摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Dr. Peter J. Sarre Department of Chemistry University of Notti ng ham University Park Nottingham NG7 2RD, UK Faraday Editorial Board Prof. I. W. M. Smith (Birmingham) (Chairman) Prof. M. N. R. Ashfold (Bristol) Dr. B. E. Hayden (Southampton) Dr. D. C. Clary (Cambridge) Prof. A. R. Hillman (Leicester) Dr. L. R. Fisher (Bristol) Prof. J. Holzwarth (Berlin) Prof. H. M. Frey (Reading) Dr. P. J. Sarre (Nottingham) Dr. R. K. Thomas (Oxford) Editorial Manager and Secretary to Faraday Editorial Board Dr. Robert J. Parker The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF, UK Senior Assistant Editors: Mrs.S. Shah, Dr. R. A. Whitelock Assistant Editor: Mrs. C. J. Seeley Editorial Secretary: Mrs. J. E. Gibbs International Advisory Editorial Board R. S. Berry (Chicago) Y. Marcus (Jerusalem) A. M. Bradshaw (Berlin) B. J. Orr (North Ryde) A. 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Dr. P. J. Sarre, Scientific Editor. Tel.: Nottingham (0602) 51 3465 (24 hours) E- Mail (JANET): PCZPSF@UK.AC.NOTT.VAX Fax: (0602) 51 3466 Telex: 37346 UNINOT G Dr. R. J. Parker, Editorial Manager. Tel. : Cambridge (0223) 420066 E-Mail (INTERNET): RSCl @RSC.ORG (For access from JANET use RSCl %RSC.ORG@UK.AC.NSF NET-R ELAY) Fax: (0223) 423623 or 420247 Telex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX021

出版商:RSC    

年代:1994

数据来源: RSC

ISSN:0956-5000    

DOI:10.1039/FT99490BX023

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(6) 817-934 (1994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 817 Photolysis of HOBr and DOBr at 266 nm: OH and OD product-state distributions N. Shaw, A. J. Bell, M. J. Crawford and J. G. Frey 825 Chemiluminescent reaction of oxygen atoms with dimethyl disulfide and dimethyl sulfide U. B. Pavanaja, H. P. Upadhyaya, A. V. Sapre, K. V. S. Rama Rao and J. P. Mittal 831 Pulse radiolysis of iodate in aqueous solution S. P. Mezyk and A. J. Elliot 837 Temperature dependence of the rate constant for the reaction ea; + OH in water up to 150°C A. J. Elliot and D. C. Ouellette 843 Far-IR study of the hydrogen-bond vibration of intramolecular bonds in substituted 2-diethylaminomethylphenol N-oxides, as a function of the pK, of the phenolic group B. Brzezinski, A.Rabold and G. Zundel 845 Cyclodextrin-monosaccharide interactions in water A. F. Dad de Namor, P. M. Blackett, M. C. Cabaleiro and J. M. A. A1 Rawi 849 Study of the structure-breaking effect in aqueous CsCl solutions based on H,O/D,O isotope effects on transport coefficients and microdynamical properties A. Sacco, H. Weingartner, B. M. Braun and M. Holz 855 Transfer Gibbs Energies for ClO, ,BrO; ,10, ,ClO, and 10, anions for water-acetonitrile and water-tert-butyl alcohol mixtures J. Benko and 0.Vollarova 859 2-Methoxyethanol-water solvent system: Static relative permittivity from -10 to +80 "C F. Corradini, L. Marcheselli, A. Marchetti, M. Tagliazucchi, L.Tassi and G. Tosi 865 Preferential solvation of a fi-sensitive dye in binary mixtures of a non-protic and a hydroxylic solvent M. Scremin, S. P. Zanotto, V. G. Machado and M. C. Rezende 869 Cationic micellar effect on the kinetics of the protolysis of aromatic carboxylic acids studied by the ultrasonic absorp- tion method T. Ida, M. Yamasaki, H. Yano, T. Sano and S. Harada 875 Forces of inertia acting on the aqueous pore fluid of anionic polyelectrolyte gels N-T. Dang and D. Woermann 879 IR and submillimetre-wave spectra of doped poly(p-phenylene vinylene) S. El-Atawy and K. Davidsoo 883 Dielectric and steric hindrance effects on step-polymerization of a diepoxide with monoamines G. P. Johari 889 Theory of monolayers of non-Gaussian polymer chains grafted onto a surface.Part 1.-General theory V. M. Amoskov and V. A. Pryamitsyn 895 Effect of potassium on the surface potential of titania D. Courcot, L. Geagembre, M. Guelton, Y. Barbaux and B. Grzybowska 899 High-temperature diffusion of hydrogen and deuterium in palladium T. Maeda, S. Naito, M. Yamamoto, M. Mabuchi and T. Hashino 905 Surface characterization and catalytic activity of Co,Mg, -,Al,O, solid solutions. Oxidation of carbon monoxide by oxygen F. Pep and M. Occhiuzzi 911 Mechanism of branched carbon-chain formation from CO and H, over oxide catalysts. Part 1.-Adsorbed species on ZrO, and CeO, during CO hydrogenation K-I. Maruya, A. Takasawa, M. Aikawa, T. Haraoka, K. Domen and T. Onishi 919 FTIR spectroscopic study of the zeolitic adsorption of hydrogen cyanide on acidic sites C.J. Blower and T. D. Smith 931 FTIR study of the interaction of hydrogen cyanide with alkali-metal ion, silver(1) and nickel@) ion-exchanged near- faujasite zeolites C. J. Blower and T. D. Smith Note: Where an asterisk appears against the name of one or more of the authors, it is included with the authors' approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles. For further details contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W 1V OBN, UK Tel: +44 (0)71437 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/FT99490FP057

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Afanasiev, P., 193 Aikawa, M., 911 Aldaz, A., 609 Alfimov, M. V., 109 AI-Ghefaili, K. M., 383 Ali, V., 579,583 Allegrini, P., 333 Allen, N. S., 83 A1 Rawi, J. M. A., Amorim da Costa, A. M., Amoskov, V. M., 889 Aragno, A., 787 Aramaki, K., 321 Aravindakumar, C. T., 597 Asai, Y., 797 Avila, V., 69 Baba,T., 187 Ball, S. M., 523 Barbaux, Y., 895 Barthomeuf, D., 667,675 Basini, L., 787 Bassoli, M., 363 Bauer, C., 517 845 689 Dang, N-T., 875 Danil de Namor, A. F., Davidson, K., 879 Demeter, A., 41 1 Demri, D., 501 Derrick, P. J., 239 Diagne, C., 501 Dickinson, E., 173 Doblhofer, K., 745 Domen, K., 911 Doughty, A., 541 Douglas, C. B., 471 Dwyer, J., 383 Dyke, J. M., 17 Eastoe, J., 487 Easton, C. J., 739 Ebitani, K., 377 El-Atawy, S., 879 Elisei, F., 279 Elliot, A.J., 831, 837 Engberts,J. B. F. N., Eustaquio-Rincbn, R., 113 Fantola Lazzarini, A. L., 845 727 423 Hosoi, K., 349 Hutchings, G. J., 203 Hutton, R. S., 345 Ikawa, S-i., 103 Ikonnikov, I. A., 219 Indovina, V., 207 Inoue, Y., 797,815 Ishigure, K., 93, 591 Isoda, T., 869 Ito, O., 571 Iwasaki, K., 121 Jakubov, T., 783 Jameel, A. T., 625 Jayakumar, R., 161 Jenneskens, L. W., 327 Jennings, B. J., 55 Jiang, P-Y., 591 Jiang, P. Y., 93 Johansson, L. B.-A., 305 Johari, G. P., 883 Joseph, E. M., 387 Joshi, P. N., 387 Kagawa, S., 349 Kaler, E. W., 471 MatijeviC, E., 167 Matsuda, J., 321 May, I. P., 751 Maaucato, U., 333 Mchedlov-Petrossyan, N. O., Merga, G., 597 Meunier, F., 369 Mezyk, S. P., 831 Mittal, J. P., 597, 703,711, Mohan, H., 597,703 Moriguichi, I., 349 Morikawa, A., 377 Morokuma, M., 377 Momson, C.A., 755 Muir, A. V.G., 459 Mukherjee, T., 711 Nagaishi, R., 93,591 Nagaoka, H., 349 Naito, S., 899 Naito, T., 763 Navaratnam, S., 83 Neoh, K. G., 355 629 825 Rosmus, P., 517 Rossi, P. F., 363 Rudham, R., 809 Ryde,N., 167 Sacco,A., 849 Sachtler, W. M. H., 233 Saitoh, T., 479 Salmon, G. A., 75 Sano, T., 869 Sapre, A. V., 825 Same, P. J., 517 Sato, K., 797 Sbriziolo, C., 31 1 Schedel-Niedrig, Th., 403 Schlo& R., 403 Schnabel, W., 287 Scremin,M., 865 Seddon, B. J., 605 Shahid, G., 507,513 Sharma, A., 625 Shaw, N., 17,817 Sheil, M. M., 239 Sheppard, N., 507,513 Shiao, J-C., 429 Bell, A. J., 17, 817 Bendig, J., 287 Bengtsson, L. A., 559 Benko, J., 855 Bensalem, A., 653 %rces,T., 411 Bergeret, G., 773 Bickelhaupt, F., 327 Biczok, L., 41 1 Blackett, P.M., 845 Blandamer, M. J., 727 Blower, C. J., 919,931 Boggis, S. A,, 17 Borisenko, V. N., 109 Fausto, R., 689 Favaro, G., 279,333 Feliu, J. M., 609 Filimonov, I. N., 219,227 Fogden, A., 263 Fornks, V., 213 Franck, R., 667,675 Freeman, N. J., 751 FrCty, R., 773 Frey, J. G., 17, 817 Frostemark, F., 559 Gans, P., 315 Gao,Y., 803 Garcia,R., 339 Kalugin, 0.N., 297 Kato, R., 763 Katsumura, Y., 93, 591 Kaur,T., 579 Kawashima, T., 127 Keil, M., 403 Kemball, C., 659 Kida, I., 103 Kiennemann, A., 501 Kim,J-H., 377 King,F., 203 Kirschner, J., 403 Kita,H., 803 Klein, M. L., 253 Nerukh, D. A., 297 Nicholson, D., 181 Nickel, U., 617 Ninomiya, J., 103 Nishihara, H., 321 Nogami, T., 763 Nonaka, O., 121 Nuiiez Delgado, J., 553 Nyholm, L., 149 Occhiuzzi, M., 207,905 Ohtsu, K., 127 Okamura, A., 803 Oliveri, G., 363 Onishi, T., 91 1 Shihara, Y., 549 Shiralkar, V.P., 387 Shishido, T., 803 Shizuka, H., 533 Silva, C. J., 143 Silva, F., 143 Simkiss, K., 641 Singh, J., 579, 583 Singh, R., 583 Smith, T. D., 919,931 Soares, V. A. M., 649 soria,v., 339 Spiro, M., 617 Stoeckli, F., 783 Bozon-Verduraz, F., 653 Bradley, C. D., 239 Bradshaw, A. M., 403 Braun, B. M., 849 Breysse, M., 193 Briggs,B., 727 Brocklehurst, B., 271 Brown, R. G., 59 Brown, S. E., 739 Bruna, P. J., 683 Brzezinskj B., 843 Butt, M. D., 727 Byatt-Smith, J. G., 493 Cabaleiro, M. C., 845 Caceres Alonso, M., 553 Calado, J. C. G., 649 Caldararu, H., 213 Calvente, J. J., 95 Camacho, J. J., 23 Campa, M. C., 207 Capos, A., 339 Capobianco, J.A., 755 Caragheorgheopol, A., Carvill, B. T., 233 Catalina, F., 83 Cavasino, F. P., 3 11 Chen, J-S., 429,717 Chen, Y-H., 617 Cheng, A., 253 Cherqaoui, D., 97 Chesta, C. A., 69 Chevalier, S., 667,675 Cho,T., 103 Christensen, P., 459 Climent, M. A., 609 Coates, J. H., 739 Cordischi, D., 207 Corma,A., 213 Cornier, G., 755 Corradini, F., 859 Corrales, T., 83 Cosa, J. J., 69 Coudurier, G., 193 Courcot, D., 895 Crawford, M. J., 817 Cullis, P. M., 727 Curtis, J. M., 239 213 Garcia Baonza, V., 553 Garcia-Paiieda, E., 575 Gautam, P., 697 Geantet, C., 193 Gengembre, L., 895 Gil, F. P. S. C., 689 Gill, D. S., 579, 583 Gill, J. B., 315 Goede, S. J., 327 Gomez, C. M., 339 Gonplves da Silva, A. M., Gray, P. G., 369 Green, W. A., 83 Grein, F., 683 Grimshaw, J., 75 Grzybowska, B., 895 Guelton, M., 895 Hachey, M., 683 Haeberlein, M., 263 Hall, D.I., 517 Hall, G., 1 Hallbrucker, A., 293 Halpern, A., 721 Hamnett, A., 459 Hancock, G., 523 Handa,H., 187 Hann,K., 733 Hao, L., 133 Harada, S., 869 Haraoka, T., 911 Harper, R. J., 659 Harriman, A., 697 Harrison, N. J., 55 Hashino, T., 899 Hattori, H., 803 Heal, M. R., 523 Heenan, R. K., 487 Helmer, M., 31, 395 Herein, D., 403 HerzogB., 403 Higgins, S., 459 Hindermann, J-P., 501 Hirst, D. M., 517 Hoekstra, D., 727 Holmberg, B., 559 Holz,M., 849 Hoshino, H., 479 649 Kleshchevnikova, V. N., Kobayashi, A., 763 Kobayashi, H., 763 Kondo, Y., 121 Kossanyi, J., 41 1 Kurrat, R., 587 Kuwamoto, T., 121 Laachir, A., 773 Lambert, J-F., 667, 675 Langan, J. R., 75 Lavanchy, A., 783 Lawarini, E., 423 Leaist, D.G., 133 Lei,G-D., 233 Lerner, B. A., 233 Leslie, M., 641 Li, J., 39 Li, P., 605 Lin, J., 355 Lincoln, S. F., 739 Lindblom, G., 305 Liu,C-W., 39 Liu,X., 249 Loginov, A. Yu., 219,227 Longdon, P. J., 315 Lunelli, B., 137 Mabuchi, M., 899 Machado, V. G., 865 Mackie, J. C., 541 Maeda,T., 899 Maestre, A., 575 Mahy, J. W. G., 327 Maity, D. K., 703 Makarova, M. A., 383 Maksymiuk, K., 745 Malatesta, V., 333 Malcolm, B. R., 493 Mallon, D., 83 Mandal, A. B., 161 Marcheselli, L., 859 Marchetti, A., 859 Mariani, M., 423 Martins, A., 143 Maruya, K-i., 911 Masetti, F., 333 Massucci, M., 445 629 Lu, J-X., 39 Ono, Y., 187 Oradd, G., 305 Ortica, F., 279 Ota, K-i., 155 Otlejkina, E. G., 297 Otsuka, K., 451 Ottavi, G., 333 Ouellette, D. C., 837 Ozutsumi, K., 127 Padley, M.B., 203 Pal, H., 711 Palleschi, A., 435 Paradisi, C., 137 Pardo, A., 23 Parsons, B. J., 83 Pavanaja, U. B., 825 Pedulli, G. F., 137 Peng, W., 605 Pepe,F., 905 Pereira, C. M., 143 PCrez, J. M., 609 Perrichon, V., 773 Peter, L. M., 149 Petrov, N. Kh., 109 Pispisa, B., 435 Pivnenko, N. S., 297 Plane, J. M. C., Porcar, I., 339 Potter, C. A. S., 59 Poyato, J. M. L., 23 Prenosil, J. E., 587 Previtali, C. M., 69 Pryamitsyn, V. A., 889 Rabold, A., 843 Rama Rao, K. V. S., Ramsden, J. J., 587 Rao, B. S. M., 597 Rehani, S. K., 583 Rettig, W., 59 Rey,F., 213 Rezende, M. C., 865 Rhodes, N. P., 809 Richter, R., 17 Rocha,M., 143 Rochester, C. H., 203 Rodes, A., 609 Rofia,S., 137 Rosenholm, J. B., 733 31, 395 825 Sun, L. M., 369 Suquet, H., 667,675 Surov, Y. N., 297 SuzukiT., 549 Tabrizchi, M., 17 Tagliazucchi, M., 859 TakagiT., 121 Takahashi, K., 155 Takasawa, A., 9 11 Tamura, K-i., 533 Tanaka,I., 349 Tassi, L., 859 Tateno, A., 763 Taylor, M.G., 641 Teixeira-Dias, J. J. C., 689 Teo, W. K., 355 Teraoka, Y., 349 Timms, A. W., 83 Timney, J. A., 459 Tosi, G., 859 Touret, O., 773 Tournayan, L., 773 Trejo, A., 113 TsujiH., 803 Tsunashima, S., 549 Turco Liveri, M.L., Turco Liveri, V., 3 11 Udagawa, T., 763 Umemoto, H., 549 Unayarna, S-i., 549 Upadhyaya, H.P., 825 Valat, P., 411 Valls, M. J., 609 Vedrine, J. C., 193 Venanzi, M., 435 Villarnagna, F., 47 Villemin, D., 97 Vlietstra, E. J., 327 Vollkovii, O., 855 Vollmer, F., 59 Vyunnik, I. N., 297 Wang,C.F., 605 Watanabe, H., 571 Waters, M., 727 Weckstrom, K., 733 Weingktner, H., 849 Weir, D.J., 751 Werner, H., 403 311 i Whitaker, B. J., 1 Wintgens, V, 411 YamajiM., 533 Yano,H., 869 Zhang,X., 605 Whitehead,M.A., 47 Woermann,D, 875 Yamamoto. M., 899 Yoshitake, H., 155 Zholobenko,V.L., 233 Wikander, G., 305 WohlemM, 403 Yamanaka,I., 451 Yotsuyanagi, T., 93,479 Zhong, G. M., 369 WilIiams,D.E., 345 Wormald, C. J., 445 YamasakLM., 869 Young, R.N., 271 Zubarev, V. E., 721 Wilpert, A., 287 Yagci,Y., 287 YaneS,C, 575 Zanotto, S. P., 865 Zundel,G., 843 11 The following papers were accepted for publication between 1st and 31st January 1994: '*O Tracer studies of CO oxidation with 0, on MOO,. Part 1.-Diffusion of l80 atoms from active sites during the catalysis and the determination of the number of active sites Y.Iizuka Activation of surface lattice oxygen in the oxidation of carbon monoxide on silica J.B. Moffat, Y. Matsumura and K. Hashimoto Enzymatic reaction in water-in-oil microemulsions. Part 2.-Rate of hydrolysis of a hydrophobic substrate, 2- naphthyl acetate Y. Miyake, T. Owari, F. Ishiga and M.Teramoto Cyclodextrin-monosaccharide interactions in water A. F. Danil de Namor, P. M. Blackett, M. C. Cabaleiro and J. M. A. A1 Rawi Dielectric behaviour of the N,N-dimethylformamide-2-methoxyethanol-1,2-dimethoxyethaneternary solvent system from -10 to + 20 C L. Tassi, F.Corradini, A. Marchetti, M. Tagliazucchi and G. Tosi Mossbauer study of oxygen-deficient Zn(I1)-bearing ferrites (ZnJ3e,-x0,s, 0 Ix I 1) and their reactivity toward CO, decomposition to carbon M.Tsuji, M. Tabata, Y. Tamaura, K. Akanuma and T. Togawa The 2-methoxyethanol-water solvent system. Static relative permittivity from -10 to + 80 "C L. Tassi, F. Corradini, L. Marcheselli, A. Marchetti, M. Tagliazucchi and G. Tosi Photophysical studies of substituted porphyrins T. G. Truscott, P. Charlesworth, D. Kessel, C. J. Medforth and K. M.Smith Evanescent wave spectroscopy: Application to the study of the spatial distribution of charged groups on an adsorbed polyelectrolyte at the silicdwater interface L. R. White, M. Trau, F. Grieser and T. W.Healy Dual transmission line with charge transfer resistance for conducting polymers W. J. Albery and A. R.Mount Dielectric and steric hindrance effects on step-polymerization of a diepoxide with monoamines G.P.Johari Solid-state conductivities of CPQ [1,l'-bis(p-cyanophenyl)-4,4'-bipyridilium]salts, redox-state mixtures and a new intervalence adduct D. R. Rosseinsky and P. M. S. Monk l8Otracer studies of CO oxidation with O2 on MOO,. Part 2.-Active sites for CO oxidation with 0, and those for oxygen isotope exchange between CO, and MOO, Y. Iizuka, H.Tanigaki, M. Sanada, J. Tsunetushi, N. Yamauchi and S.Arai Electropolymerisation of indole-5-carboxylic acid A. R. Mount and J. G. Mackintosh Appearance energies of small cluster ions and their fragments P. W.Harland, B. R. Cameron and C. G. Aitken Relaxation and crystallization of water in a hydrogel G. P. Johari and K. Pathmanathan Active side of bacteriorhodopsin. FI'IR and 'HNMR studies using models G.Zundel, B. Brzezinski and J. Olejnik Comparison of thermal stability, acidity, catalytic properties and deactivation behaviour of novel aluminophosphate-based molecular sieves of type 36 D. B. Akolekar Influence of polymer structure on the electrochemistry of phenothiazine dyes incorporated into Nafion films R. Ramaraj and S.A. John Modification of the electronic structure of Pd by U films. Chemisorption of CO C. A. Colmenares and T. H. Gouder Photophysical properties of Merocyanine 540 derivatives A. Harriman, A. C. Bemiston and K. S. Gulliya Enhancement in the optical CO sensitivity of NiO film by the deposition of ultrafine gold particles M. Ando, T.Kobayashi and M.Haruta Spectroscopic characterization of magnesium vanadate catalysts. Part 1 .-Vibrational characterization of Mg,(VO,),, Mg,V,O, and MgV,O, powders G.Busca, G. Ricchiardi, D. S.H. Sam and J-C. Volta Brownian dynamics simulations of concentrated dispersions: Viscoelasticity and near-Newtonian behaviour D. M. Heyes, P.J. Mitchell, P. B. Visscher and J. R. Melrose Characterization of supported-palladium catalysts by deuterium NMR spectroscopy T-H. Chang, C. P.Cheng and C-T. Yeh ... 111 Excess volumes of ternary mixtures butylamin+cyclohexane-benzene and tributylamine-cyclohexanebenzene S. L. Oswal and S. G. Pate1 Spectroscopic characterization of magnesium vanadate catalysts. Part 2.-FTIR study of the surface properties of pure and mixed phase powders G. Ramis, G. Busca and V. Lorenzelli Intramolecular photodimerization of 2-naphthoates.Successful application of hydrophobic forces to preparation of large-ring compounds C-H. Tung, Y. Li and Z-Q. Yang Inelastic neutron scattering study of NH,Y zeolites W. P. J. H. Jacobs, H. Jobic and R. A. Van Santen Solid-state hydrolysis of aspirin M. C. Ball Proton transfer to the fluorine atom in fluorobenzene. Temperature and pressure dependence R. Mason, A. Parry and D. M. P. Milton FTIR study of adsorption and transformation of methanethiol and dimethylsulfide on zirconia J-C. Lavalley, M. Ziolek, 0. Saur and J. Lamotte Conformational and vibrational properties of a,mdihalogenoalkane/urea inclusion compounds: A Raman scattering investigation K. D. M. Harris, S. P. Smart, A. El Baghdadi and F.Guillaume Formation and structure of Langmuir-Blodgett films of C, and arachidic acid B. Stewart and C. Ewins Statistical thermodynamics of hard spheres in a narrow cylindrical pore P.Siders, Q. Xin and I. Hiyane Oxidation of carbon monooxide on LaMn,-,Cu,O, perovskite-type mixed oxides M. Misono, H. Yasuda, Y. Fujiwara and N. Mizuno 'H NMR relaxation time studies of the hydration of the barley protein C-hordein P. S. Belton, A. M. Gil and A. Tatham High-resolution FTIR-jet spectroscopy of CCl,F, D. McNaughton, D. McGilvery and E. G. Robertson Fourier-transform luminescence spectroscopy of solvated singlet oxygen T. G. Truscott, A. N. Macpherson and P. H. Turner FTIR study of carbon monoxide adsorption onto ceria: CO:' carbonite dianion adsorbed species C.Binet, A. Badri, M. Boutonnet-=ling and J-C. Lavalley Mechanism of bleaching by peroxides. Part3.--Kinetics of the bleaching of phenolphthalein by transition-metal salts in high-pH peroxide solutions M. Spiro, K. M. Thompson and W. P.Griffith Rotational excitations of NH4+ions in dilute solutions in alkali-metal halide lattices J. Tomkinson, C. Carlie, J. Gilchrist, R.Mukhopadhyay and B. A. Dasannacharya Model calculations of chemical interactions. Part7.-Role of vicinal delocalization in the regiochemical control of the cycloaddition of diazomethane and formonitrile oxide to methyl vinyl ether A. Rastelli, M. Bagatti, R. Gandolfi and M. Burdisso Electrostatic acceleration of the 13-Hshifts in cyclopentadiene and in penta- 1,3-diene by Li+ complexation.Aromaticity of the transition structures P. Von Rape Schleyer and H. Jiao Adsorption in energetically heterogeneous slit-like pores: Comparison of density functional theory and computer simulations S. Sokolowski, G. Chmiel, L. Lajtar and A. Patrykiejew iv FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Annual Congress: The Reactive Interface in Electrochemistry and Catalysis To be held at the University of Liverpool on 12-15 April 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN Neutron Scattering Group Neutron Scattering Data Analysis To be held at the Rutherford Appleton Laboratory on 13-15 April 1994 Further information from Mrs S.Humphreys, The Rutherford Appleton Laboratory, Chilton, Didcot 0x11 ORA Colloid and Integace Science Group Theoretical Modelling and Simulation in Colloid and Interface Science To be held at the University of Bristol on 18-20 April 1994 Further information from Dr R. Buscall, ICI Corporate Science Group, PO Box 11, The Heath, Runcorn WA7 4QE ~~ Division Autumn Meeting: Reactions and Mechanisms for Fine Chemicals To be held at the University of Glasgow on 6-9 September 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Gas Kinetics Group 13th International Symposium on Gas Kinetics To be held at University College, Dublin on 11-15 September 1994 Further information from Dr H.Sidebottom, Department of Chemistry, University College, Dublin Electrochemistry Group with the SCI ELECTROCHEM 94 To be held in Edinburgh on 12-16 September 1994 Further information from Professor D. E. Williams, Department of Chemistry, University College London, 20 Gordon Street, London WClH OM V THEROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 97 Structure and Dynamics of Van der Waals Complexes University of Durham, 6-8 April 1994 Organising Committee: Dr B. J. Howard (Chairman) Dr P. Hamilton Dr J. M. Hutson Dr D. C. Clary Professor A. C. Legon Dr B. Soep Dr P. R. R. Langridge-Smith Since Faraday Discussion No. 73 on Van der Wds molecules, in 1982, the study of weakly bound molecular complexes has developed rapidly.Spectroscopic studies can now yield detailed information on intermolecular potential-energy surfaces in molecular systems. Studies of trimers, tetramers and higher clusters are giving insight into solvation effects and providing information on many-body forces, which are important in understanding the properties of condensed phases. Investigations of photodissociation and predissociation processes are helping us to understand the dynamics of fundamental chemical processes such as molecular rearrangement and energy transfer. In addition, Van der Waals complexes provide an opportunity to control the orientation of colliding molecules and the energies and impact parameters of reactive collisions, and have added significantly to our understanding of the pathways of simple chemical reactions.This discussion will bring together experimentalists and theoreticians who are involved in the study of Van der Waals molecules. The final programme and application form may be obtained fiom Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly, London W 1V OBN. THEROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 98 Polymers at Surfaces and Interfaces University of Bristol, 12-14 September 1994 Organising Committee: Professor Sir Sam Edwards (Chairman) Dr R. Buscall Professor R. H. Ottewill Dr T. Cosgrove Professor J. S.Higgins Dr R. W. Richards Dr R. A. L. Jones New experimental methods and new theoretical and computational techniques have recently I d to gre t progress in understanding the difficult but technologically important problems associated with the conformation of polymer molecules at surfaces and interfaces.The purpose of this Discussion is to bring together experimentalists and theoreticians working towards a molecular understanding of polymers at surfaces and interactions to survey the progress in the area to date and to indicate future directions of research. The meeting will attempt to bring a unified approach to the problem, encompassing problems of the structure of surfaces and interfaces in polymer melts, the conformation of polymers at solifliquid and liquidniquid interfaces, and extensions towards more complicated biological systems.The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly, London WlV OBN. vi THE ROYAL,SOCIETY OF CHEMISTRY, FARADAY DMSION, GENERALDISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Organising Committee Professor L. D. Barron (Chairman) DrA. F.Drake Dr D. L. Andrews Professor R. E.Hester Professor A. D. Buckingham Traditional optical activity measurements such as CD are confined to the visible and near-ultraviolet spectral regions where they provide stereochemical information on chiral molecules via polarized electronic transitions. Thanks to prompting from theory and new developments in instrumentation, optical measurements are now being made in the vibrational spectrum using both infrared and Raman methods.Studies over the past decade on a large range of chkal 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 ub initio computational methods, which can relate the observations to stereochemical details. The increasing importance now being attached to molecular chirality and solution conformation in the life sciences should also encourage the partipation of biomolecular scientists.The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, London W 1V OBH. vii Are you looking fora source of international research results covering all branches of chemistryyet in a coi easy-to-read format? JOURNALOF CHEMICAL RESEARCH is the answer! Sponsored jointly by the Royal Society of Chemistry, the Gesellschaft Deutscher Chemiker and Societe Franqaise de Chimie, the Journalof Chemical Research publishes papers from around the world on all developing areas .of chemistry. Unique in format, it consists of two parts: Part S contains brief, browsable synopses and short papers. Part M reproduces the full texts of the above synopses for reference and is available in miniprint or microfiche versions. * Quick to scan * * All International research topics covered * Follow-uptexts* Rapid publication * Competitive subscription rates Don’t waste time in the library when you want to be in the lab -subscribe to the Journalof Chemical Research. For further information complete and return the attached enquiry form. 0Please send me further information on the Journal of Chemical Research Name: Position: SOCIETY OFAddress: ROYAL CHEMISTRY &%& Please return to: Sales and Promotion Department, Sim I( i+Royal Society of Chemistry, Thomas Graham House, Inromi,ilion Science Park, Milton Road, Cambridge, CB4 4WF, UK. viii

ISSN:0956-5000    

DOI:10.1039/FT99490BP059

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 817-823 Photolysis of HOBr and DOBr at 266 nm: OH and OD Product-state Distributions Nebil Shaw, Andrew J. Bell, Michael J. Crawford and Jeremy G. Frey* Department of Chemistry, University of Southampton, Southampton, UK SO95NH ~ The OH and OD rotational and vibrational product-state distributions have been recorded following the 266 nm photolysis of HOBr and DOBr, respectively, in a molecular beam. The photodissociation dynamics are similar to those observed for HOCI. The OH and OD distributions are rotationally cold and, for almost all of the states invesfigated, Gaussian in shape. Preferential population of the OH (or OD) 2113,2spin-orbit state was observed, together with a strong preference for the II(A') A-doublet levels.The alignment, 8:(02), tends towards the limit- ing value of -0.5 at the highest N levels that could be observed. In contrast to the photolysis of HOCI, about 10% of the OH fragment was observed in the first excited vibra- tional state, with a similar rotational distribution to the OH(v = 0) products. The OD product distributions from DOBr were found to be sensitive to the molecular-beam expansion conditions with a bimodal distribution obtained for the colder expansions. The photodissociation is consistent with excitation of a non-bonding electron to a t~ antibonding orbital which promotes rapid and direct bond fission via an upper state of A' symmetry. Relatively few results are available for the dissociation dynamics of the triatomic molecules HOX (X = F, C1, Br and I).However, an example of what is possible from studying the photodissociation of a triatomic molecule is provided by the impressive and extensive series of experiments and theoretical calculations that have been brought to bear on the direct and rapid photodissociation of H20 following excitation to its first electronically excited state.' The difficulty of performing accurate ab initio calculations on the HOX series makes comparison with experiments much more difficult than for H20. In the case of HOCl, on the theoretical side, only the most recent large-scale ab initio calculations have brought reasonable agreement between the observed and calculated UV absorption spectr~m.~,~ On the experimental side, the difficulties in producing pure HOCl led to considerable doubt in the experimentally observed absorp- tion spectrum at wavelengths >300 nm1.4 There is an even greater dearth of information on HOBr for which the recom- mended gas-phase absorption cross-sections are based on those for HOCl and a comparison with the solution spectra.Some of the complexities of the investigation of the HOX series compared to H20 are mitigated by the very fact that they are a series of similar compounds to observe and compare. For example, the photolysis of the HOX com-pounds is complicated by the spin-orbit states of the halogen atom; the dissociation is expected to populate both the X(2P3/2) and X(2P1,2)states, in contrast to the production of only H(2S) from water.Work on HOCl suggests that the pro- duction of a particular C1 atom spin-orbit state is correlated with the OH spin-orbit state.' The larger spin-orbit splitting in Br atoms may help to highlight these correlations. As well as being of fundamental interest in the photo- dissociation dynamics of small molecules, the study of the hypohalous acids is important in assessing the impact these species have in the environment. HOCl and HOBr are pro- posed as important species in stratospheric ozone depletion especially in the Antarctic ozone h~le.~.~ In principle, both species can act as a temporary reservoir for the reactive OH, C1 and Br radicals, but the importance of the reservoir will depend on the photochemical stability of HOX.The results presented in this article are part of our ongoing study of the H(D)OX series of Experimental The photolysis of low-pressure free-jet expansions of HOBr and DOBr was investigated by passing the vapour above the aqueous solutions through a 500 pm glass nozzle. The HOBr(aq) was formed by the reaction of liquid bromine with water in the presence of a suspension of red mercury(I1) oxide in an analogous manner to the preparation of HOC1;" DOBr was prepared in the same manner using D20. Experi-ments on DOBr were conducted using 100 Torr of both Ar and He driving gases to produce a colder supersonic expan- sion through a 200 pm glass nozzle. The apparatus used in these experiments is similar to that described previously for the 266 and 248 nm photolysis experiments on HOCl.7-9 Great care was taken to ensure that the OH fluorescence signals were linear with probe laser power.The probe laser energy was <1 pJ pulse-' at the molecular beam and verti- cally polarized. No fluorescence was observed with a pure water sample and the signal intensities recorded from the HOBr samples were determined to be linear with respect to the 266 nm laser power. The fluorescence signals were cor- rected for variations in the laser powers, the sample vapour pressure and the change in detection efficiency across the band due to the filters and the PMT response. Examples of the OH and OD laser excitation spectra obtained are shown in Fig. 1. At each laser wavelength the fluorescence signals from 20 laser shots were recorded, allowing an estimate of the error at each point to be made.This was carried forward to allow an estimate of the uncertainty in the peak areas and rotational level populations to be determined. Fragment Rotational Alignment The Ab2)(J)m~mentl~,~~can be estimated from the ratio of the intensities of the main transition and accompanying satel- lite line originating from the same lower level. However, the resolution of our dye laser (0.5 cm-'), in conjunction with the relatively large Doppler width (0.7 crn-l) arising from the high degree of translational excitation of the OH radical, means that it was very difficult to resolve satellite transitions for branches other than R,.The lowest ground-state rota- tional level for which these satellites can be resolved in our experiment is N" = 2, and by N" = 6 the intensities of the lines are too small to allow an estimate of 8:(02) to be made; the values obtained for N" = 2-6 are shown in Fig. 2. The value of Dg(02) tends towards ca. -0.5 as N" increases. In the subsequent analysis the N-dependent values of Dg(O2) were used, together with tabulated Einstein B coefficient^,'^ to extract populations from the line intensities, with the same values used for both spin-orbit components and A-doublet J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 t 306 307 308 309 310 31 1 312 wavelengthjnm Fig. 1 OH (a)and OD (b) laser excitation spectra recorded in the A-X band, 0-0 vibronic transitions.The OH and OD were generated by the 266 nm photolysis of HOBr and DOBr, respectively. Both these spectra were recorded using the vapour above an aqueous solution of hypo-bromous acid expanded through a 500 pm glass nozzle with no driving gas. The spectra cover the P, Q and R branches originating from both OH (OD) spin-orbit states. states. The OD fluorescence spectrum has many more blended lines and no value of /?;(02) could be reliably deter- mined. OH Vibrational and Rotational Populations The OH(u = 0) rotational-level populations from the pho- tolysis of HOBr are shown in Fig. 3. They can be seen to be well represented by Gaussian distributions, with the OH(211,/2) spin-orbit state more populated than the OH(211,,2) state.The n(A)state is increasingly favoured over the II(A‘) A-doublet state as N increases and the maximum of the Gaussian distribution occurs at larger N for the ll(A’) than the n(A’’) states. 1 2 3 4 5 6 7 N” Fig. 2 The OH alignment /3;(02) as a function of the rotational quantum number N in the OH(’H3,J spin-orbit state measured by comparison of the main R branch lines with their associated satel- lites. Bi(02) could not be determined for the other hdoublet com- ponent or spin-orbit state owing to the limited resolution of the laser and poor signal-to-noise ratios. Weak transitions were observed originating from OH(u = 1, N). To obtain significant intensities these lines were recorded using a much higher probe laser power (ca.50 p.I pulse-l), sufficient to saturate the transitions. The popu- lations for OH(u = 1, N”) are shown in Fig. 4. The fits in Table 1 show that for the H(A’) A-doublet levels the u = 0 and u = 1 rotational distributions are identical within the error bars. However, the II(A”) A-doublet component of the 2113/2 spin-orbit state has a slightly colder and broader dis- tribution than the equivalent u = 0 levels. A number of OH(u = 0) and OH(u = 1) line intensities were recorded under non-saturating conditions and this enabled us to esti- mate that CQ. 10% of the OH was produced in OH(v = 1). OD Rotational Populations The OD(u= 0) rotational population distributions from the photolysis of DOBr under three different expansion condi- tions (no driving gas, 100 Torr He and 100 Torr Ar) are shown in Fig.5 with parameters from the Gaussian fits listed in Table 2. The overlapped lines in the OD spectrum mean that it is not possible to derive populations from the Q2 lines. In all three cases the ’lI312 spin-orbit state is more populated than the and the II(A’) A-doublet states are favoured over the lI(A’’) as N increases. The population distributions obtained in the absence of driving gas in the expansion and those obtained with 100 Torr of He are very similar and are fitted quite well by a Gaussian distribution. The distribution in the ’llIl2 state ll(A‘) A-doublet component is an exception, having rather a flat top, suggesting possible bimodality. The results of the experiments using 100 Torr Ar are significantly different.The II(A”) Adoublet levels observed using the Ar expansion can be fitted by a Gaussian, though the peak is shifted to lower N compared with the He expansion. The II(A’) levels of the 2111,2spin-orbit state are clearly bimodal in the Ar expan- sion and a fit using two Gaussians is shown. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 819 78001 I a I T I I I i I N" N" 10 5 0 A I 11IV Fig. 3 Rotational population distributions and Gaussian fits for OH(v = 0, J) fragments from photolysis of HOBr: (a) 2113,2spin-orbit state, II(A') A-doublet component; (6) 2113/2spin-orbit state, ll(A") A-doublet component; (c) 2111/2spin-rbit state, ll(A') A-doublet component; (42n1/2 A-doublet corn- spin+-jrbit state, n(~") ponent N" N" .O 350 300 -- I ' I ' I ' I 1 ' 1 (c)-1 250 200 150 100 50 0 N'' Fig. 4 Rotational population distributions and Gaussian fits for OH(v = 1, J) fragments from photolysis of HOBr obtained under saturation conditions : (a) 2113/2spin-orbit state, ll(A') A-doublet component; (6) 2113/2 spin-rbit state, n(A") Adoublet component ; (c) spin-orbit state, n(A')A-doublet component. Populations of the ll(A") A-doublet component of the spin-rbit state could not be extracted owing to poor signal-to-noise.Spin-Orbit Populations The OH spin-orbit population ratio from the photolysis of HOBr shows some variation with N, Fig. 6. For the II(A') states the ratio rises to a plateau of ca.2.5 between N = 2 and 6, but the ll(A") manifold reaches a ratio of 4 in the same region. It is difficult to be sure that the differences between the two A-doublet components are significant as fewer pairs of lines could be resolved in the ll(A") states than for the ll(A') states. Considering the errors involved in these mea- surements it is better to consider the average spin-orbit population ratio over the N range where there is significant population of the rotational levels (N = 2-7) and to concen- trate on the n(A)states. The observed average over all rotational levels indicates a 2.5 :1 preference for the lower-energy OH('II,/,) states and is close to the statistical limit of 2 :1 if we assume that the pro- duction of Br('II,/,) is correlated with the production of OH(2113/2)and similarly for the 'II3/2 states.This is similar to the observations in HOCl. The spin-orbit splitting in Br is ca. 3677 cm-'. Even with a much higher resolution probe laser it is unlikely that this splitting could be resolved in the OH Doppler lineshape. The OD spin-orbit population ratios for the II(A') A-doublet State are shown as a function Of N in Fig. 7 for the three different beam conditions used in these experiments. Table 1 Parameters of Gaussian fits obtained from OH rotational distribution plots following the photodissociation of HOBr conditions state 2n3/2 n(A) 2n3/2 n(Ar‘) HOBr effusive beam, u = 0 2n1/2n(A) 2n1,2WA“) 2n3/2 n(Ar)HOBr effusive beam, u = 1 2n3/2 n(A“) 2n1/2n(A7 The Gaussians were calculated using the expression A exp[ -(x -The corresponding ratios for the II(A”) levels could not be evaluated as so few of the 2111,2(A”) level populations could be determined because of blended lines.The three plots are similar with an initial spin ratio of about 3, climbing to a maximum of 5 at N z 6 or 7 before falling again. The average spin ratio is thus slightly higher for OD than for OH. In order to facilitate the spin-orbit comparison the populations, summed over the A-doublet levels, were fitted to a Gaussian distribution (Table 3). A-Doublet Populations The degree of electronic alignment (DEA), see eqn. (l), is a measure of any preferential production of a A-doublet com- ponent : where P,,,,(X = A‘, A”) correspond to the populations of the two A doublet states with the same N.The DEA shows a trend towards a limiting value of -1.0 as N increases for the photolysis of both HOBr and DOBr and for all beam condi- tions (Fig. 8 and 9). The maximum alignment observed is similar for both molecules (-0.8). The OD data from both the undriven expansion and the He driven expansion give very similar DEA distributions. For the Ar-driven expansions fewer lines were recorded but it seems that the magnitude of Table 2 Parameters of Gaussian fits obtained from OD rotational distribution plots after the photolysis of DOBr conditions state DOBr 100 Torr He DOBr 100 Torr Ar Coefficients as in Table 1. Table 3 conditions HOBr sum A-doublet DOBr sum A-doublet ~ ~ ~~ Coefficients as in Table 1.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 A m AN 1.OOO f0.039 3.88 f0.16 4.30 f0.29 0.775 f0.042 2.81 f0.20 3.37 f0.35 0.330 f0.015 4.25 f0.15 3.80 f0.26 0.222 f0.004 2.14 f0.08 3.66 f0.13 0.712 f0.039 4.09 f0.22 4.31 f0.52 1.OOO f0.054 0.00 f0.15 4.62 f0.29 0.263 f0.016 4.42 f1.00 3.80 f1.50 N)2/(AN)2]. the DEA is reduced, reaching only -0.5. This may indicate some relaxation of the nascent distribution. Discussion The heat of formation of HOBr, A H --80 kJ mol-’; the O-Br bond energy is 234 kJ mfol’?ihe O-Br bond in HOBr is slightly weaker than the corresponding 0-C1 bond in HOCl (249 kJ mol-’).15 The O-Br bond strength in HOBr is very similar to that in the OBr radical (Dg = 231.3 kJ mol-’) where the C10 bond strength (Dg = 265.4 kJ mol-‘)I6 is significantly larger than the corresponding bond in HOCl.The photodissociation of HOBr to produce ground-state OH and Br products is energetically feasible for wavelengths <511 nm. At 266 nm (NAhv= 449.7 kJ mol-’) the excess energy (ignoring the internal energy of the HOBr) is 170 kJ mol-’. The heat of formation of DOBr is not available, but the O-Br bond energy is not expected to be significantly different from that in HOBr. The majority of the OH and OD fragments from HOBr and DOBr, respectively, are pro- duced in low rotational levels of the ground vibrational state. This means that most of the available energy from the pho- tolysis is channelled into relative translational motion.The Doppler widths of the OH transitions are consistent with the high kinetic energy release. The basic photodissociation dynamics of HOBr parallel those of HOCl. Even in the impulsive limit the larger mass of the Br atom compared with C1 is not sufficient to explain the difference in A 15 AN 1.OOO f0.062 5.76 k0.20 3.90 f0.31 0.379 f0.020 2.55 f 0.77 6.90 f0.95 0.246 f0.013 6.88 f0.25 5.28 k0.50 1.OOO f0.250 6.63 f0.86 4.35 f0.71 0.274 f0.015 3.62 k0.80 6.73 f1.42 0.263 f0.022 7.52 f 0.68 6.76 f1.54 1.OOO f0.070 1.10 f2.21 9.57 f2.90 0.379 +_ 0.038 3.97 f3.17 5.69 f5.44 0.455 f0.023 0.00 f0.30 5.53 f0.48 0.302 f0.021 9.30 f0.24 2.31 f0.43 Parameters of Gaussian fits obtained from a sum over A-doublet states for OH/OD rotational population distributions state A N AN ~~ ~ 2n3/2 LOO0 f0.035 3.35 f0.16 4.04 f:0.25 2n1/2 0.308 f0.012 3.38 f0.14 3.98 f0.22 effusive 2n312 1.OOO f0.079 6.21 f0.35 4.82 & 0.61 Helium 2173,2 1.OOO f0.063 6.45 f0.43 5.50 f0.93 ~~ 40 v 0 N" N" 75 C .-0 c m 550 P d---..fopaa .z 25 c Q 9? -KZLJ0U N" N" 75 50 25 0 2 4 6 a 10 12 N" Fig. 5 Rotational population distributions and Gaussian fits for OD fragments from the photolysis of DOBr: (a) effusive beam,2113/z spin-orbit state, n(A)Adoublet component; (b) effusive beam,2n312spin-orbit state, n(A") Adoublet component; (c) effusive beam,2111/zspin-orbit state, n(A)Adoublet component; (4He backing gas, 'lljlz spin-orbit state, n(A)Adoublet component; (e) He spin-orbit state, n(A") Adoublet component; (f)He backing gas, 2111/2backing gas, *nJlz spin-orbit state, n(A)Adoublet component; (g) Ar backing gas, 'lI3 spin-orbit state, n(A)Adoublet component; (h)Ar backing gas, 'II3/2 spin-orbit state, ll(A") Adoublet component; (i) Ar backing gas, 2111/z spin-orbit state, n(A)Adoublet component fitted with two Gaussians.Populations could not be determined for the n(A") component of the 2111,2spin-orbit state owing to overlapping lines. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I I I I I I I I 1 I 12345678910 N" Fig. 6 Spin ratios 2113/f: 2111/2over both A-doublet components for OH fragments following the photolysis of HOBr: (0)ll(A), (0)n(A") the fraction of OH(v = 1) population between HOCl and HOBr.The ground-state OH bond length and the bond angles in HOBr are similar to those in HOCl. This leads us to believe that the different product vibrational-state distribu- tions arises from differences in the excited-state geometries. The relationship between parent XO-H bond length and nascent OH vibrational excitation will be further explored in the study of HOI photolysis. The bimodal nature of the population distribution obtained from the experiments with 100 Torr of Ar driving the molecular beam expansion is certainly worthy of comment. Bimodal product rotational-state distributions P 949 N" N, r F c! t? PP N" N, r F N,t? 01'2'4'6'8'lo'1'21 : 2n1,2N" Fig. 7 Spin ratios 2113/2 for OD fragments following the photolysis of DOBr: (a) effusive beam, (b) He backing gas, (c) Ar backing gas 0.0 4 -0.4 n -0.8 N" 0 1 2 3 4 5 6 7 8 9 10 N" Fig.8 Plot of degree of electron alignment (DEA) for OH frag- ments following the photolysis of HOBr and evaluated as (Pn(A,,) -Pn(A*))/(Pn(A,*) where P,(,, refers to the population of the + Pn(A,)), A-doublet component for given N. This ratio tends to +l for %-Pn(At)and to -1 for Pn(A,,lPn(A,,) Q Pn(Af).The DEA for OH('lI,,,) is shown in (a)and for OH(211,,2) in (b).The solid line in (b)is a scaled prediction for the dependence of the DEA on N arising just from angular momentum considerations. have been calculated in the photodissociation of ICN, albeit for much higher rotational excitations.17 The higher N com-ponent of the bimodal OD distribution is similar to the dis- tribution obtained from the experiments with no driving gas or with He.It is possible that the new lower N component is due to some relaxation process which is converting some of the nascent distribution to produce population on low N levels. Certainly Ar has a higher collision cross-section for rotational relaxation than He, but at the local pressure in the beam is such that even the fast-moving OD radicals will have had little chance to collide in the 30 ns between pump and probe laser pulses. We also suspect that the major source of relaxation would be the water present in the expansion.The effect of the water, which will depend on the pressure in the expansion, should be the same for 100 Torr of He as for 100 Torr of Ar. However, the He expansion is similar to the undriven expansion, ruling out any relaxation effects. The more polarizable Ar will produce a significantly colder expansion than the He beam. The reduction in the parent HOBr temperature would be expected to produce a small shift in the product rotational distribution to lower N. This effect has been observed in HOCl, but this shift would not be expected to be large enough to account for the observed low N distribution and does not explain the high N component. Bimodal distributions have been recently observed in the NO product rotational-state populations following photo-dissociation of [CH,ONO], and [(CH,),ONO], clusters.'8 This has been interpreted as being due to the superposition of two NO fragment rotational distributions, one produced from the free monomer and the other from NO production from the clusters.It is possible to form clusters of HOBr -Ar or HOBr H,O in our Ar beam, though we estimate at the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.0Lo -0.5 1 --_0.0 ---02 4.5 1 P ppP -1 .o N" 0 0B -0.5 / no -1 .o 0 2 4 6 8 10 12 N " Fig. 9 DEA plot for OD fragments following the photolysis of DOBr: (a) effusive beam, (b) He backing gas, (c) Ar backing gas. Values for the 2111,2spin-orbit state are unavailable owing to over- lapping lines in the n(A)manifold.pressures of argon that we are using that there would be <5% of the HOBr clustered. Some of the rotational excitation in OH and OD is likely to come from the small additional torque provided by the anisotropy of the excited-state potential." This additional torque will also produce a broadening of the Gaussian dis- tribution and may thus account for the larger ANoD/AN,, ratio commented upon previously. Classical trajectory studies, however, gave a distribution that is significantly colder than the observed distribution7 and a full quantum- mechanical study including the effects of the electronic angular momentum is needed. The widths of the Gaussian distributions are similar for the chlorine and bromine com- pounds, with the deuteriated species having the slightly wider distribution. The ratio of the Gaussian width for the OH and OD distributions is about 1.4, very similar to the ratio found for the photolysis of HOCl and DOCl." The preference for the A-doublet A' states seen in the pho- tolysis of both HOBr and DOBr is consistent with the initial excitation of an antibonding c orbital followed by a rapid in-plane fission of the OC1 bond.This results in the singly occupied OH (OD) p orbital being in the plane of the OH rotation. Angular momentum considerations show that for low N there will not be a significant preference for one A-doublet state over the other. For a dissociation which has a dynamical stereochemical control a preference will show up in the A-doublet populations for the higher rotational states.This general trend is observed for all the OH and OD popu-lations investigated. However, the degree of electronic align- ment does not tend to the limiting value of -1 and in most cases does not follow the angular momentum predictions. This means that the dynamical preference for the formation of the in-plane orbital is not complete. The observation of some of the vector correlations and their N dependence would be useful; unfortunately the limited probe laser resolution prevented their observation via the Doppler pro- files. Conclusions The results presented here show the similar nature of the photodissociation of (D)HOCl and (D)HOBr at 266 nm.The work indicates that photodissociation occurs following exci- tation to an electronic state of A' symmetry which ab initio calculations have shown to be strongly repulsive in the 0-X bond. However, significant differences are observed, most noticeably the observation of OH vibrational excitation in the nascent photofragment and bimodality in the Ar-cooled spectrum of OD from DOBr. N.S. and M.J.C. acknowledge the SERC for studentships and A.J.B. acknowledges the Royal Society for postdoctoral funding. References 1 V. Engel, V. Staemmler, R. L. Vander Wal, F. F. Crim, R. J. Sension, B. Hudson, P. Andresen, S. Hennig, K. Weide and R. Schinke, J. Phys. Chem., 1992,%, 3201, and references therein. 2 S. Nanbu, K.Nakata and S. Iwata, Chem. Phys., 1989,135,75. 3 S. Nanbu and S. Iwata, J. Phys. Chem., 1992,%, 2103. 4 J. B. Burkholder, J. Geophys. Res., 1993,98,2963. 5 A. J. Bell, S.Boggis, J. M.Dyke, J. G. Frey, R. Richter, N. Shaw and M. Tabrizchi, J. Chem. SOC., Faraday Trans., 1994,90,17. 6 J. P. D. Abbatt and M. J. Molina, Geophys. Res. Lett., 1992, 19, 461. 7 G. Poulet, M. Pirre, F. Maguin, R. Ramaroson and G. Le Bras, Geophys. Res. Lett., 1992, 19, 2305. 8 A. J. Bell, P. R. Pardon, C. G. Hickman and J. G. Frey, J. Chem. SOC., Faraday Trans., 1990,86,3831. 9 C. G. Hickman, A. Brickell and J. G. Frey, Chem. Phys. Lett., 1991,185,101. 10 C. G. Hickman, N. Shaw, M.J. Crawford, A. J. Bell and J. G. Frey, J. Chem. SOC., Faraday Trans., 1993,89,1623. 11 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley-Interscience, New York, 4th edn., 1980. 12 C. H.Greene and R. N. Zare, J. Chem. Phys., 1983,78,6741. 13 R. N. Dixon, J. Chem. Phys., 1986,85, 1866. 14 W. L. Dimpfl and J. L. Kinsey, J. Quant. Spectrosc. Radiat. Transfer, 1979, 21,233. 15 JANAF Thermochemical Tables, NSRDS-NBS 37, National Bureau of Standards, Washington DC, 2nd edn., 197 1. 16 K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure ZV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. 17 J. Qian, C. J. Williams and D. J. Tannor, J. Chem. Phys., 1992, 97,6300. 18 E. Kades, M. Rosslein, U. Bruhlmann and J. R. Huber, J. Phys. Chem., 1993,97,989. 19 R. Schinke and V. Engel, J. Chem. Phys., 1985,83,5068. Paper 3/05599J; Received 16th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000817

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 825-829 Chemiluminescent Reaction of Oxygen Atoms with Dimethyl Disulfide and Dimethyl Sulfide Ubaradka 6. Pavanaja, Hari P. Upadhyaya, Avinash V. Sapre, Kuchimanchi V. S. Rama Rao* and Jai P. Mittal Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay400 085,India Broad chemiluminescence spectra in the range 240-460 nm with a sharp peak at 308.5nm are obtained from the reaction between oxygen atoms [O("P)] with dimethyl disulfide (DMDS) and dimethyl sulfide (DMS). The emitting species are identified as OH (A'C') and SO, ("8, 'B). The photon yields are found to be 3.3 x lob6 and 4.4 x for DMDS and DMS, respectively. Computer simulations were carried out to elucidate the reaction mechanism. Reactions of oxygen atoms with various organosulfur com- pounds are of importance in atmospheric and environmental chemistry.' Many reduced sulfur compounds, mainly dimethyl disulfide (DMDS) and dimethyl sulfide (DMS), are released into the atmosphere from oceanic, biogenic (algae, bacteria, plants) and anthropogenic (wood, pulp mills, pet- roleum refineries, sewage treatment plants) source^.^^^ OH radicals and O(3P) are mainly responsible for the atmo-spheric oxidation of the above compounds and contribute to the formation of SO, ,thus increasing the acidity of the atmo- sphere.,~~The present work was undertaken as an extension of our earlier work on the chemiluminescent reaction of O(,P) with CS,.' Reactions of O(,P) with DMDS and DMS have been studied earlier by several workers and negative temperature dependences of the rate constants have been rep~rted.~,~Moreover, chemiluminescence can be used to detect reduced sulfur corn pound^.^^^ SO, chemiluminescence was observed in the reaction of 0, with DMDS" and in the reaction of O(3P) with DMS." The primary reactions of DMDS and DMS with O(,P) are known to be direct addi- tion of an oxygen atom to the reduced sulfur compounds, followed by rapid unimolecular decomposition.' r 0 I*IICH,SSCH, + 0 -+ LCH,SSCH,] +CH3S0 + CH3S; AH = -142 kJ mol-' (la) r o i* CH3SCH, + 0 -+ CH,SCH, +CH,SO + CH,;L " 1 AH = -129 kJ mol-' (lb) Both of these reactions are exothermic with fairly high rate constants.lo The small negative activation energy for these reactions is also consistent with an addition mechanism.' In the present work chemiluminescent emission from the reac- tion of oxygen atom with DMDS and DMS is observed.The emitting species are identified as electronically excited OH radicals and SO, molecules, formed as a result of secondary reactions, mainly that of SO and CH, with oxygen atoms. Mechanisms are proposed for the formation of the emitting species based on the experimental results. The photon yields are estimated for both reactions as described by Fontijn et ~1.'~ Experimental Most of the experiments were performed in a discharge flow tube in the pressure range 1-1.5 Torr. Some experiments were also performed in a beam-gas configuration, the apparatus being described elsewhere.' The diameter of the flow tube used was 34 mm and the distance from discharge cavity to the observation zone was ca.60 cm. The flow tube was pumped by a 20 dm3 s-l rotary vacuum pump. Oxygen atoms were generated in a microwave discharge (Raytheon 100 W, 2450 MHz) cavity. Three gas compositions were used: (1) Pure 0,, (2) Ar-0, (2-3%) mixtures and (3) Ar-N,O (2-2.5%) mixtures. All gases used were of high purity (Indian Oxygen Co., IOLAR grade, >99.99%). DMDS and DMS (Fluka Chemical Research) were distilled under vacuum. Any traces of water were removed by storing the liquids over anhydrous Na,S04 overnight and then subjecting them to several freeze-pumpthaw cycles prior to use. Before carrying out the experiment the entire flow tube was heated to ca.200 "C with a heating coil and was continuously pumped out to remove any traces of moisture in the flow tube. DMDS and DMS were injected directly into the observation zone. The total gas pressure in the observation zone was monitored by a capacitance manometer (Datametrics 1174, Edwards). The reagent inlet pressure was monitored by a strain gauge (Membranovac 1VS Leybold). NO was injected in the inter- action zone for the titration of oxygen atoms. The relative O(3P) concentrations were monitored using the standard NO + 0 reaction by measuring the NO; emission at 525 nm. NO was prepared by adding an FeSO, solution to an acidic KNO, solution, purified using trap-to-trap distillation and was used after it had been passed through a dry ice and acetone slurry trap.Light from the observation zone was col- lected and focused on the slit (0.25 mm) of a stepper motor driven monochromator (Jarrel-Ash model 82- 140,0.3 m, pro- vided with two gratings, one for the 200-400 nm range and another for the 400-800 nm range). The dispersed light was measured by a photomultiplier tube (Philips XP2254B) and an electrometer amplifier (Keithley 61 7). The resolution of the detection system was ca. 0.2 nm. The spectral data acquisi- tion was performed by an IBM-compatible personal com-puter. The photon yields were estimated by comparing them with the standard NO + 0 afterglow as described by Fontijn et aZ.I3 Results and Discussion Spectra Dimethyl Disu&de A strong emission in the near-UV was observed in the reac- tion of DMDS (10 mTorr) with O(3P)[Fig.l(a)]. The emis- sion spectrum extended from 250 to 460 nm, with a vibrationally resolved peak at 308.5 nm. The broad spectrum 826 J 26. "." I 1 III 3.0- h .In C.-2 2.5-2 Y.+2.0- 4-.- InC .-E 1.5- 1.o-Ii ' I ' I' I ' I ' I ' 1.1 r' ' 1 240 260 280 300 320 340 360 380 400 420 A/nm Fig. 1 Emission spectra from the reaction of oxygen atoms gener- ated from Ar-O, (2%) discharge, with (a) DMDS (10 mTorr), (b) DMS (3 mTorr), at a total pressure of 1.2 Torr. Inset: Vibrationally resolved spectrum in the range 305-309 nm. is very similar to that observed by Glinski and Dixon" in the reaction of DMDS with 0, and Tabares12 in the reac- tion of thiophene with oxygen atoms.Both groups attributed the observed emission to electronically excited SO, (3B, 'B). From the similarity of the spectra, it is concluded that the emitter is electronically excited SO,. The SO, spectrum is vibrationally unresolvable in our system, probably due to the high vibrational temperature of the SO, formed in the reac- tion. The emission in the range 306-309 nm shows vibra- tional features, shown in Fig. 1 (inset), which can be attributed to the 0-0 band of the A-X emission from OH (Table l).14 In the discharge of pure O2 and Ar-0, mixtures, production of singlet oxygen [O,('A,)] and small amounts of 0,cannot be ruled out and DMDS is known to give chemi- luminescence with O3,lo hence it is desirable to confirm the reacting species.For this, pure O(3P) as generated by an Ar-N,O discharge', where neither 0, nor singlet oxygen are produced. Under these conditions, the shapes of spectra are similar, confirming that the reaction of O(3P)gives che- miluminescence with DMDS and DMS. To confirm that OH* emission is purely from the reagent and not from trace amounts of moisture in the flow tube or in the argon, a blank spectrum (no reagent, i.e. DMS or DMDS was added) was recorded, which showed no signal at 308.5 nm. Dimethyl Suljide A very similar spectrum was observed in the reaction of DMS (3 mTorr) with 0 atoms generated in 2% 0, in Ar at a total pressure of 1.2 Torr [Fig. l(b)].A vibrationally resolved peak was observed in the range 306-309 nm [Fig. 1 (inset)]. Lee et al." observed SO, chemiluminescence in the reaction of DMS with O(3P),but did not report the spectrum. Wit# very similar arguments it is concluded that the emitters are electronically excited SO, and OH. Pressure Dependence Studies The dependence of chemiluminescence intensity on the DMDS, DMS and O(3P) concentrations was studied by Table 1 0-0 Bandheads of the A-X system of OH* l/nm obs. lit. assignment 306.5 306.4 Rl -306.7 R2 307.8 307.8 Q1308.5 308.9 Q2 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 keeping the concentration of one of the reactants constant and varying the others at argon flow rates of 20-30 Torr dm3 s-'. The SOY emission was measured at two wavelengths, 290 and 330 nm, and the OH* emission was followed at 308.5 nm.The results are summarised in Fig. 2 and 3 at concentra- tions from 0-300 mTorr (DMDS) and 0-12 mTorr (DMS). For lower concentrations of DMDS/DMS the chemilumin- escence intensity grows almost linearly with the DMDS/DMS concentration, but at higher substrate concentration it reach- es a maximum and then decreases. Similarly the O(3P) concentration was varied at constant DMDS and DMS con- centration [Fig, 2 (inset) and 3 (inset)]. A linear dependence on O(3P)concentration at low pressure was obtained, but at high O(3P)concentrations the intensity tended to level off. A similar pressure dependence was observed for Ar-N,O dis-charge, where 0,and singlet oxygen [O,('Ag)] were absent, on addition of DMDS or DMS. However, the work was carried out for Ar-0, (2%) discharge since signals were much better for these mixtures.These results show that the chemilumescence is very sensi- tive to the concentrations of O(3P)and DMDS or DMS. The maximum intensity is seen at a pressure of 3 mTorr for DMS and 16 mTorr for DMDS. The decreases of intensity beyond these pressures are due to complex reactions involving various intermediates. A similar type of behaviour has been I 11 lo nv) c.-C 3 -5 20 Y >. c.-In a-CI C.-I I I I 0.0 0.1 0.2 0.3 DMDSrorr Fig. 2 Chemiluminescence intensity us. DMDS pressure [Ar-O, (2%) at 1.2 Torr] at wavelength (a)290 nm, (b) 330 nm, (c) 308.5 nm.Inset : Chemiluminescence intensity us. relative O(3P) pressure (at DMDS = 10 mTorr and Ar-O, < 10%)at wavelength (a) 290 nm, (b) 330 nm, (c) 308.5 nm. 3 h .g 2 3 f W >.c.-E' c .-C i0 1 I I I 0.000 0.004 0.008 0.012 DMS/Torr Fig. 3 Chemiluminescence intensity us. DMS pressure [Ar-O, (2%) at 1.2 Torr] at wavelength (a)290 nm, (b) 330 nm, (c) 308.5 nm. Inset: Chemiluminescence intensity us. relative O(3P)pressure (at DMS = 3 mTorr and Ar-0, < 10%) at wavelength (a) 290 nm, (b) 330 nm, (c) 308.5 nm. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 reported by Tabares and Ureiia', for the OH* intensity dependence in the reaction of furan with oxygen atoms. To discover whether there was some contribution from the primary reactions to the observed emission, experiments were performed in beam-gas configuration in a molecular beam apparatus.No emission was observed under such conditions, which ruled out the possibility of production of OH* and SO; in the primary processes. CH,SCH, + 0 -+CH,SCH, + OH (i) CH,SSCH, + 0 -+ CH,SSCH, + OH (ii) These reactions are endothermic and hence the production of OH* is energetically unfavorable. The DMDS and DMS pressure dependences show a sharp increase and exponential decrease. Chemiluminescence Photon Yields As we will see later, the oxygen atoms and DMDS or DMS molecules are principally consumed in the fast bimolecular reactions (la) and (lb) with a very small fraction of oxygen atoms participating in the further reactions of the products of these reactions.The chemiluminescence is obtained from the secondary products, SO; and OH*, formed in their electronically excited state. It is of interest to determine the photon yield for total reaction, i.e. the number of chemiluminescence photons, N,, , generated for every DMDS/DMS molecule consumed in the reaction (Nreacted), For equal concentrations of DMDS/DMS and 0 atoms, we have Nreacted = (N;klz)/(1 + NOklz) "N No where No is the initial concentration of either reactant, k, is the rate constant for the reaction (la) or (lb) and z is the time domain of the flow reactor within the viewing zone. The total number of photons from SO, is given by NSO, = A,6, LnLzdt where A,*,, is the Einstein coefficient for the SO; molecules: the values for 'B and are 1.6 x lo3 and 123.5 s-', respec-tively.' ' The integration is carried over the time domain z to obtain Nsoz.While numerical integration has been carried out for the complex reaction scheme, we prefer to evaluate N,, experimentally. This was done by comparing the chemilumin- escence from the sulfide reaction with that from the standard NO + 0 reaction, adopting the method described by Fontijn et all3 The reference chemiluminescence photons, NNO2, is given by NNOz = A:,, ngo2 dt = k:,,, Ni zl where Ago, = 1.6 x lo4 s-','~ is the Einstein coefficient for NO: and Go,is the total chemiluminescent rate constant for the reaction NO + O.', At high values of A:,,, the rate of the reference reaction is essentially controlled by relatively slow NO + 0 recombination reaction and the above equa- tion holds for the equal concentration of NO and 0,as in the present case.The ratio NSoZ/NNo2 is experimentally evaluated, after due consideration of the spectrally overlapping regions. Since we have and @ = (NSOz/NNOz)k&+O NO Owing to the complex reaction mechanism, as elaborated in a later section, the chemiluminescence intensity and hence the photon yield varies with substrate concentration. However, the photon yields are 3.3 x lo-, for DMDS and 4.4 x for DMS, respectively, at their intensity maxima.t Chemiluminescence Mechanism It is well known that the formation of electronically excited SO, in the reaction of DMDS with 0, and in the reaction of DMS with O(,P) is due to reaction of the initially generated SO e.g.M0 + SO -SO;; AH = -548 kJ mol-' (iii) This channel has sufficient energy to generate the product SO, in the upper electronic level. The reaction of O(,P) atoms with episulfide also exhibits SO, afterglow," for which the following mechanism is pro- posed : O+ AH = -276 kJ mol-(iv) Reaction (iv) is followed by reaction (iii)". Lee et a!." con-sidered the direct formation of SO in the endothermic reac- tion 0 + CH,SCH, + 2CH, + SO; AH = 84 kJ mol-i (v) but ruled this out on the basis of the overall negative activa- tion energy observed for the consumption of 0 atoms in the reaction with DMS.The formation of C,H, and SO in the primary reaction 0 + CH,SCH, + C,H, + SO; AH = -293 kJ mol-' (vi) may be considered on energetic grounds but was not sup- ported by the mass-spectrometric results, which showed no evidence for C,H, at low pressure." Lee et a!." concluded that the primary step in the reaction mechanism is reaction (lb) followed by reaction (2) (later). The production of electronically excited OH cannot be accounted for as a result of reactions (i) and (ii) since the energy requirement is large. A possible reaction to explain the formation of OH* is M0 + H -OH; AH = -427 kJ mol-' (vii) where M is a third body, mainly Ar in the present experimen- tal condition. Production of H atoms was reported by Lee et al." in the reaction of O(3P)with DMS, viz.CH, + O(,P) -+ H,CO + H; AH = -293 kJ mol-' (viii) 7 The cumulative errors in the evaluation of photon yields are CQ. 10%. The H atoms generated can react with O(,P) atoms to give OH*. Based on the above considerations, we propose the follow- ing reaction scheme. CH,SSCH, + 0 +CH,SO + CH3S; k, = 1.3 x lo-'' cm3 molecule-' s-' (14 CH,SCH, + 0 +CH,SO + CH, ; k,, = 5 x lo-" cm3 molecule-' s-l l7 (W MCH,SO -CH, +SO; k, = 5 x S-' (2) CH, + 0 -,H,CO + H; k, = 1.4 x lo-'' cm3 molecule-' s-' '7p18 (3) 0 + H -OH*; k, = 2.3 x cm3molecule-' s-' l9 (4) OH* &OH + hv; k, = 1.5 x lo6 S-' 20,2' (5) MO,+H-HO, ; k6 = 3.2 x 10-l~cm3molecule-1 s-' 17v2, (6) SO+OLSOf; @=1.3; k, = 2.7 x cm3molecule-' s-' 23 (7) SO? --LSO, + hv; k8 = 123.5 S-' l5 (8) CH, + 0, LCH,O,; k, = 3.7 x cm3 molecule-' s-' l7 (9) These rate constants are the values recommended in the ref- erences mentioned.The second-order rate constants for the reactions of 0 atoms with H, and SO and 0, with CH, and H [reactions (4), (7), (9) and (6)] are dependent on the third-body (M) con-centration in the fall-off region. The rate constants for these reactions are taken from ref. 17 [the pressure of M (Ar) being 1.5 Torr here]. In the above mechanism, we consider as the primary step the reaction of O(3P) atoms with DMDS or DMS, i.e. reac-tions (la) and (lb). The primary radical products generated trigger the entire reaction sequence, resulting in the observed chemiluminescence.With both sulfide molecules, the rate- controlling step for SO formation is the slow unimolecular dissociation of CH,SO [reaction (211. Reaction (lb) for DMS gives a direct primary route for generation of CH, radicals, which are precursors for OH* [reactions (3) and (4)]. This facilitates the much higher emission intensity from OH* as observed in the DMS reaction. In the reaction scheme, the dimerization of CH, radicals is not considered because (i) their concentration is too small compared to that of 0 and 0, and (ii) the reaction rate con- stant k,,,, (1.4 x lo-'' cm3molecule-' s-')~"'~is larger than kCH, +CH3 (5.9 x 10-'' cm3 molecule-' s-1).24*25 Numerical Simulation The reactions in the above mechanism lead to various coupled differential equations. To elucidate the exact profile of the different transient species in the flow system, these coupled differential equations have been solved numerically using the program EPISODE developed by Hindmarsh and Byrne.26 All the equations have been solved taking dV/u as dt, where dV is the volume element in the flow direction of the observation zone and u is the bulk linear flow velocity.A typical concentration profile of all the species for 15 mTorr of 0 and 15 mTorr of DMDS is shown in Fig. 4. The concen- tration of CH,SO increases up to 10 ps (or the corresponding volume element) and subsequently reaches a steady state. Similarly, the steady state is reached for CH, and SO? after J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10 ms. In the case of SO, H and OH*, the steady state is not attained even after 200 ms. As the residence time in the obser- vation zone is <2 ms in the present experiments, a steady-state condition cannot be imposed. The concentration profiles in Fig. 4 show that the major depletion of 0 atoms and sulfide molecules occurs by their mutual reaction. The 0 atoms are promptly and almost entirely consumed [Fig. qb)] in ca. 100 ps, leaving very few 0 atoms to sustain the second- ary reaction sequence [Fig. 4(c)-(g)]. The unavailability of 0 atoms €or further reaction is the principal reason for the low photon yields. When the relative 0-atom concentration is increased, greater accessibility of 0 atoms for the secondary reactions becomes feasible and the photon yields do increase.The exothermic chemical system dynamics can be optimised, in principle, to achieve a better photon yield for a selected channel. Taking the DMS + 0 system as an example,27 we can take advantage of the primary generation of CH, and, under conditions of abundant supply of 0 atoms, drive the system so as to convert the major fraction of the products of the primary reaction (lb)to generate OH*. Since the concen- tration of the emitting species is entirely dependent on the decomposition channel of CH,SO, it is desirable to assess the decomposition rate of the radical under our experimental conditions. The unimolecular decomposition rate constant (5 x lo-, s-') and the bond-dissociation energy (209 kJ rnol-'), for the CH,SO radical suggest that this radical is very stable with respect to decomposition.The exothermicity of the formation reaction of CH,SO [reactions (la)and (lb), may result in some vibrational excitation of CH,SO, which in turn may enhance its decomposition rate. To discover whether the CH,SO undergoes decomposition with the above or higher rate constants, a similar simulation was per- formed taking k, in the range of 5 x 10-5-5 x lo5 s-'. The photon yields calculated for different values of k, are com- pared with the experimental observed values. The experimen- tal photon yield matches the simulated value for k, = 5 x s-'. This shows that the majority of CH,SO disso-ciates with the lower rate constant only, and the participation of vibrationally excited CH,SO can be ruled out in the light of this explanation.In the reaction scheme CH,, which is a precursor of OH*, is also formed in reaction (lb). So we may expect the ratio OH* :SO! to be higher for DMS than that for DMDS. Experimentally it has been observed that the ratio of area under the curve for OH* and SO! in the case of DMS is ca. three times higher than that for DMDS. By simulation (in the case of DMS the recombination reaction of CH, is also con- sidered as the CH, concentration is much higher in this case 1o4 tls Fig. 4 Computer simulation showing time-domain concentration of different species (at DMDS = 15 mTorr and 0 = 15 mTorr).(a) CH,SO, (b)DMDS or 0,(c)SO, (d) SO;, (e)CH,, (f)H, (9)OH*. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 4.0 15.0 4 4 10.0 2.0v 20 25 30 35 40 45 50 DMDS/mTorr 0.0 0 10 20 30 40 50 DMDS/mTorr Fig. 5 Computer simulation of SO! (a)and OH* (b)emission inten- sity us. DMDS pressure. Inset shows a magnified view of the tailing portion. than for DMDS) studies the same ratio was obtained, which further confirms the mechanism proposed. The nature of the emission intensity curve can be simulated with the help of the EPISODE program, if the intensity is considered to be proportional to the photon density of the emitting species. The simulation thus arrived at with respect to DMDS pressure is shown in Fig. 5. The curve shows a linear rise and an exponential fall, similar to that obtained experimentally.To discover the contribution of the quen- ching term, if any, in the fall-off region, a similar simulation was carried out taking a typical collisional quenching rate constant for diatomic and triatomic molecules for OH* and SO;, respectively. Both show a similar intensity pattern with variation of DMDS pressure. Conclusion Formation of electronically excited SOz and OH in the gas- phase reaction of oxygen atoms with DMDS and DMS is reported. Broad chemiluminescence spectra in the range 240-460 nm with a sharp peak at 308.5 nm are obtained. The emission intensities show a linear dependence on DMDS/ DMS/O concentration in the low-pressure region.However, at higher oxygen or reagent concentrations the emission intensity reaches a maximum and then starts to decrease. A similar trend was observed for computer simulation of the pressure dependence of the chemiluminescence intensity. The maximum photon yields are found to be 3.3 x and 4.4 x for DMDS and DMS, respectively. The authors thank Dr. V. K. Kelkar for help with the com- puter programming and one of the referees for his suggestion on evaluation of photon yields. References 1 G. S. Tyndall and A. R. Ravishankara, Znt. J. Chem. Kinet., 1991,23,483. 2 F. Yin, D. Grosjean and J. H. Seinfeld, J. Atmos. Chem., 1990, 11,309; 365. 3 T. E. Gradel, Rev. Geophys. Space Phys., 1977,15,421. 4 R. A. Cox and F. J. Sandalls, Atmos.Enuiron., 1974,8, 1269. 5 P. D. Naik, U. B. Pavanaja, A. V. Sapre, K. V. S. Rama Rao and J. P. Mittal, Chem. Phys. Lett., 1991, 186, 565. 6 I. R. Slagle, F. Balocchi and D. Gutman, J. Phys. Chem., 1978, 02, 1333. 7 W. S. Nip, D. L. Singleton and R. J. Cvetanovic, J. Am. Chem. SOC.,1981,103,3526; 3530. 8 T. J. Kelly, J. S.Gaffney, M. F. Phillips and R. L. Tanner, Anal. Chem., 1983,55,135. 9 J. K. Nelson, R. J. Getty and J. W. Birks, Anal. Chem., 1983, 55, 1767. 10 R. J. Glinski and D. A. Dixon, J. Phys. Chem., 1985,89, 33. 11 J. H. Lee, R. B. Timmons and L. J. Stief, J. Chem. Phys., 1976, 64,300. 12 F. L. Tabares and A. G. Ureiia, J. Chem. SOC., Faraday Trans. 2, 1985,81,1395. 13 A. Fontijn, C. B. Meyer and H. I. Schiff, J. Chem.Phys., 1964, 40,64. 14 R. B. W. Pearse and A. G. Gaydon, The ldentijcation of Molec-ular Spectra, Chapman and Hall, London, 3rd edn., 1965. 15 F. Su, J. W. Bottenheim, D. L. Thorsell, J. G. Calvert and E. K. Damon, Chem. Phys. Lett., 1977,49, 305. 16 V. M. Donnelley and F. Kaufmann, J. Chem. Phys., 1977, 66, 4100. 17 R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson Jr., J. A. Kerr and J. Troe, J. Phys. Chem. Ref Data, 1992,21, 1125. 18 R. Zellner, D. Hartmann, J. Kanthauses, D. Rhasa and G. Weibring, J. Chem. SOC., Faraday Trans. 2, 1988,84,549. 19 W. Tsang and R. F. Hampson, J. Phys. Chem. Ref: Data, 1986, 15, 1087. 20 K. R. German, J. Chem. Phys., 1975,63,5252. 21 W. H. Smith, J. Chem. Phys., 1970,53,792. 22 K. L. Catleton, W. J. Kesseler and W. J. Marinelli, J. Phys. Chem., 1993,97,6412. 23 I).L. Singleton and R. J. Cvetanovic, J. Phys. Chem. Ref. Data, 1988,17,1377. 24 I. R. Slagle, D. Gutman, J. W. Davies and M. J. Pilling, J. Phys. Chem., 1988,92,2455. 25 A. F. Wagner and D. M. Wardlaw, J. Phys. Chem., 1988, 92, 2462. 26 A. C. Hindmarsh, G. D. Byrne, Lawrence Liuermore Laboratory, Report UCID-30112, Rev. 1, April, 1977. 27 H. P. Upadhyaya, U. B. Pavanaja, A. V. Sapre, K. V. S. Rama Rao and J. P. Mittal, 3rd Int. Con$ on Chemical Kinetics, July 12-16, 1993, NIST, Gaithersburg, USA. Paper 3/05549C; Received 15th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000825

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 831-836 83 1 Pulse Radiolysis of Iodate in Aqueous Solution Stephen P. Mezyk* Research Chemistry Branch, AECL Research, Whiteshell Laboratories, Pina wa ,Manitoba, Canada ROE 1LO A. John Elliot System Chemistry and Corrosion, AECL Research, Chalk River Laboratories, Chalk River, Ontario, Canada KOJ 1PO The reactions of primary water radiolysis radicals with 10; have been re-investigated using electron pulse radiolysis and absorption spectroscopy. Rate constants for iodate reaction with e,,, C(5.45 f0.45) x lo9 dm3 mol-' s-'1, 0'-C(1.82 f0.04) x lo9 dm3 mol-' s-'1 and 'OH (<lo5 dm3 mol-' s-') have been measured and compared with previously determined values. The observed iodate concentration dependence of the two tran- sient absorptions produced by its reaction with the hydrated electron and the oxide radical were found to be due only to spur scavenging effects, and thus the dimerization equilibria proposed by previous workers is not neces- sary.The previously reported rate constants for the reaction of iodate with hydroxyl radicals have been shown to be due to just iodate reaction with the small concentration of 0'-present in equilibrium with the hydroxyl radical. The release of radioactive iodine to the environment follow- ing a nuclear reactor accident has been a concern in recent years.'-3 Owing to the large quantities of water also expected to be released in most accidents from water-cooled reactors, most of the iodine is assumed to be in solution within the reactor containment.As such, a good knowledge of aqueous iodine chemistry is necessary to model its behaviour accu-rately under these conditions. The formation of volatile iodine species is mediated by the aqueous chemistry that occurs. The reaction of volatile I, with water gives HO14 I, + H,OeHOI +I-+ H+ which further reacts viu the overall reaction 3HOIeIO; + 21-+ 3H+ to form the non-volatile iodate species in neutral or basic media. However, the presence of high radiation fields, which are also expected following an accident, can cause significant changes in the chemistry of the system. It has recently been shown, for example, that the radiolysis of iodate in the pres- ence of organics leads to the formation of CH,I.' Such low- molecular-weight iodoalkanes are more volatile and harder to contain than inorganic forms of iodine.A good knowledge of the aqueous radiation chemistry of iodate is needed in order to understand the mechanisms of radiolytic reactions that lead to volatile products. Unfor- tunately, the rate constants and mechanisms reported in the literature to date6-" are not in good agreement. This paper describes our investigation of the radiolytically induced oxidation and reduction reactions of iodate in aqueous solution, which resolves some of these previous dis- crepancies. Experimental Solutions of sodium iodate (Fisher Scientific, Certified) or potassium iodate (Aldrich, A.C.S. reagent) were prepared using triply distilled or Millipore water. The pH adjustments were made by using NaOH (Aldrich 99.99% semiconductor grade), KOH (Mallinckrodt Volumetric Solution, 1.0 mol dm-3), or HClO, (Baker, Analyzed), and by the addition of sodium phosphate (monobasic, Anachemica A.C.S.reagent) or sodium borate (Anachemica, A.C.S. reagent) buffers. Fisher certified sodium formate, potassium hydrogen-carbonate, potassium hexacyanoferrate(I1) and potassium thiocynate were all used as received. The electron pulse radiolysis facilities, at AECL Research, Chalk River Laboratories and the Radiation Laboratory, University of Notre Dame, were used for these experiments. Both these systems have been described in detail else-where.",12 Dosimetry was carried out using either 0,-(A = 475 nm, GE= 2.39 x lo4) or N,O-saturated (A= 472 nm, GE= 4.92 x lo4) mol dm-3 SCN- solutions, and corrections for the electron density of solutions were applied when appropriate. Throughout this paper, G is defined as the number of species produced or destroyed per 100 eV, and E is in units of dm3 mol -'cm -'.Care was taken with pH measurements throughout these experiments, with values only being recorded of solutions immediately before pulsing. It was found that in non-buffered solutions, pH changes of over one unit occurred during He or N20 bubbling, which significantly changed the chemistry that occurred. Solutions above pH 11 were made by quanti- tative dilution of 1.0 mol dm-, KOH, and their pHs calcu- lated based on the assumption that the concentration of hydroxide ions in the stock solution was 1.0 mol drn-,.All measurements were done at room temperature (21 f2°C). Results and Discussion Reaction of e& and C0;-The radiolysis of water produces the free radicals e,,,, 'H and 'OH, and the ensuing chemistry observed for iodate can be accounted for in terms of the reactions of these initial entities. By the addition of suitable scavengers, the reactions of specific species can be studied in isolation. The reduction of 10, by e;,, and C0;- may be under-stood in terms of the following mechanism: 10; + e,,, -+ IOj2-(1) 10, + c0;--b 10i2-+ CO, (2) I0i2-+ H+ =HOIO',- (34 I0i2-+ H,O=HOIO;-+ -OH (3b) There have been two assignments proposed for the absorb- ing species produced by the reduction of iodate.Buxton and Sellers'* postulated that the 425 nm species observed at higher pH was due to the species 1Oi2-, formed by the reac- tions (1) and (2), and that at lower pHs, the 490 nm absorp- tion was due to HOIO',-, formed by reaction (34. Support for this belief was obtained from pulse electron irradiated conductivity experiments, which found no conductivity change following the reduction of iodate by the hydrated electron, and also by an analysis of the pH dependence of the observed optical decays of iodate reduction by formate. This is in contrast to the findings of flash photolysis studies7*' performed in near neutral solutions. Based on a comparative study of aerated aqueous ClO, ,BrO, and 10, solutions,' the transient absorption produced for iodate photolysis was attributed to the YO2 radical, formed by primary decomposition of the iodate ion, 10, + hv+'I02 + 0-To reconcile these experimental observations, we believe that an additional reaction occurs for the photolytically pro- duced '102, '102 + H20 +HOIO',-+ H' (4) which gives the exact species created by pulse radiolysis.The rate constant for reaction (1) was measured in this study by observing the change in the decay of the hydrated electron at 600 nm as a function of iodate concentration over the range (1-10) x mol dm-3. The values obtained over the pH range 4.0-12.8 have an average value of (5.45 & 0.45) x lo9 dm3 mol-' s-'. This rate constant is about a factor of three lower than its diffusion-controlled value and, over the pH range studied, there does not appear to be any significant ionic strength effect on k,.We have no explanation for this observation; however, it is consistent with the behaviour of the self-reaction of the hydrated elec- tron.14 The individual data are shown in contrast to the pre- viously measured values in Table l,l59l6 and are seen to be about 20% lower. Evidence for pK, can be seen in Fig. 1 which shows the pH dependence of the spectra obtained from pulse irradiating helium-saturated, 10-mol dm -iodate solutions containing 1.0 mol dm-3 tert-butyl alcohol (Bu'OH) to scavenge the hydroxyl radical. These spectra show a single broad peak at Table 1 Comparison of the rate constants of this study with liter- ature values for radiolysis product reaction with iodate in aqueous soh tion reactive this work other values species pH k/dm3 mol-' s-' k/dm3 mol-' s-l (I 4.0 (5.02f0.08) x lo9 6.0 (5.03 k 0.15) x lo9 7.1 x lo9 (15) 7.0 (5.67 f0.08)x 109 7.7 x 109 (16) - 10.0 (5.70f0.15) x lo9 11.0 8.3 x lo9 (16) 12.0 (5.83 f0.25) x lo9 12.8 (5.46 f0.15) x lo9 14.0 9.6 x lo9 (16) c0;- 8.8 (8.1 f0.2) x 107 (1.3 f0.1) x lo8 (10) 'H - 9.5 x 107 (5) 0.- 14.0 (1.80f0.04)x 109 1.6 x 109 (8) 13.7 (1.85 f 0.04)x 109 3.0 x 109 (7) 2.0 x lo9 (6) 1.1 x lo7(8) 'OH < 105 <5 x lo7 (7) 9.2 x lo8 (6) ~~ a Reference number in parentheses. J.CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 4.0 -3.0 -u m-I 0 .-2.0 -1.0 - t J v' 0.01 I350 ' 400 " 450 500 " 550 ' I600 A/nm Fig. 1 End of pulse transient absorption spectra produced by the pulse electron irradiation (ca. 10 Gy) of He-saturated, mol dmW310; solutions, containing 1.0 mol dm-j Bu'OH at pH 14.0 (B),13.5 (V),13.0 (a),12.0(a),11.0 (A)and 7.0 (0) 490 nm for pH values lower than 12, whereas at higher pH values, the spectrum shifts to a peak centred at 425 nm. The dependence on pH of the measured intensity at various wavelengths across the spectrum was determined, with typical values shown in Fig. 2. The plotted E values were calculated from the measured GEvalues by assuming that the initial hydrated electron yield was enhanced by the competi- tive contribution from the hydrogen-atom reaction with the hydroxide ion at the different pHs, l7'H + -OH +e,,, + H20; k = 2.2 x lo7 dm3 mol-' s-' relative to its other scavenging reactions, l7'H + Bu'OH +products; k = 1.7 x lo5 dm3 mol-' s-' 'H + 10,+products; k = 5.9 x lo7 dm3 mol-' s-l The initial e,,, yield was taken as 2.86, based on the *OH spur scavenging effects of the 1.0 mol dm-3 Bu'OH.'' An initial 'H yield of 0.6 was assumed.The solid lines in Fig. 2 are calculated pK, curves, with an average value of 12.6 f0.1. An earlier study by Buxton and Sellers" reported 1200 -c'E 1000 -0 c -I 800 -0 600 -E'E 400 -200 -lll'lllt 11.0 . 11.5 . 12.0 . 12.5 ' 13.0 .13.5 . 14.0 I PH Fig. 2 pH dependence of the peak transient absorption coefficients observed in mol dm-3 10; ,He-saturated solutions at 375 (B), 400 (a)and 425 (A) nm. Solid lines are fitted intensity curves, corre- sponding to pK, values of 12.5, 12.6 and 12.7,respectively. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 a value for this pK, as 13.3; however, in their study, insuE- cient formate was present to scavenge all the 0'-produced at the highest pHs studied. This would allow the formation of the transient IOi2-(see later), a species that has an intense absorption peak at 360 nm, resulting in artificially high absorbances being obtained and thus a higher pK, value. The variation of the intensity of the 490 nm peak absorp- tion with iodate concentration has previously been reported to be due to the formation of a dimeric species' '10, + 10, 41,OJ-with a calculated equilibrium constant of ca.10 dm3 mol-'. Fig. 3 shows the yields determined at pH 5.2 in this study, over the concentration range 10-4-2.5 x lo-' mol drn-,. The fitted line corresponds to the increased yield expected from scavenging effects within spurs.' These theoretical values have been calculated from the equation where G(S)is the calculated scavenged electron yield, G,,, = 2.55 is the yield of electrons that escape spur recombination, Go = 4.80 is the initial electron yield and a is a constant that is related to the rate constant for iodate scavenging of spur electrons. Based on experimental observations of the reaction of hydrated electrons with CH,Cl," this constant is described by the expression a = 9.06 x lo-'' x k(e& + 10;) The calculated yields were normalized to the experimental GE data at low4mol dm-, iodate and the excellent agree- ment seen over the concentration range studied demonstrates that no such dimerization equilibrium needs to be considered.The reduction of iodate by the formate radical has also been investigated in this study. Formate acts as a scavenger of both 'H and 'OH radicals through the reactions 'H/'OH + HCO, -+H,/H,O + C0;-(5) and reduction of iodate then proceeds via reaction (2). The addition of 1.0 mol dm-3 HCO, to an N,O-saturated solu-4.5 I-=-I 251""1 ' ' "***" ' ' """' ' ' """' ' ''I 104 10-3 1o-2 10-1 [IO;]/mol dm-3 Fig.3 Iodate concentration dependence of the 490 nm absorbance in He-saturated solutions containing 5.0 x lop3 mol dmp3 NaH,PO, (pH 5.2) and 1.0 mol dm-3 Bu'OH. The solid line is the theoretical prediction of increased yield due to radiation spur scav- enging, based on the model of LaVerne and Pimblott." These two yields have been normalized at an iodate concentration of mol dm-'. tion containing mol dmF3 10, gave the same spectrum as that observed for e& reduction of iodate, with about twice the yield. The rate constant for reduction, k,, as estimated from the rate of formation of the transient absorption at 490 nm for this solution, was (8.1 f0.2) x lo7 mol dm-3 s-l, which is a little lower than previously reported" for a 0.20 mol dm-, formate solution (see Table 1).At lower formate concentrations, the observed rate of reaction was slightly slower; however, this decrease is readily explained by the expected ionic strength dependence of this reaction of two, single negatively charged, species. In this study, at pH 8.8 and at a constant iodate concentra- tion of 0.10 mol dmP3, little change in the observed decay rate at 490 nm was found when the formate concentration was changed from 1.0 mol dm-, (2k/~= 4.1 x lo6 cm s-l) to mol dm-3 (2k/c = 3.9 x lo6 cm s-'). These rate con- stants are in excellent agreement with the flash photolysis value reported previously7 (2kl.5= 4.0 & 0.7 x lo6 cm s-'). The Buxton and Sellers" analysis of the pH dependence of the observed decays of the species produced upon iodate reduction by formate was based on a pK, value that is too high.A re-analysis of these data is thus necessary. Although these data would have some interference from IOi2-forma-tion, from the incomplete formate scavenging of OD-,at worst only ca. 10%of the radicals produced would be in this form. This small contribution has been neglected in the fol- lowing kinetic treatment. The measured 2k/c values of Buxton and Sellers," at various pHs, are given in Table 2. From our experimental peak GEvalues shown in Fig. 1, absorption coefficients can be calculated, and thus these values converted to 2k decay rate constants. Under these experimental conditions, the absorp- tion decay is comprised of only two species in equilibrium, each reacting with itself and the other.Based on the Buxton and Sellers mechanism," this would be HOI0;-+ HOI0;-+products (6) IOi2-+ HOI0;-+products (7) 1Oi2-+ IOi2--+ products (8) An analysis of this decay mechanism gives the observed bimolecular rate constant as lo Refitting this equation to the newly calculated 2k data, using linear regression techniques and the value of K, deter-mined in this study, gives the values 2k, = (5.99 & 0.11) Table 2 Rate constants data taken from ref. 10 for decay of '10, at 480 nm in Ar-saturated 2.0 x mol dm-3 10; and 2.0 x lo-' mol dm-3 HCO; ~____ ~ 2k/109 dm3 mol-' s-' (WE)PH /lo6 cm spl experimental" calculatedb 14.0 2.1 f0.1 2.6 2.6 13.0 3.3 f0.1 4.1 4.1 12.0 5.1 0.1 5.8 5.8 11.0 8.7 5.1 f0.1 5.3 * 0.1 5.9 6.0 6.0 6.0 3.0 5.8 f 0.2 6.1 6.0 'Values of 2k calculated using absorption coefficient values from this work.Calculated values using the model described in the text. 834 x lo9 dm3 mol-' s-', k, = (1.19 & 0.06) x 10" dm3 mol-' s-' and 2k8 = (2.33 & 0.02) x lo9 dm3 mol-' s-'. The pH dependence of the calculated decay rate constants using this model is also shown in Table 2 and is seen to be in very good agreement with the experimental data. At both neutral and basic pHs, replacing 1.0 mol dm-, formate by 1.0 mol dm-, Bu'OH gave an increase of ca. 35% in the observed decay rate. This is in agreement with previous observations," and thus we also attribute this change to the reactions of HOIOi- and IOj2- radicals with the organic radicals produced by the scavenging of 'OH radicals.Further corroboration of this effect was obtained by com- puter simulation of the observed kinetic decays across the pH range 8.7-14. The decay of the reduced species to form non- absorbing products was found not to be purely second order. These curves were modelled by the following reaction scheme: radiation .H2O ' OH, e,,,, '€4 e(;,, + 10, -,IOj2-; k, = 5.45 x lo9 dm3 mol-' s-' H20$Hf + -OH; K,= 1.0 x IOj2-+ H+=HOIO',-; K, = 2.51 x lo-', dm3 mol-' HOI0;-+ HOI0;---* products; 2k6 = 5.99 x lo9 dm3 mol-' s-' HOI0;-+ 1Oi2--+products; k, = 1.19 x 10" dm3 mol-' s-' IOj2-+ IOj2-4products; 2k8 = 2.33 x lo9 dm3 mol-' s-' 'OH + (CH,),COH -,'CH2C(CH3),0H; l7k = 6.0 x lo8 dm3 mol-' s-' HOIO',-+ *CH,C(CH,),OH +products; k, (9) 1Oi2-+ 'CH,C(CH,),OH -,products; k,, (10) using the FACSIMILE2' modelling program and calculated absorption coefficients for the two absorbing species IOi2-and HOIOi-.The two equilibrium constants were incorpor- ated in the model by assuming rate constants for their forward and backward components; values for K, of k, = 1.11 x lop3s-' and k, = 1.11 x 10'' dm3 mol-' s-',~' and for K, of k, = 2.0 x 10" dm3 mo1-' s-' and k-, = 8.0 x lo8 s-I, were chosen. Varying the last values by an order of magnitude had no effect on the results obtained. The two unknown values, k, and klo, were optimized for each pH, and average values of k, = (2.45 f0.35) x lo8 dm3 mol-' s-' and klo = (1.59 & 0.28) x lo8 dm3 mol-' s-' were obtained.An example of the calculated fit to the experi- mental data at pH 12.7 is shown in Fig. 4. Reaction of 'H with 10, The reaction of hydrogen atoms with iodate has been pre- viously investigated by steady-state competition technique^,^ and a reaction rate constant determined as 5.9 x lo7 dm3 mol-' s-'. However, the pulse radiolysis of 0.10 mol dm-, 10, and 1.0 rnol dmV3 Bu'OH in pH 1-2 solution did not give any change in absorption; thus, no value of the reaction rate constant could be determined in this study. At present time-resolved EPR studies are underway to determine this J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3.0 -1 2.5 2.0 G7 1.5 z 1.o 0.5 0.0 1 I 1 1 1 I-0.5 I 0.0 20.0 40.0 60.0 80.0 f/P Fig. 4 Comparison of the experimental and calculated decay at 490 nm for a pulse electron irradiated (80.7 Gy), N,-saturated solution containing mol dm-3 10; and 1.0 mol dm-3 Bu'OH at pH 12.7 Reaction of 0'-with 10, At basic pHs, the 'OH radical deprotonates (pK = 11.9)22 to give O'-. This radical also reacts with iodate, according to7 0.-+ 10; -,10i2-(1 1) to produce an intense transient with a peak at 360 nm. The spectrum obtained in this study [Fig. 5(a)] agrees well with that previously determined.' By varying the 10, concentra-tion over the range (1-10) x mol drn-,, and observing the growth kinetics of the transient absorption in N20-saturated solutions, a reaction rate constant of (1.82 & 0.03) x lo9 dm3 mol-' s-' was obtained, [Fig.5(b)], in general agreement with previous determinations6-8 (see Table 1). The 360 nm transient absorption yield dependence on iodate concentration was also determined in this study and the data are shown in Fig. 6 for helium- or nitrogen-saturated solutions at pH 14. Under these experimental conditions the electron-reduced species, HOI0;-and 10;' -, have a signifi- cant contribution to the observed absorbance. Based on the spectra presented in Fig. 1, an absorption coefficient of E = 830 dm3 mol-' cm-l at 360 nm for the e;,, reduced species is calculated (it was assumed that no spur scavenging 1'1'1'1 -20 15 -$ 10-0 [10,]/10-3 rnol dm-3 5-I 0-1 200 300 400 500 600 700 800 A/nm Fig.5 (a) End of pulse transient absorption spectrum produced in N,O-saturated, rnol dm-3 10; solution at pH 14.0. (b) Deter-(.) mination of the 0'-reaction rate constant at 360 nm at pH 14 value. and 13.7 (A). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 12.0 t -1 6.01 1 t 1 104 i 0-3 10-2 lo-' [IO;]/mol drr~-~ Fig. 6 Iodate concentration dependence of the 360 nm absorbance produced in He- or N,-saturated solutions at pH 14.0 at end of pulse (I)in comparison to the calculated values for e;,, (-), 0'-(---) and total (. .) yields due to spur scavenging occurs for lop3 mol dm-3 10;).For the data in Fig. 6, this = 2780 at mol dm-3 10,.corresponds to GE,,~ A similar analysis can be performed to determine the 10;' -yield. Based on the measured23 total yield of oxidizing species, in N,O-saturated solutions at pH 14, of G = 7.65, an absorption coefficient at 360 nm of E = 2680 dm3 mol-' cm-' is determined from the data given in Fig. 5. For de- oxygenated solutions, the yield of oxidizing species has been determined as G = 3.87, which gives GE,, = 10380 for an iodate concentration of mol dm-3. The sum of these two values is GE,, = 13 160, ca. 3% higher than the experimental value of GE= 12770. To account for this small difference, each of the individual GE values was multiplied by 0.97, to give a normalized yield at mol dmP3 iodate.The effects of spur scavenging at higher iodate concentra- tions for each component were calculated using the method of Laverne and Pimbl~tt.'~ The initial yield and escape probability of 0'-have been assumed to be the same as for 'OH. This approximation was made since no direct param- eters for 0'-scavenging could be found in the literature. These values are shown in Fig. 6 and the excellent agreement between the total scavenged yield and the experimental values shows that only this process is occurring. At higher doses, the decay of the absorption at 360 nm was found to be second order to zero intensity. The rate constant for an N,O-saturated, lop3mol dm-3 10, solution at pH 14 was 2k,k = 1.37 x lo5 cm s-'. From the measured absorption coefficient of 10;' -at this wavelength, this corre- sponds to a value of 2k = 3.67 x 10' dm3 mol-' s-', in fair agreement with the previous determinationg of 1.8 x lo8 dm3 mol-' s-'.Owing to the high initial pH, a full ionic strength dependence on this decay was not performed. Reaction of 'OH + 10, The largest variation in the previously reported rate constant for iodate reaction is that for the 'OH radical. Previous deter- minations have given values in the range (1-100) x lo7 dm3 mo1-l s-' (see Table l),and although a transient peak with a maximum at 360 nm has been identified, little agreement on its intensity has been obtained. Our initial experiments, with only iodate in water, showed a weak transient spectrum with a maximum at 360 nm.The observed growth rate at this wavelength became faster at 835 higher iodate concentrations, over the range (5.0-20) x lo-, mol dm-3 and, although the data were very scattered, an apparent second-order rate constant of (1.0 0.4) x lo6 dm3 mol-' s-' was obtained. The spectrum shape obtained under these experimental conditions was essentially identical to that produced by the 0'-transient. This suggested that the reaction sequence that occurred was 'OH + -0HeO'-+ H,O (12) 0.-+ 10, +10;2-with the observed growth rate being limited by the forward component of the first reaction. This hypothesis was checked as follows. When the experiment was repeated using N,O-saturated, lo-, mol dm-3 iodate solutions, containing 5.0 x mol dm-3 phosphate buffer to ensure a constant pH of 5.2, no peak was observed.This finding is in agreement with the experiments of Buxton and Sellers." A very weak absorption rising at shorter wavelengths was seen, but there was essentially zero absorbance at 360 nm. As the reaction of 'OH with 10, did not appear to give any product species absorbance, the measurement of its rate constant by Fe(CN):- and HCO, competition kinetics was attempted. A solution containing loW4mol dm-3 Fe(CN):- and 0.10 mol dm-3 10; gave the same intensity change at 420 nm as only mol dm-3 Fe(CN):- and similarly, the transient absorbance at 600 nm in a solution of 0.10 rnol dm-3 HCO; did not change upon addition of 0.10 mol dm-3 10;. Based on the known rate constants for the reac- tion of 'OH with Fe(CN):- (k = 1.05 x 10" dm3 mol-' sW1)l7and HCO, (k = 8.5 x lo6 dm3 mol-' s-'),~ and the assumption that a 10% change in the observed intensity could be detected, we believe that the rate constant for the reaction 'OH + 10, + products has to be <lo5 dm3 mol-' SKI.This value is significantly slower than the three reported values in the In these experiments it was found that the pH of unbuffered, air-saturated, 10, solutions was ca.7.7, but this rose to >8.7 upon bubbling with He or N,O. At these more basic pHs, a greater fraction of the 'OH radical exists in its base form, Om-,which can react with iodate ions. Even though the equilibrium concentration of 0'-would typically only be 0.1% of 'OH, the only other loss of 'OH radicals is by the relatively slow process of H,O, production; thus, iodate scavenging of 0'-could severely perturb this system.The observed absorbance change at 350 nm in an N,O- saturated solution containing 5.1 x mol dm-3 10; and 1.1 x mol dm-3 sodium tetraborate is shown in Fig. 7. To test our hypothesis, this curve was modelled by the fol- lowing simplified reaction scheme (rate constants not from this study taken from ref. 17) 'OH + -OH + 0'-+ H,O; k,, = 1.3 x 10" dm3 mol-ls-' 0'-+ H,O +'OH + -OH; k-12 = 1.86 x lo6 dm3 mol-ls-' 0'-+ 10; +IOi2-; k,, = 1.84 x lo9 dm3 mol-' s-' e& + N,O +O*-+ N,; k = 8.9 x lo9 dm3 mol-' s-* 'OH + 'OH + H,O,; k = 4.5 x lo9 dm3 mol-' s-' 836 3.5 I I I I I I 113.0 -I 2.5 2.0 (5 m 1.5 0 F 1.o 0.5 0.0 1 V." 0.0 20.0 40.0 60.0 80.0 tlW Fig.7 Comparison of the experimental and calculated kinetic data at 350 nm for a pulse electron irradiated (4.90 Gy), N,O-saturated solution containing 5.1 x mol dm-3 10; and 1.1 x mol dm-' sodium tetraborate at pH 9.2 e,,, + 10, + H+ HOI0;-; k, = 5.45 x lo9 dm3 mol-' s-' HOIOi-+ HOI0;-+products; 2k6 = 5.99 x lo9 dm3 mol-' s-' 10:: -+ IOi2--+ products; 2k = 3.67 x lo8 dm3 mol-' s-l 10;' -+products ; k 13 (14) again using the FACSIMILE2' program and known absorp- tion coefficients for all the absorbing species. The k13 rate constant was fitted to the experimental data by the program, with an optimized value of 5.9 x lo3s-' obtained.This first- order decay of the IOi2-species had to be included to simu- late the experimental decay timescales. The agreement of this value to the previously measured value of 3.3 x lo3 s-' gives further support to this model. The calculated fit for this curve is also shown in Fig. 7 and is seen to be in fair agree- men t. Conclusion The major conclusion of this work is that the rate constant for 'OH reaction with iodate is much slower (<lo5 dm3 mo1-l s-') than previously The reaction inter- mediate observed at 360 nm under these conditions has been shown to be due to the formation of IOk2-by reaction of iodate with Ow-,where this species is produced by the reac- tion 'OH + -OH --+ H20 + 0'-J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 The iodate concentration dependence of the yield of the transients produced by e,,, and 0'-reaction with 10, has been shown to be due only to spur scavenging, and thus no additional dimerization equilibria need be considered. We thank Dr. R. H. Schuler and the staff of the Radiation Laboratory for the use of their facilities in this study, and Drs. N. Sagert and G. V. Buxton for helpful discussions throughout the course of this work. We also thank a referee who pointed out some inconsistencies in an earlier version of this manuscript. References 1 Proc. 1st CSNI Workshop on Iodine Chemistry in Reactor Safety, ed. A. M. Deane and P. E. Potter, Harwell Research Report, AERE-R 11974,1986. 2 Proc. 2nd CSNI Workshop on Iodine Chemistry in Reactor Safety, ed.A. C. Vikis, Atomic Energy of Canada Ltd. Research Report, AECL-9923,1989. 3 Proc. 3rd CSNI Workshop on Iodine Chemistry in Reactor Safety, ed. K. Ishigure, M. Saeki, K. Soda and J. Sugimoto, JAE Research Report, JAERI-M 92-012, 1992. 4 M. Eigen and K. Kustin, J. Am. Chem. SOC., 1962,84,1355. 5 J. Paquette and B. L. Ford, Radiat. Phys. Chem., 1990,36,353. 6 0.Amichai and A. Trenin, J. Phys. Chem., 1970,74, 830. 7 F. Barat, L. Gilles, B. Hickel and B. Lesigne, J. Phys. Chem., 1972, 76, 302. 8 Y. Tendler and M. Farraggi, J. Chem. Phys., 1973,58,848. 9 U. K. Klaning, K. Sehested and T. Wolff, J. Chem. Soc., Faraday. Trans. I, 1981,77, 1707. 10 G. V. Buxton and R. M. Sellers, J. Chem. Soc., Faraday. Trans. I, 1985,81,449. 11 A. J. Elliot and F. C. Sopchyshyn, Int. J. Chem. Kinet., 1984, 16, 1247. 12 R. H. Schuler, A. L. Hartzell and B. Behar, J. Phys. Chem., 1981, 85, 192. 13 F. Barat, L. Gilles, B. Hickel and J. Sutton, Chem. Commun., 1969,1485. 14 H. Christensen and K. Sehested,J. Phys. Chem., 1986,90,186. 15 G. Duplatre and C. D. Jonah, Rudiat. Phys. Chem., 1985, 24, 557. 16 M. Anbar and E. J. Hart, J. Phys. Chem., 1965,69,973. 17 G. V. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross, J. Phys. Chem. Ref: Data, 1988, 17, 513. 18 Z. D. Draganic and I. G. Draganic, J. Phys. Chem., 1973, 77, 765. 19 J. A. Laverne and S. M. Pimblott, J. Phys. Chem., 1991, 95, 3 196. 20 A. R. Curtis and W. P. Sweetenham, Harwell Research Report AERE-R 12805, 1988. 21 W. C. Natzle and C. B. Moore, J. Phys. Chem., 1985,89,2065. 22 J. Rabani and M. S. Matheson, J. Am. Chem. Soc., 1964, 86, 3175. 23 G. V. Buxton and F. S. Dainton, Proc. R. SOC. London, Ser. A, 1965,287,427. 24 G. V. Buxton and A. J. Elliot, Radiat. Phys. Chem., 1986,27,241. Paper 3/053221; Received 6th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000831

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 837-841 Temperature Dependence of the Rate Constant for the Reaction -+ OH in Water up to 150'C =aq A. John Elliot" and Denis C. OueHette System Chemistry & Corrosion Branch, Chalk River Laboratories, Chalk River, Ontario, Canada KOJ IJO The temperature dependence of the rate constant for the reaction, e, + OH +OH-(1) has been measured in liquid water up to 150°C. The values of k, were derived from fitting the absorbance vs. time plots of the decay of the hydrated electron in pulsed-irradiated deoxygenated 1 x mot dm-3 sodium borate solutions using the computer programme FACSIMILE. The values for k, were (2.8 0.2) x 10,' (18"C), (5.0 f0.4) x 10'' (75"C), (6.0 & 0.5) x 10" (100°C) and (7.2 & 0.3) x 10,' dm3 mol-' s-' (150°C).The tem- perature dependence of k, was found to be much less than for diffusion in water. The temperature dependence of E,,, for the hydrated electron has been determined up to 200 "C. Over the last two decades, the temperature dependence of the rate constants for most of the important reactions involving the primary species formed in the radiolysis of light water have been measured.'-'4 In this paper we report on one of the remaining important reactions, the reaction of hydrated electrons with hydroxyl radicals to form hydroxide ions e,; + OH --* OH-(1) In the course of this work we have evaluated the molar absorption coefficient of the hydrated electron as a function of temperature up to 200°C. This, in turn, allowed us to cal- culate the absolute bimolecular rate constant for dimer- ization of the hydrated electron [reaction (2), see later] from the data of Christensen and Sehested.' Experimental The water was purified by passing once-distilled water through a Millipore Milli-Q system and then redistilling from alkaline permanganate.Solutions of 1 x lo-' mol dm-3 sodium tetraborate (Anachemia) were prepared and deoxy- genated by stripping with helium gas using the syringe- bubbler technique. The high-temperature pulse radiolysis facility has been described in earlier An EG&G FNDl WQ photodiode was used for light detection for all experiments, except when a HTV R-166 photomultiplier was used to record the pulse shape by Cerenkov emission, The dosimeter was an oxygen-saturated KSCN solution (0.01 mol dm-3) ~~~ G iswhere the GE value of 2.39 x lo4 was ~sed.''~'~ defined as the number of species formed per 100 eV and the molar absorption coefficient, E, is expressed in dm' mol-' cm-'.Thus, throughout the paper GE~,~has units: (number of species) (100 eV)-' dm3 mol-' cm-'.(In this paper we have used the term g for the yield of the primary species formed in the radiolysis of water at ca. s after ioniza- tion; G is used for the experimental value.) Results The decay of the hydrated electron at its absorption maximum in a deoxygenated borate-buffered solution (pH 9.2 at room temperature) was used to monitor the rate of reac- tion (1) as shown in Fig.1-4. The time profile of the absorp- tion trace was fitted using the parameter-fitting routines in the FACSIMILE' kinetic simulation programme; the reac- tion scheme used comprised reaction (1) and the following I..0.12,. . . , . . , . . . . , time/l s Fig. 1 Absorbance at 720 nm when deoxygenated loF3mol dm-3 borate-buffered solutions are irradiated with a 22.54 Gy (upper), 11.00 Gy (middle) and 1.57 Gy (lower) pulse at 18°C. The dotted lines are the FACSIMILE fits to the experimental trace and the dash-dot lines are fits where k, is either 30% larger or smaller than the fitted value. a 0.07 5 0.06 f!$, 0.05 n 0.04 0.03 0.02 0.01 0.0OI' "2.0 " " ' ' '' 'I. 0.0 4.0 6.0 time/l 0-6 s Fig. 2 Absorbance at 780 nm when deoxygenated mol dm-3 borate-buffered solutions are irradiated with a 23.46 Gy (upper), 11.06 Gy (middle) and 1.81 Gy (lower) pulse at 75°C.The dotted lines are the FACSIMILE fits to the experimental trace. 838 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.05 0.04 Q)0 m+ 0.03 s n 0.02 0.01 0.0OI' . ' . ' . . ' time/l O-'j s 0.0 5.0 10.0 Fig. 4 Absorbance at 880 nm when deoxygenated mol dm-3 time/l s borate-buffered solutions are irradiated with an 11.00Gy (upper) and Fig. 3 Absorbance at 810 nm when deoxygenated mol dm-3 1.80 Gy (lower) pulse at 150°C. The dotted lines are the FAC- borate-buffered solutions are irradiated with a 9.86 Gy (upper) and SIMILE fits to the experimental trace and the dash-dot lines are fits 1.69 Gy (lower) pulse at 100°C.The dotted lines are the FAC- where k, is either 30% larger or smaller than the fitted value. SIMILE fits to the experimental trace. It was necessary to add reaction (12) (assumed to be a sequence : pseudo-first-order reaction) to the reaction set to allow for ea; + ea; -+ H, (2) the reaction of the hydrated electron with trace impurities. OH + OH -+ H,O, (3) This reaction will be discussed later. ea; + H,02+OH + OH-(4) The following equilibria were also used: ea; + H +H, (5) H,O H++ OH-(13) OH + H,O, -+ HO;+ H,O (6) HO;+ OH-*0;-+ H,O (14) OH + H,+H + H20 (7) B(OH), + OH-*B(OH)i (15) OH + 0;-+0,+ OH-(8) The temperature dependence for K13 was taken from Sweeton et a1.,2' and the value of k-13 was extrapolated to H+H+H, (9) higher temperatures from the data at 0-48°C of Natzle and Moore2, using the Smoluchowski equation assuming aH + OH-+e; + H,O (10) diffusion-controlled reaction.' The temperature dependence H + OH -+ H,O (1 1) of K14 was calculated from the data of Christensen and Sehested,6 Buxton et and Schwarz and Biel~ki~~ and ea; + impurity -+ products (12) from the data for K,,;,l k14 was taken to be 1.3 x 10" dm3 The temperature dependence used for the rate constants of mol-s-at 25 "C,and the temperature dependence was cal- reactions (3),14 (4)," (6),3 (7),'v4 (8),13 (9),"714 (lo)', and culated to be diffusion controlled." For K1,, the tem-.(11)14 were taken directly from the literature. The tem-perature dependence as reported by Mesmer et ~1 was ~~ perature dependence for reaction (2)' was recalculated as used; k,, was assumed to be 1 x 10" dm3 mol-' s-l at described below.The temperature dependence for reaction (5) 25 "Cwith an activation energy of 14 kJ mol- '. has not been reported; we used the value k, = 2.5 x 1O1O The borate buffer, which does not react with e,;, OH or H, dm3 mol- s-at 23 "C2o and gave it the same temperature was used to ensure that reaction (16) did not compete with dependence as diffusion in water. The values for the rate con- the above reactions. stants are given in Table 1. ea; + H++H (16) Table 1 Rate constants for the reactions k/dm3 mol-' s-' reaction number 20 "C 75 "C 100 "C 150°C 1 (2.8 f0.2) x 10" (5.0 f0.4) x 10" (6.0 f0.5) x 10" (7.2 f0.3) x 10" 2 2k = 1.1 x 10" 4.2 x 10" 6.7 x 10'' 1.4 x 10" 3 2k = 8.6 x lo9 1.6 x 10" 1.8 x 10" 2.4 x 10'' 4 1.2 x 10" 2.8 x 10" 3.6 x 10" 5.7 x 1O'O 5 2.3 x 10" 7.2 x 10'' 1.1 x 10" 1.9 x 10'' 6 2.7 x 107 6.4 x 107 8.6 x lo7 1.4 x lo8 7 3.5 x 10' 1.1 x 10' 1.7 x 10' 3.4 x lo8 8 9.0 x 109 2.1 x 10" 2.7 x 10" 4.1 x 10" 9 2k = 9.0 x lo9 2.7 x lo1' 3.8 x 10" 6.6 x lo1' 10 1.9 x 107 2.3 x 10' 5.6 x lo8 2.4 x 10'' I1 1.5 x 10" 2.7 x 10" 3.1 x 10" 4.0 x 10'' 12" (1.9 _+ 0.2) x 104 (3.1 _+ 0.4) x lo4 (3.9 f 0.2) x 104 (8.4 f0.4) x lo4 'Units, s-'. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The g values for the primary species were taken from Elliot et Since the Cerenkov radiation did not contribute signifi- cantly to the absorption profiles, the data were fitted through the pulse using the pulse shape determined from the Ceren- kov emission near 250 nm.The data were recorded at the absorption maximum for the hydrated electron at each tem- perature.' Before we can analyse the experimental data for reaction (I), the molar absorption coefficient of the hydrated electron as a function of temperature must be known. The tem- perature dependence of GE,,, for the hydrated electron has been reported by a number of laboratories but, because of a lack of data on G(eJ, the actual molar absorption Coefficient has not been reported. Likewise, E,,, is required to calculate the temperature dependence for reaction (2) from the 2k, values reported by Christensen and Sehested' where a con- stant was assumed.Molar Absorption Coefficient of ePg as a Function of Temperature The GE,,, for the hydrated electron at its absorption maximum was measured at the end of the 0.2 ps pulse (1.5-2.0 Gy) in a deoxygenated borate-buffered solution over the temperature range 20-200°C. Corrections of 2 and 7% for decay of the signal in the pulse were applied at 150 and 200"C, respectively. To calculate crnax, G(e,;) was taken as g(e,;) below 150"C, while at 200 "C, G(ea;) was taken as g(e,;) +g(H). FACSIMILE calculations indicate that, at 200 "C,all atomic hydrogen was converted to hydrated electrons during the pulse at prevailing hydroxide ion concentration (ca. 4.6 xlop3mol dmP3 as a consequence of the borate buffer present at 200°C) by reaction (10).The data of 150°C could not be used, as the conversion of hydrogen atoms to hydrated electrons is only partially completed during the pulse. The values for E,,, as a function of temperature are shown in Fig. 5. We report a value of 20000 &700 dm3 mol-' cm-' at 715-720 nm for the molar absorption coefii- cient at 20°C; it should be remembered that this is based on the thiocyanate dosimeter using a GE~,~ value of 2.39 xlo4, which was calibrated against the Fricke dosimeter.' 7,18 22000 '"'~"''~'"'""~''"'""1'"" 1 21000 20000 c I5 19000- I- 18000 m E %17000 -'e;;\Z, \ 0 -i l6Oo0 tI h-\ 15000r~""'"""'~~'~'~""''"''""'0 50 100 150 200 250 300 350 temperature/"C Fig.5 Molar absorption coefficient at i,,,as a function of tem-perature: this work (m); Jou and Freeman (A);26Michael et al. (0);25 The dotted line represents and Christensen and Sehested (0).5 the data of Shirashi et ~l.,~'and the dashed line is from eqn. (I)in the text. All molar absorption coefficients are normalized at 20000 dm3 mol--' cm-' at 20-25°C. 839 The temperature dependence of GE,,, of the hydrated elec- tron has been reported by a number of laboratories. Our data agree with those of Michael et ~l.,,~who published values for 3 xlo-' mol dm-3 sodium hydroxide in water over the tem- perature range -4 to 90°C and using the Fricke solution as the dosimeter. At 20°C, our value of GE,,, of 5.32 xlo4 is in good agreement with their value of 5.23 xlo4 at 25°C.Other laboratories have used different dosimeters and, as such, their absolute values of Gcrnax differ slightly from ours at room temperature. Jou and Freeman26 have published values for the temperature range 1-107"C, and Shiraishi et have published data at 20-250 "C, for water. Christensen and Sehested' have reported GE,,, data for solutions containing 1 xlo-' mol dmP3 sodium hydroxide and 0.12-0.2 mol dmP3 hydrogen in which all the hydroxyl radicals were con- verted to hydrogen atoms [reaction (7)], and these, in turn, were converted to hydrated electrons [reaction (lo)]. To estimate the temperature dependence of E,,, from these studies, we calculated the value of cmaxfrom the published GE,,, data and then normalized the values to 20000 dm3 mol-' cm-' at 20-25°C.For the Christensen and Sehested data,' GE,,, values uncorrected for spur scavenging were used; G(e,;) was taken to be equal to the sum g(e,;) +g(0H)+g(H) for neutral pH solutions, and it was assumed that the temperature dependence would be the same as in the basic solution. As shown in Fig. 5, there is agreement between the results from the different publications, except for the data from Shiraishi et al.27We have no explanation as to why the data of Shiraishi et a!.differ from the others. For fitting our data we have used the relationship (I), ~(trC)=20364 -20.4t (1) which is given by the dashed line in Fig. 5. With this molar absorption coefficient we can now calcu- late the true temperature dependence of the rate constant for reaction (2) from the values given in Table I1 in the paper by Christensen and Sehested,s where a constant molar absorp- tion coefficient of 18400 dm3 mol-' cm-' was assumed.From our corrected data, at 25"C, a value of 2k, =1.29 x10" dm3 mol-' s-' was calculated and the Arrhenius activation energy for k, was found to be 20.3 kJ mol-' for temperatures up to 150°C. The corrected values up to 200°C are shown in Fig. 6. Christensen and Sehested reported an activation energy of 23 kJ mol-'.' Estimation of k, from Kinetic Traces At 18 and 75"C, data were recorded at three different doses (and pulse lengths): 1.8 Gy (0.2 ps), 11 Gy (0.5 ps) and 22 Gy (0.8 ps). At 100, 150 and 200 "C, only the 1.8 and 11 Gy doses were studied.As noted earlier, it was necessary to incorporate reaction (12) to account for the reaction of the electron with trace impurities. The procedure to fit the data was to assume initially that k,, was zero, and then fit the highest dose traces for k, with FACSIMILE; this value of k, was then held fixed and k,, was varied to fit the low dose traces. The values of k,, were then fixed for the higher dose data and k, fitted again. This process was repeated until consistent values of k, and k,, were found. If intermediate dose data were available, these were then fitted with the fixed value of k12. This pro- cedure worked up to and including 150"C, and examples of the fits are shown in Fig. 1-4. The fitted values of k, and k,, are given in Table 1 ;the values of k, are also displayed as an Arrhenius plot in Fig. 6 with an apparent activation energy of 7.9 kJ mol-'.The values of k,, are consistent with the pseudo-first-order rate constants expected from (1-2) x mol dm-3 trace impurities, such as oxygen, which react with the hydrated electron at near-diffusion-controlled rates. The V vA F m .. 1 0.0020 0.0025 0.0030 0.0035 KIT Fig. 6 Rate constants for reaction (1) from this work (+);reaction (3) from ref. 14 (m);and reaction (2) from ref. 5 after correction for the temperature dependence of the molar absorption coeficient at pH 10.9 (V), pH 12 (0)and pH 13 (A). The dash-dot line is an Arrhenius fit for reaction (1).value of k, at 18 "C of (2.8 & 0.2) x lo1' dm3 mol-' s-' is in good agreement with the value of (3.0 & 0.7) x 10'' dm3 mol-' sC1 estimated by Matheson and Rabani at 23 "C20 The error bars in Table 1 are one standard deviation based on three to six determinations of the rate constant in ques- tion. In Fig. 1 and 4, the dash-dot lines represent a +30% variation in the rate constant k, for 10 Gy traces. At 18"C, this is clearly an excessive error estimate but, at 150"C, the dashed lines are only just outside the noise envelope. Material transport calculations indicate that reaction (1) accounts for 50% of the hydrated electron loss at 18 "C, while reaction (2) accounts for only 15%. At 150"C, reaction (1) accounts for only 38%, whereas reaction (2) accounts for 50%.These calculations were based on a 10 Gy pulse and after 70% of the hydrated electron had decayed. Reaction (12) accounted for 16% of the hydrated electron loss at both tem- peratures. Data were also collected at 200°C and the traces appeared to decay by first-order kinetics at that temperature. However, the observed first-order rate constant was dose dependent; it was 5.6 x lo5 s-l at 1.8 Gy and 9.0 x lo5 s-l at 9.5 Gy. Based on the values at lower temperatures, an estimate for the value of k,, of 1-2 x lo5 s-l at 200°C can be made. A number of ways of treating the kinetic data at 200°C were tried; for example, attempts were made to fit the kinetic traces using values for k, based on a continued Arrhenius dependence up to 200 "C, and on the much lower value calcu- lated from Table I1 in the paper by Christensen and Sehested' (see Fig.6). Fits could be found, but our confidence in the results was low, owing to the relatively large contribu- tion of k12, the small contribution of reaction (1) to the overall kinetics in the case of the Arrhenius dependence for k,, and the relatively large value for k, found assuming the lower reported value for k, .' Discussion The temperature dependence of reaction (1) is similar to that of the bimolecular reaction of the hydroxyl radical [reaction (3)], rather than that of the bimolecular reaction of the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 hydrated electron [reaction (2)], as shown in the Arrhenius plot in Fig.6. In fact, almost all near-diffusion-controlled reactions involving the hydroxyl radica12,'0*28 have a tem- perature dependence lower than that for self-diffusion in water (for the temperature range 20-200"C, Ediffis ca. 17 kJ mol-').lo Interestingly, the activation energy of reaction (2) up to 150°C of 20.3 kJ mol-' is essentially the same as that measured by Schmidt et aL2' (19.8 kJ mol-') for diffusion in water of the hydrated electron over the temperature range 15-90°C. If reaction (2) is assumed to be a truly diffusion- controlled reaction up to 150"C, then using the Smolu- chowski equation, a reaction radius of nearly 0.5 nm can be calculated (assuming a spin statistical factor of 0.25). However, it is interesting that the rate constants measured by Christensen and Sehested' show no strong dependence on ionic strength (see Fig.6). Reaction (1) is an important reaction in the spur, as it is the major pathway for reforming water. In a recent pub- lication, Laverne and Pimblott3' used diffusion-kinetic mod- elling to simulate reactions in the spur as a function of temperature. In the absence of any other evidence, they assumed that reaction (1) had the temperature dependence of self-diffusion in water, although some calculations were made with an activation energy of 12.6 kJ mol-'. The present results, although not linear in the Arrhenius plot (Fig. 6), have an apparent activation energy of 7.9 kJ mol-l. The yields calculated by Laverne and Pimblott are obviously strongly influenced by the catastrophic drop in the rate con- stant k, between 150 and 200°C (Fig.6). It is at this point that the calculated yields suddenly deviate sharply to lower values than the experimental yields for molecular hydrogen. It is becoming apparent that the temperature dependence of k, up to at least 200°C must be confirmed, perhaps at lower ionic strengths than the original work.' At present we are modifying our high-temperature apparatus to be able to satu- rate solutions under high pressures of hydrogen to reinvesti- gate reaction (2). This work was funded by the CANDU Owners Group under working party 15. The authors thank Dr. G. V. Buxton (University of Leeds, UK) for discussions during the course of this work.References 1 K. H. Schmidt, J. Phys. Chem., 1977,81,1257. 2 H. Christensen and K. Sehested, Radiat. Phys. Chem., 1981, 18, 723. 3 H. Christensen, K. Sehested and H. Corfitzen, J. Phys. Chem., 1982,86,1588. 4 H. Christensen and K. Sehested,J. Phys. Chem., 1983,87,118. 5 H. Christensen and K. Sehested,J. Phys. Chem., 1986,90, 186. 6 H. Christensen and K. Sehested, J. Phys. Chem., 1988,92,3007. 7 G. V. Buxton, N. D. Wood and S. Dyster, J. Chem. SOC., Faraday Trans. 1, 1988,84, 11 13. 8 A. J. Elliot and D. R. McCracken, Radiat. Phys. Chem., 1989,33, 69. 9 A. J. Elliot, Radiat. Phys. Chem., 1989,34,753. 10 A. J. Elliot, D. R. McCracken, G. V. Buxton and N. D. Wood, J. Chem. SOC.,Faraday Trans., 1990,86, 1539. 11 K. Sehested and H. Christensen, Radiat.Phys. Chem., 1990, 36, 499. 12 P. Han and D. M. Bartels, J. Phys. Chem., 1992, %, 4899. 13 A. J. Elliot and G. V. Buxton, J. Chem. Soc., Faraday Trans., 1992,88,2465. 14 G. V. Buxton and A. J. Elliot, J. Chem. SOC., Faraday Trans., 1993,89,485. 15 A. J. Elliot, D. C. Ouellette and D. R. McCracken, AECL report, AECL-10667,1992. 16 A. J. Elliot, M. P. Chenier and D. C. Ouellette, J. Chem. SOC., Faraday Trans., 1993,89, 1193. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 841 17 E. M. Fielden, in The Study of Fast Processes and Transient 25 B. D. Michael, E. J. Hart and K. H. Schmidt, J. Phys. Chem., Processes by Electron Pulse Radiolysis, ed. J. H. Baxendale and 1971,75,2798. F. Bussi, Reidel, Dordrecht, 1982, p. 49. 26 F-Y. Jou and G. R. Freeman, J. Phys. Chem., 1977,81,909. 18 G. V. Buxton and C. R. Stuart, personal communication. 27 H. Shiraishi, Y. Katsumura, D. Hiroishi, K. Ishigure and M. 19 A. R. Curtis and W. P. Sweetenham, FACSIMILE/ Washio, J. Phys. Chem., 1988,92,3011. CHECKMAT, Harwell Laboratory, AERE R 12805, 1988. 28 A. J. Elliot and A. S. Simsons, Radiat. Phys. Chem., 1984, 24, 20 M. S. Matheson and J. Rabani, J. Phys. Chem., 1965,69,1324. 229. 21 F. W. Sweeton, R. E. Mesmer and C. F. Baes, J. Solution Chem., 29 K. H. Schmidt, P. Han and D. M. Bartels, J. Phys. Chem., 1992, 1974, 3, 191. 96,200. 22 W. C. Natzle and C. B. Moore, J. Phys. Chem., 1985,89,2605. 30 J. A. Laverne and S. M. Pimblott, J. Phys. Chem., 1993, 97, 23 H. A. Schwarz and B. H. J. Bielski, J. Phys. Chem., 1986, 90, 3297. 1445. 24 R. E. Mesmer, C. F. Baes and F. H. Sweeton, Znorg. Chem., 1972, 11, 537. Paper 3/06266J ;Received 20th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000837

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS.. 1994. 94361, 843-844 843 Far-IR Study of the Hydrogen-bond Vibration of Intramolecular Bonds in Substituted 2-Diethylaminomethylphenol N-Oxides, as a Function of the pK, of the Phenolic Group Bogumil Brzezinski Faculty of Chemistry,A, Mickiewicz University, Grunwaldzka 6 PL-60-780Poznari, Poland Arno Rabold and Georg Zundel Institute of Physical Chemistry, University of Munich, Theresienstr. 4 I, 0-80333Munich, Germany The intramolecular hydrogen-bond vibration in R-substituted 2-diethylaminomethylphenol N-oxides in the far- infrared region has been studied. With decreasing pK, value of the phenolic group this vibration shifts first towards higher and then towards lower wavenumbers. The proton is transferred within the OH.* -ON-: 0-0-.H+ON hydrogen bond with increasing acidity of the phenolic group. This bond is strongest for R = 4 -NO,, and the hydrogen bond vibration is very broad in this system. It has been found that the change of the position of the hydrogen bond vibration is independent of the mass of the substituents. All results in this work agree very well with results obtained for corresponding intermolecular hydrogen bonds. Recently we have studied intermolecular hydrogen bonds between aliphatic N-oxides and phenols as a function of t!ie pK, value of the phenols by FTIR'"3as well as 'Hand '.'C NMR ~pectroscopy.~ For this family of systems the OH. .ON 0--..H 'ON equilibrium could be completelq shifted from the left- to the right-hand side by altering the acidity of the phenol.If the acidity is low, only two bands are observed in the region 3200-1700 ern-.'. With increasing acidity a continuum arises which extends increasingly toward smaller wavenumbers. With the 3,4-dini trophenol (pK, 3.42) system an intense continuum is observed, but only in the region 1500-600 cm-', extending with less intensity to 1I)O cm -1.1.3 With further increasing acidity of the phenols the intensity of the continuum in the 1500-600 cni-' regicm decreases and moves back towards higher wavenumber.' .' The changes of the continuum are discussed with regard 10 the shape of the proton potential in ref. 2. Particularly inter- esting is the behaviour of the far-infrared hydrogen-bmd vibration. This is independent of changes in the mass of the phenol as well as the mass of the aliphatic N-oxides (see Fig.2 in ref. 2).2*3The band for the hydrogen-bond vibration shifts with the decreasing pK, value of the phenol first towards higher and then lower wavenumbers. which COrri:- sponds to the changes of the infrared continuum. Analogous results were obtained from 'H and I3C NMR spectros-copy.2*4 We have already studied intramolecular OH. .ON 5; O--..H+ONhydrogen by MIR and 'H and I3C NMR spectroscopies. In this paper the hydrogen bond vibta- tion of corresponding intramolecular hydrogen bonds in t be far-infrared region are studied. Experimental Ten 2-diethylaminomethylphenols and their N-oxides were synthesized following the procedures given in ref.7 and 3. respectively. The N-oxides were dissolved in chloroform -acetonitrile (3 : 1).The solvents were dried with 3 A molecular sieves. The concentration of the solutions was 0.2 mol dm . '. All preparations and transfers of solutions were carried out in a carefully dried glovebox. The spectra were measured with an FTIR Bruker IFS 1 13v spectrometer using a cell with Si windows (mean layer thich- ness 0.26 mm). A set of 400 scans was collected with a resolution of 1 cm-using an He-cooled bolometer. Results and Discussion Ten substituted 2-diethylaminomethylphenolN-oxides were studied in chloroform-acetonitrile (3 : 1) solution in the far-infrared region [Fig. l(a)-(c)]. The systems and all data obtained are summarized in Table 1.The positions of the hydrogen-bond vibrations are shown as a function of the pK, value of the parent phenol in Fig. 2. The hydrogen-bond vibration shifts toward higher wave- numbers with increasing acidity of the phenolic group. The largest shift is observed for R = 4-N02, indicating that this system has the strongest intramolecular hydrogen bond. This is in good agreement with the 'H and I3C NMR data.6 As the acidity increases further, the hydrogen-bond vibration shifts towards lower wavenumbers. For the most symmetrical system, R = 4-NO2, i.e. the system in which the fluctuation of the proton is fa~test,~ the hydrogen-bond vibration is very broad. and the continuous absorption extends down to 100 em-'. The most interesting result is that the shift of the h>.drogen-bond vibration is independent of the mass of the phenol.i.e. for all compounds the same 'reduced mass' deter- mines the position of the hydrogen-bond vibration. This demonstrates that the shift is only determined by the force constant. An analogous result was obtained for the intermo- lecular hydrogen-bond vibration and explained by the fact that the centre of gravity of the bridged atoms is far from the hydrogen-bond axk2 Therefore. a large part of the mass of the molecules is only slightly involved in the vibration. and basically only the bridged atoms are involved in the hj drogen-bond vibrations. In the case of the intramolecular bonds it is immediately clear that the mass of the substituents does not essentially influence the position of the hydrogen- bond vibration.The position of the b(N0) vibration for each compound is also given in Table 1. This band is found in the region 460- 470 cm-'. With increasing acidity it shifts first towards higher wavenumbers and after the most symmetrical system (R = NO,) it is no longer observed. 0.8:I0.71 350 300 250 200 150 100 wavenumber/cm-’ (b) 0.7‘ 350 300 250 200 150 100 wavenum ber/cm -0.7 350 300 250 200 150 100 wavenumber/cm-Fig. 1 FTIR spectra of solutions of R-substituted 2-diethylaminomethylphenol N-oxides: (a) (-) 4-But, (---) 4-F, (* * *) 4-C6H,, (-. -) 4C1; (b) (-) 4-C02C2H5, (---) 4-NO2, (a(-. -) 4-CN, (. . -) CCO,CH,, and (c) (-) 3,4,6-c1,, .) 3,4-(No212 300 250 c 1 E -3 200 I I I150 ‘ 4 6 8 10 12 PK, Fig.2 Position of the hydrogen-bond vibration (v,) as a function of the pK, of the parent phenols J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Hydrogen-bond (v,) and 4NO) vibrations (an-’)of R-substituted 2-diethylaminomethylphenol N-oxides and the pK, values of the parent phenols compound R PK,” V, WO) 1 4-But 10.20 172 461.O 2 4-F 9.81 189 462.5 3 4-C6H5 9.55 194 463.0 4 4-c1 9.37 212 464.0 5 4-C0,C2H5 8.50 221 465.5 6 7 4-C02CH, 4-CN 8.47 7.95 237 256 465.5 466.0 8 4-N02 7.15 289 468.0 9 3,4,6-C13 6.72 255 - 10 3,WNO2)2 5.42 170 - @ From ref. 9. Conclusions The hydrogen-bond vibration of substituted 2-di-ethylaminomethylphenol N-oxides observed in the far-infrared region shifts with increasing acidity of the phenolic group first towards higher and then lower wavenumbers.Thus, with increasing transfer of the proton in the OH. .ON 0-.* .H +ON hydrogen bonds, the bonds ini- tially become stronger and then again weaker. The bond is strongest in the most symmetrical system and the hydrogen bond vibration is broadened. These results are analogous to results obtained for corresponding intermolecular hydrogen bonds. The only difference is that intramolecular hydrogen bonding is strongest for the R = 4-N02 system (pK, of the parent phenol is 7.15) whereas the intermolecular bond is strongest for the 3,4-dinitrophenol system (pK, = 5.42).Our thanks are due to the Polish Ministry of National Edu-cation, the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for providing facilities for this work. References 1 B. Brzezinski, B. Brycki, G. Zundel and Th. Keil, J. Phys. Chem., 1991,%, 8598. 2 Th.Keil, B. Brzezinski and G. Zundel, J. Phys. Chem., 1992,96, 4421. 3 B. Brzezinski, G. Schroeder, G. Zundel and Th. Keil, J. Chem. SOC.,Perkin Trans. 2, 1992,819. 4 B. Brycki, B. Brzezinski, G. Zundel and Th. Keil, Magn. Reson. Chem., 1992,30,507. 5 B. Brzezinski, J. Olejnik, G. Zundel and R. Kramer, J. Mol. Struct., 1989,212, 247. 6 B. Brzezinski, B. Brycki, H. Maciejewska-Urjasz and G. Zundel, Magn. Reson. Chem., 1993,31,642. 7 B. Brycki, B. Brzezinski, H. Maciejewska and G. Zundel, J. Mol. Struct., 1991,246, 61. 8 B. Brzezinski and G. Zundel, J. Mol. Struct., 1984,118,311. 9 G.Kortiim, W.Vogel and K. Anchussov, Dissociation Constants of Organic Acids in Aqueous Solutions, Butterworth, London, 1961. Paper 3/05929D; Received 4th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000843

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS.. 1994. 90(61. 845-84" Cyclodextrin-Monosaccharide Interactions in Water Angela F. Danil de Namor," Peter M. Blackett, Mercedes C. Cabaleirot and Jasim M. A. Al RawiS Laboratory of Thermochemistry, Department 0'' Chemistry, University of Surrey, Guildford, Surrey, UK GU25XH Stability constants and derived Gibbs energies, enthalpies and entropies of complexation of D-monosaccharides with 2-and /I-cyclodextrinll in water at 298.15 < obtained by titration microcalorimetry are reported. The results show that x-cyclodextrin interacts with D-glucose. o-fructose, D-xylose, D-mannose and o-galactose. No heat was evolved with D-arabinose. However, /I-cyclodextrin is able to recognise aldopentoses (o-xylose and D-arabinose) but not aldohexoses.13CNMR studies on these systems are discussed. Cyclodextrins are cyclic oligosaccharides constituted by Y-J-glucopyranose units and are known to interact with a ~itie variety of substrates. An important feature of cyclodextrins is that these compounds are characterised by their low degree of toxicity and therefore. these ligands have found numerotis applications in the pharmaceutical and food industries.'.' Despite the large number of contributions in this area. thermodynamic studies on the binding (A, G .A, H-,A, S I *)f these ligands with different substrates are relatively scarce It seems appropriate to emphasise that the Gibbs energy )f complexation is a most relevant parameter sincc it reflects the selectivity of a host for a given guest.However, interpretation based solely on A, C;' values c:!n be misleading and therefore it is always desirable to evalu,i!e the enthalpy and entropy associated with these processes. Stability constant data so far reported in cyclodevtrin chemistry3 are within the range required to be measured accurately by titration calorimetry and therefore this tec:i-nique offers the advantage that not only the stability but also the enthalpy can be accurately derived. particularly when highly sensitive microcalorimetric systems are used for t htse purposes. This paper reports: (i) Thermodynamic data for the con -plexation of D-monosaccharides and x-and p-cyclodext rir:s in water at 298.15 K derived from titration microcalorimetr! : (ii) 13C NMR studies on various monosaccharides and 5-cyclodextrin in D,O at the same temperature. Experimental x-(from Aldrich) and /j-(from Sigma) cyclodextrins wore dried in a vacuum oven for 3 days at 343 K prior to ujc.11-Glucose. D-, L-, and m-xylose, D-fructose, methyl-fl-u-g! L-copyranoside, methyi-/I-galactopyranoside and methyl 2-14-glucopyranoside (all from Fluka), D-mannose. 11-arabinoe and 3-O-methyl glucose (Aldrich) and 1,-galactose (BDt-i) were dried under vacuum for a few hours at 313 K before IM. Heats of complexation of cyclodextrins and monosaccha -rides were measured in a Thermal Activity Monitor micit- calorimetric system purchased from Thermonierric. Aqueou solutions of cyclodextrins (0.08--0.10rnol dm- '1 in the calcl-metric vessel were titrated with the monosaccharide n ate-t Present address: Instituto de Quimica Organica.Departamrnt!, de Quimica & Ingenieria Quimica. Universidad Nacional del Stir. Bahia Blanca 8000. R. Argentina. Present address: Department of Chemistry. University of Sanxi. Yemen. r-Cyclodextrin = cyclomaltohexaose: /I-cyclodaxtrin = cycia-maltoheptaose. solution', (1-3 rnol dm-3) contained in a 500 p1 gas-tight motor driven Hamilton syringe. All calorimetric measure- ments ivere performed at 298.15 K and the solvent was doubly distilled deionised water. The reliability of the calo- rimeter was checked using the test calibration of Ba" and 18 crwn 6 ether in water at 298.15 K suggested by Briggner and Wadso.' A value of -31.86 & 0.32 kJ mol-' was obtained.in excellent agreement with the literature value of -3 1.42 & 0.20 kJ mol- '. All calorimetric data experiments performed in triplicate. 'c' NMR experiments were carried out on a Bruker AC-300 MHz spectrometer with a wide bore magnet oper- ating in the Fourier-transform mode using proton decoupling for I 'C. Spectra were internally referenced with 1P-dioxane relative to tetramethylsilane (TMS). For the 13C NMR experiments. the spectra of r-cyclodextrin (ca. 1 x rnol dm 3, and then, with an excess (1 x 10.' rnol dm-3) of the appropmte monosaccharide were recorded. These studies were also carried out in the reverse order by recording the spectra of the monosaccharide (0.01 rnol dm-3) and then adding an excess of x-cyclodextnn (0.1 rnol dm -').Results and Discussion Thermodynamic Data of Complexation From calorimeter data, values for the stability constant (expressed as log K,) and the enthalpy of complexation A, H were calculated using a minimisation program developed by Karlson and Kullberg.' The thermodynamic data fitted a model which corresponds to a 1 : 1 (monosaccharide: cyolodext rin) stoichiometry complex. Therefore, thermodyna- mic data for the interaction of cyclodextrins (CD)with mono- saccharides (M) in water at 298.15 K are referred to the process. M(H,O) + CD(H20j-+ M *CD(H20) (1) Table 1 lists log K, and derived Gibbs energies, A,G'. enth-alpies. Ac Fi'. and entropies, A, S values for the complexation process [qn.(I)]. The individual errors in K, and Ac H' are expressed as the standard deviation (IT)of the mean using the expression (T = (Xui -.f):(n -1). where n is the number of steps (at least six) considered in each calorimetric run. The CJ values given in Table 1 are the weighted average of three calorimetric runs for each monosaccharide considered. Note that it was the availability of a highly sensitive micro- calorimetric system which made possible the quantitative evaluation of the enthalpic contribution. Indeed. enthalpy data could not be obtained by classical titration calorimetry. Thermodynamic data show the following. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Thermodynamic parameters of complexation of (a-and B-)cyclodextrins with D-monosaccharides in water at 298.15 K monosaccharide 1% K, Ac G"/kJ mol - Ac H"/kJ mol - A,So/J K-' mol-' D-glucose D-fructose 1.56 k0.23 1.72 f0.24 a-cyclodext rin -8.90 f1.31 -9.82 f1.30 -0.14 -0.05 f0.03 k0.01 29.4 f4.4 32.8 f4.6 D-xylose D-mannose 1.57 & 0.11 1.77 f0.17 -8.96 f1.37 -10.10 f0.97 -0.09 f0.01 -0.11 f0.01 29.8 f2.1 33.5 f3.2 D-galactose D-arabinose 1.19 f0.31 -6.79 f1.77 -0.32 _+ 0.12 21.7 _+ 5.9 D-arabinose D-xylose 1.21 & 0.32 1.22 * 0.10 B-cyclodextrin -6.91 f1.82 -6.96 f0.57 -0.42 -0.62 0.22 k0.06 21.8 & 6.1 21.3 f1.1 (a) In water, low-stability complexes are formed. This is conformers) in water or indeed to a very small enthalpy mainly attributed to strong host-water and guest-water associated with the binding process. This is now being inves- interactions.As far as the monosaccharides are concerned, tigated further. these molecules are able to interact with water through The most striking feature of these results is the ability of hydrogen-bond formation. Attachment of water to cyclo- /3-cyclodextrin to interact with aldopentoses but not with dextrins occurs through the hydroxy groups occupying both aldohexoses in water. This was previously reported by As a Aoyama et aL7 on the basis of 13C NMR studies ofrims of the cone as well as the inside of the ~avity.~ result of the individual attraction shown by the host and the aligosugar-sugar interactions in water. In fact, these authors guest for the solvent, the binding between the cyclodextrin attributed the lack of interaction of this macrocycle with and the monosaccharide is relatively weak. Stability con-aldohexoses to steric factors, suggesting that the presence stants for monosaccharides and /3-cyclodextrins in water have (aldohexoses) or the absence (aldopentoses) of a substituent been reported by Aoyama et aL7 However, the data reported in the C-5 of the pyranoside ring could be a key factor as far by these authors can be only regarded as tentative values as /3-cyclodextrin -monosaccharide interactions are con-since the overall error reported (520%) is relatively large cerned.In summary, the broad conclusions drawn from and the temperature at which these measurements were Table 1 are that (i) a-cyclodextrin is unable to recognise these carried out was not reported. monosaccharides selectively as assessed from stability con- (b)The complexation of monosaccharides to cyclodextrins stant data (D-galactose lies just outside of the range) and (ii) (exothermic reaction) must be accompanied by strong desol- /?-cyclodextrin distinguishes between aldopentoses and aldo- vation of both the host and the guest molecules (endothermic hexoses.reaction) upon complexation, leading to a small enthalpy Encouraged by the realisation that cyclodextrins could dis- change for this process. Positive entropies are often found for tinguish between aldohexoses and aldopentoses, enantio- reactions in which the host, guest or both undergo strong meters of xylose were investigated in order to assess if desolvation upon complexation, and this is also observed in cyclodextrins could selectively recognise one isomer from these systems (see Table 1).No heat was detected when a- another. The stability constant should determine this, and cyclodextrin was titrated with D-arabinose. This result on its these data are shown in Table 2. Also reported in this table own does not provide evidence that this monosaccharide are the contributions of the enthalpy and entropy to the does not interact with a-cyclodextrin. It could be that the Gibbs energy of these processes. In order to test whether a-heat associated with this process is too small to be measured, cyclodextrin is able to discriminate between the isomers, the in which case calorimetry is not a suitable reporter for this 'student's' t-distribution test was applied to these data and it system.However, it should be stressed that, among the was concluded that at the 95% confidence interval, a claim monosaccharides considered, the fC conformer predomi-that a-cyclodextrin is able to discriminate between these nates, except for arabinose.* For this monosaccharide, the isomers is not justified. iC conformer is the predominant species in solution. There- fore, it is most interesting to investigate whether the absence Blocked Monosaccharid4 yclodextrin Interactions of heat observed is attributed to the lack of interaction A few blocked monosaccharides (methyl-a-D-glucopyrano-between a-cyclodextrin and arabinose (in which case this side, methyl-/3-glucopyranoside, 3-0-methyl glucose and ligand could have a potential role in the recognition of methyl-/3-D-galactopyranoside)were chosen to try to locate Table 2 Thermodynamic parameters of complexation of xyloses with (a-and b-)cyclodextrins in water at 298.15 K monosaccharide 1% K, Ac Go/kJ mol -' Ac H"/kJ mol-' AcSo/J K-' mol-' a-c yclodextrin D-xylose 1.57 k0.1 1" -8.9 f0.63 -0.09 &-0.01" 29.8 k2.1 L-xyiose 2.07 5 0.37 -11.81 5 2.1 1 -0.12 f0.01 39.2 & 7.1 DL-xylose 1.42 f0.13 -8.10 f0.74 -0.11 * 0.01 26.8 & 2.5 B-cyclodextrin D-X ylose 1.22 f0.10" -6.96" f0.57 -0.62 f0.06" 21.3 f1.9 L-xylose 1.32 -t 0.13 -7.53 f0.74 -0.87 rf 0.12 22.3 f2.5 DL-xylose 1.06 -t 0.07 -6.05 f0.40 -1.12 f0.18 16.5 f 1.5 a From data listed in Table 1.J. CHEM.SOC. FARADAY TRANS., 1994, VOL 90 Table 3 A6 values in the 13CNMR of monosaccharides in the pres- ence of r-cyclodextrin at 298.15 K A6 D-mannose' D-~~UCOS~' D-fructose" D-xylose" c-1 0.10 0.11 0.03 0.13 c-2 0.11 0.12 0.04 0.11 c-3 0.11 0.14 0.06 0.13 c-4 0.07 0.02 0.02 0.11 c-5 0.14 0.12 0.07 0.14 C-6 0.07 0.07 0.06 -The chemical shifts (6) in ppm of the carbons of the monosaccha- rides are as follows: D-mannose; = 94.48; 6,, = 72.03; 6c-3~1 73.86; 6,, = 67.66; dCm5= 76.98 ; dC-6 = 6 1.79 ; D-glucose; dc-96.72; 6,, = 74.95; 6,, = 76.76; 6,, = 70.46; dc-5 = 76.57; dCa6== 61.57; D-fructose; dC-, = 98.88; aC-, = 75.27; 6c.3 = 76.20; 8c-4 = 64.70; 6c-5 = 76.20; = 63.48 and D-xylose; = 97.43; 6,, = 73.63; 6c-3 = 76.63;aC-, = 70.22 and 6c-5 = 66.00.the site of complexation on the pyranose ring for glucose. No heat was observed for the blocked monosaccharides with 3-cyclodextrin except for methyl z-D-ghcopyranoside (log K,= 1.32 k0.23; AcGo= -7.53 & 1.31 kJ mol-'; Ac H"= -0.27 f0.04 kJ mol-' and AcSo= 24.4 & 4.4 J K-'mol-'). It appears that as far as the receptor binding to the monosaccharide is concerned, the r-conformer is the important site. Indeed no heat was detected between the p-conformer or with the monosaccharide substituted at C-3. Table 4 A6 values in the I3C NMR of a-cyclodextrin in the pres- ence of monosaccharides at 298..15 K AJO D-mannose mglucose D-fructose D-xylose c-1 0.10 0.12 0.00 0.13 c-2 0.08 0.09 0.01 0.19 c-3 0.11 0.11 0.02 0.14 c-4 0.11 0.11 0.01 0.14 c-5 0.09 0.09 0.00 0.12 C-6 0.05 0.05 -0.03 0.08 'The chemical shifts (6) in ppm of the carbons of r-cyclodextrin in D,O at 298.15 K are: 6,, = 102.27; a,--, = 74.11; 6,, = 72.85; dc-4 = 82.09;6c-5= 72.49 and 6c-6= 61.16.Further studies are being carried out to investigate whether or not this is applicable to other monosaccharides. I3C NMR Studies A8 values for the I3C NMR of monosaccharides in the pres- ence of x-cyclodextrin and vice uersa are shown in Tables 3 and 4. The 13C NMR signals of the carbons of mannose, glucose and xylose (Table 4) particularly those arising from carbons 1-6 show, in the presence of z-cyclodextrin, shifts towards lower fields.The much smaller shifts observed for fructose are also reflected in the AcH" value for this mono- saccharide and x-cyclodextrin shown in Table 1. Since all these monosaccharides can exist as ;C,conformers corre- sponding to a configuration in which the 3-OH is axial, it should be reasonable to assume that the ring oxygen and the 3-OH play an important role in the monosaccharide-r- cyclodextrin interaction that affects the resonance frequencies of the carbon atoms. This suggestion seems to be supported by the lack of heat observed (see text) when the 3-OH of the monosaccharide is in equatorial position or methylated. Again, the fact that methyl-b-D-glucopyranose does not give detectable heat with a-cyclodextrin could be explained on steric grounds, since the methoxy group could prevent the interaction of the neighbouring ring oxygen. P.M.B. thanks SERC (UK) for a scholarship. References D. Duchgne, in Cyclodextrins and the Industrial b'ses, ed. D. DuchCne, Editions de Sante, Paris, 1987. ch. 6. H. Hashimoto, in Proc. 4th Int. Symp. on Cyclodextrins, ed. 0. Huber and J. Szejtli, Kluwer, Dordrecht, 1988, p. 533. R.Traboulssi, Ph.D. Thesis, University of Surrey, 1990. L. E. Brignner and I. Wadso, J. Biochem. Biophys., 1991,22, 101. R.Karlsson and L. Kullberg, Chem. Scr., 1976,9, 54. A. Cesaro, in Thermodynamic Data for Biochemistry and Biotech- nology, ed. H-J. Hinz, Springer-Verlag, 1986, ch. 6, p. 178. Y. Aoyama, Y. Nagai, J. Otsuki, K. Kobayashi and H. Toi. Anyew. Chem., Int. Ed. Engl., 1992, 31, 745. J. F. Stoddard, Stereochemistry of Carbohydrates, Wiley Inter- science, New York, 1971. Paper 3/04644C; Received 3rd August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000845

出版商:RSC    

年代:1994

数据来源: RSC