摘要:

Contents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes. A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M.Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. PlonkaContents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes.A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M. Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. Plonka

ISSN:0300-9599    

DOI:10.1039/F198783FX005

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

Electrochemistry Group Workshop on Electrochemical Techniques and Instruments To be held at the University of Warwick on 6-7 January 1988 Further information from Dr P. N. Bartlett, Department of Chemistry, University of Warwick, Coventry CV4 7AL Surface Reactivity and Catalysis Group with the Process Technology Group and the Institute of Chemical Engineers Opportunities for Innovation in the Application of Catalysis To be held at Queen Mary College, London on 6-7 January 1988 Further information from Professor J. Pritchard, Queen Mary College, London Division with the Institute of Mathematics and its Applications Mathematical Modelling of Semiconductor Devices and Processes To be held at the University of Loughborough on 7-8 January 1988 Further information from the Institute of Mathematics, Maitland House, Warrior Square, Southend-on-Sea SS1 2JY Division London Symposium: Modern Electrochemical Systems To be held at Imperial College, London on 12 January 1988 Further information from Mrs Y.A. Fish, Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group with the 3Ps Group Plastics, Packaging and Printing To be held at the Institute of Physics, 47 Belgrave Square, London on 18 February 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex l w 1 1 OLW Theoretical Chemistry Group Postgraduate Students’ Meeting To be held at University College, London on 2 March 1988 Further information from Dr G. Doggett, Department of Chemistry, University of York, York Colloid and Interface Science Group with The Society of Chemical Industry and British Radio frequency Spectroscopy Group Spectroscopy in Colloid Science To be held at the University of Bristol on 5-7 April 1988 Further information from Dr R. Buscall, ICI Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 40E Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-15 April 1988 Further information from Dr J.F. Gibson, Royal Society of Chemistry, Burlington House, London W1V OBN 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 GLI 3 9BP 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 302 (xiii)Electrochemistry Group Workshop on Electrochemical Techniques and Instruments To be held at the University of Warwick on 6-7 January 1988 Further information from Dr P.N. Bartlett, Department of Chemistry, University of Warwick, Coventry CV4 7AL Surface Reactivity and Catalysis Group with the Process Technology Group and the Institute of Chemical Engineers Opportunities for Innovation in the Application of Catalysis To be held at Queen Mary College, London on 6-7 January 1988 Further information from Professor J. Pritchard, Queen Mary College, London Division with the Institute of Mathematics and its Applications Mathematical Modelling of Semiconductor Devices and Processes To be held at the University of Loughborough on 7-8 January 1988 Further information from the Institute of Mathematics, Maitland House, Warrior Square, Southend-on-Sea SS1 2JY Division London Symposium: Modern Electrochemical Systems To be held at Imperial College, London on 12 January 1988 Further information from Mrs Y.A. Fish, Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group with the 3Ps Group Plastics, Packaging and Printing To be held at the Institute of Physics, 47 Belgrave Square, London on 18 February 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex l w 1 1 OLW Theoretical Chemistry Group Postgraduate Students’ Meeting To be held at University College, London on 2 March 1988 Further information from Dr G. Doggett, Department of Chemistry, University of York, York Colloid and Interface Science Group with The Society of Chemical Industry and British Radio frequency Spectroscopy Group Spectroscopy in Colloid Science To be held at the University of Bristol on 5-7 April 1988 Further information from Dr R.Buscall, ICI Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 40E Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-15 April 1988 Further information from Dr J. F. Gibson, Royal Society of Chemistry, Burlington House, London W1V OBN 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 GLI 3 9BP 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 302 (xiii)

ISSN:0300-9599    

DOI:10.1039/F198783BX007

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 83(2) 231-569 (1 987) 23 1 245 257 267 27 1 28 1 289 299 .?23 334 351 37 1 383 40 1 41 1 425 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS Site-transfer Conductivity in Solid Iron Hexacyanoferrates by Dielectric Relaxometry, Voltammetry and Spectroscopy. Prussian Blue, Congeners and Mixtures D. R. Rosseinsky, J. S. Tonge, J. Berthelot and J. F. Cassidy Electron-transfer Rates by Dielectric Relaxometry and the Direct-current Conductivities of Solid Homonuclear and Heteronuclear Mixed-valence Metal Cyanometallates and of the Methylene Blue-Iron Dithiolate Adduct D. R. Rosseinsky and J. S. Tonge Polarographic Evidence for the Interaction of Reduced Nitroimidazole Deriv- atives with DNA Bases P. J.Declerck and C. J. De Ranter Influence of Mixed Cerium@) Sulphate and Ferroin Catalysts on the Oscilla- tory Redox Reaction between Maloic Acid and Bromate F. D'Alba and S. Di Lorenzo Electrocatalysis under Temkin Adsorption Conditions A. Saraby-Reintjes Mobilities and Molar Volumes of Multicharged Cations in N,N-Dimethyl- formamide at 25 "C W. Grzybkowski and M. Pilarczyk Local Structure of Nickel Oxide Grown at High Temperatures in Ceramic Electrolyte Cells M. Tomellini, D. Gozzi, A. Bianconi and I. Davoli Mechanism of Oxygen Reactions at Porous Oxide Electrodes. Part 1 .-Oxygen Evolution at RuO, and Ru,Snl_,O2 Electrodes in Alkaline Solution under Vigorous Electrolysis Conditions M. E. G. Lyons and L. D. Burke Metal-organic Chemical Vapour Deposition (MOCVD) of Compound Semi- conductors.Part 2.-Preparation of ZnSe Epitaxial Layers on (100) Orientated GaAs Single-crystalline Substrates G. Fan and J. 0. Williams The Thermodynamics of Solvation of Ions. Part 2.-The Enthalpy of Hydration at 298.15 K Y. Marcus The Identification and Characterisation of Mixed Oxidation States at Oxidised Titanium Surfaces by Analysis of X-Ray Photoelectron Spectra A. F. Carley, k'' R. Chalker, J. C. Riviere and M. W. Roberts Khetics of Metal Oxide Dissolution. Oxidative Dissolution of Chromium from Mix .d Nickel-Iron-Chromium Oxides by Permanganate A. B. O'Brien, M. G. Segal and W. J. Williams Radical Cations of Trialkylphosphine Oxides, Trialkylphosphates, Hexa- methylphosphoramide, Dimethyl Sulphoxide and various Sulphones, Sulphites and Sulphates. An Electron Spin Resonance and Radiation Chemical Study M.C. R. Symons and R. Janes Radiation Damage in a Phosphated Sugar. An Electron Spin Resonance Study of Phosphorus-centred Radicals Trapped in an X-Irradiated Single Crystal of a Phenoxyphosphoryl Xylofuranose Derivative A. Celalyan- Berthier, T. Berclaz and M. Geoffroy Exchange of Oxygen Isotopes between Carbon Dioxide and Ion-exchanged Zeolites A T. Takaishi and A. Endoh Estimations of Stability Constants by Potentiometry of some Lanthanum and Erbium Dicarboxylates at Constant Ionic Strength C. B. Monk 9 F A R 143 1 439 45 1 463 477 487 495 51 1 517 527 535 547 559 Contents Solvent-exchange Reactions of Metal Ions. Diagnosis of Mechanisms in Terms of the,Bond Order of the Activated Complexes S.J. Formosinho Ionic Solvation in Water-Cosolvent Mixtures. Part 13.-Free Energies of Transfer of Single Ions from Water into Water-Tetrahydrofuran Mixtures I. M. Sidahmed and C. F. Wells 129Xe Nuclear Magnetic Resonance Study of Xenon adsorbed on Zeolite NaY Exchanged with Alkali-metal and Alkaline-earth Cations T. Ito and J. Fraissard CH Bond Activation and Radical-Surface Reactions for Propylene and Methane over a-Bi,O, S. P. Mehandru, A. B. Anderson and J. F. Brazdil Adsorption and Dissociation of Ammonia on the Hydroxylated Surface of Magnesium Oxide Powders S. Coluccia, S. Lavagnino and L. Marchese Incorporation and Stability of Iron in Molecular-sieve Structures. Ferrisilicate Analogues of Zeolite ZSM-5 R. Szostak, V. Nair and T. L. Thomas Local Polarity of Solvent Mixtures in the Field of Electronically Excited Molecules and Exciplexes P.Suppan Aluminium Distribution in the Bulk and on the Surface of Y Zeolites Dealu- minated with SiCl, Vapour. Influence of Conditions of Dealumination L. Kubelkova, L. Dudikovi, Z. Bastl, G. Borbely and H. K. Beyer A lH and 13C Nuclear Magnetic Resonance Study of the Conformation of Aerosol OT in Water and Hydrocarbon Solutions F. Heatley Study of the Conformational Equilibria between Rotational Isomers using Ultrasonic Spectroscopy. Part 1 .-1 -Chloro-2-methylpropane and 1 -Bromo- 2-methylpropane H. Nomura, S. Koda and K. Hamada Catalytic Properties of Synthetic Faujasites Modified with Fluoride Anions K. A. Becker and S. Kowalak Concentration Dependence of Spin Friction Coefficients in Suspensions of Parallel Cylinders and Spheres J. H. Masliyah and T. G. M. van de Ven Kinetics of Reaction with Hydroxide Ions and Solubilities of Iron@) Complex Cations in Aqueous Urea Solutions. Derivation of Transfer Chemical Potentials for Initial and Transition States M. J. Blandamer, J. Burgess, A. W. Hakin, P. Guardado, S. Nuttall and S. Radulovic

ISSN:0300-9599    

DOI:10.1039/F198783FP021

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions II, Issue 2,1987 Physical Chemistry in Condensed Phases 347 355 37 1 387 403 41 1 417 43 5 449 46 1 For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions 11, Issue 2, is reproduced below. Exchange Interaction in Linear Trimeric Copper(I1) Complexes with Ferromag- netic and Antiferromagnetic Ground States S. Gehring, H. Astheimer and W. Hasse Dielectric Properties of Aqueous Glycerol and a Model relating these to the Properties of Water J. B. Bateman and C. Gabriel The Orientation Order of Solutions of a Dye Molecule in Liquid-crystalline Solvents J. W. Emsley, G. R. Luckhurst, G. N. Shilstone and I. Sage Effective Collision Cross-sections for the Thermal Conductivity of a Polyatomic- Monoatomic Binary Gas Mixture F.R. W. McCourt and W-K. Liu The Stability of the Carbon Tetrahalide Ions Y. J. Kime, D. C. Driscoll and P. A. Dowben Molecular Emission in the Electron-impact Excitation of Sulphur Dioxide C. A. F. Johnson, S. D. Kelly and J. E. Parker High-resolution Studies of the Electronic Spectra of H,S and D,S C. A. Mayhew, J-P. Connerade, M. A. Baig, M. N. R. Ashfold, J. M. Bayley, R. N. Dixon and J. D. Prince Studies of Methylene Chemistry by Pulsed Laser-induced Decomposition of Ketene. Part 3 .-Recent Mechanistic Developments R. Becerra, C. E. Canosa- Mas, H. M. Frey and R. Walsh Bond Formation in Momentum Space D. L. Cooper and N. L. Allan Reviews of Books J. H. Carpenter; J. M. Hollas The following papers were accepted for publication in J.Chem. SOC., Faraday Trans- actions I during November. 6/695 61873 61 1303 61 1440 611518 611519 Metachromasy in Clay Minerals. Sorption of Pyronin Y by Montmorillonite and Laponite 2. Grauer, G. L. Grauer, D. Avnir and S. Yariv Photogeneration of Hydrogen from Water over a Alumina-supported ZnS-CdS Catalyst J. Kobayashi, K. Kitaguchi, H. Tanaka, H. Tsuiki and A. Ueno Kinetics of Pyrrole Polymerisation in Aqueous Iron Chloride Solution R. B. Bjorklund Study of the Electronic Structure of UBr, using X-ray Photoelectron Spectro- scopy G. C. Allen and J. W. Tyler The Nature of Supported-molybdena Catalysts : Evidence from Pyridine Ad- sorption H. M. Ismail, C. R. Theocharis, D. N. Waters, M. I. Zaki and R. B. Fahim Fluorescence of Ethidium in Alcohol-Water Solutions G.Baldini, G. Varani and M. Manfredi6/ 672 Measurements of Tracer-diffusion Coefficients of Lithium Ions, Chloride Ions and Water in Aqueous Lithium Chloride Solutions K. Tanaka and M. Nomura 6/ 779 Volume and Compressibility Changes in Mixed Aqueous Solutions of Electro- lytes at 25 "C K. Patil and G. Mehta 6/ 803 The Effect of Hydrogen Sulphide on the Adsorption and Thermal Desorption of Carbon Monoxide over Rhodium Catalysts S. D. Jackson, B. J. Brandretch and D. Winstanley 6/ 1852 Ultrasonic Relaxation Studies associated with the Interactions of Acetic Acid with Some Polyvinyl Pyridines in Aqueous Solution M. Stuckey, T. Akashen and E. Wyn-Jones 6/ 1947 Enthalpies of Transfer of Tetra-alkylammonium Halides from Water to Water- Propan-1-01 Mixtures at 25 "C G. Carthy, D.Feakins and W. E. Waghorne 6/2013 Methyl Orange as a Probe of the Semiconductor-Electrolyte Interfaces in CdS Suspensions A. Mills and G. Williams (ii)Cumulative Author Index 1987 Agnel, J-P. L., 225 Alberti, A., 91 Anderson, A. B., 463 Antholine, W. E., 151 Atherton, N. M., 37 Axelsen, V., 107 Barratt, M. D., 135 Basosi, R., 151 Bastl, Z., 51 1 Battesti, C. M., 225 Becker, K. A., 535 Berclaz, T., 401 Berleur, F., 177 Berthelot, J., 23 1 Beyer, H. K., 51 1 Bianconi, A., 289 Blandamer, M. J., 559 BorbCly, G., 51 1 Braquet, P., 177 Brazdil, J. F., 463 Bruce, J. M., 85 Brustolon, M., 69 Budil, D. E., 13 Burgess, J., 559 Burke, L. D., 299 Carley, A. F., 351 Cassidy, J. F., 231 Celalyan-Berthier, A., 401 Chalker, P. R., 351 Coluccia, S., 477 Corvaja, C., 57 Couillard, C., 125 Crossland, W.A., 37 D’Alba, F., 267 Davoli, I., 289 De Doncker, J., 125 De Laet, M., 125 De Ranter, C. J., 257 Declerck, P. J., 257 Delahanty, J. N., 135 1)i Lorenzo, S., 267 Dodd, N. J. F., 85 Ducret, F., 141 Ihdikovb, L., 51 1 Dusaucy, A-C., 125 €mpis, J. M. A., 43 Endoh, A., 41 1 Evans, J. C., 43, 135 Fan, G., 323 Fatome, M., 177 Flint, N. J., 167 Formosinho, S. J., 431 Forrester, A. R., 21 1 Fraissard, J., 451 Geoffroy, M., 401 Gilbert, B. C., 77 Gozzi, D., 289 Grampp, G., 161 Greci, L., 69 Grossi, L., 77 Grzybkowski, W., 28 1 Guardado, P., 559 Hakin, A. W., 559 Halpern, A., 219 Hamada, K., 527 Harrer, W., 161 Heatley, F., 517 Hemminga, M. A., 203 Herold, B. J., 43 Hudson, A., 91 Ito, T., 451 Jaenicke, W., 161 Janes, R., 383 Kerr, C W., 85 Koda, S., 527 Korth, H-G., 95 Kowalak, S., 535 Kubelkova, L., 511 Lambelet, P., 141 Lavagnino, S., 477 Lecomte, C., 177 Lin, C.P., 13 Loliger, J., 141 Lyons, M. E. G., 299 Makela, R., 51 Maniero, A. L., 57, 69 Marchese, L., 477 Marcus, Y., 339 Masliyah, J. H., 547 McLauchlan, K. A., 29 Mehandru, S. P., 463 Monk, C. B., 425 Nair, V., 487 Nomura, H., 527 Norris, J. R., 13 Nuttall, S., 559 O’Brien, A. B., 371 Parry, D. J., 77 Pedersen, J. A., 107 Pedulli, G. F., 91 Pilarczyk, M., 281 Pogni, R., 151 Priolisi O., 57 Purushotham, V., 21 1 Radulovic, S., 559 Raffi, J. J., 225 Riviere, J. C., 351 Roberts, M. W., 351 Roman, V., 177 RomBo, M. J., 43 Rosseinsky, D. R., 231, 245 Rowlands, C. C., 43, 135 Saraby-Reintjes, A., 271 Saucy, F., 141 Savoy, M-C., 141 Segal, M.G., 371 Segre, U., 69 Sidahmed, I. M., 439 Steenken, S., 113 Stevens, D. G., 29 Suppan, P., 495 Sustmann, R., 95 Swartz, H. M., 191 Symons, M. C. R., 1, 383 Szostak, R., 487 Tabner, B. J., 167 Takaishi, T., 41 1 Thiery, C. L., 225 Thomas, T. L., 487 Tilquin, B., 125 Tomellini, M., 289 Tonge, J. S., 231, 245 Trabalzini, L., 151 Vachon, A., 177 van de Ven, T. G. M., 547 Vincent, P. B., 225 Vuolle, M., 51 Wells, C. F., 439 Williams, J. O., 323 Williams, W. J., 371 (iii)NOMENCLATURE A N D 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 Chemistry in all its publications. Their basis is the ’ Systhme International d‘UnitBs’ (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 Ph ysicochemical Quantities and Units (Pergamon, Oxford, 1 979). 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, 8, C, D, E, F, and H ( Pergamon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butterworths, London, 1971, now publis- hed by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society’s editorial staff.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No.83 Brownian Motion University of Cambridge, 7-9 April 1987 Organising Committee: Dr M. La1 (Chairman) Dr R. Ball Dr E. Dickinson Dr J. S. Higgins Dr P. N. Pusey Dr D. A. Young Mrs Y. A. Fish The Faraday Discussion on Brownian Motion will be introduced by Professor J. M. Deutch of MIT and will include contributions from P. Mazur, P. Meakin, R. Jullien, D. A. Weitz, M. Fixman, P. N. Pusey, R. H. Ottewill, A. Vrij, J. A. McCammon, 6. A. Ackerson and V. Degiorgio dealing with hydrodynamics, fractals, Brownian dynamics of aggregation processes and photon correlation spectroscopy. There will be a poster session for which contributions are invited in the form of a brief abstract to be sent by 31 January 1987 to: Dr M.Lal, Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JW. The final programme and application form 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. 84 Dynamics of Elementary Gas-phase Reactions University of Birmingham, 14-16 September 1987 Organising Committee: Professor R. Grice (Chairman) Dr M. S. Child Dr J. N. L. Connor Dr M. J. Pilling Professor I. W. M. Smith Professor J. P. Simons The Discussion will focus on the development of experimental and theoretical approaches to the detailed description of elementary gas-phase reaction dynamics. Studies of reactions at high collision energy, state-to-state kinetics, non-adiabatic processes and thermal energy reactions will be included.Emphasis will be placed on systems exhibiting kinetic and dynamical behaviour which can be related to the structure of the reaction potential- energy surface or surfaces. The preliminary programme may be obtained from: Mrs. Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 23 Molecular Vibrations University of Reading, 15-16 December 1987 Organising Committee: Professor I. M. Mills (Chairman) Dr J. E. Baggott Professor A. D. Buckingham Dr M. S. Child Dr N. C. Handy Dr 6. J. Howard The Symposium will focus on recent advances in our understanding of the vibrations of polyatomic molecules.The topics to be discussed will include force field determinations by both ab initio and experimental methods, anharmonic effects in overtone spectroscopy, local modes and anharmonic resonances, intramolecular vibuational relaxation, and the frontier with molecular dynamics and reaction kinetics. 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. 8 5 Solvation University of Durham, 28-30 March 1988 Organising Committee: Professor M. C. R. Symons (Chairman) Professor J. S. Rowlinson Professor A. K. Covington Dr I. R. McDonald The purpose of the Discussion is to compare solvation of ionic and non-ionicspecies in the gas phase and in matrices with corresponding solvation in the bulk liquid phase.The aim will be to confront theory with experiment and to consider the application of these concepts to relaxation and solvolytic processes. Contributions for consideration by the organising Committee are invited in the following areas: (a) Gas phase non-ionic clusters (b) Liquid phase non-ionic clusters (c) Gas phase ionic clusters (d) Liquid phase ionic solutions (e) Dynamic processes including solvolysis Abstracts of about 300 words should be sent by 31 May 1987 to: Professor M. C. R. Symons, Department of Chemistry, The University, Leicester LE1 7RH. Dr J. Yarwood Dr A. D. Pethybridge Professor W. A. P. Luck Dr D. A. YoungJOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistrykhemical physics which have appeared recently in J.Chem. Research, The Royal Society of Chemistry’s synopsis + microform journal, include the following: Formation of Methyl Radicals from Radical-cations of Lead Tetra-acetate and Thallium Acetates Dynamic N.M.R. investigation of Rotation about Alkyl-Olefin Bonds. Barriers and Populations for Some Substituted lsopropylethylenes J. Edgar Anderson, Bernt Bettels, Parviz Gharagozloo, and Kam-Hang Koon (1 986, Issue 1 1 ) Optical Study of the ’Face-to-Face’ Complexation of Water-soluble Metalloporphyrins and Metallophthalocyanines Stanislaw Radzki, Serge Gaspard, and Charles Giannotti (1986, Issue 10) Harish Chandra and Martyn C. R. Symons (1986, Issue 11 ) Crystallographic and Physicochemical Properties of T - Electron Systems, Part 10.Ab initio Tadeusz Marek Krygowski Jehan A. Baban, Brian STO-3G Interpretation of Hammett Substituent Constants and Gunter Hafelinger (1 986, lssue 9) P. Roberts, and Alice C. H. Tsang (1 986, Issue 9) in Mixtures of Water and Ethanol E.s.r. Studies of N-AI kyl-N-dial kyl borylaminyl Radicals in Solution Hydrophobic Interactions of Gaseous Hydrocarbons derived from Studies of their Solubilities Robert W. Cargill and Donald E. MacPhee (1986,lssue8) (vii)Electrochemistry Group Spring Informal Meeting To be held at the University of Bristol on 1-3 April 1987 Further information from Dr A. R. Hillman, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 ITS Polar Solids Group Ceramic Sensors To be held at the University of Keele on 6-7 April 1987 Further information from Professor C.R. A. Catlow, Department of Chemistry, University of Keele, Staffordshire ST5 5BG Colloid and Interface Science Group Rheology of Dispersions and Suspensions To be held at the University of Bath on 9-10 April 1987 Further information from Dr R. Aveyard, Department of Chemistry, University of Hull, Hull HU6 7RX Division - Annual Congress The Chemistry and Physics of Intercalation To be held at University College, Swansea on 13-16April 1987 Further information from Professor J. H. Purnell, Department of Chemistry, University College, Singleton Park, Swansea SA2 8PP Polymer Ph ysics Group Fundamental Aspects of Polymer Flammability To be held a t Baden Powell House, London on 14-15 April 1987 Further information from Dr G.C. Stevens, CERL, Kelvin Avenue, Leatherhead KT22 7SE Division Full-day Endowed Lecture Symposium on Intramolecular Dynamics and Chemical Reactivity including the Centenary Lecture by S. A. Rice and the Tilden Lecture by M. S. Child To be held at the Scientific Societies Lecture Theatre, London on 6 May 1987 Further information from Mrs Y. A. 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Richardson, Division of Materials Applications, National Physical Laboratory, Queens Road, Teddington, Middlesex TW11 OLW Division Autumn Meeting Spectroscopy of Gas-phase Molecular Ions and Clusters To be held at the University of Nottingham on 22-24 September 1987 Further information from Professor J.P. Simons, Department of Chemistry, University of Nottingham, Nottingham NG7 2RD Polymer Physics Group with the Institute of Marine Engineers Polymers in a Marine Environment To be held in London on 14-16 October 1987 Dr G. J. Lake, MRPRA, Brickendonbury, Herts SG13 8NLIndustrial Physical Chemistry Group The Physical Chemistry of Small Carbohydrates (as part of the International Symposium on Solute-Solute-Solvent Interactions) To be held at the University of Regensburg, West Germany on 10-14 August 1987 Further information from Dr F. Franks, Pafra Ltd, 150 Science Park, Milton Road, Cambridge CB4 4GG Industrial Physical Chemistry Group The Interaction of Biologically Active Molecules and Membranes To be held at Girton College, Cambridge on 8-10 September 1987 Further information from Dr T.G. Ryan, ICI New Science Group, PO Box 11, The Heath, Runcorn WA7 4QE Polymer Physics Group Biennial Meeting To be held at University of Reading on 9-1 1 September 1987 Further information from Dr D. Bassett, Department of Physics, University of Reading, Reading RG7 2AD Neutron Scattering Group Applications of Neutron and X-Ray Optics To be held at the University of Oxford on 14-15 September 1987 Further information from Dr R. K. Thomas, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ Colloid and Interface Science Group Polydispersity in Colloid Science To be held at the University of Nottingham on 15-16 September 1987 Further information from Dr. R. Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 4QE ~~~~~~ ~~~~~~~~ ~ ~ Polymer Physics Group New Materials To be held at the University of Warwick on 22-25 September 1987 Further information from Dr M. J. Richardson, Division of Materials Applications, National Physical Laboratory, Queens Road, Teddington, Middlesex TW11 OLW Division Autumn Meeting Spectroscopy of Gas-phase Molecular Ions and Clusters To be held at the University of Nottingham on 22-24 September 1987 Further information from Professor J. P. Simons, Department of Chemistry, University of Nottingham, Nottingham NG7 2RD Polymer Physics Group with the Institute of Marine Engineers Polymers in a Marine Environment To be held in London on 14-16 October 1987 Dr G. J. Lake, MRPRA, Brickendonbury, Herts SG13 8NL

ISSN:0300-9599    

DOI:10.1039/F198783BP023

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1987,83, 231-243 Site-transfer Conductivity in Solid Iron Hexacyanoferrates by Dielectric Relaxometry, Voltammetry and Spectroscopy Prussian Blue, Congeners and Mixtures David R. Rosseinsky" and James S. Tonge and (in part) Joelle Berthelott and Joseph F. Cassidy Department of Chemistry, The University, Exeter EX4 4QD Charge-transfer processes in iron(II1) hexacyanoferrate(II), i.e. Prussian Blue (PB), have been studied by conductimetry, dielectric relaxometry (DR) and spectrophotometry. In dry PB the predominant process is sitewise electron transfer, from agreement in inferred electron-transfer frequencies, v. When wet or admixed with salts allowing ionic conduction, the latter dominates, with evident accompanying redox processes at the electrodes.The reduced and oxidised congeners of PB have higher conductivities arising from more complex mechanisms. The prediction of the d.c. conductivity of PB from optical charge-transfer parameters is reasonably successful. The origins of dielectric relaxation processes in mixed-valence solids can be manifold. Electronic polarisability being out of the accessible frequency range, and true-dipole or bound-ion motions being summarily excluded,l there remain Poole-Frenkel (field) or Schottky (contact) effects, ionic conductivity and redox processes at the electrodes as the main alternatives to the bulk electronic site-transfer process.172 If the bulk d.c. conductivity is ionic in mechanism then it must be accompanied by electron-transfer reactions at the electrodes, i.e.the observable generation of oxidised and reduced species at the respective electrode contacts; the rate of the bulk (ionic) mechanism could govern the rate of the contact process, or vice versa. If the contact process dominates, then non-ohmic d.c. responses will in general be observed, accompanying the appearance of electrode-reaction products. The absence of such products, and the observation of ohmic behaviour, indicate, first, the absence of the ionic conductivity mechanism, and, secondly, in solids of appropriate structure, electron transfer as the probable relaxation process. With both mechanisms, ionic or electronic, equally possible, fine structural composition modifications will militate in favour of one or the other, and examples of such shifts of behaviour will clearly be of interest.When the evidence indicates a dominant electronic conductivity, dielectric relaxometry then provides a means of measuring site-to-site electron-transfer rates in suitable solids since, at a formal level, the net polarisation effect of such an electron transfer simulates that of a dipole oriented from rand0mness.l This interpretation, of studies on both organic donor-acceptor adducts2 and on inorganic mixed-valents37 such as K,(MnO,) (MnOi-) or EU,S,,~ is clearly substantiated by the establishment of an impressive equality of rate in the solid4 K,(MnO,), and in solution (MnO;+MnOi-), both in frequency v and activation energy (the difference in milieux being of little consequence, from Marcusian arguments4). Furthermore, for Eu,S4 the activation energy of the rate measured by high-frequency dielectric relaxometry (i.e.time-domain reflec- tometry) matches that for electron transfer from Mossbauer spectra6 and the d.c. conductivity o . ~ That this should be so comes from the established1T2 relationship t Present address : Department of Chemistry, UniversitC de Rennes, Rennes, France. 23 1 9-2232 Site-transfer Conductivity in Solid Iron Hexacyanoferrates between conductivity rs and the transfer or hop frequency v long accepted for ionic conductance but, for curious only recently a~ceptedl-~ for electronic hopping conductance : rs = ne2a2v/6kT where a is the hop distance and n the number density of transfer sites (i.e. of redox, or donor-acceptor, pairs). Clearly the activation energy for v, if substantial, will approxi- mate that for the d.c.conductivity, rs. The combination of activation-energy identities, the predictability of rs values observed in d.c. measurements from v values obtained from dielectric relaxometry, and the variety of materials for which such identities hold, provide substantial support for the mechanistic a~signments.l-~ One question arises in the treatment of the relaxometry data, in that the usual Cole-Cole arc1 can depend in curvature on the value of rs used in calculating the imaginary permittivity E”. This is rarely critical, although at some temperatures it may sometimes be possible to adjust rs within its experimental uncertainty, to effect improved agreement with eqn (1). Provided any adjustment is made to improve arc circularity2 rather than force such agreement, and especially if there is accessible another temperature where the intrinsic dielectric parameters of the material leave the inferred value of v insensitive to the value of o used in data processing, the Cole-Cole arc analysis is quite validated and free of self-fulfilling prediction. (To find a rs value giving best circularity, then confirming that it falls within the experimental limits of direct measurement, is also permissible.) Thus Cole-Cole analyses rather than2 E” us.log(frequency) have been preferred1* 3* as providing an additional fitting criterion, that of arc circularity, although in principle the inferences are identical. Sitewise electron-transfer frequencies in mixed-valents form an important* part of the large corpus of electron-transfer ratesg commonly measured between isolated centres in solution. Hushlo established a fundamental model employing harmonic potential-energy curves which related the (dark or ‘ thermal ’) electron-transfer frequency to the spectroscopic parameters attending the equivalent photoeffected (‘ optical ’) electron transfer governed by Franck-Condon restrictions.Meyerll has used an elaboration of this theory to directly calculate v for the thermal process from solely charge-transfer spectroscopy, and this has been applied to thin-film Prussian Blue, a mixed-valence paradigm, where approximate agreement between directly measured optical and thermal (from a) parameters was found.12 An outline of the theory, not readily accessible, is given in the Appendix.It is clearly of interest to extend experimental studies from thin-film configurations to bulk material. Dielectric relaxometry on Prussian Blue (PB) is an appropriate tool to study further the charge transfer process, the optical aspect of which still ensures its pigmentary use after 272 years. The extreme insolubility makes for difficulty in obtaining exactly reproduced compositions; thus three separate micro-crystals, grown by only moderately differing techniques, yielded three distinct X-ray diffraction However, it is reasonably clear that the same Fe3+-Fe*I charge transfer chromophore, in a cubic lattice, is involved throughout. The two extremes of composition are KFe3+FeIT(CN), * 5H20, so-called soluble Prussian Blue, on the one hand, and (Fe3+),(Fe11(CN),>, * 15H20, insoluble Prussian Blue on the other.Extensive electro- chemical studies14-17 suggest that in the electrodeposition of PB films the insoluble form is first depositc:d, which on exposure to potential cycling in K+-containing solution is probably irrevcrsibly converted to the soluble form (which remains as a film: in bulk it can be dispersed colloidally, hence its misnomer). A slight shift of charge-transfer spectrum accompanies the transformation. Compressed-disc conductivities (table 1) show the variation of activation energy reported in the literature18-22 for the insoluble form. One problem is that even given the counter-cation, the interstitial H 2 0 content is variable, and as will be seen below for solid-state studies, a constant humectant/ desiccant should be present. (Conceivably, hydration changes affecting the conductivitiesD.R. Rosseinsky, J. S. Tonge, J. Berthelot and J. F. Cassidy 233 during pressure-dependence observations account for one negative value cited.22) Thus, conductimetric, spectrophotometric and dielectric relaxometry studies on PB were undertaken. Furthermore, study of the role of H20 content in the conductance mechanism, and of the addition of ionic material [A12(S0,),, or the by-products of, e.g. the Fe'ISO, + K,Fe(CN), reaction used in novel solid-state preparations of PB] in effecting a mechanistic shift, suggested the use of solid-state cyclic voltammetry. These preparative solid-state reaction products were thus examined, as were CrlI1-containing PB, and the PB congeners, namely, the all-reduced iron(I1) ferrocyanide (PW, Prussian White), the part-oxidised PB known as Prussian Green (PG), and the all-oxidised iron(rI1) ferricyanide, a brown-yellow material (PY).23 Finally, the Hush relationlo was tested on bulk spectrophotometric data and observed 0 values.Experimental The apparatus for dielectric relaxation (DR) was used as before,'* with the introduction of a Brookdeal phase-sensitive delector for added precision. All material was examined in compacted powder form, contacts being (depending on the study) Pt discs, silver paint, or hydrogen uranyl phosphate ; vacuum-deposited gold on PB rapidly tarnished then decomposed. Direct-current B were obtained from slow current-voltage scans over ca.5 V. Spectrophotometry was performed on a Beckman Acta MIV and voltammetry on a Bruker E44S potentiostat assembly. The PB was prepared by mixing 0.1 rnol dm-, K, Fe(CN), (AnalaR) with 0.1 mol dm-, FeC1, (AnalaR) (the high electro- lyte concentration allowing flocculation and settling) then washing and drying (60 "C) of the KFeFe(CN), - 5H20 solid. CHN analysis locally, and K by Butterworths, gave accord to better than 1 % . The CN stretch at 2075 cm-l and charge-transfer band at ca. 13 900 cm-l agree with published values. Discs of 13 mm diameter were pressed at 5 ton (ca. 5 Mg) for 5 min; often, exposure to water vapour was needed for disc cohesion, before vacuum drying at lo-, mmHg at 60 "C for 12 h, then mounting, with contacts, in stoppered tubes with blue silica-gel desiccant to ensure constancy of H 2 0 vapour.Such storage was found to minimise, but did not prevent, drift of B. Spectrophotometry was undertaken on powdered PB ground and compacted in KBr discs, the slight background absorbance being subtracted, and the absorptivity treated as though for true solution using the stoichiometric PB concentration within the disc. The mixing of 5 x lo-, mol dm-, chromic sulphate and rnol dm-, K, Fe(CN), in mol dm-3 H2S0, at 60 "C produced a green solution from which after 6 h a small amount of fine blue precipitate settled. The Fe: Cr ratio could be determined as 6: 1, and the CHN were internally consistent with CrFe,(Fe(CN),), - 14H20, although some 10%. uncertainty attaches to this assignment; Cr3+ putatively acts as the counter-cation in the place of Fe3+.Mossbauer and u.v.-visible spectra here confirm the presence of the PB chromophore, although the absorption intensity was appreciably low. A significant difference was the stability of deposited-gold electrodes, in contrast with those tried on soluble PB. Equal amounts of PB and A12(S0,), were ground separately and then together in a Glenn Creston mill and compacted as before. D.c. B measurements being found impossible, these samples were subjected to cyclic voltammetry only. (PB could also be intimately coprecipitated in BaSO,, but this material is too hard for compaction.) Further iron hexacyanoferrates with admixed salts could be generated by grinding together already-ground reactants as follows (oxidation numbers of reactants given in parentheses) : (a) Fe,(SO,), xH,O + K,Fe(CN), - 3H20 + yellow solid with green flecks (111,111) (b) Fe,(SO,), -xH20 + K,Fe(CN), * 3H20 -+ green solid (11, 11)234 Site-transfer Conductivity in Solid Iron Hexacyanoferrates 103 KIT 3 Fig.1. Temperature dependence of the d.c. conductivity CT of soluble PB: 0, heating; 0, cooling. (c) FeSO, - 7H20 + K,Fe(CN), - 3H20 + blue solid (11,111) (d) FeSO, - 7H20 + K,Fe(CN), * 3H20 + green solid (I1,II). The all-reduced product K2Fe2+Fe11(CN), expected from (d) would be white, the attrition in grinding and compaction clearly generating the substantially oxidised green state; (a) and (c) are as expected, the latter containing traces of reduction, but (b) is again partly oxidised, beyond the expected blue product.The electrochemistry on thin films in contact with aqueous ~ o l u t i o n ~ ~ - ~ ~ suggests that H20 could be redox coreagent in (d) and (b). Again cyclic voltammetry was performed on the discs using Pt sheet electrodes. The congeners PW, PG and PY were prepared as in the literat~re.~, (The PW had a faint blue tinge from a minute trace of the highly coloured PB, impossible to avoid in the absence of an all-reducing atmosphere.) Results and Discussion Electronic Conductivity and Dielectric Relaxation of KFeFe(CN), The ohmic o for our PB K+Fe3+Fe*I(CN);- has a temperature dependence as in fig. 1, the activation energy E being 76 k 5,68 k 6, and 66 f 6 kJ mol-l, for three samples. These values are higher than the range (table 1) quoted for (Fe3+)4 Fe3+Fer1(CN),, insoluble PB.This might be ascribed to a role of the redox counter-cation ‘(Fe3+)i’ in assisting electronic hopping not open to K+. Conductivities and DR frequency v were found to be constant only for 2 8 h, so measurements of both were taken within this time; results for DR (fig. 2) over a restricted temperature range give the activation energy to compareD. R. Rosseinsky, J . S. Tonge, J . Berthelot and J. F. Cassidy 235 Table 1. Activation energies for conductivity of insoluble PB T range/K EJkJ mol-l ref. (18) 294-337 45.6 337-384 33.5 (19) 30 1-398 33.8 398-454 61.8 ca. 300 46 (20)" ca. 300 ca. 50 (21) 297-333 ? negative (22)b a Calculated from spectrum. Pressure dependence. 1 I 1 I \ 50 100 150 200 E" 100 0 . . 50 I I I I I I 50 100 150 200 250 300 350 E' Fig.2. Dielectric relaxation of one sample of soluble PB: e' plotted against E" in Cole-Cole arcs at (top to bottom) 283, 277, 267 and 260 K. The numbers associated with encircled points are the applied (periodic) frequencies in kHz.236 Site-transfer Conductivity in Solid Iron Hexacyanoferrates E' Fig. 3. Cole-Cole arc at 298 K for CrlllFe:l* Fe"(CN), - 14H,O. Encircled points have applied (periodic) frequencies in kHz. Table 2. Electron-transfer frequencies v from DR and from conductivity expt sample T/K v/kHz vCond/kHz 5 6a 7 8 283 220 277 176 267 29.8 260 14.1 298 40.8 298 75.4 307 152 298 2.3 198 160 30.2 13.3 (5. 02)b 34.7 71.5 1.7 207 a Entry 6: 24 h after entry 5. Shoulder [fig. 2(d)l. with that from CT. The observed frequencies v match quite well those (vcond, table 2) calculated from eqn (1) using the measured d.c.0 values, with the Fe3+-Fe** distancel39 24 a = 5.6 A. The activation energies were, for v, 81 f 9 kJ mol-l and for CT, 78 6 kJ mol-l, again establishing the same mechanism for both. The agreement is achieved by use of the same sample, and a restricted duration, < 8 h, of measurement. Measurements of both v and CT, after greater lapses of time, showed observed v and calculated v,,,d to vary almost identically with this time lapse, as shown by the entries 5 and 6 in table 2. This correspondence is further confirmation of the correctness of the interpretation and mechanistic assignment, as the continuing time-dependent annealing processes confer closely identical changes on both v and Vcond.The CrlI1 containing PB had an activation energy for (ohmic) CT of 53k 1 kJ mol-l, nearer the table 1 range, as expected for a redoxible 3+ counter-cation. The DR measurements gave a Cole-Cole plot as in fig. 3, with v = 8.6 kHz, which compares very well with the vcond calculated from CT using eqn (l), 9.1 kHz, again substantiating the basic thesis. (The low-frequency value of e', 60, is lower than for pure PB, fig. 2). Possible Proton Conductivity in PB A layer of hydrogen uranyl phosphate (HUP) which is a strictly non-electronic proton conductor and so a blocking electrode for was interposed by compression between metal and PB contact: CT was found to be < 10-l2 R-l cm- l. The pure HUP had a 0 of ca. R-l cm-l, from both poorly reproducible d.c.measurements, andD. R . Rosseinsky, J . S . Tonge, J . Berthelot and J . F. Cassidy 237 1 time/min Fig. 4. The variation of 0 for a sample of PB on exposure to saturated water vapour at 298 K. (a) indicates the initial dry sample, (6) the first exposure to H,O and ( c ) the sample re-dried and re-exposed. Inset: voltammogram after 480 min. Fig. 5. 100 cycle voltammetry on 50: 5OPB-Al2(SO4),. Arrows denote sweep direction; paren- thesised numerals are cycle number. Forward maximum at ca. 0.5 V is evident. (Sweep rate 0.5 V s-l) extrapolation of complex impedance measurements;25 thus the very low 0 value for HUP-PB-HUP suggests that PB does not undergo proton conduction, provided that proton tunnelling through HUP-PB surfaces is not the current limiting factor.In order to test the possibility that H,O acquisition might engender proton conduction, a previously dried sample of 'soluble' PB was, after initial 0 measurement, exposed to water vapour in a tube containing some H,O, and o then monitored with time for 2 h (fig. 4). 0 apparently increased while the response became more and more non-ohmic. Re-drying restored the original behaviour, and exposure to water vapour was monitored again. After 8 h the response was as shown in the inset to fig. 4, which resembles typical voltammograms as in solution electrochemistry : in both solid and solution, the accretion of redox product at the electrode introduces an apparently negative resistance, or ' polarisation '. The observed polarisation peaking grew, the sample swelled visibly, and after 12 h fell off the electrodes.(The electrodes were silver-paint grids, such a network allowing rapid equilibration with hydrating water.) These observations imply growing proton conductivity with water uptake.238 Site-transfer Conductivity in Solid Iron Hexacyanoferrntes 1 E E 3 0 potential (span as in legend) Fig. 6. 100 cycle voltammetry on equimolar mixtures: (a) Fe~11(S04)3-K,Fe111(CN), (- 1.4 to 3.6 V), (b) Fe~11(S0,)3-K4Fe11(CN), (- 1.8 to 3.2 V), (c) Fe11S04-K3Fe111(CN), (- 1.8 to 3.2 V) and ( d ) Fe1*S04-K4Fe11(CN), (- 1.5 to 3.5 V). Forward maxima at ca. -0.5, 0.5, 2 and > 3 V correlate approximately with the two PB redox reactions, iron(Ir)/iron(IIr) and ferrocyanide/ ferricyanide, observable in aqueous electrochemistry. Arrows and parentheses as for fig.5 ; 0.5 V s-l. Ionic Conductivity in Intimate PB-Salt Compactions The current-potential relationship for PB-Al,(SO,), discs being severely non-ohmic, repetitive voltammetric scans were run (fig. 5); the voltammetric peaks decrease with time. The peaks have to be ascribed to PB + PG (or PY)+e with PB+e + PW concurrently occurring at either electrode. The asymmetry was induced by the asymmetric potential scan, chosen so as to enhance the voltammetric effects; the necessity for the two reactions at the respective electrodes to be in phase places a restriction on current. Similar behaviour (fig. 6) is found for the mixtures (a)-(d) containing K,SO,; the 111-111 mixture (a), nearly all PY, clearly has very restricted scope for complementary redox reactions to occur at the two electrodes [iron(rIr) ferricyanide being not readily oxidised further], and an only slightly non-ohmic loop is evident. The other three examples all contain redox material for both electrodes, and peaks appear at comparable potentials.Essentially Se S/e e PY ~f {PG) ~f PB T, PW (8 = 1 - 6) ; e e Fe3+ 7, Fe2+ and Fe(CN):- ?f Fe(CN)t-D. R. Rosseinsky, J. S. Tonge, J. Berthelot and J. F. Cassidy 239 Fen- Fen Fern - Fe" Fern- Fern Fig. 7. Approximate d.c. conductivities at 298 K of PW, PB, PG and PY, plotted as a function of composition. summarises the possible electrode processes. Inspection of the originally green disc (d) after voltammetry showed the face, exposed to predominantly anodic scanning, to be yellow-brown, the other being blue, the material between remaining green.In the discs showing redox voltammetry, the prevalence of K+ thus allows ionic conductivity in the bulk, while the appropriate redox reactions occur at the electrodes. Application of 35 V d.c. to a pure PB disc gave a slowly diminishing current, from 10+ to A over 3 days, but afterwards no colour changes were observed on either surface. This confirms the occurrence of solely electronic conductivity in the bulk, there being insufficient ionic carriers to permit dominant ionic conductivity which would necessarily accompany redox reactions at the electrodes. The mechanistic assignment for pure PB is thus further confirmed. Iron Hexacyanoferrate Congeners In contrast to K,SO,-containing discs, for the congeners linear current potential responses are obtained at > 3 V, so high potentials were employed to yield approximate resistances.The dependence of conductivity on composition is seen in fig. 7, where, curiously, PB is found to be the least conductive of the congeners. Band mechanisms in the other solids, or a combination of band and bandgap states in complex mechanisms, might prevail. DR on PG, with the applied frequencies encompassing an expected relaxation range (fig. 8) was not interpretable at any of these temperatures in Cole-Cole (thus simple mechanistic) terms, so excluding the simple hop mechanism and indirectly supporting the conjecture above. When the simple relaxation assumption for DR fails, it fails spectacularly, as here. Dielectric relaxometry on PY or PW proved irretrievably irreproducible.240 400( 300C E l 1 2ooc 1 ooc Site-transfer Conductivity in Solid Iron Hexacyanoferrates 400 300 El1 2001 1 OO( I I I I I 200 400 600 800 1000 Ef c 1000 - 900 - 800 - 700 - 600 - En 500 - 400 - 300 - 200 - 100 200 400 600 BOO 1000 2000 100 200 300 400 E f E l Fig.8. Complex permittivity plots for Prussian Green using a data for the appropriate temperatures: (a) 300, (b) 274 and (c) 262 K. Large circles: mean a; small circles: upper limits of a; dots: lower limits of 0. Optical Charge Transfer in PB With the preceding observations pointing strongly to sitewise electron transfer as the conduction mechanism in PB, except when compositions are modified so as to favour ionic conduction, the attempted use of optical charge-transfer parameters to predict the a of PB appears to be a practical proposition.The particles of PB embedded in KBr in the spectrophotometry are large enough to be deemed bulk solid for the purposes of deriving the optical parameters. The theory (see Appendix) gives the spectroscopically predicted thermal electron-transfer frequency vsp from where and &a is the interaction matrix element calculated from the bandwidth ~ i , the optical frequency at the absorption maximum vmax and corresponding absorptivity E ~ ~ ~ together with geometric and orbital occupancy factors; ESP is likewise derived. The spectrum is shown in fig. 9, and the result of the calculation, compared with thin-film and literature data, is presented in table 3. as, is calculated from vsp by eqn (I), and while oSp is very sensitive to ~4 (k 10 nm changing oSp tenfold), the agreements with the measured G values, although by no means exact, are sufficiently good in order of magnitude to support the procedure.D.R . Rosseinsky, J . S. Tonge, J . Berthelot and J . F. Cassidy 24 1 1.8 1.4 s 1.0 f i -E % 2 0.6 0.2 400 500 600 700 800 wavelength/nm Fig. 9. Visible spectrum of PB in a KBr disc at 298 K. Table 3. Optical properties of PB and calculated conductivities bulk PB bulk PB this work thin film12 (lit.)33 v/ l O3 cm-l 13.9 14.5 14.126 v , . ~ / ~ O ~ cm-l 5.6 5.0 6.026 E,,,/ lo4 dm3 mo1-I cm-l 0.123 1.55 0.9lIz6 EttlIeV 0.43 0.58 0.501° cexptl/W1 cm-' 10-7 f 1 1.7 x lopi ( l o p i ) 7&/ 1 0-2eV 0.432 1.55 1.1526 csp/W1 cm-l 2.9 x 10-6 4.3 x 10-8 7 x 10-7 Conclusions The d.c.conduction mechanism in pure PB is predominantly sitewise electron hopping, from the ohmic response and the agreement of DR frequencies with hop frequencies. CrIII containing PB also conforms with this interpretation. Ionic conduction can be induced by either H 2 0 uptake or inclusion of ionic material, and here both the appearance of redox products at the electrodes, and obviously redox voltammetry, * confirm the interpretation. The Hush theory, for the prediction from spectra of the hop frequency and hence the hopping conductivity, receives approximate support. PB congeners might conduct by band mechanisms, or a complex mechanism involving gap states, even, at low potentials, possibly encompassing ionic participation.We thank the S.E.R.C. for a studentship to J. S. T. and for equipment, and the Universite de Rennes for a travelling scholarship to J.B. Appendix Optical Properties and Thermal Charge Transfer in PB Robin26 gives a full molecular-orbital scheme for the electron charge transfer between FeII and FelI1 and associated intense visible absorption band at 14000 cm-l. The transferring electron spends 99% of its time at a FeII site;27 this weak coupling puts PB in class I1 of the Robin and Day classification.20 The two iron centres are not identical,242 Site-transfer Conductivity in Solid Iron Hexacyanoferrates and therefore the associated initial and final potential-energy curves are not symmetrical. The simple relation ESP = 4Eth was modified by Hushlo? 28 for unsymmetrical states to give (2) Es2P 4@sp - Eo) Eth = where E, is the overall energy change from initial to final state, ESP is the optical transfer energy hv and Eth is the calculated value of E,.(Note: in this section v may be an optical frequency, not an electron-transfer rate.) Using parameters derived from the spectrum,12 with Hush's perturbation theory', for the high-temperature limit of a harmonic-oscillator system, together with weak electronic coupling between the transfer 29-32 and adopting Meyer's formulation,ll one obtains an expression for the rate of thermal electron transfer vsp from solely optical parameters : vsp = (2n: TJ, ti) (L)' exp [ - (h~)~/411k TI k TA (3) = v, exp [ - (h~)~/411kT]. J [In ref. (12) eqn (3) contains a transcription error; fi is written as h.] The parameter A is a measure of the bond-length reorganisation energy, v is the frequency corresponding to the maximum molar adsorptivity cmax, and &a is the tunnelling matrix element: (4.24 x lOi4 E,,, v~~~ a hv ) ;(gc)-l (4) where, in specified8*12 units, vOe5 is the bandwidth at half the maximum absorptivity, a is the distance between transfer centres, g is the donor orbital OccupancylO and c is the coordination number of the donor.Rewriting eqn (2) gives Eth = (Eo A)2/411 ( 5 ) where 11 = ESP for the symmetrical case, when E, = 0, provided the metal-ligand vibrations of the initial and final states are identical. For the unsymmetrical case, as in The observed activation energy E, from conductivity measurements is to be compared with the corresponding predicted thermal quantity Eth given by where (A/eV) = 3.606 (Vo.5/eV)2 at 25 "C.The predicted electron-transfer frequency vsp is given by vsp = vo exp (- E t h / k T ) (8) where vo [eqn ( 3 ) ] is given by v0 = (2nT$,/R)(n/lkT)?. (9) The value of vsp can then be used in ne2 a2 vSp OSP = ~ kT where a factor of six has been omitted from the denominator, because the spectrum value of vSp refers to one specified direction.D. R. Rosseinsky, J. S. Tonge, J. Berthelot and J. F. Cassidy 243 References 1 D. R. Rosseinsky and J. S. Tonge, J. Chem. SOC., Faraday Trans. I , 1982, 78, 3595. 2 S. Bone, J. Eden, P. R. C. Gascoyne and R. Pethig, J . Chem. SOC., Faraday Trans. I , 1981, 77, 1729. 3 D. R. Rosseinsky, J. A. Stephan and J. S. Tonge, J. Chem.SOC., Faraday Trans. I , 1981, 77, 1719. 4 (a) D. R. Rosseinsky, Faraday Discuss. Chem. SOC., 1982, 74, 105; (b) D. R. Rosseinsky, Nature, 5 B. C. Bunker, R. S. Drago and M. K. Kroeger, J. Am. Chem. SOC., 1982,104, 4593. 6 B. C. Bunker, M. K. Kroeger, R. M. Richman and R. S. Drago, J. Am. Chem. SOC., 1981, 103,4254. 7 0. Berkooz, M. Malamud and S. Shtrikman, Solid State Commun., 1968, 6, 185. 8 R. D. Cannon, Mechanistic Aspects of Inorganic Reactions: ACS Symp. Ser. 198, ed. D. B. Rorabacher 9 R. D. Cannon, Electron Transfer Reactions (Butterworths, London, 1980). (London), 1986, 320, 665. and J. F. Endicott (Am. Chem. SOC., Washington, D.C., 1982). 10 N. S. Hush, Prog. Inorg. Chem., 1967, 8, 391. 11 T. J. Meyer, Chem. Phys. Lett., 1979, 64,417. 12 S. J. England, P. Kathirgamanathan and D. R. Rosseinsky, J. Chem. SOC., Chem. Commun., 1980,840. 13 H. J. Buser, A. Ludi, W. Petter and D. Schwarzenbach, Inorg. Chem., 1977, 16, 2704; J. Chem. SOC., 14 R. J. Mortimer and D. R. Rosseinsky, J. Chem. SOC., Dalton Trans., 1984, 2059. 15 R. J. Mortimer and D. R. Rosseinsky, J. Electroanal. Chem. Interfacial Electrochem., 1983, 151, 133. 16 R. M. C. Goncalves, H. Kellawi and D. R. Rosseinsky, J. Chem. Sac., Dalton Trans., 1983, 991. 17 D. Ellis, M. Ekkoff and V. D. Neff, J. Phys. Chem., 1981, 85, 1225. 18 S. Kawai, M. Uchida and R. Kiriyama, Mem. Inst. Sci. Ind. Res. Osaka Univ., 1972, 29, 103. 19 H. Inoue and S. Yanagisawa, J. Inorg. Nucl. Chem., 1974, 36, 1409. 20 M. B. Robin and P. Day, Adv. Inorg. Chem. Radiochem., 1967, 10, 247. 21 P. E. Fielding and D. P. Mellor, J. Chem. Phys., 1954, 22, 1155. 22 Y. Hara and S. Minomura, J. Chem. Phys., 1974, 61, 5339. 23 J. F. De Wet and R. Rolle, 2. Anorg. Allg. Chem., 1965, 336, 96. 24 J. F. Keggin and F. D. Miles, Nature (London), 1936, 137, 577. 25 C. M. Johnson, M. G. Shitton and A. T. Howe, J. Solid State Chem., 1981,37, 37. 26 M. B. Robin, Inorg. Chem., 1962, 1, 337. 27 B. Mayoh and P. Day, J. Chem. SOC., Dalton Trans., 1974, 846. 28 N. S. Hush, Electrochim. Acta, 1968,13, 1005; A. Ludi in Mixed-Valence Compounds, ed. D. B. Brown 29 G. C. Allen, Transition Met. Chem., 1976, 1, 143. 30 J. J. Hopfield, Proc. Natl Acad. Sci. USA, 1974, 71, 3640. 31 B. Mayoh and P. Day, J. Am. Chem. SOC., 1972,94, 2885. 32 C. Creutz and H. Taube, J. Am. Chem. SOC., 1969,91, 3988. 33 M. Dadic, Croat. Chem Acta, 1959, 31, 101. Chem. Commun., 1972, 1299. (NATO Adv. Study. Inst. Ser. D., Reidel, London, 1979), p. 151. Paper 6/433; Received 3rd March, 1986

ISSN:0300-9599    

DOI:10.1039/F19878300231

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1987,83, 245-255 Electron-transfer Rates by Dielectric Relaxometry and the Direct-current Conductivities of Solid Homonuclear and Heteronuclear Mixed-valence Metal Cyanometallates and of the Methylene Blue-Iron Dithiolate Adduct David R. Rosseinsky* and James S. Tonge Department of Chemistry, The University, Exeter EX4 4QD Dielectric relaxation (DR) frequencies for electron transfer in a series of metal hexacyanometallates have been compared with values from d.c. conductivity. If the compounds do not show evidence of proton conduction, the agreement is good, so establishing a site-transfer electronic conduction mechanism. Cu,Fe(CN), shows some disagreement, while marked anomalies with Co~'[CoI'I(CN),] and Fe:'[CrlI1(CN),], are ascribed to proton con- duction and electrode processes.Despite variation from sample to sample in the Methylene Blue-dithiolate adduct, the relaxation/conductivity relation- ships for each is also satisfactory for this single-metal-ion complex, bis[3,7- bis(dimethylaminophenazathionium)] tris(maleonitriledithio1ato) iron2-. The measurement of electron-transfer rates v within mixed-valence solids and donor- acceptor complexes by dielectric relaxometry (DR) is quite well e~tab1ished.l~~ Accord of v with the phenomenologically equation involving the d.c. conductivity 0 (a and v (1) being independently derived) a = (ne2a2/6kT)v confirms the validity of the assumptions, including that of discrete sitewise transfer of electrons in the conductivity mechanism. In such compounds no examples are known where v is readily measured but fails to accord with expression (l), i.e.with the direct-current conductivity. Complex permittivity plots as Cole-Cole arcs provide4 the clearest analyses yielding v. Iron(m) hexacyanoferrate(I1) [Prussian Blue (PB)] showed good accord in eqn (1) of separately measured values of v and 0, even though the 0 values were not constant. Despite this, variation of a was tracked by an almost exactly equivalent variation of v. PB is the best known of the metal hexacyanometallates and it is of interest to examine whether others of the group also show the conformity of v and a via eqn ( 1 ) and to what extent homonuclear and heteronuclear members6 differ in their charge-transfer behaviour. There are in fact no reported conductivities for the Co-CN or Cr-CN bonded analogues of PB, though the activation energies have been reported and rationalised.' Examples now chosen are those in table 1 .Besides the hexacyanometallates, an adduct with an organic cation Methylene Blue (MB+) as acceptor and an iron dithiolate anion as donor was also examined for contrast. Apart from comprising only one metal-ion centre, the adduct possesses the further property of having more diffusely delocalised charges in both donor and acceptor moieties than is the case with PB or the manganate-permanganate adduct,l? which makes for an equivalent dipole in the transfer model1 that is somewhat less clearly defined. The MB,Fe(mnt), adduct has been studied conductimetricallys and was found to have a d.c. conductivity 0 depending markedly on preparative condition^,^ particularly solvent.Values from to lo-'* i2-l cm-l are obtained with activation energies E, ranging from 22 to 61 kJ mol-l, which imply a quite marked variability in a key conductivity 245246 Electron Transfer Rates Table 1. The hexacyanometallates and MB adduct ~~ ~~ ~ c (%I N (%I compound K, M,[M;(CN),], * wH,O expt calcd expt calcd 1 KCoIIFelI1(CN), .14H,O 12.6 12.8 14.4 14.9 2 KVJ11Fe11(CN),-14H,0 13.4 13.0 15.6 15.2 3 Mn~TIFelll(CN),], * 14H,O 17.7 17.1 20.2 20.0 4 Cu~1Fe11(CN),.14H,0 13.3 12.2 15.1 14.2 5 CO~I[CO"'(CN),],* 12H,0a - - - - 6 Fe:l[Crl*l(CN),],. 14H,0b - - - - 7 (Cl,Hl,N,S),{Fe[S,C,(CN)z]~} 50.5 50.6 15.0 16.1 a Formula as supplied. Fe: Cr = 2.95 : 1 . determinant, possibly the fractional extent of solvent or H20 inclusion, or other comparable structural factor.The crystal structure has not yet been established, only powders hitherto being accessible.8 The Fe, of formal oxidation state (IV), has a charge more closely corresponding in X-ray photoelectron spectroscopylO to a 3 + entity. Experimental Compound 1 in table 1 was prepared by addition of 0.1 mol dmF3 aqueous CoSO, to excess 0.1 mol dm-3 K,Fe(CN),, the red-brown solid being centrifuged, washed extensively on a filter with water and vacuum dried at 25 "C for 12 h. Heating at > 100 "C gave a purple-dark-brown solid which is duel' to some decomposition forming the linkage isomers, comprising the [CoI1I(CN)FeI1] configuration. Mossbauer spectroscopy on the red solid showed the presence of normal ferricyanide.Analysis gave the results in table 1 : all H values were ca. 1 % (except for compound 3, which was ca. 2% ) rather than the expected 4.5%, representing either some loss of H20 or, equally probably, analytical error in the small amount of H. For compounds 2-4, similar preparations employing K,Fe(CN), with sulphate [or for (2), chloride] solutions were followed, ending with 40 "C vacuum drying. Compound 5 was used both in the pink hydrated form as received from Ventrom Gmbh (Karlsruhe), and as the blue dehydrated form obtained by 90 "C vacuum drying. A small amount of compound 6 was presented to us by Prof. Kellawi, who prepared it following the recipe4 for PB, but using K,Cr(CN), to provide the anion. 13 mm diameter discs from ground-up powders were pressed at 5-10 ton (5-10 Mg) for ca.20 min and, as for PB,4 silver paint discs ca. 5 mm in diameter on the opposing faces formed the electrodes. Conductimetry from current-potential plots on a Bruker EMS potentiostat4 proved convenient. The MB,Fe(mnt), adduct (7) was prepared12 from 0.5 g Na,(mnt) in 20 cm3 DMF- MeOH(1: l), added to 0.24 g FeC1;6H20 in 2 cm3 DMF. After being warmed to ca. 40 "C for 3 min on a steam bath, to this was added 0.67 g B.D.H. MB+Cl- in 30 cm3 of 1 : 1 DMF-H,O. A dark precipitate was immediately formed; after a further 5 min heating the mixture was cooled in an ice bath before filtration. The blue-violet microcrystalline powder was washed copiously with ethanol and ether, then dried at 40 "C under vacuum for 12 h. Discs were pressed from the ground powder, at 8 ton (ca. 8 Mg) for 5 min, the surfaces having a metallic copper-like lustre.A variety of contacts were tested in conductimetry up to 2 V, using the Bruker E44S. Vacuum-deposited gold, silver paint and colloidal graphite paint gave values in close agreement (ca. 1.1 x 10-l C2-l cm-l ), mercury (or rigid platinum-disc pressure contacts)D. R. Rosseinsky and J . S. Tonge 247 giving slightly higher or lower values (by factors of ca. 2), respectively. In view of the dependence of mercury surface tension on potential, we inclined to the former group of contacts and chose silver paint for electrodes. Dielectric relaxometry was performed on a Wayne-Kerr bridge and Brookdeal phase-sensitive dete~tor.l-~ DR could be used down to ca.- 10 "C in cooling baths, but encountered screening and pick-up problems in the Oxford CF4 cryostat used for lower-temperature conductimetry. Thus DR was confined to temperatures 2 0 "C. As noted earlier with K,(MnO,),, it is often best to use the 0 value inferred during DR measurement, since problems arising from drift with time are thereby obviated when comparing the DR v value and CJ., The latter must be checked against independent measurement, however, and for the pink compound 5 there was no satisfactory way of fitting a 0 value to the DR measurement, only the independent t~ thus being usable. Results and Discussion Current-voltage plots were Ohmic for compounds 1 and 3 and the blue form of compound 5, but for compound 2 were of power 1.4 in voltage (from a log-log current-voltage plot).The pink form of compound 5 often showed a plateau or peak in voltammetry, as did Prussian White,4 giving linearity after 2.5 V, from which an effective d.c. resistance was taken. Substance 6 showed a solution-like voltammetric maximum at ca. 1 V, and again linearity after ca. 3.5 V, as did water-saturated PB,4 though the sample of 6 in this case had been kept moderately dry over silica gel. Compound 1 The ohmicity at room temperature gave way to an exponent 1.4 in potential at the highest temperature (345 K) and the data at this and higher temperatures were omitted as possibly involving the decomposition to the linkage isomer (Poole-Frenkel or Schottky effects4 might also arise). DR Cole-Cole plots of E" (from frequency-dependent resistance) against E' (from frequency-dependent capacitance), ideall~l-~ circular arcs, show (fig.1) some slight deviation at low frequency and at higher temperatures (indicating an extra relaxation, summarily ascribed to the growth of a small amount of isomer). The independent 0 measurements (fig. 2 ) gave an activation energy E, = 52 & 4 kJ mol-l, which agrees adequately with the value fitted during DR and with that for v (table 2). The individual values of oobsd exceed those calculated from expression (1) by a fairly constant factor of two. While there are measurement errors of tens of percent, these do not account for the discrepancy. If a subtle shift and flattening of the Cole-Cole arc is introduced by the intrusion of small amounts of the linkage isomer into the sample, the numerical analysis could be sufficiently perturbed to account for the discrepancy (only a partial explanation, see above). Nevertheless, agreement of the activation energies does indicate the same barrier for both processes and different origins are not to be invoked because of the marginal discrepancy.The spectroscopic charge-transfer band13 occurs at 19670 cm-l, and for an 'isoener- getic' transfer (final state = initial state)4 the activation energies would be < a x 19 670 cm-l or ca. 60 kJ mol-1 ; the discrepancy arises in part from the asymmetry of the electron transfer. It is possible to infer the consequences of asymmetry, given quantitative details of the oscillator strength of the transition, as has been attempted in an approximate way with PB,4 but the parameters are not sufficiently accurately available for the present compound (1); in principle the spectra can afford even the (thermal) conductive charge-transfer rate v .~248 Electron Transfer Rates Fig. 1. Cole-Cole plots for compound 1 of imaginary permittivity E” (from frequency-dependent resistance) against real permittivity e’ (from frequency-dependent capacitance) for various temperatures. Each point is for a particular value of applied frequency in the measurement, and the frequency of the point at the maximum of the arc represents the DR frequency of the system. (The numerals on the arc are applied periodic frequencies in kHz). Temperature: (a) 296, (6) 318, (c) 325, ( d ) 332 K. Compound 2 The disc voltammetry showed a slight hysteresis on reversing the potential from positive to negative (3 V), but the extent was sufficiently small (from our experience with the PB group4) for the best Ohmic line through the origin to be used to obtain 0.But a clear indication is inferred of a second (probably ionic) contribution accompanied by electrode reactions (oxidation and reduction, respectively, as is especially possible with mixed-valence material) as a minor conduction pathway. The DR plots (fig. 3) show a second arc which contributes increasingly the lower the temperature, in accord with the preceding sample. We take the higher-frequency arc asD . R . Rosseinsky and J . S. Tonge 103 KIT I I -12 - 1 4 n - I -' -16 C e I . W E 4 -18 -20 - 2 2 249 Fig. 2. Activation-energy plot for a measurements of compound 1 independently observed (0) and fitted to DR data (O), giving 52 & 4 kJ mol-1 and 45 & 10 kJ mol-l, respectively.Table 2. The dielectric relaxation frequencies v, giving calculated d.c. conductivities adiel, for comparison with directly observed d.c. values, o,,,sd, and those fitted from DR data, ofit, for KCoFe(CN),, compound 1 T/K v/kHz" diel. obsd fit 296 11.4 1.2 2.1 2.5 318 28 2.75 4.6 5.7 325 57 5.5 10.2 11.5 332 96 9.1 17 18.5 " Activation energy for v is 46+4 kJ mol-l. representing the electron-transfer relaxation. This is indeed justified in table 3, where the calculated 0 from expression (1) agrees with the fitted and independent values very well; the activation energies are also given. The conductive and dielectric rates are thus in satisfactory agreement.Compound 3 Apart from being nearly Ohmic (potential exponent 1 . 1 9 , the charge-transfer properties of this sample (fig. 4) are like those of compound 2, a second arc occuring at lower250 Electron Transfer Rates 10 0 10 20 30 40 0 20 40 60 00 8 20 10 0 20 1 0 1 1 1 1 1 20 40 60 00 100 0.9 1.6 .1._ 1 1 0 20 40 60 80 E l Fig. 3. Cole-Cole plots for compound 2 at various temperatures; frequencies marked in kHz. Temperature: (a) 298, (b) 279, (c) 271, (d) 262 K. Table 3. DR frequencies v and d.c. .conductivities o for KVFe(CN),, compound 2, and for Mn,[Fe(CN),],, compound 3 T/K v/kHz diel. obsd fit 263 272 279 298.6 262 275 284.6 298 314 4.9 7 21 155 6.5 9.3 12.4 31 125 compound 2" 0.60 0.82 2.5 16.5 compound 3b 1.04 1.42 1.83 4.4 16.6 0.63 0.93 2.6 16.3 0.7 1 1.43 3.0 8.3 24.4 0.67 1 .o 2.8 18.6 0.71 1.43 3.0 8.3 24.4 a Activation energy for v = 65+8 kJ mol-l, for gobs* = 61 +7 kJ mol-'. Activation energy for v = 38 7 kJ rnol-l, for oobsd = 47 3 kJ mol-l.D. R .Rosseinsky and J. S. Tonge 10 25 1 - I 1 I I I I 1 20 10 € ) I - ( b ) - I 1 I 1 I 1 I0 Table 4. DR frequency v and predicted d.c. conductivity ddiel and observed conductivity a&sd for Cu,Fe(CN),, compound 4 ( C ) - I I I a/ 10-lo i2-l cm-l 20 10 T/K v/Hz die1 . obsd - - I 298 26 3.1 1 . 1 316 69 7.7 1.5 335 116 12.2 20 temperature. The fit of observed and calculated CT is good (table 3) and the site-transfer mechanism is supported. Compound 4 The conductivity was low, ca. R-l cm-l, and at the extreme of the bridge/p.s.d.technique for the DR measurement. The linear technique of ref. (1) was used and for one measurement at 298 K we had 1 h access to a Solartron frequency response252 Electron Transfer Rates 25 - 120 100 90 450 0 0 . 0 . 1 1 I 1 0 50 too 150 200 E l Fig. 5. Cole-Cole plots for the hydrated form of compound 5 showing arbitrarily resolved multiple arcs in a complex combination of relaxation processes; frequencies marked in Hz. 0.6 0.4 0.3 0.2 I 50 100 0 50 100 0 I 50 100 I 2.5 . I 50 100 0 E' Fig. 6. Cole-Cole plots for MB adduct 7. Temperature: (a) 296.13, (b) 308.63, (c) 318.63, (d) 326.14 K. analyser, which gives from a complex impedance plots an exactly equivalent analysis to the Cole-Cole method. While the values (table 4) of a(ca1cd) and a(obsd) are somewhat different at first sight, the discrepancies are not excessive, being 5 10 fold, and the activation energies are not markedly different.Again, the disagreements in a are unlikely to be real. Compound 5 The pink hydrated form shows all the complications of hydrous PB4 which are ascribable to protonic conduction. The activation energy for o of the pink form is 45f2 kJ mol-1D . R . Rosseinsky and J . S . Tonge 103 K I T 3.0 3.1 3.2 3.3 3.4 I I I 1 I 253 -1 9 h I - 5 -20 - I C \ W c 4 -2 1 - 22 0 n 3 3 1 W c 4 1 7 Fig. 7. Activation-energy plot of conductivity 0 (O), and DR frequency v (0). Sample- as in fig. 6. and for the blue form is 88 +4 kJ mol-l. The DR curve shows the possibility of an array of relaxation processes (fig. 5).If the first is responsible for part of the conductivity it gives too high a value for a(calcd), and the data must be deemed irresolvable. Relaxometry for the blue form was too time-unstable, presumably on account of the uptake of traces of water (despite precautions), and was abandoned. Compound 6 The o value at 298 K was 1.6 x lop8 Sz-l cm-l from the linear part of the disc voltammogram. The clear indication of electrode reaction indicated ionic (protonic) conduction and observations were discontinued. Compound 7 For the MB adduct, Cole-Cole arcs4 were obtained for the DR observations (fig. 6). The temperature dependences of 0 and v are shown in the activation energy plot (fig. 7). The arcs in fig. 6 are very sensitive4 to the 0 values employed in the analysis at the lower temperatures, as was found2 for K,(MnO,),, and in the present case this lends an appreciable uncertainty (< 30%) to values of v.As explained before,* the extent of this difficulty varies from material to material and is an intrinsic electronic/dielectric property of each material. Hence the agreement between the activation energies for two sets of o values on the one hand (42f2 and 4 7 f 4 kJ mol-l) and that for the v values (59 + 5 kJ mol-l), all for the same sample, is perhaps acceptable; the error limits (from least-squares standard deviation) refer to the scatter in a small set of data, but there is nevertheless an overlap, as would be required by expression (1).254 Electron Transfer Rates - 6.5 r -7.0 - h * I E d & -7.5 - \ 2 - o -8.0- a a Do W log (uexpt/f2-l cm-') Fig.8. Calculated us. experimental values of IT (log scale). Sample as for fig. 6 and 7 except for 0, the more conductive sample. Line is of unit slope. Thus it is worthwhile to examine the applicability of expression (1) directly. In the absence of direct density measurements (flotation values being inaccessible because the adduct was found to be soluble in available liquids of comparable density) the weight and volume of a compressed disc were used to obtain a value of 1.45 g ~ m - ~ , which falls very close to the average value for a number of iron and nickel dithiolates (1.5 g cmd3). The former then allows n, taken to be the number density of Fe-MB pairs, to be calculated as 8.65 x 1020 ~ m - ~ . For a, the donor-acceptor distance, resort was made to the X-ray structure14~ l5 of [(C,H,),As],Fe(mnt), for a rough estimate, and 5 A was taken as being probably representative for the MB adduct.The values odiel calculated from expression (1) are compared with the DC values in fig. 8 and it is seen that the agreement is indeed quite satisfactory. It was found for previous materials4 that cr might differ from sample to sample, but the observed v varied in quite strict accord with the corresponding cr. It was thus useful to make a further MB,Fe(mnt), sample in an attempt to get a different cr value, and this preparation indeed gave a cr at 298 K of 1.22 x lo-' R-l cm-'. The corresponding observed v value of 5.2 kHz gives the extra point on fig. 8, also in good agreement with its observed cr value.Thus expression (1) again is seen to account for the electron-transfer properties within a donor-acceptor complex conducting via a site-transfer mechanism. Regarding the hexacyanometallates, relationship (1) for v and cr appears to hold straightforwardly when the conductivity proceeds predominantly by electron transfer, as for compounds 1-3. For the CuII-FeII salt (4) some difference arises, but conductivity and dielectric relaxation still probably involve the same mechanism of electron transfer. Compounds 5 and 6 are not simple electron-transfer conductors and our DR analysis does not hold. As to why this should be so, it is probably relevant that CrlI1 and Col*I in complexes in solution are notably inert species, which would militate against electron transfer here, favouring ionic mechanisms instead.Clearly, examination by DR provides detailed insights into conductivity mechanisms, quantitively so if these are unalloyed electronic site-transfer processes. We thank the S.E.R.C. for a studentship to J.S.T., and for the provision of apparatus. We thank Dr R. E. Meads for the Mossbauer spectrum of compound 1.D . R. Rosseinsky and J . S . Tonge 255 References 1 D. R. Rosseinsky, J. A. Stephan and J. S. Tonge, J. Chem. SOC., Faraday Trans. I , 1981, 77, 1719. 2 D. R. Rosseinsky and J. S. Tonge, J. Chem. SOC., Faraday Trans. I , 1982, 78, 3595. 3 D. R. Rosseinsky, Faraday Discuss. Chem. Soc., 1982, 74, 105. 4 D. R. Rosseinsky, J. S. Tonge, J. Berthelot and J. F. Cassidy, J . Chem. SOC., Faraday Trans. I , 1987, 5 S. Bone, J. Eden, P. R. C. Gascoyne and R. Pethig, J. Chem. SOC., Faraday Trans. I , 1981, 77, 1719. 6 N. S. Hush, Prog. Znorg. Chem., 1967, 8, 391. 7 P. S. Braterman, P. B. P. Phipps and R. J. P. Williams, J. Chem. SOC., 1965, 6164. 8 D. R. Rosseinsky and R. E. Malpas, J. Chem. SOC., Dalton Trans., 1979, 749. 9 D. R. Rosseinsky, K. Kite, R. E. Malpas and R. A. Hann, J. Electroanal. Chem., 1976, 68, 120. 10 S. 0. Grim, L. J. Matienzo and W. E. Swartz Jr, Znorg. Chem., 1974, 13, 447. 11 D. F. Shriver and D. B. Brown, Znorg. Chem., 1969,8, 37. 12 A. Davison and R. H. Holm, Znorg. Synth., 1967, 10, 8. 13 P. S. Braterman, J. Chem. SOC. A , 1966, 1471. 14 E. I. Stiefel, L. E. Bennett, Z. Dori, T. H. Crawford, C. Simo and H. B. Gray, Znorg. Chem., 1970, 9, 15 A. Sequeira and I. Bernal, J. Cryst. Mol. Struct., 1973, 3, 157. 83, 231. 281. Paper 5/992; Received 13th June, 1985

ISSN:0300-9599    

DOI:10.1039/F19878300245

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1987, 83, 257-265 Polarographic Evidence for the Interaction of Reduced Nitroimidazole Derivatives with DNA Bases Paul J. Declerck and Carnie1 J. De Ranter* Instituut voor Farmaceutische Wetenschappen, taboratorium voor Analytische Chemie en Medicinale Fysicochemie, Katholieke Universiteit Leuven, Van Evenstraat 4, B-3000 Leuven, Belgium Nitroimidazole drugs require reductive activation to exert antimicrobial and mutagenic activity (e.g. antiprotozoal, antitumour). The polarographic behaviour of several nitroimidazoles has been investigated in the presence and in the absence of the potential biological targets (i.e. adenine, guanine, cytosine and thymine). Adenine, guanine and cytosine caused the half-wave potential to shift in the positive direction.Thymine, however, caused negative shifts of the half-wave potentials. Adsorption is not the cause of the positive shifts observed but is clearly the cause of the negative shift induced by thymine. We conclude that primarily adenine and guanine are susceptible for interaction with reduced nitroimidazoles. The results also showed that the stability of the responsible intermediates is related to the value of the half-wave potential of the parent drug. Nitroimidazoles are among the most important nitroheterocyclic drugs, because of their excellent antimicrobial activities. In addition to their widespread use as antiprotozoal and antibacterial agents,l> their radiosensitizing properties3y cause these compounds to be of current interest in cancer the rap^.^^^ When exerting these activities, their selectivity originates partly from the anaerobic, or at least hypoxic, conditions of organisms or tissues that are susceptible to nitroimidazoles.In addition, metabolic differences in diverse target cells play an important role in determining the ~electivity.~~ This selectivity is directly related to the prerequisite for their biological activity, i.e. reduction of the nitro g r o ~ p . ~ - l ~ During bioreductive activation reactive intermediates are generated.' The latter are responsible for lethal interactions with various targets in susceptible cells (e.g. Trichomonas vaginalis). Although reduced intermediates exhibit reactivity towards different biological processes, the lethal effects of activated nitroim- idazoles are presumably caused by an irreversible interaction with DNA.14-17 Therefore, many in vitro experiments have been performed in which nitroimidazoles are reduced in the presence of nucleic acids. Chemical reductive activation of nitroimidazoles revealed a degree of covalent binding to DNA which appeared to be proportional to the guanine and cytosine contents of the nucleic acids.l79 l8 It has also been shown that, after chemical reduction, misonidazole (a 2-nitroimidazole) binds to guanine.lg? 2o Our previous studies,21* 22 in which 5-nitroimidazoles were electrochemically activated, demonstrated that the reduced intermediates bind covalently to DNA, specifically to guanine residues. However, experiments by others, using similar electrochemical tech- niques, suggested an interaction with thymine.23* 24 In order to substantiate the guanine specificity and in order to investigate further the potential susceptibility of DNA bases other than guanine, the effect of these various DNA bases on the polarographic reduction of nitroimidazoles is described here.The results provide evidence that reduced nitroimidazoles interact mainly with guanine and adenine. The data also strongly suggest that reaction with thymine is unlikely to occur. These results have been presented in part at the Biochemical Pharmacology Symposium (Oxford, 25-26 July, 1985).25 257258 Polarography of Nitroimidazole Derivatives Experiment a1 Materials Nitroimidazoles were obtained as gifts from the following sources : metronidazole [ 1-(2-hydroxyethyl)-2-methyl-5-nitroimidazole] and ornidazole [ 1 -(3-chloro-2-hydroxy- propyl)-2-methyl-5-nitroimidazole] from Roche (Belgium) ; RP 8979 [ 1 -(2-hydroxyethyl)- 2-methyl-4-nitroimidazole] and RP 1 1 193 (2,5-dimethyl-4-nitroimidazole) from Rhhe-Poulenc (France) ; Ronidazole { 1 -methyl-2-[(carbamoyloxy)-methyl]-5-nitro- imidazole} and L 58 1,490 (2-methyl-4-nitroimidazole) from Merck Sharp and Dohme (New Jersey) ; ZK 26 1 73 { 3,4-di hydro-6-[ 2-(dime t hy1amino)e t hox yl-2-[( 1 -methyl- 5- nitroimidazol-2-yl)methine]-2-naphtyliden- 1 -one}, carnidazole {O-methyl-[2-(2-methyl- 5-nitroimidazol- 1 -yl)ethyl]-thiocarbamate} and dimetridazole [( 1,2-dimethyl-5-nitro- imidazole] from Schering A.G.(Federal Republic of Germany), Janssen Pharmaceutica (Belgium) and May and Baker (Essex), respectively. All other chemicals were of re- agent grade or Merck Suprapur quality.Quartz-distilled water, freshly prepared, was used to prepare all solutions. Apparatus Direct-current polarography was performed with a Tacussel polarograph (model PRG-4) composed of a three-electrode potentiostat (model PRT-30-01), a UAP-4 unit and a recorder (model EPL-2B) provided with a TV-1 1-GD plug-in unit. A thermostatted Tacussel cell (model CPR-3B) provided with a mechanical drop dislodger was used. A.c. polarographic experiments were carried out with similar equipment except that a model PRG-3 Tacussel polarograph, provided with a UAP-3 unit, was used. Experimental Conditions All potentials were measured relative to a saturated calomel electrode.A platinum wire served as auxiliary electrode. The working electrode was a dropping mercury electrode with a mercury flow rate (rn) of 0.595 mg s-l in the supporting electrolyte at open circuit and with a mercury column height of 50.0cm. All measurements were performed at 25.0 "C; average currents were measured; the drop time was adjusted to 1.0 s. D.c. polarograms were recorded at a scan rate of 4 mV s-l covering a potential range of 300 mV on either side of the half-wave potential. A.c. polarography was carried out at a scan rate of 5 mV s-l, on which an alternating voltage with an amplitude of 5 mV and a frequency of 350 Hz was superimposed. A phase angle of 90" with respect to the applied alternating voltage was employed to select the capacitive current. All solutions were prepared with a modified Britton-Robinson buffer obtained as follows.A solution containing 0.04 rnol dmP3 acetic acid, 0.04 rnol dm-3 phosphoric acid and 0.04 mol dm-3 boric acid was added to a solution of 0.04 mol dm-3 sodium acetate, 0.04 rnol dmP3 disodium hydrogen phosphate and 0.01 mol dm-3 sodium borate to adjust the pH to 7.40. All nitroimidazole solutions contained an accurately known concentration of ca. 1.00 x mol dmP3. When studying the influence of DNA bases or surfactants on the polarographic behaviour of nitroimidazoles, these DNA bases or surfactants were added in various concentrations ( 10-6-10-1 mol dm-3). All solutions were deoxygenated before recording the polarogram. Owing to the position of the half-wave potentials (i.e. near the electrocapillary maximum), separate residual-current curves were recorded in order to determine Ei exactly.The values thus obtained parallel the respective one-electron reduction poten- tials at pH 7 (E:) determined by pulse radiolysis.26 Half-wave potentials were obtained with an accuracy of 2 mV. Shifts in half-wave potentials were statistically significantP. J. Declerck and C. J. De Ranter 259 > E 1 PI - I 50 - Fig. 1. Shift in half-wave potential of metronidazole plotted against the logarithm of the concentration of: ., adenine; 0, guanine; 0, cytosine and 0, thymine. The solid lines were calculated according to eqn (1); for guanine a deviation is observed as indicated by the dashed lined. Insert: double-reciprocal plot [eqn (l)] for the metronidazole-adenine interaction.(t-test, 95% probability) if > 5 mV. According to the type of adsorption behaviour observed the adsorption parameters were determined by use of the Langmuir or Frumkin 28 Results D.C. Polarography Because of an apparent hyperbolic concentration dependence of the shift in half-wave potential of the nitroimidazoles in the presence of the added products, a double-reciprocal plot [eqn (l)] was used to calculate the maximum value of AEt and some other characteristic values : 1 1 K where A is the concentration of the DA base added and K = A when AE+ = BAEY"". Fitting of the experimental data gave correlation coefficients ( r ) close to 1. A tipica1 curve is shown in fig. 1. The shifts caused by a particular DNA base showed the same trend for all nitroimidazoles studied.Adenine and cytosine cause an increasing positive, shift, reaching a limiting value. Guanine causes an increasing positive shift up to a certain concentration (i.e. 6 x mol dm-3), but at higher concentrations the positive shift decreases and even becomes negative. Thymine causes the half-wave potentials to shift in the negative direction. The values obtained by calculation according to eqn (l), reflecting the influence of A, G, C or T on the half-wave potential of the nitro compounds investigated, are presented in table 1. From fig. 1 and table 1 it is obvious that the concentration required to produce a given shift is dependent on the DNA base added. Arranging them in order of decreasing ' shift capacity' gives: adenine, guanine, cytosine and thymine.With regard to table 1 it should be mentioned that for guanine only the rising portion of the curve (fig. 1) may be fitted to eqn (1). Hence AErax and K, calculated for guanine, are apparent values. This calculation procedure is allowed because the rising and descending parts of the curve are results of two independent phenomena (see below). The values thus calculated correspond to values that would be obtained in the absence of adsorption. It is also clear260 30 > E y 2 0 - 4 4 4 10 Polarography of Nitroimidazole Derivatives - - Table 1. Influence of the DNA bases on the half-wave potential of the nitrocompounds investigated DNA basea AEionc b/mV AEimaxe/mV Kc/mol dmP3 2 A G C T A G C T A G c T A G C T metronidazole, Ei = -496 mV 27 63 1.4 x 10-5 16 54 2.3 x 10-5 19 48 1.5 x 10-4 -3 - 63 1.9 x 10-3 10 60 4.9 x 10-5 10 53 4.3 x 10-4 ornidazole, Eh = -466 mV 27 51 8.7 x lov6 -1 < O - ZK 26173, Ei = -329 mV 6 58 8.8 x 5 44 7.5 x 10-5 2 17 8.0 x 10-4 - NSd NS RP 8979, Ei = -600 mV 53 86 6.4 x 29 91 2.1 x 10-5 35 73 1.1 x 10-4 -2 < O - a A, adenine; G, guanine; C , cytosine; T, thymine.given concentration (A and G, 1.00 x 1.00 x lop4 mol dmV3) calculated by eqn (1). AEi for a mol dm-3; C and T, See text [eqn (l)]. Not significant. 0 0 0 0 I I I I I 400 500 600 700 - E I o / ~ V Fig. 2. Experimental determination of AE+ for different nitroimidazoles ; [guanine] = 1.00 x mol dm-3. The value obtained is plotted against the half-wave potential of the re- spective nitroimidazole in the absence of guanine. For carnidazole, dimetridazole, ronidazole, RP 11 193 and L 581, 490 the half-wave potential is -453, - 517, -445, -717 and -648 mV, respectively; for the other nitroimidazoles see table 1.P .J . Declerck and C . J . De Ranter log,, (concentration/mol dm-3) 26 1 Fig. 3. Shift in half-wave potential of ornidazole plotted against the logarithm of the concentration of: +, cyclohexanol; 0, t-butyl alcohol and 0, Triton X-100. Table 2. Influence of the adsorbing products on the half-wave potential of the nitrocompounds investigated nitrocompound (Ei/mV) product added AEcone a/mV A E Y b/mV Kc/mol dm-3 metronidazole ( - 496) t-butyl alcohol - 40 - 1 9 ~ 10 1 x 10-1 ornidazole (- 466) cyclohexanol -7 -12x10 7 x 10-3 t-butyl alcohol - 27 - 1 8 ~ 10 1 x 10-1 Triton X-100 - 28 -30 x 10 1 x 10-4 ZK 261 73 ( - 329) c yclohexanol 0 0 Triton X-100 - 29 - 1 8 ~ 10 1 x 10-4 RP 8979 (- 600) t-butyl alcohol - 15 - 1 7 ~ 10 1 x 10-1 - a Experimentally determined AE; for a given concentration (cyclohexanol, 1.0 x t-butyl alcohol, 6 x mol dm-3; mol dm-3; Triton X-100, 3.1 x lop5 mol dm-3. See text [eqn (l)].that the maximum value of AEi appears to be related to the nitroimidazole rather than to the DNA base. To investigate this relationship further, the effect of 1 .OO x mol dm-3 guanine on the half-wave potential of a number of additional nitroimidazoles was investigated (fig. 2). The obvious differences are clearly related to the half-wave potential. ZK 261 73 shows a second polarographic wave (E; = - 804 mV) which is due to the reduction of the ketone group.DNA bases did not affect this half-wave potential. This phenomenon indicates that the shifts caused by the investigated compounds are specifically related to the reduction of the nitro group (see Discussion). In order to substantiate the distinction between shifts caused either by adsorption of added substances or by a chemical reaction between generated intermediates and the added substance, we also investigated, apart from the a.c. polarographic experiments (see below), the influence of cyclohexanol, t-butyl alcohol and Triton X-100, known to be purely adsorbing products. Cyclohexanol, t-butyl alcohol and Triton X-100 cause an obvious negative shift of the half-wave potential (fig. 3). Table 2 shows some specific characteristics of this effect.Unlike the results of the experiments in the presence of the DNA bases (adenine, guanine and cytosine), the shifts are negative and appear only at relatively high concentrations of the surfactants. In most cases the absolute values of AEi are higher than those obtained with the DNA bases. 10 FAR 1262 Polarography of Nitroimidazole Derivatives '/- I \ I I - 5.00 -4.00 Q - 3-00 -200 i loglo ([guanine]/mol dm-3) \ Fig. 4. Relative shift in half-wave potential potential AGel, (A, metronidazole; W, ornidazole; 0, ZK 26173; e, RP 8979) and relative surface coverage 8 (+) plotted against the logarithm of the concentration of guanine. Table 3. Concentration (mol dmP3), of the product added, required to obtain half of the effect investigated (6' or AElel) AEle' = 0.5 product added 6' = 0.5 metronidazole ornidazole ZK 26173 RP 8979 adenine 2.9 x 10-3 1.4 x 10-5 8.7 x 8.8 x 6.4 x guanine 1.5 x 10-3 2.3 x 10-5 4.9 x 10-5 7.5 x 10-5 2.1 x 10-5 cytosine 5.6x 10-3 1.5 x 10-4 4.3 x 10-4 8.0 x 10-4 1.1 x 10-4 thymine 2.2 x 10-3 1.9 x 10-3 t-butyl alcohol 8.5 x 1 x 10-1 1 x 10-1 n.d.1 x 10-1 U - - - n.d. cyclohexanol 4.6 x 10-3 n.d.b 6.6 x Triton X-100 8.3 x 10-5 n.d. 1 x 10-4 1 x n.d. - Could not be determined. Not determined. Table 4. Effect produced by the added product at a given concentrationa AEfel 2 product added 6' metronidazole ornidazole ZK 26173 RP 8979 adenine 0.03 0.43 0.53 0.10 0.62 guanine 0.01 0.30 0.17 0.1 1 0.32 cytosine 0.02 0.40 0.19 0.12 0.48 thymine 0.04 0.05 cyclo hexanol 0.06 n.d." 0.06 n.d.Triton X-100 0.4 n.d. 0.1 0.2 n.d. b - - - - t-butyl alcohol 0.3 0.2 1 0.15 n.d. 0.1 Adenine, guanine, 1.00 x Could not be determined mol dm-3; cytosine, thymine, 1.00 x lop4 rnol dm-3; cyclohexan- mol dm-3. Not determined. 01, 1.0 x mol dmP3; t-butyl alcohol, 6.0 x lop2 mol dmP3; Triton X-100, 3. I xP . J. Declerck and C. J . De Ranter 263 A.C. Polarography The above-mentioned DNA bases are known to adsorb on the mercury s~rface.~~-~O To distinguish unequivocally between chemical and adsorption effects of the DNA bases, their adsorption behaviour in our experimental conditions was tested. A.c. polarography enabled us to express the adsorption behaviour (relative surface coverage, 8)27928 as a function of the concentration. To make an adequate comparison with the observed shifts, the latter were converted into relative shifts, i.e.the ratio of the shift observed with a certain concentration to the maximum shift. Those results are presented in fig. 4 and in tables 3 and 4. Discussion Adenine, guanine and cytosine cause positive shifts of the half-wave potential of the investigated nitrocompounds, in contrast to the negative shifts caused by thymine, also a DNA base, and by cyclohexanol, t-butyl alcohol and Triton X-100. These differences suggest that two distinct causes underly the two types of shifts. The negative shifts are the consequence of an adsorption of the added products on the mercury surface, thereby hampering the electrode process, We suggest that the positive shifts arise as a consequence of some interaction (as has been proposed to occur in u ~ u o ) ~ ~ J ~ between reduced nitroimidazole intermediates and the DNA base added. However, adenine, guanine and cytosine can also adsorb at the mercury electrode, and it is known that products adsorbing at the working electrode can affect the reaction that is taking place.31 Hence, it could not be excluded apriori that, owing to the presence of the adsorbed aromatic molecules, the observed positive shifts arise from, e.g., a decrease in activation energy for the electron transfer.The results shown in fig. 4 and tables 3 and 4 provide conclusive evidence that the adsorption can be neglected within the concentration ranges at which the positive shifts appear. Complex formation between the bases and the respective nitrocompounds prior to reduction, facilitating the reduction process, could be ruled out by spectrophotometric control of the initial solutions. The relationship of both 8 and Aqe' to the concentration of thymine, cyclohexanol, t-butyl alcohol or Triton X-100 (curves not shown) is strikingly similar.This demon- strates that in these cases adsorption is indeed the cause of the observed negative shifts (see also tables 3 and 4). However, for adenine, guanine and cytosine the concentrations required to produce a given relative shift and those required to produce a given 8 are quite different (table 3). In these cases, high Aqe' values are obtained at concentrations which exhibit almost no adsorption (table 4), providing strong evidence that the positive shifts are not caused by any adsorption but, most probably, by a chemical reaction. The decline in positive shifts when the concentratibn of guanine exceeds a certain value (fig.1 and 4) further illustrates the appearance of the two distinct phenomena (adsorption us. chemical reaction). In this case (in contrast to adenine and cytosine) the adsorption curve is situated nearer to the curves representing the relative shifts. Indeed, the maximum relative shifts induced by guanine reach a maximum value as the adsorption curve starts to rise. Consequently the observed decrease in positive shift is due to adsorption effects. Such a maximum appears with all nitroimidazoles investigated. The hypothesis is further supported by the observation that for ZK 26173, exhibiting two reduction waves, only the half-wave potential of the nitro-group reduction is affected in the presence of adenine, guanine or cytosine.This clearly shows the specificity of the interaction, requiring the presence of an intermediate generated during reduction of the nitro group. In addition, three important remarks are appropriate. (i) We observed that the limiting current remains unaffected by the addition of adenine, guanine or cytosine (data not shown). This indicates that the DNA bases react with an 10-2264 Polarography of Nitroimidazole Derivatives end-product or with an intermediate (eg. a nitroso derivative) which in the absence of the DNA base reacts with an end-product of the reduction process. (ii) In each case the positive shifts in half-wave potential reach a limiting value (ATax, fig.1). This indicates that the measured effect is limited by a particular factor. In this case the limiting value of AE+ is probably governed by the total concentration of the reactive intermediates generated. This effective concentration is probably dependent on the stability of the intermediates and therefore dependent on the nitroimidazole. This is confirmed by the differences in A Z a X for different nitroimidazoles. As shown in table 1, RP 8979 undergoes the highest shifts and consequently forms the most stable intermediates. This is supported by the correlation suggested by Knox et ~ l . , ~ ~ which states that the nitro compound with the most negative half-wave potential forms the most stable intermediate. (iii) The results show remarkable differences with respect to the concentration of a DNA base required to produce a given shift.These differences probably arise from the different relative susceptibilities of the various DNA bases. The higher the susceptibility, the lower the concentration that is required to produce a given effect. Arranging them in order of decreasing susceptibility gives : adenine, guanine, cytosine. In conclusion, adenine and guanine interact most effectively with the intermediate(s) generated during nitroimidazole reduction. The present results clearly confirm our previous experiments2'* 22 and those of Varghese and Whitmorel9* 2o and of LaRusso et al.,179 l8 which indicated an interaction at the level of guanine. Moreover, the results described here demonstrate an interaction between the reduced nitroimidazole deriva- tives and adenine (and to a lesser extent with cytosine).In addition it is shown that thymine remains unaffected by these intermediates. These observations are also in agreement with reports of the mutagenic properties of nitroimidazoles; i.e. alkylation of guanine and adenine has been proposed to be the most likely e~planation.~~ P. J. Declerck was a Research Assistant of the National Fund for Scientific Research (Belgium). We thank Dr M. Muller (New York) for critical reading of the manuscript. References 1 M. Muller, Scand. J. Infect. Dis., 1981, suppl. 26, 31. 2 I. de Carneri, in Nitroimidazole: Chemistry, Pharmacology and Clinical Application, ed. A. Breccia, 3 I. J. Stratford, Int. J . Radiat Oncol.Biol. Phys., 1982, 8, 391. 4 D. J. Grdina, H. D. Thames and L. Milas, Inz. J . Radiat. Oncol. Biol. Phys., 1984, 10, 379. 5 T. H. Wasserman, J. Stetz and T. L. Philips, Cancer, 1981, 47, 2382. 6 G. E. Adams, Biochem. Pharmacol., 1986, 35, 71. 7 N. Yarlett, T. E. Gorrell, R. Marczak and M. Muller, Mol. Biochem. Parasitol., 1985, 14, 29. 8 M. Muller, Biochem. Pharmacol., 1986, 35, 37. 9 D. I. Edwards, M. Dye and H. Carne, J . Gen. Microbiol., 1973, 76, 135. B. Cavalleri and G. E. Adams (Plenum Press, New York, 1982), p. 115. 10 D. G. Lindmark and M. Muller, Antimicrob. Agents Chemother., 1976, 10, 476. 11 B. P. Goldstein, R. R. Vidal-Plana, B. Cavalleri, L. Zerilli, G. Carniti and L. G. Silvestri, J. Gen. 12 P. L. Olive, Br. J . Cancer, 1979, 40, 94. 13 E.J. T. Chrystal, R. L. Koch, R. L. McLafferty and P. Goldman, Antimicrob. Agents Chemother., 14 R. M. H. Ings, J. A. McFadzean and W. E. Ormerod, Biochem. Pharmacol., 1974, 23, 1421. 15 D. I. Edwards, J . Antimicrob. Chemother., 1977, 3, 43. 16 B. P. Goldstein, E. Nielsen, M. Berti, G. Bolzoni and L. G. Silvestri, J . Gen. Microbiol., 1977, 100,271. 17 N. F. LaRusso, M. Tomasz, M. Muller and R. Lipman, Mol. Pharmacol., 1977, 13, 872. 18 N. F. LaRusso, M. Tomasz, D. Kaplan and M. Muller, Antimicrob. Agents Chemother., 1978, 13, 19. 19 A. J. Varghese and G. F. Whitmore, Cancer. Res., 1980, 40, 2165. 20 A. J. Varghese and G. F. Whitmore, Cancer. Res., 1983, 43, 78. 21 P. J. Declerck, C. J. De Ranter and G. Volckaert, Arch. Int. Physiol. Biochim., 1982, 90, B183. 22 P. J. Declerck, C. J. De Ranter and G. Volckaert, FEBS Lett., 1983, 164, 145. Microbiol., 1977, 100, 283. 1980, 18, 566.P. J . Declerck and C. J . De Ranter 265 23 R. J. Knox, R. C. Knight and D. I. Edwards, IRCS J . Med. Sci., 1980, 8, 190. 24 R. J. Knox, R. C. Knight and D. I. Edwards, Int. J . Radiat. Biol., 1982, 41, 465. 25 P. J. Declerck and C. J. De Ranter, Biochem. Pharmacol., 1986, 35, 59. 26 P. Wardman and E. D. Clarke, J. Chem. SOC., Faraday Trans. I , 1976,72, 1377. 27 A. N. Frumkin and B. B. Damaskin, in Modern Aspects of Electrochemistry, ed. J. O’M. Bockris and 28 V. Vetted, Collect. Czech. Chem. Commun., 1966, 31, 2105. 29 H. Kinoshita, S. D. Christian and G . Dryhurst, J . Electroanal. Chem., 1977, 83, 151. 30 Y. M. Temerk and M. M. Kamal, Bull. SOC. Chim. Belg., 1982, 91, 1. 31 H. H. Bauer, P. J. Herman and P. J. Elving, in Modern Aspects ofElectrochemistry, ed. J. O’M. Bockris 32 R. J. Knox, R. C. Knight and D. I. Edwards, Br. J . Cancer, 1981, 44, 741. 33 T. A. Connors, in Chemical Carcinogens, ed. C. E. Searle (American Chemical Society, Washington, B. E. Conway (Butterworths, London, 1964), p. 149. and B. E. Conway (Plenum Press, New York, 1972), p. 143. 1984), vol. 2, p. 1241. Paper 511633; Received 20th September, 1985

ISSN:0300-9599    

DOI:10.1039/F19878300257

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1987, 83, 267-270 Influence of Mixed Cerium(1v) Sulphate and Ferroin Catalysts on the Oscillatory Redox Reaction between Malonic Acid and Bromate Francesca D'Alba* and Sergio Di Lorenzo Istituto di Ingegneria Chimica, Viale dele Scienze, 90100 Palermo, Italy We examine the influence of mixed cerium(rv) sulphate and ferroin catalysts in the oscillating reduction of bromate by malonic acid. The tests were made by potentiometric and spectrophotometric methods. Results are shown as number of cycles and both maximum and minimum of potential us. time. The mixed cerium(1v)-ferroin catalysts have almost the same effect as pure ferroin at the beginning and as pure cerium(1v) at the end. The number of cycles is quite steady, but the frequency of oscillations of the cerium(1v) catalyst is very low and very different from that of the mixed catalysts and of pure ferroin.The Belousov-Zhabotinsky system1* (malonic acid-potassium bromate-sulphuric or perchloric acid with ~ e r i u m ~ - ~ or ferroin as catalyst) is the most widely studied oscillatory phenomenon. The interest shown in such systems is due to their importance in biological structures, in the stability of chemical reactors and in the theoretical field. We have proposed6 an irreversible step, related to mass transfer, in the generation of oscillatory behaviour and have claimed47 the presence of bromine supersaturation in the mechanisms of the Belousov-Zhabotinsky system, catalysed by cerium salts. The aim of the present work is to compare the mechanisms in the Belousov- Zhabotinsky system catalysed by cerium with both that in the system catalysed by ferroin and that in the system catalysed by mixed catalysts in order to determine the influence of the catalyst on the oscillation parameters. Experimental Some light-absorbance measurements were carried out in an unstirred 1 cm cuvette of a double-beam spectrophotometer (Coleman model 124 D, Perkin-Elmer) at the maximum light absorbance of bromine (A = 390 nm).The reagents and the catalyst were mixed and the solution was shaken before being put in the cuvette. Most of the measurements were potentiometric and were carried out by means of a high-impedance electrometer (Keithley 61 0 C model) and recorded by a potentiometric recorder (Leeds and Northrup Speedomax recorder XL 682) connected to the electro- meter.We recorded the potential of the galvanic chain (Ptlsol I( agar-agar + KNO, 11 KCl (satd)lHg,Cl, IHglPt) during the whole life of the oscillating reaction by means of a cell, described in a previous paper.' The temperature was controlled at 308 f0.5 K by an air thermostat. Solutions were stirred at a steady rate by a magnetic stirrer. All reagents were of analytical grade. We tested several solutions with malonic acid (0.15 mol drn-,); KBrO, (0.33 mol dmP3); H,S04 (0.45 mol dm-3); catalyst 14.15 x g ion drn-,, where the catalysts are ferroin, cerium(1v) sulphate and mixtures (0.25: 0.75; 0.50: 0.50; 0.75: 0.25) of them (cerium salts were previously stabilized by sulphuric acid). The range of potentials, referred to the standard hydrogen electrode, as measured in 267268 Mixed Catalysts in the B-Z System our tests was from 1.10 to 1.36 V.For the catalyst couples, we can write the corresponding Nernst equation, using the values indicated by Pourbaix :8 Ce4+ + e- Ce3+ (1) and El = 1.44 + 0.0591 log [Ce4+]/[Ce3+] [Fe(Phen),13+ + e- $ [Fe(Phen),12 + E2 = 1.14 + 0.0591 log [Fe(Phen),I3+’/[Fe(Phen),12’ and we obtain values in agreement with our experimental data. For bromate and other species of bromine present in the corresponding potential-pH diagram, the only electrode reaction having potential values in agreement with our experimental data is the following : Br2(aq) + 2e- 2Br- (3) E3 = 1.087 + 0.0295 log [Br2],,/[Br-I2. However, the synchroneity observed visually between the maxima of the potential and the maxima of the intensity in the colour of the ferriin and the synchroneity between the minima of the potential and the maxima of the intensity in the colour of ferroin suggest that the potentials measured, although mixed, are very near to the thermodynamic potential of the [Fe(Phen)3]3+/[Fe(Phen)3]2+ catalyst couple.Similarly, in the test with pure cerium catalyst, potentials refer to the [Ce4+]/[Ce3+] catalyst couple. Results Fig. 1 shows the behaviour of a solution with pure cerium catalyst in a light-absorbance test. The bromine concentration increases until it reaches a threshold value after oscillations begin. The great irregularities, shown after a few cycles, are due to a deficiency in stirring. The anomalies in regular and irregular oscillations are thought to be due to the crossing of the beam by bubbles of liquid bromine. The results of the potentiometric tests are shown as n, number of cycles, us. time, as both Emax maximum potential and Emin minimum potential vs.time. Fig. 2 shows the different types of oscillation in relation to the catalyst. The oscillations with the pure ferroin catalyst [fig. 2(a)] are divided into two periods: (1) a rapid ferriin production period and (2) a rapid ferriin consumption period. The oscillations with the pure cerium catalyst are divided into three periods:4 (1) a rapid cerium(1v) production period, (2) a rapid cerium(1v) consumption period and (3) a slow cerium(1v) consumption period [fig. 2 (bl)], a steady cerium(1v) period [fig. 2 (b2)] and a slow cerium(1v) production period [fig.2 (b3)]. The oscillations with the mixed cerium(1v)-ferroin catalysts at the beginning are analogous to those exhibited by pure ferroin; at the end, they are analogous to those exhibited by pure cerium(1v). Fig. 3 shows the influence of the catalyst on the number of cycles. All curves exhibit an analogous number of cycles, but the pure cerium(1v) catalyst leads to a frequency which is almost a quarter of that of the others. Fig. 4 shows the influence of the catalyst on the maximum and the minimum potential. The pure ferroin catalyst leads to an almost steady maximum, minimum and average potential. The pure cerium(1v) catalyst leads to an increasing average potential. All curves with both ferroin and cerium(1v) are almost analogous, at the beginning, to that with pure ferroin and, at the end, to that with pure cerium(rv).The induction time decreases when the cerium(1v) concentration decreases in the tests with mixed cerium(1v)-ferroin catalysts and does not take place in the tests with pure ferroin. Tests with cerium(II1) salts show a longer induction time than those shown by tests with cerium(1v) salts.F. D’Alba and S. Di Lorenzo 269 a, 5 2 -2 time Fig. 1. Absorbance of bromine as a function of time in KBrO, (3.3 x 10-l mol drn-,), C,O,H, (1.5 x 10-l mol dm-3), H,SO, (4.5 x 10-1 mol drn-,) and Ce(SO,), (1.415 x lo-, g ion drn-,) at A = 390 nm. Fig. 2. Types of cycles: (a) rapid ferriin production period and rapid ferriin consumption period; ( b 1) slow cerium(rv) consumption period; (b2) steady cerium(1v) period; ( b 3 ) slow cerium(1v) production period.I P Fig. 3. time. 250 200 150 n 100 50 0 50 100 150 200 tlmin Influence of mixed cerium(1v)-ferroin catalysts. Number of cycles as [KBrO,] = 3.3 x 10-1 mol dm-3; and [C,O,H,] = 1.5 x 10-1 mol dm-3, 0 A n 0 4.5 x 10-l mol dm-,. [Ce(SO,),]/ 1 O-, g ion dm-, 0 1.4150 0.3537 1.0613 0.7075 0.7075 1.061 3 0.3537 1.41 50 0 ferroin/lO-, g ion dmP3270 Mixed Catalysts in the B-Z System 1.35 1.30 1.25 1.20 1.15 1.10 L 1 0 50 100 150 200 tlmin Fig. 4. Influence of mixed cerium(1v) - ferroin catalysts. Maxima and minima of the potential, referred to the standard hydrogen electrode, as functions of time. [KBrO,] = 3.3 x lo-’ mol dm-3; [C,O,H,] = 1.5 x 10-l mol dm-3 and [H,SO,] = 4.5 x 10-l mol dmP3. Ce(SO,),/ g ion dmP3 ferroin/ 1 0-3 g ion dm-3 n o 1.4150 A 0.3537 1.0613 A 0.7075 0.7075 1.0613 0.3537 0 1.4150 0 References 1 B. P. Belousov, J . Res. Radiat. Med., 1959, 1958, 145. 2 A. M. Zhabotinsky, BioJizika, 1964, 9, 306. 3 R. J. Field, E. Koros and R. M. Noyes, J. Am. Chem. Soc., 1972,94, 8649. 4 S. Di Lorenzo and F. D’Alba, J. Chem. Soc., Faraday Trans. 1, 1985,81, 421. 5 S. Di Lorenzo and F. D’Alba, Bioelectrochem. Bioenerg., 1984, 13, 287. 6 F. D’Alba and S. Di Lorenzo, J . Chem. Soc., Faraday Trans. I , 1983,79, 39. 7 F. D’Alba and G. Serravalle, J . Chim. Phys., 1981, 78, 131. 8 M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solution (Pergamon Press, Oxford, 1966). Paper 511634; Received 20th September, 1985

ISSN:0300-9599    

DOI:10.1039/F19878300267

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1987, 83, 271-279 Electrocatalysis under Temkin Adsorption Conditions A. Saraby-Reintjest Van ' t Hofl Laboratory for Physical and Colloid Chemistry, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands The theoretical description of steady-state current-potential curves for simple electrocatalytic reactions under Temkin adsorption conditions has been extended beyond the potential range of quasi-equilibrium. The theory predicts that current-potential curves calculated on the basis of Langmuir and Temkin adsorption conditions exhibit only minor differences at cover- ages below 0.9. The volcano curve for hydrogen evolution by the Volmer- Heyrovsky reaction mechanism proves to be independent of the nature of the adsorption isotherm if plotted against the apparent free energy of adsorption at equilibrium coverage under standard conditions.For any other value of the free energy of adsorption, however, the width of the volcano curve depends on the Temkin heterogeneity factor f. The volcano curve is not truncated under Temkin adsorption conditions. In the kinetic treatment of electrochemical reactions with adsorbed intermediates it is important to know how the fractional coverage of the surface by the intermediates depends on environmental variables. The Langmuir isotherm, which is based on the assumption that the apparent free energy of adsorption is independent of the coverage, is not likely to represent actual adsorption conditions accurately. Temkinl postulated that the free energy of adsorption may increase linearly with coverage, either due to heterogeneity of the surface' or to lateral in order to account for experimental deviations from Langmuir behaviour.The Temkin and Frumkin isotherms are based on this postulate, as is the Temkin-type isotherm suggested by Gileadi and C ~ n w a y . ~ The latter authors have calculated coverages and accompanying pseudo- capacities under Temkin adsorption conditions and have concluded that the pseudo- capacity is a sensitive function of the reaction mechanism and the manner in which the free energy of adsorption varies with the ~ o v e r a g e . ~ ~ The derivation of steady-state current-potential curves under Temkin adsorption conditions has generally been limited to conditions of quasi-equilibrium for the first electron-transfer step(s).6-12 It is clear that this constitutes a severe restriction, for many electrocatalytic reactions occur in a potential range far removed from that of quasi- equilibrium.The condition that the coverage 0 should lie in the range 0.2 < 0 < 0.8 at quasi-equilibrium constitutes a further restriction to the applicability of such a treatment. It is the aim of this paper to extend the calculation of steady-state current-potential curves under Temkin adsorption conditions beyond the potential range of quasi- equilibrium. The theory will be illustrated by considering the hydrogen evolution reaction (HER) by the Volmer-Heyrovsky reaction mechanism, which is among the simplest of electrocatalytic processes. Furthermore, the volcano curve for this mechanism under Temkin adsorption conditions will be considered.t Present address : Applied Electrostatics Research Group, Department of Electrical Engineering, The University, Southampton SO9 5NH. 27 1272 Elect r oca taly sis under Temkin A dsorp t ion Conditions Steady-state Current-Potential Curves The HER according to the Volmer-Heyrovsky (V-H) mechanism involves the formation of adsorbed hydrogen (H) as a reaction intermediate: k+ 1 H++e-+(H) k-1 k+Z (H)+H++e-eH,. k-2 According to Temkin’s postulate, the apparent free energy of adsorption of hydrogen increases linearly with the hydrogen coverage OH : Here and AG,,, stand for the apparent and true standard free energies of adsorption (kJ mol-l), f is a dimensionless proportionality constant which is normally positive and R and T have their usual meanings.Conway et al.13 have pointed out that the standard state for adsorbed hydrogen is thus defined by A standard state in which 8, andfare variables is, however, not a practical basis for investigating the influence of Temkin adsorption behaviour. It is perfectly legitimate, however, to select a value for one of the variables, which then defines the other variable according to eqn (4).7 An obvious choice is 8, = 0.50, which givesf= 0. One must keep in mind, however, that there are other combinations of OH andffor which ma,, is also If in a plot of potential energy us. the reaction coordinate the curve associated with adsorbed hydrogen shifts vertically without loss of shape when =ad, is varied,15 it follows that the heights of the activation-energy barriers for reactions (1) and (2) on a given metal will increase by p1flpTOH and p2fRTOH as a result of Temkin adsorption.The forward and reverse rates for reactions (1) and (2) are then given by equal to AG,d,.$ t An entirely analogous case is provided by the hydrogen electrode, the potential of which is given by RT [H+l2 E = E,+- In-. 2F P H z Strictly speaking, the standard state is given by [H+l2/pH2 = 1. It is customary, however, to define the standard state as [H+] = 1 and pH, = 1 atm (1 atm = 101 325 Pa). $ Parsons14 chooses a standard state of OH = 0.50 independent off. The consequence is that the standard free energies of adsorption and activation all become functions off.A. Saraby-Reintjes 273 Here E is the potential with respect to the standard hydrogen potential (V), the quantities k are the true rate constants (forf8, = 0) in eqn (1) and (2) at E = 0 (mol cm-, s-l), pH2 is the hydrogen pressure (atm), [H+] is the activity of H,O+ ions in the solution and F is the Faraday constant (C mol-l).The true rate constants have been multiplied by the terms exp ( -pfeH) and exp [( 1 -p)fiH] to yield the apparent rate constants under Temkin adsorption conditions for the adsorption and desorption processes, respectively. The symmetry factors al and a,, are, in principle, identical to p1 and (1 -p2). The condition for the steady state, diH/dt = 0, is given by u+1- u-, - u+, + u-, = 0. (9) The calculation of steady-state coverages and current-potential curves will be greatly simplified if it is assumed that all symmetry coefficients a and p equal 0.50.According to eqn (5)-(9) 8, can then be derived from in which16 and When H , is absent from the system and Q = 0, eqn (10) becomes At high E when P % 1, the right-hand side of eqn (13) approaches (k+l [H+]/k-,) exp (- FE/RT); in this case reaction (1) is at quasi-equilibrium. When P $ 1 at high cathodic potentials, the coverage approaches a maximal value which is independent of pH and E, and is only determined by the ratio of rate constants k+, and k+, and the Temkin parameter f. The evaluation of 8, from eqn (13) constitutes the first step in the derivation of steady-state current-potential curves. We shall be interested chiefly in intermediate coverages in the range 0.2 < 8, < 0.8.Under Langmuir conditions 8, < 0.1 at all potentials if k+,/k+, < 0.1, and OH > 0.9 for E < 0 under standard conditions when k+,/k+, > lo4. The HER on the iron-group metals as well as on the noble metals (Pt, Pd, Ir, Rh) lies within this relatively limited range of k+,/k+, ratios.l49 l7 Fig. 1 and 2 show OH as a function of E and f for k+,/k+, = 10 and lo3, respectively, chosen as representative for the lower and upper ranges of k+,/k+, ratios in which the variation of E,,, with 8, affects the coverage. The calculation of 8, under Temkin adsorption conditions is essentially similar to that by Conway and Gileadi4* i F The steady-state current density for the HER is given by = u+, - 21-, + u+, - u-,. (14) - It is evident from eqn (5)-(8) that Temkin-type adsorption has a two-fold effect on these partial reaction rates: the variations in the apparent rate constants and in 8, or (1 - 8,) work in opposite directions and partially compensate each other.If H , is absent and u-, = 0, it follows from eqn (9) and (14) that the current density for the HER can be written i - = 2u+, = 2k+, [H+] e, exp (-KT exp [(I +)ft?H]. F RT274 Electrocatalysis under Ternkin Adsorption Conditions 100 0 -100 -200 -300 EImV Fig. 1. Dependence of the coverage 8, on the potential E for various values off, calculated with the aid of eqn (1 1) and (13) for k+l/k+2 = 10, P = lo3 at E = 0 . f (a) 0, (b) 1, ( c ) 2, ( d ) 5, (e) 10, (f) 20, ( g ) 50. 100 0 -100 -200 -300 EImV Fig. 2. Dependence of the coverage 8, on the potential E for various values off, calculated with the aid of eqn (1 1) and (13) for k+,/k+, = lo3, P = lo3 at E = 0.(a)-(g) as for fig. 1. Current-potential curves for various values off, calculated with the aid of eqn (1 5 ) with use of the coverages in fig. 1 and 2, are shown in fig. 3 and 4. The chief variation in these i-E curves due to Temkin adsorption is observed when k+l/k+2 is large and OH > 0.9; in this case the increase in exp [(l -P)fOH] dominates over the decrease in 8,. At intermediate and low coverages, however, the deviations from Langmuir behaviour are very small. This prediction by the theory is in complete agreement with the frequently made observation that current-potential curves derived for Langmuir adsorption conditions represent experimental data surprisingly well, in spite of the fact that the Langmuir isotherm is unlikely to represent actual adsorption conditions accurately. This has sometimes been called the ‘ paradox of heterogeneous kinetics’.l*, l9 Steady-stateA.Saraby-Reintjes 275 3 2 n + - z 7 1 2 - C Y W M -1 c -L - 100 0 -100 -200 -300 EImV Fig. 3. Steady-state polarisation curves under Temkin adsorption conditions calculated for k+Jk+* = 10, P = lo3 at E = 0 and various values off. (a)-(g) as for fig. 1. current-potential curves similar to those in fig. 4 have been derived by Conway by a somewhat different method.20 It is often thought that Tafel slopes in the vicinity of - 60 mV per decade are indicative of Temkin adsorption conditions. It has been shown by Conway and 21 however, that there is no extended potential interval with a Tafel slope of -60 mV per decade under Temkin adsorption conditions unlessfor k+,/k+, are very large.The Tafel slopes which can be derived from fig. 3 and 4 support this view. Volcano Curve for the HER The volcano curve for the Volmer-Heyrovsky mechanism gives the relationship between the standard exchange current density i, for this mechanism and the standard free energy of adsorption of hydrogen.l*q 22 If the true rate constants for a metal with AGads = 0 are indicated as k:,, k",, k:2 and k", the forward and reverse rates of reactions (1) and (2) for a metal with AG,,, # 0 under Temkin adsorption conditions are given by:276 Electrocatalysis under Temkin Adsorption Conditions 100 0 -100 -200 -300 EImV Fig.4. Steady-state polarisation curves under Temkin adsorption conditions calculated for k+,/k+, = lo3, P = lo3 at E = 0 and various values off. (a)-(g) as for fig. 1. ( '2 '7 exp ( ( l - p 2 ) =ads) -~ RT U+, = k:, [H+] 8, exp R T The overall (cathodic) current density in the presence of H, can be derived from eqn (9), (11), (12) and (14): F- (Q+1) '+'* At equilibrium i = 0 and PQ = 1. The exchange current density follows from eqn (20) and equals, under standard conditions ([H+] = 1, pH2 = 1 atm, E = 0): i 2(1-PQ) (20) - - The coverage 8, at equilibrium is given by BH/( 1 - 8,) = exp ( -dGadS/R7). Since it is likely that Q + 1 at equilibrium under standard conditions,17 the volcano curve for the V-H mechanism in terms of the apparent free energy of adsorption is given byA .Saraby- Reintjes 277 I I I I I I I I I I I I I 60 40 2 0 0 -2 0 - 40 - 60 mad, /kJ mol-I Fig. 5. Volcano curves for the HER by the V-H mechanism plotted as the logarithm of the exchange current density against different values of the apparent free energy of adsorption =ads: (a) Gads at equilibrium coverage under standard conditions, (b) =ad, at maximum coverage under standard conditions, (c) dGad, at minimum coverage under standard conditions, ( d ) dGads at zero coverage. It is clear that the shape and the height of this volcano curve are independent of the manner in which =ad, varies with the coverage, in other words that the choice of adsorption isotherm is irrelevant, provided log i, is plotted against the value o f x a d , under standard conditions. The maximum of the volcano curve is obtained by differentiating eqn (22): If it is assumed that p2 =L50, it follows that the maximum of the volcano curve is characterised by OH = 0.50, AG,,, = 0 and i, = Fk",.The volcano curve given by eqn (22) whenp, = 0.50 is shown as curve (a) in fig. 5. It is symmetric about Ead, = 0 and contains linear sections of slope BRT and -8RT at either side. It is not truncated due to Temkin adsorption, as predicted by some the0~ies.l~ In the unlikely case that p2 has a constant value not equal to 0.50, the volcano curve will be asymmetric, and contain linear sections of slope (1 -P2)/RT and -p,/RT at either side of the maximum. The maximum itself will then be characterised by 8, = p2, G a d , = - RT In p2/( I -&)I and i, = 2Fk~,p2[(~,/(l -p2)](1-82).There will again be no truncation of the top of the volcano curve. The more realistic assumption that p2 depends on AGad, has been elaborated by Parsons on the basis of parabolic potential-energy curves and Langmuir a d s ~ r p t i o n . ~ ~ The linear sections of the volcano curve are steeper in this case; however, the effect in the vicinity of the maximum, where Temkin adsorption has been thought to be operative, is negligible. The experimental verification of predicted volcano curves suffers from the difficulty of estimating G a d , in aqueous solutions. It has been customary to plot logi, (or the overpotential q) against estimated values of the M-H bond energy or the heat of adsorption, derived from gas-phase data24-26 or from electrochemical 28 or against the heat of formation of metal hydrides.28 Ideally, values of =ad, on various metals should be determined in aqueous solutions278 Electrocatalysis under Temkin Adsorption Conditions from the equilibrium coverages under standard conditions; however, it has hitherto proved very difficult to determine such coverages accurately.If E,,, is determined at coverages other than the equilibrium coverage, the relevant points on the volcano curve will undergo a horizontal shift by fRTABH. Curves (b)-(d) in fig. 5 illustrate how experimental volcano curves for /I = 0.50 will be distorted if log i,, is plotted against the values of E,,, at maximum, minimum (both under standard conditions) and zero coverage instead of mads at equilibrium.These curves have been calculated for the value f= 10, on the assumption thatfis independent of the nature of the metal. In curves (b) and (c) the deviation occurs chiefly on either side of the maximum; the volcano curve is now no longer symmetric about the maximum, but the height of the maximum has remained undiminished. Trasatti28 has reported that there is no evidence in his experimental volcano curves of the presence of a horizontal region near the maximum and that the slopes on either side of the maximum can be lower or higher than the theoretical value BRT. These findings are in qualitative agreement with the theory presented above. Conclusion Since it is possible to evaluate steady-state coverages under Temkin adsorption con- ditions as a function of the potential and the heterogeneity factor f numerically, as originally indicated by Conway and Gileadi, there is no need to restrict the kinetic treatment of electrocatalytic reactions under Temkin adsorption conditions to the potential range in which the first electron-transfer step is at quasi-equilibrium.No simplifying assumptions are required, and the steady-state treatment for mixed control by the kinetics of the first and second steps is therefore likely to predict the experimental behaviour more accurately than the quasi-equilibrium approach. While the coverage and the pseudo-capacity exhibit a marked dependence on the nature of the adsorption isotherm and the value o f f , it appears that the difference between steady-state current-potential curves for Temkin and Langmuir adsorption conditions is relatively small owing to the dual effect of Temkin adsorption on the coverage and the apparent rate constants.The widespread use of the Langmuir isotherm in the kinetic treatment of electrocatalytic processes is thereby largely justified, provided one remains aware of the fact that a Temkin-type isotherm is likely to give a more realistic representation of actual adsorption conditions and a more accurate estimate of the coverage. The theory presented supports the claim by Conway et al.21 that Temkin adsorption is not characterised by Tafel slopes of - 60 mV per decade. The full kinetic treatment predicts that Temkin adsorption affects the width rather than the height of volcano curves. The equations for steady-state current-potential curves under Temkin adsorption conditions, derived above for the HER by the Volmer-Heyrovsky mechanism, can be applied to similar simple electrocatalytic processes, but also, with minor modifications, to the anodic dissolution of a number of metals whereby solution anions act as catalysts,29 though strictly speaking dissolution of metals is not ranked among electrocatalytic processes.The author thanks Dr L. M. Peter for reading the manuscript. References 1 M. I. Temkin, Zh. Fiz. Khim., 1941, 15, 296. 2 J. Horiuti, Trans. Symp. Electrode Processes, ed. E. Yeager (Wiley, New York, 1959), p. 17. 3 E. Gileadi and B. E. Conway, Modern Aspects of Electrochemistry III, ed. J. O'M. Bockris and 4 B. E. Conway and E. Gileadi, Can. J . Chem., 1964, 42, 90.5 E. Gileadi and B. E. Conway, J. Chem. Phys., 1963,39, 3420. B. E. Conway (Butterworths, London, 1964), p. 347.A . Saraby-Reintjes 279 6 J. G. N. Thomas, Trans. Faraday Soc., 1961, 57, 1603. 7 L. I. Krishtalik, Zh. Fiz. Khim., 1959, 33, 2729. 8 B. E. Conway and E. Gileadi, Trans. Faraday SOC., 1962, 58, 2493. 9 I. A. Ammar and S. Darwish, Electrochim. Acta, 1967, 12, 833. 10 N. A. Darwish, F. Hilbert, W. J. Lorenz and H . Rosswag, Electrochim. Acta, 1973, 18, 421. 11 R. J. Chin and K. Nobe, J . Electrochem. SOC., 1972, 119, 1457. 12 H. C. Kuo and K. Nobe, J . Electrochem. SOC., 1978, 125, 853. 13 B. E. Conway, H. Angerstein-Kozlowska and H . P. Dhar, Electrochim. Acta, 1974, 19, 455. 14 R. Parsons, Trans. Faraday SOC., 1958, 54, 1053. 15 J. Horiuti and M. Polanyi, Acta Physicochim. URSS, 1935, 2, 505. 16 A. Saraby-Reintjes, J . Chem. SOC., Faraday Trans. 1, 1986, 82, 3343. 17 A. Saraby-Reintjes, Electrochim. Acta, 1986, 31, 251. 18 M. Boudart, J . Am. Inyt. Chem. Eng., 1956, 2, 62. 19 E. Gileadi, Electrosorption, ed. E. Gileadi (Plenum Press, New York, 1967), p. 16. 20 B. E. Conway, M.T.P. Rev. Sci., Ser. I , ed. J . O’M. Bockris (Butterworths, London, 1973), vol. 6, p. 41. 21 B. E. Conway, D. J. MacKinnon and B. V . Tilak, Trans. Faraday SOC., 1970, 66, 1203. 22 H. Gerischer, Bull. SOC. Chim. Belg., 1958, 67, 506. 23 R. Parsons, Surf. Sci., 1969, 18, 28. 24 B. E. Conway and J. O’M. Bockris, J . Chem. Phys., 1957, 26, 532. 25 H. Kita, J . Electrochem. SOC., 1966, 113, 1095. 26 A. K. Vijh and A. Belanger, Z. Phys. Chem. N.F., 1973,83, 173. 27 L. I. Krishtalik, Zh. Fiz. Khim., 1960, 34, 117. 28 S. Trasatti, J. Electroanal. Chem., 1972, 39, 163. 29 A. Saraby-Reintjes, Electrochim. Acta, 1985, 30, 387. Paper 5 / 1758 ; Received 10th October, 1985

ISSN:0300-9599    

DOI:10.1039/F19878300271

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1987, 83, 281-287 Mobilities and Molar Volumes of Multicharged Cations in N,N-Dimethylformamide at 25 "C Waclaw Grzybkowski" and Michal Pilarczyk Department of Physical Chemistry, Institute of Inorganic Chemistry and Technology, Technical University of Gdarisk, 80-952 Gdarisk, Poland The molar conductivities and the apparent molal volumes are reported for Be(C10,),, In(ClO,), and Al(ClO,), in N,N-dimethylformamide at 25 "C. The limiting equivalent conductivities and molar volumes of the cations are derived and discussed in terms of influence of charge number on the properties of DMF-solvated cations. The electrolytic conductivity and density of an electrolyte solution are physicochemically important properties in understanding the nature of solute-solvent and solute-solute interactions.Although a number of studies on the properties are to be found in the literature on aqueous sol~tions,~-~ experimental data on non-aqueous solutions of the multicharged electrolytes are still lacking for a profound discussion of the properties of the complex-forming cations. As part of our continuing programme of measurements leading to the properties of the solvated metal cations we have studied the molar conductivities and apparent molal volumes of the divalent transition-metal perchlorates in N,N-dimethylformamide (DMF) solution. It has been shown that the cations belonging to the Mn2+-Zn2+ series exist in DMF solution exclusively as M(DMF),"+-type solvated complexes, being the real migrating units responsible for charge transport due to cations.,9 Moreover, the derived mobilities and molar volumes of the solvates remain constant to within several percent of their absolute values when the nature of the metal is changed.The small but distinct variation of the properties of the cations within the Mn2+-Zn2+ series is related to the electronic structure of the central metal ion. The present study was undertaken in order to compare the properties of the structurally inter-related complex ions differing in the charge of the central metal cation and to examine the properties of the complex cation differing in the nature of the coordination sphere. In this paper the molar conductivities are obtained for In(ClO,), and Be(ClO,), and the apparent molal volumes are reported for A1(C10,)3, In(ClO,), and Be(ClO,), in DMF solution.It has been shown by Movius and Matwiyoff,6* and Schneider8 that A13+ occurs as the hexasolvated cation, Al(DMF)i+, and the same may be expected for In3+ in DMF solution. The existence of the Be(DMF)z+ cation is well established and the lifetime of solvent molecules in the primary solvation shell is quite long89 Experimental N,N-Dimethylformamide (reagent grade) was dried by means of 4A molecular sieve and distilled under reduced pressure at 45-50 "C. The specific conductivity of the purified solvent was in the range (3-5) x S cm-l. The density at 25 "C was 0.944030 g ~ m - ~ . Literature values are 0.943 87 and 0.94407 g ~ m - ~ according to ref. (10) and (1 l), respectively. DMF-solvated Be(ClO,),, Al(ClO,), and In(ClO,), were prepared from the corre- sponding hydrates by dissolving them in DMF, followed by removing any excess of the 28 1282 Mobilities and Molar Volumes of Multicharged Cations Table 1.Molar conductivities of Be(C10,), and In(C10,), in DMF solution at 25 "C C A m C Am / 1 OP4 mol dmP3 /S cm2 mol-1 / 1 O-, mol dm+ /S cm2 mol-I 1.4965 2.1732 2.8929 3.7797 4.7606 5.8523 7.5021 8.8658 10.812 14.191 17.456 21.906 0.85201 0.97657 1.2087 1.5044 1.7743 2.0194 2.3088 2.4535 2.7989 3.2720 4.06 17 5.0663 BWlO,), 182.53 23.982 29.671 179.52 176.84 33.513 174.01 46.305 170.99 60.245 168.34 76.682 164.36 94.045 162.12 151.87 158.70 203.47 1 54.54 270.51 1 50.52 341.86 147.19 In(C104), 274.13 6.1644 271.80 9.1546 267.88 16.185 263.49 25.806 259.79 36.526 256.48 66.613 253.01 102.46 25 1.47 146.48 247.95 208.98 243.37 256.66 236.98 230.41 145.56 141.65 139.39 133.30 128.45 123.75 119.88 110.57 105.47 100.20 95.69 224.44 21 1.97 194.18 180.07 170.19 153.35 142.54 133.90 126.76 1 20.79 solvent under reduced pressure at 60 "C.The crystalline solids were recrystallized twice from anhydrous DMF. The stock solutions of A1(C104), and In(ClO,), were analysed for the respective metals using the 8-hydroxyquinoline gravimetric method. The stock solution of Be(ClO,), was analysed by conductometric titration using NaOH solution. Solutions for measurement were prepared by weighed dilutions, making vacuum corrections in all cases. Details of the procedures for measuring conductance and density values were identical to those described previo~sly.~~ All the preparations and further manipulations involving anhydrous materials were performed in a dry box.Results and Discussion The conductance data are listed in table 1. In order to compare the data for electrolytes differing in their cation charge fig. 1 shows the equivalent conductivities plotted against square root of the equivalent concentration. The solid line represents the conductimetric curve obtained previously4 for Zn(C104), in DMF at 25 "C. Our previous study has shown that the conductimetric curves for Mn(ClO,),, Co(ClO,),, Ni(C10,),, Cu(ClO,), and Zn(C104), display close similarity owing to the fact that all five perchlorates occur as the complex electrolytes of the M(DMF)g+ - 2ClO;-type involving the hexa-solvated cations, differing in the nature of the central metal ion Inspection of fig.1 shows that at lowest concentrations the line for the transition-metal perchlorates runs wellW. Grzybkowski and M . Pilarczyk 40 283 - 0 0 0 0.0 0.1 0.2 (Clequiv d ~ n - ~ ) * Fig. 1. Plots, at 25 "C, of the equivalent conductivity us. the square root of equivalent concentration for Be(C10,), (0) and In(ClO,), (0). The solid line represents the conductivity curve for Zn(C10,), . below the points found for Be(ClO,),. However, at the higher concentrations the differences gradually decrease. The equivalent conductivities of In(ClO,), are markedly lower than those of Be(ClO,),, but their variation at the lowest concentration range indicates that the limiting values display the opposite order. Moreover, both of them seem to be higher than the value for Zn(C10,),.The attempt to derive the limiting equivalent conductivities made use of the equation expressing the conductivities in terms of ionic strength A = Ao-S11/2+EIlnI+J1 (1) assuming A', E and J are adjustable parameters. Accordingly, A' values have been found by the procedure consisting of the linearization of the (A-Ao+SZ1/2)/I us. 1nI and minimization against A'. The least-squares procedure leads to the equation describing the equivalent conductivity : A = 101.30 - 368.6I1l2 + 802.71 In I + 2958.71 for the solutions of In(ClO,),, valid up to an ionic strength of 0.0015 mol dm-, (a = 0.02) and A = 97.95 - 274.1 P2 + 467.31 In I+ 1626.1 I for the solutions of Be(ClO,),, valid up to an ionic strength of 0.0022 mol dm-, (a = 0.04).The calculations were performed using literature data12 for the viscosity and dielectric constant of DMF ('1 = 0.796 CP and E = 36.71) and the known value1, of the limiting conductivity of C10, in DMF, A? = 52.4 S cm2 mol-1 at 25 "C. The results obtained for the solutions of Be(ClO,), were analysed using the Fuoss and Edelson method as described in a previous paper.5 The treatment yields Ao = 97.75 S cm2 equiv-l for the limiting equivalent conductance and K = 57 mol dm-, for the first-step association constant at 25 "C. The good agreement of the derived limiting conductivities provides support for the reliability of the treatments used in our study. The association constant for Be(C10,), is higher than the value of 25 mol dm-, determined for Zn(C10,), using the same m e t h ~ d .~ The smaller tetra-solvated (2) (3)284 Mobilities and Molar Volumes of Multicharged Cations Table 2. Relative densities and apparent molal volumes for Be(ClO,),, Al(ClO,), and In(C10,), in DMF solution at 25 "C ~ C 1000(d-d*) /mol dm-3 /g dmP3 0.023 376 0.029 43 8 0.039460 0.053 088 0.068 096 0.078 99 0.12649 0.21293 0.432 56 0.0 10078 0.01992 1 0.030 160 0.040 170 0.049 97 5 0.060 102 0.005 501 0.010038 0.020 327 0.030309 0.040 292 0.050275 0.060 4 1 2 Be(ClO,), 4.218 5.275 6.997 9.374 1 1.960 13.842 21.855 36.322 72.190 AWO,), 2.883 5.566 8.204 10.980 13.612 16.215 In(ClO,), 2.045 3.688 7.310 10.930 14.460 17.972 21.559 cbv /cm3 mol-l 29.09 30.43 32.41 33.21 34.19 34.62 37.22 39.54 43.45 41.57 48.64 56.46 55.09 56.11 58.83 43.86 48.90 56.74 55.67 57.5 1 59.00 59.65 Be(DMF)i+ cation exhibits a more pronounced ability for outer-sphere association than the hexa-solvated Zn(DMF)i+ ion.The density data are presented in table 2, where the apparent molal volumes, &, have been included. The densities can be calculated from the least-squares polynomials of the (4) form where do = 0.944030 g ~ r n - ~ is density of the solvent. The resulting equations are listed in table 3. 1000(d - do) = AC + Bc3J2 + Cc2 Apparent molal volumes of the solutes were calculated using & = M2/d0 - 1000(d - do)/do c ( 5 ) where M , is molecular weight of the salts, 207.913 for Be(ClO,),, 325.333 for Al(C10,), and 413.172 for In(ClO,),. We found that values are not a linear function of the square root of concentration and can be expressed in terms of c using a Redlich-type equation:l> l4 ( 6 ) &, = 4; + Sv c1J2 + bv C.Unfortunately, the theoretical slope, S,, cannot be calculated from the Debye-Huckel limiting law because of the lack of physical data for DMF. Thus, the data were fitted into the polynomials and the resulting equations are listed in table 4. Further discussion may be made after splitting the limiting quantities into their ionicW. Grzybkowski and M . Pilarczyk 285 Table 3. Density equations for the metal perchlorates in DMF at 25 "C salt standard best equation deviation Be(ClO,), 1000(d- do) = 186.7~ - 4 9 . 9 ~ ~ 1 ~ + 3 0 . 4 ~ ~ 0.040 Al(ClO,), 1000(d-do) = 295.9~- 1 0 8 . l ~ ~ / ~ 0.030 In(ClO,), 1000(d-do) = 376.6~ - 8 5 .3 ~ ~ 1 ~ 0.040 Table 4. Apparent molal volume equations for the metal perchlorates in DMF at 25 "C salt best equation Be(C10,), In(ClO,), #v = 22.43 + 5 2 . 8 9 ~ ~ ' ~ - 32.17~ #v = 30.79 + 21 1.45~'/~ - 384.55~ Al(ClO,), #v = 20.24+ 255.90~"~ -410.60~ Table 5. Limiting equivalent conductivities, partial molar volumes and crystallographic radii for the DMF-solvated cations at 25 "C Zn( DMF):+ 39.05 -48.1 -49.4 0.74 Al(DMF);+ 50.25" - 94.4 -94.8 0.50 In(DMF),3+ 48.30 - 84.0 -85.7 0.81 Be( DM F)i+ 45.35 - 54.0 -54.1 0.31 - 1.26 35.22b - Ag(DMF),' a From ref. (4). From ref. (12), p. 677. contributions. The limiting equivalent conductivities of the Be2+ and In3+ cations in DMF solution were calculated on the assumption that 2 = 52.4 S cm2 mol-1 for the C10; anion.13 The partial molar volumes of the cations were calculated using the value of 38.2 cm3 mol-l for anion^,^ estimated previously by means of the correspondence principle and Mukerjee's method.The resulting values are listed in table 5 along with the respective data for the Zn2+ and Ag+ cations. The Zn2+ cation was chosen as belonging to the Mn2+-Zn2+ series and adjacent to the trivalent cations studied in present paper. 'There is considerable evidence that Ag+ in the absence of coordination anions is solvated by four DMSO molecules.* Taking into account the similar solvating properties of DMF and DMSO* we infer that the same applies to the Ag+ cation in DMF solution. Moreover, the coordination number four was found for Ag+ in a~etonitri1e.l~ Thus, the Ag+ and Be2+ cations can be considered as the structurally inter-related coordination clusters, Ag(DMF)i and Be(DMF),2+.Inspection of the data shows that the limiting equivalent conductivity of the In3+ cation is closely similar to that of A13+. The same applies to their volumetric properties. These facts provide the evidence that the In3+ cation exists also as the hexa-solvated In(DMF)i+ complex cation. It has been shown by Libui that the same is true for the trivalent cations in DMSO sol~tion.~? l6286 Mobilities and Molar Volumes of Multicharged Cations 6o i Fig. 2. Plots of the limiting equivalent conductivities of the DMF-solvated cations us. the charge number. A more detailed analysis of data presented in table 5 shows that the ratios of the limiting equivalent conductivities of the structurally inter-related complex cations differing by unit in their charge number Lo [iM (D M F)i+] 'o[Ag(DMF)zl = 0.78 .= 0.79 0.03 and Lo[+M(DM F);+] lo[iBe(DMF),2+] amount to the same value of 0.8. The value reported for the hexa-solvated cations is the average one calculated for the five divalent transition-metal cations and two trivalent cations. A similar finding was reported by LibuS for acetonitrile- and DMSO-solvated cations. The preliminary observation for DMF solution was also pre~ented.~ As is seen, the conductance ratios are distinctly higher than the values of 2/3 and 1/2 resulting from Stokes law on the assumption of constant radii of the solvated cations. Fig. 2 shows plots of the equivalent conductivity of the DMF-solvated cations us.the charge number for the two groups of the structurally inter-related solvo-cations. Inspection of this figure shows that the values of 1; do not exhibit the direct proportionality to the charge number arising from the Stokes relationship. The dependence is less steep, indicating an increase of dielectric drag related to increasing ionic charge. It seems to be obvious that an influence of the charge on the size of the well defined structures should be an effect of minor importance. Inspection of the partial molar volumes shows a distinct difference between the data obtained for the di- and tri-valent hexa-solvated cations. This effect is due to the difference in electrostriction being the most important component of the partial molar volume of an ion.The most common interpretation of the p (ion) values as a function of size and charge have been developed by assuming that p (ion) is made up of two major components : To (ion) = V O (int)+ TO (elect) (7) where Vo (int) is the intrinsic partial molar volume of the ion and To (elect) is the electrostriction partial molar volume of the ion. According to Heplerl' TO (ion) = Ar3-BBZ2/r (8) where A and B are empirical constants. As is seen, the contribution due to the electrostriction is determined by the second power of charge number and the reciprocal of crystallographic radius. Vo (int) may be estimated using the values of 3.2 found for A in previous The resulting values of TO (elect) are given in table 5 along with the corresponding ionic radii.l8 As is seen, the contribution related to the size of the ionsW.Grzybkowski and M . Pilarczyk 287 is relatively small. Comparison of the radii leads to the conclusion that the difference in electrostriction is determined by the charge number. However, the influence of the ion size is evident for the trivalent cations. The difference in ionic radius is clearly reflected in their volumetric properties and mobilities. It should be noted that the Be(DMF),2+ cation exhibits the relatively high value of ro (elect). The effect is especially distinct when it is related to one molecule of solvent. The corresponding value amounts to 13.5 cm3 per molecule and is much higher than the value of 8 cm3 for Zn(DMF)i+. The significant difference is a clear manifestation of the difference in ion-solvent interaction due to the electronic structure of the central metal ion.The same is reflected in their ability for ion-pair formation. The regularity presented in this paper provides a basis for more realistic estimation of the limiting conductivities of the complex ions or ion pairs than those arising from Stokes law. The most popular approximation consists in the assumption that Ao[MX+] = 0.5;1°[+M2+]19 or mobility of the M2+ cation is twice that of MX+ ion pair.20 Our results suggest that A"[MX+] is a much higher fraction of ;1"[+M2+]. The value of 0.80 for the ratio appears very reasonable. Moreover, the results of calculations performed by Pethybridge21 using the Lee-Wheaton equation provide good support for reliability of this value. References I F.J. Millero, Chem. Rev., 1971, 71, 147. 2 B. S. Krumgalz, J . Chem. Sac., Faraday Trans. 1, 1980, 76, 1887. 3 J. W. Akitt, J . Chem. Soc., Faraday Trans. 1, 1980, 76, 2259. 4 W. LibuS, B. Chachulski, W. Grzybkowski, M; Pilarczyk and D. Puchalska, J. Solution Chem., 1981, 5 W. Grzybkowski and M. Pilarczyk, J. Chem. Soc., Faraday Trans. 1, 1983,79, 2319. 6 W. G. Movius and N. A. Matwiyoff, Znorg. Chem., 1967, 6, 847. 7 W. G. Movius and N. A. Matwiyoff, J . Phys. Chem., 1968,72, 3063. 8 H. Schneider, Electrochim. Acta, 1976, 21, 71 1 . 9 N. A. Matwiyoff and W. G. Movius, J . Am. Chem. SOC., 1967, 89, 6077. 10 M. R. J. Dack, K. J. Bird and A. J. Parker, Aust. J . Chem., 1975, 28, 955. I 1 D. Hamilton and R. H. Stokes, J . Solution Chem., 1972, 1, 213. 12 A. C. Covington and T. Dickinson, Physical Chemistry of Organic Solvent Systems (Plenum Press, 13 P. Bruno and M. Della Monica, Electrochim. Acta, 1975, 20, 1979. 14 0. Redlich and P. Rosenfeld, Z . Electrochem., 1931,37, 105. 15 T-C. Chang and D. E. Irish, J . Solution Chem., 1974,3, 161. 16 M. Pilarczyk and W. LibuS, Bull. Pol. Ac. Chem., 1972, 20, 539. 17 L. C. Hepler, J . Phys. Chem., 1957, 61, 1426. 18 A. F. Wells, Structural Inorganic Chemistry (Clarendon Press, Oxford, 1975), p. 259. 19 A. Diamond, A. Fanelli and S. Petrucci, Inorg. Chem., 1973, 12, 61 1 . 20 A. M. Hafez, M. M. Osman and M. M. El-Kholy, Electrochim. Acta, 1985, 30, 541. 21 A. D. Pethybridge, 2. Phys. Chem. N.F., 1982, 133, 143. 10, 631. London, 1973), p. 5. Paper 51221 3 ; Received 16th December, 1985

ISSN:0300-9599    

DOI:10.1039/F19878300281

出版商:RSC    

年代:1987

数据来源: RSC