摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Editorial Manager Prof. A. Robert Hillman Dr. Robert J. Parker Department of Chemistry The Royal Society of Chemistry University of Leicester Thomas Graham House University Road Science Park Leicester LEI 7RH, UK Milton Road Cambridge CB4 4WF, UK Staff Editor: Dr. R. A. Whitelock Senior Assistant Editor: Mrs. S. Shah Assistant Editors: Dr. G. F. McCann, Miss J. C. Thorn Editorial Secretary: Mrs. J. E. Gibbs Faraday Editorial Board Prof. M. N. R. Ashfold (Bristol) (Chairman) Dr. J. A. Beswick (Paris) Prof. A. R. Hillman (Leicester) Dr. D. C. Clary (Cambridge) Prof. J. Holzwarth (Berlin) Dr. L. R. Fisher (Bristol) Dr. D. Langevin (Paris) Dr.B. E. Hayden (Southampton) Dr. S. K. Scott (Leeds) Prof. J. S. Higgins (London) Dr. R. K. Thomas (Oxford) Dr. R. J. Parker (RSC, Cambridge) (Secretary) International Advisory Editorial Board R. S. Berry (Chicago) R. A. Marcus (Pasadena) A. M. 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They are designed to be topical articles of interest to a wide range of research scientists in the areas of Physical Chemistry, Biophysical Chemistry and Chemical Physics. Full details of the form of manuscripts for Articles and Faraday Communications, con- ditions for acceptance etc. are given in issue number one of Faraday Transactions, published in January of each year, or may be obtained from the Editorial Manager. There is no page charge for papers published in Faraday Transactions. Fifty reprints are supplied free of charge. Prof. A. R. Hillman, Scientific Editor. Tel. : Leicester (01 16) 2525226 (24 hours) E-Mail (JANET):ARH7@UK.AC.LElCESTER Fax: (0116) 2525227 Dr. R. J. Parker, Editorial Manager. Tel.: Cambridge (0223) 420066 E- Mail (INTERNET): RSC1 @RSC.ORG (For access from JANET use RSCI %RSC.ORG@UK.AC.NSF N ET- R ELAY) Fax: (0223) 426017

ISSN:0956-5000    

DOI:10.1039/FT99490FX085

出版商:RSC    

年代:1994

数据来源: RSC

ISSN:0956-5000    

DOI:10.1039/FT99490BX087

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(22) 3377-3471 (1 994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 3377 Ionization radii of compressed atoms J. C. A. Baeyens 3383 Conformation properties of buta-1,3-diene-1,4-diones(bisketenes): Computational and photoelectron spectroscopic studies N. H. Werstiuk, J. Ma, M. A. McAllister, T. T. Tidwell and D-c. Zhao 3391 Explosive decomposition of gaseous chlorine dioxide M. I. Lopez,A. E. Croce and J. E. Sicre 3397 Microcalorimetric titration of a-cyclodextrin with some straight-chain a,o-dicarboxylates in aqueous solution at differ- ent temperatures I. Gomez-Orellana, D. Hallen and M. Stodeman 340 1 Conductometric study of association phenomena of some metal(r1) complexes in water at different temperatures H.A. Shehata 3405 Dissolution of synthetic hydroxyapatite in the presence of lanthanum ions Ph. Schaad, Pb. Gramain, F. Gorce and J. C. Voegel 3409 EPR spectroscopic studies of haemoglobin breakdown in malarial parasite-infected erythrocytes R. Cammack, D. S. Patil and D. Linstead 341 1 Electron processes in AOT reverse micelles. Part 1 .-Absorption spectra and lifetimes of hydrated electrons as studied by pulse radiolysis J. L. Gebicki, L. Gebicka and J. Kroh 3415 Electrosteric stabilisation of colloidal zirconia with low-molecular-weight polyacrylic acid. An atomic force micros-copy study S. Biggs and T. W. Healy 3423 Description of cubic liquid-crystalline structures using simple surface foliations A.Fogden and S. Lidin 3433 Determination of orientational order parameters in liquid crystals from temperature-dependent 3C NMR experiments A. Hagemeyer, R. Tarroni and C. Zannoni 3443 Growth and structure of ZnO, overlayers on Cu(100) H. N. Kouroukli and R. M. Nix 3449 l80tracer studies of CO oxidation with 0, on MOO,. Part 3.-Reaction mechanism of oxygen isotope exchange between CO, and MOO, Y. Iizuka, M. Sanada, J. Tsunetoshi, N. Yamauchi and S. Arai 3455 IR spectroscopic study of CD,CN adsorbed on ALPO-18 molecular sieve and the solid acid catalysts SAPO-18and MeAPO-18 J. Chen, J. M. Thomas and G. Sankar 3461 Surface and subsurface acidity of faujasite-type zeolites in relation to their composition: XPS and TPD of ammonia study C. Guimon, A.Zouiten, A. Boreave, G. Pfister-Guillouzo, P. Schulz, F. Fitoussi and C. Quet FARADAY COMMUNICATIONS 3469 Chemistry of NO,+ in association with water clusters A. J. Stace, J. F. Winkel and S. R. Atrill 3471 Corrigendum to Electrostatic acceleration of the 1,5-H shifts in cyclopentadiene and in penta-1’3-diene by Li+ com- plexation :Aromaticity of the transition structures H. Jiao and P. von Rague Schleyer 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/FT99490FP236

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aas,N., 1015 Bauer, C., 517 Brown, N. M. D., 1357 Cheng, A., 253 Denkov, N. D., 2077Abadzhieva, N., 1987 Baur, W. H., 2141 Brown, R. G., 59 Cheng, C. P., 1157 Derrick, P. J., 239Abbott, A. P., 1533 Beagley, B., 2775 Brown, S. E., 739 Cheng, Y., 2517 Dewing, J., 1047Abraham, R. J., 2775 Beer, P. D., 2931 Briickner, A., 3 159 Cherqaoui, D., 97,2015 Dheer, J. M., 3261Abramowicz, T., 24 17 Beeston, M. A., 3109 Bruna, P. J., 683 Chesta, C. A., 69 Diagne, C., 501Acharya, A. N., 3293 Bell, A. J., 17, 817 Bruque, S., 3103 Chevalier, S., 667,675 Dickinson, E., 173,2737Aeby, D., 3129 Belton, P. S., 1099 Brzezinski, B., 843, 1095 Chi, Q., 2057 Diebler, H., 2359Afanasiev, P., 193 Benavente, J., 3 103 Buchachenko, A. A., 3229 Child, M.S., 1739 Dines, T. J., 1461Agren, H., 1479 Bender, B. R., 1449 Buchner, R., 2475 Chiu, S. S-L., 1575 Doblhofer, K., 745Aikawa, M., 911 Bendig, J., 287 Buckley, A. M., 1003 Chmiel, G., 1153 Domen, K., 91 1 Aitken, C. G., 935 Benfer, S., 2969 Buemi, G., 1211 Cho, T., 103 Doney, S. C., 1865Akanuma, K., 1171 Bengtsson, L. A., 559, 2401, Bujan-Nuiiez, M. C., 2737 Choi, W., 3315 Dong, S., 2057Akolekar, D. B., 1041 2531 Bulow, M., 2585 Choisnet, J., 1987 Donnamaria, M. C., 2731Alava, I., 2443 Benko, J., 855 Burdisso, M., 1077 Choudhary, V. R., 3357 Dore, J. C., 2497Albert, I. D. L., 2617 Benniston, A. C., 953,2627 Burgard, C., 3077 Chowdhry, B. Z., 1999 Dory, M., 2319Albery, W. J., 11 15 Beno, B., 1599 Burgess, J., 3071 Christensen, P., 459 Dossi, C., 1335Alcober, C., 2395 Bensalem, A., 653 Burget, D., 2481 Chung, Y-L., 2547 Doughty, A., 541Aldaz,A., 609 Bensch, W., 2791 Busca, G., 1161, 1293,3181, CiZmek, A., 1973 Douglas, C.B., 471Alfimov, M. V., 109 Berbaran-Santos, M. N., 3347 Claridge, J. B., 2799 Downing, J. W., 1653Al-Ghefaili, K. M., 383, 2623 Busch, T., 261 1 Clark, G. R., 3139 Duarte, M. L. T. S., 29531047 Btrces, T., 411,2635 Buschmann, H-J., 1507 Clark, T., 1669, 1678, 1783, Duke, M. M., 2027Ali, V., 579, 583 Bergeret, G., 773 Butler, L. J., 1581, 1612, 1807, 1808,1809,1810 Dunford, H. B., 3201Aliev, A. E., 1323 Berglund, J., 3309 1613,1614, 1671, 1677, Clegg, S. L., 1875 Dunmur, D. A., 1357Allegrini, P., 333 Bernardi, F., 1617, 1669, 1809 Cltment, R., 2001 Dunstan, D. E., 1261Allen, N.S., 83 1671,1672 Butt, M. D., 727 Climent, M. A,, 609 DuplPtre, G., 1501Almond, M. J., 3153 Berndt, H., 2837 Buttar, D., 1811 Coates, J. H., 739 Duxbury, G., 1357,3373Alonso, J. L., 2849 Bertran, J., 1679, 1757, Buxton, G. V., 3309 Coitiiio, E. L., 1745 Dwyer, J., 383, 1047 Alparone, A., 2873 1800,1806 Byatt-Smith, J. G., 493 Collett, J. H., 1961 Dyke, J. M., 17 A1 Rawi, J. M. A., 845 Beutel, T., 1335 Cabaleiro, M. C., 845 Colmenares, C. A., 1285 Dziembaj, R., 2099 Amorim da Costa, A. M., Beyer, H. K., 1329 Caceres, C., 2125,3203 Coluccia, S., 3167 Eastoe, J., 487, 2497, 3121 689 Bhuiyan, L. B., 2002 Chceres, M., 1217 Cook, J., 1999 Easton, C. J., 739Amoskov, V. M., 889 Bickelhaupt, F., 327, 1363 Caceres Alonso, M., 553 Cooney, R. P., 2579 Ebitani, K., 377 Ando, M., 1011 Bickley, R.I., 2257 Cairns, J. A., 1461 Cooper, D. L., 1643 Eder, F., 2977 Andre, J-M.,2319 Biczok, L., 411,2635 Calado, J. C. G., 649 Cordischi, D., 207 Edwards, H. A., 3341 Andreoli, R., 3241 Bielanski, A., 2099 Caldararu, H., 213,2643 Corma,A., 213 Eggen, B. R., 3029 Andris, J., 1703,2365 Biggs, P., 1197, 1205 Callens, F. J., 2541, 2653, Cormier, G., 755 Eggins, B. R., 2249 Andrews, S. J., 1003 Biggs, S., 3415 3261 Corradini, F., 859, 1089 Egsgaard, H., 941Anson, C. E., 1449 Billingham, J., 1953 Calvaruso, G., 2505 Corrales, T., 83 El-Atawy, S., 879 Antonik, T., 1973 Bilmes, S. A., 2395 Calvente, J. J., 575 Corvaja, C., 3267 El Baghdadi, A., 1313 Aragno, A., 787 Binet, C., 1023 Calvo, E., 2395 Cosa, J. J., 69 El-Basil, S., 2201 Arai, S., 1307, 3449 Binks, B.P., 2743 Calvo, E. J., 987 Costas, M., 1513 Elding, L. I., 3309 Aramaki, K., 321 Black, S. N., 1003 Camacho, J. J., 23 Cottier, D., 1003 Elisei, F., 279 Aravindakumar, C. T., 597 Blackett, P. M., 845 Cameron, B. R., 935 Coudurier, G., 193 Elliot, A. J., 831, 837 Arean, C. O., 3367 Blake, J. F., 1727 Caminati, W., 2183 Courcot, D., 895 Endregard, M., 2775 Asai, Y., 797 Blanco, M., 2125,3203 Cammack, R., 2921,3409 Coveney, P. V., 1953 Engberts, J. B. F. N., 727,Ashfold, M. N. R., 1357 Blanco, S., 1365 Campa, M. C., 207 Cox, A. P., 2171 1905,2703,2709Asmus, K-D., 1391 Blandamer, M. J., 727, Campelo, J. M., 2265 Cox, R. A., 1819 Enomoto, N., 1279 Assfield, X., 1743 1905,2703,2709 Campos, A., 339 Cracknell, R.F., 1487 Escribano, V. S., 3181 Atrill, S. R., 3469 Blaszczak, Z., 2455 Cane, E., 3213 Craig, P. J., 3153 Eustaquio-Rincon,R., 113,Attwood, D., 1961 Blower, C., 919,931 Canosa-Mas, C. E., 1197, Craig, S. L., 1663 2913 Aveyard, R., 2743 Bocherel, P., 1473 1205 Cramer, C. J., 1802,3203 Ewins, C., 969 Avila, V., 69 Boddenberg, B., 1345 Capitan, M. J., 2783 Crawford, M. J., 817 Fan, J., 3281 Axford, S. D. T., 2085 Boesman, E., 2541 Capobianco, J. A., 755 Crisafulli, C., 2809 Fantola Lazzarini, A. L.,Baas, J. M. A., 2881 Boeyens, J. C. A., 3377 Caragheorgheopol, A., 213 Croce, A. E., 3391 423 Baba,T., 187 Boggis, S. A., 17 Carley, A. F., 3341 Crowther, D., 2155 Farhoud, M., 2455 Baba, Y., 2423 Bohm, F., 2453 Carlile, C. J., 1149 Cruzeiro-Hansson, L., 1415 Fausto, R., 689,2953Back, G-H., 2283 Booth, C., 1961 Carlsen, L., 941 Cullis, P.M., 727, 1905, Favaro, G., 279,333Badia, A., 1501 Borden, W. T., 1606, 1614, Carrizosa, I., 2783 2703,2709 Favero, L. B., 2183 Badri, A., 1023 1616, 1671,1673, 1675, Carvill, B. T., 233 Curtis, J. M., 239 Favero, P. G., 2183 Bagatti, M., 1077 1689, 1733, 1734, 1735, Castaiio, F., 2443 DAlagni, M., 1523 Favre, E., 2001 Bailey, R. T., 3373 1743, 1744,1802, 1807 Castaiio, R., 1227 Damiani, D., 2183 Fawcett, W. R., 2697 Baisogolov, A. Yu., 3229 Bordiga, S., 2827, 3367 Castellani, F., 298 1 Dang, N-T., 875 Feliu, J. M., 609 Balaji, V., 1653 Bordoni, S., 2981 Castells, R. C., 2677 Danil de Namor, A. F., 845 Fenn, C., 1507 Ball, M. C., 997, 3373 Boreave, A., 3461 Castro, S., 1217 Das,D., 1993 Fernando, K.R., 1895 Ball, S. M., 523, 1467 Borello, E., 2827 Catalina, F., 83 Das, T. N., 963 Fierro, J. L. G., 2125, 3203 Bally, T., 1615, 1674, 1733, Borge, G., 1227 Cataliotti, R. S., 1397 Dasannacharya, B. A., 1149 Filimonov, I. N., 219, 227 1808 Borisenko, V. N., 109 Cavani, F., 2981 Dash, A. C., 3293 Finger, G., 2141 Ban, M. I., 1610 Borsari, M., 3241 Cavasino, F. P., 31 1,2505 Dash, K. C., 2235 Finocchio, E., 3347 Baonza, V. G., 553 Bottoni, A., 1617 Ceccarani, M. L., 1397 Datka, J., 2417 Fischer, H., 3331 Baonza, V. G., 1217 Boutonnet-Kizling, M., Cense, J-M., 2015 Davey, R. J., 1003 Fisher, I., 2425 Barbaux, Y., 895 1023 Centeno, M. A., 2783 David, G., 261 1 Fishtik, I., 3245 Barbero, C., 2061 Bowker, M., 1015 Cevc, G., 1941 Davidson, K., 879 Fitoussi, F., 3461 Barbosa, J., 3287 Bowmaker, G.A., 2579 Chakrabarty, D. K., 1993 Davies, M. J., 2643 Flamigni, L., 2331 Barczynski, P., 2489 Bozon-Verduraz, F., 653 Chang, T-h., 11 57 De Benedetto,,G. E., 1495 Fleischmann, M., 1923 Barker, S. A., 1689 Bradley, C. D., 239 Charlesworth, D., 1999 de Boer, E., 2663 Fletcher, P. D. I., 2743 Barnes, J. A., 1709 Bradshaw, A. M., 403 Charlesworth, P., 1073 de Castro, B., 307 1 Flint, C. D., 1357 Barthel, J., 2475 Branton, P. J., 2965 Chaudhry, M., 2235,2243, Deeth, R. J., 3237 Fogden, A,, 263,3423Barthomeuf, D., 667,675 Bratu, I., 2325 2683 Defrance, A., 1473 Fontanesi, C., 2925,3241Bartl, H., 2791 Braun, B. M., 849 Che,M., 2277 Dejaegere, A., 1763 Fornts, V., 213 Bartlett, P.N., 2155 Brei, V. V., 2961 Chen, J-S., 429, 717 de &ng, H. C., 2459 Fowler, P., 2865 Basini, L., 787 Breysse, M., 193 Chen, J., 3455 Delhalle, J., 2319 Fracheboud, J-M., 1197,Bassat, J. M., 1987 Briggs, B., 727, 1905, 2703, Chen, J. S., 2765 Demeter, A., 411, 2635 1205 Bassoli, M., 363 2709 Chen, K., 3089 Dempsey, P., 1003 Franci, M. M., 1605, 1740, Battaglini, F., 987 Brocklehurst, B., 271, 2001, Chen, L., 2467 Demri, D., 501 1744 Battistuvi Gavioli, G., 2897 Chen, Y-H., 617 Deng, N-J., 1961 Franck, R., 667,675324 1 Brogan, M. S., 1461 Chen, Z., 2931 Deng,Z., 2009 Franco, L., 3267 1 Franco, M. L. T. M. B., Frank, J., 3201 Franke, O., 2821 Freeman, N. J., 751 FrCty, R., 773 Frey, J. G., 17, 817 Frostemark, F., 559, 2401, 3273 253 1 HallCn, D., 3397 Halpern, A., 721 Hamnett, A., 459 Hancock, F.E., 3341 Hancock, G., 523,1467 Handa, H., 187 Hann,K., 733 Hao, L., 133,1223,1909 Harada, S., 869 Igawa, K., 2119 Iizuka, Y., 1301,1307,3449 Ikawa, S-i., 103, 3065 Ikonnikov, I. A., 219 Ilczyszyn, M., 1411 Il'ichev, Y. V., 2717 Ilyas, M., 2413 Imamura, H., 2119 Indovina, V., 207 Kim, J-H., 377 Kimura, M., 1355,2423 King, F., 203 Kingston, P. A., 2743 Kinjo, Y ., 2235,2683 Kirby, A. R., 2551 Kirchner, S., 1941 Kirschner, J., 403 Kita,H., 803 Lindner, R., 2425 Linstead, D., 3409 Lister, D. G., 2849, 3205 Liu, B-T., 1435,2945 Liu,C-W., 39 Liu,G., 2697 Liu, W., 3281 Liu,X., 249 Liu, Y-P., 1715 Fujiwara, Y., 1183 Funabiki, T., 2107 Galantini, L., 1523 Gale, J. D., 3175 Gale, P. A., 2931 Haraoka, T., 91 1 Hardy, J.A., 2171 Harland, P. W., 935 Harper, R. J., 659 Harriman, A., 697,953, Inerowicz, H. D., 2223 Inoue, Y., 797,815 Ishiga, F., 979 Ishigure, K., 93,591 Ishikawa, T., 2567 Kitchen, D. C., 1581 Klein, M. L., 253, 2009 Kleshchevnikova, V. N., Klissurski, D., 1987 629 Loginov, A. Yu., 219,227 Lohse, U., 1033 Long, A., 1547 Longdon, P. J., 315 Lopez, M. I., 3391 Gallardo Amores, J. M., 3181 Harris, K. D. M., 2627 1313, Isoda,T., 869 Ito, O., 571 Kloss, A. A., Knoche, W., 2697 1507 Lopez Agudo, A., 3203 2125, Galvagno, S., 2803,2809 Gameiro, A. P., 3071 Gandolfi, R., 1077 Gans, P., 315,2351 Gao,Y., 803 Harris, P. J. F., 2799 Harrison, N. J., 55 Haruta, M., 1011 Haselbach, E., 2481 1323 Iwamaru, S-i., 3253 Iwasaki, K., 121 Iwata, S., 3253 Jackson, S.D., 3341 Jacob, K-H., 2969 Knozinger, E., Knozinger, H., Kobayashi, A., Kobayashi, H., Kobayashi, T., 2969 1335 763 763 1011 Lorenzelli, V., 1293, 3347 Loveday, D. C., 1533 Lovejoy, E. R., 2159 Lu, J., 3281 Lu, J-X., 39 Garcia, A., 2265 Garcia, B. E., 2913 Garcia,R., 339 Garcia Fierro, J-L., 1455 Hashimoto, K., 1177 Hashino, T., 899 Hashitomi, O., 2423 Hasik, M., 2099 Jacobs, W. P. J. H., Jacques, P., 2481 Jain, S. K., 2065 Jakobsen, H. J., 2095 1191 Koga, N., 1789 Kondo, Y., 121 Kong, k'. C., 2375 Kontturi, A-K., 2037 Lubin, Z., 3007 Ludemann, H-D., Lui, Y-P., 1735 Luna, D., 2265 2071 Garda-Pafieda, E., 575 Hatchikian, E. C., 2921 Jakubov, T., 783 Kontturi, K., 2037 Lunelli, B., 137 Garrone, E., 3367 Gautam, P., 697 Gavuzzo, E., 1523 Gazzano, M., 2981 Geantet, C., 193 Gebicka, L., 341 1 Gebicki, J. L., 341 1 Hattori, H., 803 Hawkins, G.D., 1802,3203 Hayashi, H., 2133 Haymet, A. D. J., 1245 Heal, M. R., 523,1467 Healy, T. W., 1251,3301, 3415 Jameel, A. T., 625 Janchen, J., 1033,2837 Jancke, K., 2141 Jayakumar, R., 161,2725 Jayasooriya, U. A., 1265 Jeevan, R. G., 2725 Jenneskens, L. W., 327, Kornatowski, J., 2141 Kossanyi, J., 41 1, 2635 Kosslick, H., 2837 Kotz, R., 2061 Kourouklis, H. N., 3443 Kover, W. B., 2297 Kralchevsky, P. A., 2077 Luthjens, H., 2459 Ma, J., 1351, 3383 Mabuchi, M., 899,1979 MacFarlane, A. J. B., Machado, V. G., 865 Macias, M., 2265 Mackenzie, K., 1810 251 1 Gengembre, L., 895 Gerratt, J., 1643, 1672, Gerry, M. C. L., 2601,3023 Getty, S. J., 1689 Ghiggino, K. P., 2845 Giamello, E., 3 167 Giglio, E., 1523 Gil, A.M., 1099 Gil, F. P. S. C., 689 Gilbert, B. 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R., 1251 Yeh, C-t., 1157 Tanaka, T., 2107 Truscott, T. G., 1065, 1073, Vedrine, J. C., 193 Whitehead, J. C., 3373 Yonemochi, E., 31 17 Tanigaki, H., 1307 2453 Velev, 0.D., 2077 Whitehead, M. A., 47 Yoshida, H., 2107 Tanodekaew, S., 1961 Tsang, S. C., 2799 Venanzi, M., 435,1857 Wikander, G., 305 Yoshida, S., 2107 Tapia, O., 2365 Tse, J. S., 3145 Ventura, 0.N., 1745 Wild, O., 1819 Yoshitake, H., 155 Taravillo, M., 1217 Tseung, A. C. C., 3089 Venturini, A., 1617 Wilde, C. P., 1233 Yotsuyanagi, T., 93,479 Tarroni, R., 3213, 3433 Tsuchiya, S., 21 19 Venturini, M., 2359 Wilhelm, M., 1391 Young, R.N., 271,2001 Tassi, L., 859, 1089 Tsuchiyama, T., 1355 Verbeeck, R.M., 2653 Wilkie, J., 1709 Yu, J-S., 2283 Tateno, A., 763 Tsuji, H., 803 Verhaeghe, T., 2003 Willey, R. J., 3347 Zamaraev, K. I., 2147 Tatham, A., 1099 Tsuji, M., 117 1 Vigue, J., 1553 Williams, C., 2147 Zambonin, C. G., 1495 Tatibouet, J-M., 2277 Tsunashima, S., 549 Villamagna, F., 47 Williams, D. E., 345 Zannoni, C., 3433 Tawn, D. N., 2897 Tsunetoshi, J., 1307,3449 Villarreal, J. R., 2849 Williams, F., 1605, 1681, Zanotto, S. P., 865 Taylor, A., 1003 Tuan, V. A., 2837 Villemin, D., 97, 2015 1733,1810 Zecchina, A., 2827, 3167, Taylor, M. G., 641 Tung, C-H., 947 Vinckier, C., 2003 Williams, I. H., 1615, 1673, 3367 Taylor, M. J., 3139 Tuiion, I., 1757 Visscher, P. B., 1133 1679, 1709,1737,1738, Zentel, R., 3331 Tecklenburg, M.M. J., Turco Liveri, M. L., 3 11, Vizoso, S., 2337 1739,1740, 1799 Zhang, J., 2057 2849 2505 Vlietstra, E. J., 327, 1363 Williams, R., 2921 Zhang, M., 1233 Teixeira-Dias, J. J. C., 689 Turco Liveri, V., 3 11 Voegel, J. C., 3405 Wilpert, A., 287 Zhang, X., 605 Teo, W. K., 355 Turner, G. O., 2845 Vollarova, O., 855 Wilson, C., 3051 Zhang, Z. C., 1335 Teramoto, M., 979 Turner, P. H., 1065 Vollmer, F., 59 Wilson, S. A., 1616, 1674, Zhao, D-c., 3383 Teraoka, Y., 349 Turulski, J., 3061 Volta, J-C., 1161, 1441 1680 Zhao, Z., 2467 Termignone, G., 1967 Udagawa, T., 763 von der Gonna, V., 261 1 Winkel, J. F., 3469 Zhao, Z. S., 3097 Thomas, H., 2125,3203 Ueno, A., 1279 von RaguC Schleyer, P., Winstanley, A.W., 3191 Zholobenko, V. L., 233, Thomas, J. M., 2147,3455 Ugo,R., 1335 1559,1605,1606, 1671, Wintgens, V., 411,2635 1047 Thompson, K. M., 1105 Umemoto, H., 549 1673,1674,1677,1680, Wischerhoff,E., 3331 Zhong, G. M., 369 Thompson, L., 2375 Unayama, S-i., 549 1734,1743, 1744, 1801, Woermann, D., 875,2215 Zhongmin, L., 3007 Thompson, N. E., 1047 Unger, K. K., 2965 1805,3471 Wohlers, M., 403 Zielesny, A., 2215 Thorn, J. C., 1365,3205 Unnikrishnan, S., 2291 Vyunnik, I. N., 297 Wojnarovits, L., 2459 Ziolek, M., 1029 Tidwell, T. T., 3383 Unwin, P. R., 3109 Wagenaar, A., 2703,2709 Wolf, G-U., 3159 Zouiten, A., 3461 Timmins, G. S., 2643 Upadhyaya, H. P., 825 Waghorne, W. E., 2691 Wolthuizen, J. P., 1033 Zubarev, V. E., 721 Timms, A.W., 83 Valat, P., 41 1,2635 Wales, D. J., 1061, 1831, Wormald, C. J., 445 Zukal, A., 2821 Timney, J. A., 459 Valencia, E., 2555 1839 Wu,X., 2345 Zundel, G., 843,1095 iv The following papers were accepted for publication in September, 1994: Surface electrochemistry and kinetics of anodic bromine formation at platinum B. E. Conway, Y. Phillips and S. Y.Qian Relation between the surface states of oxide films at Rh electrodes and kinetics of the oxygen evolution reaction G. Jerkiewicz and J. J. Borodzinski Genesis of the ternary V-TiSi catalysts and their behaviour in the CO+NO reaction J. L. G. Fierro, M. Galhn-Fereres, R. Mariscal, L. J. Alemany and J. A. Anderson Reversible and irreversible adsorption of nitrogen monoxide on cobalt ion-exchanged ZSM-5 and mordenite zeolites at 273-523 K M.Iwamoto, W-X. Zhang, H. Yahiro and J. Izumi Creation and detection of coherence and polarization in pulsed EPR A. Schweiger Pivalic acid as combined buffer and scavenger for studies of cloud water chemistry with pulse radiolysis R.E. Buhler and T. Nauser Molecular dynamics simulations of flexible molecules. Part 1 .-Aqueous solution of ethylene glycol H. Hayashi, H. Tanaka and K. Nakanishi Some physicochemical properties of dimethyl sulfoxid-methanol liquid mixtures. Experimental and quantum chemical studies S. 0. Romanowski, C. M. Kinart and W. J. Kinart Solvation of cobalt@) and perchlorate ions in binary mixtures of donor solvents E. Kamienska-Piotrowicz Influence of oxidation and reduction conditions upon the morphology of silica-supported polycrystalline silver catalysts R.P.Cooney, G. J. Millar, J. B. Metson and G. A. Bowmaker Synthesis, photochemistry and cross-linking of visibly sensitised photopolymers of PVA based on (E)-2-(4- formylstyryl)-3,4-dimethylthiozoliummethylsulfate N.S. Allen, I. C. Barker, M. Edge, J. A. Sperry and R. J. Batten Thermodynamic behaviour of liquid p-xylene near freezing V. G. Baonza, S. Castro, M. Taravillo, M. Caiceres and J. Niiiiez Rotational Brownian motion of a two-dimensional rigid rotator: Relaxation and steady-state regimes N. M. Hounkonnou and P. Navez IR spectroscopic study of NO, adsorption on chromia K. Hadjiivanov, D. Klissurski and V. Ph. Bushev Solvent effect of the photoinduced electron-transfer reaction between a dicyanobis(polypyridine)ruthenium(II) complex and a tris(P-diketonato)ruthenium(III) complex Y.Kaizu, N.Sonoyama and 0. Karasawa Electrically induced reversible structural change of a highly swollen polymer gel network M. Hirai, T. Hirai, A. Sukumoda, H. Nemoto, Y. Amemiya, K. Kobayashi and T. Ueki Water adsorption in carbons described by the Dubinin-Astakhov and Dubinin-Serpinski equatioiis F. Stoeckli, L. Currit, A. Laederach and T. A. Centeno Reactive collisions with excited state atoms A. Gonzalez-Urena and R. Vetter In situ FTIR study of CO-H, reactions over Rh/TiO, catalysts at high pressure and temperature C. H. Rochester, J. A. Chudek, M. W. McQuire and G. W. McQuire Excimer-laser photofragmentation of boron trichloride and implications for its vapour and doping processing by laser S.Georgiou, E.Raptakis, X.Xing, E. Hontzopoulos and Y. P. Vlahoyannis Growth of a well-oriented layer of p-nitroaniline on the (100) plane of a p-cyanoaniline single crystal, studied by polarised absorption and second-harmonic generation H. Kobayashi, T. Ehara and M. Kotani Analyses for a dipolar relaxation in the featureless dielectric spectrum, and a comparison between the permittivity, modulus and impedance representations G. P. Johari and M. G. Parthun High-temperature dielectric relaxation in nylon-12, and the effects of annealing and absorbed water G. P. Johari and K. Pathmanathan Dissolution of amorphous aluminosilicate zeolite precursors in alkaline solutions.Part 3 .-Influence of temperature on the dissolution process B. Subotic, A. &mek and T. Antonic V Intramolecular hydrogen bonding in N-salicylideneanilines: X-Ray diffraction and solid-state NMR studies J. Klinowski, H. He, K. Wozniak, W. Jones, T. Dziembowska and E. Grech 27Aland 29Si solid-state NMR studies of dealuminated mordenite J. Klinowski, D. W. McClomb and J. Barras Catalytic properties of y-alumina. Ab-initio molecular orbital studies of clusters of chlorinated y-alumina J. Thomson, G. Webb, B. Webster and J. M. Winfield Liquid mixtures at high pressures referred to a pseudo-spinodal curve: Ethanol-methylcyclopentane and carbon disulfide-tetramethylsilane systems V. G. Baonza, F. Orbis, M. CQceres, J. E. Fernhdez Rubio and J.L. Niifiez Electronic spectroscopy of size-selected ionic complexes E. J. Bieske Kinetics of disproportionation of hypoiodous acid V. W. Truesdale, C. Canosa-Mas and G. W. Luther III Cadmium-exchanged Y zeolites studied with carbon monoxide and xenon as probes B. Hoddenberg and T. Sprang Room-temperature catalytic fluorination of C, and C, chlorocarbons and chlorohydrocarbons on fluorinated Fe,O, and Co,O, J. Thomson IR studies of adhesion promoters. Part 1.-Methoxytrimethylsilane on silica at the solid/liquid interface C. H. Rochester and A. Piers Modified interacting-sphere model for self-diffusion and infinite dilution mutual diffusivity of alkanes P. H. Salim and M. A. Trebble X-Ray photoelectron spectrum of glassy B,S,: Experimental and theoretical study D.Gonbeau, H. Bouih, G. Pfiiter-Guillouzo, M. Menetrier and A. Levasseur Reaction of atomic oxygen with some simple alkenes. Part 1.-Low-pressure studies on reactions involving ethene, propene and (a-but-Zene C. Anastasi and M. G. Sanderson Reaction of atomic oxygen with some simple alkenes. Part 2.-High-pressure studies on reaction pathways involving ethene, propene and (a-but-2-ene C. Anastasi, M. G. Sanderson, P. Pagsberg and A. Sillesen Resonance-enhanced multiphoton ionisation spectroscopy of thiirane M.N. R. Ashfold, R. A. Morgan, P. Puyuelo, J. D. Howe, W. J. Buma, J. B. Milan and C. A. De Lange EPR studies of the reactions of high energy copper species with hydrocarbons using a rotating cryostat B.Mile, J. A. Howard and G. McGimpsey Apparently viscoelastic responses in a quartz crystal microbalance study of an electrodeposited non-aqueous colloid film D. R. Rosseinsky and J. S. Graham Ultra-thin particulate films prepared from capped and uncapped reverse-micelle-entrapped silver particles J. H. Fendler, F. C. Meldrum and N. A. Kotov Hydrogen adsorption-desorption and oxide formation-reduction on polycrystalline platinum in unbuffered aqueous solutions D. Pletcher and S. Sotiropoulos Vapour-liquid equilibrium of the ethanol-propanal system J. A. R. Renuncio, B. Coto, C. Pando and R. G. Rubio Surface characterization of yttria-stabilized tetragonal ZrO,. Part 2.-Adsorption of CC) C. Morterra, G. Cerrato, V. Bolis, C.Lamberti, L.Ferroni and L.Montanaro Surface characterization of yttria-stabilized tetragonal ZrO,.Part 3.-co2 adsorption and the CO2-CO interaction C. Morterra, G. Cerrato and L. Ferroni FTIR study of the effects of sulfur poisons on NO adsorption on supported copper oxide catalysts C. H. Rochester, M.B. Padley, G. J. Hutchings and F. King Microwave spectroscopic investigation of thionyl chloride, SOC12: Hyperfine constants and harmonic force field H. S. P. Miiller and M. C. L. Gerry Phases in vanadium pentoxide-tungsten trioxide catalysts M. Najbar, A. Inglot and B. Borzecka-Prokop Hexapole state selection and focusing vs. brute force orientation of beam molecules J. Bulthuis, J. J. Van Leuken and S. Stolte vi Pressure and temperature dependence of the rate constants for the association reaction of OH radicals with NO between 301 and 23 K I.W. M. Smith, P. Sharkey, I. R. Sims, P. Bocherel and B. R. Rowe Spin effects on spur kinetics. Part 3.-Re-encounters and the independent pairs approximation S. M. Pimblott, N. J. B. Green and B. Brocklehurst Theoretical and experimental studies on the novel thiazylfluoroformate, FC(0)SN C. 0.Della Vkdova and H-G. Mack THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 102 Unimolecular Reaction Dynamics Exeter College, Oxford, 19-21 December 1995 Organ ising Committee: Professor M. J. Pilling (Chairman) Professor J. P. Simons Professor M. S. Child Professor I. W. M. Smith Dr D. C. Clary Professor J. Troe Professor R. Walsh Unimolecular reactions, defined as processes occurring in the ground electronic state, depend on the interplay between microcanonical dissociation or isomerisation and energy transfer.Recent years have seen developments in both experimental and theoretical techniques for probing aspects of such reactions. The Discussion will highlight these latest developments and promote interaction between a potentially wide range of fields. Topics of inclusion are: Theories of microcanonical reactions: quantum dynamics calculations, quantum resonances, developments beyond RRKM Experimental studies including dissociation of ions, clusters and Van der Waals molecules; real time and frequency domain studies; IVR; multi-channel reactions Relaxation and supercollisions .Unimolecular dynamics in condensed phases Contributions are invited for consideration by the Organising Committee. Titles and abstracts of about 300 words should be submitted by 15 January 1995 to: Professor M. J. Pilling, School of Chemistry, University of Leeds, Leeds LS2 9JT. Full papers for publication in the Faraday General Discussion 102 volume will be required by 31 August 1995. vii FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Endowed Lecture Symposium: Simulation of Condensed Matter from First Principles To be held at Cambridge University on 23 November 1994 Further information from Mrs. Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Theoretical Chemistry Group with CCPl Electronic Structure: From Molecules to Enzymes To be held at University College London on 30 November 1994 Further information from Dr.P. J. Knowles, School of Chemistry, University of Sussex, Falmer, Brighton BN1 905 Colloid and Intei$ace Science Group Surface Forces and Probe Microscopy To be held at Imperial College, London on 19 December 1994 Further information from Dr. D. Clark, Institute of Food Research, Norwich Laboratory, Norwich Research Park, Colney, Norwich NR4 7UA Division Endowed Lecture Symposium: Recent Advances in the Study and Preparation of Novel Surfaces To be held at The Royal Institution on 26 January 1995 Further information from Mrs. Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Division Endowed Lecture Symposium: Spectroscopy and Dynamics of Electronically Excited States To be held at University of Manchester on 29 March 1995 Further information from Mrs.Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN ~ ~~ Colloid and Intei$ace Science Group Concentrated Dispersions To be held at the University of Bristol on 29-31 March 1995 Further information from Dr. A. Lips, Unilever Research, Colworth Laboratory, Colworth House, Sharnbrook, Bedford MK44 1LQ Statistical Mechanics and Thermodynamics Group with the Experimental Thermodynamics Society Experimental Thermodynamics Conference To be held at the University of Reading on 5-7 April 1995 Further information from Dr. J. Henderson, School of Chemistry, University of Leeds, Leeds LS2 9JT Division Annual Congress: Lasers in Chemistry To be held at Heriot Watt University, Edinburgh on 10-13 April 1995 Further information from Dr.J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN ~ ~~ Division Endowed Lecture Symposium: Molecular Complexes and Interactions To be held at the University of Bristol on 4 May 1995 Further information from Mrs. Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN ... Vlll Division Joint Meeting with the Division de Chimie Physique de la Societe' Francaise de Chimie, Deutsche Bunsen Gesellschaft fir Physikalische Chemie and Associazione Italiana di Chimica Fisica Fast Elementary Processes in Molecular Systems To be held at the UniversitC de Lille, France on 16-30 June 1995 Further information from Dr.C. Troyanowsky, Division de Chimie Physique, Laboratoire de Chimie Physique, 11 rue Pierre et Marie Curie, 75005 Paris, France Biophysical Chemistry Group Dynamic Processes in Biophysics To be held at the University of East Anglia on 6-8 September 1995 Further information from Dr. D. C. Clark, Institute of Food Research, Norwich Laboratory, Norwich Research Park, Colney, Norwich NR4 7UA Polymer Physics Group Biennial Meeting To be held at the University of Leeds on 6-8 September 1995 Further information from Professor G. R. Davies, Department of Physics, University of Leeds, Leeds LS2 9JT British Carbon Group Carbon '96 To be held at the University of Newcastle upon Tyne on 7-12 July 1996 Further information from Dr.K. M. Thomas, Northern Carson Research Laboratories, The University, Newcastle upon Tyne NE1 7RU THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Organising Committee Professor L. D. Barron (Chairman) Dr A. F. Drake Dr D. L. Andrecvs Professor R. E. Hester Professor A. D. Buckingham Traditional optical activity measurements such as CD are confined to the visible and near-ultraviolet spectral regions where they provide stereochemical information on chiral molecules via polarized electronic transitions. Thanks to prompting from theory and new developments in instrumentation, optical measurements are now being made in the vibrational spectrum using both infrared and Raman methods.Studies over the past decade on a large range of chiral molecules, from small organics to biological macromolecules, have demonstrated that vibrational optical activity opens up a whole new world of fundamental studies and practical applications undreamt of in the realm of conventional electronic optical activity. The meeting seeks to bring together experimentalists and theoreticians to discuss the current and future experimental possibilities and the development of theories, including ab initio computational methods, which can relate the observations to stereochemical details.The increasing importance now being attached to molecular chirality and solution conformation in the life sciences should also encourage the participation of biomolecular scientists. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, London W 1V OBH. ix THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 100 Atmospheric Chemistry: Measurements, Mechanisms and Models University of East Anglia, Norwich, 19-21 April 1995 Organising Committee: Professor I. W. M. Smith and Dr J. R. Sodeau (Co-chairmen) Dr R. A. Cox Dr J. C. Plane Dr J. Pyle Professor F. Taylor The priority now given by national governments to the study of atmospheric science confirms that our understanding of global climate and compositional changes depends upon measurements in both the laboratory and the field.The data obtained by the experimentalists are then applied by modellers who provide the most significant input into legislative controls on pollution matters. However there have been few opportunities for laboratory and field workers along with the modelling community to attend an "interdisciplinary" discussion in which overall progress in our understanding of specific atmospheric problems is assessed. The object of this discussion is to bring together the researchers in the diverse disciplines that make up a1:mospheric chemistry so that their individual results and conclusions can be communicated to each other.Some of the key issues to be discussed will include: ozone balances in the atmosphere; heterogeneous processes; the interaction of chemistry and dynamics in determining atmospheric composition and change. Particular reference will be made to the input of data to global models from the use of satellite, airborne and ground-based instrumentation. Contributions are invited for consideration by the Organising Committee covering topics within the area of chemistry, dynamics and modelling in the lower and upper atmosphere. Abstracts of about 300 words should be submitted by 31 May 1994 to: Professor I. W. M.Smith OR Dr R. J. Sodeau School of Chemistry School of Chemical Sciences University of Birmingham Universify of East AngZia Edgbaston, Birmingham Norwich BI5 2lT, UK NR4 7TJ, UK Full papers for publication in the Discussion volume will be required by December 1994.THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 101 Gels Paris, France, 64 September 1995 Organising Committee: Dr J. W. Goodwin (Chairman) Dr R. Audebert Dr R. Buscall Professor M. Djabourov Dr A. M. Howe Professor J. Livage Professor J. Lyklema Professor S. B. Ross-Murphy During the last few years there has been an increase in both theoretical and experimental work on gels as new techniques have been applied to a wide range of gelling systems. Typical of these are gels formed from polymers by both physical and chemical interactions as well as gels formed by inorganic and surfactant systems.The meeting will deal with the structure and dynamics of gels with the latter heading covering both swelling and rheological behaviour. Mixed systems such as polymer/surfactant and polymer/particle gels will also be discussed. The Discussion will bring together experimentalists and theoreticians interested in different types of gelling systems and encourage them to interact and assess the current scene and provide a benchmark for future developments. Contributions are invited for consideration by the Organising Committee. Titles and abstracts of about '300 words should be submitted by 30 September 1994 to: Dr J. W. Goodwin, School of Chemistry, University of Bristol, Cuntock's Close, Bristol, BS8 1 TS, UK Full papers for publication in the Faraday General Discussion 101 volume will be required by May 1995.X ECAMP 5 CHEMICAL PHYSICS SYMPOSIUM A Chemical Physics Symposium will be held as part of the 5th European Conference on Atomic and Molecular Physics, which will take place in Edinburgh, Scotland from 3-7 April 1995. The Symposium has the support of the Faraday Society, the Bunsengesellschaft and the Soci& Francaise de Chimie and extends the range of topics in Chemical Physics covered by ECAMP 5. The programme for the Symposium is as shown below. Plenay Lecture B R Rowe (Rennes) Chemical reactions at low temperature Session on Spectroscopy K Muller-Dethlefs (Munich) ZEKE photoelectron spectroscopy W J Van der Zande Photodissociation and molecular spectroscopy (FOM, Amsterdam) Session on Dynamics J M Launay (Rennes) 3D time independent reactive scattering F Temps (Gottingen) State-selected unimolecular reactions Session on Theoretical Chemistry P W Fowler (Exeter) Fullerenes M Raimondi (Milan) Internuclear forces from the VB method Session on Biological Applications of Chemical Physics P Fromherz (Ulm) Information processing in cells D Chapman Biomembranes (Royal Free Hospital, London) To obtain further information about ECAMP 5, contact: Dr N J Mason Telephone ++44 71380 7797 The Secretary, ECAMP 5 Fax ++44 71380 7145 Department of Physics and Astronomy E-mail Ucaps7n@ucl.ac.uk University College London Gower Street LONDON WC1E 6BT xi CHEMICAL SOCIETY REVIEWS Chemical Society Reviews, recognising the importance of inter-disciplinary dialogue in scientific research, contains reviews which cover a diversity of chemistry topics.The articles provide introductions to topical areas of chemistry, enabling their import and application to adjacent fields to be readily assessed. Chemical Society Reviews is essential for advanced undergraduates and postgradu- also invaluable to chemists with wide ates who require-a broad-based knowl- subject interests, and scientists in related edge and understanding of modern disciplines who recognise the advantages chemistry, and who wish to identify a of keeping in touch with the latest career path or research direction. It is advances in chemistry. Chemical Society Reviews: Editorial Board Professor H W Kroto, FRS, Chairman,* Publishes articles of a wide appeal University of Sussex * Provides access to exciting research Professor M J Blandamer, Uniiversity of Leicester Dr A R Butler, University of St Andrews * Attracts articles from the world’s lead- developments Professor E C Constable, University of Basel, Switzerland Dr T C Gallagher, University of Bristol ing chemists Includes key reference lists that facili- Professor D M P Mingos, FRS, tate further reading Imperial College, London Professor J F Stoddart, University of Birmingham * Is attractively presented, with colour figures Has proved so successful since its re-Staff Editors: launch in 1992 that it is now pub- Mr K J Wilkinson, Dr J A Rhodes and Dr M J Sugden,lished six times a year Royal Society of Chemistry, United Kingdom.~~~ 1994 Subscription Details Published six times a year ISSN 0306-00121I EC €99.00 USA $186.00 Canada f 1 11 .OO Rest of World €1 06.00 Chemical Society Reviews will add substantially to your ROYALknowledge of modern chemistry -order your subscription SOCIETY OF CHEMISTRYtoday! To order, please contact: Turpin Distribution Services Limited, Blackhorse Road, Letchworth, Herts SG6 1HN, United Kingdom. Tel: +44 (01462 672555. Fax: +44 (O)462 480947. Telex: 825372 TURPIN G. For further information please contact: Sales and Promotion Department, Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, United Information Kingdom.Tel: +44 (01223 42006. Fax: +44 (0)223 423623.Services Edited by P.K. Datta, University of Northumbria at Newcastle J.S. Gray, University of Northumbria at Newcastle This title is published in three highly illustrated volumes and covers the latest developments in the subject. It provides a unique combination of the science of coatings and surfaces, the technologies of deposition, surface modification and analysis, and practical applications. The three volumes provide a useful and comprehensive blend of reviews and state-of-the-art papers, written by international experts, and reflect the current status of and likely future advances in surface engineering. Surface Engineering Fundamentals of CoatingsVolume I: ~ This volume considers principles of coatinghubstrate design in aqueous and high temperature corrosion, and wear properties, scanning the coatings spectrum from organic, through metallic to ceramic. The emphasis in this volume is on the sciencand design of coatings and substrate systems rather than on technology.Hardcover ISBN 0 85186 665 4 (1993) xvi + 370 pages Price f52.50 Volume 11:Surface Engineering Engineering Applications Volume II is dedicated to topics concerning the performance of coatings and surface treatments embracing four main areas: the inhibition of wear and fatigue; corrosion control; application of coatings in heat engines and machining; and qualities and properties of coatings. 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To order, please contact: Turpin Distribution Services Ltd., Blackhorse Road, Letchworth, Herts SG6 1HN, United Kingdom Tel: +44 (0)462 672555 Fax: +44 (0)462 480947 ROYAL RSC members should obtain members’ prices and order from: SOCIETY OF Membership Administration CH EMISTRY Royal Society of Chemistry Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF,Information United Kingdom Services Tel: +44 (0)223 420066 Fax: +44 (0) 223 423623 ...Xlll NOMENCLATURE AND SYMBOLISM For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. Nomenclature.The following publications provide the IUPAC nomenclature rules and guidance on their use: Nomenclature of Organic Chemistry, Sections A, B, C, 0, E, F and H, Pergamon, Oxford, 1979 edn. NOm8nClatl~f8of lnorganic Chemistry, Blackwell Scientific Publications, Oxford, 1990. Biochemical Nomenclature and Related Documents, The Biochemical Society, London, 1978. Where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society’s editorial staff. Units and Symbols. A detailed treatment of units and symbols with specific application to chemistry, based on the Systbme Internationale d’Unitbs (SI), is given in Quantities, Units and Symbols in Physical Chemistry, published for IUPAC by Blackwell Scientific Publications, Oxford (1988 edn.). A comprehensive list of IUPAC publications on nomenclature and symbolism appears in the January issue of J. Chem. SOC.,Faraday Transactions. xiv

ISSN:0956-5000    

DOI:10.1039/FT99490BP238

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(22), 3377-3381 Ionization Radii of Compressed Atoms Jan C. A. Boeyens? lnstitut fur Kristallographie , Freie Universitat Berlin, Takustr. 6, 14 195 Berlin The compression of all atoms has been modelled by changing the free-atom boundary condition obeyed by Irmelectronic wavefunctions, from r -co, Y(r)= 0 to r 3ro, Y(r)= 0,ro < m, in numerical Hartree-Fock- Slater calculations of electronic energy levels. As ro decreases, energy levels increase uniformly and by trans- ferring the excess energy, an electron escapes from the valence shell when compression reaches a critical value of ro , characteristic of each atom. These ionization radii display remarkable periodicity, commensurate with the known chemistry of the elements, and introduce a new fundamental theoretical parameter that could serve to quantify chemical reactivity.Insofar as the compression of atomic wavefunctions occurs within crowded environments that lead to chemical interactions, ionization radii provide a more realistic index of the chemical properties of atoms in the bulk, than ionization energies, which are more appropriate in spectroscopic analyses of free atoms. The problem of a compressed hydrogen atom has been con- sidered by several independent authors over a period of many years,'-5 with a view to providing a theoretical framework to analyse the spectroscopic and chemical behaviour of atoms under applied pressure. Many other atomic properties, such as electron density in condensed phases and chemical re-activity, are formally related to the same question, but the suitability of hydrogen with its solitary electron, as a repre-sentative model in these situations, is debatable.Extending the treatment to non-hydrogen atoms requires a numerical approach and is the topic of this study. The normal hydrogen problem, in summary, requires solu- tion of the Schrodinger equation, HY = EY, under the assumption that the total wavefunction is a product of radial and angular parts, which allows separation of the variables. Y is a spherical har- monic for integer n > 13 0, with n -1 -1 nodes at r > 0. The radial function is a solution of in atomic units and the boundary conditions P(n1, 0) = 0 and Jim . r -co, P(n2; I) = 0.For the compressed hydrogen atom the latter is replaced by the artificial boundary condition r Jim -r,, P(nl;I) = 0, for some ro < 00. This corresponds to an impenetrable potential barrier, spherically surrounding the atom at a radial distance ro. The symmetry of the coulomb field and hence the angular-dependent part of the wavefunction is not affected by this condition. Apart from the electron density which obviously changes, the most dramatic effect of such compression is the general increase in all electronic energy levels. Calculations are commonly divided into three regions: (i) the bound region where E < 0; (ii) the critical region around E = 0; and (iii) the ionized state, E > 0. The first region provides an insight into the spectroscopic effects of applied pressure and condensed phases, and the ionized state probably relates to chemical activation effects.It corresponds to a particle in a t On leave from the Department of Chemistry, University of the Witwatersrand, Johannesburg, South Africa. spherical box and is the simplest to solve. The solutions Rkl =j,(kr) with (tik)2= E/2m are spherical Bessel functions. The bound- ary condition becomes jl(kro)= 0, which for j, becomes sin (kr,) = 0, i.e. kr, = nn, En, = (hnn)2/2rnr, for any integer n > 0. The critical point for hydrogen (El = 0) occurs at r, = 1.835~~. Non-hydrogen Atoms The interpretation of experimentally measured electron-density functions in condensed (crystal) phases could obvi- ously benefit from information about actual atomic wavefunctions and energy levels.In the first approximation these would correspond to the wavefunctions and energy levels of spherically compressed atoms. For an N-electron atomic system of Hamiltonian formal solutions of the Schrodinger problem are not known. The most successful procedure to obtain reasonable solutions for these systems is the Hartree-Fock variational method, based on the central-field approximation. This replaces the exact Hamiltonian by where U(r,) is a single-particle potential introduced to approximate the inter-electronic interactions. The equation, H+ = E$, is now separable provided $ is a product of one- electron functions, + = $(1)4(2). ..$(i). ..$(iV),to give [-V: --+ U(ri) 4, = Ei$i "Iri for all i, which is the wavefunction for an electron in a central field.More concisely [ -(1/2)V? + V(r,)]$i= Ei$i, where V(ri)is the sum of the nuclear coulomb potential, the total electronic coulomb potential and the exchange potential. Like the hydrogen equation this also separates into radial, angular and spin parts, 1$h6, 4) = -WYlm(~,4)PS where Y is a spherical harmonic as before. The radial func- tion, for each electron, is now a solution of with boundary conditions, P(0) = P(m) = 0. The integer n now orders the solutions so that n = I + 1corresponds to the lowest eigenvalue of an eigenfunction without nodes, and so forth. It assumes the same role as the principal quantum number in the hydrogen problem.A radial equation like this must be solved for each com- bination nl (called an orbital) and in each case a different exchange potential must be calculated. This is computa- tionally the most difficult part. The Slater6 simplification of this problem replaces the exchange potentials for different orbitals by a universal exchange potential formed by suitable averaging over the individual exchange potentials. In the program by Herman and Skillman,7 chosen for the present study, self-consistency is achieved by defining the total potential as 00z 1‘V(r)= --+ -L4~(r‘)~p(r’)dr’ + 14n(r’)p(r’) dr’ rr The spherically averaged charge density is defined in terms of the radial wavefunctions where W,, is the occupation number of the nl subshell.The last term respresents the Slater6 free-electron exchange approximation. In this Hartree-Fock-Slater (HFS) scheme, Y is rep-resented by a single determinantal wavefunction built up from spin-orbitals, so that the same radial function is assign- ed to both spins with the same orbital. Open-shell configu- rations are treated like those with closed shells only and the multiplet splitting is completely ignored. The free-electron exchange approximation is used only in the interior region to ensure correct asymptotic behaviour far out. The radial equation is solved in an iterative fashion. A starting energy eigenvalue is guessed and the equation is numerically integrated in two directions. Outward integra- tion by the Noumerov method starts from the origin and meets the inward integration from infinity, using the appro- priate boundary condition in both cases.From the mismatch of the inward and outward logarithmic derivatives, a correc- tion for the initial energy eigenvalue is calculated and the procedure continued to convergence. Once all P,,are known, a charge density is calculated and used to form a new poten- tial as input for the next cycle in the self-consistent procedure. Method of Study Numeric calculations of electronic wavefunctions and energy levels was done with the standard computer program of Herman and Skillman,7 previously only used for free-atom or ion calculations, and modified to operate under the same artificial boundary condition described above for the hydro- gen atom.The published7 FORTRAN code was used with minor modifications and addition of the option to introduce a spherical barrier at ro. This is achieved by multiplying the self-consistent-field wavefunctions before normalization and during each iteration cycle by the step-function S = exp[ -(r/ro)p] J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Electronic energy levels of the free and compressed praseo- dymium atom ~~ ~ level electrons free atom compressed to 5.6 a, Is 2 -2933.4 -2933.2 2s 2 -460.82 -460.63 -431.132P 6 -437.33 3s 2 -100.12 -99.92 3P 6 -89.740 -89.545 3d 10 -70.247 -70.052 4s 2 -20.046 -19.854 4P 6 -16.093 -15.901 4d 10 -9.0790 -8.8867 5s 2 -2.9108 -2.7231 5P 6 -1.8372 -1.6544 4f 3 -0.8 102 -0.6197 6s 2 -0.3417 -0.301 1 The sharpness, whereby the function changes from 1 to 0 around ro depends on the value chosen for p % 1, as shown graphically in Fig. 1.The choice is left as an option in the program, which searches automatically, once self-consistency has been reached, for the point at which the longest wave- function fades to zero. A value of p = 20 was used for all the calculations in this study. Two different strategies were explored to avoid the possi- bility of excising far-out nodes: moving the barrier in by small steps, or imposing it directly at the final ro. The same self-consistent field is invariably found by either approach. Starting parameters for any calculation are conveniently available in the form of the published’ free-atom potentials and energies.All these data are now available, together with the program, on computer file to interested users. As a first test of the modified program, it was used to examine how the energies of occupied levels varied during compression and if the critical region for each atom could be identified from the results. The praseodymium atom, as an example, serves to illustrate most of the principles. On com- pression to 5.6 a, (ro = 5.0, p = 20) the energy levels all shift by an almost uniform amount, as shown in Table 1. Relative to their absolute values, the core levels are seen to be hardly affected. For all levels, 1s to 4s, the shift is less than 1% and only for 4f (24%) and 6s (12%) is it more than 10%.In prac- tice this means that all deep core levels could be ignored when the process of ionization is considered.r Fig. 1 Step function, S = exp[ -(r/r,JP], for values of p = 2 (a),4 (b),10 (c) and 20 (djand ro = 4 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.0I I ' I p -2.0 a, 2P -4.0 ?i-3-51 -4.5 --5.0 I 2s 1 2 3 4 5 6 7 8 9 rla0 Fig. 2 Orbital compression curves for sodium As another example, Fig. 2 graphically illustrates the behaviour of the upper levels of the sodium atom under com- pression. In this case the 3s level is virtually unaffected by compression, but a dramatic increase in the 2p level occurs. Another typical situation is shown in Fig. 3 which represents compression of the vanadium atom, one of the cases where levels cross.This should not be interpreted as an escape of an electron from the 3d level. Each curve represents the centre of gravity of a multiplet of levels and substantial mixing must obviously occur on compression. This interaction between the levels transfers energy by a process of resonance to a single electron which is pushed to the ionization limit. This process is not unlike ionization in an applied field, often visualized in terms of Fermi's golden rule.' This links the probability of resonance between levels to the density of states in the energy gap. For a separation of AE between upper and lower levels, where the latter has an occupation number of N (electrons), the probability is P = N/AE, with a maximum of P = 1, which describes fully resonating levels.Note that the probability for resonance within the multiplet at the highest level, is likewise equal to unity. The energy transferred from all lower levels to the topmost level is therefore calculated as E, = Ni &JAEi, where ci is the increase in energy due to compression. Since the N, electrons at the highest level resonate amongst them- selves in the same way, the compression produces an enhancement of the highest one-electron level by an amount E, = E, +N,E,, measured from the highest group level. 3P 2 3 4 5 6 7 0 9 10 ria 0 Fig. 3 Orbital compression curves for vanadium 3379 When E, has increased to match the binding energy of the highest valence level, ionization occurs, and the correspond- ing value of ro represents the ionization radius.This mechanism is a function of the simplifying assump- tions, made to enable integration of the radial equation and is not unique to compressed atoms. A parallel situation exists for a free atom in an ionizing field, V;.. The two systems have the same ground state, i.e. ro = co or V;. = 0. It is only in this state that the quantum numbers n and 1 are integers.'P2 In all other states they are only assumed to be integral and assigned the same ground-state orbital occupation numbers. This raises the energy levels, rather than redistributing elec- trons to excited levels. Furthermore, for ionization to occur it is necessary for one orbital energy term, E = (l/n*) to approach zero and thus for an effective quantum number n -,co.The implied, infinite number of excited states that feature in this process are never considered in spectroscopic analyses, and transitions are approximated by the FGR pro-cedure. In the present instance it is not the total energy, but only the excess-over-ground-state, or compression shift, that represents excitation and contributes to the one-electron pro- motion. This means, that the monopositive ion produced in the process has the same ground state (at ro)as the free atom, except for the loss of one electron. This is the best reason for not moving away from the free-atom orbital structure in the course of the calculation.This calculation cannot produce ionization energies because the multiplet structure is neglected. Only group potentials can be obtained and these correspond to atomic ionization energies only for singly occupied valence levelsg Alternatively the compression calculation can be repeated for monopositive ions to calculate ionization energies by differ- ence. This has not been done because the ionization radii are considered to provide a more meaningful parameter charac- teristic of the chemical behaviour of atoms. Results and Discussion The program' was finally modified to increase the compres- sion steadily and at each stage to calculate the contribution from all levels to the overall promotion energy. This enables the ionization radius to be determined by a simple graphical procedure, as shown in Fig.4 for the oxygen atom. By this procedure the ionization radii of all atoms, He-Ha, were determined, using 1985 IUPAC electronic configurations. 1.o \0.5> E \> ionization radius .... . ..,....... ..............................................0.0 C9 CI -g -0.5; O -1.0 -1.5 2 3 4 5 6 7 8 9 10 rla0 Fig. 4 One-electron compression curve for the valence level of oxygen 3380 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Calculated ionization radii/A ~~ 1H 2 He 0.98 0.30 3 Li 4 Be 5B 6C 7N 80 9F 10Ne 1.25 1.09 1.62 1.60 1.56 1.45 1.36 1.20 11 Na 2.73 12 Mg 2.36 13 A1 2.61 14 Si 2.40 15 P 2.20 16 S 2.05 17 C1 1.89 18 Ar 1.81 19 K 20 Ca 21 Sc 22 Ti 23 V 24Cr 25 Mn 26 Fe 27Co 28 Ni 29Cu 30 Zn 31 Ga 32Ge 33 As 34 Se 35 Br 36 Kr 3.74 3.26 3.13 3.01 2.95 2.98 2.94 2.87 2.85 2.86 2.85 2.78 3.29 2.94 2.62 2.40 2.28 2.12 37 Rb 4.31 38 Sr 3.83 39 Y 3.55 40 Zr 3.32 41 Nb 3.30 42 Mo 3.21 43 Tc 3.16 44 Ru 3.13 45 Rh 3.08 46 Pd 2.49 47 Ag 3.04 48 Cd 3.02 49 In 3.55 50 Sn 3.26 51 Sb 3.01 52Te 2.81 53 I 2.60 54 Xe 2.49 55 cs 4.96 56 Ba 4.48 57 La 4.13 72 Hf 3.83 73 Ta 3.57 74 W 3.42 75 Re 3.38 76 0s 3.37 77 Ir 3.23 78 Pt 3.16 79 Au 3.14 80 Hg 3.12 81 TI 3.82 82 Pb 3.47 83 Bi 3.19 84 Po 2.97 85 At 2.84 86 Rn 2.66 87 Fr 88 Ra 89 Ac 104Rf 105 Ha 5.35 4.83 4.47 4.08 3.91 58 d' 65 d' 4.13 4.18 58 Ce 59 Pr 60Nd 61 Pm 62 Sm 63 Eu 64Gd 65 Tb 66 Dy 67 Ho 68 Er 69Tm 70Yb 71 Lu 4.48 4.53 4.60 4.56 4.56 4.06 4.22 4.59 4.56 4.63 4.63 4.62 4.66 4.24 90Th 91 Pa 92 U 93 Np 94 Pu 95 Am 96 Cm 97 Bk 98 Cf 99 Es 100 Fm 101 Md 102 No 103 Lw 4.24 4.51 4.53 4.51 4.85 4.85 4.68 4.46 4.44 4.56 4.41 4.40 4.32 4.36 In addition, the following alternative configuration^,^ were also considered; Ni : [Ar](3d)'(4s)' ; Ce : [Xe](4f)'(5d)'(6~)~; Tb : [Xe](4f)8(5d)1(6s)2; Bk-No : [Rn](5f)8-13(6d)'(7s)2.The two Ni configurations gave the same result. The calculated ionization radii are given in Table 2 and are also displayed graphically in Fig. 5. An inverse relationship with first ionization energies is immediately obvious.In fact, Fig. 5 can be reproduced almost exactly using ionization energies, provided different scales are used over different intervals of atomic number, i.e. 1-4, 5-10, 10-18, 18-(21-30)- 36, 36-(39-48)-54, 55-[(57-71)71-80]-86, 78-89. The inter- vals in parentheses require different scales again. However, a plot similar to Fig. 5, but using ionization energies on a common scale, does not reveal the same periodic relation- ships as Fig. 5, which reflects known chemistry rather well. For instance, the periodic order is wrong for the alkali metals, the aluminium group and for the transition series. It would seem that ionization energies, which are derived from atomic spectroscopy, are not as reliable an index of the chem- istry of atoms in crowded environments, as the ionization radii.It is proposed that in a critical region, close to the ioniza- tion limit, environmental crowding can cause the transfer of excess energy to a single electron, which can then overcome the nuclear attraction and initiate chemical interactions. This is the promotion state of valence theory, and an atom in this 1 I 0 2 4 6 8 10 12 14 16 18 atomic number, n 97 do 98 do 99 do 100do 101 do 102 do 4.82 4.88 4.84 4.81 4.80 4.71 state is best described in terms of a valence electron, decou- pled from the nucleus, and a monopositive core. The valence electron is confined to the sphere r d ro and described by the spherical Bessel functions that tend to zero at ro.The first few of these, as normalized wavefunctions, are j, = 0.2251k3l2 sin(kr)/kr j, = 0.1928k3/2[sin(kr)-kr co~(kr)]/(kr)~ j2= 0.1742k3/2[3 sin(kr) -kr cos(kr) -(kr)2~in(kr)]/(kr)~ with first zeros at ro = h/k, where Lo = 1, ;I.1 = 1.4303 and A2 = 1.8335. The freed (valence) electron is described by a spherically symmetrical state function made up of a normalized linear combination of spherical Bessel functions, Y = N c c,j, I For states of spherical symmetry the values of c, depend on the number of spherical harmonics Yl,, uiz. N, = 21 + 1 and cl cc (Nl)-1/2.In chemistry, these are the linear combinations sp3, sp3d5, etc. The radial part of these functions is shown in Fig. 6. The range of 1, within 0 < I < 3, depends on the orbital angular momentum of the valence electron and the 0.10 I I 0.09 0.08 0.07 c 0.06 5 0.05.- 2 0.04 0.03 0.02 0.01 0.00 -0.01 I I I i 0 1 2 3 4 5 6 rla0 Fig.5 Variation in the atomic ionization radii with atomic number, Fig. 6 Linear sums of normalized spherical Bessel functions trun- n;n = 0 (a),18 (b),36 (c), 54 (d),68 (e)and 86 (f) cated at r, = 5.6 a, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 coefficients further depend on the details of the resonance that produces the valence state. The details will therefore differ between atoms, but all valence-state functions will have several features in common. They will be nodeless functions between 0 and ro ,slowly varying and thus representing states of low kinetic energy.This explains why the properties of electron-pair bonds could be calculated' rather well from atomic wavefunctions like Y = (3~/4nn)'~~(l/r~)exp[-(r/r~)P].In this expression c is a constant, n is the principal quantum number of the valence electron and ro is an empirically established atomic radius. The correspondence between these radii and those newly established here, is almost exact. It is noted in conclusion that the same procedure can now be developed without resort to any empirical parameters. The ionization radii which describe the response of electronic charge clouds to environmental crowding are calculated from first principles. No other parameters are needed in order to formulate valence-state wavefunctions, and from these, the properties of chemical bonds can be calculated directly.Further study of the wavefunctions in the critical region around ro is now required to establish the correct valence- state functions for each atom. Note how Sommerfeld and Welker2 also identified this important region for further study and showed that it could be approximated by Bessel functions. An Alexander von Humboldt Research Award made this study possible, and is gratefully acknowledged. References A. Michels, J. de Boer and A. Bijl, Physica, 1937, 4, 981. A. Sommerfeld and H. Welker, Ann. Phys., 1938,32, 56. S. R. de Groot and C. A. ten Seldam, Physica, 1946,12,669. P. Plath, Dissertation, Technische Universitat Berlin, 1972. P. 0.Froman, S. Yngne and N. Froman, J. Math. Phys., 1987, 28, 1813. 6 J. C. Slater, Phys. Rev., 1951,81, 385. 7 F. Herman and S. Skillman, Atomic Structure Calculations, Prentice-Hall, New Jersey, 1963. 8 J. L. Martin, Basic Quantum Mechanics, Clarendon Press, Oxford, 1981. 9 C. F. Fischer, The Hartree-Fock Method for Atoms. A Numeri- cal Approach, Wiley, New York, 1977. 10 Periodic Table of the Elements, VCH, Weinheim, 1986. 11 J. C. A. Boeyens,S. Afr. J. Chem., 1980,33, 14; 63. Paper 4/03868A; Received 27th June, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003377

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(22), 3383-3390 3383 Conformation Properties of Buta=l,3-diene-l,4=diones(Bisketenes) : Computational and Photoelectron Spectroscopic Studies Nick H. Werstiuk" and Jiangong Ma Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M 1 Michael A. McAIlister, Thomas T. Tidwell * and Da-chuan Zhao Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A I The highest occupied molecular orbital (HOMO) energies of a series of monoketenes RCH=C=O with a variety of representative substituents have been calculated by a6 inifio methods, and give good agreement with avail- able experimental photoelectron ionization energies. The structures and orbital energies of the monoketenes Me,SiCH=C=O (5) and ButMe2SiCH=C=0 (6), the alkene ButMe,SiCH=CH2 (7) and the bisketenes (Me,SiC=C=O), (1) and (ButMe2SiC=C=O), (4) have also been calculated by a6 inifio methods, and are compared with experimentally measured photoelectron ionization energies.The spectra of the ButMe2Si com- pounds show a characteristic band associated with the But-Si bond. Comparison of the measured and calcu- lated spectra provides strong evidence that the bisketenes 1 and 4 exist predominantly in twisted conformations, with dihedral angles 105" in the former case and 120" in the latter. Dipole moment measurements on 1 and 5 confirm this conclusion. The preparation of the unique stabilized and persistent biske- tene, 2,3-bis(trimethylsilyl)buta-1,3-diene-l,4-dione(1) from thermolysis or photolysis of 3,4-bis(trimethylsilyl)cyclobut-3-+ A€= ene-1,2-dione was recently reported [eqn.(l)]'". The reacti- 49.7kJ mi-vity of 1 has been studied experimentally,'b and the effect of substituents on the ring opening of 2 and the structures and stabilities of the resulting bis(ketene) conformations 3a-3c (R = H, SiH, ,F) [eqn. (2)] have been examined by computa- tional methods.lc The availability of bisketene 1and its analogue 2,3-bis(tert- butyldimethylsilyl)buta-1,3-diene-1,4-dione (4)lb as well as the -monoketenes (trimethylsily1)ketene (5) and (tert-butyl-I dimethylsily1)ketene (6) provided a unique opportunity tow3sKMe3Si @(4) \SiMe, gain information on the bonding and conformational behav- 0 iour of silylketenes both by calculation, using ab initio theory, 1 and experimentally by means of ultraviolet photoelectron (PE) spectroscopy.The study also is timely because the con- RYCGo formational properties of butadiene systems are topics of major current and PE spectroscopy has been of particular value in these investigations,2 including exami- nations of silyl-substituted alkenes and diene~.~ Reported 2 3a herein are the results of theoretical and PE studies of 1,4,5,6 and (tert-butyldimethylsily1)ethene (7).The alkene 7 was studied along with the ketenes in order to determine the R CGo + behaviour of this tert-butyldimethylsilyl system relative to that of other ~ilylalkenes,~ including (trimethylsilyl)ethene, + Yo y.,CQ0 and for comparison of the effects of these two substituents with those on the ketenes.3b 3c n These studies have shown that the strong ketene stabilizing Bu'Me,Si HV effect of silyl substituents on monoketeneldqe is also mani- fested in bisketenes, although the effect of the substituents on Me,SiCH=C=Othe cyclobutenedione structure 2 influences the equilibrium as well.lc The parent bisketene structure 3a (R = H) is also cal- 5 culated to be destabilized relative to monoketenes, as illus- trated by the calculated exothermic energy change of 49.7 kJ mol-' in the isodesmic reaction of eqn. (3). Photochemical decarbonylation is a characteristic reaction of ketenes and bisketenes'"Vbvf and leads to alkynes in the ButMe2SiiH= C= 0 Bu'Me,SiC H=CH2case of 1and 4.'"*b9fThermal decarbonylation of 1 and 4 has not been studied, but is not observed below 150 "C.6 7 Results and Discussion Calculations The ab initio calculations on the ketenes 1, 4, 5, 6 and the alkene 7 were carried out at McMaster University on IBM RS/6000 model 350 and 530 computers with GAUSSIAN 92.'t Because of the size of the molecules, the direct-SCF method was used in the calculations. The AM1 calculations were carried out at McMaster using AMPAC Version 2.106 running on an MIPS 2030 workstation. All optimizations were carried out using the keyword PRECISE to tighten the convergence criteria. Synthetic spectra were calculated from the MO results on a SUN 3/60 computer with a FORTRAN program PESPEC.,' The calculations on the monoketenes in Table 1 were carried out at the University of Toronto with the GAUSSIAN 907 and GAUSSIAN 92' series of programs running on Hewlett Packard 9000-750 and IBM RS/6000-530 computers.Mono ketenes The orbital energies of the highest occupied molecular orbital (HOMO) of a series of monoketenes whose structures and energies were reported previously calculated at the HF/ 6-31G*//HF/6-31G* level are listed in Table 1 along with Table 1 Calculated (HF/6-3 lG*//HF/6-31G*) HOMO orbital ener- gies (E,) of ketenes (RCH-(2-0) and calculated and observed first ionization energies (EJeV) R -EJEha E,(calc)/eV E,(obs)/eV ref. H 0.3584 9.75 9.64 Li 0.2829 7.70 BeH 0.3567 9.7 1 BH2 CH3 NH2 OH 0.3763' 0.3394 0.3446' 0.3471 10.24 9.24 9.38' 9.44 8.92, 8.95 F 0.3619 9.85 Na 0.267 1 7.27 MgHAlH 0.3348 0.3606' 9.1 1 9.8 1 SiH, 0.3606 9.8 1 PHZ SHc1 0.3630 0.3637 0.3570 9.88 9.90 9.71 9.24, 9.25 CF3 c-Pr 0.3951 0.3226 10.75 8.78 HC=C 0.3308 9.00 CH2=CH 0.3101 8.44 CHO 0.3817 10.39 C02H 0.3839 10.45 CN 0.3841 10.45 10.07 Ph NO2 0.4172 0.2928 11.35 7.97 8.06 NO 0.3897 10.60 NC 0.3646 9.92 [t=C=O 0.3379 9.19 8.78 0.3103 8.44 8.39 1 E, (hartree) z 4.35975 x lo-'' J.'Planar substituent. Pyramidal (planar 0.2877 Eh,7.83 eV). t Z-matrices of the optimized geometries of the conformers of 2,3-bis(trimethylsilyl)buta-1,3-diene-1,4-dione(la, lb and lc), 2,3-bis(di-tert-butyldimethylsilyl)buta-1,3-diene-1,4-dione (4a, 4b and 4c), (trimethylsily1)ketene(5a and 5b), (tert-butyldimethylsily1)ketene (6a, 6b and 6c), (tert-butyldimethylsily1)ethene (7a, 7b and 7c), along with labelled structural formulae, are available as supplementary material (supplementary publication SUP 57035, 13 pp.), deposited with the British Library.Details are available from the Editorial office. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 calculated and available experimental values of the ionization energy (EJ of the The HOMO of these ketenes is the ncco of the ketenyl moiety. In the case of the mono- substituted ketenes the calculated MO eigenvalues (orbital energies) give calculated Ei values that are 0.1-0.48 eV higher than the experimental values, a result that is usually seen when eigenvalues based on Koopmans' theoremg are corre- lated with the experimental Ei .lo" That the eigenvalues are higher than the Ei values has been attributed to the fact that electron correlation and molecular relaxation are not taken into account in the Hartree-Fock calculations."" Even so, the orbital energies of CH,=C=O, CH,CH=C=O and CH(CN)=C=O calculated at the HF/6-3 1G* level of theory reproduce the increase in the Ei of the HOMO found experi- mentally for the strong n-acceptor group CN.8" Thus these results permit for the first time the systematic evaluation of the effect of substituents on the ketene Ei.Many substituents give calculated values in the range of 9.1-9.9 eV (H, BeH, CH,, NH,, OH, F, MgH, AlH,, SiH,, PH, SH and Cl). Values outside this range include the strong n-acceptor groups BH,, CH=O, CO,H, CEN and NO,, which have Ei values between 10.24 and 11.35 eV, and would stabilize the HOMO.A discussion of the importance of n-acceptor effects in ketenes has been presented elsewhere."' The calculated Ei of CF,CH=C=O (10.75 eV) is also quite high. This effect evidently arises from the strong a-withdrawing effect of the CF, group which stabilizes the HOMO. Experimentally (CF,),C=C=O also has a high first Ei at 10.95 eV.'* This effect is not so great with the a-acceptor F and C1 substit- uents, since these also act as n donors which are destabi- lizing.ld The very low Ei values for LiCH=C=O and NaCH=C=O (7.70 and 7.27 eV, respectively) reflect the high ionicity of the metal-carbon bonds, and the consequent high electron density of the ketenyl n system.The groups CH,=CH, HC"-C, Ph and c-Pr are all below 9.0 eV. These groups can act as n donors and this effect is destabilizing to the HOMO. The only ketene bearing a strong a-acceptor group that has been studied with PE spectroscopy is ClCH=C=0,8"' and the stabilization of the HOMO of this ketene is reproduced by calculation at the HF/6-31G* level of theory. These trends follow the effects of substituents on the photoelectron spectra of substituted benzenes, in which the ionization energies of the HOMO generally increase with the electron-accepting ability of the substituent.'Ob*c Trimethylsilylketene (5) The energies of the monoketene Me,SiCH=C=O (5) were calculated with the STO-31G and 6-31G** basis sets for the s-cis (5a) and s-trans (5b) conformations (Table 2).The s-cis conformation was the most stable at both levels of theory. At the highest level examined (HF/3 lG**//HF/6-31G**, with forced C,symmetry) it is 3.93 kJ mol- lower in energy than 5b.It was also found that the effect on the energy of fixing the C-C-0 angle to linearity (179.99') was very small (0.1 kJ mol- '). \/.+. ,H (343%. n---d / H' 5a 5b Values of the zero-point energy (ZPE) of 5a and 5b were also calculated to ensure that the total energies are good measures of the relative thermodynamic stabilities of silylke- tene conformations and the values scaled" by 0.9 are given J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Calculated total and relative energies of conformers of 5, 6 and 7 relative energy conformer total energy/& /kJ mol-' 5a, s-c~s'~ -552.312602 0.00 5b, s-tranP -552.3 11 901 1.84 5a, s-cisbd -558.979 237f 0.00 5a, s-cisa*b*d -558.979 237/ 0.00" 5a, ~-cis'*~J -558.979 287 --0.13 5b, s-tranf"b*d -558.977 742 3.939 5a, s-c~s'.~,~ -558.974 649 5b, s-trans"*b*h -558.973 050 4.20'6ab,d.J -676.080 264 0.006bb.d.k -676.078 292 5.18 6bd*' -676.078 3 12 5.12 kb.d.rn -676.078 012 5.92 7b, (O.O)d*n -602.372 564 9.98" 7, (60.0)d*" -602.372 990 7.86" 7, (90.0)d*" -602.375 173 2.12" 7a, (1 15.6)'." -602.375 98 1 0.00" 7, (140.0d." -602.372 215 2.01" 7c, (180.0)d." -602.373 513 6.48" 'Forced C, symmetry.C=C=O bond angle fixed at 179.99'. HF/STO-3G//HF/STO-3G. dHF/6-3 lG**//HF/6-3 1 G**. " ZPE 375.7 kJ mol-'. C-C-0 bond angle optimized (178,62"). ZPE 375.4 kJ mol-'. HF/6-31G**//HF/STO-3G. Relative to s-cis. j Methyl group s-cis to C-C-0. But group s-cis to C-C-0. The C-C-0 bond angle was optimized (179.15'). rn But group s-trans to C-C-0. " The dihedral angle between the carbon of the tert-butyl group and the plane of the vinyl group. " Relative to con- former 7a. No symmetry (C,) constraint. in Table 2. That the ZPE of 5a is only 0.32 kJ mol- ' higher than the ZPE of 5b shows that it is possible to use uncor- rected total energies to study conformational properties of silylketenes. The STO-3G basis set was used in order to ascertain whether the geometries obtained with a minimal basis set are similar to those obtained with the HF/6-31G** basis set and to establish whether the AE obtained with HF/ 6-31G**//HF/STO-3G calculations is comparable to that obtained at the higher level of theory (HF/6-31G**//HF/6- 31G**).A good correlation would suggest that the conforma- tional behaviour of large bis(ketenes) such as l and 4 can be reliably studied at the HF/6-3 lG**//HF/STO-3G level of theory. The CH,-Si bond lengths (1.889 A) calculated with the 6-31G* basis are 0.024 A longer than those obtained with the minimal basis set (1.865 A). The other geometrical parameters (bond lengths, bond angles and dihedral angles) calculated with these basis sets are similar for both con-formers.The calculated C-Si bond lengths compare closely to the bond lengths of alkyl- (1.864 & 0.008 A) and aryl- silanes (1.84 & 0.01 A) obtained with X-ray crystallographic structure determinations.' These results, coupled with the fact that the AE obtained with HF/6-31G**//HF/STO-3G calculations is nearly identical to the AE obtained at the HF/ 6-31G**//HF/6-31G** level of theory, suggest that it is pos- sible to obtain reliable results for large bis(ketenes) at the HF/6-31G**//HF/STO-3G level of theory. Based on the eigenvectors (orbital coefficients), the HOMO of 5a is a n-type MO localized predominantly on the C-C bond (C,, 0.41 and C,, 0.21) and the oxygen (0.29), but with some mixing with the orbitals of the (CH,),Si group, specifi- cally the out-of-plane methyls (0.1 1) and the Si (0.08).Because the HF/6-31G** basis set is a split-valence basis set, only the largest coefficients are given. The further orbitals HOMO-1, HOMO-2 and HOMO-3 have substantial Q C-Si character. The LUMO has the largest coefficients on Si (0.30), the C=C bond (C, , 0.27 and C,, 0.55), the in-plane methyl (0.39) and the ketene C-H (0.69). It has been suggested based on a frontier molecular orbital analysis that it is the relatively low HOMO-LUMO gap and the size of the orbital coefficients at the oxygen of the HOMO and the carbonyl carbon of the LUMO (or at the methylene carbon of the HOMO and the carbonyl carbon of the LUMO) that determine the rates and regiochemistry of the dimerizations of CH,=C=O and Me,C=C=0.'3 However, the calculated HOMO-LUMO gap of Me,SiCH=C=O (5) is 13.53 eV and that of CH,=C=O is 13.67 eV at the HF/6-31G** level, so the lack of reactivity of the former cannot be ascribed to a large HOMO-LUMOgap.(tert-ButyldimethyZsilyl)ketene (6) The conformations 6adc of (tert-butyldimethylsily1)ketene(6) were studied, and the conformer 6a with a methyl group cis to the ketenyl group was found to be the most stable by more than 5 kJ mol-' with the HF/STO-3G and HF/6-31G** basis sets. The total and relative energies obtained with the HF/6-31G** basis set are given in Table 2. Here, as in the case of 5, fixing the C=C=O bond angle to 179.99" to mini- mize the cpu requirements has little effect on the energy.At this level of thoery, the But-Si-C-C dihedral angle of 6a is 115.37", and the dihedral angles between the C-C bond and the in-plane and out-of-plane methyl groups are 6.0" and 123.9", respectively. H' 6a 6b \ /c=c=oH' 6c The But-% bond lengths of all three conformations are about 0.03-0.04 8,longer than the CH,-Si bonds, yet these bond lengths (1.920, 1.921 and 1.919 A) are nearly identical. Thus the But-Si bond length is considerably longer than the CH,-Si bond, but there is no significant dependence on the Bu'-Si-CH=C torsional angle. The orbital energies of the four highest occupied molecular orbitals as a function of the Bu'-Si-C=C dihedral angle, along with the fitted curves calculated with PESPEC," are displayed in Fig.1. As found in the case of Me,SiCH=C=O (59, the HOMO of 6a is a n-type MO with the largest coefficients being located on the C=C bond (C,, 0.35 and C,, 0.19), the oxygen (0.25), the silicon (0.096), the But carbon (0.14) and the silyl-methyl carbon (0.11) anti to the ketene group. The energies of HOMO-1 of 6a4, which basically correspond to the But-Si r~ bond, range from 10.6 to 10.8 eV, whereas HOMO-1 of 5a and 5b, which correspond to the a-type MOs of the CH,-Si, are 11.8 eV, or 1 eV higher. The shift to lower Ei values reflects the normal weaker But-X bond strength compared with CH,-X. For the LUMO, the largest coefficients are found on Si (0.26), the C=C bond (C,, 0.25 and C,, 0.52), the silylmethyl anti to the ketene group (0.73), the silylmethyl syn to the ketene group (0.39) and the ketene C-H (0.69).Conformers with the But-Si-C=C dihedral angle fixed at 130", 100" and 60" were also studied with the HF/6-31G** basis set to obtain a potential for twisting about the R,Si-C bond (plot not shown). -9.0 r 1 n P. Y " <) -9.5'! HOMO L Q) -11.0 HOMO-I m c.-t -1 2.0 -1 2.5 0 20 40 60 80 100 120 140 160 180 dihedral angle/degrees Fig. 1 Angular dependences of the MO orbital energies of the HOMO, HOMO-1, HOMO-2 and HOMO-3 of 6 (tert-ButyZdirnethylsi2yl)ethene (7) Six conformers of 7 were studied at the HF/6-31G**//HF/6- 31G** level of theory.As found in the case of 6, the con- former with a methyl group cis to the vinyl group is the lowest-energy geometrical structure. CH3 FH3 v-7a 7b 7c The total energies of the six conformers and the energies relative to 7a are given in Table 2. For 7a the dihedral angle between the But group [C(4)] and C(l) of the vinyl group is 115.6'; for the cis methyl group, the dihedral angle is 6.15'. These values are virtually identical to the parameters calcu- lated for 6a. The potential for driving the Bu'-Si-C=C dihedral angle calculated from the relative energies (plot not shown) with PESPEC resembled that found for 6 except that the AE (9.98 kJ mol-') between 7a and 7b was greater than that (5.18 kJ mol- ') between the ketene analogues 6a and 6b.This difference is undoubtedly due to the fact that the steric interaction between the But group and the vinyl group, spe- cifically the vinyl hydrogen at C(1), which is syn to the But group, is greater than the steric interaction between the ketene and Bu' groups of 6a. That the calculated C-C-Si bond angle of 7b is larger (130.8') than the C-C-Si bond angles of 7c (124.3") and 6a (123.0") provides support for this proposal. Moreover, the AE between 7a and 7c is 6.48 kJ mol-', close to the value (5.92 kJ mol-') found for 6a and 6c. As found in the case of 6, the HOMO of 7a is a n-type MO with the largest coefficients being located on the double-bond carbons, the Bu' quaternary carbon, the carbon of the in- plane methyl and on silicon. HOMO-1 also is a n-type MO with the largest coefficients located on the same atoms as the HOMO.A display of the MO energies of the four highest occupied orbitals as a function of dihedral angle is given in Fig. 2. As in the case of 6, the fitted curves were calculated with PESPEC. That the HOMO-HOMO-1 gap increases smoothly to 0.91 eV from 0.48 eV when the But-Si-C=C dihedral angle is decreased to 90" from 180" suggests that there is a very weak c-n interaction between the But-Si bond and the 7c system. As found for 6, the But-Si bond of 7 is significantly longer than the CH,-Si bonds in all the con- formations studied by calculation. Even so, the Bu'-Si bond length of the 90" conformer is virtually identical (1.925 A) to J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 -9.51 .l -1 2.010 20 40 60 80 100 120 140 160 I80 dihedral angle/degrees Fig. 2 Angular dependences of the MO orbital energies of the HOMO, HOMO-1, HOMO-2 and HOMO-3 of 7 the Bu'-Si bond lengths of 6b (1.924 A)and 6c(1.922 A); the C=C bond lengths (data not given) also show no significant dependence on this dihedral angle. According to the results of the calculations, 6 and 7 exhibit similar conformational properties and orbital interactions, but based on the angular dependence of the HOMO-HOMO-1 gap, the u-n inter-action is greater in the case of 7 than for 6. Undoubtedly, this is due to the fact that the HOMO-HOMO-1 gap is smaller in 7 than in 6 (see Fig. 1 and 2). 2,3-Bis(trimethylsilyl)buta-~,3-diene-l,4-dione(1) The conformational behaviour of bisketene 1 was studied with semiempirical (AM1)6 and ab initio methods and the relative energies as a function of the C-C-C-C dihedral angles are given in Table 3.The potential obtained from the AM1 energies that were calculated with no symmetry con- straints, as displayed in Fig. 3, indicates that conformers with C=C-C=C dihedral angles ranging between 70" and 90" are expected to be the most stable geometrical structures in the gas phase. Ab initio calculations were carried out with STO-3G and 6-31G** basis sets with the C-0, C=C, C-Si, C-C and C-H bonds of one-half of 1 set equal to the corresponding bonds of the other R,Si-CH=C=O fragment, and the C=C=O bond angles fixed at 179.99'.Data obtained with these basis sets (only the 180", 120', 105', 90" and 45" conformers were studied at the 6-31G** level of theory) are given in Table 3. The ab initio potentials for twist- ing about the C(2)C(3) 0 bond of the bisketene moiety are displayed in Fig. 3. As found with AM1, twisted conformers are lower in energy than the s-transconformer la, but unlike the results obtained with AM1, at the 6-31G** level the 105' 50 I45"-.ici40 ''k. dihedral anglejdegrees Potentials for twisting about the bisketene moiety of 1calcu-lated with PESPEC: (a) AMl; (b) STO-3G; (c) 6-31G**; (d) 6-31G**//STO-3G J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Total and relative energies of conformers 1 and 4 Relative energiy/kJ mol -' Ah4 1 STO-3G//STO-3Gb 6-31G**//STO-3Gb 6-31G**//6-13G**b dihedral angle 1 1 4 1 4 1 ~~ 0 28.97 35.89 56.16 46.26 58.69 46.03 15 24.53 28.62 54.85 38.98 57.77 30 16.83 24.53 40.35 32.86 39.39 45 9.24 23.91 28.27 28.70 24.30 28.44 60 3.35 19.15 18.71 14.03 13.86 75 0.0 10.89 11.80 6.21 7.27 90 0.33 4.85 7.33 1.78 4.60 1.59 105 1.93 1.13 4.83 0.00' 2.82 0.W 120 6.91 0.00" 1.30 0.85 OW 0.87 135 15.20 1.OO 0.008 4.82 0.56 150 24.03 1.90 0.53 10.41 2.99 165 27.93 1.01 1.80 11.78 5.55 180 30.69 0.23 2.39 1 1.07 6.51 11.54 'Relative to the calculated &H of the 75" conformer (-460.23 kJ mol-I).* Relative to the calculated total energy of the lowest-energy conformer.'E = -1116.777386 Eh.d E = -1116.786744 Eh.eE = -1103.478881 Eh.f E = -1350.971966 Eh.@E = 133.934418 Eh.conformer is the lowest-energy geometrical structure; with the STO-3G basis set, the 120" conformer is the lowest- energy geometrical structure. In the case of s-cis conformer lc, in which the C-C-C=C dihedral angle is fixed at 0", a steric interaction twists the trimethylsilyl groups out of the n plane by 14.5'. Me,Si, $C50 C I Me,Si ,cI'C50 0 ~e,si, 5~5 C C5C'SiMe, 0" la lb lc Single-point calculations for 1 were also carried out with the 6-31G** basis set on the STO-3G optimized geometries and the relative energies are also compiled in Table 3. As in the case of 5, these calculations were carried out to establish whether the potential obtained at the HF/6-31G**//HF/ STO-3G level of theory parallels that obtained with the HF/ 6-31G** basis set geometry.The STO-3G potential (Fig. 3) is flatter, with the s-trans conformer similar in energy to the twisted conformers, compared with the HF/6-3 1G** potential in the region between 180" and go", while the HF/6-31G**// HF/STO-3G potential is very similar to that for the HF/6- 31G** geometry. These results are in accord with the data obtained for 5 and suggest that calculations at the 6-31G**// STO-3G level can be reliably used to study the conforma- tional properties of bisketenes with substituents on silicon larger than hydrogen, and can be usefully extended to Bu'Me,Si as well. Because bisketenes 1 (six methyls) and 4 (10 methyls) each have a large number of conformational degrees of freedom, hypersurfaces are required to describe the torsional potentials accurately.Even so, the calculated two-dimensional poten- tials trace minimum-energy paths through the hypersurfaces. Conformational isomerism about the Si-C bonds also will not affect the HOMO-HOMO-1 splitting or the low-lying Ei levels of the Si-C bonds. Based on the eigenvectors obtained with the HF/6-31G**// HF/6-31G** calculations, the HOMO of the 105" twisted conformation of the bisketene (lb) is basically a z-type MO. The largest coefficients are found on the C-C-0 carbons and oxygens; there is a small involvement of Si orbitals. HOMO-1 also has n character involving both ketene groups with a slightly larger involvement of But-Si orbitals than in the case of the HOMO.The orbitals HOMO-2, HOMO-3, HOMO-4 and HOMO-5 are basically a-type C-Si orbitals of the But-Si group. The LUMO is highly delocalized with involvement of the ketene n systems, the silicons and the two in-plane methyl groups. At the 6-31G** level, the HOMO- HOMO-1 splittings calculated for the 180', 150" and 90" conformers are 3.05, 0.88 and 0.34 eV, respectively. A display of the orbital energies of the four highest molecular orbitals as a function of the O=C=C-C=C=O dihedral angle is given in Fig. 4. The fitted curves were calculated with PESPEC, and this is the reason for the apparent avoided crossing at a torsional angle near 80'. However, it is expected that these curves should cross because the $ositive com-bination of the s-cis conformer should transform to a nega- tive combination in the s-trans conformer, and an estimated crossing is shown with dashed lines.2,3-Bis(di-tert-butyldimethylsilyl)buta-1,J-diene-l,l-dione (4) Owing to the size of bisketene 4, optimized geometries were obtained with the STO-3G basis set as the C-C-C-C dihedral angle was decreased incrementally by 15" starting at 180". The relative energies of 4 calculated at the HF/STO- 3G//HF/STO-3G and HF/6-3 lG**//HF/STO-3G levels of theory as a function of this dihedral angle are listed in Table 3, and were used to obtain the potentials displayed in Fig. 5. As found in the case of 1, the STO-3G calculations produced -1 1.51 1 - '*'Oo 20 40 60 80 100 120 140 160 180 dihedral angle/degrees Fig.4 Angular dependences of the MO orbital energies of the HOMO, HOMO-1, HOMO-2 and HOMO-3 of 1 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 dihedral angle/degrees Fig. 5 Potentials for twisting about the bisketene moiety of 4 calcu-lated with PESPEC: (a)STO-3G; (b)6-31G**/STO-3G a potential that is flat in the region between 180 and 90". However, at the HF/6-31G**//HF/STO-3G level of theory, there is a single minimum in a deeper well and the most stable conformations of 4 have C=C-C=C dihedral angles in the region of 120". In the case of 1 the most stable confor- mation has a dihedral angle of 105" at the HF/6-31G**//HF/ 6-3 1G** and HF/6-31G**//HF/STO-3G levels of theory. Displays of the eigenvalues of HOMO, HOMO-1, HOMO-2 and HOMO-3 as a function of the dihedral angle are given in Fig.6. The estimated crossing of the HOMO and HOMO-1 curves is shown as dashed lines. 4a 4b ButMezSi,c5 c50 I Bu'Me,Si Nc3C+o 4c Photoelectron Spectroscopic Studies and Synthesis of Spectra The ultraviolet photoelectron spectra of the monoketenes 5 and 6 and the bisketenes 1 and 4 are shown in Fig. 7. The PE spectrum of the alkene 7 is shown in Fig. 8, and selected ver- tical ionization energies are given in Table 4, together with b -8.5 t>, -9.0 rg -9.5'I:-10.0 i.2 I--.--I-1 2 .o'0 20 40 60 80 100 120 140 160 180 dihedraI a ng le/deg rees Fig. 6 Angular dependences of the orbital energies of the HOMO, HOMO-1, HOMO-2 and HOMO-3 of 4 8 10 12 14 16 8 10 12 14 16 binding energy/eV binding energy/eV Fig.7 UV photoelectron spectra: (a)5, (b)6,(c) 1, (d) 4 r I 8 9 10 11 12 13 14 15 16 binding energy/eV Fig. 8 UV photoelectron spectrum of 7 the calculated HOMO energies for comparison. The mono- ketene Me,SiCH=C=;O (5)exhibits a single band centred at 8.95 eV (calculated 9.43 eV) and a group of three overlapping bands centred at 11.0 eV corresponding to the three Si-CH, bonds which are resolved from the main group of c bands [Fig. 7(a)].The relative values of the MO eigenvalues cor- relate well with the relative E, values. The general features of the PE spectra of Bu'Me,SiCH=C=O (6) and of Me,SiCH=C=O (5) are the same except that there is a Table 4 Selected experimental and calculated vertical ionization energies ionization energy/eV calculated other observed compound experimental (HOMO) bands ~ ~ ~~ ~ ~ 5 8.95" 9.43 three bands centred at 11.0 eV 6 8.79" 9.85" 10.799.36 ifour or five bands centred at 11.5 eV 7 9.16" 9.77 10.lb 10.62 10.4' 1 1.2b 1 8.4' 9.1b 9.75'8.87c ifour or five bands centred at 12.0 eV 4 7.7b 8.80d two bands centred 9.6' 10.1 Id at 9.7 eV ~ Estimated error: k0.025 eV.'Estimated error: k0.05 eV. '105" conformer. 120" conformer. 'Estimated from the shoulder on the band centred at 9.7 eV. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3.5, I ionization energy/eV Fig. 9 Display of the synthetic partial UV photoelectron spectrum of 6 calculated with PESPEC second low Ei band at 9.85 eV (calculated 10.79 eV) for the former corresponding to the But-% bond, as predicted by these calculations.The MO eigenvalues correlate remarkably well with the relative values of the vertical Eis. For 1 and 4-7 the absolute values of the eigenvalues of the highest occupied MOs are, on average, 0.61 eV higher (Fig. 1 and 2) than the Ei values (Table 4).This is consistent with the results for the other monoketenes (Table 1) for the same reasons. The syn- thetic spectrum of 6 obtained with PESPEC and the poten- tial given in Fig. 1 is displayed in Fig. 9. Only the eigenvalues of six MOs (HOMO to HOMO-5) were used to synthesize the partial spectrum that was convoluted with a Gaussian lineshape with a full width at half height (FWHH) of 0.7; the temperature was set at 300 K.In the case of 7 the lowest E, (9.15 eV) is less than the E, values of SiH,CH=CH, (10.37)4a and MeSiH,CH=CH, (9.86)46 in keeping with the increase in the number of alkyl groups on silicon. That the partial synthetic spectrum of 6 accurately reproduces the low E, regions of the PE spectrum validates the application of a combination of ab initio calculations and PE spectroscopy to the study of the conformational behaviour of bisketenes 1 and 4. The PE spectrum of 1 as displayed in Fig. 7(c), shows two broad overlapping bands at 8.4 and 9.1 eV (AEi = 0.7 eV), in addition to the broad feature due to four or five bands centred at 11 eV and the relatively sharp band at 12.0 eV, both resolved from the main sigma envelope.The partial spectrum of 1 calculated from the HF/6-31G**//HF/6-31G** potential (see Fig. 3) and the eigenvalues of the eight highest occupied molecular orbitals from PESPEC is displayed in Fig. 10. A Gaussian convolution of 0.7 and a temperature of 300 K were used in the calculations. The synthetic spectrum G 5-4-.-C 4-v 8 9 10 11 12 13 14 ion iza tion ene rg y/eV Fig. 10 Display of the synthetic partial ultraviolet photoelectron spectra of 1 accurately reproduces the broad overlapping low-E, bands, the HOMO-HOMO-1 splitting of less than 1 eV, and the portion of the higher Ei band structure of the PE spectrum.Just as for the monoketenes 5 and 6 the calculated values are CQ. 0.5 eV higher in energy than those observed in the gas phase. The PE spectrum of the bisketene 4 is displayed in Fig. 7(d), and only one broad band centred at 7.7 eV is completely resolved from the main group of overlapping bands. That the E, of this band is lower than the low E, band of 1 is consis- tent with the expectation of a larger HOMO-HOMO-1 split- ting for 4 compared with the splitting of 1. This is a consequence of the fact that conformers with larger C=C-C=C dihedral angles (120") should be preferentially populated for 4 (Fig. 5) compared with 1 (105"). From the shoulder on the next series of overlapping bands, we estimate that the next band has a maximum at ca.9.6 eV yielding a HOMO-HOMO-1 splitting of 1.9 eV. This result is in accord with the splitting obtained from the partial synthetic spec- trum of 4 (not shown) which shows that the larger HOMO- HOMO-1 splitting causes the HOMO-1 band to overlap the low-Ei bands arising from the a-type C-Si molecular orbitals of the But-% groups, a result that correlates with the PE spectrum. The experimental and synthetic spectra provide strong evidence that 4 also prefers twisted conforma- tions in the gas phase, but with C=C-C=C dihedral angles that are larger than in the case of 1. An alternative explanation, suggested by a referee, is that the 7.7 eV band consists of both the negative and positive combinations of the ketenyl nccO orbitals, and that the 9.6 eV band of 4 correlates with the 9.85 eV band of 6,and arises from the But-% bond.This would require the C=C-C=C dihedral angle to be closer to 90" than our assignment of 120". Dipole Moments Dipole moments have frequently been used for the determi- nation of conformations in cqp-unsaturated and as an independent experimental test of the preferred confor- mation of the bisketene 1, dipole moments of 1 and the monoketene Me,SiCH=C=O (5) were determined 14b~c in cyclohexane as 2.7 f0.3 and 1.8 f0.3 D,? respectively. This large measured dipole moment of the bisketene rules out the anti-planar conformation 3a, which would have a zero dipole moment, but is consistent with a variety of other geometries ranging from near perpendicular to syn-coplanar.The mea- sured value of 2.7 D corresponds to a calculated dihedral angle of 80" between the dipoles of two monoketenes, but values of 0" to 120" fall witkin the uncertainty of the mea- surements. The HF/6-3 lG**//HF/6-3 1G** ab initio calcu-lated dipole moments of 5 and the 105" conformer of 1 are 1.86 and 2.33 D, respectively, and agree well with the experi- mental values. Conclusions We have shown for the monoketenes that the calculated HOMO energies and the ionization energies correlate well with available experimental values and may be understood in terms of the effects of these substituents on the ketenes. The calculated orbital energies and experimental ionization ener- gies of the Bu'Me,Si substituted ketene 6 and alkene 7 show a lowering by 1 eV relative to the Me,Si analogues of the HOMO-1 level, ascribed to weakening of the Si-C bond for the But group.The conformations of the bisketenes 1 D (Debye) z 3.33564 x lo-'' C m. 3390 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (RC=C=O), are calculated to be twisted out of linearity by 105" for R = Me,Si and 120" for R = Bu'Me,Si, and the experimental spectra are in agreement with this conclusion. The dipole moment of 1 rules out an anti-periplanar confor- mation, and is consistent with a twisted conformation. Experimental Khvostenko, L. M. Khalilov, Ya. B. Yasman, M. M. 'Timo-shenko, Yu. V. Chizhov, B. G. Zykov, I. I. Furlei and S. R. Rafikov, J. Organomet. Chem., 1979, 166, 169; (c) H.Bock and H. Seidl, J. Am. Chem. SOC., 1968, 90, 5694; (d) H. Bock and B. Solouki, in The Chemistry of Organic Silicon Compounds, ed. S. Patai and Z. Rappoport, Wiley, New York, 1989, ch. 9; (e) Yu. N. Pancheko and C. W. Bock, Struct. Chem., 1992, 3, 27; (f) W. F. Reynolds, G. K. Hamer and A. R. Bassindale, J. Chem. SOC., Perkin Trans. 2, 1977,971. Monoketenes 515=and 6lSband bisketenes 1lavb and 4" were obtained as has been described. To prepare 7 a solution in THF-diethyl ether-pentane of CH,=CHLi prepared as described15" from CH2=CHBr (2.3 g, 31.5 mmol) in a 50 ml round-bottom flask at -78 "C was added dropwise to a solu-tion of tert-butyldimethylsilyl chloride (4.1 g, 27.2 mmol, Aldrich) under N, at -78 "C. The mixture was kept at room temperature for four days, and 'H NMR analysis showed the reactant : product ratio to be 59 :41, indicating 52% conver-sion.The reaction mixture was washed with H20, dried over MgSO, and fractionally distilled. The fraction with bp 65- 5 6 (a)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. Andres, K. Ragh- avachari, 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, Gaussian Inc., Pittsburgh, PA, 1992; (b) W. J. Hehre, L. Radom, P. v. R. Schleyer and J. A. Pople, Ab Initio Molecular Orbital Theory, Wiley-Interscience, New York, 1986; (c) M. Dupuis, D. Spangler, J. J. Wendoloski, M. W. Schmidt and S. T.Elbert, GAMESS: An ab initio computational package, August 5, 1989 version. Dewar Research Group and J. J. P. Stewart, Austin Method 1 Package 1.0. QCPE 506, QCPE Bull., 1986,6,2. 75°C contained THF and the desired product 7, and the latter was collected by preparative VPC (3 m x 1 cm OV-17 column, 40°C) to give 7 as a colourless oil: 'H NMR, 6 (CDC1,) 0.027 (s, 6), 0.87 (s, 9), 5.67 (dd, 1, J 4.9, 19.5 Hz), 6.00 (dd, 1, J 4.9, 14.7 Hz), 6.15 (dd, 1, J 19.5, 14.7 Hz); 13C NMR, IR, v,,,(CDCl,)/cm-': 1592 (C=C); EIMS m/z142 (M', 4), 85 (M+-But, loo), 73 (22), 59 (54); HRMS m/z, calculated for PE spectra were measured with a locally constructed instrument16 by signal averaging 20-30 scans with argon as the calibrant. To ensure that the scale was linear over the 10 6 (CDCl,) -6.51, 16.28, 26.33, 132.27, 137.40; C8H18Si 142.1178, found 142.1175. 7 8 M.J. Frisch, M. Head-Gordon, G. W. Trucks, J. B. Foresman, H. B. Schlegel, K. Raghavachari, M. A. Robb, J. S. Binkley, C. Gonzalez, D. J. De Frees, D. J. Fox, K. A. Whiteside, R. Seeger, C. F. Melius, J. Baker, R. L. Martin, L. R. Kahn, J. J. P. Stewart, S. Topiol and A. J. Pople, GAUSSIAN 90, Gaussian Inc., Pitts- burgh, PA, 1990. (a)H. Bock, T. Hirabayashi and S. Mohmand, Chem. Ber., 1981, 150, 167; (b) D. Hall, J. P. Maier and P. Rosmus, Chem. Phys., 1977, 24, 373; (c) D. P. Chong, N. P. C. Westwood and S. R. Langhoff, J. Phys. Chem., 1984, 88, 1479; (d) D. Colbourne and N. P. C. Westwood, J. Chem. SOC., Perkin Trans. 2, 1985, 2049; (e) R. Sammynaiken and N. P.C. Westwood, J. Chem. SOC., Perkin Trans. 2, 1989,1987; (f)J. K. Terlouw, P. C. Burgers and J. L. Holmes, J. Am. Chem. SOC.,1979, 101,225; (9)F. Chuburu, eV range, a variable offset was used at the beginning of the scans and calibrations were also run with acetone (Ei = 9.70 eV) and 0, (Ei = 12.30 eV). Dipole moments were obtained with a Dekameter DK03 instrument (Wissenschaftlich-Technische, Werkstratten GmbH) and an Abbe refractometer by a reported mathodf4* 9 10 S. Lacombe, G. Pfistser-Guillouzo, A. Ben Cheik, J. Chuche and J. C. Pommelet, J. Am. Chem. SOC., 1991, 113, 1954; (h) R. Gleiter, R. W. Saalfrank, W. Paul, D. 0.Cowan and M. Eckert- Maksic, Chem. Ber., 1983, 116,2888. T. A. Koopmans, Physica, 1933,1, 104. (a) K. Kimura, S. Katsumata, Y. Achiba, T.Yamazaki and S. Iwata, in The Handbook of He1 Photoelectron Spectra of Funda- and calculated procedure. 14" A series of cyclohexane solu- tions of bisketene 1 and monoketene 5 were utilized to give dipole moments of 1.8 D for 1and 2.7 D for 5. mental Organic Molecules: Ionization Energies, Ab initio Assign-ments and Valence Electronic Structure of 200 Molecules, Japan Scientific Societies Press, Tokyo, 1981; (b)R. Edgell, J. C. Green and C. N. R. Rao, Chem. Phys. Lett., 1975, 33, 600; (c) A. D. Financial support by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. 11 Baker, D. P. May and D. W. Turner, J. Chem. SOC.B, 1968,22. J. A. Pople, H. B. Schlegel, R. Krishnan, D. J. DeFrees, J. S. Binkley, M. J. Frisch, R.A. Whiteside, R. F. Hout and W. J. Hehre, Int. J. Quantum Chem., Quantum Chem. Symp., 1981, 15, References 12 269. H. J. M. Bowen, J. Donohue, D. G. Jenkin, 0. Kennard, P. J. (a) D. Zhao and T. T. Tidwell, J. Am. Chem. SOC., 1992, 114, 10980; (b)D. Zhao, A. D. Allen and T. T. Tidwell, J. Am. Chem. SOC.,1993, 115, 10097; (c) M. A. McAllister and T. T. Tidwell, J. Am. Chem. SOC., 1994, 116, 7233; (d) L. Gong, M. A. McAllister and T. T. Tidwell, J. Am. Chem. SOC., 1991,113, 6021; (e) M. A. McAllister and T. T. Tidwell, J. Org. Chem., 1994, 59, 4506; (f) M. A. McAllister and T. T. Tidwell, J. Am. Chem. SOC., 1992, 114, 5362; (9) T. T. Tidwell, Ketenes, Wiley-Interscience, New York, 1994, in the press. (a) N. H. Werstiuk and G. Timmins, Can. J. Chem., 1988, 66, 2954; (b) N. H. Werstiuk, K. B. Clark and W. J. Leigh, Can. J. Chem., 1990, 68, 2078; (c) N. H. Werstiuk, G. Timmins, J. Ma and T. A. Wildman, Can. J. Chem., 1992,70,1971. (a) K. B. Wiberg, R. E. Rosenberg and P. R. Rablen, J. Am. Chem. SOC., 1991, 113, 2890; (b) K. B. Wiberg and R. E. Rosenberg, J. Am. Chem. SOC., 1990,112, 1509; (c)H. Gu and M. 13 14 15 16 Wheatly and P. H. Whiffen, in Tales of Interatomic Distances and Configuration Molecules and Ions, ed. L. E. Sutton, The Chemical Society Special Publication 11, Thanet Press, 1958. I. Fleming, Frontier Orbitals and Organic Chemical Reactions, Wiley, New York, 1976, ch. 4, pp. 143-146. (a)V. I. Minkin, in Stereochemistry :Fundamentals and Methods, ed. H. B. Kagan, Thieme Chemie, Stuttgart, 1977, vol. 2, ch. 1; (b) D. P. Shoemaker, C. W. Garland and J. W. Nibler, Experi-ments in Physical Chemistry, McGraw Hill, New York 5th edn., 1989, p. 396; (c) J. W. Smith, Trans. Faraday SOC., 1950,46,394. (a)Yu. K. Grishin, S. V. Ponomarev and S. A. Lebedev, Zh. Org. Khim., 1974, 404; (b) E. Valenti, M. A. Pericas and F. Serratosa, J. Org. Chem., 1990, 55, 395; (c) H. Neumann and D. Seebach, Tetrahedron Lett., 1976,4839. N. H. Werstiuk, D. N. Butler and E. Shahid, Can. J. Chem., 1986,65, 760. Karplus, J. Mol. Struct. (Theochem), 1993,260,347. (a) P. Mollere, H. Bock, G. Becker and G. Fritz, J. Organomet. Chem., 1972, 46, 89; (b) V. P. Yuriev, A. A. Panasenko, V. I. Paper 4/01980F; Received 5th April, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003383

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(22), 3391-3396 Explosive Decomposition of Gaseous Chlorine Dioxide Maria 1. Lopez, Adela E. Croce and Juan E. Sicre* lnstituto de lnvestigaciones Fisicoquimicas Teoricas y Aplicadas (INIFTA ), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Casilla de Correo 16,Sucursal4, (1900)La Plata, A rg en tina The slow thermal decomposition of gaseous OClO has been investigated between 318 and 333 K and at initial pressures between 4 and 120 Torr in vitreous containers. A slow reaction proceeds while the OClO partial pressure remains higher than a given critical pressure p,(OCIO), then an explosion occurs. p,(OCIO) depends on the total effective pressure, p,(OCIO), and follows the equation p,(OCIO) = a + b p,(OCIO).The system is in the explosive regime at temperatures >336 K and with p(OCI0) ranging between 1 and 70 Torr. Prior to explosion, practically no OClO decomposition is observed. The induction period, 7,of the explosion has been determined between 336 and 387 K, and follows the equation, z =A + B/p(OCIO). A quadratic chain-branching reaction mechanism with initiation and termination reactions on the wall is oper- ating. A bimolecular reaction between adsorbed OClO leading to formation of the chain carrier, CIO, is proposed as the initiation. The chain-branching coefficient depends on the difference between the rate constants of the wall reactions: 2 CIO -+ CI + CIO, and 2 CIO -+ CI, + 0,. The first-order diffusioncontrolled reaction: CI,O, -products, leads to chain termination.Gaseous chlorine dioxide, OC10, appears to be a stable sub- stance at room temperature and decomposes slowly in vitre- ous containers (Pyrex, quartz) at temperatures a few degrees higher if shielded from external stimulation (light, electric dis- charge, etc.). With external stimulation, the rate may enhance and eventually be of explosive character at any temperature OClO +3C12 + 0,; AHo = -102.5 kJ mol- Schumacher and Stieger' have studied the slow decompo- sition at temperatures between 312 and 318 K and at rather high OClO pressures (100-400 Torr). They observed that an explosion occurred when the major part of the OClO was decomposed in a quartz vessel. Later McHale and von Elbe, studied OClO explosive decomposition in Pyrex vessels between 327 and 407 K and at 0.2-40 Torr.They measured the induction period, that is, the time elapsed between the introduction of the reactant into the reaction vessel and its self-ignition, but did not character- ize the explosion limits. They concluded that the reaction is of degenerate chain-branching explosion type and proposed Cl,O, to be the intermediate responsible for the delayed reaction. Gray and Ip9 reinvestigated this reaction and concluded on thermodynamic grounds that the degenerate branching agent could not be Cl,O,. Moreover, their measurements of the temperature-time history of the reaction under a variety of conditions has lead them to the conclusion that the reac- tion chain is being propagated isothermally.All these studies were made in static reaction vessels and all of the authors agree that the wall plays as great a part in the initiation as in the termination process. .~More recently, Paillard et ~1 studied the pyrolysis of OClO in shock waves at high temperatures (>473 K) in the absence of wall effects. In the 436 nm photolysis of OClO (5-100 Torr) at room temperature, Cl,, O,, Cl,04 and Cl2O, are The quantum yield for OClO decomposition is much larger than two at low OClO pressures and decreases strongly as the OClO pressure increases. Doubtless this high quantum yield suggests that a chain mechanism is operating. Over recent years the chemical kinetics of the chlorine oxides have received a great deal of interest,899 particularly owing to the implication of some of these compounds in the ozone depletion observed in the Antarctic stratosphere in the spring.In this context the properties of the C10 radical and the dimer C1,0, : C10 + ClO + M eC1,02 + M (1,-1) have been extensively Also some properties of Cl,O, formed by the reaction:, C10 + OClO + M Cl,03 + M (II,-11) have been recently rep~rted.''*'~-'~ Nevertheless, to date some doubt over the participation of Cl,O, in OClO decomposition3 still remains. Moreover, not much is known about the initiation and chain-termination processes of this reaction. Therefore, there is a possibility of meeting a correlation between the total pressure of the system and the corresponding OClO pressure at which the explosion occurs.We present here a large number of experi-mental results which demonstrate this relationship and provide important kinetic information. Experimental A conventional apparatus for static gas-kinetics studies was utilized. Halocarbon grease was employed as a lubricant for the stopcocks. As a vacuum system, a mercury diffusion pump coupled with a rotary pump was used. Pressure mea- surements were made with an MKS (Baratron) manometer connected to a chart recorder where the pressure us. time plots were obtained. The induction periods were measured with a stopwatch. The temperature was kept constant ( & 0.05 "C) by circulation of a thermostatic liquid. Spherical containers were used as reaction vessels, of which six were made of Pyrex (Cristalerias Rigolleau, Argentina : two P60, two P100, one P300 and one P500 of 60, 100, 300 and 500 cm3 nominal capacity, respectively), three of fused quartz (Heraeus-Hanau, Germany: 440, 4100 and 4200 of 40,100 and 200 cm3 nominal capacity, respectively) and one of common glass [composition (oxide%): 6.6 Na,1.8 Mg, 4.4 Al, 79 Si, 1.0 K, 5.3 Ca, 1.2 Ti, 0.7 Fe; G60 of 60 cm' nominal capacity].Chlorine was obtained from a high-quality commercial cylinder bubbled through water, dried with concentrated sul- furic acid and purified by low-temperature distillation. Chlo- rine dioxide was prepared and purified as previously de~cribed.~Chlorine and chlorine dioxide were stored in the dark at liquid-nitrogen temperature.High-quality commercial oxygen and argon were passed through a 153 K cold trap and stored in 2 dm3 glass vessels. Gas Handling Stringent operative conditions were followed in order to obtain an acceptable reproducibility of the results. The reac- tion vessel had to be thoroughly evacuated. Chlorine dioxide, whether pure or mixed with inert gases, shows greater explo- siveness when stored at room temperature in Pyrex glass con- tainers. For that reason, the reagent was introduced into the reaction vessel directly from the store trap kept at an appro- priately low temperature. All the manipulations were made in the dark. The condensable reaction products were retained in a trap at liquid-nitrogen temperature.No unplanned explo- sions occurred. Results The reaction course was followed by measuring pressure changes at constant temperature and volume as a function of time. The decomposition of OClO results primarily from a chain-isothermal mechanism and the reaction shows no sig- nificant ~elf-heating,~ such that the temperature of the reac- tion mixture is assumed to be that of the thermostatic bath. Experiments with OClO pure or mixed with oxygen, chlorine or argon were performed. At temperatures below 333 K, slow decomposition of OClO is observed. Therefore, in these conditions the OClO pressure at which the reaction becomes explosive was recorded as a function of the initial OClO pressure. The initial pressure of OClO was varied between 4 and 120 Torr and the tem- perature between 318 and 333 K.The course of the experiment is similar to that obtained by Gray and IP,~ and by Schumacher and Stieger’ in transpar- ent quartz reaction vessels. After a given initial pressure of gaseous OC10, pi(OCIO), is introduced into the cell practi- cally no pressure increase is observed for some minutes. This could last for as long as 10 min, which allowed us, if neces- sary, to add a desired amount of inert gas to the OClO. The pressure then begins to rise and a practically steady rate is set up which slowly increases until suddenly an abrupt pressure jump occurs. The OClO explosion pressure, p,(OClO), can be determined fairly well from the trace of the recorder and the total pressure increase corresponds to the expected stoichi- ometry.Depending on the experimental conditions a single run could last several hours. The OClO explosion pressure, p,(OClO), increases with the initial OClO pressure, pi(OCIO), the volume of the reaction vessel, the temperature and the pressure of the added inert gas. Fig. 1 shows a schematic representation where the ordi- nate represents either pe(OCIO) or p(OClO), i.e. the partial pressure of OClO present in any gas mixture, and the abscis- sa gives p,(OClO), the initial OClO pressure, pi(OCIO), in pure OClO systems. If mixtures of chlorine and oxygen in the ratio 1 : 2 are added to OC10, p,(OClO) is taken as p,(OClO) = pi(OCIO) + 2/3b(c1,) + do,)], assuming that the produced C1, and 0, replace the decomposed OC10.When any gas is added to OC10, the effective total pressure, p,(OClO), is determined by calculation (see the Appendix). The line that passes through the origin with a slope of unity corresponds to systems composed of pure OC10, point A’ for example. The trace A‘A represents the course of an experi- ment carried out with pure OClO which is slowly decom- posed until the corresponding explosion pressure pe(OCIO) is reached. Therefore, the full line that passes through A rep-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 // /f /f pure OClO / n0 0 / ’/ slow reaction 0,/1 /f f / OClO + O2+ Cl*(+M) / explosive decomposition PO(OCI0) Fig. 1 Correlation between the effective total pressure, p,(OCIO), and the corresponding explosion pressure, p,(OCIO) resents the limit of explosion.It is also possible to reach this limit by starting as A, with an OClO pressure slightly higher than the corresponding limit point, by adding an amount of inert gas that makes the pressure equal to p,(OClO) of the point A[p,(OClO)(A’)]. In this case, the amount of OClO needed to decompose in order to reach the limit A is small, and A will be reached in much less time. Now if the partial OClO pressure corresponding to p,(OClO)(A’) < p,(OClO), for example at point A,, the explosion will occur with a neg- ligible amount of OClO decomposition after the induction period, z. Therefore, in Fig. 1 the slow reaction and the explosive decomposition regimes can be clearly distinguished.The limit between these regimes depends on the pressure, temperature, volume and nature of the reaction vessel. The experimental results can be represented by a linear equation p,(OClO) = a + b p,(OC10) (1) The results obtained in the P300 reaction vessel are shown in Fig. 2 and the fitted a and b values for the experiments -0 20 40 60 po(OCIO)/Torr Fig. 2 p,(OClO) vs. p,(OCIO) plots for the P300 reaction vessel at: 0,318; V, 321; 0,324 and 0,327 K. The lines correspond to a least-squares fit of the data to eqn. (1). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Intercept, a, and slope, b, values from eqn. (1)at different temperatures for several reaction vessels with radius R T/K -a/Torr b -a/Torr G60 R = 2.42 cm 324 2.2 f1.2 0.34 f0.10 327 0.7 k0.5 0.30 f0.05 330 0.8 f0.4 0.49 f0.04 333 1.0 f0.3 0.85 0.04 443 R = 2.17 cm Ql00 R = 324 0.9 f0.3 0.15 f0.02 0.29 If: 0.09 327 0.3 f0.2 0.21 f0.02 0.17 f0.03 330 0.7 f0.2 0.38 f0.02 0.27 f0.07 333 1.3 f0.5 0.70 & 0.05 0.11 f0.3 P60 R = 2.45 cm PlOO R = 32 1 0.9 _+ 0.5 324 0.5 f0.3 0.16 k0.03 0.7 f1.1 327 0.4 f0.6 0.28 f0.07 0.4 f0.3 330 0.7 f0.4 0.47 f0.05 0.07 f0.3 333 0.6 f0.2 0.80 f0.02 b 2.88 cm 0.179 f0.008 0.309 f0.003 0.52 f0.01 0.84 f0.05 3.04 cm 0.20 f0.02 0.29 f0.06 0.37 f0.03 0.58 k0.03 P300 R = 4.13 cm P500 R = 4.92 cm 318 1.0 f2 0.16 f0.08 0.7 f 0.3 0.17 0.02 32 1 0.9 f 0.2 0.25 f0.01 324 0.9 f0.4 0.37 50.03 327 0.6 f0.1 0.51 f0.01 carried out in the various reaction vessels are given in Table 1.At temperatures above 333 K no slow OClO decomposi- tion is observed. In this case, there is practically no OClO decomposition during the induction period of explosion, z, such that p,(OClO) = pi(OCIO). In these experiments, z may be even less than one minute, being affected by the time necessary to introduce OClO into the reaction vessel. Conse- quently, the inert gas, if necessary, has to be admitted first. In these experiments, z as a function of OClO pressure, was determined. Many series of experiments were carried out in which OClO was varied between 1 and 70 Torr over the 336-387 K range in various reaction vessels.The low and high OClO pressure results are shown in Fig. 3 and 4 as zus. 6oo r----l 500 400 300 200 100 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 ’ [1/p(0CI0)]/Torr -Fig. 3 Dependence of z on the inverse of OClO pressure at 0,336; V,345; A, 351; W, 357; 0,363; A,369; V, 375; 0,381 and 0,387 K. Some points are averages of at least 10 experiments. (-) Least-squares fit of z to eqn. (2)(---) calculation of z with eqn. (1 1). 0.2 0.3 0.21 f0.02 -0.5 f0.5 0.28 k0.03 l/p(OClO) and l/z us. p(OC10) plots, respectively. Within experimental error, these results are independent of the volume, nature of the reaction vessel and, for relatively high p(OClO), total pressure. Our results may be satisfactorily represented by the expres- sion Bz=A+-p(OC10) which transforms into: _-_1 K,p(OC10)1 -(3)t A 1 + K,p(OC10) with K, = A/B.The fitted A and B values are given in Table 2.Discussion The explosive decomposition of OClO results from a branched-chain mechanism with initiation and termination 7 0.08 v).. h t-\ Fv 0.04 0.00 0 10 20 30 40 50 60 70 p (OCIO)/Torr Fig. 4 OClO pressure dependence of l/~.Symbols as in Fig. 3, except (-) which corresponds to calculation with eqn. (3). Table 2 Intercept, A, and slope B, values from eqn. (2) at different temperatures 336 184 f10 851 & 74 345 76 f23 434 & 84 351 35 k 11 371 & 63 357 21 f14 278 & 60 363 10f 11 205 f37 369 4* 11 174 k 31 375 5f1 151 f5 38 1 3f4 122 f14 387 2f2 107 f7 reactions operating on the wall.,., According to the extensive data available on kinetic systems involving chlorine ~xides~~-'~the chain-propagating radicals are most likely to be C10 and C1, since oxygen atoms are not favoured in low- temperature thermal system^.^ The C10 radical, initially formed on the wall, may be in equilibrium with OClO forming Cl,03 [reactions (11,-11)],'2-'4 may recombine to form the dimer C1,0, [reactions (1,-1)]" or may undergo bimolecular self-reactions :8 c10 + c10 --* OClO + c1 (111) ClO + c10 --+ c10, + c1 (IV) c10 + c10 -+ c1, + 0, (V) The unstable chlorine peroxy radical C10, decomposes readily to give C1 atoms C10, + M-+Cl+ 0, + M (VI) The reaction c1+OClO --+ 2 c10 (-111) is very fast.8 Under our experimental conditions there is a large excess of OC10, such that reaction (-111) is the only fate for chlorine atoms in the gas phase. As the termination reac- tion, the unstable intermediate Cl,O, formed through reac- tion (11) is assumed to be destroyed on the wall.The explosion occurs for a minimum p(OClO), p,(OClO), and for higher p(OC10) values a slow reaction is observed (Fig. 1). Therefore, depending on p(OClO), an efficient chain-termination process may be operating C1,0, aproducts (VW Even though there are numerous experiments in which Cl,O, has been detected as an intermediate, unstable species in glass '9'reaction vessels,' ,-14 in only one experiment is the nearly stable oxide, Cl,O, ,claimed to have been Assuming the steady-state approximation for C1 and C10, radicals, and also that Cl,O, and C1,0, are in equilibrium with C10, such that dp(Cl,O,) = K,&OC10) dp(C10) and dp(C1,0,) = 2K1p(ClO) dp(C10), the C10 formation rate can be written as: dn[1 + KI,p(OC1O) + 4K, FI]-= U, + 2(k,v -kv)n2dt -kV,,~,&OCIO)n= u, + an2 -/h (4) where n denotes C10 pressure, u, is the initial rate of forma- tion of C10 on the wall and a is the quadratic branching coefficient given by a = 2(kIv-k,) and /?= k,,,K,&OClO).Taking into account that the OClO explosion pressure increases with the total pressure [eqn. (l)], it will be assumed that reaction (VII) is diffusion controlled. Semenov16 has shown that for both non-branching and linear-branching J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 chains, the diffusional term can be replaced by the product of an heterogeneous constant times the diffusing carrier mean pressure, p: khetp= ADp/R2= kV,9(Cl,O,), where D is the mean diffusion coefficient of the carrier through the gas mixture, R is the radius of the spherical vessel and A is a constant. More recently, Moin and Fagara~h'~ have found that in the case of quadratic branching, the value of A depends on the magnitude of the kinetic coefficients and for a small branching factor, the approximation of Semenov is applicable. Therefore, the B coefficient in eqn. (4) is given by p=-EADK,,p(OC10) = EAD"K,, p(OC10) ~ ~R2 R2 p,(OC10) (5) where Do is the binary coefficient of diffusion of Cl,03 through pure OClO referred to unit pressure, p,(OClO) is OClO effective total pressure (see Appendix) and E is an effi- ciency factor for Cl,O, destruction at the wall.At temperatures >333 K no slow reaction is observed and the induction period for the explosion, z, is measured. In these conditions CI,O, and Cl,O, are not likely to be formed'0,'2 and diffusion to the wall may be neglected. The rate equation is consequently reduced to dn -dt = u0 + an2 For a positive chain-branching coefficient, 2(k,, -k,) = a > 0, integration of eqn. (6) with n between 0 and infinity allows the induction period of the explosion to be deter- mined : (7) As a is independent of p(OC10) this equation takes the form of eqn.(2) if we suppose a Langmuir adsorption equi- librium : OClO(g) + S OClO(ads) (VII1,-VIII) where S is a vacant site, followed by a bimolecular surface reaction between adsorbed OC10, 2 OClO(ads) +C10 + ClO, (1x4 2 OClO(ads) -+ 2 C10 + 0, (IXb) The OClO fractional surface coverage is given by with Kv,,,= k,,,~k~,,,,.Then the initiation rate," uo results. The rate constant, k,,Jmolecule cm -3 s-', depends on the nature of the surface and the surface-to-volume ratio of the reaction vessel. By substituting uo from eqn. (8) into eqn. (7): the form of eqn. (2) is recovered. From comparison with the experimental results (Table 2) K,,,, can be calculated as: KvI1florr-' = 5 x expC(7000 k2~)/7-1 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 In order to calculate kIx, the values for k,, and k, have been estimated from the recently reported data of Sander and co-worker~,~ k,,/cm3 molecule-s-' = 2.30 x 10-'' exp( -2450/T) k,/cm3 molecule-'^-^ = 1.13 x exp(-1510/T) such that k,, = kv at 312 K, the lowest temperature at which spontaneous thermal explosion has been observed. Thus, k,Jmolecule cm -s-' --4 x * 2)/Rexp(19W +_ 2000/T) (10) where R (in cm) is the radius of the reaction vessel. The value of the pre-exponential factor of k,, may be inter-preted by assuming that the surface bimolecular reaction takes place between mobile adsorbed OClO molecules, while the activation energy of reaction (IXa) exceeds the gas-phase positive enthalpy change.20 At temperatures lower than 333 K the slow reaction is observed and p,(OClO) is measured.The whole mechanism including C1202,C1203and the diffusion of Cl2O, has to be considered in order to interpret the experimental results. In this case, if a steady state for C10 radicals is assumed, dn/ dt = 0, that is, uo + an2 -Bn = 0 leads to n = [B -(p2 -4u0~t)"~]1/2a.When (4u0a -b2)= 4 is zero, the critical pressure of the carrier is p,(ClO) = P/2a = n,, while the system will be non-stationary'6,21 for 4 =-0. From the critical condition 4 = 0, fl = (4u0 a)"' and p(OC10) = p,(OClO) follow. By replacing fl and uo from eqn. (5) and (8), respectively, the form of eqn. (i) for the limit of explosion is obtained, where a = -l/K,,,, and b = R'(4k1, a)"'/(AD0~K,,).From Table 1 it is seen that the a values have the expected sign but inherent large error bars. Consequently, only the order of magnitude of K,,,, could be determined. This result is in agreement with the value extrapolated from high tem-peratures using eqn. (9). On the other hand the b values permit calculation at &KIl. Taking k,, from eqn. (lo), A = 15 from ref. 17, Do= 5.78 x T3l2Torr cm' s-l (see Appendix), then &K,florr-' = 1.5 x expC(6200 & 1500)/T] 3395 is obtained. The exponential factor is located within the experimental error given by Hayman and Cox" (see Burk-holder et ~1.'~).Therefore, the ratio of the pre-exponential factors leads to E = 0.07. An expression for the induction period may also be derived from the complete mechanism by integrating eqn.(4) between the limits 0, t and 0, n. For the case of a non-stationary system (4 > 0) the following equation results: t = 25 log( + un2 "")00 -+ (y+ K,,p(OClO) + 1 The evaluation of z from this equation should be performed for n -+ 03. The asymptotic value of the second term is already reached for p(C10) = 0.005-0.05 Torr. For those values of p(ClO), the change in the contribution of the logarithmic term is negligible. Therefore n = 0.05 has been used in the calculation. The induction periods, z, have been calculated using the following values for rate and equilibrium constants and are represented in Fig. 3 and 4 K,,,JI"Trr-' = 2 x lo-'' exp(7000/T) k,Jmolecule cm-3 s-' = 1.6 x 1034/Rexp( -18000/T) K,/Torr-l = 4.1 x lo-'' exp(8720/T); ref.8 EK,florr-' = 1.5 x lo-'' exp(6200/T); E = 0.07 The average self-heating of the reaction mixture, AT, which is proportional to the reaction rate,22 has been estimated. The postulated reaction mechanism (I)-(IX) gives for the rate of OClO decomposition -dp(OClO)/dt x 2k,, p(Cl0)'. For p(C10) = 0.005 Torr the rate is 0.04 Torr s-l, so that AT x 4 K, if only the thermal conductivity for pure OClO in a 300 cm3 spherical vessel is considered. This value is in good agreement with AT x 3 K measured by Gray and Ip3 prior to the explosion. The induction period has also been calculated employing eqn. (11) for experiments made with dilute OClO at 336, 345 and 351 K and the results are given in Table 3.The differ-Table 3 Effect of the pressure of a mixture of C1, and 0, (1 : 2) on the induction period, z; zCalcvalues are calculated by means of eqn. (1 1) and z, by means of eqn. (2) 336 K, PlOO 1.96 19.4 456 438 617 2.17 19.3 398 409 575 2.26 30.5 361 385 560 10.4 9.8 226 228 265 20.0 48.8 161 171 226 345 K, PlOO 1.90 10.4 257 257 305 2.21 19.5 213 222 272 2.01 31.4 222 236 292 5.26 7.52 138 127 158 4.75 17.7 143 131 167 345 K, P3OO 6.56 9.71 126 127 142 11.8 9.80 101 97 113 9.31 18.5 110 106 123 11.5 19.3 90 97 114 351 K, PlOO 2.27 7.57 163 163 199 2.26 30.9 132 159 200 4.26 9.79 106 99 122 4.78 30.7 86 90 113 9.41 6.47 71 61 75 ences between zeXpand z, are larger at a high pressure of diluent gas and low temperatures when the diffusion process is more important.Owing to the fact that C1206 was found among the reac- tion products' the recombination of ClO, radicals produced in the initiation reaction (IXa) should occur. This reaction probably takes place between adsorbed ClO, molecules C~O,+ CIO, wall c~,o, (XI Nevertheless, under our experimental conditions a large part of this substance is transformed to chlorine per~hlorate.~ On the other hand, the shock-wave results obtained by Paillard et aL4 may also be interpreted by eqn. (7). They found that zw [kp(OC10)]-' with an apparent activation energy of 66.5 kJ mol-'.They have proposed the following reaction as gas-phase initiation 2 OClO --+ C10 + ClO, Therefore, replacing uo = k,p(OC10)2 in eqn. (7), k = (4kga)'''/7r results. If we assume for a an apparent activation energy of 13 kJ mol-' at 10oO K, then E, = 120 kJ mol-l. Appendix The mean diffusion coefficient, D, of species 1 in the reaction mixture can be calculated by the approximate f0rmula:~,3~~ n (1 -xJ/D = 1(Xj/D,j)j=2 where xj is the mole fraction xj = pj/P of each component and Dlj is the binary diffusion coefficient of species 1 with respect to j in the gas mixture. It is assumed that x1= 0 for the minor species C120,, and, the OClO effective total pres- sure is defined as : that is, D = PD,, oclo/po(oclo) = D0/po(0C1O) where Dois the binary diffusion coefficient at unit pressure.Binary diffusion coefficients have been calculated according to kinetic theory. The following diameters, 6.7, 5.5, 5.06, 3.38 and 3.40 (in A), for Cl,03, OC10, Cl,, 0, and Ar, respec- tively, have been evaluated from molecular Lennard-Jones potential-energy parameters obtained from viscosity data2 at 323 K. For C120, and OC10, input data for similar mol- ecules have been empolyed. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 With D,,oclo/D1,c12= 0.944 and Dl,oclo/D1,o2 = 0.522, po(OCIO)= pi(OCIO) results for the decomposition of OClO with no inert gas added. (For completeness, D,.oclo/D1,,Ar = 0.567.) This research was financially supported by the Consejo Nacional de Investigaciones Cientificas y Tknicas and the Comision de Investigaciones Cientificas de la Provincia de Buenos Aires.We are indebted to Dr. E. Castellano for helpful discussions. We thank Dr. J. B. Burkholder and Dr. S. P. Sander for communicating their results to us prior to publication. References 1 H. J. Schumacher and G. Z. Stieger, Z. Phys. Chem., 1930,7,363. 2 E. T. McHale and G. von Elbe, J. Phys. Chem., 1968,72,1849. 3 P. Gray and J. K. K. Ip, Combust. Flame, 1972,18,361. 4 C. Paillard, S. Youssefi, H. Alaiteh, N. Charpentier and G. DuprC, J. Chim. Phys. (Paris), 1987,84, 41. 5 M. I. Lopez and J. E. Sicre, to be published. 6 M. I. Lopez and J. E. Sicre, J. Phys. Chem., 1988,92, 563. 7 M. I. Lopez and J.E. Sicre, J. Phys. Chem., 1990,94,3860. 8 R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson Jr., J. A. Kerr and J. Troe, Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry : Suppl. 111, J. Phys. Chem. Re$ Data, 1989, 18,no. 2. 9 W. B. De More, S. P. Sander, D. M. Golden, M. J. Molina, R. F. Hampson, M. J. Kurylo, C. J. Howard and A. R. Ravishankara, JPL Publ. 90-1, Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, Jet Propulsion Laboratory, Cali- fornia Institute of Technology, CA, 1990. 10 J. B. Burkholder, J. J. Orlando and C. J. Howard, J. Phys. Chem., 1990,94,687. 11 F. Jensen and J. Oddershede, J. Phys. Chem., 1990,94,2235. 12 G. D. Hayman and R. A. Cox, Chem. Phys. Lett., 1989,155,l. 13 A. D. Parr, R. P. Wayne, G. D. Hayman, M. E. Jenkin and R. A. Cox, Geophys. Res. Lett., 1990, 17, 2357. 14 J. B. Burkholder, R. L. Mauldin 111, R. Yokelson, S. Solomon and A. R. Ravishankara, J. Phys. Chem., 1993,97,7597. 15 E. T. McHale, G. von Elbe, J. Am. Chem. SOC., 1967,89,2795. 16 N. N. Semenov, Chemical Kinetics and Chain Reactions, Clar-endon Press, Oxford, 1935; Some Problems of Chemical Kinetics and Reactivity, Pergamon Press, London, 1959, vol. 2. 17 F. B. Moin and M. B. Fagarash, Kinet. Katal., 1988, 29, 466 (Engl. transl.). 18 A. Clark, Theory of Adsorption and Catalysis, Academic Press, New York, 1970. 19 S. L. Nickolaisen, R. R. Fried1 and S. P. Sander, J. Phys. Chem., 1994,98, 155. 20 A. J. Colussi and M. A. Grela, J. Phys. Chem., 1993,97, 3775. 21 F. S. Dainton, Chain Reactions, Methuen, London, 2nd edn., 1966. 22 S. W. Benson, The Foundations of Chemical Kinetics, McGraw-Hill, New York, 1960. 23 R. B. Bird, W. E. Steward and E. N. Lightfoot, Transport Pheno- mena, Wiley, New York, 1960. 24 J. D. Ramshaw, J. Non-Equilib. Thermodyn., 1990,15,295. 25 J. 0. Hirschfelder, C. F. Curtiss and R. B. Bird, Molecular Theory of Gases and Liquids, Wiley, New York, 1954. Paper 4/03114H; Received 24th May, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003391

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(22), 3397-3400 3397 Microca lor imet r ic Tit ration of a-Cyclodext r in't with Some Straight-chain a,o-Dicarboxylates in Aqueous Solution at Different Temperatures Isabel Gomez-OrellanaJ Dan Hallen* and Magnus Stodeman Thermochemistry, Chemical Center, University of Lund, P.O. Box 124,S-22100Lund, Sweden The complex formation of a-cyclodextrin and straight-chain aliphatic a,co-dicarboxylates, -O,C(CH,),CO,-(n = 6, 7, 8), in aqueous solution has been studied by titration microcalorimetry at 288.15, 298.15 and 308.15 K. Apparent Gibbs energies, enthalpies, entropies and heat capacities for the 1 : 1 complex formations were derived from the calorimetric data. The thermodynamics of binding show typical enthalpy-entropy compensation effects, which result in weak temperature dependences of the Gibbs energies.The heat capacities of binding are large and negative. This confirms the view that the binding of non-polar moieties to cyclodextrins is to a large extent a result of hydrophobic dehydration. Experimental conditions causing deviations from the 1 : 1 model, are also discussed. Studies of complex formation between macrocyclic com-pounds and ions or hydrophobic molecules are important in both fundamental research and in applied areas such as ana- lytical chemistry, pharmaceutical chemistry and environ-mental ~hemistry.'-~ Cyclodextrins (CD) are one of the most studied families of macrocyclic compounds. The most common CDs are a-,/3-and y-CD, which consist of six, seven and eight D( +)-glucopyranose units, respectively.The macro- cyclic rings are formed by ~(1-4) bonds of the building units, which result in structures that can be described as short, truncated, hollow cones. The interior of the CD is largely hydrophobic and is normally considered as the binding site for the guest molecules. This makes CDs interesting to use in studies with hydrophobic or partly hydrophobic compounds as guest molecules in aqueous solution. Most of the thermodynamic studies on complex formation between CDs and hydrophobic, or partly hydrophobic, com- pounds in aqueous solution have been done at 298.15 K. However, the changes in heat capacities of these complex for- mations are large and It is thus necessary to have data from different temperatures to be able to discuss the thermodynamic properties of the binding process and the driving forces for the binding.In this respect, calorimetry is the most reliable method for studying these complexes as a function of temperature. Generally, the uncertainties are large for enthalpies and, especially, heat capacities, derived from Gibbs energy data.* Aversa et aL9 used a potentiometric method to determine equilibrium constants for the binding of dicarboxylic acids of alkanes and their anions to or-CD in the temperature range 288.15-318.15 K. They fitted the data to a 1 : 1 binding model for -O,C(CH,),CO,- (n = 5, 6), HO,C(CH,),CO,-(n = 0-6), and H0,C(CH2),C0,H (n= 0-4), whereas for HO,C(CH,),CO,H (n = 5, 6) a second complex containing two a-CD and one dicarboxylic acid was assumed.Enthalpies of complex formation were derived from the van't Hoff equa-tion, neglecting the heat capacities. Our laboratory has for a long time been interested in the so called 'hydrophobic effect,' which is of major importance in biology and technical applications of chemistry. Our aim is to obtain heat capacity data for processes in which hydro- ? Cyclomaltohexaose. $ Present address : Department of Applied Physics, Faculty of Physics, University of Santiago de Compostela, E-15706 Santiago, Spain. phobic moieties are transferred from a non-polar environ- ment to an aqueous solution or vice versa. We have in this respect done work on solution properties of hydrocarbons,' alcohols,' ' alkyl esters,12 crown ethers,' saccharide^'^ and chl~roalkanes.'~These studies have shown that there is a high degree of group additivity of partial molar heat capac- ities.' 6-18 Our earlier microcalorimetric work on the binding of hydrophobic, or partly hydrophobic, compounds to CDs in aqueous solutions-7 has shown that the thermodynamic properties of these complexes can to a large extent be explained by related processes, such as the transfer of the non-polar moieties from their pure state to aqueous solution. Experimental A sample of a-CD, obtained from Sigma, was further purified according to French et a1." to compare calorimetric binding experiments with unpurified samples from the same batch.No significant differences could be observed.The measure- ments were therefore performed with samples from the same batch as obtained from Sigma. a-CD was equilibrated for several days at room temperature over a saturated solution of Ca(NO,), providing a relative humidity of 51%.,O From a Karl Fischer titration (Metrohm Coulometer) it was found that the hydration state was a-CD. (6.561O.I8)H2O, thus the molar mass of a-CD was taken as 1090 g mol- '. Suberic, azelaic and sebacic acid were purchased from Jansen (99%), Fluka (99%) and Aldrich (99%), respectively, and were used as received. Calorimetric measurements were made using our stainless-steel microcalorimetric titration vessel, volume 1 ml, in our four-channel microcalorimetric system2' and the com- mercial version [ThermoMetric AB (LKB) 2277, Jarfalla, Sweden].The titrations were performed at 4-5 min intervals using a dynamic correction for the power re~ponse.~~.~~ The microcalorimeters were calibrated electrically, using insertion heaters immersed in water inside the vessel. Dissolution of propan-1-01 in water was used as a test process and for Cali- bration of the instrument.'',24 To minimise contributions from protonated dicarboxylates in the binding reactions, all solutions were adjusted to pH 9.5 using glycine (0.1 mol 1-') as buffer. The pH used is 3-4 pH units higher than the pK, of all the dicarboxylic acids. The a-CD solutions were adjusted to the same pH to avoid con- tributions from protonation enthalpies.No significant binding of glycine to a-CD could be observed. The titrations were performed at two different concentra- tion ranges. High range: aliquots of 12-15 injections of 14-20 pl of 0.5-0.9 mol 1-' of the dicarboxylates were added to 0.9 ml of 0.10-0.12 mol 1-' of a-CD. Low range: aliquots of 12-15 injections of 14-30 pl of 0.10-0.12 mol I-' of a-CD were added to 0.7-0.9 ml of 0.005-0.015 mol 1-' of the dicarboxylates. The calorimetric data were fitted to a 1 : 1 model (A2-refers to the a,o-dicarboxylate), [(a -CD)A~-]A'-+ a -CDe(a -CD)A2-; K: = [or -CD][A2-] (1) where K: is the apparent equilibrium constant. Data were also fitted to two other models in which two complexes are formed, (a -CD)A2- plus (a -CD)A,4-, and (a -CD)A2-plus (a -CD), A2 -, respectively. The second complexes are described by the apparent equilibrium constants for the second complexes, 8;: 2A2-+ a -CD e(a -CD)A24-; [(a -CD)A24-] 1 (2)"= [a -CD][A2-]2 and A2-+ 2a -CD +(a -CD),A2-; [(a -CD), A2-] (3)B; = [a -CD][A2-] The mean activity coefficient for the charged constituents, y*, in eqn.(2) was calculated according to the Debye-Huckel limiting-law approximation, assuming two independent point charges on the a,o-dicarboxylates. The activity coefficient for a-CD was assumed to be unity in all cases. In all other equi- libria, y* is cancelled out in the definitions of the equilibrium constants when using the Debye-Huckel limiting law approx- imation. Values for apparent equilibrium constants and apparent enthalpies of complex formation were obtained from non-linear regression according to Marq~ardt,,~ includ-ing an algorithm to eliminate linear parameters.26 Uncer- tainties in the fitted parameters were obtained from the diagonal of the variance-covariance matrix of the regressions.Results In the initial stages of the work we optimised concentrations and volumes of titrants and titrands assuming a 1 :1 model for all the a,o-dicarb~xylates.~' This resulted in concentra- tions and volumes within the high concentration range. None J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 & 0.5-\ 0 m p" 0.0 I n I-0.5---1.5 '0 -O2 _ 4I 6 81012 no. of injections Fig. 1 Typical residuals obtained from regressions of calorimetric data for a,o-dicarboxylates +a-CD at different concentration ranges assuming a 1 : 1 binding model. The figure shows the residuals for suberate, -O,C(CH,),CO,-, at pH 9.52 and 288.15 K: (a)30 ~1 per step injections of a suberate solution at 0.70 mol 1-' to 0.7 mi of an a-CD solution at 0.11 moll-', and (0)30 pl per step injections of an a-CD solution at 0.11 mol 1-' to 0.7 ml of a suberate solution at 0.011 mol 1-'.There were no significant differences in the residuals when assuming a 1 :2 or 2 : 1 model at the high concentration range. of the models, defined by eqn. (1)-(3), could be used to ration- alise the calorimetric data for any of the a,o-dicarboxylates in this concentration range. The residuals of all the regres- sions showed non-random behaviour (Fig.1). The standard error of points from the regressions was much higher than what is accepted according to the performance of the calo- rimeter used.23 Neither changing pH between 8.5 (phosphate buffer) and 11.2 (glycine buffer), nor increasing the ionic strength, by addition of NaCl, improved the regressions. We could use NaCl to increase the ionic strength without any risk of competing inclusion-complex formation of C1- with a-cyclodextrin.28 Hersey et al. have reported the binding con- stant to be @1.28 The residuals obtained in the regressions were qualitatively the same for all the different situations. In the low concentration range the calorimetric data could be rationalised to a 1 : 1 model.Experiments with pimelate (nCH2= 5) in the high concentration range showed similar behaviour to the longer-chain a,o-dicarboxylates. However, the binding constant was too low9 to allow measurements in the low concentration range (K, < 50).27*29It is impossible to resolve the equilibrium constant and the enthalpy with a statistical ~ignificance,~~ and thus the derived heat capacity change suffers from large uncertainties. The results of the apparent thermodynamic properties obtained for the binding of a,o-dicarboxylates to a-CD are shown in Table 1. The uncertainties in the entropy changes Table 1 Apparent thermodynamic properties for the binding of a,o-dicarboxylatesto a-CD in dilute aqueous solution at pH 9.5 ncH2 T/K K:/moll-' AG"'"/kJ mol-' AH"'/kJ mol-' AS"'/J K-' mol-' TAS"'/kJ mol-' ACC/J K-' mo1-I 6 288.15 155 f 2 -12.08 f0.03 -12.34 f0.03 -0.89 f 0.15 -0.26 _+ 0.04 193 f7' -12.61 & 0.09' 298.15 135 f2 -12.16 f0.04 -15.16 & 0.04 -10.06 & 0.19 -3.00 f0.06 -279 f2 163 f3' -11.36 f0.05' -16.7 & 1.3' -13.80 f4.2' 308.15 109f1 -12.02 f0.02 -17.91 f 0.03 -19.12 f0.12 -5.89 f 0.04 128 _+ 5' -12.43 f0.10' 7 288.15 591 f10 -15.29 f0.04 -15.02 f 0.04 0.93 f0.20 -0.27 f 0.06 298.15 491 & 4 -15.36 f0.02 -18.74f 0.03 -11.34 f0.12 -3.38 f0.04 -317 f4 308.15 447f4 -15.63 f0.02 -21.36 f0.04 -18.58 f0.15 -5.73 & 0.05 8 288.15 1601 f36 -17.68 f0.05 -17.66 f0.07 0.05 f0.30 -0.16 & 0.09 298.15 1515 & 23 -18.15 f0.04 -20.87 f 0.06 9.11 f0.24 -2.72 & 0.07 -327 f 3 308.15 1262 f16 -18.29 f0.03 -24.21 f0.06 -19.20 & 0.22 -5.92 & 0.07 " AGO' = -RT ln(K;lmoll-').'Values reported by Aversa et aL9 The enthalpy was derived from the van't Hoff treatment of the temperature dependence of the equilibrium constant, neglecting the heat capacity. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 were estimated by simple propagation of error calculations, treating AGO' and AH"' as independent variables, whilst being aware that they are statistically correlated. The residual heats were all 50.2 mJ. In Table 1 we have included values of the equilibrium constants, enthalpies and entropies for suberate reported by Aversa et a1.' The equilibrium constants in this 3399 340 I I I 1 330 -I 320-'310 jL 300 work are similar to the values obtained from the poten- tiometric measurements.However, the enthalpies and the entropies cannot be compared, because the van't Hoff treat-ment used to derive the enthalpy values from potentiometric measurements does not take into account the heat capacities. Fig. 2-5 show the values for -AGO', -AH"', -AS"' and -AC;' plotted against the number of methylene groups in the alkane chain, nCH2. Values for -AH' and -AGO' increase with increasing length of the alkane chain, Fig. 2 and 3, whereas -AS"' for the a,w-dicarboxylates are independent of the chain length, Fig. 4. The AC;' values are large and negative, Fig. 5. The AG"' values hardly change in the tem- perature range studied, owing to enthalpy-entropy com-pen~ation,~'which is illustrated in Fig.6. 2o k . r 18 I-E .16 7 .14 .I 12 1 I I I10 ' 5 6 7 8 9 'CH2 Fig. 2 -AGO' for the 1 : 1 binding of some a,w-dicarboxylates to a-CD at (0)288.15 K, (H) 298.15 K and (+) 308.15 K us. the number of methylene groups in the alkane chain, nCH, 25 I I I r 1 I10 1 5 6 7 8 9 'CH2 Fig. 3 -AH"' for the 1 : 1 binding of some a,w-dicarboxylates to a-CD at (0)288.15 K, (m) 298.15 K and (+) 308.15 K us. the number of methylene groups in the alkane chain, ncI,2 I I5' I I 6 7 8 9 'CH2 Fig. 4 -AS"' for the 1 : 1 binding of some a,w-dicarboxylates to a-CD at (0)288.15 K, (m) 298.15 K and (+) 308.15 K us. the number of methylene groups in the alkane chain, nCHz .;290 2 280 I 270 260 5 6 7 8 9 "CHz Fig.5 -ACi' for the 1 : 1 binding of some a,o-dicarboxylates to a-CD at 298.15 K us. the number of methylene groups in the alkane chain, nCH2 7 I-E -15 3>5 -20 -25 -285 290 295 300 305 310 TIK -12-I0 "7kE I-14 -I I -20 285 290 295 300 305 3 10 TIK Fig. 6 AH"', -TAS"' and AGO' for the binding of some a,o-dicarb- oxylates to a-CD us. the temperature, T, for (0)nCHz= 6, (H) ncH2= 7 and (+) nCH2= 8. The figure shows how the -TAS"' term and the AH"' term compensate for each other in the Gibbs energy function, resulting in weak temperature dependences of the Gibbs energies. Discussion There are three reasons which may account for the inability to apply any of the 1 : 1, 1 : 2 or 2 : 1 models in the high- concentration range: (i) The thermodynamic properties of complex formation were treated as apparent quantities because the complex is not defined in its standard state in solution.This means that the change in complex concentra- tion after each consecutive titration step may contribute to the total observed enthalpy. To correct for this one has to measure the enthalpy of dilution of the formed complex. However, this is impossible because shifts in the equilibrium would contribute to the measured enthalpy. (ii) The formation of higher-order complexes than was assumed in the models. Attempts to fit experimental data to models containing higher orders of weak complexes would not lead to sta- tistically significant parameters.However, the correlation between the fitted parameters would be significant, which 3400 means that it would be impossible to resolve the individual parameters. Another possibility is that all the 1 : 1, 1 : 2, and 2: 1 complexes are formed, although the fitting problem would also remain with this model. (iii) Non-specific inter- actions between the 1 : 1 complex and the free dicarboxylates or the a-CD molecules. The difference between (ii) and (iii) is a matter of definition, since using a model according to (ii) would result in weak additional complexes. The reason for the deviation from a 1 : 1 complex model at higher concen- tration is thus unsolved.Any attempt to try to explain the phenomena based on the calorimetric measurements is not meaningful. The large negative AC; values emphasise the danger in discussing processes in which hydrophobic moieties in aqueous solution participate as enthalpy-driven or entropy- driven ,Fig. 6. The transfer of a hydrophobic moiety from a non-polar environment to aqueous solution is characterised by an anomalously large positive heat capacity change due to hydrophobic hydration. The complex formation can be seen as the opposite process, because the cavity of CD can be con- sidered as a hydrophobic environment. Thus, the large nega- tive values of the AC;' reported in this work are to a large extent due to the transfer of the hydrophobic part of the a,o-dicarboxylates from aqueous solution into the hydrophobic cavity of a-CD.Earlier studies made on, e.g., straight-chain alcohols6 and diols' showed linear dependences of AC;' on the number of methylene groups in the alkyl chain for nCH2< 6. The nCHzincrement for these alcohols was found to be -102 J K-' mol-' and for the diols -90 J K mol-', which is significantly more negative than the values which characterise the transfer of an alkyl chain from water to a non-polar solution (about 50-60 J K-' mol-').'1*12~31,32 The excess heat capacity is probably due to conformational changes of the macromolec~le,~*~~~~ leading to changes in the hydration of the glucose units of the CD. In the binding process, water molecules inside the cavity are exchanged with the guest molecule.The expulsion of the water molecules is not expected to contribute to the excess heat capacity, since the molar hear capacity of the water inside the a-CD cavity has a value similar to that of the bulk water outside a-CD.14 References 1 Proc. 4th Znt. Symp. on Cyclodextrins, ed. 0. Huber and J. Szejtli, Kluwer Academic Press, Dordrecht, 1989. 2 M. L. Bender and M. Komiyama, Cyclodextrin Chemistry, Springer Verlag, Berlin, 1978. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3 W. Saenger, Angew. Chem., Int. Ed. Engl., 1980,19,344. 4 V. T.dSousa and M. L. Bender, Acc. Chem. Res., 1987,20, 146, 5 M. Bastos, L-E. Briggner, I. Shehatta and I. Wadso, J. Chem. Thermodyn., 1990,22,1181. 6 D. HallCn, A.Schon, I. Shehatta and I. Wadso, J. Chem. SOC., Faraday Trans., 1992,88,2859. 7 I. G6mez-Orellana and D. HallCn, Thermochim. Acta, 1993, 211, 183. 8 S. F. Dec and S. J. Gill, J. Chem. Educ., 1985,55, 765. 9 A. Aversa, W. Etter, R. I. Gelb and L. M. Schwartz, J. Inclusion Phenom., 1990,9,277. 10 G. Olofsson, A. A. Oshodj, E. Qvarnstrom and I. Wadso, J. Chem. Thermodyn., 1984,16, 1041. 11 D. HallCn, S-0. Nilsson, W. Rothschild and I. Wadso, J. Chem. Thermodyn., 1986,18,429. 12 S-0. Nilsson and I. Wadso, J. Chem. Thermodyn., 1986,18,673. 13 L-E. Briggner and I. Wadso, J. Chem. Thermodyn., 1990,22, 143. 14 L-E. Briggner and I. Wadso, J. Chem. Thermodyn., 1990, 22, 1067. 15 D. HallCn, J. Chem. Thermodyn., 1993, 25, 519. 16 S.Cabani, P. Gianni, V. Mollica and L. Lepori, J. Solution Chem., 1981,10,563. 17 J. P. Guthrie, Can. J. Chem., 1977,553700. 18 N. Nichols, R. Skold, C. Spink, J. Suurkuusk and I. Wadso, J. Chem. Thermodyn., 1976,8, 1081. 19 D. French, M. L. Levine, J. H. Pazur and E. Norberg, J. Am. Chem. SOC., 1949,71,353. 20 Handbook of Chemistry and Physics, CRC Press Inc., Boca Raton, Florida, 68th edn., 1986, pp. E-42. 21 J. Suurkuusk and I. Wadso, Chem. Scr., 1982,20, 155. 22 M. Bastos, S. Hagg, P. Lonnbro and I. Wadso, J. Biochem. Biophys. Methods, 1991,23,255. 23 P. Backman, M. Bastos, D. Hallen, P. Lonnbro and I. Wadso, J. Biochem. Biophys. Methods, 1994,28,85. 24 L-E. Briggner and I. Wadso, J. Biochern. Biophys. Methods, 1991,22, 101. 25 P. R. Bevington, Data Reduction and Error Analysis for Physical Sciences, McGraw-Hill, New York, 1969, ch. 11. 26 W. H. Lawton and E. A. Sylvestre, Technometrics, 1971, 13,461. 27 D. Hallen, Pure Appl. Chem., 1993,7, 1527. 28 A. Hersey, B. H. Robinson and H. C. Kelly, J. Chem. SOC., Faraday Trans. I, 1986,82,1271. 29 T. Wiseman, S. Williston, J. Brandts and L. N. Lin, Anal. Biochem., 1989,179,131. 30 R. Lumry and S. Rajender, Biopolymers, 1970,9, 1125. 31 N. Nichols, R. Skold, C. Spink and I. Wadso, J. Chem. Ther- modyn., 1976,8,993. 32 J. Konicek and I. Wadso, Acta Chem. Scand., 1971,25, 1541. 33 Y. Matsui and K. Mochida, Bull. Chem. SOC. Jpn., 1979, 52, 2808. Paper 3/06837D;Received 16th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949003397

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(22), 3401-3404 Conductometric Study of Association Phenomena of Some Metal(l1) Complexes in Water at Different Temperatures Hassan A. Shehatat Chemistry Department, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt The association constants, K,, for Mg, Ca, Sr and Ba salts of acetate, propionate and butyrate have been determined at 25, 35, 45 and 55°C in aqueous solutions using the conductometric technique; the data were treated by the Lee and Wheaton method. K, tends to increase with atomic number in the order Mg < Ca < Sr < Ba for all the systems under study. While for the ligands, K, tends to increase in the order acetate < propionate < butyrate. Thermodynamic parameters, Lw" and AS*,for ion-pair formation of all the salts were obtained and discussed.The thermodynamic metal gradient, p, was derived and the thermodynamic character- istics of ion-pair formation of all systems were discussed. Also, Walden products, Aoq, were determined and discussed. The association between monovalent cations and organic ligands has been studied in depth.',, In a previous paper,3 we presented data on the association of Na' with formate, acetate and benzoate in ethanol-H,O and glycerol-H,O mixtures at 25 "C using conductance measurements. For most of the organic salts we have investigated so far (formates, acetates, propionates, butyrates and benzoates) there were no systematic good measurements reported in the literature, par- ticularly for their association with bivalent cations.In the 1970s, theoretical treatments were proposed by Quint and Viallard (Q-V)" and by Lee and Wheaton (L-W)' which dealt with the conductivity in dilute electrolyte solutions having an ionic composition. Lee and Wheaton tested the Q-V theory for some simple cases.6 Pethybridge and Taba7s8 have employed the L-W theory to study the conductivity of some binary asymmetric electrolytes. Indaratna et aL9 have also employed the L-W theory to study the conductivity of some BaC1,-HCl mixtures and a solution of CdC1, ,in which charged ion pairs exist. In this paper, the association of alkaline-earth metals (Mg", Ca2+, Sr2+ and Ba2+) with some aliphatic organic ligands (acetate, propionate and butyrate) in aqueous solu- tion was studied at different temperatures using a conducto- metric technique; the data were analysed by the L-W method.Although the results were discussed in terms of the association constant, K,, we were also able to account for the linearity between the Gibbs energy and the equilibrium constants (linear Gibbs energy relationship, LGER). Neiboer et a1."*" offered a rational scheme for the derivation of eqn. (1) and (2) that represent linear relationships for the forma- tion constants of metal-ion complexes or acids (proton complexes) formed within a set of metal ions and related ligands : log KML = B log KMsL + (log KMLo -log KMsLo) (1) log KML. = c log KMLo + (log KMsL -log KMsLo) (2) where K,, is the stability constant of complexes formed by a metal ion M and a ligand L, KMLo is the stability constant of complexes formed with reference ligand Lo, KMsLis the sta- bility constant of complexes formed with reference metal ion Ms, and KMsLo is the stability constant of the protonation process.B and C are two gradients that are invariant within the range of log K values encompassed. We have used this approach for the association phenomena of the substituted t Present address : Chemistry Department, Faculty of Teachers, P.O. Box 21, Al-Qunfudah 21912, Saudi Arabia. acetate salts, and we shall also use it where the standard Gibbs energy, AG", was used instead of association or proto- nation constants, in eqn. (1). The thermodynamic characteristics of any process can be studied through the parameters AH", AS* and AG", which may be evaluated by studying the process over a range of temperatures.If we apply the Neiboer equation to the associ- ation process at different temperatures, then the constant, B, is replace by p, a constant which represents the thermal change of a process. Hence, we can use the values of to discuss the thermodynamic characteristics of metal-ion com- plexes. Experimental Materials All chemicals were either of Analytical Reagent grade (AR) or Merck grade. Water was distilled twice and then passed through a column which contains mixed resin (anion and cation exchangers). The conductances of this deionized water are 0.85 x 1.05 x lop6, 1.30 x and 1.45 x S cm-' at 25, 35, 45 and 55 "C, respectively. The exact concen- tration of stock solutions was determined by the ion-exchange resin procedure.Electric Conductance The measurements were carried out with a conductivity meter, model LBR, at a frequency of 50 Hz, corresponding to mains frequency, and at 3.00 f:0.09 Hz since the shaped distortion factor <5%. The resistance range is 0.1 R to 10 Ma. The cell modelI2 used in the measurements is LTA 100, where the cell constant value equals 1.00. The measurements were carried out at 25, 35, 45 and 55°C. The concentrations of measured solutions were in the range of 1.5 x to 2 x mol dm-3. (Note that points at low concentrations e.g. 1.5 x mol dm-3 must be included, since A. values obtained by fitting techniques are dependent upon points measured at low concentrations.) Ma thematical Model The association of M2+(Mg2+, Ca2+, Sr2+ and Ba*+) with acetate, propionate and butyrate can be mono- or bi-dentate depending on the conditions of the experiment.Since the conditions of the experiments permit only the first association to take place, then, for monodentate binding: M2+ + L-=ML+ (1) 3402 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Therefore, the association constant, KA, is given by: Table 2 Computed values of K,, A,, d and CT for propionate salts in aqueous solution at various temperatures KA = [ML+]Y +/[M2+]Y2+ [L-]Y -(3) metal T/K Ao/cm2 f2-I K,/dm3 mol-' d/A uThe conductometric data were analysed using the L-W method.The L-W equation for the equivalent conductivity of Mg 298 89.2 23 1.2 5.94 0.024 ionj in a solution containing s ions is: 308 102.7 243.6 5.91 0.025 318 116.4 256.2 5.93 0.030 2. = A? 328 130.3 270.1 5.95 0.018IJ Ca 298 96.0 245.4 5.90 0.035 1 + zj ~V~XAZ(~XP~~)+ M~xPK)~+ c:(t~~.)31} 308 109.7 258.2 5.92 0.033 p=2 v=l 318 123.6 271.1 5.96 0.014 328 137.6 284.1 5.96 0.022 Sr 298 95.8 269.8 4.96 8.018 308 109.5 283.6 4.97 0.032 318 123.4 297.4 5.00 0.052(4) 328 137.4 311.2 4.98 0.092 where the various terms are as defined in the original Ba 298 99.8 288.7 4.95 0.021 paper13. Since t = kd, the equation involves a distance of 308 112.8 302.5 4.96 0.047 closest approach d which is assumed to be the same for all 318 126.1 316.3 4.99 0.063 pairs of oppositely charged ions.Lee and Wheaton in fact 328 139.5 329.8 5.01 0.039 omitted the terms of the order k3 [those involving Cg(t)and VY)(t)] in their analyses of data for unsymmetrical electro- lytes.' The conductivity of the binary electrolyte solution is Table 3 Computed values of K,, A,, d and cr for butyrate salts in aqueous solution at various temperatures given by: A($ML2)= 1zi ciAJc (5) I Mg 298 85.9 236.1 5.97 0.039 where ciis the true molar concentration of ionic species i, cis 308 97.0 249.2 5.95 0.120 the stoichiometric molar concentration of the electrolyte and 318 108.2 263.1 5.99 0.040 Aiis the conductivity of ion i per mol of transported charge. 328 119.5 278.3 6.02 0.012 For the activity coefficients in eqn.(3), we used the Debye- Ca 298 92.5 250.3 5.94 0.098 Hiickel expression : 308 103.7 263.4 5.98 0.065 318 114.0 277.3 6.00 0.038 Y = exp[ -(zie)'k/2&KT(1+ kd)] (6) 328 125.2 292.1 5.99 0.056 92.4 273.9 4.97 0.072For an asymmetrical associated pure electrolyte there are five Sr 298 parameters, i.e. three A:, KA and d. The preliminary analysis 308 103.5 287.1 4.95 0.091 318 113.8 302.1 4.99 0.064of the conductivity data showed that three of the parameters, 328 125.0 317.4 5.03 0.025A&-, KA and d, are strongly coupled. Consequently, we first Ba 298 96.5 293.2 4.98 0.038considered A0(M2'/2), KA and d as freely adjustable param- 308 108.1 306.1 5.01 0.092eters, and Ao(ML-) was fixed. With the best-fit value 318 119.0 320.3 5.02 0.074 obtained for d, we then calculated the best values of Ao(ML-), 328 130.2 335.6 4.99 0.019 Ao(M2+/2) and KA .This cyclic iteration was repeated until there was no change in the values of the parameters (normally two or three interations were sufficient). The results It is clearly desirable to obtain the standard enthalpy obtained are reported in Tables 1-3. ~7is the standard devi- change, AH*, by studying the association constants, KA,over ation. a range of temperatures by means of van't HOE'Sisochore: dlOgKA AH" Table 1 Computed values of K,, A, , d and cr for acetate salts in dT RT2 (7) aqueous solution at various temperatures then, metal T/K Ao/cm2 R-* KJdm3 mol-' d/A cr AH"log KA = --+ a constant Mg 298 94.1 49.6 5.82 0.024 RT 308 106.4 54.8 5.84 0.025 318 118.9 59.7 5.85 0.030 (the limitations of this equation in the derivation of our data 328 131.5 65.8 5.92 0.018 are KA > 0, and 298 < T/"Cd 328), where log KA values Ca 298 100.5 51.5 5.84 0.035 were plotted against 1/T, which is a straight line with slope 308 112.8 56.6 5.80 0.040 -AH*/R.Therefore, standard enthalpy changes, AH", can be 318 125.3 61.4 5.87 0.014 evaluated. The standard Gibbs energy changes, AG", can be 328 137.6 67.6 5.93 0.022 calculated from eqn. (9). Sr 298 100.4 53.8 4.92 0.018 AGe = -RT In KA (9)308 112.7 58.9 4.91 0.032 318 125.2 63.8 4.94 0.052 Also, the standard entropy change, ASe, can be evaluated as 328 137.4 69.8 4.97 0.092 follows : Ba 298 104.7 56.2 4.94 0.021 308 116.9 61.4 4.91 0.047 AS" = (AH" -AGe)/T (10) 318 129.3 66.2 4.96 0.063 The values of AH" and AS" are given in Tables 4 and 5,328 142.5 72.4 4.95 0.039 respectively. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Standard enthalpy changes, AHe, for acetate, propionate and butyrate salts AH*/kJ mol-' ligand Mg Ca Sr Ba acetate 3.29 3.12 3.05 2.94 propionate but yrate 1.85 1.95 1.72 1.86 1.70 1.73 1.57 1.61 Table 5 Standard entropy changes, ASe, for acetate, propionate and butytate salts at 25 "C AS~/Jmol -1 K -1 ligand Mg Ca Sr Ba acetate 43.49 43.26 43.36 43.36 proprionate 5 1.48 51.51 52.25 52.38 butyrate 51.98 52.15 52.48 52.62 Results and Discussion As shown in Tables 1-3, the A. values of acetate, propionate and butyrate salts increase in the order Mg < Sr < Ca < Ba.The values are in a reasonable range expected from some lit-erature source^.^^,^^ However, the values for Mg salts are small. We are unable to find any notable trends in the A. values. However, at infinite dilution the conductance of an electrolyte, ML, depends on the independent contributions from M and L. The independence of these contributions is judged by comparison of pairs of electrolytes containing a common ion. as shown in Table 6. The differences show the good internal consistency of the data. However, the Walden products, A,q, share a common characteristic; the A,q values for ML decrease with an increase in temperature from 25 to 55 "C. The decrease in A, q is very small, ca.1.5% on going from 25 to 55°C. Therefore, the behaviour of the Walden products can be summarized by the following func- tion : d(AOq)/dT= 0.5 dAo/dT (11) The decrease in A,q with temperature, which is often found in aqueous solutions,16 can probably be interpreted as a thermal expansion of the solvent sheath (which envelopes an ion and moves with it by ion-solvent interactions), i.e., the expansion of a solvated ion, because of the activation of solvent molecules forming the sheath. A, values for the ligands increase in the order butyrate < propionate < ace-tate, i.e. as the chain length of the ligands increase, the A. values decrease. Here, we have to describe the trend in the K, values of alkaline-earth metals.As is clear in Tables 1-3 K, tends to increase with atomic number in the order Mg < Ca < Sr < Ba. Ion association decreases as the cation Table 6 Differences of pairs of electrolytes containing a common ion pair with a pair with a common ion A(A,, + -A,, +) common ion (A,, -A,, -) Ca-Mg 6.6 f0.2 acetate-propionate 4.7 & 0.2 Sr-Ca -0.1 0.1 propionate-butyrate 3.4 f0.1 Ba-Sr 4.1 f0.1 acetate-butyrate 8.1 0.1 size increases owing to weakening electrostatic interactions. Accordingly, one would have expected more stable Mg than Ba ion pairs. However, the stability of the ion pairs decreases in the order Ba2+ > Sr2+> Ca2+ > Mg2+ for all the ligands under investigation. In the presence of a basic anion, associ- ation can occur through the intermediacy of a solvent mol- ecule.This 'localized solvolysis' was used to explain the cation order Ba2+ > Sr2+> Ca2+> Mg2+ for the salts of weak acids in water, since the smaller cations polarize the solvent more, and thus cause greater association.' 'The lower stability of the Mg2+ ion pairs compared with Ba2+ is due mainly to an unfavourable enthalpy term; the enthalpies decrease in the order (Ba2+ > Sr2+> Ca2+ > Mg2+ (Table 4)-From the data it is clear that the distance of closest approach of M2+ and L- ions is larger than the sum of ionic crystallographic radii for these pairs of ions. These data suggest that the ion pair is solvent-separated and is not spherical. If we consider the ion pair to be formed only by the action of the Coulombic force in a continuum, both the values of AHe and AS" for ion-pair formation will be negative.In general, however, the solvation of ions is weakened as soon as the ion pair is formed, which causes an increase in bot! AHe and AS*. Therefore, all the experimental values of AH and ASe in Tables 4 and 5 are positive. Assocation takes place with the hydrated cations, in which the water is arranged more easily around the smallest ions, and therefore the enthalpies decrease with increasing size. Subsequently loss of hydration occurs, resulting in an increase in the disorder of the system. Entrofy changes will be greater with smaller cations. Also, AS values increase as the size of ligand increases in the order acetate < propionate < butyrate.For all the salts under investigation, the association con- stants increased as the temperature increased which indicates that these association processes are endothermic. Also, the association of ligands increases in the order acetate < pro-pionate < butyrate i.e., the association becomes stronger as the size of ligand increases. Comparing the acetate ion with a solvent molecule as a ligand, it is clear that the acetate ion has less steric hindrance, the same coordinating atom (oxygen), and is a much stronger base, owing to the negative charge on the oxygen. Thus, there is good reason to believe that the acetate ion would preferentially displace solvent from the primary coordination sphere of the alkaline-earth- metal cations." The same arguments apply to the propionate and butyrate ions.Therefore, acetate, propionate and butyrate anions should displace solvent molecules from the inner coordination sphere of the alkaline-earth-metal cations, causing greater association in the order butyrate > propionate > acetate. On the other hand, we can apply the other approach, namely the linear Gibbs energy relationship, LGER. Neiboer et al.' 'discussed the linearity between protonation constants of the derivatives of selected reference compounds and the corresponding stability constants of their derivatives with a metal using eqn. (1). However, in view of the relation between the standard Gibbs energy change and the stability or proto- nation constants, eqn.(1) can be expressed in terms of Gibbs energy."*19 where AassG is the Gibbs energy of the association process at different temperatures, AprotG is the Gibbs energy of the protonation process at any temperature,20T21 Aa,, G" is the standard Gibbs energy of the complexation (association) process at 25 "C, AproiG" is the standard Gibbs energy of the protonation process at 25°C and B is the rate of change in 1 28 30 32 -AprotG/kJ mol-' Fig. 1 Correlation between the ionic association and the proto- nation Gibbs energies for the metal butyrates at different tem- peratures: (0)Mg"; (A) Ca"; (0)Sr"; (a)Ba" the Gibbs energy of association in a series of complexes, ML, at different temperatures, of the metal ion M, with respect to the Gibbs energy for protonation of the related series of com- plexes, MsL, of the reference metal ion, Ms, at different tem- peratures.Values of p may be used to discuss the thermodynamic characteristics of metal ion complexes. Consequently we shall study the linear relationships between A,,, G and Aprot G. Eqn. (12) can be simplified as AassG = BAprotG + R (13) Fig. 1 shows ABSSGus. AprotG,for butyrate salts, which is linear with slope p. Table 7 Thermodynamic metal gradient, 8, values for acetate, pro- pionate and butyrate salts 1038 ~ ~~ ~ ligand Mg Ca Sr Ba acetate 583 57 1 571 568 propionate 579 564 566 556 butyrate 552 547 547 540 J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 The values of #Iare given in Table 7. From inspection of the values of /Ifor all systems under study, it is obvious that j3 is positive, which indicates that the change in the Gibbs energy for association is directly proportional to the proto- nation standard Gibbs energy. Also, it is clear that for each ligand, L, the j? values decrease in the order Mg > Ca xSr > Ba. Therefore, the Gibbs energy for BaL is the lowest, while that for MgL is the highest and those for CaL and SrL are very similar. However, if we apply values of fi to the reactions of a single test metal ion, M, with a series of related ligands, p values decrease in the order acetate > propionate > butyrate. That is, the rate of change of Gibbs energies of association and that of protonation is smaller where the association process takes place spontaneously in the order butyrate > propio-nate > acetate.References R. J. L. Martin, Aust. J. Chem., 1962, 15,409. M. M. Jones and E. Griswold, J. Am. Chem. SOC.,1954,76,3247. H. A. Shehata, J. Znd. Chem. SOC., 1993,70,295. J. Quint and A. Viallard, J. Sol. Chem., 1978,7, 137. W. H. Lee and R. J. Wheaton, J. Chem. SOC., Faraday Trans. 2, 1978,74, 743. 6 W. H. Lee and R. J. Wheaton, J. Chim. Phys., 1977,74,659. 7 A. D. Pethybridge, J. Chem. SOC., Faraday Trans. 1, 1982, 78, 627. 8 A. D. Pethybridge and S. S. Taba, J. Chem. SOC., Faraday Trans. 1, 1982,78, 1331. 9 K. Indaratna, A. J. McQuillan and R. A. Matheson, J. Chem. SOC.,Faraday Trans. 1,1986,82,2755. 10 E. Nieboer, W. McBryde and A. E. William, Can. J. Chem., 1970, 48,2549. 11 E. Nieboer, W. McBryde and A. E. William, Can. J. Chem., 1970, 48,2565. 12 M. M. Emara, H. A. Shehata and A. M. Wasfi, Pak. J. Sci. Ind. Res., 1987,30, 254. 13 W. H. Lee and R. J. Wheaton, J. Chem. SOC., Faraday Trans. 2, 1978,74, 1456. 14 R. A. Robinson and R. H. Stokes, Electrolyte Solutions, Pitman Press, London, 2nd edn., 1959, pp. 463-465. 15 M. Muffaruddin, M. Salahuddin and W. U. Malik, J. Znd. Chem. SOC.,1963,40,467. 16 R. Fujishiro, G. Wada and R. Tamamushi, in Yoeki-no-seishitsu, Tokyo-Kagaku-dojin, Tokyo, 1968, vol. 2, pp. 58; 121. 17 R. W. Kreis and R. H. Wood, J. Phys. Chem., 1971,75,2319. 18 S. H. Elnakhaily, Ph.D. Thesis, Al-Azhar University, 1990. 19 H. A. Shethata, S. H. Elnkhaily and M. M. Emara, J. Znd. Chem. SOC.,1993,70, 193. 20 M. M. Emara, H. A. Shehata and M. S. Abd Elsamad, Pak. J. Sci. Znd. Res., 1988,31, 104. 21 C. D. Hodgman, R. C. Weast and S. M. Selby, in Handbook of Chemistry and Physics, 1955, Chemical Rubber Publishing Co., Ohio, 37th edn., p. 1646. Paper 3/07812E; Received 22nd December, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949003401

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994,90(22), 3405-3408 Dissolution of Synthetic Hydroxyapatite in the Presence of Lanthanum Ions Ph. Schaad and Ph. Gramain" Institut Charles Sadron (CRM-€A HP) CNRS-ULP, 6 rue Boussingault, 67983 Strasbourg Cedex, France F. Gorce and J. C. Voegel Centre de Recherches Odontologiques CJF 92-04 INSERM, 1 Place de I'Hbpital, 6700 Strasbourg, France Numerous cations exhibit strong interactions with dental or synthetic hydroxyapatite (HAP) and of these, there are indications that lanthanum improves the acid resistance of these compounds. In the present study, the reaction of La3+ ions with synthetic HAP in a water suspension has been investigated. It has been demonstrated that these cations react with phosphate ions in equilibrium with HAP to form insoluble LaPO,.This reaction releases protons which induce HAP dissolution up to its solubility equilibrium and to complete precipitation of La3+ ions. The kinetics of HAP dissolution at pH 5 with samples equilibrated for 2 h in the presence of 10 or 50 ppm LaCI, show no differences compared with pure HAP. This strongly suggests that La3+ ions added to HAP in water suspension have no effect on the surface condition of this compound. Hydroxyapatite [Ca,,(PO,),(OH),] is not only a main con- stituent of mineral phosphate, but also the main inorganic constituent of bones and teeth. Its surface interactions with various inorganic and organic compounds in aqueous media have been extensively investigated because it is possible that they affect the dissolution and growth processes.In particu- lar, there are indications that lanthanum may improve the acid resistance of enarnel'v2 and, when associated with fluo- ride ions, may contribute to the prevention of ~aries.~ Kobay-ashi et aL4 suggest that lanthanum (ionic radius 1.14 A) exchanges calcium (ionic radius 1.05 A) at the HAP surface and is able to contribute the prevention of enamel disso- lution. Tanizawa et aL5 proposed a mechanism in which La3+ ions react with apatite to form stable crystalline LaPO, in acidic solution. However, Collys et aL6 showed that HAP treated with lanthanum does not influence either the indenta- tion length or calcium release. In order to clarify this point, it would be useful to known more about the chemical reaction mechanisms of La3+ with HAP.Since the dissolution mecha- nism of HAP is very sensitive to its surface composition, the influence of lanthanum on the kinetics of HAP dissolution seems an interesting area of study. In this study, the reaction between synthetic HAP powder and lanthanum chloride in neutral and acidic solutions is investigated. Two types of experiments are performed. First, the reaction of LaCl, with HAP equilibrated in solution is analysed. Secondly, the dissolution kinetics of an HAP sus-pension equilibrated in the presence of LaCl, are studied at a constant pH of 5. Experimental HAP The experiments were carried out with 100-160 pm hydroxy- apatite HAP platelets (Bio-Gel HTP, Bio-Rad Laboratories, Richmond, California) prepared and characterised as described elsewhere.' A scanning electron microscope study showed that the particles are thin platelets. The specific surface area, determined by the BET method was 51 m2 g-'.The experimental Ca: P ratio of this HAP is 1.625 f0.06 which may be compared with the theoretical value of 1.667. Its zero-point charge in 8 x lo-' mol 1-' KCl solution is measured' at pH 7.47 and 37 "C. Solutions The salts used were of analytical grade. LaC1, -xH,O (Fluka, with x = 6.5) was added as a concentrated aqueous solution. Activity factors were calculated using the Debye-Huckel approximation for calcium and phosphate ions. The contri- bution of lanthanum ions was neglected for the calculation of the activity factors because their concentration was always at least two orders of magnitude lower than that of KCl (8 x lo-' mol 1-'), which was added to maintain a constant ionic strength.For all experiments, corrections were also made systematically to take into account the variation in volume due to evaporation and acid injection. Phosphate concentrations in the solutions were determined by spectrop- hotometric analysis of molybdenum blue phosphate complex- Values were obtained to an accuracy of f10%. Reaction of Phosphate Ions with La3' The reaction of phosphate ions with La3+ was studied at 37°C by analysis of lanthanum, calcium and phosphate con- centrations using atomic emission spectrometry (AES),' ' X-ray fluorescence and pHmetric methods. For AES and X-ray fluorescence analysis, 50 ml of a solu- tion containing lop4 mol I-' CaCl,, 6.0 x mol 1-' KH,PO, and 8.0 x lo-' mol 1-' KC1 (this solution is termed the 'standard solution' in this text) and adjusted to pH 5, were stirred for ca.1 h until the equilibrium of the CaZ+ and pH electrodes was attained. Then, the pH was adjusted to 7, and 10 ml of concentrated lanthanum solution (3.12 x lo-, mol 1-' in the standard solution at pH 5) were added so that the lanthanum concentration in the bulk was precisely 7 ppm (5.20 x lo-' mol 1-'). During this step, the pH decreased to 4 and a solution of KOH was added in order to readjust the pH to 7.0. A 5 ml aliquot of the solution was withdrawn using a 0.22 pm Millex-GV Millipore filter and lanthanum, calcium and phosphate concentrations were determined (Table 1).Experiments involving the pHmetric method were carried out by adding 50 ml of 8 x lo-, mol 1-' KCl solution to give a solution containing 50 ppm (3.76 x mol 1-') of lanthanum chloride. The solution was adjusted to pH 5 and aliquots of 50 pl of CaC1, solution (lo-' mol 1-') and of 30 pl of KH,PO, solution (lo-' mol 1-') were added. Between J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Amount of ionic species in solution before and after addi- tion of 7 ppm lanthanum to a solution containing calcium and phos- phate ions in amounts corresponding to a saturated HAP solution at pH 7" Ca2+/lo6 mol phosphate/lo6 mol La3+/lo6 mol before La3+ addition 6.0 f0.15 3.6 f0.2 0 after addition of 5.7 f0.15 0.5 f0.2 <0.4 f0.4 3.2 x mol La3+ " Values are obtained by AES and X-ray fluorescence after filtration through a 0.22 pm filter.each addition, the pH was accurately adjusted to 5 until equi- librium was reached (Fig. 1). At equilibrium phosphate ions were titrated after filtration through a 0.22 pm filter. Equilibration of HAP in the Presence of La3+ 196 ml of a solution containing stoichiometric amounts of calcium and phosphate ions corresponding to the HAP equi- librium conditions at pH 7.0 (1 x lo-, mol 1-' CaCl,, 6 x lo-' mol 1-' KH,PO, and 8 x lo-, mol L-' KCl) were first adjusted to pH 5.0 for electrode stabilisation at 37°C.Then the pH was adjusted to 7 and 40 mg of HAP were introduced and slowly stirred for 2 h. 4 ml of the con- centrated lanthanum chloride solution were added so that the initial concentration of lanthanum in the bulk solution was 10 or 50 ppm (7.52 x lo-' mol 1-' or 3.76 x lo-, moll-'). After 2 h, the time necessary to attain equilibrium (pH ca. 6.5), the stirring was stopped and 150 ml of the supernatant solution were withdrawn and analysed after powder decanta- tion (Table 2). Dissolution Kinetics Dissolution kinetics measurements on HAP were performed at a constant pH of 5 with an automated set-up composed of a reactor (Mettler, type DV702, Greifensee, Switzerland) and two titrators (Mettler, type DL21, Greifensee, Switzerland) connected to a recorder and interfaces to a microcomputer.'2 The reactor, thermostatted at 37 "C, includes a stirring system working at lo00 rpm, a water-tight cover, an argon bubbling system, a combined pH electrode (Mettler, type DG111, Grei- fensee, Switzerland) and a calcium electrode (Orion Research, type 94-09, Cambridge, USA).The pH and calcium ion activ- ities within the bulk solutions were continuously monitored with time and recorded. Dissolution at pH 5 without Lanthanum 200 ml of the standard solution at pH 5 were equilibrated in the reactor for 1 h. The pH was adjusted to 7, and 40 mg HAP were added and the solution was equilibrated for 2 h. The HAP powder was decanted and 150 ml of solution were withdrawn and used to determine the phosphate concentra- tion.The aim of this step was to follow the same conditions of equilibration as in our dissolution experiments published previously7 using only 10 mg of HAP in 50 ml solution. In the reactor, the pH was then rapidly adjusted and automatically maintained at pH 5.0 for the dissolution process. During this period, the proton uptake and calcium release were contin- uously recorded until equilibrium was reached. At equi-librium, the phosphate concentration was also determined. Dissolution at pH 5 with Lanthanum The same method as described above was used with 196 ml of solution. However, after 2 h of HAP equilibration and before the decantation step, 4 ml of concentrated lanthanum solution were added to obtain a lanthanum concentration in the bulk of 10 or 50 ppm.Once the HAP equilibrium was attained (controlled by the pH stability), the precipitate of LaPO, was decanted together with the HAP suspension, so that nearly all the added lanthanum stayed in the reactor 5'0 i 4.0 -E '& 3.0 F-gm+ 2.0 L + I 1.o 0.0 -0.0 1.0 2.0 3.0 4.0 5.0 added phosphate/l O5 rnol Fig. 1 pHmetric titration of a solution containing 1.80x mol (50 ppm) lanthanum after phosphate additions [reaction (l)]. Ali-quots of a base (KOH) are added after phosphate addition to main- tain the pH at the initial value of 5. once the 150 ml of solution were withdrawn. The initial con- ditions of the dissolution step were the same as in the absence of lanthanum except that HAP is in equilibrium with a solu- tion of lower phosphate ion concentration and non-congruent (Ca/P # 1.667) (see Table 2).Solubility Product of HAP in the Presence of LaCI, The ionic product of HAP was calculated using the Debye- Huckel approximation after equilibration in the presence of LaCl, and at the end of the dissolution period. The values obtained were compared with the solubility product of HAP" (pK, = 59.5 & 1.2, Table 3). Results and Discussion When a solution of LaC1, is added to a suspension of HAP equilibrated at pH 7, the pH decreases rapidly. This decrease induces the dissolution of HAP up to equilibrium. In order to Table 2 Concentrations of ionic species present in solution at 37 "C after equilibration of HAP in the presence of 0, 10 or 50 ppm lanthanum ions and before dissolution at pH 5 La3+ added (ppm) Ca2+/mmoll-' phosphate/mmol 1-' PH" La3+/iOi1 mmol 1-' PK, 0 0.11 & 0.01 0.14f0.05 6.90 0 58.4k 1 10 0.17k0.01 0.12 k0.05 6.64 0.36 59.0f1 50 0.47 f0.01 0.02f0.05 6.57 2.17 59.6& 1 " pH at equilibrium.The lanthanum concentrations at equilibrium were calculated from the solubility product of LaPO, .15 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Concentrations observed at the equilibrium of dissolution of HAP and final HAP ionic products La3+ (PPm) Ca2+/mmolI-' phosphate/mmol 1-PH ~a3+/1011moi 1-1 0 PK, 10 3.3 f 0.1 2.0 f 0.2 4.95 3.5 59.9 f 1 50 3.5 f 0.1 2.0 f 0.2 4.95 3.5 59.7 f 1 The lanthanum concentrations at equilibrium were calculated from the solubility product of LaPO, .' characterise the system fully, we investigated (i) the nature of the reaction between phosphate and lanthanum ions, (ii) the reaction of HAP equilibrated in the presence of La3+ and (iii) the effect of lanthanum on the dissolution kinetics of HAP at pH 5.Reaction of Phosphate Ions with La3+ LaC1, was added to a solution containing lo-, mol I-' calcium and 6.0 x lo-' mol I-' phosphate, concentrations which correspond to the ionic concentrations in equilibrium with HAP at pH 7, in order to obtain a bulk concentration of 7 ppm. The decrease in observed pH was neutralised by KOH addition. Table 1 summarises the concentrations observed in solution after filtration of the suspension through a 0.22 pm filter.It is clear that calcium concentrations are not affected, but that all the La3+ ions and most of the phosphate ions disappear. The ratio between both species takes a value close to 1. This demonstrates that an insoluble LaPO, pre-cipitate is formed [reaction (l)] rather than lanthanum hydroxides such as La(OH)2+ (pK, = -8.5 at 25 OC):'" La3+ + H,PO,-+LaPO, + 2H+ LaPO, is known to be the least soluble rare-earth-metal phosphate with a pK, of 26.15 0.52 at 25°C and 26.49 at 70°C.'5 In order to confirm the release of two protons for each La3+ ion, titration experiments were carried out at pH 5. At this pH, 98% of phosphate ions are in the H,PO,- form. Fig. 1 shows that for each addition of 1 mol phosphate to a solution containing 50 ppm lanthanum, 2 mol NaOH are required to maintain the pH at its initial value of 5, thus confirming reaction (1).Equilibrium of HAP in the Presence of La3+ Table 2 shows the results of experiments in which 10 or 50 ppm lanthanum are added to an equilibrated suspension of 40 mg HAP in 200 ml water at pH 7. Addition of lanthanum provokes a decrease in pH which induces the dissolution of HAP up to the equilibrium (pH 6.6). The results of the analysis show that (i) no La3+ ions may be detected in the filtered solutions (the values in Table 2 are calculated according to the solublity product), (ii) the phosphate concen- tration decreases as the lanthanum concentration increases and (iii) the calcium concentration is high.It is clear the LaPO, precipitates until the lanthanum ions disappear. As a result, at the end of the process, HAP is in equilibrium with calcium and phosphate ions in non-congruent ratios with respect to HAP. From the data obtained, the ionic product of HAP may be calculated and compared with its solubility product. Table 2 shows that the solubility product of HAP is attained whatever the amount of lanthanum added. This is also demonstrated by the results of analysis of solutions at the end of the dissolution experiments (Table 3) carried out at pH 5 and in the presence of 10 or 50 ppm La3+. Once again the solubility product of HAP is attained with no influence from lanthanum. To summarise, the addition of La3+ ions to HAP suspen- sions induces a dissolution of HAP with LaPO, precipitation up to the non-congruent equilibrium of solubility of pure HAP.This precipitation, which occurs in the bulk, may poss- ibly also occur on the HAP surface. If such a formation occurs and can affect the HAP interface by ion adsorption or ion-exchange reaction, the HAP dissolution behaviour must be affected. Dissoiution Kinetics In order to study the effect of lanthanum in solution on the dissolution kinetics of HAP, we use the solutions of equili-brated HAP which have been previously analysed (Table 2). The pH was rapidly lowered to 5 and maintained constant up to the solubility equilibrium. Both the amounts of calcium released and protons consumed were recorded.The kinetics of HAP suspensions equilibrated in the presence of 0, 10 and 50 ppm lanthanum are shown in Fig. 2. No important differ- ences are observed. The deviation from the reference kinetics (with 0 ppm La3+), which are more important than the experimental errors, may be attributed to the slight difference of initial conditions since, as described in Table 2, the pres- ence of lanthanum induces an excess of calcium ions over phosphate ions compared with the congruent equilibrium. Such excess may slightly change the initial properties of the HAP surface. Fig. 3 presents the evolution with time of the ratios of consumed protons over released calcium. Once again no significant differences are observed and the average values are in agreement with a dissolution process following the chemical reaction (Q,JQca = 1.4 at pH 5).From these experiments, it appears that the addition of lanthanum ions in our experimental conditions has no effect on the permeability of the adherent charged layer. It may be concluded that no acid resistant precipitate of LaPO, is formed on the HAP surface. 5.0I I I 0.01 I I 1 0 200 400 600 f/min Fig. 2 Kinetics of proton uptake during the dissolution of 40 mg HAP in 50 ml solution at pH 5.0 and 37°C. HAP is equilibrated in the presence of (a) 0, (b) 10 and (c) 50 ppm of lanthanum for 2 h before the dissolution process. 3408 2.0 I I I I t 11I 1.5 +c1 m0 " L1.o 0 200 400 600 t/min Fig.3 Evolution of ratios of consumed protons and calcium rel- eased QH+/QCa2+us. time, for the kinetics of dissolution at pH 5.0 with (a)0, (b) 10 and (c)50 ppm lanthanum Conclusion The experiments described above clearly show that the inter- action between La3+ ions and HAP in solution is controlled by the precipitation of the very insoluble LaPO, salt. This reaction releases protons which induce dissolution of HAP until the lanthanum is completely consumed. Furthermore, the kinetics of the dissolution of HAP are not affected by the presence of precipitated LaPO, formed in situ and in contact with HAP for 2 h. The main effect is that the equilibrium solutions become non-congruent, but the dissolution process is still controlled by the solubility product of pure HAP.These results are in agreement with the results of Collys et aL3 showing that no recovery of the enamel surface is obtained when lanthanum, present alone, in calcifying solu- tions (containing H2P04- ions) is applied to HAP for 24 h. According to our experiments, such a solution contains lanthanum in the form of insoluble LaPO, which is unable to interact strongly with the dissolution process of HAP. If we consider the results of Tanizawa et aL5 which show that La3+ ions reacted with apatite to form stable crystalline LaPO, in acidic solution without ion-exchange, our results seem to demonstrate that this coating has no influence on the acidic resistance of HAP. However, Tanizawa used a much more J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 concentrated lanthanum concentration (1740 ppm) leading probably to a much thicker coated layer. Our conclusions also seem to contradict the results of Shimano' who showed that an exchange reaction of Ca2+ and La3+ occurred on the enamel surface when an extracted human tooth was dipped in concentrated lanthanum solution (25 680 ppm). After seven days, the surface became coated with LaPO, and is acid resistant. However, Shimano also notes that there is no effect at lower lanthanum concentrations (lower than loo00 ppm). We thank Professor M. J. F. Leroy (Plate Forme Analytique, ULP Strasbourg) for kindly placing X-ray fluorescence and AES at our disposal and M. Rastegar for his technical assist- ance.P. s. wishes to thank the Faculty of Odontology of Strasbourg for financial support. References 1 R. Shimano, J. Dent. Health (Jpn.), 1980,29, 17. 2 Y. Koayashi, M. Ozeki, T. Yagi, T. Hosoi and M. Takei, J. Dent. Health (Jpn.), 1980,30, 388. 3 K. Collys, R. Cleymaet, D. Slop, E. Quartier and D. Coomans, Trace Elem. Med., 1992,9,97. 4 Y. Kobayashi, M. Ozeki, M. Takei and R. Shimano, J. Dent. Health (Jpn.), 1979,29,276. 5 Y. Tanizawa, T. Sawamura and T. Suzuki, J. Chem. SOC., Faraday Trans., 1990,86,4025. 6 K. Collys, D. Slop, L. de Langhe and D. Coomans, J. Dent. Res., 1990,69,458. 7 J. M. Thomann, J. C. Voegel and Ph. Gramain, Calcif. Tissue Znt., 1990,54, 121. 8 Unpublished data. 9 P. S. Chen, T. Y. Toribara and H. Warner, Anal. Chem., 1956, 28, 1756. 10 B. N. Ames, Methods Enzymol., 1966,8, 115. 11 F. Dudermel-Le Cornec, 1992, Ph.D. Thesis, Universite Louis Pasteur, Strasbourg. 12 J. M. Thomann, P. Gasser, E. Brks, J. C. Voegel and Ph. Gramain, Comput. Methods Programs Biomed., 1990,31,89. 13 Ph. Gramain, J. C. Voegel, M. Gumpper and J. M. Thomann, J. Colloid Interface Sci., 1987, 118, 148. 14 C. F. Baes and E. E. Mesner, The Hydrolysis of Cations, Wiley, New York, 1976, pp. 135-139. 15 F. H. Firsching and S. N. Brune, J. Chem. Eng. Data, 1991, 36, 93. Paper 4103166K; Received 27th May, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949003405

出版商:RSC    

年代:1994

数据来源: RSC