摘要:

4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces. Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K.Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M. Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves.Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H.Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces.Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K. Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M.Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves. Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H. Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons

ISSN:0300-9599    

DOI:10.1039/F198884FX013

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications. Their basis is the 'Systeme International d'Unit6s' (9). 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, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987).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. (xiv)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications.Their basis is the 'Systeme International d'Unit6s' (9). 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, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987). 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. (xiv)

ISSN:0300-9599    

DOI:10.1039/F198884BX015

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

ISSN 0300-9238 JCFTAR 84(4) 885-1 285 (1 988) 885 899 91 7 923 93 1 94 1 95 1 959 969 979 993 1013 1025 033 047 057 065 1075 1083 30 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS Glutamic Acid-Hydrogen Phosphate Hydrogen Bonds. Proton Polarizability and Proton Transfer as a Function of the Cations Present and of the Degree of Hydration : Infrared Investigations The Interaction between Ions and the Activation Barrier of Elementary Events of Crystal Growth and Evaporation V. K. W. Cheng, B. A. W. Coller and E. R. Smith Hydrogenolysis of Ethane. Part 2.-Initial Rate Measurements over Ni and Pd Catalysts Mechanistic Aspects of Oxidative Coupling of Methane over LaAlO, T. Tagawa and H. Imai Measurements of the Electrolyte Conductivity of Alkali-metal Perchlorates and LiNO, in Acetone at 25 "C N. Schmelzer, J.Einfeldt and M. Grigo Electrochemical Regeneration of NAD+. A New Evaluation of its Actual Yield J. Bonnefoy, J. Moiroux, J-M. Lava1 and C. Bourdillon Interactions between Metal Cations and the Ionophore Lasalocid. Part 1 .--Complexation of Alkaline-earth-metal Cations by Lasalocid, Bromo- lasalocid and Salicylic Acid in Methanol J. Juillard , C. Tissier and G. Jeminet Interactions between Metal Cations and the Ionophore Lasalocid. Part 2.-Gibbs Functions, Enthalpies and Entropies for Complexation of Alkali- metal Cations by Lasalocid and Bromolasalocid Interactions betwen Metal Cations and the Ionophore Lasalocid. Part 3.-Interactions of Lasalocid with Mn2+, Fe2+, Co2+, Ni2+ and Zn2+ in Methanol P.Laubry, C. Tissier, G. Mousset and J. Juillard Solvation Thermodynamics of Ethidium Bromide in Mixed Solvents G. Varani, G. Chirico and G. Baldini The Brusselator Model of Oscillatory Reactions. Relationships between Two- variable and Four-variable Models with Rigorous Application of Mass Conservation and Detailed Balance The Brusselator: It does Oscillate all the same R. Lefever, G. Nicolis and P. Borckmans Effect of the Polymer Backbone on the Ligand Substitution Reaction of a Macromolecule-Metal Complex. Acid and Base Hydrolyses of the Polymer- bound Cobalt(rr1) Complexes Water Dynamics and Aggregate Structure in Reversed Micelles at Sub-zero Temperatures. A Deuteron Spin Relaxation Study P-0. Quist and B. Halle Molar Gibbs Energies of Transfer for Cu2+, Zn2+, Cd2+ and Pb2+ G.Gritzner Conductance Studies of Tetra-alkylammonium Bromides in 2-Methoxyethanol at 25 "C D. Das Gupta, S. Das and D. K. Hazra Ion-exchange Equilibria between Solid Aluminium Pectinates and Ca, Mn", Cu" and Fe*** Ions in Aqueous Solution R. A. Jorge and A. P. Chagas Studies on the Reactivity of 'OH Radicals in Non-aqueous Solvents using Laser Flash Photolysis T. Vidoczy, N. N. Blinov, G. Irinyi and D. Gal Preferential Solvation of the Europium(rrr) Ion in Water-Non-aqueous Solvent Mixtures. A Luminescence Lifetime Study F. Tanaka, Y. Kawasaki and S. Y amashita U. Burget and G. Zundel S. Kristyan and J. Szamosi Y. Pointud and J. Juillard P. Gray, S. K. Scott and J. H. Merkin Y. Kurimura, Y. Takagi and M. Saito FAR 1Con tents Selective Conversion of Methanol into Aromatic Hydrocarbons over Zinc- exchanged ZSM-5 Zeolites Y.Ono, H. Adachi and Y. Senoda Oxidation of Thiocyanate and Iodide Ions by Hydrogen Atoms in Acid Solutions. A Pulse Radiolysis Study A. J. Elliot, S. Geertsen and G. V. Buxton Ionisation Constants of 'OH and HO; in Aqueous Solution up to 200 "C. A Pulse Radiolysis Study Practical Limitations of Polyacetylene used as a High-power-density Cathode J. B. Schlenoff and J. C. W. Chien The Partitioning of Proteins between Water-in-oil Microemulsions and Conjugate Aqueous Phases Association Constants for the Electron-donor-Acceptor Complexes of Indole and 1 -Methylindole with 1 -(2,4,6-Trinitrophenyl)propan-2-one from Nuclear Magnetic Resonance Shift Measurements.An Anomalous Scatchard Plot J. A. Chudek, R. Foster, R. L. Mackay, F. M. Page and D. R. Twiselton Ionic Solvation in Water-Cosolvent Mixtures. Part 15.-Free Energies of Transfer of Single Ions from Water into Water-Dimethylformamide Mixtures I. M. Sidahmed and C. F. Wells Time-resolved Analysis of a ' Crystal-like ' Structure-forming Process of a Monodisperse Polystyrene Sphere as studied by Rapid-scanning Spectro- photometry T. Okubo Deformation of ' Crystal-like ' Structure of a Monodisperse Polystyrene Sphere under Shear Rate as studied by the Transmitted-light Spectrum Method T. Okubo Electron Spin Resonance Studies of the CS;- Radical Anion, and its Conjugate Acid The Formation of P-Muonium-substituted Cyclopentyl and Cycloheptyl Radicals, and the Significance of the A I / A , Isotope Ratio in Relation to the Conformations of Muonium-substituted Alkyl Radicals C. J.Rhodes and M. C. R. Symons Enhanced Dissolution of PuO, in Nitric Acid using Uranium(1v) Differences in Thermal and Mechanochemical Polymorphism. Effects of Impurity on the System Aragonite-Calcite Light-scattering Studies of Aggregation in Aqueous Solutions of Poly(viny1 alcohol co-vinyl acetate) Copolymer N. J. Crowther and D. Eagland Combined Study of Coagulation Kinetics and Close-range Aggregate Structure A. Lips and R. M. Duckworth Solubilities of Salts and Kinetics of Reactions involving Inorganic Complex Ions in Aqueous Acetone Mixtures. Derivation of Transfer Chemical Potentials for Ions in these Aqueous Mixtures at Ambient Pressure and 298.2 K M. J. Blandamer, B. Briggs, J. Burgess, P. Guardado, S. Radulovic and C. D. Hubbard Kinetics of the Dissociation Reaction of Ferricenium Tri-iodide in Benzene Solution S. R. Logan and M. R. Welsh Study of the Conformational Equilibria between Rotational Isomers using Ultrasonic Relaxation and Raman Spectroscopy. Part 2.-Di- and Tri- halogenoalkanes H. Nomura, S. Koda and K. Hamada Volumetric and Infrared Studies of the Adsorption of Gaseous Benzene, Toluene and Chlorobenzene by Zinc Oxide M. Nagao and K. Matsuoka G. V. Buxton, N. D. Wood and (in part) S. Dyster P. D. I. Fletcher and D. Parrott J. S. Lea and M. C. R. Symons A. Inoue T. Isobe and M. Senna 1091 1101 1113 1123 1131 1145 1153 1163 1171 1181 1187 1195 1199 121 1 1223 1243 1259 1267 1277

ISSN:0300-9599    

DOI:10.1039/F198884FP045

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

315 329 347 363 377 385 40 1 409 417 423 427 428 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions ll,lssue4,1988 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions II, Issue 4 is reproduced below: Temperature Dependence of Antennae Chlorophyll Fluorescence Kinetics in Photosystem I Reaction Centre Protein G. F. W. Searle, R. Tamkivi, A. van Hoek and T. J. Schaafsma The Maxwell Stress Tensor and the Thermodynamics of the Diffuse Double Layers at Surfactant-loaded Interfaces Implementation and Limitations of the Transition-state Theory for Ion- Molecule Systems with Non-spherical Dividing Surfaces J. Turulski and J. Niedzielski Kinetics and Mechanism of Photoinduced and Thermal Proton-transfer Processes in o-Hydroxybenzaldehyde and o-Hydroxyacetophenone.A Remarkable Temperature Dependence of the Reaction Rate J. Konijnenberg, A. H. Huizer and C. A. G. 0. Varma The Rate Constants for the Reaction of the Hydroxyl Radical with Benzene A Photophysical and Theoretical Study of Styrylanthracenes G. Bartocci, F. Masetti, U. Mazzucate, A. Spalletti, G. Orlandi and G. Poggi Arrhenius Parameters for the Reaction 1-C,H, + O,C,H, + HO, S. K. Gulati and R. W. Walker Ab Initio Molecular Orbital and Photoelectron Spectroscopic Study of the Pyridine-Boron Trifluoride Electron-Donor-Acceptor I. H. Hillier, M. A. Vincent, J. A. Connor, M. F. Guest, A. A. MacDowell and W. von Niessen Electronic Spectra of Dibenzo[b,h]biphenylene H. Yamaguchi and H. Baumann The Microscopic Velocity of a Salt in an Electrolyte Solution.An Alternative Characterization by a New Variation Principle F. 0. Raineri and E. 0. Timmermann Corrigendum to Velocity Correlation Functions in Different Reference Frames. Their Relation with Phenomenological and Empirical Transport Coefficients Corrigendum to Salt Velocity Correlation Functions : a Microscopic Interpretation. Part 1 .-Solution of a Single Binary Electrolyte F. 0. Raineri and E. 0. Timmermann S. Ljunggren and J. C. Eriksson D. L. Baulch, I. M. Campbell and S. M. Saunders F. 0. Raineri and E. 0. TimmermannThe following papers were accepted for publication in Faraday Transactions I during January, 1988. 6/2070 71062 71964 71 1029 711 156 711201 7/ 1369 71 1378 71 1426 711451 7 1 1524 71 1559 711561 7/ 1563 7/ 1598 7/ 1620 Thermodynamic Studies of Transfer of some Amino Acids and Peptides from Water to Aqueous Glucose and Sucrose Solutions at 288.15 K R.Bhat and J. C. Ahluwalia Effect of Micelle Formation on the Absorption Spectra of Functionalized Detergent with the Anthraquinone Moiety K. Hoshino, T. Saji, K. Suga ands M. Fujihira Correlations of Heterogeneity Parameters for Single Solute and Multi-solute Adsorption from Dilute Solutions A. W. Marczewski, A. Derylo- Marczewska and M. Jaroniec Hydrogen Effect in Chemisorption of n-Hexane over Pt Black Preparation and Electrochemical Behaviour of a Methylene Blue (MB) Modified Electrode based on Nafion Polymer Film Synthesis of a Viologen-Tetratitanate Intercalation Compound and its Photochemical Behaviour H.Miyata, Y. Sugahara, K. Kuroda and C. Kato A Model for the Mass Transfer Resistance at the Surface of Zeolite Crystals M. Kocirik, P. Struve, K. Fiedler and M. Bulow Surface Reactions of Geothite with Phosphate R. G. Jonasson, R. R. Martin, M. E. Giuliacci and K. Tazaki Conductivity of Polypyrrole Films doped with Aromatic Sulphonate Deriva- tives S. Kuwabata, K. Okamoto and H. Yoneyama Tracer Diffusion of the Tris( 1,lO-phenanthroline)iron(n) Cation in Aqueous Salt Solutions. Effect of Hydrophobic Interactions T. Torninaga, S. Matsumoto, T. Koshiba and Y. Yamamoto Salt Solutions in Supercritical Water. Some Preliminary Studies on the Influence of Gravity D. J. Turner A Correlated X-Ray Photoelectron and Electron Spin Resonance Spectroscopic Study of Rhodium-exchanged X and Y Zeolites D.Goldfarb, S. Contarini and L. Kevan The Static and Kinetic Properties of a Ligand-exchange Reaction and its Effect on Longitudinal Magnetization Recovery in Aqueous- Al"' N.M.R. Spectroscopy Catalytic Decomposition of Mercaptans of Metal Films of Iron, Nickel, Palladium, Aluminium and Copper Entropy of Transfer of Glycine, Diglycine, p-Nitroaniline and Benzoic Acid from Water to Aqueous Solutions of Polyhydroxy Compounds J. Prakash Chatterjee and 1. N. Basumallick Transfer Chemical Potentials for Ions, Solubilities of Salts and Kinetics of Reactions involving Inorganic Complex Ions at Ambient Pressure and 298.2 K in Binary Aqueous Mixtures containing Ethanol and Propan-2-01 M. J. Blandamer, B. Briggs, J. Burgess, D. Elvidge, P.Guardado, A. W. Hakin, S. Radulovic and C. D. Hubbard A. Sarkany Z. Lu and S. Dong T. Jin and K. Ichikawa Y. K. Al-Haidary and J. M. Saleh (ii)711621 7/ 1650 7/ 1655 7/ 1665 7/ 1670 7/ 1690 711691 7/ 1809 7/ 1834 7/ 1836 7/ 1854 711881 7/ 1884 7/ 1892 7/ 1939 7/ 1940 712010 The Crystallisation Kinetics of Calcite in the Presence of Magnesium Ions W. A. House, M. R. Howson and A. D. Pethybridge Multicomponent Ion Exchange in Zeolites. Part 4.-The Exchange of Magnesium Ions in Zeolites X and Y K. R. Franklin and R. P. Townsend Adsorption of Oxygen and Reactivity with HCI on a Barium-dosed Lead Surface M. Ayyoob and M. S. Hegde Dynamic Studies of the Photoinduced Metathesis of C,H, and Photoreduction of Mo With CO on Anchored and Impregnated Mo/SiO, Catalysts M.Anpo, M. Kondo, Y. Kubokawa, C. Louis and M. Che Spectrometric and Chemical Studies of 5-Acyl- and 5-Nitroso-2-(N,N- disubstituted Amino) Thiazoles T. N. Birkinshaw, G. D. Meakins and S. J. Plackett Effect of Solvent on the Reactions of Coordination Complexes. Part 5.--Kinetics of Solvolysis of the cis-Bromo-2-(2-aminothiazole)- bis(ethylenediamine)cobalt(m) in Methanol-Water, Propan-2-01-Water and Ethylene Glycol-Water Media ' Crystal-like ' Structure of Colloidal Spheres in an Alternating Electric Field T. Okubo The Construction and Characteristics of Drug-selective Electrodes ; their Applications to Determine Complexation Constants of Inclusion Complexes with a- and P-Cyclodextrins including a Kinetic Study N. Takisawa, D. G. Hall, E. Wyn-Jones and (in part) P.Brown Metal Oxides as Heterogeneous Catalysts for Oxygen Evolution under Photochemical Conditions A. Harriman, I. J. Pickering, J. M. Thomas and P. A. Christensen Electron-addition and Electron-loss Pathways for Cyanoalkanes H. Chandra and M. C. R. Symons Iridium Oxide Hydrosols as Catalysts for the Decay of Zinc Porphyrin Radical Cations in Water A. Harriman, G. S. Nahor, S. Mosseri and P. Neta Double-layer Interaction between Spheres with Unequal Surface Potentials J. Th. G. Overbeek The Deterministic Aspect of the Bray-Liebhafsky Oscillatory Reaction A. Anic and Lj. Kolar-Anic Successive Addition of Electrons to Sodium Quinizarin 2- and 6-Sulphonate in Aqueous Solution. A Pulse- and y-Radiolysis Study T. Mukherjee, E. J. Land, A. J. Swallow, P. M. Guyan and J.M. Bruce Electron Spin Resonance and Electron Nuclear Double Resonance Study on Mixed Crystals of Benzophenone and Diphenylnitroxide A. L. Maniero and M. Brustolon Reactions of 1,1,3,3-Tetramethylcyclobutane on Evaporated Metal Films J. K. A. Clarke, B. F. Hegarty and J. J. Rooney A Study on the Correlation of the Solvation Properties of Water and of Dimethylsulphoxide in Metal Nitrate-Water and Metal Nitrate-Dimethylsulphoxide Melts with the Molecular Properties of the Constituents of the Melts A. C. Dash and J. Pradhan G. A. Sacchetto and Z. Kodejs (iii)7/2050 Determination of Rate Parameters in Seeded Emulsion Polymerisation Systems. A Sensitivity Analysis I. A. Maxwell, E. D. Sudol, D. H. Napper and R. G. Gilbert 7/2057 Extraframework Aluminium in Steam and Sic1 Dealuminated Y Zeolite.A 27Al and ''Si N.M.R. Study J. Sanz, V. Fornes and A. Coma 7/2083 Fourier-transform Infrared Vibrational Circular Dichroism of Simple Carbohydrates 7/2232 Interactions between Metal Cations and the Ionophore Lasalocid. Part 5.-A Pontentiometric, Polarographic and E.S.R. Study of Cupric Ion-Lasalocid Equilibria in Methanol P. Laubry, G. Mousset, P. Martinet, M. Tissier, C. Tissier and J. Juillard 8/ 1 14 Double-layer Interaction between Spheres with Unequal Surface Potential. Response to the Critique T. Chandramouly, D. M. Back and P. L. Polavarapu R. BarouchCumulative Author Index 1988 Abe, H., 511 Abraham, M. H., 175, 865 Adachi, H., 1091 Allen, G. C., 165, 355 Anazawa, I., 275 Anpo, M., 751 Aracil, J., 539 Aveyard, R., 675 Baba, K., 459 Baglioni, P., 467 Baldini, G., 979 Barna, T., 229 Bazsa, G., 215, 229 Berei, K., 367 Berroa de Ponce, H., 255 Binks, B.P., 675 Blandamer, M. J., 1243 Blesa, M. A., 9 Blinov, N. N., 1075 Bonnefoy, J., 941 Borckmans, P., 1013 Borgarello, E., 261 Bourdillon, C., 941 Breen, J., 293 Briggs, B., 1243 Brown, M. E., 57 Brydson, R., 617, 631 Burgess, J., 1243 Burget, U., 885 Busca, G., 237 Buxton, G. V., 1101, 11 13 Caceres, M., 539 Carbone, A. I., 207 Cavani, F., 237 Cavasino, F. P., 207 Centi, G., 237 Chagas, A. P., 1065 Chandra, H., 609 Che, M., 751 Cheng, V. K. W., 899 Chien, J. C. W., 1123 Chirico, G., 979 Chudek, J. A., 1145 Clarke, R. J., 365 Clint, J. H., 675 Coates, J. H., 365 Coller, B. A. W., 899 Coluccia, S., 751 Compton, R. G., 473, 483 Crowcher, N.J., 1211 Danil de Namor, A. F., 255 Das, S., 1057 Dash, A. C., 75 Dash, N., 75 Davydov, A,, 37 Dawber, J. C., 41 Dawber, J. G., 41, 713 de Bleijser, J., 293 Diaz Peiia, M., 539 Dickinson, E., 871 Disdier, J., 261 Domen, K., 511 Dougal, J. C., 657 Duarte, M. Y., 97, 367 Duce, P. P., 865 Duckworth, R. M., 1223 Dyster, S., 1113 Eagland, D., 1211 Egawa, C., 321 Einfeldt, J., 931 Elliot, A. J., 1101 Engel, W., 617, 631 Eszterle, M., 575 Fernandez-Pineda, C., 647 Flanagan, T. B., 459 Fletcher, P. D. I., 1131 Foresti, E., 237 Foresti, M. L., 97 Forster, H., 491 Foster, R., 1145 Franklin, K. R., 687 Gal, D., 1075 Gabrail, S., 41 Galwey, A. K., 57, 729 Gans, P., 657 Geblewicz, G., 561 Geertsen, S., 1101 Gill, J. B., 657 Gilot, B., 801 Gopalakrishnan, R., 365 Grampp, G., 366 Gratzel, M., 197 Gray, P., 993 Green, S.I. E., 41 Grigo, M., 931 Gritzner, G., 1047 Guardado, P., 1243 Guarini, G. G. T., 331 Guidelli, R., 97, 367 Gupta, D. Das, 1057 Hadjiivanov, K., 37 Hall, D. G., 773 Halle, B., 1033 Hamada, K., 1267 Harrer, W., 366 Hasebe, T., 187 Hashimoto, K., 87 Hazra, D. K., 1057 Heatley, F., 343 Herley, P. J., 729 Herrmann, J-M., 261 Heyward, M. P., 815 Hidalgo, M. del V., 9 Hill, A., 255 Hubbard, C. D., 1243 Huis, D., 293 Ige, J., 1 Ikeda, S., 151 Imai, H., 923 Imamura, H., 765 Imanaka, T., 851 Inoue, A., 1195 Irinyi, G., 1075 Isobe, T., 1199 Iwasawa, Y., 321 Jaenicke, W., 366 Jeminet, G., 951 Johnson, G. R. A,, 501 Johnson, I., 551 Johnston, C., 309 Jorge, R. A., 1065 Jorgensen, N., 309 Juillard, J., 95 1, 959, 969 Kane, H., 851 Kanno, T., 281 Kasahara, S., 765 Kato, S., 151 Katz, N.E., 9 Kawasaki, Y., 1083 Keeble, D. J., 609 Kevan, L., 467 Kirby, C., 355 Kiricsi, I., 491 Kiss, I., 367 Klissurski, D., 37 Kobayashi, M., 281 Koda, S., 1267 Kondo, J., 511 Kondo, Y., 11 1 Konishi, Y., 281 Kornhauser, I., 785, 801 Krausz, E., 827 Kristyan, S., 917 Kubokawa, Y., 751 Kurimura, Y., 841, 1025 Kusabayashi, S., 11 1 tajtar, L., 19 Lambi, J. N., 1 Laubry, P., 969 Laval, J-M., 941 Lawrence, K. G., 175 Lea, J. S., 1181 Leaist, D. G., 581 Lefever, R., 1013 Lengyel, I., 229 Leyendekkers, J. V., 397 Leyte, J. C., 293 Lincoln, S. F., 365AUTHOR INDEX Lindner, Th., 631 Lips, A., 1223 Logan, S. R., 1259 Mackay, R. L., 1145 Maezawa, A., 851 Malanga, C., 97 Marcus, Y., 175 Maroto, A. J. G., 9 Maruya, K., 511 Mason, D., 473, 483 Matsumura, Y., 87 Matsuoka, K., 1277 Mayagoitia, V., 785, 801 McAleer, J.F., 441 McMurray, N., 379 Mead, J., 675 Mensch, C. T. J., 65 Merkin, J. H., 993 Mills, A., 379 Mirti, P., 29 Mitsushima, I., 851 Mohamed, M. A. A., 57, 729 Moiroux, J., 941 Morris, J. J., 865 Morton, J. R., 413 Moseley, P. T., 441 Mousset, G., 969 Muhler, M., 631 Murray, B. S., 871 Nagao, M., 1277 Nakamura, Y., 11 1 Nakao, N., 665 Nakayama, N., 665 Narayanan, S., 521 Nazhat, N. B., 501 Nicolis, G., 1013 Nishihara, C., 433 Nishikawa, S., 665 Nomura, H., 151, 1267 Norris, J. 0. W., 441 Noszticzius, Z., 575 Nucci, L., 97 Ohtani, S., 187 Okamoto, Y., 851 Okubo, T., 703, 1163, 1171 Olofsson, G., 551 Onishi, T., 511 Ono, Y., 1091 Oosawa, Y., 197 Page, F. M., 1145 Painter, D., 773 Parrott, D., 1131 Pelizzetti, E., 261 Penar, J., 739 Pezzatini, G., 367 Piccini, S., 331 Pichat, P., 261 Piekarski, H., 529, 591 Pointud, Y., 959 Pota, G., 215 Preston, K.F., 413 Prior, D. V., 865 Quist, P-O., 1033 Radulovic, S., 1243 Rajaram, R. R., 391 Renuncio, J. A. R., 539 Rhodes, C. J., 1187 Rochester, C. H., 309 Rojas, F., 785, 801 Rubio, R. G., 539 Saadalla-Nazhat, R. A., 501 Saito, M., 1025 Saito, Y., 275 Sakamoto, Y., 459 Sakata, Y., 511 Sato, T., 275 Sauer, H., 617 Sawabe, K., 321 Sayari, A., 413 Sbriziolo, C., 207 Schelly, Z. A., 575 Schiffrin, D. J., 561 Schiller, R. L., 365 Schlenoff, J. B., I123 Schlogl, R., 631 Schmelzer, N., 931 Schulz, R. A., 865 Scott, S. K., 993 Sellers, R. M., 355 Senna, M., 1199 Senoda, Y., 1091 Sermon, P.A., 391 Serpone, N., 261 Shindo, H., 433 Sidahmed, I. M., 1153 Smith, E. R., 899 Sokolowski, S., 19, 139 Somsen, G., 529 Soriyan, 0. O., 1 Stainsby, G., 871 Stevens, J. C. H., 165 Stone, W. E. E., 117 Symons, M. C. R., 609, 1181, Szamosi, J., 917 Tagawa, T., 923 Takada, T., 765 Takagi, Y., 1025 Takato, K., 841 Tanaka, F., 1083 Tanaka, K., 601 Tanaka, K-i., 601 Taylor, P. J., 865 Thomas, J. M., 617, 631 Tissier, C., 951, 969 Tofield, B. C., 441 Torres-Sanchez, R-M., 117 Townsend, R. P., 687 Trifiro, F., 237 Tsuchiya, S., 765 Twiselton, D. R., 1145 Uematsu, R., 111 Uma, K., 521 Unwin, P. R., 473, 483 van Veen, J. A. R., 65 van Wingerden, R., 65 Varani, G., 979 Vasaros, L., 367 Vizquez-Gonzalez, M. I., 647 Vidoczy, T., 1075 Viguria, E. C., 255 Vink, H., 133 Viswanathan, B., 365 Walker, R.A. C., 255 Ward, J., 713 Wells, C. F., 815, 1153 Welsh, M. R., 1259 Williams, B. G., 617, 631 Williams, D. E., 441 Williams, R. A., 713 Wood, N. D., 11 13 Wyn-Jones, E., 773 Yamada, Y., 751 Yamashita, S., 1083 Yoshida, S., 87 Zecchina, A., 751 Zeitler, E., 617, 631 Zelano, V., 29 Zielinski, R., 151 Zundel, G., 885 1187NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, 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 WIV OBN). These recommendations are applied by The Royal Society of Cemistry 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, 0, E, F, and H (Pergamon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butteworths, London, 1971, now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987). 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. (vii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No.8 6 Spectroscopy at Low Temperatures University of Exeter, 13-15 September 1988 Organising Committee: Professor A. C. Legon (Chairman) Dr P. B. Davies Dr B. J. Howard Dr P. R. R. Langridge-Smith Dr R. N. Perutz Dr M. Poliakoff The Discussion will focus on recent developments in spectroscopy of transient species (ions, radicals, clusters and complexes) in matrices or free jet expansions. The aim of the meeting is to bring together scientists interested in similar problems but viewed from the perspective of different environments. The Introductory Lecture will be given by G. C. Pirnentel and speakers include: L. Andrews, K. H. Bowen, B. J. Howard, L. B. Knight Jr, E. Knozinger, D. H. Levy, J. P. Maier, J. Michl, M. Moskovits, A.J. Stace, M. Takami, J. J. Turner, M. Poliakoff, A. J. Barnes, J. M. Hollas, M. C. R. Syrnons and P. Suppan. 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 WITH THE ASSOCIAZIONE ITALIANA DI CHlMlCA FISICA, DIVISION DE CHlMlE PHYSIQUE OF THE SOCIETE FRANCAISE DE CHlMlE AND DEUTSCHE BUNSEN GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE JOINT MEETING Structure and Reactivity of Surfaces Centro Congressi, Trieste, Italy, 13-16 September 1988 Organising Committee: M. Che V. Ponec F. S. Stone G. Ertl R. Rosei A. Zecchina The conference will cover surface reactivity and characterization by physi (i) Metals (both in single crystal and dispersed form) a l method (ii) Insulators and sem&onductors (oxides, sulphides, halides, both in single crystal and dispersed forms) (iii) Mixed systems (with special emphasis on metal-support interaction) The meeting aims to stimulate the comparison between the surface properties of dispersed and supported solids and the properties of single crystals, as well as the comparison and the joint use of chemical and physical methods.Further information may be obtained from: Professor C. Morterra, lnstituto di Chimica Fisica, Corso Massimo D'Ateglio 48, 10125 Torino, Italy. (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM Orientation and Polarization Effects in Reactive Col I isions To be held at the Physikzentrum, Bad Honnef, West Germany, 12-14 December 1988 Orga nising Committee : Dr S.Stolte Professor J. P. Simons Dr K. Burnett Dr H. Loesch Professor R. N. Dixon Professor R. A. Levine The Symposium will focus on the study of vector properties in reaction dynamics and photodissociation rather than the more traditional scalar quantities such as energy disposal, integral cross-sections and branching ratios. Experimental and theoretical advances have now reached the stage where studies of Dynamical Stereochemistry can begin to map the anisotropy of chemical interactions. The Symposium will provide an impetus to the development of 3-D theories of reaction dynamics and assess the quality and scope of the experiments that are providing this impetus. Contributions for consideration by the Organising Committee are invited in the following areas: (A) Collisions of oriented or rotationally aligned molecular reagents (6) Collisions of orbitally aligned atomic reagents (C) Photoinitiated ‘collisions’ in van der Waals complexes (D) Polarisation of the products of full and half-collisional processes 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. 87 Catalysis by Well Characterised Materials University of Liverpool, 11-13 April 1989 Organising Committee: Professor R. W. Joyner (Chairman) Professor A. K. Cheetham Professor F. S. Stone The understanding of heterogeneous catalysis is an important academic activity, which compliments industry‘s continuing search for novel and more efficient catalytic processes.The emergence of relevant, in particular in situ techniques and new developments of well established experimental approaches to catalyst characterisation are making a very significant impact on our knowledge of catalyst composition, structure, morphology and their inter-relationships. Well characterised catalysts, which will be the subject of the Faraday Discussion, include single-crystal surfaces, whether of metals, oxides or sulphides; crystalline microporous solids, such as zeolites and clays, and appropriate industrial catalysts. The elucidation of structure/function relationships and catalytic mechanism will be important aspects of the scientific programme.Contributions describing novel methods for synthesising well characterised catalysts and also reporting important advances in characterisation techniques will also be welcome. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 May 1988 to: Professor R. W. Joyner, Leverhulme Centre for Innovative Catalysis, Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool, Grove Street, P. 0. Box 147, Liverpool L69 3BX. Full papers for publication in the Discussion volume will be required by December 1988. Dr. K. C. Waugh Professor P. 6. WellsJOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistrykhemical physics which appear currently in J. Chem.Research, The Royal Society of Chemistry's synopsis + microform journal, include the following: An E.s.r. Study of the Radiolysis of Acetylenic Acids and Esters in a Freon Matrix Christopher J. Rhodes and Martyn C. R. Symons (1 988, Issue 1 ) Imbibition of Sodium Nitrate by Zeolite Na-Y at 25 "C and Kevin R. Franklin, Barrie M. Lowe Gordon H. Walters (1 988, Issue 1 ) The Solubility of Carbon Dioxide in Mixtures of Water and Acetone Robert W. Cargill, Donald E. MacPhee and Kenneth Patrick (1 988, Issue 1 ) Correlation Analysis of the Reactivity in the Oxidation of Aromatic Aldehydes by An E.s.r. Study of Azoalkane Radical Cations Electrochemical Studies of some Nickel(I1) Complexes of the Type [Ni(NNS)(Heterocycle)-1 N-Bromoacetamide Louwrier (1 988, Issue I ) and [NiZ(NNS)~-(Heterocycle)-][CIO4] (1988, Issue 1 ) Anita Gupta, Sandhya Mathur and Kalyan K.Banerji (1988, Issue 1 ) Christopher J. Rhodes and Pieter W. F. Sanat K. Mandal, Parimal Paul and Kamalaksha Nag Influence of the Acid-strength Distribution of the Zeolite Catalyst on the t-Butylation of Phenol Avelino Corma, Hermenegildo Garcia and Jaime Primo (1 988, Issue 1 ) The Effect of Nitric Oxide on the Kinetics of Decomposition of Thionitrites Garley and Geoffrey Stedman (1 988, Issue 2) Kinetics of the Solvolysis of Chlorapenta-aminecobalt(1ll) Ions in Water and in Water - Propan-2-01 Mixtures Evaluation of Broyden - Fletcher - Goldfarb - Shanno (BFGS) Variable Metric Method in Geometry Optimisation using Semi-empirical SCF-MO Procedures Dimitris K. Agrafiotis and Henry S.Rzepa (1 988, Issue 3) Michael S. Kamal H. Halawani and Cecil F. Wells (1988, Issue 2)FARADAY DIVISION INFORMAL AND GROUP MEETINGS Neutron Scattering Group Vibrational Spectroscopy To be held at Imperial College, London on 20-21 April 1988 Further information from Dr J. Howard, ICI plc, New Science Group N129, PO Box 90, Wilton, Middlesbrough Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S.P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9BP Colloid and Interface Science Group with the Biochemical Society Dynamic Properties of Biomolecular Assemblies To be held at the University of Nottingham on 20-22 July 1988 Further information from Dr S. E. Harding, School of Agriculture, Unversity of Nottingham, Department of Applied Biochemistry, Sutton Bonington LE12 5RD Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G. Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ ~ ~~ Neutron Scattering Group Postgraduate Informal Neutron Conference To be held at the University of Keele on 25-27 July 1988 Further information from Professor C.R. A. Catlow, Department of Chemistry, University of Keele, Keele, Staffs ST5 5BG Electrochemistry Group with the Electroanalytical Group and the Society of Chemcial Industry Electrochemcial Dynamics To be held at the University of Strathclyde on 5-10 September 1988 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Statistical Mechanics and Thermodynamics Group Dense Fluids To be held a t the University of Cambridge on 14-16 September 1988 Further information from Or P. Francis, Department of Chemistry, University of Hull, Hull HU6 7RX Carbon Group with the Carbon and Graphite Group of the SC1 Carbon 88 To be held at the University of Newcastle upon Tyne on 18-23 September 1988 Further information from The Conference Secretariat, Carbon 88, Society of Chemical Industry, 14/15 Belgrave Square, London SW1 X 8PS Division Autumn Meeting: Polymerisation and Polymer Behaviour To be held at the University of Birmingham on 20-22 September 1988 Further information from Professor I.W. M. Smith, Department of Chemistry, University of Birmingham, PO Box 363, Birminaham 615 2TT Colloid and Interface Science Group Structure in Colloidal Systems and its Characterisation To be held at the University of Bath on 21-23 September 1988 Further information from Dr R. Buscall, ICI plc, Corporate and Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 4QE (xi)FARADAY DIVISION INFORMAL AND GROUP MEETINGS Neutron Scattering Group Vibrational Spectroscopy To be held at Imperial College, London on 20-21 April 1988 Further information from Dr J.Howard, ICI plc, New Science Group N129, PO Box 90, Wilton, Middlesbrough Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S. P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9BP Colloid and Interface Science Group with the Biochemical Society Dynamic Properties of Biomolecular Assemblies To be held at the University of Nottingham on 20-22 July 1988 Further information from Dr S.E. Harding, School of Agriculture, Unversity of Nottingham, Department of Applied Biochemistry, Sutton Bonington LE12 5RD Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G. Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ ~ ~~ Neutron Scattering Group Postgraduate Informal Neutron Conference To be held at the University of Keele on 25-27 July 1988 Further information from Professor C. R. A. Catlow, Department of Chemistry, University of Keele, Keele, Staffs ST5 5BG Electrochemistry Group with the Electroanalytical Group and the Society of Chemcial Industry Electrochemcial Dynamics To be held at the University of Strathclyde on 5-10 September 1988 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Statistical Mechanics and Thermodynamics Group Dense Fluids To be held a t the University of Cambridge on 14-16 September 1988 Further information from Or P. Francis, Department of Chemistry, University of Hull, Hull HU6 7RX Carbon Group with the Carbon and Graphite Group of the SC1 Carbon 88 To be held at the University of Newcastle upon Tyne on 18-23 September 1988 Further information from The Conference Secretariat, Carbon 88, Society of Chemical Industry, 14/15 Belgrave Square, London SW1 X 8PS Division Autumn Meeting: Polymerisation and Polymer Behaviour To be held at the University of Birmingham on 20-22 September 1988 Further information from Professor I. W. M. Smith, Department of Chemistry, University of Birmingham, PO Box 363, Birminaham 615 2TT Colloid and Interface Science Group Structure in Colloidal Systems and its Characterisation To be held at the University of Bath on 21-23 September 1988 Further information from Dr R. Buscall, ICI plc, Corporate and Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 4QE (xi)

ISSN:0300-9599    

DOI:10.1039/F198884BP047

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(4), 885-898 Glutamic Acid-Hydrogen Phosphate Hydrogen Bonds Proton Polarizability and Proton Transfer as a Function of the Cations Present and of the Degree of Hydration : Infrared Investigations Ulrike Burget and Georg Zundel" Institut fur Physikalische Chemie der Universitat Miinchen, 0-8000 Miinchen 2, Federal Republic of Germany Polyglutamic acid (L-glu),-hydrogen phosphate (Pi) systems have been studied by i.r. spectroscopy dry and hydrated (75 % relative humidity) as a function of the Pi : glu residue ratio and as a function of the type of cations present. In all (L-glu),-Pi systems COH.*--OP~CO- - * * HOP hydrogen bonds with a high degree of proton polarizability are formed. In the Li+ system the weight of limiting structure (11) is < 0.05, whereas for the K+ system it is 0.95.This difference is explained on the basis of the different electrostatic field strength, and thus the different polarizing power, of these cations with regard to Pi. The Na+ systems show intermediate behaviour; at P,:glu = 1 : 3 the system resembles the Li+ system, but since the polarizing power of ,Na+ is less than that of Li+, the proton polarizability of the COH*.*-OP+CO- -**HOP bond is much larger than with the Li+ system. Proton-limiting structure (11), however, has a very low weight. At higher Pi : glu the system resembles the K' system. In the P,:glu = 1.6: 1 system these hydrogen bonds are completely formed and 75 YO of the glu protons have transferred to Pi. The proton polarizability of these hydrogen bonds increases in the series Li+ < Na+ -= K+.The influence of hydration and the secondary structure of (L-glu), are discussed. All results taken together show that systems with glu residue-Pi hydrogen bonds can easily be controlled by the nature of cations present and by the degree of hydration of the systems. (1) (11) In almost all proteins a large number of glutamic and aspartic acid residues are present. Furthermore, it is known that the structure and function of many proteins is determined by the interaction of these proteins with phosphates.lp2 The phosphates should form hydrogen bonds with the side chains of the proteins. Almost nothing is known about the nature of these hydrogen bonds. This is particularly true with regard to the proton- transfer processes and the proton polarizability of such hydrogen A high degree of proton polarizability is indicated by continua in the i.r.~ p e c t r a . ~ . ~ In this paper such hydrogen bonds are studied using films of poly-L-glutamic acid (L-glu), with hydrogen phosphates (Pi). The manner in which the cations influence the nature of the hydrogen bonds formed is of particular interest. Experimental (L-glu), was purchased from Miles GmbH (Miinchen) with a mean chain length of n = 275. Na,HPO, 12H20, K2HP0, - 3H,O, H,PO, and LiOH . H 2 0 were purchased from Merck (Darmstadt) and Fluka (Neu-Ulm). The Li,HPO, solutions were obtained by potentiometric titration of an aqueous H,PO, solution with LiOH. 885 30-2886 Glu tam ic Acid- Hydrogen Phosphate Hydrogen Bonds -.- ( a ) 4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 wavenumber/crn-' 0 .o .- wavenumber/cm-l -.. . - ..-.. .,.-, ;-' -.-- 1.0 - '.... 4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 wavenumber/cm -' Fig. 1. 1.r. spectra of dried (L-glu),-X,HPO, films with different phosphate : sidechain ratios. P,:glu = 1 : 3 (-), 1 : 1 (----), 2 : l (-...); pure (L-glu), (-..-); cations present: (a) Li, (b) Na, (4 K.U . Burget and G. Zundel 887 0.6 n * E 8 0.4 2 9 e 0.2 2 2 W 0) 1 :1 2:l 3:l P i : glu Fig. 2. Absorbance of the continuum at 1900 cm-I of dried (L-glu),-X,HPO, systems with different cations: 0, Li; 0, Na; 0, K. The films of the polymer systems were prepared on silicon supports, from 0.035 mol dm-3, 2 % pyridine containing aqueous solutions of (L-glu), and the respective phosphate in the ratio desired.The precipitation of the polymer films was performed by the centrifugationdrying procedure described in ref. (7), whereby instead of a laboratory centrifuge, an ultracentrifuge was used, improving the quality of the polymer films. Like the i.r. cells described in ref. (8), the cells could be evacuated, permitting adjustment of a defined humidity over the films by use of saturated aqueous solutions of different salts. The i.r. measurements weKe performed with an i.r. spectrophotometer, model 325, Bodenseewerk Perkin-Elmer (Uberlingen). Results and Discussion Fig. 1 shows the i.r. spectra of films of (L-glu),-alkali-metal hydrogen phosphate systems. In fig. 2 the absorbance (at 1900 cm-l) of the observed i.r.continua in these spectra is shown as a function of the Pi:glu ratio. (L-glu),Li,HPO, Fig. l(a) shows that i.r. continua extend from ca. 3000cm-l toward smaller wavenumbers over the whole region. The intensity of these continua increases in proportion to the Pi : glu ratio, reaching a saturation value at a ratio of ca. 1.2 : 1 (fig. 2). This result demonstrates that Pi is bound via a hydrogen bond (I) COH.-*-OPSCO-.**HOP (11) (1) to the carboxylic acid group. The occurrence of the continua demonstrates that the proton fluctuates within these hydrogen bonds and that they show large proton polarizability . In the spectrum of pure (L-glu),, v(C=O) of the monomeric carboxylic acid groups is found at 1730 cm-l and that of the dimeric groups at 1710 cm-l.The dimeric species causes the broad band with a maximum at ca. 2600 cm-' [fig. 1 (a)]. After addition of Pi, one relatively broad band at 1712 cm-l is observed. The position of v(C=O) shows that these groups are acceptors of relatively strong hydrogen bonds [see eqn (2)]. The donor group can only be the OH group of Pi. Hence, these OH groups form hydrogen bonds to the second 0 atom of the glutamic acid residues. This is confirmed by the observation that in the case of the Li+ systems the 2888 Glut am ic A c id- Hydrogen Phosphate Hydrogen Bonds - .. * . . - . . 4, r ' lbl _*. I I 18 00 1600 1400 1200 1000 800 1800 1600 1400 0.4 0.7 1.0 l.? - 1800 1600 1400 1200 1000 I I I 800 1200 1000 800 wavenumber/ cm -' Fig. 3. 1.r. spectra of dried (L-glu),-X,HPO, films with different phosphate : sidechain ratios. (--.--), 2 : 1 (.-.-).Pure (L-glu), ( - a * - ) . Cations present: (a), (b) Li; (c), ( d ) Na; (e), (f) K. (a), (b) Pi: glu = I 4 (----), 1 2 (. . * *), I : 1 (-); ( c ) - ( f ) Pi glu = 1.3 (----), 1 1 (-), 1.5: 1U. Burget and G. Zundel 889 (5 (OH) vibration, expected at ca. 2400 cm-',' that indicates hydrogen bonds between the phosphates,l0 is only observed at Pi: glu ratios > 1 : 1 [fig. 1 (a)]. Thus, the following arrangement with one hydrogen bond with high proton polarizability, and one hydrogen bond in which the proton is localized at the phosphate, showing negligible proton polarizability, occurs : R-C7 \OH* Hoe R-C, Li+ Li+ 'OH. (1') (L - glu), is a-helical under these conditions and a Corey-Pauling-Koltun molecular rnodel shows that an arrangement according to eqn (2) is highly favourable if (L-glu), is present as an a-helix. v,,(-PO:-) is found as doublet (at 1040 with shoulder at 1080 cm-l), vs(-PO!-) at 950 cm-' and vP-(OH) at 870 cm-l [fig.3(b)].11-15 The splitting of v,,(-PO:-) is caused by removal of the degeneracy owing to the asymmetrical environment of the -PO:- groups of proton-limiting structure (I'), caused by the strong interaction of the Lit ions with the -PO:- groups. This is confirmed by the fact that this splitting vanishes with increasing degree of hydration [fig. 4(a)]. Hence in the dried system the Li+ ions are arranged as shown in eqn (2). Returning to the result that the intense v(C=O) band at 1712 cm-' suggests that the proton-transfer equilibrium [eqn (1) and (2)] is largely shifted to the left, this result is confirmed by the following facts.The band pair caused by coupling of v(C-0) and iS(0H) in the -COOH groups, observed at 1250 and 1 170 cm-', remains with Pi addition [fig. 3(b)]. Finally, no bands of PO; groups of proton-limiting structure (11') arise with increasing Pi content in fig. 3 (b); only the bands of the -PO:- groups already discussed and vP-(OH) are observed. v,,(-CO;) at ca. 1560 cm-l is masked by the band observed at 1545 cm-l (the amide I1 band). v,(-CO;) at 1400 cm-' [fig. 3(e)] is superimposed and overlaps strongly with the band at 1406cm-1 [(CH,) of the glu residues]. Nevertheless, a quantitative evaluation of the integrated intensity of the band at ca. 1400 cm-l can be performed as follows.The spectrum of the pure (L-glu), system is subtracted from the spectrum of the Pi+(L-glu), system. This procedure is justified since the absorbance of (CH,) at 1406 cm-' is the same with (L-glu), and with salts of (L-glu),. This evaluation shows that c 5 % of the carboxylic acid protons are present in proton-limiting structure (11'). As we shall see later with the Na' and K+ systems, this equilibrium is usually shifted to the right. Thus, in the Li+ systems the position of this equilibrium is determined by the influence of the Li' ions. These polarize Pi, hence the increase of the negative charge density at the hydrogen-bond acceptor 0 atom of Pi is so large that the equilibrium [eqn (2)] is almost completely shifted to the left. Nevertheless, these hydrogen bonds show still proton polarizability, as indicated by the continuum [fig.1 (a) and 21. Hence, a double-minimum proton potential exists. This is understandable16. l7 since the equilibrium is determined by AHo ASo RT +R In KpT = -- whereby KpT is the equilibrium constant, AHo the standard enthalpy and ASo the standard entropy of the proton-transfer reaction. With such proton-transfer equilibria AS" is always relatively large and shifting the equilibria to the left. The proton potential determining the proton polarizability is, however, determined by AH".Glutamic Acid-Hydrogen Phosphate Hydrogen Bonds ( a ) I I L I I I I I I I I I I I I I I I I I I I00 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 wavenumber/cm-' 0.0 ," 1... /" _ ' ' -.. - '. /' ' 4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 wavenumber/cm 0.0 r /"\ .... ( c ) m4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 wavenumberl cm -' Fig. 4. 1.r. spectra of (L-glu),-X,HPO, films (Pi: giu = 1 : 1) thoroughly dried (-), at 75 YO relative humidity (----); pure dried (L-glu), (-.+-). Cations present: (a) Li, (b) Na, (c) K.U. Burget and G. Zundel 89 1 In the classical approximation the proton potential is symmetrical and the proton polarizability largest if AH" = 0. The position of the proton-transfer equilibrium is, however, determined by AGO, caused by the entropy term with this type of reaction, AGO is always larger than AH".'6*17 Thus, hydrogen bonds may already display proton polarizabilities if the proton-transfer equilibria are still almost completely on the left."T l7 For Pi: glu >, 1.2: 1 the intensity of the continuum no longer increases (fig.2). The Li+ ions that are tightly bound to the phosphates prevent the formation of hydrogen- bonded chains with large proton polarizability as observed, for instance, with (L-his), with KH2P04.22 The additional Pi ions are associated via hydrogen phosphate-hydrogen phosphate bonds [eqn (3)] in which the protons are localized at the Pi and which therefore show no proton polarizability. This is shown by the 2 d(0H) vibration*-" observed at 2400 cm-' [fig. I(;)]. Li+ Li+ Li+ 0 0 0 *HO-P;-;O* * *HO-PzO* HO-PzO 0 0 0 Li+ Li+ Li+ I: - I: - I: - 1: - I: - I: - (3) Fig. 2 shows that with the K+ systems the intensity of the continuum is much stronger than with the Li+ systems.At lower Pi : glu ratios the intensity increases in proportion to this ratio. It approaches a saturation value at a ratio of 1.6 : 1. This result shows that Pi is bound to the carboxylic acid groups via a hydrogen bond with large proton polarizability. Fig. 3 (e) shows that in the Pi: glu = 1 : 3 instead of the two v(C=O) bands of the monomer and dimer COOH groups, a broad shoulder is observed in the region 1700-1750 cm-'. At 1560 cm-' a pronounced shoulder is found, caused by v,,(-CO;). This demonstrates that the proton-transfer equilibrium [eqn (l)] in the glutamic acid-phosphate bonds is shifted to a large extent to the right. v,, (-CO,) is found as a pronounced shoulder at 1560 cm-. This shoulder can, however, not be evaluated quantitatively since this band is superimposed on the amide I1 band. The position of the equilibrium can, however, be determined from v,(-CO;) at 1400 cm-l.From the difference of the absorbance of the sample with a Pi : glu ratio = 1 : 3 and that of the pure (L-glu), [elimination of (CH,) at 1406 cm-'1 it can be estimated that ca. 95 % of the carboxylic acid protons in the hydrogen bonds [eqn (l)] have already transferred to the phosphate if K+ is present instead of Li+, i.e. the weight of proton- limiting structure (11) in eqn (1) amounts to 0.95. This result is confirmed by the bands of the phosphate groups shown in fig. 3 (f>. In the K+ systems with Pi : glu ratios of 1 : 3 or 1 : 1, v,,(-PO,) is observed at 1120 cm-l, vs(-PO,) at 1070 cm-l, v,,[-P-(OH),] at 920 cm-l and v,[-P-(OH),] at 870 cm-l.No bands of -PO:- groups are found in these spectra, confirming that the proton-transfer equilibrium is almost completely shifted to the right. The completely different behaviour of the K+ systems compared with the Li' systems is understandable using the following considerations. The electrostatic field of the K+ ions and hence their polarizing power with regard to Pi is much smaller than that of the Li+ ions. Hence, the negative charge density at the 0 atom of Pi involved in the polarizable hydrogen bonds is higher with the K+ system than with the Li+ systems. Thus, proton-limiting structure (11) in eqn (1) and (4) has noticeable weight. Hence, negatively charged carboxylate groups are present.Then one of the K+ ions may transfer from Pi to these carboxylate groups and shift the proton-transfer equilibrium almost892 Glutarnic Acid-Hydrogen Phosphate Hydrogen Bonds completely to the right. Thus the groups in eqn (4) are present and proton-limiting structure (11”) dominates. K+ H K+ fi 0 0 R-C,.; 0- -HO-P-;-;O K+ I 4 0 R-C, Q0 I OH.. .O;-;P=O K+ -:I - I:- 0 K+ H : (11”) (4) L 0 Kt H (1”) 7 I H0 0 4* 0 0 R-C: - \;‘O* -HO-!kO K+ R-C ‘OH- -0-;-;P~-r0 K+ -:I - A- 0 The 2 &OH) vibration’-’* at 2370 cm-l shows [fig. 1 (c)] that these groups are cross- linked via hydrogen bonds in which the proton is localized at its Pi as shown in In the Pi:glu = 1 : 1 system [fig. 3(e) and (f)] v(C=O) in the region 1700-1750 cm-’ vanishes completely. v,,(-CO,) is observed as very intense band at 1560 cm-’. A comparison of the intensity of the continuum (fig.2) in the Pi : glu = 1 : 1 system with that in the 1.6 : 1 system shows that in the 1 : 1 system ca. 90 % of the hydrogen bonds with large proton polarizability are formed. Fig. 2 shows that 100 % are formed at the Pi : glu ratio 1.6:l. An evaluation of v,(-CO,) as described above (observed at 1400 cm-’) shows that 95 +5 % of the carboxylic acid protons have transferred to the phosphate groups. The weight of proton-limiting structure (11”) in eqn (4) amounts to 0.95 with the K+ systems, whereas in the Li+ systems it was < 0.05. The intensity of the continuum shows that the proton polarizability of the hydrogen bonds is much larger with the K+ system than with the Li+ systems.With regard to the Pi which is not involved in the groups [eqn (4)], the spectra of samples with Pi : glu > 1 : 1 [fig. 3 (f)] show that v,,(-PO:-) arises at 1070 cm-l with a shoulder at 1120 cm-l, v,(-PO:-) appears at 970 cm-l and vP-(OH) at 850 cm-’. These bands mask the bands of the Pi in the groups [eqn (4)]. The intense 2 d(OH) band with a maximum at ca. 2370cm-’ shows that these additional hydrogen phosphate molecules are bound between the phosphates of the groups [eqn (4)] via hydrogen bonds in which the protons are localized at one The v(0H) stretching vibration of these bonds is observed as a broad shoulder in the region 3200-2750 cm-l [fig. 1 (c)]. eqn (4). (L-glu),Na,HPO, The continuum shows that with the Na+ systems (I) COH.---OP-LCO--.-HOP (11) bonds are formed (fig.2). The intensity of the continuum is only slightly smaller than with the K+ systems, thus the proton polarizability of these hydrogen bonds is only slightly smaller than with the K+-systems. Nevertheless, especially at Pi : glu < 1.6 : 1, the Na+ systems show an intermediate behaviour between the Li+ and K+ systems. In the case of the P,:glu = 1 : 3 system [figs. 3(c) and (d)] no shoulder of v,,(-CO,) is found at 1560 cm-’. Furthermore, the evaluation of v,(-CO,) described in the case of the K+ system shows that the transfer of the carboxylic acid protons to Pi is at leastU . Burget and G. ZundeI 893 5 YO. Thus, the proton-transfer equilibrium remains almost completely on the left-hand side. vas(-POi-) of proton-limiting structure (I) is found at 1070 cm-l, v,(-POi-) as a relatively intense band at 925 cm-’ and vP-(OH) at 870 cm-’.v(C=O) of -COOH monomers at 1730 cm-’ vanishes completely. A pronounced sharp shoulder at ca. 1710 cm-’ is, however, observed. These results show that the -COOH groups not associated with Pi are still preferentially present as dimers. Finally, in the region 2600- 2200 cm-’ a broad band is observed. The asymmetric shape of this band shows that two different vibrations contribute to this band. All these results are understandable as follows: besides domains in which the carboxylic acid groups are still present as dimers, regions are present in which the Pi are attached to the carboxylic groups similar than those in the Li+ systems. R-C HO** ‘OH- R--C\ HO* * OH- -*o* - P Y .HO’ (I,’,) Na+ .p ‘ Na+ 0.‘OH* (I,’’,) The nature of the hydrogen bonds is, however, different. Owing to the weaker field (smaller polarizing power) of the Na+ ion compared to the Li+ ion, the negative charge density at the 0 atom of Pi remains higher. Thus, the H+ is slightly loosened from the carboxylic group and may fluctuate within the hydrogen bond, but it is still preferentially present at the carboxylic group. At ca. 2350 cm-’ a broad band arises by the 2 d(0H) vibrations from some Pi-Pi hydrogen bonds in which the proton is localized at one Pi.*-’’ This result taken together with those below suggest that besides domains in which the carboxylic acid groups are still present as dimers, domains in which the groups are arranged as shown in eqn (6) are present.The molecular situation changes completely going from the Pi : glu = 1 : 3 to the 1 : 1 system [fig. 1 and fig. 3(c) and ( d ) ] . In the spectrum of the 1 : 1 system a pronounced shoulder of v,,(-CO;) is found at 1560 cm-’. This demonstrates that proton-limiting -:I-- 0 L 1894 Glut am ic Acid- Hydrogen Phosphate Hydrogen Bonds structure (11) now has considerable weight. A quantitative evaluation of v,(-CO,) shows that ca. 45 YO of the carboxylic acid protons have transferred to the phosphates. Hence, the weight of proton-limiting structure (11) in eqn (1) amounts to 0.45. This big shift of the equilibrium occurs since one of the Na+ ions of Pi moves to the carboxylate group. Thus, a structure as shown in eqn (6) is present. The Pi are now cross-linked to a large extent via hydrogen bonds in which the proton is localized at one Pi, as shown by the 2 6(OH) vibration at 2370 cm-1.8-10 The position of the proton-transfer equilibrium in the hydrogen bond with proton polarizability is confirmed by the bands of the phosphate groups.v,,(-PO~-) of proton- limiting structure (I””) is strongly split owing to removal of degeneracy, caused by the highly asymmetric environment of these groups. One component of this doublet is now found at 1165 and the other at 1060 cm-‘. The band found at 1120 cm-’ is caused by v,,(-PO;) of proton-limiting structure (11””). The fact that a v(C=O) vibration of proton-limiting structure (I””) at ca. 1730 cm-’ is no longer observed may be for the following reasons. A comparison of the intensity of the continuum (fig.2) in the Pi:glu = 1 : 1 system with that in the 1.6: 1 system (see below) shows that in the 1 : 1 system only 80 % of the Pi form hydrogen bonds according to eqn 1. The remaining 20% Pi may form hydrogen bonds to the C=O groups. Owing to these hydrogen bonds and the influence of the Na+ ions, v(C-0) is shifted slightly more towards smaller wavenumbers and is strongly broadened, and therefore masked. This explanation is confirmed by results obtained with the hydrated systems (later). The intensity of the continuum shows a saturation value at a Pi : glu ratio of 1.6 : 1. Thus, COH - - - -0P $ CO- - - - HOP bonds with large proton polarizability are completely formed, and under these conditions a network of hydrogen bonds according to eqn (6) is present.The quantitative evaluation of v,(-CO;) at 1400 cm-l shows that 75 k5 % of the glutamic acid protons are present at the phosphate, i.e. the weight of proton- limiting structure (I””) now amounts to 0.25 and that of structure (11””) to 0.75. When the Pi:glu ratio increases further, the hydrogen phosphates are bound via Pi- Pi hydrogen bonds in which the proton is localized at one Pi. These hydrogen bonds are indicated by the intense band at 2370cm-l [fig. l(b)]. The vibrations of these Pi molecules are now observed [fig. 3(d)]: v,,(-POi-) is a doublet with a maximum at 1070 cm-’ and a shoulder at 1120 cm-’, v,(-POi-) is at 970 cm-’ and vP-(OH) at 850 cm-’. These vibrations mask the phosphate bands of Pi in eqn (6), but at 1170 cm-I a weak shoulder is still found due to the high-wavenumber component of va,(-POi-) of proton-limiting structure (11””) [fig.3 ( d ) ] . Influence of Hydration Even in the thoroughly dried systems a few water molecules are present, as indicated by a shoulder at 3500 cm-’ (fig. 1). The amount of water increases in the series Li+ < Na+ < K+. Furthermore, it increases with increasing Pi content. The fact that the amount of water increases in this series of cations opposite to the change of their hydration e n t h a l ~ y ~ ~ is understandable when one considers that the sidechain, phosphates and cations form a tightly bound network. It is tightest with cations having the strongest field, i.e. with the Li+ ions. Fig. 4 shows that with hydration at 75% relative humidity of the air this shoulder increases strongly, indicating that under these conditions the amount of adsorbed water is much higher.In Fig. 5 the absorbance of the continuum at 1900 cm-’ is given as a function of the Pi:glu ratio. This figure shows that with the hydrated systems the intensity of the continua also increases in proportion to the P,:glu ratio. In the case of the Li+ systems this curve reaches a saturation value at Pi : glu = 1.2 : 1, and in the cases of the Na+ and K+ systems at a ratio of 1.7: 1. This demonstrates that similar to the dried systemsU. Burget and G. Zundel 895 " I 3 0.4 2 0 0 W 8 0.2 -e 0, 2 1:l 2: 1 3:l Pi: glu Fig. 5. Absorbance of the continuum at 1900 cm-' of (L-glu),-X,HPO, systems at 75 YO relative humidity with various cations: 0, Li; 0, Na; 0, K.hydrogen bonds with 1arge.proton polarizability as shown in eqn (2), (4) and (6) are formed between the sidechains and the phosphates. (L-glu),Li,HPO, Fig. 4(a) shows that in the hydrated systems instead of the v,,(-PO~-) doublet only one broad band at ca. 1045 cm-l is observed. Hence, the degeneracy of v,,(-PO,2-) is no longer removed. This result demonstrates that the Li+-Pi bonds are loosened by the attachment of water molecules to the Li+ ions.8*'7*24 The comparison of the intensity of the i.r. continuum in the hydrated Li+ systems (fig. 5 ) with that of the dried ones (fig. 2) shows that with hydration the intensity of the continuum increases by ca. 20 %. This result demonstrates that the proton polarizability of the COH-.--OP~CO-.-.HOP bonds increases owing to the loosening of the Li' ions from Pi, resulting in a larger negative charge density of the hydrogen-bond acceptor 0 atom at Pi.Hence, the proton potential well at this 0 atom is lowered, favouring charge fluctuation and increasing the proton polarizability of these hydrogen bonds. In spite of this fact the proton-transfer equilibrium is not noticeably shifted to the right since [fig. 4(a)J with hydration no changes of the carboxylic acid and of the phosphate bands are observed. Hence, the weight of proton-limiting structure (11') remains < 0.05. For v(C=O) an additional shoulder is found at 1730 cm-'. This shows that the hydrogen bonds in which the C=O groups act as acceptors are weaker in the hydrated system than in the dried system.This is understandable since with some groups in the hydrated systems instead of the strong C=O---HOP bonds in which the proton is localized, similar but considerably weaker C=O . - HOH bonds are formed. (~-glu),Na,HP0 , and (L-glu)"-K ,HPO The limiting value of the intensity of the continuum in fig. 5 shows that in the case of the hydrated systems, sidechain-phosphate hydrogen bonds with large proton polarizability are completely formed at Pi : glu = 1.7 : 1. Comparison of the intensities of the continua in the 1 : 1 systems with those in the 1.7: 1 systems in fig. 5 shows that in the hydrated 1 : 1 Na+ system as well as in the dried system ca. 80 YO of the hydrogen bonds with large proton polarizability are formed. After hydration in the 1 : 1 K+ systems instead of 90% only ca.80% of the polarizable hydrogen bonds are present, i.e. owing to hydration in the 1 : 1 K+ system ca. 10 YO of these bonds are broken. Comparison of the intensity of the continuum at Pi : glu > 1.7 : 1 of the hydrated and dried Na+ and K+ systems (cJ fig. 2 and fig. 5 ) shows that the intensity of the continuum decreases by ca. 25 YO. Thus, the proton polarizability of the COH - * . -0PSCO- * . .896 Glu tam ic Acid- Hydrogen Phosphate Hydrogen Bonds HOP bonds decreases owing to the presence of water molecules. Hence, the interaction of water molecules with hydrogen bonds with large proton polarizability reduces the charge fluctuation and thus the proton polarizability of these bonds, a result already known from other The following changes in the spectra were also observed after hydration. In the case of the Na’ systems [fig.4(b)] at 1730 cm-l a v(C=O) band arises. This shows that the strong C=O...HO-C or C=O...HOP bonds in which the protons are localized are broken owing to the presence of water molecules. These hydrogen bonds are replaced by C=O - * - HOH bonds, which are much weaker and therefore v(C=O) is shifted to higher wavenumbers. Furthermore, in the case of the Na+ system the band at 1165 cm-l {one component of the doublet caused by removal of the degeneracy of vas(-POi-) due to a strongly asymmetrical environment [eqn (6)]) decreases and vanishes with further hydration (spectrum not shown). This change demonstrates that hydration causes the environment of the -PO:- groups in eqn (6) of proton-limiting structure (I””) to become symmetrical because the water molecules loosen the Na+ ion from the phosphate group.In the case of the Pi: glu = 1 : 1 K+ system no v(C=O) band arises with increasing hydration [fig. 4(c)] since the weight of proton-limiting structure (I”) in eqn (4) is too small. The band at 1120 cm-l decreases. As already discussed, this band is partially caused by v,,(-PO~) of proton-limiting structure (11”) and partially by one of the two components of v,,(-PO~-) not involved in the glu-Pi groups [eqn (4)]. In the dried systems this vibration is split owing to the removal of degeneracy. On hydration, the K+ ions are loosened from the -PO:- groups and only one band of the degenerate v,,(-PO:-) vibration is observed at ca.1065 cm-l. This band is superimposed on v, (-PO,) of proton-limiting structure (11”). Thus, the removal of degeneracy of v,, (-PO:-) causes a decrease of the band in the region around 1120 cm-l. In the hydrated Pi : glu = 1 : 1 system v,(-PO:-) appears at 975 cm-’, whereas at 923 cm-’ vaS [-P-(OH),] of proton-limiting structure (11”) decreases. With Pi : glu 2 1.7 : 1 in which the Pi:glu bonds are completely formed, no change of vas[-P-(OH),] at 923 cm-l occurs with hydration (spectrum not shown). The 2 6(OH) band at 2370 cm-l, indicating the asymmetrical hydrogen bonds between the phosphate molecules, decreases only slightly with hydration of the Pi:glu = 1 : 1 systems [fig. 4(b) and (c)] and also remains relatively intense in the 1.7: 1 systems (spectrum not shown). These results show that after hydration a considerable amount of Pi is still cross-linked via Pi-Pi hydrogen bonds in which the proton is localized at one of the Pi and which show no considerable proton polarizability.The Secondary Structure of (L-glu), The secondary structure of (L-glu), can be determined from the amide bands3, Pure (L-glu), is present as an a-helix in dried as well as in hydrated film~.,~-~l Deprotonated (L-glu-), is, however, only present as an a-helix in the films if they are hydrated at 90% of air h ~ r n i d i t y . ~ ~ Less strongly hydrated and dried films of deprotonated (L-glu-), are present as the antiparallel P-c~nformation.~~ The conformation of (L-glu), in (L-glu),-N-base films is discussed in ref. (31). In the case of (L-glu),-Li,HPO, films [fig.6(a)] (L-glu), is almost completely protonated. The amide-A band is found at 3290 cm-l and is relatively sharp, indicating an ordered s t r ~ c t u r e . ~ ~ The amide I band at 1651 cm-l and the amide I1 band at 1547 cm-l are relatively sharp, confirming that an ordered structure is present. They show that (L-glu), is present in these films as an a-helix. When these films are hydrated at 75% relative air humidity no noticeable change of these bands is observed. Thus (L-glu), remains a-helical on hydration. In the case of the (L-glu),-Na,HPO, [fig. 6(6)] and (L-glu),-K,HPO, films [fig. 6(c)]U. Burget and G. Zundel 897 0.2 8 5 2 0.4 0.7 1 . 0 3250 wavenumber/cm-’ . (c1 ‘. \ ’. e , . . . . I 3250 Fig. 6. 1.r. spectra of dried (L-glu),-X,HPO, films with different phosphate : sidechain ratios.P,:glu = 1 : 3 (-), 1 : 1 (----), 1.5: 1 (--.--), 2 : 1 (---.); pure (L-glu), (--ap). Cations present: (a) Li, (b) Na, (c) K. the amide A is also found as a sharp band at 3290 cm-l if the Pi : glu ratio is small. With increasing Pi : glu this band broadens strongly and shifts a little to smaller wavenumbers. The amide I band is found at 1649 cm-l and broadens with increasing Pi: glu also. All these results taken together ~ h o ~ ~ ~ - ~ ~ that (L-glu), changes with increasing Pi content from an a-helical structure to a coil. Fig. 6(c) shows that in the K+ systems (L-glu), loses the a-helical conformation at Pi : glu = 1 : 1, whereas with the Na+ systems this transition occurs only at Pi : glu = 1.5 : 1.This shows that transfer of the glu protons to Pi induces the conformational change since this transfer is more complete with the K+ systems than with the Na+ systems. Hydration at 75% relative air humidity does not influence the structure of (L-glu), in these Pi-containing films, hence the structure is mainly determined by the degree of deprotonation of (L-glu),. Conclusions Hydrogen phosphates form highly polarizable hydrogen bonds of the type shown in eqn (1) with the sidechains of (L-glu),. The formation of these hydrogen bonds is complete in the Li+ systems at Pi: glu = 1.2: 1 and in the Na+ and K+ systems at P,:glu = 1.6: 1. In the case of the Li+ systems Pi is bound by a second C=O - - - HOP hydrogen bond to a neighbouring glu residue. In these bonds the proton cannot fluctuate, it remains at Pi [see eqn (2)].In the hydrogen bonds with large proton polarizability < 5% of the glu protons are present at Pi. The weight of proton-limiting structure (11”) is so small since owing to the strong fields, i.e. the large polarizing power of the Li’ ions, the negative charge density at the acceptor 0 atom is strongly reduced. In the case of the K’ systems Pi is also bound via bonds with large proton polarizability [eqn (4)]. 95 +5 YO of the glutamic acid protons shift in this case to the Pi, i.e. proton-limiting structure (11”) has a weight of 0.95. This large difference, compared with the Li+ systems, is caused by the much weaker field of K+, i.e. the smaller polarizing power of these ions. Therefore, a proton can transfer to Pi and then a K+ ion can move to the carboxylate group being formed, shifting the proton-transfer equilibrium to the right-hand side.The Na+ systems are an intermediate case. At a low Pi : glu the arrangement is the same [eqn (5)] as in the case of the Li+ systems, but the proton polarizability of the glu-Pi bonds is much larger than with the Li+ systems. The reason of this is that the polarizing power of Na+ is much smaller than that of Li+. Therefore, the negative charge density898 Glutamic Acid-Hydrogen Phosphate Hydrogen Bonds at the hydrogen-bond acceptor 0 atom of Pi is much higher with the Na+ systems. Hence, the proton is slightly loosened from the glu residue and can fluctuate much more freely in the hydrogen bond. At higher P,:glu the arrangement is the same as in the K+ systems [eqn (6)], but even if the hydrogen bonds are completely formed (P,:glu = 1.6 : 1) only 75 5 O/O of the glutamic acid protons have transferred to Pi.Hence, proton- limiting structure (I””) in eqn (6) has considerable weight and the Pi bands show that in this structure PO:- groups are in an extremely asymmetrical environment. The proton polarizability increases in the series Li+ < N+ < K+. In the hydrated Li+ systems the proton polarizability of the COH * * -0P CO- - - - HOP bonds is a little larger than in the dried systems since the water moleculs loosen the Li+ ions from Pi, and hence lower the proton potential well at the 0 atom of Pi. For the Na+ and K+ systems the proton polarizability decreases slightly with increasing degree of hydration since the water molecules hinder the charge fluctuation to a small degree.We thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for providing the facilities for this work. References 1 E. D. Korn, Physiol. Rev., 1982, 62, 672, and references therein. 2 K. Ch. Holmes, in Biophysics, ed. W. Hoppe, W. Lohmann, H. Markl and H. Ziegler (Springer, Berlin, 3 E. G. Weidemann and G. Zundel, Z. Naturforsch., Teil A, 1970, 25, 627. 4 R. Janoschek, E. G. Weidemann, H. Pfeiffer and G. Zundel, J. Am. Chem. SOC., 1972, 94, 2387. 5 G. Zundel, in The Hydrogen Bonds-Recent Developments in Theory and Experiments, ed. P. Schuster, 6 R. Lindemann and G. Zundel, J. Chem. SOC., Faraday Trans. 2, 1977, 73, 788. 7 K. P. Hofmann and G.Zundel, Rev. Sci. Instrum., 1971, 42, 1726. 8 G. Zundel, Hydration and Intermolecular Interaction (Academic Press, New York, 1969; Mir Moscva, 9 D. Hadii and N. Kobilarov, J. Chem. SOC. A., 1966,439. 1983), p. 566. G. Zundel and C. Sandorfy (North Holland, Amsterdam, 1976) vol. 11, chap. 15, p. 681. Moscow, 1972). 10 E. Steger and K. Herzog, 2. Anorg. Allg. Chem., 1964, 331, 169. 11 A. Chapman and L. Thirlwell, Spectrochim. Acta, 1964, 20, 937. 12 L. C . Thomas and R. A. Chittenden, Spectrochim. Acta, 1964, 20, 467. 13 A. Chapman, D. A. Long and D. T. L. Jones, Spectrochim. Acta, 1965, 21, 633. 14 R. W. Lovejoy and G. L. Wagner, J. Phys. Chem., 1964, 68, 544. 15 L. C. Thomas and R. A. Chittenden, Spectrochim. Acta, Part A, 1970, 26, 781. 16 G. Zundel and J. Fritsch, J. Phys. Chem., 1984, 88, 6295. 17 G. Zundel and J. Fritsch, in Chemical Physics of Solvation, ed. R. R. Dogonadze, E. Kalman, A. A. 18 H. Baba, A. Matsuyama and H. Kokubun, Spectrochim. Acta, Part A, 1969, 25, 1709. 19 G. S. Denisov and V. M. Schreiber, Vestni Leningr. Univ., 1976, 4, 61. 20 A. Koll, M. Rospenk and L. Sobczyk, J. Chem. SOC., Faraday Trans. 1 , 1981, 77, 2309. 21 R. Kramer and G. Zundel, 2. Phys. Chem. (Frankfurt), 1985, 144, 265. 22 U. Burget and G. Zundel, Biopolymers, 1987, 26, 95. 23 Landolt-Bornstein, ed. K-H. Hellwege (Springer, Berlin, 1976), Group IV, vol. 2, p. 10. 24 A. Murr and G. Zundel, Electrochim. Acta, 1967, 12, 1147. 25 U. Burget and G. Zundel, J. Mol. Struct., 1986, 145, 93. 26 P. Doty, A. Wada, A. Young and E. R. Blout, J. Polym. Sci., 1957, 23, 851. 27 H. Lenorment, A. Boudras and E. R. Blout, J. Am. Chem. SOC., 1958, 80, 6191. 28 L. Goldstein and E. Katchalski, Bull. Res. Council. Isr., 1960, 9A, 138. 29 E. Eizuha and J. T. Young, Biochemistry, 1965, 4, 1249. 30 M. L. Tiffany and S. Krimm, Biopolymers, 1969, 8, 347. 31 R. Lindemann and G. Zundel, Biopolymers, 1977, 16, 2407. 32 G. Zundel, in Biophysics, ed. W. Hoppe, W. Lohmann, H. Markl and H. Ziegler (Springer, Berlin, 1983), p. 249. 33 Yu. N. Chirgadze and E. V. Brazhnikov, Biopolymers, 1974, 13, 1701. 34 Yu. N. Chirgadze, E. V. Brazhnikov and N. A. Nevskaya, J. Mol. Biol., 1976, 102, 781. Kornyshev and J. Ulstrup (Elsevier, Amsterdam, 1986), vol. 11, chap. 2. Paper 6/1572; Received 1st August, 1986

ISSN:0300-9599    

DOI:10.1039/F19888400885

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

.J. Chem. SOC., Faraday Trans. 1, 1988, 84(4), 899-916 The Interaction between Ions and the Activation Barrier of Elementary Events of Crystal Growth and Evaporation Vincent K. W. Cheng*'f and Bruce A. W. Coller Department of Chemistry, Monash University, Clayton 3168, Victoria, Australia Edgar R. Smith$ Department of Mathematics, University of Melbourne, Parkville 3052, Victoria, Australia The activation barrier of the elementary processes of the growth and evaporation of an NaC1-type ionic crystal in contact with its vapour such as surface diffusion, incorporation and detachment, are examined in terms of the sum of the pair potential energy between adions and lattice ions. Lattice- sum studies of the Coulombic part of the potential energy was evaluated exactly using integral transform which is equivalent to the solution of the Poisson equation derived earlier.The Coulombic potential-energy function experienced by adions decays rapidly to zero within a distance less than one interionic separation above the surface. Three forms of non-Coulombic potential energy were evaluated by direct lattice summation. Both the pair potential-energy function derived from ab initio quantum mechanics and the effective Born-Mayer potential-energy function are short-ranged. More- over, at a binding site, the ab initio potential energy experienced by an adion is attractive and slightly larger than that of the Coulombic part, whilst the Born-Mayer potential function gives rise to a small but repulsive contribution to the total potential energy. The total potential energy of an adion was shown not to be proportional to the number of nearest neighbours, despite being effectively short-ranged.The non-Coulombic potential energy derived from a combination of the Mott-Littleton polarisation energy and overlap repulsion is long-ranged, and at an equilibrium adsorption site it is attractive and is very large compared with the Coulombic potential-energy function. All non-Coulombic potential energies are dependent on the type of charge of the adion. The activation barrier to incorporation, either directly from the vapour phase or after surface diffusion, is zero, whilst the barrier to detachment is determined by the total potential energy of the surface ion. The activation barrier for surface diffusion, as calculated from the variation of adion potential energy along a diffusion path, is shown to be contributed almost exclusively from the Coulombic part.Of the two possible diffusion paths, the lateral path along the unit cell (path 1) has a larger activation barrier than the preferred diffusion path diagonal to the unit cell (path 2) which has a barrier of almost half the size. The corresponding diffusion will be more restricted on the surface of bivalent metal sulphates because of the larger charges of lattice ions. For both mathematical and computational simplicity, the dependence of the rate of growth and dissolution on concentration derived from the one-component Kossel crystal model is considered to be adequate for the analysis of experimental rate law for ionic crystals.Such analysis has been reported with apparent success in identifying the growth mechanism, such as nucleation at a large distance from eq~ilibrium.l-~ The rate- ? Present address: Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong. 1 Present address : Department of Mathematics, La Trobe University, Bundoora 3083, Victoria, Australia. 899900 Crystal Growth and Evaporation determining step, however, has shown to be inconclusively established on the basis of the rate law a10ne.~ It has been shown that crystal growth and dissolution dominated by surface diffusion or by direct transfer have the same form of dependence of rate on c~ncentration.~.~ Thus recourse to other data is essential in the establishment of the rate- determining step.The rate-determining step in a complex reaction sequence may sometimes be identified in terms of the relative magnitudes of the activation energies of the elementary steps. In a sequence of reactions it is supposed to have the largest activation energy, and in parallel, competitive reactions it is supposed to have the smallest. In crystal growth and dissolution the Arrhenius activation energy has been a useful indicator to discern volume (solvent) diffusion-controlled kinetics from those of surface control.6 Volume diffusion- controlled kinetics would have an activation energy of ca. 20 kJ mol-1 and that for surface-controlled process would be considerably higher. The same idea is expected to be a useful guide to the establishment of the rate-determining step among the various interfacial processes. We can justify our assumption from the following relationships’ [eqn (1) and (2)] for the choice of rate-determining step for crystal growth in solution in terms of the activation free energies for incorporation at kink, AGhink, deadsorption, AGieads, and surface diffusion, dGHdiff.If the rate-determining step is surface diffusion, then If the integration or detachment at kink is the rate-determining step, then The comparison of activation free energies can be reduced approximately to a comparison of activation energies of these elementary processes by neglecting or cancelling the various T A S and RTcontributions to AGI and A#. The numerical values of the activation energies of the elementary events, such as surface diffusion, incorporation and detachment at kinks, can be calculated from the interaction between ions, and they would provide some insight into the rate-determining step for the surface-controlled kinetics. When the BCF theory for crystal growth* is extended to ionic crystals there will be a barrier for surface diffusion arising from the repulsion between like charges along the lateral direction (path 1) of the surface unit cells or along the path diagonal to these unit cells (path 2) as shown in fig.1. An assessment of the activation energies of these interfacial elementary events can similarly be made in terms of the interactions between solid and fluid units in the simulation model. The fluid units, which may either be the molten liquid or the solvated lattice unit, are assumed to occupy the same volume as a solid cell.’ Activation is necessary in surface diffusion and detachment.* For example, the activation energy for a surface diffusion jump along (100) surface is simply the binding energy mq5, where m is the total number of neighbours at the departure site and g5 is the bond energy between a solid-fluid pair.’ In terms of the pair potential energy between the solid and fluid units, (3) # is given as The maximum in the barrier for the diffusion jump along either path 1 or path 2 is located midway between a kink site and its neighbour.If the solid/vapour interface is considered, g5 is a function of only the bond energy between a solid-solid pair. The potential energy of an ion adsorbed on a KC1 (100) crystal/vapour interface and its relation to the activation barriers for the elementary growth and dissolution events are studied in this paper using simple formulae.The total ion-ion potential is the sum of both Coulombic and non-Coulombic contributions. Furthermore the usefulness of the one-component crystal as a model for a multicomponent ionic crystal will be clarified with detailed knowledge of potential energies of ions at the crystal surface. The results will be generalised to the case of a bivalent metal sulphate. Although the vapour phase is known to contain a large number of ion pairs, whose interionic separation is less than $ = A f - + ( A s + Af).2T 7' @ad-ion 90 1 Fig. 1. Surface diffusion paths of an adion. that in the crystal lattice, the activation barrier of the abovementioned elementary events involving ion pairs and the influence of ion pairs on the kinetics of crystal growth and evaporation are not considered in this study.Hovel3 showed that at room temperature the lifetime of an ion pair adsorbed on the (100) KCl crystal surface is extremely short. Potential Energy of an Ion near an Ionic Crystal Surface The potential energy of an ion inside and outside an ionic crystal can be calculated as the sum of all the pair potential energies between the ion in question and the rest. Similarly the potential energy of the crystal can be given as the sum of all possible pair potential energies. This pair potential energy as a function of distance can be determined experimentally from gas-phase data or calculated from ab initio quantum mechanics.lO*l' It has long been recognised12v13 that these pair potentials can be split into a number of contributions such as Coulombic, polarisation, repulsion etc.However, it has been found that this pair potential-energy function needs to be replaced by a slightly different effective pair potential-energy function in order to account adequately for bulk cry st a1 proper ties. lo The Coulombic energy of a finite crystal can be accurately evaluated by adding all Coulombic interactions between (point) ion pairs of charge qi and qj and separation rij within the lattice: where a is the unit-cell parameter. Similarly the Coulombic potential energy V(r) for an ion near the surface of an ionic crystal can be calculated by the direct summation involving individual pair Coulombic interactions between the given ion of charge q and the ion of charge qi in the lattice at separation r - ri, where ri is the displacement of the ion relative to the same origin as r : Both r and ri are scaled by a.The alternating addition and subtraction of terms dictated by the arrangement of ions in unit cells results in a slowly and conditionally convergent series.14 Rapid convergence of the sum can be achieved by the integral transform method.15 One of us has developed analytical methods for such a calculation on the basis902 Crystal Growth and Evaporation of the integral transform. ''-la Alternatively, the Coulombic potential energy can be calculated from the Poisson equation approach.12 The polarisation potential energy is the result of the polarisation of the adsorbed ion by the electric field of the crystal and that due to the polarised crystal in the presence of the adsorbed ion.van der Waals forces between neighbouring ions give rise to the repulsive potential energy. Contributions due to polarisation and repulsion have been studied by Hove.13 Only crystal polarisation (by the adsorbed ion) and overlap repulsion are considered to be important. The lattice sum potential energy (in eV) for polarisation and repulsion for the KCl lattice were stated as, respectively, eqn (6) and (7): Vr = (81.20) (1 + 9.122) exp (-9.122) + (75) (1 + 10.15~) (cos nx) (cos ny) exp (10.152) + (90.1) (1 + 1 1 1.072) (cos 27rx + cos 2ny) exp (- 1 1.072) +(119)(1+ 12.73z)(cos2nx)(cos27ry)exp(- 12.732).(7) In eqn (6) a,( = a/2) is the interionic distance within the crystal and the lattice sum of the pair potential energy due to polarisation over all ions in the semi-infinite crystal. S , = M , c (a0/r,)4, is spilt (according to the sign of the ionic charge given) into contributions due to cations and anions in the lattice. The constants M , and M- (in short M + ) are derived in terms of the in-crystal ion polarisabilities a+ and dielectric constant-& according to the theory of Mott and Littleton." Here the positive subscript refers to a cation and negative subscript refers to an anion in the lattice. M+ - is given M , = - 47r(a,+aT) and the dielectric constant of the crystal E is related to a+ - by 8-1 4n a,+a- ~ + 2 - 3 a i 2 ' (9) We note that in our calculation, M , and E are dimensionless.Bulakh and Chernov2' and Hovel3 gives M , and M- as, respechely, 0.01 56 and 0.0690 for a rigid KCl lattice. These values can be calculated from the in-crystal ion polarisabilities given later in table 4. For an elastic lattice, M , and M- are, respectively, 0.0488 and 0.0771, which implies a small change in a+. The polarisation contribution to the binding energy of the adsorbed ion was considered to be significant in previous calculation^.^^^^^ However, the factor of 1/2 in eqn (6) was not present in its original derivation but appeared in the calculation of the potential energy experienced by a vacancy in the bulk of a crystal." More often the energy and static properties of an ionic solid, such as interionic separation and isothermal bulk modulus, are successfully calculated using an effective pair potential-energy function2' which contains the Coulombic part, qyl(r), in which the ions are assumed as point charges, and the short-ranged non-Coulombic part, and C .. D . . I/sf(r) = B . . exp ( - a i i r ) - 3 - - 2 i,j = +, - , r6 r8 The coefficients B,,, aii, Ci, and D , for various ionic crystals are given in tables 1 and 2. The abovementioned contributions to the pair potential energy due to Coulombic, repulsion and polarisation forces are incorporated into eqn (1 2), but the dipoledipole polarisation potential energy now has a 1 / r 6 dependence.2' Thus it is not as long-rangedV. K. W. Cheng, B. A . W. Coller and E. R. Smith 903 Table 1. Coefficients (in atomic units) for ab initio non-Coulombic potential-energy functionsll pair interactions A B C D E F Na-Na 140.383 99 -4.611 12 -0.257 01 1.181 66 13.190 09 5.571 62 Na-CI" 100.788 12 -3.042 14 -0.116 15 0.684 38 0.022 70 3.889 75 Na-C112 76.822 55 -3.333 44 -0.254 08 0.663 91 16.758 16 3.994 29" c1-Cl 2.141 67 - 1.034 75 -0.061 19 0.586 59 627 519.578 40 11.349 35 "The inclusion of a negative sign in ref.(12) gives an unphysical potential-energy curve. Table 2. Mayer coefficients for Born-Mayer non-Coulombic potential-energy functions'' Na-Na 3.155 25 16.08 1.68 0.8 Na-C1 11.2 233.0 Cl-Cl 116.0 13.9 K-K 2.967 3575.47 24.3 24.0 K-Cl 48.0 73.0 Cl-c1 124.5 250.0 as assumed by Mott and Litt1et0n.l~ Various sets of van der Waal coefficients Cii and D , have been proposed,21 but those reported by Born-Mayer are regarded to be most reliable.Subsequently Catlow et a1.22 proposed an improvement of C , and suggested that it would be suitable for simulation The effective pair potential-energy function in fact takes into account of the many-body interactions, e.g. polarisation of the ion pair in question by surrounding ions2* The calculations of ionic crystal static properties using accurately determined (from ah initio quantum mechanics) pair potential-energy function for NaCl has shown discrepancies with crystal properties obtained from experiment." This pair potential- energy function has a form similar to that of the effective pair potential-energy function of eqn (lo)-( 12), which contains both Coulombic and non-Coulombic parts. The non- Coulombic part is given as y;(r) = (Aij r-Bii + Cij r) exp (- D , r ) + Eij r-Fij i, j = + , - .The coefficients Aij to Fij, given in table 1 are fitting parameters and are not related to the various types of interaction mentioned above. This pair potential energy accurately reproduces experimental data for ion pairs in the gas phase.l0.l1 However, it predicts a smaller interionic separation in the crystal and hence the existence of covalent inter- action between ions. A more negative lattice energy but a dynamically less stable 1a.ttice was also predicted using this pair potential-energy function. lo Ions of opposite charges near the surface will relax from their bulk equilibrium positions in directions opposite to each other but perpendicular to the surface. It has been shown both e~perirnentally~~ and theoretically26 that in the few layers near the surface of NaCl, the anions are displaced upwards and cations downwards.The corresponding shift of equilibrium adsorption positions is expected. Lattice summations involving relaxed surfaces have been extensively reviewed,27 but no agreeable method has been identified. It has been suggested that in the NaCI-type surface lattice the change in position alone is expected to lower the surface energy by 2040 %. Relaxation from bulk lattice structure at surfaces may cause some degree of polarisation. We would expect, as a result, that higher total binding energies at equilibrium adsorption sites should arise904 Crystal Growth and Evaporation from the relaxation of the surface lattice structure.Any polarisation of surface ions as a result of the relaxation was ~uggested~~ to be mostly cancelled out and thus not to contribute to the surface energy. For simplicity of calculation, these additional contributions to the adion potential energy due to surface relaxation are neglected. Calculations of Coulombic Potential Energy of Ions near Ionic Crystal Surfaces In the present study the Coulombic potential energy V(r) of a univalent ion near the unrelaxed (100) KCl surface [eqn (5)] was calculated using a number of general half- space lattice Recently we derived a potential-energy expression by transforming l / x by the identity (hereafter defined as method 1) - 1 - $lom t-f exp (- t x 2 ) dt 1 " = Jq, t-f exp ( - t x 2 ) dt + t-f exp ( - tx2) dt.The integral in eqn (14) was actually split into two parts at the arbitrarily chosen point of q2, which allows us to control the convergence rate of the resulting lattice sums. The sum involving the first integral Vl(r) can be evaluated in terms of the complementary error function (16) to give a simple sum over the lattice A The remaining part of the potential V2(r) as a result of summing terms containing the second integral in eqn (15) is numerically cumbersome.'7v18 It depends on the unit-cell dipole moment and the shape of the crystal, and if the unit cell dipole moment is not zero, it diverges.l' More recently, we developed a different way of evaluating a half-space lattice sum for the special case of the surface containing (100) NaC1-type unit cells,lS which has a zero unit-cell dipole moment (hereafter defined as method 2).Instead of splitting the transformation of l / r [eqn (14)] into two terms, the integrand in eqn (14) was transformed using the identity 1 f a exp (- tyz) = 1 J exp [ - (uz/t + 2iuy)l du. d n -a The resulting integral over t in eqn (1 8) effectively transformed all space variables to its reciprocal lattice space and could be evaluated directly. The resulting lattice sum was then split into one containing only the reference reciprocal lattice and the other containing rest. The first sum needed to be evaluated with recourse to the (100) NaC1- type unit-cell structure and the second sum could be evaluated readily to give the Coulombic potential at a distance z above the surface as'' x cos (2n[k,(x - X i ) + k,Cy -&)I} (1 9) where N = 8 is the number of ions in a unit cell.In all the lattice sums, the origin of the position vector Y = (x, y , z ) and the two-dimensional reciprocal lattice vector k = (k,,k,) is taken at the centre of a unit cell in reciprocal space. It is sufficient to take the x and y components (along the surface) to within + 5 during the actual computation.V. K. W. Cheng, B. A . W. Coller and E. R. Smith 905 'The positions of the univalent cation are (a/4, a/4, a/4), (a/4, - a / 4 , - a/4), ( - a / 4 , - a/4, a/4), ( - a/4, a/4, - a / 4 ) and those for the anion are ( - a/4, a/4, a/4), (a/4, - a / 4 , a/4), ( - a / 4 , - a / 4 , - a / 4 ) . As a comparison, the Coulombic potential of an ion above the (100) KC1 surface was also calculated from the solution to the Poisson equation12.When the adsorption position is taken as a variable, these methods also provide the barriers to surface diffusion. The Coulombic activation barriers for surface diffusion .were calculated at a fixed distance of a / 2 (a,) above the surface for paths both parallel to a row of ions (path 1) and parallel to the diagonal of the top of the unit cell (path 2). 'The Coulombic potential above this surface was also calculated at various distances measured from the centre of a surface unit cell. The unit-cell parameter for the KC1 lattice was taken as 630 pm13 and that for NaCl as 564 pm.l0 For the purpose of comparison, the Coulombic energy of a monovalent ion at a lattice site at the end of a semi-infinite one-dimensional (non-polar and non-relaxed) KC1-type lattice array with interionic distance a, was also calculated using the direct summation formula Calculations of Non-Coulombic Potential Energy of Ions near Ionic Crystal Surfaces Non-Coulombic attraction, such as polarisation, increases the binding energy of an adion at its equilibrium adsorption site, and this attraction is expected to vary as the adion moves from, say, its initial equilibrium ad-site to another along a given surface diffusion path.We can assume, as in previous work,lg that the pair potential energy (in e.s.u.) experienced by an adion in the presence of another ion in the neighbourhood at a distance ri from it-takes the form - Two sets of in-crystal polarisabilities (table 4, see later) are used in this calculation: (1) those reported by Tessman et aLZ8 and used by Hovel3 and (2) recently improved data proposed by Fowler.29 In the latter set the polarisabilities of group I and I1 ions are similar to that of ref.(28), but the polarisability of an anion and a do ion such as Ag+ depends on its neighbouring ionic environment. We also assumed that the values of the polarisabilities for these ions are not significantly changed near the surface, although the surface ions may experience a different electric field than that in the bulk crystal. The lattice sum for the pair polarisation interaction given above is not a conditional sum because terms due to the positive and negative charges are not cancelling each other (all interactions are attractive), unlike the case of the Coulombic potential.Therefore if the pair potential is a long-range interaction, the nett potential function will remain long-range. This half-space sum was evaluated directly. The surface repulsion (for KCl only) was evaluated using eqn (7), and the total potential energy of an adion will be given as the difference between the total attraction and the repulsion. The direct lattice sum is also used to evaluate the non-Coulombic potential energy of adions on crystal surfaces, assuming that the ion-pair interaction V:i(r) follows that of the Born-Mayer theory eqn (12) or, for the NaCl surface, that proposed from ab initio quantum mechanics [eqn (1 3)]. Results Coulombic Potential Energy Among the methods adopted to calculate the Coulombic potential of an ion near the (100) KCl/vacuum interface, the summation formula developed by Hoskins et a1.16 was906 Crystal Growth and Evaporation Table 3.Contributions to the binding energy (eV) of univalent ions at the equilibrium adsorption site on the (100) KCI surface this work adion method 1 eqn (17) ref. (12) ref. (19) ref. (1 1) ref. (30)" electrostatic -0.47 -0.30 -0.30 -0.30 -0.30 - 0.60 repulsion 0.1 6b 0.1 6b 0.16b 0.16b 0.18 0.32 polarisation K+ -1.07 -1.07 -0.53 -0.53 - - c1- -0.97 -0.97 -0.48 -0.48 - - total K' -1.38 -1.21 -0.67 -0.67 -0.12 - 0.28 c1- -1.28 -1.10 -0.62 -0.62 -0.12 - 0.28 " Nearest-neighbour interactions only. Ref. (1 3). found to be divergent. The failure of the summation formula was later found to be caused by the inadequate treatment of the contour integration and the asymptotic expansion.Absolutely convergent numerical results for the Coulombic potentials near the surface were obtained from methods 1 and 2.17~ la However, the Coulombic potential calculated from method 1 is still very time consuming. The Coulombic potential from method 2 showed significant improvements in this aspect but it was still found to be less efficient than the solution to the Poisson equation." The Coulombic potential above the (100) KCl surface calculated from methods 1 and 217,18 is short-ranged (decaying to zero within one layer distance). The potential from method 2 is identical to that from the Poisson equation.12 Along a line immediately above the adsorption site, the potential calculated from the first method was found to oscillate slightly." A small activation barrier of ca.0.02 eV (2 kJ mol-l) would be associated with the deposition of monovalent ions from the neighbouring vacuum. Method 2 gives potentials above the surface only, and oscillation of potentials is not found. Furthermore, from both methods 1 and 2, the surface potential at a position equidistant from two neighbouring ions (of opposite charge) is zero. The Coulombic potential energies for a univalent ion in the vacuum space at a presumed equilibrium adsorption site (distance a,) above a lattice site of the (100) KCl crystal surface is given in table 3, together with other major contributions to the total potential energy such as polarisation [eqn (6)] and repulsive interactions [eqn (7)]. The Coulombic binding energy calculated by method 1 for a univalent adion at a supposed equilibrium adsorption site above the plane surface of a semi-infinite lattice array is found to be 0.47 eV (45 kJ mol-'), which is higher than those found in a number of previous works but lower than the nearest-neighbour value of Stranski3' (table 3).The smaller Coulombic potential of 0.30 eV (28.8 kJ mol-l) was found in method 2 [eqn (1 9)] and in the Poisson-equation approach.12 The results reported by Hovel3 indicate that it is reasonable to take just the leading term of the fast convergent expression derived from the Poisson-equation approach.12 On the other hand the Coulombic potential energy of a univalent ion at the lattice site next to a one-dimensional semi- infinite KC1 array [eqn (I 8)] is 0.22 eV (20 kJ mol-').The potential energy of the same ion inside an infinite linear KCl array would be 0.44 eV (40 kJ mol-l). The surface Coulombic potential calculated from method 1 gradually converts into the bulk potential (2.6 eV at a lattice site), which is many times larger than the surface potential (obtained from both methods 1 and 2), over a distance of two ionic layers beneath the surface, as predicted.17 The Coulombic potential of an ion at the surface layer (2.3 eV) is slightly smaller than that of the bulk ion. Thus a proportionality between the Coulombic potential and the number of neighbours cannot be established on the basis of the value of the surface and bulk Coulombic potential. The bulk potentialV. K . W. Cheng, B.A . W. Coller and E. R. Smith 907 1 .o >, 2 3 - 0.5 5 \ x .r( Y * a - C -0.5 Fig. 2. Variation of non-Coulombic potential energies of an Na-Cl ion pair with distance. The distance z is scaled by the crystal interionic separation, a, = 282 pm (-) Born-Mayer, (----) tzb initio (-.--.-), Mott-Littleton anion next to cation (-. .-- .-), Mott-Littleton cation next to anion. (scaled by 1/4mo) calculated with method 1 at (a/6, a/6, a/6) was found to obey the exact result of 2/3 for unit cells with a = 2.31 Also, at the centre of a unit cell, where complete cancellation of charges occurs, the Coulombic potential is zero. The magnitude of surface and bulk Coulombic potential energy for other NaC1-type (100) surface are determined by the value of the unit-cell parameter chosen.Thus the Coulombic binding energy of an adion above the NaCl (100) surface is larger than that for KC1 because of its smaller value of a. Non-Coulombic Energy 'The variation of the non-Coulombic potential energy of an NaCl pair with interionic separation as given by Mott and Littleton eqn (21). Born-Mayer eqn (12) and Clementi et al. eqn (1 3 ) are presented in fig. 2. All these potential functions decay to zero or a small value at large interionic separations. However, the Mott-Littleton potential-energy function calculation with both sets of ion polarisabilities cannot be regarded as being as short-ranged as those at large interionic separations ( r = 4 . 5 4 , the potential function remains at a small but almost constant value. The choice of a different set of ion polarisabilities only cause a small difference in the potential energy. At a given interionic separation, the attractive part of the Born-Mayer potential energy is the smallest and the Mott-Littleton potential energy is the largest.The magnitude of the effective pair potential with larger van der Waals coefficients given by Catlow (not shown in fig. 2) is only slightly larger than that with Mayer coefficients. The position of the potential- energy minimum occurs at r = 1 .35a0, whereas for the ab initio potential-energy function the minimum occurs at r = 1. la,. At a small interionic separation as shown in fig. 2, the Mott-Littleton polarisation pair potential-energy function for a cation in the neighbourhood of an anion is smaller than that experienced by an anion next to a cation.When the interionic separation gets larger, both potential functions converge to the same value. On the other hand, the potential energy of an ion next to another of opposite908 Crystal Growth and Evaporation charge as determined by the effective pair potential-energy function or the ab initio pair potential-energy function is the same regardless of the type of charge it carries. The attractive part of all these potentials are, however, much smaller than the Coulombic pair potential energy at a given interionic separation. The potential energy of an ion as a function of the distance above the crystal surface, as given in fig. 3, have largely the same shape as the pair potential-energy curve. The Mott-Littleton potential energy of an adion on a NaC1-type (100) surface using eqn (21) and polarisabilities given in table 4 proves to be very time-consuming.The summation of a considerably large number of terms is needed for good convergence. The potential energy as a function of the distance above the (100) NaCl surface is long-ranged, as shown in fig. 3. This form of the polarisation potential energy at an adsorption site, as given in table 3, appears to provide a major contribution to the binding energy of an adion. The polarisation energy of an adion above (1 00) NaCl was found to be larger than that above (100) KCI. Although the polarisability of Na+ is smaller than that of K+, the smaller interionic distance for crystalline NaCl seems to have more influence on the magnitude of the polarisation energy.Furthermore, this contribution to the adsorption energy is larger than that obtained by HoveL3 and Bulakh and Chernov20 using eqn (6) by a factor of 2. Fig. 4 and 5 show the variation of the van der Waals repulsion potential [eqn (7)], which is the same for both cation and anion, for an adion on a (100) KCI surface as it moves along diffusion paths 1 and 2, respectively. This repulsion contribution to the binding energy of the adion at an equilibrium adsorption site is given in table 3. Such a large repulsion, at the given ionic separation, suggest that there is an appreciable overlap of electron clouds between neighbouring ions in the crystal. On the other hand, the potential energy of adions as calculated by the Born-Mayer potential-energy function and the ab initio pair potential-energy function are short- ranged and thus only require the sum of a few terms for good convergence.The magnitudes of these potential energies are larger than that given by their respective pair potential-energy functions. They are even larger than the Coulomb interaction [eqn (19)], which was significantly reduced because of the cancellation of contributions due to opposite charges (table 3). The equilibrium binding energy of a negatively charged adion was found to be larger than that of a positively charged adion if it is determined by the effective pair potential-energy function (with either Mayer or Catlow coefficients) or the ab initio pair potential-energy function. On the contrary, the Mott-Littleton potential energy of a positively charged adion is larger.The potential-energy minimum of an adion due to both the effective and the ab initio potential-energy function again occurs, respectively, at r = 1 .35a0 and 1. la,. Activation Barriers to Surface Diffusion The overall barriers for both diffusion paths were calculated from the difference between the total binding energies at the equilibrium adsorption site and midway between two equilibrium adsorption sites. At these midway locations the adion of binding energies were found to be a minimum. The Coulombic contribution to the activation barriers to surface diffusion for the paths across the plane (100) KCl surface under consideration (paths 1 and 2) calculated from method 117 has a number of maxima and minima (fig. 4 and 5).On the other hand, the barrier obtained from method 218 appear to be physically more reasonable. The barriers found by method 2 for paths 1 and 2 are 0.52 and 0.26 eV (50 and 25.4 kJ mol-l), respectively, for the (100) KC1 surface. The barriers are comparable to Hove’s values of 0.60 and 0.19 eV,I3 without consideration of the polarisation contribution. The variation of all non-Coulombic potential energy along the two different paths are smooth and very small, as shown in tables 5 and 6. The barrier to surface diffusion for the Mott-Littleton potential energy appears to be larger, but it is only a small fraction of the corresponding equilibrium binding energy.V. K. W. Cheng, B. A . W. Coller and E. R. Smith 909 0.5 - 0 0.5 1.0 1.5 I Fig. 3. Variation of potential energy of an adion (Na+ or C1-) with distance above a surface lattice ion of opposite charge.The distance z is scaled by the crystal interionic separation, a, = 282 pm. A, Born-Mayer, anion; A, Born-Mayer, cation; I, ab initio, anion; 0, ab initio, cation; e, Mott-Littleton, anion ; 0, Mott-Littleton, cation. Table 4. Contributions to the potential energy (eV) of an ion at an equilibrium adsorption site a above a (100) NaC1-type surface surface adion ion polarisability Coulombic Born-Mayerb ab initio polarisation NaCl Na' 0.148 C1- 3.13 KCl K+ 0.79 (0.75)' C1- 3.39 (3.66)" -0.34 0.22 -0.63 -1.22 -0.34 0.12 - 1.03 -1.00 -0.30 0.16 - -1.07(-1.11)" -0.30 0.12 - -0.97 (- 1 .OO)' ~ a At a distance a, above the surface lattice ions. value stands for repulsion.Use polarisabilities given by Hove.13 Positive Table 5. Contributions to the activation energy (eV) for surface diffusion of an ion along the lateral path on a (100) NaC1-type surfacea surface adion Coulombic Born-Mayer ab initio polarisationb NaCl Na+ 0.68 - 0.006 -0.18 0.40 c1- 0.68 -0.016 -0.18 0.52 KCI K+ 0.60 -0.012 - 0.35 (0.13)' c1- 0.60 - 0.045 - 0.44 (0.17)c a At a distance a, above the surface lattice ions. " Use polarisabilities given by Hove.13 Positive value stands for positive barrier.910 Crystal Growth and Evaporation 0.4 I I *-*, 0.2 p-' 7A +/--* 0.5 1 1 1 1 1 1 Y I I 1 I 1 1 \ c t + r c,- Fig. 4. Various contributions to the potential energy of an ion above the KCl surface moving along the lateral diffusion path (path 1). The distance r is scaled by the crystal interionic separation, u, = 315 pm.A, Coulombic, method 1 ; A, Coulombic, method 2; 0, overlap repulsion, eqn (7). 0.2 + -0' \ '-. . . I Y [ -0.4 Fig. 5. Various contributions to the potential energy of an ion above the KC1 surface moving along the diagonal diffusion path (path 2). The distance Y is scaled by the crystal interionic separation, a, = 315 pm. A, Coulombic, method 1; A, Coulombic, method 2; 0, overlap repulsion, eqn (7).V. K. W. Cheng, B. A . W. Coller and E. R. Smith 91 1 Table 6. Contributions to the activation energy (eV) for surface diffusion of an ion along the diagonal path on a (100) NaC1-type surfacea surface adion Coulombic Born-Mayer ab initio polarisationb NaCl Na' 0.34 - 0.034 -0.176 0.34 c1- 0.34 - 0.073 -0.10 0.40 KCl K+ 0.30 -0.10 - 0.29 (0.22)' c1- 0.30 -0.12 - 0.32 (0.40)" a At a distance a, above the surface lattice ions.polarisabilities given by Hove.13 Positive value stands for positive barrier. " Use Furthermore, the barrier derived from both the effective potential-energy function and the ab initio pair potential-energy function is negative (i.e. the non-Coulombic potential energy midway between two equilibrium adsites along a given diffusion path is a minimum) and that from the Mott-Littleton potential-energy function is positive. Discussion The short-ranged nature of the Coulombic potential experienced by an adsorbed ion on an unrelaxed (100) NaC1-type surface is the result of the charge neutrality of the crystal lattice. Its strength is thus much less than the pair Coulomb interaction assumed by Stranski3' (fig.2 and table 3). The zero potential found at positions equidistant from two neighbouring ions arises because of cancellation of almost equal contributions due to ions of opposite charges. The small potential barrier near the crystal surface found using method 117 is likely to be of unphysical origin and not due to incomplete charge cancellation. Since the surface Coulombic potential calculated from method 1 differs from that of method 2 and from the Poisson equation, the surface potential derived from method 1 is likely to be in error. The accuracy of the sum cannot be checked experimentally. However, the Coulombic potential of an ion at a bulk lattice site as calculated by method 1 is expected to be reliable because it was shown to be capable of reproducing agreeable results obtained from exact calculations.31 The identical results obtained from method 2 and from the Poisson equation, which is a physically equivalent approach, validates method 2. The integral transform method has the advantage over the direct lattice sums in that it does not require a large number of terms for good convergence, although its rate of convergence is still lower than the expression derived from the Poisson equation.12 The ions between a lattice ion deep in the bulk and the surface ion would become a dielectric medium in our calculation of the surface Coulombic potential energy using eqn (19). However, the dielectric constant of the crystal was not taken into account. Because of the almost effective cancellation of the Coulombic potential energy due to opposite charges, the Coulombic potential calculated from eqn (19) is effectively contributed by the first layer of ions.This deduction has been validated'' using the Poisson equation. l2 For an NaCl ion pair, the total attractive potential energy at large separations is determined by the Coulombic contribution. On the other hand, the exact form of the non-Coulombic part determines the repulsive end and the shape of the potential well and hence ion pair properties such as vibrational force constant and dissociation energy. lo At a site on the (100) NaCl surface and within the crystal the cancellation of both the surface and bulk Coulombic potential due to opposite charges provides the possibility for the non-Coulombic potential energy to play a dominant role in determining the binding energy of an ion either at the crystal surface or inside the crystal.Whilst the ab initio potential-energy function can provide a large attractive influence on the912 Crystal Growth and Evaporation adsorption site binding energy, the corresponding influence due to the effective potential energy is repulsive. The dependence of the Mott-Littleton potential energy of an ion, either in an ion pair or on the crystal surface, on the charges of its neighbour(s) (and more precisely their polarisability) is consistent with that reported for the bulk crysta1.l' In the latter case, the Mott-Littleton potential energy of an electron at the cation site and a hole at an anion site were considered.Although such charge dependence is not found for the pair potential-energy calculation using either the effective potential function or the ab initio function, a neighbouring charge dependence of adion potential energy using these two potential functions in the lattice sum was found (fig. 3) because of the additional involvement of cation-cation or anion-anion pair interaction. Furthermore, the binding energy of a positively charged adion relative to that of a negatively charged adion as calculated using these two potential functions is opposite to that predicted from the Mott-Littleton potential function. This comparison is not expected to alter even if the binding energy of an adion include, in addition to the polarisation energy, the charge- independent repulsive potential energy [eqn (7)].We can therefore consider the Mott- Littleton potential together with the repulsive part adopted by Hovel3 and Bulakh and Chernov2' as an inappropriate representation of the non-Coulombic potential between ions in both the gas and crystal phase. One way to improve this potential function is to modify M , and one such modification has been shown to be necessary in the application of the M6tt-Littleton method to lattice-dynamical problems.21 We have shown that simply a small change to a different set of a+, and hence M , , values does not significantly change the potential energy calculated. Our polarisation-energy expression and results differ from those previously p~blishedl~*~' because we did not include the factor of 1/2 in eqn (6).This factor is absent in the original derivation by Mott and Littleton and it was only included in their calculation of the energy at a vacant site in the bulk lattice. On the contrary, with appropriately chosen coefficients, the Born-Mayer potential function is known to be able to predict agreeable lattice energy and other static lattice properties.1° Therefore we would expect it to provide a more accurate measure for the potential energy of an ion on the surface or in the lattice. As a result of this small potential-energy minimum at z > a,, we would expect a repulsive contribution to the binding energy of an adion from non-Coulombic interaction. The large adion binding energy calculated using the ab initio pair potential-energy function (Coulombic + non- Coulombic) relative to that obtained from the Born-Mayer effective pair potential- energy function is consistent with the larger lattice energy calculated with the ab initio pair potential-energy function.lo It has been suggested that a many-body interaction correction is needed to describe the crystal properties accurately. The lattice sum expressions for calculating the Coulombic and non-Coulombic potential energy of an ion in a bulk lattice site, in the surface layer or at an adsorption site can be separated into mutually exclusive but unequal components of half space one- dimensional and two-dimensional sums in the same way as that shown in eqn (19) and (20). At kink and step sites at large two-dimensional clusters or along infinitely long steps or spirals, we can similarly treat additional contributions to the lattice sums arising from those ions above a complete lattice plane separately as one- and two-dimensional sums.Again the Coulombic potential resulting from the contribution of these one- and/ or two-dimensional sums involving partial cancellation of charges of a large number of ions is expected to be short-ranged and similar in magnitude to those for an ion next to an infinitely long array of ions [eqn (20)] or above the surface of a lattice [eqn (19)]. Thus, for the crystal growth/dissolution kinetic models for non-polar surfaces, such as the (100) face of NaCl and KC1, we can still justifiably approximate the overall potential energy of ions at both bulk and surface lattice sites into contributions from nearest neighbours, but we cannot exactly establish the proportionality dependence of the short-V.K . W. Cheng, B. A . W. Coller and E. R. Smith 913 Table 7. Binding energy (eV) at a lattice and surface site for NaCl site lattice ion Coulombic Born-Mayer" ab initio polarisation lattice Na+ -2.70 1.25 -2.39 - 3.25 c1- -2.70 0.96 - 2.75 -2.21 surface Na+ - 2.36 1.03 - 1.76 - 2.03 c1- -2.36 0.84 - 1.72 - 1.21 " Positive value stands for repulsion. Table 8. Lattice parameters (in pm) for a number of sparingly soluble ionic crystals at room temperat~re,~ crystal a, a!l a, geometric mean CaCO, (calcite) 499.0 499.0 1706.4 751.8 BaSO, (barite) 887.8 545.0 715.2 702.1 SrSO, (celestite) 835.9 535.2 686.6 674.7 PbSO, (anglesite) 848.0 539.8 695.8 682.9 CaSO, (anhydrite) 699.1 699.6 623.8 673.2 530.0 720.0 840.0 680.0" 2(H,O) a Ref.(36). ranged potential energy on the number of them. On the other hand, this approximate picture may be invalid for ions on the (1 11) polar surfaces. They may experience a long- range contribution to the Coulombic potential energy depending on crystal 33 The activation barriers to surface diffusion along the two paths chosen for an adion on the unrelaxed (100) KCI surface are calculated from the difference in its maximum and minimum binding energy along the path. The preferred surface diffusion path is shown to be the one along path 2 (the diagonal path). The adion is assumed to move at a distance of a/2 (a,) above the surface lattice ions. It is not expected to be any closer to the surface lattice because of the strong overlap repulsion which determines the ionic radii.Furthermore, because of the short-range characteristic potential energy experienced by the adion, any upward displacement of the adion from its adsorption site would result in a reduction in its binding energy. The adion can be regarded as completely desorbed when the distance between it and its nearest neighbour is ca. a,,. The variations of the Coulombic and non-Coulombic potential energy, as determined by the Born-Mayer effective pair potential-energy function and the ab initio pair potential-energy function, with distance along a diffusion path indicate that, although the binding energy of an adion is determined to some extent by the non-Coulombic contribution, the activation barrier it experienced originates from the Coulombic part.Thus both types of non-Coulombic interactions show another consistency with each other : they both predict a negative but negligible surface diffusion barrier, because adion at the supposed site of minimum binding energy has in fact a slightly higher non- Coulombic binding energy. On the other hand the Mott-Littleton potential-energy function gives a positive and large barrier, which however, is still small compared with the binding energy. The small contribution from the non-Coulombic potential energy to the surface diffusion activation energy is a consequence of its attractive or repulsive nature throughout the surface. When the charge factors are considered for the BaS0,-type lattice, the Coulombic energy should be increased by a factor of 4.Unfortunately, the extension of the present lattice sum to calculate Coulombic energy for the barium sulphate crystal is complicated914 Crystal Growth and Evaporation because of the latter's orthorhombic lattice Because of the differences between lattice parameters at various faces, the electrostatic binding energies of ions on these crystal faces will be different. However, as shown from the small difference in the Coulombic potential energy of ions on a KC1 and an NaCl lattice (tables 4 and 7), we can assume, from a comparison of the geometric mean lattice parameter for a number of sparingly soluble ionic crystals (table 8) with that of KC1 (630 pm), that the mean surface Coulombic binding energy of ions on these crystal faces are not affected significantly by the difference in lattice parameters. Thus the larger Coulombic energy and activation barrier for surface diffusion expected for these crystals will be largely contributed by the charge factor.Similarly, the perturbation to the short-ranged Coulombic potential energy of surface ions as a result of the relaxation from the uniform lattice structure near the surface is expected to be small. Contributions to the Activation Energies of Elementary Events and the Rate- determining Step The Coulombic interaction between ions at the crystal/vapour interface has been shown to be the determining factor to the activation barrier for surface diffusion and, to some extent, detachment. Because of the presence of lateral neighbours at steps and kinks, the activation energy for detachment would be higher than that for surface diffusion and would be influenced by the presence of, for example, dislocation at the surface site under consideration.However, when an ion is adsorbed onto an equilibrium adsorption site of a plane ionic crystal surface from the vapour, the short-ranged net ion-ion attraction it experienced will not give rise to an activation barrier. Similarly, there is no activation barrier due to ion-ion interactions for the integration at a step or kink after the adion overcomes its last surface diffusion barrier or when it is incorporated directly from the vapour. The large activation barriers for surface diffusion are expected to reduce the average surface diffusion distance along either path.Additional contributions to the surface- diffusion activation barrier along a step from ions at steps make the overall barrier even larger than the corresponding barrier to movement along the plane ionic surface. Thus diffusion of ions along and away from steps are even less frequent, and adions would prefer to remain attached to a step or kink site, once they reached it. Kink generation would be more favoured than the one-component Kossel crystal model would indicate. It can be considered that ions at steps have been successfully incorporated into the lattice. Nevertheless, this does not suggest any dependence of the kink density along a step on the distance from equilibrium. The less frequent diffusion of growth units along steps into and out of kinks would slow down the growth and dissolution rate.According to the BCF theory,' however, only the rate constant is reduced to an extent determined by retardation factors. A comparison of the magnitude of the activation energy calculated for surface diffusion and incorporation at kink would support the former as the rate-determining step in crystal growth involving these two in sequence. On the other hand, surface diffusion is not the preferred rate-determining step compared with detachment at kinks during evaporation because the latter process have a higher activation energy. This comparison is consistent with the criteria given in eqn (1) and (2). When the surface diffusion-controlled growth process is competing with direct incorporation at kinks from the vapour, the high activation energy of surface diffusion would make it an unfavourable choice as the rate-determining step.However, such a choice of the rate- determining step between these two competing processes according to their activation energies is inconsistent with that stated in eqn (1) and (2), which is derived from a full consideration of the rate theory.lVi Eqn (1) and (2) would suggest that surface diffusion, by virtue of its large activation energy, can still be an important process when it isV. K. W. Cheng, B. A . W. Coller and E. R. Smith 915 competing with direct incorporation. For these competing processes there are other factors, such as the difference in the driving forces (distance from equilibrium) near steps and kinks and at the rest of the interface, which determine the mean lifetime of surface units and the other components in the activation free energy of surface diffusion and incorporation.Therefore this approximation of comparing the activation free energies by the activation energies is apparently only valid in the sequential process in which the continuity of the diffusion flux is implied in the derivation of eqn (1) and (2).4 Conclusion In this study we provide an alternative way of understanding the relative significance of various elementary events in the growth and evaporation of ionic crystals by considering the nature of their activation barriers in relation to the potential energy of ions at the surface. We have also shown the importance of the form of the non-Coulombic potential energy, and hence their influence, in determining the potential energy of ions at the unrelaxed (100) NaC1-type crystal surface and, perhaps, inside the crystal lattice.The non-Coulombic pair potential-energy functions considered in this work are different from each other in terms of both their origin and their actual value at a given interionic separation, although they have common features, particularly those between the Born- Mayer function and the ab initio function. Because the Born-Mayer function takes into account the complexity of the many-body interaction, we would regard the lattice sum of the Born-Mayer function with appropriately chosen coefficients to provide the most suitable description of the potential energy of ions at the crystal surface, and these coefficients are not expected to differ from those for the bulk crystal.Lattice-sum studies have indicated that the barrier to detachment reflects the total binding energy of an adion, and that there is no barrier to attachment. The ionic nature of the crystal surface gives rise to activation barriers to surface diffusion of adions by virtue of the repulsion between adions and lattice ions of like charges. The surface- diffusion activation energy was shown to depend on the migration path and is determined by the electrostatic potential energy of the adion. It is appreciably larger for crystals such as barium sulphate because of the charge factor in the Coulombic potential expression [eqn (1 9)]. The rate-determining step needs to be established from the theory of crystal growth.However, as a result of the approximation of more general a rite ria,^ it can be successfully determined in terms of a comparison of the activation energy only in the case of the sequential process of surface diffusion, incorporation or detachment. Other factors, such as the difference in the driving force factor for the competing processes of surface diffusion and direct transfer, must be considered before such a comparison is made in this case. The large Arrhenius activation energy measured for the growth of a number of ionic crystals in ~ o l u t i o n ~ ~ ~ ~ ~ appears to support the proposition of the importance of surface diffusion in the growth of ionic crystals. The influence of the detachment activation during evaporation has been illustrated in our previous work on the dependence of lattice stress (reduction in binding energy of adions) on the Arrhenius activation energy for dissol~tion.~~ In that case detachment at the defect sites is essential to promote dissolution.The influence of stress-field parameters on the binding energy (and hence activation energy of detachment) for KC1-type crystals has been studied.37 We thank the referees for valuable suggestions on the revision of the manuscript. 31 FAR 1916 Crystal Growth and Evaporation References 1 P. Bennema, J. Cryst. Growth, 1967, 1, 278; 287. 2 J. Christoffersen, J. Cryst. Growth, 1980, 49, 29. 3 G. M. van Rosmalen, M. C. van der Leeden and J. Gorman, KristaII Techn., 1980, 15, 4 P. Bennema, J. Cryst. Growth, 1969, 5, 331. 5 G. H. Gilmer, R. Ghez and N. Cabrera, J. Cryst. Growth, 1971, 8, 79. 6 L. L. Bircumshaw and A. C. Riddiford, Q. Rev. Chem. Soc., 1952, 7, 157. 7 P. Bennema, J. Cryst. Growth, 1969, 5, 29. 8 W. K. Burton, N. Cabrera and F. C. Frank, Philos. Trans. R. SOC. London, Ser. A , 195 21 3. , 243, 299. 9 J. P. van der Eerden, P. Bennema and T. A. Cherepanova, Prog. Cryst. Growth Charct., 1978, 1, 219. 10 A. Laaksonen and E. Clementi, Mol. Phys., 1985, 56, 495. 11 P. K. Swaminathan, A. Laaksonen, G. Corongin and E. Clementi, J . Chem. Phys., 1986, 84, 867. 12 J. E. Lennard-Jones and B. M. Dent, Trans. Faraday SOC., 1928, 24, 92. 13 J. E. Hove, Phys. Rev., 1955, 99, 430. 14 W. A. Schwalm, Am. J . Phys., 1982, 50, 444. 15 M. Born and K. Huang, Dynamical Theories of Crystal Lattices (Oxford University Press, Oxford, 16 C. S. Hoskins, M. L. Glaser and E. R. Smith, J . Phys. A., 1978, 10, 879. 17 E. R. Smith, Physica A , 1983, 120, 327. 18 V. K. W. Cheng and E. R. Smith, J . Phys. A , 1987, 20, 2773. 19 N. F. Mott and M. J. Littleton, Trans. Faraday SOC., 1983, 34, 485. 20 B. M. Bulakh and A. A. Chernov, J. Cryst. Growth, 1981, 52, 39. 21 M. J. L. Sangster and M. Dixon, Adv. Phys., 1976, 25, 247. 22 C. R. A. Catlow, K. M. Diller and M. J. Norgett, J. Phys. C, 1977, 10, 1395. 23 P. W. Tasker, Computer Simulation of Solids, ed. C. R. A. Catlow and W. C. Mackrodt (Springer 24 J. E. Inglesfield, Computer Simulation of Solids, ed. C. R. A. Catlow and W. C. Mackrodt (Springer 25 E. G. McRae and C. W. Caldwell, Surf. Sci., 1964, 2, 509. 26 E. J. W. Verwey, Recl. Trav. Chim. Pays-Bas, 1946, 65, 521. 27 C. G. Benson and K. S . Yun, in The SoIidlGas Interface, ed. E. A. Flood (Marcel Dekker, New York, 28 J. R. Tessman, A. H. Kahn and W. Shockley, Phys. Rev., 1953, 92, 980. 29 P. W. Fowler and N. C. Pyper, Proc. R. SOC. London, Ser. A, 1985, 398, 377. 30 I. N. Stranski, Z Phys. Chem., 1928, 136, 259. 31 P. J. Forrester and M. L. Glaser, Research Report No. 20 (University of Melbourne, 1981), 32 E. R. Smith, Proc. R. SOC. London, Ser. A , 1982, 375, 475; 1983, 381, 241. 33 E. R. Smith, Mol. Phys., 1986, 57, 793. 34 E. R. Smith, unpublished results (1982). 35 CRC Handbook (Chemical Rubber Company, Cleveland, 60th edn, 1979), pp. B186-B205. 36 V. K. Cheng, B. A. W. Coller and J. L. Powell, Faraday Discuss. Chem. SOC., 1984, 77, 243. 37 H. B. Hungtington, J. E. Dickie and R. Thomson, Phys. Rev., 1961, 100, 1117. 38 G. L. Gardner and G. H. Nancollas, J . Phys. Chem., 1983, 87,4699. 39 G. E. Cassford, W. A. House and A. D. Pethybridge, J. Chem. SOC., Faraday Trans. 1, 1983, 79, 1954), p. 388. Verlag, Berlin, 1982), p. 288. Verlag, Berlin, 1982), p. 115. 1967), vol. 1, p. 203. unpublished. 1617. Paper 6/ 1688 ; Received 19th August, 1986

ISSN:0300-9599    

DOI:10.1039/F19888400899

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(4), 917-921 Hydrogenolysis of Ethane Part 2.-Initial Rate Measurements over Ni and Pd Catalysts Sandor Kristyan Department of Atomic Physics, Institute of Physics, Technical University of Budapest, H- 1521, Budapest, Hungary Janos Szamosi" Department of Chemistry, Western Illinois University, Macomb, Illinois 61455, U.S.A. The initial rate of the hydrogenolysis of ethane was measured over Ni and Pd catalysts at different partial pressures of C,H, and H,. The exper- imentally obtained rate is in excellent agreement with a recently proposed mechanism, in which both ethane and hydrogen are adsorbed dissociatively, and the kinetically slow rupture of the C-C bond occurs in an interaction with molecular hydrogen. The kinetics of the hydrogenolysis of ethane, C,H, + H, -+ 2CH,, and other aliphatic hydrocarbons over Ni,1-3 Pt,, Co5 and Pd3 has been the subject of numerous experimental investigations owing to the importance of such reactions in petroleum reforming. The rate of methane production depends on the partial pressure of both ethane (PE) and hydrogen (pH); it passes through a maximum with increasing PH and is proportional to PE up to a limit.Based on earlier experimental results and theoretical considerations a detailed mechanism' has been proposed recently for the hydrogenolysis of ethane. The first step is the kinetically fast adsorption of the ethane-hydrogen gas mixture: K E KH C,H6+(7-m)* + C2H,*+(6-m)H* H2+2* 2H* where m is the number of hydrogen atoms remaining on the two-carbon surface compound, C2H,*, * denotes a free site and K , and KH are the equilibrium constants.The second step is the kinetically slow rupture of the C-C bond in an irreversible reaction : C,H,* + B + CH,* where B is the bond splitting agent, either a free site (*), adsorbed hydrogen (H*) or molecular hydrogen. The last step is the hydrogenation of the one-carbon surface species, CH,*, with adsorbed or molecular hydrogen : CH,* + H2(or H*) -+ CH,. Assuming that the third step is fast and the coverage of CH,* is negligibly small, so that steady-state approximations can be applied, the rate of hydrogenolysis will be proportional to the concentration of the bond-splitting agent, B, and the coverage by the two-carbon surface compound : r = k[B] [C,H,*]. Depending on the identity of B there are three possible rate equations which have been 917 31-2918 Hydrogenolysis of Ethane 4 , f-- manometer 2 I chroma t o graph r t-- gases *--tube pump Ct H6, H *e tc.4 * + d=10mm 1 ' ,- (glass cylinder) ,mixer Fig. 1. The circulation apparatus. investigated in detaiL6 In case of a Langmuir-Hinshelwood' mechanism (B = H*), or if the bond-splitting agent is a free site (B = *), the initial rate curves cross each other owing to an inverse rate dependence on PE at low hydrogen pressures; however, this does not happen if Rideal's' mechanism is applied, i.e. if [B] = P,. In this article the results of initial rate measurements over Ni and Pd catalysts are presented and compared with the aforementioned mechanism. Experimental The experiments were carried out in a circulating apparatus (fig.1) connected to a gas- chromatograph. The apparatus had a total volume of 170 cm3, of which 6.5 and 7.8 cm3 were the volumes of the reactor containing Ni and Pd catalysts, respectively. The glass tubes had a diameter of ca. 1 cm, and the rate of circulation was kept at 20.0 cm3 s-l. The initial rate of CH, formation was measured in the pressure range 0.5-10.0 kPa, and at temperatures between 220 and 250 "C with the Ni catalyst and 300 and 350 "C with Pd, using electrical temperature control. All the employed gases were 99.9% pure. The total pressure was adjusted to 13.33 kPa with He. Both catalysts were used in powder form without support material, (0.050g in the case of Ni and 0.141 g in the case of Pd). Ni was prepared from its hydroxide by reduction with hydrogen, while Pd was obtained from its chloride salt with formaldehyde, The former had a surface area of 16.6m2 g-', the latter 4.5m2 g-', measured before the reactions.Before each series of experiments the catalysts were treated for 2 x 8 h at 300 "C with circulating hydrogen, and for 10 min after every measurement. Control runs were regularly performed in order to verify that there had been no loss in catalytic activity. As expected, there were no indications that methane had an effect on the initial rate.S. Kristyan and J. Szarnosi 919 1 .o 3.0 0 0 0 0 2.5 5 7.5 10 PH /kPa 0 d 12.5 2.5 5 75 10 12.5 PH/kPa Fig. 2. Initial rates of the hydrogenolysis of ethane on Ni catalyst at different partial pressures of hydrogen and ethane (a) and at different temperatures (b).(a) PE = a, 9.41; El, 4.70; 0,2.94; A, 1.17, +, 0.58 kPa; T = 250 "C. (b) T = a, 220; +, 230; 0, 240; 0, 250 "C; PE = 4.70Pa. Results and Discussion The kinetics of the hydrogenolysis of ethane was studied over a wide range of pressures, where all the major characteristics of the process were already apparent. The catalysts were employed in powder form in order to avoid possible side effects associated with support materials. Since the volume and the temperature of every part of the circulating apparatus were known, the amount of methane formed in the reaction could be calculated from the real gas law. In addition, the methane production was sufficiently920 1.6 1.4 r( I ; 1.2 5 1.0 5 x" n + W bo 0.8 2 0.6 $ 0.4 0.2 n CI L Hydrogenolysis + I / * \ of Ethane 7- 1 .2 g 1.0 - n Y CI + .2 0.0 8 0.6 2 0.4 0.2 MI n W - 3 0 2.5 5 7.5 10 12.5 P, /kPa Fig.3. Initial rates of the hydrogenolysis of ethane on Pd catalyst at different partial pressures of hydrogen and ethane (a) and at different temperatures (b). (a) PE = .,7.74; 0, 5.35; 0 , 2 . 9 7 ; +, 0.58 kPa; T = 350 "C. (b) T = 0, 300; +, 320; 0, 335; 0, 350 "C; PE = 5.35 kPa. slow (the conversion did not exceed 5 % in 20 min) that the slope of its time evolution was taken as the initial rate. The rate of hydrogenolysis was found to go through a maximum with increasing partial pressure of hydrogen; furthermore, the maximum is shifted toward higher PH as PE is increased (fig. 2 and 3). The rate is proportional to the partial pressure of ethane, i.e.the rate us. PH curves belonging to different ethane pressures do not cross each other. The temperature dependence of the rate proved to be as expected; however, the temperature was kept at or below 250 "C when using Ni. Above 250 "C Ni becomes increasingly susceptible to surface poisoning : rates above 500 "C are lower than those below that temperature. (Recently a new method has been developed concerning catalytic activation and regeneration using electrostatic field gradient^,^ and one of the systems employed for the purpose of demonstration was the hydrogenolysis of ethane on Ni wire catalyst. It is assumed that parallel competitive poisoning takes place, andS. Kristyan and J. Szamosi 92 1 Table 1. Calculated values" __ k/,umol dm-3 cat a1 y s t m K/dm3 kPa-' K,/dm3 kPa-' g-I kPa-l s-' Ni (250 "C) 2.36f0.1 0.0131 f0.0012 (5.85f0.08) x lC4 0.78 k 0.082 Pd (350 "C) 3.33 f O .l 0.735 k 0.045 0.727 & 0.075 0.585 f 0.052 - " m is the number of hydrogen atoms on the two-carbon-atom surface species; KH andK, are the equilibrium constants of the chemisorption of hydrogen and ethane, respectively; k is the rate constant of the rate-determining step, related to 1 g of catalyst. a model, based on the presence of two distinct types of active surface sites, has been suggested. lo) Table 1 contains the parameters calculated by using Simplex curve-fitting, i.e. the step- by-step minimization of the sum of the squares of the differences between the estimated and the measured values.(The possible alternatives were also tested but were found to give unsatisfactory fits to the data.) Of special interest is m, the number of hydrogens on C,H,*. In both cases m is far from being an integer, suggesting either that the hydrogen content of the surface compound is not constant5 or that the surface compound must pass a conversion between a ' hydrogen-rich ' and a more strongly bonded form4 before hydrogenolysis can take place. The above experimental results clearly substantiate our previously suggested model which is based on the dissociative adsorption of the gases, and molecular hydrogen being the sole agent for the rupture of the C-C bond in the rate-determining step. A non- dissociative model in which ethane does not lose hydrogens upon adsorption, but a surface compound containing weaker bonds develops instead, was also considered ear lie^.^ Even though it does exhibit most of the observed phenomena at low and medium PH values, the non-dissociative model is incompatible with the experimentally obtained data at higher pressures.Current work is focussed on the determination of the heat of adsorption of hydrocarbons on metal surfaces. Under serious consideration is a mechanistic investigation concerning self-poisoning at higher temperatures. This research was supported by grants from the National Academy of Sciences of Hungary, the R. A. Welch Foundation, and the University Research Council of Western Illinois University. Discussions with Professors P. Tetenyi, A. Sarkany and L. Guczi have been most helpful. References 1 J. H. Sinfelt and W. F. Taylor, Trans. Faraday SOC., 1968, 64, 3086. 2 G. A. Martin, C. R. Acad. Sci. Paris, Ser. C, 1977, 284, 479. 3 S. Kristyan, D.Sc. (Techn.) Dissertation (Technical University of Budapest, 1982). 4 L. Guczi, A. Sarkhy and P. Titinyi, J. Chem. SOC., Faraday Trans. 1, 1974,70, 1971. 5 H. Forster and H.-J. Otto, 2. Phys. Chem. N . F., 1980, 120, 223. 6 S. Kristyan and J. Szamosi, J. Chem. SOC., Faraday Trans. I , 1984, 80, 1645. 7 A. W. Adamson, Physical Chemistry of Surfaces (Wiley, New York, 1974). 8 E. K. Rideal, Concepts in Cufalysis, Academic Press, New York, 1968). 9 S. Kristyan and R. B. Timmons, J . Catal., 1986, 101, 331. 10 S. Kristyan and R. B. Timmons, J. Chem. SOC., Faraday Trans. I , 1987, 83, 2825. Paper 7/27 1 ; Received 16th February, 1987

ISSN:0300-9599    

DOI:10.1039/F19888400917

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1988, 84(4), 923-929 Mechanistic Aspects of Oxidative Coupling of Methane over LaAlO, Tomohiko Tagawa" and Hisao Imai Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi 227, Japan Mechanistic aspects of the oxidative coupling reaction of methane over an LaAlO, catalyst (La : A1 = 1 : 1) prepared by the mist decomposition method have been studied, using both continuous-flow and pulsed-flow techniques. The delayed pulse technique together with temperature-programmed desorption reveal that an adsorbed oxygen species is effective in the formation of the C, compounds, while a gaseous or weakly adsorbed oxygen species is involved in the combustion reaction. Methane cannot stay on the surface stably.Comparing these results with those obtained using the continuous flow reactor, mechanistic aspects are considered from the viewpoint of oxygen activity. The stability of this catalyst is also discussed. .- __-_ The oxidative coupling of methane is of considerable interest with regard to the efficient use of natural gas. As summarized in our recent papers, most reports on this subject deal with the screening of the catalyst for the efficient production of ethane and ethene.lV2 However, several papers have reported the mechanistic aspects of this Different kinds of sites, such as PbO,, [Li+O-],* 0; on La20,,5 Sm3+O-,6 0; or 0;- on Sm20,' and a bifunctional redox site comprising Bi-Al-O', have been suggested as the active sites, but the nature of these oxygen-containing species is not yet clear.The pulse reaction technique, a non-steady-state method, is often useful in investigating the surface species involved in a reaction. When oxygen and the reactant hydrocarbon are pulsed with an interval (a delayed pulse), the nature of the active surface species can be specified. For example, this technique has been applied to the oxidation of etheneg and in oxidative dehydrogenation. lo We have reported that an LaA10, catalyst prepared by the mist decomposition method has demonstrated a high activity and a high selectivity.' By comparison with a set of La,Al,-,O, catalysts, the active site is suggested to be an amorphous phase of LaAlO, which can adsorb oxyygen reversibly.2 However, details of the nature of the oxygen species remain unknown.The present paper reports a study of the catalytic behaviour of LaA10,. The pulse reaction technique has been used mainly to investigate the adsorption species which are responsible for the production of C , compounds. The mechanistic aspects of this reaction are discussed on the basis of the oxygen species adsorbed on the catalyst. Experiment a1 The LaAlO, catalyst (La : A1 = 1 : 1) was prepared by the mist decomposition method. A mixed solution ( 5 wt %) of lanthanum nitrate and aluminium nitrate was atomized by a supersonic vibrator. The mist of raw materials thus formed was treated successively through three furnaces with a flow of air ( I dm3 min-l). The temperatures of the furnaces were maintained at 443, 573 and 1073 K, respectively.The resulting fine particles of the composite oxides were collected on a glass filter at 390 K, pressed and their size was determined. The B.E.T. surface area of the catalyst was 1.93 m2 g-l. Details of the apparatus and the mist decomposition method have been given elsewhere.l1P l2 923924 Oxidative Coupling of Methane over LaA10, The flow reaction was conducted with the system reported previous1y.'v2 The pulse reaction was conducted as follows unless otherwise noted. The catalyst (60-100 mesh, 0.025 g) was placed in a quartz tube reactor of 6 mm i.d. and set in the furnace. After pretreatment with air at 983 K for 16 h, the reactor was purged with a flow of purified helium carrier (15 cm3 min-l) at 983 K and connected to the gas chromatograph by a gas-handling valve. Another gas-handling valve was provided to switch the analysing columns of molecular sieve 5A and Porapak Q.A mixture of methane (0.6cm3) and oxygen (0.3 cm3) was pulsed at the reaction temperature. Two successive pulses were injected for each analysing column and the results were combined for further calculations. The yield of each product was calculated in terms of a C , base and expressed as pmol per unit surface area of the catalyst. The C, selectivity represents the percentage of ethane and ethene in the total amount of converted methane. A delayed pulse technique was also applied by providing intervals between the oxygen pulse (1.0 cm3) and the methane pulse (0.6cm3) at 983 K. The reactivity of the adsorbed oxygen species was tested using the modified pulse reaction system with 0.056 g of the sample.After the pretreatment with air at 983 K the catalyst bed was adjusted to the 'outgassing temperature' and purged with a flow of purified helium for 0.5 h; then the catalyst was cooled to the room temperature. The reactor was purged with a mixture of methane and helium (1 : 1) and isolated by a stainless-steel four-way valve. The temperature was then increased to 823 K and held for 0.5 h. The contents of the reactor was introduced to the gas-chromatograph by opening the four-way valve while the reactor was at 823 K. As CO cannot be separated by the Porapak Q column, the yield of CO was estimated from the water peak. The temperature-programmed desorption (t.p.d.) spectrum of the sample was obtained as follows.0.58 g of the sample was pretreated in purified air at 983 K for 16 h; then the temperature was lowered to 323 K and kept constant for 30 min in the air stream. Next helium gas was passed through the sample at a rate of 20 cm3 min-l. After 10 min the temperature was increased at a rate of 6.25 K min-l. The composition of the exit gas was monitored by a thermal conductivity cell. A gas sampler situated immediately after the cell was used for a detailed analysis of the desorbed gas by gas chromatography. X-Ray diffraction and Fourier-transform infrared spectroscopy were also used for the characterization. Oxygen (Research Grade) was obtained from Kayama Oxygen Co. Ltd, and used without further purification. All other gases, including the purified helium carrier gas, and materials, together with the methods of their purification, have been mentioned in our previous paper. Results The stability of the catalyst was tested by the continuous-flow reaction system under the following conditions ; T = 983 K, W/F = 59.4 g h dmP3, CH,/air = 1.The initial activity of 21.1 YO of methane conversion and C, selectivity of 40% decreased by 10-20% over 20 h on stream, but thereafter remained essentially constant for > 80 h. The X.r.d. spectra of the deactivated catalyst showed development of crystalline perovs kite. A similar test was conducted by the pulse reaction. Six consecutive standard pulses at 983 K showed no significant changes both in the conversion (35%) and the selectivity (45 YO). Activity and selectivity were still maintained after operations for more than 50 h (100 pulses), using various reaction conditions.Thus deactivation occurred only under continuous-flow conditions. The effects of the partial pressures of methane [P(CH,)] and oxygen [P(O,)] were also studied on the fresh catalyst. Fig. 1 (a) summarizes the effect of varying P(CH,) under the continuous-flow conditions at 983 K. P(CH,) was changed from 5 to 75 kPa keeping- P(0,) constant at 10.1 kPa. Fig. 1 (b) shows the effect of varying P(0,) from 2 to 25 kPaT. Tagawa and H. Imai 925 1 , - 2 5 4 h r 4 E 7Y) 3 - 3 2 c, - 0 2 W E 4 1 0 1 2 3 4 5 0 1 2 3 4 In [P(CHdIkPal In [P(O,)lkPaI Fig. 1. (a) Effect of partial pressure of methane. P(CH,) was varied with a constant P(0,) of 10.1 kPa. (b) Effect of partial pressure of oxygen.P(0,) was varied with a constant P(CH,) of 50.5 kPa. T = 983 K, W = 0.039 g, F = 40 cm3 min-l, N, balance. 0, CO; 0, CO,; a, ethane; @, ethene. Table 1. Reaction rates of the oxidative coupling of methanea r = kP(CH,)Z (P(0,)’ CH,/O, < 4 4 c CH,/O, product x y x y C,H, 1.7 -0.8 1.0 0.5 C,H, 1.9 -0.7 0.9 1.2 CO 0.7 0.9 0.7 1.4 CO, 0.5 1.2 0.1 1.7 a From continuous-flow reactions at 983 K. For the reaction conditions see fig. 1. with a constant P(CH,) of 50.6 kPa at 983 K. In both cases the apparent reaction orders were greatly changed at CH,/O, = 4, which corresponds to the stoichiometric ratio of methane to oxygen for ethane production (4CH, + 0, + 2C,H, + 2H,O). With higher P(O,), the C, formations showed negative reaction orders on P(0,).Table 1 summarizes the simple power-law rate expression of this reaction obtained from fig. 1. Two different sets of power-law kinetics were applicable, depending on the CH,/O, ratio. Similar effects were observed in case of a pulse reaction in which the amount of one reactant was changed while the other was kept constant. Thik suggests that the state of the catalyst during the pulse reaction is similar to that during the flow reaction. Table 2 shows the results of the pulse reaction of ethane and ethene with oxygen. Ethene was oxidized to form CO, (CO + CO,) and with a sufficient amount of oxygen; the oxidation proceeded further to CO,. The product distribution of the ethane oxidation was similar to that of methane, while methane gave a slightly higher CO,/C, ratio and C-C/C=C ratio.The delayed pulse technique was applied to two sequences : ( a ) methane + interval + oxygen, and (b) oxygen -+ interval + methane. In the case of sequence (a), fig. 2(a) shows926 Oxidative Coupling of Methane over LaAlO, Table 2. Pulse reaction at 983 K" yield/pmol ratio ~~ _ ~ ~ _ _ _ _ _ ~ ~- reactant/pmol C,H, C,H, CO CO, C,H,/C,H, CO,/CO COJC, CH, (49.4) 1.67 0.76 0.31 4.57 2.18 14.5 2.03 - 1.1 C,H, (49.4) 2.02 20.32 3.20 3.57 - (3.30) nil 0.07 nil 2.48 - - - C,H, (3.30) 0.80 0.52 0.13 1.88 1.54 14.8 1.53 i Q? 2 1 1 0 a W = 0.025 g, 0, = 12.3 pmol in a purified He carrier. / O O / O &-e----. 0 - 0 20 40 60 in terval/s 0- 0 - 0 - = , PP\t_. , , 0 0 20 40 60 interval/s Fig. 2. (a) Change in retention time from a methane pulse in sequence (a) at 983 K.RJR, is the ratio of the retention time from the methane pulse following an interval x to the retention time of a simultaneous pulse. 0, CO,; 0, ethane; a, ethene. (6) Product distribution, 0, C, and 0, cox. the changes in the retention time from the methane pulse as a function of the interval. Ethane and ethene were produced at the methane pulse and CO, was produced in the presence of gaseous oxygen. Fig. 2(b) shows the product distribution in this case. The formation of CO, greatly decreased when the interval was increased, while the formation of C , products showed little change. In the case of sequence (b), fig. 3(a) shows the changes in the retention time from the oxygen pulse as a function of the interval. Both C , and CO, were formed at the methane pulse.No products were observed at the oxygen pulse, suggesting that no carbonaceous deposit is present on the surface. Fig. 3(b) shows the product distribution in this case. The formation of CO, greatly decreased in the absence of gaseous oxygen; on the other hand the formation of C , increased more than three times with an interval of 10 s. Increasing the interval further caused a gradual decrease of C , formation. Fig. 4 shows the reactivity of adsorbed oxygen species together with the t.p.d. spectrum of this sample. As shown fig. 4(b), two oxygen desorption peaks were observed at temperatures below 983 K. When the catalyst was outgassed with helium at room temperature, a large amount of CO, was formed, together with a slight amount of ethane and ethene. The outgassing at 823 K caused a decrease in CO,, together with remarkable increases in ethane (eight-fold) and ethene (three-fold).After outgassing at 983 and 1033 K, only a small amount of C , was formed, with a further decrease in CO, formation. The C ,T. Tagawa and H. Imai 1.3 ( a ) 0.9 I 1 I I 0 20 40 60 interval/s 927 200 N E - 0 \ % 100 2 $ x 0 in terval/s Fig. 3. (a) Change in retention time from an oxygen pulse in sequence (b) at 983 K. RJR, is the ratio of the retention time from the oxygen pulse following an interval x to the retention time of a simultaneous pulse. (b) Product distribution. For s mbols, see fig. 2. 0.75 - 0 E, E 3 0.50 1 -0 0.25 0 200 6 00 1000 TIK Fig. 4. (a) Activity of adsorbed oxygen species against outgassing temperature.The activity was measured by the reaction with methane at 823 K. 0, CO,; 0 , CO; 0, ethane; 0, ethene. (b) T.p.d. spectrum of the sample. selectivities were calculated as 10.7, 67.2, 65.0 and 64.2 % at outgassing temperatures of 300, 823, 983 and 1033 K, respectively. Discussion Stability of the Catalyst Previously we have found that activity and selectivity depend on the crystal structure of the catalyst. Formation of the perovskite phase decreases both the activity and selectivity, but this requires a pretreatment temperature higher than 1073 K2 The decrease in the activity and the selectivity observed during the flow reaction may be explained by the development of the perovskite crystallites, except for the temperature928 Oxidative Coupling of Methane over LaA10, factor.The experiments under pulsed-flow conditions revealed that this could not take place under a flow of helium at 983 K within at least 50 h, including > 100 intermittent contacts with the reactants. Therefore one possible reason for the development of the perovskite crystallites is a local increase in temperature due to the heat of the reaction. Thus the effective dissipation of the heat of the reaction under pulsed-flow conditions could prevent the catalyst from undergoing deactivation. Adsorbed Species Providing intervals between the pulses of the two reactants can permit one to distinguish some reactive adsorbed species from gaseous s p e c i e ~ . ~ * l ~ Fig. 2 shows that C, products are formed by the methane pulse by reaction with surface oxygen species remaining from the previous reaction and that CO, species are formed only when gaseous oxygen is available.The product distribution reveals that the methane species on the surface are removed so quickly that CO, was no longer produced after 30 s. The Fourier-transform infrared study of the catalyst at room temperature after heating in the presence of methane and oxygen revealed gaseous methane but no adsorbed hydrocarbon species. Thus it is concluded that no strongly adsorbed hydrocarbon species are involved in this reaction. On the other hand, when intervals were provided after the oxygen pulse [sequence (b)] all the products were formed by the subsequent methane pulse and none were formed at the oxygen pulse. These results, shown in fig.3, again suggest that no stable methane adspecies exists on the surface but that adsorbed oxygen can react with the pulsed methane. The product distribution shows that CO, are mainly formed with gaseous or weakly adsorbed oxygen. In spite of this, the yield of C, products increased with the disappearance of gaseous oxygen. The yield of C, products also decreased with as the intervals were lengthened,'suggesting that the active oxygen species on the surface is also desorbed slowly. Thus it is concluded that C , is formed by the reaction with the adsorbed oxygen species, which is strongly bonded to the catalyst. On the other hand, the combustion reaction occurs with either gaseous oxygen or weakly bound oxygen adspecies. Reactivity of Adsorbed Oxygen Species Fig.4(b) shows the t.p.d. spectrum of LaAlO,., The reactivity of the oxygen species which remains after outgassing above 983 K is very small compared to that desorbed at lower temperatures. The oxygen which is desorbed giving the higher t.p.d. peak shows a much greater activity and a higher selectivity to C, than does that arising from the continuous-flow reaction. Without outgassing (total absorbed oxygen) the combustion reactions are dominant and C, yields are decreased, suggesting the further oxidation of C, products. Thus weakly bound oxygen or gaseous oxygen which has been desorbed as a result of increasing the catalyst temperature from 300 to 823 K (the first t.p.d. peak) is active for the combustion reaction, which is inconsistent with the discussion given above.This can also be supported by the nature of non-selective catalysts such as CeAlO, or PrAlO,, which only showed a t.p.d. peak in the lower-temperature region. The estimated amount of oxygen involved in the oxidation (total adsorbed oxygen) is 9.7 pmol m-,, this is less than monolayer coverage (1 1 pmol m-,), suggesting that surface oxygen is used in the oxidation. Also, the amount of oxygen that has reacted below 910 K (estimated as 5.7 pmol m-2) is close to the amount of desorbed oxygen obtained in the t.p.d. measurements (4.9 pmol m-,), suggesting that the oxygen reversibly adsorbed on the amorphous LaA10, is reactive.T. Tagawa and H. Imai 929 Mechanistic Aspects As shown in table 1, two different sets of power-law kinetics are applicable, depending on the CH,/O, ratio.This suggests that the concentration of oxygen may modify the reaction mechanism. The change in contact time in the continuous-flow reaction at 983 K indicated that ethane and CO are the primary products obtained via a parallel path and that ethene and CO, are the secondary products.13 However, the possibility that these secondary reactions occur in the gas phase has not been checked under these conditions. The reactivities of ethane and ethene, shown in table 2, may provide further details. As methane and ethane gave similar product distributions, the reaction path to produce COX and ethene via a secondary reaction of ethane is supported. Oxidative dehydrogenation can convert ethane into ethene, and ethene is oxidized to form CO,, so that, with an excess of oxygen, the C, products are further oxidized to CO,.The results shown in fig. 4 also suggest that C, products formed by the reaction with strongly bonded oxygen are further oxidized to CO, by weakly bonded oxygen. This would explain the negative dependence of C , formation on the partial pressure of oxygen when the CH,/O, < 4. Table 1 also shows that the reaction order for methane in ethane formation is doubled with the higher partial pressure of oxygen. This suggests the possibility of a competitive adsorption of methane with either oxygen or the oxidation products. With an oxygen deficiency the oxidation of ethene may be inhibited and the oxidation of ethane dominates as a side-reaction. In this situation most of the adsorbed oxygen species in the low-temperature region may be absent on the surface.This situation was simulated by the reaction of methane with adsorbed oxygen species which remained after outgassing at higher temperatures. This resulted in the formation of a considerable amount of C, compounds with a higher C, selectivity. At present stage no information has been obtained about the relationship between the adsorbed species and the structural properties of LaA10,. However, as far as the LaAlO, catalyst prepared by the mist decomposition method is concerned, the oxygen active for C , formation is concluded to be a reversibly adsorbed species which is stable after outgassing at 823 K. On the other hand, the combustion reaction takes place with an excess of oxygen in the gas phase or in a weakly adsorbed state. References 1 H. Imai and T. Tagawa, J. Chem. SOC., Chem. Commun., 1986, 52. 2 H. Imai and T. Tagawa, J. Catal., 1987, 106, 394. 3 W. Hinsen, W. Bytyn and M. Baerns, Proc. 8th Int. Congr. Catal., 1984, 3, 581. 4 T. Ito, J-X. Wang, C-H. Lin and J. H. Lunsford, J. Am. Chem. SOC., 1985, 107, 5062. 5 C-H. Lin, K. D. Campbell, J-X. Wang and J. H. Lunsford, J. Phys. Chem., 1986, 90, 534. 6 K. Otsuka, K. Jinno and A. Morikawa, J. Catal., 1986, 100, 353. 7 K. Otsuka and K. Jinno, Inorg. Chim. Acta, 1986, 121, 237. 8 I. T. A. Emesh and Y. Amenomiya, J. Phys. Chem., 1986, 906,4785. 9 A. Verma and S. Kaliaguine, J. Catal., 1973, 30, 430. 10 T. Tagawa, T. Hattori and Y. Murakami, J. Catal., 1982, 75, 56. 11 H. Imai and F. Orito, Nippon Kagakukai si, 1984, 851. 13 T. Tagawa and H. Imai, unpublished data. - 12 H. Imai, K. Takami and M. Naito, Muter. Res. Bull., 1984, 19, 1293. Paper 71328; Received 23rd February, 1987

ISSN:0300-9599    

DOI:10.1039/F19888400923

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(4), 931-939 Measurements of the Electrolyte Conductivity of Alkali-metal Perchlorates and LiNO, in Acetone at 25 OC Natalija Schmelzer,* Jurgen Einfeldt and Manfred Grigo Wilhelm- Pieck- Universitat Rostock, Sektion Physik, Universitatsplatz, 3, Rostock, DDR-2500, German Democratic Republic The molar conductivities of solutions of LiClO,, NaClO,, KClO,, RbClO,, CsClO, and LiNO, have been measured in acetone at 25 "C. We have used the conductance equation KKE3-AL to describe the measured data. We have converted the fitting parameter R, which is connected with the association constant KF, into a potential parameter d+/kT, - and we discuss the ion-solvent interactions for the given systems. In previous papers's we investigated systematically the behaviour of several simple ions in alcohols, using conductivity measurements with the KKE3-MAL theory3 in order to study ion-solvent interactions uia the potential parameter of the effective ion-ion interactions.Ref. (4) extends these investigations with our own measurements to solvents with small dielectric constants (e.g. tetrahydrofuran and ethyl acetate) and tests KKE3-MAL theory in systems with a high degree of association of the ions. In the present paper we report the results of measurements in acetone and extend the abovementioned investigations to the class of aprotic protophobic solvent^.^ Experiment a1 Acetone (purum speciale) was distilled over activated A3 molecular sieves under argon. The specific conductance of the different batches, 0, varied from 0.8 x lo-' to 2 x lo-' W'cm-l.LiNO,, LiCIO,, NaClO, and KC10, (Merck, very high purity) were recrystallized once from conductivity water and dried to constant weight under an argon atmosphere at temperatures between 170 and 300 "C. RbC10, and CsClO, were recrystallized twice and dried at 250 "C. The salts were stored in vacuum desiccators over P,O, and were dried once more before each measurement. The concentrations were determined as where G, and G,, are the masses of salt and solvent, respectively (vacuum corrected) and M is the molecular weight of the salt. The conductivity measurements for the concentrations used were carried out beginning with the pure solvent by adding the stock solutions. The conductivity cells have a lock-system and operate with a small overpressure of argon.Even during a run taking over 40 h there was no indication of evaporation of the solvent. The resistances were measured with a IST bridge to an accuracy of 0.002%.6 The resistances of the electrolytes were measured for frequencies (f) between 0.2 and 10 kHz and were determined in the usual way by an extrapolation of the resistances for l/f+ 0. The total precision was estimated as k0.03 %. Some measurements were repeated with other charges of salt and of solvent and other cells. The cell constants were 0.18395, 0.055 367 and 0.010642 cm-'. 93 1932 Molar Conductivities of Alkali-metal Perchlorates Table 1. Dependence of the density on con- centration for alkali-metal perchlorates and LiNO, in acetone at 25 OCa A T , 4 = p,(T) + salt d is valid for LiCIO, 0.08297 & < 0.1 mol kg-l NaCIO, 0.091 03 & < 0.38 rnol kg-' KCIO, 0.1022 #z < 0.01 mol kg-l RbCIO, 0.1586 tj? < 0.004 rnol kg-' CsCIO, 0.1734 tj? < 0.0 1 rnol kg-I LiNO, 0.045 54 6 < 0.1 mol kg-l a rjz is the concentration in rnol kg (solution)-l, po is the density of the solvent and p is the density of solution.The densities were measured using equipment from Paar-AG, Austria (model DM02) and are summarized in table 1 as p(%, T ) = po( T ) + dfi. Here po is the density of acetone at 25 "C. The molarity c is then calculated as c = p(%, T)%. The measured density of acetone was 0.784 48 g ~ m - ~ . From the literature known density values of acetone are 0.7840 g cm-3,19 0.7845 g cm-3,17 0.784 43 g cm-3,18 and 0.784 53 g Theory An analysis of the measurements are made in a chemical model on the basis of RPM since the DK of acetone is 20.47, and association of the ions in this solvent is expected.It is clear that RPM is a simplification of physical reality, but it has been treated consistently'* and takes into account, beyond the Coulomb interaction, short-range interactions via R, the potential parameter between the ions. The limits of RPM result from a neglect of the specific properties of the ion-ion interaction and from an overemphasising of the hard-sphere structure. However, a number of investigations'? 2 * show that RPM is suitable for an analytical discussion of a broad range of chemical classes of electrolytes. For a description equations :l the mass-action law with the association where of the experimental points (A us.c curves) we used the following (MAL) : 1 -a -- a2 - 'f ",'> K A ( T ) constant given by Ebeling :lo w2 b = 4ngoDkTR is the Bjerrum parameter and fySA - are the activity coefficients of the free ions in the MSA approximation, given by [ 1 + 2 d R - ( 1 + 2~cr!R);] 103K~ In f+ - = lnfySA = - 4ncNR3af (3)N . Schrnelzer, J . Einfeldt and M . Grigo 933 Fig. 1. < Rab R,, < r < Rib kT r > R;, R,, = r:r + r,; R:, = R,, + s where s is the size of the solvent molecule and q = bR/2. with the Debye parameter 8ncNR3b The conductivity theory according to Kremp, Kraeft and Ebeling (the KKE3 theory), which was expanded to an empirical J; term and which is consistently reformulated in terms of a chemical m0de1,~ results in A(c) = a(c)A, - (S, A, + S,) (ac)i + (El A, + E,) ac In (ac) + ( J1 A, + J,) ac + A, aREST with the coefficients S, and J , as given by ref.(1 1) and JREsT = J; (ac);. By analogy with the work of Justice12 and Elshazly et a1.lS2 we convert the R parameter obtained from fitting the experimental data into the potential parameter d , - / k T , which characterizes the ' square-mound potential model ' (fig. 1). (4) Results Table 2 reproduces the results of A us. c measurements ordered with respect to the concentration. Column (1) shows several runs. Column (4) contains the fraction of free ions, a(c). Fig. 2 shows graphically for the measuredlsystems the change in the experimental points with A*(ac) = A/a as a function of (ac)z. The parameters A, and R determined by a least-squares fit and the KA values calculated from eqn (2) with known values of b and R are summarized in the table 3.Column (5) enables us to estimate the accuracy of the fitting of the data. Since the applied theory is valid only for small concentrations one can draw conclusions concerning the limit of validity of the theory by a comparison of the fitted parameter at the minimum and maximum fitted concentrations (cmin and cmax); the fitted parameters must stay independent of the fit concentration if the theory is valid and the measured data are accurate. Fig. 3 shows that this demand is fulfilled for A, in the range 8 x lop6 d c/mol dmP3 d 2 x It is important to measure the conductivity data at a sufficiently small concentration.934 Molar Conductivities of Alkali-metal Perchlorates Table 2.The measured molar conductivities of alkali-metal perchlorates and LiNO, in acetone at - c/ 10-4 A/n-l run mol dm-, cm2 mol-' 25 "C c/ 10-4 A p - 1 a(c) run mol dm-, cm2 mo1-1 a(c) LiC10, 0.3106 190.80 0.4987 189.20 0.7856 187.30 2.143 180.77 3.285 177.32 4.042 174.46 5.446 170.84 6.802 167.51 8.398 164.28 10.52 160.50 15.17 153.90 20.89 147.52 39.68 133.54 71.71 119.95 NaClO, 0.0795 1 195.35 0.1594 194.08 0.2970 192.63 0.5080 191.02 2.260 181.43 4.0 10 175.32 5.383 171.66 6.604 168.90 7.328 167.23 8.682 164.83 13.43 157.54 13.45 157.41 18.28 151.70 22.79 147.25 26.22 144.55 35.08 138.33 36.12 137.52 47.14 131.80 65.97 124.14 124.70 109.15 196.70 99.30 412.10 84.14 0.996 0.994 0.991 0.979 0.970 0.964 0.955 0.947 0.938 0.928 0.999 0.997 0.995 0.992 0.972 0.955 0.944 0.935 0.930 0.92 1 0.895 0.895 KClO, 194.30 0.2227 0.4359 192.46 0.6441 190.97 0.7466 189.88 1.0585 187.90 1.1867 187.33 1.6047 184.70 2.3010 181.26 3.1600 177.67 4.327 173.47 6.577 166.66 11.115 157.05 CSClO, 0.1765 * 198.83 0.4008 196.15 0.7456 193.17 1.3226 188.98 2.0507 184.44 3.2836 178.26 5.1829 170.90 6.7880 165.86 RbClO, 0.3259 193.42 0.5948 191.04 1.0469 187.76 1.6129 184.3 1 2.5364 179.30 3.9864 173.40 LiNO, 0.0722 165.75 0.1537 144.03 0.2665 125.62 0.484 1 105.31 0.7758 90.02 1.2192 76.83 2.7275 55.76 5.7216 40.22 6.0537 40.06 12.987 28.637 13.9150 28.360 24.871 22.146 43.315 18.156 79.23 1 15.070 143.84 13.035 325.74 11.31 1 635.25 10.382 1383.02 9.413 0.996 0.992 0.989 0.987 0.982 0.980 0.974 0.965 0.955 0.943 0.923 0.996 0.991 0.983 0.972 0.960 0.942 0.919 0.902 0.994 0.989 0.981 0.972 0.960 0.943 0.8 15 0.707 0.618 0.518 0.444 0.378N .Schmelzer, J . Einfeldt and M . Grigo 200 1KI 160 935 - ( a 1 --. (iii). ( i ) - I I I I I I I I 1 0 0.01 0.03 0.05 .. (iii) . . I 1 1 I 1 0 0.01 0.03 0.05 ((rc)j /mol) dm-3 Fig. 2. The molar conductivity of alkali-metal perchlorates in acetone at 25 "C (a) KClO,, (b) RbClO,, (c) CsC10, (i) A(c), (ii) A*(ac) and (iii) A. - S(nc)i. Table 3. Conductance parameters of alkali-metal perchlorates and LiNO, in acetone at 25 "C for c < 1 x mol dm-l LiNO, ?05.28 f 0.60 1.66 fO.OO 40 338.70 0.24 -5.15 LiClO, 195.43 f 0.15 4.44 & 0.10 132.30 0.19 0.83 NaClO, 197.51 f0.06 3.99k0.02 170.95 0.1 1 0.1 1 KClO, 198.83f0.11 3.71 k0.04 207.60 0.16 - 0.45 RbClO, 198.93f0.17 3.62k0.09 233.65 0.12 -0.64 CSCIO, 202.81 f0.08 3.41 f0.02 268.63 0.098 - 1.01 A comparison of our measurements with the literature data for conductivities of alkali-metal perchlorates in acetone at 25 "C (fig.4) shows that there are essential differences from the data of Accascina13 and Shkodinl, but not from the data for CsClO, of Pistoia.15 In contrast to the literature data, we have found a systematic dependence of the fitted parameters of eqn (4) on the reciprocal value of the sum of crystallographic radii. Because of similarities between the cations with respect to their electronic structures we can expect such a dependence.936 7 197 4 Molar Conductivities of Alkali-metal Perchlorates --o-o-o- - I I 1 1 I 198 0 5 E 0.4 w 1 0.3 - I I I I I 1 -6 -5 -4 -3 -2 - 1 log Cmax Fig.3. Dependence of the fitted parameter A,, on (a) cmin and (b) cmax for NaClO, in acetone at 25 "C. 0.2 <" 190 N$ L i t - I 1 I I I I 0 0 0 0 1 I I I I 0.24 0.28 0.32 1 I r I Cs' Rb' K + The ionic conductance of K+ ions in acetone was calculated from the transference number for the SCN- ion, t(SCN-) = 0.6237,16 and from the A, value for KSCN, A,(KSCN) = 201.76 R-' cm2 mol-', and is equal to A,(K+) = 72.92 S2-l cm2 mol-l. The ionic conductivities of the alkali-metal salts of ClO, and NO, are determined on the basis of the value from our own measurements and from literature data; they are shownN . Schmelzer, J . Einfeldt and M . Grigo 937 2oo[ O 0 0 0 0 0 0 O 0 0 0 c+/lO-* rnolidm-3 Fig.5. Molar conductivity of LiClO, and LiNO, in acetone at 25 "C: 0, LICIO,; 0, LiNO,. Table 4. Single ionic conductances of some ions in acetone at 25 "C Li+ 72.52 67.71 73.46 Na+ 74.60 73.42 74.67 K' 75.92 75.92 75.92 75.92 Rb' 76.02 CS' 79.90 80.70 78.17 ClO; 122.91 120.83 121.17 NO, 132.10 125.16 a A,, is the ionic conductance in R-l cm2 mol-' (this work). (17). ' Ref. (18). Ref. (19). Ref. (15). Ref. in the table 4. The ionic conductivities, and therefore ionic mobilities, of the anions ClO, and NO, are significantly larger in acetone than the mobilities of alkali-metal cations. In general, the anionic mobilities in acetone are larger than the ionic mobilities of cations of comparable size; this indicates a specific interaction of the cations with acet0ne.l' Fig.6 shows the dependence of d+/kT of some electrolytes in acetone at 25 "C on the reciprocal sum of the crystallographic radii of the ions. The positive value of d+/kT can be interpreted by additional repulsive short-range interactions between- two oppositely charged ions at distances of the size of solvent molecule, e.g. by the solvate shell of an ion or of both ions. The negative d+/kT parameters point to the existence of additional attractive short-range interactions between two ions. It is interesting to follow the variation of d+/kT with the exchange of cations and anions. (i) A change in the cation of the elecfiolyte while keeping the same anion does not cause such a strong alteration of the potential parameter as does a change in the anion in comparable electrolytes, e.g.lithium halides, alkali-metal iodides and alkali-metal perchlorates. (ii) The parameter d+/kT, and therefore the ion-solvent interaction, decreases from Li+ to Cs+ in alkali-me61 iodides and alkali-metal perchlorates ; this confirms the observation938 Molar Conductivities of Alkali-metal Perchlorates I I 4 LiBr c10; I I I I I I 0.1 0.3 I 0.5 I I I I LiCl Fig. 6. Dependence of d+/kT parameter on the reciprocal sum of the Pauling radii for some electrolytes in acetone at 25 "C : lithium halides18 and alkali-metal iodide^."^^^ of Minc and Werblan on alkali-metal cations in acetonitrile (which also belongs to the aprotic protophobic group of solvents).20 These authors20 conclude, on the basis of differences observed in the activation parameters of the abovementioned ions, that large ions such as ClO;, Et,N+ and Cs+ move through the solution without a primary solvation layer, while small cations such as Li+ and Na+ have a primary layer.(iii) The large positive values of d+/kT for LiI, NaI and KI indicate interactions between the I- ion and the acetone molecule. According to Parkerz1 the anions are solvated in the dipolar aprotic solvents by iondipole interactions upon which is superimposed an interaction due to the mutual polarizability of the ion and the solvent molecules. Small anions with localized charges, e.g. CI- and F-, are not polarizable, they are solvated by dipolar aprotic solvents very poorly or not at all. In contrast, the interaction of the polarizable anions ClO,, I- and SCN- is favoured by strongly polar molecules of aprotic protophobic solvents.Parker21 gives the following polarizabilities for these anions : anion a/cm3 mo1-' c1- 9.07 Br- 12.66 I- 19.21 SCN- 16.54 ClO, 13.24 the appropriate d+/kT polarizable anions have If one compares the polarizabilities of these anions with parameters, -one finds some degree of dependence: the more - stronger interactions with the solvent, resulting in an increase in d+/kT - for electrolytes with the same cation, e.g. LEI, LiBr, LiI; KClO,, KI. The essentially different behaviour of LiNO, compared with LiClO, (fig. 5 ) can be interpreted by a smaller solvation of the NO, ion in contrast to the ClO, ion. Qualitatively the same behaviour is shown by the measurements of AgNO, and AgClO, in acetone at 25 0C.22 In fig.7 the dependence of d+/kT on the reciprocal sum of the Pauling radii of the ions is presented for alkali-metal perchlorates in four solventsN . Schmelzer, J . Einfeldt and M. Grigo - 2 939 0 - - I I 1 I I *t Li + Fig. 7. Dependence of d , / k T on the reciprocal sum of the Pauling radii for alkali-metal perchlorates in: e, acetone (our measurements); 0, acetonitrile ;25 0, y-butyrolactone;23 A, sulpholane (30 0C).24 of the aprotic protophobic group: acetone, acetonitrile, sulpholane (30 "C) and y- butyrolactone. The variation of d, / k T alkali perchlorates in acetonitrile and sulpholane is qualitatively the same as fofacetone. This statement is not conclusive for the behaviour of alkali-metal perchlorates in y-butyrolactone, because the margin of error of the fitted R parameter for LiCIO, (R = 6.7 & 3.2) and KC10, (R = 7.5 f 1.8) is very large. References 1 S.Elshazly, M. Grigo and J. Einfeldt, Z . Phys. Chem. (Leipzig), 1983, 264, 1041. 2 S. Elshazly and M. Grigo, Z . Phys. Chem. (Leipzig), 1984, 265, 417. 3 D. Kremp, W. D. Kraeft and W. Ebeling, Ann. Phys. (Leipzig), 1966, 18, 246; W. Ebeling, W. D. Kraeft and D. Kremp, J . Phys. Chem., 1966,70, 3338; W. D. Kraeft and W. Ebeling, Z . Phys. Chem. (Leipzig), 1969, 240, 141; W. Ebeling and M. Grigo, Ann. Phys. (Leipzig), 1981, 37, 21. 4 S. Elshazly, M. Grigo, N. Schmelzer and J. Einfeldt, to be published. 5 I. M. Kolthoff, in Non-Aqueous Electrochemistry, ed. J. C. Marchon (Butterworth, London, 1971). 6 J. Einfeldt, N. Schmelzer, P.Grobbeker and B. Fichtelmann, Exp. Techn. Phys., 1981, 29, 413. 7 H. Falkenhagen, Theorie der Elektrolyte ( S . Hirzel Verlag, Leipzig, 1971). 8 H. Falkenhagen and W. Ebeling, in ed. S . Petrucci, Ionic Interactions (Academic Press, New York, 9 J. Einfeldt, Dissertation B (University of Rostock, 1985). 1971), vol. 1. 10 W. Ebeling, Z . Phys. Chem. (Leipzig), 1968, 238, 400. 11 M. Grigo and R. Sandig, Wiss. Z . Univ. Rostock, 1982, 31, 7. 12 J-C. Justice and M. C. Justice, Faraday Discuss. Chem. Soc., 1978, 64, 265. 13 F. Accascina and S. Shiavo, Ann. Chim. (Rome), 1953, 43, 695. 14 A. M. Shkodin and I. A. Sergeeva, Elektrochimiya, 1971, 7, 552. 15 G. Pistoia and G . Pecci, J. Phys. Chem., 1970, 74, 1450. 16 H. C. Brookes, M. C. B. Hotz and A. N. Spong, J. Chem. SOC. A , 1971, 15, 2415. 17 M. B. Reynolds and Ch. A. Kraus, J . Am. Chem. SOC., 1948, 70, 1709. 18 L. G. Savedoff, J . Am. Chem. SOC., 1966, 88, 664. 19 D. F. Evans, H. J. Thomas, J. A. Nadas and M. A. Malesich, J . Phys. Chem., 1971, 75, 1714. 20 S. Minc and L. Werblan, Roczn. Chem., 1966, 40, 1537; 1735; 1989. 21 ( a ) A. J. Parker, Q. Reu. Chem. SOC. 1962, 16, 163; (h) A. J. Parker, Chem. Rev., 1969, 69, 1. 22 V. S. Griffiths and K. S . Lawrence, J . Chem. Soc., 1955, 1208; 2797; V. S. Griffiths and M. L. Pearce, 23 M. Salomon and E. J. Plichta, Electrochim. Acta, 1984, 29, 731. 24 R. Fernandez-Prini and J. E. Prue, Trans. Faraday SOC., 1965, 62, 1257. 25 R. L. Kay, B. J. Hales and G. P. Cunningham, J. Phys. Chem., 1967,71,3926; F. Conti and G. Pistoia, J . Phys. Chem., 1968, 72, 2245; F. Accascina, H. G. Pistoia and S. Schiavo, Ric. Sci., 1966, 36, 560. J. Chem. SOC., 1957, 3243. Paper 71375; Received 27th February, 1987

ISSN:0300-9599    

DOI:10.1039/F19888400931

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chern. Soc., Furuduy Trans. I, 1988, 84(4), 941-950 Electrochemical Regeneration of NAD+ A New Evaluation of its Actual Yield Jacques Bonnefoy Laboratoire de Chimie, UA CNRS No 484, ENS Cachan, 61 Avenue Pdt Wilson, 94230 Cachan, France Jacques Moiroux" Faculte' de Pharmacie, Universite' de Picardie, 3 Place Dewailly , 80037 Amiens, France Jean-Marc Lava1 and Christian Bourdillon Laboratoire de Technologie Enzymatique, UA CNRS No 523, Universite' de Technologie de Compiggne, BP 233, 60206 Compiggne Cedex, France Owing to its actual yield being better than 99.99 YO, i.e. a coenzyme turnover number > 10000, the electrochemical oxidation of NADH must be considered as one of the best methods of regeneration of enzymatically active NAD' in terms of efficiency. Owing to its intrinsic simplicity of execution it could be very useful in the development of selective oxidations involving dehydrogenases.The sole cause of deactivation of the coenzyme is the acid-catalysed hydrolysis of NADH, a reaction not related to the method of regeneration. Enzyme-catalysed reactions that require oxidized nicotinamide cofactors (NAD', NADP+) are not widely used in preparative chemistry. Although many of these reactions are potentially valuable in synthesis, the cofactors required are expensive and cannot be used economically in stoichiometric reactions. Therefore the cofactors must be recycled, and the criteria by which the usefulness of the regeneration methods are measured are cost, rate of regeneration, turnover number achieved for NAD+ and simplicity of execution.' Turnover numbers as high as 40000 (i.e.a molecule of coenzyme can be used 40000 times in order to produce 40000 molecules of product) have been reported for enzymatic regeneration schemes2 which, however, introduce a second enzyme system (except for the so-called 'coupled-substrate ' regeneration3), additional reactants and products, and complex separation problems. The present work concerns the electrochemical regeneration of NAD+. Considering a turnover number of 10000 as a correct target, the yield of the electrochemical transformation NADH -+ NAD+ must be 99.99 YO. In a previous work4 we showed that the actual yield increases with increasing recycling frequency (rate of regeneration) proving that the deactivation of the coenzyme is only time-dependent. Therefore the deactivation is not related to the total number of redox cycles actually performed and does not result from the use of the electrochemical regeneration in itself.Our conclusion is that the observed coenzyme deactivation is due to a chemical process not related to the electrochemical reaction, and we obtained an actual yield of 99.97%. This paper describes the experimental conditions under which it is possible to obtain an effective yield of at least 99.99 YO for the electrochemical regeneration, and shows that the cause of the deactivation of the coenzyme can be identified since the sole hydrolysis of the cofactor accounts for the observed deactivation. Improving the yield from 99.97 to 99.99% is of decisive importance since it means improving the coenzyme turnover number from 3000 to 10000.94 1942 Electrochemical Regeneration of NAD+ Table 1. Definition of the symbols used in the text buffer concentration concentration of the acidic component of the buffer NADH concentration NAD' concentration current apparent rate constant for the deactivation of the coenzyme during the experiment of regeneration assimilated to a hypothetical first-order reaction first-order rate constant for the disappearance of NADH moles of enzymatically active coenzyme (NAD+ + NADH) remaining at the end of the regeneration experiment moles of D-gluconate produced at the end of the experiment moles of deactivated coenzyme as a result of the hydrolysis of NADH moles of NADH initially introduced in the reactor = n,/nj = ng/ni = number of regenerating cycles actually performed global diffusion reaction parameter [see ref.( S ) ] = (P - 1 + N , ) / P = yield per electrochemical regenerating cycle time volume of solution = c,+c, The enzymatic reaction taking place in the reactor was: P-D-glucose + NAD+ i, D-glucono-d-lactone + NADH + H+ - glucose dehydrogenase (GDW IHz0 D-gluconate + H+ Our main reason for choosing this reaction was tllat the enzymatic equilibrium is shifted toward the right-hand side at neutral pH, thus avoiding the complexity of a situation in which the reversibility of the enzymatic process should be taken into consideration. A glossary of symbols used in the text is given in table 1. Experimental Materials Enzymes and biochemicals, including gluconic acid, were purchased from Sigma and used without further purification.Glucose dehydrogenase (E. C . 1 . l . 1 .47) was from Bacillus species. All other chemicals were Prolabo (reagent grade). The reticulated non-microporous vitreous carbon (RVC) was obtained from the Fluorocarbon Company (RVC 4 type). Apparatus The NADH concentration was determined by measuring the absorbances of solutions at 340 nm ( E = 6220 dm3 mol-1 cm-l) with a Beckman Acta C 111 spectrophotometer. The electrochemical equipment consisted of a Tacussel PRT 200- 1 X potentiostat and a Tacussel IGS-N integrator. Current vs. time curves were recorded on a Sefram TGM- 164 X-Y recorder (with time-base).J. Bonnefoy, J . Moiroux, J-M. Lava1 and C. Bourdillon 943 Reactor The reactor consisted of a two-compartment water-jacketted Tacussel MCT cell thermostatted at 20 "C.The working electrode was a cylinder of RVC (diameter ca. 1.4 cm, height ca. 2 cm, effective area ca. 150 cm2) screwed to the tip of a rotating Tacussel ED1 mechanical drive system, whose precise control of rotation speed (2000 r.p.m.) was ensured by a Tacussel CONTROVIT independent servocontrol electronic amplifier. The working-electrode compartment of the cell was made vapour-leak proof and had the shape of a cylinder slightly larger than the working electrode. The counter- electrode, which was made of platinum foil (area ca. 0.25 cm2), was introduced into this compartment. The KC1-saturated calomel reference electrode (SCE) was introduced into a second compartment filled with the background solution (0.5 mol dm-3 phosphate buffer, pH 7.5, plus 0.3 mol dmP3 glucose), communicating with the working-electrode compartment through ceramics which did not allow diffusion of the coenzyme. During the experiment the controlled potential of the working electrode was held at 800 mV us.SCE. Hydrogen produced at the counter-electrode formed bubbles which stuck onto the working electrode and were regularly removed by periodically and carefully creating a small depression in the working-electrode compartment. Procedures Pretreatment of the Working Electrode After sonication the rotating electrode was put into a solution of 0.5 mol dm-3 phosphate buffer, pH 7.5, and the pretreatment consisted of applying a potential of 1.250 mV us. SCE for 2 min, followed by 1 min with no applied potential and then a potential of - 1.250 mV us.SCE for 2 min, followed again by 1 min with no applied potential. This cycle was repeated twice. Before use in the reactor the electrochemically pretreated electrode was stored for the night in a solution of 5 x mol dmP3 NAD+. The following day the electrode was rinsed thoroughly with water and dried by rotation in air at 5000 r.p.m. for 30 s, then it was ready for use. Determination of p,* The volume of background solution initially introduced into the working-electrode compartment was 5 cm3. Once the current was stabilized an aliquot of 0.1 cm3 of a stock solution of NADH was added. Then the current us. time curve was recorded until the achievement of the exhaustive electrolysis of NADH.The -In i us. t plot is almost linear at short times ( t < 20 s), the corresponding slope being^,*.^ Without pretreatment of the working electrode, p t is ca. ten times smaller. Regeneration, Determinations of ni and n, Once the exhaustive electrolysis of NADH was achieved it was possible to deduce ni from the quantity of electricity which had been used. Incidentally another determination of ni was also easy since the NADH concentration of the stock solution could be evaluated spectrophotometrically. Then as the current had reached its background value, the working-electrode potential being permanently applied, 0.5 cm3 of the solution contained in the working-electrode compartment was sucked with a syringe in order to dissolve the enzyme and were then reintroduced in the same compartment.The addition of the enzyme into the reactor provoked the beginning of the experiment of regeneration. At the end of the experiment an aliquot of 3 cm3 (ufJ of the solution contained in the working-electrode compartment was taken out in order to proceed to944 Electrochemical Regeneration of NAD+ 0 0.1 0.2 0.3 CHA/mol dm-3 Fig. 1. Dependences of the NADH hydrolysis rate constant, kobsd, on pH and concentration of the acidic component of the phosphate buffer CHA. (a) pH 7.0, (b) pH 7.5, ( c ) pH 8.0. the gluconate assay (after elimination of glucose dehydrogenase by means of ultrafiltration) and the measurements of pH, and final absorbance at 340 nm. As a result we could determine the final concentrations of gluconate and NADH. Then it was necessary to know u, to a good accuracy in order to calculate n,. The solution which still wetted the RVC cylinder was retrieved against the walls of the working-electrode compartment after rotation of the RVC cylinder at 5000 r.p.m.above the level of the solution remaining in the reactor. Then the final volume u,, retrieved in the working- electrode compartment was determined as follows : 2 cm3 of a solution containing chlorophenol red at a known concentration were added to uf2 and the measurement of the absorbance of the resulting solution at 572 nm allowed the determinations of u,, and u, since u, = u,, + uf2. Enzymatic Assays Assays for the enzyme and biochemicals were taken directly from ref. (6): glucose dehydrogenase (p. 650, the assay for glucose-6-phosphate dehydrogenase was used for glucose dehydrogenase, substituting glucose and NAD+ for glucose-6-phosphate and NADP+, respectively), NAD+ (p.2048) and gluconate (p. 1243). Results and Discussion Stability of Nicotinamide Cofactors in Solution Several of the reactions that transform NADH and NAD+ into enzymatically inactive substances have been studied : in particular, NADH undergoes generalized acid- catalysed hydration and, ultimately, transformation to a cyclic ether ;' NAD+ reacts with hydroxide ion and with other nucleophiles.' Moreover, the factors which influence the lifetimes of these intrinsically unstable species in solution have been also examined in more detail in order to develop practical protocols for using and regenerating cofact~rs.~ However, the scope of the latter work was limited to buffer concentrations smaller than 0.1 mol dm-3, i.e.a range which does not extend to the concentration values (ca. 0.5 mol dm-3) we used in the present work. The rate of disappearance of NADH was followed spectrophotometrically : the absorbance at 340 nm decreases and the absorbance at 260 nm increases with increasingJ . Bonnefoy, J. Moiroux, J-M. Lava1 and C. Bourdillon I -1 I ( a ' ) I \ * 945 10 8 6 4 E -- 4 \ 2 0 4 Fig. 2. Experiment of regeneration: i us. t Curves. The initial and final conditions are those given in table 2. The working electrode is a rotating (2000 r.p.m.) RVC cylinder (effective area : 150 cm2). (a) Experimental curve, (a') electrochemical oxidation of NADH (recorded before addition of enzyme), (b) theoretical curve taking into account the effects of electrode fouling and coenzyme hydrolysis, (c) same as (b) plus enzyme deactivation.(b) and (c) were computed according to assumptions discussed in the text. time, an isosbestic point being observed at 305 nm (E = 2920 dm3 mol-' cm-'). At fixed pH and Cb this rate obeys first-order kinetics according to the equation: -d(NADH)/dt = kobsd(NADH). The experiments were carried out at 20 "C in phosphate buffers ( c b < 0.5 rnol dm-3), the pH varying between 6 and 8. As already reported,' kobsd depends both on the pH and the concentration of the acidic component of the buffer CHAa Extrapolation at C,, = 0 gives a proportionality coefficient of (15.0f0.4) x lo3 dm3 mol-' h-' between kobsd and [H+], which is roughly twice greater than that given in the literatureg (9.4 x lo3 dm3 mol-' h).Moreover, the dependence of kobsd on C,, is not linear at a given pH, and at a given C,, > 0 kobsd is no longer a linear function of [H+] and decreases markedly with increasing pH (fig. 1). As a consequence it appears that the hydrolysis of NADH in relatively concentrated phosphate buffers is much slower than would be expected if kobsdwere a linear function of both [H+] and CHA. Such a result could not be deduced from previous Finally, we also checked that kobsd does not depend noticeably on daylight irradiation, gluconate concentration up to 0.1 mol dm-3 and the presence of glucose (0.3 mol dmP3) when the buffer is 0.5 mol dm-3 phosphate at pH 7.5, and that the rate of degradation of NAD+ can be neglected under those conditions as reported earlierg [we found kobsd = 8.4 x h-' for NAD+, NAD+ being assayed enzymatically (see the Experimental section)]. Cofactor Regeneration In order to reach the highest regenerating frequency obtainable without making radical alterations in the reactor we used for previous we chose to proceed under the following conditions : the enzyme was not immobilized and its concentration was rather large (ca.5.3 units per cm3); the working-electrode area was made as great as possible, while the volume of solution contained in the working-electrode cell compartment was made as small as possible (see the experimental section); the working- electrode potential was held at the highest possible value of 800 mV us.the KCl SCE,946 Electrochemical Regeneration of NAD+ Table 2. Data obtained in a typical regeneration experiment" tlh PHib P W enzyme concentration/ uni ts ~ r n - ~ P P, = (P/t)/cycles h-l 10' ni/molesd 10' n,/molesd Nf 4 7.5 7.1 5.3 900 k 30 225 & 7 4.4 4.0 k 0.1 0.91 f 0.03 Initial glucose concentration = 0.3 mol dm-3. " The definitions of the symbols are given in table I . Initial pH (0.5 mol dm-3 phosphate buffer). Final pH. Initial volume of the solution oi = 5.1 cm3, final volume of = 4.5 cm3. beyond which value oxidation of the background would generate large amounts of reactive oxidizing species that could deactivate the enzyme and its substrates; both the working and auxiliary electrodes were introduced into the same compartment, thus reducing markedly the internal resistance of the cell and partially neutralizing the excess of protons (see later in the text). It can be shown theoretically that these conditions correspond to the optimization of the regenerating frequency and the resulting steady- state current can be calculated easily.'' The procedures used for the determination of n, and n, (enzymatic assay) are given in the Experimental section. Knowing n,, it is possible to calculate NF and P. P can be deduced either from the assay of D-gluconate, the corresponding value being denoted P,, or from the amount of electricity, Q, which was used during the experiment (allowing for background current), the corresponding value being noted P,. According to the stoichiometry of the electrochemical reaction : NADH --+ NAD+ + H+ + 2e.P = Q/2Fni, where Q is in C , ni in moles and F is the Faraday constant. The values found for P, and P, are always in agreement with an uncertainty of < 3 %. The resulting common value will be denoted P further in the text. Under our experimental conditions the best NAD+ recycling frequency we can achieve over 4 h is an average of ca. 250 cycles h-l. A typical experiment, repeated five times, yields the set of data gathered together in table 2 and the corresponding current vs. time curve is shown in fig. 2(a). These data allow the calculation of R = 0.9999+0.0001 or 99.99k0.01 %, but does not allow a better precision since: AR = AP/P2 + ANF/P+ (N,/P2)AP = 0.0001 a result which does not completely fulfill our requirement since we want to ascertain that the yield of the electrochemical reaction by itself is at least 99.99 YO.Therefore the only way to reach that target is to prove that the deactivation of the coenzyme can be justified by the occurrence of a chemical reaction not related to the electrochemical process. Justification of the Cofactor Deactivation (l-NF) represents the amount of coenzyme deactivated during the experiment. If we assimilate this deactivation to a hypothetical reaction obeying first-order kinetics, the corresponding rate constant is kapp = (2.4k0.8) x lo-' h-l, a value which lies in the interval whose lower and upper limits are the rate constants kobsd of the hydrolysis ofJ. Bonnefoy, J. Moiroux, J-M. Lava1 and C. Bourdillon 947 ,u conate cos e + working electrode ( R V C ) auxiliary electrode (Pt) Fig.3. Schematic representation of the reactions occurring in the reactor. NADH in 0.5 mol dm-3 phosphate buffer at pH, 7.5 and pH, 7.1, (2.05-0.2) x lop2 and (4.3 0.4) x h-l, respectively. In order to estimate the amount of enzymatically active cofactor which is lost due to the hydrolysis of NADH, we need to know the concentration C, of NADH in the bulk of the reactor at any time during the experiment. This can be deduced from the intensity i since i = 2Fvp*C,, under the conditions of controlled potential electrolysis." Under our experimental conditions, owing to the complexity of the hydrodynamics resulting from the rotation of a cylinder of RVC (see Experimental section) p* is constant only when C, reaches a quasi-steady state.At the beginning of the experiment p* is greater, and a starting value of p* of 0.1 s-l is experimentally accessible when there is no catalytic coupling, as described in the Experimental section. Since p* < 0.1 s-l and v < ui = 5.1 cm3 the current us. time curve [fig. 2(a)] shows that C , is always greater than 4.8 x mol dm-3, so that at any time during the experiment the reduced form, NADH, amounts to > 55 ?h of the total quantity of coenzyme introduced initially in the reactor. As we already noticed, the rate of NADH hydrolysis at a given buffer concentration is pH-dependent. There are two causes for the observed pH decrease during the regeneration experiment. The first is inherent in the system due to the stoichiometries of the reactions involved in the process whose net balance of materials corresponds to the overall equation : glucose + gluconate + 2e + 3H+ so that the production of each molecule of gluconate is accompanied by the production of two electrons and three protons.At the same time the consumption of the two electrons at the auxiliary or counter-electrode is accompanied by the consumption of two protons and that is the reason why we chose to introduce the working and auxiliary electrodes in the same compartment (fig. 3). However, only two thirds of the protons produced anodically are neutralized cathodically . The second cause of the acidification of the solution lies in the small oxidation of the background which occurs at 800 mV and yields protons. In order to minimize the effect of these productions of protons we used a concentrated phosphate buffer whose initial pH is slightly higher than the second pK, (7.2) of phosphoric acid.The discussion related to the acidification of the solution shows that the rate of production of protons is proportional to i and, at any time, the number of protons formed since the beginning is proportional to the quantity of electricity which has circulated through the circuit. As pH, and pH, are known, the calculation of the total number of protons produced during 4 h, i.e. for a total quantity of electricity Q = 79.2 C, is easy. Then the intensity us. time curve allows the calculation of the quantities of 32 FAR 1948 Electrochemical Regeneration of NAD’ electricity delivered to the system during regular and successive time intervals (for example, every hour), and the corresponding productions of protons and resulting decreases in pH.Knowing the pH and the composition of the buffer, in particular the concentration of its acidic component, we can determine the value of kobsd at various times during the experiment. We found that kobsd is ca. 0.020, 0.024, 0.031, 0.039 and 0.043 h-l at times of 0, 1, 2, 3 and 4 h, respectively. kobsd = 0.031 h-l can be considered as a good average over the total duration (4 h) of the experiment. This yields an estimate of the number of moles of coenzyme, nh, which are deactivated in 4 h due to the hydrolysis of NADH : In [(0.55ni-nh)/0.55n] = 0.031 x 4; i.e. n h = 0.3 x lop7 mol. This value is almost certainly underestimated since the factor 0.55 (percentage of reduced form of the coenzyme) in the above equation is an underestimation, as follows clearly from the discussion concerning its determination (see above).As our purpose is the determination of the yield of the electrochemical reaction by itself the new calculation of R implies that NF should be corrected as follows: NF = (n,+nh)/ni = 0.98k0.3, so that R > 0.9999 or 99.99%. There is no intrinsic limitation to the use of the electrochemical regeneration of NAD+ in biotechnological processes. Theoretically there is the possibility for side reactions at the anode and cathode. One could imagine direct reduction of NAD+ to NADH, direct reduction or oxidation (Kolbe reaction) of gluconate or direct oxidation of glucose. These side reactions do not occur significantly under our experimental conditions because the corresponding potentials lie outside the potential interval used for this regeneration.NAD+ cannot be reduced at a platinum cathode whose potential is controlled by the discharge of protons.12 It is much the same for the electrochemical reduction of g l u c ~ n a t e , ~ ~ ~ which is not oxidizable at 800 mV.13’ Moreover, the fact that the values found for Pg and P, are in very good agreement proves that gluconate does not undergo any destruction through side processes. At 800 mV the direct oxidation of glucose does not occur ~ignificant1y.l~ Our experiments confirm this point since the current remains at the background level as long as there is no enzyme introduced in the reactor.The Shape of the Current us. Time Curve The justification of the shape of the i us. t curve [fig. 2(a)] can improve our understanding of the processes involved in the regeneration, especially with the purpose of determining the parameters which limit the current, i.e. the rate of production of gluconate. It has been established previously that the calculation of i is not cumbersome once we reach a steady state which corresponds to the equality between the rate of the electrochemical production of NAD+, whose kinetics are governed by parameter p* and the rate of its enzymatic consumption (which is controlled by the enzyme activityll). Such a steady state is obtained in a relatively short time,” < 1 h in the case of our experiments. The steady-state concentration C , of NADH in the reactor is given by the equation: where VmaX is the maximum rate of the enzymatic reaction when both NAD+ and glucose concentrations, C, and C,, respectively, are considerably higher than the Michaelis constants Km,(NAD+) and K,,(glucose) and when the product C, C, is also much greater than KmGKdO, Kdo being the dissociation constant of the enzymatic complex for NAD+.Vmax is proportional to the amount of active enzyme used in the experiment. Kl (= l+KmG/CG) may be considered to be constant under ourJ . Bonnefoy, J. Moiroux, J-M. Lava1 and C. Bourdillon 949 experimental conditions since glucose is in large excess and the effect of the change in CG occurring during the experiment can be neglected ; K2( = Kmo + Kdo KmG/CG) may also be assumed to be constant; and CT (= C,+C,) is the total concentration of coenzyme, in oxidized and reduced forms.mol dm-3), Kdo (ca. low3 mol dm-3) and KmG (ca. 7 x mol dmP3), therefore Kl x 1.0 and K2 x 1.5 x As already mentioned, p* cannot be measured at the steady state but its initial value (at t = 0) can be determined experimentally. After the regeneration experiment the initial value of p* was redetermined according to the same procedure and it appears that the pretreatment of the working electrode prevents it from fast inhibition, since the observed decrease in the initial value ofp* never exceeds 10 Yo. Therefore it is reasonable to assume that the quasi-steady-state value of p* roughly obeys a law of the type p* = p z exp (-- 0.0263t), which predicts an exponential decrease of 10 YO over 4 h for an initial value of ca.0.095 s-'. At any time t, CT is given by the equation C, = CTo exp ( -kappt) and k roughly obeys the empirical law kapp = (0.018+0.0015t) h-l. Thus it is possible to plot i us. t ( i = 2Fup C,) assuming that Vmax does not vary during the experiment. The theoretical i us. t curve [fig. 2(b)] is obtained under these circumstances. The obvious discrepancy appearing in fig. 2 between the experimental and theoretical curves [(a) and (b), respectively] implies that neither the loss of electrode activity nor the deactivation of the coenzyme can account for the experimentally observed decrease in i which develops rather markedly with increasing time and is almost linear when t > 1 h, i.e. under the quasi-steady-state conditions.Therefore a loss of activity of the enzyme must be taken into consideration. The loss of activity of the enzyme implies a decrease in Vmax with increasing time. Its occurrence was confirmed by the following experiment : when a significant new amount of enzyme was added at time t = 3 h, i was enhanced appreciably and actually reached the value given by fig. 2(b) for t = 3 h. There are two main causes for the loss of activity of the enzyme. First, the enzyme undergoes a spontaneous deactivation even at 20 oC.2d Secondly, the progressive acidification of the medium, accompanying the production of gluconate, probably affects the enzymatic activity. 15, l6 Assuming that Vmax decreases exponentially with increasing time, it is then possible to obtain a satisfactory fit between the experimental i us.t curve and the theoretical curve [fig. 2(c)]. The only noticeable discrepancy appears at short times when the system does not operate under quasi- steady-state conditions. Fig. 2(c) is obtained when the variation of Vmax obeys the equation Vmax = Vmo exp (-- 0.3248t). At the end of the experiment this equation gives a value of 0.27 for the ratio V,,,/ Vmo. Such a value implies a loss of activity of the enzyme of the order of 70 %, showing that the decrease in the enzymatic activity undoubtedly constitutes the limiting factor in the process, a conclusion identical to that drawn in studies devoted to enzymatic regeneration of NAD+.l As the main purpose of this work was the determination of the actual yield of the electrochemical regeneration of NAD+ we did not try to find out how the experimental conditions should be modified in order to minimize the loss of activity of the enzyme, especially since this enzyme was chosen only as a convenient model without any practical application in mind.For practical use in organic synthesis the whole design of the reactor must be reconsidered. The regenerating frequency must be improved once again, therefore the ratio of the active area of the working electrode over the volume of the cell must be dramatically increased and the acidification of the medium must be compensated during the process. With our very simple reactor we tried to perform as many regenerating cycles as possible, allowing the experiment to last more than 50 h.We had to refuel the reactor with enzyme several times, but this cannot be repeated indefinitely. Besides we Values can be found in the l i t e r a t ~ r e ' ~ . ~ ~ for Kmo (ca. 1.5 x mol dm-3. 32-2950 Electrochemical Regeneration of NAD+ did not neutralize the protons produced. As a consequence, the completion of the experiment was dictated by the exhaustion of the coenzyme. However, even under those poorly adjusted conditions, we obtained actual coenzyme turnover numbers greater than 7000. Conclusion Owing to its actual yield being better than 99.99%, i.e. a coenzyme turnover number > 10000, the electrochemical regeneration of NAD+ must be considered as one of the best methods of regeneration of this cofactor in terms of efficiency. Having established such a result should greatly enhance the standing of this method since its potentialities have been misjudged, hastily ignored and rejected too many times in recent years.We think that it deserves much more serious consideration. The sole cause of deactivation of the coenzyme observed in the present work is the acid-catalysed hydrolysis of its reduced form, NADH, a reaction whose occurrence is not related to the method of regeneration. With respect to the criteria concerning the usefulness of a regeneration method which have been listed recently in the literature1 it appears that the electrochemical regeneration offers very good opportunities : a very satisfactory coenzyme turnover number, simplicity of execution since there is no need of a second enzymatic system, reasonable rate of regeneration of ca.200 cycles h-’ which can surely be improved markedly and, unlike many organic oxidants of potential use in enzyme-catalysed oxidations, the electrode is still stable at the highest pH values required for maximum activity of the enzymatic cat a1 y st s. We acknowledge support of this work by the C.N.R.S. (Unites Associees 523 ‘ Laboratoire de Technologie Enzymatique ’ and 484 ‘ Synthese et Electrochimie des Compost% d’Intiret Pharmacologique ’), especially for the grant for one of us (J. B.). References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 L. G. Lee and G. M. Whitesides, J. Am. Chem. SOC., 1985, 107, 6999. D. J. Fink and V. W. Rodwell, Biotechnol. Bioeng., 1975, 17, 1029; (6) W. H. Baricos, R. P. Chambers and W. Cohen, Anal. Lett., 1976, 9, 257; (c) R. Wichmann, C . Wandrey, A. F. Buchmann and M. R. Kula, Biotechnol. Bioeng., 1981, 23, 2789; (d) C. H. Wong, D. G. Drueckhammer and H. M. Sweers, J. Am. Chem. SOC., 1985, 107, 4028. M. 0. Mansson, P. 0. Larsson and K. Mosbach, FEBS Lett., 1979, 98, 309. J. M. Laval, C. Bourdillon and J. Moiroux, Biotechnol. Bioeng., 1987, 30, 157. A. J. Bard and K. S. Santhanam, in Electroanalytical Chemistry, ed. A. J. Bard (Marcel Dekker, New York, 1970), vol. 4, p. 215. H. U. Bergmeyer, Methods of Enzymatic Analysis (Academic Press, New York, 2nd edn, 1974). (a) S. L. Johnson and P. T. Tuazon, Biochemistry, 1977, 16, 1175; (b) N. J. Oppenheimer and N. 0. Kaplan, Biochemistry, 1974, 13, 4675. S. L. Johnson and D. L. Morrison, Biochemistry, 1970, 9, 1460. C. H. Wong and G. M. Whitesides, J. Am. Chem. SOC., 1981, 103, 4890. J. M. Laval, C. Bourdillon and J. Moiroux, J . Am. Chem. SOC., 1984, 106, 4701. C. Bourdillon, J. M. Laval and D. Thomas, J . Electrochem. SOC., 1986, 133, 706. P. J. Elving, W. T. Bresnaham, J. Moiroux and Z. Samec, Bioelectrochem. Bioenerg., 1982, 9, 365. L. Eberson, in Organic Electrochemistry, ed. M. M. Baizer (Marcel Dekker, New York, 1973), (a) p. 414; (b) p. 470. E. Skou, Electrochim. Acta, 1977, 22, 313. H. E. Pauly and G. Pfleiderer, 2. Physiol. Chem., 1975, 356, 1613. J. A. Bach and H. L. Sadoff, J. Bacteriol., 1962, 83, 699. Paper 71436; Received 9th March, 1987

ISSN:0300-9599    

DOI:10.1039/F19888400941

出版商:RSC    

年代:1988

数据来源: RSC