摘要:

physicochemical topics, thereby encouraging scientists of different disciplines to contribute their varied viewpoints to a coiiinion theme. A recent Discussion is :- The Royal Soci- of Chemistry- No.75 lntraamolecwlar Kinetics No. 75 in the series, this publication is the result of a general discussion held at the University of Warwick in April 1983. Contents: The Spiers Meniorlal Lecture; Vibrational Redistribution within Excited Electronic States of Polyatomic Molecules Inrraniolecular R e h u t i o n o f 1. vcited States lsomerization of Intcrnal~ncrgy-selected Ions Kinetics of Ion-Molecule Collision Coinple\es in the Gas Phase, E\periinent and Theory lntrainolccular Decay 01' Soinc Open-shell Pulya t o niic Ca lions On tlic Theory u i Iiitrdniolccul~r I n e r g y Transfer Pulsed Laser Preparation and Ouaniuin Superposition Statc Evolution in ReguLtr and Irregular Systems A Ouantuiii-iiicclianical Internal-collision Model for State-sclcctcd Uniinolccular Decoiiiposilio n The Correspondence Principle and Intramolecular Dynamics lntrainoleculdr Dcphasiiig.t'icusecond Evolution of Wavepacket States in a Molecule with Int erinediate-casc level Struct urc Energy Conversion in van der Waals C'u~~iplc\c\ ol s-Tetrarine and Argon Tim-dependent Processes in Polyatuinic Molecules During and After Intense Intrarcd Irradiation Energy Distributions in tlic (.N(X'L+) bragnient froiii tlie Infrared Multiplepholun Dissociation ol' CI. ICN. A Coinparison between 1:xperiiiiental Results and the Predictions ot Statistical Theories of ChFO + Product Energy Partitioning in the Decoiii- position of State-selectively Excited HOON and IIOOD Low-power Inl-rarcd Laser I'hoiolysis o f Tetramethy ldioxetan Uniinolecular Reactions lnduccd by Vibrational Overtone Excitation Uniiiiolecular Decomposition of t-Butylhydro- peroxide by Direct Excitation of the 6-0 0-11 Stretching Overtone I'icosecond-jet Spectroscopy and Photoclieinistry.Energy Redistribution and its Iiiipact'on Coherence, Isoincrization, Ihssociatiun and Solvalioii knergy Redistribution in Large Molecules. Duect St ud y o f In1 rainolucular Rehxa lion in the Gas Phase with Picosecond Gating Rotation-dependent Intrainolecuhr I'r~)cessc.sofSO:(A'A.) in a Superwnic Jct Role of Rotation-Vibration Interaction in Vibrational Keh\ation. Energy Kcdistribution in k,xcitcd Singlet I~'ornialdc1iyde Sub-lhppler.Spectroscopy of Benrcnc in the "('liaiinel-lliree" Region Intraiiiulccular 1:lectronic Kclau~tion and I'liotois~)iiieruati[)n Processes in tlie lsuhted Azabenrene Molecules Pyridinc, Pyrazinc and I'yriiiiidinc Softcover 434pp 0 85186 658 1 Price f25.00 ($48.00) Rest of the World f26.00 RSC Members f 16.25 Faraday Discussions of the Chemical Society 7< lnrruniolei u h r Kincrit I Faraday Symposia are usually held annually and are confined to more specialiscd topics than Discussions, with particular reference to recent rapidly developing lines of rescuch. A recent Symposium is :- No.l?The Hydrophobic Interadion No. 17 in the series, this publication is the result of a symposium on The Hydrophobic Interaction held at the Uiiiversity of Reading in December 1982.Contents: Hydrophobic Interdctionr a llistaric.11 Per spect ivr llydrupliobic Ilydration Geometric Kelaution in Water. Its Role in Precise Vapour-pressure Measureiiients of the SolubilkdtiorI of Benzene by Aqueous Sodiuiii Octylsulphate Solutions Nuclear Magnetic Resonance R e b u t i o n Investigation of Tetrahydrofuran and Methyl Iodide Clathrdtes Infrared and Nuckar Magnetic Kcwnance Studies Pertaining to the (age Model t o r Solutions oS Acetone in Water Irothernial Transport Properties in Solutions o f S y mmet r ica I Tet ra-alk y hmnioniuiii Bromides Thermodynamics of Cavity I'oriiiaiion in Water. A Molecular Dynamics Study Molecular Librations and Solvent Oricnt- ational Correlations in Hydrophobic Phenomena Monte Carlo Computer Siniulation Study of the Hydrophobic Effect.Potential ot Mean Force for ECfir)gaq at 25 and SOv C Hydroplicibic Moments and Protein Structure Application 01' the Kirkwood-Buff Theory to the t'roblcin 01 Hydrophobic Interactions Ihentangleinent of Ilydrophubic and IFlcctrosi~tic Contributions t o the I.ilni Pressures O i Ionic Surfactants llydrophobir. Intcracliuns in Dilute Su lut io ns u t 1'0 1 y (vin y I a Ico lio I) ('onioriii;tiionaI and 1:unc.i ional I'ropertics of tiaeiiwglobin in Water+Alcohol Mixtures. Dependence o f Bull. Electrostatic and tlydrupliohic I n t c r x t i o n s upon ptl and KCI concentrations Softcover 24Opp 0 85186 668 9 Price f36.50 ($70.00) Rest of the World f38.50 RSC Members f 23.75 ORDERING RSC Members should send their orders to: The Royal Society of Chemistry.The Membership Officer. 30 Russell Square, Non-RSC Members The Royal Society of Chemistry, Distribution Centre, Blackhorse Road, L London WC1 B 5DT. Letchworth, Herts SO6 IHN, England. Faradaj Symposia of the Chemical Society hGi 17 I hc HI drophohr' Inrcrm rron 1 9 X ? (viii)physicochemical topics, thereby encouraging scientists of different disciplines to contribute their varied viewpoints to a coiiinion theme. A recent Discussion is :- The Royal Soci- of Chemistry- No.75 lntraamolecwlar Kinetics No. 75 in the series, this publication is the result of a general discussion held at the University of Warwick in April 1983. Contents: The Spiers Meniorlal Lecture; Vibrational Redistribution within Excited Electronic States of Polyatomic Molecules Inrraniolecular R e h u t i o n o f 1.vcited States lsomerization of Intcrnal~ncrgy-selected Ions Kinetics of Ion-Molecule Collision Coinple\es in the Gas Phase, E\periinent and Theory lntrainolccular Decay 01' Soinc Open-shell Pulya t o niic Ca lions On tlic Theory u i Iiitrdniolccul~r I n e r g y Transfer Pulsed Laser Preparation and Ouaniuin Superposition Statc Evolution in ReguLtr and Irregular Systems A Ouantuiii-iiicclianical Internal-collision Model for State-sclcctcd Uniinolccular Decoiiiposilio n The Correspondence Principle and Intramolecular Dynamics lntrainoleculdr Dcphasiiig. t'icusecond Evolution of Wavepacket States in a Molecule with Int erinediate-casc level Struct urc Energy Conversion in van der Waals C'u~~iplc\c\ ol s-Tetrarine and Argon Tim-dependent Processes in Polyatuinic Molecules During and After Intense Intrarcd Irradiation Energy Distributions in tlic (.N(X'L+) bragnient froiii tlie Infrared Multiplepholun Dissociation ol' CI.ICN. A Coinparison between 1:xperiiiiental Results and the Predictions ot Statistical Theories of ChFO + Product Energy Partitioning in the Decoiii- position of State-selectively Excited HOON and IIOOD Low-power Inl-rarcd Laser I'hoiolysis o f Tetramethy ldioxetan Uniinolecular Reactions lnduccd by Vibrational Overtone Excitation Uniiiiolecular Decomposition of t-Butylhydro- peroxide by Direct Excitation of the 6-0 0-11 Stretching Overtone I'icosecond-jet Spectroscopy and Photoclieinistry. Energy Redistribution and its Iiiipact'on Coherence, Isoincrization, Ihssociatiun and Solvalioii knergy Redistribution in Large Molecules.Duect St ud y o f In1 rainolucular Rehxa lion in the Gas Phase with Picosecond Gating Rotation-dependent Intrainolecuhr I'r~)cessc.sofSO:(A'A.) in a Superwnic Jct Role of Rotation-Vibration Interaction in Vibrational Keh\ation. Energy Kcdistribution in k,xcitcd Singlet I~'ornialdc1iyde Sub-lhppler. Spectroscopy of Benrcnc in the "('liaiinel-lliree" Region Intraiiiulccular 1:lectronic Kclau~tion and I'liotois~)iiieruati[)n Processes in tlie lsuhted Azabenrene Molecules Pyridinc, Pyrazinc and I'yriiiiidinc Softcover 434pp 0 85186 658 1 Price f25.00 ($48.00) Rest of the World f26.00 RSC Members f 16.25 Faraday Discussions of the Chemical Society 7< lnrruniolei u h r Kincrit I Faraday Symposia are usually held annually and are confined to more specialiscd topics than Discussions, with particular reference to recent rapidly developing lines of rescuch.A recent Symposium is :- No.l?The Hydrophobic Interadion No. 17 in the series, this publication is the result of a symposium on The Hydrophobic Interaction held at the Uiiiversity of Reading in December 1982. Contents: Hydrophobic Interdctionr a llistaric.11 Per spect ivr llydrupliobic Ilydration Geometric Kelaution in Water. Its Role in Precise Vapour-pressure Measureiiients of the SolubilkdtiorI of Benzene by Aqueous Sodiuiii Octylsulphate Solutions Nuclear Magnetic Resonance R e b u t i o n Investigation of Tetrahydrofuran and Methyl Iodide Clathrdtes Infrared and Nuckar Magnetic Kcwnance Studies Pertaining to the (age Model t o r Solutions oS Acetone in Water Irothernial Transport Properties in Solutions o f S y mmet r ica I Tet ra-alk y hmnioniuiii Bromides Thermodynamics of Cavity I'oriiiaiion in Water.A Molecular Dynamics Study Molecular Librations and Solvent Oricnt- ational Correlations in Hydrophobic Phenomena Monte Carlo Computer Siniulation Study of the Hydrophobic Effect. Potential ot Mean Force for ECfir)gaq at 25 and SOv C Hydroplicibic Moments and Protein Structure Application 01' the Kirkwood-Buff Theory to the t'roblcin 01 Hydrophobic Interactions Ihentangleinent of Ilydrophubic and IFlcctrosi~tic Contributions t o the I.ilni Pressures O i Ionic Surfactants llydrophobir. Intcracliuns in Dilute Su lut io ns u t 1'0 1 y (vin y I a Ico lio I) ('onioriii;tiionaI and 1:unc.i ional I'ropertics of tiaeiiwglobin in Water+Alcohol Mixtures. Dependence o f Bull. Electrostatic and tlydrupliohic I n t c r x t i o n s upon ptl and KCI concentrations Softcover 24Opp 0 85186 668 9 Price f36.50 ($70.00) Rest of the World f38.50 RSC Members f 23.75 ORDERING RSC Members should send their orders to: The Royal Society of Chemistry. The Membership Officer. 30 Russell Square, Non-RSC Members The Royal Society of Chemistry, Distribution Centre, Blackhorse Road, L London WC1 B 5DT. Letchworth, Herts SO6 IHN, England. Faradaj Symposia of the Chemical Society hGi 17 I hc HI drophohr' Inrcrm rron 1 9 X ? (viii)

ISSN:0300-9599    

DOI:10.1039/F198480FX013

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

AUTHOR INDEX xxxv Sabbatini, L., 1029 Sacco, A., 2669 Sanders, J. V., 571 Sangster, D. F., 291 1 Sarka, K., 521 Sasahira, A., 473 Sasse, W. H. F., 571 Satchell, P. W., 2395 Sato, K., 841 Sato, Y., 341 Savino, V., 759 Sayers, C. M., 1217 Schiller, R. L., 1257 Schmidt, J., 1 Schmidt, P. P., 2017 Schneider, H., 3275, 3285 Schulz, R. A., 489, 1323 Scott, J. M. W., 739, 1651, 2287, Scott, S. K., 3409 Segall, R. L., 2609 Sehested, K., 2929, 2969 Seidl, V., 1367 Sem, P., 297 Serratosa, J. M., 2225 Seyama, H., 237 Seyedmonir, S. R., 87, 2269 Shanahan, M. E. R., 37 Sheppard, A., 2999 Sherwood, P. M. A., 135, 2099, Shindo, Y., 879, 2199 Shiotani, H., 2145 Shizuka, H., 383, 341 Siekhaus, W. J., 61 Sircar, S., 1101, 2489 Smart, R. St C., 2957, 2609 Smith, I. M., 3021 Smith, R., 3233 Snow, R.L., 3463 Solar, S., 2929 Solar, W., 2929 Solymosi, F., 1841 Soma, M., 237 Soupart, J-B., 3209 Sourisseau, C., 3257 Spink, J. A., 3469 Spoto, G., 1875, 1891 Spotswood, T. M., 3147 Staricco, E. H., 2631 Stassinopoulou, K., 3095 Stedman, D. H., 285 Stout, D. R., 3481 Strohbusch, F., 1757 Strumolo, D., 1479 Struve, P., 813, 2167 Styring, M. G., 3051 Subramanian, R., 2405 2881, 3359 2549, 2867 Sundar, H. G. K., 3491 Sutcliffe, L. H., 669, 3021 Sutton, H. C., 2301 Sutton, L. E., 635 Suzuki, H., 803 Suzuki, T., 1925, 3157 Symons, M. C. R., 423, 1005, Szamosi, J., 1645 Szczepaniak, W., 2935 Takagi, Y., 1925 Takahashi, K., 803 Takahashi, N., 629 Takanaka, J., 941 Takao, S., 993 Takasaki, S., 803 Takegami, H., 1221 Tam, S-C., 2255 Tamamushi, R., 2751 Tamaru, K., 29, 1567, 1595 Tamilarasan, R., 2405 Tanabe, S., 803 Tanaka, K., 2563,2981 Tanaka, T., 119 Taniewska-Osinska, S., 1409 Tascon, J.M. D., 1089 Teo, H. H., 981, 1787 Tetenyi, P., 3037 Thomas, J. K., 1163 Thompson, L., 1673 Thomson, M., 1867 Thomson, S. J., 1689 Tiddy, G. J. T., 789, 3339 Tittarelli, P., 2209 Tominaga, T., 941 Tomkinson, J., 225 Tonelli, C., 1605 Toprakcioglu, C., 13,413 Tran, T., 1867 Trasatti, S., 913 Tripathi, A. D., 1517 Tronc, E., 2619 Troncoso, G., 2127 Truscott, T. G., 2293 Tsurusaki, T., 879 Tuck, J. J., 309 Turner, P. S., 2609 Tusk, M., 1757 Tvarbikova, Z., 2639 Tyrrell, H. J. V., 1279 Ueki, Y.. 341 Ueno, A., 803 Unno, H., 1059 Valencia, E., 2127 van de Ven, T. G. M., 2677 van Ommen, J. G., 2479 van Truong, N., 3275, 3285 Vargas, I., 1947 2767, 2803, 21 1, 1999 Vedrine, J.C., 1017 Veith, J., 2313 Velasco, J. R., 3429 Vesala, A., 2439 Vickerman, J. C., 1903 Vincent, B., 2599 Vinek, H., 1239 Vink, H., 507, 1297 Waghorne, W. E., 1267 Wagley, D. P., 47 Walker, R. W., 435, 3187, 3195, Wallington, T. J., 2737 Wang, G-W., 1039 Watkins, P. E., 2323 Watkiss, P. J., 1279 Watt, R. A. C., 489 Webb, G., 1689 Webster, B. C., 255, 267 Weiner, E. R., 1491 Wells, C. F., 2155. 2445 Wells, J. D., 1233 Whang, B. C. Y., 291 1 Whittle, E., 2323 Wichterlova, B., 2639 Wiesner, S., 3021 Wilhelmy, D. M., 563 Williams E. H., 3147 Williams, P. A., 403 Williams, R. J. P., 2255 Wokaun, A., 1305 Wolff, T., 2969 Wood, S. W., 3419 Woolf, L. A., 549, 1287 Wright, C. J., 1217 Wu, D. C., 1795 Wiirflinger, A., 3221 Wyn-Jones, E., 1915 Yamabe, M., 1059 Yamamoto, S., 941 Yamashita, H., 1435 Yamauchi, H., 2033 Yamazaki, A., 3245 Yariv, S., 1705 Yasumori, I., 841 Yeates, S.G., 1787 Yide, X., 969, 3103 Ylikoski, J., 2439 Yokokawa, T., 473 Yoneda, N., 879 Yonezawa, T., 1435 Yoshida, S., 119, 1435 Zambonin, P. G., 1029 Zanderighi, L., 1605 Zecchina. A., 2209, 2723, 1875, Zipelli, C., 1777 Zundel, G., 553 348 1, 2827 1891AUTHOR INDEX xxxv Sabbatini, L., 1029 Sacco, A., 2669 Sanders, J. V., 571 Sangster, D. F., 291 1 Sarka, K., 521 Sasahira, A., 473 Sasse, W. H. F., 571 Satchell, P. W., 2395 Sato, K., 841 Sato, Y., 341 Savino, V., 759 Sayers, C. M., 1217 Schiller, R. L., 1257 Schmidt, J., 1 Schmidt, P. P., 2017 Schneider, H., 3275, 3285 Schulz, R. A., 489, 1323 Scott, J. M. W., 739, 1651, 2287, Scott, S.K., 3409 Segall, R. L., 2609 Sehested, K., 2929, 2969 Seidl, V., 1367 Sem, P., 297 Serratosa, J. M., 2225 Seyama, H., 237 Seyedmonir, S. R., 87, 2269 Shanahan, M. E. R., 37 Sheppard, A., 2999 Sherwood, P. M. A., 135, 2099, Shindo, Y., 879, 2199 Shiotani, H., 2145 Shizuka, H., 383, 341 Siekhaus, W. J., 61 Sircar, S., 1101, 2489 Smart, R. St C., 2957, 2609 Smith, I. M., 3021 Smith, R., 3233 Snow, R. L., 3463 Solar, S., 2929 Solar, W., 2929 Solymosi, F., 1841 Soma, M., 237 Soupart, J-B., 3209 Sourisseau, C., 3257 Spink, J. A., 3469 Spoto, G., 1875, 1891 Spotswood, T. M., 3147 Staricco, E. H., 2631 Stassinopoulou, K., 3095 Stedman, D. H., 285 Stout, D. R., 3481 Strohbusch, F., 1757 Strumolo, D., 1479 Struve, P., 813, 2167 Styring, M. G., 3051 Subramanian, R., 2405 2881, 3359 2549, 2867 Sundar, H.G. K., 3491 Sutcliffe, L. H., 669, 3021 Sutton, H. C., 2301 Sutton, L. E., 635 Suzuki, H., 803 Suzuki, T., 1925, 3157 Symons, M. C. R., 423, 1005, Szamosi, J., 1645 Szczepaniak, W., 2935 Takagi, Y., 1925 Takahashi, K., 803 Takahashi, N., 629 Takanaka, J., 941 Takao, S., 993 Takasaki, S., 803 Takegami, H., 1221 Tam, S-C., 2255 Tamamushi, R., 2751 Tamaru, K., 29, 1567, 1595 Tamilarasan, R., 2405 Tanabe, S., 803 Tanaka, K., 2563,2981 Tanaka, T., 119 Taniewska-Osinska, S., 1409 Tascon, J. M. D., 1089 Teo, H. H., 981, 1787 Tetenyi, P., 3037 Thomas, J. K., 1163 Thompson, L., 1673 Thomson, M., 1867 Thomson, S. J., 1689 Tiddy, G. J. T., 789, 3339 Tittarelli, P., 2209 Tominaga, T., 941 Tomkinson, J., 225 Tonelli, C., 1605 Toprakcioglu, C., 13,413 Tran, T., 1867 Trasatti, S., 913 Tripathi, A.D., 1517 Tronc, E., 2619 Troncoso, G., 2127 Truscott, T. G., 2293 Tsurusaki, T., 879 Tuck, J. J., 309 Turner, P. S., 2609 Tusk, M., 1757 Tvarbikova, Z., 2639 Tyrrell, H. J. V., 1279 Ueki, Y.. 341 Ueno, A., 803 Unno, H., 1059 Valencia, E., 2127 van de Ven, T. G. M., 2677 van Ommen, J. G., 2479 van Truong, N., 3275, 3285 Vargas, I., 1947 2767, 2803, 21 1, 1999 Vedrine, J. C., 1017 Veith, J., 2313 Velasco, J. R., 3429 Vesala, A., 2439 Vickerman, J. C., 1903 Vincent, B., 2599 Vinek, H., 1239 Vink, H., 507, 1297 Waghorne, W. E., 1267 Wagley, D. P., 47 Walker, R. W., 435, 3187, 3195, Wallington, T. J., 2737 Wang, G-W., 1039 Watkins, P. E., 2323 Watkiss, P. J., 1279 Watt, R. A. C., 489 Webb, G., 1689 Webster, B. C., 255, 267 Weiner, E. R., 1491 Wells, C. F., 2155. 2445 Wells, J. D., 1233 Whang, B. C. Y., 291 1 Whittle, E., 2323 Wichterlova, B., 2639 Wiesner, S., 3021 Wilhelmy, D. M., 563 Williams E. H., 3147 Williams, P. A., 403 Williams, R. J. P., 2255 Wokaun, A., 1305 Wolff, T., 2969 Wood, S. W., 3419 Woolf, L. A., 549, 1287 Wright, C. J., 1217 Wu, D. C., 1795 Wiirflinger, A., 3221 Wyn-Jones, E., 1915 Yamabe, M., 1059 Yamamoto, S., 941 Yamashita, H., 1435 Yamauchi, H., 2033 Yamazaki, A., 3245 Yariv, S., 1705 Yasumori, I., 841 Yeates, S. G., 1787 Yide, X., 969, 3103 Ylikoski, J., 2439 Yokokawa, T., 473 Yoneda, N., 879 Yonezawa, T., 1435 Yoshida, S., 119, 1435 Zambonin, P. G., 1029 Zanderighi, L., 1605 Zecchina. A., 2209, 2723, 1875, Zipelli, C., 1777 Zundel, G., 553 348 1, 2827 1891

ISSN:0300-9599    

DOI:10.1039/F198480BX015

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY FARADAY TRANSACTIONS, PARTS I A N D I 1 The Journal of the Chemicalsociety is published in six sections, ofwhich five are termed Transoctions; these are distinguished by their subject matter, as follows: Dalton Transactions (Inorganic Chemistry). All aspects of the chemistry of inorganic and organometallic compounds; including bioinorganic chemistry and solid-state inorganic chemistry; of their structures, properties, and reactions, including kinetics and mechanisms; new or improved experimental techniques and syntheses. Furaday Transactions I (Physical Chemistry). Radiation chemistry, gas-phase kinetics, electrochemistry (other than preparative), surface and interfacial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions, and kinetics of polymerization, etc.Faraday Transactions II (Chemical Physics). Theoretical chemistry, especially valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenomena, spectroscopic studies (including i.r., e.s.r., n.m.r., and kinetic spec- troscopy, etc.) leading to assignments of quantum states, and fundamental theory. Studies of impurities in solid systems. Perkin Transactions I (Organic Chemistry). All aspects of synthetic and natural product organic, organometallic and bio-organic chemistry, including aliphatic, alicyclic, and aromatic systems (carbocyclic and heterocyclic). Perkin Transactions II (Physical Organic Chemistry). Kinetic and mechanistic studies of organic, organometallic and bio-organic reactions.The description and application of physicochemical, spectroscopic, and theoretical procedures to organic chemistry, including structure-activity relationships. Physical aspects of bio-organic chemistry and of organic compounds, including polymers and biopolymers. Authors are requested to indicate, at the time they submit a typescript, the journal for which it is intended. Should this seem unsuitable, the Editor will inform the author. The sixth section of the Journal of the Chemical Society is Chemical Communications, which is intended as a forum for preliminary accounts of original and significant work, in any area of chemistry that is likely to prove of wide general appeal or exceptional specialist interest. Such preliminary reports should be followed up eventually by full papers in other journals (e.g.the five Transactions) providing detailed accounts of the work. NOTES I t has always been the policy of the Faraday Transactions that brevity should not be a factor influencing acceptability for publication. In addition however to full papers both sections carry at the end of each issue a section headed ‘Notes’, which are short self-contained accounts of experimental observations, results, or theory that will not require enlargement into ‘full’ papers. The Notes section is not used for preliminary communications. The layout of a Note is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of Notes is the same as for full papers.However, Notes are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors’ wishes as to whether an article is a full paper or a Note, but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong Notes to the full papers section. As a guide a Note should not exceed I500 words or word-equivalents. (i)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participatingmember, has produced a set of recommendations in a pamphlet ‘Quantities, Units, and Symbols’ (1975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W 1V OBN).These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘ Systeme International d’Unites’ (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A , B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 197 1, now published by Pergamon).Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society’s editorial staff.(ii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 78 ~ Laser Studies in Reaction Kinetics Radicals in Condensed Phases University of Leicester, 4-6 September 1984 I Evangelische Akademie, Tutzing, West Germany, 24-27 September 1984 Organising Committee Professor M. C. R. Symons (Chairman) Dr G. B. Buxton Dr T. A. Claxton Dr K. A. McLauchlan Professor Lord Tedder Dr R. L. Willson ~ Organising Committee I R. Ben Aim (Gif sur Yvette) G. Giacometti (Padova) P. Rigny (Gif sur Yvette) E. W. Schlag (Munchen) The discussion will be primarily concerned with the structure and reactions of radicals in liquids and solids. It is designed to bring together theoretical work on structure, environmental effects and reactivity with spectroscopic and mechanistic studies directly concerned with radicals.Fundamental aspects will be stressed, and particular attention will be given to new developments including measurement at short time intervals, special solvent effects, and the effects of external fields. A special area for inclusion will be electron gain and loss processes including trapped and solvated electrons, electrochemical reactions, and specific electron capture and electron loss in low-temperature systems. Photochemical charge-transfer processes will also be included. The aim of this meeting is the discussion of the latest experiments and related theories in the field of laser studies of elementary chemical reactions in molecular beams, in the gas phase, and in the condensed phase.The discussion will include oral contributions and poster presentations. , Contributions are invited and a very brief preliminary abstract (less than 50 words) should 1 be submitted by 15 May 1984 to: Professor Dr J. Troe, lnstitut fur Physikalische , Chemie, Universitat Gottingen, Tammannstrasse 6, 03400 Gottingen, West Germany. Authors of accepted contributions will be required to provide a manuscript for publication in a special issue of the Berichte der Bunsengesellschaft fur Physikalische Chemie. The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN I THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY D EUTSC H E B U NS EN G ES E LLSCH AFT FU R PHY S I KA L I SCH E C H EM I E SOClETC DE CHlMlE PHYSIQUE ASSOCIAZIONE ITALIANA DI CHIMICA FISICA Joint Discussion Meeting on: I.W. M. Smith (Cambridge) J. Troe (Gottingen) K. Welge (Bielefeld) (iii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 19 Molecular Electronic Structure Calculations-Methods and Applications University of Cambridge, 12-1 3 December 1984 N.B. Please note change of date Molecular electronic structure calculations have now developed into a powerful predictive tool and are necessary in several different fields t o aid the understanding and interpretation of experimental observations. The meeting will review the current state of this rapidly developing discipline and will bring together experts on some of the most advanced methods and their applications.The meeting will provide an opportunity for discussion and comparison of the various techniques currently in use. It will therefore not only be a valuable forum for discussion among research workers in the field, but should also show the non-specialist what theoretical calculations can be expected to achieve now and in the near future. The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 79 Polymer Liquid Crystals University of Cambridge, 1-3 April 1985 The object of the meeting will be t o discuss all aspects of the developing subject of polymeric liquid crystals. The hope is to bring together scientists from the fields of conventional polymer science and monomeric liquid crystals who are active in this field.The discussion is aimed at understanding the following facets: (a) The chemical characteristics that give rise to polymer liquid crystalline behaviour. (b) The nature of the high local anisotropy of these systems and their structural organisation at the molecular, micron and macroscopic levels. (c) The physical properties and their industrial exploitation, with particular reference to the influence of external force fields such as flow, electric and magnetic fields. (d) The inter-relations of polymer liquid crystals with small-molecule mesophases, conventional flexible polymers and biopolymers which exhibit liquid-crystalline behaviour. Contributions are invited for consideration by the Organising Committee.A title and 300- word abstract should be submitted as soon as convenient and not later than 31 May 1984 to : Professor B. R. Jennings, Electro-optics Group, Department of Physics, Brunel University, Uxbridge UB8 3PH.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 80 Physical Interactions and Energy Exchange a t the Gas-Solid Interface McMaster University, Hamilton, Ontario, Canada, 23-25 July 1985 Organising Committee : Professor J. A. Morrison (Chairman) Dr M. L. Klein Professor G. Scoles Professor W. A. Steele Professor F. S. Stone Dr R. K. Thomas The discussion will be concerned with certain aspects of current research on the gas-solid interface: elastic, inelastic and dissipative scattering of atoms and molecules from crystal surfaces, and the structure and dynamics of physisorbed species, including overlayers.Emphasis will be placed on the themes of physical interactions and energy exchange rather than on molecular-beam technology or the phenomenology of phase transitions on overlayers. The interplay between theory and experiment will be stressed as they relate to the nature of atom and molecule surface interaction potentials, including many- body effects. Contributions for consideration by the Organising Committee are invited; titles should be submitted as soon as possible, and abstracts of about 200 words by 31 May 1984, to: Professor J. A. Morrison, Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4M1FARADAY DIVISION INFORMAL AND GROUP MEETINGS Statistical Mechanics and Thermodynamics Group Thermodynamics of Mixed Polymer Systems To be held at the University of Sheffield on 4-5 April 1984 Further information from Dr G. C.Maitland, Department of Chemical Engineering, Imperial College, London SW7 2BY Surface Reactivity and Catalysis Group Studies of Model Catalysts -Their Impact on Applied Catalysis To be held at UMIST on 8-1 0 April 1984 Further information from Dr J. C. 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A recent Discussion is :- The Royal Society of Chemistry No.75 lntramolecular Kinetics No. 75 in the series, this publication is the result of a general discussion held at the University of Warwick in April 1983. Contents: The Spiers Memorial Lecture; Vibrational Redistribution within Excited Electronic States of Polyatomic Molecules Intramolecular Relaxation of Excited States Isomerization of Internal-energy-selected Ions Kinetics of Ion-Molecule Colhsion Complexes in the Gas Phase, Experiment and Theory Intramolecular Decay of Some Open-shell Polyatomic Cations On the Theory of Intramolecular Energy Transfer Pulsed Laser Preparation and Quantum Superposition State Evolution in Regular and Irregular Systems A Quantum-mechanical Internal-cohsion Model for State-selected Unimolecular Decomposition The Correspondence Principle and Intramolecular Dynamics Intramolecular Dephasing. Picosecond Evolution of Wavepacket States in a Molecule with Intermediate-case level Structure Energy Conversion in van der Waals Complexes of s-Tetrazine and Argon Tune-dependent Processes in Polyatomic Molecules During and After Intense Intrared Irradiation Energy Distributions in the CN(X’Z+) Fragment from the Infrared Multiplephoton Dissociation of CFICN.A Comparison between Experimental Results and the Predictions of Statistical Theories O f C6Fb + Product Energy Partitioning in the Decom- position of State-selectively Excited HOON and HOOD Low-power Infrared Laser Photolysis of Tetramethyldioxetan Unimolecular Reactions Induced by Vibrational Overtone Excitation Unmolecular Decomposition of t-Butylhydro- peroxide by Direct Excitation of the 6-0 0-H Stretching Overtone Picosecond j e t Spectroscopy and Photochemistry. Energy Redistribution and its Impact‘on Coherence, Isomerization, Dissociation and Solvation Energy Redistribution in Large Molecules.Direct Study of Intramolecular Relaxation in the Gas Phase with Picosecond Gating Rotation-dependent Intramolecular Processes of SO?(A’A:) in a Supersonic Jet Role of Rotation-Vibration Interaction in Vibrational Relaxation. Energy Redistribution in Excited Singlet Formaldehyde Sub-Doppler, Spectroscopy of Benzene in the “Channel-three” Region Intramolecular Electronic Relaxation and Photoisomerization Processes in the Isolated Azabenzene Molecules Pyridine, Pyrazrne and Pyrimidine Softcover 434 p 0 85186 658 1 Price f25.00 (b8.00) Rest of the World f26.00 RSC Members f 16.25 Faraday Discussions of the Chemical Society No. 7 $ Inrrarnoleculur Kincrrrs Faraday Symposia are usually held annually and are confined to more specialised topics than Discussions, with particular reference to recent rapidly developing lines of research.A recent Symposium is :- No.17 The Hydrophobic hteractlon No. 17 in the series, this publication is the result of a symposium on The Hydrophobic Interaction held at the University of Reading in December 1982. Contents: Hydrophobic Interactions-a Historical Per spec t ive Precise Vapour-pressure Measurements of the Solubilization of Benzene by Aqueous Sodium Oc t y lsulp hate Solutions Nuclear Magnetic Resonance Relaxation Investigation of Tetrahydrofuran and Methyl Iodide Clathrates Infrared and Nuclear Magnetic Resonance Studies Pertaining to the Cage Model for Solutions of Acetone in Water Isothermal Transport Properties in Solutions of Symmetrical Tetra-alkylammonium Bromides Thermodynamics of Cavity Formation in Water. A Molecular Dynamics Study Molecular Librations and Solvent Orient- ational Correlations in Hydrophobic Phenomena Monte Carlo Computer Simulation Study of the Hydrophobic Effect. Potentlal of Mean Force forDCHr)gaq at 25 and SOc C Geometric Relaxation in Water. Its Role in Hydrophobic Hydration Hydrophobic Moments and Protein Structure Application of the Kirkwood-Buff Theory to the Problem of Hydrophobic Interactions Disentanglement of Hydrophobic and Electrostatic Contributions to the Film Pressures of Ionic Surfactants Hydrophobic Interactions in Dilute Solutions of Poly(viny1 alcohol) Conformational and Functional Properties of Haenloglobin in Water+Alcohol Mixtures. Dependence of Bulk Electrostatic and Hydrophobic Interactions upon pH and KCI concentrations Softcover 24 Price f36.50%70.001 Rest of the World f38.50 RSC Members f 23.75 p 0 85186 668 9 ORDERING RSC Members should send their orders to: The Royal Society of Chemistry, The Membership Officer, 30 Russell Square. Non-RSC Members The Royal Society of Chemistry, Distribution Centre. Blackhorse Road, L London WClB5DT. Letchworth. Herts SG6 IHN, England. Faraday Symposia of the Chemical Society No 17 The H b drophobic lnterurrron 19x2 (viii)

ISSN:0300-9599    

DOI:10.1039/F198480FP029

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1984,80, 769-780 Average Vibrational Energy Transfer during a Single Collision of Excited Molecules with Heat-bath Molecules BY IZHACK OREF* Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel AND B. SEYMOUR RABINOVITCH Department of Chemistry BG10, University of Washington, Seattle, Washington 98195, U.S.A. Received 10th May, 1983 A collisional energy-transfer probability of the form B(E+ AE) B'(E') B'(E - AE) dE' 0 is assumed where B(E) is the Boltzmann distribution and A E is the incremental energy transferred and which can display negative as well as positive values. Single collisions between vibrationally excited substrate molecules with heat-bath molecules are considered. The depen- dence of the average energy per collision transferred up, down and overall on the initial energy content, on the temperature and on the size of the bath and substrate molecules is calculated and compared with experimental data in the literature.Vibrational-relaxation and energy-transfer studies are of current interest in photo- physics and molecular dynamics. At low levels of excitation energy transfer between polyatomic molecules by resonant vibrational-vibrational (V-V) energy transfer is frequently the most important mechanism. Studies have been done in the So manifold using single-mode excitation by CO, laser1-' and shock-tube heating.8qg In the S, manifold relaxation studies were made by tuned laser excitation followed by resolved S, + So fluorescence."+l* At high levels of excitation, recent photophysi~al,l~-~' multiphoton excitationl8.l9 and shock-tube studies20 support the finding from chemical- activation studies,' that vibrational energy transfer by highly vibrationally excited polyatomic molecules takes place in the gas phase on virtually every collision. For weak colliders an important mechanism is vibrational-translational, rotational (V-T, R) transfer. The small contribution of the latter processes can be judged from the small values of the collisional efficiencies of the rare gases and diatoms and weak colliders in general. The pragmatic numerical value of the average energy which is transferred depends on the nature and energetics of the collision partners as well as on the collisional transition probability model which applies to the system.In a thermal system at equilibrium, the average amount of energy gained by the substrate molecule equals the average and the equilibrium assumption forces the distribution to remain Boltzmann after any (statistical) sequence of collisional events. In a unimolecular reacting model system at high pressures, the Boltzmann distribution is essentially maintained over all energy space. At low pressures, and for the case Eo % RT, an operational definition of a strong collision is that the Boltzmann vibrational distribution is maintained up to E,. The above definitions are used in conjunction with weak-collision models to obtain 769770 VIBRATIONAL ENERGY TRANSFER the degree of ‘weakness’ of a collision, i.e. the collisional efficiency relative to strong collision. There are several empirical weak-collider energy-transfer models,21* 28+ 27 such as exponential or Poisson-analytic forms.These models are used to evaluate the average vibrational energy transferred to the substrate, (AE, ) i.e. up transition, or from the: substrate, (AE,) i.e. down transition, or the overall average, (AE). The conservation relations are maintained by imposing detailed balance. The models are helpful in the correlation of experimental data. In a non-equilibrium (frequently reacting) system the situation is more complicated. Such a system may correspond to chemical, photochemical28* 29 and l a ~ e r ~ ~ - l ~ activation of reaction. Here a molecule excited to a high vibrational level is allowed to collide with a thermal heat-bath molecule.In the process a quantity of energy is transferred. The <AE) transferred depends not only on the energy content of the reactant molecule and the temperature of the bath but on the energy-transfer probability model which is used. Detailed balance is not maintained in the non-equilibrium system, and an alternative requirement to the conservation relation is that a non-reacting system reaches a Boltzmann distribution after a sufficient number of collisions. The purpose of the present work was to evaluate (AE,), (AE,) and ( A E ) in a non-equilibrium system with the use of a proposed model for strong-collision energy-transfer cases which allows the approach to the vibrational equilibrium condition to be attained more readily. Such a model applies in a number of experimental cases (see later), and the detailed mechanism of V-V, T, R need not enter explicitly. For the purpose of gaining an understanding of how the size and temperature of the heat bath and the size and energy content of the excited molecule affect the magnitudes of the various (AE), classical densities may be used.This does not detract from the general conclusions since for this purpose the behaviour of a real molecule may be simulated by s classical oscillators. The results are compared with some existing experimental data. PROBABILITY TRANSFER RELATIONS Consider a dilute system of substrate molecules all excited to a single value of internal energy, E, in a bath gas of temperature T. The probability of transferring an amount of energy from the excited molecule to the bath P(E, AE) can be evaluated in the following manner.25 When a collision takes place between a substrate molecule and a bath molecule an amount of energy AE is exchanged in the process.The probability that any molecule will start at Ei and reach the Ei + AE level while its partner will start at Ej and end at the Ej-AE level is F(Ei, E;, AE) = B(E,) B(E, + AE) B(E;) B(E; - AE). (1) + AE is for an up transition and - AE is for a down transition; B(E) is the Boltzmann distribution and the primed quantities indicate the bath molecules. The first term in eqn (l), B(E,), denotes the probability of obtaining a molecule in state Ei, while the rest of the expression is the transition probability, P(E,, AE), from state Ei to state Et+AE.B(E) has the form B(E) = ES-lexp(-E/RT)/(s- l)!(RT)s (2) and for the bath molecule s’ replaces s in eqn (2). The use of s does not detract from the generality of the exposition. When the results of the calculations are compared with experiments, the effective value of s’ is calculated from the value of the average energy as described later.I. OREF AND B. S. RABINOVITCH 77 1 The probability that a substrate with initial state i will exchange a given AE regardless of the initial state j of the colliding bath molecule is P(Ei,AEU)dAE = B(Ei+AE)dAE B’(E;)B‘(E;-AE)dE;/Q (3) P( Ei, AED ) dAE = B( Ei - AE) d AE (4) E E for the substrate up transition, and B’(E;) B’(E; + AE) dE;/Q for the down transition. AE is the lower limit in the integral expression in eqn (3) since the bath molecule is losing energy and has to have at least energy E’ equal to the amount transferred AE.In eqn (4) the lower limit is 0 since the bath is gaining energy and therefore can have any value. The value of the normalization parameter Q’is obtained from the conservation of probability relation 00 Q-l dAE[P(Ei, AE)] = 1 ( 5 ) -00 Detailed balance follows naturally from the form of eqn (3) and (4). For example, the equality B(Ei) P(Ei, AEU) = B(Ei+AE) P(Ei, AED) (6) is obtained by making the transformation E” = E’ - AE and substituting it in eqn (3) and replacing Ei by Ei+AE (the initial state) in eqn (4). It should be stressed again that detailed balance obtains only in an equilibrium (thermalized) system. The average quantities which are sought in this work are I (AE, ) = JOm d(AE) AE P(Ei, AE,) (AE,) = J” d(AE)AEP(E,,AE,) -00 (7) where P(Ei, AEu) and P(E,, AED) obey the normalization condition of eqn (5).RESULTS The form of the collisional probability, eqn (3) and (4), for classical oscillators is given in fig. 1 for four values of the internal energy of the reactant molecule with s = s’ = 15 and T = 1000 K. The normalized curves have a regular quasisymmetrical shape. The principal features are as follows. (a) The location of the maximum of the probability curve depends on the internal energy of the reactant molecule; the lower the value of E the higher the value of AE at which the probability curve peaks. (b) The higher the value of E, the broader the peak; at E = 0 kcal mol-lt the width at half height is ca.11 kcal mol-l while at E = 60 kcal mol-1 it is ca. 21 kcal mol-l. (c) The average energy transferred up is 21.6 kcal mol-l for the E = 0 kcal mol-1 case and down is - 18.8 kcal mol-l for the E = 60 kcal mol-1 case, i.e. the value of I(AE)I has to do with the location of the maximum and not with its width. (d) The probability t 1 kcal = 4.184 kJ.772 VIBRATIONAL ENERGY TRANSFER -40 -30 -20 -10 O 10 20 30 40 AElkcal mol-’ Fig. 1. Collisional energy-transfer probability P(E, AE) plotted against AE at 1000 K for reactant and bath molecules with 15 degrees of freedom. The curves shown are for internal energies of 0, 10, 28 and 60 kcal mol-l. curve peaks at values of AE that are different from the original value of E. The same behaviour is obtained in trajectory-type calculation^^^^ 31 where a potential is assumed and a detailed study of the energy exchange is made.By contrast, weak collisions as evaluated by the simple exponential and stepladder models probe the regions close to E wherever it may be. The dependence of the average energy up (A&, ) and down (A& ) and total energy transferred in a collision ( A E ) on the internal energy is given in fig. 2 for 400, 1000 and 2000 K and s = s’ = 15. The major points which emerge are as follows. (a) At very low internal energy up collisions predominate and ( A E ) x (A&). (b) At high internal energies down collisions predominate and (AE) x (A&). (c) At internal energies around the average energy at equilibrium (sRT), up and down collisions are both significant and ( A E ) is the average of the two.( d ) The higher the temperature the larger is (A&) at lower values of E and the smaller is I(AE,)I at higher values of E. This is so because the model requires that a Boltzmann distribution be obtained in one collision for s’ = co (a few collisions when s’ # 00). As the temperature in- creases, larger up collisions are needed for a substrate with low E to obtain the value of ( E ) which increases with the temperature. By the same token, if the molecule is energy-rich it takes smaller jumps for the high-temperature system to get to ( E ) than for the lower-temperature case, all other things being equal. Note that the intercept at E = 0 is not sRT. The intercept will approach the limiting value more closely as s’ -+ 00 ; the latter case is that of gas/wall strong-collision intera~tion.~~ The effect of the number of normal modes of the bath molecules on the average energy transferred is shown explicitly for T = 1000 K in fig.3. (a) The larger the heat bath the larger the absolute value of (AE) (up and down). (b) The incremental increase in AEis not directly proportional to the value of s’ (see discussion below); an infinitely large bath molecule (s’ = co) which collides with a substrate with s = 15 transfers on the average only a little more than a molecule with s’ = 30. The size of the reactantI. OREF AND B. S. RABINOVITCH 773 0 10 20 30 40 50 60 70 80 90 Elkcal mol-’ Fig. 2. Average energy transferred per collision ( A E ) plotted against internal energy E at 400, 1000 and 2000 K for s = s’ = 15.The solid line above ( A E ) = 0 is for (AE,) and below the line is for (AE,). The barred line indicates the overall energy transferred ( A E ) . limits the amount of energy which can be transferred up and down the energy scale. (c) At lower values of internal energy up collisions predominate and at high internal energies down collisions are the most important. ( d ) At values of internal energies close to the average thermal energy up and down collisions take place at the same time and the curves all cross at E x 30 kcal mol-l (i.e. sRT). There are two effects to a change in the temperature of a real molecular system with quantum oscillators instead of the classical ones used here. At low temperature the average internal energy of the bath molecule is low and therefore the number of equivalent classical oscillators s’ as given by ( E ) = s’RTis small.Therefore, ( A E ) should be small (fig. 3). However, at low values of T, (AE) should be larger since the bath molecule is colder (fig. 2). The two effects tend to counteract each other. The effect of the number of the vibrational modes of the bath on the average energy transferred can also be seen in fig. 4(a). The average energy of the substrate with s = 15 at 1000 K is 30 kcal mol-l; if the internal energy is below that value up col- lisions will prevail; above this value down collisions are important, as can indeed be seen from fig. 4(b). The effect of s and s’ on ( A E ) is introduced by using a reduced number of degrees of freedom parameter,25 s,, which is defined as sr = 2(s - 1) (s’ - l)/[(s - 1) + (s’ - l)].(c) A plot of ( A E ) against A$ yields a family of straight lines, each belonging to a different value of the internal energy, E; the linear774 VIBRATIONAL ENERGY TRANSFER 30 20 10 - L o E 2 -10 - 0 3 2 W -20 -30 I I I 1 0 10 20 30 40 50 60 70 80 -40 Elkcal mol-‘ Fig. 3. The average energy transferred (AE) plotted against the internal energy E at 1000 K and s = 15 for four values of the number of degrees of freedom of the bath molecule s’ = 5, 15, 30 and a. Full lines indicate (AE,) and (AE,). The barred lines indicate {AE). correlation over such a large spread in the values of s’, and therefore of s,, is a great simplification and enables easy correlation of various combinations of reactant and bath molecules.The limiting value of s,, for s’ = m, is 2(s- l), or s, = 5.3 for the case s = 15 (see fig. 4). The dependence of the overall average energy transferred during a collision on the temperature and the internal energy is given in fig. 5 for s = s’ = 15. The slightly concave lines are almost parallel. The spacing between them decreases by a constant increment as the internal energy increases. The spacing between the lines of E = 10 and E = 20 kcal mol-l is ca. 7.4 kcal mol-l, while between the lines E = 40 and E = 50 kcal mol-l is ca. 6 kcal mol-l. Over a limited range, however, the curves can be approximated as straight lines. One may examine collisional energy transfer in another systematic way. One may increase the size of the reactant while keeping the size of the bath molecule constant.Fig. 6 shows a graph of ( A E ) against E for s = 10 and 15 and s’ = 15. The results are very interesting; (AE,) increases as s increases, while (AE,) for s = 10 is larger than that for s = 15. In the up collision less energy is needed to bring the small molecule to its average energy while in the case of the down collisions (AE,) is larger for the small molecule since it is a smaller heat bath than the larger molecule and can retain less energy. This is precisely the prediction of the statistical which saysI. OREF AND B. S. RABINOVITCH 775 I 4 I 30 Fig. 4. The average energy (AE) plotted against the square root of the reduced number of modes, s,, for various values of E (in kcal mol-1) at 1000 K and s = 15.the larger the reactant molecule the more energy it can retain and therefore the smaller (AE,,) in a collision with a constant size heat bath. Fig. 7 shows the dependence of (AE) on the reduced number of degrees of freedom s,. Here s, increases because s increases while s’ remains constant. The average energy transferred in a collision with a bath molecule (s’ = 15 and T = 1000 K) increases as the number of degrees of freedom of the reactant increases. The reasons for such behaviour stem from the following facts. At low energy content (e.g. E = 10 kcal mol-l), as s increases more energy is needed to bring the molecule to its average energy and hence (AE) increases and is positive, i.e. up collisions predominate. At high levels of excitation (e.g. 70 kcal mol-l), as s increases the absolute value of A E decreases and is negative.In this case down collisions take place to bring the molecule to its equilibrium average energy but as s increases its equilibrium average energy increases and a smaller down step is needed. The model which is presented here anticipates the limiting energy-transfer behaviour of various experimental systems and does so in a simple and a consistent fashion. COMPARISONS WITH EXPERIMENT AND OTHER THEORIES How do the results which were obtained here compare with experiment? One system available for comparison is chemical activation. There, a substrate molecule is excited by the insertion of an atom or a radical into a double bond. For example, it is possible to oWain a butyl radical with average excess energy of 43 kcal mol-l by776 30 20 10 - I - g o 3 V .Y 2 4 4 v -10 -20 -30 VIBRATIONAL ENERGY TRANSFER 3 500 I000 1500 2000 TIK Fig.5. The average energy (AE) plotted against the temperature of the bath for s = s’ = 15 for the various values of E (in kcal mol-I) indicated on the curves. the reaction C,H, + . H C,H,. The energy spectrum of the molecule is not a delta function since the butene possesses initial thermal energy. Nevertheless, the energy spectrum is narrow and energy-transfer studies on such systems are instructive. When an excited butyl radical collides with a series of bath molecules, the larger the collider the larger The inert gases transfer ca. 2 kcal mol-l, the diatomics a little more and the polyatomics cis-butene and SF, ca.9 kcal mol-l. All the experi- mental values of (AE) which are quoted here were calculated by using the expo- nential transition probability model for the inert gases and the stepladder model for polyatomic colliders. Also, cyclopropane excited by chemical activation by lCH, addition to C2H4 has been reported to transfer ca. 4 kcal mol-l in a He bath, ca. 6 kcal mob1 in N, and ca. 10 kcal mol-l in a C,H, bath.,l Many more examples of such systems can be found in ref. (21), but the trend is clear: the larger the bath (s’), the larger is (AE). This result agrees with statistical-model r e s ~ l t s ~ ~ - ~ ~ and with our findings. Of course two effects influence collisional efficiency: one is the size of the heat bath and the other is the potential for the collisional interaction.Insofar as the former effects operate, the results in this series follow qualitative statistical expectation. However, it is possible to make a more quantitative comparison between theory and experiment for the case of the polyatomic butyl radical and SF, that function operationally in that study21 as strong colliders. Thus, at E = 43 kcal mol-l and s z 15 for the butyl radical777 E/kcal mol-’ Fig. 6. (AE) plotted against the internal energy E for s = 10 and 15, s’ = 15 and T = 1000 K. The dashed line indicates the average value of the energy transferred while the full lines indicate (AE,) and (AED). 20- 10- ‘i 0 - -z -10- - ; dq -20- A4 2 W -30 - -40 - 10 20 30 40 50 60 70 I I -50; 3 4 s t Fig. 7. (AE) plotted against the square root of the reduced number of degrees of freedom, s,.Values of initial energy E as indicated in the figure (in kcal mol-l) for s’ = 15 and T = 1000 K.778 VIBRATIONAL ENERGY TRANSFER and s’ = 3 for SF, (found by calculating the average energy of SF, and then calculating s’ from ( E ) = s’kT when T = 300 K), the value of ( A E ) calculated from eqn (7) is 6.2 kcal mol-l. This prediction is in reasonable agreement with the experimental value of ca. 9 kcal mol-l. Not many data exist for the temperature effect on the magnitude of ( A E ) in chemically activated systems. The little there is covers the low-temperature range 200-700 K and seems contradictory. Cyclopropane colliding with C2H4 shows21 an increase and then a decrease in ( A E ) on going from 300 to 700 K. For excited C,H,F,,O colliding with CH,ClF, ( A E ) remains constant at 300 and 475 K.For excited C,H,F colliding with N, there is a five-fold increase in ( A E ) on going from 315 to 560 K, an unexpectedly large temperature effect.21 The present model predicts a moderate increase in ( A E ) with temperature rise (fig. S), and reliable experimental data are clearly needed to verify this point. In recent experiments,’ azulene was photoexcited by laser and its energy-transfer behaviour was investigated by allowing it to collide with 17 bath gases. Molecules with an energy content of 17500 cm-l transferred to the bath molecules less energy than molecules excited to 30600 cm-l. The trend is similar to the one shown in fig. 2 and 3. In addition, the larger s’ the larger the value of (AE), in agreement with fig.3 and 4. In other experiments,, the energy-transfer behaviour of laser-excited cycloheptatriene was studied for a variety of bath gases, where it was found that there is practically no energy dependence of (AE), in contradiction to the results reported in ref. (41). Another type of experiment involves changing the size of the substrate (increase in the value of s) at constant s’. This type of experiment is harder to interpret since differ- ent substrate molecules have different threshold energies for decomposition, E,, and different working temperatures, usually. Statistical theory predicts that the larger the substrate the smaller is (AE), since more energy remains in the substrate The best way to obtain reliable results is to change s in a homologous series where E, and the activated complex remain unchanged.One such system is the alkyl-radical system, where excited butyl, pentyl, hexyl and octyl radicals were allowed to collide with various di- and poly-atomic gases.43 A slight increase in the magnitude of (AE) was found going from butyl to octyl. This is opposed to the prediction of the present and statistical model given in ref. (39). Cyclopropane transfers21 CQ. 10 kcal mol-l with C,H, as a bath and 9 3 kcal mol-l with n-C5Hlo.17 Dimethylcyclopropane transfers2’ 1 1.4 kcal mol-l with cis-butene as bath.* Methylcyclopropane is reported to 7 & 1 kcal mot1 in a collision with n-C,H,,, while ethylcyclopropane is reported to 7 f 4 kcal mob1 with 2-methylpentane as bath.Clearly, a definite correlation is difficult to make. 2-Pentyl and dimethyl-2-pentyl are reported2l9 45 to transfer 4.6 kcal mot1 in collisions with CF,. Again a change in the number of modes of the substrate does not appear to cause a change in (AE). More systematic and reliable experiments must be performed in order to understand the effect that increasing s has on the magnitude of (AE). calculations give good agreement with the experimental results. However, a cut-off energy in the transitional stretching mode correlating with relative translational motion along the line of centres is introduced and empirically adjusted. The empirical adjustments were done in such a way as to force the calculations for the value of (AE) for methylcyclopropane to reproduce the experimental one.The empirical values were then used in other collision-pair calculations. It is useful here to compare other statistical models. The transition-modes * Ref. (39) quotes a value of 6 kcal mol-I from ref. (44) instead of 11.4 kcal mol-’.I. OREF AND B. S. RABINOVITCH 779 The ergodic-collision theoryg6 predicts values which are generally larger than the reported experimental values. For dimethylcyclopropane colliding with cis-but-2-ene, instead of the reported value of 11.4 kcal mol-1 it predicts values in the range 27-51 kcal mol-l. For the 2-pentyl radical colliding with CF, it predicts 9.6 kcal mol-1 instead of 4.5 kcal mol-l. Generally, agreement to about a factor of two or better is obtained between theoretical and experimentally reported values.An improvement of the results calculated by the previous theory is obtained by the impulsive-collision theory.,' In this theory the collisional period is very short and therefore only kinetic energy is available for redistribution. The value of ( A E ) for the collision between dimethylcyclopropane and C,H, is now reduced to the range 14-27 kcal mol-l compared with the experimental 11.4 kcal mol-l. If one makes in the present treatment the common assumption that the classical s is half the number of modes, good agreement is obtained with the ergodic-collision theory4s as one would expect. The advantage of simplicity and lack of empiricism makes it as a useful tool in understanding the dependence of ( A E ) on the size and temperature of the collision partners.CONCLUSIONS (a) The collisional energy-transfer probability P(E, AE) as given by eqn (3) and (4) is a smooth function which obeys the conservation of probability [eqn (5)] and detailed balance [eqn (6)]. (b) The width at half of P(E, AE) is smaller at lower values of E (fig. 1). (c) At lower values of E most of the collisions are up transition and ( A E ) = ( A E , ) . At higher values of E most of the collisions are down transitions and ( A E ) = (A&) (fig. 1,2 and 5). At intermediate regions up and down collisions are operative. ( d ) Increasing the temperature of the bath effects larger (AE,) and smaller (AE,,) (fig. 2). (e) Increasing the size of the bath molecule increases the size of the average energy jump up and down (fig. 3). (f) The reduced number of degrees of freedom is a good parameter to use in order to show the effective molecular size dependence of the energy jumps, fig.4. (g) At low level of excitation ( A E ) is larger, the greater the size of the substrate, s. The collisions are up transitions (fig. 6). (h) At high levels of excitation (AE) is smaller, the larger s. The collisions are down transitions (fig. 6). (i) As s increases for a given level of excitation the value of ( A E ) increases. It is still negative at high levels of excitation and positive at lower ones. This work is supported by the United States-Israel Binational Science Foundation. 1.0. thanks the Berenstein Fund for the Advancement of Science for its assistance. B.S.R. thanks the Office of Naval Research and the National Science Foundation for their assistance.B. S. R. also thanks Prof. John Albery for his hospitality at the Imperial College of Science and Technology, London, while on leave there. 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Vandersall, D. A. Chernoff and S. A. Rice, J. Chem. Phys., 1981, 74,4888. l5 C. S. Parmenter, J. Phys. Chem., 1982,86, 1735. l6 K. Y. Tang and C. S. Parmenter, J. Chem. Phys., 1983,78, 3923. M. J. Rossi and J. R. Barker, Chem. Phys. Lett., 1982, 85, 21. R. Duperrex and H. Van den Bergh, J. Chem. Phys., 1979,71, 3613. R. Duperrex and H. Van den Bergh, Proc. 2nd Int. Con$ Infrared Phys., 1979, p.217. D. C. Tardy and B. S. Rabinovitch, Chem. Rev., 1977, 77, 369. 2o A. Lifshitz, A. Bar Nun, A. Burcat, A. Ofir and R. D. Levine, J. Phys. Chem., 1982,86,791. 22 I. Oref, J. Chem. Phys., 1982, 77, 5146. 23 0. Herscovitz and I. Oref, J. Phys. Chem., 1982, 86, 1495. 24 0. Herscovitz, E. Tzidoni and I. Oref, Chem. Phys., 1982, 71, 221. 25 I. Oref, 0. Herscovitz and E. Tzidoni, J. Phys. Chem., 1983, 87, 98. 26 R. E. Harnngton, B. S. Rabinovitch and M. Hoare, J. Chem. Phys., 1960,33, 744. 27 J. Troe, J. Chem. Phys., 1977,66,4745; 4758; J. Phys. Chem., 1979,83, 114. 28 H. Hippler, K. Luther and J. Troe, Faraday Discuss. Chem. SOC., 1979,67, 173. 2g T. F. Hunter, M. G. Stock and N. Webb, J. Chem. Soc., Faraday Trans. 2, 1979,75, 738. 30 I. Oref and B. S. Rabinovitch, Chem. Phys., 1977, 26, 385. 31 R. C. Bhattacharjee and W. Forst, Chem. Phys., 1978, 30, 217. 32 D. F. Kelley, T. Kasai and B. S. Rabinovitch, J. Phys. Chem., 1981, 85, 1 100. 33 I. Oref and B. S. Rabinovitch, Ace. Chem. Res., 1979, 12, 166. 34 Y. N. Lin and B. S. Rabinovitch, J. Phys. Chem., 1970, 74, 315. 35 I. Oref, Int. J. Chem. Kiner., 1977, 9, 751. 36 I. Oref, J. Phys. Chem., 1977, 81, 1967. 37 1. Oref, J. Chem. Phys., 1981, 75, 131; 1982, 77, 1253. 38 R. J. McCluskey and R. W. Carr, J. Phys. Chem., 1978, 82, 2637. 39 R. W. Carr, Chem. Phys. Lett., 1980, 74, 437. 4o H. W. Chang and D. W. Setser, J. Am. Chem. Soc., 1969,91, 648. 41 M. J. Rossi, J. R. Pladziewicz and J. R. Barker, J. Chem. Phys., 1983,78, 6695. 42 H. Hippler, J. Troe and H. J. Wendelken, 7th Int. Symp. Gas Kinet., Gottingen, Germany, 23-27 Aug. 43 D. C. Tardy and B. S. Rabinovitch, J. Chem. Phys., 1968,48, 5194. 44 J. D. Rynbrandt and B. S. Rabinovitch, J. Phys. Chem., 1970,74, 1679. 45 J. H. Georgakakos, B. S. Rabinovitch and E. J. McAlduff, J. Chem. Phys., 1970, 52, 2143. 46 S. Nordholm, B. C. Freasier and D. L. Jolly, Chem. Phys., 1977, 25,433. 47 H. W. Shranz and S. Nordholm, Int. J. Chem. Kinet., 1981, 13, 1051. 1982. (PAPER 3/74)

ISSN:0300-9599    

DOI:10.1039/F19848000769

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1984,80, 781-788 Square-wave Amperometric Monitoring of Reaction Rates BY BRIAN G. Cox* Chemistry Department, University of Stirling, Stirling FK9 4LA AND WOJCIECH JEDRAL* Chemistry Department, Warsaw University, 02-093 Warsaw, Poland Received 25th May, 1983 The application of square-wave amperometry, an analytical technique involving the use of two indicator electrodes polarized by a square wave, in the determination of reaction rates is described. Kinetic measurements have been made on the bromination of anisole in aqueous solution. Square-wave amperometry in conjunction with a stopped-flow apparatus equipped with Pt electrodes in the observation tube was used to monitor the disappearance of bromine in dilute solutions. Reaction rates with half-lives down to ca.5 ms can be measured using relatively high-frequency square waves (ca. 800-1000 Hz). The results are in good agreement with values measured spectrophotometrically at higher bromine concentrations. The rate of dissociation of Ag+ from its macrobicyclic cryptand 21 1 complex was also measured, using a pair of silver indicator electrodes. Free Ag+ may be monitored selectively in the presence of its cryptate complex by using a low-amplitude square wave (& 150 mV). The advantages and limitations of the amperometric technique for kinetic measurements are discussed. Square-wave amperometry is potentially a very useful technique for conventional analytical applications and for monitoring reactions over a wide range of reaction rates.l The principle of the method is illustrated in fig.1. When an alternating potential [fig. 1 (a)] is applied to a pair of identical polarizing electrodes immersed in a solution containing a suitable reversible redox couple (e.g. Br2/Br-) the resulting current is the sum of the faradaic current and the capacitance or charging current [fig. 1 (b)]. The capacitance current depends upon the rate of change of potential, dE/dt, and with a square-wave potential a large capacitance current is observed immediately following the change in sign of the polarizing potential. This current decays rapidly during the time of constant polarizing potential. The faradaic current requires the presence of both (or all) species involved in the electrode reaction and depends upon the concentration of the limiting reagent governing the electrode reactions, e.g.Br, in the presence of excess Br- (anode : Br- + $Br2 + e ; cathode : $Brz + e + Br-). The faradaic current also decays with time, owing to the depletion of depolarizer (Br, in the above example) in the vicinity of the electrode, but more slowly than the capacitance current [fig. 1 (b)]. It remains at a significant level prior to change in sign of E, and if the current is sampled at the end of each square-wave period [fig. 1 (c)] the measured value will be predominantly equal to the faradaic current. Both the capacitance and faradaic current depend upon the amplitude of the applied alternating potential. Full details of the technique have been discussed ear1ier.l A major advantage of the square-wave method over conventional d.c.amperometric methods is that when the square-wave potential is symmetrical (i.e. no d.c. bias is present) the measured current is independent of stirring. This is essential for use in 78 1782 MONITORING OF REACTION RATES (4 + .+ kg 0 I I I I I I time I (cl I I time Fig. 1. (a) Square-wave signal, generator output; (b) square-wave signal, cell output: (-) total current, (. . . . .) capacitance current; (c) delay (id) and sampling time (2,) of measuring circuit . conjunction with techniques which require measurements on still solutions. For example, in the stopped-flow technique measurements are made on virtually still solutions, generated by stopping rapidly flowing solutions just after mixing. The use of high-frequency square waves should, in principle, allow very fast reactions to be monitored.In practice, however, the usable frequency range will be limited by the decay of the capacitance current, because of an increasingly high background level relative to the faradiac current as the frequency increases. It is also possible that the establishment of a steady current proportional to the depolarizer concentration will take a finite time and hence influence the observed kinetic behaviour. The present paper describes a study of the use of the square-wave amperometric method to monitor the bromination of anisole in aqueous solution with an all-glass stopped-flow apparatus containing a pair of Pt electrodes in the observation tube. Reactions with half-lives down to ca. 5 ms could be followed using square-wave frequencies of up to 800 Hz.This is very close to the lower limit (1-3 ms) possible from rapid-mixing techniques, the application of which has until now been limited almost exclusively to reactions that may be followed using optical or conductimetric detection. The application of the method to monitoring free Ag+ in rapid equilibrium with its complex (cryptate) with the macrobicyclic cryptand 21 1 ligand (cryp) (I) has also been studied using two Ag electrodes: kP Ag+ + cryp t Ag(cryp)+. kdB. G. COX AND W. JEDRAL 783 It was found that Ag+ may be measured selectively in the presence of its cryptate complex, and this has been used to monitor the rate of dissociation of Ag+ from Ag(cryp)+ on the addition of excess acid : H+ Ag(cryp)+ -+ Ag+ + (cryp)H$+.(2) H20 Rates were measured at total Ag+ concentrations down to mol dmP3. (1) rO1 L o *J N L O ~ N U EXPERIMENTAL AND RESULTS MATERIALS Anisole was treated with FeSO,, washed with aqueous sodium hydroxide, dried over calcium sulphate and fractionally distilled under reduced pressure.2 No impurities were detectable in the n.m.r. and i.r. spectra. Cryptand 211 (Merck) was used without further purification. All inorganic chemicals were high-purity commercial grades. APPARATUS Square-wave signals were provided by a Farnell LFM4 Sine-Square generator, and the cell signal was measured using a sample-and-hold integrated circuit triggered by a delayed signal from the generator, as previously described.l The stopped-flow system was an all-glass apparatus, originally constructed for conductimetric dete~tion,~ containing a pair of Pt electrodes ca.0.2 cm2 in area. The signal amplitude from the generator was normally +250 mV for bromine detection and + 150 mV for Ag+ detection, at a frequency in the range 50-800Hz. Measured rate constants were independent of the generator frequency. It is clear from fig. 1 (c) that the sampling frequency is determined by the frequency output of the generator, and thus ideally the frequency should be high relative to the rate constant to be measured. Rate constants up to ca. 100 s-l were measured (4 x 7 ms) and on this time-scale signals recorded at 800 Hz (1.25 ms per sample) the output appears as an exponentially decaying step function. This, together with the increasingly high background capaci- tance current sets a lower limit of ca.5 ms on the half-life of reactions that can be conveniently followed using the present electronic circuit. Bromination reactions were also followed by observing the decrease in absorbance at 350 nm due to Br,/Br; using a Durrum Gibson stopped-flow apparatus. This equipment, when fitted with the temperature-jump cell containing a pair of stainless- steel electrodes, could also be used for current measurements on the Br2/Br- system. However, it was difficult to avoid small leaks from the drive and stop syringes providing an independent connection between the reaction solution and earth, which led to severe distortion of the kinetic traces. Output signals from the sample-and-hold circuit (current measurements) or the photomultiplier (optical measurements) were fed into a Commodore 30 16 microcom- puter via a Data Lab DL 901 transient recorder. The data were analysed by standard methods to yield the required first-order rate constants (see below).Silver-ion concentrations were monitored using a pair of silver wire electrodes sealed through soda glass (electrode area ca. 0.3 cm2) dipping into a glass, jacketed cell. Reactions were sufficiently slow (ti x 20 s) to be recorded with a conventional chart recorder.784 MONITORING OF REACTION RATES 0 1 .o 2 .o 3.0 4 .O [ anisole] /lo-' rnol dm-' [Br-] = 0.1 mol dm-3. Fig. 2. Rate of bromination of anisole in water at 25 "C: initial [Br,] x 7 x mol dm-3, All kinetic measurements were carried out at 25.0 f 0.2 "C ( & 0.5 "C for the Durum Gibson stopped-flow apparatus).BROMINATION OF ANISOLE Initial measurements using amperometric detection were carried out with 5 x lo-, < [anisole]/mol dm-3 d 4 x lo-,, ionic strength = 0.2 mol dmh3 (NaClO,) and [Br,] = 5 x 10-5-1 x lo-, mol dm-3. Under these conditions, pseudo-first-order kinetics were observed : [Br-] = 0.1 mol dm-3, - d[Br,]*/dt = k, [Br,]* (3) where [Br,]* = [Br,] + [Br,] represents the total bromine concentration. The observed first-order rate constant, k,, was directly proportional to the anisole concentration : k, = k,[anisole] (4) as shown in fig. 2. The initial bromination products are much less reactive towards bromine than the original so that no correction is necessary for further bromination. Product analysis was not attempted, but there is good that bromination takes place predominantly (> 90%) in the para position.Rate constants were then measured at varying concentrations of Br- over the range 0.02 d [Br-]/mol dm-3 d 0.3. Provided that Br, is much more reactive towards anisole than Br;,, the observed second-order rate constant, k,, should be represented by k2 = k,r2/(l +K[Br-I) ( 5 ) where kPr2 is the second-order rate constant for reaction of anisole with Br, and Kis the equilibnum constant [Br;]/[Br,] [Br-1. Values of k, measured at various bromide concentrations are listed in table 1, together with kBr, values obtained from eqn (9, using K = 16 dm3 mol-l.' The kBr, values show no systematic trend with [Br-1, inB. G. COX AND W. JEDRAL 785 Table 1.Rate of bromination of anisole in water at 25 'Ca WBrl k2 k2( 1 + nBr-1) kk( 1 + qBr-1) /mol dm-3 / 1 O4 dm3 mo1-l s-l / 1 O4 dm3 mol-1 s-l / l O4 dm3 mol-1 s-lb - 0.02 2.24 2.95 0.05 1.69 3.05 3.55 0.10 1 . 1 1 2.87 2.90 0.15 0.851 2.91 3.02 0.20 0.681 2.87 3.08 O.3Oc 0.524 3.04 3.15 a Ionic strength, I = 0.20 (NaC10,). k;l is the rate constant measured spectrophoto- metrically. I = 0.3. agreement with the assumption that Br; is much less reactive towards anisole than Br,. The average value [kBr, = (2.95 0.07) x lo4 dm3 mol-1 s-l] is in good agreement with earlier reported values: kBr, z 2.6 x lo4 dm3 mol-l s-l (measured using ring-disc electrodes);* kBr, = 2.7 x lo4 dm3 mol-1 s-' (electrochemical generation of Br, at low anisole concentration^);^ kBr2 z 4 x lo4 dm3 mol-1 s-I (approximate extrapolation from results obtained at 0 "C using cell-potential measurements at anisole concentra- tions of ca.lod5 mol dm-3).4 The reactions were also checked using optical measure- ments as described above and the results are included in table 1 for comparison. The agreement between the two sets of data is satisfactory. At low bromide concentrations, < 0.05 mol dm-3, it was very difficult to observe the reaction optically because of the low concentrations of the more highly absorbing Br; species present. However, note that difficulty was also experienced with ampero- metric measurements at low bromide concentrations (< 0.02 mol dm-3). Separate measurements suggest that at these low bromide levels linear response to bromine concentration is limited to bromine concentrations < 5 x mol dm-3 (cf.linear response to [Br,] 2 lop3 mol dm-3 at [Br-] = 0.1 mol dm-3).1 DISSOCIATION OF Ag(cryp)+ In the presence of an excess of acid, metal cations may be displaced quantitatively from cryptate complexes [reaction (1)].1°9 l1 It can be readily shown that for a reaction scheme represented by kd Ag(cryp)+ f Ag+ + cryp kr cryp + H+ 4 (cryp)H+ (7) i.e. the acid is acting as a scavenger for free cryptand in equilibrium with Ag(cryp)+, the observed rate law is given by - d[Ag(cryp)+l/dt = k, [Ag(cryp)+I k, = k, k[H+]/(k,[Ag+] + k[H+]). Provided k[H+] & kf[Ag+] eqn (9) simplifies to k , = k,786 MONITORING OF REACTION RATES 3 . 6 r Fig. 3. Dissociation of Ag+ from Ag(cryp)+ in the presence of excess acid: initial [Ag(cryp)+] = 3.2 x mol dmW3, [HClO,] = 2.0 x mol dm-3, [NaNO,] = 0.10 rnol dm-3. Table 2.Dissociation of Ag(cryp)+ in water at 25 "C" k,/ 10-2 s-1 [NaN03]/mol dm-3 . . . 0.008 0.028 0.1 0.3 1.0 0.1 [HClO,]/ loV3 mol dm-3 . . . 2.0 2.0 2.0 2.0 2.0 20.0 ~ ~~~~~ initial [Ag(cryp)+]/mol dm-3 9.0 x 10-5 4.6b 3.50b 3.34b 2.99 2.80 3.30 1.0 x 10-5 3.12 3.19 3.17 2.90 3.15 3.17 3 . 6 ~ - - 3.2 x 10-5 3.15 3.14 3.18 - 3.19 - - a Generator amplitude _+ 150 mV, frequency 200 Hz; silver electrode area 0.3 cm2. * Non- linear response of measured signal with concentration. i.e. the dissociation of the cryptate complex is the rate-determining step in the overall reaction. The stability of Ag(cryp)+ in water is relatively high, log K, = 8.5 l2 [corresponding to a difference in standard reduction potentials of free Ag+ and Ag(cryp)+of 500 m y , and the kinetic characteristics of the reduction of Ag+ and Ag(cryp)+ are presumably rather different. By applying a low potential difference across two silver electrodes (e.g. Ag (anode) -+ Ag+ + e Ag+ (cathode) + e -+ Ag 150 mV) a current controlled by the electrode reactions may be used to monitor Ag+ selectively in the presence of Ag(cryp)+.Reactions were initiated by adding a small aliquot of a concentrated solution of Ag(cryp)+ to an aqueous HClO, solution (normally 2 x mol dm-3) at varying ionic strengths (NaClO,). Fig. 3 shows a typical reaction trace. The increase in signalB. G. COX AND W. JEDRAL 787 corresponds to the increase in [Ag+] as Ag+ is displaced from Ag(cryp)+ by H+.Rates were measured for a variety of Ag(cryp)+ concentrations down to 3 x mol dm-3 and several different ionic strengths. The observed rate constant, k,, was independent of acid concentration, the ionic strength and the initial concentration of Ag(cryp)+ [table 2, cf. eqn (lo)]. The value obtained, k , = (3.1 kO.2) x s-l, may be identified with the dissociation rate constant, k,, for Ag(cryp)+ and compares favourably with an independent determination of this rate at low ionic strength with conductimetric detection, which gives (2.9 & 0.1) x s-l.13 When combined with K, (= k f / k d ) this gives a value of kf = 1.1 x lo7 dm3 mol-l s-l. It is expected that the use of silver-plated Pt electrodes in the stopped-flow apparatus will enable much faster reactions involving Ag+ to be monitored. DISCUSSION The stopped-flow study of the bromination of anisole shows that the amperometric method utilizing a square-wave polarizing potential can be used to measure reaction rates close to the stopped-flow limit.In this case we have used without modification a stopped-flow apparatus designed for conductimetric detection. It is clear from the agreement between results obtained with optical and electrochemical detection that the establishment of a faradaic current proportional to concentration is rapid on the stopped-flow time scale. The rapid response time combined with the very high sensitivity of amperometric methods1, means that it should be possible to measure second-order rate constants covering a very wide range.The time range of the present technique is limited by the decay rate of the capacitance current, which typically had a time constant of the order of 1 ms in our system (z = RC, where R is the resistance between the electrodes and C is the capacitance of the double layer). Experiments using pulsed potentials have reently been used to monitor reactions with half-lives approaching s,15 further emphasising the rapid response times possible using amperometric detection. This has been achieved in pulse polarographic experiments using i.r. compensation or charge-injection techniquesl5' l6 to charge the double layer in < s, allowing delays between charging the double layer and measuring the faradaic current as low as 7 ,us. This technique is of course limited to measuring rates of systems at equilibrium, rather than the irreversible reactions that we have described.Equivalent double-layer charging rates may be very difficult to achieve during continuous monitoring of concentration levels. Compared with conductimetric methods, a significant advantage of electrochemical detection is the ability to monitor selectively a given electroactive species in the presence of high concentrations of other ions. This is illustrated clearly in reaction (2), in which the concentration of Ag+ is measured in the presence of Ag(cryp)+, excess HClO, and NaC10,. Because the electrode processes at the two electrodes are identical (Ag+ + e =t Ag), square-wave potentials with very low amplitudes can be used, thus allowing great selectivity. A disadvantage is the requirement of reasonably high background electrolyte levels ( to 10-l mol dm-3) to ensure rapid decay of the capacitance current. In practice the background capacitance current is relatively insensitive to electrolyte concentration.This is because as the electrolyte concentration decreases the fraction of capacitance current remaining after a given time increases (z increases) but the absolute level of the initial current decreases. An important distinction may be made between the two reactions described here. In the dissociation of Ag(cryp)+, the species monitored (Ag+) is not a reactant and its concentration does not affect the reaction rate. On the other hand the bromination of anisole is first order in bromine concentration and alteration in concentration as788 MONITORING OF REACTION RATES a result of the electrode reactions will in principle influence the rate.For such a first-order reaction, however, the depletion of the species (Br, in this case) during one half cycle will be cancelled by the increase during the second half cycle. The cancellation will not be exact for a reaction which is, for example, second order with respect to a particular component, except in the limit of low potential (and hence low current). It can be readily shown that such effects should be small; e.g. variations by as much as a factor of two in the concentration of the monitored species in the vicinity of the electrode during one a.c. cycle result in an error of only 1 1 % in the measured rate constant. It may also be noted that the stopped-flow technique is in general inconvenient for the determination of rates of reactions which are second order in the monitored component, irrespective of the analytical technique used.The principle of the amperometric method described is identical to that used in the familiar dead-stop titration method based on a d.c. polarizing potential. However, analytical work involving d.c. polarizing potentials is hampered by the stirrer-dependent decay of the faradaic current. Quantitative measurements using rotating indicator electrodess? 14? 1 7 7 l8 have been described, but these are often inconvenient and cannot be used in some situations, such as stopped-flow measurements of fast reactions. By contrast, when symmetrical alternating potentials with frequencies as low as a few Hz are used the measured currents are independent of stirring.Furthermore, the required faradaic current may be conveniently measured using a simple and inexpensive sample-and-hold circuit-l This should greatly enhance the applicability of amperimetric measurements to the measurement of reaction rates. We thank the Nuffield Foundation for a grant to W. J. and Prof. H. Schneider of the Max-Planck-Institut fur biophysikalische Chemie in Gottingen for assistance in the construction of the stopped-flow apparatus. B. G. Cox and W. Jedral, J , Electroanal. Chem., 1982, 136, 93. Press, New York, 1966), p. 74. B. G. Cox, D. Knop and H. Schneider, J . Phys. Chem., 1980,84, 320. R. P. Bell and D. J. Rawlinson, J . Chem. SOC., 1961, 63. L. M. Stock and H. C. Brown, J . Am. Chem. SOC., 1960,82, 1942. 1. Tanigucki, M. Yans, H. Yamaguchi and K. Yasukouchi, J . Electroanal. Chem., 1982, 132, 233. R. 0. Griffith, A. McKeown and A. G. Winn, Trans. Faraday SOC., 1928, 24, 101. W. J. Albery, M. L. Hitchman and J. Ulstrup, Trans. Faraday SOC., 1968,64, 2831. J. E. Dubois and J. J. Aaron, C.R. Acad. Sci., 1964, 258, 2313. lo B. G. Cox and H. Schneider, J . Am. Chem. SOC., 1977,99, 2809. l 1 B. G. Cox, J. Garcia-Rosas and H. Schneider, J . Am. Chem. Soc., 1981, 103, 1054. l2 F. Arnaud-Neu, B. Spiess and M. Schwing-Weill, Helu. Chem. Acta, 1977, 60, 2633. l 3 B. G. Cox, H. Schneider and J. Stroka, unpublished results. l4 A. M. Bond, Modern Polarographic Methods in Analytical Chemistry (Marcel Dekker, New York, l5 M. Krizan, H. Schmidtpott and H. Strehlow, J . Electroanal. Chem., 1977, 80, 345. l6 M. Krizan, J . Electroanal. Chem., 1977, 80, 337. l 7 R. P. Bell and T. Spencer, J. Chem. SOC., 1959, 1156. * D. D. Perrin, W. L. F. Armarego and D. R. Perrin, PurrJication of Laboratory Chemicals (Pergamon 1980). J. M. Kolthoff and W. L. Reynolds, Discuss. Faraday Soc., 1954, 17, 167.

ISSN:0300-9599    

DOI:10.1039/F19848000781

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1984,80, 789-801 Gel and Liquid-crystal Phase Structures of the Trioxyethylene Glycol Monohexadecyl Ethert/Water System BY CRAIG D. ADAM, JAMES A. DURRANT, MICHAEL R. WRY AND GORDON J. T. TIDDY* Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Merseyside L63 3JW Received 6th June, 1983 Optical microscopy, differential scanning calorimetry, small-angle X-ray diffraction and nuclear magnetic resonance spectroscopy have been used to study the lyotropic phases formed by trioxyethylene glycol monohexadecyl ether with water (,H,O). The phase diagram exhibits regions corresponding to lamellar (La), inverse cubic [v,( l), V, (211, gel (Lp) and isotropic liquid phases (W, L,, L3). The two V, phases were identified by both optical microscopy and small-angle X-ray measurements.The gel phase exhibits a large melting entropy, and part of the molecule has a much restricted mobility. Low-angle X-ray data indicate that the alkyl chains are ordered, with a thickness equivalent to one hydrocarbon chain length. Taken together these data confirm a monolayer ‘interdigitated’ alkyl chain structure for the gel. In the presence of water the gel is stable (at thermodynamic equilibrium) below 40 “C. Anhydrous C,,EO, exhibits polymorphic phase behaviour. After melting the crystalline surfactant, a metastable gel structure forms on cooling and the crystalline solid reforms only after lengthy storage at low temperature. The maximum water-layer thickness of the La phase is larger than that of Ls despite the expected weaker inter-layer attractions in the latter case.This implies the existence of an additional repulsion in L,, possibly arising from elastic undulations of liquid bilayers. 1. INTRODUCTION Lyotropic gel phases formed by amphiphile +water mixtures comprise surfactant layers, made up of rotating, stiff, alkyl chains, which are separated by water layers. Thus they are similar to the lamellar phases, but have much less chain mobility. Bilayer gel phases occur fairly frequently with dialkyl surfactants, having been reported for zwitterionic,l? anionic3 and cationic4+ derivatives. Similar structures occur for monoglycerides which are monoalkyl lipid^.^-^ Here the gel phase is metastable in pure compounds,6 although it appears to be thermodynamically stable in mixtures of With monoalkyl ionic surfactants, gel phases have not been widely studied, although Vincent and Skoulios reported their existence in a range of alkanoate/water systems.The gels were observed for potassium, rubidium and caesium derivatives having C14 or longer chains, but not for lithium and sodium salts. The one cationic surfactant studied, hexadecyl trimethyl ammonium bromide, also formed a gel. For all of this group of gels, the surfactant layers consist of interdigited single chains with head groups ‘up’ or ‘down’ in an irregular manner [fig. 1 (a)]. Duringrecent studies of lyotropic liquid-crystal formation by alkyl poly(oxyethy1ene) glycol alkyl ethers,l13 l2 a ‘solid’ was observed to occur on addition of water to liquid t Systematic name : 3,6,9-trioxapentacosan- 1-01.789790 c 6 E03 /WATER MESOPHASES Fig. 1. Schematic representation of gel phase structures: (a) monolayer, (6) bilayer. trioxyethylene glycol monohexadecyl ether [n-C16H33(OCH2CH2)3 OH ; C,,EO,]. It was suspected that this might be a gel phase. Thus it seemed timely to undertake a thorough investigation of the mesophases formed by C16E03 + water mixtures. From optical microscopy and X-ray diffraction measurements we were able to study the gel and liquid-crystal structures. Differential scanning calorimetry (d.s.c.) gave transition enthalpies, from which molecular motion and structure could be inferred, while nuclear magnetic resonance (n.m.r.) spectroscopy was employed to monitor molecular orientation and mobility.The results of this investigation are detailed herein. 2. EXPERIMENTAL Optical microscopy and n.m.r. measurements were made as described previo~sly.~~ Differential scanning calorimetry was carried out using a Perkin-Elmer DSC-2 calorimeter, which was calibrated using materials with known temperatures of fusion (e.g. ice) and transition heats (e.g. high-purity indium). Transition enthalpies (AH) were estimated using standard procedures, with a reproducibility of & 2%. Measurements were made over the temperature range from - 30 to 50 "C using a heating rate of 5 "C min-l and a range of 5 mcal-l (1 cal = 4.1855 J) on ca. 5-1 0 mg samples sealed into aluminium pans. Temperatures > 50 "C were avoided because of problems of water loss. The small-angle X-ray scattering (SAXS) data were obtained using a Kratky camera with proportional-counter detection operated in continuous-scan mode.Spectra were not corrected for slit-smearing effects as these were estimated to have little effect on the d-spacings in this work. Samples were mounted either in 1 mm glass capillaries or sandwiched between pieces of mylar film to about the same thickness. High-temperature work utilised the Anton-Paar Kratky variable-temperature cell which is continuously adjustable up to ca. 70 "C, to a precision of ca. 0.5 "C. All high-angle diffraction work was performed on a Philips vertical diffractometer also using a continuous-scan mode. Deuterium oxide ( > 99.7 %) was commercially available, while the sample of trioxyethylene glycol monohexadecyl ether was that used previously.ll Samples were prepared by mixing the weighed constituents in sealed tubes above 40 "C. Sufficient of each sample was prepared so that n.m.r., microscopy, d.s.c.and X-ray diffraction measurements could be made on identical compositions. A sample of C,,EO, with a 2H-labelled terminal hydroxy group was prepared by dispersing the surfactant in 2H20, followed by removal of water under vacuum.c. D . ADAM, J . A. DURRANT, M. R. LOWRY AND G. J. T. TIDDY I 1 1 I 79 1 I I I I !- 20 I/ I 3. RESULTS 3.1. THE PHASE DIAGRAM The C,,EO,/water (,H,O) phase diagram is shown in fig. 2. It exhibits regions corresponding to lamellar (La), inverse cubic [V, (l), V, (2)], gel (Lg ) and isotropic liquid phases (W, L,, L3) (nomenclature according to previous publications'l, la).Phase boundaries were determined by visual examination of single-composition samples (ca. 20) using the hot-stage polarising microscope, together with d.s.c. runs on the same samples. The presence of a narrow central n.m.r. line, together with constancy of n.m.r. A values (see below), was used to estimate the water-rich La and Lg boundaries. Additional information, particularly on the temperature limits of the two V,-cubic phases, was obtained using the microscope ' penetration ' technique.15 A concentration gradient is set up by contacting pure surfactant with water on the microscope slide. The various phases form as separate ' bands' with distant boundaries, so that their appearance or disappearance can be monitored as the temperature varies.For the L, liquid region, the boundaries were obtained from visual examination of ca. 10 additional samples placed in an accurately thermostatted water bath ( _+ 0.05 "C). The boundaries correspond to the clearing or clouding of stirred samples as the temperature was slowly varied (0.5 "C min-l). While a number of general comments are made here some particular features, such as polymorphism of the anhydrous surfactant, are discussed in the following sections. Detection of the two V, phases was by both optical microscopy and small-angle X-ray measurements (see section 3.3). The 'penetration' experiment evidence has been792 C,,EO,/WATER MESOPHASES described previous1y.12 On single-composition samples the V2( 1)/V2(2) transition was visible without polarisers on heating by the formation of 'cracks' in the texture to give a 'crazy-paving' appearance, followed by their widening to occupy the whole field of view.A similar texture was observed at the V,/L2 transition. All the lyotropic phase transitions appeared to be reversible on cooling, provided that the rate of temperature change was sufficiently slow. Only the V,/L, transition showed a slight tendency to supercool. The L, region has tie-lines to both La and V2(2). We were unable to detect any difference in the lowest temperature at which the two V, phases occur. Thus we have no evidence for the presence of a V2(l)/L3 two-phase area. However, since this relies on penetration observations, and the two V, phases are difficult to distinguish when the cubic 'band' is narrow, we cannot discount the possibility that coexistence of L, and V2(1) occurs over a temperature range of ca.0.5 "C. Two-phase areas containing surfactant-rich phases and W or L, were easy to detect. Similarly, L,/mesophase or L,/gel regions were fairly wide. However, the La/LB and mesophase/mesophase coexistence regions were small and are shown as single lines on the diagram. Polymorphism of the pure surfactant made determination of equilibrium boundaries rather uncertain at very high surfactant concentrations, hence these are shown as dotted lines. The maximum water contents of V, and La phases are shown to be identical at the transition temperature. While this is unlikely to be correct in general, we were unable to detect any difference in this case.More difficult is the maximum water content of the La phase in equilibrium with W. The narrowness of this region prevented accurate measurements, but we saw no evidence to suggest a marked increase in water concentration compared with the La/L3 region. The phase diagram of C1,E0,/2H,0 strongly resembles that of C,,EO,/water already published.ll Minor differences are that the mesophases generally exist at lower temperatures and higher surfactant concentrations, as expected with a shorter oxyethylene (EO) chain. We have observed small differences between the transitions with ,H,O and those with normal water.ll While we could observe no change in the LB/La transition temperature, the other regions were shifted to lower temperatures with ,H,O by ca. 2 "C. This is similar to the reduction of the lower consolute temperatures (the cloud point) for C,EO, and C,EO, in ,H,O.l6 3.2.DIFFERENTIAL SCANNING CALORIMETRY A summary of the d.s.c. data is given in table 1, while typical heating scans are given in fig. 3. No measurements were made above 50 "C to avoid problems with both water loss and sample demixing owing to passage into the L, + W region. Only a few samples were studied at low temperatures (< - 10 "C) because of possible inhomogeneity problems caused by ice formation. The first heating scan on the anhydrous surfactant (dried for several days under vacuum over silica gel) gave the major peak at 34 "C with a small shoulder at 29 "C [fig. 3(a)]. On all subsequent scans the single major peak occurred at 29-30 "C with a much reduced intensity [fig.3(b)]. This polymorphism was easily observed on the optical microscope, where the first scan corresponded to the melting of the crystalline solid (ca. 35 "C) with formation of a gel texture on cooling. The gel structure was confirmed by X-ray and n.m.r. measurements (see below). Subsequent melting was at a lower temperature (31 "C). The crystalline solid reformed only after lengthy storage at low temperatures (days or weeks at -20 "C). Attempts to dry the sample further caused no change in behaviour, thus we do not think that small amounts of water are responsible for these results. We were unable to eliminate completely the ' gel' shoulder from the main melting peak, although low-temperature storage did reduce it to < 5% of the total heat.In the previous report" on C1,E03 the gel melting point is mistaken for that of the crystalline surfactant.C. D. ADAM, J. A. DURRANT, M. R. LOWRY AND G. J. T. TIDDY 793 Table 1. Summary of d.s.c. data C16E03 scan transition transition ASa (wt %) range/"C temperature/"C type AHu/kJ mol-l /mol-l K-l loob 100 - - 90.51 83.67 82.65 - - 79.89 78.37 - - - 73.53 - - - - 69.76 63.90 63.56 - - - - - hexadecane - 30-50 - - - 10-50 10-50 - 30-50 - - 10-50 - 30-50 - - - - 30-50 - - - - 10-50 10-50 - - 30-50 - - - - - 34.0 28.6 4.0 - 19.0 38.0 37.7 39.0 1.2 - 24.9 38.4 39.0 1 .o - 17.0 - 24.0 47.8 40.4 0.7 - 14.0 - 22.0 47.0 38.6 38.1 48.0 39.0 1 .o - 12.4 -21.5 18.1 76.7 34.7 0.9 1.8 31.8 32.0 32.0 1.8 2.9 29.2 29.2 1 .o 3.6 2.1 0.7 23.2 0.3 7.8 2.9 1.9 29.4 25.7 1.1 25.8 0.4 18.0 I .9 53.4 242 115 3 7 102 103 103 7 12 94 94 4 14 8 2 74 1 30 12 6 94 83 3 83 1 69 8 183 a Enthalpy and entropy per mole of surfactant.First heating scan after prolonged storage at -20 "C. cooling temperature/OC 47 37 27 17 I I I I J I I I I I I I I I 7 17 27 37 17 7 17 27 37 47 heating temperature/OC Fig. 3. Typical d.s.c. scans for anhydrous C,,EO,: (a) first heating from 5 to 50 "C, (b) cooling from 50 to 15 "C and (c) subsequent heating from 15 to 50 "C. 21 FAR 1794 C16E03/WATER MESOPHASES The magnitudes of transition heats (and entropies) given in table 1 can be compared with the values for melting of hexadecane.17 The value of AH for gel melting is significantly lower than the value for hexadecane, while that of the crystalline surfactant is larger.This is consistent with the hypothesis that the gel structure contains stiff, rotating, paraffin chains while the EO groups have conformational freedom, but spectroscopic evidence is required for proof. Two further transitions were observed for the gel at ca. 1 and -20 "C. These probably arise from minor modifications of the alkyl-chain packing since the A H values are too small to be due to crystallisation of the trioxyethylene group. No supercooling was observed with any of the gel transitions. In the presence of water we observed a small peak corresponding to the La/V2 phase change, while the major peak is due to the Ls/La or Ls/L2 transitions according to composition. This peak shifts to higher temperatures with initial addition of water.A peak growing in strength with water concentration appeared at - 12 to - 19 "C on cooling for samples containing < ca. 80% surfactant. This was attributed to ice formation by an excess of water and some water within the Ls phase, since a corresponding transition occurred at 1 "C on heating. One could regard the non-freezing water as 'bound', giving ca. 8 molecules per surfactant. Storage at low temperature (- 20 "C) failed to produce any sign of the crystalline surfactant, even at the lowest water concentration (90% C16E03), implying that the gel is the equilibrium state above this level. If water were regarded merely as a non-interacting solute in C16E03 liquid, only 3.5% is required to depress the freezing point by 4 "C, so this limit is not unreasonable.(A more stringent test of gel stability was made using X-ray measure- ments, see below.) Two further small transitions at 1 and -23 "C corresponding to those at similar temperatures with the anhydrous gel were also observed. The 1 "C transition was observed in cooling runs for high-water-content samples but it was obscured by water melting on heating. Again, we attribute these to alterations in the state of the alkyl chains, since the AH values are proportional to the C1,E03 concentration. Moreover, these transitions and the Ls/La or Ls/L, transitions were reversible as for the anhy- drous gel, supporting this conclusion. A few measurements were made on samples con- taining normal water. No difference in the transitions was observed, except that the ice melting peak occurred at 0 "C.In particular, the AH values for the small -23 and 1 "C transitions and for the Ls/La or Ls/L2 transitions were unaltered. 3.3. X-RAY DIFFRACTION MEASUREMENTS Small-angle X-ray diffraction measurements were made on samples in the La and Ls regions as a function of composition at 25 and 42 "C, respectively. Both gave spacings in the ratio 1 :a: g, as expected for layer structures.lo~ l8 Making the assumptions that discrete layers do exist, and that densities of hydrocarbon and water + EO layers are 0.773 x lo3 and 1 .O x lo3 kg m-3, respectively, the dimensions of the layers and area per surfactant molecule (a) can be calculated using standard formulae.18 Fig. 4 shows the unit-cell spacing (do), the amphiphile thickness (d,) and the water-layer thickness (dw) as a function of the water/surfactant weight ratio (1 - Ca)/Ca.The values of a are independent of surfactant concentration for both the gel and lamellar phases, being 46 Az for the gel phase and 40 A2 for the lamellar phase. Within the lamellar phase the do values increase with water concentration. The constancy of d, and a with increasing water content indicates that the repulsive interaction between the first one or two EO residues next to the alkyl chain is the dominant factor in determining these values, since they do not alter at higher water concentrations where the chains have the freedom to adopt more extendedC. D. ADAM, J. A. DURRANT, M. R. LOWRY AND G. J. T. TIDDY 795 60 4 5 0 0 do - 0 o o 0 00 00 * * d w + $ + + d , i t * ++ ++ + ** * 0 0.15 0.3 0.45 0.6 6 o r i 15 0 0.15 0.3 0.45 0.6 (1 -Ca>lC, Fig.4. X-ray diffraction data for C,,E0,/2H20 samples. 0, Bragg spacing (do); *, water + EO- layer thickness (dJ, and +, alkyl-chain-layer thickness (d,) plotted as a function of water/ surfactant weight ratio [( 1 - C,)/CJ : (a) gel phase, 25 "C; (b) lamellar phase 42 "C. configurations. Fully extended, a trioxyethylene chain is ca. 12.5 A long. Thus the maximum water-layer thickness in the L, phase (31 A) is ca. 20% larger than the extent of two all-trans EO groups. The thickness of the hydrocarbon region (24 A) is also longer than a single all-trans C,, chain (21 A), indicating that this region has a conventional liquid-bilayer arrangement with disordered chains. Little significance can be attached to the minimum water thickness of the oxyethylene layer, since it is determined by the L,/L2 transition rather than any particular property of the layers alone.In the region Lg the constant value of d, (21 A) is in excellent agreement with that expected for an hydrocarbon layer of single all-trans paraffin chains. Measurements on a few samples (67-83 % C,,EO,) at high angles gave a single, strong line at 4.18 A. These observations are identical to those of Vincent and Skoulios on monoalkyl ionic surfactants,lo demonstrating that the Ls phase consists of rotating, interdigited. all-trans alkyl chains with head groups almost randomly arranged in ' up ' or ' down ' positions [fig. 1 (a)]. The maximum water/EO swelling is ca. 25 A, equal to the full extension of two EO groups.27-2796 C,&O,/WATER MESOPHASES With the gel phase measurements were extended to the anhydrous surfactant. At the lowest water content (0.61 %) there was no sign of any spacing at higher distances (low angles) than 34 A. However, the crystalline surfactant gave a do value of 58 A, which was partially replaced by the gel line at 34A after heating (melting) and recooling to 25 "C. It proved impossible to obtain a sample (by heating and rapid cooling) where the gel do line was more intense than that of the crystalline solid. This implies that the absolute intensity of the do reflection is much lower in the gel than in the crystal, perhaps as a result of the disorder associated with the random alternation of the EO groups. Note that the presence of as little as 0.61 % water was sufficient to remove completely the crystalline surfactant.A single sample (63.9% C,,EO,) that exhibited two V, phases on the microscope was examined by X-ray diffraction at 52 "C and at 1 "C temperature intervals over the range 55-58 "C. At 52 "C a major peak at 61 A and a second at 50.5 A were observed. At 55 and 56 "C multiple reflections were observed, while at 57 "C a clear spectrum with peaks at 75A (major) and 52A was obtained. This is strong confirmation that a transition occurs between two phases additional to L, (do = 56 A). Unfortunately we were unable to obtain sufficient reflections to begin to determine the unit cells of the cubic phases present. At 58 "C a broad peak was obtained, corresponding to the V,/L, transition. 3.4.NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY The advantage of n.m.r. measurements on samples containing ,H,O is that ,H resonance can be used to monitor the state of the water, while lH gives information on the surfactant mobility. Nuclei such as ,H with spin quantum number I > 4 possess an electric quadrupole moment which interacts with electric-field gradients. In anisotropic media this results in a splitting of the resonance into 21 peaks. The frequency difference between neighbouring peaks ( A ) for a powder sample, where the electric-field gradient is axially symmetric, is given by19 where EQ is the quadrupole coupling constant, which is dependent on both nuclear properties and the magnitude of the electric-field gradient, while S is an order parameter depending on the time-average angle between the electric-field gradient and the mesophase axis (OD,) and is given by s = ( 3 cos2 - 1).(2) For ,H,O molecules in lyotropic liquid crystals the values of A are determined by the fraction of water molecules bound to the surfactant head group (Pb). These water moleculesareinrapidexchangewitheachotherandwith'free'water(z&, x lo7 s-l).,O Where a number of different binding sites (i) having different values of the order parameter (Si) are involved, as for the three different EO groups, the A value results from a weighted average of these: It is normally assumed that A = 0 for 'free' water. In the present case, since the terminal hydroxy group is expected to be in rapid exchange with that of water, this must be treated as an additional site.At high water concentrations the fraction of bound water is proportional to surfactant concentration. Hence we can writeC. D. ADAM, J. A. DURRANT, M. R. LOWRY AND G. J. T. TIDDY 797 Table 2. Values of q,eff for various C,,E03 phases crystalline solid 25 1 4c 1 4c Lp (dry) 25 24 170 Lp (1&33% ,H,O) 25 30f.3 175-260 La (1 7-39 % ,H,O) 42 150+30 (i.of0.2)x 103 La (sodium dodecanoate)d 82 120 - a Value of q,eff for paraffin chains. Value of q, eff for oxyethylene groups. Single- component decay. d Value for sodium dodecanoate taken from ref. (13). where C,/C, is the molar ratio of surfactant to water, n is the number of water molecules bound per surfactant molecule and A, is the weighted average A value of bound water.The values of A are expected to change abruptly at phase boundaries because of alterations in both pb and s b . In multiphase regions overlapping spectra are observed. In particular, the spectrum from a liquid crystal + W (or L,) dispersion consists of a singlet from W or L, and a doublet from the mesophase. In a binary system at equilibrium the compositions of two coexisting phases are invariant at a given temperature and pressure. Hence the lamellar or gel boundary with L,, W or L, can be detected by the appearance of a single peak which increases in intensity on appropriate alteration of concentration, while the liquid-crystal A value remains constant. For the lH n.m.r. of the surfactant we monitored the free induction decay (f.i.d.) following a n/2 pulse.In mesophases this is determined by residual dipolar proton- proton interactions that remain because the chain motion is not isotropic. The large number of possible dipolar interactions between one proton and every other one in C,,EO, makes impossible any analysis in terms of the individual CH, motions without more information. However, the overall lineshape is related to the degree of order of the m01ecule.~~-~~ Since C,,EO, contains two distinct parts (alkyl chain and head group) we anticipated that a two-component f.i.d. could be observed. We have chosen to divide the f.i.d. into two parts with intensities in proportion to the ratio of protons in each part. Each of these is characterised by an 'effective T, value' (T,,eff), which is the time taken for each decay height to reach l/e of the original value. This ignores much of the information contained within the decay, but has the merit of simplicity.Values of &,eff are expected to be longer for lower CH, order parameters (more disordered chains). In both the gel and L, phases the EO groups are expected to be more disordered than the paraffin chains, hence T,, eff (CH,) is assumed to be shorter than &,eff (EO). In practice the proton f.i.d. for these phases was clearly a two-component decay, with the faster-decaying component having the large intensity, confirming this assumption. Only a single % decay was obtained from crystalline C,,EO, at 25 "C so the same T, eff value is listed in table 2 for both the head group and the tail. Melting and cooling resulted in the appearance of two-component signals with longer decays.The value of 24 ps for the alkyl chains is consistent with an all-trans rotator phase, since it is longer than that of the crystalline solid, but much less than typical La phase values where the chains undergo rapid trans-gauche isomerisation. The value for the EO groups is a little longer than a typical La value, indicating a rather disordered arrangement of the head groups with rapid conformational exchange. After storage at -20 "C for 4 days the original 14 ps decay was obtained.798 C1&O3/WATER MESOPHASES Addition of water increased the q, eff value, a small increase being observed for the C,, chain, with a larger increase occurring for the EO groups. This suggests a further disordering of the chain on addition of water.Thus while an increased EO/water thickness might allow the EO groups to take the all-trans conformation more readily, it also makes 'off-axis' conformations more available, hence the EO group motion becomes more isotropic. As expected, both c16 and EO &,eff values increase on a transformation to the La phase. The value for the alkyl chains is only slightly larger than that of a typical ionic surfactant, but that of the EO groups indicates almost isotropic motion, suggesting a very disordered comformation of the head group. There is no variation of decay times with composition (within the limited range of stability of the La region). Water A values are shown in fig. 5 for the Lg (25 "C) and La (42 "C) phases. No differences in Lg values were observed between measurements made at 35 or 25 "C. First we observe that the A values are approximately twice as large in the Ls phase as in the La region.Mely and Charvolin observed a similar change at the lamellar/gel transition of potassium stearate +water Secondly, note that the A values at high water concentration in the La phase fit the 'free/bound' model described above, while much more limited agreement with this simple model occurs in the Lg phase. [The invariance of A with composition below a surfactant/water molar ratio of ca. 0.1 in fig. 5(a) is due to the occurrence of bulk water.] Deviation from the free/bound model occurs at ca. 75% C16E03 (mole ratio 0.19, where X-ray results show that the EO groups on opposing surfactant layers are close enough to interact.The decrease in A close to the La/L2 boundary has been checked several times. A similar decrease is with several C,, oxyethylene surfactants where it is thought to reflect an expected" increase in a (hence lower s) on passing to the L, region. Before discussing these results further we require an estimate of the contribution from the surfactant terminal hydroxy group to the observed A values. For the La phase, the fraction of ,H nuclei present in this site lies in the range 0.035-0.12. Their highest possible A value is that measured for the anhydrous- gel phase (8.82 kHz), reduced by the ratio of &, eff (EO) values for the La phase and anhydrous gel (0.17) (these being taken as a measure of the relative order parameters). This gives an enhancement of A in the range 50-180 Hz.However, since the terminal group is likely to have an order parameter that is even lower than this 'average', the actual contribution is likely to be lower. Hence alterations in A for the L, phase arise from changes in Pb, EQ or S of bound water. The difference between the A values in La or Lg is only a factor of ca. 2, while &, eff (EO) varies by a factor of 4-6. The latter value reflects changes in S for all the EO groups, and in this case is strongly influenced by the group having the lowest S value, which is likely to be the terminal OCH,CH,OH residue. Thus we conclude that A is dominated by the contribution of water molecules bound to the EO group next to the head group. This is confirmed by the observation that water A values for lamellar C1,EO,, C1,EO,, Cl2EO4 and C,,EO, are all similar to those reported here.25* 26 Since &, eff (EO) does not change markedly with alteration of water concentration, we conclude that the levelling off of A as a function of C,/C, reflects changes in the numbers of bound water molecules rather than changes in S due to interacting EO groups on opposing layers.In this composition region (> 70% C,,E03) all the water molecules are bound. Reduction of water content removes bound water molecules from all the EO groups equally. Thus in the La phase there is no evidence for any one EO binding water more strongly than the others. For the gel phase we have A values covering the range &33% 2H,0. They show a non-linear increase with surfactant mole ratio, suggesting that Pb, S and probablyC.D. ADAM, J . A. DURRANT, M. R. LOWRY AND G. J. T. TIDDY 6000 5500 5000 4500 4000 3500 2 3000 Q 2500 1 199 - - - 0 - 0 (C) - 0 - - - Om I3 0 v gel phase 5000 4500 4000 3500 - - - - I I 1 0.0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 v v lamellar phase P 2000 1800 1600 - - V - z --- a 1000- I I I I 1 0.0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 o gel 0 phase v lamellar phase 500L 0 I I I I I I I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 C16E03/water (molar ratio) Fig. 5. Water (2H) quadrupole splittings (A) as a function of surfactant/water mole ratio for: (a) gel phase, high water content (25 "C); (b) lamellar phase (42 "C); (c) comparison of gel and lamellar phase. [Continuous lines in (a) and (b) illustrate fit of values for high water content samples to ' bound/free' model.]800 C16E03/ WATER MESOPHASES EQ are concentration dependent (i.e.no ‘free’ water is present). The contribution from the terminal OH is one cause of this, since at the highest molar ratio (0.993) it could contribute up to one third of the observed value (for the anhydrous surfactant A = 8.82 kHz). However, this still indicates a rather large A value for bound water (5 kHz). The fact that the contribution from the terminal OH cannot be ignored implies that A values from water molecules bound to the second and third EO groups may also be significant. Hence the absence of a levelling off of the A values with increasing C,,EO, concentration cannot be taken to indicate preferential binding of water to one or other of the EO groups. Finally, we note that the gel-phase A values are two orders of magnitude less than expected for a fully ordered showing the considerable disorder present, 4.DISCUSSION From the evidence listed above, the gel phase has a melting entropy which is much less than that of the crystalline surfactant but nevertheless is still large. The gel part of the molecule has a much restricted mobility which is higher than that of the crystal. There is a long-range structure of one-dimensional periodicity and layer thickness equivalent to a single molecule. Taken together, these data confirm the monolayer ‘interdigitated’ alkyl-chain structure [fig. 1 (a)] for the gel. Because the aqueous gel melts at a higher temperature than the crystalline surfactant, it is a lower free-energy state than the liquid surfactant, i.e.it is at thermodynamic equilibrium. The stability of the gel is presumably due to energetically favourable hydrocarbon chain-chain interactions which are compatible with continued hydration of the head group. Thus the gel state does not occur with C,, surfactants over a similar temperature range because the paraffin chains are too short. Whether or not it occurs with longer EO surfactants remains to be investigated. However, if it occurs with C,,EO, and CI6EOl2 it is replacedll by hexagonal or cubic (I1) mesophases at higher water concentrations (> 35%), presumably because of the steric requirement of the hydrated head groups. It appears to be a common feature that gel phases in uncharged surfactant have smaller water-layer thicknesses than the neighbouring lamellar phases.This is observed both here and with lecithin,,'^ 28 but the origin of the behaviour has not been fully established. The maximum swelling of surfactant-layer phases in water can be represented as a balance between an attractive van der Waals force and repulsive forces.2g The repulsive forces have been attributed to hydration 30 elastic undulation modes of bilayers31 or zwitterion/dipole-induced image forces.32 Hydration forces arise from strong binding of water to head groups. It is that this changes the orientation of the bound-water layer, which in turn alters the positions of subsequent layers but to a lesser extent. It leads to a force with an exponential decay. For poly(oxyethy1ene) surfactants steric repulsive interactions between hydrated head groups obviously become important when the surfactant layers are sufficiently close that opposing EO groups touch each other.However, when comparing La and Ls phases in the present case, since the Ls phase has thinner bilayers than La, the attractive force is weaker. Hence an equivalent hydration for both phases would be expected to swell water layers of the L, phase to a larger extent than in the La phase. Also, any hydration force should operate beyond the length of fully extended EO groups, i.e. the water layers of both phases should swell beyond 25 A. This does not occur for the Ls phase. Thus, while we may regard hydration forces as operating up to the length of extended EO groups we have no evidence for longer-range forces.Similarly, if a dipole-induced image force3, operates, it also should swell the water layer of the Ls phase beyond 25 A. This force is likely to be very weak in our systemC. D. ADAM, J. A. DURRANT, M. R. LOWRY AND G. J. T. TIDDY 80 1 because of the very disordered EO chains and their weak dipoles compared with that of lecithin. There remains the mechanism suggested by Helfri~h,~' that hard-wall repulsions between fluid layers could give rise to a long-range force because of the layer elastic modes. Any liquid layer will undergo spontaneous bending. Since bending in neighbouring layers is not correlated, the layers pack at distances larger than hard-wall separation. The bending will be easier, the more fluid the layer is, while it will not occur at all in a rigid layer.Thus this mechanism operates for L, and not for Ls. Obviously it can account for the water thickness of L, being larger than that of LB in the present case. The Ls water layers swell under the influence of repulsion between hydrated EO groups, while the possibility of hydrocarbon-layer undulations is an additional repulsion for La. Further work on the effect of a and alkyl-chain thickness is required to substantiate this explanation. D. Chapman, R. M. Williams and B. D. Ladbrooke, Chem. Phys. Lipids, 1967,1,445. M. J. Ruocco and G. G. Shipley, Biochim. Biophys. Acta, 1982, 684, 59. M. Kodama, M. Kuwabara and S. Seki, Thermochim. Acta, 1981, 50, 81. M. Kodama, M. Kuwabara and S. Seki, Mol. Cryst. Liq. Cryst., 1981, 64, 277.K. Larsson, 2. Phys. Chem. (N.F.), 1967,56, 173. ' N. Krog and K. Larsson, Chem. Phys. Lipid, 1968,2, 129. K. Larsson and N. Krog, Chem. Phys. Lipids, 1973,10, 177. 13 N. Krog and A. P. Borup, J. Sci. Food Agric., 1973,24,691. lo J. M. Vincent and A. Skoulios, Acta Crystallogr., 1966, 20, 432; 441; 447. 3 H. Hauser, F. Paltauf and G. G. Shipley, Biochemistry, 1982, 21, 1061. D. J. Mitchell, G. J. T. Tiddy, L. Waring, T. Bostock and M. P. McDonald, J. Chem. Soc., Faraday Trans. 1, 1983, 79, 975. l2 G. J. T. Tiddy, K. Rendall and P. Galsworthy, Mol. Cryst. Liquid Cryst., 1982, 72, 147. l3 K. Rendall, G. J. T. Tiddy and M. A. Trevethan, J. Chem. Soc., Faraday Trans. 1, 1983,79, 637. l4 G. J. T. Tiddy, Phys. Rep., 1980, 57, 1. l5 A. S. C. Lawrence, in Liquid Crystals 2, ed. G. H. Brown (Gordon and Breach, London 1969), vol. 1, p. 1. M. Zulauf and J. P. Rosenbusch, J. Phys. Chem., 1983, 87, 856. H. L. Finke, M. E. Cross, G. Waddington and H. M. Huffman, J. Am. Chem. SOC., 1954, 76, 333; C Tanford, The Hydrophobic Eflect (John Wiley and Sons, New York, 2nd edn, 1980), p. 51. lS V. Luzzati, in Biological Membranes, ed. D. Chapman (Academic Press, London, 1968), chap. 3, p. 7. Is A. Johansson and B. Lindman, Liquidcrystals and Plastic Crystals, ed. G. W. Gray and P. A. Winsor (Ellis Horwood, Chichester, 1974), vol. 2, p. 192. 2o G. J. T. Tiddy, M. Walsh and E. Wyn-Jones, J. Chem. SOC., Faraday Trans. 1, 1982, 78, 389. 21 H. Wennerstrom, Chem. Phys. Lett., 1973, 18, 41, 2z J. Ulmius, H. Wennerstrom, G. Lindblom and G. Arvidson, Biochem. Biophys. Acta, 1975,389, 187. 23 M. Bloom, E. E. Burnell, S. B. W. Roeder and M. I. Valic, J. Chem. Phys., 1977,66, 3012. 24 B. Mely and J. Charvolin, Chem. Phys. Lipids, 1977, 19, 43. 25 K. Rendall and G. J. T. Tiddy, to be published. 26 T. Klason and U. Henriksson, Conference Report, July 1982, ed. K. Mittal 1983, to be published. 27 Y. Inoko and T. Mitsui, J. Phys. SOC. Jpn, 1978, 44, 1918. 28 J. Ulmius, H. Wennerstrom, G. Lindblom and G. Arvidson, Biochemistry, 1977, 16, 5742. 28 D. M. LeNeveu, R. P. Rand, V. A. Parsegian and D. Gingell, Biophys. J., 1977, 18,209. 30 S. Marcelja and N. Radic, Chem. Phys. Lett., 1976, 42, 129. 31 W. Helfrich, 2. Naturforsch., Teil A , 1978, 33, 305. 32 L. Guldbrand, B. Jonsson and H. Wennerstrom, J. Colloid interface Sci., 1982, 89, 532. (PAPER 3/9 19)

ISSN:0300-9599    

DOI:10.1039/F19848000789

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1984,80, 803-81 1 Redispersion of Cobalt Particles Supported on Titanium Dioxide BY SEIJI TAKASAKI, HIDEO SUZUKI, QORU TAKAHASHI, SHUJI TANABE, AKIFUMI UENO* AND YOSHIHIDE KOTERA Department of Materials Science, Toyohashi University of Technology, Tempakucho, Toyohashi-shi 440, Japan Received 6th June, 1983 The particle size of cobalt in a Co/TiO, catalyst increased with increasing reduction temperature up to 600 OC. At 700 OC, where the phase transformation of TiO, from anatase to rutile occurred, the cobalt particles redispersed to individual crystallites. This redispersion has been confirmed by measuring the specific activity of the cobalt catalyst for the hydrogenation of propene. The relation between the phase transformation of TiO, and the redispersion of cobalt has been studied kinetically using high-temperature X-ray diffraction spectroscopy.By adapting the one-electron approximation to metallic electrons, Kubol has re- vealed that the spacing between quantized electronic states becomes large in very fine particles. From his calculations the thermal and electronic properties may show considerable deviation from the normal bulk values when the size of the metal particle is < 100 A. Hence, studies have been focused on the relationship between the size of the metal particle and its catalytic behaviour. Many studies concerned with this relationship have been published and Boudart, has proposed that the catalytic reaction is either structure sensitive or structure insensitive. The hydrogenation of ethylene, cyclopropane or benzene on Pt/Al,033 is considered to be structure insensitive, while the hydrocracking, isomerization or dehydrocyclization of 2- methylpentane4 would be structure sensitive.Adsorption on size-controlled metal catalysts has also been studied extensively. Tauster was the first to observe a strong metal-support interaction (SMSI) when he studied the adsorption of hydrogen on platinum dispersed on various metal oxides. Bakers tried to clarify the nature of the interaction between platinum and titanium dioxide during the adsorption of hydrogen on platinum. He observed that the platinum particles increased in size with increasing reduction temperature up to 500 "C, but they redispersed to smaller sizes when they were reduced at 600 "C.This redispersion was explained in terms of the platinum- catalysed formation of Ti40,.6* ' Dumesic8 has observed the redispersion of iron particles supported on titanium dioxide when the catalyst was reduced by hydrogen at 600 "C. He assumed that the iron atoms migrated into the bulk oxide, which is reduced to Ti,O,. In the present work we have also observed the redispersion of cobalt particles in a Co/TiO, catalyst prepared by an alkoxide techniq~e.~ Redispersion was observed when the catalyst was reduced by H, at 7OO0C, the temperature of the phase transformation of TiO, from anatase to rutile. A catalyst prepared by a conventional impregnation method did not show redispersion at any reduction temperature, although the phase transformation of the support occurred at 700 OC. The sizes of the cobalt particles and the crystallites were measured using a transmission electron 803804 REDISPERSION OF C O PARTICLES SUPPORTED ON TiO, microscope and X-ray diffraction spectroscopy, respectively.The results show that the redispersion of the cobalt particles is in fact decomposition of the particles to the individual crystallites which make up the particle. The specific activities of the reduced catalysts for the hydrogenation of propene also indicate that the cobalt particles in the catalyst reduced at 700 OC redispersed to become much finer particles. The activation energies for the anatase to rutile phase transformations in the catalysts prepared by alkoxide and impregnation methods and in pure TiO, were measured by high-temperature X-ray diffraction spectroscopy.There was a consider- able difference between the activation energies of the two catalysts. This difference is attributed to the activation energy of the redispersion of cobalt particles in the catalyst prepared by the alkoxide technique. EXPERIMENTAL CATALYST PREPARATION The catalyst used was Co/TiO, and was prepared by hydrolysis of a mixed solution of tetraisopropyl titanate and cobalt nitrate dissolved in ethylene glycol. The cobalt nitrate was heated at 90°C to remove water, i.e. 90 O C Co(NO,), 6H20 - Co(NO,), * H,O and the tetraisopropyl titanate and ethylene glycol were distilled before use. Cu. 4.0 g of cobalt nitrate was dissolved in 100cm3 of ethylene glycol at 9OOC. Depending upon the desired concentration of Co in the catalyst, an appropriate amount of this solution was poured into the tetraisopropyl titanate in an atmosphere of dry N,.The solution was then stirred at 90 "C for several hours. Water was then added to the mixed solution in the ratio 4: 1 by volume to the amount of propyl titanate. A gel was obtained and it was dried by heating under reduced pressure. The fine powder obtained was then calcined at 500 OC in air for 4 h, followed by reduction for 4 h at various temperatures in a stream of hydrogen. The loading of Co ions in the catalyst could be controlled by varying the concentration of cobalt nitrate. The concentration of cobalt ions in the catalyst was measured by X-ray fluorescence spectroscopy after the extraction of the cobalt ions with hot nitric acid.In the present experiment the concentrations were 1.96, 4.11, 6.02 and 9.68 wt %. The catalyst containing 4.25 wt %cobalt was also prepared by an impregnation method using an aqueous solution of cobalt nitrate and titanium dioxide powder obtained by hydrolysis of the tetraisopropyl titanate. COBALT PARTICLE AND CRYSTALLITE SIZES The size of the cobalt particles and the size distribution were monitored by a transmission electron microscope (TEM, Hitachi H-300) operated at an accelerating voltage of 75 kV. The sample was first ground in an agate mortar and then smpended in water or ethyl alcohol using a supersonic wave. Some of the finest part of the suspension was pipetted onto a microgrid covered with a collodion film (400 mesh, Nissin Film Co.). The micrographs were obtained using an instrumental magnification of 50000 and they were enlarged 4 times when printed.The size distribution curves were obtained by measuring the dimensions of ca. 500 particles for each catalyst. The size of the cobalt crystallites was also measured using an X-ray diffractometer (Rigaku Denki Co., Geigerflex) operated at 40 kV with a filament current of 15 mA using a Zr filter for Mo Ka radiation. The average size of a cobalt crystallite was determined using Scherrer's equation for the diffraction peak at 28 = 20.0" assigned to (1 1 I) planes of cobalt. For these measurements the scanning speed was lowered to 0.25O min-'.J. Chem. Soc., Faraday Trans. I , Vol. 80, part 4 Plate 1 Plate 1. Transmission electron micrographs of Co/TiO, catalyst prepared by the alkoxide technique.The catalyst has been reduced by hydrogen at 500 OC. Wt % Co: (a) 1.96, (b) 4.11, (c) 6.02 and (d) 9.86. TAKASAKI, SUZUKI, TAKAHASHI, TANABE, UENO AND KOTERA (Facing p . 804)J. Chem. Sue., Faraday Trans. 1, Vol. 80, part 4 Plate 2 Plate 2. Transmission electron micrographs of 4.1 1 wt % Co/TiB, catalysts reduced at various temperatures: (a) 400, (b) 500, (c) 600 and (4 700 "C. TAKASAKI, SUZUKI, TAKAHASHI, TANABE, UENO AND KOTERAs. TAKASAKI et al. 805 HYDROGENATION OF PROPENE ON A SIZE-CONTROLLED COBALT CATALYST The hydrogenation of propene was taken as a model reaction to investigate the relationship between the size of the metal particle and its catalytic behaviour. The reaction was carried out using ca.1 .O g of the catalyst packed into a quartz reactor connected to a closed circulating system. The initial pressures of both hydrogen and propene were 100 mmHg and the gases were analysed by gas chromatography using a column packed with Polapak Q. The hydrogenation of propene was carried out at 50, 100 and 150 "C. In this temperature range catalytic hydrogenation often accompanies the hydrocracking to produce hydrocarbons with lower carbon numbers. Therefore, the rate of propane formation was measured for the initial 5 or 10 min of the reaction, where hydrocracking was not observed. Before the hydrogenation, the catalyst in the reactor was reduced by hydrogen at the desired temperature for 4 h and then evacuated at the same temperature for 2 h, followed by cooling to the reaction temperature.PHASE TRANSFORMATION OF TiO, FROM ANATASE TO RUTILE The activation energies of the phase transformation of titanium dioxide from anatase to rutile, both in the presence and the absence of cobalt ions, were measured by high-temperature X-ray diffraction spectroscopy in the temperature range 650-720 "C. The sample, prereduced by hydrogen at 500 "C, was placed in a sample holder made of stainless steel.lo The holder had a thermocouple in it and it was placed inside the oven of the X-ray apparatus. The oven was sealed by mica or aluminium foil through which the X-ray passed. Hydrogen gas was introduced into the oven at the flow rate of 100 cm3 min-l. For the kinetic measurements of the phase transformation, the changes in peak heights at 28 = 29.4" assigned to anatase and 28 = 32.0" assigned to rutile were monitored using Co Ka radiation with an Fe filter.The X-ray diffracto- meter was operated at 30 kV with a filament current of 10 mA. At the desired temperature, the gas introduced into the oven was changed from N, to H, and the measurements were started, although only a few percent of the anatase had been transformed to rutile. RESULTS PARTICLE SIZE AND THE SIZE DISTRIBUTION Some of the photographs obtained for the catalysts-prepared by the alkoxide technique and reduced at 500 OC are shown in plate 1. The concentration of cobalt in the catalyst was varied from 1.96 to 9.86 wt %. The size distributions obtained by measuring ca. 500 particles in each photograph are shown in fig. 1. The effect of the reduction temperature on the size distribution was studied using the 4.1 1 wt % Co/TiO, catalyst in the temperature range 400-700 OC.The photographs obtained are shown in plate 2 and the particle size distributions are given in fig. 2. The mean size of the cobalt particles in the reduced catalysts was calculated using the equation d = Eni di/Cni where ni is the number of particles of size di(A). The results are plotted against the reduction temperature in fig. 3. The size of the cobalt crystallite was also measured using the X-ray diffractometer. The catalyst was dipped into collodion oil immediately after reduction to prevent exposure to air and it then underwent X-ray analysis. The crystalline size was calculated using the equation D = O.~L/COSP where A is the wavelength of the radiation from Mo Ka and 2/? is the width of the peak at half height, following diffraction from the (1 11) plane of the cobalt crystallite. The results are shown in fig.3. However, the catalyst prepared by a conventional impregnation method contains cobalt particles with sizes from 20 to several hundred ingstroms. The size distributions of the catalysts reduced by hydrogen at 500 and 700 O C are shown in fig. 4.806 50 0 !! 30 6 2 ti 10 0 50- a 2 5 30- 2 c.’ a 10- 0 L REDISPERSION OF C O PARTICLES SUPPORTED ON TiO, 200 400 600 0 particle size/A ( b ) 20 10 H 600 bL 800 1000 particle size/A ( d ) n 200 400 600 0 200 400 600 800 1000 1200 particle size/A particle size/A Fig. 1. Particle size distributions of cobalt in Co/TiO, catalysts prepared by the alkoxide technique.Wt % Co: (a) 1.96, (b) 4.1 1, (c) 6.02 and (4 9.86. SIZE EFFECTS ON THE HYDROGENATION OF PROPENE After prolonged reaction, ethane and methane were formed by the cracking of propene and/or propane, and so the rate of propene hydrogenation was estimated by measuring the amount of propane formed in the initial stage of the reaction. The rate of hydrogenation is expressed in terms of a turnover number: the amount of propane formed per minute and per unit surface area of cobalt metal in the catalyst. The specific surface area of cobalt was estimated using the equation S = 5/pd where p is the density of metallic cobalt (i.e. 8.96 g ~ m - ~ ) , d is the size of the cobalt particle observed by TEM, and it is assumed that the cobalt crystallites are cubes. The rates thus obtained are plotted against the cobalt particle size for 4.1 1 wt % Co/TiO, in fig.5 . From the rates thus calculated the activation energies for hydrogenation were obtained (fig. 6). PHASE TRANSFORMATION OF TiO, FROM ANATASE TO RUTILE Kinetic studies of the phase transformation of TiO, from anatase to rutile were carried out using a high-temperature X-ray diffractometer. The rates of the phase transformation were estimated by measuring changes in the anatase and rutile peaks. TAKASAKI et al. 807 Q) 2 2 30 a, 20 10 W DD c.' c 2 a 50 401 0 particle sizelA particle sizelii 40 501 ( d ) 0 200 4 00 particle size/A particle sizelii Fig. 2. Cobalt particle size distribution in 4.1 1 wt % Co/TiO, catalyst reduced at various temperatures: (a) 400, (b) 500, (c) 600 and (d) 700 OC.5 00 4 00 5 N Q) ." Z 300 3 5 200 8 -u a, - .- c1 m a 100 0 1 I I I 4 00 500 600 700 reduction temperature/°C Fig. 3. Changes in mean particle size of cobalt (m) and cobalt crystalline size (0) with varying reduction temperature.808 F .- 1- - - - REDISPERSION OF Co PARTICLES SUPPORTED ON TiO, - -. 7 30 20 E 2 *-) a 10 w g 4 10 0 30-- & 20- 10- I 200 400 600 800 particle size/A ( 4 I 1 I 1 -1 10 \ '\ 0 I00 200 300 400 mean particle size/A Fig. 5. Effects of mean particle size of cobalt upon the specific activity of the cobalt catalyst for propene hydrogenation. Reaction at a, 50; 0, 100 and a, 150 OC.s. TAKASAKI et al. 809 40 d I - 2 6 2 20 \ 5 c m .- I u 0 I I I 0 / / i 200 400 600 mean particle sizelA Fig.6. Activation energy for propene hydrogenation catalysed by Co/TiO, catalysts. - 2 -Y E: - -4 -6 1 1- I I I I 1 I I 0 1.04 1.08 lo3 KIT Fig. 7. Arrhenius plots for anatase to rutile trznsformations in m, pure TiO, and in catalysts prepared by e, the alkoxide method and +, the impregnation method.810 REDISPERSION OF CO PARTICLES SUPPORTED ON TiO, heights. The height of the peak is assumed to be proportional to the concentration of the species to which it corresponds. The catalysts used were the 4.1 1 wt % Co/TiO, sample prepared by the alkoxide technique and the 4.25 wt% Co/TiO, sample prepared by the impregnation method. The phase transformation of TiO, in pure titanium dioxide was also investigated. From the rates thus obtained the activation energies for the phase transformation were estimated (fig.7). DISCUSSION As has been mentioned in a previous paper,g the size of nickel particles in a Ni/SiO, catalyst prepared by the alkoxide technique was controllable by changing the concentration of nickel in the catalyst. In the present work, the size of cobalt particles in the Co/TiO, catalyst prepared by the alkoxide technique was uniform only when the concentration of cobalt was < ca. 6 wt %. As can be seen in fig. 1, the particle size distribution becomes broad for 6.02 and 9.86 wt % Co/TiO, catalysts. Accordingly, this discussion will be concerned with the 4.1 1 wt % Co/TiO, catalyst, which has a relatively sharp particle size distribution curve. The reduction temperature is observed to have a significant effect on the size distribution.The size of the cobalt particles increased with increasing reduction temperature up to 600 OC, while at 700 OC the particle size decreased suddenly from ca. 425 to 100 A (see fig. 3). This phenomenon is called redispersion and is not unique to our work. Bakers and Dumesic8 have already reported the redispersion of platinum and iron particles, respectively, supported on TiO,. The redispersion of cobalt particles appears to be decomposition of the cobalt particle to individual crystallites, since the size of the particle is almost the same as the size of the crystallite when the redispersion occurs at 7OOOC (see fig. 3). The mechanism of the redispersion is still ambiguous. According to Baker et aZ.s the redispersion is caused by the platinum catalysed formation of Ti,O,.The formation of Ti,O, was shown by electron diffraction and, later, by e.s.r. rneas~rements.~ In the present work, no Ti,O, formation was observed by electron diffraction but the phase transformation of TiO, from anatase to rutile was observed using X-ray diffraction when the catalyst was reduced at 700 OC. The relationship between the redispersion of cobalt particles and the phase transformation will be discussed later. The redispersion of cobalt particles was confirmed by measuring the catalytic activity for propene hydrogenation. As can be seen in fig. 5, the catalyst reduced at 700 "C shows the highest turnover number of all the catalysts, suggesting that the cobalt particles in the catalyst reduced at 700 OC are the finest.The fraction of cobalt reduced to metal does not depend upon the reduction temperature used. Magnetic measurements have shown that the fraction of cobalt metal in the catalyst reduced at 500 "C was ca. 77%, while that in the catalyst reduced at 700 "C was ca. 78%. Fig. 6 gives the activation energies of the catalyst reduced at various temperatures for the propene hydrogenation. Again, the lowest activation energy was observed for the catalyst reduced at 700 "C. This leads to the conclusion that the cobalt particles in Co/TiO, catalyst prepared by the alkoxide technique are redispersed when the catalyst is reduced by hydrogen at 700 "C and the redispersion consists of decomposition of the cobalt particle into the individual crystallites of which it is composed.Note that the catalyst prepared by the conventional impregnation method did not redisperse when the catalyst was reduced at 700 OC. As suggested by Baker et aL6 the redispersion of the metal particles is involved with the change in the structure of the TiO,. In the present work the phase transformation of TiO, from anatase to rutile was observed when the catalyst was reduced at 700 OC, the temperature at which the redispersion of cobalt particles occurred. Therefore, thes. TAKASAKI et at. 81 1 activation energies of the phase transformation were measured for the catalysts prepared by the alkoxide technique and the impregnation method and for pure titanium dioxide. The results are shown in fig. 7, indicating that the values of the activation energy decreased in the sequence pure TiO, > alkoxide technique > impreg- nation method.In a previous paper, it was reported1' that the rate equation for the transformation is expressed in terms of the first order of the anatase concentration and the activation energy for pure TiO, was ca. 504.0 kJ mol-l. In the present experiments the activation energy for pure TiO, is 550.6 kJ mol-l, which is in good agreement with the earlier value; the activation energies for the catalysts prepared by the alkoxide technique and impregnation method are 500.6 and 350.7 kJ mol-l, respectively. According to Shannon et a1.12 the activation energy for the transformation is governed by the nature and amount of impurities introduced. In the present case, the added cobalt ions seem to decrease the activation energy for the phase transformation. Although the amount of cobalt ions added to both catalysts is almost the same, there are considerable difference in the activation energies. We believe that the difference in the activation energies can be attributed to the activation energy for the redispersion of the cobalt particles in the catalyst prepared by the alkoxide technique, since in the catalyst prepared by the impregnation method no redispersion occurred. We thank Dr J. Tasaki, Y. Murase and T. Mori of the Japanese Government Industrial Institute, Nagoya for measuring the magnetization and for the transmission electron micrographs. R. Kubo, J. Phys. SOC. Jpn, 1962, 17,975. M. Boudart, Proc. 6th Int. Congr. Cutul. (The Chemical Society, London, 1976), p. 1 . J. M. Dartigues, A. Chambellan and F. G. Gault, J. Am. Chem. SOC., 1976, 98, 856. S. J. Tauster and S. C. Fung, J. Catal., 1978, 55, 29. R. T. Baker, E. B. Prestridge and R. L. Carten, J. Catal., 1979,56, 390; 1982, 59, 293. * M. Boudart, Adv. Catal., 1960, 20, 153. ' T. Huizinga and R. Prins, J . Phys. Chem., 1981,85, 216. * B. J. Tatarchulk and J. A. Dumesic, J. Catal., 1981, 70, 308. l o A. Ueno, S. Okuda and Y. Kotera, 4th Int. Con$ on the Chemistry and Uses of Molybdenum, l1 Y. Suzuki and Y. Kotera, Bull. Chem. SOC. Jpn, 1962, 35, 1353. l* R. D. Shannon and J. A. Pask, J. Am. Chem. SOC., 1965,77, 391. A. Ueno, H. Suzuki and Y. Kotera, J . Chem. SOC., Faraday Trans. I , 1983, 79, 127. Colorado, 1982, p. 250. (PAPER 3/933)

ISSN:0300-9599    

DOI:10.1039/F19848000803

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. 1, 1984, 80, 813-822 Experimental Evidence of the Influence of Sorption-heat Release Processes on the Sorption Kinetics of Benzene in NaX Zeolite Crystals BY MARTIN BULOW,* PETER STRUVE AND WOLFGANG MIETK Central Institute of Physical Chemistry, Academy of Sciences of the G.D.R., 1 199 Berlin-Adlershof, Rudower Chaussee 5, German Democratic Republic AND MILAN KOCIRIK J. Heyrovskjr Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 12 138 Prague, Machova 7, Czechoslovakia Received 8th June. 1983 Under constant-volume-variable-pressure conditions the sorption uptake of benzene by NaX zeolite crystals has been investigated with respect to the influence of external thermal conditions on the uptake rate. The information obtained permits quantitative conclusions concerning the influence of the sorption heat generated on the uptake rate, i.e.the decrease in apparent diffusivities calculated under the erroneous presumption of the isothermicity of the sorption process considered. A corresponding tentative mechanism which takes into account the generation of thermal interface barriers is proposed. Furthermore, there are experimental conditions for which the uptake of benzene by NaX zeolite crystals should be considered as an isothermic process. Within that parameter region agreement was obtained between the sorption diffusion data (corrected by the Darken equation) and the n.m.r. self-diffusivities, as reported earlier. Both the n.m.r. field-gradient impulse technique' and the sorption-kinetic method2* have shown the high intracrystalline mobility of adsorbed species in large-port microporous sorbents.Therefore, if one investigates the rate of uptake of hydrocarbons on NaX zeolites, attention should be paid to external (apparatus) effects, e.g. the limited rate of sorbate supply through certain parts of the equipment4 and the finite response time of the data-logging system used [cf. ref. (5)], as well as to the non-isothermicity which may arise from the generation of sorption h e a t ~ . ~ - l ~ The latter phenomenon requires theoretical as well as experimental studies. In addition to our efforts to develop a (hitherto missing) theoretical model for simultaneous mass and heat transfer under constant-volume-variable-pressure conditions,14 we endeavoured to observe directly the influence of the sorption heat generated on the uptake rate.To obtain the necessary information we consider the sorption kinetics of benzene on NaX zeolite, a system whose intracrystalline molecular mobility is described in ref. (3) and (15). EXPERIMENTAL Uptake measurements were carried out by means of a constant-volume-variable-pressure method as described previously.2. 3, 4 9 l6 Details of the improved apparatus used have been reported in ref. (3). The main parts of the apparatus are the dosing volume and the sorption vessel, which are connected by a bakeable ultrahigh-vacuum valve. The experimental pressure (p), temperature (T) and volume ( V ) which characterize these volumes are denoted by the subscripts 0 and v, respectively. 813814 SORPTION OF BENZENE IN NaX ZEOLITE Table 1.Temperature difference AT = T, - T, between volumes V , and V, for dead-time curve moments z at T, = 353 K and related temperatures T, 2.27 2.27 4.40 5.20 8.40 9.46 31.33 32.26 40.66 53.32 166.63 173.96 233.28 100 72 52 10 0 100 -41 100 72 52 -41 100 72 0.245 0.24 0.24 0.24 0.21 0.20 0.14 0.135 0.13 0.10 0.085 0.08 0.075 Table 2. Parameter regions covered in measurements of the uptake of benzene on NaX zeolite at T, = 353 K and different temperatures T, 307 343 353 355 363 parameter p(m)/Pa 0.03-180.54 0.03-335.3 0.03-180.54 0.05-173.22 0.12-0.93 pressure step width 10.66-44.70 24.59-53.70 20.65-35.59 20.03-26.87 49.8574.33 in particular uptake runs/Pa concentration/ mmol g-l concentration step width in particular uptake runs/mmol g-l sorbate 0.15O-2.54O 0.245-2.290 0.245-2.280 1.03-2.24 0.365-1.240 sorbate 0.09-0.20 0.090-0.245 0.090.245 0.08-0.22 0.22-0.34 The time dependence of the pressure drop in the system after opening the forementioned valve was measured automatically in the volume V,.In accordance with the aim of this investigation, attention was mainly directed to the influence of temperature differences between those parts of the apparatus on the uptake rate. The dead-time behaviour of the equipment is characterized by a temperature (T,)-invariant dependence of the first reduced statistical moment z of the expansion curves of benzene vapour pressure measured as a function of the pressure po(.o) (i.e. po at a time t -+ co) in the absence of sorbent [cf.fig. 2 in ref. (3)]. Table 1 shows that parameter to be unaffected by the temperature difference A T = T, - 3. This paper presents and discusses uptake data for two senes of experiments carried out at the following temperatures (q): series (i):3 353 (307), 373 (313), 403 (319) K; series (ii): 353 (307, 343, 353, 355 or 363) K. In almost all experiments the ranges of sorbate concentration covered by particular uptake runs were sufficiently narrow and comparable (cf. table 2) to allowM. BULOW, P. STRUVE, w. MIETK AND M. KOCI~IK 815 investigation of the influence of the external thermal conditions on the uptake rate. To obtain the sorbate concentration c, the various temperatures T,, T,, TT and TM (= q) of the different parts of the apparatus were taken into account [the subscripts T and M denote the tubing and the valve connecting V, and V .and the pressure sensor (located in the dosing volume), respectively]. NaX zeolite crystals, with a mean radius R = 60 pm of spheroids circumscribing the crystals, were used as the sorbent. Their main properties have been reported in ref. (2). A few milligrams (8.08 mg of a dehydrated sample) of crystals were spread over the flat glassy bottom of the sorption vessel in the form of a one-crystal monolayer. Thus it could be assumed that in tercrystalline mass and heat transfer, which generally affect uptake curves, should not significantly interfere with the intracrystalline transport processes. Consequently, if there is any effect of the sorption heat on the sorption uptake, it should influence the intracrystalline mobility region.DATA EVALUATION To evaluate apparent diffusivities it is assumed that the sorption kinetics investigated reflect molecular transport in the zeolite crystals. Furthermore, the segment of the sorption isotherm which is covered by an individual uptake process is presumed to be linear and the sorption process to be isothermic. The latter presumption is required owing to the lack of necessary equations for the evaluation of simultaneous heat and mass transfer under constant-volume- variable-pressure conditions. We thus used the equations of statistical-momentum theory, which are valid under the conditions stated. This formalism also enabled us to take into account the finite mass flow rate through the valve and the tubing between the sorbent and the pressure enso or.^ These formulae have already been described in the l i t e r a t ~ r e .~ ~ ~ RESULTS AND DISCUSSION As a detailed discussion of the results of series (i) has been given previo~sly,~ these results will only be outlined below. The measurements of series (ii) led to the sorption isotherm of benzene on NaX zeolite at 353 K shown in fig. 1. This isotherm is evaluated under various temperatures in sections of apparatus (which were taken into account). The particular symbols used indicate different temperatures (T,) chosen for separate uptake series (cf. table 2). For most values the coincidence of the isotherms observed shows that, despite the variation in T, of the benzene vapour dosed into the vessel K, there is the expected invariant equilibrium state. Consequently, external temperature effects related to the sorption equilibrium are excluded.By contrast to the equilibrium state, on varying the temperature T, the uptake behaviour of the sorption system under consideration is changed significantly. For example, table 3 shows the variation in half-life of sorption uptake arising from changes in the temperature T, at T, = 353 K. This picture is complemented by the information obtained from fig. 2, which gives the sorbate concentration dependences of the apparent diffusivities D a p p evaluated by means of the isothermal diffusion model and assuming the external parameter z to be zero, i.e. that the valve has no influence on the uptake curve. After applying the procedure of valve-effect correction we treated the data obtained by the Darken equation, R P P = D a p p (a In c/a lnp),, (for evaluating the so-called corrected diffusivities DtPP to compare them with n.m.r.self-diffusion data, DinEr.). The results are given in fig. 3, in which the values evaluated at sorbate concentrations c < 1.5 mmol g-l and temperatures T, < T, = 353 K have been omitted owing to their large error range. The latter is due to similar values for the first reduced statistical moment of the plot of po against t at decreasing sorbate pressure po(t) on the one side and for the contribution of the valve effect during the uptake process on the other side.816 SORPTION OF BENZENE IN NaX ZEOLITE Po (-)/Pa 10- ' 10 O 10 I an - g 2.0 E \ 1 .o I O - ~ 10- ' I0 O P o (-)/Ton Fig. 1. Sorption isotherm of benzene on NaX zeolite at T, = 353 K and different temperatures T,/K: A, 307; 0, 343; V, 353; V, 355; @, 363. Table 3. Half-lives t(po(t)/[po(0) + pO(m)] = 0.5) of pressure decrease in the volume V, during the sorption uptake of benzene on NaX zeolite at T, = 353 K and various T, (values averaged over all uptake steps) 307 0.20-0.25 343 0.40.50 353 1-3 355 7-1 1 363 32-34 Within certain error ranges the values D t P P at T, > T, (307 and 343 K) are similar, although some dependence on the temperature T, seems to be significant. At T, = T, a remarkable decrease in the values D t P P over the entire concentration range considered is obtained, whereas at T, > the values DtPP are greatly reduced in comparison with those derived for T, < T,.In all experiments at varying T, the sorbate step widths are similar (excluding those at T, = 363 K, where the step width is about twice the value of that for the other series). So far the experimental conditions are comparable. Owing to sufficiently narrow concentration step widths we can consider the sorption-isotherm sections covered by particular uptake runs as being linear, so that any influence from this factor on the sorption rate may be disregarded in most cases. Consequently, one may assume that the uptake behaviour observed reflects the mass-transport phenomena in or on the zeolite crystals. As discussed elsewhere3 for the sorption system under consideration heat-release effects might be superimposed upon molecular diffusion.Thus further proof is necessary to decide between intracrystalline diffusion and other possible influences onM. BULOW, P. STRUVE, w. MIETK AND M. KOCI~IK 1 o-'O 817 0 . 0 . - - 0 J ~ I I ~ I I I 1 I ~ ~ I I ) I I I ' ' ' ~ l o - 6 / V V V l . . I . . . l I . . . . I . . . 1 1 . 1 , 0.5 1.0 1.5 2.0 clmmol g-' Fig. 3. Corrected apparent sorption diffusivities, D t P P , of benzene on NaX zeolite at T, = 353 K and different temperatures T,. Symbols as fig. 1 . the uptake rate measured. For this one can use the following tests: (i) comparison of uptake data with n.m.r. self-diffusion data,17-19 (ii) analysis of the shape of uptake curves20 and (iii) variation of the size of the zeolite crystals.ls, 2 1 ~ 22 The latter possibility must be ruled out because the size of crystals at our disposal is restricted.Owing to the hitherto available knowledgez3* 24 concerning the correlation between diffusion and self-diffusion, test (i) is the most reliable at the present. In fig. 4 a comparison like818 SORPTION OF BENZENE IN NaX ZEOLITE c a 0 0 nr A 0 0 0 0 0 . 4v 0.5 1.0 1.5 2 .O c/mmol g-' Fig. 4. Comparison of corrected sorption diffusivities, Do (closed symbols), with n.m.r. self-diffusivities, DEEr. (open symbols), of benzene on NaX zeolite in the intracrystalline region. T/K = 0, D, 353; A, A, 373; 0, 0 , 4 0 3 . that reported in ref. (3) is used. To obtain reliable evidence, sorption and n.m.r. data are compared at three different temperatures [series (i), cf. above]. Owing to the fact that the agreement between the sorption and n.m.r.data is attained within the region of the smallest half-lives of sorption uptake observed and taking into account the proofs given above of various aspects of the response-time behaviour of the apparatus used, a further essential conclusion can be drawn. This is that significant influences of external (apparatus) effects, external diffusion, intercrystalline heat and mass transfer as well as structural crystal interface resistances on all diffusivities Dapp and DtPP reported here can be ruled out. This must be more valid at T, 2 T,, where the half-life of the sorption uptake extends to larger values. Thus one might assume that under these conditions the sorption heat transfer from the zeolite crystal interface to the surroundings could be the rate-limiting process.For this heat convection might become the responsible mechanism because its direction is reversed by changing the temperature T, at the fixed temperature T, (heat radiation and conduction through the walls of the apparatus should remain approximately unchanged on varying T,). Additional evidence for such a mechanism is given by a comparison of the shapes of uptake curves obtained at similar concentrations but different temperatures To. In fig. 5 are shown typical uptake curves in plots of the pressure, po, against t, the relative uptake, y, against t and y against h. The picture obtained distinctly complements the conclusions derived above from the values DtPP. Whereas prevailing intracrystalline diffusion is seen at T, < T,, the uptake curves in plots of y against & obtained at T, > T, show shapes typical of interface (barrier)-limited sorption.This discrimination is significant owing to the extremely low response time of the equipment used in connection with the high time resolution of the data-logging system (all points in fig. 5 are evaluated; the primary kinetic curves in plots of po against t were continuously recorded at a time resolution of ca. 0.005 s). Consequently one can state that the thermal effects being superimposed upon the intracrystalline diffusion might be interpreted as some kind of interface (barrier) resistance. At present we may give only a qualitative explanation of the phenomenon observed. Owing to the strong dependence of the uptake rate on the temperature of the dosed gas, mass- and heat-transport processes in opposite directions should be accounted for. Whereas at = T, heat convection takes place between the dosing volume and the sorption vessel only due to the sorption heat generated, at T, < T, a stronger heatM.BULOW, P. STRUVE, w. METK AND M. KOCIRIK 819 40 30 i: I 5 10 4 0 30 I 20 t ( i i ) 10 I l l l ~ l l l l ~ l 5 10 30 20 10 a" --. 0 4. 40 t 20 30L 20 3 0 e--- 'O L-2L- 10 5 t l s 0 3.5 0.5 Y : 0.5 5 n SJ 1 2 3 4 5 1 2 3 4 5 r l s td lB K Fig. 5. Typical examples of uptake curves of benzene on NaX zeolite obtained at T, = 353 and different temperatures T, (c x 1.2 mmol g-l, if another sorbate concentration is not given). (A) po as a function of t : (i) p0(m) x p , (36 s) = 0.96 Pa, pi = 0.609 S; (ii) po(.o) M po (57 s) = 0.92 Pa, pi = 0.662 s; (iii) po(.o) x po (84 s) = 2.28 Pa, pi = 1.54 s; (iv) p o ( .o ) * po (325 s) = 5.93 Pa, pi = 16.1 s; p o ( . o ) x po (1000 s) = 0.01 Pa, pi = 205 s. (B) y plotted against t: (i) T, = 307 K; (ii) & = 343 K; (iii) T, = 353 K; (iv) T, = 355 K; V, 0.8155; V , 1.2433; V, 2.0452 mmol g-l; (v) T, = 363 K; (>, 0.6231; 0, 0.8944; 0, 1.2411 mmol g-l; (C) y plotted against ti, symbols as in (B) except for: (iv) (---) 0.8155, (-) 1.2433, (-. a-) 1.4665, (-- - --) 2.0452, (- . -) 2.1673 mmol g-l.820 SORPTION OF BENZENE IN NaX ZEOLITE Fig. 6. Probable time dependences of the temperature of (a) the crystal interface and (b) the surrounding gas phase. convection occurs directed into the volume YO.In the case of a linear gas expansion from into K, the latter being free of the sorbent, one can obviously formulate a mixed pressure po(O+) as follows : Furthermore, a mixed temperature q(O+) of the sorption vessel can be derived from the heat- and mass-balance conditions (cf. the Appendix) : where n denotes the mole number of the sorbate in the bulk phase. The arguments 0- and 0, given in parentheses represent the times characterizing the state of the system before and after the gas expansion, respectively, i.e. if considering the time t = 0 from the left-hand side and the right-hand side, respectively. Under real conditions the temperature T, should be constant. The temperature C(O+) is a limiting value; it changes and approximates as shown schematically in fig.6. During the sorption process the sorption heat generated raises the temperature of the crystal interface. The gas temperature simultaneously increases from z(0,) to T, due to the heat balance between the sorption vessel and the thermostat. The heat transfer from the sorbent into the gas phase should be proportional to the temperature difference AT = Grystal interface - z, where the relation holds. The true temperature of the crystal interface is obtained from the superposition of both curves represented in fig. 6. Isothermicity, being a necessary condition for the coincidence between uptake and n.m.r. data, might be approached by this way. In contrast, at T, > T, the heat convection should be directed into the sorption vessel K (onto the crystal interface) and in this way retards the sorption-heat dissipation from the crystal interface.At the beginning of the uptake process there possibly occurs a short-time superheating of the crystal interface layer followed by rapid equalizing of the temperature through the crystal. Owing to the continuing uptake process this superheating of the interface should be successively renewed and superimposed upon the heat and mass transfer into the zeolite crystal as well as upon the heat transferM. BULOW, P. STRUW, w. MIETK AND M. KOCIRIK 82 1 to the surroundings, the latter process being delayed by external temperature conditions. As a consequence, within the interfacial layer a local decrease in the sorbate concentration might take place delaying the sorption uptake. For T, < T, the supposed superheating of the crystal interface should be minimized sufficiently fast due to additional heat convection directed into the volume V,.In this way, isothermal conditions might be approached and thus data evaluation by the isothermal model would be justified. The latter conclusion is convincingly shown by the coincidence between the corrected sorption diffusion and the n.m.r. self-diffusion data (cf. fig. 4). Assuming this tentative interpretation one can conclude the following. Under the conditions that the half-lives of the sorption uptake and the sorption-heat dissipation are comparable and that the sorption heat amounts to some minimal value (as a starting point for the evidence of heat-release influences on the sorption rate) at T, 2 T, and sufficiently low gas pressures there will be no possibility of the direct observation of true intracrystalline diffusivities by sorption uptake techniques. On the other hand, the variation of T, at a fixed provides further proof of intracrystalline diffusion, widening the possible use of sorption methods for the investigation of molecular transport processes in zeolites.We thank the referees for their valuable suggestions. APPENDIX Eqn (1) and (2) result from the mass and heat balance between the dosing volume, V,, and the sorption vessel, Vv, assuming an ideal dose process. For the mass balance - where V, RT, An0 = - [Po (0,) --Po (0-)I and denote the difference in the mole number of sorbate in the gaseous bulk phase of the dosing volume and the sorption vessel, respectively.The temperature T, of the dosing volume is assumed to be constant and independent of the sorption uptake and the mass flow into the sorption vessel. The heat balance can be expressed by c p [no (0,) - no (0-11 [C (0,) - To (0-11 = cp nv (O-) [T, (0,) - T, ( O J l (A 4) where cp denotes the molar heat capacity of the sorbate. Eqn (2) for the mean temperature (0,) of the gas phase in V, being established after the sorbate is transferred from V, into V , is obtained from eqn (A 4) by simple rearrangement. Considering together eqn (2) and eqn (A 1HA 3), eqn (1) for the mixed pressure ~~(0,) is available. J. Karger, H. Pfeifer, M. Rauscher and A. Walter, J. Chem. SOC., Faraday Trans. I , 1980, 76, 717. M. Bulow, P. Lorenz, W. Mietk, P. Struve and N.N. SamuleviE, J. Chem. SOC., Faraday Trans. 1, 1983,79, 1099. M. Bulow, W. Mietk, P. Struve and P. Lorenz, J. Chem. Sac., Faraday Trans. 1,1983, 79, 2457. P. Struve, M. KoEifik, M. Bulow, A. Zikanova and A. G. Bezus, Z . Phys. Chern. (Leipzig), 1983, 264,49. M. Bulow, W. Mietk, P. Struve and A. Zikanova, 2. Phys. Chem. (Leipzig), 1983,264, 598. K. Chihara, M. Suzuki and K. Kawazoe, Chem. Eng. Sci., 1976, 31, 505. ’ M. KoEifik, J. Karger and A. Zikanova, J. Chem. Tech. Biotechnol., 1979, 29, 339. L-K. Lee and D. M. Ruthven, J. Chem. SOC., Faraday Trans. I, 1979,75, 2406.822 SORPTION OF BENZENE IN NaX ZEOLITE H-J. Doelle and L. Riekert, Angew. Chem., 1979, 91, 309. lo A. Zikanovh, M. KoEiiik, A. G. Bezus, A. A. VlZek, K. Bulow, W. Schirmer, J. Karger, H. Heifer and S. P. Zdanov, in The Properties and Applications of Zeolites, ed. R. P. Townsend (The Chemical Society, London, 1980), p. 580. l1 M. KoEifk, A. Zikanova, M. Smutek and A. G. Bezus, Collect. Czech. Chem. Commun., 1980, 45, 3392; 1981,46, 678. l2 J. Karger, M. Bulow, V. 1. Ulin, A. M. VoloSEuk, P. P. Zolotarev, M. Kdiiik and A. Zikanova, J. Chem. Tech. Biotechnol., 1982, 32, 376. l3 I. T. Eraiko, V. A. Gorlov, A. M. VoloEuk and R. Broddak, Prepr. Workshop-ZZ on Ahorption of Hydrocarbons in Microporous Ahorbents (Eberswalde, G.D.R., 1982), vol. 2, p. 154. l4 M. KoEiiik, P. Struve and M. Bulow, J. Chem. SOC., Faraday Trans. I , 1984, 80, in press. l5 P. Lorenz, M. Bulow and J. Karger, Zzv. Akad. Nauk SSSR, Ser. Khim., 1980, 1741. l6 M. Bulow, P. Struve, G. Finger, C. Redszus, K. Ehrhardt, W. Schirmer and J. Karger, J. Chem. SOC., l7 M. Bulow, J. Karger, M. Kdiiik and A. M. VoloEuk, Z . Chem. (Leipzig), 1981, 21, 175. Faraday Trans. 1, 1980, 76, 597. R. M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves (Academic Press, London, 1978). ID J. Karger, H. Heifer and M. Bulow, 2. Chem. (Leipzig), 1976, 16, 85. 2o M. Biilow, J. Karger, N. van Phat and W. Schirmer, Z . Phys. Chem. (Leipzig), 1976, 257, 1205. 21 J. Karger and J. Caro, J. Chem. Soc., Faraday Trans. 1, 1977, 73, 1363. 22 H. Yucel and D. M. Ruthven, J. Chem. Soc., Faraday Trans. I , 1980,76, 60; 71. 23 R. Ash and R. M. Barrer, Surf. Sci., 1967, 8, 461. 24 J. Karger, SurJ Sci., 1973, 36, 797; 1976, 57, 749. (PAPER 3/957)

ISSN:0300-9599    

DOI:10.1039/F19848000813

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1984,80, 823-829 A Thermodynamic Model of the Mixed-salt Cobalt(@-Hexacyanoferrate(I1) Ion Exchanger BY TATJANA S. CERANIC Institute of Physical Chemistry, Faculty of Science, University of Beograd, 1 100 1 Beograd, Yugoslavia Received 10th June, 1983 A thermodynamic model for the ion exchange of K, NH, and Cs on the ion exchanger cobalt(~r)-hexacyanoferrate(~~)-M where M = K,NH, and Cs, has been postulated. Based on the experimentally determined thermodynamic functions AGg,, AS& and AH&, and on the calculated values of excess functions, the deviations from ideality of the investigated reactions have been explained. Cobalt(Ir)-hexacyanoferrate(II)-M, M = K,NH, or Cs, has been used as an + NH,,Cs; m, +K,Cs and + K,NH,. The different ionic forms of this ion exchanger are i~ostructural~ with inorganic ion exchanger for the reactions'? an f.c.c.lattice. The chemical composition of the unit cell is [Fe(CN),Co],M, - nH,O. (1) In the ion-exchange process only M+ ions which are situated in the cases of the crystallite, denoted (CoFC), take part. The effective radius of the cage opening, dw, is 147, 149 and 160 pm for K,NH, and Cs, re~pectively.~ If the ion-exchange reaction in dilute solutions is being followed, it is seen that these ion exchangers are more selective for the ion with the larger crystal radius1 This is confirmed by the selectivity series obtained : dilute concentrated solution solution I( + NH,,Cs Cs > NH, NH, > Cs Cs + K,NH, K > NH, K x NH, However, the ion with the smaller crystal radius has priority when concentrated solutions are used.' This selectivity inversion results from the physicochemical properties of competing ions in the solution, where ion hydration is a dominant effect determining the course of the ion-exchange reaction.The experimentally determined thermodynamic quantities for the ion-exchange reaction cited indicate the overall changes in this two-phase system. In the present work an attempt is made to define and describe all processes in terms of thermodynamic quantities using the results obtained in previous work., - NH,+K,Cs C s > K K > C s - THERMODYNAMIC MODEL The ion-exchange process (CoFC)Ml + Mi(aq) (CoFC)M, + M:(aq) 823824 ION EXCHANGE ON [Fe(CN),Co],M,, M = K,NH,,Cs can be represented by a sequence of intermediate reactions in which competing ions are taking part (CoFC)M, -+ (CoFC)- + Mt (dissociation) (4 Mz(aq) -+ Mz + H20 (dehydration) (b) M:+mt+ii8t+Mt (ion exchange) (4 Mj: + H 2 0 + Mf(aq) (hydration) (4 +o- + (CoFC)M, (association). (4 Processes (b) and (d) are related to the behaviour of competing ions in the solution,l whereas processes (a), (c) and (e) take place in the ion exchanger and will therefore be considered in more detail.From the ion-exchange isotherms3 it is seen that in the reactions which were studied total exchange was not attained, i.e. the ion exchanger became a mixture of two different ionic forms, a binary system which could be an ideal or a real solid sol~tion.~. The free energy of an ion in the ion exchanger, if only one ionic species is present (NM = 1 is the chosen standard state), is given by (2) and M2) the - G@ = nMRTlni@ = 0 because the thermodynamic ion activity under these conditions is @ = 1.standard free-energy change is If the ion exchanger appears as a mixture of two ionic forms From eqn (2) and (3) it follows that - Ge-Gmixture = A c e = -AM1 RT In el +ZM, RT In 28,. (4) Since a* = N M f M (where NM is the mole fraction of the M+ ion in the ion exchanger a n d f , is its activity coefficient) and is the number of moles of the M ion per unit cell, eqn (4) takes the form AGe = (AM, +AM,) RT(xMP In N M z - R M l In NMl) + @M1 +2MP) RT(xM2 1n7Mz -xM, lnfMl>. ( 5 ) The first term on the right-hand side can easily be calculated from the ion-exchange isotherm^;^ it represents the standard free-energy change (AGid) of an ideal solid solution of two ionic forms where TM, = jMz = 1.The second term in eqn (5) is the 'non-ideality ' contribution to the free-energy change (AGE) in the mixture; therefore Ace = A@d + AGE. (6) In an ideal mixture the enthalpy of mixing is always zero, thus A P & ~ , will depend only on the temperature and entropy of the system: AGGzfgl = - TASzZ/pl. (7) The calculated AP&m1 (table 1) and ASEz,Rl (table 2) values show only a change in the randomness of the mixture relative to the ion exchanger in one given form. The question as to which of the intermediate reactions in an ion-exchange process has a predominant influence on the reaction trend is most frequently answered by an analysis of the excess thermodynamic functions.T. s.CERANIC 825 Table 1. Values - of the free energy and excess functions for the free energy for the reactions M, + M, (M = K,NH, or Cs) in the ion-exchanger phase at 298 K ~ ~~~~~ ~~~ - K + NH, 11.3 -4.6 -13.5 20.2 58.8 -38.6 38.8 20.0 K+Cs 14.6 61.9 -13.4 89.9 49.2 40.7 70.8 -21.6 m , + K 20.3 4.6 -10.2 35.1 16.5 18.6 -13.5 30.0 - NH, --* c s 110 66.5 -6.9 183.4 97.1 86.3 37.1 60.0 - Cs+K 116 -61.9 -6.4 60.5 85.7 -25.2 -47.8 133.5 Cs --+ NH, 111 -66.5 -6.0 50.5 73.4 -22.9 -74.0 147.4 - The energy of interaction, w, of two different ions situated in adjacent cages of the unit cell in the non-ideal systems has a non-zero value and is made up from the mutual repulsion and interaction with framework of the ion exchanger AW = ~ W M , M * -WM,M, - w M ~ M ~ - (8) Barrer4 correlated Aw values with the contribution to the standard free energy From eqn (9) it follows that the arrangement of M, and M, ions in the ion exchanger is controlled by the factor X M F M , , i.e.by the probability. Partial differentiation of AG,E over R, in combination with eqn (6) gives the correlation between the coefficient of activity in the ion exchanger and the energy Aw where C is Kielland’s constant.6 The values of Kielland’s constant, and thus of Aw, can be determined for all the investigated reactions since the apparent thermodynamic constant K, showed a linear dependence on the mole fraction of the competing ion in the ion-exchanger phase, i.e. it obeyed Kielland’s relation where K, is the thermodynamic constant (table 3).By using eqn (9) or eqn (5) and (10) it was possible to calculate the total contribution, AGF, to the free-energy change in the ion-exchange reaction (table 1). The experimentally determined standard free-energy change, AGgp, for the reaction studied contains terms for all the processes in both phases in which the competing ions took part (12) where Gh is the hydration free energy of the competing ions in the solution [values taken from ref. (7)]. By using eqn (12) the total contribution to the free-energy change in the ion exchanger, E A @ z / ~ l , can be calculated: AGZp = (Qdz + (GEz + GEl) - (GkP - Gh,) 28Table 2. Excess functions for the entropy change in the ideal system and enthalpy contribution to the ion-exchange reactions at 298 K M - 2 : mean TI CABE,,w,/J K-' mol-' 0 - ASk21M, Mz/M1 A H ~ p AHk2,H, ~A%2,R1 n ASid - CD M, 3 M, /J K-1 mol-1 /J K-' mol-l /J K-' mo1-I /kJ rno1-I /kJ mo1-I /kJ rno1-I eqn (20) eqn (21) h 478 K+Cs 528.0 3.6 45.0 172 57.7 229.7 487 469 478 Z K + NH, 51 1.0 - 5.5 45.3 1 64 3.8 167.8 460 495 n -3 u NH, + K 42.5 5.5 34.2 33 - 3.8 29.2 14 - 20 NH, Cs 91.0 9.1 23.2 83 54.0 137.0 77 - 156 -40 '?g Cs + K - 648.0 - 3.6 21.5 - 77 - 57.7 - 134.7 - 673 - 655 - 664 z z Cs 3 NH, - 570.0 -9.1 20.1 - 59 - 54.0 - 113.0 - 599 - 548 - 573 - U - 0 - - - -T.s. CERANIC 827 Table 3. Kielland's constant C and the interaction energy, Aw, of ions in adjacent cages of the ion exchanger for the ion-exchange reactions at 298 K - K -+ NH, - 5.3 kO.1 30.2 k 0.6 K+Cs - 4.5 f 0.1 25.4 f 0.5 NH, -+ K -2.3 kO.1 12.4 f 0.2 NH, --* CS - 21.8 f 2.0 124f 10 G - + K - 20.8 & 2.0 119+ 10 - - - - Cs -+ NH, - 19.7 f 2.0 112k10 The calculated ZAG&Rl values are presented in table 1.This contribution com- prises the effects defined by GF [eqn (9)] and also the cF term, which is related to the different hydration free energies of M, and M2 ions in the ion exchanger (table 1) AG? = ZAG&M1 - AGF. (14) Using the idealized and simplified model of Eisenman,8 the ions in an ion exchanger interact only through electrostatic forces at the moment of exchange. The Born-Lande9 relation for the electrostatic interaction of monovalent ions in the solid phase can thus be applied to calculate the free-energy change, AG$21~l, where rc is the crystal radius of the corresponding cation and r- is the radius of the ion-exchange centre3 (radius of the cage in the crystallite) with negative unit charge. The contribution to the free energy from the non-ideal electrostatic interaction between two given ions can be calculated from ACT [eqn (lo)] is related to the mutal interaction of the competing ions in the exchanger and to their interaction with the framework; thus AC," = AGF - AGF (17) represents the contribution to the free energy from the non-electrostatic forces (table 1).The contribution to the entropy, SE, and enthalpy, HE, from the non-ideal conditions in an ion exchanger composed of a mixture of two ionic forms can be (18) defined as where As&,, is defined by eqn (7) and the calculated values are presented in table 2, and since AHg?/H1 = 0.For the investigated reaction, the total contribution to the entropy change in the ideal system has been calculated first from the experimental values2 of AS& and the tabulated data for the hydration entropy of competing ions in solution7 AS&=, = ASE2,~l + ZAS~&R~ AR&gl = ZAH,E2~gl (19) ZAS&g1 = AS,$& + A S & 2 / ~ l - AS&ml (20) 28-2828 ION EXCHANGE ON [Fe(CN),Co],M,, M = K,NH,,Cs I EA GE NH&-K+ &NH*, E-K+ - 100 -60 -40 -20 ( 1 /ArJnm-* Fig. 1. Dependence of excess functions (0, CAGE, 0, AGF; a, A@; 0. AC,E> in an ideal system on the reciprocal of the difference in crystal radii of competing ions in the ion exchanger at 298 K. (a) M, < M, and (b) M, > MI. where Regardless of the method used, i.e.whether ZASE was calculated using eqn (20) or (21), the values obtained agreed within the limits of experimental error (with the exception in the reaction NH, + Cs). The values obtained using these two methods are given in table 2. DISCUSSION From the results presented above it follows that the excess function for the free energy in an ideal system depends on a number of factors ZAG&B, = AG,E+AG,E+AG,E (22) where ACF is the contribution from the difference in the hydration energy of competing ions in the ion-exchanger phase, AG,E is from the electrostatic interaction of the given ions with the framework and AGqE is related to the action of the short- range forces and the steric limitations in the exchanger. All these contributions are plotted against the reciprocal of the difference of crystal radii of the competing ions in fig.1. At first glance it appears that the favoured reactions are those in which a larger ion in the exchanger, R,, is replaced by a smaller one from the solution, M,, because X A P are less positive than if M, > M, (table 1). However, the answer as to which stage in the exchange process has the greatest influence can only be obtained after considering all the excess functions (table 1 and fig. 1).T. s. CERANIC 829 The free-energy change for the hydration of competing ions in the exchanger shows the same trend as in solution (AGF is of the same algebraic sign as AGh, table 1). In the exchanger, however, the influence of hydration is more pronounced when M, > M, than when M, < MI.In the K + NH,,Cs reactions, for example, the hydra- tion process in the exchanger is more ‘allowed’ than in solution, whereas in the m, + K,Cs and + K,NH, reactions it is more ‘forbidden’ than in solution. It is due to the more ‘open’ structure of (CoFC)K (dw > as compared with the (CoFC)NH, and (CoFC)Cs crystallites (dw M rc,NH,; dw < rc,cs).3 The electrostatic excess functions, A@, in the case where M, > m, (‘forbidden’ processes) behave in an opposite fashion to the case where M, < HI (‘allowed’ processes). This agrees with the fact that the ionic character of the competing ions is proportional to l/rc. The excess function A@ is related to the steric limitations in the exchanger phase. They can be caused by the geometry as well as the method of bonding of the competing ions to the framework.The steric limitations are pronounced if the ions in the exchanger, MI, have a larger crystal radius than the competing ions, M,, in the solution. The large positive Ac,E values for the reactions CS + K,NH, point to a ‘forbidden’ process (fig. 1) which is related to the built-in CS ions; owing to their large crystal radius they fill up the volume of the cage, superimposing the short-range forces on the interaction with the framework. Note also that the effective cage radius, dw, in (CoFC)Cs crystallite is smaller than the crystal radius of Cs. When M, > m, the steric influence is smaller, thus the excess function AG,E should be smaller than ACF and All excess functions for reactions in which NH, ions are taking part, either from the solution or from the exchanger, deviate considerably from the ACE values of the other ions (fig.1). NH, ions only formally have the properties of alkaline cations; however, they interact with the framework mainly via hydrogen bonds, thus causing the observed deviations. The obtained EAS&B~ values differ only slightly from ASg, (table 2), which means that the influence of the hydration of ions, ASh, in the solution on ASg, is very small as compared with the processes in the exchanger. We presume, therefore, that the transfer of water in the course of the exchange reaction does not influence the configurational entropy in the exchanger. The XAS&M~ values lead to the conclusion that the configurationally allowed processes are those in which a small ion is being replaced by a larger one, whereas the opposite processes are ‘forbidden’. The unexpected negative CASE value for t h e m , + Cs reaction is because of’lhe hydrogen bonding of NH, in the exchanger. The structural model described earlier3 and the thermodynamic model presented have confirmed that these ion exchangers are similar to the zeolites, which may give some ideas as to their possible applications. AG?. T. Ceranid, D. Trifunovid and R. Adamovid, 2. Naturforsch., Teil B, 1978, 33, 1099. T. S. Ceranid and R. Adamovid, Z. Naturforsch., Teil B, 1979, 34, 127. T. CeraniC, 2. Naturforsch., Teil B, 1978, 33, 1484. R. M. Barrer and J. D. Falconer, Proc. R. SOC. London, Ser. A , 1956, 236, 227. D. H. Freeman, J. Chem. Phys., 1961,35, 189. J. Kielland, J. SOC. Chem. Ind. (London), 1934, 54, 232. Publishing Company, London, 1969), p. 13. G. Eisenman, J. Biophys., 1962, 2, 259. M. Born and A. Lande, Ber. Phys. Ges., 1918, 20, 210. ’ Y. Marcus and A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes (International (PAPER 3/969)

ISSN:0300-9599    

DOI:10.1039/F19848000823

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. Soc., Faraahy Trans. 1, 198480, 831-839 Aldol Condensation of Butan-2-one and Pentan-3-one on an Activated Alumina as Monitored via in-situ Carbon- 13 Nuclear Magnetic Resonance Spectroscopy BY VALERIE A. BELL, ROBERT F. CARVER, CECIL DYBOWSKI AND HARVEY S. GOLD* Center for Catalytic Science and Technology, Department of Chemistry, University of Delaware, Newark, Delaware 197 16, U.S.A. Received 22nd June, 1983 Conventional 13C nuclear magnetic resonance spectroscopy is used to monitor the reactions of butan-2-one and pentan-3-one adsorbed on alumina. Both reaction sequences proceed via successive aldol condensations. The products observed for butan-2-one (5-methyl-4-hepten-3-one and 4-hydroxy-4-methylpentan-Zone) are those expected for homogeneous base catalysis. A J-modulated spin-echo sequence demonstrates that 5-ethyl-4-methyl-5-hepten-3-one, the 8, y condensation product, is obtained using pentan-3-one.The results demonstrate that conventional 13C n.m.r. can be effectively used to distinguish reaction pathways and products for surface- catalysed reactions. Nuclear magnetic resonance (n.m.r.) spectroscopic studies of species adsorbed on solids are becoming increasingly common, particularly in the area of catalyst characterization. At present, n.m.r. of adsorbed-state species may be accomplished using one of three techniques. These are (1) conventional n.m.r., (2) solid-state, broad-line n.m.r. and (3) magic-angle spinning (m.a.s.) n.m.r. Broad-line n.m.r. is limited with respect to resolution and is therefore constrained to studies of systems with a minimal number of species present.M.a.s. n.m.r. requires a carefully balanced sample rotor, which limits the sample preparation procedure. Use of conventional n.m.r. to study compounds on solid supports has proved to be a viable alternative to infrared (i.r.) and gas-chromatographic (g.c.) methods. The traditional limitation of n.m.r. to study interactions of reacting adsorbates on supports can easily be overcome, as has been demonstrated by Derouane and coworkers in the context of product formation and kinetics,l-s and by Bell and Gold in the study of acetone on a l ~ m i n a . ~ Conventional 13C n.m.r. studies of reactive systems include methanol on H-ZSM-5 zeolite,lo the isomerization of but- 1 -ene to but-2-ene using alumina,ll CaNaY-type zeolite12 and tin-antimony oxide,13 propene dimerization on NiO/Si0214 and formic acid dehydration on TiO,? In general, peak widths have been found to vary according to the surface area of the support, the inherent inhomogeneities of the instrumentation and of the particular sample, the degree of attachment of the adsorbate to the support, the number of monolayers or pore fillings present and the temperature of the system.These experimental variables must be carefully considered when n.m.r. of physisorbed species is contemplated. In the present study the use of conventional 13C n.m.r. is extended to butan-2-one and pentan-3-one on y-alumina. Aluminas are extensively used as catalysts, as well as supports for catalysts. Indeed, the alumina-catalysed dehydration of ethanol was recognized as early as 1797.l6 While alumina is generally classified as alpha, eta or gamma, a range of catalytic activity is encountered within each crystalline form.In 83 1832 ALDOL CONDENSATION ON ACTIVATED ALUMINA the case of y-alumina eight distinct forms have been reported, although no general agreement regarding individual structures yet exists.16 A criterion for selection of a particular sample of y-alumina for use in n.m.r. studies is the absence of iron and other paramagnetic impurities, which contribute to n.m.r. relaxation processes and attendant line broadening.17 Use of conventional 13C n.m.r. is broadly applicable to adsorbed-state heterogeneous catalytic reactions. Although species which are tightly adsorbed (chemisorbed) to the surface are not readily monitored via this method, owing to the relaxation times involved, species which are less tightly adsorbed are easily observed.Application of this technique to both the butan-2-one and the pentan-3-one/alumina reactions revealed unexpected reaction products ; butan-2-one reacts via the less hindered reaction mechanism, and pentan-3-one reacts to form a 8, y unsaturated ketone. EXPERIMENTAL Butan-Zone (Burdick and Jackson, spectral quality) was used as received following a 13C n.m.r. determination of purity. Pentan-3-one (Alpha/Ventron) was distilled before use. The alumina used was Catapal y-A1203 (Conoco), a product selected for its exceptionally low iron content (0.005%). The alumina was calcined at 775 K in a flow of oxygen for 24 h, and subsequently degassed at 673 K and 1 x Torr (ca.1.33 N m-2) for 0.5 h. The details of sample preparation and instrumental operating parameters have been described previou~ly.~ All spectra were acquired at 295 K using a Bruker WM 250 n.m.r. spectrometer operating at 62.9 MHz, with proton decoupling (1 W) and an external deuterobenzene lock solvent. The J-modulated spin-echo sequence used is that found in the Bruker instrument manual; however, the value of 1/2J was assumed constant at 0.008 s (where Jequals the carbon-proton coupling constant). J-modulated spin-echo sequences cause resonances corresponding to carbons with an odd number of attached protons to be inverted. The use of the J-modulated spin-echo sequence for adsorbed-state species was verified via use with the known butan-2-one/alumina reaction mixture before use with the pentan-2-one/alumina reaction mixture.Two distinct types of butan-2-one samples were prepared and monitored. Samples I and I1 each contain 3 x mol (2.5 statistical monolayers based upon a surface area of 280 f 10 m2 g-l for the alumina) of butan-2-one vapour deposited at 77 K utilizing an external liquid-nitrogen bath. Sample I was flame-sealed and kept at 77 K until inserted into the n.m.r. probe. Sample I1 was warmed to 295 K with gentle mixing. After 20 min the sample was cooled in liquid nitrogen and 0.8 statistical monolayer of water was added to the sample. It too was kept at 77 K until being inserted into the n.m.r. probe. The procedure used for the pentan-2-one sample parallels that for sample I of butan-2-one. RESULTS BUTAN-2-ONE SAMPLE I Within 30 min of removal of the sample from the liquid nitrogen, reaction products were readily apparent.Fig. 1 shows the spectrum resulting from the coaddition of 75 scans taken over the first 25 min that the sample was in the n.m.r. probe at 298 K. The original butan-2-one resonances at 6 8, 29, 37 and 209 ppm are evident, along with new resonances at 6 19, 123, 159 and 203 ppm, with a shoulder at 6 13 ppm. SAMPLE I1 Fig. 2 shows the spectrum which results immediately upon insertion of the sample into the probe, and shows strong new resonances at 6 73 and 52 ppm, produced byV. A. BELL, R. F. CARVER, C. DYBOWSKI AND H. S. GOLD 833 - 1 I 1 I I I I 4 I I 1 ~- 2 00 100 0 6 (PPm) Fig.1. n.m.r. spectrum of butan-2-one reacted on alumina for 30-35 min after removal from liquid nitrogen (sample I). The spectrum represents the coaddition of 75 scans, using a relaxation delay of 20 s. Chemical shifts are relative to tetramethylsilane (TMS). I I I I I 1 I I I I I I 2 00 100 0 6 (ppm) Fig. 2. I3C n.m.r. spectrum of butan-2-one reacted on alumina wherein the reaction has been quenched by addition of 0.8 statistical monolayer of water (sample 11). The intermediate ketol is desorbed. Chemical shifts are relative to TMS.834 ALDOL CONDENSATION ON ACTIVATED ALUMINA I I I I I I I I 1 1 I I 2 00 100 0 6 (PPm) Fig. 3. 13C n.m.r. spectrum of pentan-3-one reacted on alumina after standing for 6 days at 298 K. Chemical shifts are relative to TMS.The spectrum represents the coaddition of 1 16 scans, using a relaxation delay of 20 s and a 4 5 O pulse. compounds which were desorbed upon addition of water because of competition for surface sites on the alumina.* During the time it takes to acquire the spectrum of fig. 1, the reaction has gone almost to completion, as evidenced by the fact that a spectrum acquired 2 h later reveals little change. A spectrum acquired 6 days later shows additional resonances at 6 46 and 54 ppm, both of which are believed to be due to secondary condensation products, paralleling the behaviour observed for a~etone.~ Following this, the overall relative intensities changed little over the course of 1 month at 298 K. PENTAN-3-ONE Initial pentan-3-one resonances are observed at 6 8, 36 and 21 1 ppm.The reaction is observed to occur much more slowly than the corresponding butan-2-one reaction. A shoulder appears on the downfield side of the 6 8 ppm resonance after 3 h, and two resonances at 6 54 and 122 ppm appear simultaneously after 6 h, together with additional resonances in the methyl resonance region. Heating the sample to 350 K in the probe did not produce any additional detectable products. After 6 days at room temperature the spectrum of fig. 3 was obtained, in which there is sufficient intensity to identify an additional resonance at 6 142.5 ppm. Addition of 0.8 statistical monolayer of water did not produce any additional resonances. A similar sample (containing no added water) kept at room temperature for one month showed no additional resonances, although the relative intensities of the 6 54, 122 and 142.5 ppm resonances were much greater.This sample was used in the J-modulated spin-echo sequence experiment. * This competitive process effectively quenches the reaction and allows observation of tightly bound products.V. A. BELL, R. F. CARVER, C . DYBOWSKI AND H. S . GOLD 835 (2) Fig. 4. The acid- and base-catalysed reaction pathways of butan-2-one in solution.ll DISCUSSION BUTAN-2-ONE The aldol condensation of butan-2-one has two possible pathways, each involving an enol intermediate, as shown in fig. 4. Steric hindrance would be expected to cause a preference for pathway A, wherein the attacking carbanion is the more exposed a-carbon. Abbott et aZ.l* have characterized this reaction as catalysed by homogeneous acids or bases.All ketones except butanone are found to react via pathway A, the sterically favoured pathway, for both acid and base ~ata1ysis.l~ Butanone, however, is found to react via pathway B, the hindered mechanism, when a homogeneous acid catalyst is used. To determine the major product obtained in the presence of an alumina catalyst, the characteristic resonances of sample I at 6 123 and 159 ppm must be assigned. These resonances correspond to the olefinic carbons. For compound (2) (3,4-dimethylhex- 3-en-2-one), the resonance of C3 occurs at 6 134 ppm and of C4 at 6 145 ppm.20 For compound (1) (5-methylhept-4-en-3-one), peaks can be assigned by analogy to adsorbed mesityl oxide9 and by comparison with the shifts of similar unsaturated ketones upon substitution.21 Thus the resonance of C4 is determined to be 6 124+ 3 ppm.The resonance of C5 is determined to be 6 159+ 3 ppm. The resonances observed in fig. 1 indicate formation of product (l), the base-catalysed product. This agrees with the product identified by Kozima and Katsuno using i.r. detection of the desorbed products of the same reaction of butan-2-one on y-alumina.21 Resonances at 6 54 and 73 ppm in fig. 2 can be assigned by analogy to the spectrum of diacetone alcohol (4-hydroxy-4-methylpentan-2-one) adsorbed on alumina, which has resonances at 6 54 and 69 ppm. Thus the resonances at 6 54 and 73 ppm are assigned to the C4 and C5 carbons, respectively, of the ketol(5-hydroxy-5-methylheptan-3-one) formed in pathway A. The products found are those expected for a homogeneous base catalyst.This is of interest, since Lewis-acid sites are the most commonly cited cause of catalytic reactions on alumina.22 Ketones are known to interact with Lewis-acid sites via the carbonyl group.22 Also, for the aldol condensation of acetone on rutile, Griffiths and Rochester23 reported evidence of attachment of the carbonyl group to the exposed titanium ion, and concluded that this Lewis-acid site was the catalyst for the condensation. In the butan-2-one system the product formed indicates that the catalyst is a basic site in the alumina. This cannot be definitely concluded, however,836 ALDOL CONDENSATION ON ACTIVATED ALUMINA 0 OH 11 0 CH 3 Fig. 5. Reaction sequence showing the condensation of pentan-3-one to yield or,#l-conjugated unsaturated ketone.since catalysis on alumina includes the additional consideration of increased steric hindrance due to the alumina surface. It may be that the reaction on alumina is in fact catalysed by an acid site, but that the acid catalysis follows a different mechanism than that found in solution, resulting in the product expected for basic catalysis. Thus it may be that the relative steric hindrance of the two a-carbons is the governing factor when catalysis is carried out on a surface. Unfortunately other ketones cannot be used for comparison, since all other ketones react via the less hindered a carbon for either acid or base catalysis. PENTAN-3 -ONE The aldol condensation of pentan-3-one on alumina to form an unsaturated ketone22 would occur by the pathway shown in fig.5. The dehydration of the ketol (5-ethyl-4-methyl-5-hydroxyheptan-3-one) occurs via an enol intermediate.24 Characteristic shift assignments for this product can be obtained by analogy with published values for 3,4-dimethylhex-3-en-2-one, and are 134 f 5 ppm for C4 and 145_+5 ppm for C5.25 The spectrum obtained (fig. 3) shows resonances at 6 54, 122 and 142.5ppm. Thus the conjugated a,/?-unsaturated ketone is not the product obtained when pentan-3-one reacts on y-alumina. Evidence suggests that alumina stabilizes the enol configuration. If a dehydration product is formed in which the enol intermediate is conjugated, the final unsaturated ketone is the /?,y-unsaturated ketone (shown in fig. 6). Abbott and coworkers found that the /?,y-product is preferred in homogeneous catalysis when there is substitution a to the carbonyl in the original ketone.Since literature values for the /?,y-product, 5-ethyl-4-methylhept-5-en-3-one, are unavailable, the expected characteristic chemical shifts were calculated. Using the Clerc and Pretsch26 technique for saturated carbons, the shift for C4 is 6 55.1 ppm (trans). Using the method of Dorman2' for acyclic alkenes, the shift for C5 is 6 142.5 ppm, and for C6 it is 6 115.7. These calculated values agree quite well with those observed experimentally in the cases of C4 and C5, but C6 is outside the range of uncertainty. As an additional verification of resonance assignments, a J-modulated spin-echo sequence was used, yielding the spectrum in fig.7. The inversion of the resonances at 6 54 and 122 ppm is in accord with the presence of a single proton attached to the corresponding carbons. Likewise, the lack of inversion of the resonance at 6 142.5 ppm indicates that an even number of protons is attached; in this case this is consistent with the presence of zero protons at the C4 position.V. A. BELL, R. F. CARVER, C. DYBOWSKI AND H. S. GOLD 837 b H3C - CH,/ ‘CH = CH I CH3 Fig. 6. Reaction sequence showing the condensation of pentan-3-one to yield a product corresponding to conjugation in the enol intermediate. Fig. 7. 13C n.m.r., J-modulated spin-echo sequence spectrum. Using this pulse sequence the resonances of carbons with an odd number of attached protons are inverted. The spectrum represents the coaddition of 405 scans.TMS was utilized as the chemical-shift reference. A large number of other possible reaction products can be postulated. The expected chemical shifts of these theoretical products, including those resulting from re- arrangements typical of ketone reaction intermediates and cyclics, were compared with the spectrum obtained. No other possible product agrees with this spectrum.838 ALDOL CONDENSATION ON ACTIVATED ALUMINA CONCLUSION Conventional 13C n.m.r. can thus be utilized effectively to distinguish reaction products and pathways for surface-catalysed reactions. This method benefits from the ease of sample preparation, which use of a standard n.m.r. tube attached to a vacuum manifold provides. The ability to control carefully surface conditions, sample loading and the introduction of other species (such as poisons) is crucial to elucidation of the catalytic system.Temperature and time may also be easily varied, making the conventional n.m.r. study of physisorbed molecules a valuable addition to surface analytical techniques. The case of pentan-3-one is particularly interesting, since the chemical shifts, stability of the enol and the inversion spectrum all indicate that the /?,y-unsaturated ketone is the product obtained when pentan-3-one reacts on alumina. While it is indeed possible that differences in the reaction conditions used in this study (as compared with those of Kozima and Katsumo21) lead to formation of the p,y-unsaturated ketone instead of the a,/?-unsaturated ketone, both studies were performed at room temperature, and the calcining conditions for both are quite similar.It is unlikely, during the n.m.r. study, that the alumina contained adsorbed CO,, since other work2* indicates that any CO, initially present is desorbed when the sample is heated to 675 K under vacuum. In the case of the aldol condensation of acetone on Catapal, previous n.m.r. studies show that adsorption of CO, in amounts from 0 to 4 statistical monolayers before or after heating has no detectable effect upon the reaction. It therefore seems unlikely that adsorbed CO, would affect the pentan-3-one reaction. In a recent publication Pines29 discusses a significant difference in catalytic activity of two aluminas he has synthesized. One alumina was doped with potassium to a level of 0.08 % : the other remained undoped.Despite this difference, conventional X-ray powder pattern analysis shows no significant differences and surface-area measure- ments were also quite similar for both. The Catapal alumina (y, neutral) used in this n.m.r. study may contain active sites other than the neutral y-alumina used by Kozima and Katsumo, leading to differential stabilization of the enol intermediates. An intriguing possibility is that the low levels of paramagnetics and other foreign metal atoms present in other aluminas, but not in Catapal, may play a significant role. The suggestions of Roger Crecely are greatly appreciated. This work was sup- ported by the Center for Catalytic Science and Technology at the University of Delaware. Purchase of the Bruker WM 250 n.m.r.spectrometer was made possible by grant GM-27616 from the National Institutes of Health. J. B. Nagy, M. Gigot, A. Gourgue and E. G. Derouane, J . Mol. 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ISSN:0300-9599    

DOI:10.1039/F19848000831

出版商:RSC    

年代:1984

数据来源: RSC