摘要:

Ordinary Members PROFESSOR R. J. DONOVAN 1983 PROFESSOR M. C. R. SYMONS 1983 DR G. J. HILLS 1984 PROFESSOR J. M. THOMAS 1983 PROFESSOR A. J. LEADBETTER 1984 DR J. ULSTRUP 1985 DR I . W. M. SMITH 1985 PROFESSOR G. WILLIAMS 1985 PROFESSOR F. L. SWINTON 1983 DR D. A. YOUNG 1984 Honorarj, Secretarj-: DR G. J. HILLS Honorarj- Treasurer : PROFESSOR P. GRAY The President thanked the retiring members of Council, Vice-presidents Professor Sheppard and Professor Wagner, and Ordinary Members Professor King and Professor Purnell, for their services. 5. Reriew of Futurr Acfirifies A programme of future activities of the Division had been tabled and the President drew attention to the forthcoming General Discussions and Symposia. xiOrdinary Members PROFESSOR R. J. DONOVAN 1983 PROFESSOR M. C. R. SYMONS 1983 DR G. J. HILLS 1984 PROFESSOR J. M. THOMAS 1983 PROFESSOR A. J. LEADBETTER 1984 DR J. ULSTRUP 1985 DR I . W. M. SMITH 1985 PROFESSOR G. WILLIAMS 1985 PROFESSOR F. L. SWINTON 1983 DR D. A. YOUNG 1984 Honorarj, Secretarj-: DR G. J. HILLS Honorarj- Treasurer : PROFESSOR P. GRAY The President thanked the retiring members of Council, Vice-presidents Professor Sheppard and Professor Wagner, and Ordinary Members Professor King and Professor Purnell, for their services. 5. Reriew of Futurr Acfirifies A programme of future activities of the Division had been tabled and the President drew attention to the forthcoming General Discussions and Symposia. xi

ISSN:0300-9599    

DOI:10.1039/F198278FX045

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

3 708 REVIEW OF BOOKS is the absence of any reference to possible new and potentially significant applications for polymer latices. Novel applications may well be found in at least two directions, namely, those which exploit the large polymer-aqueous-phase specific surface area of latices, and those which exploit the electrical dissymmetry which is present at the interface between polymer and aqueous phase in the case of electrostatically stabilised latices. No reference is made in this book to the efforts which have so far been made to exploit for medical purposes the adsorptive and binding potentialities of the large area of polymer-aqueous-phase interface in latices. Nor is there any mention of possible catalytic applications of this large interfacial area. So far, catalytic applictions have been confined to those which rely essentially upon enhancement of the counter-ion concentration in regions of the electrical double layer which are near to the polymer surface.However, it is at least possible that the adsorptive capacity of the interface may also be useful in catalytic applications. Some discussion of possibilities such as these would have been welcome. D. C. BLACKLEY Received 14th April, 1982 Shock Waves in Chemistry. Ed. by ASSA LIFSHITZ. (Marcel Dekker, New York, 1981). Pp. ix + 390. Price SFr 182. After a somewhat hesitant start, the use of shock waves to study chemical and physical processes at high temperatures has become an accepted technique and reliable kinetic data can be obtained in this way. Several books have been written, notably by Bradley and by Gaydon and Hurle, which describe not only the underlying principles and the experimental procedures but also give some account of the early results obtained using shock waves to provide high temperatures for short, well defined times in the reactant gases.Inevitably, these books have become rather dated. This new book, edited by Lifshitz, is rather different. It is a collection of self-contained review articles on various aspects of shock waves. The first (by Khandelwal and Skinner) is concerned with hydrocarbon oxidation, and the next (by Tsang) describes the results obtained using the comparative rate technique which he has pioneered. Both these articles include extensive lists of references and represent useful summaries of the present situation.Boyd and Burns have contributed a chapter on dissociation-recombination reactions, while Kiefer describes the laser-schlieren method which he has done so much to develop. There is another chapter by an acknowledged expert, Just, on atomic resonance absorption spectrometry. Under shock-tube conditions it is very seldom that the concentrations of radicals and other species reach a steady state, and so the classical Bodenstein steady-state approximation cannot be used. Instead, it is necessary to integrate the differential equations describing the time-variation of species concentration, and Gardiner, Walker and Wakefield have provided a useful guide to the computational procedures available in this and other aspects of shock-tube work.In addition to these contributions there is another by Bar-Nun on Chemical Aspects of Shock Waves in Planetary Atmospheres which, although interesting in itself, fits rather uneasily with its companions. As is inevitable in a book of this type the standard and style of the chapters varies and there is some overlapping material; none of this, however. represents a serious drawback. What is more difficult to understand is the audience for whom the book is intended. Each chapter is a useful and interesting review which will appeal to a fairly restricted readership, but, in the opinion of this reviewer, the whole volume lacks coherence. The time-honoured phrase ‘should be on the shelves of every library’ probably applies, though the price, over &50 at the current exchange rate, must cause all university librarians to flinch in these days of U.G.C.cuts. There is still room for the definitive up-to-date book to be written on shock waves in chemistry. J. A. BARNARD Received 5th April, 19823 708 REVIEW OF BOOKS is the absence of any reference to possible new and potentially significant applications for polymer latices. Novel applications may well be found in at least two directions, namely, those which exploit the large polymer-aqueous-phase specific surface area of latices, and those which exploit the electrical dissymmetry which is present at the interface between polymer and aqueous phase in the case of electrostatically stabilised latices. No reference is made in this book to the efforts which have so far been made to exploit for medical purposes the adsorptive and binding potentialities of the large area of polymer-aqueous-phase interface in latices.Nor is there any mention of possible catalytic applications of this large interfacial area. So far, catalytic applictions have been confined to those which rely essentially upon enhancement of the counter-ion concentration in regions of the electrical double layer which are near to the polymer surface. However, it is at least possible that the adsorptive capacity of the interface may also be useful in catalytic applications. Some discussion of possibilities such as these would have been welcome. D. C. BLACKLEY Received 14th April, 1982 Shock Waves in Chemistry. Ed. by ASSA LIFSHITZ. (Marcel Dekker, New York, 1981). Pp. ix + 390.Price SFr 182. After a somewhat hesitant start, the use of shock waves to study chemical and physical processes at high temperatures has become an accepted technique and reliable kinetic data can be obtained in this way. Several books have been written, notably by Bradley and by Gaydon and Hurle, which describe not only the underlying principles and the experimental procedures but also give some account of the early results obtained using shock waves to provide high temperatures for short, well defined times in the reactant gases. Inevitably, these books have become rather dated. This new book, edited by Lifshitz, is rather different. It is a collection of self-contained review articles on various aspects of shock waves. The first (by Khandelwal and Skinner) is concerned with hydrocarbon oxidation, and the next (by Tsang) describes the results obtained using the comparative rate technique which he has pioneered.Both these articles include extensive lists of references and represent useful summaries of the present situation. Boyd and Burns have contributed a chapter on dissociation-recombination reactions, while Kiefer describes the laser-schlieren method which he has done so much to develop. There is another chapter by an acknowledged expert, Just, on atomic resonance absorption spectrometry. Under shock-tube conditions it is very seldom that the concentrations of radicals and other species reach a steady state, and so the classical Bodenstein steady-state approximation cannot be used. Instead, it is necessary to integrate the differential equations describing the time-variation of species concentration, and Gardiner, Walker and Wakefield have provided a useful guide to the computational procedures available in this and other aspects of shock-tube work.In addition to these contributions there is another by Bar-Nun on Chemical Aspects of Shock Waves in Planetary Atmospheres which, although interesting in itself, fits rather uneasily with its companions. As is inevitable in a book of this type the standard and style of the chapters varies and there is some overlapping material; none of this, however. represents a serious drawback. What is more difficult to understand is the audience for whom the book is intended. Each chapter is a useful and interesting review which will appeal to a fairly restricted readership, but, in the opinion of this reviewer, the whole volume lacks coherence. The time-honoured phrase ‘should be on the shelves of every library’ probably applies, though the price, over &50 at the current exchange rate, must cause all university librarians to flinch in these days of U.G.C. cuts. There is still room for the definitive up-to-date book to be written on shock waves in chemistry. J. A. BARNARD Received 5th April, 1982

ISSN:0300-9599    

DOI:10.1039/F198278BX047

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY FARADAY TRANSACTIONS, PARTS I AND I1 The Journal of The Chemical Society is issued in six sections: Journal of The Chemical Society, Chemical Communications Journal of The Chemical Society, Dalton Transactions Journal of The Chemical Society, Faraday Transactions, I Journal of The Chemical Society, Faraday Transactions, II Journal of The Chemical Society, Perkin Transactions, I Journal of The Chemical Society, Perkin Transactions, 11 Thus, five of the sections are directly associated with three of the Divisions of The Royal Society of Chemistry: the sixth is Chemical Communications. This continues to be the medium for the publication of urgent, novel results from all branches of chemistry. Communications should not normally exceed one printed page in length and authors are required to submit three copies of the typescript and two copies of a statement of the reasons and justification for seeking urgent publication of the work.This Section is intended to be essentially a journal for inorganic chemists containing papers on the structure and reactions of inorganic compounds and the application of physical chemistry techniques to, e.g. the study of inorganic and organometallic compounds and problems, including work on the kinetics and mechanisms of inorganic reactions and equilibria, and spectroscopic and crystallographic studies of inorganic compounds. Journal of the Chemical Society, Faraday Transactions, I and II These are, respectively, physical chemistry and chemical physics journals. P A R T I (physical chemistry) includes papers on such topics as radiation chemistry, gas-phase kinetics, electrochemistry (other than preparative), surface and interfacial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions and kinetics of polymerization, etc.P A R T I I (chemical physics) contains theoretical papers, especially those on valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenom- ena, spectroscopic studies (including i.r., e.s.r., n.m.r., and kinetic spectroscopy, etc.) leading to assignments of quantum states, and fundamental theory, and also studies of impurities in solid systems, etc. These are, respectively, the organic chemistry and the physical organic chemistry sections of the Journal.P A R T I (organic and bio-organic chemistry) is designed to contain papers on all aspects of synthetic, and natural product organic and bio-organic chemistry and to deal with aliphatic, alicyclic, aromatic, carboncyclic and heterocyclic compounds. Papers on organometallic topics are considered for either the Dalton or the Perkin Transactions. Journal of The Chemical Society, Chemical Communications Journal of The Chemical Society, Dalton Transactions Journal of The Chemical Society, Perkin Transactions, I and 11 1P A R T I I (physical organic chemistry) is for papers on reaction kinetics and mechanistic studies of organic systems and the use of physico-chemical. spectroscopic. and crystallographic techniques in the solution of organic problems. ,Yotict> t o Author.v ( I ) Although authors need not be members of the Royal Society of Chemistrq, i t is hoped that they will be.( 2 ) Authors must indicate the Part of the Jourriul they wish their paper to appear in. This preference will be respected unless i t is obviously erroneous in terms of the scientific con tent of the paper. (3) Since all papers will be subjected to refereeing, in parallel, by two independent referees. the original typescript (quarto or A4 size) and two good-quality copies should be provided. (4) All papers should be sent to the Director of Publications, The Royal Society of Chemistry, Burlington House, Piccadilly, London W 1 V OBN. ( 5 ) For details of manuscript preparation. preferred usages, etc. the In.struc~tion.s to author.^. previously available from the Faraday Society.and now obtainable from The Royal Society of Chemistry, should be consulted. (6) The Society will adopt the following abbreviations for the new journals in all its publications. J. Chtwi. Soc.. Cheni. Commuri. J. Cheni. Soc., Dulton Truns. J. Choni. Soc., Furudu?. Trunx. I J . Clieni. Soc., Furudu-\, Truns. 2 J . Cheni. Soc.. Perkin Trans. I J . Cheni. Soc.. Pcrkin Truns. 2 * The author to whom correspondence should be addressed is indicated by an asterisk after his name in the heading of each paper.THE FARADAY DIVISION OF THE ROYAL SOCIETY O F CHEMISTRY Marlow Medal and Prize Applications are invited for the award of the Marlow Medal for 1983 and prize of fl00. The award will be open to any member of the Faraday Division of The Royal Society of Chemistry who, by the age of 32, had made in the judgement of the Council of the Faraday Division, the most meritorious contribution to physical chemistry or chemical physics.The award will be made on the basis of publications (not necessarily in the Transactions) on any subject normally published in J. Chem. SOC., Faraday Transactions I and 11, that carry a date of receipt for publication not later than the candidate's 32nd birthday. Candidates should be members and under 34 on 1 st January 1983, the closing date for applications, which may be made either by the candidate himself or on his behalf by another member of the Society. Copies of the rules of the award and application forms 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 S Y M P O S I U M NO.1 7 The Hydrophobic Interaction University of Readrng, 15-16 December 1982 This term refers to interactions between chemically inert residues arising froni perturbdtions in the unique spdtial and orientatlonal correlations in liquid water These effccts provide a major contribution to many of the non covdlently bonded structures that form the basis of life processes Current advances in the statistical mechanics of polar fluids Internioleculdr forces computer simulation and membrane physics are providing a new basis for the r e examination of various aspects of hydrophobic effects their oriqin and their qiiantitatii e description Such theoretical treatments will be confronted with recerlt experimental work on simple model systems which i t i s hoped will lead to a better understanding of hLdrophobic interactions in more complex processes The following have agreed to contribute to the symposium A Ben-Naim, H J C Berendsen, D L Beveridge, S D Christian, L Cordone, D Eagland, D Eisenberg, R Lumry, P J Rossky, M C R Symons, H Weingartner, M D Zeidler The programme and application form may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN ... 111THE FARADAY DIVISION OF THE ROYAL S O C I E T Y O F CHEMISTRY GENERAL DISCUSSION N O 75 I n t ra mo I ecu I a r K i net i cs University of Warwick, 1S20 April 1983 Organising Committee Professor J P Simons (Chairman) Dr D M Hirst Dr M S Child Professor R J Donovan Dr R Walsh Dr G Hancock Professor K R Jenninps Experimental and theoretical interest in the time-dependent behaviour of isolated molecules radicals or ions is strong and increasing The Discussion will be concerned with the kinetics of processes which occur in isolated species following their preparation in states with non-equilibrium energy distributions ( e g by photon absorption or collisional activation) Topics covered will include ( a ) theoretical and experimental studies of energy redistribution in isolated species ( b ) observation and theoretical modelling of the competition between intramolecular energy redistribution and radiative decay or radiationless processes ( e y internal conversion, fragmentation.isomerisation) The preliminary programme may be obtained from Mrs Y. A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN T H E FARADAY DIVISION O F THE ROYAL SOCIETY O F CHEMISTRY GENERAL DISCUSSION NO. 76 Concentrated Colloidal Dispersions Loughborough University of Technology, 1 6 1 6 September 1983 The meeting will discuss the experimental investigation and the theoretical description of the properties of concentrated colloidal dispersions, I e those systems in which the particle-particle interactions are strong enough t o cause significant deviations from ideal behaviour Both the structural and dynamic features of concentrated systems as determined by scattering, rheological and other techniques will be considered It is anticipated that a range of dispersion types will be discussed These will include both 'model' systems and dispersions of importance to industry provided that the data from the measurements can be interpreted Further information may be obtained from Professor R .H. Ottewill, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS ivT H E F A R A D A Y D I V I S I O N O F T H E R O Y A L SOCIETY OF CHEMISTRY S Y M P O S I U M NO. 18 Molecular and Microstructural Basis of Viscoelast icity and Related Phenomena Robinson College, Cambridge, 8-9 December 1983 Organising Committee Sir Geoffrey Allen (Chairman) Professor Sir Sam Edwards Dr R. A. Pethrick Dr P. Richmond Dr D. A . Young (Editor) 1 Dr M. La1 The past few years have witnessed the development of new concepts which provide a deeper understanding of the relationship between molecular dynamic and microstructural features of systems and their viscoelastic behaviour.This Symposium is designed to bring together original contributions involving theoretical, computational and experimental studies which represent significant advances in this important field of current activity. It is hoped that such contributions, together with the discussion that they will generate, will lead to new insights into the molecular mechanisms underlying the viscoelastic/rheological behaviour of, for example, flexible and rigid rod-like polymer molecules, liquid crystals and composites. In addition to three oral sessions (at which the main papers will be presented and discussed), the Symposium may include a poster session.Such poster papers will not be published in the Symposium volume. Further information may be obtained from: Or M. Lal. Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JW THE F A R A D A Y D I V I S I O N O F THE R O Y A L SOCIETY O F CHEMISTRY GENERAL D I S C U S S I O N N O . 77 Interfacial Kinetics in Solution University of Hull, 9-11 April 1984 This Discussion will focus attention on reactions involving liquid-gas, liquid-liquid and liquid-solid interfaces (but it will not include electrode kinetics as such) The subject encompasses processes of fundamental, industrial and environmental importance and includes such topics as the rate of dissolution of reactive gases, kinetics at liquid membranes, metal and solvent extraction, Marangoni effects, heterogeneous catalysis and photocatalysis in soiution, and the kinetics of dissolution of minerals and drugs The aim of the meeting is to bring together workers in these diverse fields to highlight the complementary nature of the problems encountered and of the results obtained, and to disseminate ideas concerning new and effective experimental techniques and novel theoretical approaches Contributions for consideration by the organising committee are invited Titles should be submitted as soon as possible, and abstracts of about 300 words by 15th April 1983, to Professor D .H. Everett, Department of Physical Chemistry, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS VFARADAY DIVISION INFORMAL AND GROUP MEETINGS Division with the Institute of Physics Applications of Electron Microscopy in Chemistry To be held at the Geological Society, London on 10 January 1983 Further information from: Mrs J.Cegielka, Institute of Physics, 47 Belgrave Square, London SW1X 8QX Electrochemistry Group Spring Informal Meeting To be held at the University of Newcastle on 29-30 March 1983 Further information from Dr R. D. Armstrong, Department of Chemistry, University of Newcastle, Newcastle upon Tyne NE1 7RU Theoretical Chemistry Group - Half-da y Spring Meeting To be held at King's College, London on 2 March 1983 Further information from Dr G. G. Balint-Kurti, School of Chemistry, University of Bristol, Bristol BS8 1TS Division - Half - da y Symposium Including the Faraday Lecture: J.S. Rowlinson To be held at Imperial College, London on 10 March 1983 Further information from Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W7 V OBN Statistical Mechanics and Thermodynamics Group Liquids and Liquid Mixtures To be held at the University of Hull on 28-29 March 1983 Further information from Dr P. G. Francis, Department of Chemistry, The University, Hull HU6 7RX Division with Macrogroup UK and Polymer Physics Group Annual Chemical Congress: Copolymers To be held at the University of Lancaster on 11-1 3 April 1983 Further information from Dr J. F. Gibson, The Royal Society of Chemisrry, Burlington House, London W1 V OBN SCI Electrochemical Technology Group and Electrochemistry Group Ion Exchange Membranes To be held in Chester on 13-1 5 April 1983 Further information from Society of Chemical Industry, 14 and 15 Belgrave Square, London SWl 8PS Colloid and Interface Science Group Proteins and Colloidal Systems To be held at the University of Leeds on 14-1 5 April 1983 Further information from Dr E.Dickinson, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT Polymer Physics Group, Macrogroup UK and Plastics and Rubber Institute Polyethylenes 1933-1 983 To be held in London on 8-1 0 June 1983 Further information from The Plastics and Rubber Institute. 11 Hobart Place, London SW1 W OH2 Industrial Physical Chemistry Group Crystallization Processes in Condensed Phases To be held at Girton College, Cambridge on 5 7 July 1983 Further information from Dr I.D. Robb, Port Sunlight Laboratory, Bebington, Wirral, Merseyside L63 3.JW Polymer Physics Group Physical Aspects of Polymer Science To be held at the University of Reading on 14-1 6 September 1983 Further information from Dr D. Bassett, University of Reading, Whiteknights, Reading RG6 2AH~~~ ~ Evaluated Kinetic Data for High Temperature Reactions, Volume 4: Homogeneous Gas Phase Reactions of Halogen- & Cyanide-Containing Species D.L. Baulch, J. Duxbury, SJ. Grant, D.C. Montague Department of Physical Cbenzistyl, Unizenity of Lee&, U.K The results of decades of research are now located in this one convenient publication! Published by the American Chemical Society and the American Institute of Physics for the National Bureau of Standards, this monograph presents in 721 pages, kinetic data for 300 homogeneous gas phase reactions involving halogens, the cyanide radical, and their compounds.Wherever possible, the data have been critically evaluated to give the best estimates of reaction rate parameters and their associated limits and temperature ranges. The supplement also offers relevant thermodynamic data, discusses it thoroughly, and lists recommended rate constants for each reaction i n tabular form. A comprehensive bibliography lists all pertinent literature in the field. American Chemical Society, B&J Business Operations 1155 16th St., N.W., Washington, D.C. 20036 Please send me ~ hardcover copies of Evaluated Kinetic Data for High Temperature Reactions, Volume 4: Homogeneous Gas Phase Reactions of Halogen- and Cyanide-Containing Species at $80.00 each.Please include payment with order.* Name City State Zip *Foreign orders :icIcI $i 00 e:ich tor post:ige :ind I~anclling. I;oreign p+meiit iiiiist I>e m;icle in U.S. ciiri.eiicy. b y iiitei.n;rtiona1 money 01-cler. UNESCO coupons, or orcler through yoni- siil>scriptioii agent!. Cdifornia residents, xld 6% state tibe t;ix.NOTES I t has always been the policy of the Faraday Transactions that brevity should not be a factor influencing acceptability for publication. In addition however to full papers both sections carry at the end of each issue a section headed “Notes”, which are short self-contained accounts of experimental observations. results, or theory that will not require enlargement into “full” papers.The “Notes” section is not used for preliminary communications. The layout of a “Note” is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of “Notes” is the same as for “full” papers. However, “Notes” are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors’ wishes as to whether an article is a full paper or a “Note”, but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong“ Notes” to the “full papers” section. As a guide a “Note” should not exceed I500 words or word-equivalents.NOMENCLATURE AND SYMBOLISM 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 rules themselves and guidance on their use are given. Physicochemical Quantities and Units. Manual of Symbols and Terminology for Physicochemical Quantities and Units. (Pure and Appl. Chem., Vol. 51, Nq. I , 1979, pp. 1-41. Also available as a soft-cover booklet from Pergamon Press, Oxford.) Surface Chemistry. ‘ Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - I.‘ (Pure and Appl.Chem., Vol. 31, No. 4, 1972, pp. 577-638.) ‘ Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - 11. Heterogenous Catalysis.’ (Pure and Appl. Chem., Vol. 46, No. I , 1976, In addition, the terminology and symbols for the following subject areas are available either in the form of soft-cover booklets from Pergamon Press (denoted by *) or have been the subject of articles in Pure and Applied Chemistry in recent years: activities;* chromatography; electrochemistry; electron spectroscopy; equilibria, fluid flow; ion exchange; liquid-liquid distribution; molecular force constants; Mossbauer spectra; nuclear chemistry; pH ; polymers; quantum chemistry; radiation;* Raman spectra; reference materials (recommended reference materials for the realization of physico- chemical properties: general introduction, enthalpy, optical rotation, surface tension, optical refraction, molecular weight, absorbance and wavelength, pressure-volume- temperature rela tionships, reflectance, potent iome t ric ion activities, testing distilla tion columns); solution chemistry; spectrochemical analysis; surface chemistry; thermo- dynamics, and zeolites.Finally, the rules for the naming of organic and inorganic compounds are dealt with in the following publications from Pergamon Press: ‘Nomenclature of Organic Chemistry, Sections A, B, C , D, E, F, and H’, 1979. ‘Nomenclature of Inorganic Chemistry’, 1971. pp. 71-90.) A complete listing of all IUPAC nomenclature publications appears in the 198 1 Index issues of J.Chem. SOC. ... VlllMINUTES OF THE TENTH ANNUAL GENERAL MEETING OF THE FARADAY DIVISION The Tenth Annual General Meeting of the Faraday Division of the Royal Society of Chemistry was held at 9.00 a.m. on Tuesday 6 April 1982 at St Catherine’s College, Oxford with Professor D. H. Whiffen, M.A., D.Phi1. D.Sc.. C.Chem., F.R.S.C., F.R.S. in the Chair. 1 . Minutes The Minutes of the 9th Annual General Meeting were tabled and were approved. 2. Annual Report THE 1981 ANNUAL REPORT OF THE FARADAY DIVISION General Discussion No. 71 on ‘High Resolution Spectroscopy’ was held at the University of Bristol on 13-1 5 April and attracted 150 participants including more than forty from overseas representing nine countries. The Discussion was introduced by the 20th Spiers Memorial Lecture given by Dr K .M. Evenson (National Bureau of Standards, Boulder. U.S.A.). Professor A. Carrington was Chairman of the Organising Committee and the Discussion was supported by the High Resolution Spectroscopy Group. General Discussion No. 72, on Selectivity in Heterogeneous Catalysis’, was held at the University of Nottingham on 14-16 September and attracted a large attendance of over 230, of whom more than half were from overseas representing 23 countries. The meeting opened with the second Rideal Memorial Lecture given by Professor W. M. H. Sachtler (Shell-Amsterdam, The Netherlands). Professor F. S. Stone was Chairman of the Organising Committee and the Discussion was supported by the Surface Reactivity and Catalysis Group. Symposium No.16. on ‘Structurc of the Interfacial Region‘. was held on 16 17 December at the Physical Chemistry Laboratory, Oxford and attracted 140 participants, 49 of whom were from 12 overseas countries. D r M. La1 was the Chairman of the Organising Committee and the meeting was supported by the Statistical Mechanics and Thermodynamics Group. The Faraday Division contribution to the Annual Chemical Congress at the University of Surrey, 7-9 April, comprised two symposia: ‘Thermodynamics of Metal Ion Complexing with Crown Ethers and Cryptands’ convened by Dr M. H. Abraham and ‘Aqueous Solutions of Electrolytes at High Temperature and Pressure’ convened by Dr J. I. Bullock. The Autumn Meeting was held at the University of Leeds on 22-24 September when the Division organised a symposium on ‘Pyrolysis, Cracking and Degradation’jointly with the Industrial Physical Chemistry Group.The meeting was convened by Dr N. Taylor. The first R. A. Robinson Memorial Lectures were given in 1981 by Professor R. H. Stokes (University of New England, Armidale, Australia) on ‘The Thermodynamics of Hydrogen Bonded Liquids’ at the Universities of Newcastle and St Andrews and at Imperial College, London. The Bourke Lectures were given by Professor R. M. Hochstrasser (University of Pennsylvania, U.S.A.) on ‘Picosecond Laser Techniques and Applications in Chemistry and Biology’ at the Universities of Edinburgh and Birmingham and at the Royal Institution, London. Two London Symposia were held during the year, the first on 12 March was arranged jointly with Perkin Division and was held at University College, London on the subject of ‘Molecular Association and Reactivity’ and included the Centenary Lecture by Professor J-M.Lehn (Universiti Louis Pasteur, Strasbourg, France) and the Ingold Lecture by Professor W. P. Jencks (Brandeis University. Massachusetts, U.S.A.). The second London Symposium was held on 29 October at the Scientific Societies Lecture Theatre on ‘Advances in Spectroscopic Techniques for Detecting Un- stable Molecules’ and included the Tilden Lecture by Dr H. Kroto. The 5th National Quantum Electronics Conferencc, 23-25 September, at the University of Hull was sponsored jointly with the Institute of Physics, and a one-day Symposium marking the contribution of the late George M.Burnett to Macromolecular Chemistry was jointly sponsored with the G. M. Burnett Lecture Trustees at the University of Aberdeen. ixThe twelve Subject Groups affiliated to the Division continucd to be active. organising scientific meetings catering for their specialist interests. Most Groups also communicated with their members through Group Newsletters. Group Meetings included : Kinetics Involving Halogen-containing Compounds (Gas Kinetics Group) Molecular Motion in Liquids (Statistical Mechanics and Thermodynamics Group) Throwing Light on Electrodes (Electrochemistry Group) Physical Ageing in Polymers (Polymer Physics Group) Special Carbons and Liquid Fuels from Coal (Carbon Group) Theoretical Chemistry (reports by post-graduate students) (Theoretical Chemistry Group) Biennial Informal Meeting and A.G.M.(Electrochemistry Group) Constitution and Structure of Active Sites of Catalysts (Surface Reactivity and Catalysis Group) The Structure of Molecular Liquids (Statistical Mechanics and Thermodynamics Group) Thermal. Mechanical and Electrical Properties of Oriented Polymers (Polymer Physics Group) Modern Methods in Neutron Diffraction (Neutron Scattering Groupj Equipment and Microprocessors for Electrochemistry (Electrochemistry Group) Relaxation Processes in Colloidal Systems (Colloid and Interface Science Group) Recent Advances in the Physics and Chemistry of Carbon and its Applications (Carbon Group) New Advances of Computer Simulation in Statistical Mechanics (Statistical Mechanics and Molecular Beams in Chemical Physics (Molecular Beams Group) Biennial Conference: Physical Aspects of Polymer Science (Polymer Physics Group) 5th Anglo-Czech Symposium : Fundamental Aspects and Analytical Applications of Electro- Gas-Phase Reactions Involving Free Radicals (Gas Kinetics Group) Multiphoton Excitation and Molecules in Strong Laser Fields (Theoretical Chemistry Group) Electrochemical Energy Storage: Electrode and Separator Structures (Electrochemistry Group) Small-angle Scattering from Heterophase Systems (Neutron Scattering Group) Quantitative Surface Analysis (Surface Reactivity and Catalysis Group) Emulsion Polymerisation (Colloid and Interface Science Group) Thermodynamics Group) chemical Detectors (Electrochemistry Group) In 1981 the Marlow Medal was awarded jointly to Dr G.S. Beddard of the Royal Institution, London and Professor G. R. Fleming of the University of Chicago, U.S.A., for their collaborative contribution to picosecond photochemistry and photophysics and its application to the fundamental problem of light-harvesting in photosynthesis and for their independent contributions to these fields. Professor Joel H . Hildebrand. Honorary Fellow of the Faraday Society and consequently Honorary Fellow of the Royal Society of Chemistry, celebrated his 100th Birthday on 16 November and a scroll congratulating him on the occasion was presented to him by the Faraday Division. Newsletter No. 8 was distributed to members in February. The membership of the Division in 1981 was U.K. 3049, Overseas 1298 making a total of 4347, a small increase on 1980.3 . Treusurer’s Report A financial statement for 1981 was tabled and was accepted. 4. Elections to Council Members of Council elected to take office from the Society’s Annual General Meeting in July 1982 were as follows: Presidmt : PROFESSOR D. H. WHIFFEU 1983 Vice-presidents who hatle held ofice as President PROFESSOR SIR GEORGE PORTER DR T. M. SUGDEN PROFESSOR D. H. EVERETT PROFESSOR F. C. TOMPKINS PROFESSOR J. S. ROWLINSON Vice- Presiden t s PROFESSOR A. D. BUCKINGHAM 1983 PROFESSOR J. H. PURNELL 1985 PROFESSOR A. CARRINGTON 1984 PROFESSOR J. P. SIMONS 1985 PROFESSOR P. GRAY 1983 PROFESSOR F. S. STONE 1984 PROFESSOR I. M. MILLS 1984 XOrdinary Members PROFESSOR R. J. DONOVAN 1983 PROFESSOR M. C. R. SYMONS 1983 DR G. J. HILLS 1984 PROFESSOR J. M. THOMAS 1983 PROFESSOR A. J. LEADBETTER 1984 DR J. ULSTRUP 1985 DR I . W. M. SMITH 1985 PROFESSOR G. WILLIAMS 1985 PROFESSOR F. L. SWINTON 1983 DR D. A. YOUNG 1984 Honorarj, Secretarj-: DR G. J. HILLS Honorarj- Treasurer : PROFESSOR P. GRAY The President thanked the retiring members of Council, Vice-presidents Professor Sheppard and Professor Wagner, and Ordinary Members Professor King and Professor Purnell, for their services. 5. Reriew of Futurr Acfirifies A programme of future activities of the Division had been tabled and the President drew attention to the forthcoming General Discussions and Symposia. xiOrdinary Members PROFESSOR R. J. DONOVAN 1983 PROFESSOR M. C. R. SYMONS 1983 DR G. J. HILLS 1984 PROFESSOR J. M. THOMAS 1983 PROFESSOR A. J. LEADBETTER 1984 DR J. ULSTRUP 1985 DR I . W. M. SMITH 1985 PROFESSOR G. WILLIAMS 1985 PROFESSOR F. L. SWINTON 1983 DR D. A. YOUNG 1984 Honorarj, Secretarj-: DR G. J. HILLS Honorarj- Treasurer : PROFESSOR P. GRAY The President thanked the retiring members of Council, Vice-presidents Professor Sheppard and Professor Wagner, and Ordinary Members Professor King and Professor Purnell, for their services. 5. Reriew of Futurr Acfirifies A programme of future activities of the Division had been tabled and the President drew attention to the forthcoming General Discussions and Symposia. xi

ISSN:0300-9599    

DOI:10.1039/F198278FP089

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78, 3433-3445 Photoelectrochemical Study of the Amorphous-W0,-semiconductor-Electrolyte Junction BY FRANCESCO DI QUARTO,* GIUSEPPE Russo, CARMELO SUNSERI AND AGATINO DI PAOLA Istituto di Ingegneria Chimica, Universita di Palermo, Wale delle Scienze, 90 134 Palermo, Italy Received 2nd November, 198 1 The photoelectrochemical behaviour of amorphous anodic films grown on tungsten has been studied. The wavelength of the incident light is shown to influence the photoresponse of the amorphous films. The experimental results are interpreted on the basis of the semiconducting properties of the film and by taking into account the various mechanisms of transport occurring in amorphous materials. At longer wavelengths a Poole-Frenkel mechanism of electrical conduction in the non-extended states of the amorphous semiconductor is invoked in explaining the transport of photoinjected carriers.At the shortest wavelengths a 'free-carrier-like' mechanism of transport of the photogenerated carriers is suggested. In each case a different electrode-potential dependence of the photocurrent is obtained experimentally. We have previously made a preliminary study of the photoelectrochemical behaviour of amorphous WO, anodic films.' Its main aim was to improve our knowledge of the physico-chemical properties of the anodic films grown on tungsten by anodic polarization in different acid solutions. The experimental results could be explained reasonably well using the existing theory of the electrochemical behaviour of single-crystal semiconducting electrodes.Some discrepancies in the flat-band potential values obtained using different experimental methods remained unresolved because of various experimental inaccuracies, such as the use of polychromatic light, as well as through the lack of an adequate model of the amorphous-semiconductor-electrolyte interface. On the other hand, the solid-state properties of films grown on an electrode surface are ultimately the factors which control the electrochemical behaviour of electrodes during their use in several practical applications (electrocatalysis, corrosion, solar cells). Within this framework a more detailed study of the photoelectrochemical behaviour of the amorphous semiconducting films grown on tungsten by anodic polarization has been undertaken, in order to obtain more information on the influence of the amorphous nature of these films on their photoelectrochemical behaviour.We propose that different mechanisms of transport of the photogenerated carriers can operate in an amorphous electrode, depending on the energy of the incident photons, and thus different relationships between measured photocurrent and electrode potential have been obtained for the amorphous-semiconductor-electrolyte interface. EXPERIMENTAL Spectrographically pure tungsten foils or rods were anodized in 0.1 normal H,PO, solutions at a constant current density of 8 mA cm-2, until various final voltages were reached. Before anodization the electrode surface was prepared as previously described.'? The corresponding 34333434 PHOTOELECTROCHEMISTRY OF AMORPHOUS wo, ELECTRODES thicknesses of the anodic films were estimated by assuming an anodizing ratio of 17.0 8, V-1.3 After anodization the electrodes were inserted into a quartz electrochemical cell, where both photoelectrochemical and differential capacitance measurements were performed. The differential capacitance of the semiconducting films was measured by a lock-in technique by using a P.A.R.124 A/ 1 16 lock-in amplifier in connection with a P.A.R. 173/ 179 potentiostat equipped with a P.A.R. 175 universal programmer. The modulating a.c. voltage was a 10 mV peak-to-peak sinewave at 160 Hz, and the scanning rate was 10 mV s-l. The same apparatus was used in the linear-potential-sweep experiments. Monochromatic light was obtained by filtering light from a Bausch-Lomb 150 W Xenon lamp using a hgh-intensity Bausch-Lomb no.5 u.v.-visible monochromator equipped with focusing lenses. The exit slit-width was 3 mm, but a variable entrance slit-width was used in order to change the light intensity. A P.A.R. 125A mechanical light chopper was used in the chopped-light experiments. All capacitance and photoelectrochemical measurements were performed in 0.5 mol dm-3 H,SO, solution; the counter-electrode was a 10 cm2 Pt foil and the reference electrode was Hg I Hg2S0, 10.5 mol dmP3 H2S0, (mercurous sulphate electrode, MSE). Solutions were pre- pared from distilled water and analytical-grade reagents. All the experiments were performed at room temperature (25 & 1 "C). RESULTS The photoelectrochemical behaviour of the amorphous WO, electrodes was studied by investigating in linear-potential-sweep experiments the photoresponse of the oxidized electrodes illuminated with light of different wavelengths.The use of focusing lenses as well as the very low dark current (idark < 0.1 pA cmd2) under anodic polarization allowed steady-state photocurrent measurements without chopping the light. Because of the slow dissolution of the films, after each experiment with illumination the values of the dark current were verified. No significative change in these values were measured, provided that a relatively thick film of oxide (ca. 500 %.) was still on the electrode surface. The good reproducibility of the films as well as of the experimental data for the same experiments performed with different films was carefully checked.Unless explicitly stated, experiments reported in the various figures were performed with different films. It is significant that no influence on the shape of the photocurrent against potential curves was seen if films grown to different thicknesses were employed in the same experiments. Thus most of the investigations were performed with films grown at two different voltages, 70 and 100 V, in order to avoid experimental complications. All potentiodynamic experiments were performed by starting from + 3.0 V us. MSE and decreasing the electrode potential until the dark-current value was attained. For the various samples the value of the onset photocurrent potential was Uon = - 0.1 & 0.05 V.The same value was measured by performing the experiments with chopped light (chopping frequency 89.5 Hz) at a very low light intensity and by measuring the photocurrent using the lock-in technique. In the latter experiments the same functional dependence in the Iph against UE curves was seen. Slight hysteresis was observed in the reverse scan, especially in the experiments performed at 230 nm, where saturation effects were recorded at higher potentials ( U , 2 2 V). Following this finding and in order to reduce the dissolution of the films under illumination, a standard procedure was followed in carrying out the potentiodynamic experiments. Before each experiment under illumination the electrodes were kept at 3 V for 10 min in the dark and then swept under light to -0.2 V.In this way, for the thicker electrodes anodized to 100 V it was possible to perform five different sweeps without any appreciable change in the dark-current values.F. D I QUARTO, G. RUSSO, C. SUNSERI AND A. DI PAOLA 3435 In fig. 1 two curves are reported for different wavelengths, 340 and 230 nm, respectively. The curves, obtained with two different electrodes, show the general behaviour of the photocurrent against electrode-potential curves at the shortest and longest wavelengths. The shapes of the curves were not influenced by changing the intensity of the incident light (see also fig. 4). The influence of film thickness on the values of the photocurrent will be discussed in detail later. From the two curves, different relationships between photocurrent and electrode potential were obtained. In particular, from 380 to 270 nm a Iph against u& law was observed, whilst at 230 nm a square-root dependence was found.I ’ 1 I I 1 /I 5t Y’/l I I I I 0.0 0.5 i.0 1.5 2.0 U,lV us. MSE FIG. 1.-Photoresponse of amorphous WO, anodic films at two different wavelengths in 0.5 mol dm-3 H,SO,. Initial film thickness, 1700 A; scan rate, 10 mV s-l. Solid curve: 340 nm (electrode area, 0.13 cm2); hatched curve: 230 nm (electrode area, 0.16 cm2). In fig. 2 are reported I$h against UE: plots for a film anodized to 100 V and swept potentiodynamically at three different wavelengths, starting with A = 270 nm. A straight line was observed for the I$! against UE plots. Furthermore, the electrode potential at zero photocurrent was in fair agreement with the onset photocurrent potential and the previously reported flat-band potential obtained for Mott-Schottky plots.1 In fig. 3 we report & against UE plots for three consecutive sweeps at 300 nm. The thicknesses of the films were different because of the continuous dissolution process, which occurred evenly provided that the illumination of the electrode surface was uniform. Although a different slope is obtained for each sweep, all the straight lines have a common intersection voltage. The different slopes of the plots seem to be related to the change in the distribution of donors occurring in the films during the experiment, rather than to the thinning of the oxide films by dissolution. This aspect will be discussed below on the basis of the proposed model.In fig. 4 and 5 are reported IEh against UE plots at II = 230 nm. As shown for the experiments performed at 300 nm, both the light intensity and the film thickness influence only the slope of the lines. All the previous results did not change appreciably if the experiments were performed by starting with electrodes anodized to 70 V, or if different current densities or solutions were employed during the anodic formation of the films. Measurements of differential capacitance in the dark or under illumination were performed in order to gain a better knowledge of the changes which occurred in the films during both the dissolution process and illumination.7 I I 0 1 2 3 FIG. 2 UElV VS. MSE I I UE/V VS. MSE FIG. 3 FIG. 2.-Plots of I$, against U$: obtained at different wavelengths.Initial film thickness, 1700 A; scan rate, 10 mV s-l; electrode area, 0.16 cm2. Slit width: entrance 6 mm, exit 3 mm. Wavelengths: 0, 270 nm (1st sweep); 0, 340 nm (2nd sweep); A, 380 nm (3rd sweep). FIG. 3.-Influence of dissolution process on plots of & against U , at constant wavelength and light intensity. 2 = 300BAA nm; initial film thickness, 1700 A; scan rate, 10 mV s-l; electrode area, 0.16 cm2. Sweeps: 0, 1st; n, 2nd; A, 3rd.I ' I I 40 30 c1 \ 9 @ 20 10 0 UEIV US. MSE FIG. 4 UE{V us. MSE FIG. 5 FIG. 4.-Influence of the light intensity on plots I& against U,. 1 = 230 nm; initial film thickness, 1700 A; scan rate, 10 mV s-'; electrode area, 0.16 cm2. Slit width: 0, entrance 6 mm, exit 3 mm; n, entrance 2 mm, exit 3 mm.Both expenments are relative to the first sweep on two virgin samples. FIG. 5.-Influence of the dissolution process on plots of Gh against U , at constant wavelength and light intensity. 1 = 230 nm; initial film thickness, 1700 A; scan rate, 10 mV s-l; electrode area, 0.16 cm2. Sweeps: 0, 1st; R, 3rd; A, 5th.3438 PHOTOELECTROCHEMISTRY OF AMORPHOUS WO, ELECTRODES Fig. 6 shows the Mott-Schottky plots obtained from the differential capacitance measurements performed with the electrode used in the experiments of fig. 3, before and after the sweeps under illumination. There is a change in the semiconducting properties of the electrode which leads to an increase in the density of donors and a change in their spatial distribution.These results are in agreement with the ageing effects reported by Butler4 for TiO, electrodes. Fig. 7 and 8 show the influence of illumination and the dissolution process on both the capacitance measurements and the Mott-Schottky plots, for two electrodes anodized at 70 and 100 V, respectively. Although some doubts have been raised very recently as to the application of Mott-Schottky theory to amorphous semiconductor^,^ both these figures show unequivocally that under illumination the semiconductor electrodes exhibit behaviour which is in accordance with the existence of a depletion layer much thinner than the thickness of the anodic films. Moreover these figures agree in showing that during an experiment the distribution of donors in the films is changing as a result of both the dissolution process and ageing under electrical polarization.By comparing fig. 6 and 8 it is possible to see that under illumination the space-charge region of the electrode decreases substantially with respect to the dark conditions. Analogous effects on amorphous silicon have been reported by Wronsky.6 By assuming a direct proportionality between the absorption coefficient a and the photocurrent, we have plotted in fig. 9 the (Iph hv)i values as a function of the photon energy hv, in order to obtain a measure of the optical band-gap E g p t of the amorphous WO, films. The measured Egpt values for the various films usually ranged between 3.0 and 3.1 eV, and this scattering of values does not seem to be related to the thickness of the film or to anodizing parameters, such as current density or nature of the anion, provided that the anodization process is stopped before the onset of film breakdown.For amorphous semiconductors7 other relationships between the absorption coefficient and the energy of the incident photons have been proposed. In our case, however, the best fitting of the experimental data collected for several specimens of different thicknesses and at various light intensities was obtained by using the square-root interpolation. Although the same square-root dependence has been reported for both a m o r p h o u ~ ~ - ~ and crystalline materials having indirect optical transitions, the nature of the transitions is different in the two cases. In fact, in the case of amorphous materials no intervention of phonons is required in order that the optically induced transitions take place, whilst phonon-assisted transitions are involved in crystalline materials.For this reason transitions in amorphous materials have been defined as n~n-direct.~ With these differences in mind, fig. 9 shows a plot relative to a thermally crystallized anodic film. The crystallization process was performed under an argon atmosphere at 350 OC. The details of this process and the properties of the crystallized electrodes will be discussed in a separate paper. Owing to the crystallization, a decrease in the optical band-gap of ca. 0.3 eV is observed. The good agreement between our Egpt values and those reported in the 1iterature1O-l2 for both polycrystalline and amorphous WO, oxides is evidence of the direct propor- tionality which exists at each wavelength between the measured photocurrent and the absorption coefficient.This aspect will be discussed further below. DISCUSSION The experimental results previously outlined cannot be explained, in our opin on the basis of Gartner’s modeP3 formerly employed by various authors crystalline-semiconductor-electrolyte junctions under illurnination.l2-l8 In fact, on, for theU,lV us. MSE FIG. 6 0 1 2 3 U,/V US. MSE FIG. 7 FIG. 6.-Plots of l/Cz against UE obtained in the dark. The experiments were performed on the same electrode as fig. 3, before (0) and after (A) the three consecutive sweeps under illumination. Scan rate, 10 mV s-l; a.c. frequency, 160 Hz. FIG. 7.-Influence of the dissolution process on plots of differential capacitance against UE at constant wavelength and light intensity.,I = 300 nm; initial film thickness, 1260 A; scan rate, 10 mV s-l; electrode area, 0.16 cm2; ax. frequency, 160 Hz. Sweeps: 0, 1st; 0, 3rd.3440 PHOTOELECTROCHEMISTRY OF AMORPHOUS wo, ELECTRODES UE/V us. MSE FIG. 8.-Influence of the dissolution process on plots of l/cZ against UE obtained under constant illumination at /1 = 300 nm. Initial film thickness, 1700 A; scan rate, 10 mV s-l; a.c. frequency, 160 Hz. Sweeps: 0, 1st; A, 3rd. sub-linear Iph against UB law in the range 270-380 nm, coupled with the change to a square-root dependence at shorter wavelengths (A = 230 nm), cannot be justified by the above models. This disagreement is not surprising if we take into account that transport of the photogenerated carriers can be noticeably different in amorphous materials when compared with the crystalline case.In the latter, the photoinjected carriers can move under the action of an electric field in the delocalized states of the valence or conduction band of the semiconductor, regardless of the existence of possible mechanisms of recombination in the various regions of the semiconductor (bulk, depletion layer, surface). For amorphous semiconductors different mechanisms of transport have been invoked depending on whether the carriers move in the extended-state or in the non-extended-state 19-25 In the first case an activationless ' free-carrier-like ' transport mechanism will be operating; in the second case a field-assisted hopping mechanism with a related activation energy has been suggested.The most usual hopping mechanism proposed by various authors in explaining the electrical behaviour of amorphous semiconductors in the dark is a generalized Poole-Frenkel me~hanism.l~-*~ Moreover, a dependence of the photocurrent on the electric field following the Poole-Frenkel mechanism has been reported by different authors for344 1 a number of amorphous films,26-28 and very recently also for single-crystal semi- conductors illuminated with photons of sub-band-gap energy.16* 29 According to Hill’s theoretical analysis of the Poole-Frenkel mechanism of electrical conduction in amorphous materials, the following low-field expressions, depending on the assump- tions made, can be obtained for the dark current: iPpF = A F ~ (1) iPpF = BF (2) iPwF = CF: (3) where P is the electric field inside the electrode, and A , B and C can be considered as constants at a fixed temperature and for a given material.According to Hill, eqn (1)-(3) can be obtained as a result of the substitution of sinh /?Fi by exp /?FB in the expression for the probability of the carriers being emitted from the coulombic trapping centres.20 When this substitution is performed, a field dependence for crystalline materials analogous to that reported in eqn (3) for semicrystalline materials is obtained. On the basis of the previous considerations an interpretative model of the experimental results can be proposed according to the following assumptions. (i) The existence under constant illumination of a steady-state distribution of the photoexcited carriers among the localized states of the conduction (electrons) or valence (holes) band.These states will be assumed to have, respectively, donor-like or acceptor-like characteristics so that a field-lowering of the activation energy barriers, in accordance with the Poole-Frenkel effect, takes place under electric polarization.201 28 (ii) The absence of any contributions to the photocurrent from the carriers generated outside3442 PHOTOELECTROCHEMISTRY OF AMORPHOUS wo, ELECTRODES the space-charge region. (iii) The existence under illumination of a depletion layer whose thickness is less than the film thickness over the whole range of electrode potentials used. The first assumption is strictly related to the use of eqn (1) according to Hill's theoretical analysis of the Poole-Frenkel mechanism in amorphous semiconductors.This assumption will be relaxed in the discussion of the experiments performed at A = 230 nm, where the hypothesis of. transport in the extended states will be made for the photogenerated carriers. The second assumption is explained by considering the amorphous nature of WO, as well as the experimental finding of a negative value for the diffusion length of the holes, Lh, as obtained by extrapolation to zero photocurrent of the iph against l/C plots.30 A negative value of &,, which is physically meaningless, can be taken as evidence of a strong recombination process in the field-free region of the semiconductor, so that a negligible contribution to the photocurrent will come from this region.The third assumption is in agreement with the results described in fig. 6-8. The use of the same formulae for crystalline semiconductors in the depletion region will be considered as a simplifying good approximation. According to previous assumptions, the final expression for the photocurrent in the absence of kinetic control by the electrode-electrolyte interface can be written as iph = ip-F Gl(x,,) = 74, El - exp (- 2a x,,)] (4) where ip-F is given by eqn (1) and GA(xsc) is the total density of photogenerated carriers in the space-charge region x,,. In eqn (4) the existence of a possible reflection at the metal-oxide interface has been taken into account, and q50 represents the photon flux incident on the surface of the anodic films after reflection losses.z is the lifetime of the excess carriers photogenerated in the space-charge region and F,, is the average electric field in this region. The use of eqn (1) rests only on the experimental fitting of the photocurrent curves for WO, anodic films, but the use of eqn (2) or ( 3 ) could be more correct in other systems. According to the a values reported in the literaturelo? le for amorphous evaporated films or single-crystal WO, semiconductors, and on the basis of the experimental results reported in fig. 6-8, it seemed a good approximation to expand the exponential term in eqn (4) by retaining the first two terms, In such a way the final expression for the photocurrent becomes iph = 2Az4, Fhv a xsc.By taking into account the semiconducting nature of our films and by substitution of the mathematical expressions for Cv and x,, usually employed with crystalline semiconductors in the depletion region 32 eqn ( 5 ) becomes where E,, is the dielectric constant of the film, E , the permittivity of free space, Nd the density of donors in the film, and #sc the potential drop in the semiconductor; the remaining symbols have their usual meaning. By identifying the parameters which play the most important role in determining the photocurrent behaviour, it is possible to write e iph = constant x aN;, (7)F. DI QUARTO, G. RUSSO, C. SUNSERI A N D A. DI PAOLA 3443 where bSc has been expressed as a function of the electrode potential UE and the flat-band potential UFB.317 32 In agreement with eqn (7), a plot of i$ against U, should give a straight line, whose intercept with the voltage axis should be a measure of the flat-band potential.In fig. 2 and 3 the experimental data seem to fit both provisions. In fact good straight- line behaviour is observed at the various wavelengths in the range 270-380 nm. The intercepts with the voltage axis (0.0+0.1 V us. MSE) are not dependent on the wavelengths used and are in good agreement with the flat-band potentials previously reported.' This shows that the assumption 2axs, 6 1 made in the expansion of the exponential term in eqn ( 5 ) is valid over the range of wavelengtbs used. In eqn (7) note that, at a fixed wavelength, the slopes of the ibh against UE plots are inversely proportional to the cube root of the density of donors in the film.This seems qualitatively in accordance with the experimental results reported in fig. 3, 7 and 8 showing the influence of thickness and ageing under polarization on the photocurrent behaviour as well as on the distribution of donors in the oxide films. In fact, both the photocurrent behaviour and the capacitance data could be interpreted on the basis of a donor density which grows during the experiment. The dependence of iph on a shown in eqn (7) indicates that in fig. 9 the use of iph instead of a at every wavelength is a good approximation, provided that the experiments are performed at constant potential, and the eventual change of donor distribution is negligible over the time-scale of the experiment, It is useful at this point to observe that the use of eqn (2) or (3) rather than eqn (1) would give, respectively, the following final expressions after the mathematical treatment outlined above : e i,, = constant x a or iph = constant x aNa e (9) Preliminary experiments performed with amorphous Nb,O, anodic oxide films show a photocurrent against potential behaviour which seems in aqeement with eqn (8).Further investigations are, however, necessary before reaching a final conclusion. Eqn (9) shows a dependence of the photocurrent on the electrode potential analogous to that reported for single-crystal semiconductors, in which a Poole-Frenkel mechanism has been invoked to explain the photocurrent characteristic^.^^^ 29 This is due to the use in Hill's analysis of a relationship between velocity and mobility of carriers in semicrystalline materials which is analogous to that existing between mobility and drift velocity in crystalline materials.If this is true, the result of eqn (9) would be more general than the equation i,, = CV1-28 reported by Lemasson et al. for the zinc-selenide-electrode-electrolyte junction. 16t 29 The experimental results obtained at 3, = 230 nm can be interpreted in a straight- forward manner by relaxing assumption (iii) and by assuming that the electron-hole pairs generated by photons absorbed in the depletion layer are swept away by the electric field existing in this region in such a short time that the thermalization process in the non-extended states does not occur. The appearance of this conduction process in the extended states of higher energy agrees with the results reported in fig.4 and 5. In fact the square-root dependence of the photocurrent on the electrode potential can be explained by taking into account the dependence of the space-charge region of the3444 PHOTOELECTROCHEMISTRY OF AMORPHOUS WO, ELECTRODES semiconductor on the electrode potential and by assuming no recombination in the depletion layer, in accordance with Butler’s mode1.12 In this hypothesis the expression for the photocurrent becomes e iph = constant x aN;: In this case the slopes of the straight lines reported in fig. 5 should be in qualitative agreement with the hypothesis previously made concerning the change in donor distribution in the films during the experiment.CONCLUSIONS On the basis of the experimental results and the existing theories concerning the Poole-Frenkel mechanism of electrical conduction in amorphous materials, we have suggested an interpretative model of the photocurrent characteristics observed at the amorphous-WO,-electrolyte interface. It has also been shown that a change in the photocurrent behaviour with wavelength could be ascribed to different mechanisms of electrical conduction in the amorphous materials : hopping in localized states or ‘ free-carrier-like ’ transport in extended states at higher energies. According to the experimental data, a mobility gap ca. 0.3 eV larger than the band-gap energy of crystalline WO, has been attributed to the WO, amorphous anodic films. The existence of non-extended states below the mobility gap, where the hopping mechanism of electrical conduction follows a Poole-Frenkel law, suggests an over- lapping of these states by a distribution of defects having a donor-like or acceptor-like behaviour in the conduction or valence bands, re~pectively.~ The n-type semiconducting behaviour, as well as the non-stoichiometry usually reported for polycrystalline or single-crystal WO, electr~des,~~ suggest that the oxygen vacancies produced during the anodization process could behave as donor-like centres. Finally, we stress that a photoelectrochemical study of the amorphous- semiconductor-electrolyte interface can be a valuable tool in obtaining further knowledge of the electronic properties of these materials.F. Di Quarto, A. Di Paola and C.Sunseri, Electrochim. Acta, 1981, 26, 1177. S. Roy Morrison, Electrochemistry at Semiconductors and Oxidized Electrodes (Plenum Press, New York, 1980). F. Di Quarto, A. Di Paola and C. Sunseri, J . Electrochem. Soc., 1980, 127, 1016. M. A. Butler, J . Electrochem. Soc., 1979, 126, 338. W. E. Spear, P. G. Le Comber and A. J. Snell, Philos. Mag., 1978, 38, 303. C. R. Wronski, IEEE Trans. Electron Devices, 1977, ED-24, 351. N. F. Mott and E. A. Davies, Electronic Processes in Non-crystalline Materials (Clarendon Press, Oxford, 1971), p. 237 ff. * S. Ikonopisov, E. Klein, A. Stanchev and T. S. Nikolov, Thin Solid Films, 1975, 26, 99. S. Kapusta and N. Hackermann, Electrochim. Acta, 1980, 25, 1001. l o S. K. Deb, Philos. Mag., 1973, 27, 801. l1 Z. M. Hanafi and M. A. Khilla, 2. Phys. Chem. (N.F.), 1974, 89, 230. l2 M. A. Butler, J . Appl. Phys., 1977, 48, 1914. l 3 W. W. Gartner, Phys. Rev., 1959, 116, 84. l4 R. H. Wilson, J . Appl. Phys., 1977, 48, 4292. l 5 H. Reiss, J . Electrochem. Soc., 1978, 125, 937. l6 P. Lemasson and J. Gautron, Phys. Status Solidi (A), 1979, 53, 303. l7 J. Reichmann, Appl. Phys. Lett., 1980, 36, 574.F. DI QUARTO, G. RUSSO, C. SUNSERI AND A. DI PAOLA 3445 W. J. Albery, P. N. Bartlett, A. Hamnett and M. P. Dare-Edwards, J. Electrochem. SOC., 1981, 128, 1492. Iy A. K. Jonscher and R. M. Hill, in Physics of Thin Films, ed. G. Hass. M. H. Francombe and R. W. Hoffman (Academic Press, New York, 1975), vol. 8, p. 170 ff. 2o R. M. Hill, Philos. Mag., 1971, 23, 59. 21 R. M. Hill, Philos. Mug., 1971, 24, 1307. 22 N. F. Mott, Philos. Mag., 1971, 24, 911. 23 E. A. Owen, J. Non-Cryst. Solids, 1977, 25, 372. 24 P. G. Le Comber, W. E. Spear and D. Allan, J. Nun-Cryst. Solids, 1979, 32, 1. 25 R. M. Hill and A. K. Jonscher, J. Non-Cryst. Solidy, 1979, 32, 53. 26 D. M. Pai and S. W. Ing Jr, Phys. Rev., 1968, 173, 729. 27 M. D. Tabak and P. J. Warter Jr, Phys. Rev., 1968, 173, 899. 28 A. K. Jonscher and A. A. Ansari, Philos. Mag., 1971, 23, 205. 29 J. Gautron, P. Lemasson, F. Rabago and R. Triboulet, J. Electrochem. SOC., 1979, 126, 186. 30 S. Roy Morrison, J. Vm. Sci. Technol., 1978, 15, 1417. 31 H. Gerischer, in Advances in Electrochemistry and Electrochemical Engineering, ed. P. Delahay and 32 V. A. Miamlin and Yu. V. Pleskov, Electrochemistry of Semiconductors (Plenum Press, New York, 33 M. J. Sienko and J. M. Berak, in The Chemistry of Extended Defects in Non-metallic Solids, ed. C. W. Tobias (Interscience, New York. 1961), vol. 1, p. 139. 1967). L. Eyring and M. O’Keeffe (North-Holland, Amsterdam, 1970), p. 541. (PAPER 1 / 1706)

ISSN:0300-9599    

DOI:10.1039/F19827803433

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I , 1982, 78, 3447-3460 Rates of Extraction of Zinc into Organic Single Drops Containing Dithizone BY MICHAEL A. HUGHES* Schools of Chemical Engineering, University of Bradford, Bradford, West Yorkshire BD7 1DP AND Hou SUNGSHOU General Research Institute of Non-Ferrous Metals, Ministry of Metallurgical Industry, Peking, China Received 16th November, 1981 The rate of extraction of zinc from an aqueous phase, pH 4.0-5.2, by single drops of chloroform containing dithizone is reported. Rates for both drop travel and drop formation are given. The rates are compared with those of previous workers, but in this study the interfacial area is controlled and measured. Arguments are presented to demonstrate that the locale of the reaction is at the liquid-liquid interface and not in the hydrodynamic layer or bulk phase on the aqueous side of the interface as reported by previous workers.Criteria are presented which can be used to test for the locale of the general reaction between any extractant and metal in liquid-liquid contacting. The present rate data may be accounted for by Chapman’s model of chemical reaction at the interface coupled with diffusional transfer of all the species taking part in the overall reaction. Dithizone systems have served as the main model systems from which the basic ideas of the kinetic extraction mechanism of metal chelateslt have been developed. Irving et aL3 have suggested that the extraction rate is determined by the chemical reaction Zn2+(aq) + R-(aq) -+ ZnR+(aq) in which R- is the dithizone anion.The extraction rate increases almost in proportion to the equilibrium concentration of dithizone in the aqueous phase, the partition coefficient varying with diluent type. In almost all metal dithizone systems examined by F r e i ~ e r ~ - ~ a first-order rate of reaction with respect to metal and extractant concentrations was observed, together with a reciprocal first-order rate of reaction with respect to the hydrogen-ion concentration which corresponds to the equation above and an assumed aqueous-phase reaction. The rates for different metals follow the rates of substitution of the coordinated water. For the particular concentration conditions of the system of extraction used by Freiser we conclude that the formation of the 1 : 1 complex is a rate-limiting step.Fomin’s7 general approach to this problem allows for either 1 : 1 and 1 : 2 complex formation steps to be rate-limiting. Freiser contacted the two phrases at high intensity and found that increasing the rate of mixing did not increase the extraction rate. This was considered as proof of the homogeneous character of the limiting step. The unreliability of such a method of demonstrating the homogeneous character of a limiting step has been remarked upon several times in the literature. Probably the most important defect in Freiser’s work is the lack of interfacial area data, which makes the interpretation of the extraction mechanism difficult. 34473448 EXTRACTION OF ZINC Nitsch et al.sl rejected the homogeneous aqueous-phase mechanism of reaction, suggesting that reaction with zinc takes place at the interface.Extensive research involving various chelates and metals has been carried out by Kletenic et a1.1°-12 The heterogeneous character of the chemical reaction was demonstrated by the fact that the extraction rate is in proportion to the interfacial area and this proportionality is preserved on changing the interfacial area by more than 1000 times. However, Fomin7 considered that the surface reaction mechanism for the extraction with dithizone was not proved. These facts caused us to re-examine the extraction process of zinc by dithizone but using a technique where interfacial area is measured. We now include the generalised approach of mass transfer with chemical reaction coupled with a consideration of the solubility of the extractant in the aqueous phase.EXPERIMENTAL APPARATUS The falling-single-drop apparatus was standard has been previously described by Hanson et in an investigation to study the extraction of copper by chelating hydroxyoximes. The data were treated as in ref. (14) so as to remove the end effects due to mass transfer during drop formation. Four different column lengths were used in each run. The temperature of the experiments was 25 & 0.5 O C . CHEMICALS Dithizone was of A.R. grade (> 99% pure from B.D.H. Ltd). This extractant was also purified by the method used by Kletenic and Vinokurova,lo but in any case no significant difference was observed between the behaviour of the A.R. grade specimen and the purified form.The organic phase was prepared just before starting an experiment. Zinc solutions were made from zinc oxide (A.R.) dissolved in hydrochloric acid, evaporated to near dryness and then redissolved in the required amount of perchloric acid and then mixed with acetic acid and sodium acetate to the required ionic strength and pH. The diluent, chloroform, was of special high purity (A.R. grade) and described as ‘suitable for dithizone analysis’. All other chemicals were of A.R. grade, and water distilled from glass was used in all experiments. TECHNIQUE Ca. 80 drops per minute of the appropriate organic phase were made to fall through the appropriate aqueous phase. The drop size is ca. 2 mm in diameter when formed at the tip of a 33 gauge stainless-steel needle. The first 1 cm3 of organic loaded phase was discarded and then the next cubic centimetre(s) was retained for zinc analysis. The zinc in the solvent was analysed by a spectroscopic method13 at wavelengths 535 and 524 nm; a Pye Unicam SP800 spectrophotometer was used.The pH of the aqueous phase was kept constant by an acetate buffer. Before extraction the aqueous phase was saturated with chloroform and the organic phase was saturated with the aqueous phase but without zinc present. The ionic strength of the aqueous phase was kept constant (p = 0.25). Interfacial tensions were measured by use of the drop-volume method and viscosities were measured using an Ubbelohde viscometer.M. A. HUGHES A N D H. SUNGSHOU 3449 RESULTS PROOF THAT THE REACTION CONTROL STEP IS NOT I N THE AQUEOUS HOMOGENEOUS BULK PHASE OR A N AQUEOUS ZONE The rates of extraction per unit area during drop travel are calculated from a least-squares treatment of the mass-transfer data from the different column lengths after ref.(14). The value of the intercept obtained from the plot of mass transfer against time was considered to be the amount of metal extracted during drop formation per unit interfacial area. A typical treatment of results is given in table 1. In all these experiments the HR concentration is always higher than the Zn concentration in the bulk phases. The rate of mass transfer, D, = 3.833 x loA7 rnol m-2 s-l, was calculated from the plot of mass $ransfer against time from table 1. The amount of metal extracted during the drop formation is 5.35 x mol mP2, calculated from the intercept.TABLE 1 .-TYPICAL RESULTS C,,, organic phase = 1 x loF3 mol dmP3, pH 4.25, Czn, aqueous phase = 1 x mol dmP3, ionic strength = 0.25, temperature = 25 "C. column v, s, length time volume of surface of (s/v) [Zn],/105 mass transfer/106 /cm / s drop/cm3 drop/cm2 /cm-l mol dm-3 rnol m-2 45 3.0 0.003 85 0.1187 30.83 2.0 6.48 75 5.1 0.003 77 0.1170 31.08 2.3 7.41 125 8.7 0.003 67 0.1149 31.31 2.6 8.30 180 12.73 0.003 38 0.1087 32.19 3.3 10.25 TABLE 2.-sUMMARY OF RATES OF EXTRACTION UNDER VARIOUS CONDITIONS results of experiments rate of during drop during experimental conditions mass transfer extraction [H+l [HWO [Zn2+] formation drop falling pH /mol dmP3 / 1 OP3 rnol dmP3 / 1 0-4 mol dm-3 / 1 0-9 rnol cm-2 /mol cmP2 s-' 4.75 5.157 4.154 4.701 5.099 4.186 4.47 5.090 4.52 4.52 5.099 5.14 1.78 x 6.79 x 7.01 x 10-5 1.99 x 10-5 6.51 x 10-5 3.39 x 10-5 3.0 x 10-5 3 .o ~ 10-5 7.96 x lop6 8.02 x 01-6 7.96 x lop6 7.69 x 1.06 1.06 1.06 1.06 1.06 1.06 1.06 1.06 0.496 1.829 0.496 1.829 1 .o 1 .o 2.5 2.5 2.5 5.0 5.0 5.0 2.5 2.5 2.5 2.5 1.246 0.826 0.73 0.86 1.02 1.01 1.04 1.14 0.56 1.31 0.54 1.33 7.0 x lo-" 1.06 x 10-lo 5.1 x lo-" 1.01 x 10-'O 1.09 x 10-lo 7.5 x 10-1' 9.55 x lo-" 1.29 x 10-lo 3.30 x 10-l' 1.20 x 10-10 5.40 x lo-" 2.19 x3450 EXTRACTION OF ZINC TABLE 3.-INTERFACIAL TENSIONS AT 25 & 0.1 OC IN mN Ill-' aqueous zinc solution, pH 4.66, zinc solution, pH 5.1, [Zn] = 2.5 x mol dm-, [Zn] = 2.5 x mol dm-, chloroform 23.0 - 0.496 x lo-, mol dm-, - 22.29 1.06 x lop3 mol dm-, - 23.40 1.829 x lop3 mol dm-, - 22.68 dithizone in CHC1, dithizone in CHC1, dithizone in CHC1, - 23.27 22.26 6(av) = 22.78 mN m-'.TABLE VI VISCOSITIES AT 25 & 0.1 OC IN mPa s ~~ ~ ~ ~~ sample viscosity CHCI, 0.536 1.06 x lo-, mol dm-, HDz in CHCl, 0.5814 1.829 x lop3 mol dmp3 HDz in CHCl, 0.5852 H2O 0.895 Zn aqueous solution 0.91 11 TABLE 5.-DENSITIES AT 25 f 0.1 OC IN gCmP3 sample density CHC1, 1.4753 1.06 mol dm-, HDz in CHC1, 0.496 mol dm-, HDz in CHCl, 1.4766 1.4757 1.829 mol dm-, HDz in CHC1, I .4752 Zn aqueous solution 1.0056 TABLE 6.-DIFFUSIVITIES IN Cm2 S-' (CALCULATED USING THE WILKE-CHANG AND REII~SHERWOOD EQUATIONS) sample diffusivity dithizone in CHCl, Zn(C10,), in H,O ZnDz, in CHCl, Zn2+ in CHCl, H+ in H,O dithizone in H20 1.39 x 1.072 x 9.19 x lop6 9.79 x 10-6 5.08 x 9.31 x 10-5M.A. HUGHES AND H. SUNGSHOU 345 1 More extensive runs were carried out, and in table 2 we report the rates of extraction per unit area for different conditions of pH, CHR(o) and C,,!,,. The interfacial tensions, viscosities and densities required for the model are given in tables 3-5, respectively. The diffusivities of different species were calculated using the Wilke-Chang and the Reid-Sherwood equations and are listed in table 6. organic phase DISCUSSION I film, thickness A/ . I I 1 I I I LOCALE OF THE CHEMICAL REACTION According to the conventional mechanism of metal extraction by chelating extractants, the process involves : (i) partition of the extractants between the organic phase and the aqueous phase; (ii) dissociation of the extractants in the aqueous phase; (iii) chelation and formation of metal complexes in the aqueous phase; (iv) partition of the metal complex so formed between the aqueous phase and the organic phase.The necessary requirement for the bulk aqueous-phase reaction is that the chemical reaction rate is slow and the solubility of extractant in the aqueous bulk phase is sufficiently large, as described by Astarita15 under ‘the slow reaction regime ’. This process involves the mass transfer of the extractant from the bulk organic phase to the interface and then from the interface to the bulk aqueous phase; it should also involve the mass transfer of all other species. An ‘aqueous film’ exists which is part of the aqueous phase adjacent to the interface and is usually 10-3-10-4 cm thick, see fig.1 . The concentration of extractants in this film, like the concentration in the bulk aqueous phase FIG. 1.-Concentration profiles in the aqueous film adjacent to the interface. aqueous phase, has to be related to the concentration in the organic phase via the partition coefficient. In terms of film theory, the mass transfer of any species in this film is governed by molecular diff~si0n.l~ The treatment in this payer is based on the mass transfer of the extractant HR rather than the mass transfer of the metal. PROOF THAT THE REACTION IS NOT IN THE BULK AQUEOUS PHASE Consider the mass transfer of extractant, HR, from the interface to the bulk aqueous phase. For a quasi-stationary state the rate of chemical reaction is equal to the rate of mass transfer:3452 EXTRACTION OF Z I N C where r is the rate of extraction in mol cm2 s-l, koHR, is the mass-transfer coefficient of the extractant in the aqueous phase and CI-IR(a)i is the concentration of extractant on the aqueous side of the interface.CHR(a)i = C H R ( o ) / P H R assuming that there is no resistance to the mass transfer of HR in the organic phase. CWR(a) is the concentration of extractant in the bulk aqueous phase; CHR(o) is the concentration of extractant in the bulk organic phase (here CHR(o) = 1.06 x lop3 mol dmp3), PHR is the partition coefficient of extractant between the two phases (for dithizone PHR = 7.94 x lo5), k is the reaction rate constant if the concentrations of metal and the pH in the aqueous phase are considered to be constants and CHS!eq) is the equilibrium concentration of extractant.Here we assume CHR(eq) = 0. Finally v is the volume of the reaction zone in the aqueous phase and s is the interfacial area. Let us assume that CHR(a) = 0, i.e. the rate of extraction is controlled by a mass-transfer process involving a diffusion layer where only molecular diffusion exists. According to the film theory where DHR(&) is the diffusion coefficient of extractant HR in the aqueous phase (0.508 x cm2 s-l, see table 6), 3, is the thickness of the diffusion zone, r = 0.38 x 10-lo mol cm2 s-l, 1.00 x 10-3 7.94 x 105 mol dm-3 = I .26 x 1 0-l2 mol ~ m - ~ CHR(a)i = and a is the specific interfacial area in cm-l. Thus kgR = 60.84 cm s-l (which is an unlikely high value) and 3, = 1.23 x lopR cm.The calculated value of R is ca. ten times less that the diameter of dithizone molecule (ca. 1 x lo-’ cm) calculated from the value of the molecular volume of dithizone and assuming a sphere. If the assumed model is correct then in order to maintain the supply of the extractant to match the rate of consumption in the chemical reacion the mass transfer coefficient k& has to be as big as 60 cm s-l and the diffusion film becomes ten times less than the diameter of a dithizone molecule. Note that in this calculation we have assumed that CHR(a) = 0 and also the equilibrium concentration CHR(eq) = 0, but if the rate is controlled by the chemical reaction in the bulk aqueous phase, as suggested by Freiser, implying that (CHR(a)i - CHR(a)) 6 (CHR(a) - CHR(eq)), then k& should be > 120 cm s-l and 2 should be -= 0.62 x cm.Such values of k& and 3, are unlikely to be true even in a shake-out or stirring apparatus. In other words, the model involving the reaction in the bulk phase does not account for the observed rate. PROOF T H A T THE REACTION IS NOT I N THE AQUEOUS FILM The chemical reaction could take place in the aqueous film because according to Astarita’s ‘fast reaction regime’ theory15 the chemical reaction is so fast that all the species are at equilibrium in the bulk aqueous phase; the reaction zone coincides with the diffusion zone in the aqueous film. Rewriting Astarita’s expression for a first-order reaction in which the rate of zinc reaction, Y , is expressed in terms of CHR(a), we obtainM.A. HUGHES AND H. SUNGSHOU 3453 When the equilibrium concentration CHR(eq) in the aqueous phase is assumed to be equal to zero and the experimental values of extraction rate are substituted for r in eqn (3) then k = 7.16 x lo8 s-l. Two reaction schemes might be considered, namely and kf &'+(a) + HR(a) -+ ZnR+(a) + H+ Znz+(a) + R- -+ ZnR+. k" and in which Ka is the dissociation constant of dithizone in the aqueous phase (Freiser gives K, = 5.0 x Czn(a) is the concentration of Zn2+ in the aqueous phase and C, is the concentration of hydrogen ion in the aqueous phase. = 7.16 x 10l2 s-l mol-1 dm3 k f = ~ Czn(a) (6) k It follows that and (7) Thus the calculated value of k" is 6 orders of magnitude higher than the value reported by Freiser (6.1 x lo6 dm3 mol-1 s-l) and 5 orders of magnitude higher than the v.alue of the rate constant for water replacement around the Zn ion (5.0 x lo7 dm3 mol-1 s-l).Therefore the locale of the reaction cannot be in the aqueous film. GENERAL CRITERION FOR LOCALE OF THE CHEMICAL REACTION The following criteria for the location of the site of the chemical reaction in the liquid-liquid extraction of metals exist. CRITERION 1 Examination of eqn (2) shows that if then a bulk aqueous-phase locale is impossible. vary between taken as 1 cm s-l, then eqn (8) becomes The value of k t R varies according to the contacting experiment: typically it may and lop2 cm s-l.15 For a single-drop experiment the maximum is 1 Examination of eqn (3) shows that if (10) 1 rPHR 9 2 (D€iR k)' CHR(o) then an aqueous-film locale is impossible.The partition coefficient enters into the left-hand side of eqn ( 9 ) and (10) and is especially important. 112 F A R 13454 EXTRACTION OF ZINC The low partitioning of the dithizone extractant into the aqueous phase is a very important reason for the interfacial location of the chemical reaction. This partitioning phenomenon was neglected in the argument put forward by Kletenic. MODELLING THE RATE BASED O N A CHEMICAL REACTION AT THE INTERFACE COUPLED WITH DIFFUSION OF HR ONLY From the experimental results of table 2 the following dependence of the zinc extraction rate on the concentrations of the extractant, the metal and the hydrogen ions was observed: where n = 0.25, rn = 1.0 and p = 0.35. Such a dependence is quite different from that reported by Freiser and Kletenic, although these workers used a more acid range of pH and a different contacting technique.Since the extraction rate is proportional to the concentration of extractant and is much less dependent on the concentrations of hydrogen and metals ions, a one- dimensional mathematical model describing the diffusion of the extractant from the bulk organic phase to the interface where the chemical reaction then takes place is tenable. The rate of chemical reaction at the interface was assumed to be a function of the interfacial concentration of the extractant, HR, only. For this work, the differential equation is The initial and the boundary conditions are CER(o) = CHRo(o) = constant, t = 0 (13) CHR(o) = CHRo(o) = constant, x = 00.(15) Note that for eqn (15) the change in CHR(o) during extraction is considered to be negligible. Also k , = kiii C,,/C, = constantlo for each experiment, kiii = 1.05 x cm s-l, and C&R(o) is the initial concentration of HR in organic phase; DHR(o) is the diffusivity of HR in the organic phase and is equal to 1.39 x Here the concentrations of Zn2+ and H+ in the continuous bulk of the aqueous phase are considered to be constant and equal to the concentrations at the interface. Then an analytical solution of these differential equations can be obtained:16? l9 cm2 s-l (see table 6).M. A. HUGHES AND H. SUNGSHOU 3455 r = r(av) = M/t (19) where t is the time at moment t, rt the instantaneous rate of extraction at time t, M the mass transfer per unit area during the period from t = 0 to t = t and r(av) the average extraction rate during the period from t = 0 to t = t .in which rint is the interfacial reaction rate. In other words, the rate is then controlled by chemical reaction at the interface. then and the rate of extraction is controlled by a diffusion process. The results calculated from eqn (18) and (19) are listed in table 7 together with experimental results and the values of extraction rate based on interfacial reaction only The results in table 7 demonstrate a fairly good agreement between the experimental and calculated rates, especially for those cases where the interfacial chemical reaction rates are low, i.e. where the pH or the concentrations of Zn2+ and the extractant are low. Also the differences between the values calculated from eqn (18) and (19) and values calculated from eqn (21) are smaller.In this range of experimental conditions the extraction rate is controlled by chemical reaction at the interface. But at higher pH and Zn concentrations the difference between the calculated and experimental rates is quite obvious; we then have a mixed regime of mass transfer with chemical reaction. The pH values in Kletenic's experiments were varied from 3 to 4; in this region the reaction takes place at the interface so the value of constant kiii suggested by himlo has been confirmed in a general way by our experiments. However, this value is probably only some kind of empirical constant valid only for a certain step of the process and for certain experimental conditions. These chemical rate constants are in fact physicochemical constants and they are quite different from such parameters as mass-transfer coefficients because the latter will vary with the differing hydrodynamic conditions.If the constant kiii was a constant having some physical meaning then it should be valid during drop formation as well as drop travel. That is to say during drop formation the extraction rate should not go beyond the limits allowed by the interfacial chemical reaction the rate constant of which is kiii. It is known that in the single-drop experiments appreciable extraction occurs during drop formation (we assume here that mass transfer during coalescence is negligible in comparison). The experimental values of the amount of mass transfer during drop formation may be compared with those calculated by assuming that the process is controlled by chemical reaction only during drop formation (see table 8).The Heertjes modell8* l9 was used to calculate the total amount extracted during drop formation per unit interfacial area, Mf,19 and gives [eqn (21)l. kiii = 1.05 x lop5 cm s-l. A difference of about two orders of magnitude was found between the calculated and experimental values of Mf. Such a difference cannot be due to experimental error or to errors in the coefficients of the chosen model. On the other hand, the big 112-2TABLE 7.---THEORETICAL VALUES OF EXTRACTION RATE CALCULATED USING A ONE-DIMENSIONAL DIFFERENTIAL-EQUATION METHOD experimental conditions extraction rate/mol cm-2 s-' [H+l [HWO [Znl no.pH / 1 0-5 mol dm-3 / 1 0-6 mol dm-3 / 1 OV4 mol dm-3 calculated, eqn (18) and (19) interfacial reaction, eqn (21) experimental 1 2 3 4 5 6 7 8 9 10 11 12 4.75 5.157 4. I54 4.701 5.099 4.186 4.47 5.096 4.52 4.52 5.099 5.14 1.78 0.697 7.01 1.99 0.796 6.5 1 3.39 0.802 3.00 3 .OO 0.796 0.769 1.06 1.06 1.06 1.06 1.06 1.06 1.06 1.06 0.496 1.829 0.496 1.829 1 .o 1 .o 2.5 2.5 2.5 5.0 5.0 5.0 2.5 2.5 2.5 2.5 5 . 6 0 ~ 1 * 10 x 10-10 3.80 x lo-" 1.20 x 10-lO 2.51 x 7.81 x 1.38 x 3.81 x 3.94 x lo-" 1.45 x 10-lo 4.33 x lO-'O 1.175 x 6.25 x lo-" 1.60 x 10-lo 3.97 x lo-" 1.40 x 10-lo 3 . 5 0 ~ 8.55 x lo-" 1 . 6 4 ~ 6.94 x 4.34 x 10-11 I .60 x loplo 1.64 x 6.03 x 10-lo 7.0 x 10-l' 1.06 x loplo 5.1 x lo-" 1.01 x 10-10 1.09 x 7.50 x 10-l' 0.955 x 1.29 x 3.30 x lo-" 1.20 x 10-'O 5.4 x lo-" 2 91 x 10-loM.A. HUGHES AND H. SUNGSHOU 3457 difference between the amount of mass transfer during drop formation and drop travel (see table 2) implies that the whole process involving drop formation and drop travel involves a very significant contribution from mass-transfer mechanisms. Thus the chemical rate constant suggested by Kletenic seems to be an average value valid only for certain experimental conditions related to the pH range of his experiments. A new approach must be made based on the mass transfer of all species involved in the extraction system. MODELLING THE RATE BASED ON THE MASS TRANSFER OF ALL SPECIES2' The interfacial reaction is assumed to be very fast and the concentrations of all species involved in the extraction process at the interface are assumed to be at equilibrium, i.e.(25) cZnR2i (cHi)2 Czn(a)i (CHrt(o)i)2 Kex = where Kex is a mass-action equilibrium constant, Cii is the concentration of species j at the interface, CZnRZi and CHR(,,)i are the concentrations of ZnR, and HR on the organic side of the interface, and Czn(a)i and CHi are the concentrations of Zn2+ and H+ on the aqueous side of the interface. The rate will be decided by the mass transfer of all the species between the bulk and the interface. The analysis of the problem is based upon the transfer of the extractant HR rather than Zn and related to the transfer of other species. We therefore rewrite Chapman's equation for the expression of Kex in terms of the fluxes, NHRi, as follows : or where k: is an individual film mass-transfer coefficient of species j , NNRi is the flux of species HR across the interface and Ci is the concentration of speciesj in the bulk Eqn (28) is a cubic equation for N , in terms of the bulk concentrations, individual film mass-transfer coefficients and the equilibrium constant.Thus the rate of extraction r can be calculated through the calculated value of N , since - NHRi = 2r. As in Chapman's equation the mass-transfer coefficient ratios (kRR/k&), (k&/k&) and (k&/kg) are assumed to be in proportion to the square roots of the individual species diffusivities. The equilibrium constant Kex was chosen to be 2.0 x according to ref (1 7). Calculation in this way shows that the contribution of the mass transfer of metal to the extraction rate is too high.It is true in a falling-drop experiment that the phase ratio of the continuous phase3458 EXTRACTION OF ZINC to the dispersed phase tends to infinity; this will probably cause a decrease in the value of the ratio (kgR/kgn). On the other hand, the counter-current flux of Zn2+ and H+ at the interface also causes an increase in the value of kin (for the hydrogen ion the value of diffusivity is some ten times larger than that of Zn2+). Therefore (k'&/kin) must be calculated in another way. The Handlos-Baron equation21 (for fully circulating drops) is used for the calculation of kgn and the equation from the Kronig-Brink modeI2l for the calculation Of kLR; then -- - 20. kOZn kOHR The values of the other ratios k&/k; eqn (28) were taken as before in proportion to roots of the individual species' diffusivities.From the calculated values of N,, the average value of kGR(av) was obtained, then the calculated values of Nzni(calc), ( Z J ( ~ ~ ~ ~ ) ) , were obtained through the k&R(av). In TABLE 8.-AMOUNT OF METAL EXTRACTED DURING DROP FORMATION Mf (calculated) (experimental) no. /mol cm-2 /mol cmP2 1 2 3 4 5 6 7 8 9 10 11 12 2.345 x lo-" 5.99 x 10-11 1.49 x 10-l' 5.24 x 10-l' 1.31 x 10-lo 3.20 x 6.16 x 2.60 x 10-lo 1.65 x lo-" 5.81 x 6.13 x lo-" 2.34 x 1.24 x 10-9 0.73 x 10-9 0.86 x 10-9 1.02 x 10-9 1.01 x 10-9 1.04 x 10-9 i . i 4 x 10-9 0.56 x 10-9 1.31 x 10-9 0.55 x 10-9 0.826 x lov9 1.33 x TABLE 9.-cOMPARISON OF EXTRACTION RATES BETWEEN THE EXPERIMENTAL VALUES AND THOSE CALCULATED USING CHAPMAN'S MODEL 1 2 3 4 5 6 7 8 9 10 11 12 -0.1867 - 0.2045 - 0.2000 -0.2579 - 0.2701 - 0.2629 -0.2974 - 0.3273 -0.2675 -0.2193 -0.3219 - 0.2362 6.914 6.84 x 10-lo 7.0 x 7.50 x lo-" 1.06 x 7.33 x lo-" 9.45 x 10-1' 9.90 x 10-l' 9.63 x lo-" 1.09 x 0.955 x 1.20 x 10-lo 4.59 x lo-" 1.39 x 10-lo 1.20 x 5.52 x lo-" 1.89 x 2.19 x 0.51 x 10-lo 1.01 x 10-10 1.09 x 10-lo 7.50 x 10-l' 1.29 x 10-lo 3.30 x lo-" 5.40 x lo-"M.A. HUGHES AND H. SUNGSHOU 3459 all cases eqn (28) exhibited only one real root, Values of Nzni(calc) were compared with the values of NZni(expt), taking rexpt as the experimental value of extraction rate. It is seen from table 9 and fig. 2 that a better agreement was obtained between theoretical and observed rates than was obtained in table 7.Thus the mass-transfer model suggested by Chapman fits, in general, our falling-drop experiments except at the data points no. 3, 6 and 9, where the differences between the experimental and calculated values amount to ca. 30%. This disagreement can be explained by the lower pH or lower extractant concentration when the species at the interface are probably not at equilibrium. So the process of extraction in this region shows more chemical- control character. A’z,,;+/l O-’O mol cm-2 s-l (calculated) FIG. 2.-Comparison of the values of experimental extraction rates and the theoretical rates based on Chapman’s It might be noted that in the range 5 < kgR/k&, < 20 the results of calculation are almost the same; in this range the calculation is not sensitive to the value of this ratio.Now we should return to the problem which was discussed before, i.e. does Chapman’s model of mass transfer also fit the experimental data observed during drop formation? At the first moment of drop formation the concentrations of the various species at the interface should be equal to zero, since CZnRi I- 0. Thus the concentration of HR at the interface, CHRi is also zero. Then according to the model of Heertjes and Newman the mass transfer of metal at the interface during drop formation, Mf,19 becomes : 1 4 Mf = 7 (DHR lf/.)’ CHR(o) (29) where t, is the time for drop formation (0.755 s in our experiments) and CHR(o) is the concentration of HR in bulk organic phase. The calculated values of M, are 2.2 x mol cm-2 at CgR(o) = 1.829 x lop6 mol cmP3, 1.29 x mol cmL2 at CgR(o) = 1.06 x 10+ mol cm-3 and 6.02 x mol cm-2 at CHR(o) = 0.496 x 10+ mol cmP3.These calculated values of Mf are in the range of the experimental data (see table 8). A more quantitative calculation of M, under different experimental conditions is not possible by this last method unless the changing value of the difference between the bulk and interfacial value of HR during drop formation is taken into account. In the comparison of the experimental rates with the rates calculated by this last mass-transfer model, better agreement is obtained than when eqn (18) and (19) are3460 EXTRACTION OF ZINC used. Also the agreement with the Chapman model covers both the drop-travel and drop-formation experiments. CONCLUSIONS The solvent-extraction process of Zn2+ by dithizone occurs by means of an interfacial reaction. The locale of chemical reaction cannot be in the bulk aqueous phase nor in the aqueous film, because of the large value of the partition coefficient of the extractant.The process of extraction under most experimental conditions studied here can be explained by Chapman’s mass-transfer model. In the lower ranges of pH extractant concentration the species at the interface are not at equilibrium, and the process then shows more chemical-control character. 1 P. R. Danesi and R. Chiarizia, Critical Reviews in Analytical Chemistry, ed. B. Campbell (C.R.C. Press, Cleveland, Ohio, 1980), chap. 10, p. 1. J. M. Kolthoff and E. B. Sandell, J. Am. Chem. Soc., 1941, 63, 1960. H. Irving and R. J. P. Williams, J . Chem. Soc., 1949, 1841. B. McClellan and H. Freiser, Anal. Chern., 1964, 36, 2262. C. Honaker and H. Freiser, J. Phys. Chem., 1962, 66, 127. J. Oh and H. Freiser, Anal. Chem., 1967, 39, 295 and 1671. M. Fomin, Kinetics of Extraction (Atomizdat, Moscow, 1978). W. Nitsch, Chem. Ing. Tech., 1970, 42, 1229. W. Nitsch and K. Hillekamp, Chem Z . , 1972, 96. 254. lo Yu. B. Kletenic and 0. B. Vinokurova, Zh. Anal. Khim., 1976, 31, 871. l1 Yu. B. Kletenic, B. A. Navrotskay and A. I. Potavova, Izu. Sibirsk, Otd. Akad. Nauk SSSR, Ser. Khim. l 2 Yu. B. Kletenic, B. A. Navrotskay, A. 1. Potavova, S. A. Sedova and 0. B. Vinokurova, Izu. Sibirsk. l 3 H. Irving, J. Chem. SOC., 1952, 1, 365. l4 R. J. Whewell, M. A. Hughes and C. Hanson, J . Inorg. Nucl. Chem., 1975, 37, 2323. l5 G. Astarita. Mass Transfer with Chemical Reaction (Elsevier, Amsterdam, 1967). l6 J. Crank, The Mathematics of Drflusion (Oxford University Press, Oxford, 1975). l 7 G. K. Schwetzer and C. B. Honaker, Anal. Chim. Acta, 1958, 19, 224. l8 P. Heertjes and L. H. deNie, Chem. Eng. Sci., 1966, 21, 755. Nauk, 1970, 2, no. 4, 17. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk, 1979, 2, no. 4, 17. F. Nakashio, T. Tsuneyaki, K. Inoue and W. Sakai, International Conference on Solvent Extraction, ISEC 197 1, The Hague, Holland (SOC. Chem. Ind., London, 197 l), paper 87. G. Laddha and T. E. Degaleesen, Transport Phenomena in Liquid Extraction (Tata McGraw-Hill, New Delhi, 1976). 2o W. Chapman, R. Caban and M. Tunison, AIChE Symp. Ser., 1975, 71, 151. (PAPER 1 / 1775)

ISSN:0300-9599    

DOI:10.1039/F19827803447

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

f. Chem. SOC., Faraday Trans. I , 1982, 78, 3461-3466 Conductance and Viscosity Measurements of Te trabutylammonium Te t raphen ylboride in Non-aqueous Solvents at 25 OC BY DIP SINGH GILL,* MOHINDER SINGH CHAUHAN AND MADHU BALA SEKHRI Department of Chemistry, Himachal Pradesh University, Simla- 17 1005, India Received 8th December, 198 1 The equivalent conductance and viscosity of tetrdbutylammonium tetraphenylboride (Bu,NBPh,) have been measured in a large number of non-aqueous solvents at 25 O C and the data in all cases have been analysed by the Shedlovsky and Jones-Dole equations, respectively. The experimental A. value of Bu,NBPh, obtained from the conductance data in each solvent has been compared with the corresponding value obtained by adding together the ionic conductances of Bu,N+ and Ph,B-, which have been theoretically calculated using an equation proposed by Gill.The agreement between these two sets of A. values is good. The viscosity A coefficient of the Jones-Dole equation for Bu,NBPh, in each case is positive and is in good agreement with the limiting theoretical value calculated using the Falkenhagen-Vernon equation. The viscosity B coefficient is also positive and large in each case. The B+ and B- coefficients for Bu,N+ and Ph,B- have been calculated employing a method proposed in our previous paper and a justification based on conductance results has also been gwen for this method of separation. In one of our previous papers1-+ we reported the viscosities of some 1 : 1 electrolytes in NN-dimethylformamide + acetone mixtures3 The Jones-dole viscosity Bcoefficients for the electrolytes were split into the contributions due to individual ions using an assumption based on Bu,NBPh, as a reference salt.We found that such an assumption could be both simple and very useful in the evaluation of ionic B, and B- coefficients in pure and mixed non-aqueous solvent^.^ A detailed survey of the literature revealed that the viscosity of Bu,NBPh, has been measured in only a few non-aqueous solvent^.^ In order to emphasize the use of Bu,NBPh, as a reference salt for the evaluation. of ionic B, and B- coefficients from the B values of the electrolytes, we have measured the viscosity of this salt in acetonitrile (AN), acetone (Ac), ethyl methyl ketone (EMK), nitromethane (NM), nitrobenzene (NB), NN-dimethylformamide (DMF), NN-dimethylacetamide (DMA), dimethylsulphoxide (DMSO), propylene carbonate (PC), 1,1’,3,3’-tetramethylurea (TMU) and hexamethylphosphotriamide (HMPT).The equivalent conductance of Bu,NBPh, in most of these solvents has also not been available in the literature; we therefore measured the equivalent conductance of this salt, and the results are presented in this paper. EXPERIMENTAL All the solvents were purified by methods already reported.’. Tetrabutylammonium tetra- phenylboride (Bu,NBPh,) was prepared by the method of Accascina et af.8 Conductances were measured at 1000 Hz with a digital conductivity meter type NDC-732 supplied by Naina Electronics, Chandigarh. The details of the conductance cell and the experimental procedure for the conductance measurements have been reported earlier.2.4 , 346 13462 CONDUCTANCE A N D VISCOSITY MEASUREMENTS OF Bu,NBPh, An Ubbelohde suspended-level viscometer with flow time 756.4 s for water at 25 OC was used for the viscosity measurements. The method of calibration of the viscometer and the procedure for the viscosity measurements are given in ref. (3). The overall accuracy of the viscosity measurements was estimated as f 0.1 % and that of the conductance measurements as & 0.2%. RESULTS AND DISCUSSION CONDUCTANCE MEASUREMENTS The equivalent conductance of Bu,NBPh, has been measured in AN, Ac, DMF, DMA, DMSO, PC, TMU and methanol (MeOH) in the concentration range (1 - 80) x lo-, mol dm-3 at 25 OC. The equivalent conductance at infinite dilution (A,,) and the association constant KA in each solvent studied (reported in table 1) have been TABLE 1 .-EQUIVALENT CONDUCTANCE AT INFINITE DILUTION, A,(expt), THEORETICALLY CALCULATED EQUIVALENT CONDUCTANCE AT INFINITE DILUTION, A,(calc), THEIR PERCENTAGE DIFFERENCE AND THE ASSOCIATION CONSTANTS, KA, FOR Bu,NBPh, IN VARIOUS NON-AQUEOUS SOLVENTS AT 25 OC 120 (expt) KA A.(calc) Ao(expt) - Ao(calc) x 100 Ao(exPt) solvent /ap1 cm2 mol-l /mol dm-3 /a-l cm2 mol-1 AN Ac EMK NM NB DMF DMA DMSO PC TMU HMPT MeOH 120.0" 127.8" 102.8b 67.3c 22.3d 50.7" 45.3" 22.4" 1 7.6" 30.2" 12.2e 76.3" 11 15 - 17 30 - 121.8 131.4 105.2 68.2 22.5 52.4 45.4 23.1 18.1 30.8 12.6 75.4 - 1.5 - 1.9 - 2.4 - 1.4 -0.8 - 3.4 - 0.3 - 2.9 - 2.9 - 2.0 - 3.3 1.2 a Present work; R. M. Fuoss and E.Hirsch, J. Am. Chem. SOC., 1960, 82, 1013; S. R. C. Hughes and D. H. Price, J. Chem. SOC. A , 1967,1093; ref. (1 5). C. Atlani, J-C. Justice, M. Quintin and J. E . Dubois, J . Chim. Phys., 1969, 66, 180. calculated iteratively by a least-squares treatment with an IBM- 1620 computer using Shedlovsky's method,l09 l1 as discussed in detail in our previous papem27 The viscosity (q) and the dielectric constant ( E ) for the analysis of conductance data were taken from our previous papers.l* The mean ion activity coefficients for that purpose were calculated using the equation suggested by Justice.12 Activity-coefficient measurements by Gill and Malhotra13 in DMF show that such an equation for the evaluation of mean ion activity coefficients of electrolytes is justifiable.The standard deviations in A, and KA values given in table 1 obtained by applying standard statistical equations14 were found to be always less than +0.2% and The root-mean-square deviations oA calculated from the standard deviations of the individual points in no case exceeded the experimental uncertainty of the present conductance measurements, i.e. + 0.2%. This shows the good applicability of the Shedlovsky equation to our conductance data. As the precision of our conductance lo%, respectively.D . S. GILL, M. S. CHAUHAN A N D M. B. SEKHRI 3463 data is ca. +0.2%, the use of a conductance equation which demands a precision in the conductance data much better than +0.1% was not thought appropriate to analyse the present conductance data. To indicate the precision of the present conductance data, our A, values from table 1 are compared with values already available in the literature.Our A, values for Bu,NBPh, of 120.0 in AN, 76.3 in MeOH and 22.4 in DMSO are in good agreement with the A. values 119.7, 76.0 and 22.2 S2-l cm2 mol-l reported by Coetzee and Cunningham,15 Coplan and FUOSS~~ and calculated from the A: values of Arrington and Griswold17 in AN, MeOH and DMSO, respectively. PREDICTION OF A. VALUES FOR Bu,NBPh, I N NON-AQUEOUS SOLVENTS In our previous papers2? we proposed an equation which theoretically predicts limiting ion conductances of Bu,N+ and Ph,B- in pure and mixed non-aqueous solvents with an average uncertainty of _+ 2% in comparison with the values obtained from direct transport numbers when ri values for Bu,N+ and Ph,B- are taken in this equation to be 5.00 and 5.35 A, respectively.This equation can be written as A: = 121 F2/6nr N [ri-(O.O103~+r,)] (1) where all the symbols have their usual significance2. 6. Using ri equal to 5.00 and 5.35 A for Bu,N+ and Ph,B-, respectively, and rY equal to 0.85 A for all the solvents reported in table 1 , A: values for these two ions have been theoretically calculated from eqn (1). The q and E values for various solvents were taken from our previous papers.lV By adding these A: values for Bu,N+ and Ph,B-, A, values for Bu,NBPh, in various solvents have been calculated and are reported as A,(calc) values in table 1 . A comparison between the theoretically calculated A,(calc) values and the experimentally measured Ao(expt) values shows fairly good agreement (see the percentage difference between these two sets of values reported in the last column of table 1).VISCOSITY MEASUREMENTS The viscosity of Bu,NBPh, has been measured in AN, Ac, EMK, NM, NB, DMF, DMA, DMSO, PC, TMU and HMPT in the concentration range (30-250) x lo-, mol dm-3 at 25 OC. Plots of the Jones-Dole equation18 qr = l + A Cg+B C (2) in the form (qr - 1)/Ci against Cg were linear in all cases over the whole concentration range studied and the A coefficients for Bu,NBPh, in all the solvents, obtained from the intercept of these plots, were positive (table 2) and in good agreement with the corresponding A , coefficients calculated from the Falkenhagen-Vernon equationlg where A, = A: + A;, A: and At are the limiting ion conductances for Bu,N+ and Ph,B-, q and E are the solvent viscosity and the dielectric constant of the solvent, respectively, and Tis the absolute temperature.For the calculation of A , from eqn (3), A, values for Bu,NBPh, were taken from table 1 and the A; values for Ph,B- were calculated using A: values for Bu,N+ reported in our previous paper.6 For the solvents in which Bu,NBPh, was unassociated (table 1) the B coefficients (reported in table 2) were obtained from the average value of the (practically constant) apparent B coefficients calculated at each concentration obtained using the3464 CONDUCTANCE AND VISCOSITY MEASUREMENTS OF Bu,NBPh, THE FALKENHAGEN-VERNON EQUATION FOR Bu,NBPh, IN NON-AQUEOUS SOLVENTS AT 25 O C TABLE 2.-A AND B COEFFICIENTSa OF THE JONES-DOLE EQUATION AND A,, COEFFICIENT OF solvent lo2 A,/(dm3 mo1-l): A / (dm3 mo1-l); AN Ac EMK NM NB DMF DMA DMSO PC TMU HMPT 2.43 3.39 3.63 2.40 2.52 2.40 2.33 1.96 1.70 2.97 2.77 2.30 3.20 3.70 2.00 2.40 2.80 2.40 1.80 1.60 3.00 3.20 B/dm3 mol-l 1.32 f 0.02 1.54 f 0.02 2.02 f 0.03 1.29 50.06 1.40 f 0.02 1.96 _t 0.03 1.93 & 0.02 1.45 f 0.04 1.46 f 0.02 2.01 f 0.04 2.99 & 0.06 - a The A coefficients in this table have a maximum uncertainty of f 10%.Falkenhagen-Vernon A, coefficient from table 2 and the equation r,-l-A,Ct c * B = (4) However, for the solvents in which Bu,NBPh, showed ion association, the B coefficients (reported in table 2) were obtained using the following equation3 vr - 1 - A , (Ca)e Ca = B + B KA Ca fg and a graphical method whose details are discussed in our previous paper.3 The viscosity B coefficient for Bu,NBPh, in all the solvents is large and positive.This is a common feature of most non-aqueous solvents. In non-aqueous solvents, the structure-breaking contribution is negligible, with the result that the B coefficient is always positive and large. Our B coefficients for Bu,NBPh, in AN (1.32+0.02), in Ac (1.54+0.02) and in DMF (1.96k0.03) dm3 mol-l from table 2 are in good agreement with the values 1.35, 1.56 and 1.86 reported by Tuan and FUOSS' in AN and by Gill and Sharma3 in Ac and DMF, respectively. Our Bcoefficient for Bu,NBPh, in HMPT 2.99 0.06 is, however, higher than the value of 2.58 dm3 mol-l reported by Sacco et a/.20 in this solvent. B+ AND B- COEFFICIENTS FOR BU,N+ AND Ph,B- I N VARIOUS NON-AQUEOUS SOLVENTS The splitting of the B coefficient of electrolytes into the contributions due to individual ions cannot be made in the same way as the division of limiting equivalent conductances, since there is no quantity corresponding to the transfer numbers.Accordingly, the separation of the observed B coefficients has been an arbitrary p r o c e ~ s . ~ ~ - ~ ~ In a previous paper3 we proposed a method for splitting the B coefficient of electrolytes into the contribution due to individual ions on the basis of the following equations : and (7)D. S. GILL, M. S. CHAUHAN AND M. B. SEKHRI 3465 The present conductance measurements in various solvents have also confirmed that the experimental A, values for Bu,NBPh, are in good agreement with the theoretically calculated A, values when the ri values for Bu,N+ and Ph,B- in eqn (1) are taken to be 5.00 and 5.35 A, respectively, in all the solvents. This justifies our approach to the evaluation of the ionic B, and B- coefficients from eqn (6) and (7) using Bu,NBPh, as a reference salt.Using eqn (6) and (7), the B coefficients of Bu,NBPh, in various solvents given in table 2 have been resolved into the contributions from Bu,N+ and Ph,B-, and the values thus obtained are reported in table 3. The maximum uncertainty in the B, and B- values of table 3 is k0.06 dm3 mol-l. TABLE 3.-lONIC B, AND B- COEFFICIENTS' FOR BU,N+ AND Ph,B- IN NON-AQUEOUS SOLVENTS AT 25 "C EVALUATED FROM EQN (6) AND (7) solvent B+/dm3 mol-l B-/dm3 mol-l AN Ac EMK NM NB DMF DMA DMSO PC TMU HMPT 0.59 0.69 0.91 0.58 0.63 0.88 0.87 0.65 0.66 0.90 1.34 0.73 0.85 1 .1 1 0.71 0.77 1.08 1.06 0.80 0.80 1 . 1 1 1.65 The B,. and B- coefficients in this table have a maximum uncertainty of & 0.06 dm3 mo1-l. There are practically no viscosity data available in the literature for electrolytes in all these non-aqueous solvents. Therefore, a comparison of our B, and B- values from table 3 cannot be made. Some precise viscosity measurements of electrolytes have been recently reported by Bicknell et a1.26 in DMSO. Our B- value for Ph,B- [equal to 0.80_+0.06 in DMSO from the present method of separation (table 3)] is in good agreement with the value of 0.72 dm3 mol-l reported by Bicknell et al.26 using an independent approach to the separation.M.S.C. and M.B.S. are grateful to the C.S.I.R., New Delhi, for the award of a Research Fellowship. The authors thank Mr Amar Nath Sharma for the computer analysis of the conductance data. A research grant from the U.G.C., New Delhi, is gratefully acknowledged. ' D. S. Gill, J . Solution Chem., 1979, 8, 691. D. S. Gill and M. B. Sekhri, J . Chem. SOC., Faraday Trans. 1, 1982, 78, 119. D. S. Gill and A. N. Sharma, J. Chem. Soc., Faraday Trans. I , 1982, 78, 475. D. S. Gill and J. S. Cheema, Electrochim. Acta, in press. D. S. Gill, A. N. Sharma and H. Schneider, J . Chem. SOC., Faraday Trans. I , 1982, 78, 465. D. S. Gill, J . Chem. SOC., Faraday Trans. I , 1981, 77, 751. ' D. F-T. Tuan and R. M. Fuoss, J. Phys. Chem., 1963, 67, 1343. F. Accascina, S. Petrucci and R.M. Fuoss, J . Am. Chem. SOC., 1959, 81, 1301. D. S. Gill and J. S. Cheema, Electrochim. Acta, 1982, 27, 755. lo R. M. Fuoss and T. Shedlovsky, J . Am. Chem. SOC., 1949, 71, 1496.3466 CONDUCTANCE AND VISCOSITY MEASUREMENTS OF Bu,NBPh, l 1 R. M. Fuoss and F. Accascina, Electrolytic Conductance (Interscience, New York, 1959). l3 D. S. Gill and K. Malhotra, Indian J. Chem., 1980, 19A, 65. l4 W. J. Youden, Statistical Methods for Chemists (John Wiley, New York, 1951), p. 42. l5 J. F. Coetzee and G. P. Cunningham, J . Am. Chem. Soc., 1965, 87, 2529. J-C. Justice, Electrochim. Acta, 197 1, 16, 701. M. A. Coplan and R. M. FUOSS, J. Phys. Chem., 1964,68, 1 177. D. E. Arrington and E. Griswold, J. Phys. Chem., 1970, 74, 123. G. Jones and M. Dole, J . Am. Chem. SOC., 1929, 51, 2950. J. Falkenhagen and E. L. Vernon, Phys. Z., 1932, 33, 1401; Philos. Mag., 1932, 14, 537. 1936. W. M. Cox and J. F. Wolfenden, Proc. R. SOC. London, 1934, 145, 475. 2o A. Sacco, G. Petrella, M. D. Monica and M. Castagnolo, J. Chem. SOC., Faraday Trans. I , 1977, 73, 22 M. Kaminsky, Discuss. Faraday SOC., 1957, 24, 171. 23 R. W. Gurney, Ionic Processes in Solution (McGraw-Hill, New York, 1953). 24 B. S. Krumgalz, J. Chem. Soc., Faraduy Trans. I , 1980, 76, 1275. 25 J. M. Gordon, N. Martinus and C. A. Vincent, J. Chem. Soc., Chem. Commun., 1978, 56. R. T. M. Bicknell, K. G. Lawrence and D. Feakins, J. Chem. Soc., Faraday Trans. I , 1980, 76, 637. (PAPER 1 / 1905)

ISSN:0300-9599    

DOI:10.1039/F19827803461

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1982, 78, 3467-3476 Conformational Effects in Fluorescent Excited Charge-transfer Complex Formation BY XIU-JIN Luo,? GODFREY S. BEDDARD AND GEORGE PORTER Davy Faraday Laboratory, The Royal Institution, 2 1 Albemarle Street, London AND R. STEPHEN DAVIDSON* Department of Chemistry, The City University, Northampton Square, London EClV OHB AND TERENCE D. WHELAN Department of Chemistry, The University, Leicester LE 1 7RH Received 1 1 th December, 198 1 From a study of the variation in fluorescence quantum yield and lifetime with change in solvent polarity for some o-( 1 -naphthyl)-N-alkylpyrroles it is concluded that the conformation of the excited complex has little effect upon the wavelength of fluorescence but does have an effect upon the relative efficiencies of the radiative and non-radiative decay processes.It has been shown that there are fairly strict conformational requirements for fluorescent excimer formation to occur,l whereas this is not the case for fluorescent exciplex We and others2? have shown that the naphthylalkylamines Q C N ( 3 ) [compound (l)] exhibit fluorescent exciplex formation even when n = 1 . With only one methylene group interposed between the aryl and amino groups, the fluorescence cannot emanate from a complex having a sandwich conformation, as is required for fluorescent excimer formation. Similar results have been obtained with compound (2).4 Furthermore 9-(4-dimethylaminophenyl)anthracene also exhibits fluorescence which is more characteristic of excited charge-transfer complex formation than a localised anthracene excited ~ t a t e .~ In a definitive piece of work on compound (3) and related t Visiting research fellow, permanent address: Department of Chemistry, Jilin University, Changchun, People’s Republic of China. 34673468 FLUORESCENT COMPLEX FORMATION compounds Grabowski and Cowley et a1.6 have shown that excited charge-transfer fluorescence comes from an excited state in which the dimethylamino and cyanophenyl groups are not co-planar.6 Thus, it would appear that there are no strict conformational requirements for excited charge-transfer complex formation. We now report upon a study of compounds (4)-(6) and the corresponding intermolecular system involving compounds (7) and (8). (4) CH3 =CHI I EXPERIMENTAL SYNTHESIS COMPOUND (4) A mixture of acetonylacetone (2.92 g) and 1-naphthylamine (3.24 g) to which had been added toluene-4-sulphonic acid (0.005 g) was heated under reflux for 2 h.After co- ling, the mixture was extracted with ether (4 x 30 cm3), washed with water and dried. Evaporation of the solvent gave the crude product which was recrystallised from ethanol containing some decolourising charcoal, m.p. (from ethanol) 116-118 "C. (Found: C, 86.87; H, 6.95; N, 6.05; C,,H,,N requires: C , 86.83; H, 6.83; N, 6.32%.) r = 1.95-2.8 (7H, m), 3.95 (2H, s), 8.0 (6H, s); v,,, = 706-8 10, 1595 cm-l. COMPOUND (5) 1 -(Aminomethyl)naphthalene was prepared by the reduction of 1 -cyanonaphthalene with lithium aluminium hydride in ether solution. The crude product was mixed with acetonylacetone (4.5 g) and the mixture heated at 73 OC for 2 h.After cooling the mixture was extracted with ether (3 x 50 cm3). The ethereal layer was washed with water, dried and evaporated to give crude compound (5). This was purified by distillation in vacuo; b.p. 110-120 "C (0.001 mmHg);* m.p. (from methanol) 78-8OOC. (Found: C , 86.54; H, 7.3; N, 5.77; C,,H,,N requires: C, 86.8; H, 7.28; N, 5.95%.) z = 1.9-2.8 (6H, m), 3.65 (lH, d, J = 8 Hz), 3.95 (2H, s), 4.45 (2H, s), 7.8 (6H, s); vmax = 720-790, 1595 cm-l. COMPOUND (6) 1-(Bromomethy1)naphthalene was prepared by reaction of 1 -methylnaphthalene (7.0 g) with N-bromosuccinimide (8.0 g) in carbon tetrachloride (1 50 cm3) at 80 OC and with radiation from * 1 mmHg 'v 133.322 Pa.LUO, BEDDARD, PORTER, DAVIDSON A N D WHELAN 3469 a 150 Watt tungsten bulb for 4 h.Removal of succinimide by filtration and evaporation of the carbon tetrachloride gave the crude bromomethyl compound. This crude material was dissolved in dimethylsulphoxide (75 cm3) containing potassium cyanide (3.3 g) and a catalytic amount of 18-crown-6. The mixture was stirred under an atmosphere of nitrogen for 20 h. Addition of ether (50 cm3) and water (50 cm3) followed by separation, drying and evaporation of the ethereal layer gave crude 1 -(cyanomethyl)fiaphthalene. This crude material had a satisfactory i.r. and n.m.r. spectrum: 7 = 2.0-2.7 (7H, m), 5.95 (2H, s); vmax = 730-790, 1595, 2250 cm-l. The crude material was reduced by lithium aluminium hydride to give 1 -(2-aminoethyl)- napthalene: 7 = 1.95-2.8 (7H, m), 6.9 (4H, tt, J = 5 Hz), 7.3 (2H, s); vmax = 730-800, 1595, 3 100-3350 cm-l.Reaction of this material with acetonylacetone gave compound (6): b.p. 110-120 OC (0.001 mmHg); m.p. (from methanol) 52.5-54 "C. (Found: C, 86.67; H, 7.73; N, 5.69; C,,H,,N requires: C, 86.7; H, 7.68; N, 5.62%.) v,,, = 740-790, 1595 cm-l. COMPOUND (7) Acetonylacetone (5.0 g) was slowly added to n-propylamine (9.66 g ) . The mixture was heated under reflux for 1.5 h. The reaction mixture was distilled in uclcuo to give compound (7): b-p. 60-65 OC (0.015 mmHg) (Found: C, 78.55; H, 11.0; C,H,,N requires: C, 78.77; H, 11.02%.) 7 = 4.2 (2H, s), 6.3 (2H, t, J = 7 Hz), 7.8 (6H, s), 8.4 (2H, S, J = 7 Hz), 9.1 (3H, t, J = 7 Hz). MATERIALS Naphthalene (Aldrich) was zone-refined prior to use.Acetonitrile (Aldrich), cyclohexane (B.D.H.) and benzene (B.D.H.) were of spectrofluorimetric grade and were used as received. SPECTROSCOPIC MEASUREMENTS Quantum yields of fluorescence were determined using 9, 1 0-diphenylanthracene as standard (& = 1 in cyclohexane).' Measurements were made in 1 cm quartz cells with the excitation beam being at right angles to the position for recording the fluorescence. All solutions had an OD of < 0.1 at the excitation wavelength of 295 nm. Solutions were degassed in high vacuum ( mmHg) prior to the measurements being taken. Corrected fluorescence spectra were recorded on a Perkin-Elmer MPF-4 spectrofluorimeter and absorption spectra were recorded on a Perkin-Elmer Hitachi 200 instrument. Fluorescence decay times were measured using the technique of single-photon counting.Excitation was at 295 mm delivered from a synchronously pumped frequency doubled dye laser excited by a mode-locked CR6 (Coherent Radiation) ion laser at 514 nm.B 'H n.m.r. and 13C n.m.r. were recorded on Jeol PS 100 and FX 60 instruments and i.r. spectra were recorded on a Perkin-Elmer 257 grating instrument (solid samples in Nujol liquids as films). Melting points (uncorrected) were determined with a Kofler hot-stage apparatus. RESULTS The absorption spectra of compounds (4)-(6) were recorded in cyclohexane, benzene and acetonitrile as solvent. Some representative spectra are shown in fig. 1. As can be seen from fig. 1, the absorption spectra of these compounds are virtually identical and correspond to a summation of the spectra of the pyrrole nucleus and naphthalene.Solvent has little effect upon the adsorption spectra. The emission spectra of compounds (4)-(6) and that of the complex formed between compounds (7) and (8) are highly solvent dependent and these are shown in fig. 2-5. Where appropriate the quantum yields of fluorescence and the fluorescence lifetimes are given in tables 1 and 2. The fluorescence spectra observed for mixtures of compounds (7) and (8) in which the concentration of compound (7) is gradually increased show an iso-emissive point (fig. 6). The fluorescence lifetimes of compounds (4), (5) and ( 6 ) show little variation with change in solvent polarity. The quantum yield of fluorescence of compound (6)3470 FLUORESCENT COMPLEX FORMATION wavelength/nm FIG.1 .-Absorption spectra of compounds (4)-(6) in cyclohexane and acetonitrile. Compound (4): (--) in cyclohexane, (--)in acetonitrile; compound ( 5 ) : (- - - -) in acetonitrile, ( * . * . .)in cyclohexane; compound ( 6 ) : (- .-) in acetonitrile, (--. .-) in cyclohexane. (NB. Some spectra are slightly displaced for the sake of clarity.) 1 I I 300 350 400 450 500 550 wavelength/nm FIG. 2.-Fluorescence spectra of compound (4) in different solvents (,Iexcit. = 295 nm): (- - - -) acetonitrile, (- .-) cyclohexane, (-) benzene. drops sharply on change of solvent from benzene to acetonitrile whereas a similar solvent change only causes a small decrease in quantum yield for compounds (4) and (5). DISCUSSION The finding that the absorption spectra of compounds (4)-(6) are essentially the same and correspond to a summation of the spectra of the pyrrole nucleus and naphthalene indicates that there is little if any charge-transfer contribution in the ground state.LUO, BEDDARD, PORTER, DAVIDSON AND WHELAN 5 - h +-I .- c 4 - 347 1 1 I I I 300 350 4 0 0 4 5 0 500 550 wavelength/nm FIG.3.-Fluorescence spectra of compound ( 5 ) in different solvents (Aexcit. = 295 nm): (-) acetonitrile, (- - - -) cyclohexane, (-.--) benzene. ->’ ’.- I 6 1 3 0 0 350 400 450 500 550 boo wavelength/nm FIG. 4.-Fluorescence spectra of compound (6) in different solvents (Aexcit. = 295 nm): (----) acetonitrile, (- - - -) cyclohexane, (---) benzene. wavelength/nm FIG. 5.-Fluorescence spectra for solutions of compound (8) containing compound (7) (Aexcit, = 295 nm): (- - - -) cyclohexane ([7] = 8.7 x mol dmd3, [8] = 1.3 x mol dm-3); (--) propan-1-01 mol dm-3, [8] = 2.3 x ([7] = 6.6 x mol dm-3).3472 FLUORESCENT COMPLEX FORMATION TABLE 1 .-QUANTUM YIELDS OF FLUORESCENCE, WAVELENGTHS FOR MAXIMAL EXCIPLEX FLUORESCENCE AND FLUORESCENCE LIFETIMES FOR COMPOUNDS (4), (5) AND (6) IN DIFFERENT SOLVENTS cyclohexane benzene propan- 1-01 acetonitrile cyclohexane benzene propan- 1-01 acetonitrile cyclohexane benzene propan- 1-01 acetonitrile compound (4) 0.03 7.2 0.02 5.0 0.015 7.6 0.01 6.9 0.03 15.8 0.03 10.2 0.02 14.3 0.02 14.05 compound (5) compound (6) 0.36 36.1 0.28 75.6 0.14 40.6 0.12 41.0 5.7 4.65 5.8 6.2 6.4 6.8 8.0 6.3 15.3 14.3 14.6 11.3 405 440 470 505 398 412 429 490 396 414 455 485 a Degassed solutions; aerated solutions.TABLE 2.-LIFETIMES EXTRACTED FROM FLUORESCENCE DECAYS OF MIXTURES OF NAPHTHALENE WITH N-( l-PROPYL)-2,5-DIMETHYLPYRROLE (MEASUREMENTS MADE AT 1 > 400 nm) lifetimes/ns concentration/mol dmP3 71 7 2 naphthalene pyrrole solvent ila/nm degassed aerated degassed aerated 7.02 x 1.2 x cyclohexane 392 16.7 8.5 73.1 14.4 2.5 x 7.9 x benzene 417 10.7 8.2 86.4 15.7 3.34 x 5.8 x propan-1-01 452 14.9 1.9 29.3 16.5 3.12 x lo-" 6.15 x acetonitrile 439 10.46 3.2b a Wavelength of maximum emission of wavelength. Mono-exponential decay observed in this solvent. This view is further substantiated by the finding that the absorption spectra are insensitive to change in solvent polarity. The lack of charge transfer and conjugation between the pyrrole and naphthalene rings in compound (4) is indicative of the lack of coplanarity between the two ring systems.Molecular models indicate that compound (4) cannot adopt a planar structure in the ground state due to the bad steric interaction of the pyrrolic methyl groups and the 8 hydrogens of the naphthalene ring. The tremendous sensitivity of the wavelength of maximal fluorescence intensity of compounds (4)-(6) to solvent polarity is indicative that the fluorescence emantes from a charge-transfer state. Since in all cases the spectra were recorded for dilute solutions (< 1 x lo-* mol dm-3) the emission must be coming from an intramolecular excited charge-transfer complex. Inspection of tables 1 and 2 shows that the compounds (4)-(6)LUO, BEDDARD, PORTER, DAVIDSON AND WHELAN 7r 3473 wavelength/ nm FIG.6.-Fluorescence spectra of benzene solutions of compound (8) containing varying amounts of compound (7) (in mol ~ I r n - ~ ) : (-), 7.9 x (--), 2.4 x (----), 4.95 x (--.-), 7.1 x ( . . . . . . A ) , 1.OX 10-2. and the intermolecular complex formed between compounds (7) and (8) give rise to fluorescence having maximal emission at a wavelength which, for a particular solvent, is independent of whether the complex be inter- or intra-molecular and the conformation of intra-molecular complex. In each case, the stabilisation of the complex which occurs on changing the solvent from cyclohexane to acetonitrile is ca. 56.43 kJ mol-l. The similarity in wavelength of fluorescence for compounds (4)-(6) in a particular solvent indicates that there is a similar amount of charge transfer in the excited states of these compounds.The energy of an excited complex is given by the following equation Epc = I- E A - C- AHS where E,, is the energy of the complex, Iis the ionisation potential, C is the coulombic interaction and AHs is the solvation energy. Compounds (4)-(6) possess the same donor group (2,5-dimethylpyrrole) and the same acceptor (naphthalene). Since these compounds have similar values of 1 and Ea the only differences one might find in the energies of the complex must be due to either the C or AHs terms. These differences are apparently very small. The values for compound (4) may be a little perturbed due to the operation of inductive effects since the pyrrole and naphthalene rings are linked by a single o bond.The exciplex emission from mixtures of compounds (7) and (8) is very similar to that for compounds (4)-(6). For this system there are no strong steric factors to oppose the exciplex adopting the sandwich conformation. Compound (6) may also adopt a sandwich conformation (fig. 7) although this does involve the eclipsing of the methylene protons on the adjacent carbon atoms. Neither compound (4) nor compound (5) can adopt a sandwich conformation. It is likely that compound (4) has the twisted conformation shown in fig. 7 and also that compound ( 5 ) may adopt the conformation (shown in fig. 7) which does at least allow some degree of overlap between the naphthalene and pyrrole n systems. Thus, although the intramolecular systems compounds (4)-(6) can adopt an array of conformations, none of which are common to each other, they display similar fluorescence spectra, which is strongly suggestive of the conformation of the excited complex not playing a crucial3474 FLUORESCENT COMPLEX FORMATION t increasing importance of non-radiative decay FIG.7.-Conformations of compounds (4)-(6) in solution which can give rise to exciplex fiuorescence. role in determining the extent of charge transfer in the complex. What is probably of significance is the relative proximity of the positive and negative charges in the . excited complex since this will determine the magnitude of the Coulombic interaction term. Both the quantum yield of fluorescence and fluorescence lifetime appear to be highly dependent upon the conformation of the excited complex.Thus compound (6), which can most easily adopt a conformation approaching that preferred by intermolecular complexes, shows, in cyclohexane and benzene, quantum yields of excited complex formation which are far higher than those for compounds (4) and (5). That the low quantum yields exhibited by compounds (4) and (5) are not due to lack of interaction between the pyrrole and naphthalene rings is shown by the fact that the fluorescence due to the naphthalene system is virtually completely quenched. The low quantum yields of fluoresence for compounds (4) and (5) and the short fluorescence lifetimes show that the inability of these compounds to adopt a favourable conformation for the excited complex leads to efficient non-radiative decay.It has been previously shown that the efficiency of intramolecular quenching in some naphthylalkylamines is independent of the length of the chain linking the aryl to the amino group.3 Presumably an important non-radiative process for compounds (4) and (5) is back electron transfer since the donor and acceptor groups are located so close to each other. An interesting feature of the fluorescence of compounds (4) and (6) is that the change of solvent from cyclohexane to acetonitrile does not cause a significant decrease in fluorescence lifetimes. Only in compound (6) does the quantum yield drop to any marked extent. We have previously pointed out9 that these two findings are not incompatible. The quantum yield of exciplex fluorescence reflects the relative efficiency of the encounter complex to give a fluorescent complex compared with decay to ground species (including the formation of radical ions), whereas the lifetime is dependent upon the ability of the solvent to stabilise the exciplex.Increasing the solvent polarity will lead, on the one hand, to stabilisation (uia its ability to solvate a charged species) and, on the other hand, to de-stabilisation (owing to its ability to aid the dissociation of the exciplex into radical ions). The relatively small effect of solvent upon the fluorescence lifetime of the complexLUO, B E D D A R D , PORTER, D A V I D S O N A N D W H E L A N 3475 formed from compound (6) shows that the stabilising effect of acetonitrile is markedly compensating for its help in aiding total charge separation.The fluorescence lifetimes measurements of compounds (4)-(6) are of interest in that they illustrate how conformational effects control the dynamics of exciplex formation. For compounds (4)-(6) the exciplex emission could be resolved free from any naphthalene fluorescence, due to the low quantum yield of the latter. In the case of compounds (4) and ( 5 ) the rise in the fluorescence of the exciplex followed the lamp profile, i.e. the fluorescent exciplex formation is faster than 300 ps. This is not the case for compound (6), which in degassed cyclohexane shows a rise-time of 5.1 ns. Other systems, e.g. the 1 -napthylalkylamineslO [compound (l)], exhibit a similar phenomenon. In solvents more polar than cyclohexane, the growth of exciplex fluorescence from compound (6) cannot be observed. The very fast rise-time in the exciplex fluorescence from compounds (4) and (5) can be attributed to the proximity of the two groups.Because of the closeness of the groups, electron transfer should be very rapid. The greater separation between the donor and acceptor groups in compounds (6) will slow this process down. There are probably a number of populated ground-state conformations of compound (6) in which the pyrrole is not sufficiently near to the naphthalene group so that on excitation of the latter exciplex formation requires the occurrence of several bond rotations. On increasing the solvent polarity, the distance over which the donor and acceptor groups can interact increases12 and consequently in solvents more polar than cyclohexane growth of the exciplex fluorescence is not observed.This is probably the reason why 'grow-ins' are not seen in propan-1-01 which is more viscous (2-5 times greater) but more polar than cyclohexane. For compounds (4)-(6) exciplex formation should be little affected by fairly small changes in viscosity since the donor and acceptor do not have to diffuse together. However, when very long chains separate donor and acceptor groups, this is not the case. For the intramolecular quenching of anthracene fluorescence by the tribromomethyl groups it was found that the quenching rate was lo7 s-l for a chain of ten methylene groups interposed between the donor and acceptor whereas for short linking chains (two to three methylene groups) the process can be as much as 100 times faster.13 Formation of the intermolecular exciplex between compounds (7) and (8) occurs relatively slowly because of the necessity for the two components to diffuse together.Furthermore in a non-polar solvent such as cyclohexane the dissociation of the exciplex to give excited naphthalene and pyrrole affects the kinetics and this leads to a lengthening of the rise-time for the exciplex emission. In the wavelength region where the decay of the naphthalene fluorescence could be measured free from any exciplex emission, bi-exponential decay kinetics were observed. Measurement of the decay of the exciplex fluorescence showed that it decayed as the difference of two exponentials but with the same decay time as the naphthalene; this is as expected: [lAD*] = A exp (- t / z J - B exp (- t / z z ) where A and B are constants.The results are shown in table 2. From table 1 it can be seen that the lifetime of compound (4) is hardly affected by the presence of oxygen whereas the presence of oxygen has a larger effect upon the lifetimes of the excited states of compound (5). The rapid formation of the exciplex in the case of compound (4) and its short radiative lifetime successfully competes with bimolecular quenching by oxygen. This is not the case for compound (5). For compound (6), where exciplex formation in cyclohexane takes a measurable time, oxygen quenching of the initially excited naphthalene chromophore and of the exciplex lead to a drastic reduction in the fluorescence lifetime of the exciplex.Not3476 FLUORESCENT COMPLEX FORMATION surprisingly, oxygen has a drastic effect upon the decay profiles for the monomer and exciplex decays of the bimolecular system compound (7) plus compound (8) (table 2). In conclusion, the results show that the conformation of exciplexes has little effect upon the degree of charge transfer but that it markedly affects the relative efficiencies of radiative and non-radiative processes. X-J.L. acknowledges the financial support of the Ministry of High Education, People’s Republic of China and Jilin University. We also thank the S.E.R.C. for financial support. E. A. Chandross and C. J. Dempster, J. Am. Chem. SOC., 1970, 92, 3586; R. S. Davidson and T. D. Whelar I. Chem. SOC., Chem. Commun., 1977, 361 ; N. Mataga and M. Ottolenghi, Molecular Association, ed. R. Foster (Academic Press, New York, 1979), vol. 2, p. 1. R. S. Davidson and K. R. Trethewey, J. Chem. SOC., Chem. Commun., 1976, 827. R. Ide, Y. Sakata, S. Misumi, T. Okada and N. Mataga, J. Chem. SOC., Chem. Commun. 1972, 1009. T. Okada, T. Fujita and N. Mataga, 2. Phys. Chem. (N.F.), 1976, 101, 57. Z. R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D. J. Cowley and W. Baumann, Nouv. J. Chim. 1979, 3, 443. W. R. Ware and W. Rothman, Chem. Phys. Lett., 1976, 39, 449. G. S. Beddard, G. R. Fleming, G. Porter and R. J. Robbins, Philos. Trans. R . SOC. London, Ser. A., 1980, 298, 111. G. S. Beddard, R. S. Davidson and A. Lewis, J. Photochem. 1972/3, 1, 491. G. S. Beddard, S. E. Carlin and C. Lewis, J. Chem. SOC., Faruday Trans. 2, 1975, 71, 1894. * E. A. Chandross and H. T. Thomas, Chem. Phys. Lett., 1971, 9, 393. lo G. S. Beddard, Ph.D. Thesis (University of London, 1974). l 2 M. K. Crawford, Y. Wang and K. B. Eisenthal, Chem. Phys. Lett., 1981, 79, 529. l 3 J. A. Nairn, C. L. Braun, P. Caluwe and M. Szwarc, Chem. Phys. Lett., 1978, 54, 469. (PAPER 1 / 1922)

ISSN:0300-9599    

DOI:10.1039/F19827803467

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Furaduy Trans. I , 1982, 78, 3411-3484 Photophysics of 1,l’-Binaphthyl and its Formation of a Complex with N-( 1 -propyl)-2,5-dimethylpyrrole BY XIU-JIN Luo,? GODFREY S. BEDDARD AND GEORGE PORTER Davy Faraday Laboratory, The Royal Institution, 2 1 Albemarle Street, London AND R. STEPHEN DAVIDSON* Department of Chemistry, The City University, Northampton Square, London EClV OHB Received I 1 th December, I98 1 Fluorescence lifetimes for 1,l ’-binaphthyl in cyclohexane, benzene, propan- 1-01 and acetonitrile have been measured and found to lie between 2.5 and 3.5 ns. The conformational changes which occur in the excited singlet state of 1 ,l’-binaphthyl are so rapid that they elude detection by sub-nanosecond fluorescence spectroscopy. 1,l ’-Binaphthyl forms an exciplex with N-( 1 -propyl)-2,5-dimethylpyrrole.This is weakly fluorescent in non-polar solvents. The kinetics of formation of the exciplex were examined. The radiative decay rate constant for the exciplex apppears to be relatively small. It is suggested that the pyrrole complexes with one of the naphthalene rings of the 1,l’-binaphthyl to give a complex which resembles that formed between the pyrrole and naphthalene. Interest has been shown in the photochemistry of biaryls, e.g. chlorobiphenyls,l 1 ,l’-binaphthy12v and 9,9’-bianthryl,** owing to the fact that in the ground state the two aryl rings are not co-planar, and this can affect both the photochemical and photophysical properties of these compounds. 2-Chlorobiphenyl dechlorinates far more readily than 4-chlorobiphenyl and this has been attributed to the loss of chlorine from the 2-substituted compound relieving steric strain which in turn reduces the overall free-energy change for the dechlorination process.’ The photophysics of 9,9’-bianthryl has been studied in detail.In the excited singlet state the interplanar angle between the two rings is reduced compared with the ground state. However, in polar solvents the excited singlet state of bianthryl undergoes intramolecular electron tran~fer.~ Early studies of the 1,l’-binaphthyl system indicated that the first excited singlet state of this compound is planar.2 However, this view has been chal- lenged and the suggestion made that the interplanar angle between the two naphthyl rings is reduced in the excited singlet state but is not reduced to zero.3 We have addressed ourselves to the question as to how fast the conformational changes in the first excited singlet state of 1,l’-binaphthyl occur and whether the relaxed species can form an exciplex with tertiary amines.The latter point is of interest in the light of the recent work by Yorozu et aL6 They found that the efficiency of quenching the fluorescence of optically active 1, I ’-binaphthyl in non-polar solvents by chiral amines is dependent upon the absolute configuration of the amine, indicating that in such solvents the exciplex formed between the aromatic hydrocarbon and the amine requires the partners to be correctly orientated with respect to each other and in close proximity. t Visiting research fellow, permanent address: Department of Chemistry, Jilin University, Changchun, People’s Republic of China. 34773478 PHOTOPHYSICS OF I J I - B I N A P H T H Y L RESULTS The fluorescence spectrum of 1 , 1 ’-binaphthyl shows some dependence upon solvent polarity (fig.1) but to nothing like the same degree as that of 9,9’-bianthr~l.~ Fluorescence lifetimes were measured by the technique of time-correlated single-photon counting having an instrumental response time of 280 ps. The results are shown in table 1 . In all cases the fluorescence decays were found to be exponential and showed I I 300 400 500 wavelength/nm FIG. 1 .-Fluorescence spectra of 1,l’-binaphthyl in various solvents (excitation wavelength 295 nm) : (---) propan-1-01, (--) benzene, (- - - -) acetonitrile, (-) cyclohexane.TABLE 1 .-FLUORESCENCE LIFETIMES OF 1,l ’-BINAPHTHYL IN VARIOUS DEGASSED SOLVENTS solvent quantum yield lifetime/ns kF/ lo8 s-l cyclohexane 0.79 2.70 2.9 benzene 0.76 2.50 3.0 propan- 1-01 0.79 3.14 2.5 acet oni trile 0.67 3.22 2.0 no grow-in. This also proved to be the case when a highly viscous solvent such as ethylene glycol (> 26 cP) and liquid paraffin (20 cP) was used. In another experiment the fluorescence decay of 1,I’-binaphthyl in poly(methy1 methacrylate) was examined and again no grow-in was observed. Thus the conformational changes occurring in the excited singlet state of 1,l’-binaphthyl do so in less than 280 ps. Addition of N-( 1 -propyl)-2,5-dimethylpyrrole to cyclohexane solutions of 1,l I - binaphthyl quenched the fluorescence of the biaryl.The quenching was accompanied by a diminutive broadening of the binaphthyl fluorescence (fig. 2). The broadeningLUO, BEDDARD, PORTER A N D DAVIDSON 3479 wavelength/nm FIG. 2.-Fluorescence spectra of 1 , 1'-binaphthyl + pyrrole in cyclohexane (excitation wavelength 295 nm). Cl , 1 -BIN cPDMP (-1 1.406 x 0 (--) 1.403 x 10-4 7.579 x 10-3 (---) 1.399 x 2.572 x (- - - -) 1.329 x 4.598 x (--. .--) 1.381 x 6.114 x lo-' (. . . . . . . .) 1.379 x lop4 9.648 x lo-' is attributed to exciplex formation. When very high concentrations of the pyrrole are employed the emission owing to the exciplex becomes more discernible, but even under these conditions no discrete wavelength maximum for the exciplex is observable. A Stern-Volmer plot, based upon the results shown in fig.2, deviates from linearity being concave in shape. Unlike many systems exhibiting intermolecular exciplex f o r m a t i ~ n , ~ ~ , ~ increasing the solvent polarity did not give rise to a structureless fluorescence band, red-shifted from the hydrocarbon fluorescence, but instead little or no emission attributable to the exciplex could be observed (fig. 3). Only in benzene was some exciplex emission discernible. The pyrrole quenched the binaphthyl fluorescence in acetonitrile, the quenching process obeying simple Stern-Volmer kinetics. The rate constant for quenching was evaluated as being 3.4 x 1O1O dm3 mol-l s-l; i.e. at the diffusion-controlled limit (fig. 4). Quenching of the binaphthyl fluorescence by the pyrrole in various solvents was also examined by the technique of time-correlated single-photon counting in which all the light emitted to the red of 400 nm was collected.Since the quantum yield of exciplex fluorescence is3480 PHOTOPHYSICS OF ~,I’-BINAPHTHYL 01 1 1 300 400 xx) wavelength/nm FIG. 3.-Fluorescence spectra of 1, 1’-binaphthyl + pyrrole in various solvents (excitation wavelength 295 nm): (-) cyclohexane, (--) benzene, (---) propan-1-01, (- - -) acetonitrile. 2 2.00 0 + 1 .I* 1.00.’ / 20 [pyrrole] / 1 0-3 mol dmW3 FIG. 4.-Plot of Z,/Z against concentration of pyrrole for quenching of 1,l’-binaphthyl fluorescence in acetonitrile by the pyrrole. so small, it was considered that the percentage of counts owing to exciplex fluorescence will be minimal and would not affect analysis of the decay curves.In all cases the fluorescence decays were not mono-exponential, but they could be resolved into the sum of two exponentials. The fact that the decays are the sum of two exponentials substantiates the view that most of the emission emanates from the binaphthyl rather than the e~ciplex.‘~ The fluorescence decays were fitted to the equation Y(t) = F exp (- t/.rl) + (1 - F ) exp (- t / z JLUO, BEDDARD, PORTER A N D DAVIDSON 348 1 TABLE 2.-FLUORESCENCE LIFETIMES AND OTHER PARAMETERS OBTAINED FROM ANALYSIS OF THE DECAY CURVES FOR MIXTURES OF 1,l '-BINAPHTHYL AND N-( 1 -PROPYL)-2,5-DIMETHYLPYRROLE IN VARIOUS DEGASSED SOLVENTS concentration/mol dm-3 solvent 1 ,l'-binaphthyl pyrrole F a x 100 z,/ns z,/ns cyclohexane 3.77 x 10-4 3.64 x lo-, 0.083 10.96 1.67 benzene 1.73 x 10-4 3.30 x lop2 0.035 10.50 1.47 propan- 1-01 3.54 x 10-4 5.09 x lou2 0.028 8.61 1.78 acetonitrile i 3.30 x 10-4 3.97 x lo-, 0.008 5.65 1.16 a Fis the percentage of the decay component with the lifetime z, present at the onset of the decay.TABLE 3 .-FLUORESCENCE LIFETIMES AND OTHER PARAMETERS OBTAINED FROM ANALYSIS OF THE DECAY CURVES FOR MIXTURES OF l,l'-BINAPHTHYL AND N-( l-PROPYL)-2,5-DIMETHYLPYRROLE IN DEGASSED CYCLOHEXANE SOLUTION concentration/mol dmP3 1, 1'-binaphthyl pyrrole F'x 100 zJns z,/ns 1.38 x 10-4 9.65 x lo-, 0.457 11.19 1.05 1.38 x 10-4 6.11 x 0.367 11.26 1.31 1.39 x 10-4 4.60 x lo-, 0.319 11.12 1.46 1.40 x 10-4 2.57 x lo-, 0.236 1 1.03 1.24 1.40 x 10-4 7.58 x 10-3 0.122 10.33 2.16 1.41 x 10-4 0 0 2.69b a See footnote to table 2; Mono-exponential decay.where F is a constant and z, and z, are lifetimes. The results are shown in table 2. A more detailed study was made of the quenching process in cyclohexane by examining the effect of varying the concentration of pyrrole upon the decay profiles. The results are shown in table 3. Kinetic analysis of the following reaction scheme enabled values for k,, k , and k , to be extracted from the lifetimes given in table 3:76 k3 kproduct BN + PY -JL BN* + PY . " ( B N PY) * + product k A 2 k , k d b6 BN+Py+hv BN + Py BN + Py+hv,,. BN + Py Here Bn is 1 ,l'-binaphthyl, Py is N-( 1 -propyl)-2,5-dimethylpyrrole, hex is the fluorescence from the exciplex,3482 PHOTOPHYSICS OF I,I’-BINAPHTHYL and 1 1 - x - = (k, + k2) (k, + k,) + k , k , [Py].71 7 2 k , is a composite rate constant containing the terms kproduct,kB and k6 k p = k5 + k6 + kprOduCt. From the fluorescence lifetime data in table 1, k , + k , = 3.7 x lo8 s-l. From the appropriate plots of z1 and z, (values in table 3) against concentration (fig. 5 and 6) the following rate constants were obtained : k, = 5.4 x lo9 dm3 mol-l s-l k4 = 6.5 x lo7 s-l k, = 8.45 x lo7 s - l . 1 1 5.0 10.0 [ pyrrolel / 1 0-2 mol dm-3 FIG. 5.-Plot of 1 /zl + 1 /z2 against pyrrole concentration for quenching of 1,l’-binaphthyl fluorescence in cyclohexane by the pyrrole. [pyrrole] mol dm-3 FIG. 6.-Plot of (1 /z,) x (1 /z2) against pyrrole concentration for quenching of 1, 1’-binaphthyl fluorescence in cyclohexane by the pyrrole.LUO, BEDDARD, PORTER A N D DAVIDSON 3483 DISCUSSION Although the absorption spectrum of 1,l'-binaphthyl shows some fine-structure, the fluorescence spectrum is virtually structureless (fig. 1).The lack of structure supports the view that the initially created excited state of binaphthyl undergoes a conform- ational change. This must be a particularly rapid process since it occurs even in viscous solvents within 280 ps. The lack of an appreciable solvent effect upon the fluorescence emission (fig. 2) suggests that little stabilisation of the excited state accrues from charge transfer. The conformational changes presumably lead to a species exhibiting greater delocalisation and it was anticipated that this species should form an exciplex with N-( 1 -propyl)-2,5-dimethylpyrrole which is considerably different to that formed between the pyrrole and naphthalene.8 In particular, it was expected that the exciplex formed from the binaphthyl should fluoresce to the red of the exciplex formed with naphthalene.This proved not to be the case. In cyclohexane solution, exciplex fluorescence produced by quenching of the binaphthyl fluorescence by the pyrrole is barely detectable, manifesting itself only as a broadening of the fluorescence band owing to the binaphthyl. The quantum yield of exciplex emission is extremely small. In polar solvents such as propan- 1-01 and acetonitrile the only detectable fluorescence is that owing to the unquenched binaphthyl. That exciplex formation is occurring in all the solvents is attested by the fact that the fluorescence decays show two distinct components (table 2).The occurrence of two decays is caused by (1) unquenched monomer fluorescence and (2) formation of excited monomer via break-up of the exciplex (having rate constant = k,). For acetonitrile solution, steady-state measure- ments show the quenching process to be diffusion controlled, which indicates that it is exothermic. Thus the lack of appreciable exciplex fluorescence in any of the solvents cannot be caused by inefficient exciplex formation. Indeed, in all solvents, the fluorescence decays contain a measurable amount of the longer-lived species, indicating that even in a polar solvent such as acetonitrile there is some feedback from the exciplex to give the excited monomer. The analysis of the kinetics for exciplex formation and decay in cyclohexane solution is informative.It shows that exciplex formation is favoured over exciplex ,dissociation to give excited monomer by a factor of ca. 100. The rate constant k, is a composite rate constant and contains contributions from kproduct, k , and k,. The rate constants k, and k, appear to be comparable, and this will account to some extent for the lack of emission from the exciplex. If k , (which could include intersystem crossing to give the triplet binaphthyl) is significantly larger than k,, this will also have a marked effect upon the quantum yield for exciplex formation. Radiative decay constants for exciplexes formed between styrenes and triethylamineg are all substantially less than 5 x lo7 s-l, which suggests that k, for the binaphthyl system may well be less than k, and k,.It is difficult to give an estimated value for k , since such factors as the tightness of the exciplex and the orientation of the partners, both of which are difficult to quantify, affect k,.l0 The lack of any exciplex emission in polar solvents such as acetonitrile is not so surprising, since electron transfer to give radical ions will also compete with the fluorescent decay processe~.~~9 Since the quenching of 1,l '-binaphthyl fluorescence by the pyrrole is a dynamic process, and therefore subject to diffusional control, the pyrrole must quench the conformationally relaxed excited singlet state of the binaphthyl. Surprisingly, what little exciplex emission that can be observed resembles that produced by the interaction of the excited singlet state of naphthalene with the pyrrole.With binaphthyl, the fluorescence maximum of the exciplex appears to be obscured by the fluorescence of the binaphthyl, which is red-shifted from naphthalene fluorescence. These fluorescent3484 PHOTOPHYSICS OF ~,~'-BINAPHTHYL properties of the exciplex indicate that the pyrrole is mainly associated with one of the naphthalene rings of the binaphthyl system, which suggests that there is not extensive delocalisation in the excited singlet state of 1 , 1'-binaphthyl. Furthermore, extensive delocalisation between the two rings should reduce the reduction potential of the binaphthyl, and consequently exciplexes formed with such a system should be red-shifted relative to the complex formed with naphthalene, since (from Ecomplex cc reduction potential Ecomplex = ID - E, +constant where ID is the ionisation potential of the donor and EA is the electron affinity of the a c ~ e p t o r ) .~ ~ ~ ~ Since this is not observed we conclude that in the conformationally relaxed excited singlet state of 1,l '-binaphthyl, which forms a complex with the pyrrole, there is not extensive electron delocalisation. EXPERIMENTAL N-( 1 -propyl)-2,5-dimethylpyrrole has been previously described.* 1 , 1'-Binaphthyl was syn- thesised from 1 -bromonoaphthalene according to the procedure described in the literature. l1 Fluorescence lifetimes were measured by the method previously described.8 Analysis of the fluorescence decay curves was carried out using an iterative non-linear least-squares program.12 X-J.L. acknowledges the financial support of the Ministry of High Education, People's Republic of China and Jilin University. We also thank the S.E.R.C. for financial support. N. J. Bunce, Y. Kumar, L. Ravanal and S. Safe, J. Chem. SOC., Perkin Trans. 2, 1978, 880. M. F. M. Post, J. Langelaar and J. D. W. Van Voorst, Chem. Phys. Lett., 1975,32,59; 1977,46,331; Chem. Phys., 1976, 14, 165. M. Irie, K. Yoshida and K. Hayashi, J. Phys. Chem., 1977, 81, 969; 1977, 81, 973. F. Schneider and E. Lippert, Ber. Bunsenges. Phys. Chem., 1968, 72, 1155; 1970, 74, 624. N. Nakashima, M. Murakawa and N. Mataga, Bull. Chem. SOC. Jpn, 1976, 49, 854. T. Yorozu, K. Hayashi and M. Irie, J. Am. Chem. SOC., 1981, 103, 5480. (a) R. S. Davidson, in Molecular Association, ed. R. Foster (Academic Press, New York, 1975), vol. 1, p. 215; (b) N. Mataga and M. Ottolenghi, in Molecular Association, ed. R. Foster (Academic Press, New York, 1979), vol. 2, p. 1. X-J. Luo, G. S. Beddard, G. Porter, R. S. Davidson and T. D. Whelan, J . Chem. SOC., Faraday Trans. 1, 1982, 78, 3467. R. L. Brentnall and K. Salisbury, unpublished results. 1970). E. Sa.kellarios and T. Kyrimis, Chem. Ber., 1924, 57, 324. 1980, 298, 11 1 . lo N. Mataga and T. Kubota, Molecular Interactions and Electronic Spectra (Marcel Dekker, New York, l2 G. S. Beddard, G. R. Fleming, G. Porter and R. J. Robbins, Philos. Trans. R. SOC. London, Ser. A , (PAPER 1 / 1923)

ISSN:0300-9599    

DOI:10.1039/F19827803477

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1982, '78, 3485-3492 Thermodynamics of a Silver, Silver Azide Electrode in Water and Water + Dioxan at Different Temperatures BY REBATI C. DAS,* MIHIR K. MISRA AND BATA K. NANDA Department of Chemistry, University College of Engineering, Burla 768 01 8, Orissa State, India Received 2 1 st December, 198 1 Ag, AgCl(s) I NaCl(m) : NaN,(m) I AgN,(s), Ag The e.m.f. of the cell in water and water + 10, + 20, + 30 and + 40 mass % dioxan has been measured at 5 OC intervals from 5 to 45 OC. The values of the standard potentials of the silver, silver azide electrode have been determined in water and the mixtures at these temperatures. The change in standard thermodynamic quantities (AG*, A H e and A@) for the electrode reaction have been evaluated. The thermodynamic parameters for the transfer of HN, from water to the mixtures have also been evaluated.The results are discussed in terms of the preferential solvation of ions. This paper is a part of our systematic work on secondary silver electrodes in water and water + dioxan mixtures at different temperat~res.~.~ Cell A was studied in water and water + 10, + 20, + 30 and + 40 mass % dioxan at temperatures between 278.15 an? 308.15 K and the changes in the thermodynamic quantities for the electrode reaction were calculated. (A) Ag, AgCW I NaCl(aq) (m) : NaN,(aq) (m) I AgN,(s), Ag EXPERIMENTAL Dioxan was purified as described previously.2 Sodium azide (GR) was purified by repeated crystallisation from an aqueous solution saturated at 90 "C by cooling it to 10 O C and adding an equal volume of alcohol.The resulting crystals were washed with acetone and the product was dried at room temperature. A freshly prepared solution of sodium azide was used in each experiment. Sodium chloride (GR) was recrystallised twice from water. Preparation of the silver, silver chloride electrode and cell solutions, the setting of the cells and the e.m.f. and conductivity measurements were essentially similar to those described previously.2 The silver, silver azide electrode was prepared essentially by the method of Taylor and Nims.* Platinum spirals coated with spongy silver were prepared as described previously.2 Silver azide was deposited by electrolysing a 0.1 mol dm-, solution of sodium azide for 20-30 min using a current of 5 mA with the silver bases as anodes and platinum wires as cathodes.The electrodes were then washed and kept in contact with water saturated with AgN, in order to age them. The electrodes were dipped in cell solutions for ca. 1-2 h before use. RESULTS AND DISCUSSION A summary of e.m.f. data for cell A at different temperatures is given in table 1. The e.m.f. of the cell is given by the equation + EJ E = Eg,,+klog--- mYN, my,,- 113 3485 FAR 13486 Ag, AgN, ELECTRODE I N WATER + DIOXAN TABLE I.--E-E, FOR CELL A FROM 278.15 TO 318.15 K IN WATER AND FROM 278.15 TO 308.15 K IN WATER -I- DIOXAN MIXTURES water T / K m/ 10-2 mol dm-3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 318.15 0.5 1 .o 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 0.062 0 0.062 3 0.062 29 0.062 31 0.062 87 0.624 6 0.063 04 0.062 72 0.062 85 0.062 78 0.062 74 0.064 72 0.064 80 0.064 85 0.064 90 0.064 53 0.065 00 0.065 10 0.065 22 0.065 23 0.064 98 0.065 30 0.066 58 0.066 60 0.066 70 0.066 80 0.066 83 0.066 95 0.066 71 0.066 94 0.066 98 0.067 10 0.067 09 0.068 30 0.068 37 0.068 40 0.068 89 0.068 60 0.068 70 0.068 80 0.068 90 0.068 80 0.068 31 0.069 10 0.070 40 0.070 49 0.070 46 0.070 47 0.070 48 0.070 48 0.070 63 0.070 50 0.070 52 0.070 33 0.070 52 0.072 60 0.072 65 0.072 37 0.072 90 0.072 72 0.072 73 0.072 78 0.072 82 0.072 60 0.072 87 0.072 90 0.074 64 0.074 70 0.074 65 0.074 66 0.074 68 0.074 70 0.074 90 0.074 72 0.074 74 0.074 75 0.074 37 0.078 00 0.078 05 0.078 08 0.078 15 0.078 17 0.078 80 0.078 20 0.078 24 0.078 28 0.078 60 0.078 34 water + 10 % dioxan T/K m/ 1 0-2 mol dm-3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 0.5 1 .o 2.0 3.0 4.0 5.0 6.0 8.0 9.0 10.0 0.068 10 0.068 14 0.068 14 0.068 40 0.068 50 0.068 46 0.068 58 0.068 59 0.068 75 0.068 84 0.070 10 0.070 25 0.070 23 0.070 32 0.070 34 0.070 40 0.070 44 0.070 92 0.070 12 0.070 60 0.072 40 0.072 50 0.072 30 0.072 53 0.072 56 0.072 60 0.072 68 0.072 70 0.073 06 0.073 35 0.074 50 0.074 65 0.074 40 0.074 68 0.074 76 0.074 88 0.075 02 0.075 03 0.075 12 0.075 17 0.076 30 0.076 45 0.076 32 0.076 36 0.076 42 0.076 31 0.076 51 0.076 76 0.076 66 0.076 70 0.078 30 0.078 45 0.078 50 0.078 60 0.078 74 0.078 77 0.078 83 0.078 93 0.079 40 0.079 30 0.080 20 0.080 30 0.080 10 0.080 35 0.080 40 0.080 48 0.080 55 0.080 68 0.080 72 - water + 20% dioxan T/K m/ 1 0-2 mol dm-3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 0.5 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 0.075 20 0.075 30 0.075 12 0.075 37 0.075 37 0.075 41 0.075 43 0.075 50 0.075 52 0.075 73 0.077 70 0.077 80 0.077 74 0.077 97 0.077 99 0.078 13 0.078 21 0.078 30 0.078 37 0.078 49 0.078 35 0.078 48 0.078 30 0.078 60 0.078 71 0.078 85 0.078 94 0.079 02 0.079 12 0.079 30 0.081 00 0.081 10 0.081 22 0.081 31 0.081 51 0.081 53 0.081 56 0.081 66 0.081 82 0.081 92 0.082 40 0.082 50 0.082 64 0.082 70 0.082 92 0.082 98 0.083 02 0.083 08 0.083 31 0.083 37 0.084 38 0.084 40 0.084 43 0.084 62 0.084 64 0.084 66 0.084 71 0.084 69 0.084 80 0.084 88 0.086 40 0.086 50 0.086 54 0.086 57 0.086 70 0.086 78 0.086 88 0.086 89 0.086 94 0.086 973487 R. C.DAS, M. K. MISRA A N D B. K. N A N D A TABLE 1 .-(cont.) water + 30 % dioxan ~ ~ ~ _ _ _ _ _ _ _ _ ~ _____ T/K m/ 1 O2 mol dmP3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 0.5 1 .o 2.0 3 .O 4.0 5.0 6.0 8.0 9.0 10.0 0.082 40 0.082 45 0.082 50 0.082 57 0.082 58 0.082 61 0.082 64 0.082 65 0.082 70 0.082 74 0.084 90 0.084 98 0.084 88 0.085 00 0.085 07 0.085 10 0.085 13 0.085 16 0.085 19 0.085 39 0.085 90 0.086 00 0.085 95 0.086 16 0.086 20 0.086 25 0.086 30 0.086 37 0.086 48 0.086 45 0.087 90 0.087 95 0.087 99 0.088 10 0.088 12 0.088 17 0.088 24 0.088 29 0.088 30 0.088 32 0.089 05 0.089 10 0.089 00 0.089 20 0.089 28 0.089 38 0.089 42 0.089 45 0.089 55 0.089 57 0.090 90 0.091 00 0.091 05 0.091 17 0.091 19 0.091 30 0.091 33 0.091 42 0.091 50 0.091 53 0.093 00 0.093 10 0.093 00 0.093 18 0.093 20 0.093 29 0.093 30 0.093 40 0.093 49 0.093 55 water + 40 % dioxan T/K m/102 mol dmP3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 0.5 1 .o 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 0.090 00 0.090 10 0.090 22 0.090 35 0.090 43 0.090 54 0.090 85 0.090 76 0.090 86 0.091 00 0.092 40 0.092 50 0.092 63 0.092 70 0.092 80 0.092 82 0.092 93 0.093 00 0.093 20 0.093 34 0.092 90 0.092 95 0.092 97 0.093 00 0.093 06 0.093 18 0.093 10 0.093 36 0.093 42 - 0.095 00 0.095 08 0.095 19 0.095 14 0.095 35 0.095 42 0.095 46 0.095 52 0.095 68 - 0.095 95 0.096 00 0.096 11 0.096 17 0.096 20 0.096 24 0.096 30 0.096 32 0.096 40 - 0.097 90 0.097 95 0.098 01 0.098 07 0.098 16 0.098 18 0.098 28 0.098 35 0.098 42 - 0.099 40 0.099 45 0.099 51 0.099 59 0.099 63 0.099 67 0.099 70 0.099 80 0.100 00 - where k is 2.3026 RT/F and EaT is the liquid junction potential. Substituting the ~~ Azf.\/I Debye-Hiickel equation -1ogyi = ~ 1+.\/1+J we obtain = -+ bm.(3) All the terms in eqn (1)-(3) have their usual significance and b is (BN,-&l-). Extrapolation of the plot of E--EJ against m to m = 0 gives the value of Fell. Knowing the standard electrode potential of silver, silver chloride electrode,5* that of the silver, silver azide electrode can be calculated from the experimental value of Eqn (1) assumes that mcl- = mNy. This will be so if the two electrolytes (NaC1 and NaN,) are completely dissociated. We have observed in conductivity studies (unpublished) that both sodium chloride and sodium azide are almost completely dissociated in the range of concentrations up to 30 mass % dioxan.But in water + 40 :< dioxan there is some amount of association between the Na+ ion and the C1- or N; %ll- 113-23488 Ag, AgN, ELECTRODE IN WATER 4- DIOXAN ion. The error due to this is probably eliminated by our extrapolation procedure, because at rn -P 0 the electrolytes are completely dissociated. Furthermore, there is little uncertainty in the extrapolated values of cell because the plots of E - EJ against m were observed to be good straight lines. The values of EJ were calculated from equivalent conductivity values of the cell solutions of sodium azide and sodium chloride using Lewis and Sargent equation. The values of EJ varied between 0.001 77 and 0.00257 in water, 0.001 425 and 0.002956 in water+ lo%, dioxan, 0.001 301 and 0.002570 in water+20% dioxan, 0.001 190 and 0.002 798 in water + 30 % dioxan and 0.00 1 064 and 0.002 61 0 in water + 40% dioxan at all temperatures.The standard electrode potentials of silver, silver azide electrode (P) at all working temperatures and composition are shown in table 2. The Ee values can be represented as a function of temperature with a maximum deviation of 0.5 mV by the equation where t is the temperature in OC and a, b and c are empirical constants whose values are shown in table 3. Our E4 values of the silver, silver azide electrode in water can be compared with the values of an earlier determination by Taylor and N i r n ~ . ~ They used the same cell as we did (cell A), but also used cell B E@ = ~+b(t-25)+c(t-25)~ (4) Ag, AgCW I NaCl(aq) (4 II KCl(aq) I1 NaN,(aq) (m) I AgN,(s), Ag (B) with varying concentrations of the bridge solutions and they concluded that the liquid junction potential between the two equimolar solutions of sodium chloride and sodium azide in cell (A) is very small and thus neglected it.This is a clear approximation. In spite of that their values of Ee in water agree reasonably well with our values, as can be seen in table 4. Our determined values are more reliable because we have directly determined the liquid junction potentials instead of assuming them to be negligible. The standard free-energy, enthalpy and entropy changes ( A G e , A H 0 and A P ) for the reaction were calculated from the standard electrode potential values. These quantities are A G e = A + BT+ C T 2 represented by AgN3(s) + iH2(g) = + H+(aq) + N,(aq) ( 5 ) AH@ = A’+C‘T2 A S = D+B’T over the temperature range 278.15-308.15 K.In eqn (5)-(7), T is the temperature in K and the terms A , B, C, D , A’, B’ and C‘ are empirical parameters recorded in table 5. The change in the thermodynamic properties associated with transfer of 1 mole of HN, from water to the mixed solvent (AG?, AH? and ASP) are convenient quantities for the study of solvent e f f e ~ t . ~ These quantities were evaluated using the following relationships AGP = - F(,E$- ,EF) (8) where ,E$ and ,E$ are the standard electrode potentials in the mole-fraction scaleTABLE 2.-sTANDARD ELECTRODE POTENTIALSa (IN v) FOR THE SILVER, SILVER AZIDE ELECTRODE IN WATER AND IN DIOXAN+WATER FROM 278.15 TO 308.15 K T / K dioxan (%I 278.15 283.15 288.15 293.15 298.15 303.15 308.15 318.15 0 0.2963 ~0.0001 10 0.2930 f 0.0001 20 0.2919 f 0.0001 30 0.2885 f 0.0003 - + 0.0002 A 0.2957 f 0.0002 0.2927 f 0.0002 0.2906 f 0.0006 0.2868 k 0.0005 - + 0.0007 0.2949 If: 0.0002 0.2920 - + 0.0002 0.2891 f 0.0006 0.2849 f 0.0002 & 0.0004 0.2770 0.2940 f 0.0003 0.2912 - + 0.0002 0.2875 +0.0001 0.2828 - + 0.0002 0.2738 & 0.0002 0.2928 f 0.0001 0.2900 & 0.0001 0.2855 & 0.0001 0.2805 k 0.0004 0.2704 f 0.0003 0.2915 f 0.0002 0.2885 f 0.0001 0.2835 f 0.0001 0.2780 f 0.0002 0.2668 f 0.0001 0.2864 0.2900 - + 0.0002 If: 0.0002 0.2868 - f 0.000 1 - 0.28 12 - * 0.0002 - 0.2753 - f 0.0003 - 0.2629 - f 0.0003 - a The electrode potentials are expressed in the molal scale.The error limits were subjectively assessed considering the uncertainties in the e.m.f.readings (duplicate readings within & 0.4 mV) and the liquid junction potentials. w P 00 \D3490 Ag, AgN, ELECTRODE I N WATER + DIOXAN TABLE 3.-cONSTANTS OF EQN (4) dioxan (%I a 104b 10% 0 0.292 89 - 2.46 12 - 3.77 10 0.290 04 - 2.6246 - 5.66 20 0.285 58 - 3.9754 - 3.84 30 0.280 53 - 4.799 I - 3.99 40 0.270 47 - 7.0567 -4.41 TABLE 4.-cOMPARISON OF OUR VALUES FOR WITH THOSE OF NIMS AND TAYLOR4 278.15 0.2963 0.2959 288.15 0.2949 0.2942 298.15 0.2928 0.2919 308.15 0.2900 0.2889 318.15 0.2864 0.2854 for the electrode in water and in the mixed solvent, respectively. The thermodynamic transfer quantities were calculated from the standard electrode potentials in the mole-fraction scale because the effects of solvent on these thermodynamic quantities are more clearly reflected in this scale.’ A perusal of table 6 shows that the values of AH? and ASP are negative and increase with the increasing dioxan content, suggesting that the transfer is a structure-making process involving increased solvation of the ion.* It has been shown by Franks and Ives9 that the addition of a small quantity of an organic cosolvent to water enhances the three-dimensional hydrogen-bonded polymeric form of water.The present study of water+dioxan mixtures supports the view that for the transfer of HN, from water to highly structured mixtures of low dioxan content the net amount of order created by NH, in the mixed solvent is more than that in pure solvent. The changes in the thermodynamic quantities in the transfer process can further be separated into electrical and chemical parts where Q* = G e , H e or P.The electrostatic contribution to the AGP value can be calculated from Born’s equation and A p e , can similarly be calculated from the temperature coefficient of AGeel where D, and D, are the bulk dielectric constants and r , and r- are the radii of the cation and anion, respectively. The Y+ and Y- values are taken as 2.76lo9l1 and 1.17 respectively. Table 7 gives the chemical and electrical parts of the thermo- dynamic transfer quantities at 298.15 K.TABLE 5.-cONSTANTS OF EQN (5)-(7) dioxan (%) A B 104c D A’ i04B 104c 0 - 373 1.08 - 188.40 3559.74 180.43 - 4580.5 1 - 678 1.84 - 3395.60 - 5095.52 - 8754.20 - 3670.91 3860.95 172.32 - 6977.86 - 7262.16 - 3742.58 - 3822.50 10 12325.7 - 295.80 5385.64 285.21 10683.6 - 10310.6 20 - 5403.30 - 187.04 3780.62 225.68 - 5785.55 30 - 6565.65 - 183.90 40 - 93 16.01 - 180.64 4170.19 208.0 1 -11689.1 -91 36.20 TABLE 6.-TRANSFER THERMODYNAMIC QUANTITIES (MOLE-FRACTION SCALE) IN VARIOUS WATER + DIOXAN MIXTURES AT 298.15 K dioxan (%) AGy/kJ mol-l AH ?/kJ mol-l A@/J mol-1 K-l 10 -0.141 -0.917 - 2.605 20 -0.154 - 3.376 - 10.81 30 -0.173 - 5.753 - 18.72 40 + 0.263 - 11.13 - 38.213492 Ag, AgN, ELECTRODE I N WATER + DIOXAN PARAMETERS AT 298.15 K TABLE 7.-ELECTRICAL AND CHEMICAL CONTRIBUTIONS TO THERMODYNAMIC TRANSFER dioxan AGFe, AG&, AH& AH&,, A q e l AS& (%) /kJ mo1-l /kJ mol-' /kJ mol-1 /kJ mol-l /J mo1-I K-l/J mol-l K-l I0 - 0.564 0.423 0.842 - 1.760 4.717 - 7.322 20 - 1.268 1.114 0.475 -3.851 5.846 - 16.65 30 - 2.233 2.060 -0.225 - 5.529 6.735 - 25.45 40 - 3.599 3.863 - 1.317 -9.811 7.658 - 45.86 Further it is of interest to examine the primary medium effect which results from a difference of the ion-solvent interaction at infinite dilution in each so1vent.l39 l4 Thermodynamically it can be represented by (,* - s w 2k lim (logky,) = m + o where k is 2.3026 RT/F and the limit term indicates the primary medium effect.The activity coefficient ky refers to a value of unity for infinitely dilute solution with water as solvent. The values of lim, ~ (log $ y k ) at 298.15 K are given in table 8. This is a positive quantity increasing with increasing dioxan content. TABLE 8.-PRIMARY MEDIUM EFFECT OF N3 ION IN WATER+DIOXAN MEDIA AT 298.15 K 1 2 3 4 5 6 7 8 9 10 11 12 13 14 10 0.047 35 20 0.012 34 30 0.2080 40 0.3788 R. C. Das, G. Sahu and S. N. Mishra, Electrochim. Acta, 1974, 19, 887. R. C. Das, U. N. Dash and K. N. Panda, Electrochim. Acta, 1979, 24, 99. R. C. Das, U. N. Dash and K. N. Panda, Acta Chim. Acad. Sci. Hung., 1979, 99, 295. A. C. Taylor and L. F. Nims, J . Am. Chem. Soc., 1938, 60, 262. H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions (Reinhold, London, 1967). P. K. Das and U. C. Mishra, Electrochim. Acta, 1977, 22, 59. Physico-chemical Processes in Mixed Aqueous Solvents, ed. F. Franks (Heinmann, London, 1969). H. S. Frank and M. W. Evans, J . Chem. Phys., 1945, 13, 507. F. Franks and D. J. G. Ives, Q. Retl., 1966, 20, 1. M. Paabo. R. G. Bates and R. A. Robinson, J. Phys. Chem., 1966,70, 247. R. N. Roy, M. Vernon, A. Bothwell and J. Gibbons, J. Electrochem. SOC., 1972, 119, 694. U. N. Dash, Fluid Phase Equilibria, 1981, 5, 323. R. A. Robinson and R. G. Stokes, Electrolytic Solutions (Butterworths, London, 1959). B. B. Owen, J . Am. Chem. Soc., 1932, 54, 1758. (PAPER 1/1972)

ISSN:0300-9599    

DOI:10.1039/F19827803485

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Furuduy Trans. I , 1982, 18, 3493-3498 Reaction of Pentafluoroethyl Radicals with Cyanogen Chloride B Y CECILIA M. DE VOHRINGER AND EDUARDO H. STARICCO* Departamento de Fisico Quimica, Facultad de Ciencias Quimicas, Universidad Nacional de Cordoba, Estafeta 32, 5000 Cordoba, Argentina Received 7th January, 1982 The reaction of C,F, radicals with cyanogen chloride was studied between 293 and 573 K, using perfluoroethyl iodide as the free-radical source. CJ,. The reactions involved are The main product, C2F,C1, is tormed via an addition reaction or C,F, + ClCN -+ C,F,Cl f CN C,F, + ClCN -+ C2F,Cl + CN kc C,F, + ClCN C,F,ClCN C2F5 + C2F5 -+ C,F,,. by abstraction of a chlorine atom by The Arrhenius plot shows pronounced curvature. The following rate constants were obtained for reactions (2) and (4) log [(s) /cm$ mol-4 s-11 = 6.91 -57.10 kJ mo1-'/2.303 RT log [($),/cmi mol-4 s-4 = 2.32-20.55 kJ mol-l/2.303 RT k!. i 1 where k , is the rate constant for C,F, combination.The results are compared with those for the reaction of CF, with CICN. Comparisons between the reactivities of CF, and C,F, radicals have been obtained mainly for hydrogen abstfactions in both polar and non-polar substrates. Whittle and coworkers studied the reaction CF, + RC1+ CF,C1 + R where RCl = chloromethane' or an aromatic halide,, but no results involving transfer of a chlorine atom have been reported for C,F, radicals. The reaction of CF, with ClCN has also been ~tudied.~ It was shown that CF3C1 formation is via an addition reaction or by the abstraction of C1 by CF,, leading to a strongly curved Arrhenius plot.We have now extended this study to the reaction of C,F, radicals with ClCN, with the purpose of establishing mechanistic analogies of both reactions and comparing the Arrhenius parameters with those reported previously. EXPERIMENTAL Perfluoroethyl iodide (PCR Chem. Co.) was purified by fractional distillation and stored in liquid air. No impurities were detected by i.r. spectroscopy and gas chromatography. Cyanogen chloride was prepared and puriiied as in ref. (3). Perfluoromethane (ICN Pharm. Inc.) was distilled twice, the middle fraction being retained. 34933494 REACTION OF C2F5 RADICALS WITH ClCN APPARATUS AND PROCEDURE The reaction was performed in a cylindrical quartz vessel. This cell was in an oven, the temperature of which was controlled to kO.4 K by a PTR R52 regulator (Lauda).Between 293 and 353 K, the temperature of the reaction cell was maintained by water circulation from a thermostat. The light from an Osram HBO 500 W high pressure mercury lamp was filtered by a Corning 7740 plate. In this way, only wavelengths > 3000 reached the reaction ~essel.~ The cell was connected to a high vacuum system, in which the section used for storing and measuring gases was free of both grease and mercury. After each run the total content of the reaction vessel was transferred quantitatively to a Varian 200 chromatograph equipped with a Gow Mac density balance. The analysis was performed on a 3 m copper column packed with silica gel. Perfluoroethyl iodide was used as the C,F, radical source.RESULTS When C,F,I was photolysed in the presence of ClCN, the only products were C,F,Cl, C4FI0, C,F,CN, ICN and I,. The first three products were identified by i.r. spectroscopy and by comparing their retention times in different chromatographic columns with those of authentic samples. ICN and I, were characterized by their absorptions in the U.V. or visible. Despite thorough searching, no addition products of ClCN were found by i.r. spectroscopy or chromatography on suitable columns, at any degree of conversion. The reaction was studied in the temperature range 293-573 K and the ratio RC,Y,/l&F,q was determined at different concentrations of ClCN. The experimental results are given in table I . RC2F5C1/RL4F10 [ClCN] = kexptl at a given temperature was independent of a 5-fold variation in ClCN pressure and of a 2-fold variation in C,F51 pressure.Neither the percentage conversion nor pressure of inert gas affected kexptl. The possibility of a dark reaction between ClCN and C2F5 was tested for as follows. First, 70 Torr* of ClCN plus 60 Torr C,F,I were left in the reaction vessel for 20 h at 323 K. No products were detected by gas chromatography. Then 50 Torr of ClCN and 60 Torr of C,F,I were left for 20 min at 533 K with a similar result. However, for reaction times longer than ca. 2 h at 533 K the main products appeared owing to thermal decomposition of C,F,I. kexptl at each temperature was calculated from the slope of a plot of R~2F,Cl/&4F10 against [ClCN]. Consumption of reactants was usually negligible, but in those temperatures where the conversion was of a low percentage, an average value of ClCN was used.An Arrhenius plot of kexptl is shown in fig. 1. DISCUSSION When C,F,I is photolysed alone, the important reactions are C,F51 + hv --+ C,F, + I k, C2F5 + C2F5 --+ C4F10 I+I+M+I,+M. In the presence of cyanogen chloride two reactions could occur C2F5 + ClCN --+ C,F,CN + Cl C2F5 + ClCN -+ C,F,Cl+ CN. * 1 Torr = (101 325/760) Pa.C. M. DE VOHRINGER AND E. H. STARICCO TABLE ~.-PHOTOLYSIS OF C,F,I WITH CICNa 3495 reactant pressure/Torr photolysis conversion T / K time/s C,FJ ClCN R c ~ F ~ ~ RC~F~CN (%I kexptl 293 32 400 58.9 61.4 324 25 200 60.1 13.0 324 25 200 60.6 60.7 353 21 690 61.5 32.2 373 25 260 63.9 59.3 373 24000 127.7 59.9 383 43 860 54.3 16.6 383 25 200 66.3 60.4 400 28 920 68.5 70.7 413 51 900 49.1 14.7 413 16200 71.6 73.8 433 14400 71.2 52.1 443 8 100 68.5 52.6 457 10 800 63.4 47.7 473 14 400 70.9 9.8 473 10 800 69.5 41.0 503 10 800 76.6 16.5 533 7 200 84.8 4.0 533 1 200 62.3 5.4 533 1 200 63.8 5.5 533 7 200 85.0 17.4 573 1980 90.9 10.3 0.746 0.339 1.548 1.662 1.885 1.787 0.929 3.187 2.423 1.400 8.103 7.324 13.51 11.52 7.79 23.07 32.29 26.13 40.54 39.27 84.57 231.5 2.080 2.301 2.394 2.619 0.448 0.420 0.803 0.798 0.184 0.448 0.673 0.529 0.914 0.485 1.653 0.945 2.282 10.36 10.65 9.52 5.59 27.4 0.279 0.209 0.965 1.129 1.353 1.156 0.879 2.820 2.373 0.721 7.378 6.105 12.07 11.17 7.49 21.95 32.00 26.09 40.51 39.03 83.45 230.2 0.07 0.13 0.13 0.25 0.19 0.17 0.59 0.32 0.25 1.27 0.46 0.55 0.57 0.74 3.43 1.81 6.85 16.95 2.98 2.84 12.33 17.30 0.049 0.110 0.105 0.222 0.349 0.339 0.472 0.446 0.630 1.16 1.09 1.65 2.3 1 3.12 5.86 5.45 13.3 23.1 24.4 24.5b 22.9 52.7 a Volume of reaction vessel: 105 cm3 (between 293 and 353 K) and 100 cm3 (between 373 and 573 K).Rates of formation of products, R, in units of lOI3 mol cmP3 s-I.kexptl = RCzF5C1/R&Flo [CICN] in units of cmz mol-i s-i. 300 Torr of CF, added. 1.6 1.8 2.0 2.2 2 . L 2.6 2.8 3.0 3.2 3.L 103 KIT FIG. I .-Arrhenius plot for reaction of C,F, radicals with CICN. (--), kexpt, = k,/kt +k,/k%, from eqn (11) and (111); (---), k,/kL from eqn (11); (-.-.-), k,/k$ from eqn (111).3496 REACTION OF C2F5 RADICALS WITH ClCN If reaction (1) occurs, the C,F,Cl could arise from C2F5 + Cl + C,F,Cl.However, experiments carried out in our laboratory5 have shown that chlorine atoms readily add to ClCN giving compounds with a CN double bond. Furthermore, reaction of C1 with ClCN would produce less C,F,Cl than the amount expested if every chlorine atom formed in reaction (1) is consumed by reaction with C,F5. Accordingly, the C,F,CN/C,F,Cl ratio would always be > 1. We disregard the CN abstraction, reaction (l), because: (a) no addition product could be detected and (b) the C,F,CN/C,F,Cl ratio was never > 1 (and is indeed < 1 at low temperatures). Therefore, it seems probable that the reaction between C2F5 radicals and ClCN involves chlorine-atom transfer. As was found in the CF, + ClCN reaction, the curvature in the Arrhenius plot can be ascribed to two reactions with different A factors and activation energies which produce C,F,Cl, one of which is the genuine chlorine abstraction C2F5 + ClCN -P C,F,Cl + CN C2F5 + ClCN + C2F5ClCN (2) (3) C,F,ClCN + C,F5Cl+ CN.(4) If these are the only sources of C,F,Cl, we have where R represents the rate of formation of the products. At a given temperature, RCIFSC1/ficIF1O depends only on the concentration of ClCN, in agreement with data in table 1. At high temperatures the abstraction process becomes much more important than the addition, giving kexptl z k,/kt. On the other hand, at low temperatures, the contribution of reaction (2) to the production of C,F,Cl would be negligible. Probably k-, is much less than k, under the same conditions leading to kexIltl E k,/ki at low temperatures.The Arrhenius parameters for reactions (2) and (3) were obtained by the Prony method6 for the sum of two exponentials. The following equations were obtained log [($)/cmi mol-4 s(] = 6.91 -57.10 kJ mol-l/2.303 RT kc log [($)/cmi m o b s 2 = 2.32-20.55 kJ mol-l/2.303 RT. -1 (111) The experimental and calculated values using eqn (11) and (111) in eqn (I) are compared in table 2. The agreement is acceptable, with a deviation of 10%. This is shown in fig. 1, together with log (k,/kk) and log (k,/ki) from eqn (11) and (111). The curve represents the experimental points. For the reaction of CF, with ClCN, the Arrhenius plot was fitted taking into account the reversibility of the addition, reaction (-3), and a difference E-,-E4 of 8.37 kJ mol-1 was obtained by an iterative method.Similar calculations carried out in this work did not lead to such a result. A good fit was achieved by using Arrhenius parameters very close to those already obtained from eqn (11) and (111). This suggests that the ratio k-,/k, in eqn (I) is < 1 for the complete temperature range.C. M. DE VOHRINGER AND E. H. STARICCO 3497 TABLE 2 .-OBSERVED AND CALCULATED RATE CONSTANTSa 293 324 353 373 383 400 413 0.047 0.108 0.220 0.362 0.466 0.721 1.02 kexptl 0.047 0.107 0.227 0.348 0.453 0.638 1.09 T / K kcale. T / K kcalc. 433 443 457 473 503 533 573 1.75 2.29 3.35 5.13 11.0 22.5 52.8 kexptl 1.70 2.30 3.34 5.63 12.6 23.2 51.8 a Calculated from eqn (11) and (111); in units of cmf mol-i s-5. TABLE 3.-vARIATION OF ( & z ~ 5 C ~ / & z F 5 C I ) = A WITH TEMPERATURE 29 3 324 353 373 383 400 0.44 0.56 0.75 0.72 0.92 0.96 0.06 0.06 0.06 0.05 0.03 0.04 TIK A 41 3 433 443 457 473 533 573 0.87 0.83 0.94 0.98 0.97 0.99 0.94 TIK %a 0.07 0.12 0.05 0.02 0.03 0.0 1 0.0 1 A a Standard deviation.If the ratio of pre-exponential factors &/A, were about the same as in the CF,+ClCN system, the difference E-,-E4 would be at least 16 kJ mol-l. The CN radicals formed by reactions (2) and (4) could be removed by reaction with I,, C,F,I or by recombination with C2F5 CN+I, -+ ICN+I ( 5 ) (6) (7) Due to analytical limitations, a quantitative determination of cyanogen iodide could not be performed. Nevertheless, as can be seen in table 1, the RC2F5CN/RC2P5CI ratio was constant at each temperature, within experimental error.The variation of this ratio with temperature is shown in table 3. Near 433 K, RCzFSCN/RCzF,CI approaches unity and maintains this value up to the highest temperature studied. Cyanogen iodide was not found above 433 K. Thermal decomposition7 of ICN could explain this feature since the steady-state concentration of CN increases as the temperature increases, making the production of C,F,CN important. When hexafluoroacetone was the radical source in the CF, + ClCN reaction,, the CF,CN/CF,CI ratio was always close to unity (between 408 and 473 K). This provides more evidence for good CN trapping by I, or C,F,I, at least in the low-temperature range. A value of k , is required in order to put k , and k, on an absolute basis. Skorobogatov et a1.8 obtained k, = 3.0 x lo1, cm3 m o t 1 s-l independent of tempera- ture for C2F5 combination.However, we assume that k , for C,F, is the same as k , for CF, radicals, for which Ayscough’s value of k , = 2.3 x lo1, cm3 mol-1 s-l has usually been used.lo We also assume that Ec = 0. There is ample evidence that E, = 0 for most combination reactions89l0 (but see also Ogawa et all1), in which case our values of E would be correct. Acceptance of Skorobogatov’s value of k , would require adjustment of our A factors. CN + C,F,I -+ ICN + C,F, CN + C2F5 --+ C,F,CN.3498 REACTION OF C2F5 RADICALS WITH ClCN The Arrhenius parameters for the addition and abstraction reactions from eqn (11) and (111) using k , = 2.3 x lo1, cm3 mol-l s-l are, respectively, A , = 3.9 x lo1, cm3 mol-1 s-l, A , = 1.0 x lo9 cm3 mol-1 s-l, E, = 57.1 kJ mol-l E3 = 20.6 kJ mot1.The values can be compared with those for the CF,+ClCN reaction reported previously3 A, = 1.3 x 1013 cm3 mol-1 s-l, A , = 2.7 x 1O1O cm3 mol-1 s-l, E, = 48.6 kJ mo1-1 E3 = 26.6 kJ mol-l. Although it seems probable that there is some self-compensation of Arrhenius parameters for the addition as well as for the abstraction reaction, the difference in reactivities between CF, and C2F5 radicals become apparent when we consider the rate constants. For example, at 295 K, where addition is the main process, ICcp,+cIcN = 0.100 cmt mol-4 S-4 kC,F,+CICN = 0.047 cml mol-4 s-i. Chamberlain and Whittle12 have reported a similar difference in reactivity when CF, or C2F, radicals add to benzene. At 573 K kCF,+CICN = 102 cmi mol-+ S-4 kC2F6+CICN = 51.8 c d mol-4 s-4 and chlorine abstraction from ClCN by CF, radicals is about twice as fast as that by C2F5 radicals.Further work on the reactions of other radicals with ClCN could provide additional data for a more complete comparison of the Arrhenius parameters for chlorine abstraction, and it could also be determined if the reaction scheme is general for reactions of different radicals with ClCN. We thank CONICET (Argentina) for partial financial support through Programa de Investigacion Fisicoquimica. W. G. Alcock and E. Whittle, Trans. Faraday SOC., 1966, 62, 134, 664. R. D. Giles and E. Whittle, Trans. Faraday SOC., 1966, 62, 128. F. Cosa, E. V. Oexler and E. H. Staricco, J. Chem. SOC., Faraday Trans. I , 1981, 77, 253. J. G. Calvert and J. N. Pitts, Photochemistry (John Wiley, New York, 1967), p. 742. (a) C. M. de Vohringer, E. R. de Staricco and E. H. Staricco, unpublished results; (b) W. Durrell and R. Eckert, U S . Patent 3,864,104, 1975. F. B. Hildebrand, Introduction to Numerical Analysis (McGraw-Hill, New York, 1956). G. A. Skorobogatov, V. G. Seleznev and 0. N. Slesar, Dokl. Akud. Nauk SSSR, 1976, 231, 1407. P. B. Ayscough, J . Chem. Phys., 1956, 24, 1944. ' G. N. Lewis and D. B. Keyes, J. Am. Chem. SOC., 1981, 40,472. lo C. J. Stock and E. Whittle, J. Chem. SOC., Faraday Trans. 1, 1980, 76, 496. l1 T. Ogawa, G. A. Carlson and G. C. Pimentel, J . Phys. Chem., 1970, 74, 2090. l2 G. A. Chamberlain and E. Whittle, Inr. J . Chem. Kinet., 1972, 4, 79. (PAPER 2/029)

ISSN:0300-9599    

DOI:10.1039/F19827803493

出版商:RSC    

年代:1982

数据来源: RSC