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/F198278FX041

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/F198278BX043

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY FARADAY TRANSACTIONS. PARTS I AND I 1 The Journal of The Chemical Soeietj? is issued in six sections: Journal of' The Chemical Society, Chemical Communications Journal of The Chemical Society, Dalton Transactions Journal of The Chemical Society, Faraday Transactions, I Journal of' The Chemical Society, Faraday Transactions, I I Journal of The Chemicul Society, Perkin Transactions, I Journal of' The Chemical. Society, Perk in Transactions, I I 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 thg reasons and justification for seeking urgent publication of the work.This Section is intended to be essentially a journal for inorganic chemists containing papers on the structure and reactions of inorganic compounds and the application of physical chemistry techniques to, e . g . the study of inorganic and organometallic compounds and problems, including work on the kinetics and mechanisms of inorganic reactions and equilibria, and spectroscopic and crystallographic studies of inorganic com pounds. Journal of the Chemical Society, Faraday Transactions, I and I I These are. respectively, physical chemistry and chemical physics journals.P A R T I (physical chemistry) includes papers on such topics as radiation chemistry, gas-phase kinetics, electrochemistry (other than preparative), surface and interfacial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions and kinetics of polymerization, etc. P A R T I I (chemical physics) contains theoretical papers, especially those on valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenom- ena, spectroscopic studies (including i.r., e.s.r., n.m.r., and kinetic spectroscopy, etc.) leading to- assignments of quantum states, and fundamental theory, and also studies of impurities in solid systems, etc. Journal of The Chemical Society, Chemical Communications Journal of The Chemical Society, Dalton Transactions Journal of The Chemical Society, Perkin Transactions, I and I I 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 Transact ions.P A R T I I (physical organic chemistry) is for papers on reaction kinetics and mechanistic studies of organic systems and the use of physico-chemical, spectroscopic, and crystallographic techniques in the solution of organic problems.Notice to Authors ( I ) Although authors need not be members of the Royal Society of Chemistry i t i s 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 1 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. 1 J. Chem. SOC., Faraday Trans. 2 J. Chem. SOC., Perkin Trans. 1 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. 11THE FARADAY DIVISION O F THE ROYAL SOCIETY OF CHEMISTRY Marlow Medal and Prize Applications are invited for the award of the Marlow Medal for 1983 and prize of f 100. The award will be open to any member of the Faraday Division of The Royal Society of Chemistry who, by the age of 32, had made in the judgement of the Council of the Faraday Division, the most meritorious contribution to physical chemistry or chemical physics.The award will be made on the basis of publications (not necessarily in the Transactions) on any subject normally published in J. Chem. SOC., Faraday Transactions I and /I, that carry a date of receipt for publication not later than the candidate's 32nd birthday. Candidates should be members and under 34 on 1 st January 1983, the closing date for applications, which may be made either by the candidate himself or on his behalf by another member of the Society. Copies of the rules of the award and application forms may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY S Y M P O S I U M NO.1 7 The Hydrophobic Interaction University of Reading, 15-16 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 prmiding 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 is hoped, will lead to a better understanding of hydrophobic interactions in more complex processes.The following have agreed to contribute to the symposium: A. Ben-Naim, H. J. C. Berendsen, D. L. Beveridge, S. D. Christian, L. Cordone, D. Eagland, D. Eisenberg, R. Lumry, P. J. Rossky, M. C. R. Symons, H. Weingartner, M. D. Zeidler The programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN . . . IllTHE FARADAY DIVISION OF THE ROYAL SOCIETY O F CHEMISTRY GENERAL DISCUSSION NO. 75 Int ramolecu la r 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 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), The preliminary programme may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN Dr D. M. Hirst Professor K. R. Jennings Dr R. Walsh THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 76 Concentrated Colloidal Dispersions Loughborough University of Technology, 1 4 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 particleparticle 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. Further information may be obtained from: Professor R. H. Ottewill, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS i vTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 18 Molecular and Microstructural Basis of Viscoelasticity and Related Phenomena Robinson College, Cambridge, 8-9 December 1983 Organising Committee Sir Geoffrey Allen (Chairman) Professor Sir Sam Edwards Dr M. La1 The past few years have witnessed the development of new concepts which provide deeper understanding of the relationship between molecular dynamic and microstructural features of systems and their viscoelastic behaviour.This Symposium is designed to bring together original contributions involving theoretical, computational and experimental studies which represent significant advances in this important field of current activity. It is hoped that such contributions, together with the discussion that they will generate, will lead to new insights into the molecular mechanisms underlying the viscoelastic/rheological behaviour of, for example, flexible and rigid rod-like polymer molecules, liquid crystals and composites. In addition to three oral sessions (at which the main papers will be presented and discussed), the Symposium may include a poster session. Such poster papers will not be published in the Symposium volume.Further information may be obtained from: Dr M. Lal, Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JW Dr R. A. Pethrick Dr P. Richmond Dr D. A. Young (Editor THE FARADAY DIVISION O F THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 77 Interfacial Kinetics in Solution University of Hull, 9-11 April 1984 This Discussion will focus attention on reactions involving liquid-gas, liquid-liquid and liquid-solid interfaces (but it will not include electrode kinetics as such). The subject encompasses processes of fundamental, industrial and environmental importance and includes such topics as the rate of dissolution of reactive gases, kinetics at liquid membranes, metal and solvent extraction, Marangoni effects, heterogeneous catalysis and photocatalysis in solution, and the kinetics of dissolution of minerals and drugs.The aim of the meeting is to bring together workers in these diverse fields to highlight the complementary nature of the problems encountered and of the results obtained, and to disseminate ideas concerning new and effective experimental techniques and novel theoretical approaches. Contributions for consideration by the organising committee are invited. Titles should be submitted as soon as possible, and abstracts of about 300 words by 15th April 1983 to: Professor D. H. Everett, Department of Physical Chemistry, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS VFARADAY DIVISION INFORMAL AND GROUP MEETINGS Pojymer Physics Group Measurement Techniques for Polymeric Solids To be held at NPL, Teddington on 1-2 December 1982 Further information from Or M.J. Richardson, NPL, Teddington, Middlesex Wll OLW ~ Division - Half - day Symposium Photochemical Reaction Dynamics to include the Tilden Lecture: J. P. Simons To be held at the Scientific Societies Lecture Theatre, London on 7 December 1982 Further information from Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN E1ectrochemis:r y Group Spectroscopic Studies of Electrode Surfaces To be held at Oxfcrd on 13-1 4 December 1982 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College, London SW7 2AZ Colloid and Interface Science Group Physical and Biological Aspects of Insoluble Monolayers and Multilayers To be held at the Scientific Societies Lecture Theatre, London on 14 December 1982 Further information from Dr R.Aveyard, Department of Chemistry, The University, Hull HU6 7RX Polymer Physics Group Gels and Gelation To be held in London on 21-22 December 1982 Further information from Dr M. Miles, Food Research Institute, Norwich NR4 7UA Division with the Institute of Physics Applications of Electron Microscopy in Chemistry To be held at the Geological Society, London on 10 January 1983 Further information from: Mrs J. Cegielka, Institute of Physics, 47 Belgrave Square, London SW1X 8QX Electrochemistry Group Spring Informal Meeting To be held at the University of Newcastle on 29-30 March 1983 Further information from Dr R.D. Armstrong, Department of Chemistry, University of Newcastle, Newcastle upon Tyne NE1 6RU Theoretical Chemistry Group - Half-day Spring Meeting To be held at King's College, London on 2 March 1983 Further information from Dr G. G. Balint-Kurti, School of Chemistry, University of Bristol, Bristol BS8 1TS Division - Half - da y Symposium Including the Faraday Lecture: J. S. Rowlinson To be held at Imperial College, London on 10 March 1983 Further information from Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Statistical Mechanics and Thermodynamics Group Liquids and Liquid Mixtures To be held at the University of Hull on 28-29 March 1983 Further information from Dr P. G. Francis, Department of Chemistry, The University, Hull HU6 7RX ~ Division with Macrogroup UK and Polymer Physics Group Annual Chemical Congress : Copolymers To be held at the University of Lancaster on 11 -1 3 April 1983 Further information from Dr J.F. Gibson, The Royal Society of Chemistry, Burlington House, London W1 V OBN viSCI Electrochemical Technology Group and Electrochemistry Group Ion Exchange Membranes To be held in Chester on 13-1 5 April 1983 Further information from Society of Chemical Industry, 14 and 15 Belgrave Square, London SW1 8PS Colloid and Interface Science Group Proteins and Colloidal Systems To be held at the University of Leeds on 14-1 5 April 1983 Further information from Dr E. Dickinson, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT Polymer Physics Group, Macrogroup UK and Plastics and Rubber Institute Polyethylenes 1933-1 983 To be held in London on 8-10 June 1983 Further information from The Plastics and Rubber Institute, 11 Hobart Place, London SW1 W OH2 Industrial Physical Chemistry Group Crystallization Processes in Condensed Phases To be held at Girton College, Cambridge on 5-7 July 1983 Further information from Dr I.D. Robb, Port Sunlight Laboratory, Bebington, Wirral, Merseyside L63 3JW Polymer Physics Group Physical Aspects of Polymer Science To be held at the University of Reading on 14-1 6 September 1983 Further information from Or D. Bassett, University of Reading, Whiteknights, Reading RG6 2AH viiNOTES I t has always been the policy of the Faraday-Transactions that brevity should not be a facror 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. (Purr and Appl. Chem., Vol. 51, No. I , 1979, pp. 1 4 1 . Also available as a soft-cover booklet from Pergamon Press, Oxford.) Surface Chemistry. ’ Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - I . ’ (Pure und Appl. Chem.. Vol. 31, No. 4, 1972, pp. 577-638.) ’ Definitions. Terminology, and Symbols in Colloid and Surface Chemistry - I I . Heterogenous Catalysis. ’ (Pure and Appl. Chem., Vol. 46, No. I , 1976, In addition. the terminology and symbols for the following subject areas are available either in the form of soft-cover booklets from Pergamon Press (denoted by *) or have been the subject of articles in Pure and Applied Chemisiry in recent years: activities;* chromatography ; elect roc hem i s t ry ; electron spectroscopy ; eq ui 1 i bri a, 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- dyriamics, 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/F198278FP081

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I , 1982, 78, 3153-3161 Electronic Interactions in Triatomic Cyclic Compounds Charge-transfer Complexes between Epoxide Donors and ICl as Acceptor BY SERGIO SANTINI" AND SALVATORE SORRISO Dipartimento di Chimica, Laboratorio di Chimica Fisica, Universita di Perugia, Via Elce di Sotto 10, 06100 Perugia, Italy Received 8th October, 198 1 Charge-transfer (c.t.) complexes between epoxide donors and ICI as acceptor have been studied spectrophotometrically. It was found that the basicity in the 3-, 4-, 5- and 6-membered rings depends on the size of the ring, following in the order 4 > 5 > 6 > 3 atoms. This is principally caused by the different LCOC bond angles and the consequent different types of hybridization of the oxygen orbitals in the various compounds.K,, data show that in the styrene oxides a small conjugative effect is present between the phenyl and oxiran rings. However, this classic conjugation is completely absent in both cis- and trans-stilbene oxides. The problem of conjugation between triatomic cycles acd the adjacent p or n systems has been confronted both theoretically1 and experimentally.2aa-c The majority of these studies refer to cyclopropane and not to its heteroatomic homologues. We have undertaken additional work directed towards highlighting the effects of resonance and of classical and non-classical electronic transmission across the triatomic ring, using theoretical3 and e~perimental*~-f approaches. Other authors have also debated the conjugative properties of the oxiran ring, obtaining contradictory results.5a In the present paper we have extended the investigations to the charge-transfer complexes between some oxirans (donors) and ICl (an acceptor) with the aim of obtaining further information about the presence of electronic effects between the heteroatomic ring and one or two adjacent aryl systems.We have furthermore extended the study to 4-, 5- and 6-membered cyclic systems which contain oxygen as a heteroatom. EXPERIMENTAL MATERIALS The propylene oxide, trimethylene oxide, tetrahydrofuran and tetrahydropyran used were commercial products and were purified as indicated in the literature.6* ' Styrene oxide (a commercial product) was purified by distillation. The styrene oxide derivatives p-chloro-, p-nitro- and rn-nitro- were available from a previous study3d and generously given by the authors; p-methyl- and rn-methyl-styrene oxides were prepared as described in ref.(8); p-fluoro- and rn-fluoro-styrene oxides were prepared as described in ref. (9)- The unsubstituted trans- and cis-stilbene oxides were prepared according to the method outlined in the 1iterature;lO thep-nitro- andpp'-dichloro-derivatives of the trans- and cis-stilbene oxides were available from a previous and given by the authors ; thepp'-dinitro-derivatives of the trans- and cis-stilbene oxides were prepared and purified as described in ref. (1 1). The carbon tetrachloride was a Carlo Erba RP product and was dried over P,O, and distilled. Iodine monochloride was prepared by a standard method (m.p. 27.2 O C ) .1 2 31533154 MOLECULAR COMPLEXES OF EPOXIDE DONORS MEASUREMENTS The spectra were recorded by an Optica CF4-DR double-beam spectrophotometer. Equilib- rium measurements at fixed wavelengths were taken using a single-beam Unicam SP 500 spectropho t ometer . The stability constants (&) were determined spectrophotometrically at 20 OC, both in the region of the halogen absorption band (500-600 nm) and in that of the blue-shifted perturbed halogen band (300-450 nm). A second new band (the c.t. band) appears on complexation at shorter wavelengths (< 300 nm); but this spectral region was not used for analysis owing to its overlapping with the donor absorption band. The stoichiometry of the complexes was assumed to be 1 : 1 as described later. Two different methods were used in determining Krt: (1) from the decrease in the free halogen absorption measured in the 500-600 nm region; (2) from the Benesi-Hildebrand procedure13 in the region of the perturbed halogen band (300-450 nm).The K,, values reported are averages of 4 or 5 values obtained in different experiments and have a maximum deviation of 4-5%. RESULTS AND DISCUSSION STOIC H I 0 MET RY AND S T R U C T URE 0 F CHAR G E-TR ANSFER COMPLEXES BETWEEN ETHERS, EPOXIDES AND THIOETHERS AND OACCEPTORS (I, AND Icl) o acceptors, such as iodine and iodine monochloride molecules, form c.t. complexes having a 1 : 1 stoichiometryl* when they react with oxygen-containing organic compounds, particularly with alcohols, ethers, cyclic ethers and heteroaromatic oxygen compounds.The iodine monochloride molecule forms stronger c. t. complexes than does the iodine molecule because of the greater basicity of the iodine atom in IC1 compared with the corresponding I, molecule, owing to the electron affinity of the iodine atom, which is higher in ICl than in I,.15 Oxygen has two hybridized electron lone-pairs, one of which is engaged in the formation of one c.t. bond.16 The halogen molecule is directly linked to the oxygen atom in a geometry that affords maximum overlap between the lone-pair donor orbital and the antibonding acceptor molecular orbital. As regards the ether-halogen complexes, structural data17 have shown the presence of a linear arrangement : In the c.t. complex of 1,4-dioxan with IC1 an arrangement in which two molecules of iodine monochloride are attached to the oxygen atoms of 1,4-dioxan in an equatorial position has been proposed :I8 n In view of the absence of experimental evidence to the contrary or clearly contradictory theoretical reasons, it seems fairly probable that the same geometry is characteristic of these complexes in solution.X-ray studies on thioether-halogen complexes show a structural situation analogous to that found for ether complexes, i.e. 1 : 1 stoichiometry and a linear arrangementS. SANTINI AND S. SORRISO 3155 of the complex 's- --x-x. / For our c.t. complexes of cyclic ether and thioether donors with ICl we can suppose, on the grounds of the previous considerations, that complexes of a 1 : 1 stoichiometry are formed in which the iodine atom is directly linked to the donor atom in a linear arrangement : O(S)---I-Cl.\ / The hybridization of the oxygen and sulphur atoms in these complexes is an important factor in c.t. complexation.16 OXYGEN BASICITY I N THE 3-, 4-, 5- AND 6-ATOM RINGS K,, data for the cyclic ether series show that the electron-donating ability of the oxygen atom is dependent on the size of the ring, the basicity being in the order 4 > 5 > 6 > 3 atoms. An order of this type has previously been reported in studies of c.t. complexes with iodine' as well as in other studies.lg TABLE 1 .-STABILITY CONSTANTS OF MOLECULAR CHARGE-TRANSFER COMPLEXES BETWEEN IC1 AND VARIOUS CYCLIC OXIDES IN cc1, AT 20 OC TOGETHER WITH ABSORPTION MAXIMA OF THE PERTURBED HALOGEN BANDS Kc t compound i/nm /dm3 mol-' propylene oxide 388 30.0 trimethylene oxide 398 84.0 tetrahydrofuran 402 56.5 tetrahydropyran 405 48.0 The steric and inductive effects of the methyl groups do not appear to be important in determining this order of basicity for the oxygen atom in different rings.It thus seems reasonable to assume that the availability of the lone-pair oxygen electrons in the various cyclic ethers differs with the size of the ring, and in particular with the LCOC angle. The change in this angle must result in an altered hybridization of the orbitals, which in turn affects the electron distribution on the oxygen atom. This explanation may be valid for the 3-membered ring, in which the hybridization of the oxygen is fundamentally different from that in other rings.2o According to theoretical calculations, however, the difference in hybridization in 4-, 5- and 6-membered rings is very Undoubtedly other factors are involved.Arnett and Wu22 have explained the difference in electron-donating abilities of tetrahydrofuran and tetra- hydropyran on the basis of interactions between the electrons in non-bonded oxygen orbitals and in the adjacent C-H bonds, these being greater when the ring is planar than when it is puckered. Since the tetrahydrofuran ring is, in turn, probably slightly less nearly planar than the trimethylene oxide ring, the explanation may be extended in some degree to the latter. The basicity of the cyclic sulphides towards the iodine molecule in c.t. complexes varies with ring size in the order 5 > 6 > 4 > 3 members.z3? 23 This further establishes that the ring-size effects differ for two heteroatoms even if they both belong to the same family in the periodic table.Steric factors alone do not account adequately for31 56 MOLECULAR COMPLEXES OF EPOXIDE DONORS the results of the interaction of cyclic sulphides with molecular iodine. It has been suggested that the basicity differences were due rather to differences in electron availability caused by different ring sizes; i.e. the electron distribution is altered by ring size. It is also interesting to note how the basicity of the oxygen atom varies in the series of epoxides shown in table 2. (The parent compound, ethylene oxide, was not TABLE 2.-sTABILITY CONSTANTS OF MOLECULAR CHARGE-TRANSFER COMPLEXES BETWEEN Icl AND VARIOUS EPOXIDES IN eel, AT 20 O C TOGETHER WITH ABSORPTION MAXIMA OF THE PERTURBED HALOGEN BANDS compound Kct l/nm /dm3 mol-l propylene oxide 388 30.0 styrene oxide 390 25.4 cis-stilbene oxide 410 16.2 trans-s tilbene oxide 420 10.2 included because its boiling point is below the temperature at which spectroscopic measurements were made.) The data show how the introduction of a methyl or phenyl group in place of a hydrogen atom in ethylene oxide produces a small variation in the K,, value, indicating that the methyl and phenyl groups have an almost similar effect on the basicity of the oxygen atom.A greater decrease in the value of K,, is, instead, observed in the cis- and trans-stilbene oxides. Before giving an explanation of this behaviour on the basis of steric and electronic effects, we must consider the molecular and electronic structure of the ethylene and styrene oxide molecules.Ethylene oxide is a molecule having the following structure (a = 61' 24'):25 The H-C-H plane that of the ring. formed by the carbon and hydrogen atoms is perpendicular The crystal structure of p-nitrostyrene oxide has been determined by Williams and is depicted below (where = nitrogen, @ = oxygen and a = 60" 7'): to et The dihedral angle between the plane of the phenyl ring and the plane of the oxiran ring is 80'2'. The relative arrangement of the two rings should essentially beS. SANTINI AND S. SORRISO 31 57 determined by the interaction between the two sp5 hybridized orbitals of the oxiran ring and a p orbital on an adjacent carbon atom, in accordance with the bent-bond model developed for cyclopropane.26 The optimum geometry for this interaction has been shown experimentally to be that where the plane of the oxiran ring and the axis of the p orbital are parallel, as shown below: In the case of p-nitrostyrene oxide the configuration of the molecule does not coincide exactly with that necessary for maximum interaction.This is caused by non-bonding interactions of the ortho-hydrogens of the phenyl ring with the hydrogens of the oxiran ring. One may consider that the small difference in basicity between propylene and stryene oxides, as illustrated by their K,,, values, is caused by the presence in the latter (in addition to the small electron-withdrawing inductive effect of the phenyl ring) of a small electron-donating conjugative effect between the phenyl and oxiran rings, as expected from the bent-bond theory described above.This is confirmed by the values of ionization potential (i.p.) assigned to the oxiran ring in ethylene and styrene oxides, which are slightly lower in styrene oxide,*' thus indicating that the phenyl group has a greater electron-donating capacity than hydrogen, evidently caused by the prevalence in this compound of the conjugative electron-donating effect over the inductive electron-withdrawing effect. In the cis- and trans-stilbene oxides the lower electron- donating capacity of the oxygen may be explained not only on the basis of a probable loss of conjugation between the phenyl groups and the oxiran ring, for which only the inductive electron-withdrawing effect of the two phenyls would remain operative, but also on that of the presence of steric hindrance of the relatively big IC1 molecule.EFFECT OF SUBSTITUENT ON THE COMPLEXATION REACTION OF STYRENE AND STILBENE OXIDES WITH I c l The data in table 3 show the existence of a small effect produced by the substituents on the complexation centre formed by the oxygen atom. The small variations obtained in K,,, are insufficient to allow any appropriate considerations to be made as to which type of electronic effect (inductive and/or resonance) operates in these systems. One can, in any case, note that the substituents produce little effect on the K,, values. An attempt was made to correlate the values of K,, with the op and om values obtained by McDaniel and Brown2X using the Hammett method.A value of p = - 0.47 (r = 0.991 ; s = 0.02) was obtained (see fig. 1). The Hammett plot has been used in previous studies for donor substrates of various types whose complexation centre was conjugated to an adjacent 7c system, in particular for substituted thiophenols complexed with I, ( p = - 1.1 l),2s for mono- and di-substituted diphenyl sulphides complexed with I, (p,,,, = -0.64; pdi = -0.43)30 and for aryl diphenylmethyl sulphide complexes with I, ( p = - 1 .04).31 In our case, the small value of p is a clear indication of the overall small size of the electronic (inductive and/or resonance) transmission effect between the two rings. This result is confirmed by p.e.s. measurements on the aryloxiran molecule, where a similarity between the3158 MOLECULAR COMPLEXES OF EPOXIDE DONORS 1.5 1.4 h I .- - 0 E 1.3 m E -0 .1.2 2 00 - 1.1 1.0 TABLE 3.-sTABILITY CONSTANTS OF MOLECULAR CHARGE-TRANSFER COMPLEXES BETWEEN Icl AND SUBSTITUTED STYRENE OXIDES IN cc1, AT 20 " c TOGETHER WITH ABSORPTlON MAXIMA OF THE PERTURBED HALOGEN BANDS - - - - - - Kr t subs ti tuen t A/nm /dm3 mol-l H m-C1 p-Me m-Me m-NO, m-F p-c1 P-NO, P-F 390 395 410 385 390 420 420 395 412 25.4 20.2 16.2 27.2 26.7 10.2 11.2 22.0 17.2 0.4 -0.2 0.0 0.2 0.4 0.6 0.8 FIG. 1 .-Hammett plot for the stability constants of charge-transfer complexes between IC1 and substituted styrene oxides in CCI, at 20 OC. U n systems in the aryloxiran and benzene is indicating a small interaction within the aryloxiran molecule between the n system of the aromatic hydrocarbon part and the lone-pair orbitals of the ethoxy group.In the trans-stilbene oxides the substituent still has a weak effect on the K,, value (table 4), but the size of this effect is smaller than that for the corresponding styrene oxide derivatives. One might think that in this case only the inductive effect would remain operative. Structural data on trans-stilbene oxide are not available in the literature, and therefore the hypothesis suggested here cannot be confirmed, but it is supported by the results obtained for the cis-derivatives. In the cis-stilbene oxides the effect of the substituent is nearly analogous to that observed in the trans-derivatives, and is therefore of a smaller size with respect to the effect observed in the styrene oxides.It thus seems that in these derivatives also, onlyS. SANTINI A N D S. SORRISO 3159 TABLE 4.-sTABILITY CONSTANTS OF MOLECULAR CHARGE-TRANSFER COMPLEXES BETWEEN Icl AND SUBSTITUTED Cis- AND lranS-STILBENE OXIDES IN cc1, AT 20 “c TOGETHER WITH ABSORPTION MAXIMA OF THE PERTURBED HALOGEN BANDS compound cis-stilbene oxide P-NO, PP’-(NO,)2 PP’-Cl, P-NO, PP’-(NO2) 2 PP’-CL trans-stilbene oxide 410 41 8 432 422 420 428 43 5 425 16.2 12.2 9.4 11.4 10.2 8.4 6 . 6 8.0 the inductive effect is operative. A similar result has been obtained in a previous study for the cis-phenylpyridylcyclopropanes X For these compounds the hypothesis of a direct electronic interaction between the two rings was suggested. Supposing that an interaction of this type is also present in the cis-stilbene oxides, one might think that the conjugative effect of the substituents (and of the phenyl groups linked to them) would be transmitted from one ring to the other, bypassing oxiran ring so that it would experience only weak inductive effects. In regard to this, identical behaviour with respect to the transmission of electronic effects is found between the cyclopropane ring and the oxiran ring when they are linked to two aromatic systems.In the trans-derivative of both types of compound an absence of conjugative effects between the phenyl groups and the triatomic cycle is noted; in the cis-derivatives of both types of substrates one may hypothesize the presence of direct electronic interactions between the two aromatic systems, with no conjugation between these systems and the triatomic ring.ABSORPTION I N THE VISIBLE REGION When a halogen molecule is complexed with a donor molecule to give a c.t. complex, the absorption band of the halogen in the visible region is shifted towards the blue. For complexes of the same acceptor with analogous donors, the size of the blue-shift is a measure of the strength of the complex. Fig. 2 shows a plot of log,, K,, against the difference in energy of electron promotion between the free and perturbed halogen [A,,,(free) = 459 nm in CCl,]. The plot shows a good correlation for all the oxiran compounds, indicating a structural analogy among the relative complexes for which the shift of the ICl band is a rough measure of the strength of the c.t.interaction. The deviation observed for 4-, 5- and 6-membered oxides is then due to different steric and electronic requirements for the complexation reaction.3160 MOLECULAR COMPLEXES OF EPOXIDE DONORS 3000 4ooo! - - I E 2 2000- 2 1000- 0 ’ 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 See e.g. R. Hoffmann and R. B. Davidson, J. Am. Chem. Soc., 1971,93, 5699 and references therein; W. J. E. Parr and T. Schaefer, J. Am. Chem. SOC., 1977,99, 1033 and references therein; R. C. Hahn, P. H. Howard and G. A. Lorenzo, J. Am. Chem. Soc., 1971, 93, 5816. (a) R. S . Brown and T. G. Taylor, J . Am. Chem. SOC., 1973, 95, 8025 and references therein; (b) Y. Kuyusama and Y. Ikeda, Bull. Chem. SOC. Jpn, 1973,46,204; (c) D. J. Williams, P. Crotti, B. Macchia and F.Macchia, Tetrahedron, 1975, 31, 993. S. Sorriso, F. Stefani, E. Semprini and A. Flamini, J. Chem. SOC., Perkin Trans. 2, 1976, 374. (a) V. Mancini, P. Passini and S . Santini, J . Chem. SOC., Chem. Commun., 1978, 100; (6) V. Mancini, P. Passini and S . Santini, J. Chem. SOC., Perkin Trans. 2, 1980, 463; ( c ) V. Mancini, G. Morelli and L. Standoli, Gazz. Chim. Ital., 1977,47, 107; ( d ) S . Sorriso, C. Battistini, B. Macchia and F. Macchia, Z . Naturforsch., Teil B, 1977, 32, 1467; (e) R. S . Cataliotti, G. Paliani, S. Sorriso, B. Macchia and F. Macchia, Z . Phys. Chem., N.F., 1977, 105, 1; ( f ) M. T. Foffani, G. Innorta, S. Sorriso and S . Torroni, J. Org. Mass Spectrom., in press. See e.g. (a) L. A. Strait, R. Ketcham, D. Jambotkar and V. P. Shoh, J. Am. Chem.SOC., 1964, 86, 4628; (b) L. A. Strait, D. Jambotkar, R. Ketcham and M. Hrenoff, J. Org. Chem., 1966, 31, 3976; (c) R. G. Pews and N. D. Ojha, J. Chem. SOC., Chem. Commun., 1970, 1033. S . Searles, J. Am. Chem. SOC., 1951, 73, 124. M. Brandon, M. Tamres and S. Searles Jr, J. Am. Chem. Soc., 1960, 82, 2129. A. C. Knipe, J. Chem. Soc., Perkin Trans. 2, 1973, 589. A. Huth and F. Neubauer, Liebigs Ann. Chem., 1979, 56. G. Berti, F. Bottari, P. L. Ferrarini and B. Macchia, J. Org. Chem., 1965, 30, 4091. F. M. Fonad and P. G. Farrel, J. Org. Chem., 1975, 40, 3881. J. Cornog and R. A. Karges, J . Am. Chem. Soc., 1932, 54, 1882. H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1950, 72, 600. R. Foster, Organic Charge-transfer Complexes (Academic Press, London, 1969), chap.8 and references therein. Se e.g. R. L. Strong and J. Perano, J. Am. Chem. Soc., 1961, 83, 2843 and references therein; A. K. Chandra and D. C . Mukherjee, Trans. Faraday SOC., 1964,60,62; A. I. Popov, C. Castellani Bisi and W. B. Person, J. Phys. Chem., 1960, 64, 691. R. S. Mulliken and W, B. Person, Molecular Complexes (Wiley, New York, 1969), chap. 5 and 1 1 . R. Foster, Molecular Complexes (Elek Science, London, 1973), vol. 1, chap. 4. 0. Hassel and J. Hvoslef, Acta Chem. Scand., 1956, 10, 138.S. SANTINI AND S. SORRISO 3161 l9 A. Weissberger, Heterocyclic Compounds with Three- and Four-membered Rings (Wiley, New York, 4o S . Searles Jr, M. Tamres and E. R. Lippincott, J. Am. Chem. SOC., 1953, 75, 2775. *I C. A. Coulson and W. E. Moffitt, Philos. Mag., 1949, 40, 1. p 2 E. M. Arnett and C. Y. Wu, J. Am. Chem. SOC., 1962, 84, 1684. 23 M. Tamres and S . Searles Jr, J. Am. Chem. SOC., 1962, 66, 1099. 24 J. D. McCullough and D. Mulvey, J . Am. Chem. SOC., 1959, 81, 1291 and references therein. 25 A. Weissberger, Heterocyclic Compounds with Three- and Four-membered Rings (Wiley, New York, 1964), part I, p. 4 and references therein. 26 W. A. Bernett, J . Chem. Educ., 1967, 44, 17. 27 E. J. McAlduff and K. N. Houk, Can. J. Chem., 1977, 55, 318. 28 D. H. McDaniel and H. C . Brown, J . Org. Chem., 1958, 23, 420. 29 G. Reichenbach, S . Santini and U. Mazzucato, J . Chem. SOC., Faraday Trans. I, 1973, 69, 143. 3o S. Santini, G. Reichenbach and U. Mazzucato, J . Chem. SOC., Perkin Trans. 2, 1974, 494. 31 S . Santini, G. Reichenbach, S . Sorriso and A. Ceccon, J. Chem. SOC., Perkin Trans. 2, 1974, 1056. 32 K. C. Li Akiyama, P. R. Le Breton, P. P. Fu and R. G. Harvey, J . Phys. Chem., 1979,83, 2997. 1964), part 11, p. 987. (PAPER 1 / 1569)

ISSN:0300-9599    

DOI:10.1039/F19827803153

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1982, 78, 3163-3175 Heterogeneity Analysis of the Silica Surface by Gas Adsorption BY CHRISTINE A. LENG* AND ALEC T. CLARK Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Merseyside L63 3JW Received 18th November, 198 1 In recent years computerised analysis methods have been developed for the determination of the distribution of adsorption energies on the surfaces of powders and precipitates from highly accurate experimental adsorption isotherms. An apparatus for the measurement of such isotherms has been constructed at Port Sunlight, and the iterative analysis method HILDA of House and Jaycock has been applied to the investigation of surface modifications of silica. The samples studied, Gasil I and TK800, are IUPAC surface-area standards and are available from the National Physical Laboratory.Results are given for a series of krypton isotherms, and the technique has been extended by the use of the nitrogen gas probe in the low-pressure rangeplp, z lo-*- 1 OW3, enabling the detection of the very high-energy regions of the surface. The high-energy heterogeneity analysis approach is shown to be sensitive to the surface changes of Gasil I induced by heat treatment in the range 423-1 170 K, and yields distinctive surface-energy distribution functions for the xerogel and Aerosil silica types. Many powders and precipitates important in industrial processing have non-uniform surfaces. Non-uniformities are due to the presence of composite structures such as catalysts, local variations in the structure of amorphous materials, non-uniform distributions of surface groups, defects, impurities, edges and corners and different types of crystal planes.' Characterisation of these surfaces is a difficult but compelling problem owing to the interest in the materials and their applications. A considerable body of work has accrued which, at times, has a highly empirical content.In the absence of more rigorous methods, conventions and rules are sought to classify experimental data, and to reveal trends without achieving absolute values. This is the case in the application of gas-adsorption techniques to the investigation of hetero- geneous samples. For uniform surfaces, the calculations of Langmuir,2 Brunauer, Emmett and Teller,3 Hill,4 de Boer,5 and Fowler and Guggenheim6 provide theoretical models for the adsorption isotherm. These models assume that the surface has a single adsorption potential energy, U, and have been applied to surfaces approaching this condition such as graphitised carbons.However, the models have also been widely applied to heterogeneous samples which clearly violate the above condition, resulting in a purely empirical description of the adsorption process. This approach, nevertheless, has yielded much useful information for a wide range of gas-solid systems, particularly in showing trends from a series of measurements such as nitrogen B.E.T. surface areas. Progress has been made in the last decade with the development of computerised methods of analysis of the adsorption isotherm.*.Such methods explicitly include surface heterogeneity, and determine the distribution of adsorption energy,f( U ) , over the sample surface. Ross and Morrisong demonstrated the narrow, single-peaked energy distributions resulting from the isotherms of nitrogen and argon on graphitised carbon and boron 31633164 HETEROGENEITY ANALYSIS OF THE si SURFACE nitride, and presented a very interesting series of energy distributions showing the increasing uniformity of the carbon black surface as the graphitisation process proceeds. Very detailed adsorption isotherms for krypton adsorbed onto silver iodide were analysed by Sidebottom et d.l0 to give energy distributions showing a series of sharp peaks; the peaks were discussed in terms of the crystal planes present and the technique was able to distinguish between four different preparative methods for silver iodide.Further applications include an investigation of annealing of precipitated sodium chloride crystals,ll and the analysis of the data of Aristov and KiselevI2 for a silica ~amp1e.l~ This type of computer analysis requires very high quality data; the vast quantity of isotherms in the literature are not suitable for analysis owing to limitations of accuracy, number of points and the pressure range of the data. For this reason, developments of the analysis methodP have been more rapid than applications of the technique. We have constructed a gas-adsorption apparatus for the measurement of accurate, extensive and detailed submonolayer isotherms. This first investigation deals with modifications of silica surfaces.Previous accurate heterogeneity studies have involved ionic crystals or graphitised carbon samples; here we test the sensitivity of the technique for an amorphous sample. Silicas are naturally abundant, and are manu- factured by two main processes to produce types of silica known as xerogels, resulting from the mixing of acid and silicate, and pyrogenic or high-temperature silicas such as Aero~i1.l~ The silica samples used in this study, Gasil I (xerogel) and TK800 (Aerosil), are surface area standardsI6 and are available from NPL. The Gasil I surface was modified by heat treatment in the temperature range 423-1 170 K. Heating alters the surface structure by the removal of hydroxy groups,17 and hence changes the powder properties.Adsorption isotherms were measured in the range 10-5-1 Torr* using krypton and nitrogen adsorbates; the isotherms were analysed using the HILDA method of House and Jaycock* to derive the surface adsorption energy distributions. In the case of krypton, the isotherms were measured from the lowest pressures attainable up to the completion of the B.E.T. monolayer. The use of the nitrogen gas probe in the same pressure range is a very different proposition, and represents a significant extension of the application of this technique. For nitrogen, the available p / p o range extends from lop8 to ( p o is the saturation vapour pressure), so that at the highest pressures measured, the surface coverage is well below that of the monolayer. Attention is focused on the very low pressure range of the isotherm, and thus information is provided on the very high energy regions of the surface.This is a particularly interesting part of the energy distribution, as the higher-energy sites are expected to be very sensitive to surface changes. Energy distributions are reported for Gasil I heated at 423,623 and 870 K using krypton as the adsorbate, and for Gasil I heated at 423, 673, 870 and 1170 K with nitrogen as the adsorbate. Classification of the different types of silica is an important problem, and towards this end we have determined the energy distribution of the Aerosil sample, TK800, to enable a comparison with the Gasil I results. EXPERIMENTAL The gas-adsorption apparatus, shown schematically in fig. 1, utilises the volumetric method.Gas is leaked from the dose region through valve A into the sample tube, distributing itself in the gas phase in the warm and cold parts of the sample tube, and adsorbing onto the sample. * 1 Torr = 133.33 Pa.C. A. L E N G A N D A. T. C L A R K 3165 The change in pressure recorded by the pressure gauge P, gives the amount of gas entering the sample region, (the relevant volumes are calibrated), and the equilibrium pressure above the sample is measured by P,. The amount of gas adsorbed per gram of sample is calculated from these readings, taking in to account thermal transpiration. lH He I I I A I x c r---------’ I vapo u r dose 6 A I I I I I -----I pressure momet er sample I J 1 i q N2/02 T1 FIG. 1 .-Schematic diagram of the volumetric adsorption apparatus.TI denotes the sample temperature (77.8 K in this work). The dashed box indicates a cabinet thermostatted at a temperature T2 slightly above room temperature. ‘ x ’ marks the position of a Springham greaseless vacuum valve with a Viton A diaphragm. The ‘dose’ region between taps A and B supplies the adsorbate for a given run. The bulb below valve C is of known volume and enables the calibration of experimental volumes required for the calculation of the amount of gas adsorbed, and further provides flexibility in the isotherm measurements by increasing the dose volume. IG indicates an ionisation gauge; an Edwards Diffstak with a 63 mm diffusion pump utilising Santovac 5 fluid and an EDM2 rotary pump provides the pumping system. The pressure is measured by Baratron transducers with ranges 1 0-4- 10 and 1 Torr.The use of two pressure gauges, rather than one, gives a considerable improvement in accuracy by eliminating cumulative errors. The system is free from mercury or tap grease contamination utilising an Edwards ‘Diffstak’ with Santovac 5 fluid and Springham valves with Won A diaphragms. Temperature control is particularly important and a nitrogen vapour pressure thermometer is used to maintain the sample temperature constant to within f O . O 1 K. The glassline is housed in a thermostatted cabinet. Research grade gases are supplied by B.O.C. A silver iodide sample was studied initially as a calibration. The krypton adsorption isotherm for this sample has been very carefully measured,’O and shows three steps in the submonolayer region.The steps indicate the condensation of a two-dimensional gas-like layer of krypton on parts of the sample with very similar energies, to a two-dimensional liquid-like film, and provide a stiff test of experimental reproducibility. ANALYSIS A straightforward extension of adsorption onto a homogeneous surface is to consider a collection of such surfaces with different energies, which are of sufficient size to neglect boundary interactions. For this system, the adsorption process is described by the following equation:19 At constant temperature, OT,T(p) is the fractional coverage of gas over the total surface at a pressure p of adsorbate above the sample, and 8 ( p , U ) is the fractional coverage on a uniform region of the surface with energy U.The distribution function f(U) denotes the frequency of regions of energy U per unit energy interval, i.e. the fraction of surface with energies in the range U and U+ dU; f ( U) is normalised.3166 HETEROGENEITY A N A L Y S I S OF THE sl SURFACE Analytical transforms have been found for eqn (1) to link a homogeneous isotherm 0 (e.g. Langmuir) to an empirical equation for the total isotherm OTOT such as the Dubinin-Radushkevich or Freundlich isotherms, via an energy distribution function.20 Such investigations are very satisfying in providing a basis for these often observed isotherm shapes; however, the transform methods severely limit the complexity of the functions involved. Ross and Olivier21 chose a Gaussian, or a sum of Gaussians, to describe the energy distribution; for a range of adsorbants this is a good approximation. Using the Hill- de Boer model for 0, they calculated total isotherms from eqn (1) which compared well, in many cases, with experiment.Samples were characterised by heterogeneity parameters such as the position and width of the Gaussian. The method used in this work was the iterative scheme initiated by Adamson and Lingz2 and refined and computerised by House and Jaycock.8 0 ( p ) is calculated using a homogeneous adsorption model, and a trial function f o ( U ) is used to calculate O&,,(p) from the equation. The latter value is compared with the experimental result at each point, and iterations continue until fn( U ) gives @GOT( p ) in agreement with experiment.The extent of agreement is measured by the root mean-square deviation ( o ~ , ~ . ~ . ) : where O%& is the experimental result, and the isotherm has N points. Equations of this type are difficult to handle,z3 and care is required with their numerical solution. The numerical stability of the analysis necessitates high-quality data with many experimental points and low scatter. Given good data, the iterative method is straightforward if the initial function f o ( U ) is close to the exact value so that convergence is rapid. It is desirable to aim for this situation, and this aspect of the analysis supports the use of krypton gas at liquid-nitrogen temperatures. Two-dimensional condensation takes place in this case, and O(p) approaches the step-function form assumed in the calculation offo( U ) .s Experience is needed to clarify the importance of this consideration with respect to other factors. Data-handling is an important feature of the analysis, and specialised computer- graphics programs have greatly assisted in this respect. The first step of the numerical procedure is the reduction of the scatter of the experimental points by a smoothing routine. In general it is a difficult task to follow data closely without reproducing, or even magnifying, irregularities in the data. The method currently used is a double rn-point, least-squares quadratic fit, with rn between 5 and 13. For rn = 5 this smoothing routine takes the first 5 data points (points 1-5), finds the best quadratic curve through the points, and calculates one fitted point.Points 2-6 are similarly fitted, and the procedure is continued over the 70-100 points of the isotherm, and then repeated throughout once more. The smoothing results must be closely checked for each set of data. A second smoothing option performs this operation on the energy distribution function between iterations. Parameters specifying the result of the analysis include the number of data points, N , the number of iterations, n, required for convergence (minimum r.m.s.), the computer C.P.U. time taken, smoothing factors indicating the quality of the smoothing of the raw data (SMl), and of the energy distribution on the final iteration (SM2), and the r.m.s. value.C. A. LENG AND A. T. CLARK 3167 RESULTS Kr/GAsIL I : 423, 623 AND 870 K Three krypton adsorption isotherms were measured on a sample of G a d I which was outgassed at 423 K, then heated at 623 K and further at 870 K in a vacuum.Outgassing at 423 K removes the physically adsorbed water without chemically modifying the surface, while calcination at higher temperatures removes surface hydroxy groups.17 The adsorption isotherms were measured at a temperature of 77.8 K in all cases; this value was calculated from the vapour-pressure measurement using loglop = a+b/T+cT the relationship with constants given by Friedman and White.26 Fig. 2 shows details of the experimental results. Within experimental error, the isotherms are generally smooth; this very amorphous sample does not show the isotherm steps present for AgI. The experimental points were measured in groups of 5-20 per run. It is important to demonstrate agreement of points from different groups, as errors between runs are greater, unless care is taken, than the variation of points of a single experiment.For this reason, measurements were taken between pairs of points of a previous run, and considerable overlap ensured as points were taken throughout the pressure range. The reproducibility of the isotherms is very good; over most of the pressure range the error is of the order of The experimental points were smoothed using a double 9-point least-squares quadratic fit to give points lying on the curve shown with the raw data (*) in fig. 2. The smoothed data were analysed to give the energy distributions shown in fig. 3, without further smoothing during the numerical calculations.The Hill-de Boer isotherm was used for 8 ( p , T ) . The factor A , relating the measured pressures to the adsorption energy U s has the value 2.373 x lo6 Torr. Termination of the energy distribution at the high-energy limit corresponds to the lowest pressure measureable; the low-energy limit is given by the B.E.T. monolayer capacity. Table 1 gives details of the B.E.T. analysis. The latter value is a reasonable and well recognised choice, although arbitrary, as is common for krypton B.E.T. plots.25 The main requirement for a meaningful comparison of the energy distribution as a function of heat treatment is a consistent termination limit. The continued rise off( U ) at low energies seen in fig. 3 indicates that, in this case, multilayer adsorption intervenes before the completion of the monolayer.Results for the 623 and 870 K samples were taken at lower pressures than for the 423 K sample, and fig. 4 compares the resulting wider-ranging energy distributions. The general form of the krypton/Gasil I energy distribution is roughly exponential. This result may be compared with the energy distribution for the adsorption of argon onto a fully hydroxylated silica;13 the measurements of Aristov and Kiselev,12 were not taken for this purpose so that the energy distribution has lower resolution than those shown in fig. 3 and 4; however, the overall shape is in agreement. The 423 K Gasil I energy distribution shows a maximum at 8.16 kJ mol-l, and a ‘hump’ at 9.62 and 10.3 kJ mol-l. The 623 K energy distribution shows more structure, with peaks at 8.41, 8.62 and 9.08 kJ mol-’ and an indication of structure at 10.3 kJ mol-l.The 870 K result is almost linear. As the calcination temperature increases, the concavity of the energy distributions decreases, with a reduction of low-energy regions in the range 8.0-8.9 kJ mol-l, and an increase of intermediate energies in the range 8.9- 1 1.3 kJ mol-l. Fig. 4shows that the higher-energy regions, 11.3-12.6 kJ mol-l, have decreased on heating from 623 to 870 K. 1 %.3168 HETEROGENEITY ANALYSIS OF THE si SURFACE 0.70 r 0.65 - 0.60 - - 0 . 5 5 - 0.50 - E I w 0.05 I I I I 0 .ooo 0.005 0.01 0 0.015 0.020 FIG. the I I I I 0 0.1 0.2 0.3 0-4 pressure/Torr 2.-Krypton adsorption isotherms on the silica sample Gasil 1 heated at 423,623 and 870 K; (a) shows low-pressure region and (b) shows the full isotherms.*, Experimental points; (-) smoothed data. In terms of the analysis, the general smoothness of the energy distribution functions is a clear indication of the quality of the data, and the stability of the numerical methods. Table 2 shows that convergence of the iterative analysis is rapid, computer time low, and the r.m.s. values are of the order of Further discussion on the stability of the energy distributions is given in the Discussion. N,/GASIL I : 423, 673, 870 AND 1170 K Nitrogen is the standard gas used for silica B.E.T. surface-area measurements which involve the portion of the isotherm near the completion of the monolayer (in the rangeC. A. LENG A N D A. T.CLARK 0.4 0.0 3169 - - I I I I3170 1 . 4 - 1 . 6 - h s? 1 . 2 - + 1.0 0.8 0.6 0 . 4 0.2 0.0 HETEROGENEITY ANALYSIS OF THE si SURFACE 5 . -. - - - - - .'.--\ ---__ J I I I TABLE 1 .-Kr/GAsIL I B E T . ANALYSIS samplea S.A.b/m2 g-l Vmb/m2 g-l Cb 423 K 178.9 34.1 14.3 623 K 179.8 34.3 14.0 870 K 182.8 34.9 15.1 a The sample is identified by the temperature of heat treatment. S.A. represents the krypton surface area, V, the monolayer capacity and the B.E.T. c parameter is also given. po was taken to be 2.46 Torr and the value of 19.5 A2 was used for the krypton molecular area. The trend is similar to that shown by the nitrogen and argon B.E.T. results for Gasil I reported by Rouquerol et aZ.24 TABLE ~.-K~/GAsIL I T/K N m SMl SM2 nu r.m.s. C.P.U. time/min 423 69 9 0.0025 - 15 L O X 10-9 2.5 623 73 9 0.0057 - 15 L O X 10-9 2.8 870 86 9 0.0071 - 15 4 .2 ~ 2.8 a These analyses were set to terminate at 15 iterations. All isotherms were measured at 77.8 K determined from the nitrogen vapour pressure measurement and the relationship of Friedman and White.2s The calculations were carried out on a Harris series 500 computer. 40-300 Torr or p/po : 0.05-0.35). In the pressure range 1 0-5- 1 Torr @/p0 : l O-s- l 0-3), the entire measured isotherm is well below the monolayer capacity, and we seek to determine the effect of the heat treatment on the very high-energy regions of the Gasil I surface. The isotherms for this gas probe were again smooth within experimental error. In contrast to the krypton results, analysis of the nitrogen data after double 9-point smoothing yielded a noisy distribution function with many sharp peaks.TheC . A . LENG A N D A. T. CLARK 3171 additional smoothing option was used between iterations to produce the energy distributions shown in fig. 5, again using the Hill-de Boer model for adsorption on a uniform region of the surface. Considerable detail of structure is shown, and the effects of heat treatment are clearly observable. For nitrogen, A , = 1.390 x lo6 Torr.8 At 423 K we see a very peaked energy distribution, showing that nitrogen is preferentially adsorbed onto some parts of the surface with respect to others. More structure is expected than for the krypton adsorbate as the quadrupole moment of nitrogen enhances the polar contributions to the adsorption energy. f ( U ) has sizeable values throughout the energy range, indicating the presence of significant regions of the Gasil I surface with these energies.After heating at 673 K the surface has a much smoother energy distribution. The surface is no longer very specific to nitrogen adsorption, and the fraction of surface with a given energy decreases almost linearly with energy. The result for 870 K is also smooth but no longer linear; the downward convexity shows a significant loss of surface with energies in the range 10.8-14.2 kJ mol-l. At 1170 K a marked change occurs: there is a return of surface specificity with peaks at 8.8 and 9.6 kJ mol-l, and a further substantial reduction of high-energy surface. The structure at 1170 K is very similar to that at 423 K with a strong damping of f ( U ) with increasing U ; comparing peak positions, there is agreement at 8.8 and 9.6 kJ mol-l, a slightly altered structure in the range 10.4-12.1 kJ mol-l, and a peak at 12.5 kJ mol-1 in both cases.The integral off( U ) over an energy interval is proportional to the fraction of surface present with energies in that interval. Thus we can monitor the reduction of surface with energies in the range, say, 10.8-14.2 kJ mol-l compared with that in the interval 8.3-10.8 kJ mol-l, by calculating the ratio of the respective areas A , and A , , under thef(U) curve. These values are shown in table 3. TABLE ~.--N,JGASIL I 423 0.41 673 0.32 870 0.25 1170 0.19 TABLE ~.-N,/GAsIL I T/K N rn SM1 SM2 n r.m.s./10-3 C.P.U. time/min 423 76 9 0.0091 0.0011 52 1.53 14.5 673 94 9 0.0058 0.0010 52 1.30 15.3 870 88 9 0.0034 0.0014 52 1.92 14.7 1170 78 9 0.0014 0.0097 52 1.41 15.1 The parameters shown in tables 2 and 4 confirm the predictions based on the analysis details.The nitrogen analysis results in a higher r.m.s., longer computing time and more iterations. However, these values are well within practical limits, and the nitrogen gas probe in this high-energy range should provide very interesting results over a range of samples.3172 HETEROGENEITY ANALYSIS OF THE si SURFACE I . 6 H 1.6 Ll . 1 . 6 - 1.4 1.2 h 1.0 b - - - 0.8- 0 . 6 - 0 . 4 0.2 0.0 - - 10 12 U/kJ mol-’ FIG. 5.-High-energy nitrogen energy distributions for Gasil I heated at (a) 423, (b) 623, (c) 870 and ( d ) 1170 K.C. A. LENG A N D A.T. CLARK 3173 N2/TK800: 423 K Results were taken for the Kr/TK800 system, but the energy distribution was not very different from that of the Gasil I sample. The nitrogen probe in the higher-energy range, however, gave a distinctive energy distribution for TK800. This result is shown in fig. 6; a series of 4 closely spaced peaks occurs between 8.3 and 10.2 kJ mol-I, with a further peak at 1 1.1 kJ mol-l and slight structure at higher energies. This spectrum is very different from that for Gasil I outgassed at the same temperature; the general shape of the energy distribution function is more like that of the Gasil I sample heated at 673 K. This result is in agreement with i.r. measurements which show a lower density of hydroxy groups for Aerosils than xerogels outgassed at 423 K.DISCUSSION We now discuss some aspects of the stability of the energy distributions shown in fig. 3-6. The experimental reproducibility is incorporated into the data as each isotherm is a compilation of several experiments, and care was taken to represent measurement errors evenly throughout the pressure range. Repetition of the whole isotherm for a given sample would merely give a different arrangement of the data points within experimental error. We consider here the possibility that this arrangement might affect the energy distribution, with perhaps particular groupings of points on the isotherm giving rise to structure on the energy distribution. For example, could the peak at 9.08 kJ rnol-' on the Kr/Gasil I 623 K energy distribution be dependent on the positions of a few points? We answered this question by using a random-number generator to move the data points within the experimental error, 1 %.The energy distributior, from the isotherm generated in this way was unchanged from that in fig. 4 except for very slight changes in the shape, but not position, of the small peaks at 8.41 and 8.62 kJ mol-l. The peak at 9.08 kJ mol-I and the remainder of the fine structure were unaltered. The highly structured energy distribution for the N2/Gasil I 423 K system was similarly tested, and no change resulted. We conclude from these tests, and from the values of the analysis parameters, that the energy distribution functions reported are very stable and that the details of these spectra may be considered closely.The differences in the krypton energy distributions are not large, but are well outside the errors involved in the experiment and the analysis. The energy distributions before and after randomisation are in agreement because the smoothed data points from both isotherms are in agreement. The smoothing routine is thus successfully eliminating experimental scatter. Uncertainty about the validity of a peak in the energy distribution occurs when the resolution of the peak is of the same order as that of the data. The smoothing level used here is such that the information extracted from the data is of a lower resolution than that of the measurements; thus the features present in the energy distribution are very stable. This also suggests that valid fine structure may appear from a lower level of smoothing, particularly in the case of the nitrogen results which were calculated using the two smoothing options, and are stable to randomisation of the data in a range greater than the experimental error.Investigation of alternative smoothing methods is an important aspect for future work. At this stage, where relatively few systems have been investigated by heterogeneity analysis, we feel that the highly stable features provide sufficient information, and present our results in this form. We conclude that the heterogeneity analysis technique is sensitive to the surface modifications of Gasil I due to the effects of heat treatment, and shows distinct energy distributions for an Aerosil and a xerogel. Two probes have been investigated.The3174 1 . 6 1 . 4 1.2 h 1.0 HETEROGENEITY ANALYSIS OF THE si SURFACE 0.6 0 . 4 0.2 0.0 10 12 U/kJ mol-' 14 FIG. 6.-High-energy nitrogen energy distribution for the Aerosil sample TK800 outgassed at 423 K. use of krypton optimised the conditions for rapid and stable analysis for the energy distribution. Generally, krypton enables the investigation of very low surface-area materials and is particularly useful for samples which have ' patchy ' non-uniformity , such as different crystal planes. For these amorphous samples, the nitrogen-gas probe at low pressures yielded structured energy distribution for high-energy surface regions. The analysis was less rapid than for krypton, but remains well within practical limits. We are currently investigating the effect of different isotherm models for 8 on the form of the energy distribution, and of particular interest is the analysis of these results using the approach recently reported by Sacher and Further experimental study is directed towards the consideration of sample variability.Many thanks are due to Mr F. R. Morgan for his considerable efforts in the construction and development of the apparatus. We thank Dr M. J. Jaycock for consultations on the technique, Dr A. L. Love11 for useful discussions on silica and Mrs D. A. Patchett for graphics programming. M. J. Jaycock and G. D. Parfitt, Chemistry of Interfaces (Ellis Horwood, Chichester, 1981), chap. 4. S. Brunauer, P. H. Emmett and E. Teller, J . Am. Chem. SOC., 1938, 60, 309. T. L. Hill, Introduction to Statistical Thermodynamics (Addison-Wesley, Reading, Mass., 1960).J. H. de Boer, The Dynamical Character of Adsorption (Clarendon Press, Oxford, 1953). R. Fowler and E. A. Guggenheim, Statistical Thermodynamics (Cambridge University Press, 1939), chap. 8. K. S. W. Sing, in Characterisation of Powder Surfaces, ed. G. D. Parfitt and K. S. W. Sing (Academic Press, New York, 1976), p. 1. W. A. House and M. J. Jaycock, Colloid Polym. Sci., 1978, 256, 52. S. Ross and I. D. Morrison, Sur- Sci., 1975, 52, 103. * I. Langmuir, J . Am. Chem. SOC., 1918, 40, 1361. lo E. W. Sidebottom, W. A. House and M. J. Jaycock, J . Chem. SOC., Faraday Trans. I , 1976,72,2709. l1 W. A. House and M. J. Jaycock, J . Colloid Interface Sci., 1977, 59, 252. l2 B. G. Aristov and A. V.Kiselev, Colloid J . USSR, 1965, 27, 246.C. A. LENG A N D A. T. CLARK 3175 l 3 W. A. House, J . Chem. Soc., Faraday Trans. I , 1978, 74, 1045. l4 M. Jaroniec, Surf. Sci., 1975, 50, 553; W. Rudzinksi, L. Lajtar and A. Patrykiejew, Surf. Sci., 1977, 67, 195; R. R. Zolandz and A. L. Myers, Progress in Filtration and Separation, 1979, 1, 1; W. A. House, J . Colloid Interface Sci., 1978, 67, 166. l 5 D. Barby, in Characterisation of Powder Surfaces, ed. G. D. Parfitt and K. S. W. Sing (Academic Press, New York, 1976), p. 353. D. H. Everett, G. D. Parfitt, K. S. W. Sing and R. Wilson, J . Appl. Chem. Biotechnol., 1974, 24, 199. R. K. Iler, The Chemistry of Silica (John Wiley, New York, 1979). S. C. Liang, Can. J . Chem., 1955, 33, 279; A. J. Rosenberg, J . Am. Chem. Soc., 1956, 78, 2929; G. A. Miller, J . Phys. Chem., 1963, 67, 1359; W. A. House and M. J. Jaycock, J . Appl. Chem. Biotechnol., 1975, 25, 327. l 9 A. W. Adamson, Physical Chemistry of Surfaces (John Wiley, New York, 3rd edn, 1976), p. 607. 2o G. F. Cerofolini, Surf. Sci., 1971, 24, 391; M. Jaroniec, W. Rudzinski, S. Sokolowski and 21 S. Ross and J. P. Olivier, On Physical Adsorption (Wiley Interscience, New York, 1974). 22 A. W. Adamson and I. Ling, Adv. Chem. Ser., 1961, 33, 51. 23 D. L. Phillips, J . Assoc. Comput. Mach., 1962, 9, 84; S . Twomey, J . Assoc. Comput. Mach., 1963, 24 J. Rouquerol, F. Rouquerol, C. Peres, Y. Griilet and M. Soudellal, in Characterisation of Porous 25 K. S. W. King and D. Swallow, Proc. Brit. Ceram. SOC., 1965, 5, 39; M. J. Jaycock, in Particle Size 26 A. S. Friedman and D. White, J . Am. Chem. SOC., 1950, 72, 3931. 27 R. S. Sacher and I. D. Morrison, J . Colloid Interface Sci., 1979, 70, 153. R. Smarzowski, Colloid Polym. Sci., 1975, 253, 164. 10, 97. Solids, ed. S. J. Gregg, K. S. W. Sing and H. F. Stoeckli (S.C.I. Publications, London, 1980). Analysis, ed. M. J. Groves (Heyden, London, 1978), p. 308. (PAPER 1 / 1798)

ISSN:0300-9599    

DOI:10.1039/F19827803163

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1 , 1982, 78, 3177-3185 Coordination and Dispersion of Co2+ Ions in Coo-MgO Solid Solutions BY KRYSTYNA DYREK AND ZBIGNIEW SOJKA Institute of Chemistry, Jagiellonian University, Cracow, Poland Received 30th November, 198 1 COO-MgO solid solutions of COO concentrations 0.25-1 5.3 mol % have been investigated by e.s.r. spectroscopy in the temperature range 4-77 K. The distribution of isolated Co2+ ions between octahedral and tetrahedral sites has been estimated quantitatively. In the most dilute solid solutions (up to 3.00 mol % COO) Co2+ ions occupy predominantly trigonally distorted tetrahedral sites. The exchange interactions extend up to the 3rd coordination sphere in CoSMgO. The dependence of the number of paramagnetic 0; radicals formed in the course of oxygen adsorption on COO-MgO at room temperature is correlated with the analogous dependence of the number of isolated Co2+ ions of tetrahedral symmetry with trigonal distortion.Two kinds of adsorption centres are postulated in CoCMgO solid solutions: tetrahedrally coordinated Co2+ ions with trigonal distortion isolated or coupled to the paramagnetic neighbours. The occupation by Co2+ ions of both octahedral and tetrahedral sites in Coo-MgO solid solutions was postulated by one of us on the basis of magnetic- susceptibility measurements and reflectance spectra. This interpretation was also made by Stone et aL39 in thecase of the dilute COO-MgO solid solutions. The non-octahedral coordination of Co2+ in MgO was also postulated by Krylov et aL5 Stone et aL3v4 represent the opinion that Co2+ in tetrahedral coordination appears only in the surface layers of the Coo-MgO crystallites whereas, according to our data, in the dilute solid solutions (containing 30.9 mol % COO or less) tetrahedrally coordinated Co2+ is also present in the bulk crystal.To clarify this problem we have attempted in the present work to determine quantitatively the distribution of Co2+ ions between octahedral and tetrahedral sites in dilute Coo-MgO solid solutions. A more precise determination of the symmetry of Co2+ ions in tetrahedral surroundings was another object of our investigations, together with the problem of cobalt dispersion in the MgO matrix. EXPERIMENTAL MATERIALS Coo-MgO solid solutions were obtained by the thermal decomposition in U ~ C U O (1 0-3- 1 OP4 Pa) at 773 K of coprecipitated carbonates of Co2+ and Mg2+ as described previously.' The only difference in the procedure was a shortening of the calcination time from 20 to 10 h.The starting materials supplied by Polskie Odczynniki Chemiczne (Gliwice, Poland) were analytically pure. All operations on the samples were carried out in sealed glass ampoules in such a way that the samples were not allowed to come into contact with air. CHEMICAL ANALYSIS The concentration of Co and Mg in the investigated solid solutions was determined by complexometric titration with EDTA after dissolving the samples in HC1.I The absence of Co3+ impurities was checked by an iodometric method.6 103 3177 FAR 783178 COORDINATION AND DISPERSION OF Co2+ I N Coo-MgO E.S.R.SPECTRA E.s.r. spectra were recorded in the temperature range 4-300K using a Bruker ER200 spectrometer and an X-25 e.s.r. spectrometer (constructed at the Department of Electronics, Technical University, Wrodaw, Poland) both operating in the X-band mode with modulation frequency 100 kHz. Ultramarine and Co2+ in MgO (0.39 mol % COO), calcined at 1273 K, were used as standards of spin concentration. ADSORPTION Oxygen adsorption was carried out at room temperature under a pressure of 5 hPa. After 10 min of adsorption any excess oxygen was removed by evacuation. RESULTS AND DISCUSSION two kinds of e.s.r. signals were expected for COO-MgO: those of Co2+ ions in octahedral and tetrahedral surroundings. In an octahedral crystal field the ground state 4qg of Co2+ is split by spin-orbit coupling to yield a Kramers doublet between which the e.s.r.transitions are observed. The resulting g factor is given by the formula' On the basis of our previous g = 1O/3 + k, - 15/2 (L/A) (1) where k, is the parameter of covalency (0.85-0.90 for CoO-Mg07), 13. is the spin-orbit coupling constant ( - 178 cm-l for Co2+ 8, and A is the octahedral crystal-field splitting (ca. 9500 cm-l for COO-MgOg). The g value calculated from formula (1) is equal to ca. 4.3. The lowest excited state has an energy only 305 cm-l higher than the Kramers doublet,1° and so the spin-lattice relaxation time for Co2+ in octahedral surroundings is very short and the corresponding e.s.r. signal may be observed exclusively at low temperatures. In fact the signal of CO;&~ has been observed by several authors at liquid-helium and liquid-nitrogen temperatures.'* At 77 K the hyperfine splitting resulting from the nuclear spin of Co ( I = 7/2) is unresolved, and only a broad single line is observed.In a tetrahedral crystal field the ground-state level 4A2 of Co2+ is separated from the lowest excited level by ca. 4200 cm-l l4 and so the relaxation time is long enough to observe the e.s.r. signal of tetrahedral cobalt at room temperature. The g factor is given by the formula g = 2-811/A (2) which leads to a g value of 2.34. In the e.s.r. spectra of dilute Coo-MgO solid solutions (containing 15.3 mol % of COO or less) recorded at 4 K two signals are observed (fig. 1); a broad line with g = 4.23, which may be attributed to isolated Co2+ ions in octahedral coordination, and a sharp symmetric signal of much lower intensity, g = 2.00, characteristic of defects in MgO.I5 The signal ascribed to Co2+ ions vanished at ca.100 K whereas the signal arising from defects was also observed at room temperature. No signal due to tetrahedrally coordinated cobalt was observed even at 4 K, in spite of the fact that the dilute solid solutions according to magnetic-susceptibility data and reflectance spectral? were believed to contain predominantly or even exclusively tetrahedrally coordinated Co2+ ions. The number of spins associated with the signal attributed to the octahedrally coordinated Co2+ ions is considerably lower than the total amount of cobalt calculated from the weight of the sample and known concentration of COO (table 1, columnsK.DYREK AND Z. SOJKA 3179 FIG. 1.-E.s.r. spectrum of COO-MgO (0.39 mol % COO) at 4 K. TABLE 1 .-DISTRIBUTION OF Co2+ IONS IN Coo-MgO SOLID SOLUTIONS~ no. of isolated no. of isolated total no. of CoEzta ions (from CO;;~, ions (from Co2+ ions e.s.r.) before e.s.r.) after isolated ions in clusters mol % in the sample calcination at calcination at Co,Z,',,,. undetectable in COO x 10-19 1273 K x 1273 K x 10-19 ions x e.s.r. x no. of no. of Co2+ (1) (2) (3) (4) ( 5 ) (6) 0.25 3.7 1.4 & 0.2 3.5k 1.4 2.1 0.2 0.39 5.8 3.4 f. 1.2 5.6 1.7 2.2 0.2 2.14 31.4 8.5+ 1.0 19.2 f. 3.2 10.7 12.2 3.00 52.3 10.4& 1.5 25.6 f 2.3 15.2 26.7 10.39 142.5 4.1 & 1.1 5.9 & 2.4 1.8 136.6 15.30 202.0 4.4f0.1 8.4 k 3.2 4.0 193.6 a All the values in table 1 are calculated from three independent experiments.3 and 2, respectively). After the samples had been heated in vaczio at 1273 K for 7 h the intensity of the signal due to the octahedrally coordinated Co2+ increased considerably (table I , column 4, and fig. 2). Simultaneously the samples changed colour from blue to pink. These results indicate the presence in the dilute solid solutions of two kinds of Co2+ paramagnetic species, those detectable and not detectable by e.s.r. As has been shown already the e.s.r. signal of g value 4.23 observed in the temperature range 4- 100 K may be attributed to octahedrally coordinated Co2+ ions. The species not detectable by e.s.r. predominate in the dilute solid solutions of high specific surface area,' i.e.in crystallites revealing a high concentration of Schottky defects.16 The magnetic properties and optical absorption of the Co2+ ions in these solid solutions are typical of tetrahedrally coordinated cobalt, which strongly suggests that the deficit in the number of spins determined by e.s.r. spectroscopy corresponds to Co2+ ions in tetrahedral coordination. The change of colour from blue to pink and simultaneous increase in intensity of the signal attributed to octahedrally coordinated Co2+ ions which occur upon heating the samples in vacuo at 1273 K indicate an ordering of the lattice and migration of Co2+ ions from tetrahedral to octahedral sites typical of the B, type crystal lattice. These results agree with those of previous investigations of reflectance spectra2 which showed that the change in Co coordination in the COO-MgO solid solutions is a result of calcination.103-23180 COORDINATION A N D DISPERSION OF Co2+ I N Coo-MgO 0 L 8 12 16 niol ? COO FIG. 2.-Plot of e.s.r. signal intensity of isolated CoE&,, ions as a function of Coo concentration: 1, before calcination; 2, after calcination at 1273 K for 7 h. The lack of an e.s.r. signal from tetrahedrally coordinated cobalt may be explained by the assumption of a trigonal distortion of the tetrahedron, as shown by Kazansky et a[." In the case of a shortening of the tetrahedron along the trigonal axis the e.s.r. signal may be described by a spin Hamiltonian of the form H = Bkil Sz f f z + gdSy H y + s, w1+ m% + S(S + 111 (3) with a negative D value and with gll > gl > 2.At liquid-helium temperature (kT = 3 cm-l) for D < 0 and 101 $= kT (strong trigonal distortion) the population of the doublet 1/2 taking part in the resonance is very small and so observation of the e.s.r. signal for polycrystalline samples is difficult. With increasing temperature the total population of the doublet increases but simultaneously the difference in the population of levels of the doublet decreases. Competition between these two effects precludes the observation of the e.s.r. signal of Co2+ ions in tetrahedral coordination having a sufficiently strong trigonal distortion (at both low and high temperatures). The lack of an e.s.r. signal for Co2+ in tetrahedral surrounditlgs has been observed by Graber et for spinel CdIn2S, and by Kazansky et all9 for Co-exchanged zeolites, in spite of the observation of tetrahedral Coz+ in the reflectance spectra.The width of the e.s.r. signal for g = 4.23 attributed to octahedrally coordinated Co2+ ions changes with Co concentration, as shown in fig. 3. We suppose that two main factors control the width of the e.s.r. line: dipole-dipole interactions, which increase with increasing Co concentration, and imperfections of the crystal lattice, which increase as the Co concentration decreases. The important contribution of the lattice imperfections to the total linewidth is indicated by a reduction in width of the e.s.r. signal upon calcination of the sample. On the other hand in the concentration range up to 15.3 mol % COO the exchange interactions do not narrow the signal significant 1 y , The dependence of signal intensity of CO:&~ on the temperature at which measure-K.D Y R E K A N D Z. SOJKA 3181 I 0 4 8 12 16 mol % COO FIG. 3.-Dependence of the width of the Co& e.s.r. line on COO concentration at 77 K. 0 20 4 0 60 80 100 temperature/K FIG. 4.-E.s.r. signal intensity of Co2+ in COO-MgO plotted against temperature: 1, 0.39; 2, 3.00; 3, 15.3 mol % COO. ments are made is shown in fig. 4. The intensity of the signal for a given sample increases on lowering the temperature down to a temperature just above the Neel point. Around the Neel temperature and below it a decrease in the signal intensity owing to increasing antiferromagnetic interactions is observed. The Neel temperature decreases with decreasing cobalt concentration, and so for 15.3 mol % COO a maximum occurs at 20 K, whereas for more dilute solid solutions a continuous increase in the signal intensity is observed, indicating that the Neel temperature is beyond the range in which the measurements were carried out.Solid solutions containing > 15.3 mol % COO do not give an e.s.r. signal at low temperatures because the Neel point of these preparations lies above 77 K.' At temperatures > 77 K there is also no e.s.r. signal owing to the short relaxation time of the Co2+ ions in octahedral surroundings. The dependence of the signal intensity on Co concentration at 77 K is shown in fig. 2. The shape of this dependence suggests that it is determined by two opposing factors.An increase in the Co concentration (i.e. an increase in the total number of paramagnetic centres) causes an increase in the signal intensity. However, a simulta- neous increase in metal-metal exchange interactions leads to a decrease in the number of isolated Co2+ ions and hence a decrease in their e.s.r. signal intensity. These two3182 COORDINATION AND DISPERSION OF C02+ IN COO-MgO opposing effects cause the appearance of a maximum in the plot of I = f ( x ) (where x is the mole fraction of COO) at x = 0.03. As shown by Gesmundo20 the value of x corresponding to the maximum e.s.r. signal intensity can be used to estimate the distance within which the exchange interactions occur. The number of isolated Co2+ ions distributed randomly in a solid solution is proportional to x( 1 - x ) ~ . The value of rn equal to the number of cationic sites around a given ion which should not be occupied by other Co2+ ions in order that the given ion is isolated is related to the mole fraction of COO corresponding to the e.s.r.signal of maximum intensity by the formula m = (1 --xmax)/~,,,. In the MgO crystal lattice (B, type) there are 18 cationic sites around a given cation including the next-nearest neighbours and 42 including the third-nearest neighbours. A value of rn = 32 was found from the experimental dependence of the e.s.r. signal intensity on Co concen- tration. The experimental data shown in fig. 5 fit the curve corresponding to m = 42 better than that corresponding to rn = 18, and so we conclude that the exchange interactions extend to the third-nearest neighbours. 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 x (mole fraction of Coo) FIG. 5.-Comparison of the experimental (solid line) and calculated (dashed line) data for extent of exchange interactions: 1, up to the second coordination sphere (rn = 18); 2, up to the third coordination sphere (rn = 42); 3, experimental data.As has already been shown the number of spins associated with the signal attributed to the octahedrally coordinated isolated Co2+ ions is considerably lower than the total amount of cobalt in the sample (table 1 and fig. 2). We assume that the deficit in the number of Co2+ ions determined by e.s.r. spectroscopy corresponds to the tetrahedrally coordinated cobalt with trigonal distortion not observable in the e.s.r.spectra under the conditions in which the measurements were carried out and/or to the Co2+ in clusters coupled by exchange interactions. The difference between the number of spins corresponding to the signal due to isolated Co2+ ions in octahedral surroundings before and after heating of the sample at 1273 K gives the number of isolated Co2+ ions tetrahedrally coordinated with trigonal distortion. The amount of Co2+ in clusters not detectable by e.s.r. spectroscopy can then be estimated as the difference between the total content of cobalt, calculated from the weight of a given sample and known concentration of COO, and the total number of isolated Co2+ ions, as found in the sample after additional calcination. The results presented in table 1 reveal that in mostK.DYREK AND Z. SOJKA 3183 of the dilute solid solutions isolated Co2+ ions predominate in distorted tetrahedral coordination. Evidently cobalt having this coordination also enters the crystal bulk and is not restricted to the surface, as postulated by other author^.^^^ The number of Co2+ ions in clusters was calculated assuming that they do not contribute to the e.s.r. signal. Indeed, the Co2+-Co2+ pairs exhibit resonance outside the signal due to isolated Co2+ ions and may contribute only to the tails of the line.20 However, the clusters consisting of several Co2+ ions give a signal of Lorenzian shape and with a g factor close to that of the isolated ions.21 Such clusters may contribute to the total intensity of the signal at g = 4.23, which reduces the accuracy of the estimation.This effect may be considerable in the case of the solid solutions containing 10.4 and 15.3 mol % COO, which explains the anomalous intensity of their e.s.r. signals as compared with that calculated on the basis of the isolated-ion approximation. mol % COO FIG. 6.-E.s.r. signal intensity of 0; species plotted against concentration. Adsorption at room temperature. Spectra registered at 77 K. As shown previously1 the mechanism of oxygen adsorption is different for solid solutions of various COO concentrations; paramagnetic 0; species appear in the dilute solid solutions whereas diamagnetic 02- ions are formed in the concentrated solid solutions. In order to check the role of cobalt dispersion in the determination of the adsorptive properties of the solid solutions we determined quantitatively the amount of oxygen adsorbed in a paramagnetic form.The plot of the e.s.r. signal intensity of 0; radical as a function of COO concentration (fig. 6 ) passes through a maximum at 3.00 mol % COO; this is undoubtedly correlated to the maximum concentration of isolated Co2+ ions. Similar behaviour is observed for dilute solid solutions of Mn0-Mg0.22 On all the samples showing hyperfine splitting, i.e. containing isolated Mn2+ ions, oxygen is adsorbed in the form of paramagnetic species giving a signal in the centre of the sextet hyperfine structure. The intensity of this signal increases monotonically with Mn concentration up to ca. 3 mol % MnO. For the more concentrated solid solutions giving only a single e.s.r.line from coupled Mn2+ ions oxygen is adsorbed in form of diamagnetic 02- ions. The formation of paramagnetic species of adsorbed oxygen 0- and 0; requires the3184 COORDINATION AND DISPERSION OF Co2+ I N Coo-MgO transfer of only one electron per atom or molecule, respectively. The conditions for such a transfer are fulfilled only in the case of isolated Co2+ ions. The exchange interactions in COO-MgO are extended up to the third coordination sphere. There are three principal contributions to the exchange interaction : correlation, polarisation and delo~alisation.~~ In the latter effect the electron is assumed to drift from one cation to another and so enables the transfer of more than one electron from other cations to the cation which plays the role of adsorption centre.The distance from which the next electron may be supplied via delocalisation effects is determined by the extent of the exchange interactions. In the Coo-MgO solid solutions the Co2+ adsorption centre may be considered as isolated if the three neighbouring spheres are unoccupied by other Co2+ cations. However, if this condition is not satisfied, i.e. if there are other cobalt cations in the vicinity of the Co2+ ion, transfer of more than one electron to the adsorption centre is possible and oxygen is adsorbed in a diamagnetic form. CONCLUSIONS The mechanism of oxygen adsorption on solid solutions of transition-metal oxides in MgO is determined by two factors: coordination of the transition-metal ion and metal-metal interactions. The change of coordination from octahedral to tetrahedral may influence significantly the adsorption properties of the solid solutions, as the metal ions of reduced coordination number show enhanced ability for adsorption in order to complete their coordination spheres.On the other hand the metal-metal interactions control the form in which oxygen is adsorbed. The existence of two kinds of adsorption centres in COO-MgO solid solutions is postulated, both containing tetrahedrally coordinated cobalt with trigonal distortion. One of them is isolated, the other coupled to its paramagnetic neighbours. The authors are greatly indebted to Prof. A. Bielanski, Jagiellonian University, for helpful discussions. We also thank Prof. P. Hagenmuller and Prof. M. Pouchard for allowing measurements of the e.s.r.spectra at low temperatures in the Institute of Solid State Chemistry of the University Bordeaux I. The assistance of Mr J. M. Dance during e.s.r. experiments is gratefully acknowledged. K. Dyrek, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1973, 21, 675. K. Dyrek and V. A. Shvets, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1974, 22, 315. A. P. Hagan, C. 0. Arean and F. S. Stone, Proc. 8th Int. Symp. Reactivity of Solids, Gothenburg, 1976 (Plenum Press, New York, 1977), p. 69. A. P. Hagan, M. G. Lofthouse, F. S. Stone and M. A. Trevethan, Studies in Surface Science and Catalysis, vol. 3, Preparation of Catalysts I i , ScientlJc Basis for the Preparation of Catalysts. Proceedings of the Second International Synposium, Louvain-la-Neuve, 1978, ed.B. Delmon, P. Grange, P. Jacobs and G. Poucelet (Elsevier, Amsterdam, 1979), p. 417. G. N. Asmelov, V. A. Matyshak, A. A. Kadushin and 0. V. Krylov, Kinet. Katal., 1977, 18, 1506. A. Bielanski and M. Najbar, J. Catal., 1972, 25, 398. ’ W. Low, Phys. Rev., 1957, 109, 256. * A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Metal Ions (Clarendon Press, Oxford, 1970), p. 399. N. A. Mironova and U. A. Ulmanis, Izv. Akad. Nauk Latv. SSR, Ser. Fiz. Tekh. Nauk, 1973,4, 39. lo M. H. L. Pryce, Proc. R. SOC. London, 1965, 283, 433. l 1 D. Cordischi, V. Indovina, M. Occhiuzzi and A. Arieti, J . Chem. Soc., Faraday Trans. 1, 1979, 75, 533. V. G. Anufriev and U. A. Ulmanis, Zzv. Akad. Nauk Latv. SSR, Ser. Fiz. Tekh. Nauk, 1979, 3, 70. l3 J. S. Thorp, M. D. Hossain, L. C. Bluck and T. G. Burhell, J . Mater. Sci., 1980, 15, 903. l4 T. P. P. Hall and W. Hayes, J. Chem. Phys., 1960, 32, 1871.K . D Y R E K AND Z . SOJKA 3185 l 5 J. H. Lunsford and J. P. Jayne, J. Phys. Chem., 1965, 69, 2182. l6 A. Bielanski, Z. Kluz, M. Jagielo and I. Waclawska, Z. Phys. Chem. ( N . F.), 1975, 97, 207. l 7 I. D. Mikhejkin, G. M. Zhidomirov and V. B. Kazansky, Usp. Khim., 1972, 41, 909. l * N. Graber, H. J. Wagner and C. F. Schwerdtfeger, J. Phys. Soc. Jpn, 1979, 46, 1953. l9 1. D. Mikhejkin, 0. I. Brotikovskii, G. M. Zhidomirov and V. B. Kazansky, Kinet. Katal., 1971, 12, *O F. Gesmundo and P. F. Rossi, J . Solid State Chem., 1973, 8, 287. * l A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Metal ions (Clarendon Press, Oxford, 1970), p. 51.7. 2 2 K. Dyrek, unpublished results. 23 J. D. Goodenough, Magnetism and the Chemical Bond (Interscience, New York, 1963), p. 171. 1442. (PAPER 1 / 1830)

ISSN:0300-9599    

DOI:10.1039/F19827803177

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1982, 78, 3187-3202 Pyrolysis of Ethylbenzene BY C. TERENCE BROOKS? AND STANLEY J. PEACOCK British Gas Corporation, London Research Station, Michael Road, London SW6 2AD AND BRYAN G. REUBEN* Department of Chemical Engineering, Polytechnic of the South Bank, Borough Road, London SEI OAA Received 3rd December, 198 1 The pyrolysis of ethylbenzene has been studied using a static reactor. At low conversion hydrogen and styrene are the major products together with methane, toluene, ethylene, ethane and benzene plus traces of higher molecular-weight hydrocarbons. The pyrolysis is a chain reaction with a chain length of the order PhC,H, -+ PhCH, + CH, of 10 initiated by for which k/s-' = 1014.4*1.1 exp (- 293 k 18 kJ mol-'/RT) based on an average of toluene and methane yields.This agrees well with previous work involving toluene and aniline carriers. The results may be explained by a complex mechanism involving free radicals, CH,, PhCH,, PhCHCH,, PhCH,CH,, C,H,, Ph and H. Termination appears to occur mainly by the reaction 2PhCHCH, + PhCH: CH, +PhC,H,. PhCHCH, -+ PhCH : CH, + H For the reaction a rate constant k/s-' = 1015.9 exp (-217 kJ mol-'/RT) was deduced. The hydrogenation of coal and oil is a potentially important route to substitute natural gas. In these processes the cracking and subsequent hydrogenation of aromatic compounds, such as ethylbenzene, play a part. Although there have been many studies of the pyrolyses of aromatic hydrocarbons, their mechanism is still uncertain. Studies of the pyrolysis of ethylbenzene date back to Szwarc's work' in 1949.Szwarc suggested that the reaction be described by a free-radical chain mechanism. Initiation occurs by fission of the aliphatic C-C bond to yield benzyl and methyl radicals. PhCH,CH, --+ PhCH, + CH,. CH, + PhCH,CH, -, CH, + PhCHCH, CH, + PhCH,CH, -+ CH, + PhCH,CH, PhCHCH, -, H + PhCHCH, H + PhCH,CH, + H, + PhCH,CH, H + PhCH,CH, -+ H, + PhCHCH, H + PhCH,CH, -, PhH + C,H, H + PhCH,CH, -+ CH, + PhCH, (1) (2 4 (2) ( 5 ) (6) (6 4 (6 b) (6 c ) This was followed by the propagation reactions Present Address: British Gas Corporation, Westfield Development Centre, Cardenden, Fife. 31873188 PYROLYSIS OF ETHYLBENZENE C2H5 + PhCHZCH, -+ C2H6 + PhCHZCH, C,H, + PhCH,CH, -+ C2H6 + PhCHCH,. (8) (8 a) Possible chain-termination processes were H + PhCH, -+ PhCH, 2C2H5 --* C2H4 -t- C2H6 H+H+M-+H,+M.Szwarc suggested that the reaction was too complicated to be suitable for a conventional study and proposed a technique employing an excess of toluene as a carrier. Thus the reaction was modified to PhCH,CH, + PhCH, + CH, PhCH, + CH, -+ PhCH, + CH, 2 PhCH, -, (PhCH,),. Szwarc obtained a rate constant for reaction (I) k/s-l = lo1, exp (-263.3 kJ mol-l/RT). The carrier method has been widely used in one form or another in subsequent Estban et al., used aniline as a carrier in a plug flow apparatus. They obtained the rate constant for reaction (1) from the methane yield. Crown et aL3 used a stirred flow reactor to study the pyrolysis in the presence of an excess of toluene. Their findings show that the experimental first-order rate constant is dependent on the pressure of toluene and that the rate of methane production falls with a reduction in carrier pressure for a given partial pressure of reactant.k/s-l = 1014a6 exp (- 292.6 kJ mol-l/RT) The rate constant for reaction (1) was Clarke and Price4 have carried out a detailed investigation using a toluene carrier and have suggested that the pyrolysis occurs between 910 and 1089 K by 3 main reactions k/s-l = exp (- 289.2 kJ mol-l/RT). PhCH,CH, -+ PhCH, + CH, PhCHZCH, + PhH + C2H4 PhCH,CH, -+ PhC,H, + H,. (1) (13) (14) For the initiation reaction (1) a rate constant of k/s-l = exp (- 293 kJ mol-l/RT) was obtained. quartz vessel a rate constant of for reaction (1 3) was obtained. Experiments showed ethylene yield to be surface-dependent but in a conditioned k/s-l = exp (-216.1 kJ mol-l/RT)C. T.BROOKS, S. J. PEACOCK A N D B. G. REUBEN 3189 A rate constant for reaction (14) of k/s-l = exp (-267.5 kJ mol-l/RT) was obtained from styrene yield. Experiments without a carrier gas have been performed by Lee and Oliver5 and Hausmann and Kings who found the order of reaction for styrene formation to be one-half. A rate expression k:/mol: dm-g s-l = 1015 exp (-292.6 kJ mol-lRT) where was proposed. Considerable scatter on the Arrhenius plot makes the value unreliable. The following chain mechanism was proposed d[styrene]/dt = k+[PhCH,CH,]g (9 PhCH,CH, -+ PhCH, +CH, (1) CH, + PhCH,CH, -+ PhCHCH, + CH, (2 4 PhCHCH, -+ PhCHCH, + H ( 5 ) H + PhCH,CH, -+ PhCHCH, + H, (6 a) 2 PhCHCH, -+ various products.( W For long chains the expression d[styrene]/dt = (k, k: /2k,,,)~[PhCH,CH,]~ (ii) was found to be consistent with the experimental results. Thus the bulk of the kinetic studies have been carried out with carriers in flow systems. As better experimental techniques became available the limitations of these techniques became apparent. For a more complete investigation from which a reaction mechanism can be proposed it is desirable that the pyrolysis be carried out in the absence of carriers. Modern methods enable accurate results to be obtained for vapours in a static system.' This paper sets out (a) to confirm that the rate data obtained for the initiation reaction by carrier techniques also apply to the system in the absence of a carrier and (b) to obtain a satisfactory reaction mechanism.EXPERIMENTAL A conventional static system described previously was used.' RESULTS PRELIMINARY EXPERIMENTS The major reaction products are hydrogen and styrene. To minimise side reactions and secondary product formation, the reaction was studied at low conversions. A typical experiment at 783 K with 4 min residence time gave ca. 3% conversion. Thus ca. 0.55 Torrt of hydrogen and styrene were produced from an initial concentration of 30 Torr of ethylbenzene. The other products are methane and toluene formed in t 1 Torr = 101 325/760 Pa.3190 PYROLYSIS OF ETHYLBENZENE approximately an order of magnitude lower concentration together with ethylene, ethane and benzene. At these low conversions no other gaseous or liquid products were seen. After evaporating a sample of the condensate only trace quantities of solid products were identified by mass spectrometry.Hence we assume that only the primary pyrolysis reactions are taking place and accordingly for most of the kinetic experiments conversions were kept at these low levels. Reaction (12) giving bibenzyl is an attractive chain termination step but repeated attempts to detect it in the reaction products were unsuccessful. This was surprising but it is significant that the most recent study of the pyrolysis in the absence of carrier gas also failed to detect bibenzyl.* It is possible that it is unstable at the temperatures of these experiments. Oxygen impurity has a profound effect on hydrocarbon pyrolysis.Rates of pyrolysis were measured from samples of ethylbenzene which had been degassed a different number of times. The rate was independent of the number of degassing cycles which suggests that the ethylbenzene is oxygen-free. The variation of gaseous product yield with time is shown in fig. 1. For all products a linear relationship is obtained and hence initial rate was taken as the yield after 4 min divided by time. 0 2 4 6 8 10 residence time/min FIG. 1.-Progress of reaction with time. Hydrogen 0, methane 0, ethane and ethylene (> yield at 788 K for a mixture of 30 Torr of ethylbenzene and 300 Torr of nitrogen. Most experiments were carried out with a 6 cm diameter, 20 cm long silica reaction vessel with a surface-to-volume ratio of 0.077 mm-l. Initial rates were methane 2.5 x Torr s-l, hydrogen 2.29 x Torr s-l, ethylene 3.54 x Torr s-l and ethane 3.75 x lou4 Torr s-l for a reaction temperature of 792 K and ethylbenzene concentration of 34 Torr.Under similar conditions in a reaction vessel packed with silica tubes (1.105 mm-l surface-to-volume ratio) initial rates were methane 3.12 x Torr s-l, hydrogen 2.17 x Torr s-l, ethylene 3.96 x Torr s-l and ethane 3.12 x Torr s-l. Therefore under the conditions of our experiments the effects of surface were assumed to be insignificant.C. T. BROOKS, S. J. PEACOCK AND B. G. REUBEN 3191 Fig. 2 shows the effect of added nitrogen on reaction rate. Varying amounts of nitrogen were added to 30 Torr of ethylbenzene with a reaction temperature of 788 K and residence time of 4 min.Nitrogen increases ethylene yield and may decrease the benzene yield slightly but the other products are apparently unaffected. 0.7 0-5 k - O Q A 8 Q - 00 0'0 0" U U 0 0.31 I I I I , , 2- t- ; 2 0-08 0.06 2 5 0.07 2 a a 0.05 0.03 I : 0 I I I 1 I I B O m m o # U Y 0 0 0 0 n o - 8 B -. I I I I 1 1 100 200 3 00 4 00 500 600 nitrogen pressure/Torr FIG. 2.-Effect of added nitrogen on rate. Hydrogen 0, methane 0, ethane a, styrene 0, toluene D, ethylene c) and benzene A yields after 4 min. FORMATION OF METHANE AND TOLUENE If methane and toluene are formed only in an initiation reaction then their yields should be equal and the rate constant derived from both products should be the same. The rates of formation of these products showed a first-order dependence on ethylbenzene concentration (fig.3). The temperature dependences for methane and toluene formation are shown in fig. 4. The slopes of the lines yield values of k / s s 1 = 1014.48+1.1 exp (-293.8+ 18 kJ mol-l/RT) k/s-l = 1014.35*1.2 exp (-291.8+ 19 kJ mol-l/RT) and3192 PYROLYSIS OF ETHYLBENZENE 0 10 20 30 40 50 ethylbenzene pressure/Torr FIG. 3.-Dependence of methane 0 and toluene yield on ethylbenzene pressure. Reaction - 4.2 - 4 . 4 -4.6 h -1.8 . - 5 0 b~ -5.0 - - 5 . 2 - 5 . 4 -5.6 temperature = 719 K, residence time = 4 min. a 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 1.30 103 KIT FIG. 4.-Temperature dependence of methane 0 and toluene m yield. respectively, which are identical within experimental error. These rate data agree well with those obtained by carrier techniques and we conclude that methane and toluene are indeed produced only in an initiation stage.FORMATION OF HYDROGEN A N D STYRENE Hydrogen and styrene account for 90 % of the total reaction products. Fig. 5 shows the dependence of these products on ethylbenzene pressure and a least-squares analysis of a log-log line yields 0.6 reaction order for both products. The temperature dependence plot (fig. 6) yields a rate expression (in mol dme3 s-l) of d[H,]/dt = 1019.8*0.5 exp (- 377.4_+ 16.8 kJ rnol-'/RT) [PhC,H5]o-6 d[styrene]/dt = 1018.2+0.3 exp (-359.5 16.0 kJ mol-l/RT) [PhC,H5]o.6.0.6 t 0 . 5 0 + ---. 22 0 . 4 2 0.3 2 a 0.2 0 . 1 .- x I -0 0 3. T. BROOKS, S. J . PEACOCK A N D B. G. REUBEN 3 193 I 1 I I e t hy 1 benzene pressu re/Torr 10 20 30 LO FIG. 5.-Dependence of hydrogen 0 and styrene @ yield on ethylbenzene pressure.Reaction temperature = 719 K, residence time = 4 min. - 7.0 h ? -7.2 E a 0 - .g -7.L 1” E: -7.6 0 - I a a 3 v w 2 - 7 . 8 -7.0 I I 1 I I 1.24 1.25 1.26 1.27 1.28 1.29 103 K I T FIG. 6.-Temperature dependence of hydrogen 0 and styrene 0 yield. Ethylbenzene concentration = 6.05 x mol drnw3, residence time = 4 min. FORMATION OF ETHANE, ETHYLENE A N D BENZENE Fig. 7 shows the effect of ethylbenzene pressure on the yields of ethane, ethylene and benzene and reaction orders of 1 . 1 , 0.9 and 1 .O, respectively, are obtained. The temperature dependences (fig. 8) give rise to the rate expressions (all in mol dm-3 s-l) d[C,H,]/dt = l Ozl.osf 0-4 exp ( - 389.9 f 16.4 kJ ~ o ~ - ~ ) / R T [ P ~ C , H , ] ~ .~ d[C,H,]/dt = 1021-48*0.2 exp (-405.5 f 6.0 kJ ~ O ~ - ~ ) / R T [ P ~ C , H , ] ~ . ~ d[PhH]/dt = 1016.59*0-2 exp (- 321.4+ 18.0 kJ ~ o ~ - ~ ) / R T [ P ~ C , H , ] .3194 PYROLYSIS OF ETHYLBENZENE I 0.30 0.20 0.10 0 10 20 30 4 0 ethyl benzene pressure/Torr FIG. 7.-Dependence of ethane 0 , ethylene 0 and benzene A yield on ethylbenzene pressure. Reaction temperature = 719 K, residence time = 4 min. - 4 . 4 - 4 . 6 h T - 4 , 8 c, c - 5 . c m -0 Q) 2) ;h W - 5 -5.2 s. Y w " 5 . 4 - - 5 . 6 - 5.E 0 1.24 1.25 1.26 1.27 1.28 1.29 1.30 1 0 3 K I T - 4 . 4 - 4 . 5 -4.6 n -4.7 v al N P -4.8 5 s. u 0 -4.9 2 -5.0 -5.1 FIG. 8.-Temperature dependence of ethane ethylene 0 and benzene yield. Ethylbenzene concentration = 6.05 x lo-* mol dm-3, residence time = 4 min.C. T.BROOKS, S. J. PEACOCK A N D B. G. REUBEN 3195 TABLE 1 .-SOLID COMPOUNDS FOUND BY MASS SPECTROMETRIC ANALYSIS OF THE RESIDUE FROM THE PYROLYSIS OF ETHYLBENZENE AT HIGH CONVERSIONS accurate mass relative molecular (measured) molecular intensity weight formula (%I compounda 128 142 152 154 166 168 178 180 192 194 202 204 206 216 228 230 252 254 278 280 300 302 304 330 342 3 50 378 380 402 502 - 142.0770 (CllHlo) 154.0793 (CI2Hl0) 166.0782 (C13Hlo) 168.0939 (C13H12) 178.0782 (C14Hlo) 180.0939 (CI4Hl2) 192.0939 (C15H12) 194.1095 (C15H14) 202.078 3 ( C16Hlo) 204.0939 (C16H12) 206.1096 (C16H14) 216.0938 (Cl,Hl2) - - 5 trace 1 1 4 5 2 100 20 14 2 26 29 5 4 4 5 10 17 9 6 trace 2 2 3 trace trace trace trace trace trace naphthalene methylnaphthalene biphenylene biphenyl fluorene biphenylmethane anthracene stilbene met hylan t hracene ethylfluorene pyrene dihydropyrene ethylanthracene methylpyrene chr y sene p-terphenyl benzop yrene dihydrobenzopyrene benzochrysenes dihydrobenzochrysenes coronene benzopery lenes di hydrobenzopery lenes methylbenzonaphthoperinaphthenes methyldibenzochrysene pyrenopyrene dibenzopicenes dihydro benzopicenes tetrabenzopyrene benzanthracenopyrene a Other isomeric forms are possible.FORMATION OF HIGHER MOLECULAR WEIGHT PRODUCTS Samples of the solid products of reaction in the condensate were prepared in a simple flow apparatus operating at 778 K and 833 K with 2 min residence time. The electron impact mass spectrum of the volatile matter in the sample was obtained using a Varian MAT 31 1 A mass spectrometer linked to an on-line data system.At low conversions obtained from the experiment carried out at 778 K, very few products other than styrene, toluene and benzene are observed. However, at higher conversions a range of polynuclear aromatic hydrocarbons was found. The m/z values for the parent peaks together with relative abundance and possible structures are shown in table 1 . It also shows examples of some of the components of the solid products identified by accurate mass measurements.3196 PYROLYSIS OF ETHYLBENZENE DISCUSSION STOICHIOMETRY A N D ANALYTICAL ERROR The reaction products obtained by all workers suggest that the integrity of the benzene ring is retained in all reactions in this system at temperatures up to ca. 800 K.If multi-ring products are discounted, then stoichiometry demands that styrene yield must equal hydrogen + methane + ethane yields, methane yield must equal toluene yield and benzene yield must equal ethane +ethylene yields. Previous workers, for example Shirazi,* have not concentrated on this point, presumably because of analytical difficulties that were also observed in this work. Toluene and methane yields were found to be identical within experimental error, as mentioned earlier and shown in fig. 4. Hydrogen yields typically appeared 1 5 % higher than styrene yields as illustrated in fig. 6, though at the low temperature used for the run shown in fig. 5 they appear equal as would be expected in view of the methane and ethane yields being small compared with the styrene yield.The styrene peak on the gas chromatograph trace occurred on the side of a very large ethylbenzene peak and though its size was estimated by a consistent technique, we are inclined to attribute the discrepancy largely to a systematic error in this measurement. Nonetheless, note that formation of coking precursors such as polynuclear aromatic hydrocarbons would lead to an excess of hydrogen over styrene and that this would increase with temperature. Yields of benzene were approximately in balance with ethane + ethylene (fig. 7) considering the errors involved in measurement of such small concentrations. The activation energy for benzene production (fig. 8) is slightly less than for ethane and ethylene which would be consistent with a loss of benzene at higher temperatures to give polynuclear aromatic hydrocarbons.The effect of added nitrogen is most difficult to explain (fig. 2) in that benzene yields drop while ethane remains constant and ethylene increases. This would also be consistent with the formation of polynuclear aromatic hydrocarbons and the amounts that would need to be formed are very small. There is no obvious reason, however, why added nitrogen should have this effect. POSSIBLE REACTION MECHANISMS To account for all the products, any reaction mechanism is necessarily complicated. Initiation is via reaction (1) followed in Szwarc’s scheme by reactions (2) and (20) which give rise to PhCH,CH, and PhCHCH,. Whether both these radicals are involved in the mechanism is open to discussion. On the one hand, abstraction reactions between free radicals and olefins containing allylic hydrogen atoms are generally accepted to involve only the latter.Reaction (2a) might thus be expected to be at least an order of magnitude faster than reaction (2) and subsequent reactions of PhCH,CH, might reasonably be ignored. On the other hand, the added stability of the PhCHCH, radical which makes it the preferred product also leads to its being less reactive. ShirazP has calculated that in certain cases PhCH,CH, reacts lo7 times as fast as PhCHCH, and we have consequently felt it justifiable to retain both species in our mechanism. Formation of toluene and methane in equal amounts suggests that the benzyl radical is more reactive than originally proposed by Szwarc and suggests the need for reactions (3) and ( 3 a ) : PhCH, + PhCH,CH, + PhCH,CH, + PhCH, PhCH, + PhCH,CH, --+ PhCHCH, + PhCH,.(3) (3 4C. T. BROOKS, S. J. PEACOCK A N D B . G. REUBEN 3197 Szwarc suggested that toluene might also be formed by the reaction We have not felt it necessary to include it because our rate constants for toluene formation agree with those involving carriers where reaction (1 5 ) is not thought to take place. Decomposition of PhCHCH, was proposed to account for styrene formation in reaction ( 5 ) . The radical PhCH,CH, might similarly react as in reaction (4) to give ethylene CH,+PhC,H, -+ C,H5+PhCH,. (15) PhCH,CH, -+ Ph + C2H4. (4) The phenyl radical can then react as in reactions (7) and (7a) to yield benzene Ph + PhC,H, ---* PhH + PhCH,CH, Ph + PhC,H, + PhH + PhCHCH,.C2H5 + PhCZH, -+ PhCHZCH, + C2H6 C,H, + PhC,H, -+ PhCHCH, + C2H6. (7) (7 4 (8) (8 4 Ethylene formation is dependent on nitrogen concentration and under the conditions (9) Ethane, the remaining product, can be formed by attack of an ethyl radical on e thy1 benzene of this work the reaction C,H,+M -+C,H,+H+M is in its pressure-dependent region and hence it probably also occurs. The mechanism is therefore made up of a number of cycles. The major cycle yields hydrogen and styrene and this accounts for 90% of the reaction product. Benzene and ethylene are formed in another cycle whilst ethane is formed in the third cycle. In addition some ethylene is formed separately by the decomposition of ethyl radicals. Fig. 9 represents the scheme diagrammatically. It illustrates the interlocking cyclic nature of the reactions.The solid circles represent reactions and reactants are shown as entering and products as emerging from them. Free radicals are shown as molecular formulae and neutral molecules as names. The ‘crossroads’ at the open circle reflect the fact that reactions generating PhCH,CH, might also generate PhCHCH,. To summarise, the propagation reactions considered are reactions (2)-(9) below : PhCH,CH, -+ PhCH, + CH, (11 (2) (2 4 (3) (3 4 (4) ( 5 ) (6) (6 4 (6 4 CH, + PhCH,CH, -+ PhCH,CH, + CH, CH, + PhCH,CH, -+ PhCHCH, + CH, PhCH, + PhCH,CH, -+ PhCH,CH, + PhCH, PhCH, + PhCH,CH, -+ PhCHCH, + PhCH, PhCH,CH, -+ Ph + C,H, PhCHCH, -+ H + PhCH :CH, H + PhCH,CH, -+ PhCH,CH, + H, H + PhCH,CH, -+ PhCHCH, + H, H + PhCH,CH, -+ C,H, + PhH3198 PYROLYSIS OF ETHYLBENZENE ethylbenzene I I G PhCHCH, I\ 5 styrene I 1 11 ethylbenzene 1 I I H Ph \ ethylbenzene ethylene + M b 6b benzene M FIG.9.-Diagrammatic representation of the proposed ethylbenzene pyrolysis mechanism. Ph + PhCH,CH, + PhCH,CH, + PhH Ph + PhCH,CH, -, PhCHCH, + PhH C,H, + PhCH,CH, -+ PhCH,CH, + C,H, C,H, + PhC,H, + PhCHCH, + C,H, C2H5 + (M) -+ H + C2H.4 + (M) (7) ( 7 4 ( 8 ) ( 8 4 (9) (10) 2 PhC,H, -+ PhCH,CH, + PhCH : CH,. Szwarc reported a chain length of ca. 15. The ratio initiation/overall rate in this work suggests a chain length of just over 10. No single high-molecular-weight product predominates in the mass spectrometric analysis of the solid products and hence we assume that the major termination reaction is via disproportionation 2 PhCHCH, -+ PhCH,CH, + PhCHCH,.(10) The rate expression for styrene is d[PhCH : CH,]/dt = 2k,[PhC2H5] + k5(k1/k10)' [PhC2H,]g. (iii) Hence a plot of rate of styrene formation/[ethylbenzene] against [ethylbenzene$ should be a straight line with the intercept equal to k,. Fig. 10 shows that this is approximately true. From the methane and toluene yield 2k, = 19.42 x lop8 s-l. The least-squares intercept is 2.5 x s-' but there is considerable scatter and the line inC. T. BROOKS, S. J. PEACOCK A N D B. G. REUBEN 3199 effect goes through the origin. If 2k1 [PhC,H,] is assumed to be small compared with k5( k 1 / k 10); [ PhC , H ,I+ then d[PhCH : CH,]/dt = k,(kl/kl,)~[PhC,H5]~ (iv) and half-order kinetics are predicted.In fact the chain length is ca. 10; so the first term cannot be neglected and hence an order slightly greater than 0.5 is predicted by the mechanism. Experiment gives a value of ca. 0.6. 3 1 . 4 v1 . h c ; 1.2 $ 1.0 N OJ 2 c) Y . h c .s 0.8 2 2 J 5 a 0 - € c x c) 0.4 - E v X 5 0.2 v I I I I 0 0.1 0.2 0.3 0.4 0.5 [ ethylbenzene 1 -4 FIG. 10.-Function plot to test the proposed mechanism. If a value for the rate constant of the termination reaction (10) is assumed, then it is possible to estimate a value for the rate constant for reaction ( 5 ) PhCHCH, + H + PhCH : CH,. Now kstyrene = k5(k1/k10)t (v) k5ls-l = (Astyrene) (~lo/AA'ex~ [( -Estyrene +iQ/RTI ( 5 ) (vi) where kstyrene and Astyrene are the experimentally measured rate constants and Arrhenius parameters.Substituting for k, and kstyrene and assuming a value for k,, allows k, to be estimated. Benson9 suggests that termination reactions involving large radicals have a rate constant of ca. k5/s-l N 1015.9 exp (-217 kJ mol-l/RT). The high value for the activation energy is a result of the resonance stabilisation of the radical PhCHCH,. Thermochemical calculation suggests a value of ca. 205 kJ mol-1 for the activation energy. The rate expression for hydrogen yield is more complicated as hydrogen is produced in reactions ( 6 ) and (6a) and hydrogen atoms are consumed in reactions (6), (6a) and dm3 mol-1 s-l. Hence using this figure (6 b).3200 PYROLYSIS OF ETHYLBENZENE Eqn (vii) describes hydrogen yield (vii) where 8 = (k, + kau) [PhC2H5] + k,. If ksb 6 (k, + k6u) then an expression identical to that for styrene yield is obtained.d[H,]/dt = k,(kl/klo)g [PhC,H,]g. (viii) By carrying out an analysis as described earlier a further check of the rate constant for reaction ( 5 ) can be obtained. This yields k5/s-l = 1016.5 exp (-229.9 kJ mol-l/RT). This value is less accurate than that obtained from styrene yield because in addition to the assumptions made there it also assumes that k6b is very small compared with (k6 + k6~). A comparison of hydrogen yield with ethane yield gives the rate expression (ix) d[H21/dt - - (k6 + k6U) + (k6 + '6U) kg [phC,H5]-1. d[C2H61/dt k6b (kt3 + '8U) k6b A plot of (d[H,]/dt)/(d[C,H,]/dt) against [PhC,HJ1 should yield a straight line with an intercept of (k6 + k,,)/k,b and a gradient of (k6 + k6J k,/(k, + kau) kSb.Fig. 1 1 shows that this holds true for a number of temperatures between 758 and 945 K. The ratio of gradient to intercept gives a value of k,/(k, + k8u). Thus an Arrhenius plot of log(gradient/intercept) against inverse temperature will yield an activation energy and pre-exponential factor for this ratio of rate constants. However a small .: 2 -: M E O 1 I 1 I I 1 I 0 0.02 0.04 0.06 0.08 0.10 0.12 [ethylbenzene] -1 /Tom-' FIG. 11.-Comparison of hydrogen and ethane yields with [ethylbenzene]-' at 845 K 0 , 8 1 5 K 0, 791 K 0, 764 K 0, 825 K 0, 798.5 K and 758 K A.C. T. BROOKS, S. J. PEACOCK AND B. G. REUBEN 320 1 error in the value of the intercept has a large effect on the function (gradient/intercept) and leads to a meaningless plot.A more productive approach is to plot log(gradient) against inverse temperature to yield Arrhenius parameters for the function (k,+k6,)kg/(k,+k,,)k6b. The plot is shown in fig. 12. It yields a value of 47.2 1 1.7 kJ mol-l for the activation energy. This is equal to &, 6a + E, - E,, - E6b where &, 6, and E,, 8a are approximate activation energies applicable to reactions (6) and (6a) and (8) and @a), respectively. The scattered intercepts in fig. 11 suggest that &, 6a - &b is Ca. 0. If so Eg - E,, = 47.2 kJ m01-l. c 2 -3.2 % 2 +, -3.0 2, % Y . m 3 - 2 . 8 + - 2 . 4 1.18 1.20 1.22 1.24 1.26 1.28 1.30 1.32 1 O3 KIT FIG. 12.-Arrhenius plot for the function (k, + k6u) k,/(k, + kEU) kBb derived from the pyrolysis reaction scheme. Arrhenius parameters for reaction (9) are well documented.Lin and Backlo suggest kco/s-l = 1013-6 exp (- 158.8 kJ mol-l/RT). Arrhenius parameters for the ethane-forming reactions (8) and @a) are more difficult to obtain. However, by analogy with ethyl radical reactions an activation energy of 58.5 kJ mol-l seems reasonable. This means that Eg-E,,8u is ca. 100 kJ mol-l compared with the experimental value of 47+ 11 kJ mol-l. Reaction (9) is, however, in its pressure-dependent region and Lin and Back report the activation energy for the bimolecular limit to be 135.4 kJ mol-l. Using this value E, - Fa, ,, becomes 76.1 kJ mol-l giving better but still only moderate agreement with experiment where 0 = (k, + kaa) [PhC,H5] + k,. If k, b (k,+k,,) [PhC,H,] then an order of 2 is predicted if reaction (9) is taken as second order. In this pressure region its order will be 2 so an order of $ is predicted. If the reverse is true and k , is small then3202 PYROLYSIS OF ETHYLBENZENE and an order of $ is predicted. This does not agree with the experimental value of 1.1 and suggests that the two terms are of similar value, and if anything reaction (9) is the more important. If so, a considerable quantity of ethylene is produced via reaction (9) and its inclusion in the mechanism is justified. We thank the referees for helpful and constructive comments. M. Szwarc, J. Chem. Phys., 1949, 17, 431. G. L. E. Estban, J. A. Kerr and A. F. Trotmann-Dickenson, J . Chem. SOC., 1963, 3873. C. W. P. Crowne, V. J. Grigulis and J. J. Throssell, Trans. Faraday Soc., 1969, 65, 1051. W. D. Clark and S. J. Price, Can. J. Chem., 1970, 48, 1059. E. H. Lee and G. D. Oliver, Ind. Eng. Chem., 1959, 51, 1351. 13 E. D. Hausmann and C. J. King, Ind. Eng. Chem., Fundam., 1966, 5, 295. ’ C. T. Brooks, S. J. Peacock and B. G. Reuben, J . Chem. SOC., Faraday Trans. I , 1979, 75, 652. Z. H. Shirazi, Ph.D. Thesis (University College of Wales, Swansea, 1973). S. W. Benson, Thermochemical Kinetics (Wiley, New York, 1967). lo M. C. Lin and M. H. Back, Can. J. Chem., 1966, 44, 2357. (PAPER 1 / 1879)

ISSN:0300-9599    

DOI:10.1039/F19827803187

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 3203-3212 Ultrasonic Investigations of Mixtures of n-Octane with Isomeric Octanols Isoentropic Compressibility and Excess Volumes of Mixing BY AKL M. AWWAD AND RICHARD A. PETHRICK* Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow G1 1XL Received 2 1 st December, 198 1 Volumes of mixing, isoentropic compressibilities, acoustic attenuations and viscosities are reported on mixtures of various isomeric octanols and octane at 298.15 K. The data are interpreted in terms of competing effects of the alkane chain taking up the ‘free’ volume in the system and the disturbance of the distribution between cyclic and linear structures and monomer forms in the mixtures. Recent studies of the behaviour of n-alkanol + n-alkane mixtures have been undertaken in an attempt to interpret the way in which structure in the associated alcohols is modified by the addition of alkanes.’.’ Such investigations are crucial if we are ever to understand the processes which occur when water is added to alcohol.s-lo In this latter situation, large excess acoustic attenuations are often reported and attributed to the effect of the sound wave on the distribution of structural forms present at different concentrations.Returning to the ‘ simpler’ system of an alcohol in its homologous hydrocarbon, the stability of the hydrogen-bonded structures will depend on the accessibility of the hydroxy group. It has been suggested by Staveley2 that the behaviour observed in dilute solutions in benzene is associated with the ‘ interaction between the hydroxy group and the surrounding benzene molecules which underwent a change when the hydrocarbon group of the alcohol became sufficiently long to be capable of screening the hydroxy group’.In an earlier study of isomeric octanols with methanol it was observed that excesses in the attenuation similar to those reported for aqueous solutions were observed.ll This present study is an extension of these investigations. EXPERIMENTAL MATERIALS The isomeric octanols : octan- 1-01, octan-2-01, 2-ethylhexan- 1-01, 2,4-dimethylhexan-3-01, 2,3-dimethylhexan-3-01 and the n-octane were all obtained from Aldrich Chemical Co Ltd as better than 99% pure. The purity was confirmed using gas-liquid chromatography and the structuraI integrity using I3C n.m.r.spectroscopy. The liquids were all stored over type 4A molecular sieves (B.D.H.) and filtered before use. Densities and refractive indices of the pure components were measured (see table 1) and found to be in good agreement with values published in the DENSITY MEASUREMENTS The densities were determined using an Anton Paar densimeter (DMA 60) thermostatted with a precision of & 0.01 K. The overall precision of the densities measured was estimated to 32033204 ULTRASONIC STUDIES OF OCTANOL+OCTANE MIXTURES TABLE ~.-DENSITIES, p, REFRACTIVE INDICES, n,, VELOCITY OF SOUND, c, OF THE PURE COMPONENTS AT 298.15 K molecule n-octane octan-1-01 octan-2;ol 2-ethylhexan- 1-01 2,4-dimethylhexan-3-01 2,3-dimethylhexan-3-01 Plg cm-3 obs.lit. 0.698 76 0.698 67 - 0.698 61 0.821 13 0.821 39 - 0.82 157 0.81627 0.8162 0.83205 0.83272 0.834 18 0.834 8 I 0.82906 - c/ms-l ~~~ obs. lit. obs. lit. 1.395 22 1.395 18 1.395 05 I .427 24 1.427 57 - 1.427 5 1.424 34 1.424 1.42921 - 1.43022 -- - 1.42920 -- I 177.85 1177.25 1349.86 1348.83 1323.45 1321.80 1309.75 1307.75 - - 1288.50 - 1286.52 - be better than +_ 3 x of the components. kg-3. The mixtures were prepared by the addition of weighted amounts ULTRASONIC MEASUREMENTS The attenuation and velocity data reported in this paper were obtained using a swept-frequency acoustic resonator.1s- l9 The smnd velocity was determined from the separation of the resonance peaks in the frequency range 1.5-3 MHz. The attenuation was determined from the variation of the widths of the resonances in this region.A detailed study of the frequency dependence on the pure components has been reported e1se~here.I~ VISCOSITY MEASUREMENTS The viscosities of the pure components and the mixtures were determined using a suspended level viscometer. The flow times were determined electronically using an electronic stop watch with a precision of +_ 0.5 s and the temperature of the water bath was controlled to better than kO.01 K. RESULTS VOLUMES OF MIXING Values of the excess volumes of mixing VE of the binary mixtures of the isomeric octanols and octane were calculated using the following equation where X , , X2, M , , M,, V;T and V,* are, respectively, the mole fraction, molecular weight and molar volumes of the pure components designated 1 and 2.The density, p, is the measured value for the binary mixture. Values of VE for the isomeric octanol + octane mixtures were calculated using the data presented in fig. I t and fitted to an empirical equation of the form m VE =X(1-X) a, Xn-l (2) n-1 where a, is the fitting coefficient of order n obtained from a least-squares fit of the data presented in table 2. The standard deviations associated with this analysis are also presented in table 2, along with the coefficients for a,. Comparison of the plots t The data presented in the figures are available on request from the authors.TABLE 2.-cOEFFICIENTS a, AND STANDARD DEVIATIONS d VE/CM3 m0l-l FOR LEAST SQUARES REPRESENTATIONS OF EXCESS VOLUMES AT 298.15 K BY EQN (2) Qrl octan- l-ol 2-ethylhexan- l-ol octan-2-01 2,4-dirnethylhexan-3-01 2,3-dimethylhexan-3-01 0.02441 - 0.93847 - 5.94620 24.282 60 21.17675 - 32.909 25 - 5.69659 0.0004 0.00404 1.241 37 1 8.700 92 18.39394 0.0005 - 7.099 36 - 26.08026 - 5.16065 0.004 76 2.5 1494 3 1 .1 44 67 28.491 33 0.0009 - 12.91642 - 41.284 85 - 7.05422 0.095 82 3.39620 33.694 05 42.73724 0.0002 - 13.787 89 - 52.33445 - 13.808 27 0.062 16 3.997 39 - 23.419 8 1 65.892 23 76.38988 - 99.855 47 - 23.077 38 0.0007 > ?3206 ULTRASONIC STUDIES OF OCTANOL+OCTANE MIXTURES 1 I 0 0.5 X FIG. 1.-Excess volumes for [xC,H,,OH+(l -x)n-octane] at 298.15 K. 0, octan-1-01; 0, octan-2-01; A, 2-ethylhexan-1-01; A, 2,3-dirnethylhexan-3-01; a, 2,4-dimethylhexan-3-01. of the actual data and the predictions of eqn (2) indicates that the deviation is in all cases < 0.1 %.The excess volumes for the binary mixtures of branched-chain octanols with octane are all positive over the whole mole-fraction range (fig. 1). In the case of the straight-chain octan-1-01, VE values are negative except at low mole fractions, where they are positive. ACOUSTIC VELOCITY A N D ISOENTROPIC COMPRESSIBILITY The velocities of sound measured at ca. 2 MHz for the pure components are listed in table 1. The excess isoentropic compressibility was calculated using an empirical equation of the form KF = Ks(obs)-lX1 V;" KS,+X2 c Ks21 1x1 v+xz V 3 - l (3) where Xl X , c Ks, and Ks2 are, respectively, the mole fractions, molar volumes and isoentropic compressibilities of the pure components and K, (obs) is the isoentropic compressibility of the binary mixture calculated using the Laplace relation K, = be2)-' where cis the measured velocity of sound in the mixture.The isoentropic compressibility excesses so obtained are plotted in fig. 2. ULTRASONIC ATTENUATION A N D VISCOSITY MEASUREMENTS Attenuation data ( a / f 2 ) obtained over the frequency range 1.5-3 MHz are presented in fig. 3 as a function of mole fraction of the mixture. The viscosities of the mixtures are presented in fig. 4. The attenuation of an organic liquid can be separated into two parts : a relaxational element associated with the disturbance of rotational isomeric equilibria or hydrogen-bonded associated forms of the alcohol,A. M. AWWAD A N D R. A. PETHRICK 3207 -'t 0 0-5 X 1 FIG.2.-Excess isentropic compressibility K," for [xC,H,,OH +( 1 - x)n-octane] at 298.15 K. 0, octan-1-01; A, octan-2-01; 0, 2-ethylhexan-1-01; ., 2,3-dimethylhexan-3-01; 0, 2,4-dimethylhexan-3-01. 0 0 - 5 1 X FIG. 3.-Ultrasonic absorption coefficient for [xC,H,,OH +( 1 - x)n-octane] at 298.15 K and 2MHz. 0, octan-1-01; 0 , octan-2-01; A, 2-ethylhexan-1-01; 0, 2,3-dimethylhexan-3-01; ., 2,4-dimethylhexan-3-01.3208 ULTRASONIC STUDIES OF OCTANOL-kOCTANE MIXTURES 0 0.5 X 1 FIG. 4.-Variation of viscosity (poise) as a function of mole fraction, x, of octanols for [xC,H,,OH + (1 -x)n- octane] at 298.15 K. A, octan-1-01; 0, octan-2-01; A, 2-ethylhexan-1-01; ., 2,3-dimethyl hexan-3-01; 0, 2,4-dimethylhexan-3-01. I A 0 0-5 X 1 FIG. 5.-Ultrasonic absorption contribution from shear viscosity (a/p)s for [xC,H,,OH + ( 1 - x) n-octane] at 2 MHz and 298.15 K.A, octan-1-01; A, octan-2-01; *, 2ethylhexan-1-01; 0, 2,3-dimethylhexan-2-01; ., 2,4-dimethylhexan-3-01.A. M. A W W A D A N D R. A. PETHRICK 3209 and a classical contribution associated with viscous and thermal contributions. The contribution from shear relaxation can be calculated from the viscosity using the relation where vs is the shear viscosity. The calculated shear contribution is presented in fig. 5. The viscous contribution contains a volume-viscosity element, and the total attenuation contains also a thermal-conductivity loss. The latter for most organic liquids makes a contribution of 4 1 % to the observed attenuation. Estimates based (a/P)s = g7%/3pc3 (4) on very imprecise literature data indicate that this is also true in the which case the volume viscosity can be calculated from (a/P)v = (27%/PC3)V = (a/P)o,s - (a/P)s where vv is the volume viscosity of the mixture.Calculated values presented in fig. 6. present case, in ( 5 ) for (alp)" are 1 I I 0 0 . 5 1 X FIG. 6.-Ultrasonic absorption contribution from volume viscosity (alp)" for [xC,H,,OH +( 1 -x) octane] at 2 MHz and 298.15 K. e, octan-1-01; 0, octan-2-01; A, 2-ethylhexan-1-01; 0, 2,3-dimethylhexan-3-01; m, 2,4-dirnethylhexan-3-01. DISCUSSION A survey of the literature indicates that few measurements have been performed in a systematic manner of the changes in the structure of alcohols with the addition of non-polar molecules. This is in contrast to the considerable attention which has been given to alcohols with other hydrogen-bonding molecules. In this present study the isomeric octanols allow the effects of chain isomerism, hydrogen-bond accessibility and configurational mobility of the second molecule to be studied.VOLUME OF MIXING The V E values in the case of the straight-chain octan-1-01 are negative except for mole fractions < 0.02, where they become positive (fig. 1). The maximum values of the excess volumes for branched octanols decreased in the order 2,3- dimethylhexan-3-01, 2,4-dimethylhexan-3-01, octan-2-01, 2-ethylhexan- 1 -01. Accord- ing to Tresczanowicz and Benson13 the VE data for similar binary mixtures can be I04 F A R 783210 ULTRASONIC STUDIES OF OCTANOL+OCTANE MIXTURES explained qualitatively by postulating that the excess is the result of two opposing effects : self-association of the octanol and physical dipole-dipole interactions between octanol monomers and multimers, leading to increases in volume.Negative contri- butions arise from changes of ‘free volume’ in the real binary mixture. Additional contributions can be envisaged as arising from the restrictions in rotational motion15 which arise when the octane molecule is accommodated interstitially within the branched-oc tanol structure. The data presented in fig. 1 indicated that the negative contribution reaches a maximum at ca. 0.4 mole fraction of octan-1-01 and at very low dilution octan-1-01 it becomes positive owing to hydrogen-bond breaking. The positive values of VE for the branched-chain octanols are a consequence of steric interactions associated with the branched alkyl chains reducing the extent of hydrogen bonding in the system.For instance, in the case of 2,3-dimethylhexan-3-01 the hydroxy group is highly hindered by the alkyl group and gives the highest maximum excess volume values. As the steric effect of the alkyl group is increased so the positive contribution becomes more important than the negative. In octan-1-01 the octane molecules are accommodated in the octanol structure and a negative direction is observed. Similar effects have been reported by Benson for other alcohol + alkane systems. ACOUSTIC VELOCITY AND ISOENTROPIC COMPRESSIBILITIES As indicated above, the main effect of addition of octane is a change in the ‘free’ volume in the mixture compared with that in the pure components.Disruption of the alcohol structure and restriction of the rotational motion has been described as the condensation 21 Interstitial accommodation and orientational order lead to a more compact structure and to an observed decrease in the excess compressibility, fig. 2. At low alkanol concentrations the negative deviations observed are attributed to the effects of break-up of the hydrogen-bond structure and this tends to increase the compressibility and leads ultimately to the positive trend. The behaviour reported here is similar to that reported by Kiyohara and Benson2 on various n-alkanol + n- octane systems. The main conclusion with regard to the low mole-fraction data is that breaking of multimer structure leads to a positive value of K,E and a concomitant increase in the ability for interstitial accommodation of the alkane.This behaviour is modulated by the steric interactions of the alkyl group both on the stability of the multimer structure but also on the ability of the liquid to accommodate the straight-chain alkane. The infrared spectrum of these mixtures is very complex and does not allow either positive identificatior, or estimation of the concentrations of the various multilinear structures present. VISCOTHERMAL ACOUSTIC ATTENUATION COEFFICIENT The frequency dependence of the acoustic attenuation in the isomeric octanols has been reported previ0us1y.l~ In the following analysis account has been taken of the contribution associated with rotational isomerism in octan-2-01, 2,3-dimethylhexan- 3-01 and 2,4-dimethylhexan-3-01 to the acoustic attenuation.In all cases the rotational isomeric contribution decreased linearly with the concentrations of the octanol allowing the viscothermal acousticattenuation to be defined as a frequency-independent contribution. The concentration variation of the viscosity reflects the complex manner in which the ‘ hydrogen-bonded structure’ varies with concentration and reflects the extent to which ‘ associated ’ cyclic and linear hydrogen-bonded structures are destabilized by the addition of non-polar molecules. The observed deviations are negative, implyingA. M. A W W A D AND R. A. PETHRICK 321 1 break up of associated structure in all cases. The ultrasonic attenuation is observed to increase rapidly on the addition of alcohol to the alkane and a maximum in the attenuation coefficient is observed in the dilute region for octan- 1-01, octan-2-01 and 2-ethylhexan-1 -01 and implies that as the alcohol concentration is increased so the probability of formation of cyclic forms increases.The maximum is probably of similar origin to that observed in water alkanol systems, where the excess is attributed to disturbance of the distribution between cyclic and linear pure forms and mixed associated forms.s The height of the absorption maximum decreases as the steric effects of the alkyl group increase. The height decreases in the order octan- 1-01,2-ethylhexan- 1-01, octan-2-ol,2,4-dimethylhexan-3-ol and 2,3-dimethylhexan-3-01. The steric effects in the 2,4- and 2,3-dimethylhexan-3-01 lead to less self-association in these systems than in the corresponding linear systems. Introduction of n-octane to 2,3-dimethylhexan-3-01 leads to a slow increase in the attenuation, and no maximum at low concentrations is observed, hydrogen bonding between these molecules being insufficiently strong to lead to association, and hence the mechanism of relaxation observed in the other systems is excluded here.The excess absorption at low concentrations appears to be due to a structural relaxation contribution to the volume viscosity rather than to the classical shear-viscosity absorption (fig. 5-7). 0 0.05 Y 0.1 FIG. 7.-Ultrasonic absorption coefficient for [xC,H,,OH+(l -x)n-octane] at 298.15 K. 2 MHz and low mole fraction, x, of octanols < 0.1 .0, octan-1-01; A, octan-2-01; ., 2-ethylhexan-1-01; A, 2,3- dimethylhexan-3-01; 0, 2,4-dimethylhexan-3-01. As the concentration of the octanols increases, the disordering action of the n-octane molecules becomes less important in comparison with the effects of hydrogen bonding between neighbouring molecules. Studies of the temperature and frequency dependence of the attenuation in the pure alcohols have indicated that the total attenuation of the pure alcohol is a combination of contributions from rotation isomerism and processes associated with structural relaxation of hydrogen-bonded clusters and dynamics of the structural association. This analysis is further supported by the trends observed at high mole fractions of alcohol, the rotational isomeric contribution being a linear function of the concentration of alcohol.104-23212 ULTRASONIC STUDIES OF OCTANOL+OCTANE MIXTURES CONCLUSIONS This study indicates that in mixtures of alcohol and alkane the observed excesses are a consequence of a number of competing effects. In the sterically unhindered alcohols cyclic and linear associated forms can be formed. Addition of alkane allows the 'free' volume in the lattice to be filled and also will perturb the degree of clustering present. In the sterically hindered alcohols the cyclic associated forms cannot be formed and no excess features associated with critical association are observed. These latter effects are observed in the region of 0.1 mole fraction for the majority of systems studied.A lack of definitive spectroscopic data precludes a detailed analysis of these data at the present time in terms of linear and cyclic associated structures. A.M.A. thanks the Petroleum Research Institute, Iraq for sabbatical leave and financial support during the period of study. K. N. Marsh and C. Burfitt, J. Chem. Thermodyn., 1975, 7, 955. 0. Kiyohara and G. C. Benson, J. Chem. Thermodyn., 1979, 11, 861. L. A. K. Staveley and B. Spice, J . Chem. Soc., 1952, 406. G. Dharmaraju, G. Nayayanaswamy and G. K. Raman, J . Chem. Thermodvn., 1979, 11, 861. H. C. Van Ness, C. A. Soczek and N. K. Kochar, J. Chem. Eng. Data, 1962, 12, 346. M. Diaz Pena and D. Rodriguez Cheda, An. R. Soc. ESP. Fis. Quim. Ser. B, 1970, LXVI, 637. G. N. Swamy, G. Dharmaraju and G. K. Raman, Can. J. Chem., 1980, 58, 229. J. Emery and S. Gasse, Acustica, 1979, 43, 205. S. Nishikawa, M. Mashima and T. Yasunaga, Bull. Chem. Soc. Jpn, 1976, 49, 1413. '" M. J. Blandamer and 0. Waddington, Adv. Mol. Relaxation Processes, 1970, 2, 1. l 1 C. Dugue, J. Emery and R. A. Pethrick, Mol. Phys., 1981, 42, 1453. l 2 A. J. Tresczanowicz and G. C. Benson, J . Chem. Thermodyn., i978, 10, 967. l 3 A. J. Tresczanowicz and G. C. Benson, J . Chem. Thermodyn., 1980 12, 173. l4 J. L. Hales and J. H. Ellender, J. Chem. Thermodyn., 1976, 8, 1177. 15 C. Dugue, J. Emery and R. A. Pethrick, Mol. Phys., 1980, 41, 703. l 6 E. Alcart, G. Tardajou and M. Diaz Pena, J . Chem. Eng. Data, 1980, 25, 140. l 7 Selected Values of Properties of Hydrocarbons and Related Compounds, API Project 44 (Thermodyn- amics Research Centre, Texas A & M University, College Station, Texas, October 31, 1952 and October 3 1, 1963). F. Eggers, Acustica, 1967, 19, 323. I 9 R. A. Pethrick, J . Phys. E, 1972, 5, 571. 2" D. Patterson, Pure Appl. Chem., 1976, 47, 305. 2 1 P. Trancrede, P. Bothorel, P. de St Romain and D. Patterson, J. Chem. Soc., Faraday Trans. 2, 1977, 73, 15. (PAPER 1 / 1968)

ISSN:0300-9599    

DOI:10.1039/F19827803203

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1982, 78, 3213-3222 Kinetics of the Photopolymerization of 2,5-Distyrylpyrazine in Solution BY EL-ZEINY M. EBEID, MAHMOUD H. ABCEL-KADER A N D SALAH E. MORSI* Centre for Graduate Studies and Research, University of Alexandria, P.O. Box 832, Alexandria and Chemistry Department, Faculty of Science, University of Tanta, Tanta, Egypt Received 22nd December, 198 1 The photoreactivity of 2,5-distyrylpyrazine (DSP) in methanolic solutions has been investigated using various spectroscopic and kinetic methods. The effect of the excitation wavelength has also been studied : light of wavelength i = 403 nm induces a photo-oligomerization process whereas light of wavelength A = 365 nm causes both photo-oligomerization and photopolymerization processes. These two processes are manifested as two stages in the reaction.The photo-oligomerization stage is characterized by higher values of quantum yield for chemical reaction (4 = 1.8 at 30 "C) and a higher energy of activation compared with the photopolymerization stage. The pattern of change in the absorption spectra of DSP solutions as a result of excitation with light of I = 365 nm depends upon the exciting light intensity. A sharp isosbestic point at 287 nm is obtained when high light fluxes (ca. 7.57 x lo-' ein min-') are applied, whereas three isosbestic points are obtained at 350, 325 and 287 nm on applying low light fluxes (ca. 5 x lo-* ein min-I) or by applying 403 nm light regardless of its intensity. The photo-oligomerization stage undergoes a power kinetic law and has an activation energy E, = 38.4k 1.5 kJ mol-I, an activation enthalpy AH* = 36.9k 1.5 kJ mol-' and an activation entropy AS* = - 184.3k0.3 J K-' mol-I.The photopolymerization stage obeys second-order kinetics with activation parameters : E, = 16.0k 1.0 kJ mol-', AH* = 13.6-t 1.0 kJ mol-' and AS* = -240.2k0.3 J K-' mol-I. The energy of activation in both cases accounts for a diffusion-controlled process that brings the monomer molecules to a spatial configuration conductive for the photocycloaddition of the olefinic double bonds. The highly negative values of the entropies of activation in both stages indicate that the activated states are highly ordered compared with the ground states. A mechanism for the reaction has been postulated that is based on a two-centre attack by an excited DSP monomer molecule on two ground-state molecules during the photo-oligomerization stage.A stepwise mechanism prevails in the photopolymerization stage. Both the fluorescence and excitation spectra of DSP in methanolic solution decrease in intensity as the U.V. irradiation time increases, indicating that the photoproducts have no detectable fluorescence in this spectral region. 2,5-Distyrylpyrazine (DSP) is a prototype compound in the field of lattice-controlled photopolymerization reactions. Its photoreactivity has been observed previously' but a comprehensive study of this photoreaction has recently been initiated by Hasegawa et al.,'. who showed via various analytical techniques that a photopolymerization process takes place both in solution and the solid state upon U.V.irradiation. The photochromism observed4 in DSP single crystals has prompted us to study the effect of light on DSP solutions in order to investigate this reaction at the isolated molecular level. This effect has been studied qualitatively,j? and a photo-oligomeriza- tion process has been found to occur as a result of light irradiation (3, 3 380 nm) of DSP-tetrahydrofuran solutions. The resulting photo-oligomer has been reported:' to consist mainly of pentamers. Apart from the kinetic studies previously made on the solid-state photoreactivity 32133214 PHOTOPOLYMERIZATION OF 2,5-DISTYRYLPYRAZINE of DSP,6*7 very little information is presently available on the kinetics of photopoly- merization in solution.In this study various spectroscopic techniques have been used to study the photoreaction of DSP in methanolic solutions. EXPERIMENTAL DSP was prepared using the method of Hasegawa et a1.8 The monomer was recrystallized from xylene, chromatographed on basic silica gel 60-80 mesh using AnalaR methylene chloride as eluent followed by vacuum sublimation of the resulting batch. The purity of the yield was verified by matching both the absorption and excitation spectra of 1.02 x mol dm-3 methanolic solution. The U.V. spectra were recorded using a Unicam SP 8000 spectrophotometer. Samples were thermostatted using apparatus similar to that described in ref. (3), consisting of a rotor, distilled water as a thermoregulating medium, a Pyrex beaker, a magnetic stirrer, a thermoregulator and a metallic cell-holder fixed vertically such that it is covered by the thermoregulating medium. The thermoregulator used was a copper coil in which thermostatted circulating water is pumped by an ultrathermostat (Haake model FE).The temperature fluctuations were found to be d 0.5 "C. The reaction vessel was a well stoppered silica cell of 10 mm optical path-length which was immersed in the thermoregulating medium by mounting in the cell holder. Irradiation did not begin until the prescribed temperature was reached. The solutions were deoxygenated by flushing with pure nitrogen gas for ca. 30 min before kinetic runs. To obtain the spectral runs the cell was removed, dried well and put into the spectrophotometer. The light source used was a mercury high-pressure lamp (HBO 200 W/2, Osram) combined with interference filters (maximum transmission at 365 and 403 nm with spectral width at half transmission of 8 nm).The incident light intensity was measured using ferrioxalate actinometry as described by Hatchard and P a r k e ~ . ~ Both fluorescence and excitation spectra were recorded on a Shimadzu RF 510 spectrofluorophotometer. The kinetic studies were performed within the range of concentrations where Beer's law holds, (i.e. absorbance changing linearly with concentration). RESULTS A N D DISCUSSION EFFECT OF EXCITATION WAVELENGTH Fig. 1 shows the effect of 403 nm irradiation on a 1.01 x mol dm-3 DSP methanolic solution with increasing time. Three isosbestic points are obtained at 3 5 0 , 325 and 287 nm.The reaction product is believed to be a lower chemical aggregation of DSP molecules and it exhibits absorption (besides a band at ca. 333 nm) in the form of two bands at ca. 225 and 260 nm. A linear correlation has been found between the absorbances at the two wavelengths ?L = 378 and 1 = 260 nm, indicatinglO that the observed spectral change is due to a single photoreaction. The remaining bands at ca. 333 and 225 nm overlap too much to be monitored. Since the irradiation conditions in this case compare favourably with those of Hasegawa et a1.5*6 we expect this photoproduct to be a DSP oligomer. The pattern of change in the spectrum was independent of the light intensity; however, increasing the light intensity leads to an increase in the rate of photoreaction.Upon irradiation with light of 3, = 365 nm (rather than 3, = 403 nm) the reaction was found to occur via two clearly distinguishable stages with different rates, as indicated from the analysis of the absorption spectra (see fig. 7). The earlier stage involves the formation of an oligomer and can be produced by applying low light intensities (ca. 0.5 x lop7 ein min-l). In this case, spectra similar to those shown in fig. 1 are obtained as the period of irradiation is increased, but a 'stationary absorption'E-2. M. EBEID, M. H. ABDEL-KADER A N D S. E. MORSI 3215 1 0 v) * 2 0 8 f b) 0 5 0 6 e 0, - 0 4 13 x c * v) - ; 0 2 200 225 250 275 300 325 350 400 h/nm FIG. 1 -Effect of irradiation with light of wavelength A = 403 nm on the absorption spectrum of a 1.01 x 10-5 mol dm-3 DSP methanolic solution at 30 O C : (-) fresh sample; (-.-*-.) after 17 min irradiation; ( * .. . . - ) after 52 min irradiation (intensity = 5.8 x lo-* ein min-l). 0.0 8 - I c .* E 0.06 I m vl + .d c 3 w 2 0.04 e 0, -2 W . Y 0.02 0 50 100 relative intensity (arb. units) FIG. 2.-Effect of light intensity on the rate constants of the photo-oligomerization process. stage is obtained after reaching ca. 10% reaction. At this ‘stationary absorption’ the reaction rates fall dramatically and the reaction becomes too slow to be followed. This early stage exhibits a relatively high initial quantum yield (4 = 1.8 at 30 “C). We attribute this ‘stationary absorption’ to the formation of a stationary concen- tration of the oligomer, since the rate of its generation equals the rate of its consumption either by photodissociation to the monomer or by photopolymerization to higher molecular aggregates.The reason why the photo-oligomer reacts is apparent from its considerable absorption component in the 365 nm region of the spectrum.3216 PHOTO POL Y M ERI Z A TI ON OF 2 3 - D ISTY RY L PY R A Z INE .g 0.8 .- 2 ' Y 0.21 .r: I 0 225 250 275 300 325 350 LOO X/nm FIG. 3.-Effect of irradiation on a 1.25 x mol dm-3 DSP methanolic solution with light of wavelength A = 365 nm (intensity = 7.57 x lo-' ein min-I): (-) fresh; (---.-) after 45 min irradiation; (. . . . . .) after 105 min irradiation ; (- - - -) after 195 min irradiation. U SCHEME 1 .-(Q) Photopolymerization of DSP; (6) activated state during the photo-oligomerization stage.E-Z.M. EBEID, M. H. ABDEL-KADER AND S. E. MORSI 3217 Fig. 2 shows the effect of 365 nm light intensities (in arbitrary units) on the rate constants for this stage at 30 O C . The curvature in the plot is attributed to the multiphoton processes occurring during this stage. The method of extracting the rate constants for this stage is mentioned in the kinetic part of this work. On applying higher intensities of the 365 nm light (ca. 7.6 x ein min-') only one sharp isosbestic point at 207 nm is obtained. Fig. 3 shows the effect of intense 365 nm light on the absorption spectrum of a 1.25 x 10-5 mol dm-3 DSP methanolic solution. The sharpness of the isosbestic point at 287 nm indicates that the polymer being formed has a very low polydispersity. The initial quantum yield for chemical reaction during this stage is relatively low (4 = 0.1 1 at 30 "C).The irradiation product in this case has bands at ca. 230 and 287 nm, the band at 378 nm being due to the original species. There is also a linear correlation between the absorbances at the two wavelengths ;C = 378 and R = 230 nm, indicating', that the observed spectral change is due to a single photoreaction. The decrease in wavelength of the absorption maximum is attributed to the loss of extended conjugation along the DSP molecule owing to the consumption of the double bonds in forming cyclobutane rings according to scheme (1 ;I). The percentage of these consumed double bonds increases as the number of the polymerized monomeric units ( n ) increases. This is accompanied by a greater blue shift of the absorption maxima.The same argument has been proved3? l1 for the photoproducts of DSP in the solid state. EMISSION AND EXCITATION SPECTRA Fig. 4 shows the effect of 365 nm light on the excitation spectrum of a 0.430 x lop5 mol dm-3 DSP solution. The uniform decrease in intensity of the excitation bands with no change in the spectral pattern indicates that the product has no detectable fluorescence in this range of the ,pectrum. This is also confirmed by the uniform decrease in intensity of the fluorescence band at 450 nm as a result of excitation with light of R = 365 nm. The fluorescence spectrum of DSP in solution is shown in fig. 5 (&,, = 365 nm, concentration 1.24 x lop6 mol dm-"). KINETIC ANALYSIS OF THE FIRST STAGE The absorbance change (x) at the A = 378 nm is monitored as a function of irradiation time with 365 nm light of very low intensity (ca.0.7 x ein min-l). The value of x corresponds to ( A , - A , ) where A , is the initial absorbance and A , is the absorbance at time t. The A, was kept the same for all kinetic runs. Simple kinetics based on orders of reactionl29l3 was not applicable to this stage, but a power law has been applied. Fig. 6 shows a plot of x' against t. The applicability of this power law is due to the complexity of this stage, which does not account for a simple process. The complexity of this stage is also demonstrated by the relation between the rate constants and light intensity as shown in fig. 2. The slopes of the lines in fig. 6 give the rate constants at various temperatures.Thermodynamic data were obtained from a least-squares fit14+ l5 to four data points, giving an activation enthalpy for the photochemical process of AH* = 36.9_+ 1.5 kJ mo1-l. The entropy of activation, AS*, is - 184.3 k0.3 J K-I mol-I. The energy of activation E, = 38.4+ 1.5 kJ mo1-l. The highly negative value of the entropy of activation indicates that the activated state is highly ordered compared with the ground state.3218 PHOTO POLYMER I Z A T I ON OF 2 3 - D ISTY RY L PY R A Z INE J. i ' I I I I 250 300 3 50 4 00 h/nm FIG. 4.-Changes in the excitation spectrum of a 0.43 x 1 0-5 mol dmP3 DSP methanolic solution as a result of irradiation with exciting light of wavelength E. = 365 nm: (---) fresh sample; ( . . .. .) after 1 min irradiation; (-.-.-.-) after 20 min irradiation. Note that the spectrum was obtained by following the 450 nm fluorescence band. KINETIC ANALYSIS OF THE SECOND STAGE Fig. 7 shows a plot of the fractional change a, where a = (A, - A , ) / A , , against the time ofirradiation with intense 365 nm light (ca. 7.0 x ein min-l). The dotted parts of the curves correspond to the first stage, which is not well resolved under conditions of intense irradiation. A second-order rate equation applies to the second stage, and is given by16 where a is the number of moles of the reactant at the beginning of the reaction and x is the number of moles which have reacted at time t. Taking the initial absorbance of the DSP monomer, A,, to be proportional to a, A, to be proportional to (a-x) and (A,-A,) to be proportional to x, we can write the second-order rate equation in the form 1 A , - A , A , - A , A, ( A, )=!=)* k t = - ~E-Z.M. E B E I D , M. H. ABDEL-KADER A N D S. E. MORSI 3219 h/nm FIG. 5.--Changes in the fluorescence spectrum of a 1.24 x lop6 mol dmT3 DSP methanolic solution as a result of irradiation with 365 nm light (A = 365 nm): (-) fresh sample; (-.-.-.-) after 10 min irradiation; (-----) after 25 min irradiation; ( . . . . .) after 58 min irradiation. 0 5 10 15 tlmin FIG. 6.-Plot of x2 values as a function of time t: 0, 25; 6, 34; 0, 43, and 0, 49 OC.3220 PHOTO POLY MER I Z A TI ON OF 2 3 - D ISTY RY L PY R A Z I NE 0. 0 T . h j- 0. 0 s 0 . 0 10 20 30 40 50 60 t/min FIG. 7.-Piot of a against t showing the fractional change in the absorption maximum as a result of irradiation with intense (ca.7 x lop7 ein min-I) light of wavelength 1 = 365 nm: 0 = 13; 0 , 21 ; 0, 30 and 0, 40.5 "C. Fig. 8 shows a plot of the values ( A , - A , ) / A , A , against t after correction by subtracting values of the ordinates corresponding to the first stage. The slopes of the straight lines give the rate constants at the cited temperatures. A statistical analysis gives the energy of activation Ea = 16.0 1 kJ mol-l, the enthalpy of activation AH* = 13.6+ 1 kJ mol-l and the entropy of activation AS* = -240.2f0.3 J K-l mol-l. This highly negative value of AS* indicates that the activated state is highly ordered compared with the ground state. GENERAL DISCUSSION AND CONCLUSIONS In the light of the kinetic data we can propose a mechanism for the photoreaction of DSP in solution.DSP monomer molecules are electronically excited by both 365 and 403 nm light. The excited monomer molecule then makes a simultaneous two-centre attack on two monomer molecules, forming an activated state that might be represented by scheme (1 6). This activated state then leads to chemically bonded oligomers. The high monomer concentrations at the beginning of the reaction may allow this process to occur. The highly negative value of entropy of activation (AS* = - 184.3 0.3 J K-l mol-l) is an indication of this ordered activated state. ThisE-Z. M. EBEID, M. H. ABDEL-KADER A N D S. E. MORSI 322 1 0 I 0 10 20 30 40 50 60 70 80 tlmin FIG. 8.-Application of a second-order kinetic law to the change in intensity of the absorption maximum as a function of irradiation time ( t ) using intense (ca.7 x ein min-I) light of wavelength I. = 365 nm. Note that the parts of the ordinate corresponding to the first stage are subtracted. 0, 13, 0, 21, 0, 30 and 0, 40.5 "C. model explains the relatively high rates and high energy of activation characterizing this stage. On exciting with light of a wavelength that these oligomers can absorb (e.g. 365 nm) the oligomer molecules continue incorporating other monomer molecules at both ends in a stepwise mechanism forming a relatively high molecular-weight polymer. This latter type of reaction constitutes the second stage described above. Because of the pronounced difference in resonance stabilization energies of the olefinic double bonds, the resulting photo-oligomer exhibits photoreactivity which is different from that of the monomer. This explains the low quantum yield and the low rates of reaction during the second stage compared with the first stage.The relatively low values characterizing the energies of activation for this photo- reaction account for a diffusion-controlled process and correspond to the energy necessary to bring the reacting molecules to a spatial configuration suitable for the addition of the olefinic double bonds. For a two-centre attack to occur (e.g. the first stage) molecular diffusion involves two monomeric molecules, whereas a stepwise mechanism (e.g. the second stage) involves the diffusion of only one molecule. This explains why the energy of activation for the first stage is nearly twice that of the second stage.The study also shows that the photoproducts in both the first and second stages are non-fluorescent. This is attributed to the loss of extended conjugation and coplanarity in case of oligomer and polymer molecules. We acknowledge with gratitude the financial support of the Egyptian Academy of Scientific Research and Technology.3222 PHOTOPOLYMERIZATION OF 2,5-DISTYRYLPYRAZINE C. F. Koelsch and W. H. Gumprecht, J. Org. Chem., 1958, 23, 1603. M. Hasegawa and Y. Suzuki, J . Polym. Sci., Part B, 1967, 5, 813. M. Hasegawa, Y. Suzuki, M. Nakanishi and F. Nakanishi, Prog. Polym. Sci. Jpn, 1973, 5, 143. E. M. Ebeid, S . E. Morsi and J. 0. Williams. unpublished results. M. Hasegawa, Y. Suzuki and T. Tamaki, Bull. Chem. Soc. Jpn, 1970, 43, 3020. H. Nakanishi, Y. Suzuki, F. Suzuki and M. Hasegawa, J. Polym. Sci., Part A-I, 1969, 7, 753. M. Hasegawa, Y. Suzuki, F. Suzuki and N. Nakanishi, J. Polym. Sci. Part A - I , 1969, 7, 743. J. G. Hatchard and C. A. Parker, Proc. R . SOC. London, Ser. A , 1956, 235, 518. T. Tamaki, Y. Suzuki and M. Hasegawa, Bull. Chem. Soc. Jpn, 1972, 45, 1988. G. M. Harris, Chemical Kinetics (D. C. Heath and Co., Chicago, 1966). ’ C. H. Bamford, G. C. Eastmond and J. C. Ward, Proc. R . Soc. London, 1963, 271, 357. lo H. Mauser, Z . Naturforsch., Teil B, 1968, 23, 1025. l 3 C. R. Metz, Theory and Problems of Physical Chemistry (McGraw Hill, New York, 1976), chap. 10. l4 E. M. Ebeid, S. E. Morsi and J. 0. Williams, J. Chem. Soc., Faraday Trans. I , 1979, 75, 1111. l5 C. I. Chase, Elementary Statistical Procedures (McGraw Hill, New York, 1967), p. 91. l6 P. J. F. Griffiths and J. D. R. Thomas, Calculations in Adtunced Physical Chemistry (Edward Arnold, London, 1962), p. 140. (PAPER 1 /1974)

ISSN:0300-9599    

DOI:10.1039/F19827803213

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1982, 78, 3223-3234 Ion-Ion-Solvent Interactions in Solution Part 3.-Aqueous Solutions of Sodium Nitrate BY RAY L. FROST Chemistry Department, Queensland Institute of Technology, Brisbane. Australia 4001 AND DAVID W. JAMES* Chemistry Department, University of Queensland, Brisbane, Australia 4067 Received 2 1 st December, 198 I The profile of the band due to the symmetric stretching vibration in the Raman spectrum of aqueous solutions of NaNO, has been studied as a function of concentration. Analysis based on the Fourier transform indicates that there are at least three principal components in the band. Analysis gives the positions of the three components as 1047.6, 1050.0 and 1052.0 cm-I, with a fourth component appearing at 1070.0 cm-l in the most concentrated solutions.On the basis of concentration dependence and band shape these components have been assigned to the free aquated nitrate ion (1047.6), the solvent-separated ion pair (lOSO.O), the contact ion pair (1052.0) and ion aggregates (1070.0). Association quotients K, and K2 for the two equilibria K , K2 Na+(aq)+NO; (as)+ Na+-H,O.NO; (aq)eNa+'NO;.(aq) are K , = 2.7 dm3 mol-l (0.5 mol drn-,) and K , = 3.3 (6 mol drn-,). There have been a variety of spectroscopic studies of nitrate solutions having a monovalent cation and these have been extensively reviewed.' The nitrate ion has proved to be sensitive to variations in environment and has been used as a probe in studies of solids, glasses, melts and solutions4 in order to provide information about electrolyte environment.*> In an extensive study of the Raman spectra of aqueous solutions of alkali-metal nitrates it is found that even at low concentrations the antisymmetric stretching vibrational band of the NO; (v,), centred at ca.1390 cm-l, is split into two components. It is concluded that this splitting is due to the perturbation of the nitrate ion when it is aquated, and n.m.r. StudieslO have suggested that anion solvation is asymmetric, in accord with this idea. A lowering of anion symmetry from DBh to C,, would split the E' mode (D3h) into an A, and B, component.l1*l2 Similar splittings have been observed for dilute solutions of nitrates in liquid arnmonia.l2>l3 In more concentrated solutions of sodium nitrate the components of v, vary in energy and in the later report this band was separated into two pairs of components.I4 The spectral band due to the in-plane bending vibration v, was a single band at low concentrations at 720 cm-1 but at high concentrations, > 7 mol dm-3, a second component was resolved at 740 cm-l in H2015 and 728 cm-I in D20.14 The appearance of this second component was attributed to the existence of ion pairs at high concentrations.The band due to the symmetric stretching vibration (vl) was reported as a symmetrical band sh<,wing a small concentration dependent shift.l4,I5 In a later report the band was shown to develop asymmetry at high c0ncentrations.l The contour of the v1 vibration was extensively studied16-18 and values for the 32233224 I ON-I ON-SO L V E N T INTER A C T I 0 N S vibrational relaxation time and reorientional relaxation time were calculated. Both the relaxation times showed strong concentration dependence which was interpreted loosely in terms of ion association.It was pointed out that the analysis was uncertain due to the asymmetry of the band at higher concentration^.^^ We have made a systematic study of the Raman spectra of aqueous solutions of metal nitrates which will be reported in this and following papers. This paper develops the techniques which are used to analyse the spectra and applies them to the spectra of solutions of sodium nitrate. The results obtained 'are rationalised with previous spectroscopic and transport studies for this system. Subsequent papers will examine the influence of cation size, temperature, deuteration of solvent, changing anion species, cation charge and presence of partially filled valence shells on the cation.EXPERIMENTAL The sodium nitrate was laboratory reagent grade which was recrystallised twice from purified water, oven dried at 380 K for 48 h and stored over P,O,. The water was triply distilled, the final distillation being from alkaline permanganate. Spectra were run on a Cary 82 spectro- photometer using 90' scattering geometry with a restricted collection angle and constant energy band pass slits of < 0.8 cm-l. As reported elsewherelG it was established through an examination of bandwidth as a function of slitwidth that a correction for slit function was not necessary in this study. Radiation (514.5 nm) from a Coherent Radiation CR3 laser (400-800 mW at sample) was passed through a Glan-Thompson prism to ensure a high degree of polarization.The two polarized components of the spectra were recorded using a Polaroid sheet and quartz wedge scrambler and the system was checked using the v1 band of CD,CN which gave a value p = 0.002, in good agreement with published values.zo The sample was held in a 1 cm glass curvette which had all edges masked and which was held in a temperature-controlled assembly. The laser was weakly focussed and passed once through the solution. Spectra were recorded on both chart and paper tape. The digitised spectra were recorded at a 0.2 cm-l interval and several recordings of each spectrum were made and averaged. The spectra for two polarizations of the v1 symmetrical stretching band of the nitrate ion were combined to give the isotropic and anisotropic profiles21 for the band.These profiles, which were the average of several experimental determinations, were smoothed using a Savitsky-Golay smoothing routine and were then Fourier transformed using standard routines. The Raman spectrum peaked near 1050 cm-l. The Fourier transform was made about 1050.445 cm-l as origin with the band being analysed to ca. 50cm-' on each side of the origin. This was the effective limit as beyond this the band due to the nitrate ion antisymmetric stretch starts to make appreciable contribution and at lower energies the scattering due to water becomes significant. This truncation of the spectrum causes a slight curvature to appear at short times in the transformed spectrum.2z Band-component analysis was performed interactively using a suite of programsz3, 24 including a non-linear least-squares routine.25 The bands were described by a Lorentzian- Gaussian product function of the form I , e x p [ - - ~ ( ~ - ~ , ) ~ ] 1 + G(v - 9,)2 r ( q = where I, is the band intensity, vo is the position of the band maximum and X , and X , are half-width parameters for Lorentzian and Gaussian bands.Band fitting was carried out interactively with X , and X , allowed to vary. The criteria for the selection of starting values for band parameters and the number of bands will be described later.R. L. FROST A N D D. W. JAMES 3225 RESULTS A N D DISCUSSION B A N D MOMENTS A N D MODULATION TIMES In a stochastic line-shape theoryz6 which has been applied to the vibrational correlation function G(v)(t) may be determined by the measurement of the vibrational second moment [M,(a)] and the modulation time t, which characterises the correlation decay of the stochastic perturbation Hamiltorian.For slow modulation the perturbation remains for a long time; the initial phase coherence of individual oscillators is rapidly lost. Conversely for fast modulation the original phase persists and the band contour is narrow. The vibrational correlation function may be expressed as d&) = exp (- <[q(O)I*) N exp [ - ( t / z c - 1 >I + r,, t>). For short times ( t <. z,) this reduces to which is valid at all times in the slow modulation limit z,(o*)~ % 1.This expression is independent of modulation time and describes the rigid lattice approximation of vibrational dephasing. For long times, t 9 z,, we have which corresponds to extreme motional narrowing (z, (O")f < 1). The modulation times are calculated by using [ml(O)l2 from the experimentally measured second moment and adjusting T, to give agreement between theoretical and experimental correlation functions. The second moments which we calculated are much smaller than those previously reportedla and this reflects the uncertainty in the previous measurement owing to the use of excessively large The second moments and modulation times are listed in table 1. The quantity M, z,/271c gives a measure of modulation in the correlation function. The values for sodium nitrate solutions lie between 0.9 and 1.26, which are intermediate between the values in CHCl, (0.2) where modulation is fast and D,O in H,O ( 5 ) where modulation is This intermediate modulation reflects the influence of hydrogen bonding between the nitrate ion and water.TIME CORRELATION FUNCTIONS The time correlation functions (t.c.f.) obtained by Fourier transforming the energy spectra are given in fig. 1. They are all curved with the slope increasing with increasing concentration. The slope at the exp (- I ) point gives a measure of the relaxation time which for a pure Lorentzian band will correspond to the value obtained from the band half-width.16 These values are both listed in table 1 and are seen to be appreciably different but to follow the same trends. The shape and slope of the t.c.f.are dependent on the nature of the spectroscopic band. If a band is composed of two components of the same half-width the t.c.f. will display minima, the positions of which are related to the separation of the components. If there are two components of different half- width the slope of the t.c.f. will change as the relative intensity of the two components change and minima will appear at times corresponding to the peak separation.22 For sodium nitrate solutions the band is close to Lorentzian in shape for dilute solutions and hence the curvature reflects the presence of a minimum outside the range of the time scale shown (the t.c.f. becomes very noisy at long times). This will correspondw N N o\ v 7 /PS /PS 0 TABLE 1 .-BAND PARAMETERS FOR AQUEOUS SOLUTIONS OF NaNO, I - 0 concentration v, vs M2 (a) MZ (D) z, z,, * zv(w9 z&lj rf: v, /mol dm-3 /cm-l /cm-l /(cm-1)2 /(cm-l j2 (b/c) /PS /2nc ~~ 1 .o 1048.4 1048.4 56.0 144.0 - 1.8 0.94 0.94 1.36 1.07 2.0 1049.1 1049.1 64.0 144.0 - 3.8 0.98 1.04 1.31 0.89 4.0 1049.9 1050.5 64.0 152.0 + 3.7 1.14 1.21 1.15 0.77 6.0 1050.8 1051.2 69.0 169.0 + 3.2 1.07 1.18 1.12 0.78 8.0 1051.1 1051.2 72.0 176.0 + 6.5 1.03 1.16 0.98 0.8 1 M,(a), second moment of the isotropic band; M,(J?), second moment of the anisotropic band; C(h/cr), band asymmetry parameter; T~,, modulation ime; z,, vibrational/relaxation time.I 0 zR. L. FROST AND D. W. J A M E S 3227 FIG. 1.- t i p s -Time correlation functions from the A ; symmetric stretching vibration band of NO; in solutions of NaNO,.Concentration/mol dmp3: (a) 0.05; ( b ) 1 ; (c) 2; (d) 4; ( e ) 6; (f) 8. aqueous to a peak separation between two components of < 2 cm-l. The change in slope as the concentration increases indicates that the half-width of the components which grow at higher concentrations is greater than those dominant at low concentrations. The above discussion of the t.c.f. refers to the modulus of the Fourier transform. A single Lorentzian profile which is transformed about an origin different from the band maximum, or any band consisting of more than two components, has both real and imaginary parts to the Fourier transform. If la1 is the magnitude of the real part of the transform and (bl that of the imaginary part, tan-' Ib/al describes an angle 8 which is time dependent.For a single Lorentzian band, a plot of 8 against t gives a straight line, the slope of which depends on the separation between the transformation origin and the band maximum. For a band having two components, a plot of B against t gives a curve which oscillates about the straight line describing the major component. The curve crosses the straight line at a time t, which depends on the separation of the two components. The magnitude of 8 for the curve and straight line at a time tn/2 gives a measure of the relative intensity of the two peaks.28 The development described previously28 used synthetic bands which were noise free. The application to experimental bands will be influenced by the signal-to-noise ratio of the spectrum.For solutions above 1 mol dm-, concentration the SINratio for individual bands was at least 100 : 1, and since up to 10 spectra were averaged to give the profile to be analysed the ratio was better than this. In fig. 3 the experimental curve is shown together with the composite curve so the noise level can be gauged. The time resolution of the t.c.f. and 8 functions is 0.3 ps and the time range is tens of picoseconds. This makes use of the 8 function method feasible for the solution spectra recorded. The variation of 8 with time for solutions of sodium nitrate are shown in fig. 2. The straight line represents the 8 function for a curve which we have identified in dilute solutions (0.01 mol dm-,) of KNO,, LiNO, and Mg(NO,),. This band, having a peak position of 1047.6cm-l, a half-width (h.w.h.h.) of 3.395cm-l and a shape ratio [x3/(.x3+x4)] of 0.85, we identify with the nitrate ion aquated by water in an environment where interaction with the cation can be ignored.As the concentration3228 I 0 N-I 0 N-S 0 L V E N T I N T E R A C T I 0 N S -1.34 -2.04 1.6 3 . 2 4.8 t i p s (4 4; (el 6 ; (1) 8. FIG. 2.-Theta functions for transforms in fig. 1. Concentration/mol dm-3: (u) 0.05; (6) 1 ; (c) 2; of NO; is increased the slope of the 0 function becomes more positive. There is evidence, particularly in the 1 mol dm-3 solution spectrum, of an intersection between 7 and 8 ps. The 0 function becomes noisy in this region and this prevents accurate estimation of the cross-over point. An intersection at 7-8 ps would correspond to a second band at ca.2cm-' higher energy than 1047.6, i.e. 1049.6cm-l. The 2 and 4 mol dm-3 solution 0 functions appear to oscillate about the position of this second band, indicating that it is the dominant component. The functions for the highest concentrations appear to oscillate about a line of more positive slope and have inflections at times corresponding to separations of 2 and 4.5 cm-l. The curves are consistent with an initial peak at 1047.6 cm-l, a second peak growing in intensity at ca. 1050 cm-l and at high concentrations a third component at ca. 1052 cm-l. In the band-component analysis to be described below the plots of 8 against t for the original and synthetic bands were compared to give a goodness-of-fit criterion. It was found that this comparison was a much more sensitive measure of the fit than the more common e.m.s.value. However, in the curves presented the e.m.s. curve is presented as being a familiar criterion. The isotropic band profiles for the symmetrical stretching frequency of the nitrate ion were analysed for component bands using a non-linear least-squares routine in an interactive mode. Three band components having initial peak energies of 1047.6, 1049.6 and 1052 cm-l were used to fit the band. For each band the four parameters of peak energy, peak height, X3 and X , were allowed to vary. There must be a question of the uniqueness-of-fit in a procedure of this sort. Since there are a total of twelve parameters it is not surprising that excellent fitting was attained. However, there are a number of criteria by which the validity of the results can be assessed.The e.m.s. between the experimental and calculated spectrum must be small, the 0 function for the composite spectrum must agree with that for the experimental band at least toR. L. FROST A N D D . W. JAMES 3229 4 ps, the position of the bands and the concentration variation of the band intensity must make chemical sense, agreement with previous work must be examined, and the band parameters for the component bands must be reasonable. In this analysis a band component at 1047 6 cm-I identified with the aquated nitrate was fixed (peak position, X , and X,) with only its peak height allowed to vary. Two additional peaks having initial peak positions of 1049.6 and 1052 cm-I with X3 and X , the same as the initial peak were allowed to vary in all four parameters (peak position, X3, X , and peak height).At the highest concentrations Lhere was a persistent residual at appreciably higher energy so an addition band was permitted. The results for the band-component analyses are presented in table 2, the fitted spectra at two representative concentrations are shown in fig. 3 and the variation of band area of the component bands with concentration change is shown in fig. 4. Although the band parameters were not fixed it is evident that a band component at I050 cm-1 has essentially constant parameters at all concentrations and bands at 1052 and 1070 cm-1 have reasonably constant parameters for all concentrations in which they appear with reasonably intensity.The e.m.s. values at all concentrations are excellent. It might be expected that the band parameters for a given component will be concentration dependent. Indeed the half-width and band shape do show appreciable variation. In a study of solutions of perchlorate saltsz9 it has been found that the band position may also change. This is more extensively discussed el~ewhere.~~ ION ASSOCIATION IN NaNO, SOLUTIONS The conclusions which can be drawn from the analysis presented above are clearly different from those which have been previously made. On the basis of analysis of the Raman bands due to the antisymmetric stretching vibration and the appearance of a component in the plane bending vibration at ca. 730 cm-I it was suggested that contact ion pairs were formed at 8 mol dm-, concentration.It was also suggested that although solvent-separated ion pairs might contribute to a shift in the symmetrical stretching vibration, their presence could not be positively identified. Although ion association will vary with concentration in a continuous manner our results are consistent with the series of equilibria K , K2 K : , Na+(aq)+NO;(aq) + Na+-H,O*NO;(aq) eNa+-NO;(aq)e (Na+*NO;), (as) where (aq) denotes species existing as aquated species. The band centred at 1047.6 cm-' we associate with NO; (aq), the band at 1050.0 cm-I with Na+-H,O*NO; (aq), the band at 1052.0 cm-I with Na+-NO; (as) and the band at 1070.0 cm-I with the ion aggregate (Na+*NO,), (aq). If it assumed that the molar intensity for each species is the same, the association quotients for the various association equilibria can be calculated.Kl in a 0.5 mol dm3 solution of NaNO, is 2.7 dm3 mol-I. The second equilibrium is always in competition either with the first or third equilibria and is also undoubtedly dependent on the water concentration. However on the simple assumption that "a+ NO;(aq)] I (band 3 ) - [Na+-H,O*NO;(aq)] - I (band 2) _____ - K - where I is a measure of band area, values of K , of 1.1 at 4 rnol dm-3 and 3.3 at 6 mol dm-, were calculated and these may be compared with the value of 2.98 estimated previously.' For NaNO, in D,O the K , value for 4 mol dm-, solution is 0.8, which compares with a value of 0.6 reported previously.14 However, since the basis for the band analysis is quite different in the two studies the correspondence must be regarded as fortuitous.w w w 0 TABLE Z.-cOMPONENT-BAND ANALYSIS FOR THE ISOTROPIC BAND OF NaNO, SOLUTIONS aquated NO; solvent-separated ion-pair band contact ion-pair band ion aggregate band concentration V", 04 shape V , wi shape vm w: shape 'm 4 shape /mol dm-3 /cm 1 /cm 1 ratio area /cm-l /cm-' ratio area /cm-l /cm-1 ratio area /cm-l /cm-' ratio area K , K2 ___ - 0.001 2.74 - 0.5 1047.6 3.395 0.851 0.560 1050.0 3.61 0.851 0.43 1052.0 3.98 0.72 0.01 1070.0 - 1 .o 1047.6 3.395 0.851 0.420 - 3.5 0.850 0.58 1052.0 3.98 0.72 0.01 1070.0 - - 0.001 3.3 - 6.0 1047.6 3.395 0.851 0.098 ~- 3.50 0.840 0.202 1052.0 5.07 0.655 0.660 1070.0 9.38 0.790 0.028 - 3.3 8.0 1047.6 3.395 0.851 0.05 ~- 3.50 0.840 0.168 1052.0 5.13 0.613 0.731 1070.0 5.13 0.790 0.040 ~ 4.3 2.0 1047.6 3.395 0.851 0.375 - 3.627 0.842 0.475 1052.0 4.00 0.723 0.128 1070.0 8.52 0.689 0.013 1.7 .27 4.0 1047.6 3.395 0.851 0.250 - 3.488 0.840 0.342 1052.0 4.40 0.696 0.384 1070.0 9.14 0.757 0.021 1.34 1.12 In each section: v, is the band maximum (cm-I); (dm3 mol-I); K , is the second equilibrium constant.is the band half-width (h.w.h.h.) (cm-'); shape ratio is x3/(x,+x4); area is the integrated band area; K , is the first equilibrium constant U 0 7 U 7 0 m 0 r c m z rl z 4 m w 9 cl rl e( c( 0 z mR. L. FROST AND D. W. JAMES 323 1 7. 6 x c G .3 4 . 0 * .- X 1.00 1.02 1.04 1.06 1.08 1.1 0 wavenumber/ 1 O3 cm-' l b l wavenumber/103 cm-' FIG. 3.-Band-component analysis of the A ; symmetric stretching vibration band of NO; in aqueous solutions of NaNO,: ( a ) 2 mol dmP3 solution, (6) 4 mol dm-3 solution.The differences in the parameters for the component bands we assign to the four solution species may be rationalised in terms of possible solution interactions. The band at 1047.6cm-' is highly Lorentzian with a halfwidth of 3.4 cm-I. This corresponds to a value of z, of 1.56 ps. In liquids where the relaxation is controlled by collisional interactions the value of z, lies between 2 and 4.5 ps.21q27 It could be suggested that the measured bandwidth is instrument limited. In solutions of NaNO, in [2H,]DMS0 we have measured the width of this band as 1.65 cm-l (z, = 3.2 ps).3232 I 0 N-ION-SO L VENT INTERACT IONS 0.8 i 0.2 \ 1050.0 I \ I I 1070.0 1047.6 1 2 3 1 5 6 7 8 cation concentration/mol ~ i r n - ~ FIG. 4.-Variation in component-band intensities with concentration.It is this latter value which we feel is an indication of the relaxation in which hydrogen-bonded interactions play little part, i.e. the collisions are essentially elastic. In aqueous solution the anion is hydrated through a hydrogen-bonding mechanism and this will change the nature of the ‘collisions’. The reduction in the relaxation time in aqueous solution means that the dephasing process is more efficient and this may occur through a symmetry perturbation when the nitrate ion is aquated. This suggestion supports the previous observation that even in dilute aqueous solution the antisymmetric stretching vibration gives rise to a band showing loss of degeneracy. Because solution in [2H,]DMS0 yields a very different bandwidth it is probable that in the hydration process the symmetry of the nitrate ion is perturbed either through a solvent cage of different symmetry or through preferential solvation at one or more of the oxygen atoms on the nitrate ion.The band positions for all associated species in the solutions studied are at higher energy than the band for the dilute solution species. This shift reflects an increasing coulombic perturbation of the nitrate ion by the cation. There appear to be two mechanisms contributing to the change in energy of the v , band. A symmetrical polarization (space and time averaged) increases the energy whereas an unsymmetrical polarization decreases the energy. This can be understood in terms of changes in the electron distribution and is discussed in detail el~ewhere.~” 32 The position of the band maximum attributed to the contact ion pair species is close to that observed in molten NaN03,6 while the position of the highest energy component assigned to the ion aggregate is close to the value observed for the anhydrous The band attributed to the solvent-separated ion pair has a shape ratio essentially the same, and a half-width only marginally greater than the dilute solution species.This indicates that the relaxation processes are similar for the the two species and so although the formation of the species increases the coulombic field on the nitrate ion it does not lead to an increased symmetry perturbation. This requires that the cation exchanges rapidly and the coulombic influence is averaged over the whole of the hydration shell of the anion.R.L. FROST AND D. W. JAMES 3233 The band attributed to the contact ion pair species shows both a significant increase in Gaussian character and a considerable increase in half-width. The shorter relaxation (z, = 1.18 ps) which this indicates will arise from the greater symmetry perturbation caused by the directional nature of the close approach of the cation. The Gaussian character of the band reflects that the interactions in the associated com- plex are increasingly solid-like (Gaussian). The band associated with the ion aggregate shows all the characteristics expected of a vibrating disordered solid. The linewidth is broad (z, = 0.55 ps), indicating a high level of symmetry perturbation probably due to the dynamic disordered nature of the aggregate.At the same time the band shows appreciable Gaussian character indicating that the vibration is overdamped by the fluctuating field of the environment. As the concentration of salt solution increases, the dynamic continuum of states which characterises the liquid changes, and it may be appropriate to treat the system as a continuum. We feel, however, that there are probably ion-ion and ion-solvent associated species which occupy minima in the potential-energy surface. The analysis which we present is an attempt to characterise these minima and give rational description to the species associated with them. A study of this sort is only acceptable if the premises involved can be applied to a wide range of systems.To this end we present in the following papers an analysis of the Raman spectra of an extensive set of salt systems under a variety of conditions. We thank the Australian Research Grants Committee for grants enabling the purchase of Cary 82 spectrometer system. Dr R. Appleby is thanked for helpful discussions. D. E. Irish and M. H. Brooker, in Advances in Infrared and Raman Spectroscopy, ed. R. J. H. Clark and R. E. Hester (Heyden, London, 1976), vol. 2, p. 212. R. D. Tobias, in The Ramun Eflect, ed. A. Anderson (Marcel Dekker, New York, 1973), vol. 2, p. 405. R. E. Verrall, in Water, A Comprehensive Trearise, ed. F. Franks (Plenum Press, New York, 1973), C. C. Addison, N. Logan, S. C. Wallwork and C. D. Garner, Q. Rev., 1971, 25, 289.D. E. Irish, A. R. Davis and R. A. Plane, J. Chem. Phys., 1969, 50, 2262. G. J. Janz and D. W. James, J . Chem. Phys., 1961,35,739; D. W. James and W. H. Leong, J . Chem. Phys., 1968, 49, 5089. 0. Redlick, Chem. Rev., 1946, 39, 333. ti R. E. Hester, Anal. Chem., 1972, 46, 490R. W. E. L. Grossman, Anal. Chem., 1974, 48, 345R. lo H. G. Hertz, J . Solution Chem., 1973, 2, 239. D. E. Irish, in Physical Chemistry of Organic Solvent Systems, ed. A. K. Covington and T. Dickinson (Plenum Press, London, 1973), p. 433. H. Brintsinger and R. E. Hester, Znorg. Chem., 1966, 5, 980; R. E. Hester and W. E. L. Grossman, Znorg. Chem., 1966, 5, 1308. l 3 K. R. Plowman and J. J. Lagowski, J . Phys. Chem., 1974,78, 143; A. T. Lemley and J. J. Lagowski. J . Phys. Chem., 1974, 78, 708. l4 J. D. Riddell, D. J. Lockwood and D. E. Irish, Can. J . Chem., 1972, 50, 2957. l5 D. E. Irish and A. R. Davis, Can. J . Chem., 1968,46,943; D. E. Irish and G. E. Walrafen, J. Chem. vol. 3, p. 21 1. Phys., 1967, 46, 378. D. W. James and R. L. Frost, Faraday Discuss. Chem. Soc., 1978, 64, 48. M. Koubaa and M. Perrot, C . R. Acad. Sci., Ser. C, 1978 286, 99. T. Kato, J. Unemura and T. Takenaka, Mol. Phys., 1978,36,621; T. Kato and T. Takenaka, Chem. Phys. Lett., 1979, 62, 77. l9 D. E. Irish and T. Jarv, Faraday Discuss. Chem. Soc., 1978, 64, 95. 2o J. E. Griffiths, J . Chem. Phys., 1967, 47, 1836. 21 J. E. Griffiths, in Advances in Raman Spectroscopy, ed. J. P. Mathieu (Heyden, London, 1973), vol. 22 R. L. Frost, R. Appleby, M. T. Carrick and D. W. James, Can. J . Spectrosc., in press. 1, p. 444.3234 I ON-I0 N-S 0 L V E N T INTER ACT I 0 N S 23 D. Sweatman and W. Garrett, unpublished results. 24 R. L. F. Frost and R. Appleby, unpublished results. 25 P. Sampson, Program B.M.D.O7R, in Biomedical Computer Programs, ed. W. J. Dixon (University 26 R. Kubo, in Fluctuations, Relaxation and Resonance in Magnetic Systems, ed. D. T. Haas (Plenum 27 W. G. Rothschild, J . Chem. Phys., 1976, 65, 455. 28 D. W. James, M. T. Carrick and R. L. Frost, J . Raman Spectrosc., in press; D. W. James and R. L. Frost, Can. J. Spectrosc., 1978, 1, 1. 29 R. L. Frost, D. W. James, R. Appleby and R. E. Mayes, J. Phys. Chem., in press. 30 J. E. Griffiths, J . Chem. Phys., 1973, 59, 751. 31 D. W. James and R. L. Frost, Aust. J. Chern., 1982, in press. 32 M. T. Carrick, D. W. James and W. H. Leong, Aust. J. Chem., in press. 33 D. W. James and W. H. Leong, J. Chem. Phys., 1968 49, 5089. of California Press, 1974), p. 387. Press, New York, 1962), p. 27. (PAPER 1 / 1999)

ISSN:0300-9599    

DOI:10.1039/F19827803223

出版商:RSC    

年代:1982

数据来源: RSC