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/F198278FX009

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/F198278BX011

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY FARADAY TRANSACTIONS, PARTS I A N D I 1 The Journal of The Chemical Society is issued in six Journal of The Chemical Society, Journal of The Chemical Society, Journal of The Chemical Society, Journal of The Chemical Society, Journal of The Chemical Society, Journal of The Chemical Society, sections : Chemical Communications Dalton Transactions Faraday Transactions, I Faraday Transactions, 11 Perkin Transactions, I 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 Ijroblems, 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 11 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.PART I I (chemical physics) contains theoretical papers, especially those on valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenom- ena, spectroscopic studies (including Lr., 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. Journal of The Chemical Society, Chemical Communications Journal of The Chemical Society, Dalton Transactions Journal of The Chemical Society, Perkin Transactions, I and 11 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. 1PART 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. Notice to Authors (1) Although authors need not be members of the Royal Society of Chemistry it is hoped that they will be.(2) Authors must indicate the Part of the Journal they wish their paper to appear in. This preference will be respected unless it is obviously erroneous in terms of the scientific content 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 I V OBN. ( 5 ) For details of manuscript preparation, preferred usages, etc. the Instructions to Authors, 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 . Chem. SOC., Chem. Commun. J. Chem. SOC., Dalton Trans. J. Chem. SOC., Faraday Trans. I J. Chem. SOC., Faraday Trans. 2 J. Chem. Soc., Perkin Trans. I J . Chem. Soc., Perkin Trans. 2 * The author to whom correspondence should be addressed is indicated by an asterisk after his name in the heading of each paper. I1THE 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 NO 73 V A N D E R WAALS MOLECULES St Catherine’s College. Oxford, 5-7 April 1982 Organising Committee Dr E B Smith (Chairman) Professor A D Buckingharn Dr G Duxbury Dr 8 J Howard Dr G C Maitland There has been increasing interest in recent years in molecules whose components are bound only by relatively weak Van der Waals interactions The study of their structure and properties, and of the binding forces involved, has been stimulated both by new developments in spectroscopy and molecular beams specifically designed to study such molecules, and by the concerted application of a range of more standard techniques to these molecules high resolution spectroscopy, inelastic scattering, theoretical calculation.measurement of thermophysical properties The aim of this discussion is to bring together workers in these diverse fields, to highlight the complementary nature of the information obtained using the various techniques and t o examine the direction that future work should take to increase our understanding of these molecules The programme and application form may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry Burlington House, London W1 V OBN 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 S O C I E T Y O F C H E M I S T R Y A S S O C I A Z I O N E I T A L I A N A D I C H I M I C A F l S l C A S O C l i T i D E C H l M l E P H Y S I Q U E DEUTSCHE BUNSEN GESELLSCHAFT FUR P H Y S I K A L I S C H E C H E M I E F A R A D A Y D I S C U S S I O N N O . 7 4 Electron and Proton Transfer University of Southampton, 14-1 6 September 1982 This meeting will be concerned with fundamental aspects of the chemical kinetics of electron and proton transfer reactions in solution and with particular reference to well defined biological systems.Attention will be focused on (i) the theory of charge transfer, (ii) critical experiments designed to test those theories and (iii) their application to the understanding of charge transfer reactions in molecules of biological interest. The meeting will encompass well characterised reactions in solution, redox reactions and elementary biochemical reactions; particular attention will be paid to isotope effects, to electron and proton tunnelling, to intermolecular and intramolecular transfers and to related questions concerning the organisation of biological systems. Among those who have agreed to take part are R. A. Marcus, R. R. Dogonadze, H. Gerischer, J. Jortner, R. M. Kuznetsov, N.Sutin, R. J. P. Williams, H. L. Friedman, J. M. Saveant, J. F. Holzwarth, F. Willig, J. C. Mialocq, M. Kosower, L. I. Krishtalik, E. F. Caldin, H. H. Limbach, W. J. Albery, M. M. Kreevoy, J. J. Hopfield, P. Rich, H. A. 0. Hill, K. Heremans, C. Gavach and D. B. Kell. The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry Burlington House, London Wl V OBNFARADAY DIVISION O F THE ROYAL SOCIETY O F CHEMISTRY SYMPOSIUM NO. 1 7 The Hydrophobic Interaction University of Reading, 15-1 6 December 1982 This term refers to interactions between chemically inert residues arising from perturbations in the unique spatial and orientational correlations in liquid water. These effects provide a major contribution to many of the non-covalently bonded structures that form the basis of life processes. Current advances in the statistical mechanics of polar fluids, intermolecular forces, computer simulation, and membrane physics are providing a new basis for the re-examination of various aspects of hydrophobic effects, their origin and their quantitative description.Such theoretical treatments will be confronted with recent experimental work on simple model systems which, it i s hoped, will lead to a better understanding of hydrophobic interactions in more complex processes. The following have provisionally agreed to contribute to the symposium : A. Ben-Naim Hebrew University, Jerusalem 0. J. Berne Columbia University, New York S. D. Christian Oklahoma University The preliminary programme may be obtained from : Mrs Y.A. Fish, The Royal Society of Chemistry Burlington House, London W1 V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 75 Intramolecular Kinetics University of Warwick, 18-20 April 1983 Organising Committee Professor J. P. Simons (Chairman) Dr M. S. Child Professor R. J. Donovan Dr G. Hancock Dr D. M. Hirst Professor K. R. Jennings Dr R. Walsh 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.g.internal conversion, fragmentation, isomerisation). Contributions for consideration by the Organising Committee are invited. Titles should be submitted as soon as possible and abstracts of 300 words by 31 May 1982. Full papers for publication in the Discussion Volume will be required by 15 December 1982. Titles and abstracts should be sent to: Professor J. P. Simons, Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD. ivTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO.76 Concent rated CoI loidal Dispersions Loughborough University of Technology, 14-1 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 particl+particle interactions are strong enough to 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. Contributions for consideration by the organising committee are invited and abstracts of about 300 words should be sent by 31st August 1982 to: Professor R.H. Ottewill. School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division : Half day symposium Laser Spectroscopy (including the Centenary Lecture by T. Oka) To be held at University College, London on 28 April 1982 Further information from Mr S. S. Langer, The Royal Society of Chemistry, Burlington House, London W1 V OBN Gas Kinetics Group Seventh International Symposium on Gas Kinetics To be held at the University of Gottingen, West Germany on 23-27 August 1982 Further information from Or R. Walsh, Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD Colloid and Interface Science Group with the Colloid and Surface Chemistry Group of the SCl Adsorption from Solution To be held at the University of Bristol on 8-1 0 September 1982 Further information from Or W.D. Cooper, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ ~~~ Industrial Physical Chemistry Group Supercritical Fluids: Their Chemistry and Application To be held at Girton College, Cambridge on 13-1 5 September 1982 Further infOrmation from Or W. R. Ladner, National Coal Board, Coal Research Establishment, Stoke Orchard, Cheltenham GL52 4RZ Neutron Scattering Group and Polymer Ph ysics Group with the Institute of Physics The Neutron and its Applications To be held in Cambridge on 13-1 7 September 1982 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SW1 X 8QX Molecular Beams Group Molecular Beams and Molecular Structure To be held at the University of Bristol on 16-1 7 September 1982 Further information from Dr J.C. Whitehead, Department of Chemistry, University of Manchester, Manchester M13 9PL VFARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Autumn Meeting: Energy and Chemistry To be held at Heriot-Watt University, Edinburgh on 21 -23 September 1982 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1 V OBN Statistical Mechanics and Thermodynamics Group with the British Society of Rheology Microstructure and Rheology To be held at Trinity Hall, Cambridge on 21-24 September 1982 Further information from Dr P.Richmond, Unilever Research, Port Sunlight, Wirral, Merseyside L62 3JW High Resolution Spectroscopy Group High Resolution Fourier Transform, Laser Infrared and Electronic Spectroscopy To be held at the University of Newcastle-upon-Tyne on 22-24 September 1982 Further information from Dr P. J. Sarre, Department of Chemistry, University of Nottingham, Nottingham NG7 2RD Polymer Physics Group Polymer Electronics To be held in London on 20 October 1982 Further information from the Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SWlX 8QX Division with Polymer Physics Group and Macrogroup UK 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 Chemistry, Burlington House, London W1V OBN Polymer Physics Group, Macrogroup UK and the Plastics and Rubber Institute Polyethylenes 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 OHZ V iI I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I The Journal of Physical Chemistry reporting the most recent experimental and theoretical research in fundamental aspects of physical chemistry and chemical physics. Over twenty papers of original research by the world’s leading physical chemists in each biweekly issue keep you up-to-date in this dynamic field. PLUS a newly expanded format now includes - 0 Long Letters - Short papers in active research fields that deserve rapid dissemination are handled by the Editors expeditiously.scientists are invited to contribute critique articles in specialized areas of research - written for the nonspecialist. 0 Symposia Issues - Special issues include the publishing of signijicant selected symposia. 0 Feature Articles - Leading This internationally respected journal, edited by Dr. Mostafa El-Sayed, is a vital aid in keeping current in developments in all facets of your discipline. Order your copy of the new JOURNAL OF PHYSICAL CHEMISTRY today! Thi Journal of Physical Chemistry American Chemical Society 1155 Sixteenth St., N.W. Washington, D.C. 20036 U.S.A. Yes, I would like t o receive the JOURNAL OF PHYSICAL CHEMISTRY at the one year rate checked below: ---------------------------.~-.~-~~.--.----------.-.--~--- 1982 Foreign (Surface Foreign (Air Nonmembers 0 $198.00 0 $252.00 Postage Included) Freight Included) 0 Payment enclosed (Payable to American Chemical Society). Bill me Charge my 0 Mastercard 0 VISA 0 Bill company Card No.Interbnk No. (MasterCard only) Expire Date Signature Address City StateICount~y Zip Journal subscriptions start danuary 1982. Allow 60 days for your first copy to be mailed. Foreign payment must be made in US. currency by international money order, UNESCO coupons, US. bank draft or order through your subscription agency. viiNOTES It 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, No. 1, 1979, pp. 141. Also availablc 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. 1, 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 relationships, reflectance, potentiometric ion activities, testing distillation 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. A complete listing of all IUPAC nomenclature publications appears in the 198 1 Index issues of J. Chem. SOC. pp. 71-90.) ... Vlll

ISSN:0300-9599    

DOI:10.1039/F198278FP017

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 657-676 Chlorine-ca t a1 y sed Pyr ol y sis of 1,2-Dichloroe t hane Part 1 .-Experimental Results and Proposed Mechanism BY PHILIP G . ASHMORE,* JOHN W. GARDNER, ANTHONY J . OWEN, BARBARA SMITH AND PHILIP R . SUTTON Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD Received 30th December, 1980 Experimental studies of the pyrolysis and chlorination of l,2-C,H,C12 in the presence of small proportions of chlorine, or of chlorine plus nitric oxide, confirm that the main propagating steps between 520 and 620 K are 1 el+ C,H,CI,(DCE) + ezH3C12(k) + HCI 2 t,H,Cl, C,H,Cl(VC) + el -2 C1, + e2H3Clz A C,H,CI,(TCE) +el. The initial rates (d[VC]/dt), and (d[TCE]/dt), decrease together as the vessels age, but k,/k, = y remains constant at constant PDCE. y increases as PDCE is increased, or if inert gases are added, as expected from unimolecular behaviour of k,, and the Arrhenius parameters of y increase together as PDCE is increased.y(P) is evaluated for several ranges of PDCE at five temperatures to allow study of k i p ) by unimolecular theory in Part 2. The addition of VC lowers (d[VC]/dt), through reaction (-2), and the results are used to evaluate k-,/k, ; k-, is also pressure-dependent. In vessels with fresh surfaces, d[VC]/dt is proportional to PDCE xp:f* for low pel, and is independent of added inert gases; in aged vessels, the order in PDCE becomes 0.62, very close to the dependence of y(P) on PDCE. Calculations of [k], and [el], from the observed rates and rate constants point to an initiation step C12+S -,clS+el and termination by the reverse reaction in fresh-surfaced vessels, where S is a surface site; with aged surfaces, the results point to the same initiation step combined with termination by k + SCI + C,H,CI, + S especially at lower temperatures.The pyrolysis of 1,2-dichloroethane (DCE) to hydrogen chloride and vinyl chloride (VC) at temperatures between 670 and 770 K is thought to proceed by a radical chain mechanism with the propagation steps Cl + C,H,Cl, -+ C2H3C12 + HCl C,H,Cl, -+ C,H,Cl+ e l . The early investigations' 9 were interpreted in terms of homogeneous initiation and termination steps, but there are many difficulties with such an interpretation. The reaction rate generally increases in a run to a maximum rate, and widely different Arrhenius parameters have been reported for the overall rate constant.The differences seem to arise in part from changes to the vessel surface, suggesting that initiation 657658 CHLORINE-CATALYSED PYROLYSIS OF 1,2-C2H4C12 and/or termination may occur on the ~ u r f a c e s . ~ ? ~ Holbrook et aL5 also pointed out that the autocatalysis of the thermal decomposition might be caused by chlorine formed by surface reactions C,H,Cl, + surface(s) C,H,Cl,(s) C,H,Cl,(s) + C,H,(g) + Cl,(s) + els + Cl(g). Catalysis by ca. 1% of chlorine had been reported by Barton,, using a Pyrex flow system at 623 K, and by Takahashi et aL6 who used a stainless-steel reactor and proposed that the chlorine initiated the chains by the reaction metal + CI, + metal-Cl+ O(g).Huybrechts et al.' investigated the chlorine-sensitised photo-decomposition, and from the relative rates of formation of VC by reaction (2) and of 1,1,2-trichloroethane (TCE) by reaction (3) (3) Cl, + C2H3C12 -+ C,H,Cl, + el k 2 / k 3 = (d[VC1/dt) [C121/(d[TCEl/dt) they determined the ratio k2/k3 at five temperatures between 433 and 510 K. From the published* Arrhenius parameters for k , they found (1) However, a contemporary investigation by Gardner (J.W.G.) of the chlorine-catalysed thermal decomposition, at temperatures between 525 and 630 K, showed much lower values of k , than predicted by eqn (I), and also smaller Arrhenius parameters.@ In a short extension of J.W.G.'s work, Sutton (P.S.) showed that k, falls with total pressure as expected for a unimolecular rate constant at appropriate pressures,1o a possibility not considered in the photolysis studies.The effects of different pressures of DCE and of added inert gases upon k , have later been investigated in more detail by Smith (B.S.) and Owen (A.J.O.), who also investigated the inhibitory effects of VC through reaction log,,, k,/s-' = 14.33f0.47-(10710+630) K/4.576 T. Cl+ C,H,Cl + C2H3C12. (-2) (-2) This paper summarises the essential features of the dependence of the rates d[VC]/dt, d[TCE]/dt and - d[DCE]/dt on reaction conditions, and our evaluations of khP)/k3 = y ( P ) and kL%) at different temperatures and pressures. These results, and published data on k , and k,, are used to evaluate [Cl] and [C2H3C12] for different reaction conditions, and hence to provide information about the initiation and termination steps in fresh- and aged-surface vessels.In the following paperll the predictions of various theories of unimolecular fall-off are tested against A. J.O.'s extensive observations on the variations of kip) and kL%) with pDCE in order to obtain the best model for reaction (2). EXPERIMENTAL The cylindrical vessels were maintained at temperatures controlled by a Sirect controller and measured by a Pt/13% Rh-Pt thermocouple, in a tubular electric furnace (J.W.G., P.S.) or an air-thermostat with forced circulation (B.S., A.J.O.). Mixtures were made up in heated, blackened vessels and fed to the reaction vessel through heated tubing and greaseless stopcocks (Young's).Vessels A and B were of quartz with plane ends and s/v ratios 1.4 and 6.6 cm-l, respectively.B Pyrex vessel C, of dimensions similar to A, was used uncoated and,then with coatings of KCl, AgCl and Teflon to investigate the effects of different surfaces. Pressures wereASHMORE, GARDNER, OWEN, SMITH A N D SUTTON 659 measured (J.W.G., P.S.) by an all-glass transducer, developed and kindly supplied by Dr P. J. Thomas of I.C.I. Mond Division, or by an S.E. transducer (B.S., A.J.O.), and recorded on a Servoscribe chart recorder. The reactants and samples of products were purified as described el~ewhere.~ To check our 1 ,2-C2H,Cl,, Dr G. Martens kindly supplied some unstabilised material; both samples gave the same results. The organic compounds were identified and determined qlantitatively by gas chromatography, using a Pye Unicam model 104 dual-column instrument, flame ionisation detector and a temperature programming unit.The columns used and the calibrations are detailed elsewhere.g The product HC1 was determined by titrating samples from the reaction vessel and also by determining the pH of a solution prepared by a standard routine from the gas-chromatography sample. Pressures ofchlorine were determined with a single-beam photometer, using a monochromator (J.W.G.) or a Carl-Zeiss filter (B.S., A.J.O.) passing wavelengths c1os.- to 330 nm. The exit beam was monitored using an RCA 1P28 photomultiplier (J.W.G.) or an EM1 photomultiplier (B.S., A.J.O.), the output being fed to a Servoscribe recorder. Frequent calibration checks against chlorine pressures were carried out with both systems.In experiments using chlorine plus nitric oxide to catalyse the decomposition by the reaction NO + C1, -+ NOCl + e l the nitrosyl chloride formed (in small amounts, ca. l0-15% of the initial nitric oxide) was monitored9 using 250 nm radiation from the monochromator. RESULTS OVERALL REACTIONS A N D RATES Early work by J.W.G., confirmed by later checks, established that the primary products of the decomposition catalysed by chlorine were hydrogen chloride, vinyl chloride (VC) and 1,1,2-trichloroethane (TCE). The secondary products 1,l- dichloroethylene and cis- and trans- 1,2-dichloroethylene are formed by pyrolysis of the TCE. Typical results are shown in fig. 1 (a). A careful search was made for other C, and C, chloroalkanes and chloroalkenes, ethane, ethylene and methane; it can be asserted that none were present in concentrations > of typical initial DCE concentrations.It was also established by gas-chromatographic analysis that no detectable amounts of organic derivatives of NO were formed during the experiments with NO and Cl, present. High-molecular-weight products would not have been detected, but sample mass balances of observed reactants and products agreed within & 1 % at all extents of reaction in both vessels. Initial mixtures of DCE, C1, and NOCl gave much slower initial rates than with chlorine alone; initial mixtures of DCE, C1, and NO gave much faster rates, but these fell off rapidly as NOCl was formed. Later in each run with NO plus Cl,, however, the pressure increases were closely equal to the pressure of VC formed, as shown in fig.l(b). The amount of chlorine that disappeared was again closely equal to the amount of TCE f ~ r m e d . ~ The changes were the same in the fresh vessels A and B, as shown, and were found to be the same for pressures of NO between 0.5 and 5.0 T ~ r r . ~ * The absence of surface effects clearly indicates homogeneous initiation and homogeneous termination, probably by the reactions NO + C1, + NOCl + e l . At suitable points during various runs, determinations were made of k,/k, from (d[VC]/dt) [Cl,]/(d[TCE]/dt) and these are discussed in a later section. * 1 Torr = 101 325/760 N m-2.660 CHLORINE-CATALYSED PYROLYSIS OF 1,2-C,H4Cl, time/s LO 50 60 70 L O 30 t 20 --.3 5 a b \ 10 0 time/s ,d FIG. l.-(u) Correlations between pressure change (0) and VC formed (0) and between HCl formed (0) and VC plus TCE plus dichloroethylenes formed (+). Relationships between C1, lost (V), TCE formed (A) and total dichloroethylenes formed ( x). pDCE,O = 82.5 Torr at 584 K. Vessel B. (b) Correlations of pressure changes in vessel B (0) and in vessel A [a] with pvc formed (A) for initial pressurespcll = 10.8 Torr, pNO = 2.0 Torr and PDCE = 120.5 Torr (curve I) or 73.5 Torr (curve 11). 544 K. The curves are independent of pNO between 0.5 and 5.0 Torr. In the chlorine-catalysed reaction the overall initial reactions are and 1 ,2-C2H4Cl2 + CH2=CHC1 + HCl 1,2-C,H,Cl, + C1, -+ 1,l ,2-C2H3C13 + HCl. We have repeatedly confirmed by comparison between pressure increase, photometry of C1, and analyses of reactants and products, as illustrated in fig.l(a), that (dptotal/dt)o = (dp,,/dt), and - (dpClp/dt),, = (dpTCE/dt),, and hence that the initial rates could be determined satisfactorily from the continuous recording of total pressure (transducer) and chlorine partial pressure (photometer). The rates were corrected for the dead space;12 this is very important, because the corrections for dptotal/dt and - dpCl2/dr are in opposite senses.ASHMORE, GARDNER, OWEN, SMITH A N D SUTTON 66 1 INITIAL RATES (dp,,/dt)o After ca. 100 successive runs J.W.G. found the initial rates settled to nearly constant values for fixed initial pressures of DCE and C1, at fixed T. These rates, in the still-fresh vessels A (quartz, s/v = 1.4 cm-l) and B (quartz, s/v = 6.6 cm-l) were strictly proportional9 to PDCE,O with fixed pclz, o.Defining k’ by (d~vc/dt)o = ~’PDCE, o k’ was found to vary with pel,, as shown in fig. 2 for the fresh vessels A and B at / Y I I I I I P C l , l T O ~ 0 1.0 2.0 FIG. 2.-The experimental rate constants k’l,4 (0) in fresh vessel A (s/v = 1.4 cm-l) and k’6.6 (0) in fresh vessel B (s/v = 6.6 anF1) against pel, at 584 K. The solid curves is based on point + with k’6.6 proportional to PEf2. 584 K. In vessel A, k’l.4 appeared to reach a limiting value as pc12,0 increased to moderate values. In vessel B, however, a limiting rate was not achieved even at much higher values ofpClz, o. Indeed, in the lower range ofpClz, o, the rates were proportional to ptf:,, o.This is illustrated in fig. 2, where the line for k’6.6 is based on point + with k’6.6 OC Podfz, 0’ In the aged vessel B (i.e. after ca. 500 runs) the initial rates remained proportional to over a wide range, as shown in fig. 3 for three pressures of DCE. Moreover, in other vessels with different surface coatings the rates with fixed PDCE were found by A.J.O. to be proportional top$fz, or to a slightly higher order. Thus the behaviour of fresh vessel A seems in retrospect to be anomalous. As vessel B aged (i.e. during the experiments of J.W.G. and B.S.) several significant changes in the kinetics of the decomposition occurred. In the first place the rates for chosen conditions of T, pDCE and pclz decreased significantly as shown in table 1.Secondly, the order of reaction in pDCE decreased significantly, falling well below unity in later experiments by B.S.; from the slopes in fig. 3 it was deduced that in the aged vessel B (d~vcldt), = Q(~ci,, (PDCE, o / T ~ r r ) ~ . ~ ~662 CHLORINE-CATALYSED PYROLYSIS OF 1,2-C,H4C1, 0.9 / / / Y 4 0.3- s I 0 1.0 2.0 3.0 (pa,/Torr)t’So / I L .O A / I 1, 5.0 FIG. 3.-Plots of (dp,,/dt), against (pc1,)g.6 in aged vessel B with pDCE,o equal to x , 35; A, 65 and 0, 135 Torr. 572 K. TABLE l.--(dp,,/dt),/Torr s-l IN FRESH AND AGED VESSEL B AT 572 K PClZ, ,/Tom state of B 35 65 135 1 .o fresh 0.37 0.70 1.43 aged 0.10 0.13 0.19 2.0 fresh 0.53 0.98 2.02 aged 0.14 0.19 0.29 where B is a specific rate. Thirdly, at a fixed chlorine pressure (1 .O Torr) the activation energies for k’1.4 (vessel A, fresh), k’6.6 (vessel B, fresh) and for /I (vessel B, aged) were found to be 27, 32 and 47 kcal mol-l, respectively.Fourthly, while investigating the effect of adding inert gases J.W.G. found that large pressures of added argon had no effect on (dp,,/dt),, whereas B.S. found that adding inert gases such as CO,, CCl,, cyclo-C4F, and C,Fl, all increased (dp,,/dt),, the increases being larger for the gases with larger molecules. Table 2 illustrates the change for adding CO, to 41 Torr of DCE at 532 K (the significance of yexp is explained in the next section of this paper). Fifthly, the order in chlorine in fresh vessel A was not simple, as the rates reached limiting values at moderate pcl,,o; in the fresh vessel B it was 0.5 at low values of pc12, ,, but not simple at higher pressures; in aged vessel B, it was 0.5 over a wide range of pressures; in other vessels, with different surface coatings, it was 0.5 or (at some temperatures) slightly higher.ASHMORE, GARDNER, OWEN, SMITH AND SUTTON 663 TABLE 2.-&LATIVE MAGNITUDE OF (dpvc/dt), ON ADDING co, TO 41 TORR OF DCE (VESSEL B, 532 K) relative magnitude relative magnitude PCO*/PDCE dPvc/dt Yexp 0 1 .oo 1.80 2.60 1 .oo 1.39 1.63 1.80 1 .oo 1.41 1.61 1.74 In summary, it appears that the rate (dpvc/dt)o depends on the state of the vessel as well as on T, pDCE,O and pClz,,.STUDIES OF THE PROPAGATING STEPS (2) AND (3) I N THE CHLORINE-CATALYSED DECOMPOSITION If VC and TCE are formed predominantly by steps (1)-(3), where R = CH,ClCHCl, C~+DCE -, R+Hc~ (1) R+VC+Cl (2) R+C12 + TCE+Cl (3) and the reverse of reaction (2) is ignored while [VC] is small then where yexp is calculated from the rates and pclo.J.W.G. measured the rates (dptOtal/dt) and - (dpClz/dt) and pc12 at various points during each of many runs. On plotting (d&,,,,/dt) x pcle against - (dp,,Jdt) it was found that points for runs in fresh vessels A and B at the same temperature and the samep,,., fitted the same straight line, as exemplified in fig. 4 forp,,,, = 24 Torr. These results suggested that yexp was independent of the vessel and of the percentage of chlorine, and did represent the ratio k2/k3 of the rate constants of two homogeneous reactions. It was thought at that time that yexp was also independent ofp,,, (which was usually in the range 40-90 Torr) so the values were averaged at each temperature.The averages for five temperatures are shown on an Arrhenius plot in fig. 5, with the weightings of the averages in brackets, together with four values derived from the experiments9 on the homogeneous catalysis of the decomposition by chlorine + nitric oxide. Line I11 in fig. 5 was fitted by a least-squares procedure to the chlorine-catalysed results. Regression analysis of the individual experimental variances shows the 95 % confidence limits on E2 - E3 and on log[(A2/A,)/dm3 mol-'1 are as shown in eqn (111) k,/s-l 17 100 & 350 4.576 T/K ' = (3.57 0.13)- loglo k3/dm3 mol-l s-l These results seem to confirm that the principal reactions forming VC and TCE in the chlorine-catalysed pyrolysis are the homogeneous reactions (2) and (3).However, doubt was cast on these results or their interpretation when it became known that Huybrechts et a1.' had studied the apparently similar chlorine-sensitised photo-664 CHLORINE-CATALYSED PYROLYSIS OF l,2-C2H,C1, ;7 I/" XO I 0 1 2 3 4 5 6 7 6 9 -(dpa,/dt)/Torr min-' FIG. 4.-Plots for 578 and 561 K which show (dptotal/df)pC1, is proportional to - (dp,,,/dt), independently of the vessel. Vessel A, s/v = 1.4 cm-' ( x ) ; vessel B, s/v = 6.6 cm-' (0); pDCE.0 = 24 Torr. -2.5 7 -3.0 E 'c1 - E 1 n * -3.5 2 Y 0 - - 3 -4.0 -4.5 I I I I I lo3 K I T M 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 FIG. 5.-Arrhenius plots of log (k,/k,) for chlorine catalysed (0 with weightings in brackets) and nitric oxide+chlorine catalysed (+) decompositions (line 111) and for eqn (IV), (VI) and (VII).ASHMORE, GARDNER, OWEN, SMITH A N D SUTTON 665 decomposition of 1,2-dichloroethane over the temperature range 433-5 10 K and found k,/s-l 19970f 580 4.576 T/K .= 5.58 f 0.27 - loglo k3/dm3 mol-l s-l Eqn (IV) gave considerably higher values of k,/k, than eqn (111) as shown in fig. 5. In searching for reasons for the discrepancies, it was considered that they could not be attributed to the onset of reactions other than (2) or (3) at our higher temperature!, nor to other effects of the photolysing beam such as enhanced decomposition of R. In view of the unimolecular nature of reaction (2), the most likely explanation of our lower results at lower total pressures than those of Huybrechts et al. was the unimolecular fall-off of k , at pressures around 10-100 Torr.The apparently similar reaction c2H5 + C,H, + H shows fall-off in that (total) pressure range.13 In a short project, P.S. found a clear dependence of k, on pressure of DCE by exploring a wider pressure range and lower temperatures than used by J.W.G., as reported in ref. (10). Those experiments were extended by B.S., who found further evidence for unimolecular behaviour of reaction (2) in experiments with selected pressures of DCE at temperatures between 520 and 575 K in vessel B. Typical fall-off curves were found for yexp, illustrated by curve (1) through the points 0 in fig. 6 I/- 0 I 0 LO I I I 80 120 160 PDCE + pco2/Torr FIG. &-Relative values of y as pDCE, is increased (0, curve 1) and as CO, is added to 41 Torr of DCE (0, mean of 9 runs, curve 2).532 K. Vessel B, intermediate age. which shows yexp relative to the value forp,,, = 41 Torr. Fig. 6 also shows the effects on yrelative of adding CO, to 41 Torr of DCE at 532 K as points 0, each being the mean of 9 runs. The increases in yrelative as CO, was added are shown in table 2, together with relative changes in (dp,,/dt). The increases in yrelative closely parallel the increases in dp,,/dt; there was very little increase in dpTCE/dt. Inspection of the rate equations for reactions (2) and (3) shows that these results can only be explained666 CHLORINE-CATALYSED PYROLYSIS OF 1,2-C2H,C1, by changes in k,, and not by changes in [R], as CO, is added. B.S. also observed that adding inert gases such as cyclo-C,F, and C,F,, increased yexp to a greater extent than did CO,, bringing the fall-off curves close to that of DCE itself.These 'inert gas' effects are in keeping with unimolecular rate theory. Chlorine itself would be expected to have less effect than CO,; in addition, the proportion of chlorine used was rarely > 10% of the total pressure, and all later experiments (A.J.O.) at fixedp,,, showed that yexp was independent of pclo in those proportions. A.J.O. obtained extensive results with aged vessel B and also with a Pyrex vessel C , of dimensions close to those of A, when uncoated (CP) and when coated with Teflon (CT). While running-in the vessels CP and CT, A.J.O. observed that the overall rates fell but yexp remained constant as shown in fig. 7.Table 3 summarises data at 572 K 1 . 2 1 . a 0 - 0.8 I 2 y" \ a 0.6 0.4 100 4 b t- 80 --.. 60 1 4 0 12 16 number of admission FIG. 7.-Changes in kexp = (d In PDCE/df)O (0) and constancy of yexp (A) with successive admissions in a fresh Teflon-coated vessel, s/v = 1.4 cm-l, 572 K. pDCE.0 x 62f2 Torr, 7.8-8.1 % C1,. for the three vessels after running-in, and shows that yexp is substantially independent of the particular vessel. Over the higher pressure range the results are a reasonable fit to the empirical equation (V) log,,(y/Torr) = 0.64+0.61 log,, (pDcE/Torr). The last line of table 3 gives the predicted values of y for the mean pressures in vessel B. The dependence of y UponpDCE can only be attributed to changes in k , withp,,,, k, K (PDCE/Torr)o'gl. so It is significant that this proportionality correlates very closely with the equation established for (dpvc/dt), in aged vessel B, viz.(dpVC/dt)O = k2[k10 = B(pcl,/Torr)~'5((pDCE/Torr),o.s2.TABLE 3.-MEAN VALUES p , 7 (WITH STANDARD ERRORS OF MEANS AND NUMBER OF RUNS) OF PDcE, YExp FOR DIFFERENT VESSELS AT 572 K quartz B p f S.E./Torr s/v = 6.6 cm-l jjf S.E./Torr no. of runs Pyrex C (CP) p f S.E./Torr s/v = 1.4 cm-l jjf S.E./Torr no. of runs Teflon C (CT) p f S.E./Torr s/v = 1.4 cm-l jjf S.E./Torr no. of runs prediction from eqn (V) of y for p values in aged vessel B 9.02 f 0.08 13.1 k0.49 19 9.05 f 0.13 12.6 f 1.20 4 8.97 f 0 . 16 12.3 f 0.89 6 22.8 f 0.33 29.1 f 0.78 18 23.4 f 0.35 32.8 f 1.20 4 23.3 f 0.25 32.6 f 2.93 8 30.1 36.5 f 0.29 40.1 f 0.48 19 36.4 & 0.58 41 .Of 1.07 3 36.7 f 0.3 1 44.7f 1.80 9 40.2 63.4 f 0.61 57.4 f 1.12 24 65.0 f 0.47 63.1 f 1.76 8 64.3 f 0.3 1 60.0 f 0.63 27 56.4 10 1.2 & 0.58 75.4 f 1.29 23 103.0 f 2.23 73.0 & 3.58 4 104.0& 1.11 70.0k3.12 8 75.2 TABLE 4.-MEAN VALUES p, 7 (WITH STANDARD ERRORS OF MEANS AND NUMBERS OF RUNS) OF PDCE, YExxp IN VESSEL B AT THE TEMPERATURES INDICATED 572 K p f S.E./Torr jjk S.E./Torr no.of runs 560 K p+_ S.E./Torr 7f S.E./Torr no. of runs 547 K p & S.E./Torr jjk S.E./Torr no. of runs 534 K p & S.E./Torr jjfS.E./Torr no. of runs 521 K pfS.E./Torr jjf S.E./Torr no. of runs 9.0 f 0.08 13.1 k0.49 19 9.2f0.13 12.5 f 0.54 5 9.3 f 0.33 8.9 f0.89 4 9.4f 0.12 5.3 f 0.16 6 9.6 f 0.46 3.6 f 0.36 5 22.8 & 0.33 29.1 f 0.78 18 24.5 f 0.66 26.1 f 0.92 8 22.5 k0.38 18.4f 0.83 4 23.0 f 0.26 11.9 f 0.41 6 18.6 f 0.25 5.2 f0.18 9 36.5 f 0.29 40.1 f 0.48 19 36.8 f 0.48 31.4f 1.40 7 36.8 f 0.76 24.7f 1.13 5 35.6 f 0.78 16.0f0.37 6 27.3 f 0.46 8.1 f 0.40 11 63.4f 0.61 57.4f 1.12 24 64.3 f 0.96 44.1 f 1.73 6 65.0 f 1.26 32.4 f 1.74 4 65.2 f 1.35 23.1 f 0.33 6 55.0 f 0.38 1 1.6 f 0.25 11 101.2 f 0.58 75.4 f 1.29 23 100.4f 1.51 54.0 f 2.00 6 97.3 f 0.90 38.3 & 1.58 6 100.2+_ 1.35 27.7 f 0.92 6 91.8 f0.77 16.8 f 0.62 11 134.6 f 0.90 87.2 f 1.80 11 138.3 +_ 1.82 63.8 f 1.55 4 - 131.7 f2.35 30.4 f 1.05 4 134.8 f 1.57 18.9 f 0.24 4 z X c3 4 0668 CHLORINE-CATALYSED PYROLYSIS OF l,2-C2H,Cl, The correlation means that [R], is independent of pDCE, in aged vessel B; this point is discussed in detail in a later section of this paper.Table 4 summarises the results from many runs in the aged vessel B, and shows the mean values of yig; for groups of pressures at five different temperatures. In Part 211 of this series RRKM calculations are described that identify the model of unimolecular fall-off that best fits these mean 7, p values, and the appropriate fall-off 90 80- 70 - 8 60- a3 * 50- 40- 30 - *O- b $ t- n5 I * 1Q - PLXE/Tofl FIG. 8.-Mean-values y' of yipb at 0, 572; 0, 560; A, 547; A, 534 and 0, 521 K with the number of runs which determined each mean. The curves show the RRKM predictions for model G (see Part 211 of this series). curves are shown in fig. 8. From these curves the variations of k2/k, with temperature was found to depend on the pressure range according to eqn (VI) and (VII) 16500 (100 Torr) 4.58 T/K = 2.86- 4.58 l5 550 T/K (25 Torr).= 3.65- k2/s1 loglo ( k3/drn3 mol-l s-l These equations are plotted in fig. 5 , and fit very satisfactorily around line I11 which represents J.W.G.'s mean values for the range 40-90 Torr. It therefore appears certain that the experimental results shown by lines I11 and IV are in keeping with predictions from RRKM theory illustrated by lines VI and VII. In Part 211 the high-pressure parameters A? and E p of kp are evaluated. EFFECTS OF THE PRODUCT vc ON THE RATE OF THE CATALYSED DECOMPOSITIONS J.W.G. showedg that the addition of VC to mixtures of DCE and C1, reduced the initial rate of decomposition, because reaction (-2) is then important. A full stationary-state treatment of reactions (l), (2), (- 2) and (3) shows that expression (11)ASHMORE, GARDNER, OWEN, SMITH AND SUTTON 669 for yexp is modified to give yvc in the presence of VC: vc dptotaddt yvc = ( -dpclz/dt) x p c l z (VIII) where a = k-,[VC]/k,[DCE].Thus if values of k,/k, are known it is possible to calculate k-,/k, for selected points in any run. Using this method J.W.G. found k-,/k, = 0.75 at 630 K. A.J.O. investigated this effect in greater detail in order to study the temperature variation of k J k , and to see whether k-, was pressure-dependent, as would be 1.61 . , 0 ( b ) 15 .5 10 b V >O + 1 5 0 0 , 0.4 0.8 1.2 1.6 2.0 2.4 @VC/PDCE) I 1 I I I I 1 I L 5 10 15 20 25 30 35 40 45 Pvc, 0 /Torn FIG. 9.-(a) Experimental values (0) of y r c dotted against pvc, with (pDCE +pvc)o = 65 Torr, at 520 K.The curves are calculated from eqn (VIII) for k _ , / k , = 1 .O, 1.3 or 1.6. (b) Plots of k _ , / k , at 520 K calculated from eqn (VIII) assuming yvc = 15.0 Torr for pDcE,o = 65 Torr, pVc,, = 0. The line is the least- mean-squares fit. Vessel B. expected by analogy with the pressure dependence of k,. Fig. 9(a) shows the experimental values of y y c determined from plotted against pvc, for runs at 520 K with pDCE, +pvc, kept constant at 65 Torr. The three lines show y y c calculated from eqn (VIII) assuming (a) k-,/k, has values 1 .O, 1.3 or 1.6 and calculating a from appropriate values of pvc, PDCE and pClz, and (b) k,/k, is constant and has the value for pvc = 0, which would only be true if DCE670 CHLORINE-CATALYSED PYROLYSIS OF 1,2-C2H4C12 and VC have equal efficiencies in energising or de-energising the radical t2H3Cl2.The best fit is for k-,/kl x 1.3, but the lines do not curve sufficiently. This may be because k 2 / k 3 changes as pvc, OIPDCE, o changes. A practical way round the difficulty is to use the same basic data and eqn (VIII) to calculate a and hence k-,/k, point by point, and to investigate whether the ratio changes systematically with pvc, O/pDCE, ,. Fig. 9(b) shows the plot of k-,/kl against pvc, O/pDCE, (with pDCE, , +pvc, , = 65 Torr) with the line fitted by least-mean- squares procedures. Similar systematic variation was found at other pressures (100, 35 and 25 Torr) at 520 K, and also from experiments at 544, 570 and 590 K. For comparison of results we take the intercept values of k-,/k,, so that the only energising/de-energising species is DCE.TABLE VALUES OF k-,/k, AND 0 = log,, (k-,/dm3 mo1-l s1). (21, IS THE MEAN OF THE 0 AT EACH TEMPERATURE. ~ PDCEITOrr x 25 x 35 x 65 x 100 0 m 595 K k-,/k, 0.79 0 9.56 570 K k-,/k, 0.76 0 9.49 544K k-,/k, 0.91 % 9.51 520K k-,/k, 1 .oo 0 9.50 0.87 9.60 0.8 1 9.52 0.89 9.50 1.15 9.56 1.12 9.71 1.45 9.77 1.41 9.70 1.35 9.63 1.66 - 9.88 9.69 1.74 - 9.85 9.66 1.90 9.83 9.64 1.75 - 9.74 9.61 - Table 5 shows the intercept (pvc = 0) values of k-,/kl, and the corresponding values of 0 = log,, (k-,/dm3 mol-l s-l), using the published data14 for k,. At all four temperatures the values of k-,/k, fall with decrease in pressure, showing that k-, is in a fall-off region, just as k , is, in the pressure range investigated.These results are examined in more detail in Part 2.11 DISCUSSION OVERALL MECHANISMS OF THE CATALYSED DECOMPOSITIONS The experimental results confirm that the propagating steps in the early stages of the catalysed decompositions are (1)-(3), with reaction ( - 2) affecting the rates later in each run or when VC is added to initial mixtures. The changes in rates and rate laws in vessel B as it aged, and differences from vessel to vessel, point to changes and differences in the control of the chain-centre concentrations by the initiation and termination steps. Many overall rate laws, derived from different combinations of heterogeneous and homogeneous initiation and termination steps, were tested against the experimental results but did not prove very discriminating. Examination of chain-centre concentrations calculated from measured rates and the rate constants k,, kip) and k3 has revealed much more about the nature of the initiation and termination steps under different reaction conditions.ASHMORE, GARDNER, OWEN, SMITH AND SUTTON 67 1 CHAIN-CENTRE CONCENTRATIONS BASED ON RATE MEASUREMENTS THE RE LA T I VE CON C E N TR A TIONS [R],/[el], These depend solely on the propagating reactions (1)-(3) in the initial stages when VC is low and reaction (- 2) can be neglected.For steady rates they will adjust to give irrespective of the initiation and termination reactions. Taking the published date for k, and k, and using the data for kip) reported in Part 2,11 the ratio takes the value TABLE TH THE RATIO [kl0/[tl], FOR VARIOUS REACTION CONDITIONS 520 572 629 2 5 2 5 2 5 T/K PClZ, o/Torr p,,,, o/Torr 35 43 34 14 13 4.7 4.6 65 57 48 18 17 5.8 5.7 135 88 77 25 24 7.6 7.5 shown in table 6.It is therefore likely that termination reactions involving R will be more im ortant at lower temperatures, lower pel, and higher pDCE; and those in- volving 8 1 will be relatively more important at higher temperatures and lower pDCE. ABSOLUTE VALUES OF [el], AND [R], IN VESSEL B [el], was calculated from - (d[DCE]/dt),/k,[DCE], with1* log,,(k,/dm3 mol-l s-l) = 10.80 -(3100 K/4.576 T). Fig. 10 shows how [el], depends on [Cl,], at 572 K. In the fresh vessel [el], is proportional to [Cl,]o*5, is independent of [DCE],, and lies very close to [Cl],, = (&[c12]o)o*5. In the aged vessel [el], depends on a power > 0.5, and is smaller with higher values of [DCE],.Similar differences were found at other temperatures. log,,(k,/dm3 mol-l s-l) = 8.76-(920 K/4.576 T ) ; check calculations from (d[VC]/dt),/kip) confirmed the values. The left-hand lines in fig. 11 show that in the fresh vessel at 572 K [R], is proportional to [C1,]8.5 but is larger for higher values of [DCE],. The points show experimental results in the aged vessel at 572 K; [k], is then closely proportional to [C1,]8.5 (more accurately it fits a slightly higher power) and is clearly independent of [DCE], and the values are much lower than in the fresh vessel. Similar results were found at other temperatures. [R], was calculated from (d[TCE]/dt),/k,[Cl,], with8 DEDUCTIONS ABOUT INITIATION AND TERMINATION REACTIONS IN VESSEL B FRESH VESSEL B The simplest explanation of the observations summarised in fig.10 and 11 is that the initiation and termination reactions in fresh vessel B controlled [el],, especially at higher temperatures where [cl]/[R] is higher.672 CHLORINE - c AT ALY SED PYROLYSIS OF 1 ,2-C2H,Cl, FIG. 10.-Plots of [el],, and [el], against (pcl /T~rr)$.~ for the fresh vessel B (0) at all PDCE,O and for the aged vessel B at pDCE.0 equal to x , 35; A, 65 and 0, 135 Torr, 572 K. 36r 6' 18 16 I4 I2 10 8 6 4 2 FIG. 11.-Plots of [k], against (pclo/Torr)~.5 at 572 K for the fresh vessel withpDCE,o x x , 35; A, 65 and 0, 135 Torr (left-hand axis); and for the aged vessel B with pDCE.0 x +, 25; x , 35; A, 65 and 0, 135 Torr (right-hand axis).ASHMORE, GARDNER, OWEN, SMITH AND SUTTON 673 Since [ello is proportional to [C1,]8.5 and close to [el],, = d(K,,[Cl,],), the gas-phase dissociation and recombination steps (d) and (r) must be examined as candidates for the initiation and termination reactions d r C1, + M e 2el+ M, There are several objections to this assignment. It has been shown15 previously that the attainment of 90% of [el],, when C1, is admitted to vessels at 650 K would take more than 100 s by homogeneous dissociation of Cl,(qcl, = 20 Torr, ptotal = 760 Torr).It would take longer with ptotal z 100 Torr, especially at lower temperatures, but no induction periods were observed in the catalysed pyrolysis. Reaction (r) is much slower than the termination step (t) t c l + R + RCl (9 for which the rateconstant has been estimatedls as dm3 mol-1 s-l; experimentally measured values for e l + other chloroethylene radicals are lower ( 1010.9 dm3 mol-l s-l for CHCl,CCl, and 10ll.O dm3 mol-l s-l for C,C15).A recent experimental meas~rementl~ of the rate constant for path (r) suggests a maximum value of 5 x lo9 dm6 rnol-, s-l at 572 K. Combining these rate constants with typical values of [ell0 and [k], for fresh-surfaced vessels at 572 K shows that reaction (t) is > lo4 times as effective as reaction (r) for all reactant pressures used. Similarly, calculations from published data1* on reaction (d) show that it produces chlorine atoms at a much slower rate than they would be removed by reaction (t) were it operative under those conditions.We therefore reject reactions (d) and (r) as effective initiation and termination reactions. Another pair of reactions that could result in [el], = [el],, are reactions of chlorine molecules and atoms with surface sites S such as the pair (i,s) that would be balanced at true equilibrium (from the principle of microscopic reversibility) i c1, + s * SCl +el. s Collision theory shows that surface removal of el in vessel B would have to have an efficiency &(el) > to compete with reaction (t) at the calculated levels of [el] and of [A]. The lowest measured19 limit of &(el) is ca. on freshly acid-washed walls (conventionally described as 'poisoned' for the removal of el). After heating to 100 OC, the efficiency rises to ca. 3 x on salt-coated or flamed Pyrex surfaces.It therefore seems likely that the surface removal can compete successfully with gas-termination steps in the vessel B under the conditions used by J.W.G., although it is unlikely that it completely overwhelms step (t). There is a good deal of evidence from previous studies of temperature distribution in vessels where thermal chlorinations are taking place that the reaction takes place close to the walls,,O with chains initiated and terminated on the walls. If we assume that [el], M (KD[C1,]o)o.5 through the reversible pair (i, s) then for low [Cl,], in fresh B and is higher (> This describes the observed kinetics. It also suggests the overall activation energy674 CHLORINE-CATALYSED PYROLYSIS OF 1,2-C2H,C1, should be close to El +AH/2 where AH is the enthalpy change of the reaction c12 (8) f 2 a (g).As El = 3 kcal mol-1 and AH = 58 kcal mol-l, the predicted overall activation energy is 3 + 29 = 32 kcal mol-l. J.W.G.'s resultsg gave 33 kcal mol-1 for the fresh vessel B. Stationary-state treatment also gives k [DCEI, [ell0 k l [ ~ ~ ~ l , (G [cl,~,)~ * 50 k2 x [RIo = k , + k3[C1,], because k, % k,[Cl,], under the experimental conditions. If this is correct, [R],/[C1,]8.50 should be proportional to [DCE]8.38 because experi- mentally over this pressure range it was found that k , was proportional to [DCE];as2. The slopes of the left-hand lines in fig. 11 should then be proportional to [DCE];-38. They are closely so, as shown in table 7. TABLE 7.-DEPENDENCE OF [R], ON PDCE (aged vessel B, 572 K) 135 65 35 22.5 16.8 13.2 3.49 3.44 3.42 Adding inert gases would increase k,, but if [el], is fixed [k], must fall proportionately from the above equation.Hence (d[VC]/dt) = k,[k], remains constant, as found9 experimentally for the fresh vessel B. AGED VESSEL B In the aged vessel B, the experimental evidence shows that at the same temperature the rates are lower than in the fresh vessel and centre concentrations are correspondingly lower by factors of 4-8. This further favours a first-order surface termination against a second-order gas-phase termination. In the aged vessel the addition of inert gases increases (d[VC]/dt), (table 2) but not (d[TCE]/dt),. The increase in (d[VC]/dt), = k,[R], is entirely attributable to the increase in k,, as [k], = (d[TCE]/dt),/k,[Cl,], remains constant.This points to control of [k] by the initiating and terminating steps in the aged vessel B. Calculations of the rates of the gas-phase termination reactionslG and R+R+R2 R + e l + RCl show that in the aged vessel B surface removal of R should be more effective provided ~ ( k ) b 2 x which is very likely. The surface removal of k by the reaction SCl+R-+ RCl+S (s') combined with the initiation step (i) and the equation 6 = K[Cl,]0.5/( 1 + K[C1,]0-5)ASHMORE, GARDNER, OWEN, SMITH A N D SUTTON 675 where K is the Langmuir coefficient, for the fraction 8 of sites occupied by C1 leads [R], = ki[Cl,]~.5/k,, K. to the prediction Thus [R], is proportional to [C1,]8.5 and is independent of [DCE], as required by the experimental results in fig. 11. If [R], is controlled, then by eqn (IX) [el], has to follow Recalling that k , cc [DCE]o.S2 and that k3[C1,], < k , when [Cl,], is low, the expression for [el],, correctly describes the shape of the curves through the experimental points x , A, and 0 in fig.10, and their slopes at fixed [Cl,], are closely proportional to [DCE]-o.38. Other assumptions about surface termination reactions, such as removal of R by empty sites S, do not lead to the correct relationships for [R], and [el],. Accepting path (i) as the initiating step and path (s') as the terminating step, (d [ VC] / d t)$P) = k gP) ki [ Cl ,] 8.5 / k, I K and the overall activation energy is given by Eoverall = B2P) + Ei - E,, - AH' where AH', the enthalpy change of the reaction s + 3c1, e SCl is equal to +D(cl-cl) -D(s-cl). Ei is not less than the enthalpy change of the reaction (i) which is D~cl-cl)-D~s-cl). E, is probably small.Therefore Eoverall w EP) + D(cl-cl) - D(s-cl) - P ( c l - C l ) + D(s-cl) w EP) + iD(c1-a)- Now D(cl-cl) M 58 kcal mol-1 and EgP) w 17 kcal mol-l at p w 100 Torr (see eqn (VI), with E3 w 0.9 kcal mol-l]. Therefore Eoverall M 46 kcal mol-l. This predicted value compares very favourably with the experimental value for the aged vessel B of 47 kcal mob1 (A.J.O.). CONCLUSIONS FOR VESSEL B We therefore have self-consistent and reasonably quantitative interpretations of the experimental results for the chlorine-catalysed pyrolysis in the fresh-surface vessel B and in the aged vessel B, on the basis that the chain-termination reaction in the fresh vessel is essentially removal of el on the surface, while in aged B it is removal of k on the surface.This change-over is helped, undoubtedly, by the relatively higher values of [k]/[el] at the lower temperatures used for many of the experiments in the aged vessel. Unfortunately, we lack information about the ageing process that might point to chemical reasons for the aged quartz surface to favour reaction (s') with k rather than (s) with e l , and the fresh quartz surface to favour reaction with el rather than with R.676 CHLORINE-CA TALY SED PYROLYSIS OF 1 ,2-C,H4Cl, INITIATION A N D TERMINATION REACTIONS IN OTHER VES.SELS It is not possible to provide a satisfactory explanation of the limiting rate shown in fig. 2 for the quartz vessel A, s/v = 1.4 cm-l, as pclp is increased. Inclusion of the gas-phase termination step (t) as well as the surface termination, which would be reduced at the lower s/v, makes the rate less dependent on pclz than one-half order, and this effect increases as pclz and the radical concentrations increase.However, detailed analysis shows that including (t) makes the order in pDCE lower than the observed unity. In the uncoated Pyrex vessel C , with s/v = 1.4 cm-l, [el] and [R] behaved like the results found for fresh vessel B, with some evidence for gas-phase as well as surface termination. After coating with Teflon the concentrations fell to even lower levels than in aged vessel B. This suggests that coating with Teflon, and to a less extent ageing in B, might reduce the rate of initiation through blocking chemisorption sites for Cl,, while retaining the ability of the walls to adsorb or remove R. D. H. R. Barton, J. Chem. SOC., 1949, 148. D. H. R. Barton and K. E. Howlett, J. Chem. SOC., 1949, 155. K. E. Howlett, Trans. Faraday SOC., 1952, 48, 25. G. A. Kapralova and N. N. Semenov, Russ. J. Phys. Chem. (Engl. Transl.), 1963, 37, 35, 156, 258. K. A. Holbrook, R. W. Walker and W. R. Watson, J. Chem. SOC. B, 1968, 1089. K. A. Holbrook, R. W. Walker and W. R. Watson, J. Chem. SOC. B, 1971, 577. T. Takahashi, T. Abe, Y. Migkoshi and S. Asano, Kogyo Kagaka Zasshi, 1968, 71, 504. 1972, 81, 65. F. S. Dainton, D. A. Lomax and M. Weston, Trans. Faraday SOC., 1962, 58, 308. J. W. Gardner, Thesis (University of Manchester, 1975). (This includes a full summary of earlier work on the catalysed and uncatalysed decompositions). lo P. G. Ashmore, J. W. Gardner and P. Sutton, Chem. Wetenschap, Belgische Chemische Industrie, June 1973, p. 11 (abstract of paper, Third International Symposium on Gas Kinetics, Brussels, 1973). l1 P. G. Ashmore, A. J. Owen and P. J. Robinson, J. Chem. SOC., Faraday Trans. 1, 1982, 78, 677. l2 P. J. Robinson, Trans. Faraday Soc., 1965, 61, 1655. l3 P. J. Robinson and K. A. Holbrook, Unimolecular Reactions (Wiley Interscience, New York, 1972), l4 C. Cillien, P. Goldfinger, G. Huybrechts and G. Martens, Trans. Faraday SOC., 1967, 63, 1631. l5 S. W. Benson and J. H. Buss, J. Chem Phys., 1957, 27, 301. l7 M. A. A. Clyne and D. H. Stedman, Trans. Faraday SOC., 1968, 64, 2698. la R. A. Carabetta and H. N. Palmer, J. Chem. Phys., 1967, 46, 1333. l9 P. G. Ashmore, A. J. Parker and D. E. Stearne, Trans. Faraday SOC., 1971, 67, 3081. 2o G. A. Kapralova and N. N. Semenov, Russ. J. Phys. Chem. (Engl. Transl.), 1963, 37, 35, 156, 258. 'I G. Huybrechts, J. Katihabwa, G. Martens, M. Nejszaten and J. Olbregts, Bull. SOC. Chim. Belg., pp. 262-263. G. Chiltz, P. Goldfinger, G. Huybrechts, G. Martens and G. Verbeke, Chem. Rev., 1963, 63, 355. (PAPER O/ 1986)

ISSN:0300-9599    

DOI:10.1039/F19827800657

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1982, 78, 677-693 Chlorine-catalysed Pyrolysis of 1,2-Dichloroethane Part 2.-Unimolecular Decomposition of the 172-Dichloroethyl Radical and its Reverse Reaction PHILIP G. ASHMORE* AND ANTHONY J. OWEN Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 80, Manchester M60 1QD AND PETER J. ROBINSON Department of Chemistry, Manchester Polytechnic, Manchester M 1 5GD Received 30th December, 1980 Fall-off curves for the unimolecular rate constant k , of the reaction C2H3C12 C,H,Cl + C1 have been calculated by the Forst and RRKM methods and compared with the experimental results reported in Part 1 and by Huybrechts et al. The Forst calculations can be fitted to the Part 1 results, but then predict kern) values that lie below the experimental results of Huybrechts et al.which refer to lower temperatures and higher pressures. In contrast RRKM calculations using higher Arrhenius parameters give fall-off curves that are compatible with all available experimental data. The preferred RRKM models without allowance for the centrifugal effect have Edrn) x 19.6 kcal mol-l (82 kJ mol-l) and Aim) x 1014.0 s-l. When a reasonable centrifugal effect is allowed for the early transition state (Z+/Z x 2) the preferred models have E2 x 20.0 kcal mol-I (84 kJ mol-’) and Aim) x s-l. Our experimental evaluations of kLP,, are used in conjunction with earlier experimental results and RRKM-based calculations for the reverse reaction Cl + C,H,Clz C2H3C12 to evaluate Ai?) and EL?). The decomposition of 1,2-dichloroethane (DCE) catalysed by chlorine or by chlorine plus nitric oxide proceeds by the chain-propagating steps (1) and (2) with competition from step (3) (1) C2H3C12 + C2H3Cl+ el (2) (3) The competitive reactions allow evaluation of k , / k , and experimental studies’ have shown that k , is in the unimolecular fall-off region for pressures p between 8 and 150 Torr.? Extensive measurements of yi,)p = k$”)/k3 as a function of p were made at five temperatures between 520 and 572 K.The mean values @-’) were calculated at chosen levels of p using the published2 Arrhenius parameters for k , and are here shown in table 1 with the standard errors of the means. It is clearly of importance to see how these results conform with various theories Cl + C,H,Cl, + C2H3C12 + HCl C,H,Cl, + Cl, -+ C2H,C1, + Cl.t 1 Torr = 101 325/760 N m-*. 677c z TABLE ME MEAN VALUES OF p, kip) (WITH STANDARD ERRORS OF MEANS AND NUMBERS OF RUNS) OF pDCE, M VESSEL I3 AT THE TEMPERATURES INDICATED 572 K p f s.s./Torr ~ p ) + ~ . ~ . / 1 0 5 S-1 k p ) f ~ . ~ . / 1 0 5 s-1 kip) +s.E./~o~ s-1 ~ p + ~ . ~ . / 1 0 5 s-1 k p k ~ . ~ . / 1 0 5 s-1 no. of runs 560 K p f s.E./Torr no. of runs 547 K p f s.E./Torr no. of runs 534 K p f s.e./Torr no. of runs 521 K p f s.E./Torr no. of runs 9.0 f0.08 0.92 f 0.03 19 9.2 f0.13 0.88 f 0.04 5 9.3 f0.33 0.63 f 0.66 4 9.4 f0.12 0.38 f 0.01 6 9.6 f0.46 0.26 f 0.03 5 22.8 f0.33 2.04 f 0.05 18 24.5 k0.66 1.84 f 0.06 8 22.5 f0.38 1.3 1 f 0.06 4 23.0 f0.26 0.85 f 0.03 6 18.6 f0.25 0.37 f 0.01 9 36.5 f0.29 2.82 f 0.03 19 36.8 f0.48 2.22 f 0.10 7 36.8 f0.76 1.75 f 0.08 5 35.6 f0.78 1.14f 0.03 6 27.3 k0.46 0.58 f 0.03 11 63.4 f0.61 4.03 f 0.08 24 64.3 f0.96 3.12 f 0.12 6 65.0 1.26 2.30 0.12 4 65.2 & 1.35 1.65 f 0.03 6 55.0 f0.38 0.83 f 0.02 11 101.2 f0.58 5.30 f 0.09 23 100.4 f 1.51 3.82f0.14 6 97.3 f0.90 2.72 f 0 .11 6 100.2 f 1.35 1.97 f 0.07 6 91.8 f0.77 1.20 f 0.04 11 134.6 f0.90 6.13 f 0.13 11 138.3 f 1.82 4.51 fO.ll 4 131.7 f2.33 2.17 f 0.08 4 134.8 f 1.57 1.35 f 0.02 4 z 0 r 0 x 0 Z 1P. G. ASHMORE, A. J. OWEN A N D P. J. ROBINSON 679 of unimolecular fall-off. Our first theoretical calculations were based on the Forst approach3 which requires a detailed knowledge of molecular parameters only for the reacting species. These did not result in a completely satisfactory explanation of the experimental results.However, full RRKM calculations gave a much more satisfactory explanation of our own and of other investigators' results reported in Part 1. The agreement between theory and experiment led to the evaluation of the parameters A and Eiw) of the high-pressure rate constant ki") which is inaccessible by direct experiment. CALCULATIONS OF FALL-OFF CURVES AND FIT TO THE EXPERIMENTAL kp) DATA FORST CALCULATIONS Programs were written by P. J. R. to calculate the collision rate constant kcoll, the density of states N(Ev,) by the Hoare and Ruijgrok4 method of steepest descent, the partition function Q, for the radical, and hence k(E) = A ia)N(E- Ei"))/N(E) (with E > Ei")) for suitable pairs of A 8") and Ei").Finally k i p ) was calculated from eqn with M, the collision partner, at p = 5, 10, 25, 50, 75, 100, 125, 150 and 200 Torr. The first calculations ignored the centrifugal effe~t,~ partly for simplicity, partly because the low level of Aim), ELw) values led to the view that the transition state was formed early in the dissociation (2) of the radical, with some tightening due to rapid formation of the n-bond before the C-C1 bond is much extended. This kind of transition state would give rise to comparatively low values for I z / I and hence a small centrifugal effect. This point is taken up later in this paper. Molecular parameters for the dichloroethyl radical have been proposed by previous As exemplified later, the two sets of parameters gave very similar results, and once this was settled, the Beadle, Knox, Placid0 and Waugh (BKPW)6 parameters were used in the Forst calculations. In later calculations, convenient groupings of frequencies, based on the BKPW values, were used to simplify the computations.The BKPW and other required parameters for the radical C2H3C12 are given in table 2. Calculations of k i p ) with a wide range of Aim) and ELa) values identified a comparatively small network of values that gave kip) close to the experimental kip) means at 572 K. From this network pairs of A ia), EL") values could be selected that gave kpoo) for p = 100 Torr equal to the value obtained from short interpolation to p = 100 Torr between the experimental means. These pairs lie on the line lipoo) shown in fig. 1 (a), and show a compensation effect (high A , goes with high E,, and low A , with low E,) similar to that in pairs of A$"), EL") required to produce a given /tia).However, the higher pairs produce a fall-off curve that lies below the experimental E L P ) value at p < 100 Torr and above them at p > 100 Torr, i.e. is too straight. In contrast, the lower pairs lead to kip) above the experimental values for p < 100 Torr and below those at p > 100 Torr, i.e. is too bent. The result is, of course, that each pair gives a different value of the high-pressure rate constant kiw). These changes of shape of the calculated curves passing through k&loo) can be applied to identify A iw) and and hence ki") within closer limits. A simple way is to repeat the procedure used for the kgoo) curve for a much lower pressure. Thus the line kp5) in fig.1 (a) represents values of A and ESm) that give ki25) equal to the experimental * Equations with lower numbers are in Part 1.'680 UNIMOLECULAR DECOMPOSITION e2H3C12 -P C2H3Cl+e1 TABLE 2.-PARAMETERS FOR FORST AND RRKM CALCULATIONS (a) Data for radical internal rotation I = 33 x vibration frequencies/cm-l [ref. (6)] grouped frequencies/cm-l collision diameter 0.55 nm collision bath C,H4C12 g cm2, degeneracy 1, symmetry number 1 3005 (2), 2957, 1450, 1304, 1264, 1230, 3000 (3), 1280 (4), 1020 (2), 754 (3), 301, 220 1052, 986, 768, 754, 709, 301, 220 (b) Data for Forst models model A B C D 16.00 E'&.)/kcal mol-1 17.15 17.40 17.80 log,o(A 6.p,,/s-') 12.60 12.80 13.00 12.00 (c) Data for RRKM complexes grouped log10 A $72 / AS&,/ EO/ EL%)/ frequencies complex S-' cal mol-1 K-' kcal mol-1 kcal mol-l /cm-l F 13.70 0.864 16.30 19.12 3008 (3), 958 (3), 777 (21, 447 (3)Y 777 (2), 431 (3), 737 (2), 379 (3), 199 (2), 131 (1) G 14.00 2.241 16.60 19.57 3025 (3), 957 (3), 175 (2), 93 (1) H 14.30 3.613 16.81 20.03 3028 (3), 957 (3), 157 (2), 89 (1) (d) Data for RRKM complexes which allow for the centrifugal effect with Iz/I= 2.0 Eo/kcal mo1-l 16.5 17.0 17.5 Eim)/kcal mol-l 19.5 20.0 20.5 A p/ 1014 S-1 Xl x, x3 y, y 2 y 3 Zl z2 z3 0.7 0.8 0.9 1.7 1.8 2.0 4.0 4.4 5.0 value obtained by short interpolations of the experimental means t o p = 25 Torr.The intersection of the two lines k$25) and k$lo0) gives the values of A am) (close to 1012.7 s-l) and Elm) (close to 17.5 kcal mol-l) that give a fall-off curve passing through the experimental means 6 $ p ) close to p = 25 and 100 Torr.Two more sophisticated procedures confirmed the results of the simple procedure. Method (l), based on Szirovica and Walsh,8 involved comparison of the locus of A am), Earn) values fitting the slope of the Lindemann reciprocal plot (1 / k i p ) against l/p) at a given temperature with the locus of the values fitting k i p ) at a specific pressure. Lindemann plots of two Forst calculations for 572 K are shown in fig. 2; they are nearly linear over the pressure range of interest. A network of values of theoretical slopes can be constructed for suitable ranges of A 6") and Earn), and pairsP. G. ASHMORE, A. J. OWEN AND P. J .ROBINSON 68 1 1L.O n I - 2 13.5- 5.. - s 2 13.0- 0 - 12.5 12.0 - - I I I I 1.0 4.0 7.0 10.0 100 TO=/PDCE FIG. 2.-Lindemann plots of l/k$p) for Forst calculations with model B (0) and with model D (A) (see table 2) at 572 K. of ASw), ESm) values can be found, by interpolation, that give slopes equal to that of the line fitted (by least-mean-square procedures) to the Lindemann plot of l/kiP) against 1 / p . These pairs are plotted as line L in fig. 1 (a). The three lines intersect within a small range of Aim), Edm) values which are therefore consistent with the shape of the fall-off curve at 572 K. Table 3 shows the 572 K values using method (1) and BKPW parameters. Method (2) involved the direct fitting ofempirical equations for the theoretical fall-off 23 FAR I682 UNIMOLECULAR DECOMPOSITION C,H3Cl, -+ C,H,Cl+Cl TABLE 3.-A am), Earn) VALUES HTTING EXPERIMENTAL DATA AND FORST CALCULATIONS.UNLESS NOTED OTHERWISE, METHOD (1) AND BKPW PARAMETERS WERE USED. 52 1 534 547 560 572 572" 572b 12.85 12.90 12.60 12.60 12.80 12.70 12.80 74.2 73.8 71.9 71.9 73.0 72.3 73.4 17.7 17.6 17.2 17.2 17.4 17.3 17.5 a Method (2) and BKPWs parameters; method (2) and SR7 parameters. curves to the experimental k i p ) , p points. No simple function will describe these curves over a wide range of pressures. However, quartic polynomials in p , developed over the range 1-250 Torr, could be fitted accurately to k$p)(calc) over the more limited range 5-1 50 Torr for each pair of A am), Eim) values. The sum of the squares of the residuals (s.s.r.) of the experimental points from the quartic curve could then be computed, and a small network of A Ei") pairs was identified with the lowest s.s.r.An additional discrimination was obtained by examining the distribution of the signs of the residuals over the pressure range. The results for 572 K, shown in table 3 for both BKPW and SR parameters, are very close to those found by method (1) or the simple method. Method (1) was applied, using the BKPW parameters, at four other temperatures with results also shown in table 3. Giving equal weightings to these values at the five temperatures, the mean values found for the Arrhenius parameters are Eim) = 72.9 f 1.1 kJ mol-1 = 17.42 f 0.26 kcal mol-l and i.e. log,,(A $")/s-') = 12.75 & 0.25 17420 4.576 T/K' 10glo(k$")/s-l) = 12.75 - In fig.3 the values of 6 $ p ) are plotted for each temperature with the theoretical Forst curve (full line) appropriate to the overall mean A $"), Ei") values. The overall pattern from fig. 3 indicates that the mean Arrhenius parameters Aim) and Earn) give a reasonable fit to the fall-off at all five temperatures. However, they are much lower than the values found by Huybrechts et aL9 for fairly high pressures [E, = 20.7 kcal mol-1 and log,,(A,/s-l) = 14.331. Moreover, eqn (XI) predicts values of kam) that lie below those found experimentally by Huybrechts et al. until very low temperatures are reached, as illustrated in fig. 4 by the positions of the lines (XI) and (XII)g 20 700 4.576 T / K ' 10glo(k$"~/S-') = 14.33 - This conflict might arise from errors in our fitting procedures, which is unlikely because of the cross-checks from different procedures, or from systematic errors in the experimental results (those of Huybrechts et aL9 and our own1) or from the acknowledged3 decrease in accuracy of the, Forst method at lower pressures.Incorpor-P. G. ASHMORE, A. J. OWEN A N D P. J. ROBINSON 683 FIG. 3.-Calculated Forst curves for the mean Aim), Egm) optimum values at the five temperatures (see table 3) with mean kip) for 0 572; 0, 560; V, 547; A, 534 and 0, 521 K. 7.0 6.0 n I \ I 4 v 2 M - 5.0 4 .O 1.6 1.8 2.0 2.2 2.4 lo3 KIT FIG. 4.-Arrhenius plots of k, from the experiments of Huybrechts et a1.O (XII), of /cia)) from our Forst from our RRKM calculations (XIII). calculations (XI) and of 23-2684 UNIMOLECULAR DECOMPOSITION C,H,Cl, + C,H,Cl+Cl ation of the centrifugal effect would make only minor changes in the calculations.The Forst estimates of ki") suggest that our experimental pressure range lies just below pi, where the Forst method would still be reliable. On the other hand, extrapolation of the Huybrechts results to our temperatures indicates k, values well above the predicted Forst ki"), which would mean that our experimental pressure range is well below the true pi. It is therefore possible that the discrepancies result from inaccuracies of the Forst method under these conditions. A reaction of the same kind as (2) is the decomposition of the ethyl radical for which the experimental value of k is given by log,,(k/s-l) = 14.4-31 800 K/4.576T [ref.(lo)] log,,(k/s-l) = 13.6- 32400 K/4.576T [ref. (1 l)]. or Benson12 has pointed out that for the ethyl radical there is a positive contribution to AS+- of 3.6 cal mol-1 K-l by loss of symmetry; this loss does not occur in forming the complex from C2H3C12, and for reaction (2) A S z might be 3.0 cal mol-1 K-l lower, i.e. A , might be expected to lie between 1013.6 and lo1,.* s-l. This appears to favour lower A("), E(") values. On the other hand, the higher A,, E, values from Huybrechts' relatively high-pressure experiments appear to fit well with accepted bond energies (such as R-CHC1-H where R = CH,, CH,Cl, CHCl,, CCl,) and the estimated standard (1 atm) entropies of the radicals R.C.HCLg We therefore conclude that our estimation of A$"), Ei") values from the fall-off curves using Forst calculations is not satisfactory, and it is desirable to see whether full RRKM calculations give a better description of our own and the Huybrechts results.RRKM CA LCU L A T I 0 NS Full RRKM calculations were first done for 572 K neglecting the centrifugal effect. A series of activated complexes were constructed to give selected values of Aim) at 572 K. Details of the assignments are not critical since RRKM fall-off calculations are insensitive to the detailed structure of the activated complex, the shape and position of the fall-off curve being determined essentially by the resulting AS+- and by Pa) [ref. (13), pp. 152 and 178].* State sums for the complexes were generated by exact count and densities for the radical by the Whitten-Rabinovitch method.14 and values of ELm) were investigated to locate those that gave fall-off curves close to the experimental mean values kip), using the procedure tested during the Forst calculations.Fig. 1 (b) shows pairs of Aim), values that gave calculated k i p ) in agreement with the experimental values for p = 100, 25 and 10 Torr. As can be seen, the intersection indicated Aim) and Eb") values substantially higher than in the Forst calculations, uiz. close to logl,(A$"3)/s-1) = 14.0 and Eim) = 19.6 kcal mol-1 at 572 K. These parameters define complex G (table 2). The differences between kip) values calculated for this complex and the individual experimental values are expressed in table 4 as the sum CRlp) of the residuals Rip) = [k&P)(exp) - k$p)(model i)] and the sums C Vi(p) of the corresponding variances for the experimental pressure ranges at 572 K.The results of applying a t-test at the 1% level of significance are also shown; model G predictions are not significantly Suitable complexes ( A AS$ In (Akm)/s-') = In [(ekT/L)/s-']+----. R * Note Ekm' = E,,+RT+(E#)-(E) andP. G. ASHMORE, 1. J . wi +I 4 ]WEN A N U l'. J. K U B I N S U N M m m m m m m E 000000 2 ooooo+ 68 5686 UNIMOLECULAR DECOMPOSITION e2H3cl2 -+ C2H3C1+c1 different from the experimental results in each range, but are on the borderline for the lowest pressure range. Table 4 shows that model H, with higher Aim) and ESm) than G, has residuals that are more positive than those of G at the lower pressures, whereas model F, with lower Aim) and Eim), has residuals that are more negative.The t-tests also show that models F and H give less satisfactory fits to the experimental results at 572 K than does G. FIG. 5.-RRKM calculations of kip) for model G with mean kip) for 0 , 5 7 2 ; 0 , 5 6 0 ; V, 547; 534 and 0, 521 K. To investigate how G and other models behave at other temperatures, E, for each model was kept constant and Elm), Aim) were calculated using the equations in the footnote, page 684. The calculations for model G are compared in fig. 5 with the experimental means (shown with the number of contributory runs). At 572 K, the model gives an excellent fit at all except the lowest point. At 560 and 534 K it gives good fits, although a model with slightly lower parameters (but not as low as those of F) gives better fits.At 547 K model F is better than G, and a model with even lower E,, A would fit still better; at 521 K a model with slightly higher E,, Aim) is better than G. In view of the minor scale of these differences, the choice of a single representative model would fall on G, admitting that a small range of values around EJG) (say 16.6kO.3 kcal mol-l) and a correlated (compensating) range of its l~g,,[A(~)(G)/s-~] (say 14.0 If: 0.2) would give very similar fits. Having decided the preferred model without allowance for the centrifugal effect, it remains to match with its predictions a model allowing for centrifugal effects. This is more important than with the Forst calculations, as the larger A-factor suggests that the activated complex is ‘looser’ than had appeared from the Forst value of the A-factor.Simple models for the radical and activated complex suggested that I # / I would probably be ca. 2. A range of I z / I values from 1 to 4 was examined using the approach of Waage and Rabin~vitch.~ The factor bR was calculated for the radical from [for symbols, see ref. (1 3), p. 911 (s- l ) ( I + / I - l)kT)-’ E, + aE,P. G. ASHMORE, A. J. OWEN A N D P. J. ROBINSON Z 7.0- 6.0- 5.0- 4.0- 3.0- 2.0- 1 .o- 0 - 1 60 687 WI vr 2 1 3 to give the values: I + / I 1 1.5 2.0 3 .O 4.0 %R 1 .oo 0.72 0.56 0.39 0.29. The computed fall-off curve for a given Aim), ELm) with no centrifugal effects was then treated as follows: (a) k i p ) was multiplied by P / I , (b) the pressurep was divided by GR and (c) Aim) correspondingly became Aim) x I # / I , with Eim) unchanged.For some of the calculations, the starting data were generated by interpolation between models F, G and H rather than by explicit RRKM computations. After trial comparisons with the experimental results for 572 K, attention was focused on the models X I . . .Z, listed in table 2 with I # / I = 2.0. Note that the final line gives the final A i"), i.e. the initial A im) x I # / I . The predicted curves at 572 K for the nine models are shown with the experimental means in fig. 6. In each triplet the centre curves pass very close to the experimental 0 20 40 60 80 100 120 140 PDCE/TO~ - 1 FIG. 6.-Fall-off curves at 572 K for models X, Y and Z which allow for centrifugal effect with I f / l = 2.0. Parameters of the models are given in table 2.688 UNIMOLECULAR DECOMPOSITION C,H,Cl, + C,H,Cl+t]l means at p = 101 Torr (kip) = 5.30 x lo5 s-l).However, closer inspection shows that curve Y, is a close fit to all the experimental means; curve Z, passes very close to the experimental mean at p = 101 Torr but lies well below the experimental means at p < 100 Torr and above them at p > 100 Torr; curve X, lies above the experimental means at p < 80 Torr, and below them at p > 80 Torr. This is shown more quanti- tatively in table 5, which lists the residuals [6$P)(exp)-kip)(model i)] for each mean pressure. The residuals for Y, are in all cases very much smaller than the standard errors of the experimental means; for Z,, the residuals are very much larger than the standard errors of the mean; for X,, they are comparable or larger and there is a systematic change in sign along the curve.Application of the t-test to the lower pressure ranges gives the significance results at the 1 % level shown in table 5, and these confirm that model Y, is to be preferred to models X, and 2, for representing the experimental results. TABLE 5.-EXPERIMENTAL MEANS p AND Eip) WITH THE RESIDUALS OF THE MEANS ([6ip'(eXp) - kLp'(mode1 i)]/103 s-l> FOR RRKM MODELS X,, Y, AND Z, WHICH ALLOW FOR CENTRIFUGAL EFFECTS. THE RESIDUAL ARE INDICATED AS SIGNIFICANT (s) OR NOT SIGNIFICANT (ns) USING THE GTESTS AT THE 1 % LEVEL OF SIGNIFICANCE. 572 K, VESSEL B. residuals of means/103 s-' 19 9.0 f 0.08 0.92 f 0.03 - 12.0 s 0.0 ns + 14.0 s 18 22.8 & 0.33 2.04 f 0.05 - 1.4 ns + 1.4 ns +18.3 s 19 36.5 f 0.29 2.82 & 0.03 -9.8 s 0.0 ns +10.5 s 63.4 f 0.61 4.03 & 0.08 -4.2 ns 0.0 ns + 11.2 ns 24 5.30 f 0.09 +2.8 ns 0.0 ns + 1.4 ns 23 101.2 & 0.58 1 1 134.6 f 0.90 6.13 f 0.13 +4.9 ns -4.9 ns -14.8 ns We conclude that the preferred model after allowing for centrifugal effects is Y, with Eim) = 20.0 kcal mol-l and A $") = 1.8 x 1014 s-l; other models with parameters close to these, but with A$") and Eim) compensating, would give similar fits.Rather surprisingly, a similar study of models with I it / I = 4.0 pointed to parameters close to those of Y,, with EL") = 20.0 kcal mol-l and Aim) = 1.7 x 1014 s-l. The Arrhenius parameters for Y, lead to the prediction 20 000 4.576 T / K 10glo(k~")/S-l) = 14.26 - (XIII) and this is plotted as line (XIII) in fig.4. The Arrhenius plot for model G would lie slightly below line (XIII). DISCUSSION OF THE RRKM RESULTS Line (XIII) in fig. 4, and that for model G, lie well above line (XII) for the results of Huybrechts et aZ.9 Their results were obtained in the pressure range ofp,,, between 150 to 450 Torr. The general position of their points and of line (XII) are very satisfactory in relation to the RRKM plot (XIII) - much more so than with line (XI) from the Forst calculations. They did not report any fall-off, but the predicted fall-off is, of course, much smaller at the lower temperatures of their work, e.g. at 454 K k&p)(calc) falls by ca. 25% when p changes from 450 to 150 Torr; at 572 K, kip)(calc)P. G. ASHMORE, A.J. OWEN A N D P. J. ROBINSON 689 changes by a factor of nearly two. There is slight evidence for a fall from the spread of their results at 490 K, and rather stronger evidence from their (few) results at 510 K. It is also possible that photolytically generated C1 atoms, comprising states, thermal kip). It therefore appears that the RRKM calculations not only fit our own experimental observations' at five temperatures, but can also resolve the apparent major conflict between our early results and those of Huybrechts et aL9 What appeared to be systematic errors in one or both of the experimental determinations of k,/k, from the thermal- or the photo-sensitised decompositions are now seen to be a natural consequence of the unimolecular behaviour of k, at different pressures.Our evaluation of A $a) and Eia), and the statistical treatment of our experimental data, have of necessity ignored possible systematic errors, for example in the experimental evaluation, of k, or in the parameters quoted in table 2. The direct experimental investigation of kip) at high pressures, in the absence of competitive chlorination, is very difficult as evidenced by earlier work referenced in Part 1. The thermal decomposition of l,2-C,H4C1, in the absence of chlorine shows complicated rate relationships (as referenced in Part 1) and cannot provide a check on k i p ) at high pressures. A further check on our evaluation of Aim) and Eim) may be provided by applying the methods advocated by Troe15 and this will be set in motion. In the meantime the satisfactory explanation based on RRKM theory of experimental results from different laboratories over a wide range of temperatures would seem to justify our evaluation of Eia) within a few kJ mol-l, with corresponding limits on Aim), and the resulting eqn (XIII).Some further support comes from consideration of the rate constant kL$) for the reverse reaction (- 2). ARRHENIUS PARAMETERS FOR THE REACTION (-2) Our experimental investigations' of k-, through the inhibitory effects of VC on the decomposition of DCE clearly showed that k-, is pressure-dependent and increases with increase in temperature. These results did not allow accurate determinations of A L$) and ELg), but by combining them with data from studies of reaction (- 2) at lower temperatures, reasonably accurate values of the Arrhenius parameters of reaction (- 2) and their variation with pressure can be obtained.Reaction (- 2) has been studied experimentally in the course of extensive investi- gations of the chlorination of ethylene and the chloroethylenes. Knox and WaughlG found difficulties with heterogeneous reactions in studying v c + a -+ R and produce excited k in reaction (l), which results in a k, value rather f arger than the but evaluated k-, indirectly as 3.5 x 1O1O dm3 mol-' s-' at all temperatures. Ayscough et al." quoted an earlier value18 (1010.2 dm3 mol-1 s-l) but preferred their own assessments over the range 20-50 OC, with low pressures of VC around 5-30 Torr, of loglo(k~,/dm3 mol-' s-l) = 10.3 - 1500 K/4.576 T. In a theoretical study of several related reactions, BKPW5 calculated rate constants for the following detailed mechanism (using our nomenclature) : Cl+VC 5 R* kbi R*+M R+M R* 2 vc+c1 which combine to give d[kl - kbi [vcl dt ka + kbi[MlTABLE 6.-CALCULATED AND EXPERIMENTAL VALUES OF k-, AT VARIOUS CONCENTRATIONS AND TEMPERATURES equivalent calculated experimental pressure loglo(k~;b~e)/dm3 mol-l s-l) l ~ g ~ ~ ( k l _ a j ~ ~ ~ ) / d r n ~ mol-1 s-l) ~ ( a .0 ) A ( a , b , c ) M/10-3 mol at 560 K h:al fik3 label dmF3 /Ton 308 K 352 K 406 K 520 K 544 K 570 K 595 K mol-l mol-l s-l - - - - - - BKPW +co +co 9.51 9.64 9.69 2.87 100 9.43 9.53 9.54 9.74 9.83 9.85 9.88 1.37 1010.38 109.98 (4 (b) (c) 1.60 55 9.38 9.48 9.48 9.61 9.64 9.66 9.69 0.80 0.73 25 9.32 9.4 1 9.39 9.50 9.51 9.49 9.56 0.46 109.70 c) "3: 4P.G. ASHMORE, A. J . OWEN A N D P. J. ROBINSON 69 1 and hence From this expression it is clear that kL$) = A , and BKPW gave the values of A at 308, 352 and 406K shown here in the top line of table 6 . They also gave kbi = 1.23 x dm3 mo1-l s-l at 352 K, from their fig. 5 it is possible to estimate k , at the three temperatures and at chosen pressures or concentrations, so that kL%) can be calculated for their temperatures from eqn (XIV). The results of these calculations are listed in table 6. It seemed more appropriate to consider constant concentration rather than constant pressure, in view of the wide temperature difference between those calculated and our experimental results. Accordingly, kLadbJ) was calculated for three concentrations which are equivalent to pressures of (a) 100, (b) 55 and (c) 25 Torr at 560 K, the mid-temperature of A.J. O.'s experimental values which are given in the right half of table 6. Before comparing the calculated and experimental results, a fundamental difficulty must be mentioned. It arises from the complex relations between the reversible reactions (2, - 2) and the function of DCE and VC as collision partners (M). In studies of the photochlorination of VC, VC itself is usually the effective collision parameter (chlorine is less effective); in studies of the thermal decomposition of DCE, DCE is the effective collision partner. Our experiments summarised in fig. 9 of Part 1 show that VC and DCE are of very similar but not identical efficiency as M. Unfortunately we can see no rigorous way of allowing for this difference in the data at present available for reaction (2) and for reaction (- 2), so that our comparisons that follow are limited by that consideration.When the results in table 6 are plotted as an Arrhenius diagram (fig. 7) the points for each concentration are reasonable fits to straight lines. The activation energy E-, 9 . 0 t , , , , 2.0 2.5 3.0 3.5 1.5 1 O3 KIT FIG. 7.-Arrhenius plots of kL$) for concentrations that are equivalent to (a) 100 Torr at 560 K with A (this work) and from BKPW;5 (c) 25 Torr at 560 K with (this work) and dfrom BKPW;5 ( d ) very high pressures, estimated from this work, with + from BKPW5 table 3. etc. from BKPW;5 (6) 55 Torr at 560 K with 0 (this work) and692 UNIMOLECULAR DECOMPOSITION C2H3C12 + C2H3Cl+C1 falls as the concentration falls, and so does the pre-exponential factor A-,.These changes would be expected from the corresponding falls in E, and A , as the pressure is lowered, by the requirements of microscopic reversibility. As a result eqn (XV) and (XVI) bear the same relation to each other as do eqn (VIA) and (VIIA) [derived from the eqn (VI) and (VII) of Part 1 using the Arrhenius expression2 for k,] 1370 (100 Torr) 4.576 T/K loglo(k-,/dm3 mol-1 s-l) = 10.38 - 460 (25 Torr) 4.576 T/K loglo(k~,/dm3 mol-1 s-l) = 9.70- 10g~~(k,/~-~) = 12.40 - 4.576 420 T/K (1 00 Torr) (VIA) 16470 (25 Torr). (VII A) 4.576 T/K loglo(k2/~-l) = 1 1.61 - These equations for kL%) and k i p ) have been derived from quite separate experiments and separate calculations. They correspond within very close limits to an equilibrium constant for reactions (2, -2) given by eqn (VXII), where Keq = k,/k-, for R+VC+Cl 16 000 4.576 T/K logl,(Keq/mol dmb3) = 2.00 - (XVII) If the concept of microscopic reversibility can be extended to kim) and kLT) then by combining eqn (XVII) with the Arrhenius equation for ki") there emerges eqn (XVIII) 4000 4.576 T / K log (k'lf',/dm3 mol-1 s-l) = 12.26 - (XVIII) This equation gives the line ( d ) in fig. 7.It passes close to Knox's calculated kLT) at the lower temperatures (+). The large changes in A+, E-, with increasing pressure are of course, reflections of the correspondingly large changes in A,, E, over the same pressure range. It may be recalled that the values of E&lo0) and were derived from RRKM calculations that fit the experimental results at these pressures; however, they are substantially independent of whether one chooses model G, or the model Y, with centrifugal effects. On the other hand, ELm) does depend on which model one uses, but the difference Eim)(Y,) - Eim)(model G) is only 400 cal mol-l, and if the Arrhenius equation for kim) for model G were used in place of eqn (XVIII) the plot would be only very slightly displaced from the line ( d ) in fig. 7. P. G. Ashmore, J. W. Gardner, A. J. Owen, B. S. Smith and P. Sutton, J . Chem. SOC., Faraday Trans. I , 1982, 78, 657. F. S. Dainton, D. A. Lomax and M. Weston, Trans. Faraday SOC., 1962, 58, 308. W. Forst, J . Phys. Chem., 1972, 76, 342. M. R. Hoare and T. W. Ruijgrok, J. Chem. Phys., 1970, 52, 113. E. V. Waage and B. S. Rabinovitch, Chem. Rev., 1970, 70, 377. P. C. Beadle, J. H. Knox, F. Placid0 and K. C. Waugh, Trans. Faraday SOC., 1969, 65, 1571. ' G. B. Skinner and B. S. Rabinovitch, Bull. SOC. Chim. Belg., 1973, 82, 305. L. Szirovica and R. Walsh, J, Chem. SOC., Faraday Trans. I , 1974, 70, 35. G. Huybrechts, J. Katihabwa, G. Martens, M. Nejszaten and J. Olbregts, Bull. SOC. Chim. Belg., 1972, 81, 65. lo L. F. Loucks and K. J. Laidler, Can. J. Chem., 1967, 45, 2795.P. G . ASHMORE, A. J. OWEN AND P. J. ROBINSON 693 l1 M. C. Lin and M. H. Back, Can. J . Chem., 1966,44, 505, 2357. l3 P. J. Robinson and K. A. Holbrook, Unimolecular Reactions (Wiley Interscience, New York, 1972). l4 G. Z. Whitten and B. S. Rabinovitch, J. Chem. Phys., 1963, 38, 2466. l5 J. Troe, Ber. Bunsenges. Phys. Chem., 1974, 78, 478. l6 J. H. Knox and K. C. Waugh, Trans. Faraday Soc., 1969, 65, 1585. l7 P. B. Ayscough, F. S. Dainton and B. E. Fleischfresser, Trans. Faraday Soc., 1966, 62, 1838. S. W. Benson, Thermochemical Kinetics (Wiley, New York, 1968), pp. 67 and 68. G. Chiltz, P. Goldfinger, G. Huybrechts, G. Martens and G. Verbeke, Chem. Rev., 1963, 63, 355. (PAPER 0/1987)

ISSN:0300-9599    

DOI:10.1039/F19827800677

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1982, 78, 695-71 1 Reactivity of Semiquinone Radicals and its Relation to the Biochemical Role of Superoxide BY HARRY C. SUTTON* Institute of Nuclear Sciences, Department of Scientific and Industrial Research, Lower Hutt, New Zealand AND DAVID F. SANGSTER Australian Atomic Energy Commission Research Establishment, Sutherland, New South Wales 2232, Australia Received 16th February, 198 1 Semiquinone radicals derived from 9, 10-anthraquinone-2-sodium sulphonate (AQS), menadione (MD), duroquine (DQ) and 2,5-dimethylbenzoquinone (DMBQ) by pulse radiolysis of aqueous solutions containing propan-2-01 and quinone are shown to reduce methaemoglobin and cytochrome c at rates which greatly exceed those of the corresponding reductions by superoxide. Reasons for this are suggested.Rate constants are reported for these and related reactions, including dismutation of the semiquinones. From studies of the y radiolysis of similar quinone-haem solutions containing oxygen, it is shown that the equilibrium 0; +Q e 0, + Q- (4) is maintained in these systems, and in similar ones in which superoxide is generated by enzymes, thereby converting superoxide to the more reactive radical, semiquinone. Addition of superoxide dismutase to these systems suppresses reactions caused by semiquinone, a conclusion of current biochemical significance. Measurements of the equilibrium constant K4, and hence of redox properties of the semiquinones, have been obtained from studies of this suppression, and agree with previous measurements by other methods.The presence of the enzyme superoxide dismutase (SOD) in aerobic cells in which superoxide is produced is generally taken as evidence of some harmful property of superoxide which is prevented or controlled by this enzyme.l However, although superoxide can act as a reducing agent 0, +e- G 0; (1) with Eo = -0.155 V, or as an oxidising agent 2H+ + 0; + e- f H,O, with Eo = 0.92 V at pH 7, it is in fact chemically rather unreactive and, for example, reduces the haem groups in methaemoglobin (metHb) 0; +metHb + oxyHb (2) or oxidises those in oxyhaemoglobin (oxyHb) with rate constants of only ca. 4 x lo3 dm3 mo1-1 s-l at pH 7.2 These reactions are so slow that the uncatalysed dismutation of superoxide with 2k, = 9 x lo5 dm3 mol-l s-l at pH 7.23 usually occurs more rapidly.2H+ + 20; -+ H,O, (3) It is possible, therefore, that superoxide may be a biochemical precursor of more 695696 REACTIVITY OF SEMIQUINONE RADICALS reactive species, and Winterbo~rn~.~ has suggested that these may include semiquinone radicals formed from superoxide and naturally occurring quinones in the rapid (4) equilibrium reaction The role of superoxide dismutase might be viewed then as one of indirect control of reactions caused by free radicals such as semiquinones. The experiments described below were designed to investigate these possibilities, using the rate of reduction of metHb and of cytochrome c (cyt c) as measures of reactivity. Superoxide reduces cyt c 0; + Q e Q- +O,. 0; +cyt c + reduced cyt c+O, ( 5 ) with k , = 1.1 x lo6 dm3 mol-1 s-l at pH 7.2.' In the first part of this paper we report measurements by radiation-chemical methods of the yields and rates of relevant reactions of semiquinone radicals in oxygen-free solution; the second part uses these data to interpret effects observed in oxygen-containing solutions in which equilibrium (4) is maintained.EXPERIMENTAL Methaemoglobin prepared as described previously2 was treated with iodoacetamide to prevent its sulphydryl groups from reacting with q~inones,~ and was dialysed before use. No significant differences were observed in the results from four such preparations. Horse-heart cytochrome c (Sigma type VI), superoxide dismutase from bovine erythrocytes, ox-liver catalase, menadione and duroquinone were obtained from Sigma Chemical Co.; 2,5- dimethylbenzoquinone was supplied by ICN Pharmaceuticals. The results obtained with menadione (Sigma or Fluka) and 9,1O-anthraquinone-2-sodium sulphonate (B.D.H.) were not influenced by prior recrystallisation from ethanol and water, respectively, nor by addition of aqueous solutions of the corresponding hydroquinone, prepared by electrolytic reduction of the quinone. Sunlight and ultraviolet radiation (1 < 400 nm) strongly affected the quinone solutions and were excluded. Solutions were prepared from water redistilled from alkaline KMnO,, buffered to pH 7.2 with AnalaR Na,HP04 (10 mmol dm-3) and KH2P0, (5.5 mmol dm-3). Propan-2-01 of B.D.H. spectroscopic grade and B.D.H. (AnalaR), found analytically to be free of peroxides, gave identical results.Acetone (B.D.H.) and sodium formate (Merck) were of AnalaR grade. The molar extinction coefficients per haem group of our haemoglobin solutions at pH 7.2 were determined at 576,555 and 630 nm as follows. An aerated solution of metHb was reduced to deoxyHb in a filled and stoppered spectrophotometer cell by injecting freshly prepared oxygen-free Na2S204 solution to ca. 600 pmol dm-3 concentration. The deoxyHb was converted to oxyHb by opening to air, subsequently to metHb by adding a crystal of K,Fe(CN), and finally to ferrihaemoglobin cyanide by adding KCN. Spectra at each stage were reproducible provided that [Na,S,04] < 1 mmol dm-3 and led to the following data, based on 6540 = 11.4 x lo4 dm3 mol-l cm-l for ferrihaemoglobin cyanide:* E,,,(metHb) = 4000 dm3 mol-l cm-l; Acdm = &,,,(deoxyHb) -&,,,(metHb) = 8900 dm3 mol-l cm-l; Acorn = &,,,(oxyHB) -E5,,(metHb) = 10600 dm3 mol-l cm-l.These were checked for consistency by analysing the products of an irradiated oxygen-free solution for [oxyHb] after admitting air. The result (G = 5.8) was the same as in normal analysis for [deoxyHb]. Analyses of cyt c were based on measurements at the sharp 550 nm peak of reduced cyt c, adopting Acre = &(reduced cyt c) -&(cyt c) = 2 1000 dm3 mol-l cm-l for a conventional spectrophotometer and 20000 dm3 mol-l cm-l in pulse-radiolysis studies, because of the wider slit-width employed. Concentrations of superoxide dismutase (SOD) are quoted on the assumption that our preparation was 100% pure with Mr = 32200. Its absolute activity was determined as described in part I1 of the Results and Discussion section.Oxygen concentrations estimated from solubility data at 2 1 & 2 "C were 270 or 1300 pmol dm-3 for solutions equilibrated with air or oxygen. Oxygen-free protein solutions were prepared either by evacuation with repeated cooling and thawing cycles or, forH. C. SUTTON AND D. F. SANGSTER 697 pulse-radiolysis studies, by prior bubbling with oxygen-free nitrogen or argon to give an estimated concentration < 0.1 ymol dm-3. The spectra of initially oxyHb solutions so treated or of radiolysis products from metHb confirmed [O,] < 1 pmol dm-3. Irradiations with ‘j0Co y rays were performed in a ‘Gammacell’ unit at dose rates measured by the Fricke dosimeter of generally 1.3 Gy s-l.The pulse radiolysis facility at the Australian Atomic Energy Commission’s Research Establishment consisted of a 1 MeV Van de Graaff electron accelerator delivering single pulses (3.4 p s ) of fast electrons through the thin wall (0.3 mm) of a ‘Suprasil’ cell with an optical path length of I cm. A remotely operated peristaltic pump provided fresh solution for each pulse. Concentrations of light absorbing species were monitored by a beam of light from a 250 W high-pressure xenon lamp, filtered to remove U.V. This beam traversed the irradiated portion of the solution before entering a Bausch and Lomb high-intensity monochromator with a 250-700 nm grating. Changes in light transmission were measured with a 1P28 photomultiplier and then amplified and processed by a Biomation 610 B analogue-to-digital converter and storage unit, and a PDP 1 1 /03 minicomputer.The pulse trigger/amplifier unit incorporated variable delays, signal back-off and sample-hold facilities to enable the initial light level to be measured with minimum exposure of the solution to light. Absorbance values and kinetic rate constants could be readily calculated by the software program developed by Thornton and Laurence.l0 The dose per pulse was monitored by the current induced in a loop of wire near the end of the accelerator flight tube, and displayed on a storage cathode-ray oscilloscope. This sensor was calibrated before and after each series of experiments using an aerated solution of KCNS (5 mmol dm-3) as dosimeter, and taking &(CNS); = 7600 dm3 mol-l cm-l at 480 nmll and G(CNS), = 2.8.Calibrations usually agreed within f 10% but individual estimates of dose per pulse could occasionally be in error by f 20%. The doses ranged from 5 to 20 Gy per pulse and were generally 10 Gy. RESULTS AND DISCUSSION I. OXYGEN-FREE SOLUTIONS Semiquinone radicals were produced by the method of Pate1 and Willson12 by irradiating oxygen-free aqueous solutions of propan-2-01, acetone and quinones in which the following reactions occur at pH 7. (Literature values13 of rate constants are given in units of dm3 mol-1 s-l.) H,O -+-+ eLq, H, OH CH3COCH3 + eiq + (CH3)2 CO- k, = 6 x lo9 (CH3)2 CHOH + OH + (CH3)2 COH + H,O k , = lo9 (CH3)2 CHOH + H + (CH3)2 COH + H, k, = 5 x lo7 (6) (7) (8) (CH,),COH (CH,), COP+ H+ pK = 12.2 l4 (9) 2(CH3)COH -+ products (10) (CH3)2 COH + Q + Q- + CH3COCH3 + €3’ Q+e- aq +Q- 2H+ + 24- -+ Q2-+ QH,.In the presence of methaemoglobin or of cytochrome c, the following reactions also (14) occur : (15) (CH,),COH +metHb --+ CH3COCH3 + H+ + deoxyHb (CH3)2 COH + cyt c + CH3COCH3 + H+ + reduced cyt c698 REACTIVITY OF SEMIQUINONE RADICALS Q-+metHb + Q+deoxyHb Q-+cyt c + Q+reduced cyt c. A. STEADY-STATE 7 RADIOLYSIS STUDIES For these studies, evacuated or nitrogen-bubbled solutions of metHb (40- 100 pmol dm-3 haem concentration) were prepared in phosphate buffer at pH 7.2 containing a standard mixture of propan-2-01 (0.2 mol dm-3), acetone (0.005-0.01 mol dm-3) and catalase (6 mg dm-3) to suppress slow thermal reactions of radiolytically generated hydrogen peroxide.MetHb was reduced forming deoxyHb in concentrations directly proportional to radiation dose up to > 60% reduction, and reduction continued to completion on prolonged irradiation. The initial yield was 5.8 f 0.3 haem groups reduced per 100 eV, which agrees within experimental uncer- tainties with the primary yield of water radicals, for which G z 5.8 is generally accepted.15 The yield of reduction was not appreciably altered by adding the chelating agent EDTA (1 00 pmol dm-3) before irradiation to remove possible metallic impurities or by omitting catalase or acetone (implying that solvated electrons reduce metHb nearly as efficiently as propan-2-01 radicals do), but was approximately halved by omitting propan-2-01. Similarly, oxygen-free solutions of cyt c (20-60 pmol dm-3) were also reduced with the same yield, i.e.G( -cyt c) = 5.8 & 0.3. Similar behaviour and yields were observed on irradiating oxygen-free solutions of cyt c in the standard solvent containing, additionally, 100 pmol dm-3 concentrations of the quinones AQS, DQ, MD or DMBQ. The cyt c in all of these was reduced with the initial yield G = 5.8 & 0.3, in the presence or absence of EDTA. MetHb-quinone solutions gave identical results except for DMBQ, for which a slow thermal reaction occurred in oxygen-free solutions a few minutes after irradiation, causing reoxidation of radiolytically produced deoxyHb : 2H++Q+2 deoxyHb --+ 2 metHb+QH,. This reaction is thermodynamically favourable for DMBQ and deoxyHb (AE = +0.04 V at pH 7) but not for cyt c (AE = -0.07 V) or for any of the other quinones studied, all of which were stable in the solutions used for 10 min or more.This reaction was not observed in pulse-radiolysis studies (complete in < 1 s) or in the presence of oxygen. B. PULSE RADIOLYSIS: FORMATION A N D DISMUTATION OF SEMIQUINONES From studies of the pulse radiolysis of oxygen-free solutions of quinones in propan-2-01 (1 mol dm-3)-acetone (I mol dm-3) solutions, Pate1 and Willson12 showed that semiquinone radicals are produced from propan-2-01 radicals in reaction (1 1) in yields assumed to be 6 per 100 eV with rate constants of 4.2, 4.0 and 3.9 x lo9 dm3 mol-1 s-l, respectively, for MD, DQ and DMBQ. These rates were confirmed within 40 % or less under our conditions for protein-free quinone solutions of concentration 100 or 300 pmol dmW3 in the standard solvent at pH 7.2 described in section A above.For AQS we found k,, = (3+ 1 ) x lo9. Yields of semiquinones observed 50 ps or less after a pulse were in the range 5.0-5.6; these were calculated from the following literature values of their extinction coefficients; 12 500 dm3 mol-l cm-' for MD at 390 nm,12 6800 for DMBQ at 435 nm,12 7600 for DQ at 440 nm,12 8000 for AQS at 390 nm and 7900 at 505 nm.16* l7 These yields are less than the value G = 6 assumed in most of the measurements of the E values quoted above, and this is partly attributed to dismutation within the first 5 0 ~ s . In later experiments, the yield G = 5.8 has been assumed when estimating concentrations of semiquinone precursors (i.e.propan-2-01 radicals) formed during a pulse.H. C. SUTTON A N D D. F. SANGSTER 699 The dismutation of semiquinones, measured from the decay of absorption at the peak wavelengths, was second-order for > 90% of the decay. For MD the radiation induced absorption decayed to zero, but for AQS at 505 nm, DQ at 440 nm and DMBQ at 435 nm a small permanent absorption remained (presumably due to dismutation products), equivalent to 6% or less of that initially induced. Values of 2k13 were calculated adopting the E values given above with a small correction for the residual absorption; these are listed in table 1. The results were reproducible within TABLE ~.-SEMIQUINONE AND SUPEROXIDE RATE CONSTANTS AT pH 7.2 2k13 k1,b k17b semiquinone" E,"(Q/Q-)/V /dm3 mol-l s-l /dm3 mol-l s-' /dm3 mol-l s-' AQS- -0.380 (1.8f 1) x losc 9~ 107 1 .9 ~ 109 DQ- -0.235 (2.3 f0.6) x losd 5.5 x los 1.2 x lo8 DMBQ- - 0.074 (1.2f0.3) x lo8 3 x 105 L O X 107 MD- -0.203 (0.4f 1) x lose 1.5 x lo7 2 . 7 ~ los EO(O,lO,) 2k3 k2 k5 super oxide -0.155 9~ 105 5 x 103 1 x lo6 a In aqueous solution with propan-2-01 (0.2 mol dm-3); error z 30%; literature 1.5 x lo8 in 0.6 mol dm--3 propan- values are 3.2 x log at pH 1.717 and 1.3 x lo9 at pH 4;27 2-01; e 1.6 x lo8 in 0.6 mol dm-3 propan-2-01. ca. 30% for three-fold variations of dose per pulse and three-fold variations of quinone concentration. For AQS, the residual absorption at the second peak at 390 nm was much larger and most weight was given to the data at 505 nm. Increase of propan-2-01 concentration reduced 2k,, appreciably, as shown in table 1 .c . HAEM REDUCTION BY PROPAN-2-0L RADICAL: REACTIONS (14) AND (15) After irradiating a solution of metHb in the standard solvent at pH 7.2 with a pulse of electrons, deoxyHb was produced as shown by increase of absorbance at 555 nm and a concomitant but smaller (ca. 5) decrease at 630 nm. The growth of absorbance at 555 nm was approximately exponential and corresponded to a pseudo-first- order rate constant, R, (obtained from computerised analysis of the data), of 1.1 k0.2 x lo4 s-l for 38 pmol dm-, metHb and of 5.0k0.5 x lo4 s-l for 152 pmol dmP3 metHb, thus indicating that k,, = R,/[metHb] z 3 x lo8 mol-l s-l. However, the final yields of deoxyHb obtained on completion of these reactions were only 47 & 10% and 69 f lo%, respectively, of the expected yields of propan-2-01 radicals (G = 5.8) deduced from the metHb reduction yields (G = 5.8) observed on y radiolysis of similar oxygen-free solutions.The value of k,, must therefore be less than estimated above, owing to competing reactions. Dismutation of propan-2-01 radicals in reaction (10) must compete with reaction (14) at the high radical concentrations occurring under pulse conditions; on this basis the observed yields and R, values can be accounted for by methods described later if 2k1, and k,, are assigned the values (4$- 1) x lo9 and (2.4k0.5) x lo8 dm3 mol-1 s-l, respectively. Cyt c solutions in the standard solvent were also reduced by propan-2-01 radicals, as shown by increased absorbance at 550 nm after a pulse and a concomitant but much smaller decrease at 535 nm.For 33 and 72 pmol dm-, solutions, the reduction rates and yields were R, = 1.3 x lo4 and 2.5 x lo4 s-l and G(reduction)/5.8 = 0.54 and 0.65,700 REACTIVITY OF SEMIQUINONE RADICALS respectively. These data are consistent with the occurrence of the competing reactions (10) and (15) if 2k10 and k,, are (4f 1) x lo9 and (2.6k0.5) x lo8 dm3 mol-1 s-l, respectively. Although these values of 2k10 agree, they exceed that reported from pulse radiolysis measurements at ;1 < 300 nm, namely, 2k,, = 1.4f0.3 x lo9 at pH 6.18 We cannot account for this discrepancy, but by putting most weight on the R, data we conclude that k,, and k,, have the values reported above. These are similar to the rate constant of 3.7 x lo8 dm3 mol-1 s-l reported for the one-electron reduction of ferrideutero- porphyrin IX by propan-2-01 radi~a1s.l~ D.HAEM REDUCTION B Y SEMIQUINONES: REACTIONS (16) AND (17) Most of this work employed oxygen-free solutions of metHb or cyt c in the standard propan-2-01 (0.2 mol dm-3)-acetone (7 mmol dmP3) solvent at pH 7.2, containing quinones of typically 100 or 300 pmol dm-3 concentration, which were more than twice that of metHb or cyt c. Pulse radiolysis of metHb-quinone solutions gave the following results. (a) At 555 nm (the deoxyHb peak) an initial increase of absorbance complete within 15 p s was followed by a larger and slower increase, typically requiring milliseconds, to a final value which was constant for several seconds provided that the light source used to monitor it was filtered to exclude ;1 < 500 nm.The initial increase was very small for DQ and AQS. (b) At 630 nm (the metHb peak) the absorbance decreased to approximately a- third of the increase in (a), ai a similar rate. (c) MD and AQS 0 n a O ' O 2 I Ot..' 0 n 8 -0.05 0 12 mls 0 12.5 mls 0.05 n 0 0 0 n 8 -0.03 - d I I 0 12.5 m Is I I 0 12.5 mls FIG. 1 .-Optical density changes after pulse radiolysis of oxygen-free quinone-haem solutions : (a) DQ-metHb at 555 nm, dose 8.6 Gy; (b) DQ-metHb at 630 nm, dose = 8.6 Gy; (c) DMBQ-cyt c at 550 nm, dose 6.8 Gy; ( d ) DMBQ-cyt c at 440 nm, dose = 9.9 Gy.H. C. SUTTON AND D. F. SANGSTER 70 1 i 1 I i 0 2 4 time after pulse/ms FIG. 2.-Pulse radiolysis of oxygen-free metHbmenadione solutions + propan-2-01 (0.2 mol dm-3) at pH 7.2: (a) [metHb] = 115 pmol dmP3, [MD] = 200 pmol dmP3, [pr2] = 8.0 pmol dn-3; (b) [metHb] = 50 pmol dm-3, [MD] = 150 pmol dm-3, [pr2], = 4.0 pmol dm-3.The ordinate in (a) has been displaced upwards by 0.01 absorbance units for clarity. Each point is an average of the data for 0.1 ms. Curves were calculated from eqn (20) adopting k , , = 1.5 x lo7 dm3 mol-I s-', 2k,, = 4 x los and other parameters as in text. solutions showed a rapid ( < 15 ps) increase in absorbance at 390 nm (the semiquinone peak) followed by a slow decrease concomitant with (a). Fig. 1 and 2 illustrate (a) and (b). The rapid increase of absorbance (OD,,) in (a) and (c) is attributed to formation of semiquinone radicals in reaction (1 1) and deoxyHb in reaction (14).Values observed at 555 nm were in reasonable agreement with those calculated from (18) where [pr2], is the initial concentration of propan-2-01 radicals estimated from the dose per pulse, assuming that G(pr2) = 5.8 and ignoring reaction (lo), f is the fraction of (CH,),COH radicals which produce semiquinone (obtained from f= k,, [QJ/(kll [QJ + k,, [metHb]}), E&- is the extinction coefficient of the semiquinone radical ( E ~ ~ ~ ~ ~ ~ ~ is negligible by comparison), and has the meaning and value (8900 dm3 molt1 cm-l) given in the experimental section. Pulse radiolysis of haem-free quinone solutions gave the values E&- at 555 nm = 300,250 and 280 dm3 mol-l cm-l, respectively, for AQS, DQ and DMBQ and 1600 for MD, which accounts for the large initial absorbance for MD solutions illustrated in fig.2. Conditions were chosen so OD15 = [ ~ Q l o { f & Q - + (1 -f) AEdrnI702 REACTIVITY OF SEMIQUINONE RADICALS that the calculated value off(based on the k,, and k,, values reported above) exceeded 0.97. The subsequent slower absorbance changes are attributed to the reduction of metHb in reaction (16) in competition with the dismutation of semiquinone in reaction (1 3). The final spectral changes noted above agree at least approximately with this view since = 8900 dm3 mol-1 cm-l at 555 nm, - 2700 at 630 nm and ca. - 35000 at 390 nm, but the data were too noisy at 630 and 390 nm for useful quantitative treatment. The increase in absorbance at 555 nm was not totally first- or second-order but could be expressed approximately as first-order for the first 80% of the reaction.Values of R,, the apparent first-order rate constant, obtained by computerised analysis of the data, increased with increasing [metHb] but not in direct proportion to it. R, also increased in the sequence R,(DMBQ) < R,(DQ) < R,(MD) < R,(AQS). The final yields of deoxyHb were < G(pr2) and increased with increasing [metHb] and decreasing [pr2], from typically 0.6 to 1.2 for DMBQ, 2.3 to 4.5 for DQ, 2.5 to 3.5 for MD and 4.2 to 5.2 for AQS. Values of k,, were obtained from these data as follows: (i) The competition assumed for Q- leads to where GI5, the yield at 15 ps, = (1 -f) G(pr2), G(Q-) = fG(pr2), yI5, the concentration of semiquinone after 15 p s , =f[pr2],, p = k,,[metHb], and q = 2k13.Thus, k16 may be obtained from the measured yields and values of 2k13. (ii) The observed absorbance, OD,, at time t (p 15 p s ) after a pulse was compared with that calculated from (20) where x15, the concentration of deoxyHb formed in the first 15 p s , = (1 -f) [pr2],, and x,C and yt are the concentrations at time t of deoxyHb and of Q- which result from the assumed competition of reactions (1 3) and (1 6) for the Q- formed in the first 15 p s . These are given by OD, = (xi5 + x:) A ~ d m + ~ t EQ- These relations express the observed growth of absorbance satisfactorily using the values given above for 2k13 and other parameters, together with values of the adjustable parameter k16, which were constant within f 20% for each quinone (except DMBQ which gave very approximate data) for three-fold variations of [metHb], two-fold variations of dose per pulse and three-fold variations of [quinone].Fig. 2 illustrates the data for MD solutions. Allowance was made for 10% or higher uncertainties in dosimetry and hence in estimates of initial semiquinone concentrations when fitting calculated growth curves to the data, by adjusting y,, within these limits to make the calculated final absorbance agree with that observed. A variation of this method was based on comparisons of the observed 'first-order' growth rates of absorbance (R, values) with those calculated from appropriate semi-log plots of the results of eqn (20). This puts less weight on uncertain 2k13 values and was useful where G(deoxyHb) > 2.5. Mean values of k,, obtained by the yield and growth methods were (3 f 1) x lo5 dm3 mol-1 s-l and (3 f 1.5) x lo5, respectively, for DMBQ, (5.5 f 1.5) x 10, and (5.5f 1.4) x lO6forDQ,(l.5f0.5) x 107and(1.5+0.4) x 107forMDand(4+ 10) x lo7 and (9 1 2 ) x 107 for AQS.Uncertainties in 2k13 contribute significantly to the errors quoted. For AQS the high yields and uncertain dosimetry make the yield method unsuitable.H. C. SUTTON AND D. F. SANGSTER 703 The reduction of cyt c in similar solutions occurred more rapidly after pulse radiolysis, and with higher yields than for metHb. Irradiated solutions displayed increased absorbance at the sharp 550 nm peak of reduced cyt c as illustrated in fig. 1 (c) for DMBQ, and a concomitant but much smaller decrease at 535 nm which is consistent with the spectrum of reduced cyt c.The semiquinone of DMBQ has an absorption peak at 440 nm where the absorption of cyt c is near a minimum, so it was possible in this case to demonstrate clearly the initial rapid formation of semiquinone and subsequent slower reduction of cyt c, for which &(reduced cyt c) -&(cyt c) = - 7300 at 440 nm. This is illustrated in fig. 1 ( d ) , in which the reduction is concomitant with the increase of absorbance in fig. l(c). Rapid initial increases of absorbance observed at 550 nm were analogous to those for metHb at 555, but these were relatively small compared with the later increase because of the large value of Acre (21 000 at 550 nm). For AQS, the semiquinone dismutation reaction (1 3) was almost completely suppressed by the very rapid reaction (17), and reduction yields approached G(pr2).The resulting first-order reduction rates (R, values) were proportional to [cyt c] as shown in fig. 3, from which k,, = 1.9 x lo9 dm3 mol-l s-l was obtained directly. ov I I I 50 100 150 [ cytochrome c] /pmol dm-3 FIG. 3.-First-order growth rates ( R , values, see text) for reduction of cytochrome c by AQS semiquinone radicals, measured by pulse radiolysis. [AQS]/[cyt c] = 1 in experiment +, and 2 3 in others. EDTA (1 00 pmol dm-3) added in experiment . Kinetic analyses of the data at 550 nm for all the quinones were performed as described for metHb with most emphasis on the growth method because of the higher yields. Results are given in table 1. 11. SYSTEMS CONTAINING OXYGEN The standard solvent used in these y radiolysis studies employed propan-2-01 (0.2 mol dm-3) to convert OH to superoxide, and was made up in pH 7.2 buffer containing catalase (6 mg dm-3) to remove radiolytic hydrogen peroxide. Fig.4 illustrates a typical experiment with metHb in which the small yield [G(oxyHb) = 0.51 observed in the absence of quinones and reported previously at comparable dose rates2 increases with increasing concentration of menadione. Thermal reactions are negligible over the time-scale of these experiments (< 5 min for irradiation and analysis).704 REACTIVITY OF SEMIQUINONE RADICALS dose/ lozo eV dm-3 FIG. 4.-Radiolysis of metHb (38 pmol dm-3)-menadione mixtures in aerated solution at pH 7.2 containing propan-2-01 (0.2 mol dmP3) and catalase (6 mg dmP3).Menadione concentration 0, 130; 0, 10; a, 2.8; A, 0 pmol dmP3. *VQ 0 0 0 --- u s 4 - 0,” / 0 0,’ 0 O L I I I I 0.001 0.1 10 [Ql/[ozl FIG. 5.-MetHb reduction yields in quinone-oxygen mixtures. Initial [metHb] = 45 pmol dm-3; dose rate 1.3 Gy s-l; pH 7.2. 0, aerated solution+propan-2-01; W, oxygen saturated+propan-2-01; A, aerated solution+sodium formate (see text). Filled symbols (0) with EDTA; open symbols (0) without EDTA; *, [metHb] = 100 pmol dm-3; **, equilibrated with 2% oxygen. Continuous and interrupted curves have been calculated from eqn (25) and its modified form, respectively, as explained in the text.H. C. SUTTON AND D. F. SANGSTER 705 Fig. 5 shows how the initial reduction yield determined from such data varies with quinone concentration. The observations may be summarised as follows.(a) At constant initial [metHb], G(oxyHb) for each quinone is determined by [Q]/[O,] for five-fold or more variations of [Q] and of [O,]. (b) G(oxyHb) increases with [Q]/[O,] to a limiting value of 4.8 0.4 for AQS, DQ and MD, which is unaltered by increasing [metHb]. This is significantly less than G(radica1s) = 5.8, and analytical uncertainties cannot account for the difference (see Experimental section). (c) For G(oxyHb) > 2, prolonged irradiation causes complete reduction of metHb in contrast with quinone- free solutions in which reduction is only partial., ( d ) Addition of EDTA (100 pmol ~ l m - ~ ) has no significant effect except for DMBQ. (e) Use of sodium formate (0.01 mol dm-3) instead of propan-2-01 gives similar results.(f) For G(oxyHb) < 3 the slope of yield-dose curves as in fig. 4 declines with increasing dose, and initial yields decrease with decreasing [metHb]. Addition of superoxide dismutase (SOD) to these solutions decreases G(oxyHb). This is illustrated in fig. 6 for an aerated MD solution for which GO, the oxyHb yield [SOD] /[metHbl FIG. 6.-Effect of superoxide dismutase on G(oxyHb), plotted according to eqn (29). Aerated solutions with propan-2-01 (0.2 mol dm-3), menadione (80 pmol dmP3), and EDTA (100 pmol dm-3) at pH 7.2. without SOD, was 4.7, and G(oxyHb) in the presence of SOD is denoted as GSoD. Values of [SOD14 (the concentration of SOD which halves Go) were obtained from such data and these varied with five-fold variations of [O,], two-fold variations of [metHb] and more than five-fold variations of [Q] in such a way as to maintain the quantity 2 = [SOD]+ [O,]/[Q] [metHb] constant within ca. 30%.Table 2 illustrates this behaviour for MD and shows that the results are not significantly altered by adding EDTA or by exchanging sodium formate for propan-2-01. SOD had no effect on oxygen-free systems at the concentrations used above, although much higher con- centrations are reported to react non-catalytically with some semiquinones.20 The radiolysis of cyt c in similar solutions differs from that of metHb because 0; reduces cyt c much more rapidly, giving nearly quantitative reduction yields of G(-cyt c) = 5.6 in aerated solution without quinone. However, the [SOD]) values obtained from experiments similar to those in fig.6 increased with addition of quinones. Typical values for 20 pmol dm-3 cyt c were 17 nmol dmd3 in the standard aerated solution at pH 7.2 containing either propan-2-01 or sodium formate, 24 nmol dm-3 with added AQS (100 pmol dm-3), 40 nmol dm-3 with [AQS] = 4 mmol dm-3 in oxygen-saturated solution, 80 nmol dm-3 for DQ (100 pmol dm-3) in aerated solution, 200 nmol dm-3 for MD (100 pmol dm-3) in aerated solution and706 REACTIVITY OF SEMIQUINONE RADICALS TABLE 2.-[SODIi VALUES FOR metHb-MD SOLUTIONS EDTA IPAor [me tH b] [MDI lo21 [SOD]ia present formate (F) /pmol dm-3 /pmol dm-3 /pmol dmd3 /nmol dmP3 Zb IPA IPA IPA IPA IPA F IPA F 48 45 48 115 46 46 43 40 80 320 320 80 10 90 120 100 270 270 1300 270 270 270 1300 270 26 0.0018 80 0.0014 22 0.00 19 70 0.0020 3.2 0.00 19 20 0.0013 4.9 0.0012 21 0.0014 a Calculated from concentration by weight assuming 100% purity (see text); Z = [SOD], [O,]/[metHb] [MD]. TABLE 3.-SEMIQUINONE-SUPEROXIDE EQUILIBRIUM DATA EXQ/Q-)/V -0.380a -0.235b - 0.203' - 0.074" G'(Q-/Q2-)/V 0.04d 0.36" 0.19" 0.42" Ze 1.3 x 10-5 2.2 x 10-4 1.6 x 10-3 1.4 x 1.9 x 10-4 5.2 x 0.14 61 we m q n (291 - 7.0 x 0.13 42 K4(literature) 1.7 x 10-4f 4.6 x 0.1@ 50' - 6.5 x 10-3 2.7 x 3 .2 ~ 10-l K*[eqn (2811 a Ref. (16); ref. (26); " ref. (25); calculated from E"(Q/Q-)and Eo(Q/Q2-); see text for definition and method of expressing SOD concentration; f calculated from K4 (DQ) using Eo (Q/Q-) values in ref. (16). 600 nmol dm-3 for DMBQ (25 pmol dmP3) in aerated solution. The quantity W = [SOD14 [O,]/[cyt c] [Q] (analogous to 2 above) was approximately constant for five-fold variations of [O,] and [Q] for MD and DQ but not for AQS, nor were the competition plots as in fig.6 always linear for AQS. For DMBQ, 2 was constant over five-fold [Q] variations but its dependence on [O,] could not be studied reliably owing to a thermal reaction in oxygen-saturated solution causing the re-oxidation of reduced cyt c in the presence of SOD. Averaged values of W are reported in table 3. EDTA (100 pmol dm-3) was used in this work, and yields were lower without it. The SOD concentrations reported here are nominal values based on the assumption that our preparation was 100% pure with M, = 32200. For later purposes we needed to know its absolute activity, i.e.the quantity k,, [SOD] for the catalysed reaction SOD 2H++20; + H,O,+O,. To determine this, a portion of our stock aqueous solution (kept frozen) was air-freighted to a laboratory in U.K. and analysed by pulse,radiolysis. Pulse radiolysisH. C. SUTTON AND D. F. SANGSTER 707 of an aerated solution in pyrophosphate buffer (2 mmol dm-3) containing ethanol (0.1 mol dm-3) produced excess superoxide which was decomposed in reaction (23) with a pseudo-first-order rate constant, k23 [SOD], which was found to be proportional to [SOD]. The data gave k,, = 1.45 x lo9 dm-3 mol-1 s-l. k,, is independent of pH from 5.3 to 9.5,,l but decreases with increasing ionic strength,, to an estimated value of 1.14 x lo9 in our solutions. This determination was checked by comparing k23 with k, in competition experiments with cyt c similar to those illustrated for metHb in fig.6. Repeated measurements over four months gave k23/k5 = 3050 300. Values are available for k, (1.1 x lo6 at pH 7.2 at low ionic strength) and its variation with ionic strength (two-fold decrease to 40 mmol dm-3) which are consistent with three recent studies.' For our conditions we estimate k, = 0.55 x lo6 and hence k,, = 1.7 x lo9. We adopt 1.3 x lo9 as a weighted mean of this and the more direct value reported above. The value k23 = 3.7 x lo9 has been reported for very pure SOD preparations,22 but the significant quantity in this work is k23 [SOD]. REACTION MECHANISM The reducing species eiq and H formed during radiolysis of our solutions react rapidly either with oxygen to form superoxide or with quinone to form semiquinone radicals.In solutions containing formate, OH is rapidly converted to 0; by the fast reactions OH + HCOO- + COT + H,O co, + 0, -b 0, + c o , . The similarity of our results on substituting propan-2-01 for formate (necessitated by quinone solubility problems) indicates that the following rapid reactions23 of propan- 2-01 radicals formed from OH are quantitative under our conditions, thus converting OH to 0;: (CH,),COH + 0, --+ (CH,),COHO, (CH,),COHO, + HPOf --+ (CH,),(CO)-O, + H,PO; (CH,),(CO)-0, --+ (CH,),CO + 0;. The major subsequent reactions of 0; and Q- are the following: metHb I-+ oxyHb If the sum of [O;]+[Q-] is denoted by [S] and it is assumed that the equilibrium (4) is maintained, then r, the fraction present as Q-, is given by r = [Q-l/[Sl = 1/(1+ [02I/K*[Ql). (24)708 REACTIVITY OF SEMIQUINONE RADICALS The yield of oxyHb predicted by this mechanism is then given by (25) G(radica1s) {drkl, + (1 - r)k,) [metHb] {rk16 + (1 - r)k,) [metHb] + 2k13 r2[S] + 2k3( 1 - r),[S] G(0xyHb) = where d is an adjustable parameter which is assigned the value 0.83 (i.e.4.8/5.8) to account for the limiting yields in fig. 5. This implies that 17% of the semiquinone radicals reacting with metHb in the presence of oxygen form products other than oxyHb. Using the rate constants reported above together with literature values of K4 (table 3), k , (5 x lo3 at pH 7),, 2k3 (9 x lo5 at pH 7.2),3 [S] and hence G(oxyHb) can be evaluated for our conditions by a steady-state treatment, taking G(radica1s) = 5.8.The continuous curves in fig. 5 have been calculated in this way and account very approximately for the observed trend of the results, although the observed yields are less than those calculated, especially for DMBQ and where [Q]/[02] approaches zero. This indicates either that k , is less than the value adopted or that some impurity removes 0; in a pseudo-first-order manner, as reported in other studies.,? 24 It is also probable that superoxide reacts with semiquinone as follows : 2HS +Q- +O- 2 +Q+H202 (or QH2+02) (26) and this could account for the behaviour of DMBQ. Inclusion of this reaction adds the term k26 r( I - r ) [S] to the denominator of eqn (27), and the interrupted curves in fig. 5 have been calculated on this basis, assuming that k26 = 2 x lo8 dm3 mol-1 s-l for all semiquinones and that k, = 2.5 x lo3 (half the value adopted above).These curves correspond more closely with the observations, especially for DMBQ and DQ, which have small values of k,. The experiments in the presence of SOD were carried out at high [Q]/[O,] where G(oxyHb) approaches dG(radica1s). For AQS, MD and DQ the term 2k13r2[S] in eqn (25) is insignificant, so that in the presence of SOD reaction (23) competes with reaction (16) for the equilibrium mixture of 0; and Q-. This leads to and (28) where Go, GSoD and 2 have been defined above. For DMBQ the term 2k13r2[S] is appreciable, causing G(oxyHb) to be less than dG(radica1s) at high [Q]/[O,], so eqn (27) and (28) are only approximations in this case. Fig. 6 and the nearly constant values of 2 in table 2 confirm the validity of this treatment.Values of K4 for each quinone have been calculated using eqn (28) and are listed in table 3. There may be considerable errors in these owing to the uncertainties in k16 (ca. f 50% for DMBQ and 30% for other quinones), in 2 (similar values) and in k23 (ca. f 30%, see above), but they should not exceed 60% standard deviation in total. Within these limits, the results agree with those obtained by other methods, thus supporting the proposed mechanism and assumption of equilibrium and the rate constants reported above. The conditions for maintenance of equilibrium may be estimated from the rates of reactions (4) and (-4), using rate constants available from the literature.l2? 17, 25$ 26 For metHb solutions under our conditions k4[Q][0;] exceeds the rate of the fastest competing reaction, k23 [SOD] [Oil, by more than 20-fold at [SOD]+ concentrations for AQS and by more than 80-fold for other quinones.For the reverse reaction k-, [O,] [Q-] exceeds k16 [metHb] [Q-] by similar factors. The maintenance of equilib- K4 = k23 [SOD]$ [021/k16 [metHbl [QI = k23Z/k16H. C . SUTTON AND D. F. SANGSTER 709 rium observed in the metHb experiments is thus understandable. However, for cyt c the corresponding factors are reduced to typically 2- and %fold for AQS and other quinones, respectively, owing to the increased values of kl,[cyt c] over those of k,,[metHb]. Thus one might expect equilibrium to be maintained for MD, DQ and DMBQ, giving [SOD]+ values in accordance with but not for AQS.The observations reported above confirm this view and give values for K4 reported in table 3 which are in reasonable agreement with those obtained from metHb. 111. CONCLUSIONS The data in table 1 demonstrate greatly increased rate constants for reduction of metHb and of cyt c by semiquinone radicals over those for superoxide. For the menadione semiquinone, which has similar one-electron reducing properties [E"(Q/Q-), table 13 to that of superoxide, this increase is 3000-fold for metHb and 300-fold for cyt c, whilst the rates for other semiquinones reflect their reduction potentials and approach the diffusion-controlled limit for the most reducing, i.e. for AQS semiquinone reacting with cyt c. The values found for these rate constants are supported by the agreement of K4 values determined from them with those previously obtained by independent methods.The oxygen produced in reaction (5) between cyt c and 0; must be triplet oxygen (3Z02) for which AG,,, x - 39 kJ mol-l, rather than singlet oxygen (lA 0,) for which AG,,, x + 5 5 kJ mol-l. For orbital-symmetry reasons similar to those discussed for the 0; dismutation reaction,28 Koppen01~~ suggests that this might explain why k, (lo6 dm3 mol-1 s-l) is so much less than the rate constant for the corresponding reduction of cyt c by CO; (lo9 dm3 mol-1 s-l 30). He argues that the production of 3Z 0, in reaction (9, although not spin-forbidden, would require overlap between the filled nz orbital of 0; and a vacant orbital of the iron-porphyrin system, thus requiring appreciable activation energy.In contrast, reduction of cyt c by CO; permits overlap between the half-filled 7cz orbital of COT and the half-filled orbital of the low-spin iron-porphyrin system of cyt c, leading readily to the formation of singlet CO,. Similar reasoning might account for the high rates of cyt c reduction by semiquinone radicals reported in table 1, since these also produce singlet-state products (quinones). The slow reduction of metHb by 0; relative to that by Q- might also be regarded similarly if the former is considered formally as a reaction with iron-porphyrin leading initially to deoxyHb and oxygen (necessarily 3X 0, for thermodynamic reasons), subsequently converted to oxyHb : 0; + haem-Fe3+ -+ 0, (") + haem-Fe2+ oxyHb.This requires overlap between the filled nz orbital of 0; and a vacant orbital in the iron-porphyrin system which, for high-spin metHb with all five d-type orbitals half-filled, must be of higher energy than is the case for cyt c, and so require considerable activation energy. However, these speculations must be viewed with some reserve since related arguments would lead one to expect the equilibrium reaction (4) to have appreciable activation energy, involving as it does the production of triplet oxygen and doublet semiquinone from singlet quinone and doublet superoxide. In fact, equilibrium (4) is very rapidly attained, forming reactive semiquinone from less reactive superoxide. The reason for this apparently paradoxical situation is not clear. Semiquinones, like superoxide, may also exhibit oxidising properties arising from710 REACTIVITY OF SEMIQUINONE RADICALS their one-electron reduction to hydroquinones.The redox values for this process [Eo(Q-/Q2-)] are listed in table 3, and those for DQ (+0.36 V at pH 7) and DMBQ (+0.42 V) are sufficiently high to permit the reaction Q- + Fe2+ - - - haem -+ Fe3+ * - haem + Q2- (30) to occur with deoxyHb ( E O = +0.15 V at pH 7) and oxyHb ( E O c +0.27 V when in equilibrium with deoxyHb in oxygen saturated solution). Experimentally this was not observed ; both semiquinones displayed reducing properties only, leading to complete reduction of metHb. This contrasts with superoxide for which Eo(O;/Oi-) = +0.9, and which oxidises and reduces oxyHb + metHb mixtures at comparable slow rates at pH 7.2 A possible reason for this observation is that the mixed potential resulting from Q/Q- and Q-/Q2- reactions would be expected to be somewhere between the potential of each and perhaps approximate to their mean, as observed for s~peroxide.~~ If so, the resulting mixed potential may approximate 0.16 V, which would not favour reaction (3 1).The major biochemical conclusion from this work confirms Winterbourn’s suggestion that naturally occurring quinones such as menadione can convert the superoxide formed in aerobic metabolism to semiquinones which are much more reactive, and that the enzyme superoxide dismutase can control or suppress their reactions. Using enzymatic sources of superoxide to reduce cyt c, she showed that the concentration of SOD required for 50% suppression of this reduction was increased in the presence of quinones.s This increase agrees approximately with that calculated from the W values reported in table 4, as shown below. TABLE 4.-[SOD], VALUES IN AERATED ENZYME SOLUTIONS calc.from obs.a W values [CY t CI [QI quinone /pmol dm-3 /pmol dm-3 none AQS DQ MD DMBQ 14 14 14 14 14 ~ ~~~ - 0 5.4 100 7.5 100 12 33 50 50 70 50 610 820 b a Ref. (6), SOD concentrations calculated from manufacturer’s specification; not rele- vant, equilibrium not attained. We conclude that the equilibrium discussed above also occurs in certain biochemical situations, and that the reactions suppressed by SOD in such cases are not those of superoxide but of semiquinones. This conclusion may have more general significance, as Winterbourn has pointed We thank Dr Christine Winterbourn (Clinical Biochemistry Department, Christ- church Hospital) and Dr B.Halton (Victoria University of Wellington) for helpful discussions. We thank Dr P. O’Neill (Institute of Cancer Research, Sutton) for the pulse-radiolysis measurements on our SOD preparation, Dr M. Manning for assistance with computer programming, Mr D. Williamson for preparing our haemoglobin, Mr G. Watt for designing the pulse trigger/amplifier unit, and Mr S. Kanard, Mrs F. McGinnity and Miss S. Bolton for technical assistance.H. C. SUTTON AND D. F. SANGSTER 71 1 I. Fridovich, Annu. Rev. Biochem., 1975, 44, 147. H. C. Sutton, P. B. Roberts and Christine C. Winterbourn, Biochem. J., 1976, 155, 503. D. Behar, G. Czapski, J. Rabani, L. M. Dorfman and H. A. Schwarz, J. Phys. Chem., 1970,74,3209. C. C. Winterbourn, J. K. French and R. C. Claridge, Biochem. J., 1979, 179, 665. C. C. Winterbourn, in Chemical and Biochemical Aspects of Superoxide and Superoxide Dismutase, ed. J . V. Bannister and H. A. 0. Hill (Elsevier, New York, 1980), p. 372. C. C. Winterbourn, Arch. Biochem. Biophys., in press. W. H. Koppenol, K. J. H. Van Buuren, J. Butler and R. Braams, Biochim. Biophys. Acta, 1976, 449, 157. Ruth F. Benesch, R. Benesch and Suzanna Yung, Anal. Biochem., 1973, 55, 245. V. Massey, Biochim. Biophys. Acta, 1959, 34, 255. lo A. T. Thornton and G. S. Laurence, Radiat. Phys. Chem., 1978, 11, 3 1 1 . l 1 J. H. Baxendale, P. L. T. Bevan and D. A. Stott, Trans. Faraday Soc., 1968, 64, 2389. l 2 K. B. Pate1 and R. L. Willson, J. Chem. Soc., Faraday Trans. I , 1973, 69, 814. l 3 M. Anbar and P. Neta, Int. J. Appl. Radiat. Isot., 1967, 18, 493. l4 K. D. Asmus, A. Henglein, A. Wigger and G. Beck, Ber. Bunsenges. Phys. Chem., 1966, 70, 756. l5 J. K. Thomas, Adv. Radiat. Chem., 1969, 1, 103. D. Meisel and P. Neta, J. Am. Chem. Soc., 1975, 97, 5198. B. E. Hulme, E. J. Land and G. 0. Phillips, J. Chem. Soc., Faraday Trans. I , 1972, 68, 1992. M. Simic, P. Neta and E. Hayon, J. Phys. Chem., 1969, 11, 3794. Soc.,,1980, 102, 1015. l9 D. Brault, C. Bizet, P. Morliere, M. Rougee, E. J. Land, R. Santus and A. J. Swallow, J. Am. Chem. 2o P. Wardman, Proc. Meet. Assoc. Radiat. Res., 1979, 189. 21 G. Rotilio, R. C. Bray and E. Martin Fielden, Biochim. Biophys. Acta, 1972, 268, 605. 2 2 M. E. McAdam, Biochem. J., 1977, 161, 697. 23 Y. Ilan, J. Rabani and A. Henglein, J. Phys. Chem., 1976, 80, 1558. 24 G. Czapski, Annu. Rev. Phys. Chem., 1971, 22, 171. 25 Y. A. Ilan, G. Czapski and D. Meisel, Biochim. Biophys. Acta, 1976, 430, 209. 26 D. Meisel and G. Czapski, J. Phys. Chem., 1975, 79, 1503. K. P. Clark and H. I. Stonehill, J. Chem. Soc., Faraday Trans. I , 1977, 5, 722. 28 W. H. Koppenol and J. Butler, FEBS Lett., 1977, 83, 1 . 2g W. H. Koppenol, The Reactivity of the Superoxide Anion Radical in Biochemical Systems (Thesis) 30 E. J. Land and A. J. Swallow, Arch. Biochem. Biophys., 1971, 145, 365. 31 P. L. Airey and H. C. Sutton, J. Chem. Soc., Faraday Trans. I , 1976, 72, 2441 and 2452. (University of Utrecht, Utrecht, 1978). (PAPER 1 /252)

ISSN:0300-9599    

DOI:10.1039/F19827800695

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 713-724 Classification of the Mesophase of Di-isobutylsilanediol BY JOHN D. BUNNING AND JOHN E. LYDON Astbury Department of Biophysics, University of Leeds, Leeds LS2 9JT AND COLIN EABORN AND PATRICIA M. JACKSON School of Molecular Sciences, University of Sussex, Brighton BN1 8QJ AND JOHN W. GOODBY AND GEORGE W. GRAY* Department of Chemistry, University of Hull, Hull HU6 7RX Received 25th February, 198 1 The thermotropic liquid crystal mesophase formed by di-isobutylsilanediol has been known for over twenty-five years, but its nature and structure have remained uncertain and the physical characteristics of the phase appeared unique and quite different from those of any other recognised type of liquid crystal. In a previous preliminary publication, it was suggested that the mesophase of di-isobutylsilanediol is in fact of the discotic type only very recently characterised in certain organic systems.Here we describe more fully our grounds for this suggestion. A model is proposed for the structure of this mesophase; this offers an explanation as to why other dialkylsilanediols do not behave similarly. A reinterpretation of earlier X-ray crystallographic data led us to suggest that the mesophase consists of hydrogen-bonded dimers. The model is supported by data from optical microscopy, thermal analysis and miscibility studies. The optical textures shown by the mesophase and those observed at the crystal mesophase boundary are discussed and compared with those of the discotic phases of the benzene hexa-n-alkanoates.The structure of the thermotropic mesophase of di-isobutylsilanediol, (i-Bu),Si(OH), (1) must present one of the longest-standing problems in the field of liquid crystal research. This compound was first prepared by Eabornl in 1952. He noticed that it had a ‘double melting point ’, suspected liquid crystal behaviour, and passed a sample to Hartshorne for optical examination. Hartshorne found that the mesophase was puzzling : microscopic textures unlike those of any previously encountered mesophase type were formed and the situation was further complicated because the molecular structure (plate 1) offered no clue as to which type of mesophase was likely. A preliminary X-ray diffraction study of the crystalline solid was carried out by Bernal et aL2 but this did not give any clear implications concerning the molecular ordering in the mesophase.Eaborn and Hartshorne reported3 the optical observations in detail and made a tentative assignment of the phase as smectic, but they could offer no convincing explanation of the peculiar features of the microscopic texture. Their model incor- porated the extended hydrogen-bonded chain shown in fig. 1 which at that time was thought to exist in the crystal structure of another silanediol derivative. Although many thousands of liquid crystal phases must have been examined in the meantime, none has been reported with textures identical to those of (I) and no closely related compounds have been reported with comparable mesogenic properties. For twenty-five years, the problem of the structure of this mesophase has lain dormant.We have attempted a comprehensive structural study of the mesophase of (I) using a range ofphysical techniques. In addition to repeating Hartshorne’s optical microscopy, FAR 1 713 24714 ME SO PHASE OF D 1-1 SOB U TY L S I L ANED I 0 L we have extended the X-ray investigation to a study of the mesophase itself and we have undertaken differential thermal analysis and miscibility tests. On the basis of these investigations, we have come to the conclusion that this mesophase is discotic4 and it must therefore be regarded, in retrospect, as the first example of this type of phase to be reported. We have already made a preliminary report5 suggesting that the phase is discotic, but now describe fully the grounds for the conclusion that this is indeed the case.i-Bu \ Y-? I 1 I t I 1 ?-Y \ 1 8 ' : A-0 \ / ii i-Bu i-Bu FIG. 1.-Extended system of hydrogen bonding suggested by Eaborn and Hartshorne3 for both the crystalline solid and the mesophase of di-isobutylsilanediol. A scheme of association of hydroxyl groups in this manner was proposed by Kakudo and Watase (Technul. Rep. Osaka Univ., 1952, 2, 247) for the crystalline solid of diethylsilanediol and by Kakudo, Kasai and Watase (J. Chem. Phys., 1953, 21, 1894) for diallylsilanediol. In both cases their models were based on a combination of preliminary X-ray studies and infrared absorption spectra and cannot be regarded as unequivocal. A full crystal structure determination of diphenylsilanediol has recently been reported by Parkanyi and Bocelli (Cryst.Struct. Cumrnun., 1978, 7, 335) and in this case all of the hydrogen bonds are of the usual -OH. .O- type, and there is no evidence of OH groups lying as opposed dipoles. RESULTS MESOPHASE FORMATION I N DIALKYLSILANEDIOLS As reported by Eaborn and Hartshorne3 the behaviour of di-isobutylsilanediol in forming a mesophase is apparently unique and in their paper they give a substantial list of dialkylsilanediols which either they or others had observed to give normal melting behaviour. In the course of this work dicyclohexylsilanediol has been reprepared and found to give no mesophase and this is also the case for the following silanediols prepared at the University of Sussex (m.p./"C in parenthesis) : di-n-hexyl- (86-87) ; methyl-n-pentyl-( 57) ; methyl-n-octyl-( 57) ; methyl-n-decyl-( 59-60) ; methyl- n-dodecyl-(67-68); methyl-n-octadecyl-(87-88); methylcyclohexyl-(97-98); di-o-tolyl- (138-139).In some cases, decomposition of the sample is a problem and this was particularly true with methyl-n-pentylsilanediol and methyl-n-octylsilanediol, but even in these instances it seems clear that no mesophases are formed. Perhaps dialkylsilanediols having a secondary carbon p to the silicon would be closer analogues to di-isobutylsilanediol and these may be worthy of examination in the future. OPTICAL MICROSCOPY The transition temperatures observed for di-isobutylsilanediol were : crystal - mesophase - isotropic liquid crystal - mesophase - isotropic liquid. 88.4 O C 98.7 O C 77 O C 95.6 O C Eaborn and Hartshorne3 have given a detailed description of the changes observed at the isotropic liquid -+ mesophase transition and at the crystal + mesophase andBUNNING, LYDON, EABORN, JACKSON, GOODBY A N D GRAY 715 mesophase + crystal transitions.We have repeated these observations and we concur with their descriptions. A brief summary of these is given below. There are two major additions to their description which we shall make. First, we shall distinguish between the effects of rapid cooling and of slow cooling of the isotropic liquid, since the cooling rate dramatically affects the texture of the mesophase formed. Secondly, we shall describe the appearance of homeotropic* dendrites of mesophase not reported previously. ISOTROPIC LIQUID -+ MESOPHASE TRANSITION Some degree of supercooling always occurs at this transition.When the mesophase is formed by rapid cooling the mosaic fan texture shown in plate 2 arises. When viewed with crossed polars, this always appeared grey or pale yellow in colour, indicating that the degree of birefringence was very low. By contrast, slow cooling of the isotropic liquid produces a star-like or fern-like dendritic growth of areas of mesophase as shown in plate 3(a)-(c).t Most of these growths are homeotropic and are difficult to see against the isotropic background (and can be seen most clearly with the analyser removed, by virtue of the Becke line effect at their edges), but there are occasional birefringent regions. We suggest that the homeotropic areas have grown on the glass surfaces (and are therefore parallel to the stage) and are being viewed down the axis of a uniaxial indicatrix, whereas the birefringent areas are growing in the bulk of the sample and are randomly aligned. Although the dendrites have rounded corners they are very similar in appearance to dendrites of crystalline solid phases, having branching at specific angles and showing a high degree of mirror symmetry along the dendrite arms.The strong suggestion of six-fold symmetry at the centres of these stars coupled with the prevalence of branching angles of 60’ implies that each entire growth is a single domain of mesophase and that it is being viewed down a six-fold axis. (In such a situation, one should perhaps refer to the optical property as pseudo-homeotropic, since the viewing direction is a six-fold rotation axis rather than one of infinite order.) On further cooling, when all of the sample has been converted to mesophase, the rod texture shown in plate 4(a) or (b) develops. At first sight, the appearance of this texture of bright rods running through a dark background suggests an open lattice framework of some kind.However, the dark areas are homeotropic (not isotropic) and because the intensity of the light regions gradually fades into the background it would appear that the ‘rods’ are actually curved regions at the edges of extended homeotropic domains. This hypothesis is reinforced by the observation that when viewed with polarised light, in the absence of an analyser, the contrast between these ‘rods’ and the surrounding mesophase regions is lost once in every 180’ of rotation of the stage.When viewed between crossed polars, they extinguish every 90’. Using a specially designed thin hot stage,g Eaborn and Hartshorne made a conoscopic examination of this texture. The homeotropic regions were found to be optically negative and the uniaxial indicatrix was found to tilt at an increasing angle as the axis of a rod was approached. SOLID -+ MESOPHASE TRANSITIONS Rapid cooling of the mesophase produces the solid in the form of spherulites of radial, acicular crystals (plate 5). When the temperature of a sample in this state is raised, the broad pattern of the spherulites is maintained by the mesophase, but the * Optically extinct because the dendritic area is being viewed along the uniaxial optical axis.? Note that although Eaborn and Hartshorne used the word ‘dendritic’ in their descriptions of optical textures of (I), they were not describing this texture, but rather the branching pattern of the rod texture mentioned below. 24-2716 MES 0 PHASE OF D 1-1 SOB U TY L S I L ANE D I 0 L fine detail changes dramatically. The radial pattern of individual crystals gives place to a tangential pattern of bands (plate 6). This was termed the striated band texture by Hartshorne. If a single crystal (grown from solvent) is heated, it also gives a striated band and on cooling reverts to more or less its original form. The polarisation colours of the mesophase are noticeably lower than those of the corresponding single crystal. COMPARISON OF THE MESOPHASES OF BENZENE HEXA-n-HEPTANOATE AND DI-ISOBUTY LSI L ANEDIOL The textures which we have observed for the mesophase of (I) were totally dissimilar to textures shown by any known smectic, nematic or cholesteric phases.There are how- ever, some points of resemblance with those described by Chandrasekhar et aL4 for the new discotic phases of certain esters of hexahydroxybenzene. Moreover, like the discotic phases, and unlike smectic and nematic phases, the phase exhibited by the silanediol was shown to be optically negative uniaxial. Therefore, a comparison was made between these textures and those of the discotic phase exhibited by benzene- hexa-n-heptanoate (11). For comparison, plates 7-9 show the typical textures exhibited by the discotic phase of benzene hexa-n-heptanoate (11).Plate 7 shows the mosaic-fan texture of (11). This texture is clearly very similar to plate 2 for the silanediol and this texture appears to be typical of both the ester and the diol. Plate 8 shows the pseudo-fan texture of the discotic phase. This texture was coloured pale yellow and grey as was that shown in plate 2 for the silanediol. The striated pattern in plate 8 is also very similar to the striated band structure shown in fig. 2 of the paper by Eaborn and Hart~horne.~ Plate 9 shows the feather texture of the discotic phase of (11). This texture was not exhibited by (I) and therefore may be typical only of (11). Thus, it can be seen that there are some points of similarity in the optical properties of the mesophases of (I) and (11).Moreover, it is now clear that other discogenic systems form discotic phases with the rod texture' and also the homeotropic dendritic texture. DIFFERENTIAL THERMAL ANALYSIS Differential thermal analysis was used to confirm the transition temperatures observed with hot-stage optical microscopy and to measure the enthalpy changes for the solid + mesophase and mesophase -+ isotropic liquid transitions. The differential thermal analysis trace for a heating cycle is shown in fig. 2. The trace for cooling is considerably broader than that for heating, and we found that for a second and subsequent heating cycles the traces continue to broaden. It appears therefore that the compound is prone to thermal decomposition and that even the first heating of a sample to observe the mesophase is accompanied by an appreciable degree of decomposition.The enthalpy values obtained are: AH(crystal1ine solid -+ mesophase) = 7.6 kJ mol-l, AH(mesophase -+ isotropic liquid) = 7.2 kJ molt'. The magnitude of AH(mesophase -+ isotropic liquid) is unusually large, as is the ratio of the enthalpy change at the mesophase-+isotropic transition to that at the crystal -+ mesophase transition (ca. 0.94).* We take these facts to indicate that a considerable fraction of the intermolecular bonding of the crystal is retained in this mesophase. * In systems involving smectic and nematic phases, this ratio is very low (0.2 or less), whereas for the discotic system benzene hexaheptanoate it is 0.67.J. Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 PLATE 1 .-Model of di-isobutylsilanediol (I).PLATE 2.-Mosaic fan texture of (I); crossed polars ( x 100). BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAY Plates 1 and 2 (Facing p . 716)J . Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 Plate 3 PLATE 3.-Star-like or fern-like dendritic growth areas of the mesophase of (I), obtained on slow cooling. Most areas are homeotropic, as shown by the dotted white lines in (b), which have been drawn to make the boundaries clear [ x 75 for (b) and (c); x 50 for (a)]; crossed polarisers ( x 150). BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYJ . Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 Plate 4 PLATE 4.-Rod texture of the mesophase of (I); crossed polars [ x 100 for (a) and (b)]. The structure around the sharp boundaries where the rods intersect (such as the arrowed regions) is discussed under fig.10. BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYJ . Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 Plates 5 and 6 PLATE 5.-Crystalline texture of (I) obtained by rapid cooling of the mesophase; crossed polars ( x 100). PLATE 6.-Mesophase texture of (I) obtained by heating the crystalline texture shown in plate 5 ; this is the striated band texture of Hartshorne; crossed polars ( x 100). BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYJ. Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 Plates 7 and 8 PLATE 7.-Discotic mesophase of benzene hexa-n-heptanoate (11) ; mosaic fan texture; crossed polars ( x 73, compare with plate 2. PLATE 8.-Striated texture of the discotic mesophase of (11); crossed polars ( x 75); similar to the striated band texture of plate 6.BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYJ . Chem. Soc., Faraday Trans. 1, Vol. 78,part 3 Plates 9 and 10 PLATE 9.-Feather texture of the discotic mesophase of (11); crossed polars ( x 75). PLATE 10.-(a) Model of the hydrogen-bonded dimer of di-isobutylsilanediol. (b) Model showing the stacking of two hydrogen-bonded dimers of di-isobutylsilanediol. BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYBUNNING, LYDON, EABORN, JACKSON, GOODBY A N D GRAY t I M E S O P H A S E *-- I I I I CRYSTALLINE I I ISOTROPIC 717 increasing temperature FIG. 2.-Differential thermal analysis trace for di-isobutylsilanediol obtained on heating a sample of the material, initially as the crystalline solid, at a heating rate of 10 O C min-I.X-RAY DIFFRACTION The X-ray diffraction pattern of the mesophase of (I) is very simple. Only two factors are apparent: an outer diffuse ring corresponding to a repeat distance of 4.7 A and an inner ring corresponding to a repeat distance of ca. 11-12 A. This inner ring is of an intermediate type. It is by no means as sharp as those found for smectic mesophases, but it is not as diffuse as the inner rings given by isotropic samples or by nematic mesophases. The only type of mesophase which has been observed to give a diffraction pattern of this type is the recently characterised discotic phase. MISCIBILITY STUDIES To test our suspicion that the mesophase of (I) was of the discotic type, a miscibility study was carried out with benzene hexa-n-heptanoate as the standard reference discotic material.The diagram of state for binary mixtures of these two materials is shown in fig. 3 : it can be seen that there is indeed a region of continuous miscibility connecting components (I) and (11). This is shown by the darker shaded region and means that a single, continuous phase consisting of varying proportions of two molecular species [ranging from 100% of (I) to 100% of (II)] exists across the diagram of state. Thus, for every composition from left to right across the diagram of state, there is a temperature range, admittedly narrow in places, over which the texture of the liquid crystal phase is typically of the discotic type. This temperature range is that between the temperature at which the last trace of crystalline solid disappears and that at which the first sign of isotropic liquid appears, i.e. the commencement of the two-phase region consisting of isotropic liquid and discotic phase denoted by the lighter shading in fig.3. On the basis of Sackmann’s miscibility principle, the phases of (I) and (11) are therefore of the same type. It is apparent that the mesophase -, isotropic liquid transition temperatures are considerably depressed and there may be a number of factors responsible for this. First, since it is necessary to preheat mixtures to ensure thorough mixing of the isotropic liquids, the thermal instability of (I) must give rise to some contaminants.718 ME SO PHASE OF D 1-1 SOBU TY L SI L AN ED I 0 L I" Q) * 80 .. . . . . . 100 % I 100 % II composition - FIG. 3.-Phase diagram for binary mixtures of di-isobutylsilanediol (I) and benzene hexa-n-heptanoate. (11). The range over which the one-phase discotic region exists is shown by the darker tone. The paler area indicates the two-phase region where the discotic phase and the isotropic liquid co-exist. At high concentrations of (11) the occurrence of the crystalline solid + mesophase transition is difficult to detect and the dotted line represents where the eutectic might be expected to lie. Numerous compositions in the narrow one-phase region [25-70% of (I)] were in fact examined; only a few points have been indicated in the figure. The continuity of phase was confirmed by contact preparations (G. W.Gray and D. G. Mc- Donnell, Mol. Cryst. Liq. Cryst. Lett., 1977,34,211). On cooling such a preparation to 90 OC, two discotic regions were separated by the isotropic liquid and two-phase regions, but below 86OC, through supercooling effects, a single discotic phase of uniform texture extended across the preparation. Secondly, the two molecular species have very different structures. As will be discussed later, we suggest that the molecular units in (I) have approximate 4-fold rotational symmetry whereas the molecules of (11) have 6-fold symmetry. This difference may however not be significant in the context of discotic mesophase formation since discogenic molecules have now been discovered with 2, 3, 4 and 6-fold rotational symmetry. See, for example, the recent review of discotic phases by Billard.' Nevertheless, in spite of the departure from ideality, we consider that the miscibility continuum observed affords strong support for the hypothesis that the mesophase of (I) is discotic.DISCUSSION MOLECULAR ORDERING I N THE MESOPHASE AND THE CRYSTALLINE SOLID We suggest that the basic structural unit of the mesophase is the hydrogen-bonded dimer shown in fig. 4 and plate lO(a), rather than the extended chain shown in fig. 1. We picture these dimers as stacking on top of one another [plate lO(b)] giving columns which pack together in a hexagonal array. We propose a model for the crystalline solid where further hydrogen bonding links the dimers together, as shown in fig. 5, and causes them to lie tilted with respect to the stack axis.Crystallographic studies to test this hypothesis are at present being undertaken. The preliminary X-ray investigation of the crystalline solid carried out by Bernal et aL2 was reported by Eaborn and Hart~horne.~ The unit cell is triclinic with a = 14.8 A, b = 5.06 A, c = 28.8 A, a = 90°, #I = 121°, y = 96O. At first sight, this does not suggest any obvious model for the molecular arrangement in the mesophase. However, we note that two of the angles are close to 90' and the third is near to 120O. This suggests that it might be possible to redraw the latticeas a distorted hexagonal array:BUNNING, LYDON, EABORN, JACKSON, GOODBY A N D GRAY 719 i-Bu i-Bu \ / 0 -H 0 7\ I I H Y I I I I I I I H - 0 \ /O ii i-Bu i-Bu FIG. 4.-Hydrogen-bonded dimer of di-isobutylsilanediol, which we suggest is the basic structural unit of both the mesophase and the crystalline solid.1 -- HO,si,O--- H HO,,i 'I ,O H - - - HO,si,O 'i H - - - L -I-- - - w-w- FIG. 5.-Extended scheme of hydrogen bonding which we suggest occurs in the crystalline solid of di-isobutylsilanediol. Upper: perspective sketch showing the way the molecules stack in the crystalline solid. Lower: A view perpendicular to the stack axis. Note that symmetry considerations do not require that the dimers should lie normal to the stack axis. this can be done if additional lattice points are included half way along the 28 A axis, as shown in fig. 6. The predominant spacing of such a lattice would be 28/2 x 2/3/2 = 12 A, and the molecules are pictured as lying in the array shown in fig.7. This arrangement appears to be compatible with the partial crystallographic information at present available. This transition from solid to mesophase is pictured as involving untilting the molecular dimers to bring them perpendicular to the stack720 MESOPHASE OF DI-ISOBUTYLSILANEDIOL 14 FIG. 6. FIG. 7. FIG. 6.-Unit cell of the crystalline state of di-isobutylsilanediol. The shaded parallelogram represents the ac face of the unit cell. If additional lattice points are added half-way along the c axis as shown, a distorted hexagonal arrangement is produced. FIG. 7.-Stacking of columns in the pseudo-hexagonal lattice suggested for the crystalline solid. FIG. 8.-Alternative orientations which we suggest each molecule can adopt in the mesophase by virtue of its approximate 4-fold symmetry. axis; presumably this will allow an increase in their rotational motion about this axis.The dimer has approximate 4-fold symmetry and if we add the possibility that each assembly can take up either of two orientations (as shown in fig. 8), this would apparently introduce a satisfactory degree of randomness to account for the partial diffuseness of the 11-12 A ring, If the isobutyl group were to be replaced by a different alkyl residue, say a straight chain, the dimers could no longer have a compact disc-like shape unless the alkyl chain adopted a very non-extended conformation, unusual for a thermotropic mesophase. We can see, therefore, why mesophase formation is not a general property of the alkyl- silanediols and may be restricted to the isobutyl compound.As discussed below, a number of observations of Eaborn and Hartshorne3 are explicable if we assume that the dimers are inclined at an angle to the needle axis in the crystal and if they lie normally (or at an angle more close to 90°) to the stack axis in the mesophase. The crystal + mesophase transition would therefore involve a realignment of the molecules and the breaking of the inter-dimer (but not the intra-dimer) hydrogen bonds. By analogy with other hydrogen bonding for molecules of comparable size (such as alkanols and carboxylic acids), we would have expected an appreciable amount of intra-dimer hydrogen bonding to persist into the isotropic liquid. However, since the mesophase -, isotropic liquid transition enthalpy is only slightly less than that of the crystal mesophase transition, we conclude that the majority of the intra-dimer hydrogen bonds are broken at the mesophase -+ isotropic liquid transition.BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAY 72 1 FIG.9.-Form of disclination which may occur in the rod texture of the mesophase. The curved surfaces in this figure have been drawn to indicate the alignment of the dimer molecules. It is not intended to imply that the mesophase is divided into layers. In some instances the disclinations lie on the surfaces of the slide and cover slip (rather than in the bulk of the phase). In these cases, the pattern of the molecular alignments corresponds to the upper or lower half of this figure [cf. fig. 12(B) and the discussion of conoscopic figures near to the disclinations in ref.(3)]. FIG. 10.-Boundary where two disclination rods intersect. Examples of intersections of this type can be seen in plates 4(u) and (b). Since we are dealing with an area of interface rather than a line of intersection, this boundary remains in focus when the microscope is raised and lowered. OPTICAL TEXTURES Eaborn and Hartshorne3 devoted a considerable portion of their 1955 paper to a description of the optical textures of the mesophase of (I). In the absence of any satisfactory model for the molecular ordering in the mesophase, they were, however, at a loss for an explanation of these textures. In the discussion below, we shall attempt to show that the discotic model offers a satisfactory explanation for the textures of the mesophase itself and, coupled with the model for the crystalline solid given above, it appears to offer a convincing explanation of the changes observed on both heating and cooling the crystalline solid/mesophase boundary.As mentioned above, the appearance of the dendritic islands of mesophase indicates an underlying 6-fold symmetry and this is clearly compatible with the mesophase model suggested. The ‘rods’ in the rod texture are disclination lines where the planes of the dimers lie radially, as shown in fig. 9. Wherever these disclinations intersect, a clear line of intersection can be seen [see plate 4(a) and (b)]. This line remains in focus as the microscope is raised and lowered and, as indicated in fig. 10, represents a plane of interface.As described by Eaborn and Hart~horne,~ the light lines of the rod texture often have a notched appearance and the extinction directions near to these are often oblique (rather than strictly parallel and perpendicular to the disclination). We suggest that these two phenomena are related and have their explanation in terms of the pattern of molecular orientation shown in fig. 11, where the planes of the dimeric molecules describe conical surfaces which approach the disclination line obliquely. We suggest that the reason why these textures are so unlike those of conventional smectic phases must lie in the different relative values of the elastic constants of this722 MESOPHASE OF DI-ISOBUTYLSILANEDIOL FIG. I 1 .-Conical pattern of molecular orientation which we suggest explains the notched appearance of some areas of the rod texture and the oblique extinction in regions adjacent to these disclinations. FIG.12.-Two alternative models proposed for the rod texture by Eaborn and Hartshorne3 (redrawn from their paper). A, The mesophase is visualized as being nematic in type. The vertical and tilted lines represent Si-OH chains of the type shown in fig. 1. B, Here the mesophase is smectic. The curved layers shown are composed of Si-OH chains lying side-by-side. There are objections to both models. Eaborn and Hartshorne preferred B, although, as they stated, it is not easy to see how a structure of this type could give large perfectly homeotropic regions. mesophase (which may not be shared by all discotic phases).The phase has the same mechanical characteristics as a sheet of flexible card: it can be easily bent in one direction, but once it is bent it is much more difficult to bend in a direction at right angles. For comparison, the original postulates (A and B) for the rod texture made by Eaborn and Hartshorne3 are in fig. 12. Model B is essentially a smectic structure with layers of Si-OH chains (of the type shown in fig. 1) running parallel to the layers. A major problem of the model, as discussed by Eaborn and Hart~horne,~ is the difficulty in explaining the perfectly homeotropic nature of the bulk of the sample. Eaborn and Hartshorne3 referred to the areas of the rod texture which appear light under crossed polars as biitonnets. We would however prefer to avoid the use of this term completely because of possible confusion.We suggest that the term should be used solely to describe islands of smectic phase separating out of the isotropic liquid. CHANGES OBSERVED AT THE CRYSTAL/MESOPHASE BOUNDARY It appears that the texture changes observed at the solid + mesophase transition are explicable in terms of a realignment of the dimers from a tilted to a more normal arrangement. At the transition therefore, the structure contracts along the stack axis and expands along a perpendicular direction. For a single isolated crystal, this causesBUNNING, LYDON, EABORN, JACKSON, GOODBY A N D GRAY 723 transverse breaks to occur giving the striated band appearance as described by Eaborn and Hartshome. For spherulites the effect is slightly different.The contraction along the stack axis (i.e. along the radii of the spherulite) causes tangential cracks to occur and the structure changes from a radial array of needles to a tangential array of mesophase domains. The tangential expansion of the phase cannot be accommodated by the spherulite and the mesophase domains are forced to tilt out of a strictly tangential alignment giving patterns as shown in plate 6 . Eaborn and Hartshorne have described a further phenomenon at the solid + meso- phase transition. This was observed when a single crystal was laid across a hole in a cover slip and heated. The parts of the crystal which were in contact with the glass adopted a convoluted scalloped structure and an explanation was offered in terms of a specific interaction between the mesophase and the glass surface.We suggest that the differential expansion of the sample at the solid + mesophase transition offers an alternative explanation. The parts of the crystal in contact with the glass surface will be hotter than the region over the hole and the transition will occur there first. Thus the outer regions of the sample are attempting to contract along one direction and expand along another before any dimension changes occur in the central region; this strain results in a scalloped undulation at the edges. EXPERIMENTAL MATERIALS Di-isobutylsilanediol was made as described by E a b o d and the other dialkylsilanediols mentioned were prepared and purified by the general procedures described by Harris.8 The dichlorodiorganosilanes used in the preparations were provided by Petrarch Systems.OPTICAL MICROSCOPY A N D MISCIBILITY STUDIES Microscopic studies were carried out using a Nikon LKe polarising microscope equipped with a Mettler FP52 heating stage and FP5 control unit. DIFFERENTIAL THERMAL ANALYSIS D.t.a. was carried out using a Stanton Redcroft low-temperature differential thermal analyser. The observed transition temperatures agreed with those obtained by optical microscopy. The system was calibrated using indium metal as standard and enthalpy values are believed to be accurate to f 10%. X-RAY D I FFR A c T ION The X-ray diffraction patterns were obtained with a camera specially constructed for liquid crystal studies in the Astbury Department of Biophysics, Leeds University.The sample was contained in a 0.3 mm diameter Lindemann glass tube and Cu Ka radiation was used. The diffraction pattern was recorded on a flat photographic film. CONCLUSIONS We may summarise our findings and hypotheses in the following form: (1) We concur with the experimental observations of Eaborn and Hartshorne3 but we draw radically different conclusions about the nature of the mesophase. (2) We suggest that the mesophase is discotic and that the basic unit is a dimer in which two molecules are held together by perfectly conventional hydrogen bonds. (3) We suggest that in the crystalline solid the dimer units are hydrogen-bonded together in tilted stacks in an approximately hexagonal array and that at the crystal to mesophase transition the inter-dimer hydrogen bonds break and the dimers take up an untilted orientation. (4) We suggest that the elastic constants of this mesophase are radically different from those of smectic or nematic phases and this gives rise to the distinctive optical textures.724 ME SO PHASE OF D I-ISOB U TY L S I LA NED IOL The authors gratefully acknowledge financial support of their research work by the S.R.C. We also thank Dr N. H. Hartshorne for a critical reading of the manuscript. C. Eaborn, J. Chem. SOC., 1952, 2840. Results obtained by J. D. Bernal, C. H. Carlisle and A. de Rahim and reported in ref. (3). C. Eaborn and N. H. Hartshorne, J. Chem. SOC., 1955, 549. * S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh, Pramana, 1977, 9, 471; a comparison with photomicrographs of discotic textures in the article by J. Billard, J. C. Dubois, N. H. Tinh and A. Zann in Nouv. J. Chim., 1978, 2, 535 is also of interest. J. D. Bunning, J. W. Goodby, G. W. Gray and J. E. Lydon, Springer Series in Chemical Physics 11, Liquid Crystals of One- and Two-Dimensional Order, ed. W. Helfrich and G. Heppke (Springer-Verlag, Berlin, 1980), p. 397. N. H. Hartshorne and A. Stuart, Crystals and the Polarising Microscope (Edward Arnold, London, 4th edn, 1970), p. 447 et seq. 'I J. Billard, Springer Series in Chemical Physics 11, Liquid Crystals of One- and Two-Dimensional Order, ed. W. Helfrich and G. Heppke (Springer-Verlag, Berlin, 1980), p. 383. G. I. Harris, J. Chem. SOC., 1963, 5978; J. Chem. SOC. B, 1970, 488 and 492. (PAPER 1/327)

ISSN:0300-9599    

DOI:10.1039/F19827800713

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. 1, 1982, 78, 725-734 Triplet-state Electron Spin Resonance Studies of Aryl Cations Part 4.l-Low-temperature Kinetics of the Decay of 3Ar+ in Crystalline and Glassy Media BY HANNA B. AMBROZ~ AND TERENCE J. KEMP* Department of Chemistry and Molecular Sciences, University of Warwick, Coventry CV4 7AL Received 17th March, 198 1 The rates and kinetics of decay of triplet-state aryl cations (3Ar+) in the temperature range 82-120 K are highly dependent on both the substituent groups and the nature of the environment. Amino-substituted 3Ar+ decays by second-order kinetics far more rapidly in microcrystalline samples at all temperatures than RO- or RS-substituted analogues, although the activation energies all fall in the range 22.1 & 1.7 kJ mol-I. In LiCl-H,O glasses, 2,5-di-n-butoxy-4-morpholinophenyl cation decays at temperatures below the matrix T, value according to the Dole ‘ Q-function’, which corresponds to two parallel second-order processes taking place in different reaction zones (probably without diffusion between zones): these decays also exhibit ‘stepwise’ behaviour if the temperature is raised during the very slow final section of the decay at a given temperature.At least part of the decay of 3Ar+ results in formation of the species Ar . A large number of kinetic studies of reactions of free radicals in solid-state samples have been obtained by e.s.r. methods, usually at low temperatures and following generation by photolysis and, especially, radiolysis.2 These studies have revealed a number of unique features, such as relatively fast hydrogen-atom transfer reactions at 4.2 K,3 primary kinetic-isotope effects of the order of lo4 (both indicative of quantum-mechanical t~nnelling)~ and a frequent lack of adherence of the observed kinetic processes to simple rate laws.One particularly characteristic feature is the so-called ‘stepwise i.e. the radical concentration falls quite sharply at a certain temperature but then the decay tails off quickly until a near-plateau concen- tration is reached; however, raising the temperature a few degrees will prompt a second sharp fall followed by a new near-plateau region: this behaviour can be stimulated at a series of increasing temperatures until the radical e.s.r. signal finally vanishes. The data for each individual decay often fail to fit classical simple first-, second- or higher-order decays, and the concept of time-dependent rate constants is often invoked.Among the reasons cited for this type of effect is the initial spatial inhomogeneity of the free radicals under investigation following their generation according to the track and spur models of radiolysis. Inhomogeneity of radical products is also to be expected in the photolysis of solid-state samples, especially crystalline materials where most damage will occur at or near the surfaces and at lattice defect centres. While ‘ stepwise ’ kinetics have been demonstrated and analysed for radical pairs in a number of X-irradiated azoxybenzene derivative^,^?^ to our knowledge no comparable behaviour has been reported for ground-state organic triplet species, and rather few kinetic data for these highly reactive entities are available, with the t Permanent address : Institute of Nuclear Research, Warsaw, Poland.725726 E.S.R. STUDIES OF ARYL CATIONS exceptions (i) of the recent, very detailed survey of the decay kinetics of diphenylcarbene and fluorenylidene in various organic glasses9 a and (ii) briefer studies of these carbenes in isobutylene and fluorolube mat rice^.^ In view of the considerable interest attaching to the aryl cation species as an extremely reactive organic intermediate,1° and its x existence as a triplet species (n5) (sp,))', denoted 3Ar+ (species I), when suitably substituted,lJO we have carried out an e.s.r. investigation of its kinetics of decay in crystalline and glassy matrices, noting a profound affect of environment upon the character and rate of the decay process in the temperature regime 77-130 K.As before,' we have generated 3Ar+ by the reaction hv(Hgarc) ArN,+BF,- 3Ar+ + N, + BF,- EXPERIMENTAL Details of materials, sample preparation and photolysis have been given before.' (Samples in LiC1-H,O-acetone are thermally unstable, and were frozen immediately after dissolution without deoxygenation). The conditions of irradiation were reproduced as closely as possible throughout the series of experiments. At an early point in this study, it was found that the rate of radical decay depended on the individual crystalline sample (i.e. on size and manner of preparation). Microcrystalline powder of varying degrees of subdivision were prepared by adjusting the speed of precipitation of the arenediazonium salt from acetone solution on addition of diethyl ether (by varying either the proportion ofether or the temperature).LiCl-H,O glassy samples were prepared by mixing together an acetone solution of the diazonium salt with an equal volume of concentrated aqueous LiCl to give final concentrations of LiCl recorded in the tables. The aryl cations selected for study, and their respective zero-field parameters," were as follows : 2,5-di-n-butoxy-4-morpholinophenyl (4MPh+); D = 0.2346, E z 0 cm-l; 2,4,5-trimethoxyphenyl cation (TMPh+); D = 0.1779, E = 0.0032 cm-l; 2,5-di-n-ethoxy-4-n-butylthiophenyl cation (4BTPh+); D = 0.1684, E = 0.0029 cm-l. Kinetic experiments were performed using the kinetic mode of the Bruker ER 200 tt e.s.r.spectrometer while the magnetic field was locked on the maximum of the Hmin feature of 3Ar+. At the lower temperatures, decays of 3Ar+ were followed for ca. 100 min or until a plateau was reached. In all cases, the sample was irradiated outside the e.s.r. cavity at 77 K and was then transferred into the cavity which was temperature-stabilised at T < 79 K with an Oxford Instruments model E.S.R. 9A cryostat cooled by a flow of cold N,. The position of Hmin having been located, the temperature was adjusted upwards to a value from 82 to 120 K to an accuracy of k 0.1 K and with a stability of k 0.1 K effected by an automatic electric heating unit. The first few minutes of each decay were 'lost' because of the time needed to achieve thermal equilibrium. Concentrations of 3Ar+ in different samples were estimated by comparison of the Hmin feature with weighed samples of diphenylpicrylhydrazyl (DPPH) under identical instrumental conditions; we estimate that (initial) C3Ar+] is typically (0.5-1.5) x low3 mol dm-3.(Hmin is the strongest peak in the triplet spectrum because it features absorption by all the triplet states except those in the exact canonical orientations.) No temperature- or time-dependence of linewidth of the Hmin feature was apparent in the temperature regime utilised.H. B. AMBROZ AND T. J. KEMP 727 RESULTS AND DISCUSSION PRINCIPAL TRENDS The rate constants for decay of 3Ar+ show great sensitivity towards both the pattern of ring substitution and the nature of the matrix (fig.1): the latter is particularly significant in that while kinetic decays in microcrystalline powders of the arenediazo- 25 20 " I v) m I 1 E a I d & ; \ Y 10 E 82 85 90 95 100 105 110 115 120 TIK FIG. 1.-Second-order rate constants for the decay of Ar+ at different temperatures. 0, 4MPh+ (fine powder); ., 4MPh+ (powder); x , TMPh+ (fine powder); +, 4BThPh+ (fine powder); A, 4MPh+ (dilute solution in acetone glass); A, 4MPh+ (concentrated solution in acetone glass); 0, 4MPh+ (1.2 mol dm-3 LiC1-water); 0, 4MPh+ (6.0 mol dm-3 LiC1-water). nium salt are second-order, and those in acetone solution approximately so, those of the same salts dissolved in LiC1-water glasses (containing ca. 50% v/v acetone as co-solvent) are much slower, and are not of simple kinetic order (see below).(The examples of decay in LEI-H,O shown in fig. 1 refer only to those sections of the total decay process which fit to a second-order plot).728 E. S. R. STUDIES OF ARYL CATIONS RATES AND ENERGETICS OF DECAY The sequence of rates in different matrices is, for 4-MPh+: rapidly > concentrated > slowly > dilute precipitated solution in precipitated solution in crystals acetone glass crystals acetone glass (fine powder) (powder) > LiCl-H,O glass (1.2 mol dm-3 LiCl) b LiCl-H,O glass (6.0 mol dm-3 LEI) and rate constants at different temperatures are summarised in table 1 and 2. The effect of ring substitution on the decay rate is also clear from table 1; thus the decay rate TABLE RATE CONSTANTS k (mol-l dm3 s-l) AND ACTIVATION ENERGIES EA (kJ mol-l) FOR THE DECAY OF 3Ar+ GENERATED IN MICROCRYSTALLINE POWDERS TMPh+ 4BTPh+ 4MPh+ k at ~ temp./K fine powders powder 82 85 90 95 100 105 110 120 E A - - 0.119 - - - 0.319 - 0.104 0.019 7.18 0.436 0.214 0.126 24.10 2.18 0.447 0.464 72.54 7.62 2.08 2.92 - 27.47 19.64 14.34 - - 20.8 23.6 20.4 23.7 - 1.57 - - TABLE 2.-APPARENT RATE CONSTANTS k (m0l-l dm3 S-l) AND ACTIVATION ENERGIES E A (kJ mol-l) FOR THE SECTIONS OF THE SECOND-ORDER DECAYS OF 4MPh+ IN GLASS MATRICES parent diazo salt concentrations/mol dmT3 ca.0.1 ca. 0.5 LiCl glass k at ca. 1.2 mol dmP3 ca. 6 mol dmP3 temp./K acetone glass LiCl LiCl 90 0.098 0.712 0.042 0.058 95 0.287 1.446 0.165 0.104 1 00 1.49 4.28 105 6.94 13.86 0.330 0.253 115 - - 1.23 1.21 120 - - 4.31 2.75 EA 22.2 14.2 12.6 11.5 - -H.B. AMBROZ A N D T. J. KEMP 729 of 4MPh+ in fine powders is ca. 10, times faster than those of either TMPh+ or 4BTPhS at all temperatures, although the activation energies for all three species lie in the range 22.0f 1.6 kJ mol-l. The notably slower decay of 4MPh+ in ‘powder’ samples compared with ‘fine powder’ is associated with a small difference in activation energy (table 1). In a ‘fine powder’, 4MPh+ decays perceptibly even at 82 K, while this is the case for the other two aryl cations only at 95 K: at 110 K the decay of 4MPh+ in a ‘fine powder’ is too fast for observation using the chart recorder, while at 120 K the decays of TMPh+ and 4BTPh+ are followed easily. 4MPh+ was chosen for more detailed study as the representative species, the majority of 3Ar+ species known bearing a 4-amino substituent.l? lo* l1 The energetics of decay in LiCl-H,O and acetone glasses are generally of considerably lower values (with the exception of the 0.1 mol dm-3 solution in acetone), all falling in the range 13 f 1.5 kJ mol-l.(Diphenylcarbene decays in organic glasses with E A in the range 2-20 kJ m01-l.~) The larger E A value for the more dilute solution implies an increased role of diffusion in this molecular glass. tl s FIG. 2.-Second-order plots (in terms of spin concentration) for the decay of Ar+ in microcrystals at 100 K. A, 4MPh+ (fine powder); x ,4MPh+ (powder:concentration-’ units x 2); .,4BThPh+ (powder:concen- tration-’ units x 50); 0, 4BThPh+ (powder: concentration-’ units x lo).REACTION ORDERS OF DECAY All three arenediazonium salts gave good second-order kinetics in crystalline powders of all state subdivision (fig. 2). Approximately second-order behaviour was also shown in acetone glasses. In LiCl-H,O glasses, however, the kinetic situation proved much more complex indicating, we believe, a determining role of the matrix (fig. 3). Attempts to fit the data to a simple kinetic law -dc/dt = kcn gave seemingly nonsensical results, e.g. fits to plots of [Ar+]-n against time with n varying from n > 4 to n = 0 varying within a single run (i.e. n is time-dependent), and recourse was made to more complex models. A typical decay is shown in fig. 4. (N.B. some portions of these decays gave an approximate but inexact fit to [Ar+]-l against time plots, yielding the apparent second-order rate constants listed in tables 1 and 2 solely for comparison with those measured in other media, e.g.the sections of fig. 3 at t > 2400 s).730 E.S.R. STUDIES OF ARYL CATIONS Altogether five kinetic expressions previously developed for solid-state reactions (i) Waite's equation12 for a second-order diffusion-controlled reaction : were utilised, viz. c-l - co-1 = IC[ 1 + 2ro/(nDt)q t where c, co are the concentrations of Ar+ at times t, to; IC is 4nr0D; D is the diffusion coefficient of the reactant species and r,, is the Smoluchowski 'reaction radius' within which reaction occurs with 100% probability. This equation is often tested13 in the plot of (c-l- c0-l) t* against ti, to give a line slope IC and an intercept 2rOk-/(nD)k I I I I I I 1 600 1200 1800 2400 3000 3600 4200 tls FIG.3.-Plots of [spins]-' against time for glassy solutions of 4MPh+ at 105 K. A, Concentrated 4MPh+ in acetone glass (spins-' units x 2); A, dilute 4MPh+ in acetone glass (spins-' units x 2); 0, 6.0 mol dm-3 LiC1-water; 0, 1.2 mol dm-3 LiC1-water. :: initial intensity (77 K) 16 ' 25 50 75 100 tlmin FIG. 4.--Kinetic decay of Hmin absorption for 4MPh+ in 6.0 mol dm-3 LiC1-water glass at 105 K. (Note initial rapid decay before attaining thermal equilibrium.)H. B. AMBROZ A N D T. J. KEMP 73 1 (ii) Dole’s expression14 (the so-called ‘ Q-function ’, a development of Waite’s equation) which applies to two simultaneous second-order decays with or without diffusion control and without exchange between reaction zones : Q G (c-l-co-l)-lt where l+CoXfxs(kf+k,)t = - 1 (1 +(K-BQ)t) = xf2kf+x~ks+coxfxskfk,t A and kf and k, refer to the second-order rate constants for the ‘fast’ and ‘slow’ zones, respectively, and x, and x, are the mole fractions of the radicals in ‘fast’ and ‘slow’ I I 1 I I I I I I 1 5 10 15 20 25 30 35 LO L5 50 55 FIG.5.-Test of the Dole Q-function for the decay of 4MPh+ in 1.2 mol dm-3 LiC1-water glass. Numbers refer to temperature (K), bracketed figures to scaling factor. zones, respectively, at time zero. Discrimination between the diffusion control and non-diffusion control cases can be achieved with a plot of Q against h: at very short times Q extrapolates either to a non-zero value (when non-diffusion control is operative) or to zero (when the process is diffusion-controlled). (iii) Two composite first-order reactions.15* l6 Designating c, and cf as the concen- trations of slowly- and rapidly-decaying triplets, respectively, then with c = c, + cf, In (c - cs)/c, = In (C,~/C,~) - (k, - k,)t.(iv) The so-called ‘root t’ law, uiz.17 c = co exp (- kti)). (v) The plot of c against 1nt.l8 The best fit of the decay kinetics of 4MPh+ in LiC1-H,O-acetone glasses was unquestionably with the Dole function14 (fig. 5 and 6). This seems to exhibit a positive intercept at some temperatures, implying non-diffusion control. However, there732 E.S.R. STUDIES OF ARYL CATIONS remains the possibility that in sample manipulation we have missed a critical early stage of the decay which would have led to an intercept of zero.Linear plots were obtained in the temperature range 95-1 15 K, while outside this range the more typical14 curved (or two-stage) plots were found. Consequently it appears that 4MPh+ disappears in two distinct, non-overlapping reaction zones, which correspond to ~~ ~ reaction within two different microenvironments 7 5 120 [ x 1021 at 4MPh+. 1 - 1 I I I I 1 I 1 I I I 5 10 15 20 25 30 3 5 40 45 50 55 FIG. 6.-Test of the Dole @function for the decay of 4MPh+ in 6.0 mol dm-3 LiC1-water glass. Numbers refer to temperature (K), bracketed figures to scaling factor. SUBSTITUENT EFFECTS Comparison of the decay rates of the three arenediazonium salts was restricted to 'fine powder' specimens (table 1). Decay of 4MPh+ was always more than an order of magnitude faster than those of TMPh+ and 4BTPh+ (table 1 and fig.I), although the activation energies follow no clear trend, being rather similar for all three cations. The kinetic lability of aryl cations with N-donor substituents in the 4-position explains why no triplet signal was obtained from powdered irradiated 4-aminobenzene- diazonium tetrafluoroboratell (although clear triplet resonance was achievedl using a LiCl-H,O medium). Evidently while the p-NR, substituent provides the greatest electronic stabilisation of the triplet state of the phenyl cation (as opposed to its singlet co~nterparf~~), and the highest zero-field splitting parameters,' this substituent renders the cation more highly reactive than p-OR and p-SR substituents. EFFECT OF MATRIX We have noted above the profound effect of the matrix on the rate and kinetic order of the decay process.These differences can arise from either one or a combination of physical effects as follows. (i) The crystalline samples will undergo photolysis primarily at the surface to give high local concentrations of the resulting ionic radical products, which will favour reaction with neighbouring fragments. The reactive species will not be solvated, but will experience a local crystal field manifest in the site-splitting of the Hmin and other components of the triplet spectrum.20H. B. AMBROZ A N D T. J. KEMP 733 (ii) The glassy sample will undergo photolysis throughout the bulk of the sample, to give a fairly homogeneous distribution of the photofragments which will be situated in a rigid solvent environment.The reactions of the fragments will be controlled by the solvent (either through direct reaction or as a result of diffusion to a counter-ion or other ion). The resulting triplet e.s.r. spectrum is much broader than in the crystalline samples and features no site-splitting. There are also some detailed differences within the two classes of medium, as given below. CRYSTALLINE MATRICES We have noted above that while the decay kinetics are strictly second-order in all types of crystalline matrix, the rates are greatly affected by the mode of preparation of the crystals. Rapidly-precipitated (and therefore smaller) crystals feature a ca. ten-fold faster decay of 4MPh+ than more slowly precipitated material. The origins of this difference may be (i) the state of subdivision of the sample, (ii) the possibility that different samples may have different crystal structures (or may consist of different combination of co-existing, different structures), (iii) the level of defect centres in different samples.We have noted before20 the site-splitting of the triplet resonance spectrum of several 3Ar+, and that different samples of the same material feature different ratios of the well-defined sites following photolysis (some sites even being absent in some samples and present in others). Note that while the effect of crystal size is important, it is less significant than that of the 4-substituent group. The decay process is presumably charge neutralisation of 3Ar+ by a local source of electrons (presumably the counter-ion).Significantly, as [3Ar+] falls during a controlled warming experiment, [Ar ] actually increases slightly, but reproducibly . This situation is rather similar to that seen for LiCl glassy solutions. GLASSES The two types of glass utilised, namely acetone and LiCl-H,O, differ in several important ways. Acetone forms a molecular glass, which is typically characterised by a weak network,,l (relatively) low viscosity and a low value (probably ca. 60 K) for the T, parameter in the VTF (or Fulcher) equation21*22 W( 7) = A exp [ - B / ( T - T,)] where W( 7) is the fluidity (or reciprocal relaxation time), T is the temperature, A and B are characteristic parameters and T, is the temperature at which mass transport ceases. LEI-H,O glass by contrast has a strong, ionic a high viscosity and a high value for T,, e.g.129 K for a 9.5 mol dm-3 sample.24 Our experiments were confined to the temperature range 80-120 K and presumably refer to ‘non-diffusional’ conditions. Kinetic decays of 3Ar+ in the acetone glass were approximately second-order and refer presumably to a bimolecular process. The presence of large molar concentrations of acetone has little effect on the decay rate of 3Ar+, implying little role for any type of C-H fission process, viz. 3Ar+ + Me,CO + ArH + +CH, COMe. The order in LiCl-H,O glass is complex (see above) and fits most satisfactorily to the Dole Q-function (see above), although we cannot be sure that the non-zero intercepts of fig. 5 and 6 are real because of the difficulty of obtaining data at very short times, and consequently cannot discriminate between the ‘ diffusion ’ and734 E.S.R.STUDIES OF ARYL CATIONS ‘non-diffusion’ situations. The decay of 3Ar+ is probably due to a multiplicity of reactions, but careful study in a critical temperature range of the decay of 3Ar+ in a number of samples revealed a small, but definite and reproducible increase in the concentration of Ar species, implying electron-transfer to 3Ar+ from its environment, the main candidate as the source of the electron being Cl-. Such a process would explain the comparatively fast initial stages of the decay of 3Ar+ exemplified in fig. 4 for these samples. Clearly the decay of 3Ar+ is a highly complex process, depending on a variety of factors including the substitution pattern of 3Ar+, the nature of the matrix, the temperature and the concentration of the parent arenediazonium salt.While at very low temperatures electron-transfer dominates, at higher temperatures other processes became significant. The main differences between our study and the parallel one concerning neutral carbenes9 is that these decay by hydrogen-atom transfer processes involving, it seems, some degree of tunnelling. The kinetics involved are also complex, with fits to log c against or b plots, although the process is regarded as essentially pseudo-first-order, with different sites being regarded as the source of different reactivity. Note that plots of this type give rather good fits to second- or third-order reactions and are consequently not very discriminating.It may be that attempts to explain the complex kinetics features of these systems (and of 3Ar+) in terms of a single mechanism are an over-simplication. We thank the S.R.C. for financial support of H.B.A. and for purchase of the e.s.r. spectrometer, the magnetometer and the cryostat. Part 3. H. B. Ambroz and T. J. Kemp, J. Chem. SOC., Perkin Trans. 2, 1980, 768. T. J. Kemp, in Electron Spin Resonance, senior reporter P. B. Ayscough (Specialist Periodical Report, The Chemical Society, London, 1979), vol. 5, chap. 9. E. D. Sprague, J. Phys. Chem., 1977, 81, 516. J. J. Tria and R. H. Johnsen, J. Phys. Chem., 1977, 81, 1274 and references given therein. R. Bensasson, M. Durup, A. Dworkin, M. Magat, R. Marx and H. Szwarc, Discuss. Faraday SOC., 1963, 36, 177. A. I. Mikhailov, Ya. S. Lebedev and N. Ya. Buben, Kinet. Katal., 1964, 5, 1020; 1965, 6, 48. (a) J. J. Tria and R. H. Johnsen, J. Phys. Chem., 1977, 81, 1279; (b) J. J. Tria and R. H. Johnsen, J. Phys. Chem., 1978,82, 1235. @ (a) V. P. Senthilnathan and M. S . Platz, J. Am. Chem. SOC., 1980, 102, 7637; (b) C-T. Lin and P. P. Gaspar, Tetrahedron Lett., 1980, 3553. lo H. B. Ambroz and T. J. Kemp, Chem. SOC. Rev., 1979, 8, 353. l1 H. B. Ambroz and T. J. Kemp, J. Chem. SOC., Perkin Trans. 2, 1979, 1420. l2 T. R. Waite, Phys. Rev., 1957, 107, 463, 471. W. Y. Wen, D. R. Johnson and M. Dole, J. Phys. Chem., 1974,78, 1798. l4 M. Dole, C. S. Hsu, V. M. Patel and G. N. Patel, J. Phys. Chem., 1975, 79, 2473. l6 W. G. French and J. E. Willard, J. Phys. Chem., 1968, 72, 4604. !3 K. Tonyama, K. Nunome and M. Iwasaki, J. Am. Chem. SOC., 1977,99, 5823. D. R. Johnson, W. Y. Wen and M. Dole, J. Phys. Chem., 1973 77, 2174. B. V. Bol’shakov, A. A. Stepanov and V. A. Tolkatchev, Int. J. Chem. Kinet., 1980, 12, 271 and references given therein. V. Attarian, H. Szwarc and L. Ter Missian, J. Chim. Phys., 1961, 58, 837. J. D. Dill, P. v. R. Schleyer and J. A. Pople, J. Am. Chem. SOC., 1977, 99, 1. 2o H. B. Ambroz and T. J. Kemp, Chem. Phys. Lett., 1980. 21 C. A. Angel1 and K. J. Rao, J. Chem. Phys., 1972, 57, 470. p 2 R. J. Greet and D. Turnbull, J. Chem. Phys., 1967,47, 2185. 23 G. W. Brady, J. Chem. Phys., 1958, 28,464. 24 G. V. Buxton, F. C. R. Cattell and F. S. Dainton, J. Chem. Suc., Faraday Trans. I , 1975, 71, 11 5. (PAPER 1/436)

ISSN:0300-9599    

DOI:10.1039/F19827800725

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1982, 70, 735-745 Mechanism of the Catalytic Reduction of Nitric Oxide with Ammonia by a Solution of Dinitro-(alkyldiamine) ColI1 Complexes BY SHUICHI NAITO* AND KENZI TAMARU Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 23rd March, 1981 It is demonstrated that the reduction of nitric oxide by dinitro-(alkyldiamine) ColI1 complexes proceeds through the dissociation of the amine group of the alkyldiamine ligand, followed by the formation of mononitrosyl and dinitrosyl intermediate complexes from which N, and N,O are produced, respectively. In aqueous solution, the coordination of NO and NH, to the catalytically active intermediate complex is the rate-determining step, which shifts to the reaction of coordinated nitric oxide with ammonia in DMF solution. The stabilities of these intermediate complexes depend on the structure of the alkyldiamine ligands, which cause different selectivities for N, and N,O formation.Heterogeneous catalytic reduction of nitric oxide by CO, H, or NH, over various noble metals and metal oxides has been extensively studied and several mechanisms have been proposed.lP2 Some of these mechanisms involve the dissociation of nitric oxide as an initial step,, but in other cases it is considered that surface complexes of nitric oxide and reducing agents are formed.* On metal oxides, oxidation of the surface by nitric oxide and its subsequent reduction by reducing agents have been proposed as the catalytic cycle.5 However, in homogeneous systems this reaction is not well known and only a few systems for the NO-CO reaction have been reported, these including the RhI-RhIII and PdII-CuI redox cycles.Recently, we reported for the first timea that COII~(LL)~_,(NO~)~ ions (LL = ethyl- enediamine, triethylene tetramine, etc.) catalyse the reduction of nitric oxide with various amines, including ammonia, to form nitrogen and nitrous oxide in water, DMF, DMSO, etc. Although some mechanisms have been proposed for the NO-NH, reaction in heterogeneous system^,^ it has not been investigated in homogeneous systems. We now report the mechanisms of this reaction in aqueous and DMF solutions obtained using electronic spectra and n.m.r. during the reaction and also kinetic effects due to changing the amine ligands and the solvents.EXPERIMENTAL REACTION PROCEDURE 0.1-1 mmol of the complex was dissolved in 100-150 cm3 of freshly distilled H,O or DMF and poured into the reaction vessel (250 cm3 round-bottom flask), which was connected to the closed gas circulation system (total volume ca. 400 cm3). Before the reaction, the complex solution was thoroughly degassed with repeated freezing by liquid nitrogen. After warming to room temperature, the solution was stirred vigorously using a magnetic stirrer and a mixture of NO+NH, gases was introduced onto the catalyst 735736 CATALYTIC REDUCTION OF NO WITH NH, solution. In most experiments with aqueous solutions, concentrated aqueous ammonia was used instead of ammonia gas, the aqueous ammonia being introduced into the complex solution using a pipette in a side arm of the reaction vessel whilst purging the whole system with nitrogen gas.Nitric oxide was purified by repeated distillation between a liquid-nitrogen cold trap and an ethanol slush bath. NH, was purified by methanol + dry ice and liquid-nitrogen cold traps. For the kinetic experiments, a freshly prepared catalyst solution was used for each run and the initial rates of N, and N,O formation were measured, due to the deactivation of the complexes at later stages of the reaction. The analysis of the composition in the gas phase was performed by gas chromatography (molecular sieve 13X for N, and NO and Chromosorb 103 for N,O and NH,). For isotope-labelled experiments, 15N0 (nitrogen-1 5 , purity 96%, from Prochem.B.O.C. Limited) was used without further purification. The isotopic distributions of the products N, and N,O were analysed using a quadrupole mass spectrometer (UTI) with an ionization voltage of 70 eV. ELECTRONIC SPECTRA A N D N . M . R . DURING THE REACTION For the measurements of electronic spectra during the reaction, another small reaction vessel was used, with a side arm connected through a stopcock to a qualtz cell. The reaction was carried out with the same procedure mentioned above and the spectra at each stage were recorded using a Hitachi 340 spectrophotometer after pouring a part of the solution into the quartz cell. For n.m.r. experiments, another small vessel was used with several side arms connected to n.m.r. tubes. At each step of the reaction some of the catalyst solution was poured into one of the n.m.r.tubes, which was then cut off from the vessel using a gas burner. N.m.r. measurements were carried out using a R-24B Hitachi n.m.r. spectrometer. PREPARATION OF THE COMPLEXES The complexes employed in this study were prepared using a method similar to that reported by Holtzclaw et d.lo In most cases, stoichiometric amounts of the amine solution were added dropwise to an aqueous solution of Na,Co(NO,),. After stirring for several hours, the precipitate was filtered, recrystallized from water and checked by infrared and n.m.r. spectroscopy and elemental analyses. Calculated for Co(en),(NO,), : N, 30.9; H, 5.1 ; C, 15.2; found: N, 30.7; H, 5.4; C, 14.8. Calculated for Co(trien) (NO,),: N, 27.2; H, 5.5; C, 19.9; found: N, 27.2; H, 6.0; C, 19.6.RESULTS REACTIONS OF AQUEOUS SOLUTIONS OF THE COMPLEXES WITH VARIOUS AMINE LIGANDS The NO-NH, reaction was investigated in aqueous solutions of CoI" (LL)(NO,): complexes with different amine ligands (LL). It was recognized that there are two types of reactions with different diamine or tetramine ligands. When such diamine ligands as H,NCH,CH,NH,, H,NCH,CH,NH(CH,) and H,NCH,CH,N(CH,),, which have at least one primary amine group, were employed in the complex, both N, and N,O were formed without appreciable absorption of NO, as shown in fig. 1. However, when such ligands as (CH,)HNCH,CH,NH(CH,), (CH,),NCH,CH,N(CH,), and triethylenetetramine were used, considerable amounts of NO were absorbed into the solution accompanied by the production of N,O with a very small amount of N,, as shown in fig.2. In the case of the triethylenetetramine complex, a dinitrosyl complex was precipitated due to evaporation of the solution during the reaction. The precipitate had infrared bands at 1880 and 1800 cm-l, characteristic of coordinated dinitrosyl species. However, it could not be purified by recrystallization because of its insolubility after precipitation (analysis, found: N, 25.9; H, 4.7; C, 15.6). As shown in fig. 1, the reaction did not proceed when only NO or NH, was present, which suggests that coordinated NO, groups or diamine do not react directly with NH, or NO to form N, or N,O. However, to initiate the reaction, NH, could beS. NAITO AND K. TAMARU L 737 4 . A reaction time/h FIG.1 .-NO-NH, reaction in aqueous solution of the cis-CO(en),(NO,), complex. T = 298 K, 1.6 mmol complex in 100 m3 H20, NH, = 0.3 mol. (a) Only NO was introduced; (b) after the evacuation of NO, NH, added to (a); (c) NO introduced to (b). 0, NO; 0, N,; A, N,O. 0 10 20 30 reaction timelh FIG. 2.-NO-NH3 reaction in aqueous solution of the Co(trien) (NO,), complex. T = 298 K, 1.5 mmol complex in 100 cm3 H,O, NH, = 0.5 mol. 0, NO; 0, N,; A, N 2 0 ; x , total amount of N species fixed in the solution. substituted for some other amine such as excess ethylenediamine, methylamine, ethylamine, n-butylamine or butylamine. In fig. 3 we see the linear dependence of the initial rates of N, and N,O formation upon the basicity of the amine, which indicates an important role of OH-.Even NaOH solution could initiate the reaction with nitric oxide to produce N, and N,O but the decomposition of the complex itself caused some unknown black materials to be precipitated in this case. REACTION KINETICS I N AQUEOUS SOLUTIONS The initial rates of N, and N,O formation by cis-Co(en),(NO,), complex in aqueous solution depend linearly upon the concentration of the complex and the concentration738 80 70 0, !i.$50- 5 ,840- .z 2 2; a;., 60- o h E m Z .+ 5 3 0 - *- 20 10 CATALYTIC REDUCTION OF NO WITH NH, - - .A ' 2;* , 3 y" a5 , J - ; ..'i 4 ...' I 0 1 2 3 amount of the complex/ 1 (r3 mol FIG. 4.-Dependences of the inititial rates of N, and N,O formation upon the concentration of cis- or trans-Co(en),(NO,),. T = 299 K, 100 m3 H,O solution, NH, = 0.3 mol, PNo = 4 x lo4 Pa.0, VN,, and A, VNx0 for [cis-Co(en),(NO,),)NO,. a, V N , and A, VN,O for [cis-Co(en),(N0,),]NO2. 0, V N 2 and A, VN,O for [ trans-Co(en), (NO,),]NO,. of NH,, as shown in fig. 3 and 4. Fig. 4 also shows that the counter-anions, NO; and NO;, have little effect upon the rate of reaction. On the other hand, fig. 4 shows that there exists a large trans effect in this reaction, that is the cis-Co(en),(NO,)~ complex has a catalytic activity 3-4 times larger than the trans-Co(en),(NO,)$ complex for the initial rates of N, and N20 formation. This effect may be explained by the electron- withdrawing nature of the nitro group when it is in the trans position, which facilitates the nucleophilic attack of OH- to dissociate the coordinated amine.Dependence of the reaction rate upon the partial pressure of NO was also examined. As shown in fig. 5, the reaction order was unity with respect to the intial rate of N, formation and 1.3-1.5 for N,O formation. As the reaction proceeded, the complex gradually lost its catalytic activity and over a period of ten catalytic cycles the reaction rate decreased to one-fifth of its initial value. After a long period of reaction, the Co(en)(NH,)(H,O)(NO,), complex (analysis, calculated : N, 28.8 ; H,S. NAITO AND K. TAMARU 15 .s - 2 E 10- c1 41 \ "c1 c: g 5 - 739 - +yA+4' 1 ,+40 @ I . . . , . . , . . 4 .O 4.5 FIG. 5-Dependences of the initial rates of N, and N,O formation upon the partial pressure of NO. T = 299 K, 1.5 mmol cis-Co(en),(NO,), in 100 cm3 H,O, NH, = 0.6 mol.0, VN,; A, VN,O. log pN, 4.5; C, 8.2; found: N, 29.7; H, 4.8; C, 8.9) was obtained from the catalytic solution, which suggests that the dissociation of ethylenediamine is an important process for this reaction. But the Co(en)(NO,)i complex itself showed very low catalytic activity. REACTION I N DMF SOLUTION When the Co(en),(NO,), complex was dissolved in DMF, the catalytic behaviour for the reduction of nitric oxide by ammonia was similar to that in aqueous solution. When only NO or NH, was present the reaction did not proceed, but when both of them were introduced into the system, N, and N,O were formed in the manner shown fig. 1. Dependences of the initial rate of N, formation upon the concentrations of Co3+, NO and NH, were also examined. The rate is proportional to the concentra- tion of the complex and the reciprocal of the rate is inversely proportional to the concentration of NH, and to the partial pressure of NO (fig.6).740 CATALYTIC REDUCTION OF NO WITH NH, The reaction rate in DMF solution is shown much faster than that in aqueous solution when we consider the difference of ammonia concentration in fig. 3 and 6. The effect of added water on the reaction rate in DMF solution was studied and the results are shown in fig. 7. The reciprocal of the rate of N, formation is proportional to the amount of H,O added. ~ 0 5 10 concentration of added water (volume %) FIG. 7.-Effect of added water on the initial rate of N, formation by the cis-Co(en),(NO,), complex in DMF solution. T = 299 K, 0.2 mmol complex in (50 -x) cm3 of DMF and x cm3 of H,O, PNo = 4 x lo4 Pa, NH, = 280 cm3.ELECTRONIC SPECTRA AND N . M . R . DURING THE REACTION The electronic spectra of the cis-Co(en),(NO,), complex during the NO-NH, reaction in aqueous and DMF solutions were studied. In aqueous solution, the complex has an absorption band around 330 nm, which did not change on addition of NO and NH,. This is consistent with the kinetic results that the rate-determining step would be the coordination of NO and NH, to the original Co(en),(NO,), complex. However, in DMF solution the complex possesses an absorption band at 324 nm, as shown in fig. 8. On the addition of only NO or NH,, this band did not change, but when both NO and NH, were added its intensity decreased considerably in 20 min and a new band around 340 nm appeared slowly as the reaction proceeded.Considering the kinetic results, it is reasonable to suppose that the first decrease in intensity of the band at 324 nm corresponds to the formation of some intermediate complex coordinated with NO and NH,, while the increase in intensity of the bond at 340 nm corresponds to the deactivation process of the complex, which will be discussed again later. Fig. 8 shows how the spectrum of the trans-Co(en),(NO,), complex changes; the spectrum initially has an absorption band at 352nm and undergoes similar changes to those found for the spectrum of the cis complex. Proton n.m.r. spectra of the trans-Co(en),(NO,), complex in [2H,]-DMS0 solution during the NO-ND, reaction are given in fig. 9. The resonance peak at S = 2.4 can be assigned to a -CH,- proton of coordinated ethylenediamine and 6 = 4.7 to -NH,. The peak at 6 = 3.3 is due to the residual HDO in the solvent.When ND,S. NAITO A N D K. TAMARU 74 1 cis - t r a n s - Co (e n12(N0,1, 300 LOO 500 wavelength/ nm FIG. 8.-Electronic spectra of the cis- and trans-Co(en),(NO,), complex in DMF solution during the NO-NH, reaction. T = 299 K. I, Spectra of the complexes in DMF solution (no spectrum change was observed when only NO or NH, was added); 11, P,, = 4 x lo4 Pa, NH, = 200 cm3 added to I, spectrum taken after 10 min; 111, after 30 min; IV, after 900 min. 5 L 3 2 1 0 6 (PPm) FIG. 9.-N.m.r. spectra (60 MHz) of trans-Co(en),(NO,), in [2H,]-DMS0 solution during the N@NH, reaction (internal reference: Me,SiC,H,SO,Na).I, Spectra of the complex in [zH,]-DMSO; 11, ND, introduced into I ; 111, NO+ND, reaction for 1 h; IV, after exposure to air.742 CATALYTIC REDUCTION OF NO WITH NH, was added to the system, the spectra did not change, except for the disappearance of the peak at 6 = 3.3 due to the isotopic dilution of HDO with ND,. After the addition of NO, a new peak appeared at 6 = 3.1, a position intermediate between that for the -NH, proton of coordinated ethylenediamine (6 = 4.7) and that of free ethylenediamine (6 = 1.5). On exposure of this solution to air, this new peak disappeared completely suggesting that it comes from some intermediate complex formed in the NO-NH, reation. ISOTOPE-LABELLED EXPERIMENTS To examine the nitrogen sources of the products N, and N,O, the reaction between 15N0 and 14NH, was studied in a DMF solution of the Co(en),(NO,), complex.As shown in table 1, the N, produced was mainly 15N14N, indicating reaction between 15N0 and 14NH, molecules. On the other hand, N20 was mainly 15N15N0, indicating TABLE 1 .-YIELDS OF 15NCG4NH, REACTION WHEN CATALYSED BY cis-CO(en),(NO,), COMPLEX IN DMF SOLUTION NO conversion 14N14N 14N15N 15N15N 14N14N0 l4NI5NO 15N15N0 0.18 0.05 0.95 0.00 0.04 0.02 0.94 0.35 0.04 0.96 0.00 0.02 0.02 0.96 0.55 0.03 0.96 0.0 1 0.01 0.02 0.97 0.73 0.03 0.96 0.0 1 0.0 1 0.02 0.97 reaction between two nitric oxide molecules. These results also suggest that the coordinated nitro group does not participate in the formation of N,, because the amount of 14N14N formed was negligible, considering the purity of 15N0 employed (15N, 96%).DISCUSSION Since the starting Co(LL),.,(NO,)~ complex is a coordinatively saturated octahedral complex, it is necessary that the dissociaton of some ligand precedes the coordination of NO or NH,. Because of the necessity of the presence of NH, or other bases to initiate the reaction, OH- seems to play an important role in the dissociation step. The process may be similar to the one postulated in the case of the base hydrolysis of the Co (en),(NO,)Cl+ complex1lYl2, cited as a S,lCB mechanism: Co(en),(NO,)Cl+ + OH- Co(en)(en-H)(NO,)Cl + + H,O (1) Co(en)(en-H)(NO,)Cl+ Co(en)(en-H)(NO,)+ + C1- (2) (en-H = H, NCH, CH,NH-). It is reasonable to suppose that step (1) is also present in the case of the Co(en),(NO,)z complex.However, since Co(en),(NO,)Cl+ did not exhibit any catalytic activity for the NO-NH, reaction, Co(en)(en-H)(NO,)+ cannot be the catalytically active species, which may be formed by the dissociation of one of the NO; groups Co(en)(en-H)(NO,): complex [step (2)]. Consequently, it is necessaryS. NAITO A N D K. TAMARU 743 to consider some other leaving ligands, which should be one of the coordinated amine groups of ethylenediamine as follows: (1’) (2’) (NO,), (S)+ + OH- (3) Co(en),(NO,): +OH- $ Co(en)(en - H)(NO,): + H,O Co(en)(en - H)(NO,): + S + Co(en)(H,NC,H,NH-)(NO,),(S)+ Co(en)(H,NC,H,NH-)(NO,),(S)+ + H,O Co(en)(H,NC,H,NH,) ( S = solvent molecule; N = dangling amino group). However, the other amine group of the ethylenediamine should stay on the cobalt cation to maintain the catalytic activity, because Co(en)(NO,): showed very low activity for the NO-NH, reaction.The proton n.m.r. peak at 6 = 3.1, which appeared during the NO-NH, reaction in DMSO solution, can be assigned to this half-dissociated dangling - NH, proton, if a rapid exchange motion of coordinated and uncoordinated -NH, groups is assumed. However, it was impossible to investigate the temperature dependence of the peak due to the DMSO solvent. Another possibility as regards this n.m.r. peak is that hydrogen bonding takes place between the dissociated -NH, groups and coordinated - NO,, which would reduce the electronic shielding of the - NH, proton. When only NH, was introduced into the catalyst solution, no such peak was observed, which suggests the concentration of this catalytically active Co(en) (en’) (S)+ species (en’ = half-dissociated dangling ethylenediamine) is very small under these conditions.The next step would be the coordination of NO and NH, to this half-dissociated ethylenediamine complex, which requires two coordinatively vacant sites. Accordingly it is necessary to suppose the formation of the Co(en’),(NO,),(S): complex by the same procedure discussed above : (4) In aqueous solution, this step seems to be very slow, because the electronic spectra of the complex do not change after the introduction of NO and NH,. Kinetic data show that the following rate equations are applicable in this case and also support the above consideration : Co(en’), (NO,), (S): + NO + NH, + Co(en’), (NO,), (NO) (NH,)+ .d[N,]/dt = k[CO3’][NH,]PNo ( 5 ) d[N,O]/dt = k’[Co3+][NH,]PNo. (6) However, in DMF or DMSO solutions, step (4) takes place on the addition of NO and NH,, as shown by the change in the n.m.r. spectra. The electronic spectra also changed considerably during the NO-NH, reaction, suggesting the rapid formation of a reaction intermediate. From the results of fig. 6, we propose the following scheme for the reaction in DMF solution (the scheme can also explain the effect of added water): K , Co(en),(NO,)~+DMF~Co(en’),(NO,),(DMF);t K2 Co(en), (NO,): + H,O e Co(en’), (NO,), (H,O);t Co(en’),(NO,),(DMF)~+NO+NH,~Co(en’),(NO,),(NO)(NH,)S kl. k-1744 CATALYTIC REDUCTION OF NO WITH NH, where PH,] and [Co3+] represent the concentrations of NH, and the complex in the system, P,, is the partial pressure of NO and [H,O] is the concentration of added water in DMF solution.From the results of isotope-labelled experiments, it is recognized that N, is formed by the reaction between NO and NH, and N,O is formed from the reaction between two NO molecules, suggesting the participation of both mononitrosyl and dinitrosyl intermediate complexes. In the case of the CO (tien),+ complex, some dinitrosyl species could be obtained from the solution during the reaction. But in the case of the CO(en)i+ complex, only the starting complex and Co(en),+ could be isolated from the solution. When these complexes were impregnated into NaY zeolite, both mononitrosyl and dinitrosyl adsorbed species could be observed by infrared spectroscopy.13 Consequently, it is reasonable to suppose the existence of these intermediate complexes in the solution.A plausible reaction mechanism can be proposed as follows: CO(L), (NO,); + s * CO(L‘), (NO,), (s); (8) Co(L’),(NO,),(S): +NO + NH, S Co(L’),(NO,),(NO)(NH,)+ (9) Co(L’), (NO,),(S): + 2NO Co(L’), (NO,), (NO): (10) Co(L’), (NO,),(NO): + 2(H) + Co(L), (NO,): + N,O + H,O (12) where L representsethylenediamine (n = 2) or triethylenetetramine (n = l), S represents an H,O or DMF molecule and L’ represents the half-dissociated dangling state. From this mechanism, the ratio of N, and N,O formed should be 2 : 1, but experimental results indicate that in the case of the Co(en)i+ complex this ratio is almost 1 : 1, and that in the case of the Co (trien)3+ complex N,O was the main product and very little N, was formed.In order to elucidate these results, it is necessary to introduce another pathway to produce N,O, which would be the disproportionation of NO to form N,O and coordinated NO, : Co(L‘),(NH,)(NO,)3 + (H) + Co(L),(NO,)$ + N, + 2H20. (14) Step (1 3) may be similar to the disproportionation of NO by Co(en), (NO)Cl, to form N,O and coordinated N0,.14 Step (14) seems to be very slow and corresponds to the deactivation process of the complexes, giving rise to an absorption peak at 340 nm in fig. 8. In aqueous solution, step (9) is slow and would be the rate-determining step, consistent with the kinetic data, n.m.r. and electronic spectra. In DMF solution, steps (9) and (10) are now in equilibrium and steps (1 1) and (12) would be the rate- determining steps at the initial stage of the reaction. The contributions of steps (12) and (1 3) for the formation of N,O are comparable in the case of the Co(en),+ complex. However, for the Co(trien),+ complex the Co(trien)(NO,)(NO)i intermediate is stable and step (13) is the main route for the formation of N,O.S. NAITO A N D K. TAMARU 745 M. Shelef and H. S. Gandhi, Znd. Eng. Chem., Prod. Res. Deu., 1972, 11, 393. T. P. Kobylinski and B. W. Taylor, J. Catal., 1974, 33, 376. G. Ertl, J. Kuppers and E. Latta, Surf. Sci., 1977, 65, 235. J. D. Butler and D. R. Davis, J. Chem. SOC., Dalton Trans., 1976, 2249. J. W. London and A. T. Bell, J. Catal., 1973, 31, 96. R. Eisenberg and C. D. Meyer, Acc. Chem. Res., 1975, 8, 26. M. Kubota, K. J. Evans, C. A. Koerntger and J. C. Marsters Jr, J. Am. Chem. SOC., 1978,100, 342. * S . Naito, J. Chem. SOC., Chem. Commun., 1978, 175. K. Otto and M. Shelef, J. Phys. Chem., 1972, 76, 37. lo H. F. Holtzclaw Jr, D. P. Sheetz and B. D. McCarty, Znorg. Synth., 1953, IV, 176. l1 R. G. Pearson, H. H. Schmidtke and F. Basolo, J. Am. Chem. SOC., 1960, 82, 4434. l2 F. Basolo and R. G. Pearson, Mechanism of Inorganic Reactions (John Wiley, New York, 1958). l3 S. Naito and K. Tamaru, in preparation. l4 D. Gwost and K. G. Caulton, Znorg. Chem., 1974, 13, 414. (PAPER 1 /473) 25

ISSN:0300-9599    

DOI:10.1039/F19827800735

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 747-760 Effects of Self-heating During the Thermal Decomposition of di-t-Butyl Peroxide Anomalous Reaction Orders and Activation Energies, and their Correction BY JOHN F. GRIFFITHS* AND HARI J. S I N G H ~ Department of Physical Chemistry, University of Leeds, Leeds LS2 9JT Received 23rd March, 1981 Self-heating accompanying an exothermic reaction that has a positive activation energy enhances the reaction rate. This means that measured reaction orders and rate constants (and hence the effective activation energy) are no longer the true, isothermal values. Theories have been formulated to assess the magnitude of the discrepancies and the way in which they are controlled by conditions. Now it is possible to write down an approximate, but very precise, algebraic expression for the proportionate errors (AE/E and An/n) in terms of the measured, centre-temperature excesses that are generated within a spherically shaped reacting mass.This is an important development because the sphere has only recently yielded to satisfactory analytical interpretations of self-heating and criticality, yet its finite dimensions make it the shape, amongst class A geometries, that is an archetype for most practical cases. The objective in the present study is thus to show whether or not the theoretical predictions for anomalous orders and activation energies are quantitatively satisfactory. The subject for experimental investigation is di-t-butyl peroxide. Its thermal decomposition is studied over the temperature range 420-510 K at low pressures of the pure vapour (< 10 Torr) in a spherical vessel (1 dm3).Pressure increases are monitored continuously by transducer and internal temperature changes measured by a very fine thermocouple. At the lowest temperature, decomposition remains virtually isothermal; we observe a first-order dependence of initial rate on initial pressure, and we measure an overall isothermal activation energy consistent with reported values. At higher temperatures, approaching but still below those at which ignition occurs (T, < 460 K), self-heating accompanies reaction and centre-temperature excesses of up to 15 K are measured. They are quasi-steady maxima, achieved within 2 s of entry of reactant to the vessel; there is a subsequent decline, and from ca. 20 s on the reaction is virtually isothermal.Reaction times, characterized by successive ' quarter-lives', are considerably shorter during the first interval than the subsequent isothermal periods. Where self-heating occurs, enhanced reaction orders are measured, rising as increases, and in carefully chosen circumstances curvature of a log (dpldt), against logp, plot is seen. The same is true for effective activation energies, and we show a curved Arrhenius plot (In k against l / q ) . The measured activation energy can easily exceed the isothermal value by 50% because of self-heating. The anomalous reaction orders and activation energies predicted from theory are in very good agreement with the experimental values. We show that retrospective correction of kinetic data, influenced by self-heating, can be very successful.When an exothermic reaction proceeds, it is accompanied by self-heating, although swift rates may be necessary for the temperature rise to be substantial. Rate constants, and through them reaction rates, are strongly temperature-dependent and so they increase as the reactant temperature rises. Modest temperature changes (several K) are influential and this means that measured Arrhenius parameters (E, A ) and effective reaction orders are enhanced. If the reactant temperature is being measured routinely then the experimentalist is made aware of the influences being imposed on his results, and he can take steps to correct them. Except in the case of combustion t Permanent address: Department of Chemistry, University of Gorakhpur, Gorakhpur, India.747 25-2748 SELF-HEATING DURING DI-t-BUTYL PEROXIDE DECOMPOSITION studies it is not common to find continuous monitoring of reactant temperature during ‘isothermal ’ kinetic studies. Thus significant errors may be introduced. Theoretical approaches have been made to yield expressions that assess the size of theseeffects and their dependence onconditions.l Interpretations follow the steady-state analysis for thermal explosions and criticality. The first treatments were able to provide solutions only in terms of spatially averaged temperatures. The next develop- ment was to predict the effects of distributed temperatures in systems for which simple analytical solutions were available-the infinite cylinder or the infinite slab.From a practical point of view, finite vessels are more important: the sphere is the shape that can yield to theoretical treatments and in which experimental studies can be made with precision. Numerical solutions to the classical, steady-state equations have been known for many year^,^.^ and recently analytical approaches have been de~ised.~ The virtue of an analytical interpretation is that we can see explicitly what factors are in fluen tial. There are already experiments6’7 which show that anomalous k values can arise through self-heating, and what is now required are tests to show whether or not the theoretical predictions are quantitatively satisfactory. The subject for the most convenient investigation needs to be a well-behaved, simple, exothermic, gaseous reaction that is capable of ignition at low pressures.Di-t-butyl peroxide (DTBP) decomposition appears to be a satisfactory example; it has long been used as a thermal source of methyl radicals, and the need for controlled rates of generation has prompted more than ten independent determinations of the kinetic and Arrhenius parameters. Overall, wide ranges of temperature (360-626 K), pressure (5-500 Torr) and extents of dilution with inert gases have been investigated., Although a critical appraisal of the results leads to recommended ‘best’ values for E = 157.9+ 1.3 kJ mol-l and log (Als-l) = 15.80k0.13, in fact the margins of discrepancies between different determinations are wider. Activation energies span the range 138-167 kJ mol-l, and accordingly log ( A l s - l ) varies between 14 and 17.4.The exothermicity for decompositionlo AH,,, = - 180 kJ mol-l (AU460 z - 196 kJ mol-l). Decomposition is first-order and it does not appear to be susceptible to unimolecular fall-off even at pressures as low as 1 Torr.ll The extent of decomposition can be followed by pressure change in a closed vessel. We investigate the decomposition of DTBP throughout the isothermal regime from 420 K, through non-isothermal decomposition up to 500 K, and in conditions where ignition occurs. The po-T, ignition boundary is located for the first time; our vessel is spherical (1.1 dm3). Decomposition is followed by monitoring pressure changes, and in all experiments the temperature change is measured using a very fine thermocouple.From these measurements, quasi-steady temperature excesses which accompany decomposition are mapped throughout the po-T, region. Conditions at which non-isothermal reaction is accompanied by convection are avoided. The temperature measurements provide the main route to assessing how and when self-heating influences the measured reaction order and activation energy. We also show that even without knowledge of temperature change, retrospective correction of a rate constant to that for isothermal reaction is viable from data obtained in non-isothermal circumstances. In what follows the symbols to be used are: p/Torr total pressure PolTorr initial reactant pressure CO initial concentration C reactant concentrationJ. F. GRIFFITHS AND H. J. SINGH 749 x l a t l s q l s T / K T,IK y = (1 - x / a ) AT/K = ( T - T,) R/J mol-l K-l E/J mol-1 q/J mol-l k1s-I A / s Ic/W m-l K-l r/m n f 6 w B The correction fraction of DTBP decomposed fraction of DTBP remaining time quarter-life for the decomposition of DTBP reactant temperature vessel temperature temperature rise gas constant overall activation energy [d In k/d( 1 / G)] exothermicity first-order rate constant pre-exponential factor thermal conductivity vessel radius reaction order rate correction factor Frank-Kamenetskii dimensionless heat-release rate qr2A Ec, exp (- E/RT,) KRT,” Semenov dimensionless heat-release rate dimensionless temperature rise ( = ATE/RT,2) central dimensionless temperature rise spatially-averaged dimensionless temperature rise geometric factor.THEORETICAL BACKGROUND of kinetic rate data for non-isothermal conditions and negligible reactant consumption has been considered by Boddington and Gray.l They show how errors in activation energy E and in reaction order n may be expressed in terms of the same dimensionless group as is already used to discuss the critical boundary between stable and unstable behaviour from the point of view of thermal explosion theory. For non-uniform heating resulting from purely conductive control of heat transport, this is the Frank-Kamenetskii 6; for uniform internal heating it is the Semenov number ly. Although either of the terms 6 or ly is sufficient to determine the errors, they are more conveniently set out in other ways. In particular, if the rate correction-factor f is used, where ideal rate (all reactant at T,) - V(isotherma1) - observed rate V (0 bse rved) f= then dln f - r v - - - - - or -~ (Semenov conditions). AE An dlnf E - n dln6 dln ly To proceed further, we need to know f and 6 orfand w in terms of some suitable quantity such as reactant temperature 8.Under Semenov conditions this is easy. In terms of the dimensionless, average excess temperature @ we have AE An 0 E n 1-6’ -=-=-750 SE L F-HE A T I N G DURING D I-t-BU TY L PEROXIDE DECOMPOSITION For distributed temperatures the solution depends on the shape. For the simplest shapes analytical solutions exist: in parametric form for the infinite slab and in explicit form for the infinite cylinder. For the sphere progress has hitherto had to be guided by numerical solutions tabulated by Chandrasekhar and Wares.2 Proceeding from these, Boddington et a2.12 argued that, although the dependence off upon 6 might not be identical in the cylinder and the sphere, differences between the two quotients (1 -f)/( 1 -fcrit.) would be negligible.Subsequently, Archer and Tylers generated numerical solutions for f(0,) and hence for dln f/dln 6. Recently Boddington et aZ.5 have provided two new, analytical solutions for the sphere. They use a reversion of the known, infinite-series solution for the exponential approximation to the Arrhenius rate law. Although not exact, errors are around only 1%, so the precision is very satisfactory. In terms of centre temperature they write 6 = ![ 1 + 9 exp ( - 0,/2)] [ 1 - exp (- 6,/2)], f= exp(-8,/2)+g and AE/E z An/n = 2[exp (6,/2) - I] [9 + exp (8,/2)]/[4 + exp (8,/2)] [9 - 4exp (8,/2)].The solution is applicable right up to criticality and (just as in the Semenov case) errors grow without limit as 8, approaches its critical value. The value Gcrit. = 10/3 is in error only by 1 part in 300. (They extend this treatment to generalised boundary conditions so that all cases between Frank-Kamenetskii and Semenov extremes are now soluble algebraically.) When 6 is small, errors for the slab, cylinder and sphere (j = 0, 1 , 2, respectively) may be put in common form: . AE . An 6 - 200 lim - = lim - = s+, E a+, n (j+l)(j+3)-j-' For the sphere, the limiting fractional errors are thus (6/15) or (20,/5). These limiting forms are excellent rough guides, being only 10% in error for 6 as large as 1.5.EXPERIMENTAL The reaction vessel (Pyrex glass, spherical, volume 1.1 dm3) was located in an electrically heated furnace which had a spherical cavity in order to maintain a uniform temperature (k0.8 K) over the vessel surface. The furnace was thermostatted over long periods to & 1 K; stability over short intervals was better than this. The temperature was measured with a chromel-alumel thermocouple fixed to the outer surface of the reactor. A reference junction was placed in ice at O°C and the e.m.f. developed between them was measured using a potentiometer (sensitivity & 0.01 mV). The reaction vessel was connected to a conventional, glass vacuum line. DTBP vapour was admitted to the vessel from a roughly equal sharing volume via an electromagnetic valve (Leybold-Haraeus).The valve could be opened for short, reproducible time intervals (0.1-1 s). Initial reactant pressures and pressure changes were measured using a pressure transducer (SE labs SE42, sensitivity 19.09 Torr mV-l). Its signal was recorded using a potentiometric chart recorder (Smiths Servoscribe, maximum sensitivity 0.5 mV f.s.d.). The transducer was mounted externally but as close as possible to the furnace by a side-arm to the vessel. The total dead volume (including that to the entry valve) was less than 1% of the reactor volume. Measurement of temperature rises accompanying reaction were made using a very fine thermocouple (Pt-Pt/l3% Rh, 25 pm wire) mounted at the tip of a probe which was inserted concentrically through the gas-entry arm to the vessel.Measurements in the present study were made with the junction located at the centre. A reference junction was attached to the outerJ. F. GRIFFITHS AND H. J. SINGH 75 1 vessel surface and the signal between the two was amplified and recorded on an oscilloscope (Tektronix 564B). The highest sensitivity compatible with low noise was 10 K cm-l displace- ment. Temperature changes were measured to kO.5 K from these records. DTBP (Fluka AG, 99.9%) was further purified by trap-to-trap distillation and. its vapour stored at S.V.P. in reservoirs (2 x 5 dm3) on the vacuum line. The storage volumes were painted black to prevent photochemical change. The decomposition of DTBP in the reaction vessel was investigated in the pressure range 0.5-10 Torr at temperatures in the range 420-503 K.RESULTS po-Ta IGNITION DIAGRAM Fig. 1 displays a po-l;, ignition diagram for the spontaneous decomposition of DTBP in a 1.1 dm3 spherical vessel. With the exception of that for methyl nitrate7? l3 the present diagram is more comprehensive than those usually found in the literature. Not only do we locate the ignition boundary, but also we show the position of isotherms which join points where equal extents of self-heating are measured during 9 8 7 6 g 5 g 4 b --. 3 2 1 10 b, 430 450 470 490 510 Tal K FIG. 1 .-p,,-T, ignition diagram for DTBP decomposition in a spherical vessel ( 1 . 1 dm3). The solid lines join experimental conditions at which the same quasi-steady, central-temperature excess is reached.decomposition (see below). The lines in fig. 1 relate to quasi-steady temperature rises of 0.5, 1, 2, 5 and 10 K; these data are important because they show (i) the extent of non-isothermal decomposition within the subcritical region and (ii) the magni- tude of self-heating achieved and the way it is controlled by the initial (p,-T,) conditions. The principal purpose of measuring quasi-steady centre-temperature excesses is to be able to compare predicted deviations of reaction orders and activation energies due to non-isothermal reaction with experimentally measured anomalies. However, the p,-T, location of the ignition boundary determines the pressure and752 SE L F-HE A T I N G DURING D I-t-B U T Y L PER 0 X I D E D E C 0 M P 0 S I TI ON temperature ranges to be investigated, and the locations of the isotherms show us where heating effects are expected to influence the measured kinetic parameters.When < 435 K, reaction is virtually isothermal throughout the entire pressure range studied (0-10 Torr). Between 435 and 462 K a similar range of pressures may still be investigated, but now the non-isothermal regime is invaded and quasi-steady tempera- ture excesses increase as the initial pressure is raised. At higher vessel temperatures (462-480 K) the accessible pressure range becomes limited by the location of the ignition boundary. Moreover, the non-isothermal regime of decomposition extends to lower reactant pressures, and this means that at all initial pressures above ca. 1 Torr self-heating accompanies the early stages of reaction (see below).NON-EX P LO S I V E D E C 0 M POSIT I ON : TEMPER A TU RE-T I M E RECORDS AND QU AS I-STEAD Y TEM PER A TU R E EXCESSES An initial cooling is first registered as cold reactant enters the vessel [fig. 2(a)]. In circumstances where non-isothermal decomposition follows, the temperature then rises through ambient, and continues upward to approach a low, quasi-steady g -10 :"r"-w . ;1, i"- t . , . , . , ' time/s 0 20 40 60 80 100 time/s FIG. 2.-Typical pressure and temperature histories during the non-isothermal decomposition of DTBP, po = 3.8 Torr, T, = 466 K. The maximum temperature change is reached very early during decomposition. During much of the later period, reaction is isothermal. maximum before decaying to ambient. The maximum temperature is always achieved within a very brief interval after admission to the vessel (< 4 s), and the system is restored to virtually isothermal conditions in less than ca.20 s. In the present study all temperature measurements (> 100 experiments) were obtained at the centre of the vessel; accordingly the isotherms of fig. 1. are unique to the centrally measured temperature rises.J. F. GRIFFITHS A N D H. J. SINGH 753 N ON-E X P LO SI V E D E COM P 0s IT I ON : PRESS URE-T I ME RE C OR D S, 0 VE R A L L STOICHIOMETRY AND EXTENTS OF DECOMPOSITION The pressure record always shows an initial jump as the gas enters the vessel. It is followed by a further rise due to thermal decomposition of DTBP. At sub-critical conditions the pressure increases smoothly to its final value.To measure p a , reaction is allowed to continue for more than 6 half-lives, when decomposition is virtually complete. A typical record is shown in fig. 2(b); the conditions chosen here are close to criticality, but even so, more than 60 s elapses before p ceases to rise perceptibly. This distinction from the early rise and fall of T is important because it means that the bulk of reaction is occurring when reaction is isothermal. When T, < 460 K, ti (isothermal) > 70 s. In gas-phase studies under closed-vessel conditions, pressure-time records are potentially a more precise and certainly a more convenient route to extents of reaction than are direct concentration measurements. Provided that the overall stoichiometry does not vary during reaction, the isothermal pressure change is proportional to the measurements. The ratio of the final pressure to initial reactant pressure, measured at many points throughout the slow-decomposition region, is 2.98 kO.02. This is the value to be expected on the basis of the stoichiometry C,H,OOC,H, = 2(CH,),CO + C,H,.The fraction of reactant remaining at any time t is thus given by the relationship C/C, = (2.98 p o - p ) / 1.98 po. In principle the measured pressure must be corrected for the effect of internal temperature rise. A useful relationship is p(isotherma1) = p(measured) q/( T, + 0.4 A&) in which 0.4 AG assumes that the average temperature rise is t of that measured at the centre.6 This correction is always < 1 %. Extents of decomposition and velocity constants were derived from pressure changes in the present study. REACTION ORDER FROM INITIAL RATES The pressure-time records obtained enable dp/dt at t = 0 to be deduced.Thus the sensitivity of rate- to concentration is determined appropriately from the variation of initial reaction rate with initial concentration: log (dp/dt), -, against logp, at constant temperature leads to reaction order from its gradient. The initial rates of pressure change for various initial pressures determined at different vessel temperatures are given in tables 1 and 2 and displayed in fig. 3. The enhanced gradients of lines drawn through points that represent data at vessel temperatures in excess of 450 K is to be attributed to non-isothermal effects. The solid lines in fig.3 are not arbitrarily selected: they are derived from a theoretical interpretation. This point is taken up in the Discussion. VELOCITY CONSTANTS The decomposition of DTBP is implicitly first-order and the gradient of the isothermal graph in fig. 3 shows this. Moreover, log(c/c,) plotted against time is expected to be linear, and in isothermal circumstances [e.g. fig. 4(a)] this is found to be so. The isothermal rate constant is obtained from the gradient. Provided that reactant pressures are in excess of those at which unimolecular fall-off in rate is754 SELF-HEATING DURING DI-t-BUTYL PEROXIDE DECOMPOSITION TABLE 1 .-INITIAL RATES, QUARTER-LIVES AND ISOTHERMAL RATE CONSTANTS (dP/dt)o po/Torr /lo3 Torr s-I T,/K 423 2.54 4.18 4.54 4.96 6.45 6.9 7.3 423 427 2.86 3.86 4.05 4.77 5.15 6.1 1 6.29 6.5 6.68 8.4 427 430 1.53 1.74 2.77 3.9 4.6 6.32 8.4 10.9 430 - - -_ - - - - mean values - - - - - - - - - - mean values 1.59 1.58 2.41 3.56 4.67 6.26 8.74 11.1 mean values 965 1069 937 1049 1183 1329 1183 1 loo+ 130 720 648 612 810 594 690 630 780 738 708 690 f 60 512 562 600 540 510 600 52 1 510 544 & 36 2.98 2.69 3.07 2.74 2.43 2.18 2.43 2.6 2 0.3 3.99 4.44 4.70 3.55 4.84 4.16 4.56 3.69 3.89 4.06 4.2 f 0.4 5.62 5.12 4.79 5.33 5.64 4.79 5.52 5.64 5.3 f 0 .3 observed, the first-order dependence should also be reflected by rate constants that are independent of initial reactant pressures (constant K ) . This is confirmed in table 1 at three vessel temperatures, in which are included rate constants obtained from (i) the gradient of the least-mean-squares line of the respective log (c/c,) against time graph and (ii) the time to 25% decomposition of DTBP.The scatter (k 10%) of these values is not pressure-dependent; it reflects variations of vessel temperature (k 1 K) between successive experiments when the duration of reaction is very long (> 1 h). At vessel temperatures > 450 K and at sufficiently high pressures for substantial self-heating to occur, first-order plots [log (c/c,) against t] show a distinct curvature [e.g. fig. 4(6)]. The interval over which curvature is found (i.e. up to ca. 30% reaction) corresponds to the time during which a temperature rise accompanies decomposition. At constant vessel temperature the time to 25% decomposition of DTBP now diminishes consistently as reactant pressure is raised, and the rate constant is enhanced accordingly (table 2).In circumstances when the quasi-steady temperature excess is found to increase substantially (e.g. at T, = 460 and 466 K, where po can be varied over a wide range) the characteristic reaction time ( t i , 100-75% DTBP) is almostJ. F. GRIFFITHS AND H. J. SINGH 755 TABLE 2.-TEMPERATURE RISES, QUARTER-LIVES AND NON-ISOTHERMAL RATE CONSTANTS 460 0.8 1 .o 1.9 3.6 4.0 5.4 7.7 1.72 1.78 2.29 2.48 3.02 4.77 5.54 0.94 1 .oo 1.10 1.13 1.29 466 480 isothermal prediction 0.5 1 2 5 5 6 8 isothermal prediction 3 2.5 4 4 5.5 10 14.5 isothermal prediction 6.5 6 7 9 10.5 1.35 1.97 4.41 8.61 1.17 1.59 2.32 6.68 4.99 5.06 5.95 9.60 11.32 35.3 10.7 10.7 12.7 12.1 14.8 32 31 27 23.5 21 22 19.5 19.5 19.0 17.5 17.8 15.7 14.8 15 11.5 9.5 9.5 5.2 5.3 5.3 4.6 5.0 90.6 92.8 106.5 122.4 137.0 130.7 143.8 147.5 151.2 164.4 161.6 183.2 194.3 191.8 250.1 302.9 302 553.2 542.7 542.7 625.3 575.3 -0.6 - 1 .4 n U z \ 2 M 2 -2.2 -3.0 -0.2 0 0.2 0.4 0.6 0.8 1 .o log &/To") FIG. 3.-Determination of reaction order from the dependence of initial rate of pressure change on the initial pressure. Enhanced reaction orders are measured when self-heating is substantial. The points are determined experimentally. The solid lines are obtained from theoretical interpretation. 0, 430; 0, 466 and A, 480 K.756 SELF-HEATING DURING DI-f-BUTYL PEROXIDE DECOMPOSITION O.61 b 0 10 20 30 40 time/s FIG. 4.-Function plots for the first order decomposition of DTBP at T, = 460 K.(a) po = 1.0 Torr: In (1 - x / a ) against time; (b) pa = 7.7 Torr; In (1 - x / a ) against time; (c) pa = 7.7 Torr; eqn (I) against time. TABLE 3.-cONSECUTIVE QUARTER-LIVES DURING NON-ISOTHERMAL REACTION time intervals/s for change of concentration of DTBP t;/s T,/K p,/Torr (A7Jss/K 100-75% 75-56% 5 4 2 % (isothermal) 460 1.9 4.0 7.7 466 1.75 4.77 5.54 473 1.28 1.72 480 1 .o 1.13 1.29 2 5 8 3 10.5 14.5 4 8 6 9 10.5 23.5 22 19.5 17 11.5 10 9 7 5.5 5 5 34 34 32 19 15.5 14 11 9 6.5 5 6.5 21 19 16 10 9.5 9 7 9 32 32 32 19 19 19 11.5 11.5 9.5 9.5 9.5 halved and the rate constant derived from it increases nearly two-fold. However, the shorter characteristic times are not maintained throughout reaction ; the second and third ' quarter-lives' representing the change in concentration of DTBP 7556% and 56-42 %, respectively, revert to that broadly characteristic of the isothermal reation (table 3).J.F. GRIFFITHS A N D H. J. SINGH 757 DISCUSSION CORRECTION OF RATE DATA FOR THE INFLUENCE OF SELF-HEATING Boddington et a1.12 have discussed three approaches to counter non-isothermal effects. The first is avoidance, and it requires a criterion for the maximum acceptable error due to self-heating. We might regard values forfin the range 0.95-1.00 to be tolerable; this means that for spherical geometries 8, 6 0.1290 and 6 < 0.7078. Thus if the isothermal rate is 5 % less than the observed rate, the error in the activation energy is = 0.052. . AE 28, - 2 ~ 0 . 1 2 9 0 lim - = ~ 5 d - i , E 0'+3>- For DTBP decomposition a typical temperature rise would be ca.1.5 K and sufficient to generate an error in E of 7.5 kJ mol-l. Method two is allowance, in which observed rate constants are corrected to their isothermal values on the basis of JT0,); 0, is derived from measured internal temperature rises. Archer and Tyler6 utilize this approach to correct rate data for the decomposition of 3-methyl-3-chlorodiazimine. Here we exploit the converse, namely to predict non-isothermal rate constants from k(T,) and to compare them with measured values (see below). Method three is retrospective correction: it is suggested that a function plot [e.g. log (1 - a / x ) against t] influenced by heating effects may be corrected, even without knowledge of the temperature changes involved. The procedure has yet to be tested, and so we investigate the consequences here, taking the data of fig.4(b) as an example in which non-isothermal effects are prominent. The corrected function obtained by integration of the rate equation dx/dt = [k( T,)/fl (a - x ) ~ is, for a first-order reaction,12 )I [l-(1 -yd)i) [l + ( I -yd)i] --In( where y = 1 - x / a and d = 6/dCrit.. For spherical geometry Gcrit. = 3.32, and 6 can be calculated, although the enthalpy of reaction and the thermal conductivity of reactant are required. Here d is evaluated most conveniently from d = Tc3rit. ~ X P (-E/RT,)/T,3 ex~(-E/RT,rit.) where Grit. is the vessel temperature for ignition at a corresponding pressure to that for decomposition at T,. f, = for a spherical reacting mass at criticality. A valid assumption6 in this analysis is that for a first-order reaction at the quasi-steady state f = [ ?'(isothermal)/ V(observed)] = [k(isothermal)/k(observed)]. Representative values for eqn (I), applied to the kinetic data which yield fig.4(b) (Po = 7.7 Torr, T, = 460 K) are given in table 4. These revised data are plotted in fig. 4(c). A linear relationship exists where previously a curve was found, and its758 SELF-HEATING DURING DI-t-BUTYL PEROXIDE DECOMPOSITION TABLE 4.-RETROSPECTIVE CORRECTION OF DATA 0 4 8 12 16 20 24 28 32 36 40 1 0.941 0.889 0.833 0.793 0.749 0.712 0.674 0.642 0.613 0.586 0 0.0608 0.1176 0.1827 0.23 19 0.2890 0.3397 0.3827 0.4432 0.4894 0.5344 0 0.047 1 0.092 1 5 0.1448 0.1854 0.2331 0.2523 0.3229 0.365 1 0.4054 0.4450 gradient, k(T,), corresponds to that for isothermal reaction at 460 K [fig.4(a)]. The retrospective correction of a first-order functibn plot thus offers a very satisfactory route to isothermal rate constants. ANOMALOUS REACTION ORDERS The quasi-steady temperature increases that accompany the decomposition of DTBP are the closest realization of idealized steady-state conditions. They are the route to 8, and f via the relation f= exp (-8,/2)+4 and thence predicted dependences for the initial rate of pressure change on initial reactant pressure, uiz. Typical values forfare given in table 5 based on our experimental values for AT, when T, = 460, 466 and 480 K. We see that when AT = 5 K, the isothermal rate constant is at least 15% less than that measured. The discrepancy is still 6.5% when AT, is only 2 K, and it is noteworthy that these conditions can prevail when T, = 20 K or more below the critical vessel temperature.We may derive log (dpldt), against log po in graphical form and these are the solid lines drawn in fig. 3. There is excellent agreement between the predicted and TABLE 5.-RELATIONSHIPS BETWEEN ISOTHERMAL AND NON-ISOTHERMAL RATE CONSTANTS 460 2 5 8 466 2.5 10.5 14.5 480 6 9 10.5 0.1728 0.4320 0.6912 0.2105 0.8840 1.2207 0.476 1 0.7141 0.833 1 0.9338 0.8446 0.7662 0.9201 0.7142 0.6345 0.8324 0.7598 0.7275 9.06 9.70 - 10.71 - 11.82 15.12 16.52 - 21.17 - 23.83 30.2 36.28 - 39.75 - 41.51J. F. GRIFFITHS A N D H. J . SINGH 759 experimental relationships. At = 430 K reaction is isothermal throughout the pressure range and thus a straight line, gradient (n) = 1, is found.As 8, rises Anln increases and so an enhanced gradient, giving curvature for log (dpldt), against log po, is to be expected. This is a distinct feature when T, = 466 K and it matches our experimental observations : the gradient at low pressure is 1 , rising to ca. 2.3 when p o exceeds 5 Torr. At T, = 480 K, only a very restricted pressure range (0.75-1.3 Torr) is accessible and in it AT, varies from 6 to 10.5 K. It is extremely difficult to interpret curvature over these limited ranges: the overall order appears to be 1.43. Even here the predicted enhancement of reaction order as a result of self-heating is supported by experimental observations; one would not normally assess reaction order over such a limited pressure range. ACTIVATION ENERGY AND THE ERRORS IN IT ARISING FROM SELF-HEATING The overall activation energy for decomposition of DTBP is of course derived from the dependence of the measured rate constant on vessel temperature via the gradient of an Arrhenius plot (In k against l/T,).k is independent of initial pressure when reaction is isothermal and a mean value can be assigned at each vessel temperature. An average value is not approprate to reaction in the non-isothermal decomposition region since measured values for k are now dependent on initial pressures (see table 2). The .graph of In k against 1 / T , shown in fig. 5 corresponds to data obtained at the initial pressure 4.5 0.5 Torr. 2.1 2.2 2.3 2.4 2.5 lo3 KIT, FIG. 5.-Arrhenius activation energy plot for the decomposition of DTBP over the temperature range T, = 403-470 K.The points are experimental values for Ink. The linear portion of the solid line and the extrapolated dashed line are obtained from a least-mean-squares analysis of isothermal rate data. The curved part of the solid line is obtained by correction for non-isothermal effects viuf. Over a range of vessel temperatures where self-heating during decomposition does not rise beyond 1 K (i.e. T, = 403-445 K), the graph is linear and its gradient from a least-mean-squares analysis corresponds to 152 & 0.6 kJ mol-l, and log (Als-l) = 15.3 k0.3, broadly consistent with accepted value^.^ On the basis of these Arrhenius parameters we may now calculate isothermal rate760 SE LF-HE AT I N G DURING DI-t-B U TY L PEROXIDE DECOMPOSITION constants for higher vessel temperatures: the extrapolation of the linear portion of fig.5 (dashed line) represents these values. Non-isothermal values fork( 7) = k( &)/‘7) are thus accessible and the solid line in fig. 5 represents them. The experimental data are remarkably consistent with it throughout the temperature range (q = 403-470 K). The graph is, of course, linear in the isothermal regime where f= 1. The major implication for the apparent activation energy in non-isothermal circumstances is the substantial over-estimation of E. When 6 is not small, the error in E, expressed in the terms of x(0) used by Boddington et af.,5 is given by AE - 4x/5(1 +x/10) - E (1 + ~ / 5 ) ( 1 - 4 ~ / 5 ) but this can be rationalized in terms off as AE E(observed) - E(isotherma1) - :( 1 -f) (9f- 1) - - E E( isot hermal) 1 OfV- f) .The limiting conditions are explicit. Whenf = 1, E(observed) = E(isotherma1). When f= 1 (i.e. at Gcrit.), the error is infinite. Some values offfor decomposition of DTBP at 4.5 Torr through a range of temperatures are given in table 6, and the errors due to them are calculated. TABLE ~.-QUASI-STEADY TEMPERATURE RISES, f AND ERRORS IN E AT 4.5 Torr f AE/E 448 1.5 0.1366 0.9472 0.06 456 3 0.2637 0.9012 0.12 460 5.5 0.4752 0.8308 0.26 466 10 0.8206 0.7307 0.62 These errors are not strictly those measured experimentally because they represent tangents to the Ink against l/T, curve. Nonetheless, the apparent activation energy can be substantially greater than the isothermal value. For example, the gradient from Ink against 1/T, in the temperature range T, = 446-466 K is equivalent to an activation energy of ca. 240 kJ mol-l, an error of nearly 50%. The authors thank the Indian Government for a Visiting Fellowship awarded to Dr Singh, and Prof. P. Gray and Mr S. K. Scott for helpful discussions. T. Boddington and P. Gray, Proc. R. SOC. London, Ser. A, 1970, 320, 71. S. Chandrasekhar and G. W. Wares, Astrophys. J . , 1949, 109, 581. P. L. Chambrk, J . Chem. Phys. 1952, 20, 1795. V. V. Barzykin and A. G. Merzhanov, Dokl. Akad. Nauk SSSR, 1958, 120, 1271. T. Boddington, P. Gray and S. K. Scott, Proc. R. SOC. London, Ser. A , 1981, 378, 26. W. Archer and B. J. Tyler, J. Chem. SOC., Faraday Trans. I , 1976, 72, 1448. ’ P. Gray, J. F. Griffiths, K. Hasegawa, Int. J. Chem. Kinet. 1981, in press. * Z. G. Szabo, Advances in the Kinetics of Homogeneous Gas Reactions (Methuen, London, 1961), p. 118. D. H. Shaw and H. 0. Pritchard, Can. J . Chem., 1968,46, 2721. 21, (U.S. Govt. Printing Office, Washington, D.C., 1970), p. 430. T. Boddington, P. Gray and B. J. Tyler, Int. J. Chem. Kinet., 1974, VI, 531. lo S. W. Benson and H. E. ONeal, Kinetic Data on Gas Phase Unimolecular Reactions, NSRDS-NBS l1 P. J. Robinson and K. A. Holbrook, Unimolecular Reactions (Wiley, London, 1972). l3 T. Boddington, P. Gray and J. F. Griffiths, Archivum Termodynamiki i Spalania, 1978, 9, 537. (PAPER 1 /474)

ISSN:0300-9599    

DOI:10.1039/F19827800747

出版商:RSC    

年代:1982

数据来源: RSC