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THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Dr. Peter J. Sarre Department of Chemistry U n iversi ty of N otti ng ham University Park Nottingham NG7 2RD, UK Faraday Editorial Board Prof. M. N. R. Ashfold (Bristol) (Chairman) Dr. J. A. Beswick (Paris) Prof. A. R. Hillman (Leicester) Dr. D. C. Clary (Cambridge) Prof. J. Holzwarth (Berlin) Dr. L. R. Fisher (Bristol) Dr. D. Langevin (Paris) Dr. B. E. Hayden (Southampton) Dr. P. J. Sarre (Nottingham) Prof. J. S. Higgins (London) Dr. R. K. Thomas (Oxford) Editorial Manager and Secretary to Faraday Editorial Board Dr. Robert J. Parker The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF, UK Staff Editor: Dr.R. A. Whitelock Senior Assistant Editor: Mrs. S. Shah Assistant Editors: Dr. L. Milne, Mrs. C. J. Seeley Editorial Secretary: Mrs. J. E. Gibbs International Advisory Editorial Board R. S. Berry (Chicago) Y. Marcus (Jerusalem) A. M. Bradshaw (Berlin) B. J. Orr (North Ryde) A. Carrington (Southampton) R. H. OttewiII (Bristol) M. Che (Paris) R. Parsons (Southampton) M. S. Child (Oxford) S. L. Price (London) B. E. Conway (Ottawa) F. Rondelez (Paris) G. R. Fleming (Chicago) D. K. Russell (Auckland) R. Freeman (Cambridge) J. P. Simons (Oxford) H. L. Friedman (Stony Brook) S. Stolte (Amsterdam) H. H. J. Girault (Lausanne) J. Troe (Gottingen) H. lnokuchi (Okazaki) J. Wolfe (Kensington, NSW) J. N. lsraelachvili (Santa Barbara) C.Zannoni (Bologna) M. L. Klein (Philadelphia) R. N. Zare (Stanford) A. C. Legon (Exeter) A. Zecchina (Turin) R. A. Marcus (Pasadena) C. Zhang (Dalian) Journal of the Chemical Society, Faraday Transactions (ISSN 0956-5000) is published twice monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd., Black- horse Road, Letchworth, Herts. SG6 1 HN, UK. NB Turpin Distribution Services Ltd., dis- tributors, is wholly owned by the Royal Society of Chemistry. 1994 Annual subscription rate EC f744.00, Rest of World f800.00, USA $1400.00, Canada f840 (excl.GST). Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Second class postage is paid at Rahway, NJ. Airfreight and mailing in the USA by Mercury Airfreight International Ltd. Inc., 2323 Randolph Avenue, Avenel, NJ 07001, USA and at additional mailing offices. USA Postmaster: send address changes to Journal of the Chemical Society, Faraday Trans- actions, c/o Mercury Airfreight International Ltd. Inc., 2323 Randolph Avenue, Avenel, NJ 07001. All despatches outside the UK by consolidated Airfreight. 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They are designed to be topical articles of interest to a wide range of research scientists in the areas of Physical Chemistry, Biophysical Chemistry and Chemical Physics. Full details of the form of manuscripts for Articles and Faraday Communications, con- ditions for acceptance etc. are given in issue number one of Faraday Transactions, published in January of each year, or may be obtained from the Editorial Manager. There is no page charge for papers published in Faraday Transactions. Fifty reprints are supplied free of charge. 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 YoRSC.0R G@U K.AC. N SF N ET- R ELAY) Fax: (0223) 423623 or 420247 Telex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX053

出版商:RSC    

年代:1994

数据来源: RSC

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DICTIONARY OF SUBSTANCES AND THEIR EFFECTS Dictionary of Substances and their Effects (DOSE) is a new, unique, user-friendly guide to 5,000 chemicals and the impact they have on life forms and the environment across the globe. Compiled with the aid of official lists from the EC, UK, USA and Canada, DOSE is being published in seven alphabetical volumes, which will be completed in 1994. Each volume contains an index of chemical names, CAS registry numbers and molecular formulae, as well as a glossary of biological organisms. A separate volume containing cumulative indices of all volumes will be published after Volume 7. 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ISSN:0956-5000    

DOI:10.1039/FT99490BX055

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(14) 2003-2157 (1994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 2003 Kinetic study in a microwave-induced plasma afterglow of the CU(~~S)atom reaction with N,O from 458 to 980 K and with NO, from 303 to 762 K C. Vinckier, T. Verhaeghe and I. Vanhees 2009 Electronic states and the metal-insulator transition in caesium-ammonia solutions Z. Deng, M. L. Klein and G. J. Martyna 2015 Use of a neural network to determine the normal boiling points of acyclic ethers, peroxides, acetals and their sulfur analogues D. Cherqaoui, D. Villemin, A. Mesbah, J-M. Cense and V. Kvasnicka 2021 Ligand exchanges between ethanol and some amines in excited mercury complexes in the gas phase S.Yamamoto, T. Nagaoka, Y. Sueishi and N. Nishimura 2027 Some thermodynamic properties of aqueous amino acid systems at 288.15, 298.15, 313.15 and 328.15 K;group addi- tivity analyses of standard-state volumes and heat capacities A. W. Hakin, M. M. Duke, J. L. Marty and K. E. Preuss 203 7 Effect of preferential solvation on Gibbs energies of ionic transfer A-K. Kontturi, K. Kontturi, L. Murtomaki and D. J. Schifliin 2043 Water distillation through poly(tetrafluoroethy1ene) hydrophobic membranes in a stirred cell M. I. Vazquez-Gonzalez and L. Martinez 2047 Mechanistic aspects of biological redox reactions involving NADH Part 5.-AM 1 transition-state studies for the pyruvate-L-lactate interconversion in L-lactate dehydrogenase S. Ranganathan and J. E.Gready 2057 Direct observation of native and unfolded glucose oxidase structures by scanning tunnelling microscopy Q. Chi, J. Zhang, S. Dong and E. Wang 2061 Influence of immobilising anions on the redox switching of polyaniline V. W. Jones, M. Kalaji, G. Walker, C. Barbero and R. Kotz 2065 Conductivity and dielectric relaxation in hydrated fused salts G. P. Johari, D. A. Wasylyshyn and S. K. Jain 207 1 Structure and transport in concentrated micellar solutions with a lower consolute boundary J. M. Keller, H- D. Ludemann and G. G. Warr 2077 Formation of two-dimensional structures from colloidal particles on fluorinated oil substrate G. S. Lazarov, N. D. Denkov, 0.D. Velev, P. A. Kralchevsky and K. Nagayama 2085 Determination of aggregate structures by combined light-scattering and rheological studies S.D. T. Axford and T. M. Herrington 2095 Direct observation of aluminium guest ions in the silicate phases of cement minerals by 27Al MAS NMR spectroscopy J. Skibsted, H. J. Jakobsen and C. Hall 2099 Physiochochemical and catalytic properties of polyaniline protonated with 12-molybdophosphoric acid M. Hasik, A. Pron, J. Pozniczek, A. Bielabski, Z. Piwowarska, K. Kruczala and R. Dziembaj 2107 Photoluminescsence spectra resulting from hydroxy groups on magnesium oxide supported on silica H. Yoshida, T. Tanaka, T. Funabiki and S. Yoshida 21 13 Raman band shifts of y-Bi,MoO, and a-Bi,Mo,O,, exchanged with "0 tracer at active sites for reoxidation T. Ono and N. Ogata 21 19 Hydrogenation behaviour over Si0,-supported lanthanide-palladium bimetallic catalysts with considerable hydrogen uptake H.Imamura, K. Igawa, Y. Kasuga, Y. Sakata and S. Tsuchiya 2125 Surface distribution and heteroatom removal activity of equilibrium adsorption prepared, doubly promoted (Zn,CO) Mo/Al,O, catalysts H. Thomas, C. Caceres, M. Blanco, J. L. G. Fierro and A. Lopez Agudo 2 133 Selective oxidative coupling of methane catalysed over hydroxyapatite ion-exchanged with lead Y. Matsumura, J. B. Moffat, S. Sugiyama, H. Hayashi, N. Shigemoto and K. Saitoh 2141 Group behaviour of SAPO-11 molecular sieves containing various metals (Mg, Zn, Mn or Cd, Ni, Cr) J. Kornatowski, G. Finger, K. Jancke, J. Richter-Mendau, D. Schultze, W. Joswig and W. H.Baur 2147 Mechanistic study of sec-butyl alcohol dehydration on zeolite H-ZSM-5 and amorphous aluminosilicate M. A. Makarova, C. Williams, K. I. Zamaraev and J. M. Thomas 2155 Book reviews P. N. Bartlett; D. Crowther; A. R. Hillman; G. Martin; N. G. Parsonage Note: Where an asterisk appears against the name of one or more of the authors, it is included with the authors’ approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles. For further details contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W 1V OBN, UK Tel: +44 (0)71-437 8656 Fax: +44 (0)71-287 9798 Telecom Gold 84: BUR210 Electronic Mailbox (Internet) LIBRARY @RSC.ORG. If the material is not available from the Society’s Library, the staff will be pleased to advise on its availability from other sources. Please note that copies are not available from the RSC at Thomas Graham House, Cambridge.

ISSN:0956-5000    

DOI:10.1039/FT99490FP150

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aas,N., 1015 Black, S. N., 1003 Chiu, S.S-L., 1575 Enomoto, N., 1279 Harper, R. J., 659 Abadzhieva, N., 1987 Blackett, P. M., 845 Chmiel, G., 1153 Eustaquio-Rincon, R., 113 Harriman, A., 697,953 Abbott, A. P., 1533 Blake, J. F., 1727 Cho, T., 103 Ewins, C., 969 Harris, K. D. M., 1313, Afanasiev, P., 193 Blanco, M., 2125 Choisnet, J., 1987 Fantola Lazzarini, A. L., 1323 digren, H., 1479 Blanco, S., 1365 Chowdhry, B. Z., 1999 423 Harrison, N. J., 55 Aikawa, M., 911 Blandamer, M. J., 727, 1905 Christensen, P., 459 Fausto, R., 689 Haruta, M., 1011 Aitken, C. G., 935 Blower, C., 919,931 CiZmek, A., 1973 Favaro, G., 279,333 Hashimoto, K., 1177 Akanuma, K., 1171 Bocherel, P., 1473 Clark, T., 1669, 1678, 1783, Favre, E., 2001 Hashino, T., 899 Akolekar, D.B., 1041 Boddenberg, B., 1345 1807,1808,1809, 1810 Feliu, J. M., 609 Hasik, M., 2099 Albery, W. J., 1115 Boggis, S. A., 17 Clegg, S. L., 1875 Fenn, C., 1507 Hattori, H., 803 Aldaz,A., 609 Booth, C., 1961 Clement, R., 2001 Fernando, K. R., 1895 Hawkins, C. D., 1802 Alfimov, M. V., 109 Borden, W. T., 1606,1614, Climent, M. A., 609 Fierro, J. L. G., 2125 Hayashi, H., 2133 Al-Ghefaili, K. M., 383, 1616,1671,1673, 1675, Coates, J. H., 739 Filimonov, I. N., 219, 227 Haymet, A. D. J., 1245 1047 1689,1733,1734, 1735, Coitiiio, E. L., 1745 Finger, G., 2141 Heal, M.R., 523, 1467 Ali, V., 579, 583 1743,1744,1802,1807 Collett, J. H., 1961 Fleischmann, M., 1923 Healy, T.W., 1251 Aliev, A. E., 1323 Borge, G., 1227 Colmenares, C. A., 1285 Flint, C. D., 1357 Heatley, F., 1961 Allegrini, P., 333 Borisenko, V. N., 109 Cook, J., 1999 Fogden, A., 263 Heenan, R. K., 487 Allen, N. S., 83 Bottoni, A., 1617 Cooper, D. L., 1643 Fornes, V., 213 Hefter, G., 1899 A1 Rawi, J. M. A., 845 Boutonnet-filing, M., Cordischi, D., 207 Fracheboud, J-M., 1197, Helmer, M., 31,395 Amorim da Costa, A. M., 1023 Corma,A., 213 1205 Herein, D., 403 689 Bowker, M., 1015 Cormier, G., 755 Franci, M. M., 1605, 1740, Herod, A. A., 1357 Amoskov, V. M., 889 Bozon-Verduraz, F., 653 Corradini, F., 859, 1089 1744 Herrington, T. M., 2085 Ando, M., 1011 Bradley, C. D., 239 Corrales, T., 83 Franck, R., 667,675 Herrmann, J-M., 1441 Andres, J., 1703 Bradshaw, A.M., 403 Cosa, J. J., 69 Freeman, N. J., 751 HerzogB., 403 Andrews, S.J., 1003 Braun, B. M., 849 Costas, M., 1513 Frkty, R., 773 Heyes, D. M., 1133,1931 Anson, C. E., 1449 Breysse, M., 193 Cottier, D., 1003 Frey, J. G., 17, 817 Higgins, S., 459 AntoniC, T., 1973 Briggs, B., 727, 1905 Coudurier, G., 193 Frostemark, F., 559 Hillier, I. H., 1575 Aragno, A., 787 Brocklehurst, B., 271, 2001 Courcot, D., 895 Fujiwara, Y ., 1183 Hillman, A. R., 1533,2155 Arai, S., 1307 Brogan, M. S., 1461 Coveney, P. V., 1953 Funabiki, T., 2107 Hindermann, J-P., 501 Aramaki, K., 321 Brown, N. M. D., 1357 Cox, R. A., 1819 Galantini, L., 1523 Hirst, D. M., 517, 1811 Aravindakumar, C. T., 597 Brown, R.G., 59 Cracknell, R.F., 1487 Gandolfi, R., 1077 Hiyane, I., 973 Asai,Y., 797 Brown, S.E., 739 Craig, S.L., 1663 Gans, P., 315 Hoekstra, D., 727,1905 Ashfold, M. N. R., 1357 Bruna, P. J., 683 Cramer, C. J., 1802 Gao,Y., 803 Hoffmann, R., 1507 Asmus, K-D., 1391 Brzezinski, B., 843, 1095 Crawford, M. J., 817 Garcia,R., 339 Holmberg, B., 559 Assfield, X., 1743 Buckley, A. M., 1003 Crowther, D., 2155 Garcia Fierro, J-L., 1455 Holz,M., 849 Attwood, D., 1961 Buemi, G., 1211 Cruzeiro-Hansson, L., 1415 Garcia-Paiieda, E., 575 Hoshino, H., 479 Avila, V., 69 Burdisso, M., 1077 Cullis, P. M., 727, 1905 Gautam, P., 697 Hosoi, K., 349 Axford, S.D. T., 2085 Busca, G., 1161,1293 Curtis, J. M., 239 Gavuzzo, E., 1523 Houk, K.N., 1599,1605, Baba,T., 187 Buschmann, H-J., 1507 DAlagni, M., 1523 Geantet, C., 193 1614,1615, 1616,1672, Badia, A., 1501 Butler, L. J., 1581, 1612, Dang, N-T., 875 Gengembre, L., 895 1678,1680, 1810 Badri, A., 1023 1613,1614,1671,1677, Danil de Namor, A. F., 845 Gerratt, J., 1643, 1672, Hrovat, D. A., 1689 Bagatti, M., 1077 1809 Das,D., 1993 1673,1801 HSU, J-P., 1435 Balaji, V., 1653 Butt, M. D., 727 Das, T. N., 963 Getty, S. J., 1689 Hu, W. P., 1715 Ball, M. C., 997 Buttar, D., 1811 Dasannacharya, B. A., 1149 Giglio, E., 1523 Hungerbuhler, H., 1391 Ball, S. M., 523, 1467 Byatt-Smith, J. G., 493 Davey, R. J., 1003 Gil, A. M., 1099 Hutchings, G. J., 203 Bally, T., 1615, 1674, 1733, Cabaleiro, M. C., 845 Davidson, K., 879 Gil, F.P. S. C., 689 Hutton, R. S., 345 1808 Caceres, C., 2125 De Benedetto, G. E., 1495 Gilchrist, J., 1149 Igawa, K., 21 19 Ban, M. I., 1610 Caceres, M., 1217 Defrance, A., 1473 Gill, D. S., 579, 583 Iizuka, Y., 1301,1307 Baonza, V. G., 553 Caceres Alonso, M., 553 Dejaegere, A., 1763 Gill, J. B., 315 Ikawa, S-i., 103 Baonza, V. G., 1217 Cairns, J. A., 1461 Demeter, A., 411 Goede, S. J., 327, 1363 Ikonnikov, I. A., 219 Barbaux, Y., 895 Calado, J. C. G., 649 Dempsey, P., 1003 Gomez, C. M., 339 Ilczyszyn, M., 1411 Barbero, C., 2061 Caldararu, H., 213 Demri, D., 501 Gonplves da Silva, A. M., Imamura, H., 21 19 Barker, S. A., 1689 Calvente, J. J., 575 Deng, N-J., 1961 649 Indovina, V., 207 Barnes, J.A., 1709 Calvo, E. J., 987 Deng, Z., 2009 Goodfellow, J. M., 1415 Inoue, Y., 797,815 Barthomeuf, D., 667,675 Camacho, J. J., 23 Denkov, N. D., 2077 Gordillo, G. J., 1913 Ishiga, F., 979 Bartlett, P. N., 2155 Cameron, B. R., 935 Derrick, P. J., 239 Gouder, T. H., 1285 Ishigure, K., 93,591 Basini,L., 787 Campa, M. C., 207 DewingJ., 1047 Goworek, T., 1501 Isoda,T., 869 Bassat, J. M., 1987 Carnpos, A., 339 Diagne, C., 501 Gray, P. G., 369 Ito, O., 571 Bassoli, M., 363 Canosa-Mas, C. E., 1197, Dickinson, E., 173 Gready, J. E., 2047 IwasabK., 121 Battaglini, F., 987 1205 Dines, T. J., 1461 Green, W. A., 83 Jacobs, W.P. J. H., 1191 Bauer, C., 517 Capobianco, J. A., 755 Doblhofer, K., 745 Grein, F., 683 Jain, S. K., 2065 Baur, W.H., 2141 Caragheorgheopol, A., 213 Domen, K., 911 Grieser, F., 1251 Jakobsen, H. J., 2095 Bell, A. J., 17,817 Carlile, C. J., 1149 Doney, S. C., 1865 GrXith, W. P., 1105 Jakubov, T., 783 Belton, P. S., 1099 Carlsen, L., 941 Dong, S., 2057 Grimshaw, J., 75 Jameel, A. T., 625 Bender, B. R., 1449 Carvill, B. T., 233 Dossi, C., 1335 Gnybowska, B., 895 Jghchen, J., 1033 Bendig, J., 287 Castaiio, R., 1227 Doughty, A., 541 Guelton, M., 895 Jancke, K., 2141 Bengtsson, L. A., 559 Castro, S., 1217 Douglas, C. B., 471 Guilhaume, N., 1541 Jayakumar, R., 161 Benko, J., 855 Catalina, F., 83 Downing, J. W., 1653 Guillaume, F., 1313 Jayasooriya, U. A., 1265 Benniston, A. C., 953 Cataliotti, R. S., 1397 Duke, M.M., 2027 Guldi, D. M., 1391 Jenneskens, L. W., 327, Beno, B., 1599 Cavasino, F. P., 3 11 Dunmur, D. A,, 1357 Gulliya, K. S., 953 1363 Bensalem, A., 653 Ceccarani, M. L., 1397 Dunstan, D. E., 1261 Hachey, M., 683 Jennings, B. 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Y., 287 ... 111 The following papers were accepted for publication between 1st and 31st May 1994: State-to-state photoionisation dynamics probed by zero kinetic energy (EKE)photoelectron spectroscopy K Muller-Dethlefs, I. Fischer and R. Lindner Hydroxy groups in ferrisilicates studied by IR spectroscopy J.Datka and T. Abramowicz Intracrystalline diffusion of benzene in microporous gallosilicate with MFI structure M. Bulow and A. Micke Conductance studies of acid-base equilibria between 4-methoxy-2,6-dimethylpyridineN-oxide and trifluoracetic acid in nitrobenzene M. Szafran and P.Barczynski Thermodynamic studies on the interaction of rz-alkyltrimethyl ammonium bromides with anionic polypeptides in aqueous solutions M. N. Jones, A. J. B. Macfarlane, M. I. P. Andrade and F. Sarmiento Enthalpies of transfer of tetrabutylammonium bromide from water to highly aqueous water-methanol, ethanol, -propan-1-01 and acetonitrile mixtures at 298 K; consideration of the extended coordination model solvation parameters W.E. Waghorne, P. Hogan, I. McStravick and J. Mullally Calculation of the adsorption potential energy of water vapour on a-cristobalite E. Valencia Marcus inverted region: Nature of donor-acceptor pairs and free ion yields P. Jacques, D. Burget, E. Vauthey, P. Suppan and E. Haselbach Effects of complexation by cyclodextrins on the photoreactivity of Rose Bengal and Erythrosin B. A laser flash photolysis investigation L. Flamigni Spectrochemistry of solutions. Part 27.-Formation of [Mg(NCS)]+ in solutions of Mg(NCS), in methanol P. Gans, J. B. Gill and K. M. L. Holden Vibrational relaxation of the v(C=O) mode of ethyl trichloroacetate in solution A. Hernanz, R. Navarro and I. Bratu Coordination sequences of zeolites revisited: Asymptotic behaviour for large distances C.P.Herrero Kinetic study of double-helix formation and double-helix dissociation of polyadenylic acid F. Secco, R. Maggini, M. Venturini and H. Diebler Distribution of electrolytes between membraneous and bulk phases, and the dielectric properties of membraneous water, studied by impedance spectroscopy measurements on dense cellulose acetate membranes I. W. Plesner, B.Malmgren-Hansen and T. S. Sdrensen Scanning tunnelling microscopy of various alkanols and an alkanethiol adsorbed onto graphite V. J. Morris, A. P. Gunning, A. R. Kirby and X. Mallard Conversion of isopropyl alcohol to acetone catalysed by Cr,O, at 473 K: Role of molecular oxygen M. Ilyas, S. Shah, R.Nigar and H. Khan Two-step reduction of Indigo Carmine by dithionite: A stopped-flow study P.Ramamurthy, N. Srividya, G. Paramasivan and K. Seetharaman X-Ray photoelectron spectroscopy characterization of the reduction and oxidation behaviour of Ni-containing HZSM-5 zeolites S. Xiao and Z. Meng Theoretical studies of the electronic structure, conformations, spectra and hyperpolarisabilities of squarates and related molecules J. 0. Morley, M. Dory, J-M. Andre and J. Delhalle One-electron oxidation of iron (11) imidazole and iron (11) bis(imidazo1-2-yl) methane complexes: A pulse radiolysis study B. J. Parsons, S. Navaratnam, Z. Zhao and L. Chen EXAFS data analysis for lanthanide sesquioxides P. Malet, M. J. Capitan, M. A. Centeno, J. A. Odriozola and I. Carrizosa Structure of cobalt-aerosol-OT reversed micelles studied by small-angle scattering methods J.Eastoe, D. C. Steytler, B. H. Robinson, R. K. Heenan, A. N. North and J. C. Dore Laser-induced fluorescence spectroscopy of ethyl and methyl p-aminobenzoate van der Waals complexes with non-polar solvents (CH,, C,H, and CF,) F. Castano, R. Pereira, I. Alava and M. T.Martinez Spectroscopic investigation of the polymerisation of pyrrole and thiophene within zeolite channels R. P. Cooney, G. J. Millar, G. F. McCann, C. M. Hobbis and G. A. Bowmaker iv Large surface area maintained at elevated temperatures by decreasing bulk density of alumina aerogel T. Horiuchi, T. Sago, T. Usaki, T. Sugiyama, H. hlasuda, M. Horio, K. Suzuki and T. Mori Exchange of strong carbon dioxide O=C bonds on an MgO surface Y.Yanagisawa, K. Takoaka and S. Yamabe Further studies on the polarizabilities and hyperpolarizabilities of the substituted polyenes and polyphenyls D. Pugh, I. D. L. Albert and J.O. Morley Microwave spectrum of suluryl chloride fluoride. S0,CIF: Structure, hyperfine constants and harmonic force field H. S. P. Muller and M. C. L. Gerry Singlet and triplet excited-state formation in the high-energy electron tracks in liquid cis-and trans-decalin as studied by product formation in radiolysis and photolysis A. Hummel, H. C. De Leng and L. H. Luthjens Textures of tetradecahedron 6-FeOOH particle\ md their thermal decomposition products T. Ishikawa, R.Orii, A. Yasukawa and K. Kandori Fourier-transform detection of singlet oxygen and fluorescence from cell membrane bound porphyrins T.G. Truscott, F. Bohm, G. Marston and R. P. Wayne Redox electrocatalysis by tetracyanoquinodimethane in phospholipid monolayers adsorbed at a liquiaiquid interface Y. Cheng and D. J. Schiffrin Formation and structure of mono- and di-lead hydroxide and fluoride complexes in molten NH,NO3*1.5H,O at 50 "C B. Holmberg, F. Frostemark and L. Bengtsson Excess molar enthalpies of nitrous oxide-toluene in the liquid and supercritical regions J. A. R. Renuncio, R. C. Castells, C. Munduina and C. Pando Application of matched spin probes to the investigation of the overall motion of model lubricant molecules D. G. Gillies, L. H. Sutcliffe and Y-L. Chung Electron nuclear double resonance study of RbCl:Sz S.van Doorslaer, F. Maes, F. Callens, P. Moens and E. Boesman Studies of light-induced nickel EPR signals in Demlfovibrio gigas hydrogenase M. Medina, R. Williams, R. Cammack and E. C. Hatchikian Single crystal structure of C6()at 300 K: Evidence for the presence of oxygen in a statical y disordered model R. Schlogl, W. Bensch, H. Werner and H. Bart1 Effects of surfactant charge and structure on excited-state protolytic dissociation of naphth 1-01 in vesicles H. Lemmetyinen, Y. V. Il'ichev, K. M. Sulntsev md M. (I. Kuzmin V FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Autumn Meeting: Reactions and Mechanisms for Fine Chemicals To be held at the University of Glasgow on 6-9 September 1994 Further information from Dr J.F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Gas Kinetics Group 13th International Symposium on Gas Kinetics To be held at University College, Dublin on 11-15 September 1994 Further information from Dr H. Sidebottom, Department of Chemistry, University College, Dublin Electrochemistry Group with the SCI ELECTROCHEM 94 To be held in Edinburgh on 12-16 September 1994 Further information from Professor D. E. Williams, Department of Chemistry, University College London, 20 Gordon Street, London WClH OAJ ~~ ~ ~ ~~ Biophysical Chemistry Group with the Industrial Division Biotechnology Group Peptide + Water = Protein To be held at University College, London on 19 September 1994 Further information from Professor J.L. Finney, Department of Physics and Astronomy, University College London, Gower Street, London WClE 6BT ~~ ~ ~~ British Carbon Group Applications of Microporous Carbons To be held at the University of Leeds on 28 and 29 September 1994 Further information from Professor B. Rand, Department of Chemistry, The University, Leeds LS2 9JT Theoretical Chemistry Group with CCPl Electronic Structure: From Molecules to Enzymes To be held at University College London on 30 November 1994 Further information from Dr P. J. Knowles, School of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ Division Annual Congress: Lasers in Chemistry To be held at Heriot Watt University, Edinburgh on 10-13 April 1995 Further information from Dr J. F.Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN ~~ Division Joint Meeting with the Division de Chirnie Physique de la Societe' Francaise de Chimie, Deutsche Bunsen Gesellschaft fur Physikalische Chernie and Associazione Italiana di Chimica Fisica Fast Elementary Processes in Molecular Systems To be held at the Universite De Lille, France on 16-30 June 1995 Further information from Dr C. Troyanowsky , Division de Chimie Physique, Laboratoire de Chimie Physique, 11 rue Pierre et Marie Curie, 75005 Paris, France British Carbon Group Carbon '96 To be held at the University of Newcastle upon Tyne on 7-12 July 1996 Further information from Dr K. M. Thomas, Northern Carson Research Laboratories, The University, Newcastle upon Tyne NE1 7RU vi THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 98 Polymers at Surfaces and Interfaces University of Bristol, 12-14 September 1994 Organ ising Conirzi ittee: 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 theoretiml and computational techniques have recently led to great progress in understanding the difficult but technologically important problems associated with the conformation of polymer molecules at surfiices 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 solidliquid and liquidliquid interfaces, and extensions towards more complicated biological systems. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly, London W 1V OBN. THE ROYAL SOCIETY OF CHEMISTRY, FARAIIAY DIVISION. GENERAL DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Organising Conmiittee Professor L.D. Barron (Chairman) Dr A. F. Drake Dr D. L. Andrews Professor R. E. Hester Professor A. D. Buckingham Traditional optical activity measurements such as CI> are confined to the visible and near-ultraviolet spectral regions where they provide stereochemical inforination on chiral molecules via polarized electronic transitions. Thanks to prompting from theory and new developments in instrumentation, optical measurements are now being made iri the vibrational spectrum using both infrared and Raman methods. Studies over the past decade on a large range of chiral molecules, from small organics to biological macromolecules, have demonstrated that vibrational optical activity opens up a whole new world of fundamental studies and practical .ipplications undreamt of in the realm of conventional electronic optical activity.The meeting seeks to bring together experimcntalists and theoreticians to discuss the current and future experimental possibilities and the development of theories, including ab initio computational methods, which can relate the observations to stereochemical detailh. 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 THE ROYAL SOCIETY OF CHEMISTRYy FARADAY DIVISION, GENERAL DISCUSSION 100 Atmospheric Chemistry: Measurements, Mechanisms and Models University of East Anglia, Norwich, 19-21 April 1995 Organising Committee: Professor I.W. M. Smith and Dr J. R. Sodeau (Co-chairmen) Dr R. A. Cox Dr J. C. Plane Dr J. Pyle Professor F. Taylor The priority now given by national governments to the study of atmospheric science confirms that our understanding of global climate and compositional changes depends upon measurements in both the laboratory and the field. The data obtained by the experimentalists are then applied by modellers who provide the most significant input into legislative controls on pollution matters. However there have been few opportunities for laboratory and field workers along with the modelling community to attend an "interdisciplinary" discussion in which overall progress in our understanding of specific atmospheric problems is assessed.The object of this discussion is to bring together the researchers in the diverse disciplines that make up atmospheric chemistry so that their individual results and conclusions can be communicated to each other. Some of the key issues to be discussed will include: ozone balances in the atmosphere; heterogeneous processes; the interaction of chemistry and dynamics in determining atmospheric composition and change. Particular reference will be made to the input of data to global models from the use of satellite, airborne and ground-based instrumentation. Contributions are invited for consideration by the Organising Committee covering topics within the area of chemistry, dynamics and modelling in the lower and upper atmosphere.Abstracts of about 300 words should be submitted by 31 May 1994 to: Professor I. W. M.Smith OR Dr R. J. Sodeau School of Chemistry School of Chemical Sciences University of Birmingham University of East Anglia Edgbaston, Birmingham B15 2lT, UK Norwich NR4 7TJ,UK Full papers for publication in the Discussion volume will be required by December 1994. THE ROYAL SOCIETY OF CHEMISTRYy FARADAY DIVISIONy GENERAL DISCUSSION 101 Gels Paris, France, 6-8 September 1995 Organising Committee: Dr J. W. Goodwin (Chairman) Dr R. Audebert Dr R. Buscall Professor M. Djabourov Dr A.M. Howe Professor J. Livage Professor J. Lyklema Professor S.B. Ross-Murphy During the last few years there has been an increase in both theoretical and experimental work on gels as new techniques have been applied to a wide range of gelling systems. Typical of these are gels formed from polymers by both physical and chemical interactions as well as gels formed by inorganic and surfactant systems. The meeting will deal with the structure and dynamics of gels with the latter heading covering both swelling and rheological behaviour. Mixed systems such as polymer/surfactant and polymer/particle gels will also be discussed. The Discussion will bring together experimentalists and theoreticians interested in different types of gelling systems and encourage them to interact and assess the current scene and provide a benchmark for future developments.Contributions are invited for consideration by the Organising Committee. Titles and abstracts of about 300 words should be submitted by 30 September 1994 to: Dr J. W. Goodwin, School of Chemistry, University of Bristol, Cuntock's Close, Bristol, BS8 ITS, UK Full papers for publication in the Faraday General Discussion 101 volume will be required by May 1995. ... 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ISSN:0956-5000    

DOI:10.1039/FT99490BP152

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2003-2007 Kinetic Study in a Microwave-induced Plasma Afterglow of the CU(~~S)Atom Reaction with N20from 458 to 980 K and with NO, from 303 to 762 K Chris Vinckier,* Tom Verhaeghe and lnge Vanhees Laboratory for Analytical and Inorganic Chemistry, Department of Chemistry, K. U. Leuven Celestijnenlaan 200F 300 1 Heverlee , Belgium The rate constants for the reaction of ground-state copper atoms (4 ’S) with N,O and NO, have been derived in a fast-flow reactor. The microwave-induced plasma (MIP) afterglow technique was used for the generation of copper atoms in the gas phase. The rate constant for the reaction Cu(4’S) + N,O CuO + N, at temperatures between 458 and 980 K is found to be (2.4 k0.6) x lo-’’ exp -(48.6 +_ 1.4 kJ mol-’/RT) cm3 molecule-’ s-’.The reaction Cu(4’S) + NO, -+ products, was followed in the temperature range from 303 to 762 K and has a rate constant (1.3 0.6) x lo-’’ exp -(0.2 f1.3 kJ mol-’/RQ cm3 molecule-’ s-’. From the copper atom decays in argon at 528 K, a diffusion coefficient, DCu,Ar 543.2 f60.2 cm2 Torr s-’, could be derived. = The introduction of the high-temperature fast-flow reactor (HTFFR) technique by Fontijn has allowed the determi- nation of a large number of kinetic parameters of refractory metal atom-gas reactions over a temperature range of between 300 and 2000 K.’ Recently the high-temperature photochemistry (HTP) technique, has been developed. While in the former technique metal atoms were generated mostly by thermal heating of the metal, in the HTP technique, metal atoms are produced photolytically and are monitored in a real-time detection mode.An alternative method used to vaporize non-volatile metals is the plasma-afterglow atomization technique3v4 where the metals are generated in the gas phase by a reaction of a vola- tile metal salt, MeX(g), with hydrogen atoms. In this work the reaction between copper atoms, dinitrogen oxide and nitrogen dioxide has been investigated in a fast- flow reactor. The reaction with dinitrogen oxide Cu(4 ’S) + N,O -+ CuO + N, has been studied in the temperature range from 458 to 980 K and the Arrhenius expression for k, will be derived and com- pared with the recent measurements by Narayan et ul.’ on the same reaction in the somewhat broader temperature range of 470 to 1340 K.The other reaction with nitrogen dioxide Cu(4 ,S) + NO, -+ products was followed between 303 and 762 K. No other kinetic data on reaction (2) are yet available in the literature for compari- son. Experimental A schematic view of the slightly modified experimental technique3 is shown in Fig. 1. It is basically a quartz fast-flow reactor with an internal diameter of 5.7 cm and a length of 1 m. At the upstream end the water-cooled flange carries a carrier-gas inlet (GI)and a sample holder (SH) with an exter- nal diameter of 1.9 cm. A Kanthal resistance wire allows the solid CuCl pellet to heat up to a temperature, T,, of cu. 800 K independently of the reactor temperature.A shielded chromel-alumel thermocouple (TC in contact with the CuCl(s) pellet allows the temperature, T,, of the solid to be monitored. At a distance of 13.5 cm downstream of the sample holder, a second gas inlet (G,) is equipped with an air-cooled microwave cavity, type 216 L, powered by an Electro Medical Supplies (EMS) microwave generator oper- ating at 2450 & 25 MHz and with a maximum power.of 200 W. Between the cavity and the reactor a Wood’s horn traps the UV light from the microwave-induced plasma (MIP). Downstream of G, the reactor oven (RO) allows the gas tem- perature, T, to be varied in the range 300-1000 K. Con-stant temperature can be maintained within 2.5% over a distance of ca. 25 cm. At the downstream end the water-cooled flange carries two quartz probes : an along-the-axis movable thermocouple (TC,), to measure the gas temperature, and the additive inlet (AI) for the introduction of the co-reagent, N,O or NO,.An Alcatel-type 2033 double-stage oil rotary pump with a nominal pump capacity of 35 m3 h- ’ results in a flow veloc- ity for argon of 309 cm s-’at 295 K. Pressures are measured by means of a Datametrics-type 1018 electronic manometer powering a sensor (P) which measures pressures from to 13 Torr. In the kinetic zone the copper atoms are detected by atomic absorption spectroscopy at 324.7 nm. The light path of the hollow cathode lamp HCL is focussed perpendicular to the main reactor axis through a slit of 1 cm height in the reactor oven construction.The light exiting the reactor is then focussed on the entrance slit of a McPherson model 270 monochromator (M). A photomultiplier (PM) Hamamatsu R955 with a wavelength range 160-900 nm is used as a detector. While the detection system remains at a fixed posi- tion, the fast-flow reactor assembly is mounted on a carriage C 0HCL Fig. 1 Schematic view of the experimental set-up: G, and G,, gas inlets; AI, N,O or NO, inlet; SH, sample holder for the CuCl(s) pellet; TC, , thermocouple for the CuCl(s) pellet; C, microwave cavity; HW, horn of Wood; RO, reactor oven; P, pressure sensor; RP, rotary pump; TC, , thermocouple for gas temperature; HCL, hollow cathode lamp; M, monochromator; PM, photomultiplier; A, amplifier; R, recorder.which allows a horizontal displacement so that copper absorbances can be measured along the reactor axis. Typical initial absorbances in the kinetic zone are 50.3, corresponding to an upper limit for the copper atom concen- tration of 4.3 x lolo atoms cmP3. In this way the concentra- tion of copper atoms remains much lower than the additive N20 or NO, concentration so that pseudo-first order condi- tions for copper atom decays are fully established. The gases used are argon and dinitrogen oxide from UCAR with a purity in excess of 99.999% and 99.5%, respec-tively. Hydrogen is from L'Air Liquide with a quality of 99.9997%. Nitrogen dioxide was added as a 0.92% mixture in UHP helium (UCAR). Gas flows were regulated via Brook's precision needle valves of ELF type.Microwave-induced Plasma (MIP) Afterglow Atomization of CuCl A number of aspects of the MIP-afterglow atomization of CuCl have been discussed previ~usly.~.~ In principle, the copper chloride oligomers, Cu,Cl,(g), produced by the vapor- ization of the CuCl pellet are mixed with the reaction pro- ducts of the MIP-afterglow downstream of G,. A complex and unknown reaction sequence converts a fraction of Cu,Cl,(g) into Cu atoms through subsequent atomic hydro- gen reactions : Cu,CI,(g) + nH +Cu(g) + products (3) Owing to a loss process on the reactor wall, the copper con- centration reaches a maximum within a timescale of less than 30 ms which corresponds to a distance of about 10 cm down- stream of the MIP inlet, G,.Since one can assume that the sticking coefficient, ycu ,on the reactor wall approaches unity, the decay of Cu(g) is diffusion controlled and the observed decay constant k, is given by:778 where DCu,Aris the binary diffusion coefficient of copper atoms in argon and r the reactor radius. As an example, the values of k, averaged over four runs are given in Table 1 for various initial hydrogen concentrations. The initial hydrogen concentration has no systematic effect on the value of k,. As a weighted average for the five deter- minations, one obtains k, = 30.7 & 3.4 s-' at a reactor press- ure P, = 8 Torr and temperature = 528 _+ 2 K. From eqn. (I) one can calculate the binary diffusion coefficient D,,, Ar = 67.9 f7.5 cm2 s-' which can be reduced to a reference pres- sure of 1 Torr : Dcu,Ar = 543.2 f60.2 cm2 Torr s-'.With our earlier value,4 D,,, = 219.9 _+ 9.2 cm2 Torr s-' determined at Tp = 300 K, one can calculate the temperature dependence of the diffusion coefficient from the relation D,,, = D3,,(528/300)". From the measurements at 300 and 528 K one obtains n = 1.6 & 0.25 which is somewhat larger than n = 1.5 calculated on the basis of pure kinetic gas consider- Table 1 Decay constants, k,, of the Cu atoms as a function of the initial hydrogen concentration" [H ,]/mTorr T,/K TJK k&-' 5 530 533 26.5 f1.5 15 527 534 33.3 f0.7 50 527 532 23.6 11.5 100 527 534 28.9 f3.8 200 530 534 21.4 f6.1 a Reactor pressure P, = 8 Torr and the microwave power P, = 40 W. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ations, but which is in qualitative agreement with the average temperature dependence, derived from the more elabo- rate Lennard- Jones f~rmalisrn.~ Results and Discussion Cu + N,O Reaction The rate constant for reaction (1) can now be determined from the copper atom decay as a function of the reaction time under various amounts of added N,O. The mathemati- cal treatment is the same as used in our previous work on the kinetics of the Mg + Cl,, N,O reactions:"." ~ +In A,, = -{k1c:201 7*34DCu, Ar}i + B (11)2r2 in which A,, is the copper absorbance, t, the reaction time, B, an integration constant and q, a correction factor. A complete discussion on the mathematics behind eqn.(11) and the influ- ence of the various flow characteristics on the magnitude of q is presented in the earlier work of Talcott et aL7 and Fontijn and Felder,' respectively. The value of k, may be determined by following the copper absorbance A,, as a function of the reaction time, t, at various amounts of added N,O. When the slopes, S, derived from eqn. (11) are plotted against [N,O], straight lines are obtained with an intercept of 7.34Dc,, Ar/2r2 and a slope equal to kl/q. For the measurements in argon the correction factor, q, is set equal to 1.3 with an associated sys- tematic error of lo%, and the overall accuracy is estimated to be in the range 40-70%.' All plots and calculations are made using the SAS-607 statistical package.', The uncertainties are given as standard deviations.As an example to illustrate the procedure used the natural logarithm of the copper absorbance In A,, is plotted against the reaction time for various N,O concentrations, Fig. 2, at a reaction temperature = 609 K and a pressure P, = 10 Torr and with the MIP-afterglow parameters set at T, = 539 K and [H2] = 200 mTorr. When the slope, S, of these lines is plotted us. the N,O concentration, Fig. 3, a straight line is obtained from which a weighted linear regression yields a value for k, = (1.1 & 0.2) x cm3 molecule-' s-'. -2.5 -3.0 -3.5 TI V 7 E -4.0 -4.5 -5.0 -5.5 0 10 20 30 4b 50 E I reaction time/ms Fig.2 Natural logarithm of the Cu absorbance as a function of the reaction time. The experimental conditions are: P, = 10 Torr; T, = 609 K; [HJ = 200 mTorr; T, = 539 K and P, = 40 W. The N,O concentrations are (0)0; (+) 1.6; (0)3.2; (A) 4.0; (x) 4.8 and (V) 6.4 each in units of 10l5molecules crnp3. J. CHEM. SOC. FARADAY TRANS., 1994,VOL. 90 80 60701 $ 4050j 30v *O/10 0 2 4 6 [N,Ol/l Owl5 molecules ~rn-~ Fig. 3 Observed slope, S, of eqn. (11) plotted against the N,O con-centration. The experimental conditions are the same as in Fig. 2. The five points shown at the ordinate are the observed copper decays in the five blank experiments in the absence of the co-reagent N20. Influence of the MIP-afterglow Parameters In order to verify that the MIP-afterglow conditions have no effect on the magnitude of the derived rate constants, various parameters such as the hydrogen content, temperature of the CuCl pellet and reactor pressure have been varied and their influence on k, checked.The experimental conditions are summarized in Table 2. While part of the scatter in the values of k, is due to the fact that neither < nor T, could be kept rigorously constant, one cannot see a systematic effect of the hydrogen content on the derived value of k, . Also a variation of T, between 530 and 551 K does not show an effect on k, . It is known that the sublimation of CuCl occurs via the for- mation of oligomers Cu,Cl,.'3 On the basis of their subli- mation heat one can calculate that the gas-phase concentration increases by a factor of three in the narrow temperature range 530-551 K.3-6 Variation in both the hydrogen concentration and T, seriously affects the MIP- afterglow composition but this does not seem critical for the kinetic measurements.Table 2 Influence of the MIP-afterglow parameters on the value of k, for the Cu +N,O reactiona T,/K T,/K [H,]/mTorr P,/Torr k1/cm3 molecule-' s-' hydrogen content 530 527 527 527 530 533 534 532 534 534 5 100 200 15 50 8 8 8 8 8 (3.4 0.5) x 10-15 (2.1f0.6)x 10-15 (4.4f 1.1) x 10-15 (3.0f0.5)x (2.8f0.5)x temperature, T,, of the solid 682 682 682 68 2 530 539 544 551 20 10 10 10 8 8 8 8 (4.3k 0.9)x 10-14 (3.7f0.8)x 10-14 (4.0k 0.8)x (5.4f 1.3)x reactor pressure, P, 483 485 486 485 529 532 532 529 2.5 2.5 2.5 2.5 6 6.5 8 11 (2.0 f0.6) x 10-15 (2.1k 0.5)x 10-15 (2.4f0.4)x (1.7k 0.4)x Microwave power, P, = 40W.Another MIP-afterglow parameter is the reactor pressure, P,, which was varied between 6 and 11 Torr. Even in this narrow range all MIP-afterglow phenomena related to the diffusion of the reagents and the copper atom wall loss are enhanced at a lower pressure. The initial absorbance Aocu at 11 Torr was indeed a factor of two larger than at 6 Torr. Table 2 shows that P, has no effect on the derived value of kl . Temperature Dependence of k A summary of the experimental conditions and derived values for k, is given in Table 3 at temperatures between 458 and 980 K. A weighted non-linear regression yields the expression k, = (2.4 & 0.6) x lo-'' exp -(48.6 & 1.4 kJ mol-'/RT) cm3 molecule-' s-' (111) The only other determination of k, has recently been carried out by Narayan et al.' using the metals-HTP technique in which copper atoms are photochemically produced at 248 nm by excimer laser dissociation of thermally generated Cu2C12 or CuF, molecules.The copper atom decays were monitored in real time by means of resonance fluorescence. The overall fit in the entire temperature range 470-1340 K yielded k, = 3.04 x T2.97 exp-(3087 K/T) cm3 molecule-' s-'. In view of the observed non-Arrhenius behaviour at temperatures above 1190 K, the classical Arrhe- nius expression was only derived at temperatures below 1190 K and was given by: k, = 1.70 x lo-'' exp -(5129 K/T) cm3 molecule-' s-' (IV) An Arrhenius plot of our results as In k, us.1/T is shown in Fig. 4 and gives the relation k, = 1.8tg:z x lo-'' exp -(46.9 & 1.2 kJ mol-'/RT) cm3 molecule-' s-'. In view of the symmetric error on the value of the pre-exponential factor, eqn. (111)will be used in the further discussion. When these data are compared with the work of Narayan et ~l.,~ Table 3 k, for the Cu +N,O reaction as a function of temperature T,/K T,/K [H,]/mTorr P,,Torr k1/cm3molecule-' s-l 458 468 483 485 485 486 527 527 527 530 530 546 609 621 627 682 682 682 682 704 728 774 797 818 858 919 980 524 542 529 532 529 532 532 534 534 534 533 529 539 543 539 5 30 544 539 551 529 539 532 544 532 527 529 532 2.5 5 2.5 2.5 2.5 2.5 50 100 15 200 5 2.75 200 20 20 20 10 10 10 4.5 4 20 20 5 5 5 5 8 8 6 6.5 11 8 8 8 8 8 8 8 10 8 8 8 8 8 8 8 8 8 8 8 8 8 8 (9.1f3.4)x (1.6k 0.3)x (2.2f0.5)x 10-' (1.7k0.4)x (2.4f0.4)x (2.8 0.5) x (3.0f0.5)x (2.0_+ 0.6) x 10-15 (2.1 0.6) x 10-15 (4.4f 1.1) x 10-15 (3.4f0.5)x 10-15 (1.1 f0.2) x 10-14(6.3f 1.4)x (1.9f0.4)x (1.9 f0.4)x (4.3f0.9)x (4.1f0.8)x (3.7 0.8)x 10-14 (5.4 1.3) x 10-14 (4.7f0.7) x 10-14 (7.0f 1.1) x 10-14 (1.4f0.2)x 10-13 (1.8f0.3)x (1.8f0.3)x (2.8f0.5)x (6.6f1.3)x (5.3 1.1) x 10-13 2006 -27 '. '.-35 \1 --, I. I, 1. I. I I I-%! I I I I 1.0 1.2 1.4 1.6 1.8 2.0 2.2 103 KIT Fig. 4 Arrhenius plot of In k, us. 1/T. (-) Non-linear regression of k, us. 1/T; (---) linear regression of In k, us.1/T; (-. -) Narayan et al.' one sees that our values of k, are systematically lower over the entire temperature range. This is mainly due to the lower Arrhenius activation energy of 42.6 kJ mol- ' compared with 48.6 kJ mol-' derived from our work. In the middle of the temperature range at 720 K, the value of k, determined by Narayan et ~l.,~is a factor of two higher, which falls outside the systematic uncertainty level of the correction factor, q, in eqn. (11). Therefore, there seems to be no direct explanation for the observed discrepancy between both data sets. It is interesting to compare the values of the observed acti- vation energy of k, with the barrier heights for a number of alkali-metal atom-N,O reactions.For these reactions much smaller activation energies, in the range of 5.1 and 14.6 kJ mol-',have been mea~ured.'~.'~ This has been explained by semi-empirical correlation between the so-called promotion energy of the metal atom and the observed energy barrier of the rea~tions.~,'~"' The former has been defined in two ways, being either the excitation energy to the lowest excited state of the metal atom or the excitation energy plus its ionization energy. On the basis of this latter definition one would expect a barrier for the Cu + N,O reaction at around 35 kJ mol-' which is about 20-40% lower than the experimental values. Note that by extrapolating to room temperature (300 K) k, = 8.3 x lo-'' cm3 molecule-' s-', which is almost an order of magnitude lower than 6.4 x cm3 molecule-' s-' calculated from eqn.(IV). In view of the large value of the activation energy, an uncertainty of only 5% leads to a spread of a factor of seven for k, when extrapolated to 300 K. This reinforces the need for kinetic measurements of atom reactions over a wide tem- perature range. In this way correlations between the metal atom-N,O rate constants and the promotion energy can be established at e.g. 600 K, avoiding the enormous uncer-tainties when extrapolating to room temperature. Concerning the products of reaction (1) there is no doubt that CuO is formed in its electronic ground state X21-I. With an exothermicity18 of 113.2 kJ mol-l for reaction (l), the lowest electronically excited state A' 2X+at 181.1 kJ mol-' above the ground statelg will not be accessible.Cu + NO, Reaction The experimental procedure described above is also followed to determine the Arrhenius expression of reaction (2) in the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 temperature range 303 to 762 K. It was again checked that no systematic effect on k, of the molecular hydrogen concen- tration, the temperature, T,, of the solid and the reactor pres- sure, P,, could be observed. A summary of the experimental conditions and the derived values for k, are shown in Table 4. At 305 K the average value of k, was found to be (1.8 0.8) x lo-'' cm3 molecule-' s-'. With helium as carrier gas, k, was found to be (1.9 f0.6) x lo-'' cm3 molecule-' s-l and hence the nature of the carrier gas also has no influence on the value of k, .A weighted non-linear regression gives the Arrhenius expression : k, = (1.3 f0.6) x lo-'' exp -(0.2 & 1.3 kJ mol-'/RT) cm3 molecule- ' s-' (V) The Arrhenius plot of In k, vs. 1/T is shown in Fig. 5. From the slope the very slightly different expression k, = 1.0:;:: x lo-'' exp(l.2 f1.2 kJ mol-'/RT) cm3 molecule-' s-' (VI) can be derived. Eqn. (V) shows a slightly positive, and eqn. (VI), a negative activation energy, but taking into account the rather large uncertainty on k, , k, is virtually temperature independent between 303 and 762 K. This is in sharp contrast to reaction (1) which has a large energy barrier of 48.6 kJ mol-'. In view of the large value of 2.36 eV for the electron affinity20*2' of NO,, the metal atom-NO, reaction is likely to occur via the 'electron jump' mechanism,, similar to a number of other metal atom-halogen reactions.The distance rc, where the potential-energy curve of the two approaching Cu and NO, Table 4 k, for the Cu + N,O reaction as a function of temperature ~~ ~ ~~ T,/K T,/K [H,]/mTorr P,/Torr k1/cm3 molecule-' s-' 305 551 200 7.5 (0.7 f0.1) x lo-" 305 559 25 7.5 (1.0 f0.1) x lo-" 305 552 6 7 (1.2+_ 0.2)x lo-" 305 557 3.5 7.5 (2.9 f0.6) x lo-" 303 516 10 9 (0.8 f0.2) x lo-" 303 528 10 9 (1.4 f0.3) x lo-" 303 537 15 9 (2.0 f0.4) x lo-" 303 541 30 5 (2.9 f0.4) x lo-" 303 535 200 6 (1.8 +_ 0.3) x lo-" 303 529 100 7 (2.0 f0.2) x lo-" 303 535 200 7.5 (2.7 f0.4) x lo-" 303 521 200 8 (1.6 f0.3) x lo-'' 303 515 30 10 (1.3 f0.4) x lo-" 305 573 10 6 (3.3 f0.5) x lo-" 332 539 3 7 (1.6 f0.4) x lo-" 335 549 5 7.6 (2.5 & 0.5)x lo-" 353 542 4 12 (2.1 f0.3) x lo-" 355 545 3 8.5 (1.3 f0.2) x lo-" 357 542 3 10 (1.7 f0.3) x lo-" 357 522 5 6.5 (1.5 f0.3) x lo-'' 362 539 3 5.5 (0.8 f0.1) x lo-" 397 527 3.5 7 (1.8 & 0.3)x lo-" 413 554 6 6 (1.6 f0.3) x lo-" 513 532 2 5.5 (1.0 f0.2) x lo-" 624 532 4.5 7 (1.1 f0.2) x lo-" 694 529 3 10 (1.4f0.2)x lo-" 762 55 1 9.5 7 (1.7 f0.4) x lo-" 303" 530 5 9 (1.3 & 0.1) x lo-" 303" 53 1 30 9 (2.6 f0.2) x lo-" 303" 526 10 9 (2.6 & 0.5) x lo-" 303" 545 170 10 (1.6 f0.1) x lo-" 303" 526 10 12 (1.6 f0.3) x lo-" 303" 543 10 13 (1.9 f0.3) x lo-" Measurements in helium carrier gas.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -20 -21 --22 --23 --37 I I 2007 conditions. It is indeed very unusual that a reaction involving only four atoms has already reached its high-pressure limit at around 10 Torr. Finally, it should be mentioned that the error quoted for the heat of formation,18 Af H" (298 K), of CuO is 41.8 kJ mol-l. This leaves the possibility that reac- tion (2) may be slightly exothermic in which case a common second-order reaction is dealt with. The authors are grateful to the Joint Fund for Basic Research (FKFO) for funding this project.T.V. acknowledges a docto- ral fellowship from the Institute for Scientific Research in Agriculture and Industry. I.V. and C.V. are, respectively, Research Assistant and Research Director of the National Fund for Scientific Research (NFWO), Belgium. References-2gd-30 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 1 A. Fontijn and W. Felder, in Reactiue Intermediates in the Gas 103 KIT Fig. 5 Arrhenius plot of In k, us. 1/T. (-) Non-linear regression of k, us. 1/T; (---) linear regression of In k, us. 1/T. species crosses the Coulomb interaction curve of the Cu' and NO,-ions, is given by: rc = 14.35/[Ei(Cu) -E,,(NO,)] The distance rc is given in A, the ionization energy, Ei, and electron affinity, E,,, are expressed in eV and 14.35 is a con- stant.Inserting an ionization energy of 7.73 eV for Cu atoms, eqn. (VII) yields the value of rc = 2.7 A. With this value of rc , the rate constant for the reaction, nr:E, where C is the average thermal speed of the reaction partners, can be calculated. At 300 K k, is calculated to be 1.12 x lo-'' cm3 molecule-' s-'. Table 4 shows that this calculated rate constant is about a factor of five larger than the experimentally determined value so that the pure 'electron jump' mechanism for reac- tion (2) seems unlikely. Instead it would be better to call this a 'close range' charge-transfer mechanism. An interesting point concerns the possible products of reaction (2).With a formation enthalpy A,H" (298 K) of 306.27 kJ mol-' for CuO in the gas phase, the reaction Cu + NO, -+ CuO + NO is endothermic" by 25.86 kJ mol-'. If reaction (2) was to yield CuO one should expect an energy barrier of at least 25.86 kJ mol-' which is not observed. In view of the absence of neither a pressure nor a carrier gas effect on the observed decay constant, a third-order reaction can be excluded. A remaining possibility is the formation of a so called 'sticky Cu+ NO,- collision ~omplex'.~~.~~ While most of the metal-NO, reactions lead to the formation of the metal o~ide,,~-'~ the only major exception seems to be the Cs + NO, reaction. This reaction, which is endothermic by 43.45 kJ mol-', has been shown to have a reaction cross-section exceeding23 100 A2, which is substantially larger than 3.89 A2 determined experimentally for reaction (2).This again confirms that the 'electron jump' mechanism for the reaction Cu + NO, certainly does not prevail. However, from a fun- damental kinetic point of view it is not clear how the complex Cu/NO, can be stabilized under our experimental Phasc., Generation and Monitoring, ed. D. W. Setser, Academic Press, New York, 1979, ch. 2. 2 P. Marshall, A. Narayan and A. Fontijn, J. Phys. Chem., 1990, 94, 2998. 3 C. Vinckier, A. Dumoulin, J. Corthouts and S. De Jaegere, J. Chem. SOC., Faraday Trans. 2, 1988,84, 1725. 4 C. Vinckier, J. Corthouts and S. de Jaegere, J. Chem. SOC., Faraday Trans. 2, 1988,84, 1951.5 A. Narayan, P. Futerko and A. Fontijn, J. Phys. Chem., 1992,%, 290. 6 C. Vinckier, P. Christiaens and M. Hendrickx, in Gas-Phase Metal Reactions, ed. A. Fontijn, Elsevier, Amsterdam, 1992, ch. 4. 7 C. Talcott, J. Ager I11 and C. Howard, J. Chem. Phys., 1986, 84, 6161. 8 J. Silver, J. Chem. Phys., 1984,81, 5125. 9 J. Hirschfelder, C. Curtiss and R. Bird, Molecular Theory of Gases and Liquids, Wiley, New York, 1954, ch. 8. 10 C. Vinckier and P. Christiaens J. Phys. Chem., 1992,%, 2146. 11 C. Vinckier and P. Christiaens, J. Phys. Chem., 1992,%, 8423. 12 SAS statistical package, SAS Institute Inc., Cary, NC, 1992. 13 M. Guido, G. Balducci, G. Gigli and M. Spoliti, J. Chem. Phys., 1971,55,4566. 14 D. Husain and Ji Bing, Combust. Flame, 1990,79,250. 15 J. M. Plane, in Gas-Phase Metal Reactions, ed. A. Fontijn, Else- vier, Amsterdam, 1992, ch. 3. 16 J. M. Plane, C-F. Nien and B. Rajasekhar, J. Phys. Chem., 1992, %, 1296. 17 P. Futerko and A. Fontijn, J. Chem. Phys., 1991,95,8065. 18 M. W. Chase, C. H. Davis, J. R. Downey and D. Y. Frurip, J. Phys. Chem. Ref. Data, 1985, 14, Suppl. No. 1. 19 J. M. Delaval, F. David, Y. Lefebre, P. Bernage, P. Niay and J. Schamps, J. Mol. Spectrosc., 1983, 101, 358. 20 E. Herbst, T. A. Patterson and W. C. Lineberger, J. Chem. Phys., 1974,61,1300. 21 E. P. Grimsrud, G. Caldwell, S. Chowdhury and P. Kebarle, J. Am. Chem. Soc., 1985,107,4627. 22 D. R. Herschbach, Adu. Chem. Phys., 1966,10, 319. 23 D. 0. Ham, J. L. Kinsey and F. S. Klein, Discuss. Faraday Soc., 1967,44,174. 24 R. R. Herm and D. R. Herschbach, J. Chem. Phys., 1970, 52, 5783. 25 D. D. Parrish and R. R. Herm, J. Chem. Phys., 1971,54,2518. 26 H. F. Davis, A. G. Suits, H. Hou and Y. T. Lee, Ber. Bunsenges. Phys. Chem., 1990,94, 1193. Paper 4/01443J; Received 1 lth March, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002003

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2009-2013 Electronic States and the Metal-Insulator Transition in Caesium-Ammonia Solutions Zhihong Deng and Michael L. Klein* Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA Glenn J. Martyna Department of Chemistry, Indiana University, Bloomington, IN 474054001, USA The nature of the electronic states in caesium-ammonia solutions is examined from the insulating to the metallic regime using two different microscopic models (Z. Deng, G. J. Martyna and M. L. Klein, J. Chem. Phys., 1994, 100, 7590.). In the first model, the ammonia molecules are treated via a classical point-charge model and the cations as a positive neutralizing background. In the second model, the ammonia solvent is made fully polariz-able and the cations, here caesium, are explicitly included.The solvent and ions are treated classically and the electronic degrees of freedom are handled using the Car-Parrinello method and density functional theory. At 260 K, the models give the following picture of the electronic states as a function of caesium/electron concentra- tion: The dilute solution behaves like an electrolyte in which the electrons exist as polarons, on average spher- ical states localized in solvent cavities, far from the counter ions. At ca. 0.5 mol% metal (MPM), the solvated electrons are spin-paired and form large peanut-shaped species called bipolarons. The electrons still exist as bipolarons at ca.1 MPM but well separated from each other. The bipolarons exhibit no strong tendency to be oriented either parallel or perpendicular to each other.We have investigated the effect of system size on the structure and dynamics of the bipolarons at ca. 1 MPM and found it to be small. At higher concentration, ca. 2 MPM, the electrons exhibit a tendency to cluster and the electron density oscillates between localized and delocalized states. At much higher concentration, ca. 9 MPM, the solution behaves as a good liquid metal in which the electron density forms multi-tunnel-like extended states. The caesium cations are always solvated by the ammonia and are thereby isolated from close contact with the electron density. The role of the cations is assessed through a comparison of the results of the two models. At low metal concentration, the effect of the cations turns out to be rather small.However, the explicit inclusion of the ions is found to increase the metallic character of the solution at ca. 9 MPM. Our findings rationalize a large body of experimental data on this system. Metal-ammonia solutions are a classic chemical system that has attracted much interest and study.'-' The solutions exhibit a wide variety of phenomena as a function of metal concentration. However, for the most part, the effect of differ- ent metal counter ions is found to be rather small. At very low concentration, i.e. MPM, the solutions are char- acterized by isolated excess electrons called polarons. At higher concentrations, <2 MPM, the solutions are domi- nated by localized spin-paired species, referred to as bipol- arons.' The metal-insulator transition occurs at ca.4 MPM and, for concentrations greater than ca. 9 MPM, the solution behaves like a good liquid metal. Unlike other metal-ammonia solutions, caesium and ammonia are completely miscible and there is no two-phase coexistence region (miscibility gap). The solubility of caesium in ammonia is the largest among different metal solutes. Although the available experimental information is extensive, the microscopic under- standing of the electronic states of the system is mostly qual- itati~e.~-'~Little is known about the specific role played by the metal cations. The evolution of spin-pairing in dilute metal-ammonia volume of an isolated electron,' observations that suggest that the association is weak.In order to explain these results, Mott has proposed an electron distribution resembling that of a hydrogen molecule with a binding energy of a few tenths of an eV above that of two isolated electrons.' Previous simulation studies employing path integral Monte Carlo (PIMC) and Car-Parrinello local spin density functional (CP-LSDA) meth~ds'~" have confirmed Mott's inferences about the electronic states at ca. 1 MPM. In the singlet state, the electrons spin pair and form peanut-shaped cavities with peaks in the electron density about 6.5 8, apart.l4.' These spin-paired species are the so-called 'bipolarons'. A representative configuration of a singlet-state bipolaron in a periodic box of 256 ammonia molecules is shown in Plate 1.Despite this success, several issues remain to be explored. For example, how is the stability of the bipol- aron affected when the metal counter ions are explicitly included? Is there a finite size effect on the structure and dynamics of the bipolaron? To what extent are the bipol- arons associated at 1 MPM? Is there any difference in the properties of bipolarons at different metal concentrations? solutions has been extensively studied e~perimentally.',~,~,~ For metal concentrations above lop3 MPM, there is a marked drop in the spin susceptibility per electron. The con- ductivity per electron also falls off and reaches a minimum at ca. 0.1 MPM.' These observations have been interpreted to indicate that the electrons form associated spin-paired species such that at ca.0.1 MPM, spin-pairing is essentially complete with almost 80% of the electrons spin-paired.' However, the optical absorption spectrum of the solution is relatively unchanged from that at low metal concentration and the volume of the spin-paired species is approximately twice the In this paper, these issues are addressed with specific refer- ence to caesium-ammonia solutions. Two different models of caesium-ammonia solution are considered. l6 In the first model (I), the ammonia solvent is treated as a partially pol- arizable simple point-charge model and the cations as a uniform positive background.I6 In the second model (11), the ammonia solvent is made fully polarizable and the cations, here caesium, are explicitly included.The Car-Parrinello LSDA method is used to examine the electronic states in caesium-ammonia solutions at four different concentrations : 0.5, 1, 2 and 9 MPM. This detailed study, which complements our previous work using the first m~del,'~*'~*~~ will allow us to assess the role of the alkali-metal cations in determining the properties of the metal-ammonia solution. In addition, a possible system size effect on our previous low-concentration st~dies'~,'~is examined. To this end, a larger simulation cell (ca. 29 8, edge) containing 512 ammonia molecules plus two or four caesium atoms is employed to study the 0.5 and 1 MPM solutions. The structure, energetics and dynamics of the bipolarons obtained at these concentrations are com- pared to those obtained in earlier at 1 MPM using a smaller cell (ca.22 8, edge) containing 256 ammonia mol- ecules and two excess electrons. The optical conductivity and the qualitative nature of the electronic states for the two models are compared to each other and experiment. Antici- pating our results we will see that the most detailed model gives an excellent account of properties of the metallic solu- tion. Potential Models and Methodology In the two models of caesium-ammonia solution presented in this paper, the ammonia molecules are treated using the simple rigid point-charge ansatz.'6" This is a reasonable approximation as the ammonia molecule is known to retain its gas-phase geometry throughout the metal-ammonia phase diagram.The electronic degrees of freedom associated with the valence electrons are taken to interact with the ammonia and the alkali-metal cations via pseudo-potentials. Specifi- cally, the pseudo-potential used to describe electron-ammonia interactions contains the appropriate electrostatic interactions and both polarization and repulsion contribu- tions from the nitrogen.'6*20 For the e--Cs+ interaction, the s-wave pseudo-potential compilation of Bachelet is used as a local potential.'6.2 ' This simplification is justified because, in caesium-ammonia solutions, the caesium cations are solvated by ammonia molecules and isolated from close contact with the electron density.' LSDA is used to treat electronic exchange and correlation.' 6*22-24 The dipole polarizability of the ammonia nitrogen is treated in two ways.16 In the par- tially polarizable model, the electric field due to all the other ammonia molecules at a given nitrogen atom is assumed to vanish.The electronic contribution to the polarization energy is treated as first order in the polarizability using a pairwise additive scheme. In the fully polarizable model, both the ammonia-ammonia and ammonia-metal-ion contributions to the polarization energy are treated fully self-consistently and the electronic part is, again, treated using the pair approximation. In both cases, electron-electron terms in the polarization energy are neglected.The details of the poten- tials, as well as the treatment of the many-body polarization energy, are described in detail elsewhere.16 As in previous work, the Cs+-ammonia pair potential is taken to be of the following form2' The potential parameters were obtained from a fit to the position and value of the minimum total energy of the Cs+(NH3) complex determined by ab initio calculation^.^^^^^ The parameters used in eqn. (1) for both partially and fully polarizable ammonia can be found in Table 1. The Cs+-Cs+ interaction is described by the familiar Tosi-Fumi potential27 zi zj e2 vM+-M+(rij) = -+ B exp(-olrij) --c6--c8 (2)r6. r?.'ij V V J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Cs+-ammonia potential parameters model ion rM+&A V(r)/K C,,/Ehah2 C,/E,ag ro/aoa I Cs+ 3.0444 -8428 12764282.14 297.45 0.5 I1 Cs+ 3.0444 -8428 12683035.01 249.09 0.5 Table 2 Cs+-Cs+ potential parameters BIG u/a; C6/Eh C8/Eh 1923.9351 1.8765 158.845 1037.463 where the parameters are listed in Table 2.The polarizability of the caesium cations themselves is neglected. The effect of this approximation remains to be investigated. The electronic states of the system were determined using LSDA.22y23 In this scheme, the energy of an N-electron system is written as t2 r where n(r) = nl(r) + n-,(r), n,(r) = 1;:,I t+biU(r) and N,1' + N-, = N. The self-consistent equations for the Kohn- Sham (KS) orbitals are then =Ei $ia(r) (4) where the exchange correlation energy is defined as &(r) = drn(r)[e,(r) + ec(r)].The forms of the spin-polarized e,(r) and E,(r) are taken from Perdew and Z~nger.~~ The lowest spin state was studied in the present paper. The single-particle orbitals have been expanded in a plane- wave basis set using a cut-off of k,, = 124L for the small simulation box (L x 22 A), and k,,, = 18n/L for the large simulation box (L z29 A), where L is the edge of the simula- tion cell. Only the r point is used in the calculations because of the large size of the unit cell and the liquid nature of the system. The plane-wave cut-off was tested on a few represen- tative configurations and found to give energies to within a few per cent of the fully converged results. LSDA gives an expression for the ground-state energy.Thus, in all the calculations presented in this paper, the properties of the system on the ground-state electronic surface were determined. Clearly, this is an approximation in the metallic regime where the energy gap goes to zero. However, the present scheme has been shown to give good results in a variety of similar application^.^^.^^ Additional approximations are used in the evaluation of the optical con- ductivity. Here, we employ the Kubo-Greenwood (KG) rela-ti~n,~'*~l a(o) = -x IM[ XR)I'S[&P(R)-Ep) -ho] ) (5) where o is the spin state, f; is the occupation number of the spin state (a,i), R is the volume of the simulation cell, and J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 Plate 1 Electron density of a representative configuration of the singlet-state bipolaron at ca. 1 MPE taken from our previous CP-LSDA calculations using model I.14.15 The simulation cell (ca. 22 8, edge) consists of 256 ammonia molecules and two excess electrons. The outermost contour contains 95% of the electron density. Plate 2 Electron density of a representative configuration of a caesium-ammonia solution at ca. 0.5 MPM taken from the present Car- Parrinello LSDA calculations using model 11. The simulation cell (ca. 29 A edge) consists of 512 ammonia molecules and two caesium atoms. The Cs+ ions are shown with covalent radii (pink balls) and ammonia molecules with a ball-and-stick representation. The outermost contour contains 95% of the electron density.Deng et al. (Facing p. 2010) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Plate 3 Electron density of representative configurations of a caesium-ammonia solution at ca. 1 MPM taken from the present CP-LSDA calculations using model 11: in (a)the two bipolarons are parallel to each other while in (b)they are perpendicular. The simulation cell (ca. 29 A edge) consists of 512 ammonia molecules and four caesium atoms. The Csf ions are shown with covalent radii (pink balls) and ammonia molecules with a ball-and-stick representation. The outermost contour contains 95% of the electron density. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Plate 4 Electron density of representative configurations of a caesium-ammonia solution at ca.2 MPM taken from the present CP-LSDA calculations using model I1: (a) dimerized bipolaronic structure; (b)amoeba-like more extended structure. The simulation cell (ca. 22 A edge) consists of 256 ammonia molecules and four caesium atoms. The Cs+ ions are shown with covalent radii (pink balls) and ammonia molecules with a ball-and-stick representation. The outermost contour contains 95% of the electron density. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Plate 5 Electron density of representative configurations of caesium-ammonia solution at ca. 9 MPM taken from the present CP-LSDA calculations using model 11: (a) Cs+ ions are shown with covalent radii and ammonia molecules with a ball-and-stick representation. (b)As in (a)but with space-filling ammonia molecules.The system consists of 256 ammonia molecules and 24 caesium atoms. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 201 1 Table 3 Simulation parametersa number of concentration /MPM number of valence electrons ammonia molecules box length /A 0.5 2 512 28.4 1 2 256 22.4 1 4 512 28.6 2 4 256 22.4 2 4 256 22.9 9 24 256 23.4 9 24 256 25.1 ' 1 au (mass) = 9.106 x kg. 1 au (time step) = 2.42 x lo-'' s. MT = (Y: I@IYq) is the momentum-operator matrix element between states (0,i) and (a,j). The latter is evaluated using the spin-up and spin-down single-particle KS states instead of the 'true' many-body eigenfunctions and eigen- values. Clearly, the use of the LSDA single-particle 'excited' states, represents an additional approximation as LSDA is a theory for the ground state.The average is over a canonical distribution of the nuclear positions (R)on the ground-state electronic surface. The structure and dynamics of the system were determined In ourusing the Car-Parrinello meth~d.~~,~~model of metal-ammonia solutions, the ammonia molecules and metal cations move on a Born-Oppenheimer (BO) energy surface generated by the electrons and the induced dipoles UdiRiI) = min (V;,C(R,), PJI + min {E,sC{Ri), $ill (6) Pi(a) Jli That is, the electrons must be in the instantaneous ground state and the induced dipoles must satisfy the minimization condition at every solvent configuration.16 The Car-Parrinello approach is applied to this system by treating the plane-wave expansion coefficients of the orbitals in the LSDA expression for the ground-state energy [cf.eqn. (3)] as dynamical variables and assigning them a small fictitious mass, me,and a very low temperature, K.32*33Similarly, the components of the induced dipoles on the nitrogen atoms are also treated as dynamical variables and assigned another small fictitious mass, mp, and a low temperature, Tp.34*35 The system is propagated according to a Hamiltonian consisting of the kinetic energy of the ions and ammonia molecules, the kinetic energy of the basis-set parameters, the kinetic energy of the induced dipoles, the pair additive portion of the ammonia-ammonia, ion-ion and ammonia-ion interactions, the electronic energy and the many-body polarization energy of the solvent.16 The fictitious masses, me and mp,have no physical meaning, but control the timescale of the motion of the electrons and the induced dipoles, respectively.In prac- tice, the fictitious masses of the basis-set parameters and the induced dipoles are adjusted for each concentration studied until an adiabatic separation of the dynamics of these fast variables and solvent is attained (see Table 3). Simulations were run for 30 ps for each state point studied using the velocity Verlet integration alg~rithm.~~ The large temperature differences between the slow ammonia molecules and metal ions (at 260 K) and fast but cold (T < 5 K) degrees of freedom associated with the induced dipoles and basis set parameters, are maintained using a modification of the Nose-Hoover canonical dynamics scheme, the MKT chain meth~d.'~*~~*~~Here, independent chains of thermostats are placed on each orbital, the induced dipole moments, the metal cations and the translational and rotational degrees of solvent molecules.16 The MKT chains were found to main- tain temperature control very well and, as a result, deviations model time step/au' mass, m,/aub mass, m,,/sub T, Tp K/K I1 8 64 600 0.02 5.0 I 16 256 600 0.02 5.0 I1 8 64 600 0.02 5.0 I 8 64 600 0.02 5.0 I1 8 64 600 0.02 5.0 I 4 16 600 0.02 5.0 I1 2 4 600 0.02 5.0 from the Born-Oppenheimer surface were observed to be <5% during a typical 30 ps trajectory.For higher concentrations (2 and 9 MPM), a smaller simu- lation cell (ca. 22 A edge) which consists of 256 ammonia molecules plus four and 24 caesium atoms, respectively, was used. Previous employed the simple model and this smaller simulation cell. The volume of the simulation cell is taken to be:' v,= VNHs + n, v,+ n, v, (7) where VNHJis the volume of 256 (512) ammonia molecules for the smaller (larger) simulation cell at a typical liquid density, pNHJ= 0.023 A-3, and the parameter V, = 65 A3 is the excess volume per electron and r/; = 125 A3 is the excess volume per Cs+ ion.' Results 0.5 and 1 MPM The caesium-ammonia solutions were examined at concen- trations corresponding to ca.0.5 and 1 MPM using the second model (fully polarizable ammonia and explicit caesium cations)16 and a large simulation box (ca. 29 A). At ca. 0.5 MPM, the valence electrons of the caesium atoms ionize and then associate to form a peanut-shaped bipolaron (see Plate 2). The distance between the average position of spin-up and spin-down density (see Fig. 1) is about 6.5 A tips Fig. 1 Distance between the average position of spin-up and spin- down density, as a function of time; for the singlet-state bipolaron in CQ. 0.5 MPM caesium-ammonia solution 2012 Table 4Average energes per bipolaron/E, 1 MPM I -0.19 0.18 0.06 -0.16 0.5 MPM I1 -0.20 0.19 0.06 -0.16 1 MPM I1 -0.19 0.18 0.06 -0.16 which is about the same as that of the 1 MPM singlet-state bipolaron previously studied.' 49L The bipolaron is observed to fluctuate and reorient.At times, the two electrons move closer than 6.5 8, with a concomitant shape change from peanut-like to spherical. However, when the electrons 'rebound' to the peanut shape, the bipolaron is oriented dif- ferently. The electrons at ca. 0.5 MPM never collapse to a single spherical cavity, but remain at least 3.5 8, apart (see Fig. 1). The caesium cations are solvated in the bulk ammonia solvent and are not in close contact with the elec- tron density. The average distance between the two caesium cations is about 12 A. The self-diffusion coefficient of caesium ions is about 0.2 A2 ps-l, less than half of that of the ammonia solvent, 0.5 A* ps-'.At ca. 1 MPM, the electron density is still localized. The electrons exist as separate bipolarons, see Plate 3. Each bipol- aron is very similar to those found using the simple model and a smaller simulation cell. 14*' The distance between the average position of spin-up and spin-down density in each bipolaron is about 6.5 A. The two bipolarons are well separated and constantly change their shape and orientation, moderated by the solvent fluctuation. The bipolarons sample all possible relative orientations, including parallel [Plate 3(a)] and perpendicular [Plate 3(b)] configurations. The caesium cations are, again, solvated in ammonia and isolated from close contact of the electron density. In order to quantify and compare the bipolarons obtained from different systems further, the average energy of the bipolaron and its component parts, V&3, V,,, V,, and kinetic energy (Ek)are presented in Table 4.The energetics of different bipolarons are almost identical.2and 9 MPM Higher-concentration caesium-ammonia solutions were also studied using the CP-LSDA method. At ca. 2 MPM, the electrons begin to associate and form clusters. Two types of cluster are observed. The first, shown in Plate qa),is a local- ized form, a dimer of bipolarons. This state occurs in 80% of the configurations. In Plate 4(b), the second cluster type which accounts for the remaining configurations is shown. Here, the electron density is delocalized in a tube-like struc- ture which spans the simulation cell.At ca. 9 MPM, the elec- tronic structure of the system is radically different (see Plate 5). The electrons are now delocalized in a multi-tunnel-like structure which spans the entire simulation cell. That is, the electron density remains excluded from the ammonia solvent, as at the low concentrations, and forms a bicontinuous con- struct. As tested by methods described elsewhere,' the7728 individual orbitals are also delocalized. No localized bipol- aronic structures are observed at this concentration. The single-particle electronic density of states, N(E), calcu-lated by averaging over KS eigenvalues [cf: eqn. (4)] of 40 configurations well separated in time, is presented as a func- tion of concentration in Fig.2. The gap narrows and closes as the metal concentration of the solution is increased. In Fig. 3, the optical conductivity is shown as a function of concentra- tion. At ca. 2 MPM, the extrapolated dc conductivity is very small (recall that the metal-insulator transition occurred at ca. 4 MPM). However, at ca. 9 MPM, a dc conductivity of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 LV I OEb!6'' IOi4' ' $!2' ' ' !I ' ' "012' 'Oi4"I EIeV Fig. 2 Single-particle electronic density of states, N(E), calculated by averaging over the Kohn-Sham eigenvalues of 40 configurations well separated in time using model 11. The Fermi energy defined as the eigen energy of the highest occupied orbital has been set to zero for each configuration.(a)2, (b)9 MPM. I-k 12345 E/eV EP Fig. 3 Optical conductivity of caesium-ammonia solutions at 260 K. (a)2, (b)9 MPM. (-) model 11, (---) model 1. about 2000 R-' cm-' is obtained by extrapolating our results to zero frequency, which is in reasonable agreement with the experimental value, 1500 R-' cm-'.' As studied above, at both 2 and 9 MPM, the Cs+ ions are, always, solvated by the ammonia and isolated from close contact with the electron density. The Csf-N radial distribu- tion function, which is presented in Fig. 4 (solid curve), indi- 4 6 8 10 12 r/A Fig. 4 Radial distribution functions for caesium-ammonia solution at ca. 9 MPM using model 11. (-) Cs+-N and (---) Cs+-Cs+ g(r) functions. J. CHEM. SOC.FARADAY TRANS., 1994, VOL. 90 cates that the first solvation shell for Cs' consists of about eight ammonia molecules. It was found that the local many- body polarization effects reduce the coordination number of Cs+ by about one ammonia molecule. The Cs+-Cs+ radial distribution function for caesium- ammonia solution at ca. 9 MPM is also shown in Fig. 4 (dashed curve). The first peak at about 4.5 A is a contact ion pair and the second peak at about 9 A is a solvent-separated ion pair. The area under the first peak gives, on average, one contact ion pair per Cs' ion, which suggests that the Cs' ions are dimerized. It remains to be ascertained whether or not this is a real effect or an artifact arising from neglect of the polarization of the Cs+ cation.Conclusion In summary, CP-LSDA calculations have been performed on caesium-ammonia solutions for a wide range of concentra- tions using two different microscopic models.16 In model I, the ammonia molecules are treated via a classical point- charge model and the cations as a positive neutralizing back- ground. In model 11, the ammonia solvent is made fully polarizable and the cations, here caesium, are explicitly included. The results were used to determine the role of the cations in the solutions and to assess finite size effects in our studies. The effect of the counter ions on the solutions was deter- mined by a comparison of the two models described above. At low concentrations (1 and 2 MPM), only minor differences were found.However, the effect of the counter ions was quite visible at high concentrations. Explicitly including the Cs + cations increases the metallic character of solutions at ca. 9 MPM. Specifically, the 'energy gap' decreases by a factor of seven and the extrapolated dc conductivity increases by about a factor of three. In fact, the optical conductivity calcu- lated for the system with the explicit ions is in reasonable accord with e~perirnent,'.~~ a finding which adds credibility to the present calculations. Finite size effects at low concentrations were examined using the most detailed model. The bipolarons at ca. 0.5 MPM are very similar to those at ca. 1 MPM. Essentially, no difference was found between studies at 1 MPM using a simulation cell with 256 ammonia molecules plus two excess electrons and a larger cell with 512 ammonia molecules and four caesium atoms.We would like to thank John Shelley for indispensable help in creating the three-dimensional graphics. Also, Z.D. thanks Cray for a Research Fellowship and G.J.M. thanks NSF for a Postdoctoral Research Associateship in Computational Science and Engineering (ASC 90-08812). The research described herein was supported by the National Science Foundation under CHE 92-24536. Some of the computations benefited from use of facilities provided by NSF/DMR 91- 20668. The bulk of the computing was performed at the Pitts- burgh Super Computing Center under grant MCA 933020. 201 3 References 1 J. C. Thompson, Electrons in Liquid Ammonia, Oxford Uni- versity Press, London, 1976.2 Physics and Chemistry of Electrons and Ions in Condensed Matter, ed. J. Acrivos, D. Reidel, New York, 1984. 3 Metal-Ammonia Solutions, ed. G. Lepoutre and M. J. Sienko, W. A. Benjamin, New York, 1963. 4 Electrons in Fluids, ed. J. Jortner and N. R. Kestner, Springer- Verlag, Berlin, 1973. 5 Colloque Weyl IV, J. Phys. Chem., 1975,79,2789. 6 Colloque Weyl V, J. Phys. Chem., 1980,84, 1065. 7 Colloque Weyl VI, J. Phys. Chem., 1984,88,3699. 8 Colloque Weyl VII, J. Phys. ZV,1991, 1. 9 N. F. Mott. Metal-Insulator Transition, Taylor & Francis, London, 2nd edn., 1990. 10 J. Jortner and M. H. Cohen, Phys. Rev. B, 1976,13, 1548. 11 K. Rapcewics and N. W. Ashcroft, Phys.Rev. B, 1991,44,4032. 12 M. Sprik, R. Impey and M. L. Klein, Phys. Rev. Lett., 1986, 56, 2326. 13 M. Marchi, M. Sprik and M. L. Klein, J. Phys. C, 1990,2, 5833. 14 Z. Deng, G. J. Martyna and M. L. Klein, Phys. Rev. Lett., 1992, 68,2496. 15 G. J. Martyna, Z. Deng and M. L. Klein, J. Chem. Phys., 1993, 98, 555. 16 Z. Deng, G. J. Martyna and M. L. Klein, J. Chem. Phys., 1994, loo. 17 Z. Deng, G. J. Martyna and M. L. Klein, Phys. Rev. Lett., 1993, 71, 267. 18 M. Sprik, R. Impey and M. L. Klein, J. Am. Chem. SOC., 1986, 56,2326. 19 F. Leclerq, P. Damay and P. Chieux, J. Phys. C, 1991,5, 150. 20 G.J. Martyna and M. L. Klein, J. Phys. Chem., 1991,95, 515. 21 G. Bachelet, D. Hamann and M. Schluter, Phys. Rev. B, 1982, 26,4 199. 22 Theory of the Inhomogeneous Electron Gas, ed.S. Lundqvist and N. H. March, Plenum Press, New York, 1983. 23 R. 0.Jones and 0.Gunnarsson, Rev. Mod. Phys., 1989,61,689. 24 J. P. Perdew and A. Zunger, Phys. Rev. B, 1981,23,5048. 25 S. F. Smith, J. Chandrasekhar and W. L. Jorgensen, J. Phys. Chem., 1982,86,3308. 26 J. Tse, personal communication, 1985. 27 M. J. L. Sangster and M. Dixon, Adv. Phys., 1976,25,247. 28 E. Fois, A. Selloni and M. Parrinello Phys. Rev. B, 1989, 39, 48 12. 29 R. Car G. Galli, R. Martin and M. Parrinello, Phys. Rev. Lett., 1989,63,988. 30 F. Wooten, Optical Properties of Solids, Academic Press, New York, 1972. 31 E. J. Mele, personal communication, 1992. 32 R. Car and M. Parrinello, Phys. Rev. Lett., 1985,55, 2471. 33 D. K. Remler and P. A. Madden, Mol. Phys., 1990,70,921. 34 M. Sprik and M. L. Klein, J. Chem. Phys., 1988,89,7556. 35 M. Sprik, J. Phys. Chem., 1991,95,2283. 36 W. C. Swope, H. C. Andersen, P. H. Berens and K. R. Wilson, J. Chem. Phys., 1982,76,637. 37 G. J. Martyna, M. Tuckerman and M. L. Klein, J. Chem. Phys., 1992,97,2635. 38 G. J. Martyna and M. L. Klein, Liouville Operator Analysis of the Car-Parrinello Method, preprint, 1994. 39 M. Schlauf and R. Winter, 2. Phys. Chem., 1988,156,207. Paper 4/00759J; Received 7th February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002009

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2015-2019 Use of a Neural Network to determine the Normal Boiling Points of Acyclic Ethers, Peroxides, Acetals and their Sulfur Analogues Driss Cherqaoui, Didier Villemin* and Abdelhalim Mesbah Ecole Nationale Superieure d'lngenieurs de Caen (E.N.S.I. de Caen), I.S.M.RA., U.R.A. 480 CNRS, 6 boulevard du Marechal Juin, 14050 Caen Cedex, France Jean-Michel Cense Ecole Nationale Superieure de Chimie de Paris, 11 rue P. et M. Curie, 75005 Paris, France Vladi mi r Kvasn icka Department of Mathematics, Faculty of Chemical Technology, Slovak Technical University, 8 1237 Bra tisla va , Slova kia Models of relationships between structure and boiling point (bp) of 185 acyclic ethers, peroxides, acetals and their sulfur analogues have been constructed by means of a multilayer neural network (NN) using the back- propagation algorithm.The ability of a neural network to predict the boiling point of acyclic molecules containing polar atoms is outlined. The usefulness of the so-called embedding frequencies for the characterization of chemical structures in quantitative structureproperty studies has been shown. NNs proved to give better results than multiple linear regression and other models in the literature. NNs have recently'T2 become the focus of much attention, largely owing to their wide range of applicability and the ease with which they can handle complex and non-linear prob- lems. A leading reference book3 on the application and the meaning of NN in chemistry has recently been published in which an extensive list of references can be found.NNs have been applied to the identification of proton-NMR ~pectra,~ to the interpretation of IR ~pectra,~.~ to the prediction of 13C chemical shifts,' to the classification of mass spectra,* to the estimation of aqueous ~olubilities,~ to the determination of protein structure,".' ' to the investigation of quantitative structure-activity relationships (QSAR)' 2-'4 and to the pre- diction of chemical reactivity.' '*16 Boiling point (bp) is one of the properties used to charac- terize organic compounds. However, it may happen that this property is not available in the literature or difficult to evalu- ate experimentally. It appears obvious that the usefulness of quantitative structure-property relationships (QSPR) cannot be denied in those cases.Several method^'^,'^ for prediction of the bp of organic compounds have been described in the literature. We have recently used NNs to predict the bp of alkane~.'~These compounds were chosen because they are simple, easy to code and do not have polarized atoms nor intramolecular bonds. The goals of the current work are: (a) To provide an appli- cation of the NN theory (developed in our earlier paper") to acyclic ethers, peroxides, acetals and their sulfur analogues. (b) To show the NNs ability to predict the bp of acyclic molecules containing heteroatoms. (c) To call attention to the interest of molecular descriptors such as the embedding frequencies in the presence of hetero- atoms.(6)To compare the results obtained by an NN to those given by multiple linear regression (MLR) and to those given in the literature. Neural Networks Artificial NNs are mathematical models of biological neural systems. Three components constitute an NN : the processing elements, the topology of the connections between the nodes (vertices),20 and the learning rule. In this paper, the specific algorithm used is the back-propagation (BP) system. Its ' goal is to minimize an error function. A description of the BP algorithm was given previo~sly'~ with a simple example of application and a more extensive description can be found in other works.21,22 Embedding Frequencies In a BP NN the input layer contains information concerning the data samples under study.In chemistry this information is represented by molecular codes (molecular descriptors). In our study the molecular codes correspond to the embedding frequencie~.~~These integer entities determine to some extent the structure of acyclic compounds composed of carbon, oxygen and sulfur atoms (hydrogen atoms are ignored). Their simple graph-theoretical construction has been described in our recent p~blication.~~ Let T be a tree with vertices evalu- ated by symbols C, 0 or S. T is assigned to any acyclic molecule with skeleton composed of carbon, oxygen, and sulfur atoms. Let T' be a subtree of the tree T. T' corre-sponds to a connected cluster of atoms. The embedding frequen~y~~?~~of 7" in T,denoted by n(T, T'),is then defined as the number of appearance of the cluster T' in the tree- molecule T.In Table 1, 20 clusters used in the construction of input activities, are listed. The input activities correspond to 20 embedding frequencies assigned to these clusters, di= n(T, TJ, for i = 1, 2, . .., 20, where T formally treated as a tree corresponds to a molecule determined by these 20 descriptors. Examples of descriptors for three molecules are listed in Table 2. Table 1 List of 20 clusters used for the construction of embedding frequencies no. cluster no. cluster ~~ 1c 20 4 c-c 5 c-0 7 0-0 8 S-S 10 c-c-0 11 c-0-c 13 C-C-S 14 C-S-C 16 C-C-C-C 17 C-(C)3" 19 C-C-(C)3' 20 C-(C)4" no.3 6 9 12 15 18 cluster s C-S c-c-c0-c-0 S-C-S C-C-C-C-C 17: isobutyl; 19: isopentyl; 20: neopentyl. 2016 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Three examples of 20 descriptors assigned to acyclic mol- Table 3 (continued) ecules no. name bpcrp bPprcd 55 methyl tert-pentyl ether 86.30 80.03 6.27 56 1,2-dimethylpropyl methyl ether 82.00 85.08 -3.08 dimethyl 2 2 0 0 2 0 1 0 0 0 57 1,l-diethoxyethane 103.00 103.38 -0.38 58 l,l-dimethoxy-2-methylpropane 103.50 103.49 0.01peroxide 0 0 0 0 0 0 0 0 0 0 59 2-ethoxy-2-methoxypropane 96.00 96.67 -0.67 dipropyl 6 0 1 4 0 2 0 0 2 0 60 1,l-diaethoxybutane 112.00 114.82 -2.82 61 1-methoxy- 1-propoxyethane 104.00 107.02 -3.02sulfide 0 0 2 1 0 0 0 0 0 0 62 1,4-dimethoxybutane 132.50 131.04 1.46 dibutyl 8 0 2 6 0 2 0 1 4 0 63 1,2diethoxyet hane 123.50 120.49 3.01 disulfide 0 0 2 0 0 2 0 0 0 0 64 1,3-dimet hoxybutane 120.30 123.54 -3.24 65 methyl pentyl sulfide 145.00 146.94 -1.94 66 butyl ethyl sulfide 144.20 143.05 1.15 67 dipropyl sulfide 142.80 142.02 0.78 68 isopropyl propyl sulfide 132.00 131.01 0.99Method 69 ethyl isobutyl sulfide 134.20 132.84 1.36 137.00 138.59 -1.59The set of 185 compounds (Table 3) used in the present paper 70 isopentyl methyl sulfide 71 methyl 2-methylbutyl sulfide 139.00 138.21 0.79has been studied by Balaban et This set essentially con- 72 sec-butyl ethyl sulfide 133.60 131.66 1.94 sists of two basic types of molecules: (1) Acyclic ethers, per- 73 tert-butyl ethyl sulfide 120.40 116.57 3.83 74 diisopropyl sulfide 120.00 119.84 0.16oxides and acetals (73 ethers, 17 diethers, 21 acetals and 6 137.00 135.40 1.60peroxides).(2) Acyclic sulfide, disulfide and thioacetal (45 sul-75 1-ethylpropyl methyl sulfide 76 dipropyl disulfide 195.80 191.77 4.03 fides, 6 bis-sulfides, 4 thioacetals and 13 disulfides). 77 diisopropyl disulfide 177.20 176.05 1.15 78 sec-butyl ethyl disulfide 181.00 185.93 -4.93 79 isopropyl propyl disulfide 185.90 185.14 0.76 80 tert-butyl ethyl disulfide 175.70 172.93 2.77Table 3 Compounds studied with their experimental (exp) bps, pre- 81 1,l-bis(ethy1thio)ethane 186.00 185.68 0.32 dicted (pred) bps and corresponding residuals (res) (all in "C) 82 1,2-bis(ethylthio)ethane 211.00 210.96 0.04 ~~ 83 hexyl methyl ether 125.00 122.66 2.34 no. name bpcxp bPprcd 84 ethyl pentyl ether 118.00 115.99 2.01 85 butyl propyl ether 117.10 117.97 --0.87 dimethyl ether -23.70 -4.80 -18.90 86 butyl isopropyl ether 107.00 106.03 0.97 dimethyl peroxide 14.00 9.8 1 4.19 87 isobutyl propyl ether 102.50 106.13 -3.63 dimethyl sulfide 37.30 40.64 -3.34 88 ethyl isopentyl ether 112.00 108.48 3.52 dimethyl disulfide 109.70 112.31 -2.61 89 tert-butyl propyl ether 97.40 92.68 4.72 ethyl methyl ether 10.80 7.50 3.30 90 2,2-dimethylpropyl ethyl ether 91.50 97.97 -6.47 ethyl methyl peroxide 39.00 39.24 -0.24 91 tert-butyl isopropyl ether 87.60 87.88 -0.28 dimethoxymethane 42.00 36.41 5.59 92 ethyl 1-methylbutyl ether 106.50 103.06 3.44 ethyl methyl sulfide 66.60 66.95 -0.35 93 ethyl tert-pentyl ether 101.00 98.75 2.25 ethyl methyl disulfide 135.00 134.93 0.07 94 1,2-dimethylpropyl ethyl ether 99.30 104.49 -5.19 bis(meth ylt hio)met hane 148.50 150.48 -1.98 95 ethyl 1-ethylpropyl ether 90.00 105.45 -15.45 methyl propyl ether 40.00 35.63 4.37 96 dipropoxymethane 137.00 134.89 2.1 1 diethyl ether 34.60 34.98 -0.38 97 2,2-diethoxypropane 114.00 109.58 4.42 isopropyl methyl ether 32.00 31.41 0.59 98 1 -ethox y-1 -propoxyet hane 126.00 122.16 3.84 diethyl peroxide 63.00 58.15 4.85 99 1,l-diethoxypropane 124.00 122.39 1.61 isopropyl methyl peroxide 53.50 59.66 -6.16 100 1,3-diethoxypropane 140.50 139.22 1.28 ethoxymethoxyethane 67.00 69.19 -2.19 101 1,5-dimethoxypentane 157.50 152.06 5.44 1,l -dimethoxyethane 64.40 64.48 -0.08 102 1-ethoxy-4-methoxybutane 146.00 146.66 -0.66 1,2-dimethoxyethane 84.70 74.53 10.17 103 1,4-dimethoxypentane 145.00 143.88 1.12 methyl propyl sulfide 95.50 94.82 0.68 104 1,3-dimethoxypentane 141.00 144.78 --3.78 diethyl sulfide 92.00 90.89 1.11 105 hexyl methyl sulfide 171.00 169.07 1.93 isopropyl methyl sulfide 84.40 88.01 -3.61 106 butyl propyl sulfide 166.00 166.13 -0.13 diethyl disulfide 154.00 152.98 1.02 107 isobutyl propyl sulfide 155.00 155.18 -0.18 1,l -bis(methylthio)ethane 156.00 152.97 3.03 108 isobutyl isopropyl sulfide 145.00 147.67 -2.67 ethylthiomethylthiomethane 166.00 167.16 -1.16 109 ethyl 2-methylbutyl sulfide 159.00 153.68 5.32 1,2-bis(methylthio)ethane 183.00 187.30 -4.30 110 tert-butyl propyl sulfide 138.00 139.41 -1.41 butyl methyl ether 70.30 72.38 -2.08 111 sec-butyl isopropyl sulfide 142.00 144.8 1 -2.81 ethyl propyl ether 63.60 62.14 1.46 112 ethyl isopentyl sulfide 159.00 154.27 4.73 ethyl isopropyl ether 52.50 54.75 -2.25 113 butyl isopropyl sulfide 163.50 154.47 9.03 isobutyl methyl ether 59.00 61.79 -2.79 114 1,3-bis(ethylthio)propane 229.50 225.03 4.47 sec-butyl methyl ether 59.50 65.29 -5.79 115 dibutyl ether 142.00 142.18 -0.18 tert-butyl methyl ether 55.20 53.56 1.64 116 isopentyl propyl ether 125.00 130.53 -5.53 diet hoxymethane 88.00 92.65 -4.65 117 butyl isobutyl ether 132.00 129.77 2.23 2,2-dime thoxypropane 83.00 77.85 5.15 118 butyl sec-butyl ether 130.50 130.10 0.40 1,3-dimethoxypropane 104.50 105.09 -0.59 119 butyl tert-butyl ether 125.00 1 15.23 9.77 lethoxy-2-methoxyethane 102.00 104.25 -2.25 120 sec-butyl isobutyl ether 122.00 122.66 -0.66 1,2-dime thoxypropane 92.00 99.99 -7.99 121 1,3-dimethylpentyl methyl ether 121.00 133.12 -12.12 ethyl isopropyl sulfide 107.40 106.47 0.93 122 diisobutyl ether 122.20 119.50 2.70 butyl methyl sulfide 123.20 124.26 -1.06 123 isobutyl tert-butyl ether 112.00 115.69 --3.69 isobutyl methyl sulfide 112.50 114.89 -2.39 124 di-tert-butyl ether 106.00 113.24 -7.24 ethyl propyl sulfide 118.50 116.96 1.54 125 isopropyl tert-pentyl ether 114.50 114.79 -0.29 tert-butyl methyl sulfide 101.50 102.34 -0.84 126 heptyl methyl ether 151.00 148.31 2.69 ethyl propyl disulfide 173.70 174.22 -0.52 127 1-ethylpropyl propyl ether 128.50 126.77 1.73 ethyl isopropyl disulfide 165.50 165.57 -0.07 128 di-tert-butyl peroxide 109.50 101.08 8.42 bis(ethy1thio)methane 181.00 183.98 -2.98 129 1,1 -diisopropoxyethane 126.00 130.9 1 -4.9 1 methyl pentyl ether 99.50 97.25 2.25 130 1,1 -dipropoxyethane 147.00 141.33 5.67 ethyl butyl ether 92.30 93.88 -1.58 131 1,3-dirnethoxyet hane 158.00 157.66 0.34 dipropyl ether 90.10 89.07 1.03 132 2,4-dimethoxy-2-methylpentane 147.00 146.48 0.52 isopropyl propyl ether 80.20 79.21 0.99 133 1,4-diethoxybutane 165.00 157.69 7.31 ethyl isobutyl ether 82.00 84.03 -2.03 134 dibutylsulfide 188.90 187.68 1.22 isopentyl methyl ether 9 1.20 86.77 4.43 135 diisobutyl sulfide 170.00 169.06 0.94 methyl 2-methylbutyl ether 91.50 87.01 4.49 136 butyl isobutyl sulfide 178.00 177.68 0.32 ethyl sec-butyl ether 81.20 83.83 -2.63 137 di-tert-butyl sulfide 148.50 147.73 0.77 methyl 1-methylbutyl ether 93.00 85.81 7.19 138 di-sec-butyl sulfide 165.00 167.32 -2.32 diisopropyl ether 69.00 69.74 -0.74 139 butyl sec-butyl sulfide 177.00 177.83 -0.83 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2017 Table 3 (continued) Table 4 Comparison of standard error of learning (SEL) and corre- lation coefficient (R) of NNs, MLR, eqn. (l),eqn. (2) and eqn. (3) method SEL R 140 sec-butyl isobutyl sulfide 167.00 170.87 -3.87 141 heptyl methyl sulfide 195.00 191.54 3.46 3.507 0.997142 dibutyl disulfide 226.00 225.07 0.93 143 diisobutyl disulfide 215.00 216.22 -1.22 3.31 1 0.998 144 di-tert-butyl disulfide 201.00 202.29 -1.29 2.942 0.998 145 1,1-bis(isopropylt hiokthane 205.00 215.31 -10.31 2.685 0.998 146 l-ethyl-1,3-dimethylbutylmethyl 2.800 0.998 ether 151.50 154.31 -2.81 2.948 0.998 147 ethyl heptyl ether 165.50 161.94 3.56 6.350 0.992 148 butyl isopentyl ether 157.00 151.68 5.32 9.0 0.982149 tert-butyl isopentyl ether 139.00 142.02 -3.02 10.5 0.977150 butyl pentyl ether 163.00 163.76 -0.76 8.2 0.986151 1,5-dimethylhexyl methyl ether 153.50 155.94 -2.44 152 isobutyl isopentyl ether 139.00 148.07 -9.07 153 methyl I-methylheptyl ether 162.00 160.11 1.89 "3 .. . "8 is for 3 ... 8 neurons in the hidden layer. 154 methyl octyl ether 173.00 175.51 -2.51 155 2-ethylhexyl methyl ether 159.50 162.75 -3.25 According to Zupan and Gasteiger2' 'a good rule of 156 methyl 1,1,4-trirnethylpentyl ether 159.50 144.30 15.20 thumb is that the number of data values taken for training 157 3,5-dimethylhexyl methyl ether 155.50 165.14 -9.64 158 ethyl 1,1,3-trimethylbutyl ether 141.00 142.65 -1.65 should be equal to or greater than the number of weights to 159 tert-butyl tert-pentyl peroxide 126.00 136.20 -10.20 be determined in the network' (i.e.p 2 1). In this paper, six 160 1,l-dimethoxy-2,2-architectures of NN (20-x-1; x = 3, 4, 5, 6, 7, 8; i.e. p E Cl.05,dimethylpentane 164.00 145.47 18.53 161 1,l-diethoxypentane 163.00 175.06 -12.06 2.761) have been tried, and two studies have been achieved: 162 1,1 -dipropoxypropane 166.50 158.68 7.82 learning and prediction. The term learning is used when the 163 1,l-diisopropoxypropane 146.00 149.80 -3.80 NN estimates bp values for molecules in the training set.164 1,3-dipropoxypropane 165.00 185.08 -20.08 165 1,3-diisopropoxypropane 159.00 152.69 6.8 1 When it estimates bp values for molecules not included in the 166 ethyl heptyl sulfide 195.00 211.74 -16.74 training set, this is prediction.167 methyl octyl sulfide 218.00 215.22 2.78 168 bis(buty1thio)methane 250.00 257.62 -7.62 Learning169 2,2-bis(propylthio)propane 235.00 225.10 9.90 170 ethyl octyl ether 186.50 187.80 -1.30 NNs 171 ethyl 1,1,3,3-tetramethylbutyl 156.50 161.80 -5.30 In order to determine the best architecture, six different ones ether 172 bis(1-ethylpropyl) ether 162.00 160.46 1.54 have been tried (20-x-1; x = 3, 4, 5, 6, 7, 8). The criteria used 173 bis(1-methylbutyl) ether 162.00 160.46 1.54 for the comparison of the six architectures are the correlation 174 butyl 1-methylpropyl ether 170.00 173.67 -3.67 175 diisopentyl ether 173.20 168.25 4.95 coefficient (R) and the standard error of learning (SEL) 176 dipentyl ether 186.80 185.45 1.35 defined by: 177 isopropyl heptyl ether 173.00 172.71 0.29 178 heptyl propyl ether 187.00 185.97 1.03 179 isopentyl pentyl ether 174.00 176.40 -2.40 180 methyl I-methyloctyl ether 188.50 186.00 2.50 181 di-tert-pentyl sulfide 199.00 195.40 3.60 182 dipentyl sulfide 228.00 227.28 0.72 183 disopentyl sulfide 215.00 210.35 4.65 184 isobutyl 4-methylpentyl sulfide 216.00 216.52 -0.52 185 methyl nonyl sulfide 240.00 232.63 7.37 where bprnea, stands for the arithmetic mean of all N observed values of the bp.We used a network with 20 units and a bias in the input The results obtained are given in Table 4. Fig. 1 clearly layer, a variable hidden layer including bias, and one unit in indicates that the SEL goes down to a minimum correspond- the output layer. Input and output data were normalized ing to six neurons in the hidden layer. It can be seen that the between 0.1 and 0.9. The weights were initialized to random SEL increases slightly (i.e. the learning performance values between -0.5 and +0.5 and no momentum was decreases) for seven and eight neurons. That is due to the fact added. The learning rate was initially set to 1 and was grad- that the number of weights is nearly equal to the number of ually decreased until the error function could no longer be molecules in the training set.Thus, the information brought minimized. All computations were performed on an Iris Indigo (Silicon Graphics) workstation using our own programs, written in C language. 3.4 Results and Discussion In a BP NN the input and output neurons are known since 3.2 they present, respectively, the embedding frequencies and the bp of the molecules. Unfortunately, there are neither theoreti- cal results available, nor satisfying empirical rules that would . enable us to determine the number of hidden layers and of neurons contained in these layers. However, for most of the applications of NNs to chemistry, one hidden layer seems to be sufficient.For the determination of the number of hidden neurons, we have recently" discussed the usefulness of the p parameter, defined as : no. of hidden neurons number of data point in the training set Fig. 1 SEL as a function of the number of neurons in the hidden = sum of the number of connections in the NN layer 201 8 to the training set is not sufficient to train correctly the NN with the architecture 20-x-1 (x = 7 and 8). MLR The most widely used mathematical method in QSAR or QSPR is MLR. The objective of such an analysis is to find an equation that relates a dependent variable (such as the bp property) to one or more independent variables (such as molecular descriptors). The solution to the problem consists in determining the coefficients a, and the constant term a, of the following equation: bp = a, + 1aidi It is helpful to note some inherent difficulties or* MLR in particular, arising from the interdependence of molecular descriptors.In this study MLR was used to correlate bp with only 15 independent molecular descriptors (d6, d,, d,, d,, and d,, are removed). The correlation coefficient and the standard error of learning are 0.992 and 6.350, respectively. Other Models in the Literature Bps of the 185 compounds studied were correlated by .~~Balaban et ~1with chemical structures using two or three topological descriptors. Three equations were found : bp = -59.10 + 44.30 'X + 42.88Ns; R = 0.982; S = 9.0 (1) bp = -11.23 -7.21She,+ 35.04'~' -18.30TMe; R = 0.977; S = 10.5 (2) bp = -41.75 + 43.79 'X + 45.03Ns -2.905he,; R = 0.986; S = 8.2 (3) All the results given by NNs, MLR, eqn. (l), eqn.(2) and eqn. (3) are shown in Table 4. We see that in all cases the NN approach gives the best results. However, the learning abil- ities of the models are not completely comparable since the descriptors used are not the same. In this study NNs show an interesting ability to extract information about cyclic com- pounds directly from the embedding frequencies. Prediction The predictive ability of an NN is its ability to give a satisfying output to a molecule not included in the examples the NN learned. To determine that predictive ability, cross- validation has been used.In this procedure one compound is removed from the data set, the network is trained with the remaining compounds and used to predict the discarded compound. The process is repeated in turn for each com- pound in the data set. After cross-validation, the predictive ability of different networks was assessed by the standard error of prediction (SEP) and the cross-validated R2 (R,2,). Table 5 Comparison of predictive ability for NNs and MLR method SEP R:v NN3 5.223 0.988 NN4 5.152 0.988 NN5 5.102 0.989 NN6 5.946 0.985 NN7 6.215 0.983 MLR 6.710 0.98 1 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 input layer bias hidden layer d20 di W W Fig. 2 Architecture of a BP network with three layers. The configu- ration shown is 20-5-1.Table 5 shows the results obtained with five different archi- tectures and with MLR. This table shows that the NN per-formance is a function of the number of hidden neurons. NNs give a superior performance to that given by MLR. In MLR the relationship between bp and molecular descriptors is expressed by a linear combination of the contributing terms. On the contrary the NN owes its predictive ability to its non- linear power. This does not mean that the NN is a poly- nomial model but it is able to learn by example how to make predictions for cases not belonging to the training set. It can be seen that the best architecture is 20-5-1 (p= 1.67; Fig. 2). It is interesting to note the variation of the SEP according to the number of iterations. Fig.3 shows this varia- tion for the NN with an architecture 20-5-1. The learning performance of the NN increases with the number of iter- ations, but its predictive ability slowly decreases after 4000 iterations. This is known as the overtraining effect, due to a too long learning time. Indeed, the weights obtained after the overtraining contain more information specific to the training set. Therefore, prediction on the test set will not really be satisfying. Thus, when a very low error in the training set is I\ !,,,21 '>''',' ,'I. 0 2000 4000 6000 8000 10000 no. of iterations Fig. 3 Predictive ability of NN (top curve). Learning ability of NN (bottom curve). J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 2019 sought, the predictive ability of an NN is less successful. The 6 M. E. Munk, M. S. Madison and E. W. Robb, Mikrochim Acta ability to predict being an essential quality of an NN, the overtraining effect must be avoided. The full results of cross- validation for 4000 iterations and with the NN architecture 20-5-1 are gathered in Table 3. Those results are satisfying and show that the embedding frequencies are very useful 7 8 9 (Wien), 1991,11, 505. V. Kvasnicka, J. Math. Chem., 1991,6,63. B. Curry and D. E. Rumelhart, Tetrahedron Comput. Methodol., 1990, 3,213. N. Bodor, A. Harget and M. J. Huang, J. Am. Chem. SOC., 1991, 113,9480. descriptors for the compounds studied. Nevertheless, six out- liers can be seen (compounds 1, 95, 156, 160, 164 and 166 with residuals between 15 and 20°C).For dimethyl ether, a large deviation is expected because it is the only one to have a negative experimental bp. It should be noted that the NN predicted a negative value for this compound. Since the bp is one of the physical properties that are difficult to measure,’* 10 11 12 13 14 15 L. H. Holley and M. Karplus, Proc. Natl. Acad. Sci. USA, 1989, 86, 152. N. Qian and T. J. Sejnowski, J. Mol. Biol., 1988,202, 865. D. Villemin, D. Cherqaoui and J-M. Cense, J. Chim. Phys., 1993, 90, 1505. T. Aoyama and H. Ichikawa, Chem. Pharm. Bull., 1991,39,358. T. Aoyama and H. Ichikawa, Chem. Pharm. Bull., 1991,39,372. V. Simon, J. Gasteiger and J. Zupan, J. Am. Chem. SOC., 1993, the experimental bps of the other outliers may be in error.16 115,9148. D. W. Elrod, G. M. Maggiora and R. G. Trenary, J. Chem. Znf: Comput. Sci., 1990,30, 477. Conclusion 17 18 D. E. Pearson, J. Chem. Educ., 1957,28,60. R. D. Cramer, J. Am. Chem. SOC.,1980,102, 1837. This paper has discussed the use of BP NN to predict the boiling point of acyclic ethers, peroxides, acetals and their sulfur analogues. The performances of NN were compared 19 20 D. Cherqaoui and D. Villemin, J. Chem. SOC., Faraday Trans., 1994,90,97. N. Trinajstic, in Chemical Graph Theory, CRC Press, Boca Raton, FL, 1992. with those given by MLR and those of other models in the literature, and proved to be better. It is interesting to note that the performances of the NNs decrease when overtraining occurs.The embedding frequencies provide enough informa- tion to an NN for prediction of the bp of the compounds studied. The approach using the embedding frequencies is adapted to the modelling of compounds containing hetero- 21 22 23 J. L. McClelland, D. E. Rumelhart and the PDP Research Group, in Parallel Distributed Processing, ed. J. L. McClelland and D. E. Rumelhart, MIT Press, Cambridge, MA, 1988, vol. I, p. 319. J. A. Freeman and D. M. Skapura, in Neural Networks Algo- rithms, Applications, and Programming Techniques, Addison-Wesley, Reading, 1991, p. 89. R. D. Poshusta and M. C. McHugues, J. Math. Chem., 1989, 3, atoms, which is not the case for descriptors based on topo- logical in dice^.^' 24 193. D. Cherqaoui, D. Villemin and V. Kvasnicka, Chemom. Zntell. tab. Syst., in the press. 25 V. Kvasnicka, D. Cherqaoui and D. Villemin, J. Cornput. Chem., References 1 J. Zupan and J. Gasteiger, Anal. Chim. Acta, 1991, 248, 1. 2 M. Tusar, J. Zupan and J. Gasteiger, J. Chim. Phys.,. 1992, 89, 1517. 3 J. Zupan and J. Gasteiger, in Neural Networks for Chemists, VCH, New York, 1993. 4 J. U. Thomsen and B. Meyer, J. Magn. Reson., 1989,84,212. 26 27 28 29 in the press. A. T. Balaban, L. B. Kier and N. Joshi, J. Chem. Znf: Comput. Sci., 1992, 32, 237. Ref. 3, p. 263. M. Randic, Croat. Chem. Acta, 1993,66,289. M. Randic and N. Trinajstic, J. Mol. Struct. (Theochem), 1993, 284,209. 5 E. W. Robb and M. E. Munk, Mikrochim Acta (Wien), 1990, I, 131. Paper 31073296; Received 13th December, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002015

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2021-2026 2021 Ligand Exchanges between Ethanol and some Amines in Excited Mercury Complexes in the Gas Phase Shunzo Yamamoto," Toshiro Nagaoka, Yoshimi Sueishi and Norio Nishimura Department of Chemistry, Faculty of Science, Okayama University, 3-1-1, Tsushima-naka, Okayama700,Japan The Hg(6 3P,)-photosensitized luminescence of amine(AM)-alcohol(AL) mixtures and some aminoalcohols has been investigated under conditions of steady illumination at room temperature. For AM-AL mixtures, the inten- sity of luminescence at 300 nm from an HgAL* complex decreased with increasing amine pressure, while the emission intensity at 360 nm from an HgAM* complex increased upon addition of AL. For aminoalcohols only emission from exciplexes formed between Hg* and an amino group was observed.In order to explain these results, we proposed that a ligand-exchange reaction, HgAL* + AM --* HgAM* + AL, effectively occurs in these systems. It is well known that mixtures of mercury vapour and ammonia or aliphatic amines, and those of mercury vapour and water or aliphatic alcohols give sensitized luminescence at about 350 and 300 nm, respectively, when subjected to irradiation with 253.7 nm mercury resonance radiation.'-7 In spite of there having been many investigations of the mercury-photosensitized luminescence of aliphatic amines and alcohols, there have been almost no investigations of amine(AM)-alcohol(AL) mixtures. We have measured the mercury-photosensitized emission of some AM-AL mixtures to obtain a more comprehensive understanding of molecular attachment to the excited mercury atoms.We observed that for AM-AL mixtures the intensity of the sensitized luminescence of ethanol at 300 nm decreased drastically with increasing amine pressure, while the emission intensity from an excited mercury-amine complex at 360 nm increased by addition of ethanol. In a previous paper,' we have discussed briefly the possi-bility of the following ligand-exchange reaction : HgAL* + AM + HgAM* + AL. The ligand-exchange reac-tion in excited mercury complexes was first observed for the Hg-NH,-diethylamine system by Callear and Devonport. Since the emission bands for NH, and diethylamine appear at similar wavelength regions, they confirmed this reaction using the resonance flash technique.In the present system, however, the emission bands for ethanol and the amines are well separated and their intensities were easy to measure. The present paper reports the details of mercury-photosensitized luminescence of ethanol-amine mixtures. Experimental Ethylamine, propylamine, butylamine, diethylamine, ethanol, 2-aminoethanol and 3-aminopropan- 1-01 were obtained from commercial sources (G.R. grade). These reagents were used after drying and repeated trap-to-trap distillation. Amine- ethanol mixtures of various proportions were prepared by circulating gas mixtures of known amounts of ethanol and amines with a magnetically operated fan around a closed loop.The experimental apparatus and the procedures for the measurement of resonance radiation at 253.7 nm and sensi- tized luminescence were similar to those used previously. The emission spectra were obtained with a Hitachi 203 fluo-rescence spectrophotometer equipped with a Hamamatsu R-446 photomultiplier tube. The reaction cell was a 10 mm quartz cuvette with four transparent planes. The cell, contain- ing a mercury droplet, was connected to a vacuum system. The 253.7 nm resonance line from a low-pressure mercury lamp (Toshiba Electric Co., germicidal lamp) was isolated with a monochromator and used to excite ground-state mercury atoms in the cell to the 6,P, state. Sample gases were introduced to the cell and permitted to stand for a few minutes before measurement of the luminescence.All experi- ments were conducted at room temperature. The mercury-photosensitized reaction of AM-AL mixtures for product analysis was carried out at room temperature in a conventional cylindrical quartz vessel (5 cm long and 4 cm in diameter) fitted with plane quartz windows at each end. A low-pressure lamp (germicidal lamp) was used. The amounts of hydrogen, which is the only non-condensable product at 77 K, were determined volumetrically using a Toepler pump and gas burette. Results Fig. 1 shows the emission spectra obtained in the mercury- photosensitized reactions of ethanol, propylamine, and the ethanol-propylamine (1 : 1)mixture at room temperature and about 400 Pa. The shapes and position of emission bands for ethanol and propylamine are consistent with those reported previo~sly.',~As is shown in Fig.1, the ethanol-propylamine mixture gives only the emission band at 360 nm. Since the quenching rate constant of the 253.7 nm resonance line and the quantum yield for the sensitized luminescence for ethanol are similar to those for propylamine, it seems very strange that the emission from Hg*-ethanol exciplex was not observed for the mixture. Only the emission band at 360 nm was also observed for the aminoalcohols (Fig. 2). In Fig. 3, the pressure dependences of the intensities of the emission bands for ethanol, propylamine and the 1 : 1 mixture are shown. The intensities at 300 and 360 nm for pure ethanol and propylamine increase with increasing pres- sure of the substrates.The emission intensity at 360 nm increases more markedly for the mixture than that for pure propylamine, even though the partial pressure of the amine is smaller for the mixture than for pure propylamine at the same total pressure, while the emission at 300 nm is not observed over the pressure range examined in the mixture. Fig. 4 shows the pressure dependence of the intensity of the 253.7 nm resonance line. The quenching efficiency of the mixture is similar to those of ethanol and propylamine. The effect of propylamine on the emission intensity of the mercury-photosensitized luminescence of ethanol is shown in Fig. 5. The intensity decreases drastically with increasing amine : ethanol ratio.In Fig. 6, the effect of ethanol on the emission intensity of the sensitized luminescence of propyl-amine is shown. When ethanol was added to constant J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (a1 0 00 0. oo 0 0 0 0 0 0 00 0 0 0 0 00 0 0 D. 00 00 I 0. I I0 0 . 0I I lo UI. I (b1 0OO 00 0 0 0 0 0 amounts of the amine, the intensity at 360 nm increased with 0 increasing ethanol pressure. The effect of the addition of0 ethanol is more marked in the case of a smaller amount of amine. 300 350 400 wavelength/nm Discussion Fig. 1 Emission bands obtained in the Hg(3P,)-sensitized reactions The above findings cannot be explained by the superposition of (a)ethanol (0)and propylamine (0)and (b)ethanol-propylamine of the mechanisms for amines and alcohols, which are essen- (1 : 1) mixture at room temperature and at about 400 Pa 0°0t I h v) 0 c.-C 3 0 4 0 v 0 0 cr5 .-v) 0 0 Q,c 0.-C C 0 .-0 0 v) 00.-000 I I O n -wavelengt h/n m Fig.2 Emission band obtained in the Hg(3P,)-sensitized reaction of 3-aminopropan-1-01 0' I I 0 0.1 0.2 propylamine : ethanol Fig. 5 Effect of propylamine on the emission intensity for ethanol at 300 nm; Pethano]= 1333(0)and 1760 Pa (0) 10 0 \0-:5 \ n "0 500 1000 P/Pa I 1 1.o 2.0Fig. 3 Pressure dependences of the intensities of the sensitized lumi- OO ethanol : propylaminenescence at 300 and 360 nm for ethanol (a),propylamine (O), respectively, and the ethanol-propylamine (1 : 1) mixture at (m) 300 Fig.6 Effect of ethanol on the emission intensity for propylamine at and (0)360 nm 360 nm; Ppropylamine = 157 (Oh 207 (01,272 (m) and 357 Pa (0) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 tially the same as those for ammonia and One might think that association of alcohol and amine molecules through hydrogen bonding causes the observed phenomena. As mentioned above, however, the quenching efficiency of the AL-AM mixture is similar to those of ethanol and propyl- amine. Furthermore, triethylamine-diethyl ether mixtures, where hydrogen bonding is not possible, show a similar enhancement amine luminescence, although the magnitude of the increase is small.These results indicate that any special interaction between the amine and alcohol (or ether) in the ground states is not important. In a previous paper,* we proposed that the ligand-exchange reaction, HgAL* + AM + HgAM* + AL, is involved in the mercury-photosensitized reaction of amine- alcohol mixtures. The following mechanism will be applied to that case. Hg + hv(253.7 nm) + Hg* ; Hg* -+ Hg + hv(253.7 nm); Hg* + AL + Hg"; -, other reaction; Hg* + AM + Hg"; + other reaction; Hg" + AL -+ HgAL*; + other reaction; Hg" + AM + HgAM*; -+ other reaction; Hg" + AL + M -+ HgAL*; + other reaction; Hg" + AM + M -+ HgAM*; +other reaction; HgAL* -+ Hg + AL + hv(300 nm); 6k4 (15) + other reaction; (1 - w4 (16) HgAM* -+ Hg + AM + hv(360 nm) 6'k4 (17) -+ other reaction; (1 - 6')kk (18) HgAL* + AM -+ HgAM* + AL; k, (19) where Hg, Hg* and Hg" are Hg('S,), Hg(,P,) and Hg(3P,) and M is the third body. Reactions other than reaction (19) have already been proposed for the mercury-alcohol and mercury-amine systems.A steady-state treatment gives the following equation for the intensity of the sensitized luminescence of ethanol: -= ABC(1 + 2:,[AM] )I1 where, c= + (k;/k2XCAMI/CALI) + (kJk2)CMI + (~~/k~XCAMlCMI/CALl))/(1 + (k,/kJCMI} and I; and I, represent the intensities in the absence and presence of amines. The values of k2, k; , k, and kj involved in C are available in the literature. Although the values of k, and k; are also available, k,/k, and kl/ko must depend on the experimental conditions because of the imprisonment effect.We estimated them from the quenching of the resonance line by ethanol and amines. In order to obtain values of a and a' the effect of nitrogen on the intensities of the sensitized lumi- nescences of ethanol and amines was examined. Nitrogen is known to produce the 3P, state effectively from the 3P1 state of mercury.".' ' The emission intensities increase with increasing nitrogen pressure. The effect of nitrogen on the intensity of the sensitized luminescence is expressed by the following equation. This equation is derived by adding the reaction to the above mechanism. Here, I; and I, represent the intensities in the absence and presence of nitrogen; k, is the rate constant of reaction (20) (k,/cm3 molecule-' s-' = 6.3 x lo-'' ') and S represents ethanol or amines.Since k, can be neglected in high-pressure regions, eqn. (11) can be transformed into Fig. 7 shows the experimental plots for eqn. (111). The straight lines show the validity of this equation. From the slopes of the straight lines in Fig. 7 and the ratio of kN/kl, values of a (or a') can be obtained. The rate constants and other con- stants involved in A, B and C are summarized in Table 1. Eqn. (I)is transformed to The right-hand side of this equation is plotted against the aminepressure(P,,)in Fig.8. From theslopesofthestraightlines, lines, values of k, for ethylamine, propylamine and butyl- amine were obtained using the reported value of k, for ethanol, and are listed in Table 1.As is shown in Table 1, the ligand-exchange reactions in the excited mercury complexes between ethanol and some amines are very rapid and occur at about the gas-kinetic collision rate (or a little faster). k, decreases with increasing molecular complexity of the amine. This may result from the difficulty of access of the amine to the active site of the exciplex between an excited mercury atom and an ethanol molecule because of the steric hindrance of the alkyl group. Since ethanol is less strongly bound to an excited mercury atom than the amines, this ligand-exchange 1.5 1.0,0 20 Fig. 7 Effect of N, on the intensities of the sensitized luminescence of ethanol (O),ethylamine (a),propylamine (0)and butylamine ( W) 2024 J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Constants involved in eqn. (I) '' /Pa-(k l/kS' kJ10-cm3molecule-s-la k3/10-30 cm6 molecule-' s-la k,/106s-la k,/10-" cm3 molecule- ' S-' U ~ ethanol 0.063 6.0 f 1.5 8f4 8.3 f 2.8 - 0.304 ethylamine propylamine butylamine 0.040 0.051 0.097 1.8 f0.6 3.7 f0.6 4.8 f0.6 4.0 f 1.4 ca. 2 3.0 f 1.0 0.54 f0.02 1.23 f0.14 0.99 0.06 15.8 f5.3 7.5 k 2.5 4.4 f 1.5 0.354 0.490 0.288 a Data of Phillips et UI.'*~~' reaction must be considerably exothermic. This type of ligand-exchange reaction was observed in the Hg-NH,-di- ethylamine system, and a slightly smaller rate constant was rep~rted.~Exothermic ligand-switching reactions, in which H20 is replaced by NO or SO, molecules clustered to 0,-ions, were found to have large rate constants (10-'o-10-9 cm3 molecule-' s-l) for O,-(H,O),, where n = 1-4.12 The following equation for the intensity of the emission from an Hg*-amine exciplex is derived by steady-state treat- ment of reactions (1)-( 19).The ratios of the rate constants and those of other constants involved in A', B', C' and E' are all known (Table 1). We obtained the ratio of constants involved in D' by the method given in the Appendix, and we can calculate the value of D' under various conditions. The solid lines in Fig. 9 show the values calculated by using the constants in Tables 1 and 2. As is shown in Fig.9, the agreement between the observed and the calculated values is not very good for the cases of the lowest and highest amine pressures, but the calculated values reproduce the general trends observed (the emission intensity at 360 nm increases with increasing ethanol pressure (Pethanol) and the effect of ethanol addition is more marked for smaller amounts of amine). The dotted line in Fig. 9 was obtained by using 20% higher values of Bk,/y'kj and yk3/y'k;, and the dashed line was drawn by using 10% lower values (these are within the uncertainty). These lines are in good agreement with the observed values. From the above reaction mechanism, the quantum yield of the sensitized luminescences for ethanol and amines at low pressures, where reactions (1 1)-( 14) are neglected, are 1 OO 200 400 600 PethanollPa Fig.9 Experimental plots of eqn. (AI) (later) for ethanol-207 (a), (.),157= 272 (0)andpropylamine mixtures; Ppropylamine 357 Pa (m) Table 2 Values of constants and their ratios .-5QGI? ~~ ~~~~ ~~~~ @I@' 0.72"= /?I/?'= 17.2 f2.4 S/S'= 0.068 /? = 0.62-1.0 6 = 0.042-0.068 &/a' = 0.62 1 I 50 100 150 y/y' = 1.8 f0.4 = 0.036-0.058 P,,IPa 6' = 0.63-1.0 Fig. 8 Experimental plots of eqn. (IV) for ethanol-thylamine (O), ethanol-propylamine (0)and ethanol-butylamine (0)mixtures a Data of Phillips.' J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Intercepts and slopes of straight lines of Fig. A1 157 207 272 357 intercept 7.3 5.3 3.8 2.8 slope/Pa - 0.0443 0.0310 0.0212 0.0155 (intercept/slope)/Pa 165 171 179 181 expressed as follows : Q, = ap6 cp’ = ,’pa’ We can calculate the ratio of 6/6’ using the reported values of Q, and 0’ and the ratios of ct/cr’ and p/p’ obtained here.This value is also shown in Table 2. Since 0 < p, 6, p‘, and 6’ < 1, the values of p, /?’,6 and 6’ were estimated and are listed in Table 3. The quantum yields of the sensitized luminescence of ethanol and propylamine are low and very similar to one another, but the step which lowers the quantum yield for ethanol is different from that for propylamine. That is, the step for ethanol is the process of emission from the exciplex, while that for propylamine is the formation process of the exciplex.For the amine-ethanol mixture, the amine exciplex is effectively produced uia the ethanol exciplex by the ligand- exchange reaction. This formation process of the amine exci- plex is much more efficient than its direct formation from an excited mercury atom and an amine molecule. A similar ligand-exchange reaction must occur for aminoalcohols. Callear and Connor measured relative intensities of the HgH,O* and HgNH: emissions from the mercury-photosensitized reaction of H,O-NH, mixtures.’, They observed that the ratio of the intensities, I(NH3)/I(H20), is proportional to [NH,]/[H,O] and explained this relation in terms of the competition between NH, and H,O in the removal of Hg(,P,).Since the quantum yield of the lumines- cence of NH, is very large (close to unity3), the emission from the HgNHj exciplex is not enhanced by the addition of water. This is different from the findings for the amine- alcohol mixtures. From the variation of the luminescence efficiency with the structure of the amine, Newman et aL5 concluded that the main process competing with the luminescence is the abstraction of an a-hydrogen atom. They also pointed out that for secondary and tertiary amines the absence of the sen- sitized luminescence is due to the abundance of a-hydrogen atoms or steric effects during collisions with excited mercury atoms. Indeed, we could not observe any emission band in the wavelength region around 360 nm for pure diethylamine and triethylamine.However, when ethanol was added to the amines, emission bands, whose shapes and positions are very similar to those for some primary amines, appeared at around 360 nm, and their intensities increased with increas- ing ethanol pressure. As mentioned above, the additional process that becomes possible upon addition of ethanol effec- tively produces amine exciplexes ; that is, the ligand-exchange reaction. Since the excitation energy of the exciplex (HgAL*) is smaller than that of an excited mercury atom [Hg(3P1) and Hg(3Po)], the amine decomposition seems to be suppressed. Fig. 10 shows the effect of ethanol on the yield of hydrogen in the mercury-photosensitized decomposition of amine+thanol mixtures. The yield of hydrogen decreases with increasing AL: AM ratio.This result is in accord with expectation based on the hypothesis mentioned above. Appendix Since [MI = [AL] + [AM], eqn. (V) can be modified as follows: The left-hand side of eqn. (AI) is plotted against [AL] in Fig. Al. Straight lines are obtained when [AM] is constant. This shows that the third term of the denominator can be neglected. The intercepts and slopes of the straight lines and the intercept/slope ratios are shown in Table 3. The intercept/ slope ratio is given by (Pk2 + yk,[AM])/yk,. As is shown in Table 3, this ratio is almost independent of [AM], thus demonstrating that the term yk,[AM] can be neglected. Therefore, the intercept and slope can be expressed as follows; Yk3 (AIII)slope = P’k; + y‘k;[AM] From the relationship between the intercept and slope and the amine pressure, values of Bk2/y’kj, P’k;/yk,, /3k2/Fk; and yk3/y’k; were obtained and are listed in Table Al.P/p’ and y/y’ can be determined from /3k,/pk; and yk,/y’kj obtained here and the reported values of k,, k;, k, and k; . P/P’ and y/y’ are 17.2 & 2.4 and 1.8 f0.4. 20 I U 0 1.o 2.0 I [ethanol]/[AM] 200 400 600 Fig. 10 Effect of ethanol on the yield of hydrogen from the Pethanom mercury-sensitized reactions of diethylamine (0)and propylamine Fig. A1 Experimental plots of eqn. (V) for ethanol-propylamine = 157 (O),207 (O),272 (0)(0) mixtures; Pethano, and 357 Pa (D) 2026 J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 Table A1 Ratios of rate constants and other constants 3 4 R. H. Newman, C. G. Freeman, M. J. McEwan, R. F. C. Clar- idge and L. F. Phillips, Trans. Faraday SOC.,1970,66,2827. A. B. Callear, J. H. Connor and J. Koskikallio, J. Chem. SOC., (bk,/yk,)/Pa = 174 f9 (bk,/y’k;)/Pa = 1230 k250 y‘k;/yk, = 0.14 f0.03 (/?’k;/yk,)/Pa = 6.3 f0.8 /Yk;//?k, = 0.036 f0.005 The equation for D’ can be rewritten as follows: 5 6 7 8 9 Faraday Trans. 2, 1974,70, 1542. R. H. Newman, C. G. Freeman, M. J. McEwan, R. F. C. Clar- idge and L. F. Phillips, Trans. Faraday SOC.,1971,67, 1360. C. G. Freeman, M. J. McEwan, R. F. C. Claridge and L. F. Phillips, Trans. Faraday SOC., 1970,66,2974. C. G. Freeman, M. J. McEwan, R. F. C. Claridge and L. F. Phillips, Trans. Faraday SOC.,1971,67, 67. S. Yamamoto, T. Nagaoka, Y. Sueishi and N. Nishimura, Chem. Lett., 1992, 621. A. B. Callear and C. P. Devonport, J. Chem. Phys., 1974, 78, 3738. 10 H. Horiguchi and S. Tsuchiya, Bull. Chem. SOC. Jpn., 1971, 44, 1213. and the value of D’ under various conditions can be calcu- 11 J. Pitre, K. Hammond and L. Krause, Phys. Reo. A, 1972, 6, 1210. lated by using the ratios given in Table Al. References 12 13 D. W. Fahey, H. Bohringer, F. C. Fehsonfeld and E. E. Fergu- son, J. Chem. Phys., 1982,76, 1799. A. B. Callear and J. H. Connor, J. Chem. SOC.,Faraday Trans. 2, 1974,70, 1767. 1 L. F. Phillips, Acc. Chem. Res., 1974,7, 135. 2 A. B. Callear, Chem. Rev., 1987,87, 355. Paper 3/06022E; Received 8th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002021

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2027-2035 Some Thermodynamic Properties of Aqueous Amino Acid Systems at 288.15, 298.15, 313.15 and 328.15 K: Group Additivity Analyses of Standard-state Volumes and Heat Capacities Andrew W. Hakin,* Michelle M. Duke, Jocelyn L. Marty and Kathryn E. Preuss Department of Chemistry, University of Lethbridge,4401 University Drive, Lethbridge, Alberta , Canada TIK 3M4 Densities and heat capacities have been measured for aqueous solutions of L-aspartic acid, L-glutamic acid and a-aminobutyric acid at 288.15, 298.15, 313.15 and 328.15 K. These data have been used to calculate apparent molar volumes, V,,,, and apparent molar heat capacities, Cp,,,,2 which in turn have been used to calculate standard-state volumes, Vg, and standard-state heat capacities, Ci, ,.Helgeson, Kirkham and Flowers equa- tions, for neutral organic species in water, have been used to model the calculated standard-state volumes and heat capacities of the amino acids as a function of temperature at constant pressure. These data, and data previously reported for amino acid systems, have been used as input for a group additivity type analysis. The merits of the additivity scheme are discussed, and attempts are made to interpret the predicted trends in the group contributions as a function of temperature. For several years we have been involved with experimental research projects involving the chemistry of Canada's oil sand deposits.'.2 These projects have coincided with our interests in the field of thermodynamics.Indeed, we have often used our knowledge of chemical thermodynamics to elucidate the complicated chemistry of the oil sand deposits and the chemistry of oil extraction processes. The current paper reflects these described interests and, in part, was stimu- lated by a paper from Shock3 in which the stability of aqueous peptide systems was investigated at elevated tem- peratures. Several authors4*' have discussed the role of micro-organisms in the biodegradation of oil deposits. The geology of these deposits dictates that any indigenous life forms must survive a wide range of temperature and pressure conditions. To understand how these organisms are able to exist in such a hostile environment requires the investigation of the ther- modynamic properties of aqueous solutions of organic species which are essential to life, namely aqueous protein systems.Accepting the complexity of such an approach, we have commenced our investigations by focusing our attention on the simple building blocks of protein systems, i.e. aqueous solutions of amino acids and simple peptides. Thermodynamic data reported in the literature usually refer to the standard conditions of 298.15 K and 1 atmo- sphere.6-' * This paper reports densities, p, apparent molar volumes, V2,+, and apparent molar heat capacities, Cp,2, ,, for L-aspartic acid, L-glutamic acid and a-aminobutyric acid in water at 288.15, 298.15, 313.15 and 328.15 K. In addition, we have combined standard-state volume and heat capacity data for L-aspartic acid, L-glutamic acid and a-aminobutyric acid with standard-state thermodynamic data for aqueous amino acid systems reported in our previous to form a thermodynamic data base that covers our experimen- tal temperature range.We have increased the utility of the volume and heat capacity data reported in this paper by using them as input to a semi-empirical modelling procedure proposed by Helge- son, Kirkham and Flowers (HKF).2' The HKF equations allow for the prediction of standard-state thermodynamic properties at elevated temperatures and pressures. However, in the current paper we will restrict our attention to the con- stant pressure variants of these equations. The semi-empirical nature of HKF equations dictates that estimates of standard- state properties at elevated temperatures and pressures are based upon experimental thermodypamic data collected at ambient temperature and pressure.To date, there are only very limited experimental thermodynamic data available for aqueous amino acid solutions with which to compare the estimates produced from these models. However, we are cur- rently working on the construction of a high temperature and pressure densitometer capable of making precise density mea- surements up to 730 K and 40 MPa that should, in part, address this shortfall. The HKF equations reported in this paper provide estimates of standard-state properties which will serve as useful reference points in the future evaluation of this instrument. Thermodynamic data reported in this paper are also used as input to a group additivity scheme that estimates the tem- perature dependences of chemical group contributions to standard-state volumes and heat capacities for aqueous amino acid systems.Although there are several examples in the literature of group additivity schemes being applied to aqueous amino acid systems, these schemes are invariably restricted to 298.15 K, with the notable exception of the work of Privalov and co-worker~.~~*~~ Experimental L-Aspartic acid (99 +%), L-glutamic acid (99+ Yo),and r-aminobutyric acid (99+ YO),were obtained from Sigma Chemicals Ltd. The acids were purified by repeated rec-rystallization from water and were dried and stored over phosphorus pentoxide in a vacuum oven set at 50°C.The NMR spectra of the amino acids were obtained on a Briiker 250 MHz instrument using D20as an internal standard. The recorded spectra were in excellent agreement with reference spectra.24 All solutions were prepared by weight on the molality con- centration scale using bi-distilled, degassed water. Solutions were stored in sealable 125 ml Nalgene bottles. Densities were measured relative to water with a Sodev 02D vibrating tube, flow den~itometer.~' This instrument routinely measures densities with part-per-million precision. Volumetric heat capacities were measured relative to water using a Picker flow microcalorimeter.26 Small residual heat leaks were corrected for by a calorimetric heat loss correction factor, or f factor.27 This factor was evaluated as J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 f= 0.996 727 2 using standard solutions of sodium chloride in Specific heat capacity data for the aqueous acid systems water. The reported value is assumed to be independent of were calculated from heat capacity ratios, [cpp/(ci,lpy) -13, temperature. at each temperature investigated. Apparent molar heat capac- Details of the densitometer and the flow microcalorimeter, ities, Cp,2, +, were calculated from specific heat data using the and procedures used in their calibration, have been described equation: previously.' 9320,28 Results and Discussion Densities at 288.15, 298.15, 313.15 and 328.15 K for solutions where cpis the specific heat capacity of the solution, and ci, of L-aspartic acid, L-glutamic acid and a-aminobutyric acid in is the specific heat capacity of pure water at the temperature Apparent molar heat capacity data as a function water are reported in Tables 1, 2 and 3, respectively.Appar- of intere~t.~' ent molar volumes, V2,&, were calculated using the equation: of temperature are also contained in Tables 1,2 and 3. Procedures for estimating the uncertainties in apparent molar volumes, 6V2,ql , and apparent molar heat capacities, 6Cp,2, ql have been described previously.' 993' where m is the molality of the solution, M is the molecular The apparent molar volumes and heat capacities of a-weight of the solute, py is the density of pure water at the aminobutyric acid were found to be well modelled by the temperature of interest,29 and p is the density of the solution linear equation : under study.Calculated apparent molar volumes are report- (3)ed for aqueous solutions of L-aspartic acid, L-glutamic acid and a-aminobutyric acid in Tables 1,2 and 3, respectively. where Y represents the extensive thermodynamic property of Table 1 Densities, p, apparent molar volumes, V2,+, heat capacity ratios and apparent molar heat capacities, C,2. + , of aqueous solutions of L-aspartic acid at 288.15, 298.15, 313.15 and 328.15 K" rn/mol kg-Cp,2,+/JK-' mol-' 288.15 K 0.036 27 1.001 287 72.69 (0.14) -1.659 112.6 (1.9) 0.032 63 1.001 075 72.46 (0.15) -1.505 110.2 (2.2) 0.030 42 1 .000 945 72.36 (0.16) -1.403 109.9 (2.3) 0.026 70 1.000714 72.57 (0.19) -1.253 107.3 (2.6) 0.023 97 1.000 553 72.43 (0.21) -1.161 100.4 (2.9) 0.037 47 1.001 357 72.75 (0.13) -1.717 112.5 (1.9) 0.034 55 1.001 187 72.58 (0.15) -1.556 115.1 (2.0) 0.031 21 1.000985 72.62 (0.16) -1.447 109.8 (2.3) 0.027 94 1.000791 72.50 (0.18) -1.309 107.4 (2.5) 0.019 24 1.000 272 72.13 (0.26) -0.9208 101.8 (3.6) 298.15 K 0.036 27 0.999 190 74.00 (0.14) -1.467 139.5 (1.9) 0.032 63 0.998 973 73.98 (0.15) -1.340 136.8 (2.2) 0.030 42 0.998 843 73.99 (0.17) -1.260 135.4 (2.3) 0.026 70 0.998 624 73.94 (0.19) -1.104 132.7 (2.6) 0.023 97 0.998 462 74.02 (0.21) -1.000 134.3 (2.9) 0.037 47 0.999 257 74.01 (0.13) -1.520 139.0 (1.9) 0.034 55 0.999 084 74.03 (0.15) -1.411 137.9 (2.0) 0.031 2 1 0.998 890 73.96 (0.16) -1.280 136.9 (2.3) 0.027 94 0.998 699 73.91 (0.18) -1.161 134.6 (2.5) 0.019 24 0.998 189 73.67 (0.26) -0.8196 129.3 (3.6) 313.15 K 0.036 27 0.994310 75.53 (0.14) -1.298 163.7 (1.9) 0.032 63 0.994 098 75.50 (0.16) -1.214 157.7 (2.2) 0.030 42 0.993 971 75.50 (0.17) -1.063 167.2 (2.3) 0.026 70 0.993 757 75.51 (0.19) -0.942 1 165.8 (2.6) 0.023 97 0.993 607 75.22 (0.21) -0.8735 159.8 (2.9) 0.037 47 0.994 372 75.62 (0.14) -1.372 160.7 (1.9) 0.034 55 0.994 2 13 75.37 (0.15) -1.247 161.7 (2.0) 0.03 1 2 1 0.994 02 1 75.36 (0.16) -1.144 159.4 (2.3) 0.027 94 0.993 831 75.40 (0.18) -1.040 157.3 (2.5) 0.019 24 0.993 336 75.10 (0.26) -0.7273 153.7 (3.6) 328.15 K 0.032 63 0.987 545 76.53 (0.16) -0.9898 188.7 (2.2) 0.030 42 0.987417 76.64 (0.17) -0.9821 181.0 (2.3) 0.026 70 0.987 210 76.51 (0.19) -0.8963 175.2 (2.6) 0.023 97 0.987 058 76.39 (0.21) -0.7990 175.7 (2.9) 0.037 47 0.987 8 13 76.70 (0.14) -1.168 185.9 (1.9) 0.034 55 0.987 655 76.48 (0.15) -1.084 184.1 (2.0) 0.03 1 2 1 0.987 468 76.43 (0.16) -1.022 178.2 (2.3) 0.027 94 0.987 282 76.44 (0.18) -0.8 172 192.9 (2.5) 0.019 24 0.986 792 76.24 (0.27) -0.6637 170.3 (3.6) * Estimated uncertainties for V2,+ and Cp,2, + appear in parentheses.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Densities, p, apparent molar volumes, V2,@,heat capacity ratios and apparent molar heat capacities, Cp,2,4, of aqueous solutions of L-glutamic acid at 288.15, 298.15, 313.15 and 328.15 K" m/mol kg -Cp,2,4/J K-' mol-' ~ 288.15 K 0.061 30 1.002 687 88.36 (0.08) -3.012 163.4 (1.2) 0.053 74 1.002251 88.27 (0.09) -2.620 164.8 (1.3) 0.047 82 1.001 906 88.26 (0.10) -2.341 164.0 (1.5) 0.043 02 1.001 628 88.19 (0.12) -2.213 153.4 (1.6) 0.040 49 1.001 478 88.25 (0.12) -2.045 157.6 (1.7) 0.059 55 1.002 582 88.39 (0.08) -2.947 162.1 (1.2) 0.049 83 1.002 02 1 88.29 (0.10) -2.454 162.9 (1.4) 0.044 17 1.001 691 88.28 (0.1 1) -2.227 158.1 (1.6) 0.038 36 1.001355 88.19 (0.13) -1.947 156.4 (1.8) 0.033 3 1 1.001 059 88.21 (0.15) -1.699 155.5 (2.1) 298.15 K 0.061 30 1.O00 545 89.87 (0.08) -2.687 190.8 (1.2) 0.053 74 1.O00 117 89.83 (0.09) -2.373 189.5 (1.3) 0.047 82 0.999 784 89.75 (0.11) -2.210 180.7 (1.5) 0.043 02 0.999 5 11 89.72 (0.12) -1.979 181.5 (1.6) 0.040 49 0.999 366 89.73 (0.12) -1.824 185.6 (1.7) 0.059 55 1.Ooo 447 89.82 (0.08) -2.671 186.4 (1.2) 0.049 83 0.999 897 89.77 (0.10) -2.259 184.4 (1.4) 0.044 17 0.999 576 89.74 (0.1 1) -1.981 186.4 (1.6) 0.038 36 0.999 246 89.70 (0.13) -1.709 187.6 (1.8) 0.033 31 0.998 956 89.75 (0.15) -1.551 179.4 (2.1) 313.15 K 0.061 30 0.995 625 91.55 (0.08) -2.295 222.7 (1.2) 0.053 74 0.995 210 91.47 (0.09) -2.124 213.8 (1.3) 0.047 82 0.994 885 91.41 (0.11) -1.897 213.1 (1.5) 0.043 02 0.994 620 91.37 (0.12) -1.693 214.3 (1.6) 0.040 49 0.994 480 91.35 (0.13) -1.611 212.4 (1.7) 0.059 55 0.995 523 91.61 (0.09) -2.417 209.9 (1.2) 0.049 83 0.994 992 91.50 (0.10) -2.025 209.4 (1.4) 0.044 17 0.994 682 91.42 (0.12) -1.806 199.7 (1.6) 0.038 36 0.994 358 91.43 (0.13) -1.647 208.0 (1.8) 0.033 31 0.994 080 91.36 (0.15) -1.371 206.8 (2.1) 328.15 K 0.061 30 0.989 041 92.79 (0.08) -2.075 240.5 (1.2) 0.053 74 0.988 633 92.71 (0.10) -1.827 239.8 (1.3) 0.047 82 0.988 304 92.86 (0.1 1) -1.672 236.5 (1.5) 0.043 02 0.988 043 92.19 (0.12) -1.512 235.8 (1.6) 0.040 49 0.987 91 1 92.71 (0.13) -1.430 234.5 (1.7) 0.059 55 0.988 945 92.78 (0.09) -1.813 254.9 (1.2) 0.049 83 0.988 422 92.68 (0.10) -1.699 239.3 (1.4) 0.044 17 0.988 110 92.77 (0.12) -1.529 237.6 (1.6) 0.038 36 0.987 799 92.61 (0.13) -1.394 229.7 (1.8) 0.033 3 1 0.987 523 92.62 (0.15) -1.069 247.6 (2.1) 11 Estimated uncertainties for V,, and Cp,,, appear in parentheses.interest, Y&, is the value of Y2,&at infinite dilution, m is the lined by King.'6*34 This method may be summarized using molality of the solution, and S, is the experimental slope.eqn. (4): The appropriate form of eqn. (3) was fitted to our calculated Y2,+-UAY"= y; + Sum (4)V2, and C,2, & data using weighted linear least-squares regression analyses. Weights used in these analyses were cal- where a is the degree of dissociation, AFo is the difference culated as the reciprocals of the square in the uncertainties in between the standard-state thermodynamic properties of the the apparent molar volumes and heat capacities. ionized and the un-ionized species, S, is a constant, and is The acid groups on the side chains of L-glutamic and L-the standard-state thermodynamic property of the non-aspartic acid are known to undergo partial dissociation in dissociated acid.With respect to volume this equation aqueous solution. Over the investigated concentration ranges, requires estimates of the temperature dependence of A-Vo for L-glutamic acid is calculated to show a maximum of ca. 5% each acid. Owing to the unavailability of these data, we have dissociation whilst L-aspartic acid shows a maximum of ca. selected an average value for L-aspartic and L-glutamic acid 8% dissociation. Ignoring the contributions of this ionization of -11.5 cm3 mol-'. This value is based upon the volumes process could lead to the calculation of erroneous standard- of ionization of several other amino acid systems.16 For heat state properties. Correcting calculated standard-state proper- capacities, the temperature dependence of AC; for each acid ties for the ionization contribution requires acid dissociation was estimated from the reported temperature dependence of constants as a function of temperature.These data were the heats of proton i~nization.~~ Our analyses of heat capac- obtained from ref 32 and 33. Standard-state properties for the ity data do not take into account contributions made by the un-ionized (in terms of the side-chain acid group) L-glutamic temperature-dependent state of the involved equilibria or the and L-aspartic acids were calculated using the method out- related thermal effects. In all of our calculations 'relaxation' 2030 J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 Table 3 Densities, p, apparent molar volumes, V2,+, heat capacity ratios and apparent molar heat capacities, Cp,2, +, of aqueous solutions of a-aminobutyric acid at 288.15, 298.15, 313.15 and 328.15 K" lo(+ cp, 1P; -1)m/mol kg -288.15 K 0.2486 1.005 972 74.99 (0.02) -5.703 216.1 (0.3) 0.1809 1.004 139 74.92 (0.03) -4.181 215.6 (0.4) 0.1614 1.003 598 74.94 (0.03) -3.748 215.4 (0.4) 0.1445 1.003 145 74.86 (0.03) -3.397 214.0 (0.5) 0.1158 1.002 372 74.64 (0.04) -2.743 212.5 (0.6) 0.09325 1.001 718 74.88 (0.05) -2.135 217.0 (0.8) 0.06709 1.OOO 986 74.86 (0.07) -1.535 217.0 (1.1) 0.04556 1.OOO 395 74.65 (0.1 1) -1.086 213.4 (1.5) 298.15 K 0.2486 1.003 771 75.69 (0.02) -5.056 229.1 (0.3) 0.1809 1.001 964 75.70 (0.03) -3.745 228.0 (0.4) 0.1614 1.001 454 75.61 (0.03) -3.345 227.7 (0.4) 0.1445 1.000991 75.65 (0.04) -3.019 227.3 (0.5) 0.1158 1.OOO 220 75.59 (0.04) -2.447 226.2 (0.6) 0.09325 0.999 6 10 75.56 (0.05) -1.948 227.2 (0.8) 0.06709 0.998 894 75.53 (0.08) -1.421 225.9 (1.1) 0.04556 0.998 305 75.54 (0.1 1) -0.9962 223.4 (1.5) 313.15 K 0.2486 0.998 8 15 76.45 (0.02) -4.408 241.7 (0.3) 0.1809 0.997 039 76.47 (0.03) -3.214 242.0 (0.4) 0.1614 0.996 534 76.42 (0.03) -2.889 241.4 (0.4) 0.1445 0.996 092 76.39 (0.04) -2.601 240.9 (0.5) 0.1158 0.995 331 76.36 (0.04) -2.068 242.5 (0.6) 0.09325 0.994 726 76.40 (0.05) -1.678 242.3 (0.8) 0.06709 0.994 028 76.34 (0.08) -1.227 239.9 (1.1) 0.04556 0.993 451 76.35 (0.11) -0.8200 240.3 (1.5) 328.15 K 0.2486 0.1809 0.992 229 0.990 469 77.04 (0.02) 77.06 (0.03) 0.1614 0.1445 0.989 967 0.989 526 77.02 (0.03) 77.02 (0.04) 0.1158 0.09325 0.06709 0.04556 0.988 768 0.988 177 0.987 482 0.986 9 1 1 77.03 (0.04) 77.01 (0.06) 77.01 (0.08) 77.04 (0.1 1) Estimated uncertainties for V2,+ and C,2, + appear in parentheses.contribution^^^ have been assumed to be negligible. The concentration dependence of a was calculated for each acid using eqn. (5) and (6). Initial estimates of a were fed into the Davies equation,32 eqn. (5), to produce estimates of the activity coefficients, y. The computed value of y was in turn entered into eqn. (6) to produce a new estimate of a. This successive approximation procedure was repeated until consistent values of a and y were obtained.The Debye-Huckel constant, A, used in eqn. (5) is the one reported by Robinson and Stokes32 (A = 0.5115 at 298.15 K). Standard-state volumes and heat capacities for the investi- gated aqueous amino acid systems are reported and com- pared with available literature data in Table 4. Table 4 identifies the severe lack of standard-state volume and heat capacity data available in the literature for the amino acid systems investigated in this paper, especially at temperatures removed from 298.15 K. Our reported vg value for a-aminobutyric acid at 298.15 K is in excellent agreement with the data of previous authors,' '-16 with the possible exception of the early work of -3.747 253.5 (0.3) -2.840 251.4 (0.4) -2.455 253.3 (0.4) -2.08 1 256.9 (0.5) -1.845 250.5 (0.6) -1.361 256.2 (0.8) -1.064 250.9 (1.1) -0.7207 251.4 (1.5) Cohn et a1.' Also, our reported v; value at 288.15 K for this system is in good agreement with the result reported by Wadi et d.," but is slightly lower than the value of 75.06 cm3 mol -'reported by Cabani et al.' As one might expect, our reported value for the standard- state volume of L-aspartic acid at 298.15 K is larger than those reported by authors who did not account for the degree of ionization of the acid in their calculations.If the ionization correction is omitted from our calculations, then the V2,& data reported in Table 1 yield vjo2 = 73.63 cm3 mol-'. It therefore appears sensible to conclude that such a correction should not be discounted as being negligible.We also note that the ionization corrected v; value for this system at 298.15 K reported by Mishra and AhluwaliaI6 is in excellent agreement with our value. The value of vg = 71.79 cm3 mol-'reported by Jolicoeur et d6does not appear to agree with other values in the literature. We are unable to account for this discrepancy. Turning to our standard-state volume data for L-glutamic acid at 298.15 K we once again note that our reported value is larger than those reported by authors6*8 who did not account for the ionization of the acid in their calculations. However, our value is in good agreement with the ionization corrected value of Mishra and Ahluwalia.16 If the effects of ionization on the V2,&data reported in Table 2 are dis- counted then we obtain rg = 89.48 cm3 mol- '.This value is in good agreement with the value reported by Jolicoeur et J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 203 1 Table 4 Comparison of calculated P; and Ci, data with literature data T/K V;/crn3 mol-' S,/cm3 mol-2 kg" c,2/J K-' mol-' Scm/JK-' mol-2 kg" L-aspartic acid 288.15 72.84 f0.22 13.63 f6.73 98.8 f4.0 601.2 f125.6 298.15 74.78 f0.1 1 -2.09 f3.51 134.4 & 1.7 358.9 f52.7 C71.79: 74.1,' (D, L) 73.83; 74.8e] (127')313.15 75.98 0.19 7.16 k 6.02 172.3 k7.8 1.0 f242.5 328.15 77.1 1 f0.20 6.05 f6.44 180.4 k12.3 495.7 f392.4 ~-glutamic acid 288.15 88.52 f0.07 3.04 & 1.28 152.3 f5.1 269.0 f100.1 298.15 90.06 f0.10 2.50 f1.90 180.6 f5.8 210.7 f113.0 189.36,' 85.88," 89.85e] (1 77') 313.15 91.62 f0.07 4.90 f1.39 202.3 & 8.9 308.5 f177.1 328.15 92.84 f0.30 4.85 f5.98 227.0 & 12.5 384.8 f244.6 a-aminobutyric acid 288.15 74.67 f0.09 1.33 f0.47 213.4 f1.5 10.5 f7.7 [74.78,/ 75.06g] 298.15 75.51 f0.04 0.76 & 0.19 224.6 f0.5 18.5 f2.6 [76.5,h 75.85,' 75.50,' 75.63' C222.2,' 234.1' 227.2"] 75.54," 75.92,' 75.64"] 313.15 76.34 f0.03 0.50 f0.16 241.3 f0.7 1.8 f3.9 328.15 77.01 f0.02 0.13 f0.09 253.1 f2.6 0.8 f13.8 " For L-aspartic acid and L-glutamic acid S, and Sc refer to S, in eqn.(4) whilst for a-aminobutyric acid S, and Sc, refer to S, in eqn. (3). 'Ref. 6. Ref. 7. Ref. 8. Ref. 16. Ref. 17. Ref. 18. Rgf. 9. Ref. 11. j Ref. 10. Ref. 12. Ref. 14. " Ref. 13." Ref. 15. aL6 We are unable to offer an explanation for the very low effective Born coefficient for the neutral organic aqueous value reported by Millero and co-workers.* species of interest, 8 represents a solvent-dependent param- The e;, values at 298.15 K for L-aspartic and L-glutamic eter, and Q and X are defined using the derivatives of the acid, reported by Jolicoeur et uL,~were not corrected for the relative permittivity for water as a function of temperature ionization of the acids. If the Cp,2, data reported in Tables 1 and pressure, eqn. (9) and (10): and 2 for L-aspartic acid and L-glutamic acid are treated in a similar fashion, then we obtain values of ci, = 120.21 J K-' Q=:($) T (9)mol-' and ci, = 172.79 J K-' mol-',respectively. These values are in reasonable agreement with those reported by In E aIn E Jolicoeur et aL6 Literature value^'^^'^^'^ for the standard- state heat capacity of a-aminobutyric acid at 298.15 K are in good agreement with the value that can be calculated from Estimates for Q and X,contained in Table 5, were calculated the Cp,2, data reported in Table 3.at 1 atm and the temperature of interest using a BASIC com-Equations of state for standard-state heat capacities and puter program that followed the procedures described by volumes are reported in the literature in the form of revised Helgeson and Ki~-kharn.~'*~~ Values for 0,c,c1 and c2 were HKF equations for neutral organics in The data obtained by fitting eqn. (7) and (8) to our standard-state data reported in Table 4 have been modelled using constant pres- using multiple regression analysis procedures.To remain sure variants of these equations: consistent with investigations of Shock and Helgeson, we have used the effective Born coefficients reported in ref. 36 V; = 0 + -5: -u),Q (7) and have set 8 = 228 K. The results of our fitting procedures T-8 are reported in Table 5. c;,2 = C' + -c2 + weTX (8) Analysis of and cp,Data for Amino Acids in Water (T-el2 Standard-state volume and heat capacity data reported in where 0,t, c1 and c2 are fitting parameters, o,defines the this paper comprise contributions which shed light on the Table 5 Calculated parameters for eqn. (7) and (8) for aqueous solutions of aspartic acid, glutamic acid and a-aminobutyric acid (4 amino acid 0/cm3 mol-' &m3 K mol-' c,/J K-' mol-' c,/105 J K-' mol-' wJ105 J mol-' " aspartic acid 81.43 f 0.74 -574.91 f53.88 210.41 f8.10 -4.670 f0.4231 -2.049 74 glutamic acid 96.34 f0.15 -571.79 f11.48 230.27 f5.88 -3.8551 f0.3071 -3.044 28 a-aminobutyric acid 79.00 f0.10 -311.10 f7.43 256.57 f4.07 -2.1365 f0.2125 -1.536 36 288.15 6.49 x 10-7 -3.16 x 10-7 298.15 6.69 x 10-~ -3.14 x 10-~ 313.15 7.17 x 10-7 -3.12 x 10-~ 328.15 7.88 x 10-7 -3.17 x 10-7 " Data obtained from ref.36. solvation of the solute species and which give us structural information. The HKF equations utilised in this paper are one type of semi-empirical modelling procedure that has been used to gain access to these contributions.Many other such models have been utilised and dis~ussed.**~~,~~ In the HKF model, the standard-state apparent molar properties defined by eqn. (7) and (8), can be subdivided into two contribution^,^' eqn. (11): Yo,= Y,"+ Y; where subscripts s and e define a non-solvation and a solva-tion contribution, respectively. Further, the non-solvation, or structural, term is taken to represent an intrinsic contribution and a contribution arising from the disruption in the solvent media caused by the presence of the solute species. The solva- tion, or electrostatic, contribution to standard-state proper- ties represents interactions between the solvent and the solute. These contributions are defined using eqn. (12) and (13): vz = -weQ c;, = weTX where we,Q and X have been defined previously.If chemical group additivity schemes are to be applied to our amino acid data then it would appear appropriate to focus our attention on structural contributions to standard- state properties. This may be done by removing the contribu- tions made by solute-solvent interactions to standard-state properties. These contributions are solute specific. Group additivity schemes constructed using this procedure offer the potential of being able to estimate the temperature depen- dence of standard-state volumes and heat capacities of aqueous solutions of peptide and protein systems. To avoid the problems of dealing with the ionic com- ponents of amino acids, previous a~thors~'-~~ have utilised additivity schemes based on neutral N-acetyl amino acid amides, cyclic dipeptides, or small zwitterionic peptides in which the ionic groups are a long way removed from the side chains of interest.The ability to construct an additivity scheme based on the volumetric and thermochemical proper- ties of simple amino acids is therefore very appealing. Group Additivity Analysis We have explored several group additivity schemes for aqueous amino acid systems. In brief, obtaining reliable esti- mates for group contributions relies on maximizing the number degrees of freedom in each analysis. In practice, this leads to some restrictions in the number of group contribu- tions and amino acids which can be analysed at one time. Also, the manner in which one separates amino acids into group contributions must be carefully considered.Our analyses are based on a multiple regression procedure that uses matrix mathematics to produce estimates of the required group contributions. In our adopted scheme the structural standard-state volumes and heat capacities are broken down into four group contributions. These contribu- tions are identified in eqn. (14): In addition, two simplifying assumption^^'-^^ are employed to decrease the number of unknowns in our analyses. These assumptions are defined by eqn. (1 5) and (16): (15) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 6 Calculated group contributions of structural components of standard-state volumes and heat capacities at 288.15, 298.15, 313.15 and 328.15 K" ~~ structural group P:/cm3 mol-' C;]J K-' mol-' 288.15 K CH(NH,)CO,H OH C0,H CH2 34.73 (0.34) 15.57 (0.11) 8.74 (0.36) 20.94 (0.36) -22.3 (4.8) 86.2 (1.5) 18.1 (5.0) -4.6 (5.0) 298.15 K CH(NH ,)CO,H OH C0,H CH2 35.54 (0.31) 15.56 (0.10) 8.74 (0.32) 21.83 (0.32) -0.4 (2.8) 83.1 (0.9) 19.3 (2.9) 9.5 (2.9) 313.15 K CH(NH,)CO,H OH C0,H CH, 36.03 (0.29) 15.69 (0.09) 8.96 (0.30) 22.40 (0.30) 20.5 (1.7) 80.1 (0.5) 26.7 (1.8) 21.8 (1.8) 328.15 K CH(NH ,)CO,H CH, OH- 36.28 (0.21') 15.85 (0.07) 8.95 (0.23) 37.5 (1.2) 77.5 (0.4) 27.2 (1.2) C0,H 22.91 (0.23) 23.4 (1.2) Calculated standard errors are contained in parentheses.For example, the non-solvation contributions to thermody- namic standard-state properties of L-threonine can be rep- resented by eqn.(17): P:(L-threonine) = P'(L-threonine) -Y~(L-threonine) Standard-state volume and heat capacity data for glycine, L-alanine, L-serine, L-threonine, L-valine, L-leucine, L-isoleucine, L-glutamic acid, L-aspartic acid and a-aminobutyric acid at 288.15, 298.15, 313.15 and 328.15 K were used as the input data set for this scheme.19920 The resultant estimates of group contributions to volumes and heat capacities at each tem- perature are reported in Table 6 together with their calcu- lated standard errors. Tables 7 and 8 compare experimental standard-state volumes and heat capacities with those which can be calcu- lated for our data set of ten amino acids. The differences between the experimental and the calculated values are similar in magnitude to the estimated uncertainties in our standard-state properties, and therefore afford us a high degree of confidence in our analysis procedures.In comparing our group contributions to those obtained in other studies, we caution the reader that the present paper looks at structural group contributions to standard-state heat capacities. In other words, the y: contribution to of eqn. (11). In previous investigations, Yg itself was used for group contributions and so any comparisons that we make inher- ently assume that contributions made by Yz to for each group are small. We have been unable to calculate the elec- trostatic contributions of each group that we investigated.However, based on the magnitude of calculated 7: values for the amino acids our assumption appears valid, at least to a first approximation. Group contributions to standard-state heat capacities of aqueous organic species have been calculated by several However, with the exception of the temperature-dependent study of Makhatadze and Priva10v~~ these studies are restricted to the standard temperature of 298.15 K. Hedwig et report group contributions to standard-state heat capacities at 298.15 K. These values were J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 7 Comparison of experimental and calculated standard-state volumes (cm3 mol-') at 288.15, 298.15, 313.15 and 328.15 K T = 288.15 K T = 298.15 K T = 313.15 K T = 328.15 K amino acid expt.calc. expt. calc. expt. calc. expt. calc. L-alanine" 59.67 58.80 60.47 59.62 61.14 60.37 61.53 60.92 L-serine" 59.70 60.02 60.66 60.86 61.63 61.78 62.22 62.28 L-threonine" 76.18 75.86 76.90 76.70 77.93 77.78 78.52 78.46 L-valineb 90.08 90.04 90.80 90.85 91.67 91.88 92.57 92.75 L-leucineb 106.81 105.97 107.73 106.78 109.00 107.97 110.18 109.03 L-glutamic acid 88.52 88.78 90.06 90.5 3 9 1.62 92.00 92.84 93.29 L-aspartic acid 72.84 72.57 74.78 74.30 75.98 75.60 77.11 76.66 L-isoleucineb 104.9 106.02 105.76 106.84 107.04 108.03 108.09 109.10 glycine" 42.48 43.14 43.26 43.97 44.01 44.58 44.5 1 44.96 a-aminobutyric acid 74.67 74.64 75.51 75.46 76.34 76.37 77.01 77.1 1 " Experimental data from ref. 19. Experimental data from ref.20. Table 8 Comparison of experimental and calculated standard-state heat capacities (J K-' mol-') at 288.15, 298.15, 313.15 and 328.15 K T = 288.15 K T = 298.15 K T = 313.15 K T = 328.15 K amino acid expt. calc. expt. calc. expt. calc. expt. calc. ~ L-alanine" 126.3 117.1 141.2 134.6 153.7 151.4 166.5 165.4 L-serine" 89.0 95.9 114.1 116.2 140.0 142.1 157.9 158.1 L-threonine" 192.8 185.9 205.3 203.2 228.5 226.4 240.2 240.0 L-valineb 294.4 29 1 .O 305.8 302.2 3 16.3 3 13.2 325.2 322.1 L-leucineb 388.1 382.4 397.5 390.6 406.8 398.8 411.9 405.5 L-glutamic acid 152.3 173.2 180.6 203.7 202.3 232.2 227.0 247.6 L-aspartic acid 98.8 77.9 134.4 11 1.3 172.3 142.4 180.4 159.8 L-isoleucineb 372.9 383.1 383.8 391.3 392.8 399.6 399.5 406.3 glycine" 15.2 29.6 37.6 50.2 57.8 70.0 76.8 86.4 a-aminobutyric acid 213.4 207.2 224.6 221.6 241.3 235.7 253.1 247.3 " Experimental data from ref.19. Experimental data from ref. 20. obtained from an analysis of several amino acids, peptides The methylene group contribution to standard-state heat and N-acetylamides. In addition, Nichols et a1." have report- capacities decreases with increasing temperature. As dis-ed group contributions to standard-state heat capacities cussed previously, with side-chain contributions,20 this trend based on an analysis of amides and other relatively simple reflects the apolar nature of the methylene group and its compounds. Group contributions reported by Makhatadze hydrophobic character. With increasing temperature the and Priva10v~~ are calculated from the partial molar heat structure-promoting ability of this group is decreased.Use of capacities of various peptides and organic compounds that the assumptions defined by eqn. (15) and (16) dictates that the model amino acid side chains. All of these authors agree that trends in the contributions of H atoms and CH, groups to a methylene group should contribute ca. 90 J K-' mol-' to standard-state heat capacities mirror that displayed by the observed standard-state heat capacities. This value is in very methylene group. Hydrophobic solvation is most dominant good agreement with our calculated value of 83.1 J K-' with the methyl group. mol-'. Also, our estimates for the contributions of a hydro- Group contributions to standard-state heat capacities for gen atom and a methyl group to observed standard-state the CH(NH2)C02H, OH and C02H groups are seen to heat capacities, 41.55 J K-' mol-' and 124.7 J K-' mol-', respectively, are in good agreement with the values reported 1oo.oo(tin ref.42, but are noticeably less than the values reported in ref. 50. It has been suggested4' that these differences can be attributed to the chemical homogeneity of the compounds 80.00 i tt rstudied in ref. 50 which leads to a non-unique set of coeffi- 60.00-Icients. Estimates of the H atom and methyl group contribu- -tions calculated in ref. 23 are also larger than those we have ,-40.00-calculated in our analysis. At present this apparent discrep- L 20.00-ancy cannot be resolved because our group-analysis scheme 7 is restricted by the number of unknowns that can be solved 2 0.00-for at one time, i.e. insufficient degrees of freedom.With an 'c, increased data base of thermodynamic data for aqueous -20.00-amino acids we should, in time, be able to modify our group I 1 I 1 Ianalysis to solve directly for the H atom and the CH, group -40.00 contributions. This would remove the requirement of the assumptions defined by eqn. (1 5) and (16). The temperature dependences of group contributions to the structural components of standard-state heat capacities are shown in Fig. 1. 2034 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I I t40.00 I I I1 our current data set is too limited and does not permit the estimation of structural contributions of the peptide group 35.004 (CONH) to standard-state volumes and heat capacities.Hence, we are currently working on extending the data sets I 30.001 t used in our analyses by measuring the volumetric and ther- mochemical properties of several other amino acid and dipeptide systems. The results of these studies will be report- ed in future publications. 15.00 A.W.H. is grateful to the Natural Sciences and Engineering 10.0 ] 0 -I-Research Council of Canada (NSERC) for operating support and to the University of Lethbridge for equipment funding. r7 u K.E.P. is grateful to NSERC for the award of an Under- 5.00.) 280.00 290.00 300.00 graduate Student Research Award. 310.00 320.00 330.00 TI References Fig.2 Temperature dependence of the group contributions to struc- 1 L. Barta, A. W. Hakin and L. G. Hepler, in Oil Field Chemistry, tural components of standard-state volumes. (0)CH(NH,)CO,H, OH,(.)(0)(*I Enhanced Recovery and Production Stimulation, ACS Symp. Ser. CH, 7 COP. 396, ed. J. K. Borchardt and T. F. Yen, American Chemical Society, Washington DC, 1989, ch. 23. A. W. Hakin and M. M. Duke, in Proceedings Fine Tails Sympo- increase with increasing temperature. These trends are indica- 2 tive of the polar nature of these groups and reflect the role of hydrophilic hydration. Although there are some variations in specific values, we note that our group contributions to standard-state heat capacities follow the same general tem- sium, ed.J. K. Liu, Fine Tails Fundamentals Consortium, Edmonton, 1993. 3 E. L. Shock, Geochim. Cosmochim. Acta, 1992,56,3481. 4 L. R. Brown, Chem. Eng. Prog., 1987,83,35. 5 I. Rubinstein, C. Spykerelle and D. W. S. Westlake, Geochim. Cosmochim. Acta, 1977,41, 1341. perature dependences as those reported by Makhatadze and ~rivalov.~~ 6 C. Jolicoeur, B. Riedl, D. Desrochers, L. L. Lemelin, R. The temperature dependences of group contributions to the structural components of standard-state volumes are shown in Fig. 2. The methylene group contribution to standard-state volumes at 298.15 K is in good agreement 7 8 Zamojska and 0.Enea, J. Solution Chem., 1986, 15, 109. L. G. Longsworth, in Electrochemistry in Biology and Medicine, ed. T.S. Hedlovsky, Wiley-Interscience, New York, 1955, ch. 12. F. J. Millero, A. LoSurdo and C. Shin, J. Phys. Chem., 1978, 82, 784. with the value of 15.9 cm3 mol-' reported in ref. 11 and is in 9 E. J. Cohn, T. L. McMeekin, J. T. Edsall and M. H. Blanchard, fair agreement with the value of 16.9 cm3 mol-' reported by Millero et a1.* The value of 16.9 cm3 mol-' may also be cal- culated from the standard-state volume data reported by Makhatadze et al.22We also note that our calculated contri- butions for the OH and C02H groups at 298.15 K are in good agreement with the values of 8.1 and 21.6 cm3 mol-' that were calculated in ref. 11 using a molecular modelling 10 11 12 13 J. Am. Chem. Soc., 1934,56,784. C. H. Spink and I. Wadso, J. Chem. Thermodynam., 1975,7,561. S.Cabani, G. Conti, E. Matteoli and M. R. Tine, J. Chem. Soc., Faraday Trans. I, 1981,77, 2377. H. D. Ellerton, G. Reinfields, D. E. Mulcahy and P. J. Dunlop, J. Phys. Chem., 1964,68, 398. J. C. Ahluwalia, C. Ostiguy, G. Perron and J. E. Desnoyers, Can. J. Chem., 1977,55,3364. procedure. Agreement is less satisfactory with the same group contributions that can be calculated from the data contained in ref. 22. The trends in these data as a function of temperature are more difficult to interpret. However, for the CH(NH2)C02H 14 15 16 17 18 G. DiPaola and B. Belleau, Can. J. Chem., 1978,56, 1827. M. R. Tine, Ph.D. Thesis, Universita di Pisa, 1977. A. K. Mishra and J. C. Ahluwalia, J. Phys. Chem., 1984,88,86. R. K. Wadi, M. N. Islam and R.K. Goyal, Indian J. Chem. A., 1990,29,1055. S. Cabani, G. Conti, E. Matteoli and M. R. Tine, J. Chem. Soc., and C02H groups the data appear to follow previously docu- mented behaviour, i.e. the volume contributions of these groups increase with increasing temperature, a feature indica- tive of hydrophilic hydration. However, the volume contribu- tion of the hydrophobic methylene group and the hydrophilic OH group are almost independent of temperature. These 19 20 21 Faraday Trans. I, 1981,77,2385. A. W. Hakin, M. M. Duke, S. A. Klassen, R. M. McKay and K. E. Preuss, Can. J. Chem., in the press. M. M. Duke, A. W. Hakin, R. McKay and K. E. Preuss, Can. J. Chem., in the press. H. C. Helgeson, D. H. Kirkham and G. C. Flowers, Am. J. Sci., 1981,281,1249. trends are not so readily explained. Once again we arrive at 22 G.I. Makhatadze, V. N. Medvedkin and P. L. Privalov, Bio- the conclusion that obtaining reliable structural information from volume data is a more difficult proposition than obtain- ing structural information from heat capacity data. This con- clusion is re-emphasised by closer inspection of the temperature dependences of group contributions to standard- state volumes which can be calculated from the data con- 23 24 25 polymers, 1990,30, 1001. G. I. Makhatadze and P. L. Privalov, J. Mol. Biol., 1990, 213, 375. C. J. Pouchert and J. R. Campbell, The AIdrzch Library ofNMR Spectra, The Aldrich Chemical Co., Gillingham, 1974, vol. 111. P. Picker, E. Tremblay and C. Jolicoeur, J. Solution Chem., 1974, 3, 377.tained in ref. 22. In the 288.15-328.15 K temperature range, data amassed by the latter authors predict volume decreases for the C02H and OH groups with increasing temperature whilst the reverse trend is shown for the methylene group with increasing temperature. Such patterns appear to contra- dict those that one would predict based on the hydrophilic 26 27 28 29 P. Picker, P.-A. Leduc, P. R. Philip and J. E. Desnoyers, J. Chem. Thermodynam., 1971,3,631. J. E. Desnoyers, C. DeVisser, G. Perron and P. Picker, J. Solu-tion Chem., 1976,5,605. A. W. Hakin, S. A. M. Mudrack and C. L. Beswick, Can. J. Chem., 1993,71,925. G.S. Kell, J. Chem. Eng. Data, 1967, 12, 66. and hydrophobic characters of these groups. One goal of our investigations into the thermodynamic properties of aqueous amino acid systems is to develop an analysis procedure that will allow us to estimate standard- state volumes and heat capacities for peptide and perhaps protein systems, as a function of temperature.Unfortunately, 30 31 32 33 G. S. Kell, in Water-A Comprehensive Treatise, ed. F. Franks, Plenum Press, New York, 1972, vol. I, pp. 363-412. G. R. Hedwig, J. Solution Chem., 1988, 17, 383. R. A. Robinson and R. H. Stokes, Electrolyte Solutions, Butter-worths, London, 2nd edn. (revised), 1965. Handbook of Biochemistry Selected Data for Molecular Biology, ed. H. A. Sober CRC Press, Boca Raton, 2nd. edn., 1970. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2035 34 E. J. King, J. Phys. Chem., 1969,73, 1220. 43 T. E. Leslie and T. H. Lilley, Biopolymers, 1985,24, 695. 35 J. W. Larson, K. G. Zeeb and L. G. Hepler, Can. J. Chem., 1982, 44 S. Cabani, G. Conti and E. Matteoli, Biopolymers, 1977, 16, 465. 60,2141. 45 K. P. Murphy and S. J. Gill, J. Chem. Thermodynam., 1989, 21, 36 E. L. Shock and H. C. Helgeson, Geochim. Cosmochim. Acta, 903. 1990,54,915. 46 G. R. Hedwig, Biopolymers, 1992,32, 537. 37 E. L. Shock, E. H. Oelkers, J. W. Johnson, D. A. Sverjensky and 47 J. J. Spitzer, S. K. Suri and R. H. Wood, J. Solution Chem., 1985, H. C. Helgeson, J. Chem. SOC.,Faraday Trans., 1992,88,803. 14, 571. 38 H. C. Helgeson and D. H. Kirkham, Am. J. Sci., 1974,274, 1089. 48 S. K. Suri and R. H. Wood, J. Solution Chem., 1986,15, 705. 39 H. C. Helgeson and D. H. Kirkham, Am. J. Sci., 1976,276,97. 49 M. J. Blandamer, J. Burgess, M. R. Cottrell and A. W. Hakin, J. 40 G. R. Hedwig, J. Chem. SOC.,Faraday Trans., 1993,89,2761. Chem. SOC.,Faraday Trans 1, 1987,83,3039. 41 D. P. Kharakoz, Biophys. Chem., 1989,34,115. 50 N. Nichols, R. Skold, C. Spink, J. Suurskuusk and I. Wadso, J. 42 G. R. Hedwig, J. F. Reading and T. H. Lilley, J. Chem. SOC., Chem. Thermodynam., 1976,8, 1081. Faraday Trans., 1991,87, 1751. Paper 4/00629A; Received 1st February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002027

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM.SOC. FARADAY TRANS., 1994, 90(14), 2037-2041 Effect of Preferential Solvation on Gibbs Energies of Ionic Transfer Anna-Kaisa Kontturi, Kyosti Kontturi and Lasse Murtornaki Laboratory of Physical Chemistry and Electrochemistry, Helsinki University of Technology, Kemistintie 1, SF-02150,Espoo, Finland David J. Schiff rin Department of Chemistry, University of Liverpool, P.O. Box 147,Liverpool, UK L693BX It is shown that the transfer potential of the Rb+ ion between water and 1,2-dichloroethane is strongly influenced by ion pairing in the organic phase. A6 initio calculations of the energy of the Rb+-TPB- ion pair for different interionic distances leads to a contact ion-pair interionic distance of 0.36 nm. The very large differences in the transfer potential of Rb+ observed between organic solutions containing tetraphenylborate (TPB-) and tetrakis(4-chlorophenyl)borate (TPBCI-) are due to preferential solvation of the phenyl rings of the p-chloro derivative.These effects are likely to be present in systems where the size of the ionic components is greater than that of the solvent. There has been, recently, a renewed interest in the measure- ment of ionic Gibbs energies of transfer between immiscible electrolyte^.'-^ This has relevance both to problems of elec- troanalysis and separation science, and for the understanding of solvation effects. The electrochemical determination of these quantities presents many advantages over conventional methods based on solubility or partition measurements. However, the indiscriminate use of voltammetric techniques can lead to serious errors.The purpose of the present work is to clarify the limitations that can be encountered in their use and to highlight the effect that preferential solvation and ion pairing have on ionic transfer potentials. Voltammetry at Liquidbiquid Interfaces Linear sweep voltammetry has been extensively used for the determination of Gibbs energies of transfer. For singly charged ions (M+), the transfer reaction considered is M+(w)+M+(o) (1) where (w) and (0)refer to the aqueous and organic phases, respectively. Two experimental approaches have been used : (a) trace-ion transfer and (b) transfer in the absence of a base electrolyte. The difference between these is whether the poten- tial window is defined by the ion being investigated or by some other more hydrophobic base electrolyte ions.These techniques are straightforward transpositions of those generally used for reactions at metallic electrodes pro- vided that the relative permittivity of the organic phase is sufficiently large to avoid ion pairing of the transferred species with base electrolyte ions. However, this is not the case for the solvents commonly employed in ion-transfer studies at liquid/liquid interfaces. The effect of ion pairing in the low relative permittivity receiving phase has been taken into account in different ways. When analysing the transfer of associated electrolytes,6 the solution of the Nernst-Planck equation must be carried out considering all species present and the corresponding ionic association equilibria.For a cation-transfer process that can lead to ion pairing, the equilibrium condition M+(o)+ A-(o)sM+A-(o) (11) must be considered in solving the diffusion and migration equations. A-is the ion-pairing anion present in the organic phase. The current measured is due to ions crossing the inter- face;6 this appears as a boundary condition in the transport equation. In the organic side of the interface the current cor- responds to the total flux of the metal ion taken as the ion constituent [ref. 4, eqn. (6) and (711, as defined in ref. 7, instead of that of free ions. This is why the flux equations describing transport in the organic phase must always be written with respect to the metal ion constituent.Although the division of the flux in the organic phase into contribu- tions due to free and ion-paired species is possible, this is of little value. The Galvani potential difference across the inter- face is, however, determined by the interfacial thermodyna- mic activities of the free ions. The distinction is important for the calculation of standard ionic-transfer potentials. The solution of the diffusional problem in the case of trace-ion transport in the organic phase and binary aqueous electro- lytes has already been pre~ented.~ From the convolution inte- the current function x(ot) can be calculated, where 5 = a[Dy)/Dy)]1/2, = zFv/RT,D is the diffusion coefficient, v is the sweep rate, ciw) is the concentration in the aqueous phase of the transferred ion and the other symbols have their usual meaning.8 is given by : where y'+") and y$) are the activity coefficients of the cation corresponding to its surface concentration on the water and oil sides of the interface, respectively; a+ is the degree of dis- sociation of the Rb+ ion pair in the organic phase and A;Qi is the starting potential; it is assumed that there is no ion pairing in the aqueous phase. The current is given by: i = F~~~)J(naD~))x(at) (3) The standard ionic-transfer potential calculated from eqn. (1)-(3), A:@:, corresponds to the ionic equilibrium (I).Great care must be taken when calculating A:@,"+ from the half- wave potential (A:Q1,2) for associated electrolyte^.'*^ What is actually measured refers always to the ionic constituent being transferred and not to the free ion.The half-wave potential corresponds to a situation where the concentration of the constituent7 is equal on both sides of the interface. This condi- tion is not given by the equality of the ionic and ion-pair contributions to the total current.' For example, for the transfer of Rb+ ion from water to 1,2-DCE containing TPB- as the base electrolyte anion, the equilibrium condition : Rb+(o)+ TPB-(o)s Rb+TPB-(o) (111) when [Rb+(o)] + [Rb+TPB-(o)] 4 [TPB-(o)] results in the following relationship between free Rb+ and its total concen- tration as a constituent: (4) where C,(x) and CRb+(x)are the total and the free ion con- centrations at a distance x from the interface, Ka is the association constant for Rb+TPB- in 1,ZDCE and C;ppB-is the bulk concentration of TPB- in the organic phase.For the trace ion case, the concentration of the free ion is directly proportional to the total concentration, since the activity coefficients will be constant. In this case, the half wave poten- tial is given by: where is the standard transfer potential of the free ion. These considerations have been used by Samec et d.,' Wandlowski et d2and Sabela et aL3 for the calculation of for a series of ions based on an internal calibration using the transfer of the tetramethylammonium ion. Although medium effects were considered, the transfer poten- tial calculations were based on the assumption that the current crossing the interface could be divided into a free and an ion-paired component, with the half-wave potential being determined by the condition that these two currents should be equal.' This appears to contradict the conditions resulting from thermodynamic considerations.Eqn. (4) implies that in the trace-ion case, the ratio of free to total Rb+ ion concen- tration is constant and therefore, the only way by which the condition proposed in ref. 1 could apply is when either the electroneutrality condition is not fulfilled or the local activ- ities are strongly altered by the transferred species. The origin of this problem is that the contribution of the free ions to the current, as the authors in ref.1 defined it, is in fact the total (measured) current. Otherwise, the total current would be larger for a higher degree of association. This is probably one of the reasons for the difference in the measured value of A:@&+ between the results in ref. 1 and 4. Another approach to the measurement of transfer poten- tials makes use of the potential limit of polarisation when the aqueous base electrolyte consists only of a salt of the metal ion. The solution to this transport problem has already been pre~ented.~An alternative approach has been discussed by Shao et a/.,' who considered the integrated form of the cyclic voltammogram for the transfer of a trace ion. A calibration curve expressing the ratio of currents in the forward and return scans as a function of the difference in switching potential and half-wave potential was used.The value of Ar@ib+ obtained, 0.475 V, was significantly different from that previously determined (0.274 V)4 by convolution voltam- metry for the binary case?. There are several reasons that could account for this dis- crepancy. First, the integrated Nicholson and Shain current function used corresponds to the trace-ion case and cannot be used for a binary aqueous electrolyte, especially at the low t The value of A:@);;,,+ given in ref. 4 corresponded actually to the Galvani potential difference between the organic solvent and water; thus, the values for A;@);;,,+ are positive. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 concentrations investigated, for which the migration terms are significant. Secondly, the calculation is based on the assumption that the double-layer capacitance has little dependence on potential at the limit of the polarisation window. This, however, is not the case' and different non- faradaic contributions at the end of the forward scan and the current peak in the return scan will introduce significant uncertainties. Thirdly, the high currents employed in ref. 5 result in very large uncompensated IR potential drops and, hence, the elementary calibration based on the idealised tracer case breaks down. The IR drop problem is inherent to transport reactions at high currents, when significant migra- tion effects result in changes in the value of the uncompen- sated resistance throughout the potential scan.Finally, no ionic association corrections appeared to have been made in the work of Shao et al.' Although in the previous work4 the currents measured were always sufficiently small to avoid the problems pre- viously discussed, the convolution analysis was carried out for the binary-ion transfer case and the aqlieous electrolyte concentrations used were always very high in order to avoid the simultaneous transfer of TPB-. The difference between the values of the transfer potentials measured by Shao and Girault' are too large to be due only to different experimen- tal approaches. The purpose of the present work was to understand the origin of the above discrepancies and to establish a reliable method for the analysis of ion-transfer potentials derived from voltammetric results.Experimental In order to distinguish between anion (TPB-) or cation (Rb+) transfers at the positive potential limit of the polarisa- tion window (A:@ > 0), a micropipette technique was used." This allows a distinction to be made between transfer of the anion from the organic to the aqueous phase from that of the aqueous cation in the opposite direction. Micropipettes were prepared from borosilicate glass capil- laries GC15OF-15 (Clark Electromedical Instruments) using a vertical pipette puller type L/M-3p-A (List-Medical, Germany). Settings in the pipette puller were adjusted to obtain tip diameters of ca.25 pm. The pipettes were used without polishing or other treatment. They were mounted in a pipette holder with a side tube (Clark Electromedical Instruments) and filled with the aqueous solution by suction through the side tube. The electrochemical cell consisted of a U-tube (3 mm id) inside which the organic solution was placed." The aqueous reference solution was placed on top of the organic phase in one branch of the U-tube and the micropipette was then immersed in the organic solution in the other. Linear voltage sweeps were applied to the aqueous reference electrode with a waveform generator (PPR1, Hi-Tek). The current was mea- sured with a current amplifier (Keithley, 428) connected to the electrode inside the micropipette and digitally recorded (Advantest R 921 1E Digital Spectrum Analyser).During the experiments, the interface was monitored with a microscope to ensure that this was located at the tip of the micropipette. The measured potentials corresponded to the cell: Ag I AgClIO.1 mol dm-3 RbCl(w), J 0.01 mol dm-3 TBATP(o) (or TBATPBCl) I mol dm-3 TBACl(w) I AgCl IAg (or LiCl(w)) The potentials were corrected to the absolute Galvani poten- tial scale as described el~ewhere.~ Tetrabutyl ammonium tetrakis(4-chloropheny1)borate (TBATPBCl) was prepared by mixing an excess of aqueous J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 15' 10; -20:-300 -200 -100 0 100 200 300 400 Ay@/mV Fig. 1 Voltammograms of (....) 0.1 mol dm-3 LiCl(w)-0.01 mol dmP3 TBATPB(o) and (-) 0.1 mol dm-3 LiCl(w)-0.01 mol dm-3 TBATPBCl(o).Sweep rate 100 mV s-'; micropipette 25 pm. (TBACl (Fluka) with an ethanol solution of KTPBCl (Lancaster Synthesis). The product was filtered and re-crystallised from acetone. Tetrabutyl ammonium tetra-phenylborate (TBATPB) was prepared as described earlier.4 RbCl, LiCl, (Merck pro analysis), TBACl (Fluka) and 1,2- DCE (Rathburn HPLC) were used without further purifi- cation. Water was purified with a millipore system (Milli-Q). The ab initio calculation of the structure of the Rb'-TPB- ion pair was carried out using Gaussian 92 (HF/STO-3G) in the Cray computer of the Centre for Scientific Computing, Helsinki. The calculation took 40 h of CPU time. The com- parison of relative sizes and orientations of 1,2-DCE and the TPBCl- anion were made with a desktop molecular model- ling package.' ' Results and Discussion In all the experiments, the aqueous phase was always present inside the micropipette.When the anion of the organic phase is transferred at the positive potential limit, spherical diffu- sion prevails in the forward sweep and linear diffusion is present during the return sweep. This results in a character- istic voltammogram showing a peak in the current on sweep reversal. If the species transferred is the aqueous cation, no such peak is observed during the return sweep. Thus, the shape of the voltammetric response is diagnostic of the nature of the ionic species being transferred. An example of the advantages of this technique can be seen in Fig.1, where the transfer of TPB- occurs in preference to that of Li+. The hemispherical diffusion due to Li+ transfer can be observed when the highly hydrophobic TPBCl- anion is employed in place of TPB -. Fig. 2 shows the voltammetric response for Rb+ transfer when TBATPB is used as the organic base electrolyte. It can be clearly seen that, contrary to reports in the literature,' the species transferred is the Rb+ ion and not the organic anion, as indicated by the absence of a peak during the reverse sweep at the positive end of the polarisation window. This is an important point that should be stressed, since it clearly shows that the convolution voltammetry technique pre-viously developed gave transfer data for Rb' and not for TPB-. In order to confirm these results, the same experiments were carried out using TPBCl- as the base electrolyte anion and the corresponding voltammograms are also shown in Fig.2. It is quite clear that in this case the species transferred is the Rb+ ion, in agreement with previous results by Shao and Gira~lt.~ The difference in A:@:,,+ in ref. 4 and 5 results /'/---20 1-400 -300 -200 -100 0 100 200 300 400 500 600 AyO/mV Fig. 2 As Fig. 1 but for RbCl(w) mainly from the different electrolytes used in the organic phase. However, the very large change in the transfer potential or Rb', ca. 210 mV, observed when the anion is changed from TPB- to TPBC1- is very unusual. If the difference were due to ion pairing of Rb+ in the organic phase, it would be very difficult to rationalise it in terms of distances of closest approach effects for a dielectric continuum model.The ion-pairing association constant is given by :' zlz2e2 K, = N, E,Eok, T Q(b)16x2 (-) with IZlZZ I e2b= 4ne, E~ k, Tr (7) where E, is the relative permittivity of the solvent, c0 is the permittivity of vacuum, z1 and z2 are the charge numbers of the cation and anion forming the ion pair, e is the electronic charge, N, is the Avogadro constant, k, is the Boltzmann constant, T is the absolute temperature and r is the sum of the ionic radii of the components of the contact ion-pair. Q(b) is an ion separation function, the values which are given in ref.12. Considering that the van der Waals radius (rVdw)of chlo- rine is ca. 0.175 nm,13 the change in the radius when TPB- is replaced by TPBCl- as the base electrolyte anion is of the order of 0.1 nm (taking rvdw for hydrogen as 0.12 nm). The hard-sphere diameter of TPB- is ca. 0.49 nm14 and, hence, the slight increase in molecular dimensions when a para-hydrogen in TPB- is replaced by chlorine cannot account for the large difference in association constant of the anion with Rb'. Certainly, there is no reason for assuming that the elec- trostatic contribution to the Gibbs energy of solvation should be altered significantly by the small change in ionic radius. The origin of this interesting effect can be understood when analysing in detail possible preferential solvation contribu- tions to the solvation energy of TPBCL-.Fig. 3 shows a molecular model of an Rb+-TPB- ion pair derived from the results of the ab initio calculation using Gaussian 92. This calculation gave a distance of closest approach between Rb' and the boron atom in TPB- of 0.36 nm. This is in reason- able agreement with crystallographic data of solid RbTPB, of 0.404 nm." The larger value in the latter case is expected due to ion-ion interactions in the crystal. As can be seen, in forming an ion pair, the Rb+ ion can approach the centre of positive charge in the molecule, the limitation being the n: Fig. 3 Minimum potential-energy configuration of the ion pair Rb+-TpB-from initiocalculations,showing the formation of the contact ion The atoms are drawn using their van der Waals radii.ring systems in the phenyl groups. This type of specific inter- action in ion-pair formation has been previously observed in solvents of low relative permittivity.' When the para-hydrogen atom in the phenyl rings of TPB- is replaced by chlorine, a large local dipole moment appears in each of the phenyl rings, of the order of 5.0 x lop3' C m.17 The C-Cl dipole moment contribution in 1,ZDCE is 4.90 x lop3' C m l8 and therefore, preferential solvation of the phenyl rings containing the dipolar C-Cl bond is expected to occur by dipoledipole interactions. Fig. 4 shows a simple visualisation of possible preferential solvation struc- tures." The configuration of the TPBCl- anion is favourable for strong short-range dipole-dipole interactions which results in the known increase of solvation energy observed when comparing TPB- with TPBCl-.From Fig. 1, it can be seen that this is at least 20 kJ mol-'. In fact, a larger differ- ence is expected since in this experiment, the positive limit is determined by Li+(w) transfer rather than by that of TPBCl-. The origin of this effect cannot be due simply to the small increase in ionic radius and the model show fn in Fig. 4 -Fig. 4 Molecular model showing a possible relative orientation of the 1,2-DCE molecules with respect to the TPBCl anion leading to large dipole-dipole interactions between the solvent and the C-Cl bond in the phenyl rings J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 clearly indicates that the increased solvation energy must be due mainly to preferential solvation of the phenyl rings by 1,2-DCE by dipole-dipole interactions. For Rb+-TPBCl- ion-pair formation to occur, the solvent molecules interacting with the C-C1 dipole in the phenyl ring must be displaced (cf:Fig. 4 and 3) and therefore the distance of closest approach will be strongly influenced by preferential solvation effects. These can be quantified by considering dis- tance effects on the ion-pairing constant. The association constant of Rb+-TPB- in 1,ZDCE has been estimated as 1700 considering E = 10.23;19 from eqn. (6) this value corre- sponds to a distance of closest approach of 0.794 nm.The radius of the Rb+ ion is 0.147 nm and from the molecular modelling results shown in Fig. 3, it is quite clear that the value of K, reported in the literature" must be wrong con- sidering the dimensions of the ion pair. The calculation of the ion-pair association constant cannot be carried out simply from the results of the ab initio calcu-lation, which is only applicable in vacuum. Although it is not feasible at present to Carry Out these CalCUlatiOnS including solvent molecules, the value obtained gives an indication of the expected distance range if electrostatic interactions over- ride anion solvation energy. Attempts to measure the associ- ation constant from the conductivity of saturated RbTPB solutions in 1,2-DCE were unsuccessful.The solubility of RbTPB in 1,ZDCE is 9.9 x mol dm-3 at 25"C,19 and with K, = 1700, this should have given a detectable conduc- tivity of K z 0.6 pS. However, K was less than the detection limit of 0.1 pS. Using eqn. (6) and (7) and r z 0.36 nm gives a value of ca. lo5 for K, ,and following the procedure described in ref. 4, A:@' = 0.360 V. However, the reason for observing the transfer of Rb+ and not that of TPB- is the very large association constant for the Rb+-TPB- ion pair. The above arguments are applicable to all cases where the size of the organic solvent is smaller than that of the ions; ion-pair association constants will be strongly influenced by local solvation effects. The authors thank Mr. Raimo Uusvuori, of the Centre for Scientific Computing, Helsinki, for carrying out the ab initio calculations.The support of the European Community, Human Capital and Mobility Programme (Contract Number ERB CHRXCT 920076) is gratefully acknowledged. References 1 Z. Samec, V. MareEek and M. P. Colombini, J. Electroanal. Chem., 1988,257, 147. 2 T. Wandlowski, V. MareEek and Z. Samec, Electrochim. Acta, 1990,35,1173. 3 A. Sabela, V. MaraEek, Z. Samec and R. Fuoco, Electrochim. Acta, 1992,37, 23 1. 4 A-K. Kontturi, K. Kontturi, L. Murtomaki and D. J. Schiffrin, J. Chem. SOC., Faraday Trans., 1990,86,819. 5 Y. Shao, A. A. Stewart and H. H. Girault, J. Chem. SOC., Faraday Trans., 1991,87,2593. 6 K. Kontturi, T. Ojala and P. Forssell, J. Chem. SOC., Faraday Trans.I, 1984,80, 3379. 7 R. Haase, Thermodynamics of Irreversible Processes, Addison-Wesley, London, 1969, p. 268. 8 A. J. Bard and L. R. Faulkner, Electrochemical Methods, Wiley, New York, 1980. 9 Y. Cheng, V. J. Cunnane, L. Murtomaki, K. Kontturi and D. J. Schiffrin, J. Chem. SOC., Faraday Trans., 1991,87,107. 10 G. Taylor and H. H. Girault, J. Electroanal. Chem., 1986, 208, 179. 11 Desktop Molecular Modeller (Version 1.2), Oxford University Press, 1989. 12 R. A. Robinson and R. H. Stokes, Electrolyte Solutions, Butter-worth, London, 1959, p. 396. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2041 13 14 B. E. Douglas, D. H. McDaniel and J. J. Alexander, Concepts and Models of Inorganic Chemistry, Wiley, New York, 2nd edn., 1982. AX. Kontturi, K.Kontturi, L. Murtomaki and D. J. Schiffrin, 17 18 19 R.J. W. Le Fkvre, Dipole Moments, Methuen, London, 1938. G. J. Moody and J. D. R. Thomas, Dipole Moments in Organic Chemistry, Edward Arnold, London, 1971, p. 43. M. H.Abraham and A. F. Dad de Namor, J. Chem. SOC., 15 J. Chem. SOC., Faraday Trans., 1990,86,931. Ya. Ozols, S.Vimba and A. Ievins, KristollograJiya, 1962,7, 362. Faraday Trans. I, 1976,72,955. 16 A. Abbott and D. J. Schiffrin, J. Chem. SOC., Faraday Trans., 1990,86,1453. Paper 4/00556B;Received 28th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002037

出版商:RSC    

年代:1994

数据来源: RSC