exceed the total number of distinct fre- quencies expected for the fundamental vibrations of a single conformer (3N -6 = 33).This observation is consonant with the large population calculated for the most stable conformer at room temperature (92%). Hence, only the frequencies of the relatively intense Raman and IR bands of the pure liquid (Fig. 2) and the calculated frequencies for the most stable conformer were considered in the vexp vs. vcalc linear regres- sion.3200-2700 cm-'Region This region includes the v(CH,) and v(CH,) vibrations which correspond, for a single molecule, to seven CH oscillators and as many distinct frequencies since no degeneracies occur. The four highest frequencies should be ascribed to the anti-symmetric modes (two for the CH, group, one for each CH, group), the lowest three frequencies should correspond to the symmetric modes. This expected general pattern is observed for the tGg', gGg' and tTt conformers. The antisymmetric components of the CH, groups, in par- ticular of (0)CH2,change appreciably, both in frequency and composition, with conformation. Both v(CH, as A') and v(CH, as A") calculated non-scaled frequencies increase with the number of gauche arrangements along the series tTt < tTg < tGg' < gGg'.In addition, these frequencies (cm-') present reasonable linear correlations with methyl CH J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3200 2400 1600 800 I I I I - bond lengths (pm): v(A’) = -134.99r(C1H,)+ 17 537 (correlation coefficient R2 = 0.892); v(A”) = -100.54r(C1H4)+ 13 812 (R2= 0.973). Hence, it can be concluded that the methyl CH bond lengths, in particular r(C,H,) and r(C1H4), are appreciably sensitive to conformation. For isolated CH stretching vibrations of alkanes, good linear correlations of v(CH) against r(CH) have already been con~idered.~~ 1500-800 an-’Region This region includes the CH, and CH, bending vibrations, the CC and CO stretching modes, and the COH bending vibration, corresponding to a total of 18 fundamental vibra- tions in the monomer.In particular, 14 bending vibrations (5 involving the CH, group, 4 for each CH, group, 1 COH bending), and 4 stretching vibrations (1 CC and 3 CO) are considered in this region. The modes which exhibit the largest sensitivities, both in frequency and composition, to conformational changes are 6(COH), v(0C) and v(CC), as well as the wagging, twisting and rocking vibrations of the CH, groups. In the most stable conformer, an extensive mixture of coordinates occurs for tCH,, for the CH, group adjacent to OH. In addition, the calculated 6COH frequency exhibits a decrease of ca. 150 cm-’ on going from the conformations with a gauche arrangement in the CC-OH axis (tGg’, gGg’, tTg) to the all- t trans conformer.This large frequency shift is consonant with the calculated profile of the COH angle variation us. CC-OH, as discussed above. Region below 600cm-’ For a single molecule, this region includes a total of seven fundamental vibrations, namely, three skeletal bending vibra- tions (CCO, COC, OCC) and four torsional modes (COCC, OCCO, CCOH, HCOC). Among the conformers considered herein, only for the most stable conformer does S(0CC) mix with z(OCC0) and to an appreciable extent (27%).It is interesting to point out that, for n-butane, a similar vibrational coupling has been previously observed and discussed between S(CCC) and z(CCCC), also for the gauche c~nformation.~’ For z(COCC), z(OCC0) and z(HCOC),the frequency shifts due to conformational changes are larger whenever the con- formational coordinate corresponds to the predominant tor- sional mode involved.Generally speaking, z(CC0H) and z(C0CC) are pure modes in all the considered conformers, except for gGg’, where z(C0CC) mixes appreciably with the remaining torsional coordinates. For the tTt and tTg conformers, z(HC0C) does not change appreciably, either in frequency (ca. 150 cm-’)or in composition [over 86% of PED is from t(HC0C) and approximately 12% from z(0CCO)l. In gGg', z(HC0C) occurs at the lowest frequency value among all the studied conformers, 125 cm- ',and mixes appreciably with z(C0CC) and z(OCC0). For this all-gauche form, an appreciable steric interaction is expected to occur between H atoms of the methyl and of the C(9)-methylene groups.Finally, for tGg', z(HC0C) occurs at 146 cm-', with a significant 10% PED element from S(0CC). On the whole, z(HC0C) is not expected to be particularly sensitive to conformational changes, except through an indirect mechanism, whenever repulsive steric interactions involving at least one of the methyl H atoms occur. v(0H) Stretching Region This region displays various important spectral features, occurring, both in the FTIR spectra (Fig. 3) and in the Raman spectra (Fig. 4). Among these, two are ascribed to monomeric species and occur at ca. 3640 (shoulder) and 3608 cm-'(hereafter referred to as A and B, respectively).In addi- tion, a very wide and convoluted complex of bands, referred to as C, occurs in the range 3550-3200 cm- Band B is the most sharp feature which becomes progres- sively more prominent at lowering concentrations of C,E, in CCl,, both in the FTIR and Raman spectra. In paricular, for a 0.01 mole fraction, the IR peak absorbance of B is twice that of C (Fig. 3). It is interesting to mention that the same approximate ratio of intensities (B/C x 2) is observed, in the Raman spectra (Fig. 4), for a much less dilute solution (0.09 mole fraction in CCl,). In addition, for the same concentra- tion, the Raman spectrum of feature C clearly shows two maxima at ca. 3460 and 3310 cm-', probably ascribed to different degrees of aggregation, whereas the same feature C shows a single maximum in the FTIR spectrum.wavenu m ber/cm -' 1.o h cn w.-C P Y \-3 0.0 3600 3500 3400 3300 3200 wavenumber/cm-' Fig. 3 FTIR spectra of 2-methoxyethanol in CCl, solutions, after smoothing:(a)0.02 mole fraction solution spectra at 1,263; 2,283; 3, 303 and 4, 323 K; (b)room temperature spectra with mole fraction: 1,0.02; 2,O.Ol; 3,0.002 and 4,0.0002 J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 4 3600 3520 3440 3360 3280 3200 wavenumber/cm-Fig. 4 Raman spectra of 2-methoxyethanol in CC1, solutions at room temperature, after smoothing. Mole fraction: 1, pure liquid; 2, 0.50;3,0.17 and 4,O.m The absorbance ratio A/B (A is the high-frequency shoul- der of B) increases with temperature [Fig.3(u)], but is rela- tively insensitive to concentration variation [Fig. 3(6)], at least for the range of concentrations considered (mole frac- tions varying from 0.02 to 0.0002). By contrast, C is a very broad band whose width increases with concentration, its absorbance and peak frequency being dependent both on temperature and concentration. In particular, the absorbance of C increases and its peak frequency decreases with both reduction in temperature and increase in concentration (Fig. 3)-Previous papers on C,E, based their assign- 1*2*4*798~13-1 ments of IR bands A and B on assumptions of the more probable molecular structures, and estimated enthalpy differ- ences from the inverse temperature dependence of the logarithm of the ratio of intensities.To the best of our know-ledge, it is the first time that ab initio calculations at the levels considered here are used to provide a theoretical and quanti- tative assessment of the vibrational spectra and to shed light on the interpretation of the above-mentioned spectral fea- tures. The order of the a6 initio scaled v(0H) frequencies (cm- ') for the various conformers is gGg'(3659) < tGg'(3665) < tTg(3678) < tTt(3690). These results indicate an increase of v(0H) with the number of trans arrangements, a trend opposite to that observed for v(CH,,). In addition, the hydrogen-bonded conformers (tGg' and gGg') have close v(0H) scaled frequencies (Av = 6 an-'), a result which suggests the assignment of these conformers to the same observed vibrational band.As expected, the T-type conformers have higher v(0H) frequencies than the G-type conformers, since the OH oscillators of the latter conformers are intramolecularly hydrogen bonded. It is also interesting to notice that the v(0H) frequency of the all-trans conformer J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 is shifted ca. 30 cm-’ from the centre of the tGg’-gGg’ pair, to higher frequencies. The same approximate frequency differ- ence between bands A and B is observed both in the FTIR (Fig. 3) and Raman spectra (Fig. 4). The ab initio results clearly point to the tGg’ conformer as the best candidate for band B, since this conformer corre- sponds to the highest monomer population (92% at room temperature).A small contribution to band B from the gGg’ conformer is also expected, since the vibrational calculations yield close v(0H) frequencies for the tGg’ and gGg’ con- formers (Av = 6 cm-’), and the MP2/6-31G* calculations yield a significant population for the gGg’ conformer (ca. 7%). The proximity of the calculated v(0H) frequencies for the tGg’ and gGg’ conformers is not surprising, since the different CO-CC arrangements in these two conformers are not expected to affect the v(0H) frequency appreciably. For the all-trans conformer, the calculated v(0H) frequency is shifted by ca. 30 cm- to higher frequencies with respect to the centre of v(0H) for the G-type conformers (3662 cm-’), in accord with the observed frequency separations between the centres of bands A and B in the FTIR and Raman spectra (ca. 32 cm-’; Fig.3 and 4). This agreement points clearly to the tTt conformer as the best candidate for band A, though a negligible contribution from the t Tg conformer should not be totally excluded as this latter conformer is very close in energy to the all-trans conformer (Table 1)and the calculated v(0H) frequencies for the tTt and t Tg conformers fall within the observed width of band A. In addition, while the barrier for the tTt+tTg transition has not been evaluated, it is unlikely to exceed the value of thermal energy (RT = 2.5 kJ mol-’ at room temperature), hence providing a mechanism for interconversion between the tTt and t Tg conformers.The deconvolution of bands A and B is difficult and subject to large errors at least for the weak shoulder, A. However, the relative intensity of this shoulder, A/B, was found to be approximately insensitive to concentration varia- tion in a non-polar solvent (CCl,), in agreement with the very low dipole moment of the all-trans conformer (Table 1). In addition, from the temperature dependence of the intensities ratio ZJZB, it is possible to estimate a conformational energy difference which falls in the range 7-10 kJ mol-’, in accord with the energy difference between the gGg’ conformer and the T-type conformers. This observation gives further support to the above assignment of conformers to bands A and B. While the calculated intensities should not be considered totally reliable and refer to the isolated molecule as, in fact, all the calculated quantities, it is worth pointing out that the calculated v(0H) Raman intensity for the t Tt conformer approximately doubles the corresponding intensities for the remaining conformers studied, whereas all the conformers exhibit approximately the same calculated IR line intensities.Apparently, these theoretical results do not agree with experi- ment, since band A is observed both in the Raman and FTIR spectra. However, as the concentration of C,E, is progres- sively reduced, band A becomes clearly distinguishable in the Raman spectra for much less dilute solutions than those which enable observation of the same feature in the FTIR spectra.In fact, for band A to become evident in the FTIR spectra, a more dilute solution (roughly, one tenth of the mole fractions ratio) had to be prepared. These findings lead to the conclusion that the v(0H) band is a more sensitive probe to the presence of monomers in the Raman spectra than in the FTIR spectra. Since the theoretical results refer to the isolated molecule, care should be exercised in extrapolating the calculated quan- tities to the liquid-phase spectra, especially to the pure liquid or to concentrated solutions. In particular, the formation of intermolecular hydrogen-bonded oligomers is likely to affect both the geometries and energies of the monomers present in the oligomeric species, hence leading to changes in the observed IR and Raman spectra.Moreover, non-specific intermolecular interactions may also affect the structures and vibrational spectra of monomers. However, both the structur- al and vibrational results presented herein point to a clear distinction between the hydrogen bonded G-type conformers (tGg’ and gGg’) and the higher energy T-type conformers (tTg and tTt). The authors thank JNICT (Junta Nacional de Investiga@o Cientifica), Lisboa, for financial support, and F.P.S.C.G. thanks Instituto Pedro Nunes, Coimbra, for a research grant. References 1 L. P. Kuhn and R. A. Wires, J. Am. Chem. Soc., 1964,86,2161. 2 P. J. Krueger and H. D. Mettee, J. Mol. Spectrosc., 1965, 18, 131. 3 J. Feeney and S. M. Walker, J. Chem.SOC.A, 1966,1148. 4 H. Sait6, T. Yonezawa, S. Matsuoka and K. Fukui, Bull. Chem. SOC.Jpn., 1966,39,989. 5 R. Iwamoto, Spectrochim. Acta, Part A, 1971,27,2385. 6 P. Buckley and M. Brochu, Can. J. Chem., 1972,50, 1149. 7 L. S. Prabhumirashi and C. I. Jose, J. Chem. Soc., Faraday Trans.2,1975,71,1545. 8 L. S. Prabhumirashi and C. I. Jose, J. Chem. SOC., Faraday Trans. 2, 1976,72, 1721. 9 G. Roux, G. Perron and J. E. Desnoyers, J. Solution Chem., 1978, 7,639. 10 S. Vazquez, R. A. Mosquera, M. A. Rios and C. Van Alsenoy, J. Mol. Struct. (Theochem), 1989,188,95. 11 G. Douheret, A. Pal and M. I. Davis, J. Chem. Soc., Faraday Trans. I, 1989,%5,2723. 12 G. Douheret, A. Pal and M. I. Davis, J. Chem. Thermodyn, 1990, 22,99. 13 I. Suryanarayana, G. P. Someswar and B. Subrahmanyam, 2. Phys. Chem., 1990,271,621. 14 F. A. J. Singelenberg and J. H. Van der Maas, J. Mol. Struct., 1991, 243, 111. 15 F. A. J. Singelenberg, E. T. G. Lutz and J. H. Van der Maas, J. Mol. Struct., 1991, 245, 173. 16 F. A. J. Singelenberg, J. H. Van der Maas and L. M. J. Kroon-Batenburg, J. Mol. Struct., 1991, 245, 183. 17 G. Douheret and M. I. Davis, Chem. SOC.Rev., 1993,22,43. 18 M. I. Davis, Chem. SOC.Rev., 1993, 22, 127. 19 M. J. Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. W. Wong, J. B. Foresman, B. G.Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R. Gomperts, J. L. Anders, K. Raghava-chari, J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. Defrees, J. Baker, J. J. P. Stewart and J. A. Pople, GAUSSIAN 92, revision A, Gaussian Inc., Pittsburgh PA, 1992. 20 R. G. Snyder and J. H. Schachtschneider, Spectrochim. Acta, 1965,21, 165. 21 P. Pulay, J-G. Lee and J. E. Boggs, J. Chem. Phys., 1983, 79, 3382. 22 R. Ditchfield and K. Seidman, Chem. Phys. Lett., 1978,54, 57. 23 D. J. Defrees, B. A. Levi, S. K. Pollack, W. J. Hehre, J. S. Binkley and J. A. Pople, J. Am. Chem. Soc., 1979,101,4085. 24 R. G. Snyder, A. L. Aljibury, H. L. Strauss, H. L. Casal, K. M. Gough and W. F. Murphy, J. Chem. Phys., 1984,81,5352. 25 R. A. MacPhail and R. G. Snyder, J. Chem. Phys., 1989, 91, 3895. Paper 3/05810G; Received 27th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000689

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 697-701 Internal Rotation in Auramine 0 Pennathur Gautam and Anthony Harriman" Center for Fast Kinetics Research, The University of Texas at Austin, Austin, TX 78712,USA The fluorescence quantum yield and excited singlet state lifetime measured for Auramine 0 in different alcohols increase with increasing microviscosity of the solvent. The results can be explained quantitatively in terms of current theoretical models in which internal rotation of the N,N-dimethyaniline groups is an activationless process controlled entirely by viscous flow. Auramine 0 binds to deoxyribonucleic acid (DNA) and horse liver alcohol dehydrogenase (HLADH) in neutral aqueous solution. Binding to DNA is accompanied by modest increases in fluorescence yield and lifetime of the dye whereas binding to HLADH facilitates a dramatic increase in fluorescence.There appear to be two binding sites for Auramine 0 on the protein, and in one particular site the conformation of the dye is essentially frozen in a form unfavourable for rotational diffusion. Internal rotation in small molecules can provide an impor- tant route for non-radiative deactivation of the first excited singlet state and, in certain cases, may result in formation of a twisted intramolecular charge-transfer state (TICT). Accord- ing to the size of the rotor and the amplitude of its motion, the rate of rotation may depend on frictional forces with adjacent solvent molecules. However, if excited state deacti- vation involves a substantial change in dipole moment it may be necessary to distinguish between 'polarity' and 'viscosity' effects induced by changes in solvent and/or temperat~re.'-~ On considering only viscosity effects it is common practice to express the rate of rotation in terms of Kramers' the~ry.~ Although few systems give an exact correspondence with the original theory, when subjected to close scrutiny, there have been numerous modifications and extensions which have sig- nificantly improved the agreement between experiment and theory.'-'' One particular improvement has involved the use of microviscosity in place of bulk or shear viscosity as a means of expressing specific solute-solvent interactions.' '-I4 We now show that the concept of microviscosity allows a good description of internal rotation in Auramine 0.77 N+ " ''1'''' CH3I CH3 -CH, CH 3 Auramine 0, a cationic dye, is used as a stain for DNA and HLADH.I5 The dye is essentially non-fluorescent in water but fluoresces upon binding to a biological substrate. Oster and NishijimaI6 observed that the fluorescence yield of Auramine 0 in glycerol was markedly dependent on tem- perature. This was explained in terms of internal rotation of the N,N-dimethylaniline groups providing an effective means for non-radiative decay to the ground state. It was further claimed that fluorescence quenching occurred if the group rotated by more than 2" during the period of excitation.I6 In fact, the N,N-dimethylaniline group is an important constitu- ent of many TICT molecules" and information regarding its rotational diffusion is of considerable current interest.We have, therefore, re-examined the photophysics of Auramine 0 in a series of normal alcohols at various temperatures. Our results indicate that rotation of the N,N-dimethylaniline group is an activationless process which depends on the vis- cosity of the surrounding medium without an apparent polarity effect. Binding the dye to a biological substrate can cause a substantial decrease in the rate of internal rotation, in certain cases the effect is far more pronounced than that which can be achieved by a viscous solvent. Experimental Auramine 0 [4,4'-(imidocarbonyl)bis(N,N-dimethylaniline) monohydrochloride] was obtained from Aldrich Chemicals and purified by repeated recrystallization from warm 0.02 mol dm-3 NaCl solution before being dried over P205.I5 The purified material gave a linear Beer's law plot in water (pH 7) containing 0.5 mol dm-3 KCl at concentrations below 5 x low3mol dm-3, from which the molar absorption coefi- cient at 431 nm was determined to be 43200 dm3 mol-' cm-'.'H NMR (D,O): S = 2.89 (s, 6 H); 6.59 (d, 2 H, J = 9.1 Hz); 7.26 (d, 2 H,J = 9.0 Hz). Elemental analysis: Calculated for C,,H,,N,Cl-H20: C = 63.44; H = 7.52; N = 13.05%. Found: C = 63.32; H = 7.60; N = 13.01%. Sol- vents, obtained from Aldrich Chemicals, were of the highest available purity and were used as received. DNA (calf thymus) and single-stranded DNA (calf thymus) were obtained from Sigma Chemicals and were used as received; concentrations were assessed by absorption spectroscopy." HLADH was obtained from Sigma Chemicals and was dia- lysed against 0.1 mol dm-3 phosphate buffer (pH 7.4), its concentration was determined by absorption spectroscopy using a molar absorption coefficient at 280 nm at 35 400 dm3 rno1-l cm-'.l5 Absorption spectra were recorded with a Hitachi U3210 spectrophotometer and fluorescence spectra were recorded with a fully corrected Perkin-Elmer LS5 spectrofluorimeter. For fluorescence studies the solution was adjusted to possess an absorbance of ca. 0.05 at the excitation wavelength of 370 nm. Solutions were purged thoroughly with nitrogen and the sample cell was thermostatted with a constant-flow circulat- ing water bath.Temperatures were measured with a thermo- couple in direct contact with the solution and were accurate to within & 0.2"C. Fluorescence lifetimes were measured by time-correlated, single-photon counting using a frequency-doubled, mode- locked, synchronously pumped, cavity dumped Styryl-9 dye laser. Fluorescence was isolated from scattered laser light using a high radiance monochromator. The excitation wave- length was 417 nm and fluorescence was detected at 540 nm via a pinhole attachment and a Hamamatsu R2809U micro-channel plate phototube. Approximately 40 O00 counts were collected in the peak channel and, for each measurement, four different time bases were used.After deconvolution of the instrument response function, the ultimate time resolution of this instrument was cu. 20 ps. Titrations of Auramine 0 with DNA or HLADH were made in aqueous solution containing 5 x mol dm-3 sodium sulfate and 1 x mol dm-3 phosphate pH 7 buffer or 0.1 mol dm-3 phosphate pH 7.4 buffer, respectively. Each titration was made by adding successive aliquots of dye to fixed concentrations of substrate and by adding increasing amounts of substrate to a fixed concentration of dye. The course of the reaction was followed by absorption and fluo- rescence spectroscopy. Time-resolved fluorescence measure- ments were made at various ratios of dye to substrate. Values for the shear viscosity (q)of the solvent at a particu-lar temperature were extrapolated from data available in the literature.’ Microscoviscosity (qp) values were calculated by the semi-empirical method of Spernol and Wirtz,” as detailed by Sun and Saltiel.14 Thus, microviscosity is defined as =fq (1) where the microviscosity factorfis expres sed as: f= (0.16 + 0.4r/r,)(0.9 + 0.4T,, -0.25T,) (2) Here, r and rL are the solute and solvent molecular radii, respectively, and the terms rLand T, refer, respectively, to the reduced solvent and solute temperatures.T -T,KL = -(3)Tb -Tm In this expression, T is the experimental temperature, and Tm are the boiling and melting points of the solvent for TL and solute for T,. Molecular radii for the various alcohols were estimated from molar volumes assuming a space-filling factor of 0.74.All calculations were made on the basis of the solute being N,N-dimethylaniline, for which the molar volume is equal to 126.4 A3 according to X-ray crystallo- graphic determination. Further details on the calculation and multifarious applications of microviscosity are provided by Sun and Saltiel.14 Results and Discussion Fluorescence in Alcohols Absorption and fluorescence spectra of Auramine 0 in decan-1-01 are shown in Fig. 1. There is a relatively large Stokes’ shift (2800 an-’)and poor mirror symmetry between the lowest-energy absorption band and the fluorescence band. This suggests to us that there might be a significant change in geometry upon promotion to the first excited singlet state.Even so, the corrected excitation spectrum gave a good match to the absorption spectrum across the entire visible region, including the absorption peak at 370 nm. The fluorescence quantum yield (a,) in decan-1-01, measured rela- tive to Rhodamine 101 in ethanol” and corrected for changes in refractive index,” was found to be 0.0045 & 0.0005 while the fluorescence lifetime (2,) was mea- sured to be 92 3 ps. The derived radiative lifetime (zo = z$QDf = 20.4 ns) was in agreement with that calculated from the Strickler-Berg expression (zo = 18.6 n~).’~ The absorption and fluorescence spectra of Auramine 0 did not change with solvent and, in particular, there was no indication of a polarity effect. It was observed, however, that (D, was markedly dependent on the nature of the solvent for a series of alcohols at 23 “C,after correction for changes in the J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 A/nm Fig. 1 Absorption and fluorescence spectra recorded for Auramine 0 in decan-1-01 refractive index of the solvent (Table 1). For solvents in which fluorescence could be resolved from the instrumental response function, there was good agreement between z, and QD, (Table 1) and the fluorescence decay profiles could be analysed satisfactorily in terms of a single exponential com- ponent. Measurements made in pentan-1-01 and nonan-1-01 showed that Qf increased with decreasing temperature (Table 1). Additional studies carried out under isoviscosity condi- tions using different alcohols at varying temperatures, selec- ted to give a constant viscosity of 5 cP,~ indicated that both a, and z, were essentially independent of temperature over the range investigated (-41 < T/”C< 26).In general, both QD, and z, increase with increasing solvent viscosity (q), as observed first by Oster and Nishijima.16 However, as shown in Fig. 2, a plot of (l/QDf)us. (T/q)deviates from linearity. A linear plot might be expected if the rate of internal conversion is controlled by rotational diffusion of one of the N,N-dimethyaniline groups. Thus, the reciprocal of the fluorescence quantum yield can be related to the solvent Table 1 Effect of (micro)viscosity on the fluorescence quantum yield and lifetime of Auramine 0 in alcohols solvent T/K tl/cP VP/cP l/qa 7sa /ps CH30H 296.0 0.58 0.48 600 <20 C,H,OH 296.0 1.16 0.87 330 <20 C3H,0H 296.0 2.09 1.43 200 24 C,H90H 296.0 2.85 1.78 168 27 C5H110H 284.9 5.61 3.27 97 52 289.8 4.74 2.78 113 38 294.1 4.09 2.40 130 32 296.0 4.00 2.34 132 35 298.5 3.59 2.11 143 32 303.9 3.04 1.79 166 30 308.6 2.62 1.55 191 25 313.5 2.28 1.35 215 22 322.6 1.78 1.06 276 <20 296.0 4.93 2.67 108 45 296.0 6.60 3.38 98 50 296.0 8.17 3.89 78 64 282.5 16.24 7.41 36 136 288.2 13.97 6.41 42 115 292.4 11.77 5.41 51 95 296.0 10.40 4.73 62 83 297.6 9.70 4.47 65 78 303.0 8.24 3.80 72 67 308.6 6.90 3.19 86 58 312.5 5.94 2.75 100 49 320.5 4.75 2.21 123 40 296.0 13.47 5.96 53 92 a f5%.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0,' o II 1I I 1I I I I m2oo3oo4oo5oomm (TI'1)IK cp-' Fig. 2 1/Qf us. (0)T/q or (A)T/q, viscosity via eqn. (4): 1/@r= 1 + (Tokc) + Ca7o(T/v)l (4) Here, kisc refers to the rate constant for intersystem crossing to the triplet manifold and a is a proportionality constant. The form of Fig. 2 suggests that shear viscosity does not give an accurate measure of frictional forces with adjacent solvent molecules for this system, especially at low viscosity. Indeed, replacing shear viscosity with microviscosity (q,) resulted in a substantial improvement in the correlation between the experimental data and eqn.(4) (Fig. 2). From the intercept of this latter plot, we estimate that kisc has a value of (4 f1) x lo8 s-' and the slope (uT,) is numerically equal to 4.03 CPK-I. The intercept can be employed to estimate that in the absence of internal rotation @, would have a value of cu. 0.12. Assuming that internal conversion from the first excited singlet state occurs exclusively by way of internal rotation, the rate constant (k,J can be expressed in terms of the Stokes-Einstein relati~nship.~~ Here, k, is. the Boltzmann constant and V is the hydrody-namic volume of the rotating group. Combining eqn. (4) and (5) indicates that the proportionality constant a can be equated to kB/V and, therefore, the hydrodynamic volume of the rotor has a value of 166 A3.This derived value for the hydrodynamic volume seems quite reasonable since, from X-ray crystallographic, data, the molar volume of N,N-dimethylaniline is 126.4 A3. The photophysical properties of Auramine 0,therefore, appear to be controlled by rotational diffusion of one of the N,N-dimethylaniline groups. The diffu-sion coefficient of the rotor (D = RT/6mq,, where T is the radius of the rotor) has a value of 1 x lo-'' m2 s-' in decan-1-01at 296 K, where the microviscosity is 6 cP. On the basis of an isomicroviscosity plot made at )I, = 2.8 CP (Fig. 3), there is a small negative activation energy for internal rotation (EA= -3.8 kJ mol-I). The results obtained in different solvents at fixed temperature can be expressed in terms of Kramers' theory assuming the Smolukowski limit and using microviscosity in place of shear vi~cosity'~[Fig.Wl. krot = A/qi (6) Least-squares analysis of the experimental data gave ct = (0.97 f0.02), which confirms that the rate of rotation is controlled almost exclusively by viscous flow. Results obtained in the same solvent at different temperatures can be expressed in terms of the 'medium-enhanced barrier model' introduced by Fischer and co-workers2' [Fig. 4(b)].Again, 1 1 I I I23.70 ! I r I 3.10 320 3.3 3.40 3.50 103 KIT Fig. 3 Isomicroviscosity(2.8 cP) plot of log k,,, us. 1/T linear plots were observed with slopes close to unity for both pentan-1-01 and nonan-1-01.Individual k,,, values used for these plots were derived from the measured 7s values (k,,, x l/rJ where possible and, in all other cases, were calculated from the fluorescence quantum yields (k,,, cc l&). In the medium-enhanced barrier model, k,,, is related to microviscosity according to In k,,, = In k, + ASJR + b In B,, -b In q,, (7) where k, is the limiting rate constant for rotation at zero microviscosity, AS, is the medium-imposed entropy change, Y ACL -c 24.5 24.0 23.5 23.0 ! -m I 1 I I-3 om 1 Ias0 I I too ILsa UK] In '1, 25.0 I I 1 I 1 24.5 24.0 Y 0 aL -c 23.5 22.0 am 0.50 100 150 200 250 In ftp Fig. 4 (a)Kramers' plot for the experimental data collected in dif-ferent alcohols at 296 K.(b) Medium-enhanced barrier model plot for the experimental data collected in (m)pentan-1-01 and (A)nonan-1-01 at different temperatures. and B,, is the microviscosity at 0 K. The constant b refers specifically to the fraction of the activation energy of viscous flow that gives the enthalpy increment by which the medium augments the intrinsic barrier for solute rearrangement. l4 From Fig. qb), b = 1.00 & 0.05 for both pentan-1-01 and nonan-1-01. This finding is consistent with an activationless structural change that is controlled exclusively by viscous flow. The slight change in intercept between the two solvents can be related to changes in the entropy term due to the difference in molar volume of the solvent.In a given solvent, k,,, can also be expressed in terms of transition state theory In (k,,JT) = ln(ick,/h)exp(ASS/R) -(AHS/RT) (8) where AH' is the enthalpy of activation change, AS$ the entropy of activation and IC an adiabaticity factor. From the corresponding plots for results obtained in pentan-1-01 and nonan-1-01 (Fig. 5), AH' values of 18.4 and 22.1 kJ mol-', respectively, were found for pentan- l-ol and nonan- l-ol. Indi- vidual AH' values can be expressed in the form AH' = AH: + AHv (9) where AH: refers to the activation enthalpy change for twist- ing of the molecule and AHv refers to the medium-imposed enthalpy change. For this system, AHv is equal to the activa- tion energy for viscous flow of the solvent, as obtained from the Andrade expre~sion,'~ since b = 1, and has values of 23.0 and 27.2 kJ mol- ',respectively, for pentan-1-01 and nonan-1- 01.' Consequently, the activation enthalpy change for twist- ing of the molecule has a value of ca.-5 kJ mol-'. This derived value is in excellent accord with the activation energy obtained from the isomicroviscosity plot (Fig. 3). Fluorescence of Auramine 0 Bound to Biological Substrates Fluorescence from Auramine 0 in water was difficult to detect and time-resolved fluorescence studies indicated that z, was less than 20 ps. On addition of DNA to a neutral aqueous solution of Auramine 0 containing Na2S04 (5 x lop3 mol drnp3) the fluorescence intensity increased markedly and the peak maximum was shifted from 485 to 533 nm (Fig.6). The absorption peak maximum was red- shifted by only ca. 4 nm, such that upon binding to the poly- nucleotide the Stokes' shift increases to 4225 cm-'. The fluorescence spectral changes gave a good fit to the Scatchard with a binding constant of (1.4 & 0.2) x lo6 dm3 mol-' and a saturation number 0.9 +_ 0.1 molecules of dye per phosphate group. Similar properties were observed for binding to single-stranded DNA. It seems likely, therefore, E92, II I I I I I 4 I J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 m 0.80 0 (D2 0.60-s 0.40 --0.20 0.m 300350400450500550m650 Llnm Fig. 6 Absorption and fluorescence spectra recorded for Auramine 0 in the presence of excess DNA (phosphate :dye ratio = 60) that binding to the polynucleotide involves primarily electro- static interactions with the phosphate groups without inter- calation between base-pairs.At a phosphate: dye ratio of 60, under conditions where ca. 99% of the dye is bound to DNA, the fluorescence quantum yield was determined to be 0.0014 +_ 0.0005. This corresponds to a 15-fold increase in the fluorescence quantum yield upon binding to DNA relative to aqueous solution. Time-resolved fluorescence studies indicated that the decay profile could be analysed satisfactorily in terms of a single exponential component of lifetime 60 & 4 ps. Thus, binding to the polynucleotide surface hinders rotation about the N,N-dimethyaniline groups and thereby decreases the rate of internal conversion.The bound molecule still under- goes rapid internal conversion, however, and it is clear that such surface binding does not present a major barrier to internal rotation. Previous work has established that Auramine 0 binds selectively to HLADH in neutral aqueous sol~tion.'~ Addi-tion of HLADH to an aqueous solution of the dye containing phosphate buffer (0.1 mol dm-3, pH 7.4) had little effect on the absorption spectrum but enhanced the fluorescence inten- sity (Fig. 7). The fluorescence peak for bound dye was located at 523 nm. Analysis of the titration data according to the Scatchard gave an association constant of (5.6 _+ 0.5) x lo6 dm3 mol- ' and a saturation number corre- sponding to a maximum loading of 1.5 dye molecules per 16.8 ' "40 4h & &I & 6;o I I 1 1 1 I 3ba 3.b 3h 3.30 3.k 3io 3L L/nm Fig.7 Fluorescence spectra recorded for (a) Auramine 0 in waterlo3 KIT containing 0.1 mol dm-3 phosphate pH 7.4 buffer and (b) after addi-Fig. 5 Plot to the transition state theory for experimental data col- tion of HLADH (protein :dye molar ratio = 60).The spectrum lected in (a)pentan-1-01 and (m) nonan-1-01 at different tem- recorded in water is shown at x 20 amplification and the sharp peak peratures centred at 427 nm is a Raman excitation band. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 molecule of protein. Similar parameters were derived by fitting the titration data to the Hill expression2’ and earlier work by Conrad et a1.” also observed a non-integer satura- tion number (n = 1.6-1.7).Since HLADH exists in solution in the form of a dimer composed of two identical subunits28 it appears that three molecules of Auramine 0 bind to a single dimer molecule. The binding sites appear to exhibit comparable association constants without displaying c~operativity~~and it should be noted that previous studies showed that binding of dye does not inhibit the normal enzy- matic functions of the protein.’ Time-resolved fluorescence studies made over a range of dye : protein molar ratios required analysis in terms of three exponential components. A very short-lived component (7 z 20 ps) was dominant at high dye : protein ratios but its fractional contribution to the total signal decreased progres- sively with increasing moIe fraction of protein.This com- ponent is attributed to Auramine 0 free in solution; its apparent lifetime of 20 ps is, in fact, broadened by the instru- ment response. The other two components had lifetimes of 400 & 50 ps and 1.0 & 0.1 ns and occurred at a fixed ratio of 2 : 1, with the shorter-lived component predominating, throughout the titration. Both lifetimes are attributed to dye molecules bound to the protein. The shorter lifetime is assign- ed to dye bound in a relatively flexible site, and the longer lifetime is assigned to dye molecules localized at a tight binding site where rotation is severely hindered. It is clear that binding of Auramine 0 to HLADH causes a dramatic reduction in the rate of internal conversion and, on the assumption that this process involves rotational diffusion, the protein is able to lock the conformation in a geometry unfavourable for internal conversion.There are two binding sites on the protein, distinguishable by the fluorescence life- time of bound Auramine 0, but even dye molecules located at the less tightly bound site are in a rigid environment. Thus, the fluorescence lifetime of Auramine 0 localized at such sites is 400 f50 ps while the measured fluorescence lifetime in glycerol at 22°C (q = 12 P) was 300 f20 ps. Dye molecules located at the more tightly bound site are effectively frozen since the observed fluorescence lifetime is 1.0 k0.1 ns. Our work does not permit identification of either binding site but it indicates that at least one of the sites provides a cavity whose dimensions almost exactly match those of Auramine 0.Concluding Remarks This work has considered two methods for restricting inter- nal rotation in Auramine 0; namely, increasing solvent vis- cosity and binding to a biological macromolecule. The effects of solvent viscosity are readily interpreted in terms of current theoretical models since the rate of internal rotation is con- trolled uniquely by viscous flow. The effects of binding to a macromolecule are both more pronounced and more difficult to interpret. Binding to the surface of DNA via electrostatic forces can be related to a decrease in the rotational diffusion coefficient and the observed fluorescence spectral shifts imply structural distortion in the excited singlet state manifold. Binding to HLADH causes a dramatic reduction in the rate 70 1 of internal rotation which is equivalent to freezing the mol-ecule into a set conformation. The rotational diffusion coeffi- cients for bound forms of the dye appear to be extremely low.However, according to the medium-enhanced barrier model [eqn. (7)] and transition state theory [eqn. (S)], a low k,,, can also be caused by a large negative entropy change. Thus, a dye molecule bound to HLADH may be considered to reside in a conformation unfavourable for rapid internal rotation. Twisting to a more favourable conformation, which has to occur within the excited state lifetime, may involve a signifi-cant decrease in entropy.This work was supported by the National Institutes of Health (CA53619). The CFKR is supported by The Uni- versity of Texas at Austin. References 1 J. Hicks, M. T. Vandersall, E. V. Sitzmann and K. B. Eisenthal, Chem. Phys. Lett., 1987, 135, 413. 2 J. D. Simon and S-G. Su J. Phys. Chem., 1990,94,3656. 3 A. Harriman, J. Photochem. Photobiol. A: Chem., 1992,65,79. 4 J. E. I. Korppi-Tommola, A. Hakkarainen, T. Hukka and J. Subbi, J. Phys. Chem., 1991,95,8482. 5 R. F. Grote and J. T. Hynes, J. Chem. Phys., 1980,73,2715. 6 J. L. Skinner and P. G. Wolynes, J. Chem. Phys., 1978,69,2143. 7 B. Bagchi and D. W. Oxtoby, J. Chem. Phys., 1983,78,2735. 8 S. P. Velsko, D. H. Waldeck and G.R. Fleming, J. Chem. Phys., 1983, 78, 249. 9 D. Ben-Amotz and J. M. Drake, J. Chem. Phys., 1988,89,1019. 10 G. Ponterini and F. Momicchioli, Chem. Phys., 1991,151, 111. 11 J. L. Dote, D. Kivelson and R. N. Schwartz, J. Phys. Chem., 1981,85,2169. 12 A. Gierer and K. Wirtz, 2. Naturforsch, Teil A, 1953,8, 532. 13 Y-P. Sun, J. Saltiel, N. S. Park, E. A. Hoburg, and D. H. Waldeck, J. Phys. Chem., 1991,%, 10336. 14 Y-P.Sun and J. Saltiel, J. Phys. Chem., 1989,93, 8310. 15 R. H. Conrad, J. R. Heitz and L. Brand, Biochemistry, 1970, 9, 1540. 16 G. Oster and Y. Nishijima, J. Am. Chem. SOC.,1956,78, 1581. 17 E. W. Lippert, W. Rettig, V. Bonacic-Koutecky, F. Heisel and J. A. Miehe, Adv. Chem. Phys., 1987,68, 1. 18 S. J. Atherton and A. Harriman, J. Am. Chem. Soc., 1993, 115, 1816. 19 Landolt-Bornstein, Zahlenwerte und Functionen., Secheste Auflage, Band 11, Teil 5, Springer Verlag, Berlin, 1969, pp. 208-230. 20 A. Spernol and K. Wirtz, Naturforsch., Teil A, 1953,8,522. 21 S. R. Meech and D. Phillips, J. Photochem, 1983,23, 193. 22 N. Nakashima, S. R. Meech, A. R. Auty, A. C. Jones and D. Phillips, J. Photochem., 1985,30,207. 23 S. J. Strickler and R. A. Berg, J. Chem. Phys., 1962,37,814. 24 E. Akesson, A. Hakkarainen, E. Laitinen, V. Helenius, T. Gillbro, J. Korppi-Tommola and V. Sundstrom, J. Chern.Phys., 1991,95,6508. 25 D. Gegiou, K. A. Muszkat and E. Fischer, J. Am. Chem. SOC., 1968,90,12. 26 I. Klotz, Science, 1982,217,1247. 27 T. G. Traylor, M. J. Mitchell, J. P. Ciccone and S. Melson, J. Am. Chem. SOC., 1982,104,4986. 28 C-I. Branden, H. Ekland, B. Nordstrom, Y. Boiwe, G. Soder- lund, E. Zeppezauer, I. Ohlsson and A. Akeson, Proc. Natl. Acad. Sci. USA, 1973,70,2439. Paper 3/06039J; Received 11th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000697

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 703-709 Characterization of Transients Formed in Aqueous Solutions of Substituted Alkyl Sulfides: A Pulse Radiolysis Studyt Dilip K. Maity, Hari Mohan and Jai P. Mittal Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085,India ~ ~~~ The transient optical absorption band (A,,, = 340 nm) formed on reaction of OH radicals with substituted alkyl sulfides, S(CH,COpR),, R = CH,, C,H,, C,H,, C,H, in neutral aqueous solutions has been assigned to the OH adduct. The yield, lifetime and molar absorption coefficient of this band is found to decrease with increasing chain length of R. The OH adduct of S(CH2C02CH3), is able to undergo an electron transfer reaction with Br-, I-and SCN-. In acidic solutions, dimer radical cations are formed at high solute concentration.The observed rate constant for the decay of dimer radical cations is deconvoluted to different processes and the deprotonation is found to be the rate-determining step. The yield and lifetime are found to decrease with increasing chain length of R. The high concentration of acid required for their formation is discussed. Cli-and SO;-are able to undergo one-electron transfer reactions with S(CH2C0,CH3), with rate constants of 1.9 x lo9 and 1.4 x 10’’ dm3 mol-’ s-’, respectively. The dimer radical cation is found to be a strong oxidizing agent and is able to oxidize Br-with a rate constant of 2.2 x lo9 dm3 mol-’ s-’. Hydroxyl radical induced reactions with alkyl sulfides (R2S) in aqueous solutions have shown the formation of sulfur- centred radical cations.’ Simple sulfur-centred radical cations (R2S’+) are observed only in cases where the unpaired p- electron of oxidized sulfur is stabilised by the adjacent system or by steric influence^.^.^ These cations absorb at ca.300 nm. In other cases, simple sulfur-centred radical cations are converted to dimer radical cations (R,S);+, as oxidized sulfur has a high tendency to stabilise by co-ordination with a lone pair from the second sulfur at~m.~-~ The dimer radical cations absorb in the region 450-550 nm. The oxi- dized sulfur also has a high tendency to stabilise by coordi- nation with a lone pair of other hetero atoms such as N, 0,P and Such interactions are represented by a two-centre three-electron bond containing two bonding Q electrons and one antibonding o* electron and can take place by both inter- and intra-molecular association.In addition to electron-transfer reactions, OH radicals also react by H-atom abstraction and form a-thioalkyl radicals which absorb in the region 280-300 nm.4-7 Analysis of the transient signal obtained during and immediately after the pulse, on pulse radiolysis of an aqueous solution of ethyl sulfide showed a short-lived transient band (i,,,= 340 nm, t,,, < 1 p),which was assigned to an OH add~ct.~ This band could not be resolved as it is formed within the pulse. Because of the electrophilic nature of OH radicals and the high electron density at sulfur, the OH adduct may be imme- diately converted either to dimer radical cations or the a-thioalkyl radical via a complex sequence of reaction^.^ It is possible that lowering of the electron density at sulfur by the presence of groups of high electron-withdrawing power may increase the lifetime of the OH adduct.Pulse radiolysis of neutral aqueous solutions of dimethyl 3,3’-thiodipropionate gave a well resolved transient band (A,,, = 345 nm, tl,, = 4 p),which was assigned to the OH adduct.” On the other hand, OH radical induced reactions with thiodiglycolic acid (TDGA) led to the formation of a transient band (A,,, = 285 nm), which was assigned to a-thioalkyl radicals.12 This may, perhaps, be due to reduced electron density at sulfur as the C0,H group has very high electron-withdrawing power (Q* = +2.94).13 Re~ently,’~ it has been shown that OH radical induced reactions in aqueous solutions of methyl- ? Preliminary results were presented at the 3rd International Con- ference on Chemical Kinetics, July 12-16, 1993, Gaithersburg, USA.thiomethyl acetate lead to the formation of a transient band which has been assigned to the OH adduct formed on stabili- zation with an internal hydrogen bond between the hydroxyl hydrogen and an oxygen located either in an adjacent car- bony1 or methoxy group. Therefore, it appears that substit- uents can affect the nature of OH radical induced reactions with organic sulfur compounds. With this objective, pulse radiolysis studies have been carried out on substituted alkyl sulfides in which the electron density at sulfur is expected to be reduced by the presence of substituents of higher electron- withdrawing power.Such studies have gained importance as sulfur-centred transients are considered as possible interme- diates in redox reactions of biom~lecules~~*~ and are helpful in understanding the physico-chemical processes and redox properties in sulfur drugs, amino acids and other biological systems containing sulfur. A quantum-chemical calculation has been performed at the semiempirical (AM1) level to find the structures of different alkyl sulfides and their respective radical cations.16 The com- puted data for the net atomic charge on sulfur and ionization potential values for different sulfur compounds are used to analyse the present experimental results.Experimental Esters (dimethyl, diethyl, dipropyl, dibutyl) of 2,2’-thiodietha- noic acid were prepared by esterification of 2,2’-thiodiethanoic acid (TDGA) with the appropriate alcohols by a standard method.17 The purity of these esters was checked by thin- layer chromatography and IR analysis and they were found to be free from 2,2’-thiodiethanoic acid. Thiodipropionic acid dilauryl ester (C,0H5,04S, Registry No. 123-28-4) and 3,3’- thiodipropionic acid dioctadecyl ester (C4, H ,O,S, Regis-try No. 693-36-7) were obtained from Sigma Chemicals and used without further purification. All solutions were prepared in deionized ‘nanopure’ water. Fresh solutions were used in each experiment.The pH of the solution was adjusted with NaOH and HC104. Indian Oxygen, ‘iolar’ grade N,O, O,, N, gases were used for purgmg the solutions. The reaction of OH radicals in neutral aqueous solutions was carried out in N,O-saturated solutions where ea; are quantitatively con-verted to OH radicals (N,O + e,; -+ ‘OH + OH-+ N,) with G(0H) = 5.6. In acidic solutions, the reaction of OH radicals was carried out in 0,-saturated solutions where e- and H atoms are converted to HO, radicals (e,; + H+ + H“? , H' + 0, -+ HO;) and G(0H) = 2.9 (pH = 0-3.0). The reac- tion of 0'-was carried out in N,O-saturated solution at pH 12.5 where OH radicals are converted to 0'-('OH+ OH--+ 0'-+ H,O). Pulse radiolysis experiments were carried out with high- energy electron pulses (7 MeV, 50 ns), from a linear acceler- ator whose details are described elsewhere.'* The dose delivered per pulse (ca.1.0 x 1017 eV cm-3) was determined by use of an aerated aqueous solution of KSCN (10 mmol dm-3).19 The ac conductivity changes produced on pulse radiolysis were monitored in the dual-function cell arrange- ment and electronic detection system obtained from the Hahn-Meitener-Institut, Berlin.20 All the experiments were carried out at 25°C. The rate constant values were the average of at least three experiments and the variation was within *lo%. The atomic charge over the sulfur atom and the ionization potential of the different sulfides are computed in their fully optimized geometry.' The geometry optimization was by semiempirical molecular-orbital calculations with AM 1 para-metrization,2' as this appears to be the most successful method.Results and Discussion Reaction of OH Radicals with Dimethyl 2,2'-thiodiethaooate at pH 6.0 Fig. l(a) shows the transient optical absorption spectrum obtained on pulse radiolysis of N,O-saturated aqueous solu- tion of dimethyl 2,2'-thiodiethanoate (DMTE) (1.3 x lop3 mol dm-3, pH 6.0). It exhibits an absorption band with A,,, = 340 nm. In N,O-saturated aqueous solutions at pH 6.0, the primary reactive species produced on pulse radiolysis would be OH radicals and H atoms, The AA value at 340 nm decreased from 0.029 to 0.0028 in the presence of 0.2 mol dmV3 tert-butyl alcohol, a strong OH-radical and weak H-atom scavenger.Therefore, this band [Fig. l(a)] is attrib- uted to the reaction of OH radicals with DMTE. The small absorption observed in the presence of tert-butyl alcohol [Fig. l(b)] is attributed to the reaction of H atoms with DMTE as independent studies on the pulse radiolysis of N,-saturated aqueous solutions of DMTE (1.0 x mol dm-3, pH = 1.5, [tert-butyl alcohol] = 0.2 mol dm-3) also showed a similar absorption band. The rate constant for the reaction of OH radicals with DMTE is determined to be 2.3 x 10" dm3 mol-' SKI.The molar absorption coefficient of this band [Fig. l(a)], determined by competition kinetics using an N,O-saturated solution of KSCN (2.0 x mol dmP3, E~~~ = 7200 dm3 mol-' cm-l) is 3150+ 200 dm3 mol-' cm-'.The band is observed to decay by first-order kinetics with tl,, = 8.6 ps. J. CHEM. SOC. FARADAY TRANS., 1994,VOL. 90 300 4 00 500 600 wavelengthlnm Fig. 1 Transient optical absorption spectrum obtained immediately after pulse radiolysis of an aqueous solution of DMTE: (a) N,O-saturated, [DMTE] = 1.3 x mol dm-3, pH 6.0; (b) N,O-saturated, [DMTE] = 1.3 x lo-' mol dm-3, pH 6.0, [tert-butyl alcohol] = 0.2 mol dm-3; (c) N,O-saturated, [DMTE] = 1.3 x lo-' mol drnp3, pH = 6.0; (d) N,O-saturated, [DMTE] = 1.3 x rnol dmP3, pH = 12.5, (e) 0,-saturated, [DMTE] = 6.6 x rnol dmP3, [HClO,] = 4.5mol dm-3 The conductivity studies showed this band to be due to a neutral species as there was no change in the relative conduc- tivity of the solution before and after the pulse.Considering the neutral nature, high rate constant and previous experi- mental observations:.' ',I4 the band is assigned to the OH adduct formed according to reaction (1) (Scheme 1). It is also possible that the OH adduct may form a six-membered ring configuration with internal hydrogen bonding.I4 The OH adduct (A = 340 nm) may decay to a-thioalkyl radicals [reaction (2)] or to solute radical cations [reaction (3)]. The ambiguity between these two modes of decay was resolved when the decay of the 340 nm band was studied in the pres- ence of a proton acceptor. The decay of this band remained unaffected in the presence of 5.0 x mol dm-3 1 :1 mixture of HPOZ--H,PO,, suggesting that the decay may be reaction (2).The absence of conductivity changes would also support this process as the most probable mode of decay. The intensity of this band remained independent of solute concentration in the region (1.0-6.5) x lop3mol dm-3. At higher concentrations, the intensity of the 340 nm band reduced slightly and decay became faster. Simultaneously, another band of small intensity was observed to grow in the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 region of 485 nm [Fig. l(c)]. The decrease in the intensity and the faster decay of the 340 nm band at higher solute concen- tration could be due to the reaction of solute molecules with the OH adduct [reaction (7)] to form a solute dimer radical cation. The small decrease in the absorbance at 340 nm and corre- sponding increase at 485 nm [Fig.l(a) and (c)]is attributed to reaction (7). From these results, the molar absorption coef- ficient and the G value of the transient species at 485 nm are determined to be 1.6 x lo4 dm3 mol-' cm-' and 0.28, respectively. The absence of an absorption band in the region of 400 nm shows that intra-molecular orbital overlap between oxidized sulfur and oxygen is not taking place. This may be due to the unstable nature of the four-membered ring configuration. The 485 nm band is attributed to a dimeric species as it was observed only at high solute concentration. Reaction of OH Radicals with Different Esters In order to investigate the effect of the chain length of the ester group, similar studies have been carried out on different esters of 2,2'-thiodiethanoic acid (Table 1).These studies have been carried out at the same solute concentration (2.0 x 10-mol dmW3) and same dose (1.0 x lo" eV per pulse). In each case, a transient band was observed at the same wave- length (340 nm). The G value, E, and tl,, of the OH adduct decreases with increasing chain length showing that bulkier groups lower the stability of the OH adduct owing to steric influence. The G value (Table l),of the OH adduct is much less than that of G(0H) = 5.6 in N,O-saturated solutions. Therefore, the remaining fraction of OH radicals may be reacting to form the a-thioalkyl radicals, absorbing in the region 280- 300 nm. This band may lie within the broad absorption of the 340 nm band.The remaining fraction of OH radicals may not -0.02 nU300 400 500 600 wavelengt h/n m Fig. 2 Transient optical absorption spectrum obtained immediately after pulse radiolysis of an N,O-saturated solution of (a) dilauryl ester; (b) dioctadecyl ester; (c) N,-saturated DCE and (d) N,-be reacting to form the solute radical cations as the conduc- tivity studies have shown the formation of a neutral species. When the chain length between sulfur and the ester group is increased (as in the case of 3,3'-thiodipropionic acid), OH radical induced reactions showed somewhat different results. Fig. 2(a) shows the transient optical absorption spectrum obtained on pulse radiolysis of an N,O-saturated aqueous solution of thiodipropionic acid dilauryl ester.It exhibits an absorption band at 410 nm with a small shoulder at 340 nm. The 340 nm band may be due to the OH adduct and the 410 nm band may be due to radical cation 1 formed on p-orbital overlap of oxidized sulfur with oxygen making a two-centre three-electron bonding. The five-membered ring configu- ration obtained in this case is expected to increase the stabil- ity of this intra-molecular radical cation because of less angular strain. O=C-CH;!II 1 Owing to limited solubility, variation in the absorbance of transient bands with solute concentration could not be carried out. When the ester side chain is further increased from dilauryl to dioctadecyl, absorption of the transient band formed on pulse radiolysis is further reduced [Fig.2(6)]. The decreased intensity may be due to lower stability introduced by the bulky side chain. Reaction of OH Radicals with DMTE at Different pH In the pH range 0-10, there was no change in the intensity of the 340 and 485 nm bands formed on reaction of OH radicals with DMTE at low and high solute concentrations, respec- tively. Even the decay remained unaffected. At pH > 12.0, the OH radicals would be converted to 0.-.Fig. l(6) shows the transient optical absorption spectrum obtained on pulse radiolysis of an N,O-saturated aqueous solution of DMTE (1.3 x mol dm-3) at pH 12.5. This exhibits an absorption band with A,,, = 285 nm that may be due to a-thioalkyl radicals formed on H-atom abstraction by 0'-. This was observed to decay by second-order kinetics 2k/ d = 9.0 x lo5 s-'. The formation rate constant was deter- mined to be 3.0 x lo9 dm3 mol- 's-'. When the HClO, concentration was >1.0 mol dm-3, the intensity of the 340 nm band decreased and another absorp- tion band appeared in the region of 485 nm.This shows that the OH adduct is reacting with H+ only when the HClO, concentration is ~1.0mol dm-3. Fig. l(e) shows the tran- sient optical absorption spectrum obtained on pulse radiolysis of 0,-saturated solution of DMTE (6.6 x mol dm-3) in 4.5 mol dm-3 HClO,. This band was not observed in the presence of the OH radical scavenger (tert-saturated, [DMTE] = 2.5 x Table 1 ~max solute /nm DMTE" 340 DETE~ 340 DPTE' 340 DBTEd 340 rnol dm-3 in DCE Physical parameters of the OH-adduct formed from various esters of 2,2'-thoidiethanoic acid formation rate constant /1O'O dm3 mol-ls-' G 8.6 3.65 f0.2 6.5 2.85 0.2 1.7 3.27 0.3 1.1 1.95 0.4 E /dm3 mol-' cm-' 3150 f200 3130 f250 2620 f 300 2210 * 500 ~~ Dose = 1.0 x lo" eV cm-3 per pulse.Solute concentration = 2.0 x mol dm-3. Dimethyl 2,2'-thiodiethanoate. * Diethyl 2,2'-thiodietha- noate. Dipropyl 2,2'-thiodiethanoate. Dibutyl 2,2'-thiodiethanoate. 706 [HClO,]/mol dm-3 0 4.0 I I 1 3.0 0 0 2.0 4.0 6.0 8.0 10.0 [DMTE]/1O3 rnol dm-3 Fig. 3 Variation in the absorbance of 485 nm band as a function of (a) HClO, concentration, [DMTE] = 6.6 x mol dm-3; (b) DMTE concentration, [HClO,] = 9.8 mol dmP3 butyl alcohol) or in the absence of DMTE showing that the band is due to reaction of OH radicals with the solute and not to any transient species formed from radiolysis of HClO,.Fig. 3(a) shows the variation in the absorbance of the 485 nm band as a function of HClO, concentration. This may not represent the true variation of absorbance with [HClO,] as the dose absorbed by water would not remain the same at higher [HCIO,]. The rate constant for the reac-tion of OH radicals with DMTE ([HCIO,] = 9.2 mol dmP3, A = 485 nm) was determined to be 1.4 x lo9 dm3 mol-I s-'. The intensity of this band decreased with decrease in the solute concentration without any change in the position of A,,,.The absence of the transient band in the region of 400 nm at low solute concentrations again suggests that the intra-molecular radical cation is not formed, possibly because of the unstable nature of the four-membered ring configuration. The reactive species produced on radiolysis of aerated aqueous solutions containing a high concentration of HCIO, are HO,, OH and ClO, radicals. The HO, radicals could not be the source of the transient band absorbing at 485 nm as (i) the redox potential value of the HO, radical is quite low (+1.0).,, Stronger one-electron oxidants such as Bri-were unable to produce this band (see text). (ii) The intensity of this band remained unaffected in N,-saturated solutions where HO, radicals would not be formed.C10, radicals also could not be responsible for the tran-sient band observed at 485 nm as (i) they have no absorption at i> 400 nm23 and ClO, radicals generated on pulse radiolysis of a neutral aqueous solution of NaClO, (7.5 mol drnp3)containing 5 x lop3mol dmP3 DMTE do not show transient absorption similar to that observed in the presence of HClO,. The OH radicals are shown to undergo acid-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 dation of the OH adduct [reaction (5)J If the formation of the 485 nm band is due to acid-catalysed oxidation of the OH adduct, then a similar band should also be observed in the presence of another acid. In fact the pulse radiolysis studies in aerated aqueous solutions of DMTE (5.0 x mol drnp3)containing 5.0 mol dm-3 H,SO, showed the for-mation of a transient band at 485 nm.The absorbance at 485 nm was slightly lower in the case of H,SO, than that observed in 5.0 mol dm-3 HClO,. This should be due to the lower Hammett acidity function (H,) value for H,SO, .,' In H,SO, solution, the 485 nm band could not be due to SO;-as (i) it absorbs at 450 nm30 and (ii) the rate constant for the reaction of OH radicals with HSO, is very low (8 x lo5 dm3 mol-1 s-1 30).The increase in the intensity and lifetime of the transient band with solute concentration suggest the existence of an equilibrium [reaction (6)]. Therefore, the transient band observed in the presence of high acid and solute concentra-tion is assigned to the dimer radical cation of DMTE formed via reactions (l),(5) and (6).The absorbance of the transient band at 485 nm [Fig. 3(b)] has reached saturation at a solute concentration of 6 x mol dm-3 showing that all the solute radical cations are con-verted to dimer radical cations. Taking the molar absorption coefficient value of 1.6 x lo4 dm3 mol-' cm-', and the absorbance at 485 nm of 0.039, the G value of the transient species absorbing at 485 nm is determined to be 1.45, much less than G(0H) = 2.9 in neat water at pH = 1.0. This must be due to the much lower value of G(0H) in the presence of a high acid concentration as all the radiation energy would not be absorbed by water alone. The product of the G and E (2.32 x lo4 dm3 mol-I cm-') obtained for DMTE in this way, is close to that determined directly by independent experiment (2.26 x lo4 dm3 mol-' cm-').These studies have also been performed on the other esters (Table 2). In order to compare the stability and yield of dimer radical cation of these esters, pulse radiolysis studies have been carried out at identical conditions of dose and concen-tration. It can be seen that the yield (GE) and lifetime decreases as the chain length of the ester group is increased. It is suggested that the extra methylene group may hinder a good orbital overlap of the oxidized sulfur atom with the unoxidized molecule to form a strong two-centre three-electron bond. A red shift in the transient absorption band (Table 2) is observed with increasing chain length in the ester group.The decreased separation between o-o* orbitals due to increased electron-releasing power of the substituted alkyl group,13 would cause a red shift in A,,,. On the other hand, a large separation between these orbitals would correspond to a stronger interaction. The increased electron-withdrawing power would in turn reduce the electron density over the sulfur, and so the net atomic charge over the sulfur atom will increase (become more positive). Our quantum-chemically catalysed oxidation of and bromo-alkanes,26 and computed results (Table 3) provide quantitative data for the a number of other organic corn pound^.^^^^^ Therefore, it is net atomic charge over the sulfur atom for different com-possible that this band (485 nm) is due to H+-catalysed oxi-pounds and the data support our conclusions.Table 2 Physical parameters of the dimer radical cation formed from various esters of 2,2'-thiodiethanoic acid formation stability LX rate constant tlj2 constant solute /nm /lo9 dm3 mol-' s-' /P /dm3 mol-' DMTE 485 1.4 DETE 500 1.2 DPTE 510 1 .o DBTE 520 - [solute] = 4.0 x lop3mol dm-3. [HClO,] = 150 105 63 450 22 433 9.2 mol dm-3. Dose = 1.0 x 1017 eV ~rn-~per pulse. GE k -. 6a /lo3 dm3 mol-' /lo7 s-1 22.6 1.3 17.1 0.26 13.4 0.23 7.6 __ See Scheme 1. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Net atomic charge at sulfur and ionization potential for various substituted alkyl sulfides Taft ionization net atomic charge parameter, potential acid conc.for formation of solute at sulfur o* lev dimer cations/mol dm -3 -0.068 0.0 9.54 -0.099 -0.100 9.36 FH2C02CH3 s\ +0.066 +2.00 10.21 > 1.0 CH2C02CH3 /CH2C02C2H 5 s, +0.065 10.09 >1.0 CH,C02C2H5 FH2C02H +0.13 +2.94 10.48 >4.0s\CH2C02H Decay Kinetics The 485 nm band was observed to decay by first-order kinetics with a lifetime dependent on the solute and HClO, concentration. Asmus and co-worker~,~' have shown that the rate-determining step for the decay of the dimer radical cation of alkyl sulfides can be either the back reaction of the equilibrium (k -6) or the deprotonation of solute radical cation (k4). Using different concentrations of proton acceptor to accelerate the deprotonation of solute radical cations, they have shown from the plots of kobs us. kobs [solute] that the deprotonation of radical cation would be the rate-determining step if plots have same slope (k6)and different intercepts (k, + deprotonation rate constant x [proton acceptor]).The decay by back reaction of the equilibrium would yield the same intercept (k-6) and different slopes (k,/(k, + deprotonation rate constant x [proton acceptor])}. This method of estimation of the various rate constants associated with the decay of dimer radical cations is not pos- sible in the present case as dimer radical cations are observed at high acid concentration and a proton acceptor could not be used. However, plots of kobs us.kobs [solute] for various acid concentrations gave straight lines (Fig. 4) with the same slope and different intercept. Therefore, deprotonation reac- 0' I I I 0.1 0.2 0.3 0.4 0.5 k,,,[solute]/mol dm-3 s-' Fig. 4 kobaDS. k,, [DMTE] for various HClO, concentrations; (a) 9.2, (b) 8.3 and (c) 7.4 mol dm-3 tion should be the rate-determining step and the stability constant of the dimer radical cation would be equal to the slope of straight lines (Fig. 4) and the values are shown in the Table 2. From the formation rate constant value of 1.4 x lo9 dm3 mol-' s-', for DMTE, k-, is calculated to be 1.3 x lo7 s-l. The stability constants for the dimer radical cation gener- ated from other esters has also been evaluated by a similar procedure and the values are shown in Table 2.The stability constants for the dimer radical cation of dimethyl sulfide and dipropyl sulfide has been reported to be 2 x lo5 and 540 dm3 mol-', re~pectively.~'The lower value of the stability con- stant for DMTE explains the instability. The stability con- stant for the dimer radical cation of thiodiglycolic acid (68 dm3 mol-') was still lower than that of DMTE (105 dm3 mol-') and explains the higher concentration of acid (>4.0 mol dm-3) required for the formation of dimer radical cations of TDGA12* as compared with DMTE (> 1.0 mol dm-3). The intercept is seen to depend on H+ concentration (Fig. 4). This may be due to the fact that at a lower H+ concentra-tion (<1.0 mol dm-3), the OH adduct would decay to a- thioalkyl radicals and at a higher concentration of H+, the OH adduct can react with H+ to form radical cations.It is also possible that a-thioalkyl radicals may react with H+ at higher acid concentration. Therefore, the intercept would be represented by a complex function and it would be difficult to determine the deprotonation rate constant. The ratio of the intercept for two values of HClO, increases at a much higher rate than the increase in the ratio of the HClO, concentra-tion. This may be due to an increased contribution of reac- tion (2) at lower H concentration.+ Variation of Atomic Charge at Sulfur with Substituents It has been shown that the nature of the OH radical reaction depends on the nature of the substituents present in the mol- ecule.The net atomic charge at sulfur has been calculated (Table 3) and the results suggest that the amount of acid required for acid-catalysed oxidation of substituted sulfides by OH radicals can be related to the net atomic charge on the sulfur. The ionization potential of substituted alkyl sul- 708 I -2.5 22 E 9.0 I I I 12.0 -0.1 0.0 0.1 0.2 net atomic charge at S Fig. 5 Variation of the ionization potential of alkyl sulfides and the energy corresponding to A,,, of the corresponding dimer radical cation with net atomic charge at sulfur for various alkyl sulfides fides is also observed to increase with electron-withdrawing power or net atomic charge on the sulfur (Fig.5). This is expected as reduced electron density at the sulfur would make it difficult to ionize the compound. With simple alkyl sulfides, such as dimethyl sulfide, the dimer radical cations are observed at neutral pH. The electron-releasing power of the C,H, group in diethyl sulfide, increases the electron density (lowers the net atomic charge) at sulfur and thereby lowers the ionization potential (Table 3). As the alkyl group is replaced by substituents with high electron-withdrawing groups, the electron density at sulfur is lowered and the net atomic charge increases. The sulfide radical cation formation required higher acid concentration. The net atomic charge at sulfur in TDGA is very high, showing very low electron density at sulfur and an acid concentration >4.0 mol dm- ' was required for the electron-transfer reaction. The ionization potential is also very high.When C0,H groups are replaced by C0,CH3 groups as in S(CH,C0,CH3)2 ,electron density at sulfur is relatively higher owing to the electron-releasing power of the CH, group and OH radicals are able to form an OH adduct and dimer radical cations are formed at relatively lower H+ concentration (> 1.0 mol dm-3). The ionization potential calculated for this compound also supports the atomic charge at sulfur. For other substituted alkyl sulfides (Table 3), with net atomic charge on sulfur lower than S(CH2C0,CH3),, the amount of acid required for dimer radical cation formation and the ionization potentials are lower.Therefore, it can be concluded that the nature of the OH radical reaction is greatly influenced by the electron- releasing/-withdrawing power of the substituents. The ioniza- tion potential and amount of acid required for acid-catalysed oxidation by OH radicals increases with net atomic charge on sulfur. The position of the energy corresponding to Amax (eV) for the dimer radical cation of the substituted alkyl sul- fides is also observed to depend on the net atomic charge at sulfur (Fig. 5), which suggests that the strength of the sulfur- sulfur three-electron bond decreases with the presence of electron-withdrawing substituents in substituted alkyl sul- fides. Stability of Esters in High Acid Concentration The optical absorption spectra of an acidic aqueous solution of DMTE remained unchanged with time, suggesting that the solution is stable and does not undergo hydrolysis.The OH radicals failed to form dimer radical cations at HClO, < 4.0 mol dm3 in acidic aqueous solutions of TDGA. It is also reported3' that in presence of H+, hydrolysis of esters would be difficult and an adduct with H+ at the sulfur may be formed. Hydrolysis is expected to be faster in highly basic solutions.32 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Reactions with Specific Oneelectron Oxidants The decay of Cli-, formed on pulse radiolysis of an 0,-saturated solution of C1- (2.0 x lo-, mol dm-3, pH 1.5) was observed (A,,, = 345 nm) to become faster and of first order in the presence of small amounts of DMTE [(l-7) x mol dm-3], showing that Cl;-is able to undergo an electron-transfer reaction with DMTE.The second-order rate constant for the reaction was determined from the slope of the linear plot of the pseudo-first-order rate Cl;-+ DMTE +2C1-+ DMTE+ (8) vs. DMTE concentration and the value is 1.9 x lo9 dm3 mol-' s-'. Time-resolved studies showed the formation of a new band with A,,, = 380 nm. This band may be due to a neutral three-electron-bonded species formed between DMTE+ and C1 -, 2. Three-electron-bonded species between halogen and alkyl sulfides are known to absorb in this region.33 The increased absorption in the region of 500 nm was not observed even at high solute concentration. C H2C02CH3 CI:.S / \ CHZCOZC H3 2 SO;-is also a strong oxidizing agent.The decay of SO;-, formed on pulse radiolysis of the N,-saturated solution of S20i-(2.0 x lo-, mol dm-3, tert-butyl alcohol, 0.5 mol dm-3, A = 460 nm) was found to become faster in the pres- ence of a small concentration of DMTE. The rate constant for the oxidation of DMTE by SO;-was determined to be 1.4 x 10" dm3 mol-I s-'. Time-resolved studies did not show the formation of a new band in the 350-600 nm region. This shows that although the electron transfer is taking place, the dimer radical cation is not formed even at high solute concentration. This suggests that the radical cation is not stable at this pH. At neutral pH, deprotonation of the radical cation to a-thioalkyl radicals is very fast.Br;- and 1;-were not able to undergo an electron-transfer reaction with DMTE, suggesting that the redox potential for the formation of the radical cation of DMTE is more than that of the Br;-/2Br-couple (+ 1.6 V). The transient (A = 485 nm) formed on reaction of OH radicals with DMTE was observed to oxidize Br- with a rate constant of 2.2 x lo9 dm3 mol-I s-and time-resolved studies showed the formation of a new band with A,= = 380 nm. This could be due to a Br adduct as Bri-, if formed, would have been evidenced by a transient band at 360 nm. These studies suggest that the redox poten- tial for the DMTE/DMTE+ couple is roughly between 1.6 and 2.0 V. The OH adduct was also observed to react with I-, Br-and SCN-. The time-resolved studies showed the formation of a new band (8 ps after the pulse) with A,,, = 380 nm.Detailed kinetic studies could not be carried out as the absorption bands of the OH adduct and the oxidized species are close to each other. The new band formed at 8 ps after the pulse could be due to the adduct of an oxidized atom with DMTE. Formation of Radical Cations in l,%Dichloroethane lY2-dichloroethane (DCE) has been used frequently as a solvent for the study of solute radical cations of organic com- pound~~~because of its high ionization potential. Therefore, complementary studies have been carried out in DCE to investigate the formation of radical cations of DMTE, which could be formed by the following mechanism : CH,CICH,CI --+ CH,CICH,CI+ + e-(9) CH,ClCH,Cl + e--+ CH,ClCH, + C1-(10) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 CH2ClCH2C1++ DMTE 4DMTE" + CH,ClCH,CI Fig. 2(4 shows the transient optical absorption spectrum obtained on pulse radiolysis of an N,-saturated solution of DMTE (2.5 x mol dm-3) in DCE.-It-exhibits absorp- tion bands at 370 nm. Pulse radiolysis of DCE does not produce this band [Fig. 2(c)J.The broad absorption in the region of 500 nm was observed at high solute concentration. Therefore, it should be due to dimer radical cations. The band at 370 nm may be due to a species of the type Cl:. S(CH,CO,CH,), formed between C1- and DMTE". The band at 370 nm was observed to decay by first-order kinetics with tl,2 = 36 p. The complex between C1- and radical cations of alkyl sulfides is reported to absorb in the region of 380 nm.33 Conclusions The OH radicals react with esters of 2,2'-thiodiethanoic acid to form an OH adduct in neutral solution and dimer radical cations in acidic solution.The deprotonation is the rate- determining step for the decay of dimer radical cations. A high concentration of acid is required for the formation of dimer radical cations owing to the reduced electron density at the sulfur. References K-D. Asmus, in Sulfur-centered Reactive Intermediates in Chem- istry and Biology, ed. C. Chatgilialoglu and K-D. Asmus, NATO AS1 Series A: Life Sciences, vol. 197, Plenum Press, New York, 1990, p. 155. K. Kim, S. R. Mani and H. J. Shine, J. Org.Chem., 1975, 40, 3857. K-D. Asmus, D. Bahnemann, Ch. H. Fischer and V. Veltwisch, J. Am. Chem. SOC., 1979,101,5322. M. Bonifacic, H. Mockel, D. Bahnemann and K-D. Asmus, J. Chem. SOC.,Perkin Tmns. 2, 1975, 675. M. Gobl and K-D. Asmus, J. Chem. SOC., Perkin Trans. 2, 1984, 691. K. Bobrowski and J. Holcman, J. Phys. Chem., 1989,93,6381. M. Gobl, M. Bonifacic and K-D. Asmus, J. Am. Chem. SOC., 1984,106,5984. R. S. Glass, M. Hojatie, G. S. Wilson, S. Mahling, M. Gobl and K-D. Asmus, J. Am. Chem. SOC., 1984,106,5382. H. Hungerbuhler, S. N. Guha and K-D. Asmus, J. Chem. SOC., Chem. Commun., 199 1,999. 10 E. Anklam, H. Mohan and K-D. Asmus, J. Chem. SOC., Perkin Trans. 2, 1988, 1297. 11 H. Mohan and J. P. Mittal, J. Chem. SOC., Perkin Trans. 2, 1992, 207.12 (a) H. Mohan and P. N. Moorthy, J. Chem. SOC., Perkin Trans. 2, 1990, 413; (b) D. K. Maity and H. Mohan, J. Chem. SOC., Perkin Trans. 2, 1993, 2229. 13 R. W. Taft, J. Chem. Phys., 1957,26,93;J. Am. Chem. SOC., 1953, 75, 423 1. 14 (a) K. Bobrowski and C. Schoneich, J. Chem. SOC., Chem. Commun., 1993, 795; (b) C. Schonich and K. Bobrowski, J. Am. Chem. SOC., 1993,115,6538. 15 C. von Sonntag, Chemical Basis of Radiation Biology, Taylor and Francis, New York, 1978, p. 353. 16 D. K. Maity, Hari Mohan and J. P. Mittal, unpublished results. 17 A. I. Vogel, Text Book of Practical Organic Chemistry, Longman, London, 1987, p. 501. 18 (a) S. N. Guha, P. N. Moorthy, K. Kishore, D. B. Naik and K. N. Rao, Proc. Id. Acad.Sci., (Chem. Sci) 1987,99,261; (b)K. I. Priyadarsini, D. B. Naik, P. N. Moorthy and J. P. Mittal, Proc. 7th Tihany Symp. on Radiation Chemistry, Hungarian Chemical Society, Budapest, 1991, p.105. 19 E. M. Fielden, The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis, ed. J. H. Baxendale and F. Busi, Reidel, Boston, 1984, p. 59. 20 K-D. Asmus and E. Janata, The Study of Fast Process and Tran- sient Species by Electron Pulse Radiolysis, ed. J. H. Baxendale and F. Busi, Reidel, Boston, 1984, p.91. 21 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J. Am. Chem. SOC.,1985,107,3902. 22 B. H. J. Bielski, R. L. Arudi and M. W. Sutherland, J. Bid. Chem., 1983,258,1748. 23 S. A. Choudhri, M. Gobl, T. Freyholdt and K-D. Asmus, J. Am. Chem. SOC., 1984,106,5988. 24 H. Mohan and K-D. Asmus, J. Chem. SOC., Perkin Trans. 2, 1987,1795. 25 H. Mohan and J. P. Mittal, J. Chem. SOC., Perkin Trans. 2, 1992, 1731. 26 H. Mohan, D. K. Maity and J. P. Mittal, J. Chem. SOC.,Faraday Trans., 1993,89, 477. 27 K-D. Asmus, P. S. Williams, B. C. Gilbert, J. N. Winter, J. Chem. SOC., Chem. Commun., 1987,208. 28 M. Bonifacic and K-D. Asmus, J. Phys. Chem., 1976,80,2426. 29 M. A. Paul and F. A. Long, Chem. Rev., 1957,57,1. 30 E. Heckel, A. Henglein and G. Beck, Ber. Bunsenges. Phys. Chem., 1966,70,149. 31 J. Monig, R. Goslich and K-D. Asmus, Ber. Bunsenges. Phys. Chem., 1986,90, 115. 32 J. March, Advanced Organic Chemistry, Wiley Eastern Ltd., 1986, p. 334. 33 M. Bonifacic and K-D. Asmus, J. Chem. SOC., Perkin Trans. 2, 1980,758. 34 T. Sumiyoshi, N. Sugita, K. Watanal and M. Katayama, Bull. Chem. SOC.Jpn., 1988,61,3055. Paper 3/04990F; Received 17th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000703

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 711-716 71 1 Pulse Radiolytic One-electron Reduction of 2-Hydroxy- and 2,6=Dihydroxy=9,1O=anthraquinones Haridas Pal,* Tulsi Mukherjee and Jai P. Mittal Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085,India The semiquinone free radicals produced by one-electron reduction of 2-hydroxy-9,lO-anthraquinone (2HAQ) and 2,6-dihydroxy-9,10-anthraquinone (26DHAQ) in aqueous formate solution, water-isopropyl alcohol-acetone mixed solvent and isopropyl alcohol have been studied using the pulse radiolysis technique. The absorption characteristics, kinetic parameters of formation and decay, acid-base behaviour and redox characteristics of the semiquinones have been investigated and compared with the corresponding characteristics of a few intramole- cularly hydrogen-bonded anthrasemiquinone derivatives.The non-hydrogen-bonded semiquinones show two pK, values (4.7 and 10.7 for 2HAQ and 5.4 and 8.7 for 26DHAQ, respectively) within the pH range 1-14, whereas other intramolecularly hydrogen-bonded semiquinones show only one pK, . The one-electron reduction potential (€,) values for 2HAQ (-440 mV) and 26DHAQ (-400 mV) are more negative than those of the intramolecularly hydrogen-bonded systems. Pulse radiolysis is an important technique for studying the electron-accepting properties of the quinones by character- ising their semiquinone radicals formed by one-electron reduction.'-24 A difference in substitution in simple quinones may cause different characteristic changes in their proper- tie^.^'-^' In the present paper we report on the character- istics of the semiquinones of 2HAQ and 26DHAQ (where no intramolecular hydrogen bonding exists) in various solvents and compare the results with those of other similar systems with intramolecular hydrogen bonding. Since both 2HAQ and 26DHAQ are not sufficiently soluble in water below pH x 8, a water-isopropyl alcohol-acetone mixed-solvent system (30.2 : 5 :1 molar ratio) was mostly used in the present st~dy.'~,'~,~~ At pH > 8, however, the reduced semiquinone radicals were also produced and investigated in aqueous solutions containing 10-mol dm-3 sodium formate as an additive. The reduced semiquinone radicals were also studied in pure isopropyl alcohol solution for comparison purposes.In the above aqueous-organic mixed-solvent system, since the bulk of the medium remains essentially aqueous, reporting of the pH of the solutions is j~stified.~~,~',~ Experimental 26DHAQ (Aldrich) was purified by repeated crystallisation from methanol. 2HAQ was prepared from 2-amino-9,lO- anthraquinone (TCI, Japan) by diazotization followed by hydrolysis in hot acidic water. 2HAQ thus prepared was purified by crystallisation from a water-methanol mixture. The crystals showed a melting point of 306"C, the reported value. Isopropyl alcohol and acetone were of spectroscopic grade from Spectrochem India. Other chemicals used were of the purest grade available from Fluka, Merck or BDH and were used without further purification.Triply distilled water was used for making all aqueous solutions and in the aqueous-organic mixed solvent. Solutions were made alka- line to 1 mol dmP3 NaOH for studies at pH x 14. Details of the pulse radiolysis and other experimental arrangements are as described earlier. 15-'7 The absorbed dose was measured by thiocyanate dosimetry (5 x mol dm-air-saturated KSCN sol~tion~~.~~)following the absorbance of the (SCN);- radicals at 500 nm, produced under isodosage conditions. Results and Discussion Absorption Spectra and pK, Values of 2HAQ and 26DHAQ In the pH range (1-14) of our investigation, 2HAQ can exist either in neutral or in monoanionic form and 26DHAQ can exist either in neutral, monoanionic or dianionic form, depending on the pH of the solutions, as the phenolic OH groups participate in the protolytic equilibria.For the conve- nience of presentation, the neutral, monoanionic and dia- nionic forms are generalised as QH2, QH- and Q2-, respectively. Spectroscopic data and the acid dissociation constants of 2HAQ and 26DHAQ are listed in Table 1, along with those of unsubstituted anthaquinone (AQ)34 and other anthraquinone derivatives, namely 1,4-dihydroxy-9,10-anthraquinone (14DHQ),24 1,5-dihydroxy-9,10-anthra-quinone (15DHAQ)I6 and 1,8-dihydroxy-9,10-anthraquinone (18DHAQ).16 From Table 1 it is seen that the pK,, and pK,, values for the present systems are much lower than those of 14DHAQ, lSDHAQ and 18DHAQ, as the last three quin- ones have strong intramolecularly hydrogen-bonded struc- tures, resulting in deprotonation at higher pH.Considering the position and intensity of the absorption peaks of all the hydroxyquinones listed in Table 1, it is seen that for 2HAQ and 26DHAQ the charge-transfer (CT) of the longest-wavelength absorption band is much weaker (lower absorption maxima and low molar absorption coefficient for the CT band) than with Table 1 Absorption characteristics of 2HAQ, 26DHAQ and other related quinones in aqueous 5 mol dm-3 isopropyl alcohol-1 mol dm -acetone A/nm (&/lo2m2mol-') quinone neutral monoanion dianion pK,, pK,, ' AQb2HAQ 26DHAQ 330 (5.01) 375 (1.77) 332 (2.50) 350 (8.45) 400 (1.80) -481 (2.60) 348 (4.12) 333 (18.7) 422 (8.90) 480 (2.78) --344 (27.0) 420 (15.4) 500 (1.56) -7.4 7.1 -8.7 - 14DHAQ' 15DHAQd 18DHAQd 465 (8.32) 477 (8.43) 420 (9.60) 430 (12.2) 547 (9.74) 581 (8.63) 480 (12.2) 495 (11.0) 562 (10.4) 600 (10.8) 475 (13.3) 495 (12.4) 9.9 10.6 9.7 12.7 12.5 12.1 (I Error limits in pK, values are k0.l; ref.34; 'ref. 24; ref. 16. those for 14DHAQ, l5DHAQ and 18DHAQ, of which 14DHAQ has the strongest CT character indicated by the longest-wavelength absorption maximum for the CT band. This is due to the quasi-aromatic structure of 14DHAQ resulting from intramolecular hydrogen bonding.,' Difference (Semiquinone -Quinone) Absorption Spectra and the Acid Dissociation Constants of the Semiquinone Radicals On delivery of an electron pulse to a nitrogen-bubbled solu- tion of a quinone (ca.lop4mol drn-,) in an aqueous-organic mixed solvent, semiquinone radicals are formed as follows : H,O4 H', e,;, OH', other products (1) OH'(H') + CH,CHOHCH, -, H,O(H,) + CH,tOHCH, (11) e,; + CH,COCH, +(CH,COCH,)'-(111) (CH,COCH,)'-+ H20+CH,tOHCH, + OH-(IV) CH,eOHCH, + QH,(QH-or Q2-) + QH;(QH;-or QH*'-) + CH,COCH, (V) In N,O-saturated aqueous solution of quinones mol drn-,) containing lo-' mol dmP3 sodium formate, the semi- quinone radicals are formed as follows : Hz eai + N,O -N, + OH' + OH-(VI)OH'(H') + HCO, ___* H20 + C0;-(VW C0;-+ QH-(Q2-) -QH;-(QHo2-) + CO, (VIII) In reactions (V) and (VIII) the product semiquinone radicals can be either of neutral (QH;), monoanionic (QHi-) or dia- nionic (QHo2-) form depending on the pH of the solutions.In pure isopropyl alcohol some solvated electrons are gen- erated, G(es-) = 100 nmol J-l, which may reduce the quin- ones .directly.? The main reducing species, however, are the CH,COHCH, radicals [reaction (IX)] formed either from the higher excited states, or, by a neutralisation reaction involving es- to give H' followed by reaction (11)and also by ion-molecule reactions. CH,eOHCH, + QH, +QH; + CH,COCH, (IX) The semiquinone-quinone difference absorption spectra obtained in aqueous-organic mixed solvent [after completion of reaction (V)] for 2HAQ and 26DHAQ at different pH are shown in Fig. 1 and 2, respectively. A careful investigation of these spectra clearly suggests that for both the systems the semiquinone exists in three different forms of protonation at pH 1.5, 7 and 13.The pK, values associated with the different forms of the semiquinones of 2HAQ and 26DHAQ have been estimated following the changes in AA with pH at suitable wavelengths where there is no ground-state absorption, and fitting the experimental points according to eqn. (1). t The G values for the free radicals is given by the number of molecules formed per 100 eV of energy absorbed, or, in SI units, by the number of mol formed upon absorption of 1 J of energy per kg. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 32 , 2 0 300 400 500 600 700 800 A/nm Fig. 1 Semiquinone -quinone difference absorption spectra of the reduced semiquinone radical of 2HAQ in aqueous-organic mixed solvent at pH 1.5 (O),7.0 (*) and 13 (0).Dose, ca.8.7 Gy. where AA is the observed absorbance at any pH and A,, A, and A, are the absorbances if the semiquinones exist exclu- sively in forms 1, 2 and 3, respectively. pK,, and pK,, are the first and second acid dissociation constants of the radicals, respectively. Fig. 3 shows the AA us. pH plot for the semi- quinones of 2HAQ (at 615 nm) and 26DHAQ (at 600 nm) along with the respective computer-fitted curves. The pK,, and pK,, values thus obtained for the semiquinones of 2HAQ and 26DHAQ are listed in Table 2. From comparing the semiquinone-quinone difference absorption spectra in aqueous-organic mixed solvent at pH 1.5 with those in pure isopropyl alcohol (Fig.4) it is suggested that the neutral semi- quinone radical (QH;) exists in the acidic pH region (pH z 1.5) for both of the q~inones.'~~'~ The pK,, and pK,, values, as listed in Table 2, are therefore associated with the following equilibria for the semiquinones of both 2HAQ and 26DHAQ. pK.1 QH; QH;-+ H+ (X) PK~Z QH;-QHo2-+ Hf (XI)Note from Table 2 that the pK,, values for the semiquinones of 2HAQ and 26DHAQ are a little higher than those of the semiquinones of 14DHAQ, l5DHAQ and 18DHAQ. Because 50 R 30 -nv) 44.-C 3 uj 10 -% WI 2 -10 -Y 2 -30 -300 400 500 600 700 800 A/n m Fig. 2 Semiquinone -quinone difference absorption spectra of the reduced semiquinone radical of 26DHAQ in aqueous-organic mixed solvent at pH 1.5 (O),7.0 (*) and 13 (0).Dose, ca.8.7 Gy. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I 11 h .-u)4-C ujam8 m 0 FY I2 I I I I 2 5 8 11 14 PH Fig. 3 Variation of AA with pH for the semiquinone radicals of 2HAQ (0)at 615 nm and 26DHAQ (*) at 600nm. Dose, ca. 16 Gy. of the intramolecular hydrogen bonding, the semiquinone monoanions of 14DHAQ, 15DHAQ and 18DHAQ become more stable than the corresponding monoanions of the semi- quinones of 2HAQ and 26DHAQ. This stability effect causes Table 2 Absorption characteristics and pK, values of the semi- quinone radicals of 2HAQ, 26DHAQ and other related quinones in aqueous 5 rnol dmP3 isopropyl alcohol-1 rnol dmP3 acetone quinone neutral monoanion dianion pK,, pK,, a AQb 389 (8.9) 385 (6.7) 4.4 -490 (5.2) 2HAQ 380 (6.6) 400 (6.6) 405 (6.5) 4.7 10.7 450 (4.4) 460 (5.1) 26DHAQ 390 (1 1.0) 410 (9.0) 420 (14.7) 5.4 8.7 450 (4.5) 500 (5.7) 14DHAQ' 410 (1 1.6) 388 (5.8) 3.3 >14 475 (13.7) 15DHAQd 410 (12.4) 390 (8.0) 3.65 >14 440 (13.0) 18DHAQdse 400 (15.8) 380 (8.5) 3.95 >14 450 (14.7) a Error limits in the pK, values are kO.1; ref.34; ref. 24; ref. 16; in ref. 16 the A,,, and E values were wrongly quoted in Table 2 for 18DHAQ semiquinone. I + Ii 300 500 700 A/n rn Fig. 4 Semiquinone -quinone difference absorption spectra of the reduced semiquinone radicals of 2HAQ (0)and 26DHAQ (0)in pure isopropyl alcohol.Dose, ca. 10Gy. differences in the pK,, values for the two sets of semi- quinones. The lower pK,, values (Table 2) for the semiquinones of 2HAQ and 26DHAQ are also due to the absence of intra- molecular hydrogen bonding. The monoanionic forms of the semiquinones of 14DHAQ, l5DHAQ and 18DHAQ have stronger intramolecularly hydrogen-bonded structures than the corresponding neutral forms. This causes the second stage of deprotonation [reaction (XI)] to be very difficult for the semiquinones of 14DHAQ, l5DHAQ and 18DHAQ, making the pK,, values of these semiquinones exceptionally high compared with those of the semiquinones of 2HAQ and 26DHAQ, as represented in Scheme 1. Note that for hydroxy-substituted anthrasemiquinones, having no intramolecular hydrogen bonding, the pK,, values are a little higher than that of unsubstituted anthra-semiquinone (Table 2).Apparently, the electrondonating substituent (hydroxy group) increases the effective electron density of the semiquinonoid moiety, making the semi-quinonoid hydroxy group less acidic compared with that in unsubstituted anthrasemiquinone. This hypothesis is further supported by the observation that substitution of the hydroxy group by an amino group, having a stronger electrondonating effect, causes an increase in the pK,, value of the semiq~inones.'~ In the case of the semiquinones of 14DHAQ, 15DHAQ and MDHAQ, the effect of intramolecu-lar hydrogen bonding predominates over the electron-donating effect of the substituents, resulting in a lower pK,, than the pK, of the unsubstituted anthrasemiquinone.The semiquinone -quinone difference absorption spectra for both 2HAQ and 26DHAQ at pH x 13 were also obtained in N,O-saturated aqueous solutions of the quinones containing lo-' mol dm- formate. The absorption spectra (not shown in the figures) were qualitatively similar to the corresponding spectra in aqueous-organic mixed solvent at pH x 13. Therefore, the same radical species are formed by the reaction of either CH,COHCH,, e,; or C0;- with the quinones. Corrected Absorption Spectra of the Semiquinone Radicals The corrected absorption spectra of the different ionic forms of the semiquinones were obtained by correcting the corre- sponding difference (semiquinone -quinone) absorption spectra.' 6*1 Different GR values were chosen for different experimental conditions. Thus for mixed aqueous-organic ~olvent,'~.'~,~~G, is taken to be 6.2 molecules (100 ev)-' (or 6.42 x mol J-'); for pure isopropyl alcoh01,~~~"~~~ GR is taken to be 5.1 molecules (100 eV)-' (or 5.28 x mol J-'); and for aqueous formate,'5-'6*24*36~37 GR is taken to be 6.5 molecules (100 eV)-' (or 6.73 x lop7mol J-').The corrected absorption spectra for the neutral (pH 1.5), monoanionic (pH 7) and dianionic (pH x 13) forms of the semiquinones of 2HAQ and 26HAQ in aqueous-organic mixed solvent are shown in Fig. 5 and 6, respectively. The corrected spectra obtained in pure isopropyl alcohol and in aqueous formate solution in pH x 13 (not shown in the figure) are similar to those obtained in mixed aqueous- organic solvent at pH 1.5 and 13, respectively, for both of the quinones.The spectroscopic parameters of the semiquinones of 2HAQ and 26DHAQ are listed in Table 2 along with those of the semiquinones of AQ, 14DHAQ, l5DHAQ and 18DHAQ for comparison. From the table it is seen that the neutral forms of the semiquinones of both AQ and its hydroxy derivatives absorb at the same wavelength region, i.e. ca. 375-410 nm. Therefore, hydroxy substitution has very little effect on the position of the absorption peak for the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2HAQ 0' 0' 0'm0-OH 0-0-neutral monoanion dianion 26DHAQ 0' 0' ?* OH 0-0-neutral monoanion dianion 14DHAQ 0 .O..H.0 -H.o..H'o O.."'O neutral monoanion dianion Scheme 1 neutral semiquinone radicals except that it causes a large solvent or in pure isopropyl alcohol were used to estimate the increment in the molar absorption coefficients. The effect is reaction rates of CH,eOHCH, radicals with the quinones. considerably high for semiquinone radicals where intramole- The rate constants were obtained by monitoring the growth cular hydrogen bonding is possible. The absorption charac- of the semiquinone absorbance at wavelengths >600 nm teristics of the monoanionic form of the semiquinone radicals where the ground-state quinones do not have any absorption. of hydroxy anthraquinones are also qualitatively similar to In isopropyl alcohol the reaction of es- with the quinones is those of the unsubstituted anthrasemiquinone monoanion, very fast and does not interfere with the reaction of except that the absorption peaks are much stronger for the CH,eOHCH, with the quinones.Rate constants for the former systems, especially when the derivatives have intra- reaction of C0;-with 2HAQ and 26DHAQ were obtained molecularly hydrogen-bonded structures. A quantitative by monitoring the growth of the semiquinone absorbances at correlation between the strength of hydrogen bonding and >600 nm, using N,O-saturated aqueous solutions of the the corresponding change in the absorption characteristics of quinones (5 x to 4 x mol dm-3) containing lo-' the anthrasemiquinones is beyond the scope of this report.mol dm-3 sodium formate. The rate constants for the reac- tions between ea; and the quinones were obtained by observ- Kinetic Parameters of tbe Semiquioooe Radicals ing the rate of disappearance of absorbance for ea; at 700 nm in aqueous solutions containing different concentrations Nitrogen-bubbled solutions of the quinones (5 x to 4 x mol dm-3) either in aqueous-organic mixed 16 IIf n z 3 2i:_0 300 400 500 600 700 800 300 400 500 600 700 800 A/nm Ifnm Fig. 5 Absorption spectra of the reduced semiquinone radicals of Fig. 6 Absorption spectra of the reduced semiquinone radicals of 2HAQ in aqueous-organic mixed solvent, obtained from the data 26DHAQ in aqueous-organic mixed solvent, obtained from the data given in Fig.1 after correction for the parent depletion; pH 1.5 (O), given in Fig. 2 after correction for the parent depletion; pH 1.5 (O), 7.0 (*) and 13 (0) 7.0 (*) and 13 (0) Pure isopropyl alcohol; ,, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Kinetic parameters of the semiquinone radicals of 2HAQ and 26DHAQ in aqueous 5 mol dm-, isopropyl alcohol-1 mol dm-' acetone rate constant reaction PH /dm3 rno1-ls-l ~~~ 2HAQ + CH,tOHCH, 26DHAQ + CH,COHCH, 2HAQ + CH,tOHCH, a a --1.5 7 (1.3 f0.1) x 109 (0.7 f 0.1) x 109 (1.3 f0.1) x 109 (1.3 rt 0.1) x 109 26DHAQ + CH,tOHCH, 2HAQ + ea; 26DHAQ + ea; 2HAQ + C0;-26DHAQ + C0;-2 (2HAQ semiquinone) 2 (26DHAQ semiquinone) 13 1.5 7 13 llb 1lb llb 1lb 1.5 1.5 (0.9 f0.1) x 109 (2.3 rt 0.1) x 109 (1.2 f0.1) x 109 (0.7 f0.1) x 109 (1.0 0.1) x 109 (1.0 f0.1) x 109 (0.4 f0.2) x 109 (0.8 f0.1) x 10" (1.4 f0.1) x 10" (0.5 f0.3) x lo9 aqueous lo-' rnol dm-3 sodium formate solution.0.05 h v).z 0.03 3 ujn v a O,O1 (5 x to 4 x mol dm-3) of the quinones and 5 x lo-' mol dm-3 tert-butyl alcohol at appropriate pH. Corrections were applied for the much slower decay of eag absorbance in solutions containing no quinones. The second- order rate constants (Table 3) were obtained for all the above reactions from the linear dependence of the pseudo-first-order rate constants with quinone concentration. The semiquinone absorbances are seen to decay in the acidic pH region (pH z 1.5) following bimolecular second- order kinetics, obviously due to the disproportionation of the semiquinone into the parent quinone and the corresponding hydroquinone [reaction (XII)], as is observed for all other anthrasemiquinone radicals.7*24 2(semiquinone) +quinone + hydroquinone (XII) The second-order rate constants for the semiquinone decay are listed in Table 3. At higher pH (~6) the decay of the semiquinones is too slow to extract meaningful kinetic parameters. The semiquinone to quinone and hydroquinone equilibrium, as observed for other substituted anthra-semiquinones,' 5-' 7,24 was not observed for the present systems over the entire pH range (1-14) covered in this study. This could be because either the attainment of the equili- bration is very slow for the present systems or there is no equilibration at all.One-electron Reduction Potential (E') The one-electron reduction potentials (El) of 2HAQ and 26DHAQ were measured in water-isopropyl alcohol-acetone mixed solvent at pH 7 and 11, following the electron-transfer equilibria between the semiquinone and a suitable redox ref- erence,15-1 7.2 2-24 1,l'-dimethyl-4,4'-bipyridylium dichloride -0.01 i(MBP2+; E' = -330 mV3' us. NHE at 25°C). We have -0.01 0 100 200 300 400 500 tirnejps Fig. 7 Oscilloscopic traces showing attainment of electron-transfer equilibrium in aqueous-organic mixed solvent at pH 11. Wavelength, 610 nm; dose, ca. 6.3 Gy. (a) [MBP2+] = mol drn-,. A (b) [2HAQ] = 1.9 x lop4 mol dm-,. B (b) [26DHAQ] = 1.4 x mol dm-,.A (c) [2HAQ] = 1.56 x mol dm-, and [MBP2+] = 7.4 x mol drn-,. B (c) [26DHAQ] = 1.22 x lo4 mol dm-, and [MBP2+] = 5.5 x mol dm-,. Table 4 One-electron reduction potentials of 2HAQ and 26DHAQ in aqueous 5 mol dm-, isopropyl alcohol-1 mol dm-, acetone E l/mVa quinone PH 7 pH 11 AQb -445 -445 2HAQ -440 -440 26DHAQ -400 -400 14DHAQ -249 -304 1 5DHAQd -306 -340 1 8DHAQd -325 -405 a f20 mV; ref. 34; ref. 24; ref. 16. recently e~tablished~~ that, owing to a change in ion solva- tion, the E' value of the MBP2+/MBP'+ couple changes from -450 mV vs. NHE in aqueous formate solution39 to -330 mV us. NHE in aqueous 5 mol dmP3 isopropyl alcohol-1 mol dm-3 acetone. Most of the measurements for redox calculations were made at 610 nm (the weaker absorp- tion peak of the MBP*+ radical38) where the interference from the parent quinone and semiquinone absorption is nil and minimal, respectively.Owing to the very high molar absorption coefficient of MBP'+ at 395 nm,38*39 the redox measurements were also made at this wavelength, in addition to the 610 nm measurements. Single pulses of low dose (ca. 6 Gy) were given to N2-bubbled solutions of the quinones (1 x to 2 x mol dmd3) and MBP2+ redox reference (1 x lop5 to 2 x lop4 mol dmp3) for all redox studies. Within a few hundred ms, the parents and radicals came into an equilibrium, semiquinone + MBP2+squinone + MBP" (XIII) Fig. 7 shows some typical redox equilibration traces for 2HAQ-MBP2+ and 26DHAQ-MBP2+ pairs at pH 11.0.The E values in mV us. NHE at 25 "Cwere calculated using eqn. (3) E'(quinone/semiquinone) = E~(MBP~+/MBP*+)-59 log K (3) where K is the equilibrium constant for reaction (XIII) and is expressed as [quinone],, [MBP' +Ieq (4)K= [semiquinone],,[MBP2 + leq The El values thus obtained for 2HAQ and 26DHAQ at pH 7 and 11 are listed in Table 4 along with the E' values of 716 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 unsubstituted anthraquinone and other anthraquinone deriv- atives for comparison. The latter values have been cor-re~ted,~~wherever necessary, for E'(MBP2+/MBP'+) = -330 mV us. NHE at 25 "C. It is seen from Table 4 that the E' values of 2HAQ and 26DHAQ are similar to that of the 5 6 7 8 A. J. Swallow, A.B. Ross and W. P. Helman, Radiat. Phys. Chem., 1981,17, 127. R. L. Willson, Chem. Commun., 1971,1249. K. B. Pate1 and R. L. Willson, J. Chem. SOC., Faraday Trans. I, 1973,69, 814. P. S. Rao and E. Hayon, J. Phys. Chem., 1973,77,2274. unsubstituted anthraquinone, but are more negative than those of the other hydroxy anthraquinones for which intra- molecular hydrogen bonding is possible. Thus the semi- quinones of AQ, 2HAQ and 26DHAQ are stronger reducing agents than the other intramolecularly hydrogen-bonded hydroxy-anthrasemiquinones.This observation is in accord- 9 10 11 12 E. Hayon, T. Ibata, N. N. Lichtin and M. Simic, J. Phys. Chem., 1972,76,2072. D. Meisel and P. Neta, J. Am. Chem. SOC., 1975,97,5198. P. Wardman and E. D. Clarke, J. Chem. SOC., Faraday Trans., 1976,72, 1377.K. P. Clark and H. I. J. Stonehill, J. Chem. SOC., Faraday Trans., 1977,73, 722. ance with expectation, because the intramolecular hydrogen bonding will introduce a +6 charge on the carbonyl oxygen, making the quinonoid moiety more electron afinic than that of simple AQ and its derivatives where there is no intramole- cular hydrogen bonding. 13 14 15 B. E. Hulme, E. J. Land and G. 0. Phillips, Chem. Commun., 1969,518. B. E. Hulme, E. J. Land and G. 0. Phillips, J. Chem. SOC., Faraday Trans., 1972,68,1992. H. Pal, D. K. Palit, T. Mukherjee and J. P. Mittal, Radiat. Phys. Chem., 1991,37,227. 16 H. Pal, D. K. Palit, T. Mukherjee and J. P. Mittal, J. Chem. SOC., Conclusion 17 Faraday Trans., 1991,87,1109. H. Pal, D. K. Palit, T. Mukherjee and J.P. Mittal, Radiat. Phys. The characteristics of the semiquinone radicals formed by one-electron reduction of 2HAQ and 26DHAQ have been found to be quite dissimilar to those of 14DHAQ, l5DHAQ and 18DHAQ. The dissimilarities have been interpreted in terms of the presence and absence of intramolecular hydro- 18 19 20 Chem., 1992,40,529. E. McAlpine, R. S. Sinclair, T. G. Truscott and E. J. Land, J. Chem. SOC., Faraday Trans. I, 1978,74,597. E. J. Land, E. McAlpine, R. S. Sinclair and T. G. Truscott, J. Chem. SOC., Faraday Trans. I, 1976,72,2091. E. J. Land, T. Mukherjee, A. J. Swallow and J. M. Bruce, J. gen bonding in these systems. Where the semiquinones of 14DHAQ, 1 5DHAQ and 18DHAQ disproportionate quickly, establishing an equilibrium among the semiquinone, the parent quinone and the hydroquinone over a pH range of ca.6 to 12, the semiquinones of 2HAQ and 26DHAQ do not show the attainment of such equilibrium, at least within the longest time period of our observation (5 ms). Regarding the redox characteristics, the semiquinones of 2HAQ and 26DHAQ are very similar to that of AQ and are stronger reducing agents than the other hydroxy-substituted anthra- semiquinones where intramolecular hydrogen bonds are 21 22 23 24 25 26 Chem. SOC., Faraday Trans. 1, 1983,79,391. N. J. F. Dodd and T. Mukherjee, Biochem. Pharmacol., 1984,33, 379. T. Mukherjee, B. Cercek, N. J. F. Dodd and A. J. Swallow, Radiat. Phys. Chem., 1987,30,271. T. Mukherjee, E. J. Land, A. J. Swallow, P. M. Guyan and J. M. Bruce, J. Chem.SOC., Faraday Trans. I, 1988,842855. T. Mukherjee, A. J. Swallow, P. M. Guyan and J. M. Bruce, J. Chem. SOC., Faraday Trans., 1990,86,1483. D. K. Palit, H. Pal, T. Mukherjee and J. P. Mittal, J. Photochem. Photobiol., A: Chem., 1990,52,375. D. K. Palit, H. Pal, T. Mukherjee and J. P. Mittal, J. Chem. SOC., Faraday Trans., 1990,86,3861. present. 27 H. Pal, D. K. Palit, T. Mukherjee and J. P. Mittal, J. Photochem. The acid dissociation constants of the semiquinones of 2HAQ and 26DHAQ are very different from those of the semiquinones of other hydroxyanthraquinones having intra- molecular hydrogen bonding. It is also seen that for the present hydroxyanthrasemiquinones, where no intramolecu- lar hydrogen bonding is possible, the molar absorption coeffi- 28 29 30 31 Photobiol., A: Chem., 1991,62, 183.S. R. Flom and P. F. Barbara, J. Phys. Chem., 1985,89,4489. G. Smulevich, J. Chem. Phys., 1985,83, 14. C. C. Westcott, pH Measurements, Academic Press, New York, 1978. R. G. Bates, M. Paabo and R. A. Robinson, J. Phys. Chem., 1963,67, 1833. cients for their absorption peaks are much lower than those for hydroxyanthrasemiquinones with intramolecularly hydrogen-bonded structures. 32 33 J. W. T. Spinks and R. J. Woods, An Introduction to Radiation Chemistry, Wiley, New York, 2nd edn., 1976. G. E. Adams, J. W. Boag, J. Currant and B. D. Michael, in Pulse Radiolysis, ed. M. Ebert, J. P. Keene, A. J. Swallow and J. H. We are grateful to Mr. T. N. Das of Chemistry Division, Bhabha Atomic Research Centre, for his kind help in the computational fitting of the AA us. pH curves. References 34 35 36 Baxendale, Academic Press, London, 1965, p. 117. H. Pal, T. Mukherjee and J. P. Mittal, Radiat. Phys. Chern., in the press. E. A. Gastilovich, L. V. Golitsina, G. T. Kryuchkova and D. N. Shigorin, Opt. Spektrosk., 1976,40,45. R. H. Schuler, L. K. Patterson and E. Janata, J. Phys. Chem., 1980,84,2088. 1 2 3 P. Neta, in The Chemistry of Quinonoid Compounds, ed. S. Patai and Z. Rappoport, Wiley, New York, 1991, vol. 11, p. 879. A. J. Swallow,Prog. React, Kinet., 1978,9, 238. A. J. Swallow, in The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis, ed. J. H. Baxendale and F. 37 38 39 R. H. Schuler, A. Hartzell and B. Behar, J. Phys. Chem., 1981, 85, 192. H. Pal and T. Mukherjee, J. Indian Chem. SOC., in the press. J. A. Farrington, M. Ebert and E. J. Land, J. Chem. SOC., Faraday Trans. I, 1978,74,665. Busi, Reidel, Dordrecht, Holland, 1982, p. 289. 4 A. J. Swallow, in Functions of Quinones in Energy Conserving Systems, ed. B. L. Trumpower, Academic Press, London, 1982. Paper 3/04979E; Received 17th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000711

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 717-720 NMR Study of 8-Caprolactam in various Solvents Graphical Determination of Monomer Shift, Dimer Shift and Dimerization Constant from the Dilution Shift Data Jenn-Shing Chen Department of Applied Chemistry, National Chiao Tung University, Hsin Chu, Taiwan, 30050, Republic of China A graphical method for the simultaneous determination of the monomer shift, dimer shift and dimerization constant of a self-association system from the dilution NMR shift data is proposed. This method avoids the conventional extrapolation to infinite dilution to obtain the monomer shift. Three self-association systems: E-caprolactam in [*H,]chloroform, [2H,]acetone and [2H,]acetonitrile were studied at various temperatures. The enthalpy and entropy of dimerization were also determined from van't Hoff plots.The self-association of two monomers to give a dimer is the first step toward nucleation and colloidal flocculation.'*2 It is also one of the factors that cause the thermodynamic proper- ties of solution to deviate from ideal it^.^,^ A variety of theo- retical and experimental methods has been employed to investigate the equilibium and kinetic behaviour related to this phen~menon.~-~ The advent of NMR spectroscopy has emerged as one of the most effective methods for those studies.'w8 Conventionally, in an NMR experiment, an estimated value of the monomer shift, Av, ,is first obtained by extrapo- lating the dilution shift data to infinite dilution. A standard procedure is then followed to obtain a dimerization constant, K,and a dimer shift, Avd ,by fitting an equation containing K and Avd as parameters.Unfortunately, the behaviour of the shift us. concentration in the diluted region is inherently non- linear, and the curve is so steep (especially in the case of strong association) that a precise determination of Av, is impossible. As a rule, such a determination is very sensitive to the assessed value of Avm, i.e. is prone to large errors in the determination of K and Av, because of an inaccurate esti- mation of Av,. In order to have a more accurate determination of Avm, Av, and K, we propose a novel graphical method in which the conventional extrapolation is avoided. This graphical method is based on two physically equivalent, albeit different in form, equations governing the dependence of the observed chemical shift, Avobs, on the initial concentration of the solute,.CAIo, as a function of the parameters Avm, Av, and K.An accurate determination of K at various temperatures is considered to be a crucial step toward the accurate determi- nation of the standard enthalpy and entropy change for self- association by using a van't Hoff plot. Thus our proposed graphical method would provide an additional powerful tool for a reliable thermodynamic investigation of self-association. Tbeory In a monomer-dimer equilibrium system, at a temperature where there is a rapid interconversion A + AeA, (1) the observed chemical shift, Avobs, is the weighted average of the shifts of the monomer and dimer and was given by Gutowsky and Saikag as where CAIo is the initial concentration of solute and [A] and [Az] are the concentrations of monomer and dimer at equi- librium.Av, and Avd are the monomer shift and dimer shift, respectively. f, and fd, the mole fractions of monomer and dimer, stand for their respective statistical weights. With the relationf, +fd = 1, one obtains (3) Furthermore, the dimerization constant K can be expressed in terms of mole fractions as (4) Substitution of eqn. (2) and (3) into eqn. (4) followed by taking the square root gives, after rearrangement For a simpler notation we denote the last square root term by X: Thus, eqn.(5) explicitly expresses the observed chemical shifts as a linear function of X.In order to use eqn. (5), Av, should be known in advance in order to calculate X.A plot of Avobs us. X then gives Avd and K from the intercept and slope. Replacing 1 -fd by f, in eqn. (4), and solving for fd gives (1+ 8K[A]o)'/z -1 fd = (1 + 8K[A]o)'/z + 1 (7) The dependence of AvObson CAIocan then be written: AVobs = AV, +fd(Avd -AV,,,) = Av, + (Av~-Av,) (1+ 8K[A]o)1/2 -1 (8)(1 + 8K[A]o)'/2 + 1 Eqn. (8) explicitly expresses the dependence of Avobs onfd in a linear form. With the knowledge of K to calculate fd, one may obtain AV, and Avd from the intercept of the plot of Avobsm.fd atfd = 0 andf, = 1, respectively. Eqn. (5) and (8), albeit in different forms, carry the same physical meaning.Both equations, however, are our pivotal results and are the basis of the following proposed graphical method. Proposed Graphical Method Purcell et a!. have studied a similar self-association system of 6-valerolactam in CDCl, .lo They applied the direct-search procedure" in which an initial monomer shift is assumed, and then the dimer shift and dimerization constant are varied independently until the standard deviation for non-linear fitting (the shift us. concentration data) reach a minimum. They concluded that the regressed dimer shift is fairly inde- pendent of the initial trial monomer shift, within the range of the latter. However, the regressed dimerization constant depends strongly on the initial trial monomer shift.The uncertainty in final values of monomer shift and dimerization constant is large because a wide range of pairs of (monomer shift, dimerization constant) values are found to have nearly equal least-square fitting errors. Such a formidable short- coming prompts us to propose an alternative algorithm to treat the dilution shift data associated with the monomer- dimer equilibrium. In an earlier investigation,12 we employed an iteration scheme to determine Av,, Av, and K simultaneously from the dilution shift data. This scheme starts with a guessed AV, to calculate X = {(Avobs-AV,)/[A]~)'/' in eqn. (5). Data of Avobsus. X are then regressed in terms of a quadratic (instead of a linear) polynomial to calculate a tentative value of Av, from the intercept and a tentative value of K from the fore- going Av, and limiting slope (slope at X = 0) of the regressed curve.The K thus obtained is then inserted into eqn. (7) to calculate fd which is used for regressing data of Avobs us. fd based on eqn. (8) in terms of a quadratic polynomial, to obtain an improved value of Av, from the intercept, and an improved value of Avd from the foregoing AV, and limiting slope. This newly obtained AV, is then inserted in eqn. (5) to initiate the second iteration to obtain further improved values of Av, and K. The iteration procedure is repeated until the difference of two successive monomer shifts is less than a preset tolerance. Based on eqn. (5) and (8), we propose an alternative graphi- cal method.The primary results have been published else- where.', It should be noted in advance that the validity of eqn. (5) and (8) is tacitly based on the following assumptions: only dimerization takes place and the thermodynamic behav- iour of the solution is so ideal over the concentration range studied that only one value of the dimerization constant pre- vails. In this ideal case, if a value of AV, in X of eqn. (6) is correctly guessed, a hear regression on the data of Avobs us. X based on eqn. (5) gives a line of perfect fitting to the experi- mental data i.e. its correlation coefficient is unity. The 'true' values of Av, and K can then be derived from the intercept and slope of this line. If, on the other hand, a quadratic instead of a linear equation is used in the regression, the same perfectly fitted line is restored, i.e.the quadratic term becomes identically zero. If, however, the value of Av, is not guessed correctly, the curves from linear and quadratic regressions are not the same. In particular, the linear part of the regressed quadratic equation is not equal to the regressed linear equation as before. Nevertheless, it still provides a pair of regressed values of Av, and K from the linear part of the quadratic equation. This is tantamount to saying that guess- ing a correct value of Avm results in a correct value of K irrespective of whether a linear or quadratic form is used in the regression. Conversely, an incorrect guessed value of AV, leads to different regressed values of K for linear and quadra- tic regressions.If we plot regressed values of K us. guessed J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 values of AV, generated from linear and quadratic regres- sions, respectively, two distinct curves will be obtained. The intersection P1, having the common values of Av, and K from linear and quadratic regressions, determines the 'true' values of AV, and K for the self-association system based on eqn. (5). Similarly, putting a guessed value of K into thef, of eqn. (7) followed by regressing Avobs us.f, using linear and quadra- tic equations based on eqn. (8) will produce, from the inter- cept, the same value of Av,, if the guessed value is correct, or different values if not.Plots of guessed values of K us. regressed values of Av, for linear and quadratic regressions, generate two distinct curves. Again, the intersection P2 deter- mines the true values of AV, and K based on eqn (8). Ideally, the two intersections, P1 and P2, should coincide, since they represent the same physical entities. The departure, however, may reflect (1) the experimental error of shift and/or solute concentration measurement ;(2) extended association beyond dimerization and (3) the non-ideal thermodynamic behaviour of the solution. The last possibility arises from the fact that the equilibrium constant depends on the solute concentration in a non-ideal thermodynamic solution. This behaviour is inconsistent with the underlying assumption that the equi- librium constant should be a constant value throughout the concentration range studied.In practical applications, this non-coincidence of P1 and P2 would provide a consistency check and a method of estimating errors for this graphical determination. We then give equal weight to both determi- nations from P1 and P2. Accordingly, the average and stan- dard deviation of AV, and/or K from two determinations are taken as the finally determined values and errors of AV, and K. The finally determined value of Av, is then put into X, followed by a linear regression on Avobs us. X based on eqn. (5) to obtain the true value of Av, from the intercept. The finally determined value of K is also put intof, followed by linear regression on the data of Bobs m.fd based on eqn.(8) to obtain the true value of Avd by evaluating the foregoing reg- ressed linear equation at fd = 1. The average and standard deviation of Av, from two determinations are then taken as the finally determined value and error of Av, . Our previous scheme of iteration' 'determines the intersec- tion of the curve plotted for regressed values of K us. guessed values of Av, based on eqn. (9,and that plotted for guessed values of K us. regressed values of Av, based on eqn. (8); both are in terms of quadratic regression. Hence, the iteration scheme takes only the intersection of Q1 and 42 into account. An extensive comparison of fitting the experimental data to the theoretical curves of eqn.(5) and (S), using the parameters Av,, Av, and K obtained from the two methods, demonstrates that the new graphical method is superior to the previous iteration method. Results and Discussion We have carried out NMR spectroscopy on three self-association systems : &-caprolactam in [2H,]chloroform, [2H,]acetone and [2H,]acetonitrile at various temperatures. The 300 MHz proton spectra were taken on a Varian Unity- 300 NMR spectrometer equipped with a variable tem-perature controller, model VTC4, whose precision was calibrated, using ethylene glycol as standard, to be within f1 K. All chemicals were the highest grade from Aldrich and were used without further purification. Samples of different concentration (expressed in units of molality) of solute, with a trace of TMS as a reference, were prepared gravimetrically in small vials with the help of microsyringes.The samples were J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 then transferred into 5 mm od NMR tubes and sealed without further degassing. The peak for monitoring shift change with concentration is from the proton in the amine group of E-caprolactam. The average of readings from five spectra taken repeatedly without removing the tube from the magnet were referred to as the observed value of each shift. The measured NH shift may include errors induced by a trace of water in the sol- vents. The nominal water level in various solvents is within the range 0.01-0.05%. We estimated that there was only a negligible effect on the NH shift measurement, since the sol- vents are overwhelming predominant and, moreover, have a strong capability for hydrogen bonding with water molecules. Fig.1 illustrates the graphical determination of Av, and K for the system E-caprolactam in [2HlJchloroform at 298 K. The curves L1 and Q1 represent the plots of regressed values of K us. guessed values of Av, for linear and quadratic regres- sions based on eqn. (5). The intersection P1 then corresponds to the determined values of Av, and K based on eqn. (5). Likewise, the curves L2 and 42 represent the plots of guessed values of K vs. regressed values of Av, for linear and quadra- tic regressions based on eqn. (8). The intersection P2 then corresponds to the determined values of Avm and K based on eqn.(8).The difference in P1 and P2 is indicative of the errors discussed earlier. From P1 and P2, the values of Avm, K and Avd and their errors can then be determined. The results for the three systems at various temperatures are listed in Table 1. As seen from this table, K for any system decreases with increase in temperature. This indicates that the dimerization is an exothermic process. Also from Table 1, decrease of Av, and Avd with increase in temperature implies that increasing temperature will weaken the molecular association within a dimer or a monomer-solvent complex. This phenomenon has also been observed by other author^.'^.' In the systems studied it is more appropriate to represent the equilibrium (I) as A-solvent + A-solvent == A,.A near- infrared investigation' of similar systems of 8-valerolactam in various solvents with strong hydrogen-bonding capability led to the same conclusion. We also observed that in the system of E-caprolactam in CDCl, the proton shift from a trace of undeuteriated chloroform increases with the concen- tration of the solute. An NMR study of lactam<hloroform complexation is now underway in our laboratory. The 'goodness' of the determination can be appraised by comparing the experimental values of Avobs with its theoreti- cal counterpart calculated from the final determined values of 2.2 I II I I 2.0 .. 1.8 1.4 1.2 ' . '1.0 1800 1820 1840 1860 1880 Av,,,/Hz Fig.1 Graphical determination of monomer shift and dimerization constant for E-caprolactam in [2H,]chloroform at 298 K Table 1 Monomer shift, dimer shift and dimerization constant for E-caprolactam in deuteriated chloroform, acetone and acetonitrile at various temperatures solvent T/K Avfiz Avaz K/kg mol-' [2H,]chlor~form 268 1900.8 f1.3 2745.4 f0.1 2.698 f0.016 283 1846.0 f 1.2 2697.8 f0.2 2.080 f0.012 298 1817.2 f2.8 2644.5 f0.3 1.513 f0.025 313 1797.7 f2.5 2602.8 f0.5 1.158 f0.039 328 1772.8 f2.1 2565.0 0.6 0.905 f0.015 [ZH,]acetone 298 1956.4 f1.8 2520.7 f0.1 0.456 f0.010 305 1942.5 f0.6 2500.2 f0.1 0.416 f0.003 313 1919.7 f0.3 2474.5 f0.2 0.409f 0.001 328 1889.6 f0.6 2430.8 f0.0 0.353 f 0.031 ['HJacetonitrile 298 1858.2 f1.8 2600.0 f2.9 0.143 f0.005 318 1829.9 f2.3 2526.1 f6.1 0.124 f0.007 338 1803.0 & 1.5 2483.9 f8.9 0.097 & 0.004 348 1795.8 f1.4 2466.7 f8.2 0.086 f0.001 Av,, Avd and K by the different methods.Fig. 2 shows the plots of Avobs vs. X for E-caprolactam in [2H,]chloroform at various temperatures. Similar plots for Avobs vs.fd, and AV,bs DS. initial concentration of solute for the same system at various temperatures are shown in Fig. 3 and 4. With dimerization constants known at different tem-peratures, a van't Hoff plot of log K us. 1/T can then be employed to determine the standard enthalpy, AH', and stan- dard entropy, ASo, of dimerization. Such plots are shown in Fig. 5 for the three systems.The determined values and errors of AHo and ASo for each system are listed in Table 2. The errors are calculated by a standard procedure described else- where." A dimerization process which involves attracting two separate entities together usually reduces the intermolec- ular potential energy (i.e.it is exothermic) and the randomness 28003000 r----l 1400 10 20 30 40 50 60 X/H 2' /2kg'l2 mol-'l2 Fig. 2 Comparison of theoretical curves with experimental data for AvObsus. X for 6-caprolactam in [2H,]chloroform at (a) 268, (b)283, (c) 298, (d) 313 and (e) 328 K Table 2 Standard enthalpy and entropy of dimerization for E-caprolactam in various solvents solvent AHo/kJ mol-' ASo/J mol-' K-I [.'H ,]chloroform -13.52 f0.38 -41.91 f1.34 [2H,]acetone -6.57 f0.92 -28.68 & 3.01 [ZH,]acetonitrile -8.83 +_ 1.05 -45.64 & 3.18 720 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2800 I 1 permittivity, E, ranging from 2.25 to 9.13. They obtained at 298 K in chloroform a value for the dimerization constant, K, of 1.55 dm3 mol- 'or 1.05 kg mol-'. This is in fair agreement 2600 with our result of 1.51 kg mol-'. They also found that within the range of relative permittivity studied K can be expressed as a linear function of 1/~.While we have also observed the 2400 tendency of K to increase with l/r, a good linear relation is I> not obtained. This may be explained by the fact that, in addi- tion to providing a dielectric environment for the dimer- g 2200 ization process, the solvents may have some specific interaction with the monomer of the solute.2000 1800 Conclusion ~ 0.2 0.4 0.6 0.8 1 .o We have proposed an effective graphical method for a simul- f* taneous determination of the monomer shift, dimer shift and Fig. 3 Comparison of theoretical curves with experimental data for dimerization constant from the dilution shift data of self- AvObs us. fd for 6-caprolactam in [*H,]chloroform at temperatures associating systems. The conventional extrapolation to infin- (a)-(e)as in Fig. 2 ite dilution for an estimation of the monomer shift is deliberately avoided. This new method, in addition to a con- of the system. In this regard, the negative values of AHo and sistency check, provides a means of estimating errors.ASo seem to be physically justified.Because it can offer a more accurate determination of equi- Franzen and Stephens' * used dielectric polarization to librium constant and spectrometric parameters for self-study the dimerization of E-caprolactam in solvents of relative association, we believe this graphical method will lend itself 2800 1to reliable thermodynamic investigations of such a system. Financial support from the National Science Council of 2600 Taiwan under the grant number NSC 83-0208-M-009-036, and the starting fund for new faculties administered by 2400 National Chiao-Tung University, Hsin-Chu, Taiwan, is grate- fully acknowledged. NI3 2200 60 2000 References i 1 J. Frankel, Kinetic Theory of Liquids, Oxford University Press, Oxford, 1946.2 Aggregation Processes in Solutions, ed. E. Wyn-Jones and J. 1800 Gormally, Elsevier, Amsterdam, 1983. I1600 1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 3 P. Hobza and R. Zahradnik, Weak Intermolecular Interactions in Chemistry and Biology, Elsevier, Amsterdam, 1978. [A],/mol kg-' 4 J. M. Praunitz, R. N. Lichtenthaler and E. G. de Azevedo, Fig. 4 Comparison of theoretical curves with experimental data for Molecular Thermodynamics of Fluid Phase Equilibrium, Prentice-AV,~ us. initial concentration fd .for c-caprolactam in Hall, Englewood Cliffs, New Jersey, 2nd edn., 1986. [ZHl]chloroform at temperatures (a)-(e)as in Fig. 2 5 J. C. Dore and J. Teixeira, Hydrogen Bonded Liquids, Kluwer, Dordrecht, 1989. 6 P. Hobza and R. Zahradnik, Intermolecular Complexes, Elsevier, Amsterdam, 1988. 7 J. S. Davies and K. K. Deb, Adv. Magn. Reson., 1970,4,201. 8 Molecular Association, ed. R. Foster, Academic Press, London, 1975. 9 H. S. Gutowsky and A. Saika, J. Chem. Phys., 1953,21,1688. 10 J. M. Purcell, H. Susi and J. R. Cavanaugh, Can. J. Chem., 1969, 49, 3655. 11 R. Hooke and J. A. Jeeves, J. Assoc. Comput. Much, 1961,8,212. 12 J. S. Chen and R. B. Shirts, J. Phys. Chem., 1985,89, 1643. 13 J. S. Chen and F. Rosenberger, Tetrahedron Lett., 1990,31,3975. 14 N. Muller and R. C. Reiter, J. Chem. Phys., 1965,42, 3265. 15 Soon Ng, Spectrochim. Acta, Part A, 1980,36,927. 16 H. Susi and J. S. Ard, Arch. Biochem. Biophys., 1966,117, 147. 17 R. Bevington, Data Reduction and Error Analysis for Physical Science, McGraw-Hill, New York, 1969, ch. 6.2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 18 J. S. Franzen and R. E. Stephens, Biochemistry, 1963,2, 1321. 103 KIT Fig. 5 Plot of In K us. 1/T for dimerization of e-caprolactam in deuteriated (a)chloroform, (b)acetone and (c)acetonitrile Paper 3/06231G; Received 18th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000717

出版商:RSC    

年代:1994

数据来源: RSC